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

This document describes the stabs debugging symbol tables.

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.

This document is one of the few published sources of documentation on stabs.  It is believed to be completely comprehensive for stabs used by C.  The lists of symbol descriptors (see Symbol Descriptors) and type descriptors (see Type Descriptors) are believed to be completely comprehensive.  There are known to be stabs for C++ and COBOL which are poorly documented here.  Stabs specific to other languages (e.g. Pascal, Modula-2) are probably not as well documented as they should be.

Other sources of information on stabs are dbx and dbxtool interfaces, 2nd edition, by Sun, circa 1988, and AIX Version 3.2 Files Reference, Fourth Edition, September 1992, "dbx Stabstring Grammar" in the a.out section, page 2-31.  This document is believed to incorporate the information from those two sources except where it explictly directs you to them for more information.

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).  IBM's xcoff uses .stabx (and some other directives such as .file and .bi) instead of .stabs, .stabn or .stabd.

The overall format of each class of stab is:

     .stabs "STRING",TYPE,0,DESC,VALUE
     .stabx "STRING",VALUE,TYPE,SDB-TYPE
     .stabn TYPE,0,DESC,VALUE
     .stabd TYPE,0,DESC

For .stabn and .stabd, there is no string (the n_strx field is zero, see Symbol Tables).  For .stabd the value field is implicit and has the value of the current file location.  The SDB-TYPE field to .stabx is unused for stabs and can always be set to 0.

The number in the type field gives some basic information about what type of stab this is (or whether it is a stab, as opposed to an ordinary symbol).  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-INFORMATION"

NAME is the name of the symbol represented by the stab. NAME can be omitted, which means the stab represents an unnamed object.  For example, :t10=*2 defines type 10 as a pointer to type 2, but does not give the type a name.  Omitting the NAME field is supported by AIX dbx and GDB after about version 4.8, but not other debuggers.  GCC sometimes uses a single space as the name instead of omitting the name altogether; apparently that is supported by most debuggers.

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.

The c symbol descriptor is an exception in that it is not followed by type information.  See Constants.

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.

There is an AIX extension for type attributes.  Following the = is any number of type attributes.  Each one starts with @ and ends with ;.  Debuggers, including AIX's dbx, skip any type attributes they do not recognize.  GDB 4.9 does not do this---it will ignore the entire symbol containing a type attribute.  Hopefully this will be fixed in the next GDB release.  Because of a conflict with C++ (see Cplusplus), new attributes should not be defined which begin with a digit, (, or -; GDB may be unable to distinguish those from the C++ type descriptor @.  The attributes are:

aBOUNDARY
     BOUNDARY is an integer specifying the alignment.  I assume it applies to all variables of this type.

sSIZE
     Size in bits of a variable of this type.

pINTEGER
     Pointer class (for checking).  Not sure what this means, or how INTEGER is interpreted.

P
     Indicate this is a packed type, meaning that structure fields or array elements are placed more closely in memory, to save memory at the expense of speed.

All this can make the "STRING" field quite long.  All versions of GDB, and some versions of DBX, can handle arbitrarily long strings.  But many versions of DBX cretinously limit the strings to about 80 characters, so compilers which must work with such DBX's need to split the .stabs directive into several .stabs directives.  Each stab duplicates exactly all but the "STRING" field.  The "STRING" field of every stab except the last is marked as continued with a double-backslash at the end.  Removing the backslashes and concatenating the "STRING" fields of each stab produces the original, long 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 files
=====================================

Before any other stabs occur, there must be a stab specifying the source file.  This information is contained in a symbol of stab type N_SO; the string contains the name of the file.  The value of the symbol is the start address of portion of the text section corresponding to that file.

Some compilers (for example, gcc2 and SunOS4 /bin/cc) also include the directory in which the source was compiled, in a second N_SO symbol preceding the one containing the file name.  This symbol can be distinguished by the fact that it ends in a slash. According to a comment in GDB's partial-stab.h, other compilers (especially unnamed C++ compilers) put out useless N_SO's for nonexistent source files (after the N_SO for the real source file).

For example:

     .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0	; 100 is N_SO
     .stabs "hello.c",100,0,0,Ltext0
             .text
     Ltext0:

Instead of N_SO symbols, XCOFF uses a .file assembler directive which assembles to a standard COFF .file symbol; explaining this in detail is outside the scope of this document.

There are several different schemes for dealing with include files: the traditional N_SOL approach, Sun's N_BINCL scheme, and the XCOFF C_BINCL (which despite the similar name has little in common with N_BINCL).

An N_SOL symbol specifies which include file subsequent symbols refer to.  The string field is the name of the file and the value is the text address corresponding to the start of the previous include file and the start of this one.  To specify the main source file again, use an N_SOL symbol with the name of the main source file.

A N_BINCL symbol specifies the start of an include file.  In an object file, only the name is significant.  The Sun linker puts data into some of the other fields.  The end of the include file is marked by a N_EINCL symbol of the same name.  In an ojbect file, there is no significant data in the N_EINCL symbol; the Sun linker puts data into some of the fields.  N_BINCL and N_EINCL can be nested.  If the linker detects that two source files have identical stabs with a N_BINCL and N_EINCL pair (as will generally be the case for a header file), then it only puts out the stabs once. Each additional occurance is replaced by an N_EXCL symbol.  I believe the Sun (SunOS4, not sure about Solaris) linker is the only one which supports this feature.

For the start of an include file in XCOFF, use the .bi assembler directive which generates a C_BINCL symbol.  A .ei directive, which generates a C_EINCL symbol, denotes the end of the include file.  Both directives are followed by the name of the source file in quotes, which becomes the string for the symbol.  The value of each symbol, produced automatically by the assembler and linker, is an offset into the executable which points to the beginning (inclusive, as you'd expect) and end (inclusive, as you would not expect) of the portion of the COFF linetable which corresponds to this include file.  C_BINCL and C_EINCL do not nest.

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

A N_SLINE symbol represents the start of a source line.  The DESC field contains the line number and the VALUE field contains the code address for the start of that source line.

GNU documents N_DSLINE and N_BSLINE symbols for line numbers in the data or bss segments, respectively.  They are identical to N_SLINE but are relocated differently by the linker.  They were intended to be used to describe the source location of a variable declaration, but I believe that gcc2 actually puts the line number in the desc field of the stab for the variable itself.  GDB has been ignoring these symbols (unless they contain a string field) at least since GDB 3.5.

XCOFF uses COFF line numbers instead, which are outside the scope of this document, ammeliorated by adequate marking of include files (see Source Files).

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

Procedures
==========

All of the following stabs use the N_FUN symbol type.

A function is represented by a F symbol descriptor for a global (extern) function, and f for a static (local) function.  The next N_SLINE symbol can be used to find the line number of the start of the function.  The value field is the address of the start of the function.  The type information of the stab represents the return type of the function; thus foo:f5 means that foo is a function returning type 5.

The AIX documentation also defines symbol descriptor J as an internal function.  I assume this means a function nested within another function.  It also says Symbol descriptor m is a module in Modula-2 or extended Pascal.

Procedures (functions which do not return values) are represented as functions returning the void type in C.  I don't see why this couldn't be used for all languages (inventing a void type for this purpose if necessary), but the AIX documentation defines I, P, and Q for internal, global, and static procedures, respectively. These symbol descriptors are unusual in that they are not followed by type information.

For any of the above symbol descriptors, after the symbol descriptor and the type information, there is optionally a comma, followed by the name of the procedure, followed by a comma, followed by a name specifying the scope.  The first name is local to the scope specified.  I assume then that the name of the symbol (before the :), if specified, is some sort of global name.  I assume the name specifying the scope is the name of a function specifying that scope.  This feature is an AIX extension, and this information is based on the manual; I haven't actually tried it.

The stab representing a procedure is located immediately following the code of the procedure.  This 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

Constants
*********

The c symbol descriptor indicates that this stab represents a constant.  This symbol descriptor is an exception to the general rule that symbol descriptors are followed by type information.  Instead, it is followed by = and one of the following:

b VALUE
     Boolean constant.  VALUE is a numeric value; I assume it is 0 for false or 1 for true.

c VALUE
     Character constant.  VALUE is the numeric value of the constant.

e TYPE-INFORMATION , VALUE
     Constant whose value can be represented as integral. TYPE-INFORMATION is the type of the constant, as it would appear after a symbol descriptor (see Stabs Format).  VALUE is the numeric value of the constant.  GDB 4.9 does not actually get the right value if VALUE does not fit in a host int, but it does not do anything violent, and future debuggers could be extended to accept integers of any size (whether unsigned or not).  This constant type is usually documented as being only for enumeration constants, but GDB has never imposed that restriction; I don't know about other debuggers.

i VALUE
     Integer constant.  VALUE is the numeric value.  The type is some sort of generic integer type (for GDB, a host int); to specify the type explicitly, use e instead.

r VALUE
     Real constant.  VALUE is the real value, which can be INF (optionally preceded by a sign) for infinity, QNAN for a quiet NaN (not-a-number), or SNAN for a signalling NaN.  If it is a normal number the format is that accepted by the C library function atof.

s STRING
     String constant.  STRING is a string enclosed in either ' (in which case ' characters within the string are represented as \' or " (in which case " characters within the string are represented as \").

S TYPE-INFORMATION , ELEMENTS , BITS , PATTERN
     Set constant.  TYPE-INFORMATION is the type of the constant, as it would appear after a symbol descriptor (see Stabs Format). ELEMENTS is the number of elements in the set (is this just the number of bits set in PATTERN?  Or redundant with the type?  I don't get it), BITS is the number of bits in the constant (meaning it specifies the length of PATTERN, I think), and PATTERN is a hexadecimal representation of the set.  AIX documentation refers to a limit of 32 bytes, but I see no reason why this limit should exist. This form could probably be used for arbitrary constants, not just sets; the only catch is that PATTERN should be understood to be target, not host, byte order and format.

The boolean, character, string, and set constants are not supported by GDB 4.9, but it will ignore them.  GDB 4.8 and earlier gave an error message and refused to read symbols from the file containing the constants.

This information is followed by ;.

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 INFORMATION",
            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 INFORMATION",
             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

The symbol descriptor is omitted for automatic variables.  Since type information should being with a digit, -, or (, only digits, -, and ( are precluded from being used for symbol descriptors by this fact.  However, the Acorn RISC machine (ARM) is said to get this wrong: it puts out a mere type definition here, without the preceding TYPENUMBER=.  This is a bad idea; there is no guarantee that type descriptors are distinct from symbol descriptors.

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.

Register variables
==================

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.

The value is the register number.

AIX defines a separate symbol descriptor d for floating point registers.  This seems incredibly stupid---why not just just give floating point registers different register numbers?  I have not verified whether the compiler actually uses d.

If the register is explicitly allocated to a global variable, but not initialized, as in

     register int g_bar asm ("%g5");

the stab may be emitted at the end of the object file, with the other bss symbols.

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
==========

The symbol descriptor p is used to refer to parameters which are in the arglist.  Symbols have symbol type N_PSYM.  The value of the symbol is the offset relative to the argument list.

If the parameter is passed in a register, then the traditional way to do this is to provide two symbols for each argument:

     .stabs "arg:p1" . . .		; N_PSYM
     .stabs "arg:r1" . . .		; N_RSYM

Debuggers are expected to use the second one to find the value, and the first one to know that it is an argument.

Because this is kind of ugly, some compilers use symbol descriptor P or R to indicate an argument which is in a register. The symbol value is the register number.  P and R mean the same thing, the difference is that P is a GNU invention and R is an IBM (xcoff) invention.  As of version 4.9, GDB should handle either one.  Symbol type C_RPSYM is used with R and N_RSYM is used with P.

AIX, according to the documentation, uses D for a parameter passed in a floating point register.  This strikes me as incredibly bogus---why doesn't it just use R with a register number which indicates that it's a floating point register?  I haven't verified whether the system actually does what the documentation indicates.

There is at least one case where GCC uses a p/r pair rather than P; this is where the argument is passed in the argument list and then loaded into a register.

On the sparc and hppa, for a P symbol whose type is a structure or union, the register contains the address of the structure.  On the sparc, this is also true of a p/r pair (using Sun cc) or a p symbol.  However, if a (small) structure is really in a register, r is used.  And, to top it all off, on the hppa it might be a structure which was passed on the stack and loaded into a register and for which there is a p/r pair!  I believe that symbol descriptor i is supposed to deal with this case, (it is said to mean "value parameter by reference, indirect access", I don't know the source for this information) but I don't know details or what compilers or debuggers use it, if any (not GDB or GCC).  It is not clear to me whether this case needs to be dealt with differently than parameters passed by reference (see below).

There is another case similar to an argument in a register, which is an argument which is actually stored as a local variable.  Sometimes this happens when the argument was passed in a register and then the compiler stores it as a local variable.  If possible, the compiler should claim that it's in a register, but this isn't always done.  Some compilers use the pair of symbols approach described above ("arg:p" followed by "arg:"); this includes gcc1 (not gcc2) on the sparc when passing a small structure and gcc2 (sometimes) when the argument type is float and it is passed as a double and converted to float by the prologue (in the latter case the type of the "arg:p" symbol is double and the type of the "arg:" symbol is float).  GCC, at least on the 960, uses a single p symbol descriptor for an argument which is stored as a local variable but uses N_LSYM instead of N_PSYM.  In this case the value of the symbol is an offset relative to the local variables for that function, not relative to the arguments (on some machines those are the same thing, but not on all).

If the parameter is passed by reference (e.g. Pascal VAR parameters), then type symbol descriptor is v if it is in the argument list, or a if it in a register.  Other than the fact that these contain the address of the parameter other than the parameter itself, they are identical to p and R, respectively.  I believe a is an AIX invention; v is supported by all stabs-using systems as far as I know.

Conformant arrays refer to a feature of Modula-2, and perhaps other languages, in which the size of an array parameter is not known to the called function until run-time.  Such parameters have two stabs, a x for the array itself, and a C, which represents the size of the array.  The value of the x stab is the offset in the argument list where the address of the array is stored (it this right? it is a guess); the value of the C stab is the offset in the argument list where the size of the array (in elements? in bytes?) is stored.

The following are also said to go with N_PSYM:

     "name" -> "param_name:#type"
                            -> pP (<<??>>)
                            -> pF FORTRAN function parameter
                            -> X  (function result variable)
                            -> b  (based variable)

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

As a simple example, the code

     main (argc, argv)
          int argc;
          char **argv;
     {

produces the stabs

     .stabs "main:F1",36,0,0,_main                 ; 36 is N_FUN
     .stabs "argc:p1",160,0,0,68                   ; 160 is N_PSYM
     .stabs "argv:p20=*21=*2",160,0,0,72

The type definition of argv is interesting because it contains several type definitions.  Type 21 is pointer to type 2 (char) and argv (type 20) is pointer to type 21.

Type definitions
****************

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.

Builtin types
=============

Certain types are built in (int, short, void, float, etc.); the debugger recognizes these types and knows how to handle them.  Thus don't be surprised if some of the following ways of specifying builtin types do not specify everything that a debugger would need to know about the type---in some cases they merely specify enough information to distinguish the type from other types.

The traditional way to define builtin types is convolunted, so new ways have been invented to describe them.  Sun's ACC uses the b and R type descriptors, and IBM uses negative type numbers.  GDB can accept all three, as of version 4.8; dbx just accepts the traditional builtin types and perhaps one of the other two formats.

Traditional Builtin types
-------------------------

Often types are defined as subranges of themselves.  If the array bounds can fit within an int, then they are given normally.  For example:

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

Builtin types can also be described as subranges of int:

     .stabs "unsigned short:t6=r1;0;65535;",128,0,0,0

If the lower bound of a subrange is 0 and the upper bound is -1, it means that the type is an unsigned integral type whose bounds are too big to describe in an int.  Traditionally this is only used for unsigned int and unsigned long; GCC also sometimes uses it for long long and unsigned long long, and the only way to tell those types apart is to look at their names.  On other machines GCC puts out bounds in octal, with a leading 0.  In this case a negative bound consists of a number which is a 1 bit followed by a bunch of 0 bits, and a positive bound is one in which a bunch of bits are 1.

     .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0
     .stabs "long long int:t7=r1;0;-1;",128,0,0,0

If the lower bound of a subrange is 0 and the upper bound is negative, it means that it is an unsigned integral type whose size in bytes is the absolute value of the upper bound.  I believe this is a Convex convention for unsigned long long.

If the lower bound of a subrange is negative and the upper bound is 0, it means that the type is a signed integral type whose size in bytes is the absolute value of the lower bound.  I believe this is a Convex convention for long long.  To distinguish this from a legitimate subrange, the type should be a subrange of itself.  I'm not sure whether this is the case for Convex.

If the upper bound of a subrange is 0, it means that this is a floating point type, and the lower bound of the subrange indicates the number of bytes in the type:

     .stabs "float:t12=r1;4;0;",128,0,0,0
     .stabs "double:t13=r1;8;0;",128,0,0,0

However, GCC writes long double the same way it writes double; the only way to distinguish them is by the name:

     .stabs "long double:t14=r1;8;0;",128,0,0,0

Complex types are defined the same way as floating-point types; the only way to distinguish a single-precision complex from a double-precision floating-point type is by the name.

The C void type is defined as itself:

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

I'm not sure how a boolean type is represented.

Defining Builtin Types using Builtin Type Descriptors
-----------------------------------------------------

There are various type descriptors to define builtin types:

b SIGNED CHAR-FLAG WIDTH ; OFFSET ; NBITS ;
     Define an integral type.  SIGNED is u for unsigned or s for signed.  CHAR-FLAG is c which indicates this is a character type, or is omitted.  I assume this is to distinguish an integral type from a character type of the same size, for example it might make sense to set it for the C type wchar_t so the debugger can print such variables differently (Solaris does not do this).  Sun sets it on the C types signed char and unsigned char which arguably is wrong.  WIDTH and OFFSET appear to be for small objects stored in larger ones, for example a short in an int register.  WIDTH is normally the number of bytes in the type.  OFFSET seems to always be zero.  NBITS is the number of bits in the type.

     Note that type descriptor b used for builtin types conflicts with its use for Pascal space types (see Miscellaneous Types); they can be distinguished because the character following the type descriptor will be a digit, (, or - for a Pascal space type, or u or s for a builtin type.

w
     Documented by AIX to define a wide character type, but their compiler actually uses negative type numbers (see Negative Type Numbers).

R FP_TYPE ; BYTES ;
     Define a floating point type.  FP_TYPE has one of the following values:

     1 (NF_SINGLE)
          IEEE 32-bit (single precision) floating point format.

     2 (NF_DOUBLE)
          IEEE 64-bit (double precision) floating point format.

     3 (NF_COMPLEX)

     4 (NF_COMPLEX16)

     5 (NF_COMPLEX32)
          These are for complex numbers.  A comment in include/aout/stab_gnu.h describes them as Fortran complex, double complex, and complex*16, respectively, but what does that mean?  (i.e. Single precision?  Double precison?).

     6 (NF_LDOUBLE)
          Long double.  It would be cleaner to define a different code for every possible format of long double.

     BYTES is the number of bytes occupied by the type.  This allows a debugger to perform some operations with the type even if it doesn't understand FP_CODE.

g TYPE-INFORMATION ; NBITS
     Documented by AIX to define a floating type, but their compiler actually uses negative type numbers (see Negative Type Numbers).

c TYPE-INFORMATION ; NBITS
     Documented by AIX to define a complex type, but their compiler actually uses negative type numbers (see Negative Type Numbers).

The C void type is defined as a signed integral type 0 bits long:

     .stabs "void:t19=bs0;0;0",128,0,0,0

I'm not sure how a boolean type is represented.

Negative Type numbers
---------------------

Since the debugger knows about the builtin types anyway, the idea of negative type numbers is simply to give a special type number which indicates the built in type.  There is no stab defining these types.

I'm not sure whether anyone has tried to define what this means if int can be other than 32 bits (or other types can be other than their customary size).  If int has exactly one size for each architecture, then it can be handled easily enough, but if the size of int can vary according the compiler options, then it gets hairy. I guess the consistent way to do this would be to define separate negative type numbers for 16-bit int and 32-bit int; therefore I have indicated below the customary size (and other format information) for each type.  The information below is currently correct because AIX on the RS6000 is the only system which uses these type numbers.  If these type numbers start to get used on other systems, I suspect the correct thing to do is to define a new number in cases where a type does not have the size and format indicated below.

Also note that part of the definition of the negative type number is the name of the type.  Types with identical size and format but different names have different negative type numbers.

-1
     int, 32 bit signed integral type.

-2
     char, 8 bit type holding a character.   Both GDB and dbx on AIX treat this as signed.  GCC uses this type whether char is signed or not, which seems like a bad idea.  The AIX compiler (xlc) seems to avoid this type; it uses -5 instead for char.

-3
     short, 16 bit signed integral type.

-4
     long, 32 bit signed integral type.

-5
     unsigned char, 8 bit unsigned integral type.

-6
     signed char, 8 bit signed integral type.

-7
     unsigned short, 16 bit unsigned integral type.

-8
     unsigned int, 32 bit unsigned integral type.

-9
     unsigned, 32 bit unsigned integral type.

-10
     unsigned long, 32 bit unsigned integral type.

-11
     void, type indicating the lack of a value.

-12
     float, IEEE single precision.

-13
     double, IEEE double precision.

-14
     long double, IEEE double precision.  The compiler claims the size will increase in a future release, and for binary compatibility you have to avoid using long double.  I hope when they increase it they use a new negative type number.

-15
     integer.  32 bit signed integral type.

-16
     boolean.  Only one bit is used, not sure about the actual size of the type.

-17
     short real.  IEEE single precision.

-18
     real.  IEEE double precision.

-19
     stringptr.  See Strings.

-20
     character, 8 bit unsigned type.

-21
     logical*1, 8 bit unsigned integral type.

-22
     logical*2, 16 bit unsigned integral type.

-23
     logical*4, 32 bit unsigned integral type.

-24
     logical, 32 bit unsigned integral type.

-25
     complex.  A complex type consisting of two IEEE single-precision floating point values.

-26
     complex.  A complex type consisting of two IEEE double-precision floating point values.

-27
     integer*1, 8 bit signed integral type.

-28
     integer*2, 16 bit signed integral type.

-29
     integer*4, 32 bit signed integral type.

-30
     wchar.  Wide character, 16 bits wide (Unicode format?).  This is not used for the C type wchar_t.

Miscellaneous Types
===================

b TYPE-INFORMATION ; BYTES
     Pascal space type.  This is documented by IBM; what does it mean?

     Note that this use of the b type descriptor can be distinguished from its use for builtin integral types (see Builtin Type Descriptors) because the character following the type descriptor is always a digit, (, or -.

B TYPE-INFORMATION
     A volatile-qualified version of TYPE-INFORMATION.  This is a Sun extension.  A volatile-qualified type means that references and stores to a variable of that type must not be optimized or cached; they must occur as the user specifies them.

d TYPE-INFORMATION
     File of type TYPE-INFORMATION.  As far as I know this is only used by Pascal.

k TYPE-INFORMATION
     A const-qualified version of TYPE-INFORMATION.  This is a Sun extension.  A const-qualified type means that a variable of this type cannot be modified.

M TYPE-INFORMATION ; LENGTH
     Multiple instance type.  The type seems to composed of LENGTH repetitions of TYPE-INFORMATION, for example character*3 is represented by M-2;3, where -2 is a reference to a character type (see Negative Type Numbers).  I'm not sure how this differs from an array.  This appears to be a FORTRAN feature. LENGTH is a bound, like those in range types, see Subranges.

S TYPE-INFORMATION
     Pascal set type.  TYPE-INFORMATION must be a small type such as an enumeration or a subrange, and the type is a bitmask whose length is specified by the number of elements in TYPE-INFORMATION.

* TYPE-INFORMATION
     Pointer to TYPE-INFORMATION.

Cross-references to other types
===============================

If a type is used before it is defined, one common way to deal with this is just to use a type reference to a type which has not yet been defined.  The debugger is expected to be able to deal with this.

Another way is with the x type descriptor, which is followed by s for a structure tag, u for a union tag, or e for a enumerator tag, followed by the name of the tag, followed by :. for example the following C declarations:

     struct foo;
     struct foo *bar;

produce

     .stabs "bar:G16=*17=xsfoo:",32,0,0,0

Not all debuggers support the x type descriptor, so on some machines GCC does not use it.  I believe that for the above example it would just emit a reference to type 17 and never define it, but I haven't verified that.

Modula-2 imported types, at least on AIX, use the i type descriptor, which is followed by the name of the module from which the type is imported, followed by :, followed by the name of the type.  There is then optionally a comma followed by type information for the type (This differs from merely naming the type (see Typedefs) in that it identifies the module; I don't understand whether the name of the type given here is always just the same as the name we are giving it, or whether this type descriptor is used with a nameless stab (see Stabs Format), or what).  The symbol ends with ;.

Subrange types
==============

The r type descriptor defines a type as a subrange of another type.  It is followed by type information for the type which it is a subrange of, a semicolon, an integral lower bound, a semicolon, an integral upper bound, and a semicolon.  The AIX documentation does not specify the trailing semicolon, in an effort to specify array indexes more cleanly, but a subrange which is not an array index has always included a trailing semicolong (see Arrays).

Instead of an integer, either bound can be one of the following:

A OFFSET
     The bound is passed by reference on the stack at offset OFFSET from the argument list.  See Parameters, for more information on such offsets.

T OFFSET
     The bound is passed by value on the stack at offset OFFSET from the argument list.

a REGISTER-NUMBER
     The bound is pased by reference in register number REGISTER-NUMBER.

t REGISTER-NUMBER
     The bound is passed by value in register number REGISTER-NUMBER.

J
     There is no bound.

Subranges are also used for builtin types, see Traditional Builtin Types.

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

Arrays use the a type descriptor.  Following the type descriptor is the type of the index and the type of the array elements.  If the index type is a range type, it will end in a semicolon; if it is not a range type (for example, if it is a type reference), there does not appear to be any way to tell where the types are separated.  In an effort to clean up this mess, IBM documents the two types as being separated by a semicolon, and a range type as not ending in a semicolon (but this is not right for range types which are not array indexes, see Subranges).  I think probably the best solution is to specify that a semicolon ends a range type, and that the index type and element type of an array are separated by a semicolon, but that if the index type is a range type, the extra semicolon can be omitted.  GDB (at least through version 4.9) doesn't support any kind of index type other than a range anyway; I'm not sure about dbx.

The type of the index is often a range type, expressed as the letter r and some parameters.  It defines the size of the array.  In the example below, the range r1;0;2; defines an index type which is a subrange of type 1 (integer), with a lower bound of 0 and an upper bound of 2.  This defines the valid range of subscripts of a three-element C array.

For example, the definition

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

produces the output

     .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
          .global _char_vec
          .align 4
     _char_vec:
          .byte 97
          .byte 98
          .byte 99

If an array is packed, it means that the elements are spaced more closely than normal, saving memory at the expense of speed.  For example, an array of 3-byte objects might, if unpacked, have each element aligned on a 4-byte boundary, but if packed, have no padding. One way to specify that something is packed is with type attributes (see Stabs Format), in the case of arrays another is to use the P type descriptor instead of a.  Other than specifying a packed array, P is identical to a.

An open array is represented by the A type descriptor followed by type information specifying the type of the array elements.

An N-dimensional dynamic array is represented by

     D DIMENSIONS ; TYPE-INFORMATION

DIMENSIONS is the number of dimensions; TYPE-INFORMATION specifies the type of the array elements.

A subarray of an N-dimensional array is represented by

     E DIMENSIONS ; TYPE-INFORMATION

DIMENSIONS is the number of dimensions; TYPE-INFORMATION specifies the type of the array elements.

Strings
=======

Some languages, like C or the original Pascal, do not have string types, they just have related things like arrays of characters.  But most Pascals and various other languages have string types, which are indicated as follows:

n TYPE-INFORMATION ; BYTES
     BYTES is the maximum length.  I'm not sure what TYPE-INFORMATION is; I suspect that it means that this is a string of TYPE-INFORMATION (thus allowing a string of integers, a string of wide characters, etc., as well as a string of characters).  Not sure what the format of this type is.  This is an AIX feature.

z TYPE-INFORMATION ; BYTES
     Just like n except that this is a gstring, not an ordinary string.  I don't know the difference.

N
     Pascal Stringptr.  What is this?  This is an AIX feature.

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

Enumerations are defined with the e type descriptor.

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. The type definition 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.  Therefore it does not describe a block local symbol, but a file local one.

The source line:

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

generates the following stab

     .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 ;.

There is no standard way to specify the size of an enumeration type; it is determined by the architecture (normally all enumerations types are 32 bits).  There should be a way to specify an enumeration type of another size; type attributes would be one way to do this see Stabs Format.

Structures
==========

Directive:
     .stabs

Type:
     N_LSYM or C_DECL

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(array)
     	index_type(range of int from 0 to 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.

Giving a type a name
====================

To give a type a name, use the t symbol descriptor.  For example,

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

specifies that s_typedef refers to type number 16.  Such stabs have symbol type N_LSYM or C_DECL.

If instead, you are giving a name to a tag for a structure, union, or enumeration, use the T symbol descriptor instead.  I believe C is the only language with this feature.

If the type is an opaque type (I believe this is a Modula-2 feature), AIX provides a type descriptor to specify it.  The type descriptor is o and is followed by a name.  I don't know what the name means---is it always the same as the name of the type, or is this type descriptor used with a nameless stab (see Stabs Format)?  There optionally follows a comma followed by type information which defines the type of this type.  If omitted, a semicolon is used in place of the comma and the type information, and, the type is much like a generic pointer type---it has a known size but little else about it is specified.

Unions
======

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
==============

There are various types for function variables.  These types are not used in defining functions; see symbol descriptor f; they are used for things like pointers to functions.

The simple, traditional, type is type descriptor f is followed by type information for the return type of the function, followed by a semicolon.

This does not deal with functions the number and type of whose parameters are part of their type, as found in Modula-2 or ANSI C.  AIX provides extensions to specify these, using the f, F, p, and R type descriptors.

First comes the type descriptor.  Then, if it is f or F, this is a function, and the type information for the return type of the function follows, followed by a comma.  Then comes the number of parameters to the function and a semicolon.  Then, for each parameter, there is the name of the parameter followed by a colon (this is only present for type descriptors R and F which represent Pascal function or procedure parameters), type information for the parameter, a comma, 0 if passed by reference or 1 if passed by value, and a semicolon.  The type definition ends with a semicolon.

For example,

     int (*g_pf)();

generates the following code:

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

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
*************

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

#
     method type (two ## if minimal debug)

@
     Member (class and variable) type.  It is followed by type information for the offset basetype, a comma, and type information for the type of the field being pointed to.  (FIXME: this is acknowledged to be gibberish.  Can anyone say what really goes here?).

     Note that there is a conflict between this and type attributes (see Stabs Format); both use type descriptor @. Fortunately, the @ type descriptor used in this C++ sense always will be followed by a digit, (, or -, and type attributes never start with those things.

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 `this.'  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 "this: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 a range of integers between 0 and 1, and whose elements are 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(range of int from 0 to 1);
     	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.  The `this' pointer for a class is a pointer to the class type.

     .stabs "this: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"

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
***************************

(DIGIT)
(
-
     Local variable, see Automatic variables.

a
     Parameter passed by reference in register, see Parameters.

c
     Constant, see Constants.

C
     Conformant array bound (Pascal, maybe other languages), see Parameters.  Name of a caught exception (GNU C++).  These can be distinguished because the latter uses N_CATCH and the former uses another symbol type.

d
     Floating point register variable, see Register variables.

D
     Parameter in floating point register, see Parameters.

f
     Static function, see Procedures.

F
     Global function, see Procedures.

G
     Global variable, see Global Variables.

i
     See Parameters.

I
     Internal (nested) procedure, see Procedures.

J
     Internal (nested) function, see Procedures.

L
     Label name (documented by AIX, no further information known).

m
     Module, see Procedures.

p
     Argument list parameter, see Parameters.

pP
     See Parameters.

pF
     FORTRAN Function parameter, see Parameters.

P
     Unfortunately, three separate meanings have been independently invented for this symbol descriptor.  At least the GNU and Sun uses can be distinguished by the symbol type.  Global Procedure (AIX) (symbol type used unknown), see Procedures.  Register parameter (GNU) (symbol type N_PSYM), see Parameters.  Prototype of function referenced by this file (Sun acc) (symbol type N_FUN).

Q
     Static Procedure, see Procedures.

R
     Register parameter see Parameters.

r
     Register variable, see Register variables.

S
     Static file scope variable see Initialized statics, see Un-initialized statics.

t
     Type name, see Typedefs.

T
     enumeration, struct or union tag, see Typedefs.

v
     Parameter passed by reference, see Parameters.

V
     Static procedure scope variable see Initialized statics, see Un-initialized statics.

x
     Conformant array, see Parameters.

X
     Function return variable, see Parameters.

Table D: Type Descriptors
*************************

DIGIT
(
     Type reference, see Stabs Format.

-
     Reference to builtin type, see Negative Type Numbers.

#
     Method (C++), see Cplusplus.

*
     Pointer, see Miscellaneous Types.

&
     Reference (C++).

@
     Type Attributes (AIX), see Stabs Format.  Member (class and variable) type (GNU C++), see Cplusplus.

a
     Array, see Arrays.

A
     Open array, see Arrays.

b
     Pascal space type (AIX), see Miscellaneous Types.  Builtin integer type (Sun), see Builtin Type Descriptors.

B
     Volatile-qualified type, see Miscellaneous Types.

c
     Complex builtin type, see Builtin Type Descriptors.

C
     COBOL Picture type.  See AIX documentation for details.

d
     File type, see Miscellaneous Types.

D
     N-dimensional dynamic array, see Arrays.

e
     Enumeration type, see Enumerations.

E
     N-dimensional subarray, see Arrays.

f
     Function type, see Function types.

g
     Builtin floating point type, see Builtin Type Descriptors.

G
     COBOL Group.  See AIX documentation for details.

i
     Imported type, see Cross-references.

k
     Const-qualified type, see Miscellaneous Types.

K
     COBOL File Descriptor.  See AIX documentation for details.

n
     String type, see Strings.

N
     Stringptr, see Strings.

M
     Multiple instance type, see Miscellaneous Types.

o
     Opaque type, see Typedefs.

P
     Packed array, see Arrays.

r
     Range type, see Subranges.

R
     Builtin floating type, see Builtin Type Descriptors.

s
     Structure type, see Structures.

S
     Set type, see Miscellaneous Types.

u
     Union, see Unions.

v
     Variant record.  This is a Pascal and Modula-2 feature which is like a union within a struct in C.  See AIX documentation for details.

w
     Wide character, see Builtin Type Descriptors.

x
     Cross-reference, see Cross-references.

z
     gstring, see Strings.

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 (see Procedures) or text segment variable (see Variables).

     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)

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>>

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.

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.

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. (Bugs gcc/1063, gdb/1066.)

  * 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).  This appendix only covers those differences which are not covered in the main body of this document.

  * 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

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

(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.

  * 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.  dbx seems not to care.

  * 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.