2 @setfilename stabs.info
9 * Stabs: (stabs). The "stabs" debugging information format.
15 This document describes the stabs debugging symbol tables.
17 Copyright 1992,1993,1994,1995,1997,1998,2000,2001
18 Free Software Foundation, Inc.
19 Contributed by Cygnus Support. Written by Julia Menapace, Jim Kingdon,
22 Permission is granted to copy, distribute and/or modify this document
23 under the terms of the GNU Free Documentation License, Version 1.1 or
24 any later version published by the Free Software Foundation; with no
25 Invariant Sections, with the Front-Cover Texts being ``A GNU Manual,''
26 and with the Back-Cover Texts as in (a) below.
28 (a) The FSF's Back-Cover Text is: ``You have freedom to copy and modify
29 this GNU Manual, like GNU software. Copies published by the Free
30 Software Foundation raise funds for GNU development.''
33 @setchapternewpage odd
36 @title The ``stabs'' debug format
37 @author Julia Menapace, Jim Kingdon, David MacKenzie
38 @author Cygnus Support
41 \def\$#1${{#1}} % Kluge: collect RCS revision info without $...$
42 \xdef\manvers{\$Revision$} % For use in headers, footers too
44 \hfill Cygnus Support\par
46 \hfill \TeX{}info \texinfoversion\par
50 @vskip 0pt plus 1filll
51 Copyright @copyright{} 1992,1993,1994,1995,1997,1998,2000,2001 Free Software Foundation, Inc.
52 Contributed by Cygnus Support.
54 Permission is granted to copy, distribute and/or modify this document
55 under the terms of the GNU Free Documentation License, Version 1.1 or
56 any later version published by the Free Software Foundation; with no
57 Invariant Sections, with the Front-Cover Texts being ``A GNU Manual,''
58 and with the Back-Cover Texts as in (a) below.
60 (a) The FSF's Back-Cover Text is: ``You have freedom to copy and modify
61 this GNU Manual, like GNU software. Copies published by the Free
62 Software Foundation raise funds for GNU development.''
68 @top The "stabs" representation of debugging information
70 This document describes the stabs debugging format.
73 * Overview:: Overview of stabs
74 * Program Structure:: Encoding of the structure of the program
75 * Constants:: Constants
77 * Types:: Type definitions
78 * Symbol Tables:: Symbol information in symbol tables
79 * Cplusplus:: Stabs specific to C++
80 * Stab Types:: Symbol types in a.out files
81 * Symbol Descriptors:: Table of symbol descriptors
82 * Type Descriptors:: Table of type descriptors
83 * Expanded Reference:: Reference information by stab type
84 * Questions:: Questions and anomalies
85 * Stab Sections:: In some object file formats, stabs are
87 * Symbol Types Index:: Index of symbolic stab symbol type names.
91 @c TeX can handle the contents at the start but makeinfo 3.12 can not
97 @chapter Overview of Stabs
99 @dfn{Stabs} refers to a format for information that describes a program
100 to a debugger. This format was apparently invented by
102 the University of California at Berkeley, for the @code{pdx} Pascal
103 debugger; the format has spread widely since then.
105 This document is one of the few published sources of documentation on
106 stabs. It is believed to be comprehensive for stabs used by C. The
107 lists of symbol descriptors (@pxref{Symbol Descriptors}) and type
108 descriptors (@pxref{Type Descriptors}) are believed to be completely
109 comprehensive. Stabs for COBOL-specific features and for variant
110 records (used by Pascal and Modula-2) are poorly documented here.
112 @c FIXME: Need to document all OS9000 stuff in GDB; see all references
113 @c to os9k_stabs in stabsread.c.
115 Other sources of information on stabs are @cite{Dbx and Dbxtool
116 Interfaces}, 2nd edition, by Sun, 1988, and @cite{AIX Version 3.2 Files
117 Reference}, Fourth Edition, September 1992, "dbx Stabstring Grammar" in
118 the a.out section, page 2-31. This document is believed to incorporate
119 the information from those two sources except where it explicitly directs
120 you to them for more information.
123 * Flow:: Overview of debugging information flow
124 * Stabs Format:: Overview of stab format
125 * String Field:: The string field
126 * C Example:: A simple example in C source
127 * Assembly Code:: The simple example at the assembly level
131 @section Overview of Debugging Information Flow
133 The GNU C compiler compiles C source in a @file{.c} file into assembly
134 language in a @file{.s} file, which the assembler translates into
135 a @file{.o} file, which the linker combines with other @file{.o} files and
136 libraries to produce an executable file.
138 With the @samp{-g} option, GCC puts in the @file{.s} file additional
139 debugging information, which is slightly transformed by the assembler
140 and linker, and carried through into the final executable. This
141 debugging information describes features of the source file like line
142 numbers, the types and scopes of variables, and function names,
143 parameters, and scopes.
145 For some object file formats, the debugging information is encapsulated
146 in assembler directives known collectively as @dfn{stab} (symbol table)
147 directives, which are interspersed with the generated code. Stabs are
148 the native format for debugging information in the a.out and XCOFF
149 object file formats. The GNU tools can also emit stabs in the COFF and
150 ECOFF object file formats.
152 The assembler adds the information from stabs to the symbol information
153 it places by default in the symbol table and the string table of the
154 @file{.o} file it is building. The linker consolidates the @file{.o}
155 files into one executable file, with one symbol table and one string
156 table. Debuggers use the symbol and string tables in the executable as
157 a source of debugging information about the program.
160 @section Overview of Stab Format
162 There are three overall formats for stab assembler directives,
163 differentiated by the first word of the stab. The name of the directive
164 describes which combination of four possible data fields follows. It is
165 either @code{.stabs} (string), @code{.stabn} (number), or @code{.stabd}
166 (dot). IBM's XCOFF assembler uses @code{.stabx} (and some other
167 directives such as @code{.file} and @code{.bi}) instead of
168 @code{.stabs}, @code{.stabn} or @code{.stabd}.
170 The overall format of each class of stab is:
173 .stabs "@var{string}",@var{type},@var{other},@var{desc},@var{value}
174 .stabn @var{type},@var{other},@var{desc},@var{value}
175 .stabd @var{type},@var{other},@var{desc}
176 .stabx "@var{string}",@var{value},@var{type},@var{sdb-type}
179 @c what is the correct term for "current file location"? My AIX
180 @c assembler manual calls it "the value of the current location counter".
181 For @code{.stabn} and @code{.stabd}, there is no @var{string} (the
182 @code{n_strx} field is zero; see @ref{Symbol Tables}). For
183 @code{.stabd}, the @var{value} field is implicit and has the value of
184 the current file location. For @code{.stabx}, the @var{sdb-type} field
185 is unused for stabs and can always be set to zero. The @var{other}
186 field is almost always unused and can be set to zero.
188 The number in the @var{type} field gives some basic information about
189 which type of stab this is (or whether it @emph{is} a stab, as opposed
190 to an ordinary symbol). Each valid type number defines a different stab
191 type; further, the stab type defines the exact interpretation of, and
192 possible values for, any remaining @var{string}, @var{desc}, or
193 @var{value} fields present in the stab. @xref{Stab Types}, for a list
194 in numeric order of the valid @var{type} field values for stab directives.
197 @section The String Field
199 For most stabs the string field holds the meat of the
200 debugging information. The flexible nature of this field
201 is what makes stabs extensible. For some stab types the string field
202 contains only a name. For other stab types the contents can be a great
205 The overall format of the string field for most stab types is:
208 "@var{name}:@var{symbol-descriptor} @var{type-information}"
211 @var{name} is the name of the symbol represented by the stab; it can
212 contain a pair of colons (@pxref{Nested Symbols}). @var{name} can be
213 omitted, which means the stab represents an unnamed object. For
214 example, @samp{:t10=*2} defines type 10 as a pointer to type 2, but does
215 not give the type a name. Omitting the @var{name} field is supported by
216 AIX dbx and GDB after about version 4.8, but not other debuggers. GCC
217 sometimes uses a single space as the name instead of omitting the name
218 altogether; apparently that is supported by most debuggers.
220 The @var{symbol-descriptor} following the @samp{:} is an alphabetic
221 character that tells more specifically what kind of symbol the stab
222 represents. If the @var{symbol-descriptor} is omitted, but type
223 information follows, then the stab represents a local variable. For a
224 list of symbol descriptors, see @ref{Symbol Descriptors}. The @samp{c}
225 symbol descriptor is an exception in that it is not followed by type
226 information. @xref{Constants}.
228 @var{type-information} is either a @var{type-number}, or
229 @samp{@var{type-number}=}. A @var{type-number} alone is a type
230 reference, referring directly to a type that has already been defined.
232 The @samp{@var{type-number}=} form is a type definition, where the
233 number represents a new type which is about to be defined. The type
234 definition may refer to other types by number, and those type numbers
235 may be followed by @samp{=} and nested definitions. Also, the Lucid
236 compiler will repeat @samp{@var{type-number}=} more than once if it
237 wants to define several type numbers at once.
239 In a type definition, if the character that follows the equals sign is
240 non-numeric then it is a @var{type-descriptor}, and tells what kind of
241 type is about to be defined. Any other values following the
242 @var{type-descriptor} vary, depending on the @var{type-descriptor}.
243 @xref{Type Descriptors}, for a list of @var{type-descriptor} values. If
244 a number follows the @samp{=} then the number is a @var{type-reference}.
245 For a full description of types, @ref{Types}.
247 A @var{type-number} is often a single number. The GNU and Sun tools
248 additionally permit a @var{type-number} to be a pair
249 (@var{file-number},@var{filetype-number}) (the parentheses appear in the
250 string, and serve to distinguish the two cases). The @var{file-number}
251 is 0 for the base source file, 1 for the first included file, 2 for the
252 next, and so on. The @var{filetype-number} is a number starting with
253 1 which is incremented for each new type defined in the file.
254 (Separating the file number and the type number permits the
255 @code{N_BINCL} optimization to succeed more often; see @ref{Include
258 There is an AIX extension for type attributes. Following the @samp{=}
259 are any number of type attributes. Each one starts with @samp{@@} and
260 ends with @samp{;}. Debuggers, including AIX's dbx and GDB 4.10, skip
261 any type attributes they do not recognize. GDB 4.9 and other versions
262 of dbx may not do this. Because of a conflict with C++
263 (@pxref{Cplusplus}), new attributes should not be defined which begin
264 with a digit, @samp{(}, or @samp{-}; GDB may be unable to distinguish
265 those from the C++ type descriptor @samp{@@}. The attributes are:
268 @item a@var{boundary}
269 @var{boundary} is an integer specifying the alignment. I assume it
270 applies to all variables of this type.
273 Pointer class (for checking). Not sure what this means, or how
274 @var{integer} is interpreted.
277 Indicate this is a packed type, meaning that structure fields or array
278 elements are placed more closely in memory, to save memory at the
282 Size in bits of a variable of this type. This is fully supported by GDB
286 Indicate that this type is a string instead of an array of characters,
287 or a bitstring instead of a set. It doesn't change the layout of the
288 data being represented, but does enable the debugger to know which type
292 Indicate that this type is a vector instead of an array. The only
293 major difference between vectors and arrays is that vectors are
294 passed by value instead of by reference (vector coprocessor extension).
298 All of this can make the string field quite long. All versions of GDB,
299 and some versions of dbx, can handle arbitrarily long strings. But many
300 versions of dbx (or assemblers or linkers, I'm not sure which)
301 cretinously limit the strings to about 80 characters, so compilers which
302 must work with such systems need to split the @code{.stabs} directive
303 into several @code{.stabs} directives. Each stab duplicates every field
304 except the string field. The string field of every stab except the last
305 is marked as continued with a backslash at the end (in the assembly code
306 this may be written as a double backslash, depending on the assembler).
307 Removing the backslashes and concatenating the string fields of each
308 stab produces the original, long string. Just to be incompatible (or so
309 they don't have to worry about what the assembler does with
310 backslashes), AIX can use @samp{?} instead of backslash.
313 @section A Simple Example in C Source
315 To get the flavor of how stabs describe source information for a C
316 program, let's look at the simple program:
321 printf("Hello world");
325 When compiled with @samp{-g}, the program above yields the following
326 @file{.s} file. Line numbers have been added to make it easier to refer
327 to parts of the @file{.s} file in the description of the stabs that
331 @section The Simple Example at the Assembly Level
333 This simple ``hello world'' example demonstrates several of the stab
334 types used to describe C language source files.
338 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
339 3 .stabs "hello.c",100,0,0,Ltext0
342 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
343 7 .stabs "char:t2=r2;0;127;",128,0,0,0
344 8 .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
345 9 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
346 10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
347 11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
348 12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
349 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
350 14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
351 15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
352 16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
353 17 .stabs "float:t12=r1;4;0;",128,0,0,0
354 18 .stabs "double:t13=r1;8;0;",128,0,0,0
355 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
356 20 .stabs "void:t15=15",128,0,0,0
359 23 .ascii "Hello, world!\12\0"
374 38 sethi %hi(LC0),%o1
375 39 or %o1,%lo(LC0),%o0
386 50 .stabs "main:F1",36,0,0,_main
387 51 .stabn 192,0,0,LBB2
388 52 .stabn 224,0,0,LBE2
391 @node Program Structure
392 @chapter Encoding the Structure of the Program
394 The elements of the program structure that stabs encode include the name
395 of the main function, the names of the source and include files, the
396 line numbers, procedure names and types, and the beginnings and ends of
400 * Main Program:: Indicate what the main program is
401 * Source Files:: The path and name of the source file
402 * Include Files:: Names of include files
405 * Nested Procedures::
407 * Alternate Entry Points:: Entering procedures except at the beginning.
411 @section Main Program
414 Most languages allow the main program to have any name. The
415 @code{N_MAIN} stab type tells the debugger the name that is used in this
416 program. Only the string field is significant; it is the name of
417 a function which is the main program. Most C compilers do not use this
418 stab (they expect the debugger to assume that the name is @code{main}),
419 but some C compilers emit an @code{N_MAIN} stab for the @code{main}
420 function. I'm not sure how XCOFF handles this.
423 @section Paths and Names of the Source Files
426 Before any other stabs occur, there must be a stab specifying the source
427 file. This information is contained in a symbol of stab type
428 @code{N_SO}; the string field contains the name of the file. The
429 value of the symbol is the start address of the portion of the
430 text section corresponding to that file.
432 With the Sun Solaris2 compiler, the desc field contains a
433 source-language code.
434 @c Do the debuggers use it? What are the codes? -djm
436 Some compilers (for example, GCC2 and SunOS4 @file{/bin/cc}) also
437 include the directory in which the source was compiled, in a second
438 @code{N_SO} symbol preceding the one containing the file name. This
439 symbol can be distinguished by the fact that it ends in a slash. Code
440 from the @code{cfront} C++ compiler can have additional @code{N_SO} symbols for
441 nonexistent source files after the @code{N_SO} for the real source file;
442 these are believed to contain no useful information.
447 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0 # @r{100 is N_SO}
448 .stabs "hello.c",100,0,0,Ltext0
454 Instead of @code{N_SO} symbols, XCOFF uses a @code{.file} assembler
455 directive which assembles to a @code{C_FILE} symbol; explaining this in
456 detail is outside the scope of this document.
458 @c FIXME: Exactly when should the empty N_SO be used? Why?
459 If it is useful to indicate the end of a source file, this is done with
460 an @code{N_SO} symbol with an empty string for the name. The value is
461 the address of the end of the text section for the file. For some
462 systems, there is no indication of the end of a source file, and you
463 just need to figure it ended when you see an @code{N_SO} for a different
464 source file, or a symbol ending in @code{.o} (which at least some
465 linkers insert to mark the start of a new @code{.o} file).
468 @section Names of Include Files
470 There are several schemes for dealing with include files: the
471 traditional @code{N_SOL} approach, Sun's @code{N_BINCL} approach, and the
472 XCOFF @code{C_BINCL} approach (which despite the similar name has little in
473 common with @code{N_BINCL}).
476 An @code{N_SOL} symbol specifies which include file subsequent symbols
477 refer to. The string field is the name of the file and the value is the
478 text address corresponding to the end of the previous include file and
479 the start of this one. To specify the main source file again, use an
480 @code{N_SOL} symbol with the name of the main source file.
485 The @code{N_BINCL} approach works as follows. An @code{N_BINCL} symbol
486 specifies the start of an include file. In an object file, only the
487 string is significant; the linker puts data into some of the other
488 fields. The end of the include file is marked by an @code{N_EINCL}
489 symbol (which has no string field). In an object file, there is no
490 significant data in the @code{N_EINCL} symbol. @code{N_BINCL} and
491 @code{N_EINCL} can be nested.
493 If the linker detects that two source files have identical stabs between
494 an @code{N_BINCL} and @code{N_EINCL} pair (as will generally be the case
495 for a header file), then it only puts out the stabs once. Each
496 additional occurrence is replaced by an @code{N_EXCL} symbol. I believe
497 the GNU linker and the Sun (both SunOS4 and Solaris) linker are the only
498 ones which supports this feature.
500 A linker which supports this feature will set the value of a
501 @code{N_BINCL} symbol to the total of all the characters in the stabs
502 strings included in the header file, omitting any file numbers. The
503 value of an @code{N_EXCL} symbol is the same as the value of the
504 @code{N_BINCL} symbol it replaces. This information can be used to
505 match up @code{N_EXCL} and @code{N_BINCL} symbols which have the same
506 filename. The @code{N_EINCL} value, and the values of the other and
507 description fields for all three, appear to always be zero.
511 For the start of an include file in XCOFF, use the @file{.bi} assembler
512 directive, which generates a @code{C_BINCL} symbol. A @file{.ei}
513 directive, which generates a @code{C_EINCL} symbol, denotes the end of
514 the include file. Both directives are followed by the name of the
515 source file in quotes, which becomes the string for the symbol.
516 The value of each symbol, produced automatically by the assembler
517 and linker, is the offset into the executable of the beginning
518 (inclusive, as you'd expect) or end (inclusive, as you would not expect)
519 of the portion of the COFF line table that corresponds to this include
520 file. @code{C_BINCL} and @code{C_EINCL} do not nest.
523 @section Line Numbers
526 An @code{N_SLINE} symbol represents the start of a source line. The
527 desc field contains the line number and the value contains the code
528 address for the start of that source line. On most machines the address
529 is absolute; for stabs in sections (@pxref{Stab Sections}), it is
530 relative to the function in which the @code{N_SLINE} symbol occurs.
534 GNU documents @code{N_DSLINE} and @code{N_BSLINE} symbols for line
535 numbers in the data or bss segments, respectively. They are identical
536 to @code{N_SLINE} but are relocated differently by the linker. They
537 were intended to be used to describe the source location of a variable
538 declaration, but I believe that GCC2 actually puts the line number in
539 the desc field of the stab for the variable itself. GDB has been
540 ignoring these symbols (unless they contain a string field) since
543 For single source lines that generate discontiguous code, such as flow
544 of control statements, there may be more than one line number entry for
545 the same source line. In this case there is a line number entry at the
546 start of each code range, each with the same line number.
548 XCOFF does not use stabs for line numbers. Instead, it uses COFF line
549 numbers (which are outside the scope of this document). Standard COFF
550 line numbers cannot deal with include files, but in XCOFF this is fixed
551 with the @code{C_BINCL} method of marking include files (@pxref{Include
557 @findex N_FUN, for functions
559 @findex N_STSYM, for functions (Sun acc)
560 @findex N_GSYM, for functions (Sun acc)
561 All of the following stabs normally use the @code{N_FUN} symbol type.
562 However, Sun's @code{acc} compiler on SunOS4 uses @code{N_GSYM} and
563 @code{N_STSYM}, which means that the value of the stab for the function
564 is useless and the debugger must get the address of the function from
565 the non-stab symbols instead. On systems where non-stab symbols have
566 leading underscores, the stabs will lack underscores and the debugger
567 needs to know about the leading underscore to match up the stab and the
568 non-stab symbol. BSD Fortran is said to use @code{N_FNAME} with the
569 same restriction; the value of the symbol is not useful (I'm not sure it
570 really does use this, because GDB doesn't handle this and no one has
574 A function is represented by an @samp{F} symbol descriptor for a global
575 (extern) function, and @samp{f} for a static (local) function. For
576 a.out, the value of the symbol is the address of the start of the
577 function; it is already relocated. For stabs in ELF, the SunPRO
578 compiler version 2.0.1 and GCC put out an address which gets relocated
579 by the linker. In a future release SunPRO is planning to put out zero,
580 in which case the address can be found from the ELF (non-stab) symbol.
581 Because looking things up in the ELF symbols would probably be slow, I'm
582 not sure how to find which symbol of that name is the right one, and
583 this doesn't provide any way to deal with nested functions, it would
584 probably be better to make the value of the stab an address relative to
585 the start of the file, or just absolute. See @ref{ELF Linker
586 Relocation} for more information on linker relocation of stabs in ELF
587 files. For XCOFF, the stab uses the @code{C_FUN} storage class and the
588 value of the stab is meaningless; the address of the function can be
589 found from the csect symbol (XTY_LD/XMC_PR).
591 The type information of the stab represents the return type of the
592 function; thus @samp{foo:f5} means that foo is a function returning type
593 5. There is no need to try to get the line number of the start of the
594 function from the stab for the function; it is in the next
595 @code{N_SLINE} symbol.
597 @c FIXME: verify whether the "I suspect" below is true or not.
598 Some compilers (such as Sun's Solaris compiler) support an extension for
599 specifying the types of the arguments. I suspect this extension is not
600 used for old (non-prototyped) function definitions in C. If the
601 extension is in use, the type information of the stab for the function
602 is followed by type information for each argument, with each argument
603 preceded by @samp{;}. An argument type of 0 means that additional
604 arguments are being passed, whose types and number may vary (@samp{...}
605 in ANSI C). GDB has tolerated this extension (parsed the syntax, if not
606 necessarily used the information) since at least version 4.8; I don't
607 know whether all versions of dbx tolerate it. The argument types given
608 here are not redundant with the symbols for the formal parameters
609 (@pxref{Parameters}); they are the types of the arguments as they are
610 passed, before any conversions might take place. For example, if a C
611 function which is declared without a prototype takes a @code{float}
612 argument, the value is passed as a @code{double} but then converted to a
613 @code{float}. Debuggers need to use the types given in the arguments
614 when printing values, but when calling the function they need to use the
615 types given in the symbol defining the function.
617 If the return type and types of arguments of a function which is defined
618 in another source file are specified (i.e., a function prototype in ANSI
619 C), traditionally compilers emit no stab; the only way for the debugger
620 to find the information is if the source file where the function is
621 defined was also compiled with debugging symbols. As an extension the
622 Solaris compiler uses symbol descriptor @samp{P} followed by the return
623 type of the function, followed by the arguments, each preceded by
624 @samp{;}, as in a stab with symbol descriptor @samp{f} or @samp{F}.
625 This use of symbol descriptor @samp{P} can be distinguished from its use
626 for register parameters (@pxref{Register Parameters}) by the fact that it has
627 symbol type @code{N_FUN}.
629 The AIX documentation also defines symbol descriptor @samp{J} as an
630 internal function. I assume this means a function nested within another
631 function. It also says symbol descriptor @samp{m} is a module in
632 Modula-2 or extended Pascal.
634 Procedures (functions which do not return values) are represented as
635 functions returning the @code{void} type in C. I don't see why this couldn't
636 be used for all languages (inventing a @code{void} type for this purpose if
637 necessary), but the AIX documentation defines @samp{I}, @samp{P}, and
638 @samp{Q} for internal, global, and static procedures, respectively.
639 These symbol descriptors are unusual in that they are not followed by
642 The following example shows a stab for a function @code{main} which
643 returns type number @code{1}. The @code{_main} specified for the value
644 is a reference to an assembler label which is used to fill in the start
645 address of the function.
648 .stabs "main:F1",36,0,0,_main # @r{36 is N_FUN}
651 The stab representing a procedure is located immediately following the
652 code of the procedure. This stab is in turn directly followed by a
653 group of other stabs describing elements of the procedure. These other
654 stabs describe the procedure's parameters, its block local variables, and
657 If functions can appear in different sections, then the debugger may not
658 be able to find the end of a function. Recent versions of GCC will mark
659 the end of a function with an @code{N_FUN} symbol with an empty string
660 for the name. The value is the address of the end of the current
661 function. Without such a symbol, there is no indication of the address
662 of the end of a function, and you must assume that it ended at the
663 starting address of the next function or at the end of the text section
666 @node Nested Procedures
667 @section Nested Procedures
669 For any of the symbol descriptors representing procedures, after the
670 symbol descriptor and the type information is optionally a scope
671 specifier. This consists of a comma, the name of the procedure, another
672 comma, and the name of the enclosing procedure. The first name is local
673 to the scope specified, and seems to be redundant with the name of the
674 symbol (before the @samp{:}). This feature is used by GCC, and
675 presumably Pascal, Modula-2, etc., compilers, for nested functions.
677 If procedures are nested more than one level deep, only the immediately
678 containing scope is specified. For example, this code:
690 return baz (x + 2 * y);
692 return x + bar (3 * x);
700 .stabs "baz:f1,baz,bar",36,0,0,_baz.15 # @r{36 is N_FUN}
701 .stabs "bar:f1,bar,foo",36,0,0,_bar.12
702 .stabs "foo:F1",36,0,0,_foo
705 @node Block Structure
706 @section Block Structure
710 @c For GCC 2.5.8 or so stabs-in-coff, these are absolute instead of
711 @c function relative (as documented below). But GDB has never been able
712 @c to deal with that (it had wanted them to be relative to the file, but
713 @c I just fixed that (between GDB 4.12 and 4.13)), so it is function
714 @c relative just like ELF and SOM and the below documentation.
715 The program's block structure is represented by the @code{N_LBRAC} (left
716 brace) and the @code{N_RBRAC} (right brace) stab types. The variables
717 defined inside a block precede the @code{N_LBRAC} symbol for most
718 compilers, including GCC. Other compilers, such as the Convex, Acorn
719 RISC machine, and Sun @code{acc} compilers, put the variables after the
720 @code{N_LBRAC} symbol. The values of the @code{N_LBRAC} and
721 @code{N_RBRAC} symbols are the start and end addresses of the code of
722 the block, respectively. For most machines, they are relative to the
723 starting address of this source file. For the Gould NP1, they are
724 absolute. For stabs in sections (@pxref{Stab Sections}), they are
725 relative to the function in which they occur.
727 The @code{N_LBRAC} and @code{N_RBRAC} stabs that describe the block
728 scope of a procedure are located after the @code{N_FUN} stab that
729 represents the procedure itself.
731 Sun documents the desc field of @code{N_LBRAC} and
732 @code{N_RBRAC} symbols as containing the nesting level of the block.
733 However, dbx seems to not care, and GCC always sets desc to
739 For XCOFF, block scope is indicated with @code{C_BLOCK} symbols. If the
740 name of the symbol is @samp{.bb}, then it is the beginning of the block;
741 if the name of the symbol is @samp{.be}; it is the end of the block.
743 @node Alternate Entry Points
744 @section Alternate Entry Points
748 Some languages, like Fortran, have the ability to enter procedures at
749 some place other than the beginning. One can declare an alternate entry
750 point. The @code{N_ENTRY} stab is for this; however, the Sun FORTRAN
751 compiler doesn't use it. According to AIX documentation, only the name
752 of a @code{C_ENTRY} stab is significant; the address of the alternate
753 entry point comes from the corresponding external symbol. A previous
754 revision of this document said that the value of an @code{N_ENTRY} stab
755 was the address of the alternate entry point, but I don't know the
756 source for that information.
761 The @samp{c} symbol descriptor indicates that this stab represents a
762 constant. This symbol descriptor is an exception to the general rule
763 that symbol descriptors are followed by type information. Instead, it
764 is followed by @samp{=} and one of the following:
768 Boolean constant. @var{value} is a numeric value; I assume it is 0 for
772 Character constant. @var{value} is the numeric value of the constant.
774 @item e @var{type-information} , @var{value}
775 Constant whose value can be represented as integral.
776 @var{type-information} is the type of the constant, as it would appear
777 after a symbol descriptor (@pxref{String Field}). @var{value} is the
778 numeric value of the constant. GDB 4.9 does not actually get the right
779 value if @var{value} does not fit in a host @code{int}, but it does not
780 do anything violent, and future debuggers could be extended to accept
781 integers of any size (whether unsigned or not). This constant type is
782 usually documented as being only for enumeration constants, but GDB has
783 never imposed that restriction; I don't know about other debuggers.
786 Integer constant. @var{value} is the numeric value. The type is some
787 sort of generic integer type (for GDB, a host @code{int}); to specify
788 the type explicitly, use @samp{e} instead.
791 Real constant. @var{value} is the real value, which can be @samp{INF}
792 (optionally preceded by a sign) for infinity, @samp{QNAN} for a quiet
793 NaN (not-a-number), or @samp{SNAN} for a signalling NaN. If it is a
794 normal number the format is that accepted by the C library function
798 String constant. @var{string} is a string enclosed in either @samp{'}
799 (in which case @samp{'} characters within the string are represented as
800 @samp{\'} or @samp{"} (in which case @samp{"} characters within the
801 string are represented as @samp{\"}).
803 @item S @var{type-information} , @var{elements} , @var{bits} , @var{pattern}
804 Set constant. @var{type-information} is the type of the constant, as it
805 would appear after a symbol descriptor (@pxref{String Field}).
806 @var{elements} is the number of elements in the set (does this means
807 how many bits of @var{pattern} are actually used, which would be
808 redundant with the type, or perhaps the number of bits set in
809 @var{pattern}? I don't get it), @var{bits} is the number of bits in the
810 constant (meaning it specifies the length of @var{pattern}, I think),
811 and @var{pattern} is a hexadecimal representation of the set. AIX
812 documentation refers to a limit of 32 bytes, but I see no reason why
813 this limit should exist. This form could probably be used for arbitrary
814 constants, not just sets; the only catch is that @var{pattern} should be
815 understood to be target, not host, byte order and format.
818 The boolean, character, string, and set constants are not supported by
819 GDB 4.9, but it ignores them. GDB 4.8 and earlier gave an error
820 message and refused to read symbols from the file containing the
823 The above information is followed by @samp{;}.
828 Different types of stabs describe the various ways that variables can be
829 allocated: on the stack, globally, in registers, in common blocks,
830 statically, or as arguments to a function.
833 * Stack Variables:: Variables allocated on the stack.
834 * Global Variables:: Variables used by more than one source file.
835 * Register Variables:: Variables in registers.
836 * Common Blocks:: Variables statically allocated together.
837 * Statics:: Variables local to one source file.
838 * Based Variables:: Fortran pointer based variables.
839 * Parameters:: Variables for arguments to functions.
842 @node Stack Variables
843 @section Automatic Variables Allocated on the Stack
845 If a variable's scope is local to a function and its lifetime is only as
846 long as that function executes (C calls such variables
847 @dfn{automatic}), it can be allocated in a register (@pxref{Register
848 Variables}) or on the stack.
850 @findex N_LSYM, for stack variables
852 Each variable allocated on the stack has a stab with the symbol
853 descriptor omitted. Since type information should begin with a digit,
854 @samp{-}, or @samp{(}, only those characters precluded from being used
855 for symbol descriptors. However, the Acorn RISC machine (ARM) is said
856 to get this wrong: it puts out a mere type definition here, without the
857 preceding @samp{@var{type-number}=}. This is a bad idea; there is no
858 guarantee that type descriptors are distinct from symbol descriptors.
859 Stabs for stack variables use the @code{N_LSYM} stab type, or
860 @code{C_LSYM} for XCOFF.
862 The value of the stab is the offset of the variable within the
863 local variables. On most machines this is an offset from the frame
864 pointer and is negative. The location of the stab specifies which block
865 it is defined in; see @ref{Block Structure}.
867 For example, the following C code:
877 produces the following stabs:
880 .stabs "main:F1",36,0,0,_main # @r{36 is N_FUN}
881 .stabs "x:1",128,0,0,-12 # @r{128 is N_LSYM}
882 .stabn 192,0,0,LBB2 # @r{192 is N_LBRAC}
883 .stabn 224,0,0,LBE2 # @r{224 is N_RBRAC}
886 See @ref{Procedures} for more information on the @code{N_FUN} stab, and
887 @ref{Block Structure} for more information on the @code{N_LBRAC} and
888 @code{N_RBRAC} stabs.
890 @node Global Variables
891 @section Global Variables
895 @c FIXME: verify for sure that it really is C_GSYM on XCOFF
896 A variable whose scope is not specific to just one source file is
897 represented by the @samp{G} symbol descriptor. These stabs use the
898 @code{N_GSYM} stab type (C_GSYM for XCOFF). The type information for
899 the stab (@pxref{String Field}) gives the type of the variable.
901 For example, the following source code:
908 yields the following assembly code:
911 .stabs "g_foo:G2",32,0,0,0 # @r{32 is N_GSYM}
918 The address of the variable represented by the @code{N_GSYM} is not
919 contained in the @code{N_GSYM} stab. The debugger gets this information
920 from the external symbol for the global variable. In the example above,
921 the @code{.global _g_foo} and @code{_g_foo:} lines tell the assembler to
922 produce an external symbol.
924 Some compilers, like GCC, output @code{N_GSYM} stabs only once, where
925 the variable is defined. Other compilers, like SunOS4 /bin/cc, output a
926 @code{N_GSYM} stab for each compilation unit which references the
929 @node Register Variables
930 @section Register Variables
934 @c According to an old version of this manual, AIX uses C_RPSYM instead
935 @c of C_RSYM. I am skeptical; this should be verified.
936 Register variables have their own stab type, @code{N_RSYM}
937 (@code{C_RSYM} for XCOFF), and their own symbol descriptor, @samp{r}.
938 The stab's value is the number of the register where the variable data
940 @c .stabs "name:type",N_RSYM,0,RegSize,RegNumber (Sun doc)
942 AIX defines a separate symbol descriptor @samp{d} for floating point
943 registers. This seems unnecessary; why not just just give floating
944 point registers different register numbers? I have not verified whether
945 the compiler actually uses @samp{d}.
947 If the register is explicitly allocated to a global variable, but not
951 register int g_bar asm ("%g5");
955 then the stab may be emitted at the end of the object file, with
956 the other bss symbols.
959 @section Common Blocks
961 A common block is a statically allocated section of memory which can be
962 referred to by several source files. It may contain several variables.
963 I believe Fortran is the only language with this feature.
969 A @code{N_BCOMM} stab begins a common block and an @code{N_ECOMM} stab
970 ends it. The only field that is significant in these two stabs is the
971 string, which names a normal (non-debugging) symbol that gives the
972 address of the common block. According to IBM documentation, only the
973 @code{N_BCOMM} has the name of the common block (even though their
974 compiler actually puts it both places).
978 The stabs for the members of the common block are between the
979 @code{N_BCOMM} and the @code{N_ECOMM}; the value of each stab is the
980 offset within the common block of that variable. IBM uses the
981 @code{C_ECOML} stab type, and there is a corresponding @code{N_ECOML}
982 stab type, but Sun's Fortran compiler uses @code{N_GSYM} instead. The
983 variables within a common block use the @samp{V} symbol descriptor (I
984 believe this is true of all Fortran variables). Other stabs (at least
985 type declarations using @code{C_DECL}) can also be between the
986 @code{N_BCOMM} and the @code{N_ECOMM}.
989 @section Static Variables
991 Initialized static variables are represented by the @samp{S} and
992 @samp{V} symbol descriptors. @samp{S} means file scope static, and
993 @samp{V} means procedure scope static. One exception: in XCOFF, IBM's
994 xlc compiler always uses @samp{V}, and whether it is file scope or not
995 is distinguished by whether the stab is located within a function.
997 @c This is probably not worth mentioning; it is only true on the sparc
998 @c for `double' variables which although declared const are actually in
999 @c the data segment (the text segment can't guarantee 8 byte alignment).
1001 @c 2.4.5 has a bug in that it uses @code{N_FUN}, so neither dbx nor GDB can
1002 @c find the variables)
1005 @findex N_FUN, for variables
1007 In a.out files, @code{N_STSYM} means the data section, @code{N_FUN}
1008 means the text section, and @code{N_LCSYM} means the bss section. For
1009 those systems with a read-only data section separate from the text
1010 section (Solaris), @code{N_ROSYM} means the read-only data section.
1012 For example, the source lines:
1015 static const int var_const = 5;
1016 static int var_init = 2;
1017 static int var_noinit;
1021 yield the following stabs:
1024 .stabs "var_const:S1",36,0,0,_var_const # @r{36 is N_FUN}
1026 .stabs "var_init:S1",38,0,0,_var_init # @r{38 is N_STSYM}
1028 .stabs "var_noinit:S1",40,0,0,_var_noinit # @r{40 is N_LCSYM}
1034 In XCOFF files, the stab type need not indicate the section;
1035 @code{C_STSYM} can be used for all statics. Also, each static variable
1036 is enclosed in a static block. A @code{C_BSTAT} (emitted with a
1037 @samp{.bs} assembler directive) symbol begins the static block; its
1038 value is the symbol number of the csect symbol whose value is the
1039 address of the static block, its section is the section of the variables
1040 in that static block, and its name is @samp{.bs}. A @code{C_ESTAT}
1041 (emitted with a @samp{.es} assembler directive) symbol ends the static
1042 block; its name is @samp{.es} and its value and section are ignored.
1044 In ECOFF files, the storage class is used to specify the section, so the
1045 stab type need not indicate the section.
1047 In ELF files, for the SunPRO compiler version 2.0.1, symbol descriptor
1048 @samp{S} means that the address is absolute (the linker relocates it)
1049 and symbol descriptor @samp{V} means that the address is relative to the
1050 start of the relevant section for that compilation unit. SunPRO has
1051 plans to have the linker stop relocating stabs; I suspect that their the
1052 debugger gets the address from the corresponding ELF (not stab) symbol.
1053 I'm not sure how to find which symbol of that name is the right one.
1054 The clean way to do all this would be to have a the value of a symbol
1055 descriptor @samp{S} symbol be an offset relative to the start of the
1056 file, just like everything else, but that introduces obvious
1057 compatibility problems. For more information on linker stab relocation,
1058 @xref{ELF Linker Relocation}.
1060 @node Based Variables
1061 @section Fortran Based Variables
1063 Fortran (at least, the Sun and SGI dialects of FORTRAN-77) has a feature
1064 which allows allocating arrays with @code{malloc}, but which avoids
1065 blurring the line between arrays and pointers the way that C does. In
1066 stabs such a variable uses the @samp{b} symbol descriptor.
1068 For example, the Fortran declarations
1071 real foo, foo10(10), foo10_5(10,5)
1073 pointer (foo10p, foo10)
1074 pointer (foo105p, foo10_5)
1082 foo10_5:bar3;1;5;ar3;1;10;6
1085 In this example, @code{real} is type 6 and type 3 is an integral type
1086 which is the type of the subscripts of the array (probably
1089 The @samp{b} symbol descriptor is like @samp{V} in that it denotes a
1090 statically allocated symbol whose scope is local to a function; see
1091 @xref{Statics}. The value of the symbol, instead of being the address
1092 of the variable itself, is the address of a pointer to that variable.
1093 So in the above example, the value of the @code{foo} stab is the address
1094 of a pointer to a real, the value of the @code{foo10} stab is the
1095 address of a pointer to a 10-element array of reals, and the value of
1096 the @code{foo10_5} stab is the address of a pointer to a 5-element array
1097 of 10-element arrays of reals.
1102 Formal parameters to a function are represented by a stab (or sometimes
1103 two; see below) for each parameter. The stabs are in the order in which
1104 the debugger should print the parameters (i.e., the order in which the
1105 parameters are declared in the source file). The exact form of the stab
1106 depends on how the parameter is being passed.
1110 Parameters passed on the stack use the symbol descriptor @samp{p} and
1111 the @code{N_PSYM} symbol type (or @code{C_PSYM} for XCOFF). The value
1112 of the symbol is an offset used to locate the parameter on the stack;
1113 its exact meaning is machine-dependent, but on most machines it is an
1114 offset from the frame pointer.
1116 As a simple example, the code:
1127 .stabs "main:F1",36,0,0,_main # @r{36 is N_FUN}
1128 .stabs "argc:p1",160,0,0,68 # @r{160 is N_PSYM}
1129 .stabs "argv:p20=*21=*2",160,0,0,72
1132 The type definition of @code{argv} is interesting because it contains
1133 several type definitions. Type 21 is pointer to type 2 (char) and
1134 @code{argv} (type 20) is pointer to type 21.
1136 @c FIXME: figure out what these mean and describe them coherently.
1137 The following symbol descriptors are also said to go with @code{N_PSYM}.
1138 The value of the symbol is said to be an offset from the argument
1139 pointer (I'm not sure whether this is true or not).
1143 pF Fortran function parameter
1144 X (function result variable)
1148 * Register Parameters::
1149 * Local Variable Parameters::
1150 * Reference Parameters::
1151 * Conformant Arrays::
1154 @node Register Parameters
1155 @subsection Passing Parameters in Registers
1157 If the parameter is passed in a register, then traditionally there are
1158 two symbols for each argument:
1161 .stabs "arg:p1" . . . ; N_PSYM
1162 .stabs "arg:r1" . . . ; N_RSYM
1165 Debuggers use the second one to find the value, and the first one to
1166 know that it is an argument.
1169 @findex N_RSYM, for parameters
1170 Because that approach is kind of ugly, some compilers use symbol
1171 descriptor @samp{P} or @samp{R} to indicate an argument which is in a
1172 register. Symbol type @code{C_RPSYM} is used in XCOFF and @code{N_RSYM}
1173 is used otherwise. The symbol's value is the register number. @samp{P}
1174 and @samp{R} mean the same thing; the difference is that @samp{P} is a
1175 GNU invention and @samp{R} is an IBM (XCOFF) invention. As of version
1176 4.9, GDB should handle either one.
1178 There is at least one case where GCC uses a @samp{p} and @samp{r} pair
1179 rather than @samp{P}; this is where the argument is passed in the
1180 argument list and then loaded into a register.
1182 According to the AIX documentation, symbol descriptor @samp{D} is for a
1183 parameter passed in a floating point register. This seems
1184 unnecessary---why not just use @samp{R} with a register number which
1185 indicates that it's a floating point register? I haven't verified
1186 whether the system actually does what the documentation indicates.
1188 @c FIXME: On the hppa this is for any type > 8 bytes, I think, and not
1189 @c for small structures (investigate).
1190 On the sparc and hppa, for a @samp{P} symbol whose type is a structure
1191 or union, the register contains the address of the structure. On the
1192 sparc, this is also true of a @samp{p} and @samp{r} pair (using Sun
1193 @code{cc}) or a @samp{p} symbol. However, if a (small) structure is
1194 really in a register, @samp{r} is used. And, to top it all off, on the
1195 hppa it might be a structure which was passed on the stack and loaded
1196 into a register and for which there is a @samp{p} and @samp{r} pair! I
1197 believe that symbol descriptor @samp{i} is supposed to deal with this
1198 case (it is said to mean "value parameter by reference, indirect
1199 access"; I don't know the source for this information), but I don't know
1200 details or what compilers or debuggers use it, if any (not GDB or GCC).
1201 It is not clear to me whether this case needs to be dealt with
1202 differently than parameters passed by reference (@pxref{Reference Parameters}).
1204 @node Local Variable Parameters
1205 @subsection Storing Parameters as Local Variables
1207 There is a case similar to an argument in a register, which is an
1208 argument that is actually stored as a local variable. Sometimes this
1209 happens when the argument was passed in a register and then the compiler
1210 stores it as a local variable. If possible, the compiler should claim
1211 that it's in a register, but this isn't always done.
1213 If a parameter is passed as one type and converted to a smaller type by
1214 the prologue (for example, the parameter is declared as a @code{float},
1215 but the calling conventions specify that it is passed as a
1216 @code{double}), then GCC2 (sometimes) uses a pair of symbols. The first
1217 symbol uses symbol descriptor @samp{p} and the type which is passed.
1218 The second symbol has the type and location which the parameter actually
1219 has after the prologue. For example, suppose the following C code
1220 appears with no prototypes involved:
1229 if @code{f} is passed as a double at stack offset 8, and the prologue
1230 converts it to a float in register number 0, then the stabs look like:
1233 .stabs "f:p13",160,0,3,8 # @r{160 is @code{N_PSYM}, here 13 is @code{double}}
1234 .stabs "f:r12",64,0,3,0 # @r{64 is @code{N_RSYM}, here 12 is @code{float}}
1237 In both stabs 3 is the line number where @code{f} is declared
1238 (@pxref{Line Numbers}).
1240 @findex N_LSYM, for parameter
1241 GCC, at least on the 960, has another solution to the same problem. It
1242 uses a single @samp{p} symbol descriptor for an argument which is stored
1243 as a local variable but uses @code{N_LSYM} instead of @code{N_PSYM}. In
1244 this case, the value of the symbol is an offset relative to the local
1245 variables for that function, not relative to the arguments; on some
1246 machines those are the same thing, but not on all.
1248 @c This is mostly just background info; the part that logically belongs
1249 @c here is the last sentence.
1250 On the VAX or on other machines in which the calling convention includes
1251 the number of words of arguments actually passed, the debugger (GDB at
1252 least) uses the parameter symbols to keep track of whether it needs to
1253 print nameless arguments in addition to the formal parameters which it
1254 has printed because each one has a stab. For example, in
1257 extern int fprintf (FILE *stream, char *format, @dots{});
1259 fprintf (stdout, "%d\n", x);
1262 there are stabs for @code{stream} and @code{format}. On most machines,
1263 the debugger can only print those two arguments (because it has no way
1264 of knowing that additional arguments were passed), but on the VAX or
1265 other machines with a calling convention which indicates the number of
1266 words of arguments, the debugger can print all three arguments. To do
1267 so, the parameter symbol (symbol descriptor @samp{p}) (not necessarily
1268 @samp{r} or symbol descriptor omitted symbols) needs to contain the
1269 actual type as passed (for example, @code{double} not @code{float} if it
1270 is passed as a double and converted to a float).
1272 @node Reference Parameters
1273 @subsection Passing Parameters by Reference
1275 If the parameter is passed by reference (e.g., Pascal @code{VAR}
1276 parameters), then the symbol descriptor is @samp{v} if it is in the
1277 argument list, or @samp{a} if it in a register. Other than the fact
1278 that these contain the address of the parameter rather than the
1279 parameter itself, they are identical to @samp{p} and @samp{R},
1280 respectively. I believe @samp{a} is an AIX invention; @samp{v} is
1281 supported by all stabs-using systems as far as I know.
1283 @node Conformant Arrays
1284 @subsection Passing Conformant Array Parameters
1286 @c Is this paragraph correct? It is based on piecing together patchy
1287 @c information and some guesswork
1288 Conformant arrays are a feature of Modula-2, and perhaps other
1289 languages, in which the size of an array parameter is not known to the
1290 called function until run-time. Such parameters have two stabs: a
1291 @samp{x} for the array itself, and a @samp{C}, which represents the size
1292 of the array. The value of the @samp{x} stab is the offset in the
1293 argument list where the address of the array is stored (it this right?
1294 it is a guess); the value of the @samp{C} stab is the offset in the
1295 argument list where the size of the array (in elements? in bytes?) is
1299 @chapter Defining Types
1301 The examples so far have described types as references to previously
1302 defined types, or defined in terms of subranges of or pointers to
1303 previously defined types. This chapter describes the other type
1304 descriptors that may follow the @samp{=} in a type definition.
1307 * Builtin Types:: Integers, floating point, void, etc.
1308 * Miscellaneous Types:: Pointers, sets, files, etc.
1309 * Cross-References:: Referring to a type not yet defined.
1310 * Subranges:: A type with a specific range.
1311 * Arrays:: An aggregate type of same-typed elements.
1312 * Strings:: Like an array but also has a length.
1313 * Enumerations:: Like an integer but the values have names.
1314 * Structures:: An aggregate type of different-typed elements.
1315 * Typedefs:: Giving a type a name.
1316 * Unions:: Different types sharing storage.
1321 @section Builtin Types
1323 Certain types are built in (@code{int}, @code{short}, @code{void},
1324 @code{float}, etc.); the debugger recognizes these types and knows how
1325 to handle them. Thus, don't be surprised if some of the following ways
1326 of specifying builtin types do not specify everything that a debugger
1327 would need to know about the type---in some cases they merely specify
1328 enough information to distinguish the type from other types.
1330 The traditional way to define builtin types is convoluted, so new ways
1331 have been invented to describe them. Sun's @code{acc} uses special
1332 builtin type descriptors (@samp{b} and @samp{R}), and IBM uses negative
1333 type numbers. GDB accepts all three ways, as of version 4.8; dbx just
1334 accepts the traditional builtin types and perhaps one of the other two
1335 formats. The following sections describe each of these formats.
1338 * Traditional Builtin Types:: Put on your seat belts and prepare for kludgery
1339 * Builtin Type Descriptors:: Builtin types with special type descriptors
1340 * Negative Type Numbers:: Builtin types using negative type numbers
1343 @node Traditional Builtin Types
1344 @subsection Traditional Builtin Types
1346 This is the traditional, convoluted method for defining builtin types.
1347 There are several classes of such type definitions: integer, floating
1348 point, and @code{void}.
1351 * Traditional Integer Types::
1352 * Traditional Other Types::
1355 @node Traditional Integer Types
1356 @subsubsection Traditional Integer Types
1358 Often types are defined as subranges of themselves. If the bounding values
1359 fit within an @code{int}, then they are given normally. For example:
1362 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0 # @r{128 is N_LSYM}
1363 .stabs "char:t2=r2;0;127;",128,0,0,0
1366 Builtin types can also be described as subranges of @code{int}:
1369 .stabs "unsigned short:t6=r1;0;65535;",128,0,0,0
1372 If the lower bound of a subrange is 0 and the upper bound is -1,
1373 the type is an unsigned integral type whose bounds are too
1374 big to describe in an @code{int}. Traditionally this is only used for
1375 @code{unsigned int} and @code{unsigned long}:
1378 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
1381 For larger types, GCC 2.4.5 puts out bounds in octal, with one or more
1382 leading zeroes. In this case a negative bound consists of a number
1383 which is a 1 bit (for the sign bit) followed by a 0 bit for each bit in
1384 the number (except the sign bit), and a positive bound is one which is a
1385 1 bit for each bit in the number (except possibly the sign bit). All
1386 known versions of dbx and GDB version 4 accept this (at least in the
1387 sense of not refusing to process the file), but GDB 3.5 refuses to read
1388 the whole file containing such symbols. So GCC 2.3.3 did not output the
1389 proper size for these types. As an example of octal bounds, the string
1390 fields of the stabs for 64 bit integer types look like:
1392 @c .stabs directives, etc., omitted to make it fit on the page.
1394 long int:t3=r1;001000000000000000000000;000777777777777777777777;
1395 long unsigned int:t5=r1;000000000000000000000000;001777777777777777777777;
1398 If the lower bound of a subrange is 0 and the upper bound is negative,
1399 the type is an unsigned integral type whose size in bytes is the
1400 absolute value of the upper bound. I believe this is a Convex
1401 convention for @code{unsigned long long}.
1403 If the lower bound of a subrange is negative and the upper bound is 0,
1404 the type is a signed integral type whose size in bytes is
1405 the absolute value of the lower bound. I believe this is a Convex
1406 convention for @code{long long}. To distinguish this from a legitimate
1407 subrange, the type should be a subrange of itself. I'm not sure whether
1408 this is the case for Convex.
1410 @node Traditional Other Types
1411 @subsubsection Traditional Other Types
1413 If the upper bound of a subrange is 0 and the lower bound is positive,
1414 the type is a floating point type, and the lower bound of the subrange
1415 indicates the number of bytes in the type:
1418 .stabs "float:t12=r1;4;0;",128,0,0,0
1419 .stabs "double:t13=r1;8;0;",128,0,0,0
1422 However, GCC writes @code{long double} the same way it writes
1423 @code{double}, so there is no way to distinguish.
1426 .stabs "long double:t14=r1;8;0;",128,0,0,0
1429 Complex types are defined the same way as floating-point types; there is
1430 no way to distinguish a single-precision complex from a double-precision
1431 floating-point type.
1433 The C @code{void} type is defined as itself:
1436 .stabs "void:t15=15",128,0,0,0
1439 I'm not sure how a boolean type is represented.
1441 @node Builtin Type Descriptors
1442 @subsection Defining Builtin Types Using Builtin Type Descriptors
1444 This is the method used by Sun's @code{acc} for defining builtin types.
1445 These are the type descriptors to define builtin types:
1448 @c FIXME: clean up description of width and offset, once we figure out
1450 @item b @var{signed} @var{char-flag} @var{width} ; @var{offset} ; @var{nbits} ;
1451 Define an integral type. @var{signed} is @samp{u} for unsigned or
1452 @samp{s} for signed. @var{char-flag} is @samp{c} which indicates this
1453 is a character type, or is omitted. I assume this is to distinguish an
1454 integral type from a character type of the same size, for example it
1455 might make sense to set it for the C type @code{wchar_t} so the debugger
1456 can print such variables differently (Solaris does not do this). Sun
1457 sets it on the C types @code{signed char} and @code{unsigned char} which
1458 arguably is wrong. @var{width} and @var{offset} appear to be for small
1459 objects stored in larger ones, for example a @code{short} in an
1460 @code{int} register. @var{width} is normally the number of bytes in the
1461 type. @var{offset} seems to always be zero. @var{nbits} is the number
1462 of bits in the type.
1464 Note that type descriptor @samp{b} used for builtin types conflicts with
1465 its use for Pascal space types (@pxref{Miscellaneous Types}); they can
1466 be distinguished because the character following the type descriptor
1467 will be a digit, @samp{(}, or @samp{-} for a Pascal space type, or
1468 @samp{u} or @samp{s} for a builtin type.
1471 Documented by AIX to define a wide character type, but their compiler
1472 actually uses negative type numbers (@pxref{Negative Type Numbers}).
1474 @item R @var{fp-type} ; @var{bytes} ;
1475 Define a floating point type. @var{fp-type} has one of the following values:
1479 IEEE 32-bit (single precision) floating point format.
1482 IEEE 64-bit (double precision) floating point format.
1484 @item 3 (NF_COMPLEX)
1485 @item 4 (NF_COMPLEX16)
1486 @item 5 (NF_COMPLEX32)
1487 @c "GDB source" really means @file{include/aout/stab_gnu.h}, but trying
1488 @c to put that here got an overfull hbox.
1489 These are for complex numbers. A comment in the GDB source describes
1490 them as Fortran @code{complex}, @code{double complex}, and
1491 @code{complex*16}, respectively, but what does that mean? (i.e., Single
1492 precision? Double precision?).
1494 @item 6 (NF_LDOUBLE)
1495 Long double. This should probably only be used for Sun format
1496 @code{long double}, and new codes should be used for other floating
1497 point formats (@code{NF_DOUBLE} can be used if a @code{long double} is
1498 really just an IEEE double, of course).
1501 @var{bytes} is the number of bytes occupied by the type. This allows a
1502 debugger to perform some operations with the type even if it doesn't
1503 understand @var{fp-type}.
1505 @item g @var{type-information} ; @var{nbits}
1506 Documented by AIX to define a floating type, but their compiler actually
1507 uses negative type numbers (@pxref{Negative Type Numbers}).
1509 @item c @var{type-information} ; @var{nbits}
1510 Documented by AIX to define a complex type, but their compiler actually
1511 uses negative type numbers (@pxref{Negative Type Numbers}).
1514 The C @code{void} type is defined as a signed integral type 0 bits long:
1516 .stabs "void:t19=bs0;0;0",128,0,0,0
1518 The Solaris compiler seems to omit the trailing semicolon in this case.
1519 Getting sloppy in this way is not a swift move because if a type is
1520 embedded in a more complex expression it is necessary to be able to tell
1523 I'm not sure how a boolean type is represented.
1525 @node Negative Type Numbers
1526 @subsection Negative Type Numbers
1528 This is the method used in XCOFF for defining builtin types.
1529 Since the debugger knows about the builtin types anyway, the idea of
1530 negative type numbers is simply to give a special type number which
1531 indicates the builtin type. There is no stab defining these types.
1533 There are several subtle issues with negative type numbers.
1535 One is the size of the type. A builtin type (for example the C types
1536 @code{int} or @code{long}) might have different sizes depending on
1537 compiler options, the target architecture, the ABI, etc. This issue
1538 doesn't come up for IBM tools since (so far) they just target the
1539 RS/6000; the sizes indicated below for each size are what the IBM
1540 RS/6000 tools use. To deal with differing sizes, either define separate
1541 negative type numbers for each size (which works but requires changing
1542 the debugger, and, unless you get both AIX dbx and GDB to accept the
1543 change, introduces an incompatibility), or use a type attribute
1544 (@pxref{String Field}) to define a new type with the appropriate size
1545 (which merely requires a debugger which understands type attributes,
1546 like AIX dbx or GDB). For example,
1549 .stabs "boolean:t10=@@s8;-16",128,0,0,0
1552 defines an 8-bit boolean type, and
1555 .stabs "boolean:t10=@@s64;-16",128,0,0,0
1558 defines a 64-bit boolean type.
1560 A similar issue is the format of the type. This comes up most often for
1561 floating-point types, which could have various formats (particularly
1562 extended doubles, which vary quite a bit even among IEEE systems).
1563 Again, it is best to define a new negative type number for each
1564 different format; changing the format based on the target system has
1565 various problems. One such problem is that the Alpha has both VAX and
1566 IEEE floating types. One can easily imagine one library using the VAX
1567 types and another library in the same executable using the IEEE types.
1568 Another example is that the interpretation of whether a boolean is true
1569 or false can be based on the least significant bit, most significant
1570 bit, whether it is zero, etc., and different compilers (or different
1571 options to the same compiler) might provide different kinds of boolean.
1573 The last major issue is the names of the types. The name of a given
1574 type depends @emph{only} on the negative type number given; these do not
1575 vary depending on the language, the target system, or anything else.
1576 One can always define separate type numbers---in the following list you
1577 will see for example separate @code{int} and @code{integer*4} types
1578 which are identical except for the name. But compatibility can be
1579 maintained by not inventing new negative type numbers and instead just
1580 defining a new type with a new name. For example:
1583 .stabs "CARDINAL:t10=-8",128,0,0,0
1586 Here is the list of negative type numbers. The phrase @dfn{integral
1587 type} is used to mean twos-complement (I strongly suspect that all
1588 machines which use stabs use twos-complement; most machines use
1589 twos-complement these days).
1593 @code{int}, 32 bit signed integral type.
1596 @code{char}, 8 bit type holding a character. Both GDB and dbx on AIX
1597 treat this as signed. GCC uses this type whether @code{char} is signed
1598 or not, which seems like a bad idea. The AIX compiler (@code{xlc}) seems to
1599 avoid this type; it uses -5 instead for @code{char}.
1602 @code{short}, 16 bit signed integral type.
1605 @code{long}, 32 bit signed integral type.
1608 @code{unsigned char}, 8 bit unsigned integral type.
1611 @code{signed char}, 8 bit signed integral type.
1614 @code{unsigned short}, 16 bit unsigned integral type.
1617 @code{unsigned int}, 32 bit unsigned integral type.
1620 @code{unsigned}, 32 bit unsigned integral type.
1623 @code{unsigned long}, 32 bit unsigned integral type.
1626 @code{void}, type indicating the lack of a value.
1629 @code{float}, IEEE single precision.
1632 @code{double}, IEEE double precision.
1635 @code{long double}, IEEE double precision. The compiler claims the size
1636 will increase in a future release, and for binary compatibility you have
1637 to avoid using @code{long double}. I hope when they increase it they
1638 use a new negative type number.
1641 @code{integer}. 32 bit signed integral type.
1644 @code{boolean}. 32 bit type. GDB and GCC assume that zero is false,
1645 one is true, and other values have unspecified meaning. I hope this
1646 agrees with how the IBM tools use the type.
1649 @code{short real}. IEEE single precision.
1652 @code{real}. IEEE double precision.
1655 @code{stringptr}. @xref{Strings}.
1658 @code{character}, 8 bit unsigned character type.
1661 @code{logical*1}, 8 bit type. This Fortran type has a split
1662 personality in that it is used for boolean variables, but can also be
1663 used for unsigned integers. 0 is false, 1 is true, and other values are
1667 @code{logical*2}, 16 bit type. This Fortran type has a split
1668 personality in that it is used for boolean variables, but can also be
1669 used for unsigned integers. 0 is false, 1 is true, and other values are
1673 @code{logical*4}, 32 bit type. This Fortran type has a split
1674 personality in that it is used for boolean variables, but can also be
1675 used for unsigned integers. 0 is false, 1 is true, and other values are
1679 @code{logical}, 32 bit type. This Fortran type has a split
1680 personality in that it is used for boolean variables, but can also be
1681 used for unsigned integers. 0 is false, 1 is true, and other values are
1685 @code{complex}. A complex type consisting of two IEEE single-precision
1686 floating point values.
1689 @code{complex}. A complex type consisting of two IEEE double-precision
1690 floating point values.
1693 @code{integer*1}, 8 bit signed integral type.
1696 @code{integer*2}, 16 bit signed integral type.
1699 @code{integer*4}, 32 bit signed integral type.
1702 @code{wchar}. Wide character, 16 bits wide, unsigned (what format?
1706 @code{long long}, 64 bit signed integral type.
1709 @code{unsigned long long}, 64 bit unsigned integral type.
1712 @code{logical*8}, 64 bit unsigned integral type.
1715 @code{integer*8}, 64 bit signed integral type.
1718 @node Miscellaneous Types
1719 @section Miscellaneous Types
1722 @item b @var{type-information} ; @var{bytes}
1723 Pascal space type. This is documented by IBM; what does it mean?
1725 This use of the @samp{b} type descriptor can be distinguished
1726 from its use for builtin integral types (@pxref{Builtin Type
1727 Descriptors}) because the character following the type descriptor is
1728 always a digit, @samp{(}, or @samp{-}.
1730 @item B @var{type-information}
1731 A volatile-qualified version of @var{type-information}. This is
1732 a Sun extension. References and stores to a variable with a
1733 volatile-qualified type must not be optimized or cached; they
1734 must occur as the user specifies them.
1736 @item d @var{type-information}
1737 File of type @var{type-information}. As far as I know this is only used
1740 @item k @var{type-information}
1741 A const-qualified version of @var{type-information}. This is a Sun
1742 extension. A variable with a const-qualified type cannot be modified.
1744 @item M @var{type-information} ; @var{length}
1745 Multiple instance type. The type seems to composed of @var{length}
1746 repetitions of @var{type-information}, for example @code{character*3} is
1747 represented by @samp{M-2;3}, where @samp{-2} is a reference to a
1748 character type (@pxref{Negative Type Numbers}). I'm not sure how this
1749 differs from an array. This appears to be a Fortran feature.
1750 @var{length} is a bound, like those in range types; see @ref{Subranges}.
1752 @item S @var{type-information}
1753 Pascal set type. @var{type-information} must be a small type such as an
1754 enumeration or a subrange, and the type is a bitmask whose length is
1755 specified by the number of elements in @var{type-information}.
1757 In CHILL, if it is a bitstring instead of a set, also use the @samp{S}
1758 type attribute (@pxref{String Field}).
1760 @item * @var{type-information}
1761 Pointer to @var{type-information}.
1764 @node Cross-References
1765 @section Cross-References to Other Types
1767 A type can be used before it is defined; one common way to deal with
1768 that situation is just to use a type reference to a type which has not
1771 Another way is with the @samp{x} type descriptor, which is followed by
1772 @samp{s} for a structure tag, @samp{u} for a union tag, or @samp{e} for
1773 a enumerator tag, followed by the name of the tag, followed by @samp{:}.
1774 If the name contains @samp{::} between a @samp{<} and @samp{>} pair (for
1775 C++ templates), such a @samp{::} does not end the name---only a single
1776 @samp{:} ends the name; see @ref{Nested Symbols}.
1778 For example, the following C declarations:
1789 .stabs "bar:G16=*17=xsfoo:",32,0,0,0
1792 Not all debuggers support the @samp{x} type descriptor, so on some
1793 machines GCC does not use it. I believe that for the above example it
1794 would just emit a reference to type 17 and never define it, but I
1795 haven't verified that.
1797 Modula-2 imported types, at least on AIX, use the @samp{i} type
1798 descriptor, which is followed by the name of the module from which the
1799 type is imported, followed by @samp{:}, followed by the name of the
1800 type. There is then optionally a comma followed by type information for
1801 the type. This differs from merely naming the type (@pxref{Typedefs}) in
1802 that it identifies the module; I don't understand whether the name of
1803 the type given here is always just the same as the name we are giving
1804 it, or whether this type descriptor is used with a nameless stab
1805 (@pxref{String Field}), or what. The symbol ends with @samp{;}.
1808 @section Subrange Types
1810 The @samp{r} type descriptor defines a type as a subrange of another
1811 type. It is followed by type information for the type of which it is a
1812 subrange, a semicolon, an integral lower bound, a semicolon, an
1813 integral upper bound, and a semicolon. The AIX documentation does not
1814 specify the trailing semicolon, in an effort to specify array indexes
1815 more cleanly, but a subrange which is not an array index has always
1816 included a trailing semicolon (@pxref{Arrays}).
1818 Instead of an integer, either bound can be one of the following:
1821 @item A @var{offset}
1822 The bound is passed by reference on the stack at offset @var{offset}
1823 from the argument list. @xref{Parameters}, for more information on such
1826 @item T @var{offset}
1827 The bound is passed by value on the stack at offset @var{offset} from
1830 @item a @var{register-number}
1831 The bound is passed by reference in register number
1832 @var{register-number}.
1834 @item t @var{register-number}
1835 The bound is passed by value in register number @var{register-number}.
1841 Subranges are also used for builtin types; see @ref{Traditional Builtin Types}.
1844 @section Array Types
1846 Arrays use the @samp{a} type descriptor. Following the type descriptor
1847 is the type of the index and the type of the array elements. If the
1848 index type is a range type, it ends in a semicolon; otherwise
1849 (for example, if it is a type reference), there does not
1850 appear to be any way to tell where the types are separated. In an
1851 effort to clean up this mess, IBM documents the two types as being
1852 separated by a semicolon, and a range type as not ending in a semicolon
1853 (but this is not right for range types which are not array indexes,
1854 @pxref{Subranges}). I think probably the best solution is to specify
1855 that a semicolon ends a range type, and that the index type and element
1856 type of an array are separated by a semicolon, but that if the index
1857 type is a range type, the extra semicolon can be omitted. GDB (at least
1858 through version 4.9) doesn't support any kind of index type other than a
1859 range anyway; I'm not sure about dbx.
1861 It is well established, and widely used, that the type of the index,
1862 unlike most types found in the stabs, is merely a type definition, not
1863 type information (@pxref{String Field}) (that is, it need not start with
1864 @samp{@var{type-number}=} if it is defining a new type). According to a
1865 comment in GDB, this is also true of the type of the array elements; it
1866 gives @samp{ar1;1;10;ar1;1;10;4} as a legitimate way to express a two
1867 dimensional array. According to AIX documentation, the element type
1868 must be type information. GDB accepts either.
1870 The type of the index is often a range type, expressed as the type
1871 descriptor @samp{r} and some parameters. It defines the size of the
1872 array. In the example below, the range @samp{r1;0;2;} defines an index
1873 type which is a subrange of type 1 (integer), with a lower bound of 0
1874 and an upper bound of 2. This defines the valid range of subscripts of
1875 a three-element C array.
1877 For example, the definition:
1880 char char_vec[3] = @{'a','b','c'@};
1884 produces the output:
1887 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
1896 If an array is @dfn{packed}, the elements are spaced more
1897 closely than normal, saving memory at the expense of speed. For
1898 example, an array of 3-byte objects might, if unpacked, have each
1899 element aligned on a 4-byte boundary, but if packed, have no padding.
1900 One way to specify that something is packed is with type attributes
1901 (@pxref{String Field}). In the case of arrays, another is to use the
1902 @samp{P} type descriptor instead of @samp{a}. Other than specifying a
1903 packed array, @samp{P} is identical to @samp{a}.
1905 @c FIXME-what is it? A pointer?
1906 An open array is represented by the @samp{A} type descriptor followed by
1907 type information specifying the type of the array elements.
1909 @c FIXME: what is the format of this type? A pointer to a vector of pointers?
1910 An N-dimensional dynamic array is represented by
1913 D @var{dimensions} ; @var{type-information}
1916 @c Does dimensions really have this meaning? The AIX documentation
1918 @var{dimensions} is the number of dimensions; @var{type-information}
1919 specifies the type of the array elements.
1921 @c FIXME: what is the format of this type? A pointer to some offsets in
1923 A subarray of an N-dimensional array is represented by
1926 E @var{dimensions} ; @var{type-information}
1929 @c Does dimensions really have this meaning? The AIX documentation
1931 @var{dimensions} is the number of dimensions; @var{type-information}
1932 specifies the type of the array elements.
1937 Some languages, like C or the original Pascal, do not have string types,
1938 they just have related things like arrays of characters. But most
1939 Pascals and various other languages have string types, which are
1940 indicated as follows:
1943 @item n @var{type-information} ; @var{bytes}
1944 @var{bytes} is the maximum length. I'm not sure what
1945 @var{type-information} is; I suspect that it means that this is a string
1946 of @var{type-information} (thus allowing a string of integers, a string
1947 of wide characters, etc., as well as a string of characters). Not sure
1948 what the format of this type is. This is an AIX feature.
1950 @item z @var{type-information} ; @var{bytes}
1951 Just like @samp{n} except that this is a gstring, not an ordinary
1952 string. I don't know the difference.
1955 Pascal Stringptr. What is this? This is an AIX feature.
1958 Languages, such as CHILL which have a string type which is basically
1959 just an array of characters use the @samp{S} type attribute
1960 (@pxref{String Field}).
1963 @section Enumerations
1965 Enumerations are defined with the @samp{e} type descriptor.
1967 @c FIXME: Where does this information properly go? Perhaps it is
1968 @c redundant with something we already explain.
1969 The source line below declares an enumeration type at file scope.
1970 The type definition is located after the @code{N_RBRAC} that marks the end of
1971 the previous procedure's block scope, and before the @code{N_FUN} that marks
1972 the beginning of the next procedure's block scope. Therefore it does not
1973 describe a block local symbol, but a file local one.
1978 enum e_places @{first,second=3,last@};
1982 generates the following stab:
1985 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
1988 The symbol descriptor (@samp{T}) says that the stab describes a
1989 structure, enumeration, or union tag. The type descriptor @samp{e},
1990 following the @samp{22=} of the type definition narrows it down to an
1991 enumeration type. Following the @samp{e} is a list of the elements of
1992 the enumeration. The format is @samp{@var{name}:@var{value},}. The
1993 list of elements ends with @samp{;}. The fact that @var{value} is
1994 specified as an integer can cause problems if the value is large. GCC
1995 2.5.2 tries to output it in octal in that case with a leading zero,
1996 which is probably a good thing, although GDB 4.11 supports octal only in
1997 cases where decimal is perfectly good. Negative decimal values are
1998 supported by both GDB and dbx.
2000 There is no standard way to specify the size of an enumeration type; it
2001 is determined by the architecture (normally all enumerations types are
2002 32 bits). Type attributes can be used to specify an enumeration type of
2003 another size for debuggers which support them; see @ref{String Field}.
2005 Enumeration types are unusual in that they define symbols for the
2006 enumeration values (@code{first}, @code{second}, and @code{third} in the
2007 above example), and even though these symbols are visible in the file as
2008 a whole (rather than being in a more local namespace like structure
2009 member names), they are defined in the type definition for the
2010 enumeration type rather than each having their own symbol. In order to
2011 be fast, GDB will only get symbols from such types (in its initial scan
2012 of the stabs) if the type is the first thing defined after a @samp{T} or
2013 @samp{t} symbol descriptor (the above example fulfills this
2014 requirement). If the type does not have a name, the compiler should
2015 emit it in a nameless stab (@pxref{String Field}); GCC does this.
2020 The encoding of structures in stabs can be shown with an example.
2022 The following source code declares a structure tag and defines an
2023 instance of the structure in global scope. Then a @code{typedef} equates the
2024 structure tag with a new type. Separate stabs are generated for the
2025 structure tag, the structure @code{typedef}, and the structure instance. The
2026 stabs for the tag and the @code{typedef} are emitted when the definitions are
2027 encountered. Since the structure elements are not initialized, the
2028 stab and code for the structure variable itself is located at the end
2029 of the program in the bss section.
2036 struct s_tag* s_next;
2039 typedef struct s_tag s_typedef;
2042 The structure tag has an @code{N_LSYM} stab type because, like the
2043 enumeration, the symbol has file scope. Like the enumeration, the
2044 symbol descriptor is @samp{T}, for enumeration, structure, or tag type.
2045 The type descriptor @samp{s} following the @samp{16=} of the type
2046 definition narrows the symbol type to structure.
2048 Following the @samp{s} type descriptor is the number of bytes the
2049 structure occupies, followed by a description of each structure element.
2050 The structure element descriptions are of the form
2051 @samp{@var{name}:@var{type}, @var{bit offset from the start of the
2052 struct}, @var{number of bits in the element}}.
2054 @c FIXME: phony line break. Can probably be fixed by using an example
2055 @c with fewer fields.
2058 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;
2059 s_char_vec:17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
2062 In this example, the first two structure elements are previously defined
2063 types. For these, the type following the @samp{@var{name}:} part of the
2064 element description is a simple type reference. The other two structure
2065 elements are new types. In this case there is a type definition
2066 embedded after the @samp{@var{name}:}. The type definition for the
2067 array element looks just like a type definition for a stand-alone array.
2068 The @code{s_next} field is a pointer to the same kind of structure that
2069 the field is an element of. So the definition of structure type 16
2070 contains a type definition for an element which is a pointer to type 16.
2072 If a field is a static member (this is a C++ feature in which a single
2073 variable appears to be a field of every structure of a given type) it
2074 still starts out with the field name, a colon, and the type, but then
2075 instead of a comma, bit position, comma, and bit size, there is a colon
2076 followed by the name of the variable which each such field refers to.
2078 If the structure has methods (a C++ feature), they follow the non-method
2079 fields; see @ref{Cplusplus}.
2082 @section Giving a Type a Name
2084 @findex N_LSYM, for types
2085 @findex C_DECL, for types
2086 To give a type a name, use the @samp{t} symbol descriptor. The type
2087 is specified by the type information (@pxref{String Field}) for the stab.
2091 .stabs "s_typedef:t16",128,0,0,0 # @r{128 is N_LSYM}
2094 specifies that @code{s_typedef} refers to type number 16. Such stabs
2095 have symbol type @code{N_LSYM} (or @code{C_DECL} for XCOFF). (The Sun
2096 documentation mentions using @code{N_GSYM} in some cases).
2098 If you are specifying the tag name for a structure, union, or
2099 enumeration, use the @samp{T} symbol descriptor instead. I believe C is
2100 the only language with this feature.
2102 If the type is an opaque type (I believe this is a Modula-2 feature),
2103 AIX provides a type descriptor to specify it. The type descriptor is
2104 @samp{o} and is followed by a name. I don't know what the name
2105 means---is it always the same as the name of the type, or is this type
2106 descriptor used with a nameless stab (@pxref{String Field})? There
2107 optionally follows a comma followed by type information which defines
2108 the type of this type. If omitted, a semicolon is used in place of the
2109 comma and the type information, and the type is much like a generic
2110 pointer type---it has a known size but little else about it is
2124 This code generates a stab for a union tag and a stab for a union
2125 variable. Both use the @code{N_LSYM} stab type. If a union variable is
2126 scoped locally to the procedure in which it is defined, its stab is
2127 located immediately preceding the @code{N_LBRAC} for the procedure's block
2130 The stab for the union tag, however, is located preceding the code for
2131 the procedure in which it is defined. The stab type is @code{N_LSYM}. This
2132 would seem to imply that the union type is file scope, like the struct
2133 type @code{s_tag}. This is not true. The contents and position of the stab
2134 for @code{u_type} do not convey any information about its procedure local
2137 @c FIXME: phony line break. Can probably be fixed by using an example
2138 @c with fewer fields.
2141 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
2145 The symbol descriptor @samp{T}, following the @samp{name:} means that
2146 the stab describes an enumeration, structure, or union tag. The type
2147 descriptor @samp{u}, following the @samp{23=} of the type definition,
2148 narrows it down to a union type definition. Following the @samp{u} is
2149 the number of bytes in the union. After that is a list of union element
2150 descriptions. Their format is @samp{@var{name}:@var{type}, @var{bit
2151 offset into the union}, @var{number of bytes for the element};}.
2153 The stab for the union variable is:
2156 .stabs "an_u:23",128,0,0,-20 # @r{128 is N_LSYM}
2159 @samp{-20} specifies where the variable is stored (@pxref{Stack
2162 @node Function Types
2163 @section Function Types
2165 Various types can be defined for function variables. These types are
2166 not used in defining functions (@pxref{Procedures}); they are used for
2167 things like pointers to functions.
2169 The simple, traditional, type is type descriptor @samp{f} is followed by
2170 type information for the return type of the function, followed by a
2173 This does not deal with functions for which the number and types of the
2174 parameters are part of the type, as in Modula-2 or ANSI C. AIX provides
2175 extensions to specify these, using the @samp{f}, @samp{F}, @samp{p}, and
2176 @samp{R} type descriptors.
2178 First comes the type descriptor. If it is @samp{f} or @samp{F}, this
2179 type involves a function rather than a procedure, and the type
2180 information for the return type of the function follows, followed by a
2181 comma. Then comes the number of parameters to the function and a
2182 semicolon. Then, for each parameter, there is the name of the parameter
2183 followed by a colon (this is only present for type descriptors @samp{R}
2184 and @samp{F} which represent Pascal function or procedure parameters),
2185 type information for the parameter, a comma, 0 if passed by reference or
2186 1 if passed by value, and a semicolon. The type definition ends with a
2189 For example, this variable definition:
2196 generates the following code:
2199 .stabs "g_pf:G24=*25=f1",32,0,0,0
2200 .common _g_pf,4,"bss"
2203 The variable defines a new type, 24, which is a pointer to another new
2204 type, 25, which is a function returning @code{int}.
2207 @chapter Symbol Information in Symbol Tables
2209 This chapter describes the format of symbol table entries
2210 and how stab assembler directives map to them. It also describes the
2211 transformations that the assembler and linker make on data from stabs.
2214 * Symbol Table Format::
2215 * Transformations On Symbol Tables::
2218 @node Symbol Table Format
2219 @section Symbol Table Format
2221 Each time the assembler encounters a stab directive, it puts
2222 each field of the stab into a corresponding field in a symbol table
2223 entry of its output file. If the stab contains a string field, the
2224 symbol table entry for that stab points to a string table entry
2225 containing the string data from the stab. Assembler labels become
2226 relocatable addresses. Symbol table entries in a.out have the format:
2228 @c FIXME: should refer to external, not internal.
2230 struct internal_nlist @{
2231 unsigned long n_strx; /* index into string table of name */
2232 unsigned char n_type; /* type of symbol */
2233 unsigned char n_other; /* misc info (usually empty) */
2234 unsigned short n_desc; /* description field */
2235 bfd_vma n_value; /* value of symbol */
2239 If the stab has a string, the @code{n_strx} field holds the offset in
2240 bytes of the string within the string table. The string is terminated
2241 by a NUL character. If the stab lacks a string (for example, it was
2242 produced by a @code{.stabn} or @code{.stabd} directive), the
2243 @code{n_strx} field is zero.
2245 Symbol table entries with @code{n_type} field values greater than 0x1f
2246 originated as stabs generated by the compiler (with one random
2247 exception). The other entries were placed in the symbol table of the
2248 executable by the assembler or the linker.
2250 @node Transformations On Symbol Tables
2251 @section Transformations on Symbol Tables
2253 The linker concatenates object files and does fixups of externally
2256 You can see the transformations made on stab data by the assembler and
2257 linker by examining the symbol table after each pass of the build. To
2258 do this, use @samp{nm -ap}, which dumps the symbol table, including
2259 debugging information, unsorted. For stab entries the columns are:
2260 @var{value}, @var{other}, @var{desc}, @var{type}, @var{string}. For
2261 assembler and linker symbols, the columns are: @var{value}, @var{type},
2264 The low 5 bits of the stab type tell the linker how to relocate the
2265 value of the stab. Thus for stab types like @code{N_RSYM} and
2266 @code{N_LSYM}, where the value is an offset or a register number, the
2267 low 5 bits are @code{N_ABS}, which tells the linker not to relocate the
2270 Where the value of a stab contains an assembly language label,
2271 it is transformed by each build step. The assembler turns it into a
2272 relocatable address and the linker turns it into an absolute address.
2275 * Transformations On Static Variables::
2276 * Transformations On Global Variables::
2277 * Stab Section Transformations:: For some object file formats,
2278 things are a bit different.
2281 @node Transformations On Static Variables
2282 @subsection Transformations on Static Variables
2284 This source line defines a static variable at file scope:
2287 static int s_g_repeat
2291 The following stab describes the symbol:
2294 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
2298 The assembler transforms the stab into this symbol table entry in the
2299 @file{.o} file. The location is expressed as a data segment offset.
2302 00000084 - 00 0000 STSYM s_g_repeat:S1
2306 In the symbol table entry from the executable, the linker has made the
2307 relocatable address absolute.
2310 0000e00c - 00 0000 STSYM s_g_repeat:S1
2313 @node Transformations On Global Variables
2314 @subsection Transformations on Global Variables
2316 Stabs for global variables do not contain location information. In
2317 this case, the debugger finds location information in the assembler or
2318 linker symbol table entry describing the variable. The source line:
2328 .stabs "g_foo:G2",32,0,0,0
2331 The variable is represented by two symbol table entries in the object
2332 file (see below). The first one originated as a stab. The second one
2333 is an external symbol. The upper case @samp{D} signifies that the
2334 @code{n_type} field of the symbol table contains 7, @code{N_DATA} with
2335 local linkage. The stab's value is zero since the value is not used for
2336 @code{N_GSYM} stabs. The value of the linker symbol is the relocatable
2337 address corresponding to the variable.
2340 00000000 - 00 0000 GSYM g_foo:G2
2345 These entries as transformed by the linker. The linker symbol table
2346 entry now holds an absolute address:
2349 00000000 - 00 0000 GSYM g_foo:G2
2354 @node Stab Section Transformations
2355 @subsection Transformations of Stabs in separate sections
2357 For object file formats using stabs in separate sections (@pxref{Stab
2358 Sections}), use @code{objdump --stabs} instead of @code{nm} to show the
2359 stabs in an object or executable file. @code{objdump} is a GNU utility;
2360 Sun does not provide any equivalent.
2362 The following example is for a stab whose value is an address is
2363 relative to the compilation unit (@pxref{ELF Linker Relocation}). For
2364 example, if the source line
2370 appears within a function, then the assembly language output from the
2376 .stabs "ld:V(0,3)",0x26,0,4,.L18-Ddata.data # @r{0x26 is N_STSYM}
2383 Because the value is formed by subtracting one symbol from another, the
2384 value is absolute, not relocatable, and so the object file contains
2387 Symnum n_type n_othr n_desc n_value n_strx String
2388 31 STSYM 0 4 00000004 680 ld:V(0,3)
2391 without any relocations, and the executable file also contains
2394 Symnum n_type n_othr n_desc n_value n_strx String
2395 31 STSYM 0 4 00000004 680 ld:V(0,3)
2399 @chapter GNU C++ Stabs
2402 * Class Names:: C++ class names are both tags and typedefs.
2403 * Nested Symbols:: C++ symbol names can be within other types.
2404 * Basic Cplusplus Types::
2407 * Methods:: Method definition
2408 * Method Type Descriptor:: The @samp{#} type descriptor
2409 * Member Type Descriptor:: The @samp{@@} type descriptor
2411 * Method Modifiers::
2414 * Virtual Base Classes::
2419 @section C++ Class Names
2421 In C++, a class name which is declared with @code{class}, @code{struct},
2422 or @code{union}, is not only a tag, as in C, but also a type name. Thus
2423 there should be stabs with both @samp{t} and @samp{T} symbol descriptors
2426 To save space, there is a special abbreviation for this case. If the
2427 @samp{T} symbol descriptor is followed by @samp{t}, then the stab
2428 defines both a type name and a tag.
2430 For example, the C++ code
2433 struct foo @{int x;@};
2436 can be represented as either
2439 .stabs "foo:T19=s4x:1,0,32;;",128,0,0,0 # @r{128 is N_LSYM}
2440 .stabs "foo:t19",128,0,0,0
2446 .stabs "foo:Tt19=s4x:1,0,32;;",128,0,0,0
2449 @node Nested Symbols
2450 @section Defining a Symbol Within Another Type
2452 In C++, a symbol (such as a type name) can be defined within another type.
2453 @c FIXME: Needs example.
2455 In stabs, this is sometimes represented by making the name of a symbol
2456 which contains @samp{::}. Such a pair of colons does not end the name
2457 of the symbol, the way a single colon would (@pxref{String Field}). I'm
2458 not sure how consistently used or well thought out this mechanism is.
2459 So that a pair of colons in this position always has this meaning,
2460 @samp{:} cannot be used as a symbol descriptor.
2462 For example, if the string for a stab is @samp{foo::bar::baz:t5=*6},
2463 then @code{foo::bar::baz} is the name of the symbol, @samp{t} is the
2464 symbol descriptor, and @samp{5=*6} is the type information.
2466 @node Basic Cplusplus Types
2467 @section Basic Types For C++
2469 << the examples that follow are based on a01.C >>
2472 C++ adds two more builtin types to the set defined for C. These are
2473 the unknown type and the vtable record type. The unknown type, type
2474 16, is defined in terms of itself like the void type.
2476 The vtable record type, type 17, is defined as a structure type and
2477 then as a structure tag. The structure has four fields: delta, index,
2478 pfn, and delta2. pfn is the function pointer.
2480 << In boilerplate $vtbl_ptr_type, what are the fields delta,
2481 index, and delta2 used for? >>
2483 This basic type is present in all C++ programs even if there are no
2484 virtual methods defined.
2487 .stabs "struct_name:sym_desc(type)type_def(17)=type_desc(struct)struct_bytes(8)
2488 elem_name(delta):type_ref(short int),bit_offset(0),field_bits(16);
2489 elem_name(index):type_ref(short int),bit_offset(16),field_bits(16);
2490 elem_name(pfn):type_def(18)=type_desc(ptr to)type_ref(void),
2491 bit_offset(32),field_bits(32);
2492 elem_name(delta2):type_def(short int);bit_offset(32),field_bits(16);;"
2497 .stabs "$vtbl_ptr_type:t17=s8
2498 delta:6,0,16;index:6,16,16;pfn:18=*15,32,32;delta2:6,32,16;;"
2503 .stabs "name:sym_dec(struct tag)type_ref($vtbl_ptr_type)",N_LSYM,NIL,NIL,NIL
2507 .stabs "$vtbl_ptr_type:T17",128,0,0,0
2510 @node Simple Classes
2511 @section Simple Class Definition
2513 The stabs describing C++ language features are an extension of the
2514 stabs describing C. Stabs representing C++ class types elaborate
2515 extensively on the stab format used to describe structure types in C.
2516 Stabs representing class type variables look just like stabs
2517 representing C language variables.
2519 Consider the following very simple class definition.
2525 int Ameth(int in, char other);
2529 The class @code{baseA} is represented by two stabs. The first stab describes
2530 the class as a structure type. The second stab describes a structure
2531 tag of the class type. Both stabs are of stab type @code{N_LSYM}. Since the
2532 stab is not located between an @code{N_FUN} and an @code{N_LBRAC} stab this indicates
2533 that the class is defined at file scope. If it were, then the @code{N_LSYM}
2534 would signify a local variable.
2536 A stab describing a C++ class type is similar in format to a stab
2537 describing a C struct, with each class member shown as a field in the
2538 structure. The part of the struct format describing fields is
2539 expanded to include extra information relevant to C++ class members.
2540 In addition, if the class has multiple base classes or virtual
2541 functions the struct format outside of the field parts is also
2544 In this simple example the field part of the C++ class stab
2545 representing member data looks just like the field part of a C struct
2546 stab. The section on protections describes how its format is
2547 sometimes extended for member data.
2549 The field part of a C++ class stab representing a member function
2550 differs substantially from the field part of a C struct stab. It
2551 still begins with @samp{name:} but then goes on to define a new type number
2552 for the member function, describe its return type, its argument types,
2553 its protection level, any qualifiers applied to the method definition,
2554 and whether the method is virtual or not. If the method is virtual
2555 then the method description goes on to give the vtable index of the
2556 method, and the type number of the first base class defining the
2559 When the field name is a method name it is followed by two colons rather
2560 than one. This is followed by a new type definition for the method.
2561 This is a number followed by an equal sign and the type of the method.
2562 Normally this will be a type declared using the @samp{#} type
2563 descriptor; see @ref{Method Type Descriptor}; static member functions
2564 are declared using the @samp{f} type descriptor instead; see
2565 @ref{Function Types}.
2567 The format of an overloaded operator method name differs from that of
2568 other methods. It is @samp{op$::@var{operator-name}.} where
2569 @var{operator-name} is the operator name such as @samp{+} or @samp{+=}.
2570 The name ends with a period, and any characters except the period can
2571 occur in the @var{operator-name} string.
2573 The next part of the method description represents the arguments to the
2574 method, preceded by a colon and ending with a semi-colon. The types of
2575 the arguments are expressed in the same way argument types are expressed
2576 in C++ name mangling. In this example an @code{int} and a @code{char}
2579 This is followed by a number, a letter, and an asterisk or period,
2580 followed by another semicolon. The number indicates the protections
2581 that apply to the member function. Here the 2 means public. The
2582 letter encodes any qualifier applied to the method definition. In
2583 this case, @samp{A} means that it is a normal function definition. The dot
2584 shows that the method is not virtual. The sections that follow
2585 elaborate further on these fields and describe the additional
2586 information present for virtual methods.
2590 .stabs "class_name:sym_desc(type)type_def(20)=type_desc(struct)struct_bytes(4)
2591 field_name(Adat):type(int),bit_offset(0),field_bits(32);
2593 method_name(Ameth)::type_def(21)=type_desc(method)return_type(int);
2594 :arg_types(int char);
2595 protection(public)qualifier(normal)virtual(no);;"
2600 .stabs "baseA:t20=s4Adat:1,0,32;Ameth::21=##1;:ic;2A.;;",128,0,0,0
2602 .stabs "class_name:sym_desc(struct tag)",N_LSYM,NIL,NIL,NIL
2604 .stabs "baseA:T20",128,0,0,0
2607 @node Class Instance
2608 @section Class Instance
2610 As shown above, describing even a simple C++ class definition is
2611 accomplished by massively extending the stab format used in C to
2612 describe structure types. However, once the class is defined, C stabs
2613 with no modifications can be used to describe class instances. The
2623 yields the following stab describing the class instance. It looks no
2624 different from a standard C stab describing a local variable.
2627 .stabs "name:type_ref(baseA)", N_LSYM, NIL, NIL, frame_ptr_offset
2631 .stabs "AbaseA:20",128,0,0,-20
2635 @section Method Definition
2637 The class definition shown above declares Ameth. The C++ source below
2642 baseA::Ameth(int in, char other)
2649 This method definition yields three stabs following the code of the
2650 method. One stab describes the method itself and following two describe
2651 its parameters. Although there is only one formal argument all methods
2652 have an implicit argument which is the @code{this} pointer. The @code{this}
2653 pointer is a pointer to the object on which the method was called. Note
2654 that the method name is mangled to encode the class name and argument
2655 types. Name mangling is described in the @sc{arm} (@cite{The Annotated
2656 C++ Reference Manual}, by Ellis and Stroustrup, @sc{isbn}
2657 0-201-51459-1); @file{gpcompare.texi} in Cygnus GCC distributions
2658 describes the differences between GNU mangling and @sc{arm}
2660 @c FIXME: Use @xref, especially if this is generally installed in the
2662 @c FIXME: This information should be in a net release, either of GCC or
2663 @c GDB. But gpcompare.texi doesn't seem to be in the FSF GCC.
2666 .stabs "name:symbol_descriptor(global function)return_type(int)",
2667 N_FUN, NIL, NIL, code_addr_of_method_start
2669 .stabs "Ameth__5baseAic:F1",36,0,0,_Ameth__5baseAic
2672 Here is the stab for the @code{this} pointer implicit argument. The
2673 name of the @code{this} pointer is always @code{this}. Type 19, the
2674 @code{this} pointer is defined as a pointer to type 20, @code{baseA},
2675 but a stab defining @code{baseA} has not yet been emitted. Since the
2676 compiler knows it will be emitted shortly, here it just outputs a cross
2677 reference to the undefined symbol, by prefixing the symbol name with
2681 .stabs "name:sym_desc(register param)type_def(19)=
2682 type_desc(ptr to)type_ref(baseA)=
2683 type_desc(cross-reference to)baseA:",N_RSYM,NIL,NIL,register_number
2685 .stabs "this:P19=*20=xsbaseA:",64,0,0,8
2688 The stab for the explicit integer argument looks just like a parameter
2689 to a C function. The last field of the stab is the offset from the
2690 argument pointer, which in most systems is the same as the frame
2694 .stabs "name:sym_desc(value parameter)type_ref(int)",
2695 N_PSYM,NIL,NIL,offset_from_arg_ptr
2697 .stabs "in:p1",160,0,0,72
2700 << The examples that follow are based on A1.C >>
2702 @node Method Type Descriptor
2703 @section The @samp{#} Type Descriptor
2705 This is used to describe a class method. This is a function which takes
2706 an extra argument as its first argument, for the @code{this} pointer.
2708 If the @samp{#} is immediately followed by another @samp{#}, the second
2709 one will be followed by the return type and a semicolon. The class and
2710 argument types are not specified, and must be determined by demangling
2711 the name of the method if it is available.
2713 Otherwise, the single @samp{#} is followed by the class type, a comma,
2714 the return type, a comma, and zero or more parameter types separated by
2715 commas. The list of arguments is terminated by a semicolon. In the
2716 debugging output generated by gcc, a final argument type of @code{void}
2717 indicates a method which does not take a variable number of arguments.
2718 If the final argument type of @code{void} does not appear, the method
2719 was declared with an ellipsis.
2721 Note that although such a type will normally be used to describe fields
2722 in structures, unions, or classes, for at least some versions of the
2723 compiler it can also be used in other contexts.
2725 @node Member Type Descriptor
2726 @section The @samp{@@} Type Descriptor
2728 The @samp{@@} type descriptor is used together with the @samp{*} type
2729 descriptor for a pointer-to-non-static-member-data type. It is followed
2730 by type information for the class (or union), a comma, and type
2731 information for the member data.
2733 The following C++ source:
2736 typedef int A::*int_in_a;
2739 generates the following stab:
2742 .stabs "int_in_a:t20=*21=@@19,1",128,0,0,0
2745 Note that there is a conflict between this and type attributes
2746 (@pxref{String Field}); both use type descriptor @samp{@@}.
2747 Fortunately, the @samp{@@} type descriptor used in this C++ sense always
2748 will be followed by a digit, @samp{(}, or @samp{-}, and type attributes
2749 never start with those things.
2752 @section Protections
2754 In the simple class definition shown above all member data and
2755 functions were publicly accessible. The example that follows
2756 contrasts public, protected and privately accessible fields and shows
2757 how these protections are encoded in C++ stabs.
2759 If the character following the @samp{@var{field-name}:} part of the
2760 string is @samp{/}, then the next character is the visibility. @samp{0}
2761 means private, @samp{1} means protected, and @samp{2} means public.
2762 Debuggers should ignore visibility characters they do not recognize, and
2763 assume a reasonable default (such as public) (GDB 4.11 does not, but
2764 this should be fixed in the next GDB release). If no visibility is
2765 specified the field is public. The visibility @samp{9} means that the
2766 field has been optimized out and is public (there is no way to specify
2767 an optimized out field with a private or protected visibility).
2768 Visibility @samp{9} is not supported by GDB 4.11; this should be fixed
2769 in the next GDB release.
2771 The following C++ source:
2785 generates the following stab:
2789 .stabs "vis:T19=s12priv:/01,0,32;prot:/12,32,8;pub:12,64,32;;",128,0,0,0
2792 @samp{vis:T19=s12} indicates that type number 19 is a 12 byte structure
2793 named @code{vis} The @code{priv} field has public visibility
2794 (@samp{/0}), type int (@samp{1}), and offset and size @samp{,0,32;}.
2795 The @code{prot} field has protected visibility (@samp{/1}), type char
2796 (@samp{2}) and offset and size @samp{,32,8;}. The @code{pub} field has
2797 type float (@samp{12}), and offset and size @samp{,64,32;}.
2799 Protections for member functions are signified by one digit embedded in
2800 the field part of the stab describing the method. The digit is 0 if
2801 private, 1 if protected and 2 if public. Consider the C++ class
2805 class all_methods @{
2807 int priv_meth(int in)@{return in;@};
2809 char protMeth(char in)@{return in;@};
2811 float pubMeth(float in)@{return in;@};
2815 It generates the following stab. The digit in question is to the left
2816 of an @samp{A} in each case. Notice also that in this case two symbol
2817 descriptors apply to the class name struct tag and struct type.
2820 .stabs "class_name:sym_desc(struct tag&type)type_def(21)=
2821 sym_desc(struct)struct_bytes(1)
2822 meth_name::type_def(22)=sym_desc(method)returning(int);
2823 :args(int);protection(private)modifier(normal)virtual(no);
2824 meth_name::type_def(23)=sym_desc(method)returning(char);
2825 :args(char);protection(protected)modifier(normal)virtual(no);
2826 meth_name::type_def(24)=sym_desc(method)returning(float);
2827 :args(float);protection(public)modifier(normal)virtual(no);;",
2832 .stabs "all_methods:Tt21=s1priv_meth::22=##1;:i;0A.;protMeth::23=##2;:c;1A.;
2833 pubMeth::24=##12;:f;2A.;;",128,0,0,0
2836 @node Method Modifiers
2837 @section Method Modifiers (@code{const}, @code{volatile}, @code{const volatile})
2841 In the class example described above all the methods have the normal
2842 modifier. This method modifier information is located just after the
2843 protection information for the method. This field has four possible
2844 character values. Normal methods use @samp{A}, const methods use
2845 @samp{B}, volatile methods use @samp{C}, and const volatile methods use
2846 @samp{D}. Consider the class definition below:
2851 int ConstMeth (int arg) const @{ return arg; @};
2852 char VolatileMeth (char arg) volatile @{ return arg; @};
2853 float ConstVolMeth (float arg) const volatile @{return arg; @};
2857 This class is described by the following stab:
2860 .stabs "class(A):sym_desc(struct)type_def(20)=type_desc(struct)struct_bytes(1)
2861 meth_name(ConstMeth)::type_def(21)sym_desc(method)
2862 returning(int);:arg(int);protection(public)modifier(const)virtual(no);
2863 meth_name(VolatileMeth)::type_def(22)=sym_desc(method)
2864 returning(char);:arg(char);protection(public)modifier(volatile)virt(no)
2865 meth_name(ConstVolMeth)::type_def(23)=sym_desc(method)
2866 returning(float);:arg(float);protection(public)modifier(const volatile)
2867 virtual(no);;", @dots{}
2871 .stabs "A:T20=s1ConstMeth::21=##1;:i;2B.;VolatileMeth::22=##2;:c;2C.;
2872 ConstVolMeth::23=##12;:f;2D.;;",128,0,0,0
2875 @node Virtual Methods
2876 @section Virtual Methods
2878 << The following examples are based on a4.C >>
2880 The presence of virtual methods in a class definition adds additional
2881 data to the class description. The extra data is appended to the
2882 description of the virtual method and to the end of the class
2883 description. Consider the class definition below:
2889 virtual int A_virt (int arg) @{ return arg; @};
2893 This results in the stab below describing class A. It defines a new
2894 type (20) which is an 8 byte structure. The first field of the class
2895 struct is @samp{Adat}, an integer, starting at structure offset 0 and
2898 The second field in the class struct is not explicitly defined by the
2899 C++ class definition but is implied by the fact that the class
2900 contains a virtual method. This field is the vtable pointer. The
2901 name of the vtable pointer field starts with @samp{$vf} and continues with a
2902 type reference to the class it is part of. In this example the type
2903 reference for class A is 20 so the name of its vtable pointer field is
2904 @samp{$vf20}, followed by the usual colon.
2906 Next there is a type definition for the vtable pointer type (21).
2907 This is in turn defined as a pointer to another new type (22).
2909 Type 22 is the vtable itself, which is defined as an array, indexed by
2910 a range of integers between 0 and 1, and whose elements are of type
2911 17. Type 17 was the vtable record type defined by the boilerplate C++
2912 type definitions, as shown earlier.
2914 The bit offset of the vtable pointer field is 32. The number of bits
2915 in the field are not specified when the field is a vtable pointer.
2917 Next is the method definition for the virtual member function @code{A_virt}.
2918 Its description starts out using the same format as the non-virtual
2919 member functions described above, except instead of a dot after the
2920 @samp{A} there is an asterisk, indicating that the function is virtual.
2921 Since is is virtual some addition information is appended to the end
2922 of the method description.
2924 The first number represents the vtable index of the method. This is a
2925 32 bit unsigned number with the high bit set, followed by a
2928 The second number is a type reference to the first base class in the
2929 inheritance hierarchy defining the virtual member function. In this
2930 case the class stab describes a base class so the virtual function is
2931 not overriding any other definition of the method. Therefore the
2932 reference is to the type number of the class that the stab is
2935 This is followed by three semi-colons. One marks the end of the
2936 current sub-section, one marks the end of the method field, and the
2937 third marks the end of the struct definition.
2939 For classes containing virtual functions the very last section of the
2940 string part of the stab holds a type reference to the first base
2941 class. This is preceded by @samp{~%} and followed by a final semi-colon.
2944 .stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8)
2945 field_name(Adat):type_ref(int),bit_offset(0),field_bits(32);
2946 field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)=
2947 sym_desc(array)index_type_ref(range of int from 0 to 1);
2948 elem_type_ref(vtbl elem type),
2950 meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
2951 :arg_type(int),protection(public)normal(yes)virtual(yes)
2952 vtable_index(1);class_first_defining(A);;;~%first_base(A);",
2956 @c FIXME: bogus line break.
2958 .stabs "A:t20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2959 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2963 @section Inheritance
2965 Stabs describing C++ derived classes include additional sections that
2966 describe the inheritance hierarchy of the class. A derived class stab
2967 also encodes the number of base classes. For each base class it tells
2968 if the base class is virtual or not, and if the inheritance is private
2969 or public. It also gives the offset into the object of the portion of
2970 the object corresponding to each base class.
2972 This additional information is embedded in the class stab following the
2973 number of bytes in the struct. First the number of base classes
2974 appears bracketed by an exclamation point and a comma.
2976 Then for each base type there repeats a series: a virtual character, a
2977 visibility character, a number, a comma, another number, and a
2980 The virtual character is @samp{1} if the base class is virtual and
2981 @samp{0} if not. The visibility character is @samp{2} if the derivation
2982 is public, @samp{1} if it is protected, and @samp{0} if it is private.
2983 Debuggers should ignore virtual or visibility characters they do not
2984 recognize, and assume a reasonable default (such as public and
2985 non-virtual) (GDB 4.11 does not, but this should be fixed in the next
2988 The number following the virtual and visibility characters is the offset
2989 from the start of the object to the part of the object pertaining to the
2992 After the comma, the second number is a type_descriptor for the base
2993 type. Finally a semi-colon ends the series, which repeats for each
2996 The source below defines three base classes @code{A}, @code{B}, and
2997 @code{C} and the derived class @code{D}.
3004 virtual int A_virt (int arg) @{ return arg; @};
3010 virtual int B_virt (int arg) @{return arg; @};
3016 virtual int C_virt (int arg) @{return arg; @};
3019 class D : A, virtual B, public C @{
3022 virtual int A_virt (int arg ) @{ return arg+1; @};
3023 virtual int B_virt (int arg) @{ return arg+2; @};
3024 virtual int C_virt (int arg) @{ return arg+3; @};
3025 virtual int D_virt (int arg) @{ return arg; @};
3029 Class stabs similar to the ones described earlier are generated for
3032 @c FIXME!!! the linebreaks in the following example probably make the
3033 @c examples literally unusable, but I don't know any other way to get
3034 @c them on the page.
3035 @c One solution would be to put some of the type definitions into
3036 @c separate stabs, even if that's not exactly what the compiler actually
3039 .stabs "A:T20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
3040 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
3042 .stabs "B:Tt25=s8Bdat:1,0,32;$vf25:21,32;B_virt::26=##1;
3043 :i;2A*-2147483647;25;;;~%25;",128,0,0,0
3045 .stabs "C:Tt28=s8Cdat:1,0,32;$vf28:21,32;C_virt::29=##1;
3046 :i;2A*-2147483647;28;;;~%28;",128,0,0,0
3049 In the stab describing derived class @code{D} below, the information about
3050 the derivation of this class is encoded as follows.
3053 .stabs "derived_class_name:symbol_descriptors(struct tag&type)=
3054 type_descriptor(struct)struct_bytes(32)!num_bases(3),
3055 base_virtual(no)inheritance_public(no)base_offset(0),
3056 base_class_type_ref(A);
3057 base_virtual(yes)inheritance_public(no)base_offset(NIL),
3058 base_class_type_ref(B);
3059 base_virtual(no)inheritance_public(yes)base_offset(64),
3060 base_class_type_ref(C); @dots{}
3063 @c FIXME! fake linebreaks.
3065 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:
3066 1,160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt:
3067 :32:i;2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;
3068 28;;D_virt::32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
3071 @node Virtual Base Classes
3072 @section Virtual Base Classes
3074 A derived class object consists of a concatenation in memory of the data
3075 areas defined by each base class, starting with the leftmost and ending
3076 with the rightmost in the list of base classes. The exception to this
3077 rule is for virtual inheritance. In the example above, class @code{D}
3078 inherits virtually from base class @code{B}. This means that an
3079 instance of a @code{D} object will not contain its own @code{B} part but
3080 merely a pointer to a @code{B} part, known as a virtual base pointer.
3082 In a derived class stab, the base offset part of the derivation
3083 information, described above, shows how the base class parts are
3084 ordered. The base offset for a virtual base class is always given as 0.
3085 Notice that the base offset for @code{B} is given as 0 even though
3086 @code{B} is not the first base class. The first base class @code{A}
3089 The field information part of the stab for class @code{D} describes the field
3090 which is the pointer to the virtual base class @code{B}. The vbase pointer
3091 name is @samp{$vb} followed by a type reference to the virtual base class.
3092 Since the type id for @code{B} in this example is 25, the vbase pointer name
3095 @c FIXME!! fake linebreaks below
3097 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1,
3098 160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i;
3099 2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt:
3100 :32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
3103 Following the name and a semicolon is a type reference describing the
3104 type of the virtual base class pointer, in this case 24. Type 24 was
3105 defined earlier as the type of the @code{B} class @code{this} pointer. The
3106 @code{this} pointer for a class is a pointer to the class type.
3109 .stabs "this:P24=*25=xsB:",64,0,0,8
3112 Finally the field offset part of the vbase pointer field description
3113 shows that the vbase pointer is the first field in the @code{D} object,
3114 before any data fields defined by the class. The layout of a @code{D}
3115 class object is a follows, @code{Adat} at 0, the vtable pointer for
3116 @code{A} at 32, @code{Cdat} at 64, the vtable pointer for C at 96, the
3117 virtual base pointer for @code{B} at 128, and @code{Ddat} at 160.
3120 @node Static Members
3121 @section Static Members
3123 The data area for a class is a concatenation of the space used by the
3124 data members of the class. If the class has virtual methods, a vtable
3125 pointer follows the class data. The field offset part of each field
3126 description in the class stab shows this ordering.
3128 << How is this reflected in stabs? See Cygnus bug #677 for some info. >>
3131 @appendix Table of Stab Types
3133 The following are all the possible values for the stab type field, for
3134 a.out files, in numeric order. This does not apply to XCOFF, but
3135 it does apply to stabs in sections (@pxref{Stab Sections}). Stabs in
3136 ECOFF use these values but add 0x8f300 to distinguish them from non-stab
3139 The symbolic names are defined in the file @file{include/aout/stabs.def}.
3142 * Non-Stab Symbol Types:: Types from 0 to 0x1f
3143 * Stab Symbol Types:: Types from 0x20 to 0xff
3146 @node Non-Stab Symbol Types
3147 @appendixsec Non-Stab Symbol Types
3149 The following types are used by the linker and assembler, not by stab
3150 directives. Since this document does not attempt to describe aspects of
3151 object file format other than the debugging format, no details are
3154 @c Try to get most of these to fit on a single line.
3164 File scope absolute symbol
3166 @item 0x3 N_ABS | N_EXT
3167 External absolute symbol
3170 File scope text symbol
3172 @item 0x5 N_TEXT | N_EXT
3173 External text symbol
3176 File scope data symbol
3178 @item 0x7 N_DATA | N_EXT
3179 External data symbol
3182 File scope BSS symbol
3184 @item 0x9 N_BSS | N_EXT
3188 Same as @code{N_FN}, for Sequent compilers
3191 Symbol is indirected to another symbol
3194 Common---visible after shared library dynamic link
3197 @itemx 0x15 N_SETA | N_EXT
3198 Absolute set element
3201 @itemx 0x17 N_SETT | N_EXT
3202 Text segment set element
3205 @itemx 0x19 N_SETD | N_EXT
3206 Data segment set element
3209 @itemx 0x1b N_SETB | N_EXT
3210 BSS segment set element
3213 @itemx 0x1d N_SETV | N_EXT
3214 Pointer to set vector
3216 @item 0x1e N_WARNING
3217 Print a warning message during linking
3220 File name of a @file{.o} file
3223 @node Stab Symbol Types
3224 @appendixsec Stab Symbol Types
3226 The following symbol types indicate that this is a stab. This is the
3227 full list of stab numbers, including stab types that are used in
3228 languages other than C.
3232 Global symbol; see @ref{Global Variables}.
3235 Function name (for BSD Fortran); see @ref{Procedures}.
3238 Function name (@pxref{Procedures}) or text segment variable
3242 Data segment file-scope variable; see @ref{Statics}.
3245 BSS segment file-scope variable; see @ref{Statics}.
3248 Name of main routine; see @ref{Main Program}.
3251 Variable in @code{.rodata} section; see @ref{Statics}.
3254 Global symbol (for Pascal); see @ref{N_PC}.
3257 Number of symbols (according to Ultrix V4.0); see @ref{N_NSYMS}.
3260 No DST map; see @ref{N_NOMAP}.
3262 @c FIXME: describe this solaris feature in the body of the text (see
3263 @c comments in include/aout/stab.def).
3265 Object file (Solaris2).
3267 @c See include/aout/stab.def for (a little) more info.
3269 Debugger options (Solaris2).
3272 Register variable; see @ref{Register Variables}.
3275 Modula-2 compilation unit; see @ref{N_M2C}.
3278 Line number in text segment; see @ref{Line Numbers}.
3281 Line number in data segment; see @ref{Line Numbers}.
3284 Line number in bss segment; see @ref{Line Numbers}.
3287 Sun source code browser, path to @file{.cb} file; see @ref{N_BROWS}.
3290 GNU Modula2 definition module dependency; see @ref{N_DEFD}.
3293 Function start/body/end line numbers (Solaris2).
3296 GNU C++ exception variable; see @ref{N_EHDECL}.
3299 Modula2 info "for imc" (according to Ultrix V4.0); see @ref{N_MOD2}.
3302 GNU C++ @code{catch} clause; see @ref{N_CATCH}.
3305 Structure of union element; see @ref{N_SSYM}.
3308 Last stab for module (Solaris2).
3311 Path and name of source file; see @ref{Source Files}.
3314 Stack variable (@pxref{Stack Variables}) or type (@pxref{Typedefs}).
3317 Beginning of an include file (Sun only); see @ref{Include Files}.
3320 Name of include file; see @ref{Include Files}.
3323 Parameter variable; see @ref{Parameters}.
3326 End of an include file; see @ref{Include Files}.
3329 Alternate entry point; see @ref{Alternate Entry Points}.
3332 Beginning of a lexical block; see @ref{Block Structure}.
3335 Place holder for a deleted include file; see @ref{Include Files}.
3338 Modula2 scope information (Sun linker); see @ref{N_SCOPE}.
3341 End of a lexical block; see @ref{Block Structure}.
3344 Begin named common block; see @ref{Common Blocks}.
3347 End named common block; see @ref{Common Blocks}.
3350 Member of a common block; see @ref{Common Blocks}.
3352 @c FIXME: How does this really work? Move it to main body of document.
3354 Pascal @code{with} statement: type,,0,0,offset (Solaris2).
3357 Gould non-base registers; see @ref{Gould}.
3360 Gould non-base registers; see @ref{Gould}.
3363 Gould non-base registers; see @ref{Gould}.
3366 Gould non-base registers; see @ref{Gould}.
3369 Gould non-base registers; see @ref{Gould}.
3372 @c Restore the default table indent
3377 @node Symbol Descriptors
3378 @appendix Table of Symbol Descriptors
3380 The symbol descriptor is the character which follows the colon in many
3381 stabs, and which tells what kind of stab it is. @xref{String Field},
3382 for more information about their use.
3384 @c Please keep this alphabetical
3386 @c In TeX, this looks great, digit is in italics. But makeinfo insists
3387 @c on putting it in `', not realizing that @var should override @code.
3388 @c I don't know of any way to make makeinfo do the right thing. Seems
3389 @c like a makeinfo bug to me.
3393 Variable on the stack; see @ref{Stack Variables}.
3396 C++ nested symbol; see @xref{Nested Symbols}.
3399 Parameter passed by reference in register; see @ref{Reference Parameters}.
3402 Based variable; see @ref{Based Variables}.
3405 Constant; see @ref{Constants}.
3408 Conformant array bound (Pascal, maybe other languages); @ref{Conformant
3409 Arrays}. Name of a caught exception (GNU C++). These can be
3410 distinguished because the latter uses @code{N_CATCH} and the former uses
3411 another symbol type.
3414 Floating point register variable; see @ref{Register Variables}.
3417 Parameter in floating point register; see @ref{Register Parameters}.
3420 File scope function; see @ref{Procedures}.
3423 Global function; see @ref{Procedures}.
3426 Global variable; see @ref{Global Variables}.
3429 @xref{Register Parameters}.
3432 Internal (nested) procedure; see @ref{Nested Procedures}.
3435 Internal (nested) function; see @ref{Nested Procedures}.
3438 Label name (documented by AIX, no further information known).
3441 Module; see @ref{Procedures}.
3444 Argument list parameter; see @ref{Parameters}.
3450 Fortran Function parameter; see @ref{Parameters}.
3453 Unfortunately, three separate meanings have been independently invented
3454 for this symbol descriptor. At least the GNU and Sun uses can be
3455 distinguished by the symbol type. Global Procedure (AIX) (symbol type
3456 used unknown); see @ref{Procedures}. Register parameter (GNU) (symbol
3457 type @code{N_PSYM}); see @ref{Parameters}. Prototype of function
3458 referenced by this file (Sun @code{acc}) (symbol type @code{N_FUN}).
3461 Static Procedure; see @ref{Procedures}.
3464 Register parameter; see @ref{Register Parameters}.
3467 Register variable; see @ref{Register Variables}.
3470 File scope variable; see @ref{Statics}.
3473 Local variable (OS9000).
3476 Type name; see @ref{Typedefs}.
3479 Enumeration, structure, or union tag; see @ref{Typedefs}.
3482 Parameter passed by reference; see @ref{Reference Parameters}.
3485 Procedure scope static variable; see @ref{Statics}.
3488 Conformant array; see @ref{Conformant Arrays}.
3491 Function return variable; see @ref{Parameters}.
3494 @node Type Descriptors
3495 @appendix Table of Type Descriptors
3497 The type descriptor is the character which follows the type number and
3498 an equals sign. It specifies what kind of type is being defined.
3499 @xref{String Field}, for more information about their use.
3504 Type reference; see @ref{String Field}.
3507 Reference to builtin type; see @ref{Negative Type Numbers}.
3510 Method (C++); see @ref{Method Type Descriptor}.
3513 Pointer; see @ref{Miscellaneous Types}.
3519 Type Attributes (AIX); see @ref{String Field}. Member (class and variable)
3520 type (GNU C++); see @ref{Member Type Descriptor}.
3523 Array; see @ref{Arrays}.
3526 Open array; see @ref{Arrays}.
3529 Pascal space type (AIX); see @ref{Miscellaneous Types}. Builtin integer
3530 type (Sun); see @ref{Builtin Type Descriptors}. Const and volatile
3531 qualified type (OS9000).
3534 Volatile-qualified type; see @ref{Miscellaneous Types}.
3537 Complex builtin type (AIX); see @ref{Builtin Type Descriptors}.
3538 Const-qualified type (OS9000).
3541 COBOL Picture type. See AIX documentation for details.
3544 File type; see @ref{Miscellaneous Types}.
3547 N-dimensional dynamic array; see @ref{Arrays}.
3550 Enumeration type; see @ref{Enumerations}.
3553 N-dimensional subarray; see @ref{Arrays}.
3556 Function type; see @ref{Function Types}.
3559 Pascal function parameter; see @ref{Function Types}
3562 Builtin floating point type; see @ref{Builtin Type Descriptors}.
3565 COBOL Group. See AIX documentation for details.
3568 Imported type (AIX); see @ref{Cross-References}. Volatile-qualified
3572 Const-qualified type; see @ref{Miscellaneous Types}.
3575 COBOL File Descriptor. See AIX documentation for details.
3578 Multiple instance type; see @ref{Miscellaneous Types}.
3581 String type; see @ref{Strings}.
3584 Stringptr; see @ref{Strings}.
3587 Opaque type; see @ref{Typedefs}.
3590 Procedure; see @ref{Function Types}.
3593 Packed array; see @ref{Arrays}.
3596 Range type; see @ref{Subranges}.
3599 Builtin floating type; see @ref{Builtin Type Descriptors} (Sun). Pascal
3600 subroutine parameter; see @ref{Function Types} (AIX). Detecting this
3601 conflict is possible with careful parsing (hint: a Pascal subroutine
3602 parameter type will always contain a comma, and a builtin type
3603 descriptor never will).
3606 Structure type; see @ref{Structures}.
3609 Set type; see @ref{Miscellaneous Types}.
3612 Union; see @ref{Unions}.
3615 Variant record. This is a Pascal and Modula-2 feature which is like a
3616 union within a struct in C. See AIX documentation for details.
3619 Wide character; see @ref{Builtin Type Descriptors}.
3622 Cross-reference; see @ref{Cross-References}.
3625 Used by IBM's xlC C++ compiler (for structures, I think).
3628 gstring; see @ref{Strings}.
3631 @node Expanded Reference
3632 @appendix Expanded Reference by Stab Type
3634 @c FIXME: This appendix should go away; see N_PSYM or N_SO for an example.
3636 For a full list of stab types, and cross-references to where they are
3637 described, see @ref{Stab Types}. This appendix just covers certain
3638 stabs which are not yet described in the main body of this document;
3639 eventually the information will all be in one place.
3643 The first line is the symbol type (see @file{include/aout/stab.def}).
3645 The second line describes the language constructs the symbol type
3648 The third line is the stab format with the significant stab fields
3649 named and the rest NIL.
3651 Subsequent lines expand upon the meaning and possible values for each
3652 significant stab field.
3654 Finally, any further information.
3657 * N_PC:: Pascal global symbol
3658 * N_NSYMS:: Number of symbols
3659 * N_NOMAP:: No DST map
3660 * N_M2C:: Modula-2 compilation unit
3661 * N_BROWS:: Path to .cb file for Sun source code browser
3662 * N_DEFD:: GNU Modula2 definition module dependency
3663 * N_EHDECL:: GNU C++ exception variable
3664 * N_MOD2:: Modula2 information "for imc"
3665 * N_CATCH:: GNU C++ "catch" clause
3666 * N_SSYM:: Structure or union element
3667 * N_SCOPE:: Modula2 scope information (Sun only)
3668 * Gould:: non-base register symbols used on Gould systems
3669 * N_LENG:: Length of preceding entry
3675 @deffn @code{.stabs} N_PC
3677 Global symbol (for Pascal).
3680 "name" -> "symbol_name" <<?>>
3681 value -> supposedly the line number (stab.def is skeptical)
3685 @file{stabdump.c} says:
3687 global pascal symbol: name,,0,subtype,line
3695 @deffn @code{.stabn} N_NSYMS
3697 Number of symbols (according to Ultrix V4.0).
3700 0, files,,funcs,lines (stab.def)
3707 @deffn @code{.stabs} N_NOMAP
3709 No DST map for symbol (according to Ultrix V4.0). I think this means a
3710 variable has been optimized out.
3713 name, ,0,type,ignored (stab.def)
3720 @deffn @code{.stabs} N_M2C
3722 Modula-2 compilation unit.
3725 "string" -> "unit_name,unit_time_stamp[,code_time_stamp]"
3727 value -> 0 (main unit)
3731 See @cite{Dbx and Dbxtool Interfaces}, 2nd edition, by Sun, 1988, for
3739 @deffn @code{.stabs} N_BROWS
3741 Sun source code browser, path to @file{.cb} file
3744 "path to associated @file{.cb} file"
3746 Note: N_BROWS has the same value as N_BSLINE.
3752 @deffn @code{.stabn} N_DEFD
3754 GNU Modula2 definition module dependency.
3756 GNU Modula-2 definition module dependency. The value is the
3757 modification time of the definition file. The other field is non-zero
3758 if it is imported with the GNU M2 keyword @code{%INITIALIZE}. Perhaps
3759 @code{N_M2C} can be used if there are enough empty fields?
3765 @deffn @code{.stabs} N_EHDECL
3767 GNU C++ exception variable <<?>>.
3769 "@var{string} is variable name"
3771 Note: conflicts with @code{N_MOD2}.
3777 @deffn @code{.stab?} N_MOD2
3779 Modula2 info "for imc" (according to Ultrix V4.0)
3781 Note: conflicts with @code{N_EHDECL} <<?>>
3787 @deffn @code{.stabn} N_CATCH
3789 GNU C++ @code{catch} clause
3791 GNU C++ @code{catch} clause. The value is its address. The desc field
3792 is nonzero if this entry is immediately followed by a @code{CAUGHT} stab
3793 saying what exception was caught. Multiple @code{CAUGHT} stabs means
3794 that multiple exceptions can be caught here. If desc is 0, it means all
3795 exceptions are caught here.
3801 @deffn @code{.stabn} N_SSYM
3803 Structure or union element.
3805 The value is the offset in the structure.
3807 <<?looking at structs and unions in C I didn't see these>>
3813 @deffn @code{.stab?} N_SCOPE
3815 Modula2 scope information (Sun linker)
3820 @section Non-base registers on Gould systems
3822 @deffn @code{.stab?} N_NBTEXT
3823 @deffnx @code{.stab?} N_NBDATA
3824 @deffnx @code{.stab?} N_NBBSS
3825 @deffnx @code{.stab?} N_NBSTS
3826 @deffnx @code{.stab?} N_NBLCS
3832 These are used on Gould systems for non-base registers syms.
3834 However, the following values are not the values used by Gould; they are
3835 the values which GNU has been documenting for these values for a long
3836 time, without actually checking what Gould uses. I include these values
3837 only because perhaps some someone actually did something with the GNU
3838 information (I hope not, why GNU knowingly assigned wrong values to
3839 these in the header file is a complete mystery to me).
3842 240 0xf0 N_NBTEXT ??
3843 242 0xf2 N_NBDATA ??
3853 @deffn @code{.stabn} N_LENG
3855 Second symbol entry containing a length-value for the preceding entry.
3856 The value is the length.
3860 @appendix Questions and Anomalies
3864 @c I think this is changed in GCC 2.4.5 to put the line number there.
3865 For GNU C stabs defining local and global variables (@code{N_LSYM} and
3866 @code{N_GSYM}), the desc field is supposed to contain the source
3867 line number on which the variable is defined. In reality the desc
3868 field is always 0. (This behavior is defined in @file{dbxout.c} and
3869 putting a line number in desc is controlled by @samp{#ifdef
3870 WINNING_GDB}, which defaults to false). GDB supposedly uses this
3871 information if you say @samp{list @var{var}}. In reality, @var{var} can
3872 be a variable defined in the program and GDB says @samp{function
3873 @var{var} not defined}.
3876 In GNU C stabs, there seems to be no way to differentiate tag types:
3877 structures, unions, and enums (symbol descriptor @samp{T}) and typedefs
3878 (symbol descriptor @samp{t}) defined at file scope from types defined locally
3879 to a procedure or other more local scope. They all use the @code{N_LSYM}
3880 stab type. Types defined at procedure scope are emitted after the
3881 @code{N_RBRAC} of the preceding function and before the code of the
3882 procedure in which they are defined. This is exactly the same as
3883 types defined in the source file between the two procedure bodies.
3884 GDB over-compensates by placing all types in block #1, the block for
3885 symbols of file scope. This is true for default, @samp{-ansi} and
3886 @samp{-traditional} compiler options. (Bugs gcc/1063, gdb/1066.)
3889 What ends the procedure scope? Is it the proc block's @code{N_RBRAC} or the
3890 next @code{N_FUN}? (I believe its the first.)
3894 @appendix Using Stabs in Their Own Sections
3896 Many object file formats allow tools to create object files with custom
3897 sections containing any arbitrary data. For any such object file
3898 format, stabs can be embedded in special sections. This is how stabs
3899 are used with ELF and SOM, and aside from ECOFF and XCOFF, is how stabs
3903 * Stab Section Basics:: How to embed stabs in sections
3904 * ELF Linker Relocation:: Sun ELF hacks
3907 @node Stab Section Basics
3908 @appendixsec How to Embed Stabs in Sections
3910 The assembler creates two custom sections, a section named @code{.stab}
3911 which contains an array of fixed length structures, one struct per stab,
3912 and a section named @code{.stabstr} containing all the variable length
3913 strings that are referenced by stabs in the @code{.stab} section. The
3914 byte order of the stabs binary data depends on the object file format.
3915 For ELF, it matches the byte order of the ELF file itself, as determined
3916 from the @code{EI_DATA} field in the @code{e_ident} member of the ELF
3917 header. For SOM, it is always big-endian (is this true??? FIXME). For
3918 COFF, it matches the byte order of the COFF headers. The meaning of the
3919 fields is the same as for a.out (@pxref{Symbol Table Format}), except
3920 that the @code{n_strx} field is relative to the strings for the current
3921 compilation unit (which can be found using the synthetic N_UNDF stab
3922 described below), rather than the entire string table.
3924 The first stab in the @code{.stab} section for each compilation unit is
3925 synthetic, generated entirely by the assembler, with no corresponding
3926 @code{.stab} directive as input to the assembler. This stab contains
3927 the following fields:
3931 Offset in the @code{.stabstr} section to the source filename.
3937 Unused field, always zero.
3938 This may eventually be used to hold overflows from the count in
3939 the @code{n_desc} field.
3942 Count of upcoming symbols, i.e., the number of remaining stabs for this
3946 Size of the string table fragment associated with this source file, in
3950 The @code{.stabstr} section always starts with a null byte (so that string
3951 offsets of zero reference a null string), followed by random length strings,
3952 each of which is null byte terminated.
3954 The ELF section header for the @code{.stab} section has its
3955 @code{sh_link} member set to the section number of the @code{.stabstr}
3956 section, and the @code{.stabstr} section has its ELF section
3957 header @code{sh_type} member set to @code{SHT_STRTAB} to mark it as a
3958 string table. SOM and COFF have no way of linking the sections together
3959 or marking them as string tables.
3961 For COFF, the @code{.stab} and @code{.stabstr} sections may be simply
3962 concatenated by the linker. GDB then uses the @code{n_desc} fields to
3963 figure out the extent of the original sections. Similarly, the
3964 @code{n_value} fields of the header symbols are added together in order
3965 to get the actual position of the strings in a desired @code{.stabstr}
3966 section. Although this design obviates any need for the linker to
3967 relocate or otherwise manipulate @code{.stab} and @code{.stabstr}
3968 sections, it also requires some care to ensure that the offsets are
3969 calculated correctly. For instance, if the linker were to pad in
3970 between the @code{.stabstr} sections before concatenating, then the
3971 offsets to strings in the middle of the executable's @code{.stabstr}
3972 section would be wrong.
3974 The GNU linker is able to optimize stabs information by merging
3975 duplicate strings and removing duplicate header file information
3976 (@pxref{Include Files}). When some versions of the GNU linker optimize
3977 stabs in sections, they remove the leading @code{N_UNDF} symbol and
3978 arranges for all the @code{n_strx} fields to be relative to the start of
3979 the @code{.stabstr} section.
3981 @node ELF Linker Relocation
3982 @appendixsec Having the Linker Relocate Stabs in ELF
3984 This section describes some Sun hacks for Stabs in ELF; it does not
3985 apply to COFF or SOM.
3987 To keep linking fast, you don't want the linker to have to relocate very
3988 many stabs. Making sure this is done for @code{N_SLINE},
3989 @code{N_RBRAC}, and @code{N_LBRAC} stabs is the most important thing
3990 (see the descriptions of those stabs for more information). But Sun's
3991 stabs in ELF has taken this further, to make all addresses in the
3992 @code{n_value} field (functions and static variables) relative to the
3993 source file. For the @code{N_SO} symbol itself, Sun simply omits the
3994 address. To find the address of each section corresponding to a given
3995 source file, the compiler puts out symbols giving the address of each
3996 section for a given source file. Since these are ELF (not stab)
3997 symbols, the linker relocates them correctly without having to touch the
3998 stabs section. They are named @code{Bbss.bss} for the bss section,
3999 @code{Ddata.data} for the data section, and @code{Drodata.rodata} for
4000 the rodata section. For the text section, there is no such symbol (but
4001 there should be, see below). For an example of how these symbols work,
4002 @xref{Stab Section Transformations}. GCC does not provide these symbols;
4003 it instead relies on the stabs getting relocated. Thus addresses which
4004 would normally be relative to @code{Bbss.bss}, etc., are already
4005 relocated. The Sun linker provided with Solaris 2.2 and earlier
4006 relocates stabs using normal ELF relocation information, as it would do
4007 for any section. Sun has been threatening to kludge their linker to not
4008 do this (to speed up linking), even though the correct way to avoid
4009 having the linker do these relocations is to have the compiler no longer
4010 output relocatable values. Last I heard they had been talked out of the
4011 linker kludge. See Sun point patch 101052-01 and Sun bug 1142109. With
4012 the Sun compiler this affects @samp{S} symbol descriptor stabs
4013 (@pxref{Statics}) and functions (@pxref{Procedures}). In the latter
4014 case, to adopt the clean solution (making the value of the stab relative
4015 to the start of the compilation unit), it would be necessary to invent a
4016 @code{Ttext.text} symbol, analogous to the @code{Bbss.bss}, etc.,
4017 symbols. I recommend this rather than using a zero value and getting
4018 the address from the ELF symbols.
4020 Finding the correct @code{Bbss.bss}, etc., symbol is difficult, because
4021 the linker simply concatenates the @code{.stab} sections from each
4022 @file{.o} file without including any information about which part of a
4023 @code{.stab} section comes from which @file{.o} file. The way GDB does
4024 this is to look for an ELF @code{STT_FILE} symbol which has the same
4025 name as the last component of the file name from the @code{N_SO} symbol
4026 in the stabs (for example, if the file name is @file{../../gdb/main.c},
4027 it looks for an ELF @code{STT_FILE} symbol named @code{main.c}). This
4028 loses if different files have the same name (they could be in different
4029 directories, a library could have been copied from one system to
4030 another, etc.). It would be much cleaner to have the @code{Bbss.bss}
4031 symbols in the stabs themselves. Having the linker relocate them there
4032 is no more work than having the linker relocate ELF symbols, and it
4033 solves the problem of having to associate the ELF and stab symbols.
4034 However, no one has yet designed or implemented such a scheme.
4036 @node Symbol Types Index
4037 @unnumbered Symbol Types Index
4041 @c TeX can handle the contents at the start but makeinfo 3.12 can not