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 uses COFF line numbers, which are outside the scope of this
497 document, ammeliorated by adequate marking of include files
498 (@pxref{Include Files}).
505 @findex N_STSYM, for functions (Sun acc)
506 @findex N_GSYM, for functions (Sun acc)
507 All of the following stabs normally use the @code{N_FUN} symbol type.
508 However, Sun's @code{acc} compiler on SunOS4 uses @code{N_GSYM} and
509 @code{N_STSYM}, which means that the value of the stab for the function
510 is useless and the debugger must get the address of the function from
511 the non-stab symbols instead. BSD Fortran is said to use @code{N_FNAME}
512 with the same restriction; the value of the symbol is not useful (I'm
513 not sure it really does use this, because GDB doesn't handle this and no
516 A function is represented by an @samp{F} symbol descriptor for a global
517 (extern) function, and @samp{f} for a static (local) function. The
518 value is the address of the start of the function (absolute
519 for @code{a.out}; relative to the start of the file for Sun's
520 stabs-in-ELF). The type information of the stab represents the return
521 type of the function; thus @samp{foo:f5} means that foo is a function
522 returning type 5. There is no need to try to get the line number of the
523 start of the function from the stab for the function; it is in the next
524 @code{N_SLINE} symbol.
526 @c FIXME: verify whether the "I suspect" below is true or not.
527 Some compilers (such as Sun's Solaris compiler) support an extension for
528 specifying the types of the arguments. I suspect this extension is not
529 used for old (non-prototyped) function definitions in C. If the
530 extension is in use, the type information of the stab for the function
531 is followed by type information for each argument, with each argument
532 preceded by @samp{;}. An argument type of 0 means that additional
533 arguments are being passed, whose types and number may vary (@samp{...}
534 in ANSI C). GDB has tolerated this extension (parsed the syntax, if not
535 necessarily used the information) since at least version 4.8; I don't
536 know whether all versions of dbx tolerate it. The argument types given
537 here are not redundant with the symbols for the formal parameters
538 (@pxref{Parameters}); they are the types of the arguments as they are
539 passed, before any conversions might take place. For example, if a C
540 function which is declared without a prototype takes a @code{float}
541 argument, the value is passed as a @code{double} but then converted to a
542 @code{float}. Debuggers need to use the types given in the arguments
543 when printing values, but when calling the function they need to use the
544 types given in the symbol defining the function.
546 If the return type and types of arguments of a function which is defined
547 in another source file are specified (i.e., a function prototype in ANSI
548 C), traditionally compilers emit no stab; the only way for the debugger
549 to find the information is if the source file where the function is
550 defined was also compiled with debugging symbols. As an extension the
551 Solaris compiler uses symbol descriptor @samp{P} followed by the return
552 type of the function, followed by the arguments, each preceded by
553 @samp{;}, as in a stab with symbol descriptor @samp{f} or @samp{F}.
554 This use of symbol descriptor @samp{P} can be distinguished from its use
555 for register parameters (@pxref{Register Parameters}) by the fact that it has
556 symbol type @code{N_FUN}.
558 The AIX documentation also defines symbol descriptor @samp{J} as an
559 internal function. I assume this means a function nested within another
560 function. It also says symbol descriptor @samp{m} is a module in
561 Modula-2 or extended Pascal.
563 Procedures (functions which do not return values) are represented as
564 functions returning the @code{void} type in C. I don't see why this couldn't
565 be used for all languages (inventing a @code{void} type for this purpose if
566 necessary), but the AIX documentation defines @samp{I}, @samp{P}, and
567 @samp{Q} for internal, global, and static procedures, respectively.
568 These symbol descriptors are unusual in that they are not followed by
571 The following example shows a stab for a function @code{main} which
572 returns type number @code{1}. The @code{_main} specified for the value
573 is a reference to an assembler label which is used to fill in the start
574 address of the function.
577 .stabs "main:F1",36,0,0,_main # @r{36 is N_FUN}
580 The stab representing a procedure is located immediately following the
581 code of the procedure. This stab is in turn directly followed by a
582 group of other stabs describing elements of the procedure. These other
583 stabs describe the procedure's parameters, its block local variables, and
586 @node Nested Procedures
587 @section Nested Procedures
589 For any of the symbol descriptors representing procedures, after the
590 symbol descriptor and the type information is optionally a scope
591 specifier. This consists of a comma, the name of the procedure, another
592 comma, and the name of the enclosing procedure. The first name is local
593 to the scope specified, and seems to be redundant with the name of the
594 symbol (before the @samp{:}). This feature is used by GCC, and
595 presumably Pascal, Modula-2, etc., compilers, for nested functions.
597 If procedures are nested more than one level deep, only the immediately
598 containing scope is specified. For example, this code:
610 return baz (x + 2 * y);
612 return x + bar (3 * x);
620 .stabs "baz:f1,baz,bar",36,0,0,_baz.15 # @r{36 is N_FUN}
621 .stabs "bar:f1,bar,foo",36,0,0,_bar.12
622 .stabs "foo:F1",36,0,0,_foo
625 @node Block Structure
626 @section Block Structure
630 The program's block structure is represented by the @code{N_LBRAC} (left
631 brace) and the @code{N_RBRAC} (right brace) stab types. The variables
632 defined inside a block precede the @code{N_LBRAC} symbol for most
633 compilers, including GCC. Other compilers, such as the Convex, Acorn
634 RISC machine, and Sun @code{acc} compilers, put the variables after the
635 @code{N_LBRAC} symbol. The values of the @code{N_LBRAC} and
636 @code{N_RBRAC} symbols are the start and end addresses of the code of
637 the block, respectively. For most machines, they are relative to the
638 starting address of this source file. For the Gould NP1, they are
639 absolute. For Sun's stabs-in-ELF, they are relative to the function in
642 The @code{N_LBRAC} and @code{N_RBRAC} stabs that describe the block
643 scope of a procedure are located after the @code{N_FUN} stab that
644 represents the procedure itself.
646 Sun documents the desc field of @code{N_LBRAC} and
647 @code{N_RBRAC} symbols as containing the nesting level of the block.
648 However, dbx seems to not care, and GCC always sets desc to
654 The @samp{c} symbol descriptor indicates that this stab represents a
655 constant. This symbol descriptor is an exception to the general rule
656 that symbol descriptors are followed by type information. Instead, it
657 is followed by @samp{=} and one of the following:
661 Boolean constant. @var{value} is a numeric value; I assume it is 0 for
665 Character constant. @var{value} is the numeric value of the constant.
667 @item e @var{type-information} , @var{value}
668 Constant whose value can be represented as integral.
669 @var{type-information} is the type of the constant, as it would appear
670 after a symbol descriptor (@pxref{String Field}). @var{value} is the
671 numeric value of the constant. GDB 4.9 does not actually get the right
672 value if @var{value} does not fit in a host @code{int}, but it does not
673 do anything violent, and future debuggers could be extended to accept
674 integers of any size (whether unsigned or not). This constant type is
675 usually documented as being only for enumeration constants, but GDB has
676 never imposed that restriction; I don't know about other debuggers.
679 Integer constant. @var{value} is the numeric value. The type is some
680 sort of generic integer type (for GDB, a host @code{int}); to specify
681 the type explicitly, use @samp{e} instead.
684 Real constant. @var{value} is the real value, which can be @samp{INF}
685 (optionally preceded by a sign) for infinity, @samp{QNAN} for a quiet
686 NaN (not-a-number), or @samp{SNAN} for a signalling NaN. If it is a
687 normal number the format is that accepted by the C library function
691 String constant. @var{string} is a string enclosed in either @samp{'}
692 (in which case @samp{'} characters within the string are represented as
693 @samp{\'} or @samp{"} (in which case @samp{"} characters within the
694 string are represented as @samp{\"}).
696 @item S @var{type-information} , @var{elements} , @var{bits} , @var{pattern}
697 Set constant. @var{type-information} is the type of the constant, as it
698 would appear after a symbol descriptor (@pxref{String Field}).
699 @var{elements} is the number of elements in the set (does this means
700 how many bits of @var{pattern} are actually used, which would be
701 redundant with the type, or perhaps the number of bits set in
702 @var{pattern}? I don't get it), @var{bits} is the number of bits in the
703 constant (meaning it specifies the length of @var{pattern}, I think),
704 and @var{pattern} is a hexadecimal representation of the set. AIX
705 documentation refers to a limit of 32 bytes, but I see no reason why
706 this limit should exist. This form could probably be used for arbitrary
707 constants, not just sets; the only catch is that @var{pattern} should be
708 understood to be target, not host, byte order and format.
711 The boolean, character, string, and set constants are not supported by
712 GDB 4.9, but it ignores them. GDB 4.8 and earlier gave an error
713 message and refused to read symbols from the file containing the
716 The above information is followed by @samp{;}.
721 Different types of stabs describe the various ways that variables can be
722 allocated: on the stack, globally, in registers, in common blocks,
723 statically, or as arguments to a function.
726 * Stack Variables:: Variables allocated on the stack.
727 * Global Variables:: Variables used by more than one source file.
728 * Register Variables:: Variables in registers.
729 * Common Blocks:: Variables statically allocated together.
730 * Statics:: Variables local to one source file.
731 * Parameters:: Variables for arguments to functions.
734 @node Stack Variables
735 @section Automatic Variables Allocated on the Stack
737 If a variable's scope is local to a function and its lifetime is only as
738 long as that function executes (C calls such variables
739 @dfn{automatic}), it can be allocated in a register (@pxref{Register
740 Variables}) or on the stack.
743 Each variable allocated on the stack has a stab with the symbol
744 descriptor omitted. Since type information should begin with a digit,
745 @samp{-}, or @samp{(}, only those characters precluded from being used
746 for symbol descriptors. However, the Acorn RISC machine (ARM) is said
747 to get this wrong: it puts out a mere type definition here, without the
748 preceding @samp{@var{type-number}=}. This is a bad idea; there is no
749 guarantee that type descriptors are distinct from symbol descriptors.
750 Stabs for stack variables use the @code{N_LSYM} stab type.
752 The value of the stab is the offset of the variable within the
753 local variables. On most machines this is an offset from the frame
754 pointer and is negative. The location of the stab specifies which block
755 it is defined in; see @ref{Block Structure}.
757 For example, the following C code:
767 produces the following stabs:
770 .stabs "main:F1",36,0,0,_main # @r{36 is N_FUN}
771 .stabs "x:1",128,0,0,-12 # @r{128 is N_LSYM}
772 .stabn 192,0,0,LBB2 # @r{192 is N_LBRAC}
773 .stabn 224,0,0,LBE2 # @r{224 is N_RBRAC}
776 @xref{Procedures} for more information on the @code{N_FUN} stab, and
777 @ref{Block Structure} for more information on the @code{N_LBRAC} and
778 @code{N_RBRAC} stabs.
780 @node Global Variables
781 @section Global Variables
784 A variable whose scope is not specific to just one source file is
785 represented by the @samp{G} symbol descriptor. These stabs use the
786 @code{N_GSYM} stab type. The type information for the stab
787 (@pxref{String Field}) gives the type of the variable.
789 For example, the following source code:
796 yields the following assembly code:
799 .stabs "g_foo:G2",32,0,0,0 # @r{32 is N_GSYM}
806 The address of the variable represented by the @code{N_GSYM} is not
807 contained in the @code{N_GSYM} stab. The debugger gets this information
808 from the external symbol for the global variable. In the example above,
809 the @code{.global _g_foo} and @code{_g_foo:} lines tell the assembler to
810 produce an external symbol.
812 @node Register Variables
813 @section Register Variables
816 @c According to an old version of this manual, AIX uses C_RPSYM instead
817 @c of C_RSYM. I am skeptical; this should be verified.
818 Register variables have their own stab type, @code{N_RSYM}, and their
819 own symbol descriptor, @samp{r}. The stab's value is the
820 number of the register where the variable data will be stored.
821 @c .stabs "name:type",N_RSYM,0,RegSize,RegNumber (Sun doc)
823 AIX defines a separate symbol descriptor @samp{d} for floating point
824 registers. This seems unnecessary; why not just just give floating
825 point registers different register numbers? I have not verified whether
826 the compiler actually uses @samp{d}.
828 If the register is explicitly allocated to a global variable, but not
832 register int g_bar asm ("%g5");
836 then the stab may be emitted at the end of the object file, with
837 the other bss symbols.
840 @section Common Blocks
842 A common block is a statically allocated section of memory which can be
843 referred to by several source files. It may contain several variables.
844 I believe Fortran is the only language with this feature.
848 A @code{N_BCOMM} stab begins a common block and an @code{N_ECOMM} stab
849 ends it. The only field that is significant in these two stabs is the
850 string, which names a normal (non-debugging) symbol that gives the
851 address of the common block.
854 Each stab between the @code{N_BCOMM} and the @code{N_ECOMM} specifies a
855 member of that common block; its value is the offset within the
856 common block of that variable. The @code{N_ECOML} stab type is
857 documented for this purpose, but Sun's Fortran compiler uses
858 @code{N_GSYM} instead. The test case I looked at had a common block
859 local to a function and it used the @samp{V} symbol descriptor; I assume
860 one would use @samp{S} if not local to a function (that is, if a common
861 block @emph{can} be anything other than local to a function).
864 @section Static Variables
866 Initialized static variables are represented by the @samp{S} and
867 @samp{V} symbol descriptors. @samp{S} means file scope static, and
868 @samp{V} means procedure scope static.
870 @c This is probably not worth mentioning; it is only true on the sparc
871 @c for `double' variables which although declared const are actually in
872 @c the data segment (the text segment can't guarantee 8 byte alignment).
874 @c 2.4.5 has a bug in that it uses @code{N_FUN}, so neither dbx nor GDB can
875 @c find the variables)
878 In a.out files, @code{N_STSYM} means the data segment, @code{N_FUN}
879 means the text segment, and @code{N_LCSYM} means the bss segment.
881 For example, the source lines:
884 static const int var_const = 5;
885 static int var_init = 2;
886 static int var_noinit;
890 yield the following stabs:
893 .stabs "var_const:S1",36,0,0,_var_const # @r{36 is N_FUN}
895 .stabs "var_init:S1",38,0,0,_var_init # @r{38 is N_STSYM}
897 .stabs "var_noinit:S1",40,0,0,_var_noinit # @r{40 is N_LCSYM}
900 In XCOFF files, each symbol has a section number, so the stab type
901 need not indicate the segment.
903 In ECOFF files, the storage class is used to specify the section, so the
904 stab type need not indicate the segment.
906 @c In ELF files, it apparently is a big mess. See kludge in dbxread.c
907 @c in GDB. FIXME: Investigate where this kludge comes from.
909 @c This is the place to mention N_ROSYM; I'd rather do so once I can
910 @c coherently explain how this stuff works for stabs-in-ELF.
915 Formal parameters to a function are represented by a stab (or sometimes
916 two; see below) for each parameter. The stabs are in the order in which
917 the debugger should print the parameters (i.e., the order in which the
918 parameters are declared in the source file). The exact form of the stab
919 depends on how the parameter is being passed.
922 Parameters passed on the stack use the symbol descriptor @samp{p} and
923 the @code{N_PSYM} symbol type. The value of the symbol is an offset
924 used to locate the parameter on the stack; its exact meaning is
925 machine-dependent, but on most machines it is an offset from the frame
928 As a simple example, the code:
939 .stabs "main:F1",36,0,0,_main # @r{36 is N_FUN}
940 .stabs "argc:p1",160,0,0,68 # @r{160 is N_PSYM}
941 .stabs "argv:p20=*21=*2",160,0,0,72
944 The type definition of @code{argv} is interesting because it contains
945 several type definitions. Type 21 is pointer to type 2 (char) and
946 @code{argv} (type 20) is pointer to type 21.
948 @c FIXME: figure out what these mean and describe them coherently.
949 The following are also said to go with @code{N_PSYM}:
952 "name" -> "param_name:#type"
954 -> pF Fortran function parameter
955 -> X (function result variable)
956 -> b (based variable)
958 value -> offset from the argument pointer (positive).
962 * Register Parameters::
963 * Local Variable Parameters::
964 * Reference Parameters::
965 * Conformant Arrays::
968 @node Register Parameters
969 @subsection Passing Parameters in Registers
971 If the parameter is passed in a register, then traditionally there are
972 two symbols for each argument:
975 .stabs "arg:p1" . . . ; N_PSYM
976 .stabs "arg:r1" . . . ; N_RSYM
979 Debuggers use the second one to find the value, and the first one to
980 know that it is an argument.
983 @findex N_RSYM, for parameters
984 Because that approach is kind of ugly, some compilers use symbol
985 descriptor @samp{P} or @samp{R} to indicate an argument which is in a
986 register. Symbol type @code{C_RPSYM} is used with @samp{R} and
987 @code{N_RSYM} is used with @samp{P}. The symbol's value is
988 the register number. @samp{P} and @samp{R} mean the same thing; the
989 difference is that @samp{P} is a GNU invention and @samp{R} is an IBM
990 (XCOFF) invention. As of version 4.9, GDB should handle either one.
992 There is at least one case where GCC uses a @samp{p} and @samp{r} pair
993 rather than @samp{P}; this is where the argument is passed in the
994 argument list and then loaded into a register.
996 According to the AIX documentation, symbol descriptor @samp{D} is for a
997 parameter passed in a floating point register. This seems
998 unnecessary---why not just use @samp{R} with a register number which
999 indicates that it's a floating point register? I haven't verified
1000 whether the system actually does what the documentation indicates.
1002 @c FIXME: On the hppa this is for any type > 8 bytes, I think, and not
1003 @c for small structures (investigate).
1004 On the sparc and hppa, for a @samp{P} symbol whose type is a structure
1005 or union, the register contains the address of the structure. On the
1006 sparc, this is also true of a @samp{p} and @samp{r} pair (using Sun
1007 @code{cc}) or a @samp{p} symbol. However, if a (small) structure is
1008 really in a register, @samp{r} is used. And, to top it all off, on the
1009 hppa it might be a structure which was passed on the stack and loaded
1010 into a register and for which there is a @samp{p} and @samp{r} pair! I
1011 believe that symbol descriptor @samp{i} is supposed to deal with this
1012 case (it is said to mean "value parameter by reference, indirect
1013 access"; I don't know the source for this information), but I don't know
1014 details or what compilers or debuggers use it, if any (not GDB or GCC).
1015 It is not clear to me whether this case needs to be dealt with
1016 differently than parameters passed by reference (@pxref{Reference Parameters}).
1018 @node Local Variable Parameters
1019 @subsection Storing Parameters as Local Variables
1021 There is a case similar to an argument in a register, which is an
1022 argument that is actually stored as a local variable. Sometimes this
1023 happens when the argument was passed in a register and then the compiler
1024 stores it as a local variable. If possible, the compiler should claim
1025 that it's in a register, but this isn't always done.
1027 @findex N_LSYM, for parameter
1028 Some compilers use the pair of symbols approach described above
1029 (@samp{@var{arg}:p} followed by @samp{@var{arg}:}); this includes GCC1
1030 (not GCC2) on the sparc when passing a small structure and GCC2
1031 (sometimes) when the argument type is @code{float} and it is passed as a
1032 @code{double} and converted to @code{float} by the prologue (in the
1033 latter case the type of the @samp{@var{arg}:p} symbol is @code{double}
1034 and the type of the @samp{@var{arg}:} symbol is @code{float}). GCC, at
1035 least on the 960, uses a single @samp{p} symbol descriptor for an
1036 argument which is stored as a local variable but uses @code{N_LSYM}
1037 instead of @code{N_PSYM}. In this case, the value of the symbol
1038 is an offset relative to the local variables for that function, not
1039 relative to the arguments; on some machines those are the same thing,
1042 @node Reference Parameters
1043 @subsection Passing Parameters by Reference
1045 If the parameter is passed by reference (e.g., Pascal @code{VAR}
1046 parameters), then the symbol descriptor is @samp{v} if it is in the
1047 argument list, or @samp{a} if it in a register. Other than the fact
1048 that these contain the address of the parameter rather than the
1049 parameter itself, they are identical to @samp{p} and @samp{R},
1050 respectively. I believe @samp{a} is an AIX invention; @samp{v} is
1051 supported by all stabs-using systems as far as I know.
1053 @node Conformant Arrays
1054 @subsection Passing Conformant Array Parameters
1056 @c Is this paragraph correct? It is based on piecing together patchy
1057 @c information and some guesswork
1058 Conformant arrays are a feature of Modula-2, and perhaps other
1059 languages, in which the size of an array parameter is not known to the
1060 called function until run-time. Such parameters have two stabs: a
1061 @samp{x} for the array itself, and a @samp{C}, which represents the size
1062 of the array. The value of the @samp{x} stab is the offset in the
1063 argument list where the address of the array is stored (it this right?
1064 it is a guess); the value of the @samp{C} stab is the offset in the
1065 argument list where the size of the array (in elements? in bytes?) is
1069 @chapter Defining Types
1071 The examples so far have described types as references to previously
1072 defined types, or defined in terms of subranges of or pointers to
1073 previously defined types. This chapter describes the other type
1074 descriptors that may follow the @samp{=} in a type definition.
1077 * Builtin Types:: Integers, floating point, void, etc.
1078 * Miscellaneous Types:: Pointers, sets, files, etc.
1079 * Cross-References:: Referring to a type not yet defined.
1080 * Subranges:: A type with a specific range.
1081 * Arrays:: An aggregate type of same-typed elements.
1082 * Strings:: Like an array but also has a length.
1083 * Enumerations:: Like an integer but the values have names.
1084 * Structures:: An aggregate type of different-typed elements.
1085 * Typedefs:: Giving a type a name.
1086 * Unions:: Different types sharing storage.
1091 @section Builtin Types
1093 Certain types are built in (@code{int}, @code{short}, @code{void},
1094 @code{float}, etc.); the debugger recognizes these types and knows how
1095 to handle them. Thus, don't be surprised if some of the following ways
1096 of specifying builtin types do not specify everything that a debugger
1097 would need to know about the type---in some cases they merely specify
1098 enough information to distinguish the type from other types.
1100 The traditional way to define builtin types is convolunted, so new ways
1101 have been invented to describe them. Sun's @code{acc} uses special
1102 builtin type descriptors (@samp{b} and @samp{R}), and IBM uses negative
1103 type numbers. GDB accepts all three ways, as of version 4.8; dbx just
1104 accepts the traditional builtin types and perhaps one of the other two
1105 formats. The following sections describe each of these formats.
1108 * Traditional Builtin Types:: Put on your seatbelts and prepare for kludgery
1109 * Builtin Type Descriptors:: Builtin types with special type descriptors
1110 * Negative Type Numbers:: Builtin types using negative type numbers
1113 @node Traditional Builtin Types
1114 @subsection Traditional Builtin Types
1116 This is the traditional, convoluted method for defining builtin types.
1117 There are several classes of such type definitions: integer, floating
1118 point, and @code{void}.
1121 * Traditional Integer Types::
1122 * Traditional Other Types::
1125 @node Traditional Integer Types
1126 @subsubsection Traditional Integer Types
1128 Often types are defined as subranges of themselves. If the bounding values
1129 fit within an @code{int}, then they are given normally. For example:
1132 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0 # @r{128 is N_LSYM}
1133 .stabs "char:t2=r2;0;127;",128,0,0,0
1136 Builtin types can also be described as subranges of @code{int}:
1139 .stabs "unsigned short:t6=r1;0;65535;",128,0,0,0
1142 If the lower bound of a subrange is 0 and the upper bound is -1,
1143 the type is an unsigned integral type whose bounds are too
1144 big to describe in an @code{int}. Traditionally this is only used for
1145 @code{unsigned int} and @code{unsigned long}:
1148 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
1151 For larger types, GCC 2.4.5 puts out bounds in octal, with a leading 0.
1152 In this case a negative bound consists of a number which is a 1 bit
1153 followed by a bunch of 0 bits, and a positive bound is one in which a
1154 bunch of bits are 1. All known versions of dbx and GDB version 4 accept
1155 this, but GDB 3.5 refuses to read the whole file containing such
1156 symbols. So GCC 2.3.3 did not output the proper size for these types.
1157 @c FIXME: How about an example?
1159 If the lower bound of a subrange is 0 and the upper bound is negative,
1160 the type is an unsigned integral type whose size in bytes is the
1161 absolute value of the upper bound. I believe this is a Convex
1162 convention for @code{unsigned long long}.
1164 If the lower bound of a subrange is negative and the upper bound is 0,
1165 the type is a signed integral type whose size in bytes is
1166 the absolute value of the lower bound. I believe this is a Convex
1167 convention for @code{long long}. To distinguish this from a legitimate
1168 subrange, the type should be a subrange of itself. I'm not sure whether
1169 this is the case for Convex.
1171 @node Traditional Other Types
1172 @subsubsection Traditional Other Types
1174 If the upper bound of a subrange is 0 and the lower bound is positive,
1175 the type is a floating point type, and the lower bound of the subrange
1176 indicates the number of bytes in the type:
1179 .stabs "float:t12=r1;4;0;",128,0,0,0
1180 .stabs "double:t13=r1;8;0;",128,0,0,0
1183 However, GCC writes @code{long double} the same way it writes
1184 @code{double}, so there is no way to distinguish.
1187 .stabs "long double:t14=r1;8;0;",128,0,0,0
1190 Complex types are defined the same way as floating-point types; there is
1191 no way to distinguish a single-precision complex from a double-precision
1192 floating-point type.
1194 The C @code{void} type is defined as itself:
1197 .stabs "void:t15=15",128,0,0,0
1200 I'm not sure how a boolean type is represented.
1202 @node Builtin Type Descriptors
1203 @subsection Defining Builtin Types Using Builtin Type Descriptors
1205 This is the method used by Sun's @code{acc} for defining builtin types.
1206 These are the type descriptors to define builtin types:
1209 @c FIXME: clean up description of width and offset, once we figure out
1211 @item b @var{signed} @var{char-flag} @var{width} ; @var{offset} ; @var{nbits} ;
1212 Define an integral type. @var{signed} is @samp{u} for unsigned or
1213 @samp{s} for signed. @var{char-flag} is @samp{c} which indicates this
1214 is a character type, or is omitted. I assume this is to distinguish an
1215 integral type from a character type of the same size, for example it
1216 might make sense to set it for the C type @code{wchar_t} so the debugger
1217 can print such variables differently (Solaris does not do this). Sun
1218 sets it on the C types @code{signed char} and @code{unsigned char} which
1219 arguably is wrong. @var{width} and @var{offset} appear to be for small
1220 objects stored in larger ones, for example a @code{short} in an
1221 @code{int} register. @var{width} is normally the number of bytes in the
1222 type. @var{offset} seems to always be zero. @var{nbits} is the number
1223 of bits in the type.
1225 Note that type descriptor @samp{b} used for builtin types conflicts with
1226 its use for Pascal space types (@pxref{Miscellaneous Types}); they can
1227 be distinguished because the character following the type descriptor
1228 will be a digit, @samp{(}, or @samp{-} for a Pascal space type, or
1229 @samp{u} or @samp{s} for a builtin type.
1232 Documented by AIX to define a wide character type, but their compiler
1233 actually uses negative type numbers (@pxref{Negative Type Numbers}).
1235 @item R @var{fp-type} ; @var{bytes} ;
1236 Define a floating point type. @var{fp-type} has one of the following values:
1240 IEEE 32-bit (single precision) floating point format.
1243 IEEE 64-bit (double precision) floating point format.
1245 @item 3 (NF_COMPLEX)
1246 @item 4 (NF_COMPLEX16)
1247 @item 5 (NF_COMPLEX32)
1248 @c "GDB source" really means @file{include/aout/stab_gnu.h}, but trying
1249 @c to put that here got an overfull hbox.
1250 These are for complex numbers. A comment in the GDB source describes
1251 them as Fortran @code{complex}, @code{double complex}, and
1252 @code{complex*16}, respectively, but what does that mean? (i.e., Single
1253 precision? Double precison?).
1255 @item 6 (NF_LDOUBLE)
1256 Long double. This should probably only be used for Sun format
1257 @code{long double}, and new codes should be used for other floating
1258 point formats (@code{NF_DOUBLE} can be used if a @code{long double} is
1259 really just an IEEE double, of course).
1262 @var{bytes} is the number of bytes occupied by the type. This allows a
1263 debugger to perform some operations with the type even if it doesn't
1264 understand @var{fp-type}.
1266 @item g @var{type-information} ; @var{nbits}
1267 Documented by AIX to define a floating type, but their compiler actually
1268 uses negative type numbers (@pxref{Negative Type Numbers}).
1270 @item c @var{type-information} ; @var{nbits}
1271 Documented by AIX to define a complex type, but their compiler actually
1272 uses negative type numbers (@pxref{Negative Type Numbers}).
1275 The C @code{void} type is defined as a signed integral type 0 bits long:
1277 .stabs "void:t19=bs0;0;0",128,0,0,0
1279 The Solaris compiler seems to omit the trailing semicolon in this case.
1280 Getting sloppy in this way is not a swift move because if a type is
1281 embedded in a more complex expression it is necessary to be able to tell
1284 I'm not sure how a boolean type is represented.
1286 @node Negative Type Numbers
1287 @subsection Negative Type Numbers
1289 This is the method used in XCOFF for defining builtin types.
1290 Since the debugger knows about the builtin types anyway, the idea of
1291 negative type numbers is simply to give a special type number which
1292 indicates the builtin type. There is no stab defining these types.
1294 I'm not sure whether anyone has tried to define what this means if
1295 @code{int} can be other than 32 bits (or if other types can be other than
1296 their customary size). If @code{int} has exactly one size for each
1297 architecture, then it can be handled easily enough, but if the size of
1298 @code{int} can vary according the compiler options, then it gets hairy.
1299 The best way to do this would be to define separate negative type
1300 numbers for 16-bit @code{int} and 32-bit @code{int}; therefore I have
1301 indicated below the customary size (and other format information) for
1302 each type. The information below is currently correct because AIX on
1303 the RS6000 is the only system which uses these type numbers. If these
1304 type numbers start to get used on other systems, I suspect the correct
1305 thing to do is to define a new number in cases where a type does not
1306 have the size and format indicated below (or avoid negative type numbers
1309 Part of the definition of the negative type number is
1310 the name of the type. Types with identical size and format but
1311 different names have different negative type numbers.
1315 @code{int}, 32 bit signed integral type.
1318 @code{char}, 8 bit type holding a character. Both GDB and dbx on AIX
1319 treat this as signed. GCC uses this type whether @code{char} is signed
1320 or not, which seems like a bad idea. The AIX compiler (@code{xlc}) seems to
1321 avoid this type; it uses -5 instead for @code{char}.
1324 @code{short}, 16 bit signed integral type.
1327 @code{long}, 32 bit signed integral type.
1330 @code{unsigned char}, 8 bit unsigned integral type.
1333 @code{signed char}, 8 bit signed integral type.
1336 @code{unsigned short}, 16 bit unsigned integral type.
1339 @code{unsigned int}, 32 bit unsigned integral type.
1342 @code{unsigned}, 32 bit unsigned integral type.
1345 @code{unsigned long}, 32 bit unsigned integral type.
1348 @code{void}, type indicating the lack of a value.
1351 @code{float}, IEEE single precision.
1354 @code{double}, IEEE double precision.
1357 @code{long double}, IEEE double precision. The compiler claims the size
1358 will increase in a future release, and for binary compatibility you have
1359 to avoid using @code{long double}. I hope when they increase it they
1360 use a new negative type number.
1363 @code{integer}. 32 bit signed integral type.
1366 @code{boolean}. 32 bit type. How is the truth value encoded? Is it
1367 the least significant bit or is it a question of whether the whole value
1368 is zero or non-zero?
1371 @code{short real}. IEEE single precision.
1374 @code{real}. IEEE double precision.
1377 @code{stringptr}. @xref{Strings}.
1380 @code{character}, 8 bit unsigned character type.
1383 @code{logical*1}, 8 bit type. This Fortran type has a split
1384 personality in that it is used for boolean variables, but can also be
1385 used for unsigned integers. 0 is false, 1 is true, and other values are
1389 @code{logical*2}, 16 bit type. This Fortran type has a split
1390 personality in that it is used for boolean variables, but can also be
1391 used for unsigned integers. 0 is false, 1 is true, and other values are
1395 @code{logical*4}, 32 bit type. This Fortran type has a split
1396 personality in that it is used for boolean variables, but can also be
1397 used for unsigned integers. 0 is false, 1 is true, and other values are
1401 @code{logical}, 32 bit type. This Fortran type has a split
1402 personality in that it is used for boolean variables, but can also be
1403 used for unsigned integers. 0 is false, 1 is true, and other values are
1407 @code{complex}. A complex type consisting of two IEEE single-precision
1408 floating point values.
1411 @code{complex}. A complex type consisting of two IEEE double-precision
1412 floating point values.
1415 @code{integer*1}, 8 bit signed integral type.
1418 @code{integer*2}, 16 bit signed integral type.
1421 @code{integer*4}, 32 bit signed integral type.
1424 @code{wchar}. Wide character, 16 bits wide, unsigned (what format?
1428 @node Miscellaneous Types
1429 @section Miscellaneous Types
1432 @item b @var{type-information} ; @var{bytes}
1433 Pascal space type. This is documented by IBM; what does it mean?
1435 This use of the @samp{b} type descriptor can be distinguished
1436 from its use for builtin integral types (@pxref{Builtin Type
1437 Descriptors}) because the character following the type descriptor is
1438 always a digit, @samp{(}, or @samp{-}.
1440 @item B @var{type-information}
1441 A volatile-qualified version of @var{type-information}. This is
1442 a Sun extension. References and stores to a variable with a
1443 volatile-qualified type must not be optimized or cached; they
1444 must occur as the user specifies them.
1446 @item d @var{type-information}
1447 File of type @var{type-information}. As far as I know this is only used
1450 @item k @var{type-information}
1451 A const-qualified version of @var{type-information}. This is a Sun
1452 extension. A variable with a const-qualified type cannot be modified.
1454 @item M @var{type-information} ; @var{length}
1455 Multiple instance type. The type seems to composed of @var{length}
1456 repetitions of @var{type-information}, for example @code{character*3} is
1457 represented by @samp{M-2;3}, where @samp{-2} is a reference to a
1458 character type (@pxref{Negative Type Numbers}). I'm not sure how this
1459 differs from an array. This appears to be a Fortran feature.
1460 @var{length} is a bound, like those in range types; see @ref{Subranges}.
1462 @item S @var{type-information}
1463 Pascal set type. @var{type-information} must be a small type such as an
1464 enumeration or a subrange, and the type is a bitmask whose length is
1465 specified by the number of elements in @var{type-information}.
1467 @item * @var{type-information}
1468 Pointer to @var{type-information}.
1471 @node Cross-References
1472 @section Cross-References to Other Types
1474 A type can be used before it is defined; one common way to deal with
1475 that situation is just to use a type reference to a type which has not
1478 Another way is with the @samp{x} type descriptor, which is followed by
1479 @samp{s} for a structure tag, @samp{u} for a union tag, or @samp{e} for
1480 a enumerator tag, followed by the name of the tag, followed by @samp{:}.
1481 For example, the following C declarations:
1492 .stabs "bar:G16=*17=xsfoo:",32,0,0,0
1495 Not all debuggers support the @samp{x} type descriptor, so on some
1496 machines GCC does not use it. I believe that for the above example it
1497 would just emit a reference to type 17 and never define it, but I
1498 haven't verified that.
1500 Modula-2 imported types, at least on AIX, use the @samp{i} type
1501 descriptor, which is followed by the name of the module from which the
1502 type is imported, followed by @samp{:}, followed by the name of the
1503 type. There is then optionally a comma followed by type information for
1504 the type. This differs from merely naming the type (@pxref{Typedefs}) in
1505 that it identifies the module; I don't understand whether the name of
1506 the type given here is always just the same as the name we are giving
1507 it, or whether this type descriptor is used with a nameless stab
1508 (@pxref{String Field}), or what. The symbol ends with @samp{;}.
1511 @section Subrange Types
1513 The @samp{r} type descriptor defines a type as a subrange of another
1514 type. It is followed by type information for the type of which it is a
1515 subrange, a semicolon, an integral lower bound, a semicolon, an
1516 integral upper bound, and a semicolon. The AIX documentation does not
1517 specify the trailing semicolon, in an effort to specify array indexes
1518 more cleanly, but a subrange which is not an array index has always
1519 included a trailing semicolon (@pxref{Arrays}).
1521 Instead of an integer, either bound can be one of the following:
1524 @item A @var{offset}
1525 The bound is passed by reference on the stack at offset @var{offset}
1526 from the argument list. @xref{Parameters}, for more information on such
1529 @item T @var{offset}
1530 The bound is passed by value on the stack at offset @var{offset} from
1533 @item a @var{register-number}
1534 The bound is pased by reference in register number
1535 @var{register-number}.
1537 @item t @var{register-number}
1538 The bound is passed by value in register number @var{register-number}.
1544 Subranges are also used for builtin types; see @ref{Traditional Builtin Types}.
1547 @section Array Types
1549 Arrays use the @samp{a} type descriptor. Following the type descriptor
1550 is the type of the index and the type of the array elements. If the
1551 index type is a range type, it ends in a semicolon; otherwise
1552 (for example, if it is a type reference), there does not
1553 appear to be any way to tell where the types are separated. In an
1554 effort to clean up this mess, IBM documents the two types as being
1555 separated by a semicolon, and a range type as not ending in a semicolon
1556 (but this is not right for range types which are not array indexes,
1557 @pxref{Subranges}). I think probably the best solution is to specify
1558 that a semicolon ends a range type, and that the index type and element
1559 type of an array are separated by a semicolon, but that if the index
1560 type is a range type, the extra semicolon can be omitted. GDB (at least
1561 through version 4.9) doesn't support any kind of index type other than a
1562 range anyway; I'm not sure about dbx.
1564 It is well established, and widely used, that the type of the index,
1565 unlike most types found in the stabs, is merely a type definition, not
1566 type information (@pxref{String Field}) (that is, it need not start with
1567 @samp{@var{type-number}=} if it is defining a new type). According to a
1568 comment in GDB, this is also true of the type of the array elements; it
1569 gives @samp{ar1;1;10;ar1;1;10;4} as a legitimate way to express a two
1570 dimensional array. According to AIX documentation, the element type
1571 must be type information. GDB accepts either.
1573 The type of the index is often a range type, expressed as the type
1574 descriptor @samp{r} and some parameters. It defines the size of the
1575 array. In the example below, the range @samp{r1;0;2;} defines an index
1576 type which is a subrange of type 1 (integer), with a lower bound of 0
1577 and an upper bound of 2. This defines the valid range of subscripts of
1578 a three-element C array.
1580 For example, the definition:
1583 char char_vec[3] = @{'a','b','c'@};
1587 produces the output:
1590 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
1599 If an array is @dfn{packed}, the elements are spaced more
1600 closely than normal, saving memory at the expense of speed. For
1601 example, an array of 3-byte objects might, if unpacked, have each
1602 element aligned on a 4-byte boundary, but if packed, have no padding.
1603 One way to specify that something is packed is with type attributes
1604 (@pxref{String Field}). In the case of arrays, another is to use the
1605 @samp{P} type descriptor instead of @samp{a}. Other than specifying a
1606 packed array, @samp{P} is identical to @samp{a}.
1608 @c FIXME-what is it? A pointer?
1609 An open array is represented by the @samp{A} type descriptor followed by
1610 type information specifying the type of the array elements.
1612 @c FIXME: what is the format of this type? A pointer to a vector of pointers?
1613 An N-dimensional dynamic array is represented by
1616 D @var{dimensions} ; @var{type-information}
1619 @c Does dimensions really have this meaning? The AIX documentation
1621 @var{dimensions} is the number of dimensions; @var{type-information}
1622 specifies the type of the array elements.
1624 @c FIXME: what is the format of this type? A pointer to some offsets in
1626 A subarray of an N-dimensional array is represented by
1629 E @var{dimensions} ; @var{type-information}
1632 @c Does dimensions really have this meaning? The AIX documentation
1634 @var{dimensions} is the number of dimensions; @var{type-information}
1635 specifies the type of the array elements.
1640 Some languages, like C or the original Pascal, do not have string types,
1641 they just have related things like arrays of characters. But most
1642 Pascals and various other languages have string types, which are
1643 indicated as follows:
1646 @item n @var{type-information} ; @var{bytes}
1647 @var{bytes} is the maximum length. I'm not sure what
1648 @var{type-information} is; I suspect that it means that this is a string
1649 of @var{type-information} (thus allowing a string of integers, a string
1650 of wide characters, etc., as well as a string of characters). Not sure
1651 what the format of this type is. This is an AIX feature.
1653 @item z @var{type-information} ; @var{bytes}
1654 Just like @samp{n} except that this is a gstring, not an ordinary
1655 string. I don't know the difference.
1658 Pascal Stringptr. What is this? This is an AIX feature.
1662 @section Enumerations
1664 Enumerations are defined with the @samp{e} type descriptor.
1666 @c FIXME: Where does this information properly go? Perhaps it is
1667 @c redundant with something we already explain.
1668 The source line below declares an enumeration type at file scope.
1669 The type definition is located after the @code{N_RBRAC} that marks the end of
1670 the previous procedure's block scope, and before the @code{N_FUN} that marks
1671 the beginning of the next procedure's block scope. Therefore it does not
1672 describe a block local symbol, but a file local one.
1677 enum e_places @{first,second=3,last@};
1681 generates the following stab:
1684 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
1687 The symbol descriptor (@samp{T}) says that the stab describes a
1688 structure, enumeration, or union tag. The type descriptor @samp{e},
1689 following the @samp{22=} of the type definition narrows it down to an
1690 enumeration type. Following the @samp{e} is a list of the elements of
1691 the enumeration. The format is @samp{@var{name}:@var{value},}. The
1692 list of elements ends with @samp{;}.
1694 There is no standard way to specify the size of an enumeration type; it
1695 is determined by the architecture (normally all enumerations types are
1696 32 bits). There should be a way to specify an enumeration type of
1697 another size; type attributes would be one way to do this. @xref{Stabs
1703 The encoding of structures in stabs can be shown with an example.
1705 The following source code declares a structure tag and defines an
1706 instance of the structure in global scope. Then a @code{typedef} equates the
1707 structure tag with a new type. Seperate stabs are generated for the
1708 structure tag, the structure @code{typedef}, and the structure instance. The
1709 stabs for the tag and the @code{typedef} are emited when the definitions are
1710 encountered. Since the structure elements are not initialized, the
1711 stab and code for the structure variable itself is located at the end
1712 of the program in the bss section.
1719 struct s_tag* s_next;
1722 typedef struct s_tag s_typedef;
1725 The structure tag has an @code{N_LSYM} stab type because, like the
1726 enumeration, the symbol has file scope. Like the enumeration, the
1727 symbol descriptor is @samp{T}, for enumeration, structure, or tag type.
1728 The type descriptor @samp{s} following the @samp{16=} of the type
1729 definition narrows the symbol type to structure.
1731 Following the @samp{s} type descriptor is the number of bytes the
1732 structure occupies, followed by a description of each structure element.
1733 The structure element descriptions are of the form @var{name:type, bit
1734 offset from the start of the struct, number of bits in the element}.
1736 @c FIXME: phony line break. Can probably be fixed by using an example
1737 @c with fewer fields.
1740 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;
1741 s_char_vec:17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
1744 In this example, the first two structure elements are previously defined
1745 types. For these, the type following the @samp{@var{name}:} part of the
1746 element description is a simple type reference. The other two structure
1747 elements are new types. In this case there is a type definition
1748 embedded after the @samp{@var{name}:}. The type definition for the
1749 array element looks just like a type definition for a standalone array.
1750 The @code{s_next} field is a pointer to the same kind of structure that
1751 the field is an element of. So the definition of structure type 16
1752 contains a type definition for an element which is a pointer to type 16.
1755 @section Giving a Type a Name
1757 To give a type a name, use the @samp{t} symbol descriptor. The type
1758 is specified by the type information (@pxref{String Field}) for the stab.
1762 .stabs "s_typedef:t16",128,0,0,0 # @r{128 is N_LSYM}
1765 specifies that @code{s_typedef} refers to type number 16. Such stabs
1766 have symbol type @code{N_LSYM} (or @code{C_DECL} for XCOFF).
1768 If you are specifying the tag name for a structure, union, or
1769 enumeration, use the @samp{T} symbol descriptor instead. I believe C is
1770 the only language with this feature.
1772 If the type is an opaque type (I believe this is a Modula-2 feature),
1773 AIX provides a type descriptor to specify it. The type descriptor is
1774 @samp{o} and is followed by a name. I don't know what the name
1775 means---is it always the same as the name of the type, or is this type
1776 descriptor used with a nameless stab (@pxref{String Field})? There
1777 optionally follows a comma followed by type information which defines
1778 the type of this type. If omitted, a semicolon is used in place of the
1779 comma and the type information, and the type is much like a generic
1780 pointer type---it has a known size but little else about it is
1794 This code generates a stab for a union tag and a stab for a union
1795 variable. Both use the @code{N_LSYM} stab type. If a union variable is
1796 scoped locally to the procedure in which it is defined, its stab is
1797 located immediately preceding the @code{N_LBRAC} for the procedure's block
1800 The stab for the union tag, however, is located preceding the code for
1801 the procedure in which it is defined. The stab type is @code{N_LSYM}. This
1802 would seem to imply that the union type is file scope, like the struct
1803 type @code{s_tag}. This is not true. The contents and position of the stab
1804 for @code{u_type} do not convey any infomation about its procedure local
1807 @c FIXME: phony line break. Can probably be fixed by using an example
1808 @c with fewer fields.
1811 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
1815 The symbol descriptor @samp{T}, following the @samp{name:} means that
1816 the stab describes an enumeration, structure, or union tag. The type
1817 descriptor @samp{u}, following the @samp{23=} of the type definition,
1818 narrows it down to a union type definition. Following the @samp{u} is
1819 the number of bytes in the union. After that is a list of union element
1820 descriptions. Their format is @var{name:type, bit offset into the
1821 union, number of bytes for the element;}.
1823 The stab for the union variable is:
1826 .stabs "an_u:23",128,0,0,-20 # @r{128 is N_LSYM}
1829 @samp{-20} specifies where the variable is stored (@pxref{Stack
1832 @node Function Types
1833 @section Function Types
1835 Various types can be defined for function variables. These types are
1836 not used in defining functions (@pxref{Procedures}); they are used for
1837 things like pointers to functions.
1839 The simple, traditional, type is type descriptor @samp{f} is followed by
1840 type information for the return type of the function, followed by a
1843 This does not deal with functions for which the number and types of the
1844 parameters are part of the type, as in Modula-2 or ANSI C. AIX provides
1845 extensions to specify these, using the @samp{f}, @samp{F}, @samp{p}, and
1846 @samp{R} type descriptors.
1848 First comes the type descriptor. If it is @samp{f} or @samp{F}, this
1849 type involves a function rather than a procedure, and the type
1850 information for the return type of the function follows, followed by a
1851 comma. Then comes the number of parameters to the function and a
1852 semicolon. Then, for each parameter, there is the name of the parameter
1853 followed by a colon (this is only present for type descriptors @samp{R}
1854 and @samp{F} which represent Pascal function or procedure parameters),
1855 type information for the parameter, a comma, 0 if passed by reference or
1856 1 if passed by value, and a semicolon. The type definition ends with a
1859 For example, this variable definition:
1866 generates the following code:
1869 .stabs "g_pf:G24=*25=f1",32,0,0,0
1870 .common _g_pf,4,"bss"
1873 The variable defines a new type, 24, which is a pointer to another new
1874 type, 25, which is a function returning @code{int}.
1877 @chapter Symbol Information in Symbol Tables
1879 This chapter describes the format of symbol table entries
1880 and how stab assembler directives map to them. It also describes the
1881 transformations that the assembler and linker make on data from stabs.
1884 * Symbol Table Format::
1885 * Transformations On Symbol Tables::
1888 @node Symbol Table Format
1889 @section Symbol Table Format
1891 Each time the assembler encounters a stab directive, it puts
1892 each field of the stab into a corresponding field in a symbol table
1893 entry of its output file. If the stab contains a string field, the
1894 symbol table entry for that stab points to a string table entry
1895 containing the string data from the stab. Assembler labels become
1896 relocatable addresses. Symbol table entries in a.out have the format:
1898 @c FIXME: should refer to external, not internal.
1900 struct internal_nlist @{
1901 unsigned long n_strx; /* index into string table of name */
1902 unsigned char n_type; /* type of symbol */
1903 unsigned char n_other; /* misc info (usually empty) */
1904 unsigned short n_desc; /* description field */
1905 bfd_vma n_value; /* value of symbol */
1909 If the stab has a string, the @code{n_strx} field holds the offset in
1910 bytes of the string within the string table. The string is terminated
1911 by a NUL character. If the stab lacks a string (for example, it was
1912 produced by a @code{.stabn} or @code{.stabd} directive), the
1913 @code{n_strx} field is zero.
1915 Symbol table entries with @code{n_type} field values greater than 0x1f
1916 originated as stabs generated by the compiler (with one random
1917 exception). The other entries were placed in the symbol table of the
1918 executable by the assembler or the linker.
1920 @node Transformations On Symbol Tables
1921 @section Transformations on Symbol Tables
1923 The linker concatenates object files and does fixups of externally
1926 You can see the transformations made on stab data by the assembler and
1927 linker by examining the symbol table after each pass of the build. To
1928 do this, use @samp{nm -ap}, which dumps the symbol table, including
1929 debugging information, unsorted. For stab entries the columns are:
1930 @var{value}, @var{other}, @var{desc}, @var{type}, @var{string}. For
1931 assembler and linker symbols, the columns are: @var{value}, @var{type},
1934 The low 5 bits of the stab type tell the linker how to relocate the
1935 value of the stab. Thus for stab types like @code{N_RSYM} and
1936 @code{N_LSYM}, where the value is an offset or a register number, the
1937 low 5 bits are @code{N_ABS}, which tells the linker not to relocate the
1940 Where the value of a stab contains an assembly language label,
1941 it is transformed by each build step. The assembler turns it into a
1942 relocatable address and the linker turns it into an absolute address.
1945 * Transformations On Static Variables::
1946 * Transformations On Global Variables::
1949 @node Transformations On Static Variables
1950 @subsection Transformations on Static Variables
1952 This source line defines a static variable at file scope:
1955 static int s_g_repeat
1959 The following stab describes the symbol:
1962 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
1966 The assembler transforms the stab into this symbol table entry in the
1967 @file{.o} file. The location is expressed as a data segment offset.
1970 00000084 - 00 0000 STSYM s_g_repeat:S1
1974 In the symbol table entry from the executable, the linker has made the
1975 relocatable address absolute.
1978 0000e00c - 00 0000 STSYM s_g_repeat:S1
1981 @node Transformations On Global Variables
1982 @subsection Transformations on Global Variables
1984 Stabs for global variables do not contain location information. In
1985 this case, the debugger finds location information in the assembler or
1986 linker symbol table entry describing the variable. The source line:
1996 .stabs "g_foo:G2",32,0,0,0
1999 The variable is represented by two symbol table entries in the object
2000 file (see below). The first one originated as a stab. The second one
2001 is an external symbol. The upper case @samp{D} signifies that the
2002 @code{n_type} field of the symbol table contains 7, @code{N_DATA} with
2003 local linkage. The stab's value is zero since the value is not used for
2004 @code{N_GSYM} stabs. The value of the linker symbol is the relocatable
2005 address corresponding to the variable.
2008 00000000 - 00 0000 GSYM g_foo:G2
2013 These entries as transformed by the linker. The linker symbol table
2014 entry now holds an absolute address:
2017 00000000 - 00 0000 GSYM g_foo:G2
2023 @chapter GNU C++ Stabs
2026 * Basic Cplusplus Types::
2029 * Methods:: Method definition
2031 * Method Modifiers::
2034 * Virtual Base Classes::
2038 Type descriptors added for C++ descriptions:
2042 method type (@code{##} if minimal debug)
2045 Member (class and variable) type. It is followed by type information
2046 for the offset basetype, a comma, and type information for the type of
2047 the field being pointed to. (FIXME: this is acknowledged to be
2048 gibberish. Can anyone say what really goes here?).
2050 Note that there is a conflict between this and type attributes
2051 (@pxref{String Field}); both use type descriptor @samp{@@}.
2052 Fortunately, the @samp{@@} type descriptor used in this C++ sense always
2053 will be followed by a digit, @samp{(}, or @samp{-}, and type attributes
2054 never start with those things.
2057 @node Basic Cplusplus Types
2058 @section Basic Types For C++
2060 << the examples that follow are based on a01.C >>
2063 C++ adds two more builtin types to the set defined for C. These are
2064 the unknown type and the vtable record type. The unknown type, type
2065 16, is defined in terms of itself like the void type.
2067 The vtable record type, type 17, is defined as a structure type and
2068 then as a structure tag. The structure has four fields: delta, index,
2069 pfn, and delta2. pfn is the function pointer.
2071 << In boilerplate $vtbl_ptr_type, what are the fields delta,
2072 index, and delta2 used for? >>
2074 This basic type is present in all C++ programs even if there are no
2075 virtual methods defined.
2078 .stabs "struct_name:sym_desc(type)type_def(17)=type_desc(struct)struct_bytes(8)
2079 elem_name(delta):type_ref(short int),bit_offset(0),field_bits(16);
2080 elem_name(index):type_ref(short int),bit_offset(16),field_bits(16);
2081 elem_name(pfn):type_def(18)=type_desc(ptr to)type_ref(void),
2082 bit_offset(32),field_bits(32);
2083 elem_name(delta2):type_def(short int);bit_offset(32),field_bits(16);;"
2088 .stabs "$vtbl_ptr_type:t17=s8
2089 delta:6,0,16;index:6,16,16;pfn:18=*15,32,32;delta2:6,32,16;;"
2094 .stabs "name:sym_dec(struct tag)type_ref($vtbl_ptr_type)",N_LSYM,NIL,NIL,NIL
2098 .stabs "$vtbl_ptr_type:T17",128,0,0,0
2101 @node Simple Classes
2102 @section Simple Class Definition
2104 The stabs describing C++ language features are an extension of the
2105 stabs describing C. Stabs representing C++ class types elaborate
2106 extensively on the stab format used to describe structure types in C.
2107 Stabs representing class type variables look just like stabs
2108 representing C language variables.
2110 Consider the following very simple class definition.
2116 int Ameth(int in, char other);
2120 The class @code{baseA} is represented by two stabs. The first stab describes
2121 the class as a structure type. The second stab describes a structure
2122 tag of the class type. Both stabs are of stab type @code{N_LSYM}. Since the
2123 stab is not located between an @code{N_FUN} and an @code{N_LBRAC} stab this indicates
2124 that the class is defined at file scope. If it were, then the @code{N_LSYM}
2125 would signify a local variable.
2127 A stab describing a C++ class type is similar in format to a stab
2128 describing a C struct, with each class member shown as a field in the
2129 structure. The part of the struct format describing fields is
2130 expanded to include extra information relevent to C++ class members.
2131 In addition, if the class has multiple base classes or virtual
2132 functions the struct format outside of the field parts is also
2135 In this simple example the field part of the C++ class stab
2136 representing member data looks just like the field part of a C struct
2137 stab. The section on protections describes how its format is
2138 sometimes extended for member data.
2140 The field part of a C++ class stab representing a member function
2141 differs substantially from the field part of a C struct stab. It
2142 still begins with @samp{name:} but then goes on to define a new type number
2143 for the member function, describe its return type, its argument types,
2144 its protection level, any qualifiers applied to the method definition,
2145 and whether the method is virtual or not. If the method is virtual
2146 then the method description goes on to give the vtable index of the
2147 method, and the type number of the first base class defining the
2150 When the field name is a method name it is followed by two colons rather
2151 than one. This is followed by a new type definition for the method.
2152 This is a number followed by an equal sign and the type descriptor
2153 @samp{#}, indicating a method type, and a second @samp{#}, indicating
2154 that this is the @dfn{minimal} type of method definition used by GCC2,
2155 not larger method definitions used by earlier versions of GCC. This is
2156 followed by a type reference showing the return type of the method and a
2159 The format of an overloaded operator method name differs from that of
2160 other methods. It is @samp{op$::@var{operator-name}.} where
2161 @var{operator-name} is the operator name such as @samp{+} or @samp{+=}.
2162 The name ends with a period, and any characters except the period can
2163 occur in the @var{operator-name} string.
2165 The next part of the method description represents the arguments to the
2166 method, preceeded by a colon and ending with a semi-colon. The types of
2167 the arguments are expressed in the same way argument types are expressed
2168 in C++ name mangling. In this example an @code{int} and a @code{char}
2171 This is followed by a number, a letter, and an asterisk or period,
2172 followed by another semicolon. The number indicates the protections
2173 that apply to the member function. Here the 2 means public. The
2174 letter encodes any qualifier applied to the method definition. In
2175 this case, @samp{A} means that it is a normal function definition. The dot
2176 shows that the method is not virtual. The sections that follow
2177 elaborate further on these fields and describe the additional
2178 information present for virtual methods.
2182 .stabs "class_name:sym_desc(type)type_def(20)=type_desc(struct)struct_bytes(4)
2183 field_name(Adat):type(int),bit_offset(0),field_bits(32);
2185 method_name(Ameth)::type_def(21)=type_desc(method)return_type(int);
2186 :arg_types(int char);
2187 protection(public)qualifier(normal)virtual(no);;"
2192 .stabs "baseA:t20=s4Adat:1,0,32;Ameth::21=##1;:ic;2A.;;",128,0,0,0
2194 .stabs "class_name:sym_desc(struct tag)",N_LSYM,NIL,NIL,NIL
2196 .stabs "baseA:T20",128,0,0,0
2199 @node Class Instance
2200 @section Class Instance
2202 As shown above, describing even a simple C++ class definition is
2203 accomplished by massively extending the stab format used in C to
2204 describe structure types. However, once the class is defined, C stabs
2205 with no modifications can be used to describe class instances. The
2215 yields the following stab describing the class instance. It looks no
2216 different from a standard C stab describing a local variable.
2219 .stabs "name:type_ref(baseA)", N_LSYM, NIL, NIL, frame_ptr_offset
2223 .stabs "AbaseA:20",128,0,0,-20
2227 @section Method Defintion
2229 The class definition shown above declares Ameth. The C++ source below
2234 baseA::Ameth(int in, char other)
2241 This method definition yields three stabs following the code of the
2242 method. One stab describes the method itself and following two describe
2243 its parameters. Although there is only one formal argument all methods
2244 have an implicit argument which is the @code{this} pointer. The @code{this}
2245 pointer is a pointer to the object on which the method was called. Note
2246 that the method name is mangled to encode the class name and argument
2247 types. Name mangling is described in the @sc{arm} (@cite{The Annotated
2248 C++ Reference Manual}, by Ellis and Stroustrup, @sc{isbn}
2249 0-201-51459-1); @file{gpcompare.texi} in Cygnus GCC distributions
2250 describes the differences between GNU mangling and @sc{arm}
2252 @c FIXME: Use @xref, especially if this is generally installed in the
2254 @c FIXME: This information should be in a net release, either of GCC or
2255 @c GDB. But gpcompare.texi doesn't seem to be in the FSF GCC.
2258 .stabs "name:symbol_desriptor(global function)return_type(int)",
2259 N_FUN, NIL, NIL, code_addr_of_method_start
2261 .stabs "Ameth__5baseAic:F1",36,0,0,_Ameth__5baseAic
2264 Here is the stab for the @code{this} pointer implicit argument. The
2265 name of the @code{this} pointer is always @code{this}. Type 19, the
2266 @code{this} pointer is defined as a pointer to type 20, @code{baseA},
2267 but a stab defining @code{baseA} has not yet been emited. Since the
2268 compiler knows it will be emited shortly, here it just outputs a cross
2269 reference to the undefined symbol, by prefixing the symbol name with
2273 .stabs "name:sym_desc(register param)type_def(19)=
2274 type_desc(ptr to)type_ref(baseA)=
2275 type_desc(cross-reference to)baseA:",N_RSYM,NIL,NIL,register_number
2277 .stabs "this:P19=*20=xsbaseA:",64,0,0,8
2280 The stab for the explicit integer argument looks just like a parameter
2281 to a C function. The last field of the stab is the offset from the
2282 argument pointer, which in most systems is the same as the frame
2286 .stabs "name:sym_desc(value parameter)type_ref(int)",
2287 N_PSYM,NIL,NIL,offset_from_arg_ptr
2289 .stabs "in:p1",160,0,0,72
2292 << The examples that follow are based on A1.C >>
2295 @section Protections
2298 In the simple class definition shown above all member data and
2299 functions were publicly accessable. The example that follows
2300 contrasts public, protected and privately accessable fields and shows
2301 how these protections are encoded in C++ stabs.
2303 @c FIXME: What does "part of the string" mean?
2304 Protections for class member data are signified by two characters
2305 embedded in the stab defining the class type. These characters are
2306 located after the name: part of the string. @samp{/0} means private,
2307 @samp{/1} means protected, and @samp{/2} means public. If these
2308 characters are omited this means that the member is public. The
2309 following C++ source:
2323 generates the following stab to describe the class type all_data.
2326 .stabs "class_name:sym_desc(type)type_def(19)=type_desc(struct)struct_bytes
2327 data_name:/protection(private)type_ref(int),bit_offset,num_bits;
2328 data_name:/protection(protected)type_ref(char),bit_offset,num_bits;
2329 data_name:(/num omited, private)type_ref(float),bit_offset,num_bits;;"
2334 .stabs "all_data:t19=s12
2335 priv_dat:/01,0,32;prot_dat:/12,32,8;pub_dat:12,64,32;;",128,0,0,0
2338 Protections for member functions are signified by one digit embeded in
2339 the field part of the stab describing the method. The digit is 0 if
2340 private, 1 if protected and 2 if public. Consider the C++ class
2344 class all_methods @{
2346 int priv_meth(int in)@{return in;@};
2348 char protMeth(char in)@{return in;@};
2350 float pubMeth(float in)@{return in;@};
2354 It generates the following stab. The digit in question is to the left
2355 of an @samp{A} in each case. Notice also that in this case two symbol
2356 descriptors apply to the class name struct tag and struct type.
2359 .stabs "class_name:sym_desc(struct tag&type)type_def(21)=
2360 sym_desc(struct)struct_bytes(1)
2361 meth_name::type_def(22)=sym_desc(method)returning(int);
2362 :args(int);protection(private)modifier(normal)virtual(no);
2363 meth_name::type_def(23)=sym_desc(method)returning(char);
2364 :args(char);protection(protected)modifier(normal)virual(no);
2365 meth_name::type_def(24)=sym_desc(method)returning(float);
2366 :args(float);protection(public)modifier(normal)virtual(no);;",
2371 .stabs "all_methods:Tt21=s1priv_meth::22=##1;:i;0A.;protMeth::23=##2;:c;1A.;
2372 pubMeth::24=##12;:f;2A.;;",128,0,0,0
2375 @node Method Modifiers
2376 @section Method Modifiers (@code{const}, @code{volatile}, @code{const volatile})
2380 In the class example described above all the methods have the normal
2381 modifier. This method modifier information is located just after the
2382 protection information for the method. This field has four possible
2383 character values. Normal methods use @samp{A}, const methods use
2384 @samp{B}, volatile methods use @samp{C}, and const volatile methods use
2385 @samp{D}. Consider the class definition below:
2390 int ConstMeth (int arg) const @{ return arg; @};
2391 char VolatileMeth (char arg) volatile @{ return arg; @};
2392 float ConstVolMeth (float arg) const volatile @{return arg; @};
2396 This class is described by the following stab:
2399 .stabs "class(A):sym_desc(struct)type_def(20)=type_desc(struct)struct_bytes(1)
2400 meth_name(ConstMeth)::type_def(21)sym_desc(method)
2401 returning(int);:arg(int);protection(public)modifier(const)virtual(no);
2402 meth_name(VolatileMeth)::type_def(22)=sym_desc(method)
2403 returning(char);:arg(char);protection(public)modifier(volatile)virt(no)
2404 meth_name(ConstVolMeth)::type_def(23)=sym_desc(method)
2405 returning(float);:arg(float);protection(public)modifer(const volatile)
2406 virtual(no);;", @dots{}
2410 .stabs "A:T20=s1ConstMeth::21=##1;:i;2B.;VolatileMeth::22=##2;:c;2C.;
2411 ConstVolMeth::23=##12;:f;2D.;;",128,0,0,0
2414 @node Virtual Methods
2415 @section Virtual Methods
2417 << The following examples are based on a4.C >>
2419 The presence of virtual methods in a class definition adds additional
2420 data to the class description. The extra data is appended to the
2421 description of the virtual method and to the end of the class
2422 description. Consider the class definition below:
2428 virtual int A_virt (int arg) @{ return arg; @};
2432 This results in the stab below describing class A. It defines a new
2433 type (20) which is an 8 byte structure. The first field of the class
2434 struct is @samp{Adat}, an integer, starting at structure offset 0 and
2437 The second field in the class struct is not explicitly defined by the
2438 C++ class definition but is implied by the fact that the class
2439 contains a virtual method. This field is the vtable pointer. The
2440 name of the vtable pointer field starts with @samp{$vf} and continues with a
2441 type reference to the class it is part of. In this example the type
2442 reference for class A is 20 so the name of its vtable pointer field is
2443 @samp{$vf20}, followed by the usual colon.
2445 Next there is a type definition for the vtable pointer type (21).
2446 This is in turn defined as a pointer to another new type (22).
2448 Type 22 is the vtable itself, which is defined as an array, indexed by
2449 a range of integers between 0 and 1, and whose elements are of type
2450 17. Type 17 was the vtable record type defined by the boilerplate C++
2451 type definitions, as shown earlier.
2453 The bit offset of the vtable pointer field is 32. The number of bits
2454 in the field are not specified when the field is a vtable pointer.
2456 Next is the method definition for the virtual member function @code{A_virt}.
2457 Its description starts out using the same format as the non-virtual
2458 member functions described above, except instead of a dot after the
2459 @samp{A} there is an asterisk, indicating that the function is virtual.
2460 Since is is virtual some addition information is appended to the end
2461 of the method description.
2463 The first number represents the vtable index of the method. This is a
2464 32 bit unsigned number with the high bit set, followed by a
2467 The second number is a type reference to the first base class in the
2468 inheritence hierarchy defining the virtual member function. In this
2469 case the class stab describes a base class so the virtual function is
2470 not overriding any other definition of the method. Therefore the
2471 reference is to the type number of the class that the stab is
2474 This is followed by three semi-colons. One marks the end of the
2475 current sub-section, one marks the end of the method field, and the
2476 third marks the end of the struct definition.
2478 For classes containing virtual functions the very last section of the
2479 string part of the stab holds a type reference to the first base
2480 class. This is preceeded by @samp{~%} and followed by a final semi-colon.
2483 .stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8)
2484 field_name(Adat):type_ref(int),bit_offset(0),field_bits(32);
2485 field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)=
2486 sym_desc(array)index_type_ref(range of int from 0 to 1);
2487 elem_type_ref(vtbl elem type),
2489 meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
2490 :arg_type(int),protection(public)normal(yes)virtual(yes)
2491 vtable_index(1);class_first_defining(A);;;~%first_base(A);",
2495 @c FIXME: bogus line break.
2497 .stabs "A:t20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2498 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2502 @section Inheritence
2504 Stabs describing C++ derived classes include additional sections that
2505 describe the inheritence hierarchy of the class. A derived class stab
2506 also encodes the number of base classes. For each base class it tells
2507 if the base class is virtual or not, and if the inheritence is private
2508 or public. It also gives the offset into the object of the portion of
2509 the object corresponding to each base class.
2511 This additional information is embeded in the class stab following the
2512 number of bytes in the struct. First the number of base classes
2513 appears bracketed by an exclamation point and a comma.
2515 Then for each base type there repeats a series: two digits, a number,
2516 a comma, another number, and a semi-colon.
2518 The first of the two digits is 1 if the base class is virtual and 0 if
2519 not. The second digit is 2 if the derivation is public and 0 if not.
2521 The number following the first two digits is the offset from the start
2522 of the object to the part of the object pertaining to the base class.
2524 After the comma, the second number is a type_descriptor for the base
2525 type. Finally a semi-colon ends the series, which repeats for each
2528 The source below defines three base classes @code{A}, @code{B}, and
2529 @code{C} and the derived class @code{D}.
2536 virtual int A_virt (int arg) @{ return arg; @};
2542 virtual int B_virt (int arg) @{return arg; @};
2548 virtual int C_virt (int arg) @{return arg; @};
2551 class D : A, virtual B, public C @{
2554 virtual int A_virt (int arg ) @{ return arg+1; @};
2555 virtual int B_virt (int arg) @{ return arg+2; @};
2556 virtual int C_virt (int arg) @{ return arg+3; @};
2557 virtual int D_virt (int arg) @{ return arg; @};
2561 Class stabs similar to the ones described earlier are generated for
2564 @c FIXME!!! the linebreaks in the following example probably make the
2565 @c examples literally unusable, but I don't know any other way to get
2566 @c them on the page.
2567 @c One solution would be to put some of the type definitions into
2568 @c separate stabs, even if that's not exactly what the compiler actually
2571 .stabs "A:T20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2572 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2574 .stabs "B:Tt25=s8Bdat:1,0,32;$vf25:21,32;B_virt::26=##1;
2575 :i;2A*-2147483647;25;;;~%25;",128,0,0,0
2577 .stabs "C:Tt28=s8Cdat:1,0,32;$vf28:21,32;C_virt::29=##1;
2578 :i;2A*-2147483647;28;;;~%28;",128,0,0,0
2581 In the stab describing derived class @code{D} below, the information about
2582 the derivation of this class is encoded as follows.
2585 .stabs "derived_class_name:symbol_descriptors(struct tag&type)=
2586 type_descriptor(struct)struct_bytes(32)!num_bases(3),
2587 base_virtual(no)inheritence_public(no)base_offset(0),
2588 base_class_type_ref(A);
2589 base_virtual(yes)inheritence_public(no)base_offset(NIL),
2590 base_class_type_ref(B);
2591 base_virtual(no)inheritence_public(yes)base_offset(64),
2592 base_class_type_ref(C); @dots{}
2595 @c FIXME! fake linebreaks.
2597 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:
2598 1,160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt:
2599 :32:i;2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;
2600 28;;D_virt::32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2603 @node Virtual Base Classes
2604 @section Virtual Base Classes
2606 A derived class object consists of a concatination in memory of the data
2607 areas defined by each base class, starting with the leftmost and ending
2608 with the rightmost in the list of base classes. The exception to this
2609 rule is for virtual inheritence. In the example above, class @code{D}
2610 inherits virtually from base class @code{B}. This means that an
2611 instance of a @code{D} object will not contain its own @code{B} part but
2612 merely a pointer to a @code{B} part, known as a virtual base pointer.
2614 In a derived class stab, the base offset part of the derivation
2615 information, described above, shows how the base class parts are
2616 ordered. The base offset for a virtual base class is always given as 0.
2617 Notice that the base offset for @code{B} is given as 0 even though
2618 @code{B} is not the first base class. The first base class @code{A}
2621 The field information part of the stab for class @code{D} describes the field
2622 which is the pointer to the virtual base class @code{B}. The vbase pointer
2623 name is @samp{$vb} followed by a type reference to the virtual base class.
2624 Since the type id for @code{B} in this example is 25, the vbase pointer name
2627 @c FIXME!! fake linebreaks below
2629 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1,
2630 160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i;
2631 2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt:
2632 :32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2635 Following the name and a semicolon is a type reference describing the
2636 type of the virtual base class pointer, in this case 24. Type 24 was
2637 defined earlier as the type of the @code{B} class @code{this} pointer. The
2638 @code{this} pointer for a class is a pointer to the class type.
2641 .stabs "this:P24=*25=xsB:",64,0,0,8
2644 Finally the field offset part of the vbase pointer field description
2645 shows that the vbase pointer is the first field in the @code{D} object,
2646 before any data fields defined by the class. The layout of a @code{D}
2647 class object is a follows, @code{Adat} at 0, the vtable pointer for
2648 @code{A} at 32, @code{Cdat} at 64, the vtable pointer for C at 96, the
2649 virtual base pointer for @code{B} at 128, and @code{Ddat} at 160.
2652 @node Static Members
2653 @section Static Members
2655 The data area for a class is a concatenation of the space used by the
2656 data members of the class. If the class has virtual methods, a vtable
2657 pointer follows the class data. The field offset part of each field
2658 description in the class stab shows this ordering.
2660 << How is this reflected in stabs? See Cygnus bug #677 for some info. >>
2663 @appendix Table of Stab Types
2665 The following are all the possible values for the stab type field, for
2666 @code{a.out} files, in numeric order. This does not apply to XCOFF, but
2667 it does apply to stabs in ELF. Stabs in ECOFF use these values but add
2668 0x8f300 to distinguish them from non-stab symbols.
2670 The symbolic names are defined in the file @file{include/aout/stabs.def}.
2673 * Non-Stab Symbol Types:: Types from 0 to 0x1f
2674 * Stab Symbol Types:: Types from 0x20 to 0xff
2677 @node Non-Stab Symbol Types
2678 @appendixsec Non-Stab Symbol Types
2680 The following types are used by the linker and assembler, not by stab
2681 directives. Since this document does not attempt to describe aspects of
2682 object file format other than the debugging format, no details are
2685 @c Try to get most of these to fit on a single line.
2695 File scope absolute symbol
2697 @item 0x3 N_ABS | N_EXT
2698 External absolute symbol
2701 File scope text symbol
2703 @item 0x5 N_TEXT | N_EXT
2704 External text symbol
2707 File scope data symbol
2709 @item 0x7 N_DATA | N_EXT
2710 External data symbol
2713 File scope BSS symbol
2715 @item 0x9 N_BSS | N_EXT
2719 Same as @code{N_FN}, for Sequent compilers
2722 Symbol is indirected to another symbol
2725 Common---visible after shared library dynamic link
2728 Absolute set element
2731 Text segment set element
2734 Data segment set element
2737 BSS segment set element
2740 Pointer to set vector
2742 @item 0x1e N_WARNING
2743 Print a warning message during linking
2746 File name of a @file{.o} file
2749 @node Stab Symbol Types
2750 @appendixsec Stab Symbol Types
2752 The following symbol types indicate that this is a stab. This is the
2753 full list of stab numbers, including stab types that are used in
2754 languages other than C.
2758 Global symbol; see @ref{Global Variables}.
2761 Function name (for BSD Fortran); see @ref{Procedures}.
2764 Function name (@pxref{Procedures}) or text segment variable
2768 Data segment file-scope variable; see @ref{Statics}.
2771 BSS segment file-scope variable; see @ref{Statics}.
2774 Name of main routine; see @ref{Main Program}.
2776 @c FIXME: discuss this in the Statics node where we talk about
2777 @c the fact that the n_type indicates the section.
2779 Variable in @code{.rodata} section; see @ref{Statics}.
2782 Global symbol (for Pascal); see @ref{N_PC}.
2785 Number of symbols (according to Ultrix V4.0); see @ref{N_NSYMS}.
2788 No DST map; see @ref{N_NOMAP}.
2790 @c FIXME: describe this solaris feature in the body of the text (see
2791 @c comments in include/aout/stab.def).
2793 Object file (Solaris2).
2795 @c See include/aout/stab.def for (a little) more info.
2797 Debugger options (Solaris2).
2800 Register variable; see @ref{Register Variables}.
2803 Modula-2 compilation unit; see @ref{N_M2C}.
2806 Line number in text segment; see @ref{Line Numbers}.
2809 Line number in data segment; see @ref{Line Numbers}.
2812 Line number in bss segment; see @ref{Line Numbers}.
2815 Sun source code browser, path to @file{.cb} file; see @ref{N_BROWS}.
2818 GNU Modula2 definition module dependency; see @ref{N_DEFD}.
2821 Function start/body/end line numbers (Solaris2).
2824 GNU C++ exception variable; see @ref{N_EHDECL}.
2827 Modula2 info "for imc" (according to Ultrix V4.0); see @ref{N_MOD2}.
2830 GNU C++ @code{catch} clause; see @ref{N_CATCH}.
2833 Structure of union element; see @ref{N_SSYM}.
2836 Last stab for module (Solaris2).
2839 Path and name of source file; see @ref{Source Files}.
2842 Stack variable (@pxref{Stack Variables}) or type (@pxref{Typedefs}).
2845 Beginning of an include file (Sun only); see @ref{Include Files}.
2848 Name of include file; see @ref{Include Files}.
2851 Parameter variable; see @ref{Parameters}.
2854 End of an include file; see @ref{Include Files}.
2857 Alternate entry point; see @ref{N_ENTRY}.
2860 Beginning of a lexical block; see @ref{Block Structure}.
2863 Place holder for a deleted include file; see @ref{Include Files}.
2866 Modula2 scope information (Sun linker); see @ref{N_SCOPE}.
2869 End of a lexical block; see @ref{Block Structure}.
2872 Begin named common block; see @ref{Common Blocks}.
2875 End named common block; see @ref{Common Blocks}.
2878 Member of a common block; see @ref{Common Blocks}.
2880 @c FIXME: How does this really work? Move it to main body of document.
2882 Pascal @code{with} statement: type,,0,0,offset (Solaris2).
2885 Gould non-base registers; see @ref{Gould}.
2888 Gould non-base registers; see @ref{Gould}.
2891 Gould non-base registers; see @ref{Gould}.
2894 Gould non-base registers; see @ref{Gould}.
2897 Gould non-base registers; see @ref{Gould}.
2900 @c Restore the default table indent
2905 @node Symbol Descriptors
2906 @appendix Table of Symbol Descriptors
2908 The symbol descriptor is the character which follows the colon in many
2909 stabs, and which tells what kind of stab it is. @xref{String Field},
2910 for more information about their use.
2912 @c Please keep this alphabetical
2914 @c In TeX, this looks great, digit is in italics. But makeinfo insists
2915 @c on putting it in `', not realizing that @var should override @code.
2916 @c I don't know of any way to make makeinfo do the right thing. Seems
2917 @c like a makeinfo bug to me.
2921 Variable on the stack; see @ref{Stack Variables}.
2924 Parameter passed by reference in register; see @ref{Reference Parameters}.
2927 Constant; see @ref{Constants}.
2930 Conformant array bound (Pascal, maybe other languages); @ref{Conformant
2931 Arrays}. Name of a caught exception (GNU C++). These can be
2932 distinguished because the latter uses @code{N_CATCH} and the former uses
2933 another symbol type.
2936 Floating point register variable; see @ref{Register Variables}.
2939 Parameter in floating point register; see @ref{Register Parameters}.
2942 File scope function; see @ref{Procedures}.
2945 Global function; see @ref{Procedures}.
2948 Global variable; see @ref{Global Variables}.
2951 @xref{Register Parameters}.
2954 Internal (nested) procedure; see @ref{Nested Procedures}.
2957 Internal (nested) function; see @ref{Nested Procedures}.
2960 Label name (documented by AIX, no further information known).
2963 Module; see @ref{Procedures}.
2966 Argument list parameter; see @ref{Parameters}.
2972 Fortran Function parameter; see @ref{Parameters}.
2975 Unfortunately, three separate meanings have been independently invented
2976 for this symbol descriptor. At least the GNU and Sun uses can be
2977 distinguished by the symbol type. Global Procedure (AIX) (symbol type
2978 used unknown); see @ref{Procedures}. Register parameter (GNU) (symbol
2979 type @code{N_PSYM}); see @ref{Parameters}. Prototype of function
2980 referenced by this file (Sun @code{acc}) (symbol type @code{N_FUN}).
2983 Static Procedure; see @ref{Procedures}.
2986 Register parameter; see @ref{Register Parameters}.
2989 Register variable; see @ref{Register Variables}.
2992 File scope variable; see @ref{Statics}.
2995 Type name; see @ref{Typedefs}.
2998 Enumeration, structure, or union tag; see @ref{Typedefs}.
3001 Parameter passed by reference; see @ref{Reference Parameters}.
3004 Procedure scope static variable; see @ref{Statics}.
3007 Conformant array; see @ref{Conformant Arrays}.
3010 Function return variable; see @ref{Parameters}.
3013 @node Type Descriptors
3014 @appendix Table of Type Descriptors
3016 The type descriptor is the character which follows the type number and
3017 an equals sign. It specifies what kind of type is being defined.
3018 @xref{String Field}, for more information about their use.
3023 Type reference; see @ref{String Field}.
3026 Reference to builtin type; see @ref{Negative Type Numbers}.
3029 Method (C++); see @ref{Cplusplus}.
3032 Pointer; see @ref{Miscellaneous Types}.
3038 Type Attributes (AIX); see @ref{String Field}. Member (class and variable)
3039 type (GNU C++); see @ref{Cplusplus}.
3042 Array; see @ref{Arrays}.
3045 Open array; see @ref{Arrays}.
3048 Pascal space type (AIX); see @ref{Miscellaneous Types}. Builtin integer
3049 type (Sun); see @ref{Builtin Type Descriptors}.
3052 Volatile-qualified type; see @ref{Miscellaneous Types}.
3055 Complex builtin type; see @ref{Builtin Type Descriptors}.
3058 COBOL Picture type. See AIX documentation for details.
3061 File type; see @ref{Miscellaneous Types}.
3064 N-dimensional dynamic array; see @ref{Arrays}.
3067 Enumeration type; see @ref{Enumerations}.
3070 N-dimensional subarray; see @ref{Arrays}.
3073 Function type; see @ref{Function Types}.
3076 Pascal function parameter; see @ref{Function Types}
3079 Builtin floating point type; see @ref{Builtin Type Descriptors}.
3082 COBOL Group. See AIX documentation for details.
3085 Imported type; see @ref{Cross-References}.
3088 Const-qualified type; see @ref{Miscellaneous Types}.
3091 COBOL File Descriptor. See AIX documentation for details.
3094 Multiple instance type; see @ref{Miscellaneous Types}.
3097 String type; see @ref{Strings}.
3100 Stringptr; see @ref{Strings}.
3103 Opaque type; see @ref{Typedefs}.
3106 Procedure; see @ref{Function Types}.
3109 Packed array; see @ref{Arrays}.
3112 Range type; see @ref{Subranges}.
3115 Builtin floating type; see @ref{Builtin Type Descriptors} (Sun). Pascal
3116 subroutine parameter; see @ref{Function Types} (AIX). Detecting this
3117 conflict is possible with careful parsing (hint: a Pascal subroutine
3118 parameter type will always contain a comma, and a builtin type
3119 descriptor never will).
3122 Structure type; see @ref{Structures}.
3125 Set type; see @ref{Miscellaneous Types}.
3128 Union; see @ref{Unions}.
3131 Variant record. This is a Pascal and Modula-2 feature which is like a
3132 union within a struct in C. See AIX documentation for details.
3135 Wide character; see @ref{Builtin Type Descriptors}.
3138 Cross-reference; see @ref{Cross-References}.
3141 gstring; see @ref{Strings}.
3144 @node Expanded Reference
3145 @appendix Expanded Reference by Stab Type
3147 @c FIXME: This appendix should go away; see N_PSYM or N_SO for an example.
3149 For a full list of stab types, and cross-references to where they are
3150 described, see @ref{Stab Types}. This appendix just duplicates certain
3151 information from the main body of this document; eventually the
3152 information will all be in one place.
3156 The first line is the symbol type (see @file{include/aout/stab.def}).
3158 The second line describes the language constructs the symbol type
3161 The third line is the stab format with the significant stab fields
3162 named and the rest NIL.
3164 Subsequent lines expand upon the meaning and possible values for each
3165 significant stab field. @samp{#} stands in for the type descriptor.
3167 Finally, any further information.
3170 * N_PC:: Pascal global symbol
3171 * N_NSYMS:: Number of symbols
3172 * N_NOMAP:: No DST map
3173 * N_M2C:: Modula-2 compilation unit
3174 * N_BROWS:: Path to .cb file for Sun source code browser
3175 * N_DEFD:: GNU Modula2 definition module dependency
3176 * N_EHDECL:: GNU C++ exception variable
3177 * N_MOD2:: Modula2 information "for imc"
3178 * N_CATCH:: GNU C++ "catch" clause
3179 * N_SSYM:: Structure or union element
3180 * N_ENTRY:: Alternate entry point
3181 * N_SCOPE:: Modula2 scope information (Sun only)
3182 * Gould:: non-base register symbols used on Gould systems
3183 * N_LENG:: Length of preceding entry
3189 @deffn @code{.stabs} N_PC
3191 Global symbol (for Pascal).
3194 "name" -> "symbol_name" <<?>>
3195 value -> supposedly the line number (stab.def is skeptical)
3199 @file{stabdump.c} says:
3201 global pascal symbol: name,,0,subtype,line
3209 @deffn @code{.stabn} N_NSYMS
3211 Number of symbols (according to Ultrix V4.0).
3214 0, files,,funcs,lines (stab.def)
3221 @deffn @code{.stabs} N_NOMAP
3223 No DST map for symbol (according to Ultrix V4.0). I think this means a
3224 variable has been optimized out.
3227 name, ,0,type,ignored (stab.def)
3234 @deffn @code{.stabs} N_M2C
3236 Modula-2 compilation unit.
3239 "string" -> "unit_name,unit_time_stamp[,code_time_stamp]"
3241 value -> 0 (main unit)
3249 @deffn @code{.stabs} N_BROWS
3251 Sun source code browser, path to @file{.cb} file
3254 "path to associated @file{.cb} file"
3256 Note: N_BROWS has the same value as N_BSLINE.
3262 @deffn @code{.stabn} N_DEFD
3264 GNU Modula2 definition module dependency.
3266 GNU Modula-2 definition module dependency. The value is the
3267 modification time of the definition file. The other field is non-zero
3268 if it is imported with the GNU M2 keyword @code{%INITIALIZE}. Perhaps
3269 @code{N_M2C} can be used if there are enough empty fields?
3275 @deffn @code{.stabs} N_EHDECL
3277 GNU C++ exception variable <<?>>.
3279 "@var{string} is variable name"
3281 Note: conflicts with @code{N_MOD2}.
3287 @deffn @code{.stab?} N_MOD2
3289 Modula2 info "for imc" (according to Ultrix V4.0)
3291 Note: conflicts with @code{N_EHDECL} <<?>>
3297 @deffn @code{.stabn} N_CATCH
3299 GNU C++ @code{catch} clause
3301 GNU C++ @code{catch} clause. The value is its address. The desc field
3302 is nonzero if this entry is immediately followed by a @code{CAUGHT} stab
3303 saying what exception was caught. Multiple @code{CAUGHT} stabs means
3304 that multiple exceptions can be caught here. If desc is 0, it means all
3305 exceptions are caught here.
3311 @deffn @code{.stabn} N_SSYM
3313 Structure or union element.
3315 The value is the offset in the structure.
3317 <<?looking at structs and unions in C I didn't see these>>
3323 @deffn @code{.stabn} N_ENTRY
3325 Alternate entry point.
3326 The value is its address.
3333 @deffn @code{.stab?} N_SCOPE
3335 Modula2 scope information (Sun linker)
3340 @section Non-base registers on Gould systems
3342 @deffn @code{.stab?} N_NBTEXT
3343 @deffnx @code{.stab?} N_NBDATA
3344 @deffnx @code{.stab?} N_NBBSS
3345 @deffnx @code{.stab?} N_NBSTS
3346 @deffnx @code{.stab?} N_NBLCS
3352 These are used on Gould systems for non-base registers syms.
3354 However, the following values are not the values used by Gould; they are
3355 the values which GNU has been documenting for these values for a long
3356 time, without actually checking what Gould uses. I include these values
3357 only because perhaps some someone actually did something with the GNU
3358 information (I hope not, why GNU knowingly assigned wrong values to
3359 these in the header file is a complete mystery to me).
3362 240 0xf0 N_NBTEXT ??
3363 242 0xf2 N_NBDATA ??
3373 @deffn @code{.stabn} N_LENG
3375 Second symbol entry containing a length-value for the preceding entry.
3376 The value is the length.
3380 @appendix Questions and Anomalies
3384 @c I think this is changed in GCC 2.4.5 to put the line number there.
3385 For GNU C stabs defining local and global variables (@code{N_LSYM} and
3386 @code{N_GSYM}), the desc field is supposed to contain the source
3387 line number on which the variable is defined. In reality the desc
3388 field is always 0. (This behavior is defined in @file{dbxout.c} and
3389 putting a line number in desc is controlled by @samp{#ifdef
3390 WINNING_GDB}, which defaults to false). GDB supposedly uses this
3391 information if you say @samp{list @var{var}}. In reality, @var{var} can
3392 be a variable defined in the program and GDB says @samp{function
3393 @var{var} not defined}.
3396 In GNU C stabs, there seems to be no way to differentiate tag types:
3397 structures, unions, and enums (symbol descriptor @samp{T}) and typedefs
3398 (symbol descriptor @samp{t}) defined at file scope from types defined locally
3399 to a procedure or other more local scope. They all use the @code{N_LSYM}
3400 stab type. Types defined at procedure scope are emited after the
3401 @code{N_RBRAC} of the preceding function and before the code of the
3402 procedure in which they are defined. This is exactly the same as
3403 types defined in the source file between the two procedure bodies.
3404 GDB overcompensates by placing all types in block #1, the block for
3405 symbols of file scope. This is true for default, @samp{-ansi} and
3406 @samp{-traditional} compiler options. (Bugs gcc/1063, gdb/1066.)
3409 What ends the procedure scope? Is it the proc block's @code{N_RBRAC} or the
3410 next @code{N_FUN}? (I believe its the first.)
3413 @c FIXME: This should go with the other stuff about global variables.
3414 Global variable stabs don't have location information. This comes
3415 from the external symbol for the same variable. The external symbol
3416 has a leading underbar on the _name of the variable and the stab does
3417 not. How do we know these two symbol table entries are talking about
3418 the same symbol when their names are different? (Answer: the debugger
3419 knows that external symbols have leading underbars).
3421 @c FIXME: This is absurdly vague; there all kinds of differences, some
3422 @c of which are the same between gnu & sun, and some of which aren't.
3423 @c In particular, I'm pretty sure GCC works with Sun dbx by default.
3425 @c Can GCC be configured to output stabs the way the Sun compiler
3426 @c does, so that their native debugging tools work? <NO?> It doesn't by
3427 @c default. GDB reads either format of stab. (GCC or SunC). How about
3431 @node XCOFF Differences
3432 @appendix Differences Between GNU Stabs in a.out and GNU Stabs in XCOFF
3434 @c FIXME: Merge *all* these into the main body of the document.
3435 The AIX/RS6000 native object file format is XCOFF with stabs. This
3436 appendix only covers those differences which are not covered in the main
3437 body of this document.
3441 BSD a.out stab types correspond to AIX XCOFF storage classes. In general
3442 the mapping is @code{N_@var{stabtype}} becomes @code{C_@var{stabtype}}.
3443 Some stab types in a.out are not supported in XCOFF; most of these use
3446 @c FIXME: Get C_* types for the block, figure out whether it is always
3447 @c used (I suspect not), explain clearly, and move to node Statics.
3448 Exception: initialised static @code{N_STSYM} and un-initialized static
3449 @code{N_LCSYM} both map to the @code{C_STSYM} storage class. But the
3450 distinction is preserved because in XCOFF @code{N_STSYM} and
3451 @code{N_LCSYM} must be emited in a named static block. Begin the block
3452 with @samp{.bs s[RW] data_section_name} for @code{N_STSYM} or @samp{.bs
3453 s bss_section_name} for @code{N_LCSYM}. End the block with @samp{.es}.
3455 @c FIXME: I think they are trying to say something about whether the
3456 @c assembler defaults the value to the location counter.
3458 If the XCOFF stab is an @code{N_FUN} (@code{C_FUN}) then follow the
3459 string field with @samp{,.} instead of just @samp{,}.
3462 I think that's it for @file{.s} file differences. They could stand to be
3463 better presented. This is just a list of what I have noticed so far.
3464 There are a @emph{lot} of differences in the information in the symbol
3465 tables of the executable and object files.
3467 Mapping of a.out stab types to XCOFF storage classes:
3470 stab type storage class
3471 -------------------------------
3507 @node Sun Differences
3508 @appendix Differences Between GNU Stabs and Sun Native Stabs
3510 @c FIXME: Merge all this stuff into the main body of the document.
3514 GNU C stabs define @emph{all} types, file or procedure scope, as
3515 @code{N_LSYM}. Sun doc talks about using @code{N_GSYM} too.
3518 Sun C stabs use type number pairs in the format (@var{a},@var{b}) where
3519 @var{a} is a number starting with 1 and incremented for each sub-source
3520 file in the compilation. @var{b} is a number starting with 1 and
3521 incremented for each new type defined in the compilation. GNU C stabs
3522 use the type number alone, with no source file number.
3526 @appendix Using Stabs With The ELF Object File Format
3528 The ELF object file format allows tools to create object files with
3529 custom sections containing any arbitrary data. To use stabs in ELF
3530 object files, the tools create two custom sections, a section named
3531 @code{.stab} which contains an array of fixed length structures, one
3532 struct per stab, and a section named @code{.stabstr} containing all the
3533 variable length strings that are referenced by stabs in the @code{.stab}
3534 section. The byte order of the stabs binary data matches the byte order
3535 of the ELF file itself, as determined from the @code{EI_DATA} field in
3536 the @code{e_ident} member of the ELF header.
3538 @c Is "source file" the right term for this concept? We don't mean that
3539 @c there is a separate one for include files (but "object file" or
3540 @c "object module" isn't quite right either; the output from ld -r is a
3541 @c single object file but contains many source files).
3542 The first stab in the @code{.stab} section for each source file is
3543 synthetic, generated entirely by the assembler, with no corresponding
3544 @code{.stab} directive as input to the assembler. This stab contains
3545 the following fields:
3549 Offset in the @code{.stabstr} section to the source filename.
3555 Unused field, always zero.
3558 Count of upcoming symbols, i.e., the number of remaining stabs for this
3562 Size of the string table fragment associated with this source file, in
3566 The @code{.stabstr} section always starts with a null byte (so that string
3567 offsets of zero reference a null string), followed by random length strings,
3568 each of which is null byte terminated.
3570 The ELF section header for the @code{.stab} section has its
3571 @code{sh_link} member set to the section number of the @code{.stabstr}
3572 section, and the @code{.stabstr} section has its ELF section
3573 header @code{sh_type} member set to @code{SHT_STRTAB} to mark it as a
3576 Because the linker does not process the @code{.stab} section in any
3577 special way, none of the addresses in the @code{n_value} field of the
3578 stabs are relocated by the linker. Instead they are relative to the
3579 source file (or some entity smaller than a source file, like a
3580 function). To find the address of each section corresponding to a given
3581 source file, the (compiler? assembler?) puts out symbols giving the
3582 address of each section for a given source file. Since these are normal
3583 ELF symbols, the linker can relocate them correctly. They are
3584 named @code{Bbss.bss} for the bss section, @code{Ddata.data} for
3585 the data section, and @code{Drodata.rodata} for the rodata section. I
3586 haven't yet figured out how the debugger gets the address for the text
3589 @node Symbol Types Index
3590 @unnumbered Symbol Types Index