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 All this can make the @code{"@var{string}"} field quite long. All
212 versions of GDB, and some versions of DBX, can handle arbitrarily long
213 strings. But many versions of DBX cretinously limit the strings to
214 about 80 characters, so compilers which must work with such DBX's need
215 to split the @code{.stabs} directive into several @code{.stabs}
216 directives. Each stab duplicates exactly all but the
217 @code{"@var{string}"} field. The @code{"@var{string}"} field of the
218 every stab except the last is marked as continued with a
219 double-backslash at the end. Removing the backslashes and concatenating
220 the @code{"@var{string}"} fields of each stab produces the original,
224 @section A simple example in C source
226 To get the flavor of how stabs describe source information for a C
227 program, let's look at the simple program:
232 printf("Hello world");
236 When compiled with @samp{-g}, the program above yields the following
237 @file{.s} file. Line numbers have been added to make it easier to refer
238 to parts of the @file{.s} file in the description of the stabs that
242 @section The simple example at the assembly level
246 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
247 3 .stabs "hello.c",100,0,0,Ltext0
250 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
251 7 .stabs "char:t2=r2;0;127;",128,0,0,0
252 8 .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
253 9 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
254 10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
255 11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
256 12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
257 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
258 14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
259 15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
260 16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
261 17 .stabs "float:t12=r1;4;0;",128,0,0,0
262 18 .stabs "double:t13=r1;8;0;",128,0,0,0
263 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
264 20 .stabs "void:t15=15",128,0,0,0
267 23 .ascii "Hello, world!\12\0"
282 38 sethi %hi(LC0),%o1
283 39 or %o1,%lo(LC0),%o0
294 50 .stabs "main:F1",36,0,0,_main
295 51 .stabn 192,0,0,LBB2
296 52 .stabn 224,0,0,LBE2
299 This simple ``hello world'' example demonstrates several of the stab
300 types used to describe C language source files.
302 @node Program structure
303 @chapter Encoding for the structure of the program
306 * Source file:: The path and name of the source file
313 @section The path and name of the source file
322 The first stabs in the .s file contain the name and path of the source
323 file that was compiled to produce the .s file. This information is
324 contained in two records of stab type N_SO (100).
327 .stabs "path_name", N_SO, NIL, NIL, Code_address_of_program_start
328 .stabs "file_name:", N_SO, NIL, NIL, Code_address_of_program_start
332 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
333 3 .stabs "hello.c",100,0,0,Ltext0
339 @section Line Numbers
348 The start of source lines is represented by the @code{N_SLINE} (68) stab
352 .stabn N_SLINE, NIL, @var{line}, @var{address}
355 @var{line} is a source line number; @var{address} represents the code
356 address for the start of that source line.
373 @item Symbol Descriptors:
374 @code{f} (local), @code{F} (global)
377 Procedures are described by the @code{N_FUN} stab type. The symbol
378 descriptor for a procedure is @samp{F} if the procedure is globally
379 scoped and @samp{f} if the procedure is static (locally scoped).
381 The @code{N_FUN} stab representing a procedure is located immediately
382 following the code of the procedure. The @code{N_FUN} stab is in turn
383 directly followed by a group of other stabs describing elements of the
384 procedure. These other stabs describe the procedure's parameters, its
385 block local variables and its block structure.
392 The @code{.stabs} entry after this code fragment shows the @var{name} of
393 the procedure (@code{main}); the type descriptor @var{desc} (@code{F},
394 for a global procedure); a reference to the predefined type @code{int}
395 for the return type; and the starting @var{address} of the procedure.
397 Here is an exploded summary (with whitespace introduced for clarity),
398 followed by line 50 of our sample assembly output, which has this form:
402 @var{desc} @r{(global proc @samp{F})}
403 @var{return_type_ref} @r{(int)}
409 50 .stabs "main:F1",36,0,0,_main
412 @node Block Structure
413 @section Block Structure
419 @code{N_LBRAC}, @code{N_RBRAC}
422 The program's block structure is represented by the @code{N_LBRAC} (left
423 brace) and the @code{N_RBRAC} (right brace) stab types. The following code
424 range, which is the body of @code{main}, is labeled with @samp{LBB2:} at the
425 beginning and @samp{LBE2:} at the end.
429 38 sethi %hi(LC0),%o1
430 39 or %o1,%lo(LC0),%o0
438 The @code{N_LBRAC} and @code{N_RBRAC} stabs that describe the block
439 scope of the procedure are located after the @code{N_FUNC} stab that
440 represents the procedure itself. The @code{N_LBRAC} uses the
441 @code{LBB2} label as the code address in its value field, and the
442 @code{N_RBRAC} uses @code{LBE2}.
445 50 .stabs "main:F1",36,0,0,_main
449 .stabn N_LBRAC, NIL, NIL, @var{left-brace-address}
450 .stabn N_RBRAC, NIL, NIL, @var{right-brace-address}
454 51 .stabn 192,0,0,LBB2
455 52 .stabn 224,0,0,LBE2
459 @chapter Simple types
462 * Basic types:: Basic type definitions
463 * Range types:: Range types defined by min and max value
464 * Float "range" types:: Range type defined by size in bytes
468 @section Basic type definitions
475 @item Symbol Descriptor:
479 The basic types for the language are described using the @code{N_LSYM} stab
480 type. They are boilerplate and are emited by the compiler for each
481 compilation unit. Basic type definitions are not always a complete
482 description of the type and are sometimes circular. The debugger
483 recognizes the type anyway, and knows how to read bits as that type.
485 Each language and compiler defines a slightly different set of basic
486 types. In this example we are looking at the basic types for C emited
487 by the GNU compiler targeting the Sun4. Here the basic types are
488 mostly defined as range types.
492 @section Range types defined by min and max value
495 @item Type Descriptor:
499 When defining a range type, if the number after the first semicolon is
500 smaller than the number after the second one, then the two numbers
501 represent the smallest and the largest values in the range.
508 @var{descriptor} @r{(type)}
515 N_LSYM, NIL, NIL, NIL
517 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
518 7 .stabs "char:t2=r2;0;127;",128,0,0,0
521 Here the integer type (@code{1}) is defined as a range of the integer
522 type (@code{1}). Likewise @code{char} is a range of @code{char}. This
523 part of the definition is circular, but at least the high and low bound
524 values of the range hold more information about the type.
526 Here short unsigned int is defined as type number 8 and described as a
527 range of type @code{int}, with a minimum value of 0 and a maximum of 65535.
530 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
533 @node Float "range" types
534 @section Range type defined by size in bytes
537 @item Type Descriptor:
541 In a range definition, if the first number after the semicolon is
542 positive and the second is zero, then the type being defined is a
543 floating point type, and the number after the first semicolon is the
544 number of bytes needed to represent the type. Note that this does not
545 provide a way to distinguish 8-byte real floating point types from
546 8-byte complex floating point types.
557 N_LSYM, NIL, NIL, NIL
559 17 .stabs "float:t12=r1;4;0;",128,0,0,0
560 18 .stabs "double:t13=r1;8;0;",128,0,0,0
561 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
564 Cosmically enough, the @code{void} type is defined directly in terms of
574 20 .stabs "void:t15=15",128,0,0,0
579 @chapter A Comprehensive Example in C
581 Now we'll examine a second program, @code{example2}, which builds on the
582 first example to introduce the rest of the stab types, symbol
583 descriptors, and type descriptors used in C.
584 @xref{Example2.c} for the complete @file{.c} source,
585 and @pxref{Example2.s} for the @file{.s} assembly code.
586 This description includes parts of those files.
588 @section Flow of control and nested scopes
594 @code{N_SLINE}, @code{N_LBRAC}, @code{N_RBRAC} (cont.)
597 Consider the body of @code{main}, from @file{example2.c}. It shows more
598 about how @code{N_SLINE}, @code{N_RBRAC}, and @code{N_LBRAC} stabs are used.
602 21 static float s_flap;
604 23 for (times=0; times < s_g_repeat; times++)@{
606 25 printf ("Hello world\n");
611 Here we have a single source line, the @samp{for} line, that generates
612 non-linear flow of control, and non-contiguous code. In this case, an
613 @code{N_SLINE} stab with the same line number proceeds each block of
614 non-contiguous code generated from the same source line.
616 The example also shows nested scopes. The @code{N_LBRAC} and
617 @code{N_LBRAC} stabs that describe block structure are nested in the
618 same order as the corresponding code blocks, those of the for loop
619 inside those for the body of main.
622 This is the label for the @code{N_LBRAC} (left brace) stab marking the
623 start of @code{main}.
630 In the first code range for C source line 23, the @code{for} loop
631 initialize and test, @code{N_SLINE} (68) records the line number:
638 58 .stabn 68,0,23,LM2
642 62 sethi %hi(_s_g_repeat),%o0
644 64 ld [%o0+%lo(_s_g_repeat)],%o0
649 @exdent label for the @code{N_LBRAC} (start block) marking the start of @code{for} loop
652 69 .stabn 68,0,25,LM3
654 71 sethi %hi(LC0),%o1
655 72 or %o1,%lo(LC0),%o0
658 75 .stabn 68,0,26,LM4
661 @exdent label for the @code{N_RBRAC} (end block) stab marking the end of the @code{for} loop
667 Now we come to the second code range for source line 23, the @code{for}
668 loop increment and return. Once again, @code{N_SLINE} (68) records the
672 .stabn, N_SLINE, NIL,
676 78 .stabn 68,0,23,LM5
684 86 .stabn 68,0,27,LM6
687 @exdent label for the @code{N_RBRAC} (end block) stab marking the end of the @code{for} loop
690 89 .stabn 68,0,27,LM7
695 94 .stabs "main:F1",36,0,0,_main
696 95 .stabs "argc:p1",160,0,0,68
697 96 .stabs "argv:p20=*21=*2",160,0,0,72
698 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
699 98 .stabs "times:1",128,0,0,-20
703 Here is an illustration of stabs describing nested scopes. The scope
704 nesting is reflected in the nested bracketing stabs (@code{N_LBRAC},
708 .stabn N_LBRAC,NIL,NIL,
709 @var{block-start-address}
711 99 .stabn 192,0,0,LBB2 ## begin proc label
712 100 .stabs "inner:1",128,0,0,-24
713 101 .stabn 192,0,0,LBB3 ## begin for label
717 @code{N_RBRAC} (224), ``right brace'' ends a lexical block (scope).
720 .stabn N_RBRAC,NIL,NIL,
721 @var{block-end-address}
723 102 .stabn 224,0,0,LBE3 ## end for label
724 103 .stabn 224,0,0,LBE2 ## end proc label
731 * Automatic variables:: locally scoped
733 * Register variables::
734 * Initialized statics::
735 * Un-initialized statics::
739 @node Automatic variables
740 @section Locally scoped automatic variables
747 @item Symbol Descriptor:
752 In addition to describing types, the @code{N_LSYM} stab type also
753 describes locally scoped automatic variables. Refer again to the body
754 of @code{main} in @file{example2.c}. It allocates two automatic
755 variables: @samp{times} is scoped to the body of @code{main}, and
756 @samp{inner} is scoped to the body of the @code{for} loop.
757 @samp{s_flap} is locally scoped but not automatic, and will be discussed
762 21 static float s_flap;
764 23 for (times=0; times < s_g_repeat; times++)@{
766 25 printf ("Hello world\n");
771 The @code{N_LSYM} stab for an automatic variable is located just before the
772 @code{N_LBRAC} stab describing the open brace of the block to which it is
776 @exdent @code{N_LSYM} (128): automatic variable, scoped locally to @code{main}
781 @var{frame-pointer-offset}
783 98 .stabs "times:1",128,0,0,-20
784 99 .stabn 192,0,0,LBB2 ## begin `main' N_LBRAC
786 @exdent @code{N_LSYM} (128): automatic variable, scoped locally to the @code{for} loop
791 @var{frame-pointer-offset}
793 100 .stabs "inner:1",128,0,0,-24
794 101 .stabn 192,0,0,LBB3 ## begin `for' loop N_LBRAC
797 Since the character in the string field following the colon is not a
798 letter, there is no symbol descriptor. This means that the stab
799 describes a local variable, and that the number after the colon is a
800 type reference. In this case it a a reference to the basic type @code{int}.
801 Notice also that the frame pointer offset is negative number for
805 @node Global Variables
806 @section Global Variables
813 @item Symbol Descriptor:
817 Global variables are represented by the @code{N_GSYM} stab type. The symbol
818 descriptor, following the colon in the string field, is @samp{G}. Following
819 the @samp{G} is a type reference or type definition. In this example it is a
820 type reference to the basic C type, @code{char}. The first source line in
828 yields the following stab. The stab immediately precedes the code that
829 allocates storage for the variable it describes.
832 @exdent @code{N_GSYM} (32): global symbol
837 N_GSYM, NIL, NIL, NIL
839 21 .stabs "g_foo:G2",32,0,0,0
846 The address of the variable represented by the @code{N_GSYM} is not contained
847 in the @code{N_GSYM} stab. The debugger gets this information from the
848 external symbol for the global variable.
850 @node Register variables
851 @section Global register variables
858 @item Symbol Descriptor:
862 The following source line defines a global variable, @code{g_bar}, which is
863 explicitly allocated in global register @code{%g5}.
866 2 register int g_bar asm ("%g5");
869 Register variables have their own stab type, @code{N_RSYM}, and their own
870 symbol descriptor, @code{r}. The stab's value field contains the number of
871 the register where the variable data will be stored. Since the
872 variable was not initialized in this compilation unit, the stab is
873 emited at the end of the object file, with the stabs for other
874 uninitialized globals (@code{bcc}).
877 @exdent @code{N_RSYM} (64): register variable
885 133 .stabs "g_bar:r1",64,0,0,5
889 @node Initialized statics
890 @section Initialized static variables
897 @item Symbol Descriptors:
898 @code{S} (file scope), @code{V} (procedure scope)
901 Initialized static variables are represented by the @code{N_STSYM} stab
902 type. The symbol descriptor part of the string field shows if the
903 variable is file scope static (@samp{S}) or procedure scope static
904 (@samp{V}). The source line
907 3 static int s_g_repeat = 2;
911 yields the following code. The stab is located immediately preceding
912 the storage for the variable it represents. Since the variable in
913 this example is file scope static the symbol descriptor is @samp{S}.
916 @exdent @code{N_STSYM} (38): initialized static variable (data seg w/internal linkage)
924 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
931 @node Un-initialized statics
932 @section Un-initialized static variables
939 @item Symbol Descriptors:
940 @code{S} (file scope), @code{V} (procedure scope)
943 Un-initialized static variables are represented by the @code{N_LCSYM}
944 stab type. The symbol descriptor part of the string shows if the
945 variable is file scope static (@samp{S}) or procedure scope static
946 (@samp{V}). In this example it is procedure scope static. The source
947 line allocating @code{s_flap} immediately follows the open brace for the
948 procedure @code{main}.
952 21 static float s_flap;
955 The code that reserves storage for the variable @code{s_flap} precedes the
956 body of body of @code{main}.
959 39 .reserve _s_flap.0,4,"bss",4
962 But since @code{s_flap} is scoped locally to @code{main}, its stab is
963 located with the other stabs representing symbols local to @code{main}.
964 The stab for @code{s_flap} is located just before the @code{N_LBRAC} for
968 @exdent @code{N_LCSYM} (40): uninitialized static var (BSS seg w/internal linkage)
976 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
977 98 .stabs "times:1",128,0,0,-20
978 99 .stabn 192,0,0,LBB2 # N_LBRAC for main.
981 @c ............................................................
991 @item Symbol Descriptor:
995 Procedure parameters are represented by the N_PSYM stab type. The
996 following source lines show the parameters of the main routine.
1005 The N_PSYM stabs describing parameters to a function directly follow
1006 the N_FUN stab that represents the procedure itself. The N_FUN stab
1007 immediately follows the code of the procedure it describes. Following
1008 the N_PSYM parameter stabs are any N_LSYM stabs representing local
1012 @exdent <36> N_FUN - describing the procedure main
1014 94 .stabs "main:F1",36,0,0,_main
1016 @exdent <160> N_PSYM - parameters
1017 @exdent .stabs "name:sym_desc(value_param)type_ref(int)", N_PSYM,
1018 @exdent NIL, NIL, frame_ptr_offset
1020 95 .stabs "argc:p1",160,0,0,68
1022 @exdent <160> N_PSYM - parameter
1023 @exdent .stabs "name:sym_desc(value_param)type_def(20)=ptr_to type_def(21)=
1024 @exdent ptr_to type_ref(char)
1026 96 .stabs "argv:p20=*21=*2",160,0,0,72
1029 The type definition of argv is interesting because it defines two new
1030 types in terms of an existing one. The array argv contains character
1031 pointers. The type of the array name is a pointer to the type the
1032 array holds. Thus the type of argv is ptr to ptr to char. The stab
1033 for argv contains nested type_definitions. Type 21 is ptr to type 2
1034 (char) and argv (type 20) is ptr to type 21.
1036 @node Aggregate Types
1037 @chapter Aggregate Types
1039 Now let's look at some variable definitions involving complex types.
1040 This involves understanding better how types are described. In the
1041 examples so far types have been described as references to previously
1042 defined types or defined in terms of subranges of or pointers to
1043 previously defined types. The section that follows will talk about
1044 the various other type descriptors that may follow the = sign in a
1057 @section Array types
1063 @code{N_GSYM}, @code{N_LSYM}
1064 @item Symbol Descriptor:
1066 @item Type Descriptor:
1070 As an example of an array type consider the global variable below.
1073 15 char char_vec[3] = @{'a','b','c'@};
1076 Since the array is a global variable, it is described by the N_GSYM
1077 stab type. The symbol descriptor G, following the colon in stab's
1078 string field, also says the array is a global variable. Following the
1079 G is a definition for type (19) as shown by the equals sign after the
1082 After the equals sign is a type descriptor, ar, which says that the
1083 type being defined is an array. Following the type descriptor for an
1084 array is the type of the index, the lower bound for array indexing
1085 (always 0 in C), the upper bound for array indexing (in C: one less
1086 than the length of the array), and the type of the array elements.
1088 The array definition above generates the assembly language that
1092 @exdent <32> N_GSYM - global variable
1093 @exdent .stabs "name:sym_desc(global)type_def(19)=type_desc(array)
1094 @exdent index_type_ref(int);NIL;high_bound(2);element_type_ref(char)";
1095 @exdent N_GSYM, NIL, NIL, NIL
1097 32 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
1098 33 .global _char_vec
1107 @section Enumerations
1114 @item Symbol Descriptor:
1116 @item Type Descriptor:
1120 The source line below declares an enumeration type. It is defined at
1121 file scope between the bodies of main and s_proc in example2.c.
1122 Because the N_LSYM is located after the N_RBRAC that marks the end of
1123 the previous procedure's block scope, and before the N_FUN that marks
1124 the beginning of the next procedure's block scope, the N_LSYM does not
1125 describe a block local symbol, but a file local one. The source line:
1128 29 enum e_places @{first,second=3,last@};
1132 generates the following stab, located just after the N_RBRAC (close
1133 brace stab) for main. The type definition is in an N_LSYM stab
1134 because type definitions are file scope not global scope.
1137 <128> N_LSYM - local symbol
1138 .stab "name:sym_dec(type)type_def(22)=sym_desc(enum)
1139 enum_name:value(0),enum_name:value(3),enum_name:value(4),;",
1140 N_LSYM, NIL, NIL, NIL
1144 104 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
1147 The symbol descriptor (T) says that the stab describes a structure,
1148 enumeration, or type tag. The type descriptor e, following the 22= of
1149 the type definition narrows it down to an enumeration type. Following
1150 the e is a list of the elements of the enumeration. The format is
1151 name:value,. The list of elements ends with a ;.
1153 @node Structure tags
1154 @section Structure Tags
1161 @item Symbol Descriptor:
1163 @item Type Descriptor:
1167 The following source code declares a structure tag and defines an
1168 instance of the structure in global scope. Then a typedef equates the
1169 structure tag with a new type. A seperate stab is generated for the
1170 structure tag, the structure typedef, and the structure instance. The
1171 stabs for the tag and the typedef are emited when the definitions are
1172 encountered. Since the structure elements are not initialized, the
1173 stab and code for the structure variable itself is located at the end
1174 of the program in .common.
1180 9 char s_char_vec[8];
1181 10 struct s_tag* s_next;
1184 13 typedef struct s_tag s_typedef;
1187 The structure tag is an N_LSYM stab type because, like the enum, the
1188 symbol is file scope. Like the enum, the symbol descriptor is T, for
1189 enumeration, struct or tag type. The symbol descriptor s following
1190 the 16= of the type definition narrows the symbol type to struct.
1192 Following the struct symbol descriptor is the number of bytes the
1193 struct occupies, followed by a description of each structure element.
1194 The structure element descriptions are of the form name:type, bit
1195 offset from the start of the struct, and number of bits in the
1200 <128> N_LSYM - type definition
1201 .stabs "name:sym_desc(struct tag) Type_def(16)=type_desc(struct type)
1203 elem_name:type_ref(int),bit_offset,field_bits;
1204 elem_name:type_ref(float),bit_offset,field_bits;
1205 elem_name:type_def(17)=type_desc(dynamic array) index_type(int);NIL;
1206 high_bound(7);element_type(char),bit_offset,field_bits;;",
1209 30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;
1210 s_char_vec:17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
1213 In this example, two of the structure elements are previously defined
1214 types. For these, the type following the name: part of the element
1215 description is a simple type reference. The other two structure
1216 elements are new types. In this case there is a type definition
1217 embedded after the name:. The type definition for the array element
1218 looks just like a type definition for a standalone array. The s_next
1219 field is a pointer to the same kind of structure that the field is an
1220 element of. So the definition of structure type 16 contains an type
1221 definition for an element which is a pointer to type 16.
1231 @item Symbol Descriptor:
1235 Here is the stab for the typedef equating the structure tag with a
1239 <128> N_LSYM - type definition
1240 .stabs "name:sym_desc(type name)type_ref(struct_tag)",N_LSYM,NIL,NIL,NIL
1244 31 .stabs "s_typedef:t16",128,0,0,0
1247 And here is the code generated for the structure variable.
1250 <32> N_GSYM - global symbol
1251 .stabs "name:sym_desc(global)type_ref(struct_tag)",N_GSYM,NIL,NIL,NIL
1255 136 .stabs "g_an_s:G16",32,0,0,0
1256 137 .common _g_an_s,20,"bss"
1259 Notice that the structure tag has the same type number as the typedef
1260 for the structure tag. It is impossible to distinguish between a
1261 variable of the struct type and one of its typedef by looking at the
1262 debugging information.
1273 @item Symbol Descriptor:
1275 @item Type Descriptor:
1279 Next let's look at unions. In example2 this union type is declared
1280 locally to a procedure and an instance of the union is defined.
1290 This code generates a stab for the union tag and a stab for the union
1291 variable. Both use the N_LSYM stab type. Since the union variable is
1292 scoped locally to the procedure in which it is defined, its stab is
1293 located immediately preceding the N_LBRAC for the procedure's block
1296 The stab for the union tag, however is located preceding the code for
1297 the procedure in which it is defined. The stab type is N_LSYM. This
1298 would seem to imply that the union type is file scope, like the struct
1299 type s_tag. This is not true. The contents and position of the stab
1300 for u_type do not convey any infomation about its procedure local
1305 .stabs "name:sym_desc(union tag)type_def(22)=type_desc(union)
1307 elem_name:type_ref(int),bit_offset(0),bit_size(32);
1308 elem_name:type_ref(float),bit_offset(0),bit_size(32);
1309 elem_name:type_ref(ptr to char),bit_offset(0),bit_size(32);;"
1310 N_LSYM, NIL, NIL, NIL
1314 105 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
1318 The symbol descriptor, T, following the name: means that the stab
1319 describes an enumeration struct or type tag. The type descriptor u,
1320 following the 23= of the type definition, narrows it down to a union
1321 type definition. Following the u is the number of bytes in the union.
1322 After that is a list of union element descriptions. Their format is
1323 name:type, bit offset into the union, and number of bytes for the
1326 The stab for the union variable follows. Notice that the frame
1327 pointer offset for local variables is negative.
1330 <128> N_LSYM - local variable (with no symbol descriptor)
1331 .stabs "name:type_ref(u_tag)", N_LSYM, NIL, NIL, frame_ptr_offset
1335 130 .stabs "an_u:23",128,0,0,-20
1338 @node Function types
1339 @section Function types
1345 The last type descriptor in C which remains to be described is used
1346 for function types. Consider the following source line defining a
1347 global function pointer.
1353 It generates the following code. Since the variable is not
1354 initialized, the code is located in the common area at the end of the
1358 <32> N_GSYM - global variable
1359 .stabs "name:sym_desc(global)type_def(24)=ptr_to(25)=
1360 type_def(func)type_ref(int)
1364 134 .stabs "g_pf:G24=*25=f1",32,0,0,0
1365 135 .common _g_pf,4,"bss"
1368 Since the variable is global, the stab type is N_GSYM and the symbol
1369 descriptor is G. The variable defines a new type, 24, which is a
1370 pointer to another new type, 25, which is defined as a function
1374 @chapter Symbol information in symbol tables
1376 This section examines more closely the format of symbol table entries
1377 and how stab assembler directives map to them. It also describes what
1378 transformations the assembler and linker make on data from stabs.
1380 Each time the assembler encounters a stab in its input file it puts
1381 each field of the stab into corresponding fields in a symbol table
1382 entry of its output file. If the stab contains a string field, the
1383 symbol table entry for that stab points to a string table entry
1384 containing the string data from the stab. Assembler labels become
1385 relocatable addresses. Symbol table entries in a.out have the format:
1388 struct internal_nlist @{
1389 unsigned long n_strx; /* index into string table of name */
1390 unsigned char n_type; /* type of symbol */
1391 unsigned char n_other; /* misc info (usually empty) */
1392 unsigned short n_desc; /* description field */
1393 bfd_vma n_value; /* value of symbol */
1397 For .stabs directives, the n_strx field holds the character offset
1398 from the start of the string table to the string table entry
1399 containing the "string" field. For other classes of stabs (.stabn and
1400 .stabd) this field is null.
1402 Symbol table entries with n_type fields containing a value greater or
1403 equal to 0x20 originated as stabs generated by the compiler (with one
1404 random exception). Those with n_type values less than 0x20 were
1405 placed in the symbol table of the executable by the assembler or the
1408 The linker concatenates object files and does fixups of externally
1409 defined symbols. You can see the transformations made on stab data by
1410 the assembler and linker by examining the symbol table after each pass
1411 of the build, first the assemble and then the link.
1413 To do this use nm with the -ap options. This dumps the symbol table,
1414 including debugging information, unsorted. For stab entries the
1415 columns are: value, other, desc, type, string. For assembler and
1416 linker symbols, the columns are: value, type, string.
1418 There are a few important things to notice about symbol tables. Where
1419 the value field of a stab contains a frame pointer offset, or a
1420 register number, that value is unchanged by the rest of the build.
1422 Where the value field of a stab contains an assembly language label,
1423 it is transformed by each build step. The assembler turns it into a
1424 relocatable address and the linker turns it into an absolute address.
1425 This source line defines a static variable at file scope:
1428 3 static int s_g_repeat
1432 The following stab describes the symbol.
1435 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
1439 The assembler transforms the stab into this symbol table entry in the
1440 @file{.o} file. The location is expressed as a data segment offset.
1443 21 00000084 - 00 0000 STSYM s_g_repeat:S1
1447 in the symbol table entry from the executable, the linker has made the
1448 relocatable address absolute.
1451 22 0000e00c - 00 0000 STSYM s_g_repeat:S1
1454 Stabs for global variables do not contain location information. In
1455 this case the debugger finds location information in the assembler or
1456 linker symbol table entry describing the variable. The source line:
1466 21 .stabs "g_foo:G2",32,0,0,0
1469 The variable is represented by the following two symbol table entries
1470 in the object file. The first one originated as a stab. The second
1471 one is an external symbol. The upper case D signifies that the n_type
1472 field of the symbol table contains 7, N_DATA with local linkage (see
1473 Table B). The value field following the file's line number is empty
1474 for the stab entry. For the linker symbol it contains the
1475 rellocatable address corresponding to the variable.
1478 19 00000000 - 00 0000 GSYM g_foo:G2
1479 20 00000080 D _g_foo
1483 These entries as transformed by the linker. The linker symbol table
1484 entry now holds an absolute address.
1487 21 00000000 - 00 0000 GSYM g_foo:G2
1489 215 0000e008 D _g_foo
1492 @node GNU Cplusplus stabs
1493 @chapter GNU C++ stabs
1496 * Basic Cplusplus types::
1499 * Methods:: Method definition
1501 * Method Modifiers:: (const, volatile, const volatile)
1504 * Virtual Base Classes::
1509 @subsection Symbol descriptors added for C++ descriptions:
1512 P - register parameter.
1515 @subsection type descriptors added for C++ descriptions
1519 method type (two ## if minimal debug)
1526 @node Basic Cplusplus types
1527 @section Basic types for C++
1529 << the examples that follow are based on a01.C >>
1532 C++ adds two more builtin types to the set defined for C. These are
1533 the unknown type and the vtable record type. The unknown type, type
1534 16, is defined in terms of itself like the void type.
1536 The vtable record type, type 17, is defined as a structure type and
1537 then as a structure tag. The structure has four fields, delta, index,
1538 pfn, and delta2. pfn is the function pointer.
1540 << In boilerplate $vtbl_ptr_type, what are the fields delta,
1541 index, and delta2 used for? >>
1543 This basic type is present in all C++ programs even if there are no
1544 virtual methods defined.
1547 .stabs "struct_name:sym_desc(type)type_def(17)=type_desc(struct)struct_bytes(8)
1548 elem_name(delta):type_ref(short int),bit_offset(0),field_bits(16);
1549 elem_name(index):type_ref(short int),bit_offset(16),field_bits(16);
1550 elem_name(pfn):type_def(18)=type_desc(ptr to)type_ref(void),
1551 bit_offset(32),field_bits(32);
1552 elem_name(delta2):type_def(short int);bit_offset(32),field_bits(16);;"
1557 .stabs "$vtbl_ptr_type:t17=s8
1558 delta:6,0,16;index:6,16,16;pfn:18=*15,32,32;delta2:6,32,16;;"
1563 .stabs "name:sym_dec(struct tag)type_ref($vtbl_ptr_type)",N_LSYM,NIL,NIL,NIL
1567 .stabs "$vtbl_ptr_type:T17",128,0,0,0
1570 @node Simple classes
1571 @section Simple class definition
1573 The stabs describing C++ language features are an extension of the
1574 stabs describing C. Stabs representing C++ class types elaborate
1575 extensively on the stab format used to describe structure types in C.
1576 Stabs representing class type variables look just like stabs
1577 representing C language variables.
1579 Consider the following very simple class definition.
1585 int Ameth(int in, char other);
1589 The class baseA is represented by two stabs. The first stab describes
1590 the class as a structure type. The second stab describes a structure
1591 tag of the class type. Both stabs are of stab type N_LSYM. Since the
1592 stab is not located between an N_FUN and a N_LBRAC stab this indicates
1593 that the class is defined at file scope. If it were, then the N_LSYM
1594 would signify a local variable.
1596 A stab describing a C++ class type is similar in format to a stab
1597 describing a C struct, with each class member shown as a field in the
1598 structure. The part of the struct format describing fields is
1599 expanded to include extra information relevent to C++ class members.
1600 In addition, if the class has multiple base classes or virtual
1601 functions the struct format outside of the field parts is also
1604 In this simple example the field part of the C++ class stab
1605 representing member data looks just like the field part of a C struct
1606 stab. The section on protections describes how its format is
1607 sometimes extended for member data.
1609 The field part of a C++ class stab representing a member function
1610 differs substantially from the field part of a C struct stab. It
1611 still begins with `name:' but then goes on to define a new type number
1612 for the member function, describe its return type, its argument types,
1613 its protection level, any qualifiers applied to the method definition,
1614 and whether the method is virtual or not. If the method is virtual
1615 then the method description goes on to give the vtable index of the
1616 method, and the type number of the first base class defining the
1619 When the field name is a method name it is followed by two colons
1620 rather than one. This is followed by a new type definition for the
1621 method. This is a number followed by an equal sign and then the
1622 symbol descriptor `##', indicating a method type. This is followed by
1623 a type reference showing the return type of the method and a
1626 The format of an overloaded operator method name differs from that
1627 of other methods. It is "op$::XXXX." where XXXX is the operator name
1628 such as + or +=. The name ends with a period, and any characters except
1629 the period can occur in the XXXX string.
1631 The next part of the method description represents the arguments to
1632 the method, preceeded by a colon and ending with a semi-colon. The
1633 types of the arguments are expressed in the same way argument types
1634 are expressed in C++ name mangling. In this example an int and a char
1637 This is followed by a number, a letter, and an asterisk or period,
1638 followed by another semicolon. The number indicates the protections
1639 that apply to the member function. Here the 2 means public. The
1640 letter encodes any qualifier applied to the method definition. In
1641 this case A means that it is a normal function definition. The dot
1642 shows that the method is not virtual. The sections that follow
1643 elaborate further on these fields and describe the additional
1644 information present for virtual methods.
1648 .stabs "class_name:sym_desc(type)type_def(20)=type_desc(struct)struct_bytes(4)
1649 field_name(Adat):type(int),bit_offset(0),field_bits(32);
1651 method_name(Ameth)::type_def(21)=type_desc(method)return_type(int);
1652 :arg_types(int char);
1653 protection(public)qualifier(normal)virtual(no);;"
1658 .stabs "baseA:t20=s4Adat:1,0,32;Ameth::21=##1;:ic;2A.;;",128,0,0,0
1660 .stabs "class_name:sym_desc(struct tag)",N_LSYM,NIL,NIL,NIL
1662 .stabs "baseA:T20",128,0,0,0
1665 @node Class instance
1666 @section Class instance
1668 As shown above, describing even a simple C++ class definition is
1669 accomplished by massively extending the stab format used in C to
1670 describe structure types. However, once the class is defined, C stabs
1671 with no modifications can be used to describe class instances. The
1681 yields the following stab describing the class instance. It looks no
1682 different from a standard C stab describing a local variable.
1685 .stabs "name:type_ref(baseA)", N_LSYM, NIL, NIL, frame_ptr_offset
1689 .stabs "AbaseA:20",128,0,0,-20
1693 @section Method defintion
1695 The class definition shown above declares Ameth. The C++ source below
1700 baseA::Ameth(int in, char other)
1707 This method definition yields three stabs following the code of the
1708 method. One stab describes the method itself and following two
1709 describe its parameters. Although there is only one formal argument
1710 all methods have an implicit argument which is the `this' pointer.
1711 The `this' pointer is a pointer to the object on which the method was
1712 called. Note that the method name is mangled to encode the class name
1713 and argument types. << Name mangling is not described by this
1714 document - Is there already such a doc? >>
1717 .stabs "name:symbol_desriptor(global function)return_type(int)",
1718 N_FUN, NIL, NIL, code_addr_of_method_start
1720 .stabs "Ameth__5baseAic:F1",36,0,0,_Ameth__5baseAic
1723 Here is the stab for the `this' pointer implicit argument. The name
1724 of the `this' pointer is always `this.' Type 19, the `this' pointer is
1725 defined as a pointer to type 20, baseA, but a stab defining baseA has
1726 not yet been emited. Since the compiler knows it will be emited
1727 shortly, here it just outputs a cross reference to the undefined
1728 symbol, by prefixing the symbol name with xs.
1731 .stabs "name:sym_desc(register param)type_def(19)=
1732 type_desc(ptr to)type_ref(baseA)=
1733 type_desc(cross-reference to)baseA:",N_RSYM,NIL,NIL,register_number
1735 .stabs "this:P19=*20=xsbaseA:",64,0,0,8
1738 The stab for the explicit integer argument looks just like a parameter
1739 to a C function. The last field of the stab is the offset from the
1740 argument pointer, which in most systems is the same as the frame
1744 .stabs "name:sym_desc(value parameter)type_ref(int)",
1745 N_PSYM,NIL,NIL,offset_from_arg_ptr
1747 .stabs "in:p1",160,0,0,72
1750 << The examples that follow are based on A1.C >>
1753 @section Protections
1756 In the simple class definition shown above all member data and
1757 functions were publicly accessable. The example that follows
1758 contrasts public, protected and privately accessable fields and shows
1759 how these protections are encoded in C++ stabs.
1761 Protections for class member data are signified by two characters
1762 embeded in the stab defining the class type. These characters are
1763 located after the name: part of the string. /0 means private, /1
1764 means protected, and /2 means public. If these characters are omited
1765 this means that the member is public. The following C++ source:
1779 generates the following stab to describe the class type all_data.
1782 .stabs "class_name:sym_desc(type)type_def(19)=type_desc(struct)struct_bytes
1783 data_name:/protection(private)type_ref(int),bit_offset,num_bits;
1784 data_name:/protection(protected)type_ref(char),bit_offset,num_bits;
1785 data_name:(/num omited, private)type_ref(float),bit_offset,num_bits;;"
1790 .stabs "all_data:t19=s12
1791 priv_dat:/01,0,32;prot_dat:/12,32,8;pub_dat:12,64,32;;",128,0,0,0
1794 Protections for member functions are signified by one digit embeded in
1795 the field part of the stab describing the method. The digit is 0 if
1796 private, 1 if protected and 2 if public. Consider the C++ class
1800 class all_methods @{
1802 int priv_meth(int in)@{return in;@};
1804 char protMeth(char in)@{return in;@};
1806 float pubMeth(float in)@{return in;@};
1810 It generates the following stab. The digit in question is to the left
1811 of an `A' in each case. Notice also that in this case two symbol
1812 descriptors apply to the class name struct tag and struct type.
1815 .stabs "class_name:sym_desc(struct tag&type)type_def(21)=
1816 sym_desc(struct)struct_bytes(1)
1817 meth_name::type_def(22)=sym_desc(method)returning(int);
1818 :args(int);protection(private)modifier(normal)virtual(no);
1819 meth_name::type_def(23)=sym_desc(method)returning(char);
1820 :args(char);protection(protected)modifier(normal)virual(no);
1821 meth_name::type_def(24)=sym_desc(method)returning(float);
1822 :args(float);protection(public)modifier(normal)virtual(no);;",
1827 .stabs "all_methods:Tt21=s1priv_meth::22=##1;:i;0A.;protMeth::23=##2;:c;1A.;
1828 pubMeth::24=##12;:f;2A.;;",128,0,0,0
1831 @node Method Modifiers
1832 @section Method Modifiers (const, volatile, const volatile)
1836 In the class example described above all the methods have the normal
1837 modifier. This method modifier information is located just after the
1838 protection information for the method. This field has four possible
1839 character values. Normal methods use A, const methods use B, volatile
1840 methods use C, and const volatile methods use D. Consider the class
1846 int ConstMeth (int arg) const @{ return arg; @};
1847 char VolatileMeth (char arg) volatile @{ return arg; @};
1848 float ConstVolMeth (float arg) const volatile @{return arg; @};
1852 This class is described by the following stab:
1855 .stabs "class(A):sym_desc(struct)type_def(20)=type_desc(struct)struct_bytes(1)
1856 meth_name(ConstMeth)::type_def(21)sym_desc(method)
1857 returning(int);:arg(int);protection(public)modifier(const)virtual(no);
1858 meth_name(VolatileMeth)::type_def(22)=sym_desc(method)
1859 returning(char);:arg(char);protection(public)modifier(volatile)virt(no)
1860 meth_name(ConstVolMeth)::type_def(23)=sym_desc(method)
1861 returning(float);:arg(float);protection(public)modifer(const volatile)
1862 virtual(no);;", @dots{}
1866 .stabs "A:T20=s1ConstMeth::21=##1;:i;2B.;VolatileMeth::22=##2;:c;2C.;
1867 ConstVolMeth::23=##12;:f;2D.;;",128,0,0,0
1870 @node Virtual Methods
1871 @section Virtual Methods
1873 << The following examples are based on a4.C >>
1875 The presence of virtual methods in a class definition adds additional
1876 data to the class description. The extra data is appended to the
1877 description of the virtual method and to the end of the class
1878 description. Consider the class definition below:
1884 virtual int A_virt (int arg) @{ return arg; @};
1888 This results in the stab below describing class A. It defines a new
1889 type (20) which is an 8 byte structure. The first field of the class
1890 struct is Adat, an integer, starting at structure offset 0 and
1893 The second field in the class struct is not explicitly defined by the
1894 C++ class definition but is implied by the fact that the class
1895 contains a virtual method. This field is the vtable pointer. The
1896 name of the vtable pointer field starts with $vf and continues with a
1897 type reference to the class it is part of. In this example the type
1898 reference for class A is 20 so the name of its vtable pointer field is
1899 $vf20, followed by the usual colon.
1901 Next there is a type definition for the vtable pointer type (21).
1902 This is in turn defined as a pointer to another new type (22).
1904 Type 22 is the vtable itself, which is defined as an array, indexed by
1905 integers, with a high bound of 1, and elements of type 17. Type 17
1906 was the vtable record type defined by the boilerplate C++ type
1907 definitions, as shown earlier.
1909 The bit offset of the vtable pointer field is 32. The number of bits
1910 in the field are not specified when the field is a vtable pointer.
1912 Next is the method definition for the virtual member function A_virt.
1913 Its description starts out using the same format as the non-virtual
1914 member functions described above, except instead of a dot after the
1915 `A' there is an asterisk, indicating that the function is virtual.
1916 Since is is virtual some addition information is appended to the end
1917 of the method description.
1919 The first number represents the vtable index of the method. This is a
1920 32 bit unsigned number with the high bit set, followed by a
1923 The second number is a type reference to the first base class in the
1924 inheritence hierarchy defining the virtual member function. In this
1925 case the class stab describes a base class so the virtual function is
1926 not overriding any other definition of the method. Therefore the
1927 reference is to the type number of the class that the stab is
1930 This is followed by three semi-colons. One marks the end of the
1931 current sub-section, one marks the end of the method field, and the
1932 third marks the end of the struct definition.
1934 For classes containing virtual functions the very last section of the
1935 string part of the stab holds a type reference to the first base
1936 class. This is preceeded by `~%' and followed by a final semi-colon.
1939 .stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8)
1940 field_name(Adat):type_ref(int),bit_offset(0),field_bits(32);
1941 field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)=
1942 sym_desc(array)index_type_ref(int);NIL;elem_type_ref(vtbl elem type);
1944 meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
1945 :arg_type(int),protection(public)normal(yes)virtual(yes)
1946 vtable_index(1);class_first_defining(A);;;~%first_base(A);",
1951 .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
1955 @section Inheritence
1957 Stabs describing C++ derived classes include additional sections that
1958 describe the inheritence hierarchy of the class. A derived class stab
1959 also encodes the number of base classes. For each base class it tells
1960 if the base class is virtual or not, and if the inheritence is private
1961 or public. It also gives the offset into the object of the portion of
1962 the object corresponding to each base class.
1964 This additional information is embeded in the class stab following the
1965 number of bytes in the struct. First the number of base classes
1966 appears bracketed by an exclamation point and a comma.
1968 Then for each base type there repeats a series: two digits, a number,
1969 a comma, another number, and a semi-colon.
1971 The first of the two digits is 1 if the base class is virtual and 0 if
1972 not. The second digit is 2 if the derivation is public and 0 if not.
1974 The number following the first two digits is the offset from the start
1975 of the object to the part of the object pertaining to the base class.
1977 After the comma, the second number is a type_descriptor for the base
1978 type. Finally a semi-colon ends the series, which repeats for each
1981 The source below defines three base classes A, B, and C and the
1989 virtual int A_virt (int arg) @{ return arg; @};
1995 virtual int B_virt (int arg) @{return arg; @};
2001 virtual int C_virt (int arg) @{return arg; @};
2004 class D : A, virtual B, public C @{
2007 virtual int A_virt (int arg ) @{ return arg+1; @};
2008 virtual int B_virt (int arg) @{ return arg+2; @};
2009 virtual int C_virt (int arg) @{ return arg+3; @};
2010 virtual int D_virt (int arg) @{ return arg; @};
2014 Class stabs similar to the ones described earlier are generated for
2017 @c FIXME!!! the linebreaks in the following example probably make the
2018 @c examples literally unusable, but I don't know any other way to get
2019 @c them on the page.
2021 .stabs "A:T20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32;
2022 A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0
2024 .stabs "B:Tt25=s8Bdat:1,0,32;$vf25:21,32;B_virt::26=##1;
2025 :i;2A*-2147483647;25;;;~%25;",128,0,0,0
2027 .stabs "C:Tt28=s8Cdat:1,0,32;$vf28:21,32;C_virt::29=##1;
2028 :i;2A*-2147483647;28;;;~%28;",128,0,0,0
2031 In the stab describing derived class D below, the information about
2032 the derivation of this class is encoded as follows.
2035 .stabs "derived_class_name:symbol_descriptors(struct tag&type)=
2036 type_descriptor(struct)struct_bytes(32)!num_bases(3),
2037 base_virtual(no)inheritence_public(no)base_offset(0),
2038 base_class_type_ref(A);
2039 base_virtual(yes)inheritence_public(no)base_offset(NIL),
2040 base_class_type_ref(B);
2041 base_virtual(no)inheritence_public(yes)base_offset(64),
2042 base_class_type_ref(C); @dots{}
2045 @c FIXME! fake linebreaks.
2047 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:
2048 1,160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt:
2049 :32:i;2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;
2050 28;;D_virt::32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2053 @node Virtual Base Classes
2054 @section Virtual Base Classes
2056 A derived class object consists of a concatination in memory of the
2057 data areas defined by each base class, starting with the leftmost and
2058 ending with the rightmost in the list of base classes. The exception
2059 to this rule is for virtual inheritence. In the example above, class
2060 D inherits virtually from base class B. This means that an instance
2061 of a D object will not contain it's own B part but merely a pointer to
2062 a B part, known as a virtual base pointer.
2064 In a derived class stab, the base offset part of the derivation
2065 information, described above, shows how the base class parts are
2066 ordered. The base offset for a virtual base class is always given as
2067 0. Notice that the base offset for B is given as 0 even though B is
2068 not the first base class. The first base class A starts at offset 0.
2070 The field information part of the stab for class D describes the field
2071 which is the pointer to the virtual base class B. The vbase pointer
2072 name is $vb followed by a type reference to the virtual base class.
2073 Since the type id for B in this example is 25, the vbase pointer name
2076 @c FIXME!! fake linebreaks below
2078 .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1,
2079 160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i;
2080 2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt:
2081 :32:i;2A*-2147483646;31;;;~%20;",128,0,0,0
2084 Following the name and a semicolon is a type reference describing the
2085 type of the virtual base class pointer, in this case 24. Type 24 was
2086 defined earlier as the type of the B class `this` pointer. The
2087 `this' pointer for a class is a pointer to the class type.
2090 .stabs "this:P24=*25=xsB:",64,0,0,8
2093 Finally the field offset part of the vbase pointer field description
2094 shows that the vbase pointer is the first field in the D object,
2095 before any data fields defined by the class. The layout of a D class
2096 object is a follows, Adat at 0, the vtable pointer for A at 32, Cdat
2097 at 64, the vtable pointer for C at 96, the virtual ase pointer for B
2098 at 128, and Ddat at 160.
2101 @node Static Members
2102 @section Static Members
2104 The data area for a class is a concatenation of the space used by the
2105 data members of the class. If the class has virtual methods, a vtable
2106 pointer follows the class data. The field offset part of each field
2107 description in the class stab shows this ordering.
2109 << How is this reflected in stabs? See Cygnus bug #677 for some info. >>
2112 @appendix Example2.c - source code for extended example
2116 2 register int g_bar asm ("%g5");
2117 3 static int s_g_repeat = 2;
2123 9 char s_char_vec[8];
2124 10 struct s_tag* s_next;
2127 13 typedef struct s_tag s_typedef;
2129 15 char char_vec[3] = @{'a','b','c'@};
2131 17 main (argc, argv)
2135 21 static float s_flap;
2137 23 for (times=0; times < s_g_repeat; times++)@{
2139 25 printf ("Hello world\n");
2143 29 enum e_places @{first,second=3,last@};
2145 31 static s_proc (s_arg, s_ptr_arg, char_vec)
2147 33 s_typedef* s_ptr_arg;
2161 @appendix Example2.s - assembly code for extended example
2165 2 .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
2166 3 .stabs "example2.c",100,0,0,Ltext0
2169 6 .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
2170 7 .stabs "char:t2=r2;0;127;",128,0,0,0
2171 8 .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
2172 9 .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
2173 10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
2174 11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
2175 12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
2176 13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
2177 14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
2178 15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
2179 16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
2180 17 .stabs "float:t12=r1;4;0;",128,0,0,0
2181 18 .stabs "double:t13=r1;8;0;",128,0,0,0
2182 19 .stabs "long double:t14=r1;8;0;",128,0,0,0
2183 20 .stabs "void:t15=15",128,0,0,0
2184 21 .stabs "g_foo:G2",32,0,0,0
2189 26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
2193 @c FIXME! fake linebreak in line 30
2194 30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;s_char_vec:
2195 17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
2196 31 .stabs "s_typedef:t16",128,0,0,0
2197 32 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
2198 33 .global _char_vec
2204 39 .reserve _s_flap.0,4,"bss",4
2208 43 .ascii "Hello world\12\0"
2213 48 .stabn 68,0,20,LM1
2216 51 save %sp,-144,%sp
2223 58 .stabn 68,0,23,LM2
2227 62 sethi %hi(_s_g_repeat),%o0
2229 64 ld [%o0+%lo(_s_g_repeat)],%o0
2234 69 .stabn 68,0,25,LM3
2236 71 sethi %hi(LC0),%o1
2237 72 or %o1,%lo(LC0),%o0
2240 75 .stabn 68,0,26,LM4
2243 78 .stabn 68,0,23,LM5
2251 86 .stabn 68,0,27,LM6
2254 89 .stabn 68,0,27,LM7
2259 94 .stabs "main:F1",36,0,0,_main
2260 95 .stabs "argc:p1",160,0,0,68
2261 96 .stabs "argv:p20=*21=*2",160,0,0,72
2262 97 .stabs "s_flap:V12",40,0,0,_s_flap.0
2263 98 .stabs "times:1",128,0,0,-20
2264 99 .stabn 192,0,0,LBB2
2265 100 .stabs "inner:1",128,0,0,-24
2266 101 .stabn 192,0,0,LBB3
2267 102 .stabn 224,0,0,LBE3
2268 103 .stabn 224,0,0,LBE2
2269 104 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
2270 @c FIXME: fake linebreak in line 105
2271 105 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
2276 109 .stabn 68,0,35,LM8
2279 112 save %sp,-120,%sp
2285 118 .stabn 68,0,41,LM9
2288 121 .stabn 68,0,41,LM10
2293 126 .stabs "s_proc:f1",36,0,0,_s_proc
2294 127 .stabs "s_arg:p16",160,0,0,0
2295 128 .stabs "s_ptr_arg:p18",160,0,0,72
2296 129 .stabs "char_vec:p21",160,0,0,76
2297 130 .stabs "an_u:23",128,0,0,-20
2298 131 .stabn 192,0,0,LBB4
2299 132 .stabn 224,0,0,LBE4
2300 133 .stabs "g_bar:r1",64,0,0,5
2301 134 .stabs "g_pf:G24=*25=f1",32,0,0,0
2302 135 .common _g_pf,4,"bss"
2303 136 .stabs "g_an_s:G16",32,0,0,0
2304 137 .common _g_an_s,20,"bss"
2308 @node Quick reference
2309 @appendix Quick reference
2312 * Stab types:: Table A: Symbol types from stabs
2313 * Assembler types:: Table B: Symbol types from assembler and linker
2314 * Symbol descriptors:: Table C
2315 * Type Descriptors:: Table D
2319 @section Table A: Symbol types from stabs
2321 Table A lists stab types sorted by type number. Stab type numbers are
2322 32 and greater. This is the full list of stab numbers, including stab
2323 types that are used in languages other than C.
2325 The #define names for these stab types are defined in:
2326 devo/include/aout/stab.def
2329 type type #define used to describe
2330 dec hex name source program feature
2331 ------------------------------------------------
2332 32 0x20 N_GYSM global symbol
2333 34 0X22 N_FNAME function name (for BSD Fortran)
2334 36 0x24 N_FUN function name or text segment variable for C
2335 38 0x26 N_STSYM static symbol (data segment w/internal linkage)
2336 40 0x28 N_LCSYM .lcomm symbol(BSS-seg variable w/internal linkage)
2337 42 0x2a N_MAIN Name of main routine (not used in C)
2338 48 0x30 N_PC global symbol (for Pascal)
2339 50 0x32 N_NSYMS number of symbols (according to Ultrix V4.0)
2340 52 0x34 N_NOMAP no DST map for sym (according to Ultrix V4.0)
2341 64 0x40 N_RSYM register variable
2342 66 0x42 N_M2C Modula-2 compilation unit
2343 68 0x44 N_SLINE line number in text segment
2344 70 0x46 N_DSLINE line number in data segment
2346 72 0x48 N_BSLINE line number in bss segment
2347 72 0x48 N_BROWS Sun source code browser, path to .cb file
2349 74 0x4a N_DEFD GNU Modula2 definition module dependency
2351 80 0x50 N_EHDECL GNU C++ exception variable
2352 80 0x50 N_MOD2 Modula2 info "for imc" (according to Ultrix V4.0)
2354 84 0x54 N_CATCH GNU C++ "catch" clause
2355 96 0x60 N_SSYM structure of union element
2356 100 0x64 N_SO path and name of source file
2357 128 0x80 N_LSYM automatic var in the stack
2358 (also used for type desc.)
2359 130 0x82 N_BINCL beginning of an include file (Sun only)
2360 132 0x84 N_SOL Name of sub-source (#include) file.
2361 160 0xa0 N_PSYM parameter variable
2362 162 0xa2 N_EINCL end of an include file
2363 164 0xa4 N_ENTRY alternate entry point
2364 192 0xc0 N_LBRAC beginning of a lexical block
2365 194 0xc2 N_EXCL place holder for a deleted include file
2366 196 0xc4 N_SCOPE modula2 scope information (Sun linker)
2367 224 0xe0 N_RBRAC end of a lexical block
2368 226 0xe2 N_BCOMM begin named common block
2369 228 0xe4 N_ECOMM end named common block
2370 232 0xe8 N_ECOML end common (local name)
2372 << used on Gould systems for non-base registers syms >>
2373 240 0xf0 N_NBTEXT ??
2374 242 0xf2 N_NBDATA ??
2380 @node Assembler types
2381 @section Table B: Symbol types from assembler and linker
2383 Table B shows the types of symbol table entries that hold assembler
2386 The #define names for these n_types values are defined in
2387 /include/aout/aout64.h
2391 n_type n_type name used to describe
2392 ------------------------------------------
2393 1 0x0 N_UNDF undefined symbol
2394 2 0x2 N_ABS absolute symbol -- defined at a particular address
2395 3 0x3 extern " (vs. file scope)
2396 4 0x4 N_TEXT text symbol -- defined at offset in text segment
2397 5 0x5 extern " (vs. file scope)
2398 6 0x6 N_DATA data symbol -- defined at offset in data segment
2399 7 0x7 extern " (vs. file scope)
2400 8 0x8 N_BSS BSS symbol -- defined at offset in zero'd segment
2401 9 extern " (vs. file scope)
2403 12 0x0C N_FN_SEQ func name for Sequent compilers (stab exception)
2405 49 0x12 N_COMM common sym -- visable after shared lib dynamic link
2406 31 0x1f N_FN file name of a .o file
2409 @node Symbol descriptors
2410 @section Table C: Symbol descriptors
2414 -------------------------------------------------
2415 (empty) local variable
2421 S static global variable
2423 T enumeration, struct or type tag
2424 V static local variable
2427 @node Type Descriptors
2428 @section Table D: Type Descriptors
2432 -------------------------------------
2433 (empty) type reference
2439 u union specifications
2444 @node Expanded reference
2445 @appendix Expanded reference by stab type.
2449 The first line is the symbol type expressed in decimal, hexadecimal,
2450 and as a #define (see devo/include/aout/stab.def).
2452 The second line describes the language constructs the symbol type
2455 The third line is the stab format with the significant stab fields
2456 named and the rest NIL.
2458 Subsequent lines expand upon the meaning and possible values for each
2459 significant stab field. # stands in for the type descriptor.
2461 Finally, any further information.
2464 * N_GSYM:: Global variable
2465 * N_FNAME:: Function name (BSD Fortran)
2466 * N_FUN:: C Function name or text segment variable
2467 * N_STSYM:: Initialized static symbol
2468 * N_LCSYM:: Uninitialized static symbol
2469 * N_MAIN:: Name of main routine (not for C)
2470 * N_PC:: Pascal global symbol
2471 * N_NSYMS:: Number of symbols
2472 * N_NOMAP:: No DST map
2473 * N_RSYM:: Register variable
2474 * N_M2C:: Modula-2 compilation unit
2475 * N_SLINE:: Line number in text segment
2476 * N_DSLINE:: Line number in data segment
2477 * N_BSLINE:: Line number in bss segment
2478 * N_BROWS:: Path to .cb file for Sun source code browser
2479 * N_DEFD:: GNU Modula2 definition module dependency
2480 * N_EHDECL:: GNU C++ exception variable
2481 * N_MOD2:: Modula2 information "for imc"
2482 * N_CATCH:: GNU C++ "catch" clause
2483 * N_SSYM:: Structure or union element
2484 * N_SO:: Source file containing main
2485 * N_LSYM:: Automatic variable
2486 * N_BINCL:: Beginning of include file (Sun only)
2487 * N_SOL:: Name of include file
2488 * N_PSYM:: Parameter variable
2489 * N_EINCL:: End of include file
2490 * N_ENTRY:: Alternate entry point
2491 * N_LBRAC:: Beginning of lexical block
2492 * N_EXCL:: Deleted include file
2493 * N_SCOPE:: Modula2 scope information (Sun only)
2494 * N_RBRAC:: End of lexical block
2495 * N_BCOMM:: Begin named common block
2496 * N_ECOMM:: End named common block
2497 * N_ECOML:: End common
2498 * Gould:: non-base register symbols used on Gould systems
2499 * N_LENG:: Length of preceding entry
2503 @section 32 - 0x20 - N_GYSM
2508 .stabs "name", N_GSYM, NIL, NIL, NIL
2512 "name" -> "symbol_name:#type"
2516 Only the "name" field is significant. the location of the variable is
2517 obtained from the corresponding external symbol.
2520 @section 34 - 0x22 - N_FNAME
2521 Function name (for BSD Fortran)
2524 .stabs "name", N_FNAME, NIL, NIL, NIL
2528 "name" -> "function_name"
2531 Only the "name" field is significant. The location of the symbol is
2532 obtained from the corresponding extern symbol.
2535 @section 36 - 0x24 - N_FUN
2536 Function name or text segment variable for C.
2539 .stabs "name", N_FUN, NIL, desc, value
2543 @exdent @emph{For functions:}
2544 "name" -> "proc_name:#return_type"
2545 # -> F (global function)
2547 desc -> line num for proc start. (GCC doesn't set and DBX doesn't miss it.)
2548 value -> Code address of proc start.
2550 @exdent @emph{For text segment variables:}
2551 <<How to create one?>>
2555 @section 38 - 0x26 - N_STSYM
2556 Initialized static symbol (data segment w/internal linkage).
2559 .stabs "name", N_STSYM, NIL, NIL, value
2563 "name" -> "symbol_name#type"
2564 # -> S (scope global to compilation unit)
2565 -> V (scope local to a procedure)
2566 value -> Data Address
2570 @section 40 - 0x28 - N_LCSYM
2571 Unitialized static (.lcomm) symbol(BSS segment w/internal linkage).
2574 .stabs "name", N_LCLSYM, NIL, NIL, value
2578 "name" -> "symbol_name#type"
2579 # -> S (scope global to compilation unit)
2580 -> V (scope local to procedure)
2581 value -> BSS Address
2585 @section 42 - 0x2a - N_MAIN
2586 Name of main routine (not used in C)
2589 .stabs "name", N_MAIN, NIL, NIL, NIL
2593 "name" -> "name_of_main_routine"
2597 @section 48 - 0x30 - N_PC
2598 Global symbol (for Pascal)
2601 .stabs "name", N_PC, NIL, NIL, value
2605 "name" -> "symbol_name" <<?>>
2606 value -> supposedly the line number (stab.def is skeptical)
2612 global pascal symbol: name,,0,subtype,line
2617 @section 50 - 0x32 - N_NSYMS
2618 Number of symbols (according to Ultrix V4.0)
2621 0, files,,funcs,lines (stab.def)
2625 @section 52 - 0x34 - N_NOMAP
2626 no DST map for sym (according to Ultrix V4.0)
2629 name, ,0,type,ignored (stab.def)
2633 @section 64 - 0x40 - N_RSYM
2637 .stabs "name:type",N_RSYM,0,RegSize,RegNumber (Sun doc)
2641 @section 66 - 0x42 - N_M2C
2642 Modula-2 compilation unit
2645 .stabs "name", N_M2C, 0, desc, value
2649 "name" -> "unit_name,unit_time_stamp[,code_time_stamp]
2651 value -> 0 (main unit)
2656 @section 68 - 0x44 - N_SLINE
2657 Line number in text segment
2660 .stabn N_SLINE, 0, desc, value
2665 value -> code_address (relocatable addr where the corresponding code starts)
2668 For single source lines that generate discontiguous code, such as flow
2669 of control statements, there may be more than one N_SLINE stab for the
2670 same source line. In this case there is a stab at the start of each
2671 code range, each with the same line number.
2674 @section 70 - 0x46 - N_DSLINE
2675 Line number in data segment
2678 .stabn N_DSLINE, 0, desc, value
2683 value -> data_address (relocatable addr where the corresponding code
2687 See comment for N_SLINE above.
2690 @section 72 - 0x48 - N_BSLINE
2691 Line number in bss segment
2694 .stabn N_BSLINE, 0, desc, value
2699 value -> bss_address (relocatable addr where the corresponding code
2703 See comment for N_SLINE above.
2706 @section 72 - 0x48 - N_BROWS
2707 Sun source code browser, path to .cb file
2710 "path to associated .cb file"
2712 Note: type field value overlaps with N_BSLINE
2715 @section 74 - 0x4a - N_DEFD
2716 GNU Modula2 definition module dependency
2718 GNU Modula-2 definition module dependency. Value is the modification
2719 time of the definition file. Other is non-zero if it is imported with
2720 the GNU M2 keyword %INITIALIZE. Perhaps N_M2C can be used if there
2721 are enough empty fields?
2724 @section 80 - 0x50 - N_EHDECL
2725 GNU C++ exception variable <<?>>
2727 "name is variable name"
2729 Note: conflicts with N_MOD2.
2732 @section 80 - 0x50 - N_MOD2
2733 Modula2 info "for imc" (according to Ultrix V4.0)
2735 Note: conflicts with N_EHDECL <<?>>
2738 @section 84 - 0x54 - N_CATCH
2739 GNU C++ "catch" clause
2741 GNU C++ `catch' clause. Value is its address. Desc is nonzero if
2742 this entry is immediately followed by a CAUGHT stab saying what
2743 exception was caught. Multiple CAUGHT stabs means that multiple
2744 exceptions can be caught here. If Desc is 0, it means all exceptions
2748 @section 96 - 0x60 - N_SSYM
2749 Structure or union element
2751 Value is offset in the structure.
2753 <<?looking at structs and unions in C I didn't see these>>
2756 @section 100 - 0x64 - N_SO
2757 Path and name of source file containing main routine
2760 .stabs "name", N_SO, NIL, NIL, value
2764 "name" -> /path/to/source/file
2765 -> source_file_terminal_name
2767 value -> the starting text address of the compilation.
2770 These are found two in a row. The name field of the first N_SO
2771 contains the path to the source file. The name field of the second
2772 N_SO contains the terminal name of the source file itself.
2775 @section 128 - 0x80 - N_LSYM
2776 Automatic var in the stack (also used for type descriptors.)
2779 .stabs "name" N_LSYM, NIL, NIL, value
2783 @exdent @emph{For stack based local variables:}
2785 "name" -> name of the variable
2786 value -> offset from frame pointer (negative)
2788 @exdent @emph{For type descriptors:}
2790 "name" -> "name_of_the_type:#type"
2793 type -> type_ref (or) type_def
2795 type_ref -> type_number
2796 type_def -> type_number=type_desc etc.
2799 Type may be either a type reference or a type definition. A type
2800 reference is a number that refers to a previously defined type. A
2801 type definition is the number that will refer to this type, followed
2802 by an equals sign, a type descriptor and the additional data that
2803 defines the type. See the Table D for type descriptors and the
2804 section on types for what data follows each type descriptor.
2807 @section 130 - 0x82 - N_BINCL
2809 Beginning of an include file (Sun only)
2811 Beginning of an include file. Only Sun uses this. In an object file,
2812 only the name is significant. The Sun linker puts data into some of
2816 @section 132 - 0x84 - N_SOL
2818 Name of a sub-source file (#include file). Value is starting address
2823 @section 160 - 0xa0 - N_PSYM
2828 stabs. "name", N_PSYM, NIL, NIL, value
2832 "name" -> "param_name:#type"
2833 # -> p (value parameter)
2834 -> i (value parameter by reference, indirect access)
2835 -> v (variable parameter by reference)
2836 -> C ( read-only parameter, conformant array bound)
2837 -> x (confomant array value parameter)
2840 -> X (function result variable)
2841 -> b (based variable)
2843 value -> offset from the argument pointer (positive).
2846 On most machines the argument pointer is the same as the frame
2850 @section 162 - 0xa2 - N_EINCL
2852 End of an include file. This and N_BINCL act as brackets around the
2853 file's output. In an ojbect file, there is no significant data in
2854 this entry. The Sun linker puts data into some of the fields.
2858 @section 164 - 0xa4 - N_ENTRY
2860 Alternate entry point.
2861 Value is its address.
2865 @section 192 - 0xc0 - N_LBRAC
2867 Beginning of a lexical block (left brace). The variable defined
2868 inside the block precede the N_LBRAC symbol. Or can they follow as
2869 well as long as a new N_FUNC was not encountered. <<?>>
2872 .stabn N_LBRAC, NIL, NIL, value
2876 value -> code address of block start.
2880 @section 194 - 0xc2 - N_EXCL
2882 Place holder for a deleted include file. Replaces a N_BINCL and
2883 everything up to the corresponding N_EINCL. The Sun linker generates
2884 these when it finds multiple indentical copies of the symbols from an
2885 included file. This appears only in output from the Sun linker.
2889 @section 196 - 0xc4 - N_SCOPE
2891 Modula2 scope information (Sun linker)
2895 @section 224 - 0xe0 - N_RBRAC
2897 End of a lexical block (right brace)
2900 .stabn N_RBRAC, NIL, NIL, value
2904 value -> code address of the end of the block.
2908 @section 226 - 0xe2 - N_BCOMM
2910 Begin named common block.
2912 Only the name is significant.
2916 @section 228 - 0xe4 - N_ECOMM
2918 End named common block.
2920 Only the name is significant and it should match the N_BCOMM
2924 @section 232 - 0xe8 - N_ECOML
2926 End common (local name)
2932 @section Non-base registers on Gould systems
2933 << used on Gould systems for non-base registers syms, values assigned
2934 at random, need real info from Gould. >>
2938 240 0xf0 N_NBTEXT ??
2939 242 0xf2 N_NBDATA ??
2946 @section - 0xfe - N_LENG
2948 Second symbol entry containing a length-value for the preceding entry.
2949 The value is the length.
2952 @appendix Questions and anomalies
2956 For GNU C stabs defining local and global variables (N_LSYM and
2957 N_GSYM), the desc field is supposed to contain the source line number
2958 on which the variable is defined. In reality the desc field is always
2959 0. (This behavour is defined in dbxout.c and putting a line number in
2960 desc is controlled by #ifdef WINNING_GDB which defaults to false). Gdb
2961 supposedly uses this information if you say 'list var'. In reality
2962 var can be a variable defined in the program and gdb says `function
2966 In GNU C stabs there seems to be no way to differentiate tag types:
2967 structures, unions, and enums (symbol descriptor T) and typedefs
2968 (symbol descriptor t) defined at file scope from types defined locally
2969 to a procedure or other more local scope. They all use the N_LSYM
2970 stab type. Types defined at procedure scope are emited after the
2971 N_RBRAC of the preceding function and before the code of the
2972 procedure in which they are defined. This is exactly the same as
2973 types defined in the source file between the two procedure bodies.
2974 GDB overcompensates by placing all types in block #1 the block for
2975 symbols of file scope. This is true for default, -ansi and
2976 -traditional compiler options. (p0001063-gcc, p0001066-gdb)
2979 What ends the procedure scope? Is it the proc block's N_RBRAC or the
2980 next N_FUN? (I believe its the first.)
2983 The comment in xcoff.h says DBX_STATIC_CONST_VAR_CODE is used for
2984 static const variables. DBX_STATIC_CONST_VAR_CODE is set to N_FUN by
2985 default, in dbxout.c. If included, xcoff.h redefines it to N_STSYM.
2986 But testing the default behaviour, my Sun4 native example shows
2987 N_STSYM not N_FUN is used to describe file static initialized
2988 variables. (the code tests for TREE_READONLY(decl) &&
2989 !TREE_THIS_VOLATILE(decl) and if true uses DBX_STATIC_CONST_VAR_CODE).
2992 Global variable stabs don't have location information. This comes
2993 from the external symbol for the same variable. The external symbol
2994 has a leading underbar on the _name of the variable and the stab does
2995 not. How do we know these two symbol table entries are talking about
2996 the same symbol when their names are different?
2999 Can gcc be configured to output stabs the way the Sun compiler
3000 does, so that their native debugging tools work? <NO?> It doesn't by
3001 default. GDB reads either format of stab. (gcc or SunC). How about
3005 @node xcoff-differences
3006 @appendix Differences between GNU stabs in a.out and GNU stabs in xcoff
3008 (The AIX/RS6000 native object file format is xcoff with stabs)
3012 Instead of .stabs, xcoff uses .stabx.
3015 The data fields of an xcoff .stabx are in a different order than an
3016 a.out .stabs. The order is: string, value, type. The desc and null
3017 fields present in a.out stabs are missing in xcoff stabs. For N_GSYM
3018 the value field is the name of the symbol.
3021 BSD a.out stab types correspond to AIX xcoff storage classes. In general the
3022 mapping is N_STABTYPE becomes C_STABTYPE. Some stab types in a.out
3023 are not supported in xcoff. See Table E. for full mappings.
3026 initialised static N_STSYM and un-initialized static N_LCSYM both map
3027 to the C_STSYM storage class. But the destinction is preserved
3028 because in xcoff N_STSYM and N_LCSYM must be emited in a named static
3029 block. Begin the block with .bs s[RW] data_section_name for N_STSYM
3030 or .bs s bss_section_name for N_LCSYM. End the block with .es
3033 xcoff stabs describing tags and typedefs use the N_DECL (0x8c)instead
3034 of N_LSYM stab type.
3037 xcoff uses N_RPSYM (0x8e) instead of the N_RSYM stab type for register
3038 variables. If the register variable is also a value parameter, then
3039 use R instead of P for the symbol descriptor.
3042 xcoff uses negative numbers as type references to the basic types.
3043 There are no boilerplate type definitions emited for these basic
3044 types. << make table of basic types and type numbers for C >>
3047 xcoff .stabx sometimes don't have the name part of the string field.
3050 xcoff uses a .file stab type to represent the source file name. There
3051 is no stab for the path to the source file.
3054 xcoff uses a .line stab type to represent source lines. The format
3055 is: .line line_number.
3058 xcoff emits line numbers relative to the start of the current
3059 function. The start of a function is marked by .bf. If a function
3060 includes lines from a seperate file, then those line numbers are
3061 absolute line numbers in the <<sub-?>> file being compiled.
3064 The start of current include file is marked with: .bi "filename" and
3065 the end marked with .ei "filename"
3068 If the xcoff stab is a N_FUN (C_FUN) then follow the string field with
3069 ,. instead of just ,
3072 The symbol descriptor for register parameters is P for a.out and R for
3077 (I think that's it for .s file differences. They could stand to be
3078 better presented. This is just a list of what I have noticed so far.
3079 There are a *lot* of differences in the information in the symbol
3080 tables of the executable and object files.)
3082 Table E: mapping a.out stab types to xcoff storage classes
3085 stab type storage class
3086 -------------------------------
3095 N_RPSYM (0x8e) C_RPSYM
3105 N_DECL (0x8c) C_DECL
3122 @node Sun-differences
3123 @appendix Differences between GNU stabs and Sun native stabs.
3127 GNU C stabs define *all* types, file or procedure scope, as
3128 N_LSYM. Sun doc talks about using N_GSYM too.
3131 @c FIXME: are you sure? The 'a' is usually followed by an 'r' which means
3132 @c "range type" but that doesn't mean the type descriptor is 'ar'.
3133 @c A (non-C) array could be indexed by an enum, for example. -kingdon
3134 GNU C stabs use `ar' as type descriptor when defining arrays vs. just
3138 Stabs describing block scopes, N_LBRAC and N_RBRAC are supposed to
3139 contain the nesting level of the block in the desc field, re Sun doc.
3140 GNU stabs always have 0 in that field.
3143 Sun C stabs use type number pairs in the format (a,b) where a is a
3144 number starting with 1 and incremented for each sub-source file in the
3145 compilation. b is a number starting with 1 and incremented for each
3146 new type defined in the compilation. GNU C stabs use the type number
3147 alone, with no source file number.