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, Jim Kingdon,
21 Permission is granted to make and distribute verbatim copies of
22 this manual provided the copyright notice and this permission notice
23 are preserved on all copies.
26 Permission is granted to process this file through Tex and print the
27 results, provided the printed document carries copying permission
28 notice identical to this one except for the removal of this paragraph
29 (this paragraph not being relevant to the printed manual).
32 Permission is granted to copy or distribute modified versions of this
33 manual under the terms of the GPL (for which purpose this text may be
34 regarded as a program in the language TeX).
37 @setchapternewpage odd
40 @title The ``stabs'' debug format
41 @author Julia Menapace, Jim Kingdon, David MacKenzie
42 @author Cygnus Support
45 \def\$#1${{#1}} % Kluge: collect RCS revision info without $...$
46 \xdef\manvers{\$Revision$} % For use in headers, footers too
48 \hfill Cygnus Support\par
50 \hfill \TeX{}info \texinfoversion\par
54 @vskip 0pt plus 1filll
55 Copyright @copyright{} 1992, 1993 Free Software Foundation, Inc.
56 Contributed by Cygnus Support.
58 Permission is granted to make and distribute verbatim copies of
59 this manual provided the copyright notice and this permission notice
60 are preserved on all copies.
66 @top The "stabs" representation of debugging information
68 This document describes the stabs debugging format.
71 * Overview:: Overview of stabs
72 * Program Structure:: Encoding of the structure of the program
73 * Constants:: Constants
75 * Types:: Type definitions
76 * Symbol Tables:: Symbol information in symbol tables
77 * Cplusplus:: Appendixes:
78 * Stab Types:: Symbol types in a.out files
79 * Symbol Descriptors:: Table of symbol descriptors
80 * Type Descriptors:: Table of type descriptors
81 * Expanded Reference:: Reference information by stab type
82 * Questions:: Questions and anomolies
83 * XCOFF Differences:: Differences between GNU stabs in a.out
84 and GNU stabs in XCOFF
85 * Sun Differences:: Differences between GNU stabs and Sun
87 * Stabs In ELF:: Stabs in an ELF file.
88 * Symbol Types Index:: Index of symbolic stab symbol type names.
94 @chapter Overview of Stabs
96 @dfn{Stabs} refers to a format for information that describes a program
97 to a debugger. This format was apparently invented by
98 @c FIXME! <<name of inventor>> at
99 the University of California at Berkeley, for the @code{pdx} Pascal
100 debugger; the format has spread widely since then.
102 This document is one of the few published sources of documentation on
103 stabs. It is believed to be comprehensive for stabs used by C. The
104 lists of symbol descriptors (@pxref{Symbol Descriptors}) and type
105 descriptors (@pxref{Type Descriptors}) are believed to be completely
106 comprehensive. Stabs for COBOL-specific features and for variant
107 records (used by Pascal and Modula-2) are poorly documented here.
109 Other sources of information on stabs are @cite{Dbx and Dbxtool
110 Interfaces}, 2nd edition, by Sun, 1988, and @cite{AIX Version 3.2 Files
111 Reference}, Fourth Edition, September 1992, "dbx Stabstring Grammar" in
112 the a.out section, page 2-31. This document is believed to incorporate
113 the information from those two sources except where it explictly directs
114 you to them for more information.
117 * Flow:: Overview of debugging information flow
118 * Stabs Format:: Overview of stab format
119 * String Field:: The string field
120 * C Example:: A simple example in C source
121 * Assembly Code:: The simple example at the assembly level
125 @section Overview of Debugging Information Flow
127 The GNU C compiler compiles C source in a @file{.c} file into assembly
128 language in a @file{.s} file, which the assembler translates into
129 a @file{.o} file, which the linker combines with other @file{.o} files and
130 libraries to produce an executable file.
132 With the @samp{-g} option, GCC puts in the @file{.s} file additional
133 debugging information, which is slightly transformed by the assembler
134 and linker, and carried through into the final executable. This
135 debugging information describes features of the source file like line
136 numbers, the types and scopes of variables, and function names,
137 parameters, and scopes.
139 For some object file formats, the debugging information is encapsulated
140 in assembler directives known collectively as @dfn{stab} (symbol table)
141 directives, which are interspersed with the generated code. Stabs are
142 the native format for debugging information in the a.out and XCOFF
143 object file formats. The GNU tools can also emit stabs in the COFF and
144 ECOFF object file formats.
146 The assembler adds the information from stabs to the symbol information
147 it places by default in the symbol table and the string table of the
148 @file{.o} file it is building. The linker consolidates the @file{.o}
149 files into one executable file, with one symbol table and one string
150 table. Debuggers use the symbol and string tables in the executable as
151 a source of debugging information about the program.
154 @section Overview of Stab Format
156 There are three overall formats for stab assembler directives,
157 differentiated by the first word of the stab. The name of the directive
158 describes which combination of four possible data fields follows. It is
159 either @code{.stabs} (string), @code{.stabn} (number), or @code{.stabd}
160 (dot). IBM's XCOFF assembler uses @code{.stabx} (and some other
161 directives such as @code{.file} and @code{.bi}) instead of
162 @code{.stabs}, @code{.stabn} or @code{.stabd}.
164 The overall format of each class of stab is:
167 .stabs "@var{string}",@var{type},@var{other},@var{desc},@var{value}
168 .stabn @var{type},@var{other},@var{desc},@var{value}
169 .stabd @var{type},@var{other},@var{desc}
170 .stabx "@var{string}",@var{value},@var{type},@var{sdb-type}
173 @c what is the correct term for "current file location"? My AIX
174 @c assembler manual calls it "the value of the current location counter".
175 For @code{.stabn} and @code{.stabd}, there is no @var{string} (the
176 @code{n_strx} field is zero; see @ref{Symbol Tables}). For
177 @code{.stabd}, the @var{value} field is implicit and has the value of
178 the current file location. For @code{.stabx}, the @var{sdb-type} field
179 is unused for stabs and can always be set to zero. The @var{other}
180 field is almost always unused and can be set to zero.
182 The number in the @var{type} field gives some basic information about
183 which type of stab this is (or whether it @emph{is} a stab, as opposed
184 to an ordinary symbol). Each valid type number defines a different stab
185 type; further, the stab type defines the exact interpretation of, and
186 possible values for, any remaining @var{string}, @var{desc}, or
187 @var{value} fields present in the stab. @xref{Stab Types}, for a list
188 in numeric order of the valid @var{type} field values for stab directives.
191 @section The String Field
193 For most stabs the string field holds the meat of the
194 debugging information. The flexible nature of this field
195 is what makes stabs extensible. For some stab types the string field
196 contains only a name. For other stab types the contents can be a great
199 The overall format of the string field for most stab types is:
202 "@var{name}:@var{symbol-descriptor} @var{type-information}"
205 @var{name} is the name of the symbol represented by the stab.
206 @var{name} can be omitted, which means the stab represents an unnamed
207 object. For example, @samp{:t10=*2} defines type 10 as a pointer to
208 type 2, but does not give the type a name. Omitting the @var{name}
209 field is supported by AIX dbx and GDB after about version 4.8, but not
210 other debuggers. GCC sometimes uses a single space as the name instead
211 of omitting the name altogether; apparently that is supported by most
214 The @var{symbol-descriptor} following the @samp{:} is an alphabetic
215 character that tells more specifically what kind of symbol the stab
216 represents. If the @var{symbol-descriptor} is omitted, but type
217 information follows, then the stab represents a local variable. For a
218 list of symbol descriptors, see @ref{Symbol Descriptors}. The @samp{c}
219 symbol descriptor is an exception in that it is not followed by type
220 information. @xref{Constants}.
222 @var{type-information} is either a @var{type-number}, or
223 @samp{@var{type-number}=}. A @var{type-number} alone is a type
224 reference, referring directly to a type that has already been defined.
226 The @samp{@var{type-number}=} form is a type definition, where the
227 number represents a new type which is about to be defined. The type
228 definition may refer to other types by number, and those type numbers
229 may be followed by @samp{=} and nested definitions.
231 In a type definition, if the character that follows the equals sign is
232 non-numeric then it is a @var{type-descriptor}, and tells what kind of
233 type is about to be defined. Any other values following the
234 @var{type-descriptor} vary, depending on the @var{type-descriptor}.
235 @xref{Type Descriptors}, for a list of @var{type-descriptor} values. If
236 a number follows the @samp{=} then the number is a @var{type-reference}.
237 For a full description of types, @ref{Types}.
239 There is an AIX extension for type attributes. Following the @samp{=}
240 are any number of type attributes. Each one starts with @samp{@@} and
241 ends with @samp{;}. Debuggers, including AIX's dbx and GDB 4.10, skip
242 any type attributes they do not recognize. GDB 4.9 and other versions
243 of dbx may not do this. Because of a conflict with C++
244 (@pxref{Cplusplus}), new attributes should not be defined which begin
245 with a digit, @samp{(}, or @samp{-}; GDB may be unable to distinguish
246 those from the C++ type descriptor @samp{@@}. The attributes are:
249 @item a@var{boundary}
250 @var{boundary} is an integer specifying the alignment. I assume it
251 applies to all variables of this type.
254 Size in bits of a variable of this type.
257 Pointer class (for checking). Not sure what this means, or how
258 @var{integer} is interpreted.
261 Indicate this is a packed type, meaning that structure fields or array
262 elements are placed more closely in memory, to save memory at the
266 All of this can make the string field quite long. All
267 versions of GDB, and some versions of dbx, can handle arbitrarily long
268 strings. But many versions of dbx cretinously limit the strings to
269 about 80 characters, so compilers which must work with such dbx's need
270 to split the @code{.stabs} directive into several @code{.stabs}
271 directives. Each stab duplicates exactly all but the
272 string field. The string field of
273 every stab except the last is marked as continued with a
274 double-backslash at the end. Removing the backslashes and concatenating
275 the string fields of each stab produces the original,
279 @section A Simple Example in C Source
281 To get the flavor of how stabs describe source information for a C
282 program, let's look at the simple program:
287 printf("Hello world");
291 When compiled with @samp{-g}, the program above yields the following
292 @file{.s} file. Line numbers have been added to make it easier to refer
293 to parts of the @file{.s} file in the description of the stabs that
297 @section The Simple Example at the Assembly Level
299 This simple ``hello world'' example demonstrates several of the stab
300 types used to describe C language source files.
304 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
305 3 .stabs "hello.c",100,0,0,Ltext0
308 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
309 7 .stabs "char:t2=r2;0;127;",128,0,0,0
310 8 .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
311 9 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
312 10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
313 11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
314 12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
315 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
316 14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
317 15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
318 16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
319 17 .stabs "float:t12=r1;4;0;",128,0,0,0
320 18 .stabs "double:t13=r1;8;0;",128,0,0,0
321 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
322 20 .stabs "void:t15=15",128,0,0,0
325 23 .ascii "Hello, world!\12\0"
340 38 sethi %hi(LC0),%o1
341 39 or %o1,%lo(LC0),%o0
352 50 .stabs "main:F1",36,0,0,_main
353 51 .stabn 192,0,0,LBB2
354 52 .stabn 224,0,0,LBE2
357 @node Program Structure
358 @chapter Encoding the Structure of the Program
360 The elements of the program structure that stabs encode include the name
361 of the main function, the names of the source and include files, the
362 line numbers, procedure names and types, and the beginnings and ends of
366 * Main Program:: Indicate what the main program is
367 * Source Files:: The path and name of the source file
368 * Include Files:: Names of include files
371 * Nested Procedures::
376 @section Main Program
379 Most languages allow the main program to have any name. The
380 @code{N_MAIN} stab type tells the debugger the name that is used in this
381 program. Only the string field is significant; it is the name of
382 a function which is the main program. Most C compilers do not use this
383 stab (they expect the debugger to assume that the name is @code{main}),
384 but some C compilers emit an @code{N_MAIN} stab for the @code{main}
388 @section Paths and Names of the Source Files
391 Before any other stabs occur, there must be a stab specifying the source
392 file. This information is contained in a symbol of stab type
393 @code{N_SO}; the string field contains the name of the file. The
394 value of the symbol is the start address of the portion of the
395 text section corresponding to that file.
397 With the Sun Solaris2 compiler, the desc field contains a
398 source-language code.
399 @c Do the debuggers use it? What are the codes? -djm
401 Some compilers (for example, GCC2 and SunOS4 @file{/bin/cc}) also
402 include the directory in which the source was compiled, in a second
403 @code{N_SO} symbol preceding the one containing the file name. This
404 symbol can be distinguished by the fact that it ends in a slash. Code
405 from the @code{cfront} C++ compiler can have additional @code{N_SO} symbols for
406 nonexistent source files after the @code{N_SO} for the real source file;
407 these are believed to contain no useful information.
412 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0 # @r{100 is N_SO}
413 .stabs "hello.c",100,0,0,Ltext0
418 Instead of @code{N_SO} symbols, XCOFF uses a @code{.file} assembler
419 directive which assembles to a standard COFF @code{.file} symbol;
420 explaining this in detail is outside the scope of this document.
423 @section Names of Include Files
425 There are several schemes for dealing with include files: the
426 traditional @code{N_SOL} approach, Sun's @code{N_BINCL} approach, and the
427 XCOFF @code{C_BINCL} approach (which despite the similar name has little in
428 common with @code{N_BINCL}).
431 An @code{N_SOL} symbol specifies which include file subsequent symbols
432 refer to. The string field is the name of the file and the
433 value is the text address corresponding to the start of the
434 previous include file and the start of this one. To specify the main
435 source file again, use an @code{N_SOL} symbol with the name of the main
441 The @code{N_BINCL} approach works as follows. An @code{N_BINCL} symbol
442 specifies the start of an include file. In an object file, only the
443 string is significant; the Sun linker puts data into some of the
444 other fields. The end of the include file is marked by an
445 @code{N_EINCL} symbol (which has no string field). In an object
446 file, there is no significant data in the @code{N_EINCL} symbol; the Sun
447 linker puts data into some of the fields. @code{N_BINCL} and
448 @code{N_EINCL} can be nested.
450 If the linker detects that two source files have identical stabs between
451 an @code{N_BINCL} and @code{N_EINCL} pair (as will generally be the case
452 for a header file), then it only puts out the stabs once. Each
453 additional occurance is replaced by an @code{N_EXCL} symbol. I believe
454 the Sun (SunOS4, not sure about Solaris) linker is the only one which
455 supports this feature.
456 @c What do the fields of N_EXCL contain? -djm
460 For the start of an include file in XCOFF, use the @file{.bi} assembler
461 directive, which generates a @code{C_BINCL} symbol. A @file{.ei}
462 directive, which generates a @code{C_EINCL} symbol, denotes the end of
463 the include file. Both directives are followed by the name of the
464 source file in quotes, which becomes the string for the symbol.
465 The value of each symbol, produced automatically by the assembler
466 and linker, is the offset into the executable of the beginning
467 (inclusive, as you'd expect) or end (inclusive, as you would not expect)
468 of the portion of the COFF line table that corresponds to this include
469 file. @code{C_BINCL} and @code{C_EINCL} do not nest.
472 @section Line Numbers
475 An @code{N_SLINE} symbol represents the start of a source line. The
476 desc field contains the line number and the value
477 contains the code address for the start of that source line. On most
478 machines the address is absolute; for Sun's stabs-in-ELF, it is relative
479 to the function in which the @code{N_SLINE} symbol occurs.
483 GNU documents @code{N_DSLINE} and @code{N_BSLINE} symbols for line
484 numbers in the data or bss segments, respectively. They are identical
485 to @code{N_SLINE} but are relocated differently by the linker. They
486 were intended to be used to describe the source location of a variable
487 declaration, but I believe that GCC2 actually puts the line number in
488 the desc field of the stab for the variable itself. GDB has been
489 ignoring these symbols (unless they contain a string field) since
492 For single source lines that generate discontiguous code, such as flow
493 of control statements, there may be more than one line number entry for
494 the same source line. In this case there is a line number entry at the
495 start of each code range, each with the same line number.
497 XCOFF does not use stabs for line numbers. Instead, it uses COFF line
498 numbers (which are outside the scope of this document). Standard COFF
499 line numbers cannot deal with include files, but in XCOFF this is fixed
500 with the C_BINCL method of marking include files (@pxref{Include
508 @findex N_STSYM, for functions (Sun acc)
509 @findex N_GSYM, for functions (Sun acc)
510 All of the following stabs normally use the @code{N_FUN} symbol type.
511 However, Sun's @code{acc} compiler on SunOS4 uses @code{N_GSYM} and
512 @code{N_STSYM}, which means that the value of the stab for the function
513 is useless and the debugger must get the address of the function from
514 the non-stab symbols instead. BSD Fortran is said to use @code{N_FNAME}
515 with the same restriction; the value of the symbol is not useful (I'm
516 not sure it really does use this, because GDB doesn't handle this and no
519 A function is represented by an @samp{F} symbol descriptor for a global
520 (extern) function, and @samp{f} for a static (local) function. The
521 value is the address of the start of the function (absolute
522 for @code{a.out}; relative to the start of the file for Sun's
523 stabs-in-ELF). The type information of the stab represents the return
524 type of the function; thus @samp{foo:f5} means that foo is a function
525 returning type 5. There is no need to try to get the line number of the
526 start of the function from the stab for the function; it is in the next
527 @code{N_SLINE} symbol.
529 @c FIXME: verify whether the "I suspect" below is true or not.
530 Some compilers (such as Sun's Solaris compiler) support an extension for
531 specifying the types of the arguments. I suspect this extension is not
532 used for old (non-prototyped) function definitions in C. If the
533 extension is in use, the type information of the stab for the function
534 is followed by type information for each argument, with each argument
535 preceded by @samp{;}. An argument type of 0 means that additional
536 arguments are being passed, whose types and number may vary (@samp{...}
537 in ANSI C). GDB has tolerated this extension (parsed the syntax, if not
538 necessarily used the information) since at least version 4.8; I don't
539 know whether all versions of dbx tolerate it. The argument types given
540 here are not redundant with the symbols for the formal parameters
541 (@pxref{Parameters}); they are the types of the arguments as they are
542 passed, before any conversions might take place. For example, if a C
543 function which is declared without a prototype takes a @code{float}
544 argument, the value is passed as a @code{double} but then converted to a
545 @code{float}. Debuggers need to use the types given in the arguments
546 when printing values, but when calling the function they need to use the
547 types given in the symbol defining the function.
549 If the return type and types of arguments of a function which is defined
550 in another source file are specified (i.e., a function prototype in ANSI
551 C), traditionally compilers emit no stab; the only way for the debugger
552 to find the information is if the source file where the function is
553 defined was also compiled with debugging symbols. As an extension the
554 Solaris compiler uses symbol descriptor @samp{P} followed by the return
555 type of the function, followed by the arguments, each preceded by
556 @samp{;}, as in a stab with symbol descriptor @samp{f} or @samp{F}.
557 This use of symbol descriptor @samp{P} can be distinguished from its use
558 for register parameters (@pxref{Register Parameters}) by the fact that it has
559 symbol type @code{N_FUN}.
561 The AIX documentation also defines symbol descriptor @samp{J} as an
562 internal function. I assume this means a function nested within another
563 function. It also says symbol descriptor @samp{m} is a module in
564 Modula-2 or extended Pascal.
566 Procedures (functions which do not return values) are represented as
567 functions returning the @code{void} type in C. I don't see why this couldn't
568 be used for all languages (inventing a @code{void} type for this purpose if
569 necessary), but the AIX documentation defines @samp{I}, @samp{P}, and
570 @samp{Q} for internal, global, and static procedures, respectively.
571 These symbol descriptors are unusual in that they are not followed by
574 The following example shows a stab for a function @code{main} which
575 returns type number @code{1}. The @code{_main} specified for the value
576 is a reference to an assembler label which is used to fill in the start
577 address of the function.
580 .stabs "main:F1",36,0,0,_main # @r{36 is N_FUN}
583 The stab representing a procedure is located immediately following the
584 code of the procedure. This stab is in turn directly followed by a
585 group of other stabs describing elements of the procedure. These other
586 stabs describe the procedure's parameters, its block local variables, and
589 @node Nested Procedures
590 @section Nested Procedures
592 For any of the symbol descriptors representing procedures, after the
593 symbol descriptor and the type information is optionally a scope
594 specifier. This consists of a comma, the name of the procedure, another
595 comma, and the name of the enclosing procedure. The first name is local
596 to the scope specified, and seems to be redundant with the name of the
597 symbol (before the @samp{:}). This feature is used by GCC, and
598 presumably Pascal, Modula-2, etc., compilers, for nested functions.
600 If procedures are nested more than one level deep, only the immediately
601 containing scope is specified. For example, this code:
613 return baz (x + 2 * y);
615 return x + bar (3 * x);
623 .stabs "baz:f1,baz,bar",36,0,0,_baz.15 # @r{36 is N_FUN}
624 .stabs "bar:f1,bar,foo",36,0,0,_bar.12
625 .stabs "foo:F1",36,0,0,_foo
628 @node Block Structure
629 @section Block Structure
633 The program's block structure is represented by the @code{N_LBRAC} (left
634 brace) and the @code{N_RBRAC} (right brace) stab types. The variables
635 defined inside a block precede the @code{N_LBRAC} symbol for most
636 compilers, including GCC. Other compilers, such as the Convex, Acorn
637 RISC machine, and Sun @code{acc} compilers, put the variables after the
638 @code{N_LBRAC} symbol. The values of the @code{N_LBRAC} and
639 @code{N_RBRAC} symbols are the start and end addresses of the code of
640 the block, respectively. For most machines, they are relative to the
641 starting address of this source file. For the Gould NP1, they are
642 absolute. For Sun's stabs-in-ELF, they are relative to the function in
645 The @code{N_LBRAC} and @code{N_RBRAC} stabs that describe the block
646 scope of a procedure are located after the @code{N_FUN} stab that
647 represents the procedure itself.
649 Sun documents the desc field of @code{N_LBRAC} and
650 @code{N_RBRAC} symbols as containing the nesting level of the block.
651 However, dbx seems to not care, and GCC always sets desc to
657 The @samp{c} symbol descriptor indicates that this stab represents a
658 constant. This symbol descriptor is an exception to the general rule
659 that symbol descriptors are followed by type information. Instead, it
660 is followed by @samp{=} and one of the following:
664 Boolean constant. @var{value} is a numeric value; I assume it is 0 for
668 Character constant. @var{value} is the numeric value of the constant.
670 @item e @var{type-information} , @var{value}
671 Constant whose value can be represented as integral.
672 @var{type-information} is the type of the constant, as it would appear
673 after a symbol descriptor (@pxref{String Field}). @var{value} is the
674 numeric value of the constant. GDB 4.9 does not actually get the right
675 value if @var{value} does not fit in a host @code{int}, but it does not
676 do anything violent, and future debuggers could be extended to accept
677 integers of any size (whether unsigned or not). This constant type is
678 usually documented as being only for enumeration constants, but GDB has
679 never imposed that restriction; I don't know about other debuggers.
682 Integer constant. @var{value} is the numeric value. The type is some
683 sort of generic integer type (for GDB, a host @code{int}); to specify
684 the type explicitly, use @samp{e} instead.
687 Real constant. @var{value} is the real value, which can be @samp{INF}
688 (optionally preceded by a sign) for infinity, @samp{QNAN} for a quiet
689 NaN (not-a-number), or @samp{SNAN} for a signalling NaN. If it is a
690 normal number the format is that accepted by the C library function
694 String constant. @var{string} is a string enclosed in either @samp{'}
695 (in which case @samp{'} characters within the string are represented as
696 @samp{\'} or @samp{"} (in which case @samp{"} characters within the
697 string are represented as @samp{\"}).
699 @item S @var{type-information} , @var{elements} , @var{bits} , @var{pattern}
700 Set constant. @var{type-information} is the type of the constant, as it
701 would appear after a symbol descriptor (@pxref{String Field}).
702 @var{elements} is the number of elements in the set (does this means
703 how many bits of @var{pattern} are actually used, which would be
704 redundant with the type, or perhaps the number of bits set in
705 @var{pattern}? I don't get it), @var{bits} is the number of bits in the
706 constant (meaning it specifies the length of @var{pattern}, I think),
707 and @var{pattern} is a hexadecimal representation of the set. AIX
708 documentation refers to a limit of 32 bytes, but I see no reason why
709 this limit should exist. This form could probably be used for arbitrary
710 constants, not just sets; the only catch is that @var{pattern} should be
711 understood to be target, not host, byte order and format.
714 The boolean, character, string, and set constants are not supported by
715 GDB 4.9, but it ignores them. GDB 4.8 and earlier gave an error
716 message and refused to read symbols from the file containing the
719 The above information is followed by @samp{;}.
724 Different types of stabs describe the various ways that variables can be
725 allocated: on the stack, globally, in registers, in common blocks,
726 statically, or as arguments to a function.
729 * Stack Variables:: Variables allocated on the stack.
730 * Global Variables:: Variables used by more than one source file.
731 * Register Variables:: Variables in registers.
732 * Common Blocks:: Variables statically allocated together.
733 * Statics:: Variables local to one source file.
734 * Parameters:: Variables for arguments to functions.
737 @node Stack Variables
738 @section Automatic Variables Allocated on the Stack
740 If a variable's scope is local to a function and its lifetime is only as
741 long as that function executes (C calls such variables
742 @dfn{automatic}), it can be allocated in a register (@pxref{Register
743 Variables}) or on the stack.
746 Each variable allocated on the stack has a stab with the symbol
747 descriptor omitted. Since type information should begin with a digit,
748 @samp{-}, or @samp{(}, only those characters precluded from being used
749 for symbol descriptors. However, the Acorn RISC machine (ARM) is said
750 to get this wrong: it puts out a mere type definition here, without the
751 preceding @samp{@var{type-number}=}. This is a bad idea; there is no
752 guarantee that type descriptors are distinct from symbol descriptors.
753 Stabs for stack variables use the @code{N_LSYM} stab type.
755 The value of the stab is the offset of the variable within the
756 local variables. On most machines this is an offset from the frame
757 pointer and is negative. The location of the stab specifies which block
758 it is defined in; see @ref{Block Structure}.
760 For example, the following C code:
770 produces the following stabs:
773 .stabs "main:F1",36,0,0,_main # @r{36 is N_FUN}
774 .stabs "x:1",128,0,0,-12 # @r{128 is N_LSYM}
775 .stabn 192,0,0,LBB2 # @r{192 is N_LBRAC}
776 .stabn 224,0,0,LBE2 # @r{224 is N_RBRAC}
779 @xref{Procedures} for more information on the @code{N_FUN} stab, and
780 @ref{Block Structure} for more information on the @code{N_LBRAC} and
781 @code{N_RBRAC} stabs.
783 @node Global Variables
784 @section Global Variables
787 A variable whose scope is not specific to just one source file is
788 represented by the @samp{G} symbol descriptor. These stabs use the
789 @code{N_GSYM} stab type. The type information for the stab
790 (@pxref{String Field}) gives the type of the variable.
792 For example, the following source code:
799 yields the following assembly code:
802 .stabs "g_foo:G2",32,0,0,0 # @r{32 is N_GSYM}
809 The address of the variable represented by the @code{N_GSYM} is not
810 contained in the @code{N_GSYM} stab. The debugger gets this information
811 from the external symbol for the global variable. In the example above,
812 the @code{.global _g_foo} and @code{_g_foo:} lines tell the assembler to
813 produce an external symbol.
815 @node Register Variables
816 @section Register Variables
819 @c According to an old version of this manual, AIX uses C_RPSYM instead
820 @c of C_RSYM. I am skeptical; this should be verified.
821 Register variables have their own stab type, @code{N_RSYM}, and their
822 own symbol descriptor, @samp{r}. The stab's value is the
823 number of the register where the variable data will be stored.
824 @c .stabs "name:type",N_RSYM,0,RegSize,RegNumber (Sun doc)
826 AIX defines a separate symbol descriptor @samp{d} for floating point
827 registers. This seems unnecessary; why not just just give floating
828 point registers different register numbers? I have not verified whether
829 the compiler actually uses @samp{d}.
831 If the register is explicitly allocated to a global variable, but not
835 register int g_bar asm ("%g5");
839 then the stab may be emitted at the end of the object file, with
840 the other bss symbols.
843 @section Common Blocks
845 A common block is a statically allocated section of memory which can be
846 referred to by several source files. It may contain several variables.
847 I believe Fortran is the only language with this feature.
851 A @code{N_BCOMM} stab begins a common block and an @code{N_ECOMM} stab
852 ends it. The only field that is significant in these two stabs is the
853 string, which names a normal (non-debugging) symbol that gives the
854 address of the common block.
857 Each stab between the @code{N_BCOMM} and the @code{N_ECOMM} specifies a
858 member of that common block; its value is the offset within the
859 common block of that variable. The @code{N_ECOML} stab type is
860 documented for this purpose, but Sun's Fortran compiler uses
861 @code{N_GSYM} instead. The test case I looked at had a common block
862 local to a function and it used the @samp{V} symbol descriptor; I assume
863 one would use @samp{S} if not local to a function (that is, if a common
864 block @emph{can} be anything other than local to a function).
867 @section Static Variables
869 Initialized static variables are represented by the @samp{S} and
870 @samp{V} symbol descriptors. @samp{S} means file scope static, and
871 @samp{V} means procedure scope static.
873 @c This is probably not worth mentioning; it is only true on the sparc
874 @c for `double' variables which although declared const are actually in
875 @c the data segment (the text segment can't guarantee 8 byte alignment).
877 @c 2.4.5 has a bug in that it uses @code{N_FUN}, so neither dbx nor GDB can
878 @c find the variables)
881 In a.out files, @code{N_STSYM} means the data segment, @code{N_FUN}
882 means the text segment, and @code{N_LCSYM} means the bss segment.
884 For example, the source lines:
887 static const int var_const = 5;
888 static int var_init = 2;
889 static int var_noinit;
893 yield the following stabs:
896 .stabs "var_const:S1",36,0,0,_var_const # @r{36 is N_FUN}
898 .stabs "var_init:S1",38,0,0,_var_init # @r{38 is N_STSYM}
900 .stabs "var_noinit:S1",40,0,0,_var_noinit # @r{40 is N_LCSYM}
903 In XCOFF files, each symbol has a section number, so the stab type
904 need not indicate the segment.
906 In ECOFF files, the storage class is used to specify the section, so the
907 stab type need not indicate the segment.
909 @c In ELF files, it apparently is a big mess. See kludge in dbxread.c
910 @c in GDB. FIXME: Investigate where this kludge comes from.
912 @c This is the place to mention N_ROSYM; I'd rather do so once I can
913 @c coherently explain how this stuff works for stabs-in-ELF.
918 Formal parameters to a function are represented by a stab (or sometimes
919 two; see below) for each parameter. The stabs are in the order in which
920 the debugger should print the parameters (i.e., the order in which the
921 parameters are declared in the source file). The exact form of the stab
922 depends on how the parameter is being passed.
925 Parameters passed on the stack use the symbol descriptor @samp{p} and
926 the @code{N_PSYM} symbol type. The value of the symbol is an offset
927 used to locate the parameter on the stack; its exact meaning is
928 machine-dependent, but on most machines it is an offset from the frame
931 As a simple example, the code:
942 .stabs "main:F1",36,0,0,_main # @r{36 is N_FUN}
943 .stabs "argc:p1",160,0,0,68 # @r{160 is N_PSYM}
944 .stabs "argv:p20=*21=*2",160,0,0,72
947 The type definition of @code{argv} is interesting because it contains
948 several type definitions. Type 21 is pointer to type 2 (char) and
949 @code{argv} (type 20) is pointer to type 21.
951 @c FIXME: figure out what these mean and describe them coherently.
952 The following symbol descriptors are also said to go with @code{N_PSYM}.
953 The value of the symbol is said to be an offset from the argument
954 pointer (I'm not sure whether this is true or not).
958 pF Fortran function parameter
959 X (function result variable)
964 * Register Parameters::
965 * Local Variable Parameters::
966 * Reference Parameters::
967 * Conformant Arrays::
970 @node Register Parameters
971 @subsection Passing Parameters in Registers
973 If the parameter is passed in a register, then traditionally there are
974 two symbols for each argument:
977 .stabs "arg:p1" . . . ; N_PSYM
978 .stabs "arg:r1" . . . ; N_RSYM
981 Debuggers use the second one to find the value, and the first one to
982 know that it is an argument.
985 @findex N_RSYM, for parameters
986 Because that approach is kind of ugly, some compilers use symbol
987 descriptor @samp{P} or @samp{R} to indicate an argument which is in a
988 register. Symbol type @code{C_RPSYM} is used with @samp{R} and
989 @code{N_RSYM} is used with @samp{P}. The symbol's value is
990 the register number. @samp{P} and @samp{R} mean the same thing; the
991 difference is that @samp{P} is a GNU invention and @samp{R} is an IBM
992 (XCOFF) invention. As of version 4.9, GDB should handle either one.
994 There is at least one case where GCC uses a @samp{p} and @samp{r} pair
995 rather than @samp{P}; this is where the argument is passed in the
996 argument list and then loaded into a register.
998 According to the AIX documentation, symbol descriptor @samp{D} is for a
999 parameter passed in a floating point register. This seems
1000 unnecessary---why not just use @samp{R} with a register number which
1001 indicates that it's a floating point register? I haven't verified
1002 whether the system actually does what the documentation indicates.
1004 @c FIXME: On the hppa this is for any type > 8 bytes, I think, and not
1005 @c for small structures (investigate).
1006 On the sparc and hppa, for a @samp{P} symbol whose type is a structure
1007 or union, the register contains the address of the structure. On the
1008 sparc, this is also true of a @samp{p} and @samp{r} pair (using Sun
1009 @code{cc}) or a @samp{p} symbol. However, if a (small) structure is
1010 really in a register, @samp{r} is used. And, to top it all off, on the
1011 hppa it might be a structure which was passed on the stack and loaded
1012 into a register and for which there is a @samp{p} and @samp{r} pair! I
1013 believe that symbol descriptor @samp{i} is supposed to deal with this
1014 case (it is said to mean "value parameter by reference, indirect
1015 access"; I don't know the source for this information), but I don't know
1016 details or what compilers or debuggers use it, if any (not GDB or GCC).
1017 It is not clear to me whether this case needs to be dealt with
1018 differently than parameters passed by reference (@pxref{Reference Parameters}).
1020 @node Local Variable Parameters
1021 @subsection Storing Parameters as Local Variables
1023 There is a case similar to an argument in a register, which is an
1024 argument that is actually stored as a local variable. Sometimes this
1025 happens when the argument was passed in a register and then the compiler
1026 stores it as a local variable. If possible, the compiler should claim
1027 that it's in a register, but this isn't always done.
1029 @findex N_LSYM, for parameter
1030 Some compilers use the pair of symbols approach described above
1031 (@samp{@var{arg}:p} followed by @samp{@var{arg}:}); this includes GCC1
1032 (not GCC2) on the sparc when passing a small structure and GCC2
1033 (sometimes) when the argument type is @code{float} and it is passed as a
1034 @code{double} and converted to @code{float} by the prologue (in the
1035 latter case the type of the @samp{@var{arg}:p} symbol is @code{double}
1036 and the type of the @samp{@var{arg}:} symbol is @code{float}).
1038 GCC, at least on the 960, has another solution to the same problem. It
1039 uses a single @samp{p} symbol descriptor for an argument which is stored
1040 as a local variable but uses @code{N_LSYM} instead of @code{N_PSYM}. In
1041 this case, the value of the symbol is an offset relative to the local
1042 variables for that function, not relative to the arguments; on some
1043 machines those are the same thing, but not on all.
1045 @c This is mostly just background info; the part that logically belongs
1046 @c here is the last sentence.
1047 On the VAX or on other machines in which the calling convention includes
1048 the number of words of arguments actually passed, the debugger (GDB at
1049 least) uses the parameter symbols to keep track of whether it needs to
1050 print nameless arguments in addition to the formal parameters which it
1051 has printed because each one has a stab. For example, in
1054 extern int fprintf (FILE *stream, char *format, @dots{});
1056 fprintf (stdout, "%d\n", x);
1059 there are stabs for @code{stream} and @code{format}. On most machines,
1060 the debugger can only print those two arguments (because it has no way
1061 of knowing that additional arguments were passed), but on the VAX or
1062 other machines with a calling convention which indicates the number of
1063 words of arguments, the debugger can print all three arguments. To do
1064 so, the parameter symbol (symbol descriptor @samp{p}) (not necessarily
1065 @samp{r} or symbol descriptor omitted symbols) needs to contain the
1066 actual type as passed (for example, @code{double} not @code{float} if it
1067 is passed as a double and converted to a float).
1069 @node Reference Parameters
1070 @subsection Passing Parameters by Reference
1072 If the parameter is passed by reference (e.g., Pascal @code{VAR}
1073 parameters), then the symbol descriptor is @samp{v} if it is in the
1074 argument list, or @samp{a} if it in a register. Other than the fact
1075 that these contain the address of the parameter rather than the
1076 parameter itself, they are identical to @samp{p} and @samp{R},
1077 respectively. I believe @samp{a} is an AIX invention; @samp{v} is
1078 supported by all stabs-using systems as far as I know.
1080 @node Conformant Arrays
1081 @subsection Passing Conformant Array Parameters
1083 @c Is this paragraph correct? It is based on piecing together patchy
1084 @c information and some guesswork
1085 Conformant arrays are a feature of Modula-2, and perhaps other
1086 languages, in which the size of an array parameter is not known to the
1087 called function until run-time. Such parameters have two stabs: a
1088 @samp{x} for the array itself, and a @samp{C}, which represents the size
1089 of the array. The value of the @samp{x} stab is the offset in the
1090 argument list where the address of the array is stored (it this right?
1091 it is a guess); the value of the @samp{C} stab is the offset in the
1092 argument list where the size of the array (in elements? in bytes?) is
1096 @chapter Defining Types
1098 The examples so far have described types as references to previously
1099 defined types, or defined in terms of subranges of or pointers to
1100 previously defined types. This chapter describes the other type
1101 descriptors that may follow the @samp{=} in a type definition.
1104 * Builtin Types:: Integers, floating point, void, etc.
1105 * Miscellaneous Types:: Pointers, sets, files, etc.
1106 * Cross-References:: Referring to a type not yet defined.
1107 * Subranges:: A type with a specific range.
1108 * Arrays:: An aggregate type of same-typed elements.
1109 * Strings:: Like an array but also has a length.
1110 * Enumerations:: Like an integer but the values have names.
1111 * Structures:: An aggregate type of different-typed elements.
1112 * Typedefs:: Giving a type a name.
1113 * Unions:: Different types sharing storage.
1118 @section Builtin Types
1120 Certain types are built in (@code{int}, @code{short}, @code{void},
1121 @code{float}, etc.); the debugger recognizes these types and knows how
1122 to handle them. Thus, don't be surprised if some of the following ways
1123 of specifying builtin types do not specify everything that a debugger
1124 would need to know about the type---in some cases they merely specify
1125 enough information to distinguish the type from other types.
1127 The traditional way to define builtin types is convolunted, so new ways
1128 have been invented to describe them. Sun's @code{acc} uses special
1129 builtin type descriptors (@samp{b} and @samp{R}), and IBM uses negative
1130 type numbers. GDB accepts all three ways, as of version 4.8; dbx just
1131 accepts the traditional builtin types and perhaps one of the other two
1132 formats. The following sections describe each of these formats.
1135 * Traditional Builtin Types:: Put on your seatbelts and prepare for kludgery
1136 * Builtin Type Descriptors:: Builtin types with special type descriptors
1137 * Negative Type Numbers:: Builtin types using negative type numbers
1140 @node Traditional Builtin Types
1141 @subsection Traditional Builtin Types
1143 This is the traditional, convoluted method for defining builtin types.
1144 There are several classes of such type definitions: integer, floating
1145 point, and @code{void}.
1148 * Traditional Integer Types::
1149 * Traditional Other Types::
1152 @node Traditional Integer Types
1153 @subsubsection Traditional Integer Types
1155 Often types are defined as subranges of themselves. If the bounding values
1156 fit within an @code{int}, then they are given normally. For example:
1159 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0 # @r{128 is N_LSYM}
1160 .stabs "char:t2=r2;0;127;",128,0,0,0
1163 Builtin types can also be described as subranges of @code{int}:
1166 .stabs "unsigned short:t6=r1;0;65535;",128,0,0,0
1169 If the lower bound of a subrange is 0 and the upper bound is -1,
1170 the type is an unsigned integral type whose bounds are too
1171 big to describe in an @code{int}. Traditionally this is only used for
1172 @code{unsigned int} and @code{unsigned long}:
1175 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
1178 For larger types, GCC 2.4.5 puts out bounds in octal, with a leading 0.
1179 In this case a negative bound consists of a number which is a 1 bit
1180 followed by a bunch of 0 bits, and a positive bound is one in which a
1181 bunch of bits are 1. All known versions of dbx and GDB version 4 accept
1182 this, but GDB 3.5 refuses to read the whole file containing such
1183 symbols. So GCC 2.3.3 did not output the proper size for these types.
1184 @c FIXME: How about an example?
1186 If the lower bound of a subrange is 0 and the upper bound is negative,
1187 the type is an unsigned integral type whose size in bytes is the
1188 absolute value of the upper bound. I believe this is a Convex
1189 convention for @code{unsigned long long}.
1191 If the lower bound of a subrange is negative and the upper bound is 0,
1192 the type is a signed integral type whose size in bytes is
1193 the absolute value of the lower bound. I believe this is a Convex
1194 convention for @code{long long}. To distinguish this from a legitimate
1195 subrange, the type should be a subrange of itself. I'm not sure whether
1196 this is the case for Convex.
1198 @node Traditional Other Types
1199 @subsubsection Traditional Other Types
1201 If the upper bound of a subrange is 0 and the lower bound is positive,
1202 the type is a floating point type, and the lower bound of the subrange
1203 indicates the number of bytes in the type:
1206 .stabs "float:t12=r1;4;0;",128,0,0,0
1207 .stabs "double:t13=r1;8;0;",128,0,0,0
1210 However, GCC writes @code{long double} the same way it writes
1211 @code{double}, so there is no way to distinguish.
1214 .stabs "long double:t14=r1;8;0;",128,0,0,0
1217 Complex types are defined the same way as floating-point types; there is
1218 no way to distinguish a single-precision complex from a double-precision
1219 floating-point type.
1221 The C @code{void} type is defined as itself:
1224 .stabs "void:t15=15",128,0,0,0
1227 I'm not sure how a boolean type is represented.
1229 @node Builtin Type Descriptors
1230 @subsection Defining Builtin Types Using Builtin Type Descriptors
1232 This is the method used by Sun's @code{acc} for defining builtin types.
1233 These are the type descriptors to define builtin types:
1236 @c FIXME: clean up description of width and offset, once we figure out
1238 @item b @var{signed} @var{char-flag} @var{width} ; @var{offset} ; @var{nbits} ;
1239 Define an integral type. @var{signed} is @samp{u} for unsigned or
1240 @samp{s} for signed. @var{char-flag} is @samp{c} which indicates this
1241 is a character type, or is omitted. I assume this is to distinguish an
1242 integral type from a character type of the same size, for example it
1243 might make sense to set it for the C type @code{wchar_t} so the debugger
1244 can print such variables differently (Solaris does not do this). Sun
1245 sets it on the C types @code{signed char} and @code{unsigned char} which
1246 arguably is wrong. @var{width} and @var{offset} appear to be for small
1247 objects stored in larger ones, for example a @code{short} in an
1248 @code{int} register. @var{width} is normally the number of bytes in the
1249 type. @var{offset} seems to always be zero. @var{nbits} is the number
1250 of bits in the type.
1252 Note that type descriptor @samp{b} used for builtin types conflicts with
1253 its use for Pascal space types (@pxref{Miscellaneous Types}); they can
1254 be distinguished because the character following the type descriptor
1255 will be a digit, @samp{(}, or @samp{-} for a Pascal space type, or
1256 @samp{u} or @samp{s} for a builtin type.
1259 Documented by AIX to define a wide character type, but their compiler
1260 actually uses negative type numbers (@pxref{Negative Type Numbers}).
1262 @item R @var{fp-type} ; @var{bytes} ;
1263 Define a floating point type. @var{fp-type} has one of the following values:
1267 IEEE 32-bit (single precision) floating point format.
1270 IEEE 64-bit (double precision) floating point format.
1272 @item 3 (NF_COMPLEX)
1273 @item 4 (NF_COMPLEX16)
1274 @item 5 (NF_COMPLEX32)
1275 @c "GDB source" really means @file{include/aout/stab_gnu.h}, but trying
1276 @c to put that here got an overfull hbox.
1277 These are for complex numbers. A comment in the GDB source describes
1278 them as Fortran @code{complex}, @code{double complex}, and
1279 @code{complex*16}, respectively, but what does that mean? (i.e., Single
1280 precision? Double precison?).
1282 @item 6 (NF_LDOUBLE)
1283 Long double. This should probably only be used for Sun format
1284 @code{long double}, and new codes should be used for other floating
1285 point formats (@code{NF_DOUBLE} can be used if a @code{long double} is
1286 really just an IEEE double, of course).
1289 @var{bytes} is the number of bytes occupied by the type. This allows a
1290 debugger to perform some operations with the type even if it doesn't
1291 understand @var{fp-type}.
1293 @item g @var{type-information} ; @var{nbits}
1294 Documented by AIX to define a floating type, but their compiler actually
1295 uses negative type numbers (@pxref{Negative Type Numbers}).
1297 @item c @var{type-information} ; @var{nbits}
1298 Documented by AIX to define a complex type, but their compiler actually
1299 uses negative type numbers (@pxref{Negative Type Numbers}).
1302 The C @code{void} type is defined as a signed integral type 0 bits long:
1304 .stabs "void:t19=bs0;0;0",128,0,0,0
1306 The Solaris compiler seems to omit the trailing semicolon in this case.
1307 Getting sloppy in this way is not a swift move because if a type is
1308 embedded in a more complex expression it is necessary to be able to tell
1311 I'm not sure how a boolean type is represented.
1313 @node Negative Type Numbers
1314 @subsection Negative Type Numbers
1316 This is the method used in XCOFF for defining builtin types.
1317 Since the debugger knows about the builtin types anyway, the idea of
1318 negative type numbers is simply to give a special type number which
1319 indicates the builtin type. There is no stab defining these types.
1321 I'm not sure whether anyone has tried to define what this means if
1322 @code{int} can be other than 32 bits (or if other types can be other than
1323 their customary size). If @code{int} has exactly one size for each
1324 architecture, then it can be handled easily enough, but if the size of
1325 @code{int} can vary according the compiler options, then it gets hairy.
1326 The best way to do this would be to define separate negative type
1327 numbers for 16-bit @code{int} and 32-bit @code{int}; therefore I have
1328 indicated below the customary size (and other format information) for
1329 each type. The information below is currently correct because AIX on
1330 the RS6000 is the only system which uses these type numbers. If these
1331 type numbers start to get used on other systems, I suspect the correct
1332 thing to do is to define a new number in cases where a type does not
1333 have the size and format indicated below (or avoid negative type numbers
1336 Part of the definition of the negative type number is
1337 the name of the type. Types with identical size and format but
1338 different names have different negative type numbers.
1342 @code{int}, 32 bit signed integral type.
1345 @code{char}, 8 bit type holding a character. Both GDB and dbx on AIX
1346 treat this as signed. GCC uses this type whether @code{char} is signed
1347 or not, which seems like a bad idea. The AIX compiler (@code{xlc}) seems to
1348 avoid this type; it uses -5 instead for @code{char}.
1351 @code{short}, 16 bit signed integral type.
1354 @code{long}, 32 bit signed integral type.
1357 @code{unsigned char}, 8 bit unsigned integral type.
1360 @code{signed char}, 8 bit signed integral type.
1363 @code{unsigned short}, 16 bit unsigned integral type.
1366 @code{unsigned int}, 32 bit unsigned integral type.
1369 @code{unsigned}, 32 bit unsigned integral type.
1372 @code{unsigned long}, 32 bit unsigned integral type.
1375 @code{void}, type indicating the lack of a value.
1378 @code{float}, IEEE single precision.
1381 @code{double}, IEEE double precision.
1384 @code{long double}, IEEE double precision. The compiler claims the size
1385 will increase in a future release, and for binary compatibility you have
1386 to avoid using @code{long double}. I hope when they increase it they
1387 use a new negative type number.
1390 @code{integer}. 32 bit signed integral type.
1393 @code{boolean}. 32 bit type. How is the truth value encoded? Is it
1394 the least significant bit or is it a question of whether the whole value
1395 is zero or non-zero?
1398 @code{short real}. IEEE single precision.
1401 @code{real}. IEEE double precision.
1404 @code{stringptr}. @xref{Strings}.
1407 @code{character}, 8 bit unsigned character type.
1410 @code{logical*1}, 8 bit type. This Fortran type has a split
1411 personality in that it is used for boolean variables, but can also be
1412 used for unsigned integers. 0 is false, 1 is true, and other values are
1416 @code{logical*2}, 16 bit type. This Fortran type has a split
1417 personality in that it is used for boolean variables, but can also be
1418 used for unsigned integers. 0 is false, 1 is true, and other values are
1422 @code{logical*4}, 32 bit type. This Fortran type has a split
1423 personality in that it is used for boolean variables, but can also be
1424 used for unsigned integers. 0 is false, 1 is true, and other values are
1428 @code{logical}, 32 bit type. This Fortran type has a split
1429 personality in that it is used for boolean variables, but can also be
1430 used for unsigned integers. 0 is false, 1 is true, and other values are
1434 @code{complex}. A complex type consisting of two IEEE single-precision
1435 floating point values.
1438 @code{complex}. A complex type consisting of two IEEE double-precision
1439 floating point values.
1442 @code{integer*1}, 8 bit signed integral type.
1445 @code{integer*2}, 16 bit signed integral type.
1448 @code{integer*4}, 32 bit signed integral type.
1451 @code{wchar}. Wide character, 16 bits wide, unsigned (what format?
1455 @node Miscellaneous Types
1456 @section Miscellaneous Types
1459 @item b @var{type-information} ; @var{bytes}
1460 Pascal space type. This is documented by IBM; what does it mean?
1462 This use of the @samp{b} type descriptor can be distinguished
1463 from its use for builtin integral types (@pxref{Builtin Type
1464 Descriptors}) because the character following the type descriptor is
1465 always a digit, @samp{(}, or @samp{-}.
1467 @item B @var{type-information}
1468 A volatile-qualified version of @var{type-information}. This is
1469 a Sun extension. References and stores to a variable with a
1470 volatile-qualified type must not be optimized or cached; they
1471 must occur as the user specifies them.
1473 @item d @var{type-information}
1474 File of type @var{type-information}. As far as I know this is only used
1477 @item k @var{type-information}
1478 A const-qualified version of @var{type-information}. This is a Sun
1479 extension. A variable with a const-qualified type cannot be modified.
1481 @item M @var{type-information} ; @var{length}
1482 Multiple instance type. The type seems to composed of @var{length}
1483 repetitions of @var{type-information}, for example @code{character*3} is
1484 represented by @samp{M-2;3}, where @samp{-2} is a reference to a
1485 character type (@pxref{Negative Type Numbers}). I'm not sure how this
1486 differs from an array. This appears to be a Fortran feature.
1487 @var{length} is a bound, like those in range types; see @ref{Subranges}.
1489 @item S @var{type-information}
1490 Pascal set type. @var{type-information} must be a small type such as an
1491 enumeration or a subrange, and the type is a bitmask whose length is
1492 specified by the number of elements in @var{type-information}.
1494 @item * @var{type-information}
1495 Pointer to @var{type-information}.
1498 @node Cross-References
1499 @section Cross-References to Other Types
1501 A type can be used before it is defined; one common way to deal with
1502 that situation is just to use a type reference to a type which has not
1505 Another way is with the @samp{x} type descriptor, which is followed by
1506 @samp{s} for a structure tag, @samp{u} for a union tag, or @samp{e} for
1507 a enumerator tag, followed by the name of the tag, followed by @samp{:}.
1508 For example, the following C declarations:
1519 .stabs "bar:G16=*17=xsfoo:",32,0,0,0
1522 Not all debuggers support the @samp{x} type descriptor, so on some
1523 machines GCC does not use it. I believe that for the above example it
1524 would just emit a reference to type 17 and never define it, but I
1525 haven't verified that.
1527 Modula-2 imported types, at least on AIX, use the @samp{i} type
1528 descriptor, which is followed by the name of the module from which the
1529 type is imported, followed by @samp{:}, followed by the name of the
1530 type. There is then optionally a comma followed by type information for
1531 the type. This differs from merely naming the type (@pxref{Typedefs}) in
1532 that it identifies the module; I don't understand whether the name of
1533 the type given here is always just the same as the name we are giving
1534 it, or whether this type descriptor is used with a nameless stab
1535 (@pxref{String Field}), or what. The symbol ends with @samp{;}.
1538 @section Subrange Types
1540 The @samp{r} type descriptor defines a type as a subrange of another
1541 type. It is followed by type information for the type of which it is a
1542 subrange, a semicolon, an integral lower bound, a semicolon, an
1543 integral upper bound, and a semicolon. The AIX documentation does not
1544 specify the trailing semicolon, in an effort to specify array indexes
1545 more cleanly, but a subrange which is not an array index has always
1546 included a trailing semicolon (@pxref{Arrays}).
1548 Instead of an integer, either bound can be one of the following:
1551 @item A @var{offset}
1552 The bound is passed by reference on the stack at offset @var{offset}
1553 from the argument list. @xref{Parameters}, for more information on such
1556 @item T @var{offset}
1557 The bound is passed by value on the stack at offset @var{offset} from
1560 @item a @var{register-number}
1561 The bound is pased by reference in register number
1562 @var{register-number}.
1564 @item t @var{register-number}
1565 The bound is passed by value in register number @var{register-number}.
1571 Subranges are also used for builtin types; see @ref{Traditional Builtin Types}.
1574 @section Array Types
1576 Arrays use the @samp{a} type descriptor. Following the type descriptor
1577 is the type of the index and the type of the array elements. If the
1578 index type is a range type, it ends in a semicolon; otherwise
1579 (for example, if it is a type reference), there does not
1580 appear to be any way to tell where the types are separated. In an
1581 effort to clean up this mess, IBM documents the two types as being
1582 separated by a semicolon, and a range type as not ending in a semicolon
1583 (but this is not right for range types which are not array indexes,
1584 @pxref{Subranges}). I think probably the best solution is to specify
1585 that a semicolon ends a range type, and that the index type and element
1586 type of an array are separated by a semicolon, but that if the index
1587 type is a range type, the extra semicolon can be omitted. GDB (at least
1588 through version 4.9) doesn't support any kind of index type other than a
1589 range anyway; I'm not sure about dbx.
1591 It is well established, and widely used, that the type of the index,
1592 unlike most types found in the stabs, is merely a type definition, not
1593 type information (@pxref{String Field}) (that is, it need not start with
1594 @samp{@var{type-number}=} if it is defining a new type). According to a
1595 comment in GDB, this is also true of the type of the array elements; it
1596 gives @samp{ar1;1;10;ar1;1;10;4} as a legitimate way to express a two
1597 dimensional array. According to AIX documentation, the element type
1598 must be type information. GDB accepts either.
1600 The type of the index is often a range type, expressed as the type
1601 descriptor @samp{r} and some parameters. It defines the size of the
1602 array. In the example below, the range @samp{r1;0;2;} defines an index
1603 type which is a subrange of type 1 (integer), with a lower bound of 0
1604 and an upper bound of 2. This defines the valid range of subscripts of
1605 a three-element C array.
1607 For example, the definition:
1610 char char_vec[3] = @{'a','b','c'@};
1614 produces the output:
1617 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
1626 If an array is @dfn{packed}, the elements are spaced more
1627 closely than normal, saving memory at the expense of speed. For
1628 example, an array of 3-byte objects might, if unpacked, have each
1629 element aligned on a 4-byte boundary, but if packed, have no padding.
1630 One way to specify that something is packed is with type attributes
1631 (@pxref{String Field}). In the case of arrays, another is to use the
1632 @samp{P} type descriptor instead of @samp{a}. Other than specifying a
1633 packed array, @samp{P} is identical to @samp{a}.
1635 @c FIXME-what is it? A pointer?
1636 An open array is represented by the @samp{A} type descriptor followed by
1637 type information specifying the type of the array elements.
1639 @c FIXME: what is the format of this type? A pointer to a vector of pointers?
1640 An N-dimensional dynamic array is represented by
1643 D @var{dimensions} ; @var{type-information}
1646 @c Does dimensions really have this meaning? The AIX documentation
1648 @var{dimensions} is the number of dimensions; @var{type-information}
1649 specifies the type of the array elements.
1651 @c FIXME: what is the format of this type? A pointer to some offsets in
1653 A subarray of an N-dimensional array is represented by
1656 E @var{dimensions} ; @var{type-information}
1659 @c Does dimensions really have this meaning? The AIX documentation
1661 @var{dimensions} is the number of dimensions; @var{type-information}
1662 specifies the type of the array elements.
1667 Some languages, like C or the original Pascal, do not have string types,
1668 they just have related things like arrays of characters. But most
1669 Pascals and various other languages have string types, which are
1670 indicated as follows:
1673 @item n @var{type-information} ; @var{bytes}
1674 @var{bytes} is the maximum length. I'm not sure what
1675 @var{type-information} is; I suspect that it means that this is a string
1676 of @var{type-information} (thus allowing a string of integers, a string
1677 of wide characters, etc., as well as a string of characters). Not sure
1678 what the format of this type is. This is an AIX feature.
1680 @item z @var{type-information} ; @var{bytes}
1681 Just like @samp{n} except that this is a gstring, not an ordinary
1682 string. I don't know the difference.
1685 Pascal Stringptr. What is this? This is an AIX feature.
1689 @section Enumerations
1691 Enumerations are defined with the @samp{e} type descriptor.
1693 @c FIXME: Where does this information properly go? Perhaps it is
1694 @c redundant with something we already explain.
1695 The source line below declares an enumeration type at file scope.
1696 The type definition is located after the @code{N_RBRAC} that marks the end of
1697 the previous procedure's block scope, and before the @code{N_FUN} that marks
1698 the beginning of the next procedure's block scope. Therefore it does not
1699 describe a block local symbol, but a file local one.
1704 enum e_places @{first,second=3,last@};
1708 generates the following stab:
1711 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
1714 The symbol descriptor (@samp{T}) says that the stab describes a
1715 structure, enumeration, or union tag. The type descriptor @samp{e},
1716 following the @samp{22=} of the type definition narrows it down to an
1717 enumeration type. Following the @samp{e} is a list of the elements of
1718 the enumeration. The format is @samp{@var{name}:@var{value},}. The
1719 list of elements ends with @samp{;}.
1721 There is no standard way to specify the size of an enumeration type; it
1722 is determined by the architecture (normally all enumerations types are
1723 32 bits). There should be a way to specify an enumeration type of
1724 another size; type attributes would be one way to do this. @xref{Stabs
1730 The encoding of structures in stabs can be shown with an example.
1732 The following source code declares a structure tag and defines an
1733 instance of the structure in global scope. Then a @code{typedef} equates the
1734 structure tag with a new type. Seperate stabs are generated for the
1735 structure tag, the structure @code{typedef}, and the structure instance. The
1736 stabs for the tag and the @code{typedef} are emited when the definitions are
1737 encountered. Since the structure elements are not initialized, the
1738 stab and code for the structure variable itself is located at the end
1739 of the program in the bss section.
1746 struct s_tag* s_next;
1749 typedef struct s_tag s_typedef;
1752 The structure tag has an @code{N_LSYM} stab type because, like the
1753 enumeration, the symbol has file scope. Like the enumeration, the
1754 symbol descriptor is @samp{T}, for enumeration, structure, or tag type.
1755 The type descriptor @samp{s} following the @samp{16=} of the type
1756 definition narrows the symbol type to structure.
1758 Following the @samp{s} type descriptor is the number of bytes the
1759 structure occupies, followed by a description of each structure element.
1760 The structure element descriptions are of the form @var{name:type, bit
1761 offset from the start of the struct, number of bits in the element}.
1763 @c FIXME: phony line break. Can probably be fixed by using an example
1764 @c with fewer fields.
1767 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;
1768 s_char_vec:17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
1771 In this example, the first two structure elements are previously defined
1772 types. For these, the type following the @samp{@var{name}:} part of the
1773 element description is a simple type reference. The other two structure
1774 elements are new types. In this case there is a type definition
1775 embedded after the @samp{@var{name}:}. The type definition for the
1776 array element looks just like a type definition for a standalone array.
1777 The @code{s_next} field is a pointer to the same kind of structure that
1778 the field is an element of. So the definition of structure type 16
1779 contains a type definition for an element which is a pointer to type 16.
1782 @section Giving a Type a Name
1784 To give a type a name, use the @samp{t} symbol descriptor. The type
1785 is specified by the type information (@pxref{String Field}) for the stab.
1789 .stabs "s_typedef:t16",128,0,0,0 # @r{128 is N_LSYM}
1792 specifies that @code{s_typedef} refers to type number 16. Such stabs
1793 have symbol type @code{N_LSYM} (or @code{C_DECL} for XCOFF).
1795 If you are specifying the tag name for a structure, union, or
1796 enumeration, use the @samp{T} symbol descriptor instead. I believe C is
1797 the only language with this feature.
1799 If the type is an opaque type (I believe this is a Modula-2 feature),
1800 AIX provides a type descriptor to specify it. The type descriptor is
1801 @samp{o} and is followed by a name. I don't know what the name
1802 means---is it always the same as the name of the type, or is this type
1803 descriptor used with a nameless stab (@pxref{String Field})? There
1804 optionally follows a comma followed by type information which defines
1805 the type of this type. If omitted, a semicolon is used in place of the
1806 comma and the type information, and the type is much like a generic
1807 pointer type---it has a known size but little else about it is
1821 This code generates a stab for a union tag and a stab for a union
1822 variable. Both use the @code{N_LSYM} stab type. If a union variable is
1823 scoped locally to the procedure in which it is defined, its stab is
1824 located immediately preceding the @code{N_LBRAC} for the procedure's block
1827 The stab for the union tag, however, is located preceding the code for
1828 the procedure in which it is defined. The stab type is @code{N_LSYM}. This
1829 would seem to imply that the union type is file scope, like the struct
1830 type @code{s_tag}. This is not true. The contents and position of the stab
1831 for @code{u_type} do not convey any infomation about its procedure local
1834 @c FIXME: phony line break. Can probably be fixed by using an example
1835 @c with fewer fields.
1838 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
1842 The symbol descriptor @samp{T}, following the @samp{name:} means that
1843 the stab describes an enumeration, structure, or union tag. The type
1844 descriptor @samp{u}, following the @samp{23=} of the type definition,
1845 narrows it down to a union type definition. Following the @samp{u} is
1846 the number of bytes in the union. After that is a list of union element
1847 descriptions. Their format is @var{name:type, bit offset into the
1848 union, number of bytes for the element;}.
1850 The stab for the union variable is:
1853 .stabs "an_u:23",128,0,0,-20 # @r{128 is N_LSYM}
1856 @samp{-20} specifies where the variable is stored (@pxref{Stack
1859 @node Function Types
1860 @section Function Types
1862 Various types can be defined for function variables. These types are
1863 not used in defining functions (@pxref{Procedures}); they are used for
1864 things like pointers to functions.
1866 The simple, traditional, type is type descriptor @samp{f} is followed by
1867 type information for the return type of the function, followed by a
1870 This does not deal with functions for which the number and types of the
1871 parameters are part of the type, as in Modula-2 or ANSI C. AIX provides
1872 extensions to specify these, using the @samp{f}, @samp{F}, @samp{p}, and
1873 @samp{R} type descriptors.
1875 First comes the type descriptor. If it is @samp{f} or @samp{F}, this
1876 type involves a function rather than a procedure, and the type
1877 information for the return type of the function follows, followed by a
1878 comma. Then comes the number of parameters to the function and a
1879 semicolon. Then, for each parameter, there is the name of the parameter
1880 followed by a colon (this is only present for type descriptors @samp{R}
1881 and @samp{F} which represent Pascal function or procedure parameters),
1882 type information for the parameter, a comma, 0 if passed by reference or
1883 1 if passed by value, and a semicolon. The type definition ends with a
1886 For example, this variable definition:
1893 generates the following code:
1896 .stabs "g_pf:G24=*25=f1",32,0,0,0
1897 .common _g_pf,4,"bss"
1900 The variable defines a new type, 24, which is a pointer to another new
1901 type, 25, which is a function returning @code{int}.
1904 @chapter Symbol Information in Symbol Tables
1906 This chapter describes the format of symbol table entries
1907 and how stab assembler directives map to them. It also describes the
1908 transformations that the assembler and linker make on data from stabs.
1911 * Symbol Table Format::
1912 * Transformations On Symbol Tables::
1915 @node Symbol Table Format
1916 @section Symbol Table Format
1918 Each time the assembler encounters a stab directive, it puts
1919 each field of the stab into a corresponding field in a symbol table
1920 entry of its output file. If the stab contains a string field, the
1921 symbol table entry for that stab points to a string table entry
1922 containing the string data from the stab. Assembler labels become
1923 relocatable addresses. Symbol table entries in a.out have the format:
1925 @c FIXME: should refer to external, not internal.
1927 struct internal_nlist @{
1928 unsigned long n_strx; /* index into string table of name */
1929 unsigned char n_type; /* type of symbol */
1930 unsigned char n_other; /* misc info (usually empty) */
1931 unsigned short n_desc; /* description field */
1932 bfd_vma n_value; /* value of symbol */
1936 If the stab has a string, the @code{n_strx} field holds the offset in
1937 bytes of the string within the string table. The string is terminated
1938 by a NUL character. If the stab lacks a string (for example, it was
1939 produced by a @code{.stabn} or @code{.stabd} directive), the
1940 @code{n_strx} field is zero.
1942 Symbol table entries with @code{n_type} field values greater than 0x1f
1943 originated as stabs generated by the compiler (with one random
1944 exception). The other entries were placed in the symbol table of the
1945 executable by the assembler or the linker.
1947 @node Transformations On Symbol Tables
1948 @section Transformations on Symbol Tables
1950 The linker concatenates object files and does fixups of externally
1953 You can see the transformations made on stab data by the assembler and
1954 linker by examining the symbol table after each pass of the build. To
1955 do this, use @samp{nm -ap}, which dumps the symbol table, including
1956 debugging information, unsorted. For stab entries the columns are:
1957 @var{value}, @var{other}, @var{desc}, @var{type}, @var{string}. For
1958 assembler and linker symbols, the columns are: @var{value}, @var{type},
1961 The low 5 bits of the stab type tell the linker how to relocate the
1962 value of the stab. Thus for stab types like @code{N_RSYM} and
1963 @code{N_LSYM}, where the value is an offset or a register number, the
1964 low 5 bits are @code{N_ABS}, which tells the linker not to relocate the
1967 Where the value of a stab contains an assembly language label,
1968 it is transformed by each build step. The assembler turns it into a
1969 relocatable address and the linker turns it into an absolute address.
1972 * Transformations On Static Variables::
1973 * Transformations On Global Variables::
1976 @node Transformations On Static Variables
1977 @subsection Transformations on Static Variables
1979 This source line defines a static variable at file scope:
1982 static int s_g_repeat
1986 The following stab describes the symbol:
1989 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
1993 The assembler transforms the stab into this symbol table entry in the
1994 @file{.o} file. The location is expressed as a data segment offset.
1997 00000084 - 00 0000 STSYM s_g_repeat:S1
2001 In the symbol table entry from the executable, the linker has made the
2002 relocatable address absolute.
2005 0000e00c - 00 0000 STSYM s_g_repeat:S1
2008 @node Transformations On Global Variables
2009 @subsection Transformations on Global Variables
2011 Stabs for global variables do not contain location information. In
2012 this case, the debugger finds location information in the assembler or
2013 linker symbol table entry describing the variable. The source line:
2023 .stabs "g_foo:G2",32,0,0,0
2026 The variable is represented by two symbol table entries in the object
2027 file (see below). The first one originated as a stab. The second one
2028 is an external symbol. The upper case @samp{D} signifies that the
2029 @code{n_type} field of the symbol table contains 7, @code{N_DATA} with
2030 local linkage. The stab's value is zero since the value is not used for
2031 @code{N_GSYM} stabs. The value of the linker symbol is the relocatable
2032 address corresponding to the variable.
2035 00000000 - 00 0000 GSYM g_foo:G2
2040 These entries as transformed by the linker. The linker symbol table
2041 entry now holds an absolute address:
2044 00000000 - 00 0000 GSYM g_foo:G2
2050 @chapter GNU C++ Stabs
2053 * Basic Cplusplus Types::
2056 * Methods:: Method definition
2058 * Method Modifiers::
2061 * Virtual Base Classes::
2065 Type descriptors added for C++ descriptions:
2069 method type (@code{##} if minimal debug)
2072 Member (class and variable) type. It is followed by type information
2073 for the offset basetype, a comma, and type information for the type of
2074 the field being pointed to. (FIXME: this is acknowledged to be
2075 gibberish. Can anyone say what really goes here?).
2077 Note that there is a conflict between this and type attributes
2078 (@pxref{String Field}); both use type descriptor @samp{@@}.
2079 Fortunately, the @samp{@@} type descriptor used in this C++ sense always
2080 will be followed by a digit, @samp{(}, or @samp{-}, and type attributes
2081 never start with those things.
2084 @node Basic Cplusplus Types
2085 @section Basic Types For C++
2087 << the examples that follow are based on a01.C >>
2090 C++ adds two more builtin types to the set defined for C. These are
2091 the unknown type and the vtable record type. The unknown type, type
2092 16, is defined in terms of itself like the void type.
2094 The vtable record type, type 17, is defined as a structure type and
2095 then as a structure tag. The structure has four fields: delta, index,
2096 pfn, and delta2. pfn is the function pointer.
2098 << In boilerplate $vtbl_ptr_type, what are the fields delta,
2099 index, and delta2 used for? >>
2101 This basic type is present in all C++ programs even if there are no
2102 virtual methods defined.
2105 .stabs "struct_name:sym_desc(type)type_def(17)=type_desc(struct)struct_bytes(8)
2106 elem_name(delta):type_ref(short int),bit_offset(0),field_bits(16);
2107 elem_name(index):type_ref(short int),bit_offset(16),field_bits(16);
2108 elem_name(pfn):type_def(18)=type_desc(ptr to)type_ref(void),
2109 bit_offset(32),field_bits(32);
2110 elem_name(delta2):type_def(short int);bit_offset(32),field_bits(16);;"
2115 .stabs "$vtbl_ptr_type:t17=s8
2116 delta:6,0,16;index:6,16,16;pfn:18=*15,32,32;delta2:6,32,16;;"
2121 .stabs "name:sym_dec(struct tag)type_ref($vtbl_ptr_type)",N_LSYM,NIL,NIL,NIL
2125 .stabs "$vtbl_ptr_type:T17",128,0,0,0
2128 @node Simple Classes
2129 @section Simple Class Definition
2131 The stabs describing C++ language features are an extension of the
2132 stabs describing C. Stabs representing C++ class types elaborate
2133 extensively on the stab format used to describe structure types in C.
2134 Stabs representing class type variables look just like stabs
2135 representing C language variables.
2137 Consider the following very simple class definition.
2143 int Ameth(int in, char other);
2147 The class @code{baseA} is represented by two stabs. The first stab describes
2148 the class as a structure type. The second stab describes a structure
2149 tag of the class type. Both stabs are of stab type @code{N_LSYM}. Since the
2150 stab is not located between an @code{N_FUN} and an @code{N_LBRAC} stab this indicates
2151 that the class is defined at file scope. If it were, then the @code{N_LSYM}
2152 would signify a local variable.
2154 A stab describing a C++ class type is similar in format to a stab
2155 describing a C struct, with each class member shown as a field in the
2156 structure. The part of the struct format describing fields is
2157 expanded to include extra information relevent to C++ class members.
2158 In addition, if the class has multiple base classes or virtual
2159 functions the struct format outside of the field parts is also
2162 In this simple example the field part of the C++ class stab
2163 representing member data looks just like the field part of a C struct
2164 stab. The section on protections describes how its format is
2165 sometimes extended for member data.
2167 The field part of a C++ class stab representing a member function
2168 differs substantially from the field part of a C struct stab. It
2169 still begins with @samp{name:} but then goes on to define a new type number
2170 for the member function, describe its return type, its argument types,
2171 its protection level, any qualifiers applied to the method definition,
2172 and whether the method is virtual or not. If the method is virtual
2173 then the method description goes on to give the vtable index of the
2174 method, and the type number of the first base class defining the
2177 When the field name is a method name it is followed by two colons rather
2178 than one. This is followed by a new type definition for the method.
2179 This is a number followed by an equal sign and the type descriptor
2180 @samp{#}, indicating a method type, and a second @samp{#}, indicating
2181 that this is the @dfn{minimal} type of method definition used by GCC2,
2182 not larger method definitions used by earlier versions of GCC. This is
2183 followed by a type reference showing the return type of the method and a
2186 The format of an overloaded operator method name differs from that of
2187 other methods. It is @samp{op$::@var{operator-name}.} where
2188 @var{operator-name} is the operator name such as @samp{+} or @samp{+=}.
2189 The name ends with a period, and any characters except the period can
2190 occur in the @var{operator-name} string.
2192 The next part of the method description represents the arguments to the
2193 method, preceeded by a colon and ending with a semi-colon. The types of
2194 the arguments are expressed in the same way argument types are expressed
2195 in C++ name mangling. In this example an @code{int} and a @code{char}
2198 This is followed by a number, a letter, and an asterisk or period,
2199 followed by another semicolon. The number indicates the protections
2200 that apply to the member function. Here the 2 means public. The
2201 letter encodes any qualifier applied to the method definition. In
2202 this case, @samp{A} means that it is a normal function definition. The dot
2203 shows that the method is not virtual. The sections that follow
2204 elaborate further on these fields and describe the additional
2205 information present for virtual methods.
2209 .stabs "class_name:sym_desc(type)type_def(20)=type_desc(struct)struct_bytes(4)
2210 field_name(Adat):type(int),bit_offset(0),field_bits(32);
2212 method_name(Ameth)::type_def(21)=type_desc(method)return_type(int);
2213 :arg_types(int char);
2214 protection(public)qualifier(normal)virtual(no);;"
2219 .stabs "baseA:t20=s4Adat:1,0,32;Ameth::21=##1;:ic;2A.;;",128,0,0,0
2221 .stabs "class_name:sym_desc(struct tag)",N_LSYM,NIL,NIL,NIL
2223 .stabs "baseA:T20",128,0,0,0
2226 @node Class Instance
2227 @section Class Instance
2229 As shown above, describing even a simple C++ class definition is
2230 accomplished by massively extending the stab format used in C to
2231 describe structure types. However, once the class is defined, C stabs
2232 with no modifications can be used to describe class instances. The
2242 yields the following stab describing the class instance. It looks no
2243 different from a standard C stab describing a local variable.
2246 .stabs "name:type_ref(baseA)", N_LSYM, NIL, NIL, frame_ptr_offset
2250 .stabs "AbaseA:20",128,0,0,-20
2254 @section Method Defintion
2256 The class definition shown above declares Ameth. The C++ source below
2261 baseA::Ameth(int in, char other)
2268 This method definition yields three stabs following the code of the
2269 method. One stab describes the method itself and following two describe
2270 its parameters. Although there is only one formal argument all methods
2271 have an implicit argument which is the @code{this} pointer. The @code{this}
2272 pointer is a pointer to the object on which the method was called. Note
2273 that the method name is mangled to encode the class name and argument
2274 types. Name mangling is described in the @sc{arm} (@cite{The Annotated
2275 C++ Reference Manual}, by Ellis and Stroustrup, @sc{isbn}
2276 0-201-51459-1); @file{gpcompare.texi} in Cygnus GCC distributions
2277 describes the differences between GNU mangling and @sc{arm}
2279 @c FIXME: Use @xref, especially if this is generally installed in the
2281 @c FIXME: This information should be in a net release, either of GCC or
2282 @c GDB. But gpcompare.texi doesn't seem to be in the FSF GCC.
2285 .stabs "name:symbol_desriptor(global function)return_type(int)",
2286 N_FUN, NIL, NIL, code_addr_of_method_start
2288 .stabs "Ameth__5baseAic:F1",36,0,0,_Ameth__5baseAic
2291 Here is the stab for the @code{this} pointer implicit argument. The
2292 name of the @code{this} pointer is always @code{this}. Type 19, the
2293 @code{this} pointer is defined as a pointer to type 20, @code{baseA},
2294 but a stab defining @code{baseA} has not yet been emited. Since the
2295 compiler knows it will be emited shortly, here it just outputs a cross
2296 reference to the undefined symbol, by prefixing the symbol name with
2300 .stabs "name:sym_desc(register param)type_def(19)=
2301 type_desc(ptr to)type_ref(baseA)=
2302 type_desc(cross-reference to)baseA:",N_RSYM,NIL,NIL,register_number
2304 .stabs "this:P19=*20=xsbaseA:",64,0,0,8
2307 The stab for the explicit integer argument looks just like a parameter
2308 to a C function. The last field of the stab is the offset from the
2309 argument pointer, which in most systems is the same as the frame
2313 .stabs "name:sym_desc(value parameter)type_ref(int)",
2314 N_PSYM,NIL,NIL,offset_from_arg_ptr
2316 .stabs "in:p1",160,0,0,72
2319 << The examples that follow are based on A1.C >>
2322 @section Protections
2325 In the simple class definition shown above all member data and
2326 functions were publicly accessable. The example that follows
2327 contrasts public, protected and privately accessable fields and shows
2328 how these protections are encoded in C++ stabs.
2330 @c FIXME: What does "part of the string" mean?
2331 Protections for class member data are signified by two characters
2332 embedded in the stab defining the class type. These characters are
2333 located after the name: part of the string. @samp{/0} means private,
2334 @samp{/1} means protected, and @samp{/2} means public. If these
2335 characters are omited this means that the member is public. The
2336 following C++ source:
2350 generates the following stab to describe the class type all_data.
2353 .stabs "class_name:sym_desc(type)type_def(19)=type_desc(struct)struct_bytes
2354 data_name:/protection(private)type_ref(int),bit_offset,num_bits;
2355 data_name:/protection(protected)type_ref(char),bit_offset,num_bits;
2356 data_name:(/num omited, private)type_ref(float),bit_offset,num_bits;;"
2361 .stabs "all_data:t19=s12
2362 priv_dat:/01,0,32;prot_dat:/12,32,8;pub_dat:12,64,32;;",128,0,0,0
2365 Protections for member functions are signified by one digit embeded in
2366 the field part of the stab describing the method. The digit is 0 if
2367 private, 1 if protected and 2 if public. Consider the C++ class
2371 class all_methods @{
2373 int priv_meth(int in)@{return in;@};
2375 char protMeth(char in)@{return in;@};
2377 float pubMeth(float in)@{return in;@};
2381 It generates the following stab. The digit in question is to the left
2382 of an @samp{A} in each case. Notice also that in this case two symbol
2383 descriptors apply to the class name struct tag and struct type.
2386 .stabs "class_name:sym_desc(struct tag&type)type_def(21)=
2387 sym_desc(struct)struct_bytes(1)
2388 meth_name::type_def(22)=sym_desc(method)returning(int);
2389 :args(int);protection(private)modifier(normal)virtual(no);
2390 meth_name::type_def(23)=sym_desc(method)returning(char);
2391 :args(char);protection(protected)modifier(normal)virual(no);
2392 meth_name::type_def(24)=sym_desc(method)returning(float);
2393 :args(float);protection(public)modifier(normal)virtual(no);;",
2398 .stabs "all_methods:Tt21=s1priv_meth::22=##1;:i;0A.;protMeth::23=##2;:c;1A.;
2399 pubMeth::24=##12;:f;2A.;;",128,0,0,0
2402 @node Method Modifiers
2403 @section Method Modifiers (@code{const}, @code{volatile}, @code{const volatile})
2407 In the class example described above all the methods have the normal
2408 modifier. This method modifier information is located just after the
2409 protection information for the method. This field has four possible
2410 character values. Normal methods use @samp{A}, const methods use
2411 @samp{B}, volatile methods use @samp{C}, and const volatile methods use
2412 @samp{D}. Consider the class definition below:
2417 int ConstMeth (int arg) const @{ return arg; @};
2418 char VolatileMeth (char arg) volatile @{ return arg; @};
2419 float ConstVolMeth (float arg) const volatile @{return arg; @};
2423 This class is described by the following stab:
2426 .stabs "class(A):sym_desc(struct)type_def(20)=type_desc(struct)struct_bytes(1)
2427 meth_name(ConstMeth)::type_def(21)sym_desc(method)
2428 returning(int);:arg(int);protection(public)modifier(const)virtual(no);
2429 meth_name(VolatileMeth)::type_def(22)=sym_desc(method)
2430 returning(char);:arg(char);protection(public)modifier(volatile)virt(no)
2431 meth_name(ConstVolMeth)::type_def(23)=sym_desc(method)
2432 returning(float);:arg(float);protection(public)modifer(const volatile)
2433 virtual(no);;", @dots{}
2437 .stabs "A:T20=s1ConstMeth::21=##1;:i;2B.;VolatileMeth::22=##2;:c;2C.;
2438 ConstVolMeth::23=##12;:f;2D.;;",128,0,0,0
2441 @node Virtual Methods
2442 @section Virtual Methods
2444 << The following examples are based on a4.C >>
2446 The presence of virtual methods in a class definition adds additional
2447 data to the class description. The extra data is appended to the
2448 description of the virtual method and to the end of the class
2449 description. Consider the class definition below:
2455 virtual int A_virt (int arg) @{ return arg; @};
2459 This results in the stab below describing class A. It defines a new
2460 type (20) which is an 8 byte structure. The first field of the class
2461 struct is @samp{Adat}, an integer, starting at structure offset 0 and
2464 The second field in the class struct is not explicitly defined by the
2465 C++ class definition but is implied by the fact that the class
2466 contains a virtual method. This field is the vtable pointer. The
2467 name of the vtable pointer field starts with @samp{$vf} and continues with a
2468 type reference to the class it is part of. In this example the type
2469 reference for class A is 20 so the name of its vtable pointer field is
2470 @samp{$vf20}, followed by the usual colon.
2472 Next there is a type definition for the vtable pointer type (21).
2473 This is in turn defined as a pointer to another new type (22).
2475 Type 22 is the vtable itself, which is defined as an array, indexed by
2476 a range of integers between 0 and 1, and whose elements are of type
2477 17. Type 17 was the vtable record type defined by the boilerplate C++
2478 type definitions, as shown earlier.
2480 The bit offset of the vtable pointer field is 32. The number of bits
2481 in the field are not specified when the field is a vtable pointer.
2483 Next is the method definition for the virtual member function @code{A_virt}.
2484 Its description starts out using the same format as the non-virtual
2485 member functions described above, except instead of a dot after the
2486 @samp{A} there is an asterisk, indicating that the function is virtual.
2487 Since is is virtual some addition information is appended to the end
2488 of the method description.
2490 The first number represents the vtable index of the method. This is a
2491 32 bit unsigned number with the high bit set, followed by a
2494 The second number is a type reference to the first base class in the
2495 inheritence hierarchy defining the virtual member function. In this
2496 case the class stab describes a base class so the virtual function is
2497 not overriding any other definition of the method. Therefore the
2498 reference is to the type number of the class that the stab is
2501 This is followed by three semi-colons. One marks the end of the
2502 current sub-section, one marks the end of the method field, and the
2503 third marks the end of the struct definition.
2505 For classes containing virtual functions the very last section of the
2506 string part of the stab holds a type reference to the first base
2507 class. This is preceeded by @samp{~%} and followed by a final semi-colon.
2510 .stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8)
2511 field_name(Adat):type_ref(int),bit_offset(0),field_bits(32);
2512 field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)=
2513 sym_desc(array)index_type_ref(range of int from 0 to 1);
2514 elem_type_ref(vtbl elem type),
2516 meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
2517 :arg_type(int),protection(public)normal(yes)virtual(yes)
2518 vtable_index(1);class_first_defining(A);;;~%first_base(A);",
2522 @c FIXME: bogus line break.
2524 .stabs "A:t20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2525 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2529 @section Inheritence
2531 Stabs describing C++ derived classes include additional sections that
2532 describe the inheritence hierarchy of the class. A derived class stab
2533 also encodes the number of base classes. For each base class it tells
2534 if the base class is virtual or not, and if the inheritence is private
2535 or public. It also gives the offset into the object of the portion of
2536 the object corresponding to each base class.
2538 This additional information is embeded in the class stab following the
2539 number of bytes in the struct. First the number of base classes
2540 appears bracketed by an exclamation point and a comma.
2542 Then for each base type there repeats a series: two digits, a number,
2543 a comma, another number, and a semi-colon.
2545 The first of the two digits is 1 if the base class is virtual and 0 if
2546 not. The second digit is 2 if the derivation is public and 0 if not.
2548 The number following the first two digits is the offset from the start
2549 of the object to the part of the object pertaining to the base class.
2551 After the comma, the second number is a type_descriptor for the base
2552 type. Finally a semi-colon ends the series, which repeats for each
2555 The source below defines three base classes @code{A}, @code{B}, and
2556 @code{C} and the derived class @code{D}.
2563 virtual int A_virt (int arg) @{ return arg; @};
2569 virtual int B_virt (int arg) @{return arg; @};
2575 virtual int C_virt (int arg) @{return arg; @};
2578 class D : A, virtual B, public C @{
2581 virtual int A_virt (int arg ) @{ return arg+1; @};
2582 virtual int B_virt (int arg) @{ return arg+2; @};
2583 virtual int C_virt (int arg) @{ return arg+3; @};
2584 virtual int D_virt (int arg) @{ return arg; @};
2588 Class stabs similar to the ones described earlier are generated for
2591 @c FIXME!!! the linebreaks in the following example probably make the
2592 @c examples literally unusable, but I don't know any other way to get
2593 @c them on the page.
2594 @c One solution would be to put some of the type definitions into
2595 @c separate stabs, even if that's not exactly what the compiler actually
2598 .stabs "A:T20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2599 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2601 .stabs "B:Tt25=s8Bdat:1,0,32;$vf25:21,32;B_virt::26=##1;
2602 :i;2A*-2147483647;25;;;~%25;",128,0,0,0
2604 .stabs "C:Tt28=s8Cdat:1,0,32;$vf28:21,32;C_virt::29=##1;
2605 :i;2A*-2147483647;28;;;~%28;",128,0,0,0
2608 In the stab describing derived class @code{D} below, the information about
2609 the derivation of this class is encoded as follows.
2612 .stabs "derived_class_name:symbol_descriptors(struct tag&type)=
2613 type_descriptor(struct)struct_bytes(32)!num_bases(3),
2614 base_virtual(no)inheritence_public(no)base_offset(0),
2615 base_class_type_ref(A);
2616 base_virtual(yes)inheritence_public(no)base_offset(NIL),
2617 base_class_type_ref(B);
2618 base_virtual(no)inheritence_public(yes)base_offset(64),
2619 base_class_type_ref(C); @dots{}
2622 @c FIXME! fake linebreaks.
2624 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:
2625 1,160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt:
2626 :32:i;2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;
2627 28;;D_virt::32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2630 @node Virtual Base Classes
2631 @section Virtual Base Classes
2633 A derived class object consists of a concatination in memory of the data
2634 areas defined by each base class, starting with the leftmost and ending
2635 with the rightmost in the list of base classes. The exception to this
2636 rule is for virtual inheritence. In the example above, class @code{D}
2637 inherits virtually from base class @code{B}. This means that an
2638 instance of a @code{D} object will not contain its own @code{B} part but
2639 merely a pointer to a @code{B} part, known as a virtual base pointer.
2641 In a derived class stab, the base offset part of the derivation
2642 information, described above, shows how the base class parts are
2643 ordered. The base offset for a virtual base class is always given as 0.
2644 Notice that the base offset for @code{B} is given as 0 even though
2645 @code{B} is not the first base class. The first base class @code{A}
2648 The field information part of the stab for class @code{D} describes the field
2649 which is the pointer to the virtual base class @code{B}. The vbase pointer
2650 name is @samp{$vb} followed by a type reference to the virtual base class.
2651 Since the type id for @code{B} in this example is 25, the vbase pointer name
2654 @c FIXME!! fake linebreaks below
2656 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1,
2657 160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i;
2658 2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt:
2659 :32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2662 Following the name and a semicolon is a type reference describing the
2663 type of the virtual base class pointer, in this case 24. Type 24 was
2664 defined earlier as the type of the @code{B} class @code{this} pointer. The
2665 @code{this} pointer for a class is a pointer to the class type.
2668 .stabs "this:P24=*25=xsB:",64,0,0,8
2671 Finally the field offset part of the vbase pointer field description
2672 shows that the vbase pointer is the first field in the @code{D} object,
2673 before any data fields defined by the class. The layout of a @code{D}
2674 class object is a follows, @code{Adat} at 0, the vtable pointer for
2675 @code{A} at 32, @code{Cdat} at 64, the vtable pointer for C at 96, the
2676 virtual base pointer for @code{B} at 128, and @code{Ddat} at 160.
2679 @node Static Members
2680 @section Static Members
2682 The data area for a class is a concatenation of the space used by the
2683 data members of the class. If the class has virtual methods, a vtable
2684 pointer follows the class data. The field offset part of each field
2685 description in the class stab shows this ordering.
2687 << How is this reflected in stabs? See Cygnus bug #677 for some info. >>
2690 @appendix Table of Stab Types
2692 The following are all the possible values for the stab type field, for
2693 @code{a.out} files, in numeric order. This does not apply to XCOFF, but
2694 it does apply to stabs in ELF. Stabs in ECOFF use these values but add
2695 0x8f300 to distinguish them from non-stab symbols.
2697 The symbolic names are defined in the file @file{include/aout/stabs.def}.
2700 * Non-Stab Symbol Types:: Types from 0 to 0x1f
2701 * Stab Symbol Types:: Types from 0x20 to 0xff
2704 @node Non-Stab Symbol Types
2705 @appendixsec Non-Stab Symbol Types
2707 The following types are used by the linker and assembler, not by stab
2708 directives. Since this document does not attempt to describe aspects of
2709 object file format other than the debugging format, no details are
2712 @c Try to get most of these to fit on a single line.
2722 File scope absolute symbol
2724 @item 0x3 N_ABS | N_EXT
2725 External absolute symbol
2728 File scope text symbol
2730 @item 0x5 N_TEXT | N_EXT
2731 External text symbol
2734 File scope data symbol
2736 @item 0x7 N_DATA | N_EXT
2737 External data symbol
2740 File scope BSS symbol
2742 @item 0x9 N_BSS | N_EXT
2746 Same as @code{N_FN}, for Sequent compilers
2749 Symbol is indirected to another symbol
2752 Common---visible after shared library dynamic link
2755 Absolute set element
2758 Text segment set element
2761 Data segment set element
2764 BSS segment set element
2767 Pointer to set vector
2769 @item 0x1e N_WARNING
2770 Print a warning message during linking
2773 File name of a @file{.o} file
2776 @node Stab Symbol Types
2777 @appendixsec Stab Symbol Types
2779 The following symbol types indicate that this is a stab. This is the
2780 full list of stab numbers, including stab types that are used in
2781 languages other than C.
2785 Global symbol; see @ref{Global Variables}.
2788 Function name (for BSD Fortran); see @ref{Procedures}.
2791 Function name (@pxref{Procedures}) or text segment variable
2795 Data segment file-scope variable; see @ref{Statics}.
2798 BSS segment file-scope variable; see @ref{Statics}.
2801 Name of main routine; see @ref{Main Program}.
2803 @c FIXME: discuss this in the Statics node where we talk about
2804 @c the fact that the n_type indicates the section.
2806 Variable in @code{.rodata} section; see @ref{Statics}.
2809 Global symbol (for Pascal); see @ref{N_PC}.
2812 Number of symbols (according to Ultrix V4.0); see @ref{N_NSYMS}.
2815 No DST map; see @ref{N_NOMAP}.
2817 @c FIXME: describe this solaris feature in the body of the text (see
2818 @c comments in include/aout/stab.def).
2820 Object file (Solaris2).
2822 @c See include/aout/stab.def for (a little) more info.
2824 Debugger options (Solaris2).
2827 Register variable; see @ref{Register Variables}.
2830 Modula-2 compilation unit; see @ref{N_M2C}.
2833 Line number in text segment; see @ref{Line Numbers}.
2836 Line number in data segment; see @ref{Line Numbers}.
2839 Line number in bss segment; see @ref{Line Numbers}.
2842 Sun source code browser, path to @file{.cb} file; see @ref{N_BROWS}.
2845 GNU Modula2 definition module dependency; see @ref{N_DEFD}.
2848 Function start/body/end line numbers (Solaris2).
2851 GNU C++ exception variable; see @ref{N_EHDECL}.
2854 Modula2 info "for imc" (according to Ultrix V4.0); see @ref{N_MOD2}.
2857 GNU C++ @code{catch} clause; see @ref{N_CATCH}.
2860 Structure of union element; see @ref{N_SSYM}.
2863 Last stab for module (Solaris2).
2866 Path and name of source file; see @ref{Source Files}.
2869 Stack variable (@pxref{Stack Variables}) or type (@pxref{Typedefs}).
2872 Beginning of an include file (Sun only); see @ref{Include Files}.
2875 Name of include file; see @ref{Include Files}.
2878 Parameter variable; see @ref{Parameters}.
2881 End of an include file; see @ref{Include Files}.
2884 Alternate entry point; see @ref{N_ENTRY}.
2887 Beginning of a lexical block; see @ref{Block Structure}.
2890 Place holder for a deleted include file; see @ref{Include Files}.
2893 Modula2 scope information (Sun linker); see @ref{N_SCOPE}.
2896 End of a lexical block; see @ref{Block Structure}.
2899 Begin named common block; see @ref{Common Blocks}.
2902 End named common block; see @ref{Common Blocks}.
2905 Member of a common block; see @ref{Common Blocks}.
2907 @c FIXME: How does this really work? Move it to main body of document.
2909 Pascal @code{with} statement: type,,0,0,offset (Solaris2).
2912 Gould non-base registers; see @ref{Gould}.
2915 Gould non-base registers; see @ref{Gould}.
2918 Gould non-base registers; see @ref{Gould}.
2921 Gould non-base registers; see @ref{Gould}.
2924 Gould non-base registers; see @ref{Gould}.
2927 @c Restore the default table indent
2932 @node Symbol Descriptors
2933 @appendix Table of Symbol Descriptors
2935 The symbol descriptor is the character which follows the colon in many
2936 stabs, and which tells what kind of stab it is. @xref{String Field},
2937 for more information about their use.
2939 @c Please keep this alphabetical
2941 @c In TeX, this looks great, digit is in italics. But makeinfo insists
2942 @c on putting it in `', not realizing that @var should override @code.
2943 @c I don't know of any way to make makeinfo do the right thing. Seems
2944 @c like a makeinfo bug to me.
2948 Variable on the stack; see @ref{Stack Variables}.
2951 Parameter passed by reference in register; see @ref{Reference Parameters}.
2954 Based variable; see @ref{Parameters}.
2957 Constant; see @ref{Constants}.
2960 Conformant array bound (Pascal, maybe other languages); @ref{Conformant
2961 Arrays}. Name of a caught exception (GNU C++). These can be
2962 distinguished because the latter uses @code{N_CATCH} and the former uses
2963 another symbol type.
2966 Floating point register variable; see @ref{Register Variables}.
2969 Parameter in floating point register; see @ref{Register Parameters}.
2972 File scope function; see @ref{Procedures}.
2975 Global function; see @ref{Procedures}.
2978 Global variable; see @ref{Global Variables}.
2981 @xref{Register Parameters}.
2984 Internal (nested) procedure; see @ref{Nested Procedures}.
2987 Internal (nested) function; see @ref{Nested Procedures}.
2990 Label name (documented by AIX, no further information known).
2993 Module; see @ref{Procedures}.
2996 Argument list parameter; see @ref{Parameters}.
3002 Fortran Function parameter; see @ref{Parameters}.
3005 Unfortunately, three separate meanings have been independently invented
3006 for this symbol descriptor. At least the GNU and Sun uses can be
3007 distinguished by the symbol type. Global Procedure (AIX) (symbol type
3008 used unknown); see @ref{Procedures}. Register parameter (GNU) (symbol
3009 type @code{N_PSYM}); see @ref{Parameters}. Prototype of function
3010 referenced by this file (Sun @code{acc}) (symbol type @code{N_FUN}).
3013 Static Procedure; see @ref{Procedures}.
3016 Register parameter; see @ref{Register Parameters}.
3019 Register variable; see @ref{Register Variables}.
3022 File scope variable; see @ref{Statics}.
3025 Type name; see @ref{Typedefs}.
3028 Enumeration, structure, or union tag; see @ref{Typedefs}.
3031 Parameter passed by reference; see @ref{Reference Parameters}.
3034 Procedure scope static variable; see @ref{Statics}.
3037 Conformant array; see @ref{Conformant Arrays}.
3040 Function return variable; see @ref{Parameters}.
3043 @node Type Descriptors
3044 @appendix Table of Type Descriptors
3046 The type descriptor is the character which follows the type number and
3047 an equals sign. It specifies what kind of type is being defined.
3048 @xref{String Field}, for more information about their use.
3053 Type reference; see @ref{String Field}.
3056 Reference to builtin type; see @ref{Negative Type Numbers}.
3059 Method (C++); see @ref{Cplusplus}.
3062 Pointer; see @ref{Miscellaneous Types}.
3068 Type Attributes (AIX); see @ref{String Field}. Member (class and variable)
3069 type (GNU C++); see @ref{Cplusplus}.
3072 Array; see @ref{Arrays}.
3075 Open array; see @ref{Arrays}.
3078 Pascal space type (AIX); see @ref{Miscellaneous Types}. Builtin integer
3079 type (Sun); see @ref{Builtin Type Descriptors}.
3082 Volatile-qualified type; see @ref{Miscellaneous Types}.
3085 Complex builtin type; see @ref{Builtin Type Descriptors}.
3088 COBOL Picture type. See AIX documentation for details.
3091 File type; see @ref{Miscellaneous Types}.
3094 N-dimensional dynamic array; see @ref{Arrays}.
3097 Enumeration type; see @ref{Enumerations}.
3100 N-dimensional subarray; see @ref{Arrays}.
3103 Function type; see @ref{Function Types}.
3106 Pascal function parameter; see @ref{Function Types}
3109 Builtin floating point type; see @ref{Builtin Type Descriptors}.
3112 COBOL Group. See AIX documentation for details.
3115 Imported type; see @ref{Cross-References}.
3118 Const-qualified type; see @ref{Miscellaneous Types}.
3121 COBOL File Descriptor. See AIX documentation for details.
3124 Multiple instance type; see @ref{Miscellaneous Types}.
3127 String type; see @ref{Strings}.
3130 Stringptr; see @ref{Strings}.
3133 Opaque type; see @ref{Typedefs}.
3136 Procedure; see @ref{Function Types}.
3139 Packed array; see @ref{Arrays}.
3142 Range type; see @ref{Subranges}.
3145 Builtin floating type; see @ref{Builtin Type Descriptors} (Sun). Pascal
3146 subroutine parameter; see @ref{Function Types} (AIX). Detecting this
3147 conflict is possible with careful parsing (hint: a Pascal subroutine
3148 parameter type will always contain a comma, and a builtin type
3149 descriptor never will).
3152 Structure type; see @ref{Structures}.
3155 Set type; see @ref{Miscellaneous Types}.
3158 Union; see @ref{Unions}.
3161 Variant record. This is a Pascal and Modula-2 feature which is like a
3162 union within a struct in C. See AIX documentation for details.
3165 Wide character; see @ref{Builtin Type Descriptors}.
3168 Cross-reference; see @ref{Cross-References}.
3171 gstring; see @ref{Strings}.
3174 @node Expanded Reference
3175 @appendix Expanded Reference by Stab Type
3177 @c FIXME: This appendix should go away; see N_PSYM or N_SO for an example.
3179 For a full list of stab types, and cross-references to where they are
3180 described, see @ref{Stab Types}. This appendix just duplicates certain
3181 information from the main body of this document; eventually the
3182 information will all be in one place.
3186 The first line is the symbol type (see @file{include/aout/stab.def}).
3188 The second line describes the language constructs the symbol type
3191 The third line is the stab format with the significant stab fields
3192 named and the rest NIL.
3194 Subsequent lines expand upon the meaning and possible values for each
3195 significant stab field. @samp{#} stands in for the type descriptor.
3197 Finally, any further information.
3200 * N_PC:: Pascal global symbol
3201 * N_NSYMS:: Number of symbols
3202 * N_NOMAP:: No DST map
3203 * N_M2C:: Modula-2 compilation unit
3204 * N_BROWS:: Path to .cb file for Sun source code browser
3205 * N_DEFD:: GNU Modula2 definition module dependency
3206 * N_EHDECL:: GNU C++ exception variable
3207 * N_MOD2:: Modula2 information "for imc"
3208 * N_CATCH:: GNU C++ "catch" clause
3209 * N_SSYM:: Structure or union element
3210 * N_ENTRY:: Alternate entry point
3211 * N_SCOPE:: Modula2 scope information (Sun only)
3212 * Gould:: non-base register symbols used on Gould systems
3213 * N_LENG:: Length of preceding entry
3219 @deffn @code{.stabs} N_PC
3221 Global symbol (for Pascal).
3224 "name" -> "symbol_name" <<?>>
3225 value -> supposedly the line number (stab.def is skeptical)
3229 @file{stabdump.c} says:
3231 global pascal symbol: name,,0,subtype,line
3239 @deffn @code{.stabn} N_NSYMS
3241 Number of symbols (according to Ultrix V4.0).
3244 0, files,,funcs,lines (stab.def)
3251 @deffn @code{.stabs} N_NOMAP
3253 No DST map for symbol (according to Ultrix V4.0). I think this means a
3254 variable has been optimized out.
3257 name, ,0,type,ignored (stab.def)
3264 @deffn @code{.stabs} N_M2C
3266 Modula-2 compilation unit.
3269 "string" -> "unit_name,unit_time_stamp[,code_time_stamp]"
3271 value -> 0 (main unit)
3279 @deffn @code{.stabs} N_BROWS
3281 Sun source code browser, path to @file{.cb} file
3284 "path to associated @file{.cb} file"
3286 Note: N_BROWS has the same value as N_BSLINE.
3292 @deffn @code{.stabn} N_DEFD
3294 GNU Modula2 definition module dependency.
3296 GNU Modula-2 definition module dependency. The value is the
3297 modification time of the definition file. The other field is non-zero
3298 if it is imported with the GNU M2 keyword @code{%INITIALIZE}. Perhaps
3299 @code{N_M2C} can be used if there are enough empty fields?
3305 @deffn @code{.stabs} N_EHDECL
3307 GNU C++ exception variable <<?>>.
3309 "@var{string} is variable name"
3311 Note: conflicts with @code{N_MOD2}.
3317 @deffn @code{.stab?} N_MOD2
3319 Modula2 info "for imc" (according to Ultrix V4.0)
3321 Note: conflicts with @code{N_EHDECL} <<?>>
3327 @deffn @code{.stabn} N_CATCH
3329 GNU C++ @code{catch} clause
3331 GNU C++ @code{catch} clause. The value is its address. The desc field
3332 is nonzero if this entry is immediately followed by a @code{CAUGHT} stab
3333 saying what exception was caught. Multiple @code{CAUGHT} stabs means
3334 that multiple exceptions can be caught here. If desc is 0, it means all
3335 exceptions are caught here.
3341 @deffn @code{.stabn} N_SSYM
3343 Structure or union element.
3345 The value is the offset in the structure.
3347 <<?looking at structs and unions in C I didn't see these>>
3353 @deffn @code{.stabn} N_ENTRY
3355 Alternate entry point.
3356 The value is its address.
3363 @deffn @code{.stab?} N_SCOPE
3365 Modula2 scope information (Sun linker)
3370 @section Non-base registers on Gould systems
3372 @deffn @code{.stab?} N_NBTEXT
3373 @deffnx @code{.stab?} N_NBDATA
3374 @deffnx @code{.stab?} N_NBBSS
3375 @deffnx @code{.stab?} N_NBSTS
3376 @deffnx @code{.stab?} N_NBLCS
3382 These are used on Gould systems for non-base registers syms.
3384 However, the following values are not the values used by Gould; they are
3385 the values which GNU has been documenting for these values for a long
3386 time, without actually checking what Gould uses. I include these values
3387 only because perhaps some someone actually did something with the GNU
3388 information (I hope not, why GNU knowingly assigned wrong values to
3389 these in the header file is a complete mystery to me).
3392 240 0xf0 N_NBTEXT ??
3393 242 0xf2 N_NBDATA ??
3403 @deffn @code{.stabn} N_LENG
3405 Second symbol entry containing a length-value for the preceding entry.
3406 The value is the length.
3410 @appendix Questions and Anomalies
3414 @c I think this is changed in GCC 2.4.5 to put the line number there.
3415 For GNU C stabs defining local and global variables (@code{N_LSYM} and
3416 @code{N_GSYM}), the desc field is supposed to contain the source
3417 line number on which the variable is defined. In reality the desc
3418 field is always 0. (This behavior is defined in @file{dbxout.c} and
3419 putting a line number in desc is controlled by @samp{#ifdef
3420 WINNING_GDB}, which defaults to false). GDB supposedly uses this
3421 information if you say @samp{list @var{var}}. In reality, @var{var} can
3422 be a variable defined in the program and GDB says @samp{function
3423 @var{var} not defined}.
3426 In GNU C stabs, there seems to be no way to differentiate tag types:
3427 structures, unions, and enums (symbol descriptor @samp{T}) and typedefs
3428 (symbol descriptor @samp{t}) defined at file scope from types defined locally
3429 to a procedure or other more local scope. They all use the @code{N_LSYM}
3430 stab type. Types defined at procedure scope are emited after the
3431 @code{N_RBRAC} of the preceding function and before the code of the
3432 procedure in which they are defined. This is exactly the same as
3433 types defined in the source file between the two procedure bodies.
3434 GDB overcompensates by placing all types in block #1, the block for
3435 symbols of file scope. This is true for default, @samp{-ansi} and
3436 @samp{-traditional} compiler options. (Bugs gcc/1063, gdb/1066.)
3439 What ends the procedure scope? Is it the proc block's @code{N_RBRAC} or the
3440 next @code{N_FUN}? (I believe its the first.)
3443 @c FIXME: This should go with the other stuff about global variables.
3444 Global variable stabs don't have location information. This comes
3445 from the external symbol for the same variable. The external symbol
3446 has a leading underbar on the _name of the variable and the stab does
3447 not. How do we know these two symbol table entries are talking about
3448 the same symbol when their names are different? (Answer: the debugger
3449 knows that external symbols have leading underbars).
3451 @c FIXME: This is absurdly vague; there all kinds of differences, some
3452 @c of which are the same between gnu & sun, and some of which aren't.
3453 @c In particular, I'm pretty sure GCC works with Sun dbx by default.
3455 @c Can GCC be configured to output stabs the way the Sun compiler
3456 @c does, so that their native debugging tools work? <NO?> It doesn't by
3457 @c default. GDB reads either format of stab. (GCC or SunC). How about
3461 @node XCOFF Differences
3462 @appendix Differences Between GNU Stabs in a.out and GNU Stabs in XCOFF
3464 @c FIXME: Merge *all* these into the main body of the document.
3465 The AIX/RS6000 native object file format is XCOFF with stabs. This
3466 appendix only covers those differences which are not covered in the main
3467 body of this document.
3471 BSD a.out stab types correspond to AIX XCOFF storage classes. In general
3472 the mapping is @code{N_@var{stabtype}} becomes @code{C_@var{stabtype}}.
3473 Some stab types in a.out are not supported in XCOFF; most of these use
3476 @c FIXME: Get C_* types for the block, figure out whether it is always
3477 @c used (I suspect not), explain clearly, and move to node Statics.
3478 Exception: initialised static @code{N_STSYM} and un-initialized static
3479 @code{N_LCSYM} both map to the @code{C_STSYM} storage class. But the
3480 distinction is preserved because in XCOFF @code{N_STSYM} and
3481 @code{N_LCSYM} must be emited in a named static block. Begin the block
3482 with @samp{.bs s[RW] data_section_name} for @code{N_STSYM} or @samp{.bs
3483 s bss_section_name} for @code{N_LCSYM}. End the block with @samp{.es}.
3485 @c FIXME: I think they are trying to say something about whether the
3486 @c assembler defaults the value to the location counter.
3488 If the XCOFF stab is an @code{N_FUN} (@code{C_FUN}) then follow the
3489 string field with @samp{,.} instead of just @samp{,}.
3492 I think that's it for @file{.s} file differences. They could stand to be
3493 better presented. This is just a list of what I have noticed so far.
3494 There are a @emph{lot} of differences in the information in the symbol
3495 tables of the executable and object files.
3497 Mapping of a.out stab types to XCOFF storage classes:
3500 stab type storage class
3501 -------------------------------
3537 @node Sun Differences
3538 @appendix Differences Between GNU Stabs and Sun Native Stabs
3540 @c FIXME: Merge all this stuff into the main body of the document.
3544 GNU C stabs define @emph{all} types, file or procedure scope, as
3545 @code{N_LSYM}. Sun doc talks about using @code{N_GSYM} too.
3548 Sun C stabs use type number pairs in the format (@var{a},@var{b}) where
3549 @var{a} is a number starting with 1 and incremented for each sub-source
3550 file in the compilation. @var{b} is a number starting with 1 and
3551 incremented for each new type defined in the compilation. GNU C stabs
3552 use the type number alone, with no source file number.
3556 @appendix Using Stabs With The ELF Object File Format
3558 The ELF object file format allows tools to create object files with
3559 custom sections containing any arbitrary data. To use stabs in ELF
3560 object files, the tools create two custom sections, a section named
3561 @code{.stab} which contains an array of fixed length structures, one
3562 struct per stab, and a section named @code{.stabstr} containing all the
3563 variable length strings that are referenced by stabs in the @code{.stab}
3564 section. The byte order of the stabs binary data matches the byte order
3565 of the ELF file itself, as determined from the @code{EI_DATA} field in
3566 the @code{e_ident} member of the ELF header.
3568 @c Is "source file" the right term for this concept? We don't mean that
3569 @c there is a separate one for include files (but "object file" or
3570 @c "object module" isn't quite right either; the output from ld -r is a
3571 @c single object file but contains many source files).
3572 The first stab in the @code{.stab} section for each source file is
3573 synthetic, generated entirely by the assembler, with no corresponding
3574 @code{.stab} directive as input to the assembler. This stab contains
3575 the following fields:
3579 Offset in the @code{.stabstr} section to the source filename.
3585 Unused field, always zero.
3588 Count of upcoming symbols, i.e., the number of remaining stabs for this
3592 Size of the string table fragment associated with this source file, in
3596 The @code{.stabstr} section always starts with a null byte (so that string
3597 offsets of zero reference a null string), followed by random length strings,
3598 each of which is null byte terminated.
3600 The ELF section header for the @code{.stab} section has its
3601 @code{sh_link} member set to the section number of the @code{.stabstr}
3602 section, and the @code{.stabstr} section has its ELF section
3603 header @code{sh_type} member set to @code{SHT_STRTAB} to mark it as a
3606 Because the linker does not process the @code{.stab} section in any
3607 special way, none of the addresses in the @code{n_value} field of the
3608 stabs are relocated by the linker. Instead they are relative to the
3609 source file (or some entity smaller than a source file, like a
3610 function). To find the address of each section corresponding to a given
3611 source file, the (compiler? assembler?) puts out symbols giving the
3612 address of each section for a given source file. Since these are normal
3613 ELF symbols, the linker can relocate them correctly. They are
3614 named @code{Bbss.bss} for the bss section, @code{Ddata.data} for
3615 the data section, and @code{Drodata.rodata} for the rodata section. I
3616 haven't yet figured out how the debugger gets the address for the text
3619 @node Symbol Types Index
3620 @unnumbered Symbol Types Index