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 GNU stabs debugging format in a.out files.
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
78 * Example2.c:: Source code for extended example
79 * Example2.s:: Assembly code for extended example
80 * Stab Types:: Symbol types in a.out files
81 * Symbol Descriptors:: Table of Symbol Descriptors
82 * Type Descriptors:: Table of Symbol Descriptors
83 * Expanded reference:: Reference information by stab type
84 * Questions:: Questions and anomolies
85 * xcoff-differences:: Differences between GNU stabs in a.out
86 and GNU stabs in xcoff
87 * Sun-differences:: Differences between GNU stabs and Sun
94 @chapter Overview of stabs
96 @dfn{Stabs} refers to a format for information that describes a program
97 to a debugger. This format was apparently invented by
98 @c FIXME! <<name of inventor>> at
99 the University of California at Berkeley, for the @code{pdx} Pascal
100 debugger; the format has spread widely since then.
102 This document is one of the few published sources of documentation on
103 stabs. It is believed to be completely comprehensive for stabs used by
104 C. The lists of symbol descriptors (@pxref{Symbol Descriptors}) and
105 type descriptors (@pxref{Type Descriptors}) are believed to be completely
106 comprehensive. There are known to be stabs for C++ and COBOL which are
107 poorly documented here. Stabs specific to other languages (e.g. Pascal,
108 Modula-2) are probably not as well documented as they should be.
110 Other sources of information on stabs are @cite{dbx and dbxtool
111 interfaces}, 2nd edition, by Sun, circa 1988, and @cite{AIX Version 3.2
112 Files Reference}, Fourth Edition, September 1992, "dbx Stabstring
113 Grammar" in the a.out section, page 2-31. This document is believed to
114 incorporate the information from those two sources except where it
115 explictly directs you to them for more information.
118 * Flow:: Overview of debugging information flow
119 * Stabs Format:: Overview of stab format
120 * C example:: A simple example in C source
121 * Assembly code:: The simple example at the assembly level
125 @section Overview of debugging information flow
127 The GNU C compiler compiles C source in a @file{.c} file into assembly
128 language in a @file{.s} file, which is translated by the assembler into
129 a @file{.o} file, and then linked with other @file{.o} files and
130 libraries to produce an executable file.
132 With the @samp{-g} option, GCC puts additional debugging information in
133 the @file{.s} file, which is slightly transformed by the assembler and
134 linker, and carried through into the final executable. This debugging
135 information describes features of the source file like line numbers,
136 the types and scopes of variables, and functions, their parameters and
139 For some object file formats, the debugging information is
140 encapsulated in assembler directives known collectively as `stab' (symbol
141 table) directives, interspersed with the generated code. Stabs are
142 the native format for debugging information in the a.out and xcoff
143 object file formats. The GNU tools can also emit stabs in the coff
144 and ecoff object file formats.
146 The assembler adds the information from stabs to the symbol information
147 it places by default in the symbol table and the string table of the
148 @file{.o} file it is building. The linker consolidates the @file{.o}
149 files into one executable file, with one symbol table and one string
150 table. Debuggers use the symbol and string tables in the executable as
151 a source of debugging information about the program.
154 @section Overview of stab format
156 There are three overall formats for stab assembler directives
157 differentiated by the first word of the stab. The name of the directive
158 describes what combination of four possible data fields will follow. It
159 is either @code{.stabs} (string), @code{.stabn} (number), or
160 @code{.stabd} (dot). IBM's xcoff uses @code{.stabx} (and some other
161 directives such as @code{.file} and @code{.bi}) instead of
162 @code{.stabs}, @code{.stabn} or @code{.stabd}.
164 The overall format of each class of stab is:
167 .stabs "@var{string}",@var{type},0,@var{desc},@var{value}
168 .stabx "@var{string}",@var{value},@var{type},@var{sdb-type}
169 .stabn @var{type},0,@var{desc},@var{value}
170 .stabd @var{type},0,@var{desc}
173 @c what is the correct term for "current file location"? My AIX
174 @c assembler manual calls it "the value of the current location counter".
175 For @code{.stabn} and @code{.stabd}, there is no string (the
176 @code{n_strx} field is zero, @pxref{Symbol Tables}). For @code{.stabd}
177 the value field is implicit and has the value of the current file
178 location. The @var{sdb-type} field to @code{.stabx} is unused for stabs
179 and can always be set to 0.
181 The number in the type field gives some basic information about what
182 type of stab this is (or whether it @emph{is} a stab, as opposed to an
183 ordinary symbol). Each possible type number defines a different stab
184 type. The stab type further defines the exact interpretation of, and
185 possible values for, any remaining @code{"@var{string}"}, @var{desc}, or
186 @var{value} fields present in the stab. @xref{Stab Types}, for a list
187 in numeric order of the possible type field values for stab directives.
189 For @code{.stabs} the @code{"@var{string}"} field holds the meat of the
190 debugging information. The generally unstructured nature of this field
191 is what makes stabs extensible. For some stab types the string field
192 contains only a name. For other stab types the contents can be a great
195 The overall format is of the @code{"@var{string}"} field is:
198 "@var{name}:@var{symbol-descriptor} @var{type-information}"
201 @var{name} is the name of the symbol represented by the stab.
202 @var{name} can be omitted, which means the stab represents an unnamed
203 object. For example, @samp{:t10=*2} defines type 10 as a pointer to
204 type 2, but does not give the type a name. Omitting the @var{name}
205 field is supported by AIX dbx and GDB after about version 4.8, but not
206 other debuggers. GCC sometimes uses a single space as the name instead
207 of omitting the name altogether; apparently that is supported by most
210 The @var{symbol_descriptor} following the @samp{:} is an alphabetic
211 character that tells more specifically what kind of symbol the stab
212 represents. If the @var{symbol_descriptor} is omitted, but type
213 information follows, then the stab represents a local variable. For a
214 list of symbol descriptors, see @ref{Symbol Descriptors,,Table C: Symbol
217 The @samp{c} symbol descriptor is an exception in that it is not
218 followed by type information. @xref{Constants}.
220 Type information is either a @var{type_number}, or a
221 @samp{@var{type_number}=}. The @var{type_number} alone is a type
222 reference, referring directly to a type that has already been defined.
224 The @samp{@var{type_number}=} is a type definition, where the number
225 represents a new type which is about to be defined. The type definition
226 may refer to other types by number, and those type numbers may be
227 followed by @samp{=} and nested definitions.
229 In a type definition, if the character that follows the equals sign is
230 non-numeric then it is a @var{type_descriptor}, and tells what kind of
231 type is about to be defined. Any other values following the
232 @var{type_descriptor} vary, depending on the @var{type_descriptor}. If
233 a number follows the @samp{=} then the number is a @var{type_reference}.
234 This is described more thoroughly in the section on types. @xref{Type
235 Descriptors,,Table D: Type Descriptors}, for a list of
236 @var{type_descriptor} values.
238 There is an AIX extension for type attributes. Following the @samp{=}
239 is any number of type attributes. Each one starts with @samp{@@} and
240 ends with @samp{;}. Debuggers, including AIX's dbx, skip any type
241 attributes they do not recognize. GDB 4.9 does not do this---it will
242 ignore the entire symbol containing a type attribute. Hopefully this
243 will be fixed in the next GDB release. Because of a conflict with C++
244 (@pxref{Cplusplus}), new attributes should not be defined which begin
245 with a digit, @samp{(}, or @samp{-}; GDB may be unable to distinguish
246 those from the C++ type descriptor @samp{@@}. The attributes are:
249 @item a@var{boundary}
250 @var{boundary} is an integer specifying the alignment. I assume it
251 applies to all variables of this type.
254 Size in bits of a variable of this type.
257 Pointer class (for checking). Not sure what this means, or how
258 @var{integer} is interpreted.
261 Indicate this is a packed type, meaning that structure fields or array
262 elements are placed more closely in memory, to save memory at the
266 All this can make the @code{"@var{string}"} field quite long. All
267 versions of GDB, and some versions of DBX, can handle arbitrarily long
268 strings. But many versions of DBX cretinously limit the strings to
269 about 80 characters, so compilers which must work with such DBX's need
270 to split the @code{.stabs} directive into several @code{.stabs}
271 directives. Each stab duplicates exactly all but the
272 @code{"@var{string}"} field. The @code{"@var{string}"} field of
273 every stab except the last is marked as continued with a
274 double-backslash at the end. Removing the backslashes and concatenating
275 the @code{"@var{string}"} fields of each stab produces the original,
279 @section A simple example in C source
281 To get the flavor of how stabs describe source information for a C
282 program, let's look at the simple program:
287 printf("Hello world");
291 When compiled with @samp{-g}, the program above yields the following
292 @file{.s} file. Line numbers have been added to make it easier to refer
293 to parts of the @file{.s} file in the description of the stabs that
297 @section The simple example at the assembly level
301 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
302 3 .stabs "hello.c",100,0,0,Ltext0
305 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
306 7 .stabs "char:t2=r2;0;127;",128,0,0,0
307 8 .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
308 9 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
309 10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
310 11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
311 12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
312 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
313 14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
314 15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
315 16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
316 17 .stabs "float:t12=r1;4;0;",128,0,0,0
317 18 .stabs "double:t13=r1;8;0;",128,0,0,0
318 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
319 20 .stabs "void:t15=15",128,0,0,0
322 23 .ascii "Hello, world!\12\0"
337 38 sethi %hi(LC0),%o1
338 39 or %o1,%lo(LC0),%o0
349 50 .stabs "main:F1",36,0,0,_main
350 51 .stabn 192,0,0,LBB2
351 52 .stabn 224,0,0,LBE2
354 This simple ``hello world'' example demonstrates several of the stab
355 types used to describe C language source files.
357 @node Program structure
358 @chapter Encoding for the structure of the program
361 * Source Files:: The path and name of the source file
368 @section The path and name of the source files
370 Before any other stabs occur, there must be a stab specifying the source
371 file. This information is contained in a symbol of stab type
372 @code{N_SO}; the string contains the name of the file. The value of the
373 symbol is the start address of portion of the text section corresponding
376 With the Sun Solaris2 compiler, the @code{desc} field contains a
377 source-language code.
379 Some compilers (for example, gcc2 and SunOS4 @file{/bin/cc}) also
380 include the directory in which the source was compiled, in a second
381 @code{N_SO} symbol preceding the one containing the file name. This
382 symbol can be distinguished by the fact that it ends in a slash. Code
383 from the cfront C++ compiler can have additional @code{N_SO} symbols for
384 nonexistent source files after the @code{N_SO} for the real source file;
385 these are believed to contain no useful information.
390 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0 ; 100 is N_SO
391 .stabs "hello.c",100,0,0,Ltext0
396 Instead of @code{N_SO} symbols, XCOFF uses a @code{.file} assembler
397 directive which assembles to a standard COFF @code{.file} symbol;
398 explaining this in detail is outside the scope of this document.
400 There are several different schemes for dealing with include files: the
401 traditional @code{N_SOL} approach, Sun's @code{N_BINCL} scheme, and the
402 XCOFF @code{C_BINCL} (which despite the similar name has little in
403 common with @code{N_BINCL}).
405 An @code{N_SOL} symbol specifies which include file subsequent symbols
406 refer to. The string field is the name of the file and the value is the
407 text address corresponding to the start of the previous include file and
408 the start of this one. To specify the main source file again, use an
409 @code{N_SOL} symbol with the name of the main source file.
411 A @code{N_BINCL} symbol specifies the start of an include file. In an
412 object file, only the name is significant. The Sun linker puts data
413 into some of the other fields. The end of the include file is marked by
414 a @code{N_EINCL} symbol (which has no name field). In an ojbect file,
415 there is no significant data in the @code{N_EINCL} symbol; the Sun
416 linker puts data into some of the fields. @code{N_BINCL} and
417 @code{N_EINCL} can be nested. If the linker detects that two source
418 files have identical stabs with a @code{N_BINCL} and @code{N_EINCL} pair
419 (as will generally be the case for a header file), then it only puts out
420 the stabs once. Each additional occurance is replaced by an
421 @code{N_EXCL} symbol. I believe the Sun (SunOS4, not sure about
422 Solaris) linker is the only one which supports this feature.
424 For the start of an include file in XCOFF, use the @file{.bi} assembler
425 directive which generates a @code{C_BINCL} symbol. A @file{.ei}
426 directive, which generates a @code{C_EINCL} symbol, denotes the end of
427 the include file. Both directives are followed by the name of the
428 source file in quotes, which becomes the string for the symbol. The
429 value of each symbol, produced automatically by the assembler and
430 linker, is an offset into the executable which points to the beginning
431 (inclusive, as you'd expect) and end (inclusive, as you would not
432 expect) of the portion of the COFF linetable which corresponds to this
433 include file. @code{C_BINCL} and @code{C_EINCL} do not nest.
436 @section Line Numbers
438 A @code{N_SLINE} symbol represents the start of a source line. The
439 @var{desc} field contains the line number and the @var{value} field
440 contains the code address for the start of that source line.
442 GNU documents @code{N_DSLINE} and @code{N_BSLINE} symbols for line
443 numbers in the data or bss segments, respectively. They are identical
444 to @code{N_SLINE} but are relocated differently by the linker. They
445 were intended to be used to describe the source location of a variable
446 declaration, but I believe that gcc2 actually puts the line number in
447 the desc field of the stab for the variable itself. GDB has been
448 ignoring these symbols (unless they contain a string field) at least
451 XCOFF uses COFF line numbers instead, which are outside the scope of
452 this document, ammeliorated by adequate marking of include files
453 (@pxref{Source Files}).
455 For single source lines that generate discontiguous code, such as flow
456 of control statements, there may be more than one line number entry for
457 the same source line. In this case there is a line number entry at the
458 start of each code range, each with the same line number.
463 All of the following stabs use the @samp{N_FUN} symbol type.
465 A function is represented by a @samp{F} symbol descriptor for a global
466 (extern) function, and @samp{f} for a static (local) function. The next
467 @samp{N_SLINE} symbol can be used to find the line number of the start
468 of the function. The value field is the address of the start of the
469 function. The type information of the stab represents the return type
470 of the function; thus @samp{foo:f5} means that foo is a function
473 The type information of the stab is optionally followed by type
474 information for each argument, with each argument preceded by @samp{;}.
475 An argument type of 0 means that additional arguments are being passed,
476 whose types and number may vary (@samp{...} in ANSI C). This extension
477 is used by Sun's Solaris compiler. GDB has tolerated it (i.e. at least
478 parsed the syntax, if not necessarily used the information) at least
479 since version 4.8; I don't know whether all versions of dbx will
480 tolerate it. The argument types given here are not merely redundant
481 with the symbols for the arguments themselves (@pxref{Parameters}), they
482 are the types of the arguments as they are passed, before any
483 conversions might take place. For example, if a C function which is
484 declared without a prototype takes a @code{float} argument, the value is
485 passed as a @code{double} but then converted to a @code{float}.
486 Debuggers need to use the types given in the arguments when printing
487 values, but if calling the function they need to use the types given in
488 the symbol defining the function.
490 If the return type and types of arguments of a function which is defined
491 in another source file are specified (i.e. a function prototype in ANSI
492 C), traditionally compilers emit no stab; the only way for the debugger
493 to find the information is if the source file where the function is
494 defined was also compiled with debugging symbols. As an extension the
495 Solaris compiler uses symbol descriptor @samp{P} followed by the return
496 type of the function, followed by the arguments, each preceded by
497 @samp{;}, as in a stab with symbol descriptor @samp{f} or @samp{F}.
498 This use of symbol descriptor @samp{P} can be distinguished from its use
499 for register parameters (@pxref{Parameters}) by the fact that it has
500 symbol type @code{N_FUN}.
502 The AIX documentation also defines symbol descriptor @samp{J} as an
503 internal function. I assume this means a function nested within another
504 function. It also says Symbol descriptor @samp{m} is a module in
505 Modula-2 or extended Pascal.
507 Procedures (functions which do not return values) are represented as
508 functions returning the void type in C. I don't see why this couldn't
509 be used for all languages (inventing a void type for this purpose if
510 necessary), but the AIX documentation defines @samp{I}, @samp{P}, and
511 @samp{Q} for internal, global, and static procedures, respectively.
512 These symbol descriptors are unusual in that they are not followed by
515 For any of the above symbol descriptors, after the symbol descriptor and
516 the type information, there is optionally a comma, followed by the name
517 of the procedure, followed by a comma, followed by a name specifying the
518 scope. The first name is local to the scope specified. I assume then
519 that the name of the symbol (before the @samp{:}), if specified, is some
520 sort of global name. I assume the name specifying the scope is the name
521 of a function specifying that scope. This feature is an AIX extension,
522 and this information is based on the manual; I haven't actually tried
525 The stab representing a procedure is located immediately following the
526 code of the procedure. This stab is in turn directly followed by a
527 group of other stabs describing elements of the procedure. These other
528 stabs describe the procedure's parameters, its block local variables and
536 The @code{.stabs} entry after this code fragment shows the @var{name} of
537 the procedure (@code{main}); the type descriptor @var{desc} (@code{F},
538 for a global procedure); a reference to the predefined type @code{int}
539 for the return type; and the starting @var{address} of the procedure.
541 Here is an exploded summary (with whitespace introduced for clarity),
542 followed by line 50 of our sample assembly output, which has this form:
546 @var{desc} @r{(global proc @samp{F})}
547 @var{return_type_ref} @r{(int)}
553 50 .stabs "main:F1",36,0,0,_main
556 @node Block Structure
557 @section Block Structure
563 @code{N_LBRAC}, @code{N_RBRAC}
566 The program's block structure is represented by the @code{N_LBRAC} (left
567 brace) and the @code{N_RBRAC} (right brace) stab types. The following code
568 range, which is the body of @code{main}, is labeled with @samp{LBB2:} at the
569 beginning and @samp{LBE2:} at the end.
573 38 sethi %hi(LC0),%o1
574 39 or %o1,%lo(LC0),%o0
582 The @code{N_LBRAC} and @code{N_RBRAC} stabs that describe the block
583 scope of the procedure are located after the @code{N_FUNC} stab that
584 represents the procedure itself. The @code{N_LBRAC} uses the
585 @code{LBB2} label as the code address in its value field, and the
586 @code{N_RBRAC} uses @code{LBE2}.
589 50 .stabs "main:F1",36,0,0,_main
593 .stabn N_LBRAC, NIL, NIL, @var{left-brace-address}
594 .stabn N_RBRAC, NIL, NIL, @var{right-brace-address}
598 51 .stabn 192,0,0,LBB2
599 52 .stabn 224,0,0,LBE2
605 The @samp{c} symbol descriptor indicates that this stab represents a
606 constant. This symbol descriptor is an exception to the general rule
607 that symbol descriptors are followed by type information. Instead, it
608 is followed by @samp{=} and one of the following:
612 Boolean constant. @var{value} is a numeric value; I assume it is 0 for
616 Character constant. @var{value} is the numeric value of the constant.
618 @item e @var{type-information} , @var{value}
619 Constant whose value can be represented as integral.
620 @var{type-information} is the type of the constant, as it would appear
621 after a symbol descriptor (@pxref{Stabs Format}). @var{value} is the
622 numeric value of the constant. GDB 4.9 does not actually get the right
623 value if @var{value} does not fit in a host @code{int}, but it does not
624 do anything violent, and future debuggers could be extended to accept
625 integers of any size (whether unsigned or not). This constant type is
626 usually documented as being only for enumeration constants, but GDB has
627 never imposed that restriction; I don't know about other debuggers.
630 Integer constant. @var{value} is the numeric value. The type is some
631 sort of generic integer type (for GDB, a host @code{int}); to specify
632 the type explicitly, use @samp{e} instead.
635 Real constant. @var{value} is the real value, which can be @samp{INF}
636 (optionally preceded by a sign) for infinity, @samp{QNAN} for a quiet
637 NaN (not-a-number), or @samp{SNAN} for a signalling NaN. If it is a
638 normal number the format is that accepted by the C library function
642 String constant. @var{string} is a string enclosed in either @samp{'}
643 (in which case @samp{'} characters within the string are represented as
644 @samp{\'} or @samp{"} (in which case @samp{"} characters within the
645 string are represented as @samp{\"}).
647 @item S @var{type-information} , @var{elements} , @var{bits} , @var{pattern}
648 Set constant. @var{type-information} is the type of the constant, as it
649 would appear after a symbol descriptor (@pxref{Stabs Format}).
650 @var{elements} is the number of elements in the set (Does this means
651 how many bits of @var{pattern} are actually used, which would be
652 redundant with the type, or perhaps the number of bits set in
653 @var{pattern}? I don't get it), @var{bits} is the number of bits in the
654 constant (meaning it specifies the length of @var{pattern}, I think),
655 and @var{pattern} is a hexadecimal representation of the set. AIX
656 documentation refers to a limit of 32 bytes, but I see no reason why
657 this limit should exist. This form could probably be used for arbitrary
658 constants, not just sets; the only catch is that @var{pattern} should be
659 understood to be target, not host, byte order and format.
662 The boolean, character, string, and set constants are not supported by
663 GDB 4.9, but it will ignore them. GDB 4.8 and earlier gave an error
664 message and refused to read symbols from the file containing the
667 This information is followed by @samp{;}.
670 @chapter A Comprehensive Example in C
672 Now we'll examine a second program, @code{example2}, which builds on the
673 first example to introduce the rest of the stab types, symbol
674 descriptors, and type descriptors used in C.
675 @xref{Example2.c} for the complete @file{.c} source,
676 and @pxref{Example2.s} for the @file{.s} assembly code.
677 This description includes parts of those files.
679 @section Flow of control and nested scopes
685 @code{N_SLINE}, @code{N_LBRAC}, @code{N_RBRAC} (cont.)
688 Consider the body of @code{main}, from @file{example2.c}. It shows more
689 about how @code{N_SLINE}, @code{N_RBRAC}, and @code{N_LBRAC} stabs are used.
693 21 static float s_flap;
695 23 for (times=0; times < s_g_repeat; times++)@{
697 25 printf ("Hello world\n");
702 Here we have a single source line, the @samp{for} line, that generates
703 non-linear flow of control, and non-contiguous code. In this case, an
704 @code{N_SLINE} stab with the same line number proceeds each block of
705 non-contiguous code generated from the same source line.
707 The example also shows nested scopes. The @code{N_LBRAC} and
708 @code{N_LBRAC} stabs that describe block structure are nested in the
709 same order as the corresponding code blocks, those of the for loop
710 inside those for the body of main.
713 This is the label for the @code{N_LBRAC} (left brace) stab marking the
714 start of @code{main}.
721 In the first code range for C source line 23, the @code{for} loop
722 initialize and test, @code{N_SLINE} (68) records the line number:
729 58 .stabn 68,0,23,LM2
733 62 sethi %hi(_s_g_repeat),%o0
735 64 ld [%o0+%lo(_s_g_repeat)],%o0
740 @exdent label for the @code{N_LBRAC} (start block) marking the start of @code{for} loop
743 69 .stabn 68,0,25,LM3
745 71 sethi %hi(LC0),%o1
746 72 or %o1,%lo(LC0),%o0
749 75 .stabn 68,0,26,LM4
752 @exdent label for the @code{N_RBRAC} (end block) stab marking the end of the @code{for} loop
758 Now we come to the second code range for source line 23, the @code{for}
759 loop increment and return. Once again, @code{N_SLINE} (68) records the
763 .stabn, N_SLINE, NIL,
767 78 .stabn 68,0,23,LM5
775 86 .stabn 68,0,27,LM6
778 @exdent label for the @code{N_RBRAC} (end block) stab marking the end of the @code{for} loop
781 89 .stabn 68,0,27,LM7
786 94 .stabs "main:F1",36,0,0,_main
787 95 .stabs "argc:p1",160,0,0,68
788 96 .stabs "argv:p20=*21=*2",160,0,0,72
789 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
790 98 .stabs "times:1",128,0,0,-20
794 Here is an illustration of stabs describing nested scopes. The scope
795 nesting is reflected in the nested bracketing stabs (@code{N_LBRAC},
799 .stabn N_LBRAC,NIL,NIL,
800 @var{block-start-address}
802 99 .stabn 192,0,0,LBB2 ## begin proc label
803 100 .stabs "inner:1",128,0,0,-24
804 101 .stabn 192,0,0,LBB3 ## begin for label
808 @code{N_RBRAC} (224), ``right brace'' ends a lexical block (scope).
811 .stabn N_RBRAC,NIL,NIL,
812 @var{block-end-address}
814 102 .stabn 224,0,0,LBE3 ## end for label
815 103 .stabn 224,0,0,LBE2 ## end proc label
822 * Automatic variables:: locally scoped
824 * Register variables::
825 * Initialized statics::
826 * Un-initialized statics::
830 @node Automatic variables
831 @section Locally scoped automatic variables
838 @item Symbol Descriptor:
842 In addition to describing types, the @code{N_LSYM} stab type also
843 describes locally scoped automatic variables. Refer again to the body
844 of @code{main} in @file{example2.c}. It allocates two automatic
845 variables: @samp{times} is scoped to the body of @code{main}, and
846 @samp{inner} is scoped to the body of the @code{for} loop.
847 @samp{s_flap} is locally scoped but not automatic, and will be discussed
852 21 static float s_flap;
854 23 for (times=0; times < s_g_repeat; times++)@{
856 25 printf ("Hello world\n");
861 The @code{N_LSYM} stab for an automatic variable is located just before the
862 @code{N_LBRAC} stab describing the open brace of the block to which it is
866 @exdent @code{N_LSYM} (128): automatic variable, scoped locally to @code{main}
869 @var{type information}",
871 @var{frame-pointer-offset}
873 98 .stabs "times:1",128,0,0,-20
874 99 .stabn 192,0,0,LBB2 ## begin `main' N_LBRAC
876 @exdent @code{N_LSYM} (128): automatic variable, scoped locally to the @code{for} loop
879 @var{type information}",
881 @var{frame-pointer-offset}
883 100 .stabs "inner:1",128,0,0,-24
884 101 .stabn 192,0,0,LBB3 ## begin `for' loop N_LBRAC
887 The symbol descriptor is omitted for automatic variables. Since type
888 information should being with a digit, @samp{-}, or @samp{(}, only
889 digits, @samp{-}, and @samp{(} are precluded from being used for symbol
890 descriptors by this fact. However, the Acorn RISC machine (ARM) is said
891 to get this wrong: it puts out a mere type definition here, without the
892 preceding @code{@var{typenumber}=}. This is a bad idea; there is no
893 guarantee that type descriptors are distinct from symbol descriptors.
895 @node Global Variables
896 @section Global Variables
903 @item Symbol Descriptor:
907 Global variables are represented by the @code{N_GSYM} stab type. The symbol
908 descriptor, following the colon in the string field, is @samp{G}. Following
909 the @samp{G} is a type reference or type definition. In this example it is a
910 type reference to the basic C type, @code{char}. The first source line in
918 yields the following stab. The stab immediately precedes the code that
919 allocates storage for the variable it describes.
922 @exdent @code{N_GSYM} (32): global symbol
927 N_GSYM, NIL, NIL, NIL
929 21 .stabs "g_foo:G2",32,0,0,0
936 The address of the variable represented by the @code{N_GSYM} is not contained
937 in the @code{N_GSYM} stab. The debugger gets this information from the
938 external symbol for the global variable.
940 @node Register variables
941 @section Register variables
943 @c According to an old version of this manual, AIX uses C_RPSYM instead
944 @c of C_RSYM. I am skeptical; this should be verified.
945 Register variables have their own stab type, @code{N_RSYM}, and their
946 own symbol descriptor, @code{r}. The stab's value field contains the
947 number of the register where the variable data will be stored.
949 The value is the register number.
951 AIX defines a separate symbol descriptor @samp{d} for floating point
952 registers. This seems incredibly stupid---why not just just give
953 floating point registers different register numbers? I have not
954 verified whether the compiler actually uses @samp{d}.
956 If the register is explicitly allocated to a global variable, but not
960 register int g_bar asm ("%g5");
963 the stab may be emitted at the end of the object file, with
964 the other bss symbols.
966 @node Initialized statics
967 @section Initialized static variables
974 @item Symbol Descriptors:
975 @code{S} (file scope), @code{V} (procedure scope)
978 Initialized static variables are represented by the @code{N_STSYM} stab
979 type. The symbol descriptor part of the string field shows if the
980 variable is file scope static (@samp{S}) or procedure scope static
981 (@samp{V}). The source line
984 3 static int s_g_repeat = 2;
988 yields the following code. The stab is located immediately preceding
989 the storage for the variable it represents. Since the variable in
990 this example is file scope static the symbol descriptor is @samp{S}.
993 @exdent @code{N_STSYM} (38): initialized static variable (data seg w/internal linkage)
1001 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
1008 @node Un-initialized statics
1009 @section Un-initialized static variables
1016 @item Symbol Descriptors:
1017 @code{S} (file scope), @code{V} (procedure scope)
1020 Un-initialized static variables are represented by the @code{N_LCSYM}
1021 stab type. The symbol descriptor part of the string shows if the
1022 variable is file scope static (@samp{S}) or procedure scope static
1023 (@samp{V}). In this example it is procedure scope static. The source
1024 line allocating @code{s_flap} immediately follows the open brace for the
1025 procedure @code{main}.
1029 21 static float s_flap;
1032 The code that reserves storage for the variable @code{s_flap} precedes the
1033 body of body of @code{main}.
1036 39 .reserve _s_flap.0,4,"bss",4
1039 But since @code{s_flap} is scoped locally to @code{main}, its stab is
1040 located with the other stabs representing symbols local to @code{main}.
1041 The stab for @code{s_flap} is located just before the @code{N_LBRAC} for
1045 @exdent @code{N_LCSYM} (40): uninitialized static var (BSS seg w/internal linkage)
1053 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
1054 98 .stabs "times:1",128,0,0,-20
1055 99 .stabn 192,0,0,LBB2 # N_LBRAC for main.
1058 @c ............................................................
1063 The symbol descriptor @samp{p} is used to refer to parameters which are
1064 in the arglist. Symbols have symbol type @samp{N_PSYM}. The value of
1065 the symbol is the offset relative to the argument list.
1067 If the parameter is passed in a register, then the traditional way to do
1068 this is to provide two symbols for each argument:
1071 .stabs "arg:p1" . . . ; N_PSYM
1072 .stabs "arg:r1" . . . ; N_RSYM
1075 Debuggers are expected to use the second one to find the value, and the
1076 first one to know that it is an argument.
1078 Because this is kind of ugly, some compilers use symbol descriptor
1079 @samp{P} or @samp{R} to indicate an argument which is in a register.
1080 The symbol value is the register number. @samp{P} and @samp{R} mean the
1081 same thing, the difference is that @samp{P} is a GNU invention and
1082 @samp{R} is an IBM (xcoff) invention. As of version 4.9, GDB should
1083 handle either one. Symbol type @samp{C_RPSYM} is used with @samp{R} and
1084 @samp{N_RSYM} is used with @samp{P}.
1086 AIX, according to the documentation, uses @samp{D} for a parameter
1087 passed in a floating point register. This strikes me as incredibly
1088 bogus---why doesn't it just use @samp{R} with a register number which
1089 indicates that it's a floating point register? I haven't verified
1090 whether the system actually does what the documentation indicates.
1092 There is at least one case where GCC uses a @samp{p}/@samp{r} pair
1093 rather than @samp{P}; this is where the argument is passed in the
1094 argument list and then loaded into a register.
1096 On the sparc and hppa, for a @samp{P} symbol whose type is a structure
1097 or union, the register contains the address of the structure. On the
1098 sparc, this is also true of a @samp{p}/@samp{r} pair (using Sun cc) or a
1099 @samp{p} symbol. However, if a (small) structure is really in a
1100 register, @samp{r} is used. And, to top it all off, on the hppa it
1101 might be a structure which was passed on the stack and loaded into a
1102 register and for which there is a @samp{p}/@samp{r} pair! I believe
1103 that symbol descriptor @samp{i} is supposed to deal with this case, (it
1104 is said to mean "value parameter by reference, indirect access", I don't
1105 know the source for this information) but I don't know details or what
1106 compilers or debuggers use it, if any (not GDB or GCC). It is not clear
1107 to me whether this case needs to be dealt with differently than
1108 parameters passed by reference (see below).
1110 There is another case similar to an argument in a register, which is an
1111 argument which is actually stored as a local variable. Sometimes this
1112 happens when the argument was passed in a register and then the compiler
1113 stores it as a local variable. If possible, the compiler should claim
1114 that it's in a register, but this isn't always done. Some compilers use
1115 the pair of symbols approach described above ("arg:p" followed by
1116 "arg:"); this includes gcc1 (not gcc2) on the sparc when passing a small
1117 structure and gcc2 (sometimes) when the argument type is float and it is
1118 passed as a double and converted to float by the prologue (in the latter
1119 case the type of the "arg:p" symbol is double and the type of the "arg:"
1120 symbol is float). GCC, at least on the 960, uses a single @samp{p}
1121 symbol descriptor for an argument which is stored as a local variable
1122 but uses @samp{N_LSYM} instead of @samp{N_PSYM}. In this case the value
1123 of the symbol is an offset relative to the local variables for that
1124 function, not relative to the arguments (on some machines those are the
1125 same thing, but not on all).
1127 If the parameter is passed by reference (e.g. Pascal VAR parameters),
1128 then type symbol descriptor is @samp{v} if it is in the argument list,
1129 or @samp{a} if it in a register. Other than the fact that these contain
1130 the address of the parameter other than the parameter itself, they are
1131 identical to @samp{p} and @samp{R}, respectively. I believe @samp{a} is
1132 an AIX invention; @samp{v} is supported by all stabs-using systems as
1135 @c Is this paragraph correct? It is based on piecing together patchy
1136 @c information and some guesswork
1137 Conformant arrays refer to a feature of Modula-2, and perhaps other
1138 languages, in which the size of an array parameter is not known to the
1139 called function until run-time. Such parameters have two stabs, a
1140 @samp{x} for the array itself, and a @samp{C}, which represents the size
1141 of the array. The value of the @samp{x} stab is the offset in the
1142 argument list where the address of the array is stored (it this right?
1143 it is a guess); the value of the @samp{C} stab is the offset in the
1144 argument list where the size of the array (in elements? in bytes?) is
1147 The following are also said to go with @samp{N_PSYM}:
1150 "name" -> "param_name:#type"
1152 -> pF FORTRAN function parameter
1153 -> X (function result variable)
1154 -> b (based variable)
1156 value -> offset from the argument pointer (positive).
1159 As a simple example, the code
1171 .stabs "main:F1",36,0,0,_main ; 36 is N_FUN
1172 .stabs "argc:p1",160,0,0,68 ; 160 is N_PSYM
1173 .stabs "argv:p20=*21=*2",160,0,0,72
1176 The type definition of argv is interesting because it contains several
1177 type definitions. Type 21 is pointer to type 2 (char) and argv (type 20) is
1181 @chapter Type Definitions
1183 Now let's look at some variable definitions involving complex types.
1184 This involves understanding better how types are described. In the
1185 examples so far types have been described as references to previously
1186 defined types or defined in terms of subranges of or pointers to
1187 previously defined types. The section that follows will talk about
1188 the various other type descriptors that may follow the = sign in a
1192 * Builtin types:: Integers, floating point, void, etc.
1193 * Miscellaneous Types:: Pointers, sets, files, etc.
1194 * Cross-references:: Referring to a type not yet defined.
1195 * Subranges:: A type with a specific range.
1196 * Arrays:: An aggregate type of same-typed elements.
1197 * Strings:: Like an array but also has a length.
1198 * Enumerations:: Like an integer but the values have names.
1199 * Structures:: An aggregate type of different-typed elements.
1200 * Typedefs:: Giving a type a name.
1201 * Unions:: Different types sharing storage.
1206 @section Builtin types
1208 Certain types are built in (@code{int}, @code{short}, @code{void},
1209 @code{float}, etc.); the debugger recognizes these types and knows how
1210 to handle them. Thus don't be surprised if some of the following ways
1211 of specifying builtin types do not specify everything that a debugger
1212 would need to know about the type---in some cases they merely specify
1213 enough information to distinguish the type from other types.
1215 The traditional way to define builtin types is convolunted, so new ways
1216 have been invented to describe them. Sun's ACC uses the @samp{b} and
1217 @samp{R} type descriptors, and IBM uses negative type numbers. GDB can
1218 accept all three, as of version 4.8; dbx just accepts the traditional
1219 builtin types and perhaps one of the other two formats.
1222 * Traditional Builtin Types:: Put on your seatbelts and prepare for kludgery
1223 * Builtin Type Descriptors:: Builtin types with special type descriptors
1224 * Negative Type Numbers:: Builtin types using negative type numbers
1227 @node Traditional Builtin Types
1228 @subsection Traditional Builtin types
1230 Often types are defined as subranges of themselves. If the array bounds
1231 can fit within an @code{int}, then they are given normally. For example:
1234 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0 ; 128 is N_LSYM
1235 .stabs "char:t2=r2;0;127;",128,0,0,0
1238 Builtin types can also be described as subranges of @code{int}:
1241 .stabs "unsigned short:t6=r1;0;65535;",128,0,0,0
1244 If the lower bound of a subrange is 0 and the upper bound is -1, it
1245 means that the type is an unsigned integral type whose bounds are too
1246 big to describe in an int. Traditionally this is only used for
1247 @code{unsigned int} and @code{unsigned long}; GCC also sometimes uses it
1248 for @code{long long} and @code{unsigned long long}, and the only way to
1249 tell those types apart is to look at their names. On other machines GCC
1250 puts out bounds in octal, with a leading 0. In this case a negative
1251 bound consists of a number which is a 1 bit followed by a bunch of 0
1252 bits, and a positive bound is one in which a bunch of bits are 1.
1255 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
1256 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
1259 If the lower bound of a subrange is 0 and the upper bound is negative,
1260 it means that it is an unsigned integral type whose size in bytes is the
1261 absolute value of the upper bound. I believe this is a Convex
1262 convention for @code{unsigned long long}.
1264 If the lower bound of a subrange is negative and the upper bound is 0,
1265 it means that the type is a signed integral type whose size in bytes is
1266 the absolute value of the lower bound. I believe this is a Convex
1267 convention for @code{long long}. To distinguish this from a legitimate
1268 subrange, the type should be a subrange of itself. I'm not sure whether
1269 this is the case for Convex.
1271 If the upper bound of a subrange is 0, it means that this is a floating
1272 point type, and the lower bound of the subrange indicates the number of
1276 .stabs "float:t12=r1;4;0;",128,0,0,0
1277 .stabs "double:t13=r1;8;0;",128,0,0,0
1280 However, GCC writes @code{long double} the same way it writes
1281 @code{double}; the only way to distinguish them is by the name:
1284 .stabs "long double:t14=r1;8;0;",128,0,0,0
1287 Complex types are defined the same way as floating-point types; the only
1288 way to distinguish a single-precision complex from a double-precision
1289 floating-point type is by the name.
1291 The C @code{void} type is defined as itself:
1294 .stabs "void:t15=15",128,0,0,0
1297 I'm not sure how a boolean type is represented.
1299 @node Builtin Type Descriptors
1300 @subsection Defining Builtin Types using Builtin Type Descriptors
1302 There are various type descriptors to define builtin types:
1305 @c FIXME: clean up description of width and offset, once we figure out
1307 @item b @var{signed} @var{char-flag} @var{width} ; @var{offset} ; @var{nbits} ;
1308 Define an integral type. @var{signed} is @samp{u} for unsigned or
1309 @samp{s} for signed. @var{char-flag} is @samp{c} which indicates this
1310 is a character type, or is omitted. I assume this is to distinguish an
1311 integral type from a character type of the same size, for example it
1312 might make sense to set it for the C type @code{wchar_t} so the debugger
1313 can print such variables differently (Solaris does not do this). Sun
1314 sets it on the C types @code{signed char} and @code{unsigned char} which
1315 arguably is wrong. @var{width} and @var{offset} appear to be for small
1316 objects stored in larger ones, for example a @code{short} in an
1317 @code{int} register. @var{width} is normally the number of bytes in the
1318 type. @var{offset} seems to always be zero. @var{nbits} is the number
1319 of bits in the type.
1321 Note that type descriptor @samp{b} used for builtin types conflicts with
1322 its use for Pascal space types (@pxref{Miscellaneous Types}); they can
1323 be distinguished because the character following the type descriptor
1324 will be a digit, @samp{(}, or @samp{-} for a Pascal space type, or
1325 @samp{u} or @samp{s} for a builtin type.
1328 Documented by AIX to define a wide character type, but their compiler
1329 actually uses negative type numbers (@pxref{Negative Type Numbers}).
1331 @item R @var{fp_type} ; @var{bytes} ;
1332 Define a floating point type. @var{fp_type} has one of the following values:
1336 IEEE 32-bit (single precision) floating point format.
1339 IEEE 64-bit (double precision) floating point format.
1341 @item 3 (NF_COMPLEX)
1342 @item 4 (NF_COMPLEX16)
1343 @item 5 (NF_COMPLEX32)
1344 @c "GDB source" really means @file{include/aout/stab_gnu.h}, but trying
1345 @c to put that here got an overfull hbox.
1346 These are for complex numbers. A comment in the GDB source describes
1347 them as Fortran complex, double complex, and complex*16, respectively,
1348 but what does that mean? (i.e. Single precision? Double precison?).
1350 @item 6 (NF_LDOUBLE)
1351 Long double. This should probably only be used for Sun format long
1352 double, and new codes should be used for other floating point formats
1353 (NF_DOUBLE can be used if a long double is really just an IEEE double,
1357 @var{bytes} is the number of bytes occupied by the type. This allows a
1358 debugger to perform some operations with the type even if it doesn't
1359 understand @var{fp_code}.
1361 @item g @var{type-information} ; @var{nbits}
1362 Documented by AIX to define a floating type, but their compiler actually
1363 uses negative type numbers (@pxref{Negative Type Numbers}).
1365 @item c @var{type-information} ; @var{nbits}
1366 Documented by AIX to define a complex type, but their compiler actually
1367 uses negative type numbers (@pxref{Negative Type Numbers}).
1370 The C @code{void} type is defined as a signed integral type 0 bits long:
1372 .stabs "void:t19=bs0;0;0",128,0,0,0
1374 The Solaris compiler seems to omit the trailing semicolon in this case.
1375 Getting sloppy in this way is not a swift move because if a type is
1376 embedded in a more complex expression it is necessary to be able to tell
1379 I'm not sure how a boolean type is represented.
1381 @node Negative Type Numbers
1382 @subsection Negative Type numbers
1384 Since the debugger knows about the builtin types anyway, the idea of
1385 negative type numbers is simply to give a special type number which
1386 indicates the built in type. There is no stab defining these types.
1388 I'm not sure whether anyone has tried to define what this means if
1389 @code{int} can be other than 32 bits (or other types can be other than
1390 their customary size). If @code{int} has exactly one size for each
1391 architecture, then it can be handled easily enough, but if the size of
1392 @code{int} can vary according the compiler options, then it gets hairy.
1393 I guess the consistent way to do this would be to define separate
1394 negative type numbers for 16-bit @code{int} and 32-bit @code{int};
1395 therefore I have indicated below the customary size (and other format
1396 information) for each type. The information below is currently correct
1397 because AIX on the RS6000 is the only system which uses these type
1398 numbers. If these type numbers start to get used on other systems, I
1399 suspect the correct thing to do is to define a new number in cases where
1400 a type does not have the size and format indicated below.
1402 Also note that part of the definition of the negative type number is
1403 the name of the type. Types with identical size and format but
1404 different names have different negative type numbers.
1408 @code{int}, 32 bit signed integral type.
1411 @code{char}, 8 bit type holding a character. Both GDB and dbx on AIX
1412 treat this as signed. GCC uses this type whether @code{char} is signed
1413 or not, which seems like a bad idea. The AIX compiler (xlc) seems to
1414 avoid this type; it uses -5 instead for @code{char}.
1417 @code{short}, 16 bit signed integral type.
1420 @code{long}, 32 bit signed integral type.
1423 @code{unsigned char}, 8 bit unsigned integral type.
1426 @code{signed char}, 8 bit signed integral type.
1429 @code{unsigned short}, 16 bit unsigned integral type.
1432 @code{unsigned int}, 32 bit unsigned integral type.
1435 @code{unsigned}, 32 bit unsigned integral type.
1438 @code{unsigned long}, 32 bit unsigned integral type.
1441 @code{void}, type indicating the lack of a value.
1444 @code{float}, IEEE single precision.
1447 @code{double}, IEEE double precision.
1450 @code{long double}, IEEE double precision. The compiler claims the size
1451 will increase in a future release, and for binary compatibility you have
1452 to avoid using @code{long double}. I hope when they increase it they
1453 use a new negative type number.
1456 @code{integer}. 32 bit signed integral type.
1459 @code{boolean}. Only one bit is used, not sure about the actual size of the
1463 @code{short real}. IEEE single precision.
1466 @code{real}. IEEE double precision.
1469 @code{stringptr}. @xref{Strings}.
1472 @code{character}, 8 bit unsigned type.
1475 @code{logical*1}, 8 bit unsigned integral type.
1478 @code{logical*2}, 16 bit unsigned integral type.
1481 @code{logical*4}, 32 bit unsigned integral type.
1484 @code{logical}, 32 bit unsigned integral type.
1487 @code{complex}. A complex type consisting of two IEEE single-precision
1488 floating point values.
1491 @code{complex}. A complex type consisting of two IEEE double-precision
1492 floating point values.
1495 @code{integer*1}, 8 bit signed integral type.
1498 @code{integer*2}, 16 bit signed integral type.
1501 @code{integer*4}, 32 bit signed integral type.
1504 @code{wchar}. Wide character, 16 bits wide (Unicode format?). This is
1505 not used for the C type @code{wchar_t}.
1508 @node Miscellaneous Types
1509 @section Miscellaneous Types
1512 @item b @var{type-information} ; @var{bytes}
1513 Pascal space type. This is documented by IBM; what does it mean?
1515 Note that this use of the @samp{b} type descriptor can be distinguished
1516 from its use for builtin integral types (@pxref{Builtin Type
1517 Descriptors}) because the character following the type descriptor is
1518 always a digit, @samp{(}, or @samp{-}.
1520 @item B @var{type-information}
1521 A volatile-qualified version of @var{type-information}. This is a Sun
1522 extension. A volatile-qualified type means that references and stores
1523 to a variable of that type must not be optimized or cached; they must
1524 occur as the user specifies them.
1526 @item d @var{type-information}
1527 File of type @var{type-information}. As far as I know this is only used
1530 @item k @var{type-information}
1531 A const-qualified version of @var{type-information}. This is a Sun
1532 extension. A const-qualified type means that a variable of this type
1535 @item M @var{type-information} ; @var{length}
1536 Multiple instance type. The type seems to composed of @var{length}
1537 repetitions of @var{type-information}, for example @code{character*3} is
1538 represented by @samp{M-2;3}, where @samp{-2} is a reference to a
1539 character type (@pxref{Negative Type Numbers}). I'm not sure how this
1540 differs from an array. This appears to be a FORTRAN feature.
1541 @var{length} is a bound, like those in range types, @xref{Subranges}.
1543 @item S @var{type-information}
1544 Pascal set type. @var{type-information} must be a small type such as an
1545 enumeration or a subrange, and the type is a bitmask whose length is
1546 specified by the number of elements in @var{type-information}.
1548 @item * @var{type-information}
1549 Pointer to @var{type-information}.
1552 @node Cross-references
1553 @section Cross-references to other types
1555 If a type is used before it is defined, one common way to deal with this
1556 is just to use a type reference to a type which has not yet been
1557 defined. The debugger is expected to be able to deal with this.
1559 Another way is with the @samp{x} type descriptor, which is followed by
1560 @samp{s} for a structure tag, @samp{u} for a union tag, or @samp{e} for
1561 a enumerator tag, followed by the name of the tag, followed by @samp{:}.
1562 for example the following C declarations:
1572 .stabs "bar:G16=*17=xsfoo:",32,0,0,0
1575 Not all debuggers support the @samp{x} type descriptor, so on some
1576 machines GCC does not use it. I believe that for the above example it
1577 would just emit a reference to type 17 and never define it, but I
1578 haven't verified that.
1580 Modula-2 imported types, at least on AIX, use the @samp{i} type
1581 descriptor, which is followed by the name of the module from which the
1582 type is imported, followed by @samp{:}, followed by the name of the
1583 type. There is then optionally a comma followed by type information for
1584 the type (This differs from merely naming the type (@pxref{Typedefs}) in
1585 that it identifies the module; I don't understand whether the name of
1586 the type given here is always just the same as the name we are giving
1587 it, or whether this type descriptor is used with a nameless stab
1588 (@pxref{Stabs Format}), or what). The symbol ends with @samp{;}.
1591 @section Subrange types
1593 The @samp{r} type descriptor defines a type as a subrange of another
1594 type. It is followed by type information for the type which it is a
1595 subrange of, a semicolon, an integral lower bound, a semicolon, an
1596 integral upper bound, and a semicolon. The AIX documentation does not
1597 specify the trailing semicolon, in an effort to specify array indexes
1598 more cleanly, but a subrange which is not an array index has always
1599 included a trailing semicolon (@pxref{Arrays}).
1601 Instead of an integer, either bound can be one of the following:
1604 @item A @var{offset}
1605 The bound is passed by reference on the stack at offset @var{offset}
1606 from the argument list. @xref{Parameters}, for more information on such
1609 @item T @var{offset}
1610 The bound is passed by value on the stack at offset @var{offset} from
1613 @item a @var{register-number}
1614 The bound is pased by reference in register number
1615 @var{register-number}.
1617 @item t @var{register-number}
1618 The bound is passed by value in register number @var{register-number}.
1624 Subranges are also used for builtin types, @xref{Traditional Builtin Types}.
1627 @section Array types
1629 Arrays use the @samp{a} type descriptor. Following the type descriptor
1630 is the type of the index and the type of the array elements. If the
1631 index type is a range type, it will end in a semicolon; if it is not a
1632 range type (for example, if it is a type reference), there does not
1633 appear to be any way to tell where the types are separated. In an
1634 effort to clean up this mess, IBM documents the two types as being
1635 separated by a semicolon, and a range type as not ending in a semicolon
1636 (but this is not right for range types which are not array indexes,
1637 @pxref{Subranges}). I think probably the best solution is to specify
1638 that a semicolon ends a range type, and that the index type and element
1639 type of an array are separated by a semicolon, but that if the index
1640 type is a range type, the extra semicolon can be omitted. GDB (at least
1641 through version 4.9) doesn't support any kind of index type other than a
1642 range anyway; I'm not sure about dbx.
1644 It is well established, and widely used, that the type of the index,
1645 unlike most types found in the stabs, is merely a type definition, not
1646 type information (@pxref{Stabs Format}) (that is, it need not start with
1647 @var{type-number}@code{=} if it is defining a new type). According to a
1648 comment in GDB, this is also true of the type of the array elements; it
1649 gives @samp{ar1;1;10;ar1;1;10;4} as a legitimate way to express a two
1650 dimensional array. According to AIX documentation, the element type
1651 must be type information. GDB accepts either.
1653 The type of the index is often a range type, expressed as the letter r
1654 and some parameters. It defines the size of the array. In the example
1655 below, the range @code{r1;0;2;} defines an index type which is a
1656 subrange of type 1 (integer), with a lower bound of 0 and an upper bound
1657 of 2. This defines the valid range of subscripts of a three-element C
1660 For example, the definition
1663 char char_vec[3] = @{'a','b','c'@};
1670 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
1679 If an array is @dfn{packed}, it means that the elements are spaced more
1680 closely than normal, saving memory at the expense of speed. For
1681 example, an array of 3-byte objects might, if unpacked, have each
1682 element aligned on a 4-byte boundary, but if packed, have no padding.
1683 One way to specify that something is packed is with type attributes
1684 (@pxref{Stabs Format}), in the case of arrays another is to use the
1685 @samp{P} type descriptor instead of @samp{a}. Other than specifying a
1686 packed array, @samp{P} is identical to @samp{a}.
1688 @c FIXME-what is it? A pointer?
1689 An open array is represented by the @samp{A} type descriptor followed by
1690 type information specifying the type of the array elements.
1692 @c FIXME: what is the format of this type? A pointer to a vector of pointers?
1693 An N-dimensional dynamic array is represented by
1696 D @var{dimensions} ; @var{type-information}
1699 @c Does dimensions really have this meaning? The AIX documentation
1701 @var{dimensions} is the number of dimensions; @var{type-information}
1702 specifies the type of the array elements.
1704 @c FIXME: what is the format of this type? A pointer to some offsets in
1706 A subarray of an N-dimensional array is represented by
1709 E @var{dimensions} ; @var{type-information}
1712 @c Does dimensions really have this meaning? The AIX documentation
1714 @var{dimensions} is the number of dimensions; @var{type-information}
1715 specifies the type of the array elements.
1720 Some languages, like C or the original Pascal, do not have string types,
1721 they just have related things like arrays of characters. But most
1722 Pascals and various other languages have string types, which are
1723 indicated as follows:
1726 @item n @var{type-information} ; @var{bytes}
1727 @var{bytes} is the maximum length. I'm not sure what
1728 @var{type-information} is; I suspect that it means that this is a string
1729 of @var{type-information} (thus allowing a string of integers, a string
1730 of wide characters, etc., as well as a string of characters). Not sure
1731 what the format of this type is. This is an AIX feature.
1733 @item z @var{type-information} ; @var{bytes}
1734 Just like @samp{n} except that this is a gstring, not an ordinary
1735 string. I don't know the difference.
1738 Pascal Stringptr. What is this? This is an AIX feature.
1742 @section Enumerations
1744 Enumerations are defined with the @samp{e} type descriptor.
1746 @c FIXME: Where does this information properly go? Perhaps it is
1747 @c redundant with something we already explain.
1748 The source line below declares an enumeration type. It is defined at
1749 file scope between the bodies of main and s_proc in example2.c.
1750 The type definition is located after the N_RBRAC that marks the end of
1751 the previous procedure's block scope, and before the N_FUN that marks
1752 the beginning of the next procedure's block scope. Therefore it does not
1753 describe a block local symbol, but a file local one.
1758 enum e_places @{first,second=3,last@};
1762 generates the following stab
1765 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
1768 The symbol descriptor (T) says that the stab describes a structure,
1769 enumeration, or type tag. The type descriptor e, following the 22= of
1770 the type definition narrows it down to an enumeration type. Following
1771 the e is a list of the elements of the enumeration. The format is
1772 name:value,. The list of elements ends with a ;.
1774 There is no standard way to specify the size of an enumeration type; it
1775 is determined by the architecture (normally all enumerations types are
1776 32 bits). There should be a way to specify an enumeration type of
1777 another size; type attributes would be one way to do this @xref{Stabs
1787 @code{N_LSYM} or @code{C_DECL}
1788 @item Symbol Descriptor:
1790 @item Type Descriptor:
1794 The following source code declares a structure tag and defines an
1795 instance of the structure in global scope. Then a typedef equates the
1796 structure tag with a new type. A seperate stab is generated for the
1797 structure tag, the structure typedef, and the structure instance. The
1798 stabs for the tag and the typedef are emited when the definitions are
1799 encountered. Since the structure elements are not initialized, the
1800 stab and code for the structure variable itself is located at the end
1801 of the program in .common.
1807 9 char s_char_vec[8];
1808 10 struct s_tag* s_next;
1811 13 typedef struct s_tag s_typedef;
1814 The structure tag is an N_LSYM stab type because, like the enum, the
1815 symbol is file scope. Like the enum, the symbol descriptor is T, for
1816 enumeration, struct or tag type. The symbol descriptor s following
1817 the 16= of the type definition narrows the symbol type to struct.
1819 Following the struct symbol descriptor is the number of bytes the
1820 struct occupies, followed by a description of each structure element.
1821 The structure element descriptions are of the form name:type, bit
1822 offset from the start of the struct, and number of bits in the
1827 <128> N_LSYM - type definition
1828 .stabs "name:sym_desc(struct tag) Type_def(16)=type_desc(struct type)
1830 elem_name:type_ref(int),bit_offset,field_bits;
1831 elem_name:type_ref(float),bit_offset,field_bits;
1832 elem_name:type_def(17)=type_desc(array)
1833 index_type(range of int from 0 to 7);
1834 element_type(char),bit_offset,field_bits;;",
1837 30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;
1838 s_char_vec:17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
1841 In this example, two of the structure elements are previously defined
1842 types. For these, the type following the name: part of the element
1843 description is a simple type reference. The other two structure
1844 elements are new types. In this case there is a type definition
1845 embedded after the name:. The type definition for the array element
1846 looks just like a type definition for a standalone array. The s_next
1847 field is a pointer to the same kind of structure that the field is an
1848 element of. So the definition of structure type 16 contains an type
1849 definition for an element which is a pointer to type 16.
1852 @section Giving a type a name
1854 To give a type a name, use the @samp{t} symbol descriptor. For example,
1857 .stabs "s_typedef:t16",128,0,0,0
1860 specifies that @code{s_typedef} refers to type number 16. Such stabs
1861 have symbol type @code{N_LSYM} or @code{C_DECL}.
1863 If instead, you are specifying the tag name for a structure, union, or
1864 enumeration, use the @samp{T} symbol descriptor instead. I believe C is
1865 the only language with this feature.
1867 If the type is an opaque type (I believe this is a Modula-2 feature),
1868 AIX provides a type descriptor to specify it. The type descriptor is
1869 @samp{o} and is followed by a name. I don't know what the name
1870 means---is it always the same as the name of the type, or is this type
1871 descriptor used with a nameless stab (@pxref{Stabs Format})? There
1872 optionally follows a comma followed by type information which defines
1873 the type of this type. If omitted, a semicolon is used in place of the
1874 comma and the type information, and, the type is much like a generic
1875 pointer type---it has a known size but little else about it is
1881 Next let's look at unions. In example2 this union type is declared
1882 locally to a procedure and an instance of the union is defined.
1892 This code generates a stab for the union tag and a stab for the union
1893 variable. Both use the N_LSYM stab type. Since the union variable is
1894 scoped locally to the procedure in which it is defined, its stab is
1895 located immediately preceding the N_LBRAC for the procedure's block
1898 The stab for the union tag, however is located preceding the code for
1899 the procedure in which it is defined. The stab type is N_LSYM. This
1900 would seem to imply that the union type is file scope, like the struct
1901 type s_tag. This is not true. The contents and position of the stab
1902 for u_type do not convey any infomation about its procedure local
1907 .stabs "name:sym_desc(union tag)type_def(22)=type_desc(union)
1909 elem_name:type_ref(int),bit_offset(0),bit_size(32);
1910 elem_name:type_ref(float),bit_offset(0),bit_size(32);
1911 elem_name:type_ref(ptr to char),bit_offset(0),bit_size(32);;"
1912 N_LSYM, NIL, NIL, NIL
1916 105 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
1920 The symbol descriptor, T, following the name: means that the stab
1921 describes an enumeration, struct or type tag. The type descriptor u,
1922 following the 23= of the type definition, narrows it down to a union
1923 type definition. Following the u is the number of bytes in the union.
1924 After that is a list of union element descriptions. Their format is
1925 name:type, bit offset into the union, and number of bytes for the
1928 The stab for the union variable follows. Notice that the frame
1929 pointer offset for local variables is negative.
1932 <128> N_LSYM - local variable (with no symbol descriptor)
1933 .stabs "name:type_ref(u_tag)", N_LSYM, NIL, NIL, frame_ptr_offset
1937 130 .stabs "an_u:23",128,0,0,-20
1940 @node Function Types
1941 @section Function types
1943 There are various types for function variables. These types are not
1944 used in defining functions; see symbol descriptor @samp{f}; they are
1945 used for things like pointers to functions.
1947 The simple, traditional, type is type descriptor @samp{f} is followed by
1948 type information for the return type of the function, followed by a
1951 This does not deal with functions the number and type of whose
1952 parameters are part of their type, as found in Modula-2 or ANSI C. AIX
1953 provides extensions to specify these, using the @samp{f}, @samp{F},
1954 @samp{p}, and @samp{R} type descriptors.
1956 First comes the type descriptor. Then, if it is @samp{f} or @samp{F},
1957 this is a function, and the type information for the return type of the
1958 function follows, followed by a comma. Then comes the number of
1959 parameters to the function and a semicolon. Then, for each parameter,
1960 there is the name of the parameter followed by a colon (this is only
1961 present for type descriptors @samp{R} and @samp{F} which represent
1962 Pascal function or procedure parameters), type information for the
1963 parameter, a comma, @samp{0} if passed by reference or @samp{1} if
1964 passed by value, and a semicolon. The type definition ends with a
1974 generates the following code:
1977 .stabs "g_pf:G24=*25=f1",32,0,0,0
1978 .common _g_pf,4,"bss"
1981 The variable defines a new type, 24, which is a pointer to another new
1982 type, 25, which is defined as a function returning int.
1985 @chapter Symbol information in symbol tables
1987 This section examines more closely the format of symbol table entries
1988 and how stab assembler directives map to them. It also describes what
1989 transformations the assembler and linker make on data from stabs.
1991 Each time the assembler encounters a stab in its input file it puts
1992 each field of the stab into corresponding fields in a symbol table
1993 entry of its output file. If the stab contains a string field, the
1994 symbol table entry for that stab points to a string table entry
1995 containing the string data from the stab. Assembler labels become
1996 relocatable addresses. Symbol table entries in a.out have the format:
1999 struct internal_nlist @{
2000 unsigned long n_strx; /* index into string table of name */
2001 unsigned char n_type; /* type of symbol */
2002 unsigned char n_other; /* misc info (usually empty) */
2003 unsigned short n_desc; /* description field */
2004 bfd_vma n_value; /* value of symbol */
2008 For .stabs directives, the n_strx field holds the character offset
2009 from the start of the string table to the string table entry
2010 containing the "string" field. For other classes of stabs (.stabn and
2011 .stabd) this field is null.
2013 Symbol table entries with n_type fields containing a value greater or
2014 equal to 0x20 originated as stabs generated by the compiler (with one
2015 random exception). Those with n_type values less than 0x20 were
2016 placed in the symbol table of the executable by the assembler or the
2019 The linker concatenates object files and does fixups of externally
2020 defined symbols. You can see the transformations made on stab data by
2021 the assembler and linker by examining the symbol table after each pass
2022 of the build, first the assemble and then the link.
2024 To do this use nm with the -ap options. This dumps the symbol table,
2025 including debugging information, unsorted. For stab entries the
2026 columns are: value, other, desc, type, string. For assembler and
2027 linker symbols, the columns are: value, type, string.
2029 There are a few important things to notice about symbol tables. Where
2030 the value field of a stab contains a frame pointer offset, or a
2031 register number, that value is unchanged by the rest of the build.
2033 Where the value field of a stab contains an assembly language label,
2034 it is transformed by each build step. The assembler turns it into a
2035 relocatable address and the linker turns it into an absolute address.
2036 This source line defines a static variable at file scope:
2039 3 static int s_g_repeat
2043 The following stab describes the symbol.
2046 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
2050 The assembler transforms the stab into this symbol table entry in the
2051 @file{.o} file. The location is expressed as a data segment offset.
2054 21 00000084 - 00 0000 STSYM s_g_repeat:S1
2058 in the symbol table entry from the executable, the linker has made the
2059 relocatable address absolute.
2062 22 0000e00c - 00 0000 STSYM s_g_repeat:S1
2065 Stabs for global variables do not contain location information. In
2066 this case the debugger finds location information in the assembler or
2067 linker symbol table entry describing the variable. The source line:
2077 21 .stabs "g_foo:G2",32,0,0,0
2080 The variable is represented by the following two symbol table entries
2081 in the object file. The first one originated as a stab. The second
2082 one is an external symbol. The upper case D signifies that the n_type
2083 field of the symbol table contains 7, N_DATA with local linkage (see
2084 Table B). The value field following the file's line number is empty
2085 for the stab entry. For the linker symbol it contains the
2086 rellocatable address corresponding to the variable.
2089 19 00000000 - 00 0000 GSYM g_foo:G2
2090 20 00000080 D _g_foo
2094 These entries as transformed by the linker. The linker symbol table
2095 entry now holds an absolute address.
2098 21 00000000 - 00 0000 GSYM g_foo:G2
2100 215 0000e008 D _g_foo
2104 @chapter GNU C++ stabs
2107 * Basic Cplusplus types::
2110 * Methods:: Method definition
2112 * Method Modifiers:: (const, volatile, const volatile)
2115 * Virtual Base Classes::
2119 @subsection type descriptors added for C++ descriptions
2123 method type (two ## if minimal debug)
2126 Member (class and variable) type. It is followed by type information
2127 for the offset basetype, a comma, and type information for the type of
2128 the field being pointed to. (FIXME: this is acknowledged to be
2129 gibberish. Can anyone say what really goes here?).
2131 Note that there is a conflict between this and type attributes
2132 (@pxref{Stabs Format}); both use type descriptor @samp{@@}.
2133 Fortunately, the @samp{@@} type descriptor used in this C++ sense always
2134 will be followed by a digit, @samp{(}, or @samp{-}, and type attributes
2135 never start with those things.
2138 @node Basic Cplusplus types
2139 @section Basic types for C++
2141 << the examples that follow are based on a01.C >>
2144 C++ adds two more builtin types to the set defined for C. These are
2145 the unknown type and the vtable record type. The unknown type, type
2146 16, is defined in terms of itself like the void type.
2148 The vtable record type, type 17, is defined as a structure type and
2149 then as a structure tag. The structure has four fields, delta, index,
2150 pfn, and delta2. pfn is the function pointer.
2152 << In boilerplate $vtbl_ptr_type, what are the fields delta,
2153 index, and delta2 used for? >>
2155 This basic type is present in all C++ programs even if there are no
2156 virtual methods defined.
2159 .stabs "struct_name:sym_desc(type)type_def(17)=type_desc(struct)struct_bytes(8)
2160 elem_name(delta):type_ref(short int),bit_offset(0),field_bits(16);
2161 elem_name(index):type_ref(short int),bit_offset(16),field_bits(16);
2162 elem_name(pfn):type_def(18)=type_desc(ptr to)type_ref(void),
2163 bit_offset(32),field_bits(32);
2164 elem_name(delta2):type_def(short int);bit_offset(32),field_bits(16);;"
2169 .stabs "$vtbl_ptr_type:t17=s8
2170 delta:6,0,16;index:6,16,16;pfn:18=*15,32,32;delta2:6,32,16;;"
2175 .stabs "name:sym_dec(struct tag)type_ref($vtbl_ptr_type)",N_LSYM,NIL,NIL,NIL
2179 .stabs "$vtbl_ptr_type:T17",128,0,0,0
2182 @node Simple classes
2183 @section Simple class definition
2185 The stabs describing C++ language features are an extension of the
2186 stabs describing C. Stabs representing C++ class types elaborate
2187 extensively on the stab format used to describe structure types in C.
2188 Stabs representing class type variables look just like stabs
2189 representing C language variables.
2191 Consider the following very simple class definition.
2197 int Ameth(int in, char other);
2201 The class baseA is represented by two stabs. The first stab describes
2202 the class as a structure type. The second stab describes a structure
2203 tag of the class type. Both stabs are of stab type N_LSYM. Since the
2204 stab is not located between an N_FUN and a N_LBRAC stab this indicates
2205 that the class is defined at file scope. If it were, then the N_LSYM
2206 would signify a local variable.
2208 A stab describing a C++ class type is similar in format to a stab
2209 describing a C struct, with each class member shown as a field in the
2210 structure. The part of the struct format describing fields is
2211 expanded to include extra information relevent to C++ class members.
2212 In addition, if the class has multiple base classes or virtual
2213 functions the struct format outside of the field parts is also
2216 In this simple example the field part of the C++ class stab
2217 representing member data looks just like the field part of a C struct
2218 stab. The section on protections describes how its format is
2219 sometimes extended for member data.
2221 The field part of a C++ class stab representing a member function
2222 differs substantially from the field part of a C struct stab. It
2223 still begins with `name:' but then goes on to define a new type number
2224 for the member function, describe its return type, its argument types,
2225 its protection level, any qualifiers applied to the method definition,
2226 and whether the method is virtual or not. If the method is virtual
2227 then the method description goes on to give the vtable index of the
2228 method, and the type number of the first base class defining the
2231 When the field name is a method name it is followed by two colons
2232 rather than one. This is followed by a new type definition for the
2233 method. This is a number followed by an equal sign and then the
2234 symbol descriptor `##', indicating a method type. This is followed by
2235 a type reference showing the return type of the method and a
2238 The format of an overloaded operator method name differs from that
2239 of other methods. It is "op$::XXXX." where XXXX is the operator name
2240 such as + or +=. The name ends with a period, and any characters except
2241 the period can occur in the XXXX string.
2243 The next part of the method description represents the arguments to
2244 the method, preceeded by a colon and ending with a semi-colon. The
2245 types of the arguments are expressed in the same way argument types
2246 are expressed in C++ name mangling. In this example an int and a char
2249 This is followed by a number, a letter, and an asterisk or period,
2250 followed by another semicolon. The number indicates the protections
2251 that apply to the member function. Here the 2 means public. The
2252 letter encodes any qualifier applied to the method definition. In
2253 this case A means that it is a normal function definition. The dot
2254 shows that the method is not virtual. The sections that follow
2255 elaborate further on these fields and describe the additional
2256 information present for virtual methods.
2260 .stabs "class_name:sym_desc(type)type_def(20)=type_desc(struct)struct_bytes(4)
2261 field_name(Adat):type(int),bit_offset(0),field_bits(32);
2263 method_name(Ameth)::type_def(21)=type_desc(method)return_type(int);
2264 :arg_types(int char);
2265 protection(public)qualifier(normal)virtual(no);;"
2270 .stabs "baseA:t20=s4Adat:1,0,32;Ameth::21=##1;:ic;2A.;;",128,0,0,0
2272 .stabs "class_name:sym_desc(struct tag)",N_LSYM,NIL,NIL,NIL
2274 .stabs "baseA:T20",128,0,0,0
2277 @node Class instance
2278 @section Class instance
2280 As shown above, describing even a simple C++ class definition is
2281 accomplished by massively extending the stab format used in C to
2282 describe structure types. However, once the class is defined, C stabs
2283 with no modifications can be used to describe class instances. The
2293 yields the following stab describing the class instance. It looks no
2294 different from a standard C stab describing a local variable.
2297 .stabs "name:type_ref(baseA)", N_LSYM, NIL, NIL, frame_ptr_offset
2301 .stabs "AbaseA:20",128,0,0,-20
2305 @section Method defintion
2307 The class definition shown above declares Ameth. The C++ source below
2312 baseA::Ameth(int in, char other)
2319 This method definition yields three stabs following the code of the
2320 method. One stab describes the method itself and following two
2321 describe its parameters. Although there is only one formal argument
2322 all methods have an implicit argument which is the `this' pointer.
2323 The `this' pointer is a pointer to the object on which the method was
2324 called. Note that the method name is mangled to encode the class name
2325 and argument types. << Name mangling is not described by this
2326 document - Is there already such a doc? >>
2329 .stabs "name:symbol_desriptor(global function)return_type(int)",
2330 N_FUN, NIL, NIL, code_addr_of_method_start
2332 .stabs "Ameth__5baseAic:F1",36,0,0,_Ameth__5baseAic
2335 Here is the stab for the `this' pointer implicit argument. The name
2336 of the `this' pointer is always `this.' Type 19, the `this' pointer is
2337 defined as a pointer to type 20, baseA, but a stab defining baseA has
2338 not yet been emited. Since the compiler knows it will be emited
2339 shortly, here it just outputs a cross reference to the undefined
2340 symbol, by prefixing the symbol name with xs.
2343 .stabs "name:sym_desc(register param)type_def(19)=
2344 type_desc(ptr to)type_ref(baseA)=
2345 type_desc(cross-reference to)baseA:",N_RSYM,NIL,NIL,register_number
2347 .stabs "this:P19=*20=xsbaseA:",64,0,0,8
2350 The stab for the explicit integer argument looks just like a parameter
2351 to a C function. The last field of the stab is the offset from the
2352 argument pointer, which in most systems is the same as the frame
2356 .stabs "name:sym_desc(value parameter)type_ref(int)",
2357 N_PSYM,NIL,NIL,offset_from_arg_ptr
2359 .stabs "in:p1",160,0,0,72
2362 << The examples that follow are based on A1.C >>
2365 @section Protections
2368 In the simple class definition shown above all member data and
2369 functions were publicly accessable. The example that follows
2370 contrasts public, protected and privately accessable fields and shows
2371 how these protections are encoded in C++ stabs.
2373 Protections for class member data are signified by two characters
2374 embeded in the stab defining the class type. These characters are
2375 located after the name: part of the string. /0 means private, /1
2376 means protected, and /2 means public. If these characters are omited
2377 this means that the member is public. The following C++ source:
2391 generates the following stab to describe the class type all_data.
2394 .stabs "class_name:sym_desc(type)type_def(19)=type_desc(struct)struct_bytes
2395 data_name:/protection(private)type_ref(int),bit_offset,num_bits;
2396 data_name:/protection(protected)type_ref(char),bit_offset,num_bits;
2397 data_name:(/num omited, private)type_ref(float),bit_offset,num_bits;;"
2402 .stabs "all_data:t19=s12
2403 priv_dat:/01,0,32;prot_dat:/12,32,8;pub_dat:12,64,32;;",128,0,0,0
2406 Protections for member functions are signified by one digit embeded in
2407 the field part of the stab describing the method. The digit is 0 if
2408 private, 1 if protected and 2 if public. Consider the C++ class
2412 class all_methods @{
2414 int priv_meth(int in)@{return in;@};
2416 char protMeth(char in)@{return in;@};
2418 float pubMeth(float in)@{return in;@};
2422 It generates the following stab. The digit in question is to the left
2423 of an `A' in each case. Notice also that in this case two symbol
2424 descriptors apply to the class name struct tag and struct type.
2427 .stabs "class_name:sym_desc(struct tag&type)type_def(21)=
2428 sym_desc(struct)struct_bytes(1)
2429 meth_name::type_def(22)=sym_desc(method)returning(int);
2430 :args(int);protection(private)modifier(normal)virtual(no);
2431 meth_name::type_def(23)=sym_desc(method)returning(char);
2432 :args(char);protection(protected)modifier(normal)virual(no);
2433 meth_name::type_def(24)=sym_desc(method)returning(float);
2434 :args(float);protection(public)modifier(normal)virtual(no);;",
2439 .stabs "all_methods:Tt21=s1priv_meth::22=##1;:i;0A.;protMeth::23=##2;:c;1A.;
2440 pubMeth::24=##12;:f;2A.;;",128,0,0,0
2443 @node Method Modifiers
2444 @section Method Modifiers (const, volatile, const volatile)
2448 In the class example described above all the methods have the normal
2449 modifier. This method modifier information is located just after the
2450 protection information for the method. This field has four possible
2451 character values. Normal methods use A, const methods use B, volatile
2452 methods use C, and const volatile methods use D. Consider the class
2458 int ConstMeth (int arg) const @{ return arg; @};
2459 char VolatileMeth (char arg) volatile @{ return arg; @};
2460 float ConstVolMeth (float arg) const volatile @{return arg; @};
2464 This class is described by the following stab:
2467 .stabs "class(A):sym_desc(struct)type_def(20)=type_desc(struct)struct_bytes(1)
2468 meth_name(ConstMeth)::type_def(21)sym_desc(method)
2469 returning(int);:arg(int);protection(public)modifier(const)virtual(no);
2470 meth_name(VolatileMeth)::type_def(22)=sym_desc(method)
2471 returning(char);:arg(char);protection(public)modifier(volatile)virt(no)
2472 meth_name(ConstVolMeth)::type_def(23)=sym_desc(method)
2473 returning(float);:arg(float);protection(public)modifer(const volatile)
2474 virtual(no);;", @dots{}
2478 .stabs "A:T20=s1ConstMeth::21=##1;:i;2B.;VolatileMeth::22=##2;:c;2C.;
2479 ConstVolMeth::23=##12;:f;2D.;;",128,0,0,0
2482 @node Virtual Methods
2483 @section Virtual Methods
2485 << The following examples are based on a4.C >>
2487 The presence of virtual methods in a class definition adds additional
2488 data to the class description. The extra data is appended to the
2489 description of the virtual method and to the end of the class
2490 description. Consider the class definition below:
2496 virtual int A_virt (int arg) @{ return arg; @};
2500 This results in the stab below describing class A. It defines a new
2501 type (20) which is an 8 byte structure. The first field of the class
2502 struct is Adat, an integer, starting at structure offset 0 and
2505 The second field in the class struct is not explicitly defined by the
2506 C++ class definition but is implied by the fact that the class
2507 contains a virtual method. This field is the vtable pointer. The
2508 name of the vtable pointer field starts with $vf and continues with a
2509 type reference to the class it is part of. In this example the type
2510 reference for class A is 20 so the name of its vtable pointer field is
2511 $vf20, followed by the usual colon.
2513 Next there is a type definition for the vtable pointer type (21).
2514 This is in turn defined as a pointer to another new type (22).
2516 Type 22 is the vtable itself, which is defined as an array, indexed by
2517 a range of integers between 0 and 1, and whose elements are of type
2518 17. Type 17 was the vtable record type defined by the boilerplate C++
2519 type definitions, as shown earlier.
2521 The bit offset of the vtable pointer field is 32. The number of bits
2522 in the field are not specified when the field is a vtable pointer.
2524 Next is the method definition for the virtual member function A_virt.
2525 Its description starts out using the same format as the non-virtual
2526 member functions described above, except instead of a dot after the
2527 `A' there is an asterisk, indicating that the function is virtual.
2528 Since is is virtual some addition information is appended to the end
2529 of the method description.
2531 The first number represents the vtable index of the method. This is a
2532 32 bit unsigned number with the high bit set, followed by a
2535 The second number is a type reference to the first base class in the
2536 inheritence hierarchy defining the virtual member function. In this
2537 case the class stab describes a base class so the virtual function is
2538 not overriding any other definition of the method. Therefore the
2539 reference is to the type number of the class that the stab is
2542 This is followed by three semi-colons. One marks the end of the
2543 current sub-section, one marks the end of the method field, and the
2544 third marks the end of the struct definition.
2546 For classes containing virtual functions the very last section of the
2547 string part of the stab holds a type reference to the first base
2548 class. This is preceeded by `~%' and followed by a final semi-colon.
2551 .stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8)
2552 field_name(Adat):type_ref(int),bit_offset(0),field_bits(32);
2553 field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)=
2554 sym_desc(array)index_type_ref(range of int from 0 to 1);
2555 elem_type_ref(vtbl elem type),
2557 meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
2558 :arg_type(int),protection(public)normal(yes)virtual(yes)
2559 vtable_index(1);class_first_defining(A);;;~%first_base(A);",
2563 @c FIXME: bogus line break.
2565 .stabs "A:t20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2566 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2570 @section Inheritence
2572 Stabs describing C++ derived classes include additional sections that
2573 describe the inheritence hierarchy of the class. A derived class stab
2574 also encodes the number of base classes. For each base class it tells
2575 if the base class is virtual or not, and if the inheritence is private
2576 or public. It also gives the offset into the object of the portion of
2577 the object corresponding to each base class.
2579 This additional information is embeded in the class stab following the
2580 number of bytes in the struct. First the number of base classes
2581 appears bracketed by an exclamation point and a comma.
2583 Then for each base type there repeats a series: two digits, a number,
2584 a comma, another number, and a semi-colon.
2586 The first of the two digits is 1 if the base class is virtual and 0 if
2587 not. The second digit is 2 if the derivation is public and 0 if not.
2589 The number following the first two digits is the offset from the start
2590 of the object to the part of the object pertaining to the base class.
2592 After the comma, the second number is a type_descriptor for the base
2593 type. Finally a semi-colon ends the series, which repeats for each
2596 The source below defines three base classes A, B, and C and the
2604 virtual int A_virt (int arg) @{ return arg; @};
2610 virtual int B_virt (int arg) @{return arg; @};
2616 virtual int C_virt (int arg) @{return arg; @};
2619 class D : A, virtual B, public C @{
2622 virtual int A_virt (int arg ) @{ return arg+1; @};
2623 virtual int B_virt (int arg) @{ return arg+2; @};
2624 virtual int C_virt (int arg) @{ return arg+3; @};
2625 virtual int D_virt (int arg) @{ return arg; @};
2629 Class stabs similar to the ones described earlier are generated for
2632 @c FIXME!!! the linebreaks in the following example probably make the
2633 @c examples literally unusable, but I don't know any other way to get
2634 @c them on the page.
2635 @c One solution would be to put some of the type definitions into
2636 @c separate stabs, even if that's not exactly what the compiler actually
2639 .stabs "A:T20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2640 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2642 .stabs "B:Tt25=s8Bdat:1,0,32;$vf25:21,32;B_virt::26=##1;
2643 :i;2A*-2147483647;25;;;~%25;",128,0,0,0
2645 .stabs "C:Tt28=s8Cdat:1,0,32;$vf28:21,32;C_virt::29=##1;
2646 :i;2A*-2147483647;28;;;~%28;",128,0,0,0
2649 In the stab describing derived class D below, the information about
2650 the derivation of this class is encoded as follows.
2653 .stabs "derived_class_name:symbol_descriptors(struct tag&type)=
2654 type_descriptor(struct)struct_bytes(32)!num_bases(3),
2655 base_virtual(no)inheritence_public(no)base_offset(0),
2656 base_class_type_ref(A);
2657 base_virtual(yes)inheritence_public(no)base_offset(NIL),
2658 base_class_type_ref(B);
2659 base_virtual(no)inheritence_public(yes)base_offset(64),
2660 base_class_type_ref(C); @dots{}
2663 @c FIXME! fake linebreaks.
2665 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:
2666 1,160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt:
2667 :32:i;2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;
2668 28;;D_virt::32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2671 @node Virtual Base Classes
2672 @section Virtual Base Classes
2674 A derived class object consists of a concatination in memory of the
2675 data areas defined by each base class, starting with the leftmost and
2676 ending with the rightmost in the list of base classes. The exception
2677 to this rule is for virtual inheritence. In the example above, class
2678 D inherits virtually from base class B. This means that an instance
2679 of a D object will not contain it's own B part but merely a pointer to
2680 a B part, known as a virtual base pointer.
2682 In a derived class stab, the base offset part of the derivation
2683 information, described above, shows how the base class parts are
2684 ordered. The base offset for a virtual base class is always given as
2685 0. Notice that the base offset for B is given as 0 even though B is
2686 not the first base class. The first base class A starts at offset 0.
2688 The field information part of the stab for class D describes the field
2689 which is the pointer to the virtual base class B. The vbase pointer
2690 name is $vb followed by a type reference to the virtual base class.
2691 Since the type id for B in this example is 25, the vbase pointer name
2694 @c FIXME!! fake linebreaks below
2696 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1,
2697 160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i;
2698 2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt:
2699 :32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2702 Following the name and a semicolon is a type reference describing the
2703 type of the virtual base class pointer, in this case 24. Type 24 was
2704 defined earlier as the type of the B class `this` pointer. The
2705 `this' pointer for a class is a pointer to the class type.
2708 .stabs "this:P24=*25=xsB:",64,0,0,8
2711 Finally the field offset part of the vbase pointer field description
2712 shows that the vbase pointer is the first field in the D object,
2713 before any data fields defined by the class. The layout of a D class
2714 object is a follows, Adat at 0, the vtable pointer for A at 32, Cdat
2715 at 64, the vtable pointer for C at 96, the virtual ase pointer for B
2716 at 128, and Ddat at 160.
2719 @node Static Members
2720 @section Static Members
2722 The data area for a class is a concatenation of the space used by the
2723 data members of the class. If the class has virtual methods, a vtable
2724 pointer follows the class data. The field offset part of each field
2725 description in the class stab shows this ordering.
2727 << How is this reflected in stabs? See Cygnus bug #677 for some info. >>
2730 @appendix Example2.c - source code for extended example
2734 2 register int g_bar asm ("%g5");
2735 3 static int s_g_repeat = 2;
2741 9 char s_char_vec[8];
2742 10 struct s_tag* s_next;
2745 13 typedef struct s_tag s_typedef;
2747 15 char char_vec[3] = @{'a','b','c'@};
2749 17 main (argc, argv)
2753 21 static float s_flap;
2755 23 for (times=0; times < s_g_repeat; times++)@{
2757 25 printf ("Hello world\n");
2761 29 enum e_places @{first,second=3,last@};
2763 31 static s_proc (s_arg, s_ptr_arg, char_vec)
2765 33 s_typedef* s_ptr_arg;
2779 @appendix Example2.s - assembly code for extended example
2783 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
2784 3 .stabs "example2.c",100,0,0,Ltext0
2787 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
2788 7 .stabs "char:t2=r2;0;127;",128,0,0,0
2789 8 .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
2790 9 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
2791 10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
2792 11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
2793 12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
2794 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
2795 14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
2796 15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
2797 16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
2798 17 .stabs "float:t12=r1;4;0;",128,0,0,0
2799 18 .stabs "double:t13=r1;8;0;",128,0,0,0
2800 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
2801 20 .stabs "void:t15=15",128,0,0,0
2802 21 .stabs "g_foo:G2",32,0,0,0
2807 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
2811 @c FIXME! fake linebreak in line 30
2812 30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;s_char_vec:
2813 17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
2814 31 .stabs "s_typedef:t16",128,0,0,0
2815 32 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
2816 33 .global _char_vec
2822 39 .reserve _s_flap.0,4,"bss",4
2826 43 .ascii "Hello world\12\0"
2831 48 .stabn 68,0,20,LM1
2834 51 save %sp,-144,%sp
2841 58 .stabn 68,0,23,LM2
2845 62 sethi %hi(_s_g_repeat),%o0
2847 64 ld [%o0+%lo(_s_g_repeat)],%o0
2852 69 .stabn 68,0,25,LM3
2854 71 sethi %hi(LC0),%o1
2855 72 or %o1,%lo(LC0),%o0
2858 75 .stabn 68,0,26,LM4
2861 78 .stabn 68,0,23,LM5
2869 86 .stabn 68,0,27,LM6
2872 89 .stabn 68,0,27,LM7
2877 94 .stabs "main:F1",36,0,0,_main
2878 95 .stabs "argc:p1",160,0,0,68
2879 96 .stabs "argv:p20=*21=*2",160,0,0,72
2880 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
2881 98 .stabs "times:1",128,0,0,-20
2882 99 .stabn 192,0,0,LBB2
2883 100 .stabs "inner:1",128,0,0,-24
2884 101 .stabn 192,0,0,LBB3
2885 102 .stabn 224,0,0,LBE3
2886 103 .stabn 224,0,0,LBE2
2887 104 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
2888 @c FIXME: fake linebreak in line 105
2889 105 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
2894 109 .stabn 68,0,35,LM8
2897 112 save %sp,-120,%sp
2903 118 .stabn 68,0,41,LM9
2906 121 .stabn 68,0,41,LM10
2911 126 .stabs "s_proc:f1",36,0,0,_s_proc
2912 127 .stabs "s_arg:p16",160,0,0,0
2913 128 .stabs "s_ptr_arg:p18",160,0,0,72
2914 129 .stabs "char_vec:p21",160,0,0,76
2915 130 .stabs "an_u:23",128,0,0,-20
2916 131 .stabn 192,0,0,LBB4
2917 132 .stabn 224,0,0,LBE4
2918 133 .stabs "g_bar:r1",64,0,0,5
2919 134 .stabs "g_pf:G24=*25=f1",32,0,0,0
2920 135 .common _g_pf,4,"bss"
2921 136 .stabs "g_an_s:G16",32,0,0,0
2922 137 .common _g_an_s,20,"bss"
2926 @appendix Values for the Stab Type Field
2928 These are all the possible values for the stab type field, for
2929 @code{a.out} files. This does not apply to XCOFF.
2931 The following types are used by the linker and assembler; there is
2932 nothing stabs-specific about them. Since this document does not attempt
2933 to describe aspects of object file format other than the debugging
2934 format, no details are given.
2936 @c Try to get most of these to fit on a single line.
2946 File scope absolute symbol
2948 @item 0x3 N_ABS | N_EXT
2949 External absolute symbol
2952 File scope text symbol
2954 @item 0x5 N_TEXT | N_EXT
2955 External text symbol
2958 File scope data symbol
2960 @item 0x7 N_DATA | N_EXT
2961 External data symbol
2964 File scope BSS symbol
2966 @item 0x9 N_BSS | N_EXT
2970 Same as N_FN, for Sequent compilers
2973 Symbol is indirected to another symbol
2976 Common sym -- visable after shared lib dynamic link
2979 Absolute set element
2982 Text segment set element
2985 Data segment set element
2988 BSS segment set element
2991 Pointer to set vector
2993 @item 0x1e N_WARNING
2994 Print a warning message during linking
2997 File name of a .o file
3000 The following symbol types indicate that this is a stab. This is the
3001 full list of stab numbers, including stab types that are used in
3002 languages other than C.
3006 Global symbol, @xref{N_GSYM}.
3009 Function name (for BSD Fortran), @xref{N_FNAME}.
3012 Function name or text segment variable for C, @xref{N_FUN}.
3015 Static symbol (data segment variable with internal linkage), @xref{N_STSYM}.
3018 .lcomm symbol (BSS segment variable with internal linkage), @xref{N_LCSYM}.
3021 Name of main routine (not used in C), @xref{N_MAIN}.
3023 @c FIXME: discuss this in the main body of the text where we talk about
3024 @c using N_FUN for variables.
3026 Read-only data symbol (Solaris2). Most systems use N_FUN for this.
3029 Global symbol (for Pascal), @xref{N_PC}.
3032 Number of symbols (according to Ultrix V4.0), @xref{N_NSYMS}.
3035 No DST map for sym (according to Ultrix V4.0), @xref{N_NOMAP}.
3037 @c FIXME: describe this solaris feature in the body of the text (see
3038 @c comments in include/aout/stab.def).
3040 Object file (Solaris2).
3042 @c See include/aout/stab.def for (a little) more info.
3044 Debugger options (Solaris2).
3047 Register variable, @xref{N_RSYM}.
3050 Modula-2 compilation unit, @xref{N_M2C}.
3053 Line number in text segment, @xref{Line Numbers}.
3056 Line number in data segment, @xref{Line Numbers}.
3059 Line number in bss segment, @xref{Line Numbers}.
3062 Sun source code browser, path to .cb file, @xref{N_BROWS}.
3065 Gnu Modula2 definition module dependency, @xref{N_DEFD}.
3068 Function start/body/end line numbers (Solaris2).
3071 Gnu C++ exception variable, @xref{N_EHDECL}.
3074 Modula2 info "for imc" (according to Ultrix V4.0), @xref{N_MOD2}.
3077 Gnu C++ "catch" clause, @xref{N_CATCH}.
3080 Structure of union element, @xref{N_SSYM}.
3083 Last stab for module (Solaris2).
3086 Path and name of source file , @xref{Source Files}.
3089 Automatic var in the stack or type definition, @xref{N_LSYM}, @xref{Typedefs}.
3092 Beginning of an include file (Sun only), @xref{Source Files}.
3095 Name of sub-source (#include) file., @xref{Source Files}.
3098 Parameter variable, @xref{Parameters}.
3101 End of an include file, @xref{Source Files}.
3104 Alternate entry point, @xref{N_ENTRY}.
3107 Beginning of a lexical block, @xref{N_LBRAC}.
3110 Place holder for a deleted include file, @xref{Source Files}.
3113 Modula2 scope information (Sun linker), @xref{N_SCOPE}.
3116 End of a lexical block, @xref{N_RBRAC}.
3119 Begin named common block, @xref{N_BCOMM}.
3122 End named common block, @xref{N_ECOMM}.
3125 End common (local name), @xref{N_ECOML}.
3127 @c FIXME: How does this really work? Move it to main body of document.
3129 Pascal @code{with} statement: type,,0,0,offset (Solaris2).
3132 Gould non-base registers, @xref{Gould}.
3135 Gould non-base registers, @xref{Gould}.
3138 Gould non-base registers, @xref{Gould}.
3141 Gould non-base registers, @xref{Gould}.
3144 Gould non-base registers, @xref{Gould}.
3147 @c Restore the default table indent
3152 @node Symbol Descriptors
3153 @appendix Table of Symbol Descriptors
3155 @c Please keep this alphabetical
3157 @c In TeX, this looks great, digit is in italics. But makeinfo insists
3158 @c on putting it in `', not realizing that @var should override @code.
3159 @c I don't know of any way to make makeinfo do the right thing. Seems
3160 @c like a makeinfo bug to me.
3164 Local variable, @xref{Automatic variables}.
3167 Parameter passed by reference in register, @xref{Parameters}.
3170 Constant, @xref{Constants}.
3173 Conformant array bound (Pascal, maybe other languages),
3174 @xref{Parameters}. Name of a caught exception (GNU C++). These can be
3175 distinguished because the latter uses N_CATCH and the former uses
3176 another symbol type.
3179 Floating point register variable, @xref{Register variables}.
3182 Parameter in floating point register, @xref{Parameters}.
3185 Static function, @xref{Procedures}.
3188 Global function, @xref{Procedures}.
3191 Global variable, @xref{Global Variables}.
3197 Internal (nested) procedure, @xref{Procedures}.
3200 Internal (nested) function, @xref{Procedures}.
3203 Label name (documented by AIX, no further information known).
3206 Module, @xref{Procedures}.
3209 Argument list parameter, @xref{Parameters}.
3215 FORTRAN Function parameter, @xref{Parameters}.
3218 Unfortunately, three separate meanings have been independently invented
3219 for this symbol descriptor. At least the GNU and Sun uses can be
3220 distinguished by the symbol type. Global Procedure (AIX) (symbol type
3221 used unknown), @xref{Procedures}. Register parameter (GNU) (symbol type
3222 N_PSYM), @xref{Parameters}. Prototype of function referenced by this
3223 file (Sun acc) (symbol type N_FUN).
3226 Static Procedure, @xref{Procedures}.
3229 Register parameter @xref{Parameters}.
3232 Register variable, @xref{Register variables}.
3235 Static file scope variable @xref{Initialized statics},
3236 @xref{Un-initialized statics}.
3239 Type name, @xref{Typedefs}.
3242 enumeration, struct or union tag, @xref{Typedefs}.
3245 Parameter passed by reference, @xref{Parameters}.
3248 Static procedure scope variable @xref{Initialized statics},
3249 @xref{Un-initialized statics}.
3252 Conformant array, @xref{Parameters}.
3255 Function return variable, @xref{Parameters}.
3258 @node Type Descriptors
3259 @appendix Table of Type Descriptors
3264 Type reference, @xref{Stabs Format}.
3267 Reference to builtin type, @xref{Negative Type Numbers}.
3270 Method (C++), @xref{Cplusplus}.
3273 Pointer, @xref{Miscellaneous Types}.
3279 Type Attributes (AIX), @xref{Stabs Format}. Member (class and variable)
3280 type (GNU C++), @xref{Cplusplus}.
3283 Array, @xref{Arrays}.
3286 Open array, @xref{Arrays}.
3289 Pascal space type (AIX), @xref{Miscellaneous Types}. Builtin integer
3290 type (Sun), @xref{Builtin Type Descriptors}.
3293 Volatile-qualified type, @xref{Miscellaneous Types}.
3296 Complex builtin type, @xref{Builtin Type Descriptors}.
3299 COBOL Picture type. See AIX documentation for details.
3302 File type, @xref{Miscellaneous Types}.
3305 N-dimensional dynamic array, @xref{Arrays}.
3308 Enumeration type, @xref{Enumerations}.
3311 N-dimensional subarray, @xref{Arrays}.
3314 Function type, @xref{Function Types}.
3317 Pascal function parameter, @xref{Function Types}
3320 Builtin floating point type, @xref{Builtin Type Descriptors}.
3323 COBOL Group. See AIX documentation for details.
3326 Imported type, @xref{Cross-references}.
3329 Const-qualified type, @xref{Miscellaneous Types}.
3332 COBOL File Descriptor. See AIX documentation for details.
3335 Multiple instance type, @xref{Miscellaneous Types}.
3338 String type, @xref{Strings}.
3341 Stringptr, @xref{Strings}.
3344 Opaque type, @xref{Typedefs}.
3347 Procedure, @xref{Function Types}.
3350 Packed array, @xref{Arrays}.
3353 Range type, @xref{Subranges}.
3356 Builtin floating type, @xref{Builtin Type Descriptors} (Sun). Pascal
3357 subroutine parameter, @xref{Function Types} (AIX). Detecting this
3358 conflict is possible with careful parsing (hint: a Pascal subroutine
3359 parameter type will always contain a comma, and a builtin type
3360 descriptor never will).
3363 Structure type, @xref{Structures}.
3366 Set type, @xref{Miscellaneous Types}.
3369 Union, @xref{Unions}.
3372 Variant record. This is a Pascal and Modula-2 feature which is like a
3373 union within a struct in C. See AIX documentation for details.
3376 Wide character, @xref{Builtin Type Descriptors}.
3379 Cross-reference, @xref{Cross-references}.
3382 gstring, @xref{Strings}.
3385 @node Expanded reference
3386 @appendix Expanded reference by stab type.
3388 @c FIXME: This appendix should go away, see N_PSYM or N_SO for an example.
3390 For a full list of stab types, and cross-references to where they are
3391 described, @xref{Stab Types}. This appendix just duplicates certain
3392 information from the main body of this document; eventually the
3393 information will all be in one place.
3397 The first line is the symbol type expressed in decimal, hexadecimal,
3398 and as a #define (see devo/include/aout/stab.def).
3400 The second line describes the language constructs the symbol type
3403 The third line is the stab format with the significant stab fields
3404 named and the rest NIL.
3406 Subsequent lines expand upon the meaning and possible values for each
3407 significant stab field. # stands in for the type descriptor.
3409 Finally, any further information.
3412 * N_GSYM:: Global variable
3413 * N_FNAME:: Function name (BSD Fortran)
3414 * N_FUN:: C Function name or text segment variable
3415 * N_STSYM:: Initialized static symbol
3416 * N_LCSYM:: Uninitialized static symbol
3417 * N_MAIN:: Name of main routine (not for C)
3418 * N_PC:: Pascal global symbol
3419 * N_NSYMS:: Number of symbols
3420 * N_NOMAP:: No DST map
3421 * N_RSYM:: Register variable
3422 * N_M2C:: Modula-2 compilation unit
3423 * N_BROWS:: Path to .cb file for Sun source code browser
3424 * N_DEFD:: GNU Modula2 definition module dependency
3425 * N_EHDECL:: GNU C++ exception variable
3426 * N_MOD2:: Modula2 information "for imc"
3427 * N_CATCH:: GNU C++ "catch" clause
3428 * N_SSYM:: Structure or union element
3429 * N_LSYM:: Automatic variable
3430 * N_ENTRY:: Alternate entry point
3431 * N_LBRAC:: Beginning of lexical block
3432 * N_SCOPE:: Modula2 scope information (Sun only)
3433 * N_RBRAC:: End of lexical block
3434 * N_BCOMM:: Begin named common block
3435 * N_ECOMM:: End named common block
3436 * N_ECOML:: End common
3437 * Gould:: non-base register symbols used on Gould systems
3438 * N_LENG:: Length of preceding entry
3442 @section 32 - 0x20 - N_GYSM
3447 .stabs "name", N_GSYM, NIL, NIL, NIL
3451 "name" -> "symbol_name:#type"
3455 Only the "name" field is significant. The location of the variable is
3456 obtained from the corresponding external symbol.
3459 @section 34 - 0x22 - N_FNAME
3460 Function name (for BSD Fortran)
3463 .stabs "name", N_FNAME, NIL, NIL, NIL
3467 "name" -> "function_name"
3470 Only the "name" field is significant. The location of the symbol is
3471 obtained from the corresponding extern symbol.
3474 @section 36 - 0x24 - N_FUN
3476 Function name (@pxref{Procedures}) or text segment variable
3477 (@pxref{Variables}).
3479 @exdent @emph{For functions:}
3480 "name" -> "proc_name:#return_type"
3481 # -> F (global function)
3483 desc -> line num for proc start. (GCC doesn't set and DBX doesn't miss it.)
3484 value -> Code address of proc start.
3486 @exdent @emph{For text segment variables:}
3487 <<How to create one?>>
3491 @section 38 - 0x26 - N_STSYM
3492 Initialized static symbol (data segment w/internal linkage).
3495 .stabs "name", N_STSYM, NIL, NIL, value
3499 "name" -> "symbol_name#type"
3500 # -> S (scope global to compilation unit)
3501 -> V (scope local to a procedure)
3502 value -> Data Address
3506 @section 40 - 0x28 - N_LCSYM
3507 Unitialized static (.lcomm) symbol(BSS segment w/internal linkage).
3510 .stabs "name", N_LCLSYM, NIL, NIL, value
3514 "name" -> "symbol_name#type"
3515 # -> S (scope global to compilation unit)
3516 -> V (scope local to procedure)
3517 value -> BSS Address
3521 @section 42 - 0x2a - N_MAIN
3522 Name of main routine (not used in C)
3525 .stabs "name", N_MAIN, NIL, NIL, NIL
3529 "name" -> "name_of_main_routine"
3533 @section 48 - 0x30 - N_PC
3534 Global symbol (for Pascal)
3537 .stabs "name", N_PC, NIL, NIL, value
3541 "name" -> "symbol_name" <<?>>
3542 value -> supposedly the line number (stab.def is skeptical)
3548 global pascal symbol: name,,0,subtype,line
3553 @section 50 - 0x32 - N_NSYMS
3554 Number of symbols (according to Ultrix V4.0)
3557 0, files,,funcs,lines (stab.def)
3561 @section 52 - 0x34 - N_NOMAP
3562 no DST map for sym (according to Ultrix V4.0)
3565 name, ,0,type,ignored (stab.def)
3569 @section 64 - 0x40 - N_RSYM
3573 .stabs "name:type",N_RSYM,0,RegSize,RegNumber (Sun doc)
3577 @section 66 - 0x42 - N_M2C
3578 Modula-2 compilation unit
3581 .stabs "name", N_M2C, 0, desc, value
3585 "name" -> "unit_name,unit_time_stamp[,code_time_stamp]
3587 value -> 0 (main unit)
3592 @section 72 - 0x48 - N_BROWS
3593 Sun source code browser, path to .cb file
3596 "path to associated .cb file"
3598 Note: type field value overlaps with N_BSLINE
3601 @section 74 - 0x4a - N_DEFD
3602 GNU Modula2 definition module dependency
3604 GNU Modula-2 definition module dependency. Value is the modification
3605 time of the definition file. Other is non-zero if it is imported with
3606 the GNU M2 keyword %INITIALIZE. Perhaps N_M2C can be used if there
3607 are enough empty fields?
3610 @section 80 - 0x50 - N_EHDECL
3611 GNU C++ exception variable <<?>>
3613 "name is variable name"
3615 Note: conflicts with N_MOD2.
3618 @section 80 - 0x50 - N_MOD2
3619 Modula2 info "for imc" (according to Ultrix V4.0)
3621 Note: conflicts with N_EHDECL <<?>>
3624 @section 84 - 0x54 - N_CATCH
3625 GNU C++ "catch" clause
3627 GNU C++ `catch' clause. Value is its address. Desc is nonzero if
3628 this entry is immediately followed by a CAUGHT stab saying what
3629 exception was caught. Multiple CAUGHT stabs means that multiple
3630 exceptions can be caught here. If Desc is 0, it means all exceptions
3634 @section 96 - 0x60 - N_SSYM
3635 Structure or union element
3637 Value is offset in the structure.
3639 <<?looking at structs and unions in C I didn't see these>>
3642 @section 128 - 0x80 - N_LSYM
3643 Automatic var in the stack (also used for type descriptors.)
3646 .stabs "name" N_LSYM, NIL, NIL, value
3650 @exdent @emph{For stack based local variables:}
3652 "name" -> name of the variable
3653 value -> offset from frame pointer (negative)
3655 @exdent @emph{For type descriptors:}
3657 "name" -> "name_of_the_type:#type"
3660 type -> type_ref (or) type_def
3662 type_ref -> type_number
3663 type_def -> type_number=type_desc etc.
3666 Type may be either a type reference or a type definition. A type
3667 reference is a number that refers to a previously defined type. A
3668 type definition is the number that will refer to this type, followed
3669 by an equals sign, a type descriptor and the additional data that
3670 defines the type. See the Table D for type descriptors and the
3671 section on types for what data follows each type descriptor.
3674 @section 164 - 0xa4 - N_ENTRY
3676 Alternate entry point.
3677 Value is its address.
3681 @section 192 - 0xc0 - N_LBRAC
3683 Beginning of a lexical block (left brace). The variable defined
3684 inside the block precede the N_LBRAC symbol. Or can they follow as
3685 well as long as a new N_FUNC was not encountered. <<?>>
3688 .stabn N_LBRAC, NIL, NIL, value
3692 value -> code address of block start.
3696 @section 196 - 0xc4 - N_SCOPE
3698 Modula2 scope information (Sun linker)
3702 @section 224 - 0xe0 - N_RBRAC
3704 End of a lexical block (right brace)
3707 .stabn N_RBRAC, NIL, NIL, value
3711 value -> code address of the end of the block.
3715 @section 226 - 0xe2 - N_BCOMM
3717 Begin named common block.
3719 Only the name is significant.
3723 @section 228 - 0xe4 - N_ECOMM
3725 End named common block.
3727 Only the name is significant and it should match the N_BCOMM
3731 @section 232 - 0xe8 - N_ECOML
3733 End common (local name)
3739 @section Non-base registers on Gould systems
3741 These are used on Gould systems for non-base registers syms.
3743 However, the following values are not the values used by Gould; they are
3744 the values which GNU has been documenting for these values for a long
3745 time, without actually checking what Gould uses. I include these values
3746 only because perhaps some someone actually did something with the GNU
3747 information (I hope not, why GNU knowingly assigned wrong values to
3748 these in the header file is a complete mystery to me).
3751 240 0xf0 N_NBTEXT ??
3752 242 0xf2 N_NBDATA ??
3759 @section - 0xfe - N_LENG
3761 Second symbol entry containing a length-value for the preceding entry.
3762 The value is the length.
3765 @appendix Questions and anomalies
3769 For GNU C stabs defining local and global variables (N_LSYM and
3770 N_GSYM), the desc field is supposed to contain the source line number
3771 on which the variable is defined. In reality the desc field is always
3772 0. (This behavour is defined in dbxout.c and putting a line number in
3773 desc is controlled by #ifdef WINNING_GDB which defaults to false). Gdb
3774 supposedly uses this information if you say 'list var'. In reality
3775 var can be a variable defined in the program and gdb says `function
3779 In GNU C stabs there seems to be no way to differentiate tag types:
3780 structures, unions, and enums (symbol descriptor T) and typedefs
3781 (symbol descriptor t) defined at file scope from types defined locally
3782 to a procedure or other more local scope. They all use the N_LSYM
3783 stab type. Types defined at procedure scope are emited after the
3784 N_RBRAC of the preceding function and before the code of the
3785 procedure in which they are defined. This is exactly the same as
3786 types defined in the source file between the two procedure bodies.
3787 GDB overcompensates by placing all types in block #1, the block for
3788 symbols of file scope. This is true for default, -ansi and
3789 -traditional compiler options. (Bugs gcc/1063, gdb/1066.)
3792 What ends the procedure scope? Is it the proc block's N_RBRAC or the
3793 next N_FUN? (I believe its the first.)
3796 The comment in xcoff.h says DBX_STATIC_CONST_VAR_CODE is used for
3797 static const variables. DBX_STATIC_CONST_VAR_CODE is set to N_FUN by
3798 default, in dbxout.c. If included, xcoff.h redefines it to N_STSYM.
3799 But testing the default behaviour, my Sun4 native example shows
3800 N_STSYM not N_FUN is used to describe file static initialized
3801 variables. (the code tests for TREE_READONLY(decl) &&
3802 !TREE_THIS_VOLATILE(decl) and if true uses DBX_STATIC_CONST_VAR_CODE).
3805 Global variable stabs don't have location information. This comes
3806 from the external symbol for the same variable. The external symbol
3807 has a leading underbar on the _name of the variable and the stab does
3808 not. How do we know these two symbol table entries are talking about
3809 the same symbol when their names are different?
3812 Can gcc be configured to output stabs the way the Sun compiler
3813 does, so that their native debugging tools work? <NO?> It doesn't by
3814 default. GDB reads either format of stab. (gcc or SunC). How about
3818 @node xcoff-differences
3819 @appendix Differences between GNU stabs in a.out and GNU stabs in xcoff
3821 @c FIXME: Merge *all* these into the main body of the document.
3822 (The AIX/RS6000 native object file format is xcoff with stabs). This
3823 appendix only covers those differences which are not covered in the main
3824 body of this document.
3828 BSD a.out stab types correspond to AIX xcoff storage classes. In general the
3829 mapping is N_STABTYPE becomes C_STABTYPE. Some stab types in a.out
3830 are not supported in xcoff. See Table E. for full mappings.
3833 initialised static N_STSYM and un-initialized static N_LCSYM both map
3834 to the C_STSYM storage class. But the destinction is preserved
3835 because in xcoff N_STSYM and N_LCSYM must be emited in a named static
3836 block. Begin the block with .bs s[RW] data_section_name for N_STSYM
3837 or .bs s bss_section_name for N_LCSYM. End the block with .es
3840 If the xcoff stab is a N_FUN (C_FUN) then follow the string field with
3841 ,. instead of just ,
3845 (I think that's it for .s file differences. They could stand to be
3846 better presented. This is just a list of what I have noticed so far.
3847 There are a *lot* of differences in the information in the symbol
3848 tables of the executable and object files.)
3850 Table E: mapping a.out stab types to xcoff storage classes
3853 stab type storage class
3854 -------------------------------
3863 N_RPSYM (0x8e) C_RPSYM
3873 N_DECL (0x8c) C_DECL
3890 @node Sun-differences
3891 @appendix Differences between GNU stabs and Sun native stabs.
3893 @c FIXME: Merge all this stuff into the main body of the document.
3897 GNU C stabs define *all* types, file or procedure scope, as
3898 N_LSYM. Sun doc talks about using N_GSYM too.
3901 Stabs describing block scopes, N_LBRAC and N_RBRAC are supposed to
3902 contain the nesting level of the block in the desc field, re Sun doc.
3903 GNU stabs always have 0 in that field. dbx seems not to care.
3906 Sun C stabs use type number pairs in the format (a,b) where a is a
3907 number starting with 1 and incremented for each sub-source file in the
3908 compilation. b is a number starting with 1 and incremented for each
3909 new type defined in the compilation. GNU C stabs use the type number
3910 alone, with no source file number.