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* Stabs: (stabs).                 The "stabs" debugging information format.

END-INFO-DIR-ENTRY

   This document describes the stabs debugging symbol tables.

   Copyright (C) 1992,1993,1994,1995,1997,1998,2000,2001    Free

Software Foundation, Inc.  Contributed by Cygnus Support.  Written by

Julia Menapace, Jim Kingdon, and David MacKenzie.

   Permission is granted to copy, distribute and/or modify this document

under the terms of the GNU Free Documentation License, Version 1.1 or

any later version published by the Free Software Foundation; with no

Invariant Sections, with no Front-Cover Texts, and with no Back-Cover

Texts.  A copy of the license is included in the section entitled "GNU

Free Documentation License".

File: stabs.info,  Node: Top,  Next: Overview,  Up: (dir)

The "stabs" representation of debugging information

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

This document describes the stabs debugging format.

* Menu:

* Overview::                    Overview of stabs

* Program Structure::           Encoding of the structure of the program

* Constants::                   Constants

* Variables::

* Types::                       Type definitions

* Macro define and undefine::   Representation of #define and #undef

* Symbol Tables::               Symbol information in symbol tables

* Cplusplus::                   Stabs specific to C++

* Stab Types::                  Symbol types in a.out files

* Symbol Descriptors::          Table of symbol descriptors

* Type Descriptors::            Table of type descriptors

* Expanded Reference::          Reference information by stab type

* Questions::                   Questions and anomalies

* Stab Sections::               In some object file formats, stabs are

                                in sections.

* Symbol Types Index::          Index of symbolic stab symbol type names.

* GNU Free Documentation License::  The license for this documentation

File: stabs.info,  Node: Overview,  Next: Program Structure,  Prev: Top,  Up: Top

1 Overview of Stabs

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

"Stabs" refers to a format for information that describes a program to

a debugger.  This format was apparently invented by Peter Kessler at

the University of California at Berkeley, for the `pdx' Pascal

debugger; the format has spread widely since then.

   This document is one of the few published sources of documentation on

stabs.  It is believed to be comprehensive for stabs used by C.  The

lists of symbol descriptors (*note Symbol Descriptors::) and type

descriptors (*note Type Descriptors::) are believed to be completely

comprehensive.  Stabs for COBOL-specific features and for variant

records (used by Pascal and Modula-2) are poorly documented here.

   Other sources of information on stabs are `Dbx and Dbxtool

Interfaces', 2nd edition, by Sun, 1988, and `AIX Version 3.2 Files

Reference', Fourth Edition, September 1992, "dbx Stabstring Grammar" in

the a.out section, page 2-31.  This document is believed to incorporate

the information from those two sources except where it explicitly

directs you to them for more information.

* Menu:

* Flow::                        Overview of debugging information flow

* Stabs Format::                Overview of stab format

* String Field::                The string field

* C Example::                   A simple example in C source

* Assembly Code::               The simple example at the assembly level

File: stabs.info,  Node: Flow,  Next: Stabs Format,  Up: Overview

1.1 Overview of Debugging Information Flow

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

The GNU C compiler compiles C source in a `.c' file into assembly

language in a `.s' file, which the assembler translates into a `.o'

file, which the linker combines with other `.o' files and libraries to

produce an executable file.

   With the `-g' option, GCC puts in the `.s' file additional debugging

information, 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 function names, parameters, and

scopes.

   For some object file formats, the debugging information is

encapsulated in assembler directives known collectively as "stab"

(symbol table) directives, which are interspersed with the generated

code.  Stabs are the native format for debugging information in the

a.out and XCOFF object file formats.  The GNU tools can also emit stabs

in the COFF and ECOFF object file formats.

   The assembler adds the information from stabs to the symbol

information it places by default in the symbol table and the string

table of the `.o' file it is building.  The linker consolidates the `.o'

files into one executable file, with one symbol table and one string

table.  Debuggers use the symbol and string tables in the executable as

a source of debugging information about the program.

File: stabs.info,  Node: Stabs Format,  Next: String Field,  Prev: Flow,  Up: Overview

1.2 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 which combination of four possible data fields follows.  It is

either `.stabs' (string), `.stabn' (number), or `.stabd' (dot).  IBM's

XCOFF assembler uses `.stabx' (and some other directives such as

`.file' and `.bi') instead of `.stabs', `.stabn' or `.stabd'.

   The overall format of each class of stab is:

     .stabs "STRING",TYPE,OTHER,DESC,VALUE

     .stabn TYPE,OTHER,DESC,VALUE

     .stabd TYPE,OTHER,DESC

     .stabx "STRING",VALUE,TYPE,SDB-TYPE

   For `.stabn' and `.stabd', there is no STRING (the `n_strx' field is

zero; see *Note Symbol Tables::).  For `.stabd', the VALUE field is

implicit and has the value of the current file location.  For `.stabx',

the SDB-TYPE field is unused for stabs and can always be set to zero.

The OTHER field is almost always unused and can be set to zero.

   The number in the TYPE field gives some basic information about

which type of stab this is (or whether it _is_ a stab, as opposed to an

ordinary symbol).  Each valid type number defines a different stab

type; further, the stab type defines the exact interpretation of, and

possible values for, any remaining STRING, DESC, or VALUE fields

present in the stab.  *Note Stab Types::, for a list in numeric order

of the valid TYPE field values for stab directives.

File: stabs.info,  Node: String Field,  Next: C Example,  Prev: Stabs Format,  Up: Overview

1.3 The String Field

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

For most stabs the string field holds the meat of the debugging

information.  The flexible 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 of the string field for most stab types is:

     "NAME:SYMBOL-DESCRIPTOR TYPE-INFORMATION"

   NAME is the name of the symbol represented by the stab; it can

contain a pair of colons (*note Nested Symbols::).  NAME can be

omitted, which means the stab represents an unnamed object.  For

example, `:t10=*2' defines type 10 as a pointer to type 2, but does not

give the type a name.  Omitting the NAME field is supported by AIX dbx

and GDB after about version 4.8, but not other debuggers.  GCC

sometimes uses a single space as the name instead of omitting the name

altogether; apparently that is supported by most debuggers.

   The SYMBOL-DESCRIPTOR following the `:' is an alphabetic character

that tells more specifically what kind of symbol the stab represents.

If the SYMBOL-DESCRIPTOR is omitted, but type information follows, then

the stab represents a local variable.  For a list of symbol

descriptors, see *Note Symbol Descriptors::.  The `c' symbol descriptor

is an exception in that it is not followed by type information.  *Note

Constants::.

   TYPE-INFORMATION is either a TYPE-NUMBER, or `TYPE-NUMBER='.  A

TYPE-NUMBER alone is a type reference, referring directly to a type

that has already been defined.

   The `TYPE-NUMBER=' form is a type definition, where the number

represents a new type which is about to be defined.  The type

definition may refer to other types by number, and those type numbers

may be followed by `=' and nested definitions.  Also, the Lucid

compiler will repeat `TYPE-NUMBER=' more than once if it wants to

define several type numbers at once.

   In a type definition, if the character that follows the equals sign

is non-numeric then it is a TYPE-DESCRIPTOR, and tells what kind of

type is about to be defined.  Any other values following the

TYPE-DESCRIPTOR vary, depending on the TYPE-DESCRIPTOR.  *Note Type

Descriptors::, for a list of TYPE-DESCRIPTOR values.  If a number

follows the `=' then the number is a TYPE-REFERENCE.  For a full

description of types, *Note Types::.

   A TYPE-NUMBER is often a single number.  The GNU and Sun tools

additionally permit a TYPE-NUMBER to be a pair

(FILE-NUMBER,FILETYPE-NUMBER) (the parentheses appear in the string,

and serve to distinguish the two cases).  The FILE-NUMBER is 0 for the

base source file, 1 for the first included file, 2 for the next, and so

on.  The FILETYPE-NUMBER is a number starting with 1 which is

incremented for each new type defined in the file.  (Separating the

file number and the type number permits the `N_BINCL' optimization to

succeed more often; see *Note Include Files::).

   There is an AIX extension for type attributes.  Following the `='

are any number of type attributes.  Each one starts with `@' and ends

with `;'.  Debuggers, including AIX's dbx and GDB 4.10, skip any type

attributes they do not recognize.  GDB 4.9 and other versions of dbx

may not do this.  Because of a conflict with C++ (*note Cplusplus::),

new attributes should not be defined which begin with a digit, `(', or

`-'; GDB may be unable to distinguish those from the C++ type

descriptor `@'.  The attributes are:

`aBOUNDARY'

     BOUNDARY is an integer specifying the alignment.  I assume it

     applies to all variables of this type.

`pINTEGER'

     Pointer class (for checking).  Not sure what this means, or how

     INTEGER is interpreted.

`P'

     Indicate this is a packed type, meaning that structure fields or

     array elements are placed more closely in memory, to save memory

     at the expense of speed.

`sSIZE'

     Size in bits of a variable of this type.  This is fully supported

     by GDB 4.11 and later.

`S'

     Indicate that this type is a string instead of an array of

     characters, or a bitstring instead of a set.  It doesn't change

     the layout of the data being represented, but does enable the

     debugger to know which type it is.

`V'

     Indicate that this type is a vector instead of an array.  The only

     major difference between vectors and arrays is that vectors are

     passed by value instead of by reference (vector coprocessor

     extension).

   All of this can make the string field quite long.  All versions of

GDB, and some versions of dbx, can handle arbitrarily long strings.

But many versions of dbx (or assemblers or linkers, I'm not sure which)

cretinously limit the strings to about 80 characters, so compilers which

must work with such systems need to split the `.stabs' directive into

several `.stabs' directives.  Each stab duplicates every field except

the string field.  The string field of every stab except the last is

marked as continued with a backslash at the end (in the assembly code

this may be written as a double backslash, depending on the assembler).

Removing the backslashes and concatenating the string fields of each

stab produces the original, long string.  Just to be incompatible (or so

they don't have to worry about what the assembler does with

backslashes), AIX can use `?' instead of backslash.

File: stabs.info,  Node: C Example,  Next: Assembly Code,  Prev: String Field,  Up: Overview

1.4 A Simple Example in C Source

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

To get the flavor of how stabs describe source information for a C

program, let's look at the simple program:

     main()

     {

             printf("Hello world");

     }

   When compiled with `-g', the program above yields the following `.s'

file.  Line numbers have been added to make it easier to refer to parts

of the `.s' file in the description of the stabs that follows.

File: stabs.info,  Node: Assembly Code,  Prev: C Example,  Up: Overview

1.5 The Simple Example at the Assembly Level

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

This simple "hello world" example demonstrates several of the stab

types used to describe C language source files.

     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

File: stabs.info,  Node: Program Structure,  Next: Constants,  Prev: Overview,  Up: Top

2 Encoding the Structure of the Program

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

The elements of the program structure that stabs encode include the name

of the main function, the names of the source and include files, the

line numbers, procedure names and types, and the beginnings and ends of

blocks of code.

* Menu:

* Main Program::                Indicate what the main program is

* Source Files::                The path and name of the source file

* Include Files::               Names of include files

* Line Numbers::

* Procedures::

* Nested Procedures::

* Block Structure::

* Alternate Entry Points::      Entering procedures except at the beginning.

File: stabs.info,  Node: Main Program,  Next: Source Files,  Up: Program Structure

2.1 Main Program

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

Most languages allow the main program to have any name.  The `N_MAIN'

stab type tells the debugger the name that is used in this program.

Only the string field is significant; it is the name of a function

which is the main program.  Most C compilers do not use this stab (they

expect the debugger to assume that the name is `main'), but some C

compilers emit an `N_MAIN' stab for the `main' function.  I'm not sure

how XCOFF handles this.

File: stabs.info,  Node: Source Files,  Next: Include Files,  Prev: Main Program,  Up: Program Structure

2.2 Paths and Names of the Source Files

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

Before any other stabs occur, there must be a stab specifying the source

file.  This information is contained in a symbol of stab type `N_SO';

the string field contains the name of the file.  The value of the

symbol is the start address of the portion of the text section

corresponding to that file.

   Some compilers use the desc field to indicate the language of the

source file.  Sun's compilers started this usage, and the first

constants are derived from their documentation.  Languages added by

gcc/gdb start at 0x32 to avoid conflict with languages Sun may add in

the future.  A desc field with a value 0 indicates that no language has

been specified via this mechanism.

`N_SO_AS' (0x1)

     Assembly language

`N_SO_C'  (0x2)

     K&R traditional C

`N_SO_ANSI_C' (0x3)

     ANSI C

`N_SO_CC'  (0x4)

     C++

`N_SO_FORTRAN' (0x5)

     Fortran

`N_SO_PASCAL' (0x6)

     Pascal

`N_SO_FORTRAN90' (0x7)

     Fortran90

`N_SO_OBJC' (0x32)

     Objective-C

`N_SO_OBJCPLUS' (0x33)

     Objective-C++

   Some compilers (for example, GCC2 and SunOS4 `/bin/cc') also include

the directory in which the source was compiled, in a second `N_SO'

symbol preceding the one containing the file name.  This symbol can be

distinguished by the fact that it ends in a slash.  Code from the

`cfront' C++ compiler can have additional `N_SO' symbols for

nonexistent source files after the `N_SO' for the real source file;

these are believed to contain no useful information.

   For example:

     .stabs "/cygint/s1/users/jcm/play/",100,0,0,Ltext0     # 100 is N_SO

     .stabs "hello.c",100,0,0,Ltext0

             .text

     Ltext0:

   Instead of `N_SO' symbols, XCOFF uses a `.file' assembler directive

which assembles to a `C_FILE' symbol; explaining this in detail is

outside the scope of this document.

   If it is useful to indicate the end of a source file, this is done

with an `N_SO' symbol with an empty string for the name.  The value is

the address of the end of the text section for the file.  For some

systems, there is no indication of the end of a source file, and you

just need to figure it ended when you see an `N_SO' for a different

source file, or a symbol ending in `.o' (which at least some linkers

insert to mark the start of a new `.o' file).

File: stabs.info,  Node: Include Files,  Next: Line Numbers,  Prev: Source Files,  Up: Program Structure

2.3 Names of Include Files

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

There are several schemes for dealing with include files: the

traditional `N_SOL' approach, Sun's `N_BINCL' approach, and the XCOFF

`C_BINCL' approach (which despite the similar name has little in common

with `N_BINCL').

   An `N_SOL' symbol specifies which include file subsequent symbols

refer to.  The string field is the name of the file and the value is the

text address corresponding to the end of the previous include file and

the start of this one.  To specify the main source file again, use an

`N_SOL' symbol with the name of the main source file.

   The `N_BINCL' approach works as follows.  An `N_BINCL' symbol

specifies the start of an include file.  In an object file, only the

string is significant; the linker puts data into some of the other

fields.  The end of the include file is marked by an `N_EINCL' symbol

(which has no string field).  In an object file, there is no

significant data in the `N_EINCL' symbol.  `N_BINCL' and `N_EINCL' can

be nested.

   If the linker detects that two source files have identical stabs

between an `N_BINCL' and `N_EINCL' pair (as will generally be the case

for a header file), then it only puts out the stabs once.  Each

additional occurrence is replaced by an `N_EXCL' symbol.  I believe the

GNU linker and the Sun (both SunOS4 and Solaris) linker are the only

ones which supports this feature.

   A linker which supports this feature will set the value of a

`N_BINCL' symbol to the total of all the characters in the stabs

strings included in the header file, omitting any file numbers.  The

value of an `N_EXCL' symbol is the same as the value of the `N_BINCL'

symbol it replaces.  This information can be used to match up `N_EXCL'

and `N_BINCL' symbols which have the same filename.  The `N_EINCL'

value, and the values of the other and description fields for all

three, appear to always be zero.

   For the start of an include file in XCOFF, use the `.bi' assembler

directive, which generates a `C_BINCL' symbol.  A `.ei' directive,

which generates a `C_EINCL' symbol, denotes the end of the include

file.  Both directives are followed by the name of the source file in

quotes, which becomes the string for the symbol.  The value of each

symbol, produced automatically by the assembler and linker, is the

offset into the executable of the beginning (inclusive, as you'd

expect) or end (inclusive, as you would not expect) of the portion of

the COFF line table that corresponds to this include file.  `C_BINCL'

and `C_EINCL' do not nest.

File: stabs.info,  Node: Line Numbers,  Next: Procedures,  Prev: Include Files,  Up: Program Structure

2.4 Line Numbers

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

An `N_SLINE' symbol represents the start of a source line.  The desc

field contains the line number and the value contains the code address

for the start of that source line.  On most machines the address is

absolute; for stabs in sections (*note Stab Sections::), it is relative

to the function in which the `N_SLINE' symbol occurs.

   GNU documents `N_DSLINE' and `N_BSLINE' symbols for line numbers in

the data or bss segments, respectively.  They are identical to

`N_SLINE' but are relocated differently by the linker.  They were

intended to be used to describe the source location of a variable

declaration, but I believe that GCC2 actually puts the line number in

the desc field of the stab for the variable itself.  GDB has been

ignoring these symbols (unless they contain a string field) since at

least GDB 3.5.

   For single source lines that generate discontiguous code, such as

flow of control statements, there may be more than one line number

entry for the same source line.  In this case there is a line number

entry at the start of each code range, each with the same line number.

   XCOFF does not use stabs for line numbers.  Instead, it uses COFF

line numbers (which are outside the scope of this document).  Standard

COFF line numbers cannot deal with include files, but in XCOFF this is

fixed with the `C_BINCL' method of marking include files (*note Include

Files::).

File: stabs.info,  Node: Procedures,  Next: Nested Procedures,  Prev: Line Numbers,  Up: Program Structure

2.5 Procedures

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

All of the following stabs normally use the `N_FUN' symbol type.

However, Sun's `acc' compiler on SunOS4 uses `N_GSYM' and `N_STSYM',

which means that the value of the stab for the function is useless and

the debugger must get the address of the function from the non-stab

symbols instead.  On systems where non-stab symbols have leading

underscores, the stabs will lack underscores and the debugger needs to

know about the leading underscore to match up the stab and the non-stab

symbol.  BSD Fortran is said to use `N_FNAME' with the same

restriction; the value of the symbol is not useful (I'm not sure it

really does use this, because GDB doesn't handle this and no one has

complained).

   A function is represented by an `F' symbol descriptor for a global

(extern) function, and `f' for a static (local) function.  For a.out,

the value of the symbol is the address of the start of the function; it

is already relocated.  For stabs in ELF, the SunPRO compiler version

2.0.1 and GCC put out an address which gets relocated by the linker.

In a future release SunPRO is planning to put out zero, in which case

the address can be found from the ELF (non-stab) symbol.  Because

looking things up in the ELF symbols would probably be slow, I'm not

sure how to find which symbol of that name is the right one, and this

doesn't provide any way to deal with nested functions, it would

probably be better to make the value of the stab an address relative to

the start of the file, or just absolute.  See *Note ELF Linker

Relocation:: for more information on linker relocation of stabs in ELF

files.  For XCOFF, the stab uses the `C_FUN' storage class and the

value of the stab is meaningless; the address of the function can be

found from the csect symbol (XTY_LD/XMC_PR).

   The type information of the stab represents the return type of the

function; thus `foo:f5' means that foo is a function returning type 5.

There is no need to try to get the line number of the start of the

function from the stab for the function; it is in the next `N_SLINE'

symbol.

   Some compilers (such as Sun's Solaris compiler) support an extension

for specifying the types of the arguments.  I suspect this extension is

not used for old (non-prototyped) function definitions in C.  If the

extension is in use, the type information of the stab for the function

is followed by type information for each argument, with each argument

preceded by `;'.  An argument type of 0 means that additional arguments

are being passed, whose types and number may vary (`...' in ANSI C).

GDB has tolerated this extension (parsed the syntax, if not necessarily

used the information) since at least version 4.8; I don't know whether

all versions of dbx tolerate it.  The argument types given here are not

redundant with the symbols for the formal parameters (*note

Parameters::); they are the types of the arguments as they are passed,

before any conversions might take place.  For example, if a C function

which is declared without a prototype takes a `float' argument, the

value is passed as a `double' but then converted to a `float'.

Debuggers need to use the types given in the arguments when printing

values, but when calling the function they need to use the types given

in the symbol defining the function.

   If the return type and types of arguments of a function which is

defined in another source file are specified (i.e., a function

prototype in ANSI C), traditionally compilers emit no stab; the only

way for the debugger to find the information is if the source file

where the function is defined was also compiled with debugging symbols.

As an extension the Solaris compiler uses symbol descriptor `P'

followed by the return type of the function, followed by the arguments,

each preceded by `;', as in a stab with symbol descriptor `f' or `F'.

This use of symbol descriptor `P' can be distinguished from its use for

register parameters (*note Register Parameters::) by the fact that it

has symbol type `N_FUN'.

   The AIX documentation also defines symbol descriptor `J' as an

internal function.  I assume this means a function nested within another

function.  It also says symbol descriptor `m' is a module in Modula-2

or extended Pascal.

   Procedures (functions which do not return values) are represented as

functions returning the `void' type in C.  I don't see why this couldn't

be used for all languages (inventing a `void' type for this purpose if

necessary), but the AIX documentation defines `I', `P', and `Q' for

internal, global, and static procedures, respectively.  These symbol

descriptors are unusual in that they are not followed by type

information.

   The following example shows a stab for a function `main' which

returns type number `1'.  The `_main' specified for the value is a

reference to an assembler label which is used to fill in the start

address of the function.

     .stabs "main:F1",36,0,0,_main      # 36 is N_FUN

   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.

   If functions can appear in different sections, then the debugger may

not be able to find the end of a function.  Recent versions of GCC will

mark the end of a function with an `N_FUN' symbol with an empty string

for the name.  The value is the address of the end of the current

function.  Without such a symbol, there is no indication of the address

of the end of a function, and you must assume that it ended at the

starting address of the next function or at the end of the text section

for the program.

File: stabs.info,  Node: Nested Procedures,  Next: Block Structure,  Prev: Procedures,  Up: Program Structure

2.6 Nested Procedures

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

For any of the symbol descriptors representing procedures, after the

symbol descriptor and the type information is optionally a scope

specifier.  This consists of a comma, the name of the procedure, another

comma, and the name of the enclosing procedure.  The first name is local

to the scope specified, and seems to be redundant with the name of the

symbol (before the `:').  This feature is used by GCC, and presumably

Pascal, Modula-2, etc., compilers, for nested functions.

   If procedures are nested more than one level deep, only the

immediately containing scope is specified.  For example, this code:

     int

     foo (int x)

     {

       int bar (int y)

         {

           int baz (int z)

             {

               return x + y + z;

             }

           return baz (x + 2 * y);

         }

       return x + bar (3 * x);

     }

produces the stabs:

     .stabs "baz:f1,baz,bar",36,0,0,_baz.15         # 36 is N_FUN

     .stabs "bar:f1,bar,foo",36,0,0,_bar.12

     .stabs "foo:F1",36,0,0,_foo

File: stabs.info,  Node: Block Structure,  Next: Alternate Entry Points,  Prev: Nested Procedures,  Up: Program Structure

2.7 Block Structure

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

The program's block structure is represented by the `N_LBRAC' (left

brace) and the `N_RBRAC' (right brace) stab types.  The variables

defined inside a block precede the `N_LBRAC' symbol for most compilers,

including GCC.  Other compilers, such as the Convex, Acorn RISC

machine, and Sun `acc' compilers, put the variables after the `N_LBRAC'

symbol.  The values of the `N_LBRAC' and `N_RBRAC' symbols are the

start and end addresses of the code of the block, respectively.  For

most machines, they are relative to the starting address of this source

file.  For the Gould NP1, they are absolute.  For stabs in sections

(*note Stab Sections::), they are relative to the function in which

they occur.

   The `N_LBRAC' and `N_RBRAC' stabs that describe the block scope of a

procedure are located after the `N_FUN' stab that represents the

procedure itself.

   Sun documents the desc field of `N_LBRAC' and `N_RBRAC' symbols as

containing the nesting level of the block.  However, dbx seems to not

care, and GCC always sets desc to zero.

   For XCOFF, block scope is indicated with `C_BLOCK' symbols.  If the

name of the symbol is `.bb', then it is the beginning of the block; if

the name of the symbol is `.be'; it is the end of the block.

File: stabs.info,  Node: Alternate Entry Points,  Prev: Block Structure,  Up: Program Structure

2.8 Alternate Entry Points

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

Some languages, like Fortran, have the ability to enter procedures at

some place other than the beginning.  One can declare an alternate entry

point.  The `N_ENTRY' stab is for this; however, the Sun FORTRAN

compiler doesn't use it.  According to AIX documentation, only the name

of a `C_ENTRY' stab is significant; the address of the alternate entry

point comes from the corresponding external symbol.  A previous

revision of this document said that the value of an `N_ENTRY' stab was

the address of the alternate entry point, but I don't know the source

for that information.

File: stabs.info,  Node: Constants,  Next: Variables,  Prev: Program Structure,  Up: Top

3 Constants

***********

The `c' symbol descriptor indicates that this stab represents a

constant.  This symbol descriptor is an exception to the general rule

that symbol descriptors are followed by type information.  Instead, it

is followed by `=' and one of the following:

`b VALUE'

     Boolean constant.  VALUE is a numeric value; I assume it is 0 for

     false or 1 for true.

`c VALUE'

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

`e TYPE-INFORMATION , VALUE'

     Constant whose value can be represented as integral.

     TYPE-INFORMATION is the type of the constant, as it would appear

     after a symbol descriptor (*note String Field::).  VALUE is the

     numeric value of the constant.  GDB 4.9 does not actually get the

     right value if VALUE does not fit in a host `int', but it does not

     do anything violent, and future debuggers could be extended to

     accept integers of any size (whether unsigned or not).  This

     constant type is usually documented as being only for enumeration

     constants, but GDB has never imposed that restriction; I don't

     know about other debuggers.

`i VALUE'

     Integer constant.  VALUE is the numeric value.  The type is some

     sort of generic integer type (for GDB, a host `int'); to specify

     the type explicitly, use `e' instead.

`r VALUE'

     Real constant.  VALUE is the real value, which can be `INF'

     (optionally preceded by a sign) for infinity, `QNAN' for a quiet

     NaN (not-a-number), or `SNAN' for a signalling NaN.  If it is a

     normal number the format is that accepted by the C library function

     `atof'.

`s STRING'

     String constant.  STRING is a string enclosed in either `'' (in

     which case `'' characters within the string are represented as

     `\'' or `"' (in which case `"' characters within the string are

     represented as `\"').

`S TYPE-INFORMATION , ELEMENTS , BITS , PATTERN'

     Set constant.  TYPE-INFORMATION is the type of the constant, as it

     would appear after a symbol descriptor (*note String Field::).

     ELEMENTS is the number of elements in the set (does this means how

     many bits of PATTERN are actually used, which would be redundant

     with the type, or perhaps the number of bits set in PATTERN?  I

     don't get it), BITS is the number of bits in the constant (meaning

     it specifies the length of PATTERN, I think), and PATTERN is a

     hexadecimal representation of the set.  AIX documentation refers

     to a limit of 32 bytes, but I see no reason why this limit should

     exist.  This form could probably be used for arbitrary constants,

     not just sets; the only catch is that PATTERN should be understood

     to be target, not host, byte order and format.

   The boolean, character, string, and set constants are not supported

by GDB 4.9, but it ignores them.  GDB 4.8 and earlier gave an error

message and refused to read symbols from the file containing the

constants.

   The above information is followed by `;'.

File: stabs.info,  Node: Variables,  Next: Types,  Prev: Constants,  Up: Top

4 Variables

***********

Different types of stabs describe the various ways that variables can be

allocated: on the stack, globally, in registers, in common blocks,

statically, or as arguments to a function.

* Menu:

* Stack Variables::             Variables allocated on the stack.

* Global Variables::            Variables used by more than one source file.

* Register Variables::          Variables in registers.

* Common Blocks::               Variables statically allocated together.

* Statics::                     Variables local to one source file.

* Based Variables::             Fortran pointer based variables.

* Parameters::                  Variables for arguments to functions.

File: stabs.info,  Node: Stack Variables,  Next: Global Variables,  Up: Variables

4.1 Automatic Variables Allocated on the Stack

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

If a variable's scope is local to a function and its lifetime is only as

long as that function executes (C calls such variables "automatic"), it

can be allocated in a register (*note Register Variables::) or on the

stack.

   Each variable allocated on the stack has a stab with the symbol

descriptor omitted.  Since type information should begin with a digit,

`-', or `(', only those characters precluded from being used for symbol

descriptors.  However, the Acorn RISC machine (ARM) is said to get this

wrong: it puts out a mere type definition here, without the preceding

`TYPE-NUMBER='.  This is a bad idea; there is no guarantee that type

descriptors are distinct from symbol descriptors.  Stabs for stack

variables use the `N_LSYM' stab type, or `C_LSYM' for XCOFF.

   The value of the stab is the offset of the variable within the local

variables.  On most machines this is an offset from the frame pointer

and is negative.  The location of the stab specifies which block it is

defined in; see *Note Block Structure::.

   For example, the following C code:

     int

     main ()

     {

       int x;

     }

   produces the following stabs:

     .stabs "main:F1",36,0,0,_main   # 36 is N_FUN

     .stabs "x:1",128,0,0,-12        # 128 is N_LSYM

     .stabn 192,0,0,LBB2             # 192 is N_LBRAC

     .stabn 224,0,0,LBE2             # 224 is N_RBRAC

   See *Note Procedures:: for more information on the `N_FUN' stab, and

*Note Block Structure:: for more information on the `N_LBRAC' and

`N_RBRAC' stabs.

File: stabs.info,  Node: Global Variables,  Next: Register Variables,  Prev: Stack Variables,  Up: Variables

4.2 Global Variables

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

A variable whose scope is not specific to just one source file is

represented by the `G' symbol descriptor.  These stabs use the `N_GSYM'

stab type (C_GSYM for XCOFF).  The type information for the stab (*note

String Field::) gives the type of the variable.

   For example, the following source code:

     char g_foo = 'c';

yields the following assembly code:

     .stabs "g_foo:G2",32,0,0,0     # 32 is N_GSYM

          .global _g_foo

          .data

     _g_foo:

          .byte 99

   The address of the variable represented by the `N_GSYM' is not

contained in the `N_GSYM' stab.  The debugger gets this information

from the external symbol for the global variable.  In the example above,

the `.global _g_foo' and `_g_foo:' lines tell the assembler to produce

an external symbol.

   Some compilers, like GCC, output `N_GSYM' stabs only once, where the

variable is defined.  Other compilers, like SunOS4 /bin/cc, output a

`N_GSYM' stab for each compilation unit which references the variable.

File: stabs.info,  Node: Register Variables,  Next: Common Blocks,  Prev: Global Variables,  Up: Variables

4.3 Register Variables

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

Register variables have their own stab type, `N_RSYM' (`C_RSYM' for

XCOFF), and their own symbol descriptor, `r'.  The stab's value is the

number of the register where the variable data will be stored.

   AIX defines a separate symbol descriptor `d' for floating point

registers.  This seems unnecessary; why not just just give floating

point registers different register numbers?  I have not verified whether

the compiler actually uses `d'.

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

initialized, as in:

     register int g_bar asm ("%g5");

then the stab may be emitted at the end of the object file, with the

other bss symbols.

File: stabs.info,  Node: Common Blocks,  Next: Statics,  Prev: Register Variables,  Up: Variables

4.4 Common Blocks

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

A common block is a statically allocated section of memory which can be

referred to by several source files.  It may contain several variables.

I believe Fortran is the only language with this feature.

   A `N_BCOMM' stab begins a common block and an `N_ECOMM' stab ends

it.  The only field that is significant in these two stabs is the

string, which names a normal (non-debugging) symbol that gives the

address of the common block.  According to IBM documentation, only the

`N_BCOMM' has the name of the common block (even though their compiler

actually puts it both places).

   The stabs for the members of the common block are between the

`N_BCOMM' and the `N_ECOMM'; the value of each stab is the offset

within the common block of that variable.  IBM uses the `C_ECOML' stab

type, and there is a corresponding `N_ECOML' stab type, but Sun's

Fortran compiler uses `N_GSYM' instead.  The variables within a common

block use the `V' symbol descriptor (I believe this is true of all

Fortran variables).  Other stabs (at least type declarations using

`C_DECL') can also be between the `N_BCOMM' and the `N_ECOMM'.