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 section, @code{N_FUN}
882 means the text section, and @code{N_LCSYM} means the bss section. For
883 those systems with a read-only data section separate from the text
884 section (Solaris), @code{N_ROSYM} means the read-only data section.
886 For example, the source lines:
889 static const int var_const = 5;
890 static int var_init = 2;
891 static int var_noinit;
895 yield the following stabs:
898 .stabs "var_const:S1",36,0,0,_var_const # @r{36 is N_FUN}
900 .stabs "var_init:S1",38,0,0,_var_init # @r{38 is N_STSYM}
902 .stabs "var_noinit:S1",40,0,0,_var_noinit # @r{40 is N_LCSYM}
905 In XCOFF files, each symbol has a section number, so the stab type
906 need not indicate the section.
908 In ECOFF files, the storage class is used to specify the section, so the
909 stab type need not indicate the section.
911 In ELF files, for Solaris 2.1, symbol descriptor @samp{S} means that the
912 address is absolute (ld relocates it) and symbol descriptor @samp{V}
913 means that the address is relative to the start of the relevant section
914 for that compilation unit. I don't know what it does for @samp{S} stabs
915 on Solaris 2.3 (in which ld no longer relocates stabs). For more
916 information on ld stab relocation, @xref{Stabs In ELF}.
921 Formal parameters to a function are represented by a stab (or sometimes
922 two; see below) for each parameter. The stabs are in the order in which
923 the debugger should print the parameters (i.e., the order in which the
924 parameters are declared in the source file). The exact form of the stab
925 depends on how the parameter is being passed.
928 Parameters passed on the stack use the symbol descriptor @samp{p} and
929 the @code{N_PSYM} symbol type. The value of the symbol is an offset
930 used to locate the parameter on the stack; its exact meaning is
931 machine-dependent, but on most machines it is an offset from the frame
934 As a simple example, the code:
945 .stabs "main:F1",36,0,0,_main # @r{36 is N_FUN}
946 .stabs "argc:p1",160,0,0,68 # @r{160 is N_PSYM}
947 .stabs "argv:p20=*21=*2",160,0,0,72
950 The type definition of @code{argv} is interesting because it contains
951 several type definitions. Type 21 is pointer to type 2 (char) and
952 @code{argv} (type 20) is pointer to type 21.
954 @c FIXME: figure out what these mean and describe them coherently.
955 The following symbol descriptors are also said to go with @code{N_PSYM}.
956 The value of the symbol is said to be an offset from the argument
957 pointer (I'm not sure whether this is true or not).
961 pF Fortran function parameter
962 X (function result variable)
967 * Register Parameters::
968 * Local Variable Parameters::
969 * Reference Parameters::
970 * Conformant Arrays::
973 @node Register Parameters
974 @subsection Passing Parameters in Registers
976 If the parameter is passed in a register, then traditionally there are
977 two symbols for each argument:
980 .stabs "arg:p1" . . . ; N_PSYM
981 .stabs "arg:r1" . . . ; N_RSYM
984 Debuggers use the second one to find the value, and the first one to
985 know that it is an argument.
988 @findex N_RSYM, for parameters
989 Because that approach is kind of ugly, some compilers use symbol
990 descriptor @samp{P} or @samp{R} to indicate an argument which is in a
991 register. Symbol type @code{C_RPSYM} is used with @samp{R} and
992 @code{N_RSYM} is used with @samp{P}. The symbol's value is
993 the register number. @samp{P} and @samp{R} mean the same thing; the
994 difference is that @samp{P} is a GNU invention and @samp{R} is an IBM
995 (XCOFF) invention. As of version 4.9, GDB should handle either one.
997 There is at least one case where GCC uses a @samp{p} and @samp{r} pair
998 rather than @samp{P}; this is where the argument is passed in the
999 argument list and then loaded into a register.
1001 According to the AIX documentation, symbol descriptor @samp{D} is for a
1002 parameter passed in a floating point register. This seems
1003 unnecessary---why not just use @samp{R} with a register number which
1004 indicates that it's a floating point register? I haven't verified
1005 whether the system actually does what the documentation indicates.
1007 @c FIXME: On the hppa this is for any type > 8 bytes, I think, and not
1008 @c for small structures (investigate).
1009 On the sparc and hppa, for a @samp{P} symbol whose type is a structure
1010 or union, the register contains the address of the structure. On the
1011 sparc, this is also true of a @samp{p} and @samp{r} pair (using Sun
1012 @code{cc}) or a @samp{p} symbol. However, if a (small) structure is
1013 really in a register, @samp{r} is used. And, to top it all off, on the
1014 hppa it might be a structure which was passed on the stack and loaded
1015 into a register and for which there is a @samp{p} and @samp{r} pair! I
1016 believe that symbol descriptor @samp{i} is supposed to deal with this
1017 case (it is said to mean "value parameter by reference, indirect
1018 access"; I don't know the source for this information), but I don't know
1019 details or what compilers or debuggers use it, if any (not GDB or GCC).
1020 It is not clear to me whether this case needs to be dealt with
1021 differently than parameters passed by reference (@pxref{Reference Parameters}).
1023 @node Local Variable Parameters
1024 @subsection Storing Parameters as Local Variables
1026 There is a case similar to an argument in a register, which is an
1027 argument that is actually stored as a local variable. Sometimes this
1028 happens when the argument was passed in a register and then the compiler
1029 stores it as a local variable. If possible, the compiler should claim
1030 that it's in a register, but this isn't always done.
1032 @findex N_LSYM, for parameter
1033 Some compilers use the pair of symbols approach described above
1034 (@samp{@var{arg}:p} followed by @samp{@var{arg}:}); this includes GCC1
1035 (not GCC2) on the sparc when passing a small structure and GCC2
1036 (sometimes) when the argument type is @code{float} and it is passed as a
1037 @code{double} and converted to @code{float} by the prologue (in the
1038 latter case the type of the @samp{@var{arg}:p} symbol is @code{double}
1039 and the type of the @samp{@var{arg}:} symbol is @code{float}).
1041 GCC, at least on the 960, has another solution to the same problem. It
1042 uses a single @samp{p} symbol descriptor for an argument which is stored
1043 as a local variable but uses @code{N_LSYM} instead of @code{N_PSYM}. In
1044 this case, the value of the symbol is an offset relative to the local
1045 variables for that function, not relative to the arguments; on some
1046 machines those are the same thing, but not on all.
1048 @c This is mostly just background info; the part that logically belongs
1049 @c here is the last sentence.
1050 On the VAX or on other machines in which the calling convention includes
1051 the number of words of arguments actually passed, the debugger (GDB at
1052 least) uses the parameter symbols to keep track of whether it needs to
1053 print nameless arguments in addition to the formal parameters which it
1054 has printed because each one has a stab. For example, in
1057 extern int fprintf (FILE *stream, char *format, @dots{});
1059 fprintf (stdout, "%d\n", x);
1062 there are stabs for @code{stream} and @code{format}. On most machines,
1063 the debugger can only print those two arguments (because it has no way
1064 of knowing that additional arguments were passed), but on the VAX or
1065 other machines with a calling convention which indicates the number of
1066 words of arguments, the debugger can print all three arguments. To do
1067 so, the parameter symbol (symbol descriptor @samp{p}) (not necessarily
1068 @samp{r} or symbol descriptor omitted symbols) needs to contain the
1069 actual type as passed (for example, @code{double} not @code{float} if it
1070 is passed as a double and converted to a float).
1072 @node Reference Parameters
1073 @subsection Passing Parameters by Reference
1075 If the parameter is passed by reference (e.g., Pascal @code{VAR}
1076 parameters), then the symbol descriptor is @samp{v} if it is in the
1077 argument list, or @samp{a} if it in a register. Other than the fact
1078 that these contain the address of the parameter rather than the
1079 parameter itself, they are identical to @samp{p} and @samp{R},
1080 respectively. I believe @samp{a} is an AIX invention; @samp{v} is
1081 supported by all stabs-using systems as far as I know.
1083 @node Conformant Arrays
1084 @subsection Passing Conformant Array Parameters
1086 @c Is this paragraph correct? It is based on piecing together patchy
1087 @c information and some guesswork
1088 Conformant arrays are a feature of Modula-2, and perhaps other
1089 languages, in which the size of an array parameter is not known to the
1090 called function until run-time. Such parameters have two stabs: a
1091 @samp{x} for the array itself, and a @samp{C}, which represents the size
1092 of the array. The value of the @samp{x} stab is the offset in the
1093 argument list where the address of the array is stored (it this right?
1094 it is a guess); the value of the @samp{C} stab is the offset in the
1095 argument list where the size of the array (in elements? in bytes?) is
1099 @chapter Defining Types
1101 The examples so far have described types as references to previously
1102 defined types, or defined in terms of subranges of or pointers to
1103 previously defined types. This chapter describes the other type
1104 descriptors that may follow the @samp{=} in a type definition.
1107 * Builtin Types:: Integers, floating point, void, etc.
1108 * Miscellaneous Types:: Pointers, sets, files, etc.
1109 * Cross-References:: Referring to a type not yet defined.
1110 * Subranges:: A type with a specific range.
1111 * Arrays:: An aggregate type of same-typed elements.
1112 * Strings:: Like an array but also has a length.
1113 * Enumerations:: Like an integer but the values have names.
1114 * Structures:: An aggregate type of different-typed elements.
1115 * Typedefs:: Giving a type a name.
1116 * Unions:: Different types sharing storage.
1121 @section Builtin Types
1123 Certain types are built in (@code{int}, @code{short}, @code{void},
1124 @code{float}, etc.); the debugger recognizes these types and knows how
1125 to handle them. Thus, don't be surprised if some of the following ways
1126 of specifying builtin types do not specify everything that a debugger
1127 would need to know about the type---in some cases they merely specify
1128 enough information to distinguish the type from other types.
1130 The traditional way to define builtin types is convolunted, so new ways
1131 have been invented to describe them. Sun's @code{acc} uses special
1132 builtin type descriptors (@samp{b} and @samp{R}), and IBM uses negative
1133 type numbers. GDB accepts all three ways, as of version 4.8; dbx just
1134 accepts the traditional builtin types and perhaps one of the other two
1135 formats. The following sections describe each of these formats.
1138 * Traditional Builtin Types:: Put on your seatbelts and prepare for kludgery
1139 * Builtin Type Descriptors:: Builtin types with special type descriptors
1140 * Negative Type Numbers:: Builtin types using negative type numbers
1143 @node Traditional Builtin Types
1144 @subsection Traditional Builtin Types
1146 This is the traditional, convoluted method for defining builtin types.
1147 There are several classes of such type definitions: integer, floating
1148 point, and @code{void}.
1151 * Traditional Integer Types::
1152 * Traditional Other Types::
1155 @node Traditional Integer Types
1156 @subsubsection Traditional Integer Types
1158 Often types are defined as subranges of themselves. If the bounding values
1159 fit within an @code{int}, then they are given normally. For example:
1162 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0 # @r{128 is N_LSYM}
1163 .stabs "char:t2=r2;0;127;",128,0,0,0
1166 Builtin types can also be described as subranges of @code{int}:
1169 .stabs "unsigned short:t6=r1;0;65535;",128,0,0,0
1172 If the lower bound of a subrange is 0 and the upper bound is -1,
1173 the type is an unsigned integral type whose bounds are too
1174 big to describe in an @code{int}. Traditionally this is only used for
1175 @code{unsigned int} and @code{unsigned long}:
1178 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
1181 For larger types, GCC 2.4.5 puts out bounds in octal, with a leading 0.
1182 In this case a negative bound consists of a number which is a 1 bit
1183 followed by a bunch of 0 bits, and a positive bound is one in which a
1184 bunch of bits are 1. All known versions of dbx and GDB version 4 accept
1185 this, but GDB 3.5 refuses to read the whole file containing such
1186 symbols. So GCC 2.3.3 did not output the proper size for these types.
1187 @c FIXME: How about an example?
1189 If the lower bound of a subrange is 0 and the upper bound is negative,
1190 the type is an unsigned integral type whose size in bytes is the
1191 absolute value of the upper bound. I believe this is a Convex
1192 convention for @code{unsigned long long}.
1194 If the lower bound of a subrange is negative and the upper bound is 0,
1195 the type is a signed integral type whose size in bytes is
1196 the absolute value of the lower bound. I believe this is a Convex
1197 convention for @code{long long}. To distinguish this from a legitimate
1198 subrange, the type should be a subrange of itself. I'm not sure whether
1199 this is the case for Convex.
1201 @node Traditional Other Types
1202 @subsubsection Traditional Other Types
1204 If the upper bound of a subrange is 0 and the lower bound is positive,
1205 the type is a floating point type, and the lower bound of the subrange
1206 indicates the number of bytes in the type:
1209 .stabs "float:t12=r1;4;0;",128,0,0,0
1210 .stabs "double:t13=r1;8;0;",128,0,0,0
1213 However, GCC writes @code{long double} the same way it writes
1214 @code{double}, so there is no way to distinguish.
1217 .stabs "long double:t14=r1;8;0;",128,0,0,0
1220 Complex types are defined the same way as floating-point types; there is
1221 no way to distinguish a single-precision complex from a double-precision
1222 floating-point type.
1224 The C @code{void} type is defined as itself:
1227 .stabs "void:t15=15",128,0,0,0
1230 I'm not sure how a boolean type is represented.
1232 @node Builtin Type Descriptors
1233 @subsection Defining Builtin Types Using Builtin Type Descriptors
1235 This is the method used by Sun's @code{acc} for defining builtin types.
1236 These are the type descriptors to define builtin types:
1239 @c FIXME: clean up description of width and offset, once we figure out
1241 @item b @var{signed} @var{char-flag} @var{width} ; @var{offset} ; @var{nbits} ;
1242 Define an integral type. @var{signed} is @samp{u} for unsigned or
1243 @samp{s} for signed. @var{char-flag} is @samp{c} which indicates this
1244 is a character type, or is omitted. I assume this is to distinguish an
1245 integral type from a character type of the same size, for example it
1246 might make sense to set it for the C type @code{wchar_t} so the debugger
1247 can print such variables differently (Solaris does not do this). Sun
1248 sets it on the C types @code{signed char} and @code{unsigned char} which
1249 arguably is wrong. @var{width} and @var{offset} appear to be for small
1250 objects stored in larger ones, for example a @code{short} in an
1251 @code{int} register. @var{width} is normally the number of bytes in the
1252 type. @var{offset} seems to always be zero. @var{nbits} is the number
1253 of bits in the type.
1255 Note that type descriptor @samp{b} used for builtin types conflicts with
1256 its use for Pascal space types (@pxref{Miscellaneous Types}); they can
1257 be distinguished because the character following the type descriptor
1258 will be a digit, @samp{(}, or @samp{-} for a Pascal space type, or
1259 @samp{u} or @samp{s} for a builtin type.
1262 Documented by AIX to define a wide character type, but their compiler
1263 actually uses negative type numbers (@pxref{Negative Type Numbers}).
1265 @item R @var{fp-type} ; @var{bytes} ;
1266 Define a floating point type. @var{fp-type} has one of the following values:
1270 IEEE 32-bit (single precision) floating point format.
1273 IEEE 64-bit (double precision) floating point format.
1275 @item 3 (NF_COMPLEX)
1276 @item 4 (NF_COMPLEX16)
1277 @item 5 (NF_COMPLEX32)
1278 @c "GDB source" really means @file{include/aout/stab_gnu.h}, but trying
1279 @c to put that here got an overfull hbox.
1280 These are for complex numbers. A comment in the GDB source describes
1281 them as Fortran @code{complex}, @code{double complex}, and
1282 @code{complex*16}, respectively, but what does that mean? (i.e., Single
1283 precision? Double precison?).
1285 @item 6 (NF_LDOUBLE)
1286 Long double. This should probably only be used for Sun format
1287 @code{long double}, and new codes should be used for other floating
1288 point formats (@code{NF_DOUBLE} can be used if a @code{long double} is
1289 really just an IEEE double, of course).
1292 @var{bytes} is the number of bytes occupied by the type. This allows a
1293 debugger to perform some operations with the type even if it doesn't
1294 understand @var{fp-type}.
1296 @item g @var{type-information} ; @var{nbits}
1297 Documented by AIX to define a floating type, but their compiler actually
1298 uses negative type numbers (@pxref{Negative Type Numbers}).
1300 @item c @var{type-information} ; @var{nbits}
1301 Documented by AIX to define a complex type, but their compiler actually
1302 uses negative type numbers (@pxref{Negative Type Numbers}).
1305 The C @code{void} type is defined as a signed integral type 0 bits long:
1307 .stabs "void:t19=bs0;0;0",128,0,0,0
1309 The Solaris compiler seems to omit the trailing semicolon in this case.
1310 Getting sloppy in this way is not a swift move because if a type is
1311 embedded in a more complex expression it is necessary to be able to tell
1314 I'm not sure how a boolean type is represented.
1316 @node Negative Type Numbers
1317 @subsection Negative Type Numbers
1319 This is the method used in XCOFF for defining builtin types.
1320 Since the debugger knows about the builtin types anyway, the idea of
1321 negative type numbers is simply to give a special type number which
1322 indicates the builtin type. There is no stab defining these types.
1324 There are several subtle issues with negative type numbers.
1326 One is the size of the type. A builtin type (for example the C types
1327 @code{int} or @code{long}) might have different sizes depending on
1328 compiler options, the target architecture, the ABI, etc. This issue
1329 doesn't come up for IBM tools since (so far) they just target the
1330 RS/6000; the sizes indicated below for each size are what the IBM
1331 RS/6000 tools use. To deal with differing sizes, either define separate
1332 negative type numbers for each size (which works but requires changing
1333 the debugger, and, unless you get both AIX dbx and GDB to accept the
1334 change, introduces an incompatibility), or use a type attribute
1335 (@pxref{String Field}) to define a new type with the appropriate size
1336 (which merely requires a debugger which understands type attributes,
1337 like AIX dbx). For example,
1340 .stabs "boolean:t10=@@s8;-16",128,0,0,0
1343 defines an 8-bit boolean type, and
1346 .stabs "boolean:t10=@@s64;-16",128,0,0,0
1349 defines a 64-bit boolean type.
1351 A similar issue is the format of the type. This comes up most often for
1352 floating-point types, which could have various formats (particularly
1353 extended doubles, which vary quite a bit even among IEEE systems).
1354 Again, it is best to define a new negative type number for each
1355 different format; changing the format based on the target system has
1356 various problems. One such problem is that the Alpha has both VAX and
1357 IEEE floating types. One can easily imagine one library using the VAX
1358 types and another library in the same executable using the IEEE types.
1359 Another example is that the interpretation of whether a boolean is true
1360 or false can be based on the least significant bit, most significant
1361 bit, whether it is zero, etc., and different compilers (or different
1362 options to the same compiler) might provide different kinds of boolean.
1364 The last major issue is the names of the types. The name of a given
1365 type depends @emph{only} on the negative type number given; these do not
1366 vary depending on the language, the target system, or anything else.
1367 One can always define separate type numbers---in the following list you
1368 will see for example separate @code{int} and @code{integer*4} types
1369 which are identical except for the name. But compatibility can be
1370 maintained by not inventing new negative type numbers and instead just
1371 defining a new type with a new name. For example:
1374 .stabs "CARDINAL:t10=-8",128,0,0,0
1377 Here is the list of negative type numbers. The phrase @dfn{integral
1378 type} is used to mean twos-complement (I strongly suspect that all
1379 machines which use stabs use twos-complement; most machines use
1380 twos-complement these days).
1384 @code{int}, 32 bit signed integral type.
1387 @code{char}, 8 bit type holding a character. Both GDB and dbx on AIX
1388 treat this as signed. GCC uses this type whether @code{char} is signed
1389 or not, which seems like a bad idea. The AIX compiler (@code{xlc}) seems to
1390 avoid this type; it uses -5 instead for @code{char}.
1393 @code{short}, 16 bit signed integral type.
1396 @code{long}, 32 bit signed integral type.
1399 @code{unsigned char}, 8 bit unsigned integral type.
1402 @code{signed char}, 8 bit signed integral type.
1405 @code{unsigned short}, 16 bit unsigned integral type.
1408 @code{unsigned int}, 32 bit unsigned integral type.
1411 @code{unsigned}, 32 bit unsigned integral type.
1414 @code{unsigned long}, 32 bit unsigned integral type.
1417 @code{void}, type indicating the lack of a value.
1420 @code{float}, IEEE single precision.
1423 @code{double}, IEEE double precision.
1426 @code{long double}, IEEE double precision. The compiler claims the size
1427 will increase in a future release, and for binary compatibility you have
1428 to avoid using @code{long double}. I hope when they increase it they
1429 use a new negative type number.
1432 @code{integer}. 32 bit signed integral type.
1435 @code{boolean}. 32 bit type. How is the truth value encoded? Is it
1436 the least significant bit or is it a question of whether the whole value
1437 is zero or non-zero?
1440 @code{short real}. IEEE single precision.
1443 @code{real}. IEEE double precision.
1446 @code{stringptr}. @xref{Strings}.
1449 @code{character}, 8 bit unsigned character type.
1452 @code{logical*1}, 8 bit type. This Fortran type has a split
1453 personality in that it is used for boolean variables, but can also be
1454 used for unsigned integers. 0 is false, 1 is true, and other values are
1458 @code{logical*2}, 16 bit type. This Fortran type has a split
1459 personality in that it is used for boolean variables, but can also be
1460 used for unsigned integers. 0 is false, 1 is true, and other values are
1464 @code{logical*4}, 32 bit type. This Fortran type has a split
1465 personality in that it is used for boolean variables, but can also be
1466 used for unsigned integers. 0 is false, 1 is true, and other values are
1470 @code{logical}, 32 bit type. This Fortran type has a split
1471 personality in that it is used for boolean variables, but can also be
1472 used for unsigned integers. 0 is false, 1 is true, and other values are
1476 @code{complex}. A complex type consisting of two IEEE single-precision
1477 floating point values.
1480 @code{complex}. A complex type consisting of two IEEE double-precision
1481 floating point values.
1484 @code{integer*1}, 8 bit signed integral type.
1487 @code{integer*2}, 16 bit signed integral type.
1490 @code{integer*4}, 32 bit signed integral type.
1493 @code{wchar}. Wide character, 16 bits wide, unsigned (what format?
1497 @node Miscellaneous Types
1498 @section Miscellaneous Types
1501 @item b @var{type-information} ; @var{bytes}
1502 Pascal space type. This is documented by IBM; what does it mean?
1504 This use of the @samp{b} type descriptor can be distinguished
1505 from its use for builtin integral types (@pxref{Builtin Type
1506 Descriptors}) because the character following the type descriptor is
1507 always a digit, @samp{(}, or @samp{-}.
1509 @item B @var{type-information}
1510 A volatile-qualified version of @var{type-information}. This is
1511 a Sun extension. References and stores to a variable with a
1512 volatile-qualified type must not be optimized or cached; they
1513 must occur as the user specifies them.
1515 @item d @var{type-information}
1516 File of type @var{type-information}. As far as I know this is only used
1519 @item k @var{type-information}
1520 A const-qualified version of @var{type-information}. This is a Sun
1521 extension. A variable with a const-qualified type cannot be modified.
1523 @item M @var{type-information} ; @var{length}
1524 Multiple instance type. The type seems to composed of @var{length}
1525 repetitions of @var{type-information}, for example @code{character*3} is
1526 represented by @samp{M-2;3}, where @samp{-2} is a reference to a
1527 character type (@pxref{Negative Type Numbers}). I'm not sure how this
1528 differs from an array. This appears to be a Fortran feature.
1529 @var{length} is a bound, like those in range types; see @ref{Subranges}.
1531 @item S @var{type-information}
1532 Pascal set type. @var{type-information} must be a small type such as an
1533 enumeration or a subrange, and the type is a bitmask whose length is
1534 specified by the number of elements in @var{type-information}.
1536 @item * @var{type-information}
1537 Pointer to @var{type-information}.
1540 @node Cross-References
1541 @section Cross-References to Other Types
1543 A type can be used before it is defined; one common way to deal with
1544 that situation is just to use a type reference to a type which has not
1547 Another way is with the @samp{x} type descriptor, which is followed by
1548 @samp{s} for a structure tag, @samp{u} for a union tag, or @samp{e} for
1549 a enumerator tag, followed by the name of the tag, followed by @samp{:}.
1550 For example, the following C declarations:
1561 .stabs "bar:G16=*17=xsfoo:",32,0,0,0
1564 Not all debuggers support the @samp{x} type descriptor, so on some
1565 machines GCC does not use it. I believe that for the above example it
1566 would just emit a reference to type 17 and never define it, but I
1567 haven't verified that.
1569 Modula-2 imported types, at least on AIX, use the @samp{i} type
1570 descriptor, which is followed by the name of the module from which the
1571 type is imported, followed by @samp{:}, followed by the name of the
1572 type. There is then optionally a comma followed by type information for
1573 the type. This differs from merely naming the type (@pxref{Typedefs}) in
1574 that it identifies the module; I don't understand whether the name of
1575 the type given here is always just the same as the name we are giving
1576 it, or whether this type descriptor is used with a nameless stab
1577 (@pxref{String Field}), or what. The symbol ends with @samp{;}.
1580 @section Subrange Types
1582 The @samp{r} type descriptor defines a type as a subrange of another
1583 type. It is followed by type information for the type of which it is a
1584 subrange, a semicolon, an integral lower bound, a semicolon, an
1585 integral upper bound, and a semicolon. The AIX documentation does not
1586 specify the trailing semicolon, in an effort to specify array indexes
1587 more cleanly, but a subrange which is not an array index has always
1588 included a trailing semicolon (@pxref{Arrays}).
1590 Instead of an integer, either bound can be one of the following:
1593 @item A @var{offset}
1594 The bound is passed by reference on the stack at offset @var{offset}
1595 from the argument list. @xref{Parameters}, for more information on such
1598 @item T @var{offset}
1599 The bound is passed by value on the stack at offset @var{offset} from
1602 @item a @var{register-number}
1603 The bound is pased by reference in register number
1604 @var{register-number}.
1606 @item t @var{register-number}
1607 The bound is passed by value in register number @var{register-number}.
1613 Subranges are also used for builtin types; see @ref{Traditional Builtin Types}.
1616 @section Array Types
1618 Arrays use the @samp{a} type descriptor. Following the type descriptor
1619 is the type of the index and the type of the array elements. If the
1620 index type is a range type, it ends in a semicolon; otherwise
1621 (for example, if it is a type reference), there does not
1622 appear to be any way to tell where the types are separated. In an
1623 effort to clean up this mess, IBM documents the two types as being
1624 separated by a semicolon, and a range type as not ending in a semicolon
1625 (but this is not right for range types which are not array indexes,
1626 @pxref{Subranges}). I think probably the best solution is to specify
1627 that a semicolon ends a range type, and that the index type and element
1628 type of an array are separated by a semicolon, but that if the index
1629 type is a range type, the extra semicolon can be omitted. GDB (at least
1630 through version 4.9) doesn't support any kind of index type other than a
1631 range anyway; I'm not sure about dbx.
1633 It is well established, and widely used, that the type of the index,
1634 unlike most types found in the stabs, is merely a type definition, not
1635 type information (@pxref{String Field}) (that is, it need not start with
1636 @samp{@var{type-number}=} if it is defining a new type). According to a
1637 comment in GDB, this is also true of the type of the array elements; it
1638 gives @samp{ar1;1;10;ar1;1;10;4} as a legitimate way to express a two
1639 dimensional array. According to AIX documentation, the element type
1640 must be type information. GDB accepts either.
1642 The type of the index is often a range type, expressed as the type
1643 descriptor @samp{r} and some parameters. It defines the size of the
1644 array. In the example below, the range @samp{r1;0;2;} defines an index
1645 type which is a subrange of type 1 (integer), with a lower bound of 0
1646 and an upper bound of 2. This defines the valid range of subscripts of
1647 a three-element C array.
1649 For example, the definition:
1652 char char_vec[3] = @{'a','b','c'@};
1656 produces the output:
1659 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
1668 If an array is @dfn{packed}, the elements are spaced more
1669 closely than normal, saving memory at the expense of speed. For
1670 example, an array of 3-byte objects might, if unpacked, have each
1671 element aligned on a 4-byte boundary, but if packed, have no padding.
1672 One way to specify that something is packed is with type attributes
1673 (@pxref{String Field}). In the case of arrays, another is to use the
1674 @samp{P} type descriptor instead of @samp{a}. Other than specifying a
1675 packed array, @samp{P} is identical to @samp{a}.
1677 @c FIXME-what is it? A pointer?
1678 An open array is represented by the @samp{A} type descriptor followed by
1679 type information specifying the type of the array elements.
1681 @c FIXME: what is the format of this type? A pointer to a vector of pointers?
1682 An N-dimensional dynamic array is represented by
1685 D @var{dimensions} ; @var{type-information}
1688 @c Does dimensions really have this meaning? The AIX documentation
1690 @var{dimensions} is the number of dimensions; @var{type-information}
1691 specifies the type of the array elements.
1693 @c FIXME: what is the format of this type? A pointer to some offsets in
1695 A subarray of an N-dimensional array is represented by
1698 E @var{dimensions} ; @var{type-information}
1701 @c Does dimensions really have this meaning? The AIX documentation
1703 @var{dimensions} is the number of dimensions; @var{type-information}
1704 specifies the type of the array elements.
1709 Some languages, like C or the original Pascal, do not have string types,
1710 they just have related things like arrays of characters. But most
1711 Pascals and various other languages have string types, which are
1712 indicated as follows:
1715 @item n @var{type-information} ; @var{bytes}
1716 @var{bytes} is the maximum length. I'm not sure what
1717 @var{type-information} is; I suspect that it means that this is a string
1718 of @var{type-information} (thus allowing a string of integers, a string
1719 of wide characters, etc., as well as a string of characters). Not sure
1720 what the format of this type is. This is an AIX feature.
1722 @item z @var{type-information} ; @var{bytes}
1723 Just like @samp{n} except that this is a gstring, not an ordinary
1724 string. I don't know the difference.
1727 Pascal Stringptr. What is this? This is an AIX feature.
1731 @section Enumerations
1733 Enumerations are defined with the @samp{e} type descriptor.
1735 @c FIXME: Where does this information properly go? Perhaps it is
1736 @c redundant with something we already explain.
1737 The source line below declares an enumeration type at file scope.
1738 The type definition is located after the @code{N_RBRAC} that marks the end of
1739 the previous procedure's block scope, and before the @code{N_FUN} that marks
1740 the beginning of the next procedure's block scope. Therefore it does not
1741 describe a block local symbol, but a file local one.
1746 enum e_places @{first,second=3,last@};
1750 generates the following stab:
1753 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
1756 The symbol descriptor (@samp{T}) says that the stab describes a
1757 structure, enumeration, or union tag. The type descriptor @samp{e},
1758 following the @samp{22=} of the type definition narrows it down to an
1759 enumeration type. Following the @samp{e} is a list of the elements of
1760 the enumeration. The format is @samp{@var{name}:@var{value},}. The
1761 list of elements ends with @samp{;}.
1763 There is no standard way to specify the size of an enumeration type; it
1764 is determined by the architecture (normally all enumerations types are
1765 32 bits). There should be a way to specify an enumeration type of
1766 another size; type attributes would be one way to do this. @xref{Stabs
1772 The encoding of structures in stabs can be shown with an example.
1774 The following source code declares a structure tag and defines an
1775 instance of the structure in global scope. Then a @code{typedef} equates the
1776 structure tag with a new type. Seperate stabs are generated for the
1777 structure tag, the structure @code{typedef}, and the structure instance. The
1778 stabs for the tag and the @code{typedef} are emited when the definitions are
1779 encountered. Since the structure elements are not initialized, the
1780 stab and code for the structure variable itself is located at the end
1781 of the program in the bss section.
1788 struct s_tag* s_next;
1791 typedef struct s_tag s_typedef;
1794 The structure tag has an @code{N_LSYM} stab type because, like the
1795 enumeration, the symbol has file scope. Like the enumeration, the
1796 symbol descriptor is @samp{T}, for enumeration, structure, or tag type.
1797 The type descriptor @samp{s} following the @samp{16=} of the type
1798 definition narrows the symbol type to structure.
1800 Following the @samp{s} type descriptor is the number of bytes the
1801 structure occupies, followed by a description of each structure element.
1802 The structure element descriptions are of the form @var{name:type, bit
1803 offset from the start of the struct, number of bits in the element}.
1805 @c FIXME: phony line break. Can probably be fixed by using an example
1806 @c with fewer fields.
1809 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;
1810 s_char_vec:17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
1813 In this example, the first two structure elements are previously defined
1814 types. For these, the type following the @samp{@var{name}:} part of the
1815 element description is a simple type reference. The other two structure
1816 elements are new types. In this case there is a type definition
1817 embedded after the @samp{@var{name}:}. The type definition for the
1818 array element looks just like a type definition for a standalone array.
1819 The @code{s_next} field is a pointer to the same kind of structure that
1820 the field is an element of. So the definition of structure type 16
1821 contains a type definition for an element which is a pointer to type 16.
1824 @section Giving a Type a Name
1826 To give a type a name, use the @samp{t} symbol descriptor. The type
1827 is specified by the type information (@pxref{String Field}) for the stab.
1831 .stabs "s_typedef:t16",128,0,0,0 # @r{128 is N_LSYM}
1834 specifies that @code{s_typedef} refers to type number 16. Such stabs
1835 have symbol type @code{N_LSYM} (or @code{C_DECL} for XCOFF).
1837 If you are specifying the tag name for a structure, union, or
1838 enumeration, use the @samp{T} symbol descriptor instead. I believe C is
1839 the only language with this feature.
1841 If the type is an opaque type (I believe this is a Modula-2 feature),
1842 AIX provides a type descriptor to specify it. The type descriptor is
1843 @samp{o} and is followed by a name. I don't know what the name
1844 means---is it always the same as the name of the type, or is this type
1845 descriptor used with a nameless stab (@pxref{String Field})? There
1846 optionally follows a comma followed by type information which defines
1847 the type of this type. If omitted, a semicolon is used in place of the
1848 comma and the type information, and the type is much like a generic
1849 pointer type---it has a known size but little else about it is
1863 This code generates a stab for a union tag and a stab for a union
1864 variable. Both use the @code{N_LSYM} stab type. If a union variable is
1865 scoped locally to the procedure in which it is defined, its stab is
1866 located immediately preceding the @code{N_LBRAC} for the procedure's block
1869 The stab for the union tag, however, is located preceding the code for
1870 the procedure in which it is defined. The stab type is @code{N_LSYM}. This
1871 would seem to imply that the union type is file scope, like the struct
1872 type @code{s_tag}. This is not true. The contents and position of the stab
1873 for @code{u_type} do not convey any infomation about its procedure local
1876 @c FIXME: phony line break. Can probably be fixed by using an example
1877 @c with fewer fields.
1880 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
1884 The symbol descriptor @samp{T}, following the @samp{name:} means that
1885 the stab describes an enumeration, structure, or union tag. The type
1886 descriptor @samp{u}, following the @samp{23=} of the type definition,
1887 narrows it down to a union type definition. Following the @samp{u} is
1888 the number of bytes in the union. After that is a list of union element
1889 descriptions. Their format is @var{name:type, bit offset into the
1890 union, number of bytes for the element;}.
1892 The stab for the union variable is:
1895 .stabs "an_u:23",128,0,0,-20 # @r{128 is N_LSYM}
1898 @samp{-20} specifies where the variable is stored (@pxref{Stack
1901 @node Function Types
1902 @section Function Types
1904 Various types can be defined for function variables. These types are
1905 not used in defining functions (@pxref{Procedures}); they are used for
1906 things like pointers to functions.
1908 The simple, traditional, type is type descriptor @samp{f} is followed by
1909 type information for the return type of the function, followed by a
1912 This does not deal with functions for which the number and types of the
1913 parameters are part of the type, as in Modula-2 or ANSI C. AIX provides
1914 extensions to specify these, using the @samp{f}, @samp{F}, @samp{p}, and
1915 @samp{R} type descriptors.
1917 First comes the type descriptor. If it is @samp{f} or @samp{F}, this
1918 type involves a function rather than a procedure, and the type
1919 information for the return type of the function follows, followed by a
1920 comma. Then comes the number of parameters to the function and a
1921 semicolon. Then, for each parameter, there is the name of the parameter
1922 followed by a colon (this is only present for type descriptors @samp{R}
1923 and @samp{F} which represent Pascal function or procedure parameters),
1924 type information for the parameter, a comma, 0 if passed by reference or
1925 1 if passed by value, and a semicolon. The type definition ends with a
1928 For example, this variable definition:
1935 generates the following code:
1938 .stabs "g_pf:G24=*25=f1",32,0,0,0
1939 .common _g_pf,4,"bss"
1942 The variable defines a new type, 24, which is a pointer to another new
1943 type, 25, which is a function returning @code{int}.
1946 @chapter Symbol Information in Symbol Tables
1948 This chapter describes the format of symbol table entries
1949 and how stab assembler directives map to them. It also describes the
1950 transformations that the assembler and linker make on data from stabs.
1953 * Symbol Table Format::
1954 * Transformations On Symbol Tables::
1957 @node Symbol Table Format
1958 @section Symbol Table Format
1960 Each time the assembler encounters a stab directive, it puts
1961 each field of the stab into a corresponding field in a symbol table
1962 entry of its output file. If the stab contains a string field, the
1963 symbol table entry for that stab points to a string table entry
1964 containing the string data from the stab. Assembler labels become
1965 relocatable addresses. Symbol table entries in a.out have the format:
1967 @c FIXME: should refer to external, not internal.
1969 struct internal_nlist @{
1970 unsigned long n_strx; /* index into string table of name */
1971 unsigned char n_type; /* type of symbol */
1972 unsigned char n_other; /* misc info (usually empty) */
1973 unsigned short n_desc; /* description field */
1974 bfd_vma n_value; /* value of symbol */
1978 If the stab has a string, the @code{n_strx} field holds the offset in
1979 bytes of the string within the string table. The string is terminated
1980 by a NUL character. If the stab lacks a string (for example, it was
1981 produced by a @code{.stabn} or @code{.stabd} directive), the
1982 @code{n_strx} field is zero.
1984 Symbol table entries with @code{n_type} field values greater than 0x1f
1985 originated as stabs generated by the compiler (with one random
1986 exception). The other entries were placed in the symbol table of the
1987 executable by the assembler or the linker.
1989 @node Transformations On Symbol Tables
1990 @section Transformations on Symbol Tables
1992 The linker concatenates object files and does fixups of externally
1995 You can see the transformations made on stab data by the assembler and
1996 linker by examining the symbol table after each pass of the build. To
1997 do this, use @samp{nm -ap}, which dumps the symbol table, including
1998 debugging information, unsorted. For stab entries the columns are:
1999 @var{value}, @var{other}, @var{desc}, @var{type}, @var{string}. For
2000 assembler and linker symbols, the columns are: @var{value}, @var{type},
2003 The low 5 bits of the stab type tell the linker how to relocate the
2004 value of the stab. Thus for stab types like @code{N_RSYM} and
2005 @code{N_LSYM}, where the value is an offset or a register number, the
2006 low 5 bits are @code{N_ABS}, which tells the linker not to relocate the
2009 Where the value of a stab contains an assembly language label,
2010 it is transformed by each build step. The assembler turns it into a
2011 relocatable address and the linker turns it into an absolute address.
2014 * Transformations On Static Variables::
2015 * Transformations On Global Variables::
2016 * ELF Transformations:: In ELF, things are a bit different.
2019 @node Transformations On Static Variables
2020 @subsection Transformations on Static Variables
2022 This source line defines a static variable at file scope:
2025 static int s_g_repeat
2029 The following stab describes the symbol:
2032 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
2036 The assembler transforms the stab into this symbol table entry in the
2037 @file{.o} file. The location is expressed as a data segment offset.
2040 00000084 - 00 0000 STSYM s_g_repeat:S1
2044 In the symbol table entry from the executable, the linker has made the
2045 relocatable address absolute.
2048 0000e00c - 00 0000 STSYM s_g_repeat:S1
2051 @node Transformations On Global Variables
2052 @subsection Transformations on Global Variables
2054 Stabs for global variables do not contain location information. In
2055 this case, the debugger finds location information in the assembler or
2056 linker symbol table entry describing the variable. The source line:
2066 .stabs "g_foo:G2",32,0,0,0
2069 The variable is represented by two symbol table entries in the object
2070 file (see below). The first one originated as a stab. The second one
2071 is an external symbol. The upper case @samp{D} signifies that the
2072 @code{n_type} field of the symbol table contains 7, @code{N_DATA} with
2073 local linkage. The stab's value is zero since the value is not used for
2074 @code{N_GSYM} stabs. The value of the linker symbol is the relocatable
2075 address corresponding to the variable.
2078 00000000 - 00 0000 GSYM g_foo:G2
2083 These entries as transformed by the linker. The linker symbol table
2084 entry now holds an absolute address:
2087 00000000 - 00 0000 GSYM g_foo:G2
2092 @node ELF Transformations
2093 @subsection Transformations of Stabs in ELF Files
2095 For ELF files, use @code{objdump --stabs} instead of @code{nm} to show
2096 the stabs in an object or executable file. @code{objdump} is a GNU
2097 utility; Sun does not provide any equivalent.
2099 The following example is for a stab whose value is an address is
2100 relative to the compilation unit (@pxref{Stabs In ELF}). For example,
2107 appears within a function, then the assembly language output from the
2113 .stabs "ld:V(0,3)",0x26,0,4,.L18-Ddata.data # @r{0x26 is N_STSYM}
2120 Because the value is formed by subtracting one symbol from another, the
2121 value is absolute, not relocatable, and so the object file contains
2124 Symnum n_type n_othr n_desc n_value n_strx String
2125 31 STSYM 0 4 00000004 680 ld:V(0,3)
2128 without any relocations, and the executable file also contains
2131 Symnum n_type n_othr n_desc n_value n_strx String
2132 31 STSYM 0 4 00000004 680 ld:V(0,3)
2136 @chapter GNU C++ Stabs
2139 * Basic Cplusplus Types::
2142 * Methods:: Method definition
2144 * Method Modifiers::
2147 * Virtual Base Classes::
2151 Type descriptors added for C++ descriptions:
2155 method type (@code{##} if minimal debug)
2158 Member (class and variable) type. It is followed by type information
2159 for the offset basetype, a comma, and type information for the type of
2160 the field being pointed to. (FIXME: this is acknowledged to be
2161 gibberish. Can anyone say what really goes here?).
2163 Note that there is a conflict between this and type attributes
2164 (@pxref{String Field}); both use type descriptor @samp{@@}.
2165 Fortunately, the @samp{@@} type descriptor used in this C++ sense always
2166 will be followed by a digit, @samp{(}, or @samp{-}, and type attributes
2167 never start with those things.
2170 @node Basic Cplusplus Types
2171 @section Basic Types For C++
2173 << the examples that follow are based on a01.C >>
2176 C++ adds two more builtin types to the set defined for C. These are
2177 the unknown type and the vtable record type. The unknown type, type
2178 16, is defined in terms of itself like the void type.
2180 The vtable record type, type 17, is defined as a structure type and
2181 then as a structure tag. The structure has four fields: delta, index,
2182 pfn, and delta2. pfn is the function pointer.
2184 << In boilerplate $vtbl_ptr_type, what are the fields delta,
2185 index, and delta2 used for? >>
2187 This basic type is present in all C++ programs even if there are no
2188 virtual methods defined.
2191 .stabs "struct_name:sym_desc(type)type_def(17)=type_desc(struct)struct_bytes(8)
2192 elem_name(delta):type_ref(short int),bit_offset(0),field_bits(16);
2193 elem_name(index):type_ref(short int),bit_offset(16),field_bits(16);
2194 elem_name(pfn):type_def(18)=type_desc(ptr to)type_ref(void),
2195 bit_offset(32),field_bits(32);
2196 elem_name(delta2):type_def(short int);bit_offset(32),field_bits(16);;"
2201 .stabs "$vtbl_ptr_type:t17=s8
2202 delta:6,0,16;index:6,16,16;pfn:18=*15,32,32;delta2:6,32,16;;"
2207 .stabs "name:sym_dec(struct tag)type_ref($vtbl_ptr_type)",N_LSYM,NIL,NIL,NIL
2211 .stabs "$vtbl_ptr_type:T17",128,0,0,0
2214 @node Simple Classes
2215 @section Simple Class Definition
2217 The stabs describing C++ language features are an extension of the
2218 stabs describing C. Stabs representing C++ class types elaborate
2219 extensively on the stab format used to describe structure types in C.
2220 Stabs representing class type variables look just like stabs
2221 representing C language variables.
2223 Consider the following very simple class definition.
2229 int Ameth(int in, char other);
2233 The class @code{baseA} is represented by two stabs. The first stab describes
2234 the class as a structure type. The second stab describes a structure
2235 tag of the class type. Both stabs are of stab type @code{N_LSYM}. Since the
2236 stab is not located between an @code{N_FUN} and an @code{N_LBRAC} stab this indicates
2237 that the class is defined at file scope. If it were, then the @code{N_LSYM}
2238 would signify a local variable.
2240 A stab describing a C++ class type is similar in format to a stab
2241 describing a C struct, with each class member shown as a field in the
2242 structure. The part of the struct format describing fields is
2243 expanded to include extra information relevent to C++ class members.
2244 In addition, if the class has multiple base classes or virtual
2245 functions the struct format outside of the field parts is also
2248 In this simple example the field part of the C++ class stab
2249 representing member data looks just like the field part of a C struct
2250 stab. The section on protections describes how its format is
2251 sometimes extended for member data.
2253 The field part of a C++ class stab representing a member function
2254 differs substantially from the field part of a C struct stab. It
2255 still begins with @samp{name:} but then goes on to define a new type number
2256 for the member function, describe its return type, its argument types,
2257 its protection level, any qualifiers applied to the method definition,
2258 and whether the method is virtual or not. If the method is virtual
2259 then the method description goes on to give the vtable index of the
2260 method, and the type number of the first base class defining the
2263 When the field name is a method name it is followed by two colons rather
2264 than one. This is followed by a new type definition for the method.
2265 This is a number followed by an equal sign and the type descriptor
2266 @samp{#}, indicating a method type, and a second @samp{#}, indicating
2267 that this is the @dfn{minimal} type of method definition used by GCC2,
2268 not larger method definitions used by earlier versions of GCC. This is
2269 followed by a type reference showing the return type of the method and a
2272 The format of an overloaded operator method name differs from that of
2273 other methods. It is @samp{op$::@var{operator-name}.} where
2274 @var{operator-name} is the operator name such as @samp{+} or @samp{+=}.
2275 The name ends with a period, and any characters except the period can
2276 occur in the @var{operator-name} string.
2278 The next part of the method description represents the arguments to the
2279 method, preceeded by a colon and ending with a semi-colon. The types of
2280 the arguments are expressed in the same way argument types are expressed
2281 in C++ name mangling. In this example an @code{int} and a @code{char}
2284 This is followed by a number, a letter, and an asterisk or period,
2285 followed by another semicolon. The number indicates the protections
2286 that apply to the member function. Here the 2 means public. The
2287 letter encodes any qualifier applied to the method definition. In
2288 this case, @samp{A} means that it is a normal function definition. The dot
2289 shows that the method is not virtual. The sections that follow
2290 elaborate further on these fields and describe the additional
2291 information present for virtual methods.
2295 .stabs "class_name:sym_desc(type)type_def(20)=type_desc(struct)struct_bytes(4)
2296 field_name(Adat):type(int),bit_offset(0),field_bits(32);
2298 method_name(Ameth)::type_def(21)=type_desc(method)return_type(int);
2299 :arg_types(int char);
2300 protection(public)qualifier(normal)virtual(no);;"
2305 .stabs "baseA:t20=s4Adat:1,0,32;Ameth::21=##1;:ic;2A.;;",128,0,0,0
2307 .stabs "class_name:sym_desc(struct tag)",N_LSYM,NIL,NIL,NIL
2309 .stabs "baseA:T20",128,0,0,0
2312 @node Class Instance
2313 @section Class Instance
2315 As shown above, describing even a simple C++ class definition is
2316 accomplished by massively extending the stab format used in C to
2317 describe structure types. However, once the class is defined, C stabs
2318 with no modifications can be used to describe class instances. The
2328 yields the following stab describing the class instance. It looks no
2329 different from a standard C stab describing a local variable.
2332 .stabs "name:type_ref(baseA)", N_LSYM, NIL, NIL, frame_ptr_offset
2336 .stabs "AbaseA:20",128,0,0,-20
2340 @section Method Definition
2342 The class definition shown above declares Ameth. The C++ source below
2347 baseA::Ameth(int in, char other)
2354 This method definition yields three stabs following the code of the
2355 method. One stab describes the method itself and following two describe
2356 its parameters. Although there is only one formal argument all methods
2357 have an implicit argument which is the @code{this} pointer. The @code{this}
2358 pointer is a pointer to the object on which the method was called. Note
2359 that the method name is mangled to encode the class name and argument
2360 types. Name mangling is described in the @sc{arm} (@cite{The Annotated
2361 C++ Reference Manual}, by Ellis and Stroustrup, @sc{isbn}
2362 0-201-51459-1); @file{gpcompare.texi} in Cygnus GCC distributions
2363 describes the differences between GNU mangling and @sc{arm}
2365 @c FIXME: Use @xref, especially if this is generally installed in the
2367 @c FIXME: This information should be in a net release, either of GCC or
2368 @c GDB. But gpcompare.texi doesn't seem to be in the FSF GCC.
2371 .stabs "name:symbol_desriptor(global function)return_type(int)",
2372 N_FUN, NIL, NIL, code_addr_of_method_start
2374 .stabs "Ameth__5baseAic:F1",36,0,0,_Ameth__5baseAic
2377 Here is the stab for the @code{this} pointer implicit argument. The
2378 name of the @code{this} pointer is always @code{this}. Type 19, the
2379 @code{this} pointer is defined as a pointer to type 20, @code{baseA},
2380 but a stab defining @code{baseA} has not yet been emited. Since the
2381 compiler knows it will be emited shortly, here it just outputs a cross
2382 reference to the undefined symbol, by prefixing the symbol name with
2386 .stabs "name:sym_desc(register param)type_def(19)=
2387 type_desc(ptr to)type_ref(baseA)=
2388 type_desc(cross-reference to)baseA:",N_RSYM,NIL,NIL,register_number
2390 .stabs "this:P19=*20=xsbaseA:",64,0,0,8
2393 The stab for the explicit integer argument looks just like a parameter
2394 to a C function. The last field of the stab is the offset from the
2395 argument pointer, which in most systems is the same as the frame
2399 .stabs "name:sym_desc(value parameter)type_ref(int)",
2400 N_PSYM,NIL,NIL,offset_from_arg_ptr
2402 .stabs "in:p1",160,0,0,72
2405 << The examples that follow are based on A1.C >>
2408 @section Protections
2411 In the simple class definition shown above all member data and
2412 functions were publicly accessable. The example that follows
2413 contrasts public, protected and privately accessable fields and shows
2414 how these protections are encoded in C++ stabs.
2416 @c FIXME: What does "part of the string" mean?
2417 Protections for class member data are signified by two characters
2418 embedded in the stab defining the class type. These characters are
2419 located after the name: part of the string. @samp{/0} means private,
2420 @samp{/1} means protected, and @samp{/2} means public. If these
2421 characters are omited this means that the member is public. The
2422 following C++ source:
2436 generates the following stab to describe the class type all_data.
2439 .stabs "class_name:sym_desc(type)type_def(19)=type_desc(struct)struct_bytes
2440 data_name:/protection(private)type_ref(int),bit_offset,num_bits;
2441 data_name:/protection(protected)type_ref(char),bit_offset,num_bits;
2442 data_name:(/num omited, private)type_ref(float),bit_offset,num_bits;;"
2447 .stabs "all_data:t19=s12
2448 priv_dat:/01,0,32;prot_dat:/12,32,8;pub_dat:12,64,32;;",128,0,0,0
2451 Protections for member functions are signified by one digit embeded in
2452 the field part of the stab describing the method. The digit is 0 if
2453 private, 1 if protected and 2 if public. Consider the C++ class
2457 class all_methods @{
2459 int priv_meth(int in)@{return in;@};
2461 char protMeth(char in)@{return in;@};
2463 float pubMeth(float in)@{return in;@};
2467 It generates the following stab. The digit in question is to the left
2468 of an @samp{A} in each case. Notice also that in this case two symbol
2469 descriptors apply to the class name struct tag and struct type.
2472 .stabs "class_name:sym_desc(struct tag&type)type_def(21)=
2473 sym_desc(struct)struct_bytes(1)
2474 meth_name::type_def(22)=sym_desc(method)returning(int);
2475 :args(int);protection(private)modifier(normal)virtual(no);
2476 meth_name::type_def(23)=sym_desc(method)returning(char);
2477 :args(char);protection(protected)modifier(normal)virual(no);
2478 meth_name::type_def(24)=sym_desc(method)returning(float);
2479 :args(float);protection(public)modifier(normal)virtual(no);;",
2484 .stabs "all_methods:Tt21=s1priv_meth::22=##1;:i;0A.;protMeth::23=##2;:c;1A.;
2485 pubMeth::24=##12;:f;2A.;;",128,0,0,0
2488 @node Method Modifiers
2489 @section Method Modifiers (@code{const}, @code{volatile}, @code{const volatile})
2493 In the class example described above all the methods have the normal
2494 modifier. This method modifier information is located just after the
2495 protection information for the method. This field has four possible
2496 character values. Normal methods use @samp{A}, const methods use
2497 @samp{B}, volatile methods use @samp{C}, and const volatile methods use
2498 @samp{D}. Consider the class definition below:
2503 int ConstMeth (int arg) const @{ return arg; @};
2504 char VolatileMeth (char arg) volatile @{ return arg; @};
2505 float ConstVolMeth (float arg) const volatile @{return arg; @};
2509 This class is described by the following stab:
2512 .stabs "class(A):sym_desc(struct)type_def(20)=type_desc(struct)struct_bytes(1)
2513 meth_name(ConstMeth)::type_def(21)sym_desc(method)
2514 returning(int);:arg(int);protection(public)modifier(const)virtual(no);
2515 meth_name(VolatileMeth)::type_def(22)=sym_desc(method)
2516 returning(char);:arg(char);protection(public)modifier(volatile)virt(no)
2517 meth_name(ConstVolMeth)::type_def(23)=sym_desc(method)
2518 returning(float);:arg(float);protection(public)modifer(const volatile)
2519 virtual(no);;", @dots{}
2523 .stabs "A:T20=s1ConstMeth::21=##1;:i;2B.;VolatileMeth::22=##2;:c;2C.;
2524 ConstVolMeth::23=##12;:f;2D.;;",128,0,0,0
2527 @node Virtual Methods
2528 @section Virtual Methods
2530 << The following examples are based on a4.C >>
2532 The presence of virtual methods in a class definition adds additional
2533 data to the class description. The extra data is appended to the
2534 description of the virtual method and to the end of the class
2535 description. Consider the class definition below:
2541 virtual int A_virt (int arg) @{ return arg; @};
2545 This results in the stab below describing class A. It defines a new
2546 type (20) which is an 8 byte structure. The first field of the class
2547 struct is @samp{Adat}, an integer, starting at structure offset 0 and
2550 The second field in the class struct is not explicitly defined by the
2551 C++ class definition but is implied by the fact that the class
2552 contains a virtual method. This field is the vtable pointer. The
2553 name of the vtable pointer field starts with @samp{$vf} and continues with a
2554 type reference to the class it is part of. In this example the type
2555 reference for class A is 20 so the name of its vtable pointer field is
2556 @samp{$vf20}, followed by the usual colon.
2558 Next there is a type definition for the vtable pointer type (21).
2559 This is in turn defined as a pointer to another new type (22).
2561 Type 22 is the vtable itself, which is defined as an array, indexed by
2562 a range of integers between 0 and 1, and whose elements are of type
2563 17. Type 17 was the vtable record type defined by the boilerplate C++
2564 type definitions, as shown earlier.
2566 The bit offset of the vtable pointer field is 32. The number of bits
2567 in the field are not specified when the field is a vtable pointer.
2569 Next is the method definition for the virtual member function @code{A_virt}.
2570 Its description starts out using the same format as the non-virtual
2571 member functions described above, except instead of a dot after the
2572 @samp{A} there is an asterisk, indicating that the function is virtual.
2573 Since is is virtual some addition information is appended to the end
2574 of the method description.
2576 The first number represents the vtable index of the method. This is a
2577 32 bit unsigned number with the high bit set, followed by a
2580 The second number is a type reference to the first base class in the
2581 inheritence hierarchy defining the virtual member function. In this
2582 case the class stab describes a base class so the virtual function is
2583 not overriding any other definition of the method. Therefore the
2584 reference is to the type number of the class that the stab is
2587 This is followed by three semi-colons. One marks the end of the
2588 current sub-section, one marks the end of the method field, and the
2589 third marks the end of the struct definition.
2591 For classes containing virtual functions the very last section of the
2592 string part of the stab holds a type reference to the first base
2593 class. This is preceeded by @samp{~%} and followed by a final semi-colon.
2596 .stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8)
2597 field_name(Adat):type_ref(int),bit_offset(0),field_bits(32);
2598 field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)=
2599 sym_desc(array)index_type_ref(range of int from 0 to 1);
2600 elem_type_ref(vtbl elem type),
2602 meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
2603 :arg_type(int),protection(public)normal(yes)virtual(yes)
2604 vtable_index(1);class_first_defining(A);;;~%first_base(A);",
2608 @c FIXME: bogus line break.
2610 .stabs "A:t20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2611 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2615 @section Inheritence
2617 Stabs describing C++ derived classes include additional sections that
2618 describe the inheritence hierarchy of the class. A derived class stab
2619 also encodes the number of base classes. For each base class it tells
2620 if the base class is virtual or not, and if the inheritence is private
2621 or public. It also gives the offset into the object of the portion of
2622 the object corresponding to each base class.
2624 This additional information is embeded in the class stab following the
2625 number of bytes in the struct. First the number of base classes
2626 appears bracketed by an exclamation point and a comma.
2628 Then for each base type there repeats a series: two digits, a number,
2629 a comma, another number, and a semi-colon.
2631 The first of the two digits is 1 if the base class is virtual and 0 if
2632 not. The second digit is 2 if the derivation is public and 0 if not.
2634 The number following the first two digits is the offset from the start
2635 of the object to the part of the object pertaining to the base class.
2637 After the comma, the second number is a type_descriptor for the base
2638 type. Finally a semi-colon ends the series, which repeats for each
2641 The source below defines three base classes @code{A}, @code{B}, and
2642 @code{C} and the derived class @code{D}.
2649 virtual int A_virt (int arg) @{ return arg; @};
2655 virtual int B_virt (int arg) @{return arg; @};
2661 virtual int C_virt (int arg) @{return arg; @};
2664 class D : A, virtual B, public C @{
2667 virtual int A_virt (int arg ) @{ return arg+1; @};
2668 virtual int B_virt (int arg) @{ return arg+2; @};
2669 virtual int C_virt (int arg) @{ return arg+3; @};
2670 virtual int D_virt (int arg) @{ return arg; @};
2674 Class stabs similar to the ones described earlier are generated for
2677 @c FIXME!!! the linebreaks in the following example probably make the
2678 @c examples literally unusable, but I don't know any other way to get
2679 @c them on the page.
2680 @c One solution would be to put some of the type definitions into
2681 @c separate stabs, even if that's not exactly what the compiler actually
2684 .stabs "A:T20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2685 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2687 .stabs "B:Tt25=s8Bdat:1,0,32;$vf25:21,32;B_virt::26=##1;
2688 :i;2A*-2147483647;25;;;~%25;",128,0,0,0
2690 .stabs "C:Tt28=s8Cdat:1,0,32;$vf28:21,32;C_virt::29=##1;
2691 :i;2A*-2147483647;28;;;~%28;",128,0,0,0
2694 In the stab describing derived class @code{D} below, the information about
2695 the derivation of this class is encoded as follows.
2698 .stabs "derived_class_name:symbol_descriptors(struct tag&type)=
2699 type_descriptor(struct)struct_bytes(32)!num_bases(3),
2700 base_virtual(no)inheritence_public(no)base_offset(0),
2701 base_class_type_ref(A);
2702 base_virtual(yes)inheritence_public(no)base_offset(NIL),
2703 base_class_type_ref(B);
2704 base_virtual(no)inheritence_public(yes)base_offset(64),
2705 base_class_type_ref(C); @dots{}
2708 @c FIXME! fake linebreaks.
2710 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:
2711 1,160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt:
2712 :32:i;2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;
2713 28;;D_virt::32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2716 @node Virtual Base Classes
2717 @section Virtual Base Classes
2719 A derived class object consists of a concatination in memory of the data
2720 areas defined by each base class, starting with the leftmost and ending
2721 with the rightmost in the list of base classes. The exception to this
2722 rule is for virtual inheritence. In the example above, class @code{D}
2723 inherits virtually from base class @code{B}. This means that an
2724 instance of a @code{D} object will not contain its own @code{B} part but
2725 merely a pointer to a @code{B} part, known as a virtual base pointer.
2727 In a derived class stab, the base offset part of the derivation
2728 information, described above, shows how the base class parts are
2729 ordered. The base offset for a virtual base class is always given as 0.
2730 Notice that the base offset for @code{B} is given as 0 even though
2731 @code{B} is not the first base class. The first base class @code{A}
2734 The field information part of the stab for class @code{D} describes the field
2735 which is the pointer to the virtual base class @code{B}. The vbase pointer
2736 name is @samp{$vb} followed by a type reference to the virtual base class.
2737 Since the type id for @code{B} in this example is 25, the vbase pointer name
2740 @c FIXME!! fake linebreaks below
2742 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1,
2743 160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i;
2744 2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt:
2745 :32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2748 Following the name and a semicolon is a type reference describing the
2749 type of the virtual base class pointer, in this case 24. Type 24 was
2750 defined earlier as the type of the @code{B} class @code{this} pointer. The
2751 @code{this} pointer for a class is a pointer to the class type.
2754 .stabs "this:P24=*25=xsB:",64,0,0,8
2757 Finally the field offset part of the vbase pointer field description
2758 shows that the vbase pointer is the first field in the @code{D} object,
2759 before any data fields defined by the class. The layout of a @code{D}
2760 class object is a follows, @code{Adat} at 0, the vtable pointer for
2761 @code{A} at 32, @code{Cdat} at 64, the vtable pointer for C at 96, the
2762 virtual base pointer for @code{B} at 128, and @code{Ddat} at 160.
2765 @node Static Members
2766 @section Static Members
2768 The data area for a class is a concatenation of the space used by the
2769 data members of the class. If the class has virtual methods, a vtable
2770 pointer follows the class data. The field offset part of each field
2771 description in the class stab shows this ordering.
2773 << How is this reflected in stabs? See Cygnus bug #677 for some info. >>
2776 @appendix Table of Stab Types
2778 The following are all the possible values for the stab type field, for
2779 @code{a.out} files, in numeric order. This does not apply to XCOFF, but
2780 it does apply to stabs in ELF. Stabs in ECOFF use these values but add
2781 0x8f300 to distinguish them from non-stab symbols.
2783 The symbolic names are defined in the file @file{include/aout/stabs.def}.
2786 * Non-Stab Symbol Types:: Types from 0 to 0x1f
2787 * Stab Symbol Types:: Types from 0x20 to 0xff
2790 @node Non-Stab Symbol Types
2791 @appendixsec Non-Stab Symbol Types
2793 The following types are used by the linker and assembler, not by stab
2794 directives. Since this document does not attempt to describe aspects of
2795 object file format other than the debugging format, no details are
2798 @c Try to get most of these to fit on a single line.
2808 File scope absolute symbol
2810 @item 0x3 N_ABS | N_EXT
2811 External absolute symbol
2814 File scope text symbol
2816 @item 0x5 N_TEXT | N_EXT
2817 External text symbol
2820 File scope data symbol
2822 @item 0x7 N_DATA | N_EXT
2823 External data symbol
2826 File scope BSS symbol
2828 @item 0x9 N_BSS | N_EXT
2832 Same as @code{N_FN}, for Sequent compilers
2835 Symbol is indirected to another symbol
2838 Common---visible after shared library dynamic link
2841 Absolute set element
2844 Text segment set element
2847 Data segment set element
2850 BSS segment set element
2853 Pointer to set vector
2855 @item 0x1e N_WARNING
2856 Print a warning message during linking
2859 File name of a @file{.o} file
2862 @node Stab Symbol Types
2863 @appendixsec Stab Symbol Types
2865 The following symbol types indicate that this is a stab. This is the
2866 full list of stab numbers, including stab types that are used in
2867 languages other than C.
2871 Global symbol; see @ref{Global Variables}.
2874 Function name (for BSD Fortran); see @ref{Procedures}.
2877 Function name (@pxref{Procedures}) or text segment variable
2881 Data segment file-scope variable; see @ref{Statics}.
2884 BSS segment file-scope variable; see @ref{Statics}.
2887 Name of main routine; see @ref{Main Program}.
2890 Variable in @code{.rodata} section; see @ref{Statics}.
2893 Global symbol (for Pascal); see @ref{N_PC}.
2896 Number of symbols (according to Ultrix V4.0); see @ref{N_NSYMS}.
2899 No DST map; see @ref{N_NOMAP}.
2901 @c FIXME: describe this solaris feature in the body of the text (see
2902 @c comments in include/aout/stab.def).
2904 Object file (Solaris2).
2906 @c See include/aout/stab.def for (a little) more info.
2908 Debugger options (Solaris2).
2911 Register variable; see @ref{Register Variables}.
2914 Modula-2 compilation unit; see @ref{N_M2C}.
2917 Line number in text segment; see @ref{Line Numbers}.
2920 Line number in data segment; see @ref{Line Numbers}.
2923 Line number in bss segment; see @ref{Line Numbers}.
2926 Sun source code browser, path to @file{.cb} file; see @ref{N_BROWS}.
2929 GNU Modula2 definition module dependency; see @ref{N_DEFD}.
2932 Function start/body/end line numbers (Solaris2).
2935 GNU C++ exception variable; see @ref{N_EHDECL}.
2938 Modula2 info "for imc" (according to Ultrix V4.0); see @ref{N_MOD2}.
2941 GNU C++ @code{catch} clause; see @ref{N_CATCH}.
2944 Structure of union element; see @ref{N_SSYM}.
2947 Last stab for module (Solaris2).
2950 Path and name of source file; see @ref{Source Files}.
2953 Stack variable (@pxref{Stack Variables}) or type (@pxref{Typedefs}).
2956 Beginning of an include file (Sun only); see @ref{Include Files}.
2959 Name of include file; see @ref{Include Files}.
2962 Parameter variable; see @ref{Parameters}.
2965 End of an include file; see @ref{Include Files}.
2968 Alternate entry point; see @ref{N_ENTRY}.
2971 Beginning of a lexical block; see @ref{Block Structure}.
2974 Place holder for a deleted include file; see @ref{Include Files}.
2977 Modula2 scope information (Sun linker); see @ref{N_SCOPE}.
2980 End of a lexical block; see @ref{Block Structure}.
2983 Begin named common block; see @ref{Common Blocks}.
2986 End named common block; see @ref{Common Blocks}.
2989 Member of a common block; see @ref{Common Blocks}.
2991 @c FIXME: How does this really work? Move it to main body of document.
2993 Pascal @code{with} statement: type,,0,0,offset (Solaris2).
2996 Gould non-base registers; see @ref{Gould}.
2999 Gould non-base registers; see @ref{Gould}.
3002 Gould non-base registers; see @ref{Gould}.
3005 Gould non-base registers; see @ref{Gould}.
3008 Gould non-base registers; see @ref{Gould}.
3011 @c Restore the default table indent
3016 @node Symbol Descriptors
3017 @appendix Table of Symbol Descriptors
3019 The symbol descriptor is the character which follows the colon in many
3020 stabs, and which tells what kind of stab it is. @xref{String Field},
3021 for more information about their use.
3023 @c Please keep this alphabetical
3025 @c In TeX, this looks great, digit is in italics. But makeinfo insists
3026 @c on putting it in `', not realizing that @var should override @code.
3027 @c I don't know of any way to make makeinfo do the right thing. Seems
3028 @c like a makeinfo bug to me.
3032 Variable on the stack; see @ref{Stack Variables}.
3035 Parameter passed by reference in register; see @ref{Reference Parameters}.
3038 Based variable; see @ref{Parameters}.
3041 Constant; see @ref{Constants}.
3044 Conformant array bound (Pascal, maybe other languages); @ref{Conformant
3045 Arrays}. Name of a caught exception (GNU C++). These can be
3046 distinguished because the latter uses @code{N_CATCH} and the former uses
3047 another symbol type.
3050 Floating point register variable; see @ref{Register Variables}.
3053 Parameter in floating point register; see @ref{Register Parameters}.
3056 File scope function; see @ref{Procedures}.
3059 Global function; see @ref{Procedures}.
3062 Global variable; see @ref{Global Variables}.
3065 @xref{Register Parameters}.
3068 Internal (nested) procedure; see @ref{Nested Procedures}.
3071 Internal (nested) function; see @ref{Nested Procedures}.
3074 Label name (documented by AIX, no further information known).
3077 Module; see @ref{Procedures}.
3080 Argument list parameter; see @ref{Parameters}.
3086 Fortran Function parameter; see @ref{Parameters}.
3089 Unfortunately, three separate meanings have been independently invented
3090 for this symbol descriptor. At least the GNU and Sun uses can be
3091 distinguished by the symbol type. Global Procedure (AIX) (symbol type
3092 used unknown); see @ref{Procedures}. Register parameter (GNU) (symbol
3093 type @code{N_PSYM}); see @ref{Parameters}. Prototype of function
3094 referenced by this file (Sun @code{acc}) (symbol type @code{N_FUN}).
3097 Static Procedure; see @ref{Procedures}.
3100 Register parameter; see @ref{Register Parameters}.
3103 Register variable; see @ref{Register Variables}.
3106 File scope variable; see @ref{Statics}.
3109 Type name; see @ref{Typedefs}.
3112 Enumeration, structure, or union tag; see @ref{Typedefs}.
3115 Parameter passed by reference; see @ref{Reference Parameters}.
3118 Procedure scope static variable; see @ref{Statics}.
3121 Conformant array; see @ref{Conformant Arrays}.
3124 Function return variable; see @ref{Parameters}.
3127 @node Type Descriptors
3128 @appendix Table of Type Descriptors
3130 The type descriptor is the character which follows the type number and
3131 an equals sign. It specifies what kind of type is being defined.
3132 @xref{String Field}, for more information about their use.
3137 Type reference; see @ref{String Field}.
3140 Reference to builtin type; see @ref{Negative Type Numbers}.
3143 Method (C++); see @ref{Cplusplus}.
3146 Pointer; see @ref{Miscellaneous Types}.
3152 Type Attributes (AIX); see @ref{String Field}. Member (class and variable)
3153 type (GNU C++); see @ref{Cplusplus}.
3156 Array; see @ref{Arrays}.
3159 Open array; see @ref{Arrays}.
3162 Pascal space type (AIX); see @ref{Miscellaneous Types}. Builtin integer
3163 type (Sun); see @ref{Builtin Type Descriptors}.
3166 Volatile-qualified type; see @ref{Miscellaneous Types}.
3169 Complex builtin type; see @ref{Builtin Type Descriptors}.
3172 COBOL Picture type. See AIX documentation for details.
3175 File type; see @ref{Miscellaneous Types}.
3178 N-dimensional dynamic array; see @ref{Arrays}.
3181 Enumeration type; see @ref{Enumerations}.
3184 N-dimensional subarray; see @ref{Arrays}.
3187 Function type; see @ref{Function Types}.
3190 Pascal function parameter; see @ref{Function Types}
3193 Builtin floating point type; see @ref{Builtin Type Descriptors}.
3196 COBOL Group. See AIX documentation for details.
3199 Imported type; see @ref{Cross-References}.
3202 Const-qualified type; see @ref{Miscellaneous Types}.
3205 COBOL File Descriptor. See AIX documentation for details.
3208 Multiple instance type; see @ref{Miscellaneous Types}.
3211 String type; see @ref{Strings}.
3214 Stringptr; see @ref{Strings}.
3217 Opaque type; see @ref{Typedefs}.
3220 Procedure; see @ref{Function Types}.
3223 Packed array; see @ref{Arrays}.
3226 Range type; see @ref{Subranges}.
3229 Builtin floating type; see @ref{Builtin Type Descriptors} (Sun). Pascal
3230 subroutine parameter; see @ref{Function Types} (AIX). Detecting this
3231 conflict is possible with careful parsing (hint: a Pascal subroutine
3232 parameter type will always contain a comma, and a builtin type
3233 descriptor never will).
3236 Structure type; see @ref{Structures}.
3239 Set type; see @ref{Miscellaneous Types}.
3242 Union; see @ref{Unions}.
3245 Variant record. This is a Pascal and Modula-2 feature which is like a
3246 union within a struct in C. See AIX documentation for details.
3249 Wide character; see @ref{Builtin Type Descriptors}.
3252 Cross-reference; see @ref{Cross-References}.
3255 gstring; see @ref{Strings}.
3258 @node Expanded Reference
3259 @appendix Expanded Reference by Stab Type
3261 @c FIXME: This appendix should go away; see N_PSYM or N_SO for an example.
3263 For a full list of stab types, and cross-references to where they are
3264 described, see @ref{Stab Types}. This appendix just duplicates certain
3265 information from the main body of this document; eventually the
3266 information will all be in one place.
3270 The first line is the symbol type (see @file{include/aout/stab.def}).
3272 The second line describes the language constructs the symbol type
3275 The third line is the stab format with the significant stab fields
3276 named and the rest NIL.
3278 Subsequent lines expand upon the meaning and possible values for each
3279 significant stab field. @samp{#} stands in for the type descriptor.
3281 Finally, any further information.
3284 * N_PC:: Pascal global symbol
3285 * N_NSYMS:: Number of symbols
3286 * N_NOMAP:: No DST map
3287 * N_M2C:: Modula-2 compilation unit
3288 * N_BROWS:: Path to .cb file for Sun source code browser
3289 * N_DEFD:: GNU Modula2 definition module dependency
3290 * N_EHDECL:: GNU C++ exception variable
3291 * N_MOD2:: Modula2 information "for imc"
3292 * N_CATCH:: GNU C++ "catch" clause
3293 * N_SSYM:: Structure or union element
3294 * N_ENTRY:: Alternate entry point
3295 * N_SCOPE:: Modula2 scope information (Sun only)
3296 * Gould:: non-base register symbols used on Gould systems
3297 * N_LENG:: Length of preceding entry
3303 @deffn @code{.stabs} N_PC
3305 Global symbol (for Pascal).
3308 "name" -> "symbol_name" <<?>>
3309 value -> supposedly the line number (stab.def is skeptical)
3313 @file{stabdump.c} says:
3315 global pascal symbol: name,,0,subtype,line
3323 @deffn @code{.stabn} N_NSYMS
3325 Number of symbols (according to Ultrix V4.0).
3328 0, files,,funcs,lines (stab.def)
3335 @deffn @code{.stabs} N_NOMAP
3337 No DST map for symbol (according to Ultrix V4.0). I think this means a
3338 variable has been optimized out.
3341 name, ,0,type,ignored (stab.def)
3348 @deffn @code{.stabs} N_M2C
3350 Modula-2 compilation unit.
3353 "string" -> "unit_name,unit_time_stamp[,code_time_stamp]"
3355 value -> 0 (main unit)
3363 @deffn @code{.stabs} N_BROWS
3365 Sun source code browser, path to @file{.cb} file
3368 "path to associated @file{.cb} file"
3370 Note: N_BROWS has the same value as N_BSLINE.
3376 @deffn @code{.stabn} N_DEFD
3378 GNU Modula2 definition module dependency.
3380 GNU Modula-2 definition module dependency. The value is the
3381 modification time of the definition file. The other field is non-zero
3382 if it is imported with the GNU M2 keyword @code{%INITIALIZE}. Perhaps
3383 @code{N_M2C} can be used if there are enough empty fields?
3389 @deffn @code{.stabs} N_EHDECL
3391 GNU C++ exception variable <<?>>.
3393 "@var{string} is variable name"
3395 Note: conflicts with @code{N_MOD2}.
3401 @deffn @code{.stab?} N_MOD2
3403 Modula2 info "for imc" (according to Ultrix V4.0)
3405 Note: conflicts with @code{N_EHDECL} <<?>>
3411 @deffn @code{.stabn} N_CATCH
3413 GNU C++ @code{catch} clause
3415 GNU C++ @code{catch} clause. The value is its address. The desc field
3416 is nonzero if this entry is immediately followed by a @code{CAUGHT} stab
3417 saying what exception was caught. Multiple @code{CAUGHT} stabs means
3418 that multiple exceptions can be caught here. If desc is 0, it means all
3419 exceptions are caught here.
3425 @deffn @code{.stabn} N_SSYM
3427 Structure or union element.
3429 The value is the offset in the structure.
3431 <<?looking at structs and unions in C I didn't see these>>
3437 @deffn @code{.stabn} N_ENTRY
3439 Alternate entry point.
3440 The value is its address.
3447 @deffn @code{.stab?} N_SCOPE
3449 Modula2 scope information (Sun linker)
3454 @section Non-base registers on Gould systems
3456 @deffn @code{.stab?} N_NBTEXT
3457 @deffnx @code{.stab?} N_NBDATA
3458 @deffnx @code{.stab?} N_NBBSS
3459 @deffnx @code{.stab?} N_NBSTS
3460 @deffnx @code{.stab?} N_NBLCS
3466 These are used on Gould systems for non-base registers syms.
3468 However, the following values are not the values used by Gould; they are
3469 the values which GNU has been documenting for these values for a long
3470 time, without actually checking what Gould uses. I include these values
3471 only because perhaps some someone actually did something with the GNU
3472 information (I hope not, why GNU knowingly assigned wrong values to
3473 these in the header file is a complete mystery to me).
3476 240 0xf0 N_NBTEXT ??
3477 242 0xf2 N_NBDATA ??
3487 @deffn @code{.stabn} N_LENG
3489 Second symbol entry containing a length-value for the preceding entry.
3490 The value is the length.
3494 @appendix Questions and Anomalies
3498 @c I think this is changed in GCC 2.4.5 to put the line number there.
3499 For GNU C stabs defining local and global variables (@code{N_LSYM} and
3500 @code{N_GSYM}), the desc field is supposed to contain the source
3501 line number on which the variable is defined. In reality the desc
3502 field is always 0. (This behavior is defined in @file{dbxout.c} and
3503 putting a line number in desc is controlled by @samp{#ifdef
3504 WINNING_GDB}, which defaults to false). GDB supposedly uses this
3505 information if you say @samp{list @var{var}}. In reality, @var{var} can
3506 be a variable defined in the program and GDB says @samp{function
3507 @var{var} not defined}.
3510 In GNU C stabs, there seems to be no way to differentiate tag types:
3511 structures, unions, and enums (symbol descriptor @samp{T}) and typedefs
3512 (symbol descriptor @samp{t}) defined at file scope from types defined locally
3513 to a procedure or other more local scope. They all use the @code{N_LSYM}
3514 stab type. Types defined at procedure scope are emited after the
3515 @code{N_RBRAC} of the preceding function and before the code of the
3516 procedure in which they are defined. This is exactly the same as
3517 types defined in the source file between the two procedure bodies.
3518 GDB overcompensates by placing all types in block #1, the block for
3519 symbols of file scope. This is true for default, @samp{-ansi} and
3520 @samp{-traditional} compiler options. (Bugs gcc/1063, gdb/1066.)
3523 What ends the procedure scope? Is it the proc block's @code{N_RBRAC} or the
3524 next @code{N_FUN}? (I believe its the first.)
3527 @c FIXME: This should go with the other stuff about global variables.
3528 Global variable stabs don't have location information. This comes
3529 from the external symbol for the same variable. The external symbol
3530 has a leading underbar on the _name of the variable and the stab does
3531 not. How do we know these two symbol table entries are talking about
3532 the same symbol when their names are different? (Answer: the debugger
3533 knows that external symbols have leading underbars).
3535 @c FIXME: This is absurdly vague; there all kinds of differences, some
3536 @c of which are the same between gnu & sun, and some of which aren't.
3537 @c In particular, I'm pretty sure GCC works with Sun dbx by default.
3539 @c Can GCC be configured to output stabs the way the Sun compiler
3540 @c does, so that their native debugging tools work? <NO?> It doesn't by
3541 @c default. GDB reads either format of stab. (GCC or SunC). How about
3545 @node XCOFF Differences
3546 @appendix Differences Between GNU Stabs in a.out and GNU Stabs in XCOFF
3548 @c FIXME: Merge *all* these into the main body of the document.
3549 The AIX/RS6000 native object file format is XCOFF with stabs. This
3550 appendix only covers those differences which are not covered in the main
3551 body of this document.
3555 BSD a.out stab types correspond to AIX XCOFF storage classes. In general
3556 the mapping is @code{N_@var{stabtype}} becomes @code{C_@var{stabtype}}.
3557 Some stab types in a.out are not supported in XCOFF; most of these use
3560 @c FIXME: Get C_* types for the block, figure out whether it is always
3561 @c used (I suspect not), explain clearly, and move to node Statics.
3562 Exception: initialised static @code{N_STSYM} and un-initialized static
3563 @code{N_LCSYM} both map to the @code{C_STSYM} storage class. But the
3564 distinction is preserved because in XCOFF @code{N_STSYM} and
3565 @code{N_LCSYM} must be emited in a named static block. Begin the block
3566 with @samp{.bs s[RW] data_section_name} for @code{N_STSYM} or @samp{.bs
3567 s bss_section_name} for @code{N_LCSYM}. End the block with @samp{.es}.
3569 @c FIXME: I think they are trying to say something about whether the
3570 @c assembler defaults the value to the location counter.
3572 If the XCOFF stab is an @code{N_FUN} (@code{C_FUN}) then follow the
3573 string field with @samp{,.} instead of just @samp{,}.
3576 I think that's it for @file{.s} file differences. They could stand to be
3577 better presented. This is just a list of what I have noticed so far.
3578 There are a @emph{lot} of differences in the information in the symbol
3579 tables of the executable and object files.
3581 Mapping of a.out stab types to XCOFF storage classes:
3584 stab type storage class
3585 -------------------------------
3621 @node Sun Differences
3622 @appendix Differences Between GNU Stabs and Sun Native Stabs
3624 @c FIXME: Merge all this stuff into the main body of the document.
3628 GNU C stabs define @emph{all} types, file or procedure scope, as
3629 @code{N_LSYM}. Sun doc talks about using @code{N_GSYM} too.
3632 Sun C stabs use type number pairs in the format
3633 (@var{file-number},@var{type-number}) where @var{file-number} is a
3634 number starting with 1 and incremented for each sub-source file in the
3635 compilation. @var{type-number} is a number starting with 1 and
3636 incremented for each new type defined in the compilation. GNU C stabs
3637 use the type number alone, with no source file number.
3641 @appendix Using Stabs With The ELF Object File Format
3643 The ELF object file format allows tools to create object files with
3644 custom sections containing any arbitrary data. To use stabs in ELF
3645 object files, the tools create two custom sections, a section named
3646 @code{.stab} which contains an array of fixed length structures, one
3647 struct per stab, and a section named @code{.stabstr} containing all the
3648 variable length strings that are referenced by stabs in the @code{.stab}
3649 section. The byte order of the stabs binary data matches the byte order
3650 of the ELF file itself, as determined from the @code{EI_DATA} field in
3651 the @code{e_ident} member of the ELF header.
3653 The first stab in the @code{.stab} section for each compilation unit is
3654 synthetic, generated entirely by the assembler, with no corresponding
3655 @code{.stab} directive as input to the assembler. This stab contains
3656 the following fields:
3660 Offset in the @code{.stabstr} section to the source filename.
3666 Unused field, always zero.
3669 Count of upcoming symbols, i.e., the number of remaining stabs for this
3673 Size of the string table fragment associated with this source file, in
3677 The @code{.stabstr} section always starts with a null byte (so that string
3678 offsets of zero reference a null string), followed by random length strings,
3679 each of which is null byte terminated.
3681 The ELF section header for the @code{.stab} section has its
3682 @code{sh_link} member set to the section number of the @code{.stabstr}
3683 section, and the @code{.stabstr} section has its ELF section
3684 header @code{sh_type} member set to @code{SHT_STRTAB} to mark it as a
3687 To keep linking fast, it is a bad idea to have the linker relocating
3688 stabs, so (except for Solaris 2.2 and earlier, see below) none of the
3689 addresses in the @code{n_value} field of the stabs are relocated by the
3690 linker. Instead they are relative to the source file (or some entity
3691 smaller than a source file, like a function). To find the address of
3692 each section corresponding to a given source file, the compiler puts out
3693 symbols giving the address of each section for a given source file.
3694 Since these are ELF (not stab) symbols, the linker can relocate them
3695 correctly. They are named @code{Bbss.bss} for the bss section,
3696 @code{Ddata.data} for the data section, and @code{Drodata.rodata} for
3697 the rodata section. For the text section, there is no such symbol. For
3698 an example of how these symbols work, @xref{ELF Transformations}. GCC
3699 does not provide these symbols; it instead relies on the stabs getting
3700 relocated, which loses for Solaris 2.3 (see below). Thus address which
3701 would normally be relative to @code{Bbss.bss}, etc., are absolute. The
3702 linker provided with Solaris 2.2 and earlier relocates stabs using
3703 relocation information from a @code{.rela.stab} section, which means
3704 that the value of an @code{N_FUN} stab in an executable is the actual
3705 address. I think this is just standard ELF relocations, as it would do
3706 for any section, rather than a special-purpose stabs hack. For Solaris
3707 2.3 and later, the linker ignores relocations for the stabs section.
3708 The value of a @code{N_FUN} stab is zero and the address of a function
3709 can be obtained from the ELF (non-stab) symbols. Sun, in reference to
3710 bug 1142109, has verified that this is intentional. Because looking
3711 things up in the ELF symbols would probably be slow, and this doesn't
3712 provide any way to deal with nested functions, it would probably be
3713 better to use a @code{Ttext.text} symbol for stabs-in-elf on non-Solaris
3714 machines, and make the address in the @code{N_FUN} relative to the
3715 @code{Ttext.text} symbol. In addition to @code{N_FUN} symbols, whether
3716 the linker relocates stabs also affects some @code{N_ROSYM},
3717 @code{N_STSYM}, and @code{N_LCSYM} symbols; see @ref{Statics}.
3719 @node Symbol Types Index
3720 @unnumbered Symbol Types Index