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 * Source Files:: The path and name of the source file
367 @section The path and name of the source files
369 Before any other stabs occur, there must be a stab specifying the source
370 file. This information is contained in a symbol of stab type
371 @code{N_SO}; the string contains the name of the file. The value of the
372 symbol is the start address of portion of the text section corresponding
375 With the Sun Solaris2 compiler, the @code{desc} field contains a
376 source-language code.
378 Some compilers (for example, gcc2 and SunOS4 @file{/bin/cc}) also
379 include the directory in which the source was compiled, in a second
380 @code{N_SO} symbol preceding the one containing the file name. This
381 symbol can be distinguished by the fact that it ends in a slash. Code
382 from the cfront C++ compiler can have additional @code{N_SO} symbols for
383 nonexistent source files after the @code{N_SO} for the real source file;
384 these are believed to contain no useful information.
389 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0 ; 100 is N_SO
390 .stabs "hello.c",100,0,0,Ltext0
395 Instead of @code{N_SO} symbols, XCOFF uses a @code{.file} assembler
396 directive which assembles to a standard COFF @code{.file} symbol;
397 explaining this in detail is outside the scope of this document.
399 There are several different schemes for dealing with include files: the
400 traditional @code{N_SOL} approach, Sun's @code{N_BINCL} scheme, and the
401 XCOFF @code{C_BINCL} (which despite the similar name has little in
402 common with @code{N_BINCL}).
404 An @code{N_SOL} symbol specifies which include file subsequent symbols
405 refer to. The string field is the name of the file and the value is the
406 text address corresponding to the start of the previous include file and
407 the start of this one. To specify the main source file again, use an
408 @code{N_SOL} symbol with the name of the main source file.
410 A @code{N_BINCL} symbol specifies the start of an include file. In an
411 object file, only the name is significant. The Sun linker puts data
412 into some of the other fields. The end of the include file is marked by
413 a @code{N_EINCL} symbol (which has no name field). In an ojbect file,
414 there is no significant data in the @code{N_EINCL} symbol; the Sun
415 linker puts data into some of the fields. @code{N_BINCL} and
416 @code{N_EINCL} can be nested. If the linker detects that two source
417 files have identical stabs with a @code{N_BINCL} and @code{N_EINCL} pair
418 (as will generally be the case for a header file), then it only puts out
419 the stabs once. Each additional occurance is replaced by an
420 @code{N_EXCL} symbol. I believe the Sun (SunOS4, not sure about
421 Solaris) linker is the only one which supports this feature.
423 For the start of an include file in XCOFF, use the @file{.bi} assembler
424 directive which generates a @code{C_BINCL} symbol. A @file{.ei}
425 directive, which generates a @code{C_EINCL} symbol, denotes the end of
426 the include file. Both directives are followed by the name of the
427 source file in quotes, which becomes the string for the symbol. The
428 value of each symbol, produced automatically by the assembler and
429 linker, is an offset into the executable which points to the beginning
430 (inclusive, as you'd expect) and end (inclusive, as you would not
431 expect) of the portion of the COFF linetable which corresponds to this
432 include file. @code{C_BINCL} and @code{C_EINCL} do not nest.
435 @section Line Numbers
437 A @code{N_SLINE} symbol represents the start of a source line. The
438 @var{desc} field contains the line number and the @var{value} field
439 contains the code address for the start of that source line. On most
440 machines the address is absolute; for Sun's stabs-in-elf, it is relative
441 to the function in which the @code{N_SLINE} symbol occurs.
443 GNU documents @code{N_DSLINE} and @code{N_BSLINE} symbols for line
444 numbers in the data or bss segments, respectively. They are identical
445 to @code{N_SLINE} but are relocated differently by the linker. They
446 were intended to be used to describe the source location of a variable
447 declaration, but I believe that gcc2 actually puts the line number in
448 the desc field of the stab for the variable itself. GDB has been
449 ignoring these symbols (unless they contain a string field) at least
452 XCOFF uses COFF line numbers instead, which are outside the scope of
453 this document, ammeliorated by adequate marking of include files
454 (@pxref{Source Files}).
456 For single source lines that generate discontiguous code, such as flow
457 of control statements, there may be more than one line number entry for
458 the same source line. In this case there is a line number entry at the
459 start of each code range, each with the same line number.
464 All of the following stabs use the @samp{N_FUN} symbol type.
466 A function is represented by a @samp{F} symbol descriptor for a global
467 (extern) function, and @samp{f} for a static (local) function. The next
468 @samp{N_SLINE} symbol can be used to find the line number of the start
469 of the function. The value field is the address of the start of the
470 function. The type information of the stab represents the return type
471 of the function; thus @samp{foo:f5} means that foo is a function
474 The type information of the stab is optionally followed by type
475 information for each argument, with each argument preceded by @samp{;}.
476 An argument type of 0 means that additional arguments are being passed,
477 whose types and number may vary (@samp{...} in ANSI C). This extension
478 is used by Sun's Solaris compiler. GDB has tolerated it (i.e. at least
479 parsed the syntax, if not necessarily used the information) at least
480 since version 4.8; I don't know whether all versions of dbx will
481 tolerate it. The argument types given here are not merely redundant
482 with the symbols for the arguments themselves (@pxref{Parameters}), they
483 are the types of the arguments as they are passed, before any
484 conversions might take place. For example, if a C function which is
485 declared without a prototype takes a @code{float} argument, the value is
486 passed as a @code{double} but then converted to a @code{float}.
487 Debuggers need to use the types given in the arguments when printing
488 values, but if calling the function they need to use the types given in
489 the symbol defining the function.
491 If the return type and types of arguments of a function which is defined
492 in another source file are specified (i.e. a function prototype in ANSI
493 C), traditionally compilers emit no stab; the only way for the debugger
494 to find the information is if the source file where the function is
495 defined was also compiled with debugging symbols. As an extension the
496 Solaris compiler uses symbol descriptor @samp{P} followed by the return
497 type of the function, followed by the arguments, each preceded by
498 @samp{;}, as in a stab with symbol descriptor @samp{f} or @samp{F}.
499 This use of symbol descriptor @samp{P} can be distinguished from its use
500 for register parameters (@pxref{Parameters}) by the fact that it has
501 symbol type @code{N_FUN}.
503 The AIX documentation also defines symbol descriptor @samp{J} as an
504 internal function. I assume this means a function nested within another
505 function. It also says Symbol descriptor @samp{m} is a module in
506 Modula-2 or extended Pascal.
508 Procedures (functions which do not return values) are represented as
509 functions returning the void type in C. I don't see why this couldn't
510 be used for all languages (inventing a void type for this purpose if
511 necessary), but the AIX documentation defines @samp{I}, @samp{P}, and
512 @samp{Q} for internal, global, and static procedures, respectively.
513 These symbol descriptors are unusual in that they are not followed by
516 For any of the above symbol descriptors, after the symbol descriptor and
517 the type information, there is optionally a comma, followed by the name
518 of the procedure, followed by a comma, followed by a name specifying the
519 scope. The first name is local to the scope specified. I assume then
520 that the name of the symbol (before the @samp{:}), if specified, is some
521 sort of global name. I assume the name specifying the scope is the name
522 of a function specifying that scope. This feature is an AIX extension,
523 and this information is based on the manual; I haven't actually tried
526 The stab representing a procedure is located immediately following the
527 code of the procedure. This stab is in turn directly followed by a
528 group of other stabs describing elements of the procedure. These other
529 stabs describe the procedure's parameters, its block local variables and
537 The @code{.stabs} entry after this code fragment shows the @var{name} of
538 the procedure (@code{main}); the type descriptor @var{desc} (@code{F},
539 for a global procedure); a reference to the predefined type @code{int}
540 for the return type; and the starting @var{address} of the procedure.
542 Here is an exploded summary (with whitespace introduced for clarity),
543 followed by line 50 of our sample assembly output, which has this form:
547 @var{desc} @r{(global proc @samp{F})}
548 @var{return_type_ref} @r{(int)}
554 50 .stabs "main:F1",36,0,0,_main
557 @node Block Structure
558 @section Block Structure
560 The program's block structure is represented by the @code{N_LBRAC} (left
561 brace) and the @code{N_RBRAC} (right brace) stab types. The variables
562 defined inside a block preceded the @code{N_LBRAC} symbol for most
563 compilers, including GCC. Other compilers, such as the Convex, Acorn
564 RISC machine, and Sun acc compilers, put the variables after the
565 @code{N_LBRAC} symbol. The values of the @code{N_LBRAC} and
566 @code{N_RBRAC} symbols are the start and end addresses of the code of
567 the block, respectively. For most machines, they are relative to the
568 starting address of this source file. For the Gould NP1, they are
569 absolute. For Sun's stabs-in-elf, they are relative to the function in
572 The @code{N_LBRAC} and @code{N_RBRAC} stabs that describe the block
573 scope of a procedure are located after the @code{N_FUN} stab that
574 represents the procedure itself.
576 Sun documents the @code{desc} field of @code{N_LBRAC} and
577 @code{N_RBRAC} symbols as containing the nesting level of the block.
578 However, dbx seems not to care, and GCC just always set @code{desc} to
584 The @samp{c} symbol descriptor indicates that this stab represents a
585 constant. This symbol descriptor is an exception to the general rule
586 that symbol descriptors are followed by type information. Instead, it
587 is followed by @samp{=} and one of the following:
591 Boolean constant. @var{value} is a numeric value; I assume it is 0 for
595 Character constant. @var{value} is the numeric value of the constant.
597 @item e @var{type-information} , @var{value}
598 Constant whose value can be represented as integral.
599 @var{type-information} is the type of the constant, as it would appear
600 after a symbol descriptor (@pxref{Stabs Format}). @var{value} is the
601 numeric value of the constant. GDB 4.9 does not actually get the right
602 value if @var{value} does not fit in a host @code{int}, but it does not
603 do anything violent, and future debuggers could be extended to accept
604 integers of any size (whether unsigned or not). This constant type is
605 usually documented as being only for enumeration constants, but GDB has
606 never imposed that restriction; I don't know about other debuggers.
609 Integer constant. @var{value} is the numeric value. The type is some
610 sort of generic integer type (for GDB, a host @code{int}); to specify
611 the type explicitly, use @samp{e} instead.
614 Real constant. @var{value} is the real value, which can be @samp{INF}
615 (optionally preceded by a sign) for infinity, @samp{QNAN} for a quiet
616 NaN (not-a-number), or @samp{SNAN} for a signalling NaN. If it is a
617 normal number the format is that accepted by the C library function
621 String constant. @var{string} is a string enclosed in either @samp{'}
622 (in which case @samp{'} characters within the string are represented as
623 @samp{\'} or @samp{"} (in which case @samp{"} characters within the
624 string are represented as @samp{\"}).
626 @item S @var{type-information} , @var{elements} , @var{bits} , @var{pattern}
627 Set constant. @var{type-information} is the type of the constant, as it
628 would appear after a symbol descriptor (@pxref{Stabs Format}).
629 @var{elements} is the number of elements in the set (Does this means
630 how many bits of @var{pattern} are actually used, which would be
631 redundant with the type, or perhaps the number of bits set in
632 @var{pattern}? I don't get it), @var{bits} is the number of bits in the
633 constant (meaning it specifies the length of @var{pattern}, I think),
634 and @var{pattern} is a hexadecimal representation of the set. AIX
635 documentation refers to a limit of 32 bytes, but I see no reason why
636 this limit should exist. This form could probably be used for arbitrary
637 constants, not just sets; the only catch is that @var{pattern} should be
638 understood to be target, not host, byte order and format.
641 The boolean, character, string, and set constants are not supported by
642 GDB 4.9, but it will ignore them. GDB 4.8 and earlier gave an error
643 message and refused to read symbols from the file containing the
646 This information is followed by @samp{;}.
649 @chapter A Comprehensive Example in C
651 Now we'll examine a second program, @code{example2}, which builds on the
652 first example to introduce the rest of the stab types, symbol
653 descriptors, and type descriptors used in C.
654 @xref{Example2.c} for the complete @file{.c} source,
655 and @pxref{Example2.s} for the @file{.s} assembly code.
656 This description includes parts of those files.
658 @section Flow of control and nested scopes
664 @code{N_SLINE}, @code{N_LBRAC}, @code{N_RBRAC} (cont.)
667 Consider the body of @code{main}, from @file{example2.c}. It shows more
668 about how @code{N_SLINE}, @code{N_RBRAC}, and @code{N_LBRAC} stabs are used.
672 21 static float s_flap;
674 23 for (times=0; times < s_g_repeat; times++)@{
676 25 printf ("Hello world\n");
681 Here we have a single source line, the @samp{for} line, that generates
682 non-linear flow of control, and non-contiguous code. In this case, an
683 @code{N_SLINE} stab with the same line number proceeds each block of
684 non-contiguous code generated from the same source line.
686 The example also shows nested scopes. The @code{N_LBRAC} and
687 @code{N_LBRAC} stabs that describe block structure are nested in the
688 same order as the corresponding code blocks, those of the for loop
689 inside those for the body of main.
692 This is the label for the @code{N_LBRAC} (left brace) stab marking the
693 start of @code{main}.
700 In the first code range for C source line 23, the @code{for} loop
701 initialize and test, @code{N_SLINE} (68) records the line number:
708 58 .stabn 68,0,23,LM2
712 62 sethi %hi(_s_g_repeat),%o0
714 64 ld [%o0+%lo(_s_g_repeat)],%o0
719 @exdent label for the @code{N_LBRAC} (start block) marking the start of @code{for} loop
722 69 .stabn 68,0,25,LM3
724 71 sethi %hi(LC0),%o1
725 72 or %o1,%lo(LC0),%o0
728 75 .stabn 68,0,26,LM4
731 @exdent label for the @code{N_RBRAC} (end block) stab marking the end of the @code{for} loop
737 Now we come to the second code range for source line 23, the @code{for}
738 loop increment and return. Once again, @code{N_SLINE} (68) records the
742 .stabn, N_SLINE, NIL,
746 78 .stabn 68,0,23,LM5
754 86 .stabn 68,0,27,LM6
757 @exdent label for the @code{N_RBRAC} (end block) stab marking the end of the @code{for} loop
760 89 .stabn 68,0,27,LM7
765 94 .stabs "main:F1",36,0,0,_main
766 95 .stabs "argc:p1",160,0,0,68
767 96 .stabs "argv:p20=*21=*2",160,0,0,72
768 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
769 98 .stabs "times:1",128,0,0,-20
773 Here is an illustration of stabs describing nested scopes. The scope
774 nesting is reflected in the nested bracketing stabs (@code{N_LBRAC},
778 .stabn N_LBRAC,NIL,NIL,
779 @var{block-start-address}
781 99 .stabn 192,0,0,LBB2 ## begin proc label
782 100 .stabs "inner:1",128,0,0,-24
783 101 .stabn 192,0,0,LBB3 ## begin for label
787 @code{N_RBRAC} (224), ``right brace'' ends a lexical block (scope).
790 .stabn N_RBRAC,NIL,NIL,
791 @var{block-end-address}
793 102 .stabn 224,0,0,LBE3 ## end for label
794 103 .stabn 224,0,0,LBE2 ## end proc label
801 * Automatic variables:: Variables allocated on the stack.
802 * Global Variables:: Variables used by more than one source file.
803 * Register variables:: Variables in registers.
804 * Common Blocks:: Variables statically allocated together.
805 * Initialized statics:: Static variables with values.
806 * Un-initialized statics:: Static variables initialialized to 0.
807 * Parameters:: Passing variables to functions.
810 @node Automatic variables
811 @section Locally scoped automatic variables
818 @item Symbol Descriptor:
822 In addition to describing types, the @code{N_LSYM} stab type also
823 describes locally scoped automatic variables. Refer again to the body
824 of @code{main} in @file{example2.c}. It allocates two automatic
825 variables: @samp{times} is scoped to the body of @code{main}, and
826 @samp{inner} is scoped to the body of the @code{for} loop.
827 @samp{s_flap} is locally scoped but not automatic, and will be discussed
832 21 static float s_flap;
834 23 for (times=0; times < s_g_repeat; times++)@{
836 25 printf ("Hello world\n");
841 The @code{N_LSYM} stab for an automatic variable is located just before the
842 @code{N_LBRAC} stab describing the open brace of the block to which it is
846 @exdent @code{N_LSYM} (128): automatic variable, scoped locally to @code{main}
849 @var{type information}",
851 @var{frame-pointer-offset}
853 98 .stabs "times:1",128,0,0,-20
854 99 .stabn 192,0,0,LBB2 ## begin `main' N_LBRAC
856 @exdent @code{N_LSYM} (128): automatic variable, scoped locally to the @code{for} loop
859 @var{type information}",
861 @var{frame-pointer-offset}
863 100 .stabs "inner:1",128,0,0,-24
864 101 .stabn 192,0,0,LBB3 ## begin `for' loop N_LBRAC
867 The symbol descriptor is omitted for automatic variables. Since type
868 information should being with a digit, @samp{-}, or @samp{(}, only
869 digits, @samp{-}, and @samp{(} are precluded from being used for symbol
870 descriptors by this fact. However, the Acorn RISC machine (ARM) is said
871 to get this wrong: it puts out a mere type definition here, without the
872 preceding @code{@var{typenumber}=}. This is a bad idea; there is no
873 guarantee that type descriptors are distinct from symbol descriptors.
875 @node Global Variables
876 @section Global Variables
883 @item Symbol Descriptor:
887 Global variables are represented by the @code{N_GSYM} stab type. The symbol
888 descriptor, following the colon in the string field, is @samp{G}. Following
889 the @samp{G} is a type reference or type definition. In this example it is a
890 type reference to the basic C type, @code{char}. The first source line in
898 yields the following stab. The stab immediately precedes the code that
899 allocates storage for the variable it describes.
902 @exdent @code{N_GSYM} (32): global symbol
907 N_GSYM, NIL, NIL, NIL
909 21 .stabs "g_foo:G2",32,0,0,0
916 The address of the variable represented by the @code{N_GSYM} is not contained
917 in the @code{N_GSYM} stab. The debugger gets this information from the
918 external symbol for the global variable.
920 @node Register variables
921 @section Register variables
923 @c According to an old version of this manual, AIX uses C_RPSYM instead
924 @c of C_RSYM. I am skeptical; this should be verified.
925 Register variables have their own stab type, @code{N_RSYM}, and their
926 own symbol descriptor, @code{r}. The stab's value field contains the
927 number of the register where the variable data will be stored.
929 The value is the register number.
931 AIX defines a separate symbol descriptor @samp{d} for floating point
932 registers. This seems unnecessary---why not just just give floating
933 point registers different register numbers? I have not verified whether
934 the compiler actually uses @samp{d}.
936 If the register is explicitly allocated to a global variable, but not
940 register int g_bar asm ("%g5");
943 the stab may be emitted at the end of the object file, with
944 the other bss symbols.
947 @section Common Blocks
949 A common block is a statically allocated section of memory which can be
950 referred to by several source files. It may contain several variables.
951 I believe @sc{fortran} is the only language with this feature. A
952 @code{N_BCOMM} stab begins a common block and an @code{N_ECOMM} stab
953 ends it. The only thing which is significant about these two stabs is
954 their name, which can be used to look up a normal (non-debugging) symbol
955 which gives the address of the common block. Each variable in the
956 common block has a @code{N_ECOML} stab, whose value is the offset within
957 the common block of that variable. I'm not sure what symbol descriptor
958 is used for the @code{N_ECOML} stabs.
960 @node Initialized statics
961 @section Initialized static variables
968 @item Symbol Descriptors:
969 @code{S} (file scope), @code{V} (procedure scope)
972 Initialized static variables are represented by the @code{N_STSYM} stab
973 type. The symbol descriptor part of the string field shows if the
974 variable is file scope static (@samp{S}) or procedure scope static
975 (@samp{V}). The source line
978 3 static int s_g_repeat = 2;
982 yields the following code. The stab is located immediately preceding
983 the storage for the variable it represents. Since the variable in
984 this example is file scope static the symbol descriptor is @samp{S}.
987 @exdent @code{N_STSYM} (38): initialized static variable (data seg w/internal linkage)
995 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
1002 @node Un-initialized statics
1003 @section Un-initialized static variables
1010 @item Symbol Descriptors:
1011 @code{S} (file scope), @code{V} (procedure scope)
1014 Un-initialized static variables are represented by the @code{N_LCSYM}
1015 stab type. The symbol descriptor part of the string shows if the
1016 variable is file scope static (@samp{S}) or procedure scope static
1017 (@samp{V}). In this example it is procedure scope static. The source
1018 line allocating @code{s_flap} immediately follows the open brace for the
1019 procedure @code{main}.
1023 21 static float s_flap;
1026 The code that reserves storage for the variable @code{s_flap} precedes the
1027 body of body of @code{main}.
1030 39 .reserve _s_flap.0,4,"bss",4
1033 But since @code{s_flap} is scoped locally to @code{main}, its stab is
1034 located with the other stabs representing symbols local to @code{main}.
1035 The stab for @code{s_flap} is located just before the @code{N_LBRAC} for
1039 @exdent @code{N_LCSYM} (40): uninitialized static var (BSS seg w/internal linkage)
1047 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
1048 98 .stabs "times:1",128,0,0,-20
1049 99 .stabn 192,0,0,LBB2 # N_LBRAC for main.
1052 @c ............................................................
1057 The symbol descriptor @samp{p} is used to refer to parameters which are
1058 in the arglist. Symbols have symbol type @samp{N_PSYM}. The value of
1059 the symbol is the offset relative to the argument list.
1061 If the parameter is passed in a register, then the traditional way to do
1062 this is to provide two symbols for each argument:
1065 .stabs "arg:p1" . . . ; N_PSYM
1066 .stabs "arg:r1" . . . ; N_RSYM
1069 Debuggers are expected to use the second one to find the value, and the
1070 first one to know that it is an argument.
1072 Because this is kind of ugly, some compilers use symbol descriptor
1073 @samp{P} or @samp{R} to indicate an argument which is in a register.
1074 The symbol value is the register number. @samp{P} and @samp{R} mean the
1075 same thing, the difference is that @samp{P} is a GNU invention and
1076 @samp{R} is an IBM (xcoff) invention. As of version 4.9, GDB should
1077 handle either one. Symbol type @samp{C_RPSYM} is used with @samp{R} and
1078 @samp{N_RSYM} is used with @samp{P}.
1080 AIX, according to the documentation, uses @samp{D} for a parameter
1081 passed in a floating point register. This strikes me as incredibly
1082 bogus---why doesn't it just use @samp{R} with a register number which
1083 indicates that it's a floating point register? I haven't verified
1084 whether the system actually does what the documentation indicates.
1086 There is at least one case where GCC uses a @samp{p}/@samp{r} pair
1087 rather than @samp{P}; this is where the argument is passed in the
1088 argument list and then loaded into a register.
1090 On the sparc and hppa, for a @samp{P} symbol whose type is a structure
1091 or union, the register contains the address of the structure. On the
1092 sparc, this is also true of a @samp{p}/@samp{r} pair (using Sun cc) or a
1093 @samp{p} symbol. However, if a (small) structure is really in a
1094 register, @samp{r} is used. And, to top it all off, on the hppa it
1095 might be a structure which was passed on the stack and loaded into a
1096 register and for which there is a @samp{p}/@samp{r} pair! I believe
1097 that symbol descriptor @samp{i} is supposed to deal with this case, (it
1098 is said to mean "value parameter by reference, indirect access", I don't
1099 know the source for this information) but I don't know details or what
1100 compilers or debuggers use it, if any (not GDB or GCC). It is not clear
1101 to me whether this case needs to be dealt with differently than
1102 parameters passed by reference (see below).
1104 There is another case similar to an argument in a register, which is an
1105 argument which is actually stored as a local variable. Sometimes this
1106 happens when the argument was passed in a register and then the compiler
1107 stores it as a local variable. If possible, the compiler should claim
1108 that it's in a register, but this isn't always done. Some compilers use
1109 the pair of symbols approach described above ("arg:p" followed by
1110 "arg:"); this includes gcc1 (not gcc2) on the sparc when passing a small
1111 structure and gcc2 (sometimes) when the argument type is float and it is
1112 passed as a double and converted to float by the prologue (in the latter
1113 case the type of the "arg:p" symbol is double and the type of the "arg:"
1114 symbol is float). GCC, at least on the 960, uses a single @samp{p}
1115 symbol descriptor for an argument which is stored as a local variable
1116 but uses @samp{N_LSYM} instead of @samp{N_PSYM}. In this case the value
1117 of the symbol is an offset relative to the local variables for that
1118 function, not relative to the arguments (on some machines those are the
1119 same thing, but not on all).
1121 If the parameter is passed by reference (e.g. Pascal VAR parameters),
1122 then type symbol descriptor is @samp{v} if it is in the argument list,
1123 or @samp{a} if it in a register. Other than the fact that these contain
1124 the address of the parameter other than the parameter itself, they are
1125 identical to @samp{p} and @samp{R}, respectively. I believe @samp{a} is
1126 an AIX invention; @samp{v} is supported by all stabs-using systems as
1129 @c Is this paragraph correct? It is based on piecing together patchy
1130 @c information and some guesswork
1131 Conformant arrays refer to a feature of Modula-2, and perhaps other
1132 languages, in which the size of an array parameter is not known to the
1133 called function until run-time. Such parameters have two stabs, a
1134 @samp{x} for the array itself, and a @samp{C}, which represents the size
1135 of the array. The value of the @samp{x} stab is the offset in the
1136 argument list where the address of the array is stored (it this right?
1137 it is a guess); the value of the @samp{C} stab is the offset in the
1138 argument list where the size of the array (in elements? in bytes?) is
1141 The following are also said to go with @samp{N_PSYM}:
1144 "name" -> "param_name:#type"
1146 -> pF FORTRAN function parameter
1147 -> X (function result variable)
1148 -> b (based variable)
1150 value -> offset from the argument pointer (positive).
1153 As a simple example, the code
1165 .stabs "main:F1",36,0,0,_main ; 36 is N_FUN
1166 .stabs "argc:p1",160,0,0,68 ; 160 is N_PSYM
1167 .stabs "argv:p20=*21=*2",160,0,0,72
1170 The type definition of argv is interesting because it contains several
1171 type definitions. Type 21 is pointer to type 2 (char) and argv (type 20) is
1175 @chapter Type Definitions
1177 Now let's look at some variable definitions involving complex types.
1178 This involves understanding better how types are described. In the
1179 examples so far types have been described as references to previously
1180 defined types or defined in terms of subranges of or pointers to
1181 previously defined types. The section that follows will talk about
1182 the various other type descriptors that may follow the = sign in a
1186 * Builtin types:: Integers, floating point, void, etc.
1187 * Miscellaneous Types:: Pointers, sets, files, etc.
1188 * Cross-references:: Referring to a type not yet defined.
1189 * Subranges:: A type with a specific range.
1190 * Arrays:: An aggregate type of same-typed elements.
1191 * Strings:: Like an array but also has a length.
1192 * Enumerations:: Like an integer but the values have names.
1193 * Structures:: An aggregate type of different-typed elements.
1194 * Typedefs:: Giving a type a name.
1195 * Unions:: Different types sharing storage.
1200 @section Builtin types
1202 Certain types are built in (@code{int}, @code{short}, @code{void},
1203 @code{float}, etc.); the debugger recognizes these types and knows how
1204 to handle them. Thus don't be surprised if some of the following ways
1205 of specifying builtin types do not specify everything that a debugger
1206 would need to know about the type---in some cases they merely specify
1207 enough information to distinguish the type from other types.
1209 The traditional way to define builtin types is convolunted, so new ways
1210 have been invented to describe them. Sun's ACC uses the @samp{b} and
1211 @samp{R} type descriptors, and IBM uses negative type numbers. GDB can
1212 accept all three, as of version 4.8; dbx just accepts the traditional
1213 builtin types and perhaps one of the other two formats.
1216 * Traditional Builtin Types:: Put on your seatbelts and prepare for kludgery
1217 * Builtin Type Descriptors:: Builtin types with special type descriptors
1218 * Negative Type Numbers:: Builtin types using negative type numbers
1221 @node Traditional Builtin Types
1222 @subsection Traditional Builtin types
1224 Often types are defined as subranges of themselves. If the array bounds
1225 can fit within an @code{int}, then they are given normally. For example:
1228 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0 ; 128 is N_LSYM
1229 .stabs "char:t2=r2;0;127;",128,0,0,0
1232 Builtin types can also be described as subranges of @code{int}:
1235 .stabs "unsigned short:t6=r1;0;65535;",128,0,0,0
1238 If the lower bound of a subrange is 0 and the upper bound is -1, it
1239 means that the type is an unsigned integral type whose bounds are too
1240 big to describe in an int. Traditionally this is only used for
1241 @code{unsigned int} and @code{unsigned long}; GCC also sometimes uses it
1242 for @code{long long} and @code{unsigned long long}, and the only way to
1243 tell those types apart is to look at their names. On other machines GCC
1244 puts out bounds in octal, with a leading 0. In this case a negative
1245 bound consists of a number which is a 1 bit followed by a bunch of 0
1246 bits, and a positive bound is one in which a bunch of bits are 1.
1249 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
1250 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
1253 If the lower bound of a subrange is 0 and the upper bound is negative,
1254 it means that it is an unsigned integral type whose size in bytes is the
1255 absolute value of the upper bound. I believe this is a Convex
1256 convention for @code{unsigned long long}.
1258 If the lower bound of a subrange is negative and the upper bound is 0,
1259 it means that the type is a signed integral type whose size in bytes is
1260 the absolute value of the lower bound. I believe this is a Convex
1261 convention for @code{long long}. To distinguish this from a legitimate
1262 subrange, the type should be a subrange of itself. I'm not sure whether
1263 this is the case for Convex.
1265 If the upper bound of a subrange is 0, it means that this is a floating
1266 point type, and the lower bound of the subrange indicates the number of
1270 .stabs "float:t12=r1;4;0;",128,0,0,0
1271 .stabs "double:t13=r1;8;0;",128,0,0,0
1274 However, GCC writes @code{long double} the same way it writes
1275 @code{double}; the only way to distinguish them is by the name:
1278 .stabs "long double:t14=r1;8;0;",128,0,0,0
1281 Complex types are defined the same way as floating-point types; the only
1282 way to distinguish a single-precision complex from a double-precision
1283 floating-point type is by the name.
1285 The C @code{void} type is defined as itself:
1288 .stabs "void:t15=15",128,0,0,0
1291 I'm not sure how a boolean type is represented.
1293 @node Builtin Type Descriptors
1294 @subsection Defining Builtin Types using Builtin Type Descriptors
1296 There are various type descriptors to define builtin types:
1299 @c FIXME: clean up description of width and offset, once we figure out
1301 @item b @var{signed} @var{char-flag} @var{width} ; @var{offset} ; @var{nbits} ;
1302 Define an integral type. @var{signed} is @samp{u} for unsigned or
1303 @samp{s} for signed. @var{char-flag} is @samp{c} which indicates this
1304 is a character type, or is omitted. I assume this is to distinguish an
1305 integral type from a character type of the same size, for example it
1306 might make sense to set it for the C type @code{wchar_t} so the debugger
1307 can print such variables differently (Solaris does not do this). Sun
1308 sets it on the C types @code{signed char} and @code{unsigned char} which
1309 arguably is wrong. @var{width} and @var{offset} appear to be for small
1310 objects stored in larger ones, for example a @code{short} in an
1311 @code{int} register. @var{width} is normally the number of bytes in the
1312 type. @var{offset} seems to always be zero. @var{nbits} is the number
1313 of bits in the type.
1315 Note that type descriptor @samp{b} used for builtin types conflicts with
1316 its use for Pascal space types (@pxref{Miscellaneous Types}); they can
1317 be distinguished because the character following the type descriptor
1318 will be a digit, @samp{(}, or @samp{-} for a Pascal space type, or
1319 @samp{u} or @samp{s} for a builtin type.
1322 Documented by AIX to define a wide character type, but their compiler
1323 actually uses negative type numbers (@pxref{Negative Type Numbers}).
1325 @item R @var{fp_type} ; @var{bytes} ;
1326 Define a floating point type. @var{fp_type} has one of the following values:
1330 IEEE 32-bit (single precision) floating point format.
1333 IEEE 64-bit (double precision) floating point format.
1335 @item 3 (NF_COMPLEX)
1336 @item 4 (NF_COMPLEX16)
1337 @item 5 (NF_COMPLEX32)
1338 @c "GDB source" really means @file{include/aout/stab_gnu.h}, but trying
1339 @c to put that here got an overfull hbox.
1340 These are for complex numbers. A comment in the GDB source describes
1341 them as Fortran complex, double complex, and complex*16, respectively,
1342 but what does that mean? (i.e. Single precision? Double precison?).
1344 @item 6 (NF_LDOUBLE)
1345 Long double. This should probably only be used for Sun format long
1346 double, and new codes should be used for other floating point formats
1347 (NF_DOUBLE can be used if a long double is really just an IEEE double,
1351 @var{bytes} is the number of bytes occupied by the type. This allows a
1352 debugger to perform some operations with the type even if it doesn't
1353 understand @var{fp_code}.
1355 @item g @var{type-information} ; @var{nbits}
1356 Documented by AIX to define a floating type, but their compiler actually
1357 uses negative type numbers (@pxref{Negative Type Numbers}).
1359 @item c @var{type-information} ; @var{nbits}
1360 Documented by AIX to define a complex type, but their compiler actually
1361 uses negative type numbers (@pxref{Negative Type Numbers}).
1364 The C @code{void} type is defined as a signed integral type 0 bits long:
1366 .stabs "void:t19=bs0;0;0",128,0,0,0
1368 The Solaris compiler seems to omit the trailing semicolon in this case.
1369 Getting sloppy in this way is not a swift move because if a type is
1370 embedded in a more complex expression it is necessary to be able to tell
1373 I'm not sure how a boolean type is represented.
1375 @node Negative Type Numbers
1376 @subsection Negative Type numbers
1378 Since the debugger knows about the builtin types anyway, the idea of
1379 negative type numbers is simply to give a special type number which
1380 indicates the built in type. There is no stab defining these types.
1382 I'm not sure whether anyone has tried to define what this means if
1383 @code{int} can be other than 32 bits (or other types can be other than
1384 their customary size). If @code{int} has exactly one size for each
1385 architecture, then it can be handled easily enough, but if the size of
1386 @code{int} can vary according the compiler options, then it gets hairy.
1387 I guess the consistent way to do this would be to define separate
1388 negative type numbers for 16-bit @code{int} and 32-bit @code{int};
1389 therefore I have indicated below the customary size (and other format
1390 information) for each type. The information below is currently correct
1391 because AIX on the RS6000 is the only system which uses these type
1392 numbers. If these type numbers start to get used on other systems, I
1393 suspect the correct thing to do is to define a new number in cases where
1394 a type does not have the size and format indicated below.
1396 Also note that part of the definition of the negative type number is
1397 the name of the type. Types with identical size and format but
1398 different names have different negative type numbers.
1402 @code{int}, 32 bit signed integral type.
1405 @code{char}, 8 bit type holding a character. Both GDB and dbx on AIX
1406 treat this as signed. GCC uses this type whether @code{char} is signed
1407 or not, which seems like a bad idea. The AIX compiler (xlc) seems to
1408 avoid this type; it uses -5 instead for @code{char}.
1411 @code{short}, 16 bit signed integral type.
1414 @code{long}, 32 bit signed integral type.
1417 @code{unsigned char}, 8 bit unsigned integral type.
1420 @code{signed char}, 8 bit signed integral type.
1423 @code{unsigned short}, 16 bit unsigned integral type.
1426 @code{unsigned int}, 32 bit unsigned integral type.
1429 @code{unsigned}, 32 bit unsigned integral type.
1432 @code{unsigned long}, 32 bit unsigned integral type.
1435 @code{void}, type indicating the lack of a value.
1438 @code{float}, IEEE single precision.
1441 @code{double}, IEEE double precision.
1444 @code{long double}, IEEE double precision. The compiler claims the size
1445 will increase in a future release, and for binary compatibility you have
1446 to avoid using @code{long double}. I hope when they increase it they
1447 use a new negative type number.
1450 @code{integer}. 32 bit signed integral type.
1453 @code{boolean}. Only one bit is used, not sure about the actual size of the
1457 @code{short real}. IEEE single precision.
1460 @code{real}. IEEE double precision.
1463 @code{stringptr}. @xref{Strings}.
1466 @code{character}, 8 bit unsigned type.
1469 @code{logical*1}, 8 bit unsigned integral type.
1472 @code{logical*2}, 16 bit unsigned integral type.
1475 @code{logical*4}, 32 bit unsigned integral type.
1478 @code{logical}, 32 bit unsigned integral type.
1481 @code{complex}. A complex type consisting of two IEEE single-precision
1482 floating point values.
1485 @code{complex}. A complex type consisting of two IEEE double-precision
1486 floating point values.
1489 @code{integer*1}, 8 bit signed integral type.
1492 @code{integer*2}, 16 bit signed integral type.
1495 @code{integer*4}, 32 bit signed integral type.
1498 @code{wchar}. Wide character, 16 bits wide (Unicode format?). This is
1499 not used for the C type @code{wchar_t}.
1502 @node Miscellaneous Types
1503 @section Miscellaneous Types
1506 @item b @var{type-information} ; @var{bytes}
1507 Pascal space type. This is documented by IBM; what does it mean?
1509 Note that this use of the @samp{b} type descriptor can be distinguished
1510 from its use for builtin integral types (@pxref{Builtin Type
1511 Descriptors}) because the character following the type descriptor is
1512 always a digit, @samp{(}, or @samp{-}.
1514 @item B @var{type-information}
1515 A volatile-qualified version of @var{type-information}. This is a Sun
1516 extension. A volatile-qualified type means that references and stores
1517 to a variable of that type must not be optimized or cached; they must
1518 occur as the user specifies them.
1520 @item d @var{type-information}
1521 File of type @var{type-information}. As far as I know this is only used
1524 @item k @var{type-information}
1525 A const-qualified version of @var{type-information}. This is a Sun
1526 extension. A const-qualified type means that a variable of this type
1529 @item M @var{type-information} ; @var{length}
1530 Multiple instance type. The type seems to composed of @var{length}
1531 repetitions of @var{type-information}, for example @code{character*3} is
1532 represented by @samp{M-2;3}, where @samp{-2} is a reference to a
1533 character type (@pxref{Negative Type Numbers}). I'm not sure how this
1534 differs from an array. This appears to be a FORTRAN feature.
1535 @var{length} is a bound, like those in range types, @xref{Subranges}.
1537 @item S @var{type-information}
1538 Pascal set type. @var{type-information} must be a small type such as an
1539 enumeration or a subrange, and the type is a bitmask whose length is
1540 specified by the number of elements in @var{type-information}.
1542 @item * @var{type-information}
1543 Pointer to @var{type-information}.
1546 @node Cross-references
1547 @section Cross-references to other types
1549 If a type is used before it is defined, one common way to deal with this
1550 is just to use a type reference to a type which has not yet been
1551 defined. The debugger is expected to be able to deal with this.
1553 Another way is with the @samp{x} type descriptor, which is followed by
1554 @samp{s} for a structure tag, @samp{u} for a union tag, or @samp{e} for
1555 a enumerator tag, followed by the name of the tag, followed by @samp{:}.
1556 for example the following C declarations:
1566 .stabs "bar:G16=*17=xsfoo:",32,0,0,0
1569 Not all debuggers support the @samp{x} type descriptor, so on some
1570 machines GCC does not use it. I believe that for the above example it
1571 would just emit a reference to type 17 and never define it, but I
1572 haven't verified that.
1574 Modula-2 imported types, at least on AIX, use the @samp{i} type
1575 descriptor, which is followed by the name of the module from which the
1576 type is imported, followed by @samp{:}, followed by the name of the
1577 type. There is then optionally a comma followed by type information for
1578 the type (This differs from merely naming the type (@pxref{Typedefs}) in
1579 that it identifies the module; I don't understand whether the name of
1580 the type given here is always just the same as the name we are giving
1581 it, or whether this type descriptor is used with a nameless stab
1582 (@pxref{Stabs Format}), or what). The symbol ends with @samp{;}.
1585 @section Subrange types
1587 The @samp{r} type descriptor defines a type as a subrange of another
1588 type. It is followed by type information for the type which it is a
1589 subrange of, a semicolon, an integral lower bound, a semicolon, an
1590 integral upper bound, and a semicolon. The AIX documentation does not
1591 specify the trailing semicolon, in an effort to specify array indexes
1592 more cleanly, but a subrange which is not an array index has always
1593 included a trailing semicolon (@pxref{Arrays}).
1595 Instead of an integer, either bound can be one of the following:
1598 @item A @var{offset}
1599 The bound is passed by reference on the stack at offset @var{offset}
1600 from the argument list. @xref{Parameters}, for more information on such
1603 @item T @var{offset}
1604 The bound is passed by value on the stack at offset @var{offset} from
1607 @item a @var{register-number}
1608 The bound is pased by reference in register number
1609 @var{register-number}.
1611 @item t @var{register-number}
1612 The bound is passed by value in register number @var{register-number}.
1618 Subranges are also used for builtin types, @xref{Traditional Builtin Types}.
1621 @section Array types
1623 Arrays use the @samp{a} type descriptor. Following the type descriptor
1624 is the type of the index and the type of the array elements. If the
1625 index type is a range type, it will end in a semicolon; if it is not a
1626 range type (for example, if it is a type reference), there does not
1627 appear to be any way to tell where the types are separated. In an
1628 effort to clean up this mess, IBM documents the two types as being
1629 separated by a semicolon, and a range type as not ending in a semicolon
1630 (but this is not right for range types which are not array indexes,
1631 @pxref{Subranges}). I think probably the best solution is to specify
1632 that a semicolon ends a range type, and that the index type and element
1633 type of an array are separated by a semicolon, but that if the index
1634 type is a range type, the extra semicolon can be omitted. GDB (at least
1635 through version 4.9) doesn't support any kind of index type other than a
1636 range anyway; I'm not sure about dbx.
1638 It is well established, and widely used, that the type of the index,
1639 unlike most types found in the stabs, is merely a type definition, not
1640 type information (@pxref{Stabs Format}) (that is, it need not start with
1641 @var{type-number}@code{=} if it is defining a new type). According to a
1642 comment in GDB, this is also true of the type of the array elements; it
1643 gives @samp{ar1;1;10;ar1;1;10;4} as a legitimate way to express a two
1644 dimensional array. According to AIX documentation, the element type
1645 must be type information. GDB accepts either.
1647 The type of the index is often a range type, expressed as the letter r
1648 and some parameters. It defines the size of the array. In the example
1649 below, the range @code{r1;0;2;} defines an index type which is a
1650 subrange of type 1 (integer), with a lower bound of 0 and an upper bound
1651 of 2. This defines the valid range of subscripts of a three-element C
1654 For example, the definition
1657 char char_vec[3] = @{'a','b','c'@};
1664 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
1673 If an array is @dfn{packed}, it means that the elements are spaced more
1674 closely than normal, saving memory at the expense of speed. For
1675 example, an array of 3-byte objects might, if unpacked, have each
1676 element aligned on a 4-byte boundary, but if packed, have no padding.
1677 One way to specify that something is packed is with type attributes
1678 (@pxref{Stabs Format}), in the case of arrays another is to use the
1679 @samp{P} type descriptor instead of @samp{a}. Other than specifying a
1680 packed array, @samp{P} is identical to @samp{a}.
1682 @c FIXME-what is it? A pointer?
1683 An open array is represented by the @samp{A} type descriptor followed by
1684 type information specifying the type of the array elements.
1686 @c FIXME: what is the format of this type? A pointer to a vector of pointers?
1687 An N-dimensional dynamic array is represented by
1690 D @var{dimensions} ; @var{type-information}
1693 @c Does dimensions really have this meaning? The AIX documentation
1695 @var{dimensions} is the number of dimensions; @var{type-information}
1696 specifies the type of the array elements.
1698 @c FIXME: what is the format of this type? A pointer to some offsets in
1700 A subarray of an N-dimensional array is represented by
1703 E @var{dimensions} ; @var{type-information}
1706 @c Does dimensions really have this meaning? The AIX documentation
1708 @var{dimensions} is the number of dimensions; @var{type-information}
1709 specifies the type of the array elements.
1714 Some languages, like C or the original Pascal, do not have string types,
1715 they just have related things like arrays of characters. But most
1716 Pascals and various other languages have string types, which are
1717 indicated as follows:
1720 @item n @var{type-information} ; @var{bytes}
1721 @var{bytes} is the maximum length. I'm not sure what
1722 @var{type-information} is; I suspect that it means that this is a string
1723 of @var{type-information} (thus allowing a string of integers, a string
1724 of wide characters, etc., as well as a string of characters). Not sure
1725 what the format of this type is. This is an AIX feature.
1727 @item z @var{type-information} ; @var{bytes}
1728 Just like @samp{n} except that this is a gstring, not an ordinary
1729 string. I don't know the difference.
1732 Pascal Stringptr. What is this? This is an AIX feature.
1736 @section Enumerations
1738 Enumerations are defined with the @samp{e} type descriptor.
1740 @c FIXME: Where does this information properly go? Perhaps it is
1741 @c redundant with something we already explain.
1742 The source line below declares an enumeration type. It is defined at
1743 file scope between the bodies of main and s_proc in example2.c.
1744 The type definition is located after the N_RBRAC that marks the end of
1745 the previous procedure's block scope, and before the N_FUN that marks
1746 the beginning of the next procedure's block scope. Therefore it does not
1747 describe a block local symbol, but a file local one.
1752 enum e_places @{first,second=3,last@};
1756 generates the following stab
1759 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
1762 The symbol descriptor (T) says that the stab describes a structure,
1763 enumeration, or type tag. The type descriptor e, following the 22= of
1764 the type definition narrows it down to an enumeration type. Following
1765 the e is a list of the elements of the enumeration. The format is
1766 name:value,. The list of elements ends with a ;.
1768 There is no standard way to specify the size of an enumeration type; it
1769 is determined by the architecture (normally all enumerations types are
1770 32 bits). There should be a way to specify an enumeration type of
1771 another size; type attributes would be one way to do this @xref{Stabs
1781 @code{N_LSYM} or @code{C_DECL}
1782 @item Symbol Descriptor:
1784 @item Type Descriptor:
1788 The following source code declares a structure tag and defines an
1789 instance of the structure in global scope. Then a typedef equates the
1790 structure tag with a new type. A seperate stab is generated for the
1791 structure tag, the structure typedef, and the structure instance. The
1792 stabs for the tag and the typedef are emited when the definitions are
1793 encountered. Since the structure elements are not initialized, the
1794 stab and code for the structure variable itself is located at the end
1795 of the program in .common.
1801 9 char s_char_vec[8];
1802 10 struct s_tag* s_next;
1805 13 typedef struct s_tag s_typedef;
1808 The structure tag is an N_LSYM stab type because, like the enum, the
1809 symbol is file scope. Like the enum, the symbol descriptor is T, for
1810 enumeration, struct or tag type. The symbol descriptor s following
1811 the 16= of the type definition narrows the symbol type to struct.
1813 Following the struct symbol descriptor is the number of bytes the
1814 struct occupies, followed by a description of each structure element.
1815 The structure element descriptions are of the form name:type, bit
1816 offset from the start of the struct, and number of bits in the
1821 <128> N_LSYM - type definition
1822 .stabs "name:sym_desc(struct tag) Type_def(16)=type_desc(struct type)
1824 elem_name:type_ref(int),bit_offset,field_bits;
1825 elem_name:type_ref(float),bit_offset,field_bits;
1826 elem_name:type_def(17)=type_desc(array)
1827 index_type(range of int from 0 to 7);
1828 element_type(char),bit_offset,field_bits;;",
1831 30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;
1832 s_char_vec:17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
1835 In this example, two of the structure elements are previously defined
1836 types. For these, the type following the name: part of the element
1837 description is a simple type reference. The other two structure
1838 elements are new types. In this case there is a type definition
1839 embedded after the name:. The type definition for the array element
1840 looks just like a type definition for a standalone array. The s_next
1841 field is a pointer to the same kind of structure that the field is an
1842 element of. So the definition of structure type 16 contains an type
1843 definition for an element which is a pointer to type 16.
1846 @section Giving a type a name
1848 To give a type a name, use the @samp{t} symbol descriptor. For example,
1851 .stabs "s_typedef:t16",128,0,0,0
1854 specifies that @code{s_typedef} refers to type number 16. Such stabs
1855 have symbol type @code{N_LSYM} or @code{C_DECL}.
1857 If instead, you are specifying the tag name for a structure, union, or
1858 enumeration, use the @samp{T} symbol descriptor instead. I believe C is
1859 the only language with this feature.
1861 If the type is an opaque type (I believe this is a Modula-2 feature),
1862 AIX provides a type descriptor to specify it. The type descriptor is
1863 @samp{o} and is followed by a name. I don't know what the name
1864 means---is it always the same as the name of the type, or is this type
1865 descriptor used with a nameless stab (@pxref{Stabs Format})? There
1866 optionally follows a comma followed by type information which defines
1867 the type of this type. If omitted, a semicolon is used in place of the
1868 comma and the type information, and, the type is much like a generic
1869 pointer type---it has a known size but little else about it is
1875 Next let's look at unions. In example2 this union type is declared
1876 locally to a procedure and an instance of the union is defined.
1886 This code generates a stab for the union tag and a stab for the union
1887 variable. Both use the N_LSYM stab type. Since the union variable is
1888 scoped locally to the procedure in which it is defined, its stab is
1889 located immediately preceding the N_LBRAC for the procedure's block
1892 The stab for the union tag, however is located preceding the code for
1893 the procedure in which it is defined. The stab type is N_LSYM. This
1894 would seem to imply that the union type is file scope, like the struct
1895 type s_tag. This is not true. The contents and position of the stab
1896 for u_type do not convey any infomation about its procedure local
1901 .stabs "name:sym_desc(union tag)type_def(22)=type_desc(union)
1903 elem_name:type_ref(int),bit_offset(0),bit_size(32);
1904 elem_name:type_ref(float),bit_offset(0),bit_size(32);
1905 elem_name:type_ref(ptr to char),bit_offset(0),bit_size(32);;"
1906 N_LSYM, NIL, NIL, NIL
1910 105 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
1914 The symbol descriptor, T, following the name: means that the stab
1915 describes an enumeration, struct or type tag. The type descriptor u,
1916 following the 23= of the type definition, narrows it down to a union
1917 type definition. Following the u is the number of bytes in the union.
1918 After that is a list of union element descriptions. Their format is
1919 name:type, bit offset into the union, and number of bytes for the
1922 The stab for the union variable follows. Notice that the frame
1923 pointer offset for local variables is negative.
1926 <128> N_LSYM - local variable (with no symbol descriptor)
1927 .stabs "name:type_ref(u_tag)", N_LSYM, NIL, NIL, frame_ptr_offset
1931 130 .stabs "an_u:23",128,0,0,-20
1934 @node Function Types
1935 @section Function types
1937 There are various types for function variables. These types are not
1938 used in defining functions; see symbol descriptor @samp{f}; they are
1939 used for things like pointers to functions.
1941 The simple, traditional, type is type descriptor @samp{f} is followed by
1942 type information for the return type of the function, followed by a
1945 This does not deal with functions the number and type of whose
1946 parameters are part of their type, as found in Modula-2 or ANSI C. AIX
1947 provides extensions to specify these, using the @samp{f}, @samp{F},
1948 @samp{p}, and @samp{R} type descriptors.
1950 First comes the type descriptor. Then, if it is @samp{f} or @samp{F},
1951 this is a function, and the type information for the return type of the
1952 function follows, followed by a comma. Then comes the number of
1953 parameters to the function and a semicolon. Then, for each parameter,
1954 there is the name of the parameter followed by a colon (this is only
1955 present for type descriptors @samp{R} and @samp{F} which represent
1956 Pascal function or procedure parameters), type information for the
1957 parameter, a comma, @samp{0} if passed by reference or @samp{1} if
1958 passed by value, and a semicolon. The type definition ends with a
1968 generates the following code:
1971 .stabs "g_pf:G24=*25=f1",32,0,0,0
1972 .common _g_pf,4,"bss"
1975 The variable defines a new type, 24, which is a pointer to another new
1976 type, 25, which is defined as a function returning int.
1979 @chapter Symbol information in symbol tables
1981 This section examines more closely the format of symbol table entries
1982 and how stab assembler directives map to them. It also describes what
1983 transformations the assembler and linker make on data from stabs.
1985 Each time the assembler encounters a stab in its input file it puts
1986 each field of the stab into corresponding fields in a symbol table
1987 entry of its output file. If the stab contains a string field, the
1988 symbol table entry for that stab points to a string table entry
1989 containing the string data from the stab. Assembler labels become
1990 relocatable addresses. Symbol table entries in a.out have the format:
1993 struct internal_nlist @{
1994 unsigned long n_strx; /* index into string table of name */
1995 unsigned char n_type; /* type of symbol */
1996 unsigned char n_other; /* misc info (usually empty) */
1997 unsigned short n_desc; /* description field */
1998 bfd_vma n_value; /* value of symbol */
2002 For .stabs directives, the n_strx field holds the character offset
2003 from the start of the string table to the string table entry
2004 containing the "string" field. For other classes of stabs (.stabn and
2005 .stabd) this field is null.
2007 Symbol table entries with n_type fields containing a value greater or
2008 equal to 0x20 originated as stabs generated by the compiler (with one
2009 random exception). Those with n_type values less than 0x20 were
2010 placed in the symbol table of the executable by the assembler or the
2013 The linker concatenates object files and does fixups of externally
2014 defined symbols. You can see the transformations made on stab data by
2015 the assembler and linker by examining the symbol table after each pass
2016 of the build, first the assemble and then the link.
2018 To do this use nm with the -ap options. This dumps the symbol table,
2019 including debugging information, unsorted. For stab entries the
2020 columns are: value, other, desc, type, string. For assembler and
2021 linker symbols, the columns are: value, type, string.
2023 There are a few important things to notice about symbol tables. Where
2024 the value field of a stab contains a frame pointer offset, or a
2025 register number, that value is unchanged by the rest of the build.
2027 Where the value field of a stab contains an assembly language label,
2028 it is transformed by each build step. The assembler turns it into a
2029 relocatable address and the linker turns it into an absolute address.
2030 This source line defines a static variable at file scope:
2033 3 static int s_g_repeat
2037 The following stab describes the symbol.
2040 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
2044 The assembler transforms the stab into this symbol table entry in the
2045 @file{.o} file. The location is expressed as a data segment offset.
2048 21 00000084 - 00 0000 STSYM s_g_repeat:S1
2052 in the symbol table entry from the executable, the linker has made the
2053 relocatable address absolute.
2056 22 0000e00c - 00 0000 STSYM s_g_repeat:S1
2059 Stabs for global variables do not contain location information. In
2060 this case the debugger finds location information in the assembler or
2061 linker symbol table entry describing the variable. The source line:
2071 21 .stabs "g_foo:G2",32,0,0,0
2074 The variable is represented by the following two symbol table entries
2075 in the object file. The first one originated as a stab. The second
2076 one is an external symbol. The upper case D signifies that the n_type
2077 field of the symbol table contains 7, N_DATA with local linkage (see
2078 Table B). The value field following the file's line number is empty
2079 for the stab entry. For the linker symbol it contains the
2080 rellocatable address corresponding to the variable.
2083 19 00000000 - 00 0000 GSYM g_foo:G2
2084 20 00000080 D _g_foo
2088 These entries as transformed by the linker. The linker symbol table
2089 entry now holds an absolute address.
2092 21 00000000 - 00 0000 GSYM g_foo:G2
2094 215 0000e008 D _g_foo
2098 @chapter GNU C++ stabs
2101 * Basic Cplusplus types::
2104 * Methods:: Method definition
2106 * Method Modifiers::
2109 * Virtual Base Classes::
2113 @subsection type descriptors added for C++ descriptions
2117 method type (two ## if minimal debug)
2120 Member (class and variable) type. It is followed by type information
2121 for the offset basetype, a comma, and type information for the type of
2122 the field being pointed to. (FIXME: this is acknowledged to be
2123 gibberish. Can anyone say what really goes here?).
2125 Note that there is a conflict between this and type attributes
2126 (@pxref{Stabs Format}); both use type descriptor @samp{@@}.
2127 Fortunately, the @samp{@@} type descriptor used in this C++ sense always
2128 will be followed by a digit, @samp{(}, or @samp{-}, and type attributes
2129 never start with those things.
2132 @node Basic Cplusplus types
2133 @section Basic types for C++
2135 << the examples that follow are based on a01.C >>
2138 C++ adds two more builtin types to the set defined for C. These are
2139 the unknown type and the vtable record type. The unknown type, type
2140 16, is defined in terms of itself like the void type.
2142 The vtable record type, type 17, is defined as a structure type and
2143 then as a structure tag. The structure has four fields, delta, index,
2144 pfn, and delta2. pfn is the function pointer.
2146 << In boilerplate $vtbl_ptr_type, what are the fields delta,
2147 index, and delta2 used for? >>
2149 This basic type is present in all C++ programs even if there are no
2150 virtual methods defined.
2153 .stabs "struct_name:sym_desc(type)type_def(17)=type_desc(struct)struct_bytes(8)
2154 elem_name(delta):type_ref(short int),bit_offset(0),field_bits(16);
2155 elem_name(index):type_ref(short int),bit_offset(16),field_bits(16);
2156 elem_name(pfn):type_def(18)=type_desc(ptr to)type_ref(void),
2157 bit_offset(32),field_bits(32);
2158 elem_name(delta2):type_def(short int);bit_offset(32),field_bits(16);;"
2163 .stabs "$vtbl_ptr_type:t17=s8
2164 delta:6,0,16;index:6,16,16;pfn:18=*15,32,32;delta2:6,32,16;;"
2169 .stabs "name:sym_dec(struct tag)type_ref($vtbl_ptr_type)",N_LSYM,NIL,NIL,NIL
2173 .stabs "$vtbl_ptr_type:T17",128,0,0,0
2176 @node Simple classes
2177 @section Simple class definition
2179 The stabs describing C++ language features are an extension of the
2180 stabs describing C. Stabs representing C++ class types elaborate
2181 extensively on the stab format used to describe structure types in C.
2182 Stabs representing class type variables look just like stabs
2183 representing C language variables.
2185 Consider the following very simple class definition.
2191 int Ameth(int in, char other);
2195 The class baseA is represented by two stabs. The first stab describes
2196 the class as a structure type. The second stab describes a structure
2197 tag of the class type. Both stabs are of stab type N_LSYM. Since the
2198 stab is not located between an N_FUN and a N_LBRAC stab this indicates
2199 that the class is defined at file scope. If it were, then the N_LSYM
2200 would signify a local variable.
2202 A stab describing a C++ class type is similar in format to a stab
2203 describing a C struct, with each class member shown as a field in the
2204 structure. The part of the struct format describing fields is
2205 expanded to include extra information relevent to C++ class members.
2206 In addition, if the class has multiple base classes or virtual
2207 functions the struct format outside of the field parts is also
2210 In this simple example the field part of the C++ class stab
2211 representing member data looks just like the field part of a C struct
2212 stab. The section on protections describes how its format is
2213 sometimes extended for member data.
2215 The field part of a C++ class stab representing a member function
2216 differs substantially from the field part of a C struct stab. It
2217 still begins with `name:' but then goes on to define a new type number
2218 for the member function, describe its return type, its argument types,
2219 its protection level, any qualifiers applied to the method definition,
2220 and whether the method is virtual or not. If the method is virtual
2221 then the method description goes on to give the vtable index of the
2222 method, and the type number of the first base class defining the
2225 When the field name is a method name it is followed by two colons
2226 rather than one. This is followed by a new type definition for the
2227 method. This is a number followed by an equal sign and then the
2228 symbol descriptor `##', indicating a method type. This is followed by
2229 a type reference showing the return type of the method and a
2232 The format of an overloaded operator method name differs from that
2233 of other methods. It is "op$::XXXX." where XXXX is the operator name
2234 such as + or +=. The name ends with a period, and any characters except
2235 the period can occur in the XXXX string.
2237 The next part of the method description represents the arguments to
2238 the method, preceeded by a colon and ending with a semi-colon. The
2239 types of the arguments are expressed in the same way argument types
2240 are expressed in C++ name mangling. In this example an int and a char
2243 This is followed by a number, a letter, and an asterisk or period,
2244 followed by another semicolon. The number indicates the protections
2245 that apply to the member function. Here the 2 means public. The
2246 letter encodes any qualifier applied to the method definition. In
2247 this case A means that it is a normal function definition. The dot
2248 shows that the method is not virtual. The sections that follow
2249 elaborate further on these fields and describe the additional
2250 information present for virtual methods.
2254 .stabs "class_name:sym_desc(type)type_def(20)=type_desc(struct)struct_bytes(4)
2255 field_name(Adat):type(int),bit_offset(0),field_bits(32);
2257 method_name(Ameth)::type_def(21)=type_desc(method)return_type(int);
2258 :arg_types(int char);
2259 protection(public)qualifier(normal)virtual(no);;"
2264 .stabs "baseA:t20=s4Adat:1,0,32;Ameth::21=##1;:ic;2A.;;",128,0,0,0
2266 .stabs "class_name:sym_desc(struct tag)",N_LSYM,NIL,NIL,NIL
2268 .stabs "baseA:T20",128,0,0,0
2271 @node Class instance
2272 @section Class instance
2274 As shown above, describing even a simple C++ class definition is
2275 accomplished by massively extending the stab format used in C to
2276 describe structure types. However, once the class is defined, C stabs
2277 with no modifications can be used to describe class instances. The
2287 yields the following stab describing the class instance. It looks no
2288 different from a standard C stab describing a local variable.
2291 .stabs "name:type_ref(baseA)", N_LSYM, NIL, NIL, frame_ptr_offset
2295 .stabs "AbaseA:20",128,0,0,-20
2299 @section Method defintion
2301 The class definition shown above declares Ameth. The C++ source below
2306 baseA::Ameth(int in, char other)
2313 This method definition yields three stabs following the code of the
2314 method. One stab describes the method itself and following two
2315 describe its parameters. Although there is only one formal argument
2316 all methods have an implicit argument which is the `this' pointer.
2317 The `this' pointer is a pointer to the object on which the method was
2318 called. Note that the method name is mangled to encode the class name
2319 and argument types. << Name mangling is not described by this
2320 document - Is there already such a doc? >>
2323 .stabs "name:symbol_desriptor(global function)return_type(int)",
2324 N_FUN, NIL, NIL, code_addr_of_method_start
2326 .stabs "Ameth__5baseAic:F1",36,0,0,_Ameth__5baseAic
2329 Here is the stab for the `this' pointer implicit argument. The name
2330 of the `this' pointer is always `this.' Type 19, the `this' pointer is
2331 defined as a pointer to type 20, baseA, but a stab defining baseA has
2332 not yet been emited. Since the compiler knows it will be emited
2333 shortly, here it just outputs a cross reference to the undefined
2334 symbol, by prefixing the symbol name with xs.
2337 .stabs "name:sym_desc(register param)type_def(19)=
2338 type_desc(ptr to)type_ref(baseA)=
2339 type_desc(cross-reference to)baseA:",N_RSYM,NIL,NIL,register_number
2341 .stabs "this:P19=*20=xsbaseA:",64,0,0,8
2344 The stab for the explicit integer argument looks just like a parameter
2345 to a C function. The last field of the stab is the offset from the
2346 argument pointer, which in most systems is the same as the frame
2350 .stabs "name:sym_desc(value parameter)type_ref(int)",
2351 N_PSYM,NIL,NIL,offset_from_arg_ptr
2353 .stabs "in:p1",160,0,0,72
2356 << The examples that follow are based on A1.C >>
2359 @section Protections
2362 In the simple class definition shown above all member data and
2363 functions were publicly accessable. The example that follows
2364 contrasts public, protected and privately accessable fields and shows
2365 how these protections are encoded in C++ stabs.
2367 Protections for class member data are signified by two characters
2368 embeded in the stab defining the class type. These characters are
2369 located after the name: part of the string. /0 means private, /1
2370 means protected, and /2 means public. If these characters are omited
2371 this means that the member is public. The following C++ source:
2385 generates the following stab to describe the class type all_data.
2388 .stabs "class_name:sym_desc(type)type_def(19)=type_desc(struct)struct_bytes
2389 data_name:/protection(private)type_ref(int),bit_offset,num_bits;
2390 data_name:/protection(protected)type_ref(char),bit_offset,num_bits;
2391 data_name:(/num omited, private)type_ref(float),bit_offset,num_bits;;"
2396 .stabs "all_data:t19=s12
2397 priv_dat:/01,0,32;prot_dat:/12,32,8;pub_dat:12,64,32;;",128,0,0,0
2400 Protections for member functions are signified by one digit embeded in
2401 the field part of the stab describing the method. The digit is 0 if
2402 private, 1 if protected and 2 if public. Consider the C++ class
2406 class all_methods @{
2408 int priv_meth(int in)@{return in;@};
2410 char protMeth(char in)@{return in;@};
2412 float pubMeth(float in)@{return in;@};
2416 It generates the following stab. The digit in question is to the left
2417 of an `A' in each case. Notice also that in this case two symbol
2418 descriptors apply to the class name struct tag and struct type.
2421 .stabs "class_name:sym_desc(struct tag&type)type_def(21)=
2422 sym_desc(struct)struct_bytes(1)
2423 meth_name::type_def(22)=sym_desc(method)returning(int);
2424 :args(int);protection(private)modifier(normal)virtual(no);
2425 meth_name::type_def(23)=sym_desc(method)returning(char);
2426 :args(char);protection(protected)modifier(normal)virual(no);
2427 meth_name::type_def(24)=sym_desc(method)returning(float);
2428 :args(float);protection(public)modifier(normal)virtual(no);;",
2433 .stabs "all_methods:Tt21=s1priv_meth::22=##1;:i;0A.;protMeth::23=##2;:c;1A.;
2434 pubMeth::24=##12;:f;2A.;;",128,0,0,0
2437 @node Method Modifiers
2438 @section Method Modifiers (const, volatile, const volatile)
2442 In the class example described above all the methods have the normal
2443 modifier. This method modifier information is located just after the
2444 protection information for the method. This field has four possible
2445 character values. Normal methods use A, const methods use B, volatile
2446 methods use C, and const volatile methods use D. Consider the class
2452 int ConstMeth (int arg) const @{ return arg; @};
2453 char VolatileMeth (char arg) volatile @{ return arg; @};
2454 float ConstVolMeth (float arg) const volatile @{return arg; @};
2458 This class is described by the following stab:
2461 .stabs "class(A):sym_desc(struct)type_def(20)=type_desc(struct)struct_bytes(1)
2462 meth_name(ConstMeth)::type_def(21)sym_desc(method)
2463 returning(int);:arg(int);protection(public)modifier(const)virtual(no);
2464 meth_name(VolatileMeth)::type_def(22)=sym_desc(method)
2465 returning(char);:arg(char);protection(public)modifier(volatile)virt(no)
2466 meth_name(ConstVolMeth)::type_def(23)=sym_desc(method)
2467 returning(float);:arg(float);protection(public)modifer(const volatile)
2468 virtual(no);;", @dots{}
2472 .stabs "A:T20=s1ConstMeth::21=##1;:i;2B.;VolatileMeth::22=##2;:c;2C.;
2473 ConstVolMeth::23=##12;:f;2D.;;",128,0,0,0
2476 @node Virtual Methods
2477 @section Virtual Methods
2479 << The following examples are based on a4.C >>
2481 The presence of virtual methods in a class definition adds additional
2482 data to the class description. The extra data is appended to the
2483 description of the virtual method and to the end of the class
2484 description. Consider the class definition below:
2490 virtual int A_virt (int arg) @{ return arg; @};
2494 This results in the stab below describing class A. It defines a new
2495 type (20) which is an 8 byte structure. The first field of the class
2496 struct is Adat, an integer, starting at structure offset 0 and
2499 The second field in the class struct is not explicitly defined by the
2500 C++ class definition but is implied by the fact that the class
2501 contains a virtual method. This field is the vtable pointer. The
2502 name of the vtable pointer field starts with $vf and continues with a
2503 type reference to the class it is part of. In this example the type
2504 reference for class A is 20 so the name of its vtable pointer field is
2505 $vf20, followed by the usual colon.
2507 Next there is a type definition for the vtable pointer type (21).
2508 This is in turn defined as a pointer to another new type (22).
2510 Type 22 is the vtable itself, which is defined as an array, indexed by
2511 a range of integers between 0 and 1, and whose elements are of type
2512 17. Type 17 was the vtable record type defined by the boilerplate C++
2513 type definitions, as shown earlier.
2515 The bit offset of the vtable pointer field is 32. The number of bits
2516 in the field are not specified when the field is a vtable pointer.
2518 Next is the method definition for the virtual member function A_virt.
2519 Its description starts out using the same format as the non-virtual
2520 member functions described above, except instead of a dot after the
2521 `A' there is an asterisk, indicating that the function is virtual.
2522 Since is is virtual some addition information is appended to the end
2523 of the method description.
2525 The first number represents the vtable index of the method. This is a
2526 32 bit unsigned number with the high bit set, followed by a
2529 The second number is a type reference to the first base class in the
2530 inheritence hierarchy defining the virtual member function. In this
2531 case the class stab describes a base class so the virtual function is
2532 not overriding any other definition of the method. Therefore the
2533 reference is to the type number of the class that the stab is
2536 This is followed by three semi-colons. One marks the end of the
2537 current sub-section, one marks the end of the method field, and the
2538 third marks the end of the struct definition.
2540 For classes containing virtual functions the very last section of the
2541 string part of the stab holds a type reference to the first base
2542 class. This is preceeded by `~%' and followed by a final semi-colon.
2545 .stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8)
2546 field_name(Adat):type_ref(int),bit_offset(0),field_bits(32);
2547 field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)=
2548 sym_desc(array)index_type_ref(range of int from 0 to 1);
2549 elem_type_ref(vtbl elem type),
2551 meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
2552 :arg_type(int),protection(public)normal(yes)virtual(yes)
2553 vtable_index(1);class_first_defining(A);;;~%first_base(A);",
2557 @c FIXME: bogus line break.
2559 .stabs "A:t20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2560 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2564 @section Inheritence
2566 Stabs describing C++ derived classes include additional sections that
2567 describe the inheritence hierarchy of the class. A derived class stab
2568 also encodes the number of base classes. For each base class it tells
2569 if the base class is virtual or not, and if the inheritence is private
2570 or public. It also gives the offset into the object of the portion of
2571 the object corresponding to each base class.
2573 This additional information is embeded in the class stab following the
2574 number of bytes in the struct. First the number of base classes
2575 appears bracketed by an exclamation point and a comma.
2577 Then for each base type there repeats a series: two digits, a number,
2578 a comma, another number, and a semi-colon.
2580 The first of the two digits is 1 if the base class is virtual and 0 if
2581 not. The second digit is 2 if the derivation is public and 0 if not.
2583 The number following the first two digits is the offset from the start
2584 of the object to the part of the object pertaining to the base class.
2586 After the comma, the second number is a type_descriptor for the base
2587 type. Finally a semi-colon ends the series, which repeats for each
2590 The source below defines three base classes A, B, and C and the
2598 virtual int A_virt (int arg) @{ return arg; @};
2604 virtual int B_virt (int arg) @{return arg; @};
2610 virtual int C_virt (int arg) @{return arg; @};
2613 class D : A, virtual B, public C @{
2616 virtual int A_virt (int arg ) @{ return arg+1; @};
2617 virtual int B_virt (int arg) @{ return arg+2; @};
2618 virtual int C_virt (int arg) @{ return arg+3; @};
2619 virtual int D_virt (int arg) @{ return arg; @};
2623 Class stabs similar to the ones described earlier are generated for
2626 @c FIXME!!! the linebreaks in the following example probably make the
2627 @c examples literally unusable, but I don't know any other way to get
2628 @c them on the page.
2629 @c One solution would be to put some of the type definitions into
2630 @c separate stabs, even if that's not exactly what the compiler actually
2633 .stabs "A:T20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2634 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2636 .stabs "B:Tt25=s8Bdat:1,0,32;$vf25:21,32;B_virt::26=##1;
2637 :i;2A*-2147483647;25;;;~%25;",128,0,0,0
2639 .stabs "C:Tt28=s8Cdat:1,0,32;$vf28:21,32;C_virt::29=##1;
2640 :i;2A*-2147483647;28;;;~%28;",128,0,0,0
2643 In the stab describing derived class D below, the information about
2644 the derivation of this class is encoded as follows.
2647 .stabs "derived_class_name:symbol_descriptors(struct tag&type)=
2648 type_descriptor(struct)struct_bytes(32)!num_bases(3),
2649 base_virtual(no)inheritence_public(no)base_offset(0),
2650 base_class_type_ref(A);
2651 base_virtual(yes)inheritence_public(no)base_offset(NIL),
2652 base_class_type_ref(B);
2653 base_virtual(no)inheritence_public(yes)base_offset(64),
2654 base_class_type_ref(C); @dots{}
2657 @c FIXME! fake linebreaks.
2659 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:
2660 1,160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt:
2661 :32:i;2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;
2662 28;;D_virt::32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2665 @node Virtual Base Classes
2666 @section Virtual Base Classes
2668 A derived class object consists of a concatination in memory of the
2669 data areas defined by each base class, starting with the leftmost and
2670 ending with the rightmost in the list of base classes. The exception
2671 to this rule is for virtual inheritence. In the example above, class
2672 D inherits virtually from base class B. This means that an instance
2673 of a D object will not contain it's own B part but merely a pointer to
2674 a B part, known as a virtual base pointer.
2676 In a derived class stab, the base offset part of the derivation
2677 information, described above, shows how the base class parts are
2678 ordered. The base offset for a virtual base class is always given as
2679 0. Notice that the base offset for B is given as 0 even though B is
2680 not the first base class. The first base class A starts at offset 0.
2682 The field information part of the stab for class D describes the field
2683 which is the pointer to the virtual base class B. The vbase pointer
2684 name is $vb followed by a type reference to the virtual base class.
2685 Since the type id for B in this example is 25, the vbase pointer name
2688 @c FIXME!! fake linebreaks below
2690 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1,
2691 160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i;
2692 2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt:
2693 :32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2696 Following the name and a semicolon is a type reference describing the
2697 type of the virtual base class pointer, in this case 24. Type 24 was
2698 defined earlier as the type of the B class `this` pointer. The
2699 `this' pointer for a class is a pointer to the class type.
2702 .stabs "this:P24=*25=xsB:",64,0,0,8
2705 Finally the field offset part of the vbase pointer field description
2706 shows that the vbase pointer is the first field in the D object,
2707 before any data fields defined by the class. The layout of a D class
2708 object is a follows, Adat at 0, the vtable pointer for A at 32, Cdat
2709 at 64, the vtable pointer for C at 96, the virtual ase pointer for B
2710 at 128, and Ddat at 160.
2713 @node Static Members
2714 @section Static Members
2716 The data area for a class is a concatenation of the space used by the
2717 data members of the class. If the class has virtual methods, a vtable
2718 pointer follows the class data. The field offset part of each field
2719 description in the class stab shows this ordering.
2721 << How is this reflected in stabs? See Cygnus bug #677 for some info. >>
2724 @appendix Example2.c - source code for extended example
2728 2 register int g_bar asm ("%g5");
2729 3 static int s_g_repeat = 2;
2735 9 char s_char_vec[8];
2736 10 struct s_tag* s_next;
2739 13 typedef struct s_tag s_typedef;
2741 15 char char_vec[3] = @{'a','b','c'@};
2743 17 main (argc, argv)
2747 21 static float s_flap;
2749 23 for (times=0; times < s_g_repeat; times++)@{
2751 25 printf ("Hello world\n");
2755 29 enum e_places @{first,second=3,last@};
2757 31 static s_proc (s_arg, s_ptr_arg, char_vec)
2759 33 s_typedef* s_ptr_arg;
2773 @appendix Example2.s - assembly code for extended example
2777 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
2778 3 .stabs "example2.c",100,0,0,Ltext0
2781 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
2782 7 .stabs "char:t2=r2;0;127;",128,0,0,0
2783 8 .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
2784 9 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
2785 10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
2786 11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
2787 12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
2788 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
2789 14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
2790 15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
2791 16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
2792 17 .stabs "float:t12=r1;4;0;",128,0,0,0
2793 18 .stabs "double:t13=r1;8;0;",128,0,0,0
2794 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
2795 20 .stabs "void:t15=15",128,0,0,0
2796 21 .stabs "g_foo:G2",32,0,0,0
2801 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
2805 @c FIXME! fake linebreak in line 30
2806 30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;s_char_vec:
2807 17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
2808 31 .stabs "s_typedef:t16",128,0,0,0
2809 32 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
2810 33 .global _char_vec
2816 39 .reserve _s_flap.0,4,"bss",4
2820 43 .ascii "Hello world\12\0"
2825 48 .stabn 68,0,20,LM1
2828 51 save %sp,-144,%sp
2835 58 .stabn 68,0,23,LM2
2839 62 sethi %hi(_s_g_repeat),%o0
2841 64 ld [%o0+%lo(_s_g_repeat)],%o0
2846 69 .stabn 68,0,25,LM3
2848 71 sethi %hi(LC0),%o1
2849 72 or %o1,%lo(LC0),%o0
2852 75 .stabn 68,0,26,LM4
2855 78 .stabn 68,0,23,LM5
2863 86 .stabn 68,0,27,LM6
2866 89 .stabn 68,0,27,LM7
2871 94 .stabs "main:F1",36,0,0,_main
2872 95 .stabs "argc:p1",160,0,0,68
2873 96 .stabs "argv:p20=*21=*2",160,0,0,72
2874 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
2875 98 .stabs "times:1",128,0,0,-20
2876 99 .stabn 192,0,0,LBB2
2877 100 .stabs "inner:1",128,0,0,-24
2878 101 .stabn 192,0,0,LBB3
2879 102 .stabn 224,0,0,LBE3
2880 103 .stabn 224,0,0,LBE2
2881 104 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
2882 @c FIXME: fake linebreak in line 105
2883 105 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
2888 109 .stabn 68,0,35,LM8
2891 112 save %sp,-120,%sp
2897 118 .stabn 68,0,41,LM9
2900 121 .stabn 68,0,41,LM10
2905 126 .stabs "s_proc:f1",36,0,0,_s_proc
2906 127 .stabs "s_arg:p16",160,0,0,0
2907 128 .stabs "s_ptr_arg:p18",160,0,0,72
2908 129 .stabs "char_vec:p21",160,0,0,76
2909 130 .stabs "an_u:23",128,0,0,-20
2910 131 .stabn 192,0,0,LBB4
2911 132 .stabn 224,0,0,LBE4
2912 133 .stabs "g_bar:r1",64,0,0,5
2913 134 .stabs "g_pf:G24=*25=f1",32,0,0,0
2914 135 .common _g_pf,4,"bss"
2915 136 .stabs "g_an_s:G16",32,0,0,0
2916 137 .common _g_an_s,20,"bss"
2920 @appendix Values for the Stab Type Field
2922 These are all the possible values for the stab type field, for
2923 @code{a.out} files. This does not apply to XCOFF.
2925 The following types are used by the linker and assembler; there is
2926 nothing stabs-specific about them. Since this document does not attempt
2927 to describe aspects of object file format other than the debugging
2928 format, no details are given.
2930 @c Try to get most of these to fit on a single line.
2940 File scope absolute symbol
2942 @item 0x3 N_ABS | N_EXT
2943 External absolute symbol
2946 File scope text symbol
2948 @item 0x5 N_TEXT | N_EXT
2949 External text symbol
2952 File scope data symbol
2954 @item 0x7 N_DATA | N_EXT
2955 External data symbol
2958 File scope BSS symbol
2960 @item 0x9 N_BSS | N_EXT
2964 Same as N_FN, for Sequent compilers
2967 Symbol is indirected to another symbol
2970 Common sym -- visable after shared lib dynamic link
2973 Absolute set element
2976 Text segment set element
2979 Data segment set element
2982 BSS segment set element
2985 Pointer to set vector
2987 @item 0x1e N_WARNING
2988 Print a warning message during linking
2991 File name of a .o file
2994 The following symbol types indicate that this is a stab. This is the
2995 full list of stab numbers, including stab types that are used in
2996 languages other than C.
3000 Global symbol, @xref{N_GSYM}.
3003 Function name (for BSD Fortran), @xref{N_FNAME}.
3006 Function name or text segment variable for C, @xref{N_FUN}.
3009 Static symbol (data segment variable with internal linkage), @xref{N_STSYM}.
3012 .lcomm symbol (BSS segment variable with internal linkage), @xref{N_LCSYM}.
3015 Name of main routine (not used in C), @xref{N_MAIN}.
3017 @c FIXME: discuss this in the main body of the text where we talk about
3018 @c using N_FUN for variables.
3020 Read-only data symbol (Solaris2). Most systems use N_FUN for this.
3023 Global symbol (for Pascal), @xref{N_PC}.
3026 Number of symbols (according to Ultrix V4.0), @xref{N_NSYMS}.
3029 No DST map for sym (according to Ultrix V4.0), @xref{N_NOMAP}.
3031 @c FIXME: describe this solaris feature in the body of the text (see
3032 @c comments in include/aout/stab.def).
3034 Object file (Solaris2).
3036 @c See include/aout/stab.def for (a little) more info.
3038 Debugger options (Solaris2).
3041 Register variable, @xref{N_RSYM}.
3044 Modula-2 compilation unit, @xref{N_M2C}.
3047 Line number in text segment, @xref{Line Numbers}.
3050 Line number in data segment, @xref{Line Numbers}.
3053 Line number in bss segment, @xref{Line Numbers}.
3056 Sun source code browser, path to .cb file, @xref{N_BROWS}.
3059 Gnu Modula2 definition module dependency, @xref{N_DEFD}.
3062 Function start/body/end line numbers (Solaris2).
3065 Gnu C++ exception variable, @xref{N_EHDECL}.
3068 Modula2 info "for imc" (according to Ultrix V4.0), @xref{N_MOD2}.
3071 Gnu C++ "catch" clause, @xref{N_CATCH}.
3074 Structure of union element, @xref{N_SSYM}.
3077 Last stab for module (Solaris2).
3080 Path and name of source file , @xref{Source Files}.
3083 Automatic var in the stack or type definition, @xref{N_LSYM}, @xref{Typedefs}.
3086 Beginning of an include file (Sun only), @xref{Source Files}.
3089 Name of include file, @xref{Source Files}.
3092 Parameter variable, @xref{Parameters}.
3095 End of an include file, @xref{Source Files}.
3098 Alternate entry point, @xref{N_ENTRY}.
3101 Beginning of a lexical block, @xref{Block Structure}.
3104 Place holder for a deleted include file, @xref{Source Files}.
3107 Modula2 scope information (Sun linker), @xref{N_SCOPE}.
3110 End of a lexical block, @xref{Block Structure}.
3113 Begin named common block, @xref{Common Blocks}.
3116 End named common block, @xref{Common Blocks}.
3119 Member of a common block, @xref{Common Blocks}.
3121 @c FIXME: How does this really work? Move it to main body of document.
3123 Pascal @code{with} statement: type,,0,0,offset (Solaris2).
3126 Gould non-base registers, @xref{Gould}.
3129 Gould non-base registers, @xref{Gould}.
3132 Gould non-base registers, @xref{Gould}.
3135 Gould non-base registers, @xref{Gould}.
3138 Gould non-base registers, @xref{Gould}.
3141 @c Restore the default table indent
3146 @node Symbol Descriptors
3147 @appendix Table of Symbol Descriptors
3149 @c Please keep this alphabetical
3151 @c In TeX, this looks great, digit is in italics. But makeinfo insists
3152 @c on putting it in `', not realizing that @var should override @code.
3153 @c I don't know of any way to make makeinfo do the right thing. Seems
3154 @c like a makeinfo bug to me.
3158 Local variable, @xref{Automatic variables}.
3161 Parameter passed by reference in register, @xref{Parameters}.
3164 Constant, @xref{Constants}.
3167 Conformant array bound (Pascal, maybe other languages),
3168 @xref{Parameters}. Name of a caught exception (GNU C++). These can be
3169 distinguished because the latter uses N_CATCH and the former uses
3170 another symbol type.
3173 Floating point register variable, @xref{Register variables}.
3176 Parameter in floating point register, @xref{Parameters}.
3179 Static function, @xref{Procedures}.
3182 Global function, @xref{Procedures}.
3185 Global variable, @xref{Global Variables}.
3191 Internal (nested) procedure, @xref{Procedures}.
3194 Internal (nested) function, @xref{Procedures}.
3197 Label name (documented by AIX, no further information known).
3200 Module, @xref{Procedures}.
3203 Argument list parameter, @xref{Parameters}.
3209 FORTRAN Function parameter, @xref{Parameters}.
3212 Unfortunately, three separate meanings have been independently invented
3213 for this symbol descriptor. At least the GNU and Sun uses can be
3214 distinguished by the symbol type. Global Procedure (AIX) (symbol type
3215 used unknown), @xref{Procedures}. Register parameter (GNU) (symbol type
3216 N_PSYM), @xref{Parameters}. Prototype of function referenced by this
3217 file (Sun acc) (symbol type N_FUN).
3220 Static Procedure, @xref{Procedures}.
3223 Register parameter @xref{Parameters}.
3226 Register variable, @xref{Register variables}.
3229 Static file scope variable @xref{Initialized statics},
3230 @xref{Un-initialized statics}.
3233 Type name, @xref{Typedefs}.
3236 enumeration, struct or union tag, @xref{Typedefs}.
3239 Parameter passed by reference, @xref{Parameters}.
3242 Static procedure scope variable @xref{Initialized statics},
3243 @xref{Un-initialized statics}.
3246 Conformant array, @xref{Parameters}.
3249 Function return variable, @xref{Parameters}.
3252 @node Type Descriptors
3253 @appendix Table of Type Descriptors
3258 Type reference, @xref{Stabs Format}.
3261 Reference to builtin type, @xref{Negative Type Numbers}.
3264 Method (C++), @xref{Cplusplus}.
3267 Pointer, @xref{Miscellaneous Types}.
3273 Type Attributes (AIX), @xref{Stabs Format}. Member (class and variable)
3274 type (GNU C++), @xref{Cplusplus}.
3277 Array, @xref{Arrays}.
3280 Open array, @xref{Arrays}.
3283 Pascal space type (AIX), @xref{Miscellaneous Types}. Builtin integer
3284 type (Sun), @xref{Builtin Type Descriptors}.
3287 Volatile-qualified type, @xref{Miscellaneous Types}.
3290 Complex builtin type, @xref{Builtin Type Descriptors}.
3293 COBOL Picture type. See AIX documentation for details.
3296 File type, @xref{Miscellaneous Types}.
3299 N-dimensional dynamic array, @xref{Arrays}.
3302 Enumeration type, @xref{Enumerations}.
3305 N-dimensional subarray, @xref{Arrays}.
3308 Function type, @xref{Function Types}.
3311 Pascal function parameter, @xref{Function Types}
3314 Builtin floating point type, @xref{Builtin Type Descriptors}.
3317 COBOL Group. See AIX documentation for details.
3320 Imported type, @xref{Cross-references}.
3323 Const-qualified type, @xref{Miscellaneous Types}.
3326 COBOL File Descriptor. See AIX documentation for details.
3329 Multiple instance type, @xref{Miscellaneous Types}.
3332 String type, @xref{Strings}.
3335 Stringptr, @xref{Strings}.
3338 Opaque type, @xref{Typedefs}.
3341 Procedure, @xref{Function Types}.
3344 Packed array, @xref{Arrays}.
3347 Range type, @xref{Subranges}.
3350 Builtin floating type, @xref{Builtin Type Descriptors} (Sun). Pascal
3351 subroutine parameter, @xref{Function Types} (AIX). Detecting this
3352 conflict is possible with careful parsing (hint: a Pascal subroutine
3353 parameter type will always contain a comma, and a builtin type
3354 descriptor never will).
3357 Structure type, @xref{Structures}.
3360 Set type, @xref{Miscellaneous Types}.
3363 Union, @xref{Unions}.
3366 Variant record. This is a Pascal and Modula-2 feature which is like a
3367 union within a struct in C. See AIX documentation for details.
3370 Wide character, @xref{Builtin Type Descriptors}.
3373 Cross-reference, @xref{Cross-references}.
3376 gstring, @xref{Strings}.
3379 @node Expanded reference
3380 @appendix Expanded reference by stab type.
3382 @c FIXME: This appendix should go away, see N_PSYM or N_SO for an example.
3384 For a full list of stab types, and cross-references to where they are
3385 described, @xref{Stab Types}. This appendix just duplicates certain
3386 information from the main body of this document; eventually the
3387 information will all be in one place.
3391 The first line is the symbol type expressed in decimal, hexadecimal,
3392 and as a #define (see devo/include/aout/stab.def).
3394 The second line describes the language constructs the symbol type
3397 The third line is the stab format with the significant stab fields
3398 named and the rest NIL.
3400 Subsequent lines expand upon the meaning and possible values for each
3401 significant stab field. # stands in for the type descriptor.
3403 Finally, any further information.
3406 * N_GSYM:: Global variable
3407 * N_FNAME:: Function name (BSD Fortran)
3408 * N_FUN:: C Function name or text segment variable
3409 * N_STSYM:: Initialized static symbol
3410 * N_LCSYM:: Uninitialized static symbol
3411 * N_MAIN:: Name of main routine (not for C)
3412 * N_PC:: Pascal global symbol
3413 * N_NSYMS:: Number of symbols
3414 * N_NOMAP:: No DST map
3415 * N_RSYM:: Register variable
3416 * N_M2C:: Modula-2 compilation unit
3417 * N_BROWS:: Path to .cb file for Sun source code browser
3418 * N_DEFD:: GNU Modula2 definition module dependency
3419 * N_EHDECL:: GNU C++ exception variable
3420 * N_MOD2:: Modula2 information "for imc"
3421 * N_CATCH:: GNU C++ "catch" clause
3422 * N_SSYM:: Structure or union element
3423 * N_LSYM:: Automatic variable
3424 * N_ENTRY:: Alternate entry point
3425 * N_SCOPE:: Modula2 scope information (Sun only)
3426 * Gould:: non-base register symbols used on Gould systems
3427 * N_LENG:: Length of preceding entry
3431 @section 32 - 0x20 - N_GYSM
3436 .stabs "name", N_GSYM, NIL, NIL, NIL
3440 "name" -> "symbol_name:#type"
3444 Only the "name" field is significant. The location of the variable is
3445 obtained from the corresponding external symbol.
3448 @section 34 - 0x22 - N_FNAME
3449 Function name (for BSD Fortran)
3452 .stabs "name", N_FNAME, NIL, NIL, NIL
3456 "name" -> "function_name"
3459 Only the "name" field is significant. The location of the symbol is
3460 obtained from the corresponding extern symbol.
3463 @section 36 - 0x24 - N_FUN
3465 Function name (@pxref{Procedures}) or text segment variable
3466 (@pxref{Variables}).
3468 @exdent @emph{For functions:}
3469 "name" -> "proc_name:#return_type"
3470 # -> F (global function)
3472 desc -> line num for proc start. (GCC doesn't set and DBX doesn't miss it.)
3473 value -> Code address of proc start.
3475 @exdent @emph{For text segment variables:}
3476 <<How to create one?>>
3480 @section 38 - 0x26 - N_STSYM
3481 Initialized static symbol (data segment w/internal linkage).
3484 .stabs "name", N_STSYM, NIL, NIL, value
3488 "name" -> "symbol_name#type"
3489 # -> S (scope global to compilation unit)
3490 -> V (scope local to a procedure)
3491 value -> Data Address
3495 @section 40 - 0x28 - N_LCSYM
3496 Unitialized static (.lcomm) symbol(BSS segment w/internal linkage).
3499 .stabs "name", N_LCLSYM, NIL, NIL, value
3503 "name" -> "symbol_name#type"
3504 # -> S (scope global to compilation unit)
3505 -> V (scope local to procedure)
3506 value -> BSS Address
3510 @section 42 - 0x2a - N_MAIN
3511 Name of main routine (not used in C)
3514 .stabs "name", N_MAIN, NIL, NIL, NIL
3518 "name" -> "name_of_main_routine"
3522 @section 48 - 0x30 - N_PC
3523 Global symbol (for Pascal)
3526 .stabs "name", N_PC, NIL, NIL, value
3530 "name" -> "symbol_name" <<?>>
3531 value -> supposedly the line number (stab.def is skeptical)
3537 global pascal symbol: name,,0,subtype,line
3542 @section 50 - 0x32 - N_NSYMS
3543 Number of symbols (according to Ultrix V4.0)
3546 0, files,,funcs,lines (stab.def)
3550 @section 52 - 0x34 - N_NOMAP
3551 no DST map for sym (according to Ultrix V4.0)
3554 name, ,0,type,ignored (stab.def)
3558 @section 64 - 0x40 - N_RSYM
3562 .stabs "name:type",N_RSYM,0,RegSize,RegNumber (Sun doc)
3566 @section 66 - 0x42 - N_M2C
3567 Modula-2 compilation unit
3570 .stabs "name", N_M2C, 0, desc, value
3574 "name" -> "unit_name,unit_time_stamp[,code_time_stamp]
3576 value -> 0 (main unit)
3581 @section 72 - 0x48 - N_BROWS
3582 Sun source code browser, path to .cb file
3585 "path to associated .cb file"
3587 Note: type field value overlaps with N_BSLINE
3590 @section 74 - 0x4a - N_DEFD
3591 GNU Modula2 definition module dependency
3593 GNU Modula-2 definition module dependency. Value is the modification
3594 time of the definition file. Other is non-zero if it is imported with
3595 the GNU M2 keyword %INITIALIZE. Perhaps N_M2C can be used if there
3596 are enough empty fields?
3599 @section 80 - 0x50 - N_EHDECL
3600 GNU C++ exception variable <<?>>
3602 "name is variable name"
3604 Note: conflicts with N_MOD2.
3607 @section 80 - 0x50 - N_MOD2
3608 Modula2 info "for imc" (according to Ultrix V4.0)
3610 Note: conflicts with N_EHDECL <<?>>
3613 @section 84 - 0x54 - N_CATCH
3614 GNU C++ "catch" clause
3616 GNU C++ `catch' clause. Value is its address. Desc is nonzero if
3617 this entry is immediately followed by a CAUGHT stab saying what
3618 exception was caught. Multiple CAUGHT stabs means that multiple
3619 exceptions can be caught here. If Desc is 0, it means all exceptions
3623 @section 96 - 0x60 - N_SSYM
3624 Structure or union element
3626 Value is offset in the structure.
3628 <<?looking at structs and unions in C I didn't see these>>
3631 @section 128 - 0x80 - N_LSYM
3632 Automatic var in the stack (also used for type descriptors.)
3635 .stabs "name" N_LSYM, NIL, NIL, value
3639 @exdent @emph{For stack based local variables:}
3641 "name" -> name of the variable
3642 value -> offset from frame pointer (negative)
3644 @exdent @emph{For type descriptors:}
3646 "name" -> "name_of_the_type:#type"
3649 type -> type_ref (or) type_def
3651 type_ref -> type_number
3652 type_def -> type_number=type_desc etc.
3655 Type may be either a type reference or a type definition. A type
3656 reference is a number that refers to a previously defined type. A
3657 type definition is the number that will refer to this type, followed
3658 by an equals sign, a type descriptor and the additional data that
3659 defines the type. See the Table D for type descriptors and the
3660 section on types for what data follows each type descriptor.
3663 @section 164 - 0xa4 - N_ENTRY
3665 Alternate entry point.
3666 Value is its address.
3670 @section 196 - 0xc4 - N_SCOPE
3672 Modula2 scope information (Sun linker)
3676 @section Non-base registers on Gould systems
3678 These are used on Gould systems for non-base registers syms.
3680 However, the following values are not the values used by Gould; they are
3681 the values which GNU has been documenting for these values for a long
3682 time, without actually checking what Gould uses. I include these values
3683 only because perhaps some someone actually did something with the GNU
3684 information (I hope not, why GNU knowingly assigned wrong values to
3685 these in the header file is a complete mystery to me).
3688 240 0xf0 N_NBTEXT ??
3689 242 0xf2 N_NBDATA ??
3696 @section - 0xfe - N_LENG
3698 Second symbol entry containing a length-value for the preceding entry.
3699 The value is the length.
3702 @appendix Questions and anomalies
3706 For GNU C stabs defining local and global variables (N_LSYM and
3707 N_GSYM), the desc field is supposed to contain the source line number
3708 on which the variable is defined. In reality the desc field is always
3709 0. (This behavour is defined in dbxout.c and putting a line number in
3710 desc is controlled by #ifdef WINNING_GDB which defaults to false). Gdb
3711 supposedly uses this information if you say 'list var'. In reality
3712 var can be a variable defined in the program and gdb says `function
3716 In GNU C stabs there seems to be no way to differentiate tag types:
3717 structures, unions, and enums (symbol descriptor T) and typedefs
3718 (symbol descriptor t) defined at file scope from types defined locally
3719 to a procedure or other more local scope. They all use the N_LSYM
3720 stab type. Types defined at procedure scope are emited after the
3721 N_RBRAC of the preceding function and before the code of the
3722 procedure in which they are defined. This is exactly the same as
3723 types defined in the source file between the two procedure bodies.
3724 GDB overcompensates by placing all types in block #1, the block for
3725 symbols of file scope. This is true for default, -ansi and
3726 -traditional compiler options. (Bugs gcc/1063, gdb/1066.)
3729 What ends the procedure scope? Is it the proc block's N_RBRAC or the
3730 next N_FUN? (I believe its the first.)
3733 The comment in xcoff.h says DBX_STATIC_CONST_VAR_CODE is used for
3734 static const variables. DBX_STATIC_CONST_VAR_CODE is set to N_FUN by
3735 default, in dbxout.c. If included, xcoff.h redefines it to N_STSYM.
3736 But testing the default behaviour, my Sun4 native example shows
3737 N_STSYM not N_FUN is used to describe file static initialized
3738 variables. (the code tests for TREE_READONLY(decl) &&
3739 !TREE_THIS_VOLATILE(decl) and if true uses DBX_STATIC_CONST_VAR_CODE).
3742 Global variable stabs don't have location information. This comes
3743 from the external symbol for the same variable. The external symbol
3744 has a leading underbar on the _name of the variable and the stab does
3745 not. How do we know these two symbol table entries are talking about
3746 the same symbol when their names are different?
3749 Can gcc be configured to output stabs the way the Sun compiler
3750 does, so that their native debugging tools work? <NO?> It doesn't by
3751 default. GDB reads either format of stab. (gcc or SunC). How about
3755 @node xcoff-differences
3756 @appendix Differences between GNU stabs in a.out and GNU stabs in xcoff
3758 @c FIXME: Merge *all* these into the main body of the document.
3759 (The AIX/RS6000 native object file format is xcoff with stabs). This
3760 appendix only covers those differences which are not covered in the main
3761 body of this document.
3765 BSD a.out stab types correspond to AIX xcoff storage classes. In general the
3766 mapping is N_STABTYPE becomes C_STABTYPE. Some stab types in a.out
3767 are not supported in xcoff. See Table E. for full mappings.
3770 initialised static N_STSYM and un-initialized static N_LCSYM both map
3771 to the C_STSYM storage class. But the destinction is preserved
3772 because in xcoff N_STSYM and N_LCSYM must be emited in a named static
3773 block. Begin the block with .bs s[RW] data_section_name for N_STSYM
3774 or .bs s bss_section_name for N_LCSYM. End the block with .es
3777 If the xcoff stab is a N_FUN (C_FUN) then follow the string field with
3778 ,. instead of just ,
3782 (I think that's it for .s file differences. They could stand to be
3783 better presented. This is just a list of what I have noticed so far.
3784 There are a *lot* of differences in the information in the symbol
3785 tables of the executable and object files.)
3787 Table E: mapping a.out stab types to xcoff storage classes
3790 stab type storage class
3791 -------------------------------
3800 N_RPSYM (0x8e) C_RPSYM
3810 N_DECL (0x8c) C_DECL
3827 @node Sun-differences
3828 @appendix Differences between GNU stabs and Sun native stabs.
3830 @c FIXME: Merge all this stuff into the main body of the document.
3834 GNU C stabs define *all* types, file or procedure scope, as
3835 N_LSYM. Sun doc talks about using N_GSYM too.
3838 Sun C stabs use type number pairs in the format (a,b) where a is a
3839 number starting with 1 and incremented for each sub-source file in the
3840 compilation. b is a number starting with 1 and incremented for each
3841 new type defined in the compilation. GNU C stabs use the type number
3842 alone, with no source file number.
3846 @appendix Using stabs with the ELF object file format.
3848 The ELF object file format allows tools to create object files with custom
3849 sections containing any arbitrary data. To use stabs in ELF object files,
3850 the tools create two custom sections, a ".stab" section which contains
3851 an array of fixed length structures, one struct per stab, and a ".stabstr"
3852 section containing all the variable length strings that are referenced by
3853 stabs in the ".stab" section. The byte order of the stabs binary data
3854 matches the byte order of the ELF file itself, as determined from the
3855 EI_DATA field in the e_ident member of the ELF header.
3857 The first stab in the ".stab" section for each object file is a "synthetic
3858 stab", generated entirely by the assembler, with no corresponding ".stab"
3859 directive as input to the assembler. This stab contains the following
3864 Offset in the ".stabstr" section to the source filename.
3870 Unused field, always zero.
3873 Count of upcoming symbols. I.E. the number of remaining stabs for this
3877 Size of the string table fragment associated with this object module, in
3882 The ".stabstr" section always starts with a null byte (so that string
3883 offsets of zero reference a null string), followed by random length strings,
3884 each of which is null byte terminated.
3886 The ELF section header for the ".stab" section has it's sh_link member set
3887 to the section number of the ".stabstr" section, and the ".stabstr" section
3888 has it's ELF section header sh_type member set to SHT_STRTAB to mark it as