Fri Sep 4 18:53:57 1992 Roland H. Pesch (pesch@fowanton.cygnus.com)

[deliverable/binutils-gdb.git] / gdb / doc / stabs.texinfo

\input texinfo
@setfilename stabs.info

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

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

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

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

@ignore
Permission is granted to process this file through Tex and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
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).
@end ifinfo

@setchapternewpage odd
@settitle STABS
@titlepage
@title The ``stabs'' debug format
@author Julia Menapace
@author Cygnus Support
@page
@tex
\def\$#1${{#1}}  % Kluge: collect RCS revision info without $...$
\xdef\manvers{\$Revision$}  % For use in headers, footers too
{\parskip=0pt
\hfill Cygnus Support\par
\hfill \manvers\par
\hfill \TeX{}info \texinfoversion\par
}
@end tex

@vskip 0pt plus 1filll
Copyright @copyright{} 1992 Free Software Foundation, Inc.
Contributed by Cygnus Support.

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.

@end titlepage

@ifinfo
@node Top
@top The "stabs" representation of debugging information

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

@menu
* Overview::                    Overview of stabs
* Program structure::           Encoding of the structure of the program
* Simple types::
* Example::                     A comprehensive example in C 
* Variables::
* Aggregate types::
* Symbol tables::               Symbol information in symbol tables
* GNU C++ stabs::

Appendixes:
* Example2.c::                  Source code for extended example
* Example2.s::                  Assembly code for extended example
* Quick reference::             Various refernce tables
* Expanded reference::          Reference information by stab type
* Questions::                   Questions and anomolies
* xcoff-differences::           Differences between GNU stabs in a.out
                                and GNU stabs in xcoff
* Sun-differences::             Differences between GNU stabs and Sun
                                native stabs
@end menu
@end ifinfo


@node Overview
@chapter Overview of stabs

@dfn{Stabs} refers to a format for information that describes a program
to a debugger.  This format was apparently invented by
@c FIXME! <<name of inventor>> at
the University of California at Berkeley, for the @code{pdx} Pascal
debugger; the format has spread widely since then.

@menu
* Flow:: Overview of debugging information flow
* Stabs format:: Overview of stab format
* C example:: A simple example in C source
* Assembly code:: The simple example at the assembly level
@end menu

@node Flow
@section Overview of debugging information flow

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

With the @samp{-g} option, GCC puts additional debugging information in
the @file{.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
@file{.o} file it is building.  The linker consolidates the @file{.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.

@node Stabs format
@section 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 @code{.stabs} (string), @code{.stabn} (number), or
@code{.stabd} (dot).

The overall format of each class of stab is:

@example
.stabs "@var{string}",@var{type},0,@var{desc},@var{value}
.stabn          @var{type},0,@var{desc},@var{value}
.stabd          @var{type},0,@var{desc}
@end example

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

The real key to decoding the meaning of a stab is the number in its type
field.  Each possible type number defines a different stab type.  The
stab type further defines the exact interpretation of, and possible
values for, any remaining @code{"@var{string}"}, @var{desc}, or
@var{value} fields present in the stab.  Table A (@pxref{Stab
types,,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 @code{.stabs} the @code{"@var{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 @code{"@var{string}"} field is:

@example
"@var{name}@r{[}:@var{symbol_descriptor}@r{]}
     @r{[}@var{type_number}@r{[}=@var{type_descriptor} @r{@dots{}]]}"
@end example

@var{name} is the name of the symbol represented by the stab.

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

Type information is either a @var{type_number}, or a
@samp{@var{type_number}=}.  The @var{type_number} alone is a type
reference, referring directly to a type that has already been defined.

The @samp{@var{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 @samp{=} and nested definitions.

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

@c FIXME! "too long" below introduced at J Gilmore's request; used to
@c say "more than 80 chars".  Why is vaguer better?
All this can make the @code{"@var{string}"} field quite long.  When the
@code{"@var{string}"} part of a stab is too long, the compiler splits
the @code{.stabs} directive into two @code{.stabs} directives.  Both
stabs duplicate exactly all but the @code{"@var{string}"} field.  The
@code{"@var{string}"} field of the first stab contains the first part of
the overlong string, marked as continued with a double-backslash at the
end.  The @code{"@var{string}"} field of the second stab holds the
second half of the overlong string.

@node C example
@section 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:

@example
main() 
@{
        printf("Hello world");
@}
@end example

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

@node Assembly code
@section The simple example at the assembly level

@example
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
@end example

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

@node Program structure
@chapter Encoding for the structure of the program

@menu
* Source file:: The path and name of the source file
* Line numbers::
* Procedures::
* Block structure::
@end menu

@node Source file
@section The path and name of the source file

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_SO} 
@end table      

The first stabs in the .s file contain the name and path of the source
file that was compiled to produce the .s file.  This information is
contained in two records of stab type N_SO (100).
 
@example
   .stabs "path_name", N_SO, NIL, NIL, Code_address_of_program_start
   .stabs "file_name:", N_SO, NIL, NIL, Code_address_of_program_start
@end example

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

@node Line numbers
@section Line Numbers 

@table @strong
@item Directive:
@code{.stabn}
@item Type:
@code{N_SLINE}
@end table

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

@example
.stabn N_SLINE, NIL, @var{line}, @var{address}
@end example

@var{line} is a source line number; @var{address} represents the code
address for the start of that source line.

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

@node Procedures
@section Procedures 

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_FUN}
@item Symbol Descriptors:
@code{f} (local), @code{F} (global)
@end table

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

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

@example
48      ret
49      restore
@end example

The @code{.stabs} entry after this code fragment shows the @var{name} of
the procedure (@code{main}); the type descriptor @var{desc} (@code{F},
for a global procedure); a reference to the predefined type @code{int}
for the return type; and the starting @var{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:

@example
.stabs "@var{name}:
        @var{desc}  @r{(global proc @samp{F})}
        @var{return_type_ref}  @r{(int)}
       ",N_FUN, NIL, NIL,
       @var{address}
@end example

@example
50 .stabs "main:F1",36,0,0,_main
@end example

@node Block Structure
@section Block Structure

@table @strong
@item Directive:
@code{.stabn}
@item Types:
@code{N_LBRAC}, @code{N_RBRAC}
@end table

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

@example
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:
@end example

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

@example
50 .stabs "main:F1",36,0,0,_main
@end example

@example
   .stabn N_LBRAC, NIL, NIL, @var{left-brace-address}
   .stabn N_RBRAC, NIL, NIL, @var{right-brace-address}
@end example

@example
51 .stabn 192,0,0,LBB2
52 .stabn 224,0,0,LBE2
@end example

@node Simple types
@chapter Simple types

@menu
* Basic types:: Basic type definitions
* Range types:: Range types defined by min and max value 
* Bit-ranges:: Range type defined by number of bits
@end menu

@node Basic types
@section Basic type definitions

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_LSYM}
@item Symbol Descriptor:
@code{t}
@end table

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

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


@node Range types
@section Range types defined by min and max value 

@table @strong
@item Type Descriptor:
@code{r}
@end table

When defining a range type, if the number after the first semicolon is
smaller than the number after the second one, then the two numbers
represent the smallest and the largest values in the range.
  
@example
4  .text
5  Ltext0:

.stabs "@var{name}:
        @var{descriptor}  @r{(type)}
        @var{type-def}=
        @var{type-desc}
        @var{type-ref};
        @var{low-bound};
        @var{high-bound};
       ",
       N_LSYM, NIL, NIL, NIL

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

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

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

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

@node Bit-ranges
@section Range type defined by number of bits

@table @strong
@item Type Descriptor:
@code{r}
@end table

In a range definition, if the number after the second semicolon is 0,
then the number after the first semicolon is the number of bits needed
to represent the type.

@example
.stabs "@var{name}:
        @var{desc}
        @var{type-def}=
        @var{type-desc}
        @var{type-ref};
        @var{bit-count};
        0;
       ",
       N_LSYM, NIL, NIL, NIL

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

Cosmically enough, the @code{void} type is defined directly in terms of
itself.

@example
.stabs "@var{name}:
        @var{symbol-desc}
        @var{type-def}=
        @var{type-ref}
       ",N_LSYM,NIL,NIL,NIL

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


@node Example
@chapter A Comprehensive Example in C 

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

@section Flow of control and nested scopes 

@table @strong
@item Directive:
@code{.stabn}
@item Types:
@code{N_SLINE}, @code{N_LBRAC}, @code{N_RBRAC} (cont.)
@end table

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

@example
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 @};
@end example

Here we have a single source line, the @samp{for} line, that generates
non-linear flow of control, and non-contiguous code.  In this case, an
@code{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 @code{N_LBRAC} and
@code{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.

@noindent
This is the label for the @code{N_LBRAC} (left brace) stab marking the
start of @code{main}.
 
@example
57 LBB2:
@end example

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

@example
.stabn N_SLINE, NIL,
       @var{line},
       @var{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

@exdent label for the @code{N_LBRAC} (start block) marking the start of @code{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:

@exdent label for the @code{N_RBRAC} (end block) stab marking the end of the @code{for} loop

77 LBE3:
@end example

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

@example
.stabn, N_SLINE, NIL,
        @var{line},
        @var{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:

@exdent label for the @code{N_RBRAC} (end block) stab marking the end of the @code{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
@end example

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

@example
.stabn N_LBRAC,NIL,NIL,
       @var{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
@end example

@noindent
@code{N_RBRAC} (224), ``right brace'' ends a lexical block (scope).

@example
.stabn N_RBRAC,NIL,NIL,
       @var{block-end-address}

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

@node Variables
@chapter Variables

@menu
* Automatic variables:: locally scoped
* Global variables::
* Register variables::
* Initialized statics::
* Un-initialized statics::
* Parameters::
@end menu

@node Automatic variables
@section Locally scoped automatic variables

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_LSYM} 
@item Symbol Descriptor:
none
@end table


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

@example
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 @};
@end example

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

@example
@exdent @code{N_LSYM} (128): automatic variable, scoped locally to @code{main}

.stabs "@var{name}:
        @var{type-ref}",
       N_LSYM, NIL, NIL,
       @var{frame-pointer-offset}

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

@exdent @code{N_LSYM} (128): automatic variable, scoped locally to the @code{for} loop

.stabs "@var{name}:
        @var{type-ref}",
        N_LSYM, NIL, NIL,
       @var{frame-pointer-offset}

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

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


@node Global Variables
@section Global Variables

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_GSYM}
@item Symbol Descriptor:
@code{G}
@end table

Global variables are represented by the @code{N_GSYM} stab type.  The symbol
descriptor, following the colon in the string field, is @samp{G}.  Following
the @samp{G} is a type reference or type definition.  In this example it is a
type reference to the basic C type, @code{char}.  The first source line in
@file{example2.c},
  
@example
1  char g_foo = 'c';
@end example

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

@example
@exdent @code{N_GSYM} (32): global symbol

.stabs "@var{name}:
        @var{descriptor}
        @var{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
@end example

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

@node Register variables
@section Global register variables 

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_RSYM}
@item Symbol Descriptor:
@code{r}
@end table

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

@example
2  register int g_bar asm ("%g5");
@end example

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

@example
@exdent @code{N_RSYM} (64): register variable

.stabs "@var{name}:
        @var{descriptor}
        @var{type-ref}",
       N_RSYM, NIL, NIL,
       @var{register}
 
133 .stabs "g_bar:r1",64,0,0,5
@end example


@node Initialized statics
@section Initialized static variables 

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_STSYM}
@item Symbol Descriptors:
@code{S} (file scope), @code{V} (procedure scope)
@end table

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

@example
3  static int s_g_repeat = 2; 
@end example

@noindent
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 @samp{S}.

@example
@exdent @code{N_STSYM} (38): initialized static variable (data seg w/internal linkage)

.stabs "@var{name}:
        @var{descriptor}
        @var{type-ref}",
       N_STSYM,NIL,NIL,
       @var{address}
        
26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
27      .align 4
28 _s_g_repeat:
29      .word 2
@end example


@node Un-initialized statics
@section Un-initialized static variables

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_LCSYM}
@item Symbol Descriptors:
@code{S} (file scope), @code{V} (procedure scope)
@end table

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

@example
20 @{
21      static float s_flap;
@end example

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

@example
39      .reserve _s_flap.0,4,"bss",4
@end example

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

@example
@exdent @code{N_LCSYM} (40): uninitialized static var (BSS seg w/internal linkage)

.stabs "@var{name}:
        @var{descriptor}
        @var{type-ref}",
        N_LCSYM, NIL, NIL,
        @var{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.
@end example

@c ............................................................

@node Parameters
@section Parameters 

@table @strong
@item Directive: 
@code{.stabs}
@item Type:
@code{N_PSYM}
@item Symbol Descriptor:
@code{p}
@end table

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

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

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

@example
@exdent <36> N_FUN - describing the procedure main

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

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

95 .stabs "argc:p1",160,0,0,68
    
@exdent <160> N_PSYM - parameter
@exdent .stabs "name:sym_desc(value_param)type_def(20)=ptr_to type_def(21)=
@exdent ptr_to type_ref(char)

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

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

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

@menu
* Arrays::
* Enumerations::
* Structure tags::
* Typedefs::
* Unions::
* Function types::
@end menu

@node Arrays
@section Array types 

@table @strong
@item Directive:
@code{.stabs}
@item Types:
@code{N_GSYM}, @code{N_LSYM}
@item Symbol Descriptor:
@code{T}
@item Type Descriptor:
@code{ar}
@end table

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

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

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

After the equals sign is a type descriptor, ar, which says that the
type being defined is an array.  Following the type descriptor for an
array is the type of the index, a null field, the upper bound of the
array indexing, and the type of the array elements.

The array definition above generates the assembly language that
follows.

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

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

@node Enumerations
@section Enumerations 

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_LSYM}
@item Symbol Descriptor:
@code{T}
@item Type Descriptor:
@code{e}
@end table

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

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

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

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

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

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 ;.

@node Structure tags
@section Structure Tags

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_LSYM}
@item Symbol Descriptor:
@code{T}
@item Type Descriptor:
@code{s}
@end table

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.

@example
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;
@end example

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.


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

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

@node Typedefs
@section Typedefs

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_LSYM}
@item Symbol Descriptor:
@code{t}
@end table

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

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

@example
31 .stabs "s_typedef:t16",128,0,0,0
@end example

And here is the code generated for the structure variable.

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

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

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


@node Unions
@section Unions 

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_LSYM}
@item Symbol Descriptor:
@code{T}
@item Type Descriptor:
@code{u}
@end table

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.

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

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.

@display
     <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
@end display

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

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.

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

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

@node Function types
@section Function types

@display
type descriptor f
@end display

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

@example
4  int (*g_pf)();
@end example

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

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

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

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

@node Symbol tables
@chapter 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:

@example
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 */
@};
@end example

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:

@example
3  static int s_g_repeat
@end example

@noindent
The following stab describes the symbol.

@example
26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
@end example

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

@example
21 00000084 - 00 0000 STSYM s_g_repeat:S1
@end example

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

@example
22 0000e00c - 00 0000 STSYM s_g_repeat:S1
@end example

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:

@example
1 char g_foo = 'c';
@end example

@noindent
generates the stab:

@example
21 .stabs "g_foo:G2",32,0,0,0
@end example

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.

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

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

@example
21 00000000 - 00 0000  GSYM g_foo:G2
@dots{}
215 0000e008 D _g_foo
@end example

@node GNU C++ stabs
@chapter GNU C++ stabs

@menu
* Basic C++ types::
* Simple classes::
* Class instance::
* Methods:: Method definition
* Protections::
* Method modifiers:: (const, volatile, const volatile)
* Virtual methods::
* Inheritance::
* Virtual base classes::
* Static members::
* Nested types::
@end menu


@subsection Symbol descriptors added for C++ descriptions:

@display
P - register parameter.
@end display

@subsection type descriptors added for C++ descriptions

@table @code
@item #
method type (two ## if minimal debug)

@item xs
cross-reference
@end table

@node Basic C++ types
@section 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.

@display
.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
@end display
        
@smallexample
.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
@end smallexample

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

@example
.stabs "$vtbl_ptr_type:T17",128,0,0,0
@end example

@node Simple classes
@section 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.

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

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.


@display
.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
@end display

@smallexample
.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
@end smallexample

@node Class instance
@section 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:

@example
main () @{
        baseA AbaseA;
@}
@end example

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

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

@example
.stabs "AbaseA:20",128,0,0,-20
@end example

@node Methods
@section Method defintion

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

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


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? >>

@example
.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
@end example

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

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

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

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.

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

.stabs "in:p1",160,0,0,72
@end example

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

@node Protections
@section 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:

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

@noindent
generates the following stab to describe the class type all_data.

@display
.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
@end display

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

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:

@example
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;@};
@};
@end example

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.

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

@node Method Modifiers
@section 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:

@example
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; @};
@};
@end example

This class is described by the following stab:

@display
.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);;", @dots{}
@end display
        
@example
.stabs "A:T20=s1ConstMeth::21=##1;:i;2B.;VolatileMeth::22=##2;:c;2C.;
             ConstVolMeth::23=##12;:f;2D.;;",128,0,0,0
@end example

@node Virtual Methods
@section 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:

@example
class A @{
public:
        int Adat;
        virtual int A_virt (int arg) @{ return arg; @};
@};
@end example
 
This results in the stab below describing class A.  It defines a new
type (20) which is an 8 byte structure.  The first field of the class
struct is Adat, an integer, starting at structure offset 0 and
occupying 32 bits.  

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

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

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

The bit offset of the vtable pointer field is 32.  The number of bits
in the field are not specified when the field is a vtable pointer.
 
Next is the method definition for the virtual member function A_virt.
Its description starts out using the same format as the non-virtual
member functions described above, except instead of a dot after the
`A' there is an asterisk, indicating that the function is virtual.
Since is is virtual some addition information is appended to the end
of the method description.  

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

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

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

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

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

@example
.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
@end example

@node Inheritance
@section Inheritance

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.


@example
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; @};
@};
@end example

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

@c FIXME!!! the linebreaks in the following example probably make the
@c examples literally unusable, but I don't know any other way to get
@c them on the page.
@smallexample
.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
@end smallexample

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

@display
.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); @dots{}
@end display
        
@c FIXME! fake linebreaks.
@smallexample
.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
@end smallexample

@node Virtual base classes
@section 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.

@c FIXME!! fake linebreaks below
@smallexample
.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
@end smallexample

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

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

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.


@node Static Members
@section Static Members


<< re-arrange - this has nothing to do with 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? >>

@node Nested types
@section Nested types

C++ allows a type to be defined nested "inside" a class.
Such types follow the same naming rule as class members:
The name of a nested type is only visible inside the class,
or when qualified using @code{::} notation.  In that respect,
a nested type "member" is rather like a static member.
In fact, the stabs syntax used for nested types is similar to
that used for static members.

@example
class ios @{
  public:
    enum io_state @{
	goodbit = 0,
	eofbit = 1,
	failbit = 2,
	badbit = 4 @};
    io_state state;
@};

ios::io_state Fail()
@{
    return ios::failbit;
@}

ios my_ios;
@end example

The relevant part of the assembly code is:
@example
.stabs ":t20=ebadbit:4,failbit:2,eofbit:1,goodbit:0,;",128,0,0,0
.stabs "ios:T21=s4state:20,0,32;io_state:/220:!'ios::io_state';;",128,0,0,0
.stabs "ios:Tt21",128,0,0,0
.stabs "Fail__Fv:F20",36,0,0,_Fail__Fv
.stabs "my_ios:G21",32,0,0,0
        .common _my_ios,4,"bss"
@end example

The first line declares type 20 to be an enum.  It gives it the
name @code{ios::io_state}.  The name is suppressed because @code{io_state}
is not a globally visible name.)

The second line defines the @code{ios} type.
The text @code{io_state:/220:!'ios::io_state';} declares that
@code{io_state} is a type "member".  The @code{/2} specifies
public visibility, just like a regular member.
This is followed by the type being defined (type 20), the
magic characters @code{:!} to indicate that we're declaring a nested
type, followed by the complete name of the type.
Single quotes surrond the name, because of the embedded @code{::}.

Teh debugger uses the name @code{ios::io_state} to back-patch the name
of type 20.

@node Example2.c
@appendix Example2.c - source code for extended example

@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 
@end example

@node Example2.s
@appendix Example2.s - assembly code for extended example

@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
@c FIXME! fake linebreak in line 30
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
@c FIXME: fake linebreak in line 105
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"
@end example


@node Quick reference
@appendix Quick reference

@menu
* Stab types:: Table A: Symbol types from stabs
* Assembler types:: Table B: Symbol types from assembler and linker
* Symbol descriptors:: Table C
* Type Descriptors:: Table D
@end menu

@node Stab types
@section 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 

@smallexample
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   ??
@end smallexample

@node Assembler types
@section 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

@smallexample
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
@end smallexample

@node Symbol descriptors
@section Table C: Symbol descriptors

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

@node Type Descriptors
@section Table D: Type Descriptors 

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


@node Expanded reference
@appendix 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.

@menu
* N_GSYM::      Global variable
* N_FNAME::     Function name (BSD Fortran)
* N_FUN::       C Function name or text segment variable
* N_STSYM::     Initialized static symbol
* N_LCSYM::     Uninitialized static symbol
* N_MAIN::      Name of main routine (not for C)
* N_PC::        Pascal global symbol
* N_NSYMS::     Number of symbols
* N_NOMAP::     No DST map
* N_RSYM::      Register variable
* N_M2C::       Modula-2 compilation unit
* N_SLINE::     Line number in text segment
* N_DSLINE::    Line number in data segment
* N_BSLINE::    Line number in bss segment
* N_BROWS::     Path to .cb file for Sun source code browser
* N_DEFD::      GNU Modula2 definition module dependency
* N_EHDECL::    GNU C++ exception variable
* N_MOD2::      Modula2 information "for imc"
* N_CATCH::     GNU C++ "catch" clause
* N_SSYM::      Structure or union element
* N_SO::        Source file containing main 
* N_LSYM::      Automatic variable
* N_BINCL::     Beginning of include file (Sun only)
* N_SOL::       Name of include file
* N_PSYM::      Parameter variable
* N_EINCL::     End of include file
* N_ENTRY::     Alternate entry point
* N_LBRAC::     Beginning of lexical block
* N_EXCL::      Deleted include file
* N_SCOPE::     Modula2 scope information (Sun only)
* N_RBRAC::     End of lexical block
* N_BCOMM::     Begin named common block
* N_ECOMM::     End named common block
* N_ECOML::     End common
* Gould::       non-base register symbols used on Gould systems
* N_LENG::      Length of preceding entry
@end menu

@node N_GSYM
@section 32 - 0x20 - N_GYSM       

@display
Global variable.

.stabs "name", N_GSYM, NIL, NIL, NIL
@end display

@example
"name" -> "symbol_name:#type"
                       # -> G
@end example

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

@node N_FNAME
@section 34 - 0x22 - N_FNAME 
Function name (for BSD Fortran)

@display
.stabs "name", N_FNAME, NIL, NIL, NIL
@end display

@example
"name" -> "function_name" 
@end example

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

@node N_FUN
@section 36 - 0x24 - N_FUN        
Function name or text segment variable for C.

@display
.stabs "name", N_FUN, NIL, desc, value
@end display

@example
@exdent @emph{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.

@exdent @emph{For text segment variables:}
<<How to create one?>>
@end example

@node N_STSYM
@section 38 - 0x26 - N_STSYM   
Initialized static symbol (data segment w/internal linkage).

@display
.stabs "name", N_STSYM, NIL, NIL, value
@end display

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

@node N_LCSYM
@section 40 - 0x28 - N_LCSYM    
Unitialized static (.lcomm) symbol(BSS segment w/internal linkage).

@display
.stabs "name", N_LCLSYM, NIL, NIL, value
@end display

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

@node N_MAIN
@section 42 - 0x2a - N_MAIN       
Name of main routine (not used in C)

@display
.stabs "name", N_MAIN, NIL, NIL, NIL
@end display

@example
"name" -> "name_of_main_routine"  
@end example

@node N_PC
@section 48 - 0x30 - N_PC               
Global symbol (for Pascal)

@display
.stabs "name", N_PC, NIL, NIL, value
@end display

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

@display
stabdump.c says: 

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

@node N_NSYMS
@section 50 - 0x32 - N_NSYMS      
Number of symbols (according to Ultrix V4.0)

@display
        0, files,,funcs,lines (stab.def)
@end display

@node N_NOMAP
@section 52 - 0x34 - N_NOMAP   
no DST map for sym (according to Ultrix V4.0)

@display
        name, ,0,type,ignored (stab.def)
@end display

@node N_RSYM
@section 64 - 0x40 - N_RSYM      
 register variable

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

@node N_M2C
@section 66 - 0x42 - N_M2C        
Modula-2 compilation unit

@display
.stabs "name", N_M2C, 0, desc, value
@end display

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

@node N_SLINE
@section 68 - 0x44 - N_SLINE      
Line number in text segment

@display
.stabn N_SLINE, 0, desc, value
@end display

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

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

@node N_DSLINE
@section 70 - 0x46 - N_DSLINE 
Line number in data segment

@display
.stabn N_DSLINE, 0, desc, value
@end display

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

See comment for N_SLINE above.

@node N_BSLINE
@section 72 - 0x48 - N_BSLINE  
Line number in bss segment

@display
.stabn N_BSLINE, 0, desc, value
@end display

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

See comment for N_SLINE above.

@node N_BROWS
@section 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

@node N_DEFD
@section 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?

@node N_EHDECL
@section 80 - 0x50 - N_EHDECL  
GNU C++ exception variable <<?>>

"name is variable name"

Note: conflicts with N_MOD2.

@node N_MOD2
@section 80 - 0x50 - N_MOD2
Modula2 info "for imc" (according to Ultrix V4.0)

Note: conflicts with N_EHDECL  <<?>>

@node N_CATCH
@section 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.

@node N_SSYM
@section 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>>

@node N_SO
@section 100 - 0x64 - N_SO        
Path and name of source file containing main routine

@display
.stabs "name", N_SO, NIL, NIL, value
@end display

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

value  -> the starting text address of the compilation.
@end example

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

@node N_LSYM
@section 128 - 0x80 - N_LSYM      
Automatic var in the stack (also used for type descriptors.)

@display
.stabs "name" N_LSYM, NIL, NIL, value
@end display

@example
@exdent @emph{For stack based local variables:}

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

@exdent @emph{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.
@end example

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.

@node N_BINCL
@section 130 - 0x82 - N_BINCL     

Beginning of an include file (Sun only)

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

@node N_SOL
@section 132 - 0x84 - N_SOL

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

@node N_PSYM
@section 160 - 0xa0 - N_PSYM    
   
Parameter variable

@display
stabs. "name", N_PSYM, NIL, NIL, value
@end display

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

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

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

@node N_EINCL
@section 162 - 0xa2 - N_EINCL  

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

@node N_ENTRY
@section 164 - 0xa4 - N_ENTRY   

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

@node N_LBRAC
@section 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. <<?>>

@display
.stabn N_LBRAC, NIL, NIL, value
@end display

@example
value -> code address of block start.
@end example

@node N_EXCL
@section 194 - 0xc2 - N_EXCL    

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

@node N_SCOPE
@section 196 - 0xc4 - N_SCOPE   

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

@node N_RBRAC
@section 224 -  0xe0 - N_RBRAC  

End of a lexical block (right brace)

@display
.stabn N_RBRAC, NIL, NIL, value
@end display

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

@node N_BCOMM
@section 226 - 0xe2 - N_BCOMM     

Begin named common block.  

Only the name is significant.
<<?>>

@node N_ECOMM
@section 228 - 0xe4 - N_ECOMM     

End named common block.  

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

@node N_ECOML
@section  232 - 0xe8 - N_ECOML   

End common (local name) 

value is address.
<<?>>

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

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

@node N_LENG
@section    - 0xfe - N_LENG

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

@node Questions
@appendix Questions and anomalies

@itemize @bullet
@item
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'

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

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

@item
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).
 
@item
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?

@item
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?
@end itemize

@node xcoff-differences
@appendix Differences between GNU stabs in a.out and GNU stabs in xcoff

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

@itemize @bullet
@item
Instead of .stabs, xcoff uses .stabx.

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

@item
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

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

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

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

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

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

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

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

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

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

@item
The symbol descriptor for register parameters is P for a.out and R for
xcoff.
@end itemize


(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

@example
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
@end example

@node Sun-differences
@appendix Differences between GNU stabs and Sun native stabs.

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

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

@item
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.

@item
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.  
@end itemize

@contents
@bye