     [file gdb-cfg.texi not included]

This file documents the GNU debugger GDBN.

This is the Tenth Edition, of Debugging with
GDBN: the GNU Source-Level Debugger for GDBN Version GDBVN.

Copyright © 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being ``Free Software'' and ``Free Software Needs Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,'' and with the Back-Cover Texts as in (a) below.

(a) The FSF's Back-Cover Text is: ``You are free to copy and modify this GNU Manual.  Buying copies from GNU Press supports the FSF in developing GNU and promoting software freedom.''

Debugging with GDBN
*******************

This file describes GDBN, the GNU symbolic debugger.

This is the Tenth Edition, for GDBN Version GDBVN.

Copyright (C) 1988-2010 Free Software Foundation, Inc.

This edition of the GDB manual is dedicated to the memory of Fred Fish.  Fred was a long-standing contributor to GDB and to Free software in general.  We will miss him.

Summary of GDBN
***************

The purpose of a debugger such as GDBN is to allow you to see what is going on ``inside'' another program while it executes---or what another program was doing at the moment it crashed.

GDBN can do four main kinds of things (plus other things in support of these) to help you catch bugs in the act:

  * Start your program, specifying anything that might affect its behavior.

  * Make your program stop on specified conditions.

  * Examine what has happened, when your program has stopped.

  * Change things in your program, so you can experiment with correcting the effects of one bug and go on to learn about another.

You can use GDBN to debug programs written in C and C++. For more information, see Supported Languages. For more information, see C and C++.

Support for D is partial.  For information on D, see D.

Support for Modula-2 is partial.  For information on Modula-2, see Modula-2.

Support for OpenCL C is partial.  For information on OpenCL C, see OpenCL C.

Debugging Pascal programs which use sets, subranges, file variables, or nested functions does not currently work.  GDBN does not support entering expressions, printing values, or similar features using Pascal syntax.

GDBN can be used to debug programs written in Fortran, although it may be necessary to refer to some variables with a trailing underscore.

GDBN can be used to debug programs written in Objective-C, using either the Apple/NeXT or the GNU Objective-C runtime.

Free Software
=============

GDBN is free software, protected by the GNU General Public License (GPL).  The GPL gives you the freedom to copy or adapt a licensed program---but every person getting a copy also gets with it the freedom to modify that copy (which means that they must get access to the source code), and the freedom to distribute further copies. Typical software companies use copyrights to limit your freedoms; the Free Software Foundation uses the GPL to preserve these freedoms.

Fundamentally, the General Public License is a license which says that you have these freedoms and that you cannot take these freedoms away from anyone else.

Free Software Needs Free Documentation
======================================

The biggest deficiency in the free software community today is not in the software---it is the lack of good free documentation that we can include with the free software.  Many of our most important programs do not come with free reference manuals and free introductory texts.  Documentation is an essential part of any software package; when an important free software package does not come with a free manual and a free tutorial, that is a major gap.  We have many such gaps today.

Consider Perl, for instance.  The tutorial manuals that people normally use are non-free.  How did this come about?  Because the authors of those manuals published them with restrictive terms---no copying, no modification, source files not available---which exclude them from the free software world.

That wasn't the first time this sort of thing happened, and it was far from the last.  Many times we have heard a GNU user eagerly describe a manual that he is writing, his intended contribution to the community, only to learn that he had ruined everything by signing a publication contract to make it non-free.

Free documentation, like free software, is a matter of freedom, not price.  The problem with the non-free manual is not that publishers charge a price for printed copies---that in itself is fine.  (The Free Software Foundation sells printed copies of manuals, too.)  The problem is the restrictions on the use of the manual.  Free manuals are available in source code form, and give you permission to copy and modify.  Non-free manuals do not allow this.

The criteria of freedom for a free manual are roughly the same as for free software.  Redistribution (including the normal kinds of commercial redistribution) must be permitted, so that the manual can accompany every copy of the program, both on-line and on paper.

Permission for modification of the technical content is crucial too. When people modify the software, adding or changing features, if they are conscientious they will change the manual too---so they can provide accurate and clear documentation for the modified program.  A manual that leaves you no choice but to write a new manual to document a changed version of the program is not really available to our community.

Some kinds of limits on the way modification is handled are acceptable.  For example, requirements to preserve the original author's copyright notice, the distribution terms, or the list of authors, are ok.  It is also no problem to require modified versions to include notice that they were modified.  Even entire sections that may not be deleted or changed are acceptable, as long as they deal with nontechnical topics (like this one).  These kinds of restrictions are acceptable because they don't obstruct the community's normal use of the manual.

However, it must be possible to modify all the technical content of the manual, and then distribute the result in all the usual media, through all the usual channels.  Otherwise, the restrictions obstruct the use of the manual, it is not free, and we need another manual to replace it.

Please spread the word about this issue.  Our community continues to lose manuals to proprietary publishing.  If we spread the word that free software needs free reference manuals and free tutorials, perhaps the next person who wants to contribute by writing documentation will realize, before it is too late, that only free manuals contribute to the free software community.

If you are writing documentation, please insist on publishing it under the GNU Free Documentation License or another free documentation license.  Remember that this decision requires your approval---you don't have to let the publisher decide.  Some commercial publishers will use a free license if you insist, but they will not propose the option; it is up to you to raise the issue and say firmly that this is what you want.  If the publisher you are dealing with refuses, please try other publishers.  If you're not sure whether a proposed license is free, write to licensing@gnu.org.

You can encourage commercial publishers to sell more free, copylefted manuals and tutorials by buying them, and particularly by buying copies from the publishers that paid for their writing or for major improvements.  Meanwhile, try to avoid buying non-free documentation at all.  Check the distribution terms of a manual before you buy it, and insist that whoever seeks your business must respect your freedom. Check the history of the book, and try to reward the publishers that have paid or pay the authors to work on it.

The Free Software Foundation maintains a list of free documentation published by other publishers, at http://www.fsf.org/doc/other-free-books.html.

Contributors to GDBN
====================

Richard Stallman was the original author of GDBN, and of many other GNU programs.  Many others have contributed to its development.  This section attempts to credit major contributors.  One of the virtues of free software is that everyone is free to contribute to it; with regret, we cannot actually acknowledge everyone here.  The file ChangeLog in the GDBN distribution approximates a blow-by-blow account.

Changes much prior to version 2.0 are lost in the mists of time.

     Plea: Additions to this section are particularly welcome.  If you or your friends (or enemies, to be evenhanded) have been unfairly omitted from this list, we would like to add your names!

So that they may not regard their many labors as thankless, we particularly thank those who shepherded GDBN through major releases: Andrew Cagney (releases 6.3, 6.2, 6.1, 6.0, 5.3, 5.2, 5.1 and 5.0); Jim Blandy (release 4.18); Jason Molenda (release 4.17); Stan Shebs (release 4.14); Fred Fish (releases 4.16, 4.15, 4.13, 4.12, 4.11, 4.10, and 4.9); Stu Grossman and John Gilmore (releases 4.8, 4.7, 4.6, 4.5, and 4.4); John Gilmore (releases 4.3, 4.2, 4.1, 4.0, and 3.9); Jim Kingdon (releases 3.5, 3.4, and 3.3); and Randy Smith (releases 3.2, 3.1, and 3.0).

Richard Stallman, assisted at various times by Peter TerMaat, Chris Hanson, and Richard Mlynarik, handled releases through 2.8.

Michael Tiemann is the author of most of the GNU C++ support in GDBN, with significant additional contributions from Per Bothner and Daniel Berlin.  James Clark wrote the GNU C++ demangler.  Early work on C++ was by Peter TerMaat (who also did much general update work leading to release 3.0).

GDBN uses the BFD subroutine library to examine multiple object-file formats; BFD was a joint project of David V. Henkel-Wallace, Rich Pixley, Steve Chamberlain, and John Gilmore.

David Johnson wrote the original COFF support; Pace Willison did the original support for encapsulated COFF.

Brent Benson of Harris Computer Systems contributed DWARF 2 support.

Adam de Boor and Bradley Davis contributed the ISI Optimum V support. Per Bothner, Noboyuki Hikichi, and Alessandro Forin contributed MIPS support. Jean-Daniel Fekete contributed Sun 386i support. Chris Hanson improved the HP9000 support. Noboyuki Hikichi and Tomoyuki Hasei contributed Sony/News OS 3 support. David Johnson contributed Encore Umax support. Jyrki Kuoppala contributed Altos 3068 support. Jeff Law contributed HP PA and SOM support. Keith Packard contributed NS32K support. Doug Rabson contributed Acorn Risc Machine support. Bob Rusk contributed Harris Nighthawk CX-UX support. Chris Smith contributed Convex support (and Fortran debugging). Jonathan Stone contributed Pyramid support. Michael Tiemann contributed SPARC support. Tim Tucker contributed support for the Gould NP1 and Gould Powernode. Pace Willison contributed Intel 386 support. Jay Vosburgh contributed Symmetry support. Marko Mlinar contributed OpenRISC 1000 support.

Andreas Schwab contributed M68K GNU/Linux support.

Rich Schaefer and Peter Schauer helped with support of SunOS shared libraries.

Jay Fenlason and Roland McGrath ensured that GDBN and GAS agree about several machine instruction sets.

Patrick Duval, Ted Goldstein, Vikram Koka and Glenn Engel helped develop remote debugging.  Intel Corporation, Wind River Systems, AMD, and ARM contributed remote debugging modules for the i960, VxWorks, A29K UDI, and RDI targets, respectively.

Brian Fox is the author of the readline libraries providing command-line editing and command history.

Andrew Beers of SUNY Buffalo wrote the language-switching code, the Modula-2 support, and contributed the Languages chapter of this manual.

Fred Fish wrote most of the support for Unix System Vr4. He also enhanced the command-completion support to cover C++ overloaded symbols.

Hitachi America (now Renesas America), Ltd. sponsored the support for H8/300, H8/500, and Super-H processors.

NEC sponsored the support for the v850, Vr4xxx, and Vr5xxx processors.

Mitsubishi (now Renesas) sponsored the support for D10V, D30V, and M32R/D processors.

Toshiba sponsored the support for the TX39 Mips processor.

Matsushita sponsored the support for the MN10200 and MN10300 processors.

Fujitsu sponsored the support for SPARClite and FR30 processors.

Kung Hsu, Jeff Law, and Rick Sladkey added support for hardware watchpoints.

Michael Snyder added support for tracepoints.

Stu Grossman wrote gdbserver.

Jim Kingdon, Peter Schauer, Ian Taylor, and Stu Grossman made nearly innumerable bug fixes and cleanups throughout GDBN.

The following people at the Hewlett-Packard Company contributed support for the PA-RISC 2.0 architecture, HP-UX 10.20, 10.30, and 11.0 (narrow mode), HP's implementation of kernel threads, HP's aC++ compiler, and the Text User Interface (nee Terminal User Interface): Ben Krepp, Richard Title, John Bishop, Susan Macchia, Kathy Mann, Satish Pai, India Paul, Steve Rehrauer, and Elena Zannoni.  Kim Haase provided HP-specific information in this manual.

DJ Delorie ported GDBN to MS-DOS, for the DJGPP project. Robert Hoehne made significant contributions to the DJGPP port.

Cygnus Solutions has sponsored GDBN maintenance and much of its development since 1991.  Cygnus engineers who have worked on GDBN fulltime include Mark Alexander, Jim Blandy, Per Bothner, Kevin Buettner, Edith Epstein, Chris Faylor, Fred Fish, Martin Hunt, Jim Ingham, John Gilmore, Stu Grossman, Kung Hsu, Jim Kingdon, John Metzler, Fernando Nasser, Geoffrey Noer, Dawn Perchik, Rich Pixley, Zdenek Radouch, Keith Seitz, Stan Shebs, David Taylor, and Elena Zannoni.  In addition, Dave Brolley, Ian Carmichael, Steve Chamberlain, Nick Clifton, JT Conklin, Stan Cox, DJ Delorie, Ulrich Drepper, Frank Eigler, Doug Evans, Sean Fagan, David Henkel-Wallace, Richard Henderson, Jeff Holcomb, Jeff Law, Jim Lemke, Tom Lord, Bob Manson, Michael Meissner, Jason Merrill, Catherine Moore, Drew Moseley, Ken Raeburn, Gavin Romig-Koch, Rob Savoye, Jamie Smith, Mike Stump, Ian Taylor, Angela Thomas, Michael Tiemann, Tom Tromey, Ron Unrau, Jim Wilson, and David Zuhn have made contributions both large and small.

Andrew Cagney, Fernando Nasser, and Elena Zannoni, while working for Cygnus Solutions, implemented the original GDB/MI interface.

Jim Blandy added support for preprocessor macros, while working for Red Hat.

Andrew Cagney designed GDBN's architecture vector.  Many people including Andrew Cagney, Stephane Carrez, Randolph Chung, Nick Duffek, Richard Henderson, Mark Kettenis, Grace Sainsbury, Kei Sakamoto, Yoshinori Sato, Michael Snyder, Andreas Schwab, Jason Thorpe, Corinna Vinschen, Ulrich Weigand, and Elena Zannoni, helped with the migration of old architectures to this new framework.

Andrew Cagney completely re-designed and re-implemented GDBN's unwinder framework, this consisting of a fresh new design featuring frame IDs, independent frame sniffers, and the sentinel frame.  Mark Kettenis implemented the DWARF 2 unwinder, Jeff Johnston the libunwind unwinder, and Andrew Cagney the dummy, sentinel, tramp, and trad unwinders.  The architecture-specific changes, each involving a complete rewrite of the architecture's frame code, were carried out by Jim Blandy, Joel Brobecker, Kevin Buettner, Andrew Cagney, Stephane Carrez, Randolph Chung, Orjan Friberg, Richard Henderson, Daniel Jacobowitz, Jeff Johnston, Mark Kettenis, Theodore A. Roth, Kei Sakamoto, Yoshinori Sato, Michael Snyder, Corinna Vinschen, and Ulrich Weigand.

Christian Zankel, Ross Morley, Bob Wilson, and Maxim Grigoriev from Tensilica, Inc. contributed support for Xtensa processors.  Others who have worked on the Xtensa port of GDBN in the past include Steve Tjiang, John Newlin, and Scott Foehner.

Michael Eager and staff of Xilinx, Inc., contributed support for the Xilinx MicroBlaze architecture.

A Sample GDBN Session
*********************

You can use this manual at your leisure to read all about GDBN. However, a handful of commands are enough to get started using the debugger.  This chapter illustrates those commands.

One of the preliminary versions of GNU m4 (a generic macro processor) exhibits the following bug: sometimes, when we change its quote strings from the default, the commands used to capture one macro definition within another stop working.  In the following short m4 session, we define a macro foo which expands to 0000; we then use the m4 built-in defn to define bar as the same thing.  However, when we change the open quote string to <QUOTE> and the close quote string to <UNQUOTE>, the same procedure fails to define a new synonym baz:

     $ cd gnu/m4
     $ ./m4
     define(foo,0000)

     foo
     0000
     define(bar,defn(`foo'))

     bar
     0000
     changequote(<QUOTE>,<UNQUOTE>)

     define(baz,defn(<QUOTE>foo<UNQUOTE>))
     baz
     Ctrl-d
     m4: End of input: 0: fatal error: EOF in string

Let us use GDBN to try to see what is going on.

     $ GDBP m4
     GDBN is free software and you are welcome to distribute copies
      of it under certain conditions; type "show copying" to see
      the conditions.
     There is absolutely no warranty for GDBN; type "show warranty"
      for details.

     GDBN GDBVN, Copyright 1999 Free Software Foundation, Inc...
     (GDBP)

GDBN reads only enough symbol data to know where to find the rest when needed; as a result, the first prompt comes up very quickly. We now tell GDBN to use a narrower display width than usual, so that examples fit in this manual.

     (GDBP) set width 70

We need to see how the m4 built-in changequote works. Having looked at the source, we know the relevant subroutine is m4_changequote, so we set a breakpoint there with the GDBN break command.

     (GDBP) break m4_changequote
     Breakpoint 1 at 0x62f4: file builtin.c, line 879.

Using the run command, we start m4 running under GDBN control; as long as control does not reach the m4_changequote subroutine, the program runs as usual:

     (GDBP) run
     Starting program: /work/Editorial/gdb/gnu/m4/m4
     define(foo,0000)

     foo
     0000

To trigger the breakpoint, we call changequote.  GDBN suspends execution of m4, displaying information about the context where it stops.

     changequote(<QUOTE>,<UNQUOTE>)

     Breakpoint 1, m4_changequote (argc=3, argv=0x33c70)
         at builtin.c:879
     879         if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3))

Now we use the command n (next) to advance execution to the next line of the current function.

     (GDBP) n
     882         set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\
      : nil,

set_quotes looks like a promising subroutine.  We can go into it by using the command s (step) instead of next. step goes to the next line to be executed in any subroutine, so it steps into set_quotes.

     (GDBP) s
     set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
         at input.c:530
     530         if (lquote != def_lquote)

The display that shows the subroutine where m4 is now suspended (and its arguments) is called a stack frame display.  It shows a summary of the stack.  We can use the backtrace command (which can also be spelled bt), to see where we are in the stack as a whole: the backtrace command displays a stack frame for each active subroutine.

     (GDBP) bt
     #0  set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
         at input.c:530
     #1  0x6344 in m4_changequote (argc=3, argv=0x33c70)
         at builtin.c:882
     #2  0x8174 in expand_macro (sym=0x33320) at macro.c:242
     #3  0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30)
         at macro.c:71
     #4  0x79dc in expand_input () at macro.c:40
     #5  0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195

We step through a few more lines to see what happens.  The first two times, we can use s; the next two times we use n to avoid falling into the xstrdup subroutine.

     (GDBP) s
     0x3b5c  532         if (rquote != def_rquote)
     (GDBP) s
     0x3b80  535         lquote = (lq == nil || *lq == '\0') ?  \
     def_lquote : xstrdup(lq);
     (GDBP) n
     536         rquote = (rq == nil || *rq == '\0') ? def_rquote\
      : xstrdup(rq);
     (GDBP) n
     538         len_lquote = strlen(rquote);

The last line displayed looks a little odd; we can examine the variables lquote and rquote to see if they are in fact the new left and right quotes we specified.  We use the command p (print) to see their values.

     (GDBP) p lquote
     $1 = 0x35d40 "<QUOTE>"
     (GDBP) p rquote
     $2 = 0x35d50 "<UNQUOTE>"

lquote and rquote are indeed the new left and right quotes. To look at some context, we can display ten lines of source surrounding the current line with the l (list) command.

     (GDBP) l
     533             xfree(rquote);
     534
     535         lquote = (lq == nil || *lq == '\0') ? def_lquote\
      : xstrdup (lq);
     536         rquote = (rq == nil || *rq == '\0') ? def_rquote\
      : xstrdup (rq);
     537
     538         len_lquote = strlen(rquote);
     539         len_rquote = strlen(lquote);
     540     }
     541
     542     void

Let us step past the two lines that set len_lquote and len_rquote, and then examine the values of those variables.

     (GDBP) n
     539         len_rquote = strlen(lquote);
     (GDBP) n
     540     }
     (GDBP) p len_lquote
     $3 = 9
     (GDBP) p len_rquote
     $4 = 7

That certainly looks wrong, assuming len_lquote and len_rquote are meant to be the lengths of lquote and rquote respectively.  We can set them to better values using the p command, since it can print the value of any expression---and that expression can include subroutine calls and assignments.

     (GDBP) p len_lquote=strlen(lquote)
     $5 = 7
     (GDBP) p len_rquote=strlen(rquote)
     $6 = 9

Is that enough to fix the problem of using the new quotes with the m4 built-in defn?  We can allow m4 to continue executing with the c (continue) command, and then try the example that caused trouble initially:

     (GDBP) c
     Continuing.

     define(baz,defn(<QUOTE>foo<UNQUOTE>))

     baz
     0000

Success!  The new quotes now work just as well as the default ones.  The problem seems to have been just the two typos defining the wrong lengths.  We allow m4 exit by giving it an EOF as input:

     Ctrl-d
     Program exited normally.

The message Program exited normally. is from GDBN; it indicates m4 has finished executing.  We can end our GDBN session with the GDBN quit command.

     (GDBP) quit

Getting In and Out of GDBN
**************************

This chapter discusses how to start GDBN, and how to get out of it. The essentials are:

  * type GDBP to start GDBN.

  * type quit or Ctrl-d to exit.

Invoking GDBN
=============

Invoke GDBN by running the program GDBP.  Once started, GDBN reads commands from the terminal until you tell it to exit.

You can also run GDBP with a variety of arguments and options, to specify more of your debugging environment at the outset.

The command-line options described here are designed to cover a variety of situations; in some environments, some of these options may effectively be unavailable.

The most usual way to start GDBN is with one argument, specifying an executable program:

     GDBP PROGRAM

You can also start with both an executable program and a core file specified:

     GDBP PROGRAM CORE

You can, instead, specify a process ID as a second argument, if you want to debug a running process:

     GDBP PROGRAM 1234

would attach GDBN to process 1234 (unless you also have a file named 1234; GDBN does check for a core file first).

Taking advantage of the second command-line argument requires a fairly complete operating system; when you use GDBN as a remote debugger attached to a bare board, there may not be any notion of ``process'', and there is often no way to get a core dump.  GDBN will warn you if it is unable to attach or to read core dumps.

You can optionally have GDBP pass any arguments after the executable file to the inferior using --args.  This option stops option processing.

     GDBP --args gcc -O2 -c foo.c

This will cause GDBP to debug gcc, and to set gcc's command-line arguments (see Arguments) to -O2 -c foo.c.

You can run GDBP without printing the front material, which describes GDBN's non-warranty, by specifying -silent:

     GDBP -silent

You can further control how GDBN starts up by using command-line options.  GDBN itself can remind you of the options available.

Type

     GDBP -help

to display all available options and briefly describe their use (GDBP -h is a shorter equivalent).

All options and command line arguments you give are processed in sequential order.  The order makes a difference when the -x option is used.

Choosing Files
--------------

When GDBN starts, it reads any arguments other than options as specifying an executable file and core file (or process ID).  This is the same as if the arguments were specified by the -se and -c (or -p) options respectively.  (GDBN reads the first argument that does not have an associated option flag as equivalent to the -se option followed by that argument; and the second argument that does not have an associated option flag, if any, as equivalent to the -c/-p option followed by that argument.) If the second argument begins with a decimal digit, GDBN will first attempt to attach to it as a process, and if that fails, attempt to open it as a corefile.  If you have a corefile whose name begins with a digit, you can prevent GDBN from treating it as a pid by prefixing it with ./, e.g. ./12345.

If GDBN has not been configured to included core file support, such as for most embedded targets, then it will complain about a second argument and ignore it.

Many options have both long and short forms; both are shown in the following list.  GDBN also recognizes the long forms if you truncate them, so long as enough of the option is present to be unambiguous. (If you prefer, you can flag option arguments with -- rather than -, though we illustrate the more usual convention.)

-symbols FILE
-s FILE
     Read symbol table from file FILE.

-exec FILE
-e FILE
     Use file FILE as the executable file to execute when appropriate, and for examining pure data in conjunction with a core dump.

-se FILE
     Read symbol table from file FILE and use it as the executable file.

-core FILE
-c FILE
     Use file FILE as a core dump to examine.

-pid NUMBER
-p NUMBER
     Connect to process ID NUMBER, as with the attach command.

-command FILE
-x FILE
     Execute commands from file FILE.  The contents of this file is evaluated exactly as the source command would. See Command files.

-eval-command COMMAND
-ex COMMAND
     Execute a single GDBN command.

     This option may be used multiple times to call multiple commands.  It may also be interleaved with -command as required.

          GDBP -ex 'target sim' -ex 'load' \
             -x setbreakpoints -ex 'run' a.out

-directory DIRECTORY
-d DIRECTORY
     Add DIRECTORY to the path to search for source and script files.

-r
-readnow
     Read each symbol file's entire symbol table immediately, rather than the default, which is to read it incrementally as it is needed. This makes startup slower, but makes future operations faster.

Choosing Modes
--------------

You can run GDBN in various alternative modes---for example, in batch mode or quiet mode.

-nx
-n
     Do not execute commands found in any initialization files.  Normally, GDBN executes the commands in these files after all the command options and arguments have been processed.  See Command Files.

-quiet
-silent
-q
     ``Quiet''.  Do not print the introductory and copyright messages.  These messages are also suppressed in batch mode.

-batch
     Run in batch mode.  Exit with status 0 after processing all the command files specified with -x (and all commands from initialization files, if not inhibited with -n).  Exit with nonzero status if an error occurs in executing the GDBN commands in the command files.  Batch mode also disables pagination, sets unlimited terminal width and height see Screen Size, and acts as if set confirm off were in effect (see Messages/Warnings).

     Batch mode may be useful for running GDBN as a filter, for example to download and run a program on another computer; in order to make this more useful, the message

          Program exited normally.

     (which is ordinarily issued whenever a program running under GDBN control terminates) is not issued when running in batch mode.

-batch-silent
     Run in batch mode exactly like -batch, but totally silently.  All GDBN output to stdout is prevented (stderr is unaffected).  This is much quieter than -silent and would be useless for an interactive session.

     This is particularly useful when using targets that give Loading section messages, for example.

     Note that targets that give their output via GDBN, as opposed to writing directly to stdout, will also be made silent.

-return-child-result
     The return code from GDBN will be the return code from the child process (the process being debugged), with the following exceptions:

       * GDBN exits abnormally.  E.g., due to an incorrect argument or an internal error.  In this case the exit code is the same as it would have been without -return-child-result.

       * The user quits with an explicit value.  E.g., quit 1.

       * The child process never runs, or is not allowed to terminate, in which case the exit code will be -1.

     This option is useful in conjunction with -batch or -batch-silent, when GDBN is being used as a remote program loader or simulator interface.

-nowindows
-nw
     ``No windows''.  If GDBN comes with a graphical user interface (GUI) built in, then this option tells GDBN to only use the command-line interface.  If no GUI is available, this option has no effect.

-windows
-w
     If GDBN includes a GUI, then this option requires it to be used if possible.

-cd DIRECTORY
     Run GDBN using DIRECTORY as its working directory, instead of the current directory.

-data-directory DIRECTORY
     Run GDBN using DIRECTORY as its data directory. The data directory is where GDBN searches for its auxiliary files.  See Data Files.

-fullname
-f
     GNU Emacs sets this option when it runs GDBN as a subprocess.  It tells GDBN to output the full file name and line number in a standard, recognizable fashion each time a stack frame is displayed (which includes each time your program stops).  This recognizable format looks like two \032 characters, followed by the file name, line number and character position separated by colons, and a newline.  The Emacs-to-GDBN interface program uses the two \032 characters as a signal to display the source code for the frame.

-epoch
     The Epoch Emacs-GDBN interface sets this option when it runs GDBN as a subprocess.  It tells GDBN to modify its print routines so as to allow Epoch to display values of expressions in a separate window.

-annotate LEVEL
     This option sets the annotation level inside GDBN.  Its effect is identical to using set annotate LEVEL (see Annotations).  The annotation LEVEL controls how much information GDBN prints together with its prompt, values of expressions, source lines, and other types of output.  Level 0 is the normal, level 1 is for use when GDBN is run as a subprocess of GNU Emacs, level 3 is the maximum annotation suitable for programs that control GDBN, and level 2 has been deprecated.

     The annotation mechanism has largely been superseded by GDB/MI (see GDB/MI).

--args
     Change interpretation of command line so that arguments following the executable file are passed as command line arguments to the inferior. This option stops option processing.

-baud BPS
-b BPS
     Set the line speed (baud rate or bits per second) of any serial interface used by GDBN for remote debugging.

-l TIMEOUT
     Set the timeout (in seconds) of any communication used by GDBN for remote debugging.

-tty DEVICE
-t DEVICE
     Run using DEVICE for your program's standard input and output.

-tui
     Activate the Text User Interface when starting.  The Text User Interface manages several text windows on the terminal, showing source, assembly, registers and GDBN command outputs (see GDBN Text User Interface).  Do not use this option if you run GDBN from Emacs (see Using GDBN under GNU Emacs).

-interpreter INTERP
     Use the interpreter INTERP for interface with the controlling program or device.  This option is meant to be set by programs which communicate with GDBN using it as a back end. See Command Interpreters.

     --interpreter=mi (or --interpreter=mi2) causes GDBN to use the GDB/MI interface (see The GDB/MI Interface) included since GDBN version 6.0.  The previous GDB/MI interface, included in GDBN version 5.3 and selected with --interpreter=mi1, is deprecated.  Earlier GDB/MI interfaces are no longer supported.

-write
     Open the executable and core files for both reading and writing.  This is equivalent to the set write on command inside GDBN (see Patching).

-statistics
     This option causes GDBN to print statistics about time and memory usage after it completes each command and returns to the prompt.

-version
     This option causes GDBN to print its version number and no-warranty blurb, and exit.

What GDBN Does During Startup
-----------------------------

Here's the description of what GDBN does during session startup:

 1. Sets up the command interpreter as specified by the command line (see interpreter).

 2. Reads the system-wide init file (if --with-system-gdbinit was used when building GDBN; see System-wide configuration and settings) and executes all the commands in that file.

 3. Reads the init file (if any) in your home directory (On DOS/Windows systems, the home directory is the one pointed to by the HOME environment variable.) and executes all the commands in that file.

 4. Processes command line options and operands.

 5. Reads and executes the commands from init file (if any) in the current working directory.  This is only done if the current directory is different from your home directory.  Thus, you can have more than one init file, one generic in your home directory, and another, specific to the program you are debugging, in the directory where you invoke GDBN.

 6. If the command line specified a program to debug, or a process to attach to, or a core file, GDBN loads any auto-loaded scripts provided for the program or for its loaded shared libraries. See Auto-loading.

     If you wish to disable the auto-loading during startup, you must do something like the following:

          $ gdb -ex "set auto-load-scripts off" -ex "file myprogram"

     The following does not work because the auto-loading is turned off too late:

          $ gdb -ex "set auto-load-scripts off" myprogram

 7. Reads command files specified by the -x option.  See Command Files, for more details about GDBN command files.

 8. Reads the command history recorded in the history file. See Command History, for more details about the command history and the files where GDBN records it.

Init files use the same syntax as command files (see Command Files) and are processed by GDBN in the same way.  The init file in your home directory can set options (such as set complaints) that affect subsequent processing of command line options and operands.  Init files are not executed if you use the -nx option (see Choosing Modes).

To display the list of init files loaded by gdb at startup, you can use gdb --help.

The GDBN init files are normally called .gdbinit. The DJGPP port of GDBN uses the name gdb.ini, due to the limitations of file names imposed by DOS filesystems.  The Windows ports of GDBN use the standard name, but if they find a gdb.ini file, they warn you about that and suggest to rename the file to the standard name.

Quitting GDBN
=============

quit [EXPRESSION]
q
     To exit GDBN, use the quit command (abbreviated q), or type an end-of-file character (usually Ctrl-d).  If you do not supply EXPRESSION, GDBN will terminate normally; otherwise it will terminate using the result of EXPRESSION as the error code.

An interrupt (often Ctrl-c) does not exit from GDBN, but rather terminates the action of any GDBN command that is in progress and returns to GDBN command level.  It is safe to type the interrupt character at any time because GDBN does not allow it to take effect until a time when it is safe.

If you have been using GDBN to control an attached process or device, you can release it with the detach command (see Debugging an Already-running Process).

Shell Commands
==============

If you need to execute occasional shell commands during your debugging session, there is no need to leave or suspend GDBN; you can just use the shell command.

shell COMMAND-STRING
!COMMAND-STRING
     Invoke a standard shell to execute COMMAND-STRING. Note that no space is needed between ! and COMMAND-STRING. If it exists, the environment variable SHELL determines which shell to run.  Otherwise GDBN uses the default shell (/bin/sh on Unix systems, COMMAND.COM on MS-DOS, etc.).

The utility make is often needed in development environments. You do not have to use the shell command for this purpose in GDBN:

make MAKE-ARGS
     Execute the make program with the specified arguments.  This is equivalent to shell make MAKE-ARGS.

Logging Output
==============

You may want to save the output of GDBN commands to a file. There are several commands to control GDBN's logging.

set logging on
     Enable logging.

set logging off
     Disable logging.

set logging file FILE
     Change the name of the current logfile.  The default logfile is gdb.txt.

set logging overwrite [on|off]
     By default, GDBN will append to the logfile.  Set overwrite if you want set logging on to overwrite the logfile instead.

set logging redirect [on|off]
     By default, GDBN output will go to both the terminal and the logfile. Set redirect if you want output to go only to the log file.

show logging
     Show the current values of the logging settings.

GDBN Commands
*************

You can abbreviate a GDBN command to the first few letters of the command name, if that abbreviation is unambiguous; and you can repeat certain GDBN commands by typing just RET.  You can also use the TAB key to get GDBN to fill out the rest of a word in a command (or to show you the alternatives available, if there is more than one possibility).

Command Syntax
==============

A GDBN command is a single line of input.  There is no limit on how long it can be.  It starts with a command name, which is followed by arguments whose meaning depends on the command name.  For example, the command step accepts an argument which is the number of times to step, as in step 5.  You can also use the step command with no arguments.  Some commands do not allow any arguments.

GDBN command names may always be truncated if that abbreviation is unambiguous.  Other possible command abbreviations are listed in the documentation for individual commands.  In some cases, even ambiguous abbreviations are allowed; for example, s is specially defined as equivalent to step even though there are other commands whose names start with s.  You can test abbreviations by using them as arguments to the help command.

A blank line as input to GDBN (typing just RET) means to repeat the previous command.  Certain commands (for example, run) will not repeat this way; these are commands whose unintentional repetition might cause trouble and which you are unlikely to want to repeat.  User-defined commands can disable this feature; see dont-repeat.

The list and x commands, when you repeat them with RET, construct new arguments rather than repeating exactly as typed.  This permits easy scanning of source or memory.

GDBN can also use RET in another way: to partition lengthy output, in a way similar to the common utility more (see Screen Size).  Since it is easy to press one RET too many in this situation, GDBN disables command repetition after any command that generates this sort of display.

Any text from a # to the end of the line is a comment; it does nothing.  This is useful mainly in command files (see Command Files).

The Ctrl-o binding is useful for repeating a complex sequence of commands.  This command accepts the current line, like RET, and then fetches the next line relative to the current line from the history for editing.

Command Completion
==================

GDBN can fill in the rest of a word in a command for you, if there is only one possibility; it can also show you what the valid possibilities are for the next word in a command, at any time.  This works for GDBN commands, GDBN subcommands, and the names of symbols in your program.

Press the TAB key whenever you want GDBN to fill out the rest of a word.  If there is only one possibility, GDBN fills in the word, and waits for you to finish the command (or press RET to enter it).  For example, if you type

     (GDBP) info bre TAB

GDBN fills in the rest of the word breakpoints, since that is the only info subcommand beginning with bre:

     (GDBP) info breakpoints

You can either press RET at this point, to run the info breakpoints command, or backspace and enter something else, if breakpoints does not look like the command you expected.  (If you were sure you wanted info breakpoints in the first place, you might as well just type RET immediately after info bre, to exploit command abbreviations rather than command completion).

If there is more than one possibility for the next word when you press TAB, GDBN sounds a bell.  You can either supply more characters and try again, or just press TAB a second time; GDBN displays all the possible completions for that word.  For example, you might want to set a breakpoint on a subroutine whose name begins with make_, but when you type b make_TAB GDBN just sounds the bell.  Typing TAB again displays all the function names in your program that begin with those characters, for example:

     (GDBP) b make_ TAB
     GDBN sounds bell; press TAB again, to see:
     make_a_section_from_file     make_environ
     make_abs_section             make_function_type
     make_blockvector             make_pointer_type
     make_cleanup                 make_reference_type
     make_command                 make_symbol_completion_list
     (GDBP) b make_

After displaying the available possibilities, GDBN copies your partial input (b make_ in the example) so you can finish the command.

If you just want to see the list of alternatives in the first place, you can press M-? rather than pressing TAB twice.  M-? means META ?.  You can type this either by holding down a key designated as the META shift on your keyboard (if there is one) while typing ?, or as ESC followed by ?.

Sometimes the string you need, while logically a ``word'', may contain parentheses or other characters that GDBN normally excludes from its notion of a word.  To permit word completion to work in this situation, you may enclose words in ' (single quote marks) in GDBN commands.

The most likely situation where you might need this is in typing the name of a C++ function.  This is because C++ allows function overloading (multiple definitions of the same function, distinguished by argument type).  For example, when you want to set a breakpoint you may need to distinguish whether you mean the version of name that takes an int parameter, name(int), or the version that takes a float parameter, name(float).  To use the word-completion facilities in this situation, type a single quote ' at the beginning of the function name.  This alerts GDBN that it may need to consider more information than usual when you press TAB or M-? to request word completion:

     (GDBP) b 'bubble( M-?
     bubble(double,double)    bubble(int,int)
     (GDBP) b 'bubble(

In some cases, GDBN can tell that completing a name requires using quotes.  When this happens, GDBN inserts the quote for you (while completing as much as it can) if you do not type the quote in the first place:

     (GDBP) b bub TAB
     GDBN alters your input line to the following, and rings a bell:
     (GDBP) b 'bubble(

In general, GDBN can tell that a quote is needed (and inserts it) if you have not yet started typing the argument list when you ask for completion on an overloaded symbol.

For more information about overloaded functions, see C++ Expressions.  You can use the command set overload-resolution off to disable overload resolution; see GDBN Features for C++.

When completing in an expression which looks up a field in a structure, GDBN also tries (The completer can be
confused by certain kinds of invalid expressions.  Also, it only
examines the static type of the expression, not the dynamic type.) to limit completions to the field names available in the type of the left-hand-side:

     (GDBP) p gdb_stdout.M-?
     magic                to_fputs             to_rewind
     to_data              to_isatty            to_write
     to_delete            to_put               to_write_async_safe
     to_flush             to_read

This is because the gdb_stdout is a variable of the type struct ui_file that is defined in GDBN sources as follows:

     struct ui_file
     {
        int *magic;
        ui_file_flush_ftype *to_flush;
        ui_file_write_ftype *to_write;
        ui_file_write_async_safe_ftype *to_write_async_safe;
        ui_file_fputs_ftype *to_fputs;
        ui_file_read_ftype *to_read;
        ui_file_delete_ftype *to_delete;
        ui_file_isatty_ftype *to_isatty;
        ui_file_rewind_ftype *to_rewind;
        ui_file_put_ftype *to_put;
        void *to_data;
     }

Getting Help
============

You can always ask GDBN itself for information on its commands, using the command help.

help
h
     You can use help (abbreviated h) with no arguments to display a short list of named classes of commands:

          (GDBP) help
          List of classes of commands:

          aliases -- Aliases of other commands
          breakpoints -- Making program stop at certain points
          data -- Examining data
          files -- Specifying and examining files
          internals -- Maintenance commands
          obscure -- Obscure features
          running -- Running the program
          stack -- Examining the stack
          status -- Status inquiries
          support -- Support facilities
          tracepoints -- Tracing of program execution without
                         stopping the program
          user-defined -- User-defined commands

          Type "help" followed by a class name for a list of
          commands in that class.
          Type "help" followed by command name for full
          documentation.
          Command name abbreviations are allowed if unambiguous.
          (GDBP)

help CLASS
     Using one of the general help classes as an argument, you can get a list of the individual commands in that class.  For example, here is the help display for the class status:

          (GDBP) help status
          Status inquiries.

          List of commands:

          info -- Generic command for showing things
                  about the program being debugged
          show -- Generic command for showing things
                  about the debugger

          Type "help" followed by command name for full
          documentation.
          Command name abbreviations are allowed if unambiguous.
          (GDBP)

help COMMAND
     With a command name as help argument, GDBN displays a short paragraph on how to use that command.

apropos ARGS
     The apropos command searches through all of the GDBN commands, and their documentation, for the regular expression specified in ARGS.  It prints out all matches found.  For example:

          apropos reload

     results in:

          set symbol-reloading -- Set dynamic symbol table reloading
                                  multiple times in one run
          show symbol-reloading -- Show dynamic symbol table reloading
                                  multiple times in one run

complete ARGS
     The complete ARGS command lists all the possible completions for the beginning of a command.  Use ARGS to specify the beginning of the command you want completed.  For example:

          complete i

     results in:

          if
          ignore
          info
          inspect

     This is intended for use by GNU Emacs.

In addition to help, you can use the GDBN commands info and show to inquire about the state of your program, or the state of GDBN itself.  Each command supports many topics of inquiry; this manual introduces each of them in the appropriate context.  The listings under info and under show in the Index point to all the sub-commands.  See Index.

info
     This command (abbreviated i) is for describing the state of your program.  For example, you can show the arguments passed to a function with info args, list the registers currently in use with info registers, or list the breakpoints you have set with info breakpoints. You can get a complete list of the info sub-commands with help info.

set
     You can assign the result of an expression to an environment variable with set.  For example, you can set the GDBN prompt to a $-sign with set prompt $.

show
     In contrast to info, show is for describing the state of GDBN itself. You can change most of the things you can show, by using the related command set; for example, you can control what number system is used for displays with set radix, or simply inquire which is currently in use with show radix.

     To display all the settable parameters and their current values, you can use show with no arguments; you may also use info set.  Both commands produce the same display.

Here are three miscellaneous show subcommands, all of which are exceptional in lacking corresponding set commands:

show version
     Show what version of GDBN is running.  You should include this information in GDBN bug-reports.  If multiple versions of GDBN are in use at your site, you may need to determine which version of GDBN you are running; as GDBN evolves, new commands are introduced, and old ones may wither away.  Also, many system vendors ship variant versions of GDBN, and there are variant versions of GDBN in GNU/Linux distributions as well. The version number is the same as the one announced when you start GDBN.

show copying
info copying
     Display information about permission for copying GDBN.

show warranty
info warranty
     Display the GNU ``NO WARRANTY'' statement, or a warranty, if your version of GDBN comes with one.

Running Programs Under GDBN
***************************

When you run a program under GDBN, you must first generate debugging information when you compile it.

You may start GDBN with its arguments, if any, in an environment of your choice.  If you are doing native debugging, you may redirect your program's input and output, debug an already running process, or kill a child process.

Compiling for Debugging
=======================

In order to debug a program effectively, you need to generate debugging information when you compile it.  This debugging information is stored in the object file; it describes the data type of each variable or function and the correspondence between source line numbers and addresses in the executable code.

To request debugging information, specify the -g option when you run the compiler.

Programs that are to be shipped to your customers are compiled with optimizations, using the -O compiler option.  However, some compilers are unable to handle the -g and -O options together.  Using those compilers, you cannot generate optimized executables containing debugging information.

NGCC, the GNU C/C++ compiler, supports -g with or without -O, making it possible to debug optimized code.  We recommend that you always use -g whenever you compile a program.  You may think your program is correct, but there is no sense in pushing your luck.  For more information, see Optimized Code.

Older versions of the GNU C compiler permitted a variant option -gg for debugging information.  GDBN no longer supports this format; if your GNU C compiler has this option, do not use it.

GDBN knows about preprocessor macros and can show you their expansion (see Macros).  Most compilers do not include information about preprocessor macros in the debugging information if you specify the -g flag alone.  Version 3.1 and later of NGCC, the GNU C compiler, provides macro information if you are using the DWARF debugging format, and specify the option -g3.

See Options for Debugging Your Program or GCC (Using the GNU Compiler Collection (GCC)), for more information on NGCC options affecting debug information.

You will have the best debugging experience if you use the latest version of the DWARF debugging format that your compiler supports. DWARF is currently the most expressive and best supported debugging format in GDBN.

Starting your Program
=====================

run
r
     Use the run command to start your program under GDBN. You must first specify the program name (except on VxWorks) with an argument to GDBN (see Getting In and Out of GDBN), or by using the file or exec-file command (see Commands to Specify Files).

If you are running your program in an execution environment that supports processes, run creates an inferior process and makes that process run your program.  In some environments without processes, run jumps to the start of your program.  Other targets, like remote, are always running.  If you get an error message like this one:

     The "remote" target does not support "run".
     Try "help target" or "continue".

then use continue to run your program.  You may need load first (see load).

The execution of a program is affected by certain information it receives from its superior.  GDBN provides ways to specify this information, which you must do before starting your program.  (You can change it after starting your program, but such changes only affect your program the next time you start it.)  This information may be divided into four categories:

The arguments.
     Specify the arguments to give your program as the arguments of the run command.  If a shell is available on your target, the shell is used to pass the arguments, so that you may use normal conventions (such as wildcard expansion or variable substitution) in describing the arguments. In Unix systems, you can control which shell is used with the SHELL environment variable. See Your Program's Arguments.

The environment.
     Your program normally inherits its environment from GDBN, but you can use the GDBN commands set environment and unset
     environment to change parts of the environment that affect your program.  See Your Program's Environment.

The working directory.
     Your program inherits its working directory from GDBN.  You can set the GDBN working directory with the cd command in GDBN. See Your Program's Working Directory.

The standard input and output.
     Your program normally uses the same device for standard input and standard output as GDBN is using.  You can redirect input and output in the run command line, or you can use the tty command to set a different device for your program. See Your Program's Input and Output.

     Warning: While input and output redirection work, you cannot use pipes to pass the output of the program you are debugging to another program; if you attempt this, GDBN is likely to wind up debugging the wrong program.

When you issue the run command, your program begins to execute immediately.  See Stopping and Continuing, for discussion of how to arrange for your program to stop.  Once your program has stopped, you may call functions in your program, using the print or call commands.  See Examining Data.

If the modification time of your symbol file has changed since the last time GDBN read its symbols, GDBN discards its symbol table, and reads it again.  When it does this, GDBN tries to retain your current breakpoints.

start
     The name of the main procedure can vary from language to language. With C or C++, the main procedure name is always main, but other languages such as Ada do not require a specific name for their main procedure.  The debugger provides a convenient way to start the execution of the program and to stop at the beginning of the main procedure, depending on the language used.

     The start command does the equivalent of setting a temporary breakpoint at the beginning of the main procedure and then invoking the run command.

     Some programs contain an elaboration phase where some startup code is executed before the main procedure is called.  This depends on the languages used to write your program.  In C++, for instance, constructors for static and global objects are executed before main is called.  It is therefore possible that the debugger stops before reaching the main procedure.  However, the temporary breakpoint will remain to halt execution.

     Specify the arguments to give to your program as arguments to the start command.  These arguments will be given verbatim to the underlying run command.  Note that the same arguments will be reused if no argument is provided during subsequent calls to start or run.

     It is sometimes necessary to debug the program during elaboration.  In these cases, using the start command would stop the execution of your program too late, as the program would have already completed the elaboration phase.  Under these circumstances, insert breakpoints in your elaboration code before running your program.

set exec-wrapper WRAPPER
show exec-wrapper
unset exec-wrapper
     When exec-wrapper is set, the specified wrapper is used to launch programs for debugging.  GDBN starts your program with a shell command of the form exec WRAPPER PROGRAM.  Quoting is added to PROGRAM and its arguments, but not to WRAPPER, so you should add quotes if appropriate for your shell.  The wrapper runs until it executes your program, and then GDBN takes control.

     You can use any program that eventually calls execve with its arguments as a wrapper.  Several standard Unix utilities do this, e.g. env and nohup.  Any Unix shell script ending with exec "$@" will also work.

     For example, you can use env to pass an environment variable to the debugged program, without setting the variable in your shell's environment:

          (GDBP) set exec-wrapper env 'LD_PRELOAD=libtest.so'
          (GDBP) run

     This command is available when debugging locally on most targets, excluding DJGPP, Cygwin, MS Windows, and QNX Neutrino.

set disable-randomization
set disable-randomization on
     This option (enabled by default in GDBN) will turn off the native randomization of the virtual address space of the started program.  This option is useful for multiple debugging sessions to make the execution better reproducible and memory addresses reusable across debugging sessions.

     This feature is implemented only on certain targets, including GNU/Linux. On GNU/Linux you can get the same behavior using

          (GDBP) set exec-wrapper setarch `uname -m` -R

set disable-randomization off
     Leave the behavior of the started executable unchanged.  Some bugs rear their ugly heads only when the program is loaded at certain addresses.  If your bug disappears when you run the program under GDBN, that might be because GDBN by default disables the address randomization on platforms, such as GNU/Linux, which do that for stand-alone programs.  Use set disable-randomization off to try to reproduce such elusive bugs.

     On targets where it is available, virtual address space randomization protects the programs against certain kinds of security attacks.  In these cases the attacker needs to know the exact location of a concrete executable code.  Randomizing its location makes it impossible to inject jumps misusing a code at its expected addresses.

     Prelinking shared libraries provides a startup performance advantage but it makes addresses in these libraries predictable for privileged processes by having just unprivileged access at the target system.  Reading the shared library binary gives enough information for assembling the malicious code misusing it.  Still even a prelinked shared library can get loaded at a new random address just requiring the regular relocation process during the startup.  Shared libraries not already prelinked are always loaded at a randomly chosen address.

     Position independent executables (PIE) contain position independent code similar to the shared libraries and therefore such executables get loaded at a randomly chosen address upon startup.  PIE executables always load even already prelinked shared libraries at a random address.  You can build such executable using gcc -fPIE -pie.

     Heap (malloc storage), stack and custom mmap areas are always placed randomly (as long as the randomization is enabled).

show disable-randomization
     Show the current setting of the explicit disable of the native randomization of the virtual address space of the started program.

Your Program's Arguments
========================

The arguments to your program can be specified by the arguments of the run command. They are passed to a shell, which expands wildcard characters and performs redirection of I/O, and thence to your program.  Your SHELL environment variable (if it exists) specifies what shell GDBN uses.  If you do not define SHELL, GDBN uses the default shell (/bin/sh on Unix).

On non-Unix systems, the program is usually invoked directly by GDBN, which emulates I/O redirection via the appropriate system calls, and the wildcard characters are expanded by the startup code of the program, not by the shell.

run with no arguments uses the same arguments used by the previous run, or those set by the set args command.

set args
     Specify the arguments to be used the next time your program is run.  If set args has no arguments, run executes your program with no arguments.  Once you have run your program with arguments, using set args before the next run is the only way to run it again without arguments.

show args
     Show the arguments to give your program when it is started.

Your Program's Environment
==========================

The environment consists of a set of environment variables and their values.  Environment variables conventionally record such things as your user name, your home directory, your terminal type, and your search path for programs to run.  Usually you set up environment variables with the shell and they are inherited by all the other programs you run.  When debugging, it can be useful to try running your program with a modified environment without having to start GDBN over again.

path DIRECTORY
     Add DIRECTORY to the front of the PATH environment variable (the search path for executables) that will be passed to your program. The value of PATH used by GDBN does not change. You may specify several directory names, separated by whitespace or by a system-dependent separator character (: on Unix, ; on MS-DOS and MS-Windows).  If DIRECTORY is already in the path, it is moved to the front, so it is searched sooner.

     You can use the string $cwd to refer to whatever is the current working directory at the time GDBN searches the path.  If you use . instead, it refers to the directory where you executed the path command.  GDBN replaces . in the DIRECTORY argument (with the current path) before adding DIRECTORY to the search path.

show paths
     Display the list of search paths for executables (the PATH environment variable).

show environment [VARNAME]
     Print the value of environment variable VARNAME to be given to your program when it starts.  If you do not supply VARNAME, print the names and values of all environment variables to be given to your program.  You can abbreviate environment as env.

set environment VARNAME [=VALUE]
     Set environment variable VARNAME to VALUE.  The value changes for your program only, not for GDBN itself.  VALUE may be any string; the values of environment variables are just strings, and any interpretation is supplied by your program itself.  The VALUE parameter is optional; if it is eliminated, the variable is set to a null value.

     For example, this command:

          set env USER = foo

     tells the debugged program, when subsequently run, that its user is named foo.  (The spaces around = are used for clarity here; they are not actually required.)

unset environment VARNAME
     Remove variable VARNAME from the environment to be passed to your program.  This is different from set env VARNAME =; unset environment removes the variable from the environment, rather than assigning it an empty value.

Warning: On Unix systems, GDBN runs your program using the shell indicated by your SHELL environment variable if it exists (or /bin/sh if not).  If your SHELL variable names a shell that runs an initialization file---such as .cshrc for C-shell, or .bashrc for BASH---any variables you set in that file affect your program.  You may wish to move setting of environment variables to files that are only run when you sign on, such as .login or .profile.

Your Program's Working Directory
================================

Each time you start your program with run, it inherits its working directory from the current working directory of GDBN. The GDBN working directory is initially whatever it inherited from its parent process (typically the shell), but you can specify a new working directory in GDBN with the cd command.

The GDBN working directory also serves as a default for the commands that specify files for GDBN to operate on.  See Commands to Specify Files.

cd DIRECTORY
     Set the GDBN working directory to DIRECTORY.

pwd
     Print the GDBN working directory.

It is generally impossible to find the current working directory of the process being debugged (since a program can change its directory during its run).  If you work on a system where GDBN is configured with the /proc support, you can use the info proc command (see SVR4 Process Information) to find out the current working directory of the debuggee.

Your Program's Input and Output
===============================

By default, the program you run under GDBN does input and output to the same terminal that GDBN uses.  GDBN switches the terminal to its own terminal modes to interact with you, but it records the terminal modes your program was using and switches back to them when you continue running your program.

info terminal
     Displays information recorded by GDBN about the terminal modes your program is using.

You can redirect your program's input and/or output using shell redirection with the run command.  For example,

     run > outfile

starts your program, diverting its output to the file outfile.

Another way to specify where your program should do input and output is with the tty command.  This command accepts a file name as argument, and causes this file to be the default for future run commands.  It also resets the controlling terminal for the child process, for future run commands.  For example,

     tty /dev/ttyb

directs that processes started with subsequent run commands default to do input and output on the terminal /dev/ttyb and have that as their controlling terminal.

An explicit redirection in run overrides the tty command's effect on the input/output device, but not its effect on the controlling terminal.

When you use the tty command or redirect input in the run command, only the input for your program is affected.  The input for GDBN still comes from your terminal.  tty is an alias for set inferior-tty.

You can use the show inferior-tty command to tell GDBN to display the name of the terminal that will be used for future runs of your program.

set inferior-tty /dev/ttyb
     Set the tty for the program being debugged to /dev/ttyb.

show inferior-tty
     Show the current tty for the program being debugged.

Debugging an Already-running Process
====================================

attach PROCESS-ID
     This command attaches to a running process---one that was started outside GDBN.  (info files shows your active targets.)  The command takes as argument a process ID.  The usual way to find out the PROCESS-ID of a Unix process is with the ps utility, or with the jobs -l shell command.

     attach does not repeat if you press RET a second time after executing the command.

To use attach, your program must be running in an environment which supports processes; for example, attach does not work for programs on bare-board targets that lack an operating system.  You must also have permission to send the process a signal.

When you use attach, the debugger finds the program running in the process first by looking in the current working directory, then (if the program is not found) by using the source file search path (see Specifying Source Directories).  You can also use the file command to load the program.  See Commands to Specify Files.

The first thing GDBN does after arranging to debug the specified process is to stop it.  You can examine and modify an attached process with all the GDBN commands that are ordinarily available when you start processes with run.  You can insert breakpoints; you can step and continue; you can modify storage.  If you would rather the process continue running, you may use the continue command after attaching GDBN to the process.

detach
     When you have finished debugging the attached process, you can use the detach command to release it from GDBN control.  Detaching the process continues its execution.  After the detach command, that process and GDBN become completely independent once more, and you are ready to attach another process or start one with run. detach does not repeat if you press RET again after executing the command.

If you exit GDBN while you have an attached process, you detach that process.  If you use the run command, you kill that process. By default, GDBN asks for confirmation if you try to do either of these things; you can control whether or not you need to confirm by using the set confirm command (see Optional Warnings and Messages).

Killing the Child Process
=========================

kill
     Kill the child process in which your program is running under GDBN.

This command is useful if you wish to debug a core dump instead of a running process.  GDBN ignores any core dump file while your program is running.

On some operating systems, a program cannot be executed outside GDBN while you have breakpoints set on it inside GDBN.  You can use the kill command in this situation to permit running your program outside the debugger.

The kill command is also useful if you wish to recompile and relink your program, since on many systems it is impossible to modify an executable file while it is running in a process.  In this case, when you next type run, GDBN notices that the file has changed, and reads the symbol table again (while trying to preserve your current breakpoint settings).

Debugging Multiple Inferiors and Programs
=========================================

GDBN lets you run and debug multiple programs in a single session.  In addition, GDBN on some systems may let you run several programs simultaneously (otherwise you have to exit from one before starting another).  In the most general case, you can have multiple threads of execution in each of multiple processes, launched from multiple executables.

GDBN represents the state of each program execution with an object called an inferior.  An inferior typically corresponds to a process, but is more general and applies also to targets that do not have processes.  Inferiors may be created before a process runs, and may be retained after a process exits.  Inferiors have unique identifiers that are different from process ids.  Usually each inferior will also have its own distinct address space, although some embedded targets may have several inferiors running in different parts of a single address space.  Each inferior may in turn have multiple threads running in it.

To find out what inferiors exist at any moment, use info inferiors:

info inferiors
     Print a list of all inferiors currently being managed by GDBN.

     GDBN displays for each inferior (in this order):

      1. the inferior number assigned by GDBN

      2. the target system's inferior identifier

      3. the name of the executable the inferior is running.

     An asterisk * preceding the GDBN inferior number indicates the current inferior.

     For example,

     (GDBP) info inferiors
       Num  Description       Executable
       2    process 2307      hello
     * 1    process 3401      goodbye

To switch focus between inferiors, use the inferior command:

inferior INFNO
     Make inferior number INFNO the current inferior.  The argument INFNO is the inferior number assigned by GDBN, as shown in the first field of the info inferiors display.

You can get multiple executables into a debugging session via the add-inferior and clone-inferior commands.  On some systems GDBN can add inferiors to the debug session automatically by following calls to fork and exec.  To remove inferiors from the debugging session use the remove-inferiors command.

add-inferior [ -copies N ] [ -exec EXECUTABLE ]
     Adds N inferiors to be run using EXECUTABLE as the executable.  N defaults to 1.  If no executable is specified, the inferiors begins empty, with no program.  You can still assign or change the program assigned to the inferior at any time by using the file command with the executable name as its argument.

clone-inferior [ -copies N ] [ INFNO ]
     Adds N inferiors ready to execute the same program as inferior INFNO.  N defaults to 1.  INFNO defaults to the number of the current inferior.  This is a convenient command when you want to run another instance of the inferior you are debugging.

          (GDBP) info inferiors
            Num  Description       Executable
          * 1    process 29964     helloworld
          (GDBP) clone-inferior
          Added inferior 2.
          1 inferiors added.
          (GDBP) info inferiors
            Num  Description       Executable
            2    <null>            helloworld
          * 1    process 29964     helloworld

     You can now simply switch focus to inferior 2 and run it.

remove-inferiors INFNO...
     Removes the inferior or inferiors INFNO....  It is not possible to remove an inferior that is running with this command.  For those, use the kill or detach command first.

To quit debugging one of the running inferiors that is not the current inferior, you can either detach from it by using the detach inferior command (allowing it to run independently), or kill it using the kill inferiors command:

detach inferior INFNO...
     Detach from the inferior or inferiors identified by GDBN inferior number(s) INFNO....  Note that the inferior's entry still stays on the list of inferiors shown by info inferiors, but its Description will show <null>.

kill inferiors INFNO...
     Kill the inferior or inferiors identified by GDBN inferior number(s) INFNO....  Note that the inferior's entry still stays on the list of inferiors shown by info inferiors, but its Description will show <null>.

After the successful completion of a command such as detach, detach inferiors, kill or kill inferiors, or after a normal process exit, the inferior is still valid and listed with info inferiors, ready to be restarted.

To be notified when inferiors are started or exit under GDBN's control use set print inferior-events:

set print inferior-events
set print inferior-events on
set print inferior-events off
     The set print inferior-events command allows you to enable or disable printing of messages when GDBN notices that new inferiors have started or that inferiors have exited or have been detached.  By default, these messages will not be printed.

show print inferior-events
     Show whether messages will be printed when GDBN detects that inferiors have started, exited or have been detached.

Many commands will work the same with multiple programs as with a single program: e.g., print myglobal will simply display the value of myglobal in the current inferior.

Occasionaly, when debugging GDBN itself, it may be useful to get more info about the relationship of inferiors, programs, address spaces in a debug session.  You can do that with the maint info program-spaces command.

maint info program-spaces
     Print a list of all program spaces currently being managed by GDBN.

     GDBN displays for each program space (in this order):

      1. the program space number assigned by GDBN

      2. the name of the executable loaded into the program space, with e.g., the file command.

     An asterisk * preceding the GDBN program space number indicates the current program space.

     In addition, below each program space line, GDBN prints extra information that isn't suitable to display in tabular form.  For example, the list of inferiors bound to the program space.

          (GDBP) maint info program-spaces
            Id   Executable
            2    goodbye
                  Bound inferiors: ID 1 (process 21561)
          * 1    hello

     Here we can see that no inferior is running the program hello, while process 21561 is running the program goodbye.  On some targets, it is possible that multiple inferiors are bound to the same program space.  The most common example is that of debugging both the parent and child processes of a vfork call.  For example,

          (GDBP) maint info program-spaces
            Id   Executable
          * 1    vfork-test
                  Bound inferiors: ID 2 (process 18050), ID 1 (process 18045)

     Here, both inferior 2 and inferior 1 are running in the same program space as a result of inferior 1 having executed a vfork call.

Debugging Programs with Multiple Threads
========================================

In some operating systems, such as HP-UX and Solaris, a single program may have more than one thread of execution.  The precise semantics of threads differ from one operating system to another, but in general the threads of a single program are akin to multiple processes---except that they share one address space (that is, they can all examine and modify the same variables).  On the other hand, each thread has its own registers and execution stack, and perhaps private memory.

GDBN provides these facilities for debugging multi-thread programs:

  * automatic notification of new threads

  * thread THREADNO, a command to switch among threads

  * info threads, a command to inquire about existing threads

  * thread apply [THREADNO] [ALL] ARGS, a command to apply a command to a list of threads

  * thread-specific breakpoints

  * set print thread-events, which controls printing of messages on thread start and exit.

  * set libthread-db-search-path PATH, which lets the user specify which libthread_db to use if the default choice isn't compatible with the program.

     Warning: These facilities are not yet available on every GDBN configuration where the operating system supports threads. If your GDBN does not support threads, these commands have no effect.  For example, a system without thread support shows no output from info threads, and always rejects the thread command, like this:

          (GDBP) info threads
          (GDBP) thread 1
          Thread ID 1 not known.  Use the "info threads" command to
          see the IDs of currently known threads.

The GDBN thread debugging facility allows you to observe all threads while your program runs---but whenever GDBN takes control, one thread in particular is always the focus of debugging. This thread is called the current thread.  Debugging commands show program information from the perspective of the current thread.

Whenever GDBN detects a new thread in your program, it displays the target system's identification for the thread with a message in the form [New SYSTAG].  SYSTAG is a thread identifier whose form varies depending on the particular system.  For example, on GNU/Linux, you might see

     [New Thread 0x41e02940 (LWP 25582)]

when GDBN notices a new thread.  In contrast, on an SGI system, the SYSTAG is simply something like process 368, with no further qualifier.

For debugging purposes, GDBN associates its own thread number---always a single integer---with each thread in your program.

info threads [ID...]
     Display a summary of all threads currently in your program.  Optional argument ID... is one or more thread ids separated by spaces, and means to print information only about the specified thread or threads. GDBN displays for each thread (in this order):

      1. the thread number assigned by GDBN

      2. the target system's thread identifier (SYSTAG)

      3. the thread's name, if one is known.  A thread can either be named by the user (see thread name, below), or, in some cases, by the program itself.

      4. the current stack frame summary for that thread

     An asterisk * to the left of the GDBN thread number indicates the current thread.

     For example,

     (GDBP) info threads
       Id   Target Id         Frame
       3    process 35 thread 27  0x34e5 in sigpause ()
       2    process 35 thread 23  0x34e5 in sigpause ()
     * 1    process 35 thread 13  main (argc=1, argv=0x7ffffff8)
         at threadtest.c:68

On Solaris, you can display more information about user threads with a Solaris-specific command:

maint info sol-threads
     Display info on Solaris user threads.

thread THREADNO
     Make thread number THREADNO the current thread.  The command argument THREADNO is the internal GDBN thread number, as shown in the first field of the info threads display. GDBN responds by displaying the system identifier of the thread you selected, and its current stack frame summary:

          (GDBP) thread 2
          [Switching to thread 2 (Thread 0xb7fdab70 (LWP 12747))]
          #0  some_function (ignore=0x0) at example.c:8
          8	    printf ("hello\n");

     As with the [New ...] message, the form of the text after Switching to depends on your system's conventions for identifying threads.

     The debugger convenience variable $_thread contains the number of the current thread.  You may find this useful in writing breakpoint conditional expressions, command scripts, and so forth.  See Convenience Variables, for general information on convenience variables.

thread apply [THREADNO | all] COMMAND
     The thread apply command allows you to apply the named COMMAND to one or more threads.  Specify the numbers of the threads that you want affected with the command argument THREADNO.  It can be a single thread number, one of the numbers shown in the first field of the info threads display; or it could be a range of thread numbers, as in 2-4.  To apply a command to all threads, type thread apply all COMMAND.

thread name [NAME]
     This command assigns a name to the current thread.  If no argument is given, any existing user-specified name is removed.  The thread name appears in the info threads display.

     On some systems, such as GNU/Linux, GDBN is able to determine the name of the thread as given by the OS.  On these systems, a name specified with thread name will override the system-give name, and removing the user-specified name will cause GDBN to once again display the system-specified name.

thread find [REGEXP]
     Search for and display thread ids whose name or SYSTAG matches the supplied regular expression.

     As well as being the complement to the thread name command, this command also allows you to identify a thread by its target SYSTAG.  For instance, on GNU/Linux, the target SYSTAG is the LWP id.

          (GDBN) thread find 26688
          Thread 4 has target id 'Thread 0x41e02940 (LWP 26688)'
          (GDBN) info thread 4
            Id   Target Id         Frame
            4    Thread 0x41e02940 (LWP 26688) 0x00000031ca6cd372 in select ()

set print thread-events
set print thread-events on
set print thread-events off
     The set print thread-events command allows you to enable or disable printing of messages when GDBN notices that new threads have started or that threads have exited.  By default, these messages will be printed if detection of these events is supported by the target. Note that these messages cannot be disabled on all targets.

show print thread-events
     Show whether messages will be printed when GDBN detects that threads have started and exited.

See Stopping and Starting Multi-thread Programs, for more information about how GDBN behaves when you stop and start programs with multiple threads.

See Setting Watchpoints, for information about watchpoints in programs with multiple threads.

set libthread-db-search-path [PATH]
     If this variable is set, PATH is a colon-separated list of directories GDBN will use to search for libthread_db. If you omit PATH, libthread-db-search-path will be reset to its default value ($sdir:$pdir on GNU/Linux and Solaris systems). Internally, the default value comes from the LIBTHREAD_DB_SEARCH_PATH macro.

     On GNU/Linux and Solaris systems, GDBN uses a ``helper'' libthread_db library to obtain information about threads in the inferior process.  GDBN will use libthread-db-search-path to find libthread_db.

     A special entry $sdir for libthread-db-search-path refers to the default system directories that are normally searched for loading shared libraries.

     A special entry $pdir for libthread-db-search-path refers to the directory from which libpthread was loaded in the inferior process.

     For any libthread_db library GDBN finds in above directories, GDBN attempts to initialize it with the current inferior process. If this initialization fails (which could happen because of a version mismatch between libthread_db and libpthread), GDBN will unload libthread_db, and continue with the next directory. If none of libthread_db libraries initialize successfully, GDBN will issue a warning and thread debugging will be disabled.

     Setting libthread-db-search-path is currently implemented only on some platforms.

show libthread-db-search-path
     Display current libthread_db search path.

set debug libthread-db
show debug libthread-db
     Turns on or off display of libthread_db-related events. Use 1 to enable, 0 to disable.

Debugging Forks
===============

On most systems, GDBN has no special support for debugging programs which create additional processes using the fork function.  When a program forks, GDBN will continue to debug the parent process and the child process will run unimpeded.  If you have set a breakpoint in any code which the child then executes, the child will get a SIGTRAP signal which (unless it catches the signal) will cause it to terminate.

However, if you want to debug the child process there is a workaround which isn't too painful.  Put a call to sleep in the code which the child process executes after the fork.  It may be useful to sleep only if a certain environment variable is set, or a certain file exists, so that the delay need not occur when you don't want to run GDBN on the child.  While the child is sleeping, use the ps program to get its process ID.  Then tell GDBN (a new invocation of GDBN if you are also debugging the parent process) to attach to the child process (see Attach).  From that point on you can debug the child process just like any other process which you attached to.

On some systems, GDBN provides support for debugging programs that create additional processes using the fork or vfork functions. Currently, the only platforms with this feature are HP-UX (11.x and later only?) and GNU/Linux (kernel version 2.5.60 and later).

By default, when a program forks, GDBN will continue to debug the parent process and the child process will run unimpeded.

If you want to follow the child process instead of the parent process, use the command set follow-fork-mode.

set follow-fork-mode MODE
     Set the debugger response to a program call of fork or vfork.  A call to fork or vfork creates a new process.  The MODE argument can be:

     parent
          The original process is debugged after a fork.  The child process runs unimpeded.  This is the default.

     child
          The new process is debugged after a fork.  The parent process runs unimpeded.

show follow-fork-mode
     Display the current debugger response to a fork or vfork call.

On Linux, if you want to debug both the parent and child processes, use the command set detach-on-fork.

set detach-on-fork MODE
     Tells gdb whether to detach one of the processes after a fork, or retain debugger control over them both.

     on
          The child process (or parent process, depending on the value of follow-fork-mode) will be detached and allowed to run independently.  This is the default.

     off
          Both processes will be held under the control of GDBN. One process (child or parent, depending on the value of follow-fork-mode) is debugged as usual, while the other is held suspended.

show detach-on-fork
     Show whether detach-on-fork mode is on/off.

If you choose to set detach-on-fork mode off, then GDBN will retain control of all forked processes (including nested forks). You can list the forked processes under the control of GDBN by using the info inferiors command, and switch from one fork to another by using the inferior command (see Debugging Multiple Inferiors and Programs).

To quit debugging one of the forked processes, you can either detach from it by using the detach inferiors command (allowing it to run independently), or kill it using the kill inferiors command.  See Debugging Multiple Inferiors and Programs.

If you ask to debug a child process and a vfork is followed by an exec, GDBN executes the new target up to the first breakpoint in the new target.  If you have a breakpoint set on main in your original program, the breakpoint will also be set on the child process's main.

On some systems, when a child process is spawned by vfork, you cannot debug the child or parent until an exec call completes.

If you issue a run command to GDBN after an exec call executes, the new target restarts.  To restart the parent process, use the file command with the parent executable name as its argument.  By default, after an exec call executes, GDBN discards the symbols of the previous executable image. You can change this behaviour with the set follow-exec-mode command.

set follow-exec-mode MODE

     Set debugger response to a program call of exec.  An exec call replaces the program image of a process.

     follow-exec-mode can be:

     new
          GDBN creates a new inferior and rebinds the process to this new inferior.  The program the process was running before the exec call can be restarted afterwards by restarting the original inferior.

          For example:

               (GDBP) info inferiors
               (gdb) info inferior
                 Id   Description   Executable
               * 1    <null>        prog1
               (GDBP) run
               process 12020 is executing new program: prog2
               Program exited normally.
               (GDBP) info inferiors
                 Id   Description   Executable
               * 2    <null>        prog2
                 1    <null>        prog1

     same
          GDBN keeps the process bound to the same inferior.  The new executable image replaces the previous executable loaded in the inferior.  Restarting the inferior after the exec call, with e.g., the run command, restarts the executable the process was running after the exec call.  This is the default mode.

          For example:

               (GDBP) info inferiors
                 Id   Description   Executable
               * 1    <null>        prog1
               (GDBP) run
               process 12020 is executing new program: prog2
               Program exited normally.
               (GDBP) info inferiors
                 Id   Description   Executable
               * 1    <null>        prog2

You can use the catch command to make GDBN stop whenever a fork, vfork, or exec call is made.  See Setting Catchpoints.

Setting a Bookmark to Return to Later
=====================================

On certain operating systems (Currently, only
GNU/Linux.), GDBN is able to save a snapshot of a program's state, called a checkpoint, and come back to it later.

Returning to a checkpoint effectively undoes everything that has happened in the program since the checkpoint was saved.  This includes changes in memory, registers, and even (within some limits) system state.  Effectively, it is like going back in time to the moment when the checkpoint was saved.

Thus, if you're stepping thru a program and you think you're getting close to the point where things go wrong, you can save a checkpoint.  Then, if you accidentally go too far and miss the critical statement, instead of having to restart your program from the beginning, you can just go back to the checkpoint and start again from there.

This can be especially useful if it takes a lot of time or steps to reach the point where you think the bug occurs.

To use the checkpoint/restart method of debugging:

checkpoint
     Save a snapshot of the debugged program's current execution state. The checkpoint command takes no arguments, but each checkpoint is assigned a small integer id, similar to a breakpoint id.

info checkpoints
     List the checkpoints that have been saved in the current debugging session.  For each checkpoint, the following information will be listed:

     Checkpoint ID

     Process ID

     Code Address

     Source line, or label

restart CHECKPOINT-ID
     Restore the program state that was saved as checkpoint number CHECKPOINT-ID.  All program variables, registers, stack frames etc.  will be returned to the values that they had when the checkpoint was saved.  In essence, gdb will ``wind back the clock'' to the point in time when the checkpoint was saved.

     Note that breakpoints, GDBN variables, command history etc. are not affected by restoring a checkpoint.  In general, a checkpoint only restores things that reside in the program being debugged, not in the debugger.

delete checkpoint CHECKPOINT-ID
     Delete the previously-saved checkpoint identified by CHECKPOINT-ID.

Returning to a previously saved checkpoint will restore the user state of the program being debugged, plus a significant subset of the system (OS) state, including file pointers.  It won't ``un-write'' data from a file, but it will rewind the file pointer to the previous location, so that the previously written data can be overwritten.  For files opened in read mode, the pointer will also be restored so that the previously read data can be read again.

Of course, characters that have been sent to a printer (or other external device) cannot be ``snatched back'', and characters received from eg. a serial device can be removed from internal program buffers, but they cannot be ``pushed back'' into the serial pipeline, ready to be received again.  Similarly, the actual contents of files that have been changed cannot be restored (at this time).

However, within those constraints, you actually can ``rewind'' your program to a previously saved point in time, and begin debugging it again --- and you can change the course of events so as to debug a different execution path this time.

Finally, there is one bit of internal program state that will be different when you return to a checkpoint --- the program's process id.  Each checkpoint will have a unique process id (or PID), and each will be different from the program's original PID. If your program has saved a local copy of its process id, this could potentially pose a problem.

A Non-obvious Benefit of Using Checkpoints
------------------------------------------

On some systems such as GNU/Linux, address space randomization is performed on new processes for security reasons.  This makes it difficult or impossible to set a breakpoint, or watchpoint, on an absolute address if you have to restart the program, since the absolute location of a symbol will change from one execution to the next.

A checkpoint, however, is an identical copy of a process. Therefore if you create a checkpoint at (eg.) the start of main, and simply return to that checkpoint instead of restarting the process, you can avoid the effects of address randomization and your symbols will all stay in the same place.

Stopping and Continuing
***********************

The principal purposes of using a debugger are so that you can stop your program before it terminates; or so that, if your program runs into trouble, you can investigate and find out why.

Inside GDBN, your program may stop for any of several reasons, such as a signal, a breakpoint, or reaching a new line after a GDBN command such as step.  You may then examine and change variables, set new breakpoints or remove old ones, and then continue execution.  Usually, the messages shown by GDBN provide ample explanation of the status of your program---but you can also explicitly request this information at any time.

info program
     Display information about the status of your program: whether it is running or not, what process it is, and why it stopped.

Breakpoints, Watchpoints, and Catchpoints
=========================================

A breakpoint makes your program stop whenever a certain point in the program is reached.  For each breakpoint, you can add conditions to control in finer detail whether your program stops.  You can set breakpoints with the break command and its variants (see Setting Breakpoints), to specify the place where your program should stop by line number, function name or exact address in the program.

On some systems, you can set breakpoints in shared libraries before the executable is run.  There is a minor limitation on HP-UX systems: you must wait until the executable is run in order to set breakpoints in shared library routines that are not called directly by the program (for example, routines that are arguments in a pthread_create call).

A watchpoint is a special breakpoint that stops your program when the value of an expression changes.  The expression may be a value of a variable, or it could involve values of one or more variables combined by operators, such as a + b.  This is sometimes called data breakpoints.  You must use a different command to set watchpoints (see Setting Watchpoints), but aside from that, you can manage a watchpoint like any other breakpoint: you enable, disable, and delete both breakpoints and watchpoints using the same commands.

You can arrange to have values from your program displayed automatically whenever GDBN stops at a breakpoint.  See Automatic Display.

A catchpoint is another special breakpoint that stops your program when a certain kind of event occurs, such as the throwing of a C++ exception or the loading of a library.  As with watchpoints, you use a different command to set a catchpoint (see Setting Catchpoints), but aside from that, you can manage a catchpoint like any other breakpoint.  (To stop when your program receives a signal, use the handle command; see Signals.)

GDBN assigns a number to each breakpoint, watchpoint, or catchpoint when you create it; these numbers are successive integers starting with one.  In many of the commands for controlling various features of breakpoints you use the breakpoint number to say which breakpoint you want to change.  Each breakpoint may be enabled or disabled; if disabled, it has no effect on your program until you enable it again.

Some GDBN commands accept a range of breakpoints on which to operate.  A breakpoint range is either a single breakpoint number, like 5, or two such numbers, in increasing order, separated by a hyphen, like 5-7.  When a breakpoint range is given to a command, all breakpoints in that range are operated on.

Setting Breakpoints
-------------------

Breakpoints are set with the break command (abbreviated b).  The debugger convenience variable $bpnum records the number of the breakpoint you've set most recently; see Convenience Variables, for a discussion of what you can do with convenience variables.

break LOCATION
     Set a breakpoint at the given LOCATION, which can specify a function name, a line number, or an address of an instruction. (see Specify Location, for a list of all the possible ways to specify a LOCATION.)  The breakpoint will stop your program just before it executes any of the code in the specified LOCATION.

     When using source languages that permit overloading of symbols, such as C++, a function name may refer to more than one possible place to break. See Ambiguous Expressions, for a discussion of that situation.

     It is also possible to insert a breakpoint that will stop the program only if a specific thread (see Thread-Specific Breakpoints) or a specific task (see Ada Tasks) hits that breakpoint.

break
     When called without any arguments, break sets a breakpoint at the next instruction to be executed in the selected stack frame (see Examining the Stack).  In any selected frame but the innermost, this makes your program stop as soon as control returns to that frame.  This is similar to the effect of a finish command in the frame inside the selected frame---except that finish does not leave an active breakpoint.  If you use break without an argument in the innermost frame, GDBN stops the next time it reaches the current location; this may be useful inside loops.

     GDBN normally ignores breakpoints when it resumes execution, until at least one instruction has been executed.  If it did not do this, you would be unable to proceed past a breakpoint without first disabling the breakpoint.  This rule applies whether or not the breakpoint already existed when your program stopped.

break ... if COND
     Set a breakpoint with condition COND; evaluate the expression COND each time the breakpoint is reached, and stop only if the value is nonzero---that is, if COND evaluates as true. ... stands for one of the possible arguments described above (or no argument) specifying where to break.  See Break Conditions, for more information on breakpoint conditions.

tbreak ARGS
     Set a breakpoint enabled only for one stop.  ARGS are the same as for the break command, and the breakpoint is set in the same way, but the breakpoint is automatically deleted after the first time your program stops there.  See Disabling Breakpoints.

hbreak ARGS
     Set a hardware-assisted breakpoint.  ARGS are the same as for the break command and the breakpoint is set in the same way, but the breakpoint requires hardware support and some target hardware may not have this support.  The main purpose of this is EPROM/ROM code debugging, so you can set a breakpoint at an instruction without changing the instruction.  This can be used with the new trap-generation provided by SPARClite DSU and most x86-based targets.  These targets will generate traps when a program accesses some data or instruction address that is assigned to the debug registers.  However the hardware breakpoint registers can take a limited number of breakpoints.  For example, on the DSU, only two data breakpoints can be set at a time, and GDBN will reject this command if more than two are used.  Delete or disable unused hardware breakpoints before setting new ones (see Disabling Breakpoints). See Break Conditions. For remote targets, you can restrict the number of hardware breakpoints GDBN will use, see set remote hardware-breakpoint-limit.

thbreak ARGS
     Set a hardware-assisted breakpoint enabled only for one stop.  ARGS are the same as for the hbreak command and the breakpoint is set in the same way.  However, like the tbreak command, the breakpoint is automatically deleted after the first time your program stops there.  Also, like the hbreak command, the breakpoint requires hardware support and some target hardware may not have this support.  See Disabling Breakpoints. See also Break Conditions.

rbreak REGEX
     Set breakpoints on all functions matching the regular expression REGEX.  This command sets an unconditional breakpoint on all matches, printing a list of all breakpoints it set.  Once these breakpoints are set, they are treated just like the breakpoints set with the break command.  You can delete them, disable them, or make them conditional the same way as any other breakpoint.

     The syntax of the regular expression is the standard one used with tools like grep.  Note that this is different from the syntax used by shells, so for instance foo* matches all functions that include an fo followed by zero or more os.  There is an implicit .* leading and trailing the regular expression you supply, so to match only functions that begin with foo, use ^foo.

     When debugging C++ programs, rbreak is useful for setting breakpoints on overloaded functions that are not members of any special classes.

     The rbreak command can be used to set breakpoints in all the functions in a program, like this:

          (GDBP) rbreak .

rbreak FILE:REGEX
     If rbreak is called with a filename qualification, it limits the search for functions matching the given regular expression to the specified FILE.  This can be used, for example, to set breakpoints on every function in a given file:

          (GDBP) rbreak file.c:.

     The colon separating the filename qualifier from the regex may optionally be surrounded by spaces.

info breakpoints [N...]
info break [N...]
     Print a table of all breakpoints, watchpoints, and catchpoints set and not deleted.  Optional argument N means print information only about the specified breakpoint(s) (or watchpoint(s) or catchpoint(s)). For each breakpoint, following columns are printed:

     Breakpoint Numbers

     Type
          Breakpoint, watchpoint, or catchpoint.

     Disposition
          Whether the breakpoint is marked to be disabled or deleted when hit.

     Enabled or Disabled
          Enabled breakpoints are marked with y.  n marks breakpoints that are not enabled.

     Address
          Where the breakpoint is in your program, as a memory address.  For a pending breakpoint whose address is not yet known, this field will contain <PENDING>.  Such breakpoint won't fire until a shared library that has the symbol or line referred by breakpoint is loaded. See below for details.  A breakpoint with several locations will have <MULTIPLE> in this field---see below for details.

     What
          Where the breakpoint is in the source for your program, as a file and line number.  For a pending breakpoint, the original string passed to the breakpoint command will be listed as it cannot be resolved until the appropriate shared library is loaded in the future.

     If a breakpoint is conditional, info break shows the condition on the line following the affected breakpoint; breakpoint commands, if any, are listed after that.  A pending breakpoint is allowed to have a condition specified for it.  The condition is not parsed for validity until a shared library is loaded that allows the pending breakpoint to resolve to a valid location.

     info break with a breakpoint number N as argument lists only that breakpoint.  The convenience variable $_ and the default examining-address for the x command are set to the address of the last breakpoint listed (see Examining Memory).

     info break displays a count of the number of times the breakpoint has been hit.  This is especially useful in conjunction with the ignore command.  You can ignore a large number of breakpoint hits, look at the breakpoint info to see how many times the breakpoint was hit, and then run again, ignoring one less than that number.  This will get you quickly to the last hit of that breakpoint.

GDBN allows you to set any number of breakpoints at the same place in your program.  There is nothing silly or meaningless about this.  When the breakpoints are conditional, this is even useful (see Break Conditions).

It is possible that a breakpoint corresponds to several locations in your program.  Examples of this situation are:

  * Multiple functions in the program may have the same name.

  * For a C++ constructor, the NGCC compiler generates several instances of the function body, used in different cases.

  * For a C++ template function, a given line in the function can correspond to any number of instantiations.

  * For an inlined function, a given source line can correspond to several places where that function is inlined.

In all those cases, GDBN will insert a breakpoint at all the relevant locations.

A breakpoint with multiple locations is displayed in the breakpoint table using several rows---one header row, followed by one row for each breakpoint location.  The header row has <MULTIPLE> in the address column.  The rows for individual locations contain the actual addresses for locations, and show the functions to which those locations belong.  The number column for a location is of the form BREAKPOINT-NUMBER.LOCATION-NUMBER.

For example:

     Num     Type           Disp Enb  Address    What
     1       breakpoint     keep y    <MULTIPLE>
             stop only if i==1
             breakpoint already hit 1 time
     1.1                         y    0x080486a2 in void foo<int>() at t.cc:8
     1.2                         y    0x080486ca in void foo<double>() at t.cc:8

Each location can be individually enabled or disabled by passing BREAKPOINT-NUMBER.LOCATION-NUMBER as argument to the enable and disable commands.  Note that you cannot delete the individual locations from the list, you can only delete the entire list of locations that belong to their parent breakpoint (with the delete NUM command, where NUM is the number of the parent breakpoint, 1 in the above example).  Disabling or enabling the parent breakpoint (see Disabling) affects all of the locations that belong to that breakpoint.

It's quite common to have a breakpoint inside a shared library. Shared libraries can be loaded and unloaded explicitly, and possibly repeatedly, as the program is executed.  To support this use case, GDBN updates breakpoint locations whenever any shared library is loaded or unloaded.  Typically, you would set a breakpoint in a shared library at the beginning of your debugging session, when the library is not loaded, and when the symbols from the library are not available.  When you try to set breakpoint, GDBN will ask you if you want to set a so called pending breakpoint---breakpoint whose address is not yet resolved.

After the program is run, whenever a new shared library is loaded, GDBN reevaluates all the breakpoints.  When a newly loaded shared library contains the symbol or line referred to by some pending breakpoint, that breakpoint is resolved and becomes an ordinary breakpoint.  When a library is unloaded, all breakpoints that refer to its symbols or source lines become pending again.

This logic works for breakpoints with multiple locations, too.  For example, if you have a breakpoint in a C++ template function, and a newly loaded shared library has an instantiation of that template, a new location is added to the list of locations for the breakpoint.

Except for having unresolved address, pending breakpoints do not differ from regular breakpoints.  You can set conditions or commands, enable and disable them and perform other breakpoint operations.

GDBN provides some additional commands for controlling what happens when the break command cannot resolve breakpoint address specification to an address:

set breakpoint pending auto
     This is the default behavior.  When GDBN cannot find the breakpoint location, it queries you whether a pending breakpoint should be created.

set breakpoint pending on
     This indicates that an unrecognized breakpoint location should automatically result in a pending breakpoint being created.

set breakpoint pending off
     This indicates that pending breakpoints are not to be created.  Any unrecognized breakpoint location results in an error.  This setting does not affect any pending breakpoints previously created.

show breakpoint pending
     Show the current behavior setting for creating pending breakpoints.

The settings above only affect the break command and its variants.  Once breakpoint is set, it will be automatically updated as shared libraries are loaded and unloaded.

For some targets, GDBN can automatically decide if hardware or software breakpoints should be used, depending on whether the breakpoint address is read-only or read-write.  This applies to breakpoints set with the break command as well as to internal breakpoints set by commands like next and finish.  For breakpoints set with hbreak, GDBN will always use hardware breakpoints.

You can control this automatic behaviour with the following commands::

set breakpoint auto-hw on
     This is the default behavior.  When GDBN sets a breakpoint, it will try to use the target memory map to decide if software or hardware breakpoint must be used.

set breakpoint auto-hw off
     This indicates GDBN should not automatically select breakpoint type.  If the target provides a memory map, GDBN will warn when trying to set software breakpoint at a read-only address.

GDBN normally implements breakpoints by replacing the program code at the breakpoint address with a special instruction, which, when executed, given control to the debugger.  By default, the program code is so modified only when the program is resumed.  As soon as the program stops, GDBN restores the original instructions.  This behaviour guards against leaving breakpoints inserted in the target should gdb abrubptly disconnect.  However, with slow remote targets, inserting and removing breakpoint can reduce the performance. This behavior can be controlled with the following commands::

set breakpoint always-inserted off
     All breakpoints, including newly added by the user, are inserted in the target only when the target is resumed.  All breakpoints are removed from the target when it stops.

set breakpoint always-inserted on
     Causes all breakpoints to be inserted in the target at all times.  If the user adds a new breakpoint, or changes an existing breakpoint, the breakpoints in the target are updated immediately.  A breakpoint is removed from the target only when breakpoint itself is removed.

set breakpoint always-inserted auto
     This is the default mode.  If GDBN is controlling the inferior in non-stop mode (see Non-Stop Mode), gdb behaves as if breakpoint always-inserted mode is on.  If GDBN is controlling the inferior in all-stop mode, GDBN behaves as if breakpoint always-inserted mode is off.

GDBN itself sometimes sets breakpoints in your program for special purposes, such as proper handling of longjmp (in C programs).  These internal breakpoints are assigned negative numbers, starting with -1; info breakpoints does not display them. You can see these breakpoints with the GDBN maintenance command maint info breakpoints (see maint info breakpoints).

Setting Watchpoints
-------------------

You can use a watchpoint to stop execution whenever the value of an expression changes, without having to predict a particular place where this may happen.  (This is sometimes called a data breakpoint.) The expression may be as simple as the value of a single variable, or as complex as many variables combined by operators.  Examples include:

  * A reference to the value of a single variable.

  * An address cast to an appropriate data type.  For example, *(int *)0x12345678 will watch a 4-byte region at the specified address (assuming an int occupies 4 bytes).

  * An arbitrarily complex expression, such as a*b + c/d.  The expression can use any operators valid in the program's native language (see Languages).

You can set a watchpoint on an expression even if the expression can not be evaluated yet.  For instance, you can set a watchpoint on *global_ptr before global_ptr is initialized. GDBN will stop when your program sets global_ptr and the expression produces a valid value.  If the expression becomes valid in some other way than changing a variable (e.g. if the memory pointed to by *global_ptr becomes readable as the result of a malloc call), GDBN may not stop until the next time the expression changes.

Depending on your system, watchpoints may be implemented in software or hardware.  GDBN does software watchpointing by single-stepping your program and testing the variable's value each time, which is hundreds of times slower than normal execution.  (But this may still be worth it, to catch errors where you have no clue what part of your program is the culprit.)

On some systems, such as HP-UX, PowerPC, GNU/Linux and most other x86-based targets, GDBN includes support for hardware watchpoints, which do not slow down the running of your program.

watch [-l|-location] EXPR [thread THREADNUM] [mask MASKVALUE]
     Set a watchpoint for an expression.  GDBN will break when the expression EXPR is written into by the program and its value changes.  The simplest (and the most popular) use of this command is to watch the value of a single variable:

          (GDBP) watch foo

     If the command includes a [thread THREADNUM] argument, GDBN breaks only when the thread identified by THREADNUM changes the value of EXPR.  If any other threads change the value of EXPR, GDBN will not break.  Note that watchpoints restricted to a single thread in this way only work with Hardware Watchpoints.

     Ordinarily a watchpoint respects the scope of variables in EXPR (see below).  The -location argument tells GDBN to instead watch the memory referred to by EXPR.  In this case, GDBN will evaluate EXPR, take the address of the result, and watch the memory at that address.  The type of the result is used to determine the size of the watched memory.  If the expression's result does not have an address, then GDBN will print an error.

     The [mask MASKVALUE] argument allows creation of masked watchpoints, if the current architecture supports this feature (e.g., PowerPC Embedded architecture, see PowerPC Embedded.)  A masked watchpoint specifies a mask in addition to an address to watch.  The mask specifies that some bits of an address (the bits which are reset in the mask) should be ignored when matching the address accessed by the inferior against the watchpoint address. Thus, a masked watchpoint watches many addresses simultaneously---those addresses whose unmasked bits are identical to the unmasked bits in the watchpoint address.  The mask argument implies -location. Examples:

          (GDBP) watch foo mask 0xffff00ff
          (GDBP) watch *0xdeadbeef mask 0xffffff00

rwatch [-l|-location] EXPR [thread THREADNUM] [mask MASKVALUE]
     Set a watchpoint that will break when the value of EXPR is read by the program.

awatch [-l|-location] EXPR [thread THREADNUM] [mask MASKVALUE]
     Set a watchpoint that will break when EXPR is either read from or written into by the program.

info watchpoints [N...]
     This command prints a list of watchpoints, using the same format as info break (see Set Breaks).

If you watch for a change in a numerically entered address you need to dereference it, as the address itself is just a constant number which will never change.  GDBN refuses to create a watchpoint that watches a never-changing value:

     (GDBP) watch 0x600850
     Cannot watch constant value 0x600850.
     (GDBP) watch *(int *) 0x600850
     Watchpoint 1: *(int *) 6293584

GDBN sets a hardware watchpoint if possible.  Hardware watchpoints execute very quickly, and the debugger reports a change in value at the exact instruction where the change occurs.  If GDBN cannot set a hardware watchpoint, it sets a software watchpoint, which executes more slowly and reports the change in value at the next statement, not the instruction, after the change occurs.

You can force GDBN to use only software watchpoints with the set can-use-hw-watchpoints 0 command.  With this variable set to zero, GDBN will never try to use hardware watchpoints, even if the underlying system supports them.  (Note that hardware-assisted watchpoints that were set before setting can-use-hw-watchpoints to zero will still use the hardware mechanism of watching expression values.)

set can-use-hw-watchpoints
     Set whether or not to use hardware watchpoints.

show can-use-hw-watchpoints
     Show the current mode of using hardware watchpoints.

For remote targets, you can restrict the number of hardware watchpoints GDBN will use, see set remote hardware-breakpoint-limit.

When you issue the watch command, GDBN reports

     Hardware watchpoint NUM: EXPR

if it was able to set a hardware watchpoint.

Currently, the awatch and rwatch commands can only set hardware watchpoints, because accesses to data that don't change the value of the watched expression cannot be detected without examining every instruction as it is being executed, and GDBN does not do that currently.  If GDBN finds that it is unable to set a hardware breakpoint with the awatch or rwatch command, it will print a message like this:

     Expression cannot be implemented with read/access watchpoint.

Sometimes, GDBN cannot set a hardware watchpoint because the data type of the watched expression is wider than what a hardware watchpoint on the target machine can handle.  For example, some systems can only watch regions that are up to 4 bytes wide; on such systems you cannot set hardware watchpoints for an expression that yields a double-precision floating-point number (which is typically 8 bytes wide).  As a work-around, it might be possible to break the large region into a series of smaller ones and watch them with separate watchpoints.

If you set too many hardware watchpoints, GDBN might be unable to insert all of them when you resume the execution of your program. Since the precise number of active watchpoints is unknown until such time as the program is about to be resumed, GDBN might not be able to warn you about this when you set the watchpoints, and the warning will be printed only when the program is resumed:

     Hardware watchpoint NUM: Could not insert watchpoint

If this happens, delete or disable some of the watchpoints.

Watching complex expressions that reference many variables can also exhaust the resources available for hardware-assisted watchpoints. That's because GDBN needs to watch every variable in the expression with separately allocated resources.

If you call a function interactively using print or call, any watchpoints you have set will be inactive until GDBN reaches another kind of breakpoint or the call completes.

GDBN automatically deletes watchpoints that watch local (automatic) variables, or expressions that involve such variables, when they go out of scope, that is, when the execution leaves the block in which these variables were defined.  In particular, when the program being debugged terminates, all local variables go out of scope, and so only watchpoints that watch global variables remain set.  If you rerun the program, you will need to set all such watchpoints again.  One way of doing that would be to set a code breakpoint at the entry to the main function and when it breaks, set all the watchpoints.

In multi-threaded programs, watchpoints will detect changes to the watched expression from every thread.

     Warning: In multi-threaded programs, software watchpoints have only limited usefulness.  If GDBN creates a software watchpoint, it can only watch the value of an expression in a single thread.  If you are confident that the expression can only change due to the current thread's activity (and if you are also confident that no other thread can become current), then you can use software watchpoints as usual.  However, GDBN may not notice when a non-current thread's activity changes the expression.  (Hardware watchpoints, in contrast, watch an expression in all threads.)

See set remote hardware-watchpoint-limit.

Setting Catchpoints
-------------------

You can use catchpoints to cause the debugger to stop for certain kinds of program events, such as C++ exceptions or the loading of a shared library.  Use the catch command to set a catchpoint.

catch EVENT
     Stop when EVENT occurs.  EVENT can be any of the following:

     throw
          The throwing of a C++ exception.

     catch
          The catching of a C++ exception.

     exception
          An Ada exception being raised.  If an exception name is specified at the end of the command (eg catch exception Program_Error), the debugger will stop only when this specific exception is raised. Otherwise, the debugger stops execution when any Ada exception is raised.

          When inserting an exception catchpoint on a user-defined exception whose name is identical to one of the exceptions defined by the language, the fully qualified name must be used as the exception name.  Otherwise, GDBN will assume that it should stop on the pre-defined exception rather than the user-defined one.  For instance, assuming an exception called Constraint_Error is defined in package Pck, then the command to use to catch such exceptions is catch exception Pck.Constraint_Error.

     exception unhandled
          An exception that was raised but is not handled by the program.

     assert
          A failed Ada assertion.

     exec
          A call to exec.  This is currently only available for HP-UX and GNU/Linux.

     syscall
     syscall [NAME | NUMBER] ...
          A call to or return from a system call, a.k.a. syscall.  A syscall is a mechanism for application programs to request a service from the operating system (OS) or one of the OS system services. GDBN can catch some or all of the syscalls issued by the debuggee, and show the related information for each syscall.  If no argument is specified, calls to and returns from all system calls will be caught.

          NAME can be any system call name that is valid for the underlying OS.  Just what syscalls are valid depends on the OS.  On GNU and Unix systems, you can find the full list of valid syscall names on /usr/include/asm/unistd.h.

          Normally, GDBN knows in advance which syscalls are valid for each OS, so you can use the GDBN command-line completion facilities (see command completion) to list the available choices.

          You may also specify the system call numerically.  A syscall's number is the value passed to the OS's syscall dispatcher to identify the requested service.  When you specify the syscall by its name, GDBN uses its database of syscalls to convert the name into the corresponding numeric code, but using the number directly may be useful if GDBN's database does not have the complete list of syscalls on your system (e.g., because GDBN lags behind the OS upgrades).

          The example below illustrates how this command works if you don't provide arguments to it:

               (GDBP) catch syscall
               Catchpoint 1 (syscall)
               (GDBP) r
               Starting program: /tmp/catch-syscall

               Catchpoint 1 (call to syscall 'close'), \
               	   0xffffe424 in __kernel_vsyscall ()
               (GDBP) c
               Continuing.

               Catchpoint 1 (returned from syscall 'close'), \
               	0xffffe424 in __kernel_vsyscall ()
               (GDBP)

          Here is an example of catching a system call by name:

               (GDBP) catch syscall chroot
               Catchpoint 1 (syscall 'chroot' [61])
               (GDBP) r
               Starting program: /tmp/catch-syscall

               Catchpoint 1 (call to syscall 'chroot'), \
               		   0xffffe424 in __kernel_vsyscall ()
               (GDBP) c
               Continuing.

               Catchpoint 1 (returned from syscall 'chroot'), \
               	0xffffe424 in __kernel_vsyscall ()
               (GDBP)

          An example of specifying a system call numerically.  In the case below, the syscall number has a corresponding entry in the XML file, so GDBN finds its name and prints it:

               (GDBP) catch syscall 252
               Catchpoint 1 (syscall(s) 'exit_group')
               (GDBP) r
               Starting program: /tmp/catch-syscall

               Catchpoint 1 (call to syscall 'exit_group'), \
               		   0xffffe424 in __kernel_vsyscall ()
               (GDBP) c
               Continuing.

               Program exited normally.
               (GDBP)

          However, there can be situations when there is no corresponding name in XML file for that syscall number.  In this case, GDBN prints a warning message saying that it was not able to find the syscall name, but the catchpoint will be set anyway.  See the example below:

               (GDBP) catch syscall 764
               warning: The number '764' does not represent a known syscall.
               Catchpoint 2 (syscall 764)
               (GDBP)

          If you configure GDBN using the --without-expat option, it will not be able to display syscall names.  Also, if your architecture does not have an XML file describing its system calls, you will not be able to see the syscall names.  It is important to notice that these two features are used for accessing the syscall name database.  In either case, you will see a warning like this:

               (GDBP) catch syscall
               warning: Could not open "syscalls/i386-linux.xml"
               warning: Could not load the syscall XML file 'syscalls/i386-linux.xml'.
               GDB will not be able to display syscall names.
               Catchpoint 1 (syscall)
               (GDBP)

          Of course, the file name will change depending on your architecture and system.

          Still using the example above, you can also try to catch a syscall by its number.  In this case, you would see something like:

               (GDBP) catch syscall 252
               Catchpoint 1 (syscall(s) 252)

          Again, in this case GDBN would not be able to display syscall's names.

     fork
          A call to fork.  This is currently only available for HP-UX and GNU/Linux.

     vfork
          A call to vfork.  This is currently only available for HP-UX and GNU/Linux.

tcatch EVENT
     Set a catchpoint that is enabled only for one stop.  The catchpoint is automatically deleted after the first time the event is caught.

Use the info break command to list the current catchpoints.

There are currently some limitations to C++ exception handling (catch throw and catch catch) in GDBN:

  * If you call a function interactively, GDBN normally returns control to you when the function has finished executing.  If the call raises an exception, however, the call may bypass the mechanism that returns control to you and cause your program either to abort or to simply continue running until it hits a breakpoint, catches a signal that GDBN is listening for, or exits.  This is the case even if you set a catchpoint for the exception; catchpoints on exceptions are disabled within interactive calls.

  * You cannot raise an exception interactively.

  * You cannot install an exception handler interactively.

Sometimes catch is not the best way to debug exception handling: if you need to know exactly where an exception is raised, it is better to stop before the exception handler is called, since that way you can see the stack before any unwinding takes place.  If you set a breakpoint in an exception handler instead, it may not be easy to find out where the exception was raised.

To stop just before an exception handler is called, you need some knowledge of the implementation.  In the case of GNU C++, exceptions are raised by calling a library function named __raise_exception which has the following ANSI C interface:

         /* ADDR is where the exception identifier is stored.
            ID is the exception identifier.  */
         void __raise_exception (void **addr, void *id);

To make the debugger catch all exceptions before any stack unwinding takes place, set a breakpoint on __raise_exception (see Breakpoints; Watchpoints; and Exceptions).

With a conditional breakpoint (see Break Conditions) that depends on the value of ID, you can stop your program when a specific exception is raised.  You can use multiple conditional breakpoints to stop your program when any of a number of exceptions are raised.

Deleting Breakpoints
--------------------

It is often necessary to eliminate a breakpoint, watchpoint, or catchpoint once it has done its job and you no longer want your program to stop there.  This is called deleting the breakpoint.  A breakpoint that has been deleted no longer exists; it is forgotten.

With the clear command you can delete breakpoints according to where they are in your program.  With the delete command you can delete individual breakpoints, watchpoints, or catchpoints by specifying their breakpoint numbers.

It is not necessary to delete a breakpoint to proceed past it.  GDBN automatically ignores breakpoints on the first instruction to be executed when you continue execution without changing the execution address.

clear
     Delete any breakpoints at the next instruction to be executed in the selected stack frame (see Selecting a Frame).  When the innermost frame is selected, this is a good way to delete a breakpoint where your program just stopped.

clear LOCATION
     Delete any breakpoints set at the specified LOCATION. See Specify Location, for the various forms of LOCATION; the most useful ones are listed below:

     clear FUNCTION
     clear FILENAME:FUNCTION
          Delete any breakpoints set at entry to the named FUNCTION.

     clear LINENUM
     clear FILENAME:LINENUM
          Delete any breakpoints set at or within the code of the specified LINENUM of the specified FILENAME.

delete [breakpoints] [RANGE...]
     Delete the breakpoints, watchpoints, or catchpoints of the breakpoint ranges specified as arguments.  If no argument is specified, delete all breakpoints (GDBN asks confirmation, unless you have set
     confirm off).  You can abbreviate this command as d.

Disabling Breakpoints
---------------------

Rather than deleting a breakpoint, watchpoint, or catchpoint, you might prefer to disable it.  This makes the breakpoint inoperative as if it had been deleted, but remembers the information on the breakpoint so that you can enable it again later.

You disable and enable breakpoints, watchpoints, and catchpoints with the enable and disable commands, optionally specifying one or more breakpoint numbers as arguments.  Use info break to print a list of all breakpoints, watchpoints, and catchpoints if you do not know which numbers to use.

Disabling and enabling a breakpoint that has multiple locations affects all of its locations.

A breakpoint, watchpoint, or catchpoint can have any of four different states of enablement:

  * Enabled.  The breakpoint stops your program.  A breakpoint set with the break command starts out in this state.

  * Disabled.  The breakpoint has no effect on your program.

  * Enabled once.  The breakpoint stops your program, but then becomes disabled.

  * Enabled for deletion.  The breakpoint stops your program, but immediately after it does so it is deleted permanently.  A breakpoint set with the tbreak command starts out in this state.

You can use the following commands to enable or disable breakpoints, watchpoints, and catchpoints:

disable [breakpoints] [RANGE...]
     Disable the specified breakpoints---or all breakpoints, if none are listed.  A disabled breakpoint has no effect but is not forgotten.  All options such as ignore-counts, conditions and commands are remembered in case the breakpoint is enabled again later.  You may abbreviate disable as dis.

enable [breakpoints] [RANGE...]
     Enable the specified breakpoints (or all defined breakpoints).  They become effective once again in stopping your program.

enable [breakpoints] once RANGE...
     Enable the specified breakpoints temporarily.  GDBN disables any of these breakpoints immediately after stopping your program.

enable [breakpoints] delete RANGE...
     Enable the specified breakpoints to work once, then die.  GDBN deletes any of these breakpoints as soon as your program stops there. Breakpoints set by the tbreak command start out in this state.

Except for a breakpoint set with tbreak (see Setting Breakpoints), breakpoints that you set are initially enabled; subsequently, they become disabled or enabled only when you use one of the commands above.  (The command until can set and delete a breakpoint of its own, but it does not change the state of your other breakpoints; see Continuing and Stepping.)

Break Conditions
----------------

The simplest sort of breakpoint breaks every time your program reaches a specified place.  You can also specify a condition for a breakpoint.  A condition is just a Boolean expression in your programming language (see Expressions).  A breakpoint with a condition evaluates the expression each time your program reaches it, and your program stops only if the condition is true.

This is the converse of using assertions for program validation; in that situation, you want to stop when the assertion is violated---that is, when the condition is false.  In C, if you want to test an assertion expressed by the condition ASSERT, you should set the condition ! ASSERT on the appropriate breakpoint.

Conditions are also accepted for watchpoints; you may not need them, since a watchpoint is inspecting the value of an expression anyhow---but it might be simpler, say, to just set a watchpoint on a variable name, and specify a condition that tests whether the new value is an interesting one.

Break conditions can have side effects, and may even call functions in your program.  This can be useful, for example, to activate functions that log program progress, or to use your own print functions to format special data structures.  The effects are completely predictable unless there is another enabled breakpoint at the same address.  (In that case, GDBN might see the other breakpoint first and stop your program without checking the condition of this one.)  Note that breakpoint commands are usually more convenient and flexible than break conditions for the purpose of performing side effects when a breakpoint is reached (see Breakpoint Command Lists).

Break conditions can be specified when a breakpoint is set, by using if in the arguments to the break command.  See Setting Breakpoints.  They can also be changed at any time with the condition command.

You can also use the if keyword with the watch command. The catch command does not recognize the if keyword; condition is the only way to impose a further condition on a catchpoint.

condition BNUM EXPRESSION
     Specify EXPRESSION as the break condition for breakpoint, watchpoint, or catchpoint number BNUM.  After you set a condition, breakpoint BNUM stops your program only if the value of EXPRESSION is true (nonzero, in C).  When you use condition, GDBN checks EXPRESSION immediately for syntactic correctness, and to determine whether symbols in it have referents in the context of your breakpoint.  If EXPRESSION uses symbols not referenced in the context of the breakpoint, GDBN prints an error message:

          No symbol "foo" in current context.

     GDBN does not actually evaluate EXPRESSION at the time the condition command (or a command that sets a breakpoint with a condition, like break if ...) is given, however.  See Expressions.

condition BNUM
     Remove the condition from breakpoint number BNUM.  It becomes an ordinary unconditional breakpoint.

A special case of a breakpoint condition is to stop only when the breakpoint has been reached a certain number of times.  This is so useful that there is a special way to do it, using the ignore count of the breakpoint.  Every breakpoint has an ignore count, which is an integer.  Most of the time, the ignore count is zero, and therefore has no effect.  But if your program reaches a breakpoint whose ignore count is positive, then instead of stopping, it just decrements the ignore count by one and continues.  As a result, if the ignore count value is N, the breakpoint does not stop the next N times your program reaches it.

ignore BNUM COUNT
     Set the ignore count of breakpoint number BNUM to COUNT. The next COUNT times the breakpoint is reached, your program's execution does not stop; other than to decrement the ignore count, GDBN takes no action.

     To make the breakpoint stop the next time it is reached, specify a count of zero.

     When you use continue to resume execution of your program from a breakpoint, you can specify an ignore count directly as an argument to continue, rather than using ignore.  See Continuing and Stepping.

     If a breakpoint has a positive ignore count and a condition, the condition is not checked.  Once the ignore count reaches zero, GDBN resumes checking the condition.

     You could achieve the effect of the ignore count with a condition such as $foo-- <= 0 using a debugger convenience variable that is decremented each time.  See Convenience Variables.

Ignore counts apply to breakpoints, watchpoints, and catchpoints.

Breakpoint Command Lists
------------------------

You can give any breakpoint (or watchpoint or catchpoint) a series of commands to execute when your program stops due to that breakpoint.  For example, you might want to print the values of certain expressions, or enable other breakpoints.

commands [RANGE...]
... COMMAND-LIST ...
end
     Specify a list of commands for the given breakpoints.  The commands themselves appear on the following lines.  Type a line containing just end to terminate the commands.

     To remove all commands from a breakpoint, type commands and follow it immediately with end; that is, give no commands.

     With no argument, commands refers to the last breakpoint, watchpoint, or catchpoint set (not to the breakpoint most recently encountered).  If the most recent breakpoints were set with a single command, then the commands will apply to all the breakpoints set by that command.  This applies to breakpoints set by rbreak, and also applies when a single break command creates multiple breakpoints (see Ambiguous Expressions).

Pressing RET as a means of repeating the last GDBN command is disabled within a COMMAND-LIST.

You can use breakpoint commands to start your program up again.  Simply use the continue command, or step, or any other command that resumes execution.

Any other commands in the command list, after a command that resumes execution, are ignored.  This is because any time you resume execution (even with a simple next or step), you may encounter another breakpoint---which could have its own command list, leading to ambiguities about which list to execute.

If the first command you specify in a command list is silent, the usual message about stopping at a breakpoint is not printed.  This may be desirable for breakpoints that are to print a specific message and then continue.  If none of the remaining commands print anything, you see no sign that the breakpoint was reached.  silent is meaningful only at the beginning of a breakpoint command list.

The commands echo, output, and printf allow you to print precisely controlled output, and are often useful in silent breakpoints.  See Commands for Controlled Output.

For example, here is how you could use breakpoint commands to print the value of x at entry to foo whenever x is positive.

     break foo if x>0
     commands
     silent
     printf "x is %d\n",x
     cont
     end

One application for breakpoint commands is to compensate for one bug so you can test for another.  Put a breakpoint just after the erroneous line of code, give it a condition to detect the case in which something erroneous has been done, and give it commands to assign correct values to any variables that need them.  End with the continue command so that your program does not stop, and start with the silent command so that no output is produced.  Here is an example:

     break 403
     commands
     silent
     set x = y + 4
     cont
     end

How to save breakpoints to a file
---------------------------------

To save breakpoint definitions to a file use the save breakpoints command.

save breakpoints [FILENAME]
     This command saves all current breakpoint definitions together with their commands and ignore counts, into a file FILENAME suitable for use in a later debugging session.  This includes all types of breakpoints (breakpoints, watchpoints, catchpoints, tracepoints).  To read the saved breakpoint definitions, use the source command (see Command Files).  Note that watchpoints with expressions involving local variables may fail to be recreated because it may not be possible to access the context where the watchpoint is valid anymore.  Because the saved breakpoint definitions are simply a sequence of GDBN commands that recreate the breakpoints, you can edit the file in your favorite editing program, and remove the breakpoint definitions you're not interested in, or that can no longer be recreated.

``Cannot insert breakpoints''
-----------------------------

If you request too many active hardware-assisted breakpoints and watchpoints, you will see this error message:

     Stopped; cannot insert breakpoints.
     You may have requested too many hardware breakpoints and watchpoints.

This message is printed when you attempt to resume the program, since only then GDBN knows exactly how many hardware breakpoints and watchpoints it needs to insert.

When this message is printed, you need to disable or remove some of the hardware-assisted breakpoints and watchpoints, and then continue.

``Breakpoint address adjusted...''
----------------------------------

Some processor architectures place constraints on the addresses at which breakpoints may be placed.  For architectures thus constrained, GDBN will attempt to adjust the breakpoint's address to comply with the constraints dictated by the architecture.

One example of such an architecture is the Fujitsu FR-V.  The FR-V is a VLIW architecture in which a number of RISC-like instructions may be bundled together for parallel execution.  The FR-V architecture constrains the location of a breakpoint instruction within such a bundle to the instruction with the lowest address.  GDBN honors this constraint by adjusting a breakpoint's address to the first in the bundle.

It is not uncommon for optimized code to have bundles which contain instructions from different source statements, thus it may happen that a breakpoint's address will be adjusted from one source statement to another.  Since this adjustment may significantly alter GDBN's breakpoint related behavior from what the user expects, a warning is printed when the breakpoint is first set and also when the breakpoint is hit.

A warning like the one below is printed when setting a breakpoint that's been subject to address adjustment:

     warning: Breakpoint address adjusted from 0x00010414 to 0x00010410.

Such warnings are printed both for user settable and GDBN's internal breakpoints.  If you see one of these warnings, you should verify that a breakpoint set at the adjusted address will have the desired affect.  If not, the breakpoint in question may be removed and other breakpoints may be set which will have the desired behavior. E.g., it may be sufficient to place the breakpoint at a later instruction.  A conditional breakpoint may also be useful in some cases to prevent the breakpoint from triggering too often.

GDBN will also issue a warning when stopping at one of these adjusted breakpoints:

     warning: Breakpoint 1 address previously adjusted from 0x00010414
     to 0x00010410.

When this warning is encountered, it may be too late to take remedial action except in cases where the breakpoint is hit earlier or more frequently than expected.

Continuing and Stepping
=======================

Continuing means resuming program execution until your program completes normally.  In contrast, stepping means executing just one more ``step'' of your program, where ``step'' may mean either one line of source code, or one machine instruction (depending on what particular command you use).  Either when continuing or when stepping, your program may stop even sooner, due to a breakpoint or a signal.  (If it stops due to a signal, you may want to use handle, or use signal 0 to resume execution.  See Signals.)

continue [IGNORE-COUNT]
c [IGNORE-COUNT]
fg [IGNORE-COUNT]
     Resume program execution, at the address where your program last stopped; any breakpoints set at that address are bypassed.  The optional argument IGNORE-COUNT allows you to specify a further number of times to ignore a breakpoint at this location; its effect is like that of ignore (see Break Conditions).

     The argument IGNORE-COUNT is meaningful only when your program stopped due to a breakpoint.  At other times, the argument to continue is ignored.

     The synonyms c and fg (for foreground, as the debugged program is deemed to be the foreground program) are provided purely for convenience, and have exactly the same behavior as continue.

To resume execution at a different place, you can use return (see Returning from a Function) to go back to the calling function; or jump (see Continuing at a Different Address) to go to an arbitrary location in your program.

A typical technique for using stepping is to set a breakpoint (see Breakpoints; Watchpoints; and Catchpoints) at the beginning of the function or the section of your program where a problem is believed to lie, run your program until it stops at that breakpoint, and then step through the suspect area, examining the variables that are interesting, until you see the problem happen.

step
     Continue running your program until control reaches a different source line, then stop it and return control to GDBN.  This command is abbreviated s.

          Warning: If you use the step command while control is within a function that was compiled without debugging information, execution proceeds until control reaches a function that does have debugging information.  Likewise, it will not step into a function which is compiled without debugging information.  To step through functions without debugging information, use the stepi command, described below.

     The step command only stops at the first instruction of a source line.  This prevents the multiple stops that could otherwise occur in switch statements, for loops, etc.  step continues to stop if a function that has debugging information is called within the line.  In other words, step steps inside any functions called within the line.

     Also, the step command only enters a function if there is line number information for the function.  Otherwise it acts like the next command.  This avoids problems when using cc -gl on MIPS machines.  Previously, step entered subroutines if there was any debugging information about the routine.

step COUNT
     Continue running as in step, but do so COUNT times.  If a breakpoint is reached, or a signal not related to stepping occurs before COUNT steps, stepping stops right away.

next [COUNT]
     Continue to the next source line in the current (innermost) stack frame. This is similar to step, but function calls that appear within the line of code are executed without stopping.  Execution stops when control reaches a different line of code at the original stack level that was executing when you gave the next command.  This command is abbreviated n.

     An argument COUNT is a repeat count, as for step.

     The next command only stops at the first instruction of a source line.  This prevents multiple stops that could otherwise occur in switch statements, for loops, etc.

set step-mode
set step-mode on
     The set step-mode on command causes the step command to stop at the first instruction of a function which contains no debug line information rather than stepping over it.

     This is useful in cases where you may be interested in inspecting the machine instructions of a function which has no symbolic info and do not want GDBN to automatically skip over this function.

set step-mode off
     Causes the step command to step over any functions which contains no debug information.  This is the default.

show step-mode
     Show whether GDBN will stop in or step over functions without source line debug information.

finish
     Continue running until just after function in the selected stack frame returns.  Print the returned value (if any).  This command can be abbreviated as fin.

     Contrast this with the return command (see Returning from a Function).

until
u
     Continue running until a source line past the current line, in the current stack frame, is reached.  This command is used to avoid single stepping through a loop more than once.  It is like the next command, except that when until encounters a jump, it automatically continues execution until the program counter is greater than the address of the jump.

     This means that when you reach the end of a loop after single stepping though it, until makes your program continue execution until it exits the loop.  In contrast, a next command at the end of a loop simply steps back to the beginning of the loop, which forces you to step through the next iteration.

     until always stops your program if it attempts to exit the current stack frame.

     until may produce somewhat counterintuitive results if the order of machine code does not match the order of the source lines.  For example, in the following excerpt from a debugging session, the f (frame) command shows that execution is stopped at line 206; yet when we use until, we get to line 195:

          (GDBP) f
          #0  main (argc=4, argv=0xf7fffae8) at m4.c:206
          206                 expand_input();
          (GDBP) until
          195             for ( ; argc > 0; NEXTARG) {

     This happened because, for execution efficiency, the compiler had generated code for the loop closure test at the end, rather than the start, of the loop---even though the test in a C for-loop is written before the body of the loop.  The until command appeared to step back to the beginning of the loop when it advanced to this expression; however, it has not really gone to an earlier statement---not in terms of the actual machine code.

     until with no argument works by means of single instruction stepping, and hence is slower than until with an argument.

until LOCATION
u LOCATION
     Continue running your program until either the specified location is reached, or the current stack frame returns.  LOCATION is any of the forms described in Specify Location. This form of the command uses temporary breakpoints, and hence is quicker than until without an argument.  The specified location is actually reached only if it is in the current frame.  This implies that until can be used to skip over recursive function invocations.  For instance in the code below, if the current location is line 96, issuing until 99 will execute the program up to line 99 in the same invocation of factorial, i.e., after the inner invocations have returned.

          94	int factorial (int value)
          95	{
          96	    if (value > 1) {
          97            value *= factorial (value - 1);
          98	    }
          99	    return (value);
          100     }

advance LOCATION
     Continue running the program up to the given LOCATION.  An argument is required, which should be of one of the forms described in Specify Location. Execution will also stop upon exit from the current stack frame.  This command is similar to until, but advance will not skip over recursive function calls, and the target location doesn't have to be in the same frame as the current one.

stepi
stepi ARG
si
     Execute one machine instruction, then stop and return to the debugger.

     It is often useful to do display/i $pc when stepping by machine instructions.  This makes GDBN automatically display the next instruction to be executed, each time your program stops.  See Automatic Display.

     An argument is a repeat count, as in step.

nexti
nexti ARG
ni
     Execute one machine instruction, but if it is a function call, proceed until the function returns.

     An argument is a repeat count, as in next.

Skipping Over Functions and Files
=================================

The program you are debugging may contain some functions which are uninteresting to debug.  The skip comand lets you tell GDBN to skip a function or all functions in a file when stepping.

For example, consider the following C function:

     101     int func()
     102     {
     103         foo(boring());
     104         bar(boring());
     105     }

Suppose you wish to step into the functions foo and bar, but you are not interested in stepping through boring.  If you run step at line 103, you'll enter boring(), but if you run next, you'll step over both foo and boring!

One solution is to step into boring and use the finish command to immediately exit it.  But this can become tedious if boring is called from many places.

A more flexible solution is to execute skip boring.  This instructs GDBN never to step into boring.  Now when you execute step at line 103, you'll step over boring and directly into foo.

You can also instruct GDBN to skip all functions in a file, with, for example, skip file boring.c.

skip [LINESPEC]
skip function [LINESPEC]
     After running this command, the function named by LINESPEC or the function containing the line named by LINESPEC will be skipped over when stepping.  See Specify Location.

     If you do not specify LINESPEC, the function you're currently debugging will be skipped.

     (If you have a function called file that you want to skip, use skip function file.)

skip file [FILENAME]
     After running this command, any function whose source lives in FILENAME will be skipped over when stepping.

     If you do not specify FILENAME, functions whose source lives in the file you're currently debugging will be skipped.

Skips can be listed, deleted, disabled, and enabled, much like breakpoints. These are the commands for managing your list of skips:

info skip [RANGE]
     Print details about the specified skip(s).  If RANGE is not specified, print a table with details about all functions and files marked for skipping. info skip prints the following information about each skip:

     Identifier
          A number identifying this skip.

     Type
          The type of this skip, either function or file.

     Enabled or Disabled
          Enabled skips are marked with y.  Disabled skips are marked with n.

     Address
          For function skips, this column indicates the address in memory of the function being skipped.  If you've set a function skip on a function which has not yet been loaded, this field will contain <PENDING>.  Once a shared library which has the function is loaded, info skip will show the function's address here.

     What
          For file skips, this field contains the filename being skipped.  For functions skips, this field contains the function name and its line number in the file where it is defined.

skip delete [RANGE]
     Delete the specified skip(s).  If RANGE is not specified, delete all skips.

skip enable [RANGE]
     Enable the specified skip(s).  If RANGE is not specified, enable all skips.

skip disable [RANGE]
     Disable the specified skip(s).  If RANGE is not specified, disable all skips.

Signals
=======

A signal is an asynchronous event that can happen in a program.  The operating system defines the possible kinds of signals, and gives each kind a name and a number.  For example, in Unix SIGINT is the signal a program gets when you type an interrupt character (often Ctrl-c); SIGSEGV is the signal a program gets from referencing a place in memory far away from all the areas in use; SIGALRM occurs when the alarm clock timer goes off (which happens only if your program has requested an alarm).

Some signals, including SIGALRM, are a normal part of the functioning of your program.  Others, such as SIGSEGV, indicate errors; these signals are fatal (they kill your program immediately) if the program has not specified in advance some other way to handle the signal. SIGINT does not indicate an error in your program, but it is normally fatal so it can carry out the purpose of the interrupt: to kill the program.

GDBN has the ability to detect any occurrence of a signal in your program.  You can tell GDBN in advance what to do for each kind of signal.

Normally, GDBN is set up to let the non-erroneous signals like SIGALRM be silently passed to your program (so as not to interfere with their role in the program's functioning) but to stop your program immediately whenever an error signal happens. You can change these settings with the handle command.

info signals
info handle
     Print a table of all the kinds of signals and how GDBN has been told to handle each one.  You can use this to see the signal numbers of all the defined types of signals.

info signals SIG
     Similar, but print information only about the specified signal number.

     info handle is an alias for info signals.

handle SIGNAL [KEYWORDS...]
     Change the way GDBN handles signal SIGNAL.  SIGNAL can be the number of a signal or its name (with or without the SIG at the beginning); a list of signal numbers of the form LOW-HIGH; or the word all, meaning all the known signals.  Optional arguments KEYWORDS, described below, say what change to make.

The keywords allowed by the handle command can be abbreviated. Their full names are:

nostop
     GDBN should not stop your program when this signal happens.  It may still print a message telling you that the signal has come in.

stop
     GDBN should stop your program when this signal happens.  This implies the print keyword as well.

print
     GDBN should print a message when this signal happens.

noprint
     GDBN should not mention the occurrence of the signal at all.  This implies the nostop keyword as well.

pass
noignore
     GDBN should allow your program to see this signal; your program can handle the signal, or else it may terminate if the signal is fatal and not handled.  pass and noignore are synonyms.

nopass
ignore
     GDBN should not allow your program to see this signal. nopass and ignore are synonyms.

When a signal stops your program, the signal is not visible to the program until you continue.  Your program sees the signal then, if pass is in effect for the signal in question at that time.  In other words, after GDBN reports a signal, you can use the handle command with pass or nopass to control whether your program sees that signal when you continue.

The default is set to nostop, noprint, pass for non-erroneous signals such as SIGALRM, SIGWINCH and SIGCHLD, and to stop, print, pass for the erroneous signals.

You can also use the signal command to prevent your program from seeing a signal, or cause it to see a signal it normally would not see, or to give it any signal at any time.  For example, if your program stopped due to some sort of memory reference error, you might store correct values into the erroneous variables and continue, hoping to see more execution; but your program would probably terminate immediately as a result of the fatal signal once it saw the signal.  To prevent this, you can continue with signal 0.  See Giving your Program a Signal.

On some targets, GDBN can inspect extra signal information associated with the intercepted signal, before it is actually delivered to the program being debugged.  This information is exported by the convenience variable $_siginfo, and consists of data that is passed by the kernel to the signal handler at the time of the receipt of a signal.  The data type of the information itself is target dependent.  You can see the data type using the ptype $_siginfo command.  On Unix systems, it typically corresponds to the standard siginfo_t type, as defined in the signal.h system header.

Here's an example, on a GNU/Linux system, printing the stray referenced address that raised a segmentation fault.

     (GDBP) continue
     Program received signal SIGSEGV, Segmentation fault.
     0x0000000000400766 in main ()
     69        *(int *)p = 0;
     (GDBP) ptype $_siginfo
     type = struct {
         int si_signo;
         int si_errno;
         int si_code;
         union {
             int _pad[28];
             struct {...} _kill;
             struct {...} _timer;
             struct {...} _rt;
             struct {...} _sigchld;
             struct {...} _sigfault;
             struct {...} _sigpoll;
         } _sifields;
     }
     (GDBP) ptype $_siginfo._sifields._sigfault
     type = struct {
         void *si_addr;
     }
     (GDBP) p $_siginfo._sifields._sigfault.si_addr
     $1 = (void *) 0x7ffff7ff7000

Depending on target support, $_siginfo may also be writable.

Stopping and Starting Multi-thread Programs
===========================================

GDBN supports debugging programs with multiple threads (see Debugging Programs with Multiple Threads).  There are two modes of controlling execution of your program within the debugger.  In the default mode, referred to as all-stop mode, when any thread in your program stops (for example, at a breakpoint or while being stepped), all other threads in the program are also stopped by GDBN.  On some targets, GDBN also supports non-stop mode, in which other threads can continue to run freely while you examine the stopped thread in the debugger.

All-Stop Mode
-------------

In all-stop mode, whenever your program stops under GDBN for any reason, all threads of execution stop, not just the current thread.  This allows you to examine the overall state of the program, including switching between threads, without worrying that things may change underfoot.

Conversely, whenever you restart the program, all threads start executing.  This is true even when single-stepping with commands like step or next.

In particular, GDBN cannot single-step all threads in lockstep. Since thread scheduling is up to your debugging target's operating system (not controlled by GDBN), other threads may execute more than one statement while the current thread completes a single step.  Moreover, in general other threads stop in the middle of a statement, rather than at a clean statement boundary, when the program stops.

You might even find your program stopped in another thread after continuing or even single-stepping.  This happens whenever some other thread runs into a breakpoint, a signal, or an exception before the first thread completes whatever you requested.

Whenever GDBN stops your program, due to a breakpoint or a signal, it automatically selects the thread where that breakpoint or signal happened.  GDBN alerts you to the context switch with a message such as [Switching to Thread N] to identify the thread.

On some OSes, you can modify GDBN's default behavior by locking the OS scheduler to allow only a single thread to run.

set scheduler-locking MODE
     Set the scheduler locking mode.  If it is off, then there is no locking and any thread may run at any time.  If on, then only the current thread may run when the inferior is resumed.  The step mode optimizes for single-stepping; it prevents other threads from preempting the current thread while you are stepping, so that the focus of debugging does not change unexpectedly. Other threads only rarely (or never) get a chance to run when you step.  They are more likely to run when you next over a function call, and they are completely free to run when you use commands like continue, until, or finish.  However, unless another thread hits a breakpoint during its timeslice, GDBN does not change the current thread away from the thread that you are debugging.

show scheduler-locking
     Display the current scheduler locking mode.

By default, when you issue one of the execution commands such as continue, next or step, GDBN allows only threads of the current inferior to run.  For example, if GDBN is attached to two inferiors, each with two threads, the continue command resumes only the two threads of the current inferior.  This is useful, for example, when you debug a program that forks and you want to hold the parent stopped (so that, for instance, it doesn't run to exit), while you debug the child.  In other situations, you may not be interested in inspecting the current state of any of the processes GDBN is attached to, and you may want to resume them all until some breakpoint is hit.  In the latter case, you can instruct GDBN to allow all threads of all the inferiors to run with the set schedule-multiple command.

set schedule-multiple
     Set the mode for allowing threads of multiple processes to be resumed when an execution command is issued.  When on, all threads of all processes are allowed to run.  When off, only the threads of the current process are resumed.  The default is off.  The scheduler-locking mode takes precedence when set to on, or while you are stepping and set to step.

show schedule-multiple
     Display the current mode for resuming the execution of threads of multiple processes.

Non-Stop Mode
-------------

For some multi-threaded targets, GDBN supports an optional mode of operation in which you can examine stopped program threads in the debugger while other threads continue to execute freely.  This minimizes intrusion when debugging live systems, such as programs where some threads have real-time constraints or must continue to respond to external events.  This is referred to as non-stop mode.

In non-stop mode, when a thread stops to report a debugging event, only that thread is stopped; GDBN does not stop other threads as well, in contrast to the all-stop mode behavior.  Additionally, execution commands such as continue and step apply by default only to the current thread in non-stop mode, rather than all threads as in all-stop mode.  This allows you to control threads explicitly in ways that are not possible in all-stop mode --- for example, stepping one thread while allowing others to run freely, stepping one thread while holding all others stopped, or stepping several threads independently and simultaneously.

To enter non-stop mode, use this sequence of commands before you run or attach to your program:

     # Enable the async interface.
     set target-async 1

     # If using the CLI, pagination breaks non-stop.
     set pagination off

     # Finally, turn it on!
     set non-stop on

You can use these commands to manipulate the non-stop mode setting:

set non-stop on
     Enable selection of non-stop mode.

set non-stop off
     Disable selection of non-stop mode.

show non-stop
     Show the current non-stop enablement setting.

Note these commands only reflect whether non-stop mode is enabled, not whether the currently-executing program is being run in non-stop mode. In particular, the set non-stop preference is only consulted when GDBN starts or connects to the target program, and it is generally not possible to switch modes once debugging has started.  Furthermore, since not all targets support non-stop mode, even when you have enabled non-stop mode, GDBN may still fall back to all-stop operation by default.

In non-stop mode, all execution commands apply only to the current thread by default.  That is, continue only continues one thread. To continue all threads, issue continue -a or c -a.

You can use GDBN's background execution commands (see Background Execution) to run some threads in the background while you continue to examine or step others from GDBN. The MI execution commands (see GDB/MI Program Execution) are always executed asynchronously in non-stop mode.

Suspending execution is done with the interrupt command when running in the background, or Ctrl-c during foreground execution. In all-stop mode, this stops the whole process; but in non-stop mode the interrupt applies only to the current thread. To stop the whole program, use interrupt -a.

Other execution commands do not currently support the -a option.

In non-stop mode, when a thread stops, GDBN doesn't automatically make that thread current, as it does in all-stop mode.  This is because the thread stop notifications are asynchronous with respect to GDBN's command interpreter, and it would be confusing if GDBN unexpectedly changed to a different thread just as you entered a command to operate on the previously current thread.

Background Execution
--------------------

GDBN's execution commands have two variants:  the normal foreground (synchronous) behavior, and a background (asynchronous) behavior.  In foreground execution, GDBN waits for the program to report that some thread has stopped before prompting for another command.  In background execution, GDBN immediately gives a command prompt so that you can issue other commands while your program runs.

You need to explicitly enable asynchronous mode before you can use background execution commands.  You can use these commands to manipulate the asynchronous mode setting:

set target-async on
     Enable asynchronous mode.

set target-async off
     Disable asynchronous mode.

show target-async
     Show the current target-async setting.

If the target doesn't support async mode, GDBN issues an error message if you attempt to use the background execution commands.

To specify background execution, add a & to the command.  For example, the background form of the continue command is continue&, or just c&.  The execution commands that accept background execution are:

run
     See Starting your Program.

attach
     See Debugging an Already-running Process.

step
     See step.

stepi
     See stepi.

next
     See next.

nexti
     See nexti.

continue
     See continue.

finish
     See finish.

until
     See until.

Background execution is especially useful in conjunction with non-stop mode for debugging programs with multiple threads; see Non-Stop Mode. However, you can also use these commands in the normal all-stop mode with the restriction that you cannot issue another execution command until the previous one finishes.  Examples of commands that are valid in all-stop mode while the program is running include help and info break.

You can interrupt your program while it is running in the background by using the interrupt command.

interrupt
interrupt -a

     Suspend execution of the running program.  In all-stop mode, interrupt stops the whole process, but in non-stop mode, it stops only the current thread.  To stop the whole program in non-stop mode, use interrupt -a.

Thread-Specific Breakpoints
---------------------------

When your program has multiple threads (see Debugging Programs with Multiple Threads), you can choose whether to set breakpoints on all threads, or on a particular thread.

break LINESPEC thread THREADNO
break LINESPEC thread THREADNO if ...
     LINESPEC specifies source lines; there are several ways of writing them (see Specify Location), but the effect is always to specify some source line.

     Use the qualifier thread THREADNO with a breakpoint command to specify that you only want GDBN to stop the program when a particular thread reaches this breakpoint.  THREADNO is one of the numeric thread identifiers assigned by GDBN, shown in the first column of the info threads display.

     If you do not specify thread THREADNO when you set a breakpoint, the breakpoint applies to all threads of your program.

     You can use the thread qualifier on conditional breakpoints as well; in this case, place thread THREADNO before or after the breakpoint condition, like this:

          (GDBP) break frik.c:13 thread 28 if bartab > lim

Interrupted System Calls
------------------------

There is an unfortunate side effect when using GDBN to debug multi-threaded programs.  If one thread stops for a breakpoint, or for some other reason, and another thread is blocked in a system call, then the system call may return prematurely.  This is a consequence of the interaction between multiple threads and the signals that GDBN uses to implement breakpoints and other events that stop execution.

To handle this problem, your program should check the return value of each system call and react appropriately.  This is good programming style anyways.

For example, do not write code like this:

       sleep (10);

The call to sleep will return early if a different thread stops at a breakpoint or for some other reason.

Instead, write this:

       int unslept = 10;
       while (unslept > 0)
         unslept = sleep (unslept);

A system call is allowed to return early, so the system is still conforming to its specification.  But GDBN does cause your multi-threaded program to behave differently than it would without GDBN.

Also, GDBN uses internal breakpoints in the thread library to monitor certain events such as thread creation and thread destruction. When such an event happens, a system call in another thread may return prematurely, even though your program does not appear to stop.

Observer Mode
-------------

If you want to build on non-stop mode and observe program behavior without any chance of disruption by GDBN, you can set variables to disable all of the debugger's attempts to modify state, whether by writing memory, inserting breakpoints, etc.  These operate at a low level, intercepting operations from all commands.

When all of these are set to off, then GDBN is said to be observer mode.  As a convenience, the variable observer can be set to disable these, plus enable non-stop mode.

Note that GDBN will not prevent you from making nonsensical combinations of these settings. For instance, if you have enabled may-insert-breakpoints but disabled may-write-memory, then breakpoints that work by writing trap instructions into the code stream will still not be able to be placed.

set observer on
set observer off
     When set to on, this disables all the permission variables below (except for insert-fast-tracepoints), plus enables non-stop debugging.  Setting this to off switches back to normal debugging, though remaining in non-stop mode.

show observer
     Show whether observer mode is on or off.

set may-write-registers on
set may-write-registers off
     This controls whether GDBN will attempt to alter the values of registers, such as with assignment expressions in print, or the jump command.  It defaults to on.

show may-write-registers
     Show the current permission to write registers.

set may-write-memory on
set may-write-memory off
     This controls whether GDBN will attempt to alter the contents of memory, such as with assignment expressions in print.  It defaults to on.

show may-write-memory
     Show the current permission to write memory.

set may-insert-breakpoints on
set may-insert-breakpoints off
     This controls whether GDBN will attempt to insert breakpoints. This affects all breakpoints, including internal breakpoints defined by GDBN.  It defaults to on.

show may-insert-breakpoints
     Show the current permission to insert breakpoints.

set may-insert-tracepoints on
set may-insert-tracepoints off
     This controls whether GDBN will attempt to insert (regular) tracepoints at the beginning of a tracing experiment.  It affects only non-fast tracepoints, fast tracepoints being under the control of may-insert-fast-tracepoints.  It defaults to on.

show may-insert-tracepoints
     Show the current permission to insert tracepoints.

set may-insert-fast-tracepoints on
set may-insert-fast-tracepoints off
     This controls whether GDBN will attempt to insert fast tracepoints at the beginning of a tracing experiment.  It affects only fast tracepoints, regular (non-fast) tracepoints being under the control of may-insert-tracepoints.  It defaults to on.

show may-insert-fast-tracepoints
     Show the current permission to insert fast tracepoints.

set may-interrupt on
set may-interrupt off
     This controls whether GDBN will attempt to interrupt or stop program execution.  When this variable is off, the interrupt command will have no effect, nor will Ctrl-c. It defaults to on.

show may-interrupt
     Show the current permission to interrupt or stop the program.

Running programs backward
*************************

When you are debugging a program, it is not unusual to realize that you have gone too far, and some event of interest has already happened. If the target environment supports it, GDBN can allow you to ``rewind'' the program by running it backward.

A target environment that supports reverse execution should be able to ``undo'' the changes in machine state that have taken place as the program was executing normally.  Variables, registers etc. should revert to their previous values.  Obviously this requires a great deal of sophistication on the part of the target environment; not all target environments can support reverse execution.

When a program is executed in reverse, the instructions that have most recently been executed are ``un-executed'', in reverse order.  The program counter runs backward, following the previous thread of execution in reverse.  As each instruction is ``un-executed'', the values of memory and/or registers that were changed by that instruction are reverted to their previous states.  After executing a piece of source code in reverse, all side effects of that code should be ``undone'', and all variables should be returned to their prior values (Note that some side effects are easier to undo than others.  For instance,
memory and registers are relatively easy, but device I/O is hard.  Some
targets may be able undo things like device I/O, and some may not.

The contract between GDBN and the reverse executing target
requires only that the target do something reasonable when
GDBN tells it to execute backwards, and then report the
results back to GDBN.  Whatever the target reports back to
GDBN, GDBN will report back to the user.  GDBN
assumes that the memory and registers that the target reports are in a
consistant state, but GDBN accepts whatever it is given.).

If you are debugging in a target environment that supports reverse execution, GDBN provides the following commands.

reverse-continue [IGNORE-COUNT]
rc [IGNORE-COUNT]
     Beginning at the point where your program last stopped, start executing in reverse.  Reverse execution will stop for breakpoints and synchronous exceptions (signals), just like normal execution.  Behavior of asynchronous signals depends on the target environment.

reverse-step [COUNT]
     Run the program backward until control reaches the start of a different source line; then stop it, and return control to GDBN.

     Like the step command, reverse-step will only stop at the beginning of a source line.  It ``un-executes'' the previously executed source line.  If the previous source line included calls to debuggable functions, reverse-step will step (backward) into the called function, stopping at the beginning of the last statement in the called function (typically a return statement).

     Also, as with the step command, if non-debuggable functions are called, reverse-step will run thru them backward without stopping.

reverse-stepi [COUNT]
     Reverse-execute one machine instruction.  Note that the instruction to be reverse-executed is not the one pointed to by the program counter, but the instruction executed prior to that one.  For instance, if the last instruction was a jump, reverse-stepi will take you back from the destination of the jump to the jump instruction itself.

reverse-next [COUNT]
     Run backward to the beginning of the previous line executed in the current (innermost) stack frame.  If the line contains function calls, they will be ``un-executed'' without stopping.  Starting from the first line of a function, reverse-next will take you back to the caller of that function, before the function was called, just as the normal next command would take you from the last line of a function back to its return to its caller (Unless the code is too heavily optimized.).

reverse-nexti [COUNT]
     Like nexti, reverse-nexti executes a single instruction in reverse, except that called functions are ``un-executed'' atomically. That is, if the previously executed instruction was a return from another function, reverse-nexti will continue to execute in reverse until the call to that function (from the current stack frame) is reached.

reverse-finish
     Just as the finish command takes you to the point where the current function returns, reverse-finish takes you to the point where it was called.  Instead of ending up at the end of the current function invocation, you end up at the beginning.

set exec-direction
     Set the direction of target execution.
set exec-direction reverse
     GDBN will perform all execution commands in reverse, until the exec-direction mode is changed to ``forward''.  Affected commands include step, stepi, next, nexti, continue, and finish.  The return command cannot be used in reverse mode.

set exec-direction forward
     GDBN will perform all execution commands in the normal fashion. This is the default.

Recording Inferior's Execution and Replaying It
***********************************************

On some platforms, GDBN provides a special process record
and replay target that can record a log of the process execution, and replay it later with both forward and reverse execution commands.

When this target is in use, if the execution log includes the record for the next instruction, GDBN will debug in replay
mode.  In the replay mode, the inferior does not really execute code instructions.  Instead, all the events that normally happen during code execution are taken from the execution log.  While code is not really executed in replay mode, the values of registers (including the program counter register) and the memory of the inferior are still changed as they normally would.  Their contents are taken from the execution log.

If the record for the next instruction is not in the execution log, GDBN will debug in record mode.  In this mode, the inferior executes normally, and GDBN records the execution log for future replay.

The process record and replay target supports reverse execution (see Reverse Execution), even if the platform on which the inferior runs does not.  However, the reverse execution is limited in this case by the range of the instructions recorded in the execution log.  In other words, reverse execution on platforms that don't support it directly can only be done in the replay mode.

When debugging in the reverse direction, GDBN will work in replay mode as long as the execution log includes the record for the previous instruction; otherwise, it will work in record mode, if the platform supports reverse execution, or stop if not.

For architecture environments that support process record and replay, GDBN provides the following commands:

target record
     This command starts the process record and replay target.  The process record and replay target can only debug a process that is already running.  Therefore, you need first to start the process with the run or start commands, and then start the recording with the target record command.

     Both record and rec are aliases of target record.

     Displaced stepping (see displaced stepping) will be automatically disabled when process record and replay target is started.  That's because the process record and replay target doesn't support displaced stepping.

     If the inferior is in the non-stop mode (see Non-Stop Mode) or in the asynchronous execution mode (see Background Execution), the process record and replay target cannot be started because it doesn't support these two modes.

record stop
     Stop the process record and replay target.  When process record and replay target stops, the entire execution log will be deleted and the inferior will either be terminated, or will remain in its final state.

     When you stop the process record and replay target in record mode (at the end of the execution log), the inferior will be stopped at the next instruction that would have been recorded.  In other words, if you record for a while and then stop recording, the inferior process will be left in the same state as if the recording never happened.

     On the other hand, if the process record and replay target is stopped while in replay mode (that is, not at the end of the execution log, but at some earlier point), the inferior process will become ``live'' at that earlier state, and it will then be possible to continue the usual ``live'' debugging of the process from that state.

     When the inferior process exits, or GDBN detaches from it, process record and replay target will automatically stop itself.

record save FILENAME
     Save the execution log to a file FILENAME. Default filename is gdb_record.PROCESS_ID, where PROCESS_ID is the process ID of the inferior.

record restore FILENAME
     Restore the execution log from a file FILENAME. File must have been created with record save.

set record insn-number-max LIMIT
     Set the limit of instructions to be recorded.  Default value is 200000.

     If LIMIT is a positive number, then GDBN will start deleting instructions from the log once the number of the record instructions becomes greater than LIMIT.  For every new recorded instruction, GDBN will delete the earliest recorded instruction to keep the number of recorded instructions at the limit. (Since deleting recorded instructions loses information, GDBN lets you control what happens when the limit is reached, by means of the stop-at-limit option, described below.)

     If LIMIT is zero, GDBN will never delete recorded instructions from the execution log.  The number of recorded instructions is unlimited in this case.

show record insn-number-max
     Show the limit of instructions to be recorded.

set record stop-at-limit
     Control the behavior when the number of recorded instructions reaches the limit.  If ON (the default), GDBN will stop when the limit is reached for the first time and ask you whether you want to stop the inferior or continue running it and recording the execution log.  If you decide to continue recording, each new recorded instruction will cause the oldest one to be deleted.

     If this option is OFF, GDBN will automatically delete the oldest record to make room for each new one, without asking.

show record stop-at-limit
     Show the current setting of stop-at-limit.

set record memory-query
     Control the behavior when GDBN is unable to record memory changes caused by an instruction.  If ON, GDBN will query whether to stop the inferior in that case.

     If this option is OFF (the default), GDBN will automatically ignore the effect of such instructions on memory.  Later, when GDBN replays this execution log, it will mark the log of this instruction as not accessible, and it will not affect the replay results.

show record memory-query
     Show the current setting of memory-query.

info record
     Show various statistics about the state of process record and its in-memory execution log buffer, including:

       * Whether in record mode or replay mode.

       * Lowest recorded instruction number (counting from when the current execution log started recording instructions).

       * Highest recorded instruction number.

       * Current instruction about to be replayed (if in replay mode).

       * Number of instructions contained in the execution log.

       * Maximum number of instructions that may be contained in the execution log.

record delete
     When record target runs in replay mode (``in the past''), delete the subsequent execution log and begin to record a new execution log starting from the current address.  This means you will abandon the previously recorded ``future'' and begin recording a new ``future''.

Examining the Stack
*******************

When your program has stopped, the first thing you need to know is where it stopped and how it got there.

Each time your program performs a function call, information about the call is generated. That information includes the location of the call in your program, the arguments of the call, and the local variables of the function being called. The information is saved in a block of data called a stack frame. The stack frames are allocated in a region of memory called the call stack.

When your program stops, the GDBN commands for examining the stack allow you to see all of this information.

One of the stack frames is selected by GDBN and many GDBN commands refer implicitly to the selected frame.  In particular, whenever you ask GDBN for the value of a variable in your program, the value is found in the selected frame.  There are special GDBN commands to select whichever frame you are interested in.  See Selecting a Frame.

When your program stops, GDBN automatically selects the currently executing frame and describes it briefly, similar to the frame command (see Information about a Frame).

Stack Frames
============

The call stack is divided up into contiguous pieces called stack frames, or frames for short; each frame is the data associated with one call to one function.  The frame contains the arguments given to the function, the function's local variables, and the address at which the function is executing.

When your program is started, the stack has only one frame, that of the function main.  This is called the initial frame or the outermost frame.  Each time a function is called, a new frame is made.  Each time a function returns, the frame for that function invocation is eliminated.  If a function is recursive, there can be many frames for the same function.  The frame for the function in which execution is actually occurring is called the innermost frame.  This is the most recently created of all the stack frames that still exist.

Inside your program, stack frames are identified by their addresses.  A stack frame consists of many bytes, each of which has its own address; each kind of computer has a convention for choosing one byte whose address serves as the address of the frame.  Usually this address is kept in a register called the frame pointer register (see $fp) while execution is going on in that frame.

GDBN assigns numbers to all existing stack frames, starting with zero for the innermost frame, one for the frame that called it, and so on upward.  These numbers do not really exist in your program; they are assigned by GDBN to give you a way of designating stack frames in GDBN commands.

Some compilers provide a way to compile functions so that they operate without stack frames.  (For example, the NGCC option

     -fomit-frame-pointer

generates functions without a frame.) This is occasionally done with heavily used library functions to save the frame setup time.  GDBN has limited facilities for dealing with these function invocations.  If the innermost function invocation has no stack frame, GDBN nevertheless regards it as though it had a separate frame, which is numbered zero as usual, allowing correct tracing of the function call chain.  However, GDBN has no provision for frameless functions elsewhere in the stack.

frame ARGS
     The frame command allows you to move from one stack frame to another, and to print the stack frame you select.  ARGS may be either the address of the frame or the stack frame number.  Without an argument, frame prints the current stack frame.

select-frame
     The select-frame command allows you to move from one stack frame to another without printing the frame.  This is the silent version of frame.

Backtraces
==========

A backtrace is a summary of how your program got where it is.  It shows one line per frame, for many frames, starting with the currently executing frame (frame zero), followed by its caller (frame one), and on up the stack.

backtrace
bt
     Print a backtrace of the entire stack: one line per frame for all frames in the stack.

     You can stop the backtrace at any time by typing the system interrupt character, normally Ctrl-c.

backtrace N
bt N
     Similar, but print only the innermost N frames.

backtrace -N
bt -N
     Similar, but print only the outermost N frames.

backtrace full
bt full
bt full N
bt full -N
     Print the values of the local variables also.  N specifies the number of frames to print, as described above.

The names where and info stack (abbreviated info s) are additional aliases for backtrace.

In a multi-threaded program, GDBN by default shows the backtrace only for the current thread.  To display the backtrace for several or all of the threads, use the command thread apply (see thread apply).  For example, if you type thread
apply all backtrace, GDBN will display the backtrace for all the threads; this is handy when you debug a core dump of a multi-threaded program.

Each line in the backtrace shows the frame number and the function name. The program counter value is also shown---unless you use set print address off.  The backtrace also shows the source file name and line number, as well as the arguments to the function.  The program counter value is omitted if it is at the beginning of the code for that line number.

Here is an example of a backtrace.  It was made with the command bt 3, so it shows the innermost three frames.

     #0  m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
         at builtin.c:993
     #1  0x6e38 in expand_macro (sym=0x2b600, data=...) at macro.c:242
     #2  0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08)
         at macro.c:71
     (More stack frames follow...)

The display for frame zero does not begin with a program counter value, indicating that your program has stopped at the beginning of the code for line 993 of builtin.c.

The value of parameter data in frame 1 has been replaced by ....  By default, GDBN prints the value of a parameter only if it is a scalar (integer, pointer, enumeration, etc).  See command set print frame-arguments in Print Settings for more details on how to configure the way function parameter values are printed.

If your program was compiled with optimizations, some compilers will optimize away arguments passed to functions if those arguments are never used after the call.  Such optimizations generate code that passes arguments through registers, but doesn't store those arguments in the stack frame.  GDBN has no way of displaying such arguments in stack frames other than the innermost one.  Here's what such a backtrace might look like:

     #0  m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
         at builtin.c:993
     #1  0x6e38 in expand_macro (sym=<optimized out>) at macro.c:242
     #2  0x6840 in expand_token (obs=0x0, t=<optimized out>, td=0xf7fffb08)
         at macro.c:71
     (More stack frames follow...)

The values of arguments that were not saved in their stack frames are shown as <optimized out>.

If you need to display the values of such optimized-out arguments, either deduce that from other variables whose values depend on the one you are interested in, or recompile without optimizations.

Most programs have a standard user entry point---a place where system libraries and startup code transition into user code.  For C this is main (Note that embedded programs (the so-called ``free-standing'' environment) are not required to have a main function as the entry point.  They could even have multiple entry points.). When GDBN finds the entry function in a backtrace it will terminate the backtrace, to avoid tracing into highly system-specific (and generally uninteresting) code.

If you need to examine the startup code, or limit the number of levels in a backtrace, you can change this behavior:

set backtrace past-main
set backtrace past-main on
     Backtraces will continue past the user entry point.

set backtrace past-main off
     Backtraces will stop when they encounter the user entry point.  This is the default.

show backtrace past-main
     Display the current user entry point backtrace policy.

set backtrace past-entry
set backtrace past-entry on
     Backtraces will continue past the internal entry point of an application. This entry point is encoded by the linker when the application is built, and is likely before the user entry point main (or equivalent) is called.

set backtrace past-entry off
     Backtraces will stop when they encounter the internal entry point of an application.  This is the default.

show backtrace past-entry
     Display the current internal entry point backtrace policy.

set backtrace limit N
set backtrace limit 0
     Limit the backtrace to N levels.  A value of zero means unlimited.

show backtrace limit
     Display the current limit on backtrace levels.

Selecting a Frame
=================

Most commands for examining the stack and other data in your program work on whichever stack frame is selected at the moment.  Here are the commands for selecting a stack frame; all of them finish by printing a brief description of the stack frame just selected.

frame N
f N
     Select frame number N.  Recall that frame zero is the innermost (currently executing) frame, frame one is the frame that called the innermost one, and so on.  The highest-numbered frame is the one for main.

frame ADDR
f ADDR
     Select the frame at address ADDR.  This is useful mainly if the chaining of stack frames has been damaged by a bug, making it impossible for GDBN to assign numbers properly to all frames.  In addition, this can be useful when your program has multiple stacks and switches between them.

     On the SPARC architecture, frame needs two addresses to select an arbitrary frame: a frame pointer and a stack pointer.

     On the MIPS and Alpha architecture, it needs two addresses: a stack pointer and a program counter.

     On the 29k architecture, it needs three addresses: a register stack pointer, a program counter, and a memory stack pointer.

up N
     Move N frames up the stack.  For positive numbers N, this advances toward the outermost frame, to higher frame numbers, to frames that have existed longer.  N defaults to one.

down N
     Move N frames down the stack.  For positive numbers N, this advances toward the innermost frame, to lower frame numbers, to frames that were created more recently.  N defaults to one.  You may abbreviate down as do.

All of these commands end by printing two lines of output describing the frame.  The first line shows the frame number, the function name, the arguments, and the source file and line number of execution in that frame.  The second line shows the text of that source line.

For example:

     (GDBP) up
     #1  0x22f0 in main (argc=1, argv=0xf7fffbf4, env=0xf7fffbfc)
         at env.c:10
     10              read_input_file (argv[i]);

After such a printout, the list command with no arguments prints ten lines centered on the point of execution in the frame. You can also edit the program at the point of execution with your favorite editing program by typing edit. See Printing Source Lines, for details.

up-silently N
down-silently N
     These two commands are variants of up and down, respectively; they differ in that they do their work silently, without causing display of the new frame.  They are intended primarily for use in GDBN command scripts, where the output might be unnecessary and distracting.

Information About a Frame
=========================

There are several other commands to print information about the selected stack frame.

frame
f
     When used without any argument, this command does not change which frame is selected, but prints a brief description of the currently selected stack frame.  It can be abbreviated f.  With an argument, this command is used to select a stack frame. See Selecting a Frame.

info frame
info f
     This command prints a verbose description of the selected stack frame, including:

       * the address of the frame

       * the address of the next frame down (called by this frame)

       * the address of the next frame up (caller of this frame)

       * the language in which the source code corresponding to this frame is written

       * the address of the frame's arguments

       * the address of the frame's local variables

       * the program counter saved in it (the address of execution in the caller frame)

       * which registers were saved in the frame

     The verbose description is useful when something has gone wrong that has made the stack format fail to fit the usual conventions.

info frame ADDR
info f ADDR
     Print a verbose description of the frame at address ADDR, without selecting that frame.  The selected frame remains unchanged by this command.  This requires the same kind of address (more than one for some architectures) that you specify in the frame command. See Selecting a Frame.

info args
     Print the arguments of the selected frame, each on a separate line.

info locals
     Print the local variables of the selected frame, each on a separate line.  These are all variables (declared either static or automatic) accessible at the point of execution of the selected frame.

info catch
     Print a list of all the exception handlers that are active in the current stack frame at the current point of execution.  To see other exception handlers, visit the associated frame (using the up, down, or frame commands); then type info catch. See Setting Catchpoints.

Examining Source Files
**********************

GDBN can print parts of your program's source, since the debugging information recorded in the program tells GDBN what source files were used to build it.  When your program stops, GDBN spontaneously prints the line where it stopped.  Likewise, when you select a stack frame (see Selecting a Frame), GDBN prints the line where execution in that frame has stopped.  You can print other portions of source files by explicit command.

If you use GDBN through its GNU Emacs interface, you may prefer to use Emacs facilities to view source; see Using GDBN under GNU Emacs.

Printing Source Lines
=====================

To print lines from a source file, use the list command (abbreviated l).  By default, ten lines are printed. There are several ways to specify what part of the file you want to print; see Specify Location, for the full list.

Here are the forms of the list command most commonly used:

list LINENUM
     Print lines centered around line number LINENUM in the current source file.

list FUNCTION
     Print lines centered around the beginning of function FUNCTION.

list
     Print more lines.  If the last lines printed were printed with a list command, this prints lines following the last lines printed; however, if the last line printed was a solitary line printed as part of displaying a stack frame (see Examining the Stack), this prints lines centered around that line.

list -
     Print lines just before the lines last printed.

By default, GDBN prints ten source lines with any of these forms of the list command.  You can change this using set listsize:

set listsize COUNT
     Make the list command display COUNT source lines (unless the list argument explicitly specifies some other number).

show listsize
     Display the number of lines that list prints.

Repeating a list command with RET discards the argument, so it is equivalent to typing just list.  This is more useful than listing the same lines again.  An exception is made for an argument of -; that argument is preserved in repetition so that each repetition moves up in the source file.

In general, the list command expects you to supply zero, one or two linespecs.  Linespecs specify source lines; there are several ways of writing them (see Specify Location), but the effect is always to specify some source line.

Here is a complete description of the possible arguments for list:

list LINESPEC
     Print lines centered around the line specified by LINESPEC.

list FIRST,LAST
     Print lines from FIRST to LAST.  Both arguments are linespecs.  When a list command has two linespecs, and the source file of the second linespec is omitted, this refers to the same source file as the first linespec.

list ,LAST
     Print lines ending with LAST.

list FIRST,
     Print lines starting with FIRST.

list +
     Print lines just after the lines last printed.

list -
     Print lines just before the lines last printed.

list
     As described in the preceding table.

Specifying a Location
=====================

Several GDBN commands accept arguments that specify a location of your program's code.  Since GDBN is a source-level debugger, a location usually specifies some line in the source code; for that reason, locations are also known as linespecs.

Here are all the different ways of specifying a code location that GDBN understands:

LINENUM
     Specifies the line number LINENUM of the current source file.

-OFFSET
+OFFSET
     Specifies the line OFFSET lines before or after the current line.  For the list command, the current line is the last one printed; for the breakpoint commands, this is the line at which execution stopped in the currently selected stack frame (see Frames, for a description of stack frames.)  When used as the second of the two linespecs in a list command, this specifies the line OFFSET lines up or down from the first linespec.

FILENAME:LINENUM
     Specifies the line LINENUM in the source file FILENAME. If FILENAME is a relative file name, then it will match any source file name with the same trailing components.  For example, if FILENAME is gcc/expr.c, then it will match source file name of /build/trunk/gcc/expr.c, but not /build/trunk/libcpp/expr.c or /build/trunk/gcc/x-expr.c.

FUNCTION
     Specifies the line that begins the body of the function FUNCTION. For example, in C, this is the line with the open brace.

FUNCTION:LABEL
     Specifies the line where LABEL appears in FUNCTION.

FILENAME:FUNCTION
     Specifies the line that begins the body of the function FUNCTION in the file FILENAME.  You only need the file name with a function name to avoid ambiguity when there are identically named functions in different source files.

LABEL
     Specifies the line at which the label named LABEL appears. GDBN searches for the label in the function corresponding to the currently selected stack frame.  If there is no current selected stack frame (for instance, if the inferior is not running), then GDBN will not search for a label.

*ADDRESS
     Specifies the program address ADDRESS.  For line-oriented commands, such as list and edit, this specifies a source line that contains ADDRESS.  For break and other breakpoint oriented commands, this can be used to set breakpoints in parts of your program which do not have debugging information or source files.

     Here ADDRESS may be any expression valid in the current working language (see working language) that specifies a code address.  In addition, as a convenience, GDBN extends the semantics of expressions used in locations to cover the situations that frequently happen during debugging.  Here are the various forms of ADDRESS:

     EXPRESSION
          Any expression valid in the current working language.

     FUNCADDR
          An address of a function or procedure derived from its name.  In C, C++, Java, Objective-C, Fortran, minimal, and assembly, this is simply the function's name FUNCTION (and actually a special case of a valid expression).  In Pascal and Modula-2, this is &FUNCTION.  In Ada, this is FUNCTION'Address (although the Pascal form also works).

          This form specifies the address of the function's first instruction, before the stack frame and arguments have been set up.

     'FILENAME'::FUNCADDR
          Like FUNCADDR above, but also specifies the name of the source file explicitly.  This is useful if the name of the function does not specify the function unambiguously, e.g., if there are several functions with identical names in different source files.

Editing Source Files
====================

To edit the lines in a source file, use the edit command. The editing program of your choice is invoked with the current line set to the active line in the program. Alternatively, there are several ways to specify what part of the file you want to print if you want to see other parts of the program:

edit LOCATION
     Edit the source file specified by location.  Editing starts at that LOCATION, e.g., at the specified source line of the specified file.  See Specify Location, for all the possible forms of the LOCATION argument; here are the forms of the edit command most commonly used:

     edit NUMBER
          Edit the current source file with NUMBER as the active line number.

     edit FUNCTION
          Edit the file containing FUNCTION at the beginning of its definition.

Choosing your Editor
--------------------

You can customize GDBN to use any editor you want
 (The only restriction is that your editor (say ex), recognizes the
following command-line syntax:)

     ex +NUMBER file

The optional numeric value +NUMBER specifies the number of the line in the file where to start editing.. By default, it is /bin/ex, but you can change this by setting the environment variable EDITOR before using GDBN.  For example, to configure GDBN to use the vi editor, you could use these commands with the sh shell:

     EDITOR=/usr/bin/vi
     export EDITOR
     gdb ...

or in the csh shell,

     setenv EDITOR /usr/bin/vi
     gdb ...

Searching Source Files
======================

There are two commands for searching through the current source file for a regular expression.

forward-search REGEXP
search REGEXP
     The command forward-search REGEXP checks each line, starting with the one following the last line listed, for a match for REGEXP.  It lists the line that is found.  You can use the synonym search REGEXP or abbreviate the command name as fo.

reverse-search REGEXP
     The command reverse-search REGEXP checks each line, starting with the one before the last line listed and going backward, for a match for REGEXP.  It lists the line that is found.  You can abbreviate this command as rev.

Specifying Source Directories
=============================

Executable programs sometimes do not record the directories of the source files from which they were compiled, just the names.  Even when they do, the directories could be moved between the compilation and your debugging session.  GDBN has a list of directories to search for source files; this is called the source path.  Each time GDBN wants a source file, it tries all the directories in the list, in the order they are present in the list, until it finds a file with the desired name.

For example, suppose an executable references the file /usr/src/foo-1.0/lib/foo.c, and our source path is /mnt/cross.  The file is first looked up literally; if this fails, /mnt/cross/usr/src/foo-1.0/lib/foo.c is tried; if this fails, /mnt/cross/foo.c is opened; if this fails, an error message is printed.  GDBN does not look up the parts of the source file name, such as /mnt/cross/src/foo-1.0/lib/foo.c. Likewise, the subdirectories of the source path are not searched: if the source path is /mnt/cross, and the binary refers to foo.c, GDBN would not find it under /mnt/cross/usr/src/foo-1.0/lib.

Plain file names, relative file names with leading directories, file names containing dots, etc. are all treated as described above; for instance, if the source path is /mnt/cross, and the source file is recorded as ../lib/foo.c, GDBN would first try ../lib/foo.c, then /mnt/cross/../lib/foo.c, and after that---/mnt/cross/foo.c.

Note that the executable search path is not used to locate the source files.

Whenever you reset or rearrange the source path, GDBN clears out any information it has cached about where source files are found and where each line is in the file.

When you start GDBN, its source path includes only cdir and cwd, in that order. To add other directories, use the directory command.

The search path is used to find both program source files and GDBN script files (read using the -command option and source command).

In addition to the source path, GDBN provides a set of commands that manage a list of source path substitution rules.  A substitution rule specifies how to rewrite source directories stored in the program's debug information in case the sources were moved to a different directory between compilation and debugging.  A rule is made of two strings, the first specifying what needs to be rewritten in the path, and the second specifying how it should be rewritten. In set substitute-path, we name these two parts FROM and TO respectively.  GDBN does a simple string replacement of FROM with TO at the start of the directory part of the source file name, and uses that result instead of the original file name to look up the sources.

Using the previous example, suppose the foo-1.0 tree has been moved from /usr/src to /mnt/cross, then you can tell GDBN to replace /usr/src in all source path names with /mnt/cross.  The first lookup will then be /mnt/cross/foo-1.0/lib/foo.c in place of the original location of /usr/src/foo-1.0/lib/foo.c.  To define a source path substitution rule, use the set substitute-path command (see set substitute-path).

To avoid unexpected substitution results, a rule is applied only if the FROM part of the directory name ends at a directory separator. For instance, a rule substituting  /usr/source into /mnt/cross will be applied to /usr/source/foo-1.0 but not to /usr/sourceware/foo-2.0.  And because the substitution is applied only at the beginning of the directory name, this rule will not be applied to /root/usr/source/baz.c either.

In many cases, you can achieve the same result using the directory command.  However, set substitute-path can be more efficient in the case where the sources are organized in a complex tree with multiple subdirectories.  With the directory command, you need to add each subdirectory of your project.  If you moved the entire tree while preserving its internal organization, then set substitute-path allows you to direct the debugger to all the sources with one single command.

set substitute-path is also more than just a shortcut command. The source path is only used if the file at the original location no longer exists.  On the other hand, set substitute-path modifies the debugger behavior to look at the rewritten location instead.  So, if for any reason a source file that is not relevant to your executable is located at the original location, a substitution rule is the only method available to point GDBN at the new location.

You can configure a default source path substitution rule by configuring GDBN with the --with-relocated-sources=DIR option.  The DIR should be the name of a directory under GDBN's configured prefix (set with --prefix or --exec-prefix), and directory names in debug information under DIR will be adjusted automatically if the installed GDBN is moved to a new location.  This is useful if GDBN, libraries or executables with debug information and corresponding source code are being moved together.

directory DIRNAME ...

dir DIRNAME ...
     Add directory DIRNAME to the front of the source path.  Several directory names may be given to this command, separated by : (; on MS-DOS and MS-Windows, where : usually appears as part of absolute file names) or whitespace.  You may specify a directory that is already in the source path; this moves it forward, so GDBN searches it sooner.

     You can use the string $cdir to refer to the compilation directory (if one is recorded), and $cwd to refer to the current working directory.  $cwd is not the same as .---the former tracks the current working directory as it changes during your GDBN session, while the latter is immediately expanded to the current directory at the time you add an entry to the source path.

directory
     Reset the source path to its default value ($cdir:$cwd on Unix systems).  This requires confirmation.

set directories PATH-LIST
     Set the source path to PATH-LIST. $cdir:$cwd are added if missing.

show directories
     Print the source path: show which directories it contains.

set substitute-path FROM TO
     Define a source path substitution rule, and add it at the end of the current list of existing substitution rules.  If a rule with the same FROM was already defined, then the old rule is also deleted.

     For example, if the file /foo/bar/baz.c was moved to /mnt/cross/baz.c, then the command

          (GDBP) set substitute-path /usr/src /mnt/cross

     will tell GDBN to replace /usr/src with /mnt/cross, which will allow GDBN to find the file baz.c even though it was moved.

     In the case when more than one substitution rule have been defined, the rules are evaluated one by one in the order where they have been defined.  The first one matching, if any, is selected to perform the substitution.

     For instance, if we had entered the following commands:

          (GDBP) set substitute-path /usr/src/include /mnt/include
          (GDBP) set substitute-path /usr/src /mnt/src

     GDBN would then rewrite /usr/src/include/defs.h into /mnt/include/defs.h by using the first rule.  However, it would use the second rule to rewrite /usr/src/lib/foo.c into /mnt/src/lib/foo.c.

unset substitute-path [path]
     If a path is specified, search the current list of substitution rules for a rule that would rewrite that path.  Delete that rule if found. A warning is emitted by the debugger if no rule could be found.

     If no path is specified, then all substitution rules are deleted.

show substitute-path [path]
     If a path is specified, then print the source path substitution rule which would rewrite that path, if any.

     If no path is specified, then print all existing source path substitution rules.

If your source path is cluttered with directories that are no longer of interest, GDBN may sometimes cause confusion by finding the wrong versions of source.  You can correct the situation as follows:

 1. Use directory with no argument to reset the source path to its default value.

 2. Use directory with suitable arguments to reinstall the directories you want in the source path.  You can add all the directories in one command.

Source and Machine Code
=======================

You can use the command info line to map source lines to program addresses (and vice versa), and the command disassemble to display a range of addresses as machine instructions.  You can use the command set disassemble-next-line to set whether to disassemble next source line when execution stops.  When run under GNU Emacs mode, the info line command causes the arrow to point to the line specified.  Also, info line prints addresses in symbolic form as well as hex.

info line LINESPEC
     Print the starting and ending addresses of the compiled code for source line LINESPEC.  You can specify source lines in any of the ways documented in Specify Location.

For example, we can use info line to discover the location of the object code for the first line of function m4_changequote:

     (GDBP) info line m4_changequote
     Line 895 of "builtin.c" starts at pc 0x634c and ends at 0x6350.

We can also inquire (using *ADDR as the form for LINESPEC) what source line covers a particular address:

     (GDBP) info line *0x63ff
     Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404.

After info line, the default address for the x command is changed to the starting address of the line, so that x/i is sufficient to begin examining the machine code (see Examining Memory).  Also, this address is saved as the value of the convenience variable $_ (see Convenience Variables).

disassemble
disassemble /m
disassemble /r
     This specialized command dumps a range of memory as machine instructions.  It can also print mixed source+disassembly by specifying the /m modifier and print the raw instructions in hex as well as in symbolic form by specifying the /r. The default memory range is the function surrounding the program counter of the selected frame.  A single argument to this command is a program counter value; GDBN dumps the function surrounding this value.  When two arguments are given, they should be separated by a comma, possibly surrounded by whitespace.  The arguments specify a range of addresses to dump, in one of two forms:

     START,END
          the addresses from START (inclusive) to END (exclusive)

     START,+LENGTH
          the addresses from START (inclusive) to START+LENGTH (exclusive).

     When 2 arguments are specified, the name of the function is also printed (since there could be several functions in the given range).

     The argument(s) can be any expression yielding a numeric value, such as 0x32c4, &main+10 or $pc - 8.

     If the range of memory being disassembled contains current program counter, the instruction at that location is shown with a => marker.

The following example shows the disassembly of a range of addresses of HP PA-RISC 2.0 code:

     (GDBP) disas 0x32c4, 0x32e4
     Dump of assembler code from 0x32c4 to 0x32e4:
        0x32c4 <main+204>:      addil 0,dp
        0x32c8 <main+208>:      ldw 0x22c(sr0,r1),r26
        0x32cc <main+212>:      ldil 0x3000,r31
        0x32d0 <main+216>:      ble 0x3f8(sr4,r31)
        0x32d4 <main+220>:      ldo 0(r31),rp
        0x32d8 <main+224>:      addil -0x800,dp
        0x32dc <main+228>:      ldo 0x588(r1),r26
        0x32e0 <main+232>:      ldil 0x3000,r31
     End of assembler dump.

Here is an example showing mixed source+assembly for Intel x86, when the program is stopped just after function prologue:

     (GDBP) disas /m main
     Dump of assembler code for function main:
     5       {
        0x08048330 <+0>:    push   %ebp
        0x08048331 <+1>:    mov    %esp,%ebp
        0x08048333 <+3>:    sub    $0x8,%esp
        0x08048336 <+6>:    and    $0xfffffff0,%esp
        0x08048339 <+9>:    sub    $0x10,%esp

     6         printf ("Hello.\n");
     => 0x0804833c <+12>:   movl   $0x8048440,(%esp)
        0x08048343 <+19>:   call   0x8048284 <puts@plt>

     7         return 0;
     8       }
        0x08048348 <+24>:   mov    $0x0,%eax
        0x0804834d <+29>:   leave
        0x0804834e <+30>:   ret

     End of assembler dump.

Here is another example showing raw instructions in hex for AMD x86-64,

     (gdb) disas /r 0x400281,+10
     Dump of assembler code from 0x400281 to 0x40028b:
        0x0000000000400281:  38 36  cmp    %dh,(%rsi)
        0x0000000000400283:  2d 36 34 2e 73 sub    $0x732e3436,%eax
        0x0000000000400288:  6f     outsl  %ds:(%rsi),(%dx)
        0x0000000000400289:  2e 32 00       xor    %cs:(%rax),%al
     End of assembler dump.

Some architectures have more than one commonly-used set of instruction mnemonics or other syntax.

For programs that were dynamically linked and use shared libraries, instructions that call functions or branch to locations in the shared libraries might show a seemingly bogus location---it's actually a location of the relocation table.  On some architectures, GDBN might be able to resolve these to actual function names.

set disassembly-flavor INSTRUCTION-SET
     Select the instruction set to use when disassembling the program via the disassemble or x/i commands.

     Currently this command is only defined for the Intel x86 family.  You can set INSTRUCTION-SET to either intel or att. The default is att, the AT&T flavor used by default by Unix assemblers for x86-based targets.

show disassembly-flavor
     Show the current setting of the disassembly flavor.

set disassemble-next-line
show disassemble-next-line
     Control whether or not GDBN will disassemble the next source line or instruction when execution stops.  If ON, GDBN will display disassembly of the next source line when execution of the program being debugged stops.  This is in addition to displaying the source line itself, which GDBN always does if possible.  If the next source line cannot be displayed for some reason (e.g., if GDBN cannot find the source file, or there's no line info in the debug info), GDBN will display disassembly of the next instruction instead of showing the next source line.  If AUTO, GDBN will display disassembly of next instruction only if the source line cannot be displayed.  This setting causes GDBN to display some feedback when you step through a function with no line info or whose source file is unavailable.  The default is OFF, which means never display the disassembly of the next line or instruction.

Examining Data
**************

The usual way to examine data in your program is with the print command (abbreviated p), or its synonym inspect.  It evaluates and prints the value of an expression of the language your program is written in (see Using GDBN with Different Languages).  It may also print the expression using a Python-based pretty-printer (see Pretty Printing).

print EXPR
print /F EXPR
     EXPR is an expression (in the source language).  By default the value of EXPR is printed in a format appropriate to its data type; you can choose a different format by specifying /F, where F is a letter specifying the format; see Output Formats.

print
print /F
     If you omit EXPR, GDBN displays the last value again (from the value history; see Value History).  This allows you to conveniently inspect the same value in an alternative format.

A more low-level way of examining data is with the x command. It examines data in memory at a specified address and prints it in a specified format.  See Examining Memory.

If you are interested in information about types, or about how the fields of a struct or a class are declared, use the ptype EXP command rather than print.  See Examining the Symbol Table.

Expressions
===========

print and many other GDBN commands accept an expression and compute its value.  Any kind of constant, variable or operator defined by the programming language you are using is valid in an expression in GDBN.  This includes conditional expressions, function calls, casts, and string constants.  It also includes preprocessor macros, if you compiled your program to include this information; see Compilation.

GDBN supports array constants in expressions input by the user.  The syntax is {ELEMENT, ELEMENT...}.  For example, you can use the command print {1, 2, 3} to create an array of three integers.  If you pass an array to a function or assign it to a program variable, GDBN copies the array to memory that is malloced in the target program.

Because C is so widespread, most of the expressions shown in examples in this manual are in C.  See Using GDBN with Different Languages, for information on how to use expressions in other languages.

In this section, we discuss operators that you can use in GDBN expressions regardless of your programming language.

Casts are supported in all languages, not just in C, because it is so useful to cast a number into a pointer in order to examine a structure at that address in memory.

GDBN supports these operators, in addition to those common to programming languages:

@
     @ is a binary operator for treating parts of memory as arrays. See Artificial Arrays, for more information.

::
     :: allows you to specify a variable in terms of the file or function where it is defined.  See Program Variables.

{TYPE} ADDR
     Refers to an object of type TYPE stored at address ADDR in memory.  ADDR may be any expression whose value is an integer or pointer (but parentheses are required around binary operators, just as in a cast).  This construct is allowed regardless of what kind of data is normally supposed to reside at ADDR.

Ambiguous Expressions
=====================

Expressions can sometimes contain some ambiguous elements.  For instance, some programming languages (notably Ada, C++ and Objective-C) permit a single function name to be defined several times, for application in different contexts.  This is called overloading.  Another example involving Ada is generics.  A generic package is similar to C++ templates and is typically instantiated several times, resulting in the same function name being defined in different contexts.

In some cases and depending on the language, it is possible to adjust the expression to remove the ambiguity.  For instance in C++, you can specify the signature of the function you want to break on, as in break FUNCTION(TYPES).  In Ada, using the fully qualified name of your function often makes the expression unambiguous as well.

When an ambiguity that needs to be resolved is detected, the debugger has the capability to display a menu of numbered choices for each possibility, and then waits for the selection with the prompt >. The first option is always [0] cancel, and typing 0 RET aborts the current command.  If the command in which the expression was used allows more than one choice to be selected, the next option in the menu is [1] all, and typing 1 RET selects all possible choices.

For example, the following session excerpt shows an attempt to set a breakpoint at the overloaded symbol String::after. We choose three particular definitions of that function name:

     (GDBP) b String::after
     [0] cancel
     [1] all
     [2] file:String.cc; line number:867
     [3] file:String.cc; line number:860
     [4] file:String.cc; line number:875
     [5] file:String.cc; line number:853
     [6] file:String.cc; line number:846
     [7] file:String.cc; line number:735
     > 2 4 6
     Breakpoint 1 at 0xb26c: file String.cc, line 867.
     Breakpoint 2 at 0xb344: file String.cc, line 875.
     Breakpoint 3 at 0xafcc: file String.cc, line 846.
     Multiple breakpoints were set.
     Use the "delete" command to delete unwanted
      breakpoints.
     (GDBP)

set multiple-symbols MODE

     This option allows you to adjust the debugger behavior when an expression is ambiguous.

     By default, MODE is set to all.  If the command with which the expression is used allows more than one choice, then GDBN automatically selects all possible choices.  For instance, inserting a breakpoint on a function using an ambiguous name results in a breakpoint inserted on each possible match.  However, if a unique choice must be made, then GDBN uses the menu to help you disambiguate the expression. For instance, printing the address of an overloaded function will result in the use of the menu.

     When MODE is set to ask, the debugger always uses the menu when an ambiguity is detected.

     Finally, when MODE is set to cancel, the debugger reports an error due to the ambiguity and the command is aborted.

show multiple-symbols
     Show the current value of the multiple-symbols setting.

Program Variables
=================

The most common kind of expression to use is the name of a variable in your program.

Variables in expressions are understood in the selected stack frame (see Selecting a Frame); they must be either:

  * global (or file-static)

or

  * visible according to the scope rules of the programming language from the point of execution in that frame

This means that in the function

     foo (a)
          int a;
     {
       bar (a);
       {
         int b = test ();
         bar (b);
       }
     }

you can examine and use the variable a whenever your program is executing within the function foo, but you can only use or examine the variable b while your program is executing inside the block where b is declared.

There is an exception: you can refer to a variable or function whose scope is a single source file even if the current execution point is not in this file.  But it is possible to have more than one such variable or function with the same name (in different source files).  If that happens, referring to that name has unpredictable effects.  If you wish, you can specify a static variable in a particular function or file by using the colon-colon (::) notation:

     FILE::VARIABLE
     FUNCTION::VARIABLE

Here FILE or FUNCTION is the name of the context for the static VARIABLE.  In the case of file names, you can use quotes to make sure GDBN parses the file name as a single word---for example, to print a global value of x defined in f2.c:

     (GDBP) p 'f2.c'::x

The :: notation is normally used for referring to static variables, since you typically disambiguate uses of local variables in functions by selecting the appropriate frame and using the simple name of the variable.  However, you may also use this notation to refer to local variables in frames enclosing the selected frame:

     void
     foo (int a)
     {
       if (a < 10)
         bar (a);
       else
         process (a);    /* Stop here */
     }

     int
     bar (int a)
     {
       foo (a + 5);
     }

For example, if there is a breakpoint at the commented line, here is what you might see when the program stops after executing the call bar(0):

     (GDBP) p a
     $1 = 10
     (GDBP) p bar::a
     $2 = 5
     (GDBP) up 2
     #2  0x080483d0 in foo (a=5) at foobar.c:12
     (GDBP) p a
     $3 = 5
     (GDBP) p bar::a
     $4 = 0

These uses of :: are very rarely in conflict with the very similar use of the same notation in C++.  GDBN also supports use of the C++ scope resolution operator in GDBN expressions.

     Warning: Occasionally, a local variable may appear to have the wrong value at certain points in a function---just after entry to a new scope, and just before exit.

You may see this problem when you are stepping by machine instructions. This is because, on most machines, it takes more than one instruction to set up a stack frame (including local variable definitions); if you are stepping by machine instructions, variables may appear to have the wrong values until the stack frame is completely built.  On exit, it usually also takes more than one machine instruction to destroy a stack frame; after you begin stepping through that group of instructions, local variable definitions may be gone.

This may also happen when the compiler does significant optimizations. To be sure of always seeing accurate values, turn off all optimization when compiling.

Another possible effect of compiler optimizations is to optimize unused variables out of existence, or assign variables to registers (as opposed to memory addresses).  Depending on the support for such cases offered by the debug info format used by the compiler, GDBN might not be able to display values for such local variables.  If that happens, GDBN will print a message like this:

     No symbol "foo" in current context.

To solve such problems, either recompile without optimizations, or use a different debug info format, if the compiler supports several such formats.  See Compilation, for more information on choosing compiler options.  See C and C++, for more information about debug info formats that are best suited to C++ programs.

If you ask to print an object whose contents are unknown to GDBN, e.g., because its data type is not completely specified by the debug information, GDBN will say <incomplete
type>.  See incomplete type, for more about this.

If you append @entry string to a function parameter name you get its value at the time the function got called.  If the value is not available an error message is printed.  Entry values are available only with some compilers. Entry values are normally also printed at the function parameter list according to set print entry-values.

     Breakpoint 1, d (i=30) at gdb.base/entry-value.c:29
     29	  i++;
     (gdb) next
     30	  e (i);
     (gdb) print i
     $1 = 31
     (gdb) print i@entry
     $2 = 30

Strings are identified as arrays of char values without specified signedness.  Arrays of either signed char or unsigned char get printed as arrays of 1 byte sized integers.  -fsigned-char or -funsigned-char NGCC options have no effect as GDBN defines literal string type "char" as char without a sign. For program code

     char var0[] = "A";
     signed char var1[] = "A";

You get during debugging

     (gdb) print var0
     $1 = "A"
     (gdb) print var1
     $2 = {65 'A', 0 '\0'}

Artificial Arrays
=================

It is often useful to print out several successive objects of the same type in memory; a section of an array, or an array of dynamically determined size for which only a pointer exists in the program.

You can do this by referring to a contiguous span of memory as an artificial array, using the binary operator @.  The left operand of @ should be the first element of the desired array and be an individual object.  The right operand should be the desired length of the array.  The result is an array value whose elements are all of the type of the left argument.  The first element is actually the left argument; the second element comes from bytes of memory immediately following those that hold the first element, and so on.  Here is an example.  If a program says

     int *array = (int *) malloc (len * sizeof (int));

you can print the contents of array with

     p *array@len

The left operand of @ must reside in memory.  Array values made with @ in this way behave just like other arrays in terms of subscripting, and are coerced to pointers when used in expressions. Artificial arrays most often appear in expressions via the value history (see Value History), after printing one out.

Another way to create an artificial array is to use a cast. This re-interprets a value as if it were an array. The value need not be in memory:

     (GDBP) p/x (short[2])0x12345678
     $1 = {0x1234, 0x5678}

As a convenience, if you leave the array length out (as in (TYPE[])VALUE) GDBN calculates the size to fill the value (as sizeof(VALUE)/sizeof(TYPE):

     (GDBP) p/x (short[])0x12345678
     $2 = {0x1234, 0x5678}

Sometimes the artificial array mechanism is not quite enough; in moderately complex data structures, the elements of interest may not actually be adjacent---for example, if you are interested in the values of pointers in an array.  One useful work-around in this situation is to use a convenience variable (see Convenience Variables) as a counter in an expression that prints the first interesting value, and then repeat that expression via RET.  For instance, suppose you have an array dtab of pointers to structures, and you are interested in the values of a field fv in each structure.  Here is an example of what you might type:

     set $i = 0
     p dtab[$i++]->fv
     RET
     RET
     ...

Output Formats
==============

By default, GDBN prints a value according to its data type.  Sometimes this is not what you want.  For example, you might want to print a number in hex, or a pointer in decimal.  Or you might want to view data in memory at a certain address as a character string or as an instruction.  To do these things, specify an output format when you print a value.

The simplest use of output formats is to say how to print a value already computed.  This is done by starting the arguments of the print command with a slash and a format letter.  The format letters supported are:

x
     Regard the bits of the value as an integer, and print the integer in hexadecimal.

d
     Print as integer in signed decimal.

u
     Print as integer in unsigned decimal.

o
     Print as integer in octal.

t
     Print as integer in binary.  The letter t stands for ``two''. (b cannot be used because these format letters are also used with the x command, where b stands for ``byte''; see Examining Memory.)

a
     Print as an address, both absolute in hexadecimal and as an offset from the nearest preceding symbol.  You can use this format used to discover where (in what function) an unknown address is located:

          (GDBP) p/a 0x54320
          $3 = 0x54320 <_initialize_vx+396>

     The command info symbol 0x54320 yields similar results. See info symbol.

c
     Regard as an integer and print it as a character constant.  This prints both the numerical value and its character representation.  The character representation is replaced with the octal escape \nnn for characters outside the 7-bit ASCII range.

     Without this format, GDBN displays char, unsigned char, and signed char data as character constants.  Single-byte members of vectors are displayed as integer data.

f
     Regard the bits of the value as a floating point number and print using typical floating point syntax.

s
     Regard as a string, if possible.  With this format, pointers to single-byte data are displayed as null-terminated strings and arrays of single-byte data are displayed as fixed-length strings.  Other values are displayed in their natural types.

     Without this format, GDBN displays pointers to and arrays of char, unsigned char, and signed char as strings.  Single-byte members of a vector are displayed as an integer array.

r
     Print using the raw formatting.  By default, GDBN will use a Python-based pretty-printer, if one is available (see Pretty Printing).  This typically results in a higher-level display of the value's contents.  The r format bypasses any Python pretty-printer which might exist.

For example, to print the program counter in hex (see Registers), type

     p/x $pc

Note that no space is required before the slash; this is because command names in GDBN cannot contain a slash.

To reprint the last value in the value history with a different format, you can use the print command with just a format and no expression.  For example, p/x reprints the last value in hex.

Examining Memory
================

You can use the command x (for ``examine'') to examine memory in any of several formats, independently of your program's data types.

x/NFU ADDR
x ADDR
x
     Use the x command to examine memory.

N, F, and U are all optional parameters that specify how much memory to display and how to format it; ADDR is an expression giving the address where you want to start displaying memory. If you use defaults for NFU, you need not type the slash /. Several commands set convenient defaults for ADDR.

N, the repeat count
     The repeat count is a decimal integer; the default is 1.  It specifies how much memory (counting by units U) to display.

F, the display format
     The display format is one of the formats used by print (x, d, u, o, t, a, c, f, s), and in addition i (for machine instructions). The default is x (hexadecimal) initially.  The default changes each time you use either x or print.

U, the unit size
     The unit size is any of

     b
          Bytes.

     h
          Halfwords (two bytes).

     w
          Words (four bytes).  This is the initial default.

     g
          Giant words (eight bytes).

     Each time you specify a unit size with x, that size becomes the default unit the next time you use x.  For the i format, the unit size is ignored and is normally not written.  For the s format, the unit size defaults to b, unless it is explicitly given. Use x /hs to display 16-bit char strings and x /ws to display 32-bit strings.  The next use of x /s will again display 8-bit strings. Note that the results depend on the programming language of the current compilation unit.  If the language is C, the s modifier will use the UTF-16 encoding while w will use UTF-32.  The encoding is set by the programming language and cannot be altered.

ADDR, starting display address
     ADDR is the address where you want GDBN to begin displaying memory.  The expression need not have a pointer value (though it may); it is always interpreted as an integer address of a byte of memory. See Expressions, for more information on expressions.  The default for ADDR is usually just after the last address examined---but several other commands also set the default address: info breakpoints (to the address of the last breakpoint listed), info line (to the starting address of a line), and print (if you use it to display a value from memory).

For example, x/3uh 0x54320 is a request to display three halfwords (h) of memory, formatted as unsigned decimal integers (u), starting at address 0x54320.  x/4xw $sp prints the four words (w) of memory above the stack pointer (here, $sp; see Registers) in hexadecimal (x).

Since the letters indicating unit sizes are all distinct from the letters specifying output formats, you do not have to remember whether unit size or format comes first; either order works.  The output specifications 4xw and 4wx mean exactly the same thing. (However, the count N must come first; wx4 does not work.)

Even though the unit size U is ignored for the formats s and i, you might still want to use a count N; for example, 3i specifies that you want to see three machine instructions, including any operands.  For convenience, especially when used with the display command, the i format also prints branch delay slot instructions, if any, beyond the count specified, which immediately follow the last instruction that is within the count.  The command disassemble gives an alternative way of inspecting machine instructions; see Source and Machine Code.

All the defaults for the arguments to x are designed to make it easy to continue scanning memory with minimal specifications each time you use x.  For example, after you have inspected three machine instructions with x/3i ADDR, you can inspect the next seven with just x/7.  If you use RET to repeat the x command, the repeat count N is used again; the other arguments default as for successive uses of x.

When examining machine instructions, the instruction at current program counter is shown with a => marker. For example:

     (GDBP) x/5i $pc-6
        0x804837f <main+11>: mov    %esp,%ebp
        0x8048381 <main+13>: push   %ecx
        0x8048382 <main+14>: sub    $0x4,%esp
     => 0x8048385 <main+17>: movl   $0x8048460,(%esp)
        0x804838c <main+24>: call   0x80482d4 <puts@plt>

The addresses and contents printed by the x command are not saved in the value history because there is often too much of them and they would get in the way.  Instead, GDBN makes these values available for subsequent use in expressions as values of the convenience variables $_ and $__.  After an x command, the last address examined is available for use in expressions in the convenience variable $_.  The contents of that address, as examined, are available in the convenience variable $__.

If the x command has a repeat count, the address and contents saved are from the last memory unit printed; this is not the same as the last address printed if several units were printed on the last line of output.

When you are debugging a program running on a remote target machine (see Remote Debugging), you may wish to verify the program's image in the remote machine's memory against the executable file you downloaded to the target.  The compare-sections command is provided for such situations.

compare-sections [SECTION-NAME]
     Compare the data of a loadable section SECTION-NAME in the executable file of the program being debugged with the same section in the remote machine's memory, and report any mismatches.  With no arguments, compares all loadable sections.  This command's availability depends on the target's support for the "qCRC" remote request.

Automatic Display
=================

If you find that you want to print the value of an expression frequently (to see how it changes), you might want to add it to the automatic
display list so that GDBN prints its value each time your program stops. Each expression added to the list is given a number to identify it; to remove an expression from the list, you specify that number. The automatic display looks like this:

     2: foo = 38
     3: bar[5] = (struct hack *) 0x3804

This display shows item numbers, expressions and their current values.  As with displays you request manually using x or print, you can specify the output format you prefer; in fact, display decides whether to use print or x depending your format specification---it uses x if you specify either the i or s format, or a unit size; otherwise it uses print.

display EXPR
     Add the expression EXPR to the list of expressions to display each time your program stops.  See Expressions.

     display does not repeat if you press RET again after using it.

display/FMT EXPR
     For FMT specifying only a display format and not a size or count, add the expression EXPR to the auto-display list but arrange to display it each time in the specified format FMT. See Output Formats.

display/FMT ADDR
     For FMT i or s, or including a unit-size or a number of units, add the expression ADDR as a memory address to be examined each time your program stops.  Examining means in effect doing x/FMT ADDR.  See Examining Memory.

For example, display/i $pc can be helpful, to see the machine instruction about to be executed each time execution stops ($pc is a common name for the program counter; see Registers).

undisplay DNUMS...
delete display DNUMS...
     Remove items from the list of expressions to display.  Specify the numbers of the displays that you want affected with the command argument DNUMS.  It can be a single display number, one of the numbers shown in the first field of the info display display; or it could be a range of display numbers, as in 2-4.

     undisplay does not repeat if you press RET after using it. (Otherwise you would just get the error No display number ....)

disable display DNUMS...
     Disable the display of item numbers DNUMS.  A disabled display item is not printed automatically, but is not forgotten.  It may be enabled again later.  Specify the numbers of the displays that you want affected with the command argument DNUMS.  It can be a single display number, one of the numbers shown in the first field of the info display display; or it could be a range of display numbers, as in 2-4.

enable display DNUMS...
     Enable display of item numbers DNUMS.  It becomes effective once again in auto display of its expression, until you specify otherwise. Specify the numbers of the displays that you want affected with the command argument DNUMS.  It can be a single display number, one of the numbers shown in the first field of the info display display; or it could be a range of display numbers, as in 2-4.

display
     Display the current values of the expressions on the list, just as is done when your program stops.

info display
     Print the list of expressions previously set up to display automatically, each one with its item number, but without showing the values.  This includes disabled expressions, which are marked as such. It also includes expressions which would not be displayed right now because they refer to automatic variables not currently available.

If a display expression refers to local variables, then it does not make sense outside the lexical context for which it was set up.  Such an expression is disabled when execution enters a context where one of its variables is not defined.  For example, if you give the command display last_char while inside a function with an argument last_char, GDBN displays this argument while your program continues to stop inside that function.  When it stops elsewhere---where there is no variable last_char---the display is disabled automatically.  The next time your program stops where last_char is meaningful, you can enable the display expression once again.

Print Settings
==============

GDBN provides the following ways to control how arrays, structures, and symbols are printed.

These settings are useful for debugging programs in any language:

set print address
set print address on
     GDBN prints memory addresses showing the location of stack traces, structure values, pointer values, breakpoints, and so forth, even when it also displays the contents of those addresses.  The default is on.  For example, this is what a stack frame display looks like with set print address on:

          (GDBP) f
          #0  set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>")
              at input.c:530
          530         if (lquote != def_lquote)

set print address off
     Do not print addresses when displaying their contents.  For example, this is the same stack frame displayed with set print address off:

          (GDBP) set print addr off
          (GDBP) f
          #0  set_quotes (lq="<<", rq=">>") at input.c:530
          530         if (lquote != def_lquote)

     You can use set print address off to eliminate all machine dependent displays from the GDBN interface.  For example, with print address off, you should get the same text for backtraces on all machines---whether or not they involve pointer arguments.

show print address
     Show whether or not addresses are to be printed.

When GDBN prints a symbolic address, it normally prints the closest earlier symbol plus an offset.  If that symbol does not uniquely identify the address (for example, it is a name whose scope is a single source file), you may need to clarify.  One way to do this is with info line, for example info line *0x4537.  Alternately, you can set GDBN to print the source file and line number when it prints a symbolic address:

set print symbol-filename on
     Tell GDBN to print the source file name and line number of a symbol in the symbolic form of an address.

set print symbol-filename off
     Do not print source file name and line number of a symbol.  This is the default.

show print symbol-filename
     Show whether or not GDBN will print the source file name and line number of a symbol in the symbolic form of an address.

Another situation where it is helpful to show symbol filenames and line numbers is when disassembling code; GDBN shows you the line number and source file that corresponds to each instruction.

Also, you may wish to see the symbolic form only if the address being printed is reasonably close to the closest earlier symbol:

set print max-symbolic-offset MAX-OFFSET
     Tell GDBN to only display the symbolic form of an address if the offset between the closest earlier symbol and the address is less than MAX-OFFSET.  The default is 0, which tells GDBN to always print the symbolic form of an address if any symbol precedes it.

show print max-symbolic-offset
     Ask how large the maximum offset is that GDBN prints in a symbolic address.

If you have a pointer and you are not sure where it points, try set print symbol-filename on.  Then you can determine the name and source file location of the variable where it points, using p/a POINTER.  This interprets the address in symbolic form. For example, here GDBN shows that a variable ptt points at another variable t, defined in hi2.c:

     (GDBP) set print symbol-filename on
     (GDBP) p/a ptt
     $4 = 0xe008 <t in hi2.c>

     Warning: For pointers that point to a local variable, p/a does not show the symbol name and filename of the referent, even with the appropriate set print options turned on.

Other settings control how different kinds of objects are printed:

set print array
set print array on
     Pretty print arrays.  This format is more convenient to read, but uses more space.  The default is off.

set print array off
     Return to compressed format for arrays.

show print array
     Show whether compressed or pretty format is selected for displaying arrays.

set print array-indexes
set print array-indexes on
     Print the index of each element when displaying arrays.  May be more convenient to locate a given element in the array or quickly find the index of a given element in that printed array.  The default is off.

set print array-indexes off
     Stop printing element indexes when displaying arrays.

show print array-indexes
     Show whether the index of each element is printed when displaying arrays.

set print elements NUMBER-OF-ELEMENTS
     Set a limit on how many elements of an array GDBN will print. If GDBN is printing a large array, it stops printing after it has printed the number of elements set by the set print elements command. This limit also applies to the display of strings. When GDBN starts, this limit is set to 200. Setting  NUMBER-OF-ELEMENTS to zero means that the printing is unlimited.

show print elements
     Display the number of elements of a large array that GDBN will print. If the number is 0, then the printing is unlimited.

set print frame-arguments VALUE
     This command allows to control how the values of arguments are printed when the debugger prints a frame (see Frames).  The possible values are:

     all
          The values of all arguments are printed.

     scalars
          Print the value of an argument only if it is a scalar.  The value of more complex arguments such as arrays, structures, unions, etc, is replaced by ....  This is the default.  Here is an example where only scalar arguments are shown:

               #1  0x08048361 in call_me (i=3, s=..., ss=0xbf8d508c, u=..., e=green)
                 at frame-args.c:23

     none
          None of the argument values are printed.  Instead, the value of each argument is replaced by ....  In this case, the example above now becomes:

               #1  0x08048361 in call_me (i=..., s=..., ss=..., u=..., e=...)
                 at frame-args.c:23

     By default, only scalar arguments are printed.  This command can be used to configure the debugger to print the value of all arguments, regardless of their type.  However, it is often advantageous to not print the value of more complex parameters.  For instance, it reduces the amount of information printed in each frame, making the backtrace more readable. Also, it improves performance when displaying Ada frames, because the computation of large arguments can sometimes be CPU-intensive, especially in large applications.  Setting print frame-arguments to scalars (the default) or none avoids this computation, thus speeding up the display of each Ada frame.

show print frame-arguments
     Show how the value of arguments should be displayed when printing a frame.

set print entry-values VALUE
     Set printing of frame argument values at function entry.  In some cases GDBN can determine the value of function argument which was passed by the function caller, even if the value was modified inside the called function and therefore is different.  With optimized code, the current value could be unavailable, but the entry value may still be known.

     The default value is default (see below for its description).  Older GDBN behaved as with the setting no.  Compilers not supporting this feature will behave in the default setting the same way as with the no setting.

     This functionality is currently supported only by DWARF 2 debugging format and the compiler has to produce DW_TAG_GNU_call_site tags.  With NGCC, you need to specify -O -g during compilation, to get this information.

     The VALUE parameter can be one of the following:

     no
          Print only actual parameter values, never print values from function entry point.

               #0  equal (val=5)
               #0  different (val=6)
               #0  lost (val=<optimized out>)
               #0  born (val=10)
               #0  invalid (val=<optimized out>)

     only
          Print only parameter values from function entry point.  The actual parameter values are never printed.

               #0  equal (val@entry=5)
               #0  different (val@entry=5)
               #0  lost (val@entry=5)
               #0  born (val@entry=<optimized out>)
               #0  invalid (val@entry=<optimized out>)

     preferred
          Print only parameter values from function entry point.  If value from function entry point is not known while the actual value is known, print the actual value for such parameter.

               #0  equal (val@entry=5)
               #0  different (val@entry=5)
               #0  lost (val@entry=5)
               #0  born (val=10)
               #0  invalid (val@entry=<optimized out>)

     if-needed
          Print actual parameter values.  If actual parameter value is not known while value from function entry point is known, print the entry point value for such parameter.

               #0  equal (val=5)
               #0  different (val=6)
               #0  lost (val@entry=5)
               #0  born (val=10)
               #0  invalid (val=<optimized out>)

     both
          Always print both the actual parameter value and its value from function entry point, even if values of one or both are not available due to compiler optimizations.

               #0  equal (val=5, val@entry=5)
               #0  different (val=6, val@entry=5)
               #0  lost (val=<optimized out>, val@entry=5)
               #0  born (val=10, val@entry=<optimized out>)
               #0  invalid (val=<optimized out>, val@entry=<optimized out>)

     compact
          Print the actual parameter value if it is known and also its value from function entry point if it is known.  If neither is known, print for the actual value <optimized out>.  If not in MI mode (see GDB/MI) and if both values are known and identical, print the shortened param=param@entry=VALUE notation.

               #0  equal (val=val@entry=5)
               #0  different (val=6, val@entry=5)
               #0  lost (val@entry=5)
               #0  born (val=10)
               #0  invalid (val=<optimized out>)

     default
          Always print the actual parameter value.  Print also its value from function entry point, but only if it is known.  If not in MI mode (see GDB/MI) and if both values are known and identical, print the shortened param=param@entry=VALUE notation.

               #0  equal (val=val@entry=5)
               #0  different (val=6, val@entry=5)
               #0  lost (val=<optimized out>, val@entry=5)
               #0  born (val=10)
               #0  invalid (val=<optimized out>)

     For analysis messages on possible failures of frame argument values at function entry resolution see set debug entry-values.

show print entry-values
     Show the method being used for printing of frame argument values at function entry.

set print repeats
     Set the threshold for suppressing display of repeated array elements.  When the number of consecutive identical elements of an array exceeds the threshold, GDBN prints the string "<repeats N times>", where N is the number of identical repetitions, instead of displaying the identical elements themselves.  Setting the threshold to zero will cause all elements to be individually printed.  The default threshold is 10.

show print repeats
     Display the current threshold for printing repeated identical elements.

set print null-stop
     Cause GDBN to stop printing the characters of an array when the first NULL is encountered.  This is useful when large arrays actually contain only short strings. The default is off.

show print null-stop
     Show whether GDBN stops printing an array on the first NULL character.

set print pretty on
     Cause GDBN to print structures in an indented format with one member per line, like this:

          $1 = {
            next = 0x0,
            flags = {
              sweet = 1,
              sour = 1
            },
            meat = 0x54 "Pork"
          }

set print pretty off
     Cause GDBN to print structures in a compact format, like this:

          $1 = {next = 0x0, flags = {sweet = 1, sour = 1}, \
          meat = 0x54 "Pork"}

     This is the default format.

show print pretty
     Show which format GDBN is using to print structures.

set print sevenbit-strings on
     Print using only seven-bit characters; if this option is set, GDBN displays any eight-bit characters (in strings or character values) using the notation \NNN.  This setting is best if you are working in English (ASCII) and you use the high-order bit of characters as a marker or ``meta'' bit.

set print sevenbit-strings off
     Print full eight-bit characters.  This allows the use of more international character sets, and is the default.

show print sevenbit-strings
     Show whether or not GDBN is printing only seven-bit characters.

set print union on
     Tell GDBN to print unions which are contained in structures and other unions.  This is the default setting.

set print union off
     Tell GDBN not to print unions which are contained in structures and other unions.  GDBN will print "{...}" instead.

show print union
     Ask GDBN whether or not it will print unions which are contained in structures and other unions.

     For example, given the declarations

          typedef enum {Tree, Bug} Species;
          typedef enum {Big_tree, Acorn, Seedling} Tree_forms;
          typedef enum {Caterpillar, Cocoon, Butterfly}
                        Bug_forms;

          struct thing {
            Species it;
            union {
              Tree_forms tree;
              Bug_forms bug;
            } form;
          };

          struct thing foo = {Tree, {Acorn}};

     with set print union on in effect p foo would print

          $1 = {it = Tree, form = {tree = Acorn, bug = Cocoon}}

     and with set print union off in effect it would print

          $1 = {it = Tree, form = {...}}

     set print union affects programs written in C-like languages and in Pascal.

These settings are of interest when debugging C++ programs:

set print demangle
set print demangle on
     Print C++ names in their source form rather than in the encoded (``mangled'') form passed to the assembler and linker for type-safe linkage.  The default is on.

show print demangle
     Show whether C++ names are printed in mangled or demangled form.

set print asm-demangle
set print asm-demangle on
     Print C++ names in their source form rather than their mangled form, even in assembler code printouts such as instruction disassemblies. The default is off.

show print asm-demangle
     Show whether C++ names in assembly listings are printed in mangled or demangled form.

set demangle-style STYLE
     Choose among several encoding schemes used by different compilers to represent C++ names.  The choices for STYLE are currently:

     auto
          Allow GDBN to choose a decoding style by inspecting your program.

     gnu
          Decode based on the GNU C++ compiler (g++) encoding algorithm. This is the default.

     hp
          Decode based on the HP ANSI C++ (aCC) encoding algorithm.

     lucid
          Decode based on the Lucid C++ compiler (lcc) encoding algorithm.

     arm
          Decode using the algorithm in the C++ Annotated Reference Manual. Warning: this setting alone is not sufficient to allow debugging cfront-generated executables.  GDBN would require further enhancement to permit that.

     If you omit STYLE, you will see a list of possible formats.

show demangle-style
     Display the encoding style currently in use for decoding C++ symbols.

set print object
set print object on
     When displaying a pointer to an object, identify the actual (derived) type of the object rather than the declared type, using the virtual function table.  Note that the virtual function table is required---this feature can only work for objects that have run-time type identification; a single virtual method in the object's declared type is sufficient.

set print object off
     Display only the declared type of objects, without reference to the virtual function table.  This is the default setting.

show print object
     Show whether actual, or declared, object types are displayed.

set print static-members
set print static-members on
     Print static members when displaying a C++ object.  The default is on.

set print static-members off
     Do not print static members when displaying a C++ object.

show print static-members
     Show whether C++ static members are printed or not.

set print pascal_static-members
set print pascal_static-members on
     Print static members when displaying a Pascal object.  The default is on.

set print pascal_static-members off
     Do not print static members when displaying a Pascal object.

show print pascal_static-members
     Show whether Pascal static members are printed or not.

set print vtbl
set print vtbl on
     Pretty print C++ virtual function tables.  The default is off. (The vtbl commands do not work on programs compiled with the HP ANSI C++ compiler (aCC).)

set print vtbl off
     Do not pretty print C++ virtual function tables.

show print vtbl
     Show whether C++ virtual function tables are pretty printed, or not.

Pretty Printing
===============

GDBN provides a mechanism to allow pretty-printing of values using Python code.  It greatly simplifies the display of complex objects.  This mechanism works for both MI and the CLI.

Pretty-Printer Introduction
---------------------------

When GDBN prints a value, it first sees if there is a pretty-printer registered for the value.  If there is then GDBN invokes the pretty-printer to print the value.  Otherwise the value is printed normally.

Pretty-printers are normally named.  This makes them easy to manage. The info pretty-printer command will list all the installed pretty-printers with their names. If a pretty-printer can handle multiple data types, then its subprinters are the printers for the individual data types. Each such subprinter has its own name. The format of the name is PRINTER-NAME;SUBPRINTER-NAME.

Pretty-printers are installed by registering them with GDBN. Typically they are automatically loaded and registered when the corresponding debug information is loaded, thus making them available without having to do anything special.

There are three places where a pretty-printer can be registered.

  * Pretty-printers registered globally are available when debugging all inferiors.

  * Pretty-printers registered with a program space are available only when debugging that program. See Progspaces In Python, for more details on program spaces in Python.

  * Pretty-printers registered with an objfile are loaded and unloaded with the corresponding objfile (e.g., shared library). See Objfiles In Python, for more details on objfiles in Python.

See Selecting Pretty-Printers, for further information on how pretty-printers are selected,

See Writing a Pretty-Printer, for implementing pretty printers for new types.

Pretty-Printer Example
----------------------

Here is how a C++ std::string looks without a pretty-printer:

     (GDBP) print s
     $1 = {
       static npos = 4294967295,
       _M_dataplus = {
         <std::allocator<char>> = {
           <__gnu_cxx::new_allocator<char>> = {
             <No data fields>}, <No data fields>
           },
         members of std::basic_string<char, std::char_traits<char>,
           std::allocator<char> >::_Alloc_hider:
         _M_p = 0x804a014 "abcd"
       }
     }

With a pretty-printer for std::string only the contents are printed:

     (GDBP) print s
     $2 = "abcd"

Pretty-Printer Commands
-----------------------

info pretty-printer [OBJECT-REGEXP [NAME-REGEXP]]
     Print the list of installed pretty-printers. This includes disabled pretty-printers, which are marked as such.

     OBJECT-REGEXP is a regular expression matching the objects whose pretty-printers to list. Objects can be global, the program space's file (see Progspaces In Python), and the object files within that program space (see Objfiles In Python). See Selecting Pretty-Printers, for details on how GDBN looks up a printer from these three objects.

     NAME-REGEXP is a regular expression matching the name of the printers to list.

disable pretty-printer [OBJECT-REGEXP [NAME-REGEXP]]
     Disable pretty-printers matching OBJECT-REGEXP and NAME-REGEXP. A disabled pretty-printer is not forgotten, it may be enabled again later.

enable pretty-printer [OBJECT-REGEXP [NAME-REGEXP]]
     Enable pretty-printers matching OBJECT-REGEXP and NAME-REGEXP.

Example:

Suppose we have three pretty-printers installed: one from library1.so named foo that prints objects of type foo, and another from library2.so named bar that prints two types of objects, bar1 and bar2.

     (gdb) info pretty-printer
     library1.so:
       foo
     library2.so:
       bar
         bar1
         bar2
     (gdb) info pretty-printer library2
     library2.so:
       bar
         bar1
         bar2
     (gdb) disable pretty-printer library1
     1 printer disabled
     2 of 3 printers enabled
     (gdb) info pretty-printer
     library1.so:
       foo [disabled]
     library2.so:
       bar
         bar1
         bar2
     (gdb) disable pretty-printer library2 bar:bar1
     1 printer disabled
     1 of 3 printers enabled
     (gdb) info pretty-printer library2
     library1.so:
       foo [disabled]
     library2.so:
       bar
         bar1 [disabled]
         bar2
     (gdb) disable pretty-printer library2 bar
     1 printer disabled
     0 of 3 printers enabled
     (gdb) info pretty-printer library2
     library1.so:
       foo [disabled]
     library2.so:
       bar [disabled]
         bar1 [disabled]
         bar2

Note that for bar the entire printer can be disabled, as can each individual subprinter.

Value History
=============

Values printed by the print command are saved in the GDBN value history.  This allows you to refer to them in other expressions. Values are kept until the symbol table is re-read or discarded (for example with the file or symbol-file commands). When the symbol table changes, the value history is discarded, since the values may contain pointers back to the types defined in the symbol table.

The values printed are given history numbers by which you can refer to them.  These are successive integers starting with one. print shows you the history number assigned to a value by printing $NUM =  before the value; here NUM is the history number.

To refer to any previous value, use $ followed by the value's history number.  The way print labels its output is designed to remind you of this.  Just $ refers to the most recent value in the history, and $$ refers to the value before that. $$N refers to the Nth value from the end; $$2 is the value just prior to $$, $$1 is equivalent to $$, and $$0 is equivalent to $.

For example, suppose you have just printed a pointer to a structure and want to see the contents of the structure.  It suffices to type

     p *$

If you have a chain of structures where the component next points to the next one, you can print the contents of the next one with this:

     p *$.next

You can print successive links in the chain by repeating this command---which you can do by just typing RET.

Note that the history records values, not expressions.  If the value of x is 4 and you type these commands:

     print x
     set x=5

then the value recorded in the value history by the print command remains 4 even though the value of x has changed.

show values
     Print the last ten values in the value history, with their item numbers. This is like p $$9 repeated ten times, except that show values does not change the history.

show values N
     Print ten history values centered on history item number N.

show values +
     Print ten history values just after the values last printed.  If no more values are available, show values + produces no display.

Pressing RET to repeat show values N has exactly the same effect as show values +.

Convenience Variables
=====================

GDBN provides convenience variables that you can use within GDBN to hold on to a value and refer to it later.  These variables exist entirely within GDBN; they are not part of your program, and setting a convenience variable has no direct effect on further execution of your program.  That is why you can use them freely.

Convenience variables are prefixed with $.  Any name preceded by $ can be used for a convenience variable, unless it is one of the predefined machine-specific register names (see Registers). (Value history references, in contrast, are numbers preceded by $.  See Value History.)

You can save a value in a convenience variable with an assignment expression, just as you would set a variable in your program. For example:

     set $foo = *object_ptr

would save in $foo the value contained in the object pointed to by object_ptr.

Using a convenience variable for the first time creates it, but its value is void until you assign a new value.  You can alter the value with another assignment at any time.

Convenience variables have no fixed types.  You can assign a convenience variable any type of value, including structures and arrays, even if that variable already has a value of a different type.  The convenience variable, when used as an expression, has the type of its current value.

show convenience
     Print a list of convenience variables used so far, and their values. Abbreviated show conv.

init-if-undefined $VARIABLE = EXPRESSION
     Set a convenience variable if it has not already been set.  This is useful for user-defined commands that keep some state.  It is similar, in concept, to using local static variables with initializers in C (except that convenience variables are global).  It can also be used to allow users to override default values used in a command script.

     If the variable is already defined then the expression is not evaluated so any side-effects do not occur.

One of the ways to use a convenience variable is as a counter to be incremented or a pointer to be advanced.  For example, to print a field from successive elements of an array of structures:

     set $i = 0
     print bar[$i++]->contents

Repeat that command by typing RET.

Some convenience variables are created automatically by GDBN and given values likely to be useful.

$_
     The variable $_ is automatically set by the x command to the last address examined (see Examining Memory).  Other commands which provide a default address for x to examine also set $_ to that address; these commands include info line and info breakpoint.  The type of $_ is void * except when set by the x command, in which case it is a pointer to the type of $__.

$__
     The variable $__ is automatically set by the x command to the value found in the last address examined.  Its type is chosen to match the format in which the data was printed.

$_exitcode
     The variable $_exitcode is automatically set to the exit code when the program being debugged terminates.

$_sdata
     The variable $_sdata contains extra collected static tracepoint data.  See Tracepoint Action Lists.  Note that $_sdata could be empty, if not inspecting a trace buffer, or if extra static tracepoint data has not been collected.

$_siginfo
     The variable $_siginfo contains extra signal information (see extra signal information).  Note that $_siginfo could be empty, if the application has not yet received any signals. For example, it will be empty before you execute the run command.

$_tlb
     The variable $_tlb is automatically set when debugging applications running on MS-Windows in native mode or connected to gdbserver that supports the qGetTIBAddr request. See General Query Packets. This variable contains the address of the thread information block.

On HP-UX systems, if you refer to a function or variable name that begins with a dollar sign, GDBN searches for a user or system name first, before it searches for a convenience variable.

GDBN also supplies some convenience functions.  These have a syntax similar to convenience variables.  A convenience function can be used in an expression just like an ordinary function; however, a convenience function is implemented internally to GDBN.

help function
     Print a list of all convenience functions.

Registers
=========

You can refer to machine register contents, in expressions, as variables with names starting with $.  The names of registers are different for each machine; use info registers to see the names used on your machine.

info registers
     Print the names and values of all registers except floating-point and vector registers (in the selected stack frame).

info all-registers
     Print the names and values of all registers, including floating-point and vector registers (in the selected stack frame).

info registers REGNAME ...
     Print the relativized value of each specified register REGNAME. As discussed in detail below, register values are normally relative to the selected stack frame.  REGNAME may be any register name valid on the machine you are using, with or without the initial $.

GDBN has four ``standard'' register names that are available (in expressions) on most machines---whenever they do not conflict with an architecture's canonical mnemonics for registers.  The register names $pc and $sp are used for the program counter register and the stack pointer.  $fp is used for a register that contains a pointer to the current stack frame, and $ps is used for a register that contains the processor status.  For example, you could print the program counter in hex with

     p/x $pc

or print the instruction to be executed next with

     x/i $pc

or add four to the stack pointer (This is a way of removing one word from the stack, on machines where stacks grow downward in memory (most machines, nowadays).  This assumes that the innermost stack frame is selected; setting $sp is not allowed when other stack frames are selected.  To pop entire frames off the stack, regardless of machine architecture, use return; see Returning from a Function.) with

     set $sp += 4

Whenever possible, these four standard register names are available on your machine even though the machine has different canonical mnemonics, so long as there is no conflict.  The info registers command shows the canonical names.  For example, on the SPARC, info registers displays the processor status register as $psr but you can also refer to it as $ps; and on x86-based machines $ps is an alias for the EFLAGS register.

GDBN always considers the contents of an ordinary register as an integer when the register is examined in this way.  Some machines have special registers which can hold nothing but floating point; these registers are considered to have floating point values.  There is no way to refer to the contents of an ordinary register as floating point value (although you can print it as a floating point value with print/f $REGNAME).

Some registers have distinct ``raw'' and ``virtual'' data formats.  This means that the data format in which the register contents are saved by the operating system is not the same one that your program normally sees.  For example, the registers of the 68881 floating point coprocessor are always saved in ``extended'' (raw) format, but all C programs expect to work with ``double'' (virtual) format.  In such cases, GDBN normally works with the virtual format only (the format that makes sense for your program), but the info registers command prints the data in both formats.

Some machines have special registers whose contents can be interpreted in several different ways.  For example, modern x86-based machines have SSE and MMX registers that can hold several values packed together in several different formats.  GDBN refers to such registers in struct notation:

     (GDBP) print $xmm1
     $1 = {
       v4_float = {0, 3.43859137e-038, 1.54142831e-044, 1.821688e-044},
       v2_double = {9.92129282474342e-303, 2.7585945287983262e-313},
       v16_int8 = "\000\000\000\000\3706;\001\v\000\000\000\r\000\000",
       v8_int16 = {0, 0, 14072, 315, 11, 0, 13, 0},
       v4_int32 = {0, 20657912, 11, 13},
       v2_int64 = {88725056443645952, 55834574859},
       uint128 = 0x0000000d0000000b013b36f800000000
     }

To set values of such registers, you need to tell GDBN which view of the register you wish to change, as if you were assigning value to a struct member:

      (GDBP) set $xmm1.uint128 = 0x000000000000000000000000FFFFFFFF

Normally, register values are relative to the selected stack frame (see Selecting a Frame).  This means that you get the value that the register would contain if all stack frames farther in were exited and their saved registers restored.  In order to see the true contents of hardware registers, you must select the innermost frame (with frame 0).

However, GDBN must deduce where registers are saved, from the machine code generated by your compiler.  If some registers are not saved, or if GDBN is unable to locate the saved registers, the selected stack frame makes no difference.

Floating Point Hardware
=======================

Depending on the configuration, GDBN may be able to give you more information about the status of the floating point hardware.

info float
     Display hardware-dependent information about the floating point unit.  The exact contents and layout vary depending on the floating point chip.  Currently, info float is supported on the ARM and x86 machines.

Vector Unit
===========

Depending on the configuration, GDBN may be able to give you more information about the status of the vector unit.

info vector
     Display information about the vector unit.  The exact contents and layout vary depending on the hardware.

Operating System Auxiliary Information
======================================

GDBN provides interfaces to useful OS facilities that can help you debug your program.

When GDBN runs on a Posix system (such as GNU or Unix machines), it interfaces with the inferior via the ptrace system call.  The operating system creates a special sata structure, called struct user, for this interface.  You can use the command info udot to display the contents of this data structure.

info udot
     Display the contents of the struct user maintained by the OS kernel for the program being debugged.  GDBN displays the contents of struct user as a list of hex numbers, similar to the examine command.

Some operating systems supply an auxiliary vector to programs at startup.  This is akin to the arguments and environment that you specify for a program, but contains a system-dependent variety of binary values that tell system libraries important details about the hardware, operating system, and process.  Each value's purpose is identified by an integer tag; the meanings are well-known but system-specific. Depending on the configuration and operating system facilities, GDBN may be able to show you this information.  For remote targets, this functionality may further depend on the remote stub's support of the qXfer:auxv:read packet, see qXfer auxiliary vector read.

info auxv
     Display the auxiliary vector of the inferior, which can be either a live process or a core dump file.  GDBN prints each tag value numerically, and also shows names and text descriptions for recognized tags.  Some values in the vector are numbers, some bit masks, and some pointers to strings or other data.  GDBN displays each value in the most appropriate form for a recognized tag, and in hexadecimal for an unrecognized tag.

On some targets, GDBN can access operating-system-specific information and display it to user, without interpretation.  For remote targets, this functionality depends on the remote stub's support of the qXfer:osdata:read packet, see qXfer osdata read.

info os
     List the types of OS information available for the target.  If the target does not return a list of possible types, this command will report an error.

info os processes
     Display the list of processes on the target.  For each process, GDBN prints the process identifier, the name of the user, and the command corresponding to the process.

Memory Region Attributes
========================

Memory region attributes allow you to describe special handling required by regions of your target's memory.  GDBN uses attributes to determine whether to allow certain types of memory accesses; whether to use specific width accesses; and whether to cache target memory.  By default the description of memory regions is fetched from the target (if the current target supports this), but the user can override the fetched regions.

Defined memory regions can be individually enabled and disabled.  When a memory region is disabled, GDBN uses the default attributes when accessing memory in that region.  Similarly, if no memory regions have been defined, GDBN uses the default attributes when accessing all memory.

When a memory region is defined, it is given a number to identify it; to enable, disable, or remove a memory region, you specify that number.

mem LOWER UPPER ATTRIBUTES...
     Define a memory region bounded by LOWER and UPPER with attributes ATTRIBUTES..., and add it to the list of regions monitored by GDBN.  Note that UPPER == 0 is a special case: it is treated as the target's maximum memory address. (0xffff on 16 bit targets, 0xffffffff on 32 bit targets, etc.)

mem auto
     Discard any user changes to the memory regions and use target-supplied regions, if available, or no regions if the target does not support.

delete mem NUMS...
     Remove memory regions NUMS... from the list of regions monitored by GDBN.

disable mem NUMS...
     Disable monitoring of memory regions NUMS.... A disabled memory region is not forgotten. It may be enabled again later.

enable mem NUMS...
     Enable monitoring of memory regions NUMS....

info mem
     Print a table of all defined memory regions, with the following columns for each region:

     Memory Region Number

     Enabled or Disabled.
          Enabled memory regions are marked with y. Disabled memory regions are marked with n.

     Lo Address
          The address defining the inclusive lower bound of the memory region.

     Hi Address
          The address defining the exclusive upper bound of the memory region.

     Attributes
          The list of attributes set for this memory region.

Attributes
----------

Memory Access Mode
..................

The access mode attributes set whether GDBN may make read or write accesses to a memory region.

While these attributes prevent GDBN from performing invalid memory accesses, they do nothing to prevent the target system, I/O DMA, etc. from accessing memory.

ro
     Memory is read only.

wo
     Memory is write only.

rw
     Memory is read/write.  This is the default.

Memory Access Size
..................

The access size attribute tells GDBN to use specific sized accesses in the memory region.  Often memory mapped device registers require specific sized accesses.  If no access size attribute is specified, GDBN may use accesses of any size.

8
     Use 8 bit memory accesses.

16
     Use 16 bit memory accesses.

32
     Use 32 bit memory accesses.

64
     Use 64 bit memory accesses.

Data Cache
..........

The data cache attributes set whether GDBN will cache target memory.  While this generally improves performance by reducing debug protocol overhead, it can lead to incorrect results because GDBN does not know about volatile variables or memory mapped device registers.

cache
     Enable GDBN to cache target memory.

nocache
     Disable GDBN from caching target memory.  This is the default.

Memory Access Checking
----------------------

GDBN can be instructed to refuse accesses to memory that is not explicitly described.  This can be useful if accessing such regions has undesired effects for a specific target, or to provide better error checking.  The following commands control this behaviour.

set mem inaccessible-by-default [on|off]
     If on is specified, make  GDBN treat memory not explicitly described by the memory ranges as non-existent and refuse accesses to such memory.  The checks are only performed if there's at least one memory range defined.  If off is specified, make GDBN treat the memory not explicitly described by the memory ranges as RAM. The default value is on.

show mem inaccessible-by-default
     Show the current handling of accesses to unknown memory.

Copy Between Memory and a File
==============================

You can use the commands dump, append, and restore to copy data between target memory and a file.  The dump and append commands write data to a file, and the restore command reads data from a file back into the inferior's memory.  Files may be in binary, Motorola S-record, Intel hex, or Tektronix Hex format; however, GDBN can only append to binary files.

dump [FORMAT] memory FILENAME START_ADDR END_ADDR
dump [FORMAT] value FILENAME EXPR
     Dump the contents of memory from START_ADDR to END_ADDR, or the value of EXPR, to FILENAME in the given format.

     The FORMAT parameter may be any one of:

     binary
          Raw binary form.

     ihex
          Intel hex format.

     srec
          Motorola S-record format.

     tekhex
          Tektronix Hex format.

     GDBN uses the same definitions of these formats as the GNU binary utilities, like objdump and objcopy.  If FORMAT is omitted, GDBN dumps the data in raw binary form.

append [binary] memory FILENAME START_ADDR END_ADDR
append [binary] value FILENAME EXPR
     Append the contents of memory from START_ADDR to END_ADDR, or the value of EXPR, to the file FILENAME, in raw binary form. (GDBN can only append data to files in raw binary form.)

restore FILENAME [binary] BIAS START END
     Restore the contents of file FILENAME into memory.  The restore command can automatically recognize any known BFD file format, except for raw binary.  To restore a raw binary file you must specify the optional keyword binary after the filename.

     If BIAS is non-zero, its value will be added to the addresses contained in the file.  Binary files always start at address zero, so they will be restored at address BIAS.  Other bfd files have a built-in location; they will be restored at offset BIAS from that location.

     If START and/or END are non-zero, then only data between file offset START and file offset END will be restored. These offsets are relative to the addresses in the file, before the BIAS argument is applied.

How to Produce a Core File from Your Program
============================================

A core file or core dump is a file that records the memory image of a running process and its process status (register values etc.).  Its primary use is post-mortem debugging of a program that crashed while it ran outside a debugger.  A program that crashes automatically produces a core file, unless this feature is disabled by the user.  See Files, for information on invoking GDBN in the post-mortem debugging mode.

Occasionally, you may wish to produce a core file of the program you are debugging in order to preserve a snapshot of its state. GDBN has a special command for that.

generate-core-file [FILE]
gcore [FILE]
     Produce a core dump of the inferior process.  The optional argument FILE specifies the file name where to put the core dump.  If not specified, the file name defaults to core.PID, where PID is the inferior process ID.

     Note that this command is implemented only for some systems (as of this writing, GNU/Linux, FreeBSD, Solaris, Unixware, and S390).

Character Sets
==============

If the program you are debugging uses a different character set to represent characters and strings than the one GDBN uses itself, GDBN can automatically translate between the character sets for you.  The character set GDBN uses we call the host
character set; the one the inferior program uses we call the target character set.

For example, if you are running GDBN on a GNU/Linux system, which uses the ISO Latin 1 character set, but you are using GDBN's remote protocol (see Remote Debugging) to debug a program running on an IBM mainframe, which uses the EBCDIC character set, then the host character set is Latin-1, and the target character set is EBCDIC.  If you give GDBN the command set
target-charset EBCDIC-US, then GDBN translates between EBCDIC and Latin 1 as you print character or string values, or use character and string literals in expressions.

GDBN has no way to automatically recognize which character set the inferior program uses; you must tell it, using the set target-charset command, described below.

Here are the commands for controlling GDBN's character set support:

set target-charset CHARSET
     Set the current target character set to CHARSET.  To display the list of supported target character sets, type set target-charset TABTAB.

set host-charset CHARSET
     Set the current host character set to CHARSET.

     By default, GDBN uses a host character set appropriate to the system it is running on; you can override that default using the set host-charset command.  On some systems, GDBN cannot automatically determine the appropriate host character set.  In this case, GDBN uses UTF-8.

     GDBN can only use certain character sets as its host character set.  If you type set host-charset TABTAB, GDBN will list the host character sets it supports.

set charset CHARSET
     Set the current host and target character sets to CHARSET.  As above, if you type set charset TABTAB, GDBN will list the names of the character sets that can be used for both host and target.

show charset
     Show the names of the current host and target character sets.

show host-charset
     Show the name of the current host character set.

show target-charset
     Show the name of the current target character set.

set target-wide-charset CHARSET
     Set the current target's wide character set to CHARSET.  This is the character set used by the target's wchar_t type.  To display the list of supported wide character sets, type set target-wide-charset TABTAB.

show target-wide-charset
     Show the name of the current target's wide character set.

Here is an example of GDBN's character set support in action. Assume that the following source code has been placed in the file charset-test.c:

     #include <stdio.h>

     char ascii_hello[]
       = {72, 101, 108, 108, 111, 44, 32, 119,
          111, 114, 108, 100, 33, 10, 0};
     char ibm1047_hello[]
       = {200, 133, 147, 147, 150, 107, 64, 166,
          150, 153, 147, 132, 90, 37, 0};

     main ()
     {
       printf ("Hello, world!\n");
     }

In this program, ascii_hello and ibm1047_hello are arrays containing the string Hello, world! followed by a newline, encoded in the ASCII and IBM1047 character sets.

We compile the program, and invoke the debugger on it:

     $ gcc -g charset-test.c -o charset-test
     $ gdb -nw charset-test
     GNU gdb 2001-12-19-cvs
     Copyright 2001 Free Software Foundation, Inc.
     ...
     (GDBP)

We can use the show charset command to see what character sets GDBN is currently using to interpret and display characters and strings:

     (GDBP) show charset
     The current host and target character set is `ISO-8859-1'.
     (GDBP)

For the sake of printing this manual, let's use ASCII as our initial character set:

     (GDBP) set charset ASCII
     (GDBP) show charset
     The current host and target character set is `ASCII'.
     (GDBP)

Let's assume that ASCII is indeed the correct character set for our host system --- in other words, let's assume that if GDBN prints characters using the ASCII character set, our terminal will display them properly.  Since our current target character set is also ASCII, the contents of ascii_hello print legibly:

     (GDBP) print ascii_hello
     $1 = 0x401698 "Hello, world!\n"
     (GDBP) print ascii_hello[0]
     $2 = 72 'H'
     (GDBP)

GDBN uses the target character set for character and string literals you use in expressions:

     (GDBP) print '+'
     $3 = 43 '+'
     (GDBP)

The ASCII character set uses the number 43 to encode the + character.

GDBN relies on the user to tell it which character set the target program uses.  If we print ibm1047_hello while our target character set is still ASCII, we get jibberish:

     (GDBP) print ibm1047_hello
     $4 = 0x4016a8 "\310\205\223\223\226k@\246\226\231\223\204Z%"
     (GDBP) print ibm1047_hello[0]
     $5 = 200 '\310'
     (GDBP)

If we invoke the set target-charset followed by TABTAB, GDBN tells us the character sets it supports:

     (GDBP) set target-charset
     ASCII       EBCDIC-US   IBM1047     ISO-8859-1
     (GDBP) set target-charset

We can select IBM1047 as our target character set, and examine the program's strings again.  Now the ASCII string is wrong, but GDBN translates the contents of ibm1047_hello from the target character set, IBM1047, to the host character set, ASCII, and they display correctly:

     (GDBP) set target-charset IBM1047
     (GDBP) show charset
     The current host character set is `ASCII'.
     The current target character set is `IBM1047'.
     (GDBP) print ascii_hello
     $6 = 0x401698 "\110\145%%?\054\040\167?\162%\144\041\012"
     (GDBP) print ascii_hello[0]
     $7 = 72 '\110'
     (GDBP) print ibm1047_hello
     $8 = 0x4016a8 "Hello, world!\n"
     (GDBP) print ibm1047_hello[0]
     $9 = 200 'H'
     (GDBP)

As above, GDBN uses the target character set for character and string literals you use in expressions:

     (GDBP) print '+'
     $10 = 78 '+'
     (GDBP)

The IBM1047 character set uses the number 78 to encode the + character.

Caching Data of Remote Targets
==============================

GDBN caches data exchanged between the debugger and a remote target (see Remote Debugging).  Such caching generally improves performance, because it reduces the overhead of the remote protocol by bundling memory reads and writes into large chunks.  Unfortunately, simply caching everything would lead to incorrect results, since GDBN does not necessarily know anything about volatile values, memory-mapped I/O addresses, etc.  Furthermore, in non-stop mode (see Non-Stop Mode) memory can be changed while a gdb command is executing. Therefore, by default, GDBN only caches data known to be on the stack (In non-stop mode, it is moderately rare for a running thread to modify the stack of a stopped thread in a way that would interfere with a backtrace, and caching of stack reads provides a significant speed up of remote backtraces.). Other regions of memory can be explicitly marked as cacheable; see Memory Region Attributes.

set remotecache on
set remotecache off
     This option no longer does anything; it exists for compatibility with old scripts.

show remotecache
     Show the current state of the obsolete remotecache flag.

set stack-cache on
set stack-cache off
     Enable or disable caching of stack accesses.  When ON, use caching.  By default, this option is ON.

show stack-cache
     Show the current state of data caching for memory accesses.

info dcache [line]
     Print the information about the data cache performance.  The information displayed includes the dcache width and depth, and for each cache line, its number, address, and how many times it was referenced.  This command is useful for debugging the data cache operation.

     If a line number is specified, the contents of that line will be printed in hex.

set dcache size SIZE
     Set maximum number of entries in dcache (dcache depth above).

set dcache line-size LINE-SIZE
     Set number of bytes each dcache entry caches (dcache width above). Must be a power of 2.

show dcache size
     Show maximum number of dcache entries.  See also info dcache.

show dcache line-size
     Show default size of dcache lines.  See also info dcache.

Search Memory
=============

Memory can be searched for a particular sequence of bytes with the find command.

find [/SN] START_ADDR, +LEN, VAL1 [, VAL2, ...]
find [/SN] START_ADDR, END_ADDR, VAL1 [, VAL2, ...]
     Search memory for the sequence of bytes specified by VAL1, VAL2, etc.  The search begins at address START_ADDR and continues for either LEN bytes or through to END_ADDR inclusive.

S and N are optional parameters. They may be specified in either order, apart or together.

S, search query size
     The size of each search query value.

     b
          bytes

     h
          halfwords (two bytes)

     w
          words (four bytes)

     g
          giant words (eight bytes)

     All values are interpreted in the current language. This means, for example, that if the current source language is C/C++ then searching for the string ``hello'' includes the trailing '\0'.

     If the value size is not specified, it is taken from the value's type in the current language. This is useful when one wants to specify the search pattern as a mixture of types. Note that this means, for example, that in the case of C-like languages a search for an untyped 0x42 will search for (int) 0x42 which is typically four bytes.

N, maximum number of finds
     The maximum number of matches to print.  The default is to print all finds.

You can use strings as search values.  Quote them with double-quotes ("). The string value is copied into the search pattern byte by byte, regardless of the endianness of the target and the size specification.

The address of each match found is printed as well as a count of the number of matches found.

The address of the last value found is stored in convenience variable $_. A count of the number of matches is stored in $numfound.

For example, if stopped at the printf in this function:

     void
     hello ()
     {
       static char hello[] = "hello-hello";
       static struct { char c; short s; int i; }
         __attribute__ ((packed)) mixed
         = { 'c', 0x1234, 0x87654321 };
       printf ("%s\n", hello);
     }

you get during debugging:

     (gdb) find &hello[0], +sizeof(hello), "hello"
     0x804956d <hello.1620+6>
     1 pattern found
     (gdb) find &hello[0], +sizeof(hello), 'h', 'e', 'l', 'l', 'o'
     0x8049567 <hello.1620>
     0x804956d <hello.1620+6>
     2 patterns found
     (gdb) find /b1 &hello[0], +sizeof(hello), 'h', 0x65, 'l'
     0x8049567 <hello.1620>
     1 pattern found
     (gdb) find &mixed, +sizeof(mixed), (char) 'c', (short) 0x1234, (int) 0x87654321
     0x8049560 <mixed.1625>
     1 pattern found
     (gdb) print $numfound
     $1 = 1
     (gdb) print $_
     $2 = (void *) 0x8049560

Debugging Optimized Code
************************

Almost all compilers support optimization.  With optimization disabled, the compiler generates assembly code that corresponds directly to your source code, in a simplistic way.  As the compiler applies more powerful optimizations, the generated assembly code diverges from your original source code.  With help from debugging information generated by the compiler, GDBN can map from the running program back to constructs from your original source.

GDBN is more accurate with optimization disabled.  If you can recompile without optimization, it is easier to follow the progress of your program during debugging.  But, there are many cases where you may need to debug an optimized version.

When you debug a program compiled with -g -O, remember that the optimizer has rearranged your code; the debugger shows you what is really there.  Do not be too surprised when the execution path does not exactly match your source file!  An extreme example: if you define a variable, but never use it, GDBN never sees that variable---because the compiler optimizes it out of existence.

Some things do not work as well with -g -O as with just -g, particularly on machines with instruction scheduling.  If in doubt, recompile with -g alone, and if this fixes the problem, please report it to us as a bug (including a test case!). See Variables, for more information about debugging optimized code.

Inline Functions
================

Inlining is an optimization that inserts a copy of the function body directly at each call site, instead of jumping to a shared routine.  GDBN displays inlined functions just like non-inlined functions.  They appear in backtraces.  You can view their arguments and local variables, step into them with step, skip them with next, and escape from them with finish. You can check whether a function was inlined by using the info frame command.

For GDBN to support inlined functions, the compiler must record information about inlining in the debug information --- NGCC using the DWARF 2 format does this, and several other compilers do also.  GDBN only supports inlined functions when using DWARF 2.  Versions of NGCC before 4.1 do not emit two required attributes (DW_AT_call_file and DW_AT_call_line); GDBN does not display inlined function calls with earlier versions of NGCC.  It instead displays the arguments and local variables of inlined functions as local variables in the caller.

The body of an inlined function is directly included at its call site; unlike a non-inlined function, there are no instructions devoted to the call.  GDBN still pretends that the call site and the start of the inlined function are different instructions.  Stepping to the call site shows the call site, and then stepping again shows the first line of the inlined function, even though no additional instructions are executed.

This makes source-level debugging much clearer; you can see both the context of the call and then the effect of the call.  Only stepping by a single instruction using stepi or nexti does not do this; single instruction steps always show the inlined body.

There are some ways that GDBN does not pretend that inlined function calls are the same as normal calls:

  * You cannot set breakpoints on inlined functions.  GDBN either reports that there is no symbol with that name, or else sets the breakpoint only on non-inlined copies of the function.  This limitation will be removed in a future version of GDBN; until then, set a breakpoint by line number on the first line of the inlined function instead.

  * Setting breakpoints at the call site of an inlined function may not work, because the call site does not contain any code.  GDBN may incorrectly move the breakpoint to the next line of the enclosing function, after the call.  This limitation will be removed in a future version of GDBN; until then, set a breakpoint on an earlier line or inside the inlined function instead.

  * GDBN cannot locate the return value of inlined calls after using the finish command.  This is a limitation of compiler-generated debugging information; after finish, you can step to the next line and print a variable where your program stored the return value.

Tail Call Frames
================

Function B can call function C in its very last statement.  In unoptimized compilation the call of C is immediately followed by return instruction at the end of B code.  Optimizing compiler may replace the call and return in function B into one jump to function C instead.  Such use of a jump instruction is called tail call.

During execution of function C, there will be no indication in the function call stack frames that it was tail-called from B.  If function A regularly calls function B which tail-calls function C, then GDBN will see A as the caller of C.  However, in some cases GDBN can determine that C was tail-called from B, and it will then create fictitious call frame for that, with the return address set up as if B called C normally.

This functionality is currently supported only by DWARF 2 debugging format and the compiler has to produce DW_TAG_GNU_call_site tags.  With NGCC, you need to specify -O -g during compilation, to get this information.

info frame command (see Frame Info) will indicate the tail call frame kind by text tail call frame such as in this sample GDBN output:

     (gdb) x/i $pc - 2
        0x40066b <b(int, double)+11>: jmp 0x400640 <c(int, double)>
     (gdb) info frame
     Stack level 1, frame at 0x7fffffffda30:
      rip = 0x40066d in b (amd64-entry-value.cc:59); saved rip 0x4004c5
      tail call frame, caller of frame at 0x7fffffffda30
      source language c++.
      Arglist at unknown address.
      Locals at unknown address, Previous frame's sp is 0x7fffffffda30

The detection of all the possible code path executions can find them ambiguous. There is no execution history stored (possible Reverse Execution is never used for this purpose) and the last known caller could have reached the known callee by multiple different jump sequences.  In such case GDBN still tries to show at least all the unambiguous top tail callers and all the unambiguous bottom tail calees, if any.

set debug entry-values
     When set to on, enables printing of analysis messages for both frame argument values at function entry and tail calls.  It will show all the possible valid tail calls code paths it has considered.  It will also print the intersection of them with the final unambiguous (possibly partial or even empty) code path result.

show debug entry-values
     Show the current state of analysis messages printing for both frame argument values at function entry and tail calls.

The analysis messages for tail calls can for example show why the virtual tail call frame for function c has not been recognized (due to the indirect reference by variable x):

     static void __attribute__((noinline, noclone)) c (void);
     void (*x) (void) = c;
     static void __attribute__((noinline, noclone)) a (void) { x++; }
     static void __attribute__((noinline, noclone)) c (void) { a (); }
     int main (void) { x (); return 0; }

     Breakpoint 1, DW_OP_GNU_entry_value resolving cannot find
     DW_TAG_GNU_call_site 0x40039a in main
     a () at t.c:3
     3	static void __attribute__((noinline, noclone)) a (void) { x++; }
     (gdb) bt
     #0  a () at t.c:3
     #1  0x000000000040039a in main () at t.c:5

Another possibility is an ambiguous virtual tail call frames resolution:

     int i;
     static void __attribute__((noinline, noclone)) f (void) { i++; }
     static void __attribute__((noinline, noclone)) e (void) { f (); }
     static void __attribute__((noinline, noclone)) d (void) { f (); }
     static void __attribute__((noinline, noclone)) c (void) { d (); }
     static void __attribute__((noinline, noclone)) b (void)
     { if (i) c (); else e (); }
     static void __attribute__((noinline, noclone)) a (void) { b (); }
     int main (void) { a (); return 0; }

     tailcall: initial: 0x4004d2(a) 0x4004ce(b) 0x4004b2(c) 0x4004a2(d)
     tailcall: compare: 0x4004d2(a) 0x4004cc(b) 0x400492(e)
     tailcall: reduced: 0x4004d2(a) |
     (gdb) bt
     #0  f () at t.c:2
     #1  0x00000000004004d2 in a () at t.c:8
     #2  0x0000000000400395 in main () at t.c:9

Frames #0 and #2 are real, #1 is a virtual tail call frame. The code can have possible execution paths mainARROWaARROWbARROWcARROWdARROWf or mainARROWaARROWbARROWeARROWf, GDBN cannot find which one from the inferior state.

initial: state shows some random possible calling sequence GDBN has found.  It then finds another possible calling sequcen - that one is prefixed by compare:.  The non-ambiguous intersection of these two is printed as the reduced: calling sequence.  That one could have many futher compare: and reduced: statements as long as there remain any non-ambiguous sequence entries.

For the frame of function b in both cases there are different possible $pc values (0x4004cc or 0x4004ce), therefore this frame is also ambigous.  The only non-ambiguous frame is the one for function a, therefore this one is displayed to the user while the ambiguous frames are omitted.

There can be also reasons why printing of frame argument values at function entry may fail:

     int v;
     static void __attribute__((noinline, noclone)) c (int i) { v++; }
     static void __attribute__((noinline, noclone)) a (int i);
     static void __attribute__((noinline, noclone)) b (int i) { a (i); }
     static void __attribute__((noinline, noclone)) a (int i)
     { if (i) b (i - 1); else c (0); }
     int main (void) { a (5); return 0; }

     (gdb) bt
     #0  c (i=i@entry=0) at t.c:2
     #1  0x0000000000400428 in a (DW_OP_GNU_entry_value resolving has found
     function "a" at 0x400420 can call itself via tail calls
     i=<optimized out>) at t.c:6
     #2  0x000000000040036e in main () at t.c:7

GDBN cannot find out from the inferior state if and how many times did function a call itself (via function b) as these calls would be tail calls.  Such tail calls would modify thue i variable, therefore GDBN cannot be sure the value it knows would be right - GDBN prints <optimized out> instead.

C Preprocessor Macros
*********************

Some languages, such as C and C++, provide a way to define and invoke ``preprocessor macros'' which expand into strings of tokens. GDBN can evaluate expressions containing macro invocations, show the result of macro expansion, and show a macro's definition, including where it was defined.

You may need to compile your program specially to provide GDBN with information about preprocessor macros.  Most compilers do not include macros in their debugging information, even when you compile with the -g flag.  See Compilation.

A program may define a macro at one point, remove that definition later, and then provide a different definition after that.  Thus, at different points in the program, a macro may have different definitions, or have no definition at all.  If there is a current stack frame, GDBN uses the macros in scope at that frame's source code line.  Otherwise, GDBN uses the macros in scope at the current listing location; see List.

Whenever GDBN evaluates an expression, it always expands any macro invocations present in the expression.  GDBN also provides the following commands for working with macros explicitly.

macro expand EXPRESSION
macro exp EXPRESSION
     Show the results of expanding all preprocessor macro invocations in EXPRESSION.  Since GDBN simply expands macros, but does not parse the result, EXPRESSION need not be a valid expression; it can be any string of tokens.

macro expand-once EXPRESSION
macro exp1 EXPRESSION
     (This command is not yet implemented.)  Show the results of expanding those preprocessor macro invocations that appear explicitly in EXPRESSION.  Macro invocations appearing in that expansion are left unchanged.  This command allows you to see the effect of a particular macro more clearly, without being confused by further expansions.  Since GDBN simply expands macros, but does not parse the result, EXPRESSION need not be a valid expression; it can be any string of tokens.

info macro [-a|-all] [--] MACRO
     Show the current definition or all definitions of the named MACRO, and describe the source location or compiler command-line where that definition was established.  The optional double dash is to signify the end of argument processing and the beginning of MACRO for non C-like macros where the macro may begin with a hyphen.

info macros LINESPEC
     Show all macro definitions that are in effect at the location specified by LINESPEC,  and describe the source location or compiler command-line where those definitions were established.

macro define MACRO REPLACEMENT-LIST
macro define MACRO(ARGLIST) REPLACEMENT-LIST
     Introduce a definition for a preprocessor macro named MACRO, invocations of which are replaced by the tokens given in REPLACEMENT-LIST.  The first form of this command defines an ``object-like'' macro, which takes no arguments; the second form defines a ``function-like'' macro, which takes the arguments given in ARGLIST.

     A definition introduced by this command is in scope in every expression evaluated in GDBN, until it is removed with the macro undef command, described below.  The definition overrides all definitions for MACRO present in the program being debugged, as well as any previous user-supplied definition.

macro undef MACRO
     Remove any user-supplied definition for the macro named MACRO. This command only affects definitions provided with the macro define command, described above; it cannot remove definitions present in the program being debugged.

macro list
     List all the macros defined using the macro define command.

Here is a transcript showing the above commands in action.  First, we show our source files:

     $ cat sample.c
     #include <stdio.h>
     #include "sample.h"

     #define M 42
     #define ADD(x) (M + x)

     main ()
     {
     #define N 28
       printf ("Hello, world!\n");
     #undef N
       printf ("We're so creative.\n");
     #define N 1729
       printf ("Goodbye, world!\n");
     }
     $ cat sample.h
     #define Q <
     $

Now, we compile the program using the GNU C compiler, NGCC.  We pass the -gdwarf-2 (This is the
minimum.  Recent versions of NGCC support -gdwarf-3
and -gdwarf-4; we recommend always choosing the most recent
version of DWARF.) and -g3 flags to ensure the compiler includes information about preprocessor macros in the debugging information.

     $ gcc -gdwarf-2 -g3 sample.c -o sample
     $

Now, we start GDBN on our sample program:

     $ gdb -nw sample
     GNU gdb 2002-05-06-cvs
     Copyright 2002 Free Software Foundation, Inc.
     GDB is free software, ...
     (GDBP)

We can expand macros and examine their definitions, even when the program is not running.  GDBN uses the current listing position to decide which macro definitions are in scope:

     (GDBP) list main
     3
     4       #define M 42
     5       #define ADD(x) (M + x)
     6
     7       main ()
     8       {
     9       #define N 28
     10        printf ("Hello, world!\n");
     11      #undef N
     12        printf ("We're so creative.\n");
     (GDBP) info macro ADD
     Defined at /home/jimb/gdb/macros/play/sample.c:5
     #define ADD(x) (M + x)
     (GDBP) info macro Q
     Defined at /home/jimb/gdb/macros/play/sample.h:1
       included at /home/jimb/gdb/macros/play/sample.c:2
     #define Q <
     (GDBP) macro expand ADD(1)
     expands to: (42 + 1)
     (GDBP) macro expand-once ADD(1)
     expands to: once (M + 1)
     (GDBP)

In the example above, note that macro expand-once expands only the macro invocation explicit in the original text --- the invocation of ADD --- but does not expand the invocation of the macro M, which was introduced by ADD.

Once the program is running, GDBN uses the macro definitions in force at the source line of the current stack frame:

     (GDBP) break main
     Breakpoint 1 at 0x8048370: file sample.c, line 10.
     (GDBP) run
     Starting program: /home/jimb/gdb/macros/play/sample

     Breakpoint 1, main () at sample.c:10
     10        printf ("Hello, world!\n");
     (GDBP)

At line 10, the definition of the macro N at line 9 is in force:

     (GDBP) info macro N
     Defined at /home/jimb/gdb/macros/play/sample.c:9
     #define N 28
     (GDBP) macro expand N Q M
     expands to: 28 < 42
     (GDBP) print N Q M
     $1 = 1
     (GDBP)

As we step over directives that remove N's definition, and then give it a new definition, GDBN finds the definition (or lack thereof) in force at each point:

     (GDBP) next
     Hello, world!
     12        printf ("We're so creative.\n");
     (GDBP) info macro N
     The symbol `N' has no definition as a C/C++ preprocessor macro
     at /home/jimb/gdb/macros/play/sample.c:12
     (GDBP) next
     We're so creative.
     14        printf ("Goodbye, world!\n");
     (GDBP) info macro N
     Defined at /home/jimb/gdb/macros/play/sample.c:13
     #define N 1729
     (GDBP) macro expand N Q M
     expands to: 1729 < 42
     (GDBP) print N Q M
     $2 = 0
     (GDBP)

In addition to source files, macros can be defined on the compilation command line using the -DNAME=VALUE syntax.  For macros defined in such a way, GDBN displays the location of their definition as line zero of the source file submitted to the compiler.

     (GDBP) info macro __STDC__
     Defined at /home/jimb/gdb/macros/play/sample.c:0
     -D__STDC__=1
     (GDBP)

Tracepoints
***********

In some applications, it is not feasible for the debugger to interrupt the program's execution long enough for the developer to learn anything helpful about its behavior.  If the program's correctness depends on its real-time behavior, delays introduced by a debugger might cause the program to change its behavior drastically, or perhaps fail, even when the code itself is correct.  It is useful to be able to observe the program's behavior without interrupting it.

Using GDBN's trace and collect commands, you can specify locations in the program, called tracepoints, and arbitrary expressions to evaluate when those tracepoints are reached. Later, using the tfind command, you can examine the values those expressions had when the program hit the tracepoints.  The expressions may also denote objects in memory---structures or arrays, for example---whose values GDBN should record; while visiting a particular tracepoint, you may inspect those objects as if they were in memory at that moment.  However, because GDBN records these values without interacting with you, it can do so quickly and unobtrusively, hopefully not disturbing the program's behavior.

The tracepoint facility is currently available only for remote targets.  See Targets.  In addition, your remote target must know how to collect trace data.  This functionality is implemented in the remote stub; however, none of the stubs distributed with GDBN support tracepoints as of this writing.  The format of the remote packets used to implement tracepoints are described in Tracepoint Packets.

It is also possible to get trace data from a file, in a manner reminiscent of corefiles; you specify the filename, and use tfind to search through the file.  See Trace Files, for more details.

This chapter describes the tracepoint commands and features.

Commands to Set Tracepoints
===========================

Before running such a trace experiment, an arbitrary number of tracepoints can be set.  A tracepoint is actually a special type of breakpoint (see Set Breaks), so you can manipulate it using standard breakpoint commands.  For instance, as with breakpoints, tracepoint numbers are successive integers starting from one, and many of the commands associated with tracepoints take the tracepoint number as their argument, to identify which tracepoint to work on.

For each tracepoint, you can specify, in advance, some arbitrary set of data that you want the target to collect in the trace buffer when it hits that tracepoint.  The collected data can include registers, local variables, or global data.  Later, you can use GDBN commands to examine the values these data had at the time the tracepoint was hit.

Tracepoints do not support every breakpoint feature.  Ignore counts on tracepoints have no effect, and tracepoints cannot run GDBN commands when they are hit.  Tracepoints may not be thread-specific either.

Some targets may support fast tracepoints, which are inserted in a different way (such as with a jump instead of a trap), that is faster but possibly restricted in where they may be installed.

Regular and fast tracepoints are dynamic tracing facilities, meaning that they can be used to insert tracepoints at (almost) any location in the target.  Some targets may also support controlling static
tracepoints from GDBN.  With static tracing, a set of instrumentation points, also known as markers, are embedded in the target program, and can be activated or deactivated by name or address.  These are usually placed at locations which facilitate investigating what the target is actually doing.  GDBN's support for static tracing includes being able to list instrumentation points, and attach them with GDBN defined high level tracepoints that expose the whole range of convenience of GDBN's tracepoints support.  Namely, support for collecting registers values and values of global or local (to the instrumentation point) variables; tracepoint conditions and trace state variables. The act of installing a GDBN static tracepoint on an instrumentation point, or marker, is referred to as probing a static tracepoint marker.

gdbserver supports tracepoints on some target systems. See Tracepoints support in gdbserver.

This section describes commands to set tracepoints and associated conditions and actions.

Create and Delete Tracepoints
-----------------------------

trace LOCATION
     The trace command is very similar to the break command. Its argument LOCATION can be a source line, a function name, or an address in the target program.  See Specify Location.  The trace command defines a tracepoint, which is a point in the target program where the debugger will briefly stop, collect some data, and then allow the program to continue.  Setting a tracepoint or changing its actions takes effect immediately if the remote stub supports the InstallInTrace feature (see install tracepoint in tracing). If remote stub doesn't support the InstallInTrace feature, all these changes don't take effect until the next tstart command, and once a trace experiment is running, further changes will not have any effect until the next trace experiment starts.  In addition, GDBN supports pending tracepoints---tracepoints whose address is not yet resolved.  (This is similar to pending breakpoints.) Pending tracepoints are not downloaded to the target and not installed until they are resolved.  The resolution of pending tracepoints requires GDBN support---when debugging with the remote target, and GDBN disconnects from the remote stub (see disconnected tracing), pending tracepoints can not be resolved (and downloaded to the remote stub) while GDBN is disconnected.

     Here are some examples of using the trace command:

          (GDBP) trace foo.c:121    // a source file and line number

          (GDBP) trace +2           // 2 lines forward

          (GDBP) trace my_function  // first source line of function

          (GDBP) trace *my_function // EXACT start address of function

          (GDBP) trace *0x2117c4    // an address

     You can abbreviate trace as tr.

trace LOCATION if COND
     Set a tracepoint with condition COND; evaluate the expression COND each time the tracepoint is reached, and collect data only if the value is nonzero---that is, if COND evaluates as true. See Tracepoint Conditions, for more information on tracepoint conditions.

ftrace LOCATION [ if COND ]
     The ftrace command sets a fast tracepoint.  For targets that support them, fast tracepoints will use a more efficient but possibly less general technique to trigger data collection, such as a jump instruction instead of a trap, or some sort of hardware support.  It may not be possible to create a fast tracepoint at the desired location, in which case the command will exit with an explanatory message.

     GDBN handles arguments to ftrace exactly as for trace.

     On 32-bit x86-architecture systems, fast tracepoints normally need to be placed at an instruction that is 5 bytes or longer, but can be placed at 4-byte instructions if the low 64K of memory of the target program is available to install trampolines.  Some Unix-type systems, such as GNU/Linux, exclude low addresses from the program's address space; but for instance with the Linux kernel it is possible to let GDBN use this area by doing a sysctl command to set the mmap_min_addr kernel parameter, as in

          sudo sysctl -w vm.mmap_min_addr=32768

     which sets the low address to 32K, which leaves plenty of room for trampolines.  The minimum address should be set to a page boundary.

strace LOCATION [ if COND ]
     The strace command sets a static tracepoint.  For targets that support it, setting a static tracepoint probes a static instrumentation point, or marker, found at LOCATION.  It may not be possible to set a static tracepoint at the desired location, in which case the command will exit with an explanatory message.

     GDBN handles arguments to strace exactly as for trace, with the addition that the user can also specify -m MARKER as LOCATION.  This probes the marker identified by the MARKER string identifier.  This identifier depends on the static tracepoint backend library your program is using.  You can find all the marker identifiers in the ID field of the info static-tracepoint-markers command output. See Listing Static Tracepoint Markers.  For example, in the following small program using the UST tracing engine:

          main ()
          {
            trace_mark(ust, bar33, "str %s", "FOOBAZ");
          }

     the marker id is composed of joining the first two arguments to the trace_mark call with a slash, which translates to:

          (GDBP) info static-tracepoint-markers
          Cnt Enb ID         Address            What
          1   n   ust/bar33  0x0000000000400ddc in main at stexample.c:22
                   Data: "str %s"
          [etc...]

     so you may probe the marker above with:

          (GDBP) strace -m ust/bar33

     Static tracepoints accept an extra collect action --- collect $_sdata.  This collects arbitrary user data passed in the probe point call to the tracing library.  In the UST example above, you'll see that the third argument to trace_mark is a printf-like format string.  The user data is then the result of running that formating string against the following arguments.  Note that info static-tracepoint-markers command output lists that format string in the Data: field.

     You can inspect this data when analyzing the trace buffer, by printing the $_sdata variable like any other variable available to GDBN.  See Tracepoint Action Lists.

     The convenience variable $tpnum records the tracepoint number of the most recently set tracepoint.

delete tracepoint [NUM]
     Permanently delete one or more tracepoints.  With no argument, the default is to delete all tracepoints.  Note that the regular delete command can remove tracepoints also.

     Examples:

          (GDBP) delete trace 1 2 3 // remove three tracepoints

          (GDBP) delete trace       // remove all tracepoints

     You can abbreviate this command as del tr.

Enable and Disable Tracepoints
------------------------------

These commands are deprecated; they are equivalent to plain disable and enable.

disable tracepoint [NUM]
     Disable tracepoint NUM, or all tracepoints if no argument NUM is given.  A disabled tracepoint will have no effect during a trace experiment, but it is not forgotten.  You can re-enable a disabled tracepoint using the enable tracepoint command. If the command is issued during a trace experiment and the debug target has support for disabling tracepoints during a trace experiment, then the change will be effective immediately.  Otherwise, it will be applied to the next trace experiment.

enable tracepoint [NUM]
     Enable tracepoint NUM, or all tracepoints.  If this command is issued during a trace experiment and the debug target supports enabling tracepoints during a trace experiment, then the enabled tracepoints will become effective immediately.  Otherwise, they will become effective the next time a trace experiment is run.

Tracepoint Passcounts
---------------------

passcount [N [NUM]]
     Set the passcount of a tracepoint.  The passcount is a way to automatically stop a trace experiment.  If a tracepoint's passcount is N, then the trace experiment will be automatically stopped on the N'th time that tracepoint is hit.  If the tracepoint number NUM is not specified, the passcount command sets the passcount of the most recently defined tracepoint.  If no passcount is given, the trace experiment will run until stopped explicitly by the user.

     Examples:

          (GDBP) passcount 5 2 // Stop on the 5th execution of
                                             // tracepoint 2

          (GDBP) passcount 12  // Stop on the 12th execution of the
                                             // most recently defined tracepoint.
          (GDBP) trace foo
          (GDBP) pass 3
          (GDBP) trace bar
          (GDBP) pass 2
          (GDBP) trace baz
          (GDBP) pass 1        // Stop tracing when foo has been
                                              // executed 3 times OR when bar has
                                              // been executed 2 times
                                              // OR when baz has been executed 1 time.

Tracepoint Conditions
---------------------

The simplest sort of tracepoint collects data every time your program reaches a specified place.  You can also specify a condition for a tracepoint.  A condition is just a Boolean expression in your programming language (see Expressions).  A tracepoint with a condition evaluates the expression each time your program reaches it, and data collection happens only if the condition is true.

Tracepoint conditions can be specified when a tracepoint is set, by using if in the arguments to the trace command. See Setting Tracepoints.  They can also be set or changed at any time with the condition command, just as with breakpoints.

Unlike breakpoint conditions, GDBN does not actually evaluate the conditional expression itself.  Instead, GDBN encodes the expression into an agent expression (see Agent Expressions) suitable for execution on the target, independently of GDBN. Global variables become raw memory locations, locals become stack accesses, and so forth.

For instance, suppose you have a function that is usually called frequently, but should not be called after an error has occurred.  You could use the following tracepoint command to collect data about calls of that function that happen while the error code is propagating through the program; an unconditional tracepoint could end up collecting thousands of useless trace frames that you would have to search through.

     (GDBP) trace normal_operation if errcode > 0

Trace State Variables
---------------------

A trace state variable is a special type of variable that is created and managed by target-side code.  The syntax is the same as that for GDB's convenience variables (a string prefixed with ``$''), but they are stored on the target.  They must be created explicitly, using a tvariable command.  They are always 64-bit signed integers.

Trace state variables are remembered by GDBN, and downloaded to the target along with tracepoint information when the trace experiment starts.  There are no intrinsic limits on the number of trace state variables, beyond memory limitations of the target.

Although trace state variables are managed by the target, you can use them in print commands and expressions as if they were convenience variables; GDBN will get the current value from the target while the trace experiment is running.  Trace state variables share the same namespace as other ``$'' variables, which means that you cannot have trace state variables with names like $23 or $pc, nor can you have a trace state variable and a convenience variable with the same name.

tvariable $NAME [ = EXPRESSION ]
     The tvariable command creates a new trace state variable named $NAME, and optionally gives it an initial value of EXPRESSION.  EXPRESSION is evaluated when this command is entered; the result will be converted to an integer if possible, otherwise GDBN will report an error. A subsequent tvariable command specifying the same name does not create a variable, but instead assigns the supplied initial value to the existing variable of that name, overwriting any previous initial value. The default initial value is 0.

info tvariables
     List all the trace state variables along with their initial values. Their current values may also be displayed, if the trace experiment is currently running.

delete tvariable [ $NAME ... ]
     Delete the given trace state variables, or all of them if no arguments are specified.

Tracepoint Action Lists
-----------------------

actions [NUM]
     This command will prompt for a list of actions to be taken when the tracepoint is hit.  If the tracepoint number NUM is not specified, this command sets the actions for the one that was most recently defined (so that you can define a tracepoint and then say actions without bothering about its number).  You specify the actions themselves on the following lines, one action at a time, and terminate the actions list with a line containing just end.  So far, the only defined actions are collect, teval, and while-stepping.

     actions is actually equivalent to commands (see Breakpoint Command Lists), except that only the defined actions are allowed; any other GDBN command is rejected.

     To remove all actions from a tracepoint, type actions NUM and follow it immediately with end.

          (GDBP) collect DATA // collect some data

          (GDBP) while-stepping 5 // single-step 5 times, collect data

          (GDBP) end              // signals the end of actions.

     In the following example, the action list begins with collect commands indicating the things to be collected when the tracepoint is hit.  Then, in order to single-step and collect additional data following the tracepoint, a while-stepping command is used, followed by the list of things to be collected after each step in a sequence of single steps.  The while-stepping command is terminated by its own separate end command.  Lastly, the action list is terminated by an end command.

          (GDBP) trace foo
          (GDBP) actions
          Enter actions for tracepoint 1, one per line:
          > collect bar,baz
          > collect $regs
          > while-stepping 12
            > collect $pc, arr[i]
            > end
          end

collect[/MODS] EXPR1, EXPR2, ...
     Collect values of the given expressions when the tracepoint is hit. This command accepts a comma-separated list of any valid expressions. In addition to global, static, or local variables, the following special arguments are supported:

     $regs
          Collect all registers.

     $args
          Collect all function arguments.

     $locals
          Collect all local variables.

     $_ret
          Collect the return address.  This is helpful if you want to see more of a backtrace.

     $_sdata
          Collect static tracepoint marker specific data.  Only available for static tracepoints.  See Tracepoint Action Lists.  On the UST static tracepoints library backend, an instrumentation point resembles a printf function call.  The tracing library is able to collect user specified data formatted to a character string using the format provided by the programmer that instrumented the program.  Other backends have similar mechanisms. Here's an example of a UST marker call:

                const char master_name[] = "$your_name";
                trace_mark(channel1, marker1, "hello %s", master_name)

          In this case, collecting $_sdata collects the string hello $yourname.  When analyzing the trace buffer, you can inspect $_sdata like any other variable available to GDBN.

     You can give several consecutive collect commands, each one with a single argument, or one collect command with several arguments separated by commas; the effect is the same.

     The optional MODS changes the usual handling of the arguments. s requests that pointers to chars be handled as strings, in particular collecting the contents of the memory being pointed at, up to the first zero.  The upper bound is by default the value of the print elements variable; if s is followed by a decimal number, that is the upper bound instead.  So for instance collect/s25 mystr collects as many as 25 characters at mystr.

     The command info scope (see info scope) is particularly useful for figuring out what data to collect.

teval EXPR1, EXPR2, ...
     Evaluate the given expressions when the tracepoint is hit.  This command accepts a comma-separated list of expressions.  The results are discarded, so this is mainly useful for assigning values to trace state variables (see Trace State Variables) without adding those values to the trace buffer, as would be the case if the collect action were used.

while-stepping N
     Perform N single-step instruction traces after the tracepoint, collecting new data after each step.  The while-stepping command is followed by the list of what to collect while stepping (followed by its own end command):

          > while-stepping 12
            > collect $regs, myglobal
            > end
          >

     Note that $pc is not automatically collected by while-stepping; you need to explicitly collect that register if you need it.  You may abbreviate while-stepping as ws or stepping.

set default-collect EXPR1, EXPR2, ...
     This variable is a list of expressions to collect at each tracepoint hit.  It is effectively an additional collect action prepended to every tracepoint action list.  The expressions are parsed individually for each tracepoint, so for instance a variable named xyz may be interpreted as a global for one tracepoint, and a local for another, as appropriate to the tracepoint's location.

show default-collect
     Show the list of expressions that are collected by default at each tracepoint hit.

Listing Tracepoints
-------------------

info tracepoints [NUM...]
     Display information about the tracepoint NUM.  If you don't specify a tracepoint number, displays information about all the tracepoints defined so far.  The format is similar to that used for info breakpoints; in fact, info tracepoints is the same command, simply restricting itself to tracepoints.

     A tracepoint's listing may include additional information specific to tracing:

       * its passcount as given by the passcount N command

          (GDBP) info trace
          Num     Type           Disp Enb Address    What
          1       tracepoint     keep y   0x0804ab57 in foo() at main.cxx:7
                  while-stepping 20
                    collect globfoo, $regs
                  end
                  collect globfoo2
                  end
                  pass count 1200
          (GDBP)

     This command can be abbreviated info tp.

Listing Static Tracepoint Markers
---------------------------------

info static-tracepoint-markers
     Display information about all static tracepoint markers defined in the program.

     For each marker, the following columns are printed:

     Count
          An incrementing counter, output to help readability.  This is not a stable identifier.

     ID
          The marker ID, as reported by the target.

     Enabled or Disabled
          Probed markers are tagged with y.  n identifies marks that are not enabled.

     Address
          Where the marker is in your program, as a memory address.

     What
          Where the marker is in the source for your program, as a file and line number.  If the debug information included in the program does not allow GDBN to locate the source of the marker, this column will be left blank.

     In addition, the following information may be printed for each marker:

     Data
          User data passed to the tracing library by the marker call.  In the UST backend, this is the format string passed as argument to the marker call.

     Static tracepoints probing the marker
          The list of static tracepoints attached to the marker.

          (GDBP) info static-tracepoint-markers
          Cnt ID         Enb Address            What
          1   ust/bar2   y   0x0000000000400e1a in main at stexample.c:25
               Data: number1 %d number2 %d
               Probed by static tracepoints: #2
          2   ust/bar33  n   0x0000000000400c87 in main at stexample.c:24
               Data: str %s
          (GDBP)

Starting and Stopping Trace Experiments
---------------------------------------

tstart
     This command starts the trace experiment, and begins collecting data. It has the side effect of discarding all the data collected in the trace buffer during the previous trace experiment.  If any arguments are supplied, they are taken as a note and stored with the trace experiment's state.  The notes may be arbitrary text, and are especially useful with disconnected tracing in a multi-user context; the notes can explain what the trace is doing, supply user contact information, and so forth.

tstop
     This command stops the trace experiment.  If any arguments are supplied, they are recorded with the experiment as a note.  This is useful if you are stopping a trace started by someone else, for instance if the trace is interfering with the system's behavior and needs to be stopped quickly.

     Note: a trace experiment and data collection may stop automatically if any tracepoint's passcount is reached (see Tracepoint Passcounts), or if the trace buffer becomes full.

tstatus
     This command displays the status of the current trace data collection.

Here is an example of the commands we described so far:

     (GDBP) trace gdb_c_test
     (GDBP) actions
     Enter actions for tracepoint #1, one per line.
     > collect $regs,$locals,$args
     > while-stepping 11
       > collect $regs
       > end
     > end
     (GDBP) tstart
     	[time passes ...]
     (GDBP) tstop

You can choose to continue running the trace experiment even if GDBN disconnects from the target, voluntarily or involuntarily.  For commands such as detach, the debugger will ask what you want to do with the trace.  But for unexpected terminations (GDBN crash, network outage), it would be unfortunate to lose hard-won trace data, so the variable disconnected-tracing lets you decide whether the trace should continue running without GDBN.

set disconnected-tracing on
set disconnected-tracing off
     Choose whether a tracing run should continue to run if GDBN has disconnected from the target.  Note that detach or quit will ask you directly what to do about a running trace no matter what this variable's setting, so the variable is mainly useful for handling unexpected situations, such as loss of the network.

show disconnected-tracing
     Show the current choice for disconnected tracing.

When you reconnect to the target, the trace experiment may or may not still be running; it might have filled the trace buffer in the meantime, or stopped for one of the other reasons.  If it is running, it will continue after reconnection.

Upon reconnection, the target will upload information about the tracepoints in effect.  GDBN will then compare that information to the set of tracepoints currently defined, and attempt to match them up, allowing for the possibility that the numbers may have changed due to creation and deletion in the meantime.  If one of the target's tracepoints does not match any in GDBN, the debugger will create a new tracepoint, so that you have a number with which to specify that tracepoint.  This matching-up process is necessarily heuristic, and it may result in useless tracepoints being created; you may simply delete them if they are of no use.

If your target agent supports a circular trace buffer, then you can run a trace experiment indefinitely without filling the trace buffer; when space runs out, the agent deletes already-collected trace frames, oldest first, until there is enough room to continue collecting.  This is especially useful if your tracepoints are being hit too often, and your trace gets terminated prematurely because the buffer is full.  To ask for a circular trace buffer, simply set circular-trace-buffer to on.  You can set this at any time, including during tracing; if the agent can do it, it will change buffer handling on the fly, otherwise it will not take effect until the next run.

set circular-trace-buffer on
set circular-trace-buffer off
     Choose whether a tracing run should use a linear or circular buffer for trace data.  A linear buffer will not lose any trace data, but may fill up prematurely, while a circular buffer will discard old trace data, but it will have always room for the latest tracepoint hits.

show circular-trace-buffer
     Show the current choice for the trace buffer.  Note that this may not match the agent's current buffer handling, nor is it guaranteed to match the setting that might have been in effect during a past run, for instance if you are looking at frames from a trace file.

set trace-user TEXT

show trace-user

set trace-notes TEXT
     Set the trace run's notes.

show trace-notes
     Show the trace run's notes.

set trace-stop-notes TEXT
     Set the trace run's stop notes.  The handling of the note is as for tstop arguments; the set command is convenient way to fix a stop note that is mistaken or incomplete.

show trace-stop-notes
     Show the trace run's stop notes.

Tracepoint Restrictions
-----------------------

There are a number of restrictions on the use of tracepoints.  As described above, tracepoint data gathering occurs on the target without interaction from GDBN.  Thus the full capabilities of the debugger are not available during data gathering, and then at data examination time, you will be limited by only having what was collected.  The following items describe some common problems, but it is not exhaustive, and you may run into additional difficulties not mentioned here.

  * Tracepoint expressions are intended to gather objects (lvalues).  Thus the full flexibility of GDB's expression evaluator is not available. You cannot call functions, cast objects to aggregate types, access convenience variables or modify values (except by assignment to trace state variables).  Some language features may implicitly call functions (for instance Objective-C fields with accessors), and therefore cannot be collected either.

  * Collection of local variables, either individually or in bulk with $locals or $args, during while-stepping may behave erratically.  The stepping action may enter a new scope (for instance by stepping into a function), or the location of the variable may change (for instance it is loaded into a register).  The tracepoint data recorded uses the location information for the variables that is correct for the tracepoint location.  When the tracepoint is created, it is not possible, in general, to determine where the steps of a while-stepping sequence will advance the program---particularly if a conditional branch is stepped.

  * Collection of an incompletely-initialized or partially-destroyed object may result in something that GDBN cannot display, or displays in a misleading way.

  * When GDBN displays a pointer to character it automatically dereferences the pointer to also display characters of the string being pointed to.  However, collecting the pointer during tracing does not automatically collect the string.  You need to explicitly dereference the pointer and provide size information if you want to collect not only the pointer, but the memory pointed to.  For example, *ptr@50 can be used to collect the 50 element array pointed to by ptr.

  * It is not possible to collect a complete stack backtrace at a tracepoint.  Instead, you may collect the registers and a few hundred bytes from the stack pointer with something like *(unsigned char *)$esp@300 (adjust to use the name of the actual stack pointer register on your target architecture, and the amount of stack you wish to capture). Then the backtrace command will show a partial backtrace when using a trace frame.  The number of stack frames that can be examined depends on the sizes of the frames in the collected stack.  Note that if you ask for a block so large that it goes past the bottom of the stack, the target agent may report an error trying to read from an invalid address.

  * If you do not collect registers at a tracepoint, GDBN can infer that the value of $pc must be the same as the address of the tracepoint and use that when you are looking at a trace frame for that tracepoint.  However, this cannot work if the tracepoint has multiple locations (for instance if it was set in a function that was inlined), or if it has a while-stepping loop.  In those cases GDBN will warn you that it can't infer $pc, and default it to zero.

Using the Collected Data
========================

After the tracepoint experiment ends, you use GDBN commands for examining the trace data.  The basic idea is that each tracepoint collects a trace snapshot every time it is hit and another snapshot every time it single-steps.  All these snapshots are consecutively numbered from zero and go into a buffer, and you can examine them later.  The way you examine them is to focus on a specific trace snapshot.  When the remote stub is focused on a trace snapshot, it will respond to all GDBN requests for memory and registers by reading from the buffer which belongs to that snapshot, rather than from real memory or registers of the program being debugged.  This means that all GDBN commands (print, info registers, backtrace, etc.) will behave as if we were currently debugging the program state as it was when the tracepoint occurred.  Any requests for data that are not in the buffer will fail.

tfind N
-------

The basic command for selecting a trace snapshot from the buffer is tfind N, which finds trace snapshot number N, counting from zero.  If no argument N is given, the next snapshot is selected.

Here are the various forms of using the tfind command.

tfind start
     Find the first snapshot in the buffer.  This is a synonym for tfind 0 (since 0 is the number of the first snapshot).

tfind none
     Stop debugging trace snapshots, resume live debugging.

tfind end
     Same as tfind none.

tfind
     No argument means find the next trace snapshot.

tfind -
     Find the previous trace snapshot before the current one.  This permits retracing earlier steps.

tfind tracepoint NUM
     Find the next snapshot associated with tracepoint NUM.  Search proceeds forward from the last examined trace snapshot.  If no argument NUM is given, it means find the next snapshot collected for the same tracepoint as the current snapshot.

tfind pc ADDR
     Find the next snapshot associated with the value ADDR of the program counter.  Search proceeds forward from the last examined trace snapshot.  If no argument ADDR is given, it means find the next snapshot with the same value of PC as the current snapshot.

tfind outside ADDR1, ADDR2
     Find the next snapshot whose PC is outside the given range of addresses (exclusive).

tfind range ADDR1, ADDR2
     Find the next snapshot whose PC is between ADDR1 and ADDR2 (inclusive).

tfind line [FILE:]N
     Find the next snapshot associated with the source line N.  If the optional argument FILE is given, refer to line N in that source file.  Search proceeds forward from the last examined trace snapshot.  If no argument N is given, it means find the next line other than the one currently being examined; thus saying tfind line repeatedly can appear to have the same effect as stepping from line to line in a live debugging session.

The default arguments for the tfind commands are specifically designed to make it easy to scan through the trace buffer.  For instance, tfind with no argument selects the next trace snapshot, and tfind - with no argument selects the previous trace snapshot.  So, by giving one tfind command, and then simply hitting RET repeatedly you can examine all the trace snapshots in order.  Or, by saying tfind - and then hitting RET repeatedly you can examine the snapshots in reverse order. The tfind line command with no argument selects the snapshot for the next source line executed.  The tfind pc command with no argument selects the next snapshot with the same program counter (PC) as the current frame.  The tfind tracepoint command with no argument selects the next trace snapshot collected by the same tracepoint as the current one.

In addition to letting you scan through the trace buffer manually, these commands make it easy to construct GDBN scripts that scan through the trace buffer and print out whatever collected data you are interested in.  Thus, if we want to examine the PC, FP, and SP registers from each trace frame in the buffer, we can say this:

     (GDBP) tfind start
     (GDBP) while ($trace_frame != -1)
     > printf "Frame %d, PC = %08X, SP = %08X, FP = %08X\n", \
               $trace_frame, $pc, $sp, $fp
     > tfind
     > end

     Frame 0, PC = 0020DC64, SP = 0030BF3C, FP = 0030BF44
     Frame 1, PC = 0020DC6C, SP = 0030BF38, FP = 0030BF44
     Frame 2, PC = 0020DC70, SP = 0030BF34, FP = 0030BF44
     Frame 3, PC = 0020DC74, SP = 0030BF30, FP = 0030BF44
     Frame 4, PC = 0020DC78, SP = 0030BF2C, FP = 0030BF44
     Frame 5, PC = 0020DC7C, SP = 0030BF28, FP = 0030BF44
     Frame 6, PC = 0020DC80, SP = 0030BF24, FP = 0030BF44
     Frame 7, PC = 0020DC84, SP = 0030BF20, FP = 0030BF44
     Frame 8, PC = 0020DC88, SP = 0030BF1C, FP = 0030BF44
     Frame 9, PC = 0020DC8E, SP = 0030BF18, FP = 0030BF44
     Frame 10, PC = 00203F6C, SP = 0030BE3C, FP = 0030BF14

Or, if we want to examine the variable X at each source line in the buffer:

     (GDBP) tfind start
     (GDBP) while ($trace_frame != -1)
     > printf "Frame %d, X == %d\n", $trace_frame, X
     > tfind line
     > end

     Frame 0, X = 1
     Frame 7, X = 2
     Frame 13, X = 255

tdump
-----

This command takes no arguments.  It prints all the data collected at the current trace snapshot.

     (GDBP) trace 444
     (GDBP) actions
     Enter actions for tracepoint #2, one per line:
     > collect $regs, $locals, $args, gdb_long_test
     > end

     (GDBP) tstart

     (GDBP) tfind line 444
     #0  gdb_test (p1=0x11, p2=0x22, p3=0x33, p4=0x44, p5=0x55, p6=0x66)
     at gdb_test.c:444
     444        printp( "%s: arguments = 0x%X 0x%X 0x%X 0x%X 0x%X 0x%X\n", )

     (GDBP) tdump
     Data collected at tracepoint 2, trace frame 1:
     d0             0xc4aa0085       -995491707
     d1             0x18     24
     d2             0x80     128
     d3             0x33     51
     d4             0x71aea3d        119204413
     d5             0x22     34
     d6             0xe0     224
     d7             0x380035 3670069
     a0             0x19e24a 1696330
     a1             0x3000668        50333288
     a2             0x100    256
     a3             0x322000 3284992
     a4             0x3000698        50333336
     a5             0x1ad3cc 1758156
     fp             0x30bf3c 0x30bf3c
     sp             0x30bf34 0x30bf34
     ps             0x0      0
     pc             0x20b2c8 0x20b2c8
     fpcontrol      0x0      0
     fpstatus       0x0      0
     fpiaddr        0x0      0
     p = 0x20e5b4 "gdb-test"
     p1 = (void *) 0x11
     p2 = (void *) 0x22
     p3 = (void *) 0x33
     p4 = (void *) 0x44
     p5 = (void *) 0x55
     p6 = (void *) 0x66
     gdb_long_test = 17 '\021'

     (GDBP)

tdump works by scanning the tracepoint's current collection actions and printing the value of each expression listed.  So tdump can fail, if after a run, you change the tracepoint's actions to mention variables that were not collected during the run.

Also, for tracepoints with while-stepping loops, tdump uses the collected value of $pc to distinguish between trace frames that were collected at the tracepoint hit, and frames that were collected while stepping.  This allows it to correctly choose whether to display the basic list of collections, or the collections from the body of the while-stepping loop.  However, if $pc was not collected, then tdump will always attempt to dump using the basic collection list, and may fail if a while-stepping frame does not include all the same data that is collected at the tracepoint hit.

save tracepoints FILENAME
-------------------------

This command saves all current tracepoint definitions together with their actions and passcounts, into a file FILENAME suitable for use in a later debugging session.  To read the saved tracepoint definitions, use the source command (see Command Files).  The save-tracepoints command is a deprecated alias for save tracepoints

Convenience Variables for Tracepoints
=====================================

(int) $trace_frame
     The current trace snapshot (a.k.a. frame) number, or -1 if no snapshot is selected.

(int) $tracepoint
     The tracepoint for the current trace snapshot.

(int) $trace_line
     The line number for the current trace snapshot.

(char []) $trace_file
     The source file for the current trace snapshot.

(char []) $trace_func
     The name of the function containing $tracepoint.

Note: $trace_file is not suitable for use in printf, use output instead.

Here's a simple example of using these convenience variables for stepping through all the trace snapshots and printing some of their data.  Note that these are not the same as trace state variables, which are managed by the target.

     (GDBP) tfind start

     (GDBP) while $trace_frame != -1
     > output $trace_file
     > printf ", line %d (tracepoint #%d)\n", $trace_line, $tracepoint
     > tfind
     > end

Using Trace Files
=================

In some situations, the target running a trace experiment may no longer be available; perhaps it crashed, or the hardware was needed for a different activity.  To handle these cases, you can arrange to dump the trace data into a file, and later use that file as a source of trace data, via the target tfile command.

tsave [ -r ] FILENAME
     Save the trace data to FILENAME.  By default, this command assumes that FILENAME refers to the host filesystem, so if necessary GDBN will copy raw trace data up from the target and then save it.  If the target supports it, you can also supply the optional argument -r (``remote'') to direct the target to save the data directly into FILENAME in its own filesystem, which may be more efficient if the trace buffer is very large.  (Note, however, that target tfile can only read from files accessible to the host.)

target tfile FILENAME
     Use the file named FILENAME as a source of trace data.  Commands that examine data work as they do with a live target, but it is not possible to run any new trace experiments.  tstatus will report the state of the trace run at the moment the data was saved, as well as the current trace frame you are examining.  FILENAME must be on a filesystem accessible to the host.

Debugging Programs That Use Overlays
************************************

If your program is too large to fit completely in your target system's memory, you can sometimes use overlays to work around this problem.  GDBN provides some support for debugging programs that use overlays.

How Overlays Work
=================

Suppose you have a computer whose instruction address space is only 64 kilobytes long, but which has much more memory which can be accessed by other means: special instructions, segment registers, or memory management hardware, for example.  Suppose further that you want to adapt a program which is larger than 64 kilobytes to run on this system.

One solution is to identify modules of your program which are relatively independent, and need not call each other directly; call these modules overlays.  Separate the overlays from the main program, and place their machine code in the larger memory.  Place your main program in instruction memory, but leave at least enough space there to hold the largest overlay as well.

Now, to call a function located in an overlay, you must first copy that overlay's machine code from the large memory into the space set aside for it in the instruction memory, and then jump to its entry point there.

         Data             Instruction            Larger
     Address Space       Address Space        Address Space
     +-----------+       +-----------+        +-----------+
     |           |       |           |        |           |
     +-----------+       +-----------+        +-----------+<-- overlay 1
     | program   |       |   main    |   .----| overlay 1 | load address
     | variables |       |  program  |   |    +-----------+
     | and heap  |       |           |   |    |           |
     +-----------+       |           |   |    +-----------+<-- overlay 2
     |           |       +-----------+   |    |           | load address
     +-----------+       |           |   |  .-| overlay 2 |
                         |           |   |  | |           |
              mapped --->+-----------+   |  | +-----------+
              address    |           |   |  | |           |
                         |  overlay  | <-'  | |           |
                         |   area    |  <---' +-----------+<-- overlay 3
                         |           | <---.  |           | load address
                         +-----------+     `--| overlay 3 |
                         |           |        |           |
                         +-----------+        |           |
                                              +-----------+
                                              |           |
                                              +-----------+

                         A code overlay

The diagram (see A code overlay) shows a system with separate data and instruction address spaces.  To map an overlay, the program copies its code from the larger address space to the instruction address space. Since the overlays shown here all use the same mapped address, only one may be mapped at a time.  For a system with a single address space for data and instructions, the diagram would be similar, except that the program variables and heap would share an address space with the main program and the overlay area.

An overlay loaded into instruction memory and ready for use is called a mapped overlay; its mapped address is its address in the instruction memory.  An overlay not present (or only partially present) in instruction memory is called unmapped; its load address is its address in the larger memory.  The mapped address is also called the virtual memory address, or VMA; the load address is also called the load memory address, or LMA.

Unfortunately, overlays are not a completely transparent way to adapt a program to limited instruction memory.  They introduce a new set of global constraints you must keep in mind as you design your program:

  * Before calling or returning to a function in an overlay, your program must make sure that overlay is actually mapped.  Otherwise, the call or return will transfer control to the right address, but in the wrong overlay, and your program will probably crash.

  * If the process of mapping an overlay is expensive on your system, you will need to choose your overlays carefully to minimize their effect on your program's performance.

  * The executable file you load onto your system must contain each overlay's instructions, appearing at the overlay's load address, not its mapped address.  However, each overlay's instructions must be relocated and its symbols defined as if the overlay were at its mapped address. You can use GNU linker scripts to specify different load and relocation addresses for pieces of your program; see Overlay Description (Using ld: the GNU linker).

  * The procedure for loading executable files onto your system must be able to load their contents into the larger address space as well as the instruction and data spaces.

The overlay system described above is rather simple, and could be improved in many ways:

  * If your system has suitable bank switch registers or memory management hardware, you could use those facilities to make an overlay's load area contents simply appear at their mapped address in instruction space. This would probably be faster than copying the overlay to its mapped area in the usual way.

  * If your overlays are small enough, you could set aside more than one overlay area, and have more than one overlay mapped at a time.

  * You can use overlays to manage data, as well as instructions.  In general, data overlays are even less transparent to your design than code overlays: whereas code overlays only require care when you call or return to functions, data overlays require care every time you access the data.  Also, if you change the contents of a data overlay, you must copy its contents back out to its load address before you can copy a different data overlay into the same mapped area.

Overlay Commands
================

To use GDBN's overlay support, each overlay in your program must correspond to a separate section of the executable file.  The section's virtual memory address and load memory address must be the overlay's mapped and load addresses.  Identifying overlays with sections allows GDBN to determine the appropriate address of a function or variable, depending on whether the overlay is mapped or not.

GDBN's overlay commands all start with the word overlay; you can abbreviate this as ov or ovly.  The commands are:

overlay off
     Disable GDBN's overlay support.  When overlay support is disabled, GDBN assumes that all functions and variables are always present at their mapped addresses.  By default, GDBN's overlay support is disabled.

overlay manual
     Enable manual overlay debugging.  In this mode, GDBN relies on you to tell it which overlays are mapped, and which are not, using the overlay map-overlay and overlay unmap-overlay commands described below.

overlay map-overlay OVERLAY
overlay map OVERLAY
     Tell GDBN that OVERLAY is now mapped; OVERLAY must be the name of the object file section containing the overlay.  When an overlay is mapped, GDBN assumes it can find the overlay's functions and variables at their mapped addresses.  GDBN assumes that any other overlays whose mapped ranges overlap that of OVERLAY are now unmapped.

overlay unmap-overlay OVERLAY
overlay unmap OVERLAY
     Tell GDBN that OVERLAY is no longer mapped; OVERLAY must be the name of the object file section containing the overlay. When an overlay is unmapped, GDBN assumes it can find the overlay's functions and variables at their load addresses.

overlay auto
     Enable automatic overlay debugging.  In this mode, GDBN consults a data structure the overlay manager maintains in the inferior to see which overlays are mapped.  For details, see Automatic Overlay Debugging.

overlay load-target
overlay load
     Re-read the overlay table from the inferior.  Normally, GDBN re-reads the table GDBN automatically each time the inferior stops, so this command should only be necessary if you have changed the overlay mapping yourself using GDBN.  This command is only useful when using automatic overlay debugging.

overlay list-overlays
overlay list
     Display a list of the overlays currently mapped, along with their mapped addresses, load addresses, and sizes.

Normally, when GDBN prints a code address, it includes the name of the function the address falls in:

     (GDBP) print main
     $3 = {int ()} 0x11a0 <main>

When overlay debugging is enabled, GDBN recognizes code in unmapped overlays, and prints the names of unmapped functions with asterisks around them.  For example, if foo is a function in an unmapped overlay, GDBN prints it this way:

     (GDBP) overlay list
     No sections are mapped.
     (GDBP) print foo
     $5 = {int (int)} 0x100000 <*foo*>

When foo's overlay is mapped, GDBN prints the function's name normally:

     (GDBP) overlay list
     Section .ov.foo.text, loaded at 0x100000 - 0x100034,
             mapped at 0x1016 - 0x104a
     (GDBP) print foo
     $6 = {int (int)} 0x1016 <foo>

When overlay debugging is enabled, GDBN can find the correct address for functions and variables in an overlay, whether or not the overlay is mapped.  This allows most GDBN commands, like break and disassemble, to work normally, even on unmapped code.  However, GDBN's breakpoint support has some limitations:

  * You can set breakpoints in functions in unmapped overlays, as long as GDBN can write to the overlay at its load address.

  * GDBN can not set hardware or simulator-based breakpoints in unmapped overlays.  However, if you set a breakpoint at the end of your overlay manager (and tell GDBN which overlays are now mapped, if you are using manual overlay management), GDBN will re-set its breakpoints properly.

Automatic Overlay Debugging
===========================

GDBN can automatically track which overlays are mapped and which are not, given some simple co-operation from the overlay manager in the inferior.  If you enable automatic overlay debugging with the overlay auto command (see Overlay Commands), GDBN looks in the inferior's memory for certain variables describing the current state of the overlays.

Here are the variables your overlay manager must define to support GDBN's automatic overlay debugging:

_ovly_table:
     This variable must be an array of the following structures:

          struct
          {
            /* The overlay's mapped address.  */
            unsigned long vma;

            /* The size of the overlay, in bytes.  */
            unsigned long size;

            /* The overlay's load address.  */
            unsigned long lma;

            /* Non-zero if the overlay is currently mapped;
               zero otherwise.  */
            unsigned long mapped;
          }

_novlys:
     This variable must be a four-byte signed integer, holding the total number of elements in _ovly_table.

To decide whether a particular overlay is mapped or not, GDBN looks for an entry in _ovly_table whose vma and lma members equal the VMA and LMA of the overlay's section in the executable file.  When GDBN finds a matching entry, it consults the entry's mapped member to determine whether the overlay is currently mapped.

In addition, your overlay manager may define a function called _ovly_debug_event.  If this function is defined, GDBN will silently set a breakpoint there.  If the overlay manager then calls this function whenever it has changed the overlay table, this will enable GDBN to accurately keep track of which overlays are in program memory, and update any breakpoints that may be set in overlays.  This will allow breakpoints to work even if the overlays are kept in ROM or other non-writable memory while they are not being executed.

Overlay Sample Program
======================

When linking a program which uses overlays, you must place the overlays at their load addresses, while relocating them to run at their mapped addresses.  To do this, you must write a linker script (see Overlay Description (Using ld: the GNU linker)).  Unfortunately, since linker scripts are specific to a particular host system, target architecture, and target memory layout, this manual cannot provide portable sample code demonstrating GDBN's overlay support.

However, the GDBN source distribution does contain an overlaid program, with linker scripts for a few systems, as part of its test suite.  The program consists of the following files from gdb/testsuite/gdb.base:

overlays.c
     The main program file.

ovlymgr.c
     A simple overlay manager, used by overlays.c.

foo.c
bar.c
baz.c
grbx.c
     Overlay modules, loaded and used by overlays.c.

d10v.ld
m32r.ld
     Linker scripts for linking the test program on the d10v-elf and m32r-elf targets.

You can build the test program using the d10v-elf GCC cross-compiler like this:

     $ d10v-elf-gcc -g -c overlays.c
     $ d10v-elf-gcc -g -c ovlymgr.c
     $ d10v-elf-gcc -g -c foo.c
     $ d10v-elf-gcc -g -c bar.c
     $ d10v-elf-gcc -g -c baz.c
     $ d10v-elf-gcc -g -c grbx.c
     $ d10v-elf-gcc -g overlays.o ovlymgr.o foo.o bar.o \
                       baz.o grbx.o -Wl,-Td10v.ld -o overlays

The build process is identical for any other architecture, except that you must substitute the appropriate compiler and linker script for the target system for d10v-elf-gcc and d10v.ld.

Using GDBN with Different Languages
***********************************

Although programming languages generally have common aspects, they are rarely expressed in the same manner.  For instance, in ANSI C, dereferencing a pointer p is accomplished by *p, but in Modula-2, it is accomplished by p^.  Values can also be represented (and displayed) differently.  Hex numbers in C appear as 0x1ae, while in Modula-2 they appear as 1AEH.

Language-specific information is built into GDBN for some languages, allowing you to express operations like the above in your program's native language, and allowing GDBN to output values in a manner consistent with the syntax of your program's native language.  The language you use to build expressions is called the working language.

Switching Between Source Languages
==================================

There are two ways to control the working language---either have GDBN set it automatically, or select it manually yourself.  You can use the set language command for either purpose.  On startup, GDBN defaults to setting the language automatically.  The working language is used to determine how expressions you type are interpreted, how values are printed, etc.

In addition to the working language, every source file that GDBN knows about has its own working language.  For some object file formats, the compiler might indicate which language a particular source file is in.  However, most of the time GDBN infers the language from the name of the file.  The language of a source file controls whether C++ names are demangled---this way backtrace can show each frame appropriately for its own language.  There is no way to set the language of a source file from within GDBN, but you can set the language associated with a filename extension.  See Displaying the Language.

This is most commonly a problem when you use a program, such as cfront or f2c, that generates C but is written in another language.  In that case, make the program use #line directives in its C output; that way GDBN will know the correct language of the source code of the original program, and will display that source code, not the generated C code.

List of Filename Extensions and Languages
-----------------------------------------

If a source file name ends in one of the following extensions, then GDBN infers that its language is the one indicated.

.ada
.ads
.adb
.a
     Ada source file.

.c
     C source file

.C
.cc
.cp
.cpp
.cxx
.c++
     C++ source file

.d
     D source file

.m
     Objective-C source file

.f
.F
     Fortran source file

.mod
     Modula-2 source file

.s
.S
     Assembler source file.  This actually behaves almost like C, but GDBN does not skip over function prologues when stepping.

In addition, you may set the language associated with a filename extension.  See Displaying the Language.

Setting the Working Language
----------------------------

If you allow GDBN to set the language automatically, expressions are interpreted the same way in your debugging session and your program.

If you wish, you may set the language manually.  To do this, issue the command set language LANG, where LANG is the name of a language, such as c or modula-2. For a list of the supported languages, type set language.

Setting the language manually prevents GDBN from updating the working language automatically.  This can lead to confusion if you try to debug a program when the working language is not the same as the source language, when an expression is acceptable to both languages---but means different things.  For instance, if the current source file were written in C, and GDBN was parsing Modula-2, a command such as:

     print a = b + c

might not have the effect you intended.  In C, this means to add b and c and place the result in a.  The result printed would be the value of a.  In Modula-2, this means to compare a to the result of b+c, yielding a BOOLEAN value.

Having GDBN Infer the Source Language
-------------------------------------

To have GDBN set the working language automatically, use set language local or set language auto.  GDBN then infers the working language.  That is, when your program stops in a frame (usually by encountering a breakpoint), GDBN sets the working language to the language recorded for the function in that frame.  If the language for a frame is unknown (that is, if the function or block corresponding to the frame was defined in a source file that does not have a recognized extension), the current working language is not changed, and GDBN issues a warning.

This may not seem necessary for most programs, which are written entirely in one source language.  However, program modules and libraries written in one source language can be used by a main program written in a different source language.  Using set language auto in this case frees you from having to set the working language manually.

Displaying the Language
=======================

The following commands help you find out which language is the working language, and also what language source files were written in.

show language
     Display the current working language.  This is the language you can use with commands such as print to build and compute expressions that may involve variables in your program.

info frame
     Display the source language for this frame.  This language becomes the working language if you use an identifier from this frame. See Information about a Frame, to identify the other information listed here.

info source
     Display the source language of this source file. See Examining the Symbol Table, to identify the other information listed here.

In unusual circumstances, you may have source files with extensions not in the standard list.  You can then set the extension associated with a language explicitly:

set extension-language EXT LANGUAGE
     Tell GDBN that source files with extension EXT are to be assumed as written in the source language LANGUAGE.

info extensions
     List all the filename extensions and the associated languages.

Type and Range Checking
=======================

     Warning: In this release, the GDBN commands for type and range checking are included, but they do not yet have any effect.  This section documents the intended facilities.

Some languages are designed to guard you against making seemingly common errors through a series of compile- and run-time checks.  These include checking the type of arguments to functions and operators, and making sure mathematical overflows are caught at run time.  Checks such as these help to ensure a program's correctness once it has been compiled by eliminating type mismatches, and providing active checks for range errors when your program is running.

GDBN can check for conditions like the above if you wish. Although GDBN does not check the statements in your program, it can check expressions entered directly into GDBN for evaluation via the print command, for example.  As with the working language, GDBN can also decide whether or not to check automatically based on your program's source language. See Supported Languages, for the default settings of supported languages.

An Overview of Type Checking
----------------------------

Some languages, such as Modula-2, are strongly typed, meaning that the arguments to operators and functions have to be of the correct type, otherwise an error occurs.  These checks prevent type mismatch errors from ever causing any run-time problems.  For example,

     1 + 2 => 3
     but
     error--> 1 + 2.3

The second example fails because the CARDINAL 1 is not type-compatible with the REAL 2.3.

For the expressions you use in GDBN commands, you can tell the GDBN type checker to skip checking; to treat any mismatches as errors and abandon the expression; or to only issue warnings when type mismatches occur, but evaluate the expression anyway.  When you choose the last of these, GDBN evaluates expressions like the second example above, but also issues a warning.

Even if you turn type checking off, there may be other reasons related to type that prevent GDBN from evaluating an expression. For instance, GDBN does not know how to add an int and a struct foo.  These particular type errors have nothing to do with the language in use, and usually arise from expressions, such as the one described above, which make little sense to evaluate anyway.

Each language defines to what degree it is strict about type.  For instance, both Modula-2 and C require the arguments to arithmetical operators to be numbers.  In C, enumerated types and pointers can be represented as numbers, so that they are valid arguments to mathematical operators.  See Supported Languages, for further details on specific languages.

GDBN provides some additional commands for controlling the type checker:

set check type auto
     Set type checking on or off based on the current working language. See Supported Languages, for the default settings for each language.

set check type on
set check type off
     Set type checking on or off, overriding the default setting for the current working language.  Issue a warning if the setting does not match the language default.  If any type mismatches occur in evaluating an expression while type checking is on, GDBN prints a message and aborts evaluation of the expression.

set check type warn
     Cause the type checker to issue warnings, but to always attempt to evaluate the expression.  Evaluating the expression may still be impossible for other reasons.  For example, GDBN cannot add numbers and structures.

show type
     Show the current setting of the type checker, and whether or not GDBN is setting it automatically.

An Overview of Range Checking
-----------------------------

In some languages (such as Modula-2), it is an error to exceed the bounds of a type; this is enforced with run-time checks.  Such range checking is meant to ensure program correctness by making sure computations do not overflow, or indices on an array element access do not exceed the bounds of the array.

For expressions you use in GDBN commands, you can tell GDBN to treat range errors in one of three ways: ignore them, always treat them as errors and abandon the expression, or issue warnings but evaluate the expression anyway.

A range error can result from numerical overflow, from exceeding an array index bound, or when you type a constant that is not a member of any type.  Some languages, however, do not treat overflows as an error.  In many implementations of C, mathematical overflow causes the result to ``wrap around'' to lower values---for example, if M is the largest integer value, and S is the smallest, then

     M + 1 => S

This, too, is specific to individual languages, and in some cases specific to individual compilers or machines.  See Supported Languages, for further details on specific languages.

GDBN provides some additional commands for controlling the range checker:

set check range auto
     Set range checking on or off based on the current working language. See Supported Languages, for the default settings for each language.

set check range on
set check range off
     Set range checking on or off, overriding the default setting for the current working language.  A warning is issued if the setting does not match the language default.  If a range error occurs and range checking is on, then a message is printed and evaluation of the expression is aborted.

set check range warn
     Output messages when the GDBN range checker detects a range error, but attempt to evaluate the expression anyway.  Evaluating the expression may still be impossible for other reasons, such as accessing memory that the process does not own (a typical example from many Unix systems).

show range
     Show the current setting of the range checker, and whether or not it is being set automatically by GDBN.

Supported Languages
===================

GDBN supports C, C++, D, Objective-C, Fortran, Java, OpenCL C, Pascal, assembly, Modula-2, and Ada. Some GDBN features may be used in expressions regardless of the language you use: the GDBN @ and :: operators, and the {type}addr construct (see Expressions) can be used with the constructs of any supported language.

The following sections detail to what degree each source language is supported by GDBN.  These sections are not meant to be language tutorials or references, but serve only as a reference guide to what the GDBN expression parser accepts, and what input and output formats should look like for different languages.  There are many good books written on each of these languages; please look to these for a language reference or tutorial.

C and C++
---------

Since C and C++ are so closely related, many features of GDBN apply to both languages.  Whenever this is the case, we discuss those languages together.

The C++ debugging facilities are jointly implemented by the C++ compiler and GDBN.  Therefore, to debug your C++ code effectively, you must compile your C++ programs with a supported C++ compiler, such as GNU g++, or the HP ANSI C++ compiler (aCC).

C and C++ Operators
...................

Operators must be defined on values of specific types.  For instance, + is defined on numbers, but not on structures.  Operators are often defined on groups of types.

For the purposes of C and C++, the following definitions hold:

  * Integral types include int with any of its storage-class specifiers; char; enum; and, for C++, bool.

  * Floating-point types include float, double, and long double (if supported by the target platform).

  * Pointer types include all types defined as (TYPE *).

  * Scalar types include all of the above.

The following operators are supported.  They are listed here in order of increasing precedence:

,
     The comma or sequencing operator.  Expressions in a comma-separated list are evaluated from left to right, with the result of the entire expression being the last expression evaluated.

=
     Assignment.  The value of an assignment expression is the value assigned.  Defined on scalar types.

OP=
     Used in an expression of the form A OP= B, and translated to A = A OP B. OP= and = have the same precedence. OP is any one of the operators |, ^, &, <<, >>, +, -, *, /, %.

?:
     The ternary operator.  A ? B : C can be thought of as:  if A then B else C.  A should be of an integral type.

||
     Logical OR.  Defined on integral types.

&&
     Logical AND.  Defined on integral types.

|
     Bitwise OR.  Defined on integral types.

^
     Bitwise exclusive-OR.  Defined on integral types.

&
     Bitwise AND.  Defined on integral types.

==, !=
     Equality and inequality.  Defined on scalar types.  The value of these expressions is 0 for false and non-zero for true.

<, >, <=, >=
     Less than, greater than, less than or equal, greater than or equal. Defined on scalar types.  The value of these expressions is 0 for false and non-zero for true.

<<, >>
     left shift, and right shift.  Defined on integral types.

@
     The GDBN ``artificial array'' operator (see Expressions).

+, -
     Addition and subtraction.  Defined on integral types, floating-point types and pointer types.

*, /, %
     Multiplication, division, and modulus.  Multiplication and division are defined on integral and floating-point types.  Modulus is defined on integral types.

++, --
     Increment and decrement.  When appearing before a variable, the operation is performed before the variable is used in an expression; when appearing after it, the variable's value is used before the operation takes place.

*
     Pointer dereferencing.  Defined on pointer types.  Same precedence as ++.

&
     Address operator.  Defined on variables.  Same precedence as ++.

     For debugging C++, GDBN implements a use of & beyond what is allowed in the C++ language itself: you can use &(&REF) to examine the address where a C++ reference variable (declared with &REF) is stored.

-
     Negative.  Defined on integral and floating-point types.  Same precedence as ++.

!
     Logical negation.  Defined on integral types.  Same precedence as ++.

~
     Bitwise complement operator.  Defined on integral types.  Same precedence as ++.

., ->
     Structure member, and pointer-to-structure member.  For convenience, GDBN regards the two as equivalent, choosing whether to dereference a pointer based on the stored type information. Defined on struct and union data.

.*, ->*
     Dereferences of pointers to members.

[]
     Array indexing.  A[I] is defined as *(A+I).  Same precedence as ->.

()
     Function parameter list.  Same precedence as ->.

::
     C++ scope resolution operator.  Defined on struct, union, and class types.

::
     Doubled colons also represent the GDBN scope operator (see Expressions).  Same precedence as ::, above.

If an operator is redefined in the user code, GDBN usually attempts to invoke the redefined version instead of using the operator's predefined meaning.

C and C++ Constants
...................

GDBN allows you to express the constants of C and C++ in the following ways:

  * Integer constants are a sequence of digits.  Octal constants are specified by a leading 0 (i.e. zero), and hexadecimal constants by a leading 0x or 0X.  Constants may also end with a letter l, specifying that the constant should be treated as a long value.

  * Floating point constants are a sequence of digits, followed by a decimal point, followed by a sequence of digits, and optionally followed by an exponent.  An exponent is of the form: e[[+]|-]NNN, where NNN is another sequence of digits.  The + is optional for positive exponents. A floating-point constant may also end with a letter f or F, specifying that the constant should be treated as being of the float (as opposed to the default double) type; or with a letter l or L, which specifies a long double constant.

  * Enumerated constants consist of enumerated identifiers, or their integral equivalents.

  * Character constants are a single character surrounded by single quotes ('), or a number---the ordinal value of the corresponding character (usually its ASCII value).  Within quotes, the single character may be represented by a letter or by escape sequences, which are of the form \NNN, where NNN is the octal representation of the character's ordinal value; or of the form \X, where X is a predefined special character---for example, \n for newline.

     Wide character constants can be written by prefixing a character constant with L, as in C.  For example, L'x' is the wide form of x.  The target wide character set is used when computing the value of this constant (see Character Sets).

  * String constants are a sequence of character constants surrounded by double quotes (").  Any valid character constant (as described above) may appear.  Double quotes within the string must be preceded by a backslash, so for instance "a\"b'c" is a string of five characters.

     Wide string constants can be written by prefixing a string constant with L, as in C.  The target wide character set is used when computing the value of this constant (see Character Sets).

  * Pointer constants are an integral value.  You can also write pointers to constants using the C operator &.

  * Array constants are comma-separated lists surrounded by braces { and }; for example, {1,2,3} is a three-element array of integers, {{1,2}, {3,4}, {5,6}} is a three-by-two array, and {&"hi", &"there", &"fred"} is a three-element array of pointers.

C++ Expressions
...............

GDBN expression handling can interpret most C++ expressions.

     Warning: GDBN can only debug C++ code if you use the proper compiler and the proper debug format.  Currently, GDBN works best when debugging C++ code that is compiled with the most recent version of NGCC possible.  The DWARF debugging format is preferred; NGCC defaults to this on most popular platforms.  Other compilers and/or debug formats are likely to work badly or not at all when using GDBN to debug C++ code.  See Compilation.

 1. Member function calls are allowed; you can use expressions like

          count = aml->GetOriginal(x, y)

 2. While a member function is active (in the selected stack frame), your expressions have the same namespace available as the member function; that is, GDBN allows implicit references to the class instance pointer this following the same rules as C++.  using declarations in the current scope are also respected by GDBN.

 3. You can call overloaded functions; GDBN resolves the function call to the right definition, with some restrictions.  GDBN does not perform overload resolution involving user-defined type conversions, calls to constructors, or instantiations of templates that do not exist in the program.  It also cannot handle ellipsis argument lists or default arguments.

     It does perform integral conversions and promotions, floating-point promotions, arithmetic conversions, pointer conversions, conversions of class objects to base classes, and standard conversions such as those of functions or arrays to pointers; it requires an exact match on the number of function arguments.

     Overload resolution is always performed, unless you have specified set overload-resolution off.  See GDBN Features for C++.

     You must specify set overload-resolution off in order to use an explicit function signature to call an overloaded function, as in

          p 'foo(char,int)'('x', 13)

     The GDBN command-completion facility can simplify this; see Command Completion.

 4. GDBN understands variables declared as C++ references; you can use them in expressions just as you do in C++ source---they are automatically dereferenced.

     In the parameter list shown when GDBN displays a frame, the values of reference variables are not displayed (unlike other variables); this avoids clutter, since references are often used for large structures. The address of a reference variable is always shown, unless you have specified set print address off.

 5. GDBN supports the C++ name resolution operator ::---your expressions can use it just as expressions in your program do.  Since one scope may be defined in another, you can use :: repeatedly if necessary, for example in an expression like SCOPE1::SCOPE2::NAME.  GDBN also allows resolving name scope by reference to source files, in both C and C++ debugging (see Program Variables).

 6. GDBN performs argument-dependent lookup, following the C++ specification.

C and C++ Defaults
..................

If you allow GDBN to set type and range checking automatically, they both default to off whenever the working language changes to C or C++.  This happens regardless of whether you or GDBN selects the working language.

If you allow GDBN to set the language automatically, it recognizes source files whose names end with .c, .C, or .cc, etc, and when GDBN enters code compiled from one of these files, it sets the working language to C or C++. See Having GDBN Infer the Source Language, for further details.

C and C++ Type and Range Checks
...............................

By default, when GDBN parses C or C++ expressions, type checking is not used.  However, if you turn type checking on, GDBN considers two variables type equivalent if:

  * The two variables are structured and have the same structure, union, or enumerated tag.

  * The two variables have the same type name, or types that have been declared equivalent through typedef.

Range checking, if turned on, is done on mathematical operations.  Array indices are not checked, since they are often used to index a pointer that is not itself an array.

GDBN and C
..........

The set print union and show print union commands apply to the union type.  When set to on, any union that is inside a struct or class is also printed.  Otherwise, it appears as {...}.

The @ operator aids in the debugging of dynamic arrays, formed with pointers and a memory allocation function.  See Expressions.

GDBN Features for C++
.....................

Some GDBN commands are particularly useful with C++, and some are designed specifically for use with C++.  Here is a summary:

breakpoint menus
     When you want a breakpoint in a function whose name is overloaded, GDBN has the capability to display a menu of possible breakpoint locations to help you specify which function definition you want. See Ambiguous Expressions.

rbreak REGEX
     Setting breakpoints using regular expressions is helpful for setting breakpoints on overloaded functions that are not members of any special classes. See Setting Breakpoints.

catch throw
catch catch
     Debug C++ exception handling using these commands.  See Setting Catchpoints.

ptype TYPENAME
     Print inheritance relationships as well as other information for type TYPENAME. See Examining the Symbol Table.

set print demangle
show print demangle
set print asm-demangle
show print asm-demangle
     Control whether C++ symbols display in their source form, both when displaying code as C++ source and when displaying disassemblies. See Print Settings.

set print object
show print object
     Choose whether to print derived (actual) or declared types of objects. See Print Settings.

set print vtbl
show print vtbl
     Control the format for printing virtual function tables. See Print Settings. (The vtbl commands do not work on programs compiled with the HP ANSI C++ compiler (aCC).)

set overload-resolution on
     Enable overload resolution for C++ expression evaluation.  The default is on.  For overloaded functions, GDBN evaluates the arguments and searches for a function whose signature matches the argument types, using the standard C++ conversion rules (see C++ Expressions, for details). If it cannot find a match, it emits a message.

set overload-resolution off
     Disable overload resolution for C++ expression evaluation.  For overloaded functions that are not class member functions, GDBN chooses the first function of the specified name that it finds in the symbol table, whether or not its arguments are of the correct type.  For overloaded functions that are class member functions, GDBN searches for a function whose signature exactly matches the argument types.

show overload-resolution
     Show the current setting of overload resolution.

Overloaded symbol names
     You can specify a particular definition of an overloaded symbol, using the same notation that is used to declare such symbols in C++: type SYMBOL(TYPES) rather than just SYMBOL.  You can also use the GDBN command-line word completion facilities to list the available choices, or to finish the type list for you. See Command Completion, for details on how to do this.

Decimal Floating Point format
.............................

GDBN can examine, set and perform computations with numbers in decimal floating point format, which in the C language correspond to the _Decimal32, _Decimal64 and _Decimal128 types as specified by the extension to support decimal floating-point arithmetic.

There are two encodings in use, depending on the architecture: BID (Binary Integer Decimal) for x86 and x86-64, and DPD (Densely Packed Decimal) for PowerPC.  GDBN will use the appropriate encoding for the configured target.

Because of a limitation in libdecnumber, the library used by GDBN to manipulate decimal floating point numbers, it is not possible to convert (using a cast, for example) integers wider than 32-bit to decimal float.

In addition, in order to imitate GDBN's behaviour with binary floating point computations, error checking in decimal float operations ignores underflow, overflow and divide by zero exceptions.

In the PowerPC architecture, GDBN provides a set of pseudo-registers to inspect _Decimal128 values stored in floating point registers. See PowerPC for more details.

D
-

GDBN can be used to debug programs written in D and compiled with GDC, LDC or DMD compilers. Currently GDBN supports only one D specific feature --- dynamic arrays.

Objective-C
-----------

This section provides information about some commands and command options that are useful for debugging Objective-C code.  See also info classes, and info selectors, for a few more commands specific to Objective-C support.

Method Names in Commands
........................

The following commands have been extended to accept Objective-C method names as line specifications:

  * clear

  * break

  * info line

  * jump

  * list

A fully qualified Objective-C method name is specified as

     -[CLASS METHODNAME]

where the minus sign is used to indicate an instance method and a plus sign (not shown) is used to indicate a class method.  The class name CLASS and method name METHODNAME are enclosed in brackets, similar to the way messages are specified in Objective-C source code.  For example, to set a breakpoint at the create instance method of class Fruit in the program currently being debugged, enter:

     break -[Fruit create]

To list ten program lines around the initialize class method, enter:

     list +[NSText initialize]

In the current version of GDBN, the plus or minus sign is required.  In future versions of GDBN, the plus or minus sign will be optional, but you can use it to narrow the search.  It is also possible to specify just a method name:

     break create

You must specify the complete method name, including any colons.  If your program's source files contain more than one create method, you'll be presented with a numbered list of classes that implement that method.  Indicate your choice by number, or type 0 to exit if none apply.

As another example, to clear a breakpoint established at the makeKeyAndOrderFront: method of the NSWindow class, enter:

     clear -[NSWindow makeKeyAndOrderFront:]

The Print Command With Objective-C
..................................

The print command has also been extended to accept methods.  For example:

     print -[OBJECT hash]

will tell GDBN to send the hash message to OBJECT and print the result.  Also, an additional command has been added, print-object or po for short, which is meant to print the description of an object.  However, this command may only work with certain Objective-C libraries that have a particular hook function, _NSPrintForDebugger, defined.

OpenCL C
--------

This section provides information about GDBNs OpenCL C support.

OpenCL C Datatypes
..................

GDBN supports the builtin scalar and vector datatypes specified by OpenCL 1.1.  In addition the half- and double-precision floating point data types of the cl_khr_fp16 and cl_khr_fp64 OpenCL extensions are also known to GDBN.

OpenCL C Expressions
....................

GDBN supports accesses to vector components including the access as lvalue where possible.  Since OpenCL C is based on C99 most C expressions supported by GDBN can be used as well.

OpenCL C Operators
..................

GDBN supports the operators specified by OpenCL 1.1 for scalar and vector data types.

Fortran
-------

GDBN can be used to debug programs written in Fortran, but it currently supports only the features of Fortran 77 language.

Some Fortran compilers (GNU Fortran 77 and Fortran 95 compilers among them) append an underscore to the names of variables and functions.  When you debug programs compiled by those compilers, you will need to refer to variables and functions with a trailing underscore.

Fortran Operators and Expressions
.................................

Operators must be defined on values of specific types.  For instance, + is defined on numbers, but not on characters or other non- arithmetic types.  Operators are often defined on groups of types.

**
     The exponentiation operator.  It raises the first operand to the power of the second one.

:
     The range operator.  Normally used in the form of array(low:high) to represent a section of array.

%
     The access component operator.  Normally used to access elements in derived types.  Also suitable for unions.  As unions aren't part of regular Fortran, this can only happen when accessing a register that uses a gdbarch-defined union type.

Fortran Defaults
................

Fortran symbols are usually case-insensitive, so GDBN by default uses case-insensitive matches for Fortran symbols.  You can change that with the set case-insensitive command, see Symbols, for the details.

Special Fortran Commands
........................

GDBN has some commands to support Fortran-specific features, such as displaying common blocks.

info common [COMMON-NAME]
     This command prints the values contained in the Fortran COMMON block whose name is COMMON-NAME.  With no argument, the names of all COMMON blocks visible at the current program location are printed.

Pascal
------

Debugging Pascal programs which use sets, subranges, file variables, or nested functions does not currently work.  GDBN does not support entering expressions, printing values, or similar features using Pascal syntax.

The Pascal-specific command set print pascal_static-members controls whether static members of Pascal objects are displayed. See pascal_static-members.

Modula-2
--------

The extensions made to GDBN to support Modula-2 only support output from the GNU Modula-2 compiler (which is currently being developed).  Other Modula-2 compilers are not currently supported, and attempting to debug executables produced by them is most likely to give an error as GDBN reads in the executable's symbol table.

Operators
.........

Operators must be defined on values of specific types.  For instance, + is defined on numbers, but not on structures.  Operators are often defined on groups of types.  For the purposes of Modula-2, the following definitions hold:

  * Integral types consist of INTEGER, CARDINAL, and their subranges.

  * Character types consist of CHAR and its subranges.

  * Floating-point types consist of REAL.

  * Pointer types consist of anything declared as POINTER TO TYPE.

  * Scalar types consist of all of the above.

  * Set types consist of SET and BITSET types.

  * Boolean types consist of BOOLEAN.

The following operators are supported, and appear in order of increasing precedence:

,
     Function argument or array index separator.

:=
     Assignment.  The value of VAR := VALUE is VALUE.

<, >
     Less than, greater than on integral, floating-point, or enumerated types.

<=, >=
     Less than or equal to, greater than or equal to on integral, floating-point and enumerated types, or set inclusion on set types.  Same precedence as <.

=, <>, #
     Equality and two ways of expressing inequality, valid on scalar types. Same precedence as <.  In GDBN scripts, only <> is available for inequality, since # conflicts with the script comment character.

IN
     Set membership.  Defined on set types and the types of their members. Same precedence as <.

OR
     Boolean disjunction.  Defined on boolean types.

AND, &
     Boolean conjunction.  Defined on boolean types.

@
     The GDBN ``artificial array'' operator (see Expressions).

+, -
     Addition and subtraction on integral and floating-point types, or union and difference on set types.

*
     Multiplication on integral and floating-point types, or set intersection on set types.

/
     Division on floating-point types, or symmetric set difference on set types.  Same precedence as *.

DIV, MOD
     Integer division and remainder.  Defined on integral types.  Same precedence as *.

-
     Negative.  Defined on INTEGER and REAL data.

^
     Pointer dereferencing.  Defined on pointer types.

NOT
     Boolean negation.  Defined on boolean types.  Same precedence as ^.

.
     RECORD field selector.  Defined on RECORD data.  Same precedence as ^.

[]
     Array indexing.  Defined on ARRAY data.  Same precedence as ^.

()
     Procedure argument list.  Defined on PROCEDURE objects.  Same precedence as ^.

::, .
     GDBN and Modula-2 scope operators.

     Warning: Set expressions and their operations are not yet supported, so GDBN treats the use of the operator IN, or the use of operators +, -, *, /, =, , <>, #, <=, and >= on sets as an error.

Built-in Functions and Procedures
.................................

Modula-2 also makes available several built-in procedures and functions. In describing these, the following metavariables are used:

a
     represents an ARRAY variable.

c
     represents a CHAR constant or variable.

i
     represents a variable or constant of integral type.

m
     represents an identifier that belongs to a set.  Generally used in the same function with the metavariable S.  The type of S should be SET OF MTYPE (where MTYPE is the type of M).

n
     represents a variable or constant of integral or floating-point type.

r
     represents a variable or constant of floating-point type.

t
     represents a type.

v
     represents a variable.

x
     represents a variable or constant of one of many types.  See the explanation of the function for details.

All Modula-2 built-in procedures also return a result, described below.

ABS(N)
     Returns the absolute value of N.

CAP(C)
     If C is a lower case letter, it returns its upper case equivalent, otherwise it returns its argument.

CHR(I)
     Returns the character whose ordinal value is I.

DEC(V)
     Decrements the value in the variable V by one.  Returns the new value.

DEC(V,I)
     Decrements the value in the variable V by I.  Returns the new value.

EXCL(M,S)
     Removes the element M from the set S.  Returns the new set.

FLOAT(I)
     Returns the floating point equivalent of the integer I.

HIGH(A)
     Returns the index of the last member of A.

INC(V)
     Increments the value in the variable V by one.  Returns the new value.

INC(V,I)
     Increments the value in the variable V by I.  Returns the new value.

INCL(M,S)
     Adds the element M to the set S if it is not already there.  Returns the new set.

MAX(T)
     Returns the maximum value of the type T.

MIN(T)
     Returns the minimum value of the type T.

ODD(I)
     Returns boolean TRUE if I is an odd number.

ORD(X)
     Returns the ordinal value of its argument.  For example, the ordinal value of a character is its ASCII value (on machines supporting the ASCII character set).  X must be of an ordered type, which include integral, character and enumerated types.

SIZE(X)
     Returns the size of its argument.  X can be a variable or a type.

TRUNC(R)
     Returns the integral part of R.

TSIZE(X)
     Returns the size of its argument.  X can be a variable or a type.

VAL(T,I)
     Returns the member of the type T whose ordinal value is I.

     Warning:  Sets and their operations are not yet supported, so GDBN treats the use of procedures INCL and EXCL as an error.

Constants
.........

GDBN allows you to express the constants of Modula-2 in the following ways:

  * Integer constants are simply a sequence of digits.  When used in an expression, a constant is interpreted to be type-compatible with the rest of the expression.  Hexadecimal integers are specified by a trailing H, and octal integers by a trailing B.

  * Floating point constants appear as a sequence of digits, followed by a decimal point and another sequence of digits.  An optional exponent can then be specified, in the form E[+|-]NNN, where [+|-]NNN is the desired exponent.  All of the digits of the floating point constant must be valid decimal (base 10) digits.

  * Character constants consist of a single character enclosed by a pair of like quotes, either single (') or double (").  They may also be expressed by their ordinal value (their ASCII value, usually) followed by a C.

  * String constants consist of a sequence of characters enclosed by a pair of like quotes, either single (') or double ("). Escape sequences in the style of C are also allowed.  See C and C++ Constants, for a brief explanation of escape sequences.

  * Enumerated constants consist of an enumerated identifier.

  * Boolean constants consist of the identifiers TRUE and FALSE.

  * Pointer constants consist of integral values only.

  * Set constants are not yet supported.

Modula-2 Types
..............

Currently GDBN can print the following data types in Modula-2 syntax: array types, record types, set types, pointer types, procedure types, enumerated types, subrange types and base types.  You can also print the contents of variables declared using these type. This section gives a number of simple source code examples together with sample GDBN sessions.

The first example contains the following section of code:

     VAR
        s: SET OF CHAR ;
        r: [20..40] ;

and you can request GDBN to interrogate the type and value of r and s.

     (GDBP) print s
     {'A'..'C', 'Z'}
     (GDBP) ptype s
     SET OF CHAR
     (GDBP) print r
     21
     (GDBP) ptype r
     [20..40]

Likewise if your source code declares s as:

     VAR
        s: SET ['A'..'Z'] ;

then you may query the type of s by:

     (GDBP) ptype s
     type = SET ['A'..'Z']

Note that at present you cannot interactively manipulate set expressions using the debugger.

The following example shows how you might declare an array in Modula-2 and how you can interact with GDBN to print its type and contents:

     VAR
        s: ARRAY [-10..10] OF CHAR ;

     (GDBP) ptype s
     ARRAY [-10..10] OF CHAR

Note that the array handling is not yet complete and although the type is printed correctly, expression handling still assumes that all arrays have a lower bound of zero and not -10 as in the example above.

Here are some more type related Modula-2 examples:

     TYPE
        colour = (blue, red, yellow, green) ;
        t = [blue..yellow] ;
     VAR
        s: t ;
     BEGIN
        s := blue ;

The GDBN interaction shows how you can query the data type and value of a variable.

     (GDBP) print s
     $1 = blue
     (GDBP) ptype t
     type = [blue..yellow]

In this example a Modula-2 array is declared and its contents displayed.  Observe that the contents are written in the same way as their C counterparts.

     VAR
        s: ARRAY [1..5] OF CARDINAL ;
     BEGIN
        s[1] := 1 ;

     (GDBP) print s
     $1 = {1, 0, 0, 0, 0}
     (GDBP) ptype s
     type = ARRAY [1..5] OF CARDINAL

The Modula-2 language interface to GDBN also understands pointer types as shown in this example:

     VAR
        s: POINTER TO ARRAY [1..5] OF CARDINAL ;
     BEGIN
        NEW(s) ;
        s^[1] := 1 ;

and you can request that GDBN describes the type of s.

     (GDBP) ptype s
     type = POINTER TO ARRAY [1..5] OF CARDINAL

GDBN handles compound types as we can see in this example. Here we combine array types, record types, pointer types and subrange types:

     TYPE
        foo = RECORD
                 f1: CARDINAL ;
                 f2: CHAR ;
                 f3: myarray ;
              END ;

        myarray = ARRAY myrange OF CARDINAL ;
        myrange = [-2..2] ;
     VAR
        s: POINTER TO ARRAY myrange OF foo ;

and you can ask GDBN to describe the type of s as shown below.

     (GDBP) ptype s
     type = POINTER TO ARRAY [-2..2] OF foo = RECORD
         f1 : CARDINAL;
         f2 : CHAR;
         f3 : ARRAY [-2..2] OF CARDINAL;
     END

Modula-2 Defaults
.................

If type and range checking are set automatically by GDBN, they both default to on whenever the working language changes to Modula-2.  This happens regardless of whether you or GDBN selected the working language.

If you allow GDBN to set the language automatically, then entering code compiled from a file whose name ends with .mod sets the working language to Modula-2.  See Having GDBN Infer the Source Language, for further details.

Deviations from Standard Modula-2
.................................

A few changes have been made to make Modula-2 programs easier to debug. This is done primarily via loosening its type strictness:

  * Unlike in standard Modula-2, pointer constants can be formed by integers.  This allows you to modify pointer variables during debugging.  (In standard Modula-2, the actual address contained in a pointer variable is hidden from you; it can only be modified through direct assignment to another pointer variable or expression that returned a pointer.)

  * C escape sequences can be used in strings and characters to represent non-printable characters.  GDBN prints out strings with these escape sequences embedded.  Single non-printable characters are printed using the CHR(NNN) format.

  * The assignment operator (:=) returns the value of its right-hand argument.

  * All built-in procedures both modify and return their argument.

Modula-2 Type and Range Checks
..............................

     Warning: in this release, GDBN does not yet perform type or range checking.

GDBN considers two Modula-2 variables type equivalent if:

  * They are of types that have been declared equivalent via a TYPE T1 = T2 statement

  * They have been declared on the same line.  (Note:  This is true of the GNU Modula-2 compiler, but it may not be true of other compilers.)

As long as type checking is enabled, any attempt to combine variables whose types are not equivalent is an error.

Range checking is done on all mathematical operations, assignment, array index bounds, and all built-in functions and procedures.

The Scope Operators :: and .
............................

There are a few subtle differences between the Modula-2 scope operator (.) and the GDBN scope operator (::).  The two have similar syntax:

     MODULE . ID
     SCOPE :: ID

where SCOPE is the name of a module or a procedure, MODULE the name of a module, and ID is any declared identifier within your program, except another module.

Using the :: operator makes GDBN search the scope specified by SCOPE for the identifier ID.  If it is not found in the specified scope, then GDBN searches all scopes enclosing the one specified by SCOPE.

Using the . operator makes GDBN search the current scope for the identifier specified by ID that was imported from the definition module specified by MODULE.  With this operator, it is an error if the identifier ID was not imported from definition module MODULE, or if ID is not an identifier in MODULE.

GDBN and Modula-2
.................

Some GDBN commands have little use when debugging Modula-2 programs. Five subcommands of set print and show print apply specifically to C and C++: vtbl, demangle, asm-demangle, object, and union.  The first four apply to C++, and the last to the C union type, which has no direct analogue in Modula-2.

The @ operator (see Expressions), while available with any language, is not useful with Modula-2.  Its intent is to aid the debugging of dynamic arrays, which cannot be created in Modula-2 as they can in C or C++.  However, because an address can be specified by an integral constant, the construct {TYPE}ADREXP is still useful.

In GDBN scripts, the Modula-2 inequality operator # is interpreted as the beginning of a comment.  Use <> instead.

Ada
---

The extensions made to GDBN for Ada only support output from the GNU Ada (GNAT) compiler. Other Ada compilers are not currently supported, and attempting to debug executables produced by them is most likely to be difficult.

Introduction
............

The Ada mode of GDBN supports a fairly large subset of Ada expression syntax, with some extensions. The philosophy behind the design of this subset is

  * That GDBN should provide basic literals and access to operations for arithmetic, dereferencing, field selection, indexing, and subprogram calls, leaving more sophisticated computations to subprograms written into the program (which therefore may be called from GDBN).

  * That type safety and strict adherence to Ada language restrictions are not particularly important to the GDBN user.

  * That brevity is important to the GDBN user.

Thus, for brevity, the debugger acts as if all names declared in user-written packages are directly visible, even if they are not visible according to Ada rules, thus making it unnecessary to fully qualify most names with their packages, regardless of context.  Where this causes ambiguity, GDBN asks the user's intent.

The debugger will start in Ada mode if it detects an Ada main program. As for other languages, it will enter Ada mode when stopped in a program that was translated from an Ada source file.

While in Ada mode, you may use `--' for comments.  This is useful mostly for documenting command files.  The standard GDBN comment (#) still works at the beginning of a line in Ada mode, but not in the middle (to allow based literals).

The debugger supports limited overloading.  Given a subprogram call in which the function symbol has multiple definitions, it will use the number of actual parameters and some information about their types to attempt to narrow the set of definitions.  It also makes very limited use of context, preferring procedures to functions in the context of the call command, and functions to procedures elsewhere.

Omissions from Ada
..................

Here are the notable omissions from the subset:

  * Only a subset of the attributes are supported:

       - 'First, 'Last, and 'Length on array objects (not on types and subtypes).

       - 'Min and 'Max.

       - 'Pos and 'Val.

       - 'Tag.

       - 'Range on array objects (not subtypes), but only as the right operand of the membership (in) operator.

       - 'Access, 'Unchecked_Access, and 'Unrestricted_Access (a GNAT extension).

       - 'Address.

  * The names in Characters.Latin_1 are not available and concatenation is not implemented.  Thus, escape characters in strings are not currently available.

  * Equality tests (= and /=) on arrays test for bitwise equality of representations.  They will generally work correctly for strings and arrays whose elements have integer or enumeration types. They may not work correctly for arrays whose element types have user-defined equality, for arrays of real values (in particular, IEEE-conformant floating point, because of negative zeroes and NaNs), and for arrays whose elements contain unused bits with indeterminate values.

  * The other component-by-component array operations (and, or, xor, not, and relational tests other than equality) are not implemented.

  * There is limited support for array and record aggregates.  They are permitted only on the right sides of assignments, as in these examples:

          (GDBP) set An_Array := (1, 2, 3, 4, 5, 6)
          (GDBP) set An_Array := (1, others => 0)
          (GDBP) set An_Array := (0|4 => 1, 1..3 => 2, 5 => 6)
          (GDBP) set A_2D_Array := ((1, 2, 3), (4, 5, 6), (7, 8, 9))
          (GDBP) set A_Record := (1, "Peter", True);
          (GDBP) set A_Record := (Name => "Peter", Id => 1, Alive => True)

     Changing a discriminant's value by assigning an aggregate has an undefined effect if that discriminant is used within the record. However, you can first modify discriminants by directly assigning to them (which normally would not be allowed in Ada), and then performing an aggregate assignment.  For example, given a variable A_Rec declared to have a type such as:

          type Rec (Len : Small_Integer := 0) is record
              Id : Integer;
              Vals : IntArray (1 .. Len);
          end record;

     you can assign a value with a different size of Vals with two assignments:

          (GDBP) set A_Rec.Len := 4
          (GDBP) set A_Rec := (Id => 42, Vals => (1, 2, 3, 4))

     As this example also illustrates, GDBN is very loose about the usual rules concerning aggregates.  You may leave out some of the components of an array or record aggregate (such as the Len component in the assignment to A_Rec above); they will retain their original values upon assignment.  You may freely use dynamic values as indices in component associations.  You may even use overlapping or redundant component associations, although which component values are assigned in such cases is not defined.

  * Calls to dispatching subprograms are not implemented.

  * The overloading algorithm is much more limited (i.e., less selective) than that of real Ada.  It makes only limited use of the context in which a subexpression appears to resolve its meaning, and it is much looser in its rules for allowing type matches.  As a result, some function calls will be ambiguous, and the user will be asked to choose the proper resolution.

  * The new operator is not implemented.

  * Entry calls are not implemented.

  * Aside from printing, arithmetic operations on the native VAX floating-point formats are not supported.

  * It is not possible to slice a packed array.

  * The names True and False, when not part of a qualified name, are interpreted as if implicitly prefixed by Standard, regardless of context. Should your program redefine these names in a package or procedure (at best a dubious practice), you will have to use fully qualified names to access their new definitions.

Additions to Ada
................

As it does for other languages, GDBN makes certain generic extensions to Ada (see Expressions):

  * If the expression E is a variable residing in memory (typically a local variable or array element) and N is a positive integer, then E@N displays the values of E and the N-1 adjacent variables following it in memory as an array.  In Ada, this operator is generally not necessary, since its prime use is in displaying parts of an array, and slicing will usually do this in Ada.  However, there are occasional uses when debugging programs in which certain debugging information has been optimized away.

  * B::VAR means ``the variable named VAR that appears in function or file B.''  When B is a file name, you must typically surround it in single quotes.

  * The expression {TYPE} ADDR means ``the variable of type TYPE that appears at address ADDR.''

  * A name starting with $ is a convenience variable (see Convenience Vars) or a machine register (see Registers).

In addition, GDBN provides a few other shortcuts and outright additions specific to Ada:

  * The assignment statement is allowed as an expression, returning its right-hand operand as its value.  Thus, you may enter

          (GDBP) set x := y + 3
          (GDBP) print A(tmp := y + 1)

  * The semicolon is allowed as an ``operator,''  returning as its value the value of its right-hand operand. This allows, for example, complex conditional breaks:

          (GDBP) break f
          (GDBP) condition 1 (report(i); k += 1; A(k) > 100)

  * Rather than use catenation and symbolic character names to introduce special characters into strings, one may instead use a special bracket notation, which is also used to print strings.  A sequence of characters of the form ["XX"] within a string or character literal denotes the (single) character whose numeric encoding is XX in hexadecimal.  The sequence of characters ["""] also denotes a single quotation mark in strings.   For example,

             "One line.["0a"]Next line.["0a"]"

     contains an ASCII newline character (Ada.Characters.Latin_1.LF) after each period.

  * The subtype used as a prefix for the attributes 'Pos, 'Min, and 'Max is optional (and is ignored in any case).  For example, it is valid to write

          (GDBP) print 'max(x, y)

  * When printing arrays, GDBN uses positional notation when the array has a lower bound of 1, and uses a modified named notation otherwise. For example, a one-dimensional array of three integers with a lower bound of 3 might print as

          (3 => 10, 17, 1)

     That is, in contrast to valid Ada, only the first component has a => clause.

  * You may abbreviate attributes in expressions with any unique, multi-character subsequence of their names (an exact match gets preference). For example, you may use a'len, a'gth, or a'lh in place of  a'length.

  * Since Ada is case-insensitive, the debugger normally maps identifiers you type to lower case.  The GNAT compiler uses upper-case characters for some of its internal identifiers, which are normally of no interest to users. For the rare occasions when you actually have to look at them, enclose them in angle brackets to avoid the lower-case mapping. For example,

          (GDBP) print <JMPBUF_SAVE>[0]

  * Printing an object of class-wide type or dereferencing an access-to-class-wide value will display all the components of the object's specific type (as indicated by its run-time tag).  Likewise, component selection on such a value will operate on the specific type of the object.

Stopping at the Very Beginning
..............................

It is sometimes necessary to debug the program during elaboration, and before reaching the main procedure. As defined in the Ada Reference Manual, the elaboration code is invoked from a procedure called adainit.  To run your program up to the beginning of elaboration, simply use the following two commands: tbreak adainit and run.

Extensions for Ada Tasks
........................

Support for Ada tasks is analogous to that for threads (see Threads). GDBN provides the following task-related commands:

info tasks
     This command shows a list of current Ada tasks, as in the following example:

          (GDBP) info tasks
            ID       TID P-ID Pri State                 Name
             1   8088000   0   15 Child Activation Wait main_task
             2   80a4000   1   15 Accept Statement      b
             3   809a800   1   15 Child Activation Wait a
          *  4   80ae800   3   15 Runnable              c

     In this listing, the asterisk before the last task indicates it to be the task currently being inspected.

     ID
          Represents GDBN's internal task number.

     TID
          The Ada task ID.

     P-ID
          The parent's task ID (GDBN's internal task number).

     Pri
          The base priority of the task.

     State
          Current state of the task.

          Unactivated
               The task has been created but has not been activated.  It cannot be executing.

          Runnable
               The task is not blocked for any reason known to Ada.  (It may be waiting for a mutex, though.) It is conceptually "executing" in normal mode.

          Terminated
               The task is terminated, in the sense of ARM 9.3 (5).  Any dependents that were waiting on terminate alternatives have been awakened and have terminated themselves.

          Child Activation Wait
               The task is waiting for created tasks to complete activation.

          Accept Statement
               The task is waiting on an accept or selective wait statement.

          Waiting on entry call
               The task is waiting on an entry call.

          Async Select Wait
               The task is waiting to start the abortable part of an asynchronous select statement.

          Delay Sleep
               The task is waiting on a select statement with only a delay alternative open.

          Child Termination Wait
               The task is sleeping having completed a master within itself, and is waiting for the tasks dependent on that master to become terminated or waiting on a terminate Phase.

          Wait Child in Term Alt
               The task is sleeping waiting for tasks on terminate alternatives to finish terminating.

          Accepting RV with TASKNO
               The task is accepting a rendez-vous with the task TASKNO.

     Name
          Name of the task in the program.

info task TASKNO
     This command shows detailled informations on the specified task, as in the following example:

          (GDBP) info tasks
            ID       TID P-ID Pri State                  Name
             1   8077880    0  15 Child Activation Wait  main_task
          *  2   807c468    1  15 Runnable               task_1
          (GDBP) info task 2
          Ada Task: 0x807c468
          Name: task_1
          Thread: 0x807f378
          Parent: 1 (main_task)
          Base Priority: 15
          State: Runnable

task
     This command prints the ID of the current task.

          (GDBP) info tasks
            ID       TID P-ID Pri State                  Name
             1   8077870    0  15 Child Activation Wait  main_task
          *  2   807c458    1  15 Runnable               t
          (GDBP) task
          [Current task is 2]

task TASKNO
     This command is like the thread THREADNO command (see Threads).  It switches the context of debugging from the current task to the given task.

          (GDBP) info tasks
            ID       TID P-ID Pri State                  Name
             1   8077870    0  15 Child Activation Wait  main_task
          *  2   807c458    1  15 Runnable               t
          (GDBP) task 1
          [Switching to task 1]
          #0  0x8067726 in pthread_cond_wait ()
          (GDBP) bt
          #0  0x8067726 in pthread_cond_wait ()
          #1  0x8056714 in system.os_interface.pthread_cond_wait ()
          #2  0x805cb63 in system.task_primitives.operations.sleep ()
          #3  0x806153e in system.tasking.stages.activate_tasks ()
          #4  0x804aacc in un () at un.adb:5

break LINESPEC task TASKNO
break LINESPEC task TASKNO if ...
     These commands are like the break ... thread ... command (see Thread Stops). LINESPEC specifies source lines, as described in Specify Location.

     Use the qualifier task TASKNO with a breakpoint command to specify that you only want GDBN to stop the program when a particular Ada task reaches this breakpoint.  TASKNO is one of the numeric task identifiers assigned by GDBN, shown in the first column of the info tasks display.

     If you do not specify task TASKNO when you set a breakpoint, the breakpoint applies to all tasks of your program.

     You can use the task qualifier on conditional breakpoints as well; in this case, place task TASKNO before the breakpoint condition (before the if).

     For example,

          (GDBP) info tasks
            ID       TID P-ID Pri State                 Name
             1 140022020   0   15 Child Activation Wait main_task
             2 140045060   1   15 Accept/Select Wait    t2
             3 140044840   1   15 Runnable              t1
          *  4 140056040   1   15 Runnable              t3
          (GDBP) b 15 task 2
          Breakpoint 5 at 0x120044cb0: file test_task_debug.adb, line 15.
          (GDBP) cont
          Continuing.
          task # 1 running
          task # 2 running

          Breakpoint 5, test_task_debug () at test_task_debug.adb:15
          15               flush;
          (GDBP) info tasks
            ID       TID P-ID Pri State                 Name
             1 140022020   0   15 Child Activation Wait main_task
          *  2 140045060   1   15 Runnable              t2
             3 140044840   1   15 Runnable              t1
             4 140056040   1   15 Delay Sleep           t3

Tasking Support when Debugging Core Files
.........................................

When inspecting a core file, as opposed to debugging a live program, tasking support may be limited or even unavailable, depending on the platform being used. For instance, on x86-linux, the list of tasks is available, but task switching is not supported.  On Tru64, however, task switching will work as usual.

On certain platforms, including Tru64, the debugger needs to perform some memory writes in order to provide Ada tasking support.  When inspecting a core file, this means that the core file must be opened with read-write privileges, using the command "set write on" (see Patching). Under these circumstances, you should make a backup copy of the core file before inspecting it with GDBN.

Tasking Support when using the Ravenscar Profile
................................................

The Ravenscar Profile is a subset of the Ada tasking features, specifically designed for systems with safety-critical real-time requirements.

set ravenscar task-switching on
     Allows task switching when debugging a program that uses the Ravenscar Profile.  This is the default.

set ravenscar task-switching off
     Turn off task switching when debugging a program that uses the Ravenscar Profile.  This is mostly intended to disable the code that adds support for the Ravenscar Profile, in case a bug in either GDBN or in the Ravenscar runtime is preventing GDBN from working properly. To be effective, this command should be run before the program is started.

show ravenscar task-switching
     Show whether it is possible to switch from task to task in a program using the Ravenscar Profile.

Known Peculiarities of Ada Mode
...............................

Besides the omissions listed previously (see Omissions from Ada), we know of several problems with and limitations of Ada mode in GDBN, some of which will be fixed with planned future releases of the debugger and the GNU Ada compiler.

  * Static constants that the compiler chooses not to materialize as objects in storage are invisible to the debugger.

  * Named parameter associations in function argument lists are ignored (the argument lists are treated as positional).

  * Many useful library packages are currently invisible to the debugger.

  * Fixed-point arithmetic, conversions, input, and output is carried out using floating-point arithmetic, and may give results that only approximate those on the host machine.

  * The GNAT compiler never generates the prefix Standard for any of the standard symbols defined by the Ada language.  GDBN knows about this: it will strip the prefix from names when you use it, and will never look for a name you have so qualified among local symbols, nor match against symbols in other packages or subprograms.  If you have defined entities anywhere in your program other than parameters and local variables whose simple names match names in Standard, GNAT's lack of qualification here can cause confusion.  When this happens, you can usually resolve the confusion by qualifying the problematic names with package Standard explicitly.

Older versions of the compiler sometimes generate erroneous debugging information, resulting in the debugger incorrectly printing the value of affected entities.  In some cases, the debugger is able to work around an issue automatically. In other cases, the debugger is able to work around the issue, but the work-around has to be specifically enabled.

set ada trust-PAD-over-XVS on
     Configure GDB to strictly follow the GNAT encoding when computing the value of Ada entities, particularly when PAD and PAD___XVS types are involved (see ada/exp_dbug.ads in the GCC sources for a complete description of the encoding used by the GNAT compiler). This is the default.

set ada trust-PAD-over-XVS off
     This is related to the encoding using by the GNAT compiler.  If GDBN sometimes prints the wrong value for certain entities, changing ada trust-PAD-over-XVS to off activates a work-around which may fix the issue.  It is always safe to set ada trust-PAD-over-XVS to off, but this incurs a slight performance penalty, so it is recommended to leave this setting to on unless necessary.

Unsupported Languages
=====================

In addition to the other fully-supported programming languages, GDBN also provides a pseudo-language, called minimal. It does not represent a real programming language, but provides a set of capabilities close to what the C or assembly languages provide. This should allow most simple operations to be performed while debugging an application that uses a language currently not supported by GDBN.

If the language is set to auto, GDBN will automatically select this language if the current frame corresponds to an unsupported language.

Examining the Symbol Table
**************************

The commands described in this chapter allow you to inquire about the symbols (names of variables, functions and types) defined in your program.  This information is inherent in the text of your program and does not change as your program executes.  GDBN finds it in your program's symbol table, in the file indicated when you started GDBN (see Choosing Files), or by one of the file-management commands (see Commands to Specify Files).

Occasionally, you may need to refer to symbols that contain unusual characters, which GDBN ordinarily treats as word delimiters.  The most frequent case is in referring to static variables in other source files (see Program Variables).  File names are recorded in object files as debugging symbols, but GDBN would ordinarily parse a typical file name, like foo.c, as the three words foo . c.  To allow GDBN to recognize foo.c as a single symbol, enclose it in single quotes; for example,

     p 'foo.c'::x

looks up the value of x in the scope of the file foo.c.

set case-sensitive on
set case-sensitive off
set case-sensitive auto
     Normally, when GDBN looks up symbols, it matches their names with case sensitivity determined by the current source language. Occasionally, you may wish to control that.  The command set case-sensitive lets you do that by specifying on for case-sensitive matches or off for case-insensitive ones.  If you specify auto, case sensitivity is reset to the default suitable for the source language.  The default is case-sensitive matches for all languages except for Fortran, for which the default is case-insensitive matches.

show case-sensitive
     This command shows the current setting of case sensitivity for symbols lookups.

info address SYMBOL
     Describe where the data for SYMBOL is stored.  For a register variable, this says which register it is kept in.  For a non-register local variable, this prints the stack-frame offset at which the variable is always stored.

     Note the contrast with print &SYMBOL, which does not work at all for a register variable, and for a stack local variable prints the exact address of the current instantiation of the variable.

info symbol ADDR
     Print the name of a symbol which is stored at the address ADDR. If no symbol is stored exactly at ADDR, GDBN prints the nearest symbol and an offset from it:

          (GDBP) info symbol 0x54320
          _initialize_vx + 396 in section .text

     This is the opposite of the info address command.  You can use it to find out the name of a variable or a function given its address.

     For dynamically linked executables, the name of executable or shared library containing the symbol is also printed:

          (GDBP) info symbol 0x400225
          _start + 5 in section .text of /tmp/a.out
          (GDBP) info symbol 0x2aaaac2811cf
          __read_nocancel + 6 in section .text of /usr/lib64/libc.so.6

whatis [ARG]
     Print the data type of ARG, which can be either an expression or a name of a data type.  With no argument, print the data type of $, the last value in the value history.

     If ARG is an expression (see Expressions), it is not actually evaluated, and any side-effecting operations (such as assignments or function calls) inside it do not take place.

     If ARG is a variable or an expression, whatis prints its literal type as it is used in the source code.  If the type was defined using a typedef, whatis will not print the data type underlying the typedef.  If the type of the variable or the expression is a compound data type, such as struct or  class, whatis never prints their fields or methods.  It just prints the struct/class name (a.k.a. its tag).  If you want to see the members of such a compound data type, use ptype.

     If ARG is a type name that was defined using typedef, whatis unrolls only one level of that typedef. Unrolling means that whatis will show the underlying type used in the typedef declaration of ARG.  However, if that underlying type is also a typedef, whatis will not unroll it.

     For C code, the type names may also have the form class CLASS-NAME, struct STRUCT-TAG, union UNION-TAG or enum ENUM-TAG.

ptype [ARG]
     ptype accepts the same arguments as whatis, but prints a detailed description of the type, instead of just the name of the type. See Expressions.

     Contrary to whatis, ptype always unrolls any typedefs in its argument declaration, whether the argument is a variable, expression, or a data type.  This means that ptype of a variable or an expression will not print literally its type as present in the source code---use whatis for that.  typedefs at the pointer or reference targets are also unrolled.  Only typedefs of fields, methods and inner class typedefs of structs, classes and unions are not unrolled even with ptype.

     For example, for this variable declaration:

          typedef double real_t;
          struct complex { real_t real; double imag; };
          typedef struct complex complex_t;
          complex_t var;
          real_t *real_pointer_var;

     the two commands give this output:

          (GDBP) whatis var
          type = complex_t
          (GDBP) ptype var
          type = struct complex {
              real_t real;
              double imag;
          }
          (GDBP) whatis complex_t
          type = struct complex
          (GDBP) whatis struct complex
          type = struct complex
          (GDBP) ptype struct complex
          type = struct complex {
              real_t real;
              double imag;
          }
          (GDBP) whatis real_pointer_var
          type = real_t *
          (GDBP) ptype real_pointer_var
          type = double *

     As with whatis, using ptype without an argument refers to the type of $, the last value in the value history.

     Sometimes, programs use opaque data types or incomplete specifications of complex data structure.  If the debug information included in the program does not allow GDBN to display a full declaration of the data type, it will say <incomplete type>.  For example, given these declarations:

              struct foo;
              struct foo *fooptr;

     but no definition for struct foo itself, GDBN will say:

            (GDBP) ptype foo
            $1 = <incomplete type>

     ``Incomplete type'' is C terminology for data types that are not completely specified.

info types REGEXP
info types
     Print a brief description of all types whose names match the regular expression REGEXP (or all types in your program, if you supply no argument).  Each complete typename is matched as though it were a complete line; thus, i type value gives information on all types in your program whose names include the string value, but i type ^value$ gives information only on types whose complete name is value.

     This command differs from ptype in two ways: first, like whatis, it does not print a detailed description; second, it lists all source files where a type is defined.

info scope LOCATION
     List all the variables local to a particular scope.  This command accepts a LOCATION argument---a function name, a source line, or an address preceded by a *, and prints all the variables local to the scope defined by that location.  (see Specify Location, for details about supported forms of LOCATION.)  For example:

          (GDBP) info scope command_line_handler
          Scope for command_line_handler:
          Symbol rl is an argument at stack/frame offset 8, length 4.
          Symbol linebuffer is in static storage at address 0x150a18, length 4.
          Symbol linelength is in static storage at address 0x150a1c, length 4.
          Symbol p is a local variable in register $esi, length 4.
          Symbol p1 is a local variable in register $ebx, length 4.
          Symbol nline is a local variable in register $edx, length 4.
          Symbol repeat is a local variable at frame offset -8, length 4.

     This command is especially useful for determining what data to collect during a trace experiment, see collect.

info source
     Show information about the current source file---that is, the source file for the function containing the current point of execution:

       * the name of the source file, and the directory containing it,

       * the directory it was compiled in,

       * its length, in lines,

       * which programming language it is written in,

       * whether the executable includes debugging information for that file, and if so, what format the information is in (e.g., STABS, Dwarf 2, etc.), and

       * whether the debugging information includes information about preprocessor macros.

info sources
     Print the names of all source files in your program for which there is debugging information, organized into two lists: files whose symbols have already been read, and files whose symbols will be read when needed.

info functions
     Print the names and data types of all defined functions.

info functions REGEXP
     Print the names and data types of all defined functions whose names contain a match for regular expression REGEXP. Thus, info fun step finds all functions whose names include step; info fun ^step finds those whose names start with step.  If a function name contains characters that conflict with the regular expression language (e.g. operator*()), they may be quoted with a backslash.

info variables
     Print the names and data types of all variables that are defined outside of functions (i.e. excluding local variables).

info variables REGEXP
     Print the names and data types of all variables (except for local variables) whose names contain a match for regular expression REGEXP.

info classes
info classes REGEXP
     Display all Objective-C classes in your program, or (with the REGEXP argument) all those matching a particular regular expression.

info selectors
info selectors REGEXP
     Display all Objective-C selectors in your program, or (with the REGEXP argument) all those matching a particular regular expression.

     Some systems allow individual object files that make up your program to be replaced without stopping and restarting your program.  For example, in VxWorks you can simply recompile a defective object file and keep on running.  If you are running on one of these systems, you can allow GDBN to reload the symbols for automatically relinked modules:

     set symbol-reloading on
          Replace symbol definitions for the corresponding source file when an object file with a particular name is seen again.

     set symbol-reloading off
          Do not replace symbol definitions when encountering object files of the same name more than once.  This is the default state; if you are not running on a system that permits automatic relinking of modules, you should leave symbol-reloading off, since otherwise GDBN may discard symbols when linking large programs, that may contain several modules (from different directories or libraries) with the same name.

     show symbol-reloading
          Show the current on or off setting.

set opaque-type-resolution on
     Tell GDBN to resolve opaque types.  An opaque type is a type declared as a pointer to a struct, class, or union---for example, struct MyType *---that is used in one source file although the full declaration of struct MyType is in another source file.  The default is on.

     A change in the setting of this subcommand will not take effect until the next time symbols for a file are loaded.

set opaque-type-resolution off
     Tell GDBN not to resolve opaque types.  In this case, the type is printed as follows:

          {<no data fields>}

show opaque-type-resolution
     Show whether opaque types are resolved or not.

maint print symbols FILENAME
maint print psymbols FILENAME
maint print msymbols FILENAME
     Write a dump of debugging symbol data into the file FILENAME. These commands are used to debug the GDBN symbol-reading code.  Only symbols with debugging data are included.  If you use maint print
     symbols, GDBN includes all the symbols for which it has already collected full details: that is, FILENAME reflects symbols for only those files whose symbols GDBN has read.  You can use the command info sources to find out which files these are.  If you use maint print psymbols instead, the dump shows information about symbols that GDBN only knows partially---that is, symbols defined in files that GDBN has skimmed, but not yet read completely.  Finally, maint print msymbols dumps just the minimal symbol information required for each object file from which GDBN has read some symbols. See Commands to Specify Files, for a discussion of how GDBN reads symbols (in the description of symbol-file).

maint info symtabs [ REGEXP ]
maint info psymtabs [ REGEXP ]

     List the struct symtab or struct partial_symtab structures whose names match REGEXP.  If REGEXP is not given, list them all.  The output includes expressions which you can copy into a GDBN debugging this one to examine a particular structure in more detail.  For example:

          (GDBP) maint info psymtabs dwarf2read
          { objfile /home/gnu/build/gdb/gdb
            ((struct objfile *) 0x82e69d0)
            { psymtab /home/gnu/src/gdb/dwarf2read.c
              ((struct partial_symtab *) 0x8474b10)
              readin no
              fullname (null)
              text addresses 0x814d3c8 -- 0x8158074
              globals (* (struct partial_symbol **) 0x8507a08 @ 9)
              statics (* (struct partial_symbol **) 0x40e95b78 @ 2882)
              dependencies (none)
            }
          }
          (GDBP) maint info symtabs
          (GDBP)

     We see that there is one partial symbol table whose filename contains the string dwarf2read, belonging to the gdb executable; and we see that GDBN has not read in any symtabs yet at all. If we set a breakpoint on a function, that will cause GDBN to read the symtab for the compilation unit containing that function:

          (GDBP) break dwarf2_psymtab_to_symtab
          Breakpoint 1 at 0x814e5da: file /home/gnu/src/gdb/dwarf2read.c,
          line 1574.
          (GDBP) maint info symtabs
          { objfile /home/gnu/build/gdb/gdb
            ((struct objfile *) 0x82e69d0)
            { symtab /home/gnu/src/gdb/dwarf2read.c
              ((struct symtab *) 0x86c1f38)
              dirname (null)
              fullname (null)
              blockvector ((struct blockvector *) 0x86c1bd0) (primary)
              linetable ((struct linetable *) 0x8370fa0)
              debugformat DWARF 2
            }
          }
          (GDBP)

Altering Execution
******************

Once you think you have found an error in your program, you might want to find out for certain whether correcting the apparent error would lead to correct results in the rest of the run.  You can find the answer by experiment, using the GDBN features for altering execution of the program.

For example, you can store new values into variables or memory locations, give your program a signal, restart it at a different address, or even return prematurely from a function.

Assignment to Variables
=======================

To alter the value of a variable, evaluate an assignment expression. See Expressions.  For example,

     print x=4

stores the value 4 into the variable x, and then prints the value of the assignment expression (which is 4). See Using GDBN with Different Languages, for more information on operators in supported languages.

If you are not interested in seeing the value of the assignment, use the set command instead of the print command.  set is really the same as print except that the expression's value is not printed and is not put in the value history (see Value History).  The expression is evaluated only for its effects.

If the beginning of the argument string of the set command appears identical to a set subcommand, use the set variable command instead of just set.  This command is identical to set except for its lack of subcommands.  For example, if your program has a variable width, you get an error if you try to set a new value with just set width=13, because GDBN has the command set width:

     (GDBP) whatis width
     type = double
     (GDBP) p width
     $4 = 13
     (GDBP) set width=47
     Invalid syntax in expression.

The invalid expression, of course, is =47.  In order to actually set the program's variable width, use

     (GDBP) set var width=47

Because the set command has many subcommands that can conflict with the names of program variables, it is a good idea to use the set variable command instead of just set.  For example, if your program has a variable g, you run into problems if you try to set a new value with just set g=4, because GDBN has the command set gnutarget, abbreviated set g:

     (GDBP) whatis g
     type = double
     (GDBP) p g
     $1 = 1
     (GDBP) set g=4
     (GDBP) p g
     $2 = 1
     (GDBP) r
     The program being debugged has been started already.
     Start it from the beginning? (y or n) y
     Starting program: /home/smith/cc_progs/a.out
     "/home/smith/cc_progs/a.out": can't open to read symbols:
                                      Invalid bfd target.
     (GDBP) show g
     The current BFD target is "=4".

The program variable g did not change, and you silently set the gnutarget to an invalid value.  In order to set the variable g, use

     (GDBP) set var g=4

GDBN allows more implicit conversions in assignments than C; you can freely store an integer value into a pointer variable or vice versa, and you can convert any structure to any other structure that is the same length or shorter.

To store values into arbitrary places in memory, use the {...} construct to generate a value of specified type at a specified address (see Expressions).  For example, {int}0x83040 refers to memory location 0x83040 as an integer (which implies a certain size and representation in memory), and

     set {int}0x83040 = 4

stores the value 4 into that memory location.

Continuing at a Different Address
=================================

Ordinarily, when you continue your program, you do so at the place where it stopped, with the continue command.  You can instead continue at an address of your own choosing, with the following commands:

jump LINESPEC
jump LOCATION
     Resume execution at line LINESPEC or at address given by LOCATION.  Execution stops again immediately if there is a breakpoint there.  See Specify Location, for a description of the different forms of LINESPEC and LOCATION.  It is common practice to use the tbreak command in conjunction with jump.  See Setting Breakpoints.

     The jump command does not change the current stack frame, or the stack pointer, or the contents of any memory location or any register other than the program counter.  If line LINESPEC is in a different function from the one currently executing, the results may be bizarre if the two functions expect different patterns of arguments or of local variables.  For this reason, the jump command requests confirmation if the specified line is not in the function currently executing.  However, even bizarre results are predictable if you are well acquainted with the machine-language code of your program.

On many systems, you can get much the same effect as the jump command by storing a new value into the register $pc.  The difference is that this does not start your program running; it only changes the address of where it will run when you continue.  For example,

     set $pc = 0x485

makes the next continue command or stepping command execute at address 0x485, rather than at the address where your program stopped. See Continuing and Stepping.

The most common occasion to use the jump command is to back up---perhaps with more breakpoints set---over a portion of a program that has already executed, in order to examine its execution in more detail.

Giving your Program a Signal
============================

signal SIGNAL
     Resume execution where your program stopped, but immediately give it the signal SIGNAL.  SIGNAL can be the name or the number of a signal.  For example, on many systems signal 2 and signal SIGINT are both ways of sending an interrupt signal.

     Alternatively, if SIGNAL is zero, continue execution without giving a signal.  This is useful when your program stopped on account of a signal and would ordinary see the signal when resumed with the continue command; signal 0 causes it to resume without a signal.

     signal does not repeat when you press RET a second time after executing the command.

Invoking the signal command is not the same as invoking the kill utility from the shell.  Sending a signal with kill causes GDBN to decide what to do with the signal depending on the signal handling tables (see Signals).  The signal command passes the signal directly to your program.

Returning from a Function
=========================

return
return EXPRESSION
     You can cancel execution of a function call with the return command.  If you give an EXPRESSION argument, its value is used as the function's return value.

When you use return, GDBN discards the selected stack frame (and all frames within it).  You can think of this as making the discarded frame return prematurely.  If you wish to specify a value to be returned, give that value as the argument to return.

This pops the selected stack frame (see Selecting a Frame), and any other frames inside of it, leaving its caller as the innermost remaining frame.  That frame becomes selected.  The specified value is stored in the registers used for returning values of functions.

The return command does not resume execution; it leaves the program stopped in the state that would exist if the function had just returned.  In contrast, the finish command (see Continuing and Stepping) resumes execution until the selected stack frame returns naturally.

GDBN needs to know how the EXPRESSION argument should be set for the inferior.  The concrete registers assignment depends on the OS ABI and the type being returned by the selected stack frame.  For example it is common for OS ABI to return floating point values in FPU registers while integer values in CPU registers.  Still some ABIs return even floating point values in CPU registers.  Larger integer widths (such as long long int) also have specific placement rules.  GDBN already knows the OS ABI from its current target so it needs to find out also the type being returned to make the assignment into the right register(s).

Normally, the selected stack frame has debug info.  GDBN will always use the debug info instead of the implicit type of EXPRESSION when the debug info is available.  For example, if you type return -1, and the function in the current stack frame is declared to return a long long
int, GDBN transparently converts the implicit int value of -1 into a long long int:

     Breakpoint 1, func () at gdb.base/return-nodebug.c:29
     29        return 31;
     (GDBP) return -1
     Make func return now? (y or n) y
     #0  0x004004f6 in main () at gdb.base/return-nodebug.c:43
     43        printf ("result=%lld\n", func ());
     (GDBP)

However, if the selected stack frame does not have a debug info, e.g., if the function was compiled without debug info, GDBN has to find out the type to return from user.  Specifying a different type by mistake may set the value in different inferior registers than the caller code expects.  For example, typing return -1 with its implicit type int would set only a part of a long long int result for a debug info less function (on 32-bit architectures).  Therefore the user is required to specify the return type by an appropriate cast explicitly:

     Breakpoint 2, 0x0040050b in func ()
     (GDBP) return -1
     Return value type not available for selected stack frame.
     Please use an explicit cast of the value to return.
     (GDBP) return (long long int) -1
     Make selected stack frame return now? (y or n) y
     #0  0x00400526 in main ()
     (GDBP)

Calling Program Functions
=========================

print EXPR
     Evaluate the expression EXPR and display the resulting value. EXPR may include calls to functions in the program being debugged.

call EXPR
     Evaluate the expression EXPR without displaying void returned values.

     You can use this variant of the print command if you want to execute a function from your program that does not return anything (a.k.a. a void function), but without cluttering the output with void returned values that GDBN will otherwise print.  If the result is not void, it is printed and saved in the value history.

It is possible for the function you call via the print or call command to generate a signal (e.g., if there's a bug in the function, or if you passed it incorrect arguments).  What happens in that case is controlled by the set unwindonsignal command.

Similarly, with a C++ program it is possible for the function you call via the print or call command to generate an exception that is not handled due to the constraints of the dummy frame.  In this case, any exception that is raised in the frame, but has an out-of-frame exception handler will not be found.  GDB builds a dummy-frame for the inferior function call, and the unwinder cannot seek for exception handlers outside of this dummy-frame.  What happens in that case is controlled by the set unwind-on-terminating-exception command.

set unwindonsignal
     Set unwinding of the stack if a signal is received while in a function that GDBN called in the program being debugged.  If set to on, GDBN unwinds the stack it created for the call and restores the context to what it was before the call.  If set to off (the default), GDBN stops in the frame where the signal was received.

show unwindonsignal
     Show the current setting of stack unwinding in the functions called by GDBN.

set unwind-on-terminating-exception
     Set unwinding of the stack if a C++ exception is raised, but left unhandled while in a function that GDBN called in the program being debugged.  If set to on (the default), GDBN unwinds the stack it created for the call and restores the context to what it was before the call.  If set to off, GDBN the exception is delivered to the default C++ exception handler and the inferior terminated.

show unwind-on-terminating-exception
     Show the current setting of stack unwinding in the functions called by GDBN.

Sometimes, a function you wish to call is actually a weak alias for another function.  In such case, GDBN might not pick up the type information, including the types of the function arguments, which causes GDBN to call the inferior function incorrectly. As a result, the called function will function erroneously and may even crash.  A solution to that is to use the name of the aliased function instead.

Patching Programs
=================

By default, GDBN opens the file containing your program's executable code (or the corefile) read-only.  This prevents accidental alterations to machine code; but it also prevents you from intentionally patching your program's binary.

If you'd like to be able to patch the binary, you can specify that explicitly with the set write command.  For example, you might want to turn on internal debugging flags, or even to make emergency repairs.

set write on
set write off
     If you specify set write on, GDBN opens executable and core files for both reading and writing; if you specify set write
     off (the default), GDBN opens them read-only.

     If you have already loaded a file, you must load it again (using the exec-file or core-file command) after changing set write, for your new setting to take effect.

show write
     Display whether executable files and core files are opened for writing as well as reading.

GDBN Files
**********

GDBN needs to know the file name of the program to be debugged, both in order to read its symbol table and in order to start your program.  To debug a core dump of a previous run, you must also tell GDBN the name of the core dump file.

Commands to Specify Files
=========================

You may want to specify executable and core dump file names.  The usual way to do this is at start-up time, using the arguments to GDBN's start-up commands (see Getting In and Out of GDBN).

Occasionally it is necessary to change to a different file during a GDBN session.  Or you may run GDBN and forget to specify a file you want to use.  Or you are debugging a remote target via gdbserver (see Using the gdbserver Program).  In these situations the GDBN commands to specify new files are useful.

file FILENAME
     Use FILENAME as the program to be debugged.  It is read for its symbols and for the contents of pure memory.  It is also the program executed when you use the run command.  If you do not specify a directory and the file is not found in the GDBN working directory, GDBN uses the environment variable PATH as a list of directories to search, just as the shell does when looking for a program to run.  You can change the value of this variable, for both GDBN and your program, using the path command.

     You can load unlinked object .o files into GDBN using the file command.  You will not be able to ``run'' an object file, but you can disassemble functions and inspect variables.  Also, if the underlying BFD functionality supports it, you could use gdb -write to patch object files using this technique.  Note that GDBN can neither interpret nor modify relocations in this case, so branches and some initialized variables will appear to go to the wrong place.  But this feature is still handy from time to time.

file
     file with no argument makes GDBN discard any information it has on both executable file and the symbol table.

exec-file [ FILENAME ]
     Specify that the program to be run (but not the symbol table) is found in FILENAME.  GDBN searches the environment variable PATH if necessary to locate your program.  Omitting FILENAME means to discard information on the executable file.

symbol-file [ FILENAME ]
     Read symbol table information from file FILENAME.  PATH is searched when necessary.  Use the file command to get both symbol table and program to run from the same file.

     symbol-file with no argument clears out GDBN information on your program's symbol table.

     The symbol-file command causes GDBN to forget the contents of some breakpoints and auto-display expressions.  This is because they may contain pointers to the internal data recording symbols and data types, which are part of the old symbol table data being discarded inside GDBN.

     symbol-file does not repeat if you press RET again after executing it once.

     When GDBN is configured for a particular environment, it understands debugging information in whatever format is the standard generated for that environment; you may use either a GNU compiler, or other compilers that adhere to the local conventions. Best results are usually obtained from GNU compilers; for example, using NGCC you can generate debugging information for optimized code.

     For most kinds of object files, with the exception of old SVR3 systems using COFF, the symbol-file command does not normally read the symbol table in full right away.  Instead, it scans the symbol table quickly to find which source files and which symbols are present.  The details are read later, one source file at a time, as they are needed.

     The purpose of this two-stage reading strategy is to make GDBN start up faster.  For the most part, it is invisible except for occasional pauses while the symbol table details for a particular source file are being read.  (The set verbose command can turn these pauses into messages if desired.  See Optional Warnings and Messages.)

     We have not implemented the two-stage strategy for COFF yet.  When the symbol table is stored in COFF format, symbol-file reads the symbol table data in full right away.  Note that ``stabs-in-COFF'' still does the two-stage strategy, since the debug info is actually in stabs format.

symbol-file [ -readnow ] FILENAME
file [ -readnow ] FILENAME
     You can override the GDBN two-stage strategy for reading symbol tables by using the -readnow option with any of the commands that load symbol table information, if you want to be sure GDBN has the entire symbol table available.

core-file [FILENAME]
core
     Specify the whereabouts of a core dump file to be used as the ``contents of memory''.  Traditionally, core files contain only some parts of the address space of the process that generated them; GDBN can access the executable file itself for other parts.

     core-file with no argument specifies that no core file is to be used.

     Note that the core file is ignored when your program is actually running under GDBN.  So, if you have been running your program and you wish to debug a core file instead, you must kill the subprocess in which the program is running.  To do this, use the kill command (see Killing the Child Process).

add-symbol-file FILENAME ADDRESS
add-symbol-file FILENAME ADDRESS [ -readnow ]
add-symbol-file FILENAME ADDRESS -s SECTION ADDRESS ...
     The add-symbol-file command reads additional symbol table information from the file FILENAME.  You would use this command when FILENAME has been dynamically loaded (by some other means) into the program that is running.  ADDRESS should be the memory address at which the file has been loaded; GDBN cannot figure this out for itself.  You can additionally specify an arbitrary number of -s SECTION ADDRESS pairs, to give an explicit section name and base address for that section.  You can specify any ADDRESS as an expression.

     The symbol table of the file FILENAME is added to the symbol table originally read with the symbol-file command.  You can use the add-symbol-file command any number of times; the new symbol data thus read keeps adding to the old.  To discard all old symbol data instead, use the symbol-file command without any arguments.

     Although FILENAME is typically a shared library file, an executable file, or some other object file which has been fully relocated for loading into a process, you can also load symbolic information from relocatable .o files, as long as:

       * the file's symbolic information refers only to linker symbols defined in that file, not to symbols defined by other object files,

       * every section the file's symbolic information refers to has actually been loaded into the inferior, as it appears in the file, and

       * you can determine the address at which every section was loaded, and provide these to the add-symbol-file command.

     Some embedded operating systems, like Sun Chorus and VxWorks, can load relocatable files into an already running program; such systems typically make the requirements above easy to meet.  However, it's important to recognize that many native systems use complex link procedures (.linkonce section factoring and C++ constructor table assembly, for example) that make the requirements difficult to meet.  In general, one cannot assume that using add-symbol-file to read a relocatable object file's symbolic information will have the same effect as linking the relocatable object file into the program in the normal way.

     add-symbol-file does not repeat if you press RET after using it.

add-symbol-file-from-memory ADDRESS
     Load symbols from the given ADDRESS in a dynamically loaded object file whose image is mapped directly into the inferior's memory. For example, the Linux kernel maps a syscall DSO into each process's address space; this DSO provides kernel-specific code for some system calls.  The argument can be any expression whose evaluation yields the address of the file's shared object file header. For this command to work, you must have used symbol-file or exec-file commands in advance.

add-shared-symbol-files LIBRARY-FILE
assf LIBRARY-FILE
     The add-shared-symbol-files command can currently be used only in the Cygwin build of GDBN on MS-Windows OS, where it is an alias for the dll-symbols command (see Cygwin Native). GDBN automatically looks for shared libraries, however if GDBN does not find yours, you can invoke add-shared-symbol-files.  It takes one argument: the shared library's file name.  assf is a shorthand alias for add-shared-symbol-files.

section SECTION ADDR
     The section command changes the base address of the named SECTION of the exec file to ADDR.  This can be used if the exec file does not contain section addresses, (such as in the a.out format), or when the addresses specified in the file itself are wrong.  Each section must be changed separately.  The info files command, described below, lists all the sections and their addresses.

info files
info target
     info files and info target are synonymous; both print the current target (see Specifying a Debugging Target), including the names of the executable and core dump files currently in use by GDBN, and the files from which symbols were loaded.  The command help target lists all possible targets rather than current ones.

maint info sections
     Another command that can give you extra information about program sections is maint info sections.  In addition to the section information displayed by info files, this command displays the flags and file offset of each section in the executable and core dump files.  In addition, maint info sections provides the following command options (which may be arbitrarily combined):

     ALLOBJ
          Display sections for all loaded object files, including shared libraries.

     SECTIONS
          Display info only for named SECTIONS.

     SECTION-FLAGS
          Display info only for sections for which SECTION-FLAGS are true. The section flags that GDBN currently knows about are:

          ALLOC
               Section will have space allocated in the process when loaded. Set for all sections except those containing debug information.

          LOAD
               Section will be loaded from the file into the child process memory. Set for pre-initialized code and data, clear for .bss sections.

          RELOC
               Section needs to be relocated before loading.

          READONLY
               Section cannot be modified by the child process.

          CODE
               Section contains executable code only.

          DATA
               Section contains data only (no executable code).

          ROM
               Section will reside in ROM.

          CONSTRUCTOR
               Section contains data for constructor/destructor lists.

          HAS_CONTENTS
               Section is not empty.

          NEVER_LOAD
               An instruction to the linker to not output the section.

          COFF_SHARED_LIBRARY
               A notification to the linker that the section contains COFF shared library information.

          IS_COMMON
               Section contains common symbols.

set trust-readonly-sections on
     Tell GDBN that readonly sections in your object file really are read-only (i.e. that their contents will not change). In that case, GDBN can fetch values from these sections out of the object file, rather than from the target program. For some targets (notably embedded ones), this can be a significant enhancement to debugging performance.

     The default is off.

set trust-readonly-sections off
     Tell GDBN not to trust readonly sections.  This means that the contents of the section might change while the program is running, and must therefore be fetched from the target when needed.

show trust-readonly-sections
     Show the current setting of trusting readonly sections.

All file-specifying commands allow both absolute and relative file names as arguments.  GDBN always converts the file name to an absolute file name and remembers it that way.

GDBN supports GNU/Linux, MS-Windows, HP-UX, SunOS, SVr4, Irix, and IBM RS/6000 AIX shared libraries.

On MS-Windows GDBN must be linked with the Expat library to support shared libraries.  See Expat.

GDBN automatically loads symbol definitions from shared libraries when you use the run command, or when you examine a core file. (Before you issue the run command, GDBN does not understand references to a function in a shared library, however---unless you are debugging a core file).

On HP-UX, if the program loads a library explicitly, GDBN automatically loads the symbols at the time of the shl_load call.

There are times, however, when you may wish to not automatically load symbol definitions from shared libraries, such as when they are particularly large or there are many of them.

To control the automatic loading of shared library symbols, use the commands:

set auto-solib-add MODE
     If MODE is on, symbols from all shared object libraries will be loaded automatically when the inferior begins execution, you attach to an independently started inferior, or when the dynamic linker informs GDBN that a new library has been loaded.  If MODE is off, symbols must be loaded manually, using the sharedlibrary command.  The default value is on.

     If your program uses lots of shared libraries with debug info that takes large amounts of memory, you can decrease the GDBN memory footprint by preventing it from automatically loading the symbols from shared libraries.  To that end, type set auto-solib-add off before running the inferior, then load each library whose debug symbols you do need with sharedlibrary REGEXP, where REGEXP is a regular expression that matches the libraries whose symbols you want to be loaded.

show auto-solib-add
     Display the current autoloading mode.

To explicitly load shared library symbols, use the sharedlibrary command:

info share REGEX
info sharedlibrary REGEX
     Print the names of the shared libraries which are currently loaded that match REGEX.  If REGEX is omitted then print all shared libraries that are loaded.

sharedlibrary REGEX
share REGEX
     Load shared object library symbols for files matching a Unix regular expression. As with files loaded automatically, it only loads shared libraries required by your program for a core file or after typing run.  If REGEX is omitted all shared libraries required by your program are loaded.

nosharedlibrary
     Unload all shared object library symbols.  This discards all symbols that have been loaded from all shared libraries.  Symbols from shared libraries that were loaded by explicit user requests are not discarded.

Sometimes you may wish that GDBN stops and gives you control when any of shared library events happen.  Use the set stop-on-solib-events command for this:

set stop-on-solib-events
     This command controls whether GDBN should give you control when the dynamic linker notifies it about some shared library event. The most common event of interest is loading or unloading of a new shared library.

show stop-on-solib-events
     Show whether GDBN stops and gives you control when shared library events happen.

Shared libraries are also supported in many cross or remote debugging configurations.  GDBN needs to have access to the target's libraries; this can be accomplished either by providing copies of the libraries on the host system, or by asking GDBN to automatically retrieve the libraries from the target.  If copies of the target libraries are provided, they need to be the same as the target libraries, although the copies on the target can be stripped as long as the copies on the host are not.

For remote debugging, you need to tell GDBN where the target libraries are, so that it can load the correct copies---otherwise, it may try to load the host's libraries.  GDBN has two variables to specify the search directories for target libraries.

set sysroot PATH
     Use PATH as the system root for the program being debugged.  Any absolute shared library paths will be prefixed with PATH; many runtime loaders store the absolute paths to the shared library in the target program's memory.  If you use set sysroot to find shared libraries, they need to be laid out in the same way that they are on the target, with e.g. a /lib and /usr/lib hierarchy under PATH.

     If PATH starts with the sequence remote:, GDBN will retrieve the target libraries from the remote system.  This is only supported when using a remote target that supports the remote get command (see Sending files to a remote system). The part of PATH following the initial remote: (if present) is used as system root prefix on the remote file system. (If you want to specify a local system root using a directory that happens to be named remote:, you need to use some equivalent variant of the name like ./remote:.)

     For targets with an MS-DOS based filesystem, such as MS-Windows and SymbianOS, GDBN tries prefixing a few variants of the target absolute file name with PATH.  But first, on Unix hosts, GDBN converts all backslash directory separators into forward slashes, because the backslash is not a directory separator on Unix:

            c:\foo\bar.dll => c:/foo/bar.dll

     Then, GDBN attempts prefixing the target file name with PATH, and looks for the resulting file name in the host file system:

            c:/foo/bar.dll => /path/to/sysroot/c:/foo/bar.dll

     If that does not find the shared library, GDBN tries removing the : character from the drive spec, both for convenience, and, for the case of the host file system not supporting file names with colons:

            c:/foo/bar.dll => /path/to/sysroot/c/foo/bar.dll

     This makes it possible to have a system root that mirrors a target with more than one drive.  E.g., you may want to setup your local copies of the target system shared libraries like so (note c vs z):

           /path/to/sysroot/c/sys/bin/foo.dll
           /path/to/sysroot/c/sys/bin/bar.dll
           /path/to/sysroot/z/sys/bin/bar.dll

     and point the system root at /path/to/sysroot, so that GDBN can find the correct copies of both c:\sys\bin\foo.dll, and z:\sys\bin\bar.dll.

     If that still does not find the shared library, GDBN tries removing the whole drive spec from the target file name:

            c:/foo/bar.dll => /path/to/sysroot/foo/bar.dll

     This last lookup makes it possible to not care about the drive name, if you don't want or need to.

     The set solib-absolute-prefix command is an alias for set sysroot.

     You can set the default system root by using the configure-time --with-sysroot option.  If the system root is inside GDBN's configured binary prefix (set with --prefix or --exec-prefix), then the default system root will be updated automatically if the installed GDBN is moved to a new location.

show sysroot
     Display the current shared library prefix.

set solib-search-path PATH
     If this variable is set, PATH is a colon-separated list of directories to search for shared libraries.  solib-search-path is used after sysroot fails to locate the library, or if the path to the library is relative instead of absolute.  If you want to use solib-search-path instead of sysroot, be sure to set sysroot to a nonexistent directory to prevent GDBN from finding your host's libraries.  sysroot is preferred; setting it to a nonexistent directory may interfere with automatic loading of shared library symbols.

show solib-search-path
     Display the current shared library search path.

set target-file-system-kind KIND
     Set assumed file system kind for target reported file names.

     Shared library file names as reported by the target system may not make sense as is on the system GDBN is running on.  For example, when remote debugging a target that has MS-DOS based file system semantics, from a Unix host, the target may be reporting to GDBN a list of loaded shared libraries with file names such as c:\Windows\kernel32.dll.  On Unix hosts, there's no concept of drive letters, so the c:\ prefix is not normally understood as indicating an absolute file name, and neither is the backslash normally considered a directory separator character.  In that case, the native file system would interpret this whole absolute file name as a relative file name with no directory components.  This would make it impossible to point GDBN at a copy of the remote target's shared libraries on the host using set sysroot, and impractical with set solib-search-path.  Setting target-file-system-kind to dos-based tells GDBN to interpret such file names similarly to how the target would, and to map them to file names valid on GDBN's native file system semantics.  The value of KIND can be "auto", in addition to one of the supported file system kinds.  In that case, GDBN tries to determine the appropriate file system variant based on the current target's operating system (see Configuring the Current ABI).  The supported file system settings are:

     unix
          Instruct GDBN to assume the target file system is of Unix kind.  Only file names starting the forward slash (/) character are considered absolute, and the directory separator character is also the forward slash.

     dos-based
          Instruct GDBN to assume the target file system is DOS based. File names starting with either a forward slash, or a drive letter followed by a colon (e.g., c:), are considered absolute, and both the slash (/) and the backslash (\\) characters are considered directory separators.

     auto
          Instruct GDBN to use the file system kind associated with the target operating system (see Configuring the Current ABI). This is the default.

When processing file names provided by the user, GDBN frequently needs to compare them to the file names recorded in the program's debug info.  Normally, GDBN compares just the base names of the files as strings, which is reasonably fast even for very large programs.  (The base name of a file is the last portion of its name, after stripping all the leading directories.) This shortcut in comparison is based upon the assumption that files cannot have more than one base name.  This is usually true, but references to files that use symlinks or similar filesystem facilities violate that assumption.  If your program records files using such facilities, or if you provide file names to GDBN using symlinks etc., you can set basenames-may-differ to true to instruct GDBN to completely canonicalize each pair of file names it needs to compare.  This will make file-name comparisons accurate, but at a price of a significant slowdown.

set basenames-may-differ
     Set whether a source file may have multiple base names.

show basenames-may-differ
     Show whether a source file may have multiple base names.

Debugging Information in Separate Files
=======================================

GDBN allows you to put a program's debugging information in a file separate from the executable itself, in a way that allows GDBN to find and load the debugging information automatically. Since debugging information can be very large---sometimes larger than the executable code itself---some systems distribute debugging information for their executables in separate files, which users can install only when they need to debug a problem.

GDBN supports two ways of specifying the separate debug info file:

  * The executable contains a debug link that specifies the name of the separate debug info file.  The separate debug file's name is usually EXECUTABLE.debug, where EXECUTABLE is the name of the corresponding executable file without leading directories (e.g., ls.debug for /usr/bin/ls).  In addition, the debug link specifies a 32-bit Cyclic Redundancy Check (CRC) checksum for the debug file, which GDBN uses to validate that the executable and the debug file came from the same build.

  * The executable contains a build ID, a unique bit string that is also present in the corresponding debug info file.  (This is supported only on some operating systems, notably those which use the ELF format for binary files and the GNU Binutils.)  For more details about this feature, see the description of the --build-id command-line option in Command Line Options (The GNU Linker).  The debug info file's name is not specified explicitly by the build ID, but can be computed from the build ID, see below.

Depending on the way the debug info file is specified, GDBN uses two different methods of looking for the debug file:

  * For the ``debug link'' method, GDBN looks up the named file in the directory of the executable file, then in a subdirectory of that directory named .debug, and finally under the global debug directory, in a subdirectory whose name is identical to the leading directories of the executable's absolute file name.

  * For the ``build ID'' method, GDBN looks in the .build-id subdirectory of the global debug directory for a file named NN/NNNNNNNN.debug, where NN are the first 2 hex characters of the build ID bit string, and NNNNNNNN are the rest of the bit string.  (Real build ID strings are 32 or more hex characters, not 10.)

So, for example, suppose you ask GDBN to debug /usr/bin/ls, which has a debug link that specifies the file ls.debug, and a build ID whose value in hex is abcdef1234.  If the global debug directory is /usr/lib/debug, then GDBN will look for the following debug information files, in the indicated order:

  - /usr/lib/debug/.build-id/ab/cdef1234.debug

  - /usr/bin/ls.debug

  - /usr/bin/.debug/ls.debug

  - /usr/lib/debug/usr/bin/ls.debug.

You can set the global debugging info directory's name, and view the name GDBN is currently using.

set debug-file-directory DIRECTORIES
     Set the directories which GDBN searches for separate debugging information files to DIRECTORY.  Multiple directory components can be set concatenating them by a directory separator.

show debug-file-directory
     Show the directories GDBN searches for separate debugging information files.

A debug link is a special section of the executable file named .gnu_debuglink.  The section must contain:

  * A filename, with any leading directory components removed, followed by a zero byte,

  * zero to three bytes of padding, as needed to reach the next four-byte boundary within the section, and

  * a four-byte CRC checksum, stored in the same endianness used for the executable file itself.  The checksum is computed on the debugging information file's full contents by the function given below, passing zero as the CRC argument.

Any executable file format can carry a debug link, as long as it can contain a section named .gnu_debuglink with the contents described above.

The build ID is a special section in the executable file (and in other ELF binary files that GDBN may consider).  This section is often named .note.gnu.build-id, but that name is not mandatory. It contains unique identification for the built files---the ID remains the same across multiple builds of the same build tree.  The default algorithm SHA1 produces 160 bits (40 hexadecimal characters) of the content for the build ID string.  The same section with an identical value is present in the original built binary with symbols, in its stripped variant, and in the separate debugging information file.

The debugging information file itself should be an ordinary executable, containing a full set of linker symbols, sections, and debugging information.  The sections of the debugging information file should have the same names, addresses, and sizes as the original file, but they need not contain any data---much like a .bss section in an ordinary executable.

The GNU binary utilities (Binutils) package includes the objcopy utility that can produce the separated executable / debugging information file pairs using the following commands:

     objcopy --only-keep-debug foo foo.debug
     strip -g foo

These commands remove the debugging information from the executable file foo and place it in the file foo.debug.  You can use the first, second or both methods to link the two files:

  * The debug link method needs the following additional command to also leave behind a debug link in foo:

          objcopy --add-gnu-debuglink=foo.debug foo

     Ulrich Drepper's elfutils package, starting with version 0.53, contains a version of the strip command such that the command strip foo -f foo.debug has the same functionality as the two objcopy commands and the ln -s command above, together.

  * Build ID gets embedded into the main executable using ld --build-id or the NGCC counterpart gcc -Wl,--build-id.  Build ID support plus compatibility fixes for debug files separation are present in GNU binary utilities (Binutils) package since version 2.18.

The CRC used in .gnu_debuglink is the CRC-32 defined in IEEE 802.3 using the polynomial:

      x^{32} + x^{26} + x^{23} + x^{22} + x^{16} + x^{12} + x^{11}
      + x^{10} + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1

The function is computed byte at a time, taking the least significant bit of each byte first.  The initial pattern 0xffffffff is used, to ensure leading zeros affect the CRC and the final result is inverted to ensure trailing zeros also affect the CRC.

Note: This is the same CRC polynomial as used in handling the Remote Serial Protocol qCRC packet (see GDBN Remote Serial Protocol).  However in the case of the Remote Serial Protocol, the CRC is computed most significant bit first, and the result is not inverted, so trailing zeros have no effect on the CRC value.

To complete the description, we show below the code of the function which produces the CRC used in .gnu_debuglink.  Inverting the initially supplied crc argument means that an initial call to this function passing in zero will start computing the CRC using 0xffffffff.

     unsigned long
     gnu_debuglink_crc32 (unsigned long crc,
                          unsigned char *buf, size_t len)
     {
       static const unsigned long crc32_table[256] =
         {
           0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419,
           0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4,
           0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07,
           0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
           0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856,
           0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
           0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4,
           0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
           0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3,
           0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a,
           0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599,
           0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
           0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190,
           0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f,
           0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e,
           0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
           0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed,
           0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
           0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3,
           0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
           0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a,
           0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5,
           0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010,
           0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
           0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17,
           0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6,
           0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615,
           0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
           0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344,
           0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
           0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a,
           0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
           0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1,
           0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c,
           0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef,
           0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
           0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe,
           0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31,
           0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c,
           0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
           0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b,
           0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
           0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1,
           0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
           0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278,
           0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7,
           0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66,
           0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
           0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605,
           0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8,
           0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b,
           0x2d02ef8d
         };
       unsigned char *end;

       crc = ~crc & 0xffffffff;
       for (end = buf + len; buf < end; ++buf)
         crc = crc32_table[(crc ^ *buf) & 0xff] ^ (crc >> 8);
       return ~crc & 0xffffffff;
     }

This computation does not apply to the ``build ID'' method.

Index Files Speed Up GDBN
=========================

When GDBN finds a symbol file, it scans the symbols in the file in order to construct an internal symbol table.  This lets most GDBN operations work quickly---at the cost of a delay early on.  For large programs, this delay can be quite lengthy, so GDBN provides a way to build an index, which speeds up startup.

The index is stored as a section in the symbol file.  GDBN can write the index to a file, then you can put it into the symbol file using objcopy.

To create an index file, use the save gdb-index command:

save gdb-index DIRECTORY
     Create an index file for each symbol file currently known by GDBN.  Each file is named after its corresponding symbol file, with .gdb-index appended, and is written into the given DIRECTORY.

Once you have created an index file you can merge it into your symbol file, here named symfile, using objcopy:

     $ objcopy --add-section .gdb_index=symfile.gdb-index \
         --set-section-flags .gdb_index=readonly symfile symfile

There are currently some limitation on indices.  They only work when for DWARF debugging information, not stabs.  And, they do not currently work for programs using Ada.

Errors Reading Symbol Files
===========================

While reading a symbol file, GDBN occasionally encounters problems, such as symbol types it does not recognize, or known bugs in compiler output.  By default, GDBN does not notify you of such problems, since they are relatively common and primarily of interest to people debugging compilers.  If you are interested in seeing information about ill-constructed symbol tables, you can either ask GDBN to print only one message about each such type of problem, no matter how many times the problem occurs; or you can ask GDBN to print more messages, to see how many times the problems occur, with the set complaints command (see Optional Warnings and Messages).

The messages currently printed, and their meanings, include:

inner block not inside outer block in SYMBOL

     The symbol information shows where symbol scopes begin and end (such as at the start of a function or a block of statements).  This error indicates that an inner scope block is not fully contained in its outer scope blocks.

     GDBN circumvents the problem by treating the inner block as if it had the same scope as the outer block.  In the error message, SYMBOL may be shown as ``(don't know)'' if the outer block is not a function.

block at ADDRESS out of order

     The symbol information for symbol scope blocks should occur in order of increasing addresses.  This error indicates that it does not do so.

     GDBN does not circumvent this problem, and has trouble locating symbols in the source file whose symbols it is reading.  (You can often determine what source file is affected by specifying set verbose on.  See Optional Warnings and Messages.)

bad block start address patched

     The symbol information for a symbol scope block has a start address smaller than the address of the preceding source line.  This is known to occur in the SunOS 4.1.1 (and earlier) C compiler.

     GDBN circumvents the problem by treating the symbol scope block as starting on the previous source line.

bad string table offset in symbol N

     Symbol number N contains a pointer into the string table which is larger than the size of the string table.

     GDBN circumvents the problem by considering the symbol to have the name foo, which may cause other problems if many symbols end up with this name.

unknown symbol type 0xNN

     The symbol information contains new data types that GDBN does not yet know how to read.  0xNN is the symbol type of the uncomprehended information, in hexadecimal.

     GDBN circumvents the error by ignoring this symbol information. This usually allows you to debug your program, though certain symbols are not accessible.  If you encounter such a problem and feel like debugging it, you can debug GDBP with itself, breakpoint on complain, then go up to the function read_dbx_symtab and examine *bufp to see the symbol.

stub type has NULL name

     GDBN could not find the full definition for a struct or class.

const/volatile indicator missing (ok if using g++ v1.x), got...
     The symbol information for a C++ member function is missing some information that recent versions of the compiler should have output for it.

info mismatch between compiler and debugger

     GDBN could not parse a type specification output by the compiler.

GDB Data Files
==============

GDBN will sometimes read an auxiliary data file.  These files are kept in a directory known as the data directory.

You can set the data directory's name, and view the name GDBN is currently using.

set data-directory DIRECTORY
     Set the directory which GDBN searches for auxiliary data files to DIRECTORY.

show data-directory
     Show the directory GDBN searches for auxiliary data files.

You can set the default data directory by using the configure-time --with-gdb-datadir option.  If the data directory is inside GDBN's configured binary prefix (set with --prefix or --exec-prefix), then the default data directory will be updated automatically if the installed GDBN is moved to a new location.

The data directory may also be specified with the --data-directory command line option. See Mode Options.

Specifying a Debugging Target
*****************************

A target is the execution environment occupied by your program.

Often, GDBN runs in the same host environment as your program; in that case, the debugging target is specified as a side effect when you use the file or core commands.  When you need more flexibility---for example, running GDBN on a physically separate host, or controlling a standalone system over a serial port or a realtime system over a TCP/IP connection---you can use the target command to specify one of the target types configured for GDBN (see Commands for Managing Targets).

It is possible to build GDBN for several different target
architectures.  When GDBN is built like that, you can choose one of the available architectures with the set architecture command.

set architecture ARCH
     This command sets the current target architecture to ARCH.  The value of ARCH can be "auto", in addition to one of the supported architectures.

show architecture
     Show the current target architecture.

set processor
processor
     These are alias commands for, respectively, set architecture and show architecture.

Active Targets
==============

There are multiple classes of targets such as: processes, executable files or recording sessions.  Core files belong to the process class, making core file and process mutually exclusive.  Otherwise, GDBN can work concurrently on multiple active targets, one in each class.  This allows you to (for example) start a process and inspect its activity, while still having access to the executable file after the process finishes.  Or if you start process recording (see Reverse Execution) and reverse-step there, you are presented a virtual layer of the recording target, while the process target remains stopped at the chronologically last point of the process execution.

Use the core-file and exec-file commands to select a new core file or executable target (see Commands to Specify Files).  To specify as a target a process that is already running, use the attach command (see Debugging an Already-running Process).

Commands for Managing Targets
=============================

target TYPE PARAMETERS
     Connects the GDBN host environment to a target machine or process.  A target is typically a protocol for talking to debugging facilities.  You use the argument TYPE to specify the type or protocol of the target machine.

     Further PARAMETERS are interpreted by the target protocol, but typically include things like device names or host names to connect with, process numbers, and baud rates.

     The target command does not repeat if you press RET again after executing the command.

help target
     Displays the names of all targets available.  To display targets currently selected, use either info target or info files (see Commands to Specify Files).

help target NAME
     Describe a particular target, including any parameters necessary to select it.

set gnutarget ARGS
     GDBN uses its own library BFD to read your files.  GDBN knows whether it is reading an executable, a core, or a .o file; however, you can specify the file format with the set gnutarget command.  Unlike most target commands, with gnutarget the target refers to a program, not a machine.

          Warning: To specify a file format with set gnutarget, you must know the actual BFD name.

     See Commands to Specify Files.

show gnutarget
     Use the show gnutarget command to display what file format gnutarget is set to read.  If you have not set gnutarget, GDBN will determine the file format for each file automatically, and show gnutarget displays The current BDF target is "auto".

Here are some common targets (available, or not, depending on the GDB configuration):

target exec PROGRAM
     An executable file.  target exec PROGRAM is the same as exec-file PROGRAM.

target core FILENAME
     A core dump file.  target core FILENAME is the same as core-file FILENAME.

target remote MEDIUM
     A remote system connected to GDBN via a serial line or network connection.  This command tells GDBN to use its own remote protocol over MEDIUM for debugging.  See Remote Debugging.

     For example, if you have a board connected to /dev/ttya on the machine running GDBN, you could say:

          target remote /dev/ttya

     target remote supports the load command.  This is only useful if you have some other way of getting the stub to the target system, and you can put it somewhere in memory where it won't get clobbered by the download.

target sim [SIMARGS] ...
     Builtin CPU simulator.  GDBN includes simulators for most architectures. In general,

                  target sim
                  load
                  run

     works; however, you cannot assume that a specific memory map, device drivers, or even basic I/O is available, although some simulators do provide these.  For info about any processor-specific simulator details, see the appropriate section in Embedded Processors.

Some configurations may include these targets as well:

target nrom DEV
     NetROM ROM emulator.  This target only supports downloading.

Different targets are available on different configurations of GDBN; your configuration may have more or fewer targets.

Many remote targets require you to download the executable's code once you've successfully established a connection.  You may wish to control various aspects of this process.

set hash
     This command controls whether a hash mark # is displayed while downloading a file to the remote monitor.  If on, a hash mark is displayed after each S-record is successfully downloaded to the monitor.

show hash
     Show the current status of displaying the hash mark.

set debug monitor
     Enable or disable display of communications messages between GDBN and the remote monitor.

show debug monitor
     Show the current status of displaying communications between GDBN and the remote monitor.

load FILENAME
     Depending on what remote debugging facilities are configured into GDBN, the load command may be available.  Where it exists, it is meant to make FILENAME (an executable) available for debugging on the remote system---by downloading, or dynamic linking, for example. load also records the FILENAME symbol table in GDBN, like the add-symbol-file command.

     If your GDBN does not have a load command, attempting to execute it gets the error message ``You can't do that when your target is ...''

     The file is loaded at whatever address is specified in the executable. For some object file formats, you can specify the load address when you link the program; for other formats, like a.out, the object file format specifies a fixed address.

     Depending on the remote side capabilities, GDBN may be able to load programs into flash memory.

     load does not repeat if you press RET again after using it.

Choosing Target Byte Order
==========================

Some types of processors, such as the MIPS, PowerPC, and Renesas SH, offer the ability to run either big-endian or little-endian byte orders.  Usually the executable or symbol will include a bit to designate the endian-ness, and you will not need to worry about which to use.  However, you may still find it useful to adjust GDBN's idea of processor endian-ness manually.

set endian big
     Instruct GDBN to assume the target is big-endian.

set endian little
     Instruct GDBN to assume the target is little-endian.

set endian auto
     Instruct GDBN to use the byte order associated with the executable.

show endian
     Display GDBN's current idea of the target byte order.

Note that these commands merely adjust interpretation of symbolic data on the host, and that they have absolutely no effect on the target system.

Debugging Remote Programs
*************************

If you are trying to debug a program running on a machine that cannot run GDBN in the usual way, it is often useful to use remote debugging. For example, you might use remote debugging on an operating system kernel, or on a small system which does not have a general purpose operating system powerful enough to run a full-featured debugger.

Some configurations of GDBN have special serial or TCP/IP interfaces to make this work with particular debugging targets.  In addition, GDBN comes with a generic serial protocol (specific to GDBN, but not specific to any particular target system) which you can use if you write the remote stubs---the code that runs on the remote system to communicate with GDBN.

Other remote targets may be available in your configuration of GDBN; use help target to list them.

Connecting to a Remote Target
=============================

On the GDBN host machine, you will need an unstripped copy of your program, since GDBN needs symbol and debugging information. Start up GDBN as usual, using the name of the local copy of your program as the first argument.

GDBN can communicate with the target over a serial line, or over an IP network using TCP or UDP.  In each case, GDBN uses the same protocol for debugging your program; only the medium carrying the debugging packets varies.  The target remote command establishes a connection to the target. Its arguments indicate which medium to use:

target remote SERIAL-DEVICE
     Use SERIAL-DEVICE to communicate with the target.  For example, to use a serial line connected to the device named /dev/ttyb:

          target remote /dev/ttyb

     If you're using a serial line, you may want to give GDBN the --baud option, or use the set remotebaud command (see set remotebaud) before the target command.

target remote HOST:PORT
target remote tcp:HOST:PORT
     Debug using a TCP connection to PORT on HOST. The HOST may be either a host name or a numeric IP address; PORT must be a decimal number.  The HOST could be the target machine itself, if it is directly connected to the net, or it might be a terminal server which in turn has a serial line to the target.

     For example, to connect to port 2828 on a terminal server named manyfarms:

          target remote manyfarms:2828

     If your remote target is actually running on the same machine as your debugger session (e.g. a simulator for your target running on the same host), you can omit the hostname.  For example, to connect to port 1234 on your local machine:

          target remote :1234

     Note that the colon is still required here.

target remote udp:HOST:PORT
     Debug using UDP packets to PORT on HOST.  For example, to connect to UDP port 2828 on a terminal server named manyfarms:

          target remote udp:manyfarms:2828

     When using a UDP connection for remote debugging, you should keep in mind that the `U' stands for ``Unreliable''.  UDP can silently drop packets on busy or unreliable networks, which will cause havoc with your debugging session.

target remote | COMMAND
     Run COMMAND in the background and communicate with it using a pipe.  The COMMAND is a shell command, to be parsed and expanded by the system's command shell, /bin/sh; it should expect remote protocol packets on its standard input, and send replies on its standard output.  You could use this to run a stand-alone simulator that speaks the remote debugging protocol, to make net connections using programs like ssh, or for other similar tricks.

     If COMMAND closes its standard output (perhaps by exiting), GDBN will try to send it a SIGTERM signal.  (If the program has already exited, this will have no effect.)

Once the connection has been established, you can use all the usual commands to examine and change data.  The remote program is already running; you can use step and continue, and you do not need to use run.

Whenever GDBN is waiting for the remote program, if you type the interrupt character (often Ctrl-c), GDBN attempts to stop the program.  This may or may not succeed, depending in part on the hardware and the serial drivers the remote system uses.  If you type the interrupt character once again, GDBN displays this prompt:

     Interrupted while waiting for the program.
     Give up (and stop debugging it)?  (y or n)

If you type y, GDBN abandons the remote debugging session. (If you decide you want to try again later, you can use target
remote again to connect once more.)  If you type n, GDBN goes back to waiting.

detach
     When you have finished debugging the remote program, you can use the detach command to release it from GDBN control. Detaching from the target normally resumes its execution, but the results will depend on your particular remote stub.  After the detach command, GDBN is free to connect to another target.

disconnect
     The disconnect command behaves like detach, except that the target is generally not resumed.  It will wait for GDBN (this instance or another one) to connect and continue debugging.  After the disconnect command, GDBN is again free to connect to another target.

monitor CMD
     This command allows you to send arbitrary commands directly to the remote monitor.  Since GDBN doesn't care about the commands it sends like this, this command is the way to extend GDBN---you can add new commands that only the external monitor will understand and implement.

Sending files to a remote system
================================

Some remote targets offer the ability to transfer files over the same connection used to communicate with GDBN.  This is convenient for targets accessible through other means, e.g. GNU/Linux systems running gdbserver over a network interface.  For other targets, e.g. embedded devices with only a single serial port, this may be the only way to upload or download files.

Not all remote targets support these commands.

remote put HOSTFILE TARGETFILE
     Copy file HOSTFILE from the host system (the machine running GDBN) to TARGETFILE on the target system.

remote get TARGETFILE HOSTFILE
     Copy file TARGETFILE from the target system to HOSTFILE on the host system.

remote delete TARGETFILE
     Delete TARGETFILE from the target system.

Using the gdbserver Program
===========================

gdbserver is a control program for Unix-like systems, which allows you to connect your program with a remote GDBN via target remote---but without linking in the usual debugging stub.

gdbserver is not a complete replacement for the debugging stubs, because it requires essentially the same operating-system facilities that GDBN itself does.  In fact, a system that can run gdbserver to connect to a remote GDBN could also run GDBN locally!  gdbserver is sometimes useful nevertheless, because it is a much smaller program than GDBN itself.  It is also easier to port than all of GDBN, so you may be able to get started more quickly on a new system by using gdbserver. Finally, if you develop code for real-time systems, you may find that the tradeoffs involved in real-time operation make it more convenient to do as much development work as possible on another system, for example by cross-compiling.  You can use gdbserver to make a similar choice for debugging.

GDBN and gdbserver communicate via either a serial line or a TCP connection, using the standard GDBN remote serial protocol.

     Warning: gdbserver does not have any built-in security. Do not run gdbserver connected to any public network; a GDBN connection to gdbserver provides access to the target system with the same privileges as the user running gdbserver.

Running gdbserver
-----------------

Run gdbserver on the target system.  You need a copy of the program you want to debug, including any libraries it requires. gdbserver does not need your program's symbol table, so you can strip the program if necessary to save space.  GDBN on the host system does all the symbol handling.

To use the server, you must tell it how to communicate with GDBN; the name of your program; and the arguments for your program.  The usual syntax is:

     target> gdbserver COMM PROGRAM [ ARGS ... ]

COMM is either a device name (to use a serial line), or a TCP hostname and portnumber, or - or stdio to use stdin/stdout of gdbserver. For example, to debug Emacs with the argument foo.txt and communicate with GDBN over the serial port /dev/com1:

     target> gdbserver /dev/com1 emacs foo.txt

gdbserver waits passively for the host GDBN to communicate with it.

To use a TCP connection instead of a serial line:

     target> gdbserver host:2345 emacs foo.txt

The only difference from the previous example is the first argument, specifying that you are communicating with the host GDBN via TCP.  The host:2345 argument means that gdbserver is to expect a TCP connection from machine host to local TCP port 2345. (Currently, the host part is ignored.)  You can choose any number you want for the port number as long as it does not conflict with any TCP ports already in use on the target system (for example, 23 is reserved for telnet). (If you choose a port number that
conflicts with another service, gdbserver prints an error message
and exits.)  You must use the same port number with the host GDBN target remote command.

The stdio connection is useful when starting gdbserver with ssh:

     (gdb) target remote | ssh -T hostname gdbserver - hello

The -T option to ssh is provided because we don't need a remote pty, and we don't want escape-character handling.  Ssh does this by default when a command is provided, the flag is provided to make it explicit. You could elide it if you want to.

Programs started with stdio-connected gdbserver have /dev/null for stdin, and stdout,stderr are sent back to gdb for display through a pipe connected to gdbserver. Both stdout and stderr use the same pipe.

Attaching to a Running Program
..............................

On some targets, gdbserver can also attach to running programs. This is accomplished via the --attach argument.  The syntax is:

     target> gdbserver --attach COMM PID

PID is the process ID of a currently running process.  It isn't necessary to point gdbserver at a binary for the running process.

You can debug processes by name instead of process ID if your target has the pidof utility:

     target> gdbserver --attach COMM `pidof PROGRAM`

In case more than one copy of PROGRAM is running, or PROGRAM has multiple threads, most versions of pidof support the -s option to only return the first process ID.

Multi-Process Mode for gdbserver
................................

When you connect to gdbserver using target remote, gdbserver debugs the specified program only once.  When the program exits, or you detach from it, GDBN closes the connection and gdbserver exits.

If you connect using target extended-remote, gdbserver enters multi-process mode.  When the debugged program exits, or you detach from it, GDBN stays connected to gdbserver even though no program is running.  The run and attach commands instruct gdbserver to run or attach to a new program. The run command uses set remote exec-file (see set remote exec-file) to select the program to run.  Command line arguments are supported, except for wildcard expansion and I/O redirection (see Arguments).

To start gdbserver without supplying an initial command to run or process ID to attach, use the --multi command line option. Then you can connect using target extended-remote and start the program you want to debug.

In multi-process mode gdbserver does not automatically exit unless you use the option --once.  You can terminate it by using monitor exit (see Monitor Commands for gdbserver).  Note that the conditions under which gdbserver terminates depend on how GDBN connects to it (target remote or target extended-remote).  The --multi option to gdbserver has no influence on that.

TCP port allocation lifecycle of gdbserver
..........................................

This section applies only when gdbserver is run to listen on a TCP port.

gdbserver normally terminates after all of its debugged processes have terminated in target remote mode.  On the other hand, for target extended-remote, gdbserver stays running even with no processes left. GDBN normally terminates the spawned debugged process on its exit, which normally also terminates gdbserver in the target remote mode.  Therefore, when the connection drops unexpectedly, and GDBN cannot ask gdbserver to kill its debugged processes, gdbserver stays running even in the target remote mode.

When gdbserver stays running, GDBN can connect to it again later. Such reconnecting is useful for features like disconnected tracing.  For completeness, at most one GDBN can be connected at a time.

By default, gdbserver keeps the listening TCP port open, so that additional connections are possible.  However, if you start gdbserver with the --once option, it will stop listening for any further connection attempts after connecting to the first GDBN session.  This means no further connections to gdbserver will be possible after the first one.  It also means gdbserver will terminate after the first connection with remote GDBN has closed, even for unexpectedly closed connections and even in the target extended-remote mode.  The --once option allows reusing the same port number for connecting to multiple instances of gdbserver running on the same host, since each instance closes its port after the first connection.

Other Command-Line Arguments for gdbserver
..........................................

The --debug option tells gdbserver to display extra status information about the debugging process. The --remote-debug option tells gdbserver to display remote protocol debug output.  These options are intended for gdbserver development and for bug reports to the developers.

The --wrapper option specifies a wrapper to launch programs for debugging.  The option should be followed by the name of the wrapper, then any command-line arguments to pass to the wrapper, then -- indicating the end of the wrapper arguments.

gdbserver runs the specified wrapper program with a combined command line including the wrapper arguments, then the name of the program to debug, then any arguments to the program.  The wrapper runs until it executes your program, and then GDBN gains control.

You can use any program that eventually calls execve with its arguments as a wrapper.  Several standard Unix utilities do this, e.g. env and nohup.  Any Unix shell script ending with exec "$@" will also work.

For example, you can use env to pass an environment variable to the debugged program, without setting the variable in gdbserver's environment:

     $ gdbserver --wrapper env LD_PRELOAD=libtest.so -- :2222 ./testprog

Connecting to gdbserver
-----------------------

Run GDBN on the host system.

First make sure you have the necessary symbol files.  Load symbols for your application using the file command before you connect.  Use set sysroot to locate target libraries (unless your GDBN was compiled with the correct sysroot using --with-sysroot).

The symbol file and target libraries must exactly match the executable and libraries on the target, with one exception: the files on the host system should not be stripped, even if the files on the target system are.  Mismatched or missing files will lead to confusing results during debugging.  On GNU/Linux targets, mismatched or missing files may also prevent gdbserver from debugging multi-threaded programs.

Connect to your target (see Connecting to a Remote Target). For TCP connections, you must start up gdbserver prior to using the target remote command.  Otherwise you may get an error whose text depends on the host system, but which usually looks something like Connection refused.  Don't use the load command in GDBN when using gdbserver, since the program is already on the target.

Monitor Commands for gdbserver
------------------------------

During a GDBN session using gdbserver, you can use the monitor command to send special requests to gdbserver. Here are the available commands.

monitor help
     List the available monitor commands.

monitor set debug 0
monitor set debug 1
     Disable or enable general debugging messages.

monitor set remote-debug 0
monitor set remote-debug 1
     Disable or enable specific debugging messages associated with the remote protocol (see Remote Protocol).

monitor set libthread-db-search-path [PATH]
     When this command is issued, PATH is a colon-separated list of directories to search for libthread_db (see set libthread-db-search-path).  If you omit PATH, libthread-db-search-path will be reset to its default value.

     The special entry $pdir for libthread-db-search-path is not supported in gdbserver.

monitor exit
     Tell gdbserver to exit immediately.  This command should be followed by disconnect to close the debugging session.  gdbserver will detach from any attached processes and kill any processes it created. Use monitor exit to terminate gdbserver at the end of a multi-process mode debug session.

Tracepoints support in gdbserver
--------------------------------

On some targets, gdbserver supports tracepoints, fast tracepoints and static tracepoints.

For fast or static tracepoints to work, a special library called the in-process agent (IPA), must be loaded in the inferior process. This library is built and distributed as an integral part of gdbserver.  In addition, support for static tracepoints requires building the in-process agent library with static tracepoints support.  At present, the UST (LTTng Userspace Tracer, http://lttng.org/ust) tracing engine is supported.  This support is automatically available if UST development headers are found in the standard include path when gdbserver is built, or if gdbserver was explicitly configured using --with-ust to point at such headers.  You can explicitly disable the support using --with-ust=no.

There are several ways to load the in-process agent in your program:

Specifying it as dependency at link time

     You can link your program dynamically with the in-process agent library.  On most systems, this is accomplished by adding -linproctrace to the link command.

Using the system's preloading mechanisms

     You can force loading the in-process agent at startup time by using your system's support for preloading shared libraries.  Many Unixes support the concept of preloading user defined libraries.  In most cases, you do that by specifying LD_PRELOAD=libinproctrace.so in the environment.  See also the description of gdbserver's --wrapper command line option.

Using GDBN to force loading the agent at run time

     On some systems, you can force the inferior to load a shared library, by calling a dynamic loader function in the inferior that takes care of dynamically looking up and loading a shared library.  On most Unix systems, the function is dlopen.  You'll use the call command for that.  For example:

          (GDBP) call dlopen ("libinproctrace.so", ...)

     Note that on most Unix systems, for the dlopen function to be available, the program needs to be linked with -ldl.

On systems that have a userspace dynamic loader, like most Unix systems, when you connect to gdbserver using target remote, you'll find that the program is stopped at the dynamic loader's entry point, and no shared library has been loaded in the program's address space yet, including the in-process agent.  In that case, before being able to use any of the fast or static tracepoints features, you need to let the loader run and load the shared libraries.  The simplest way to do that is to run the program to the main procedure.  E.g., if debugging a C or C++ program, start gdbserver like so:

     $ gdbserver :9999 myprogram

Start GDB and connect to gdbserver like so, and run to main:

     $ gdb myprogram
     (GDBP) target remote myhost:9999
     0x00007f215893ba60 in ?? () from /lib64/ld-linux-x86-64.so.2
     (GDBP) b main
     (GDBP) continue

The in-process tracing agent library should now be loaded into the process; you can confirm it with the info sharedlibrary command, which will list libinproctrace.so as loaded in the process.  You are now ready to install fast tracepoints, list static tracepoint markers, probe static tracepoints markers, and start tracing.

Remote Configuration
====================

This section documents the configuration options available when debugging remote programs.  For the options related to the File I/O extensions of the remote protocol, see system-call-allowed.

set remoteaddresssize BITS
     Set the maximum size of address in a memory packet to the specified number of bits.  GDBN will mask off the address bits above that number, when it passes addresses to the remote target.  The default value is the number of bits in the target's address.

show remoteaddresssize
     Show the current value of remote address size in bits.

set remotebaud N
     Set the baud rate for the remote serial I/O to N baud.  The value is used to set the speed of the serial port used for debugging remote targets.

show remotebaud
     Show the current speed of the remote connection.

set remotebreak
     If set to on, GDBN sends a BREAK signal to the remote when you type Ctrl-c to interrupt the program running on the remote.  If set to off, GDBN sends the Ctrl-C character instead.  The default is off, since most remote systems expect to see Ctrl-C as the interrupt signal.

show remotebreak
     Show whether GDBN sends BREAK or Ctrl-C to interrupt the remote program.

set remoteflow on
set remoteflow off
     Enable or disable hardware flow control (RTS/CTS) on the serial port used to communicate to the remote target.

show remoteflow
     Show the current setting of hardware flow control.

set remotelogbase BASE
     Set the base (a.k.a. radix) of logging serial protocol communications to BASE.  Supported values of BASE are: ascii, octal, and hex.  The default is ascii.

show remotelogbase
     Show the current setting of the radix for logging remote serial protocol.

set remotelogfile FILE
     Record remote serial communications on the named FILE.  The default is not to record at all.

show remotelogfile.
     Show the current setting  of the file name on which to record the serial communications.

set remotetimeout NUM
     Set the timeout limit to wait for the remote target to respond to NUM seconds.  The default is 2 seconds.

show remotetimeout
     Show the current number of seconds to wait for the remote target responses.

set remote hardware-watchpoint-limit LIMIT
set remote hardware-breakpoint-limit LIMIT
     Restrict GDBN to using LIMIT remote hardware breakpoint or watchpoints.  A limit of -1, the default, is treated as unlimited.

set remote hardware-watchpoint-length-limit LIMIT
     Restrict GDBN to using LIMIT bytes for the maximum length of a remote hardware watchpoint.  A limit of -1, the default, is treated as unlimited.

show remote hardware-watchpoint-length-limit
     Show the current limit (in bytes) of the maximum length of a remote hardware watchpoint.

set remote exec-file FILENAME
show remote exec-file
     Select the file used for run with target extended-remote.  This should be set to a filename valid on the target system.  If it is not set, the target will use a default filename (e.g. the last program run).

set remote interrupt-sequence
     Allow the user to select one of Ctrl-C, a BREAK or BREAK-g as the sequence to the remote target in order to interrupt the execution. Ctrl-C is a default.  Some system prefers BREAK which is high level of serial line for some certain time. Linux kernel prefers BREAK-g, a.k.a Magic SysRq g. It is BREAK signal followed by character g.

show interrupt-sequence
     Show which of Ctrl-C, BREAK or BREAK-g is sent by GDBN to interrupt the remote program. BREAK-g is BREAK signal followed by g and also known as Magic SysRq g.

set remote interrupt-on-connect
     Specify whether interrupt-sequence is sent to remote target when GDBN connects to it.  This is mostly needed when you debug Linux kernel.  Linux kernel expects BREAK followed by g which is known as Magic SysRq g in order to connect GDBN.

show interrupt-on-connect
     Show whether interrupt-sequence is sent to remote target when GDBN connects to it.

set tcp auto-retry on
     Enable auto-retry for remote TCP connections.  This is useful if the remote debugging agent is launched in parallel with GDBN; there is a race condition because the agent may not become ready to accept the connection before GDBN attempts to connect.  When auto-retry is enabled, if the initial attempt to connect fails, GDBN reattempts to establish the connection using the timeout specified by set tcp connect-timeout.

set tcp auto-retry off
     Do not auto-retry failed TCP connections.

show tcp auto-retry
     Show the current auto-retry setting.

set tcp connect-timeout SECONDS
     Set the timeout for establishing a TCP connection to the remote target to SECONDS.  The timeout affects both polling to retry failed connections (enabled by set tcp auto-retry on) and waiting for connections that are merely slow to complete, and represents an approximate cumulative value.

show tcp connect-timeout
     Show the current connection timeout setting.

The GDBN remote protocol autodetects the packets supported by your debugging stub.  If you need to override the autodetection, you can use these commands to enable or disable individual packets.  Each packet can be set to on (the remote target supports this packet), off (the remote target does not support this packet), or auto (detect remote target support for this packet).  They all default to auto.  For more information about each packet, see Remote Protocol.

During normal use, you should not have to use any of these commands. If you do, that may be a bug in your remote debugging stub, or a bug in GDBN.  You may want to report the problem to the GDBN developers.

For each packet NAME, the command to enable or disable the packet is set remote NAME-packet.  The available settings are:

Command Name
Remote Packet
Related Features

fetch-register
p
info registers

set-register
P
set

binary-download
X
load, set

read-aux-vector
qXfer:auxv:read
info auxv

symbol-lookup
qSymbol
Detecting multiple threads

attach
vAttach
attach

verbose-resume
vCont
Stepping or resuming multiple threads

run
vRun
run

software-breakpoint
Z0
break

hardware-breakpoint
Z1
hbreak

write-watchpoint
Z2
watch

read-watchpoint
Z3
rwatch

access-watchpoint
Z4
awatch

target-features
qXfer:features:read
set architecture

library-info
qXfer:libraries:read
info sharedlibrary

memory-map
qXfer:memory-map:read
info mem

read-sdata-object
qXfer:sdata:read
print $_sdata

read-spu-object
qXfer:spu:read
info spu

write-spu-object
qXfer:spu:write
info spu

read-siginfo-object
qXfer:siginfo:read
print $_siginfo

write-siginfo-object
qXfer:siginfo:write
set $_siginfo

threads
qXfer:threads:read
info threads

get-thread-local-
storage-address
qGetTLSAddr
Displaying __thread variables

get-thread-information-block-address
qGetTIBAddr
Display MS-Windows Thread Information Block.

search-memory
qSearch:memory
find

supported-packets
qSupported
Remote communications parameters

pass-signals
QPassSignals
handle SIGNAL

hostio-close-packet
vFile:close
remote get, remote put

hostio-open-packet
vFile:open
remote get, remote put

hostio-pread-packet
vFile:pread
remote get, remote put

hostio-pwrite-packet
vFile:pwrite
remote get, remote put

hostio-unlink-packet
vFile:unlink
remote delete

hostio-readlink-packet
vFile:readlink
Host I/O

noack-packet
QStartNoAckMode
Packet acknowledgment

osdata
qXfer:osdata:read
info os

query-attached
qAttached
Querying remote process attach state.

traceframe-info
qXfer:traceframe-info:read
Traceframe info

install-in-trace
InstallInTrace
Install tracepoint in tracing

disable-randomization
QDisableRandomization
set disable-randomization

Implementing a Remote Stub
==========================

The stub files provided with GDBN implement the target side of the communication protocol, and the GDBN side is implemented in the GDBN source file remote.c.  Normally, you can simply allow these subroutines to communicate, and ignore the details.  (If you're implementing your own stub file, you can still ignore the details: start with one of the existing stub files.  sparc-stub.c is the best organized, and therefore the easiest to read.)

To debug a program running on another machine (the debugging target machine), you must first arrange for all the usual prerequisites for the program to run by itself.  For example, for a C program, you need:

 1. A startup routine to set up the C runtime environment; these usually have a name like crt0.  The startup routine may be supplied by your hardware supplier, or you may have to write your own.

 2. A C subroutine library to support your program's subroutine calls, notably managing input and output.

 3. A way of getting your program to the other machine---for example, a download program.  These are often supplied by the hardware manufacturer, but you may have to write your own from hardware documentation.

The next step is to arrange for your program to use a serial port to communicate with the machine where GDBN is running (the host machine).  In general terms, the scheme looks like this:

On the host,
     GDBN already understands how to use this protocol; when everything else is set up, you can simply use the target remote command (see Specifying a Debugging Target).

On the target,
     you must link with your program a few special-purpose subroutines that implement the GDBN remote serial protocol.  The file containing these subroutines is called  a debugging stub.

     On certain remote targets, you can use an auxiliary program gdbserver instead of linking a stub into your program. See Using the gdbserver Program, for details.

The debugging stub is specific to the architecture of the remote machine; for example, use sparc-stub.c to debug programs on SPARC boards.

These working remote stubs are distributed with GDBN:

i386-stub.c
     For Intel 386 and compatible architectures.

m68k-stub.c
     For Motorola 680x0 architectures.

sh-stub.c
     For Renesas SH architectures.

sparc-stub.c
     For SPARC architectures.

sparcl-stub.c
     For Fujitsu SPARCLITE architectures.

The README file in the GDBN distribution may list other recently added stubs.

What the Stub Can Do for You
----------------------------

The debugging stub for your architecture supplies these three subroutines:

set_debug_traps
     This routine arranges for handle_exception to run when your program stops.  You must call this subroutine explicitly in your program's startup code.

handle_exception
     This is the central workhorse, but your program never calls it explicitly---the setup code arranges for handle_exception to run when a trap is triggered.

     handle_exception takes control when your program stops during execution (for example, on a breakpoint), and mediates communications with GDBN on the host machine.  This is where the communications protocol is implemented; handle_exception acts as the GDBN representative on the target machine.  It begins by sending summary information on the state of your program, then continues to execute, retrieving and transmitting any information GDBN needs, until you execute a GDBN command that makes your program resume; at that point, handle_exception returns control to your own code on the target machine.

breakpoint
     Use this auxiliary subroutine to make your program contain a breakpoint.  Depending on the particular situation, this may be the only way for GDBN to get control.  For instance, if your target machine has some sort of interrupt button, you won't need to call this; pressing the interrupt button transfers control to handle_exception---in effect, to GDBN.  On some machines, simply receiving characters on the serial port may also trigger a trap; again, in that situation, you don't need to call breakpoint from your own program---simply running target remote from the host GDBN session gets control.

     Call breakpoint if none of these is true, or if you simply want to make certain your program stops at a predetermined point for the start of your debugging session.

What You Must Do for the Stub
-----------------------------

The debugging stubs that come with GDBN are set up for a particular chip architecture, but they have no information about the rest of your debugging target machine.

First of all you need to tell the stub how to communicate with the serial port.

int getDebugChar()
     Write this subroutine to read a single character from the serial port. It may be identical to getchar for your target system; a different name is used to allow you to distinguish the two if you wish.

void putDebugChar(int)
     Write this subroutine to write a single character to the serial port. It may be identical to putchar for your target system; a different name is used to allow you to distinguish the two if you wish.

If you want GDBN to be able to stop your program while it is running, you need to use an interrupt-driven serial driver, and arrange for it to stop when it receives a ^C (\003, the control-C character).  That is the character which GDBN uses to tell the remote system to stop.

Getting the debugging target to return the proper status to GDBN probably requires changes to the standard stub; one quick and dirty way is to just execute a breakpoint instruction (the ``dirty'' part is that GDBN reports a SIGTRAP instead of a SIGINT).

Other routines you need to supply are:

void exceptionHandler (int EXCEPTION_NUMBER, void *EXCEPTION_ADDRESS)
     Write this function to install EXCEPTION_ADDRESS in the exception handling tables.  You need to do this because the stub does not have any way of knowing what the exception handling tables on your target system are like (for example, the processor's table might be in ROM, containing entries which point to a table in RAM). EXCEPTION_NUMBER is the exception number which should be changed; its meaning is architecture-dependent (for example, different numbers might represent divide by zero, misaligned access, etc).  When this exception occurs, control should be transferred directly to EXCEPTION_ADDRESS, and the processor state (stack, registers, and so on) should be just as it is when a processor exception occurs.  So if you want to use a jump instruction to reach EXCEPTION_ADDRESS, it should be a simple jump, not a jump to subroutine.

     For the 386, EXCEPTION_ADDRESS should be installed as an interrupt gate so that interrupts are masked while the handler runs.  The gate should be at privilege level 0 (the most privileged level).  The SPARC and 68k stubs are able to mask interrupts themselves without help from exceptionHandler.

void flush_i_cache()
     On SPARC and SPARCLITE only, write this subroutine to flush the instruction cache, if any, on your target machine.  If there is no instruction cache, this subroutine may be a no-op.

     On target machines that have instruction caches, GDBN requires this function to make certain that the state of your program is stable.

You must also make sure this library routine is available:

void *memset(void *, int, int)
     This is the standard library function memset that sets an area of memory to a known value.  If you have one of the free versions of libc.a, memset can be found there; otherwise, you must either obtain it from your hardware manufacturer, or write your own.

If you do not use the GNU C compiler, you may need other standard library subroutines as well; this varies from one stub to another, but in general the stubs are likely to use any of the common library subroutines which NGCC generates as inline code.

Putting it All Together
-----------------------

In summary, when your program is ready to debug, you must follow these steps.

 1. Make sure you have defined the supporting low-level routines (see What You Must Do for the Stub):

          getDebugChar, putDebugChar,
          flush_i_cache, memset, exceptionHandler.

 2. Insert these lines in your program's startup code, before the main procedure is called:

          set_debug_traps();
          breakpoint();

     On some machines, when a breakpoint trap is raised, the hardware automatically makes the PC point to the instruction after the breakpoint.  If your machine doesn't do that, you may need to adjust handle_exception to arrange for it to return to the instruction after the breakpoint on this first invocation, so that your program doesn't keep hitting the initial breakpoint instead of making progress.

 3. For the 680x0 stub only, you need to provide a variable called exceptionHook.  Normally you just use:

          void (*exceptionHook)() = 0;

     but if before calling set_debug_traps, you set it to point to a function in your program, that function is called when GDBN continues after stopping on a trap (for example, bus error).  The function indicated by exceptionHook is called with one parameter: an int which is the exception number.

 4. Compile and link together: your program, the GDBN debugging stub for your target architecture, and the supporting subroutines.

 5. Make sure you have a serial connection between your target machine and the GDBN host, and identify the serial port on the host.

 6. Download your program to your target machine (or get it there by whatever means the manufacturer provides), and start it.

 7. Start GDBN on the host, and connect to the target (see Connecting to a Remote Target).

Configuration-Specific Information
**********************************

While nearly all GDBN commands are available for all native and cross versions of the debugger, there are some exceptions.  This chapter describes things that are only available in certain configurations.

There are three major categories of configurations: native configurations, where the host and target are the same, embedded operating system configurations, which are usually the same for several different processor architectures, and bare embedded processors, which are quite different from each other.

Native
======

This section describes details specific to particular native configurations.

HP-UX
-----

On HP-UX systems, if you refer to a function or variable name that begins with a dollar sign, GDBN searches for a user or system name first, before it searches for a convenience variable.

BSD libkvm Interface
--------------------

BSD-derived systems (FreeBSD/NetBSD/OpenBSD) have a kernel memory interface that provides a uniform interface for accessing kernel virtual memory images, including live systems and crash dumps.  GDBN uses this interface to allow you to debug live kernels and kernel crash dumps on many native BSD configurations.  This is implemented as a special kvm debugging target.  For debugging a live system, load the currently running kernel into GDBN and connect to the kvm target:

     (GDBP) target kvm

For debugging crash dumps, provide the file name of the crash dump as an argument:

     (GDBP) target kvm /var/crash/bsd.0

Once connected to the kvm target, the following commands are available:

kvm pcb
     Set current context from the Process Control Block (PCB) address.

kvm proc
     Set current context from proc address.  This command isn't available on modern FreeBSD systems.

SVR4 Process Information
------------------------

Many versions of SVR4 and compatible systems provide a facility called /proc that can be used to examine the image of a running process using file-system subroutines.  If GDBN is configured for an operating system with this facility, the command info proc is available to report information about the process running your program, or about any process running on your system.  info proc works only on SVR4 systems that include the procfs code. This includes, as of this writing, GNU/Linux, OSF/1 (Digital Unix), Solaris, Irix, and Unixware, but not HP-UX, for example.

info proc
info proc PROCESS-ID
     Summarize available information about any running process.  If a process ID is specified by PROCESS-ID, display information about that process; otherwise display information about the program being debugged.  The summary includes the debugged process ID, the command line used to invoke it, its current working directory, and its executable file's absolute file name.

     On some systems, PROCESS-ID can be of the form [PID]/TID which specifies a certain thread ID within a process.  If the optional PID part is missing, it means a thread from the process being debugged (the leading / still needs to be present, or else GDBN will interpret the number as a process ID rather than a thread ID).

info proc mappings
     Report the memory address space ranges accessible in the program, with information on whether the process has read, write, or execute access rights to each range.  On GNU/Linux systems, each memory range includes the object file which is mapped to that range, instead of the memory access rights to that range.

info proc stat
info proc status
     These subcommands are specific to GNU/Linux systems.  They show the process-related information, including the user ID and group ID; how many threads are there in the process; its virtual memory usage; the signals that are pending, blocked, and ignored; its TTY; its consumption of system and user time; its stack size; its nice value; etc.  For more information, see the proc man page (type man 5 proc from your shell prompt).

info proc all
     Show all the information about the process described under all of the above info proc subcommands.

set procfs-trace
     This command enables and disables tracing of procfs API calls.

show procfs-trace
     Show the current state of procfs API call tracing.

set procfs-file FILE
     Tell GDBN to write procfs API trace to the named FILE.  GDBN appends the trace info to the previous contents of the file.  The default is to display the trace on the standard output.

show procfs-file
     Show the file to which procfs API trace is written.

proc-trace-entry
proc-trace-exit
proc-untrace-entry
proc-untrace-exit
     These commands enable and disable tracing of entries into and exits from the syscall interface.

info pidlist
     For QNX Neutrino only, this command displays the list of all the processes and all the threads within each process.

info meminfo
     For QNX Neutrino only, this command displays the list of all mapinfos.

Features for Debugging DJGPP Programs
-------------------------------------

DJGPP is a port of the GNU development tools to MS-DOS and MS-Windows.  DJGPP programs are 32-bit protected-mode programs that use the DPMI (DOS Protected-Mode Interface) API to run on top of real-mode DOS systems and their emulations.

GDBN supports native debugging of DJGPP programs, and defines a few commands specific to the DJGPP port.  This subsection describes those commands.

info dos
     This is a prefix of DJGPP-specific commands which print information about the target system and important OS structures.

info dos sysinfo
     This command displays assorted information about the underlying platform: the CPU type and features, the OS version and flavor, the DPMI version, and the available conventional and DPMI memory.

info dos gdt
info dos ldt
info dos idt
     These 3 commands display entries from, respectively, Global, Local, and Interrupt Descriptor Tables (GDT, LDT, and IDT).  The descriptor tables are data structures which store a descriptor for each segment that is currently in use.  The segment's selector is an index into a descriptor table; the table entry for that index holds the descriptor's base address and limit, and its attributes and access rights.

     A typical DJGPP program uses 3 segments: a code segment, a data segment (used for both data and the stack), and a DOS segment (which allows access to DOS/BIOS data structures and absolute addresses in conventional memory).  However, the DPMI host will usually define additional segments in order to support the DPMI environment.

     These commands allow to display entries from the descriptor tables. Without an argument, all entries from the specified table are displayed.  An argument, which should be an integer expression, means display a single entry whose index is given by the argument.  For example, here's a convenient way to display information about the debugged program's data segment:

          (GDBP) info dos ldt $ds
          0x13f: base=0x11970000 limit=0x0009ffff 32-Bit Data (Read/Write, Exp-up)

     This comes in handy when you want to see whether a pointer is outside the data segment's limit (i.e. garbled).

info dos pde
info dos pte
     These two commands display entries from, respectively, the Page Directory and the Page Tables.  Page Directories and Page Tables are data structures which control how virtual memory addresses are mapped into physical addresses.  A Page Table includes an entry for every page of memory that is mapped into the program's address space; there may be several Page Tables, each one holding up to 4096 entries.  A Page Directory has up to 4096 entries, one each for every Page Table that is currently in use.

     Without an argument, info dos pde displays the entire Page Directory, and info dos pte displays all the entries in all of the Page Tables.  An argument, an integer expression, given to the info dos pde command means display only that entry from the Page Directory table.  An argument given to the info dos pte command means display entries from a single Page Table, the one pointed to by the specified entry in the Page Directory.

     These commands are useful when your program uses DMA (Direct Memory Access), which needs physical addresses to program the DMA controller.

     These commands are supported only with some DPMI servers.

info dos address-pte ADDR
     This command displays the Page Table entry for a specified linear address.  The argument ADDR is a linear address which should already have the appropriate segment's base address added to it, because this command accepts addresses which may belong to any segment.  For example, here's how to display the Page Table entry for the page where a variable i is stored:

          (GDBP) info dos address-pte __djgpp_base_address + (char *)&i
          Page Table entry for address 0x11a00d30:
          Base=0x02698000 Dirty Acc. Not-Cached Write-Back Usr Read-Write +0xd30

     This says that i is stored at offset 0xd30 from the page whose physical base address is 0x02698000, and shows all the attributes of that page.

     Note that you must cast the addresses of variables to a char *, since otherwise the value of __djgpp_base_address, the base address of all variables and functions in a DJGPP program, will be added using the rules of C pointer arithmetics: if i is declared an int, GDBN will add 4 times the value of __djgpp_base_address to the address of i.

     Here's another example, it displays the Page Table entry for the transfer buffer:

          (GDBP) info dos address-pte *((unsigned *)&_go32_info_block + 3)
          Page Table entry for address 0x29110:
          Base=0x00029000 Dirty Acc. Not-Cached Write-Back Usr Read-Write +0x110

     (The + 3 offset is because the transfer buffer's address is the 3rd member of the _go32_info_block structure.)  The output clearly shows that this DPMI server maps the addresses in conventional memory 1:1, i.e. the physical (0x00029000 + 0x110) and linear (0x29110) addresses are identical.

     This command is supported only with some DPMI servers.

In addition to native debugging, the DJGPP port supports remote debugging via a serial data link.  The following commands are specific to remote serial debugging in the DJGPP port of GDBN.

set com1base ADDR
     This command sets the base I/O port address of the COM1 serial port.

set com1irq IRQ
     This command sets the Interrupt Request (IRQ) line to use for the COM1 serial port.

     There are similar commands set com2base, set com3irq, etc. for setting the port address and the IRQ lines for the other 3 COM ports.

     The related commands show com1base, show com1irq etc. display the current settings of the base address and the IRQ lines used by the COM ports.

info serial
     This command prints the status of the 4 DOS serial ports.  For each port, it prints whether it's active or not, its I/O base address and IRQ number, whether it uses a 16550-style FIFO, its baudrate, and the counts of various errors encountered so far.

Features for Debugging MS Windows PE Executables
------------------------------------------------

GDBN supports native debugging of MS Windows programs, including DLLs with and without symbolic debugging information.

MS-Windows programs that call SetConsoleMode to switch off the special meaning of the Ctrl-C keystroke cannot be interrupted by typing C-c.  For this reason, GDBN on MS-Windows supports C-BREAK as an alternative interrupt key sequence, which can be used to interrupt the debuggee even if it ignores C-c.

There are various additional Cygwin-specific commands, described in this section.  Working with DLLs that have no debugging symbols is described in Non-debug DLL Symbols.

info w32
     This is a prefix of MS Windows-specific commands which print information about the target system and important OS structures.

info w32 selector
     This command displays information returned by the Win32 API GetThreadSelectorEntry function. It takes an optional argument that is evaluated to a long value to give the information about this given selector. Without argument, this command displays information about the six segment registers.

info w32 thread-information-block
     This command displays thread specific information stored in the Thread Information Block (readable on the X86 CPU family using $fs selector for 32-bit programs and $gs for 64-bit programs).

info dll
     This is a Cygwin-specific alias of info shared.

dll-symbols
     This command loads symbols from a dll similarly to add-sym command but without the need to specify a base address.

set cygwin-exceptions MODE
     If MODE is on, GDBN will break on exceptions that happen inside the Cygwin DLL.  If MODE is off, GDBN will delay recognition of exceptions, and may ignore some exceptions which seem to be caused by internal Cygwin DLL ``bookkeeping''.  This option is meant primarily for debugging the Cygwin DLL itself; the default value is off to avoid annoying GDBN users with false SIGSEGV signals.

show cygwin-exceptions
     Displays whether GDBN will break on exceptions that happen inside the Cygwin DLL itself.

set new-console MODE
     If MODE is on the debuggee will be started in a new console on next start. If MODE is off, the debuggee will be started in the same console as the debugger.

show new-console
     Displays whether a new console is used when the debuggee is started.

set new-group MODE
     This boolean value controls whether the debuggee should start a new group or stay in the same group as the debugger. This affects the way the Windows OS handles Ctrl-C.

show new-group
     Displays current value of new-group boolean.

set debugevents
     This boolean value adds debug output concerning kernel events related to the debuggee seen by the debugger.  This includes events that signal thread and process creation and exit, DLL loading and unloading, console interrupts, and debugging messages produced by the Windows OutputDebugString API call.

set debugexec
     This boolean value adds debug output concerning execute events (such as resume thread) seen by the debugger.

set debugexceptions
     This boolean value adds debug output concerning exceptions in the debuggee seen by the debugger.

set debugmemory
     This boolean value adds debug output concerning debuggee memory reads and writes by the debugger.

set shell
     This boolean values specifies whether the debuggee is called via a shell or directly (default value is on).

show shell
     Displays if the debuggee will be started with a shell.

Support for DLLs without Debugging Symbols
..........................................

Very often on windows, some of the DLLs that your program relies on do not include symbolic debugging information (for example, kernel32.dll).  When GDBN doesn't recognize any debugging symbols in a DLL, it relies on the minimal amount of symbolic information contained in the DLL's export table.  This section describes working with such symbols, known internally to GDBN as ``minimal symbols''.

Note that before the debugged program has started execution, no DLLs will have been loaded.  The easiest way around this problem is simply to start the program --- either by setting a breakpoint or letting the program run once to completion.  It is also possible to force GDBN to load a particular DLL before starting the executable --- see the shared library information in Files, or the dll-symbols command in Cygwin Native.  Currently, explicitly loading symbols from a DLL with no debugging information will cause the symbol names to be duplicated in GDBN's lookup table, which may adversely affect symbol lookup performance.

DLL Name Prefixes
.................

In keeping with the naming conventions used by the Microsoft debugging tools, DLL export symbols are made available with a prefix based on the DLL name, for instance KERNEL32!CreateFileA.  The plain name is also entered into the symbol table, so CreateFileA is often sufficient.  In some cases there will be name clashes within a program (particularly if the executable itself includes full debugging symbols) necessitating the use of the fully qualified name when referring to the contents of the DLL.  Use single-quotes around the name to avoid the exclamation mark (``!'')  being interpreted as a language operator.

Note that the internal name of the DLL may be all upper-case, even though the file name of the DLL is lower-case, or vice-versa.  Since symbols within GDBN are case-sensitive this may cause some confusion. If in doubt, try the info functions and info variables commands or even maint print msymbols (see Symbols). Here's an example:

     (GDBP) info function CreateFileA
     All functions matching regular expression "CreateFileA":

     Non-debugging symbols:
     0x77e885f4  CreateFileA
     0x77e885f4  KERNEL32!CreateFileA

     (GDBP) info function !
     All functions matching regular expression "!":

     Non-debugging symbols:
     0x6100114c  cygwin1!__assert
     0x61004034  cygwin1!_dll_crt0@0
     0x61004240  cygwin1!dll_crt0(per_process *)
     [etc...]

Working with Minimal Symbols
............................

Symbols extracted from a DLL's export table do not contain very much type information. All that GDBN can do is guess whether a symbol refers to a function or variable depending on the linker section that contains the symbol. Also note that the actual contents of the memory contained in a DLL are not available unless the program is running. This means that you cannot examine the contents of a variable or disassemble a function within a DLL without a running program.

Variables are generally treated as pointers and dereferenced automatically. For this reason, it is often necessary to prefix a variable name with the address-of operator (``&'') and provide explicit type information in the command. Here's an example of the type of problem:

     (GDBP) print 'cygwin1!__argv'
     $1 = 268572168

     (GDBP) x 'cygwin1!__argv'
     0x10021610:      "\230y\""

And two possible solutions:

     (GDBP) print ((char **)'cygwin1!__argv')[0]
     $2 = 0x22fd98 "/cygdrive/c/mydirectory/myprogram"

     (GDBP) x/2x &'cygwin1!__argv'
     0x610c0aa8 <cygwin1!__argv>:    0x10021608      0x00000000
     (GDBP) x/x 0x10021608
     0x10021608:     0x0022fd98
     (GDBP) x/s 0x0022fd98
     0x22fd98:        "/cygdrive/c/mydirectory/myprogram"

Setting a break point within a DLL is possible even before the program starts execution. However, under these circumstances, GDBN can't examine the initial instructions of the function in order to skip the function's frame set-up code. You can work around this by using ``*&'' to set the breakpoint at a raw memory address:

     (GDBP) break *&'python22!PyOS_Readline'
     Breakpoint 1 at 0x1e04eff0

The author of these extensions is not entirely convinced that setting a break point within a shared DLL like kernel32.dll is completely safe.

Commands Specific to GNU Hurd Systems
-------------------------------------

This subsection describes GDBN commands specific to the GNU Hurd native debugging.

set signals
set sigs
     This command toggles the state of inferior signal interception by GDBN.  Mach exceptions, such as breakpoint traps, are not affected by this command.  sigs is a shorthand alias for signals.

show signals
show sigs
     Show the current state of intercepting inferior's signals.

set signal-thread
set sigthread
     This command tells GDBN which thread is the libc signal thread.  That thread is run when a signal is delivered to a running process.  set sigthread is the shorthand alias of set signal-thread.

show signal-thread
show sigthread
     These two commands show which thread will run when the inferior is delivered a signal.

set stopped
     This commands tells GDBN that the inferior process is stopped, as with the SIGSTOP signal.  The stopped process can be continued by delivering a signal to it.

show stopped
     This command shows whether GDBN thinks the debuggee is stopped.

set exceptions
     Use this command to turn off trapping of exceptions in the inferior. When exception trapping is off, neither breakpoints nor single-stepping will work.  To restore the default, set exception trapping on.

show exceptions
     Show the current state of trapping exceptions in the inferior.

set task pause
     This command toggles task suspension when GDBN has control. Setting it to on takes effect immediately, and the task is suspended whenever GDBN gets control.  Setting it to off will take effect the next time the inferior is continued.  If this option is set to off, you can use set thread default pause on or set thread pause on (see below) to pause individual threads.

show task pause
     Show the current state of task suspension.

set task detach-suspend-count
     This command sets the suspend count the task will be left with when GDBN detaches from it.

show task detach-suspend-count
     Show the suspend count the task will be left with when detaching.

set task exception-port
set task excp
     This command sets the task exception port to which GDBN will forward exceptions.  The argument should be the value of the send rights of the task.  set task excp is a shorthand alias.

set noninvasive
     This command switches GDBN to a mode that is the least invasive as far as interfering with the inferior is concerned.  This is the same as using set task pause, set exceptions, and set signals to values opposite to the defaults.

info send-rights
info receive-rights
info port-rights
info port-sets
info dead-names
info ports
info psets
     These commands display information about, respectively, send rights, receive rights, port rights, port sets, and dead names of a task. There are also shorthand aliases: info ports for info port-rights and info psets for info port-sets.

set thread pause
     This command toggles current thread suspension when GDBN has control.  Setting it to on takes effect immediately, and the current thread is suspended whenever GDBN gets control.  Setting it to off will take effect the next time the inferior is continued. Normally, this command has no effect, since when GDBN has control, the whole task is suspended.  However, if you used set task pause off (see above), this command comes in handy to suspend only the current thread.

show thread pause
     This command shows the state of current thread suspension.

set thread run
     This command sets whether the current thread is allowed to run.

show thread run
     Show whether the current thread is allowed to run.

set thread detach-suspend-count
     This command sets the suspend count GDBN will leave on a thread when detaching.  This number is relative to the suspend count found by GDBN when it notices the thread; use set thread
     takeover-suspend-count to force it to an absolute value.

show thread detach-suspend-count
     Show the suspend count GDBN will leave on the thread when detaching.

set thread exception-port
set thread excp
     Set the thread exception port to which to forward exceptions.  This overrides the port set by set task exception-port (see above). set thread excp is the shorthand alias.

set thread takeover-suspend-count
     Normally, GDBN's thread suspend counts are relative to the value GDBN finds when it notices each thread.  This command changes the suspend counts to be absolute instead.

set thread default
show thread default
     Each of the above set thread commands has a set thread default counterpart (e.g., set thread default pause, set thread default exception-port, etc.).  The thread default variety of commands sets the default thread properties for all threads; you can then change the properties of individual threads with the non-default commands.

QNX Neutrino
------------

GDBN provides the following commands specific to the QNX Neutrino target:

set debug nto-debug
     When set to on, enables debugging messages specific to the QNX Neutrino support.

show debug nto-debug
     Show the current state of QNX Neutrino messages.

Darwin
------

GDBN provides the following commands specific to the Darwin target:

set debug darwin NUM
     When set to a non zero value, enables debugging messages specific to the Darwin support.  Higher values produce more verbose output.

show debug darwin
     Show the current state of Darwin messages.

set debug mach-o NUM
     When set to a non zero value, enables debugging messages while GDBN is reading Darwin object files.  (Mach-O is the file format used on Darwin for object and executable files.)  Higher values produce more verbose output.  This is a command to diagnose problems internal to GDBN and should not be needed in normal usage.

show debug mach-o
     Show the current state of Mach-O file messages.

set mach-exceptions on
set mach-exceptions off
     On Darwin, faults are first reported as a Mach exception and are then mapped to a Posix signal.  Use this command to turn on trapping of Mach exceptions in the inferior.  This might be sometimes useful to better understand the cause of a fault.  The default is off.

show mach-exceptions
     Show the current state of exceptions trapping.

Embedded Operating Systems
==========================

This section describes configurations involving the debugging of embedded operating systems that are available for several different architectures.

GDBN includes the ability to debug programs running on various real-time operating systems.

Using GDBN with VxWorks
-----------------------

target vxworks MACHINENAME
     A VxWorks system, attached via TCP/IP.  The argument MACHINENAME is the target system's machine name or IP address.

On VxWorks, load links FILENAME dynamically on the current target system as well as adding its symbols in GDBN.

GDBN enables developers to spawn and debug tasks running on networked VxWorks targets from a Unix host.  Already-running tasks spawned from the VxWorks shell can also be debugged.  GDBN uses code that runs on both the Unix host and on the VxWorks target.  The program GDBP is installed and executed on the Unix host.  (It may be installed with the name vxgdb, to distinguish it from a GDBN for debugging programs on the host itself.)

VxWorks-timeout ARGS
     All VxWorks-based targets now support the option vxworks-timeout. This option is set by the user, and  ARGS represents the number of seconds GDBN waits for responses to rpc's.  You might use this if your VxWorks target is a slow software simulator or is on the far side of a thin network line.

The following information on connecting to VxWorks was current when this manual was produced; newer releases of VxWorks may use revised procedures.

To use GDBN with VxWorks, you must rebuild your VxWorks kernel to include the remote debugging interface routines in the VxWorks library rdb.a.  To do this, define INCLUDE_RDB in the VxWorks configuration file configAll.h and rebuild your VxWorks kernel.  The resulting kernel contains rdb.a, and spawns the source debugging task tRdbTask when VxWorks is booted.  For more information on configuring and remaking VxWorks, see the manufacturer's manual.

Once you have included rdb.a in your VxWorks system image and set your Unix execution search path to find GDBN, you are ready to run GDBN.  From your Unix host, run GDBP (or vxgdb, depending on your installation).

GDBN comes up showing the prompt:

     (vxgdb)

Connecting to VxWorks
.....................

The GDBN command target lets you connect to a VxWorks target on the network.  To connect to a target whose host name is ``tt'', type:

     (vxgdb) target vxworks tt

GDBN displays messages like these:

     Attaching remote machine across net...
     Connected to tt.

GDBN then attempts to read the symbol tables of any object modules loaded into the VxWorks target since it was last booted.  GDBN locates these files by searching the directories listed in the command search path (see Your Program's Environment); if it fails to find an object file, it displays a message such as:

     prog.o: No such file or directory.

When this happens, add the appropriate directory to the search path with the GDBN command path, and execute the target command again.

VxWorks Download
................

If you have connected to the VxWorks target and you want to debug an object that has not yet been loaded, you can use the GDBN load command to download a file from Unix to VxWorks incrementally.  The object file given as an argument to the load command is actually opened twice: first by the VxWorks target in order to download the code, then by GDBN in order to read the symbol table.  This can lead to problems if the current working directories on the two systems differ.  If both systems have NFS mounted the same filesystems, you can avoid these problems by using absolute paths. Otherwise, it is simplest to set the working directory on both systems to the directory in which the object file resides, and then to reference the file by its name, without any path.  For instance, a program prog.o may reside in VXPATH/vw/demo/rdb in VxWorks and in HOSTPATH/vw/demo/rdb on the host.  To load this program, type this on VxWorks:

     -> cd "VXPATH/vw/demo/rdb"

Then, in GDBN, type:

     (vxgdb) cd HOSTPATH/vw/demo/rdb
     (vxgdb) load prog.o

GDBN displays a response similar to this:

     Reading symbol data from wherever/vw/demo/rdb/prog.o... done.

You can also use the load command to reload an object module after editing and recompiling the corresponding source file.  Note that this makes GDBN delete all currently-defined breakpoints, auto-displays, and convenience variables, and to clear the value history.  (This is necessary in order to preserve the integrity of debugger's data structures that reference the target system's symbol table.)

Running Tasks
.............

You can also attach to an existing task using the attach command as follows:

     (vxgdb) attach TASK

where TASK is the VxWorks hexadecimal task ID.  The task can be running or suspended when you attach to it.  Running tasks are suspended at the time of attachment.

Embedded Processors
===================

This section goes into details specific to particular embedded configurations.

Whenever a specific embedded processor has a simulator, GDBN allows to send an arbitrary command to the simulator.

sim COMMAND
     Send an arbitrary COMMAND string to the simulator.  Consult the documentation for the specific simulator in use for information about acceptable commands.

ARM
---

target rdi DEV
     ARM Angel monitor, via RDI library interface to ADP protocol.  You may use this target to communicate with both boards running the Angel monitor, or with the EmbeddedICE JTAG debug device.

target rdp DEV
     ARM Demon monitor.

GDBN provides the following ARM-specific commands:

set arm disassembler
     This commands selects from a list of disassembly styles.  The "std" style is the standard style.

show arm disassembler
     Show the current disassembly style.

set arm apcs32
     This command toggles ARM operation mode between 32-bit and 26-bit.

show arm apcs32
     Display the current usage of the ARM 32-bit mode.

set arm fpu FPUTYPE
     This command sets the ARM floating-point unit (FPU) type.  The argument FPUTYPE can be one of these:

     auto
          Determine the FPU type by querying the OS ABI.

     softfpa
          Software FPU, with mixed-endian doubles on little-endian ARM processors.

     fpa
          GCC-compiled FPA co-processor.

     softvfp
          Software FPU with pure-endian doubles.

     vfp
          VFP co-processor.

show arm fpu
     Show the current type of the FPU.

set arm abi
     This command forces GDBN to use the specified ABI.

show arm abi
     Show the currently used ABI.

set arm fallback-mode (arm|thumb|auto)
     GDBN uses the symbol table, when available, to determine whether instructions are ARM or Thumb.  This command controls GDBN's default behavior when the symbol table is not available.  The default is auto, which causes GDBN to use the current execution mode (from the T bit in the CPSR register).

show arm fallback-mode
     Show the current fallback instruction mode.

set arm force-mode (arm|thumb|auto)
     This command overrides use of the symbol table to determine whether instructions are ARM or Thumb.  The default is auto, which causes GDBN to use the symbol table and then the setting of set arm fallback-mode.

show arm force-mode
     Show the current forced instruction mode.

set debug arm
     Toggle whether to display ARM-specific debugging messages from the ARM target support subsystem.

show debug arm
     Show whether ARM-specific debugging messages are enabled.

The following commands are available when an ARM target is debugged using the RDI interface:

rdilogfile [FILE]
     Set the filename for the ADP (Angel Debugger Protocol) packet log. With an argument, sets the log file to the specified FILE.  With no argument, show the current log file name.  The default log file is rdi.log.

rdilogenable [ARG]
     Control logging of ADP packets.  With an argument of 1 or "yes" enables logging, with an argument 0 or "no" disables it.  With no arguments displays the current setting.  When logging is enabled, ADP packets exchanged between GDBN and the RDI target device are logged to a file.

set rdiromatzero
     Tell GDBN whether the target has ROM at address 0.  If on, vector catching is disabled, so that zero address can be used.  If off (the default), vector catching is enabled.  For this command to take effect, it needs to be invoked prior to the target rdi command.

show rdiromatzero
     Show the current setting of ROM at zero address.

set rdiheartbeat
     Enable or disable RDI heartbeat packets.  It is not recommended to turn on this option, since it confuses ARM and EPI JTAG interface, as well as the Angel monitor.

show rdiheartbeat
     Show the setting of RDI heartbeat packets.

target sim [SIMARGS] ...
     The GDBN ARM simulator accepts the following optional arguments.

     --swi-support=TYPE
          Tell the simulator which SWI interfaces to support. TYPE may be a comma separated list of the following values. The default value is all.

          none

          demon

          angel

          redboot

          all

Renesas M32R/D and M32R/SDI
---------------------------

target m32r DEV
     Renesas M32R/D ROM monitor.

target m32rsdi DEV
     Renesas M32R SDI server, connected via parallel port to the board.

The following GDBN commands are specific to the M32R monitor:

set download-path PATH
     Set the default path for finding downloadable SREC files.

show download-path
     Show the default path for downloadable SREC files.

set board-address ADDR
     Set the IP address for the M32R-EVA target board.

show board-address
     Show the current IP address of the target board.

set server-address ADDR
     Set the IP address for the download server, which is the GDBN's host machine.

show server-address
     Display the IP address of the download server.

upload [FILE]
     Upload the specified SREC FILE via the monitor's Ethernet upload capability.  If no FILE argument is given, the current executable file is uploaded.

tload [FILE]
     Test the upload command.

The following commands are available for M32R/SDI:

sdireset
     This command resets the SDI connection.

sdistatus
     This command shows the SDI connection status.

debug_chaos
     Instructs the remote that M32R/Chaos debugging is to be used.

use_debug_dma
     Instructs the remote to use the DEBUG_DMA method of accessing memory.

use_mon_code
     Instructs the remote to use the MON_CODE method of accessing memory.

use_ib_break
     Instructs the remote to set breakpoints by IB break.

use_dbt_break
     Instructs the remote to set breakpoints by DBT.

M68k
----

The Motorola m68k configuration includes ColdFire support, and a target command for the following ROM monitor.

target dbug DEV
     dBUG ROM monitor for Motorola ColdFire.

MicroBlaze
----------

The MicroBlaze is a soft-core processor supported on various Xilinx FPGAs, such as Spartan or Virtex series.  Boards with these processors usually have JTAG ports which connect to a host system running the Xilinx Embedded Development Kit (EDK) or Software Development Kit (SDK). This host system is used to download the configuration bitstream to the target FPGA.  The Xilinx Microprocessor Debugger (XMD) program communicates with the target board using the JTAG interface and presents a gdbserver interface to the board.  By default xmd uses port 1234.  (While it is possible to change this default port, it requires the use of undocumented xmd commands.  Contact Xilinx support if you need to do this.)

Use these GDB commands to connect to the MicroBlaze target processor.

target remote :1234
     Use this command to connect to the target if you are running GDBN on the same system as xmd.

target remote XMD-HOST:1234
     Use this command to connect to the target if it is connected to xmd running on a different system named XMD-HOST.

load
     Use this command to download a program to the MicroBlaze target.

set debug microblaze N
     Enable MicroBlaze-specific debugging messages if non-zero.

show debug microblaze N
     Show MicroBlaze-specific debugging level.

MIPS Embedded
-------------

GDBN can use the MIPS remote debugging protocol to talk to a MIPS board attached to a serial line.  This is available when you configure GDBN with --target=mips-idt-ecoff.

Use these GDBN commands to specify the connection to your target board:

target mips PORT
     To run a program on the board, start up GDBP with the name of your program as the argument.  To connect to the board, use the command target mips PORT, where PORT is the name of the serial port connected to the board.  If the program has not already been downloaded to the board, you may use the load command to download it.  You can then use all the usual GDBN commands.

     For example, this sequence connects to the target board through a serial port, and loads and runs a program called PROG through the debugger:

          host$ GDBP PROG
          GDBN is free software and ...
          (GDBP) target mips /dev/ttyb
          (GDBP) load PROG
          (GDBP) run

target mips HOSTNAME:PORTNUMBER
     On some GDBN host configurations, you can specify a TCP connection (for instance, to a serial line managed by a terminal concentrator) instead of a serial port, using the syntax HOSTNAME:PORTNUMBER.

target pmon PORT
     PMON ROM monitor.

target ddb PORT
     NEC's DDB variant of PMON for Vr4300.

target lsi PORT
     LSI variant of PMON.

target r3900 DEV
     Densan DVE-R3900 ROM monitor for Toshiba R3900 Mips.

target array DEV
     Array Tech LSI33K RAID controller board.

GDBN also supports these special commands for MIPS targets:

set mipsfpu double
set mipsfpu single
set mipsfpu none
set mipsfpu auto
show mipsfpu
     If your target board does not support the MIPS floating point coprocessor, you should use the command set mipsfpu none (if you need this, you may wish to put the command in your GDBN init file).  This tells GDBN how to find the return value of functions which return floating point values.  It also allows GDBN to avoid saving the floating point registers when calling functions on the board.  If you are using a floating point coprocessor with only single precision floating point support, as on the R4650 processor, use the command set mipsfpu single.  The default double precision floating point coprocessor may be selected using set mipsfpu double.

     In previous versions the only choices were double precision or no floating point, so set mipsfpu on will select double precision and set mipsfpu off will select no floating point.

     As usual, you can inquire about the mipsfpu variable with show mipsfpu.

set timeout SECONDS
set retransmit-timeout SECONDS
show timeout
show retransmit-timeout
     You can control the timeout used while waiting for a packet, in the MIPS remote protocol, with the set timeout SECONDS command.  The default is 5 seconds.  Similarly, you can control the timeout used while waiting for an acknowledgment of a packet with the set retransmit-timeout SECONDS command.  The default is 3 seconds. You can inspect both values with show timeout and show retransmit-timeout.  (These commands are only available when GDBN is configured for --target=mips-idt-ecoff.)

     The timeout set by set timeout does not apply when GDBN is waiting for your program to stop.  In that case, GDBN waits forever because it has no way of knowing how long the program is going to run before stopping.

set syn-garbage-limit NUM
     Limit the maximum number of characters GDBN should ignore when it tries to synchronize with the remote target.  The default is 10 characters.  Setting the limit to -1 means there's no limit.

show syn-garbage-limit
     Show the current limit on the number of characters to ignore when trying to synchronize with the remote system.

set monitor-prompt PROMPT
     Tell GDBN to expect the specified PROMPT string from the remote monitor.  The default depends on the target:

     pmon target
          PMON

     ddb target
          NEC010

     lsi target
          PMON>

show monitor-prompt
     Show the current strings GDBN expects as the prompt from the remote monitor.

set monitor-warnings
     Enable or disable monitor warnings about hardware breakpoints.  This has effect only for the lsi target.  When on, GDBN will display warning messages whose codes are returned by the lsi PMON monitor for breakpoint commands.

show monitor-warnings
     Show the current setting of printing monitor warnings.

pmon COMMAND
     This command allows sending an arbitrary COMMAND string to the monitor.  The monitor must be in debug mode for this to work.

OpenRISC 1000
-------------

See OR1k Architecture document (www.opencores.org) for more information about platform and commands.

target jtag jtag://HOST:PORT

     Connects to remote JTAG server. JTAG remote server can be either an or1ksim or JTAG server, connected via parallel port to the board.

     Example: target jtag jtag://localhost:9999

or1ksim COMMAND
     If connected to or1ksim OpenRISC 1000 Architectural Simulator, proprietary commands can be executed.

info or1k spr
     Displays spr groups.

info or1k spr GROUP
info or1k spr GROUPNO
     Displays register names in selected group.

info or1k spr GROUP REGISTER
info or1k spr REGISTER
info or1k spr GROUPNO REGISTERNO
info or1k spr REGISTERNO
     Shows information about specified spr register.

spr GROUP REGISTER VALUE
spr REGISTER VALUE
spr GROUPNO REGISTERNO VALUE
spr REGISTERNO VALUE
     Writes VALUE to specified spr register.

Some implementations of OpenRISC 1000 Architecture also have hardware trace. It is very similar to GDBN trace, except it does not interfere with normal program execution and is thus much faster.  Hardware breakpoints/watchpoint triggers can be set using:

$LEA/$LDATA
     Load effective address/data

$SEA/$SDATA
     Store effective address/data

$AEA/$ADATA
     Access effective address ($SEA or $LEA) or data ($SDATA/$LDATA)

$FETCH
     Fetch data

When triggered, it can capture low level data, like: PC, LSEA, LDATA, SDATA, READSPR, WRITESPR, INSTR.

htrace commands:

hwatch CONDITIONAL
     Set hardware watchpoint on combination of Load/Store Effective Address(es) or Data.  For example:

     hwatch ($LEA == my_var) && ($LDATA < 50) || ($SEA == my_var) && ($SDATA >= 50)

     hwatch ($LEA == my_var) && ($LDATA < 50) || ($SEA == my_var) && ($SDATA >= 50)

htrace info
     Display information about current HW trace configuration.

htrace trigger CONDITIONAL
     Set starting criteria for HW trace.

htrace qualifier CONDITIONAL
     Set acquisition qualifier for HW trace.

htrace stop CONDITIONAL
     Set HW trace stopping criteria.

htrace record [DATA]*
     Selects the data to be recorded, when qualifier is met and HW trace was triggered.

htrace enable
htrace disable
     Enables/disables the HW trace.

htrace rewind [FILENAME]
     Clears currently recorded trace data.

     If filename is specified, new trace file is made and any newly collected data will be written there.

htrace print [START [LEN]]
     Prints trace buffer, using current record configuration.

htrace mode continuous
     Set continuous trace mode.

htrace mode suspend
     Set suspend trace mode.

PowerPC Embedded
----------------

GDBN supports using the DVC (Data Value Compare) register to implement in hardware simple hardware watchpoint conditions of the form:

     (GDBP) watch ADDRESS|VARIABLE \
       if  ADDRESS|VARIABLE == CONSTANT EXPRESSION

The DVC register will be automatically used when GDBN detects such pattern in a condition expression, and the created watchpoint uses one debug register (either the exact-watchpoints option is on and the variable is scalar, or the variable has a length of one byte).  This feature is available in native GDBN running on a Linux kernel version 2.6.34 or newer.

When running on PowerPC embedded processors, GDBN automatically uses ranged hardware watchpoints, unless the exact-watchpoints option is on, in which case watchpoints using only one debug register are created when watching variables of scalar types.

You can create an artificial array to watch an arbitrary memory region using one of the following commands (see Expressions):

     (GDBP) watch *((char *) ADDRESS)@LENGTH
     (GDBP) watch {char[LENGTH]} ADDRESS

PowerPC embedded processors support masked watchpoints.  See the discussion about the mask argument in Set Watchpoints.

PowerPC embedded processors support hardware accelerated ranged breakpoints.  A ranged breakpoint stops execution of the inferior whenever it executes an instruction at any address within the range it specifies.  To set a ranged breakpoint in GDBN, use the break-range command.

GDBN provides the following PowerPC-specific commands:

break-range START-LOCATION, END-LOCATION
     Set a breakpoint for an address range. START-LOCATION and END-LOCATION can specify a function name, a line number, an offset of lines from the current line or from the start location, or an address of an instruction (see Specify Location, for a list of all the possible ways to specify a LOCATION.) The breakpoint will stop execution of the inferior whenever it executes an instruction at any address within the specified range, (including START-LOCATION and END-LOCATION.)

set powerpc soft-float
show powerpc soft-float
     Force GDBN to use (or not use) a software floating point calling convention.  By default, GDBN selects the calling convention based on the selected architecture and the provided executable file.

set powerpc vector-abi
show powerpc vector-abi
     Force GDBN to use the specified calling convention for vector arguments and return values.  The valid options are auto; generic, to avoid vector registers even if they are present; altivec, to use AltiVec registers; and spe to use SPE registers.  By default, GDBN selects the calling convention based on the selected architecture and the provided executable file.

set powerpc exact-watchpoints
show powerpc exact-watchpoints
     Allow GDBN to use only one debug register when watching a variable of scalar type, thus assuming that the variable is accessed through the address of its first byte.

target dink32 DEV
     DINK32 ROM monitor.

target ppcbug DEV

target ppcbug1 DEV
     PPCBUG ROM monitor for PowerPC.

target sds DEV
     SDS monitor, running on a PowerPC board (such as Motorola's ADS).

The following commands specific to the SDS protocol are supported by GDBN:

set sdstimeout NSEC
     Set the timeout for SDS protocol reads to be NSEC seconds.  The default is 2 seconds.

show sdstimeout
     Show the current value of the SDS timeout.

sds COMMAND
     Send the specified COMMAND string to the SDS monitor.

HP PA Embedded
--------------

target op50n DEV
     OP50N monitor, running on an OKI HPPA board.

target w89k DEV
     W89K monitor, running on a Winbond HPPA board.

Tsqware Sparclet
----------------

GDBN enables developers to debug tasks running on Sparclet targets from a Unix host. GDBN uses code that runs on both the Unix host and on the Sparclet target.  The program GDBP is installed and executed on the Unix host.

remotetimeout ARGS
     GDBN supports the option remotetimeout. This option is set by the user, and  ARGS represents the number of seconds GDBN waits for responses.

When compiling for debugging, include the options -g to get debug information and -Ttext to relocate the program to where you wish to load it on the target.  You may also want to add the options -n or -N in order to reduce the size of the sections.  Example:

     sparclet-aout-gcc prog.c -Ttext 0x12010000 -g -o prog -N

You can use objdump to verify that the addresses are what you intended:

     sparclet-aout-objdump --headers --syms prog

Once you have set your Unix execution search path to find GDBN, you are ready to run GDBN.  From your Unix host, run GDBP (or sparclet-aout-gdb, depending on your installation).

GDBN comes up showing the prompt:

     (gdbslet)

Setting File to Debug
.....................

The GDBN command file lets you choose with program to debug.

     (gdbslet) file prog

GDBN then attempts to read the symbol table of prog. GDBN locates the file by searching the directories listed in the command search path. If the file was compiled with debug information (option -g), source files will be searched as well. GDBN locates the source files by searching the directories listed in the directory search path (see Your Program's Environment). If it fails to find a file, it displays a message such as:

     prog: No such file or directory.

When this happens, add the appropriate directories to the search paths with the GDBN commands path and dir, and execute the target command again.

Connecting to Sparclet
......................

The GDBN command target lets you connect to a Sparclet target. To connect to a target on serial port ``ttya'', type:

     (gdbslet) target sparclet /dev/ttya
     Remote target sparclet connected to /dev/ttya
     main () at ../prog.c:3

GDBN displays messages like these:

     Connected to ttya.

Sparclet Download
.................

Once connected to the Sparclet target, you can use the GDBN load command to download the file from the host to the target. The file name and load offset should be given as arguments to the load command. Since the file format is aout, the program must be loaded to the starting address.  You can use objdump to find out what this value is.  The load offset is an offset which is added to the VMA (virtual memory address) of each of the file's sections. For instance, if the program prog was linked to text address 0x1201000, with data at 0x12010160 and bss at 0x12010170, in GDBN, type:

     (gdbslet) load prog 0x12010000
     Loading section .text, size 0xdb0 vma 0x12010000

If the code is loaded at a different address then what the program was linked to, you may need to use the section and add-symbol-file commands to tell GDBN where to map the symbol table.

Running and Debugging
.....................

You can now begin debugging the task using GDBN's execution control commands, b, step, run, etc.  See the GDBN manual for the list of commands.

     (gdbslet) b main
     Breakpoint 1 at 0x12010000: file prog.c, line 3.
     (gdbslet) run
     Starting program: prog
     Breakpoint 1, main (argc=1, argv=0xeffff21c) at prog.c:3
     3        char *symarg = 0;
     (gdbslet) step
     4        char *execarg = "hello!";
     (gdbslet)

Fujitsu Sparclite
-----------------

target sparclite DEV
     Fujitsu sparclite boards, used only for the purpose of loading. You must use an additional command to debug the program. For example: target remote DEV using GDBN standard remote protocol.

Zilog Z8000
-----------

When configured for debugging Zilog Z8000 targets, GDBN includes a Z8000 simulator.

For the Z8000 family, target sim simulates either the Z8002 (the unsegmented variant of the Z8000 architecture) or the Z8001 (the segmented variant).  The simulator recognizes which architecture is appropriate by inspecting the object code.

target sim ARGS
     Debug programs on a simulated CPU.  If the simulator supports setup options, specify them via ARGS.

After specifying this target, you can debug programs for the simulated CPU in the same style as programs for your host computer; use the file command to load a new program image, the run command to run your program, and so on.

As well as making available all the usual machine registers (see Registers), the Z8000 simulator provides three additional items of information as specially named registers:

cycles
     Counts clock-ticks in the simulator.

insts
     Counts instructions run in the simulator.

time
     Execution time in 60ths of a second.

You can refer to these values in GDBN expressions with the usual conventions; for example, b fputc if $cycles>5000 sets a conditional breakpoint that suspends only after at least 5000 simulated clock ticks.

Atmel AVR
---------

When configured for debugging the Atmel AVR, GDBN supports the following AVR-specific commands:

info io_registers
     This command displays information about the AVR I/O registers.  For each register, GDBN prints its number and value.

CRIS
----

When configured for debugging CRIS, GDBN provides the following CRIS-specific commands:

set cris-version VER
     Set the current CRIS version to VER, either 10 or 32. The CRIS version affects register names and sizes.  This command is useful in case autodetection of the CRIS version fails.

show cris-version
     Show the current CRIS version.

set cris-dwarf2-cfi
     Set the usage of DWARF-2 CFI for CRIS debugging.  The default is on. Change to off when using gcc-cris whose version is below R59.

show cris-dwarf2-cfi
     Show the current state of using DWARF-2 CFI.

set cris-mode MODE
     Set the current CRIS mode to MODE.  It should only be changed when debugging in guru mode, in which case it should be set to guru (the default is normal).

show cris-mode
     Show the current CRIS mode.

Renesas Super-H
---------------

For the Renesas Super-H processor, GDBN provides these commands:

regs
     Show the values of all Super-H registers.

set sh calling-convention CONVENTION
     Set the calling-convention used when calling functions from GDBN. Allowed values are gcc, which is the default setting, and renesas. With the gcc setting, functions are called using the NGCC calling convention.  If the DWARF-2 information of the called function specifies that the function follows the Renesas calling convention, the function is called using the Renesas calling convention.  If the calling convention is set to renesas, the Renesas calling convention is always used, regardless of the DWARF-2 information.  This can be used to override the default of gcc if debug information is missing, or the compiler does not emit the DWARF-2 calling convention entry for a function.

show sh calling-convention
     Show the current calling convention setting.

Architectures
=============

This section describes characteristics of architectures that affect all uses of GDBN with the architecture, both native and cross.

x86 Architecture-specific Issues
--------------------------------

set struct-convention MODE
     Set the convention used by the inferior to return structs and unions from functions to MODE.  Possible values of MODE are "pcc", "reg", and "default" (the default).  "default" or "pcc" means that structs are returned on the stack, while "reg" means that a struct or a union whose size is 1, 2, 4, or 8 bytes will be returned in a register.

show struct-convention
     Show the current setting of the convention to return structs from functions.

A29K
----

set rstack_high_address ADDRESS
     On AMD 29000 family processors, registers are saved in a separate register stack.  There is no way for GDBN to determine the extent of this stack.  Normally, GDBN just assumes that the stack is ``large enough''.  This may result in GDBN referencing memory locations that do not exist.  If necessary, you can get around this problem by specifying the ending address of the register stack with the set rstack_high_address command.  The argument should be an address, which you probably want to precede with 0x to specify in hexadecimal.

show rstack_high_address
     Display the current limit of the register stack, on AMD 29000 family processors.

Alpha
-----

See the following section.

MIPS
----

Alpha- and MIPS-based computers use an unusual stack frame, which sometimes requires GDBN to search backward in the object code to find the beginning of a function.

To improve response time (especially for embedded applications, where GDBN may be restricted to a slow serial line for this search) you may want to limit the size of this search, using one of these commands:

set heuristic-fence-post LIMIT
     Restrict GDBN to examining at most LIMIT bytes in its search for the beginning of a function.  A value of 0 (the default) means there is no limit.  However, except for 0, the larger the limit the more bytes heuristic-fence-post must search and therefore the longer it takes to run.  You should only need to use this command when debugging a stripped executable.

show heuristic-fence-post
     Display the current limit.

These commands are available only when GDBN is configured for debugging programs on Alpha or MIPS processors.

Several MIPS-specific commands are available when debugging MIPS programs:

set mips abi ARG
     Tell GDBN which MIPS ABI is used by the inferior.  Possible values of ARG are:

     auto
          The default ABI associated with the current binary (this is the default).

     o32

     o64

     n32

     n64

     eabi32

     eabi64

show mips abi
     Show the MIPS ABI used by GDBN to debug the inferior.

set mipsfpu
show mipsfpu
     See set mipsfpu.

set mips mask-address ARG
     This command determines whether the most-significant 32 bits of 64-bit MIPS addresses are masked off.  The argument ARG can be on, off, or auto.  The latter is the default setting, which lets GDBN determine the correct value.

show mips mask-address
     Show whether the upper 32 bits of MIPS addresses are masked off or not.

set remote-mips64-transfers-32bit-regs
     This command controls compatibility with 64-bit MIPS targets that transfer data in 32-bit quantities.  If you have an old MIPS 64 target that transfers 32 bits for some registers, like SR and FSR, and 64 bits for other registers, set this option to on.

show remote-mips64-transfers-32bit-regs
     Show the current setting of compatibility with older MIPS 64 targets.

set debug mips
     This command turns on and off debugging messages for the MIPS-specific target code in GDBN.

show debug mips
     Show the current setting of MIPS debugging messages.

HPPA
----

When GDBN is debugging the HP PA architecture, it provides the following special commands:

set debug hppa
     This command determines whether HPPA architecture-specific debugging messages are to be displayed.

show debug hppa
     Show whether HPPA debugging messages are displayed.

maint print unwind ADDRESS
     This command displays the contents of the unwind table entry at the given ADDRESS.

Cell Broadband Engine SPU architecture
--------------------------------------

When GDBN is debugging the Cell Broadband Engine SPU architecture, it provides the following special commands:

info spu event
     Display SPU event facility status.  Shows current event mask and pending event status.

info spu signal
     Display SPU signal notification facility status.  Shows pending signal-control word and signal notification mode of both signal notification channels.

info spu mailbox
     Display SPU mailbox facility status.  Shows all pending entries, in order of processing, in each of the SPU Write Outbound, SPU Write Outbound Interrupt, and SPU Read Inbound mailboxes.

info spu dma
     Display MFC DMA status.  Shows all pending commands in the MFC DMA queue.  For each entry, opcode, tag, class IDs, effective and local store addresses and transfer size are shown.

info spu proxydma
     Display MFC Proxy-DMA status.  Shows all pending commands in the MFC Proxy-DMA queue.  For each entry, opcode, tag, class IDs, effective and local store addresses and transfer size are shown.

When GDBN is debugging a combined PowerPC/SPU application on the Cell Broadband Engine, it provides in addition the following special commands:

set spu stop-on-load ARG
     Set whether to stop for new SPE threads.  When set to on, GDBN will give control to the user when a new SPE thread enters its main function.  The default is off.

show spu stop-on-load
     Show whether to stop for new SPE threads.

set spu auto-flush-cache ARG
     Set whether to automatically flush the software-managed cache.  When set to on, GDBN will automatically cause the SPE software-managed cache to be flushed whenever SPE execution stops.  This provides a consistent view of PowerPC memory that is accessed via the cache.  If an application does not use the software-managed cache, this option has no effect.

show spu auto-flush-cache
     Show whether to automatically flush the software-managed cache.

PowerPC
-------

When GDBN is debugging the PowerPC architecture, it provides a set of pseudo-registers to enable inspection of 128-bit wide Decimal Floating Point numbers stored in the floating point registers. These values must be stored in two consecutive registers, always starting at an even register like f0 or f2.

The pseudo-registers go from $dl0 through $dl15, and are formed by joining the even/odd register pairs f0 and f1 for $dl0, f2 and f3 for $dl1 and so on.

For POWER7 processors, GDBN provides a set of pseudo-registers, the 64-bit wide Extended Floating Point Registers (f32 through f63).

Controlling GDBN
****************

You can alter the way GDBN interacts with you by using the set command.  For commands controlling how GDBN displays data, see Print Settings.  Other settings are described here.

Prompt
======

GDBN indicates its readiness to read a command by printing a string called the prompt.  This string is normally (GDBP).  You can change the prompt string with the set prompt command.  For instance, when debugging GDBN with GDBN, it is useful to change the prompt in one of the GDBN sessions so that you can always tell which one you are talking to.

Note:  set prompt does not add a space for you after the prompt you set.  This allows you to set a prompt which ends in a space or a prompt that does not.

set prompt NEWPROMPT
     Directs GDBN to use NEWPROMPT as its prompt string henceforth.

show prompt
     Prints a line of the form: Gdb's prompt is: YOUR-PROMPT

Versions of GDBN that ship with Python scripting enabled have prompt extensions.  The commands for interacting with these extensions are:

set extended-prompt PROMPT
     Set an extended prompt that allows for substitutions. See gdb.prompt, for a list of escape sequences that can be used for substitution.  Any escape sequences specified as part of the prompt string are replaced with the corresponding strings each time the prompt is displayed.

     For example:

          set extended-prompt Current working directory: \w (gdb)

     Note that when an extended-prompt is set, it takes control of the PROMPT_HOOK hook.  See prompt_hook, for further information.

show extended-prompt
     Prints the extended prompt.  Any escape sequences specified as part of the prompt string with set extended-prompt, are replaced with the corresponding strings each time the prompt is displayed.

Command Editing
===============

GDBN reads its input commands via the Readline interface.  This GNU library provides consistent behavior for programs which provide a command line interface to the user.  Advantages are GNU Emacs-style or vi-style inline editing of commands, csh-like history substitution, and a storage and recall of command history across debugging sessions.

You may control the behavior of command line editing in GDBN with the command set.

set editing
set editing on
     Enable command line editing (enabled by default).

set editing off
     Disable command line editing.

show editing
     Show whether command line editing is enabled.

See Command Line Editing, for more details about the Readline interface.  Users unfamiliar with GNU Emacs or vi are encouraged to read that chapter.

Command History
===============

GDBN can keep track of the commands you type during your debugging sessions, so that you can be certain of precisely what happened.  Use these commands to manage the GDBN command history facility.

GDBN uses the GNU History library, a part of the Readline package, to provide the history facility. See Using History Interactively, for the detailed description of the History library.

To issue a command to GDBN without affecting certain aspects of the state which is seen by users, prefix it with server (see Server Prefix).  This means that this command will not affect the command history, nor will it affect GDBN's notion of which command to repeat if RET is pressed on a line by itself.

The server prefix does not affect the recording of values into the value history; to print a value without recording it into the value history, use the output command instead of the print command.

Here is the description of GDBN commands related to command history.

set history filename FNAME
     Set the name of the GDBN command history file to FNAME. This is the file where GDBN reads an initial command history list, and where it writes the command history from this session when it exits.  You can access this list through history expansion or through the history command editing characters listed below.  This file defaults to the value of the environment variable GDBHISTFILE, or to ./.gdb_history (./_gdb_history on MS-DOS) if this variable is not set.

set history save
set history save on
     Record command history in a file, whose name may be specified with the set history filename command.  By default, this option is disabled.

set history save off
     Stop recording command history in a file.

set history size SIZE
     Set the number of commands which GDBN keeps in its history list. This defaults to the value of the environment variable HISTSIZE, or to 256 if this variable is not set.

History expansion assigns special meaning to the character !. See Event Designators, for more details.

Since ! is also the logical not operator in C, history expansion is off by default. If you decide to enable history expansion with the set history expansion on command, you may sometimes need to follow ! (when it is used as logical not, in an expression) with a space or a tab to prevent it from being expanded.  The readline history facilities do not attempt substitution on the strings != and !(, even when history expansion is enabled.

The commands to control history expansion are:

set history expansion on
set history expansion
     Enable history expansion.  History expansion is off by default.

set history expansion off
     Disable history expansion.

show history
show history filename
show history save
show history size
show history expansion
     These commands display the state of the GDBN history parameters. show history by itself displays all four states.

show commands
     Display the last ten commands in the command history.

show commands N
     Print ten commands centered on command number N.

show commands +
     Print ten commands just after the commands last printed.

Screen Size
===========

Certain commands to GDBN may produce large amounts of information output to the screen.  To help you read all of it, GDBN pauses and asks you for input at the end of each page of output.  Type RET when you want to continue the output, or q to discard the remaining output.  Also, the screen width setting determines when to wrap lines of output.  Depending on what is being printed, GDBN tries to break the line at a readable place, rather than simply letting it overflow onto the following line.

Normally GDBN knows the size of the screen from the terminal driver software.  For example, on Unix GDBN uses the termcap data base together with the value of the TERM environment variable and the stty rows and stty cols settings.  If this is not correct, you can override it with the set height and set width commands:

set height LPP
show height
set width CPL
show width
     These set commands specify a screen height of LPP lines and a screen width of CPL characters.  The associated show commands display the current settings.

     If you specify a height of zero lines, GDBN does not pause during output no matter how long the output is.  This is useful if output is to a file or to an editor buffer.

     Likewise, you can specify set width 0 to prevent GDBN from wrapping its output.

set pagination on
set pagination off
     Turn the output pagination on or off; the default is on.  Turning pagination off is the alternative to set height 0.  Note that running GDBN with the --batch option (see -batch) also automatically disables pagination.

show pagination
     Show the current pagination mode.

Numbers
=======

You can always enter numbers in octal, decimal, or hexadecimal in GDBN by the usual conventions: octal numbers begin with 0, decimal numbers end with ., and hexadecimal numbers begin with 0x.  Numbers that neither begin with 0 or 0x, nor end with a . are, by default, entered in base 10; likewise, the default display for numbers---when no particular format is specified---is base 10.  You can change the default base for both input and output with the commands described below.

set input-radix BASE
     Set the default base for numeric input.  Supported choices for BASE are decimal 8, 10, or 16.  BASE must itself be specified either unambiguously or using the current input radix; for example, any of

          set input-radix 012
          set input-radix 10.
          set input-radix 0xa

     sets the input base to decimal.  On the other hand, set input-radix 10 leaves the input radix unchanged, no matter what it was, since 10, being without any leading or trailing signs of its base, is interpreted in the current radix.  Thus, if the current radix is 16, 10 is interpreted in hex, i.e. as 16 decimal, which doesn't change the radix.

set output-radix BASE
     Set the default base for numeric display.  Supported choices for BASE are decimal 8, 10, or 16.  BASE must itself be specified either unambiguously or using the current input radix.

show input-radix
     Display the current default base for numeric input.

show output-radix
     Display the current default base for numeric display.

set radix [BASE]
show radix
     These commands set and show the default base for both input and output of numbers.  set radix sets the radix of input and output to the same base; without an argument, it resets the radix back to its default value of 10.

Configuring the Current ABI
===========================

GDBN can determine the ABI (Application Binary Interface) of your application automatically.  However, sometimes you need to override its conclusions.  Use these commands to manage GDBN's view of the current ABI.

One GDBN configuration can debug binaries for multiple operating system targets, either via remote debugging or native emulation. GDBN will autodetect the OS ABI (Operating System ABI) in use, but you can override its conclusion using the set osabi command. One example where this is useful is in debugging of binaries which use an alternate C library (e.g. UCLIBC for GNU/Linux) which does not have the same identifying marks that the standard C library for your platform provides.

show osabi
     Show the OS ABI currently in use.

set osabi
     With no argument, show the list of registered available OS ABI's.

set osabi ABI
     Set the current OS ABI to ABI.

Generally, the way that an argument of type float is passed to a function depends on whether the function is prototyped.  For a prototyped (i.e. ANSI/ISO style) function, float arguments are passed unchanged, according to the architecture's convention for float.  For unprototyped (i.e. K&R style) functions, float arguments are first promoted to type double and then passed.

Unfortunately, some forms of debug information do not reliably indicate whether a function is prototyped.  If GDBN calls a function that is not marked as prototyped, it consults set coerce-float-to-double.

set coerce-float-to-double
set coerce-float-to-double on
     Arguments of type float will be promoted to double when passed to an unprototyped function.  This is the default setting.

set coerce-float-to-double off
     Arguments of type float will be passed directly to unprototyped functions.

show coerce-float-to-double
     Show the current setting of promoting float to double.

GDBN needs to know the ABI used for your program's C++ objects.  The correct C++ ABI depends on which C++ compiler was used to build your application.  GDBN only fully supports programs with a single C++ ABI; if your program contains code using multiple C++ ABI's or if GDBN can not identify your program's ABI correctly, you can tell GDBN which ABI to use. Currently supported ABI's include ``gnu-v2'', for g++ versions before 3.0, ``gnu-v3'', for g++ versions 3.0 and later, and ``hpaCC'' for the HP ANSI C++ compiler.  Other C++ compilers may use the ``gnu-v2'' or ``gnu-v3'' ABI's as well.  The default setting is ``auto''.

show cp-abi
     Show the C++ ABI currently in use.

set cp-abi
     With no argument, show the list of supported C++ ABI's.

set cp-abi ABI
set cp-abi auto
     Set the current C++ ABI to ABI, or return to automatic detection.

Optional Warnings and Messages
==============================

By default, GDBN is silent about its inner workings.  If you are running on a slow machine, you may want to use the set verbose command.  This makes GDBN tell you when it does a lengthy internal operation, so you will not think it has crashed.

Currently, the messages controlled by set verbose are those which announce that the symbol table for a source file is being read; see symbol-file in Commands to Specify Files.

set verbose on
     Enables GDBN output of certain informational messages.

set verbose off
     Disables GDBN output of certain informational messages.

show verbose
     Displays whether set verbose is on or off.

By default, if GDBN encounters bugs in the symbol table of an object file, it is silent; but if you are debugging a compiler, you may find this information useful (see Errors Reading Symbol Files).

set complaints LIMIT
     Permits GDBN to output LIMIT complaints about each type of unusual symbols before becoming silent about the problem.  Set LIMIT to zero to suppress all complaints; set it to a large number to prevent complaints from being suppressed.

show complaints
     Displays how many symbol complaints GDBN is permitted to produce.

By default, GDBN is cautious, and asks what sometimes seems to be a lot of stupid questions to confirm certain commands.  For example, if you try to run a program which is already running:

     (GDBP) run
     The program being debugged has been started already.
     Start it from the beginning? (y or n)

If you are willing to unflinchingly face the consequences of your own commands, you can disable this ``feature'':

set confirm off
     Disables confirmation requests.  Note that running GDBN with the --batch option (see -batch) also automatically disables confirmation requests.

set confirm on
     Enables confirmation requests (the default).

show confirm
     Displays state of confirmation requests.

If you need to debug user-defined commands or sourced files you may find it useful to enable command tracing.  In this mode each command will be printed as it is executed, prefixed with one or more + symbols, the quantity denoting the call depth of each command.

set trace-commands on
     Enable command tracing.

set trace-commands off
     Disable command tracing.

show trace-commands
     Display the current state of command tracing.

Optional Messages about Internal Happenings
===========================================

GDBN has commands that enable optional debugging messages from various GDBN subsystems; normally these commands are of interest to GDBN maintainers, or when reporting a bug.  This section documents those commands.

set exec-done-display
     Turns on or off the notification of asynchronous commands' completion.  When on, GDBN will print a message when an asynchronous command finishes its execution.  The default is off.

show exec-done-display
     Displays the current setting of asynchronous command completion notification.

set debug arch
     Turns on or off display of gdbarch debugging info.  The default is off

show debug arch
     Displays the current state of displaying gdbarch debugging info.

set debug aix-thread
     Display debugging messages about inner workings of the AIX thread module.

show debug aix-thread
     Show the current state of AIX thread debugging info display.

set debug check-physname
     Check the results of the ``physname'' computation.  When reading DWARF debugging information for C++, GDBN attempts to compute each entity's name.  GDBN can do this computation in two different ways, depending on exactly what information is present. When enabled, this setting causes GDBN to compute the names both ways and display any discrepancies.

show debug check-physname
     Show the current state of ``physname'' checking.

set debug dwarf2-die
     Dump DWARF2 DIEs after they are read in. The value is the number of nesting levels to print. A value of zero turns off the display.

show debug dwarf2-die
     Show the current state of DWARF2 DIE debugging.

set debug displaced
     Turns on or off display of GDBN debugging info for the displaced stepping support.  The default is off.

show debug displaced
     Displays the current state of displaying GDBN debugging info related to displaced stepping.

set debug event
     Turns on or off display of GDBN event debugging info.  The default is off.

show debug event
     Displays the current state of displaying GDBN event debugging info.

set debug expression
     Turns on or off display of debugging info about GDBN expression parsing.  The default is off.

show debug expression
     Displays the current state of displaying debugging info about GDBN expression parsing.

set debug frame
     Turns on or off display of GDBN frame debugging info.  The default is off.

show debug frame
     Displays the current state of displaying GDBN frame debugging info.

set debug gnu-nat
     Turns on or off debugging messages from the GNU/Hurd debug support.

show debug gnu-nat
     Show the current state of GNU/Hurd debugging messages.

set debug infrun
     Turns on or off display of GDBN debugging info for running the inferior. The default is off.  infrun.c contains GDB's runtime state machine used for implementing operations such as single-stepping the inferior.

show debug infrun
     Displays the current state of GDBN inferior debugging.

set debug jit
     Turns on or off debugging messages from JIT debug support.

show debug jit
     Displays the current state of GDBN JIT debugging.

set debug lin-lwp
     Turns on or off debugging messages from the Linux LWP debug support.

show debug lin-lwp
     Show the current state of Linux LWP debugging messages.

set debug observer
     Turns on or off display of GDBN observer debugging.  This includes info such as the notification of observable events.

show debug observer
     Displays the current state of observer debugging.

set debug overload
     Turns on or off display of GDBN C++ overload debugging info. This includes info such as ranking of functions, etc.  The default is off.

show debug overload
     Displays the current state of displaying GDBN C++ overload debugging info.

set debug parser
     Turns on or off the display of expression parser debugging output. Internally, this sets the yydebug variable in the expression parser.  See Tracing Your Parser (Bison), for details.  The default is off.

show debug parser
     Show the current state of expression parser debugging.

set debug remote
     Turns on or off display of reports on all packets sent back and forth across the serial line to the remote machine.  The info is printed on the GDBN standard output stream. The default is off.

show debug remote
     Displays the state of display of remote packets.

set debug serial
     Turns on or off display of GDBN serial debugging info. The default is off.

show debug serial
     Displays the current state of displaying GDBN serial debugging info.

set debug solib-frv
     Turns on or off debugging messages for FR-V shared-library code.

show debug solib-frv
     Display the current state of FR-V shared-library code debugging messages.

set debug target
     Turns on or off display of GDBN target debugging info. This info includes what is going on at the target level of GDB, as it happens. The default is 0.  Set it to 1 to track events, and to 2 to also track the value of large memory transfers.  Changes to this flag do not take effect until the next time you connect to a target or use the run command.

show debug target
     Displays the current state of displaying GDBN target debugging info.

set debug timestamp
     Turns on or off display of timestamps with GDBN debugging info. When enabled, seconds and microseconds are displayed before each debugging message.

show debug timestamp
     Displays the current state of displaying timestamps with GDBN debugging info.

set debugvarobj
     Turns on or off display of GDBN variable object debugging info. The default is off.

show debugvarobj
     Displays the current state of displaying GDBN variable object debugging info.

set debug xml
     Turns on or off debugging messages for built-in XML parsers.

show debug xml
     Displays the current state of XML debugging messages.

Other Miscellaneous Settings
============================

set interactive-mode
     If on, forces GDBN to assume that GDB was started in a terminal.  In practice, this means that GDBN should wait for the user to answer queries generated by commands entered at the command prompt.  If off, forces GDBN to operate in the opposite mode, and it uses the default answers to all queries. If auto (the default), GDBN tries to determine whether its standard input is a terminal, and works in interactive-mode if it is, non-interactively otherwise.

     In the vast majority of cases, the debugger should be able to guess correctly which mode should be used.  But this setting can be useful in certain specific cases, such as running a MinGW GDBN inside a cygwin window.

show interactive-mode
     Displays whether the debugger is operating in interactive mode or not.

Extending GDBN
**************

GDBN provides three mechanisms for extension.  The first is based on composition of GDBN commands, the second is based on the Python scripting language, and the third is for defining new aliases of existing commands.

To facilitate the use of the first two extensions, GDBN is capable of evaluating the contents of a file.  When doing so, GDBN can recognize which scripting language is being used by looking at the filename extension.  Files with an unrecognized filename extension are always treated as a GDBN Command Files. See Command files.

You can control how GDBN evaluates these files with the following setting:

set script-extension off
     All scripts are always evaluated as GDBN Command Files.

set script-extension soft
     The debugger determines the scripting language based on filename extension.  If this scripting language is supported, GDBN evaluates the script using that language.  Otherwise, it evaluates the file as a GDBN Command File.

set script-extension strict
     The debugger determines the scripting language based on filename extension, and evaluates the script using that language.  If the language is not supported, then the evaluation fails.

show script-extension
     Display the current value of the script-extension option.

Canned Sequences of Commands
============================

Aside from breakpoint commands (see Breakpoint Command Lists), GDBN provides two ways to store sequences of commands for execution as a unit: user-defined commands and command files.

User-defined Commands
---------------------

A user-defined command is a sequence of GDBN commands to which you assign a new name as a command.  This is done with the define command.  User commands may accept up to 10 arguments separated by whitespace.  Arguments are accessed within the user command via $arg0...$arg9.  A trivial example:

     define adder
       print $arg0 + $arg1 + $arg2
     end

To execute the command use:

     adder 1 2 3

This defines the command adder, which prints the sum of its three arguments.  Note the arguments are text substitutions, so they may reference variables, use complex expressions, or even perform inferior functions calls.

In addition, $argc may be used to find out how many arguments have been passed.  This expands to a number in the range 0...10.

     define adder
       if $argc == 2
         print $arg0 + $arg1
       end
       if $argc == 3
         print $arg0 + $arg1 + $arg2
       end
     end

define COMMANDNAME
     Define a command named COMMANDNAME.  If there is already a command by that name, you are asked to confirm that you want to redefine it. COMMANDNAME may be a bare command name consisting of letters, numbers, dashes, and underscores.  It may also start with any predefined prefix command.  For example, define target my-target creates a user-defined target my-target command.

     The definition of the command is made up of other GDBN command lines, which are given following the define command.  The end of these commands is marked by a line containing end.

document COMMANDNAME
     Document the user-defined command COMMANDNAME, so that it can be accessed by help.  The command COMMANDNAME must already be defined.  This command reads lines of documentation just as define reads the lines of the command definition, ending with end. After the document command is finished, help on command COMMANDNAME displays the documentation you have written.

     You may use the document command again to change the documentation of a command.  Redefining the command with define does not change the documentation.

dont-repeat
     Used inside a user-defined command, this tells GDBN that this command should not be repeated when the user hits RET (see repeat last command).

help user-defined
     List all user-defined commands, with the first line of the documentation (if any) for each.

show user
show user COMMANDNAME
     Display the GDBN commands used to define COMMANDNAME (but not its documentation).  If no COMMANDNAME is given, display the definitions for all user-defined commands.

show max-user-call-depth
set max-user-call-depth
     The value of max-user-call-depth controls how many recursion levels are allowed in user-defined commands before GDBN suspects an infinite recursion and aborts the command.

In addition to the above commands, user-defined commands frequently use control flow commands, described in Command Files.

When user-defined commands are executed, the commands of the definition are not printed.  An error in any command stops execution of the user-defined command.

If used interactively, commands that would ask for confirmation proceed without asking when used inside a user-defined command.  Many GDBN commands that normally print messages to say what they are doing omit the messages when used in a user-defined command.

User-defined Command Hooks
--------------------------

You may define hooks, which are a special kind of user-defined command.  Whenever you run the command foo, if the user-defined command hook-foo exists, it is executed (with no arguments) before that command.

A hook may also be defined which is run after the command you executed. Whenever you run the command foo, if the user-defined command hookpost-foo exists, it is executed (with no arguments) after that command.  Post-execution hooks may exist simultaneously with pre-execution hooks, for the same command.

It is valid for a hook to call the command which it hooks.  If this occurs, the hook is not re-executed, thereby avoiding infinite recursion.

In addition, a pseudo-command, stop exists.  Defining (hook-stop) makes the associated commands execute every time execution stops in your program: before breakpoint commands are run, displays are printed, or the stack frame is printed.

For example, to ignore SIGALRM signals while single-stepping, but treat them normally during normal execution, you could define:

     define hook-stop
     handle SIGALRM nopass
     end

     define hook-run
     handle SIGALRM pass
     end

     define hook-continue
     handle SIGALRM pass
     end

As a further example, to hook at the beginning and end of the echo command, and to add extra text to the beginning and end of the message, you could define:

     define hook-echo
     echo <<<---
     end

     define hookpost-echo
     echo --->>>\n
     end

     (GDBP) echo Hello World
     <<<---Hello World--->>>
     (GDBP)

You can define a hook for any single-word command in GDBN, but not for command aliases; you should define a hook for the basic command name, e.g.  backtrace rather than bt. You can hook a multi-word command by adding hook- or hookpost- to the last word of the command, e.g. define target hook-remote to add a hook to target remote.

If an error occurs during the execution of your hook, execution of GDBN commands stops and GDBN issues a prompt (before the command that you actually typed had a chance to run).

If you try to define a hook which does not match any known command, you get a warning from the define command.

Command Files
-------------

A command file for GDBN is a text file made of lines that are GDBN commands.  Comments (lines starting with #) may also be included.  An empty line in a command file does nothing; it does not mean to repeat the last command, as it would from the terminal.

You can request the execution of a command file with the source command.  Note that the source command is also used to evaluate scripts that are not Command Files.  The exact behavior can be configured using the script-extension setting. See Extending GDB.

source [-s] [-v] FILENAME
     Execute the command file FILENAME.

The lines in a command file are generally executed sequentially, unless the order of execution is changed by one of the flow-control commands described below.  The commands are not printed as they are executed.  An error in any command terminates execution of the command file and control is returned to the console.

GDBN first searches for FILENAME in the current directory. If the file is not found there, and FILENAME does not specify a directory, then GDBN also looks for the file on the source search path (specified with the directory command); except that $cdir is not searched because the compilation directory is not relevant to scripts.

If -s is specified, then GDBN searches for FILENAME on the search path even if FILENAME specifies a directory. The search is done by appending FILENAME to each element of the search path.  So, for example, if FILENAME is mylib/myscript and the search path contains /home/user then GDBN will look for the script /home/user/mylib/myscript. The search is also done if FILENAME is an absolute path. For example, if FILENAME is /tmp/myscript and the search path contains /home/user then GDBN will look for the script /home/user/tmp/myscript. For DOS-like systems, if FILENAME contains a drive specification, it is stripped before concatenation.  For example, if FILENAME is d:myscript and the search path contains c:/tmp then GDBN will look for the script c:/tmp/myscript.

If -v, for verbose mode, is given then GDBN displays each command as it is executed.  The option must be given before FILENAME, and is interpreted as part of the filename anywhere else.

Commands that would ask for confirmation if used interactively proceed without asking when used in a command file.  Many GDBN commands that normally print messages to say what they are doing omit the messages when called from command files.

GDBN also accepts command input from standard input.  In this mode, normal output goes to standard output and error output goes to standard error.  Errors in a command file supplied on standard input do not terminate execution of the command file---execution continues with the next command.

     gdb < cmds > log 2>&1

(The syntax above will vary depending on the shell used.) This example will execute commands from the file cmds. All output and errors would be directed to log.

Since commands stored on command files tend to be more general than commands typed interactively, they frequently need to deal with complicated situations, such as different or unexpected values of variables and symbols, changes in how the program being debugged is built, etc.  GDBN provides a set of flow-control commands to deal with these complexities.  Using these commands, you can write complex scripts that loop over data structures, execute commands conditionally, etc.

if
else
     This command allows to include in your script conditionally executed commands. The if command takes a single argument, which is an expression to evaluate.  It is followed by a series of commands that are executed only if the expression is true (its value is nonzero). There can then optionally be an else line, followed by a series of commands that are only executed if the expression was false.  The end of the list is marked by a line containing end.

while
     This command allows to write loops.  Its syntax is similar to if: the command takes a single argument, which is an expression to evaluate, and must be followed by the commands to execute, one per line, terminated by an end.  These commands are called the body of the loop.  The commands in the body of while are executed repeatedly as long as the expression evaluates to true.

loop_break
     This command exits the while loop in whose body it is included. Execution of the script continues after that whiles end line.

loop_continue
     This command skips the execution of the rest of the body of commands in the while loop in whose body it is included.  Execution branches to the beginning of the while loop, where it evaluates the controlling expression.

end
     Terminate the block of commands that are the body of if, else, or while flow-control commands.

Commands for Controlled Output
------------------------------

During the execution of a command file or a user-defined command, normal GDBN output is suppressed; the only output that appears is what is explicitly printed by the commands in the definition.  This section describes three commands useful for generating exactly the output you want.

echo TEXT
     Print TEXT.  Nonprinting characters can be included in TEXT using C escape sequences, such as \n to print a newline.  No newline is printed unless you specify one. In addition to the standard C escape sequences, a backslash followed by a space stands for a space.  This is useful for displaying a string with spaces at the beginning or the end, since leading and trailing spaces are otherwise trimmed from all arguments. To print  and foo = , use the command echo \ and foo = \ .

     A backslash at the end of TEXT can be used, as in C, to continue the command onto subsequent lines.  For example,

          echo This is some text\n\
          which is continued\n\
          onto several lines.\n

     produces the same output as

          echo This is some text\n
          echo which is continued\n
          echo onto several lines.\n

output EXPRESSION
     Print the value of EXPRESSION and nothing but that value: no newlines, no $NN = .  The value is not entered in the value history either.  See Expressions, for more information on expressions.

output/FMT EXPRESSION
     Print the value of EXPRESSION in format FMT.  You can use the same formats as for print.  See Output Formats, for more information.

printf TEMPLATE, EXPRESSIONS...
     Print the values of one or more EXPRESSIONS under the control of the string TEMPLATE.  To print several values, make EXPRESSIONS be a comma-separated list of individual expressions, which may be either numbers or pointers.  Their values are printed as specified by TEMPLATE, exactly as a C program would do by executing the code below:

          printf (TEMPLATE, EXPRESSIONS...);

     As in C printf, ordinary characters in TEMPLATE are printed verbatim, while conversion specification introduced by the % character cause subsequent EXPRESSIONS to be evaluated, their values converted and formatted according to type and style information encoded in the conversion specifications, and then printed.

     For example, you can print two values in hex like this:

          printf "foo, bar-foo = 0x%x, 0x%x\n", foo, bar-foo

     printf supports all the standard C conversion specifications, including the flags and modifiers between the % character and the conversion letter, with the following exceptions:

       * The argument-ordering modifiers, such as 2$, are not supported.

       * The modifier * is not supported for specifying precision or width.

       * The ' flag (for separation of digits into groups according to LC_NUMERIC') is not supported.

       * The type modifiers hh, j, t, and z are not supported.

       * The conversion letter n (as in %n) is not supported.

       * The conversion letters a and A are not supported.

     Note that the ll type modifier is supported only if the underlying C implementation used to build GDBN supports the long long int type, and the L type modifier is supported only if long double type is available.

     As in C, printf supports simple backslash-escape sequences, such as \n, \t, \\, \", \a, and \f, that consist of backslash followed by a single character.  Octal and hexadecimal escape sequences are not supported.

     Additionally, printf supports conversion specifications for DFP (Decimal Floating Point) types using the following length modifiers together with a floating point specifier. letters:

       * H for printing Decimal32 types.

       * D for printing Decimal64 types.

       * DD for printing Decimal128 types.

     If the underlying C implementation used to build GDBN has support for the three length modifiers for DFP types, other modifiers such as width and precision will also be available for GDBN to use.

     In case there is no such C support, no additional modifiers will be available and the value will be printed in the standard way.

     Here's an example of printing DFP types using the above conversion letters:

          printf "D32: %Hf - D64: %Df - D128: %DDf\n",1.2345df,1.2E10dd,1.2E1dl

eval TEMPLATE, EXPRESSIONS...
     Convert the values of one or more EXPRESSIONS under the control of the string TEMPLATE to a command line, and call it.

Scripting GDBN using Python
===========================

You can script GDBN using the Python programming language (http://www.python.org/).  This feature is available only if GDBN was configured using --with-python.

Python scripts used by GDBN should be installed in DATA-DIRECTORY/python, where DATA-DIRECTORY is the data directory as determined at GDBN startup (see Data Files). This directory, known as the python directory, is automatically added to the Python Search Path in order to allow the Python interpreter to locate all scripts installed at this location.

Additionally, GDBN commands and convenience functions which are written in Python and are located in the DATA-DIRECTORY/python/gdb/command or DATA-DIRECTORY/python/gdb/function directories are automatically imported when GDBN starts.

Python Commands
---------------

GDBN provides one command for accessing the Python interpreter, and one related setting:

python [CODE]
     The python command can be used to evaluate Python code.

     If given an argument, the python command will evaluate the argument as a Python command.  For example:

          (GDBP) python print 23
          23

     If you do not provide an argument to python, it will act as a multi-line command, like define.  In this case, the Python script is made up of subsequent command lines, given after the python command.  This command list is terminated using a line containing end.  For example:

          (GDBP) python
          Type python script
          End with a line saying just "end".
          >print 23
          >end
          23

set python print-stack
     By default, GDBN will print only the message component of a Python exception when an error occurs in a Python script.  This can be controlled using set python print-stack: if full, then full Python stack printing is enabled; if none, then Python stack and message printing is disabled; if message, the default, only the message component of the error is printed.

It is also possible to execute a Python script from the GDBN interpreter:

source script-name
     The script name must end with .py and GDBN must be configured to recognize the script language based on filename extension using the script-extension setting.  See Extending GDB.

python execfile ("script-name")
     This method is based on the execfile Python built-in function, and thus is always available.

Python API
----------

At startup, GDBN overrides Python's sys.stdout and sys.stderr to print using GDBN's output-paging streams. A Python program which outputs to one of these streams may have its output interrupted by the user (see Screen Size).  In this situation, a Python KeyboardInterrupt exception is thrown.

Basic Python
............

GDBN introduces a new Python module, named gdb.  All methods and classes added by GDBN are placed in this module. GDBN automatically imports the gdb module for use in all scripts evaluated by the python command.

 -- Variable: gdb.PYTHONDIR
     A string containing the python directory (see Python).

 -- Function: gdb.execute (command [, from_tty [, to_string]])
     Evaluate COMMAND, a string, as a GDBN CLI command. If a GDB exception happens while COMMAND runs, it is translated as described in Exception Handling.

     FROM_TTY specifies whether GDBN ought to consider this command as having originated from the user invoking it interactively. It must be a boolean value.  If omitted, it defaults to False.

     By default, any output produced by COMMAND is sent to GDBN's standard output.  If the TO_STRING parameter is True, then output will be collected by gdb.execute and returned as a string.  The default is False, in which case the return value is None.  If TO_STRING is True, the GDBN virtual terminal will be temporarily set to unlimited width and height, and its pagination will be disabled; see Screen Size.

 -- Function: gdb.breakpoints ()
     Return a sequence holding all of GDBN's breakpoints. See Breakpoints In Python, for more information.

 -- Function: gdb.parameter (parameter)
     Return the value of a GDBN parameter.  PARAMETER is a string naming the parameter to look up; PARAMETER may contain spaces if the parameter has a multi-part name.  For example, print object is a valid parameter name.

     If the named parameter does not exist, this function throws a gdb.error (see Exception Handling).  Otherwise, the parameter's value is converted to a Python value of the appropriate type, and returned.

 -- Function: gdb.history (number)
     Return a value from GDBN's value history (see Value History).  NUMBER indicates which history element to return. If NUMBER is negative, then GDBN will take its absolute value and count backward from the last element (i.e., the most recent element) to find the value to return.  If NUMBER is zero, then GDBN will return the most recent element.  If the element specified by NUMBER doesn't exist in the value history, a gdb.error exception will be raised.

     If no exception is raised, the return value is always an instance of gdb.Value (see Values From Inferior).

 -- Function: gdb.parse_and_eval (expression)
     Parse EXPRESSION as an expression in the current language, evaluate it, and return the result as a gdb.Value. EXPRESSION must be a string.

     This function can be useful when implementing a new command (see Commands In Python), as it provides a way to parse the command's argument as an expression.  It is also useful simply to compute values, for example, it is the only way to get the value of a convenience variable (see Convenience Vars) as a gdb.Value.

 -- Function: gdb.post_event (event)
     Put EVENT, a callable object taking no arguments, into GDBN's internal event queue.  This callable will be invoked at some later point, during GDBN's event processing.  Events posted using post_event will be run in the order in which they were posted; however, there is no way to know when they will be processed relative to other events inside GDBN.

     GDBN is not thread-safe.  If your Python program uses multiple threads, you must be careful to only call GDBN-specific functions in the main GDBN thread.  post_event ensures this.  For example:

          (GDBP) python
          >import threading
          >
          >class Writer():
          > def __init__(self, message):
          >        self.message = message;
          > def __call__(self):
          >        gdb.write(self.message)
          >
          >class MyThread1 (threading.Thread):
          > def run (self):
          >        gdb.post_event(Writer("Hello "))
          >
          >class MyThread2 (threading.Thread):
          > def run (self):
          >        gdb.post_event(Writer("World\n"))
          >
          >MyThread1().start()
          >MyThread2().start()
          >end
          (GDBP) Hello World

 -- Function: gdb.write (string [, stream])
     Print a string to GDBN's paginated output stream.  The optional STREAM determines the stream to print to.  The default stream is GDBN's standard output stream.  Possible stream values are:

     gdb.STDOUT
          GDBN's standard output stream.

     gdb.STDERR
          GDBN's standard error stream.

     gdb.STDLOG
          GDBN's log stream (see Logging Output).

     Writing to sys.stdout or sys.stderr will automatically call this function and will automatically direct the output to the relevant stream.

 -- Function: gdb.flush ()
     Flush the buffer of a GDBN paginated stream so that the contents are displayed immediately.  GDBN will flush the contents of a stream automatically when it encounters a newline in the buffer.  The optional STREAM determines the stream to flush.  The default stream is GDBN's standard output stream.  Possible stream values are:

     gdb.STDOUT
          GDBN's standard output stream.

     gdb.STDERR
          GDBN's standard error stream.

     gdb.STDLOG
          GDBN's log stream (see Logging Output).

     Flushing sys.stdout or sys.stderr will automatically call this function for the relevant stream.

 -- Function: gdb.target_charset ()
     Return the name of the current target character set (see Character Sets).  This differs from gdb.parameter('target-charset') in that auto is never returned.

 -- Function: gdb.target_wide_charset ()
     Return the name of the current target wide character set (see Character Sets).  This differs from gdb.parameter('target-wide-charset') in that auto is never returned.

 -- Function: gdb.solib_name (address)
     Return the name of the shared library holding the given ADDRESS as a string, or None.

 -- Function: gdb.decode_line [expression]
     Return locations of the line specified by EXPRESSION, or of the current line if no argument was given.  This function returns a Python tuple containing two elements.  The first element contains a string holding any unparsed section of EXPRESSION (or None if the expression has been fully parsed).  The second element contains either None or another tuple that contains all the locations that match the expression represented as gdb.Symtab_and_line objects (see Symbol Tables In Python).  If EXPRESSION is provided, it is decoded the way that GDBN's inbuilt break or edit commands do (see Specify Location).

 -- Function: gdb.prompt_hook (current_prompt)

     If PROMPT_HOOK is callable, GDBN will call the method assigned to this operation before a prompt is displayed by GDBN.

     The parameter current_prompt contains the current GDBN prompt.  This method must return a Python string, or None.  If a string is returned, the GDBN prompt will be set to that string.  If None is returned, GDBN will continue to use the current prompt.

     Some prompts cannot be substituted in GDBN.  Secondary prompts such as those used by readline for command input, and annotation related prompts are prohibited from being changed.

Exception Handling
..................

When executing the python command, Python exceptions uncaught within the Python code are translated to calls to GDBN error-reporting mechanism.  If the command that called python does not handle the error, GDBN will terminate it and print an error message containing the Python exception name, the associated value, and the Python call stack backtrace at the point where the exception was raised.  Example:

     (GDBP) python print foo
     Traceback (most recent call last):
       File "<string>", line 1, in <module>
     NameError: name 'foo' is not defined

GDBN errors that happen in GDBN commands invoked by Python code are converted to Python exceptions.  The type of the Python exception depends on the error.

gdb.error
     This is the base class for most exceptions generated by GDBN. It is derived from RuntimeError, for compatibility with earlier versions of GDBN.

     If an error occurring in GDBN does not fit into some more specific category, then the generated exception will have this type.

gdb.MemoryError
     This is a subclass of gdb.error which is thrown when an operation tried to access invalid memory in the inferior.

KeyboardInterrupt
     User interrupt (via C-c or by typing q at a pagination prompt) is translated to a Python KeyboardInterrupt exception.

In all cases, your exception handler will see the GDBN error message as its value and the Python call stack backtrace at the Python statement closest to where the GDBN error occured as the traceback.

When implementing GDBN commands in Python via gdb.Command, it is useful to be able to throw an exception that doesn't cause a traceback to be printed.  For example, the user may have invoked the command incorrectly.  Use the gdb.GdbError exception to handle this case.  Example:

     (gdb) python
     >class HelloWorld (gdb.Command):
     >  """Greet the whole world."""
     >  def __init__ (self):
     >    super (HelloWorld, self).__init__ ("hello-world", gdb.COMMAND_OBSCURE)
     >  def invoke (self, args, from_tty):
     >    argv = gdb.string_to_argv (args)
     >    if len (argv) != 0:
     >      raise gdb.GdbError ("hello-world takes no arguments")
     >    print "Hello, World!"
     >HelloWorld ()
     >end
     (gdb) hello-world 42
     hello-world takes no arguments

Values From Inferior
....................

GDBN provides values it obtains from the inferior program in an object of type gdb.Value.  GDBN uses this object for its internal bookkeeping of the inferior's values, and for fetching values when necessary.

Inferior values that are simple scalars can be used directly in Python expressions that are valid for the value's data type.  Here's an example for an integer or floating-point value some_val:

     bar = some_val + 2

As result of this, bar will also be a gdb.Value object whose values are of the same type as those of some_val.

Inferior values that are structures or instances of some class can be accessed using the Python dictionary syntax.  For example, if some_val is a gdb.Value instance holding a structure, you can access its foo element with:

     bar = some_val['foo']

Again, bar will also be a gdb.Value object.

A gdb.Value that represents a function can be executed via inferior function call.  Any arguments provided to the call must match the function's prototype, and must be provided in the order specified by that prototype.

For example, some_val is a gdb.Value instance representing a function that takes two integers as arguments.  To execute this function, call it like so:

     result = some_val (10,20)

Any values returned from a function call will be stored as a gdb.Value.

The following attributes are provided:

      -- Variable: Value.address
          If this object is addressable, this read-only attribute holds a gdb.Value object representing the address.  Otherwise, this attribute holds None.

      -- Variable: Value.is_optimized_out
          This read-only boolean attribute is true if the compiler optimized out this value, thus it is not available for fetching from the inferior.

      -- Variable: Value.type
          The type of this gdb.Value.  The value of this attribute is a gdb.Type object (see Types In Python).

      -- Variable: Value.dynamic_type
          The dynamic type of this gdb.Value.  This uses C++ run-time type information (RTTI) to determine the dynamic type of the value.  If this value is of class type, it will return the class in which the value is embedded, if any.  If this value is of pointer or reference to a class type, it will compute the dynamic type of the referenced object, and return a pointer or reference to that type, respectively.  In all other cases, it will return the value's static type.

          Note that this feature will only work when debugging a C++ program that includes RTTI for the object in question.  Otherwise, it will just return the static type of the value as in ptype foo (see ptype).

      -- Variable: Value.is_lazy
          The value of this read-only boolean attribute is True if this gdb.Value has not yet been fetched from the inferior. GDBN does not fetch values until necessary, for efficiency. For example:

               myval = gdb.parse_and_eval ('somevar')

          The value of somevar is not fetched at this time.  It will be fetched when the value is needed, or when the fetch_lazy method is invoked.

The following methods are provided:

      -- Function: Value.__init__ (VAL)
          Many Python values can be converted directly to a gdb.Value via this object initializer.  Specifically:

          Python boolean
               A Python boolean is converted to the boolean type from the current language.

          Python integer
               A Python integer is converted to the C long type for the current architecture.

          Python long
               A Python long is converted to the C long long type for the current architecture.

          Python float
               A Python float is converted to the C double type for the current architecture.

          Python string
               A Python string is converted to a target string, using the current target encoding.

          gdb.Value
               If val is a gdb.Value, then a copy of the value is made.

          gdb.LazyString
               If val is a gdb.LazyString (see Lazy Strings In Python), then the lazy string's value method is called, and its result is used.

      -- Function: Value.cast (type)
          Return a new instance of gdb.Value that is the result of casting this instance to the type described by TYPE, which must be a gdb.Type object.  If the cast cannot be performed for some reason, this method throws an exception.

      -- Function: Value.dereference ()
          For pointer data types, this method returns a new gdb.Value object whose contents is the object pointed to by the pointer.  For example, if foo is a C pointer to an int, declared in your C program as

               int *foo;

          then you can use the corresponding gdb.Value to access what foo points to like this:

               bar = foo.dereference ()

          The result bar will be a gdb.Value object holding the value pointed to by foo.

      -- Function: Value.dynamic_cast (type)
          Like Value.cast, but works as if the C++ dynamic_cast operator were used.  Consult a C++ reference for details.

      -- Function: Value.reinterpret_cast (type)
          Like Value.cast, but works as if the C++ reinterpret_cast operator were used.  Consult a C++ reference for details.

      -- Function: Value.string ([encoding[, errors[, length]]])
          If this gdb.Value represents a string, then this method converts the contents to a Python string.  Otherwise, this method will throw an exception.

          Strings are recognized in a language-specific way; whether a given gdb.Value represents a string is determined by the current language.

          For C-like languages, a value is a string if it is a pointer to or an array of characters or ints.  The string is assumed to be terminated by a zero of the appropriate width.  However if the optional length argument is given, the string will be converted to that given length, ignoring any embedded zeros that the string may contain.

          If the optional ENCODING argument is given, it must be a string naming the encoding of the string in the gdb.Value, such as "ascii", "iso-8859-6" or "utf-8".  It accepts the same encodings as the corresponding argument to Python's string.decode method, and the Python codec machinery will be used to convert the string.  If ENCODING is not given, or if ENCODING is the empty string, then either the target-charset (see Character Sets) will be used, or a language-specific encoding will be used, if the current language is able to supply one.

          The optional ERRORS argument is the same as the corresponding argument to Python's string.decode method.

          If the optional LENGTH argument is given, the string will be fetched and converted to the given length.

      -- Function: Value.lazy_string ([encoding [, length]])
          If this gdb.Value represents a string, then this method converts the contents to a gdb.LazyString (see Lazy Strings In Python).  Otherwise, this method will throw an exception.

          If the optional ENCODING argument is given, it must be a string naming the encoding of the gdb.LazyString.  Some examples are: ascii, iso-8859-6 or utf-8.  If the ENCODING argument is an encoding that GDBN does recognize, GDBN will raise an error.

          When a lazy string is printed, the GDBN encoding machinery is used to convert the string during printing.  If the optional ENCODING argument is not provided, or is an empty string, GDBN will automatically select the encoding most suitable for the string type.  For further information on encoding in GDBN please see Character Sets.

          If the optional LENGTH argument is given, the string will be fetched and encoded to the length of characters specified.  If the LENGTH argument is not provided, the string will be fetched and encoded until a null of appropriate width is found.

      -- Function: Value.fetch_lazy ()
          If the gdb.Value object is currently a lazy value (gdb.Value.is_lazy is True), then the value is fetched from the inferior.  Any errors that occur in the process will produce a Python exception.

          If the gdb.Value object is not a lazy value, this method has no effect.

          This method does not return a value.

Types In Python
...............

GDBN represents types from the inferior using the class gdb.Type.

The following type-related functions are available in the gdb module:

 -- Function: gdb.lookup_type (name [, block])
     This function looks up a type by name.  NAME is the name of the type to look up.  It must be a string.

     If BLOCK is given, then NAME is looked up in that scope. Otherwise, it is searched for globally.

     Ordinarily, this function will return an instance of gdb.Type. If the named type cannot be found, it will throw an exception.

If the type is a structure or class type, or an enum type, the fields of that type can be accessed using the Python dictionary syntax. For example, if some_type is a gdb.Type instance holding a structure type, you can access its foo field with:

     bar = some_type['foo']

bar will be a gdb.Field object; see below under the description of the Type.fields method for a description of the gdb.Field class.

An instance of Type has the following attributes:

      -- Variable: Type.code
          The type code for this type.  The type code will be one of the TYPE_CODE_ constants defined below.

      -- Variable: Type.sizeof
          The size of this type, in target char units.  Usually, a target's char type will be an 8-bit byte.  However, on some unusual platforms, this type may have a different size.

      -- Variable: Type.tag
          The tag name for this type.  The tag name is the name after struct, union, or enum in C and C++; not all languages have this concept.  If this type has no tag name, then None is returned.

The following methods are provided:

      -- Function: Type.fields ()
          For structure and union types, this method returns the fields.  Range types have two fields, the minimum and maximum values.  Enum types have one field per enum constant.  Function and method types have one field per parameter.  The base types of C++ classes are also represented as fields.  If the type has no fields, or does not fit into one of these categories, an empty sequence will be returned.

          Each field is a gdb.Field object, with some pre-defined attributes:

          bitpos
               This attribute is not available for static fields (as in C++ or Java).  For non-static fields, the value is the bit position of the field.  For enum fields, the value is the enumeration member's integer representation.

          name
               The name of the field, or None for anonymous fields.

          artificial
               This is True if the field is artificial, usually meaning that it was provided by the compiler and not the user.  This attribute is always provided, and is False if the field is not artificial.

          is_base_class
               This is True if the field represents a base class of a C++ structure.  This attribute is always provided, and is False if the field is not a base class of the type that is the argument of fields, or if that type was not a C++ class.

          bitsize
               If the field is packed, or is a bitfield, then this will have a non-zero value, which is the size of the field in bits.  Otherwise, this will be zero; in this case the field's size is given by its type.

          type
               The type of the field.  This is usually an instance of Type, but it can be None in some situations.

      -- Function: Type.array (N1 [, N2])
          Return a new gdb.Type object which represents an array of this type.  If one argument is given, it is the inclusive upper bound of the array; in this case the lower bound is zero.  If two arguments are given, the first argument is the lower bound of the array, and the second argument is the upper bound of the array.  An array's length must not be negative, but the bounds can be.

      -- Function: Type.const ()
          Return a new gdb.Type object which represents a const-qualified variant of this type.

      -- Function: Type.volatile ()
          Return a new gdb.Type object which represents a volatile-qualified variant of this type.

      -- Function: Type.unqualified ()
          Return a new gdb.Type object which represents an unqualified variant of this type.  That is, the result is neither const nor volatile.

      -- Function: Type.range ()
          Return a Python Tuple object that contains two elements: the low bound of the argument type and the high bound of that type.  If the type does not have a range, GDBN will raise a gdb.error exception (see Exception Handling).

      -- Function: Type.reference ()
          Return a new gdb.Type object which represents a reference to this type.

      -- Function: Type.pointer ()
          Return a new gdb.Type object which represents a pointer to this type.

      -- Function: Type.strip_typedefs ()
          Return a new gdb.Type that represents the real type, after removing all layers of typedefs.

      -- Function: Type.target ()
          Return a new gdb.Type object which represents the target type of this type.

          For a pointer type, the target type is the type of the pointed-to object.  For an array type (meaning C-like arrays), the target type is the type of the elements of the array.  For a function or method type, the target type is the type of the return value.  For a complex type, the target type is the type of the elements.  For a typedef, the target type is the aliased type.

          If the type does not have a target, this method will throw an exception.

      -- Function: Type.template_argument (n [, block])
          If this gdb.Type is an instantiation of a template, this will return a new gdb.Type which represents the type of the Nth template argument.

          If this gdb.Type is not a template type, this will throw an exception.  Ordinarily, only C++ code will have template types.

          If BLOCK is given, then NAME is looked up in that scope. Otherwise, it is searched for globally.

Each type has a code, which indicates what category this type falls into.  The available type categories are represented by constants defined in the gdb module:

gdb.TYPE_CODE_PTR
     The type is a pointer.

gdb.TYPE_CODE_ARRAY
     The type is an array.

gdb.TYPE_CODE_STRUCT
     The type is a structure.

gdb.TYPE_CODE_UNION
     The type is a union.

gdb.TYPE_CODE_ENUM
     The type is an enum.

gdb.TYPE_CODE_FLAGS
     A bit flags type, used for things such as status registers.

gdb.TYPE_CODE_FUNC
     The type is a function.

gdb.TYPE_CODE_INT
     The type is an integer type.

gdb.TYPE_CODE_FLT
     A floating point type.

gdb.TYPE_CODE_VOID
     The special type void.

gdb.TYPE_CODE_SET
     A Pascal set type.

gdb.TYPE_CODE_RANGE
     A range type, that is, an integer type with bounds.

gdb.TYPE_CODE_STRING
     A string type.  Note that this is only used for certain languages with language-defined string types; C strings are not represented this way.

gdb.TYPE_CODE_BITSTRING
     A string of bits.

gdb.TYPE_CODE_ERROR
     An unknown or erroneous type.

gdb.TYPE_CODE_METHOD
     A method type, as found in C++ or Java.

gdb.TYPE_CODE_METHODPTR
     A pointer-to-member-function.

gdb.TYPE_CODE_MEMBERPTR
     A pointer-to-member.

gdb.TYPE_CODE_REF
     A reference type.

gdb.TYPE_CODE_CHAR
     A character type.

gdb.TYPE_CODE_BOOL
     A boolean type.

gdb.TYPE_CODE_COMPLEX
     A complex float type.

gdb.TYPE_CODE_TYPEDEF
     A typedef to some other type.

gdb.TYPE_CODE_NAMESPACE
     A C++ namespace.

gdb.TYPE_CODE_DECFLOAT
     A decimal floating point type.

gdb.TYPE_CODE_INTERNAL_FUNCTION
     A function internal to GDBN.  This is the type used to represent convenience functions.

Further support for types is provided in the gdb.types Python module (see gdb.types).

Pretty Printing API
...................

An example output is provided (see Pretty Printing).

A pretty-printer is just an object that holds a value and implements a specific interface, defined here.

 -- Function: pretty_printer.children (self)
     GDBN will call this method on a pretty-printer to compute the children of the pretty-printer's value.

     This method must return an object conforming to the Python iterator protocol.  Each item returned by the iterator must be a tuple holding two elements.  The first element is the ``name'' of the child; the second element is the child's value.  The value can be any Python object which is convertible to a GDBN value.

     This method is optional.  If it does not exist, GDBN will act as though the value has no children.

 -- Function: pretty_printer.display_hint (self)
     The CLI may call this method and use its result to change the formatting of a value.  The result will also be supplied to an MI consumer as a displayhint attribute of the variable being printed.

     This method is optional.  If it does exist, this method must return a string.

     Some display hints are predefined by GDBN:

     array
          Indicate that the object being printed is ``array-like''.  The CLI uses this to respect parameters such as set print elements and set print array.

     map
          Indicate that the object being printed is ``map-like'', and that the children of this value can be assumed to alternate between keys and values.

     string
          Indicate that the object being printed is ``string-like''.  If the printer's to_string method returns a Python string of some kind, then GDBN will call its internal language-specific string-printing function to format the string.  For the CLI this means adding quotation marks, possibly escaping some characters, respecting set print elements, and the like.

 -- Function: pretty_printer.to_string (self)
     GDBN will call this method to display the string representation of the value passed to the object's constructor.

     When printing from the CLI, if the to_string method exists, then GDBN will prepend its result to the values returned by children.  Exactly how this formatting is done is dependent on the display hint, and may change as more hints are added.  Also, depending on the print settings (see Print Settings), the CLI may print just the result of to_string in a stack trace, omitting the result of children.

     If this method returns a string, it is printed verbatim.

     Otherwise, if this method returns an instance of gdb.Value, then GDBN prints this value.  This may result in a call to another pretty-printer.

     If instead the method returns a Python value which is convertible to a gdb.Value, then GDBN performs the conversion and prints the resulting value.  Again, this may result in a call to another pretty-printer.  Python scalars (integers, floats, and booleans) and strings are convertible to gdb.Value; other types are not.

     Finally, if this method returns None then no further operations are peformed in this method and nothing is printed.

     If the result is not one of these types, an exception is raised.

GDBN provides a function which can be used to look up the default pretty-printer for a gdb.Value:

 -- Function: gdb.default_visualizer (value)
     This function takes a gdb.Value object as an argument.  If a pretty-printer for this value exists, then it is returned.  If no such printer exists, then this returns None.

Selecting Pretty-Printers
.........................

The Python list gdb.pretty_printers contains an array of functions or callable objects that have been registered via addition as a pretty-printer.  Printers in this list are called global printers, they're available when debugging all inferiors. Each gdb.Progspace contains a pretty_printers attribute. Each gdb.Objfile also contains a pretty_printers attribute.

Each function on these lists is passed a single gdb.Value argument and should return a pretty-printer object conforming to the interface definition above (see Pretty Printing API).  If a function cannot create a pretty-printer for the value, it should return None.

GDBN first checks the pretty_printers attribute of each gdb.Objfile in the current program space and iteratively calls each enabled lookup routine in the list for that gdb.Objfile until it receives a pretty-printer object. If no pretty-printer is found in the objfile lists, GDBN then searches the pretty-printer list of the current program space, calling each enabled function until an object is returned. After these lists have been exhausted, it tries the global gdb.pretty_printers list, again calling each enabled function until an object is returned.

The order in which the objfiles are searched is not specified.  For a given list, functions are always invoked from the head of the list, and iterated over sequentially until the end of the list, or a printer object is returned.

For various reasons a pretty-printer may not work. For example, the underlying data structure may have changed and the pretty-printer is out of date.

The consequences of a broken pretty-printer are severe enough that GDBN provides support for enabling and disabling individual printers.  For example, if print frame-arguments is on, a backtrace can become highly illegible if any argument is printed with a broken printer.

Pretty-printers are enabled and disabled by attaching an enabled attribute to the registered function or callable object.  If this attribute is present and its value is False, the printer is disabled, otherwise the printer is enabled.

Writing a Pretty-Printer
........................

A pretty-printer consists of two parts: a lookup function to detect if the type is supported, and the printer itself.

Here is an example showing how a std::string printer might be written.  See Pretty Printing API, for details on the API this class must provide.

     class StdStringPrinter(object):
         "Print a std::string"

         def __init__(self, val):
             self.val = val

         def to_string(self):
             return self.val['_M_dataplus']['_M_p']

         def display_hint(self):
             return 'string'

And here is an example showing how a lookup function for the printer example above might be written.

     def str_lookup_function(val):
         lookup_tag = val.type.tag
         if lookup_tag == None:
             return None
         regex = re.compile("^std::basic_string<char,.*>$")
         if regex.match(lookup_tag):
             return StdStringPrinter(val)
         return None

The example lookup function extracts the value's type, and attempts to match it to a type that it can pretty-print.  If it is a type the printer can pretty-print, it will return a printer object.  If not, it returns None.

We recommend that you put your core pretty-printers into a Python package.  If your pretty-printers are for use with a library, we further recommend embedding a version number into the package name. This practice will enable GDBN to load multiple versions of your pretty-printers at the same time, because they will have different names.

You should write auto-loaded code (see Auto-loading) such that it can be evaluated multiple times without changing its meaning.  An ideal auto-load file will consist solely of imports of your printer modules, followed by a call to a register pretty-printers with the current objfile.

Taken as a whole, this approach will scale nicely to multiple inferiors, each potentially using a different library version. Embedding a version number in the Python package name will ensure that GDBN is able to load both sets of printers simultaneously. Then, because the search for pretty-printers is done by objfile, and because your auto-loaded code took care to register your library's printers with a specific objfile, GDBN will find the correct printers for the specific version of the library used by each inferior.

To continue the std::string example (see Pretty Printing API), this code might appear in gdb.libstdcxx.v6:

     def register_printers(objfile):
         objfile.pretty_printers.append(str_lookup_function)

And then the corresponding contents of the auto-load file would be:

     import gdb.libstdcxx.v6
     gdb.libstdcxx.v6.register_printers(gdb.current_objfile())

The previous example illustrates a basic pretty-printer. There are a few things that can be improved on. The printer doesn't have a name, making it hard to identify in a list of installed printers.  The lookup function has a name, but lookup functions can have arbitrary, even identical, names.

Second, the printer only handles one type, whereas a library typically has several types.  One could install a lookup function for each desired type in the library, but one could also have a single lookup function recognize several types.  The latter is the conventional way this is handled. If a pretty-printer can handle multiple data types, then its subprinters are the printers for the individual data types.

The gdb.printing module provides a formal way of solving these problems (see gdb.printing). Here is another example that handles multiple types.

These are the types we are going to pretty-print:

     struct foo { int a, b; };
     struct bar { struct foo x, y; };

Here are the printers:

     class fooPrinter:
         """Print a foo object."""

         def __init__(self, val):
             self.val = val

         def to_string(self):
             return ("a=<" + str(self.val["a"]) +
                     "> b=<" + str(self.val["b"]) + ">")

     class barPrinter:
         """Print a bar object."""

         def __init__(self, val):
             self.val = val

         def to_string(self):
             return ("x=<" + str(self.val["x"]) +
                     "> y=<" + str(self.val["y"]) + ">")

This example doesn't need a lookup function, that is handled by the gdb.printing module.  Instead a function is provided to build up the object that handles the lookup.

     import gdb.printing

     def build_pretty_printer():
         pp = gdb.printing.RegexpCollectionPrettyPrinter(
             "my_library")
         pp.add_printer('foo', '^foo$', fooPrinter)
         pp.add_printer('bar', '^bar$', barPrinter)
         return pp

And here is the autoload support:

     import gdb.printing
     import my_library
     gdb.printing.register_pretty_printer(
         gdb.current_objfile(),
         my_library.build_pretty_printer())

Finally, when this printer is loaded into GDBN, here is the corresponding output of info pretty-printer:

     (gdb) info pretty-printer
     my_library.so:
       my_library
         foo
         bar

Inferiors In Python
...................

Programs which are being run under GDBN are called inferiors (see Inferiors and Programs).  Python scripts can access information about and manipulate inferiors controlled by GDBN via objects of the gdb.Inferior class.

The following inferior-related functions are available in the gdb module:

 -- Function: gdb.inferiors ()
     Return a tuple containing all inferior objects.

 -- Function: gdb.selected_inferior ()
     Return an object representing the current inferior.

A gdb.Inferior object has the following attributes:

      -- Variable: Inferior.num
          ID of inferior, as assigned by GDB.

      -- Variable: Inferior.pid
          Process ID of the inferior, as assigned by the underlying operating system.

      -- Variable: Inferior.was_attached
          Boolean signaling whether the inferior was created using `attach', or started by GDBN itself.

A gdb.Inferior object has the following methods:

      -- Function: Inferior.is_valid ()
          Returns True if the gdb.Inferior object is valid, False if not.  A gdb.Inferior object will become invalid if the inferior no longer exists within GDBN.  All other gdb.Inferior methods will throw an exception if it is invalid at the time the method is called.

      -- Function: Inferior.threads ()
          This method returns a tuple holding all the threads which are valid when it is called.  If there are no valid threads, the method will return an empty tuple.

      -- Function: Inferior.read_memory (address, length)
          Read LENGTH bytes of memory from the inferior, starting at ADDRESS.  Returns a buffer object, which behaves much like an array or a string.  It can be modified and given to the gdb.write_memory function.

      -- Function: Inferior.write_memory (address, buffer [, length])
          Write the contents of BUFFER to the inferior, starting at ADDRESS.  The BUFFER parameter must be a Python object which supports the buffer protocol, i.e., a string, an array or the object returned from gdb.read_memory.  If given, LENGTH determines the number of bytes from BUFFER to be written.

      -- Function: Inferior.search_memory (address, length, pattern)
          Search a region of the inferior memory starting at ADDRESS with the given LENGTH using the search pattern supplied in PATTERN.  The PATTERN parameter must be a Python object which supports the buffer protocol, i.e., a string, an array or the object returned from gdb.read_memory.  Returns a Python Long containing the address where the pattern was found, or None if the pattern could not be found.

Events In Python
................

GDBN provides a general event facility so that Python code can be notified of various state changes, particularly changes that occur in the inferior.

An event is just an object that describes some state change.  The type of the object and its attributes will vary depending on the details of the change.  All the existing events are described below.

In order to be notified of an event, you must register an event handler with an event registry.  An event registry is an object in the gdb.events module which dispatches particular events.  A registry provides methods to register and unregister event handlers:

      -- Function: EventRegistry.connect (object)
          Add the given callable OBJECT to the registry.  This object will be called when an event corresponding to this registry occurs.

      -- Function: EventRegistry.disconnect (object)
          Remove the given OBJECT from the registry.  Once removed, the object will no longer receive notifications of events.

Here is an example:

     def exit_handler (event):
         print "event type: exit"
         print "exit code: %d" % (event.exit_code)

     gdb.events.exited.connect (exit_handler)

In the above example we connect our handler exit_handler to the registry events.exited.  Once connected, exit_handler gets called when the inferior exits.  The argument event in this example is of type gdb.ExitedEvent.  As you can see in the example the ExitedEvent object has an attribute which indicates the exit code of the inferior.

The following is a listing of the event registries that are available and details of the events they emit:

events.cont
     Emits gdb.ThreadEvent.

     Some events can be thread specific when GDBN is running in non-stop mode.  When represented in Python, these events all extend gdb.ThreadEvent.  Note, this event is not emitted directly; instead, events which are emitted by this or other modules might extend this event. Examples of these events are gdb.BreakpointEvent and gdb.ContinueEvent.

           -- Variable: ThreadEvent.inferior_thread
               In non-stop mode this attribute will be set to the specific thread which was involved in the emitted event. Otherwise, it will be set to None.

     Emits gdb.ContinueEvent which extends gdb.ThreadEvent.

     This event indicates that the inferior has been continued after a stop. For inherited attribute refer to gdb.ThreadEvent above.

events.exited
     Emits events.ExitedEvent which indicates that the inferior has exited. events.ExitedEvent has two attributes:

           -- Variable: ExitedEvent.exit_code
               An integer representing the exit code, if available, which the inferior has returned.  (The exit code could be unavailable if, for example, GDBN detaches from the inferior.) If the exit code is unavailable, the attribute does not exist.

           -- Variable: ExitedEvent inferior
               A reference to the inferior which triggered the exited event.

events.stop
     Emits gdb.StopEvent which extends gdb.ThreadEvent.

     Indicates that the inferior has stopped.  All events emitted by this registry extend StopEvent.  As a child of gdb.ThreadEvent, gdb.StopEvent will indicate the stopped thread when GDBN is running in non-stop mode.  Refer to gdb.ThreadEvent above for more details.

     Emits gdb.SignalEvent which extends gdb.StopEvent.

     This event indicates that the inferior or one of its threads has received as signal.  gdb.SignalEvent has the following attributes:

           -- Variable: SignalEvent.stop_signal
               A string representing the signal received by the inferior.  A list of possible signal values can be obtained by running the command info signals in the GDBN command prompt.

     Also emits  gdb.BreakpointEvent which extends gdb.StopEvent.

     gdb.BreakpointEvent event indicates that one or more breakpoints have been hit, and has the following attributes:

           -- Variable: BreakpointEvent.breakpoints
               A sequence containing references to all the breakpoints (type gdb.Breakpoint) that were hit. See Breakpoints In Python, for details of the gdb.Breakpoint object.

           -- Variable: BreakpointEvent.breakpoint
               A reference to the first breakpoint that was hit. This function is maintained for backward compatibility and is now deprecated in favor of the gdb.BreakpointEvent.breakpoints attribute.

events.new_objfile
     Emits gdb.NewObjFileEvent which indicates that a new object file has been loaded by GDBN.  gdb.NewObjFileEvent has one attribute:

           -- Variable: NewObjFileEvent.new_objfile
               A reference to the object file (gdb.Objfile) which has been loaded. See Objfiles In Python, for details of the gdb.Objfile object.

Threads In Python
.................

Python scripts can access information about, and manipulate inferior threads controlled by GDBN, via objects of the gdb.InferiorThread class.

The following thread-related functions are available in the gdb module:

 -- Function: gdb.selected_thread ()
     This function returns the thread object for the selected thread.  If there is no selected thread, this will return None.

A gdb.InferiorThread object has the following attributes:

      -- Variable: InferiorThread.name
          The name of the thread.  If the user specified a name using thread name, then this returns that name.  Otherwise, if an OS-supplied name is available, then it is returned.  Otherwise, this returns None.

          This attribute can be assigned to.  The new value must be a string object, which sets the new name, or None, which removes any user-specified thread name.

      -- Variable: InferiorThread.num
          ID of the thread, as assigned by GDB.

      -- Variable: InferiorThread.ptid
          ID of the thread, as assigned by the operating system.  This attribute is a tuple containing three integers.  The first is the Process ID (PID); the second is the Lightweight Process ID (LWPID), and the third is the Thread ID (TID). Either the LWPID or TID may be 0, which indicates that the operating system does not  use that identifier.

A gdb.InferiorThread object has the following methods:

      -- Function: InferiorThread.is_valid ()
          Returns True if the gdb.InferiorThread object is valid, False if not.  A gdb.InferiorThread object will become invalid if the thread exits, or the inferior that the thread belongs is deleted.  All other gdb.InferiorThread methods will throw an exception if it is invalid at the time the method is called.

      -- Function: InferiorThread.switch ()
          This changes GDBN's currently selected thread to the one represented by this object.

      -- Function: InferiorThread.is_stopped ()
          Return a Boolean indicating whether the thread is stopped.

      -- Function: InferiorThread.is_running ()
          Return a Boolean indicating whether the thread is running.

      -- Function: InferiorThread.is_exited ()
          Return a Boolean indicating whether the thread is exited.

Commands In Python
..................

You can implement new GDBN CLI commands in Python.  A CLI command is implemented using an instance of the gdb.Command class, most commonly using a subclass.

 -- Function: Command.__init__ (name, COMMAND_CLASS [, COMPLETER_CLASS [, PREFIX]])
     The object initializer for Command registers the new command with GDBN.  This initializer is normally invoked from the subclass' own __init__ method.

     NAME is the name of the command.  If NAME consists of multiple words, then the initial words are looked for as prefix commands.  In this case, if one of the prefix commands does not exist, an exception is raised.

     There is no support for multi-line commands.

     COMMAND_CLASS should be one of the COMMAND_ constants defined below.  This argument tells GDBN how to categorize the new command in the help system.

     COMPLETER_CLASS is an optional argument.  If given, it should be one of the COMPLETE_ constants defined below.  This argument tells GDBN how to perform completion for this command.  If not given, GDBN will attempt to complete using the object's complete method (see below); if no such method is found, an error will occur when completion is attempted.

     PREFIX is an optional argument.  If True, then the new command is a prefix command; sub-commands of this command may be registered.

     The help text for the new command is taken from the Python documentation string for the command's class, if there is one.  If no documentation string is provided, the default value ``This command is not documented.'' is used.

 -- Function: Command.dont_repeat ()
     By default, a GDBN command is repeated when the user enters a blank line at the command prompt.  A command can suppress this behavior by invoking the dont_repeat method.  This is similar to the user command dont-repeat, see dont-repeat.

 -- Function: Command.invoke (argument, from_tty)
     This method is called by GDBN when this command is invoked.

     ARGUMENT is a string.  It is the argument to the command, after leading and trailing whitespace has been stripped.

     FROM_TTY is a boolean argument.  When true, this means that the command was entered by the user at the terminal; when false it means that the command came from elsewhere.

     If this method throws an exception, it is turned into a GDBN error call.  Otherwise, the return value is ignored.

     To break ARGUMENT up into an argv-like string use gdb.string_to_argv.  This function behaves identically to GDBN's internal argument lexer buildargv. It is recommended to use this for consistency. Arguments are separated by spaces and may be quoted. Example:

          print gdb.string_to_argv ("1 2\ \\\"3 '4 \"5' \"6 '7\"")
          ['1', '2 "3', '4 "5', "6 '7"]

 -- Function: Command.complete (text, word)
     This method is called by GDBN when the user attempts completion on this command.  All forms of completion are handled by this method, that is, the TAB and M-? key bindings (see Completion), and the complete command (see complete).

     The arguments TEXT and WORD are both strings.  TEXT holds the complete command line up to the cursor's location. WORD holds the last word of the command line; this is computed using a word-breaking heuristic.

     The complete method can return several values:

       * If the return value is a sequence, the contents of the sequence are used as the completions.  It is up to complete to ensure that the contents actually do complete the word.  A zero-length sequence is allowed, it means that there were no completions available.  Only string elements of the sequence are used; other elements in the sequence are ignored.

       * If the return value is one of the COMPLETE_ constants defined below, then the corresponding GDBN-internal completion function is invoked, and its result is used.

       * All other results are treated as though there were no available completions.

When a new command is registered, it must be declared as a member of some general class of commands.  This is used to classify top-level commands in the on-line help system; note that prefix commands are not listed under their own category but rather that of their top-level command.  The available classifications are represented by constants defined in the gdb module:

gdb.COMMAND_NONE
     The command does not belong to any particular class.  A command in this category will not be displayed in any of the help categories.

gdb.COMMAND_RUNNING
     The command is related to running the inferior.  For example, start, step, and continue are in this category. Type help running at the GDBN prompt to see a list of commands in this category.

gdb.COMMAND_DATA
     The command is related to data or variables.  For example, call, find, and print are in this category.  Type help data at the GDBN prompt to see a list of commands in this category.

gdb.COMMAND_STACK
     The command has to do with manipulation of the stack.  For example, backtrace, frame, and return are in this category.  Type help stack at the GDBN prompt to see a list of commands in this category.

gdb.COMMAND_FILES
     This class is used for file-related commands.  For example, file, list and section are in this category. Type help files at the GDBN prompt to see a list of commands in this category.

gdb.COMMAND_SUPPORT
     This should be used for ``support facilities'', generally meaning things that are useful to the user when interacting with GDBN, but not related to the state of the inferior.  For example, help, make, and shell are in this category.  Type help support at the GDBN prompt to see a list of commands in this category.

gdb.COMMAND_STATUS
     The command is an info-related command, that is, related to the state of GDBN itself.  For example, info, macro, and show are in this category.  Type help status at the GDBN prompt to see a list of commands in this category.

gdb.COMMAND_BREAKPOINTS
     The command has to do with breakpoints.  For example, break, clear, and delete are in this category.  Type help
     breakpoints at the GDBN prompt to see a list of commands in this category.

gdb.COMMAND_TRACEPOINTS
     The command has to do with tracepoints.  For example, trace, actions, and tfind are in this category.  Type help tracepoints at the GDBN prompt to see a list of commands in this category.

gdb.COMMAND_OBSCURE
     The command is only used in unusual circumstances, or is not of general interest to users.  For example, checkpoint, fork, and stop are in this category.  Type help
     obscure at the GDBN prompt to see a list of commands in this category.

gdb.COMMAND_MAINTENANCE
     The command is only useful to GDBN maintainers.  The maintenance and flushregs commands are in this category. Type help internals at the GDBN prompt to see a list of commands in this category.

A new command can use a predefined completion function, either by specifying it via an argument at initialization, or by returning it from the complete method.  These predefined completion constants are all defined in the gdb module:

gdb.COMPLETE_NONE
     This constant means that no completion should be done.

gdb.COMPLETE_FILENAME
     This constant means that filename completion should be performed.

gdb.COMPLETE_LOCATION
     This constant means that location completion should be done. See Specify Location.

gdb.COMPLETE_COMMAND
     This constant means that completion should examine GDBN command names.

gdb.COMPLETE_SYMBOL
     This constant means that completion should be done using symbol names as the source.

The following code snippet shows how a trivial CLI command can be implemented in Python:

     class HelloWorld (gdb.Command):
       """Greet the whole world."""

       def __init__ (self):
         super (HelloWorld, self).__init__ ("hello-world", gdb.COMMAND_OBSCURE)

       def invoke (self, arg, from_tty):
         print "Hello, World!"

     HelloWorld ()

The last line instantiates the class, and is necessary to trigger the registration of the command with GDBN.  Depending on how the Python code is read into GDBN, you may need to import the gdb module explicitly.

Parameters In Python
....................

You can implement new GDBN parameters using Python.  A new parameter is implemented as an instance of the gdb.Parameter class.

Parameters are exposed to the user via the set and show commands.  See Help.

There are many parameters that already exist and can be set in GDBN.  Two examples are: set follow fork and set charset.  Setting these parameters influences certain behavior in GDBN.  Similarly, you can define parameters that can be used to influence behavior in custom Python scripts and commands.

 -- Function: Parameter.__init__ (name, COMMAND-CLASS, PARAMETER-CLASS [, ENUM-SEQUENCE])
     The object initializer for Parameter registers the new parameter with GDBN.  This initializer is normally invoked from the subclass' own __init__ method.

     NAME is the name of the new parameter.  If NAME consists of multiple words, then the initial words are looked for as prefix parameters.  An example of this can be illustrated with the set print set of parameters.  If NAME is print foo, then print will be searched as the prefix parameter.  In this case the parameter can subsequently be accessed in GDBN as set print foo.

     If NAME consists of multiple words, and no prefix parameter group can be found, an exception is raised.

     COMMAND-CLASS should be one of the COMMAND_ constants (see Commands In Python).  This argument tells GDBN how to categorize the new parameter in the help system.

     PARAMETER-CLASS should be one of the PARAM_ constants defined below.  This argument tells GDBN the type of the new parameter; this information is used for input validation and completion.

     If PARAMETER-CLASS is PARAM_ENUM, then ENUM-SEQUENCE must be a sequence of strings.  These strings represent the possible values for the parameter.

     If PARAMETER-CLASS is not PARAM_ENUM, then the presence of a fourth argument will cause an exception to be thrown.

     The help text for the new parameter is taken from the Python documentation string for the parameter's class, if there is one.  If there is no documentation string, a default value is used.

 -- Variable: Parameter.set_doc
     If this attribute exists, and is a string, then its value is used as the help text for this parameter's set command.  The value is examined when Parameter.__init__ is invoked; subsequent changes have no effect.

 -- Variable: Parameter.show_doc
     If this attribute exists, and is a string, then its value is used as the help text for this parameter's show command.  The value is examined when Parameter.__init__ is invoked; subsequent changes have no effect.

 -- Variable: Parameter.value
     The value attribute holds the underlying value of the parameter.  It can be read and assigned to just as any other attribute.  GDBN does validation when assignments are made.

There are two methods that should be implemented in any Parameter class.  These are:

 -- Function: Parameter.get_set_string (self)
     GDBN will call this method when a PARAMETER's value has been changed via the set API (for example, set foo off). The value attribute has already been populated with the new value and may be used in output.  This method must return a string.

 -- Function: Parameter.get_show_string (self, svalue)
     GDBN will call this method when a PARAMETER's show API has been invoked (for example, show foo).  The argument svalue receives the string representation of the current value.  This method must return a string.

When a new parameter is defined, its type must be specified.  The available types are represented by constants defined in the gdb module:

gdb.PARAM_BOOLEAN
     The value is a plain boolean.  The Python boolean values, True and False are the only valid values.

gdb.PARAM_AUTO_BOOLEAN
     The value has three possible states: true, false, and auto.  In Python, true and false are represented using boolean constants, and auto is represented using None.

gdb.PARAM_UINTEGER
     The value is an unsigned integer.  The value of 0 should be interpreted to mean ``unlimited''.

gdb.PARAM_INTEGER
     The value is a signed integer.  The value of 0 should be interpreted to mean ``unlimited''.

gdb.PARAM_STRING
     The value is a string.  When the user modifies the string, any escape sequences, such as \t, \f, and octal escapes, are translated into corresponding characters and encoded into the current host charset.

gdb.PARAM_STRING_NOESCAPE
     The value is a string.  When the user modifies the string, escapes are passed through untranslated.

gdb.PARAM_OPTIONAL_FILENAME
     The value is a either a filename (a string), or None.

gdb.PARAM_FILENAME
     The value is a filename.  This is just like PARAM_STRING_NOESCAPE, but uses file names for completion.

gdb.PARAM_ZINTEGER
     The value is an integer.  This is like PARAM_INTEGER, except 0 is interpreted as itself.

gdb.PARAM_ENUM
     The value is a string, which must be one of a collection string constants provided when the parameter is created.

Writing new convenience functions
.................................

You can implement new convenience functions (see Convenience Vars) in Python.  A convenience function is an instance of a subclass of the class gdb.Function.

 -- Function: Function.__init__ (name)
     The initializer for Function registers the new function with GDBN.  The argument NAME is the name of the function, a string.  The function will be visible to the user as a convenience variable of type internal function, whose name is the same as the given NAME.

     The documentation for the new function is taken from the documentation string for the new class.

 -- Function: Function.invoke (*ARGS)
     When a convenience function is evaluated, its arguments are converted to instances of gdb.Value, and then the function's invoke method is called.  Note that GDBN does not predetermine the arity of convenience functions.  Instead, all available arguments are passed to invoke, following the standard Python calling convention.  In particular, a convenience function can have default values for parameters without ill effect.

     The return value of this method is used as its value in the enclosing expression.  If an ordinary Python value is returned, it is converted to a gdb.Value following the usual rules.

The following code snippet shows how a trivial convenience function can be implemented in Python:

     class Greet (gdb.Function):
       """Return string to greet someone.
     Takes a name as argument."""

       def __init__ (self):
         super (Greet, self).__init__ ("greet")

       def invoke (self, name):
         return "Hello, %s!" % name.string ()

     Greet ()

The last line instantiates the class, and is necessary to trigger the registration of the function with GDBN.  Depending on how the Python code is read into GDBN, you may need to import the gdb module explicitly.

Program Spaces In Python
........................

A program space, or progspace, represents a symbolic view of an address space. It consists of all of the objfiles of the program. See Objfiles In Python. See program spaces, for more details about program spaces.

The following progspace-related functions are available in the gdb module:

 -- Function: gdb.current_progspace ()
     This function returns the program space of the currently selected inferior. See Inferiors and Programs.

 -- Function: gdb.progspaces ()
     Return a sequence of all the progspaces currently known to GDBN.

Each progspace is represented by an instance of the gdb.Progspace class.

 -- Variable: Progspace.filename
     The file name of the progspace as a string.

 -- Variable: Progspace.pretty_printers
     The pretty_printers attribute is a list of functions.  It is used to look up pretty-printers.  A Value is passed to each function in order; if the function returns None, then the search continues.  Otherwise, the return value should be an object which is used to format the value.  See Pretty Printing API, for more information.

Objfiles In Python
..................

GDBN loads symbols for an inferior from various symbol-containing files (see Files).  These include the primary executable file, any shared libraries used by the inferior, and any separate debug info files (see Separate Debug Files). GDBN calls these symbol-containing files objfiles.

The following objfile-related functions are available in the gdb module:

 -- Function: gdb.current_objfile ()
     When auto-loading a Python script (see Auto-loading), GDBN sets the ``current objfile'' to the corresponding objfile.  This function returns the current objfile.  If there is no current objfile, this function returns None.

 -- Function: gdb.objfiles ()
     Return a sequence of all the objfiles current known to GDBN. See Objfiles In Python.

Each objfile is represented by an instance of the gdb.Objfile class.

 -- Variable: Objfile.filename
     The file name of the objfile as a string.

 -- Variable: Objfile.pretty_printers
     The pretty_printers attribute is a list of functions.  It is used to look up pretty-printers.  A Value is passed to each function in order; if the function returns None, then the search continues.  Otherwise, the return value should be an object which is used to format the value.  See Pretty Printing API, for more information.

A gdb.Objfile object has the following methods:

 -- Function: Objfile.is_valid ()
     Returns True if the gdb.Objfile object is valid, False if not.  A gdb.Objfile object can become invalid if the object file it refers to is not loaded in GDBN any longer.  All other gdb.Objfile methods will throw an exception if it is invalid at the time the method is called.

Accessing inferior stack frames from Python.
............................................

When the debugged program stops, GDBN is able to analyze its call stack (see Stack frames).  The gdb.Frame class represents a frame in the stack.  A gdb.Frame object is only valid while its corresponding frame exists in the inferior's stack.  If you try to use an invalid frame object, GDBN will throw a gdb.error exception (see Exception Handling).

Two gdb.Frame objects can be compared for equality with the == operator, like:

     (GDBP) python print gdb.newest_frame() == gdb.selected_frame ()
     True

The following frame-related functions are available in the gdb module:

 -- Function: gdb.selected_frame ()
     Return the selected frame object.  (see Selecting a Frame).

 -- Function: gdb.newest_frame ()
     Return the newest frame object for the selected thread.

 -- Function: gdb.frame_stop_reason_string (reason)
     Return a string explaining the reason why GDBN stopped unwinding frames, as expressed by the given REASON code (an integer, see the unwind_stop_reason method further down in this section).

A gdb.Frame object has the following methods:

      -- Function: Frame.is_valid ()
          Returns true if the gdb.Frame object is valid, false if not. A frame object can become invalid if the frame it refers to doesn't exist anymore in the inferior.  All gdb.Frame methods will throw an exception if it is invalid at the time the method is called.

      -- Function: Frame.name ()
          Returns the function name of the frame, or None if it can't be obtained.

      -- Function: Frame.type ()
          Returns the type of the frame.  The value can be one of:

          gdb.NORMAL_FRAME
               An ordinary stack frame.

          gdb.DUMMY_FRAME
               A fake stack frame that was created by GDBN when performing an inferior function call.

          gdb.INLINE_FRAME
               A frame representing an inlined function.  The function was inlined into a gdb.NORMAL_FRAME that is older than this one.

          gdb.TAILCALL_FRAME
               A frame representing a tail call.  See Tail Call Frames.

          gdb.SIGTRAMP_FRAME
               A signal trampoline frame.  This is the frame created by the OS when it calls into a signal handler.

          gdb.ARCH_FRAME
               A fake stack frame representing a cross-architecture call.

          gdb.SENTINEL_FRAME
               This is like gdb.NORMAL_FRAME, but it is only used for the newest frame.

      -- Function: Frame.unwind_stop_reason ()
          Return an integer representing the reason why it's not possible to find more frames toward the outermost frame.  Use gdb.frame_stop_reason_string to convert the value returned by this function to a string. The value can be one of:

          gdb.FRAME_UNWIND_NO_REASON
               No particular reason (older frames should be available).

          gdb.FRAME_UNWIND_NULL_ID
               The previous frame's analyzer returns an invalid result.

          gdb.FRAME_UNWIND_OUTERMOST
               This frame is the outermost.

          gdb.FRAME_UNWIND_UNAVAILABLE
               Cannot unwind further, because that would require knowing the values of registers or memory that have not been collected.

          gdb.FRAME_UNWIND_INNER_ID
               This frame ID looks like it ought to belong to a NEXT frame, but we got it for a PREV frame.  Normally, this is a sign of unwinder failure.  It could also indicate stack corruption.

          gdb.FRAME_UNWIND_SAME_ID
               This frame has the same ID as the previous one.  That means that unwinding further would almost certainly give us another frame with exactly the same ID, so break the chain.  Normally, this is a sign of unwinder failure.  It could also indicate stack corruption.

          gdb.FRAME_UNWIND_NO_SAVED_PC
               The frame unwinder did not find any saved PC, but we needed one to unwind further.

          gdb.FRAME_UNWIND_FIRST_ERROR
               Any stop reason greater or equal to this value indicates some kind of error.  This special value facilitates writing code that tests for errors in unwinding in a way that will work correctly even if the list of the other values is modified in future GDBN versions.  Using it, you could write:

                    reason = gdb.selected_frame().unwind_stop_reason ()
                    reason_str =  gdb.frame_stop_reason_string (reason)
                    if reason >=  gdb.FRAME_UNWIND_FIRST_ERROR:
                        print "An error occured: %s" % reason_str

      -- Function: Frame.pc ()
          Returns the frame's resume address.

      -- Function: Frame.block ()
          Return the frame's code block.  See Blocks In Python.

      -- Function: Frame.function ()
          Return the symbol for the function corresponding to this frame. See Symbols In Python.

      -- Function: Frame.older ()
          Return the frame that called this frame.

      -- Function: Frame.newer ()
          Return the frame called by this frame.

      -- Function: Frame.find_sal ()
          Return the frame's symtab and line object. See Symbol Tables In Python.

      -- Function: Frame.read_var (variable [, block])
          Return the value of VARIABLE in this frame.  If the optional argument BLOCK is provided, search for the variable from that block; otherwise start at the frame's current block (which is determined by the frame's current program counter).  VARIABLE must be a string or a gdb.Symbol object.  BLOCK must be a gdb.Block object.

      -- Function: Frame.select ()
          Set this frame to be the selected frame.  See Examining the Stack.

Accessing frame blocks from Python.
...................................

Within each frame, GDBN maintains information on each block stored in that frame.  These blocks are organized hierarchically, and are represented individually in Python as a gdb.Block. Please see Frames In Python, for a more in-depth discussion on frames.  Furthermore, see Examining the Stack, for more detailed technical information on GDBN's book-keeping of the stack.

The following block-related functions are available in the gdb module:

 -- Function: gdb.block_for_pc (pc)
     Return the gdb.Block containing the given PC value.  If the block cannot be found for the PC value specified, the function will return None.

A gdb.Block object has the following methods:

      -- Function: Block.is_valid ()
          Returns True if the gdb.Block object is valid, False if not.  A block object can become invalid if the block it refers to doesn't exist anymore in the inferior.  All other gdb.Block methods will throw an exception if it is invalid at the time the method is called.  This method is also made available to the Python iterator object that gdb.Block provides in an iteration context and via the Python iter built-in function.

A gdb.Block object has the following attributes:

      -- Variable: Block.start
          The start address of the block.  This attribute is not writable.

      -- Variable: Block.end
          The end address of the block.  This attribute is not writable.

      -- Variable: Block.function
          The name of the block represented as a gdb.Symbol.  If the block is not named, then this attribute holds None.  This attribute is not writable.

      -- Variable: Block.superblock
          The block containing this block.  If this parent block does not exist, this attribute holds None.  This attribute is not writable.

      -- Variable: Block.global_block
          The global block associated with this block.  This attribute is not writable.

      -- Variable: Block.static_block
          The static block associated with this block.  This attribute is not writable.

      -- Variable: Block.is_global
          True if the gdb.Block object is a global block, False if not.  This attribute is not writable.

      -- Variable: Block.is_static
          True if the gdb.Block object is a static block, False if not.  This attribute is not writable.

Python representation of Symbols.
.................................

GDBN represents every variable, function and type as an entry in a symbol table.  See Examining the Symbol Table. Similarly, Python represents these symbols in GDBN with the gdb.Symbol object.

The following symbol-related functions are available in the gdb module:

 -- Function: gdb.lookup_symbol (name [, block [, domain]])
     This function searches for a symbol by name.  The search scope can be restricted to the parameters defined in the optional domain and block arguments.

     NAME is the name of the symbol.  It must be a string.  The optional BLOCK argument restricts the search to symbols visible in that BLOCK.  The BLOCK argument must be a gdb.Block object.  If omitted, the block for the current frame is used.  The optional DOMAIN argument restricts the search to the domain type.  The DOMAIN argument must be a domain constant defined in the gdb module and described later in this chapter.

     The result is a tuple of two elements. The first element is a gdb.Symbol object or None if the symbol is not found. If the symbol is found, the second element is True if the symbol is a field of a method's object (e.g., this in C++), otherwise it is False. If the symbol is not found, the second element is False.

 -- Function: gdb.lookup_global_symbol (name [, domain])
     This function searches for a global symbol by name. The search scope can be restricted to by the domain argument.

     NAME is the name of the symbol.  It must be a string. The optional DOMAIN argument restricts the search to the domain type. The DOMAIN argument must be a domain constant defined in the gdb module and described later in this chapter.

     The result is a gdb.Symbol object or None if the symbol is not found.

A gdb.Symbol object has the following attributes:

      -- Variable: Symbol.type
          The type of the symbol or None if no type is recorded. This attribute is represented as a gdb.Type object. See Types In Python.  This attribute is not writable.

      -- Variable: Symbol.symtab
          The symbol table in which the symbol appears.  This attribute is represented as a gdb.Symtab object.  See Symbol Tables In Python.  This attribute is not writable.

      -- Variable: Symbol.name
          The name of the symbol as a string.  This attribute is not writable.

      -- Variable: Symbol.linkage_name
          The name of the symbol, as used by the linker (i.e., may be mangled). This attribute is not writable.

      -- Variable: Symbol.print_name
          The name of the symbol in a form suitable for output.  This is either name or linkage_name, depending on whether the user asked GDBN to display demangled or mangled names.

      -- Variable: Symbol.addr_class
          The address class of the symbol.  This classifies how to find the value of a symbol.  Each address class is a constant defined in the gdb module and described later in this chapter.

      -- Variable: Symbol.is_argument
          True if the symbol is an argument of a function.

      -- Variable: Symbol.is_constant
          True if the symbol is a constant.

      -- Variable: Symbol.is_function
          True if the symbol is a function or a method.

      -- Variable: Symbol.is_variable
          True if the symbol is a variable.

A gdb.Symbol object has the following methods:

      -- Function: Symbol.is_valid ()
          Returns True if the gdb.Symbol object is valid, False if not.  A gdb.Symbol object can become invalid if the symbol it refers to does not exist in GDBN any longer. All other gdb.Symbol methods will throw an exception if it is invalid at the time the method is called.

The available domain categories in gdb.Symbol are represented as constants in the gdb module:

gdb.SYMBOL_UNDEF_DOMAIN
     This is used when a domain has not been discovered or none of the following domains apply.  This usually indicates an error either in the symbol information or in GDBN's handling of symbols.

gdb.SYMBOL_VAR_DOMAIN
     This domain contains variables, function names, typedef names and enum type values.

gdb.SYMBOL_STRUCT_DOMAIN
     This domain holds struct, union and enum type names.

gdb.SYMBOL_LABEL_DOMAIN
     This domain contains names of labels (for gotos).

gdb.SYMBOL_VARIABLES_DOMAIN
     This domain holds a subset of the SYMBOLS_VAR_DOMAIN; it contains everything minus functions and types.

gdb.SYMBOL_FUNCTION_DOMAIN
     This domain contains all functions.

gdb.SYMBOL_TYPES_DOMAIN
     This domain contains all types.

The available address class categories in gdb.Symbol are represented as constants in the gdb module:

gdb.SYMBOL_LOC_UNDEF
     If this is returned by address class, it indicates an error either in the symbol information or in GDBN's handling of symbols.

gdb.SYMBOL_LOC_CONST
     Value is constant int.

gdb.SYMBOL_LOC_STATIC
     Value is at a fixed address.

gdb.SYMBOL_LOC_REGISTER
     Value is in a register.

gdb.SYMBOL_LOC_ARG
     Value is an argument.  This value is at the offset stored within the symbol inside the frame's argument list.

gdb.SYMBOL_LOC_REF_ARG
     Value address is stored in the frame's argument list.  Just like LOC_ARG except that the value's address is stored at the offset, not the value itself.

gdb.SYMBOL_LOC_REGPARM_ADDR
     Value is a specified register.  Just like LOC_REGISTER except the register holds the address of the argument instead of the argument itself.

gdb.SYMBOL_LOC_LOCAL
     Value is a local variable.

gdb.SYMBOL_LOC_TYPEDEF
     Value not used.  Symbols in the domain SYMBOL_STRUCT_DOMAIN all have this class.

gdb.SYMBOL_LOC_BLOCK
     Value is a block.

gdb.SYMBOL_LOC_CONST_BYTES
     Value is a byte-sequence.

gdb.SYMBOL_LOC_UNRESOLVED
     Value is at a fixed address, but the address of the variable has to be determined from the minimal symbol table whenever the variable is referenced.

gdb.SYMBOL_LOC_OPTIMIZED_OUT
     The value does not actually exist in the program.

gdb.SYMBOL_LOC_COMPUTED
     The value's address is a computed location.

Symbol table representation in Python.
......................................

Access to symbol table data maintained by GDBN on the inferior is exposed to Python via two objects: gdb.Symtab_and_line and gdb.Symtab.  Symbol table and line data for a frame is returned from the find_sal method in gdb.Frame object. See Frames In Python.

For more information on GDBN's symbol table management, see Examining the Symbol Table, for more information.

A gdb.Symtab_and_line object has the following attributes:

      -- Variable: Symtab_and_line.symtab
          The symbol table object (gdb.Symtab) for this frame. This attribute is not writable.

      -- Variable: Symtab_and_line.pc
          Indicates the current program counter address.  This attribute is not writable.

      -- Variable: Symtab_and_line.line
          Indicates the current line number for this object.  This attribute is not writable.

A gdb.Symtab_and_line object has the following methods:

      -- Function: Symtab_and_line.is_valid ()
          Returns True if the gdb.Symtab_and_line object is valid, False if not.  A gdb.Symtab_and_line object can become invalid if the Symbol table and line object it refers to does not exist in GDBN any longer.  All other gdb.Symtab_and_line methods will throw an exception if it is invalid at the time the method is called.

A gdb.Symtab object has the following attributes:

      -- Variable: Symtab.filename
          The symbol table's source filename.  This attribute is not writable.

      -- Variable: Symtab.objfile
          The symbol table's backing object file.  See Objfiles In Python. This attribute is not writable.

A gdb.Symtab object has the following methods:

      -- Function: Symtab.is_valid ()
          Returns True if the gdb.Symtab object is valid, False if not.  A gdb.Symtab object can become invalid if the symbol table it refers to does not exist in GDBN any longer.  All other gdb.Symtab methods will throw an exception if it is invalid at the time the method is called.

      -- Function: Symtab.fullname ()
          Return the symbol table's source absolute file name.

Manipulating breakpoints using Python
.....................................

Python code can manipulate breakpoints via the gdb.Breakpoint class.

 -- Function: Breakpoint.__init__ (spec [, type [, wp_class [,internal]]])
     Create a new breakpoint.  SPEC is a string naming the location of the breakpoint, or an expression that defines a watchpoint.  The contents can be any location recognized by the break command, or in the case of a watchpoint, by the watch command.  The optional TYPE denotes the breakpoint to create from the types defined later in this chapter.  This argument can be either: gdb.BP_BREAKPOINT or gdb.BP_WATCHPOINT.  TYPE defaults to gdb.BP_BREAKPOINT.  The optional INTERNAL argument allows the breakpoint to become invisible to the user.  The breakpoint will neither be reported when created, nor will it be listed in the output from info breakpoints (but will be listed with the maint info breakpoints command).  The optional WP_CLASS argument defines the class of watchpoint to create, if TYPE is gdb.BP_WATCHPOINT.  If a watchpoint class is not provided, it is assumed to be a gdb.WP_WRITE class.

 -- Function: Breakpoint.stop (self)
     The gdb.Breakpoint class can be sub-classed and, in particular, you may choose to implement the stop method. If this method is defined as a sub-class of gdb.Breakpoint, it will be called when the inferior reaches any location of a breakpoint which instantiates that sub-class.  If the method returns True, the inferior will be stopped at the location of the breakpoint, otherwise the inferior will continue.

     If there are multiple breakpoints at the same location with a stop method, each one will be called regardless of the return status of the previous.  This ensures that all stop methods have a chance to execute at that location.  In this scenario if one of the methods returns True but the others return False, the inferior will still be stopped.

     You should not alter the execution state of the inferior (i.e., step, next, etc.), alter the current frame context (i.e., change the current active frame), or alter, add or delete any breakpoint.  As a general rule, you should not alter any data within GDBN or the inferior at this time.

     Example stop implementation:

          class MyBreakpoint (gdb.Breakpoint):
                def stop (self):
                  inf_val = gdb.parse_and_eval("foo")
                  if inf_val == 3:
                    return True
                  return False

The available watchpoint types represented by constants are defined in the gdb module:

gdb.WP_READ
     Read only watchpoint.

gdb.WP_WRITE
     Write only watchpoint.

gdb.WP_ACCESS
     Read/Write watchpoint.

 -- Function: Breakpoint.is_valid ()
     Return True if this Breakpoint object is valid, False otherwise.  A Breakpoint object can become invalid if the user deletes the breakpoint.  In this case, the object still exists, but the underlying breakpoint does not.  In the cases of watchpoint scope, the watchpoint remains valid even if execution of the inferior leaves the scope of that watchpoint.

 -- Function: Breakpoint.delete
     Permanently deletes the GDBN breakpoint.  This also invalidates the Python Breakpoint object.  Any further access to this object's attributes or methods will raise an error.

 -- Variable: Breakpoint.enabled
     This attribute is True if the breakpoint is enabled, and False otherwise.  This attribute is writable.

 -- Variable: Breakpoint.silent
     This attribute is True if the breakpoint is silent, and False otherwise.  This attribute is writable.

     Note that a breakpoint can also be silent if it has commands and the first command is silent.  This is not reported by the silent attribute.

 -- Variable: Breakpoint.thread
     If the breakpoint is thread-specific, this attribute holds the thread id.  If the breakpoint is not thread-specific, this attribute is None.  This attribute is writable.

 -- Variable: Breakpoint.task
     If the breakpoint is Ada task-specific, this attribute holds the Ada task id.  If the breakpoint is not task-specific (or the underlying language is not Ada), this attribute is None.  This attribute is writable.

 -- Variable: Breakpoint.ignore_count
     This attribute holds the ignore count for the breakpoint, an integer. This attribute is writable.

 -- Variable: Breakpoint.number
     This attribute holds the breakpoint's number --- the identifier used by the user to manipulate the breakpoint.  This attribute is not writable.

 -- Variable: Breakpoint.type
     This attribute holds the breakpoint's type --- the identifier used to determine the actual breakpoint type or use-case.  This attribute is not writable.

 -- Variable: Breakpoint.visible
     This attribute tells whether the breakpoint is visible to the user when set, or when the info breakpoints command is run.  This attribute is not writable.

The available types are represented by constants defined in the gdb module:

gdb.BP_BREAKPOINT
     Normal code breakpoint.

gdb.BP_WATCHPOINT
     Watchpoint breakpoint.

gdb.BP_HARDWARE_WATCHPOINT
     Hardware assisted watchpoint.

gdb.BP_READ_WATCHPOINT
     Hardware assisted read watchpoint.

gdb.BP_ACCESS_WATCHPOINT
     Hardware assisted access watchpoint.

 -- Variable: Breakpoint.hit_count
     This attribute holds the hit count for the breakpoint, an integer. This attribute is writable, but currently it can only be set to zero.

 -- Variable: Breakpoint.location
     This attribute holds the location of the breakpoint, as specified by the user.  It is a string.  If the breakpoint does not have a location (that is, it is a watchpoint) the attribute's value is None.  This attribute is not writable.

 -- Variable: Breakpoint.expression
     This attribute holds a breakpoint expression, as specified by the user.  It is a string.  If the breakpoint does not have an expression (the breakpoint is not a watchpoint) the attribute's value is None.  This attribute is not writable.

 -- Variable: Breakpoint.condition
     This attribute holds the condition of the breakpoint, as specified by the user.  It is a string.  If there is no condition, this attribute's value is None.  This attribute is writable.

 -- Variable: Breakpoint.commands
     This attribute holds the commands attached to the breakpoint.  If there are commands, this attribute's value is a string holding all the commands, separated by newlines.  If there are no commands, this attribute is None.  This attribute is not writable.

Finish Breakpoints
..................

A finish breakpoint is a temporary breakpoint set at the return address of a frame, based on the finish command.  gdb.FinishBreakpoint extends gdb.Breakpoint.  The underlying breakpoint will be disabled and deleted when the execution will run out of the breakpoint scope (i.e. Breakpoint.stop or FinishBreakpoint.out_of_scope triggered). Finish breakpoints are thread specific and must be create with the right thread selected.

 -- Function: FinishBreakpoint.__init__ ([frame] [, internal])
     Create a finish breakpoint at the return address of the gdb.Frame object FRAME.  If FRAME is not provided, this defaults to the newest frame.  The optional INTERNAL argument allows the breakpoint to become invisible to the user.  See Breakpoints In Python, for further details about this argument.

 -- Function: FinishBreakpoint.out_of_scope (self)
     In some circumstances (e.g. longjmp, C++ exceptions, GDBN return command, ...), a function may not properly terminate, and thus never hit the finish breakpoint.  When GDBN notices such a situation, the out_of_scope callback will be triggered.

     You may want to sub-class gdb.FinishBreakpoint and override this method:

          class MyFinishBreakpoint (gdb.FinishBreakpoint)
              def stop (self):
                  print "normal finish"
                  return True

              def out_of_scope ():
                  print "abnormal finish"

 -- Variable: FinishBreakpoint.return_value
     When GDBN is stopped at a finish breakpoint and the frame used to build the gdb.FinishBreakpoint object had debug symbols, this attribute will contain a gdb.Value object corresponding to the return value of the function.  The value will be None if the function return type is void or if the return value was not computable.  This attribute is not writable.

Python representation of lazy strings.
......................................

A lazy string is a string whose contents is not retrieved or encoded until it is needed.

A gdb.LazyString is represented in GDBN as an address that points to a region of memory, an encoding that will be used to encode that region of memory, and a length to delimit the region of memory that represents the string.  The difference between a gdb.LazyString and a string wrapped within a gdb.Value is that a gdb.LazyString will be treated differently by GDBN when printing.  A gdb.LazyString is retrieved and encoded during printing, while a gdb.Value wrapping a string is immediately retrieved and encoded on creation.

A gdb.LazyString object has the following functions:

 -- Function: LazyString.value ()
     Convert the gdb.LazyString to a gdb.Value.  This value will point to the string in memory, but will lose all the delayed retrieval, encoding and handling that GDBN applies to a gdb.LazyString.

 -- Variable: LazyString.address
     This attribute holds the address of the string.  This attribute is not writable.

 -- Variable: LazyString.length
     This attribute holds the length of the string in characters.  If the length is -1, then the string will be fetched and encoded up to the first null of appropriate width.  This attribute is not writable.

 -- Variable: LazyString.encoding
     This attribute holds the encoding that will be applied to the string when the string is printed by GDBN.  If the encoding is not set, or contains an empty string,  then GDBN will select the most appropriate encoding when the string is printed.  This attribute is not writable.

 -- Variable: LazyString.type
     This attribute holds the type that is represented by the lazy string's type.  For a lazy string this will always be a pointer type.  To resolve this to the lazy string's character type, use the type's target method.  See Types In Python.  This attribute is not writable.

Auto-loading
------------

When a new object file is read (for example, due to the file command, or because the inferior has loaded a shared library), GDBN will look for Python support scripts in several ways: OBJFILE-gdb.py and .debug_gdb_scripts section.

The auto-loading feature is useful for supplying application-specific debugging commands and scripts.

Auto-loading can be enabled or disabled, and the list of auto-loaded scripts can be printed.

set auto-load-scripts [yes|no]
     Enable or disable the auto-loading of Python scripts.

show auto-load-scripts
     Show whether auto-loading of Python scripts is enabled or disabled.

info auto-load-scripts [REGEXP]
     Print the list of all scripts that GDBN auto-loaded.

     Also printed is the list of scripts that were mentioned in the .debug_gdb_scripts section and were not found (see .debug_gdb_scripts section). This is useful because their names are not printed when GDBN tries to load them and fails.  There may be many of them, and printing an error message for each one is problematic.

     If REGEXP is supplied only scripts with matching names are printed.

     Example:

          (gdb) info auto-load-scripts
          Loaded  Script
          Yes     py-section-script.py
                  full name: /tmp/py-section-script.py
          Missing my-foo-pretty-printers.py

When reading an auto-loaded file, GDBN sets the current objfile.  This is available via the gdb.current_objfile function (see Objfiles In Python).  This can be useful for registering objfile-specific pretty-printers.

The OBJFILE-gdb.py file
.......................

When a new object file is read, GDBN looks for a file named OBJFILE-gdb.py, where OBJFILE is the object file's real name, formed by ensuring that the file name is absolute, following all symlinks, and resolving . and .. components.  If this file exists and is readable, GDBN will evaluate it as a Python script.

If this file does not exist, and if the parameter debug-file-directory is set (see Separate Debug Files), then GDBN will look for REAL-NAME in all of the directories mentioned in the value of debug-file-directory.

Finally, if this file does not exist, then GDBN will look for a file named DATA-DIRECTORY/python/auto-load/REAL-NAME, where DATA-DIRECTORY is GDBN's data directory (available via show data-directory, see Data Files), and REAL-NAME is the object file's real name, as described above.

GDBN does not track which files it has already auto-loaded this way. GDBN will load the associated script every time the corresponding OBJFILE is opened. So your -gdb.py file should be careful to avoid errors if it is evaluated more than once.

The .debug_gdb_scripts section
..............................

For systems using file formats like ELF and COFF, when GDBN loads a new object file it will look for a special section named .debug_gdb_scripts. If this section exists, its contents is a list of names of scripts to load.

GDBN will look for each specified script file first in the current directory and then along the source search path (see Specifying Source Directories), except that $cdir is not searched, since the compilation directory is not relevant to scripts.

Entries can be placed in section .debug_gdb_scripts with, for example, this GCC macro:

     /* Note: The "MS" section flags are to remove duplicates.  */
     #define DEFINE_GDB_SCRIPT(script_name) \
       asm("\
     .pushsection \".debug_gdb_scripts\", \"MS\",@progbits,1\n\
     .byte 1\n\
     .asciz \"" script_name "\"\n\
     .popsection \n\
     ");

Then one can reference the macro in a header or source file like this:

     DEFINE_GDB_SCRIPT ("my-app-scripts.py")

The script name may include directories if desired.

If the macro is put in a header, any application or library using this header will get a reference to the specified script.

Which flavor to choose?
.......................

Given the multiple ways of auto-loading Python scripts, it might not always be clear which one to choose.  This section provides some guidance.

Benefits of the -gdb.py way:

  * Can be used with file formats that don't support multiple sections.

  * Ease of finding scripts for public libraries.

     Scripts specified in the .debug_gdb_scripts section are searched for in the source search path. For publicly installed libraries, e.g., libstdc++, there typically isn't a source directory in which to find the script.

  * Doesn't require source code additions.

Benefits of the .debug_gdb_scripts way:

  * Works with static linking.

     Scripts for libraries done the -gdb.py way require an objfile to trigger their loading.  When an application is statically linked the only objfile available is the executable, and it is cumbersome to attach all the scripts from all the input libraries to the executable's -gdb.py script.

  * Works with classes that are entirely inlined.

     Some classes can be entirely inlined, and thus there may not be an associated shared library to attach a -gdb.py script to.

  * Scripts needn't be copied out of the source tree.

     In some circumstances, apps can be built out of large collections of internal libraries, and the build infrastructure necessary to install the -gdb.py scripts in a place where GDBN can find them is cumbersome.  It may be easier to specify the scripts in the .debug_gdb_scripts section as relative paths, and add a path to the top of the source tree to the source search path.

Python modules
--------------

GDBN comes with several modules to assist writing Python code.

gdb.printing
............

This module provides a collection of utilities for working with pretty-printers.

PrettyPrinter (NAME, SUBPRINTERS=None)
     This class specifies the API that makes info pretty-printer, enable pretty-printer and disable pretty-printer work. Pretty-printers should generally inherit from this class.

SubPrettyPrinter (NAME)
     For printers that handle multiple types, this class specifies the corresponding API for the subprinters.

RegexpCollectionPrettyPrinter (NAME)
     Utility class for handling multiple printers, all recognized via regular expressions. See Writing a Pretty-Printer, for an example.

FlagEnumerationPrinter (NAME)
     A pretty-printer which handles printing of enum values.  Unlike GDBN's built-in enum printing, this printer attempts to work properly when there is some overlap between the enumeration constants.  NAME is the name of the printer and also the name of the enum type to look up.

register_pretty_printer (OBJ, PRINTER, REPLACE=False)
     Register PRINTER with the pretty-printer list of OBJ. If REPLACE is True then any existing copy of the printer is replaced.  Otherwise a RuntimeError exception is raised if a printer with the same name already exists.

gdb.types
.........

This module provides a collection of utilities for working with gdb.Types objects.

get_basic_type (TYPE)
     Return TYPE with const and volatile qualifiers stripped, and with typedefs and C++ references converted to the underlying type.

     C++ example:

          typedef const int const_int;
          const_int foo (3);
          const_int& foo_ref (foo);
          int main () { return 0; }

     Then in gdb:

          (gdb) start
          (gdb) python import gdb.types
          (gdb) python foo_ref = gdb.parse_and_eval("foo_ref")
          (gdb) python print gdb.types.get_basic_type(foo_ref.type)
          int

has_field (TYPE, FIELD)
     Return True if TYPE, assumed to be a type with fields (e.g., a structure or union), has field FIELD.

make_enum_dict (ENUM_TYPE)
     Return a Python dictionary type produced from ENUM_TYPE.

deep_items (TYPE)
     Returns a Python iterator similar to the standard gdb.Type.iteritems method, except that the iterator returned by deep_items will recursively traverse anonymous struct or union fields.  For example:

          struct A
          {
              int a;
              union {
                  int b0;
                  int b1;
              };
          };

     Then in GDBN:

          (GDBP) python import gdb.types
          (GDBP) python struct_a = gdb.lookup_type("struct A")
          (GDBP) python print struct_a.keys ()
          {['a', '']}
          (GDBP) python print [k for k,v in gdb.types.deep_items(struct_a)]
          {['a', 'b0', 'b1']}

gdb.prompt
..........

This module provides a method for prompt value-substitution.

substitute_prompt (STRING)
     Return STRING with escape sequences substituted by values.  Some escape sequences take arguments.  You can specify arguments inside ``{}'' immediately following the escape sequence.

     The escape sequences you can pass to this function are:

     \\
          Substitute a backslash.

     \e
          Substitute an ESC character.

     \f
          Substitute the selected frame; an argument names a frame parameter.

     \n
          Substitute a newline.

     \p
          Substitute a parameter's value; the argument names the parameter.

     \r
          Substitute a carriage return.

     \t
          Substitute the selected thread; an argument names a thread parameter.

     \v
          Substitute the version of GDB.

     \w
          Substitute the current working directory.

     \[
          Begin a sequence of non-printing characters.  These sequences are typically used with the ESC character, and are not counted in the string length.  Example: ``\[\e[0;34m\](gdb)\[\e[0m\]'' will return a blue-colored ``(gdb)'' prompt where the length is five.

     \]
          End a sequence of non-printing characters.

     For example:

          substitute_prompt (``frame: \f,
                             print arguments: \p{print frame-arguments}'')

     will return the string:

          "frame: main, print arguments: scalars"

Creating new spellings of existing commands
===========================================

It is often useful to define alternate spellings of existing commands. For example, if a new GDBN command defined in Python has a long name to type, it is handy to have an abbreviated version of it that involves less typing.

GDBN itself uses aliases.  For example s is an alias of the step command even though it is otherwise an ambiguous abbreviation of other commands like set and show.

Aliases are also used to provide shortened or more common versions of multi-word commands.  For example, GDBN provides the tty alias of the set inferior-tty command.

You can define a new alias with the alias command.

alias [-a] [--] ALIAS = COMMAND

ALIAS specifies the name of the new alias. Each word of ALIAS must consist of letters, numbers, dashes and underscores.

COMMAND specifies the name of an existing command that is being aliased.

The -a option specifies that the new alias is an abbreviation of the command.  Abbreviations are not shown in command lists displayed by the help command.

The -- option specifies the end of options, and is useful when ALIAS begins with a dash.

Here is a simple example showing how to make an abbreviation of a command so that there is less to type. Suppose you were tired of typing disas, the current shortest unambiguous abbreviation of the disassemble command and you wanted an even shorter version named di. The following will accomplish this.

     (gdb) alias -a di = disas

Note that aliases are different from user-defined commands. With a user-defined command, you also need to write documentation for it with the document command. An alias automatically picks up the documentation of the existing command.

Here is an example where we make elms an abbreviation of elements in the set print elements command. This is to show that you can make an abbreviation of any part of a command.

     (gdb) alias -a set print elms = set print elements
     (gdb) alias -a show print elms = show print elements
     (gdb) set p elms 20
     (gdb) show p elms
     Limit on string chars or array elements to print is 200.

Note that if you are defining an alias of a set command, and you want to have an alias for the corresponding show command, then you need to define the latter separately.

Unambiguously abbreviated commands are allowed in COMMAND and ALIAS, just as they are normally.

     (gdb) alias -a set pr elms = set p ele

Finally, here is an example showing the creation of a one word alias for a more complex command. This creates alias spe of the command set print elements.

     (gdb) alias spe = set print elements
     (gdb) spe 20

Command Interpreters
********************

GDBN supports multiple command interpreters, and some command infrastructure to allow users or user interface writers to switch between interpreters or run commands in other interpreters.

GDBN currently supports two command interpreters, the console interpreter (sometimes called the command-line interpreter or CLI) and the machine interface interpreter (or GDB/MI).  This manual describes both of these interfaces in great detail.

By default, GDBN will start with the console interpreter. However, the user may choose to start GDBN with another interpreter by specifying the -i or --interpreter startup options.  Defined interpreters include:

console
     The traditional console or command-line interpreter.  This is the most often used interpreter with GDBN. With no interpreter specified at runtime, GDBN will use this interpreter.

mi
     The newest GDB/MI interface (currently mi2).  Used primarily by programs wishing to use GDBN as a backend for a debugger GUI or an IDE.  For more information, see The GDB/MI Interface.

mi2
     The current GDB/MI interface.

mi1
     The GDB/MI interface included in GDBN 5.1, 5.2, and 5.3.

The interpreter being used by GDBN may not be dynamically switched at runtime.  Although possible, this could lead to a very precarious situation.  Consider an IDE using GDB/MI.  If a user enters the command "interpreter-set console" in a console view, GDBN would switch to using the console interpreter, rendering the IDE inoperable!

Although you may only choose a single interpreter at startup, you may execute commands in any interpreter from the current interpreter using the appropriate command.  If you are running the console interpreter, simply use the interpreter-exec command:

     interpreter-exec mi "-data-list-register-names"

GDB/MI has a similar command, although it is only available in versions of GDBN which support GDB/MI version 2 (or greater).

GDBN Text User Interface
************************

The GDBN Text User Interface (TUI) is a terminal interface which uses the curses library to show the source file, the assembly output, the program registers and GDBN commands in separate text windows.  The TUI mode is supported only on platforms where a suitable version of the curses library is available.

The TUI mode is enabled by default when you invoke GDBN as GDBP -tui. You can also switch in and out of TUI mode while GDBN runs by using various TUI commands and key bindings, such as C-x C-a. See TUI Key Bindings.

TUI Overview
============

In TUI mode, GDBN can display several text windows:

command
     This window is the GDBN command window with the GDBN prompt and the GDBN output.  The GDBN input is still managed using readline.

source
     The source window shows the source file of the program.  The current line and active breakpoints are displayed in this window.

assembly
     The assembly window shows the disassembly output of the program.

register
     This window shows the processor registers.  Registers are highlighted when their values change.

The source and assembly windows show the current program position by highlighting the current line and marking it with a > marker. Breakpoints are indicated with two markers.  The first marker indicates the breakpoint type:

B
     Breakpoint which was hit at least once.

b
     Breakpoint which was never hit.

H
     Hardware breakpoint which was hit at least once.

h
     Hardware breakpoint which was never hit.

The second marker indicates whether the breakpoint is enabled or not:

+
     Breakpoint is enabled.

-
     Breakpoint is disabled.

The source, assembly and register windows are updated when the current thread changes, when the frame changes, or when the program counter changes.

These windows are not all visible at the same time.  The command window is always visible.  The others can be arranged in several layouts:

  * source only,

  * assembly only,

  * source and assembly,

  * source and registers, or

  * assembly and registers.

A status line above the command window shows the following information:

target
     Indicates the current GDBN target. (see Specifying a Debugging Target).

process
     Gives the current process or thread number. When no process is being debugged, this field is set to No process.

function
     Gives the current function name for the selected frame. The name is demangled if demangling is turned on (see Print Settings). When there is no symbol corresponding to the current program counter, the string ?? is displayed.

line
     Indicates the current line number for the selected frame. When the current line number is not known, the string ?? is displayed.

pc
     Indicates the current program counter address.

TUI Key Bindings
================

The TUI installs several key bindings in the readline keymaps (see Command Line Editing). The following key bindings are installed for both TUI mode and the GDBN standard mode.

C-x C-a
C-x a
C-x A
     Enter or leave the TUI mode.  When leaving the TUI mode, the curses window management stops and GDBN operates using its standard mode, writing on the terminal directly.  When reentering the TUI mode, control is given back to the curses windows. The screen is then refreshed.

C-x 1
     Use a TUI layout with only one window.  The layout will either be source or assembly.  When the TUI mode is not active, it will switch to the TUI mode.

     Think of this key binding as the Emacs C-x 1 binding.

C-x 2
     Use a TUI layout with at least two windows.  When the current layout already has two windows, the next layout with two windows is used. When a new layout is chosen, one window will always be common to the previous layout and the new one.

     Think of it as the Emacs C-x 2 binding.

C-x o
     Change the active window.  The TUI associates several key bindings (like scrolling and arrow keys) with the active window.  This command gives the focus to the next TUI window.

     Think of it as the Emacs C-x o binding.

C-x s
     Switch in and out of the TUI SingleKey mode that binds single keys to GDBN commands (see TUI Single Key Mode).

The following key bindings only work in the TUI mode:

PgUp
     Scroll the active window one page up.

PgDn
     Scroll the active window one page down.

Up
     Scroll the active window one line up.

Down
     Scroll the active window one line down.

Left
     Scroll the active window one column left.

Right
     Scroll the active window one column right.

C-L
     Refresh the screen.

Because the arrow keys scroll the active window in the TUI mode, they are not available for their normal use by readline unless the command window has the focus.  When another window is active, you must use other readline key bindings such as C-p, C-n, C-b and C-f to control the command window.

TUI Single Key Mode
===================

The TUI also provides a SingleKey mode, which binds several frequently used GDBN commands to single keys.  Type C-x s to switch into this mode, where the following key bindings are used:

c
     continue

d
     down

f
     finish

n
     next

q
     exit the SingleKey mode.

r
     run

s
     step

u
     up

v
     info locals

w
     where

Other keys temporarily switch to the GDBN command prompt. The key that was pressed is inserted in the editing buffer so that it is possible to type most GDBN commands without interaction with the TUI SingleKey mode.  Once the command is entered the TUI SingleKey mode is restored.  The only way to permanently leave this mode is by typing q or C-x s.

TUI-specific Commands
=====================

The TUI has specific commands to control the text windows. These commands are always available, even when GDBN is not in the TUI mode.  When GDBN is in the standard mode, most of these commands will automatically switch to the TUI mode.

Note that if GDBN's stdout is not connected to a terminal, or GDBN has been started with the machine interface interpreter (see The GDB/MI Interface), most of these commands will fail with an error, because it would not be possible or desirable to enable curses window management.

info win
     List and give the size of all displayed windows.

layout next
     Display the next layout.

layout prev
     Display the previous layout.

layout src
     Display the source window only.

layout asm
     Display the assembly window only.

layout split
     Display the source and assembly window.

layout regs
     Display the register window together with the source or assembly window.

focus next
     Make the next window active for scrolling.

focus prev
     Make the previous window active for scrolling.

focus src
     Make the source window active for scrolling.

focus asm
     Make the assembly window active for scrolling.

focus regs
     Make the register window active for scrolling.

focus cmd
     Make the command window active for scrolling.

refresh
     Refresh the screen.  This is similar to typing C-L.

tui reg float
     Show the floating point registers in the register window.

tui reg general
     Show the general registers in the register window.

tui reg next
     Show the next register group.  The list of register groups as well as their order is target specific.  The predefined register groups are the following: general, float, system, vector, all, save, restore.

tui reg system
     Show the system registers in the register window.

update
     Update the source window and the current execution point.

winheight NAME +COUNT
winheight NAME -COUNT
     Change the height of the window NAME by COUNT lines.  Positive counts increase the height, while negative counts decrease it.

tabset NCHARS
     Set the width of tab stops to be NCHARS characters.

TUI Configuration Variables
===========================

Several configuration variables control the appearance of TUI windows.

set tui border-kind KIND
     Select the border appearance for the source, assembly and register windows. The possible values are the following:

     space
          Use a space character to draw the border.

     ascii
          Use ASCII characters +, - and | to draw the border.

     acs
          Use the Alternate Character Set to draw the border.  The border is drawn using character line graphics if the terminal supports them.

set tui border-mode MODE
set tui active-border-mode MODE
     Select the display attributes for the borders of the inactive windows or the active window.  The MODE can be one of the following:

     normal
          Use normal attributes to display the border.

     standout
          Use standout mode.

     reverse
          Use reverse video mode.

     half
          Use half bright mode.

     half-standout
          Use half bright and standout mode.

     bold
          Use extra bright or bold mode.

     bold-standout
          Use extra bright or bold and standout mode.

Using GDBN under GNU Emacs
**************************

A special interface allows you to use GNU Emacs to view (and edit) the source files for the program you are debugging with GDBN.

To use this interface, use the command M-x gdb in Emacs.  Give the executable file you want to debug as an argument.  This command starts GDBN as a subprocess of Emacs, with input and output through a newly created Emacs buffer.

Running GDBN under Emacs can be just like running GDBN normally except for two things:

  * All ``terminal'' input and output goes through an Emacs buffer, called the GUD buffer.

     This applies both to GDBN commands and their output, and to the input and output done by the program you are debugging.

     This is useful because it means that you can copy the text of previous commands and input them again; you can even use parts of the output in this way.

     All the facilities of Emacs' Shell mode are available for interacting with your program.  In particular, you can send signals the usual way---for example, C-c C-c for an interrupt, C-c C-z for a stop.

  * GDBN displays source code through Emacs.

     Each time GDBN displays a stack frame, Emacs automatically finds the source file for that frame and puts an arrow (=>) at the left margin of the current line.  Emacs uses a separate buffer for source display, and splits the screen to show both your GDBN session and the source.

     Explicit GDBN list or search commands still produce output as usual, but you probably have no reason to use them from Emacs.

We call this text command mode.  Emacs 22.1, and later, also uses a graphical mode, enabled by default, which provides further buffers that can control the execution and describe the state of your program. See GDB Graphical Interface (The GNU Emacs Manual).

If you specify an absolute file name when prompted for the M-x gdb argument, then Emacs sets your current working directory to where your program resides.  If you only specify the file name, then Emacs sets your current working directory to the directory associated with the previous buffer.  In this case, GDBN may find your program by searching your environment's PATH variable, but on some operating systems it might not find the source.  So, although the GDBN input and output session proceeds normally, the auxiliary buffer does not display the current source and line of execution.

The initial working directory of GDBN is printed on the top line of the GUD buffer and this serves as a default for the commands that specify files for GDBN to operate on.  See Commands to Specify Files.

By default, M-x gdb calls the program called gdb.  If you need to call GDBN by a different name (for example, if you keep several configurations around, with different names) you can customize the Emacs variable gud-gdb-command-name to run the one you want.

In the GUD buffer, you can use these special Emacs commands in addition to the standard Shell mode commands:

C-h m
     Describe the features of Emacs' GUD Mode.

C-c C-s
     Execute to another source line, like the GDBN step command; also update the display window to show the current file and location.

C-c C-n
     Execute to next source line in this function, skipping all function calls, like the GDBN next command.  Then update the display window to show the current file and location.

C-c C-i
     Execute one instruction, like the GDBN stepi command; update display window accordingly.

C-c C-f
     Execute until exit from the selected stack frame, like the GDBN finish command.

C-c C-r
     Continue execution of your program, like the GDBN continue command.

C-c <
     Go up the number of frames indicated by the numeric argument (see Numeric Arguments (The GNU Emacs Manual)), like the GDBN up command.

C-c >
     Go down the number of frames indicated by the numeric argument, like the GDBN down command.

In any source file, the Emacs command C-x SPC (gud-break) tells GDBN to set a breakpoint on the source line point is on.

In text command mode, if you type M-x speedbar, Emacs displays a separate frame which shows a backtrace when the GUD buffer is current. Move point to any frame in the stack and type RET to make it become the current frame and display the associated source in the source buffer.  Alternatively, click Mouse-2 to make the selected frame become the current one.  In graphical mode, the speedbar displays watch expressions.

If you accidentally delete the source-display buffer, an easy way to get it back is to type the command f in the GDBN buffer, to request a frame display; when you run under Emacs, this recreates the source buffer if necessary to show you the context of the current frame.

The source files displayed in Emacs are in ordinary Emacs buffers which are visiting the source files in the usual way.  You can edit the files with these buffers if you wish; but keep in mind that GDBN communicates with Emacs in terms of line numbers.  If you add or delete lines from the text, the line numbers that GDBN knows cease to correspond properly with the code.

A more detailed description of Emacs' interaction with GDBN is given in the Emacs manual (see Debuggers (The GNU Emacs Manual)).

The GDB/MI Interface
********************

Function and Purpose
====================

GDB/MI is a line based machine oriented text interface to GDBN and is activated by specifying using the --interpreter command line option (see Mode Options).  It is specifically intended to support the development of systems which use the debugger as just one small component of a larger system.

This chapter is a specification of the GDB/MI interface.  It is written in the form of a reference manual.

Note that GDB/MI is still under construction, so some of the features described below are incomplete and subject to change (see GDB/MI Development and Front Ends).

Notation and Terminology
========================

This chapter uses the following notation:

  * | separates two alternatives.

  * [ SOMETHING ] indicates that SOMETHING is optional: it may or may not be given.

  * ( GROUP )* means that GROUP inside the parentheses may repeat zero or more times.

  * ( GROUP )+ means that GROUP inside the parentheses may repeat one or more times.

  * "STRING" means a literal STRING.

GDB/MI General Design
=====================

Interaction of a GDB/MI frontend with GDBN involves three parts---commands sent to GDBN, responses to those commands and notifications.  Each command results in exactly one response, indicating either successful completion of the command, or an error. For the commands that do not resume the target, the response contains the requested information.  For the commands that resume the target, the response only indicates whether the target was successfully resumed. Notifications is the mechanism for reporting changes in the state of the target, or in GDBN state, that cannot conveniently be associated with a command and reported as part of that command response.

The important examples of notifications are:

  * Exec notifications.  These are used to report changes in target state---when a target is resumed, or stopped.  It would not be feasible to include this information in response of resuming commands, because one resume commands can result in multiple events in different threads.  Also, quite some time may pass before any event happens in the target, while a frontend needs to know whether the resuming command itself was successfully executed.

  * Console output, and status notifications.  Console output notifications are used to report output of CLI commands, as well as diagnostics for other commands.  Status notifications are used to report the progress of a long-running operation.  Naturally, including this information in command response would mean no output is produced until the command is finished, which is undesirable.

  * General notifications.  Commands may have various side effects on the GDBN or target state beyond their official purpose.  For example, a command may change the selected thread.  Although such changes can be included in command response, using notification allows for more orthogonal frontend design.

There's no guarantee that whenever an MI command reports an error, GDBN or the target are in any specific state, and especially, the state is not reverted to the state before the MI command was processed.  Therefore, whenever an MI command results in an error, we recommend that the frontend refreshes all the information shown in the user interface.

Context management
------------------

In most cases when GDBN accesses the target, this access is done in context of a specific thread and frame (see Frames). Often, even when accessing global data, the target requires that a thread be specified.  The CLI interface maintains the selected thread and frame, and supplies them to target on each command.  This is convenient, because a command line user would not want to specify that information explicitly on each command, and because user interacts with GDBN via a single terminal, so no confusion is possible as to what thread and frame are the current ones.

In the case of MI, the concept of selected thread and frame is less useful.  First, a frontend can easily remember this information itself.  Second, a graphical frontend can have more than one window, each one used for debugging a different thread, and the frontend might want to access additional threads for internal purposes.  This increases the risk that by relying on implicitly selected thread, the frontend may be operating on a wrong one.  Therefore, each MI command should explicitly specify which thread and frame to operate on.  To make it possible, each MI command accepts the --thread and --frame options, the value to each is GDBN identifier for thread and frame to operate on.

Usually, each top-level window in a frontend allows the user to select a thread and a frame, and remembers the user selection for further operations.  However, in some cases GDBN may suggest that the current thread be changed.  For example, when stopping on a breakpoint it is reasonable to switch to the thread where breakpoint is hit.  For another example, if the user issues the CLI thread command via the frontend, it is desirable to change the frontend's selected thread to the one specified by user.  GDBN communicates the suggestion to change current thread using the =thread-selected notification. No such notification is available for the selected frame at the moment.

Note that historically, MI shares the selected thread with CLI, so frontends used the -thread-select to execute commands in the right context.  However, getting this to work right is cumbersome.  The simplest way is for frontend to emit -thread-select command before every command.  This doubles the number of commands that need to be sent.  The alternative approach is to suppress -thread-select if the selected thread in GDBN is supposed to be identical to the thread the frontend wants to operate on.  However, getting this optimization right can be tricky.  In particular, if the frontend sends several commands to GDBN, and one of the commands changes the selected thread, then the behaviour of subsequent commands will change.  So, a frontend should either wait for response from such problematic commands, or explicitly add -thread-select for all subsequent commands.  No frontend is known to do this exactly right, so it is suggested to just always pass the --thread and --frame options.

Asynchronous command execution and non-stop mode
------------------------------------------------

On some targets, GDBN is capable of processing MI commands even while the target is running.  This is called asynchronous command execution (see Background Execution).  The frontend may specify a preferrence for asynchronous execution using the -gdb-set target-async 1 command, which should be emitted before either running the executable or attaching to the target.  After the frontend has started the executable or attached to the target, it can find if asynchronous execution is enabled using the -list-target-features command.

Even if GDBN can accept a command while target is running, many commands that access the target do not work when the target is running.  Therefore, asynchronous command execution is most useful when combined with non-stop mode (see Non-Stop Mode).  Then, it is possible to examine the state of one thread, while other threads are running.

When a given thread is running, MI commands that try to access the target in the context of that thread may not work, or may work only on some targets.  In particular, commands that try to operate on thread's stack will not work, on any target.  Commands that read memory, or modify breakpoints, may work or not work, depending on the target.  Note that even commands that operate on global state, such as print, set, and breakpoint commands, still access the target in the context of a specific thread,  so frontend should try to find a stopped thread and perform the operation on that thread (using the --thread option).

Which commands will work in the context of a running thread is highly target dependent.  However, the two commands -exec-interrupt, to stop a thread, and -thread-info, to find the state of a thread, will always work.

Thread groups
-------------

GDBN may be used to debug several processes at the same time. On some platfroms, GDBN may support debugging of several hardware systems, each one having several cores with several different processes running on each core.  This section describes the MI mechanism to support such debugging scenarios.

The key observation is that regardless of the structure of the target, MI can have a global list of threads, because most commands that accept the --thread option do not need to know what process that thread belongs to.  Therefore, it is not necessary to introduce neither additional --process option, nor an notion of the current process in the MI interface.  The only strictly new feature that is required is the ability to find how the threads are grouped into processes.

To allow the user to discover such grouping, and to support arbitrary hierarchy of machines/cores/processes, MI introduces the concept of a thread group.  Thread group is a collection of threads and other thread groups.  A thread group always has a string identifier, a type, and may have additional attributes specific to the type.  A new command, -list-thread-groups, returns the list of top-level thread groups, which correspond to processes that GDBN is debugging at the moment.  By passing an identifier of a thread group to the -list-thread-groups command, it is possible to obtain the members of specific thread group.

To allow the user to easily discover processes, and other objects, he wishes to debug, a concept of available thread group is introduced.  Available thread group is an thread group that GDBN is not debugging, but that can be attached to, using the -target-attach command.  The list of available top-level thread groups can be obtained using -list-thread-groups --available. In general, the content of a thread group may be only retrieved only after attaching to that thread group.

Thread groups are related to inferiors (see Inferiors and Programs).  Each inferior corresponds to a thread group of a special type process, and some additional operations are permitted on such thread groups.

GDB/MI Command Syntax
=====================

GDB/MI Input Syntax
-------------------

COMMAND ==>
     CLI-COMMAND | MI-COMMAND

CLI-COMMAND ==>
     [ TOKEN ] CLI-COMMAND NL, where CLI-COMMAND is any existing GDBN CLI command.

MI-COMMAND ==>
     [ TOKEN ] "-" OPERATION ( " " OPTION )* [ " --" ] ( " " PARAMETER )* NL

TOKEN ==>
     "any sequence of digits"

OPTION ==>
     "-" PARAMETER [ " " PARAMETER ]

PARAMETER ==>
     NON-BLANK-SEQUENCE | C-STRING

OPERATION ==>
     any of the operations described in this chapter

NON-BLANK-SEQUENCE ==>
     anything, provided it doesn't contain special characters such as "-", NL, """ and of course " "

C-STRING ==>
     """ SEVEN-BIT-ISO-C-STRING-CONTENT """

NL ==>
     CR | CR-LF

Notes:

  * The CLI commands are still handled by the MI interpreter; their output is described below.

  * The TOKEN, when present, is passed back when the command finishes.

  * Some MI commands accept optional arguments as part of the parameter list.  Each option is identified by a leading - (dash) and may be followed by an optional argument parameter.  Options occur first in the parameter list and can be delimited from normal parameters using -- (this is useful when some parameters begin with a dash).

Pragmatics:

  * We want easy access to the existing CLI syntax (for debugging).

  * We want it to be easy to spot a MI operation.

GDB/MI Output Syntax
--------------------

The output from GDB/MI consists of zero or more out-of-band records followed, optionally, by a single result record.  This result record is for the most recent command.  The sequence of output records is terminated by (gdb).

If an input command was prefixed with a TOKEN then the corresponding output for that command will also be prefixed by that same TOKEN.

OUTPUT ==>
     ( OUT-OF-BAND-RECORD )* [ RESULT-RECORD ] "(gdb)" NL

RESULT-RECORD ==>
      [ TOKEN ] "^" RESULT-CLASS ( "," RESULT )* NL

OUT-OF-BAND-RECORD ==>
     ASYNC-RECORD | STREAM-RECORD

ASYNC-RECORD ==>
     EXEC-ASYNC-OUTPUT | STATUS-ASYNC-OUTPUT | NOTIFY-ASYNC-OUTPUT

EXEC-ASYNC-OUTPUT ==>
     [ TOKEN ] "*" ASYNC-OUTPUT

STATUS-ASYNC-OUTPUT ==>
     [ TOKEN ] "+" ASYNC-OUTPUT

NOTIFY-ASYNC-OUTPUT ==>
     [ TOKEN ] "=" ASYNC-OUTPUT

ASYNC-OUTPUT ==>
     ASYNC-CLASS ( "," RESULT )* NL

RESULT-CLASS ==>
     "done" | "running" | "connected" | "error" | "exit"

ASYNC-CLASS ==>
     "stopped" | OTHERS (where OTHERS will be added depending on the needs---this is still in development).

RESULT ==>
      VARIABLE "=" VALUE

VARIABLE ==>
      STRING

VALUE ==>
      CONST | TUPLE | LIST

CONST ==>
     C-STRING

TUPLE ==>
      "{}" | "{" RESULT ( "," RESULT )* "}"

LIST ==>
      "[]" | "[" VALUE ( "," VALUE )* "]" | "[" RESULT ( "," RESULT )* "]"

STREAM-RECORD ==>
     CONSOLE-STREAM-OUTPUT | TARGET-STREAM-OUTPUT | LOG-STREAM-OUTPUT

CONSOLE-STREAM-OUTPUT ==>
     "~" C-STRING

TARGET-STREAM-OUTPUT ==>
     "@" C-STRING

LOG-STREAM-OUTPUT ==>
     "&" C-STRING

NL ==>
     CR | CR-LF

TOKEN ==>
     any sequence of digits.

Notes:

  * All output sequences end in a single line containing a period.

  * The TOKEN is from the corresponding request.  Note that for all async output, while the token is allowed by the grammar and may be output by future versions of GDBN for select async output messages, it is generally omitted.  Frontends should treat all async output as reporting general changes in the state of the target and there should be no need to associate async output to any prior command.

  * STATUS-ASYNC-OUTPUT contains on-going status information about the progress of a slow operation.  It can be discarded.  All status output is prefixed by +.

  * EXEC-ASYNC-OUTPUT contains asynchronous state change on the target (stopped, started, disappeared).  All async output is prefixed by *.

  * NOTIFY-ASYNC-OUTPUT contains supplementary information that the client should handle (e.g., a new breakpoint information).  All notify output is prefixed by =.

  * CONSOLE-STREAM-OUTPUT is output that should be displayed as is in the console.  It is the textual response to a CLI command.  All the console output is prefixed by ~.

  * TARGET-STREAM-OUTPUT is the output produced by the target program. All the target output is prefixed by @.

  * LOG-STREAM-OUTPUT is output text coming from GDBN's internals, for instance messages that should be displayed as part of an error log.  All the log output is prefixed by &.

  * New GDB/MI commands should only output LISTS containing VALUES.

See GDB/MI Stream Records, for more details about the various output records.

GDB/MI Compatibility with CLI
=============================

For the developers convenience CLI commands can be entered directly, but there may be some unexpected behaviour.  For example, commands that query the user will behave as if the user replied yes, breakpoint command lists are not executed and some CLI commands, such as if, when and define, prompt for further input with >, which is not valid MI output.

This feature may be removed at some stage in the future and it is recommended that front ends use the -interpreter-exec command (see -interpreter-exec).

GDB/MI Development and Front Ends
=================================

The application which takes the MI output and presents the state of the program being debugged to the user is called a front end.

Although GDB/MI is still incomplete, it is currently being used by a variety of front ends to GDBN.  This makes it difficult to introduce new functionality without breaking existing usage.  This section tries to minimize the problems by describing how the protocol might change.

Some changes in MI need not break a carefully designed front end, and for these the MI version will remain unchanged.  The following is a list of changes that may occur within one level, so front ends should parse MI output in a way that can handle them:

  * New MI commands may be added.

  * New fields may be added to the output of any MI command.

  * The range of values for fields with specified values, e.g., in_scope (see -var-update) may be extended.

If the changes are likely to break front ends, the MI version level will be increased by one.  This will allow the front end to parse the output according to the MI version.  Apart from mi0, new versions of GDBN will not support old versions of MI and it will be the responsibility of the front end to work with the new one.

The best way to avoid unexpected changes in MI that might break your front end is to make your project known to GDBN developers and follow development on gdb@sourceware.org and gdb-patches@sourceware.org.

GDB/MI Output Records
=====================

GDB/MI Result Records
---------------------

In addition to a number of out-of-band notifications, the response to a GDB/MI command includes one of the following result indications:

"^done" [ "," RESULTS ]
     The synchronous operation was successful, RESULTS are the return values.

"^running"
     This result record is equivalent to ^done.  Historically, it was output instead of ^done if the command has resumed the target.  This behaviour is maintained for backward compatibility, but all frontends should treat ^done and ^running identically and rely on the *running output record to determine which threads are resumed.

"^connected"
     GDBN has connected to a remote target.

"^error" "," C-STRING
     The operation failed.  The C-STRING contains the corresponding error message.

"^exit"
     GDBN has terminated.

GDB/MI Stream Records
---------------------

GDBN internally maintains a number of output streams: the console, the target, and the log.  The output intended for each of these streams is funneled through the GDB/MI interface using stream records.

Each stream record begins with a unique prefix character which identifies its stream (see GDB/MI Output Syntax).  In addition to the prefix, each stream record contains a STRING-OUTPUT.  This is either raw text (with an implicit new line) or a quoted C string (which does not contain an implicit newline).

"~" STRING-OUTPUT
     The console output stream contains text that should be displayed in the CLI console window.  It contains the textual responses to CLI commands.

"@" STRING-OUTPUT
     The target output stream contains any textual output from the running target.  This is only present when GDB's event loop is truly asynchronous, which is currently only the case for remote targets.

"&" STRING-OUTPUT
     The log stream contains debugging messages being produced by GDBN's internals.

GDB/MI Async Records
--------------------

Async records are used to notify the GDB/MI client of additional changes that have occurred.  Those changes can either be a consequence of GDB/MI commands (e.g., a breakpoint modified) or a result of target activity (e.g., target stopped).

The following is the list of possible async records:

*running,thread-id="THREAD"
     The target is now running.  The THREAD field tells which specific thread is now running, and can be all if all threads are running.  The frontend should assume that no interaction with a running thread is possible after this notification is produced. The frontend should not assume that this notification is output only once for any command.  GDBN may emit this notification several times, either for different threads, because it cannot resume all threads together, or even for a single thread, if the thread must be stepped though some code before letting it run freely.

*stopped,reason="REASON",thread-id="ID",stopped-threads="STOPPED",core="CORE"
     The target has stopped.  The REASON field can have one of the following values:

     breakpoint-hit
          A breakpoint was reached.

     watchpoint-trigger
          A watchpoint was triggered.

     read-watchpoint-trigger
          A read watchpoint was triggered.

     access-watchpoint-trigger
          An access watchpoint was triggered.

     function-finished
          An -exec-finish or similar CLI command was accomplished.

     location-reached
          An -exec-until or similar CLI command was accomplished.

     watchpoint-scope
          A watchpoint has gone out of scope.

     end-stepping-range
          An -exec-next, -exec-next-instruction, -exec-step, -exec-step-instruction or similar CLI command was accomplished.

     exited-signalled
          The inferior exited because of a signal.

     exited
          The inferior exited.

     exited-normally
          The inferior exited normally.

     signal-received
          A signal was received by the inferior.

     solib-event
          The inferior has stopped due to a library being loaded or unloaded. This can only happen when stop-on-solib-events (see Files) is set.

     fork
          The inferior has forked.  This is reported when catch fork (see Set Catchpoints) has been used.

     vfork
          The inferior has vforked.  This is reported in when catch vfork (see Set Catchpoints) has been used.

     syscall-entry
          The inferior entered a system call.  This is reported when catch syscall (see Set Catchpoints) has been used.

     syscall-entry
          The inferior returned from a system call.  This is reported when catch syscall (see Set Catchpoints) has been used.

     exec
          The inferior called exec.  This is reported when catch exec (see Set Catchpoints) has been used.

     The ID field identifies the thread that directly caused the stop -- for example by hitting a breakpoint.  Depending on whether all-stop mode is in effect (see All-Stop Mode), GDBN may either stop all threads, or only the thread that directly triggered the stop. If all threads are stopped, the STOPPED field will have the value of "all".  Otherwise, the value of the STOPPED field will be a list of thread identifiers.  Presently, this list will always include a single thread, but frontend should be prepared to see several threads in the list.  The CORE field reports the processor core on which the stop event has happened.  This field may be absent if such information is not available.

=thread-group-added,id="ID"
=thread-group-removed,id="ID"
     A thread group was either added or removed.  The ID field contains the GDBN identifier of the thread group.  When a thread group is added, it generally might not be associated with a running process.  When a thread group is removed, its id becomes invalid and cannot be used in any way.

=thread-group-started,id="ID",pid="PID"
     A thread group became associated with a running program, either because the program was just started or the thread group was attached to a program.  The ID field contains the GDBN identifier of the thread group.  The PID field contains process identifier, specific to the operating system.

=thread-group-exited,id="ID"[,exit-code="CODE"]
     A thread group is no longer associated with a running program, either because the program has exited, or because it was detached from.  The ID field contains the GDBN identifier of the thread group.  CODE is the exit code of the inferior; it exists only when the inferior exited with some code.

=thread-created,id="ID",group-id="GID"
=thread-exited,id="ID",group-id="GID"
     A thread either was created, or has exited.  The ID field contains the GDBN identifier of the thread.  The GID field identifies the thread group this thread belongs to.

=thread-selected,id="ID"
     Informs that the selected thread was changed as result of the last command.  This notification is not emitted as result of -thread-select command but is emitted whenever an MI command that is not documented to change the selected thread actually changes it.  In particular, invoking, directly or indirectly (via user-defined command), the CLI thread command, will generate this notification.

     We suggest that in response to this notification, front ends highlight the selected thread and cause subsequent commands to apply to that thread.

=library-loaded,...
     Reports that a new library file was loaded by the program.  This notification has 4 fields---ID, TARGET-NAME, HOST-NAME, and SYMBOLS-LOADED.  The ID field is an opaque identifier of the library.  For remote debugging case, TARGET-NAME and HOST-NAME fields give the name of the library file on the target, and on the host respectively.  For native debugging, both those fields have the same value.  The SYMBOLS-LOADED field is emitted only for backward compatibility and should not be relied on to convey any useful information.  The THREAD-GROUP field, if present, specifies the id of the thread group in whose context the library was loaded.  If the field is absent, it means the library was loaded in the context of all present thread groups.

=library-unloaded,...
     Reports that a library was unloaded by the program.  This notification has 3 fields---ID, TARGET-NAME and HOST-NAME with the same meaning as for the =library-loaded notification. The THREAD-GROUP field, if present, specifies the id of the thread group in whose context the library was unloaded.  If the field is absent, it means the library was unloaded in the context of all present thread groups.

=breakpoint-created,bkpt={...}
=breakpoint-modified,bkpt={...}
=breakpoint-deleted,bkpt={...}
     Reports that a breakpoint was created, modified, or deleted, respectively.  Only user-visible breakpoints are reported to the MI user.

     The BKPT argument is of the same form as returned by the various breakpoint commands; see GDB/MI Breakpoint Commands.

     Note that if a breakpoint is emitted in the result record of a command, then it will not also be emitted in an async record.

GDB/MI Frame Information
------------------------

Response from many MI commands includes an information about stack frame.  This information is a tuple that may have the following fields:

level
     The level of the stack frame.  The innermost frame has the level of zero.  This field is always present.

func
     The name of the function corresponding to the frame.  This field may be absent if GDBN is unable to determine the function name.

addr
     The code address for the frame.  This field is always present.

file
     The name of the source files that correspond to the frame's code address.  This field may be absent.

line
     The source line corresponding to the frames' code address.  This field may be absent.

from
     The name of the binary file (either executable or shared library) the corresponds to the frame's code address.  This field may be absent.

GDB/MI Thread Information
-------------------------

Whenever GDBN has to report an information about a thread, it uses a tuple with the following fields:

id
     The numeric id assigned to the thread by GDBN.  This field is always present.

target-id
     Target-specific string identifying the thread.  This field is always present.

details
     Additional information about the thread provided by the target. It is supposed to be human-readable and not interpreted by the frontend.  This field is optional.

state
     Either stopped or running, depending on whether the thread is presently running.  This field is always present.

core
     The value of this field is an integer number of the processor core the thread was last seen on.  This field is optional.

GDB/MI Ada Exception Information
--------------------------------

Whenever a *stopped record is emitted because the program stopped after hitting an exception catchpoint (see Set Catchpoints), GDBN provides the name of the exception that was raised via the exception-name field.

Simple Examples of GDB/MI Interaction
=====================================

This subsection presents several simple examples of interaction using the GDB/MI interface.  In these examples, -> means that the following line is passed to GDB/MI as input, while <- means the output received from GDB/MI.

Note the line breaks shown in the examples are here only for readability, they don't appear in the real output.

Setting a Breakpoint
--------------------

Setting a breakpoint generates synchronous output which contains detailed information of the breakpoint.

     -> -break-insert main
     <- ^done,bkpt={number="1",type="breakpoint",disp="keep",
         enabled="y",addr="0x08048564",func="main",file="myprog.c",
         fullname="/home/nickrob/myprog.c",line="68",times="0"}
     <- (gdb)

Program Execution
-----------------

Program execution generates asynchronous records and MI gives the reason that execution stopped.

     -> -exec-run
     <- ^running
     <- (gdb)
     <- *stopped,reason="breakpoint-hit",disp="keep",bkptno="1",thread-id="0",
        frame={addr="0x08048564",func="main",
        args=[{name="argc",value="1"},{name="argv",value="0xbfc4d4d4"}],
        file="myprog.c",fullname="/home/nickrob/myprog.c",line="68"}
     <- (gdb)
     -> -exec-continue
     <- ^running
     <- (gdb)
     <- *stopped,reason="exited-normally"
     <- (gdb)

Quitting GDBN
-------------

Quitting GDBN just prints the result class ^exit.

     -> (gdb)
     <- -gdb-exit
     <- ^exit

Please note that ^exit is printed immediately, but it might take some time for GDBN to actually exit.  During that time, GDBN performs necessary cleanups, including killing programs being debugged or disconnecting from debug hardware, so the frontend should wait till GDBN exits and should only forcibly kill GDBN if it fails to exit in reasonable time.

A Bad Command
-------------

Here's what happens if you pass a non-existent command:

     -> -rubbish
     <- ^error,msg="Undefined MI command: rubbish"
     <- (gdb)

GDB/MI Command Description Format
=================================

The remaining sections describe blocks of commands.  Each block of commands is laid out in a fashion similar to this section.

Motivation
----------

The motivation for this collection of commands.

Introduction
------------

A brief introduction to this collection of commands as a whole.

Commands
--------

For each command in the block, the following is described:

Synopsis
........

      -command ARGS...

Result
......

GDBN Command
............

The corresponding GDBN CLI command(s), if any.

Example
.......

Example(s) formatted for readability.  Some of the described commands  have not been implemented yet and these are labeled N.A. (not available).

GDB/MI Breakpoint Commands
==========================

This section documents GDB/MI commands for manipulating breakpoints.

The -break-after Command
------------------------

Synopsis
........

      -break-after NUMBER COUNT

The breakpoint number NUMBER is not in effect until it has been hit COUNT times.  To see how this is reflected in the output of the -break-list command, see the description of the -break-list command below.

GDBN Command
............

The corresponding GDBN command is ignore.

Example
.......

     (gdb)
     -break-insert main
     ^done,bkpt={number="1",type="breakpoint",disp="keep",
     enabled="y",addr="0x000100d0",func="main",file="hello.c",
     fullname="/home/foo/hello.c",line="5",times="0"}
     (gdb)
     -break-after 1 3
     ~
     ^done
     (gdb)
     -break-list
     ^done,BreakpointTable={nr_rows="1",nr_cols="6",
     hdr=[{width="3",alignment="-1",col_name="number",colhdr="Num"},
     {width="14",alignment="-1",col_name="type",colhdr="Type"},
     {width="4",alignment="-1",col_name="disp",colhdr="Disp"},
     {width="3",alignment="-1",col_name="enabled",colhdr="Enb"},
     {width="10",alignment="-1",col_name="addr",colhdr="Address"},
     {width="40",alignment="2",col_name="what",colhdr="What"}],
     body=[bkpt={number="1",type="breakpoint",disp="keep",enabled="y",
     addr="0x000100d0",func="main",file="hello.c",fullname="/home/foo/hello.c",
     line="5",times="0",ignore="3"}]}
     (gdb)

The -break-commands Command
---------------------------

Synopsis
........

      -break-commands NUMBER [ COMMAND1 ... COMMANDN ]

Specifies the CLI commands that should be executed when breakpoint NUMBER is hit.  The parameters COMMAND1 to COMMANDN are the commands.  If no command is specified, any previously-set commands are cleared.  See Break Commands.  Typical use of this functionality is tracing a program, that is, printing of values of some variables whenever breakpoint is hit and then continuing.

GDBN Command
............

The corresponding GDBN command is commands.

Example
.......

     (gdb)
     -break-insert main
     ^done,bkpt={number="1",type="breakpoint",disp="keep",
     enabled="y",addr="0x000100d0",func="main",file="hello.c",
     fullname="/home/foo/hello.c",line="5",times="0"}
     (gdb)
     -break-commands 1 "print v" "continue"
     ^done
     (gdb)

The -break-condition Command
----------------------------

Synopsis
........

      -break-condition NUMBER EXPR

Breakpoint NUMBER will stop the program only if the condition in EXPR is true.  The condition becomes part of the -break-list output (see the description of the -break-list command below).

GDBN Command
............

The corresponding GDBN command is condition.

Example
.......

     (gdb)
     -break-condition 1 1
     ^done
     (gdb)
     -break-list
     ^done,BreakpointTable={nr_rows="1",nr_cols="6",
     hdr=[{width="3",alignment="-1",col_name="number",colhdr="Num"},
     {width="14",alignment="-1",col_name="type",colhdr="Type"},
     {width="4",alignment="-1",col_name="disp",colhdr="Disp"},
     {width="3",alignment="-1",col_name="enabled",colhdr="Enb"},
     {width="10",alignment="-1",col_name="addr",colhdr="Address"},
     {width="40",alignment="2",col_name="what",colhdr="What"}],
     body=[bkpt={number="1",type="breakpoint",disp="keep",enabled="y",
     addr="0x000100d0",func="main",file="hello.c",fullname="/home/foo/hello.c",
     line="5",cond="1",times="0",ignore="3"}]}
     (gdb)

The -break-delete Command
-------------------------

Synopsis
........

      -break-delete ( BREAKPOINT )+

Delete the breakpoint(s) whose number(s) are specified in the argument list.  This is obviously reflected in the breakpoint list.

GDBN Command
............

The corresponding GDBN command is delete.

Example
.......

     (gdb)
     -break-delete 1
     ^done
     (gdb)
     -break-list
     ^done,BreakpointTable={nr_rows="0",nr_cols="6",
     hdr=[{width="3",alignment="-1",col_name="number",colhdr="Num"},
     {width="14",alignment="-1",col_name="type",colhdr="Type"},
     {width="4",alignment="-1",col_name="disp",colhdr="Disp"},
     {width="3",alignment="-1",col_name="enabled",colhdr="Enb"},
     {width="10",alignment="-1",col_name="addr",colhdr="Address"},
     {width="40",alignment="2",col_name="what",colhdr="What"}],
     body=[]}
     (gdb)

The -break-disable Command
--------------------------

Synopsis
........

      -break-disable ( BREAKPOINT )+

Disable the named BREAKPOINT(s).  The field enabled in the break list is now set to n for the named BREAKPOINT(s).

GDBN Command
............

The corresponding GDBN command is disable.

Example
.......

     (gdb)
     -break-disable 2
     ^done
     (gdb)
     -break-list
     ^done,BreakpointTable={nr_rows="1",nr_cols="6",
     hdr=[{width="3",alignment="-1",col_name="number",colhdr="Num"},
     {width="14",alignment="-1",col_name="type",colhdr="Type"},
     {width="4",alignment="-1",col_name="disp",colhdr="Disp"},
     {width="3",alignment="-1",col_name="enabled",colhdr="Enb"},
     {width="10",alignment="-1",col_name="addr",colhdr="Address"},
     {width="40",alignment="2",col_name="what",colhdr="What"}],
     body=[bkpt={number="2",type="breakpoint",disp="keep",enabled="n",
     addr="0x000100d0",func="main",file="hello.c",fullname="/home/foo/hello.c",
     line="5",times="0"}]}
     (gdb)

The -break-enable Command
-------------------------

Synopsis
........

      -break-enable ( BREAKPOINT )+

Enable (previously disabled) BREAKPOINT(s).

GDBN Command
............

The corresponding GDBN command is enable.

Example
.......

     (gdb)
     -break-enable 2
     ^done
     (gdb)
     -break-list
     ^done,BreakpointTable={nr_rows="1",nr_cols="6",
     hdr=[{width="3",alignment="-1",col_name="number",colhdr="Num"},
     {width="14",alignment="-1",col_name="type",colhdr="Type"},
     {width="4",alignment="-1",col_name="disp",colhdr="Disp"},
     {width="3",alignment="-1",col_name="enabled",colhdr="Enb"},
     {width="10",alignment="-1",col_name="addr",colhdr="Address"},
     {width="40",alignment="2",col_name="what",colhdr="What"}],
     body=[bkpt={number="2",type="breakpoint",disp="keep",enabled="y",
     addr="0x000100d0",func="main",file="hello.c",fullname="/home/foo/hello.c",
     line="5",times="0"}]}
     (gdb)

The -break-info Command
-----------------------

Synopsis
........

      -break-info BREAKPOINT

Get information about a single breakpoint.

GDBN Command
............

The corresponding GDBN command is info break BREAKPOINT.

Example
.......

N.A.

The -break-insert Command
-------------------------

Synopsis
........

      -break-insert [ -t ] [ -h ] [ -f ] [ -d ] [ -a ]
         [ -c CONDITION ] [ -i IGNORE-COUNT ]
         [ -p THREAD ] [ LOCATION ]

If specified, LOCATION, can be one of:

  * function

  * filename:linenum

  * filename:function

  * *address

The possible optional parameters of this command are:

-t
     Insert a temporary breakpoint.

-h
     Insert a hardware breakpoint.

-c CONDITION
     Make the breakpoint conditional on CONDITION.

-i IGNORE-COUNT
     Initialize the IGNORE-COUNT.

-f
     If LOCATION cannot be parsed (for example if it refers to unknown files or functions), create a pending breakpoint. Without this flag, GDBN will report an error, and won't create a breakpoint, if LOCATION cannot be parsed.

-d
     Create a disabled breakpoint.

-a
     Create a tracepoint.  See Tracepoints.  When this parameter is used together with -h, a fast tracepoint is created.

Result
......

The result is in the form:

     ^done,bkpt={number="NUMBER",type="TYPE",disp="del"|"keep",
     enabled="y"|"n",addr="HEX",func="FUNCNAME",file="FILENAME",
     fullname="FULL_FILENAME",line="LINENO",[thread="THREADNO,]
     times="TIMES"}

where NUMBER is the GDBN number for this breakpoint, FUNCNAME is the name of the function where the breakpoint was inserted, FILENAME is the name of the source file which contains this function, LINENO is the source line number within that file and TIMES the number of times that the breakpoint has been hit (always 0 for -break-insert but may be greater for -break-info or -break-list which use the same output).

Note: this format is open to change.

GDBN Command
............

The corresponding GDBN commands are break, tbreak, hbreak, thbreak, and rbreak.

Example
.......

     (gdb)
     -break-insert main
     ^done,bkpt={number="1",addr="0x0001072c",file="recursive2.c",
     fullname="/home/foo/recursive2.c,line="4",times="0"}
     (gdb)
     -break-insert -t foo
     ^done,bkpt={number="2",addr="0x00010774",file="recursive2.c",
     fullname="/home/foo/recursive2.c,line="11",times="0"}
     (gdb)
     -break-list
     ^done,BreakpointTable={nr_rows="2",nr_cols="6",
     hdr=[{width="3",alignment="-1",col_name="number",colhdr="Num"},
     {width="14",alignment="-1",col_name="type",colhdr="Type"},
     {width="4",alignment="-1",col_name="disp",colhdr="Disp"},
     {width="3",alignment="-1",col_name="enabled",colhdr="Enb"},
     {width="10",alignment="-1",col_name="addr",colhdr="Address"},
     {width="40",alignment="2",col_name="what",colhdr="What"}],
     body=[bkpt={number="1",type="breakpoint",disp="keep",enabled="y",
     addr="0x0001072c", func="main",file="recursive2.c",
     fullname="/home/foo/recursive2.c,"line="4",times="0"},
     bkpt={number="2",type="breakpoint",disp="del",enabled="y",
     addr="0x00010774",func="foo",file="recursive2.c",
     fullname="/home/foo/recursive2.c",line="11",times="0"}]}
     (gdb)
     -break-insert -r foo.*
     ~int foo(int, int);
     ^done,bkpt={number="3",addr="0x00010774",file="recursive2.c,
     "fullname="/home/foo/recursive2.c",line="11",times="0"}
     (gdb)

The -break-list Command
-----------------------

Synopsis
........

      -break-list

Displays the list of inserted breakpoints, showing the following fields:

Number
     number of the breakpoint

Type
     type of the breakpoint: breakpoint or watchpoint

Disposition
     should the breakpoint be deleted or disabled when it is hit: keep or nokeep

Enabled
     is the breakpoint enabled or no: y or n

Address
     memory location at which the breakpoint is set

What
     logical location of the breakpoint, expressed by function name, file name, line number

Times
     number of times the breakpoint has been hit

If there are no breakpoints or watchpoints, the BreakpointTable body field is an empty list.

GDBN Command
............

The corresponding GDBN command is info break.

Example
.......

     (gdb)
     -break-list
     ^done,BreakpointTable={nr_rows="2",nr_cols="6",
     hdr=[{width="3",alignment="-1",col_name="number",colhdr="Num"},
     {width="14",alignment="-1",col_name="type",colhdr="Type"},
     {width="4",alignment="-1",col_name="disp",colhdr="Disp"},
     {width="3",alignment="-1",col_name="enabled",colhdr="Enb"},
     {width="10",alignment="-1",col_name="addr",colhdr="Address"},
     {width="40",alignment="2",col_name="what",colhdr="What"}],
     body=[bkpt={number="1",type="breakpoint",disp="keep",enabled="y",
     addr="0x000100d0",func="main",file="hello.c",line="5",times="0"},
     bkpt={number="2",type="breakpoint",disp="keep",enabled="y",
     addr="0x00010114",func="foo",file="hello.c",fullname="/home/foo/hello.c",
     line="13",times="0"}]}
     (gdb)

Here's an example of the result when there are no breakpoints:

     (gdb)
     -break-list
     ^done,BreakpointTable={nr_rows="0",nr_cols="6",
     hdr=[{width="3",alignment="-1",col_name="number",colhdr="Num"},
     {width="14",alignment="-1",col_name="type",colhdr="Type"},
     {width="4",alignment="-1",col_name="disp",colhdr="Disp"},
     {width="3",alignment="-1",col_name="enabled",colhdr="Enb"},
     {width="10",alignment="-1",col_name="addr",colhdr="Address"},
     {width="40",alignment="2",col_name="what",colhdr="What"}],
     body=[]}
     (gdb)

The -break-passcount Command
----------------------------

Synopsis
........

      -break-passcount TRACEPOINT-NUMBER PASSCOUNT

Set the passcount for tracepoint TRACEPOINT-NUMBER to PASSCOUNT.  If the breakpoint referred to by TRACEPOINT-NUMBER is not a tracepoint, error is emitted.  This corresponds to CLI command passcount.

The -break-watch Command
------------------------

Synopsis
........

      -break-watch [ -a | -r ]

Create a watchpoint.  With the -a option it will create an access watchpoint, i.e., a watchpoint that triggers either on a read from or on a write to the memory location.  With the -r option, the watchpoint created is a read watchpoint, i.e., it will trigger only when the memory location is accessed for reading.  Without either of the options, the watchpoint created is a regular watchpoint, i.e., it will trigger when the memory location is accessed for writing. See Setting Watchpoints.

Note that -break-list will report a single list of watchpoints and breakpoints inserted.

GDBN Command
............

The corresponding GDBN commands are watch, awatch, and rwatch.

Example
.......

Setting a watchpoint on a variable in the main function:

     (gdb)
     -break-watch x
     ^done,wpt={number="2",exp="x"}
     (gdb)
     -exec-continue
     ^running
     (gdb)
     *stopped,reason="watchpoint-trigger",wpt={number="2",exp="x"},
     value={old="-268439212",new="55"},
     frame={func="main",args=[],file="recursive2.c",
     fullname="/home/foo/bar/recursive2.c",line="5"}
     (gdb)

Setting a watchpoint on a variable local to a function.  GDBN will stop the program execution twice: first for the variable changing value, then for the watchpoint going out of scope.

     (gdb)
     -break-watch C
     ^done,wpt={number="5",exp="C"}
     (gdb)
     -exec-continue
     ^running
     (gdb)
     *stopped,reason="watchpoint-trigger",
     wpt={number="5",exp="C"},value={old="-276895068",new="3"},
     frame={func="callee4",args=[],
     file="../../../devo/gdb/testsuite/gdb.mi/basics.c",
     fullname="/home/foo/bar/devo/gdb/testsuite/gdb.mi/basics.c",line="13"}
     (gdb)
     -exec-continue
     ^running
     (gdb)
     *stopped,reason="watchpoint-scope",wpnum="5",
     frame={func="callee3",args=[{name="strarg",
     value="0x11940 \"A string argument.\""}],
     file="../../../devo/gdb/testsuite/gdb.mi/basics.c",
     fullname="/home/foo/bar/devo/gdb/testsuite/gdb.mi/basics.c",line="18"}
     (gdb)

Listing breakpoints and watchpoints, at different points in the program execution.  Note that once the watchpoint goes out of scope, it is deleted.

     (gdb)
     -break-watch C
     ^done,wpt={number="2",exp="C"}
     (gdb)
     -break-list
     ^done,BreakpointTable={nr_rows="2",nr_cols="6",
     hdr=[{width="3",alignment="-1",col_name="number",colhdr="Num"},
     {width="14",alignment="-1",col_name="type",colhdr="Type"},
     {width="4",alignment="-1",col_name="disp",colhdr="Disp"},
     {width="3",alignment="-1",col_name="enabled",colhdr="Enb"},
     {width="10",alignment="-1",col_name="addr",colhdr="Address"},
     {width="40",alignment="2",col_name="what",colhdr="What"}],
     body=[bkpt={number="1",type="breakpoint",disp="keep",enabled="y",
     addr="0x00010734",func="callee4",
     file="../../../devo/gdb/testsuite/gdb.mi/basics.c",
     fullname="/home/foo/devo/gdb/testsuite/gdb.mi/basics.c"line="8",times="1"},
     bkpt={number="2",type="watchpoint",disp="keep",
     enabled="y",addr="",what="C",times="0"}]}
     (gdb)
     -exec-continue
     ^running
     (gdb)
     *stopped,reason="watchpoint-trigger",wpt={number="2",exp="C"},
     value={old="-276895068",new="3"},
     frame={func="callee4",args=[],
     file="../../../devo/gdb/testsuite/gdb.mi/basics.c",
     fullname="/home/foo/bar/devo/gdb/testsuite/gdb.mi/basics.c",line="13"}
     (gdb)
     -break-list
     ^done,BreakpointTable={nr_rows="2",nr_cols="6",
     hdr=[{width="3",alignment="-1",col_name="number",colhdr="Num"},
     {width="14",alignment="-1",col_name="type",colhdr="Type"},
     {width="4",alignment="-1",col_name="disp",colhdr="Disp"},
     {width="3",alignment="-1",col_name="enabled",colhdr="Enb"},
     {width="10",alignment="-1",col_name="addr",colhdr="Address"},
     {width="40",alignment="2",col_name="what",colhdr="What"}],
     body=[bkpt={number="1",type="breakpoint",disp="keep",enabled="y",
     addr="0x00010734",func="callee4",
     file="../../../devo/gdb/testsuite/gdb.mi/basics.c",
     fullname="/home/foo/devo/gdb/testsuite/gdb.mi/basics.c",line="8",times="1"},
     bkpt={number="2",type="watchpoint",disp="keep",
     enabled="y",addr="",what="C",times="-5"}]}
     (gdb)
     -exec-continue
     ^running
     ^done,reason="watchpoint-scope",wpnum="2",
     frame={func="callee3",args=[{name="strarg",
     value="0x11940 \"A string argument.\""}],
     file="../../../devo/gdb/testsuite/gdb.mi/basics.c",
     fullname="/home/foo/bar/devo/gdb/testsuite/gdb.mi/basics.c",line="18"}
     (gdb)
     -break-list
     ^done,BreakpointTable={nr_rows="1",nr_cols="6",
     hdr=[{width="3",alignment="-1",col_name="number",colhdr="Num"},
     {width="14",alignment="-1",col_name="type",colhdr="Type"},
     {width="4",alignment="-1",col_name="disp",colhdr="Disp"},
     {width="3",alignment="-1",col_name="enabled",colhdr="Enb"},
     {width="10",alignment="-1",col_name="addr",colhdr="Address"},
     {width="40",alignment="2",col_name="what",colhdr="What"}],
     body=[bkpt={number="1",type="breakpoint",disp="keep",enabled="y",
     addr="0x00010734",func="callee4",
     file="../../../devo/gdb/testsuite/gdb.mi/basics.c",
     fullname="/home/foo/devo/gdb/testsuite/gdb.mi/basics.c",line="8",
     times="1"}]}
     (gdb)

GDB/MI Program Context
======================

The -exec-arguments Command
---------------------------

Synopsis
........

      -exec-arguments ARGS

Set the inferior program arguments, to be used in the next -exec-run.

GDBN Command
............

The corresponding GDBN command is set args.

Example
.......

     (gdb)
     -exec-arguments -v word
     ^done
     (gdb)

The -environment-cd Command
---------------------------

Synopsis
........

      -environment-cd PATHDIR

Set GDBN's working directory.

GDBN Command
............

The corresponding GDBN command is cd.

Example
.......

     (gdb)
     -environment-cd /kwikemart/marge/ezannoni/flathead-dev/devo/gdb
     ^done
     (gdb)

The -environment-directory Command
----------------------------------

Synopsis
........

      -environment-directory [ -r ] [ PATHDIR ]+

Add directories PATHDIR to beginning of search path for source files. If the -r option is used, the search path is reset to the default search path.  If directories PATHDIR are supplied in addition to the -r option, the search path is first reset and then addition occurs as normal. Multiple directories may be specified, separated by blanks.  Specifying multiple directories in a single command results in the directories added to the beginning of the search path in the same order they were presented in the command. If blanks are needed as part of a directory name, double-quotes should be used around the name.  In the command output, the path will show up separated by the system directory-separator character.  The directory-separator character must not be used in any directory name. If no directories are specified, the current search path is displayed.

GDBN Command
............

The corresponding GDBN command is dir.

Example
.......

     (gdb)
     -environment-directory /kwikemart/marge/ezannoni/flathead-dev/devo/gdb
     ^done,source-path="/kwikemart/marge/ezannoni/flathead-dev/devo/gdb:$cdir:$cwd"
     (gdb)
     -environment-directory ""
     ^done,source-path="/kwikemart/marge/ezannoni/flathead-dev/devo/gdb:$cdir:$cwd"
     (gdb)
     -environment-directory -r /home/jjohnstn/src/gdb /usr/src
     ^done,source-path="/home/jjohnstn/src/gdb:/usr/src:$cdir:$cwd"
     (gdb)
     -environment-directory -r
     ^done,source-path="$cdir:$cwd"
     (gdb)

The -environment-path Command
-----------------------------

Synopsis
........

      -environment-path [ -r ] [ PATHDIR ]+

Add directories PATHDIR to beginning of search path for object files. If the -r option is used, the search path is reset to the original search path that existed at gdb start-up.  If directories PATHDIR are supplied in addition to the -r option, the search path is first reset and then addition occurs as normal. Multiple directories may be specified, separated by blanks.  Specifying multiple directories in a single command results in the directories added to the beginning of the search path in the same order they were presented in the command. If blanks are needed as part of a directory name, double-quotes should be used around the name.  In the command output, the path will show up separated by the system directory-separator character.  The directory-separator character must not be used in any directory name. If no directories are specified, the current path is displayed.

GDBN Command
............

The corresponding GDBN command is path.

Example
.......

     (gdb)
     -environment-path
     ^done,path="/usr/bin"
     (gdb)
     -environment-path /kwikemart/marge/ezannoni/flathead-dev/ppc-eabi/gdb /bin
     ^done,path="/kwikemart/marge/ezannoni/flathead-dev/ppc-eabi/gdb:/bin:/usr/bin"
     (gdb)
     -environment-path -r /usr/local/bin
     ^done,path="/usr/local/bin:/usr/bin"
     (gdb)

The -environment-pwd Command
----------------------------

Synopsis
........

      -environment-pwd

Show the current working directory.

GDBN Command
............

The corresponding GDBN command is pwd.

Example
.......

     (gdb)
     -environment-pwd
     ^done,cwd="/kwikemart/marge/ezannoni/flathead-dev/devo/gdb"
     (gdb)

GDB/MI Thread Commands
======================

The -thread-info Command
------------------------

Synopsis
........

      -thread-info [ THREAD-ID ]

Reports information about either a specific thread, if the THREAD-ID parameter is present, or about all threads.  When printing information about all threads, also reports the current thread.

GDBN Command
............

The info thread command prints the same information about all threads.

Result
......

The result is a list of threads.  The following attributes are defined for a given thread:

current
     This field exists only for the current thread.  It has the value *.

id
     The identifier that GDBN uses to refer to the thread.

target-id
     The identifier that the target uses to refer to the thread.

details
     Extra information about the thread, in a target-specific format.  This field is optional.

name
     The name of the thread.  If the user specified a name using the thread name command, then this name is given.  Otherwise, if GDBN can extract the thread name from the target, then that name is given.  If GDBN cannot find the thread name, then this field is omitted.

frame
     The stack frame currently executing in the thread.

state
     The thread's state.  The state field may have the following values:

     stopped
          The thread is stopped.  Frame information is available for stopped threads.

     running
          The thread is running.  There's no frame information for running threads.

core
     If GDBN can find the CPU core on which this thread is running, then this field is the core identifier.  This field is optional.

Example
.......

     -thread-info
     ^done,threads=[
     {id="2",target-id="Thread 0xb7e14b90 (LWP 21257)",
        frame={level="0",addr="0xffffe410",func="__kernel_vsyscall",
                args=[]},state="running"},
     {id="1",target-id="Thread 0xb7e156b0 (LWP 21254)",
        frame={level="0",addr="0x0804891f",func="foo",
                args=[{name="i",value="10"}],
                file="/tmp/a.c",fullname="/tmp/a.c",line="158"},
                state="running"}],
     current-thread-id="1"
     (gdb)

The -thread-list-ids Command
----------------------------

Synopsis
........

      -thread-list-ids

Produces a list of the currently known GDBN thread ids.  At the end of the list it also prints the total number of such threads.

This command is retained for historical reasons, the -thread-info command should be used instead.

GDBN Command
............

Part of info threads supplies the same information.

Example
.......

     (gdb)
     -thread-list-ids
     ^done,thread-ids={thread-id="3",thread-id="2",thread-id="1"},
     current-thread-id="1",number-of-threads="3"
     (gdb)

The -thread-select Command
--------------------------

Synopsis
........

      -thread-select THREADNUM

Make THREADNUM the current thread.  It prints the number of the new current thread, and the topmost frame for that thread.

This command is deprecated in favor of explicitly using the --thread option to each command.

GDBN Command
............

The corresponding GDBN command is thread.

Example
.......

     (gdb)
     -exec-next
     ^running
     (gdb)
     *stopped,reason="end-stepping-range",thread-id="2",line="187",
     file="../../../devo/gdb/testsuite/gdb.threads/linux-dp.c"
     (gdb)
     -thread-list-ids
     ^done,
     thread-ids={thread-id="3",thread-id="2",thread-id="1"},
     number-of-threads="3"
     (gdb)
     -thread-select 3
     ^done,new-thread-id="3",
     frame={level="0",func="vprintf",
     args=[{name="format",value="0x8048e9c \"%*s%c %d %c\\n\""},
     {name="arg",value="0x2"}],file="vprintf.c",line="31"}
     (gdb)

GDB/MI Ada Tasking Commands
===========================

The -ada-task-info Command
--------------------------

Synopsis
........

      -ada-task-info [ TASK-ID ]

Reports information about either a specific Ada task, if the TASK-ID parameter is present, or about all Ada tasks.

GDBN Command
............

The info tasks command prints the same information about all Ada tasks (see Ada Tasks).

Result
......

The result is a table of Ada tasks.  The following columns are defined for each Ada task:

current
     This field exists only for the current thread.  It has the value *.

id
     The identifier that GDBN uses to refer to the Ada task.

task-id
     The identifier that the target uses to refer to the Ada task.

thread-id
     The identifier of the thread corresponding to the Ada task.

     This field should always exist, as Ada tasks are always implemented on top of a thread.  But if GDBN cannot find this corresponding thread for any reason, the field is omitted.

parent-id
     This field exists only when the task was created by another task. In this case, it provides the ID of the parent task.

priority
     The base priority of the task.

state
     The current state of the task.  For a detailed description of the possible states, see Ada Tasks.

name
     The name of the task.

Example
.......

     -ada-task-info
     ^done,tasks={nr_rows="3",nr_cols="8",
     hdr=[{width="1",alignment="-1",col_name="current",colhdr=""},
     {width="3",alignment="1",col_name="id",colhdr="ID"},
     {width="9",alignment="1",col_name="task-id",colhdr="TID"},
     {width="4",alignment="1",col_name="thread-id",colhdr=""},
     {width="4",alignment="1",col_name="parent-id",colhdr="P-ID"},
     {width="3",alignment="1",col_name="priority",colhdr="Pri"},
     {width="22",alignment="-1",col_name="state",colhdr="State"},
     {width="1",alignment="2",col_name="name",colhdr="Name"}],
     body=[{current="*",id="1",task-id="   644010",thread-id="1",priority="48",
     state="Child Termination Wait",name="main_task"}]}
     (gdb)

GDB/MI Program Execution
========================

These are the asynchronous commands which generate the out-of-band record *stopped.  Currently GDBN only really executes asynchronously with remote targets and this interaction is mimicked in other cases.

The -exec-continue Command
--------------------------

Synopsis
........

      -exec-continue [--reverse] [--all|--thread-group N]

Resumes the execution of the inferior program, which will continue to execute until it reaches a debugger stop event.  If the --reverse option is specified, execution resumes in reverse until it reaches a stop event.  Stop events may include

  * breakpoints or watchpoints

  * signals or exceptions

  * the end of the process (or its beginning under --reverse)

  * the end or beginning of a replay log if one is being used.

In all-stop mode (see All-Stop Mode), may resume only one thread, or all threads, depending on the value of the scheduler-locking variable.  If --all is specified, all threads (in all inferiors) will be resumed.  The --all option is ignored in all-stop mode.  If the --thread-group options is specified, then all threads in that thread group are resumed.

GDBN Command
............

The corresponding GDBN corresponding is continue.

Example
.......

     -exec-continue
     ^running
     (gdb)
     @Hello world
     *stopped,reason="breakpoint-hit",disp="keep",bkptno="2",frame={
     func="foo",args=[],file="hello.c",fullname="/home/foo/bar/hello.c",
     line="13"}
     (gdb)

The -exec-finish Command
------------------------

Synopsis
........

      -exec-finish [--reverse]

Resumes the execution of the inferior program until the current function is exited.  Displays the results returned by the function. If the --reverse option is specified, resumes the reverse execution of the inferior program until the point where current function was called.

GDBN Command
............

The corresponding GDBN command is finish.

Example
.......

Function returning void.

     -exec-finish
     ^running
     (gdb)
     @hello from foo
     *stopped,reason="function-finished",frame={func="main",args=[],
     file="hello.c",fullname="/home/foo/bar/hello.c",line="7"}
     (gdb)

Function returning other than void.  The name of the internal GDBN variable storing the result is printed, together with the value itself.

     -exec-finish
     ^running
     (gdb)
     *stopped,reason="function-finished",frame={addr="0x000107b0",func="foo",
     args=[{name="a",value="1"],{name="b",value="9"}},
     file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="14"},
     gdb-result-var="$1",return-value="0"
     (gdb)

The -exec-interrupt Command
---------------------------

Synopsis
........

      -exec-interrupt [--all|--thread-group N]

Interrupts the background execution of the target.  Note how the token associated with the stop message is the one for the execution command that has been interrupted.  The token for the interrupt itself only appears in the ^done output.  If the user is trying to interrupt a non-running program, an error message will be printed.

Note that when asynchronous execution is enabled, this command is asynchronous just like other execution commands.  That is, first the ^done response will be printed, and the target stop will be reported after that using the *stopped notification.

In non-stop mode, only the context thread is interrupted by default. All threads (in all inferiors) will be interrupted if the --all  option is specified.  If the --thread-group option is specified, all threads in that group will be interrupted.

GDBN Command
............

The corresponding GDBN command is interrupt.

Example
.......

     (gdb)
     111-exec-continue
     111^running

     (gdb)
     222-exec-interrupt
     222^done
     (gdb)
     111*stopped,signal-name="SIGINT",signal-meaning="Interrupt",
     frame={addr="0x00010140",func="foo",args=[],file="try.c",
     fullname="/home/foo/bar/try.c",line="13"}
     (gdb)

     (gdb)
     -exec-interrupt
     ^error,msg="mi_cmd_exec_interrupt: Inferior not executing."
     (gdb)

The -exec-jump Command
----------------------

Synopsis
........

      -exec-jump LOCATION

Resumes execution of the inferior program at the location specified by parameter.  See Specify Location, for a description of the different forms of LOCATION.

GDBN Command
............

The corresponding GDBN command is jump.

Example
.......

     -exec-jump foo.c:10
     *running,thread-id="all"
     ^running

The -exec-next Command
----------------------

Synopsis
........

      -exec-next [--reverse]

Resumes execution of the inferior program, stopping when the beginning of the next source line is reached.

If the --reverse option is specified, resumes reverse execution of the inferior program, stopping at the beginning of the previous source line.  If you issue this command on the first line of a function, it will take you back to the caller of that function, to the source line where the function was called.

GDBN Command
............

The corresponding GDBN command is next.

Example
.......

     -exec-next
     ^running
     (gdb)
     *stopped,reason="end-stepping-range",line="8",file="hello.c"
     (gdb)

The -exec-next-instruction Command
----------------------------------

Synopsis
........

      -exec-next-instruction [--reverse]

Executes one machine instruction.  If the instruction is a function call, continues until the function returns.  If the program stops at an instruction in the middle of a source line, the address will be printed as well.

If the --reverse option is specified, resumes reverse execution of the inferior program, stopping at the previous instruction.  If the previously executed instruction was a return from another function, it will continue to execute in reverse until the call to that function (from the current stack frame) is reached.

GDBN Command
............

The corresponding GDBN command is nexti.

Example
.......

     (gdb)
     -exec-next-instruction
     ^running

     (gdb)
     *stopped,reason="end-stepping-range",
     addr="0x000100d4",line="5",file="hello.c"
     (gdb)

The -exec-return Command
------------------------

Synopsis
........

      -exec-return

Makes current function return immediately.  Doesn't execute the inferior. Displays the new current frame.

GDBN Command
............

The corresponding GDBN command is return.

Example
.......

     (gdb)
     200-break-insert callee4
     200^done,bkpt={number="1",addr="0x00010734",
     file="../../../devo/gdb/testsuite/gdb.mi/basics.c",line="8"}
     (gdb)
     000-exec-run
     000^running
     (gdb)
     000*stopped,reason="breakpoint-hit",disp="keep",bkptno="1",
     frame={func="callee4",args=[],
     file="../../../devo/gdb/testsuite/gdb.mi/basics.c",
     fullname="/home/foo/bar/devo/gdb/testsuite/gdb.mi/basics.c",line="8"}
     (gdb)
     205-break-delete
     205^done
     (gdb)
     111-exec-return
     111^done,frame={level="0",func="callee3",
     args=[{name="strarg",
     value="0x11940 \"A string argument.\""}],
     file="../../../devo/gdb/testsuite/gdb.mi/basics.c",
     fullname="/home/foo/bar/devo/gdb/testsuite/gdb.mi/basics.c",line="18"}
     (gdb)

The -exec-run Command
---------------------

Synopsis
........

      -exec-run [--all | --thread-group N]

Starts execution of the inferior from the beginning.  The inferior executes until either a breakpoint is encountered or the program exits.  In the latter case the output will include an exit code, if the program has exited exceptionally.

When no option is specified, the current inferior is started.  If the --thread-group option is specified, it should refer to a thread group of type process, and that thread group will be started. If the --all option is specified, then all inferiors will be started.

GDBN Command
............

The corresponding GDBN command is run.

Examples
........

     (gdb)
     -break-insert main
     ^done,bkpt={number="1",addr="0x0001072c",file="recursive2.c",line="4"}
     (gdb)
     -exec-run
     ^running
     (gdb)
     *stopped,reason="breakpoint-hit",disp="keep",bkptno="1",
     frame={func="main",args=[],file="recursive2.c",
     fullname="/home/foo/bar/recursive2.c",line="4"}
     (gdb)

Program exited normally:

     (gdb)
     -exec-run
     ^running
     (gdb)
     x = 55
     *stopped,reason="exited-normally"
     (gdb)

Program exited exceptionally:

     (gdb)
     -exec-run
     ^running
     (gdb)
     x = 55
     *stopped,reason="exited",exit-code="01"
     (gdb)

Another way the program can terminate is if it receives a signal such as SIGINT.  In this case, GDB/MI displays this:

     (gdb)
     *stopped,reason="exited-signalled",signal-name="SIGINT",
     signal-meaning="Interrupt"

The -exec-step Command
----------------------

Synopsis
........

      -exec-step [--reverse]

Resumes execution of the inferior program, stopping when the beginning of the next source line is reached, if the next source line is not a function call.  If it is, stop at the first instruction of the called function.  If the --reverse option is specified, resumes reverse execution of the inferior program, stopping at the beginning of the previously executed source line.

GDBN Command
............

The corresponding GDBN command is step.

Example
.......

Stepping into a function:

     -exec-step
     ^running
     (gdb)
     *stopped,reason="end-stepping-range",
     frame={func="foo",args=[{name="a",value="10"},
     {name="b",value="0"}],file="recursive2.c",
     fullname="/home/foo/bar/recursive2.c",line="11"}
     (gdb)

Regular stepping:

     -exec-step
     ^running
     (gdb)
     *stopped,reason="end-stepping-range",line="14",file="recursive2.c"
     (gdb)

The -exec-step-instruction Command
----------------------------------

Synopsis
........

      -exec-step-instruction [--reverse]

Resumes the inferior which executes one machine instruction.  If the --reverse option is specified, resumes reverse execution of the inferior program, stopping at the previously executed instruction. The output, once GDBN has stopped, will vary depending on whether we have stopped in the middle of a source line or not.  In the former case, the address at which the program stopped will be printed as well.

GDBN Command
............

The corresponding GDBN command is stepi.

Example
.......

     (gdb)
     -exec-step-instruction
     ^running

     (gdb)
     *stopped,reason="end-stepping-range",
     frame={func="foo",args=[],file="try.c",
     fullname="/home/foo/bar/try.c",line="10"}
     (gdb)
     -exec-step-instruction
     ^running

     (gdb)
     *stopped,reason="end-stepping-range",
     frame={addr="0x000100f4",func="foo",args=[],file="try.c",
     fullname="/home/foo/bar/try.c",line="10"}
     (gdb)

The -exec-until Command
-----------------------

Synopsis
........

      -exec-until [ LOCATION ]

Executes the inferior until the LOCATION specified in the argument is reached.  If there is no argument, the inferior executes until a source line greater than the current one is reached.  The reason for stopping in this case will be location-reached.

GDBN Command
............

The corresponding GDBN command is until.

Example
.......

     (gdb)
     -exec-until recursive2.c:6
     ^running
     (gdb)
     x = 55
     *stopped,reason="location-reached",frame={func="main",args=[],
     file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="6"}
     (gdb)

GDB/MI Stack Manipulation Commands
==================================

The -stack-info-frame Command
-----------------------------

Synopsis
........

      -stack-info-frame

Get info on the selected frame.

GDBN Command
............

The corresponding GDBN command is info frame or frame (without arguments).

Example
.......

     (gdb)
     -stack-info-frame
     ^done,frame={level="1",addr="0x0001076c",func="callee3",
     file="../../../devo/gdb/testsuite/gdb.mi/basics.c",
     fullname="/home/foo/bar/devo/gdb/testsuite/gdb.mi/basics.c",line="17"}
     (gdb)

The -stack-info-depth Command
-----------------------------

Synopsis
........

      -stack-info-depth [ MAX-DEPTH ]

Return the depth of the stack.  If the integer argument MAX-DEPTH is specified, do not count beyond MAX-DEPTH frames.

GDBN Command
............

There's no equivalent GDBN command.

Example
.......

For a stack with frame levels 0 through 11:

     (gdb)
     -stack-info-depth
     ^done,depth="12"
     (gdb)
     -stack-info-depth 4
     ^done,depth="4"
     (gdb)
     -stack-info-depth 12
     ^done,depth="12"
     (gdb)
     -stack-info-depth 11
     ^done,depth="11"
     (gdb)
     -stack-info-depth 13
     ^done,depth="12"
     (gdb)

The -stack-list-arguments Command
---------------------------------

Synopsis
........

      -stack-list-arguments PRINT-VALUES
         [ LOW-FRAME HIGH-FRAME ]

Display a list of the arguments for the frames between LOW-FRAME and HIGH-FRAME (inclusive).  If LOW-FRAME and HIGH-FRAME are not provided, list the arguments for the whole call stack.  If the two arguments are equal, show the single frame at the corresponding level.  It is an error if LOW-FRAME is larger than the actual number of frames.  On the other hand, HIGH-FRAME may be larger than the actual number of frames, in which case only existing frames will be returned.

If PRINT-VALUES is 0 or --no-values, print only the names of the variables; if it is 1 or --all-values, print also their values; and if it is 2 or --simple-values, print the name, type and value for simple data types, and the name and type for arrays, structures and unions.

Use of this command to obtain arguments in a single frame is deprecated in favor of the -stack-list-variables command.

GDBN Command
............

GDBN does not have an equivalent command.  gdbtk has a gdb_get_args command which partially overlaps with the functionality of -stack-list-arguments.

Example
.......

     (gdb)
     -stack-list-frames
     ^done,
     stack=[
     frame={level="0",addr="0x00010734",func="callee4",
     file="../../../devo/gdb/testsuite/gdb.mi/basics.c",
     fullname="/home/foo/bar/devo/gdb/testsuite/gdb.mi/basics.c",line="8"},
     frame={level="1",addr="0x0001076c",func="callee3",
     file="../../../devo/gdb/testsuite/gdb.mi/basics.c",
     fullname="/home/foo/bar/devo/gdb/testsuite/gdb.mi/basics.c",line="17"},
     frame={level="2",addr="0x0001078c",func="callee2",
     file="../../../devo/gdb/testsuite/gdb.mi/basics.c",
     fullname="/home/foo/bar/devo/gdb/testsuite/gdb.mi/basics.c",line="22"},
     frame={level="3",addr="0x000107b4",func="callee1",
     file="../../../devo/gdb/testsuite/gdb.mi/basics.c",
     fullname="/home/foo/bar/devo/gdb/testsuite/gdb.mi/basics.c",line="27"},
     frame={level="4",addr="0x000107e0",func="main",
     file="../../../devo/gdb/testsuite/gdb.mi/basics.c",
     fullname="/home/foo/bar/devo/gdb/testsuite/gdb.mi/basics.c",line="32"}]
     (gdb)
     -stack-list-arguments 0
     ^done,
     stack-args=[
     frame={level="0",args=[]},
     frame={level="1",args=[name="strarg"]},
     frame={level="2",args=[name="intarg",name="strarg"]},
     frame={level="3",args=[name="intarg",name="strarg",name="fltarg"]},
     frame={level="4",args=[]}]
     (gdb)
     -stack-list-arguments 1
     ^done,
     stack-args=[
     frame={level="0",args=[]},
     frame={level="1",
      args=[{name="strarg",value="0x11940 \"A string argument.\""}]},
     frame={level="2",args=[
     {name="intarg",value="2"},
     {name="strarg",value="0x11940 \"A string argument.\""}]},
     {frame={level="3",args=[
     {name="intarg",value="2"},
     {name="strarg",value="0x11940 \"A string argument.\""},
     {name="fltarg",value="3.5"}]},
     frame={level="4",args=[]}]
     (gdb)
     -stack-list-arguments 0 2 2
     ^done,stack-args=[frame={level="2",args=[name="intarg",name="strarg"]}]
     (gdb)
     -stack-list-arguments 1 2 2
     ^done,stack-args=[frame={level="2",
     args=[{name="intarg",value="2"},
     {name="strarg",value="0x11940 \"A string argument.\""}]}]
     (gdb)

The -stack-list-frames Command
------------------------------

Synopsis
........

      -stack-list-frames [ LOW-FRAME HIGH-FRAME ]

List the frames currently on the stack.  For each frame it displays the following info:

LEVEL
     The frame number, 0 being the topmost frame, i.e., the innermost function.

ADDR
     The $pc value for that frame.

FUNC
     Function name.

FILE
     File name of the source file where the function lives.

FULLNAME
     The full file name of the source file where the function lives.

LINE
     Line number corresponding to the $pc.

FROM
     The shared library where this function is defined.  This is only given if the frame's function is not known.

If invoked without arguments, this command prints a backtrace for the whole stack.  If given two integer arguments, it shows the frames whose levels are between the two arguments (inclusive).  If the two arguments are equal, it shows the single frame at the corresponding level.  It is an error if LOW-FRAME is larger than the actual number of frames.  On the other hand, HIGH-FRAME may be larger than the actual number of frames, in which case only existing frames will be returned.

GDBN Command
............

The corresponding GDBN commands are backtrace and where.

Example
.......

Full stack backtrace:

     (gdb)
     -stack-list-frames
     ^done,stack=
     [frame={level="0",addr="0x0001076c",func="foo",
       file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="11"},
     frame={level="1",addr="0x000107a4",func="foo",
       file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="14"},
     frame={level="2",addr="0x000107a4",func="foo",
       file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="14"},
     frame={level="3",addr="0x000107a4",func="foo",
       file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="14"},
     frame={level="4",addr="0x000107a4",func="foo",
       file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="14"},
     frame={level="5",addr="0x000107a4",func="foo",
       file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="14"},
     frame={level="6",addr="0x000107a4",func="foo",
       file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="14"},
     frame={level="7",addr="0x000107a4",func="foo",
       file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="14"},
     frame={level="8",addr="0x000107a4",func="foo",
       file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="14"},
     frame={level="9",addr="0x000107a4",func="foo",
       file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="14"},
     frame={level="10",addr="0x000107a4",func="foo",
       file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="14"},
     frame={level="11",addr="0x00010738",func="main",
       file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="4"}]
     (gdb)

Show frames between LOW_FRAME and HIGH_FRAME:

     (gdb)
     -stack-list-frames 3 5
     ^done,stack=
     [frame={level="3",addr="0x000107a4",func="foo",
       file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="14"},
     frame={level="4",addr="0x000107a4",func="foo",
       file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="14"},
     frame={level="5",addr="0x000107a4",func="foo",
       file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="14"}]
     (gdb)

Show a single frame:

     (gdb)
     -stack-list-frames 3 3
     ^done,stack=
     [frame={level="3",addr="0x000107a4",func="foo",
       file="recursive2.c",fullname="/home/foo/bar/recursive2.c",line="14"}]
     (gdb)

The -stack-list-locals Command
------------------------------

Synopsis
........

      -stack-list-locals PRINT-VALUES

Display the local variable names for the selected frame.  If PRINT-VALUES is 0 or --no-values, print only the names of the variables; if it is 1 or --all-values, print also their values; and if it is 2 or --simple-values, print the name, type and value for simple data types, and the name and type for arrays, structures and unions.  In this last case, a frontend can immediately display the value of simple data types and create variable objects for other data types when the user wishes to explore their values in more detail.

This command is deprecated in favor of the -stack-list-variables command.

GDBN Command
............

info locals in GDBN, gdb_get_locals in gdbtk.

Example
.......

     (gdb)
     -stack-list-locals 0
     ^done,locals=[name="A",name="B",name="C"]
     (gdb)
     -stack-list-locals --all-values
     ^done,locals=[{name="A",value="1"},{name="B",value="2"},
       {name="C",value="{1, 2, 3}"}]
     -stack-list-locals --simple-values
     ^done,locals=[{name="A",type="int",value="1"},
       {name="B",type="int",value="2"},{name="C",type="int [3]"}]
     (gdb)

The -stack-list-variables Command
---------------------------------

Synopsis
........

      -stack-list-variables PRINT-VALUES

Display the names of local variables and function arguments for the selected frame.  If PRINT-VALUES is 0 or --no-values, print only the names of the variables; if it is 1 or --all-values, print also their values; and if it is 2 or --simple-values, print the name, type and value for simple data types, and the name and type for arrays, structures and unions.

Example
.......

     (gdb)
     -stack-list-variables --thread 1 --frame 0 --all-values
     ^done,variables=[{name="x",value="11"},{name="s",value="{a = 1, b = 2}"}]
     (gdb)

The -stack-select-frame Command
-------------------------------

Synopsis
........

      -stack-select-frame FRAMENUM

Change the selected frame.  Select a different frame FRAMENUM on the stack.

This command in deprecated in favor of passing the --frame option to every command.

GDBN Command
............

The corresponding GDBN commands are frame, up, down, select-frame, up-silent, and down-silent.

Example
.......

     (gdb)
     -stack-select-frame 2
     ^done
     (gdb)

GDB/MI Variable Objects
=======================

Introduction to Variable Objects
--------------------------------

Variable objects are "object-oriented" MI interface for examining and changing values of expressions.  Unlike some other MI interfaces that work with expressions, variable objects are specifically designed for simple and efficient presentation in the frontend.  A variable object is identified by string name.  When a variable object is created, the frontend specifies the expression for that variable object.  The expression can be a simple variable, or it can be an arbitrary complex expression, and can even involve CPU registers.  After creating a variable object, the frontend can invoke other variable object operations---for example to obtain or change the value of a variable object, or to change display format.

Variable objects have hierarchical tree structure.  Any variable object that corresponds to a composite type, such as structure in C, has a number of child variable objects, for example corresponding to each element of a structure.  A child variable object can itself have children, recursively.  Recursion ends when we reach leaf variable objects, which always have built-in types.  Child variable objects are created only by explicit request, so if a frontend is not interested in the children of a particular variable object, no child will be created.

For a leaf variable object it is possible to obtain its value as a string, or set the value from a string.  String value can be also obtained for a non-leaf variable object, but it's generally a string that only indicates the type of the object, and does not list its contents.  Assignment to a non-leaf variable object is not allowed.

A frontend does not need to read the values of all variable objects each time the program stops.  Instead, MI provides an update command that lists all variable objects whose values has changed since the last update operation.  This considerably reduces the amount of data that must be transferred to the frontend.  As noted above, children variable objects are created on demand, and only leaf variable objects have a real value.  As result, gdb will read target memory only for leaf variables that frontend has created.

The automatic update is not always desirable.  For example, a frontend might want to keep a value of some expression for future reference, and never update it.  For another example,  fetching memory is relatively slow for embedded targets, so a frontend might want to disable automatic update for the variables that are either not visible on the screen, or ``closed''.  This is possible using so called ``frozen variable objects''.  Such variable objects are never implicitly updated.

Variable objects can be either fixed or floating.  For the fixed variable object, the expression is parsed when the variable object is created, including associating identifiers to specific variables.  The meaning of expression never changes.  For a floating variable object the values of variables whose names appear in the expressions are re-evaluated every time in the context of the current frame.  Consider this example:

     void do_work(...)
     {
             struct work_state state;

             if (...)
                do_work(...);
     }

If a fixed variable object for the state variable is created in this function, and we enter the recursive call, the variable object will report the value of state in the top-level do_work invocation.  On the other hand, a floating variable object will report the value of state in the current frame.

If an expression specified when creating a fixed variable object refers to a local variable, the variable object becomes bound to the thread and frame in which the variable object is created.  When such variable object is updated, GDBN makes sure that the thread/frame combination the variable object is bound to still exists, and re-evaluates the variable object in context of that thread/frame.

The following is the complete set of GDB/MI operations defined to access this functionality:

Operation
Description

-enable-pretty-printing
enable Python-based pretty-printing
-var-create
create a variable object
-var-delete
delete the variable object and/or its children
-var-set-format
set the display format of this variable
-var-show-format
show the display format of this variable
-var-info-num-children
tells how many children this object has
-var-list-children
return a list of the object's children
-var-info-type
show the type of this variable object
-var-info-expression
print parent-relative expression that this variable object represents
-var-info-path-expression
print full expression that this variable object represents
-var-show-attributes
is this variable editable? does it exist here?
-var-evaluate-expression
get the value of this variable
-var-assign
set the value of this variable
-var-update
update the variable and its children
-var-set-frozen
set frozeness attribute
-var-set-update-range
set range of children to display on update

In the next subsection we describe each operation in detail and suggest how it can be used.

Description And Use of Operations on Variable Objects
-----------------------------------------------------

The -enable-pretty-printing Command
-----------------------------------

     -enable-pretty-printing

GDBN allows Python-based visualizers to affect the output of the MI variable object commands.  However, because there was no way to implement this in a fully backward-compatible way, a front end must request that this functionality be enabled.

Once enabled, this feature cannot be disabled.

Note that if Python support has not been compiled into GDBN, this command will still succeed (and do nothing).

This feature is currently (as of GDBN 7.0) experimental, and may work differently in future versions of GDBN.

The -var-create Command
-----------------------

Synopsis
........

      -var-create {NAME | "-"}
         {FRAME-ADDR | "*" | "@"} EXPRESSION

This operation creates a variable object, which allows the monitoring of a variable, the result of an expression, a memory cell or a CPU register.

The NAME parameter is the string by which the object can be referenced.  It must be unique.  If - is specified, the varobj system will generate a string ``varNNNNNN'' automatically.  It will be unique provided that one does not specify NAME of that format. The command fails if a duplicate name is found.

The frame under which the expression should be evaluated can be specified by FRAME-ADDR.  A * indicates that the current frame should be used.  A @ indicates that a floating variable object must be created.

EXPRESSION is any expression valid on the current language set (must not begin with a *), or one of the following:

  * *ADDR, where ADDR is the address of a memory cell

  * *ADDR-ADDR --- a memory address range (TBD)

  * $REGNAME --- a CPU register name

A varobj's contents may be provided by a Python-based pretty-printer.  In this case the varobj is known as a dynamic varobj.  Dynamic varobjs have slightly different semantics in some cases.  If the -enable-pretty-printing command is not sent, then GDBN will never create a dynamic varobj.  This ensures backward compatibility for existing clients.

Result
......

This operation returns attributes of the newly-created varobj.  These are:

name
     The name of the varobj.

numchild
     The number of children of the varobj.  This number is not necessarily reliable for a dynamic varobj.  Instead, you must examine the has_more attribute.

value
     The varobj's scalar value.  For a varobj whose type is some sort of aggregate (e.g., a struct), or for a dynamic varobj, this value will not be interesting.

type
     The varobj's type.  This is a string representation of the type, as would be printed by the GDBN CLI.

thread-id
     If a variable object is bound to a specific thread, then this is the thread's identifier.

has_more
     For a dynamic varobj, this indicates whether there appear to be any children available.  For a non-dynamic varobj, this will be 0.

dynamic
     This attribute will be present and have the value 1 if the varobj is a dynamic varobj.  If the varobj is not a dynamic varobj, then this attribute will not be present.

displayhint
     A dynamic varobj can supply a display hint to the front end.  The value comes directly from the Python pretty-printer object's display_hint method.  See Pretty Printing API.

Typical output will look like this:

      name="NAME",numchild="N",type="TYPE",thread-id="M",
       has_more="HAS_MORE"

The -var-delete Command
-----------------------

Synopsis
........

      -var-delete [ -c ] NAME

Deletes a previously created variable object and all of its children. With the -c option, just deletes the children.

Returns an error if the object NAME is not found.

The -var-set-format Command
---------------------------

Synopsis
........

      -var-set-format NAME FORMAT-SPEC

Sets the output format for the value of the object NAME to be FORMAT-SPEC.

The syntax for the FORMAT-SPEC is as follows:

      FORMAT-SPEC ==>
      {binary | decimal | hexadecimal | octal | natural}

The natural format is the default format choosen automatically based on the variable type (like decimal for an int, hex for pointers, etc.).

For a variable with children, the format is set only on the variable itself, and the children are not affected.

The -var-show-format Command
----------------------------

Synopsis
........

      -var-show-format NAME

Returns the format used to display the value of the object NAME.

      FORMAT ==>
      FORMAT-SPEC

The -var-info-num-children Command
----------------------------------

Synopsis
........

      -var-info-num-children NAME

Returns the number of children of a variable object NAME:

      numchild=N

Note that this number is not completely reliable for a dynamic varobj. It will return the current number of children, but more children may be available.

The -var-list-children Command
------------------------------

Synopsis
........

      -var-list-children [PRINT-VALUES] NAME [FROM TO]

Return a list of the children of the specified variable object and create variable objects for them, if they do not already exist.  With a single argument or if PRINT-VALUES has a value of 0 or --no-values, print only the names of the variables; if PRINT-VALUES is 1 or --all-values, also print their values; and if it is 2 or --simple-values print the name and value for simple data types and just the name for arrays, structures and unions.

FROM and TO, if specified, indicate the range of children to report.  If FROM or TO is less than zero, the range is reset and all children will be reported.  Otherwise, children starting at FROM (zero-based) and up to and excluding TO will be reported.

If a child range is requested, it will only affect the current call to -var-list-children, but not future calls to -var-update. For this, you must instead use -var-set-update-range.  The intent of this approach is to enable a front end to implement any update approach it likes; for example, scrolling a view may cause the front end to request more children with -var-list-children, and then the front end could call -var-set-update-range with a different range to ensure that future updates are restricted to just the visible items.

For each child the following results are returned:

name
     Name of the variable object created for this child.

exp
     The expression to be shown to the user by the front end to designate this child. For example this may be the name of a structure member.

     For a dynamic varobj, this value cannot be used to form an expression.  There is no way to do this at all with a dynamic varobj.

     For C/C++ structures there are several pseudo children returned to designate access qualifiers.  For these pseudo children EXP is public, private, or protected.  In this case the type and value are not present.

     A dynamic varobj will not report the access qualifying pseudo-children, regardless of the language.  This information is not available at all with a dynamic varobj.

numchild
     Number of children this child has.  For a dynamic varobj, this will be 0.

type
     The type of the child.

value
     If values were requested, this is the value.

thread-id
     If this variable object is associated with a thread, this is the thread id. Otherwise this result is not present.

frozen
     If the variable object is frozen, this variable will be present with a value of 1.

The result may have its own attributes:

displayhint
     A dynamic varobj can supply a display hint to the front end.  The value comes directly from the Python pretty-printer object's display_hint method.  See Pretty Printing API.

has_more
     This is an integer attribute which is nonzero if there are children remaining after the end of the selected range.

Example
.......

     (gdb)
      -var-list-children n
      ^done,numchild=N,children=[child={name=NAME,exp=EXP,
      numchild=N,type=TYPE},(repeats N times)]
     (gdb)
      -var-list-children --all-values n
      ^done,numchild=N,children=[child={name=NAME,exp=EXP,
      numchild=N,value=VALUE,type=TYPE},(repeats N times)]

The -var-info-type Command
--------------------------

Synopsis
........

      -var-info-type NAME

Returns the type of the specified variable NAME.  The type is returned as a string in the same format as it is output by the GDBN CLI:

      type=TYPENAME

The -var-info-expression Command
--------------------------------

Synopsis
........

      -var-info-expression NAME

Returns a string that is suitable for presenting this variable object in user interface.  The string is generally not valid expression in the current language, and cannot be evaluated.

For example, if a is an array, and variable object A was created for a, then we'll get this output:

     (gdb) -var-info-expression A.1
     ^done,lang="C",exp="1"

Here, the values of lang can be {"C" | "C++" | "Java"}.

Note that the output of the -var-list-children command also includes those expressions, so the -var-info-expression command is of limited use.

The -var-info-path-expression Command
-------------------------------------

Synopsis
........

      -var-info-path-expression NAME

Returns an expression that can be evaluated in the current context and will yield the same value that a variable object has. Compare this with the -var-info-expression command, which result can be used only for UI presentation.  Typical use of the -var-info-path-expression command is creating a watchpoint from a variable object.

This command is currently not valid for children of a dynamic varobj, and will give an error when invoked on one.

For example, suppose C is a C++ class, derived from class Base, and that the Base class has a member called m_size.  Assume a variable c is has the type of C and a variable object C was created for variable c.  Then, we'll get this output:

     (gdb) -var-info-path-expression C.Base.public.m_size
     ^done,path_expr=((Base)c).m_size)

The -var-show-attributes Command
--------------------------------

Synopsis
........

      -var-show-attributes NAME

List attributes of the specified variable object NAME:

      status=ATTR [ ( ,ATTR )* ]

where ATTR is { { editable | noneditable } | TBD }.

The -var-evaluate-expression Command
------------------------------------

Synopsis
........

      -var-evaluate-expression [-f FORMAT-SPEC] NAME

Evaluates the expression that is represented by the specified variable object and returns its value as a string.  The format of the string can be specified with the -f option.  The possible values of this option are the same as for -var-set-format (see -var-set-format).  If the -f option is not specified, the current display format will be used.  The current display format can be changed using the -var-set-format command.

      value=VALUE

Note that one must invoke -var-list-children for a variable before the value of a child variable can be evaluated.

The -var-assign Command
-----------------------

Synopsis
........

      -var-assign NAME EXPRESSION

Assigns the value of EXPRESSION to the variable object specified by NAME.  The object must be editable.  If the variable's value is altered by the assign, the variable will show up in any subsequent -var-update list.

Example
.......

     (gdb)
     -var-assign var1 3
     ^done,value="3"
     (gdb)
     -var-update *
     ^done,changelist=[{name="var1",in_scope="true",type_changed="false"}]
     (gdb)

The -var-update Command
-----------------------

Synopsis
........

      -var-update [PRINT-VALUES] {NAME | "*"}

Reevaluate the expressions corresponding to the variable object NAME and all its direct and indirect children, and return the list of variable objects whose values have changed; NAME must be a root variable object.  Here, ``changed'' means that the result of -var-evaluate-expression before and after the -var-update is different.  If * is used as the variable object names, all existing variable objects are updated, except for frozen ones (see -var-set-frozen).  The option PRINT-VALUES determines whether both names and values, or just names are printed.  The possible values of this option are the same as for -var-list-children (see -var-list-children).  It is recommended to use the --all-values option, to reduce the number of MI commands needed on each program stop.

With the * parameter, if a variable object is bound to a currently running thread, it will not be updated, without any diagnostic.

If -var-set-update-range was previously used on a varobj, then only the selected range of children will be reported.

-var-update reports all the changed varobjs in a tuple named changelist.

Each item in the change list is itself a tuple holding:

name
     The name of the varobj.

value
     If values were requested for this update, then this field will be present and will hold the value of the varobj.

in_scope
     This field is a string which may take one of three values:

     "true"
          The variable object's current value is valid.

     "false"
          The variable object does not currently hold a valid value but it may hold one in the future if its associated expression comes back into scope.

     "invalid"
          The variable object no longer holds a valid value. This can occur when the executable file being debugged has changed, either through recompilation or by using the GDBN file command.  The front end should normally choose to delete these variable objects.

     In the future new values may be added to this list so the front should be prepared for this possibility.  See GDB/MI Development and Front Ends.

type_changed
     This is only present if the varobj is still valid.  If the type changed, then this will be the string true; otherwise it will be false.

new_type
     If the varobj's type changed, then this field will be present and will hold the new type.

new_num_children
     For a dynamic varobj, if the number of children changed, or if the type changed, this will be the new number of children.

     The numchild field in other varobj responses is generally not valid for a dynamic varobj -- it will show the number of children that GDBN knows about, but because dynamic varobjs lazily instantiate their children, this will not reflect the number of children which may be available.

     The new_num_children attribute only reports changes to the number of children known by GDBN.  This is the only way to detect whether an update has removed children (which necessarily can only happen at the end of the update range).

displayhint
     The display hint, if any.

has_more
     This is an integer value, which will be 1 if there are more children available outside the varobj's update range.

dynamic
     This attribute will be present and have the value 1 if the varobj is a dynamic varobj.  If the varobj is not a dynamic varobj, then this attribute will not be present.

new_children
     If new children were added to a dynamic varobj within the selected update range (as set by -var-set-update-range), then they will be listed in this attribute.

Example
.......

     (gdb)
     -var-assign var1 3
     ^done,value="3"
     (gdb)
     -var-update --all-values var1
     ^done,changelist=[{name="var1",value="3",in_scope="true",
     type_changed="false"}]
     (gdb)

The -var-set-frozen Command
---------------------------

Synopsis
........

      -var-set-frozen NAME FLAG

Set the frozenness flag on the variable object NAME.  The FLAG parameter should be either 1 to make the variable frozen or 0 to make it unfrozen.  If a variable object is frozen, then neither itself, nor any of its children, are implicitly updated by -var-update of a parent variable or by -var-update *.  Only -var-update of the variable itself will update its value and values of its children.  After a variable object is unfrozen, it is implicitly updated by all subsequent -var-update operations. Unfreezing a variable does not update it, only subsequent -var-update does.

Example
.......

     (gdb)
     -var-set-frozen V 1
     ^done
     (gdb)

The -var-set-update-range command
---------------------------------

Synopsis
........

      -var-set-update-range NAME FROM TO

Set the range of children to be returned by future invocations of -var-update.

FROM and TO indicate the range of children to report.  If FROM or TO is less than zero, the range is reset and all children will be reported.  Otherwise, children starting at FROM (zero-based) and up to and excluding TO will be reported.

Example
.......

     (gdb)
     -var-set-update-range V 1 2
     ^done

The -var-set-visualizer command
-------------------------------

Synopsis
........

      -var-set-visualizer NAME VISUALIZER

Set a visualizer for the variable object NAME.

VISUALIZER is the visualizer to use.  The special value None means to disable any visualizer in use.

If not None, VISUALIZER must be a Python expression. This expression must evaluate to a callable object which accepts a single argument.  GDBN will call this object with the value of the varobj NAME as an argument (this is done so that the same Python pretty-printing code can be used for both the CLI and MI). When called, this object must return an object which conforms to the pretty-printing interface (see Pretty Printing API).

The pre-defined function gdb.default_visualizer may be used to select a visualizer by following the built-in process (see Selecting Pretty-Printers).  This is done automatically when a varobj is created, and so ordinarily is not needed.

This feature is only available if Python support is enabled.  The MI command -list-features (see GDB/MI Miscellaneous Commands) can be used to check this.

Example
.......

Resetting the visualizer:

     (gdb)
     -var-set-visualizer V None
     ^done

Reselecting the default (type-based) visualizer:

     (gdb)
     -var-set-visualizer V gdb.default_visualizer
     ^done

Suppose SomeClass is a visualizer class.  A lambda expression can be used to instantiate this class for a varobj:

     (gdb)
     -var-set-visualizer V "lambda val: SomeClass()"
     ^done

GDB/MI Data Manipulation
========================

This section describes the GDB/MI commands that manipulate data: examine memory and registers, evaluate expressions, etc.

The -data-disassemble Command
-----------------------------

Synopsis
........

      -data-disassemble
         [ -s START-ADDR -e END-ADDR ]
       | [ -f FILENAME -l LINENUM [ -n LINES ] ]
       -- MODE

Where:

START-ADDR
     is the beginning address (or $pc)

END-ADDR
     is the end address

FILENAME
     is the name of the file to disassemble

LINENUM
     is the line number to disassemble around

LINES
     is the number of disassembly lines to be produced.  If it is -1, the whole function will be disassembled, in case no END-ADDR is specified.  If END-ADDR is specified as a non-zero value, and LINES is lower than the number of disassembly lines between START-ADDR and END-ADDR, only LINES lines are displayed; if LINES is higher than the number of lines between START-ADDR and END-ADDR, only the lines up to END-ADDR are displayed.

MODE
     is either 0 (meaning only disassembly), 1 (meaning mixed source and disassembly), 2 (meaning disassembly with raw opcodes), or 3 (meaning mixed source and disassembly with raw opcodes).

Result
......

The output for each instruction is composed of four fields:

  * Address

  * Func-name

  * Offset

  * Instruction

Note that whatever included in the instruction field, is not manipulated directly by GDB/MI, i.e., it is not possible to adjust its format.

GDBN Command
............

There's no direct mapping from this command to the CLI.

Example
.......

Disassemble from the current value of $pc to $pc + 20:

     (gdb)
     -data-disassemble -s $pc -e "$pc + 20" -- 0
     ^done,
     asm_insns=[
     {address="0x000107c0",func-name="main",offset="4",
     inst="mov  2, %o0"},
     {address="0x000107c4",func-name="main",offset="8",
     inst="sethi  %hi(0x11800), %o2"},
     {address="0x000107c8",func-name="main",offset="12",
     inst="or  %o2, 0x140, %o1\t! 0x11940 <_lib_version+8>"},
     {address="0x000107cc",func-name="main",offset="16",
     inst="sethi  %hi(0x11800), %o2"},
     {address="0x000107d0",func-name="main",offset="20",
     inst="or  %o2, 0x168, %o4\t! 0x11968 <_lib_version+48>"}]
     (gdb)

Disassemble the whole main function.  Line 32 is part of main.

     -data-disassemble -f basics.c -l 32 -- 0
     ^done,asm_insns=[
     {address="0x000107bc",func-name="main",offset="0",
     inst="save  %sp, -112, %sp"},
     {address="0x000107c0",func-name="main",offset="4",
     inst="mov   2, %o0"},
     {address="0x000107c4",func-name="main",offset="8",
     inst="sethi %hi(0x11800), %o2"},
     [...]
     {address="0x0001081c",func-name="main",offset="96",inst="ret "},
     {address="0x00010820",func-name="main",offset="100",inst="restore "}]
     (gdb)

Disassemble 3 instructions from the start of main:

     (gdb)
     -data-disassemble -f basics.c -l 32 -n 3 -- 0
     ^done,asm_insns=[
     {address="0x000107bc",func-name="main",offset="0",
     inst="save  %sp, -112, %sp"},
     {address="0x000107c0",func-name="main",offset="4",
     inst="mov  2, %o0"},
     {address="0x000107c4",func-name="main",offset="8",
     inst="sethi  %hi(0x11800), %o2"}]
     (gdb)

Disassemble 3 instructions from the start of main in mixed mode:

     (gdb)
     -data-disassemble -f basics.c -l 32 -n 3 -- 1
     ^done,asm_insns=[
     src_and_asm_line={line="31",
     file="/kwikemart/marge/ezannoni/flathead-dev/devo/gdb/ \
       testsuite/gdb.mi/basics.c",line_asm_insn=[
     {address="0x000107bc",func-name="main",offset="0",
     inst="save  %sp, -112, %sp"}]},
     src_and_asm_line={line="32",
     file="/kwikemart/marge/ezannoni/flathead-dev/devo/gdb/ \
       testsuite/gdb.mi/basics.c",line_asm_insn=[
     {address="0x000107c0",func-name="main",offset="4",
     inst="mov  2, %o0"},
     {address="0x000107c4",func-name="main",offset="8",
     inst="sethi  %hi(0x11800), %o2"}]}]
     (gdb)

The -data-evaluate-expression Command
-------------------------------------

Synopsis
........

      -data-evaluate-expression EXPR

Evaluate EXPR as an expression.  The expression could contain an inferior function call.  The function call will execute synchronously. If the expression contains spaces, it must be enclosed in double quotes.

GDBN Command
............

The corresponding GDBN commands are print, output, and call.  In gdbtk only, there's a corresponding gdb_eval command.

Example
.......

In the following example, the numbers that precede the commands are the tokens described in GDB/MI Command Syntax.  Notice how GDB/MI returns the same tokens in its output.

     211-data-evaluate-expression A
     211^done,value="1"
     (gdb)
     311-data-evaluate-expression &A
     311^done,value="0xefffeb7c"
     (gdb)
     411-data-evaluate-expression A+3
     411^done,value="4"
     (gdb)
     511-data-evaluate-expression "A + 3"
     511^done,value="4"
     (gdb)

The -data-list-changed-registers Command
----------------------------------------

Synopsis
........

      -data-list-changed-registers

Display a list of the registers that have changed.

GDBN Command
............

GDBN doesn't have a direct analog for this command; gdbtk has the corresponding command gdb_changed_register_list.

Example
.......

On a PPC MBX board:

     (gdb)
     -exec-continue
     ^running

     (gdb)
     *stopped,reason="breakpoint-hit",disp="keep",bkptno="1",frame={
     func="main",args=[],file="try.c",fullname="/home/foo/bar/try.c",
     line="5"}
     (gdb)
     -data-list-changed-registers
     ^done,changed-registers=["0","1","2","4","5","6","7","8","9",
     "10","11","13","14","15","16","17","18","19","20","21","22","23",
     "24","25","26","27","28","30","31","64","65","66","67","69"]
     (gdb)

The -data-list-register-names Command
-------------------------------------

Synopsis
........

      -data-list-register-names [ ( REGNO )+ ]

Show a list of register names for the current target.  If no arguments are given, it shows a list of the names of all the registers.  If integer numbers are given as arguments, it will print a list of the names of the registers corresponding to the arguments.  To ensure consistency between a register name and its number, the output list may include empty register names.

GDBN Command
............

GDBN does not have a command which corresponds to -data-list-register-names.  In gdbtk there is a corresponding command gdb_regnames.

Example
.......

For the PPC MBX board:

     (gdb)
     -data-list-register-names
     ^done,register-names=["r0","r1","r2","r3","r4","r5","r6","r7",
     "r8","r9","r10","r11","r12","r13","r14","r15","r16","r17","r18",
     "r19","r20","r21","r22","r23","r24","r25","r26","r27","r28","r29",
     "r30","r31","f0","f1","f2","f3","f4","f5","f6","f7","f8","f9",
     "f10","f11","f12","f13","f14","f15","f16","f17","f18","f19","f20",
     "f21","f22","f23","f24","f25","f26","f27","f28","f29","f30","f31",
     "", "pc","ps","cr","lr","ctr","xer"]
     (gdb)
     -data-list-register-names 1 2 3
     ^done,register-names=["r1","r2","r3"]
     (gdb)

The -data-list-register-values Command
--------------------------------------

Synopsis
........

      -data-list-register-values FMT [ ( REGNO )*]

Display the registers' contents.  FMT is the format according to which the registers' contents are to be returned, followed by an optional list of numbers specifying the registers to display.  A missing list of numbers indicates that the contents of all the registers must be returned.

Allowed formats for FMT are:

x
     Hexadecimal

o
     Octal

t
     Binary

d
     Decimal

r
     Raw

N
     Natural

GDBN Command
............

The corresponding GDBN commands are info reg, info
all-reg, and (in gdbtk) gdb_fetch_registers.

Example
.......

For a PPC MBX board (note: line breaks are for readability only, they don't appear in the actual output):

     (gdb)
     -data-list-register-values r 64 65
     ^done,register-values=[{number="64",value="0xfe00a300"},
     {number="65",value="0x00029002"}]
     (gdb)
     -data-list-register-values x
     ^done,register-values=[{number="0",value="0xfe0043c8"},
     {number="1",value="0x3fff88"},{number="2",value="0xfffffffe"},
     {number="3",value="0x0"},{number="4",value="0xa"},
     {number="5",value="0x3fff68"},{number="6",value="0x3fff58"},
     {number="7",value="0xfe011e98"},{number="8",value="0x2"},
     {number="9",value="0xfa202820"},{number="10",value="0xfa202808"},
     {number="11",value="0x1"},{number="12",value="0x0"},
     {number="13",value="0x4544"},{number="14",value="0xffdfffff"},
     {number="15",value="0xffffffff"},{number="16",value="0xfffffeff"},
     {number="17",value="0xefffffed"},{number="18",value="0xfffffffe"},
     {number="19",value="0xffffffff"},{number="20",value="0xffffffff"},
     {number="21",value="0xffffffff"},{number="22",value="0xfffffff7"},
     {number="23",value="0xffffffff"},{number="24",value="0xffffffff"},
     {number="25",value="0xffffffff"},{number="26",value="0xfffffffb"},
     {number="27",value="0xffffffff"},{number="28",value="0xf7bfffff"},
     {number="29",value="0x0"},{number="30",value="0xfe010000"},
     {number="31",value="0x0"},{number="32",value="0x0"},
     {number="33",value="0x0"},{number="34",value="0x0"},
     {number="35",value="0x0"},{number="36",value="0x0"},
     {number="37",value="0x0"},{number="38",value="0x0"},
     {number="39",value="0x0"},{number="40",value="0x0"},
     {number="41",value="0x0"},{number="42",value="0x0"},
     {number="43",value="0x0"},{number="44",value="0x0"},
     {number="45",value="0x0"},{number="46",value="0x0"},
     {number="47",value="0x0"},{number="48",value="0x0"},
     {number="49",value="0x0"},{number="50",value="0x0"},
     {number="51",value="0x0"},{number="52",value="0x0"},
     {number="53",value="0x0"},{number="54",value="0x0"},
     {number="55",value="0x0"},{number="56",value="0x0"},
     {number="57",value="0x0"},{number="58",value="0x0"},
     {number="59",value="0x0"},{number="60",value="0x0"},
     {number="61",value="0x0"},{number="62",value="0x0"},
     {number="63",value="0x0"},{number="64",value="0xfe00a300"},
     {number="65",value="0x29002"},{number="66",value="0x202f04b5"},
     {number="67",value="0xfe0043b0"},{number="68",value="0xfe00b3e4"},
     {number="69",value="0x20002b03"}]
     (gdb)

The -data-read-memory Command
-----------------------------

This command is deprecated, use -data-read-memory-bytes instead.

Synopsis
........

      -data-read-memory [ -o BYTE-OFFSET ]
        ADDRESS WORD-FORMAT WORD-SIZE
        NR-ROWS NR-COLS [ ASCHAR ]

where:

ADDRESS
     An expression specifying the address of the first memory word to be read.  Complex expressions containing embedded white space should be quoted using the C convention.

WORD-FORMAT
     The format to be used to print the memory words.  The notation is the same as for GDBN's print command (see Output Formats).

WORD-SIZE
     The size of each memory word in bytes.

NR-ROWS
     The number of rows in the output table.

NR-COLS
     The number of columns in the output table.

ASCHAR
     If present, indicates that each row should include an ASCII dump.  The value of ASCHAR is used as a padding character when a byte is not a member of the printable ASCII character set (printable ASCII characters are those whose code is between 32 and 126, inclusively).

BYTE-OFFSET
     An offset to add to the ADDRESS before fetching memory.

This command displays memory contents as a table of NR-ROWS by NR-COLS words, each word being WORD-SIZE bytes.  In total, NR-ROWS * NR-COLS * WORD-SIZE bytes are read (returned as total-bytes).  Should less than the requested number of bytes be returned by the target, the missing words are identified using N/A.  The number of bytes read from the target is returned in nr-bytes and the starting address used to read memory in addr.

The address of the next/previous row or page is available in next-row and prev-row, next-page and prev-page.

GDBN Command
............

The corresponding GDBN command is x.  gdbtk has gdb_get_mem memory read command.

Example
.......

Read six bytes of memory starting at bytes+6 but then offset by -6 bytes.  Format as three rows of two columns.  One byte per word.  Display each word in hex.

     (gdb)
     9-data-read-memory -o -6 -- bytes+6 x 1 3 2
     9^done,addr="0x00001390",nr-bytes="6",total-bytes="6",
     next-row="0x00001396",prev-row="0x0000138e",next-page="0x00001396",
     prev-page="0x0000138a",memory=[
     {addr="0x00001390",data=["0x00","0x01"]},
     {addr="0x00001392",data=["0x02","0x03"]},
     {addr="0x00001394",data=["0x04","0x05"]}]
     (gdb)

Read two bytes of memory starting at address shorts + 64 and display as a single word formatted in decimal.

     (gdb)
     5-data-read-memory shorts+64 d 2 1 1
     5^done,addr="0x00001510",nr-bytes="2",total-bytes="2",
     next-row="0x00001512",prev-row="0x0000150e",
     next-page="0x00001512",prev-page="0x0000150e",memory=[
     {addr="0x00001510",data=["128"]}]
     (gdb)

Read thirty two bytes of memory starting at bytes+16 and format as eight rows of four columns.  Include a string encoding with x used as the non-printable character.

     (gdb)
     4-data-read-memory bytes+16 x 1 8 4 x
     4^done,addr="0x000013a0",nr-bytes="32",total-bytes="32",
     next-row="0x000013c0",prev-row="0x0000139c",
     next-page="0x000013c0",prev-page="0x00001380",memory=[
     {addr="0x000013a0",data=["0x10","0x11","0x12","0x13"],ascii="xxxx"},
     {addr="0x000013a4",data=["0x14","0x15","0x16","0x17"],ascii="xxxx"},
     {addr="0x000013a8",data=["0x18","0x19","0x1a","0x1b"],ascii="xxxx"},
     {addr="0x000013ac",data=["0x1c","0x1d","0x1e","0x1f"],ascii="xxxx"},
     {addr="0x000013b0",data=["0x20","0x21","0x22","0x23"],ascii=" !\"#"},
     {addr="0x000013b4",data=["0x24","0x25","0x26","0x27"],ascii="$%&'"},
     {addr="0x000013b8",data=["0x28","0x29","0x2a","0x2b"],ascii="()*+"},
     {addr="0x000013bc",data=["0x2c","0x2d","0x2e","0x2f"],ascii=",-./"}]
     (gdb)

The -data-read-memory-bytes Command
-----------------------------------

Synopsis
........

      -data-read-memory-bytes [ -o BYTE-OFFSET ]
        ADDRESS COUNT

where:

ADDRESS
     An expression specifying the address of the first memory word to be read.  Complex expressions containing embedded white space should be quoted using the C convention.

COUNT
     The number of bytes to read.  This should be an integer literal.

BYTE-OFFSET
     The offsets in bytes relative to ADDRESS at which to start reading.  This should be an integer literal.  This option is provided so that a frontend is not required to first evaluate address and then perform address arithmetics itself.

This command attempts to read all accessible memory regions in the specified range.  First, all regions marked as unreadable in the memory map (if one is defined) will be skipped.  See Memory Region Attributes.  Second, GDBN will attempt to read the remaining regions.  For each one, if reading full region results in an errors, GDBN will try to read a subset of the region.

In general, every single byte in the region may be readable or not, and the only way to read every readable byte is to try a read at every address, which is not practical.   Therefore, GDBN will attempt to read all accessible bytes at either beginning or the end of the region, using a binary division scheme.  This heuristic works well for reading accross a memory map boundary.  Note that if a region has a readable range that is neither at the beginning or the end, GDBN will not read it.

The result record (see GDB/MI Result Records) that is output of the command includes a field named memory whose content is a list of tuples.  Each tuple represent a successfully read memory block and has the following fields:

begin
     The start address of the memory block, as hexadecimal literal.

end
     The end address of the memory block, as hexadecimal literal.

offset
     The offset of the memory block, as hexadecimal literal, relative to the start address passed to -data-read-memory-bytes.

contents
     The contents of the memory block, in hex.

GDBN Command
............

The corresponding GDBN command is x.

Example
.......

     (gdb)
     -data-read-memory-bytes &a 10
     ^done,memory=[{begin="0xbffff154",offset="0x00000000",
                   end="0xbffff15e",
                   contents="01000000020000000300"}]
     (gdb)

The -data-write-memory-bytes Command
------------------------------------

Synopsis
........

      -data-write-memory-bytes ADDRESS CONTENTS

where:

ADDRESS
     An expression specifying the address of the first memory word to be read.  Complex expressions containing embedded white space should be quoted using the C convention.

CONTENTS
     The hex-encoded bytes to write.

GDBN Command
............

There's no corresponding GDBN command.

Example
.......

     (gdb)
     -data-write-memory-bytes &a "aabbccdd"
     ^done
     (gdb)

GDB/MI Tracepoint Commands
==========================

The commands defined in this section implement MI support for tracepoints.  For detailed introduction, see Tracepoints.

The -trace-find Command
-----------------------

Synopsis
........

      -trace-find MODE [PARAMETERS...]

Find a trace frame using criteria defined by MODE and PARAMETERS.  The following table lists permissible modes and their parameters.  For details of operation, see tfind.

none
     No parameters are required.  Stops examining trace frames.

frame-number
     An integer is required as parameter.  Selects tracepoint frame with that index.

tracepoint-number
     An integer is required as parameter.  Finds next trace frame that corresponds to tracepoint with the specified number.

pc
     An address is required as parameter.  Finds next trace frame that corresponds to any tracepoint at the specified address.

pc-inside-range
     Two addresses are required as parameters.  Finds next trace frame that corresponds to a tracepoint at an address inside the specified range.  Both bounds are considered to be inside the range.

pc-outside-range
     Two addresses are required as parameters.  Finds next trace frame that corresponds to a tracepoint at an address outside the specified range.  Both bounds are considered to be inside the range.

line
     Line specification is required as parameter.  See Specify Location. Finds next trace frame that corresponds to a tracepoint at the specified location.

If none was passed as MODE, the response does not have fields.  Otherwise, the response may have the following fields:

found
     This field has either 0 or 1 as the value, depending on whether a matching tracepoint was found.

traceframe
     The index of the found traceframe.  This field is present iff the found field has value of 1.

tracepoint
     The index of the found tracepoint.  This field is present iff the found field has value of 1.

frame
     The information about the frame corresponding to the found trace frame.  This field is present only if a trace frame was found. See GDB/MI Frame Information, for description of this field.

GDBN Command
............

The corresponding GDBN command is tfind.

-trace-define-variable
----------------------

Synopsis
........

      -trace-define-variable NAME [ VALUE ]

Create trace variable NAME if it does not exist.  If VALUE is specified, sets the initial value of the specified trace variable to that value.  Note that the NAME should start with the $ character.

GDBN Command
............

The corresponding GDBN command is tvariable.

-trace-list-variables
---------------------

Synopsis
........

      -trace-list-variables

Return a table of all defined trace variables.  Each element of the table has the following fields:

name
     The name of the trace variable.  This field is always present.

initial
     The initial value.  This is a 64-bit signed integer.  This field is always present.

current
     The value the trace variable has at the moment.  This is a 64-bit signed integer.  This field is absent iff current value is not defined, for example if the trace was never run, or is presently running.

GDBN Command
............

The corresponding GDBN command is tvariables.

Example
.......

     (gdb)
     -trace-list-variables
     ^done,trace-variables={nr_rows="1",nr_cols="3",
     hdr=[{width="15",alignment="-1",col_name="name",colhdr="Name"},
          {width="11",alignment="-1",col_name="initial",colhdr="Initial"},
          {width="11",alignment="-1",col_name="current",colhdr="Current"}],
     body=[variable={name="$trace_timestamp",initial="0"}
           variable={name="$foo",initial="10",current="15"}]}
     (gdb)

-trace-save
-----------

Synopsis
........

      -trace-save [-r ] FILENAME

Saves the collected trace data to FILENAME.  Without the -r option, the data is downloaded from the target and saved in a local file.  With the -r option the target is asked to perform the save.

GDBN Command
............

The corresponding GDBN command is tsave.

-trace-start
------------

Synopsis
........

      -trace-start

Starts a tracing experiments.  The result of this command does not have any fields.

GDBN Command
............

The corresponding GDBN command is tstart.

-trace-status
-------------

Synopsis
........

      -trace-status

Obtains the status of a tracing experiment.  The result may include the following fields:

supported
     May have a value of either 0, when no tracing operations are supported, 1, when all tracing operations are supported, or file when examining trace file.  In the latter case, examining of trace frame is possible but new tracing experiement cannot be started.  This field is always present.

running
     May have a value of either 0 or 1 depending on whether tracing experiement is in progress on target.  This field is present if supported field is not 0.

stop-reason
     Report the reason why the tracing was stopped last time.  This field may be absent iff tracing was never stopped on target yet.  The value of request means the tracing was stopped as result of the -trace-stop command.  The value of overflow means the tracing buffer is full.  The value of disconnection means tracing was automatically stopped when GDBN has disconnected. The value of passcount means tracing was stopped when a tracepoint was passed a maximal number of times for that tracepoint. This field is present if supported field is not 0.

stopping-tracepoint
     The number of tracepoint whose passcount as exceeded.  This field is present iff the stop-reason field has the value of passcount.

frames
frames-created
     The frames field is a count of the total number of trace frames in the trace buffer, while frames-created is the total created during the run, including ones that were discarded, such as when a circular trace buffer filled up.  Both fields are optional.

buffer-size
buffer-free
     These fields tell the current size of the tracing buffer and the remaining space.  These fields are optional.

circular
     The value of the circular trace buffer flag.  1 means that the trace buffer is circular and old trace frames will be discarded if necessary to make room, 0 means that the trace buffer is linear and may fill up.

disconnected
     The value of the disconnected tracing flag.  1 means that tracing will continue after GDBN disconnects, 0 means that the trace run will stop.

GDBN Command
............

The corresponding GDBN command is tstatus.

-trace-stop
-----------

Synopsis
........

      -trace-stop

Stops a tracing experiment.  The result of this command has the same fields as -trace-status, except that the supported and running fields are not output.

GDBN Command
............

The corresponding GDBN command is tstop.

GDB/MI Symbol Query Commands
============================

The -symbol-list-lines Command
------------------------------

Synopsis
........

      -symbol-list-lines FILENAME

Print the list of lines that contain code and their associated program addresses for the given source filename.  The entries are sorted in ascending PC order.

GDBN Command
............

There is no corresponding GDBN command.

Example
.......

     (gdb)
     -symbol-list-lines basics.c
     ^done,lines=[{pc="0x08048554",line="7"},{pc="0x0804855a",line="8"}]
     (gdb)

GDB/MI File Commands
====================

This section describes the GDB/MI commands to specify executable file names and to read in and obtain symbol table information.

The -file-exec-and-symbols Command
----------------------------------

Synopsis
........

      -file-exec-and-symbols FILE

Specify the executable file to be debugged.  This file is the one from which the symbol table is also read.  If no file is specified, the command clears the executable and symbol information.  If breakpoints are set when using this command with no arguments, GDBN will produce error messages.  Otherwise, no output is produced, except a completion notification.

GDBN Command
............

The corresponding GDBN command is file.

Example
.......

     (gdb)
     -file-exec-and-symbols /kwikemart/marge/ezannoni/TRUNK/mbx/hello.mbx
     ^done
     (gdb)

The -file-exec-file Command
---------------------------

Synopsis
........

      -file-exec-file FILE

Specify the executable file to be debugged.  Unlike -file-exec-and-symbols, the symbol table is not read from this file.  If used without argument, GDBN clears the information about the executable file.  No output is produced, except a completion notification.

GDBN Command
............

The corresponding GDBN command is exec-file.

Example
.......

     (gdb)
     -file-exec-file /kwikemart/marge/ezannoni/TRUNK/mbx/hello.mbx
     ^done
     (gdb)

The -file-list-exec-source-file Command
---------------------------------------

Synopsis
........

      -file-list-exec-source-file

List the line number, the current source file, and the absolute path to the current source file for the current executable.  The macro information field has a value of 1 or 0 depending on whether or not the file includes preprocessor macro information.

GDBN Command
............

The GDBN equivalent is info source

Example
.......

     (gdb)
     123-file-list-exec-source-file
     123^done,line="1",file="foo.c",fullname="/home/bar/foo.c,macro-info="1"
     (gdb)

The -file-list-exec-source-files Command
----------------------------------------

Synopsis
........

      -file-list-exec-source-files

List the source files for the current executable.

It will always output the filename, but only when GDBN can find the absolute file name of a source file, will it output the fullname.

GDBN Command
............

The GDBN equivalent is info sources. gdbtk has an analogous command gdb_listfiles.

Example
.......

     (gdb)
     -file-list-exec-source-files
     ^done,files=[
     {file=foo.c,fullname=/home/foo.c},
     {file=/home/bar.c,fullname=/home/bar.c},
     {file=gdb_could_not_find_fullpath.c}]
     (gdb)

The -file-symbol-file Command
-----------------------------

Synopsis
........

      -file-symbol-file FILE

Read symbol table info from the specified FILE argument.  When used without arguments, clears GDBN's symbol table info.  No output is produced, except for a completion notification.

GDBN Command
............

The corresponding GDBN command is symbol-file.

Example
.......

     (gdb)
     -file-symbol-file /kwikemart/marge/ezannoni/TRUNK/mbx/hello.mbx
     ^done
     (gdb)

GDB/MI Target Manipulation Commands
===================================

The -target-attach Command
--------------------------

Synopsis
........

      -target-attach PID | GID | FILE

Attach to a process PID or a file FILE outside of GDBN, or a thread group GID.  If attaching to a thread group, the id previously returned by -list-thread-groups --available must be used.

GDBN Command
............

The corresponding GDBN command is attach.

Example
.......

     (gdb)
     -target-attach 34
     =thread-created,id="1"
     *stopped,thread-id="1",frame={addr="0xb7f7e410",func="bar",args=[]}
     ^done
     (gdb)

The -target-detach Command
--------------------------

Synopsis
........

      -target-detach [ PID | GID ]

Detach from the remote target which normally resumes its execution. If either PID or GID is specified, detaches from either the specified process, or specified thread group.  There's no output.

GDBN Command
............

The corresponding GDBN command is detach.

Example
.......

     (gdb)
     -target-detach
     ^done
     (gdb)

The -target-disconnect Command
------------------------------

Synopsis
........

      -target-disconnect

Disconnect from the remote target.  There's no output and the target is generally not resumed.

GDBN Command
............

The corresponding GDBN command is disconnect.

Example
.......

     (gdb)
     -target-disconnect
     ^done
     (gdb)

The -target-download Command
----------------------------

Synopsis
........

      -target-download

Loads the executable onto the remote target. It prints out an update message every half second, which includes the fields:

section
     The name of the section.

section-sent
     The size of what has been sent so far for that section.

section-size
     The size of the section.

total-sent
     The total size of what was sent so far (the current and the previous sections).

total-size
     The size of the overall executable to download.

Each message is sent as status record (see GDB/MI Output Syntax).

In addition, it prints the name and size of the sections, as they are downloaded.  These messages include the following fields:

section
     The name of the section.

section-size
     The size of the section.

total-size
     The size of the overall executable to download.

At the end, a summary is printed.

GDBN Command
............

The corresponding GDBN command is load.

Example
.......

Note: each status message appears on a single line.  Here the messages have been broken down so that they can fit onto a page.

     (gdb)
     -target-download
     +download,{section=".text",section-size="6668",total-size="9880"}
     +download,{section=".text",section-sent="512",section-size="6668",
     total-sent="512",total-size="9880"}
     +download,{section=".text",section-sent="1024",section-size="6668",
     total-sent="1024",total-size="9880"}
     +download,{section=".text",section-sent="1536",section-size="6668",
     total-sent="1536",total-size="9880"}
     +download,{section=".text",section-sent="2048",section-size="6668",
     total-sent="2048",total-size="9880"}
     +download,{section=".text",section-sent="2560",section-size="6668",
     total-sent="2560",total-size="9880"}
     +download,{section=".text",section-sent="3072",section-size="6668",
     total-sent="3072",total-size="9880"}
     +download,{section=".text",section-sent="3584",section-size="6668",
     total-sent="3584",total-size="9880"}
     +download,{section=".text",section-sent="4096",section-size="6668",
     total-sent="4096",total-size="9880"}
     +download,{section=".text",section-sent="4608",section-size="6668",
     total-sent="4608",total-size="9880"}
     +download,{section=".text",section-sent="5120",section-size="6668",
     total-sent="5120",total-size="9880"}
     +download,{section=".text",section-sent="5632",section-size="6668",
     total-sent="5632",total-size="9880"}
     +download,{section=".text",section-sent="6144",section-size="6668",
     total-sent="6144",total-size="9880"}
     +download,{section=".text",section-sent="6656",section-size="6668",
     total-sent="6656",total-size="9880"}
     +download,{section=".init",section-size="28",total-size="9880"}
     +download,{section=".fini",section-size="28",total-size="9880"}
     +download,{section=".data",section-size="3156",total-size="9880"}
     +download,{section=".data",section-sent="512",section-size="3156",
     total-sent="7236",total-size="9880"}
     +download,{section=".data",section-sent="1024",section-size="3156",
     total-sent="7748",total-size="9880"}
     +download,{section=".data",section-sent="1536",section-size="3156",
     total-sent="8260",total-size="9880"}
     +download,{section=".data",section-sent="2048",section-size="3156",
     total-sent="8772",total-size="9880"}
     +download,{section=".data",section-sent="2560",section-size="3156",
     total-sent="9284",total-size="9880"}
     +download,{section=".data",section-sent="3072",section-size="3156",
     total-sent="9796",total-size="9880"}
     ^done,address="0x10004",load-size="9880",transfer-rate="6586",
     write-rate="429"
     (gdb)

GDBN Command
............

No equivalent.

Example
.......

N.A.

The -target-select Command
--------------------------

Synopsis
........

      -target-select TYPE PARAMETERS ...

Connect GDBN to the remote target.  This command takes two args:

TYPE
     The type of target, for instance remote, etc.

PARAMETERS
     Device names, host names and the like.  See Commands for Managing Targets, for more details.

The output is a connection notification, followed by the address at which the target program is, in the following form:

     ^connected,addr="ADDRESS",func="FUNCTION NAME",
       args=[ARG LIST]

GDBN Command
............

The corresponding GDBN command is target.

Example
.......

     (gdb)
     -target-select remote /dev/ttya
     ^connected,addr="0xfe00a300",func="??",args=[]
     (gdb)

GDB/MI File Transfer Commands
=============================

The -target-file-put Command
----------------------------

Synopsis
........

      -target-file-put HOSTFILE TARGETFILE

Copy file HOSTFILE from the host system (the machine running GDBN) to TARGETFILE on the target system.

GDBN Command
............

The corresponding GDBN command is remote put.

Example
.......

     (gdb)
     -target-file-put localfile remotefile
     ^done
     (gdb)

The -target-file-get Command
----------------------------

Synopsis
........

      -target-file-get TARGETFILE HOSTFILE

Copy file TARGETFILE from the target system to HOSTFILE on the host system.

GDBN Command
............

The corresponding GDBN command is remote get.

Example
.......

     (gdb)
     -target-file-get remotefile localfile
     ^done
     (gdb)

The -target-file-delete Command
-------------------------------

Synopsis
........

      -target-file-delete TARGETFILE

Delete TARGETFILE from the target system.

GDBN Command
............

The corresponding GDBN command is remote delete.

Example
.......

     (gdb)
     -target-file-delete remotefile
     ^done
     (gdb)

Miscellaneous GDB/MI Commands
=============================

The -gdb-exit Command
---------------------

Synopsis
........

      -gdb-exit

Exit GDBN immediately.

GDBN Command
............

Approximately corresponds to quit.

Example
.......

     (gdb)
     -gdb-exit
     ^exit

The -gdb-set Command
--------------------

Synopsis
........

      -gdb-set

Set an internal GDBN variable.

GDBN Command
............

The corresponding GDBN command is set.

Example
.......

     (gdb)
     -gdb-set $foo=3
     ^done
     (gdb)

The -gdb-show Command
---------------------

Synopsis
........

      -gdb-show

Show the current value of a GDBN variable.

GDBN Command
............

The corresponding GDBN command is show.

Example
.......

     (gdb)
     -gdb-show annotate
     ^done,value="0"
     (gdb)

The -gdb-version Command
------------------------

Synopsis
........

      -gdb-version

Show version information for GDBN.  Used mostly in testing.

GDBN Command
............

The GDBN equivalent is show version.  GDBN by default shows this information when you start an interactive session.

Example
.......

     (gdb)
     -gdb-version
     ~GNU gdb 5.2.1
     ~Copyright 2000 Free Software Foundation, Inc.
     ~GDB is free software, covered by the GNU General Public License, and
     ~you are welcome to change it and/or distribute copies of it under
     ~ certain conditions.
     ~Type "show copying" to see the conditions.
     ~There is absolutely no warranty for GDB.  Type "show warranty" for
     ~ details.
     ~This GDB was configured as
      "--host=sparc-sun-solaris2.5.1 --target=ppc-eabi".
     ^done
     (gdb)

The -list-features Command
--------------------------

Returns a list of particular features of the MI protocol that this version of gdb implements.  A feature can be a command, or a new field in an output of some command, or even an important bugfix.  While a frontend can sometimes detect presence of a feature at runtime, it is easier to perform detection at debugger startup.

The command returns a list of strings, with each string naming an available feature.  Each returned string is just a name, it does not have any internal structure.  The list of possible feature names is given below.

Example output:

     (gdb) -list-features
     ^done,result=["feature1","feature2"]

The current list of features is:

frozen-varobjs
     Indicates support for the -var-set-frozen command, as well as possible presense of the frozen field in the output of -varobj-create.

pending-breakpoints
     Indicates support for the -f option to the -break-insert command.

python
     Indicates Python scripting support, Python-based pretty-printing commands, and possible presence of the display_hint field in the output of -var-list-children

thread-info
     Indicates support for the -thread-info command.

data-read-memory-bytes
     Indicates support for the -data-read-memory-bytes and the -data-write-memory-bytes commands.

breakpoint-notifications
     Indicates that changes to breakpoints and breakpoints created via the CLI will be announced via async records.

ada-task-info
     Indicates support for the -ada-task-info command.

The -list-target-features Command
---------------------------------

Returns a list of particular features that are supported by the target.  Those features affect the permitted MI commands, but unlike the features reported by the -list-features command, the features depend on which target GDB is using at the moment.  Whenever a target can change, due to commands such as -target-select, -target-attach or -exec-run, the list of target features may change, and the frontend should obtain it again. Example output:

     (gdb) -list-features
     ^done,result=["async"]

The current list of features is:

async
     Indicates that the target is capable of asynchronous command execution, which means that GDBN will accept further commands while the target is running.

reverse
     Indicates that the target is capable of reverse execution. See Reverse Execution, for more information.

The -list-thread-groups Command
-------------------------------

Synopsis
--------

     -list-thread-groups [ --available ] [ --recurse 1 ] [ GROUP ... ]

Lists thread groups (see Thread groups).  When a single thread group is passed as the argument, lists the children of that group. When several thread group are passed, lists information about those thread groups.  Without any parameters, lists information about all top-level thread groups.

Normally, thread groups that are being debugged are reported. With the --available option, GDBN reports thread groups available on the target.

The output of this command may have either a threads result or a groups result.  The thread result has a list of tuples as value, with each tuple describing a thread (see GDB/MI Thread Information).  The groups result has a list of tuples as value, each tuple describing a thread group.  If top-level groups are requested (that is, no parameter is passed), or when several groups are passed, the output always has a groups result.  The format of the group result is described below.

To reduce the number of roundtrips it's possible to list thread groups together with their children, by passing the --recurse option and the recursion depth.  Presently, only recursion depth of 1 is permitted.  If this option is present, then every reported thread group will also include its children, either as group or threads field.

In general, any combination of option and parameters is permitted, with the following caveats:

  * When a single thread group is passed, the output will typically be the threads result.  Because threads may not contain anything, the recurse option will be ignored.

  * When the --available option is passed, limited information may be available.  In particular, the list of threads of a process might be inaccessible.  Further, specifying specific thread groups might not give any performance advantage over listing all thread groups. The frontend should assume that -list-thread-groups --available is always an expensive operation and cache the results.

The groups result is a list of tuples, where each tuple may have the following fields:

id
     Identifier of the thread group.  This field is always present. The identifier is an opaque string; frontends should not try to convert it to an integer, even though it might look like one.

type
     The type of the thread group.  At present, only process is a valid type.

pid
     The target-specific process identifier.  This field is only present for thread groups of type process and only if the process exists.

num_children
     The number of children this thread group has.  This field may be absent for an available thread group.

threads
     This field has a list of tuples as value, each tuple describing a thread.  It may be present if the --recurse option is specified, and it's actually possible to obtain the threads.

cores
     This field is a list of integers, each identifying a core that one thread of the group is running on.  This field may be absent if such information is not available.

executable
     The name of the executable file that corresponds to this thread group. The field is only present for thread groups of type process, and only if there is a corresponding executable file.

Example
-------

     GDBP
     -list-thread-groups
     ^done,groups=[{id="17",type="process",pid="yyy",num_children="2"}]
     -list-thread-groups 17
     ^done,threads=[{id="2",target-id="Thread 0xb7e14b90 (LWP 21257)",
        frame={level="0",addr="0xffffe410",func="__kernel_vsyscall",args=[]},state="running"},
     {id="1",target-id="Thread 0xb7e156b0 (LWP 21254)",
        frame={level="0",addr="0x0804891f",func="foo",args=[{name="i",value="10"}],
                file="/tmp/a.c",fullname="/tmp/a.c",line="158"},state="running"}]]
     -list-thread-groups --available
     ^done,groups=[{id="17",type="process",pid="yyy",num_children="2",cores=[1,2]}]
     -list-thread-groups --available --recurse 1
      ^done,groups=[{id="17", types="process",pid="yyy",num_children="2",cores=[1,2],
                     threads=[{id="1",target-id="Thread 0xb7e14b90",cores=[1]},
                              {id="2",target-id="Thread 0xb7e14b90",cores=[2]}]},..]
     -list-thread-groups --available --recurse 1 17 18
     ^done,groups=[{id="17", types="process",pid="yyy",num_children="2",cores=[1,2],
                    threads=[{id="1",target-id="Thread 0xb7e14b90",cores=[1]},
                             {id="2",target-id="Thread 0xb7e14b90",cores=[2]}]},...]

The -add-inferior Command
-------------------------

Synopsis
--------

     -add-inferior

Creates a new inferior (see Inferiors and Programs).  The created inferior is not associated with any executable.  Such association may be established with the -file-exec-and-symbols command (see GDB/MI File Commands).  The command response has a single field, thread-group, whose value is the identifier of the thread group corresponding to the new inferior.

Example
-------

     GDBP
     -add-inferior
     ^done,thread-group="i3"

The -interpreter-exec Command
-----------------------------

Synopsis
--------

     -interpreter-exec INTERPRETER COMMAND

Execute the specified COMMAND in the given INTERPRETER.

GDBN Command
------------

The corresponding GDBN command is interpreter-exec.

Example
-------

     (gdb)
     -interpreter-exec console "break main"
     &"During symbol reading, couldn't parse type; debugger out of date?.\n"
     &"During symbol reading, bad structure-type format.\n"
     ~"Breakpoint 1 at 0x8074fc6: file ../../src/gdb/main.c, line 743.\n"
     ^done
     (gdb)

The -inferior-tty-set Command
-----------------------------

Synopsis
--------

     -inferior-tty-set /dev/pts/1

Set terminal for future runs of the program being debugged.

GDBN Command
------------

The corresponding GDBN command is set inferior-tty /dev/pts/1.

Example
-------

     (gdb)
     -inferior-tty-set /dev/pts/1
     ^done
     (gdb)

The -inferior-tty-show Command
------------------------------

Synopsis
--------

     -inferior-tty-show

Show terminal for future runs of program being debugged.

GDBN Command
------------

The corresponding GDBN command is show inferior-tty.

Example
-------

     (gdb)
     -inferior-tty-set /dev/pts/1
     ^done
     (gdb)
     -inferior-tty-show
     ^done,inferior_tty_terminal="/dev/pts/1"
     (gdb)

The -enable-timings Command
---------------------------

Synopsis
--------

     -enable-timings [yes | no]

Toggle the printing of the wallclock, user and system times for an MI command as a field in its output.  This command is to help frontend developers optimize the performance of their code.  No argument is equivalent to yes.

GDBN Command
------------

No equivalent.

Example
-------

     (gdb)
     -enable-timings
     ^done
     (gdb)
     -break-insert main
     ^done,bkpt={number="1",type="breakpoint",disp="keep",enabled="y",
     addr="0x080484ed",func="main",file="myprog.c",
     fullname="/home/nickrob/myprog.c",line="73",times="0"},
     time={wallclock="0.05185",user="0.00800",system="0.00000"}
     (gdb)
     -enable-timings no
     ^done
     (gdb)
     -exec-run
     ^running
     (gdb)
     *stopped,reason="breakpoint-hit",disp="keep",bkptno="1",thread-id="0",
     frame={addr="0x080484ed",func="main",args=[{name="argc",value="1"},
     {name="argv",value="0xbfb60364"}],file="myprog.c",
     fullname="/home/nickrob/myprog.c",line="73"}
     (gdb)

GDBN Annotations
****************

This chapter describes annotations in GDBN.  Annotations were designed to interface GDBN to graphical user interfaces or other similar programs which want to interact with GDBN at a relatively high level.

The annotation mechanism has largely been superseded by GDB/MI (see GDB/MI).

What is an Annotation?
======================

Annotations start with a newline character, two control-z characters, and the name of the annotation.  If there is no additional information associated with this annotation, the name of the annotation is followed immediately by a newline.  If there is additional information, the name of the annotation is followed by a space, the additional information, and a newline.  The additional information cannot contain newline characters.

Any output not beginning with a newline and two control-z characters denotes literal output from GDBN.  Currently there is no need for GDBN to output a newline followed by two control-z characters, but if there was such a need, the annotations could be extended with an escape annotation which means those three characters as output.

The annotation LEVEL, which is specified using the --annotate command line option (see Mode Options), controls how much information GDBN prints together with its prompt, values of expressions, source lines, and other types of output.  Level 0 is for no annotations, level 1 is for use when GDBN is run as a subprocess of GNU Emacs, level 3 is the maximum annotation suitable for programs that control GDBN, and level 2 annotations have been made obsolete (see Limitations of the Annotation Interface (GDB's Obsolete Annotations)).

set annotate LEVEL
     The GDBN command set annotate sets the level of annotations to the specified LEVEL.

show annotate
     Show the current annotation level.

This chapter describes level 3 annotations.

A simple example of starting up GDBN with annotations is:

     $ gdb --annotate=3
     GNU gdb 6.0
     Copyright 2003 Free Software Foundation, Inc.
     GDB is free software, covered by the GNU General Public License,
     and you are welcome to change it and/or distribute copies of it
     under certain conditions.
     Type "show copying" to see the conditions.
     There is absolutely no warranty for GDB.  Type "show warranty"
     for details.
     This GDB was configured as "i386-pc-linux-gnu"

     ^Z^Zpre-prompt
     (GDBP)
     ^Z^Zprompt
     quit

     ^Z^Zpost-prompt
     $

Here quit is input to GDBN; the rest is output from GDBN.  The three lines beginning ^Z^Z (where ^Z denotes a control-z character) are annotations; the rest is output from GDBN.

The Server Prefix
=================

If you prefix a command with server  then it will not affect the command history, nor will it affect GDBN's notion of which command to repeat if RET is pressed on a line by itself.  This means that commands can be run behind a user's back by a front-end in a transparent manner.

The server  prefix does not affect the recording of values into the value history; to print a value without recording it into the value history, use the output command instead of the print command.

Using this prefix also disables confirmation requests (see confirmation requests).

Annotation for GDBN Input
=========================

When GDBN prompts for input, it annotates this fact so it is possible to know when to send output, when the output from a given command is over, etc.

Different kinds of input each have a different input type.  Each input type has three annotations: a pre- annotation, which denotes the beginning of any prompt which is being output, a plain annotation, which denotes the end of the prompt, and then a post- annotation which denotes the end of any echo which may (or may not) be associated with the input.  For example, the prompt input type features the following annotations:

     ^Z^Zpre-prompt
     ^Z^Zprompt
     ^Z^Zpost-prompt

The input types are

prompt
     When GDBN is prompting for a command (the main GDBN prompt).

commands
     When GDBN prompts for a set of commands, like in the commands command.  The annotations are repeated for each command which is input.

overload-choice
     When GDBN wants the user to select between various overloaded functions.

query
     When GDBN wants the user to confirm a potentially dangerous operation.

prompt-for-continue
     When GDBN is asking the user to press return to continue.  Note: Don't expect this to work well; instead use set height 0 to disable prompting.  This is because the counting of lines is buggy in the presence of annotations.

Errors
======

     ^Z^Zquit

This annotation occurs right before GDBN responds to an interrupt.

     ^Z^Zerror

This annotation occurs right before GDBN responds to an error.

Quit and error annotations indicate that any annotations which GDBN was in the middle of may end abruptly.  For example, if a value-history-begin annotation is followed by a error, one cannot expect to receive the matching value-history-end.  One cannot expect not to receive it either, however; an error annotation does not necessarily mean that GDBN is immediately returning all the way to the top level.

A quit or error annotation may be preceded by

     ^Z^Zerror-begin

Any output between that and the quit or error annotation is the error message.

Warning messages are not yet annotated.

Invalidation Notices
====================

The following annotations say that certain pieces of state may have changed.

^Z^Zframes-invalid

     The frames (for example, output from the backtrace command) may have changed.

^Z^Zbreakpoints-invalid

     The breakpoints may have changed.  For example, the user just added or deleted a breakpoint.

Running the Program
===================

When the program starts executing due to a GDBN command such as step or continue,

     ^Z^Zstarting

is output.  When the program stops,

     ^Z^Zstopped

is output.  Before the stopped annotation, a variety of annotations describe how the program stopped.

^Z^Zexited EXIT-STATUS
     The program exited, and EXIT-STATUS is the exit status (zero for successful exit, otherwise nonzero).

^Z^Zsignalled
     The program exited with a signal.  After the ^Z^Zsignalled, the annotation continues:

          INTRO-TEXT
          ^Z^Zsignal-name
          NAME
          ^Z^Zsignal-name-end
          MIDDLE-TEXT
          ^Z^Zsignal-string
          STRING
          ^Z^Zsignal-string-end
          END-TEXT

     where NAME is the name of the signal, such as SIGILL or SIGSEGV, and STRING is the explanation of the signal, such as Illegal Instruction or Segmentation fault. INTRO-TEXT, MIDDLE-TEXT, and END-TEXT are for the user's benefit and have no particular format.

^Z^Zsignal
     The syntax of this annotation is just like signalled, but GDBN is just saying that the program received the signal, not that it was terminated with it.

^Z^Zbreakpoint NUMBER
     The program hit breakpoint number NUMBER.

^Z^Zwatchpoint NUMBER
     The program hit watchpoint number NUMBER.

Displaying Source
=================

The following annotation is used instead of displaying source code:

     ^Z^Zsource FILENAME:LINE:CHARACTER:MIDDLE:ADDR

where FILENAME is an absolute file name indicating which source file, LINE is the line number within that file (where 1 is the first line in the file), CHARACTER is the character position within the file (where 0 is the first character in the file) (for most debug formats this will necessarily point to the beginning of a line), MIDDLE is middle if ADDR is in the middle of the line, or beg if ADDR is at the beginning of the line, and ADDR is the address in the target program associated with the source which is being displayed.  ADDR is in the form 0x followed by one or more lowercase hex digits (note that this does not depend on the language).

JIT Compilation Interface
*************************

This chapter documents GDBN's just-in-time (JIT) compilation interface.  A JIT compiler is a program or library that generates native executable code at runtime and executes it, usually in order to achieve good performance while maintaining platform independence.

Programs that use JIT compilation are normally difficult to debug because portions of their code are generated at runtime, instead of being loaded from object files, which is where GDBN normally finds the program's symbols and debug information.  In order to debug programs that use JIT compilation, GDBN has an interface that allows the program to register in-memory symbol files with GDBN at runtime.

If you are using GDBN to debug a program that uses this interface, then it should work transparently so long as you have not stripped the binary.  If you are developing a JIT compiler, then the interface is documented in the rest of this chapter.  At this time, the only known client of this interface is the LLVM JIT.

Broadly speaking, the JIT interface mirrors the dynamic loader interface.  The JIT compiler communicates with GDBN by writing data into a global variable and calling a fuction at a well-known symbol.  When GDBN attaches, it reads a linked list of symbol files from the global variable to find existing code, and puts a breakpoint in the function so that it can find out about additional code.

JIT Declarations
================

These are the relevant struct declarations that a C program should include to implement the interface:

     typedef enum
     {
       JIT_NOACTION = 0,
       JIT_REGISTER_FN,
       JIT_UNREGISTER_FN
     } jit_actions_t;

     struct jit_code_entry
     {
       struct jit_code_entry *next_entry;
       struct jit_code_entry *prev_entry;
       const char *symfile_addr;
       uint64_t symfile_size;
     };

     struct jit_descriptor
     {
       uint32_t version;
       /* This type should be jit_actions_t, but we use uint32_t
          to be explicit about the bitwidth.  */
       uint32_t action_flag;
       struct jit_code_entry *relevant_entry;
       struct jit_code_entry *first_entry;
     };

     /* GDB puts a breakpoint in this function.  */
     void __attribute__((noinline)) __jit_debug_register_code() { };

     /* Make sure to specify the version statically, because the
        debugger may check the version before we can set it.  */
     struct jit_descriptor __jit_debug_descriptor = { 1, 0, 0, 0 };

If the JIT is multi-threaded, then it is important that the JIT synchronize any modifications to this global data properly, which can easily be done by putting a global mutex around modifications to these structures.

Registering Code
================

To register code with GDBN, the JIT should follow this protocol:

  * Generate an object file in memory with symbols and other desired debug information.  The file must include the virtual addresses of the sections.

  * Create a code entry for the file, which gives the start and size of the symbol file.

  * Add it to the linked list in the JIT descriptor.

  * Point the relevant_entry field of the descriptor at the entry.

  * Set action_flag to JIT_REGISTER and call __jit_debug_register_code.

When GDBN is attached and the breakpoint fires, GDBN uses the relevant_entry pointer so it doesn't have to walk the list looking for new code.  However, the linked list must still be maintained in order to allow GDBN to attach to a running process and still find the symbol files.

Unregistering Code
==================

If code is freed, then the JIT should use the following protocol:

  * Remove the code entry corresponding to the code from the linked list.

  * Point the relevant_entry field of the descriptor at the code entry.

  * Set action_flag to JIT_UNREGISTER and call __jit_debug_register_code.

If the JIT frees or recompiles code without unregistering it, then GDBN and the JIT will leak the memory used for the associated symbol files.

Custom Debug Info
=================

Generating debug information in platform-native file formats (like ELF or COFF) may be an overkill for JIT compilers; especially if all the debug info is used for is displaying a meaningful backtrace.  The issue can be resolved by having the JIT writers decide on a debug info format and also provide a reader that parses the debug info generated by the JIT compiler.  This section gives a brief overview on writing such a parser.  More specific details can be found in the source file gdb/jit-reader.in, which is also installed as a header at INCLUDEDIR/gdb/jit-reader.h for easy inclusion.

The reader is implemented as a shared object (so this functionality is not available on platforms which don't allow loading shared objects at runtime).  Two GDBN commands, jit-reader-load and jit-reader-unload are provided, to be used to load and unload the readers from a preconfigured directory.  Once loaded, the shared object is used the parse the debug information emitted by the JIT compiler.

Using JIT Debug Info Readers
----------------------------

Readers can be loaded and unloaded using the jit-reader-load and jit-reader-unload commands.

jit-reader-load READER-NAME
     Load the JIT reader named READER-NAME.  On a UNIX system, this will usually load LIBDIR/gdb/READER-NAME, where LIBDIR is the system library directory, usually /usr/local/lib.  Only one reader can be active at a time; trying to load a second reader when one is already loaded will result in GDBN reporting an error.  A new JIT reader can be loaded by first unloading the current one using jit-reader-load and then invoking jit-reader-load.

jit-reader-unload
     Unload the currently loaded JIT reader.

Writing JIT Debug Info Readers
------------------------------

As mentioned, a reader is essentially a shared object conforming to a certain ABI.  This ABI is described in jit-reader.h.

jit-reader.h defines the structures, macros and functions required to write a reader.  It is installed (along with GDBN), in INCLUDEDIR/gdb where INCLUDEDIR is the system include directory.

Readers need to be released under a GPL compatible license.  A reader can be declared as released under such a license by placing the macro GDB_DECLARE_GPL_COMPATIBLE_READER in a source file.

The entry point for readers is the symbol gdb_init_reader, which is expected to be a function with the prototype

     extern struct gdb_reader_funcs *gdb_init_reader (void);

struct gdb_reader_funcs contains a set of pointers to callback functions.  These functions are executed to read the debug info generated by the JIT compiler (read), to unwind stack frames (unwind) and to create canonical frame IDs (get_Frame_id).  It also has a callback that is called when the reader is being unloaded (destroy).  The struct looks like this

     struct gdb_reader_funcs
     {
       /* Must be set to GDB_READER_INTERFACE_VERSION.  */
       int reader_version;

       /* For use by the reader.  */
       void *priv_data;

       gdb_read_debug_info *read;
       gdb_unwind_frame *unwind;
       gdb_get_frame_id *get_frame_id;
       gdb_destroy_reader *destroy;
     };

The callbacks are provided with another set of callbacks by GDBN to do their job.  For read, these callbacks are passed in a struct gdb_symbol_callbacks and for unwind and get_frame_id, in a struct gdb_unwind_callbacks. struct gdb_symbol_callbacks has callbacks to create new object files and new symbol tables inside those object files.  struct gdb_unwind_callbacks has callbacks to read registers off the current frame and to write out the values of the registers in the previous frame.  Both have a callback (target_read) to read bytes off the target's address space.

Reporting Bugs in GDBN
**********************

Your bug reports play an essential role in making GDBN reliable.

Reporting a bug may help you by bringing a solution to your problem, or it may not.  But in any case the principal function of a bug report is to help the entire community by making the next version of GDBN work better.  Bug reports are your contribution to the maintenance of GDBN.

In order for a bug report to serve its purpose, you must include the information that enables us to fix the bug.

Have You Found a Bug?
=====================

If you are not sure whether you have found a bug, here are some guidelines:

  * If the debugger gets a fatal signal, for any input whatever, that is a GDBN bug.  Reliable debuggers never crash.

  * If GDBN produces an error message for valid input, that is a bug.  (Note that if you're cross debugging, the problem may also be somewhere in the connection to the target.)

  * If GDBN does not produce an error message for invalid input, that is a bug.  However, you should note that your idea of ``invalid input'' might be our idea of ``an extension'' or ``support for traditional practice''.

  * If you are an experienced user of debugging tools, your suggestions for improvement of GDBN are welcome in any case.

How to Report Bugs
==================

A number of companies and individuals offer support for GNU products. If you obtained GDBN from a support organization, we recommend you contact that organization first.

You can find contact information for many support companies and individuals in the file etc/SERVICE in the GNU Emacs distribution.

The fundamental principle of reporting bugs usefully is this: report all the facts.  If you are not sure whether to state a fact or leave it out, state it!

Often people omit facts because they think they know what causes the problem and assume that some details do not matter.  Thus, you might assume that the name of the variable you use in an example does not matter. Well, probably it does not, but one cannot be sure.  Perhaps the bug is a stray memory reference which happens to fetch from the location where that name is stored in memory; perhaps, if the name were different, the contents of that location would fool the debugger into doing the right thing despite the bug.  Play it safe and give a specific, complete example.  That is the easiest thing for you to do, and the most helpful.

Keep in mind that the purpose of a bug report is to enable us to fix the bug.  It may be that the bug has been reported previously, but neither you nor we can know that unless your bug report is complete and self-contained.

Sometimes people give a few sketchy facts and ask, ``Does this ring a bell?''  Those bug reports are useless, and we urge everyone to refuse to respond to them except to chide the sender to report bugs properly.

To enable us to fix the bug, you should include all these things:

  * The version of GDBN.  GDBN announces it if you start with no arguments; you can also print it at any time using show version.

     Without this, we will not know whether there is any point in looking for the bug in the current version of GDBN.

  * The type of machine you are using, and the operating system name and version number.

  * What compiler (and its version) was used to compile GDBN---e.g. ``GCC--2.8.1''.

  * What compiler (and its version) was used to compile the program you are debugging---e.g.  ``GCC--2.8.1'', or ``HP92453-01 A.10.32.03 HP C Compiler''.  For NGCC, you can say GCC --version to get this information; for other compilers, see the documentation for those compilers.

  * The command arguments you gave the compiler to compile your example and observe the bug.  For example, did you use -O?  To guarantee you will not omit something important, list them all.  A copy of the Makefile (or the output from make) is sufficient.

     If we were to try to guess the arguments, we would probably guess wrong and then we might not encounter the bug.

  * A complete input script, and all necessary source files, that will reproduce the bug.

  * A description of what behavior you observe that you believe is incorrect.  For example, ``It gets a fatal signal.''

     Of course, if the bug is that GDBN gets a fatal signal, then we will certainly notice it.  But if the bug is incorrect output, we might not notice unless it is glaringly wrong.  You might as well not give us a chance to make a mistake.

     Even if the problem you experience is a fatal signal, you should still say so explicitly.  Suppose something strange is going on, such as, your copy of GDBN is out of synch, or you have encountered a bug in the C library on your system.  (This has happened!)  Your copy might crash and ours would not.  If you told us to expect a crash, then when ours fails to crash, we would know that the bug was not happening for us.  If you had not told us to expect a crash, then we would not be able to draw any conclusion from our observations.

     To collect all this information, you can use a session recording program such as script, which is available on many Unix systems. Just run your GDBN session inside script and then include the typescript file with your bug report.

     Another way to record a GDBN session is to run GDBN inside Emacs and then save the entire buffer to a file.

  * If you wish to suggest changes to the GDBN source, send us context diffs.  If you even discuss something in the GDBN source, refer to it by context, not by line number.

     The line numbers in our development sources will not match those in your sources.  Your line numbers would convey no useful information to us.

Here are some things that are not necessary:

  * A description of the envelope of the bug.

     Often people who encounter a bug spend a lot of time investigating which changes to the input file will make the bug go away and which changes will not affect it.

     This is often time consuming and not very useful, because the way we will find the bug is by running a single example under the debugger with breakpoints, not by pure deduction from a series of examples. We recommend that you save your time for something else.

     Of course, if you can find a simpler example to report instead of the original one, that is a convenience for us.  Errors in the output will be easier to spot, running under the debugger will take less time, and so on.

     However, simplification is not vital; if you do not want to do this, report the bug anyway and send us the entire test case you used.

  * A patch for the bug.

     A patch for the bug does help us if it is a good one.  But do not omit the necessary information, such as the test case, on the assumption that a patch is all we need.  We might see problems with your patch and decide to fix the problem another way, or we might not understand it at all.

     Sometimes with a program as complicated as GDBN it is very hard to construct an example that will make the program follow a certain path through the code.  If you do not send us the example, we will not be able to construct one, so we will not be able to verify that the bug is fixed.

     And if we cannot understand what bug you are trying to fix, or why your patch should be an improvement, we will not install it.  A test case will help us to understand.

  * A guess about what the bug is or what it depends on.

     Such guesses are usually wrong.  Even we cannot guess right about such things without first using the debugger to find the facts.

     [file rluser.texi not included]
     [file hsuser.texi not included]

In Memoriam
***********

The GDBN project mourns the loss of the following long-time contributors:

Fred Fish
     Fred was a long-standing contributor to GDBN (1991-2006), and to Free Software in general.  Outside of GDBN, he was known in the Amiga world for his series of Fish Disks, and the GeekGadget project.

Michael Snyder
     Michael was one of the Global Maintainers of the GDBN project, with contributions recorded as early as 1996, until 2011.  In addition to his day to day participation, he was a large driving force behind adding Reverse Debugging to GDBN.

Beyond their technical contributions to the project, they were also enjoyable members of the Free Software Community.  We will miss them.

Formatting Documentation
************************

The GDBN 4 release includes an already-formatted reference card, ready for printing with PostScript or Ghostscript, in the gdb subdirectory of the main source directory (In
gdb-GDBVN/gdb/refcard.ps of the version GDBVN
release.).  If you can use PostScript or Ghostscript with your printer, you can print the reference card immediately with refcard.ps.

The release also includes the source for the reference card.  You can format it, using TeX, by typing:

     make refcard.dvi

The GDBN reference card is designed to print in landscape mode on US ``letter'' size paper; that is, on a sheet 11 inches wide by 8.5 inches high.  You will need to specify this form of printing as an option to your DVI output program.

All the documentation for GDBN comes as part of the machine-readable distribution.  The documentation is written in Texinfo format, which is a documentation system that uses a single source file to produce both on-line information and a printed manual.  You can use one of the Info formatting commands to create the on-line version of the documentation and TeX (or texi2roff) to typeset the printed version.

GDBN includes an already formatted copy of the on-line Info version of this manual in the gdb subdirectory.  The main Info file is gdb-GDBVN/gdb/gdb.info, and it refers to subordinate files matching gdb.info* in the same directory.  If necessary, you can print out these files, or read them with any editor; but they are easier to read using the info subsystem in GNU Emacs or the standalone info program, available as part of the GNU Texinfo distribution.

If you want to format these Info files yourself, you need one of the Info formatting programs, such as texinfo-format-buffer or makeinfo.

If you have makeinfo installed, and are in the top level GDBN source directory (gdb-GDBVN, in the case of version GDBVN), you can make the Info file by typing:

     cd gdb
     make gdb.info

If you want to typeset and print copies of this manual, you need TeX, a program to print its DVI output files, and texinfo.tex, the Texinfo definitions file.

TeX is a typesetting program; it does not print files directly, but produces output files called DVI files.  To print a typeset document, you need a program to print DVI files.  If your system has TeX installed, chances are it has such a program.  The precise command to use depends on your system; lpr -d is common; another (for PostScript devices) is dvips.  The DVI print command may require a file name without any extension or a .dvi extension.

TeX also requires a macro definitions file called texinfo.tex.  This file tells TeX how to typeset a document written in Texinfo format.  On its own, TeX cannot either read or typeset a Texinfo file.  texinfo.tex is distributed with GDB and is located in the gdb-VERSION-NUMBER/texinfo directory.

If you have TeX and a DVI printer program installed, you can typeset and print this manual.  First switch to the gdb subdirectory of the main source directory (for example, to gdb-GDBVN/gdb) and type:

     make gdb.dvi

Then give gdb.dvi to your DVI printing program.

Installing GDBN
***************

Requirements for Building GDBN
==============================

Building GDBN requires various tools and packages to be available. Other packages will be used only if they are found.

Tools/Packages Necessary for Building GDBN
==========================================

ISO C90 compiler
     GDBN is written in ISO C90.  It should be buildable with any working C90 compiler, e.g. GCC.

Tools/Packages Optional for Building GDBN
=========================================

Expat
     GDBN can use the Expat XML parsing library.  This library may be included with your operating system distribution; if it is not, you can get the latest version from http://expat.sourceforge.net. The configure script will search for this library in several standard locations; if it is installed in an unusual path, you can use the --with-libexpat-prefix option to specify its location.

     Expat is used for:

       * Remote protocol memory maps (see Memory Map Format)

       * Target descriptions (see Target Descriptions)

       * Remote shared library lists (see Library List Format, or alternatively see Library List Format for SVR4 Targets)

       * MS-Windows shared libraries (see Shared Libraries)

       * Traceframe info (see Traceframe Info Format)

zlib
     GDBN will use the zlib library, if available, to read compressed debug sections.  Some linkers, such as GNU gold, are capable of producing binaries with compressed debug sections.  If GDBN is compiled with zlib, it will be able to read the debug information in such binaries.

     The zlib library is likely included with your operating system distribution; if it is not, you can get the latest version from http://zlib.net.

iconv
     GDBN's features related to character sets (see Character Sets) require a functioning iconv implementation.  If you are on a GNU system, then this is provided by the GNU C Library.  Some other systems also provide a working iconv.

     If GDBN is using the iconv program which is installed in a non-standard place, you will need to tell GDBN where to find it. This is done with --with-iconv-bin which specifies the directory that contains the iconv program.

     On systems without iconv, you can install GNU Libiconv.  If you have previously installed Libiconv, you can use the --with-libiconv-prefix option to configure.

     GDBN's top-level configure and Makefile will arrange to build Libiconv if a directory named libiconv appears in the top-most source directory.  If Libiconv is built this way, and if the operating system does not provide a suitable iconv implementation, then the just-built library will automatically be used by GDBN.  One easy way to set this up is to download GNU Libiconv, unpack it, and then rename the directory holding the Libiconv source code to libiconv.

Invoking the GDBN configure Script
==================================

GDBN comes with a configure script that automates the process of preparing GDBN for installation; you can then use make to build the gdb program.

The GDBN distribution includes all the source code you need for GDBN in a single directory, whose name is usually composed by appending the version number to gdb.

For example, the GDBN version GDBVN distribution is in the gdb-GDBVN directory.  That directory contains:

gdb-GDBVN/configure (and supporting files)
     script for configuring GDBN and all its supporting libraries

gdb-GDBVN/gdb
     the source specific to GDBN itself

gdb-GDBVN/bfd
     source for the Binary File Descriptor library

gdb-GDBVN/include
     GNU include files

gdb-GDBVN/libiberty
     source for the -liberty free software library

gdb-GDBVN/opcodes
     source for the library of opcode tables and disassemblers

gdb-GDBVN/readline
     source for the GNU command-line interface

gdb-GDBVN/glob
     source for the GNU filename pattern-matching subroutine

gdb-GDBVN/mmalloc
     source for the GNU memory-mapped malloc package

The simplest way to configure and build GDBN is to run configure from the gdb-VERSION-NUMBER source directory, which in this example is the gdb-GDBVN directory.

First switch to the gdb-VERSION-NUMBER source directory if you are not already in it; then run configure.  Pass the identifier for the platform on which GDBN will run as an argument.

For example:

     cd gdb-GDBVN
     ./configure HOST
     make

where HOST is an identifier such as sun4 or decstation, that identifies the platform where GDBN will run. (You can often leave off HOST; configure tries to guess the correct value by examining your system.)

Running configure HOST and then running make builds the bfd, readline, mmalloc, and libiberty libraries, then gdb itself.  The configured source files, and the binaries, are left in the corresponding source directories.

configure is a Bourne-shell (/bin/sh) script; if your system does not recognize this automatically when you run a different shell, you may need to run sh on it explicitly:

     sh configure HOST

If you run configure from a directory that contains source directories for multiple libraries or programs, such as the gdb-GDBVN source directory for version GDBVN, configure creates configuration files for every directory level underneath (unless you tell it not to, with the --norecursion option).

You should run the configure script from the top directory in the source tree, the gdb-VERSION-NUMBER directory.  If you run configure from one of the subdirectories, you will configure only that subdirectory.  That is usually not what you want.  In particular, if you run the first configure from the gdb subdirectory of the gdb-VERSION-NUMBER directory, you will omit the configuration of bfd, readline, and other sibling directories of the gdb subdirectory.  This leads to build errors about missing include files such as bfd/bfd.h.

You can install GDBP anywhere; it has no hardwired paths. However, you should make sure that the shell on your path (named by the SHELL environment variable) is publicly readable.  Remember that GDBN uses the shell to start your program---some systems refuse to let GDBN debug child processes whose programs are not readable.

Compiling GDBN in Another Directory
===================================

If you want to run GDBN versions for several host or target machines, you need a different gdb compiled for each combination of host and target.  configure is designed to make this easy by allowing you to generate each configuration in a separate subdirectory, rather than in the source directory.  If your make program handles the VPATH feature (GNU make does), running make in each of these directories builds the gdb program specified there.

To build gdb in a separate directory, run configure with the --srcdir option to specify where to find the source. (You also need to specify a path to find configure itself from your working directory.  If the path to configure would be the same as the argument to --srcdir, you can leave out the --srcdir option; it is assumed.)

For example, with version GDBVN, you can build GDBN in a separate directory for a Sun 4 like this:

     cd gdb-GDBVN
     mkdir ../gdb-sun4
     cd ../gdb-sun4
     ../gdb-GDBVN/configure sun4
     make

When configure builds a configuration using a remote source directory, it creates a tree for the binaries with the same structure (and using the same names) as the tree under the source directory.  In the example, you'd find the Sun 4 library libiberty.a in the directory gdb-sun4/libiberty, and GDBN itself in gdb-sun4/gdb.

Make sure that your path to the configure script has just one instance of gdb in it.  If your path to configure looks like ../gdb-GDBVN/gdb/configure, you are configuring only one subdirectory of GDBN, not the whole package.  This leads to build errors about missing include files such as bfd/bfd.h.

One popular reason to build several GDBN configurations in separate directories is to configure GDBN for cross-compiling (where GDBN runs on one machine---the host---while debugging programs that run on another machine---the target). You specify a cross-debugging target by giving the --target=TARGET option to configure.

When you run make to build a program or library, you must run it in a configured directory---whatever directory you were in when you called configure (or one of its subdirectories).

The Makefile that configure generates in each source directory also runs recursively.  If you type make in a source directory such as gdb-GDBVN (or in a separate configured directory configured with --srcdir=DIRNAME/gdb-GDBVN), you will build all the required libraries, and then build GDB.

When you have multiple hosts or targets configured in separate directories, you can run make on them in parallel (for example, if they are NFS-mounted on each of the hosts); they will not interfere with each other.

Specifying Names for Hosts and Targets
======================================

The specifications used for hosts and targets in the configure script are based on a three-part naming scheme, but some short predefined aliases are also supported.  The full naming scheme encodes three pieces of information in the following pattern:

     ARCHITECTURE-VENDOR-OS

For example, you can use the alias sun4 as a HOST argument, or as the value for TARGET in a --target=TARGET option.  The equivalent full name is sparc-sun-sunos4.

The configure script accompanying GDBN does not provide any query facility to list all supported host and target names or aliases.  configure calls the Bourne shell script config.sub to map abbreviations to full names; you can read the script, if you wish, or you can use it to test your guesses on abbreviations---for example:

     % sh config.sub i386-linux
     i386-pc-linux-gnu
     % sh config.sub alpha-linux
     alpha-unknown-linux-gnu
     % sh config.sub hp9k700
     hppa1.1-hp-hpux
     % sh config.sub sun4
     sparc-sun-sunos4.1.1
     % sh config.sub sun3
     m68k-sun-sunos4.1.1
     % sh config.sub i986v
     Invalid configuration `i986v': machine `i986v' not recognized

config.sub is also distributed in the GDBN source directory (gdb-GDBVN, for version GDBVN).

configure Options
=================

Here is a summary of the configure options and arguments that are most often useful for building GDBN.  configure also has several other options not listed here.  See What Configure Does (configure.info), for a full explanation of configure.

     configure [--help]
               [--prefix=DIR]
               [--exec-prefix=DIR]
               [--srcdir=DIRNAME]
               [--norecursion] [--rm]
               [--target=TARGET]
               HOST

You may introduce options with a single - rather than -- if you prefer; but you may abbreviate option names if you use --.

--help
     Display a quick summary of how to invoke configure.

--prefix=DIR
     Configure the source to install programs and files under directory DIR.

--exec-prefix=DIR
     Configure the source to install programs under directory DIR.

--srcdir=DIRNAME
     Warning: using this option requires GNU make, or another
     make that implements the VPATH feature.

     Use this option to make configurations in directories separate from the GDBN source directories.  Among other things, you can use this to build (or maintain) several configurations simultaneously, in separate directories.  configure writes configuration-specific files in the current directory, but arranges for them to use the source in the directory DIRNAME.  configure creates directories under the working directory in parallel to the source directories below DIRNAME.

--norecursion
     Configure only the directory level where configure is executed; do not propagate configuration to subdirectories.

--target=TARGET
     Configure GDBN for cross-debugging programs running on the specified TARGET.  Without this option, GDBN is configured to debug programs that run on the same machine (HOST) as GDBN itself.

     There is no convenient way to generate a list of all available targets.

HOST ...
     Configure GDBN to run on the specified HOST.

     There is no convenient way to generate a list of all available hosts.

There are many other options available as well, but they are generally needed for special purposes only.

System-wide configuration and settings
======================================

GDBN can be configured to have a system-wide init file; this file will be read and executed at startup (see What GDBN does during startup).

Here is the corresponding configure option:

--with-system-gdbinit=FILE
     Specify that the default location of the system-wide init file is FILE.

If GDBN has been configured with the option --prefix=$prefix, it may be subject to relocation.  Two possible cases:

  * If the default location of this init file contains $prefix, it will be subject to relocation.  Suppose that the configure options are --prefix=$prefix --with-system-gdbinit=$prefix/etc/gdbinit; if GDBN is moved from $prefix to $install, the system init file is looked for as $install/etc/gdbinit instead of $prefix/etc/gdbinit.

  * By contrast, if the default location does not contain the prefix, it will not be relocated.  E.g. if GDBN has been configured with --prefix=/usr/local --with-system-gdbinit=/usr/share/gdb/gdbinit, then GDBN will always look for /usr/share/gdb/gdbinit, wherever GDBN is installed.

Maintenance Commands
********************

In addition to commands intended for GDBN users, GDBN includes a number of commands intended for GDBN developers, that are not documented elsewhere in this manual.  These commands are provided here for reference.  (For commands that turn on debugging messages, see Debugging Output.)

maint agent EXPRESSION
maint agent-eval EXPRESSION
     Translate the given EXPRESSION into remote agent bytecodes. This command is useful for debugging the Agent Expression mechanism (see Agent Expressions).  The agent version produces an expression useful for data collection, such as by tracepoints, while maint agent-eval produces an expression that evaluates directly to a result.  For instance, a collection expression for globa + globb will include bytecodes to record four bytes of memory at each of the addresses of globa and globb, while discarding the result of the addition, while an evaluation expression will do the addition and return the sum.

maint info breakpoints
     Using the same format as info breakpoints, display both the breakpoints you've set explicitly, and those GDBN is using for internal purposes.  Internal breakpoints are shown with negative breakpoint numbers.  The type column identifies what kind of breakpoint is shown:

     breakpoint
          Normal, explicitly set breakpoint.

     watchpoint
          Normal, explicitly set watchpoint.

     longjmp
          Internal breakpoint, used to handle correctly stepping through longjmp calls.

     longjmp resume
          Internal breakpoint at the target of a longjmp.

     until
          Temporary internal breakpoint used by the GDBN until command.

     finish
          Temporary internal breakpoint used by the GDBN finish command.

     shlib events
          Shared library events.

set displaced-stepping
show displaced-stepping
     Control whether or not GDBN will do displaced stepping if the target supports it.  Displaced stepping is a way to single-step over breakpoints without removing them from the inferior, by executing an out-of-line copy of the instruction that was originally at the breakpoint location.  It is also known as out-of-line single-stepping.

     set displaced-stepping on
          If the target architecture supports it, GDBN will use displaced stepping to step over breakpoints.

     set displaced-stepping off
          GDBN will not use displaced stepping to step over breakpoints, even if such is supported by the target architecture.

     set displaced-stepping auto
          This is the default mode.  GDBN will use displaced stepping only if non-stop mode is active (see Non-Stop Mode) and the target architecture supports displaced stepping.

maint check-symtabs
     Check the consistency of psymtabs and symtabs.

maint cplus first_component NAME
     Print the first C++ class/namespace component of NAME.

maint cplus namespace
     Print the list of possible C++ namespaces.

maint demangle NAME
     Demangle a C++ or Objective-C mangled NAME.

maint deprecate COMMAND [REPLACEMENT]
maint undeprecate COMMAND
     Deprecate or undeprecate the named COMMAND.  Deprecated commands cause GDBN to issue a warning when you use them.  The optional argument REPLACEMENT says which newer command should be used in favor of the deprecated one; if it is given, GDBN will mention the replacement as part of the warning.

maint dump-me
     Cause a fatal signal in the debugger and force it to dump its core. This is supported only on systems which support aborting a program with the SIGQUIT signal.

maint internal-error [MESSAGE-TEXT]
maint internal-warning [MESSAGE-TEXT]
     Cause GDBN to call the internal function internal_error or internal_warning and hence behave as though an internal error or internal warning has been detected.  In addition to reporting the internal problem, these functions give the user the opportunity to either quit GDBN or create a core file of the current GDBN session.

     These commands take an optional parameter MESSAGE-TEXT that is used as the text of the error or warning message.

     Here's an example of using internal-error:

          (GDBP) maint internal-error testing, 1, 2
          .../maint.c:121: internal-error: testing, 1, 2
          A problem internal to GDB has been detected.  Further
          debugging may prove unreliable.
          Quit this debugging session? (y or n) n
          Create a core file? (y or n) n
          (GDBP)

maint set internal-error ACTION [ask|yes|no]
maint show internal-error ACTION
maint set internal-warning ACTION [ask|yes|no]
maint show internal-warning ACTION
     When GDBN reports an internal problem (error or warning) it gives the user the opportunity to both quit GDBN and create a core file of the current GDBN session.  These commands let you override the default behaviour for each particular ACTION, described in the table below.

     quit
          You can specify that GDBN should always (yes) or never (no) quit.  The default is to ask the user what to do.

     corefile
          You can specify that GDBN should always (yes) or never (no) create a core file.  The default is to ask the user what to do.

maint packet TEXT
     If GDBN is talking to an inferior via the serial protocol, then this command sends the string TEXT to the inferior, and displays the response packet.  GDBN supplies the initial $ character, the terminating # character, and the checksum.

maint print architecture [FILE]
     Print the entire architecture configuration.  The optional argument FILE names the file where the output goes.

maint print c-tdesc
     Print the current target description (see Target Descriptions) as a C source file.  The created source file can be used in GDBN when an XML parser is not available to parse the description.

maint print dummy-frames
     Prints the contents of GDBN's internal dummy-frame stack.

          (GDBP) b add
          ...
          (GDBP) print add(2,3)
          Breakpoint 2, add (a=2, b=3) at ...
          58	  return (a + b);
          The program being debugged stopped while in a function called from GDB.
          ...
          (GDBP) maint print dummy-frames
          0x1a57c80: pc=0x01014068 fp=0x0200bddc sp=0x0200bdd6
           top=0x0200bdd4 id={stack=0x200bddc,code=0x101405c}
           call_lo=0x01014000 call_hi=0x01014001
          (GDBP)

     Takes an optional file parameter.

maint print registers [FILE]
maint print raw-registers [FILE]
maint print cooked-registers [FILE]
maint print register-groups [FILE]
maint print remote-registers [FILE]
     Print GDBN's internal register data structures.

     The command maint print raw-registers includes the contents of the raw register cache; the command maint print cooked-registers includes the (cooked) value of all registers, including registers which aren't available on the target nor visible to user; the command maint print register-groups includes the groups that each register is a member of; and the command maint print remote-registers includes the remote target's register numbers and offsets in the `G' packets.  See Registers (GDBN Internals).

     These commands take an optional parameter, a file name to which to write the information.

maint print reggroups [FILE]
     Print GDBN's internal register group data structures.  The optional argument FILE tells to what file to write the information.

     The register groups info looks like this:

          (GDBP) maint print reggroups
           Group      Type
           general    user
           float      user
           all        user
           vector     user
           system     user
           save       internal
           restore    internal

flushregs
     This command forces GDBN to flush its internal register cache.

maint print objfiles
     Print a dump of all known object files.  For each object file, this command prints its name, address in memory, and all of its psymtabs and symtabs.

maint print section-scripts [REGEXP]
     Print a dump of scripts specified in the .debug_gdb_section section. If REGEXP is specified, only print scripts loaded by object files matching REGEXP. For each script, this command prints its name as specified in the objfile, and the full path if known. See .debug_gdb_scripts section.

maint print statistics
     This command prints, for each object file in the program, various data about that object file followed by the byte cache (bcache) statistics for the object file.  The objfile data includes the number of minimal, partial, full, and stabs symbols, the number of types defined by the objfile, the number of as yet unexpanded psym tables, the number of line tables and string tables, and the amount of memory used by the various tables.  The bcache statistics include the counts, sizes, and counts of duplicates of all and unique objects, max, average, and median entry size, total memory used and its overhead and savings, and various measures of the hash table size and chain lengths.

maint print target-stack
     A target is an interface between the debugger and a particular kind of file or process.  Targets can be stacked in strata, so that more than one target can potentially respond to a request. In particular, memory accesses will walk down the stack of targets until they find a target that is interested in handling that particular address.

     This command prints a short description of each layer that was pushed on the target stack, starting from the top layer down to the bottom one.

maint print type EXPR
     Print the type chain for a type specified by EXPR.  The argument can be either a type name or a symbol.  If it is a symbol, the type of that symbol is described.  The type chain produced by this command is a recursive definition of the data type as stored in GDBN's data structures, including its flags and contained types.

maint set dwarf2 always-disassemble

maint show dwarf2 always-disassemble
     Control the behavior of info address when using DWARF debugging information.

     The default is off, which means that GDBN should try to describe a variable's location in an easily readable format.  When on, GDBN will instead display the DWARF location expression in an assembly-like format.  Note that some locations are too complex for GDBN to describe simply; in this case you will always see the disassembly form.

     Here is an example of the resulting disassembly:

          (gdb) info addr argc
          Symbol "argc" is a complex DWARF expression:
               1: DW_OP_fbreg 0

     For more information on these expressions, see the DWARF standard (http://www.dwarfstd.org/).

maint set dwarf2 max-cache-age
maint show dwarf2 max-cache-age
     Control the DWARF 2 compilation unit cache.

     In object files with inter-compilation-unit references, such as those produced by the GCC option -feliminate-dwarf2-dups, the DWARF 2 reader needs to frequently refer to previously read compilation units. This setting controls how long a compilation unit will remain in the cache if it is not referenced.  A higher limit means that cached compilation units will be stored in memory longer, and more total memory will be used.  Setting it to zero disables caching, which will slow down GDBN startup, but reduce memory consumption.

maint set profile
maint show profile
     Control profiling of GDBN.

     Profiling will be disabled until you use the maint set profile command to enable it.  When you enable profiling, the system will begin collecting timing and execution count data; when you disable profiling or exit GDBN, the results will be written to a log file.  Remember that if you use profiling, GDBN will overwrite the profiling log file (often called gmon.out).  If you have a record of important profiling data in a gmon.out file, be sure to move it to a safe location.

     Configuring with --enable-profiling arranges for GDBN to be compiled with the -pg compiler option.

maint set show-debug-regs
maint show show-debug-regs
     Control whether to show variables that mirror the hardware debug registers.  Use ON to enable, OFF to disable.  If enabled, the debug registers values are shown when GDBN inserts or removes a hardware breakpoint or watchpoint, and when the inferior triggers a hardware-assisted breakpoint or watchpoint.

maint set show-all-tib
maint show show-all-tib
     Control whether to show all non zero areas within a 1k block starting at thread local base, when using the info w32 thread-information-block command.

maint space
     Control whether to display memory usage for each command.  If set to a nonzero value, GDBN will display how much memory each command took, following the command's own output.  This can also be requested by invoking GDBN with the --statistics command-line switch (see Mode Options).

maint time
     Control whether to display the execution time of GDBN for each command. If set to a nonzero value, GDBN will display how much time it took to execute each command, following the command's own output. Both CPU time and wallclock time are printed. Printing both is useful when trying to determine whether the cost is CPU or, e.g., disk/network, latency. Note that the CPU time printed is for GDBN only, it does not include the execution time of the inferior because there's no mechanism currently to compute how much time was spent by GDBN and how much time was spent by the program been debugged. This can also be requested by invoking GDBN with the --statistics command-line switch (see Mode Options).

maint translate-address [SECTION] ADDR
     Find the symbol stored at the location specified by the address ADDR and an optional section name SECTION.  If found, GDBN prints the name of the closest symbol and an offset from the symbol's location to the specified address.  This is similar to the info address command (see Symbols), except that this command also allows to find symbols in other sections.

     If section was not specified, the section in which the symbol was found is also printed.  For dynamically linked executables, the name of executable or shared library containing the symbol is printed as well.

The following command is useful for non-interactive invocations of GDBN, such as in the test suite.

set watchdog NSEC
     Set the maximum number of seconds GDBN will wait for the target operation to finish.  If this time expires, GDBN reports and error and the command is aborted.

show watchdog
     Show the current setting of the target wait timeout.

GDBN Remote Serial Protocol
***************************

Overview
========

There may be occasions when you need to know something about the protocol---for example, if there is only one serial port to your target machine, you might want your program to do something special if it recognizes a packet meant for GDBN.

In the examples below, -> and <- are used to indicate transmitted and received data, respectively.

All GDBN commands and responses (other than acknowledgments and notifications, see Notification Packets) are sent as a PACKET.  A PACKET is introduced with the character $, the actual PACKET-DATA, and the terminating character # followed by a two-digit CHECKSUM:

     $PACKET-DATA#CHECKSUM

The two-digit CHECKSUM is computed as the modulo 256 sum of all characters between the leading $ and the trailing # (an eight bit unsigned checksum).

Implementors should note that prior to GDBN 5.0 the protocol specification also included an optional two-digit SEQUENCE-ID:

     $SEQUENCE-ID:PACKET-DATA#CHECKSUM

That SEQUENCE-ID was appended to the acknowledgment.  GDBN has never output SEQUENCE-IDs.  Stubs that handle packets added since GDBN 5.0 must not accept SEQUENCE-ID.

When either the host or the target machine receives a packet, the first response expected is an acknowledgment: either + (to indicate the package was received correctly) or - (to request retransmission):

     -> $PACKET-DATA#CHECKSUM
     <- +

The +/- acknowledgments can be disabled once a connection is established. See Packet Acknowledgment, for details.

The host (GDBN) sends COMMANDs, and the target (the debugging stub incorporated in your program) sends a RESPONSE.  In the case of step and continue COMMANDs, the response is only sent when the operation has completed, and the target has again stopped all threads in all attached processes.  This is the default all-stop mode behavior, but the remote protocol also supports GDBN's non-stop execution mode; see Remote Non-Stop, for details.

PACKET-DATA consists of a sequence of characters with the exception of # and $ (see X packet for additional exceptions).

Fields within the packet should be separated using , ; or :.  Except where otherwise noted all numbers are represented in HEX with leading zeros suppressed.

Implementors should note that prior to GDBN 5.0, the character : could not appear as the third character in a packet (as it would potentially conflict with the SEQUENCE-ID).

Binary data in most packets is encoded either as two hexadecimal digits per byte of binary data.  This allowed the traditional remote protocol to work over connections which were only seven-bit clean. Some packets designed more recently assume an eight-bit clean connection, and use a more efficient encoding to send and receive binary data.

The binary data representation uses 7d (ASCII }) as an escape character.  Any escaped byte is transmitted as the escape character followed by the original character XORed with 0x20. For example, the byte 0x7d would be transmitted as the two bytes 0x7d 0x5d.  The bytes 0x23 (ASCII #), 0x24 (ASCII $), and 0x7d (ASCII }) must always be escaped.  Responses sent by the stub must also escape 0x2a (ASCII *), so that it is not interpreted as the start of a run-length encoded sequence (described next).

Response DATA can be run-length encoded to save space. Run-length encoding replaces runs of identical characters with one instance of the repeated character, followed by a * and a repeat count.  The repeat count is itself sent encoded, to avoid binary characters in DATA: a value of N is sent as N+29.  For a repeat count greater or equal to 3, this produces a printable ASCII character, e.g. a space (ASCII code 32) for a repeat count of 3.  (This is because run-length encoding starts to win for counts 3 or more.)  Thus, for example, 0*  is a run-length encoding of ``0000'': the space character after * means repeat the leading 0 32 - 29 = 3 more times.

The printable characters # and $ or with a numeric value greater than 126 must not be used.  Runs of six repeats (#) or seven repeats ($) can be expanded using a repeat count of only five (").  For example, 00000000 can be encoded as 0*"00.

The error response returned for some packets includes a two character error number.  That number is not well defined.

For any COMMAND not supported by the stub, an empty response ($#00) should be returned.  That way it is possible to extend the protocol.  A newer GDBN can tell if a packet is supported based on that response.

At a minimum, a stub is required to support the g and G commands for register access, and the m and M commands for memory access.  Stubs that only control single-threaded targets can implement run control with the c (continue), and s (step) commands.  Stubs that support multi-threading targets should support the vCont command.  All other commands are optional.

Packets
=======

The following table provides a complete list of all currently defined COMMANDs and their corresponding response DATA. See File-I/O Remote Protocol Extension, for details about the File I/O extension of the remote protocol.

Each packet's description has a template showing the packet's overall syntax, followed by an explanation of the packet's meaning.  We include spaces in some of the templates for clarity; these are not part of the packet's syntax.  No GDBN packet uses spaces to separate its components.  For example, a template like foo BAR BAZ describes a packet beginning with the three ASCII bytes foo, followed by a BAR, followed directly by a BAZ.  GDBN does not transmit a space character between the foo and the BAR, or between the BAR and the BAZ.

Several packets and replies include a THREAD-ID field to identify a thread.  Normally these are positive numbers with a target-specific interpretation, formatted as big-endian hex strings.  A THREAD-ID can also be a literal -1 to indicate all threads, or 0 to pick any thread.

In addition, the remote protocol supports a multiprocess feature in which the THREAD-ID syntax is extended to optionally include both process and thread ID fields, as pPID.TID. The PID (process) and TID (thread) components each have the format described above: a positive number with target-specific interpretation formatted as a big-endian hex string, literal -1 to indicate all processes or threads (respectively), or 0 to indicate an arbitrary process or thread.  Specifying just a process, as pPID, is equivalent to pPID.-1.  It is an error to specify all processes but a specific thread, such as p-1.TID.  Note that the p prefix is not used for those packets and replies explicitly documented to include a process ID, rather than a THREAD-ID.

The multiprocess THREAD-ID syntax extensions are only used if both GDBN and the stub report support for the multiprocess feature using qSupported.  See multiprocess extensions, for more information.

Note that all packet forms beginning with an upper- or lower-case letter, other than those described here, are reserved for future use.

Here are the packet descriptions.

!
     Enable extended mode.  In extended mode, the remote server is made persistent.  The R packet is used to restart the program being debugged.

     Reply:

     OK
          The remote target both supports and has enabled extended mode.

?
     Indicate the reason the target halted.  The reply is the same as for step and continue.  This packet has a special interpretation when the target is in non-stop mode; see Remote Non-Stop.

     Reply: see Stop Reply Packets, for the reply specifications.

A ARGLEN,ARGNUM,ARG,...
     Initialized argv[] array passed into program. ARGLEN specifies the number of bytes in the hex encoded byte stream ARG.  See gdbserver for more details.

     Reply:

     OK
          The arguments were set.

     E NN
          An error occurred.

b BAUD
     (Don't use this packet; its behavior is not well-defined.) Change the serial line speed to BAUD.

     JTC: When does the transport layer state change?  When it's received, or after the ACK is transmitted.  In either case, there are problems if the command or the acknowledgment packet is dropped.

     Stan: If people really wanted to add something like this, and get it working for the first time, they ought to modify ser-unix.c to send some kind of out-of-band message to a specially-setup stub and have the switch happen "in between" packets, so that from remote protocol's point of view, nothing actually happened.

B ADDR,MODE
     Set (MODE is S) or clear (MODE is C) a breakpoint at ADDR.

     Don't use this packet.  Use the Z and z packets instead (see insert breakpoint or watchpoint packet).

bc
     Backward continue.  Execute the target system in reverse.  No parameter. See Reverse Execution, for more information.

     Reply: see Stop Reply Packets, for the reply specifications.

bs
     Backward single step.  Execute one instruction in reverse.  No parameter. See Reverse Execution, for more information.

     Reply: see Stop Reply Packets, for the reply specifications.

c [ADDR]
     Continue.  ADDR is address to resume.  If ADDR is omitted, resume at current address.

     This packet is deprecated for multi-threading support.  See vCont packet.

     Reply: see Stop Reply Packets, for the reply specifications.

C SIG[;ADDR]
     Continue with signal SIG (hex signal number).  If ;ADDR is omitted, resume at same address.

     This packet is deprecated for multi-threading support.  See vCont packet.

     Reply: see Stop Reply Packets, for the reply specifications.

d
     Toggle debug flag.

     Don't use this packet; instead, define a general set packet (see General Query Packets).

D
D;PID
     The first form of the packet is used to detach GDBN from the remote system.  It is sent to the remote target before GDBN disconnects via the detach command.

     The second form, including a process ID, is used when multiprocess protocol extensions are enabled (see multiprocess extensions), to detach only a specific process.  The PID is specified as a big-endian hex string.

     Reply:

     OK
          for success

     E NN
          for an error

F RC,EE,CF;XX
     A reply from GDBN to an F packet sent by the target. This is part of the File-I/O protocol extension.  See File-I/O Remote Protocol Extension, for the specification.

g
     Read general registers.

     Reply:

     XX...
          Each byte of register data is described by two hex digits.  The bytes with the register are transmitted in target byte order.  The size of each register and their position within the g packet are determined by the GDBN internal gdbarch functions DEPRECATED_REGISTER_RAW_SIZE and gdbarch_register_name.  The specification of several standard g packets is specified below.

          When reading registers from a trace frame (see Using the Collected Data), the stub may also return a string of literal x's in place of the register data digits, to indicate that the corresponding register has not been collected, thus its value is unavailable.  For example, for an architecture with 4 registers of 4 bytes each, the following reply indicates to GDBN that registers 0 and 2 have not been collected, while registers 1 and 3 have been collected, and both have zero value:

               -> g
               <- xxxxxxxx00000000xxxxxxxx00000000

     E NN
          for an error.

G XX...
     Write general registers.  See read registers packet, for a description of the XX... data.

     Reply:

     OK
          for success

     E NN
          for an error

H OP THREAD-ID
     Set thread for subsequent operations (m, M, g, G, et.al.).  OP depends on the operation to be performed: it should be c for step and continue operations (note that this is deprecated, supporting the vCont command is a better option), g for other operations.  The thread designator THREAD-ID has the format and interpretation described in thread-id syntax.

     Reply:

     OK
          for success

     E NN
          for an error

i [ADDR[,NNN]]
     Step the remote target by a single clock cycle.  If ,NNN is present, cycle step NNN cycles.  If ADDR is present, cycle step starting at that address.

I
     Signal, then cycle step.  See step with signal packet.  See cycle step packet.

k
     Kill request.

     FIXME: There is no description of how to operate when a specific thread context has been selected (i.e. does 'k' kill only that thread?).

m ADDR,LENGTH
     Read LENGTH bytes of memory starting at address ADDR. Note that ADDR may not be aligned to any particular boundary.

     The stub need not use any particular size or alignment when gathering data from memory for the response; even if ADDR is word-aligned and LENGTH is a multiple of the word size, the stub is free to use byte accesses, or not.  For this reason, this packet may not be suitable for accessing memory-mapped I/O devices.

     Reply:

     XX...
          Memory contents; each byte is transmitted as a two-digit hexadecimal number.  The reply may contain fewer bytes than requested if the server was able to read only part of the region of memory.

     E NN
          NN is errno

M ADDR,LENGTH:XX...
     Write LENGTH bytes of memory starting at address ADDR. XX... is the data; each byte is transmitted as a two-digit hexadecimal number.

     Reply:

     OK
          for success

     E NN
          for an error (this includes the case where only part of the data was written).

p N
     Read the value of register N; N is in hex. See read registers packet, for a description of how the returned register value is encoded.

     Reply:

     XX...
          the register's value

     E NN
          for an error

          Indicating an unrecognized QUERY.

P N...=R...
     Write register N... with value R....  The register number N is in hexadecimal, and R... contains two hex digits for each byte in the register (target byte order).

     Reply:

     OK
          for success

     E NN
          for an error

q NAME PARAMS...
Q NAME PARAMS...
     General query (q) and set (Q).  These packets are described fully in General Query Packets.

r
     Reset the entire system.

     Don't use this packet; use the R packet instead.

R XX
     Restart the program being debugged.  XX, while needed, is ignored. This packet is only available in extended mode (see extended mode).

     The R packet has no reply.

s [ADDR]
     Single step.  ADDR is the address at which to resume.  If ADDR is omitted, resume at same address.

     This packet is deprecated for multi-threading support.  See vCont packet.

     Reply: see Stop Reply Packets, for the reply specifications.

S SIG[;ADDR]
     Step with signal.  This is analogous to the C packet, but requests a single-step, rather than a normal resumption of execution.

     This packet is deprecated for multi-threading support.  See vCont packet.

     Reply: see Stop Reply Packets, for the reply specifications.

t ADDR:PP,MM
     Search backwards starting at address ADDR for a match with pattern PP and mask MM.  PP and MM are 4 bytes. ADDR must be at least 3 digits.

T THREAD-ID
     Find out if the thread THREAD-ID is alive.  See thread-id syntax.

     Reply:

     OK
          thread is still alive

     E NN
          thread is dead

v
     Packets starting with v are identified by a multi-letter name, up to the first ; or ? (or the end of the packet).

vAttach;PID
     Attach to a new process with the specified process ID PID. The process ID is a hexadecimal integer identifying the process.  In all-stop mode, all threads in the attached process are stopped; in non-stop mode, it may be attached without being stopped if that is supported by the target.

     This packet is only available in extended mode (see extended mode).

     Reply:

     E NN
          for an error

     Any stop packet
          for success in all-stop mode (see Stop Reply Packets)

     OK
          for success in non-stop mode (see Remote Non-Stop)

vCont[;ACTION[:THREAD-ID]]...
     Resume the inferior, specifying different actions for each thread. If an action is specified with no THREAD-ID, then it is applied to any threads that don't have a specific action specified; if no default action is specified then other threads should remain stopped in all-stop mode and in their current state in non-stop mode. Specifying multiple default actions is an error; specifying no actions is also an error. Thread IDs are specified using the syntax described in thread-id syntax.

     Currently supported actions are:

     c
          Continue.

     C SIG
          Continue with signal SIG.  The signal SIG should be two hex digits.

     s
          Step.

     S SIG
          Step with signal SIG.  The signal SIG should be two hex digits.

     t
          Stop.

     The optional argument ADDR normally associated with the c, C, s, and S packets is not supported in vCont.

     The t action is only relevant in non-stop mode (see Remote Non-Stop) and may be ignored by the stub otherwise. A stop reply should be generated for any affected thread not already stopped. When a thread is stopped by means of a t action, the corresponding stop reply should indicate that the thread has stopped with signal 0, regardless of whether the target uses some other signal as an implementation detail.

     Reply: see Stop Reply Packets, for the reply specifications.

vCont?
     Request a list of actions supported by the vCont packet.

     Reply:

     vCont[;ACTION...]
          The vCont packet is supported.  Each ACTION is a supported command in the vCont packet.

          The vCont packet is not supported.

vFile:OPERATION:PARAMETER...
     Perform a file operation on the target system.  For details, see Host I/O Packets.

vFlashErase:ADDR,LENGTH
     Direct the stub to erase LENGTH bytes of flash starting at ADDR.  The region may enclose any number of flash blocks, but its start and end must fall on block boundaries, as indicated by the flash block size appearing in the memory map (see Memory Map Format).  GDBN groups flash memory programming operations together, and sends a vFlashDone request after each group; the stub is allowed to delay erase operation until the vFlashDone packet is received.

     The stub must support vCont if it reports support for multiprocess extensions (see multiprocess extensions).  Note that in this case vCont actions can be specified to apply to all threads in a process by using the pPID.-1 form of the THREAD-ID.

     Reply:

     OK
          for success

     E NN
          for an error

vFlashWrite:ADDR:XX...
     Direct the stub to write data to flash address ADDR.  The data is passed in binary form using the same encoding as for the X packet (see Binary Data).  The memory ranges specified by vFlashWrite packets preceding a vFlashDone packet must not overlap, and must appear in order of increasing addresses (although vFlashErase packets for higher addresses may already have been received; the ordering is guaranteed only between vFlashWrite packets).  If a packet writes to an address that was neither erased by a preceding vFlashErase packet nor by some other target-specific method, the results are unpredictable.

     Reply:

     OK
          for success

     E.memtype
          for vFlashWrite addressing non-flash memory

     E NN
          for an error

vFlashDone
     Indicate to the stub that flash programming operation is finished. The stub is permitted to delay or batch the effects of a group of vFlashErase and vFlashWrite packets until a vFlashDone packet is received.  The contents of the affected regions of flash memory are unpredictable until the vFlashDone request is completed.

vKill;PID
     Kill the process with the specified process ID.  PID is a hexadecimal integer identifying the process.  This packet is used in preference to k when multiprocess protocol extensions are supported; see multiprocess extensions.

     Reply:

     E NN
          for an error

     OK
          for success

vRun;FILENAME[;ARGUMENT]...
     Run the program FILENAME, passing it each ARGUMENT on its command line.  The file and arguments are hex-encoded strings.  If FILENAME is an empty string, the stub may use a default program (e.g. the last program run).  The program is created in the stopped state.

     This packet is only available in extended mode (see extended mode).

     Reply:

     E NN
          for an error

     Any stop packet
          for success (see Stop Reply Packets)

vStopped

     In non-stop mode (see Remote Non-Stop), acknowledge a previous stop reply and prompt for the stub to report another one.

     Reply:

     Any stop packet
          if there is another unreported stop event (see Stop Reply Packets)

     OK
          if there are no unreported stop events

X ADDR,LENGTH:XX...
     Write data to memory, where the data is transmitted in binary. ADDR is address, LENGTH is number of bytes, XX... is binary data (see Binary Data).

     Reply:

     OK
          for success

     E NN
          for an error

z TYPE,ADDR,KIND
Z TYPE,ADDR,KIND
     Insert (Z) or remove (z) a TYPE breakpoint or watchpoint starting at address ADDRESS of kind KIND.

     Each breakpoint and watchpoint packet TYPE is documented separately.

     Implementation notes: A remote target shall return an empty string for an unrecognized breakpoint or watchpoint packet TYPE.  A remote target shall support either both or neither of a given ZTYPE... and zTYPE... packet pair.  To avoid potential problems with duplicate packets, the operations should be implemented in an idempotent way.

z0,ADDR,KIND
Z0,ADDR,KIND
     Insert (Z0) or remove (z0) a memory breakpoint at address ADDR of type KIND.

     A memory breakpoint is implemented by replacing the instruction at ADDR with a software breakpoint or trap instruction.  The KIND is target-specific and typically indicates the size of the breakpoint in bytes that should be inserted.  E.g., the ARM and MIPS can insert either a 2 or 4 byte breakpoint.  Some architectures have additional meanings for KIND; see Architecture-Specific Protocol Details.

     Implementation note: It is possible for a target to copy or move code that contains memory breakpoints (e.g., when implementing overlays).  The behavior of this packet, in the presence of such a target, is not defined.

     Reply:

     OK
          success

          not supported

     E NN
          for an error

z1,ADDR,KIND
Z1,ADDR,KIND
     Insert (Z1) or remove (z1) a hardware breakpoint at address ADDR.

     A hardware breakpoint is implemented using a mechanism that is not dependant on being able to modify the target's memory.  KIND has the same meaning as in Z0 packets.

     Implementation note: A hardware breakpoint is not affected by code movement.

     Reply:

     OK
          success

          not supported

     E NN
          for an error

z2,ADDR,KIND
Z2,ADDR,KIND
     Insert (Z2) or remove (z2) a write watchpoint at ADDR. KIND is interpreted as the number of bytes to watch.

     Reply:

     OK
          success

          not supported

     E NN
          for an error

z3,ADDR,KIND
Z3,ADDR,KIND
     Insert (Z3) or remove (z3) a read watchpoint at ADDR. KIND is interpreted as the number of bytes to watch.

     Reply:

     OK
          success

          not supported

     E NN
          for an error

z4,ADDR,KIND
Z4,ADDR,KIND
     Insert (Z4) or remove (z4) an access watchpoint at ADDR. KIND is interpreted as the number of bytes to watch.

     Reply:

     OK
          success

          not supported

     E NN
          for an error

Stop Reply Packets
==================

The C, c, S, s, vCont, vAttach, vRun, vStopped, and ? packets can receive any of the below as a reply.  Except for ? and vStopped, that reply is only returned when the target halts.  In the below the exact meaning of signal number is defined by the header include/gdb/signals.h in the GDBN source code.

As in the description of request packets, we include spaces in the reply templates for clarity; these are not part of the reply packet's syntax.  No GDBN stop reply packet uses spaces to separate its components.

S AA
     The program received signal number AA (a two-digit hexadecimal number).  This is equivalent to a T response with no N:R pairs.

T AA N1:R1;N2:R2;...
     The program received signal number AA (a two-digit hexadecimal number).  This is equivalent to an S response, except that the N:R pairs can carry values of important registers and other information directly in the stop reply packet, reducing round-trip latency.  Single-step and breakpoint traps are reported this way.  Each N:R pair is interpreted as follows:

       * If N is a hexadecimal number, it is a register number, and the corresponding R gives that register's value.  R is a series of bytes in target byte order, with each byte given by a two-digit hex number.

       * If N is thread, then R is the THREAD-ID of the stopped thread, as specified in thread-id syntax.

       * If N is core, then R is the hexadecimal number of the core on which the stop event was detected.

       * If N is a recognized stop reason, it describes a more specific event that stopped the target.  The currently defined stop reasons are listed below.  AA should be 05, the trap signal.  At most one stop reason should be present.

       * Otherwise, GDBN should ignore this N:R pair and go on to the next; this allows us to extend the protocol in the future.

     The currently defined stop reasons are:

     watch
     rwatch
     awatch
          The packet indicates a watchpoint hit, and R is the data address, in hex.

     library
          The packet indicates that the loaded libraries have changed. GDBN should use qXfer:libraries:read to fetch a new list of loaded libraries.  R is ignored.

     replaylog
          The packet indicates that the target cannot continue replaying logged execution events, because it has reached the end (or the beginning when executing backward) of the log.  The value of R will be either begin or end.  See Reverse Execution, for more information.

W AA
W AA ; process:PID
     The process exited, and AA is the exit status.  This is only applicable to certain targets.

     The second form of the response, including the process ID of the exited process, can be used only when GDBN has reported support for multiprocess protocol extensions; see multiprocess extensions. The PID is formatted as a big-endian hex string.

X AA
X AA ; process:PID
     The process terminated with signal AA.

     The second form of the response, including the process ID of the terminated process, can be used only when GDBN has reported support for multiprocess protocol extensions; see multiprocess extensions.  The PID is formatted as a big-endian hex string.

O XX...
     XX... is hex encoding of ASCII data, to be written as the program's console output.  This can happen at any time while the program is running and the debugger should continue to wait for W, T, etc.  This reply is not permitted in non-stop mode.

F CALL-ID,PARAMETER...
     CALL-ID is the identifier which says which host system call should be called.  This is just the name of the function.  Translation into the correct system call is only applicable as it's defined in GDBN. See File-I/O Remote Protocol Extension, for a list of implemented system calls.

     PARAMETER... is a list of parameters as defined for this very system call.

     The target replies with this packet when it expects GDBN to call a host system call on behalf of the target.  GDBN replies with an appropriate F packet and keeps up waiting for the next reply packet from the target.  The latest C, c, S or s action is expected to be continued.  See File-I/O Remote Protocol Extension, for more details.

General Query Packets
=====================

Packets starting with q are general query packets; packets starting with Q are general set packets.  General query and set packets are a semi-unified form for retrieving and sending information to and from the stub.

The initial letter of a query or set packet is followed by a name indicating what sort of thing the packet applies to.  For example, GDBN may use a qSymbol packet to exchange symbol definitions with the stub.  These packet names follow some conventions:

  * The name must not contain commas, colons or semicolons.

  * Most GDBN query and set packets have a leading upper case letter.

  * The names of custom vendor packets should use a company prefix, in lower case, followed by a period.  For example, packets designed at the Acme Corporation might begin with qacme.foo (for querying foos) or Qacme.bar (for setting bars).

The name of a query or set packet should be separated from any parameters by a :; the parameters themselves should be separated by , or ;.  Stubs must be careful to match the full packet name, and check for a separator or the end of the packet, in case two packet names share a common prefix.  New packets should not begin with qC, qP, or qL (The qP and qL packets predate these conventions, and have arguments without any terminator for the packet name; we suspect they are in widespread use in places that are difficult to upgrade.  The qC packet has no arguments, but some existing stubs (e.g. RedBoot) are known to not check for the end of the packet.).

Like the descriptions of the other packets, each description here has a template showing the packet's overall syntax, followed by an explanation of the packet's meaning.  We include spaces in some of the templates for clarity; these are not part of the packet's syntax.  No GDBN packet uses spaces to separate its components.

Here are the currently defined query and set packets:

QAllow:OP:VAL...
     Specify which operations GDBN expects to request of the target, as a semicolon-separated list of operation name and value pairs.  Possible values for OP include WriteReg, WriteMem, InsertBreak, InsertTrace, InsertFastTrace, and Stop. VAL is either 0, indicating that GDBN will not request the operation, or 1, indicating that it may.  (The target can then use this to set up its own internals optimally, for instance if the debugger never expects to insert breakpoints, it may not need to install its own trap handler.)

qC
     Return the current thread ID.

     Reply:

     QC THREAD-ID
          Where THREAD-ID is a thread ID as documented in thread-id syntax.

     (anything else)
          Any other reply implies the old thread ID.

qCRC:ADDR,LENGTH
     Compute the CRC checksum of a block of memory using CRC-32 defined in IEEE 802.3.  The CRC is computed byte at a time, taking the most significant bit of each byte first.  The initial pattern code 0xffffffff is used to ensure leading zeros affect the CRC.

     Note: This is the same CRC used in validating separate debug files (see Debugging Information in Separate Files).  However the algorithm is slightly different.  When validating separate debug files, the CRC is computed taking the least significant bit of each byte first, and the final result is inverted to detect trailing zeros.

     Reply:

     E NN
          An error (such as memory fault)

     C CRC32
          The specified memory region's checksum is CRC32.

QDisableRandomization:VALUE
     Some target operating systems will randomize the virtual address space of the inferior process as a security feature, but provide a feature to disable such randomization, e.g. to allow for a more deterministic debugging experience.  On such systems, this packet with a VALUE of 1 directs the target to disable address space randomization for processes subsequently started via vRun packets, while a packet with a VALUE of 0 tells the target to enable address space randomization.

     This packet is only available in extended mode (see extended mode).

     Reply:

     OK
          The request succeeded.

     E NN
          An error occurred.  NN are hex digits.

          An empty reply indicates that QDisableRandomization is not supported by the stub.

     This packet is not probed by default; the remote stub must request it, by supplying an appropriate qSupported response (see qSupported). This should only be done on targets that actually support disabling address space randomization.

qfThreadInfo
qsThreadInfo
     Obtain a list of all active thread IDs from the target (OS).  Since there may be too many active threads to fit into one reply packet, this query works iteratively: it may require more than one query/reply sequence to obtain the entire list of threads.  The first query of the sequence will be the qfThreadInfo query; subsequent queries in the sequence will be the qsThreadInfo query.

     NOTE: This packet replaces the qL query (see below).

     Reply:

     m THREAD-ID
          A single thread ID

     m THREAD-ID,THREAD-ID...
          a comma-separated list of thread IDs

     l
          (lower case letter L) denotes end of list.

     In response to each query, the target will reply with a list of one or more thread IDs, separated by commas. GDBN will respond to each reply with a request for more thread ids (using the qs form of the query), until the target responds with l (lower-case ell, for last). Refer to thread-id syntax, for the format of the THREAD-ID fields.

qGetTLSAddr:THREAD-ID,OFFSET,LM
     Fetch the address associated with thread local storage specified by THREAD-ID, OFFSET, and LM.

     THREAD-ID is the thread ID associated with the thread for which to fetch the TLS address.  See thread-id syntax.

     OFFSET is the (big endian, hex encoded) offset associated with the thread local variable.  (This offset is obtained from the debug information associated with the variable.)

     LM is the (big endian, hex encoded) OS/ABI-specific encoding of the load module associated with the thread local storage.  For example, a GNU/Linux system will pass the link map address of the shared object associated with the thread local storage under consideration. Other operating environments may choose to represent the load module differently, so the precise meaning of this parameter will vary.

     Reply:

     XX...
          Hex encoded (big endian) bytes representing the address of the thread local storage requested.

     E NN
          An error occurred.  NN are hex digits.

          An empty reply indicates that qGetTLSAddr is not supported by the stub.

qGetTIBAddr:THREAD-ID
     Fetch address of the Windows OS specific Thread Information Block.

     THREAD-ID is the thread ID associated with the thread.

     Reply:

     XX...
          Hex encoded (big endian) bytes representing the linear address of the thread information block.

     E NN
          An error occured.  This means that either the thread was not found, or the address could not be retrieved.

          An empty reply indicates that qGetTIBAddr is not supported by the stub.

qL STARTFLAG THREADCOUNT NEXTTHREAD
     Obtain thread information from RTOS.  Where: STARTFLAG (one hex digit) is one to indicate the first query and zero to indicate a subsequent query; THREADCOUNT (two hex digits) is the maximum number of threads the response packet can contain; and NEXTTHREAD (eight hex digits), for subsequent queries (STARTFLAG is zero), is returned in the response as ARGTHREAD.

     Don't use this packet; use the qfThreadInfo query instead (see above).

     Reply:

     qM COUNT DONE ARGTHREAD THREAD...
          Where: COUNT (two hex digits) is the number of threads being returned; DONE (one hex digit) is zero to indicate more threads and one indicates no further threads; ARGTHREADID (eight hex digits) is NEXTTHREAD from the request packet; THREAD... is a sequence of thread IDs from the target.  THREADID (eight hex digits).  See remote.c:parse_threadlist_response().

qOffsets
     Get section offsets that the target used when relocating the downloaded image.

     Reply:

     Text=XXX;Data=YYY[;Bss=ZZZ]
          Relocate the Text section by XXX from its original address. Relocate the Data section by YYY from its original address. If the object file format provides segment information (e.g. ELF PT_LOAD program headers), GDBN will relocate entire segments by the supplied offsets.

          Note: while a Bss offset may be included in the response,
          GDBN ignores this and instead applies the Data offset
          to the Bss section.

     TextSeg=XXX[;DataSeg=YYY]
          Relocate the first segment of the object file, which conventionally contains program code, to a starting address of XXX.  If DataSeg is specified, relocate the second segment, which conventionally contains modifiable data, to a starting address of YYY.  GDBN will report an error if the object file does not contain segment information, or does not contain at least as many segments as mentioned in the reply.  Extra segments are kept at fixed offsets relative to the last relocated segment.

qP MODE THREAD-ID
     Returns information on THREAD-ID.  Where: MODE is a hex encoded 32 bit mode; THREAD-ID is a thread ID (see thread-id syntax).

     Don't use this packet; use the qThreadExtraInfo query instead (see below).

     Reply: see remote.c:remote_unpack_thread_info_response().

QNonStop:1

QNonStop:0
     Enter non-stop (QNonStop:1) or all-stop (QNonStop:0) mode. See Remote Non-Stop, for more information.

     Reply:

     OK
          The request succeeded.

     E NN
          An error occurred.  NN are hex digits.

          An empty reply indicates that QNonStop is not supported by the stub.

     This packet is not probed by default; the remote stub must request it, by supplying an appropriate qSupported response (see qSupported). Use of this packet is controlled by the set non-stop command; see Non-Stop Mode.

QPassSignals: SIGNAL [;SIGNAL]...
     Each listed SIGNAL should be passed directly to the inferior process. Signals are numbered identically to continue packets and stop replies (see Stop Reply Packets).  Each SIGNAL list item should be strictly greater than the previous item.  These signals do not need to stop the inferior, or be reported to GDBN.  All other signals should be reported to GDBN.  Multiple QPassSignals packets do not combine; any earlier QPassSignals list is completely replaced by the new list.  This packet improves performance when using handle SIGNAL nostop noprint pass.

     Reply:

     OK
          The request succeeded.

     E NN
          An error occurred.  NN are hex digits.

          An empty reply indicates that QPassSignals is not supported by the stub.

     Use of this packet is controlled by the set remote pass-signals command (see set remote pass-signals). This packet is not probed by default; the remote stub must request it, by supplying an appropriate qSupported response (see qSupported).

qRcmd,COMMAND
     COMMAND (hex encoded) is passed to the local interpreter for execution.  Invalid commands should be reported using the output string.  Before the final result packet, the target may also respond with a number of intermediate OOUTPUT console output packets.  Implementors should note that providing access to a stubs's interpreter may have security implications.

     Reply:

     OK
          A command response with no output.

     OUTPUT
          A command response with the hex encoded output string OUTPUT.

     E NN
          Indicate a badly formed request.

          An empty reply indicates that qRcmd is not recognized.

     (Note that the qRcmd packet's name is separated from the command by a ,, not a :, contrary to the naming conventions above.  Please don't use this packet as a model for new packets.)

qSearch:memory:ADDRESS;LENGTH;SEARCH-PATTERN
     Search LENGTH bytes at ADDRESS for SEARCH-PATTERN. ADDRESS and LENGTH are encoded in hex. SEARCH-PATTERN is a sequence of bytes, hex encoded.

     Reply:

     0
          The pattern was not found.

     1,address
          The pattern was found at ADDRESS.

     E NN
          A badly formed request or an error was encountered while searching memory.

          An empty reply indicates that qSearch:memory is not recognized.

QStartNoAckMode
     Request that the remote stub disable the normal +/- protocol acknowledgments (see Packet Acknowledgment).

     Reply:

     OK
          The stub has switched to no-acknowledgment mode. GDBN acknowledges this reponse, but neither the stub nor GDBN shall send or expect further +/- acknowledgments in the current connection.

          An empty reply indicates that the stub does not support no-acknowledgment mode.

qSupported [:GDBFEATURE [;GDBFEATURE]... ]
     Tell the remote stub about features supported by GDBN, and query the stub for features it supports.  This packet allows GDBN and the remote stub to take advantage of each others' features.  qSupported also consolidates multiple feature probes at startup, to improve GDBN performance---a single larger packet performs better than multiple smaller probe packets on high-latency links.  Some features may enable behavior which must not be on by default, e.g. because it would confuse older clients or stubs.  Other features may describe packets which could be automatically probed for, but are not.  These features must be reported before GDBN will use them.  This ``default unsupported'' behavior is not appropriate for all packets, but it helps to keep the initial connection time under control with new versions of GDBN which support increasing numbers of packets.

     Reply:

     STUBFEATURE [;STUBFEATURE]...
          The stub supports or does not support each returned STUBFEATURE, depending on the form of each STUBFEATURE (see below for the possible forms).

          An empty reply indicates that qSupported is not recognized, or that no features needed to be reported to GDBN.

     The allowed forms for each feature (either a GDBFEATURE in the qSupported packet, or a STUBFEATURE in the response) are:

     NAME=VALUE
          The remote protocol feature NAME is supported, and associated with the specified VALUE.  The format of VALUE depends on the feature, but it must not include a semicolon.

     NAME+
          The remote protocol feature NAME is supported, and does not need an associated value.

     NAME-
          The remote protocol feature NAME is not supported.

     NAME?
          The remote protocol feature NAME may be supported, and GDBN should auto-detect support in some other way when it is needed.  This form will not be used for GDBFEATURE notifications, but may be used for STUBFEATURE responses.

     Whenever the stub receives a qSupported request, the supplied set of GDBN features should override any previous request.  This allows GDBN to put the stub in a known state, even if the stub had previously been communicating with a different version of GDBN.

     The following values of GDBFEATURE (for the packet sent by GDBN) are defined:

     multiprocess
          This feature indicates whether GDBN supports multiprocess extensions to the remote protocol.  GDBN does not use such extensions unless the stub also reports that it supports them by including multiprocess+ in its qSupported reply. See multiprocess extensions, for details.

     xmlRegisters
          This feature indicates that GDBN supports the XML target description.  If the stub sees xmlRegisters= with target specific strings separated by a comma, it will report register description.

     qRelocInsn
          This feature indicates whether GDBN supports the qRelocInsn packet (see Relocate instruction reply packet).

     Stubs should ignore any unknown values for GDBFEATURE.  Any GDBN which sends a qSupported packet supports receiving packets of unlimited length (earlier versions of GDBN may reject overly long responses).  Additional values for GDBFEATURE may be defined in the future to let the stub take advantage of new features in GDBN, e.g. incompatible improvements in the remote protocol---the multiprocess feature is an example of such a feature.  The stub's reply should be independent of the GDBFEATURE entries sent by GDBN; first GDBN describes all the features it supports, and then the stub replies with all the features it supports.

     Similarly, GDBN will silently ignore unrecognized stub feature responses, as long as each response uses one of the standard forms.

     Some features are flags.  A stub which supports a flag feature should respond with a + form response.  Other features require values, and the stub should respond with an = form response.

     Each feature has a default value, which GDBN will use if qSupported is not available or if the feature is not mentioned in the qSupported response.  The default values are fixed; a stub is free to omit any feature responses that match the defaults.

     Not all features can be probed, but for those which can, the probing mechanism is useful: in some cases, a stub's internal architecture may not allow the protocol layer to know some information about the underlying target in advance.  This is especially common in stubs which may be configured for multiple targets.

     These are the currently defined stub features and their properties:

     Feature Name
     Value Required
     Default
     Probe Allowed

     PacketSize
     Yes
     -
     No

     qXfer:auxv:read
     No
     -
     Yes

     qXfer:features:read
     No
     -
     Yes

     qXfer:libraries:read
     No
     -
     Yes

     qXfer:memory-map:read
     No
     -
     Yes

     qXfer:sdata:read
     No
     -
     Yes

     qXfer:spu:read
     No
     -
     Yes

     qXfer:spu:write
     No
     -
     Yes

     qXfer:siginfo:read
     No
     -
     Yes

     qXfer:siginfo:write
     No
     -
     Yes

     qXfer:threads:read
     No
     -
     Yes

     qXfer:traceframe-info:read
     No
     -
     Yes

     qXfer:fdpic:read
     No
     -
     Yes

     QNonStop
     No
     -
     Yes

     QPassSignals
     No
     -
     Yes

     QStartNoAckMode
     No
     -
     Yes

     multiprocess
     No
     -
     No

     ConditionalTracepoints
     No
     -
     No

     ReverseContinue
     No
     -
     No

     ReverseStep
     No
     -
     No

     TracepointSource
     No
     -
     No

     QAllow
     No
     -
     No

     QDisableRandomization
     No
     -
     No

     EnableDisableTracepoints
     No
     -
     No

     tracenz
     No
     -
     No

     These are the currently defined stub features, in more detail:

     PacketSize=BYTES
          The remote stub can accept packets up to at least BYTES in length.  GDBN will send packets up to this size for bulk transfers, and will never send larger packets.  This is a limit on the data characters in the packet, including the frame and checksum. There is no trailing NUL byte in a remote protocol packet; if the stub stores packets in a NUL-terminated format, it should allow an extra byte in its buffer for the NUL.  If this stub feature is not supported, GDBN guesses based on the size of the g packet response.

     qXfer:auxv:read
          The remote stub understands the qXfer:auxv:read packet (see qXfer auxiliary vector read).

     qXfer:features:read
          The remote stub understands the qXfer:features:read packet (see qXfer target description read).

     qXfer:libraries:read
          The remote stub understands the qXfer:libraries:read packet (see qXfer library list read).

     qXfer:libraries-svr4:read
          The remote stub understands the qXfer:libraries-svr4:read packet (see qXfer svr4 library list read).

     qXfer:memory-map:read
          The remote stub understands the qXfer:memory-map:read packet (see qXfer memory map read).

     qXfer:sdata:read
          The remote stub understands the qXfer:sdata:read packet (see qXfer sdata read).

     qXfer:spu:read
          The remote stub understands the qXfer:spu:read packet (see qXfer spu read).

     qXfer:spu:write
          The remote stub understands the qXfer:spu:write packet (see qXfer spu write).

     qXfer:siginfo:read
          The remote stub understands the qXfer:siginfo:read packet (see qXfer siginfo read).

     qXfer:siginfo:write
          The remote stub understands the qXfer:siginfo:write packet (see qXfer siginfo write).

     qXfer:threads:read
          The remote stub understands the qXfer:threads:read packet (see qXfer threads read).

     qXfer:traceframe-info:read
          The remote stub understands the qXfer:traceframe-info:read packet (see qXfer traceframe info read).

     qXfer:fdpic:read
          The remote stub understands the qXfer:fdpic:read packet (see qXfer fdpic loadmap read).

     QNonStop
          The remote stub understands the QNonStop packet (see QNonStop).

     QPassSignals
          The remote stub understands the QPassSignals packet (see QPassSignals).

     QStartNoAckMode
          The remote stub understands the QStartNoAckMode packet and prefers to operate in no-acknowledgment mode.  See Packet Acknowledgment.

     multiprocess
          The remote stub understands the multiprocess extensions to the remote protocol syntax.  The multiprocess extensions affect the syntax of thread IDs in both packets and replies (see thread-id syntax), and add process IDs to the D packet and W and X replies.  Note that reporting this feature indicates support for the syntactic extensions only, not that the stub necessarily supports debugging of more than one process at a time.  The stub must not use multiprocess extensions in packet replies unless GDBN has also indicated it supports them in its qSupported request.

     qXfer:osdata:read
          The remote stub understands the qXfer:osdata:read packet ((see qXfer osdata read).

     ConditionalTracepoints
          The remote stub accepts and implements conditional expressions defined for tracepoints (see Tracepoint Conditions).

     ReverseContinue
          The remote stub accepts and implements the reverse continue packet (see bc).

     ReverseStep
          The remote stub accepts and implements the reverse step packet (see bs).

     TracepointSource
          The remote stub understands the QTDPsrc packet that supplies the source form of tracepoint definitions.

     QAllow
          The remote stub understands the QAllow packet.

     QDisableRandomization
          The remote stub understands the QDisableRandomization packet.

     StaticTracepoint
          The remote stub supports static tracepoints.

     InstallInTrace
          The remote stub supports installing tracepoint in tracing.

     EnableDisableTracepoints
          The remote stub supports the QTEnable (see QTEnable) and QTDisable (see QTDisable) packets that allow tracepoints to be enabled and disabled while a trace experiment is running.

     tracenz
          The remote stub supports the tracenz bytecode for collecting strings. See Bytecode Descriptions for details about the bytecode.

qSymbol::
     Notify the target that GDBN is prepared to serve symbol lookup requests.  Accept requests from the target for the values of symbols.

     Reply:

     OK
          The target does not need to look up any (more) symbols.

     qSymbol:SYM_NAME
          The target requests the value of symbol SYM_NAME (hex encoded). GDBN may provide the value by using the qSymbol:SYM_VALUE:SYM_NAME message, described below.

qSymbol:SYM_VALUE:SYM_NAME
     Set the value of SYM_NAME to SYM_VALUE.

     SYM_NAME (hex encoded) is the name of a symbol whose value the target has previously requested.

     SYM_VALUE (hex) is the value for symbol SYM_NAME.  If GDBN cannot supply a value for SYM_NAME, then this field will be empty.

     Reply:

     OK
          The target does not need to look up any (more) symbols.

     qSymbol:SYM_NAME
          The target requests the value of a new symbol SYM_NAME (hex encoded).  GDBN will continue to supply the values of symbols (if available), until the target ceases to request them.

qTBuffer

QTBuffer

QTDisconnected
QTDP
QTDPsrc
QTDV
qTfP
qTfV
QTFrame
qTMinFTPILen

     See Tracepoint Packets.

qThreadExtraInfo,THREAD-ID
     Obtain a printable string description of a thread's attributes from the target OS.  THREAD-ID is a thread ID; see thread-id syntax.  This string may contain anything that the target OS thinks is interesting for GDBN to tell the user about the thread.  The string is displayed in GDBN's info threads display.  Some examples of possible thread extra info strings are Runnable, or Blocked on Mutex.

     Reply:

     XX...
          Where XX... is a hex encoding of ASCII data, comprising the printable string containing the extra information about the thread's attributes.

     (Note that the qThreadExtraInfo packet's name is separated from the command by a ,, not a :, contrary to the naming conventions above.  Please don't use this packet as a model for new packets.)

QTNotes

qTP

QTSave

qTsP

qTsV
QTStart
QTStop
QTEnable
QTDisable
QTinit
QTro
qTStatus
qTV
qTfSTM
qTsSTM
qTSTMat
     See Tracepoint Packets.

qXfer:OBJECT:read:ANNEX:OFFSET,LENGTH
     Read uninterpreted bytes from the target's special data area identified by the keyword OBJECT.  Request LENGTH bytes starting at OFFSET bytes into the data.  The content and encoding of ANNEX is specific to OBJECT; it can supply additional details about what data to access.

     Here are the specific requests of this form defined so far.  All qXfer:OBJECT:read:... requests use the same reply formats, listed below.

     qXfer:auxv:read::OFFSET,LENGTH
          Access the target's auxiliary vector.  See auxiliary vector.  Note ANNEX must be empty.

          This packet is not probed by default; the remote stub must request it, by supplying an appropriate qSupported response (see qSupported).

     qXfer:features:read:ANNEX:OFFSET,LENGTH
          Access the target description.  See Target Descriptions.  The annex specifies which XML document to access.  The main description is always loaded from the target.xml annex.

          This packet is not probed by default; the remote stub must request it, by supplying an appropriate qSupported response (see qSupported).

     qXfer:libraries:read:ANNEX:OFFSET,LENGTH
          Access the target's list of loaded libraries.  See Library List Format. The annex part of the generic qXfer packet must be empty (see qXfer read).

          Targets which maintain a list of libraries in the program's memory do not need to implement this packet; it is designed for platforms where the operating system manages the list of loaded libraries.

          This packet is not probed by default; the remote stub must request it, by supplying an appropriate qSupported response (see qSupported).

     qXfer:libraries-svr4:read:ANNEX:OFFSET,LENGTH
          Access the target's list of loaded libraries when the target is an SVR4 platform.  See Library List Format for SVR4 Targets.  The annex part of the generic qXfer packet must be empty (see qXfer read).

          This packet is optional for better performance on SVR4 targets. GDBN uses memory read packets to read the SVR4 library list otherwise.

          This packet is not probed by default; the remote stub must request it, by supplying an appropriate qSupported response (see qSupported).

     qXfer:memory-map:read::OFFSET,LENGTH
          Access the target's memory-map.  See Memory Map Format.  The annex part of the generic qXfer packet must be empty (see qXfer read).

          This packet is not probed by default; the remote stub must request it, by supplying an appropriate qSupported response (see qSupported).

     qXfer:sdata:read::OFFSET,LENGTH

          Read contents of the extra collected static tracepoint marker information.  The annex part of the generic qXfer packet must be empty (see qXfer read).  See Tracepoint Action Lists.

          This packet is not probed by default; the remote stub must request it, by supplying an appropriate qSupported response (see qSupported).

     qXfer:siginfo:read::OFFSET,LENGTH
          Read contents of the extra signal information on the target system.  The annex part of the generic qXfer packet must be empty (see qXfer read).

          This packet is not probed by default; the remote stub must request it, by supplying an appropriate qSupported response (see qSupported).

     qXfer:spu:read:ANNEX:OFFSET,LENGTH
          Read contents of an spufs file on the target system.  The annex specifies which file to read; it must be of the form ID/NAME, where ID specifies an SPU context ID in the target process, and NAME identifes the spufs file in that context to be accessed.

          This packet is not probed by default; the remote stub must request it, by supplying an appropriate qSupported response (see qSupported).

     qXfer:threads:read::OFFSET,LENGTH
          Access the list of threads on target.  See Thread List Format.  The annex part of the generic qXfer packet must be empty (see qXfer read).

          This packet is not probed by default; the remote stub must request it, by supplying an appropriate qSupported response (see qSupported).

     qXfer:traceframe-info:read::OFFSET,LENGTH

          Return a description of the current traceframe's contents. See Traceframe Info Format.  The annex part of the generic qXfer packet must be empty (see qXfer read).

          This packet is not probed by default; the remote stub must request it, by supplying an appropriate qSupported response (see qSupported).

     qXfer:fdpic:read:ANNEX:OFFSET,LENGTH
          Read contents of loadmaps on the target system.  The annex, either exec or interp, specifies which loadmap, executable loadmap or interpreter loadmap to read.

          This packet is not probed by default; the remote stub must request it, by supplying an appropriate qSupported response (see qSupported).

     qXfer:osdata:read::OFFSET,LENGTH
          Access the target's operating system information. See Operating System Information.

     Reply:

     m DATA
          Data DATA (see Binary Data) has been read from the target.  There may be more data at a higher address (although it is permitted to return m even for the last valid block of data, as long as at least one byte of data was read). DATA may have fewer bytes than the LENGTH in the request.

     l DATA
          Data DATA (see Binary Data) has been read from the target. There is no more data to be read.  DATA may have fewer bytes than the LENGTH in the request.

     l
          The OFFSET in the request is at the end of the data. There is no more data to be read.

     E00
          The request was malformed, or ANNEX was invalid.

     E NN
          The offset was invalid, or there was an error encountered reading the data. NN is a hex-encoded errno value.

          An empty reply indicates the OBJECT string was not recognized by the stub, or that the object does not support reading.

qXfer:OBJECT:write:ANNEX:OFFSET:DATA...
     Write uninterpreted bytes into the target's special data area identified by the keyword OBJECT, starting at OFFSET bytes into the data.  DATA... is the binary-encoded data (see Binary Data) to be written.  The content and encoding of ANNEX is specific to OBJECT; it can supply additional details about what data to access.

     Here are the specific requests of this form defined so far.  All qXfer:OBJECT:write:... requests use the same reply formats, listed below.

     qXfer:siginfo:write::OFFSET:DATA...
          Write DATA to the extra signal information on the target system. The annex part of the generic qXfer packet must be empty (see qXfer write).

          This packet is not probed by default; the remote stub must request it, by supplying an appropriate qSupported response (see qSupported).

     qXfer:spu:write:ANNEX:OFFSET:DATA...
          Write DATA to an spufs file on the target system.  The annex specifies which file to write; it must be of the form ID/NAME, where ID specifies an SPU context ID in the target process, and NAME identifes the spufs file in that context to be accessed.

          This packet is not probed by default; the remote stub must request it, by supplying an appropriate qSupported response (see qSupported).

     Reply:

     NN
          NN (hex encoded) is the number of bytes written. This may be fewer bytes than supplied in the request.

     E00
          The request was malformed, or ANNEX was invalid.

     E NN
          The offset was invalid, or there was an error encountered writing the data. NN is a hex-encoded errno value.

          An empty reply indicates the OBJECT string was not recognized by the stub, or that the object does not support writing.

qXfer:OBJECT:OPERATION:...
     Requests of this form may be added in the future.  When a stub does not recognize the OBJECT keyword, or its support for OBJECT does not recognize the OPERATION keyword, the stub must respond with an empty packet.

qAttached:PID
     Return an indication of whether the remote server attached to an existing process or created a new process.  When the multiprocess protocol extensions are supported (see multiprocess extensions), PID is an integer in hexadecimal format identifying the target process.  Otherwise, GDBN will omit the PID field and the query packet will be simplified as qAttached.

     This query is used, for example, to know whether the remote process should be detached or killed when a GDBN session is ended with the quit command.

     Reply:

     1
          The remote server attached to an existing process.

     0
          The remote server created a new process.

     E NN
          A badly formed request or an error was encountered.

Architecture-Specific Protocol Details
======================================

This section describes how the remote protocol is applied to specific target architectures.  Also see Standard Target Features, for details of XML target descriptions for each architecture.

ARM
---

Breakpoint Kinds
................

These breakpoint kinds are defined for the Z0 and Z1 packets.

2
     16-bit Thumb mode breakpoint.

3
     32-bit Thumb mode (Thumb-2) breakpoint.

4
     32-bit ARM mode breakpoint.

MIPS
----

Register Packet Format
......................

The following g/G packets have previously been defined. In the below, some thirty-two bit registers are transferred as sixty-four bits.  Those registers should be zero/sign extended (which?) to fill the space allocated.  Register bytes are transferred in target byte order.  The two nibbles within a register byte are transferred most-significant - least-significant.

MIPS32

     All registers are transferred as thirty-two bit quantities in the order: 32 general-purpose; sr; lo; hi; bad; cause; pc; 32 floating-point registers; fsr; fir; fp.

MIPS64

     All registers are transferred as sixty-four bit quantities (including thirty-two bit registers such as sr).  The ordering is the same as MIPS32.

Tracepoint Packets
==================

Here we describe the packets GDBN uses to implement tracepoints (see Tracepoints).

QTDP:N:ADDR:ENA:STEP:PASS[:FFLEN][:XLEN,BYTES][-]
     Create a new tracepoint, number N, at ADDR.  If ENA is E, then the tracepoint is enabled; if it is D, then the tracepoint is disabled.  STEP is the tracepoint's step count, and PASS is its pass count.  If an F is present, then the tracepoint is to be a fast tracepoint, and the FLEN is the number of bytes that the target should copy elsewhere to make room for the tracepoint.  If an X is present, it introduces a tracepoint condition, which consists of a hexadecimal length, followed by a comma and hex-encoded bytes, in a manner similar to action encodings as described below.  If the trailing - is present, further QTDP packets will follow to specify this tracepoint's actions.

     Replies:

     OK
          The packet was understood and carried out.

     qRelocInsn
          See Relocate instruction reply packet.

          The packet was not recognized.

QTDP:-N:ADDR:[S]ACTION...[-]
     Define actions to be taken when a tracepoint is hit.  N and ADDR must be the same as in the initial QTDP packet for this tracepoint.  This packet may only be sent immediately after another QTDP packet that ended with a -.  If the trailing - is present, further QTDP packets will follow, specifying more actions for this tracepoint.

     In the series of action packets for a given tracepoint, at most one can have an S before its first ACTION.  If such a packet is sent, it and the following packets define ``while-stepping'' actions.  Any prior packets define ordinary actions --- that is, those taken when the tracepoint is first hit.  If no action packet has an S, then all the packets in the series specify ordinary tracepoint actions.

     The ACTION... portion of the packet is a series of actions, concatenated without separators.  Each action has one of the following forms:

     R MASK
          Collect the registers whose bits are set in MASK.  MASK is a hexadecimal number whose I'th bit is set if register number I should be collected.  (The least significant bit is numbered zero.)  Note that MASK may be any number of digits long; it may not fit in a 32-bit word.

     M BASEREG,OFFSET,LEN
          Collect LEN bytes of memory starting at the address in register number BASEREG, plus OFFSET.  If BASEREG is -1, then the range has a fixed address: OFFSET is the address of the lowest byte to collect.  The BASEREG, OFFSET, and LEN parameters are all unsigned hexadecimal values (the -1 value for BASEREG is a special case).

     X LEN,EXPR
          Evaluate EXPR, whose length is LEN, and collect memory as it directs.  EXPR is an agent expression, as described in Agent Expressions.  Each byte of the expression is encoded as a two-digit hex number in the packet; LEN is the number of bytes in the expression (and thus one-half the number of hex digits in the packet).

     Any number of actions may be packed together in a single QTDP packet, as long as the packet does not exceed the maximum packet length (400 bytes, for many stubs).  There may be only one R action per tracepoint, and it must precede any M or X actions.  Any registers referred to by M and X actions must be collected by a preceding R action.  (The ``while-stepping'' actions are treated as if they were attached to a separate tracepoint, as far as these restrictions are concerned.)

     Replies:

     OK
          The packet was understood and carried out.

     qRelocInsn
          See Relocate instruction reply packet.

          The packet was not recognized.

QTDPsrc:N:ADDR:TYPE:START:SLEN:BYTES
     Specify a source string of tracepoint N at address ADDR. This is useful to get accurate reproduction of the tracepoints originally downloaded at the beginning of the trace run.  TYPE is the name of the tracepoint part, such as cond for the tracepoint's conditional expression (see below for a list of types), while BYTES is the string, encoded in hexadecimal.

     START is the offset of the BYTES within the overall source string, while SLEN is the total length of the source string. This is intended for handling source strings that are longer than will fit in a single packet.

     The available string types are at for the location, cond for the conditional, and cmd for an action command. GDBN sends a separate packet for each command in the action list, in the same order in which the commands are stored in the list.

     The target does not need to do anything with source strings except report them back as part of the replies to the qTfP/qTsP query packets.

     Although this packet is optional, and GDBN will only send it if the target replies with TracepointSource see General Query Packets, it makes both disconnected tracing and trace files much easier to use.  Otherwise the user must be careful that the tracepoints in effect while looking at trace frames are identical to the ones in effect during the trace run; even a small discrepancy could cause tdump not to work, or a particular trace frame not be found.

QTDV:N:VALUE
     Create a new trace state variable, number N, with an initial value of VALUE, which is a 64-bit signed integer.  Both N and VALUE are encoded as hexadecimal values. GDBN has the option of not using this packet for initial values of zero; the target should simply create the trace state variables as they are mentioned in expressions.

QTFrame:N
     Select the N'th tracepoint frame from the buffer, and use the register and memory contents recorded there to answer subsequent request packets from GDBN.

     A successful reply from the stub indicates that the stub has found the requested frame.  The response is a series of parts, concatenated without separators, describing the frame we selected.  Each part has one of the following forms:

     F F
          The selected frame is number N in the trace frame buffer; F is a hexadecimal number.  If F is -1, then there was no frame matching the criteria in the request packet.

     T T
          The selected trace frame records a hit of tracepoint number T; T is a hexadecimal number.

QTFrame:pc:ADDR
     Like QTFrame:N, but select the first tracepoint frame after the currently selected frame whose PC is ADDR; ADDR is a hexadecimal number.

QTFrame:tdp:T
     Like QTFrame:N, but select the first tracepoint frame after the currently selected frame that is a hit of tracepoint T; T is a hexadecimal number.

QTFrame:range:START:END
     Like QTFrame:N, but select the first tracepoint frame after the currently selected frame whose PC is between START (inclusive) and END (inclusive); START and END are hexadecimal numbers.

QTFrame:outside:START:END
     Like QTFrame:range:START:END, but select the first frame outside the given range of addresses (exclusive).

qTMinFTPILen
     This packet requests the minimum length of instruction at which a fast tracepoint (see Set Tracepoints) may be placed.  For instance, on the 32-bit x86 architecture, it is possible to use a 4-byte jump, but it depends on the target system being able to create trampolines in the first 64K of memory, which might or might not be possible for that system.  So the reply to this packet will be 4 if it is able to arrange for that.

     Replies:

     0
          The minimum instruction length is currently unknown.

     LENGTH
          The minimum instruction length is LENGTH, where LENGTH is greater or equal to 1.  LENGTH is a hexadecimal number.  A reply of 1 means that a fast tracepoint may be placed on any instruction regardless of size.

     E
          An error has occurred.

          An empty reply indicates that the request is not supported by the stub.

QTStart
     Begin the tracepoint experiment.  Begin collecting data from tracepoint hits in the trace frame buffer.  This packet supports the qRelocInsn reply (see Relocate instruction reply packet).

QTStop
     End the tracepoint experiment.  Stop collecting trace frames.

QTEnable:N:ADDR
     Enable tracepoint N at address ADDR in a started tracepoint experiment.  If the tracepoint was previously disabled, then collection of data from it will resume.

QTDisable:N:ADDR
     Disable tracepoint N at address ADDR in a started tracepoint experiment.  No more data will be collected from the tracepoint unless QTEnable:N:ADDR is subsequently issued.

QTinit
     Clear the table of tracepoints, and empty the trace frame buffer.

QTro:START1,END1:START2,END2:...
     Establish the given ranges of memory as ``transparent''.  The stub will answer requests for these ranges from memory's current contents, if they were not collected as part of the tracepoint hit.

     GDBN uses this to mark read-only regions of memory, like those containing program code.  Since these areas never change, they should still have the same contents they did when the tracepoint was hit, so there's no reason for the stub to refuse to provide their contents.

QTDisconnected:VALUE
     Set the choice to what to do with the tracing run when GDBN disconnects from the target.  A VALUE of 1 directs the target to continue the tracing run, while 0 tells the target to stop tracing if GDBN is no longer in the picture.

qTStatus
     Ask the stub if there is a trace experiment running right now.

     The reply has the form:

     TRUNNING[;FIELD]...
          RUNNING is a single digit 1 if the trace is presently running, or 0 if not.  It is followed by semicolon-separated optional fields that an agent may use to report additional status.

     If the trace is not running, the agent may report any of several explanations as one of the optional fields:

     tnotrun:0
          No trace has been run yet.

     tstop[:TEXT]:0
          The trace was stopped by a user-originated stop command.  The optional TEXT field is a user-supplied string supplied as part of the stop command (for instance, an explanation of why the trace was stopped manually).  It is hex-encoded.

     tfull:0
          The trace stopped because the trace buffer filled up.

     tdisconnected:0
          The trace stopped because GDBN disconnected from the target.

     tpasscount:TPNUM
          The trace stopped because tracepoint TPNUM exceeded its pass count.

     terror:TEXT:TPNUM
          The trace stopped because tracepoint TPNUM had an error.  The string TEXT is available to describe the nature of the error (for instance, a divide by zero in the condition expression). TEXT is hex encoded.

     tunknown:0
          The trace stopped for some other reason.

     Additional optional fields supply statistical and other information. Although not required, they are extremely useful for users monitoring the progress of a trace run.  If a trace has stopped, and these numbers are reported, they must reflect the state of the just-stopped trace.

     tframes:N
          The number of trace frames in the buffer.

     tcreated:N
          The total number of trace frames created during the run. This may be larger than the trace frame count, if the buffer is circular.

     tsize:N
          The total size of the trace buffer, in bytes.

     tfree:N
          The number of bytes still unused in the buffer.

     circular:N
          The value of the circular trace buffer flag.  1 means that the trace buffer is circular and old trace frames will be discarded if necessary to make room, 0 means that the trace buffer is linear and may fill up.

     disconn:N
          The value of the disconnected tracing flag.  1 means that tracing will continue after GDBN disconnects, 0 means that the trace run will stop.

qTP:TP:ADDR
     Ask the stub for the current state of tracepoint number TP at address ADDR.

     Replies:

     VHITS:USAGE
          The tracepoint has been hit HITS times so far during the trace run, and accounts for USAGE in the trace buffer.  Note that while-stepping steps are not counted as separate hits, but the steps' space consumption is added into the usage number.

qTV:VAR
     Ask the stub for the value of the trace state variable number VAR.

     Replies:

     VVALUE
          The value of the variable is VALUE.  This will be the current value of the variable if the user is examining a running target, or a saved value if the variable was collected in the trace frame that the user is looking at.  Note that multiple requests may result in different reply values, such as when requesting values while the program is running.

     U
          The value of the variable is unknown.  This would occur, for example, if the user is examining a trace frame in which the requested variable was not collected.

qTfP
qTsP
     These packets request data about tracepoints that are being used by the target.  GDBN sends qTfP to get the first piece of data, and multiple qTsP to get additional pieces.  Replies to these packets generally take the form of the QTDP packets that define tracepoints. (FIXME add detailed syntax)

qTfV
qTsV
     These packets request data about trace state variables that are on the target.  GDBN sends qTfV to get the first vari of data, and multiple qTsV to get additional variables.  Replies to these packets follow the syntax of the QTDV packets that define trace state variables.

qTfSTM
qTsSTM
     These packets request data about static tracepoint markers that exist in the target program.  GDBN sends qTfSTM to get the first piece of data, and multiple qTsSTM to get additional pieces.  Replies to these packets take the following form:

     Reply:

     m ADDRESS:ID:EXTRA
          A single marker

     m ADDRESS:ID:EXTRA,ADDRESS:ID:EXTRA...
          a comma-separated list of markers

     l
          (lower case letter L) denotes end of list.

     E NN
          An error occurred.  NN are hex digits.

          An empty reply indicates that the request is not supported by the stub.

     ADDRESS is encoded in hex. ID and EXTRA are strings encoded in hex.

     In response to each query, the target will reply with a list of one or more markers, separated by commas.  GDBN will respond to each reply with a request for more markers (using the qs form of the query), until the target responds with l (lower-case ell, for last).

qTSTMat:ADDRESS
     This packets requests data about static tracepoint markers in the target program at ADDRESS.  Replies to this packet follow the syntax of the qTfSTM and qTsSTM packets that list static tracepoint markers.

QTSave:FILENAME
     This packet directs the target to save trace data to the file name FILENAME in the target's filesystem.  FILENAME is encoded as a hex string; the interpretation of the file name (relative vs absolute, wild cards, etc) is up to the target.

qTBuffer:OFFSET,LEN
     Return up to LEN bytes of the current contents of trace buffer, starting at OFFSET.  The trace buffer is treated as if it were a contiguous collection of traceframes, as per the trace file format. The reply consists as many hex-encoded bytes as the target can deliver in a packet; it is not an error to return fewer than were asked for. A reply consisting of just l indicates that no bytes are available.

QTBuffer:circular:VALUE
     This packet directs the target to use a circular trace buffer if VALUE is 1, or a linear buffer if the value is 0.

QTNotes:[TYPE:TEXT][;TYPE:TEXT]...
     This packet adds optional textual notes to the trace run.  Allowable types include user, notes, and tstop, the TEXT fields are arbitrary strings, hex-encoded.

Relocate instruction reply packet
---------------------------------

When installing fast tracepoints in memory, the target may need to relocate the instruction currently at the tracepoint address to a different address in memory.  For most instructions, a simple copy is enough, but, for example, call instructions that implicitly push the return address on the stack, and relative branches or other PC-relative instructions require offset adjustment, so that the effect of executing the instruction at a different address is the same as if it had executed in the original location.

In response to several of the tracepoint packets, the target may also respond with a number of intermediate qRelocInsn request packets before the final result packet, to have GDBN handle this relocation operation.  If a packet supports this mechanism, its documentation will explicitly say so.  See for example the above descriptions for the QTStart and QTDP packets.  The format of the request is:

qRelocInsn:FROM;TO

     This requests GDBN to copy instruction at address FROM to address TO, possibly adjusted so that executing the instruction at TO has the same effect as executing it at FROM.  GDBN writes the adjusted instruction to target memory starting at TO.

Replies:

qRelocInsn:ADJUSTED_SIZE
     Informs the stub the relocation is complete.  ADJUSTED_SIZE is the length in bytes of resulting relocated instruction sequence.

E NN
     A badly formed request was detected, or an error was encountered while relocating the instruction.

Host I/O Packets
================

The Host I/O packets allow GDBN to perform I/O operations on the far side of a remote link.  For example, Host I/O is used to upload and download files to a remote target with its own filesystem.  Host I/O uses the same constant values and data structure layout as the target-initiated File-I/O protocol.  However, the Host I/O packets are structured differently.  The target-initiated protocol relies on target memory to store parameters and buffers. Host I/O requests are initiated by GDBN, and the target's memory is not involved.  See File-I/O Remote Protocol Extension, for more details on the target-initiated protocol.

The Host I/O request packets all encode a single operation along with its arguments.  They have this format:

vFile:OPERATION: PARAMETER...
     OPERATION is the name of the particular request; the target should compare the entire packet name up to the second colon when checking for a supported operation.  The format of PARAMETER depends on the operation.  Numbers are always passed in hexadecimal.  Negative numbers have an explicit minus sign (i.e. two's complement is not used).  Strings (e.g. filenames) are encoded as a series of hexadecimal bytes.  The last argument to a system call may be a buffer of escaped binary data (see Binary Data).

The valid responses to Host I/O packets are:

F RESULT [, ERRNO] [; ATTACHMENT]
     RESULT is the integer value returned by this operation, usually non-negative for success and -1 for errors.  If an error has occured, ERRNO will be included in the result.  ERRNO will have a value defined by the File-I/O protocol (see Errno Values).  For operations which return data, ATTACHMENT supplies the data as a binary buffer.  Binary buffers in response packets are escaped in the normal way (see Binary Data).  See the individual packet documentation for the interpretation of RESULT and ATTACHMENT.

     An empty response indicates that this operation is not recognized.

These are the supported Host I/O operations:

vFile:open: PATHNAME, FLAGS, MODE
     Open a file at PATHNAME and return a file descriptor for it, or return -1 if an error occurs.  PATHNAME is a string, FLAGS is an integer indicating a mask of open flags (see Open Flags), and MODE is an integer indicating a mask of mode bits to use if the file is created (see mode_t Values). See open, for details of the open flags and mode values.

vFile:close: FD
     Close the open file corresponding to FD and return 0, or -1 if an error occurs.

vFile:pread: FD, COUNT, OFFSET
     Read data from the open file corresponding to FD.  Up to COUNT bytes will be read from the file, starting at OFFSET relative to the start of the file.  The target may read fewer bytes; common reasons include packet size limits and an end-of-file condition.  The number of bytes read is returned.  Zero should only be returned for a successful read at the end of the file, or if COUNT was zero.

     The data read should be returned as a binary attachment on success. If zero bytes were read, the response should include an empty binary attachment (i.e. a trailing semicolon).  The return value is the number of target bytes read; the binary attachment may be longer if some characters were escaped.

vFile:pwrite: FD, OFFSET, DATA
     Write DATA (a binary buffer) to the open file corresponding to FD.  Start the write at OFFSET from the start of the file.  Unlike many write system calls, there is no separate COUNT argument; the length of DATA in the packet is used.  vFile:write returns the number of bytes written, which may be shorter than the length of DATA, or -1 if an error occurred.

vFile:unlink: PATHNAME
     Delete the file at PATHNAME on the target.  Return 0, or -1 if an error occurs.  PATHNAME is a string.

vFile:readlink: FILENAME
     Read value of symbolic link FILENAME on the target.  Return the number of bytes read, or -1 if an error occurs.

     The data read should be returned as a binary attachment on success. If zero bytes were read, the response should include an empty binary attachment (i.e. a trailing semicolon).  The return value is the number of target bytes read; the binary attachment may be longer if some characters were escaped.

Interrupts
==========

When a program on the remote target is running, GDBN may attempt to interrupt it by sending a Ctrl-C, BREAK or a BREAK followed by g, control of which is specified via GDBN's interrupt-sequence.

The precise meaning of BREAK is defined by the transport mechanism and may, in fact, be undefined.  GDBN does not currently define a BREAK mechanism for any of the network interfaces except for TCP, in which case GDBN sends the telnet BREAK sequence.

Ctrl-C, on the other hand, is defined and implemented for all transport mechanisms.  It is represented by sending the single byte 0x03 without any of the usual packet overhead described in the Overview section (see Overview).  When a 0x03 byte is transmitted as part of a packet, it is considered to be packet data and does not represent an interrupt.  E.g., an X packet (see X packet), used for binary downloads, may include an unescaped 0x03 as part of its packet.

BREAK followed by g is also known as Magic SysRq g. When Linux kernel receives this sequence from serial port, it stops execution and connects to gdb.

Stubs are not required to recognize these interrupt mechanisms and the precise meaning associated with receipt of the interrupt is implementation defined.  If the target supports debugging of multiple threads and/or processes, it should attempt to interrupt all currently-executing threads and processes. If the stub is successful at interrupting the running program, it should send one of the stop reply packets (see Stop Reply Packets) to GDBN as a result of successfully stopping the program in all-stop mode, and a stop reply for each stopped thread in non-stop mode. Interrupts received while the program is stopped are discarded.

Notification Packets
====================

The GDBN remote serial protocol includes notifications, packets that require no acknowledgment.  Both the GDB and the stub may send notifications (although the only notifications defined at present are sent by the stub).  Notifications carry information without incurring the round-trip latency of an acknowledgment, and so are useful for low-impact communications where occasional packet loss is not a problem.

A notification packet has the form % DATA # CHECKSUM, where DATA is the content of the notification, and CHECKSUM is a checksum of DATA, computed and formatted as for ordinary GDBN packets.  A notification's DATA never contains $, % or # characters.  Upon receiving a notification, the recipient sends no + or - to acknowledge the notification's receipt or to report its corruption.

Every notification's DATA begins with a name, which contains no colon characters, followed by a colon character.

Recipients should silently ignore corrupted notifications and notifications they do not understand.  Recipients should restart timeout periods on receipt of a well-formed notification, whether or not they understand it.

Senders should only send the notifications described here when this protocol description specifies that they are permitted.  In the future, we may extend the protocol to permit existing notifications in new contexts; this rule helps older senders avoid confusing newer recipients.

(Older versions of GDBN ignore bytes received until they see the $ byte that begins an ordinary packet, so new stubs may transmit notifications without fear of confusing older clients.  There are no notifications defined for GDBN to send at the moment, but we assume that most older stubs would ignore them, as well.)

The following notification packets from the stub to GDBN are defined:

Stop: REPLY
     Report an asynchronous stop event in non-stop mode. The REPLY has the form of a stop reply, as described in Stop Reply Packets.  Refer to Remote Non-Stop, for information on how these notifications are acknowledged by GDBN.

Remote Protocol Support for Non-Stop Mode
=========================================

GDBN's remote protocol supports non-stop debugging of multi-threaded programs, as described in Non-Stop Mode.  If the stub supports non-stop mode, it should report that to GDBN by including QNonStop+ in its qSupported response (see qSupported).

GDBN typically sends a QNonStop packet only when establishing a new connection with the stub.  Entering non-stop mode does not alter the state of any currently-running threads, but targets must stop all threads in any already-attached processes when entering all-stop mode.  GDBN uses the ? packet as necessary to probe the target state after a mode change.

In non-stop mode, when an attached process encounters an event that would otherwise be reported with a stop reply, it uses the asynchronous notification mechanism (see Notification Packets) to inform GDBN.  In contrast to all-stop mode, where all threads in all processes are stopped when a stop reply is sent, in non-stop mode only the thread reporting the stop event is stopped.  That is, when reporting a S or T response to indicate completion of a step operation, hitting a breakpoint, or a fault, only the affected thread is stopped; any other still-running threads continue to run.  When reporting a W or X response, all running threads belonging to other attached processes continue to run.

Only one stop reply notification at a time may be pending; if additional stop events occur before GDBN has acknowledged the previous notification, they must be queued by the stub for later synchronous transmission in response to vStopped packets from GDBN.  Because the notification mechanism is unreliable, the stub is permitted to resend a stop reply notification if it believes GDBN may not have received it.  GDBN ignores additional stop reply notifications received before it has finished processing a previous notification and the stub has completed sending any queued stop events.

Otherwise, GDBN must be prepared to receive a stop reply notification at any time.  Specifically, they may appear when GDBN is not otherwise reading input from the stub, or when GDBN is expecting to read a normal synchronous response or a +/- acknowledgment to a packet it has sent. Notification packets are distinct from any other communication from the stub so there is no ambiguity.

After receiving a stop reply notification, GDBN shall acknowledge it by sending a vStopped packet (see vStopped packet) as a regular, synchronous request to the stub.  Such acknowledgment is not required to happen immediately, as GDBN is permitted to send other, unrelated packets to the stub first, which the stub should process normally.

Upon receiving a vStopped packet, if the stub has other queued stop events to report to GDBN, it shall respond by sending a normal stop reply response.  GDBN shall then send another vStopped packet to solicit further responses; again, it is permitted to send other, unrelated packets as well which the stub should process normally.

If the stub receives a vStopped packet and there are no additional stop events to report, the stub shall return an OK response.  At this point, if further stop events occur, the stub shall send a new stop reply notification, GDBN shall accept the notification, and the process shall be repeated.

In non-stop mode, the target shall respond to the ? packet as follows.  First, any incomplete stop reply notification/vStopped sequence in progress is abandoned.  The target must begin a new sequence reporting stop events for all stopped threads, whether or not it has previously reported those events to GDBN.  The first stop reply is sent as a synchronous reply to the ? packet, and subsequent stop replies are sent as responses to vStopped packets using the mechanism described above.  The target must not send asynchronous stop reply notifications until the sequence is complete. If all threads are running when the target receives the ? packet, or if the target is not attached to any process, it shall respond OK.

Packet Acknowledgment
=====================

By default, when either the host or the target machine receives a packet, the first response expected is an acknowledgment: either + (to indicate the package was received correctly) or - (to request retransmission). This mechanism allows the GDBN remote protocol to operate over unreliable transport mechanisms, such as a serial line.

In cases where the transport mechanism is itself reliable (such as a pipe or TCP connection), the +/- acknowledgments are redundant. It may be desirable to disable them in that case to reduce communication overhead, or for other reasons.  This can be accomplished by means of the QStartNoAckMode packet; see QStartNoAckMode.

When in no-acknowledgment mode, neither the stub nor GDBN shall send or expect +/- protocol acknowledgments.  The packet and response format still includes the normal checksum, as described in Overview, but the checksum may be ignored by the receiver.

If the stub supports QStartNoAckMode and prefers to operate in no-acknowledgment mode, it should report that to GDBN by including QStartNoAckMode+ in its response to qSupported; see qSupported. If GDBN also supports QStartNoAckMode and it has not been disabled via the set remote noack-packet off command (see Remote Configuration), GDBN may then send a QStartNoAckMode packet to the stub. Only then may the stub actually turn off packet acknowledgments. GDBN sends a final + acknowledgment of the stub's OK response, which can be safely ignored by the stub.

Note that set remote noack-packet command only affects negotiation between GDBN and the stub when subsequent connections are made; it does not affect the protocol acknowledgment state for any current connection. Since +/- acknowledgments are enabled by default when a new connection is established, there is also no protocol request to re-enable the acknowledgments for the current connection, once disabled.

Examples
========

Example sequence of a target being re-started.  Notice how the restart does not get any direct output:

     -> R00
     <- +
     target restarts
     -> ?
     <- +
     <- T001:1234123412341234
     -> +

Example sequence of a target being stepped by a single instruction:

     -> G1445...
     <- +
     -> s
     <- +
     time passes
     <- T001:1234123412341234
     -> +
     -> g
     <- +
     <- 1455...
     -> +

File-I/O Remote Protocol Extension
==================================

File-I/O Overview
-----------------

The File I/O remote protocol extension (short: File-I/O) allows the target to use the host's file system and console I/O to perform various system calls.  System calls on the target system are translated into a remote protocol packet to the host system, which then performs the needed actions and returns a response packet to the target system. This simulates file system operations even on targets that lack file systems.

The protocol is defined to be independent of both the host and target systems. It uses its own internal representation of datatypes and values.  Both GDBN and the target's GDBN stub are responsible for translating the system-dependent value representations into the internal protocol representations when data is transmitted.

The communication is synchronous.  A system call is possible only when GDBN is waiting for a response from the C, c, S or s packets.  While GDBN handles the request for a system call, the target is stopped to allow deterministic access to the target's memory.  Therefore File-I/O is not interruptible by target signals.  On the other hand, it is possible to interrupt File-I/O by a user interrupt (Ctrl-C) within GDBN.

The target's request to perform a host system call does not finish the latest C, c, S or s action.  That means, after finishing the system call, the target returns to continuing the previous activity (continue, step).  No additional continue or step request from GDBN is required.

     (GDBP) continue
       <- target requests 'system call X'
       target is stopped, GDBN executes system call
       -> GDBN returns result
       ... target continues, GDBN returns to wait for the target
       <- target hits breakpoint and sends a Txx packet

The protocol only supports I/O on the console and to regular files on the host file system.  Character or block special devices, pipes, named pipes, sockets or any other communication method on the host system are not supported by this protocol.

File I/O is not supported in non-stop mode.

Protocol Basics
---------------

The File-I/O protocol uses the F packet as the request as well as reply packet.  Since a File-I/O system call can only occur when GDBN is waiting for a response from the continuing or stepping target, the File-I/O request is a reply that GDBN has to expect as a result of a previous C, c, S or s packet. This F packet contains all information needed to allow GDBN to call the appropriate host system call:

  * A unique identifier for the requested system call.

  * All parameters to the system call.  Pointers are given as addresses in the target memory address space.  Pointers to strings are given as pointer/length pair.  Numerical values are given as they are. Numerical control flags are given in a protocol-specific representation.

At this point, GDBN has to perform the following actions.

  * If the parameters include pointer values to data needed as input to a system call, GDBN requests this data from the target with a standard m packet request.  This additional communication has to be expected by the target implementation and is handled as any other m packet.

  * GDBN translates all value from protocol representation to host representation as needed.  Datatypes are coerced into the host types.

  * GDBN calls the system call.

  * It then coerces datatypes back to protocol representation.

  * If the system call is expected to return data in buffer space specified by pointer parameters to the call, the data is transmitted to the target using a M or X packet.  This packet has to be expected by the target implementation and is handled as any other M or X packet.

Eventually GDBN replies with another F packet which contains all necessary information for the target to continue.  This at least contains

  * Return value.

  * errno, if has been changed by the system call.

  * ``Ctrl-C'' flag.

After having done the needed type and value coercion, the target continues the latest continue or step action.

The F Request Packet
--------------------

The F request packet has the following format:

FCALL-ID,PARAMETER...

     CALL-ID is the identifier to indicate the host system call to be called. This is just the name of the function.

     PARAMETER... are the parameters to the system call. Parameters are hexadecimal integer values, either the actual values in case of scalar datatypes, pointers to target buffer space in case of compound datatypes and unspecified memory areas, or pointer/length pairs in case of string parameters.  These are appended to the CALL-ID as a comma-delimited list.  All values are transmitted in ASCII string representation, pointer/length pairs separated by a slash.

The F Reply Packet
------------------

The F reply packet has the following format:

FRETCODE,ERRNO,CTRL-C FLAG;CALL-SPECIFIC ATTACHMENT

     RETCODE is the return code of the system call as hexadecimal value.

     ERRNO is the errno set by the call, in protocol-specific representation. This parameter can be omitted if the call was successful.

     CTRL-C FLAG is only sent if the user requested a break.  In this case, ERRNO must be sent as well, even if the call was successful. The CTRL-C FLAG itself consists of the character C:

          F0,0,C

     or, if the call was interrupted before the host call has been performed:

          F-1,4,C

     assuming 4 is the protocol-specific representation of EINTR.

The Ctrl-C Message
------------------

If the Ctrl-C flag is set in the GDBN reply packet (see The F Reply Packet), the target should behave as if it had gotten a break message.  The meaning for the target is ``system call interrupted by SIGINT''.  Consequentially, the target should actually stop (as with a break message) and return to GDBN with a T02 packet.

It's important for the target to know in which state the system call was interrupted.  There are two possible cases:

  * The system call hasn't been performed on the host yet.

  * The system call on the host has been finished.

These two states can be distinguished by the target by the value of the returned errno.  If it's the protocol representation of EINTR, the system call hasn't been performed.  This is equivalent to the EINTR handling on POSIX systems.  In any other case, the target may presume that the system call has been finished --- successfully or not --- and should behave as if the break message arrived right after the system call.

GDBN must behave reliably.  If the system call has not been called yet, GDBN may send the F reply immediately, setting EINTR as errno in the packet.  If the system call on the host has been finished before the user requests a break, the full action must be finished by GDBN.  This requires sending M or X packets as necessary. The F packet may only be sent when either nothing has happened or the full action has been completed.

Console I/O
-----------

By default and if not explicitly closed by the target system, the file descriptors 0, 1 and 2 are connected to the GDBN console.  Output on the GDBN console is handled as any other file output operation (write(1, ...) or write(2, ...)).  Console input is handled by GDBN so that after the target read request from file descriptor 0 all following typing is buffered until either one of the following conditions is met:

  * The user types Ctrl-c.  The behaviour is as explained above, and the read system call is treated as finished.

  * The user presses RET.  This is treated as end of input with a trailing newline.

  * The user types Ctrl-d.  This is treated as end of input.  No trailing character (neither newline nor Ctrl-D) is appended to the input.

If the user has typed more characters than fit in the buffer given to the read call, the trailing characters are buffered in GDBN until either another read(0, ...) is requested by the target, or debugging is stopped at the user's request.

List of Supported Calls
-----------------------

open
....

Synopsis:

          int open(const char *pathname, int flags);
          int open(const char *pathname, int flags, mode_t mode);

Request:
     Fopen,PATHPTR/LEN,FLAGS,MODE

     FLAGS is the bitwise OR of the following values:

     O_CREAT
          If the file does not exist it will be created.  The host rules apply as far as file ownership and time stamps are concerned.

     O_EXCL
          When used with O_CREAT, if the file already exists it is an error and open() fails.

     O_TRUNC
          If the file already exists and the open mode allows writing (O_RDWR or O_WRONLY is given) it will be truncated to zero length.

     O_APPEND
          The file is opened in append mode.

     O_RDONLY
          The file is opened for reading only.

     O_WRONLY
          The file is opened for writing only.

     O_RDWR
          The file is opened for reading and writing.

     Other bits are silently ignored.

     MODE is the bitwise OR of the following values:

     S_IRUSR
          User has read permission.

     S_IWUSR
          User has write permission.

     S_IRGRP
          Group has read permission.

     S_IWGRP
          Group has write permission.

     S_IROTH
          Others have read permission.

     S_IWOTH
          Others have write permission.

     Other bits are silently ignored.

Return value:
     open returns the new file descriptor or -1 if an error occurred.

Errors:

     EEXIST
          PATHNAME already exists and O_CREAT and O_EXCL were used.

     EISDIR
          PATHNAME refers to a directory.

     EACCES
          The requested access is not allowed.

     ENAMETOOLONG
          PATHNAME was too long.

     ENOENT
          A directory component in PATHNAME does not exist.

     ENODEV
          PATHNAME refers to a device, pipe, named pipe or socket.

     EROFS
          PATHNAME refers to a file on a read-only filesystem and write access was requested.

     EFAULT
          PATHNAME is an invalid pointer value.

     ENOSPC
          No space on device to create the file.

     EMFILE
          The process already has the maximum number of files open.

     ENFILE
          The limit on the total number of files open on the system has been reached.

     EINTR
          The call was interrupted by the user.

close
.....

Synopsis:

          int close(int fd);

Request:
     Fclose,FD

Return value:
     close returns zero on success, or -1 if an error occurred.

Errors:

     EBADF
          FD isn't a valid open file descriptor.

     EINTR
          The call was interrupted by the user.

read
....

Synopsis:

          int read(int fd, void *buf, unsigned int count);

Request:
     Fread,FD,BUFPTR,COUNT

Return value:
     On success, the number of bytes read is returned. Zero indicates end of file.  If count is zero, read returns zero as well.  On error, -1 is returned.

Errors:

     EBADF
          FD is not a valid file descriptor or is not open for reading.

     EFAULT
          BUFPTR is an invalid pointer value.

     EINTR
          The call was interrupted by the user.

write
.....

Synopsis:

          int write(int fd, const void *buf, unsigned int count);

Request:
     Fwrite,FD,BUFPTR,COUNT

Return value:
     On success, the number of bytes written are returned. Zero indicates nothing was written.  On error, -1 is returned.

Errors:

     EBADF
          FD is not a valid file descriptor or is not open for writing.

     EFAULT
          BUFPTR is an invalid pointer value.

     EFBIG
          An attempt was made to write a file that exceeds the host-specific maximum file size allowed.

     ENOSPC
          No space on device to write the data.

     EINTR
          The call was interrupted by the user.

lseek
.....

Synopsis:

          long lseek (int fd, long offset, int flag);

Request:
     Flseek,FD,OFFSET,FLAG

     FLAG is one of:

     SEEK_SET
          The offset is set to OFFSET bytes.

     SEEK_CUR
          The offset is set to its current location plus OFFSET bytes.

     SEEK_END
          The offset is set to the size of the file plus OFFSET bytes.

Return value:
     On success, the resulting unsigned offset in bytes from the beginning of the file is returned.  Otherwise, a value of -1 is returned.

Errors:

     EBADF
          FD is not a valid open file descriptor.

     ESPIPE
          FD is associated with the GDBN console.

     EINVAL
          FLAG is not a proper value.

     EINTR
          The call was interrupted by the user.

rename
......

Synopsis:

          int rename(const char *oldpath, const char *newpath);

Request:
     Frename,OLDPATHPTR/LEN,NEWPATHPTR/LEN

Return value:
     On success, zero is returned.  On error, -1 is returned.

Errors:

     EISDIR
          NEWPATH is an existing directory, but OLDPATH is not a directory.

     EEXIST
          NEWPATH is a non-empty directory.

     EBUSY
          OLDPATH or NEWPATH is a directory that is in use by some process.

     EINVAL
          An attempt was made to make a directory a subdirectory of itself.

     ENOTDIR
          A  component used as a directory in OLDPATH or new path is not a directory.  Or OLDPATH is a directory and NEWPATH exists but is not a directory.

     EFAULT
          OLDPATHPTR or NEWPATHPTR are invalid pointer values.

     EACCES
          No access to the file or the path of the file.

     ENAMETOOLONG

          OLDPATH or NEWPATH was too long.

     ENOENT
          A directory component in OLDPATH or NEWPATH does not exist.

     EROFS
          The file is on a read-only filesystem.

     ENOSPC
          The device containing the file has no room for the new directory entry.

     EINTR
          The call was interrupted by the user.

unlink
......

Synopsis:

          int unlink(const char *pathname);

Request:
     Funlink,PATHNAMEPTR/LEN

Return value:
     On success, zero is returned.  On error, -1 is returned.

Errors:

     EACCES
          No access to the file or the path of the file.

     EPERM
          The system does not allow unlinking of directories.

     EBUSY
          The file PATHNAME cannot be unlinked because it's being used by another process.

     EFAULT
          PATHNAMEPTR is an invalid pointer value.

     ENAMETOOLONG
          PATHNAME was too long.

     ENOENT
          A directory component in PATHNAME does not exist.

     ENOTDIR
          A component of the path is not a directory.

     EROFS
          The file is on a read-only filesystem.

     EINTR
          The call was interrupted by the user.

stat/fstat
..........

Synopsis:

          int stat(const char *pathname, struct stat *buf);
          int fstat(int fd, struct stat *buf);

Request:
     Fstat,PATHNAMEPTR/LEN,BUFPTR

     Ffstat,FD,BUFPTR

Return value:
     On success, zero is returned.  On error, -1 is returned.

Errors:

     EBADF
          FD is not a valid open file.

     ENOENT
          A directory component in PATHNAME does not exist or the path is an empty string.

     ENOTDIR
          A component of the path is not a directory.

     EFAULT
          PATHNAMEPTR is an invalid pointer value.

     EACCES
          No access to the file or the path of the file.

     ENAMETOOLONG
          PATHNAME was too long.

     EINTR
          The call was interrupted by the user.

gettimeofday
............

Synopsis:

          int gettimeofday(struct timeval *tv, void *tz);

Request:
     Fgettimeofday,TVPTR,TZPTR

Return value:
     On success, 0 is returned, -1 otherwise.

Errors:

     EINVAL
          TZ is a non-NULL pointer.

     EFAULT
          TVPTR and/or TZPTR is an invalid pointer value.

isatty
......

Synopsis:

          int isatty(int fd);

Request:
     Fisatty,FD

Return value:
     Returns 1 if FD refers to the GDBN console, 0 otherwise.

Errors:

     EINTR
          The call was interrupted by the user.

Note that the isatty call is treated as a special case: it returns 1 to the target if the file descriptor is attached to the GDBN console, 0 otherwise.  Implementing through system calls would require implementing ioctl and would be more complex than needed.

system
......

Synopsis:

          int system(const char *command);

Request:
     Fsystem,COMMANDPTR/LEN

Return value:
     If LEN is zero, the return value indicates whether a shell is available.  A zero return value indicates a shell is not available. For non-zero LEN, the value returned is -1 on error and the return status of the command otherwise.  Only the exit status of the command is returned, which is extracted from the host's system return value by calling WEXITSTATUS(retval).  In case /bin/sh could not be executed, 127 is returned.

Errors:

     EINTR
          The call was interrupted by the user.

GDBN takes over the full task of calling the necessary host calls to perform the system call.  The return value of system on the host is simplified before it's returned to the target.  Any termination signal information from the child process is discarded, and the return value consists entirely of the exit status of the called command.

Due to security concerns, the system call is by default refused by GDBN.  The user has to allow this call explicitly with the set remote system-call-allowed 1 command.

set remote system-call-allowed
     Control whether to allow the system calls in the File I/O protocol for the remote target.  The default is zero (disabled).

show remote system-call-allowed
     Show whether the system calls are allowed in the File I/O protocol.

Protocol-specific Representation of Datatypes
---------------------------------------------

Integral Datatypes
..................

The integral datatypes used in the system calls are int, unsigned int, long, unsigned long, mode_t, and time_t.

int, unsigned int, mode_t and time_t are implemented as 32 bit values in this protocol.

long and unsigned long are implemented as 64 bit types.

See Limits, for corresponding MIN and MAX values (similar to those in limits.h) to allow range checking on host and target.

time_t datatypes are defined as seconds since the Epoch.

All integral datatypes transferred as part of a memory read or write of a structured datatype e.g. a struct stat have to be given in big endian byte order.

Pointer Values
..............

Pointers to target data are transmitted as they are.  An exception is made for pointers to buffers for which the length isn't transmitted as part of the function call, namely strings.  Strings are transmitted as a pointer/length pair, both as hex values, e.g.

     1aaf/12

which is a pointer to data of length 18 bytes at position 0x1aaf. The length is defined as the full string length in bytes, including the trailing null byte.  For example, the string "hello world" at address 0x123456 is transmitted as

     123456/d

Memory Transfer
...............

Structured data which is transferred using a memory read or write (for example, a struct stat) is expected to be in a protocol-specific format with all scalar multibyte datatypes being big endian.  Translation to this representation needs to be done both by the target before the F packet is sent, and by GDBN before it transfers memory to the target.  Transferred pointers to structured data should point to the already-coerced data at any time.

struct stat
...........

The buffer of type struct stat used by the target and GDBN is defined as follows:

     struct stat {
         unsigned int  st_dev;      /* device */
         unsigned int  st_ino;      /* inode */
         mode_t        st_mode;     /* protection */
         unsigned int  st_nlink;    /* number of hard links */
         unsigned int  st_uid;      /* user ID of owner */
         unsigned int  st_gid;      /* group ID of owner */
         unsigned int  st_rdev;     /* device type (if inode device) */
         unsigned long st_size;     /* total size, in bytes */
         unsigned long st_blksize;  /* blocksize for filesystem I/O */
         unsigned long st_blocks;   /* number of blocks allocated */
         time_t        st_atime;    /* time of last access */
         time_t        st_mtime;    /* time of last modification */
         time_t        st_ctime;    /* time of last change */
     };

The integral datatypes conform to the definitions given in the appropriate section (see Integral Datatypes, for details) so this structure is of size 64 bytes.

The values of several fields have a restricted meaning and/or range of values.

st_dev
     A value of 0 represents a file, 1 the console.

st_ino
     No valid meaning for the target.  Transmitted unchanged.

st_mode
     Valid mode bits are described in Constants.  Any other bits have currently no meaning for the target.

st_uid
st_gid
st_rdev
     No valid meaning for the target.  Transmitted unchanged.

st_atime
st_mtime
st_ctime
     These values have a host and file system dependent accuracy.  Especially on Windows hosts, the file system may not support exact timing values.

The target gets a struct stat of the above representation and is responsible for coercing it to the target representation before continuing.

Note that due to size differences between the host, target, and protocol representations of struct stat members, these members could eventually get truncated on the target.

struct timeval
..............

The buffer of type struct timeval used by the File-I/O protocol is defined as follows:

     struct timeval {
         time_t tv_sec;  /* second */
         long   tv_usec; /* microsecond */
     };

The integral datatypes conform to the definitions given in the appropriate section (see Integral Datatypes, for details) so this structure is of size 8 bytes.

Constants
---------

The following values are used for the constants inside of the protocol.  GDBN and target are responsible for translating these values before and after the call as needed.

Open Flags
..........

All values are given in hexadecimal representation.

       O_RDONLY        0x0
       O_WRONLY        0x1
       O_RDWR          0x2
       O_APPEND        0x8
       O_CREAT       0x200
       O_TRUNC       0x400
       O_EXCL        0x800

mode_t Values
.............

All values are given in octal representation.

       S_IFREG       0100000
       S_IFDIR        040000
       S_IRUSR          0400
       S_IWUSR          0200
       S_IXUSR          0100
       S_IRGRP           040
       S_IWGRP           020
       S_IXGRP           010
       S_IROTH            04
       S_IWOTH            02
       S_IXOTH            01

Errno Values
............

All values are given in decimal representation.

       EPERM           1
       ENOENT          2
       EINTR           4
       EBADF           9
       EACCES         13
       EFAULT         14
       EBUSY          16
       EEXIST         17
       ENODEV         19
       ENOTDIR        20
       EISDIR         21
       EINVAL         22
       ENFILE         23
       EMFILE         24
       EFBIG          27
       ENOSPC         28
       ESPIPE         29
       EROFS          30
       ENAMETOOLONG   91
       EUNKNOWN       9999

EUNKNOWN is used as a fallback error value if a host system returns any error value not in the list of supported error numbers.

Lseek Flags
...........

       SEEK_SET      0
       SEEK_CUR      1
       SEEK_END      2

Limits
......

All values are given in decimal representation.

       INT_MIN       -2147483648
       INT_MAX        2147483647
       UINT_MAX       4294967295
       LONG_MIN      -9223372036854775808
       LONG_MAX       9223372036854775807
       ULONG_MAX      18446744073709551615

File-I/O Examples
-----------------

Example sequence of a write call, file descriptor 3, buffer is at target address 0x1234, 6 bytes should be written:

     <- Fwrite,3,1234,6
     request memory read from target
     -> m1234,6
     <- XXXXXX
     return "6 bytes written"
     -> F6

Example sequence of a read call, file descriptor 3, buffer is at target address 0x1234, 6 bytes should be read:

     <- Fread,3,1234,6
     request memory write to target
     -> X1234,6:XXXXXX
     return "6 bytes read"
     -> F6

Example sequence of a read call, call fails on the host due to invalid file descriptor (EBADF):

     <- Fread,3,1234,6
     -> F-1,9

Example sequence of a read call, user presses Ctrl-c before syscall on host is called:

     <- Fread,3,1234,6
     -> F-1,4,C
     <- T02

Example sequence of a read call, user presses Ctrl-c after syscall on host is called:

     <- Fread,3,1234,6
     -> X1234,6:XXXXXX
     <- T02

Library List Format
===================

On some platforms, a dynamic loader (e.g. ld.so) runs in the same process as your application to manage libraries.  In this case, GDBN can use the loader's symbol table and normal memory operations to maintain a list of shared libraries.  On other platforms, the operating system manages loaded libraries. GDBN can not retrieve the list of currently loaded libraries through memory operations, so it uses the qXfer:libraries:read packet (see qXfer library list read) instead.  The remote stub queries the target's operating system and reports which libraries are loaded.

The qXfer:libraries:read packet returns an XML document which lists loaded libraries and their offsets.  Each library has an associated name and one or more segment or section base addresses, which report where the library was loaded in memory.

For the common case of libraries that are fully linked binaries, the library should have a list of segments.  If the target supports dynamic linking of a relocatable object file, its library XML element should instead include a list of allocated sections.  The segment or section bases are start addresses, not relocation offsets; they do not depend on the library's link-time base addresses.

GDBN must be linked with the Expat library to support XML library lists.  See Expat.

A simple memory map, with one loaded library relocated by a single offset, looks like this:

     <library-list>
       <library name="/lib/libc.so.6">
         <segment address="0x10000000"/>
       </library>
     </library-list>

Another simple memory map, with one loaded library with three allocated sections (.text, .data, .bss), looks like this:

     <library-list>
       <library name="sharedlib.o">
         <section address="0x10000000"/>
         <section address="0x20000000"/>
         <section address="0x30000000"/>
       </library>
     </library-list>

The format of a library list is described by this DTD:

     <!-- library-list: Root element with versioning -->
     <!ELEMENT library-list  (library)*>
     <!ATTLIST library-list  version CDATA   #FIXED  "1.0">
     <!ELEMENT library       (segment*, section*)>
     <!ATTLIST library       name    CDATA   #REQUIRED>
     <!ELEMENT segment       EMPTY>
     <!ATTLIST segment       address CDATA   #REQUIRED>
     <!ELEMENT section       EMPTY>
     <!ATTLIST section       address CDATA   #REQUIRED>

In addition, segments and section descriptors cannot be mixed within a single library element, and you must supply at least one segment or section for each library.

Library List Format for SVR4 Targets
====================================

On SVR4 platforms GDBN can use the symbol table of a dynamic loader (e.g. ld.so) and normal memory operations to maintain a list of shared libraries.  Still a special library list provided by this packet is more efficient for the GDBN remote protocol.

The qXfer:libraries-svr4:read packet returns an XML document which lists loaded libraries and their SVR4 linker parameters.  For each library on SVR4 target, the following parameters are reported:

  - name, the absolute file name from the l_name field of struct link_map.

  - lm with address of struct link_map used for TLS (Thread Local Storage) access.

  - l_addr, the displacement as read from the field l_addr of struct link_map.  For prelinked libraries this is not an absolute memory address.  It is a displacement of absolute memory address against address the file was prelinked to during the library load.

  - l_ld, which is memory address of the PT_DYNAMIC segment

Additionally the single main-lm attribute specifies address of struct link_map used for the main executable.  This parameter is used for TLS access and its presence is optional.

GDBN must be linked with the Expat library to support XML SVR4 library lists.  See Expat.

A simple memory map, with two loaded libraries (which do not use prelink), looks like this:

     <library-list-svr4 version="1.0" main-lm="0xe4f8f8">
       <library name="/lib/ld-linux.so.2" lm="0xe4f51c" l_addr="0xe2d000"
                l_ld="0xe4eefc"/>
       <library name="/lib/libc.so.6" lm="0xe4fbe8" l_addr="0x154000"
                l_ld="0x152350"/>
     </library-list-svr>

The format of an SVR4 library list is described by this DTD:

     <!-- library-list-svr4: Root element with versioning -->
     <!ELEMENT library-list-svr4  (library)*>
     <!ATTLIST library-list-svr4  version CDATA   #FIXED  "1.0">
     <!ATTLIST library-list-svr4  main-lm CDATA   #IMPLIED>
     <!ELEMENT library            EMPTY>
     <!ATTLIST library            name    CDATA   #REQUIRED>
     <!ATTLIST library            lm      CDATA   #REQUIRED>
     <!ATTLIST library            l_addr  CDATA   #REQUIRED>
     <!ATTLIST library            l_ld    CDATA   #REQUIRED>

Memory Map Format
=================

To be able to write into flash memory, GDBN needs to obtain a memory map from the target.  This section describes the format of the memory map.

The memory map is obtained using the qXfer:memory-map:read (see qXfer memory map read) packet and is an XML document that lists memory regions.

GDBN must be linked with the Expat library to support XML memory maps.  See Expat.

The top-level structure of the document is shown below:

     <?xml version="1.0"?>
     <!DOCTYPE memory-map
               PUBLIC "+//IDN gnu.org//DTD GDB Memory Map V1.0//EN"
                      "http://sourceware.org/gdb/gdb-memory-map.dtd">
     <memory-map>
         region...
     </memory-map>

Each region can be either:

  * A region of RAM starting at ADDR and extending for LENGTH bytes from there:

          <memory type="ram" start="ADDR" length="LENGTH"/>

  * A region of read-only memory:

          <memory type="rom" start="ADDR" length="LENGTH"/>

  * A region of flash memory, with erasure blocks BLOCKSIZE bytes in length:

          <memory type="flash" start="ADDR" length="LENGTH">
            <property name="blocksize">BLOCKSIZE</property>
          </memory>

Regions must not overlap.  GDBN assumes that areas of memory not covered by the memory map are RAM, and uses the ordinary M and X packets to write to addresses in such ranges.

The formal DTD for memory map format is given below:

     <!-- ................................................... -->
     <!-- Memory Map XML DTD ................................ -->
     <!-- File: memory-map.dtd .............................. -->
     <!-- .................................... .............. -->
     <!-- memory-map.dtd -->
     <!-- memory-map: Root element with versioning -->
     <!ELEMENT memory-map (memory | property)>
     <!ATTLIST memory-map    version CDATA   #FIXED  "1.0.0">
     <!ELEMENT memory (property)>
     <!-- memory: Specifies a memory region,
                  and its type, or device. -->
     <!ATTLIST memory        type    CDATA   #REQUIRED
                             start   CDATA   #REQUIRED
                             length  CDATA   #REQUIRED
                             device  CDATA   #IMPLIED>
     <!-- property: Generic attribute tag -->
     <!ELEMENT property (#PCDATA | property)*>
     <!ATTLIST property      name    CDATA   #REQUIRED>

Thread List Format
==================

To efficiently update the list of threads and their attributes, GDBN issues the qXfer:threads:read packet (see qXfer threads read) and obtains the XML document with the following structure:

     <?xml version="1.0"?>
     <threads>
         <thread id="id" core="0">
         ... description ...
         </thread>
     </threads>

Each thread element must have the id attribute that identifies the thread (see thread-id syntax).  The core attribute, if present, specifies which processor core the thread was last executing on.  The content of the of thread element is interpreted as human-readable auxilliary information.

Traceframe Info Format
======================

To be able to know which objects in the inferior can be examined when inspecting a tracepoint hit, GDBN needs to obtain the list of memory ranges, registers and trace state variables that have been collected in a traceframe.

This list is obtained using the qXfer:traceframe-info:read (see qXfer traceframe info read) packet and is an XML document.

GDBN must be linked with the Expat library to support XML traceframe info discovery.  See Expat.

The top-level structure of the document is shown below:

     <?xml version="1.0"?>
     <!DOCTYPE traceframe-info
               PUBLIC "+//IDN gnu.org//DTD GDB Memory Map V1.0//EN"
                      "http://sourceware.org/gdb/gdb-traceframe-info.dtd">
     <traceframe-info>
        block...
     </traceframe-info>

Each traceframe block can be either:

  * A region of collected memory starting at ADDR and extending for LENGTH bytes from there:

          <memory start="ADDR" length="LENGTH"/>

The formal DTD for the traceframe info format is given below:

     <!ELEMENT traceframe-info  (memory)* >
     <!ATTLIST traceframe-info  version CDATA   #FIXED  "1.0">

     <!ELEMENT memory        EMPTY>
     <!ATTLIST memory        start   CDATA   #REQUIRED
                             length  CDATA   #REQUIRED>

     [file agentexpr.texi not included]

Target Descriptions
*******************

One of the challenges of using GDBN to debug embedded systems is that there are so many minor variants of each processor architecture in use.  It is common practice for vendors to start with a standard processor core --- ARM, PowerPC, or MIPS, for example --- and then make changes to adapt it to a particular market niche.  Some architectures have hundreds of variants, available from dozens of vendors.  This leads to a number of problems:

  * With so many different customized processors, it is difficult for the GDBN maintainers to keep up with the changes.

  * Since individual variants may have short lifetimes or limited audiences, it may not be worthwhile to carry information about every variant in the GDBN source tree.

  * When GDBN does support the architecture of the embedded system at hand, the task of finding the correct architecture name to give the set architecture command can be error-prone.

To address these problems, the GDBN remote protocol allows a target system to not only identify itself to GDBN, but to actually describe its own features.  This lets GDBN support processor variants it has never seen before --- to the extent that the descriptions are accurate, and that GDBN understands them.

GDBN must be linked with the Expat library to support XML target descriptions.  See Expat.

Retrieving Descriptions
=======================

Target descriptions can be read from the target automatically, or specified by the user manually.  The default behavior is to read the description from the target.  GDBN retrieves it via the remote protocol using qXfer requests (see qXfer).  The ANNEX in the qXfer packet will be target.xml.  The contents of the target.xml annex are an XML document, of the form described in Target Description Format.

Alternatively, you can specify a file to read for the target description. If a file is set, the target will not be queried.  The commands to specify a file are:

set tdesc filename PATH
     Read the target description from PATH.

unset tdesc filename
     Do not read the XML target description from a file.  GDBN will use the description supplied by the current target.

show tdesc filename
     Show the filename to read for a target description, if any.

Target Description Format
=========================

A target description annex is an XML (http://www.w3.org/XML/) document which complies with the Document Type Definition provided in the GDBN sources in gdb/features/gdb-target.dtd.  This means you can use generally available tools like xmllint to check that your feature descriptions are well-formed and valid. However, to help people unfamiliar with XML write descriptions for their targets, we also describe the grammar here.

Target descriptions can identify the architecture of the remote target and (for some architectures) provide information about custom register sets.  They can also identify the OS ABI of the remote target. GDBN can use this information to autoconfigure for your target, or to warn you if you connect to an unsupported target.

Here is a simple target description:

     <target version="1.0">
       <architecture>i386:x86-64</architecture>
     </target>

This minimal description only says that the target uses the x86-64 architecture.

A target description has the following overall form, with [ ] marking optional elements and ... marking repeatable elements.  The elements are explained further below.

     <?xml version="1.0"?>
     <!DOCTYPE target SYSTEM "gdb-target.dtd">
     <target version="1.0">
       [ARCHITECTURE]
       [OSABI]
       [COMPATIBLE]
       [FEATURE...]
     </target>

The description is generally insensitive to whitespace and line breaks, under the usual common-sense rules.  The XML version declaration and document type declaration can generally be omitted (GDBN does not require them), but specifying them may be useful for XML validation tools.  The version attribute for <target> may also be omitted, but we recommend including it; if future versions of GDBN use an incompatible revision of gdb-target.dtd, they will detect and report the version mismatch.

Inclusion
---------

It can sometimes be valuable to split a target description up into several different annexes, either for organizational purposes, or to share files between different possible target descriptions.  You can divide a description into multiple files by replacing any element of the target description with an inclusion directive of the form:

     <xi:include href="DOCUMENT"/>

When GDBN encounters an element of this form, it will retrieve the named XML DOCUMENT, and replace the inclusion directive with the contents of that document.  If the current description was read using qXfer, then so will be the included document; DOCUMENT will be interpreted as the name of an annex.  If the current description was read from a file, GDBN will look for DOCUMENT as a file in the same directory where it found the original description.

Architecture
------------

An <architecture> element has this form:

       <architecture>ARCH</architecture>

ARCH is one of the architectures from the set accepted by set architecture (see Specifying a Debugging Target).

OS ABI
------

This optional field was introduced in GDBN version 7.0. Previous versions of GDBN ignore it.

An <osabi> element has this form:

       <osabi>ABI-NAME</osabi>

ABI-NAME is an OS ABI name from the same selection accepted by set osabi (see Configuring the Current ABI).

Compatible Architecture
-----------------------

This optional field was introduced in GDBN version 7.0. Previous versions of GDBN ignore it.

A <compatible> element has this form:

       <compatible>ARCH</compatible>

ARCH is one of the architectures from the set accepted by set architecture (see Specifying a Debugging Target).

A <compatible> element is used to specify that the target is able to run binaries in some other than the main target architecture given by the <architecture> element.  For example, on the Cell Broadband Engine, the main architecture is powerpc:common or powerpc:common64, but the system is able to run binaries in the spu architecture as well.  The way to describe this capability with <compatible> is as follows:

       <architecture>powerpc:common</architecture>
       <compatible>spu</compatible>

Features
--------

Each <feature> describes some logical portion of the target system.  Features are currently used to describe available CPU registers and the types of their contents.  A <feature> element has this form:

     <feature name="NAME">
       [TYPE...]
       REG...
     </feature>

Each feature's name should be unique within the description.  The name of a feature does not matter unless GDBN has some special knowledge of the contents of that feature; if it does, the feature should have its standard name.  See Standard Target Features.

Types
-----

Any register's value is a collection of bits which GDBN must interpret.  The default interpretation is a two's complement integer, but other types can be requested by name in the register description. Some predefined types are provided by GDBN (see Predefined Target Types), and the description can define additional composite types.

Each type element must have an id attribute, which gives a unique (within the containing <feature>) name to the type. Types must be defined before they are used.

Some targets offer vector registers, which can be treated as arrays of scalar elements.  These types are written as <vector> elements, specifying the array element type, TYPE, and the number of elements, COUNT:

     <vector id="ID" type="TYPE" count="COUNT"/>

If a register's value is usefully viewed in multiple ways, define it with a union type containing the useful representations.  The <union> element contains one or more <field> elements, each of which has a NAME and a TYPE:

     <union id="ID">
       <field name="NAME" type="TYPE"/>
       ...
     </union>

If a register's value is composed from several separate values, define it with a structure type.  There are two forms of the <struct> element; a <struct> element must either contain only bitfields or contain no bitfields.  If the structure contains only bitfields, its total size in bytes must be specified, each bitfield must have an explicit start and end, and bitfields are automatically assigned an integer type.  The field's START should be less than or equal to its END, and zero represents the least significant bit.

     <struct id="ID" size="SIZE">
       <field name="NAME" start="START" end="END"/>
       ...
     </struct>

If the structure contains no bitfields, then each field has an explicit type, and no implicit padding is added.

     <struct id="ID">
       <field name="NAME" type="TYPE"/>
       ...
     </struct>

If a register's value is a series of single-bit flags, define it with a flags type.  The <flags> element has an explicit SIZE and contains one or more <field> elements.  Each field has a NAME, a START, and an END.  Only single-bit flags are supported.

     <flags id="ID" size="SIZE">
       <field name="NAME" start="START" end="END"/>
       ...
     </flags>

Registers
---------

Each register is represented as an element with this form:

     <reg name="NAME"
          bitsize="SIZE"
          [regnum="NUM"]
          [save-restore="SAVE-RESTORE"]
          [type="TYPE"]
          [group="GROUP"]/>

The components are as follows:

name
     The register's name; it must be unique within the target description.

bitsize
     The register's size, in bits.

regnum
     The register's number.  If omitted, a register's number is one greater than that of the previous register (either in the current feature or in a preceding feature); the first register in the target description defaults to zero.  This register number is used to read or write the register; e.g. it is used in the remote p and P packets, and registers appear in the g and G packets in order of increasing register number.

save-restore
     Whether the register should be preserved across inferior function calls; this must be either yes or no.  The default is yes, which is appropriate for most registers except for some system control registers; this is not related to the target's ABI.

type
     The type of the register.  TYPE may be a predefined type, a type defined in the current feature, or one of the special types int and float.  int is an integer type of the correct size for BITSIZE, and float is a floating point type (in the architecture's normal floating point format) of the correct size for BITSIZE.  The default is int.

group
     The register group to which this register belongs.  GROUP must be either general, float, or vector.  If no GROUP is specified, GDBN will not display the register in info registers.

Predefined Target Types
=======================

Type definitions in the self-description can build up composite types from basic building blocks, but can not define fundamental types.  Instead, standard identifiers are provided by GDBN for the fundamental types.  The currently supported types are:

int8
int16
int32
int64
int128
     Signed integer types holding the specified number of bits.

uint8
uint16
uint32
uint64
uint128
     Unsigned integer types holding the specified number of bits.

code_ptr
data_ptr
     Pointers to unspecified code and data.  The program counter and any dedicated return address register may be marked as code pointers; printing a code pointer converts it into a symbolic address.  The stack pointer and any dedicated address registers may be marked as data pointers.

ieee_single
     Single precision IEEE floating point.

ieee_double
     Double precision IEEE floating point.

arm_fpa_ext
     The 12-byte extended precision format used by ARM FPA registers.

i387_ext
     The 10-byte extended precision format used by x87 registers.

i386_eflags
     32bit EFLAGS register used by x86.

i386_mxcsr
     32bit MXCSR register used by x86.

Standard Target Features
========================

A target description must contain either no registers or all the target's registers.  If the description contains no registers, then GDBN will assume a default register layout, selected based on the architecture.  If the description contains any registers, the default layout will not be used; the standard registers must be described in the target description, in such a way that GDBN can recognize them.

This is accomplished by giving specific names to feature elements which contain standard registers.  GDBN will look for features with those names and verify that they contain the expected registers; if any known feature is missing required registers, or if any required feature is missing, GDBN will reject the target description.  You can add additional registers to any of the standard features --- GDBN will display them just as if they were added to an unrecognized feature.

This section lists the known features and their expected contents. Sample XML documents for these features are included in the GDBN source tree, in the directory gdb/features.

Names recognized by GDBN should include the name of the company or organization which selected the name, and the overall architecture to which the feature applies; so e.g. the feature containing ARM core registers is named org.gnu.gdb.arm.core.

The names of registers are not case sensitive for the purpose of recognizing standard features, but GDBN will only display registers using the capitalization used in the description.

ARM Features
------------

The org.gnu.gdb.arm.core feature is required for non-M-profile ARM targets. It should contain registers r0 through r13, sp, lr, pc, and cpsr.

For M-profile targets (e.g. Cortex-M3), the org.gnu.gdb.arm.core feature is replaced by org.gnu.gdb.arm.m-profile.  It should contain registers r0 through r13, sp, lr, pc, and xpsr.

The org.gnu.gdb.arm.fpa feature is optional.  If present, it should contain registers f0 through f7 and fps.

The org.gnu.gdb.xscale.iwmmxt feature is optional.  If present, it should contain at least registers wR0 through wR15 and wCGR0 through wCGR3.  The wCID, wCon, wCSSF, and wCASF registers are optional.

The org.gnu.gdb.arm.vfp feature is optional.  If present, it should contain at least registers d0 through d15.  If they are present, d16 through d31 should also be included. GDBN will synthesize the single-precision registers from halves of the double-precision registers.

The org.gnu.gdb.arm.neon feature is optional.  It does not need to contain registers; it instructs GDBN to display the VFP double-precision registers as vectors and to synthesize the quad-precision registers from pairs of double-precision registers. If this feature is present, org.gnu.gdb.arm.vfp must also be present and include 32 double-precision registers.

i386 Features
-------------

The org.gnu.gdb.i386.core feature is required for i386/amd64 targets.  It should describe the following registers:

  - eax through edi plus eip for i386

  - rax through r15 plus rip for amd64

  - eflags, cs, ss, ds, es, fs, gs

  - st0 through st7

  - fctrl, fstat, ftag, fiseg, fioff, foseg, fooff and fop

The register sets may be different, depending on the target.

The org.gnu.gdb.i386.sse feature is optional.  It should describe registers:

  - xmm0 through xmm7 for i386

  - xmm0 through xmm15 for amd64

  - mxcsr

The org.gnu.gdb.i386.avx feature is optional and requires the org.gnu.gdb.i386.sse feature.  It should describe the upper 128 bits of YMM registers:

  - ymm0h through ymm7h for i386

  - ymm0h through ymm15h for amd64

The org.gnu.gdb.i386.linux feature is optional.  It should describe a single register, orig_eax.

MIPS Features
-------------

The org.gnu.gdb.mips.cpu feature is required for MIPS targets. It should contain registers r0 through r31, lo, hi, and pc.  They may be 32-bit or 64-bit depending on the target.

The org.gnu.gdb.mips.cp0 feature is also required.  It should contain at least the status, badvaddr, and cause registers.  They may be 32-bit or 64-bit depending on the target.

The org.gnu.gdb.mips.fpu feature is currently required, though it may be optional in a future version of GDBN.  It should contain registers f0 through f31, fcsr, and fir.  They may be 32-bit or 64-bit depending on the target.

The org.gnu.gdb.mips.linux feature is optional.  It should contain a single register, restart, which is used by the Linux kernel to control restartable syscalls.

M68K Features
-------------

org.gnu.gdb.m68k.core
org.gnu.gdb.coldfire.core
org.gnu.gdb.fido.core
     One of those features must be always present. The feature that is present determines which flavor of m68k is used.  The feature that is present should contain registers d0 through d7, a0 through a5, fp, sp, ps and pc.

org.gnu.gdb.coldfire.fp
     This feature is optional.  If present, it should contain registers fp0 through fp7, fpcontrol, fpstatus and fpiaddr.

PowerPC Features
----------------

The org.gnu.gdb.power.core feature is required for PowerPC targets.  It should contain registers r0 through r31, pc, msr, cr, lr, ctr, and xer.  They may be 32-bit or 64-bit depending on the target.

The org.gnu.gdb.power.fpu feature is optional.  It should contain registers f0 through f31 and fpscr.

The org.gnu.gdb.power.altivec feature is optional.  It should contain registers vr0 through vr31, vscr, and vrsave.

The org.gnu.gdb.power.vsx feature is optional.  It should contain registers vs0h through vs31h.  GDBN will combine these registers with the floating point registers (f0 through f31) and the altivec registers (vr0 through vr31) to present the 128-bit wide registers vs0 through vs63, the set of vector registers for POWER7.

The org.gnu.gdb.power.spe feature is optional.  It should contain registers ev0h through ev31h, acc, and spefscr.  SPE targets should provide 32-bit registers in org.gnu.gdb.power.core and provide the upper halves in ev0h through ev31h.  GDBN will combine these to present registers ev0 through ev31 to the user.

TMS320C6x Features
------------------

The org.gnu.gdb.tic6x.core feature is required for TMS320C6x targets.  It should contain registers A0 through A15, registers B0 through B15, CSR and PC.

The org.gnu.gdb.tic6x.gp feature is optional.  It should contain registers A16 through A31 and B16 through B31.

The org.gnu.gdb.tic6x.c6xp feature is optional.  It should contain registers TSR, ILC and RILC.

Operating System Information
****************************

Users of GDBN often wish to obtain information about the state of the operating system running on the target---for example the list of processes, or the list of open files.  This section describes the mechanism that makes it possible.  This mechanism is similar to the target features mechanism (see Target Descriptions), but focuses on a different aspect of target.

Operating system information is retrived from the target via the remote protocol, using qXfer requests (see qXfer osdata read).  The object name in the request should be osdata, and the ANNEX identifies the data to be fetched.

Process list
============

When requesting the process list, the ANNEX field in the qXfer request should be processes.  The returned data is an XML document.  The formal syntax of this document is defined in gdb/features/osdata.dtd.

An example document is:

     <?xml version="1.0"?>
     <!DOCTYPE target SYSTEM "osdata.dtd">
     <osdata type="processes">
       <item>
         <column name="pid">1</column>
         <column name="user">root</column>
         <column name="command">/sbin/init</column>
         <column name="cores">1,2,3</column>
       </item>
     </osdata>

Each item should include a column whose name is pid.  The value of that column should identify the process on the target.  The user and command columns are optional, and will be displayed by GDBN.  The cores column, if present, should contain a comma-separated list of cores that this process is running on.  Target may provide additional columns, which GDBN currently ignores.

Trace File Format
*****************

The trace file comes in three parts: a header, a textual description section, and a trace frame section with binary data.

The header has the form \x7fTRACE0\n.  The first byte is 0x7f so as to indicate that the file contains binary data, while the 0 is a version number that may have different values in the future.

The description section consists of multiple lines of ASCII text separated by newline characters (0xa).  The lines may include a variety of optional descriptive or context-setting information, such as tracepoint definitions or register set size.  GDBN will ignore any line that it does not recognize.  An empty line marks the end of this section.

The trace frame section consists of a number of consecutive frames. Each frame begins with a two-byte tracepoint number, followed by a four-byte size giving the amount of data in the frame.  The data in the frame consists of a number of blocks, each introduced by a character indicating its type (at least register, memory, and trace state variable).  The data in this section is raw binary, not a hexadecimal or other encoding; its endianness matches the target's endianness.

R BYTES
     Register block.  The number and ordering of bytes matches that of a g packet in the remote protocol.  Note that these are the actual bytes, in target order and GDBN register order, not a hexadecimal encoding.

M ADDRESS LENGTH BYTES...
     Memory block.  This is a contiguous block of memory, at the 8-byte address ADDRESS, with a 2-byte length LENGTH, followed by LENGTH bytes.

V NUMBER VALUE
     Trace state variable block.  This records the 8-byte signed value VALUE of trace state variable numbered NUMBER.

Future enhancements of the trace file format may include additional types of blocks.

.gdb_index section format
*************************

This section documents the index section that is created by save gdb-index (see Index Files).  The index section is DWARF-specific; some knowledge of DWARF is assumed in this description.

The mapped index file format is designed to be directly mmapable on any architecture.  In most cases, a datum is represented using a little-endian 32-bit integer value, called an offset_type.  Big endian machines must byte-swap the values before using them.  Exceptions to this rule are noted.  The data is laid out such that alignment is always respected.

A mapped index consists of several areas, laid out in order.

 1. The file header.  This is a sequence of values, of offset_type unless otherwise noted:

      1. The version number, currently 5.  Versions 1, 2 and 3 are obsolete. Version 4 differs by its hashing function.

      2. The offset, from the start of the file, of the CU list.

      3. The offset, from the start of the file, of the types CU list.  Note that this area can be empty, in which case this offset will be equal to the next offset.

      4. The offset, from the start of the file, of the address area.

      5. The offset, from the start of the file, of the symbol table.

      6. The offset, from the start of the file, of the constant pool.

 2. The CU list.  This is a sequence of pairs of 64-bit little-endian values, sorted by the CU offset.  The first element in each pair is the offset of a CU in the .debug_info section.  The second element in each pair is the length of that CU.  References to a CU elsewhere in the map are done using a CU index, which is just the 0-based index into this table.  Note that if there are type CUs, then conceptually CUs and type CUs form a single list for the purposes of CU indices.

 3. The types CU list.  This is a sequence of triplets of 64-bit little-endian values.  In a triplet, the first value is the CU offset, the second value is the type offset in the CU, and the third value is the type signature.  The types CU list is not sorted.

 4. The address area.  The address area consists of a sequence of address entries.  Each address entry has three elements:

      1. The low address.  This is a 64-bit little-endian value.

      2. The high address.  This is a 64-bit little-endian value.  Like DW_AT_high_pc, the value is one byte beyond the end.

      3. The CU index.  This is an offset_type value.

 5. The symbol table.  This is an open-addressed hash table.  The size of the hash table is always a power of 2.

     Each slot in the hash table consists of a pair of offset_type values.  The first value is the offset of the symbol's name in the constant pool.  The second value is the offset of the CU vector in the constant pool.

     If both values are 0, then this slot in the hash table is empty.  This is ok because while 0 is a valid constant pool index, it cannot be a valid index for both a string and a CU vector.

     The hash value for a table entry is computed by applying an iterative hash function to the symbol's name.  Starting with an initial value of r = 0, each (unsigned) character c in the string is incorporated into the hash using the formula depending on the index version:

     Version 4
          The formula is r = r * 67 + c - 113.

     Version 5
          The formula is r = r * 67 + tolower (c) - 113.

     The terminating \0 is not incorporated into the hash.

     The step size used in the hash table is computed via ((hash * 17) & (size - 1)) | 1, where hash is the hash value, and size is the size of the hash table.  The step size is used to find the next candidate slot when handling a hash collision.

     The names of C++ symbols in the hash table are canonicalized.  We don't currently have a simple description of the canonicalization algorithm; if you intend to create new index sections, you must read the code.

 6. The constant pool.  This is simply a bunch of bytes.  It is organized so that alignment is correct: CU vectors are stored first, followed by strings.

     A CU vector in the constant pool is a sequence of offset_type values.  The first value is the number of CU indices in the vector. Each subsequent value is the index of a CU in the CU list.  This element in the hash table is used to indicate which CUs define the symbol.

     A string in the constant pool is zero-terminated.

     [file gpl.texi not included]

GNU Free Documentation License
******************************

     [file fdl.texi not included]