2 @setfilename stabs.info
9 * Stabs:: The "stabs" debugging information format.
15 This document describes the stabs debugging symbol tables.
17 Copyright 1992, 1993 Free Software Foundation, Inc.
18 Contributed by Cygnus Support. Written by Julia Menapace.
20 Permission is granted to make and distribute verbatim copies of
21 this manual provided the copyright notice and this permission notice
22 are preserved on all copies.
25 Permission is granted to process this file through Tex and print the
26 results, provided the printed document carries copying permission
27 notice identical to this one except for the removal of this paragraph
28 (this paragraph not being relevant to the printed manual).
31 Permission is granted to copy or distribute modified versions of this
32 manual under the terms of the GPL (for which purpose this text may be
33 regarded as a program in the language TeX).
36 @setchapternewpage odd
39 @title The ``stabs'' debug format
40 @author Julia Menapace, Jim Kingdon, David MacKenzie
41 @author Cygnus Support
44 \def\$#1${{#1}} % Kluge: collect RCS revision info without $...$
45 \xdef\manvers{\$Revision$} % For use in headers, footers too
47 \hfill Cygnus Support\par
49 \hfill \TeX{}info \texinfoversion\par
53 @vskip 0pt plus 1filll
54 Copyright @copyright{} 1992, 1993 Free Software Foundation, Inc.
55 Contributed by Cygnus Support.
57 Permission is granted to make and distribute verbatim copies of
58 this manual provided the copyright notice and this permission notice
59 are preserved on all copies.
65 @top The "stabs" representation of debugging information
67 This document describes the stabs debugging format.
70 * Overview:: Overview of stabs
71 * Program Structure:: Encoding of the structure of the program
72 * Constants:: Constants
74 * Types:: Type definitions
75 * Symbol Tables:: Symbol information in symbol tables
76 * Cplusplus:: Appendixes:
77 * Stab Types:: Symbol types in a.out files
78 * Symbol Descriptors:: Table of symbol descriptors
79 * Type Descriptors:: Table of type descriptors
80 * Expanded Reference:: Reference information by stab type
81 * Questions:: Questions and anomolies
82 * XCOFF Differences:: Differences between GNU stabs in a.out
83 and GNU stabs in XCOFF
84 * Sun Differences:: Differences between GNU stabs and Sun
86 * Stabs In ELF:: Stabs in an ELF file.
87 * Symbol Types Index:: Index of symbolic stab symbol type names.
93 @chapter Overview of Stabs
95 @dfn{Stabs} refers to a format for information that describes a program
96 to a debugger. This format was apparently invented by
97 @c FIXME! <<name of inventor>> at
98 the University of California at Berkeley, for the @code{pdx} Pascal
99 debugger; the format has spread widely since then.
101 This document is one of the few published sources of documentation on
102 stabs. It is believed to be comprehensive for stabs used by C. The
103 lists of symbol descriptors (@pxref{Symbol Descriptors}) and type
104 descriptors (@pxref{Type Descriptors}) are believed to be completely
105 comprehensive. Stabs for COBOL-specific features and for variant
106 records (used by Pascal and Modula-2) are poorly documented here.
108 Other sources of information on stabs are @cite{Dbx and Dbxtool
109 Interfaces}, 2nd edition, by Sun, 1988, and @cite{AIX Version 3.2 Files
110 Reference}, Fourth Edition, September 1992, "dbx Stabstring Grammar" in
111 the a.out section, page 2-31. This document is believed to incorporate
112 the information from those two sources except where it explictly directs
113 you to them for more information.
116 * Flow:: Overview of debugging information flow
117 * Stabs Format:: Overview of stab format
118 * String Field:: The string field
119 * C Example:: A simple example in C source
120 * Assembly Code:: The simple example at the assembly level
124 @section Overview of Debugging Information Flow
126 The GNU C compiler compiles C source in a @file{.c} file into assembly
127 language in a @file{.s} file, which the assembler translates into
128 a @file{.o} file, which the linker combines with other @file{.o} files and
129 libraries to produce an executable file.
131 With the @samp{-g} option, GCC puts in the @file{.s} file additional
132 debugging information, which is slightly transformed by the assembler
133 and linker, and carried through into the final executable. This
134 debugging information describes features of the source file like line
135 numbers, the types and scopes of variables, and function names,
136 parameters, and scopes.
138 For some object file formats, the debugging information is encapsulated
139 in assembler directives known collectively as @dfn{stab} (symbol table)
140 directives, which are interspersed with the generated code. Stabs are
141 the native format for debugging information in the a.out and XCOFF
142 object file formats. The GNU tools can also emit stabs in the COFF and
143 ECOFF object file formats.
145 The assembler adds the information from stabs to the symbol information
146 it places by default in the symbol table and the string table of the
147 @file{.o} file it is building. The linker consolidates the @file{.o}
148 files into one executable file, with one symbol table and one string
149 table. Debuggers use the symbol and string tables in the executable as
150 a source of debugging information about the program.
153 @section Overview of Stab Format
155 There are three overall formats for stab assembler directives,
156 differentiated by the first word of the stab. The name of the directive
157 describes which combination of four possible data fields follows. It is
158 either @code{.stabs} (string), @code{.stabn} (number), or @code{.stabd}
159 (dot). IBM's XCOFF assembler uses @code{.stabx} (and some other
160 directives such as @code{.file} and @code{.bi}) instead of
161 @code{.stabs}, @code{.stabn} or @code{.stabd}.
163 The overall format of each class of stab is:
166 .stabs "@var{string}",@var{type},@var{other},@var{desc},@var{value}
167 .stabn @var{type},@var{other},@var{desc},@var{value}
168 .stabd @var{type},@var{other},@var{desc}
169 .stabx "@var{string}",@var{value},@var{type},@var{sdb-type}
172 @c what is the correct term for "current file location"? My AIX
173 @c assembler manual calls it "the value of the current location counter".
174 For @code{.stabn} and @code{.stabd}, there is no @var{string} (the
175 @code{n_strx} field is zero; see @ref{Symbol Tables}). For
176 @code{.stabd}, the @var{value} field is implicit and has the value of
177 the current file location. For @code{.stabx}, the @var{sdb-type} field
178 is unused for stabs and can always be set to zero. The @var{other}
179 field is almost always unused and can be set to zero.
181 The number in the @var{type} field gives some basic information about
182 which type of stab this is (or whether it @emph{is} a stab, as opposed
183 to an ordinary symbol). Each valid type number defines a different stab
184 type; further, the stab type defines the exact interpretation of, and
185 possible values for, any remaining @var{string}, @var{desc}, or
186 @var{value} fields present in the stab. @xref{Stab Types}, for a list
187 in numeric order of the valid @var{type} field values for stab directives.
190 @section The String Field
192 For most stabs the string field holds the meat of the
193 debugging information. The flexible nature of this field
194 is what makes stabs extensible. For some stab types the string field
195 contains only a name. For other stab types the contents can be a great
198 The overall format of the string field for most stab types is:
201 "@var{name}:@var{symbol-descriptor} @var{type-information}"
204 @var{name} is the name of the symbol represented by the stab.
205 @var{name} can be omitted, which means the stab represents an unnamed
206 object. For example, @samp{:t10=*2} defines type 10 as a pointer to
207 type 2, but does not give the type a name. Omitting the @var{name}
208 field is supported by AIX dbx and GDB after about version 4.8, but not
209 other debuggers. GCC sometimes uses a single space as the name instead
210 of omitting the name altogether; apparently that is supported by most
213 The @var{symbol-descriptor} following the @samp{:} is an alphabetic
214 character that tells more specifically what kind of symbol the stab
215 represents. If the @var{symbol-descriptor} is omitted, but type
216 information follows, then the stab represents a local variable. For a
217 list of symbol descriptors, see @ref{Symbol Descriptors}. The @samp{c}
218 symbol descriptor is an exception in that it is not followed by type
219 information. @xref{Constants}.
221 @var{type-information} is either a @var{type-number}, or
222 @samp{@var{type-number}=}. A @var{type-number} alone is a type
223 reference, referring directly to a type that has already been defined.
225 The @samp{@var{type-number}=} form is a type definition, where the
226 number represents a new type which is about to be defined. The type
227 definition may refer to other types by number, and those type numbers
228 may be followed by @samp{=} and nested definitions.
230 In a type definition, if the character that follows the equals sign is
231 non-numeric then it is a @var{type-descriptor}, and tells what kind of
232 type is about to be defined. Any other values following the
233 @var{type-descriptor} vary, depending on the @var{type-descriptor}.
234 @xref{Type Descriptors}, for a list of @var{type-descriptor} values. If
235 a number follows the @samp{=} then the number is a @var{type-reference}.
236 For a full description of types, @ref{Types}.
238 There is an AIX extension for type attributes. Following the @samp{=}
239 are any number of type attributes. Each one starts with @samp{@@} and
240 ends with @samp{;}. Debuggers, including AIX's dbx and GDB 4.10, skip
241 any type attributes they do not recognize. GDB 4.9 and other versions
242 of dbx may not do this. Because of a conflict with C++
243 (@pxref{Cplusplus}), new attributes should not be defined which begin
244 with a digit, @samp{(}, or @samp{-}; GDB may be unable to distinguish
245 those from the C++ type descriptor @samp{@@}. The attributes are:
248 @item a@var{boundary}
249 @var{boundary} is an integer specifying the alignment. I assume it
250 applies to all variables of this type.
253 Size in bits of a variable of this type.
256 Pointer class (for checking). Not sure what this means, or how
257 @var{integer} is interpreted.
260 Indicate this is a packed type, meaning that structure fields or array
261 elements are placed more closely in memory, to save memory at the
265 All of this can make the string field quite long. All
266 versions of GDB, and some versions of dbx, can handle arbitrarily long
267 strings. But many versions of dbx cretinously limit the strings to
268 about 80 characters, so compilers which must work with such dbx's need
269 to split the @code{.stabs} directive into several @code{.stabs}
270 directives. Each stab duplicates exactly all but the
271 string field. The string field of
272 every stab except the last is marked as continued with a
273 double-backslash at the end. Removing the backslashes and concatenating
274 the string fields of each stab produces the original,
278 @section A Simple Example in C Source
280 To get the flavor of how stabs describe source information for a C
281 program, let's look at the simple program:
286 printf("Hello world");
290 When compiled with @samp{-g}, the program above yields the following
291 @file{.s} file. Line numbers have been added to make it easier to refer
292 to parts of the @file{.s} file in the description of the stabs that
296 @section The Simple Example at the Assembly Level
298 This simple ``hello world'' example demonstrates several of the stab
299 types used to describe C language source files.
303 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
304 3 .stabs "hello.c",100,0,0,Ltext0
307 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
308 7 .stabs "char:t2=r2;0;127;",128,0,0,0
309 8 .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
310 9 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
311 10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
312 11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
313 12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
314 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
315 14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
316 15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
317 16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
318 17 .stabs "float:t12=r1;4;0;",128,0,0,0
319 18 .stabs "double:t13=r1;8;0;",128,0,0,0
320 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
321 20 .stabs "void:t15=15",128,0,0,0
324 23 .ascii "Hello, world!\12\0"
339 38 sethi %hi(LC0),%o1
340 39 or %o1,%lo(LC0),%o0
351 50 .stabs "main:F1",36,0,0,_main
352 51 .stabn 192,0,0,LBB2
353 52 .stabn 224,0,0,LBE2
356 @node Program Structure
357 @chapter Encoding the Structure of the Program
359 The elements of the program structure that stabs encode include the name
360 of the main function, the names of the source and include files, the
361 line numbers, procedure names and types, and the beginnings and ends of
365 * Main Program:: Indicate what the main program is
366 * Source Files:: The path and name of the source file
367 * Include Files:: Names of include files
370 * Nested Procedures::
375 @section Main Program
378 Most languages allow the main program to have any name. The
379 @code{N_MAIN} stab type tells the debugger the name that is used in this
380 program. Only the string field is significant; it is the name of
381 a function which is the main program. Most C compilers do not use this
382 stab (they expect the debugger to assume that the name is @code{main}),
383 but some C compilers emit an @code{N_MAIN} stab for the @code{main}
387 @section Paths and Names of the Source Files
390 Before any other stabs occur, there must be a stab specifying the source
391 file. This information is contained in a symbol of stab type
392 @code{N_SO}; the string field contains the name of the file. The
393 value of the symbol is the start address of the portion of the
394 text section corresponding to that file.
396 With the Sun Solaris2 compiler, the desc field contains a
397 source-language code.
398 @c Do the debuggers use it? What are the codes? -djm
400 Some compilers (for example, GCC2 and SunOS4 @file{/bin/cc}) also
401 include the directory in which the source was compiled, in a second
402 @code{N_SO} symbol preceding the one containing the file name. This
403 symbol can be distinguished by the fact that it ends in a slash. Code
404 from the @code{cfront} C++ compiler can have additional @code{N_SO} symbols for
405 nonexistent source files after the @code{N_SO} for the real source file;
406 these are believed to contain no useful information.
411 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0 # @r{100 is N_SO}
412 .stabs "hello.c",100,0,0,Ltext0
417 Instead of @code{N_SO} symbols, XCOFF uses a @code{.file} assembler
418 directive which assembles to a standard COFF @code{.file} symbol;
419 explaining this in detail is outside the scope of this document.
422 @section Names of Include Files
424 There are several schemes for dealing with include files: the
425 traditional @code{N_SOL} approach, Sun's @code{N_BINCL} approach, and the
426 XCOFF @code{C_BINCL} approach (which despite the similar name has little in
427 common with @code{N_BINCL}).
430 An @code{N_SOL} symbol specifies which include file subsequent symbols
431 refer to. The string field is the name of the file and the
432 value is the text address corresponding to the start of the
433 previous include file and the start of this one. To specify the main
434 source file again, use an @code{N_SOL} symbol with the name of the main
440 The @code{N_BINCL} approach works as follows. An @code{N_BINCL} symbol
441 specifies the start of an include file. In an object file, only the
442 string is significant; the Sun linker puts data into some of the
443 other fields. The end of the include file is marked by an
444 @code{N_EINCL} symbol (which has no string field). In an object
445 file, there is no significant data in the @code{N_EINCL} symbol; the Sun
446 linker puts data into some of the fields. @code{N_BINCL} and
447 @code{N_EINCL} can be nested.
449 If the linker detects that two source files have identical stabs between
450 an @code{N_BINCL} and @code{N_EINCL} pair (as will generally be the case
451 for a header file), then it only puts out the stabs once. Each
452 additional occurance is replaced by an @code{N_EXCL} symbol. I believe
453 the Sun (SunOS4, not sure about Solaris) linker is the only one which
454 supports this feature.
455 @c What do the fields of N_EXCL contain? -djm
459 For the start of an include file in XCOFF, use the @file{.bi} assembler
460 directive, which generates a @code{C_BINCL} symbol. A @file{.ei}
461 directive, which generates a @code{C_EINCL} symbol, denotes the end of
462 the include file. Both directives are followed by the name of the
463 source file in quotes, which becomes the string for the symbol.
464 The value of each symbol, produced automatically by the assembler
465 and linker, is the offset into the executable of the beginning
466 (inclusive, as you'd expect) or end (inclusive, as you would not expect)
467 of the portion of the COFF line table that corresponds to this include
468 file. @code{C_BINCL} and @code{C_EINCL} do not nest.
471 @section Line Numbers
474 An @code{N_SLINE} symbol represents the start of a source line. The
475 desc field contains the line number and the value
476 contains the code address for the start of that source line. On most
477 machines the address is absolute; for Sun's stabs-in-ELF, it is relative
478 to the function in which the @code{N_SLINE} symbol occurs.
482 GNU documents @code{N_DSLINE} and @code{N_BSLINE} symbols for line
483 numbers in the data or bss segments, respectively. They are identical
484 to @code{N_SLINE} but are relocated differently by the linker. They
485 were intended to be used to describe the source location of a variable
486 declaration, but I believe that GCC2 actually puts the line number in
487 the desc field of the stab for the variable itself. GDB has been
488 ignoring these symbols (unless they contain a string field) since
491 For single source lines that generate discontiguous code, such as flow
492 of control statements, there may be more than one line number entry for
493 the same source line. In this case there is a line number entry at the
494 start of each code range, each with the same line number.
496 XCOFF does not use stabs for line numbers. Instead, it uses COFF line
497 numbers (which are outside the scope of this document). Standard COFF
498 line numbers cannot deal with include files, but in XCOFF this is fixed
499 with the C_BINCL method of marking include files (@pxref{Include
507 @findex N_STSYM, for functions (Sun acc)
508 @findex N_GSYM, for functions (Sun acc)
509 All of the following stabs normally use the @code{N_FUN} symbol type.
510 However, Sun's @code{acc} compiler on SunOS4 uses @code{N_GSYM} and
511 @code{N_STSYM}, which means that the value of the stab for the function
512 is useless and the debugger must get the address of the function from
513 the non-stab symbols instead. BSD Fortran is said to use @code{N_FNAME}
514 with the same restriction; the value of the symbol is not useful (I'm
515 not sure it really does use this, because GDB doesn't handle this and no
518 A function is represented by an @samp{F} symbol descriptor for a global
519 (extern) function, and @samp{f} for a static (local) function. The
520 value is the address of the start of the function (absolute
521 for @code{a.out}; relative to the start of the file for Sun's
522 stabs-in-ELF). The type information of the stab represents the return
523 type of the function; thus @samp{foo:f5} means that foo is a function
524 returning type 5. There is no need to try to get the line number of the
525 start of the function from the stab for the function; it is in the next
526 @code{N_SLINE} symbol.
528 @c FIXME: verify whether the "I suspect" below is true or not.
529 Some compilers (such as Sun's Solaris compiler) support an extension for
530 specifying the types of the arguments. I suspect this extension is not
531 used for old (non-prototyped) function definitions in C. If the
532 extension is in use, the type information of the stab for the function
533 is followed by type information for each argument, with each argument
534 preceded by @samp{;}. An argument type of 0 means that additional
535 arguments are being passed, whose types and number may vary (@samp{...}
536 in ANSI C). GDB has tolerated this extension (parsed the syntax, if not
537 necessarily used the information) since at least version 4.8; I don't
538 know whether all versions of dbx tolerate it. The argument types given
539 here are not redundant with the symbols for the formal parameters
540 (@pxref{Parameters}); they are the types of the arguments as they are
541 passed, before any conversions might take place. For example, if a C
542 function which is declared without a prototype takes a @code{float}
543 argument, the value is passed as a @code{double} but then converted to a
544 @code{float}. Debuggers need to use the types given in the arguments
545 when printing values, but when calling the function they need to use the
546 types given in the symbol defining the function.
548 If the return type and types of arguments of a function which is defined
549 in another source file are specified (i.e., a function prototype in ANSI
550 C), traditionally compilers emit no stab; the only way for the debugger
551 to find the information is if the source file where the function is
552 defined was also compiled with debugging symbols. As an extension the
553 Solaris compiler uses symbol descriptor @samp{P} followed by the return
554 type of the function, followed by the arguments, each preceded by
555 @samp{;}, as in a stab with symbol descriptor @samp{f} or @samp{F}.
556 This use of symbol descriptor @samp{P} can be distinguished from its use
557 for register parameters (@pxref{Register Parameters}) by the fact that it has
558 symbol type @code{N_FUN}.
560 The AIX documentation also defines symbol descriptor @samp{J} as an
561 internal function. I assume this means a function nested within another
562 function. It also says symbol descriptor @samp{m} is a module in
563 Modula-2 or extended Pascal.
565 Procedures (functions which do not return values) are represented as
566 functions returning the @code{void} type in C. I don't see why this couldn't
567 be used for all languages (inventing a @code{void} type for this purpose if
568 necessary), but the AIX documentation defines @samp{I}, @samp{P}, and
569 @samp{Q} for internal, global, and static procedures, respectively.
570 These symbol descriptors are unusual in that they are not followed by
573 The following example shows a stab for a function @code{main} which
574 returns type number @code{1}. The @code{_main} specified for the value
575 is a reference to an assembler label which is used to fill in the start
576 address of the function.
579 .stabs "main:F1",36,0,0,_main # @r{36 is N_FUN}
582 The stab representing a procedure is located immediately following the
583 code of the procedure. This stab is in turn directly followed by a
584 group of other stabs describing elements of the procedure. These other
585 stabs describe the procedure's parameters, its block local variables, and
588 @node Nested Procedures
589 @section Nested Procedures
591 For any of the symbol descriptors representing procedures, after the
592 symbol descriptor and the type information is optionally a scope
593 specifier. This consists of a comma, the name of the procedure, another
594 comma, and the name of the enclosing procedure. The first name is local
595 to the scope specified, and seems to be redundant with the name of the
596 symbol (before the @samp{:}). This feature is used by GCC, and
597 presumably Pascal, Modula-2, etc., compilers, for nested functions.
599 If procedures are nested more than one level deep, only the immediately
600 containing scope is specified. For example, this code:
612 return baz (x + 2 * y);
614 return x + bar (3 * x);
622 .stabs "baz:f1,baz,bar",36,0,0,_baz.15 # @r{36 is N_FUN}
623 .stabs "bar:f1,bar,foo",36,0,0,_bar.12
624 .stabs "foo:F1",36,0,0,_foo
627 @node Block Structure
628 @section Block Structure
632 The program's block structure is represented by the @code{N_LBRAC} (left
633 brace) and the @code{N_RBRAC} (right brace) stab types. The variables
634 defined inside a block precede the @code{N_LBRAC} symbol for most
635 compilers, including GCC. Other compilers, such as the Convex, Acorn
636 RISC machine, and Sun @code{acc} compilers, put the variables after the
637 @code{N_LBRAC} symbol. The values of the @code{N_LBRAC} and
638 @code{N_RBRAC} symbols are the start and end addresses of the code of
639 the block, respectively. For most machines, they are relative to the
640 starting address of this source file. For the Gould NP1, they are
641 absolute. For Sun's stabs-in-ELF, they are relative to the function in
644 The @code{N_LBRAC} and @code{N_RBRAC} stabs that describe the block
645 scope of a procedure are located after the @code{N_FUN} stab that
646 represents the procedure itself.
648 Sun documents the desc field of @code{N_LBRAC} and
649 @code{N_RBRAC} symbols as containing the nesting level of the block.
650 However, dbx seems to not care, and GCC always sets desc to
656 The @samp{c} symbol descriptor indicates that this stab represents a
657 constant. This symbol descriptor is an exception to the general rule
658 that symbol descriptors are followed by type information. Instead, it
659 is followed by @samp{=} and one of the following:
663 Boolean constant. @var{value} is a numeric value; I assume it is 0 for
667 Character constant. @var{value} is the numeric value of the constant.
669 @item e @var{type-information} , @var{value}
670 Constant whose value can be represented as integral.
671 @var{type-information} is the type of the constant, as it would appear
672 after a symbol descriptor (@pxref{String Field}). @var{value} is the
673 numeric value of the constant. GDB 4.9 does not actually get the right
674 value if @var{value} does not fit in a host @code{int}, but it does not
675 do anything violent, and future debuggers could be extended to accept
676 integers of any size (whether unsigned or not). This constant type is
677 usually documented as being only for enumeration constants, but GDB has
678 never imposed that restriction; I don't know about other debuggers.
681 Integer constant. @var{value} is the numeric value. The type is some
682 sort of generic integer type (for GDB, a host @code{int}); to specify
683 the type explicitly, use @samp{e} instead.
686 Real constant. @var{value} is the real value, which can be @samp{INF}
687 (optionally preceded by a sign) for infinity, @samp{QNAN} for a quiet
688 NaN (not-a-number), or @samp{SNAN} for a signalling NaN. If it is a
689 normal number the format is that accepted by the C library function
693 String constant. @var{string} is a string enclosed in either @samp{'}
694 (in which case @samp{'} characters within the string are represented as
695 @samp{\'} or @samp{"} (in which case @samp{"} characters within the
696 string are represented as @samp{\"}).
698 @item S @var{type-information} , @var{elements} , @var{bits} , @var{pattern}
699 Set constant. @var{type-information} is the type of the constant, as it
700 would appear after a symbol descriptor (@pxref{String Field}).
701 @var{elements} is the number of elements in the set (does this means
702 how many bits of @var{pattern} are actually used, which would be
703 redundant with the type, or perhaps the number of bits set in
704 @var{pattern}? I don't get it), @var{bits} is the number of bits in the
705 constant (meaning it specifies the length of @var{pattern}, I think),
706 and @var{pattern} is a hexadecimal representation of the set. AIX
707 documentation refers to a limit of 32 bytes, but I see no reason why
708 this limit should exist. This form could probably be used for arbitrary
709 constants, not just sets; the only catch is that @var{pattern} should be
710 understood to be target, not host, byte order and format.
713 The boolean, character, string, and set constants are not supported by
714 GDB 4.9, but it ignores them. GDB 4.8 and earlier gave an error
715 message and refused to read symbols from the file containing the
718 The above information is followed by @samp{;}.
723 Different types of stabs describe the various ways that variables can be
724 allocated: on the stack, globally, in registers, in common blocks,
725 statically, or as arguments to a function.
728 * Stack Variables:: Variables allocated on the stack.
729 * Global Variables:: Variables used by more than one source file.
730 * Register Variables:: Variables in registers.
731 * Common Blocks:: Variables statically allocated together.
732 * Statics:: Variables local to one source file.
733 * Parameters:: Variables for arguments to functions.
736 @node Stack Variables
737 @section Automatic Variables Allocated on the Stack
739 If a variable's scope is local to a function and its lifetime is only as
740 long as that function executes (C calls such variables
741 @dfn{automatic}), it can be allocated in a register (@pxref{Register
742 Variables}) or on the stack.
745 Each variable allocated on the stack has a stab with the symbol
746 descriptor omitted. Since type information should begin with a digit,
747 @samp{-}, or @samp{(}, only those characters precluded from being used
748 for symbol descriptors. However, the Acorn RISC machine (ARM) is said
749 to get this wrong: it puts out a mere type definition here, without the
750 preceding @samp{@var{type-number}=}. This is a bad idea; there is no
751 guarantee that type descriptors are distinct from symbol descriptors.
752 Stabs for stack variables use the @code{N_LSYM} stab type.
754 The value of the stab is the offset of the variable within the
755 local variables. On most machines this is an offset from the frame
756 pointer and is negative. The location of the stab specifies which block
757 it is defined in; see @ref{Block Structure}.
759 For example, the following C code:
769 produces the following stabs:
772 .stabs "main:F1",36,0,0,_main # @r{36 is N_FUN}
773 .stabs "x:1",128,0,0,-12 # @r{128 is N_LSYM}
774 .stabn 192,0,0,LBB2 # @r{192 is N_LBRAC}
775 .stabn 224,0,0,LBE2 # @r{224 is N_RBRAC}
778 @xref{Procedures} for more information on the @code{N_FUN} stab, and
779 @ref{Block Structure} for more information on the @code{N_LBRAC} and
780 @code{N_RBRAC} stabs.
782 @node Global Variables
783 @section Global Variables
786 A variable whose scope is not specific to just one source file is
787 represented by the @samp{G} symbol descriptor. These stabs use the
788 @code{N_GSYM} stab type. The type information for the stab
789 (@pxref{String Field}) gives the type of the variable.
791 For example, the following source code:
798 yields the following assembly code:
801 .stabs "g_foo:G2",32,0,0,0 # @r{32 is N_GSYM}
808 The address of the variable represented by the @code{N_GSYM} is not
809 contained in the @code{N_GSYM} stab. The debugger gets this information
810 from the external symbol for the global variable. In the example above,
811 the @code{.global _g_foo} and @code{_g_foo:} lines tell the assembler to
812 produce an external symbol.
814 @node Register Variables
815 @section Register Variables
818 @c According to an old version of this manual, AIX uses C_RPSYM instead
819 @c of C_RSYM. I am skeptical; this should be verified.
820 Register variables have their own stab type, @code{N_RSYM}, and their
821 own symbol descriptor, @samp{r}. The stab's value is the
822 number of the register where the variable data will be stored.
823 @c .stabs "name:type",N_RSYM,0,RegSize,RegNumber (Sun doc)
825 AIX defines a separate symbol descriptor @samp{d} for floating point
826 registers. This seems unnecessary; why not just just give floating
827 point registers different register numbers? I have not verified whether
828 the compiler actually uses @samp{d}.
830 If the register is explicitly allocated to a global variable, but not
834 register int g_bar asm ("%g5");
838 then the stab may be emitted at the end of the object file, with
839 the other bss symbols.
842 @section Common Blocks
844 A common block is a statically allocated section of memory which can be
845 referred to by several source files. It may contain several variables.
846 I believe Fortran is the only language with this feature.
850 A @code{N_BCOMM} stab begins a common block and an @code{N_ECOMM} stab
851 ends it. The only field that is significant in these two stabs is the
852 string, which names a normal (non-debugging) symbol that gives the
853 address of the common block.
856 Each stab between the @code{N_BCOMM} and the @code{N_ECOMM} specifies a
857 member of that common block; its value is the offset within the
858 common block of that variable. The @code{N_ECOML} stab type is
859 documented for this purpose, but Sun's Fortran compiler uses
860 @code{N_GSYM} instead. The test case I looked at had a common block
861 local to a function and it used the @samp{V} symbol descriptor; I assume
862 one would use @samp{S} if not local to a function (that is, if a common
863 block @emph{can} be anything other than local to a function).
866 @section Static Variables
868 Initialized static variables are represented by the @samp{S} and
869 @samp{V} symbol descriptors. @samp{S} means file scope static, and
870 @samp{V} means procedure scope static.
872 @c This is probably not worth mentioning; it is only true on the sparc
873 @c for `double' variables which although declared const are actually in
874 @c the data segment (the text segment can't guarantee 8 byte alignment).
876 @c 2.4.5 has a bug in that it uses @code{N_FUN}, so neither dbx nor GDB can
877 @c find the variables)
880 In a.out files, @code{N_STSYM} means the data segment, @code{N_FUN}
881 means the text segment, and @code{N_LCSYM} means the bss segment.
883 For example, the source lines:
886 static const int var_const = 5;
887 static int var_init = 2;
888 static int var_noinit;
892 yield the following stabs:
895 .stabs "var_const:S1",36,0,0,_var_const # @r{36 is N_FUN}
897 .stabs "var_init:S1",38,0,0,_var_init # @r{38 is N_STSYM}
899 .stabs "var_noinit:S1",40,0,0,_var_noinit # @r{40 is N_LCSYM}
902 In XCOFF files, each symbol has a section number, so the stab type
903 need not indicate the segment.
905 In ECOFF files, the storage class is used to specify the section, so the
906 stab type need not indicate the segment.
908 @c In ELF files, it apparently is a big mess. See kludge in dbxread.c
909 @c in GDB. FIXME: Investigate where this kludge comes from.
911 @c This is the place to mention N_ROSYM; I'd rather do so once I can
912 @c coherently explain how this stuff works for stabs-in-ELF.
917 Formal parameters to a function are represented by a stab (or sometimes
918 two; see below) for each parameter. The stabs are in the order in which
919 the debugger should print the parameters (i.e., the order in which the
920 parameters are declared in the source file). The exact form of the stab
921 depends on how the parameter is being passed.
924 Parameters passed on the stack use the symbol descriptor @samp{p} and
925 the @code{N_PSYM} symbol type. The value of the symbol is an offset
926 used to locate the parameter on the stack; its exact meaning is
927 machine-dependent, but on most machines it is an offset from the frame
930 As a simple example, the code:
941 .stabs "main:F1",36,0,0,_main # @r{36 is N_FUN}
942 .stabs "argc:p1",160,0,0,68 # @r{160 is N_PSYM}
943 .stabs "argv:p20=*21=*2",160,0,0,72
946 The type definition of @code{argv} is interesting because it contains
947 several type definitions. Type 21 is pointer to type 2 (char) and
948 @code{argv} (type 20) is pointer to type 21.
950 @c FIXME: figure out what these mean and describe them coherently.
951 The following symbol descriptors are also said to go with @code{N_PSYM}.
952 The value of the symbol is said to be an offset from the argument
953 pointer (I'm not sure whether this is true or not).
957 pF Fortran function parameter
958 X (function result variable)
963 * Register Parameters::
964 * Local Variable Parameters::
965 * Reference Parameters::
966 * Conformant Arrays::
969 @node Register Parameters
970 @subsection Passing Parameters in Registers
972 If the parameter is passed in a register, then traditionally there are
973 two symbols for each argument:
976 .stabs "arg:p1" . . . ; N_PSYM
977 .stabs "arg:r1" . . . ; N_RSYM
980 Debuggers use the second one to find the value, and the first one to
981 know that it is an argument.
984 @findex N_RSYM, for parameters
985 Because that approach is kind of ugly, some compilers use symbol
986 descriptor @samp{P} or @samp{R} to indicate an argument which is in a
987 register. Symbol type @code{C_RPSYM} is used with @samp{R} and
988 @code{N_RSYM} is used with @samp{P}. The symbol's value is
989 the register number. @samp{P} and @samp{R} mean the same thing; the
990 difference is that @samp{P} is a GNU invention and @samp{R} is an IBM
991 (XCOFF) invention. As of version 4.9, GDB should handle either one.
993 There is at least one case where GCC uses a @samp{p} and @samp{r} pair
994 rather than @samp{P}; this is where the argument is passed in the
995 argument list and then loaded into a register.
997 According to the AIX documentation, symbol descriptor @samp{D} is for a
998 parameter passed in a floating point register. This seems
999 unnecessary---why not just use @samp{R} with a register number which
1000 indicates that it's a floating point register? I haven't verified
1001 whether the system actually does what the documentation indicates.
1003 @c FIXME: On the hppa this is for any type > 8 bytes, I think, and not
1004 @c for small structures (investigate).
1005 On the sparc and hppa, for a @samp{P} symbol whose type is a structure
1006 or union, the register contains the address of the structure. On the
1007 sparc, this is also true of a @samp{p} and @samp{r} pair (using Sun
1008 @code{cc}) or a @samp{p} symbol. However, if a (small) structure is
1009 really in a register, @samp{r} is used. And, to top it all off, on the
1010 hppa it might be a structure which was passed on the stack and loaded
1011 into a register and for which there is a @samp{p} and @samp{r} pair! I
1012 believe that symbol descriptor @samp{i} is supposed to deal with this
1013 case (it is said to mean "value parameter by reference, indirect
1014 access"; I don't know the source for this information), but I don't know
1015 details or what compilers or debuggers use it, if any (not GDB or GCC).
1016 It is not clear to me whether this case needs to be dealt with
1017 differently than parameters passed by reference (@pxref{Reference Parameters}).
1019 @node Local Variable Parameters
1020 @subsection Storing Parameters as Local Variables
1022 There is a case similar to an argument in a register, which is an
1023 argument that is actually stored as a local variable. Sometimes this
1024 happens when the argument was passed in a register and then the compiler
1025 stores it as a local variable. If possible, the compiler should claim
1026 that it's in a register, but this isn't always done.
1028 @findex N_LSYM, for parameter
1029 Some compilers use the pair of symbols approach described above
1030 (@samp{@var{arg}:p} followed by @samp{@var{arg}:}); this includes GCC1
1031 (not GCC2) on the sparc when passing a small structure and GCC2
1032 (sometimes) when the argument type is @code{float} and it is passed as a
1033 @code{double} and converted to @code{float} by the prologue (in the
1034 latter case the type of the @samp{@var{arg}:p} symbol is @code{double}
1035 and the type of the @samp{@var{arg}:} symbol is @code{float}). GCC, at
1036 least on the 960, uses a single @samp{p} symbol descriptor for an
1037 argument which is stored as a local variable but uses @code{N_LSYM}
1038 instead of @code{N_PSYM}. In this case, the value of the symbol
1039 is an offset relative to the local variables for that function, not
1040 relative to the arguments; on some machines those are the same thing,
1043 @node Reference Parameters
1044 @subsection Passing Parameters by Reference
1046 If the parameter is passed by reference (e.g., Pascal @code{VAR}
1047 parameters), then the symbol descriptor is @samp{v} if it is in the
1048 argument list, or @samp{a} if it in a register. Other than the fact
1049 that these contain the address of the parameter rather than the
1050 parameter itself, they are identical to @samp{p} and @samp{R},
1051 respectively. I believe @samp{a} is an AIX invention; @samp{v} is
1052 supported by all stabs-using systems as far as I know.
1054 @node Conformant Arrays
1055 @subsection Passing Conformant Array Parameters
1057 @c Is this paragraph correct? It is based on piecing together patchy
1058 @c information and some guesswork
1059 Conformant arrays are a feature of Modula-2, and perhaps other
1060 languages, in which the size of an array parameter is not known to the
1061 called function until run-time. Such parameters have two stabs: a
1062 @samp{x} for the array itself, and a @samp{C}, which represents the size
1063 of the array. The value of the @samp{x} stab is the offset in the
1064 argument list where the address of the array is stored (it this right?
1065 it is a guess); the value of the @samp{C} stab is the offset in the
1066 argument list where the size of the array (in elements? in bytes?) is
1070 @chapter Defining Types
1072 The examples so far have described types as references to previously
1073 defined types, or defined in terms of subranges of or pointers to
1074 previously defined types. This chapter describes the other type
1075 descriptors that may follow the @samp{=} in a type definition.
1078 * Builtin Types:: Integers, floating point, void, etc.
1079 * Miscellaneous Types:: Pointers, sets, files, etc.
1080 * Cross-References:: Referring to a type not yet defined.
1081 * Subranges:: A type with a specific range.
1082 * Arrays:: An aggregate type of same-typed elements.
1083 * Strings:: Like an array but also has a length.
1084 * Enumerations:: Like an integer but the values have names.
1085 * Structures:: An aggregate type of different-typed elements.
1086 * Typedefs:: Giving a type a name.
1087 * Unions:: Different types sharing storage.
1092 @section Builtin Types
1094 Certain types are built in (@code{int}, @code{short}, @code{void},
1095 @code{float}, etc.); the debugger recognizes these types and knows how
1096 to handle them. Thus, don't be surprised if some of the following ways
1097 of specifying builtin types do not specify everything that a debugger
1098 would need to know about the type---in some cases they merely specify
1099 enough information to distinguish the type from other types.
1101 The traditional way to define builtin types is convolunted, so new ways
1102 have been invented to describe them. Sun's @code{acc} uses special
1103 builtin type descriptors (@samp{b} and @samp{R}), and IBM uses negative
1104 type numbers. GDB accepts all three ways, as of version 4.8; dbx just
1105 accepts the traditional builtin types and perhaps one of the other two
1106 formats. The following sections describe each of these formats.
1109 * Traditional Builtin Types:: Put on your seatbelts and prepare for kludgery
1110 * Builtin Type Descriptors:: Builtin types with special type descriptors
1111 * Negative Type Numbers:: Builtin types using negative type numbers
1114 @node Traditional Builtin Types
1115 @subsection Traditional Builtin Types
1117 This is the traditional, convoluted method for defining builtin types.
1118 There are several classes of such type definitions: integer, floating
1119 point, and @code{void}.
1122 * Traditional Integer Types::
1123 * Traditional Other Types::
1126 @node Traditional Integer Types
1127 @subsubsection Traditional Integer Types
1129 Often types are defined as subranges of themselves. If the bounding values
1130 fit within an @code{int}, then they are given normally. For example:
1133 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0 # @r{128 is N_LSYM}
1134 .stabs "char:t2=r2;0;127;",128,0,0,0
1137 Builtin types can also be described as subranges of @code{int}:
1140 .stabs "unsigned short:t6=r1;0;65535;",128,0,0,0
1143 If the lower bound of a subrange is 0 and the upper bound is -1,
1144 the type is an unsigned integral type whose bounds are too
1145 big to describe in an @code{int}. Traditionally this is only used for
1146 @code{unsigned int} and @code{unsigned long}:
1149 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
1152 For larger types, GCC 2.4.5 puts out bounds in octal, with a leading 0.
1153 In this case a negative bound consists of a number which is a 1 bit
1154 followed by a bunch of 0 bits, and a positive bound is one in which a
1155 bunch of bits are 1. All known versions of dbx and GDB version 4 accept
1156 this, but GDB 3.5 refuses to read the whole file containing such
1157 symbols. So GCC 2.3.3 did not output the proper size for these types.
1158 @c FIXME: How about an example?
1160 If the lower bound of a subrange is 0 and the upper bound is negative,
1161 the type is an unsigned integral type whose size in bytes is the
1162 absolute value of the upper bound. I believe this is a Convex
1163 convention for @code{unsigned long long}.
1165 If the lower bound of a subrange is negative and the upper bound is 0,
1166 the type is a signed integral type whose size in bytes is
1167 the absolute value of the lower bound. I believe this is a Convex
1168 convention for @code{long long}. To distinguish this from a legitimate
1169 subrange, the type should be a subrange of itself. I'm not sure whether
1170 this is the case for Convex.
1172 @node Traditional Other Types
1173 @subsubsection Traditional Other Types
1175 If the upper bound of a subrange is 0 and the lower bound is positive,
1176 the type is a floating point type, and the lower bound of the subrange
1177 indicates the number of bytes in the type:
1180 .stabs "float:t12=r1;4;0;",128,0,0,0
1181 .stabs "double:t13=r1;8;0;",128,0,0,0
1184 However, GCC writes @code{long double} the same way it writes
1185 @code{double}, so there is no way to distinguish.
1188 .stabs "long double:t14=r1;8;0;",128,0,0,0
1191 Complex types are defined the same way as floating-point types; there is
1192 no way to distinguish a single-precision complex from a double-precision
1193 floating-point type.
1195 The C @code{void} type is defined as itself:
1198 .stabs "void:t15=15",128,0,0,0
1201 I'm not sure how a boolean type is represented.
1203 @node Builtin Type Descriptors
1204 @subsection Defining Builtin Types Using Builtin Type Descriptors
1206 This is the method used by Sun's @code{acc} for defining builtin types.
1207 These are the type descriptors to define builtin types:
1210 @c FIXME: clean up description of width and offset, once we figure out
1212 @item b @var{signed} @var{char-flag} @var{width} ; @var{offset} ; @var{nbits} ;
1213 Define an integral type. @var{signed} is @samp{u} for unsigned or
1214 @samp{s} for signed. @var{char-flag} is @samp{c} which indicates this
1215 is a character type, or is omitted. I assume this is to distinguish an
1216 integral type from a character type of the same size, for example it
1217 might make sense to set it for the C type @code{wchar_t} so the debugger
1218 can print such variables differently (Solaris does not do this). Sun
1219 sets it on the C types @code{signed char} and @code{unsigned char} which
1220 arguably is wrong. @var{width} and @var{offset} appear to be for small
1221 objects stored in larger ones, for example a @code{short} in an
1222 @code{int} register. @var{width} is normally the number of bytes in the
1223 type. @var{offset} seems to always be zero. @var{nbits} is the number
1224 of bits in the type.
1226 Note that type descriptor @samp{b} used for builtin types conflicts with
1227 its use for Pascal space types (@pxref{Miscellaneous Types}); they can
1228 be distinguished because the character following the type descriptor
1229 will be a digit, @samp{(}, or @samp{-} for a Pascal space type, or
1230 @samp{u} or @samp{s} for a builtin type.
1233 Documented by AIX to define a wide character type, but their compiler
1234 actually uses negative type numbers (@pxref{Negative Type Numbers}).
1236 @item R @var{fp-type} ; @var{bytes} ;
1237 Define a floating point type. @var{fp-type} has one of the following values:
1241 IEEE 32-bit (single precision) floating point format.
1244 IEEE 64-bit (double precision) floating point format.
1246 @item 3 (NF_COMPLEX)
1247 @item 4 (NF_COMPLEX16)
1248 @item 5 (NF_COMPLEX32)
1249 @c "GDB source" really means @file{include/aout/stab_gnu.h}, but trying
1250 @c to put that here got an overfull hbox.
1251 These are for complex numbers. A comment in the GDB source describes
1252 them as Fortran @code{complex}, @code{double complex}, and
1253 @code{complex*16}, respectively, but what does that mean? (i.e., Single
1254 precision? Double precison?).
1256 @item 6 (NF_LDOUBLE)
1257 Long double. This should probably only be used for Sun format
1258 @code{long double}, and new codes should be used for other floating
1259 point formats (@code{NF_DOUBLE} can be used if a @code{long double} is
1260 really just an IEEE double, of course).
1263 @var{bytes} is the number of bytes occupied by the type. This allows a
1264 debugger to perform some operations with the type even if it doesn't
1265 understand @var{fp-type}.
1267 @item g @var{type-information} ; @var{nbits}
1268 Documented by AIX to define a floating type, but their compiler actually
1269 uses negative type numbers (@pxref{Negative Type Numbers}).
1271 @item c @var{type-information} ; @var{nbits}
1272 Documented by AIX to define a complex type, but their compiler actually
1273 uses negative type numbers (@pxref{Negative Type Numbers}).
1276 The C @code{void} type is defined as a signed integral type 0 bits long:
1278 .stabs "void:t19=bs0;0;0",128,0,0,0
1280 The Solaris compiler seems to omit the trailing semicolon in this case.
1281 Getting sloppy in this way is not a swift move because if a type is
1282 embedded in a more complex expression it is necessary to be able to tell
1285 I'm not sure how a boolean type is represented.
1287 @node Negative Type Numbers
1288 @subsection Negative Type Numbers
1290 This is the method used in XCOFF for defining builtin types.
1291 Since the debugger knows about the builtin types anyway, the idea of
1292 negative type numbers is simply to give a special type number which
1293 indicates the builtin type. There is no stab defining these types.
1295 I'm not sure whether anyone has tried to define what this means if
1296 @code{int} can be other than 32 bits (or if other types can be other than
1297 their customary size). If @code{int} has exactly one size for each
1298 architecture, then it can be handled easily enough, but if the size of
1299 @code{int} can vary according the compiler options, then it gets hairy.
1300 The best way to do this would be to define separate negative type
1301 numbers for 16-bit @code{int} and 32-bit @code{int}; therefore I have
1302 indicated below the customary size (and other format information) for
1303 each type. The information below is currently correct because AIX on
1304 the RS6000 is the only system which uses these type numbers. If these
1305 type numbers start to get used on other systems, I suspect the correct
1306 thing to do is to define a new number in cases where a type does not
1307 have the size and format indicated below (or avoid negative type numbers
1310 Part of the definition of the negative type number is
1311 the name of the type. Types with identical size and format but
1312 different names have different negative type numbers.
1316 @code{int}, 32 bit signed integral type.
1319 @code{char}, 8 bit type holding a character. Both GDB and dbx on AIX
1320 treat this as signed. GCC uses this type whether @code{char} is signed
1321 or not, which seems like a bad idea. The AIX compiler (@code{xlc}) seems to
1322 avoid this type; it uses -5 instead for @code{char}.
1325 @code{short}, 16 bit signed integral type.
1328 @code{long}, 32 bit signed integral type.
1331 @code{unsigned char}, 8 bit unsigned integral type.
1334 @code{signed char}, 8 bit signed integral type.
1337 @code{unsigned short}, 16 bit unsigned integral type.
1340 @code{unsigned int}, 32 bit unsigned integral type.
1343 @code{unsigned}, 32 bit unsigned integral type.
1346 @code{unsigned long}, 32 bit unsigned integral type.
1349 @code{void}, type indicating the lack of a value.
1352 @code{float}, IEEE single precision.
1355 @code{double}, IEEE double precision.
1358 @code{long double}, IEEE double precision. The compiler claims the size
1359 will increase in a future release, and for binary compatibility you have
1360 to avoid using @code{long double}. I hope when they increase it they
1361 use a new negative type number.
1364 @code{integer}. 32 bit signed integral type.
1367 @code{boolean}. 32 bit type. How is the truth value encoded? Is it
1368 the least significant bit or is it a question of whether the whole value
1369 is zero or non-zero?
1372 @code{short real}. IEEE single precision.
1375 @code{real}. IEEE double precision.
1378 @code{stringptr}. @xref{Strings}.
1381 @code{character}, 8 bit unsigned character type.
1384 @code{logical*1}, 8 bit type. This Fortran type has a split
1385 personality in that it is used for boolean variables, but can also be
1386 used for unsigned integers. 0 is false, 1 is true, and other values are
1390 @code{logical*2}, 16 bit type. This Fortran type has a split
1391 personality in that it is used for boolean variables, but can also be
1392 used for unsigned integers. 0 is false, 1 is true, and other values are
1396 @code{logical*4}, 32 bit type. This Fortran type has a split
1397 personality in that it is used for boolean variables, but can also be
1398 used for unsigned integers. 0 is false, 1 is true, and other values are
1402 @code{logical}, 32 bit type. This Fortran type has a split
1403 personality in that it is used for boolean variables, but can also be
1404 used for unsigned integers. 0 is false, 1 is true, and other values are
1408 @code{complex}. A complex type consisting of two IEEE single-precision
1409 floating point values.
1412 @code{complex}. A complex type consisting of two IEEE double-precision
1413 floating point values.
1416 @code{integer*1}, 8 bit signed integral type.
1419 @code{integer*2}, 16 bit signed integral type.
1422 @code{integer*4}, 32 bit signed integral type.
1425 @code{wchar}. Wide character, 16 bits wide, unsigned (what format?
1429 @node Miscellaneous Types
1430 @section Miscellaneous Types
1433 @item b @var{type-information} ; @var{bytes}
1434 Pascal space type. This is documented by IBM; what does it mean?
1436 This use of the @samp{b} type descriptor can be distinguished
1437 from its use for builtin integral types (@pxref{Builtin Type
1438 Descriptors}) because the character following the type descriptor is
1439 always a digit, @samp{(}, or @samp{-}.
1441 @item B @var{type-information}
1442 A volatile-qualified version of @var{type-information}. This is
1443 a Sun extension. References and stores to a variable with a
1444 volatile-qualified type must not be optimized or cached; they
1445 must occur as the user specifies them.
1447 @item d @var{type-information}
1448 File of type @var{type-information}. As far as I know this is only used
1451 @item k @var{type-information}
1452 A const-qualified version of @var{type-information}. This is a Sun
1453 extension. A variable with a const-qualified type cannot be modified.
1455 @item M @var{type-information} ; @var{length}
1456 Multiple instance type. The type seems to composed of @var{length}
1457 repetitions of @var{type-information}, for example @code{character*3} is
1458 represented by @samp{M-2;3}, where @samp{-2} is a reference to a
1459 character type (@pxref{Negative Type Numbers}). I'm not sure how this
1460 differs from an array. This appears to be a Fortran feature.
1461 @var{length} is a bound, like those in range types; see @ref{Subranges}.
1463 @item S @var{type-information}
1464 Pascal set type. @var{type-information} must be a small type such as an
1465 enumeration or a subrange, and the type is a bitmask whose length is
1466 specified by the number of elements in @var{type-information}.
1468 @item * @var{type-information}
1469 Pointer to @var{type-information}.
1472 @node Cross-References
1473 @section Cross-References to Other Types
1475 A type can be used before it is defined; one common way to deal with
1476 that situation is just to use a type reference to a type which has not
1479 Another way is with the @samp{x} type descriptor, which is followed by
1480 @samp{s} for a structure tag, @samp{u} for a union tag, or @samp{e} for
1481 a enumerator tag, followed by the name of the tag, followed by @samp{:}.
1482 For example, the following C declarations:
1493 .stabs "bar:G16=*17=xsfoo:",32,0,0,0
1496 Not all debuggers support the @samp{x} type descriptor, so on some
1497 machines GCC does not use it. I believe that for the above example it
1498 would just emit a reference to type 17 and never define it, but I
1499 haven't verified that.
1501 Modula-2 imported types, at least on AIX, use the @samp{i} type
1502 descriptor, which is followed by the name of the module from which the
1503 type is imported, followed by @samp{:}, followed by the name of the
1504 type. There is then optionally a comma followed by type information for
1505 the type. This differs from merely naming the type (@pxref{Typedefs}) in
1506 that it identifies the module; I don't understand whether the name of
1507 the type given here is always just the same as the name we are giving
1508 it, or whether this type descriptor is used with a nameless stab
1509 (@pxref{String Field}), or what. The symbol ends with @samp{;}.
1512 @section Subrange Types
1514 The @samp{r} type descriptor defines a type as a subrange of another
1515 type. It is followed by type information for the type of which it is a
1516 subrange, a semicolon, an integral lower bound, a semicolon, an
1517 integral upper bound, and a semicolon. The AIX documentation does not
1518 specify the trailing semicolon, in an effort to specify array indexes
1519 more cleanly, but a subrange which is not an array index has always
1520 included a trailing semicolon (@pxref{Arrays}).
1522 Instead of an integer, either bound can be one of the following:
1525 @item A @var{offset}
1526 The bound is passed by reference on the stack at offset @var{offset}
1527 from the argument list. @xref{Parameters}, for more information on such
1530 @item T @var{offset}
1531 The bound is passed by value on the stack at offset @var{offset} from
1534 @item a @var{register-number}
1535 The bound is pased by reference in register number
1536 @var{register-number}.
1538 @item t @var{register-number}
1539 The bound is passed by value in register number @var{register-number}.
1545 Subranges are also used for builtin types; see @ref{Traditional Builtin Types}.
1548 @section Array Types
1550 Arrays use the @samp{a} type descriptor. Following the type descriptor
1551 is the type of the index and the type of the array elements. If the
1552 index type is a range type, it ends in a semicolon; otherwise
1553 (for example, if it is a type reference), there does not
1554 appear to be any way to tell where the types are separated. In an
1555 effort to clean up this mess, IBM documents the two types as being
1556 separated by a semicolon, and a range type as not ending in a semicolon
1557 (but this is not right for range types which are not array indexes,
1558 @pxref{Subranges}). I think probably the best solution is to specify
1559 that a semicolon ends a range type, and that the index type and element
1560 type of an array are separated by a semicolon, but that if the index
1561 type is a range type, the extra semicolon can be omitted. GDB (at least
1562 through version 4.9) doesn't support any kind of index type other than a
1563 range anyway; I'm not sure about dbx.
1565 It is well established, and widely used, that the type of the index,
1566 unlike most types found in the stabs, is merely a type definition, not
1567 type information (@pxref{String Field}) (that is, it need not start with
1568 @samp{@var{type-number}=} if it is defining a new type). According to a
1569 comment in GDB, this is also true of the type of the array elements; it
1570 gives @samp{ar1;1;10;ar1;1;10;4} as a legitimate way to express a two
1571 dimensional array. According to AIX documentation, the element type
1572 must be type information. GDB accepts either.
1574 The type of the index is often a range type, expressed as the type
1575 descriptor @samp{r} and some parameters. It defines the size of the
1576 array. In the example below, the range @samp{r1;0;2;} defines an index
1577 type which is a subrange of type 1 (integer), with a lower bound of 0
1578 and an upper bound of 2. This defines the valid range of subscripts of
1579 a three-element C array.
1581 For example, the definition:
1584 char char_vec[3] = @{'a','b','c'@};
1588 produces the output:
1591 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
1600 If an array is @dfn{packed}, the elements are spaced more
1601 closely than normal, saving memory at the expense of speed. For
1602 example, an array of 3-byte objects might, if unpacked, have each
1603 element aligned on a 4-byte boundary, but if packed, have no padding.
1604 One way to specify that something is packed is with type attributes
1605 (@pxref{String Field}). In the case of arrays, another is to use the
1606 @samp{P} type descriptor instead of @samp{a}. Other than specifying a
1607 packed array, @samp{P} is identical to @samp{a}.
1609 @c FIXME-what is it? A pointer?
1610 An open array is represented by the @samp{A} type descriptor followed by
1611 type information specifying the type of the array elements.
1613 @c FIXME: what is the format of this type? A pointer to a vector of pointers?
1614 An N-dimensional dynamic array is represented by
1617 D @var{dimensions} ; @var{type-information}
1620 @c Does dimensions really have this meaning? The AIX documentation
1622 @var{dimensions} is the number of dimensions; @var{type-information}
1623 specifies the type of the array elements.
1625 @c FIXME: what is the format of this type? A pointer to some offsets in
1627 A subarray of an N-dimensional array is represented by
1630 E @var{dimensions} ; @var{type-information}
1633 @c Does dimensions really have this meaning? The AIX documentation
1635 @var{dimensions} is the number of dimensions; @var{type-information}
1636 specifies the type of the array elements.
1641 Some languages, like C or the original Pascal, do not have string types,
1642 they just have related things like arrays of characters. But most
1643 Pascals and various other languages have string types, which are
1644 indicated as follows:
1647 @item n @var{type-information} ; @var{bytes}
1648 @var{bytes} is the maximum length. I'm not sure what
1649 @var{type-information} is; I suspect that it means that this is a string
1650 of @var{type-information} (thus allowing a string of integers, a string
1651 of wide characters, etc., as well as a string of characters). Not sure
1652 what the format of this type is. This is an AIX feature.
1654 @item z @var{type-information} ; @var{bytes}
1655 Just like @samp{n} except that this is a gstring, not an ordinary
1656 string. I don't know the difference.
1659 Pascal Stringptr. What is this? This is an AIX feature.
1663 @section Enumerations
1665 Enumerations are defined with the @samp{e} type descriptor.
1667 @c FIXME: Where does this information properly go? Perhaps it is
1668 @c redundant with something we already explain.
1669 The source line below declares an enumeration type at file scope.
1670 The type definition is located after the @code{N_RBRAC} that marks the end of
1671 the previous procedure's block scope, and before the @code{N_FUN} that marks
1672 the beginning of the next procedure's block scope. Therefore it does not
1673 describe a block local symbol, but a file local one.
1678 enum e_places @{first,second=3,last@};
1682 generates the following stab:
1685 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
1688 The symbol descriptor (@samp{T}) says that the stab describes a
1689 structure, enumeration, or union tag. The type descriptor @samp{e},
1690 following the @samp{22=} of the type definition narrows it down to an
1691 enumeration type. Following the @samp{e} is a list of the elements of
1692 the enumeration. The format is @samp{@var{name}:@var{value},}. The
1693 list of elements ends with @samp{;}.
1695 There is no standard way to specify the size of an enumeration type; it
1696 is determined by the architecture (normally all enumerations types are
1697 32 bits). There should be a way to specify an enumeration type of
1698 another size; type attributes would be one way to do this. @xref{Stabs
1704 The encoding of structures in stabs can be shown with an example.
1706 The following source code declares a structure tag and defines an
1707 instance of the structure in global scope. Then a @code{typedef} equates the
1708 structure tag with a new type. Seperate stabs are generated for the
1709 structure tag, the structure @code{typedef}, and the structure instance. The
1710 stabs for the tag and the @code{typedef} are emited when the definitions are
1711 encountered. Since the structure elements are not initialized, the
1712 stab and code for the structure variable itself is located at the end
1713 of the program in the bss section.
1720 struct s_tag* s_next;
1723 typedef struct s_tag s_typedef;
1726 The structure tag has an @code{N_LSYM} stab type because, like the
1727 enumeration, the symbol has file scope. Like the enumeration, the
1728 symbol descriptor is @samp{T}, for enumeration, structure, or tag type.
1729 The type descriptor @samp{s} following the @samp{16=} of the type
1730 definition narrows the symbol type to structure.
1732 Following the @samp{s} type descriptor is the number of bytes the
1733 structure occupies, followed by a description of each structure element.
1734 The structure element descriptions are of the form @var{name:type, bit
1735 offset from the start of the struct, number of bits in the element}.
1737 @c FIXME: phony line break. Can probably be fixed by using an example
1738 @c with fewer fields.
1741 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;
1742 s_char_vec:17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
1745 In this example, the first two structure elements are previously defined
1746 types. For these, the type following the @samp{@var{name}:} part of the
1747 element description is a simple type reference. The other two structure
1748 elements are new types. In this case there is a type definition
1749 embedded after the @samp{@var{name}:}. The type definition for the
1750 array element looks just like a type definition for a standalone array.
1751 The @code{s_next} field is a pointer to the same kind of structure that
1752 the field is an element of. So the definition of structure type 16
1753 contains a type definition for an element which is a pointer to type 16.
1756 @section Giving a Type a Name
1758 To give a type a name, use the @samp{t} symbol descriptor. The type
1759 is specified by the type information (@pxref{String Field}) for the stab.
1763 .stabs "s_typedef:t16",128,0,0,0 # @r{128 is N_LSYM}
1766 specifies that @code{s_typedef} refers to type number 16. Such stabs
1767 have symbol type @code{N_LSYM} (or @code{C_DECL} for XCOFF).
1769 If you are specifying the tag name for a structure, union, or
1770 enumeration, use the @samp{T} symbol descriptor instead. I believe C is
1771 the only language with this feature.
1773 If the type is an opaque type (I believe this is a Modula-2 feature),
1774 AIX provides a type descriptor to specify it. The type descriptor is
1775 @samp{o} and is followed by a name. I don't know what the name
1776 means---is it always the same as the name of the type, or is this type
1777 descriptor used with a nameless stab (@pxref{String Field})? There
1778 optionally follows a comma followed by type information which defines
1779 the type of this type. If omitted, a semicolon is used in place of the
1780 comma and the type information, and the type is much like a generic
1781 pointer type---it has a known size but little else about it is
1795 This code generates a stab for a union tag and a stab for a union
1796 variable. Both use the @code{N_LSYM} stab type. If a union variable is
1797 scoped locally to the procedure in which it is defined, its stab is
1798 located immediately preceding the @code{N_LBRAC} for the procedure's block
1801 The stab for the union tag, however, is located preceding the code for
1802 the procedure in which it is defined. The stab type is @code{N_LSYM}. This
1803 would seem to imply that the union type is file scope, like the struct
1804 type @code{s_tag}. This is not true. The contents and position of the stab
1805 for @code{u_type} do not convey any infomation about its procedure local
1808 @c FIXME: phony line break. Can probably be fixed by using an example
1809 @c with fewer fields.
1812 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
1816 The symbol descriptor @samp{T}, following the @samp{name:} means that
1817 the stab describes an enumeration, structure, or union tag. The type
1818 descriptor @samp{u}, following the @samp{23=} of the type definition,
1819 narrows it down to a union type definition. Following the @samp{u} is
1820 the number of bytes in the union. After that is a list of union element
1821 descriptions. Their format is @var{name:type, bit offset into the
1822 union, number of bytes for the element;}.
1824 The stab for the union variable is:
1827 .stabs "an_u:23",128,0,0,-20 # @r{128 is N_LSYM}
1830 @samp{-20} specifies where the variable is stored (@pxref{Stack
1833 @node Function Types
1834 @section Function Types
1836 Various types can be defined for function variables. These types are
1837 not used in defining functions (@pxref{Procedures}); they are used for
1838 things like pointers to functions.
1840 The simple, traditional, type is type descriptor @samp{f} is followed by
1841 type information for the return type of the function, followed by a
1844 This does not deal with functions for which the number and types of the
1845 parameters are part of the type, as in Modula-2 or ANSI C. AIX provides
1846 extensions to specify these, using the @samp{f}, @samp{F}, @samp{p}, and
1847 @samp{R} type descriptors.
1849 First comes the type descriptor. If it is @samp{f} or @samp{F}, this
1850 type involves a function rather than a procedure, and the type
1851 information for the return type of the function follows, followed by a
1852 comma. Then comes the number of parameters to the function and a
1853 semicolon. Then, for each parameter, there is the name of the parameter
1854 followed by a colon (this is only present for type descriptors @samp{R}
1855 and @samp{F} which represent Pascal function or procedure parameters),
1856 type information for the parameter, a comma, 0 if passed by reference or
1857 1 if passed by value, and a semicolon. The type definition ends with a
1860 For example, this variable definition:
1867 generates the following code:
1870 .stabs "g_pf:G24=*25=f1",32,0,0,0
1871 .common _g_pf,4,"bss"
1874 The variable defines a new type, 24, which is a pointer to another new
1875 type, 25, which is a function returning @code{int}.
1878 @chapter Symbol Information in Symbol Tables
1880 This chapter describes the format of symbol table entries
1881 and how stab assembler directives map to them. It also describes the
1882 transformations that the assembler and linker make on data from stabs.
1885 * Symbol Table Format::
1886 * Transformations On Symbol Tables::
1889 @node Symbol Table Format
1890 @section Symbol Table Format
1892 Each time the assembler encounters a stab directive, it puts
1893 each field of the stab into a corresponding field in a symbol table
1894 entry of its output file. If the stab contains a string field, the
1895 symbol table entry for that stab points to a string table entry
1896 containing the string data from the stab. Assembler labels become
1897 relocatable addresses. Symbol table entries in a.out have the format:
1899 @c FIXME: should refer to external, not internal.
1901 struct internal_nlist @{
1902 unsigned long n_strx; /* index into string table of name */
1903 unsigned char n_type; /* type of symbol */
1904 unsigned char n_other; /* misc info (usually empty) */
1905 unsigned short n_desc; /* description field */
1906 bfd_vma n_value; /* value of symbol */
1910 If the stab has a string, the @code{n_strx} field holds the offset in
1911 bytes of the string within the string table. The string is terminated
1912 by a NUL character. If the stab lacks a string (for example, it was
1913 produced by a @code{.stabn} or @code{.stabd} directive), the
1914 @code{n_strx} field is zero.
1916 Symbol table entries with @code{n_type} field values greater than 0x1f
1917 originated as stabs generated by the compiler (with one random
1918 exception). The other entries were placed in the symbol table of the
1919 executable by the assembler or the linker.
1921 @node Transformations On Symbol Tables
1922 @section Transformations on Symbol Tables
1924 The linker concatenates object files and does fixups of externally
1927 You can see the transformations made on stab data by the assembler and
1928 linker by examining the symbol table after each pass of the build. To
1929 do this, use @samp{nm -ap}, which dumps the symbol table, including
1930 debugging information, unsorted. For stab entries the columns are:
1931 @var{value}, @var{other}, @var{desc}, @var{type}, @var{string}. For
1932 assembler and linker symbols, the columns are: @var{value}, @var{type},
1935 The low 5 bits of the stab type tell the linker how to relocate the
1936 value of the stab. Thus for stab types like @code{N_RSYM} and
1937 @code{N_LSYM}, where the value is an offset or a register number, the
1938 low 5 bits are @code{N_ABS}, which tells the linker not to relocate the
1941 Where the value of a stab contains an assembly language label,
1942 it is transformed by each build step. The assembler turns it into a
1943 relocatable address and the linker turns it into an absolute address.
1946 * Transformations On Static Variables::
1947 * Transformations On Global Variables::
1950 @node Transformations On Static Variables
1951 @subsection Transformations on Static Variables
1953 This source line defines a static variable at file scope:
1956 static int s_g_repeat
1960 The following stab describes the symbol:
1963 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
1967 The assembler transforms the stab into this symbol table entry in the
1968 @file{.o} file. The location is expressed as a data segment offset.
1971 00000084 - 00 0000 STSYM s_g_repeat:S1
1975 In the symbol table entry from the executable, the linker has made the
1976 relocatable address absolute.
1979 0000e00c - 00 0000 STSYM s_g_repeat:S1
1982 @node Transformations On Global Variables
1983 @subsection Transformations on Global Variables
1985 Stabs for global variables do not contain location information. In
1986 this case, the debugger finds location information in the assembler or
1987 linker symbol table entry describing the variable. The source line:
1997 .stabs "g_foo:G2",32,0,0,0
2000 The variable is represented by two symbol table entries in the object
2001 file (see below). The first one originated as a stab. The second one
2002 is an external symbol. The upper case @samp{D} signifies that the
2003 @code{n_type} field of the symbol table contains 7, @code{N_DATA} with
2004 local linkage. The stab's value is zero since the value is not used for
2005 @code{N_GSYM} stabs. The value of the linker symbol is the relocatable
2006 address corresponding to the variable.
2009 00000000 - 00 0000 GSYM g_foo:G2
2014 These entries as transformed by the linker. The linker symbol table
2015 entry now holds an absolute address:
2018 00000000 - 00 0000 GSYM g_foo:G2
2024 @chapter GNU C++ Stabs
2027 * Basic Cplusplus Types::
2030 * Methods:: Method definition
2032 * Method Modifiers::
2035 * Virtual Base Classes::
2039 Type descriptors added for C++ descriptions:
2043 method type (@code{##} if minimal debug)
2046 Member (class and variable) type. It is followed by type information
2047 for the offset basetype, a comma, and type information for the type of
2048 the field being pointed to. (FIXME: this is acknowledged to be
2049 gibberish. Can anyone say what really goes here?).
2051 Note that there is a conflict between this and type attributes
2052 (@pxref{String Field}); both use type descriptor @samp{@@}.
2053 Fortunately, the @samp{@@} type descriptor used in this C++ sense always
2054 will be followed by a digit, @samp{(}, or @samp{-}, and type attributes
2055 never start with those things.
2058 @node Basic Cplusplus Types
2059 @section Basic Types For C++
2061 << the examples that follow are based on a01.C >>
2064 C++ adds two more builtin types to the set defined for C. These are
2065 the unknown type and the vtable record type. The unknown type, type
2066 16, is defined in terms of itself like the void type.
2068 The vtable record type, type 17, is defined as a structure type and
2069 then as a structure tag. The structure has four fields: delta, index,
2070 pfn, and delta2. pfn is the function pointer.
2072 << In boilerplate $vtbl_ptr_type, what are the fields delta,
2073 index, and delta2 used for? >>
2075 This basic type is present in all C++ programs even if there are no
2076 virtual methods defined.
2079 .stabs "struct_name:sym_desc(type)type_def(17)=type_desc(struct)struct_bytes(8)
2080 elem_name(delta):type_ref(short int),bit_offset(0),field_bits(16);
2081 elem_name(index):type_ref(short int),bit_offset(16),field_bits(16);
2082 elem_name(pfn):type_def(18)=type_desc(ptr to)type_ref(void),
2083 bit_offset(32),field_bits(32);
2084 elem_name(delta2):type_def(short int);bit_offset(32),field_bits(16);;"
2089 .stabs "$vtbl_ptr_type:t17=s8
2090 delta:6,0,16;index:6,16,16;pfn:18=*15,32,32;delta2:6,32,16;;"
2095 .stabs "name:sym_dec(struct tag)type_ref($vtbl_ptr_type)",N_LSYM,NIL,NIL,NIL
2099 .stabs "$vtbl_ptr_type:T17",128,0,0,0
2102 @node Simple Classes
2103 @section Simple Class Definition
2105 The stabs describing C++ language features are an extension of the
2106 stabs describing C. Stabs representing C++ class types elaborate
2107 extensively on the stab format used to describe structure types in C.
2108 Stabs representing class type variables look just like stabs
2109 representing C language variables.
2111 Consider the following very simple class definition.
2117 int Ameth(int in, char other);
2121 The class @code{baseA} is represented by two stabs. The first stab describes
2122 the class as a structure type. The second stab describes a structure
2123 tag of the class type. Both stabs are of stab type @code{N_LSYM}. Since the
2124 stab is not located between an @code{N_FUN} and an @code{N_LBRAC} stab this indicates
2125 that the class is defined at file scope. If it were, then the @code{N_LSYM}
2126 would signify a local variable.
2128 A stab describing a C++ class type is similar in format to a stab
2129 describing a C struct, with each class member shown as a field in the
2130 structure. The part of the struct format describing fields is
2131 expanded to include extra information relevent to C++ class members.
2132 In addition, if the class has multiple base classes or virtual
2133 functions the struct format outside of the field parts is also
2136 In this simple example the field part of the C++ class stab
2137 representing member data looks just like the field part of a C struct
2138 stab. The section on protections describes how its format is
2139 sometimes extended for member data.
2141 The field part of a C++ class stab representing a member function
2142 differs substantially from the field part of a C struct stab. It
2143 still begins with @samp{name:} but then goes on to define a new type number
2144 for the member function, describe its return type, its argument types,
2145 its protection level, any qualifiers applied to the method definition,
2146 and whether the method is virtual or not. If the method is virtual
2147 then the method description goes on to give the vtable index of the
2148 method, and the type number of the first base class defining the
2151 When the field name is a method name it is followed by two colons rather
2152 than one. This is followed by a new type definition for the method.
2153 This is a number followed by an equal sign and the type descriptor
2154 @samp{#}, indicating a method type, and a second @samp{#}, indicating
2155 that this is the @dfn{minimal} type of method definition used by GCC2,
2156 not larger method definitions used by earlier versions of GCC. This is
2157 followed by a type reference showing the return type of the method and a
2160 The format of an overloaded operator method name differs from that of
2161 other methods. It is @samp{op$::@var{operator-name}.} where
2162 @var{operator-name} is the operator name such as @samp{+} or @samp{+=}.
2163 The name ends with a period, and any characters except the period can
2164 occur in the @var{operator-name} string.
2166 The next part of the method description represents the arguments to the
2167 method, preceeded by a colon and ending with a semi-colon. The types of
2168 the arguments are expressed in the same way argument types are expressed
2169 in C++ name mangling. In this example an @code{int} and a @code{char}
2172 This is followed by a number, a letter, and an asterisk or period,
2173 followed by another semicolon. The number indicates the protections
2174 that apply to the member function. Here the 2 means public. The
2175 letter encodes any qualifier applied to the method definition. In
2176 this case, @samp{A} means that it is a normal function definition. The dot
2177 shows that the method is not virtual. The sections that follow
2178 elaborate further on these fields and describe the additional
2179 information present for virtual methods.
2183 .stabs "class_name:sym_desc(type)type_def(20)=type_desc(struct)struct_bytes(4)
2184 field_name(Adat):type(int),bit_offset(0),field_bits(32);
2186 method_name(Ameth)::type_def(21)=type_desc(method)return_type(int);
2187 :arg_types(int char);
2188 protection(public)qualifier(normal)virtual(no);;"
2193 .stabs "baseA:t20=s4Adat:1,0,32;Ameth::21=##1;:ic;2A.;;",128,0,0,0
2195 .stabs "class_name:sym_desc(struct tag)",N_LSYM,NIL,NIL,NIL
2197 .stabs "baseA:T20",128,0,0,0
2200 @node Class Instance
2201 @section Class Instance
2203 As shown above, describing even a simple C++ class definition is
2204 accomplished by massively extending the stab format used in C to
2205 describe structure types. However, once the class is defined, C stabs
2206 with no modifications can be used to describe class instances. The
2216 yields the following stab describing the class instance. It looks no
2217 different from a standard C stab describing a local variable.
2220 .stabs "name:type_ref(baseA)", N_LSYM, NIL, NIL, frame_ptr_offset
2224 .stabs "AbaseA:20",128,0,0,-20
2228 @section Method Defintion
2230 The class definition shown above declares Ameth. The C++ source below
2235 baseA::Ameth(int in, char other)
2242 This method definition yields three stabs following the code of the
2243 method. One stab describes the method itself and following two describe
2244 its parameters. Although there is only one formal argument all methods
2245 have an implicit argument which is the @code{this} pointer. The @code{this}
2246 pointer is a pointer to the object on which the method was called. Note
2247 that the method name is mangled to encode the class name and argument
2248 types. Name mangling is described in the @sc{arm} (@cite{The Annotated
2249 C++ Reference Manual}, by Ellis and Stroustrup, @sc{isbn}
2250 0-201-51459-1); @file{gpcompare.texi} in Cygnus GCC distributions
2251 describes the differences between GNU mangling and @sc{arm}
2253 @c FIXME: Use @xref, especially if this is generally installed in the
2255 @c FIXME: This information should be in a net release, either of GCC or
2256 @c GDB. But gpcompare.texi doesn't seem to be in the FSF GCC.
2259 .stabs "name:symbol_desriptor(global function)return_type(int)",
2260 N_FUN, NIL, NIL, code_addr_of_method_start
2262 .stabs "Ameth__5baseAic:F1",36,0,0,_Ameth__5baseAic
2265 Here is the stab for the @code{this} pointer implicit argument. The
2266 name of the @code{this} pointer is always @code{this}. Type 19, the
2267 @code{this} pointer is defined as a pointer to type 20, @code{baseA},
2268 but a stab defining @code{baseA} has not yet been emited. Since the
2269 compiler knows it will be emited shortly, here it just outputs a cross
2270 reference to the undefined symbol, by prefixing the symbol name with
2274 .stabs "name:sym_desc(register param)type_def(19)=
2275 type_desc(ptr to)type_ref(baseA)=
2276 type_desc(cross-reference to)baseA:",N_RSYM,NIL,NIL,register_number
2278 .stabs "this:P19=*20=xsbaseA:",64,0,0,8
2281 The stab for the explicit integer argument looks just like a parameter
2282 to a C function. The last field of the stab is the offset from the
2283 argument pointer, which in most systems is the same as the frame
2287 .stabs "name:sym_desc(value parameter)type_ref(int)",
2288 N_PSYM,NIL,NIL,offset_from_arg_ptr
2290 .stabs "in:p1",160,0,0,72
2293 << The examples that follow are based on A1.C >>
2296 @section Protections
2299 In the simple class definition shown above all member data and
2300 functions were publicly accessable. The example that follows
2301 contrasts public, protected and privately accessable fields and shows
2302 how these protections are encoded in C++ stabs.
2304 @c FIXME: What does "part of the string" mean?
2305 Protections for class member data are signified by two characters
2306 embedded in the stab defining the class type. These characters are
2307 located after the name: part of the string. @samp{/0} means private,
2308 @samp{/1} means protected, and @samp{/2} means public. If these
2309 characters are omited this means that the member is public. The
2310 following C++ source:
2324 generates the following stab to describe the class type all_data.
2327 .stabs "class_name:sym_desc(type)type_def(19)=type_desc(struct)struct_bytes
2328 data_name:/protection(private)type_ref(int),bit_offset,num_bits;
2329 data_name:/protection(protected)type_ref(char),bit_offset,num_bits;
2330 data_name:(/num omited, private)type_ref(float),bit_offset,num_bits;;"
2335 .stabs "all_data:t19=s12
2336 priv_dat:/01,0,32;prot_dat:/12,32,8;pub_dat:12,64,32;;",128,0,0,0
2339 Protections for member functions are signified by one digit embeded in
2340 the field part of the stab describing the method. The digit is 0 if
2341 private, 1 if protected and 2 if public. Consider the C++ class
2345 class all_methods @{
2347 int priv_meth(int in)@{return in;@};
2349 char protMeth(char in)@{return in;@};
2351 float pubMeth(float in)@{return in;@};
2355 It generates the following stab. The digit in question is to the left
2356 of an @samp{A} in each case. Notice also that in this case two symbol
2357 descriptors apply to the class name struct tag and struct type.
2360 .stabs "class_name:sym_desc(struct tag&type)type_def(21)=
2361 sym_desc(struct)struct_bytes(1)
2362 meth_name::type_def(22)=sym_desc(method)returning(int);
2363 :args(int);protection(private)modifier(normal)virtual(no);
2364 meth_name::type_def(23)=sym_desc(method)returning(char);
2365 :args(char);protection(protected)modifier(normal)virual(no);
2366 meth_name::type_def(24)=sym_desc(method)returning(float);
2367 :args(float);protection(public)modifier(normal)virtual(no);;",
2372 .stabs "all_methods:Tt21=s1priv_meth::22=##1;:i;0A.;protMeth::23=##2;:c;1A.;
2373 pubMeth::24=##12;:f;2A.;;",128,0,0,0
2376 @node Method Modifiers
2377 @section Method Modifiers (@code{const}, @code{volatile}, @code{const volatile})
2381 In the class example described above all the methods have the normal
2382 modifier. This method modifier information is located just after the
2383 protection information for the method. This field has four possible
2384 character values. Normal methods use @samp{A}, const methods use
2385 @samp{B}, volatile methods use @samp{C}, and const volatile methods use
2386 @samp{D}. Consider the class definition below:
2391 int ConstMeth (int arg) const @{ return arg; @};
2392 char VolatileMeth (char arg) volatile @{ return arg; @};
2393 float ConstVolMeth (float arg) const volatile @{return arg; @};
2397 This class is described by the following stab:
2400 .stabs "class(A):sym_desc(struct)type_def(20)=type_desc(struct)struct_bytes(1)
2401 meth_name(ConstMeth)::type_def(21)sym_desc(method)
2402 returning(int);:arg(int);protection(public)modifier(const)virtual(no);
2403 meth_name(VolatileMeth)::type_def(22)=sym_desc(method)
2404 returning(char);:arg(char);protection(public)modifier(volatile)virt(no)
2405 meth_name(ConstVolMeth)::type_def(23)=sym_desc(method)
2406 returning(float);:arg(float);protection(public)modifer(const volatile)
2407 virtual(no);;", @dots{}
2411 .stabs "A:T20=s1ConstMeth::21=##1;:i;2B.;VolatileMeth::22=##2;:c;2C.;
2412 ConstVolMeth::23=##12;:f;2D.;;",128,0,0,0
2415 @node Virtual Methods
2416 @section Virtual Methods
2418 << The following examples are based on a4.C >>
2420 The presence of virtual methods in a class definition adds additional
2421 data to the class description. The extra data is appended to the
2422 description of the virtual method and to the end of the class
2423 description. Consider the class definition below:
2429 virtual int A_virt (int arg) @{ return arg; @};
2433 This results in the stab below describing class A. It defines a new
2434 type (20) which is an 8 byte structure. The first field of the class
2435 struct is @samp{Adat}, an integer, starting at structure offset 0 and
2438 The second field in the class struct is not explicitly defined by the
2439 C++ class definition but is implied by the fact that the class
2440 contains a virtual method. This field is the vtable pointer. The
2441 name of the vtable pointer field starts with @samp{$vf} and continues with a
2442 type reference to the class it is part of. In this example the type
2443 reference for class A is 20 so the name of its vtable pointer field is
2444 @samp{$vf20}, followed by the usual colon.
2446 Next there is a type definition for the vtable pointer type (21).
2447 This is in turn defined as a pointer to another new type (22).
2449 Type 22 is the vtable itself, which is defined as an array, indexed by
2450 a range of integers between 0 and 1, and whose elements are of type
2451 17. Type 17 was the vtable record type defined by the boilerplate C++
2452 type definitions, as shown earlier.
2454 The bit offset of the vtable pointer field is 32. The number of bits
2455 in the field are not specified when the field is a vtable pointer.
2457 Next is the method definition for the virtual member function @code{A_virt}.
2458 Its description starts out using the same format as the non-virtual
2459 member functions described above, except instead of a dot after the
2460 @samp{A} there is an asterisk, indicating that the function is virtual.
2461 Since is is virtual some addition information is appended to the end
2462 of the method description.
2464 The first number represents the vtable index of the method. This is a
2465 32 bit unsigned number with the high bit set, followed by a
2468 The second number is a type reference to the first base class in the
2469 inheritence hierarchy defining the virtual member function. In this
2470 case the class stab describes a base class so the virtual function is
2471 not overriding any other definition of the method. Therefore the
2472 reference is to the type number of the class that the stab is
2475 This is followed by three semi-colons. One marks the end of the
2476 current sub-section, one marks the end of the method field, and the
2477 third marks the end of the struct definition.
2479 For classes containing virtual functions the very last section of the
2480 string part of the stab holds a type reference to the first base
2481 class. This is preceeded by @samp{~%} and followed by a final semi-colon.
2484 .stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8)
2485 field_name(Adat):type_ref(int),bit_offset(0),field_bits(32);
2486 field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)=
2487 sym_desc(array)index_type_ref(range of int from 0 to 1);
2488 elem_type_ref(vtbl elem type),
2490 meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
2491 :arg_type(int),protection(public)normal(yes)virtual(yes)
2492 vtable_index(1);class_first_defining(A);;;~%first_base(A);",
2496 @c FIXME: bogus line break.
2498 .stabs "A:t20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2499 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2503 @section Inheritence
2505 Stabs describing C++ derived classes include additional sections that
2506 describe the inheritence hierarchy of the class. A derived class stab
2507 also encodes the number of base classes. For each base class it tells
2508 if the base class is virtual or not, and if the inheritence is private
2509 or public. It also gives the offset into the object of the portion of
2510 the object corresponding to each base class.
2512 This additional information is embeded in the class stab following the
2513 number of bytes in the struct. First the number of base classes
2514 appears bracketed by an exclamation point and a comma.
2516 Then for each base type there repeats a series: two digits, a number,
2517 a comma, another number, and a semi-colon.
2519 The first of the two digits is 1 if the base class is virtual and 0 if
2520 not. The second digit is 2 if the derivation is public and 0 if not.
2522 The number following the first two digits is the offset from the start
2523 of the object to the part of the object pertaining to the base class.
2525 After the comma, the second number is a type_descriptor for the base
2526 type. Finally a semi-colon ends the series, which repeats for each
2529 The source below defines three base classes @code{A}, @code{B}, and
2530 @code{C} and the derived class @code{D}.
2537 virtual int A_virt (int arg) @{ return arg; @};
2543 virtual int B_virt (int arg) @{return arg; @};
2549 virtual int C_virt (int arg) @{return arg; @};
2552 class D : A, virtual B, public C @{
2555 virtual int A_virt (int arg ) @{ return arg+1; @};
2556 virtual int B_virt (int arg) @{ return arg+2; @};
2557 virtual int C_virt (int arg) @{ return arg+3; @};
2558 virtual int D_virt (int arg) @{ return arg; @};
2562 Class stabs similar to the ones described earlier are generated for
2565 @c FIXME!!! the linebreaks in the following example probably make the
2566 @c examples literally unusable, but I don't know any other way to get
2567 @c them on the page.
2568 @c One solution would be to put some of the type definitions into
2569 @c separate stabs, even if that's not exactly what the compiler actually
2572 .stabs "A:T20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2573 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2575 .stabs "B:Tt25=s8Bdat:1,0,32;$vf25:21,32;B_virt::26=##1;
2576 :i;2A*-2147483647;25;;;~%25;",128,0,0,0
2578 .stabs "C:Tt28=s8Cdat:1,0,32;$vf28:21,32;C_virt::29=##1;
2579 :i;2A*-2147483647;28;;;~%28;",128,0,0,0
2582 In the stab describing derived class @code{D} below, the information about
2583 the derivation of this class is encoded as follows.
2586 .stabs "derived_class_name:symbol_descriptors(struct tag&type)=
2587 type_descriptor(struct)struct_bytes(32)!num_bases(3),
2588 base_virtual(no)inheritence_public(no)base_offset(0),
2589 base_class_type_ref(A);
2590 base_virtual(yes)inheritence_public(no)base_offset(NIL),
2591 base_class_type_ref(B);
2592 base_virtual(no)inheritence_public(yes)base_offset(64),
2593 base_class_type_ref(C); @dots{}
2596 @c FIXME! fake linebreaks.
2598 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:
2599 1,160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt:
2600 :32:i;2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;
2601 28;;D_virt::32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2604 @node Virtual Base Classes
2605 @section Virtual Base Classes
2607 A derived class object consists of a concatination in memory of the data
2608 areas defined by each base class, starting with the leftmost and ending
2609 with the rightmost in the list of base classes. The exception to this
2610 rule is for virtual inheritence. In the example above, class @code{D}
2611 inherits virtually from base class @code{B}. This means that an
2612 instance of a @code{D} object will not contain its own @code{B} part but
2613 merely a pointer to a @code{B} part, known as a virtual base pointer.
2615 In a derived class stab, the base offset part of the derivation
2616 information, described above, shows how the base class parts are
2617 ordered. The base offset for a virtual base class is always given as 0.
2618 Notice that the base offset for @code{B} is given as 0 even though
2619 @code{B} is not the first base class. The first base class @code{A}
2622 The field information part of the stab for class @code{D} describes the field
2623 which is the pointer to the virtual base class @code{B}. The vbase pointer
2624 name is @samp{$vb} followed by a type reference to the virtual base class.
2625 Since the type id for @code{B} in this example is 25, the vbase pointer name
2628 @c FIXME!! fake linebreaks below
2630 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1,
2631 160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i;
2632 2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt:
2633 :32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2636 Following the name and a semicolon is a type reference describing the
2637 type of the virtual base class pointer, in this case 24. Type 24 was
2638 defined earlier as the type of the @code{B} class @code{this} pointer. The
2639 @code{this} pointer for a class is a pointer to the class type.
2642 .stabs "this:P24=*25=xsB:",64,0,0,8
2645 Finally the field offset part of the vbase pointer field description
2646 shows that the vbase pointer is the first field in the @code{D} object,
2647 before any data fields defined by the class. The layout of a @code{D}
2648 class object is a follows, @code{Adat} at 0, the vtable pointer for
2649 @code{A} at 32, @code{Cdat} at 64, the vtable pointer for C at 96, the
2650 virtual base pointer for @code{B} at 128, and @code{Ddat} at 160.
2653 @node Static Members
2654 @section Static Members
2656 The data area for a class is a concatenation of the space used by the
2657 data members of the class. If the class has virtual methods, a vtable
2658 pointer follows the class data. The field offset part of each field
2659 description in the class stab shows this ordering.
2661 << How is this reflected in stabs? See Cygnus bug #677 for some info. >>
2664 @appendix Table of Stab Types
2666 The following are all the possible values for the stab type field, for
2667 @code{a.out} files, in numeric order. This does not apply to XCOFF, but
2668 it does apply to stabs in ELF. Stabs in ECOFF use these values but add
2669 0x8f300 to distinguish them from non-stab symbols.
2671 The symbolic names are defined in the file @file{include/aout/stabs.def}.
2674 * Non-Stab Symbol Types:: Types from 0 to 0x1f
2675 * Stab Symbol Types:: Types from 0x20 to 0xff
2678 @node Non-Stab Symbol Types
2679 @appendixsec Non-Stab Symbol Types
2681 The following types are used by the linker and assembler, not by stab
2682 directives. Since this document does not attempt to describe aspects of
2683 object file format other than the debugging format, no details are
2686 @c Try to get most of these to fit on a single line.
2696 File scope absolute symbol
2698 @item 0x3 N_ABS | N_EXT
2699 External absolute symbol
2702 File scope text symbol
2704 @item 0x5 N_TEXT | N_EXT
2705 External text symbol
2708 File scope data symbol
2710 @item 0x7 N_DATA | N_EXT
2711 External data symbol
2714 File scope BSS symbol
2716 @item 0x9 N_BSS | N_EXT
2720 Same as @code{N_FN}, for Sequent compilers
2723 Symbol is indirected to another symbol
2726 Common---visible after shared library dynamic link
2729 Absolute set element
2732 Text segment set element
2735 Data segment set element
2738 BSS segment set element
2741 Pointer to set vector
2743 @item 0x1e N_WARNING
2744 Print a warning message during linking
2747 File name of a @file{.o} file
2750 @node Stab Symbol Types
2751 @appendixsec Stab Symbol Types
2753 The following symbol types indicate that this is a stab. This is the
2754 full list of stab numbers, including stab types that are used in
2755 languages other than C.
2759 Global symbol; see @ref{Global Variables}.
2762 Function name (for BSD Fortran); see @ref{Procedures}.
2765 Function name (@pxref{Procedures}) or text segment variable
2769 Data segment file-scope variable; see @ref{Statics}.
2772 BSS segment file-scope variable; see @ref{Statics}.
2775 Name of main routine; see @ref{Main Program}.
2777 @c FIXME: discuss this in the Statics node where we talk about
2778 @c the fact that the n_type indicates the section.
2780 Variable in @code{.rodata} section; see @ref{Statics}.
2783 Global symbol (for Pascal); see @ref{N_PC}.
2786 Number of symbols (according to Ultrix V4.0); see @ref{N_NSYMS}.
2789 No DST map; see @ref{N_NOMAP}.
2791 @c FIXME: describe this solaris feature in the body of the text (see
2792 @c comments in include/aout/stab.def).
2794 Object file (Solaris2).
2796 @c See include/aout/stab.def for (a little) more info.
2798 Debugger options (Solaris2).
2801 Register variable; see @ref{Register Variables}.
2804 Modula-2 compilation unit; see @ref{N_M2C}.
2807 Line number in text segment; see @ref{Line Numbers}.
2810 Line number in data segment; see @ref{Line Numbers}.
2813 Line number in bss segment; see @ref{Line Numbers}.
2816 Sun source code browser, path to @file{.cb} file; see @ref{N_BROWS}.
2819 GNU Modula2 definition module dependency; see @ref{N_DEFD}.
2822 Function start/body/end line numbers (Solaris2).
2825 GNU C++ exception variable; see @ref{N_EHDECL}.
2828 Modula2 info "for imc" (according to Ultrix V4.0); see @ref{N_MOD2}.
2831 GNU C++ @code{catch} clause; see @ref{N_CATCH}.
2834 Structure of union element; see @ref{N_SSYM}.
2837 Last stab for module (Solaris2).
2840 Path and name of source file; see @ref{Source Files}.
2843 Stack variable (@pxref{Stack Variables}) or type (@pxref{Typedefs}).
2846 Beginning of an include file (Sun only); see @ref{Include Files}.
2849 Name of include file; see @ref{Include Files}.
2852 Parameter variable; see @ref{Parameters}.
2855 End of an include file; see @ref{Include Files}.
2858 Alternate entry point; see @ref{N_ENTRY}.
2861 Beginning of a lexical block; see @ref{Block Structure}.
2864 Place holder for a deleted include file; see @ref{Include Files}.
2867 Modula2 scope information (Sun linker); see @ref{N_SCOPE}.
2870 End of a lexical block; see @ref{Block Structure}.
2873 Begin named common block; see @ref{Common Blocks}.
2876 End named common block; see @ref{Common Blocks}.
2879 Member of a common block; see @ref{Common Blocks}.
2881 @c FIXME: How does this really work? Move it to main body of document.
2883 Pascal @code{with} statement: type,,0,0,offset (Solaris2).
2886 Gould non-base registers; see @ref{Gould}.
2889 Gould non-base registers; see @ref{Gould}.
2892 Gould non-base registers; see @ref{Gould}.
2895 Gould non-base registers; see @ref{Gould}.
2898 Gould non-base registers; see @ref{Gould}.
2901 @c Restore the default table indent
2906 @node Symbol Descriptors
2907 @appendix Table of Symbol Descriptors
2909 The symbol descriptor is the character which follows the colon in many
2910 stabs, and which tells what kind of stab it is. @xref{String Field},
2911 for more information about their use.
2913 @c Please keep this alphabetical
2915 @c In TeX, this looks great, digit is in italics. But makeinfo insists
2916 @c on putting it in `', not realizing that @var should override @code.
2917 @c I don't know of any way to make makeinfo do the right thing. Seems
2918 @c like a makeinfo bug to me.
2922 Variable on the stack; see @ref{Stack Variables}.
2925 Parameter passed by reference in register; see @ref{Reference Parameters}.
2928 Based variable; see @ref{Parameters}.
2931 Constant; see @ref{Constants}.
2934 Conformant array bound (Pascal, maybe other languages); @ref{Conformant
2935 Arrays}. Name of a caught exception (GNU C++). These can be
2936 distinguished because the latter uses @code{N_CATCH} and the former uses
2937 another symbol type.
2940 Floating point register variable; see @ref{Register Variables}.
2943 Parameter in floating point register; see @ref{Register Parameters}.
2946 File scope function; see @ref{Procedures}.
2949 Global function; see @ref{Procedures}.
2952 Global variable; see @ref{Global Variables}.
2955 @xref{Register Parameters}.
2958 Internal (nested) procedure; see @ref{Nested Procedures}.
2961 Internal (nested) function; see @ref{Nested Procedures}.
2964 Label name (documented by AIX, no further information known).
2967 Module; see @ref{Procedures}.
2970 Argument list parameter; see @ref{Parameters}.
2976 Fortran Function parameter; see @ref{Parameters}.
2979 Unfortunately, three separate meanings have been independently invented
2980 for this symbol descriptor. At least the GNU and Sun uses can be
2981 distinguished by the symbol type. Global Procedure (AIX) (symbol type
2982 used unknown); see @ref{Procedures}. Register parameter (GNU) (symbol
2983 type @code{N_PSYM}); see @ref{Parameters}. Prototype of function
2984 referenced by this file (Sun @code{acc}) (symbol type @code{N_FUN}).
2987 Static Procedure; see @ref{Procedures}.
2990 Register parameter; see @ref{Register Parameters}.
2993 Register variable; see @ref{Register Variables}.
2996 File scope variable; see @ref{Statics}.
2999 Type name; see @ref{Typedefs}.
3002 Enumeration, structure, or union tag; see @ref{Typedefs}.
3005 Parameter passed by reference; see @ref{Reference Parameters}.
3008 Procedure scope static variable; see @ref{Statics}.
3011 Conformant array; see @ref{Conformant Arrays}.
3014 Function return variable; see @ref{Parameters}.
3017 @node Type Descriptors
3018 @appendix Table of Type Descriptors
3020 The type descriptor is the character which follows the type number and
3021 an equals sign. It specifies what kind of type is being defined.
3022 @xref{String Field}, for more information about their use.
3027 Type reference; see @ref{String Field}.
3030 Reference to builtin type; see @ref{Negative Type Numbers}.
3033 Method (C++); see @ref{Cplusplus}.
3036 Pointer; see @ref{Miscellaneous Types}.
3042 Type Attributes (AIX); see @ref{String Field}. Member (class and variable)
3043 type (GNU C++); see @ref{Cplusplus}.
3046 Array; see @ref{Arrays}.
3049 Open array; see @ref{Arrays}.
3052 Pascal space type (AIX); see @ref{Miscellaneous Types}. Builtin integer
3053 type (Sun); see @ref{Builtin Type Descriptors}.
3056 Volatile-qualified type; see @ref{Miscellaneous Types}.
3059 Complex builtin type; see @ref{Builtin Type Descriptors}.
3062 COBOL Picture type. See AIX documentation for details.
3065 File type; see @ref{Miscellaneous Types}.
3068 N-dimensional dynamic array; see @ref{Arrays}.
3071 Enumeration type; see @ref{Enumerations}.
3074 N-dimensional subarray; see @ref{Arrays}.
3077 Function type; see @ref{Function Types}.
3080 Pascal function parameter; see @ref{Function Types}
3083 Builtin floating point type; see @ref{Builtin Type Descriptors}.
3086 COBOL Group. See AIX documentation for details.
3089 Imported type; see @ref{Cross-References}.
3092 Const-qualified type; see @ref{Miscellaneous Types}.
3095 COBOL File Descriptor. See AIX documentation for details.
3098 Multiple instance type; see @ref{Miscellaneous Types}.
3101 String type; see @ref{Strings}.
3104 Stringptr; see @ref{Strings}.
3107 Opaque type; see @ref{Typedefs}.
3110 Procedure; see @ref{Function Types}.
3113 Packed array; see @ref{Arrays}.
3116 Range type; see @ref{Subranges}.
3119 Builtin floating type; see @ref{Builtin Type Descriptors} (Sun). Pascal
3120 subroutine parameter; see @ref{Function Types} (AIX). Detecting this
3121 conflict is possible with careful parsing (hint: a Pascal subroutine
3122 parameter type will always contain a comma, and a builtin type
3123 descriptor never will).
3126 Structure type; see @ref{Structures}.
3129 Set type; see @ref{Miscellaneous Types}.
3132 Union; see @ref{Unions}.
3135 Variant record. This is a Pascal and Modula-2 feature which is like a
3136 union within a struct in C. See AIX documentation for details.
3139 Wide character; see @ref{Builtin Type Descriptors}.
3142 Cross-reference; see @ref{Cross-References}.
3145 gstring; see @ref{Strings}.
3148 @node Expanded Reference
3149 @appendix Expanded Reference by Stab Type
3151 @c FIXME: This appendix should go away; see N_PSYM or N_SO for an example.
3153 For a full list of stab types, and cross-references to where they are
3154 described, see @ref{Stab Types}. This appendix just duplicates certain
3155 information from the main body of this document; eventually the
3156 information will all be in one place.
3160 The first line is the symbol type (see @file{include/aout/stab.def}).
3162 The second line describes the language constructs the symbol type
3165 The third line is the stab format with the significant stab fields
3166 named and the rest NIL.
3168 Subsequent lines expand upon the meaning and possible values for each
3169 significant stab field. @samp{#} stands in for the type descriptor.
3171 Finally, any further information.
3174 * N_PC:: Pascal global symbol
3175 * N_NSYMS:: Number of symbols
3176 * N_NOMAP:: No DST map
3177 * N_M2C:: Modula-2 compilation unit
3178 * N_BROWS:: Path to .cb file for Sun source code browser
3179 * N_DEFD:: GNU Modula2 definition module dependency
3180 * N_EHDECL:: GNU C++ exception variable
3181 * N_MOD2:: Modula2 information "for imc"
3182 * N_CATCH:: GNU C++ "catch" clause
3183 * N_SSYM:: Structure or union element
3184 * N_ENTRY:: Alternate entry point
3185 * N_SCOPE:: Modula2 scope information (Sun only)
3186 * Gould:: non-base register symbols used on Gould systems
3187 * N_LENG:: Length of preceding entry
3193 @deffn @code{.stabs} N_PC
3195 Global symbol (for Pascal).
3198 "name" -> "symbol_name" <<?>>
3199 value -> supposedly the line number (stab.def is skeptical)
3203 @file{stabdump.c} says:
3205 global pascal symbol: name,,0,subtype,line
3213 @deffn @code{.stabn} N_NSYMS
3215 Number of symbols (according to Ultrix V4.0).
3218 0, files,,funcs,lines (stab.def)
3225 @deffn @code{.stabs} N_NOMAP
3227 No DST map for symbol (according to Ultrix V4.0). I think this means a
3228 variable has been optimized out.
3231 name, ,0,type,ignored (stab.def)
3238 @deffn @code{.stabs} N_M2C
3240 Modula-2 compilation unit.
3243 "string" -> "unit_name,unit_time_stamp[,code_time_stamp]"
3245 value -> 0 (main unit)
3253 @deffn @code{.stabs} N_BROWS
3255 Sun source code browser, path to @file{.cb} file
3258 "path to associated @file{.cb} file"
3260 Note: N_BROWS has the same value as N_BSLINE.
3266 @deffn @code{.stabn} N_DEFD
3268 GNU Modula2 definition module dependency.
3270 GNU Modula-2 definition module dependency. The value is the
3271 modification time of the definition file. The other field is non-zero
3272 if it is imported with the GNU M2 keyword @code{%INITIALIZE}. Perhaps
3273 @code{N_M2C} can be used if there are enough empty fields?
3279 @deffn @code{.stabs} N_EHDECL
3281 GNU C++ exception variable <<?>>.
3283 "@var{string} is variable name"
3285 Note: conflicts with @code{N_MOD2}.
3291 @deffn @code{.stab?} N_MOD2
3293 Modula2 info "for imc" (according to Ultrix V4.0)
3295 Note: conflicts with @code{N_EHDECL} <<?>>
3301 @deffn @code{.stabn} N_CATCH
3303 GNU C++ @code{catch} clause
3305 GNU C++ @code{catch} clause. The value is its address. The desc field
3306 is nonzero if this entry is immediately followed by a @code{CAUGHT} stab
3307 saying what exception was caught. Multiple @code{CAUGHT} stabs means
3308 that multiple exceptions can be caught here. If desc is 0, it means all
3309 exceptions are caught here.
3315 @deffn @code{.stabn} N_SSYM
3317 Structure or union element.
3319 The value is the offset in the structure.
3321 <<?looking at structs and unions in C I didn't see these>>
3327 @deffn @code{.stabn} N_ENTRY
3329 Alternate entry point.
3330 The value is its address.
3337 @deffn @code{.stab?} N_SCOPE
3339 Modula2 scope information (Sun linker)
3344 @section Non-base registers on Gould systems
3346 @deffn @code{.stab?} N_NBTEXT
3347 @deffnx @code{.stab?} N_NBDATA
3348 @deffnx @code{.stab?} N_NBBSS
3349 @deffnx @code{.stab?} N_NBSTS
3350 @deffnx @code{.stab?} N_NBLCS
3356 These are used on Gould systems for non-base registers syms.
3358 However, the following values are not the values used by Gould; they are
3359 the values which GNU has been documenting for these values for a long
3360 time, without actually checking what Gould uses. I include these values
3361 only because perhaps some someone actually did something with the GNU
3362 information (I hope not, why GNU knowingly assigned wrong values to
3363 these in the header file is a complete mystery to me).
3366 240 0xf0 N_NBTEXT ??
3367 242 0xf2 N_NBDATA ??
3377 @deffn @code{.stabn} N_LENG
3379 Second symbol entry containing a length-value for the preceding entry.
3380 The value is the length.
3384 @appendix Questions and Anomalies
3388 @c I think this is changed in GCC 2.4.5 to put the line number there.
3389 For GNU C stabs defining local and global variables (@code{N_LSYM} and
3390 @code{N_GSYM}), the desc field is supposed to contain the source
3391 line number on which the variable is defined. In reality the desc
3392 field is always 0. (This behavior is defined in @file{dbxout.c} and
3393 putting a line number in desc is controlled by @samp{#ifdef
3394 WINNING_GDB}, which defaults to false). GDB supposedly uses this
3395 information if you say @samp{list @var{var}}. In reality, @var{var} can
3396 be a variable defined in the program and GDB says @samp{function
3397 @var{var} not defined}.
3400 In GNU C stabs, there seems to be no way to differentiate tag types:
3401 structures, unions, and enums (symbol descriptor @samp{T}) and typedefs
3402 (symbol descriptor @samp{t}) defined at file scope from types defined locally
3403 to a procedure or other more local scope. They all use the @code{N_LSYM}
3404 stab type. Types defined at procedure scope are emited after the
3405 @code{N_RBRAC} of the preceding function and before the code of the
3406 procedure in which they are defined. This is exactly the same as
3407 types defined in the source file between the two procedure bodies.
3408 GDB overcompensates by placing all types in block #1, the block for
3409 symbols of file scope. This is true for default, @samp{-ansi} and
3410 @samp{-traditional} compiler options. (Bugs gcc/1063, gdb/1066.)
3413 What ends the procedure scope? Is it the proc block's @code{N_RBRAC} or the
3414 next @code{N_FUN}? (I believe its the first.)
3417 @c FIXME: This should go with the other stuff about global variables.
3418 Global variable stabs don't have location information. This comes
3419 from the external symbol for the same variable. The external symbol
3420 has a leading underbar on the _name of the variable and the stab does
3421 not. How do we know these two symbol table entries are talking about
3422 the same symbol when their names are different? (Answer: the debugger
3423 knows that external symbols have leading underbars).
3425 @c FIXME: This is absurdly vague; there all kinds of differences, some
3426 @c of which are the same between gnu & sun, and some of which aren't.
3427 @c In particular, I'm pretty sure GCC works with Sun dbx by default.
3429 @c Can GCC be configured to output stabs the way the Sun compiler
3430 @c does, so that their native debugging tools work? <NO?> It doesn't by
3431 @c default. GDB reads either format of stab. (GCC or SunC). How about
3435 @node XCOFF Differences
3436 @appendix Differences Between GNU Stabs in a.out and GNU Stabs in XCOFF
3438 @c FIXME: Merge *all* these into the main body of the document.
3439 The AIX/RS6000 native object file format is XCOFF with stabs. This
3440 appendix only covers those differences which are not covered in the main
3441 body of this document.
3445 BSD a.out stab types correspond to AIX XCOFF storage classes. In general
3446 the mapping is @code{N_@var{stabtype}} becomes @code{C_@var{stabtype}}.
3447 Some stab types in a.out are not supported in XCOFF; most of these use
3450 @c FIXME: Get C_* types for the block, figure out whether it is always
3451 @c used (I suspect not), explain clearly, and move to node Statics.
3452 Exception: initialised static @code{N_STSYM} and un-initialized static
3453 @code{N_LCSYM} both map to the @code{C_STSYM} storage class. But the
3454 distinction is preserved because in XCOFF @code{N_STSYM} and
3455 @code{N_LCSYM} must be emited in a named static block. Begin the block
3456 with @samp{.bs s[RW] data_section_name} for @code{N_STSYM} or @samp{.bs
3457 s bss_section_name} for @code{N_LCSYM}. End the block with @samp{.es}.
3459 @c FIXME: I think they are trying to say something about whether the
3460 @c assembler defaults the value to the location counter.
3462 If the XCOFF stab is an @code{N_FUN} (@code{C_FUN}) then follow the
3463 string field with @samp{,.} instead of just @samp{,}.
3466 I think that's it for @file{.s} file differences. They could stand to be
3467 better presented. This is just a list of what I have noticed so far.
3468 There are a @emph{lot} of differences in the information in the symbol
3469 tables of the executable and object files.
3471 Mapping of a.out stab types to XCOFF storage classes:
3474 stab type storage class
3475 -------------------------------
3511 @node Sun Differences
3512 @appendix Differences Between GNU Stabs and Sun Native Stabs
3514 @c FIXME: Merge all this stuff into the main body of the document.
3518 GNU C stabs define @emph{all} types, file or procedure scope, as
3519 @code{N_LSYM}. Sun doc talks about using @code{N_GSYM} too.
3522 Sun C stabs use type number pairs in the format (@var{a},@var{b}) where
3523 @var{a} is a number starting with 1 and incremented for each sub-source
3524 file in the compilation. @var{b} is a number starting with 1 and
3525 incremented for each new type defined in the compilation. GNU C stabs
3526 use the type number alone, with no source file number.
3530 @appendix Using Stabs With The ELF Object File Format
3532 The ELF object file format allows tools to create object files with
3533 custom sections containing any arbitrary data. To use stabs in ELF
3534 object files, the tools create two custom sections, a section named
3535 @code{.stab} which contains an array of fixed length structures, one
3536 struct per stab, and a section named @code{.stabstr} containing all the
3537 variable length strings that are referenced by stabs in the @code{.stab}
3538 section. The byte order of the stabs binary data matches the byte order
3539 of the ELF file itself, as determined from the @code{EI_DATA} field in
3540 the @code{e_ident} member of the ELF header.
3542 @c Is "source file" the right term for this concept? We don't mean that
3543 @c there is a separate one for include files (but "object file" or
3544 @c "object module" isn't quite right either; the output from ld -r is a
3545 @c single object file but contains many source files).
3546 The first stab in the @code{.stab} section for each source file is
3547 synthetic, generated entirely by the assembler, with no corresponding
3548 @code{.stab} directive as input to the assembler. This stab contains
3549 the following fields:
3553 Offset in the @code{.stabstr} section to the source filename.
3559 Unused field, always zero.
3562 Count of upcoming symbols, i.e., the number of remaining stabs for this
3566 Size of the string table fragment associated with this source file, in
3570 The @code{.stabstr} section always starts with a null byte (so that string
3571 offsets of zero reference a null string), followed by random length strings,
3572 each of which is null byte terminated.
3574 The ELF section header for the @code{.stab} section has its
3575 @code{sh_link} member set to the section number of the @code{.stabstr}
3576 section, and the @code{.stabstr} section has its ELF section
3577 header @code{sh_type} member set to @code{SHT_STRTAB} to mark it as a
3580 Because the linker does not process the @code{.stab} section in any
3581 special way, none of the addresses in the @code{n_value} field of the
3582 stabs are relocated by the linker. Instead they are relative to the
3583 source file (or some entity smaller than a source file, like a
3584 function). To find the address of each section corresponding to a given
3585 source file, the (compiler? assembler?) puts out symbols giving the
3586 address of each section for a given source file. Since these are normal
3587 ELF symbols, the linker can relocate them correctly. They are
3588 named @code{Bbss.bss} for the bss section, @code{Ddata.data} for
3589 the data section, and @code{Drodata.rodata} for the rodata section. I
3590 haven't yet figured out how the debugger gets the address for the text
3593 @node Symbol Types Index
3594 @unnumbered Symbol Types Index