2 @setfilename stabs.info
7 * Stabs:: The "stabs" debugging information format.
13 This document describes the stabs debugging symbol tables.
15 Copyright 1992 Free Software Foundation, Inc.
16 Contributed by Cygnus Support. Written by Julia Menapace.
18 Permission is granted to make and distribute verbatim copies of
19 this manual provided the copyright notice and this permission notice
20 are preserved on all copies.
23 Permission is granted to process this file through Tex and print the
24 results, provided the printed document carries copying permission
25 notice identical to this one except for the removal of this paragraph
26 (this paragraph not being relevant to the printed manual).
29 Permission is granted to copy or distribute modified versions of this
30 manual under the terms of the GPL (for which purpose this text may be
31 regarded as a program in the language TeX).
34 @setchapternewpage odd
37 @title The ``stabs'' debug format
38 @author Julia Menapace
39 @author Cygnus Support
42 \def\$#1${{#1}} % Kluge: collect RCS revision info without $...$
43 \xdef\manvers{\$Revision$} % For use in headers, footers too
45 \hfill Cygnus Support\par
47 \hfill \TeX{}info \texinfoversion\par
51 @vskip 0pt plus 1filll
52 Copyright @copyright{} 1992 Free Software Foundation, Inc.
53 Contributed by Cygnus Support.
55 Permission is granted to make and distribute verbatim copies of
56 this manual provided the copyright notice and this permission notice
57 are preserved on all copies.
63 @top The "stabs" representation of debugging information
65 This document describes the stabs debugging format.
68 * Overview:: Overview of stabs
69 * Program structure:: Encoding of the structure of the program
70 * Constants:: Constants
71 * Example:: A comprehensive example in C
73 * Types:: Type definitions
74 * Symbol Tables:: Symbol information in symbol tables
75 * Cplusplus:: Appendixes:
76 * Example2.c:: Source code for extended example
77 * Example2.s:: Assembly code for extended example
78 * Stab Types:: Symbol types in a.out files
79 * Symbol Descriptors:: Table of Symbol Descriptors
80 * Type Descriptors:: Table of Symbol Descriptors
81 * Expanded reference:: Reference information by stab type
82 * Questions:: Questions and anomolies
83 * xcoff-differences:: Differences between GNU stabs in a.out
84 and GNU stabs in xcoff
85 * Sun-differences:: Differences between GNU stabs and Sun
87 * Stabs-in-elf:: Stabs in an ELF file.
93 @chapter Overview of stabs
95 @dfn{Stabs} refers to a format for information that describes a program
96 to a debugger. This format was apparently invented by
97 @c FIXME! <<name of inventor>> at
98 the University of California at Berkeley, for the @code{pdx} Pascal
99 debugger; the format has spread widely since then.
101 This document is one of the few published sources of documentation on
102 stabs. It is believed to be completely comprehensive for stabs used by
103 C. The lists of symbol descriptors (@pxref{Symbol Descriptors}) and
104 type descriptors (@pxref{Type Descriptors}) are believed to be completely
105 comprehensive. There are known to be stabs for C++ and COBOL which are
106 poorly documented here. Stabs specific to other languages (e.g. Pascal,
107 Modula-2) are probably not as well documented as they should be.
109 Other sources of information on stabs are @cite{dbx and dbxtool
110 interfaces}, 2nd edition, by Sun, circa 1988, and @cite{AIX Version 3.2
111 Files Reference}, Fourth Edition, September 1992, "dbx Stabstring
112 Grammar" in the a.out section, page 2-31. This document is believed to
113 incorporate the information from those two sources except where it
114 explictly directs you to them for more information.
117 * Flow:: Overview of debugging information flow
118 * Stabs Format:: Overview of stab format
119 * C example:: A simple example in C source
120 * Assembly code:: The simple example at the assembly level
124 @section Overview of debugging information flow
126 The GNU C compiler compiles C source in a @file{.c} file into assembly
127 language in a @file{.s} file, which is translated by the assembler into
128 a @file{.o} file, and then linked with other @file{.o} files and
129 libraries to produce an executable file.
131 With the @samp{-g} option, GCC puts additional debugging information in
132 the @file{.s} file, which is slightly transformed by the assembler and
133 linker, and carried through into the final executable. This debugging
134 information describes features of the source file like line numbers,
135 the types and scopes of variables, and functions, their parameters and
138 For some object file formats, the debugging information is
139 encapsulated in assembler directives known collectively as `stab' (symbol
140 table) directives, interspersed with the generated code. Stabs are
141 the native format for debugging information in the a.out and xcoff
142 object file formats. The GNU tools can also emit stabs in the coff
143 and ecoff object file formats.
145 The assembler adds the information from stabs to the symbol information
146 it places by default in the symbol table and the string table of the
147 @file{.o} file it is building. The linker consolidates the @file{.o}
148 files into one executable file, with one symbol table and one string
149 table. Debuggers use the symbol and string tables in the executable as
150 a source of debugging information about the program.
153 @section Overview of stab format
155 There are three overall formats for stab assembler directives
156 differentiated by the first word of the stab. The name of the directive
157 describes what combination of four possible data fields will follow. It
158 is either @code{.stabs} (string), @code{.stabn} (number), or
159 @code{.stabd} (dot). IBM's xcoff uses @code{.stabx} (and some other
160 directives such as @code{.file} and @code{.bi}) instead of
161 @code{.stabs}, @code{.stabn} or @code{.stabd}.
163 The overall format of each class of stab is:
166 .stabs "@var{string}",@var{type},0,@var{desc},@var{value}
167 .stabx "@var{string}",@var{value},@var{type},@var{sdb-type}
168 .stabn @var{type},0,@var{desc},@var{value}
169 .stabd @var{type},0,@var{desc}
172 @c what is the correct term for "current file location"? My AIX
173 @c assembler manual calls it "the value of the current location counter".
174 For @code{.stabn} and @code{.stabd}, there is no string (the
175 @code{n_strx} field is zero, @pxref{Symbol Tables}). For @code{.stabd}
176 the value field is implicit and has the value of the current file
177 location. The @var{sdb-type} field to @code{.stabx} is unused for stabs
178 and can always be set to 0.
180 The number in the type field gives some basic information about what
181 type of stab this is (or whether it @emph{is} a stab, as opposed to an
182 ordinary symbol). Each possible type number defines a different stab
183 type. The stab type further defines the exact interpretation of, and
184 possible values for, any remaining @code{"@var{string}"}, @var{desc}, or
185 @var{value} fields present in the stab. @xref{Stab Types}, for a list
186 in numeric order of the possible type field values for stab directives.
188 For @code{.stabs} the @code{"@var{string}"} field holds the meat of the
189 debugging information. The generally unstructured nature of this field
190 is what makes stabs extensible. For some stab types the string field
191 contains only a name. For other stab types the contents can be a great
194 The overall format is of the @code{"@var{string}"} field is:
197 "@var{name}:@var{symbol-descriptor} @var{type-information}"
200 @var{name} is the name of the symbol represented by the stab.
201 @var{name} can be omitted, which means the stab represents an unnamed
202 object. For example, @samp{:t10=*2} defines type 10 as a pointer to
203 type 2, but does not give the type a name. Omitting the @var{name}
204 field is supported by AIX dbx and GDB after about version 4.8, but not
205 other debuggers. GCC sometimes uses a single space as the name instead
206 of omitting the name altogether; apparently that is supported by most
209 The @var{symbol_descriptor} following the @samp{:} is an alphabetic
210 character that tells more specifically what kind of symbol the stab
211 represents. If the @var{symbol_descriptor} is omitted, but type
212 information follows, then the stab represents a local variable. For a
213 list of symbol descriptors, see @ref{Symbol Descriptors,,Table C: Symbol
216 The @samp{c} symbol descriptor is an exception in that it is not
217 followed by type information. @xref{Constants}.
219 Type information is either a @var{type_number}, or a
220 @samp{@var{type_number}=}. The @var{type_number} alone is a type
221 reference, referring directly to a type that has already been defined.
223 The @samp{@var{type_number}=} is a type definition, where the number
224 represents a new type which is about to be defined. The type definition
225 may refer to other types by number, and those type numbers may be
226 followed by @samp{=} and nested definitions.
228 In a type definition, if the character that follows the equals sign is
229 non-numeric then it is a @var{type_descriptor}, and tells what kind of
230 type is about to be defined. Any other values following the
231 @var{type_descriptor} vary, depending on the @var{type_descriptor}. If
232 a number follows the @samp{=} then the number is a @var{type_reference}.
233 This is described more thoroughly in the section on types. @xref{Type
234 Descriptors,,Table D: Type Descriptors}, for a list of
235 @var{type_descriptor} values.
237 There is an AIX extension for type attributes. Following the @samp{=}
238 is any number of type attributes. Each one starts with @samp{@@} and
239 ends with @samp{;}. Debuggers, including AIX's dbx, skip any type
240 attributes they do not recognize. GDB 4.9 does not do this---it will
241 ignore the entire symbol containing a type attribute. Hopefully this
242 will be fixed in the next GDB release. Because of a conflict with C++
243 (@pxref{Cplusplus}), new attributes should not be defined which begin
244 with a digit, @samp{(}, or @samp{-}; GDB may be unable to distinguish
245 those from the C++ type descriptor @samp{@@}. The attributes are:
248 @item a@var{boundary}
249 @var{boundary} is an integer specifying the alignment. I assume it
250 applies to all variables of this type.
253 Size in bits of a variable of this type.
256 Pointer class (for checking). Not sure what this means, or how
257 @var{integer} is interpreted.
260 Indicate this is a packed type, meaning that structure fields or array
261 elements are placed more closely in memory, to save memory at the
265 All this can make the @code{"@var{string}"} field quite long. All
266 versions of GDB, and some versions of DBX, can handle arbitrarily long
267 strings. But many versions of DBX cretinously limit the strings to
268 about 80 characters, so compilers which must work with such DBX's need
269 to split the @code{.stabs} directive into several @code{.stabs}
270 directives. Each stab duplicates exactly all but the
271 @code{"@var{string}"} field. The @code{"@var{string}"} field of
272 every stab except the last is marked as continued with a
273 double-backslash at the end. Removing the backslashes and concatenating
274 the @code{"@var{string}"} fields of each stab produces the original,
278 @section A simple example in C source
280 To get the flavor of how stabs describe source information for a C
281 program, let's look at the simple program:
286 printf("Hello world");
290 When compiled with @samp{-g}, the program above yields the following
291 @file{.s} file. Line numbers have been added to make it easier to refer
292 to parts of the @file{.s} file in the description of the stabs that
296 @section The simple example at the assembly level
300 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
301 3 .stabs "hello.c",100,0,0,Ltext0
304 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
305 7 .stabs "char:t2=r2;0;127;",128,0,0,0
306 8 .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
307 9 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
308 10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
309 11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
310 12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
311 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
312 14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
313 15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
314 16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
315 17 .stabs "float:t12=r1;4;0;",128,0,0,0
316 18 .stabs "double:t13=r1;8;0;",128,0,0,0
317 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
318 20 .stabs "void:t15=15",128,0,0,0
321 23 .ascii "Hello, world!\12\0"
336 38 sethi %hi(LC0),%o1
337 39 or %o1,%lo(LC0),%o0
348 50 .stabs "main:F1",36,0,0,_main
349 51 .stabn 192,0,0,LBB2
350 52 .stabn 224,0,0,LBE2
353 This simple ``hello world'' example demonstrates several of the stab
354 types used to describe C language source files.
356 @node Program structure
357 @chapter Encoding for the structure of the program
360 * Main Program:: Indicate what the main program is
361 * Source Files:: The path and name of the source file
368 @section Main Program
370 Most languages allow the main program to have any name. The
371 @code{N_MAIN} stab type is used for a stab telling the debugger what
372 name is used in this program. Only the name is significant; it will be
373 the name of a function which is the main program. Most C compilers do
374 not use this stab; they expect the debugger to simply assume that the
375 name is @samp{main}, but some C compilers emit an @code{N_MAIN} stab for
376 the @samp{main} function.
379 @section The path and name of the source files
381 Before any other stabs occur, there must be a stab specifying the source
382 file. This information is contained in a symbol of stab type
383 @code{N_SO}; the string contains the name of the file. The value of the
384 symbol is the start address of portion of the text section corresponding
387 With the Sun Solaris2 compiler, the @code{desc} field contains a
388 source-language code.
390 Some compilers (for example, gcc2 and SunOS4 @file{/bin/cc}) also
391 include the directory in which the source was compiled, in a second
392 @code{N_SO} symbol preceding the one containing the file name. This
393 symbol can be distinguished by the fact that it ends in a slash. Code
394 from the cfront C++ compiler can have additional @code{N_SO} symbols for
395 nonexistent source files after the @code{N_SO} for the real source file;
396 these are believed to contain no useful information.
401 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0 ; 100 is N_SO
402 .stabs "hello.c",100,0,0,Ltext0
407 Instead of @code{N_SO} symbols, XCOFF uses a @code{.file} assembler
408 directive which assembles to a standard COFF @code{.file} symbol;
409 explaining this in detail is outside the scope of this document.
411 There are several different schemes for dealing with include files: the
412 traditional @code{N_SOL} approach, Sun's @code{N_BINCL} scheme, and the
413 XCOFF @code{C_BINCL} (which despite the similar name has little in
414 common with @code{N_BINCL}).
416 An @code{N_SOL} symbol specifies which include file subsequent symbols
417 refer to. The string field is the name of the file and the value is the
418 text address corresponding to the start of the previous include file and
419 the start of this one. To specify the main source file again, use an
420 @code{N_SOL} symbol with the name of the main source file.
422 A @code{N_BINCL} symbol specifies the start of an include file. In an
423 object file, only the name is significant. The Sun linker puts data
424 into some of the other fields. The end of the include file is marked by
425 a @code{N_EINCL} symbol (which has no name field). In an ojbect file,
426 there is no significant data in the @code{N_EINCL} symbol; the Sun
427 linker puts data into some of the fields. @code{N_BINCL} and
428 @code{N_EINCL} can be nested. If the linker detects that two source
429 files have identical stabs with a @code{N_BINCL} and @code{N_EINCL} pair
430 (as will generally be the case for a header file), then it only puts out
431 the stabs once. Each additional occurance is replaced by an
432 @code{N_EXCL} symbol. I believe the Sun (SunOS4, not sure about
433 Solaris) linker is the only one which supports this feature.
435 For the start of an include file in XCOFF, use the @file{.bi} assembler
436 directive which generates a @code{C_BINCL} symbol. A @file{.ei}
437 directive, which generates a @code{C_EINCL} symbol, denotes the end of
438 the include file. Both directives are followed by the name of the
439 source file in quotes, which becomes the string for the symbol. The
440 value of each symbol, produced automatically by the assembler and
441 linker, is an offset into the executable which points to the beginning
442 (inclusive, as you'd expect) and end (inclusive, as you would not
443 expect) of the portion of the COFF linetable which corresponds to this
444 include file. @code{C_BINCL} and @code{C_EINCL} do not nest.
447 @section Line Numbers
449 A @code{N_SLINE} symbol represents the start of a source line. The
450 @var{desc} field contains the line number and the @var{value} field
451 contains the code address for the start of that source line. On most
452 machines the address is absolute; for Sun's stabs-in-elf, it is relative
453 to the function in which the @code{N_SLINE} symbol occurs.
455 GNU documents @code{N_DSLINE} and @code{N_BSLINE} symbols for line
456 numbers in the data or bss segments, respectively. They are identical
457 to @code{N_SLINE} but are relocated differently by the linker. They
458 were intended to be used to describe the source location of a variable
459 declaration, but I believe that gcc2 actually puts the line number in
460 the desc field of the stab for the variable itself. GDB has been
461 ignoring these symbols (unless they contain a string field) at least
464 XCOFF uses COFF line numbers instead, which are outside the scope of
465 this document, ammeliorated by adequate marking of include files
466 (@pxref{Source Files}).
468 For single source lines that generate discontiguous code, such as flow
469 of control statements, there may be more than one line number entry for
470 the same source line. In this case there is a line number entry at the
471 start of each code range, each with the same line number.
476 All of the following stabs use the @samp{N_FUN} symbol type.
478 A function is represented by a @samp{F} symbol descriptor for a global
479 (extern) function, and @samp{f} for a static (local) function. The next
480 @samp{N_SLINE} symbol can be used to find the line number of the start
481 of the function. The value field is the address of the start of the
482 function. The type information of the stab represents the return type
483 of the function; thus @samp{foo:f5} means that foo is a function
486 The type information of the stab is optionally followed by type
487 information for each argument, with each argument preceded by @samp{;}.
488 An argument type of 0 means that additional arguments are being passed,
489 whose types and number may vary (@samp{...} in ANSI C). This extension
490 is used by Sun's Solaris compiler. GDB has tolerated it (i.e. at least
491 parsed the syntax, if not necessarily used the information) at least
492 since version 4.8; I don't know whether all versions of dbx will
493 tolerate it. The argument types given here are not merely redundant
494 with the symbols for the arguments themselves (@pxref{Parameters}), they
495 are the types of the arguments as they are passed, before any
496 conversions might take place. For example, if a C function which is
497 declared without a prototype takes a @code{float} argument, the value is
498 passed as a @code{double} but then converted to a @code{float}.
499 Debuggers need to use the types given in the arguments when printing
500 values, but if calling the function they need to use the types given in
501 the symbol defining the function.
503 If the return type and types of arguments of a function which is defined
504 in another source file are specified (i.e. a function prototype in ANSI
505 C), traditionally compilers emit no stab; the only way for the debugger
506 to find the information is if the source file where the function is
507 defined was also compiled with debugging symbols. As an extension the
508 Solaris compiler uses symbol descriptor @samp{P} followed by the return
509 type of the function, followed by the arguments, each preceded by
510 @samp{;}, as in a stab with symbol descriptor @samp{f} or @samp{F}.
511 This use of symbol descriptor @samp{P} can be distinguished from its use
512 for register parameters (@pxref{Parameters}) by the fact that it has
513 symbol type @code{N_FUN}.
515 The AIX documentation also defines symbol descriptor @samp{J} as an
516 internal function. I assume this means a function nested within another
517 function. It also says Symbol descriptor @samp{m} is a module in
518 Modula-2 or extended Pascal.
520 Procedures (functions which do not return values) are represented as
521 functions returning the void type in C. I don't see why this couldn't
522 be used for all languages (inventing a void type for this purpose if
523 necessary), but the AIX documentation defines @samp{I}, @samp{P}, and
524 @samp{Q} for internal, global, and static procedures, respectively.
525 These symbol descriptors are unusual in that they are not followed by
528 For any of the above symbol descriptors, after the symbol descriptor and
529 the type information, there is optionally a comma, followed by the name
530 of the procedure, followed by a comma, followed by a name specifying the
531 scope. The first name is local to the scope specified. I assume then
532 that the name of the symbol (before the @samp{:}), if specified, is some
533 sort of global name. I assume the name specifying the scope is the name
534 of a function specifying that scope. This feature is an AIX extension,
535 and this information is based on the manual; I haven't actually tried
538 The stab representing a procedure is located immediately following the
539 code of the procedure. This stab is in turn directly followed by a
540 group of other stabs describing elements of the procedure. These other
541 stabs describe the procedure's parameters, its block local variables and
549 The @code{.stabs} entry after this code fragment shows the @var{name} of
550 the procedure (@code{main}); the type descriptor @var{desc} (@code{F},
551 for a global procedure); a reference to the predefined type @code{int}
552 for the return type; and the starting @var{address} of the procedure.
554 Here is an exploded summary (with whitespace introduced for clarity),
555 followed by line 50 of our sample assembly output, which has this form:
559 @var{desc} @r{(global proc @samp{F})}
560 @var{return_type_ref} @r{(int)}
566 50 .stabs "main:F1",36,0,0,_main
569 @node Block Structure
570 @section Block Structure
572 The program's block structure is represented by the @code{N_LBRAC} (left
573 brace) and the @code{N_RBRAC} (right brace) stab types. The variables
574 defined inside a block preceded the @code{N_LBRAC} symbol for most
575 compilers, including GCC. Other compilers, such as the Convex, Acorn
576 RISC machine, and Sun acc compilers, put the variables after the
577 @code{N_LBRAC} symbol. The values of the @code{N_LBRAC} and
578 @code{N_RBRAC} symbols are the start and end addresses of the code of
579 the block, respectively. For most machines, they are relative to the
580 starting address of this source file. For the Gould NP1, they are
581 absolute. For Sun's stabs-in-elf, they are relative to the function in
584 The @code{N_LBRAC} and @code{N_RBRAC} stabs that describe the block
585 scope of a procedure are located after the @code{N_FUN} stab that
586 represents the procedure itself.
588 Sun documents the @code{desc} field of @code{N_LBRAC} and
589 @code{N_RBRAC} symbols as containing the nesting level of the block.
590 However, dbx seems not to care, and GCC just always set @code{desc} to
596 The @samp{c} symbol descriptor indicates that this stab represents a
597 constant. This symbol descriptor is an exception to the general rule
598 that symbol descriptors are followed by type information. Instead, it
599 is followed by @samp{=} and one of the following:
603 Boolean constant. @var{value} is a numeric value; I assume it is 0 for
607 Character constant. @var{value} is the numeric value of the constant.
609 @item e @var{type-information} , @var{value}
610 Constant whose value can be represented as integral.
611 @var{type-information} is the type of the constant, as it would appear
612 after a symbol descriptor (@pxref{Stabs Format}). @var{value} is the
613 numeric value of the constant. GDB 4.9 does not actually get the right
614 value if @var{value} does not fit in a host @code{int}, but it does not
615 do anything violent, and future debuggers could be extended to accept
616 integers of any size (whether unsigned or not). This constant type is
617 usually documented as being only for enumeration constants, but GDB has
618 never imposed that restriction; I don't know about other debuggers.
621 Integer constant. @var{value} is the numeric value. The type is some
622 sort of generic integer type (for GDB, a host @code{int}); to specify
623 the type explicitly, use @samp{e} instead.
626 Real constant. @var{value} is the real value, which can be @samp{INF}
627 (optionally preceded by a sign) for infinity, @samp{QNAN} for a quiet
628 NaN (not-a-number), or @samp{SNAN} for a signalling NaN. If it is a
629 normal number the format is that accepted by the C library function
633 String constant. @var{string} is a string enclosed in either @samp{'}
634 (in which case @samp{'} characters within the string are represented as
635 @samp{\'} or @samp{"} (in which case @samp{"} characters within the
636 string are represented as @samp{\"}).
638 @item S @var{type-information} , @var{elements} , @var{bits} , @var{pattern}
639 Set constant. @var{type-information} is the type of the constant, as it
640 would appear after a symbol descriptor (@pxref{Stabs Format}).
641 @var{elements} is the number of elements in the set (Does this means
642 how many bits of @var{pattern} are actually used, which would be
643 redundant with the type, or perhaps the number of bits set in
644 @var{pattern}? I don't get it), @var{bits} is the number of bits in the
645 constant (meaning it specifies the length of @var{pattern}, I think),
646 and @var{pattern} is a hexadecimal representation of the set. AIX
647 documentation refers to a limit of 32 bytes, but I see no reason why
648 this limit should exist. This form could probably be used for arbitrary
649 constants, not just sets; the only catch is that @var{pattern} should be
650 understood to be target, not host, byte order and format.
653 The boolean, character, string, and set constants are not supported by
654 GDB 4.9, but it will ignore them. GDB 4.8 and earlier gave an error
655 message and refused to read symbols from the file containing the
658 This information is followed by @samp{;}.
661 @chapter A Comprehensive Example in C
663 Now we'll examine a second program, @code{example2}, which builds on the
664 first example to introduce the rest of the stab types, symbol
665 descriptors, and type descriptors used in C.
666 @xref{Example2.c} for the complete @file{.c} source,
667 and @pxref{Example2.s} for the @file{.s} assembly code.
668 This description includes parts of those files.
670 @section Flow of control and nested scopes
676 @code{N_SLINE}, @code{N_LBRAC}, @code{N_RBRAC} (cont.)
679 Consider the body of @code{main}, from @file{example2.c}. It shows more
680 about how @code{N_SLINE}, @code{N_RBRAC}, and @code{N_LBRAC} stabs are used.
684 21 static float s_flap;
686 23 for (times=0; times < s_g_repeat; times++)@{
688 25 printf ("Hello world\n");
693 Here we have a single source line, the @samp{for} line, that generates
694 non-linear flow of control, and non-contiguous code. In this case, an
695 @code{N_SLINE} stab with the same line number proceeds each block of
696 non-contiguous code generated from the same source line.
698 The example also shows nested scopes. The @code{N_LBRAC} and
699 @code{N_LBRAC} stabs that describe block structure are nested in the
700 same order as the corresponding code blocks, those of the for loop
701 inside those for the body of main.
704 This is the label for the @code{N_LBRAC} (left brace) stab marking the
705 start of @code{main}.
712 In the first code range for C source line 23, the @code{for} loop
713 initialize and test, @code{N_SLINE} (68) records the line number:
720 58 .stabn 68,0,23,LM2
724 62 sethi %hi(_s_g_repeat),%o0
726 64 ld [%o0+%lo(_s_g_repeat)],%o0
731 @exdent label for the @code{N_LBRAC} (start block) marking the start of @code{for} loop
734 69 .stabn 68,0,25,LM3
736 71 sethi %hi(LC0),%o1
737 72 or %o1,%lo(LC0),%o0
740 75 .stabn 68,0,26,LM4
743 @exdent label for the @code{N_RBRAC} (end block) stab marking the end of the @code{for} loop
749 Now we come to the second code range for source line 23, the @code{for}
750 loop increment and return. Once again, @code{N_SLINE} (68) records the
754 .stabn, N_SLINE, NIL,
758 78 .stabn 68,0,23,LM5
766 86 .stabn 68,0,27,LM6
769 @exdent label for the @code{N_RBRAC} (end block) stab marking the end of the @code{for} loop
772 89 .stabn 68,0,27,LM7
777 94 .stabs "main:F1",36,0,0,_main
778 95 .stabs "argc:p1",160,0,0,68
779 96 .stabs "argv:p20=*21=*2",160,0,0,72
780 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
781 98 .stabs "times:1",128,0,0,-20
785 Here is an illustration of stabs describing nested scopes. The scope
786 nesting is reflected in the nested bracketing stabs (@code{N_LBRAC},
790 .stabn N_LBRAC,NIL,NIL,
791 @var{block-start-address}
793 99 .stabn 192,0,0,LBB2 ## begin proc label
794 100 .stabs "inner:1",128,0,0,-24
795 101 .stabn 192,0,0,LBB3 ## begin for label
799 @code{N_RBRAC} (224), ``right brace'' ends a lexical block (scope).
802 .stabn N_RBRAC,NIL,NIL,
803 @var{block-end-address}
805 102 .stabn 224,0,0,LBE3 ## end for label
806 103 .stabn 224,0,0,LBE2 ## end proc label
813 * Automatic variables:: Variables allocated on the stack.
814 * Global Variables:: Variables used by more than one source file.
815 * Register variables:: Variables in registers.
816 * Common Blocks:: Variables statically allocated together.
817 * Statics:: Variables local to one source file.
818 * Parameters:: Variables for arguments to functions.
821 @node Automatic variables
822 @section Locally scoped automatic variables
829 @item Symbol Descriptor:
833 In addition to describing types, the @code{N_LSYM} stab type also
834 describes locally scoped automatic variables. Refer again to the body
835 of @code{main} in @file{example2.c}. It allocates two automatic
836 variables: @samp{times} is scoped to the body of @code{main}, and
837 @samp{inner} is scoped to the body of the @code{for} loop.
838 @samp{s_flap} is locally scoped but not automatic, and will be discussed
843 21 static float s_flap;
845 23 for (times=0; times < s_g_repeat; times++)@{
847 25 printf ("Hello world\n");
852 The @code{N_LSYM} stab for an automatic variable is located just before the
853 @code{N_LBRAC} stab describing the open brace of the block to which it is
857 @exdent @code{N_LSYM} (128): automatic variable, scoped locally to @code{main}
860 @var{type information}",
862 @var{frame-pointer-offset}
864 98 .stabs "times:1",128,0,0,-20
865 99 .stabn 192,0,0,LBB2 ## begin `main' N_LBRAC
867 @exdent @code{N_LSYM} (128): automatic variable, scoped locally to the @code{for} loop
870 @var{type information}",
872 @var{frame-pointer-offset}
874 100 .stabs "inner:1",128,0,0,-24
875 101 .stabn 192,0,0,LBB3 ## begin `for' loop N_LBRAC
878 The symbol descriptor is omitted for automatic variables. Since type
879 information should being with a digit, @samp{-}, or @samp{(}, only
880 digits, @samp{-}, and @samp{(} are precluded from being used for symbol
881 descriptors by this fact. However, the Acorn RISC machine (ARM) is said
882 to get this wrong: it puts out a mere type definition here, without the
883 preceding @code{@var{typenumber}=}. This is a bad idea; there is no
884 guarantee that type descriptors are distinct from symbol descriptors.
886 @node Global Variables
887 @section Global Variables
894 @item Symbol Descriptor:
898 Global variables are represented by the @code{N_GSYM} stab type. The symbol
899 descriptor, following the colon in the string field, is @samp{G}. Following
900 the @samp{G} is a type reference or type definition. In this example it is a
901 type reference to the basic C type, @code{char}. The first source line in
909 yields the following stab. The stab immediately precedes the code that
910 allocates storage for the variable it describes.
913 @exdent @code{N_GSYM} (32): global symbol
918 N_GSYM, NIL, NIL, NIL
920 21 .stabs "g_foo:G2",32,0,0,0
927 The address of the variable represented by the @code{N_GSYM} is not contained
928 in the @code{N_GSYM} stab. The debugger gets this information from the
929 external symbol for the global variable.
931 @node Register variables
932 @section Register variables
934 @c According to an old version of this manual, AIX uses C_RPSYM instead
935 @c of C_RSYM. I am skeptical; this should be verified.
936 Register variables have their own stab type, @code{N_RSYM}, and their
937 own symbol descriptor, @code{r}. The stab's value field contains the
938 number of the register where the variable data will be stored.
940 The value is the register number.
942 AIX defines a separate symbol descriptor @samp{d} for floating point
943 registers. This seems unnecessary---why not just just give floating
944 point registers different register numbers? I have not verified whether
945 the compiler actually uses @samp{d}.
947 If the register is explicitly allocated to a global variable, but not
951 register int g_bar asm ("%g5");
954 the stab may be emitted at the end of the object file, with
955 the other bss symbols.
958 @section Common Blocks
960 A common block is a statically allocated section of memory which can be
961 referred to by several source files. It may contain several variables.
962 I believe @sc{fortran} is the only language with this feature. A
963 @code{N_BCOMM} stab begins a common block and an @code{N_ECOMM} stab
964 ends it. The only thing which is significant about these two stabs is
965 their name, which can be used to look up a normal (non-debugging) symbol
966 which gives the address of the common block. Then each stab between the
967 @code{N_BCOMM} and the @code{N_ECOMM} specifies a member of that common
968 block; its value is the offset within the common block of that variable.
969 The @code{N_ECOML} stab type is documented for this purpose, but Sun's
970 @sc{fortran} compiler uses @code{N_GSYM} instead. The test case I
971 looked at had a common block local to a function and it used the
972 @samp{V} symbol descriptor; I assume one would use @samp{S} if not local
973 to a function (that is, if a common block @emph{can} be anything other
974 than local to a function).
977 @section Static Variables
979 Initialized static variables are represented by the @samp{S} and
980 @samp{V} symbol descriptors. @samp{S} means file scope static, and
981 @samp{V} means procedure scope static.
983 In a.out files, @code{N_STSYM} means the data segment (although gcc
984 2.4.5 has a bug in that it uses @code{N_FUN}, so neither dbx nor gdb can
985 find the variables), @code{N_FUN} means the text segment, and
986 @code{N_LCSYM} means the bss segment.
988 In xcoff files, each symbol has a section number, so the stab type
989 need not indicate the segment.
991 In ecoff files, the storage class is used to specify the section, so the
992 stab type need not indicate the segment.
994 @c In ELF files, it apparently is a big mess. See kludge in dbxread.c
995 @c in GDB. FIXME: Investigate where this kludge comes from.
997 @c This is the place to mention N_ROSYM; I'd rather do so once I can
998 @c coherently explain how this stuff works for stabs-in-elf.
1000 For example, the source lines
1003 static const int var_const = 5;
1004 static int var_init = 2;
1005 static int var_noinit;
1009 yield the following stabs:
1012 .stabs "var_const:S1",36,0,0,_var_const ; @r{36 = N_FUN}
1014 .stabs "var_init:S1",38,0,0,_var_init ; @r{38 = N_STSYM}
1016 .stabs "var_noinit:S1",40,0,0,_var_noinit ; @r{40 = N_LCSYM}
1022 Parameters to a function are represented by a stab (or sometimes two,
1023 see below) for each parameter. The stabs are in the order in which the
1024 debugger should print the parameters (i.e. the order in which the
1025 parameters are declared in the source file).
1027 The symbol descriptor @samp{p} is used to refer to parameters which are
1028 in the arglist. Symbols have symbol type @samp{N_PSYM}. The value of
1029 the symbol is the offset relative to the argument list.
1031 If the parameter is passed in a register, then the traditional way to do
1032 this is to provide two symbols for each argument:
1035 .stabs "arg:p1" . . . ; N_PSYM
1036 .stabs "arg:r1" . . . ; N_RSYM
1039 Debuggers are expected to use the second one to find the value, and the
1040 first one to know that it is an argument.
1042 Because this is kind of ugly, some compilers use symbol descriptor
1043 @samp{P} or @samp{R} to indicate an argument which is in a register.
1044 The symbol value is the register number. @samp{P} and @samp{R} mean the
1045 same thing, the difference is that @samp{P} is a GNU invention and
1046 @samp{R} is an IBM (xcoff) invention. As of version 4.9, GDB should
1047 handle either one. Symbol type @samp{C_RPSYM} is used with @samp{R} and
1048 @samp{N_RSYM} is used with @samp{P}.
1050 According to the AIX documentation symbol descriptor @samp{D} is for a
1051 parameter passed in a floating point register. This seems
1052 unnecessary---why not just use @samp{R} with a register number which
1053 indicates that it's a floating point register? I haven't verified
1054 whether the system actually does what the documentation indicates.
1056 There is at least one case where GCC uses a @samp{p}/@samp{r} pair
1057 rather than @samp{P}; this is where the argument is passed in the
1058 argument list and then loaded into a register.
1060 On the sparc and hppa, for a @samp{P} symbol whose type is a structure
1061 or union, the register contains the address of the structure. On the
1062 sparc, this is also true of a @samp{p}/@samp{r} pair (using Sun cc) or a
1063 @samp{p} symbol. However, if a (small) structure is really in a
1064 register, @samp{r} is used. And, to top it all off, on the hppa it
1065 might be a structure which was passed on the stack and loaded into a
1066 register and for which there is a @samp{p}/@samp{r} pair! I believe
1067 that symbol descriptor @samp{i} is supposed to deal with this case, (it
1068 is said to mean "value parameter by reference, indirect access", I don't
1069 know the source for this information) but I don't know details or what
1070 compilers or debuggers use it, if any (not GDB or GCC). It is not clear
1071 to me whether this case needs to be dealt with differently than
1072 parameters passed by reference (see below).
1074 There is another case similar to an argument in a register, which is an
1075 argument which is actually stored as a local variable. Sometimes this
1076 happens when the argument was passed in a register and then the compiler
1077 stores it as a local variable. If possible, the compiler should claim
1078 that it's in a register, but this isn't always done. Some compilers use
1079 the pair of symbols approach described above ("arg:p" followed by
1080 "arg:"); this includes gcc1 (not gcc2) on the sparc when passing a small
1081 structure and gcc2 (sometimes) when the argument type is float and it is
1082 passed as a double and converted to float by the prologue (in the latter
1083 case the type of the "arg:p" symbol is double and the type of the "arg:"
1084 symbol is float). GCC, at least on the 960, uses a single @samp{p}
1085 symbol descriptor for an argument which is stored as a local variable
1086 but uses @samp{N_LSYM} instead of @samp{N_PSYM}. In this case the value
1087 of the symbol is an offset relative to the local variables for that
1088 function, not relative to the arguments (on some machines those are the
1089 same thing, but not on all).
1091 If the parameter is passed by reference (e.g. Pascal VAR parameters),
1092 then type symbol descriptor is @samp{v} if it is in the argument list,
1093 or @samp{a} if it in a register. Other than the fact that these contain
1094 the address of the parameter other than the parameter itself, they are
1095 identical to @samp{p} and @samp{R}, respectively. I believe @samp{a} is
1096 an AIX invention; @samp{v} is supported by all stabs-using systems as
1099 @c Is this paragraph correct? It is based on piecing together patchy
1100 @c information and some guesswork
1101 Conformant arrays refer to a feature of Modula-2, and perhaps other
1102 languages, in which the size of an array parameter is not known to the
1103 called function until run-time. Such parameters have two stabs, a
1104 @samp{x} for the array itself, and a @samp{C}, which represents the size
1105 of the array. The value of the @samp{x} stab is the offset in the
1106 argument list where the address of the array is stored (it this right?
1107 it is a guess); the value of the @samp{C} stab is the offset in the
1108 argument list where the size of the array (in elements? in bytes?) is
1111 The following are also said to go with @samp{N_PSYM}:
1114 "name" -> "param_name:#type"
1116 -> pF FORTRAN function parameter
1117 -> X (function result variable)
1118 -> b (based variable)
1120 value -> offset from the argument pointer (positive).
1123 As a simple example, the code
1135 .stabs "main:F1",36,0,0,_main ; 36 is N_FUN
1136 .stabs "argc:p1",160,0,0,68 ; 160 is N_PSYM
1137 .stabs "argv:p20=*21=*2",160,0,0,72
1140 The type definition of argv is interesting because it contains several
1141 type definitions. Type 21 is pointer to type 2 (char) and argv (type 20) is
1145 @chapter Type Definitions
1147 Now let's look at some variable definitions involving complex types.
1148 This involves understanding better how types are described. In the
1149 examples so far types have been described as references to previously
1150 defined types or defined in terms of subranges of or pointers to
1151 previously defined types. The section that follows will talk about
1152 the various other type descriptors that may follow the = sign in a
1156 * Builtin types:: Integers, floating point, void, etc.
1157 * Miscellaneous Types:: Pointers, sets, files, etc.
1158 * Cross-references:: Referring to a type not yet defined.
1159 * Subranges:: A type with a specific range.
1160 * Arrays:: An aggregate type of same-typed elements.
1161 * Strings:: Like an array but also has a length.
1162 * Enumerations:: Like an integer but the values have names.
1163 * Structures:: An aggregate type of different-typed elements.
1164 * Typedefs:: Giving a type a name.
1165 * Unions:: Different types sharing storage.
1170 @section Builtin types
1172 Certain types are built in (@code{int}, @code{short}, @code{void},
1173 @code{float}, etc.); the debugger recognizes these types and knows how
1174 to handle them. Thus don't be surprised if some of the following ways
1175 of specifying builtin types do not specify everything that a debugger
1176 would need to know about the type---in some cases they merely specify
1177 enough information to distinguish the type from other types.
1179 The traditional way to define builtin types is convolunted, so new ways
1180 have been invented to describe them. Sun's ACC uses the @samp{b} and
1181 @samp{R} type descriptors, and IBM uses negative type numbers. GDB can
1182 accept all three, as of version 4.8; dbx just accepts the traditional
1183 builtin types and perhaps one of the other two formats.
1186 * Traditional Builtin Types:: Put on your seatbelts and prepare for kludgery
1187 * Builtin Type Descriptors:: Builtin types with special type descriptors
1188 * Negative Type Numbers:: Builtin types using negative type numbers
1191 @node Traditional Builtin Types
1192 @subsection Traditional Builtin types
1194 Often types are defined as subranges of themselves. If the array bounds
1195 can fit within an @code{int}, then they are given normally. For example:
1198 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0 ; 128 is N_LSYM
1199 .stabs "char:t2=r2;0;127;",128,0,0,0
1202 Builtin types can also be described as subranges of @code{int}:
1205 .stabs "unsigned short:t6=r1;0;65535;",128,0,0,0
1208 If the lower bound of a subrange is 0 and the upper bound is -1, it
1209 means that the type is an unsigned integral type whose bounds are too
1210 big to describe in an int. Traditionally this is only used for
1211 @code{unsigned int} and @code{unsigned long}; GCC also sometimes uses it
1212 for @code{long long} and @code{unsigned long long}, and the only way to
1213 tell those types apart is to look at their names. On other machines GCC
1214 puts out bounds in octal, with a leading 0. In this case a negative
1215 bound consists of a number which is a 1 bit followed by a bunch of 0
1216 bits, and a positive bound is one in which a bunch of bits are 1.
1219 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
1220 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
1223 If the lower bound of a subrange is 0 and the upper bound is negative,
1224 it means that it is an unsigned integral type whose size in bytes is the
1225 absolute value of the upper bound. I believe this is a Convex
1226 convention for @code{unsigned long long}.
1228 If the lower bound of a subrange is negative and the upper bound is 0,
1229 it means that the type is a signed integral type whose size in bytes is
1230 the absolute value of the lower bound. I believe this is a Convex
1231 convention for @code{long long}. To distinguish this from a legitimate
1232 subrange, the type should be a subrange of itself. I'm not sure whether
1233 this is the case for Convex.
1235 If the upper bound of a subrange is 0, it means that this is a floating
1236 point type, and the lower bound of the subrange indicates the number of
1240 .stabs "float:t12=r1;4;0;",128,0,0,0
1241 .stabs "double:t13=r1;8;0;",128,0,0,0
1244 However, GCC writes @code{long double} the same way it writes
1245 @code{double}; the only way to distinguish them is by the name:
1248 .stabs "long double:t14=r1;8;0;",128,0,0,0
1251 Complex types are defined the same way as floating-point types; the only
1252 way to distinguish a single-precision complex from a double-precision
1253 floating-point type is by the name.
1255 The C @code{void} type is defined as itself:
1258 .stabs "void:t15=15",128,0,0,0
1261 I'm not sure how a boolean type is represented.
1263 @node Builtin Type Descriptors
1264 @subsection Defining Builtin Types using Builtin Type Descriptors
1266 There are various type descriptors to define builtin types:
1269 @c FIXME: clean up description of width and offset, once we figure out
1271 @item b @var{signed} @var{char-flag} @var{width} ; @var{offset} ; @var{nbits} ;
1272 Define an integral type. @var{signed} is @samp{u} for unsigned or
1273 @samp{s} for signed. @var{char-flag} is @samp{c} which indicates this
1274 is a character type, or is omitted. I assume this is to distinguish an
1275 integral type from a character type of the same size, for example it
1276 might make sense to set it for the C type @code{wchar_t} so the debugger
1277 can print such variables differently (Solaris does not do this). Sun
1278 sets it on the C types @code{signed char} and @code{unsigned char} which
1279 arguably is wrong. @var{width} and @var{offset} appear to be for small
1280 objects stored in larger ones, for example a @code{short} in an
1281 @code{int} register. @var{width} is normally the number of bytes in the
1282 type. @var{offset} seems to always be zero. @var{nbits} is the number
1283 of bits in the type.
1285 Note that type descriptor @samp{b} used for builtin types conflicts with
1286 its use for Pascal space types (@pxref{Miscellaneous Types}); they can
1287 be distinguished because the character following the type descriptor
1288 will be a digit, @samp{(}, or @samp{-} for a Pascal space type, or
1289 @samp{u} or @samp{s} for a builtin type.
1292 Documented by AIX to define a wide character type, but their compiler
1293 actually uses negative type numbers (@pxref{Negative Type Numbers}).
1295 @item R @var{fp_type} ; @var{bytes} ;
1296 Define a floating point type. @var{fp_type} has one of the following values:
1300 IEEE 32-bit (single precision) floating point format.
1303 IEEE 64-bit (double precision) floating point format.
1305 @item 3 (NF_COMPLEX)
1306 @item 4 (NF_COMPLEX16)
1307 @item 5 (NF_COMPLEX32)
1308 @c "GDB source" really means @file{include/aout/stab_gnu.h}, but trying
1309 @c to put that here got an overfull hbox.
1310 These are for complex numbers. A comment in the GDB source describes
1311 them as Fortran complex, double complex, and complex*16, respectively,
1312 but what does that mean? (i.e. Single precision? Double precison?).
1314 @item 6 (NF_LDOUBLE)
1315 Long double. This should probably only be used for Sun format long
1316 double, and new codes should be used for other floating point formats
1317 (NF_DOUBLE can be used if a long double is really just an IEEE double,
1321 @var{bytes} is the number of bytes occupied by the type. This allows a
1322 debugger to perform some operations with the type even if it doesn't
1323 understand @var{fp_code}.
1325 @item g @var{type-information} ; @var{nbits}
1326 Documented by AIX to define a floating type, but their compiler actually
1327 uses negative type numbers (@pxref{Negative Type Numbers}).
1329 @item c @var{type-information} ; @var{nbits}
1330 Documented by AIX to define a complex type, but their compiler actually
1331 uses negative type numbers (@pxref{Negative Type Numbers}).
1334 The C @code{void} type is defined as a signed integral type 0 bits long:
1336 .stabs "void:t19=bs0;0;0",128,0,0,0
1338 The Solaris compiler seems to omit the trailing semicolon in this case.
1339 Getting sloppy in this way is not a swift move because if a type is
1340 embedded in a more complex expression it is necessary to be able to tell
1343 I'm not sure how a boolean type is represented.
1345 @node Negative Type Numbers
1346 @subsection Negative Type numbers
1348 Since the debugger knows about the builtin types anyway, the idea of
1349 negative type numbers is simply to give a special type number which
1350 indicates the built in type. There is no stab defining these types.
1352 I'm not sure whether anyone has tried to define what this means if
1353 @code{int} can be other than 32 bits (or other types can be other than
1354 their customary size). If @code{int} has exactly one size for each
1355 architecture, then it can be handled easily enough, but if the size of
1356 @code{int} can vary according the compiler options, then it gets hairy.
1357 The best way to do this would be to define separate negative type
1358 numbers for 16-bit @code{int} and 32-bit @code{int}; therefore I have
1359 indicated below the customary size (and other format information) for
1360 each type. The information below is currently correct because AIX on
1361 the RS6000 is the only system which uses these type numbers. If these
1362 type numbers start to get used on other systems, I suspect the correct
1363 thing to do is to define a new number in cases where a type does not
1364 have the size and format indicated below (or avoid negative type numbers
1367 Also note that part of the definition of the negative type number is
1368 the name of the type. Types with identical size and format but
1369 different names have different negative type numbers.
1373 @code{int}, 32 bit signed integral type.
1376 @code{char}, 8 bit type holding a character. Both GDB and dbx on AIX
1377 treat this as signed. GCC uses this type whether @code{char} is signed
1378 or not, which seems like a bad idea. The AIX compiler (xlc) seems to
1379 avoid this type; it uses -5 instead for @code{char}.
1382 @code{short}, 16 bit signed integral type.
1385 @code{long}, 32 bit signed integral type.
1388 @code{unsigned char}, 8 bit unsigned integral type.
1391 @code{signed char}, 8 bit signed integral type.
1394 @code{unsigned short}, 16 bit unsigned integral type.
1397 @code{unsigned int}, 32 bit unsigned integral type.
1400 @code{unsigned}, 32 bit unsigned integral type.
1403 @code{unsigned long}, 32 bit unsigned integral type.
1406 @code{void}, type indicating the lack of a value.
1409 @code{float}, IEEE single precision.
1412 @code{double}, IEEE double precision.
1415 @code{long double}, IEEE double precision. The compiler claims the size
1416 will increase in a future release, and for binary compatibility you have
1417 to avoid using @code{long double}. I hope when they increase it they
1418 use a new negative type number.
1421 @code{integer}. 32 bit signed integral type.
1424 @code{boolean}. 32 bit type. How is the truth value encoded? Is it
1425 the least significant bit or is it a question of whether the whole value
1426 is zero or non-zero?
1429 @code{short real}. IEEE single precision.
1432 @code{real}. IEEE double precision.
1435 @code{stringptr}. @xref{Strings}.
1438 @code{character}, 8 bit unsigned character type.
1441 @code{logical*1}, 8 bit type. This @sc{fortran} type has a split
1442 personality in that it is used for boolean variables, but can also be
1443 used for unsigned integers. 0 is false, 1 is true, and other values are
1447 @code{logical*2}, 16 bit type. This @sc{fortran} type has a split
1448 personality in that it is used for boolean variables, but can also be
1449 used for unsigned integers. 0 is false, 1 is true, and other values are
1453 @code{logical*4}, 32 bit type. This @sc{fortran} type has a split
1454 personality in that it is used for boolean variables, but can also be
1455 used for unsigned integers. 0 is false, 1 is true, and other values are
1459 @code{logical}, 32 bit type. This @sc{fortran} type has a split
1460 personality in that it is used for boolean variables, but can also be
1461 used for unsigned integers. 0 is false, 1 is true, and other values are
1465 @code{complex}. A complex type consisting of two IEEE single-precision
1466 floating point values.
1469 @code{complex}. A complex type consisting of two IEEE double-precision
1470 floating point values.
1473 @code{integer*1}, 8 bit signed integral type.
1476 @code{integer*2}, 16 bit signed integral type.
1479 @code{integer*4}, 32 bit signed integral type.
1482 @code{wchar}. Wide character, 16 bits wide, unsigned (what format?
1486 @node Miscellaneous Types
1487 @section Miscellaneous Types
1490 @item b @var{type-information} ; @var{bytes}
1491 Pascal space type. This is documented by IBM; what does it mean?
1493 Note that this use of the @samp{b} type descriptor can be distinguished
1494 from its use for builtin integral types (@pxref{Builtin Type
1495 Descriptors}) because the character following the type descriptor is
1496 always a digit, @samp{(}, or @samp{-}.
1498 @item B @var{type-information}
1499 A volatile-qualified version of @var{type-information}. This is a Sun
1500 extension. A volatile-qualified type means that references and stores
1501 to a variable of that type must not be optimized or cached; they must
1502 occur as the user specifies them.
1504 @item d @var{type-information}
1505 File of type @var{type-information}. As far as I know this is only used
1508 @item k @var{type-information}
1509 A const-qualified version of @var{type-information}. This is a Sun
1510 extension. A const-qualified type means that a variable of this type
1513 @item M @var{type-information} ; @var{length}
1514 Multiple instance type. The type seems to composed of @var{length}
1515 repetitions of @var{type-information}, for example @code{character*3} is
1516 represented by @samp{M-2;3}, where @samp{-2} is a reference to a
1517 character type (@pxref{Negative Type Numbers}). I'm not sure how this
1518 differs from an array. This appears to be a FORTRAN feature.
1519 @var{length} is a bound, like those in range types, @xref{Subranges}.
1521 @item S @var{type-information}
1522 Pascal set type. @var{type-information} must be a small type such as an
1523 enumeration or a subrange, and the type is a bitmask whose length is
1524 specified by the number of elements in @var{type-information}.
1526 @item * @var{type-information}
1527 Pointer to @var{type-information}.
1530 @node Cross-references
1531 @section Cross-references to other types
1533 If a type is used before it is defined, one common way to deal with this
1534 is just to use a type reference to a type which has not yet been
1535 defined. The debugger is expected to be able to deal with this.
1537 Another way is with the @samp{x} type descriptor, which is followed by
1538 @samp{s} for a structure tag, @samp{u} for a union tag, or @samp{e} for
1539 a enumerator tag, followed by the name of the tag, followed by @samp{:}.
1540 for example the following C declarations:
1550 .stabs "bar:G16=*17=xsfoo:",32,0,0,0
1553 Not all debuggers support the @samp{x} type descriptor, so on some
1554 machines GCC does not use it. I believe that for the above example it
1555 would just emit a reference to type 17 and never define it, but I
1556 haven't verified that.
1558 Modula-2 imported types, at least on AIX, use the @samp{i} type
1559 descriptor, which is followed by the name of the module from which the
1560 type is imported, followed by @samp{:}, followed by the name of the
1561 type. There is then optionally a comma followed by type information for
1562 the type (This differs from merely naming the type (@pxref{Typedefs}) in
1563 that it identifies the module; I don't understand whether the name of
1564 the type given here is always just the same as the name we are giving
1565 it, or whether this type descriptor is used with a nameless stab
1566 (@pxref{Stabs Format}), or what). The symbol ends with @samp{;}.
1569 @section Subrange types
1571 The @samp{r} type descriptor defines a type as a subrange of another
1572 type. It is followed by type information for the type which it is a
1573 subrange of, a semicolon, an integral lower bound, a semicolon, an
1574 integral upper bound, and a semicolon. The AIX documentation does not
1575 specify the trailing semicolon, in an effort to specify array indexes
1576 more cleanly, but a subrange which is not an array index has always
1577 included a trailing semicolon (@pxref{Arrays}).
1579 Instead of an integer, either bound can be one of the following:
1582 @item A @var{offset}
1583 The bound is passed by reference on the stack at offset @var{offset}
1584 from the argument list. @xref{Parameters}, for more information on such
1587 @item T @var{offset}
1588 The bound is passed by value on the stack at offset @var{offset} from
1591 @item a @var{register-number}
1592 The bound is pased by reference in register number
1593 @var{register-number}.
1595 @item t @var{register-number}
1596 The bound is passed by value in register number @var{register-number}.
1602 Subranges are also used for builtin types, @xref{Traditional Builtin Types}.
1605 @section Array types
1607 Arrays use the @samp{a} type descriptor. Following the type descriptor
1608 is the type of the index and the type of the array elements. If the
1609 index type is a range type, it will end in a semicolon; if it is not a
1610 range type (for example, if it is a type reference), there does not
1611 appear to be any way to tell where the types are separated. In an
1612 effort to clean up this mess, IBM documents the two types as being
1613 separated by a semicolon, and a range type as not ending in a semicolon
1614 (but this is not right for range types which are not array indexes,
1615 @pxref{Subranges}). I think probably the best solution is to specify
1616 that a semicolon ends a range type, and that the index type and element
1617 type of an array are separated by a semicolon, but that if the index
1618 type is a range type, the extra semicolon can be omitted. GDB (at least
1619 through version 4.9) doesn't support any kind of index type other than a
1620 range anyway; I'm not sure about dbx.
1622 It is well established, and widely used, that the type of the index,
1623 unlike most types found in the stabs, is merely a type definition, not
1624 type information (@pxref{Stabs Format}) (that is, it need not start with
1625 @var{type-number}@code{=} if it is defining a new type). According to a
1626 comment in GDB, this is also true of the type of the array elements; it
1627 gives @samp{ar1;1;10;ar1;1;10;4} as a legitimate way to express a two
1628 dimensional array. According to AIX documentation, the element type
1629 must be type information. GDB accepts either.
1631 The type of the index is often a range type, expressed as the letter r
1632 and some parameters. It defines the size of the array. In the example
1633 below, the range @code{r1;0;2;} defines an index type which is a
1634 subrange of type 1 (integer), with a lower bound of 0 and an upper bound
1635 of 2. This defines the valid range of subscripts of a three-element C
1638 For example, the definition
1641 char char_vec[3] = @{'a','b','c'@};
1648 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
1657 If an array is @dfn{packed}, it means that the elements are spaced more
1658 closely than normal, saving memory at the expense of speed. For
1659 example, an array of 3-byte objects might, if unpacked, have each
1660 element aligned on a 4-byte boundary, but if packed, have no padding.
1661 One way to specify that something is packed is with type attributes
1662 (@pxref{Stabs Format}), in the case of arrays another is to use the
1663 @samp{P} type descriptor instead of @samp{a}. Other than specifying a
1664 packed array, @samp{P} is identical to @samp{a}.
1666 @c FIXME-what is it? A pointer?
1667 An open array is represented by the @samp{A} type descriptor followed by
1668 type information specifying the type of the array elements.
1670 @c FIXME: what is the format of this type? A pointer to a vector of pointers?
1671 An N-dimensional dynamic array is represented by
1674 D @var{dimensions} ; @var{type-information}
1677 @c Does dimensions really have this meaning? The AIX documentation
1679 @var{dimensions} is the number of dimensions; @var{type-information}
1680 specifies the type of the array elements.
1682 @c FIXME: what is the format of this type? A pointer to some offsets in
1684 A subarray of an N-dimensional array is represented by
1687 E @var{dimensions} ; @var{type-information}
1690 @c Does dimensions really have this meaning? The AIX documentation
1692 @var{dimensions} is the number of dimensions; @var{type-information}
1693 specifies the type of the array elements.
1698 Some languages, like C or the original Pascal, do not have string types,
1699 they just have related things like arrays of characters. But most
1700 Pascals and various other languages have string types, which are
1701 indicated as follows:
1704 @item n @var{type-information} ; @var{bytes}
1705 @var{bytes} is the maximum length. I'm not sure what
1706 @var{type-information} is; I suspect that it means that this is a string
1707 of @var{type-information} (thus allowing a string of integers, a string
1708 of wide characters, etc., as well as a string of characters). Not sure
1709 what the format of this type is. This is an AIX feature.
1711 @item z @var{type-information} ; @var{bytes}
1712 Just like @samp{n} except that this is a gstring, not an ordinary
1713 string. I don't know the difference.
1716 Pascal Stringptr. What is this? This is an AIX feature.
1720 @section Enumerations
1722 Enumerations are defined with the @samp{e} type descriptor.
1724 @c FIXME: Where does this information properly go? Perhaps it is
1725 @c redundant with something we already explain.
1726 The source line below declares an enumeration type. It is defined at
1727 file scope between the bodies of main and s_proc in example2.c.
1728 The type definition is located after the N_RBRAC that marks the end of
1729 the previous procedure's block scope, and before the N_FUN that marks
1730 the beginning of the next procedure's block scope. Therefore it does not
1731 describe a block local symbol, but a file local one.
1736 enum e_places @{first,second=3,last@};
1740 generates the following stab
1743 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
1746 The symbol descriptor (T) says that the stab describes a structure,
1747 enumeration, or type tag. The type descriptor e, following the 22= of
1748 the type definition narrows it down to an enumeration type. Following
1749 the e is a list of the elements of the enumeration. The format is
1750 name:value,. The list of elements ends with a ;.
1752 There is no standard way to specify the size of an enumeration type; it
1753 is determined by the architecture (normally all enumerations types are
1754 32 bits). There should be a way to specify an enumeration type of
1755 another size; type attributes would be one way to do this @xref{Stabs
1765 @code{N_LSYM} or @code{C_DECL}
1766 @item Symbol Descriptor:
1768 @item Type Descriptor:
1772 The following source code declares a structure tag and defines an
1773 instance of the structure in global scope. Then a typedef equates the
1774 structure tag with a new type. A seperate stab is generated for the
1775 structure tag, the structure typedef, and the structure instance. The
1776 stabs for the tag and the typedef are emited when the definitions are
1777 encountered. Since the structure elements are not initialized, the
1778 stab and code for the structure variable itself is located at the end
1779 of the program in .common.
1785 9 char s_char_vec[8];
1786 10 struct s_tag* s_next;
1789 13 typedef struct s_tag s_typedef;
1792 The structure tag is an N_LSYM stab type because, like the enum, the
1793 symbol is file scope. Like the enum, the symbol descriptor is T, for
1794 enumeration, struct or tag type. The symbol descriptor s following
1795 the 16= of the type definition narrows the symbol type to struct.
1797 Following the struct symbol descriptor is the number of bytes the
1798 struct occupies, followed by a description of each structure element.
1799 The structure element descriptions are of the form name:type, bit
1800 offset from the start of the struct, and number of bits in the
1805 <128> N_LSYM - type definition
1806 .stabs "name:sym_desc(struct tag) Type_def(16)=type_desc(struct type)
1808 elem_name:type_ref(int),bit_offset,field_bits;
1809 elem_name:type_ref(float),bit_offset,field_bits;
1810 elem_name:type_def(17)=type_desc(array)
1811 index_type(range of int from 0 to 7);
1812 element_type(char),bit_offset,field_bits;;",
1815 30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;
1816 s_char_vec:17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
1819 In this example, two of the structure elements are previously defined
1820 types. For these, the type following the name: part of the element
1821 description is a simple type reference. The other two structure
1822 elements are new types. In this case there is a type definition
1823 embedded after the name:. The type definition for the array element
1824 looks just like a type definition for a standalone array. The s_next
1825 field is a pointer to the same kind of structure that the field is an
1826 element of. So the definition of structure type 16 contains an type
1827 definition for an element which is a pointer to type 16.
1830 @section Giving a type a name
1832 To give a type a name, use the @samp{t} symbol descriptor. For example,
1835 .stabs "s_typedef:t16",128,0,0,0
1838 specifies that @code{s_typedef} refers to type number 16. Such stabs
1839 have symbol type @code{N_LSYM} or @code{C_DECL}.
1841 If instead, you are specifying the tag name for a structure, union, or
1842 enumeration, use the @samp{T} symbol descriptor instead. I believe C is
1843 the only language with this feature.
1845 If the type is an opaque type (I believe this is a Modula-2 feature),
1846 AIX provides a type descriptor to specify it. The type descriptor is
1847 @samp{o} and is followed by a name. I don't know what the name
1848 means---is it always the same as the name of the type, or is this type
1849 descriptor used with a nameless stab (@pxref{Stabs Format})? There
1850 optionally follows a comma followed by type information which defines
1851 the type of this type. If omitted, a semicolon is used in place of the
1852 comma and the type information, and, the type is much like a generic
1853 pointer type---it has a known size but little else about it is
1859 Next let's look at unions. In example2 this union type is declared
1860 locally to a procedure and an instance of the union is defined.
1870 This code generates a stab for the union tag and a stab for the union
1871 variable. Both use the N_LSYM stab type. Since the union variable is
1872 scoped locally to the procedure in which it is defined, its stab is
1873 located immediately preceding the N_LBRAC for the procedure's block
1876 The stab for the union tag, however is located preceding the code for
1877 the procedure in which it is defined. The stab type is N_LSYM. This
1878 would seem to imply that the union type is file scope, like the struct
1879 type s_tag. This is not true. The contents and position of the stab
1880 for u_type do not convey any infomation about its procedure local
1885 .stabs "name:sym_desc(union tag)type_def(22)=type_desc(union)
1887 elem_name:type_ref(int),bit_offset(0),bit_size(32);
1888 elem_name:type_ref(float),bit_offset(0),bit_size(32);
1889 elem_name:type_ref(ptr to char),bit_offset(0),bit_size(32);;"
1890 N_LSYM, NIL, NIL, NIL
1894 105 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
1898 The symbol descriptor, T, following the name: means that the stab
1899 describes an enumeration, struct or type tag. The type descriptor u,
1900 following the 23= of the type definition, narrows it down to a union
1901 type definition. Following the u is the number of bytes in the union.
1902 After that is a list of union element descriptions. Their format is
1903 name:type, bit offset into the union, and number of bytes for the
1906 The stab for the union variable follows. Notice that the frame
1907 pointer offset for local variables is negative.
1910 <128> N_LSYM - local variable (with no symbol descriptor)
1911 .stabs "name:type_ref(u_tag)", N_LSYM, NIL, NIL, frame_ptr_offset
1915 130 .stabs "an_u:23",128,0,0,-20
1918 @node Function Types
1919 @section Function types
1921 There are various types for function variables. These types are not
1922 used in defining functions; see symbol descriptor @samp{f}; they are
1923 used for things like pointers to functions.
1925 The simple, traditional, type is type descriptor @samp{f} is followed by
1926 type information for the return type of the function, followed by a
1929 This does not deal with functions the number and type of whose
1930 parameters are part of their type, as found in Modula-2 or ANSI C. AIX
1931 provides extensions to specify these, using the @samp{f}, @samp{F},
1932 @samp{p}, and @samp{R} type descriptors.
1934 First comes the type descriptor. Then, if it is @samp{f} or @samp{F},
1935 this is a function, and the type information for the return type of the
1936 function follows, followed by a comma. Then comes the number of
1937 parameters to the function and a semicolon. Then, for each parameter,
1938 there is the name of the parameter followed by a colon (this is only
1939 present for type descriptors @samp{R} and @samp{F} which represent
1940 Pascal function or procedure parameters), type information for the
1941 parameter, a comma, @samp{0} if passed by reference or @samp{1} if
1942 passed by value, and a semicolon. The type definition ends with a
1952 generates the following code:
1955 .stabs "g_pf:G24=*25=f1",32,0,0,0
1956 .common _g_pf,4,"bss"
1959 The variable defines a new type, 24, which is a pointer to another new
1960 type, 25, which is defined as a function returning int.
1963 @chapter Symbol information in symbol tables
1965 This section examines more closely the format of symbol table entries
1966 and how stab assembler directives map to them. It also describes what
1967 transformations the assembler and linker make on data from stabs.
1969 Each time the assembler encounters a stab in its input file it puts
1970 each field of the stab into corresponding fields in a symbol table
1971 entry of its output file. If the stab contains a string field, the
1972 symbol table entry for that stab points to a string table entry
1973 containing the string data from the stab. Assembler labels become
1974 relocatable addresses. Symbol table entries in a.out have the format:
1977 struct internal_nlist @{
1978 unsigned long n_strx; /* index into string table of name */
1979 unsigned char n_type; /* type of symbol */
1980 unsigned char n_other; /* misc info (usually empty) */
1981 unsigned short n_desc; /* description field */
1982 bfd_vma n_value; /* value of symbol */
1986 For .stabs directives, the n_strx field holds the character offset
1987 from the start of the string table to the string table entry
1988 containing the "string" field. For other classes of stabs (.stabn and
1989 .stabd) this field is null.
1991 Symbol table entries with n_type fields containing a value greater or
1992 equal to 0x20 originated as stabs generated by the compiler (with one
1993 random exception). Those with n_type values less than 0x20 were
1994 placed in the symbol table of the executable by the assembler or the
1997 The linker concatenates object files and does fixups of externally
1998 defined symbols. You can see the transformations made on stab data by
1999 the assembler and linker by examining the symbol table after each pass
2000 of the build, first the assemble and then the link.
2002 To do this use nm with the -ap options. This dumps the symbol table,
2003 including debugging information, unsorted. For stab entries the
2004 columns are: value, other, desc, type, string. For assembler and
2005 linker symbols, the columns are: value, type, string.
2007 There are a few important things to notice about symbol tables. Where
2008 the value field of a stab contains a frame pointer offset, or a
2009 register number, that value is unchanged by the rest of the build.
2011 Where the value field of a stab contains an assembly language label,
2012 it is transformed by each build step. The assembler turns it into a
2013 relocatable address and the linker turns it into an absolute address.
2014 This source line defines a static variable at file scope:
2017 3 static int s_g_repeat
2021 The following stab describes the symbol.
2024 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
2028 The assembler transforms the stab into this symbol table entry in the
2029 @file{.o} file. The location is expressed as a data segment offset.
2032 21 00000084 - 00 0000 STSYM s_g_repeat:S1
2036 in the symbol table entry from the executable, the linker has made the
2037 relocatable address absolute.
2040 22 0000e00c - 00 0000 STSYM s_g_repeat:S1
2043 Stabs for global variables do not contain location information. In
2044 this case the debugger finds location information in the assembler or
2045 linker symbol table entry describing the variable. The source line:
2055 21 .stabs "g_foo:G2",32,0,0,0
2058 The variable is represented by the following two symbol table entries
2059 in the object file. The first one originated as a stab. The second
2060 one is an external symbol. The upper case D signifies that the n_type
2061 field of the symbol table contains 7, N_DATA with local linkage (see
2062 Table B). The value field following the file's line number is empty
2063 for the stab entry. For the linker symbol it contains the
2064 rellocatable address corresponding to the variable.
2067 19 00000000 - 00 0000 GSYM g_foo:G2
2068 20 00000080 D _g_foo
2072 These entries as transformed by the linker. The linker symbol table
2073 entry now holds an absolute address.
2076 21 00000000 - 00 0000 GSYM g_foo:G2
2078 215 0000e008 D _g_foo
2082 @chapter GNU C++ stabs
2085 * Basic Cplusplus types::
2088 * Methods:: Method definition
2090 * Method Modifiers::
2093 * Virtual Base Classes::
2097 @subsection type descriptors added for C++ descriptions
2101 method type (two ## if minimal debug)
2104 Member (class and variable) type. It is followed by type information
2105 for the offset basetype, a comma, and type information for the type of
2106 the field being pointed to. (FIXME: this is acknowledged to be
2107 gibberish. Can anyone say what really goes here?).
2109 Note that there is a conflict between this and type attributes
2110 (@pxref{Stabs Format}); both use type descriptor @samp{@@}.
2111 Fortunately, the @samp{@@} type descriptor used in this C++ sense always
2112 will be followed by a digit, @samp{(}, or @samp{-}, and type attributes
2113 never start with those things.
2116 @node Basic Cplusplus types
2117 @section Basic types for C++
2119 << the examples that follow are based on a01.C >>
2122 C++ adds two more builtin types to the set defined for C. These are
2123 the unknown type and the vtable record type. The unknown type, type
2124 16, is defined in terms of itself like the void type.
2126 The vtable record type, type 17, is defined as a structure type and
2127 then as a structure tag. The structure has four fields, delta, index,
2128 pfn, and delta2. pfn is the function pointer.
2130 << In boilerplate $vtbl_ptr_type, what are the fields delta,
2131 index, and delta2 used for? >>
2133 This basic type is present in all C++ programs even if there are no
2134 virtual methods defined.
2137 .stabs "struct_name:sym_desc(type)type_def(17)=type_desc(struct)struct_bytes(8)
2138 elem_name(delta):type_ref(short int),bit_offset(0),field_bits(16);
2139 elem_name(index):type_ref(short int),bit_offset(16),field_bits(16);
2140 elem_name(pfn):type_def(18)=type_desc(ptr to)type_ref(void),
2141 bit_offset(32),field_bits(32);
2142 elem_name(delta2):type_def(short int);bit_offset(32),field_bits(16);;"
2147 .stabs "$vtbl_ptr_type:t17=s8
2148 delta:6,0,16;index:6,16,16;pfn:18=*15,32,32;delta2:6,32,16;;"
2153 .stabs "name:sym_dec(struct tag)type_ref($vtbl_ptr_type)",N_LSYM,NIL,NIL,NIL
2157 .stabs "$vtbl_ptr_type:T17",128,0,0,0
2160 @node Simple classes
2161 @section Simple class definition
2163 The stabs describing C++ language features are an extension of the
2164 stabs describing C. Stabs representing C++ class types elaborate
2165 extensively on the stab format used to describe structure types in C.
2166 Stabs representing class type variables look just like stabs
2167 representing C language variables.
2169 Consider the following very simple class definition.
2175 int Ameth(int in, char other);
2179 The class baseA is represented by two stabs. The first stab describes
2180 the class as a structure type. The second stab describes a structure
2181 tag of the class type. Both stabs are of stab type N_LSYM. Since the
2182 stab is not located between an N_FUN and a N_LBRAC stab this indicates
2183 that the class is defined at file scope. If it were, then the N_LSYM
2184 would signify a local variable.
2186 A stab describing a C++ class type is similar in format to a stab
2187 describing a C struct, with each class member shown as a field in the
2188 structure. The part of the struct format describing fields is
2189 expanded to include extra information relevent to C++ class members.
2190 In addition, if the class has multiple base classes or virtual
2191 functions the struct format outside of the field parts is also
2194 In this simple example the field part of the C++ class stab
2195 representing member data looks just like the field part of a C struct
2196 stab. The section on protections describes how its format is
2197 sometimes extended for member data.
2199 The field part of a C++ class stab representing a member function
2200 differs substantially from the field part of a C struct stab. It
2201 still begins with `name:' but then goes on to define a new type number
2202 for the member function, describe its return type, its argument types,
2203 its protection level, any qualifiers applied to the method definition,
2204 and whether the method is virtual or not. If the method is virtual
2205 then the method description goes on to give the vtable index of the
2206 method, and the type number of the first base class defining the
2209 When the field name is a method name it is followed by two colons
2210 rather than one. This is followed by a new type definition for the
2211 method. This is a number followed by an equal sign and then the
2212 symbol descriptor `##', indicating a method type. This is followed by
2213 a type reference showing the return type of the method and a
2216 The format of an overloaded operator method name differs from that
2217 of other methods. It is "op$::XXXX." where XXXX is the operator name
2218 such as + or +=. The name ends with a period, and any characters except
2219 the period can occur in the XXXX string.
2221 The next part of the method description represents the arguments to
2222 the method, preceeded by a colon and ending with a semi-colon. The
2223 types of the arguments are expressed in the same way argument types
2224 are expressed in C++ name mangling. In this example an int and a char
2227 This is followed by a number, a letter, and an asterisk or period,
2228 followed by another semicolon. The number indicates the protections
2229 that apply to the member function. Here the 2 means public. The
2230 letter encodes any qualifier applied to the method definition. In
2231 this case A means that it is a normal function definition. The dot
2232 shows that the method is not virtual. The sections that follow
2233 elaborate further on these fields and describe the additional
2234 information present for virtual methods.
2238 .stabs "class_name:sym_desc(type)type_def(20)=type_desc(struct)struct_bytes(4)
2239 field_name(Adat):type(int),bit_offset(0),field_bits(32);
2241 method_name(Ameth)::type_def(21)=type_desc(method)return_type(int);
2242 :arg_types(int char);
2243 protection(public)qualifier(normal)virtual(no);;"
2248 .stabs "baseA:t20=s4Adat:1,0,32;Ameth::21=##1;:ic;2A.;;",128,0,0,0
2250 .stabs "class_name:sym_desc(struct tag)",N_LSYM,NIL,NIL,NIL
2252 .stabs "baseA:T20",128,0,0,0
2255 @node Class instance
2256 @section Class instance
2258 As shown above, describing even a simple C++ class definition is
2259 accomplished by massively extending the stab format used in C to
2260 describe structure types. However, once the class is defined, C stabs
2261 with no modifications can be used to describe class instances. The
2271 yields the following stab describing the class instance. It looks no
2272 different from a standard C stab describing a local variable.
2275 .stabs "name:type_ref(baseA)", N_LSYM, NIL, NIL, frame_ptr_offset
2279 .stabs "AbaseA:20",128,0,0,-20
2283 @section Method defintion
2285 The class definition shown above declares Ameth. The C++ source below
2290 baseA::Ameth(int in, char other)
2297 This method definition yields three stabs following the code of the
2298 method. One stab describes the method itself and following two
2299 describe its parameters. Although there is only one formal argument
2300 all methods have an implicit argument which is the `this' pointer.
2301 The `this' pointer is a pointer to the object on which the method was
2302 called. Note that the method name is mangled to encode the class name
2303 and argument types. << Name mangling is not described by this
2304 document - Is there already such a doc? >>
2307 .stabs "name:symbol_desriptor(global function)return_type(int)",
2308 N_FUN, NIL, NIL, code_addr_of_method_start
2310 .stabs "Ameth__5baseAic:F1",36,0,0,_Ameth__5baseAic
2313 Here is the stab for the `this' pointer implicit argument. The name
2314 of the `this' pointer is always `this.' Type 19, the `this' pointer is
2315 defined as a pointer to type 20, baseA, but a stab defining baseA has
2316 not yet been emited. Since the compiler knows it will be emited
2317 shortly, here it just outputs a cross reference to the undefined
2318 symbol, by prefixing the symbol name with xs.
2321 .stabs "name:sym_desc(register param)type_def(19)=
2322 type_desc(ptr to)type_ref(baseA)=
2323 type_desc(cross-reference to)baseA:",N_RSYM,NIL,NIL,register_number
2325 .stabs "this:P19=*20=xsbaseA:",64,0,0,8
2328 The stab for the explicit integer argument looks just like a parameter
2329 to a C function. The last field of the stab is the offset from the
2330 argument pointer, which in most systems is the same as the frame
2334 .stabs "name:sym_desc(value parameter)type_ref(int)",
2335 N_PSYM,NIL,NIL,offset_from_arg_ptr
2337 .stabs "in:p1",160,0,0,72
2340 << The examples that follow are based on A1.C >>
2343 @section Protections
2346 In the simple class definition shown above all member data and
2347 functions were publicly accessable. The example that follows
2348 contrasts public, protected and privately accessable fields and shows
2349 how these protections are encoded in C++ stabs.
2351 Protections for class member data are signified by two characters
2352 embeded in the stab defining the class type. These characters are
2353 located after the name: part of the string. /0 means private, /1
2354 means protected, and /2 means public. If these characters are omited
2355 this means that the member is public. The following C++ source:
2369 generates the following stab to describe the class type all_data.
2372 .stabs "class_name:sym_desc(type)type_def(19)=type_desc(struct)struct_bytes
2373 data_name:/protection(private)type_ref(int),bit_offset,num_bits;
2374 data_name:/protection(protected)type_ref(char),bit_offset,num_bits;
2375 data_name:(/num omited, private)type_ref(float),bit_offset,num_bits;;"
2380 .stabs "all_data:t19=s12
2381 priv_dat:/01,0,32;prot_dat:/12,32,8;pub_dat:12,64,32;;",128,0,0,0
2384 Protections for member functions are signified by one digit embeded in
2385 the field part of the stab describing the method. The digit is 0 if
2386 private, 1 if protected and 2 if public. Consider the C++ class
2390 class all_methods @{
2392 int priv_meth(int in)@{return in;@};
2394 char protMeth(char in)@{return in;@};
2396 float pubMeth(float in)@{return in;@};
2400 It generates the following stab. The digit in question is to the left
2401 of an `A' in each case. Notice also that in this case two symbol
2402 descriptors apply to the class name struct tag and struct type.
2405 .stabs "class_name:sym_desc(struct tag&type)type_def(21)=
2406 sym_desc(struct)struct_bytes(1)
2407 meth_name::type_def(22)=sym_desc(method)returning(int);
2408 :args(int);protection(private)modifier(normal)virtual(no);
2409 meth_name::type_def(23)=sym_desc(method)returning(char);
2410 :args(char);protection(protected)modifier(normal)virual(no);
2411 meth_name::type_def(24)=sym_desc(method)returning(float);
2412 :args(float);protection(public)modifier(normal)virtual(no);;",
2417 .stabs "all_methods:Tt21=s1priv_meth::22=##1;:i;0A.;protMeth::23=##2;:c;1A.;
2418 pubMeth::24=##12;:f;2A.;;",128,0,0,0
2421 @node Method Modifiers
2422 @section Method Modifiers (const, volatile, const volatile)
2426 In the class example described above all the methods have the normal
2427 modifier. This method modifier information is located just after the
2428 protection information for the method. This field has four possible
2429 character values. Normal methods use A, const methods use B, volatile
2430 methods use C, and const volatile methods use D. Consider the class
2436 int ConstMeth (int arg) const @{ return arg; @};
2437 char VolatileMeth (char arg) volatile @{ return arg; @};
2438 float ConstVolMeth (float arg) const volatile @{return arg; @};
2442 This class is described by the following stab:
2445 .stabs "class(A):sym_desc(struct)type_def(20)=type_desc(struct)struct_bytes(1)
2446 meth_name(ConstMeth)::type_def(21)sym_desc(method)
2447 returning(int);:arg(int);protection(public)modifier(const)virtual(no);
2448 meth_name(VolatileMeth)::type_def(22)=sym_desc(method)
2449 returning(char);:arg(char);protection(public)modifier(volatile)virt(no)
2450 meth_name(ConstVolMeth)::type_def(23)=sym_desc(method)
2451 returning(float);:arg(float);protection(public)modifer(const volatile)
2452 virtual(no);;", @dots{}
2456 .stabs "A:T20=s1ConstMeth::21=##1;:i;2B.;VolatileMeth::22=##2;:c;2C.;
2457 ConstVolMeth::23=##12;:f;2D.;;",128,0,0,0
2460 @node Virtual Methods
2461 @section Virtual Methods
2463 << The following examples are based on a4.C >>
2465 The presence of virtual methods in a class definition adds additional
2466 data to the class description. The extra data is appended to the
2467 description of the virtual method and to the end of the class
2468 description. Consider the class definition below:
2474 virtual int A_virt (int arg) @{ return arg; @};
2478 This results in the stab below describing class A. It defines a new
2479 type (20) which is an 8 byte structure. The first field of the class
2480 struct is Adat, an integer, starting at structure offset 0 and
2483 The second field in the class struct is not explicitly defined by the
2484 C++ class definition but is implied by the fact that the class
2485 contains a virtual method. This field is the vtable pointer. The
2486 name of the vtable pointer field starts with $vf and continues with a
2487 type reference to the class it is part of. In this example the type
2488 reference for class A is 20 so the name of its vtable pointer field is
2489 $vf20, followed by the usual colon.
2491 Next there is a type definition for the vtable pointer type (21).
2492 This is in turn defined as a pointer to another new type (22).
2494 Type 22 is the vtable itself, which is defined as an array, indexed by
2495 a range of integers between 0 and 1, and whose elements are of type
2496 17. Type 17 was the vtable record type defined by the boilerplate C++
2497 type definitions, as shown earlier.
2499 The bit offset of the vtable pointer field is 32. The number of bits
2500 in the field are not specified when the field is a vtable pointer.
2502 Next is the method definition for the virtual member function A_virt.
2503 Its description starts out using the same format as the non-virtual
2504 member functions described above, except instead of a dot after the
2505 `A' there is an asterisk, indicating that the function is virtual.
2506 Since is is virtual some addition information is appended to the end
2507 of the method description.
2509 The first number represents the vtable index of the method. This is a
2510 32 bit unsigned number with the high bit set, followed by a
2513 The second number is a type reference to the first base class in the
2514 inheritence hierarchy defining the virtual member function. In this
2515 case the class stab describes a base class so the virtual function is
2516 not overriding any other definition of the method. Therefore the
2517 reference is to the type number of the class that the stab is
2520 This is followed by three semi-colons. One marks the end of the
2521 current sub-section, one marks the end of the method field, and the
2522 third marks the end of the struct definition.
2524 For classes containing virtual functions the very last section of the
2525 string part of the stab holds a type reference to the first base
2526 class. This is preceeded by `~%' and followed by a final semi-colon.
2529 .stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8)
2530 field_name(Adat):type_ref(int),bit_offset(0),field_bits(32);
2531 field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)=
2532 sym_desc(array)index_type_ref(range of int from 0 to 1);
2533 elem_type_ref(vtbl elem type),
2535 meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
2536 :arg_type(int),protection(public)normal(yes)virtual(yes)
2537 vtable_index(1);class_first_defining(A);;;~%first_base(A);",
2541 @c FIXME: bogus line break.
2543 .stabs "A:t20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2544 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2548 @section Inheritence
2550 Stabs describing C++ derived classes include additional sections that
2551 describe the inheritence hierarchy of the class. A derived class stab
2552 also encodes the number of base classes. For each base class it tells
2553 if the base class is virtual or not, and if the inheritence is private
2554 or public. It also gives the offset into the object of the portion of
2555 the object corresponding to each base class.
2557 This additional information is embeded in the class stab following the
2558 number of bytes in the struct. First the number of base classes
2559 appears bracketed by an exclamation point and a comma.
2561 Then for each base type there repeats a series: two digits, a number,
2562 a comma, another number, and a semi-colon.
2564 The first of the two digits is 1 if the base class is virtual and 0 if
2565 not. The second digit is 2 if the derivation is public and 0 if not.
2567 The number following the first two digits is the offset from the start
2568 of the object to the part of the object pertaining to the base class.
2570 After the comma, the second number is a type_descriptor for the base
2571 type. Finally a semi-colon ends the series, which repeats for each
2574 The source below defines three base classes A, B, and C and the
2582 virtual int A_virt (int arg) @{ return arg; @};
2588 virtual int B_virt (int arg) @{return arg; @};
2594 virtual int C_virt (int arg) @{return arg; @};
2597 class D : A, virtual B, public C @{
2600 virtual int A_virt (int arg ) @{ return arg+1; @};
2601 virtual int B_virt (int arg) @{ return arg+2; @};
2602 virtual int C_virt (int arg) @{ return arg+3; @};
2603 virtual int D_virt (int arg) @{ return arg; @};
2607 Class stabs similar to the ones described earlier are generated for
2610 @c FIXME!!! the linebreaks in the following example probably make the
2611 @c examples literally unusable, but I don't know any other way to get
2612 @c them on the page.
2613 @c One solution would be to put some of the type definitions into
2614 @c separate stabs, even if that's not exactly what the compiler actually
2617 .stabs "A:T20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2618 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2620 .stabs "B:Tt25=s8Bdat:1,0,32;$vf25:21,32;B_virt::26=##1;
2621 :i;2A*-2147483647;25;;;~%25;",128,0,0,0
2623 .stabs "C:Tt28=s8Cdat:1,0,32;$vf28:21,32;C_virt::29=##1;
2624 :i;2A*-2147483647;28;;;~%28;",128,0,0,0
2627 In the stab describing derived class D below, the information about
2628 the derivation of this class is encoded as follows.
2631 .stabs "derived_class_name:symbol_descriptors(struct tag&type)=
2632 type_descriptor(struct)struct_bytes(32)!num_bases(3),
2633 base_virtual(no)inheritence_public(no)base_offset(0),
2634 base_class_type_ref(A);
2635 base_virtual(yes)inheritence_public(no)base_offset(NIL),
2636 base_class_type_ref(B);
2637 base_virtual(no)inheritence_public(yes)base_offset(64),
2638 base_class_type_ref(C); @dots{}
2641 @c FIXME! fake linebreaks.
2643 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:
2644 1,160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt:
2645 :32:i;2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;
2646 28;;D_virt::32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2649 @node Virtual Base Classes
2650 @section Virtual Base Classes
2652 A derived class object consists of a concatination in memory of the
2653 data areas defined by each base class, starting with the leftmost and
2654 ending with the rightmost in the list of base classes. The exception
2655 to this rule is for virtual inheritence. In the example above, class
2656 D inherits virtually from base class B. This means that an instance
2657 of a D object will not contain it's own B part but merely a pointer to
2658 a B part, known as a virtual base pointer.
2660 In a derived class stab, the base offset part of the derivation
2661 information, described above, shows how the base class parts are
2662 ordered. The base offset for a virtual base class is always given as
2663 0. Notice that the base offset for B is given as 0 even though B is
2664 not the first base class. The first base class A starts at offset 0.
2666 The field information part of the stab for class D describes the field
2667 which is the pointer to the virtual base class B. The vbase pointer
2668 name is $vb followed by a type reference to the virtual base class.
2669 Since the type id for B in this example is 25, the vbase pointer name
2672 @c FIXME!! fake linebreaks below
2674 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1,
2675 160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i;
2676 2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt:
2677 :32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2680 Following the name and a semicolon is a type reference describing the
2681 type of the virtual base class pointer, in this case 24. Type 24 was
2682 defined earlier as the type of the B class `this` pointer. The
2683 `this' pointer for a class is a pointer to the class type.
2686 .stabs "this:P24=*25=xsB:",64,0,0,8
2689 Finally the field offset part of the vbase pointer field description
2690 shows that the vbase pointer is the first field in the D object,
2691 before any data fields defined by the class. The layout of a D class
2692 object is a follows, Adat at 0, the vtable pointer for A at 32, Cdat
2693 at 64, the vtable pointer for C at 96, the virtual ase pointer for B
2694 at 128, and Ddat at 160.
2697 @node Static Members
2698 @section Static Members
2700 The data area for a class is a concatenation of the space used by the
2701 data members of the class. If the class has virtual methods, a vtable
2702 pointer follows the class data. The field offset part of each field
2703 description in the class stab shows this ordering.
2705 << How is this reflected in stabs? See Cygnus bug #677 for some info. >>
2708 @appendix Example2.c - source code for extended example
2712 2 register int g_bar asm ("%g5");
2713 3 static int s_g_repeat = 2;
2719 9 char s_char_vec[8];
2720 10 struct s_tag* s_next;
2723 13 typedef struct s_tag s_typedef;
2725 15 char char_vec[3] = @{'a','b','c'@};
2727 17 main (argc, argv)
2731 21 static float s_flap;
2733 23 for (times=0; times < s_g_repeat; times++)@{
2735 25 printf ("Hello world\n");
2739 29 enum e_places @{first,second=3,last@};
2741 31 static s_proc (s_arg, s_ptr_arg, char_vec)
2743 33 s_typedef* s_ptr_arg;
2757 @appendix Example2.s - assembly code for extended example
2761 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
2762 3 .stabs "example2.c",100,0,0,Ltext0
2765 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
2766 7 .stabs "char:t2=r2;0;127;",128,0,0,0
2767 8 .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
2768 9 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
2769 10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
2770 11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
2771 12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
2772 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
2773 14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
2774 15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
2775 16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
2776 17 .stabs "float:t12=r1;4;0;",128,0,0,0
2777 18 .stabs "double:t13=r1;8;0;",128,0,0,0
2778 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
2779 20 .stabs "void:t15=15",128,0,0,0
2780 21 .stabs "g_foo:G2",32,0,0,0
2785 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
2789 @c FIXME! fake linebreak in line 30
2790 30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;s_char_vec:
2791 17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
2792 31 .stabs "s_typedef:t16",128,0,0,0
2793 32 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
2794 33 .global _char_vec
2800 39 .reserve _s_flap.0,4,"bss",4
2804 43 .ascii "Hello world\12\0"
2809 48 .stabn 68,0,20,LM1
2812 51 save %sp,-144,%sp
2819 58 .stabn 68,0,23,LM2
2823 62 sethi %hi(_s_g_repeat),%o0
2825 64 ld [%o0+%lo(_s_g_repeat)],%o0
2830 69 .stabn 68,0,25,LM3
2832 71 sethi %hi(LC0),%o1
2833 72 or %o1,%lo(LC0),%o0
2836 75 .stabn 68,0,26,LM4
2839 78 .stabn 68,0,23,LM5
2847 86 .stabn 68,0,27,LM6
2850 89 .stabn 68,0,27,LM7
2855 94 .stabs "main:F1",36,0,0,_main
2856 95 .stabs "argc:p1",160,0,0,68
2857 96 .stabs "argv:p20=*21=*2",160,0,0,72
2858 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
2859 98 .stabs "times:1",128,0,0,-20
2860 99 .stabn 192,0,0,LBB2
2861 100 .stabs "inner:1",128,0,0,-24
2862 101 .stabn 192,0,0,LBB3
2863 102 .stabn 224,0,0,LBE3
2864 103 .stabn 224,0,0,LBE2
2865 104 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
2866 @c FIXME: fake linebreak in line 105
2867 105 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
2872 109 .stabn 68,0,35,LM8
2875 112 save %sp,-120,%sp
2881 118 .stabn 68,0,41,LM9
2884 121 .stabn 68,0,41,LM10
2889 126 .stabs "s_proc:f1",36,0,0,_s_proc
2890 127 .stabs "s_arg:p16",160,0,0,0
2891 128 .stabs "s_ptr_arg:p18",160,0,0,72
2892 129 .stabs "char_vec:p21",160,0,0,76
2893 130 .stabs "an_u:23",128,0,0,-20
2894 131 .stabn 192,0,0,LBB4
2895 132 .stabn 224,0,0,LBE4
2896 133 .stabs "g_bar:r1",64,0,0,5
2897 134 .stabs "g_pf:G24=*25=f1",32,0,0,0
2898 135 .common _g_pf,4,"bss"
2899 136 .stabs "g_an_s:G16",32,0,0,0
2900 137 .common _g_an_s,20,"bss"
2904 @appendix Values for the Stab Type Field
2906 These are all the possible values for the stab type field, for
2907 @code{a.out} files. This does not apply to XCOFF.
2909 The following types are used by the linker and assembler; there is
2910 nothing stabs-specific about them. Since this document does not attempt
2911 to describe aspects of object file format other than the debugging
2912 format, no details are given.
2914 @c Try to get most of these to fit on a single line.
2924 File scope absolute symbol
2926 @item 0x3 N_ABS | N_EXT
2927 External absolute symbol
2930 File scope text symbol
2932 @item 0x5 N_TEXT | N_EXT
2933 External text symbol
2936 File scope data symbol
2938 @item 0x7 N_DATA | N_EXT
2939 External data symbol
2942 File scope BSS symbol
2944 @item 0x9 N_BSS | N_EXT
2948 Same as N_FN, for Sequent compilers
2951 Symbol is indirected to another symbol
2954 Common sym -- visable after shared lib dynamic link
2957 Absolute set element
2960 Text segment set element
2963 Data segment set element
2966 BSS segment set element
2969 Pointer to set vector
2971 @item 0x1e N_WARNING
2972 Print a warning message during linking
2975 File name of a .o file
2978 The following symbol types indicate that this is a stab. This is the
2979 full list of stab numbers, including stab types that are used in
2980 languages other than C.
2984 Global symbol, @xref{N_GSYM}.
2987 Function name (for BSD Fortran), @xref{N_FNAME}.
2990 Function name (@pxref{Procedures}) or text segment variable
2994 Data segment file-scope variable, @xref{Statics}.
2997 BSS segment file-scope variable, @xref{Statics}.
3000 Name of main routine, @xref{Main Program}.
3002 @c FIXME: discuss this in the main body of the text where we talk about
3003 @c using N_FUN for variables.
3005 Read-only data symbol (Solaris2). Most systems use N_FUN for this.
3008 Global symbol (for Pascal), @xref{N_PC}.
3011 Number of symbols (according to Ultrix V4.0), @xref{N_NSYMS}.
3014 No DST map for sym (according to Ultrix V4.0), @xref{N_NOMAP}.
3016 @c FIXME: describe this solaris feature in the body of the text (see
3017 @c comments in include/aout/stab.def).
3019 Object file (Solaris2).
3021 @c See include/aout/stab.def for (a little) more info.
3023 Debugger options (Solaris2).
3026 Register variable, @xref{N_RSYM}.
3029 Modula-2 compilation unit, @xref{N_M2C}.
3032 Line number in text segment, @xref{Line Numbers}.
3035 Line number in data segment, @xref{Line Numbers}.
3038 Line number in bss segment, @xref{Line Numbers}.
3041 Sun source code browser, path to .cb file, @xref{N_BROWS}.
3044 Gnu Modula2 definition module dependency, @xref{N_DEFD}.
3047 Function start/body/end line numbers (Solaris2).
3050 Gnu C++ exception variable, @xref{N_EHDECL}.
3053 Modula2 info "for imc" (according to Ultrix V4.0), @xref{N_MOD2}.
3056 Gnu C++ "catch" clause, @xref{N_CATCH}.
3059 Structure of union element, @xref{N_SSYM}.
3062 Last stab for module (Solaris2).
3065 Path and name of source file , @xref{Source Files}.
3068 Automatic var in the stack or type definition, @xref{N_LSYM}, @xref{Typedefs}.
3071 Beginning of an include file (Sun only), @xref{Source Files}.
3074 Name of include file, @xref{Source Files}.
3077 Parameter variable, @xref{Parameters}.
3080 End of an include file, @xref{Source Files}.
3083 Alternate entry point, @xref{N_ENTRY}.
3086 Beginning of a lexical block, @xref{Block Structure}.
3089 Place holder for a deleted include file, @xref{Source Files}.
3092 Modula2 scope information (Sun linker), @xref{N_SCOPE}.
3095 End of a lexical block, @xref{Block Structure}.
3098 Begin named common block, @xref{Common Blocks}.
3101 End named common block, @xref{Common Blocks}.
3104 Member of a common block, @xref{Common Blocks}.
3106 @c FIXME: How does this really work? Move it to main body of document.
3108 Pascal @code{with} statement: type,,0,0,offset (Solaris2).
3111 Gould non-base registers, @xref{Gould}.
3114 Gould non-base registers, @xref{Gould}.
3117 Gould non-base registers, @xref{Gould}.
3120 Gould non-base registers, @xref{Gould}.
3123 Gould non-base registers, @xref{Gould}.
3126 @c Restore the default table indent
3131 @node Symbol Descriptors
3132 @appendix Table of Symbol Descriptors
3134 @c Please keep this alphabetical
3136 @c In TeX, this looks great, digit is in italics. But makeinfo insists
3137 @c on putting it in `', not realizing that @var should override @code.
3138 @c I don't know of any way to make makeinfo do the right thing. Seems
3139 @c like a makeinfo bug to me.
3143 Local variable, @xref{Automatic variables}.
3146 Parameter passed by reference in register, @xref{Parameters}.
3149 Constant, @xref{Constants}.
3152 Conformant array bound (Pascal, maybe other languages),
3153 @xref{Parameters}. Name of a caught exception (GNU C++). These can be
3154 distinguished because the latter uses N_CATCH and the former uses
3155 another symbol type.
3158 Floating point register variable, @xref{Register variables}.
3161 Parameter in floating point register, @xref{Parameters}.
3164 File scope function, @xref{Procedures}.
3167 Global function, @xref{Procedures}.
3170 Global variable, @xref{Global Variables}.
3176 Internal (nested) procedure, @xref{Procedures}.
3179 Internal (nested) function, @xref{Procedures}.
3182 Label name (documented by AIX, no further information known).
3185 Module, @xref{Procedures}.
3188 Argument list parameter, @xref{Parameters}.
3194 FORTRAN Function parameter, @xref{Parameters}.
3197 Unfortunately, three separate meanings have been independently invented
3198 for this symbol descriptor. At least the GNU and Sun uses can be
3199 distinguished by the symbol type. Global Procedure (AIX) (symbol type
3200 used unknown), @xref{Procedures}. Register parameter (GNU) (symbol type
3201 N_PSYM), @xref{Parameters}. Prototype of function referenced by this
3202 file (Sun acc) (symbol type N_FUN).
3205 Static Procedure, @xref{Procedures}.
3208 Register parameter @xref{Parameters}.
3211 Register variable, @xref{Register variables}.
3214 File scope variable, @xref{Statics}.
3217 Type name, @xref{Typedefs}.
3220 enumeration, struct or union tag, @xref{Typedefs}.
3223 Parameter passed by reference, @xref{Parameters}.
3226 Procedure scope static variable, @xref{Statics}.
3229 Conformant array, @xref{Parameters}.
3232 Function return variable, @xref{Parameters}.
3235 @node Type Descriptors
3236 @appendix Table of Type Descriptors
3241 Type reference, @xref{Stabs Format}.
3244 Reference to builtin type, @xref{Negative Type Numbers}.
3247 Method (C++), @xref{Cplusplus}.
3250 Pointer, @xref{Miscellaneous Types}.
3256 Type Attributes (AIX), @xref{Stabs Format}. Member (class and variable)
3257 type (GNU C++), @xref{Cplusplus}.
3260 Array, @xref{Arrays}.
3263 Open array, @xref{Arrays}.
3266 Pascal space type (AIX), @xref{Miscellaneous Types}. Builtin integer
3267 type (Sun), @xref{Builtin Type Descriptors}.
3270 Volatile-qualified type, @xref{Miscellaneous Types}.
3273 Complex builtin type, @xref{Builtin Type Descriptors}.
3276 COBOL Picture type. See AIX documentation for details.
3279 File type, @xref{Miscellaneous Types}.
3282 N-dimensional dynamic array, @xref{Arrays}.
3285 Enumeration type, @xref{Enumerations}.
3288 N-dimensional subarray, @xref{Arrays}.
3291 Function type, @xref{Function Types}.
3294 Pascal function parameter, @xref{Function Types}
3297 Builtin floating point type, @xref{Builtin Type Descriptors}.
3300 COBOL Group. See AIX documentation for details.
3303 Imported type, @xref{Cross-references}.
3306 Const-qualified type, @xref{Miscellaneous Types}.
3309 COBOL File Descriptor. See AIX documentation for details.
3312 Multiple instance type, @xref{Miscellaneous Types}.
3315 String type, @xref{Strings}.
3318 Stringptr, @xref{Strings}.
3321 Opaque type, @xref{Typedefs}.
3324 Procedure, @xref{Function Types}.
3327 Packed array, @xref{Arrays}.
3330 Range type, @xref{Subranges}.
3333 Builtin floating type, @xref{Builtin Type Descriptors} (Sun). Pascal
3334 subroutine parameter, @xref{Function Types} (AIX). Detecting this
3335 conflict is possible with careful parsing (hint: a Pascal subroutine
3336 parameter type will always contain a comma, and a builtin type
3337 descriptor never will).
3340 Structure type, @xref{Structures}.
3343 Set type, @xref{Miscellaneous Types}.
3346 Union, @xref{Unions}.
3349 Variant record. This is a Pascal and Modula-2 feature which is like a
3350 union within a struct in C. See AIX documentation for details.
3353 Wide character, @xref{Builtin Type Descriptors}.
3356 Cross-reference, @xref{Cross-references}.
3359 gstring, @xref{Strings}.
3362 @node Expanded reference
3363 @appendix Expanded reference by stab type.
3365 @c FIXME: This appendix should go away, see N_PSYM or N_SO for an example.
3367 For a full list of stab types, and cross-references to where they are
3368 described, @xref{Stab Types}. This appendix just duplicates certain
3369 information from the main body of this document; eventually the
3370 information will all be in one place.
3374 The first line is the symbol type expressed in decimal, hexadecimal,
3375 and as a #define (see devo/include/aout/stab.def).
3377 The second line describes the language constructs the symbol type
3380 The third line is the stab format with the significant stab fields
3381 named and the rest NIL.
3383 Subsequent lines expand upon the meaning and possible values for each
3384 significant stab field. # stands in for the type descriptor.
3386 Finally, any further information.
3389 * N_GSYM:: Global variable
3390 * N_FNAME:: Function name (BSD Fortran)
3391 * N_PC:: Pascal global symbol
3392 * N_NSYMS:: Number of symbols
3393 * N_NOMAP:: No DST map
3394 * N_RSYM:: Register variable
3395 * N_M2C:: Modula-2 compilation unit
3396 * N_BROWS:: Path to .cb file for Sun source code browser
3397 * N_DEFD:: GNU Modula2 definition module dependency
3398 * N_EHDECL:: GNU C++ exception variable
3399 * N_MOD2:: Modula2 information "for imc"
3400 * N_CATCH:: GNU C++ "catch" clause
3401 * N_SSYM:: Structure or union element
3402 * N_LSYM:: Automatic variable
3403 * N_ENTRY:: Alternate entry point
3404 * N_SCOPE:: Modula2 scope information (Sun only)
3405 * Gould:: non-base register symbols used on Gould systems
3406 * N_LENG:: Length of preceding entry
3410 @section 32 - 0x20 - N_GYSM
3415 .stabs "name", N_GSYM, NIL, NIL, NIL
3419 "name" -> "symbol_name:#type"
3423 Only the "name" field is significant. The location of the variable is
3424 obtained from the corresponding external symbol.
3427 @section 34 - 0x22 - N_FNAME
3428 Function name (for BSD Fortran)
3431 .stabs "name", N_FNAME, NIL, NIL, NIL
3435 "name" -> "function_name"
3438 Only the "name" field is significant. The location of the symbol is
3439 obtained from the corresponding extern symbol.
3442 @section 48 - 0x30 - N_PC
3443 Global symbol (for Pascal)
3446 .stabs "name", N_PC, NIL, NIL, value
3450 "name" -> "symbol_name" <<?>>
3451 value -> supposedly the line number (stab.def is skeptical)
3457 global pascal symbol: name,,0,subtype,line
3462 @section 50 - 0x32 - N_NSYMS
3463 Number of symbols (according to Ultrix V4.0)
3466 0, files,,funcs,lines (stab.def)
3470 @section 52 - 0x34 - N_NOMAP
3471 no DST map for sym (according to Ultrix V4.0)
3474 name, ,0,type,ignored (stab.def)
3478 @section 64 - 0x40 - N_RSYM
3482 .stabs "name:type",N_RSYM,0,RegSize,RegNumber (Sun doc)
3486 @section 66 - 0x42 - N_M2C
3487 Modula-2 compilation unit
3490 .stabs "name", N_M2C, 0, desc, value
3494 "name" -> "unit_name,unit_time_stamp[,code_time_stamp]
3496 value -> 0 (main unit)
3501 @section 72 - 0x48 - N_BROWS
3502 Sun source code browser, path to .cb file
3505 "path to associated .cb file"
3507 Note: type field value overlaps with N_BSLINE
3510 @section 74 - 0x4a - N_DEFD
3511 GNU Modula2 definition module dependency
3513 GNU Modula-2 definition module dependency. Value is the modification
3514 time of the definition file. Other is non-zero if it is imported with
3515 the GNU M2 keyword %INITIALIZE. Perhaps N_M2C can be used if there
3516 are enough empty fields?
3519 @section 80 - 0x50 - N_EHDECL
3520 GNU C++ exception variable <<?>>
3522 "name is variable name"
3524 Note: conflicts with N_MOD2.
3527 @section 80 - 0x50 - N_MOD2
3528 Modula2 info "for imc" (according to Ultrix V4.0)
3530 Note: conflicts with N_EHDECL <<?>>
3533 @section 84 - 0x54 - N_CATCH
3534 GNU C++ "catch" clause
3536 GNU C++ `catch' clause. Value is its address. Desc is nonzero if
3537 this entry is immediately followed by a CAUGHT stab saying what
3538 exception was caught. Multiple CAUGHT stabs means that multiple
3539 exceptions can be caught here. If Desc is 0, it means all exceptions
3543 @section 96 - 0x60 - N_SSYM
3544 Structure or union element
3546 Value is offset in the structure.
3548 <<?looking at structs and unions in C I didn't see these>>
3551 @section 128 - 0x80 - N_LSYM
3552 Automatic var in the stack (also used for type descriptors.)
3555 .stabs "name" N_LSYM, NIL, NIL, value
3559 @exdent @emph{For stack based local variables:}
3561 "name" -> name of the variable
3562 value -> offset from frame pointer (negative)
3564 @exdent @emph{For type descriptors:}
3566 "name" -> "name_of_the_type:#type"
3569 type -> type_ref (or) type_def
3571 type_ref -> type_number
3572 type_def -> type_number=type_desc etc.
3575 Type may be either a type reference or a type definition. A type
3576 reference is a number that refers to a previously defined type. A
3577 type definition is the number that will refer to this type, followed
3578 by an equals sign, a type descriptor and the additional data that
3579 defines the type. See the Table D for type descriptors and the
3580 section on types for what data follows each type descriptor.
3583 @section 164 - 0xa4 - N_ENTRY
3585 Alternate entry point.
3586 Value is its address.
3590 @section 196 - 0xc4 - N_SCOPE
3592 Modula2 scope information (Sun linker)
3596 @section Non-base registers on Gould systems
3598 These are used on Gould systems for non-base registers syms.
3600 However, the following values are not the values used by Gould; they are
3601 the values which GNU has been documenting for these values for a long
3602 time, without actually checking what Gould uses. I include these values
3603 only because perhaps some someone actually did something with the GNU
3604 information (I hope not, why GNU knowingly assigned wrong values to
3605 these in the header file is a complete mystery to me).
3608 240 0xf0 N_NBTEXT ??
3609 242 0xf2 N_NBDATA ??
3616 @section - 0xfe - N_LENG
3618 Second symbol entry containing a length-value for the preceding entry.
3619 The value is the length.
3622 @appendix Questions and anomalies
3626 For GNU C stabs defining local and global variables (N_LSYM and
3627 N_GSYM), the desc field is supposed to contain the source line number
3628 on which the variable is defined. In reality the desc field is always
3629 0. (This behavour is defined in dbxout.c and putting a line number in
3630 desc is controlled by #ifdef WINNING_GDB which defaults to false). Gdb
3631 supposedly uses this information if you say 'list var'. In reality
3632 var can be a variable defined in the program and gdb says `function
3636 In GNU C stabs there seems to be no way to differentiate tag types:
3637 structures, unions, and enums (symbol descriptor T) and typedefs
3638 (symbol descriptor t) defined at file scope from types defined locally
3639 to a procedure or other more local scope. They all use the N_LSYM
3640 stab type. Types defined at procedure scope are emited after the
3641 N_RBRAC of the preceding function and before the code of the
3642 procedure in which they are defined. This is exactly the same as
3643 types defined in the source file between the two procedure bodies.
3644 GDB overcompensates by placing all types in block #1, the block for
3645 symbols of file scope. This is true for default, -ansi and
3646 -traditional compiler options. (Bugs gcc/1063, gdb/1066.)
3649 What ends the procedure scope? Is it the proc block's N_RBRAC or the
3650 next N_FUN? (I believe its the first.)
3653 @c FIXME: This should go with the other stuff about global variables.
3654 Global variable stabs don't have location information. This comes
3655 from the external symbol for the same variable. The external symbol
3656 has a leading underbar on the _name of the variable and the stab does
3657 not. How do we know these two symbol table entries are talking about
3658 the same symbol when their names are different? (Answer: the debugger
3659 knows that external symbols have leading underbars).
3661 @c FIXME: This is absurdly vague; there all kinds of differences, some
3662 @c of which are the same between gnu & sun, and some of which aren't.
3664 Can gcc be configured to output stabs the way the Sun compiler
3665 does, so that their native debugging tools work? <NO?> It doesn't by
3666 default. GDB reads either format of stab. (gcc or SunC). How about
3670 @node xcoff-differences
3671 @appendix Differences between GNU stabs in a.out and GNU stabs in xcoff
3673 @c FIXME: Merge *all* these into the main body of the document.
3674 (The AIX/RS6000 native object file format is xcoff with stabs). This
3675 appendix only covers those differences which are not covered in the main
3676 body of this document.
3680 BSD a.out stab types correspond to AIX xcoff storage classes. In general the
3681 mapping is N_STABTYPE becomes C_STABTYPE. Some stab types in a.out
3682 are not supported in xcoff. See Table E. for full mappings.
3684 @c FIXME: Get C_* types for the block, figure out whether it is always
3685 @c used (I suspect not), explain clearly, and move to node Statics.
3687 initialised static N_STSYM and un-initialized static N_LCSYM both map
3688 to the C_STSYM storage class. But the destinction is preserved
3689 because in xcoff N_STSYM and N_LCSYM must be emited in a named static
3690 block. Begin the block with .bs s[RW] data_section_name for N_STSYM
3691 or .bs s bss_section_name for N_LCSYM. End the block with .es
3693 @c FIXME: I think they are trying to say something about whether the
3694 @c assembler defaults the value to the location counter.
3696 If the xcoff stab is a N_FUN (C_FUN) then follow the string field with
3697 ,. instead of just ,
3700 (I think that's it for .s file differences. They could stand to be
3701 better presented. This is just a list of what I have noticed so far.
3702 There are a *lot* of differences in the information in the symbol
3703 tables of the executable and object files.)
3705 Table E: mapping a.out stab types to xcoff storage classes
3708 stab type storage class
3709 -------------------------------
3718 N_RPSYM (0x8e) C_RPSYM
3728 N_DECL (0x8c) C_DECL
3745 @node Sun-differences
3746 @appendix Differences between GNU stabs and Sun native stabs.
3748 @c FIXME: Merge all this stuff into the main body of the document.
3752 GNU C stabs define *all* types, file or procedure scope, as
3753 N_LSYM. Sun doc talks about using N_GSYM too.
3756 Sun C stabs use type number pairs in the format (a,b) where a is a
3757 number starting with 1 and incremented for each sub-source file in the
3758 compilation. b is a number starting with 1 and incremented for each
3759 new type defined in the compilation. GNU C stabs use the type number
3760 alone, with no source file number.
3764 @appendix Using stabs with the ELF object file format.
3766 The ELF object file format allows tools to create object files with custom
3767 sections containing any arbitrary data. To use stabs in ELF object files,
3768 the tools create two custom sections, a ".stab" section which contains
3769 an array of fixed length structures, one struct per stab, and a ".stabstr"
3770 section containing all the variable length strings that are referenced by
3771 stabs in the ".stab" section. The byte order of the stabs binary data
3772 matches the byte order of the ELF file itself, as determined from the
3773 EI_DATA field in the e_ident member of the ELF header.
3775 The first stab in the ".stab" section for each object file is a "synthetic
3776 stab", generated entirely by the assembler, with no corresponding ".stab"
3777 directive as input to the assembler. This stab contains the following
3782 Offset in the ".stabstr" section to the source filename.
3788 Unused field, always zero.
3791 Count of upcoming symbols. I.E. the number of remaining stabs for this
3795 Size of the string table fragment associated with this object module, in
3800 The ".stabstr" section always starts with a null byte (so that string
3801 offsets of zero reference a null string), followed by random length strings,
3802 each of which is null byte terminated.
3804 The ELF section header for the ".stab" section has it's sh_link member set
3805 to the section number of the ".stabstr" section, and the ".stabstr" section
3806 has it's ELF section header sh_type member set to SHT_STRTAB to mark it as