2 @setfilename stabs.info
7 * Stabs: (stabs). The "stabs" debugging information format.
13 This document describes GNU stabs (debugging symbol tables) in a.out files.
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
71 * Example:: A comprehensive example in C
74 * Symbol tables:: Symbol information in symbol tables
75 * GNU Cplusplus stabs::
78 * Example2.c:: Source code for extended example
79 * Example2.s:: Assembly code for extended example
80 * Quick reference:: Various refernce tables
81 * Expanded reference:: Reference information by stab type
82 * Questions:: Questions and anomolies
83 * xcoff-differences:: Differences between GNU stabs in a.out
84 and GNU stabs in xcoff
85 * Sun-differences:: Differences between GNU stabs and Sun
92 @chapter Overview of stabs
94 @dfn{Stabs} refers to a format for information that describes a program
95 to a debugger. This format was apparently invented by
96 @c FIXME! <<name of inventor>> at
97 the University of California at Berkeley, for the @code{pdx} Pascal
98 debugger; the format has spread widely since then.
101 * Flow:: Overview of debugging information flow
102 * Stabs format:: Overview of stab format
103 * C example:: A simple example in C source
104 * Assembly code:: The simple example at the assembly level
108 @section Overview of debugging information flow
110 The GNU C compiler compiles C source in a @file{.c} file into assembly
111 language in a @file{.s} file, which is translated by the assembler into
112 a @file{.o} file, and then linked with other @file{.o} files and
113 libraries to produce an executable file.
115 With the @samp{-g} option, GCC puts additional debugging information in
116 the @file{.s} file, which is slightly transformed by the assembler and
117 linker, and carried through into the final executable. This debugging
118 information describes features of the source file like line numbers,
119 the types and scopes of variables, and functions, their parameters and
122 For some object file formats, the debugging information is
123 encapsulated in assembler directives known collectively as `stab' (symbol
124 table) directives, interspersed with the generated code. Stabs are
125 the native format for debugging information in the a.out and xcoff
126 object file formats. The GNU tools can also emit stabs in the coff
127 and ecoff object file formats.
129 The assembler adds the information from stabs to the symbol information
130 it places by default in the symbol table and the string table of the
131 @file{.o} file it is building. The linker consolidates the @file{.o}
132 files into one executable file, with one symbol table and one string
133 table. Debuggers use the symbol and string tables in the executable as
134 a source of debugging information about the program.
137 @section Overview of stab format
139 There are three overall formats for stab assembler directives
140 differentiated by the first word of the stab. The name of the directive
141 describes what combination of four possible data fields will follow. It
142 is either @code{.stabs} (string), @code{.stabn} (number), or
145 The overall format of each class of stab is:
148 .stabs "@var{string}",@var{type},0,@var{desc},@var{value}
149 .stabn @var{type},0,@var{desc},@var{value}
150 .stabd @var{type},0,@var{desc}
153 In general, in @code{.stabs} the @var{string} field contains name and type
154 information. For @code{.stabd} the value field is implicit and has the value
155 of the current file location. Otherwise the value field often
156 contains a relocatable address, frame pointer offset, or register
157 number, that maps to the source code element described by the stab.
159 The real key to decoding the meaning of a stab is the number in its type
160 field. Each possible type number defines a different stab type. The
161 stab type further defines the exact interpretation of, and possible
162 values for, any remaining @code{"@var{string}"}, @var{desc}, or
163 @var{value} fields present in the stab. Table A (@pxref{Stab
164 types,,Table A: Symbol types from stabs}) lists in numeric order
165 the possible type field values for stab directives. The reference
166 section that follows Table A describes the meaning of the fields for
167 each stab type in detail. The examples that follow this overview
168 introduce the stab types in terms of the source code elements they
171 For @code{.stabs} the @code{"@var{string}"} field holds the meat of the
172 debugging information. The generally unstructured nature of this field
173 is what makes stabs extensible. For some stab types the string field
174 contains only a name. For other stab types the contents can be a great
177 The overall format is of the @code{"@var{string}"} field is:
180 "@var{name}@r{[}:@var{symbol_descriptor}@r{]}
181 @r{[}@var{type_number}@r{[}=@var{type_descriptor} @r{@dots{}]]}"
184 @var{name} is the name of the symbol represented by the stab.
186 The @var{symbol_descriptor} following the @samp{:} is an alphabetic
187 character that tells more specifically what kind of symbol the stab
188 represents. If the @var{symbol_descriptor} is omitted, but type
189 information follows, then the stab represents a local variable. For a
190 list of symbol_descriptors, see @ref{Symbol descriptors,,Table C: Symbol
193 Type information is either a @var{type_number}, or a
194 @samp{@var{type_number}=}. The @var{type_number} alone is a type
195 reference, referring directly to a type that has already been defined.
197 The @samp{@var{type_number}=} is a type definition, where the number
198 represents a new type which is about to be defined. The type definition
199 may refer to other types by number, and those type numbers may be
200 followed by @samp{=} and nested definitions.
202 In a type definition, if the character that follows the equals sign is
203 non-numeric then it is a @var{type_descriptor}, and tells what kind of
204 type is about to be defined. Any other values following the
205 @var{type_descriptor} vary, depending on the @var{type_descriptor}. If
206 a number follows the @samp{=} then the number is a @var{type_reference}.
207 This is described more thoroughly in the section on types. @xref{Type
208 Descriptors,,Table D: Type Descriptors}, for a list of
209 @var{type_descriptor} values.
211 @c FIXME! "too long" below introduced at J Gilmore's request; used to
212 @c say "more than 80 chars". Why is vaguer better?
213 All this can make the @code{"@var{string}"} field quite long. When the
214 @code{"@var{string}"} part of a stab is too long, the compiler splits
215 the @code{.stabs} directive into two @code{.stabs} directives. Both
216 stabs duplicate exactly all but the @code{"@var{string}"} field. The
217 @code{"@var{string}"} field of the first stab contains the first part of
218 the overlong string, marked as continued with a double-backslash at the
219 end. The @code{"@var{string}"} field of the second stab holds the
220 second half of the overlong string.
223 @section A simple example in C source
225 To get the flavor of how stabs describe source information for a C
226 program, let's look at the simple program:
231 printf("Hello world");
235 When compiled with @samp{-g}, the program above yields the following
236 @file{.s} file. Line numbers have been added to make it easier to refer
237 to parts of the @file{.s} file in the description of the stabs that
241 @section The simple example at the assembly level
245 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
246 3 .stabs "hello.c",100,0,0,Ltext0
249 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
250 7 .stabs "char:t2=r2;0;127;",128,0,0,0
251 8 .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
252 9 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
253 10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
254 11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
255 12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
256 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
257 14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
258 15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
259 16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
260 17 .stabs "float:t12=r1;4;0;",128,0,0,0
261 18 .stabs "double:t13=r1;8;0;",128,0,0,0
262 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
263 20 .stabs "void:t15=15",128,0,0,0
266 23 .ascii "Hello, world!\12\0"
281 38 sethi %hi(LC0),%o1
282 39 or %o1,%lo(LC0),%o0
293 50 .stabs "main:F1",36,0,0,_main
294 51 .stabn 192,0,0,LBB2
295 52 .stabn 224,0,0,LBE2
298 This simple ``hello world'' example demonstrates several of the stab
299 types used to describe C language source files.
301 @node Program structure
302 @chapter Encoding for the structure of the program
305 * Source file:: The path and name of the source file
312 @section The path and name of the source file
321 The first stabs in the .s file contain the name and path of the source
322 file that was compiled to produce the .s file. This information is
323 contained in two records of stab type N_SO (100).
326 .stabs "path_name", N_SO, NIL, NIL, Code_address_of_program_start
327 .stabs "file_name:", N_SO, NIL, NIL, Code_address_of_program_start
331 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
332 3 .stabs "hello.c",100,0,0,Ltext0
338 @section Line Numbers
347 The start of source lines is represented by the @code{N_SLINE} (68) stab
351 .stabn N_SLINE, NIL, @var{line}, @var{address}
354 @var{line} is a source line number; @var{address} represents the code
355 address for the start of that source line.
372 @item Symbol Descriptors:
373 @code{f} (local), @code{F} (global)
376 Procedures are described by the @code{N_FUN} stab type. The symbol
377 descriptor for a procedure is @samp{F} if the procedure is globally
378 scoped and @samp{f} if the procedure is static (locally scoped).
380 The @code{N_FUN} stab representing a procedure is located immediately
381 following the code of the procedure. The @code{N_FUN} stab is in turn
382 directly followed by a group of other stabs describing elements of the
383 procedure. These other stabs describe the procedure's parameters, its
384 block local variables and its block structure.
391 The @code{.stabs} entry after this code fragment shows the @var{name} of
392 the procedure (@code{main}); the type descriptor @var{desc} (@code{F},
393 for a global procedure); a reference to the predefined type @code{int}
394 for the return type; and the starting @var{address} of the procedure.
396 Here is an exploded summary (with whitespace introduced for clarity),
397 followed by line 50 of our sample assembly output, which has this form:
401 @var{desc} @r{(global proc @samp{F})}
402 @var{return_type_ref} @r{(int)}
408 50 .stabs "main:F1",36,0,0,_main
411 @node Block Structure
412 @section Block Structure
418 @code{N_LBRAC}, @code{N_RBRAC}
421 The program's block structure is represented by the @code{N_LBRAC} (left
422 brace) and the @code{N_RBRAC} (right brace) stab types. The following code
423 range, which is the body of @code{main}, is labeled with @samp{LBB2:} at the
424 beginning and @samp{LBE2:} at the end.
428 38 sethi %hi(LC0),%o1
429 39 or %o1,%lo(LC0),%o0
437 The @code{N_LBRAC} and @code{N_RBRAC} stabs that describe the block
438 scope of the procedure are located after the @code{N_FUNC} stab that
439 represents the procedure itself. The @code{N_LBRAC} uses the
440 @code{LBB2} label as the code address in its value field, and the
441 @code{N_RBRAC} uses @code{LBE2}.
444 50 .stabs "main:F1",36,0,0,_main
448 .stabn N_LBRAC, NIL, NIL, @var{left-brace-address}
449 .stabn N_RBRAC, NIL, NIL, @var{right-brace-address}
453 51 .stabn 192,0,0,LBB2
454 52 .stabn 224,0,0,LBE2
458 @chapter Simple types
461 * Basic types:: Basic type definitions
462 * Range types:: Range types defined by min and max value
463 * Float "range" types:: Range type defined by size in bytes
467 @section Basic type definitions
474 @item Symbol Descriptor:
478 The basic types for the language are described using the @code{N_LSYM} stab
479 type. They are boilerplate and are emited by the compiler for each
480 compilation unit. Basic type definitions are not always a complete
481 description of the type and are sometimes circular. The debugger
482 recognizes the type anyway, and knows how to read bits as that type.
484 Each language and compiler defines a slightly different set of basic
485 types. In this example we are looking at the basic types for C emited
486 by the GNU compiler targeting the Sun4. Here the basic types are
487 mostly defined as range types.
491 @section Range types defined by min and max value
494 @item Type Descriptor:
498 When defining a range type, if the number after the first semicolon is
499 smaller than the number after the second one, then the two numbers
500 represent the smallest and the largest values in the range.
507 @var{descriptor} @r{(type)}
514 N_LSYM, NIL, NIL, NIL
516 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
517 7 .stabs "char:t2=r2;0;127;",128,0,0,0
520 Here the integer type (@code{1}) is defined as a range of the integer
521 type (@code{1}). Likewise @code{char} is a range of @code{char}. This
522 part of the definition is circular, but at least the high and low bound
523 values of the range hold more information about the type.
525 Here short unsigned int is defined as type number 8 and described as a
526 range of type @code{int}, with a minimum value of 0 and a maximum of 65535.
529 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
532 @node Float "range" types
533 @section Range type defined by size in bytes
536 @item Type Descriptor:
540 In a range definition, if the first number after the semicolon is
541 positive and the second is zero, then the type being defined is a
542 floating point type, and the number after the first semicolon is the
543 number of bytes needed to represent the type. Note that this does not
544 provide a way to distinguish 8-byte real floating point types from
545 8-byte complex floating point types.
556 N_LSYM, NIL, NIL, NIL
558 17 .stabs "float:t12=r1;4;0;",128,0,0,0
559 18 .stabs "double:t13=r1;8;0;",128,0,0,0
560 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
563 Cosmically enough, the @code{void} type is defined directly in terms of
573 20 .stabs "void:t15=15",128,0,0,0
578 @chapter A Comprehensive Example in C
580 Now we'll examine a second program, @code{example2}, which builds on the
581 first example to introduce the rest of the stab types, symbol
582 descriptors, and type descriptors used in C.
583 @xref{Example2.c} for the complete @file{.c} source,
584 and @pxref{Example2.s} for the @file{.s} assembly code.
585 This description includes parts of those files.
587 @section Flow of control and nested scopes
593 @code{N_SLINE}, @code{N_LBRAC}, @code{N_RBRAC} (cont.)
596 Consider the body of @code{main}, from @file{example2.c}. It shows more
597 about how @code{N_SLINE}, @code{N_RBRAC}, and @code{N_LBRAC} stabs are used.
601 21 static float s_flap;
603 23 for (times=0; times < s_g_repeat; times++)@{
605 25 printf ("Hello world\n");
610 Here we have a single source line, the @samp{for} line, that generates
611 non-linear flow of control, and non-contiguous code. In this case, an
612 @code{N_SLINE} stab with the same line number proceeds each block of
613 non-contiguous code generated from the same source line.
615 The example also shows nested scopes. The @code{N_LBRAC} and
616 @code{N_LBRAC} stabs that describe block structure are nested in the
617 same order as the corresponding code blocks, those of the for loop
618 inside those for the body of main.
621 This is the label for the @code{N_LBRAC} (left brace) stab marking the
622 start of @code{main}.
629 In the first code range for C source line 23, the @code{for} loop
630 initialize and test, @code{N_SLINE} (68) records the line number:
637 58 .stabn 68,0,23,LM2
641 62 sethi %hi(_s_g_repeat),%o0
643 64 ld [%o0+%lo(_s_g_repeat)],%o0
648 @exdent label for the @code{N_LBRAC} (start block) marking the start of @code{for} loop
651 69 .stabn 68,0,25,LM3
653 71 sethi %hi(LC0),%o1
654 72 or %o1,%lo(LC0),%o0
657 75 .stabn 68,0,26,LM4
660 @exdent label for the @code{N_RBRAC} (end block) stab marking the end of the @code{for} loop
666 Now we come to the second code range for source line 23, the @code{for}
667 loop increment and return. Once again, @code{N_SLINE} (68) records the
671 .stabn, N_SLINE, NIL,
675 78 .stabn 68,0,23,LM5
683 86 .stabn 68,0,27,LM6
686 @exdent label for the @code{N_RBRAC} (end block) stab marking the end of the @code{for} loop
689 89 .stabn 68,0,27,LM7
694 94 .stabs "main:F1",36,0,0,_main
695 95 .stabs "argc:p1",160,0,0,68
696 96 .stabs "argv:p20=*21=*2",160,0,0,72
697 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
698 98 .stabs "times:1",128,0,0,-20
702 Here is an illustration of stabs describing nested scopes. The scope
703 nesting is reflected in the nested bracketing stabs (@code{N_LBRAC},
707 .stabn N_LBRAC,NIL,NIL,
708 @var{block-start-address}
710 99 .stabn 192,0,0,LBB2 ## begin proc label
711 100 .stabs "inner:1",128,0,0,-24
712 101 .stabn 192,0,0,LBB3 ## begin for label
716 @code{N_RBRAC} (224), ``right brace'' ends a lexical block (scope).
719 .stabn N_RBRAC,NIL,NIL,
720 @var{block-end-address}
722 102 .stabn 224,0,0,LBE3 ## end for label
723 103 .stabn 224,0,0,LBE2 ## end proc label
730 * Automatic variables:: locally scoped
732 * Register variables::
733 * Initialized statics::
734 * Un-initialized statics::
738 @node Automatic variables
739 @section Locally scoped automatic variables
746 @item Symbol Descriptor:
751 In addition to describing types, the @code{N_LSYM} stab type also
752 describes locally scoped automatic variables. Refer again to the body
753 of @code{main} in @file{example2.c}. It allocates two automatic
754 variables: @samp{times} is scoped to the body of @code{main}, and
755 @samp{inner} is scoped to the body of the @code{for} loop.
756 @samp{s_flap} is locally scoped but not automatic, and will be discussed
761 21 static float s_flap;
763 23 for (times=0; times < s_g_repeat; times++)@{
765 25 printf ("Hello world\n");
770 The @code{N_LSYM} stab for an automatic variable is located just before the
771 @code{N_LBRAC} stab describing the open brace of the block to which it is
775 @exdent @code{N_LSYM} (128): automatic variable, scoped locally to @code{main}
780 @var{frame-pointer-offset}
782 98 .stabs "times:1",128,0,0,-20
783 99 .stabn 192,0,0,LBB2 ## begin `main' N_LBRAC
785 @exdent @code{N_LSYM} (128): automatic variable, scoped locally to the @code{for} loop
790 @var{frame-pointer-offset}
792 100 .stabs "inner:1",128,0,0,-24
793 101 .stabn 192,0,0,LBB3 ## begin `for' loop N_LBRAC
796 Since the character in the string field following the colon is not a
797 letter, there is no symbol descriptor. This means that the stab
798 describes a local variable, and that the number after the colon is a
799 type reference. In this case it a a reference to the basic type @code{int}.
800 Notice also that the frame pointer offset is negative number for
804 @node Global Variables
805 @section Global Variables
812 @item Symbol Descriptor:
816 Global variables are represented by the @code{N_GSYM} stab type. The symbol
817 descriptor, following the colon in the string field, is @samp{G}. Following
818 the @samp{G} is a type reference or type definition. In this example it is a
819 type reference to the basic C type, @code{char}. The first source line in
827 yields the following stab. The stab immediately precedes the code that
828 allocates storage for the variable it describes.
831 @exdent @code{N_GSYM} (32): global symbol
836 N_GSYM, NIL, NIL, NIL
838 21 .stabs "g_foo:G2",32,0,0,0
845 The address of the variable represented by the @code{N_GSYM} is not contained
846 in the @code{N_GSYM} stab. The debugger gets this information from the
847 external symbol for the global variable.
849 @node Register variables
850 @section Global register variables
857 @item Symbol Descriptor:
861 The following source line defines a global variable, @code{g_bar}, which is
862 explicitly allocated in global register @code{%g5}.
865 2 register int g_bar asm ("%g5");
868 Register variables have their own stab type, @code{N_RSYM}, and their own
869 symbol descriptor, @code{r}. The stab's value field contains the number of
870 the register where the variable data will be stored. Since the
871 variable was not initialized in this compilation unit, the stab is
872 emited at the end of the object file, with the stabs for other
873 uninitialized globals (@code{bcc}).
876 @exdent @code{N_RSYM} (64): register variable
884 133 .stabs "g_bar:r1",64,0,0,5
888 @node Initialized statics
889 @section Initialized static variables
896 @item Symbol Descriptors:
897 @code{S} (file scope), @code{V} (procedure scope)
900 Initialized static variables are represented by the @code{N_STSYM} stab
901 type. The symbol descriptor part of the string field shows if the
902 variable is file scope static (@samp{S}) or procedure scope static
903 (@samp{V}). The source line
906 3 static int s_g_repeat = 2;
910 yields the following code. The stab is located immediately preceding
911 the storage for the variable it represents. Since the variable in
912 this example is file scope static the symbol descriptor is @samp{S}.
915 @exdent @code{N_STSYM} (38): initialized static variable (data seg w/internal linkage)
923 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
930 @node Un-initialized statics
931 @section Un-initialized static variables
938 @item Symbol Descriptors:
939 @code{S} (file scope), @code{V} (procedure scope)
942 Un-initialized static variables are represented by the @code{N_LCSYM}
943 stab type. The symbol descriptor part of the string shows if the
944 variable is file scope static (@samp{S}) or procedure scope static
945 (@samp{V}). In this example it is procedure scope static. The source
946 line allocating @code{s_flap} immediately follows the open brace for the
947 procedure @code{main}.
951 21 static float s_flap;
954 The code that reserves storage for the variable @code{s_flap} precedes the
955 body of body of @code{main}.
958 39 .reserve _s_flap.0,4,"bss",4
961 But since @code{s_flap} is scoped locally to @code{main}, its stab is
962 located with the other stabs representing symbols local to @code{main}.
963 The stab for @code{s_flap} is located just before the @code{N_LBRAC} for
967 @exdent @code{N_LCSYM} (40): uninitialized static var (BSS seg w/internal linkage)
975 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
976 98 .stabs "times:1",128,0,0,-20
977 99 .stabn 192,0,0,LBB2 # N_LBRAC for main.
980 @c ............................................................
990 @item Symbol Descriptor:
994 Procedure parameters are represented by the N_PSYM stab type. The
995 following source lines show the parameters of the main routine.
1004 The N_PSYM stabs describing parameters to a function directly follow
1005 the N_FUN stab that represents the procedure itself. The N_FUN stab
1006 immediately follows the code of the procedure it describes. Following
1007 the N_PSYM parameter stabs are any N_LSYM stabs representing local
1011 @exdent <36> N_FUN - describing the procedure main
1013 94 .stabs "main:F1",36,0,0,_main
1015 @exdent <160> N_PSYM - parameters
1016 @exdent .stabs "name:sym_desc(value_param)type_ref(int)", N_PSYM,
1017 @exdent NIL, NIL, frame_ptr_offset
1019 95 .stabs "argc:p1",160,0,0,68
1021 @exdent <160> N_PSYM - parameter
1022 @exdent .stabs "name:sym_desc(value_param)type_def(20)=ptr_to type_def(21)=
1023 @exdent ptr_to type_ref(char)
1025 96 .stabs "argv:p20=*21=*2",160,0,0,72
1028 The type definition of argv is interesting because it defines two new
1029 types in terms of an existing one. The array argv contains character
1030 pointers. The type of the array name is a pointer to the type the
1031 array holds. Thus the type of argv is ptr to ptr to char. The stab
1032 for argv contains nested type_definitions. Type 21 is ptr to type 2
1033 (char) and argv (type 20) is ptr to type 21.
1035 @node Aggregate Types
1036 @chapter Aggregate Types
1038 Now let's look at some variable definitions involving complex types.
1039 This involves understanding better how types are described. In the
1040 examples so far types have been described as references to previously
1041 defined types or defined in terms of subranges of or pointers to
1042 previously defined types. The section that follows will talk about
1043 the various other type descriptors that may follow the = sign in a
1056 @section Array types
1062 @code{N_GSYM}, @code{N_LSYM}
1063 @item Symbol Descriptor:
1065 @item Type Descriptor:
1069 As an example of an array type consider the global variable below.
1072 15 char char_vec[3] = @{'a','b','c'@};
1075 Since the array is a global variable, it is described by the N_GSYM
1076 stab type. The symbol descriptor G, following the colon in stab's
1077 string field, also says the array is a global variable. Following the
1078 G is a definition for type (19) as shown by the equals sign after the
1081 After the equals sign is a type descriptor, ar, which says that the
1082 type being defined is an array. Following the type descriptor for an
1083 array is the type of the index, a null field, the upper bound of the
1084 array indexing, and the type of the array elements.
1086 The array definition above generates the assembly language that
1090 @exdent <32> N_GSYM - global variable
1091 @exdent .stabs "name:sym_desc(global)type_def(19)=type_desc(array)
1092 @exdent index_type_ref(int);NIL;high_bound(2);element_type_ref(char)";
1093 @exdent N_GSYM, NIL, NIL, NIL
1095 32 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
1096 33 .global _char_vec
1105 @section Enumerations
1112 @item Symbol Descriptor:
1114 @item Type Descriptor:
1118 The source line below declares an enumeration type. It is defined at
1119 file scope between the bodies of main and s_proc in example2.c.
1120 Because the N_LSYM is located after the N_RBRAC that marks the end of
1121 the previous procedure's block scope, and before the N_FUN that marks
1122 the beginning of the next procedure's block scope, the N_LSYM does not
1123 describe a block local symbol, but a file local one. The source line:
1126 29 enum e_places @{first,second=3,last@};
1130 generates the following stab, located just after the N_RBRAC (close
1131 brace stab) for main. The type definition is in an N_LSYM stab
1132 because type definitions are file scope not global scope.
1135 <128> N_LSYM - local symbol
1136 .stab "name:sym_dec(type)type_def(22)=sym_desc(enum)
1137 enum_name:value(0),enum_name:value(3),enum_name:value(4),;",
1138 N_LSYM, NIL, NIL, NIL
1142 104 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
1145 The symbol descriptor (T) says that the stab describes a structure,
1146 enumeration, or type tag. The type descriptor e, following the 22= of
1147 the type definition narrows it down to an enumeration type. Following
1148 the e is a list of the elements of the enumeration. The format is
1149 name:value,. The list of elements ends with a ;.
1151 @node Structure tags
1152 @section Structure Tags
1159 @item Symbol Descriptor:
1161 @item Type Descriptor:
1165 The following source code declares a structure tag and defines an
1166 instance of the structure in global scope. Then a typedef equates the
1167 structure tag with a new type. A seperate stab is generated for the
1168 structure tag, the structure typedef, and the structure instance. The
1169 stabs for the tag and the typedef are emited when the definitions are
1170 encountered. Since the structure elements are not initialized, the
1171 stab and code for the structure variable itself is located at the end
1172 of the program in .common.
1178 9 char s_char_vec[8];
1179 10 struct s_tag* s_next;
1182 13 typedef struct s_tag s_typedef;
1185 The structure tag is an N_LSYM stab type because, like the enum, the
1186 symbol is file scope. Like the enum, the symbol descriptor is T, for
1187 enumeration, struct or tag type. The symbol descriptor s following
1188 the 16= of the type definition narrows the symbol type to struct.
1190 Following the struct symbol descriptor is the number of bytes the
1191 struct occupies, followed by a description of each structure element.
1192 The structure element descriptions are of the form name:type, bit
1193 offset from the start of the struct, and number of bits in the
1198 <128> N_LSYM - type definition
1199 .stabs "name:sym_desc(struct tag) Type_def(16)=type_desc(struct type)
1201 elem_name:type_ref(int),bit_offset,field_bits;
1202 elem_name:type_ref(float),bit_offset,field_bits;
1203 elem_name:type_def(17)=type_desc(dynamic array) index_type(int);NIL;
1204 high_bound(7);element_type(char),bit_offset,field_bits;;",
1207 30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;
1208 s_char_vec:17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
1211 In this example, two of the structure elements are previously defined
1212 types. For these, the type following the name: part of the element
1213 description is a simple type reference. The other two structure
1214 elements are new types. In this case there is a type definition
1215 embedded after the name:. The type definition for the array element
1216 looks just like a type definition for a standalone array. The s_next
1217 field is a pointer to the same kind of structure that the field is an
1218 element of. So the definition of structure type 16 contains an type
1219 definition for an element which is a pointer to type 16.
1229 @item Symbol Descriptor:
1233 Here is the stab for the typedef equating the structure tag with a
1237 <128> N_LSYM - type definition
1238 .stabs "name:sym_desc(type name)type_ref(struct_tag)",N_LSYM,NIL,NIL,NIL
1242 31 .stabs "s_typedef:t16",128,0,0,0
1245 And here is the code generated for the structure variable.
1248 <32> N_GSYM - global symbol
1249 .stabs "name:sym_desc(global)type_ref(struct_tag)",N_GSYM,NIL,NIL,NIL
1253 136 .stabs "g_an_s:G16",32,0,0,0
1254 137 .common _g_an_s,20,"bss"
1257 Notice that the structure tag has the same type number as the typedef
1258 for the structure tag. It is impossible to distinguish between a
1259 variable of the struct type and one of its typedef by looking at the
1260 debugging information.
1271 @item Symbol Descriptor:
1273 @item Type Descriptor:
1277 Next let's look at unions. In example2 this union type is declared
1278 locally to a procedure and an instance of the union is defined.
1288 This code generates a stab for the union tag and a stab for the union
1289 variable. Both use the N_LSYM stab type. Since the union variable is
1290 scoped locally to the procedure in which it is defined, its stab is
1291 located immediately preceding the N_LBRAC for the procedure's block
1294 The stab for the union tag, however is located preceding the code for
1295 the procedure in which it is defined. The stab type is N_LSYM. This
1296 would seem to imply that the union type is file scope, like the struct
1297 type s_tag. This is not true. The contents and position of the stab
1298 for u_type do not convey any infomation about its procedure local
1303 .stabs "name:sym_desc(union tag)type_def(22)=type_desc(union)
1305 elem_name:type_ref(int),bit_offset(0),bit_size(32);
1306 elem_name:type_ref(float),bit_offset(0),bit_size(32);
1307 elem_name:type_ref(ptr to char),bit_offset(0),bit_size(32);;"
1308 N_LSYM, NIL, NIL, NIL
1312 105 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
1316 The symbol descriptor, T, following the name: means that the stab
1317 describes an enumeration struct or type tag. The type descriptor u,
1318 following the 23= of the type definition, narrows it down to a union
1319 type definition. Following the u is the number of bytes in the union.
1320 After that is a list of union element descriptions. Their format is
1321 name:type, bit offset into the union, and number of bytes for the
1324 The stab for the union variable follows. Notice that the frame
1325 pointer offset for local variables is negative.
1328 <128> N_LSYM - local variable (with no symbol descriptor)
1329 .stabs "name:type_ref(u_tag)", N_LSYM, NIL, NIL, frame_ptr_offset
1333 130 .stabs "an_u:23",128,0,0,-20
1336 @node Function types
1337 @section Function types
1343 The last type descriptor in C which remains to be described is used
1344 for function types. Consider the following source line defining a
1345 global function pointer.
1351 It generates the following code. Since the variable is not
1352 initialized, the code is located in the common area at the end of the
1356 <32> N_GSYM - global variable
1357 .stabs "name:sym_desc(global)type_def(24)=ptr_to(25)=
1358 type_def(func)type_ref(int)
1362 134 .stabs "g_pf:G24=*25=f1",32,0,0,0
1363 135 .common _g_pf,4,"bss"
1366 Since the variable is global, the stab type is N_GSYM and the symbol
1367 descriptor is G. The variable defines a new type, 24, which is a
1368 pointer to another new type, 25, which is defined as a function
1372 @chapter Symbol information in symbol tables
1374 This section examines more closely the format of symbol table entries
1375 and how stab assembler directives map to them. It also describes what
1376 transformations the assembler and linker make on data from stabs.
1378 Each time the assembler encounters a stab in its input file it puts
1379 each field of the stab into corresponding fields in a symbol table
1380 entry of its output file. If the stab contains a string field, the
1381 symbol table entry for that stab points to a string table entry
1382 containing the string data from the stab. Assembler labels become
1383 relocatable addresses. Symbol table entries in a.out have the format:
1386 struct internal_nlist @{
1387 unsigned long n_strx; /* index into string table of name */
1388 unsigned char n_type; /* type of symbol */
1389 unsigned char n_other; /* misc info (usually empty) */
1390 unsigned short n_desc; /* description field */
1391 bfd_vma n_value; /* value of symbol */
1395 For .stabs directives, the n_strx field holds the character offset
1396 from the start of the string table to the string table entry
1397 containing the "string" field. For other classes of stabs (.stabn and
1398 .stabd) this field is null.
1400 Symbol table entries with n_type fields containing a value greater or
1401 equal to 0x20 originated as stabs generated by the compiler (with one
1402 random exception). Those with n_type values less than 0x20 were
1403 placed in the symbol table of the executable by the assembler or the
1406 The linker concatenates object files and does fixups of externally
1407 defined symbols. You can see the transformations made on stab data by
1408 the assembler and linker by examining the symbol table after each pass
1409 of the build, first the assemble and then the link.
1411 To do this use nm with the -ap options. This dumps the symbol table,
1412 including debugging information, unsorted. For stab entries the
1413 columns are: value, other, desc, type, string. For assembler and
1414 linker symbols, the columns are: value, type, string.
1416 There are a few important things to notice about symbol tables. Where
1417 the value field of a stab contains a frame pointer offset, or a
1418 register number, that value is unchanged by the rest of the build.
1420 Where the value field of a stab contains an assembly language label,
1421 it is transformed by each build step. The assembler turns it into a
1422 relocatable address and the linker turns it into an absolute address.
1423 This source line defines a static variable at file scope:
1426 3 static int s_g_repeat
1430 The following stab describes the symbol.
1433 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
1437 The assembler transforms the stab into this symbol table entry in the
1438 @file{.o} file. The location is expressed as a data segment offset.
1441 21 00000084 - 00 0000 STSYM s_g_repeat:S1
1445 in the symbol table entry from the executable, the linker has made the
1446 relocatable address absolute.
1449 22 0000e00c - 00 0000 STSYM s_g_repeat:S1
1452 Stabs for global variables do not contain location information. In
1453 this case the debugger finds location information in the assembler or
1454 linker symbol table entry describing the variable. The source line:
1464 21 .stabs "g_foo:G2",32,0,0,0
1467 The variable is represented by the following two symbol table entries
1468 in the object file. The first one originated as a stab. The second
1469 one is an external symbol. The upper case D signifies that the n_type
1470 field of the symbol table contains 7, N_DATA with local linkage (see
1471 Table B). The value field following the file's line number is empty
1472 for the stab entry. For the linker symbol it contains the
1473 rellocatable address corresponding to the variable.
1476 19 00000000 - 00 0000 GSYM g_foo:G2
1477 20 00000080 D _g_foo
1481 These entries as transformed by the linker. The linker symbol table
1482 entry now holds an absolute address.
1485 21 00000000 - 00 0000 GSYM g_foo:G2
1487 215 0000e008 D _g_foo
1490 @node GNU Cplusplus stabs
1491 @chapter GNU C++ stabs
1494 * Basic Cplusplus types::
1497 * Methods:: Method definition
1499 * Method Modifiers:: (const, volatile, const volatile)
1502 * Virtual Base Classes::
1507 @subsection Symbol descriptors added for C++ descriptions:
1510 P - register parameter.
1513 @subsection type descriptors added for C++ descriptions
1517 method type (two ## if minimal debug)
1524 @node Basic Cplusplus types
1525 @section Basic types for C++
1527 << the examples that follow are based on a01.C >>
1530 C++ adds two more builtin types to the set defined for C. These are
1531 the unknown type and the vtable record type. The unknown type, type
1532 16, is defined in terms of itself like the void type.
1534 The vtable record type, type 17, is defined as a structure type and
1535 then as a structure tag. The structure has four fields, delta, index,
1536 pfn, and delta2. pfn is the function pointer.
1538 << In boilerplate $vtbl_ptr_type, what are the fields delta,
1539 index, and delta2 used for? >>
1541 This basic type is present in all C++ programs even if there are no
1542 virtual methods defined.
1545 .stabs "struct_name:sym_desc(type)type_def(17)=type_desc(struct)struct_bytes(8)
1546 elem_name(delta):type_ref(short int),bit_offset(0),field_bits(16);
1547 elem_name(index):type_ref(short int),bit_offset(16),field_bits(16);
1548 elem_name(pfn):type_def(18)=type_desc(ptr to)type_ref(void),
1549 bit_offset(32),field_bits(32);
1550 elem_name(delta2):type_def(short int);bit_offset(32),field_bits(16);;"
1555 .stabs "$vtbl_ptr_type:t17=s8
1556 delta:6,0,16;index:6,16,16;pfn:18=*15,32,32;delta2:6,32,16;;"
1561 .stabs "name:sym_dec(struct tag)type_ref($vtbl_ptr_type)",N_LSYM,NIL,NIL,NIL
1565 .stabs "$vtbl_ptr_type:T17",128,0,0,0
1568 @node Simple classes
1569 @section Simple class definition
1571 The stabs describing C++ language features are an extension of the
1572 stabs describing C. Stabs representing C++ class types elaborate
1573 extensively on the stab format used to describe structure types in C.
1574 Stabs representing class type variables look just like stabs
1575 representing C language variables.
1577 Consider the following very simple class definition.
1583 int Ameth(int in, char other);
1587 The class baseA is represented by two stabs. The first stab describes
1588 the class as a structure type. The second stab describes a structure
1589 tag of the class type. Both stabs are of stab type N_LSYM. Since the
1590 stab is not located between an N_FUN and a N_LBRAC stab this indicates
1591 that the class is defined at file scope. If it were, then the N_LSYM
1592 would signify a local variable.
1594 A stab describing a C++ class type is similar in format to a stab
1595 describing a C struct, with each class member shown as a field in the
1596 structure. The part of the struct format describing fields is
1597 expanded to include extra information relevent to C++ class members.
1598 In addition, if the class has multiple base classes or virtual
1599 functions the struct format outside of the field parts is also
1602 In this simple example the field part of the C++ class stab
1603 representing member data looks just like the field part of a C struct
1604 stab. The section on protections describes how its format is
1605 sometimes extended for member data.
1607 The field part of a C++ class stab representing a member function
1608 differs substantially from the field part of a C struct stab. It
1609 still begins with `name:' but then goes on to define a new type number
1610 for the member function, describe its return type, its argument types,
1611 its protection level, any qualifiers applied to the method definition,
1612 and whether the method is virtual or not. If the method is virtual
1613 then the method description goes on to give the vtable index of the
1614 method, and the type number of the first base class defining the
1617 When the field name is a method name it is followed by two colons
1618 rather than one. This is followed by a new type definition for the
1619 method. This is a number followed by an equal sign and then the
1620 symbol descriptor `##', indicating a method type. This is followed by
1621 a type reference showing the return type of the method and a
1624 The format of an overloaded operator method name differs from that
1625 of other methods. It is "op$::XXXX." where XXXX is the operator name
1626 such as + or +=. The name ends with a period, and any characters except
1627 the period can occur in the XXXX string.
1629 The next part of the method description represents the arguments to
1630 the method, preceeded by a colon and ending with a semi-colon. The
1631 types of the arguments are expressed in the same way argument types
1632 are expressed in C++ name mangling. In this example an int and a char
1635 This is followed by a number, a letter, and an asterisk or period,
1636 followed by another semicolon. The number indicates the protections
1637 that apply to the member function. Here the 2 means public. The
1638 letter encodes any qualifier applied to the method definition. In
1639 this case A means that it is a normal function definition. The dot
1640 shows that the method is not virtual. The sections that follow
1641 elaborate further on these fields and describe the additional
1642 information present for virtual methods.
1646 .stabs "class_name:sym_desc(type)type_def(20)=type_desc(struct)struct_bytes(4)
1647 field_name(Adat):type(int),bit_offset(0),field_bits(32);
1649 method_name(Ameth)::type_def(21)=type_desc(method)return_type(int);
1650 :arg_types(int char);
1651 protection(public)qualifier(normal)virtual(no);;"
1656 .stabs "baseA:t20=s4Adat:1,0,32;Ameth::21=##1;:ic;2A.;;",128,0,0,0
1658 .stabs "class_name:sym_desc(struct tag)",N_LSYM,NIL,NIL,NIL
1660 .stabs "baseA:T20",128,0,0,0
1663 @node Class instance
1664 @section Class instance
1666 As shown above, describing even a simple C++ class definition is
1667 accomplished by massively extending the stab format used in C to
1668 describe structure types. However, once the class is defined, C stabs
1669 with no modifications can be used to describe class instances. The
1679 yields the following stab describing the class instance. It looks no
1680 different from a standard C stab describing a local variable.
1683 .stabs "name:type_ref(baseA)", N_LSYM, NIL, NIL, frame_ptr_offset
1687 .stabs "AbaseA:20",128,0,0,-20
1691 @section Method defintion
1693 The class definition shown above declares Ameth. The C++ source below
1698 baseA::Ameth(int in, char other)
1705 This method definition yields three stabs following the code of the
1706 method. One stab describes the method itself and following two
1707 describe its parameters. Although there is only one formal argument
1708 all methods have an implicit argument which is the `this' pointer.
1709 The `this' pointer is a pointer to the object on which the method was
1710 called. Note that the method name is mangled to encode the class name
1711 and argument types. << Name mangling is not described by this
1712 document - Is there already such a doc? >>
1715 .stabs "name:symbol_desriptor(global function)return_type(int)",
1716 N_FUN, NIL, NIL, code_addr_of_method_start
1718 .stabs "Ameth__5baseAic:F1",36,0,0,_Ameth__5baseAic
1721 Here is the stab for the `this' pointer implicit argument. The name
1722 of the `this' pointer is always $t. Type 19, the `this' pointer is
1723 defined as a pointer to type 20, baseA, but a stab defining baseA has
1724 not yet been emited. Since the compiler knows it will be emited
1725 shortly, here it just outputs a cross reference to the undefined
1726 symbol, by prefixing the symbol name with xs.
1729 .stabs "name:sym_desc(register param)type_def(19)=
1730 type_desc(ptr to)type_ref(baseA)=
1731 type_desc(cross-reference to)baseA:",N_RSYM,NIL,NIL,register_number
1733 .stabs "$t:P19=*20=xsbaseA:",64,0,0,8
1736 The stab for the explicit integer argument looks just like a parameter
1737 to a C function. The last field of the stab is the offset from the
1738 argument pointer, which in most systems is the same as the frame
1742 .stabs "name:sym_desc(value parameter)type_ref(int)",
1743 N_PSYM,NIL,NIL,offset_from_arg_ptr
1745 .stabs "in:p1",160,0,0,72
1748 << The examples that follow are based on A1.C >>
1751 @section Protections
1754 In the simple class definition shown above all member data and
1755 functions were publicly accessable. The example that follows
1756 contrasts public, protected and privately accessable fields and shows
1757 how these protections are encoded in C++ stabs.
1759 Protections for class member data are signified by two characters
1760 embeded in the stab defining the class type. These characters are
1761 located after the name: part of the string. /0 means private, /1
1762 means protected, and /2 means public. If these characters are omited
1763 this means that the member is public. The following C++ source:
1777 generates the following stab to describe the class type all_data.
1780 .stabs "class_name:sym_desc(type)type_def(19)=type_desc(struct)struct_bytes
1781 data_name:/protection(private)type_ref(int),bit_offset,num_bits;
1782 data_name:/protection(protected)type_ref(char),bit_offset,num_bits;
1783 data_name:(/num omited, private)type_ref(float),bit_offset,num_bits;;"
1788 .stabs "all_data:t19=s12
1789 priv_dat:/01,0,32;prot_dat:/12,32,8;pub_dat:12,64,32;;",128,0,0,0
1792 Protections for member functions are signified by one digit embeded in
1793 the field part of the stab describing the method. The digit is 0 if
1794 private, 1 if protected and 2 if public. Consider the C++ class
1798 class all_methods @{
1800 int priv_meth(int in)@{return in;@};
1802 char protMeth(char in)@{return in;@};
1804 float pubMeth(float in)@{return in;@};
1808 It generates the following stab. The digit in question is to the left
1809 of an `A' in each case. Notice also that in this case two symbol
1810 descriptors apply to the class name struct tag and struct type.
1813 .stabs "class_name:sym_desc(struct tag&type)type_def(21)=
1814 sym_desc(struct)struct_bytes(1)
1815 meth_name::type_def(22)=sym_desc(method)returning(int);
1816 :args(int);protection(private)modifier(normal)virtual(no);
1817 meth_name::type_def(23)=sym_desc(method)returning(char);
1818 :args(char);protection(protected)modifier(normal)virual(no);
1819 meth_name::type_def(24)=sym_desc(method)returning(float);
1820 :args(float);protection(public)modifier(normal)virtual(no);;",
1825 .stabs "all_methods:Tt21=s1priv_meth::22=##1;:i;0A.;protMeth::23=##2;:c;1A.;
1826 pubMeth::24=##12;:f;2A.;;",128,0,0,0
1829 @node Method Modifiers
1830 @section Method Modifiers (const, volatile, const volatile)
1834 In the class example described above all the methods have the normal
1835 modifier. This method modifier information is located just after the
1836 protection information for the method. This field has four possible
1837 character values. Normal methods use A, const methods use B, volatile
1838 methods use C, and const volatile methods use D. Consider the class
1844 int ConstMeth (int arg) const @{ return arg; @};
1845 char VolatileMeth (char arg) volatile @{ return arg; @};
1846 float ConstVolMeth (float arg) const volatile @{return arg; @};
1850 This class is described by the following stab:
1853 .stabs "class(A):sym_desc(struct)type_def(20)=type_desc(struct)struct_bytes(1)
1854 meth_name(ConstMeth)::type_def(21)sym_desc(method)
1855 returning(int);:arg(int);protection(public)modifier(const)virtual(no);
1856 meth_name(VolatileMeth)::type_def(22)=sym_desc(method)
1857 returning(char);:arg(char);protection(public)modifier(volatile)virt(no)
1858 meth_name(ConstVolMeth)::type_def(23)=sym_desc(method)
1859 returning(float);:arg(float);protection(public)modifer(const volatile)
1860 virtual(no);;", @dots{}
1864 .stabs "A:T20=s1ConstMeth::21=##1;:i;2B.;VolatileMeth::22=##2;:c;2C.;
1865 ConstVolMeth::23=##12;:f;2D.;;",128,0,0,0
1868 @node Virtual Methods
1869 @section Virtual Methods
1871 << The following examples are based on a4.C >>
1873 The presence of virtual methods in a class definition adds additional
1874 data to the class description. The extra data is appended to the
1875 description of the virtual method and to the end of the class
1876 description. Consider the class definition below:
1882 virtual int A_virt (int arg) @{ return arg; @};
1886 This results in the stab below describing class A. It defines a new
1887 type (20) which is an 8 byte structure. The first field of the class
1888 struct is Adat, an integer, starting at structure offset 0 and
1891 The second field in the class struct is not explicitly defined by the
1892 C++ class definition but is implied by the fact that the class
1893 contains a virtual method. This field is the vtable pointer. The
1894 name of the vtable pointer field starts with $vf and continues with a
1895 type reference to the class it is part of. In this example the type
1896 reference for class A is 20 so the name of its vtable pointer field is
1897 $vf20, followed by the usual colon.
1899 Next there is a type definition for the vtable pointer type (21).
1900 This is in turn defined as a pointer to another new type (22).
1902 Type 22 is the vtable itself, which is defined as an array, indexed by
1903 integers, with a high bound of 1, and elements of type 17. Type 17
1904 was the vtable record type defined by the boilerplate C++ type
1905 definitions, as shown earlier.
1907 The bit offset of the vtable pointer field is 32. The number of bits
1908 in the field are not specified when the field is a vtable pointer.
1910 Next is the method definition for the virtual member function A_virt.
1911 Its description starts out using the same format as the non-virtual
1912 member functions described above, except instead of a dot after the
1913 `A' there is an asterisk, indicating that the function is virtual.
1914 Since is is virtual some addition information is appended to the end
1915 of the method description.
1917 The first number represents the vtable index of the method. This is a
1918 32 bit unsigned number with the high bit set, followed by a
1921 The second number is a type reference to the first base class in the
1922 inheritence hierarchy defining the virtual member function. In this
1923 case the class stab describes a base class so the virtual function is
1924 not overriding any other definition of the method. Therefore the
1925 reference is to the type number of the class that the stab is
1928 This is followed by three semi-colons. One marks the end of the
1929 current sub-section, one marks the end of the method field, and the
1930 third marks the end of the struct definition.
1932 For classes containing virtual functions the very last section of the
1933 string part of the stab holds a type reference to the first base
1934 class. This is preceeded by `~%' and followed by a final semi-colon.
1937 .stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8)
1938 field_name(Adat):type_ref(int),bit_offset(0),field_bits(32);
1939 field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)=
1940 sym_desc(array)index_type_ref(int);NIL;elem_type_ref(vtbl elem type);
1942 meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
1943 :arg_type(int),protection(public)normal(yes)virtual(yes)
1944 vtable_index(1);class_first_defining(A);;;~%first_base(A);",
1949 .stabs "A:t20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
1953 @section Inheritence
1955 Stabs describing C++ derived classes include additional sections that
1956 describe the inheritence hierarchy of the class. A derived class stab
1957 also encodes the number of base classes. For each base class it tells
1958 if the base class is virtual or not, and if the inheritence is private
1959 or public. It also gives the offset into the object of the portion of
1960 the object corresponding to each base class.
1962 This additional information is embeded in the class stab following the
1963 number of bytes in the struct. First the number of base classes
1964 appears bracketed by an exclamation point and a comma.
1966 Then for each base type there repeats a series: two digits, a number,
1967 a comma, another number, and a semi-colon.
1969 The first of the two digits is 1 if the base class is virtual and 0 if
1970 not. The second digit is 2 if the derivation is public and 0 if not.
1972 The number following the first two digits is the offset from the start
1973 of the object to the part of the object pertaining to the base class.
1975 After the comma, the second number is a type_descriptor for the base
1976 type. Finally a semi-colon ends the series, which repeats for each
1979 The source below defines three base classes A, B, and C and the
1987 virtual int A_virt (int arg) @{ return arg; @};
1993 virtual int B_virt (int arg) @{return arg; @};
1999 virtual int C_virt (int arg) @{return arg; @};
2002 class D : A, virtual B, public C @{
2005 virtual int A_virt (int arg ) @{ return arg+1; @};
2006 virtual int B_virt (int arg) @{ return arg+2; @};
2007 virtual int C_virt (int arg) @{ return arg+3; @};
2008 virtual int D_virt (int arg) @{ return arg; @};
2012 Class stabs similar to the ones described earlier are generated for
2015 @c FIXME!!! the linebreaks in the following example probably make the
2016 @c examples literally unusable, but I don't know any other way to get
2017 @c them on the page.
2019 .stabs "A:T20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2020 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2022 .stabs "B:Tt25=s8Bdat:1,0,32;$vf25:21,32;B_virt::26=##1;
2023 :i;2A*-2147483647;25;;;~%25;",128,0,0,0
2025 .stabs "C:Tt28=s8Cdat:1,0,32;$vf28:21,32;C_virt::29=##1;
2026 :i;2A*-2147483647;28;;;~%28;",128,0,0,0
2029 In the stab describing derived class D below, the information about
2030 the derivation of this class is encoded as follows.
2033 .stabs "derived_class_name:symbol_descriptors(struct tag&type)=
2034 type_descriptor(struct)struct_bytes(32)!num_bases(3),
2035 base_virtual(no)inheritence_public(no)base_offset(0),
2036 base_class_type_ref(A);
2037 base_virtual(yes)inheritence_public(no)base_offset(NIL),
2038 base_class_type_ref(B);
2039 base_virtual(no)inheritence_public(yes)base_offset(64),
2040 base_class_type_ref(C); @dots{}
2043 @c FIXME! fake linebreaks.
2045 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:
2046 1,160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt:
2047 :32:i;2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;
2048 28;;D_virt::32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2051 @node Virtual Base Classes
2052 @section Virtual Base Classes
2054 A derived class object consists of a concatination in memory of the
2055 data areas defined by each base class, starting with the leftmost and
2056 ending with the rightmost in the list of base classes. The exception
2057 to this rule is for virtual inheritence. In the example above, class
2058 D inherits virtually from base class B. This means that an instance
2059 of a D object will not contain it's own B part but merely a pointer to
2060 a B part, known as a virtual base pointer.
2062 In a derived class stab, the base offset part of the derivation
2063 information, described above, shows how the base class parts are
2064 ordered. The base offset for a virtual base class is always given as
2065 0. Notice that the base offset for B is given as 0 even though B is
2066 not the first base class. The first base class A starts at offset 0.
2068 The field information part of the stab for class D describes the field
2069 which is the pointer to the virtual base class B. The vbase pointer
2070 name is $vb followed by a type reference to the virtual base class.
2071 Since the type id for B in this example is 25, the vbase pointer name
2074 @c FIXME!! fake linebreaks below
2076 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1,
2077 160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i;
2078 2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt:
2079 :32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2082 Following the name and a semicolon is a type reference describing the
2083 type of the virtual base class pointer, in this case 24. Type 24 was
2084 defined earlier as the type of the B class `this` pointer, $t. The
2085 `this' pointer for a class is a pointer to the class type.
2088 .stabs "$t:P24=*25=xsB:",64,0,0,8
2091 Finally the field offset part of the vbase pointer field description
2092 shows that the vbase pointer is the first field in the D object,
2093 before any data fields defined by the class. The layout of a D class
2094 object is a follows, Adat at 0, the vtable pointer for A at 32, Cdat
2095 at 64, the vtable pointer for C at 96, the virtual ase pointer for B
2096 at 128, and Ddat at 160.
2099 @node Static Members
2100 @section Static Members
2102 The data area for a class is a concatenation of the space used by the
2103 data members of the class. If the class has virtual methods, a vtable
2104 pointer follows the class data. The field offset part of each field
2105 description in the class stab shows this ordering.
2107 << How is this reflected in stabs? See Cygnus bug #677 for some info. >>
2110 @appendix Example2.c - source code for extended example
2114 2 register int g_bar asm ("%g5");
2115 3 static int s_g_repeat = 2;
2121 9 char s_char_vec[8];
2122 10 struct s_tag* s_next;
2125 13 typedef struct s_tag s_typedef;
2127 15 char char_vec[3] = @{'a','b','c'@};
2129 17 main (argc, argv)
2133 21 static float s_flap;
2135 23 for (times=0; times < s_g_repeat; times++)@{
2137 25 printf ("Hello world\n");
2141 29 enum e_places @{first,second=3,last@};
2143 31 static s_proc (s_arg, s_ptr_arg, char_vec)
2145 33 s_typedef* s_ptr_arg;
2159 @appendix Example2.s - assembly code for extended example
2163 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
2164 3 .stabs "example2.c",100,0,0,Ltext0
2167 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
2168 7 .stabs "char:t2=r2;0;127;",128,0,0,0
2169 8 .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
2170 9 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
2171 10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
2172 11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
2173 12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
2174 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
2175 14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
2176 15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
2177 16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
2178 17 .stabs "float:t12=r1;4;0;",128,0,0,0
2179 18 .stabs "double:t13=r1;8;0;",128,0,0,0
2180 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
2181 20 .stabs "void:t15=15",128,0,0,0
2182 21 .stabs "g_foo:G2",32,0,0,0
2187 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
2191 @c FIXME! fake linebreak in line 30
2192 30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;s_char_vec:
2193 17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
2194 31 .stabs "s_typedef:t16",128,0,0,0
2195 32 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
2196 33 .global _char_vec
2202 39 .reserve _s_flap.0,4,"bss",4
2206 43 .ascii "Hello world\12\0"
2211 48 .stabn 68,0,20,LM1
2214 51 save %sp,-144,%sp
2221 58 .stabn 68,0,23,LM2
2225 62 sethi %hi(_s_g_repeat),%o0
2227 64 ld [%o0+%lo(_s_g_repeat)],%o0
2232 69 .stabn 68,0,25,LM3
2234 71 sethi %hi(LC0),%o1
2235 72 or %o1,%lo(LC0),%o0
2238 75 .stabn 68,0,26,LM4
2241 78 .stabn 68,0,23,LM5
2249 86 .stabn 68,0,27,LM6
2252 89 .stabn 68,0,27,LM7
2257 94 .stabs "main:F1",36,0,0,_main
2258 95 .stabs "argc:p1",160,0,0,68
2259 96 .stabs "argv:p20=*21=*2",160,0,0,72
2260 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
2261 98 .stabs "times:1",128,0,0,-20
2262 99 .stabn 192,0,0,LBB2
2263 100 .stabs "inner:1",128,0,0,-24
2264 101 .stabn 192,0,0,LBB3
2265 102 .stabn 224,0,0,LBE3
2266 103 .stabn 224,0,0,LBE2
2267 104 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
2268 @c FIXME: fake linebreak in line 105
2269 105 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
2274 109 .stabn 68,0,35,LM8
2277 112 save %sp,-120,%sp
2283 118 .stabn 68,0,41,LM9
2286 121 .stabn 68,0,41,LM10
2291 126 .stabs "s_proc:f1",36,0,0,_s_proc
2292 127 .stabs "s_arg:p16",160,0,0,0
2293 128 .stabs "s_ptr_arg:p18",160,0,0,72
2294 129 .stabs "char_vec:p21",160,0,0,76
2295 130 .stabs "an_u:23",128,0,0,-20
2296 131 .stabn 192,0,0,LBB4
2297 132 .stabn 224,0,0,LBE4
2298 133 .stabs "g_bar:r1",64,0,0,5
2299 134 .stabs "g_pf:G24=*25=f1",32,0,0,0
2300 135 .common _g_pf,4,"bss"
2301 136 .stabs "g_an_s:G16",32,0,0,0
2302 137 .common _g_an_s,20,"bss"
2306 @node Quick reference
2307 @appendix Quick reference
2310 * Stab types:: Table A: Symbol types from stabs
2311 * Assembler types:: Table B: Symbol types from assembler and linker
2312 * Symbol descriptors:: Table C
2313 * Type Descriptors:: Table D
2317 @section Table A: Symbol types from stabs
2319 Table A lists stab types sorted by type number. Stab type numbers are
2320 32 and greater. This is the full list of stab numbers, including stab
2321 types that are used in languages other than C.
2323 The #define names for these stab types are defined in:
2324 devo/include/aout/stab.def
2327 type type #define used to describe
2328 dec hex name source program feature
2329 ------------------------------------------------
2330 32 0x20 N_GYSM global symbol
2331 34 0X22 N_FNAME function name (for BSD Fortran)
2332 36 0x24 N_FUN function name or text segment variable for C
2333 38 0x26 N_STSYM static symbol (data segment w/internal linkage)
2334 40 0x28 N_LCSYM .lcomm symbol(BSS-seg variable w/internal linkage)
2335 42 0x2a N_MAIN Name of main routine (not used in C)
2336 48 0x30 N_PC global symbol (for Pascal)
2337 50 0x32 N_NSYMS number of symbols (according to Ultrix V4.0)
2338 52 0x34 N_NOMAP no DST map for sym (according to Ultrix V4.0)
2339 64 0x40 N_RSYM register variable
2340 66 0x42 N_M2C Modula-2 compilation unit
2341 68 0x44 N_SLINE line number in text segment
2342 70 0x46 N_DSLINE line number in data segment
2344 72 0x48 N_BSLINE line number in bss segment
2345 72 0x48 N_BROWS Sun source code browser, path to .cb file
2347 74 0x4a N_DEFD GNU Modula2 definition module dependency
2349 80 0x50 N_EHDECL GNU C++ exception variable
2350 80 0x50 N_MOD2 Modula2 info "for imc" (according to Ultrix V4.0)
2352 84 0x54 N_CATCH GNU C++ "catch" clause
2353 96 0x60 N_SSYM structure of union element
2354 100 0x64 N_SO path and name of source file
2355 128 0x80 N_LSYM automatic var in the stack
2356 (also used for type desc.)
2357 130 0x82 N_BINCL beginning of an include file (Sun only)
2358 132 0x84 N_SOL Name of sub-source (#include) file.
2359 160 0xa0 N_PSYM parameter variable
2360 162 0xa2 N_EINCL end of an include file
2361 164 0xa4 N_ENTRY alternate entry point
2362 192 0xc0 N_LBRAC beginning of a lexical block
2363 194 0xc2 N_EXCL place holder for a deleted include file
2364 196 0xc4 N_SCOPE modula2 scope information (Sun linker)
2365 224 0xe0 N_RBRAC end of a lexical block
2366 226 0xe2 N_BCOMM begin named common block
2367 228 0xe4 N_ECOMM end named common block
2368 232 0xe8 N_ECOML end common (local name)
2370 << used on Gould systems for non-base registers syms >>
2371 240 0xf0 N_NBTEXT ??
2372 242 0xf2 N_NBDATA ??
2378 @node Assembler types
2379 @section Table B: Symbol types from assembler and linker
2381 Table B shows the types of symbol table entries that hold assembler
2384 The #define names for these n_types values are defined in
2385 /include/aout/aout64.h
2389 n_type n_type name used to describe
2390 ------------------------------------------
2391 1 0x0 N_UNDF undefined symbol
2392 2 0x2 N_ABS absolute symbol -- defined at a particular address
2393 3 0x3 extern " (vs. file scope)
2394 4 0x4 N_TEXT text symbol -- defined at offset in text segment
2395 5 0x5 extern " (vs. file scope)
2396 6 0x6 N_DATA data symbol -- defined at offset in data segment
2397 7 0x7 extern " (vs. file scope)
2398 8 0x8 N_BSS BSS symbol -- defined at offset in zero'd segment
2399 9 extern " (vs. file scope)
2401 12 0x0C N_FN_SEQ func name for Sequent compilers (stab exception)
2403 49 0x12 N_COMM common sym -- visable after shared lib dynamic link
2404 31 0x1f N_FN file name of a .o file
2407 @node Symbol descriptors
2408 @section Table C: Symbol descriptors
2412 -------------------------------------------------
2413 (empty) local variable
2419 S static global variable
2421 T enumeration, struct or type tag
2422 V static local variable
2425 @node Type Descriptors
2426 @section Table D: Type Descriptors
2430 -------------------------------------
2431 (empty) type reference
2437 u union specifications
2442 @node Expanded reference
2443 @appendix Expanded reference by stab type.
2447 The first line is the symbol type expressed in decimal, hexadecimal,
2448 and as a #define (see devo/include/aout/stab.def).
2450 The second line describes the language constructs the symbol type
2453 The third line is the stab format with the significant stab fields
2454 named and the rest NIL.
2456 Subsequent lines expand upon the meaning and possible values for each
2457 significant stab field. # stands in for the type descriptor.
2459 Finally, any further information.
2462 * N_GSYM:: Global variable
2463 * N_FNAME:: Function name (BSD Fortran)
2464 * N_FUN:: C Function name or text segment variable
2465 * N_STSYM:: Initialized static symbol
2466 * N_LCSYM:: Uninitialized static symbol
2467 * N_MAIN:: Name of main routine (not for C)
2468 * N_PC:: Pascal global symbol
2469 * N_NSYMS:: Number of symbols
2470 * N_NOMAP:: No DST map
2471 * N_RSYM:: Register variable
2472 * N_M2C:: Modula-2 compilation unit
2473 * N_SLINE:: Line number in text segment
2474 * N_DSLINE:: Line number in data segment
2475 * N_BSLINE:: Line number in bss segment
2476 * N_BROWS:: Path to .cb file for Sun source code browser
2477 * N_DEFD:: GNU Modula2 definition module dependency
2478 * N_EHDECL:: GNU C++ exception variable
2479 * N_MOD2:: Modula2 information "for imc"
2480 * N_CATCH:: GNU C++ "catch" clause
2481 * N_SSYM:: Structure or union element
2482 * N_SO:: Source file containing main
2483 * N_LSYM:: Automatic variable
2484 * N_BINCL:: Beginning of include file (Sun only)
2485 * N_SOL:: Name of include file
2486 * N_PSYM:: Parameter variable
2487 * N_EINCL:: End of include file
2488 * N_ENTRY:: Alternate entry point
2489 * N_LBRAC:: Beginning of lexical block
2490 * N_EXCL:: Deleted include file
2491 * N_SCOPE:: Modula2 scope information (Sun only)
2492 * N_RBRAC:: End of lexical block
2493 * N_BCOMM:: Begin named common block
2494 * N_ECOMM:: End named common block
2495 * N_ECOML:: End common
2496 * Gould:: non-base register symbols used on Gould systems
2497 * N_LENG:: Length of preceding entry
2501 @section 32 - 0x20 - N_GYSM
2506 .stabs "name", N_GSYM, NIL, NIL, NIL
2510 "name" -> "symbol_name:#type"
2514 Only the "name" field is significant. the location of the variable is
2515 obtained from the corresponding external symbol.
2518 @section 34 - 0x22 - N_FNAME
2519 Function name (for BSD Fortran)
2522 .stabs "name", N_FNAME, NIL, NIL, NIL
2526 "name" -> "function_name"
2529 Only the "name" field is significant. The location of the symbol is
2530 obtained from the corresponding extern symbol.
2533 @section 36 - 0x24 - N_FUN
2534 Function name or text segment variable for C.
2537 .stabs "name", N_FUN, NIL, desc, value
2541 @exdent @emph{For functions:}
2542 "name" -> "proc_name:#return_type"
2543 # -> F (global function)
2545 desc -> line num for proc start. (GCC doesn't set and DBX doesn't miss it.)
2546 value -> Code address of proc start.
2548 @exdent @emph{For text segment variables:}
2549 <<How to create one?>>
2553 @section 38 - 0x26 - N_STSYM
2554 Initialized static symbol (data segment w/internal linkage).
2557 .stabs "name", N_STSYM, NIL, NIL, value
2561 "name" -> "symbol_name#type"
2562 # -> S (scope global to compilation unit)
2563 -> V (scope local to a procedure)
2564 value -> Data Address
2568 @section 40 - 0x28 - N_LCSYM
2569 Unitialized static (.lcomm) symbol(BSS segment w/internal linkage).
2572 .stabs "name", N_LCLSYM, NIL, NIL, value
2576 "name" -> "symbol_name#type"
2577 # -> S (scope global to compilation unit)
2578 -> V (scope local to procedure)
2579 value -> BSS Address
2583 @section 42 - 0x2a - N_MAIN
2584 Name of main routine (not used in C)
2587 .stabs "name", N_MAIN, NIL, NIL, NIL
2591 "name" -> "name_of_main_routine"
2595 @section 48 - 0x30 - N_PC
2596 Global symbol (for Pascal)
2599 .stabs "name", N_PC, NIL, NIL, value
2603 "name" -> "symbol_name" <<?>>
2604 value -> supposedly the line number (stab.def is skeptical)
2610 global pascal symbol: name,,0,subtype,line
2615 @section 50 - 0x32 - N_NSYMS
2616 Number of symbols (according to Ultrix V4.0)
2619 0, files,,funcs,lines (stab.def)
2623 @section 52 - 0x34 - N_NOMAP
2624 no DST map for sym (according to Ultrix V4.0)
2627 name, ,0,type,ignored (stab.def)
2631 @section 64 - 0x40 - N_RSYM
2635 .stabs "name:type",N_RSYM,0,RegSize,RegNumber (Sun doc)
2639 @section 66 - 0x42 - N_M2C
2640 Modula-2 compilation unit
2643 .stabs "name", N_M2C, 0, desc, value
2647 "name" -> "unit_name,unit_time_stamp[,code_time_stamp]
2649 value -> 0 (main unit)
2654 @section 68 - 0x44 - N_SLINE
2655 Line number in text segment
2658 .stabn N_SLINE, 0, desc, value
2663 value -> code_address (relocatable addr where the corresponding code starts)
2666 For single source lines that generate discontiguous code, such as flow
2667 of control statements, there may be more than one N_SLINE stab for the
2668 same source line. In this case there is a stab at the start of each
2669 code range, each with the same line number.
2672 @section 70 - 0x46 - N_DSLINE
2673 Line number in data segment
2676 .stabn N_DSLINE, 0, desc, value
2681 value -> data_address (relocatable addr where the corresponding code
2685 See comment for N_SLINE above.
2688 @section 72 - 0x48 - N_BSLINE
2689 Line number in bss segment
2692 .stabn N_BSLINE, 0, desc, value
2697 value -> bss_address (relocatable addr where the corresponding code
2701 See comment for N_SLINE above.
2704 @section 72 - 0x48 - N_BROWS
2705 Sun source code browser, path to .cb file
2708 "path to associated .cb file"
2710 Note: type field value overlaps with N_BSLINE
2713 @section 74 - 0x4a - N_DEFD
2714 GNU Modula2 definition module dependency
2716 GNU Modula-2 definition module dependency. Value is the modification
2717 time of the definition file. Other is non-zero if it is imported with
2718 the GNU M2 keyword %INITIALIZE. Perhaps N_M2C can be used if there
2719 are enough empty fields?
2722 @section 80 - 0x50 - N_EHDECL
2723 GNU C++ exception variable <<?>>
2725 "name is variable name"
2727 Note: conflicts with N_MOD2.
2730 @section 80 - 0x50 - N_MOD2
2731 Modula2 info "for imc" (according to Ultrix V4.0)
2733 Note: conflicts with N_EHDECL <<?>>
2736 @section 84 - 0x54 - N_CATCH
2737 GNU C++ "catch" clause
2739 GNU C++ `catch' clause. Value is its address. Desc is nonzero if
2740 this entry is immediately followed by a CAUGHT stab saying what
2741 exception was caught. Multiple CAUGHT stabs means that multiple
2742 exceptions can be caught here. If Desc is 0, it means all exceptions
2746 @section 96 - 0x60 - N_SSYM
2747 Structure or union element
2749 Value is offset in the structure.
2751 <<?looking at structs and unions in C I didn't see these>>
2754 @section 100 - 0x64 - N_SO
2755 Path and name of source file containing main routine
2758 .stabs "name", N_SO, NIL, NIL, value
2762 "name" -> /path/to/source/file
2763 -> source_file_terminal_name
2765 value -> the starting text address of the compilation.
2768 These are found two in a row. The name field of the first N_SO
2769 contains the path to the source file. The name field of the second
2770 N_SO contains the terminal name of the source file itself.
2773 @section 128 - 0x80 - N_LSYM
2774 Automatic var in the stack (also used for type descriptors.)
2777 .stabs "name" N_LSYM, NIL, NIL, value
2781 @exdent @emph{For stack based local variables:}
2783 "name" -> name of the variable
2784 value -> offset from frame pointer (negative)
2786 @exdent @emph{For type descriptors:}
2788 "name" -> "name_of_the_type:#type"
2791 type -> type_ref (or) type_def
2793 type_ref -> type_number
2794 type_def -> type_number=type_desc etc.
2797 Type may be either a type reference or a type definition. A type
2798 reference is a number that refers to a previously defined type. A
2799 type definition is the number that will refer to this type, followed
2800 by an equals sign, a type descriptor and the additional data that
2801 defines the type. See the Table D for type descriptors and the
2802 section on types for what data follows each type descriptor.
2805 @section 130 - 0x82 - N_BINCL
2807 Beginning of an include file (Sun only)
2809 Beginning of an include file. Only Sun uses this. In an object file,
2810 only the name is significant. The Sun linker puts data into some of
2814 @section 132 - 0x84 - N_SOL
2816 Name of a sub-source file (#include file). Value is starting address
2821 @section 160 - 0xa0 - N_PSYM
2826 stabs. "name", N_PSYM, NIL, NIL, value
2830 "name" -> "param_name:#type"
2831 # -> p (value parameter)
2832 -> i (value parameter by reference, indirect access)
2833 -> v (variable parameter by reference)
2834 -> C ( read-only parameter, conformant array bound)
2835 -> x (confomant array value parameter)
2838 -> X (function result variable)
2839 -> b (based variable)
2841 value -> offset from the argument pointer (positive).
2844 On most machines the argument pointer is the same as the frame
2848 @section 162 - 0xa2 - N_EINCL
2850 End of an include file. This and N_BINCL act as brackets around the
2851 file's output. In an ojbect file, there is no significant data in
2852 this entry. The Sun linker puts data into some of the fields.
2856 @section 164 - 0xa4 - N_ENTRY
2858 Alternate entry point.
2859 Value is its address.
2863 @section 192 - 0xc0 - N_LBRAC
2865 Beginning of a lexical block (left brace). The variable defined
2866 inside the block precede the N_LBRAC symbol. Or can they follow as
2867 well as long as a new N_FUNC was not encountered. <<?>>
2870 .stabn N_LBRAC, NIL, NIL, value
2874 value -> code address of block start.
2878 @section 194 - 0xc2 - N_EXCL
2880 Place holder for a deleted include file. Replaces a N_BINCL and
2881 everything up to the corresponding N_EINCL. The Sun linker generates
2882 these when it finds multiple indentical copies of the symbols from an
2883 included file. This appears only in output from the Sun linker.
2887 @section 196 - 0xc4 - N_SCOPE
2889 Modula2 scope information (Sun linker)
2893 @section 224 - 0xe0 - N_RBRAC
2895 End of a lexical block (right brace)
2898 .stabn N_RBRAC, NIL, NIL, value
2902 value -> code address of the end of the block.
2906 @section 226 - 0xe2 - N_BCOMM
2908 Begin named common block.
2910 Only the name is significant.
2914 @section 228 - 0xe4 - N_ECOMM
2916 End named common block.
2918 Only the name is significant and it should match the N_BCOMM
2922 @section 232 - 0xe8 - N_ECOML
2924 End common (local name)
2930 @section Non-base registers on Gould systems
2931 << used on Gould systems for non-base registers syms, values assigned
2932 at random, need real info from Gould. >>
2936 240 0xf0 N_NBTEXT ??
2937 242 0xf2 N_NBDATA ??
2944 @section - 0xfe - N_LENG
2946 Second symbol entry containing a length-value for the preceding entry.
2947 The value is the length.
2950 @appendix Questions and anomalies
2954 For GNU C stabs defining local and global variables (N_LSYM and
2955 N_GSYM), the desc field is supposed to contain the source line number
2956 on which the variable is defined. In reality the desc field is always
2957 0. (This behavour is defined in dbxout.c and putting a line number in
2958 desc is controlled by #ifdef WINNING_GDB which defaults to false). Gdb
2959 supposedly uses this information if you say 'list var'. In reality
2960 var can be a variable defined in the program and gdb says `function
2964 In GNU C stabs there seems to be no way to differentiate tag types:
2965 structures, unions, and enums (symbol descriptor T) and typedefs
2966 (symbol descriptor t) defined at file scope from types defined locally
2967 to a procedure or other more local scope. They all use the N_LSYM
2968 stab type. Types defined at procedure scope are emited after the
2969 N_RBRAC of the preceding function and before the code of the
2970 procedure in which they are defined. This is exactly the same as
2971 types defined in the source file between the two procedure bodies.
2972 GDB overcompensates by placing all types in block #1 the block for
2973 symbols of file scope. This is true for default, -ansi and
2974 -traditional compiler options. (p0001063-gcc, p0001066-gdb)
2977 What ends the procedure scope? Is it the proc block's N_RBRAC or the
2978 next N_FUN? (I believe its the first.)
2981 The comment in xcoff.h says DBX_STATIC_CONST_VAR_CODE is used for
2982 static const variables. DBX_STATIC_CONST_VAR_CODE is set to N_FUN by
2983 default, in dbxout.c. If included, xcoff.h redefines it to N_STSYM.
2984 But testing the default behaviour, my Sun4 native example shows
2985 N_STSYM not N_FUN is used to describe file static initialized
2986 variables. (the code tests for TREE_READONLY(decl) &&
2987 !TREE_THIS_VOLATILE(decl) and if true uses DBX_STATIC_CONST_VAR_CODE).
2990 Global variable stabs don't have location information. This comes
2991 from the external symbol for the same variable. The external symbol
2992 has a leading underbar on the _name of the variable and the stab does
2993 not. How do we know these two symbol table entries are talking about
2994 the same symbol when their names are different?
2997 Can gcc be configured to output stabs the way the Sun compiler
2998 does, so that their native debugging tools work? <NO?> It doesn't by
2999 default. GDB reads either format of stab. (gcc or SunC). How about
3003 @node xcoff-differences
3004 @appendix Differences between GNU stabs in a.out and GNU stabs in xcoff
3006 (The AIX/RS6000 native object file format is xcoff with stabs)
3010 Instead of .stabs, xcoff uses .stabx.
3013 The data fields of an xcoff .stabx are in a different order than an
3014 a.out .stabs. The order is: string, value, type. The desc and null
3015 fields present in a.out stabs are missing in xcoff stabs. For N_GSYM
3016 the value field is the name of the symbol.
3019 BSD a.out stab types correspond to AIX xcoff storage classes. In general the
3020 mapping is N_STABTYPE becomes C_STABTYPE. Some stab types in a.out
3021 are not supported in xcoff. See Table E. for full mappings.
3024 initialised static N_STSYM and un-initialized static N_LCSYM both map
3025 to the C_STSYM storage class. But the destinction is preserved
3026 because in xcoff N_STSYM and N_LCSYM must be emited in a named static
3027 block. Begin the block with .bs s[RW] data_section_name for N_STSYM
3028 or .bs s bss_section_name for N_LCSYM. End the block with .es
3031 xcoff stabs describing tags and typedefs use the N_DECL (0x8c)instead
3032 of N_LSYM stab type.
3035 xcoff uses N_RPSYM (0x8e) instead of the N_RSYM stab type for register
3036 variables. If the register variable is also a value parameter, then
3037 use R instead of P for the symbol descriptor.
3040 xcoff uses negative numbers as type references to the basic types.
3041 There are no boilerplate type definitions emited for these basic
3042 types. << make table of basic types and type numbers for C >>
3045 xcoff .stabx sometimes don't have the name part of the string field.
3048 xcoff uses a .file stab type to represent the source file name. There
3049 is no stab for the path to the source file.
3052 xcoff uses a .line stab type to represent source lines. The format
3053 is: .line line_number.
3056 xcoff emits line numbers relative to the start of the current
3057 function. The start of a function is marked by .bf. If a function
3058 includes lines from a seperate file, then those line numbers are
3059 absolute line numbers in the <<sub-?>> file being compiled.
3062 The start of current include file is marked with: .bi "filename" and
3063 the end marked with .ei "filename"
3066 If the xcoff stab is a N_FUN (C_FUN) then follow the string field with
3067 ,. instead of just ,
3070 The symbol descriptor for register parameters is P for a.out and R for
3075 (I think that's it for .s file differences. They could stand to be
3076 better presented. This is just a list of what I have noticed so far.
3077 There are a *lot* of differences in the information in the symbol
3078 tables of the executable and object files.)
3080 Table E: mapping a.out stab types to xcoff storage classes
3083 stab type storage class
3084 -------------------------------
3093 N_RPSYM (0x8e) C_RPSYM
3103 N_DECL (0x8c) C_DECL
3120 @node Sun-differences
3121 @appendix Differences between GNU stabs and Sun native stabs.
3125 GNU C stabs define *all* types, file or procedure scope, as
3126 N_LSYM. Sun doc talks about using N_GSYM too.
3129 GNU C stabs use `ar' as type descriptor when defining arrays vs. just
3133 Stabs describing block scopes, N_LBRAC and N_RBRAC are supposed to
3134 contain the nesting level of the block in the desc field, re Sun doc.
3135 GNU stabs always have 0 in that field.
3138 Sun C stabs use type number pairs in the format (a,b) where a is a
3139 number starting with 1 and incremented for each sub-source file in the
3140 compilation. b is a number starting with 1 and incremented for each
3141 new type defined in the compilation. GNU C stabs use the type number
3142 alone, with no source file number.