     START-INFO-DIR-ENTRY
     * Stabs: (stabs).               The "stabs" debugging information format.
     END-INFO-DIR-ENTRY

This document describes GNU stabs (debugging symbol tables) in a.out files.

Copyright 1992 Free Software Foundation, Inc. Contributed by Cygnus Support.  Written by Julia Menapace.

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy or distribute modified versions of this manual under the terms of the GPL (for which purpose this text may be regarded as a program in the language TeX).

The "stabs" representation of debugging information
***************************************************

This document describes the GNU stabs debugging format in a.out files.

Overview of stabs
*****************

Stabs refers to a format for information that describes a program to a debugger.  This format was apparently invented by the University of California at Berkeley, for the pdx Pascal debugger; the format has spread widely since then.

Overview of debugging information flow
======================================

The GNU C compiler compiles C source in a .c file into assembly language in a .s file, which is translated by the assembler into a .o file, and then linked with other .o files and libraries to produce an executable file.

With the -g option, GCC puts additional debugging information in the .s file, which is slightly transformed by the assembler and linker, and carried through into the final executable.  This debugging information describes features of the source file like line numbers, the types and scopes of variables, and functions, their parameters and their scopes.

For some object file formats, the debugging information is encapsulated in assembler directives known collectively as `stab' (symbol table) directives, interspersed with the generated code.  Stabs are the native format for debugging information in the a.out and xcoff object file formats.  The GNU tools can also emit stabs in the coff and ecoff object file formats.

The assembler adds the information from stabs to the symbol information it places by default in the symbol table and the string table of the .o file it is building.  The linker consolidates the .o files into one executable file, with one symbol table and one string table.  Debuggers use the symbol and string tables in the executable as a source of debugging information about the program.

Overview of stab format
=======================

There are three overall formats for stab assembler directives differentiated by the first word of the stab.  The name of the directive describes what combination of four possible data fields will follow.  It is either .stabs (string), .stabn (number), or .stabd (dot).

The overall format of each class of stab is:

     .stabs "STRING",TYPE,0,DESC,VALUE
     .stabn          TYPE,0,DESC,VALUE
     .stabd          TYPE,0,DESC

In general, in .stabs the STRING field contains name and type information.  For .stabd the value field is implicit and has the value of the current file location.  Otherwise the value field often contains a relocatable address, frame pointer offset, or register number, that maps to the source code element described by the stab.

The real key to decoding the meaning of a stab is the number in its type field.  Each possible type number defines a different stab type.  The stab type further defines the exact interpretation of, and possible values for, any remaining "STRING", DESC, or VALUE fields present in the stab.  Table A (see Table A: Symbol types from stabs) lists in numeric order the possible type field values for stab directives.  The reference section that follows Table A describes the meaning of the fields for each stab type in detail.  The examples that follow this overview introduce the stab types in terms of the source code elements they describe.

For .stabs the "STRING" field holds the meat of the debugging information.  The generally unstructured nature of this field is what makes stabs extensible.  For some stab types the string field contains only a name.  For other stab types the contents can be a great deal more complex.

The overall format is of the "STRING" field is:

     "NAME[:SYMBOL_DESCRIPTOR]
          [TYPE_NUMBER[=TYPE_DESCRIPTOR ...]]"

NAME is the name of the symbol represented by the stab.

The SYMBOL_DESCRIPTOR following the : is an alphabetic character that tells more specifically what kind of symbol the stab represents. If the SYMBOL_DESCRIPTOR is omitted, but type information follows, then the stab represents a local variable.  For a list of symbol_descriptors, see Table C: Symbol descriptors.

Type information is either a TYPE_NUMBER, or a TYPE_NUMBER=.  The TYPE_NUMBER alone is a type reference, referring directly to a type that has already been defined.

The TYPE_NUMBER= is a type definition, where the number represents a new type which is about to be defined.  The type definition may refer to other types by number, and those type numbers may be followed by = and nested definitions.

In a type definition, if the character that follows the equals sign is non-numeric then it is a TYPE_DESCRIPTOR, and tells what kind of type is about to be defined.  Any other values following the TYPE_DESCRIPTOR vary, depending on the TYPE_DESCRIPTOR.  If a number follows the = then the number is a TYPE_REFERENCE. This is described more thoroughly in the section on types.  See Table D: Type Descriptors, for a list of TYPE_DESCRIPTOR values.

All this can make the "STRING" field quite long.  When the "STRING" part of a stab is too long, the compiler splits the .stabs directive into two .stabs directives.  Both stabs duplicate exactly all but the "STRING" field.  The "STRING" field of the first stab contains the first part of the overlong string, marked as continued with a double-backslash at the end.  The "STRING" field of the second stab holds the second half of the overlong string.

A simple example in C source
============================

To get the flavor of how stabs describe source information for a C program, let's look at the simple program:

     main()
     {
             printf("Hello world");
     }

When compiled with -g, the program above yields the following .s file.  Line numbers have been added to make it easier to refer to parts of the .s file in the description of the stabs that follows.

The simple example at the assembly level
========================================

     1  gcc2_compiled.:
     2  .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
     3  .stabs "hello.c",100,0,0,Ltext0
     4  .text
     5  Ltext0:
     6  .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
     7  .stabs "char:t2=r2;0;127;",128,0,0,0
     8  .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
     9  .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
     10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
     11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
     12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
     13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
     14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
     15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
     16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
     17 .stabs "float:t12=r1;4;0;",128,0,0,0
     18 .stabs "double:t13=r1;8;0;",128,0,0,0
     19 .stabs "long double:t14=r1;8;0;",128,0,0,0
     20 .stabs "void:t15=15",128,0,0,0
     21      .align 4
     22 LC0:
     23      .ascii "Hello, world!\12\0"
     24      .align 4
     25      .global _main
     26      .proc 1
     27 _main:
     28 .stabn 68,0,4,LM1
     29 LM1:
     30      !#PROLOGUE# 0
     31      save %sp,-136,%sp
     32      !#PROLOGUE# 1
     33      call ___main,0
     34      nop
     35 .stabn 68,0,5,LM2
     36 LM2:
     37 LBB2:
     38      sethi %hi(LC0),%o1
     39      or %o1,%lo(LC0),%o0
     40      call _printf,0
     41      nop
     42 .stabn 68,0,6,LM3
     43 LM3:
     44 LBE2:
     45 .stabn 68,0,6,LM4
     46 LM4:
     47 L1:
     48      ret
     49      restore
     50 .stabs "main:F1",36,0,0,_main
     51 .stabn 192,0,0,LBB2
     52 .stabn 224,0,0,LBE2

This simple ``hello world'' example demonstrates several of the stab types used to describe C language source files.

Encoding for the structure of the program
*****************************************

The path and name of the source file
====================================

Directive:
     .stabs

Type:
     N_SO

The first stabs in the .s file contain the name and path of the source file that was compiled to produce the .s file.  This information is contained in two records of stab type N_SO (100).

        .stabs "path_name", N_SO, NIL, NIL, Code_address_of_program_start
        .stabs "file_name:", N_SO, NIL, NIL, Code_address_of_program_start

     2  .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
     3  .stabs "hello.c",100,0,0,Ltext0
     4       .text
     5  Ltext0:

Line Numbers
============

Directive:
     .stabn

Type:
     N_SLINE

The start of source lines is represented by the N_SLINE (68) stab type.

     .stabn N_SLINE, NIL, LINE, ADDRESS

LINE is a source line number; ADDRESS represents the code address for the start of that source line.

     27 _main:
     28 .stabn 68,0,4,LM1
     29 LM1:
     30      !#PROLOGUE# 0

Procedures
==========

Directive:
     .stabs

Type:
     N_FUN

Symbol Descriptors:
     f (local), F (global)

Procedures are described by the N_FUN stab type.  The symbol descriptor for a procedure is F if the procedure is globally scoped and f if the procedure is static (locally scoped).

The N_FUN stab representing a procedure is located immediately following the code of the procedure.  The N_FUN stab is in turn directly followed by a group of other stabs describing elements of the procedure.  These other stabs describe the procedure's parameters, its block local variables and its block structure.

     48      ret
     49      restore

The .stabs entry after this code fragment shows the NAME of the procedure (main); the type descriptor DESC (F, for a global procedure); a reference to the predefined type int for the return type; and the starting ADDRESS of the procedure.

Here is an exploded summary (with whitespace introduced for clarity), followed by line 50 of our sample assembly output, which has this form:

     .stabs "NAME:
             DESC  (global proc F)
             RETURN_TYPE_REF  (int)
            ",N_FUN, NIL, NIL,
            ADDRESS

     50 .stabs "main:F1",36,0,0,_main

Block Structure
===============

Directive:
     .stabn

Types:
     N_LBRAC, N_RBRAC

The program's block structure is represented by the N_LBRAC (left brace) and the N_RBRAC (right brace) stab types.  The following code range, which is the body of main, is labeled with LBB2: at the beginning and LBE2: at the end.

     37 LBB2:
     38      sethi %hi(LC0),%o1
     39      or %o1,%lo(LC0),%o0
     40      call _printf,0
     41      nop
     42 .stabn 68,0,6,LM3
     43 LM3:
     44 LBE2:

The N_LBRAC and N_RBRAC stabs that describe the block scope of the procedure are located after the N_FUNC stab that represents the procedure itself.  The N_LBRAC uses the LBB2 label as the code address in its value field, and the N_RBRAC uses LBE2.

     50 .stabs "main:F1",36,0,0,_main

        .stabn N_LBRAC, NIL, NIL, LEFT-BRACE-ADDRESS
        .stabn N_RBRAC, NIL, NIL, RIGHT-BRACE-ADDRESS

     51 .stabn 192,0,0,LBB2
     52 .stabn 224,0,0,LBE2

Simple types
************

Basic type definitions
======================

Directive:
     .stabs

Type:
     N_LSYM

Symbol Descriptor:
     t

The basic types for the language are described using the N_LSYM stab type.  They are boilerplate and are emited by the compiler for each compilation unit.  Basic type definitions are not always a complete description of the type and are sometimes circular.  The debugger recognizes the type anyway, and knows how to read bits as that type.

Each language and compiler defines a slightly different set of basic types.  In this example we are looking at the basic types for C emited by the GNU compiler targeting the Sun4.  Here the basic types are mostly defined as range types.

Range types defined by min and max value
========================================

Type Descriptor:
     r

When defining a range type, if the number after the first semicolon is smaller than the number after the second one, then the two numbers represent the smallest and the largest values in the range.

     4  .text
     5  Ltext0:

     .stabs "NAME:
             DESCRIPTOR  (type)
             TYPE-DEF=
             TYPE-DESC
             TYPE-REF;
             LOW-BOUND;
             HIGH-BOUND;
            ",
            N_LSYM, NIL, NIL, NIL

     6  .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
     7  .stabs "char:t2=r2;0;127;",128,0,0,0

Here the integer type (1) is defined as a range of the integer type (1).  Likewise char is a range of char.  This part of the definition is circular, but at least the high and low bound values of the range hold more information about the type.

Here short unsigned int is defined as type number 8 and described as a range of type int, with a minimum value of 0 and a maximum of 65535.

     13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0

Range type defined by size in bytes
===================================

Type Descriptor:
     r

In a range definition, if the first number after the semicolon is positive and the second is zero, then the type being defined is a floating point type, and the number after the first semicolon is the number of bytes needed to represent the type.  Note that this does not provide a way to distinguish 8-byte real floating point types from 8-byte complex floating point types.

     .stabs "NAME:
             DESC
             TYPE-DEF=
             TYPE-DESC
             TYPE-REF;
             BIT-COUNT;
             0;
            ",
            N_LSYM, NIL, NIL, NIL

     17 .stabs "float:t12=r1;4;0;",128,0,0,0
     18 .stabs "double:t13=r1;8;0;",128,0,0,0
     19 .stabs "long double:t14=r1;8;0;",128,0,0,0

Cosmically enough, the void type is defined directly in terms of itself.

     .stabs "NAME:
             SYMBOL-DESC
             TYPE-DEF=
             TYPE-REF
            ",N_LSYM,NIL,NIL,NIL

     20 .stabs "void:t15=15",128,0,0,0

A Comprehensive Example in C
****************************

Now we'll examine a second program, example2, which builds on the first example to introduce the rest of the stab types, symbol descriptors, and type descriptors used in C. See Example2.c for the complete .c source, and see Example2.s for the .s assembly code. This description includes parts of those files.

Flow of control and nested scopes
=================================

Directive:
     .stabn

Types:
     N_SLINE, N_LBRAC, N_RBRAC (cont.)

Consider the body of main, from example2.c.  It shows more about how N_SLINE, N_RBRAC, and N_LBRAC stabs are used.

     20 {
     21      static float s_flap;
     22      int times;
     23      for (times=0; times < s_g_repeat; times++){
     24        int inner;
     25        printf ("Hello world\n");
     26      }
     27 };

Here we have a single source line, the for line, that generates non-linear flow of control, and non-contiguous code.  In this case, an N_SLINE stab with the same line number proceeds each block of non-contiguous code generated from the same source line.

The example also shows nested scopes.  The N_LBRAC and N_LBRAC stabs that describe block structure are nested in the same order as the corresponding code blocks, those of the for loop inside those for the body of main.

This is the label for the N_LBRAC (left brace) stab marking the start of main.

     57 LBB2:

In the first code range for C source line 23, the for loop initialize and test, N_SLINE (68) records the line number:

     .stabn N_SLINE, NIL,
            LINE,
            ADDRESS

     58 .stabn 68,0,23,LM2
     59 LM2:
     60      st %g0,[%fp-20]
     61 L2:
     62      sethi %hi(_s_g_repeat),%o0
     63      ld [%fp-20],%o1
     64      ld [%o0+%lo(_s_g_repeat)],%o0
     65      cmp %o1,%o0
     66      bge L3
     67      nop

     label for the N_LBRAC (start block) marking the start of for loop

     68 LBB3:
     69 .stabn 68,0,25,LM3
     70 LM3:
     71      sethi %hi(LC0),%o1
     72      or %o1,%lo(LC0),%o0
     73      call _printf,0
     74      nop
     75 .stabn 68,0,26,LM4
     76 LM4:

     label for the N_RBRAC (end block) stab marking the end of the for loop

     77 LBE3:

Now we come to the second code range for source line 23, the for loop increment and return.  Once again, N_SLINE (68) records the source line number:

     .stabn, N_SLINE, NIL,
             LINE,
             ADDRESS

     78 .stabn 68,0,23,LM5
     79 LM5:
     80 L4:
     81      ld [%fp-20],%o0
     82      add %o0,1,%o1
     83      st %o1,[%fp-20]
     84      b,a L2
     85 L3:
     86 .stabn 68,0,27,LM6
     87 LM6:

     label for the N_RBRAC (end block) stab marking the end of the for loop

     88 LBE2:
     89 .stabn 68,0,27,LM7
     90 LM7:
     91 L1:
     92      ret
     93      restore
     94 .stabs "main:F1",36,0,0,_main
     95 .stabs "argc:p1",160,0,0,68
     96 .stabs "argv:p20=*21=*2",160,0,0,72
     97 .stabs "s_flap:V12",40,0,0,_s_flap.0
     98 .stabs "times:1",128,0,0,-20

Here is an illustration of stabs describing nested scopes.  The scope nesting is reflected in the nested bracketing stabs (N_LBRAC, 192, appears here).

     .stabn N_LBRAC,NIL,NIL,
            BLOCK-START-ADDRESS

     99  .stabn 192,0,0,LBB2      ## begin proc label
     100 .stabs "inner:1",128,0,0,-24
     101 .stabn 192,0,0,LBB3      ## begin for label

N_RBRAC (224), ``right brace'' ends a lexical block (scope).

     .stabn N_RBRAC,NIL,NIL,
            BLOCK-END-ADDRESS

     102 .stabn 224,0,0,LBE3      ## end for label
     103 .stabn 224,0,0,LBE2      ## end proc label

Variables
*********

Locally scoped automatic variables
==================================

Directive:
     .stabs

Type:
     N_LSYM

Symbol Descriptor:
     none

In addition to describing types, the N_LSYM stab type also describes locally scoped automatic variables.  Refer again to the body of main in example2.c.  It allocates two automatic variables: times is scoped to the body of main, and inner is scoped to the body of the for loop. s_flap is locally scoped but not automatic, and will be discussed later.

     20 {
     21      static float s_flap;
     22      int times;
     23      for (times=0; times < s_g_repeat; times++){
     24        int inner;
     25        printf ("Hello world\n");
     26      }
     27 };

The N_LSYM stab for an automatic variable is located just before the N_LBRAC stab describing the open brace of the block to which it is scoped.

     N_LSYM (128): automatic variable, scoped locally to main

     .stabs "NAME:
             TYPE-REF",
            N_LSYM, NIL, NIL,
            FRAME-POINTER-OFFSET

     98  .stabs "times:1",128,0,0,-20
     99  .stabn 192,0,0,LBB2      ## begin `main' N_LBRAC

     N_LSYM (128): automatic variable, scoped locally to the for loop

     .stabs "NAME:
             TYPE-REF",
             N_LSYM, NIL, NIL,
            FRAME-POINTER-OFFSET

     100 .stabs "inner:1",128,0,0,-24
     101 .stabn 192,0,0,LBB3      ## begin `for' loop N_LBRAC

Since the character in the string field following the colon is not a letter, there is no symbol descriptor.  This means that the stab describes a local variable, and that the number after the colon is a type reference.  In this case it a a reference to the basic type int. Notice also that the frame pointer offset is negative number for automatic variables.

Global Variables
================

Directive:
     .stabs

Type:
     N_GSYM

Symbol Descriptor:
     G

Global variables are represented by the N_GSYM stab type.  The symbol descriptor, following the colon in the string field, is G.  Following the G is a type reference or type definition.  In this example it is a type reference to the basic C type, char.  The first source line in example2.c,

     1  char g_foo = 'c';

yields the following stab.  The stab immediately precedes the code that allocates storage for the variable it describes.

     N_GSYM (32): global symbol

     .stabs "NAME:
             DESCRIPTOR
             TYPE-REF",
             N_GSYM, NIL, NIL, NIL

     21 .stabs "g_foo:G2",32,0,0,0
     22      .global _g_foo
     23      .data
     24 _g_foo:
     25      .byte 99

The address of the variable represented by the N_GSYM is not contained in the N_GSYM stab.  The debugger gets this information from the external symbol for the global variable.

Global register variables
=========================

Directive:
     .stabs

Type:
     N_RSYM

Symbol Descriptor:
     r

The following source line defines a global variable, g_bar, which is explicitly allocated in global register %g5.

     2  register int g_bar asm ("%g5");

Register variables have their own stab type, N_RSYM, and their own symbol descriptor, r.  The stab's value field contains the number of the register where the variable data will be stored.  Since the variable was not initialized in this compilation unit, the stab is emited at the end of the object file, with the stabs for other uninitialized globals (bcc).

     N_RSYM (64): register variable

     .stabs "NAME:
             DESCRIPTOR
             TYPE-REF",
            N_RSYM, NIL, NIL,
            REGISTER

     133 .stabs "g_bar:r1",64,0,0,5

Initialized static variables
============================

Directive:
     .stabs

Type:
     N_STSYM

Symbol Descriptors:
     S (file scope), V (procedure scope)

Initialized static variables are represented by the N_STSYM stab type.  The symbol descriptor part of the string field shows if the variable is file scope static (S) or procedure scope static (V). The source line

     3  static int s_g_repeat = 2;

yields the following code.  The stab is located immediately preceding the storage for the variable it represents.  Since the variable in this example is file scope static the symbol descriptor is S.

     N_STSYM (38): initialized static variable (data seg w/internal linkage)

     .stabs "NAME:
             DESCRIPTOR
             TYPE-REF",
            N_STSYM,NIL,NIL,
            ADDRESS

     26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
     27      .align 4
     28 _s_g_repeat:
     29      .word 2

Un-initialized static variables
===============================

Directive:
     .stabs

Type:
     N_LCSYM

Symbol Descriptors:
     S (file scope), V (procedure scope)

Un-initialized static variables are represented by the N_LCSYM stab type.  The symbol descriptor part of the string shows if the variable is file scope static (S) or procedure scope static (V).  In this example it is procedure scope static.  The source line allocating s_flap immediately follows the open brace for the procedure main.

     20 {
     21      static float s_flap;

The code that reserves storage for the variable s_flap precedes the body of body of main.

     39      .reserve _s_flap.0,4,"bss",4

But since s_flap is scoped locally to main, its stab is located with the other stabs representing symbols local to main. The stab for s_flap is located just before the N_LBRAC for main.

     N_LCSYM (40): uninitialized static var (BSS seg w/internal linkage)

     .stabs "NAME:
             DESCRIPTOR
             TYPE-REF",
             N_LCSYM, NIL, NIL,
             ADDRESS

     97 .stabs "s_flap:V12",40,0,0,_s_flap.0
     98 .stabs "times:1",128,0,0,-20
     99 .stabn 192,0,0,LBB2                  # N_LBRAC for main.

Parameters
==========

Directive:
     .stabs

Type:
     N_PSYM

Symbol Descriptor:
     p

Procedure parameters are represented by the N_PSYM stab type.  The following source lines show the parameters of the main routine.

     17 main (argc, argv)
     18      int argc;
     19      char* argv[];
     20 {

The N_PSYM stabs describing parameters to a function directly follow the N_FUN stab that represents the procedure itself.  The N_FUN stab immediately follows the code of the procedure it describes.  Following the N_PSYM parameter stabs are any N_LSYM stabs representing local variables.

     <36> N_FUN - describing the procedure main

     94 .stabs "main:F1",36,0,0,_main

     <160> N_PSYM - parameters
     .stabs "name:sym_desc(value_param)type_ref(int)", N_PSYM,
     NIL, NIL, frame_ptr_offset

     95 .stabs "argc:p1",160,0,0,68

     <160> N_PSYM - parameter
     .stabs "name:sym_desc(value_param)type_def(20)=ptr_to type_def(21)=
     ptr_to type_ref(char)

     96 .stabs "argv:p20=*21=*2",160,0,0,72

The type definition of argv is interesting because it defines two new types in terms of an existing one.  The array argv contains character pointers.  The type of the array name is a pointer to the type the array holds. Thus the type of argv is ptr to ptr to char.  The stab for argv contains nested type_definitions.  Type 21 is ptr to type 2 (char) and argv (type 20) is ptr to type 21.

Aggregate Types
***************

Now let's look at some variable definitions involving complex types. This involves understanding better how types are described.  In the examples so far types have been described as references to previously defined types or defined in terms of subranges of or pointers to previously defined types.  The section that follows will talk about the various other type descriptors that may follow the = sign in a type definition.

Array types
===========

Directive:
     .stabs

Types:
     N_GSYM, N_LSYM

Symbol Descriptor:
     T

Type Descriptor:
     ar

As an example of an array type consider the global variable below.

     15 char char_vec[3] = {'a','b','c'};

Since the array is a global variable, it is described by the N_GSYM stab type.  The symbol descriptor G, following the colon in stab's string field, also says the array is a global variable.  Following the G is a definition for type (19) as shown by the equals sign after the type number.

After the equals sign is a type descriptor, ar, which says that the type being defined is an array.  Following the type descriptor for an array is the type of the index, the lower bound for array indexing (always 0 in C), the upper bound for array indexing (in C: one less than the length of the array), and the type of the array elements.

The array definition above generates the assembly language that follows.

     <32> N_GSYM - global variable
     .stabs "name:sym_desc(global)type_def(19)=type_desc(array)
     index_type_ref(int);NIL;high_bound(2);element_type_ref(char)";
     N_GSYM, NIL, NIL, NIL

     32 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
     33      .global _char_vec
     34      .align 4
     35 _char_vec:
     36      .byte 97
     37      .byte 98
     38      .byte 99

Enumerations
============

Directive:
     .stabs

Type:
     N_LSYM

Symbol Descriptor:
     T

Type Descriptor:
     e

The source line below declares an enumeration type.  It is defined at file scope between the bodies of main and s_proc in example2.c. Because the N_LSYM is located after the N_RBRAC that marks the end of the previous procedure's block scope, and before the N_FUN that marks the beginning of the next procedure's block scope, the N_LSYM does not describe a block local symbol, but a file local one.  The source line:

     29 enum e_places {first,second=3,last};

generates the following stab, located just after the N_RBRAC (close brace stab) for main.  The type definition is in an N_LSYM stab because type definitions are file scope not global scope.

         <128> N_LSYM - local symbol
         .stab "name:sym_dec(type)type_def(22)=sym_desc(enum)
                enum_name:value(0),enum_name:value(3),enum_name:value(4),;",
                N_LSYM, NIL, NIL, NIL

     104 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0

The symbol descriptor (T) says that the stab describes a structure, enumeration, or type tag.  The type descriptor e, following the 22= of the type definition narrows it down to an enumeration type.  Following the e is a list of the elements of the enumeration.  The format is name:value,. The list of elements ends with a ;.

Structure Tags
==============

Directive:
     .stabs

Type:
     N_LSYM

Symbol Descriptor:
     T

Type Descriptor:
     s

The following source code declares a structure tag and defines an instance of  the structure in global scope. Then a typedef equates the structure tag with a new type.  A seperate stab is generated for the structure tag, the structure typedef, and the structure instance.  The stabs for the tag and the typedef are emited when the definitions are encountered.  Since the structure elements are not initialized, the stab and code for the structure variable itself is located at the end of the program in .common.

     6  struct s_tag {
     7    int   s_int;
     8    float s_float;
     9    char  s_char_vec[8];
     10   struct s_tag* s_next;
     11 } g_an_s;
     12
     13 typedef struct s_tag s_typedef;

The structure tag is an N_LSYM stab type because, like the enum, the symbol is file scope.  Like the enum, the symbol descriptor is T, for enumeration, struct or tag type.  The symbol descriptor s following the 16= of the type definition narrows the symbol type to struct.

Following the struct symbol descriptor is the number of bytes the struct occupies, followed by a description of each structure element. The structure element descriptions are of the form name:type, bit offset from the start of the struct, and number of bits in the element.

        <128> N_LSYM - type definition
        .stabs "name:sym_desc(struct tag) Type_def(16)=type_desc(struct type)
             struct_bytes
             elem_name:type_ref(int),bit_offset,field_bits;
             elem_name:type_ref(float),bit_offset,field_bits;
             elem_name:type_def(17)=type_desc(dynamic array) index_type(int);NIL;
             high_bound(7);element_type(char),bit_offset,field_bits;;",
             N_LSYM,NIL,NIL,NIL

     30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;
                s_char_vec:17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0

In this example, two of the structure elements are previously defined types.  For these, the type following the name: part of the element description is a simple type reference.  The other two structure elements are new types.  In this case there is a type definition embedded after the name:.  The type definition for the array element looks just like a type definition for a standalone array.  The s_next field is a pointer to the same kind of structure that the field is an element of.  So the definition of structure type 16 contains an type definition for an element which is a pointer to type 16.

Typedefs
========

Directive:
     .stabs

Type:
     N_LSYM

Symbol Descriptor:
     t

Here is the stab for the typedef equating the structure tag with a type.

         <128> N_LSYM - type definition
         .stabs "name:sym_desc(type name)type_ref(struct_tag)",N_LSYM,NIL,NIL,NIL

     31 .stabs "s_typedef:t16",128,0,0,0

And here is the code generated for the structure variable.

         <32> N_GSYM - global symbol
         .stabs "name:sym_desc(global)type_ref(struct_tag)",N_GSYM,NIL,NIL,NIL

     136 .stabs "g_an_s:G16",32,0,0,0
     137     .common _g_an_s,20,"bss"

Notice that the structure tag has the same type number as the typedef for the structure tag.  It is impossible to distinguish between a variable of the struct type and one of its typedef by looking at the debugging information.

Unions
======

Directive:
     .stabs

Type:
     N_LSYM

Symbol Descriptor:
     T

Type Descriptor:
     u

Next let's look at unions.  In example2 this union type is declared locally to a procedure and an instance of the union is defined.

     36   union u_tag {
     37     int  u_int;
     38     float u_float;
     39     char* u_char;
     40   } an_u;

This code generates a stab for the union tag and a stab for the union variable.  Both use the N_LSYM stab type.  Since the union variable is scoped locally to the procedure in which it is defined, its stab is located immediately preceding the N_LBRAC for the procedure's block start.

The stab for the union tag, however is located preceding the code for the procedure in which it is defined.  The stab type is N_LSYM.  This would seem to imply that the union type is file scope, like the struct type s_tag.  This is not true.  The contents and position of the stab for u_type do not convey any infomation about its procedure local scope.

          <128> N_LSYM - type
          .stabs "name:sym_desc(union tag)type_def(22)=type_desc(union)
          byte_size(4)
          elem_name:type_ref(int),bit_offset(0),bit_size(32);
          elem_name:type_ref(float),bit_offset(0),bit_size(32);
          elem_name:type_ref(ptr to char),bit_offset(0),bit_size(32);;"
          N_LSYM, NIL, NIL, NIL

     105 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
                128,0,0,0

The symbol descriptor, T, following the name: means that the stab describes an enumeration struct or type tag.  The type descriptor u, following the 23= of the type definition, narrows it down to a union type definition.  Following the u is the number of bytes in the union. After that is a list of union element descriptions.  Their format is name:type, bit offset into the union, and number of bytes for the element;.

The stab for the union variable follows.  Notice that the frame pointer offset for local variables is negative.

         <128> N_LSYM - local variable (with no symbol descriptor)
         .stabs "name:type_ref(u_tag)", N_LSYM, NIL, NIL, frame_ptr_offset

     130 .stabs "an_u:23",128,0,0,-20

Function types
==============

     type descriptor f

The last type descriptor in C which remains to be described is used for function types.  Consider the following source line defining a global function pointer.

     4  int (*g_pf)();

It generates the following code.  Since the variable is not initialized, the code is located in the common area at the end of the file.

         <32> N_GSYM - global variable
         .stabs "name:sym_desc(global)type_def(24)=ptr_to(25)=
                                      type_def(func)type_ref(int)

     134 .stabs "g_pf:G24=*25=f1",32,0,0,0
     135     .common _g_pf,4,"bss"

Since the variable is global, the stab type is N_GSYM and the symbol descriptor is G.  The variable defines a new type, 24, which is a pointer to another new type, 25, which is defined as a function returning int.

Symbol information in symbol tables
***********************************

This section examines more closely the format of symbol table entries and how stab assembler directives map to them.  It also describes what transformations the assembler and linker make on data from stabs.

Each time the assembler encounters a stab in its input file it puts each field of the stab into corresponding fields in a symbol table entry of its output file.  If the stab contains a string field, the symbol table entry for that stab points to a string table entry containing the string data from the stab.  Assembler labels become relocatable addresses.  Symbol table entries in a.out have the format:

     struct internal_nlist {
       unsigned long n_strx;         /* index into string table of name */
       unsigned char n_type;         /* type of symbol */
       unsigned char n_other;        /* misc info (usually empty) */
       unsigned short n_desc;        /* description field */
       bfd_vma n_value;              /* value of symbol */
     };

For .stabs directives, the n_strx field holds the character offset from the start of the string table to the string table entry containing the "string" field.  For other classes of stabs (.stabn and .stabd) this field is null.

Symbol table entries with n_type fields containing a value greater or equal to 0x20 originated as stabs generated by the compiler (with one random exception).  Those with n_type values less than 0x20 were placed in the symbol table of the executable by the assembler or the linker.

The linker concatenates object files and does fixups of externally defined symbols.  You can see the transformations made on stab data by the assembler and linker by examining the symbol table after each pass of the build, first the assemble and then the link.

To do this use nm with the -ap options.  This dumps the symbol table, including debugging information, unsorted.  For stab entries the columns are: value, other, desc, type, string.  For assembler and linker symbols, the columns are: value, type, string.

There are a few important things to notice about symbol tables.  Where the value field of a stab contains a frame pointer offset, or a register number, that value is unchanged by the rest of the build.

Where the value field of a stab contains an assembly language label, it is transformed by each build step.  The assembler turns it into a relocatable address and the linker turns it into an absolute address. This source line defines a static variable at file scope:

     3  static int s_g_repeat

The following stab describes the symbol.

     26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat

The assembler transforms the stab into this symbol table entry in the .o file.  The location is expressed as a data segment offset.

     21 00000084 - 00 0000 STSYM s_g_repeat:S1

in the symbol table entry from the executable, the linker has made the relocatable address absolute.

     22 0000e00c - 00 0000 STSYM s_g_repeat:S1

Stabs for global variables do not contain location information. In this case the debugger finds location information in the assembler or linker symbol table entry describing the variable.  The source line:

     1 char g_foo = 'c';

generates the stab:

     21 .stabs "g_foo:G2",32,0,0,0

The variable is represented by the following two symbol table entries in the object file.  The first one originated as a stab.  The second one is an external symbol.  The upper case D signifies that the n_type field of the symbol table contains 7, N_DATA with local linkage (see Table B).  The value field following the file's line number is empty for the stab entry.  For the linker symbol it contains the rellocatable address corresponding to the variable.

     19 00000000 - 00 0000  GSYM g_foo:G2
     20 00000080 D _g_foo

These entries as transformed by the linker.  The linker symbol table entry now holds an absolute address.

     21 00000000 - 00 0000  GSYM g_foo:G2
     ...
     215 0000e008 D _g_foo

GNU C++ stabs
*************

Symbol descriptors added for C++ descriptions:
----------------------------------------------

     P - register parameter.

type descriptors added for C++ descriptions
-------------------------------------------

#
     method type (two ## if minimal debug)

xs
     cross-reference

Basic types for C++
===================

<< the examples that follow are based on a01.C >>

C++ adds two more builtin types to the set defined for C.  These are the unknown type and the vtable record type.  The unknown type, type 16, is defined in terms of itself like the void type.

The vtable record type, type 17, is defined as a structure type and then as a structure tag.  The structure has four fields, delta, index, pfn, and delta2.  pfn is the function pointer.

<< In boilerplate $vtbl_ptr_type, what are the fields delta, index, and delta2 used for? >>

This basic type is present in all C++ programs even if there are no virtual methods defined.

     .stabs "struct_name:sym_desc(type)type_def(17)=type_desc(struct)struct_bytes(8)
             elem_name(delta):type_ref(short int),bit_offset(0),field_bits(16);
             elem_name(index):type_ref(short int),bit_offset(16),field_bits(16);
             elem_name(pfn):type_def(18)=type_desc(ptr to)type_ref(void),
                                         bit_offset(32),field_bits(32);
             elem_name(delta2):type_def(short int);bit_offset(32),field_bits(16);;"
             N_LSYM, NIL, NIL

     .stabs "$vtbl_ptr_type:t17=s8
             delta:6,0,16;index:6,16,16;pfn:18=*15,32,32;delta2:6,32,16;;"
             ,128,0,0,0

     .stabs "name:sym_dec(struct tag)type_ref($vtbl_ptr_type)",N_LSYM,NIL,NIL,NIL

     .stabs "$vtbl_ptr_type:T17",128,0,0,0

Simple class definition
=======================

The stabs describing C++ language features are an extension of the stabs describing C.  Stabs representing C++ class types elaborate extensively on the stab format used to describe structure types in C. Stabs representing class type variables look just like stabs representing C language variables.

Consider the following very simple class definition.

     class baseA {
     public:
             int Adat;
             int Ameth(int in, char other);
     };

The class baseA is represented by two stabs.  The first stab describes the class as a structure type.  The second stab describes a structure tag of the class type.  Both stabs are of stab type N_LSYM.  Since the stab is not located between an N_FUN and a N_LBRAC stab this indicates that the class is defined at file scope.  If it were, then the N_LSYM would signify a local variable.

A stab describing a C++ class type is similar in format to a stab describing a C struct, with each class member shown as a field in the structure.  The part of the struct format describing fields is expanded to include extra information relevent to C++ class members. In addition, if the class has multiple base classes or virtual functions the struct format outside of the field parts is also augmented.

In this simple example the field part of the C++ class stab representing member data looks just like the field part of a C struct stab.  The section on protections describes how its format is sometimes extended for member data.

The field part of a C++ class stab representing a member function differs substantially from the field part of a C struct stab.  It still begins with `name:' but then goes on to define a new type number for the member function, describe its return type, its argument types, its protection level, any qualifiers applied to the method definition, and whether the method is virtual or not.  If the method is virtual then the method description goes on to give the vtable index of the method, and the type number of the first base class defining the method.

When the field name is a method name it is followed by two colons rather than one.  This is followed by a new type definition for the method.  This is a number followed by an equal sign and then the symbol descriptor `##', indicating a method type.  This is followed by a type reference showing the return type of the method and a semi-colon.

The format of an overloaded operator method name differs from that of other methods.  It is "op$::XXXX." where XXXX is the operator name such as + or +=.  The name ends with a period, and any characters except the period can occur in the XXXX string.

The next part of the method description represents the arguments to the method, preceeded by a colon and ending with a semi-colon.  The types of the arguments are expressed in the same way argument types are expressed in C++ name mangling.  In this example an int and a char map to `ic'.

This is followed by a number, a letter, and an asterisk or period, followed by another semicolon.  The number indicates the protections that apply to the member function.  Here the 2 means public.  The letter encodes any qualifier applied to the method definition.  In this case A means that it is a normal function definition.  The dot shows that the method is not virtual.  The sections that follow elaborate further on these fields and describe the additional information present for virtual methods.

     .stabs "class_name:sym_desc(type)type_def(20)=type_desc(struct)struct_bytes(4)
             field_name(Adat):type(int),bit_offset(0),field_bits(32);

             method_name(Ameth)::type_def(21)=type_desc(method)return_type(int);
             :arg_types(int char);
             protection(public)qualifier(normal)virtual(no);;"
             N_LSYM,NIL,NIL,NIL

     .stabs "baseA:t20=s4Adat:1,0,32;Ameth::21=##1;:ic;2A.;;",128,0,0,0

     .stabs "class_name:sym_desc(struct tag)",N_LSYM,NIL,NIL,NIL

     .stabs "baseA:T20",128,0,0,0

Class instance
==============

As shown above, describing even a simple C++ class definition is accomplished by massively extending the stab format used in C to describe structure types.  However, once the class is defined, C stabs with no modifications can be used to describe class instances.  The following source:

     main () {
             baseA AbaseA;
     }

yields the following stab describing the class instance.  It looks no different from a standard C stab describing a local variable.

     .stabs "name:type_ref(baseA)", N_LSYM, NIL, NIL, frame_ptr_offset

     .stabs "AbaseA:20",128,0,0,-20

Method defintion
================

The class definition shown above declares Ameth.  The C++ source below defines Ameth:

     int
     baseA::Ameth(int in, char other)
     {
             return in;
     };

This method definition yields three stabs following the code of the method.  One stab describes the method itself and following two describe its parameters.  Although there is only one formal argument all methods have an implicit argument which is the `this' pointer. The `this' pointer is a pointer to the object on which the method was called.  Note that the method name is mangled to encode the class name and argument types.  << Name mangling is not described by this document - Is there already such a doc? >>

     .stabs "name:symbol_desriptor(global function)return_type(int)",
             N_FUN, NIL, NIL, code_addr_of_method_start

     .stabs "Ameth__5baseAic:F1",36,0,0,_Ameth__5baseAic

Here is the stab for the `this' pointer implicit argument.  The name of the `this' pointer is always $t.  Type 19, the `this' pointer is defined as a pointer to type 20, baseA, but a stab defining baseA has not yet been emited.  Since the compiler knows it will be emited shortly, here it just outputs a cross reference to the undefined symbol, by prefixing the symbol name with xs.

     .stabs "name:sym_desc(register param)type_def(19)=
             type_desc(ptr to)type_ref(baseA)=
             type_desc(cross-reference to)baseA:",N_RSYM,NIL,NIL,register_number

     .stabs "$t:P19=*20=xsbaseA:",64,0,0,8

The stab for the explicit integer argument looks just like a parameter to a C function.  The last field of the stab is the offset from the argument pointer, which in most systems is the same as the frame pointer.

     .stabs "name:sym_desc(value parameter)type_ref(int)",
             N_PSYM,NIL,NIL,offset_from_arg_ptr

     .stabs "in:p1",160,0,0,72

<< The examples that follow are based on A1.C >>

Protections
===========

In the simple class definition shown above all member data and functions were publicly accessable.  The example that follows contrasts public, protected and privately accessable fields and shows how these protections are encoded in C++ stabs.

Protections for class member data are signified by two characters embeded in the stab defining the class type.  These characters are located after the name: part of the string.  /0 means private, /1 means protected, and /2 means public.  If these characters are omited this means that the member is public.  The following C++ source:

     class all_data {
     private:
             int   priv_dat;
     protected:
             char  prot_dat;
     public:
             float pub_dat;
     };

generates the following stab to describe the class type all_data.

     .stabs "class_name:sym_desc(type)type_def(19)=type_desc(struct)struct_bytes
             data_name:/protection(private)type_ref(int),bit_offset,num_bits;
             data_name:/protection(protected)type_ref(char),bit_offset,num_bits;
             data_name:(/num omited, private)type_ref(float),bit_offset,num_bits;;"
             N_LSYM,NIL,NIL,NIL

     .stabs "all_data:t19=s12
             priv_dat:/01,0,32;prot_dat:/12,32,8;pub_dat:12,64,32;;",128,0,0,0

Protections for member functions are signified by one digit embeded in the field part of the stab describing the method.  The digit is 0 if private, 1 if protected and 2 if public.  Consider the C++ class definition below:

     class all_methods {
     private:
             int   priv_meth(int in){return in;};
     protected:
             char  protMeth(char in){return in;};
     public:
             float pubMeth(float in){return in;};
     };

It generates the following stab.  The digit in question is to the left of an `A' in each case.  Notice also that in this case two symbol descriptors apply to the class name struct tag and struct type.

     .stabs "class_name:sym_desc(struct tag&type)type_def(21)=
             sym_desc(struct)struct_bytes(1)
             meth_name::type_def(22)=sym_desc(method)returning(int);
             :args(int);protection(private)modifier(normal)virtual(no);
             meth_name::type_def(23)=sym_desc(method)returning(char);
             :args(char);protection(protected)modifier(normal)virual(no);
             meth_name::type_def(24)=sym_desc(method)returning(float);
             :args(float);protection(public)modifier(normal)virtual(no);;",
             N_LSYM,NIL,NIL,NIL

     .stabs "all_methods:Tt21=s1priv_meth::22=##1;:i;0A.;protMeth::23=##2;:c;1A.;
             pubMeth::24=##12;:f;2A.;;",128,0,0,0

Method Modifiers (const, volatile, const volatile)
==================================================

<< based on a6.C >>

In the class example described above all the methods have the normal modifier.  This method modifier information is located just after the protection information for the method.  This field has four possible character values.  Normal methods use A, const methods use B, volatile methods use C, and const volatile methods use D.  Consider the class definition below:

     class A {
     public:
             int ConstMeth (int arg) const { return arg; };
             char VolatileMeth (char arg) volatile { return arg; };
             float ConstVolMeth (float arg) const volatile {return arg; };
     };

This class is described by the following stab:

     .stabs "class(A):sym_desc(struct)type_def(20)=type_desc(struct)struct_bytes(1)
             meth_name(ConstMeth)::type_def(21)sym_desc(method)
             returning(int);:arg(int);protection(public)modifier(const)virtual(no);
             meth_name(VolatileMeth)::type_def(22)=sym_desc(method)
             returning(char);:arg(char);protection(public)modifier(volatile)virt(no)
             meth_name(ConstVolMeth)::type_def(23)=sym_desc(method)
             returning(float);:arg(float);protection(public)modifer(const volatile)
             virtual(no);;", ...

     .stabs "A:T20=s1ConstMeth::21=##1;:i;2B.;VolatileMeth::22=##2;:c;2C.;
                  ConstVolMeth::23=##12;:f;2D.;;",128,0,0,0

Virtual Methods
===============

<< The following examples are based on a4.C >>

The presence of virtual methods in a class definition adds additional data to the class description.  The extra data is appended to the description of the virtual method and to the end of the class description.  Consider the class definition below:

     class A {
     public:
             int Adat;
             virtual int A_virt (int arg) { return arg; };
     };

This results in the stab below describing class A.  It defines a new type (20) which is an 8 byte structure.  The first field of the class struct is Adat, an integer, starting at structure offset 0 and occupying 32 bits.

The second field in the class struct is not explicitly defined by the C++ class definition but is implied by the fact that the class contains a virtual method.  This field is the vtable pointer.  The name of the vtable pointer field starts with $vf and continues with a type reference to the class it is part of.  In this example the type reference for class A is 20 so the name of its vtable pointer field is $vf20, followed by the usual colon.

Next there is a type definition for the vtable pointer type (21). This is in turn defined as a pointer to another new type (22).

Type 22 is the vtable itself, which is defined as an array, indexed by integers, with a high bound of 1, and elements of type 17.  Type 17 was the vtable record type defined by the boilerplate C++ type definitions, as shown earlier.

The bit offset of the vtable pointer field is 32.  The number of bits in the field are not specified when the field is a vtable pointer.

Next is the method definition for the virtual member function A_virt. Its description starts out using the same format as the non-virtual member functions described above, except instead of a dot after the `A' there is an asterisk, indicating that the function is virtual. Since is is virtual some addition information is appended to the end of the method description.

The first number represents the vtable index of the method.  This is a 32 bit unsigned number with the high bit set, followed by a semi-colon.

The second number is a type reference to the first base class in the inheritence hierarchy defining the virtual member function.  In this case the class stab describes a base class so the virtual function is not overriding any other definition of the method.  Therefore the reference is to the type number of the class that the stab is describing (20).

This is followed by three semi-colons.  One marks the end of the current sub-section, one marks the end of the method field, and the third marks the end of the struct definition.

For classes containing virtual functions the very last section of the string part of the stab holds a type reference to the first base class.  This is preceeded by `~%' and followed by a final semi-colon.

     .stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8)
             field_name(Adat):type_ref(int),bit_offset(0),field_bits(32);
             field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)=
             sym_desc(array)index_type_ref(int);NIL;elem_type_ref(vtbl elem type);
             bit_offset(32);
             meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
             :arg_type(int),protection(public)normal(yes)virtual(yes)
             vtable_index(1);class_first_defining(A);;;~%first_base(A);",
             N_LSYM,NIL,NIL,NIL

     .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

Inheritence
===========

Stabs describing C++ derived classes include additional sections that describe the inheritence hierarchy of the class.  A derived class stab also encodes the number of base classes.  For each base class it tells if the base class is virtual or not, and if the inheritence is private or public.  It also gives the offset into the object of the portion of the object corresponding to each base class.

This additional information is embeded in the class stab following the number of bytes in the struct.  First the number of base classes appears bracketed by an exclamation point and a comma.

Then for each base type there repeats a series: two digits, a number, a comma, another number, and a semi-colon.

The first of the two digits is 1 if the base class is virtual and 0 if not.  The second digit is 2 if the derivation is public and 0 if not.

The number following the first two digits is the offset from the start of the object to the part of the object pertaining to the base class.

After the comma, the second number is a type_descriptor for the base type.  Finally a semi-colon ends the series, which repeats for each base class.

The source below defines three base classes A, B, and C and the derived class D.

     class A {
     public:
             int Adat;
             virtual int A_virt (int arg) { return arg; };
     };

     class B {
     public:
             int B_dat;
             virtual int B_virt (int arg) {return arg; };
     };

     class C {
     public:
             int Cdat;
             virtual int C_virt (int arg) {return arg; };
     };

     class D : A, virtual B, public C {
     public:
             int Ddat;
             virtual int A_virt (int arg ) { return arg+1; };
             virtual int B_virt (int arg)  { return arg+2; };
             virtual int C_virt (int arg)  { return arg+3; };
             virtual int D_virt (int arg)  { return arg; };
     };

Class stabs similar to the ones described earlier are generated for each base class.

     .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

     .stabs "B:Tt25=s8Bdat:1,0,32;$vf25:21,32;B_virt::26=##1;
             :i;2A*-2147483647;25;;;~%25;",128,0,0,0

     .stabs "C:Tt28=s8Cdat:1,0,32;$vf28:21,32;C_virt::29=##1;
             :i;2A*-2147483647;28;;;~%28;",128,0,0,0

In the stab describing derived class D below, the information about the derivation of this class is encoded as follows.

     .stabs "derived_class_name:symbol_descriptors(struct tag&type)=
             type_descriptor(struct)struct_bytes(32)!num_bases(3),
             base_virtual(no)inheritence_public(no)base_offset(0),
             base_class_type_ref(A);
             base_virtual(yes)inheritence_public(no)base_offset(NIL),
             base_class_type_ref(B);
             base_virtual(no)inheritence_public(yes)base_offset(64),
             base_class_type_ref(C); ...

     .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:
             1,160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt:
             :32:i;2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;
             28;;D_virt::32:i;2A*-2147483646;31;;;~%20;",128,0,0,0

Virtual Base Classes
====================

A derived class object consists of a concatination in memory of the data areas defined by each base class, starting with the leftmost and ending with the rightmost in the list of base classes.  The exception to this rule is for virtual inheritence.  In the example above, class D inherits virtually from base class B.  This means that an instance of a D object will not contain it's own B part but merely a pointer to a B part, known as a virtual base pointer.

In a derived class stab, the base offset part of the derivation information, described above, shows how the base class parts are ordered.  The base offset for a virtual base class is always given as 0.  Notice that the base offset for B is given as 0 even though B is not the first base class.  The first base class A starts at offset 0.

The field information part of the stab for class D describes the field which is the pointer to the virtual base class B. The vbase pointer name is $vb followed by a type reference to the virtual base class. Since the type id for B in this example is 25, the vbase pointer name is $vb25.

     .stabs "D:Tt31=s32!3,000,20;100,25;0264,28;$vb25:24,128;Ddat:1,
            160,32;A_virt::32=##1;:i;2A*-2147483647;20;;B_virt::32:i;
            2A*-2147483647;25;;C_virt::32:i;2A*-2147483647;28;;D_virt:
            :32:i;2A*-2147483646;31;;;~%20;",128,0,0,0

Following the name and a semicolon is a type reference describing the type of the virtual base class pointer, in this case 24.  Type 24 was defined earlier as the type of the B class `this` pointer, $t.  The `this' pointer for a class is a pointer to the class type.

     .stabs "$t:P24=*25=xsB:",64,0,0,8

Finally the field offset part of the vbase pointer field description shows that the vbase pointer is the first field in the D object, before any data fields defined by the class.  The layout of a D class object is a follows, Adat at 0, the vtable pointer for A at 32, Cdat at 64, the vtable pointer for C at 96, the virtual ase pointer for B at 128, and Ddat at 160.

Static Members
==============

The data area for a class is a concatenation of the space used by the data members of the class.  If the class has virtual methods, a vtable pointer follows the class data.  The field offset part of each field description in the class stab shows this ordering.

<< How is this reflected in stabs?  See Cygnus bug #677 for some info.  >>

Example2.c - source code for extended example
*********************************************

     1  char g_foo = 'c';
     2  register int g_bar asm ("%g5");
     3  static int s_g_repeat = 2;
     4  int (*g_pf)();
     5
     6  struct s_tag {
     7    int   s_int;
     8    float s_float;
     9    char  s_char_vec[8];
     10   struct s_tag* s_next;
     11 } g_an_s;
     12
     13 typedef struct s_tag s_typedef;
     14
     15 char char_vec[3] = {'a','b','c'};
     16
     17 main (argc, argv)
     18      int argc;
     19      char* argv[];
     20 {
     21      static float s_flap;
     22      int times;
     23      for (times=0; times < s_g_repeat; times++){
     24        int inner;
     25        printf ("Hello world\n");
     26      }
     27 };
     28
     29 enum e_places {first,second=3,last};
     30
     31 static s_proc (s_arg, s_ptr_arg, char_vec)
     32   s_typedef s_arg;
     33   s_typedef* s_ptr_arg;
     34   char* char_vec;
     35 {
     36   union u_tag {
     37     int  u_int;
     38     float u_float;
     39     char* u_char;
     40   } an_u;
     41 }
     42
     43

Example2.s - assembly code for extended example
***********************************************

     1  gcc2_compiled.:
     2  .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0
     3  .stabs "example2.c",100,0,0,Ltext0
     4       .text
     5  Ltext0:
     6  .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
     7  .stabs "char:t2=r2;0;127;",128,0,0,0
     8  .stabs "long int:t3=r1;-2147483648;2147483647;",128,0,0,0
     9  .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
     10 .stabs "long unsigned int:t5=r1;0;-1;",128,0,0,0
     11 .stabs "short int:t6=r1;-32768;32767;",128,0,0,0
     12 .stabs "long long int:t7=r1;0;-1;",128,0,0,0
     13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
     14 .stabs "long long unsigned int:t9=r1;0;-1;",128,0,0,0
     15 .stabs "signed char:t10=r1;-128;127;",128,0,0,0
     16 .stabs "unsigned char:t11=r1;0;255;",128,0,0,0
     17 .stabs "float:t12=r1;4;0;",128,0,0,0
     18 .stabs "double:t13=r1;8;0;",128,0,0,0
     19 .stabs "long double:t14=r1;8;0;",128,0,0,0
     20 .stabs "void:t15=15",128,0,0,0
     21 .stabs "g_foo:G2",32,0,0,0
     22      .global _g_foo
     23      .data
     24 _g_foo:
     25      .byte 99
     26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
     27      .align 4
     28 _s_g_repeat:
     29      .word 2
     30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;s_char_vec:
                17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
     31 .stabs "s_typedef:t16",128,0,0,0
     32 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
     33      .global _char_vec
     34      .align 4
     35 _char_vec:
     36      .byte 97
     37      .byte 98
     38      .byte 99
     39      .reserve _s_flap.0,4,"bss",4
     40      .text
     41      .align 4
     42 LC0:
     43      .ascii "Hello world\12\0"
     44      .align 4
     45      .global _main
     46      .proc 1
     47 _main:
     48 .stabn 68,0,20,LM1
     49 LM1:
     50      !#PROLOGUE# 0
     51      save %sp,-144,%sp
     52      !#PROLOGUE# 1
     53      st %i0,[%fp+68]
     54      st %i1,[%fp+72]
     55      call ___main,0
     56      nop
     57 LBB2:
     58 .stabn 68,0,23,LM2
     59 LM2:
     60      st %g0,[%fp-20]
     61 L2:
     62      sethi %hi(_s_g_repeat),%o0
     63      ld [%fp-20],%o1
     64      ld [%o0+%lo(_s_g_repeat)],%o0
     65      cmp %o1,%o0
     66      bge L3
     67      nop
     68 LBB3:
     69 .stabn 68,0,25,LM3
     70 LM3:
     71      sethi %hi(LC0),%o1
     72      or %o1,%lo(LC0),%o0
     73      call _printf,0
     74      nop
     75 .stabn 68,0,26,LM4
     76 LM4:
     77 LBE3:
     78 .stabn 68,0,23,LM5
     79 LM5:
     80 L4:
     81      ld [%fp-20],%o0
     82      add %o0,1,%o1
     83      st %o1,[%fp-20]
     84      b,a L2
     85 L3:
     86 .stabn 68,0,27,LM6
     87 LM6:
     88 LBE2:
     89 .stabn 68,0,27,LM7
     90 LM7:
     91 L1:
     92      ret
     93      restore
     94 .stabs "main:F1",36,0,0,_main
     95 .stabs "argc:p1",160,0,0,68
     96 .stabs "argv:p20=*21=*2",160,0,0,72
     97 .stabs "s_flap:V12",40,0,0,_s_flap.0
     98 .stabs "times:1",128,0,0,-20
     99 .stabn 192,0,0,LBB2
     100 .stabs "inner:1",128,0,0,-24
     101 .stabn 192,0,0,LBB3
     102 .stabn 224,0,0,LBE3
     103 .stabn 224,0,0,LBE2
     104 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
     105 .stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
     128,0,0,0
     106     .align 4
     107     .proc 1
     108 _s_proc:
     109 .stabn 68,0,35,LM8
     110 LM8:
     111     !#PROLOGUE# 0
     112     save %sp,-120,%sp
     113     !#PROLOGUE# 1
     114     mov %i0,%o0
     115     st %i1,[%fp+72]
     116     st %i2,[%fp+76]
     117 LBB4:
     118 .stabn 68,0,41,LM9
     119 LM9:
     120 LBE4:
     121 .stabn 68,0,41,LM10
     122 LM10:
     123 L5:
     124     ret
     125     restore
     126 .stabs "s_proc:f1",36,0,0,_s_proc
     127 .stabs "s_arg:p16",160,0,0,0
     128 .stabs "s_ptr_arg:p18",160,0,0,72
     129 .stabs "char_vec:p21",160,0,0,76
     130 .stabs "an_u:23",128,0,0,-20
     131 .stabn 192,0,0,LBB4
     132 .stabn 224,0,0,LBE4
     133 .stabs "g_bar:r1",64,0,0,5
     134 .stabs "g_pf:G24=*25=f1",32,0,0,0
     135     .common _g_pf,4,"bss"
     136 .stabs "g_an_s:G16",32,0,0,0
     137     .common _g_an_s,20,"bss"

Quick reference
***************

Table A: Symbol types from stabs
================================

Table A lists stab types sorted by type number.  Stab type numbers are 32 and greater.  This is the full list of stab numbers, including stab types that are used in languages other than C.

The #define names for these stab types are defined in: devo/include/aout/stab.def

     type   type     #define   used to describe
     dec    hex      name      source program feature
     ------------------------------------------------
     32     0x20     N_GYSM    global symbol
     34     0X22     N_FNAME   function name (for BSD Fortran)
     36     0x24     N_FUN     function name or text segment variable for C
     38     0x26     N_STSYM   static symbol (data segment w/internal linkage)
     40     0x28     N_LCSYM   .lcomm symbol(BSS-seg variable w/internal linkage)
     42     0x2a     N_MAIN    Name of main routine (not used in C)
     48     0x30     N_PC      global symbol (for Pascal)
     50     0x32     N_NSYMS   number of symbols (according to Ultrix V4.0)
     52     0x34     N_NOMAP   no DST map for sym (according to Ultrix V4.0)
     64     0x40     N_RSYM    register variable
     66     0x42     N_M2C     Modula-2 compilation unit
     68     0x44     N_SLINE   line number in text segment
     70     0x46     N_DSLINE  line number in data segment

     72     0x48     N_BSLINE  line number in bss segment
     72     0x48     N_BROWS   Sun source code browser, path to .cb file

     74     0x4a     N_DEFD    GNU Modula2 definition module dependency

     80     0x50     N_EHDECL  GNU C++ exception variable
     80     0x50     N_MOD2    Modula2 info "for imc" (according to Ultrix V4.0)

     84     0x54     N_CATCH   GNU C++ "catch" clause
     96     0x60     N_SSYM    structure of union element
     100    0x64     N_SO      path and name of source file
     128    0x80     N_LSYM    automatic var in the stack
                               (also used for type desc.)
     130    0x82     N_BINCL   beginning of an include file (Sun only)
     132    0x84     N_SOL     Name of sub-source (#include) file.
     160    0xa0     N_PSYM    parameter variable
     162    0xa2     N_EINCL   end of an include file
     164    0xa4     N_ENTRY   alternate entry point
     192    0xc0     N_LBRAC   beginning of a lexical block
     194    0xc2     N_EXCL    place holder for a deleted include file
     196    0xc4     N_SCOPE   modula2 scope information (Sun linker)
     224    0xe0     N_RBRAC   end of a lexical block
     226    0xe2     N_BCOMM   begin named common block
     228    0xe4     N_ECOMM   end named common block
     232    0xe8     N_ECOML   end common (local name)

            << used on Gould systems for non-base registers syms >>
     240    0xf0     N_NBTEXT  ??
     242    0xf2     N_NBDATA  ??
     244    0xf4     N_NBBSS   ??
     246    0xf6     N_NBSTS   ??
     248    0xf8     N_NBLCS   ??

Table B: Symbol types from assembler and linker
===============================================

Table B shows the types of symbol table entries that hold assembler and linker symbols.

The #define names for these n_types values are defined in /include/aout/aout64.h

     dec     hex     #define
     n_type  n_type  name      used to describe
     ------------------------------------------
     1       0x0     N_UNDF    undefined symbol
     2       0x2     N_ABS     absolute symbol -- defined at a particular address
     3       0x3             extern " (vs. file scope)
     4       0x4     N_TEXT    text symbol -- defined at offset in text segment
     5       0x5             extern " (vs. file scope)
     6       0x6     N_DATA    data symbol -- defined at offset in data segment
     7       0x7             extern " (vs. file scope)
     8       0x8     N_BSS     BSS symbol -- defined at offset in zero'd segment
     9                       extern " (vs. file scope)

     12      0x0C    N_FN_SEQ  func name for Sequent compilers (stab exception)

     49      0x12    N_COMM    common sym -- visable after shared lib dynamic link
     31      0x1f    N_FN      file name of a .o file

Table C: Symbol descriptors
===========================

     descriptor      meaning
     -------------------------------------------------
     (empty)         local variable
        f            local function
        F            global function
        G            global variable
        p            value parameter
        r            register variable
        S            static global variable
        t            type name
        T            enumeration, struct or type tag
        V            static local variable

Table D: Type Descriptors
=========================

     descriptor      meaning
     -------------------------------------
     (empty)         type reference
        a            array type
        e            enumeration type
        f            function type
        r            range type
        s            structure type
        u            union specifications
        *            pointer type

Expanded reference by stab type.
********************************

Format of an entry:

The first line is the symbol type expressed in decimal, hexadecimal, and as a #define (see devo/include/aout/stab.def).

The second line describes the language constructs the symbol type represents.

The third line is the stab format with the significant stab fields named and the rest NIL.

Subsequent lines expand upon the meaning and possible values for each significant stab field.  # stands in for the type descriptor.

Finally, any further information.

32 - 0x20 - N_GYSM
==================

     Global variable.

     .stabs "name", N_GSYM, NIL, NIL, NIL

     "name" -> "symbol_name:#type"
                            # -> G

Only the "name" field is significant.  the location of the variable is obtained from the corresponding external symbol.

34 - 0x22 - N_FNAME
===================

Function name (for BSD Fortran)

     .stabs "name", N_FNAME, NIL, NIL, NIL

     "name" -> "function_name"

Only the "name" field is significant.  The location of the symbol is obtained from the corresponding extern symbol.

36 - 0x24 - N_FUN
=================

Function name or text segment variable for C.

     .stabs "name", N_FUN, NIL, desc, value

     For functions:
     "name" -> "proc_name:#return_type"
                          #  -> F (global function)
                                f (local function)
     desc  -> line num for proc start.  (GCC doesn't set and DBX doesn't miss it.)
     value -> Code address of proc start.

     For text segment variables:
     <<How to create one?>>

38 - 0x26 - N_STSYM
===================

Initialized static symbol (data segment w/internal linkage).

     .stabs "name", N_STSYM, NIL, NIL, value

     "name" -> "symbol_name#type"
                           # -> S (scope global to compilation unit)
                             -> V (scope local to a procedure)
     value  -> Data Address

40 - 0x28 - N_LCSYM
===================

Unitialized static (.lcomm) symbol(BSS segment w/internal linkage).

     .stabs "name", N_LCLSYM, NIL, NIL, value

     "name" -> "symbol_name#type"
                           # -> S (scope global to compilation unit)
                             -> V (scope local to procedure)
     value  -> BSS Address

42 - 0x2a - N_MAIN
==================

Name of main routine (not used in C)

     .stabs "name", N_MAIN, NIL, NIL, NIL

     "name" -> "name_of_main_routine"

48 - 0x30 - N_PC
================

Global symbol (for Pascal)

     .stabs "name", N_PC, NIL, NIL, value

     "name" -> "symbol_name"  <<?>>
     value  -> supposedly the line number (stab.def is skeptical)

     stabdump.c says:

     global pascal symbol: name,,0,subtype,line
     << subtype? >>

50 - 0x32 - N_NSYMS
===================

Number of symbols (according to Ultrix V4.0)

             0, files,,funcs,lines (stab.def)

52 - 0x34 - N_NOMAP
===================

no DST map for sym (according to Ultrix V4.0)

             name, ,0,type,ignored (stab.def)

64 - 0x40 - N_RSYM
==================

register variable

     .stabs "name:type",N_RSYM,0,RegSize,RegNumber (Sun doc)

66 - 0x42 - N_M2C
=================

Modula-2 compilation unit

     .stabs "name", N_M2C, 0, desc, value

     "name" -> "unit_name,unit_time_stamp[,code_time_stamp]
     desc   -> unit_number
     value  -> 0 (main unit)
               1 (any other unit)

68 - 0x44 - N_SLINE
===================

Line number in text segment

     .stabn N_SLINE, 0, desc, value

     desc  -> line_number
     value -> code_address (relocatable addr where the corresponding code starts)

For single source lines that generate discontiguous code, such as flow of control statements, there may be more than one N_SLINE stab for the same source line.  In this case there is a stab at the start of each code range, each with the same line number.

70 - 0x46 - N_DSLINE
====================

Line number in data segment

     .stabn N_DSLINE, 0, desc, value

     desc  -> line_number
     value -> data_address (relocatable addr where the corresponding code
     starts)

See comment for N_SLINE above.

72 - 0x48 - N_BSLINE
====================

Line number in bss segment

     .stabn N_BSLINE, 0, desc, value

     desc  -> line_number
     value -> bss_address (relocatable addr where the corresponding code
     starts)

See comment for N_SLINE above.

72 - 0x48 - N_BROWS
===================

Sun source code browser, path to .cb file

<<?>> "path to associated .cb file"

Note: type field value overlaps with N_BSLINE

74 - 0x4a - N_DEFD
==================

GNU Modula2 definition module dependency

GNU Modula-2 definition module dependency.  Value is the modification time of the definition file.  Other is non-zero if it is imported with the GNU M2 keyword %INITIALIZE.  Perhaps N_M2C can be used if there are enough empty fields?

80 - 0x50 - N_EHDECL
====================

GNU C++ exception variable <<?>>

"name is variable name"

Note: conflicts with N_MOD2.

80 - 0x50 - N_MOD2
==================

Modula2 info "for imc" (according to Ultrix V4.0)

Note: conflicts with N_EHDECL  <<?>>

84 - 0x54 - N_CATCH
===================

GNU C++ "catch" clause

GNU C++ `catch' clause.  Value is its address.  Desc is nonzero if this entry is immediately followed by a CAUGHT stab saying what exception was caught.  Multiple CAUGHT stabs means that multiple exceptions can be caught here.  If Desc is 0, it means all exceptions are caught here.

96 - 0x60 - N_SSYM
==================

Structure or union element

Value is offset in the structure.

<<?looking at structs and unions in C I didn't see these>>

100 - 0x64 - N_SO
=================

Path and name of source file containing main routine

     .stabs "name", N_SO, NIL, NIL, value

     "name" -> /path/to/source/file
            -> source_file_terminal_name

     value  -> the starting text address of the compilation.

These are found two in a row.  The name field of the first N_SO contains the path to the source file.  The name field of the second N_SO contains the terminal name of the source file itself.

128 - 0x80 - N_LSYM
===================

Automatic var in the stack (also used for type descriptors.)

     .stabs "name" N_LSYM, NIL, NIL, value

     For stack based local variables:

     "name" -> name of the variable
     value  -> offset from frame pointer (negative)

     For type descriptors:

     "name"   -> "name_of_the_type:#type"
                                   # -> t

     type     -> type_ref (or) type_def

     type_ref -> type_number
     type_def -> type_number=type_desc etc.

Type may be either a type reference or a type definition.  A type reference is a number that refers to a previously defined type.  A type definition is the number that will refer to this type, followed by an equals sign, a type descriptor and the additional data that defines the type.  See the Table D for type descriptors and the section on types for what data follows each type descriptor.

130 - 0x82 - N_BINCL
====================

Beginning of an include file (Sun only)

Beginning of an include file.  Only Sun uses this. In an object file, only the name is significant.  The Sun linker puts data into some of the other fields.

132 - 0x84 - N_SOL
==================

Name of a sub-source file (#include file).  Value is starting address of the compilation. <<?>>

160 - 0xa0 - N_PSYM
===================

Parameter variable

     stabs. "name", N_PSYM, NIL, NIL, value

     "name" -> "param_name:#type"
                          # -> p  (value parameter)
                            -> i  (value parameter by reference, indirect access)
                            -> v  (variable parameter by reference)
                            -> C  ( read-only parameter, conformant array bound)
                            -> x  (confomant array value parameter)
                            -> pP (<<??>>)
                            -> pF (<<??>>)
                            -> X  (function result variable)
                            -> b  (based variable)

     value -> offset from the argument pointer (positive).

On most machines the argument pointer is the same as the frame pointer.

162 - 0xa2 - N_EINCL
====================

End of an include file.  This and N_BINCL act as brackets around the file's output.  In an ojbect file, there is no significant data in this entry.  The Sun linker puts data into some of the fields. <<?>>

164 - 0xa4 - N_ENTRY
====================

Alternate entry point. Value is its address. <<?>>

192 - 0xc0 - N_LBRAC
====================

Beginning of a lexical block (left brace).  The variable defined inside the block precede the N_LBRAC symbol.  Or can they follow as well as long as a new N_FUNC was not encountered. <<?>>

     .stabn N_LBRAC, NIL, NIL, value

     value -> code address of block start.

194 - 0xc2 - N_EXCL
===================

Place holder for a deleted include file.  Replaces a N_BINCL and everything up to the corresponding N_EINCL.  The Sun linker generates these when it finds multiple indentical copies of the symbols from an included file.  This appears only in output from the Sun linker. <<?>>

196 - 0xc4 - N_SCOPE
====================

Modula2 scope information (Sun linker) <<?>>

224 - 0xe0 - N_RBRAC
====================

End of a lexical block (right brace)

     .stabn N_RBRAC, NIL, NIL, value

     value -> code address of the end of the block.

226 - 0xe2 - N_BCOMM
====================

Begin named common block.

Only the name is significant. <<?>>

228 - 0xe4 - N_ECOMM
====================

End named common block.

Only the name is significant and it should match the N_BCOMM <<?>>

232 - 0xe8 - N_ECOML
====================

End common (local name)

value is address. <<?>>

Non-base registers on Gould systems
===================================

<< used on Gould systems for non-base registers syms, values assigned at random, need real info from Gould. >> <<?>>

     240    0xf0     N_NBTEXT  ??
     242    0xf2     N_NBDATA  ??
     244    0xf4     N_NBBSS   ??
     246    0xf6     N_NBSTS   ??
     248    0xf8     N_NBLCS   ??

- 0xfe - N_LENG
===============

Second symbol entry containing a length-value for the preceding entry. The value is the length.

Questions and anomalies
***********************

  * For GNU C stabs defining local and global variables (N_LSYM and N_GSYM), the desc field is supposed to contain the source line number on which the variable is defined.  In reality the desc field is always 0.  (This behavour is defined in dbxout.c and putting a line number in desc is controlled by #ifdef WINNING_GDB which defaults to false). Gdb supposedly uses this information if you say 'list var'.  In reality var can be a variable defined in the program and gdb says `function var not defined'

  * In GNU C stabs there seems to be no way to differentiate tag types: structures, unions, and enums (symbol descriptor T) and typedefs (symbol descriptor t) defined at file scope from types defined locally to a procedure or other more local scope.  They all use the N_LSYM stab type.  Types defined at procedure scope are emited after the N_RBRAC of the preceding function and before the code of the procedure in which they are defined.  This is exactly the same as types defined in the source file between the two procedure bodies. GDB overcompensates by placing all types in block #1 the block for symbols of file scope.  This is true for default, -ansi and -traditional compiler options. (p0001063-gcc, p0001066-gdb)

  * What ends the procedure scope?  Is it the proc block's N_RBRAC or the next N_FUN?  (I believe its the first.)

  * The comment in xcoff.h says DBX_STATIC_CONST_VAR_CODE is used for static const variables.  DBX_STATIC_CONST_VAR_CODE is set to N_FUN by default, in dbxout.c.  If included, xcoff.h redefines it to N_STSYM. But testing the default behaviour, my Sun4 native example shows N_STSYM not N_FUN is used to describe file static initialized variables.  (the code tests for TREE_READONLY(decl) && !TREE_THIS_VOLATILE(decl) and if true uses DBX_STATIC_CONST_VAR_CODE).

  * Global variable stabs don't have location information.  This comes from the external symbol for the same variable.  The external symbol has a leading underbar on the _name of the variable and the stab does not.  How do we know these two symbol table entries are talking about the same symbol when their names are different?

  * Can gcc be configured to output stabs the way the Sun compiler does, so that their native debugging tools work? <NO?> It doesn't by default.  GDB reads either format of stab. (gcc or SunC).  How about dbx?

Differences between GNU stabs in a.out and GNU stabs in xcoff
*************************************************************

(The AIX/RS6000 native object file format is xcoff with stabs)

  * Instead of .stabs, xcoff uses .stabx.

  * The data fields of an xcoff .stabx are in a different order than an a.out .stabs.  The order is: string, value, type.  The desc and null fields present in a.out stabs are missing in xcoff stabs.  For N_GSYM the value field is the name of the symbol.

  * BSD a.out stab types correspond to AIX xcoff storage classes. In general the mapping is N_STABTYPE becomes C_STABTYPE.  Some stab types in a.out are not supported in xcoff. See Table E. for full mappings.

     exception: initialised static N_STSYM and un-initialized static N_LCSYM both map to the C_STSYM storage class.  But the destinction is preserved because in xcoff N_STSYM and N_LCSYM must be emited in a named static block.  Begin the block with .bs s[RW] data_section_name for N_STSYM or .bs s bss_section_name for N_LCSYM.  End the block with .es

  * xcoff stabs describing tags and typedefs use the N_DECL (0x8c)instead of N_LSYM stab type.

  * xcoff uses N_RPSYM (0x8e) instead of the N_RSYM stab type for register variables.  If the register variable is also a value parameter, then use R instead of P for the symbol descriptor.

     6. xcoff uses negative numbers as type references to the basic types. There are no boilerplate type definitions emited for these basic types. << make table of basic types and type numbers for C >>

  * xcoff .stabx sometimes don't have the name part of the string field.

  * xcoff uses a .file stab type to represent the source file name. There is no stab for the path to the source file.

  * xcoff uses a .line stab type to represent source lines.  The format is: .line line_number.

  * xcoff emits line numbers relative to the start of the current function.  The start of a function is marked by .bf.  If a function includes lines from a seperate file, then those line numbers are absolute line numbers in the <<sub-?>> file being compiled.

  * The start of current include file is marked with: .bi "filename" and the end marked with .ei "filename"

  * If the xcoff stab is a N_FUN (C_FUN) then follow the string field with ,. instead of just ,

  * The symbol descriptor for register parameters is P for a.out and R for xcoff.

(I think that's it for .s file differences.  They could stand to be better presented.  This is just a list of what I have noticed so far. There are a *lot* of differences in the information in the symbol tables of the executable and object files.)

Table E: mapping a.out stab types to xcoff storage classes

     stab type       storage class
     -------------------------------
     N_GSYM          C_GSYM
     N_FNAME         unknown
     N_FUN           C_FUN
     N_STSYM         C_STSYM
     N_LCSYM         C_STSYM
     N_MAIN          unkown
     N_PC            unknown
     N_RSYM          C_RSYM
     N_RPSYM (0x8e)  C_RPSYM
     N_M2C           unknown
     N_SLINE         unknown
     N_DSLINE        unknown
     N_BSLINE        unknown
     N_BROWSE        unchanged
     N_CATCH         unknown
     N_SSYM          unknown
     N_SO            unknown
     N_LSYM          C_LSYM
     N_DECL  (0x8c)  C_DECL
     N_BINCL         unknown
     N_SOL           unknown
     N_PSYM          C_PSYM
     N_EINCL         unknown
     N_ENTRY         C_ENTRY
     N_LBRAC         unknown
     N_EXCL          unknown
     N_SCOPE         unknown
     N_RBRAC         unknown
     N_BCOMM         C_BCOMM
     N_ECOMM         C_ECOMM
     N_ECOML         C_ECOML

     N_LENG          unknown

Differences between GNU stabs and Sun native stabs.
***************************************************

  * GNU C stabs define *all* types, file or procedure scope, as N_LSYM.  Sun doc talks about using N_GSYM too.

  * GNU C stabs use `ar' as type descriptor when defining arrays vs. just `a' in Sun doc.

  * Stabs describing block scopes, N_LBRAC and N_RBRAC are supposed to contain the nesting level of the block in the desc field, re Sun doc. GNU stabs always have 0 in that field.

  * Sun C stabs use type number pairs in the format (a,b) where a is a number starting with 1 and incremented for each sub-source file in the compilation.  b is a number starting with 1 and incremented for each new type defined in the compilation.  GNU C stabs use the type number alone, with no source file number.