2 @setfilename stabs.info
7 * Stabs:: The "stabs" debugging information format.
13 This document describes the stabs debugging symbol tables.
15 Copyright 1992 Free Software Foundation, Inc.
16 Contributed by Cygnus Support. Written by Julia Menapace.
18 Permission is granted to make and distribute verbatim copies of
19 this manual provided the copyright notice and this permission notice
20 are preserved on all copies.
23 Permission is granted to process this file through Tex and print the
24 results, provided the printed document carries copying permission
25 notice identical to this one except for the removal of this paragraph
26 (this paragraph not being relevant to the printed manual).
29 Permission is granted to copy or distribute modified versions of this
30 manual under the terms of the GPL (for which purpose this text may be
31 regarded as a program in the language TeX).
34 @setchapternewpage odd
37 @title The ``stabs'' debug format
38 @author Julia Menapace
39 @author Cygnus Support
42 \def\$#1${{#1}} % Kluge: collect RCS revision info without $...$
43 \xdef\manvers{\$Revision$} % For use in headers, footers too
45 \hfill Cygnus Support\par
47 \hfill \TeX{}info \texinfoversion\par
51 @vskip 0pt plus 1filll
52 Copyright @copyright{} 1992 Free Software Foundation, Inc.
53 Contributed by Cygnus Support.
55 Permission is granted to make and distribute verbatim copies of
56 this manual provided the copyright notice and this permission notice
57 are preserved on all copies.
63 @top The "stabs" representation of debugging information
65 This document describes the stabs debugging format.
68 * Overview:: Overview of stabs
69 * Program structure:: Encoding of the structure of the program
70 * Constants:: Constants
71 * Example:: A comprehensive example in C
73 * Types:: Type definitions
74 * Symbol Tables:: Symbol information in symbol tables
75 * Cplusplus:: Appendixes:
76 * Example2.c:: Source code for extended example
77 * Example2.s:: Assembly code for extended example
78 * Stab Types:: Symbol types in a.out files
79 * Symbol Descriptors:: Table of Symbol Descriptors
80 * Type Descriptors:: Table of Symbol Descriptors
81 * Expanded reference:: Reference information by stab type
82 * Questions:: Questions and anomolies
83 * xcoff-differences:: Differences between GNU stabs in a.out
84 and GNU stabs in xcoff
85 * Sun-differences:: Differences between GNU stabs and Sun
87 * Stabs-in-elf:: Stabs in an ELF file.
93 @chapter Overview of stabs
95 @dfn{Stabs} refers to a format for information that describes a program
96 to a debugger. This format was apparently invented by
97 @c FIXME! <<name of inventor>> at
98 the University of California at Berkeley, for the @code{pdx} Pascal
99 debugger; the format has spread widely since then.
101 This document is one of the few published sources of documentation on
102 stabs. It is believed to be completely comprehensive for stabs used by
103 C. The lists of symbol descriptors (@pxref{Symbol Descriptors}) and
104 type descriptors (@pxref{Type Descriptors}) are believed to be completely
105 comprehensive. There are known to be stabs for C++ and COBOL which are
106 poorly documented here. Stabs specific to other languages (e.g. Pascal,
107 Modula-2) are probably not as well documented as they should be.
109 Other sources of information on stabs are @cite{dbx and dbxtool
110 interfaces}, 2nd edition, by Sun, circa 1988, and @cite{AIX Version 3.2
111 Files Reference}, Fourth Edition, September 1992, "dbx Stabstring
112 Grammar" in the a.out section, page 2-31. This document is believed to
113 incorporate the information from those two sources except where it
114 explictly directs you to them for more information.
117 * Flow:: Overview of debugging information flow
118 * Stabs Format:: Overview of stab format
119 * C example:: A simple example in C source
120 * Assembly code:: The simple example at the assembly level
124 @section Overview of debugging information flow
126 The GNU C compiler compiles C source in a @file{.c} file into assembly
127 language in a @file{.s} file, which is translated by the assembler into
128 a @file{.o} file, and then linked with other @file{.o} files and
129 libraries to produce an executable file.
131 With the @samp{-g} option, GCC puts additional debugging information in
132 the @file{.s} file, which is slightly transformed by the assembler and
133 linker, and carried through into the final executable. This debugging
134 information describes features of the source file like line numbers,
135 the types and scopes of variables, and functions, their parameters and
138 For some object file formats, the debugging information is
139 encapsulated in assembler directives known collectively as `stab' (symbol
140 table) directives, interspersed with the generated code. Stabs are
141 the native format for debugging information in the a.out and xcoff
142 object file formats. The GNU tools can also emit stabs in the coff
143 and ecoff object file formats.
145 The assembler adds the information from stabs to the symbol information
146 it places by default in the symbol table and the string table of the
147 @file{.o} file it is building. The linker consolidates the @file{.o}
148 files into one executable file, with one symbol table and one string
149 table. Debuggers use the symbol and string tables in the executable as
150 a source of debugging information about the program.
153 @section Overview of stab format
155 There are three overall formats for stab assembler directives
156 differentiated by the first word of the stab. The name of the directive
157 describes what combination of four possible data fields will follow. It
158 is either @code{.stabs} (string), @code{.stabn} (number), or
159 @code{.stabd} (dot). IBM's xcoff uses @code{.stabx} (and some other
160 directives such as @code{.file} and @code{.bi}) instead of
161 @code{.stabs}, @code{.stabn} or @code{.stabd}.
163 The overall format of each class of stab is:
166 .stabs "@var{string}",@var{type},0,@var{desc},@var{value}
167 .stabx "@var{string}",@var{value},@var{type},@var{sdb-type}
168 .stabn @var{type},0,@var{desc},@var{value}
169 .stabd @var{type},0,@var{desc}
172 @c what is the correct term for "current file location"? My AIX
173 @c assembler manual calls it "the value of the current location counter".
174 For @code{.stabn} and @code{.stabd}, there is no string (the
175 @code{n_strx} field is zero, @pxref{Symbol Tables}). For @code{.stabd}
176 the value field is implicit and has the value of the current file
177 location. The @var{sdb-type} field to @code{.stabx} is unused for stabs
178 and can always be set to 0.
180 The number in the type field gives some basic information about what
181 type of stab this is (or whether it @emph{is} a stab, as opposed to an
182 ordinary symbol). Each possible type number defines a different stab
183 type. The stab type further defines the exact interpretation of, and
184 possible values for, any remaining @code{"@var{string}"}, @var{desc}, or
185 @var{value} fields present in the stab. @xref{Stab Types}, for a list
186 in numeric order of the possible type field values for stab directives.
188 For @code{.stabs} the @code{"@var{string}"} field holds the meat of the
189 debugging information. The generally unstructured nature of this field
190 is what makes stabs extensible. For some stab types the string field
191 contains only a name. For other stab types the contents can be a great
194 The overall format is of the @code{"@var{string}"} field is:
197 "@var{name}:@var{symbol-descriptor} @var{type-information}"
200 @var{name} is the name of the symbol represented by the stab.
201 @var{name} can be omitted, which means the stab represents an unnamed
202 object. For example, @samp{:t10=*2} defines type 10 as a pointer to
203 type 2, but does not give the type a name. Omitting the @var{name}
204 field is supported by AIX dbx and GDB after about version 4.8, but not
205 other debuggers. GCC sometimes uses a single space as the name instead
206 of omitting the name altogether; apparently that is supported by most
209 The @var{symbol_descriptor} following the @samp{:} is an alphabetic
210 character that tells more specifically what kind of symbol the stab
211 represents. If the @var{symbol_descriptor} is omitted, but type
212 information follows, then the stab represents a local variable. For a
213 list of symbol descriptors, see @ref{Symbol Descriptors,,Table C: Symbol
216 The @samp{c} symbol descriptor is an exception in that it is not
217 followed by type information. @xref{Constants}.
219 Type information is either a @var{type_number}, or a
220 @samp{@var{type_number}=}. The @var{type_number} alone is a type
221 reference, referring directly to a type that has already been defined.
223 The @samp{@var{type_number}=} is a type definition, where the number
224 represents a new type which is about to be defined. The type definition
225 may refer to other types by number, and those type numbers may be
226 followed by @samp{=} and nested definitions.
228 In a type definition, if the character that follows the equals sign is
229 non-numeric then it is a @var{type_descriptor}, and tells what kind of
230 type is about to be defined. Any other values following the
231 @var{type_descriptor} vary, depending on the @var{type_descriptor}. If
232 a number follows the @samp{=} then the number is a @var{type_reference}.
233 This is described more thoroughly in the section on types. @xref{Type
234 Descriptors,,Table D: Type Descriptors}, for a list of
235 @var{type_descriptor} values.
237 There is an AIX extension for type attributes. Following the @samp{=}
238 is any number of type attributes. Each one starts with @samp{@@} and
239 ends with @samp{;}. Debuggers, including AIX's dbx, skip any type
240 attributes they do not recognize. GDB 4.9 does not do this---it will
241 ignore the entire symbol containing a type attribute. Hopefully this
242 will be fixed in the next GDB release. Because of a conflict with C++
243 (@pxref{Cplusplus}), new attributes should not be defined which begin
244 with a digit, @samp{(}, or @samp{-}; GDB may be unable to distinguish
245 those from the C++ type descriptor @samp{@@}. The attributes are:
248 @item a@var{boundary}
249 @var{boundary} is an integer specifying the alignment. I assume it
250 applies to all variables of this type.
253 Size in bits of a variable of this type.
256 Pointer class (for checking). Not sure what this means, or how
257 @var{integer} is interpreted.
260 Indicate this is a packed type, meaning that structure fields or array
261 elements are placed more closely in memory, to save memory at the
265 All this can make the @code{"@var{string}"} field quite long. All
266 versions of GDB, and some versions of DBX, can handle arbitrarily long
267 strings. But many versions of DBX cretinously limit the strings to
268 about 80 characters, so compilers which must work with such DBX's need
269 to split the @code{.stabs} directive into several @code{.stabs}
270 directives. Each stab duplicates exactly all but the
271 @code{"@var{string}"} field. The @code{"@var{string}"} field of
272 every stab except the last is marked as continued with a
273 double-backslash at the end. Removing the backslashes and concatenating
274 the @code{"@var{string}"} fields of each stab produces the original,
278 @section A simple example in C source
280 To get the flavor of how stabs describe source information for a C
281 program, let's look at the simple program:
286 printf("Hello world");
290 When compiled with @samp{-g}, the program above yields the following
291 @file{.s} file. Line numbers have been added to make it easier to refer
292 to parts of the @file{.s} file in the description of the stabs that
296 @section The simple example at the assembly level
300 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
301 3 .stabs "hello.c",100,0,0,Ltext0
304 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
305 7 .stabs "char:t2=r2;0;127;",128,0,0,0
306 8 .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
307 9 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
308 10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
309 11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
310 12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
311 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
312 14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
313 15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
314 16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
315 17 .stabs "float:t12=r1;4;0;",128,0,0,0
316 18 .stabs "double:t13=r1;8;0;",128,0,0,0
317 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
318 20 .stabs "void:t15=15",128,0,0,0
321 23 .ascii "Hello, world!\12\0"
336 38 sethi %hi(LC0),%o1
337 39 or %o1,%lo(LC0),%o0
348 50 .stabs "main:F1",36,0,0,_main
349 51 .stabn 192,0,0,LBB2
350 52 .stabn 224,0,0,LBE2
353 This simple ``hello world'' example demonstrates several of the stab
354 types used to describe C language source files.
356 @node Program structure
357 @chapter Encoding for the structure of the program
360 * Main Program:: Indicate what the main program is
361 * Source Files:: The path and name of the source file
368 @section Main Program
370 Most languages allow the main program to have any name. The
371 @code{N_MAIN} stab type is used for a stab telling the debugger what
372 name is used in this program. Only the name is significant; it will be
373 the name of a function which is the main program. Most C compilers do
374 not use this stab; they expect the debugger to simply assume that the
375 name is @samp{main}, but some C compilers emit an @code{N_MAIN} stab for
376 the @samp{main} function.
379 @section The path and name of the source files
381 Before any other stabs occur, there must be a stab specifying the source
382 file. This information is contained in a symbol of stab type
383 @code{N_SO}; the string contains the name of the file. The value of the
384 symbol is the start address of portion of the text section corresponding
387 With the Sun Solaris2 compiler, the @code{desc} field contains a
388 source-language code.
390 Some compilers (for example, gcc2 and SunOS4 @file{/bin/cc}) also
391 include the directory in which the source was compiled, in a second
392 @code{N_SO} symbol preceding the one containing the file name. This
393 symbol can be distinguished by the fact that it ends in a slash. Code
394 from the cfront C++ compiler can have additional @code{N_SO} symbols for
395 nonexistent source files after the @code{N_SO} for the real source file;
396 these are believed to contain no useful information.
401 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0 ; 100 is N_SO
402 .stabs "hello.c",100,0,0,Ltext0
407 Instead of @code{N_SO} symbols, XCOFF uses a @code{.file} assembler
408 directive which assembles to a standard COFF @code{.file} symbol;
409 explaining this in detail is outside the scope of this document.
411 There are several different schemes for dealing with include files: the
412 traditional @code{N_SOL} approach, Sun's @code{N_BINCL} scheme, and the
413 XCOFF @code{C_BINCL} (which despite the similar name has little in
414 common with @code{N_BINCL}).
416 An @code{N_SOL} symbol specifies which include file subsequent symbols
417 refer to. The string field is the name of the file and the value is the
418 text address corresponding to the start of the previous include file and
419 the start of this one. To specify the main source file again, use an
420 @code{N_SOL} symbol with the name of the main source file.
422 A @code{N_BINCL} symbol specifies the start of an include file. In an
423 object file, only the name is significant. The Sun linker puts data
424 into some of the other fields. The end of the include file is marked by
425 a @code{N_EINCL} symbol (which has no name field). In an ojbect file,
426 there is no significant data in the @code{N_EINCL} symbol; the Sun
427 linker puts data into some of the fields. @code{N_BINCL} and
428 @code{N_EINCL} can be nested. If the linker detects that two source
429 files have identical stabs with a @code{N_BINCL} and @code{N_EINCL} pair
430 (as will generally be the case for a header file), then it only puts out
431 the stabs once. Each additional occurance is replaced by an
432 @code{N_EXCL} symbol. I believe the Sun (SunOS4, not sure about
433 Solaris) linker is the only one which supports this feature.
435 For the start of an include file in XCOFF, use the @file{.bi} assembler
436 directive which generates a @code{C_BINCL} symbol. A @file{.ei}
437 directive, which generates a @code{C_EINCL} symbol, denotes the end of
438 the include file. Both directives are followed by the name of the
439 source file in quotes, which becomes the string for the symbol. The
440 value of each symbol, produced automatically by the assembler and
441 linker, is an offset into the executable which points to the beginning
442 (inclusive, as you'd expect) and end (inclusive, as you would not
443 expect) of the portion of the COFF linetable which corresponds to this
444 include file. @code{C_BINCL} and @code{C_EINCL} do not nest.
447 @section Line Numbers
449 A @code{N_SLINE} symbol represents the start of a source line. The
450 @var{desc} field contains the line number and the @var{value} field
451 contains the code address for the start of that source line. On most
452 machines the address is absolute; for Sun's stabs-in-elf, it is relative
453 to the function in which the @code{N_SLINE} symbol occurs.
455 GNU documents @code{N_DSLINE} and @code{N_BSLINE} symbols for line
456 numbers in the data or bss segments, respectively. They are identical
457 to @code{N_SLINE} but are relocated differently by the linker. They
458 were intended to be used to describe the source location of a variable
459 declaration, but I believe that gcc2 actually puts the line number in
460 the desc field of the stab for the variable itself. GDB has been
461 ignoring these symbols (unless they contain a string field) at least
464 XCOFF uses COFF line numbers instead, which are outside the scope of
465 this document, ammeliorated by adequate marking of include files
466 (@pxref{Source Files}).
468 For single source lines that generate discontiguous code, such as flow
469 of control statements, there may be more than one line number entry for
470 the same source line. In this case there is a line number entry at the
471 start of each code range, each with the same line number.
476 All of the following stabs use the @samp{N_FUN} symbol type.
478 A function is represented by a @samp{F} symbol descriptor for a global
479 (extern) function, and @samp{f} for a static (local) function. The next
480 @samp{N_SLINE} symbol can be used to find the line number of the start
481 of the function. The value field is the address of the start of the
482 function. The type information of the stab represents the return type
483 of the function; thus @samp{foo:f5} means that foo is a function
486 The type information of the stab is optionally followed by type
487 information for each argument, with each argument preceded by @samp{;}.
488 An argument type of 0 means that additional arguments are being passed,
489 whose types and number may vary (@samp{...} in ANSI C). This extension
490 is used by Sun's Solaris compiler. GDB has tolerated it (i.e. at least
491 parsed the syntax, if not necessarily used the information) at least
492 since version 4.8; I don't know whether all versions of dbx will
493 tolerate it. The argument types given here are not merely redundant
494 with the symbols for the arguments themselves (@pxref{Parameters}), they
495 are the types of the arguments as they are passed, before any
496 conversions might take place. For example, if a C function which is
497 declared without a prototype takes a @code{float} argument, the value is
498 passed as a @code{double} but then converted to a @code{float}.
499 Debuggers need to use the types given in the arguments when printing
500 values, but if calling the function they need to use the types given in
501 the symbol defining the function.
503 If the return type and types of arguments of a function which is defined
504 in another source file are specified (i.e. a function prototype in ANSI
505 C), traditionally compilers emit no stab; the only way for the debugger
506 to find the information is if the source file where the function is
507 defined was also compiled with debugging symbols. As an extension the
508 Solaris compiler uses symbol descriptor @samp{P} followed by the return
509 type of the function, followed by the arguments, each preceded by
510 @samp{;}, as in a stab with symbol descriptor @samp{f} or @samp{F}.
511 This use of symbol descriptor @samp{P} can be distinguished from its use
512 for register parameters (@pxref{Parameters}) by the fact that it has
513 symbol type @code{N_FUN}.
515 The AIX documentation also defines symbol descriptor @samp{J} as an
516 internal function. I assume this means a function nested within another
517 function. It also says Symbol descriptor @samp{m} is a module in
518 Modula-2 or extended Pascal.
520 Procedures (functions which do not return values) are represented as
521 functions returning the void type in C. I don't see why this couldn't
522 be used for all languages (inventing a void type for this purpose if
523 necessary), but the AIX documentation defines @samp{I}, @samp{P}, and
524 @samp{Q} for internal, global, and static procedures, respectively.
525 These symbol descriptors are unusual in that they are not followed by
528 For any of the above symbol descriptors, after the symbol descriptor and
529 the type information, there is optionally a comma, followed by the name
530 of the procedure, followed by a comma, followed by a name specifying the
531 scope. The first name is local to the scope specified, and seems to be
532 redundant with the name of the symbol (before the @samp{:}). The name
533 specifying the scope is the name of a procedure specifying that scope.
534 This feature is used by @sc{gcc}, and presumably Pascal, Modula-2, etc.,
535 compilers, for nested functions.
537 If procedures are nested more than one level deep, only the immediately
538 containing scope is specified, for example:
550 return baz (x + 2 * y);
552 return x + bar (3 * x);
560 .stabs "baz:f1,baz,bar",36,0,0,_baz.15 # 36 == N_FUN
561 .stabs "bar:f1,bar,foo",36,0,0,_bar.12
562 .stabs "foo:F1",36,0,0,_foo
565 The stab representing a procedure is located immediately following the
566 code of the procedure. This stab is in turn directly followed by a
567 group of other stabs describing elements of the procedure. These other
568 stabs describe the procedure's parameters, its block local variables and
576 The @code{.stabs} entry after this code fragment shows the @var{name} of
577 the procedure (@code{main}); the type descriptor @var{desc} (@code{F},
578 for a global procedure); a reference to the predefined type @code{int}
579 for the return type; and the starting @var{address} of the procedure.
581 Here is an exploded summary (with whitespace introduced for clarity),
582 followed by line 50 of our sample assembly output, which has this form:
586 @var{desc} @r{(global proc @samp{F})}
587 @var{return_type_ref} @r{(int)}
593 50 .stabs "main:F1",36,0,0,_main
596 @node Block Structure
597 @section Block Structure
599 The program's block structure is represented by the @code{N_LBRAC} (left
600 brace) and the @code{N_RBRAC} (right brace) stab types. The variables
601 defined inside a block preceded the @code{N_LBRAC} symbol for most
602 compilers, including GCC. Other compilers, such as the Convex, Acorn
603 RISC machine, and Sun acc compilers, put the variables after the
604 @code{N_LBRAC} symbol. The values of the @code{N_LBRAC} and
605 @code{N_RBRAC} symbols are the start and end addresses of the code of
606 the block, respectively. For most machines, they are relative to the
607 starting address of this source file. For the Gould NP1, they are
608 absolute. For Sun's stabs-in-elf, they are relative to the function in
611 The @code{N_LBRAC} and @code{N_RBRAC} stabs that describe the block
612 scope of a procedure are located after the @code{N_FUN} stab that
613 represents the procedure itself.
615 Sun documents the @code{desc} field of @code{N_LBRAC} and
616 @code{N_RBRAC} symbols as containing the nesting level of the block.
617 However, dbx seems not to care, and GCC just always set @code{desc} to
623 The @samp{c} symbol descriptor indicates that this stab represents a
624 constant. This symbol descriptor is an exception to the general rule
625 that symbol descriptors are followed by type information. Instead, it
626 is followed by @samp{=} and one of the following:
630 Boolean constant. @var{value} is a numeric value; I assume it is 0 for
634 Character constant. @var{value} is the numeric value of the constant.
636 @item e @var{type-information} , @var{value}
637 Constant whose value can be represented as integral.
638 @var{type-information} is the type of the constant, as it would appear
639 after a symbol descriptor (@pxref{Stabs Format}). @var{value} is the
640 numeric value of the constant. GDB 4.9 does not actually get the right
641 value if @var{value} does not fit in a host @code{int}, but it does not
642 do anything violent, and future debuggers could be extended to accept
643 integers of any size (whether unsigned or not). This constant type is
644 usually documented as being only for enumeration constants, but GDB has
645 never imposed that restriction; I don't know about other debuggers.
648 Integer constant. @var{value} is the numeric value. The type is some
649 sort of generic integer type (for GDB, a host @code{int}); to specify
650 the type explicitly, use @samp{e} instead.
653 Real constant. @var{value} is the real value, which can be @samp{INF}
654 (optionally preceded by a sign) for infinity, @samp{QNAN} for a quiet
655 NaN (not-a-number), or @samp{SNAN} for a signalling NaN. If it is a
656 normal number the format is that accepted by the C library function
660 String constant. @var{string} is a string enclosed in either @samp{'}
661 (in which case @samp{'} characters within the string are represented as
662 @samp{\'} or @samp{"} (in which case @samp{"} characters within the
663 string are represented as @samp{\"}).
665 @item S @var{type-information} , @var{elements} , @var{bits} , @var{pattern}
666 Set constant. @var{type-information} is the type of the constant, as it
667 would appear after a symbol descriptor (@pxref{Stabs Format}).
668 @var{elements} is the number of elements in the set (Does this means
669 how many bits of @var{pattern} are actually used, which would be
670 redundant with the type, or perhaps the number of bits set in
671 @var{pattern}? I don't get it), @var{bits} is the number of bits in the
672 constant (meaning it specifies the length of @var{pattern}, I think),
673 and @var{pattern} is a hexadecimal representation of the set. AIX
674 documentation refers to a limit of 32 bytes, but I see no reason why
675 this limit should exist. This form could probably be used for arbitrary
676 constants, not just sets; the only catch is that @var{pattern} should be
677 understood to be target, not host, byte order and format.
680 The boolean, character, string, and set constants are not supported by
681 GDB 4.9, but it will ignore them. GDB 4.8 and earlier gave an error
682 message and refused to read symbols from the file containing the
685 This information is followed by @samp{;}.
688 @chapter A Comprehensive Example in C
690 Now we'll examine a second program, @code{example2}, which builds on the
691 first example to introduce the rest of the stab types, symbol
692 descriptors, and type descriptors used in C.
693 @xref{Example2.c} for the complete @file{.c} source,
694 and @pxref{Example2.s} for the @file{.s} assembly code.
695 This description includes parts of those files.
697 @section Flow of control and nested scopes
703 @code{N_SLINE}, @code{N_LBRAC}, @code{N_RBRAC} (cont.)
706 Consider the body of @code{main}, from @file{example2.c}. It shows more
707 about how @code{N_SLINE}, @code{N_RBRAC}, and @code{N_LBRAC} stabs are used.
711 21 static float s_flap;
713 23 for (times=0; times < s_g_repeat; times++)@{
715 25 printf ("Hello world\n");
720 Here we have a single source line, the @samp{for} line, that generates
721 non-linear flow of control, and non-contiguous code. In this case, an
722 @code{N_SLINE} stab with the same line number proceeds each block of
723 non-contiguous code generated from the same source line.
725 The example also shows nested scopes. The @code{N_LBRAC} and
726 @code{N_LBRAC} stabs that describe block structure are nested in the
727 same order as the corresponding code blocks, those of the for loop
728 inside those for the body of main.
731 This is the label for the @code{N_LBRAC} (left brace) stab marking the
732 start of @code{main}.
739 In the first code range for C source line 23, the @code{for} loop
740 initialize and test, @code{N_SLINE} (68) records the line number:
747 58 .stabn 68,0,23,LM2
751 62 sethi %hi(_s_g_repeat),%o0
753 64 ld [%o0+%lo(_s_g_repeat)],%o0
758 @exdent label for the @code{N_LBRAC} (start block) marking the start of @code{for} loop
761 69 .stabn 68,0,25,LM3
763 71 sethi %hi(LC0),%o1
764 72 or %o1,%lo(LC0),%o0
767 75 .stabn 68,0,26,LM4
770 @exdent label for the @code{N_RBRAC} (end block) stab marking the end of the @code{for} loop
776 Now we come to the second code range for source line 23, the @code{for}
777 loop increment and return. Once again, @code{N_SLINE} (68) records the
781 .stabn, N_SLINE, NIL,
785 78 .stabn 68,0,23,LM5
793 86 .stabn 68,0,27,LM6
796 @exdent label for the @code{N_RBRAC} (end block) stab marking the end of the @code{for} loop
799 89 .stabn 68,0,27,LM7
804 94 .stabs "main:F1",36,0,0,_main
805 95 .stabs "argc:p1",160,0,0,68
806 96 .stabs "argv:p20=*21=*2",160,0,0,72
807 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
808 98 .stabs "times:1",128,0,0,-20
812 Here is an illustration of stabs describing nested scopes. The scope
813 nesting is reflected in the nested bracketing stabs (@code{N_LBRAC},
817 .stabn N_LBRAC,NIL,NIL,
818 @var{block-start-address}
820 99 .stabn 192,0,0,LBB2 ## begin proc label
821 100 .stabs "inner:1",128,0,0,-24
822 101 .stabn 192,0,0,LBB3 ## begin for label
826 @code{N_RBRAC} (224), ``right brace'' ends a lexical block (scope).
829 .stabn N_RBRAC,NIL,NIL,
830 @var{block-end-address}
832 102 .stabn 224,0,0,LBE3 ## end for label
833 103 .stabn 224,0,0,LBE2 ## end proc label
840 * Automatic variables:: Variables allocated on the stack.
841 * Global Variables:: Variables used by more than one source file.
842 * Register variables:: Variables in registers.
843 * Common Blocks:: Variables statically allocated together.
844 * Statics:: Variables local to one source file.
845 * Parameters:: Variables for arguments to functions.
848 @node Automatic variables
849 @section Locally scoped automatic variables
856 @item Symbol Descriptor:
860 In addition to describing types, the @code{N_LSYM} stab type also
861 describes locally scoped automatic variables. Refer again to the body
862 of @code{main} in @file{example2.c}. It allocates two automatic
863 variables: @samp{times} is scoped to the body of @code{main}, and
864 @samp{inner} is scoped to the body of the @code{for} loop.
865 @samp{s_flap} is locally scoped but not automatic, and will be discussed
870 21 static float s_flap;
872 23 for (times=0; times < s_g_repeat; times++)@{
874 25 printf ("Hello world\n");
879 The @code{N_LSYM} stab for an automatic variable is located just before the
880 @code{N_LBRAC} stab describing the open brace of the block to which it is
884 @exdent @code{N_LSYM} (128): automatic variable, scoped locally to @code{main}
887 @var{type information}",
889 @var{frame-pointer-offset}
891 98 .stabs "times:1",128,0,0,-20
892 99 .stabn 192,0,0,LBB2 ## begin `main' N_LBRAC
894 @exdent @code{N_LSYM} (128): automatic variable, scoped locally to the @code{for} loop
897 @var{type information}",
899 @var{frame-pointer-offset}
901 100 .stabs "inner:1",128,0,0,-24
902 101 .stabn 192,0,0,LBB3 ## begin `for' loop N_LBRAC
905 The symbol descriptor is omitted for automatic variables. Since type
906 information should being with a digit, @samp{-}, or @samp{(}, only
907 digits, @samp{-}, and @samp{(} are precluded from being used for symbol
908 descriptors by this fact. However, the Acorn RISC machine (ARM) is said
909 to get this wrong: it puts out a mere type definition here, without the
910 preceding @code{@var{typenumber}=}. This is a bad idea; there is no
911 guarantee that type descriptors are distinct from symbol descriptors.
913 @node Global Variables
914 @section Global Variables
921 @item Symbol Descriptor:
925 Global variables are represented by the @code{N_GSYM} stab type. The symbol
926 descriptor, following the colon in the string field, is @samp{G}. Following
927 the @samp{G} is a type reference or type definition. In this example it is a
928 type reference to the basic C type, @code{char}. The first source line in
936 yields the following stab. The stab immediately precedes the code that
937 allocates storage for the variable it describes.
940 @exdent @code{N_GSYM} (32): global symbol
945 N_GSYM, NIL, NIL, NIL
947 21 .stabs "g_foo:G2",32,0,0,0
954 The address of the variable represented by the @code{N_GSYM} is not contained
955 in the @code{N_GSYM} stab. The debugger gets this information from the
956 external symbol for the global variable.
958 @node Register variables
959 @section Register variables
961 @c According to an old version of this manual, AIX uses C_RPSYM instead
962 @c of C_RSYM. I am skeptical; this should be verified.
963 Register variables have their own stab type, @code{N_RSYM}, and their
964 own symbol descriptor, @code{r}. The stab's value field contains the
965 number of the register where the variable data will be stored.
967 The value is the register number.
969 AIX defines a separate symbol descriptor @samp{d} for floating point
970 registers. This seems unnecessary---why not just just give floating
971 point registers different register numbers? I have not verified whether
972 the compiler actually uses @samp{d}.
974 If the register is explicitly allocated to a global variable, but not
978 register int g_bar asm ("%g5");
981 the stab may be emitted at the end of the object file, with
982 the other bss symbols.
985 @section Common Blocks
987 A common block is a statically allocated section of memory which can be
988 referred to by several source files. It may contain several variables.
989 I believe @sc{fortran} is the only language with this feature. A
990 @code{N_BCOMM} stab begins a common block and an @code{N_ECOMM} stab
991 ends it. The only thing which is significant about these two stabs is
992 their name, which can be used to look up a normal (non-debugging) symbol
993 which gives the address of the common block. Then each stab between the
994 @code{N_BCOMM} and the @code{N_ECOMM} specifies a member of that common
995 block; its value is the offset within the common block of that variable.
996 The @code{N_ECOML} stab type is documented for this purpose, but Sun's
997 @sc{fortran} compiler uses @code{N_GSYM} instead. The test case I
998 looked at had a common block local to a function and it used the
999 @samp{V} symbol descriptor; I assume one would use @samp{S} if not local
1000 to a function (that is, if a common block @emph{can} be anything other
1001 than local to a function).
1004 @section Static Variables
1006 Initialized static variables are represented by the @samp{S} and
1007 @samp{V} symbol descriptors. @samp{S} means file scope static, and
1008 @samp{V} means procedure scope static.
1010 In a.out files, @code{N_STSYM} means the data segment (although gcc
1011 2.4.5 has a bug in that it uses @code{N_FUN}, so neither dbx nor gdb can
1012 find the variables), @code{N_FUN} means the text segment, and
1013 @code{N_LCSYM} means the bss segment.
1015 In xcoff files, each symbol has a section number, so the stab type
1016 need not indicate the segment.
1018 In ecoff files, the storage class is used to specify the section, so the
1019 stab type need not indicate the segment.
1021 @c In ELF files, it apparently is a big mess. See kludge in dbxread.c
1022 @c in GDB. FIXME: Investigate where this kludge comes from.
1024 @c This is the place to mention N_ROSYM; I'd rather do so once I can
1025 @c coherently explain how this stuff works for stabs-in-elf.
1027 For example, the source lines
1030 static const int var_const = 5;
1031 static int var_init = 2;
1032 static int var_noinit;
1036 yield the following stabs:
1039 .stabs "var_const:S1",36,0,0,_var_const ; @r{36 = N_FUN}
1041 .stabs "var_init:S1",38,0,0,_var_init ; @r{38 = N_STSYM}
1043 .stabs "var_noinit:S1",40,0,0,_var_noinit ; @r{40 = N_LCSYM}
1049 Parameters to a function are represented by a stab (or sometimes two,
1050 see below) for each parameter. The stabs are in the order in which the
1051 debugger should print the parameters (i.e. the order in which the
1052 parameters are declared in the source file).
1054 The symbol descriptor @samp{p} is used to refer to parameters which are
1055 in the arglist. Symbols have symbol type @samp{N_PSYM}. The value of
1056 the symbol is the offset relative to the argument list.
1058 If the parameter is passed in a register, then the traditional way to do
1059 this is to provide two symbols for each argument:
1062 .stabs "arg:p1" . . . ; N_PSYM
1063 .stabs "arg:r1" . . . ; N_RSYM
1066 Debuggers are expected to use the second one to find the value, and the
1067 first one to know that it is an argument.
1069 Because this is kind of ugly, some compilers use symbol descriptor
1070 @samp{P} or @samp{R} to indicate an argument which is in a register.
1071 The symbol value is the register number. @samp{P} and @samp{R} mean the
1072 same thing, the difference is that @samp{P} is a GNU invention and
1073 @samp{R} is an IBM (xcoff) invention. As of version 4.9, GDB should
1074 handle either one. Symbol type @samp{C_RPSYM} is used with @samp{R} and
1075 @samp{N_RSYM} is used with @samp{P}.
1077 According to the AIX documentation symbol descriptor @samp{D} is for a
1078 parameter passed in a floating point register. This seems
1079 unnecessary---why not just use @samp{R} with a register number which
1080 indicates that it's a floating point register? I haven't verified
1081 whether the system actually does what the documentation indicates.
1083 There is at least one case where GCC uses a @samp{p}/@samp{r} pair
1084 rather than @samp{P}; this is where the argument is passed in the
1085 argument list and then loaded into a register.
1087 On the sparc and hppa, for a @samp{P} symbol whose type is a structure
1088 or union, the register contains the address of the structure. On the
1089 sparc, this is also true of a @samp{p}/@samp{r} pair (using Sun cc) or a
1090 @samp{p} symbol. However, if a (small) structure is really in a
1091 register, @samp{r} is used. And, to top it all off, on the hppa it
1092 might be a structure which was passed on the stack and loaded into a
1093 register and for which there is a @samp{p}/@samp{r} pair! I believe
1094 that symbol descriptor @samp{i} is supposed to deal with this case, (it
1095 is said to mean "value parameter by reference, indirect access", I don't
1096 know the source for this information) but I don't know details or what
1097 compilers or debuggers use it, if any (not GDB or GCC). It is not clear
1098 to me whether this case needs to be dealt with differently than
1099 parameters passed by reference (see below).
1101 There is another case similar to an argument in a register, which is an
1102 argument which is actually stored as a local variable. Sometimes this
1103 happens when the argument was passed in a register and then the compiler
1104 stores it as a local variable. If possible, the compiler should claim
1105 that it's in a register, but this isn't always done. Some compilers use
1106 the pair of symbols approach described above ("arg:p" followed by
1107 "arg:"); this includes gcc1 (not gcc2) on the sparc when passing a small
1108 structure and gcc2 (sometimes) when the argument type is float and it is
1109 passed as a double and converted to float by the prologue (in the latter
1110 case the type of the "arg:p" symbol is double and the type of the "arg:"
1111 symbol is float). GCC, at least on the 960, uses a single @samp{p}
1112 symbol descriptor for an argument which is stored as a local variable
1113 but uses @samp{N_LSYM} instead of @samp{N_PSYM}. In this case the value
1114 of the symbol is an offset relative to the local variables for that
1115 function, not relative to the arguments (on some machines those are the
1116 same thing, but not on all).
1118 If the parameter is passed by reference (e.g. Pascal VAR parameters),
1119 then type symbol descriptor is @samp{v} if it is in the argument list,
1120 or @samp{a} if it in a register. Other than the fact that these contain
1121 the address of the parameter other than the parameter itself, they are
1122 identical to @samp{p} and @samp{R}, respectively. I believe @samp{a} is
1123 an AIX invention; @samp{v} is supported by all stabs-using systems as
1126 @c Is this paragraph correct? It is based on piecing together patchy
1127 @c information and some guesswork
1128 Conformant arrays refer to a feature of Modula-2, and perhaps other
1129 languages, in which the size of an array parameter is not known to the
1130 called function until run-time. Such parameters have two stabs, a
1131 @samp{x} for the array itself, and a @samp{C}, which represents the size
1132 of the array. The value of the @samp{x} stab is the offset in the
1133 argument list where the address of the array is stored (it this right?
1134 it is a guess); the value of the @samp{C} stab is the offset in the
1135 argument list where the size of the array (in elements? in bytes?) is
1138 The following are also said to go with @samp{N_PSYM}:
1141 "name" -> "param_name:#type"
1143 -> pF FORTRAN function parameter
1144 -> X (function result variable)
1145 -> b (based variable)
1147 value -> offset from the argument pointer (positive).
1150 As a simple example, the code
1162 .stabs "main:F1",36,0,0,_main ; 36 is N_FUN
1163 .stabs "argc:p1",160,0,0,68 ; 160 is N_PSYM
1164 .stabs "argv:p20=*21=*2",160,0,0,72
1167 The type definition of argv is interesting because it contains several
1168 type definitions. Type 21 is pointer to type 2 (char) and argv (type 20) is
1172 @chapter Type Definitions
1174 Now let's look at some variable definitions involving complex types.
1175 This involves understanding better how types are described. In the
1176 examples so far types have been described as references to previously
1177 defined types or defined in terms of subranges of or pointers to
1178 previously defined types. The section that follows will talk about
1179 the various other type descriptors that may follow the = sign in a
1183 * Builtin types:: Integers, floating point, void, etc.
1184 * Miscellaneous Types:: Pointers, sets, files, etc.
1185 * Cross-references:: Referring to a type not yet defined.
1186 * Subranges:: A type with a specific range.
1187 * Arrays:: An aggregate type of same-typed elements.
1188 * Strings:: Like an array but also has a length.
1189 * Enumerations:: Like an integer but the values have names.
1190 * Structures:: An aggregate type of different-typed elements.
1191 * Typedefs:: Giving a type a name.
1192 * Unions:: Different types sharing storage.
1197 @section Builtin types
1199 Certain types are built in (@code{int}, @code{short}, @code{void},
1200 @code{float}, etc.); the debugger recognizes these types and knows how
1201 to handle them. Thus don't be surprised if some of the following ways
1202 of specifying builtin types do not specify everything that a debugger
1203 would need to know about the type---in some cases they merely specify
1204 enough information to distinguish the type from other types.
1206 The traditional way to define builtin types is convolunted, so new ways
1207 have been invented to describe them. Sun's ACC uses the @samp{b} and
1208 @samp{R} type descriptors, and IBM uses negative type numbers. GDB can
1209 accept all three, as of version 4.8; dbx just accepts the traditional
1210 builtin types and perhaps one of the other two formats.
1213 * Traditional Builtin Types:: Put on your seatbelts and prepare for kludgery
1214 * Builtin Type Descriptors:: Builtin types with special type descriptors
1215 * Negative Type Numbers:: Builtin types using negative type numbers
1218 @node Traditional Builtin Types
1219 @subsection Traditional Builtin types
1221 Often types are defined as subranges of themselves. If the array bounds
1222 can fit within an @code{int}, then they are given normally. For example:
1225 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0 ; 128 is N_LSYM
1226 .stabs "char:t2=r2;0;127;",128,0,0,0
1229 Builtin types can also be described as subranges of @code{int}:
1232 .stabs "unsigned short:t6=r1;0;65535;",128,0,0,0
1235 If the lower bound of a subrange is 0 and the upper bound is -1, it
1236 means that the type is an unsigned integral type whose bounds are too
1237 big to describe in an int. Traditionally this is only used for
1238 @code{unsigned int} and @code{unsigned long}; GCC also sometimes uses it
1239 for @code{long long} and @code{unsigned long long}, and the only way to
1240 tell those types apart is to look at their names. On other machines GCC
1241 puts out bounds in octal, with a leading 0. In this case a negative
1242 bound consists of a number which is a 1 bit followed by a bunch of 0
1243 bits, and a positive bound is one in which a bunch of bits are 1.
1246 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
1247 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
1250 If the lower bound of a subrange is 0 and the upper bound is negative,
1251 it means that it is an unsigned integral type whose size in bytes is the
1252 absolute value of the upper bound. I believe this is a Convex
1253 convention for @code{unsigned long long}.
1255 If the lower bound of a subrange is negative and the upper bound is 0,
1256 it means that the type is a signed integral type whose size in bytes is
1257 the absolute value of the lower bound. I believe this is a Convex
1258 convention for @code{long long}. To distinguish this from a legitimate
1259 subrange, the type should be a subrange of itself. I'm not sure whether
1260 this is the case for Convex.
1262 If the upper bound of a subrange is 0, it means that this is a floating
1263 point type, and the lower bound of the subrange indicates the number of
1267 .stabs "float:t12=r1;4;0;",128,0,0,0
1268 .stabs "double:t13=r1;8;0;",128,0,0,0
1271 However, GCC writes @code{long double} the same way it writes
1272 @code{double}; the only way to distinguish them is by the name:
1275 .stabs "long double:t14=r1;8;0;",128,0,0,0
1278 Complex types are defined the same way as floating-point types; the only
1279 way to distinguish a single-precision complex from a double-precision
1280 floating-point type is by the name.
1282 The C @code{void} type is defined as itself:
1285 .stabs "void:t15=15",128,0,0,0
1288 I'm not sure how a boolean type is represented.
1290 @node Builtin Type Descriptors
1291 @subsection Defining Builtin Types using Builtin Type Descriptors
1293 There are various type descriptors to define builtin types:
1296 @c FIXME: clean up description of width and offset, once we figure out
1298 @item b @var{signed} @var{char-flag} @var{width} ; @var{offset} ; @var{nbits} ;
1299 Define an integral type. @var{signed} is @samp{u} for unsigned or
1300 @samp{s} for signed. @var{char-flag} is @samp{c} which indicates this
1301 is a character type, or is omitted. I assume this is to distinguish an
1302 integral type from a character type of the same size, for example it
1303 might make sense to set it for the C type @code{wchar_t} so the debugger
1304 can print such variables differently (Solaris does not do this). Sun
1305 sets it on the C types @code{signed char} and @code{unsigned char} which
1306 arguably is wrong. @var{width} and @var{offset} appear to be for small
1307 objects stored in larger ones, for example a @code{short} in an
1308 @code{int} register. @var{width} is normally the number of bytes in the
1309 type. @var{offset} seems to always be zero. @var{nbits} is the number
1310 of bits in the type.
1312 Note that type descriptor @samp{b} used for builtin types conflicts with
1313 its use for Pascal space types (@pxref{Miscellaneous Types}); they can
1314 be distinguished because the character following the type descriptor
1315 will be a digit, @samp{(}, or @samp{-} for a Pascal space type, or
1316 @samp{u} or @samp{s} for a builtin type.
1319 Documented by AIX to define a wide character type, but their compiler
1320 actually uses negative type numbers (@pxref{Negative Type Numbers}).
1322 @item R @var{fp_type} ; @var{bytes} ;
1323 Define a floating point type. @var{fp_type} has one of the following values:
1327 IEEE 32-bit (single precision) floating point format.
1330 IEEE 64-bit (double precision) floating point format.
1332 @item 3 (NF_COMPLEX)
1333 @item 4 (NF_COMPLEX16)
1334 @item 5 (NF_COMPLEX32)
1335 @c "GDB source" really means @file{include/aout/stab_gnu.h}, but trying
1336 @c to put that here got an overfull hbox.
1337 These are for complex numbers. A comment in the GDB source describes
1338 them as Fortran complex, double complex, and complex*16, respectively,
1339 but what does that mean? (i.e. Single precision? Double precison?).
1341 @item 6 (NF_LDOUBLE)
1342 Long double. This should probably only be used for Sun format long
1343 double, and new codes should be used for other floating point formats
1344 (NF_DOUBLE can be used if a long double is really just an IEEE double,
1348 @var{bytes} is the number of bytes occupied by the type. This allows a
1349 debugger to perform some operations with the type even if it doesn't
1350 understand @var{fp_code}.
1352 @item g @var{type-information} ; @var{nbits}
1353 Documented by AIX to define a floating type, but their compiler actually
1354 uses negative type numbers (@pxref{Negative Type Numbers}).
1356 @item c @var{type-information} ; @var{nbits}
1357 Documented by AIX to define a complex type, but their compiler actually
1358 uses negative type numbers (@pxref{Negative Type Numbers}).
1361 The C @code{void} type is defined as a signed integral type 0 bits long:
1363 .stabs "void:t19=bs0;0;0",128,0,0,0
1365 The Solaris compiler seems to omit the trailing semicolon in this case.
1366 Getting sloppy in this way is not a swift move because if a type is
1367 embedded in a more complex expression it is necessary to be able to tell
1370 I'm not sure how a boolean type is represented.
1372 @node Negative Type Numbers
1373 @subsection Negative Type numbers
1375 Since the debugger knows about the builtin types anyway, the idea of
1376 negative type numbers is simply to give a special type number which
1377 indicates the built in type. There is no stab defining these types.
1379 I'm not sure whether anyone has tried to define what this means if
1380 @code{int} can be other than 32 bits (or other types can be other than
1381 their customary size). If @code{int} has exactly one size for each
1382 architecture, then it can be handled easily enough, but if the size of
1383 @code{int} can vary according the compiler options, then it gets hairy.
1384 The best way to do this would be to define separate negative type
1385 numbers for 16-bit @code{int} and 32-bit @code{int}; therefore I have
1386 indicated below the customary size (and other format information) for
1387 each type. The information below is currently correct because AIX on
1388 the RS6000 is the only system which uses these type numbers. If these
1389 type numbers start to get used on other systems, I suspect the correct
1390 thing to do is to define a new number in cases where a type does not
1391 have the size and format indicated below (or avoid negative type numbers
1394 Also note that part of the definition of the negative type number is
1395 the name of the type. Types with identical size and format but
1396 different names have different negative type numbers.
1400 @code{int}, 32 bit signed integral type.
1403 @code{char}, 8 bit type holding a character. Both GDB and dbx on AIX
1404 treat this as signed. GCC uses this type whether @code{char} is signed
1405 or not, which seems like a bad idea. The AIX compiler (xlc) seems to
1406 avoid this type; it uses -5 instead for @code{char}.
1409 @code{short}, 16 bit signed integral type.
1412 @code{long}, 32 bit signed integral type.
1415 @code{unsigned char}, 8 bit unsigned integral type.
1418 @code{signed char}, 8 bit signed integral type.
1421 @code{unsigned short}, 16 bit unsigned integral type.
1424 @code{unsigned int}, 32 bit unsigned integral type.
1427 @code{unsigned}, 32 bit unsigned integral type.
1430 @code{unsigned long}, 32 bit unsigned integral type.
1433 @code{void}, type indicating the lack of a value.
1436 @code{float}, IEEE single precision.
1439 @code{double}, IEEE double precision.
1442 @code{long double}, IEEE double precision. The compiler claims the size
1443 will increase in a future release, and for binary compatibility you have
1444 to avoid using @code{long double}. I hope when they increase it they
1445 use a new negative type number.
1448 @code{integer}. 32 bit signed integral type.
1451 @code{boolean}. 32 bit type. How is the truth value encoded? Is it
1452 the least significant bit or is it a question of whether the whole value
1453 is zero or non-zero?
1456 @code{short real}. IEEE single precision.
1459 @code{real}. IEEE double precision.
1462 @code{stringptr}. @xref{Strings}.
1465 @code{character}, 8 bit unsigned character type.
1468 @code{logical*1}, 8 bit type. This @sc{fortran} type has a split
1469 personality in that it is used for boolean variables, but can also be
1470 used for unsigned integers. 0 is false, 1 is true, and other values are
1474 @code{logical*2}, 16 bit type. This @sc{fortran} type has a split
1475 personality in that it is used for boolean variables, but can also be
1476 used for unsigned integers. 0 is false, 1 is true, and other values are
1480 @code{logical*4}, 32 bit type. This @sc{fortran} type has a split
1481 personality in that it is used for boolean variables, but can also be
1482 used for unsigned integers. 0 is false, 1 is true, and other values are
1486 @code{logical}, 32 bit type. This @sc{fortran} type has a split
1487 personality in that it is used for boolean variables, but can also be
1488 used for unsigned integers. 0 is false, 1 is true, and other values are
1492 @code{complex}. A complex type consisting of two IEEE single-precision
1493 floating point values.
1496 @code{complex}. A complex type consisting of two IEEE double-precision
1497 floating point values.
1500 @code{integer*1}, 8 bit signed integral type.
1503 @code{integer*2}, 16 bit signed integral type.
1506 @code{integer*4}, 32 bit signed integral type.
1509 @code{wchar}. Wide character, 16 bits wide, unsigned (what format?
1513 @node Miscellaneous Types
1514 @section Miscellaneous Types
1517 @item b @var{type-information} ; @var{bytes}
1518 Pascal space type. This is documented by IBM; what does it mean?
1520 Note that this use of the @samp{b} type descriptor can be distinguished
1521 from its use for builtin integral types (@pxref{Builtin Type
1522 Descriptors}) because the character following the type descriptor is
1523 always a digit, @samp{(}, or @samp{-}.
1525 @item B @var{type-information}
1526 A volatile-qualified version of @var{type-information}. This is a Sun
1527 extension. A volatile-qualified type means that references and stores
1528 to a variable of that type must not be optimized or cached; they must
1529 occur as the user specifies them.
1531 @item d @var{type-information}
1532 File of type @var{type-information}. As far as I know this is only used
1535 @item k @var{type-information}
1536 A const-qualified version of @var{type-information}. This is a Sun
1537 extension. A const-qualified type means that a variable of this type
1540 @item M @var{type-information} ; @var{length}
1541 Multiple instance type. The type seems to composed of @var{length}
1542 repetitions of @var{type-information}, for example @code{character*3} is
1543 represented by @samp{M-2;3}, where @samp{-2} is a reference to a
1544 character type (@pxref{Negative Type Numbers}). I'm not sure how this
1545 differs from an array. This appears to be a FORTRAN feature.
1546 @var{length} is a bound, like those in range types, @xref{Subranges}.
1548 @item S @var{type-information}
1549 Pascal set type. @var{type-information} must be a small type such as an
1550 enumeration or a subrange, and the type is a bitmask whose length is
1551 specified by the number of elements in @var{type-information}.
1553 @item * @var{type-information}
1554 Pointer to @var{type-information}.
1557 @node Cross-references
1558 @section Cross-references to other types
1560 If a type is used before it is defined, one common way to deal with this
1561 is just to use a type reference to a type which has not yet been
1562 defined. The debugger is expected to be able to deal with this.
1564 Another way is with the @samp{x} type descriptor, which is followed by
1565 @samp{s} for a structure tag, @samp{u} for a union tag, or @samp{e} for
1566 a enumerator tag, followed by the name of the tag, followed by @samp{:}.
1567 for example the following C declarations:
1577 .stabs "bar:G16=*17=xsfoo:",32,0,0,0
1580 Not all debuggers support the @samp{x} type descriptor, so on some
1581 machines GCC does not use it. I believe that for the above example it
1582 would just emit a reference to type 17 and never define it, but I
1583 haven't verified that.
1585 Modula-2 imported types, at least on AIX, use the @samp{i} type
1586 descriptor, which is followed by the name of the module from which the
1587 type is imported, followed by @samp{:}, followed by the name of the
1588 type. There is then optionally a comma followed by type information for
1589 the type (This differs from merely naming the type (@pxref{Typedefs}) in
1590 that it identifies the module; I don't understand whether the name of
1591 the type given here is always just the same as the name we are giving
1592 it, or whether this type descriptor is used with a nameless stab
1593 (@pxref{Stabs Format}), or what). The symbol ends with @samp{;}.
1596 @section Subrange types
1598 The @samp{r} type descriptor defines a type as a subrange of another
1599 type. It is followed by type information for the type which it is a
1600 subrange of, a semicolon, an integral lower bound, a semicolon, an
1601 integral upper bound, and a semicolon. The AIX documentation does not
1602 specify the trailing semicolon, in an effort to specify array indexes
1603 more cleanly, but a subrange which is not an array index has always
1604 included a trailing semicolon (@pxref{Arrays}).
1606 Instead of an integer, either bound can be one of the following:
1609 @item A @var{offset}
1610 The bound is passed by reference on the stack at offset @var{offset}
1611 from the argument list. @xref{Parameters}, for more information on such
1614 @item T @var{offset}
1615 The bound is passed by value on the stack at offset @var{offset} from
1618 @item a @var{register-number}
1619 The bound is pased by reference in register number
1620 @var{register-number}.
1622 @item t @var{register-number}
1623 The bound is passed by value in register number @var{register-number}.
1629 Subranges are also used for builtin types, @xref{Traditional Builtin Types}.
1632 @section Array types
1634 Arrays use the @samp{a} type descriptor. Following the type descriptor
1635 is the type of the index and the type of the array elements. If the
1636 index type is a range type, it will end in a semicolon; if it is not a
1637 range type (for example, if it is a type reference), there does not
1638 appear to be any way to tell where the types are separated. In an
1639 effort to clean up this mess, IBM documents the two types as being
1640 separated by a semicolon, and a range type as not ending in a semicolon
1641 (but this is not right for range types which are not array indexes,
1642 @pxref{Subranges}). I think probably the best solution is to specify
1643 that a semicolon ends a range type, and that the index type and element
1644 type of an array are separated by a semicolon, but that if the index
1645 type is a range type, the extra semicolon can be omitted. GDB (at least
1646 through version 4.9) doesn't support any kind of index type other than a
1647 range anyway; I'm not sure about dbx.
1649 It is well established, and widely used, that the type of the index,
1650 unlike most types found in the stabs, is merely a type definition, not
1651 type information (@pxref{Stabs Format}) (that is, it need not start with
1652 @var{type-number}@code{=} if it is defining a new type). According to a
1653 comment in GDB, this is also true of the type of the array elements; it
1654 gives @samp{ar1;1;10;ar1;1;10;4} as a legitimate way to express a two
1655 dimensional array. According to AIX documentation, the element type
1656 must be type information. GDB accepts either.
1658 The type of the index is often a range type, expressed as the letter r
1659 and some parameters. It defines the size of the array. In the example
1660 below, the range @code{r1;0;2;} defines an index type which is a
1661 subrange of type 1 (integer), with a lower bound of 0 and an upper bound
1662 of 2. This defines the valid range of subscripts of a three-element C
1665 For example, the definition
1668 char char_vec[3] = @{'a','b','c'@};
1675 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
1684 If an array is @dfn{packed}, it means that the elements are spaced more
1685 closely than normal, saving memory at the expense of speed. For
1686 example, an array of 3-byte objects might, if unpacked, have each
1687 element aligned on a 4-byte boundary, but if packed, have no padding.
1688 One way to specify that something is packed is with type attributes
1689 (@pxref{Stabs Format}), in the case of arrays another is to use the
1690 @samp{P} type descriptor instead of @samp{a}. Other than specifying a
1691 packed array, @samp{P} is identical to @samp{a}.
1693 @c FIXME-what is it? A pointer?
1694 An open array is represented by the @samp{A} type descriptor followed by
1695 type information specifying the type of the array elements.
1697 @c FIXME: what is the format of this type? A pointer to a vector of pointers?
1698 An N-dimensional dynamic array is represented by
1701 D @var{dimensions} ; @var{type-information}
1704 @c Does dimensions really have this meaning? The AIX documentation
1706 @var{dimensions} is the number of dimensions; @var{type-information}
1707 specifies the type of the array elements.
1709 @c FIXME: what is the format of this type? A pointer to some offsets in
1711 A subarray of an N-dimensional array is represented by
1714 E @var{dimensions} ; @var{type-information}
1717 @c Does dimensions really have this meaning? The AIX documentation
1719 @var{dimensions} is the number of dimensions; @var{type-information}
1720 specifies the type of the array elements.
1725 Some languages, like C or the original Pascal, do not have string types,
1726 they just have related things like arrays of characters. But most
1727 Pascals and various other languages have string types, which are
1728 indicated as follows:
1731 @item n @var{type-information} ; @var{bytes}
1732 @var{bytes} is the maximum length. I'm not sure what
1733 @var{type-information} is; I suspect that it means that this is a string
1734 of @var{type-information} (thus allowing a string of integers, a string
1735 of wide characters, etc., as well as a string of characters). Not sure
1736 what the format of this type is. This is an AIX feature.
1738 @item z @var{type-information} ; @var{bytes}
1739 Just like @samp{n} except that this is a gstring, not an ordinary
1740 string. I don't know the difference.
1743 Pascal Stringptr. What is this? This is an AIX feature.
1747 @section Enumerations
1749 Enumerations are defined with the @samp{e} type descriptor.
1751 @c FIXME: Where does this information properly go? Perhaps it is
1752 @c redundant with something we already explain.
1753 The source line below declares an enumeration type. It is defined at
1754 file scope between the bodies of main and s_proc in example2.c.
1755 The type definition is located after the N_RBRAC that marks the end of
1756 the previous procedure's block scope, and before the N_FUN that marks
1757 the beginning of the next procedure's block scope. Therefore it does not
1758 describe a block local symbol, but a file local one.
1763 enum e_places @{first,second=3,last@};
1767 generates the following stab
1770 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
1773 The symbol descriptor (T) says that the stab describes a structure,
1774 enumeration, or type tag. The type descriptor e, following the 22= of
1775 the type definition narrows it down to an enumeration type. Following
1776 the e is a list of the elements of the enumeration. The format is
1777 name:value,. The list of elements ends with a ;.
1779 There is no standard way to specify the size of an enumeration type; it
1780 is determined by the architecture (normally all enumerations types are
1781 32 bits). There should be a way to specify an enumeration type of
1782 another size; type attributes would be one way to do this @xref{Stabs
1792 @code{N_LSYM} or @code{C_DECL}
1793 @item Symbol Descriptor:
1795 @item Type Descriptor:
1799 The following source code declares a structure tag and defines an
1800 instance of the structure in global scope. Then a typedef equates the
1801 structure tag with a new type. A seperate stab is generated for the
1802 structure tag, the structure typedef, and the structure instance. The
1803 stabs for the tag and the typedef are emited when the definitions are
1804 encountered. Since the structure elements are not initialized, the
1805 stab and code for the structure variable itself is located at the end
1806 of the program in .common.
1812 9 char s_char_vec[8];
1813 10 struct s_tag* s_next;
1816 13 typedef struct s_tag s_typedef;
1819 The structure tag is an N_LSYM stab type because, like the enum, the
1820 symbol is file scope. Like the enum, the symbol descriptor is T, for
1821 enumeration, struct or tag type. The symbol descriptor s following
1822 the 16= of the type definition narrows the symbol type to struct.
1824 Following the struct symbol descriptor is the number of bytes the
1825 struct occupies, followed by a description of each structure element.
1826 The structure element descriptions are of the form name:type, bit
1827 offset from the start of the struct, and number of bits in the
1832 <128> N_LSYM - type definition
1833 .stabs "name:sym_desc(struct tag) Type_def(16)=type_desc(struct type)
1835 elem_name:type_ref(int),bit_offset,field_bits;
1836 elem_name:type_ref(float),bit_offset,field_bits;
1837 elem_name:type_def(17)=type_desc(array)
1838 index_type(range of int from 0 to 7);
1839 element_type(char),bit_offset,field_bits;;",
1842 30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;
1843 s_char_vec:17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
1846 In this example, two of the structure elements are previously defined
1847 types. For these, the type following the name: part of the element
1848 description is a simple type reference. The other two structure
1849 elements are new types. In this case there is a type definition
1850 embedded after the name:. The type definition for the array element
1851 looks just like a type definition for a standalone array. The s_next
1852 field is a pointer to the same kind of structure that the field is an
1853 element of. So the definition of structure type 16 contains an type
1854 definition for an element which is a pointer to type 16.
1857 @section Giving a type a name
1859 To give a type a name, use the @samp{t} symbol descriptor. For example,
1862 .stabs "s_typedef:t16",128,0,0,0
1865 specifies that @code{s_typedef} refers to type number 16. Such stabs
1866 have symbol type @code{N_LSYM} or @code{C_DECL}.
1868 If instead, you are specifying the tag name for a structure, union, or
1869 enumeration, use the @samp{T} symbol descriptor instead. I believe C is
1870 the only language with this feature.
1872 If the type is an opaque type (I believe this is a Modula-2 feature),
1873 AIX provides a type descriptor to specify it. The type descriptor is
1874 @samp{o} and is followed by a name. I don't know what the name
1875 means---is it always the same as the name of the type, or is this type
1876 descriptor used with a nameless stab (@pxref{Stabs Format})? There
1877 optionally follows a comma followed by type information which defines
1878 the type of this type. If omitted, a semicolon is used in place of the
1879 comma and the type information, and, the type is much like a generic
1880 pointer type---it has a known size but little else about it is
1886 Next let's look at unions. In example2 this union type is declared
1887 locally to a procedure and an instance of the union is defined.
1897 This code generates a stab for the union tag and a stab for the union
1898 variable. Both use the N_LSYM stab type. Since the union variable is
1899 scoped locally to the procedure in which it is defined, its stab is
1900 located immediately preceding the N_LBRAC for the procedure's block
1903 The stab for the union tag, however is located preceding the code for
1904 the procedure in which it is defined. The stab type is N_LSYM. This
1905 would seem to imply that the union type is file scope, like the struct
1906 type s_tag. This is not true. The contents and position of the stab
1907 for u_type do not convey any infomation about its procedure local
1912 .stabs "name:sym_desc(union tag)type_def(22)=type_desc(union)
1914 elem_name:type_ref(int),bit_offset(0),bit_size(32);
1915 elem_name:type_ref(float),bit_offset(0),bit_size(32);
1916 elem_name:type_ref(ptr to char),bit_offset(0),bit_size(32);;"
1917 N_LSYM, NIL, NIL, NIL
1921 105 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
1925 The symbol descriptor, T, following the name: means that the stab
1926 describes an enumeration, struct or type tag. The type descriptor u,
1927 following the 23= of the type definition, narrows it down to a union
1928 type definition. Following the u is the number of bytes in the union.
1929 After that is a list of union element descriptions. Their format is
1930 name:type, bit offset into the union, and number of bytes for the
1933 The stab for the union variable follows. Notice that the frame
1934 pointer offset for local variables is negative.
1937 <128> N_LSYM - local variable (with no symbol descriptor)
1938 .stabs "name:type_ref(u_tag)", N_LSYM, NIL, NIL, frame_ptr_offset
1942 130 .stabs "an_u:23",128,0,0,-20
1945 @node Function Types
1946 @section Function types
1948 There are various types for function variables. These types are not
1949 used in defining functions; see symbol descriptor @samp{f}; they are
1950 used for things like pointers to functions.
1952 The simple, traditional, type is type descriptor @samp{f} is followed by
1953 type information for the return type of the function, followed by a
1956 This does not deal with functions the number and type of whose
1957 parameters are part of their type, as found in Modula-2 or ANSI C. AIX
1958 provides extensions to specify these, using the @samp{f}, @samp{F},
1959 @samp{p}, and @samp{R} type descriptors.
1961 First comes the type descriptor. Then, if it is @samp{f} or @samp{F},
1962 this is a function, and the type information for the return type of the
1963 function follows, followed by a comma. Then comes the number of
1964 parameters to the function and a semicolon. Then, for each parameter,
1965 there is the name of the parameter followed by a colon (this is only
1966 present for type descriptors @samp{R} and @samp{F} which represent
1967 Pascal function or procedure parameters), type information for the
1968 parameter, a comma, @samp{0} if passed by reference or @samp{1} if
1969 passed by value, and a semicolon. The type definition ends with a
1979 generates the following code:
1982 .stabs "g_pf:G24=*25=f1",32,0,0,0
1983 .common _g_pf,4,"bss"
1986 The variable defines a new type, 24, which is a pointer to another new
1987 type, 25, which is defined as a function returning int.
1990 @chapter Symbol information in symbol tables
1992 This section examines more closely the format of symbol table entries
1993 and how stab assembler directives map to them. It also describes what
1994 transformations the assembler and linker make on data from stabs.
1996 Each time the assembler encounters a stab in its input file it puts
1997 each field of the stab into corresponding fields in a symbol table
1998 entry of its output file. If the stab contains a string field, the
1999 symbol table entry for that stab points to a string table entry
2000 containing the string data from the stab. Assembler labels become
2001 relocatable addresses. Symbol table entries in a.out have the format:
2004 struct internal_nlist @{
2005 unsigned long n_strx; /* index into string table of name */
2006 unsigned char n_type; /* type of symbol */
2007 unsigned char n_other; /* misc info (usually empty) */
2008 unsigned short n_desc; /* description field */
2009 bfd_vma n_value; /* value of symbol */
2013 For .stabs directives, the n_strx field holds the character offset
2014 from the start of the string table to the string table entry
2015 containing the "string" field. For other classes of stabs (.stabn and
2016 .stabd) this field is null.
2018 Symbol table entries with n_type fields containing a value greater or
2019 equal to 0x20 originated as stabs generated by the compiler (with one
2020 random exception). Those with n_type values less than 0x20 were
2021 placed in the symbol table of the executable by the assembler or the
2024 The linker concatenates object files and does fixups of externally
2025 defined symbols. You can see the transformations made on stab data by
2026 the assembler and linker by examining the symbol table after each pass
2027 of the build, first the assemble and then the link.
2029 To do this use nm with the -ap options. This dumps the symbol table,
2030 including debugging information, unsorted. For stab entries the
2031 columns are: value, other, desc, type, string. For assembler and
2032 linker symbols, the columns are: value, type, string.
2034 There are a few important things to notice about symbol tables. Where
2035 the value field of a stab contains a frame pointer offset, or a
2036 register number, that value is unchanged by the rest of the build.
2038 Where the value field of a stab contains an assembly language label,
2039 it is transformed by each build step. The assembler turns it into a
2040 relocatable address and the linker turns it into an absolute address.
2041 This source line defines a static variable at file scope:
2044 3 static int s_g_repeat
2048 The following stab describes the symbol.
2051 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
2055 The assembler transforms the stab into this symbol table entry in the
2056 @file{.o} file. The location is expressed as a data segment offset.
2059 21 00000084 - 00 0000 STSYM s_g_repeat:S1
2063 in the symbol table entry from the executable, the linker has made the
2064 relocatable address absolute.
2067 22 0000e00c - 00 0000 STSYM s_g_repeat:S1
2070 Stabs for global variables do not contain location information. In
2071 this case the debugger finds location information in the assembler or
2072 linker symbol table entry describing the variable. The source line:
2082 21 .stabs "g_foo:G2",32,0,0,0
2085 The variable is represented by the following two symbol table entries
2086 in the object file. The first one originated as a stab. The second
2087 one is an external symbol. The upper case D signifies that the n_type
2088 field of the symbol table contains 7, N_DATA with local linkage (see
2089 Table B). The value field following the file's line number is empty
2090 for the stab entry. For the linker symbol it contains the
2091 rellocatable address corresponding to the variable.
2094 19 00000000 - 00 0000 GSYM g_foo:G2
2095 20 00000080 D _g_foo
2099 These entries as transformed by the linker. The linker symbol table
2100 entry now holds an absolute address.
2103 21 00000000 - 00 0000 GSYM g_foo:G2
2105 215 0000e008 D _g_foo
2109 @chapter GNU C++ stabs
2112 * Basic Cplusplus types::
2115 * Methods:: Method definition
2117 * Method Modifiers::
2120 * Virtual Base Classes::
2124 @subsection type descriptors added for C++ descriptions
2128 method type (two ## if minimal debug)
2131 Member (class and variable) type. It is followed by type information
2132 for the offset basetype, a comma, and type information for the type of
2133 the field being pointed to. (FIXME: this is acknowledged to be
2134 gibberish. Can anyone say what really goes here?).
2136 Note that there is a conflict between this and type attributes
2137 (@pxref{Stabs Format}); both use type descriptor @samp{@@}.
2138 Fortunately, the @samp{@@} type descriptor used in this C++ sense always
2139 will be followed by a digit, @samp{(}, or @samp{-}, and type attributes
2140 never start with those things.
2143 @node Basic Cplusplus types
2144 @section Basic types for C++
2146 << the examples that follow are based on a01.C >>
2149 C++ adds two more builtin types to the set defined for C. These are
2150 the unknown type and the vtable record type. The unknown type, type
2151 16, is defined in terms of itself like the void type.
2153 The vtable record type, type 17, is defined as a structure type and
2154 then as a structure tag. The structure has four fields, delta, index,
2155 pfn, and delta2. pfn is the function pointer.
2157 << In boilerplate $vtbl_ptr_type, what are the fields delta,
2158 index, and delta2 used for? >>
2160 This basic type is present in all C++ programs even if there are no
2161 virtual methods defined.
2164 .stabs "struct_name:sym_desc(type)type_def(17)=type_desc(struct)struct_bytes(8)
2165 elem_name(delta):type_ref(short int),bit_offset(0),field_bits(16);
2166 elem_name(index):type_ref(short int),bit_offset(16),field_bits(16);
2167 elem_name(pfn):type_def(18)=type_desc(ptr to)type_ref(void),
2168 bit_offset(32),field_bits(32);
2169 elem_name(delta2):type_def(short int);bit_offset(32),field_bits(16);;"
2174 .stabs "$vtbl_ptr_type:t17=s8
2175 delta:6,0,16;index:6,16,16;pfn:18=*15,32,32;delta2:6,32,16;;"
2180 .stabs "name:sym_dec(struct tag)type_ref($vtbl_ptr_type)",N_LSYM,NIL,NIL,NIL
2184 .stabs "$vtbl_ptr_type:T17",128,0,0,0
2187 @node Simple classes
2188 @section Simple class definition
2190 The stabs describing C++ language features are an extension of the
2191 stabs describing C. Stabs representing C++ class types elaborate
2192 extensively on the stab format used to describe structure types in C.
2193 Stabs representing class type variables look just like stabs
2194 representing C language variables.
2196 Consider the following very simple class definition.
2202 int Ameth(int in, char other);
2206 The class baseA is represented by two stabs. The first stab describes
2207 the class as a structure type. The second stab describes a structure
2208 tag of the class type. Both stabs are of stab type N_LSYM. Since the
2209 stab is not located between an N_FUN and a N_LBRAC stab this indicates
2210 that the class is defined at file scope. If it were, then the N_LSYM
2211 would signify a local variable.
2213 A stab describing a C++ class type is similar in format to a stab
2214 describing a C struct, with each class member shown as a field in the
2215 structure. The part of the struct format describing fields is
2216 expanded to include extra information relevent to C++ class members.
2217 In addition, if the class has multiple base classes or virtual
2218 functions the struct format outside of the field parts is also
2221 In this simple example the field part of the C++ class stab
2222 representing member data looks just like the field part of a C struct
2223 stab. The section on protections describes how its format is
2224 sometimes extended for member data.
2226 The field part of a C++ class stab representing a member function
2227 differs substantially from the field part of a C struct stab. It
2228 still begins with `name:' but then goes on to define a new type number
2229 for the member function, describe its return type, its argument types,
2230 its protection level, any qualifiers applied to the method definition,
2231 and whether the method is virtual or not. If the method is virtual
2232 then the method description goes on to give the vtable index of the
2233 method, and the type number of the first base class defining the
2236 When the field name is a method name it is followed by two colons
2237 rather than one. This is followed by a new type definition for the
2238 method. This is a number followed by an equal sign and then the
2239 symbol descriptor `##', indicating a method type. This is followed by
2240 a type reference showing the return type of the method and a
2243 The format of an overloaded operator method name differs from that
2244 of other methods. It is "op$::XXXX." where XXXX is the operator name
2245 such as + or +=. The name ends with a period, and any characters except
2246 the period can occur in the XXXX string.
2248 The next part of the method description represents the arguments to
2249 the method, preceeded by a colon and ending with a semi-colon. The
2250 types of the arguments are expressed in the same way argument types
2251 are expressed in C++ name mangling. In this example an int and a char
2254 This is followed by a number, a letter, and an asterisk or period,
2255 followed by another semicolon. The number indicates the protections
2256 that apply to the member function. Here the 2 means public. The
2257 letter encodes any qualifier applied to the method definition. In
2258 this case A means that it is a normal function definition. The dot
2259 shows that the method is not virtual. The sections that follow
2260 elaborate further on these fields and describe the additional
2261 information present for virtual methods.
2265 .stabs "class_name:sym_desc(type)type_def(20)=type_desc(struct)struct_bytes(4)
2266 field_name(Adat):type(int),bit_offset(0),field_bits(32);
2268 method_name(Ameth)::type_def(21)=type_desc(method)return_type(int);
2269 :arg_types(int char);
2270 protection(public)qualifier(normal)virtual(no);;"
2275 .stabs "baseA:t20=s4Adat:1,0,32;Ameth::21=##1;:ic;2A.;;",128,0,0,0
2277 .stabs "class_name:sym_desc(struct tag)",N_LSYM,NIL,NIL,NIL
2279 .stabs "baseA:T20",128,0,0,0
2282 @node Class instance
2283 @section Class instance
2285 As shown above, describing even a simple C++ class definition is
2286 accomplished by massively extending the stab format used in C to
2287 describe structure types. However, once the class is defined, C stabs
2288 with no modifications can be used to describe class instances. The
2298 yields the following stab describing the class instance. It looks no
2299 different from a standard C stab describing a local variable.
2302 .stabs "name:type_ref(baseA)", N_LSYM, NIL, NIL, frame_ptr_offset
2306 .stabs "AbaseA:20",128,0,0,-20
2310 @section Method defintion
2312 The class definition shown above declares Ameth. The C++ source below
2317 baseA::Ameth(int in, char other)
2324 This method definition yields three stabs following the code of the
2325 method. One stab describes the method itself and following two
2326 describe its parameters. Although there is only one formal argument
2327 all methods have an implicit argument which is the `this' pointer.
2328 The `this' pointer is a pointer to the object on which the method was
2329 called. Note that the method name is mangled to encode the class name
2330 and argument types. << Name mangling is not described by this
2331 document - Is there already such a doc? >>
2334 .stabs "name:symbol_desriptor(global function)return_type(int)",
2335 N_FUN, NIL, NIL, code_addr_of_method_start
2337 .stabs "Ameth__5baseAic:F1",36,0,0,_Ameth__5baseAic
2340 Here is the stab for the `this' pointer implicit argument. The name
2341 of the `this' pointer is always `this.' Type 19, the `this' pointer is
2342 defined as a pointer to type 20, baseA, but a stab defining baseA has
2343 not yet been emited. Since the compiler knows it will be emited
2344 shortly, here it just outputs a cross reference to the undefined
2345 symbol, by prefixing the symbol name with xs.
2348 .stabs "name:sym_desc(register param)type_def(19)=
2349 type_desc(ptr to)type_ref(baseA)=
2350 type_desc(cross-reference to)baseA:",N_RSYM,NIL,NIL,register_number
2352 .stabs "this:P19=*20=xsbaseA:",64,0,0,8
2355 The stab for the explicit integer argument looks just like a parameter
2356 to a C function. The last field of the stab is the offset from the
2357 argument pointer, which in most systems is the same as the frame
2361 .stabs "name:sym_desc(value parameter)type_ref(int)",
2362 N_PSYM,NIL,NIL,offset_from_arg_ptr
2364 .stabs "in:p1",160,0,0,72
2367 << The examples that follow are based on A1.C >>
2370 @section Protections
2373 In the simple class definition shown above all member data and
2374 functions were publicly accessable. The example that follows
2375 contrasts public, protected and privately accessable fields and shows
2376 how these protections are encoded in C++ stabs.
2378 Protections for class member data are signified by two characters
2379 embeded in the stab defining the class type. These characters are
2380 located after the name: part of the string. /0 means private, /1
2381 means protected, and /2 means public. If these characters are omited
2382 this means that the member is public. The following C++ source:
2396 generates the following stab to describe the class type all_data.
2399 .stabs "class_name:sym_desc(type)type_def(19)=type_desc(struct)struct_bytes
2400 data_name:/protection(private)type_ref(int),bit_offset,num_bits;
2401 data_name:/protection(protected)type_ref(char),bit_offset,num_bits;
2402 data_name:(/num omited, private)type_ref(float),bit_offset,num_bits;;"
2407 .stabs "all_data:t19=s12
2408 priv_dat:/01,0,32;prot_dat:/12,32,8;pub_dat:12,64,32;;",128,0,0,0
2411 Protections for member functions are signified by one digit embeded in
2412 the field part of the stab describing the method. The digit is 0 if
2413 private, 1 if protected and 2 if public. Consider the C++ class
2417 class all_methods @{
2419 int priv_meth(int in)@{return in;@};
2421 char protMeth(char in)@{return in;@};
2423 float pubMeth(float in)@{return in;@};
2427 It generates the following stab. The digit in question is to the left
2428 of an `A' in each case. Notice also that in this case two symbol
2429 descriptors apply to the class name struct tag and struct type.
2432 .stabs "class_name:sym_desc(struct tag&type)type_def(21)=
2433 sym_desc(struct)struct_bytes(1)
2434 meth_name::type_def(22)=sym_desc(method)returning(int);
2435 :args(int);protection(private)modifier(normal)virtual(no);
2436 meth_name::type_def(23)=sym_desc(method)returning(char);
2437 :args(char);protection(protected)modifier(normal)virual(no);
2438 meth_name::type_def(24)=sym_desc(method)returning(float);
2439 :args(float);protection(public)modifier(normal)virtual(no);;",
2444 .stabs "all_methods:Tt21=s1priv_meth::22=##1;:i;0A.;protMeth::23=##2;:c;1A.;
2445 pubMeth::24=##12;:f;2A.;;",128,0,0,0
2448 @node Method Modifiers
2449 @section Method Modifiers (const, volatile, const volatile)
2453 In the class example described above all the methods have the normal
2454 modifier. This method modifier information is located just after the
2455 protection information for the method. This field has four possible
2456 character values. Normal methods use A, const methods use B, volatile
2457 methods use C, and const volatile methods use D. Consider the class
2463 int ConstMeth (int arg) const @{ return arg; @};
2464 char VolatileMeth (char arg) volatile @{ return arg; @};
2465 float ConstVolMeth (float arg) const volatile @{return arg; @};
2469 This class is described by the following stab:
2472 .stabs "class(A):sym_desc(struct)type_def(20)=type_desc(struct)struct_bytes(1)
2473 meth_name(ConstMeth)::type_def(21)sym_desc(method)
2474 returning(int);:arg(int);protection(public)modifier(const)virtual(no);
2475 meth_name(VolatileMeth)::type_def(22)=sym_desc(method)
2476 returning(char);:arg(char);protection(public)modifier(volatile)virt(no)
2477 meth_name(ConstVolMeth)::type_def(23)=sym_desc(method)
2478 returning(float);:arg(float);protection(public)modifer(const volatile)
2479 virtual(no);;", @dots{}
2483 .stabs "A:T20=s1ConstMeth::21=##1;:i;2B.;VolatileMeth::22=##2;:c;2C.;
2484 ConstVolMeth::23=##12;:f;2D.;;",128,0,0,0
2487 @node Virtual Methods
2488 @section Virtual Methods
2490 << The following examples are based on a4.C >>
2492 The presence of virtual methods in a class definition adds additional
2493 data to the class description. The extra data is appended to the
2494 description of the virtual method and to the end of the class
2495 description. Consider the class definition below:
2501 virtual int A_virt (int arg) @{ return arg; @};
2505 This results in the stab below describing class A. It defines a new
2506 type (20) which is an 8 byte structure. The first field of the class
2507 struct is Adat, an integer, starting at structure offset 0 and
2510 The second field in the class struct is not explicitly defined by the
2511 C++ class definition but is implied by the fact that the class
2512 contains a virtual method. This field is the vtable pointer. The
2513 name of the vtable pointer field starts with $vf and continues with a
2514 type reference to the class it is part of. In this example the type
2515 reference for class A is 20 so the name of its vtable pointer field is
2516 $vf20, followed by the usual colon.
2518 Next there is a type definition for the vtable pointer type (21).
2519 This is in turn defined as a pointer to another new type (22).
2521 Type 22 is the vtable itself, which is defined as an array, indexed by
2522 a range of integers between 0 and 1, and whose elements are of type
2523 17. Type 17 was the vtable record type defined by the boilerplate C++
2524 type definitions, as shown earlier.
2526 The bit offset of the vtable pointer field is 32. The number of bits
2527 in the field are not specified when the field is a vtable pointer.
2529 Next is the method definition for the virtual member function A_virt.
2530 Its description starts out using the same format as the non-virtual
2531 member functions described above, except instead of a dot after the
2532 `A' there is an asterisk, indicating that the function is virtual.
2533 Since is is virtual some addition information is appended to the end
2534 of the method description.
2536 The first number represents the vtable index of the method. This is a
2537 32 bit unsigned number with the high bit set, followed by a
2540 The second number is a type reference to the first base class in the
2541 inheritence hierarchy defining the virtual member function. In this
2542 case the class stab describes a base class so the virtual function is
2543 not overriding any other definition of the method. Therefore the
2544 reference is to the type number of the class that the stab is
2547 This is followed by three semi-colons. One marks the end of the
2548 current sub-section, one marks the end of the method field, and the
2549 third marks the end of the struct definition.
2551 For classes containing virtual functions the very last section of the
2552 string part of the stab holds a type reference to the first base
2553 class. This is preceeded by `~%' and followed by a final semi-colon.
2556 .stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8)
2557 field_name(Adat):type_ref(int),bit_offset(0),field_bits(32);
2558 field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)=
2559 sym_desc(array)index_type_ref(range of int from 0 to 1);
2560 elem_type_ref(vtbl elem type),
2562 meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
2563 :arg_type(int),protection(public)normal(yes)virtual(yes)
2564 vtable_index(1);class_first_defining(A);;;~%first_base(A);",
2568 @c FIXME: bogus line break.
2570 .stabs "A:t20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2571 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2575 @section Inheritence
2577 Stabs describing C++ derived classes include additional sections that
2578 describe the inheritence hierarchy of the class. A derived class stab
2579 also encodes the number of base classes. For each base class it tells
2580 if the base class is virtual or not, and if the inheritence is private
2581 or public. It also gives the offset into the object of the portion of
2582 the object corresponding to each base class.
2584 This additional information is embeded in the class stab following the
2585 number of bytes in the struct. First the number of base classes
2586 appears bracketed by an exclamation point and a comma.
2588 Then for each base type there repeats a series: two digits, a number,
2589 a comma, another number, and a semi-colon.
2591 The first of the two digits is 1 if the base class is virtual and 0 if
2592 not. The second digit is 2 if the derivation is public and 0 if not.
2594 The number following the first two digits is the offset from the start
2595 of the object to the part of the object pertaining to the base class.
2597 After the comma, the second number is a type_descriptor for the base
2598 type. Finally a semi-colon ends the series, which repeats for each
2601 The source below defines three base classes A, B, and C and the
2609 virtual int A_virt (int arg) @{ return arg; @};
2615 virtual int B_virt (int arg) @{return arg; @};
2621 virtual int C_virt (int arg) @{return arg; @};
2624 class D : A, virtual B, public C @{
2627 virtual int A_virt (int arg ) @{ return arg+1; @};
2628 virtual int B_virt (int arg) @{ return arg+2; @};
2629 virtual int C_virt (int arg) @{ return arg+3; @};
2630 virtual int D_virt (int arg) @{ return arg; @};
2634 Class stabs similar to the ones described earlier are generated for
2637 @c FIXME!!! the linebreaks in the following example probably make the
2638 @c examples literally unusable, but I don't know any other way to get
2639 @c them on the page.
2640 @c One solution would be to put some of the type definitions into
2641 @c separate stabs, even if that's not exactly what the compiler actually
2644 .stabs "A:T20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2645 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2647 .stabs "B:Tt25=s8Bdat:1,0,32;$vf25:21,32;B_virt::26=##1;
2648 :i;2A*-2147483647;25;;;~%25;",128,0,0,0
2650 .stabs "C:Tt28=s8Cdat:1,0,32;$vf28:21,32;C_virt::29=##1;
2651 :i;2A*-2147483647;28;;;~%28;",128,0,0,0
2654 In the stab describing derived class D below, the information about
2655 the derivation of this class is encoded as follows.
2658 .stabs "derived_class_name:symbol_descriptors(struct tag&type)=
2659 type_descriptor(struct)struct_bytes(32)!num_bases(3),
2660 base_virtual(no)inheritence_public(no)base_offset(0),
2661 base_class_type_ref(A);
2662 base_virtual(yes)inheritence_public(no)base_offset(NIL),
2663 base_class_type_ref(B);
2664 base_virtual(no)inheritence_public(yes)base_offset(64),
2665 base_class_type_ref(C); @dots{}
2668 @c FIXME! fake linebreaks.
2670 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:
2671 1,160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt:
2672 :32:i;2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;
2673 28;;D_virt::32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2676 @node Virtual Base Classes
2677 @section Virtual Base Classes
2679 A derived class object consists of a concatination in memory of the
2680 data areas defined by each base class, starting with the leftmost and
2681 ending with the rightmost in the list of base classes. The exception
2682 to this rule is for virtual inheritence. In the example above, class
2683 D inherits virtually from base class B. This means that an instance
2684 of a D object will not contain it's own B part but merely a pointer to
2685 a B part, known as a virtual base pointer.
2687 In a derived class stab, the base offset part of the derivation
2688 information, described above, shows how the base class parts are
2689 ordered. The base offset for a virtual base class is always given as
2690 0. Notice that the base offset for B is given as 0 even though B is
2691 not the first base class. The first base class A starts at offset 0.
2693 The field information part of the stab for class D describes the field
2694 which is the pointer to the virtual base class B. The vbase pointer
2695 name is $vb followed by a type reference to the virtual base class.
2696 Since the type id for B in this example is 25, the vbase pointer name
2699 @c FIXME!! fake linebreaks below
2701 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1,
2702 160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i;
2703 2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt:
2704 :32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2707 Following the name and a semicolon is a type reference describing the
2708 type of the virtual base class pointer, in this case 24. Type 24 was
2709 defined earlier as the type of the B class `this` pointer. The
2710 `this' pointer for a class is a pointer to the class type.
2713 .stabs "this:P24=*25=xsB:",64,0,0,8
2716 Finally the field offset part of the vbase pointer field description
2717 shows that the vbase pointer is the first field in the D object,
2718 before any data fields defined by the class. The layout of a D class
2719 object is a follows, Adat at 0, the vtable pointer for A at 32, Cdat
2720 at 64, the vtable pointer for C at 96, the virtual ase pointer for B
2721 at 128, and Ddat at 160.
2724 @node Static Members
2725 @section Static Members
2727 The data area for a class is a concatenation of the space used by the
2728 data members of the class. If the class has virtual methods, a vtable
2729 pointer follows the class data. The field offset part of each field
2730 description in the class stab shows this ordering.
2732 << How is this reflected in stabs? See Cygnus bug #677 for some info. >>
2735 @appendix Example2.c - source code for extended example
2739 2 register int g_bar asm ("%g5");
2740 3 static int s_g_repeat = 2;
2746 9 char s_char_vec[8];
2747 10 struct s_tag* s_next;
2750 13 typedef struct s_tag s_typedef;
2752 15 char char_vec[3] = @{'a','b','c'@};
2754 17 main (argc, argv)
2758 21 static float s_flap;
2760 23 for (times=0; times < s_g_repeat; times++)@{
2762 25 printf ("Hello world\n");
2766 29 enum e_places @{first,second=3,last@};
2768 31 static s_proc (s_arg, s_ptr_arg, char_vec)
2770 33 s_typedef* s_ptr_arg;
2784 @appendix Example2.s - assembly code for extended example
2788 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
2789 3 .stabs "example2.c",100,0,0,Ltext0
2792 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
2793 7 .stabs "char:t2=r2;0;127;",128,0,0,0
2794 8 .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
2795 9 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
2796 10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
2797 11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
2798 12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
2799 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
2800 14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
2801 15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
2802 16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
2803 17 .stabs "float:t12=r1;4;0;",128,0,0,0
2804 18 .stabs "double:t13=r1;8;0;",128,0,0,0
2805 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
2806 20 .stabs "void:t15=15",128,0,0,0
2807 21 .stabs "g_foo:G2",32,0,0,0
2812 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
2816 @c FIXME! fake linebreak in line 30
2817 30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;s_char_vec:
2818 17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
2819 31 .stabs "s_typedef:t16",128,0,0,0
2820 32 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
2821 33 .global _char_vec
2827 39 .reserve _s_flap.0,4,"bss",4
2831 43 .ascii "Hello world\12\0"
2836 48 .stabn 68,0,20,LM1
2839 51 save %sp,-144,%sp
2846 58 .stabn 68,0,23,LM2
2850 62 sethi %hi(_s_g_repeat),%o0
2852 64 ld [%o0+%lo(_s_g_repeat)],%o0
2857 69 .stabn 68,0,25,LM3
2859 71 sethi %hi(LC0),%o1
2860 72 or %o1,%lo(LC0),%o0
2863 75 .stabn 68,0,26,LM4
2866 78 .stabn 68,0,23,LM5
2874 86 .stabn 68,0,27,LM6
2877 89 .stabn 68,0,27,LM7
2882 94 .stabs "main:F1",36,0,0,_main
2883 95 .stabs "argc:p1",160,0,0,68
2884 96 .stabs "argv:p20=*21=*2",160,0,0,72
2885 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
2886 98 .stabs "times:1",128,0,0,-20
2887 99 .stabn 192,0,0,LBB2
2888 100 .stabs "inner:1",128,0,0,-24
2889 101 .stabn 192,0,0,LBB3
2890 102 .stabn 224,0,0,LBE3
2891 103 .stabn 224,0,0,LBE2
2892 104 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
2893 @c FIXME: fake linebreak in line 105
2894 105 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
2899 109 .stabn 68,0,35,LM8
2902 112 save %sp,-120,%sp
2908 118 .stabn 68,0,41,LM9
2911 121 .stabn 68,0,41,LM10
2916 126 .stabs "s_proc:f1",36,0,0,_s_proc
2917 127 .stabs "s_arg:p16",160,0,0,0
2918 128 .stabs "s_ptr_arg:p18",160,0,0,72
2919 129 .stabs "char_vec:p21",160,0,0,76
2920 130 .stabs "an_u:23",128,0,0,-20
2921 131 .stabn 192,0,0,LBB4
2922 132 .stabn 224,0,0,LBE4
2923 133 .stabs "g_bar:r1",64,0,0,5
2924 134 .stabs "g_pf:G24=*25=f1",32,0,0,0
2925 135 .common _g_pf,4,"bss"
2926 136 .stabs "g_an_s:G16",32,0,0,0
2927 137 .common _g_an_s,20,"bss"
2931 @appendix Values for the Stab Type Field
2933 These are all the possible values for the stab type field, for
2934 @code{a.out} files. This does not apply to XCOFF.
2936 The following types are used by the linker and assembler; there is
2937 nothing stabs-specific about them. Since this document does not attempt
2938 to describe aspects of object file format other than the debugging
2939 format, no details are given.
2941 @c Try to get most of these to fit on a single line.
2951 File scope absolute symbol
2953 @item 0x3 N_ABS | N_EXT
2954 External absolute symbol
2957 File scope text symbol
2959 @item 0x5 N_TEXT | N_EXT
2960 External text symbol
2963 File scope data symbol
2965 @item 0x7 N_DATA | N_EXT
2966 External data symbol
2969 File scope BSS symbol
2971 @item 0x9 N_BSS | N_EXT
2975 Same as N_FN, for Sequent compilers
2978 Symbol is indirected to another symbol
2981 Common sym -- visable after shared lib dynamic link
2984 Absolute set element
2987 Text segment set element
2990 Data segment set element
2993 BSS segment set element
2996 Pointer to set vector
2998 @item 0x1e N_WARNING
2999 Print a warning message during linking
3002 File name of a .o file
3005 The following symbol types indicate that this is a stab. This is the
3006 full list of stab numbers, including stab types that are used in
3007 languages other than C.
3011 Global symbol, @xref{N_GSYM}.
3014 Function name (for BSD Fortran), @xref{N_FNAME}.
3017 Function name (@pxref{Procedures}) or text segment variable
3021 Data segment file-scope variable, @xref{Statics}.
3024 BSS segment file-scope variable, @xref{Statics}.
3027 Name of main routine, @xref{Main Program}.
3029 @c FIXME: discuss this in the main body of the text where we talk about
3030 @c using N_FUN for variables.
3032 Read-only data symbol (Solaris2). Most systems use N_FUN for this.
3035 Global symbol (for Pascal), @xref{N_PC}.
3038 Number of symbols (according to Ultrix V4.0), @xref{N_NSYMS}.
3041 No DST map for sym (according to Ultrix V4.0), @xref{N_NOMAP}.
3043 @c FIXME: describe this solaris feature in the body of the text (see
3044 @c comments in include/aout/stab.def).
3046 Object file (Solaris2).
3048 @c See include/aout/stab.def for (a little) more info.
3050 Debugger options (Solaris2).
3053 Register variable, @xref{N_RSYM}.
3056 Modula-2 compilation unit, @xref{N_M2C}.
3059 Line number in text segment, @xref{Line Numbers}.
3062 Line number in data segment, @xref{Line Numbers}.
3065 Line number in bss segment, @xref{Line Numbers}.
3068 Sun source code browser, path to .cb file, @xref{N_BROWS}.
3071 Gnu Modula2 definition module dependency, @xref{N_DEFD}.
3074 Function start/body/end line numbers (Solaris2).
3077 Gnu C++ exception variable, @xref{N_EHDECL}.
3080 Modula2 info "for imc" (according to Ultrix V4.0), @xref{N_MOD2}.
3083 Gnu C++ "catch" clause, @xref{N_CATCH}.
3086 Structure of union element, @xref{N_SSYM}.
3089 Last stab for module (Solaris2).
3092 Path and name of source file , @xref{Source Files}.
3095 Automatic var in the stack or type definition, @xref{N_LSYM}, @xref{Typedefs}.
3098 Beginning of an include file (Sun only), @xref{Source Files}.
3101 Name of include file, @xref{Source Files}.
3104 Parameter variable, @xref{Parameters}.
3107 End of an include file, @xref{Source Files}.
3110 Alternate entry point, @xref{N_ENTRY}.
3113 Beginning of a lexical block, @xref{Block Structure}.
3116 Place holder for a deleted include file, @xref{Source Files}.
3119 Modula2 scope information (Sun linker), @xref{N_SCOPE}.
3122 End of a lexical block, @xref{Block Structure}.
3125 Begin named common block, @xref{Common Blocks}.
3128 End named common block, @xref{Common Blocks}.
3131 Member of a common block, @xref{Common Blocks}.
3133 @c FIXME: How does this really work? Move it to main body of document.
3135 Pascal @code{with} statement: type,,0,0,offset (Solaris2).
3138 Gould non-base registers, @xref{Gould}.
3141 Gould non-base registers, @xref{Gould}.
3144 Gould non-base registers, @xref{Gould}.
3147 Gould non-base registers, @xref{Gould}.
3150 Gould non-base registers, @xref{Gould}.
3153 @c Restore the default table indent
3158 @node Symbol Descriptors
3159 @appendix Table of Symbol Descriptors
3161 @c Please keep this alphabetical
3163 @c In TeX, this looks great, digit is in italics. But makeinfo insists
3164 @c on putting it in `', not realizing that @var should override @code.
3165 @c I don't know of any way to make makeinfo do the right thing. Seems
3166 @c like a makeinfo bug to me.
3170 Local variable, @xref{Automatic variables}.
3173 Parameter passed by reference in register, @xref{Parameters}.
3176 Constant, @xref{Constants}.
3179 Conformant array bound (Pascal, maybe other languages),
3180 @xref{Parameters}. Name of a caught exception (GNU C++). These can be
3181 distinguished because the latter uses N_CATCH and the former uses
3182 another symbol type.
3185 Floating point register variable, @xref{Register variables}.
3188 Parameter in floating point register, @xref{Parameters}.
3191 File scope function, @xref{Procedures}.
3194 Global function, @xref{Procedures}.
3197 Global variable, @xref{Global Variables}.
3203 Internal (nested) procedure, @xref{Procedures}.
3206 Internal (nested) function, @xref{Procedures}.
3209 Label name (documented by AIX, no further information known).
3212 Module, @xref{Procedures}.
3215 Argument list parameter, @xref{Parameters}.
3221 FORTRAN Function parameter, @xref{Parameters}.
3224 Unfortunately, three separate meanings have been independently invented
3225 for this symbol descriptor. At least the GNU and Sun uses can be
3226 distinguished by the symbol type. Global Procedure (AIX) (symbol type
3227 used unknown), @xref{Procedures}. Register parameter (GNU) (symbol type
3228 N_PSYM), @xref{Parameters}. Prototype of function referenced by this
3229 file (Sun acc) (symbol type N_FUN).
3232 Static Procedure, @xref{Procedures}.
3235 Register parameter @xref{Parameters}.
3238 Register variable, @xref{Register variables}.
3241 File scope variable, @xref{Statics}.
3244 Type name, @xref{Typedefs}.
3247 enumeration, struct or union tag, @xref{Typedefs}.
3250 Parameter passed by reference, @xref{Parameters}.
3253 Procedure scope static variable, @xref{Statics}.
3256 Conformant array, @xref{Parameters}.
3259 Function return variable, @xref{Parameters}.
3262 @node Type Descriptors
3263 @appendix Table of Type Descriptors
3268 Type reference, @xref{Stabs Format}.
3271 Reference to builtin type, @xref{Negative Type Numbers}.
3274 Method (C++), @xref{Cplusplus}.
3277 Pointer, @xref{Miscellaneous Types}.
3283 Type Attributes (AIX), @xref{Stabs Format}. Member (class and variable)
3284 type (GNU C++), @xref{Cplusplus}.
3287 Array, @xref{Arrays}.
3290 Open array, @xref{Arrays}.
3293 Pascal space type (AIX), @xref{Miscellaneous Types}. Builtin integer
3294 type (Sun), @xref{Builtin Type Descriptors}.
3297 Volatile-qualified type, @xref{Miscellaneous Types}.
3300 Complex builtin type, @xref{Builtin Type Descriptors}.
3303 COBOL Picture type. See AIX documentation for details.
3306 File type, @xref{Miscellaneous Types}.
3309 N-dimensional dynamic array, @xref{Arrays}.
3312 Enumeration type, @xref{Enumerations}.
3315 N-dimensional subarray, @xref{Arrays}.
3318 Function type, @xref{Function Types}.
3321 Pascal function parameter, @xref{Function Types}
3324 Builtin floating point type, @xref{Builtin Type Descriptors}.
3327 COBOL Group. See AIX documentation for details.
3330 Imported type, @xref{Cross-references}.
3333 Const-qualified type, @xref{Miscellaneous Types}.
3336 COBOL File Descriptor. See AIX documentation for details.
3339 Multiple instance type, @xref{Miscellaneous Types}.
3342 String type, @xref{Strings}.
3345 Stringptr, @xref{Strings}.
3348 Opaque type, @xref{Typedefs}.
3351 Procedure, @xref{Function Types}.
3354 Packed array, @xref{Arrays}.
3357 Range type, @xref{Subranges}.
3360 Builtin floating type, @xref{Builtin Type Descriptors} (Sun). Pascal
3361 subroutine parameter, @xref{Function Types} (AIX). Detecting this
3362 conflict is possible with careful parsing (hint: a Pascal subroutine
3363 parameter type will always contain a comma, and a builtin type
3364 descriptor never will).
3367 Structure type, @xref{Structures}.
3370 Set type, @xref{Miscellaneous Types}.
3373 Union, @xref{Unions}.
3376 Variant record. This is a Pascal and Modula-2 feature which is like a
3377 union within a struct in C. See AIX documentation for details.
3380 Wide character, @xref{Builtin Type Descriptors}.
3383 Cross-reference, @xref{Cross-references}.
3386 gstring, @xref{Strings}.
3389 @node Expanded reference
3390 @appendix Expanded reference by stab type.
3392 @c FIXME: This appendix should go away, see N_PSYM or N_SO for an example.
3394 For a full list of stab types, and cross-references to where they are
3395 described, @xref{Stab Types}. This appendix just duplicates certain
3396 information from the main body of this document; eventually the
3397 information will all be in one place.
3401 The first line is the symbol type expressed in decimal, hexadecimal,
3402 and as a #define (see devo/include/aout/stab.def).
3404 The second line describes the language constructs the symbol type
3407 The third line is the stab format with the significant stab fields
3408 named and the rest NIL.
3410 Subsequent lines expand upon the meaning and possible values for each
3411 significant stab field. # stands in for the type descriptor.
3413 Finally, any further information.
3416 * N_GSYM:: Global variable
3417 * N_FNAME:: Function name (BSD Fortran)
3418 * N_PC:: Pascal global symbol
3419 * N_NSYMS:: Number of symbols
3420 * N_NOMAP:: No DST map
3421 * N_RSYM:: Register variable
3422 * N_M2C:: Modula-2 compilation unit
3423 * N_BROWS:: Path to .cb file for Sun source code browser
3424 * N_DEFD:: GNU Modula2 definition module dependency
3425 * N_EHDECL:: GNU C++ exception variable
3426 * N_MOD2:: Modula2 information "for imc"
3427 * N_CATCH:: GNU C++ "catch" clause
3428 * N_SSYM:: Structure or union element
3429 * N_LSYM:: Automatic variable
3430 * N_ENTRY:: Alternate entry point
3431 * N_SCOPE:: Modula2 scope information (Sun only)
3432 * Gould:: non-base register symbols used on Gould systems
3433 * N_LENG:: Length of preceding entry
3437 @section 32 - 0x20 - N_GYSM
3442 .stabs "name", N_GSYM, NIL, NIL, NIL
3446 "name" -> "symbol_name:#type"
3450 Only the "name" field is significant. The location of the variable is
3451 obtained from the corresponding external symbol.
3454 @section 34 - 0x22 - N_FNAME
3455 Function name (for BSD Fortran)
3458 .stabs "name", N_FNAME, NIL, NIL, NIL
3462 "name" -> "function_name"
3465 Only the "name" field is significant. The location of the symbol is
3466 obtained from the corresponding extern symbol.
3469 @section 48 - 0x30 - N_PC
3470 Global symbol (for Pascal)
3473 .stabs "name", N_PC, NIL, NIL, value
3477 "name" -> "symbol_name" <<?>>
3478 value -> supposedly the line number (stab.def is skeptical)
3484 global pascal symbol: name,,0,subtype,line
3489 @section 50 - 0x32 - N_NSYMS
3490 Number of symbols (according to Ultrix V4.0)
3493 0, files,,funcs,lines (stab.def)
3497 @section 52 - 0x34 - N_NOMAP
3498 no DST map for sym (according to Ultrix V4.0)
3501 name, ,0,type,ignored (stab.def)
3505 @section 64 - 0x40 - N_RSYM
3509 .stabs "name:type",N_RSYM,0,RegSize,RegNumber (Sun doc)
3513 @section 66 - 0x42 - N_M2C
3514 Modula-2 compilation unit
3517 .stabs "name", N_M2C, 0, desc, value
3521 "name" -> "unit_name,unit_time_stamp[,code_time_stamp]
3523 value -> 0 (main unit)
3528 @section 72 - 0x48 - N_BROWS
3529 Sun source code browser, path to .cb file
3532 "path to associated .cb file"
3534 Note: type field value overlaps with N_BSLINE
3537 @section 74 - 0x4a - N_DEFD
3538 GNU Modula2 definition module dependency
3540 GNU Modula-2 definition module dependency. Value is the modification
3541 time of the definition file. Other is non-zero if it is imported with
3542 the GNU M2 keyword %INITIALIZE. Perhaps N_M2C can be used if there
3543 are enough empty fields?
3546 @section 80 - 0x50 - N_EHDECL
3547 GNU C++ exception variable <<?>>
3549 "name is variable name"
3551 Note: conflicts with N_MOD2.
3554 @section 80 - 0x50 - N_MOD2
3555 Modula2 info "for imc" (according to Ultrix V4.0)
3557 Note: conflicts with N_EHDECL <<?>>
3560 @section 84 - 0x54 - N_CATCH
3561 GNU C++ "catch" clause
3563 GNU C++ `catch' clause. Value is its address. Desc is nonzero if
3564 this entry is immediately followed by a CAUGHT stab saying what
3565 exception was caught. Multiple CAUGHT stabs means that multiple
3566 exceptions can be caught here. If Desc is 0, it means all exceptions
3570 @section 96 - 0x60 - N_SSYM
3571 Structure or union element
3573 Value is offset in the structure.
3575 <<?looking at structs and unions in C I didn't see these>>
3578 @section 128 - 0x80 - N_LSYM
3579 Automatic var in the stack (also used for type descriptors.)
3582 .stabs "name" N_LSYM, NIL, NIL, value
3586 @exdent @emph{For stack based local variables:}
3588 "name" -> name of the variable
3589 value -> offset from frame pointer (negative)
3591 @exdent @emph{For type descriptors:}
3593 "name" -> "name_of_the_type:#type"
3596 type -> type_ref (or) type_def
3598 type_ref -> type_number
3599 type_def -> type_number=type_desc etc.
3602 Type may be either a type reference or a type definition. A type
3603 reference is a number that refers to a previously defined type. A
3604 type definition is the number that will refer to this type, followed
3605 by an equals sign, a type descriptor and the additional data that
3606 defines the type. See the Table D for type descriptors and the
3607 section on types for what data follows each type descriptor.
3610 @section 164 - 0xa4 - N_ENTRY
3612 Alternate entry point.
3613 Value is its address.
3617 @section 196 - 0xc4 - N_SCOPE
3619 Modula2 scope information (Sun linker)
3623 @section Non-base registers on Gould systems
3625 These are used on Gould systems for non-base registers syms.
3627 However, the following values are not the values used by Gould; they are
3628 the values which GNU has been documenting for these values for a long
3629 time, without actually checking what Gould uses. I include these values
3630 only because perhaps some someone actually did something with the GNU
3631 information (I hope not, why GNU knowingly assigned wrong values to
3632 these in the header file is a complete mystery to me).
3635 240 0xf0 N_NBTEXT ??
3636 242 0xf2 N_NBDATA ??
3643 @section - 0xfe - N_LENG
3645 Second symbol entry containing a length-value for the preceding entry.
3646 The value is the length.
3649 @appendix Questions and anomalies
3653 For GNU C stabs defining local and global variables (N_LSYM and
3654 N_GSYM), the desc field is supposed to contain the source line number
3655 on which the variable is defined. In reality the desc field is always
3656 0. (This behavour is defined in dbxout.c and putting a line number in
3657 desc is controlled by #ifdef WINNING_GDB which defaults to false). Gdb
3658 supposedly uses this information if you say 'list var'. In reality
3659 var can be a variable defined in the program and gdb says `function
3663 In GNU C stabs there seems to be no way to differentiate tag types:
3664 structures, unions, and enums (symbol descriptor T) and typedefs
3665 (symbol descriptor t) defined at file scope from types defined locally
3666 to a procedure or other more local scope. They all use the N_LSYM
3667 stab type. Types defined at procedure scope are emited after the
3668 N_RBRAC of the preceding function and before the code of the
3669 procedure in which they are defined. This is exactly the same as
3670 types defined in the source file between the two procedure bodies.
3671 GDB overcompensates by placing all types in block #1, the block for
3672 symbols of file scope. This is true for default, -ansi and
3673 -traditional compiler options. (Bugs gcc/1063, gdb/1066.)
3676 What ends the procedure scope? Is it the proc block's N_RBRAC or the
3677 next N_FUN? (I believe its the first.)
3680 @c FIXME: This should go with the other stuff about global variables.
3681 Global variable stabs don't have location information. This comes
3682 from the external symbol for the same variable. The external symbol
3683 has a leading underbar on the _name of the variable and the stab does
3684 not. How do we know these two symbol table entries are talking about
3685 the same symbol when their names are different? (Answer: the debugger
3686 knows that external symbols have leading underbars).
3688 @c FIXME: This is absurdly vague; there all kinds of differences, some
3689 @c of which are the same between gnu & sun, and some of which aren't.
3691 Can gcc be configured to output stabs the way the Sun compiler
3692 does, so that their native debugging tools work? <NO?> It doesn't by
3693 default. GDB reads either format of stab. (gcc or SunC). How about
3697 @node xcoff-differences
3698 @appendix Differences between GNU stabs in a.out and GNU stabs in xcoff
3700 @c FIXME: Merge *all* these into the main body of the document.
3701 (The AIX/RS6000 native object file format is xcoff with stabs). This
3702 appendix only covers those differences which are not covered in the main
3703 body of this document.
3707 BSD a.out stab types correspond to AIX xcoff storage classes. In general the
3708 mapping is N_STABTYPE becomes C_STABTYPE. Some stab types in a.out
3709 are not supported in xcoff. See Table E. for full mappings.
3711 @c FIXME: Get C_* types for the block, figure out whether it is always
3712 @c used (I suspect not), explain clearly, and move to node Statics.
3714 initialised static N_STSYM and un-initialized static N_LCSYM both map
3715 to the C_STSYM storage class. But the destinction is preserved
3716 because in xcoff N_STSYM and N_LCSYM must be emited in a named static
3717 block. Begin the block with .bs s[RW] data_section_name for N_STSYM
3718 or .bs s bss_section_name for N_LCSYM. End the block with .es
3720 @c FIXME: I think they are trying to say something about whether the
3721 @c assembler defaults the value to the location counter.
3723 If the xcoff stab is a N_FUN (C_FUN) then follow the string field with
3724 ,. instead of just ,
3727 (I think that's it for .s file differences. They could stand to be
3728 better presented. This is just a list of what I have noticed so far.
3729 There are a *lot* of differences in the information in the symbol
3730 tables of the executable and object files.)
3732 Table E: mapping a.out stab types to xcoff storage classes
3735 stab type storage class
3736 -------------------------------
3745 N_RPSYM (0x8e) C_RPSYM
3755 N_DECL (0x8c) C_DECL
3772 @node Sun-differences
3773 @appendix Differences between GNU stabs and Sun native stabs.
3775 @c FIXME: Merge all this stuff into the main body of the document.
3779 GNU C stabs define *all* types, file or procedure scope, as
3780 N_LSYM. Sun doc talks about using N_GSYM too.
3783 Sun C stabs use type number pairs in the format (a,b) where a is a
3784 number starting with 1 and incremented for each sub-source file in the
3785 compilation. b is a number starting with 1 and incremented for each
3786 new type defined in the compilation. GNU C stabs use the type number
3787 alone, with no source file number.
3791 @appendix Using stabs with the ELF object file format.
3793 The ELF object file format allows tools to create object files with custom
3794 sections containing any arbitrary data. To use stabs in ELF object files,
3795 the tools create two custom sections, a ".stab" section which contains
3796 an array of fixed length structures, one struct per stab, and a ".stabstr"
3797 section containing all the variable length strings that are referenced by
3798 stabs in the ".stab" section. The byte order of the stabs binary data
3799 matches the byte order of the ELF file itself, as determined from the
3800 EI_DATA field in the e_ident member of the ELF header.
3802 The first stab in the ".stab" section for each object file is a "synthetic
3803 stab", generated entirely by the assembler, with no corresponding ".stab"
3804 directive as input to the assembler. This stab contains the following
3809 Offset in the ".stabstr" section to the source filename.
3815 Unused field, always zero.
3818 Count of upcoming symbols. I.E. the number of remaining stabs for this
3822 Size of the string table fragment associated with this object module, in
3827 The ".stabstr" section always starts with a null byte (so that string
3828 offsets of zero reference a null string), followed by random length strings,
3829 each of which is null byte terminated.
3831 The ELF section header for the ".stab" section has it's sh_link member set
3832 to the section number of the ".stabstr" section, and the ".stabstr" section
3833 has it's ELF section header sh_type member set to SHT_STRTAB to mark it as