* stabs.texinfo (Builtin Type Descriptors): Document the floating

[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 the stabs debugging symbol tables.

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

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

@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
* Constants::                   Constants
* Example::                     A comprehensive example in C 
* Variables::
* Types::                       Type definitions
* Symbol tables::               Symbol information in symbol tables
* Cplusplus::

Appendixes:
* Example2.c::                  Source code for extended example
* Example2.s::                  Assembly code for extended example
* 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
* 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.

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

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

@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 number in the type field gives some basic information about what
type of stab this is (or whether it @emph{is} a stab, as opposed to an
ordinary symbol).  Each possible type number defines a different stab
type.  The stab type further defines the exact interpretation of, and
possible values for, any remaining @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.
@var{name} can be omitted, which means the stab represents an unnamed
object.  For example, @samp{:t10=*2} defines type 10 as a pointer to
type 2, but does not give the type a name.  Omitting the @var{name}
field is supported by AIX dbx and GDB after about version 4.8, but not
other debuggers.

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}.

The @samp{c} symbol descriptor is an exception in that it is not
followed by type information.  @xref{Constants}.

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.

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

@table @code
@item a@var{boundary}
@var{boundary} is an integer specifying the alignment.  I assume it
applies to all variables of this type.

@item s@var{size}
Size in bits of a variable of this type.

@item p@var{integer}
Pointer class (for checking).  Not sure what this means, or how
@var{integer} is interpreted.

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

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

All of the following stabs use the @samp{N_FUN} symbol type.

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

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

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

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

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

@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 Constants
@chapter Constants

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

@table @code
@item b@var{value}
Boolean constant.  @var{value} is a numeric value; I assume it is 0 for
false or 1 for true.

@item c@var{value}
Character constant.  @var{value} is the numeric value of the constant.

@item e@var{type-information},@var{value}
Enumeration constant.  @var{type-information} is the type of the
constant, as it would appear after a symbol descriptor
(@pxref{Stabs Format}).  @var{value} is the numeric value of the constant.

@item i@var{value}
Integer constant.  @var{value} is the numeric value.

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

@item s@var{string}
String constant.  @var{string} is a string enclosed in either @samp{'}
(in which case @samp{'} characters within the string are represented as
@samp{\'} or @samp{"} (in which case @samp{"} characters within the
string are represented as @samp{\"}).

@item S@var{type-information},@var{elements},@var{bits},@var{pattern}
Set constant.  @var{type-information} is the type of the constant, as it
would appear after a symbol descriptor (@pxref{Stabs Format}).
@var{elements} is the number of elements in the set (is this just the
number of bits set in @var{pattern}?  Or redundant with the type?  I
don't get it), @var{bits} is the number of bits in the constant (meaning
it specifies the length of @var{pattern}, I think), and @var{pattern} is
a hexadecimal representation of the set.  AIX documentation refers to a
limit of 32 bytes, but I see no reason why this limit should exist.
@end table

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

This information is followed by @samp{;}.

@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 information}",
       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 information}",
        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

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

@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 Register variables 

@c According to an old version of this manual, AIX uses C_RPSYM instead
@c of C_RSYM.  I am skeptical; this should be verified.
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.

The value is the register number.

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

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

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

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

@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 

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

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

@example
.stabs "arg:p1" . . .           ; N_PSYM
.stabs "arg:r1" . . .           ; N_RSYM
@end example

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

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

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

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

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

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

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

@c Is this paragraph correct?  It is based on piecing together patchy
@c information and some guesswork
Conformant arrays refer to a feature of Modula-2, and perhaps other
languages, in which the size of an array parameter is not known to the
called function until run-time.  Such parameters have two stabs, a
@samp{x} for the array itself, and a @samp{C}, which represents the size
of the array.  The value of the @samp{x} stab is the offset in the
argument list where the address of the array is stored (it this right?
it is a guess); the value of the @samp{C} stab is the offset in the
argument list where the size of the array (in elements? in bytes?) is
stored.

The following are also said to go with @samp{N_PSYM}:

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

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

As a simple example, the code

@example
main (argc, argv)
     int argc;
     char **argv;
@{
@end example

produces the stabs

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

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

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

@menu
* Builtin types::               Integers, floating point, void, etc.
* Miscellaneous Types::         Pointers, sets, files, etc.
* Cross-references::            Referring to a type not yet defined.
* Subranges::                   A type with a specific range.
* Arrays::                      An aggregate type of same-typed elements.
* Strings::                     Like an array but also has a length.
* Enumerations::                Like an integer but the values have names.
* Structures::                  An aggregate type of different-typed elements.
* Typedefs::                    Giving a type a name
* Unions::
* Function types::
@end menu

@node Builtin types
@section Builtin types

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

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

@menu
* Traditional Builtin Types::   Put on your seatbelts and prepare for kludgery
* Builtin Type Descriptors::    Builtin types with special type descriptors
* Negative Type Numbers::       Builtin types using negative type numbers
@end menu

@node Traditional Builtin Types
@subsection Traditional Builtin types

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

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

Builtin types can also be described as subranges of @code{int}:

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

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

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

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

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

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

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

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

The C @code{void} type is defined as itself:

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

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

@node Builtin Type Descriptors
@subsection Defining Builtin Types using Builtin Type Descriptors

There are various type descriptors to define builtin types:

@table @code
@c FIXME: clean up description of width and offset, once we figure out
@c what they mean
@item b @var{signed} @var{char-flag} @var{width} ; @var{offset} ; @var{nbits} ;
Define an integral type.  @var{signed} is @samp{u} for unsigned or
@samp{s} for signed.  @var{char-flag} is @samp{c} which indicates this
is a character type, or is omitted.  I assume this is to distinguish an
integral type from a character type of the same size, for example it
might make sense to set it for the C type @code{wchar_t} so the debugger
can print such variables differently (Solaris does not do this).  Sun
sets it on the C types @code{signed char} and @code{unsigned char} which
arguably is wrong.  @var{width} and @var{offset} appear to be for small
objects stored in larger ones, for example a @code{short} in an
@code{int} register.  @var{width} is normally the number of bytes in the
type.  @var{offset} seems to always be zero.  @var{nbits} is the number
of bits in the type.

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

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

@item R @var{fp_type} ; @var{bytes} ;
Define a floating point type.  @var{fp_type} has one of the following values:

@table @code
@item 1 (NF_SINGLE)
IEEE 32-bit (single precision) floating point format.

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

@item 3 (NF_COMPLEX)
@item 4 (NF_COMPLEX16)
@item 5 (NF_COMPLEX32)
These are for complex numbers.  A comment in
@file{include/aout/stab_gnu.h} describes them as Fortran complex, double
complex, and complex*16, respectively, but what does that mean?  (i.e.
Single precision?  Double precison?).

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

@var{bytes} is the number of bytes occupied by the type.  This allows a
debugger to perform some operations with the type even if it doesn't
understand @var{fp_code}.

@item g @var{type-information} ; @var{nbits}
Documented by AIX to define a floating type, but their compiler actually
uses negative type numbers (@pxref{Negative Type Numbers}).

@item c @var{type-information} ; @var{nbits}
Documented by AIX to define a complex type, but their compiler actually
uses negative type numbers (@pxref{Negative Type Numbers}).
@end table

The C @code{void} type is defined as a signed integral type 0 bits long:
@example
.stabs "void:t19=bs0;0;0",128,0,0,0
@end example

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

@node Negative Type Numbers
@subsection Negative Type numbers

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

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

@table @code
@item -1
@code{int}, 32 bit signed integral type.

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

@item -3
@code{short}, 16 bit signed integral type.

@item -4
@code{long}, 32 bit signed integral type.

@item -5
@code{unsigned char}, 8 bit unsigned integral type.

@item -6
@code{signed char}, 8 bit signed integral type.

@item -7
@code{unsigned short}, 16 bit unsigned integral type.

@item -8
@code{unsigned int}, 32 bit unsigned integral type.

@item -9
@code{unsigned}, 32 bit unsigned integral type.

@item -10
@code{unsigned long}, 32 bit unsigned integral type.

@item -11
@code{void}, type indicating the lack of a value.

@item -12
@code{float}, IEEE single precision.

@item -13
@code{double}, IEEE double precision.

@item -14
@code{long double}, IEEE extended, RS6000 format.

@item -15
@code{integer}.  Pascal, I assume.  32 bit signed integral type.

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

@item -17
@code{short real}.  Pascal, I assume.  IEEE single precision.

@item -18
@code{real}.  Pascal, I assume.  IEEE double precision.

@item -19
A Pascal Stringptr.  @xref{Strings}.

@item -20
@code{character}, 8 bit unsigned type.

@item -21
@code{logical*1}, 8 bit unsigned integral type.

@item -22
@code{logical*2}, 16 bit unsigned integral type.

@item -23
@code{logical*4}, 32 bit unsigned integral type.

@item -24
@code{logical}, 32 bit unsigned integral type.

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

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

@item -27
@code{integer*1}, 8 bit signed integral type.

@item -28
@code{integer*2}, 16 bit signed integral type.

@item -29
@code{integer*4}, 32 bit signed integral type.

@item -30
Wide character.  AIX appears not to use this for the C type
@code{wchar_t}; instead it uses an integral type of the appropriate
size.
@end table

@node Miscellaneous Types
@section Miscellaneous Types

@table @code
@item b @var{type-information} ; @var{bytes}
Pascal space type.  This is documented by IBM; what does it mean?

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

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

@item d @var{type-information}
File of type @var{type-information}.  As far as I know this is only used
by Pascal.

@item k @var{type-information}
A const-qualified version of @var{type-information}.  This is a Sun
extension.  A const-qualified type means that a variable of this type
cannot be modified.

@item M @var{type-information} ; @var{length}
Multiple instance type.  The type seems to composed of @var{length}
repetitions of @var{type-information}, for example @code{character*3} is
represented by @samp{M-2;3}, where @samp{-2} is a reference to a
character type (@pxref{Negative Type Numbers}).  I'm not sure how this
differs from an array.  This appears to be a FORTRAN feature.
@var{length} is a bound, like those in range types, @xref{Subranges}.

@item S @var{type-information}
Pascal set type.  @var{type-information} must be a small type such as an
enumeration or a subrange, and the type is a bitmask whose length is
specified by the number of elements in @var{type-information}.

@item * @var{type-information}
Pointer to @var{type-information}.
@end table

@node Cross-references
@section Cross-references to other types

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

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

@example
struct foo;
struct foo *bar;
@end example

produce

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

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

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

@node Subranges
@section Subrange types

The @samp{r} type descriptor defines a type as a subrange of another
type.  It is followed by type information for the type which it is a
subrange of, a semicolon, an integral lower bound, a semicolon, an
integral upper bound, and a semicolon.  The AIX documentation does not
specify the trailing semicolon; I believe it is confused.

AIX allows the bounds to be one of the following instead of an integer:

@table @code
@item A @var{offset}
The bound is passed by reference on the stack at offset @var{offset}
from the argument list.  @xref{Parameters}, for more information on such
offsets.

@item T @var{offset}
The bound is passed by value on the stack at offset @var{offset} from
the argument list.

@item a @var{register-number}
The bound is pased by reference in register number
@var{register-number}.

@item t @var{register-number}
The bound is passed by value in register number @var{register-number}.

@item J
There is no bound.
@end table

Subranges are also used for builtin types, @xref{Traditional Builtin Types}.

@node Arrays
@section Array types 

Arrays use the @samp{a} type descriptor.  Following the type descriptor
is the type of the index and the type of the array elements.  The two
types types are not separated by any sort of delimiter; if the type of
the index does not end in a semicolon I don't know what is supposed to
happen.  IBM documents a semicolon between the two types.  For the
common case (a range type), this ends up as being the same since IBM
documents a range type as not ending in a semicolon, but the latter does
not accord with common practice, in which range types do end with
semicolons.

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

For example, the definition

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

@noindent
produces the output

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

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

@c FIXME-what is it?  A pointer?
An open array is represented by the @samp{A} type descriptor followed by
type information specifying the type of the array elements.

@c FIXME: what is the format of this type?  A pointer to a vector of pointers?
An N-dimensional dynamic array is represented by

@example
D @var{dimensions} ; @var{type-information}
@end example

@c Does dimensions really have this meaning?  The AIX documentation
@c doesn't say.
@var{dimensions} is the number of dimensions; @var{type-information}
specifies the type of the array elements.

@c FIXME: what is the format of this type?  A pointer to some offsets in
@c another array?
A subarray of an N-dimensional array is represented by

@example
E @var{dimensions} ; @var{type-information}
@end example

@c Does dimensions really have this meaning?  The AIX documentation
@c doesn't say.
@var{dimensions} is the number of dimensions; @var{type-information}
specifies the type of the array elements.

@node Strings
@section Strings

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

@table @code
@item n @var{type-information} ; @var{bytes}
@var{bytes} is the maximum length.  I'm not sure what
@var{type-information} is; I suspect that it means that this is a string
of @var{type-information} (thus allowing a string of integers, a string
of wide characters, etc., as well as a string of characters).  Not sure
what the format of this type is.  This is an AIX feature.

@item z @var{type-information} ; @var{bytes}
Just like @samp{n} except that this is a gstring, not an ordinary
string.  I don't know the difference.

@item N
Pascal Stringptr.  What is this?  This is an AIX feature.
@end table

@node Enumerations
@section Enumerations 

Enumerations are defined with the @samp{e} type descriptor.

@c FIXME: Where does this information properly go?  Perhaps it is
@c redundant with something we already explain.
The source line below declares an enumeration type.  It is defined at
file scope between the bodies of main and s_proc in example2.c.
The type definition is located after the N_RBRAC that marks the end of
the previous procedure's block scope, and before the N_FUN that marks
the beginning of the next procedure's block scope.  Therefore it does not
describe a block local symbol, but a file local one.  

The source line:

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

@noindent
generates the following stab

@example
.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 ;.

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

@node Structures
@section Structures

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_LSYM} or @code{C_DECL}
@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(array)
        index_type(range of int from 0 to 7);
        element_type(char),bit_offset,field_bits;;",
        N_LSYM,NIL,NIL,NIL

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

To give a type a name, use the @samp{t} symbol descriptor.  For example,

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

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

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

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

@node Unions
@section Unions 

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

@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

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

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

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

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

For example,

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

@noindent
generates the following code:

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

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 Cplusplus
@chapter GNU C++ stabs

@menu
* Basic Cplusplus types::
* Simple classes::
* Class instance::
* Methods:: Method definition
* Protections::
* Method Modifiers:: (const, volatile, const volatile)
* Virtual Methods::
* Inheritence::
* Virtual Base Classes::
* Static Members::
@end menu

@subsection type descriptors added for C++ descriptions

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

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

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

@node Basic Cplusplus 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 `this.'  Type 19, the `this' pointer is
defined as a pointer to type 20, baseA, but a stab defining baseA has
not yet been emited.  Since the compiler knows it will be emited
shortly, here it just outputs a cross reference to the undefined
symbol, by prefixing the symbol name with xs.

@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 "this: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
a range of integers between 0 and 1, and whose elements are of type
17.  Type 17 was the vtable record type defined by the boilerplate C++
type definitions, as shown earlier.

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

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

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

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

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

@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(range of int from 0 to 1);
        elem_type_ref(vtbl elem type),
        bit_offset(32);
        meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int);
        :arg_type(int),protection(public)normal(yes)virtual(yes)
        vtable_index(1);class_first_defining(A);;;~%first_base(A);",
        N_LSYM,NIL,NIL,NIL
@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 Inheritence
@section Inheritence

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

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

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

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

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

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

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


@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.  The
`this' pointer for a class is a pointer to the class type.

@example
.stabs "this: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

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

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

@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 Stab types
@appendix 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
@appendix 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
@appendix Table C: Symbol descriptors

@c Please keep this alphabetical
@table @code
@item @var{(digit)}
@itemx (
@itemx -
Local variable, @xref{Automatic variables}.

@item a
Parameter passed by reference in register, @xref{Parameters}.

@item c
Constant, @xref{Constants}.

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

@item d
Floating point register variable, @xref{Register variables}.

@item D
Parameter in floating point register, @xref{Parameters}.

@item f
Static function, @xref{Procedures}.

@item F
Global function, @xref{Procedures}.

@item G
Global variable, @xref{Global Variables}.

@item i
@xref{Parameters}.

@item I
Internal (nested) procedure, @xref{Procedures}.

@item J
Internal (nested) function, @xref{Procedures}.

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

@item m
Module, @xref{Procedures}.

@item p
Argument list parameter, @xref{Parameters}.

@item pP
@xref{Parameters}.

@item pF
FORTRAN Function parameter, @xref{Parameters}.

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

@item Q
Static Procedure, @xref{Procedures}.

@item R
Register parameter @xref{Parameters}.

@item r
Register variable, @xref{Register variables}.

@item S
Static file scope variable @xref{Initialized statics},
@xref{Un-initialized statics}.

@item t
Type name, @xref{Typedefs}.

@item T
enumeration, struct or union tag, @xref{Typedefs}.

@item v
Parameter passed by reference, @xref{Parameters}.

@item V
Static procedure scope variable @xref{Initialized statics},
@xref{Un-initialized statics}.

@item x
Conformant array, @xref{Parameters}.

@item X
Function return variable, @xref{Parameters}.
@end table

@node Type Descriptors
@appendix Table D: Type Descriptors 

@table @code
@item @var{digit}
@itemx (
Type reference, @xref{Stabs Format}.

@item -
Reference to builtin type, @xref{Negative Type Numbers}.

@item #
Method (C++), @xref{Cplusplus}.

@item *
Pointer, @xref{Miscellaneous Types}.

@item &
Reference (C++).

@item @@
Type Attributes (AIX), @xref{Stabs Format}.  Member (class and variable)
type (GNU C++), @xref{Cplusplus}.  

@item a
Array, @xref{Arrays}.

@item A
Open array, @xref{Arrays}.

@item b
Pascal space type (AIX), @xref{Miscellaneous Types}.  Builtin integer
type (Sun), @xref{Builtin Type Descriptors}.

@item B
Volatile-qualified type, @xref{Miscellaneous Types}.

@item c
Complex builtin type, @xref{Builtin Type Descriptors}.

@item C
COBOL Picture type.  See AIX documentation for details.

@item d
File type, @xref{Miscellaneous Types}.

@item D
N-dimensional dynamic array, @xref{Arrays}.

@item e
Enumeration type, @xref{Enumerations}.

@item E
N-dimensional subarray, @xref{Arrays}.

@item f
Function type, @xref{Function types}.

@item g
Builtin floating point type, @xref{Builtin Type Descriptors}.

@item G
COBOL Group.  See AIX documentation for details.

@item i
Imported type, @xref{Cross-references}.

@item k
Const-qualified type, @xref{Miscellaneous Types}.

@item K
COBOL File Descriptor.  See AIX documentation for details.

@item n
String type, @xref{Strings}.

@item N
Stringptr, @xref{Strings}.

@item M
Multiple instance type, @xref{Miscellaneous Types}.

@item o
Opaque type, @xref{Typedefs}.

@item P
Packed array, @xref{Arrays}.

@item r
Range type, @xref{Subranges}.

@item R
Builtin floating type, @xref{Builtin Type Descriptors}.

@item s
Structure type, @xref{Structures}.

@item S
Set type, @xref{Miscellaneous Types}.

@item u
Union, @xref{Unions}.

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

@item w
Wide character, @xref{Builtin Type Descriptors}.

@item x
Cross-reference, @xref{Cross-references}.

@item z
gstring, @xref{Strings}.
@end table

@node Expanded reference
@appendix Expanded reference by stab type.

@c FIXME: For most types this should be much shorter and much sweeter,
@c see N_PSYM for an example.  For stuff like N_SO where the stab type
@c really is the important thing, the information can stay here.

@c FIXME: It probably should be merged with Tables A and B.

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 (@pxref{Procedures}) or text segment variable
(@pxref{Variables}).
@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" -> /source/directory/
       -> source_file

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 directory that the source file is relative to.  The name field of
the second N_SO contains the name of the source file itself.

Only some compilers (e.g. gcc2, Sun cc) include the directory; this
symbol can be distinguished by the fact that it ends in a slash.
According to a comment in GDB's partial-stab.h, other compilers
(especially unnamed C++ compilers) put out useless N_SO's for
nonexistent source files (after the N_SO for the real source file).

@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.  @xref{Parameters}.

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

@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

@c FIXME: Merge *all* these into the main body of the document.
(The AIX/RS6000 native object file format is xcoff with stabs).  This
appendix only covers those differences which are not covered in the main
body of this document.

@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, sdb-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.  sdb-type is unused
with stabs; it can always be set to 0.

@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 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 , 
@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.

@c FIXME: Merge all this stuff into the main body of the document.

@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
Stabs describing block scopes, N_LBRAC and N_RBRAC are supposed to
contain the nesting level of the block in the desc field, re Sun doc.
GNU stabs always have 0 in that field.  dbx seems not to care.

@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