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 According to the AIX documentation symbol descriptor @samp{D} is for a
1081 parameter passed in a floating point register. This seems
1082 unnecessary---why not 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}. 32 bit type. How is the truth value encoded? Is it
1454 the least significant bit or is it a question of whether the whole value
1455 is zero or non-zero?
1458 @code{short real}. IEEE single precision.
1461 @code{real}. IEEE double precision.
1464 @code{stringptr}. @xref{Strings}.
1467 @code{character}, 8 bit unsigned character type.
1470 @code{logical*1}, 8 bit unsigned integral type.
1473 @code{logical*2}, 16 bit unsigned integral type.
1476 @code{logical*4}, 32 bit unsigned integral type.
1479 @code{logical}, 32 bit unsigned integral type.
1482 @code{complex}. A complex type consisting of two IEEE single-precision
1483 floating point values.
1486 @code{complex}. A complex type consisting of two IEEE double-precision
1487 floating point values.
1490 @code{integer*1}, 8 bit signed integral type.
1493 @code{integer*2}, 16 bit signed integral type.
1496 @code{integer*4}, 32 bit signed integral type.
1499 @code{wchar}. Wide character, 16 bits wide, unsigned (what format?
1503 @node Miscellaneous Types
1504 @section Miscellaneous Types
1507 @item b @var{type-information} ; @var{bytes}
1508 Pascal space type. This is documented by IBM; what does it mean?
1510 Note that this use of the @samp{b} type descriptor can be distinguished
1511 from its use for builtin integral types (@pxref{Builtin Type
1512 Descriptors}) because the character following the type descriptor is
1513 always a digit, @samp{(}, or @samp{-}.
1515 @item B @var{type-information}
1516 A volatile-qualified version of @var{type-information}. This is a Sun
1517 extension. A volatile-qualified type means that references and stores
1518 to a variable of that type must not be optimized or cached; they must
1519 occur as the user specifies them.
1521 @item d @var{type-information}
1522 File of type @var{type-information}. As far as I know this is only used
1525 @item k @var{type-information}
1526 A const-qualified version of @var{type-information}. This is a Sun
1527 extension. A const-qualified type means that a variable of this type
1530 @item M @var{type-information} ; @var{length}
1531 Multiple instance type. The type seems to composed of @var{length}
1532 repetitions of @var{type-information}, for example @code{character*3} is
1533 represented by @samp{M-2;3}, where @samp{-2} is a reference to a
1534 character type (@pxref{Negative Type Numbers}). I'm not sure how this
1535 differs from an array. This appears to be a FORTRAN feature.
1536 @var{length} is a bound, like those in range types, @xref{Subranges}.
1538 @item S @var{type-information}
1539 Pascal set type. @var{type-information} must be a small type such as an
1540 enumeration or a subrange, and the type is a bitmask whose length is
1541 specified by the number of elements in @var{type-information}.
1543 @item * @var{type-information}
1544 Pointer to @var{type-information}.
1547 @node Cross-references
1548 @section Cross-references to other types
1550 If a type is used before it is defined, one common way to deal with this
1551 is just to use a type reference to a type which has not yet been
1552 defined. The debugger is expected to be able to deal with this.
1554 Another way is with the @samp{x} type descriptor, which is followed by
1555 @samp{s} for a structure tag, @samp{u} for a union tag, or @samp{e} for
1556 a enumerator tag, followed by the name of the tag, followed by @samp{:}.
1557 for example the following C declarations:
1567 .stabs "bar:G16=*17=xsfoo:",32,0,0,0
1570 Not all debuggers support the @samp{x} type descriptor, so on some
1571 machines GCC does not use it. I believe that for the above example it
1572 would just emit a reference to type 17 and never define it, but I
1573 haven't verified that.
1575 Modula-2 imported types, at least on AIX, use the @samp{i} type
1576 descriptor, which is followed by the name of the module from which the
1577 type is imported, followed by @samp{:}, followed by the name of the
1578 type. There is then optionally a comma followed by type information for
1579 the type (This differs from merely naming the type (@pxref{Typedefs}) in
1580 that it identifies the module; I don't understand whether the name of
1581 the type given here is always just the same as the name we are giving
1582 it, or whether this type descriptor is used with a nameless stab
1583 (@pxref{Stabs Format}), or what). The symbol ends with @samp{;}.
1586 @section Subrange types
1588 The @samp{r} type descriptor defines a type as a subrange of another
1589 type. It is followed by type information for the type which it is a
1590 subrange of, a semicolon, an integral lower bound, a semicolon, an
1591 integral upper bound, and a semicolon. The AIX documentation does not
1592 specify the trailing semicolon, in an effort to specify array indexes
1593 more cleanly, but a subrange which is not an array index has always
1594 included a trailing semicolon (@pxref{Arrays}).
1596 Instead of an integer, either bound can be one of the following:
1599 @item A @var{offset}
1600 The bound is passed by reference on the stack at offset @var{offset}
1601 from the argument list. @xref{Parameters}, for more information on such
1604 @item T @var{offset}
1605 The bound is passed by value on the stack at offset @var{offset} from
1608 @item a @var{register-number}
1609 The bound is pased by reference in register number
1610 @var{register-number}.
1612 @item t @var{register-number}
1613 The bound is passed by value in register number @var{register-number}.
1619 Subranges are also used for builtin types, @xref{Traditional Builtin Types}.
1622 @section Array types
1624 Arrays use the @samp{a} type descriptor. Following the type descriptor
1625 is the type of the index and the type of the array elements. If the
1626 index type is a range type, it will end in a semicolon; if it is not a
1627 range type (for example, if it is a type reference), there does not
1628 appear to be any way to tell where the types are separated. In an
1629 effort to clean up this mess, IBM documents the two types as being
1630 separated by a semicolon, and a range type as not ending in a semicolon
1631 (but this is not right for range types which are not array indexes,
1632 @pxref{Subranges}). I think probably the best solution is to specify
1633 that a semicolon ends a range type, and that the index type and element
1634 type of an array are separated by a semicolon, but that if the index
1635 type is a range type, the extra semicolon can be omitted. GDB (at least
1636 through version 4.9) doesn't support any kind of index type other than a
1637 range anyway; I'm not sure about dbx.
1639 It is well established, and widely used, that the type of the index,
1640 unlike most types found in the stabs, is merely a type definition, not
1641 type information (@pxref{Stabs Format}) (that is, it need not start with
1642 @var{type-number}@code{=} if it is defining a new type). According to a
1643 comment in GDB, this is also true of the type of the array elements; it
1644 gives @samp{ar1;1;10;ar1;1;10;4} as a legitimate way to express a two
1645 dimensional array. According to AIX documentation, the element type
1646 must be type information. GDB accepts either.
1648 The type of the index is often a range type, expressed as the letter r
1649 and some parameters. It defines the size of the array. In the example
1650 below, the range @code{r1;0;2;} defines an index type which is a
1651 subrange of type 1 (integer), with a lower bound of 0 and an upper bound
1652 of 2. This defines the valid range of subscripts of a three-element C
1655 For example, the definition
1658 char char_vec[3] = @{'a','b','c'@};
1665 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
1674 If an array is @dfn{packed}, it means that the elements are spaced more
1675 closely than normal, saving memory at the expense of speed. For
1676 example, an array of 3-byte objects might, if unpacked, have each
1677 element aligned on a 4-byte boundary, but if packed, have no padding.
1678 One way to specify that something is packed is with type attributes
1679 (@pxref{Stabs Format}), in the case of arrays another is to use the
1680 @samp{P} type descriptor instead of @samp{a}. Other than specifying a
1681 packed array, @samp{P} is identical to @samp{a}.
1683 @c FIXME-what is it? A pointer?
1684 An open array is represented by the @samp{A} type descriptor followed by
1685 type information specifying the type of the array elements.
1687 @c FIXME: what is the format of this type? A pointer to a vector of pointers?
1688 An N-dimensional dynamic array is represented by
1691 D @var{dimensions} ; @var{type-information}
1694 @c Does dimensions really have this meaning? The AIX documentation
1696 @var{dimensions} is the number of dimensions; @var{type-information}
1697 specifies the type of the array elements.
1699 @c FIXME: what is the format of this type? A pointer to some offsets in
1701 A subarray of an N-dimensional array is represented by
1704 E @var{dimensions} ; @var{type-information}
1707 @c Does dimensions really have this meaning? The AIX documentation
1709 @var{dimensions} is the number of dimensions; @var{type-information}
1710 specifies the type of the array elements.
1715 Some languages, like C or the original Pascal, do not have string types,
1716 they just have related things like arrays of characters. But most
1717 Pascals and various other languages have string types, which are
1718 indicated as follows:
1721 @item n @var{type-information} ; @var{bytes}
1722 @var{bytes} is the maximum length. I'm not sure what
1723 @var{type-information} is; I suspect that it means that this is a string
1724 of @var{type-information} (thus allowing a string of integers, a string
1725 of wide characters, etc., as well as a string of characters). Not sure
1726 what the format of this type is. This is an AIX feature.
1728 @item z @var{type-information} ; @var{bytes}
1729 Just like @samp{n} except that this is a gstring, not an ordinary
1730 string. I don't know the difference.
1733 Pascal Stringptr. What is this? This is an AIX feature.
1737 @section Enumerations
1739 Enumerations are defined with the @samp{e} type descriptor.
1741 @c FIXME: Where does this information properly go? Perhaps it is
1742 @c redundant with something we already explain.
1743 The source line below declares an enumeration type. It is defined at
1744 file scope between the bodies of main and s_proc in example2.c.
1745 The type definition is located after the N_RBRAC that marks the end of
1746 the previous procedure's block scope, and before the N_FUN that marks
1747 the beginning of the next procedure's block scope. Therefore it does not
1748 describe a block local symbol, but a file local one.
1753 enum e_places @{first,second=3,last@};
1757 generates the following stab
1760 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
1763 The symbol descriptor (T) says that the stab describes a structure,
1764 enumeration, or type tag. The type descriptor e, following the 22= of
1765 the type definition narrows it down to an enumeration type. Following
1766 the e is a list of the elements of the enumeration. The format is
1767 name:value,. The list of elements ends with a ;.
1769 There is no standard way to specify the size of an enumeration type; it
1770 is determined by the architecture (normally all enumerations types are
1771 32 bits). There should be a way to specify an enumeration type of
1772 another size; type attributes would be one way to do this @xref{Stabs
1782 @code{N_LSYM} or @code{C_DECL}
1783 @item Symbol Descriptor:
1785 @item Type Descriptor:
1789 The following source code declares a structure tag and defines an
1790 instance of the structure in global scope. Then a typedef equates the
1791 structure tag with a new type. A seperate stab is generated for the
1792 structure tag, the structure typedef, and the structure instance. The
1793 stabs for the tag and the typedef are emited when the definitions are
1794 encountered. Since the structure elements are not initialized, the
1795 stab and code for the structure variable itself is located at the end
1796 of the program in .common.
1802 9 char s_char_vec[8];
1803 10 struct s_tag* s_next;
1806 13 typedef struct s_tag s_typedef;
1809 The structure tag is an N_LSYM stab type because, like the enum, the
1810 symbol is file scope. Like the enum, the symbol descriptor is T, for
1811 enumeration, struct or tag type. The symbol descriptor s following
1812 the 16= of the type definition narrows the symbol type to struct.
1814 Following the struct symbol descriptor is the number of bytes the
1815 struct occupies, followed by a description of each structure element.
1816 The structure element descriptions are of the form name:type, bit
1817 offset from the start of the struct, and number of bits in the
1822 <128> N_LSYM - type definition
1823 .stabs "name:sym_desc(struct tag) Type_def(16)=type_desc(struct type)
1825 elem_name:type_ref(int),bit_offset,field_bits;
1826 elem_name:type_ref(float),bit_offset,field_bits;
1827 elem_name:type_def(17)=type_desc(array)
1828 index_type(range of int from 0 to 7);
1829 element_type(char),bit_offset,field_bits;;",
1832 30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;
1833 s_char_vec:17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
1836 In this example, two of the structure elements are previously defined
1837 types. For these, the type following the name: part of the element
1838 description is a simple type reference. The other two structure
1839 elements are new types. In this case there is a type definition
1840 embedded after the name:. The type definition for the array element
1841 looks just like a type definition for a standalone array. The s_next
1842 field is a pointer to the same kind of structure that the field is an
1843 element of. So the definition of structure type 16 contains an type
1844 definition for an element which is a pointer to type 16.
1847 @section Giving a type a name
1849 To give a type a name, use the @samp{t} symbol descriptor. For example,
1852 .stabs "s_typedef:t16",128,0,0,0
1855 specifies that @code{s_typedef} refers to type number 16. Such stabs
1856 have symbol type @code{N_LSYM} or @code{C_DECL}.
1858 If instead, you are specifying the tag name for a structure, union, or
1859 enumeration, use the @samp{T} symbol descriptor instead. I believe C is
1860 the only language with this feature.
1862 If the type is an opaque type (I believe this is a Modula-2 feature),
1863 AIX provides a type descriptor to specify it. The type descriptor is
1864 @samp{o} and is followed by a name. I don't know what the name
1865 means---is it always the same as the name of the type, or is this type
1866 descriptor used with a nameless stab (@pxref{Stabs Format})? There
1867 optionally follows a comma followed by type information which defines
1868 the type of this type. If omitted, a semicolon is used in place of the
1869 comma and the type information, and, the type is much like a generic
1870 pointer type---it has a known size but little else about it is
1876 Next let's look at unions. In example2 this union type is declared
1877 locally to a procedure and an instance of the union is defined.
1887 This code generates a stab for the union tag and a stab for the union
1888 variable. Both use the N_LSYM stab type. Since the union variable is
1889 scoped locally to the procedure in which it is defined, its stab is
1890 located immediately preceding the N_LBRAC for the procedure's block
1893 The stab for the union tag, however is located preceding the code for
1894 the procedure in which it is defined. The stab type is N_LSYM. This
1895 would seem to imply that the union type is file scope, like the struct
1896 type s_tag. This is not true. The contents and position of the stab
1897 for u_type do not convey any infomation about its procedure local
1902 .stabs "name:sym_desc(union tag)type_def(22)=type_desc(union)
1904 elem_name:type_ref(int),bit_offset(0),bit_size(32);
1905 elem_name:type_ref(float),bit_offset(0),bit_size(32);
1906 elem_name:type_ref(ptr to char),bit_offset(0),bit_size(32);;"
1907 N_LSYM, NIL, NIL, NIL
1911 105 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
1915 The symbol descriptor, T, following the name: means that the stab
1916 describes an enumeration, struct or type tag. The type descriptor u,
1917 following the 23= of the type definition, narrows it down to a union
1918 type definition. Following the u is the number of bytes in the union.
1919 After that is a list of union element descriptions. Their format is
1920 name:type, bit offset into the union, and number of bytes for the
1923 The stab for the union variable follows. Notice that the frame
1924 pointer offset for local variables is negative.
1927 <128> N_LSYM - local variable (with no symbol descriptor)
1928 .stabs "name:type_ref(u_tag)", N_LSYM, NIL, NIL, frame_ptr_offset
1932 130 .stabs "an_u:23",128,0,0,-20
1935 @node Function Types
1936 @section Function types
1938 There are various types for function variables. These types are not
1939 used in defining functions; see symbol descriptor @samp{f}; they are
1940 used for things like pointers to functions.
1942 The simple, traditional, type is type descriptor @samp{f} is followed by
1943 type information for the return type of the function, followed by a
1946 This does not deal with functions the number and type of whose
1947 parameters are part of their type, as found in Modula-2 or ANSI C. AIX
1948 provides extensions to specify these, using the @samp{f}, @samp{F},
1949 @samp{p}, and @samp{R} type descriptors.
1951 First comes the type descriptor. Then, if it is @samp{f} or @samp{F},
1952 this is a function, and the type information for the return type of the
1953 function follows, followed by a comma. Then comes the number of
1954 parameters to the function and a semicolon. Then, for each parameter,
1955 there is the name of the parameter followed by a colon (this is only
1956 present for type descriptors @samp{R} and @samp{F} which represent
1957 Pascal function or procedure parameters), type information for the
1958 parameter, a comma, @samp{0} if passed by reference or @samp{1} if
1959 passed by value, and a semicolon. The type definition ends with a
1969 generates the following code:
1972 .stabs "g_pf:G24=*25=f1",32,0,0,0
1973 .common _g_pf,4,"bss"
1976 The variable defines a new type, 24, which is a pointer to another new
1977 type, 25, which is defined as a function returning int.
1980 @chapter Symbol information in symbol tables
1982 This section examines more closely the format of symbol table entries
1983 and how stab assembler directives map to them. It also describes what
1984 transformations the assembler and linker make on data from stabs.
1986 Each time the assembler encounters a stab in its input file it puts
1987 each field of the stab into corresponding fields in a symbol table
1988 entry of its output file. If the stab contains a string field, the
1989 symbol table entry for that stab points to a string table entry
1990 containing the string data from the stab. Assembler labels become
1991 relocatable addresses. Symbol table entries in a.out have the format:
1994 struct internal_nlist @{
1995 unsigned long n_strx; /* index into string table of name */
1996 unsigned char n_type; /* type of symbol */
1997 unsigned char n_other; /* misc info (usually empty) */
1998 unsigned short n_desc; /* description field */
1999 bfd_vma n_value; /* value of symbol */
2003 For .stabs directives, the n_strx field holds the character offset
2004 from the start of the string table to the string table entry
2005 containing the "string" field. For other classes of stabs (.stabn and
2006 .stabd) this field is null.
2008 Symbol table entries with n_type fields containing a value greater or
2009 equal to 0x20 originated as stabs generated by the compiler (with one
2010 random exception). Those with n_type values less than 0x20 were
2011 placed in the symbol table of the executable by the assembler or the
2014 The linker concatenates object files and does fixups of externally
2015 defined symbols. You can see the transformations made on stab data by
2016 the assembler and linker by examining the symbol table after each pass
2017 of the build, first the assemble and then the link.
2019 To do this use nm with the -ap options. This dumps the symbol table,
2020 including debugging information, unsorted. For stab entries the
2021 columns are: value, other, desc, type, string. For assembler and
2022 linker symbols, the columns are: value, type, string.
2024 There are a few important things to notice about symbol tables. Where
2025 the value field of a stab contains a frame pointer offset, or a
2026 register number, that value is unchanged by the rest of the build.
2028 Where the value field of a stab contains an assembly language label,
2029 it is transformed by each build step. The assembler turns it into a
2030 relocatable address and the linker turns it into an absolute address.
2031 This source line defines a static variable at file scope:
2034 3 static int s_g_repeat
2038 The following stab describes the symbol.
2041 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
2045 The assembler transforms the stab into this symbol table entry in the
2046 @file{.o} file. The location is expressed as a data segment offset.
2049 21 00000084 - 00 0000 STSYM s_g_repeat:S1
2053 in the symbol table entry from the executable, the linker has made the
2054 relocatable address absolute.
2057 22 0000e00c - 00 0000 STSYM s_g_repeat:S1
2060 Stabs for global variables do not contain location information. In
2061 this case the debugger finds location information in the assembler or
2062 linker symbol table entry describing the variable. The source line:
2072 21 .stabs "g_foo:G2",32,0,0,0
2075 The variable is represented by the following two symbol table entries
2076 in the object file. The first one originated as a stab. The second
2077 one is an external symbol. The upper case D signifies that the n_type
2078 field of the symbol table contains 7, N_DATA with local linkage (see
2079 Table B). The value field following the file's line number is empty
2080 for the stab entry. For the linker symbol it contains the
2081 rellocatable address corresponding to the variable.
2084 19 00000000 - 00 0000 GSYM g_foo:G2
2085 20 00000080 D _g_foo
2089 These entries as transformed by the linker. The linker symbol table
2090 entry now holds an absolute address.
2093 21 00000000 - 00 0000 GSYM g_foo:G2
2095 215 0000e008 D _g_foo
2099 @chapter GNU C++ stabs
2102 * Basic Cplusplus types::
2105 * Methods:: Method definition
2107 * Method Modifiers::
2110 * Virtual Base Classes::
2114 @subsection type descriptors added for C++ descriptions
2118 method type (two ## if minimal debug)
2121 Member (class and variable) type. It is followed by type information
2122 for the offset basetype, a comma, and type information for the type of
2123 the field being pointed to. (FIXME: this is acknowledged to be
2124 gibberish. Can anyone say what really goes here?).
2126 Note that there is a conflict between this and type attributes
2127 (@pxref{Stabs Format}); both use type descriptor @samp{@@}.
2128 Fortunately, the @samp{@@} type descriptor used in this C++ sense always
2129 will be followed by a digit, @samp{(}, or @samp{-}, and type attributes
2130 never start with those things.
2133 @node Basic Cplusplus types
2134 @section Basic types for C++
2136 << the examples that follow are based on a01.C >>
2139 C++ adds two more builtin types to the set defined for C. These are
2140 the unknown type and the vtable record type. The unknown type, type
2141 16, is defined in terms of itself like the void type.
2143 The vtable record type, type 17, is defined as a structure type and
2144 then as a structure tag. The structure has four fields, delta, index,
2145 pfn, and delta2. pfn is the function pointer.
2147 << In boilerplate $vtbl_ptr_type, what are the fields delta,
2148 index, and delta2 used for? >>
2150 This basic type is present in all C++ programs even if there are no
2151 virtual methods defined.
2154 .stabs "struct_name:sym_desc(type)type_def(17)=type_desc(struct)struct_bytes(8)
2155 elem_name(delta):type_ref(short int),bit_offset(0),field_bits(16);
2156 elem_name(index):type_ref(short int),bit_offset(16),field_bits(16);
2157 elem_name(pfn):type_def(18)=type_desc(ptr to)type_ref(void),
2158 bit_offset(32),field_bits(32);
2159 elem_name(delta2):type_def(short int);bit_offset(32),field_bits(16);;"
2164 .stabs "$vtbl_ptr_type:t17=s8
2165 delta:6,0,16;index:6,16,16;pfn:18=*15,32,32;delta2:6,32,16;;"
2170 .stabs "name:sym_dec(struct tag)type_ref($vtbl_ptr_type)",N_LSYM,NIL,NIL,NIL
2174 .stabs "$vtbl_ptr_type:T17",128,0,0,0
2177 @node Simple classes
2178 @section Simple class definition
2180 The stabs describing C++ language features are an extension of the
2181 stabs describing C. Stabs representing C++ class types elaborate
2182 extensively on the stab format used to describe structure types in C.
2183 Stabs representing class type variables look just like stabs
2184 representing C language variables.
2186 Consider the following very simple class definition.
2192 int Ameth(int in, char other);
2196 The class baseA is represented by two stabs. The first stab describes
2197 the class as a structure type. The second stab describes a structure
2198 tag of the class type. Both stabs are of stab type N_LSYM. Since the
2199 stab is not located between an N_FUN and a N_LBRAC stab this indicates
2200 that the class is defined at file scope. If it were, then the N_LSYM
2201 would signify a local variable.
2203 A stab describing a C++ class type is similar in format to a stab
2204 describing a C struct, with each class member shown as a field in the
2205 structure. The part of the struct format describing fields is
2206 expanded to include extra information relevent to C++ class members.
2207 In addition, if the class has multiple base classes or virtual
2208 functions the struct format outside of the field parts is also
2211 In this simple example the field part of the C++ class stab
2212 representing member data looks just like the field part of a C struct
2213 stab. The section on protections describes how its format is
2214 sometimes extended for member data.
2216 The field part of a C++ class stab representing a member function
2217 differs substantially from the field part of a C struct stab. It
2218 still begins with `name:' but then goes on to define a new type number
2219 for the member function, describe its return type, its argument types,
2220 its protection level, any qualifiers applied to the method definition,
2221 and whether the method is virtual or not. If the method is virtual
2222 then the method description goes on to give the vtable index of the
2223 method, and the type number of the first base class defining the
2226 When the field name is a method name it is followed by two colons
2227 rather than one. This is followed by a new type definition for the
2228 method. This is a number followed by an equal sign and then the
2229 symbol descriptor `##', indicating a method type. This is followed by
2230 a type reference showing the return type of the method and a
2233 The format of an overloaded operator method name differs from that
2234 of other methods. It is "op$::XXXX." where XXXX is the operator name
2235 such as + or +=. The name ends with a period, and any characters except
2236 the period can occur in the XXXX string.
2238 The next part of the method description represents the arguments to
2239 the method, preceeded by a colon and ending with a semi-colon. The
2240 types of the arguments are expressed in the same way argument types
2241 are expressed in C++ name mangling. In this example an int and a char
2244 This is followed by a number, a letter, and an asterisk or period,
2245 followed by another semicolon. The number indicates the protections
2246 that apply to the member function. Here the 2 means public. The
2247 letter encodes any qualifier applied to the method definition. In
2248 this case A means that it is a normal function definition. The dot
2249 shows that the method is not virtual. The sections that follow
2250 elaborate further on these fields and describe the additional
2251 information present for virtual methods.
2255 .stabs "class_name:sym_desc(type)type_def(20)=type_desc(struct)struct_bytes(4)
2256 field_name(Adat):type(int),bit_offset(0),field_bits(32);
2258 method_name(Ameth)::type_def(21)=type_desc(method)return_type(int);
2259 :arg_types(int char);
2260 protection(public)qualifier(normal)virtual(no);;"
2265 .stabs "baseA:t20=s4Adat:1,0,32;Ameth::21=##1;:ic;2A.;;",128,0,0,0
2267 .stabs "class_name:sym_desc(struct tag)",N_LSYM,NIL,NIL,NIL
2269 .stabs "baseA:T20",128,0,0,0
2272 @node Class instance
2273 @section Class instance
2275 As shown above, describing even a simple C++ class definition is
2276 accomplished by massively extending the stab format used in C to
2277 describe structure types. However, once the class is defined, C stabs
2278 with no modifications can be used to describe class instances. The
2288 yields the following stab describing the class instance. It looks no
2289 different from a standard C stab describing a local variable.
2292 .stabs "name:type_ref(baseA)", N_LSYM, NIL, NIL, frame_ptr_offset
2296 .stabs "AbaseA:20",128,0,0,-20
2300 @section Method defintion
2302 The class definition shown above declares Ameth. The C++ source below
2307 baseA::Ameth(int in, char other)
2314 This method definition yields three stabs following the code of the
2315 method. One stab describes the method itself and following two
2316 describe its parameters. Although there is only one formal argument
2317 all methods have an implicit argument which is the `this' pointer.
2318 The `this' pointer is a pointer to the object on which the method was
2319 called. Note that the method name is mangled to encode the class name
2320 and argument types. << Name mangling is not described by this
2321 document - Is there already such a doc? >>
2324 .stabs "name:symbol_desriptor(global function)return_type(int)",
2325 N_FUN, NIL, NIL, code_addr_of_method_start
2327 .stabs "Ameth__5baseAic:F1",36,0,0,_Ameth__5baseAic
2330 Here is the stab for the `this' pointer implicit argument. The name
2331 of the `this' pointer is always `this.' Type 19, the `this' pointer is
2332 defined as a pointer to type 20, baseA, but a stab defining baseA has
2333 not yet been emited. Since the compiler knows it will be emited
2334 shortly, here it just outputs a cross reference to the undefined
2335 symbol, by prefixing the symbol name with xs.
2338 .stabs "name:sym_desc(register param)type_def(19)=
2339 type_desc(ptr to)type_ref(baseA)=
2340 type_desc(cross-reference to)baseA:",N_RSYM,NIL,NIL,register_number
2342 .stabs "this:P19=*20=xsbaseA:",64,0,0,8
2345 The stab for the explicit integer argument looks just like a parameter
2346 to a C function. The last field of the stab is the offset from the
2347 argument pointer, which in most systems is the same as the frame
2351 .stabs "name:sym_desc(value parameter)type_ref(int)",
2352 N_PSYM,NIL,NIL,offset_from_arg_ptr
2354 .stabs "in:p1",160,0,0,72
2357 << The examples that follow are based on A1.C >>
2360 @section Protections
2363 In the simple class definition shown above all member data and
2364 functions were publicly accessable. The example that follows
2365 contrasts public, protected and privately accessable fields and shows
2366 how these protections are encoded in C++ stabs.
2368 Protections for class member data are signified by two characters
2369 embeded in the stab defining the class type. These characters are
2370 located after the name: part of the string. /0 means private, /1
2371 means protected, and /2 means public. If these characters are omited
2372 this means that the member is public. The following C++ source:
2386 generates the following stab to describe the class type all_data.
2389 .stabs "class_name:sym_desc(type)type_def(19)=type_desc(struct)struct_bytes
2390 data_name:/protection(private)type_ref(int),bit_offset,num_bits;
2391 data_name:/protection(protected)type_ref(char),bit_offset,num_bits;
2392 data_name:(/num omited, private)type_ref(float),bit_offset,num_bits;;"
2397 .stabs "all_data:t19=s12
2398 priv_dat:/01,0,32;prot_dat:/12,32,8;pub_dat:12,64,32;;",128,0,0,0
2401 Protections for member functions are signified by one digit embeded in
2402 the field part of the stab describing the method. The digit is 0 if
2403 private, 1 if protected and 2 if public. Consider the C++ class
2407 class all_methods @{
2409 int priv_meth(int in)@{return in;@};
2411 char protMeth(char in)@{return in;@};
2413 float pubMeth(float in)@{return in;@};
2417 It generates the following stab. The digit in question is to the left
2418 of an `A' in each case. Notice also that in this case two symbol
2419 descriptors apply to the class name struct tag and struct type.
2422 .stabs "class_name:sym_desc(struct tag&type)type_def(21)=
2423 sym_desc(struct)struct_bytes(1)
2424 meth_name::type_def(22)=sym_desc(method)returning(int);
2425 :args(int);protection(private)modifier(normal)virtual(no);
2426 meth_name::type_def(23)=sym_desc(method)returning(char);
2427 :args(char);protection(protected)modifier(normal)virual(no);
2428 meth_name::type_def(24)=sym_desc(method)returning(float);
2429 :args(float);protection(public)modifier(normal)virtual(no);;",
2434 .stabs "all_methods:Tt21=s1priv_meth::22=##1;:i;0A.;protMeth::23=##2;:c;1A.;
2435 pubMeth::24=##12;:f;2A.;;",128,0,0,0
2438 @node Method Modifiers
2439 @section Method Modifiers (const, volatile, const volatile)
2443 In the class example described above all the methods have the normal
2444 modifier. This method modifier information is located just after the
2445 protection information for the method. This field has four possible
2446 character values. Normal methods use A, const methods use B, volatile
2447 methods use C, and const volatile methods use D. Consider the class
2453 int ConstMeth (int arg) const @{ return arg; @};
2454 char VolatileMeth (char arg) volatile @{ return arg; @};
2455 float ConstVolMeth (float arg) const volatile @{return arg; @};
2459 This class is described by the following stab:
2462 .stabs "class(A):sym_desc(struct)type_def(20)=type_desc(struct)struct_bytes(1)
2463 meth_name(ConstMeth)::type_def(21)sym_desc(method)
2464 returning(int);:arg(int);protection(public)modifier(const)virtual(no);
2465 meth_name(VolatileMeth)::type_def(22)=sym_desc(method)
2466 returning(char);:arg(char);protection(public)modifier(volatile)virt(no)
2467 meth_name(ConstVolMeth)::type_def(23)=sym_desc(method)
2468 returning(float);:arg(float);protection(public)modifer(const volatile)
2469 virtual(no);;", @dots{}
2473 .stabs "A:T20=s1ConstMeth::21=##1;:i;2B.;VolatileMeth::22=##2;:c;2C.;
2474 ConstVolMeth::23=##12;:f;2D.;;",128,0,0,0
2477 @node Virtual Methods
2478 @section Virtual Methods
2480 << The following examples are based on a4.C >>
2482 The presence of virtual methods in a class definition adds additional
2483 data to the class description. The extra data is appended to the
2484 description of the virtual method and to the end of the class
2485 description. Consider the class definition below:
2491 virtual int A_virt (int arg) @{ return arg; @};
2495 This results in the stab below describing class A. It defines a new
2496 type (20) which is an 8 byte structure. The first field of the class
2497 struct is Adat, an integer, starting at structure offset 0 and
2500 The second field in the class struct is not explicitly defined by the
2501 C++ class definition but is implied by the fact that the class
2502 contains a virtual method. This field is the vtable pointer. The
2503 name of the vtable pointer field starts with $vf and continues with a
2504 type reference to the class it is part of. In this example the type
2505 reference for class A is 20 so the name of its vtable pointer field is
2506 $vf20, followed by the usual colon.
2508 Next there is a type definition for the vtable pointer type (21).
2509 This is in turn defined as a pointer to another new type (22).
2511 Type 22 is the vtable itself, which is defined as an array, indexed by
2512 a range of integers between 0 and 1, and whose elements are of type
2513 17. Type 17 was the vtable record type defined by the boilerplate C++
2514 type definitions, as shown earlier.
2516 The bit offset of the vtable pointer field is 32. The number of bits
2517 in the field are not specified when the field is a vtable pointer.
2519 Next is the method definition for the virtual member function A_virt.
2520 Its description starts out using the same format as the non-virtual
2521 member functions described above, except instead of a dot after the
2522 `A' there is an asterisk, indicating that the function is virtual.
2523 Since is is virtual some addition information is appended to the end
2524 of the method description.
2526 The first number represents the vtable index of the method. This is a
2527 32 bit unsigned number with the high bit set, followed by a
2530 The second number is a type reference to the first base class in the
2531 inheritence hierarchy defining the virtual member function. In this
2532 case the class stab describes a base class so the virtual function is
2533 not overriding any other definition of the method. Therefore the
2534 reference is to the type number of the class that the stab is
2537 This is followed by three semi-colons. One marks the end of the
2538 current sub-section, one marks the end of the method field, and the
2539 third marks the end of the struct definition.
2541 For classes containing virtual functions the very last section of the
2542 string part of the stab holds a type reference to the first base
2543 class. This is preceeded by `~%' and followed by a final semi-colon.
2546 .stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8)
2547 field_name(Adat):type_ref(int),bit_offset(0),field_bits(32);
2548 field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)=
2549 sym_desc(array)index_type_ref(range of int from 0 to 1);
2550 elem_type_ref(vtbl elem type),
2552 meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
2553 :arg_type(int),protection(public)normal(yes)virtual(yes)
2554 vtable_index(1);class_first_defining(A);;;~%first_base(A);",
2558 @c FIXME: bogus line break.
2560 .stabs "A:t20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2561 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2565 @section Inheritence
2567 Stabs describing C++ derived classes include additional sections that
2568 describe the inheritence hierarchy of the class. A derived class stab
2569 also encodes the number of base classes. For each base class it tells
2570 if the base class is virtual or not, and if the inheritence is private
2571 or public. It also gives the offset into the object of the portion of
2572 the object corresponding to each base class.
2574 This additional information is embeded in the class stab following the
2575 number of bytes in the struct. First the number of base classes
2576 appears bracketed by an exclamation point and a comma.
2578 Then for each base type there repeats a series: two digits, a number,
2579 a comma, another number, and a semi-colon.
2581 The first of the two digits is 1 if the base class is virtual and 0 if
2582 not. The second digit is 2 if the derivation is public and 0 if not.
2584 The number following the first two digits is the offset from the start
2585 of the object to the part of the object pertaining to the base class.
2587 After the comma, the second number is a type_descriptor for the base
2588 type. Finally a semi-colon ends the series, which repeats for each
2591 The source below defines three base classes A, B, and C and the
2599 virtual int A_virt (int arg) @{ return arg; @};
2605 virtual int B_virt (int arg) @{return arg; @};
2611 virtual int C_virt (int arg) @{return arg; @};
2614 class D : A, virtual B, public C @{
2617 virtual int A_virt (int arg ) @{ return arg+1; @};
2618 virtual int B_virt (int arg) @{ return arg+2; @};
2619 virtual int C_virt (int arg) @{ return arg+3; @};
2620 virtual int D_virt (int arg) @{ return arg; @};
2624 Class stabs similar to the ones described earlier are generated for
2627 @c FIXME!!! the linebreaks in the following example probably make the
2628 @c examples literally unusable, but I don't know any other way to get
2629 @c them on the page.
2630 @c One solution would be to put some of the type definitions into
2631 @c separate stabs, even if that's not exactly what the compiler actually
2634 .stabs "A:T20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2635 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2637 .stabs "B:Tt25=s8Bdat:1,0,32;$vf25:21,32;B_virt::26=##1;
2638 :i;2A*-2147483647;25;;;~%25;",128,0,0,0
2640 .stabs "C:Tt28=s8Cdat:1,0,32;$vf28:21,32;C_virt::29=##1;
2641 :i;2A*-2147483647;28;;;~%28;",128,0,0,0
2644 In the stab describing derived class D below, the information about
2645 the derivation of this class is encoded as follows.
2648 .stabs "derived_class_name:symbol_descriptors(struct tag&type)=
2649 type_descriptor(struct)struct_bytes(32)!num_bases(3),
2650 base_virtual(no)inheritence_public(no)base_offset(0),
2651 base_class_type_ref(A);
2652 base_virtual(yes)inheritence_public(no)base_offset(NIL),
2653 base_class_type_ref(B);
2654 base_virtual(no)inheritence_public(yes)base_offset(64),
2655 base_class_type_ref(C); @dots{}
2658 @c FIXME! fake linebreaks.
2660 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:
2661 1,160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt:
2662 :32:i;2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;
2663 28;;D_virt::32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2666 @node Virtual Base Classes
2667 @section Virtual Base Classes
2669 A derived class object consists of a concatination in memory of the
2670 data areas defined by each base class, starting with the leftmost and
2671 ending with the rightmost in the list of base classes. The exception
2672 to this rule is for virtual inheritence. In the example above, class
2673 D inherits virtually from base class B. This means that an instance
2674 of a D object will not contain it's own B part but merely a pointer to
2675 a B part, known as a virtual base pointer.
2677 In a derived class stab, the base offset part of the derivation
2678 information, described above, shows how the base class parts are
2679 ordered. The base offset for a virtual base class is always given as
2680 0. Notice that the base offset for B is given as 0 even though B is
2681 not the first base class. The first base class A starts at offset 0.
2683 The field information part of the stab for class D describes the field
2684 which is the pointer to the virtual base class B. The vbase pointer
2685 name is $vb followed by a type reference to the virtual base class.
2686 Since the type id for B in this example is 25, the vbase pointer name
2689 @c FIXME!! fake linebreaks below
2691 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1,
2692 160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i;
2693 2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt:
2694 :32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2697 Following the name and a semicolon is a type reference describing the
2698 type of the virtual base class pointer, in this case 24. Type 24 was
2699 defined earlier as the type of the B class `this` pointer. The
2700 `this' pointer for a class is a pointer to the class type.
2703 .stabs "this:P24=*25=xsB:",64,0,0,8
2706 Finally the field offset part of the vbase pointer field description
2707 shows that the vbase pointer is the first field in the D object,
2708 before any data fields defined by the class. The layout of a D class
2709 object is a follows, Adat at 0, the vtable pointer for A at 32, Cdat
2710 at 64, the vtable pointer for C at 96, the virtual ase pointer for B
2711 at 128, and Ddat at 160.
2714 @node Static Members
2715 @section Static Members
2717 The data area for a class is a concatenation of the space used by the
2718 data members of the class. If the class has virtual methods, a vtable
2719 pointer follows the class data. The field offset part of each field
2720 description in the class stab shows this ordering.
2722 << How is this reflected in stabs? See Cygnus bug #677 for some info. >>
2725 @appendix Example2.c - source code for extended example
2729 2 register int g_bar asm ("%g5");
2730 3 static int s_g_repeat = 2;
2736 9 char s_char_vec[8];
2737 10 struct s_tag* s_next;
2740 13 typedef struct s_tag s_typedef;
2742 15 char char_vec[3] = @{'a','b','c'@};
2744 17 main (argc, argv)
2748 21 static float s_flap;
2750 23 for (times=0; times < s_g_repeat; times++)@{
2752 25 printf ("Hello world\n");
2756 29 enum e_places @{first,second=3,last@};
2758 31 static s_proc (s_arg, s_ptr_arg, char_vec)
2760 33 s_typedef* s_ptr_arg;
2774 @appendix Example2.s - assembly code for extended example
2778 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
2779 3 .stabs "example2.c",100,0,0,Ltext0
2782 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
2783 7 .stabs "char:t2=r2;0;127;",128,0,0,0
2784 8 .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
2785 9 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
2786 10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
2787 11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
2788 12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
2789 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
2790 14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
2791 15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
2792 16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
2793 17 .stabs "float:t12=r1;4;0;",128,0,0,0
2794 18 .stabs "double:t13=r1;8;0;",128,0,0,0
2795 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
2796 20 .stabs "void:t15=15",128,0,0,0
2797 21 .stabs "g_foo:G2",32,0,0,0
2802 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
2806 @c FIXME! fake linebreak in line 30
2807 30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;s_char_vec:
2808 17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
2809 31 .stabs "s_typedef:t16",128,0,0,0
2810 32 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
2811 33 .global _char_vec
2817 39 .reserve _s_flap.0,4,"bss",4
2821 43 .ascii "Hello world\12\0"
2826 48 .stabn 68,0,20,LM1
2829 51 save %sp,-144,%sp
2836 58 .stabn 68,0,23,LM2
2840 62 sethi %hi(_s_g_repeat),%o0
2842 64 ld [%o0+%lo(_s_g_repeat)],%o0
2847 69 .stabn 68,0,25,LM3
2849 71 sethi %hi(LC0),%o1
2850 72 or %o1,%lo(LC0),%o0
2853 75 .stabn 68,0,26,LM4
2856 78 .stabn 68,0,23,LM5
2864 86 .stabn 68,0,27,LM6
2867 89 .stabn 68,0,27,LM7
2872 94 .stabs "main:F1",36,0,0,_main
2873 95 .stabs "argc:p1",160,0,0,68
2874 96 .stabs "argv:p20=*21=*2",160,0,0,72
2875 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
2876 98 .stabs "times:1",128,0,0,-20
2877 99 .stabn 192,0,0,LBB2
2878 100 .stabs "inner:1",128,0,0,-24
2879 101 .stabn 192,0,0,LBB3
2880 102 .stabn 224,0,0,LBE3
2881 103 .stabn 224,0,0,LBE2
2882 104 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
2883 @c FIXME: fake linebreak in line 105
2884 105 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
2889 109 .stabn 68,0,35,LM8
2892 112 save %sp,-120,%sp
2898 118 .stabn 68,0,41,LM9
2901 121 .stabn 68,0,41,LM10
2906 126 .stabs "s_proc:f1",36,0,0,_s_proc
2907 127 .stabs "s_arg:p16",160,0,0,0
2908 128 .stabs "s_ptr_arg:p18",160,0,0,72
2909 129 .stabs "char_vec:p21",160,0,0,76
2910 130 .stabs "an_u:23",128,0,0,-20
2911 131 .stabn 192,0,0,LBB4
2912 132 .stabn 224,0,0,LBE4
2913 133 .stabs "g_bar:r1",64,0,0,5
2914 134 .stabs "g_pf:G24=*25=f1",32,0,0,0
2915 135 .common _g_pf,4,"bss"
2916 136 .stabs "g_an_s:G16",32,0,0,0
2917 137 .common _g_an_s,20,"bss"
2921 @appendix Values for the Stab Type Field
2923 These are all the possible values for the stab type field, for
2924 @code{a.out} files. This does not apply to XCOFF.
2926 The following types are used by the linker and assembler; there is
2927 nothing stabs-specific about them. Since this document does not attempt
2928 to describe aspects of object file format other than the debugging
2929 format, no details are given.
2931 @c Try to get most of these to fit on a single line.
2941 File scope absolute symbol
2943 @item 0x3 N_ABS | N_EXT
2944 External absolute symbol
2947 File scope text symbol
2949 @item 0x5 N_TEXT | N_EXT
2950 External text symbol
2953 File scope data symbol
2955 @item 0x7 N_DATA | N_EXT
2956 External data symbol
2959 File scope BSS symbol
2961 @item 0x9 N_BSS | N_EXT
2965 Same as N_FN, for Sequent compilers
2968 Symbol is indirected to another symbol
2971 Common sym -- visable after shared lib dynamic link
2974 Absolute set element
2977 Text segment set element
2980 Data segment set element
2983 BSS segment set element
2986 Pointer to set vector
2988 @item 0x1e N_WARNING
2989 Print a warning message during linking
2992 File name of a .o file
2995 The following symbol types indicate that this is a stab. This is the
2996 full list of stab numbers, including stab types that are used in
2997 languages other than C.
3001 Global symbol, @xref{N_GSYM}.
3004 Function name (for BSD Fortran), @xref{N_FNAME}.
3007 Function name or text segment variable for C, @xref{N_FUN}.
3010 Static symbol (data segment variable with internal linkage), @xref{N_STSYM}.
3013 .lcomm symbol (BSS segment variable with internal linkage), @xref{N_LCSYM}.
3016 Name of main routine (not used in C), @xref{N_MAIN}.
3018 @c FIXME: discuss this in the main body of the text where we talk about
3019 @c using N_FUN for variables.
3021 Read-only data symbol (Solaris2). Most systems use N_FUN for this.
3024 Global symbol (for Pascal), @xref{N_PC}.
3027 Number of symbols (according to Ultrix V4.0), @xref{N_NSYMS}.
3030 No DST map for sym (according to Ultrix V4.0), @xref{N_NOMAP}.
3032 @c FIXME: describe this solaris feature in the body of the text (see
3033 @c comments in include/aout/stab.def).
3035 Object file (Solaris2).
3037 @c See include/aout/stab.def for (a little) more info.
3039 Debugger options (Solaris2).
3042 Register variable, @xref{N_RSYM}.
3045 Modula-2 compilation unit, @xref{N_M2C}.
3048 Line number in text segment, @xref{Line Numbers}.
3051 Line number in data segment, @xref{Line Numbers}.
3054 Line number in bss segment, @xref{Line Numbers}.
3057 Sun source code browser, path to .cb file, @xref{N_BROWS}.
3060 Gnu Modula2 definition module dependency, @xref{N_DEFD}.
3063 Function start/body/end line numbers (Solaris2).
3066 Gnu C++ exception variable, @xref{N_EHDECL}.
3069 Modula2 info "for imc" (according to Ultrix V4.0), @xref{N_MOD2}.
3072 Gnu C++ "catch" clause, @xref{N_CATCH}.
3075 Structure of union element, @xref{N_SSYM}.
3078 Last stab for module (Solaris2).
3081 Path and name of source file , @xref{Source Files}.
3084 Automatic var in the stack or type definition, @xref{N_LSYM}, @xref{Typedefs}.
3087 Beginning of an include file (Sun only), @xref{Source Files}.
3090 Name of include file, @xref{Source Files}.
3093 Parameter variable, @xref{Parameters}.
3096 End of an include file, @xref{Source Files}.
3099 Alternate entry point, @xref{N_ENTRY}.
3102 Beginning of a lexical block, @xref{Block Structure}.
3105 Place holder for a deleted include file, @xref{Source Files}.
3108 Modula2 scope information (Sun linker), @xref{N_SCOPE}.
3111 End of a lexical block, @xref{Block Structure}.
3114 Begin named common block, @xref{Common Blocks}.
3117 End named common block, @xref{Common Blocks}.
3120 Member of a common block, @xref{Common Blocks}.
3122 @c FIXME: How does this really work? Move it to main body of document.
3124 Pascal @code{with} statement: type,,0,0,offset (Solaris2).
3127 Gould non-base registers, @xref{Gould}.
3130 Gould non-base registers, @xref{Gould}.
3133 Gould non-base registers, @xref{Gould}.
3136 Gould non-base registers, @xref{Gould}.
3139 Gould non-base registers, @xref{Gould}.
3142 @c Restore the default table indent
3147 @node Symbol Descriptors
3148 @appendix Table of Symbol Descriptors
3150 @c Please keep this alphabetical
3152 @c In TeX, this looks great, digit is in italics. But makeinfo insists
3153 @c on putting it in `', not realizing that @var should override @code.
3154 @c I don't know of any way to make makeinfo do the right thing. Seems
3155 @c like a makeinfo bug to me.
3159 Local variable, @xref{Automatic variables}.
3162 Parameter passed by reference in register, @xref{Parameters}.
3165 Constant, @xref{Constants}.
3168 Conformant array bound (Pascal, maybe other languages),
3169 @xref{Parameters}. Name of a caught exception (GNU C++). These can be
3170 distinguished because the latter uses N_CATCH and the former uses
3171 another symbol type.
3174 Floating point register variable, @xref{Register variables}.
3177 Parameter in floating point register, @xref{Parameters}.
3180 Static function, @xref{Procedures}.
3183 Global function, @xref{Procedures}.
3186 Global variable, @xref{Global Variables}.
3192 Internal (nested) procedure, @xref{Procedures}.
3195 Internal (nested) function, @xref{Procedures}.
3198 Label name (documented by AIX, no further information known).
3201 Module, @xref{Procedures}.
3204 Argument list parameter, @xref{Parameters}.
3210 FORTRAN Function parameter, @xref{Parameters}.
3213 Unfortunately, three separate meanings have been independently invented
3214 for this symbol descriptor. At least the GNU and Sun uses can be
3215 distinguished by the symbol type. Global Procedure (AIX) (symbol type
3216 used unknown), @xref{Procedures}. Register parameter (GNU) (symbol type
3217 N_PSYM), @xref{Parameters}. Prototype of function referenced by this
3218 file (Sun acc) (symbol type N_FUN).
3221 Static Procedure, @xref{Procedures}.
3224 Register parameter @xref{Parameters}.
3227 Register variable, @xref{Register variables}.
3230 Static file scope variable @xref{Initialized statics},
3231 @xref{Un-initialized statics}.
3234 Type name, @xref{Typedefs}.
3237 enumeration, struct or union tag, @xref{Typedefs}.
3240 Parameter passed by reference, @xref{Parameters}.
3243 Static procedure scope variable @xref{Initialized statics},
3244 @xref{Un-initialized statics}.
3247 Conformant array, @xref{Parameters}.
3250 Function return variable, @xref{Parameters}.
3253 @node Type Descriptors
3254 @appendix Table of Type Descriptors
3259 Type reference, @xref{Stabs Format}.
3262 Reference to builtin type, @xref{Negative Type Numbers}.
3265 Method (C++), @xref{Cplusplus}.
3268 Pointer, @xref{Miscellaneous Types}.
3274 Type Attributes (AIX), @xref{Stabs Format}. Member (class and variable)
3275 type (GNU C++), @xref{Cplusplus}.
3278 Array, @xref{Arrays}.
3281 Open array, @xref{Arrays}.
3284 Pascal space type (AIX), @xref{Miscellaneous Types}. Builtin integer
3285 type (Sun), @xref{Builtin Type Descriptors}.
3288 Volatile-qualified type, @xref{Miscellaneous Types}.
3291 Complex builtin type, @xref{Builtin Type Descriptors}.
3294 COBOL Picture type. See AIX documentation for details.
3297 File type, @xref{Miscellaneous Types}.
3300 N-dimensional dynamic array, @xref{Arrays}.
3303 Enumeration type, @xref{Enumerations}.
3306 N-dimensional subarray, @xref{Arrays}.
3309 Function type, @xref{Function Types}.
3312 Pascal function parameter, @xref{Function Types}
3315 Builtin floating point type, @xref{Builtin Type Descriptors}.
3318 COBOL Group. See AIX documentation for details.
3321 Imported type, @xref{Cross-references}.
3324 Const-qualified type, @xref{Miscellaneous Types}.
3327 COBOL File Descriptor. See AIX documentation for details.
3330 Multiple instance type, @xref{Miscellaneous Types}.
3333 String type, @xref{Strings}.
3336 Stringptr, @xref{Strings}.
3339 Opaque type, @xref{Typedefs}.
3342 Procedure, @xref{Function Types}.
3345 Packed array, @xref{Arrays}.
3348 Range type, @xref{Subranges}.
3351 Builtin floating type, @xref{Builtin Type Descriptors} (Sun). Pascal
3352 subroutine parameter, @xref{Function Types} (AIX). Detecting this
3353 conflict is possible with careful parsing (hint: a Pascal subroutine
3354 parameter type will always contain a comma, and a builtin type
3355 descriptor never will).
3358 Structure type, @xref{Structures}.
3361 Set type, @xref{Miscellaneous Types}.
3364 Union, @xref{Unions}.
3367 Variant record. This is a Pascal and Modula-2 feature which is like a
3368 union within a struct in C. See AIX documentation for details.
3371 Wide character, @xref{Builtin Type Descriptors}.
3374 Cross-reference, @xref{Cross-references}.
3377 gstring, @xref{Strings}.
3380 @node Expanded reference
3381 @appendix Expanded reference by stab type.
3383 @c FIXME: This appendix should go away, see N_PSYM or N_SO for an example.
3385 For a full list of stab types, and cross-references to where they are
3386 described, @xref{Stab Types}. This appendix just duplicates certain
3387 information from the main body of this document; eventually the
3388 information will all be in one place.
3392 The first line is the symbol type expressed in decimal, hexadecimal,
3393 and as a #define (see devo/include/aout/stab.def).
3395 The second line describes the language constructs the symbol type
3398 The third line is the stab format with the significant stab fields
3399 named and the rest NIL.
3401 Subsequent lines expand upon the meaning and possible values for each
3402 significant stab field. # stands in for the type descriptor.
3404 Finally, any further information.
3407 * N_GSYM:: Global variable
3408 * N_FNAME:: Function name (BSD Fortran)
3409 * N_FUN:: C Function name or text segment variable
3410 * N_STSYM:: Initialized static symbol
3411 * N_LCSYM:: Uninitialized static symbol
3412 * N_MAIN:: Name of main routine (not for C)
3413 * N_PC:: Pascal global symbol
3414 * N_NSYMS:: Number of symbols
3415 * N_NOMAP:: No DST map
3416 * N_RSYM:: Register variable
3417 * N_M2C:: Modula-2 compilation unit
3418 * N_BROWS:: Path to .cb file for Sun source code browser
3419 * N_DEFD:: GNU Modula2 definition module dependency
3420 * N_EHDECL:: GNU C++ exception variable
3421 * N_MOD2:: Modula2 information "for imc"
3422 * N_CATCH:: GNU C++ "catch" clause
3423 * N_SSYM:: Structure or union element
3424 * N_LSYM:: Automatic variable
3425 * N_ENTRY:: Alternate entry point
3426 * N_SCOPE:: Modula2 scope information (Sun only)
3427 * Gould:: non-base register symbols used on Gould systems
3428 * N_LENG:: Length of preceding entry
3432 @section 32 - 0x20 - N_GYSM
3437 .stabs "name", N_GSYM, NIL, NIL, NIL
3441 "name" -> "symbol_name:#type"
3445 Only the "name" field is significant. The location of the variable is
3446 obtained from the corresponding external symbol.
3449 @section 34 - 0x22 - N_FNAME
3450 Function name (for BSD Fortran)
3453 .stabs "name", N_FNAME, NIL, NIL, NIL
3457 "name" -> "function_name"
3460 Only the "name" field is significant. The location of the symbol is
3461 obtained from the corresponding extern symbol.
3464 @section 36 - 0x24 - N_FUN
3466 Function name (@pxref{Procedures}) or text segment variable
3467 (@pxref{Variables}).
3469 @exdent @emph{For functions:}
3470 "name" -> "proc_name:#return_type"
3471 # -> F (global function)
3473 desc -> line num for proc start. (GCC doesn't set and DBX doesn't miss it.)
3474 value -> Code address of proc start.
3476 @exdent @emph{For text segment variables:}
3477 <<How to create one?>>
3481 @section 38 - 0x26 - N_STSYM
3482 Initialized static symbol (data segment w/internal linkage).
3485 .stabs "name", N_STSYM, NIL, NIL, value
3489 "name" -> "symbol_name#type"
3490 # -> S (scope global to compilation unit)
3491 -> V (scope local to a procedure)
3492 value -> Data Address
3496 @section 40 - 0x28 - N_LCSYM
3497 Unitialized static (.lcomm) symbol(BSS segment w/internal linkage).
3500 .stabs "name", N_LCLSYM, NIL, NIL, value
3504 "name" -> "symbol_name#type"
3505 # -> S (scope global to compilation unit)
3506 -> V (scope local to procedure)
3507 value -> BSS Address
3511 @section 42 - 0x2a - N_MAIN
3512 Name of main routine (not used in C)
3515 .stabs "name", N_MAIN, NIL, NIL, NIL
3519 "name" -> "name_of_main_routine"
3523 @section 48 - 0x30 - N_PC
3524 Global symbol (for Pascal)
3527 .stabs "name", N_PC, NIL, NIL, value
3531 "name" -> "symbol_name" <<?>>
3532 value -> supposedly the line number (stab.def is skeptical)
3538 global pascal symbol: name,,0,subtype,line
3543 @section 50 - 0x32 - N_NSYMS
3544 Number of symbols (according to Ultrix V4.0)
3547 0, files,,funcs,lines (stab.def)
3551 @section 52 - 0x34 - N_NOMAP
3552 no DST map for sym (according to Ultrix V4.0)
3555 name, ,0,type,ignored (stab.def)
3559 @section 64 - 0x40 - N_RSYM
3563 .stabs "name:type",N_RSYM,0,RegSize,RegNumber (Sun doc)
3567 @section 66 - 0x42 - N_M2C
3568 Modula-2 compilation unit
3571 .stabs "name", N_M2C, 0, desc, value
3575 "name" -> "unit_name,unit_time_stamp[,code_time_stamp]
3577 value -> 0 (main unit)
3582 @section 72 - 0x48 - N_BROWS
3583 Sun source code browser, path to .cb file
3586 "path to associated .cb file"
3588 Note: type field value overlaps with N_BSLINE
3591 @section 74 - 0x4a - N_DEFD
3592 GNU Modula2 definition module dependency
3594 GNU Modula-2 definition module dependency. Value is the modification
3595 time of the definition file. Other is non-zero if it is imported with
3596 the GNU M2 keyword %INITIALIZE. Perhaps N_M2C can be used if there
3597 are enough empty fields?
3600 @section 80 - 0x50 - N_EHDECL
3601 GNU C++ exception variable <<?>>
3603 "name is variable name"
3605 Note: conflicts with N_MOD2.
3608 @section 80 - 0x50 - N_MOD2
3609 Modula2 info "for imc" (according to Ultrix V4.0)
3611 Note: conflicts with N_EHDECL <<?>>
3614 @section 84 - 0x54 - N_CATCH
3615 GNU C++ "catch" clause
3617 GNU C++ `catch' clause. Value is its address. Desc is nonzero if
3618 this entry is immediately followed by a CAUGHT stab saying what
3619 exception was caught. Multiple CAUGHT stabs means that multiple
3620 exceptions can be caught here. If Desc is 0, it means all exceptions
3624 @section 96 - 0x60 - N_SSYM
3625 Structure or union element
3627 Value is offset in the structure.
3629 <<?looking at structs and unions in C I didn't see these>>
3632 @section 128 - 0x80 - N_LSYM
3633 Automatic var in the stack (also used for type descriptors.)
3636 .stabs "name" N_LSYM, NIL, NIL, value
3640 @exdent @emph{For stack based local variables:}
3642 "name" -> name of the variable
3643 value -> offset from frame pointer (negative)
3645 @exdent @emph{For type descriptors:}
3647 "name" -> "name_of_the_type:#type"
3650 type -> type_ref (or) type_def
3652 type_ref -> type_number
3653 type_def -> type_number=type_desc etc.
3656 Type may be either a type reference or a type definition. A type
3657 reference is a number that refers to a previously defined type. A
3658 type definition is the number that will refer to this type, followed
3659 by an equals sign, a type descriptor and the additional data that
3660 defines the type. See the Table D for type descriptors and the
3661 section on types for what data follows each type descriptor.
3664 @section 164 - 0xa4 - N_ENTRY
3666 Alternate entry point.
3667 Value is its address.
3671 @section 196 - 0xc4 - N_SCOPE
3673 Modula2 scope information (Sun linker)
3677 @section Non-base registers on Gould systems
3679 These are used on Gould systems for non-base registers syms.
3681 However, the following values are not the values used by Gould; they are
3682 the values which GNU has been documenting for these values for a long
3683 time, without actually checking what Gould uses. I include these values
3684 only because perhaps some someone actually did something with the GNU
3685 information (I hope not, why GNU knowingly assigned wrong values to
3686 these in the header file is a complete mystery to me).
3689 240 0xf0 N_NBTEXT ??
3690 242 0xf2 N_NBDATA ??
3697 @section - 0xfe - N_LENG
3699 Second symbol entry containing a length-value for the preceding entry.
3700 The value is the length.
3703 @appendix Questions and anomalies
3707 For GNU C stabs defining local and global variables (N_LSYM and
3708 N_GSYM), the desc field is supposed to contain the source line number
3709 on which the variable is defined. In reality the desc field is always
3710 0. (This behavour is defined in dbxout.c and putting a line number in
3711 desc is controlled by #ifdef WINNING_GDB which defaults to false). Gdb
3712 supposedly uses this information if you say 'list var'. In reality
3713 var can be a variable defined in the program and gdb says `function
3717 In GNU C stabs there seems to be no way to differentiate tag types:
3718 structures, unions, and enums (symbol descriptor T) and typedefs
3719 (symbol descriptor t) defined at file scope from types defined locally
3720 to a procedure or other more local scope. They all use the N_LSYM
3721 stab type. Types defined at procedure scope are emited after the
3722 N_RBRAC of the preceding function and before the code of the
3723 procedure in which they are defined. This is exactly the same as
3724 types defined in the source file between the two procedure bodies.
3725 GDB overcompensates by placing all types in block #1, the block for
3726 symbols of file scope. This is true for default, -ansi and
3727 -traditional compiler options. (Bugs gcc/1063, gdb/1066.)
3730 What ends the procedure scope? Is it the proc block's N_RBRAC or the
3731 next N_FUN? (I believe its the first.)
3734 The comment in xcoff.h says DBX_STATIC_CONST_VAR_CODE is used for
3735 static const variables. DBX_STATIC_CONST_VAR_CODE is set to N_FUN by
3736 default, in dbxout.c. If included, xcoff.h redefines it to N_STSYM.
3737 But testing the default behaviour, my Sun4 native example shows
3738 N_STSYM not N_FUN is used to describe file static initialized
3739 variables. (the code tests for TREE_READONLY(decl) &&
3740 !TREE_THIS_VOLATILE(decl) and if true uses DBX_STATIC_CONST_VAR_CODE).
3743 Global variable stabs don't have location information. This comes
3744 from the external symbol for the same variable. The external symbol
3745 has a leading underbar on the _name of the variable and the stab does
3746 not. How do we know these two symbol table entries are talking about
3747 the same symbol when their names are different?
3750 Can gcc be configured to output stabs the way the Sun compiler
3751 does, so that their native debugging tools work? <NO?> It doesn't by
3752 default. GDB reads either format of stab. (gcc or SunC). How about
3756 @node xcoff-differences
3757 @appendix Differences between GNU stabs in a.out and GNU stabs in xcoff
3759 @c FIXME: Merge *all* these into the main body of the document.
3760 (The AIX/RS6000 native object file format is xcoff with stabs). This
3761 appendix only covers those differences which are not covered in the main
3762 body of this document.
3766 BSD a.out stab types correspond to AIX xcoff storage classes. In general the
3767 mapping is N_STABTYPE becomes C_STABTYPE. Some stab types in a.out
3768 are not supported in xcoff. See Table E. for full mappings.
3771 initialised static N_STSYM and un-initialized static N_LCSYM both map
3772 to the C_STSYM storage class. But the destinction is preserved
3773 because in xcoff N_STSYM and N_LCSYM must be emited in a named static
3774 block. Begin the block with .bs s[RW] data_section_name for N_STSYM
3775 or .bs s bss_section_name for N_LCSYM. End the block with .es
3778 If the xcoff stab is a N_FUN (C_FUN) then follow the string field with
3779 ,. instead of just ,
3783 (I think that's it for .s file differences. They could stand to be
3784 better presented. This is just a list of what I have noticed so far.
3785 There are a *lot* of differences in the information in the symbol
3786 tables of the executable and object files.)
3788 Table E: mapping a.out stab types to xcoff storage classes
3791 stab type storage class
3792 -------------------------------
3801 N_RPSYM (0x8e) C_RPSYM
3811 N_DECL (0x8c) C_DECL
3828 @node Sun-differences
3829 @appendix Differences between GNU stabs and Sun native stabs.
3831 @c FIXME: Merge all this stuff into the main body of the document.
3835 GNU C stabs define *all* types, file or procedure scope, as
3836 N_LSYM. Sun doc talks about using N_GSYM too.
3839 Sun C stabs use type number pairs in the format (a,b) where a is a
3840 number starting with 1 and incremented for each sub-source file in the
3841 compilation. b is a number starting with 1 and incremented for each
3842 new type defined in the compilation. GNU C stabs use the type number
3843 alone, with no source file number.
3847 @appendix Using stabs with the ELF object file format.
3849 The ELF object file format allows tools to create object files with custom
3850 sections containing any arbitrary data. To use stabs in ELF object files,
3851 the tools create two custom sections, a ".stab" section which contains
3852 an array of fixed length structures, one struct per stab, and a ".stabstr"
3853 section containing all the variable length strings that are referenced by
3854 stabs in the ".stab" section. The byte order of the stabs binary data
3855 matches the byte order of the ELF file itself, as determined from the
3856 EI_DATA field in the e_ident member of the ELF header.
3858 The first stab in the ".stab" section for each object file is a "synthetic
3859 stab", generated entirely by the assembler, with no corresponding ".stab"
3860 directive as input to the assembler. This stab contains the following
3865 Offset in the ".stabstr" section to the source filename.
3871 Unused field, always zero.
3874 Count of upcoming symbols. I.E. the number of remaining stabs for this
3878 Size of the string table fragment associated with this object module, in
3883 The ".stabstr" section always starts with a null byte (so that string
3884 offsets of zero reference a null string), followed by random length strings,
3885 each of which is null byte terminated.
3887 The ELF section header for the ".stab" section has it's sh_link member set
3888 to the section number of the ".stabstr" section, and the ".stabstr" section
3889 has it's ELF section header sh_type member set to SHT_STRTAB to mark it as