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* Gdb-Internals: (gdbint).	The GNU debugger's internals.
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This file documents the internals of the GNU debugger GDB.

Copyright 1990-1999 Free Software Foundation, Inc.
Contributed by Cygnus Solutions.  Written by John Gilmore.
Second Edition by Stan Shebs.

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 off
@settitle GDB Internals

@titlepage
@title{GDB Internals}
@subtitle{A guide to the internals of the GNU debugger}
@author John Gilmore
@author Cygnus Solutions
@author Second Edition:
@author Stan Shebs
@author Cygnus Solutions
@page
@tex
\def\$#1${{#1}}  % Kluge: collect RCS revision info without $...$
\xdef\manvers{\$Revision$}  % For use in headers, footers too
{\parskip=0pt
\hfill Cygnus Solutions\par
\hfill \manvers\par
\hfill \TeX{}info \texinfoversion\par
}
@end tex

@vskip 0pt plus 1filll
Copyright @copyright{} 1990-1999 Free Software Foundation, Inc.

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

@node Top
@c Perhaps this should be the title of the document (but only for info,
@c not for TeX).  Existing GNU manuals seem inconsistent on this point.
@top Scope of this Document

This document documents the internals of the GNU debugger, GDB.  It
includes description of GDB's key algorithms and operations, as well
as the mechanisms that adapt GDB to specific hosts and targets.

@menu
* Requirements::
* Overall Structure::
* Algorithms::
* User Interface::
* Symbol Handling::
* Language Support::
* Host Definition::
* Target Architecture Definition::
* Target Vector Definition::
* Native Debugging::
* Support Libraries::
* Coding::
* Porting GDB::
* Testsuite::
* Hints::
@end menu

@node Requirements

@chapter Requirements

Before diving into the internals, you should understand the formal
requirements and other expectations for GDB.  Although some of these may
seem obvious, there have been proposals for GDB that have run counter to
these requirements.

First of all, GDB is a debugger.  It's not designed to be a front panel
for embedded systems.  It's not a text editor.  It's not a shell.  It's
not a programming environment.

GDB is an interactive tool.  Although a batch mode is available, GDB's
primary role is to interact with a human programmer.

GDB should be responsive to the user.  A programmer hot on the trail of
a nasty bug, and operating under a looming deadline, is going to be very
impatient of everything, including the response time to debugger
commands.

GDB should be relatively permissive, such as for expressions.  While the
compiler should be picky (or have the option to be made picky), since
source code lives for a long time usually, the programmer doing
debugging shouldn't be spending time figuring out to mollify the
debugger.

GDB will be called upon to deal with really large programs.  Executable
sizes of 50 to 100 megabytes occur regularly, and we've heard reports of
programs approaching 1 gigabyte in size.

GDB should be able to run everywhere.  No other debugger is available
for even half as many configurations as GDB supports.


@node Overall Structure

@chapter Overall Structure

GDB consists of three major subsystems: user interface, symbol handling
(the ``symbol side''), and target system handling (the ``target side'').

Ther user interface consists of several actual interfaces, plus
supporting code.

The symbol side consists of object file readers, debugging info
interpreters, symbol table management, source language expression
parsing, type and value printing.

The target side consists of execution control, stack frame analysis, and
physical target manipulation.

The target side/symbol side division is not formal, and there are a
number of exceptions.  For instance, core file support involves symbolic
elements (the basic core file reader is in BFD) and target elements (it
supplies the contents of memory and the values of registers).  Instead,
this division is useful for understanding how the minor subsystems
should fit together.

@section The Symbol Side

The symbolic side of GDB can be thought of as ``everything you can do in
GDB without having a live program running''.  For instance, you can look
at the types of variables, and evaluate many kinds of expressions.

@section The Target Side

The target side of GDB is the ``bits and bytes manipulator''.  Although
it may make reference to symbolic info here and there, most of the
target side will run with only a stripped executable available -- or
even no executable at all, in remote debugging cases.

Operations such as disassembly, stack frame crawls, and register
display, are able to work with no symbolic info at all.  In some cases,
such as disassembly, GDB will use symbolic info to present addresses
relative to symbols rather than as raw numbers, but it will work either
way.

@section Configurations

@dfn{Host} refers to attributes of the system where GDB runs.
@dfn{Target} refers to the system where the program being debugged
executes.  In most cases they are the same machine, in which case a
third type of @dfn{Native} attributes come into play.

Defines and include files needed to build on the host are host support.
Examples are tty support, system defined types, host byte order, host
float format.

Defines and information needed to handle the target format are target
dependent.  Examples are the stack frame format, instruction set,
breakpoint instruction, registers, and how to set up and tear down the stack
to call a function.

Information that is only needed when the host and target are the same,
is native dependent.  One example is Unix child process support; if the
host and target are not the same, doing a fork to start the target
process is a bad idea.  The various macros needed for finding the
registers in the @code{upage}, running @code{ptrace}, and such are all
in the native-dependent files.

Another example of native-dependent code is support for features that
are really part of the target environment, but which require
@code{#include} files that are only available on the host system.  Core
file handling and @code{setjmp} handling are two common cases.

When you want to make GDB work ``native'' on a particular machine, you
have to include all three kinds of information.


@node Algorithms

@chapter Algorithms

GDB uses a number of debugging-specific algorithms.  They are often not
very complicated, but get lost in the thicket of special cases and
real-world issues.  This chapter describes the basic algorithms and
mentions some of the specific target definitions that they use.

@section Frames

A frame is a construct that GDB uses to keep track of calling and called
functions.

@code{FRAME_FP} in the machine description has no meaning to the
machine-independent part of GDB, except that it is used when setting up
a new frame from scratch, as follows:

@example
      create_new_frame (read_register (FP_REGNUM), read_pc ()));
@end example

Other than that, all the meaning imparted to @code{FP_REGNUM} is
imparted by the machine-dependent code.  So, @code{FP_REGNUM} can have
any value that is convenient for the code that creates new frames.
(@code{create_new_frame} calls @code{INIT_EXTRA_FRAME_INFO} if it is
defined; that is where you should use the @code{FP_REGNUM} value, if
your frames are nonstandard.)

Given a GDB frame, define @code{FRAME_CHAIN} to determine the address of
the calling function's frame.  This will be used to create a new GDB
frame struct, and then @code{INIT_EXTRA_FRAME_INFO} and
@code{INIT_FRAME_PC} will be called for the new frame.

@section Breakpoint Handling

In general, a breakpoint is a user-designated location in the program
where the user wants to regain control if program execution ever reaches
that location.

There are two main ways to implement breakpoints; either as ``hardware''
breakpoints or as ``software'' breakpoints.

Hardware breakpoints are sometimes available as a builtin debugging
features with some chips.  Typically these work by having dedicated
register into which the breakpoint address may be stored.  If the PC
ever matches a value in a breakpoint registers, the CPU raises an
exception and reports it to GDB.  Another possibility is when an
emulator is in use; many emulators include circuitry that watches the
address lines coming out from the processor, and force it to stop if the
address matches a breakpoint's address.  A third possibility is that the
target already has the ability to do breakpoints somehow; for instance,
a ROM monitor may do its own software breakpoints.  So although these
are not literally ``hardware breakpoints'', from GDB's point of view
they work the same; GDB need not do nothing more than set the breakpoint
and wait for something to happen.

Since they depend on hardware resources, hardware breakpoints may be
limited in number; when the user asks for more, GDB will start trying to
set software breakpoints.

Software breakpoints require GDB to do somewhat more work.  The basic
theory is that GDB will replace a program instruction with a trap,
illegal divide, or some other instruction that will cause an exception,
and then when it's encountered, GDB will take the exception and stop the
program. When the user says to continue, GDB will restore the original
instruction, single-step, re-insert the trap, and continue on.

Since it literally overwrites the program being tested, the program area
must be writeable, so this technique won't work on programs in ROM.  It
can also distort the behavior of programs that examine themselves,
although the situation would be highly unusual.

Also, the software breakpoint instruction should be the smallest size of
instruction, so it doesn't overwrite an instruction that might be a jump
target, and cause disaster when the program jumps into the middle of the
breakpoint instruction.  (Strictly speaking, the breakpoint must be no
larger than the smallest interval between instructions that may be jump
targets; perhaps there is an architecture where only even-numbered
instructions may jumped to.)  Note that it's possible for an instruction
set not to have any instructions usable for a software breakpoint,
although in practice only the ARC has failed to define such an
instruction.

The basic definition of the software breakpoint is the macro
@code{BREAKPOINT}.

Basic breakpoint object handling is in @file{breakpoint.c}.  However,
much of the interesting breakpoint action is in @file{infrun.c}.

@section Single Stepping

@section Signal Handling

@section Thread Handling

@section Inferior Function Calls

@section Longjmp Support

GDB has support for figuring out that the target is doing a
@code{longjmp} and for stopping at the target of the jump, if we are
stepping.  This is done with a few specialized internal breakpoints,
which are visible in the @code{maint info breakpoint} command.

To make this work, you need to define a macro called
@code{GET_LONGJMP_TARGET}, which will examine the @code{jmp_buf}
structure and extract the longjmp target address.  Since @code{jmp_buf}
is target specific, you will need to define it in the appropriate
@file{tm-@var{xyz}.h} file.  Look in @file{tm-sun4os4.h} and
@file{sparc-tdep.c} for examples of how to do this.

@node User Interface

@chapter User Interface

GDB has several user interfaces.  Although the command-line interface
is the most common and most familiar, there are others.

@section Command Interpreter

The command interpreter in GDB is fairly simple.  It is designed to
allow for the set of commands to be augmented dynamically, and also
has a recursive subcommand capability, where the first argument to
a command may itself direct a lookup on a different command list.

For instance, the @code{set} command just starts a lookup on the
@code{setlist} command list, while @code{set thread} recurses
to the @code{set_thread_cmd_list}.

To add commands in general, use @code{add_cmd}.  @code{add_com} adds to
the main command list, and should be used for those commands.  The usual
place to add commands is in the @code{_initialize_@var{xyz}} routines at the
ends of most source files.

@section Console Printing

@section TUI

@section libgdb

@code{libgdb} was an abortive project of years ago.  The theory was to
provide an API to GDB's functionality.

@node Symbol Handling

@chapter Symbol Handling

Symbols are a key part of GDB's operation.  Symbols include variables,
functions, and types.

@section Symbol Reading

GDB reads symbols from ``symbol files''.  The usual symbol file is the
file containing the program which GDB is debugging.  GDB can be directed
to use a different file for symbols (with the @code{symbol-file}
command), and it can also read more symbols via the ``add-file'' and
``load'' commands, or while reading symbols from shared libraries.

Symbol files are initially opened by code in @file{symfile.c} using the
BFD library.  BFD identifies the type of the file by examining its
header.  @code{find_sym_fns} then uses this identification to locate a
set of symbol-reading functions.

Symbol reading modules identify themselves to GDB by calling
@code{add_symtab_fns} during their module initialization.  The argument
to @code{add_symtab_fns} is a @code{struct sym_fns} which contains the
name (or name prefix) of the symbol format, the length of the prefix,
and pointers to four functions.  These functions are called at various
times to process symbol-files whose identification matches the specified
prefix.

The functions supplied by each module are:

@table @code
@item @var{xyz}_symfile_init(struct sym_fns *sf)

Called from @code{symbol_file_add} when we are about to read a new
symbol file.  This function should clean up any internal state (possibly
resulting from half-read previous files, for example) and prepare to
read a new symbol file. Note that the symbol file which we are reading
might be a new "main" symbol file, or might be a secondary symbol file
whose symbols are being added to the existing symbol table.

The argument to @code{@var{xyz}_symfile_init} is a newly allocated
@code{struct sym_fns} whose @code{bfd} field contains the BFD for the
new symbol file being read.  Its @code{private} field has been zeroed,
and can be modified as desired.  Typically, a struct of private
information will be @code{malloc}'d, and a pointer to it will be placed
in the @code{private} field.

There is no result from @code{@var{xyz}_symfile_init}, but it can call
@code{error} if it detects an unavoidable problem.

@item @var{xyz}_new_init()

Called from @code{symbol_file_add} when discarding existing symbols.
This function need only handle the symbol-reading module's internal
state; the symbol table data structures visible to the rest of GDB will
be discarded by @code{symbol_file_add}.  It has no arguments and no
result.  It may be called after @code{@var{xyz}_symfile_init}, if a new
symbol table is being read, or may be called alone if all symbols are
simply being discarded.

@item @var{xyz}_symfile_read(struct sym_fns *sf, CORE_ADDR addr, int mainline)

Called from @code{symbol_file_add} to actually read the symbols from a
symbol-file into a set of psymtabs or symtabs.

@code{sf} points to the struct sym_fns originally passed to
@code{@var{xyz}_sym_init} for possible initialization.  @code{addr} is
the offset between the file's specified start address and its true
address in memory.  @code{mainline} is 1 if this is the main symbol
table being read, and 0 if a secondary symbol file (e.g. shared library
or dynamically loaded file) is being read.@refill
@end table

In addition, if a symbol-reading module creates psymtabs when
@var{xyz}_symfile_read is called, these psymtabs will contain a pointer
to a function @code{@var{xyz}_psymtab_to_symtab}, which can be called
from any point in the GDB symbol-handling code.

@table @code
@item @var{xyz}_psymtab_to_symtab (struct partial_symtab *pst)

Called from @code{psymtab_to_symtab} (or the PSYMTAB_TO_SYMTAB macro) if
the psymtab has not already been read in and had its @code{pst->symtab}
pointer set.  The argument is the psymtab to be fleshed-out into a
symtab.  Upon return, pst->readin should have been set to 1, and
pst->symtab should contain a pointer to the new corresponding symtab, or
zero if there were no symbols in that part of the symbol file.
@end table

@section Partial Symbol Tables

GDB has three types of symbol tables.

@itemize @bullet

@item full symbol tables (symtabs).  These contain the main information
about symbols and addresses.

@item partial symbol tables (psymtabs).  These contain enough
information to know when to read the corresponding part of the full
symbol table.

@item minimal symbol tables (msymtabs).  These contain information
gleaned from non-debugging symbols.

@end itemize

This section describes partial symbol tables.

A psymtab is constructed by doing a very quick pass over an executable
file's debugging information.  Small amounts of information are
extracted -- enough to identify which parts of the symbol table will
need to be re-read and fully digested later, when the user needs the
information.  The speed of this pass causes GDB to start up very
quickly.  Later, as the detailed rereading occurs, it occurs in small
pieces, at various times, and the delay therefrom is mostly invisible to
the user.
@c (@xref{Symbol Reading}.)

The symbols that show up in a file's psymtab should be, roughly, those
visible to the debugger's user when the program is not running code from
that file.  These include external symbols and types, static symbols and
types, and enum values declared at file scope.

The psymtab also contains the range of instruction addresses that the
full symbol table would represent.

The idea is that there are only two ways for the user (or much of the
code in the debugger) to reference a symbol:

@itemize @bullet

@item by its address
(e.g. execution stops at some address which is inside a function in this
file).  The address will be noticed to be in the range of this psymtab,
and the full symtab will be read in.  @code{find_pc_function},
@code{find_pc_line}, and other @code{find_pc_@dots{}} functions handle
this.

@item by its name
(e.g. the user asks to print a variable, or set a breakpoint on a
function).  Global names and file-scope names will be found in the
psymtab, which will cause the symtab to be pulled in.  Local names will
have to be qualified by a global name, or a file-scope name, in which
case we will have already read in the symtab as we evaluated the
qualifier.  Or, a local symbol can be referenced when we are "in" a
local scope, in which case the first case applies.  @code{lookup_symbol}
does most of the work here.

@end itemize

The only reason that psymtabs exist is to cause a symtab to be read in
at the right moment.  Any symbol that can be elided from a psymtab,
while still causing that to happen, should not appear in it.  Since
psymtabs don't have the idea of scope, you can't put local symbols in
them anyway.  Psymtabs don't have the idea of the type of a symbol,
either, so types need not appear, unless they will be referenced by
name.

It is a bug for GDB to behave one way when only a psymtab has been read,
and another way if the corresponding symtab has been read in.  Such bugs
are typically caused by a psymtab that does not contain all the visible
symbols, or which has the wrong instruction address ranges.

The psymtab for a particular section of a symbol-file (objfile) could be
thrown away after the symtab has been read in.  The symtab should always
be searched before the psymtab, so the psymtab will never be used (in a
bug-free environment).  Currently, psymtabs are allocated on an obstack,
and all the psymbols themselves are allocated in a pair of large arrays
on an obstack, so there is little to be gained by trying to free them
unless you want to do a lot more work.

@section Types

Fundamental Types (e.g., FT_VOID, FT_BOOLEAN).

These are the fundamental types that GDB uses internally.  Fundamental
types from the various debugging formats (stabs, ELF, etc) are mapped
into one of these.  They are basically a union of all fundamental types
that gdb knows about for all the languages that GDB knows about.

Type Codes (e.g., TYPE_CODE_PTR, TYPE_CODE_ARRAY).

Each time GDB builds an internal type, it marks it with one of these
types.  The type may be a fundamental type, such as TYPE_CODE_INT, or a
derived type, such as TYPE_CODE_PTR which is a pointer to another type.
Typically, several FT_* types map to one TYPE_CODE_* type, and are
distinguished by other members of the type struct, such as whether the
type is signed or unsigned, and how many bits it uses.

Builtin Types (e.g., builtin_type_void, builtin_type_char).

These are instances of type structs that roughly correspond to
fundamental types and are created as global types for GDB to use for
various ugly historical reasons.  We eventually want to eliminate these.
Note for example that builtin_type_int initialized in gdbtypes.c is
basically the same as a TYPE_CODE_INT type that is initialized in
c-lang.c for an FT_INTEGER fundamental type.  The difference is that the
builtin_type is not associated with any particular objfile, and only one
instance exists, while c-lang.c builds as many TYPE_CODE_INT types as
needed, with each one associated with some particular objfile.

@section Object File Formats

@subsection a.out

The @file{a.out} format is the original file format for Unix.  It
consists of three sections: text, data, and bss, which are for program
code, initialized data, and uninitialized data, respectively.

The @file{a.out} format is so simple that it doesn't have any reserved
place for debugging information.  (Hey, the original Unix hackers used
@file{adb}, which is a machine-language debugger.)  The only debugging
format for @file{a.out} is stabs, which is encoded as a set of normal
symbols with distinctive attributes.

The basic @file{a.out} reader is in @file{dbxread.c}.

@subsection COFF

The COFF format was introduced with System V Release 3 (SVR3) Unix.
COFF files may have multiple sections, each prefixed by a header.  The
number of sections is limited.

The COFF specification includes support for debugging.  Although this
was a step forward, the debugging information was woefully limited.  For
instance, it was not possible to represent code that came from an
included file.

The COFF reader is in @file{coffread.c}.

@subsection ECOFF

ECOFF is an extended COFF originally introduced for Mips and Alpha
workstations.

The basic ECOFF reader is in @file{mipsread.c}.

@subsection XCOFF

The IBM RS/6000 running AIX uses an object file format called XCOFF.
The COFF sections, symbols, and line numbers are used, but debugging
symbols are dbx-style stabs whose strings are located in the
@samp{.debug} section (rather than the string table).  For more
information, see @xref{Top,,,stabs,The Stabs Debugging Format}.

The shared library scheme has a clean interface for figuring out what
shared libraries are in use, but the catch is that everything which
refers to addresses (symbol tables and breakpoints at least) needs to be
relocated for both shared libraries and the main executable.  At least
using the standard mechanism this can only be done once the program has
been run (or the core file has been read).

@subsection PE

Windows 95 and NT use the PE (Portable Executable) format for their
executables.  PE is basically COFF with additional headers.

While BFD includes special PE support, GDB needs only the basic
COFF reader.

@subsection ELF

The ELF format came with System V Release 4 (SVR4) Unix.  ELF is similar
to COFF in being organized into a number of sections, but it removes
many of COFF's limitations.

The basic ELF reader is in @file{elfread.c}.

@subsection SOM

SOM is HP's object file and debug format (not to be confused with IBM's
SOM, which is a cross-language ABI).

The SOM reader is in @file{hpread.c}.

@subsection Other File Formats

Other file formats that have been supported by GDB include Netware
Loadable Modules (@file{nlmread.c}.

@section Debugging File Formats

This section describes characteristics of debugging information that
are independent of the object file format.

@subsection stabs

@code{stabs} started out as special symbols within the @code{a.out}
format.  Since then, it has been encapsulated into other file
formats, such as COFF and ELF.

While @file{dbxread.c} does some of the basic stab processing,
including for encapsulated versions, @file{stabsread.c} does
the real work.

@subsection COFF

The basic COFF definition includes debugging information.  The level
of support is minimal and non-extensible, and is not often used.

@subsection Mips debug (Third Eye)

ECOFF includes a definition of a special debug format.

The file @file{mdebugread.c} implements reading for this format.

@subsection DWARF 1

DWARF 1 is a debugging format that was originally designed to be
used with ELF in SVR4 systems.

@c CHILL_PRODUCER
@c GCC_PRODUCER
@c GPLUS_PRODUCER
@c LCC_PRODUCER
@c If defined, these are the producer strings in a DWARF 1 file.  All of
@c these have reasonable defaults already.

The DWARF 1 reader is in @file{dwarfread.c}.

@subsection DWARF 2

DWARF 2 is an improved but incompatible version of DWARF 1.

The DWARF 2 reader is in @file{dwarf2read.c}.

@subsection SOM

Like COFF, the SOM definition includes debugging information.

@section Adding a New Symbol Reader to GDB

If you are using an existing object file format (a.out, COFF, ELF, etc),
there is probably little to be done.

If you need to add a new object file format, you must first add it to
BFD.  This is beyond the scope of this document.

You must then arrange for the BFD code to provide access to the
debugging symbols.  Generally GDB will have to call swapping routines
from BFD and a few other BFD internal routines to locate the debugging
information.  As much as possible, GDB should not depend on the BFD
internal data structures.

For some targets (e.g., COFF), there is a special transfer vector used
to call swapping routines, since the external data structures on various
platforms have different sizes and layouts.  Specialized routines that
will only ever be implemented by one object file format may be called
directly.  This interface should be described in a file
@file{bfd/libxyz.h}, which is included by GDB.


@node Language Support

@chapter Language Support

GDB's language support is mainly driven by the symbol reader, although
it is possible for the user to set the source language manually.

GDB chooses the source language by looking at the extension of the file
recorded in the debug info; @code{.c} means C, @code{.f} means Fortran,
etc.  It may also use a special-purpose language identifier if the debug
format supports it, such as DWARF.

@section Adding a Source Language to GDB

To add other languages to GDB's expression parser, follow the following
steps:

@table @emph
@item Create the expression parser.

This should reside in a file @file{@var{lang}-exp.y}.  Routines for
building parsed expressions into a @samp{union exp_element} list are in
@file{parse.c}.

Since we can't depend upon everyone having Bison, and YACC produces
parsers that define a bunch of global names, the following lines
@emph{must} be included at the top of the YACC parser, to prevent the
various parsers from defining the same global names:

@example
#define yyparse 	@var{lang}_parse
#define yylex 	@var{lang}_lex
#define yyerror 	@var{lang}_error
#define yylval 	@var{lang}_lval
#define yychar 	@var{lang}_char
#define yydebug 	@var{lang}_debug
#define yypact  	@var{lang}_pact 
#define yyr1		@var{lang}_r1   
#define yyr2		@var{lang}_r2   
#define yydef		@var{lang}_def  
#define yychk		@var{lang}_chk  
#define yypgo		@var{lang}_pgo  
#define yyact  	@var{lang}_act  
#define yyexca  	@var{lang}_exca
#define yyerrflag  	@var{lang}_errflag
#define yynerrs  	@var{lang}_nerrs
@end example

At the bottom of your parser, define a @code{struct language_defn} and
initialize it with the right values for your language.  Define an
@code{initialize_@var{lang}} routine and have it call
@samp{add_language(@var{lang}_language_defn)} to tell the rest of GDB
that your language exists.  You'll need some other supporting variables
and functions, which will be used via pointers from your
@code{@var{lang}_language_defn}.  See the declaration of @code{struct
language_defn} in @file{language.h}, and the other @file{*-exp.y} files,
for more information.

@item Add any evaluation routines, if necessary

If you need new opcodes (that represent the operations of the language),
add them to the enumerated type in @file{expression.h}.  Add support
code for these operations in @code{eval.c:evaluate_subexp()}.  Add cases
for new opcodes in two functions from @file{parse.c}:
@code{prefixify_subexp()} and @code{length_of_subexp()}.  These compute
the number of @code{exp_element}s that a given operation takes up.

@item Update some existing code

Add an enumerated identifier for your language to the enumerated type
@code{enum language} in @file{defs.h}.

Update the routines in @file{language.c} so your language is included.
These routines include type predicates and such, which (in some cases)
are language dependent.  If your language does not appear in the switch
statement, an error is reported.

Also included in @file{language.c} is the code that updates the variable
@code{current_language}, and the routines that translate the
@code{language_@var{lang}} enumerated identifier into a printable
string.

Update the function @code{_initialize_language} to include your
language.  This function picks the default language upon startup, so is
dependent upon which languages that GDB is built for.

Update @code{allocate_symtab} in @file{symfile.c} and/or symbol-reading
code so that the language of each symtab (source file) is set properly.
This is used to determine the language to use at each stack frame level.
Currently, the language is set based upon the extension of the source
file.  If the language can be better inferred from the symbol
information, please set the language of the symtab in the symbol-reading
code.

Add helper code to @code{expprint.c:print_subexp()} to handle any new
expression opcodes you have added to @file{expression.h}.  Also, add the
printed representations of your operators to @code{op_print_tab}.

@item Add a place of call

Add a call to @code{@var{lang}_parse()} and @code{@var{lang}_error} in
@code{parse.c:parse_exp_1()}.

@item Use macros to trim code

The user has the option of building GDB for some or all of the
languages.  If the user decides to build GDB for the language
@var{lang}, then every file dependent on @file{language.h} will have the
macro @code{_LANG_@var{lang}} defined in it.  Use @code{#ifdef}s to
leave out large routines that the user won't need if he or she is not
using your language.

Note that you do not need to do this in your YACC parser, since if GDB
is not build for @var{lang}, then @file{@var{lang}-exp.tab.o} (the
compiled form of your parser) is not linked into GDB at all.

See the file @file{configure.in} for how GDB is configured for different
languages.

@item Edit @file{Makefile.in}

Add dependencies in @file{Makefile.in}.  Make sure you update the macro
variables such as @code{HFILES} and @code{OBJS}, otherwise your code may
not get linked in, or, worse yet, it may not get @code{tar}red into the
distribution!

@end table


@node Host Definition

@chapter Host Definition

With the advent of autoconf, it's rarely necessary to have host
definition machinery anymore.

@section Adding a New Host

Most of GDB's host configuration support happens via autoconf.  It
should be rare to need new host-specific definitions.  GDB still uses
the host-specific definitions and files listed below, but these mostly
exist for historical reasons, and should eventually disappear.

Several files control GDB's configuration for host systems:

@table @file

@item gdb/config/@var{arch}/@var{xyz}.mh
Specifies Makefile fragments needed when hosting on machine @var{xyz}.
In particular, this lists the required machine-dependent object files,
by defining @samp{XDEPFILES=@dots{}}.  Also specifies the header file
which describes host @var{xyz}, by defining @code{XM_FILE=
xm-@var{xyz}.h}.  You can also define @code{CC}, @code{SYSV_DEFINE},
@code{XM_CFLAGS}, @code{XM_ADD_FILES}, @code{XM_CLIBS}, @code{XM_CDEPS},
etc.; see @file{Makefile.in}.

@item gdb/config/@var{arch}/xm-@var{xyz}.h
(@file{xm.h} is a link to this file, created by configure).  Contains C
macro definitions describing the host system environment, such as byte
order, host C compiler and library.

@item gdb/@var{xyz}-xdep.c
Contains any miscellaneous C code required for this machine as a host.
On most machines it doesn't exist at all.  If it does exist, put
@file{@var{xyz}-xdep.o} into the @code{XDEPFILES} line in
@file{gdb/config/@var{arch}/@var{xyz}.mh}.

@end table

@subheading Generic Host Support Files

There are some ``generic'' versions of routines that can be used by
various systems.  These can be customized in various ways by macros
defined in your @file{xm-@var{xyz}.h} file.  If these routines work for
the @var{xyz} host, you can just include the generic file's name (with
@samp{.o}, not @samp{.c}) in @code{XDEPFILES}.

Otherwise, if your machine needs custom support routines, you will need
to write routines that perform the same functions as the generic file.
Put them into @code{@var{xyz}-xdep.c}, and put @code{@var{xyz}-xdep.o}
into @code{XDEPFILES}.

@table @file

@item ser-unix.c
This contains serial line support for Unix systems.  This is always
included, via the makefile variable @code{SER_HARDWIRE}; override this
variable in the @file{.mh} file to avoid it.

@item ser-go32.c
This contains serial line support for 32-bit programs running under DOS,
using the GO32 execution environment.

@item ser-tcp.c
This contains generic TCP support using sockets.

@end table

@section Host Conditionals

When GDB is configured and compiled, various macros are defined or left
undefined, to control compilation based on the attributes of the host
system.  These macros and their meanings (or if the meaning is not
documented here, then one of the source files where they are used is
indicated) are:

@table @code

@item GDBINIT_FILENAME
The default name of GDB's initialization file (normally @file{.gdbinit}).

@item MEM_FNS_DECLARED
Your host config file defines this if it includes declarations of
@code{memcpy} and @code{memset}.  Define this to avoid conflicts between
the native include files and the declarations in @file{defs.h}.

@item NO_STD_REGS
This macro is deprecated.

@item NO_SYS_FILE
Define this if your system does not have a @code{<sys/file.h>}.

@item SIGWINCH_HANDLER
If your host defines @code{SIGWINCH}, you can define this to be the name
of a function to be called if @code{SIGWINCH} is received.

@item SIGWINCH_HANDLER_BODY
Define this to expand into code that will define the function named by
the expansion of @code{SIGWINCH_HANDLER}.

@item ALIGN_STACK_ON_STARTUP
Define this if your system is of a sort that will crash in
@code{tgetent} if the stack happens not to be longword-aligned when
@code{main} is called.  This is a rare situation, but is known to occur
on several different types of systems.

@item CRLF_SOURCE_FILES
Define this if host files use @code{\r\n} rather than @code{\n} as a
line terminator.  This will cause source file listings to omit @code{\r}
characters when printing and it will allow \r\n line endings of files
which are "sourced" by gdb.  It must be possible to open files in binary
mode using @code{O_BINARY} or, for fopen, @code{"rb"}.

@item DEFAULT_PROMPT
The default value of the prompt string (normally @code{"(gdb) "}).

@item DEV_TTY
The name of the generic TTY device, defaults to @code{"/dev/tty"}.

@item FCLOSE_PROVIDED
Define this if the system declares @code{fclose} in the headers included
in @code{defs.h}.  This isn't needed unless your compiler is unusually
anal.

@item FOPEN_RB
Define this if binary files are opened the same way as text files.

@item GETENV_PROVIDED
Define this if the system declares @code{getenv} in its headers included
in @code{defs.h}. This isn't needed unless your compiler is unusually
anal.

@item HAVE_MMAP
In some cases, use the system call @code{mmap} for reading symbol
tables.  For some machines this allows for sharing and quick updates.

@item HAVE_SIGSETMASK
Define this if the host system has job control, but does not define
@code{sigsetmask()}.  Currently, this is only true of the RS/6000.

@item HAVE_TERMIO
Define this if the host system has @code{termio.h}.

@item HOST_BYTE_ORDER
The ordering of bytes in the host.  This must be defined to be either
@code{BIG_ENDIAN} or @code{LITTLE_ENDIAN}.

@item INT_MAX
@item INT_MIN
@item LONG_MAX
@item UINT_MAX
@item ULONG_MAX
Values for host-side constants.

@item ISATTY
Substitute for isatty, if not available.

@item LONGEST
This is the longest integer type available on the host.  If not defined,
it will default to @code{long long} or @code{long}, depending on
@code{CC_HAS_LONG_LONG}.

@item CC_HAS_LONG_LONG
Define this if the host C compiler supports ``long long''.  This is set
by the configure script.

@item PRINTF_HAS_LONG_LONG
Define this if the host can handle printing of long long integers via
the printf format directive ``ll''. This is set by the configure script.

@item HAVE_LONG_DOUBLE
Define this if the host C compiler supports ``long double''.  This is
set by the configure script.

@item PRINTF_HAS_LONG_DOUBLE
Define this if the host can handle printing of long double float-point
numbers via the printf format directive ``Lg''. This is set by the
configure script.

@item SCANF_HAS_LONG_DOUBLE
Define this if the host can handle the parsing of long double
float-point numbers via the scanf format directive directive
``Lg''. This is set by the configure script.

@item LSEEK_NOT_LINEAR
Define this if @code{lseek (n)} does not necessarily move to byte number
@code{n} in the file.  This is only used when reading source files.  It
is normally faster to define @code{CRLF_SOURCE_FILES} when possible.

@item L_SET
This macro is used as the argument to lseek (or, most commonly,
bfd_seek).  FIXME, should be replaced by SEEK_SET instead, which is the
POSIX equivalent.

@item MALLOC_INCOMPATIBLE
Define this if the system's prototype for @code{malloc} differs from the
@sc{ANSI} definition.

@item MMAP_BASE_ADDRESS
When using HAVE_MMAP, the first mapping should go at this address.

@item MMAP_INCREMENT
when using HAVE_MMAP, this is the increment between mappings.

@item NEED_POSIX_SETPGID
Define this to use the POSIX version of @code{setpgid} to determine
whether job control is available.

@item NORETURN
If defined, this should be one or more tokens, such as @code{volatile},
that can be used in both the declaration and definition of functions to
indicate that they never return.  The default is already set correctly
if compiling with GCC.  This will almost never need to be defined.

@item ATTR_NORETURN
If defined, this should be one or more tokens, such as
@code{__attribute__ ((noreturn))}, that can be used in the declarations
of functions to indicate that they never return.  The default is already
set correctly if compiling with GCC.  This will almost never need to be
defined.

@item USE_GENERIC_DUMMY_FRAMES
Define this to 1 if the target is using the generic inferior function
call code.  See @code{blockframe.c} for more information.

@item USE_MMALLOC
GDB will use the @code{mmalloc} library for memory allocation for symbol
reading if this symbol is defined.  Be careful defining it since there
are systems on which @code{mmalloc} does not work for some reason.  One
example is the DECstation, where its RPC library can't cope with our
redefinition of @code{malloc} to call @code{mmalloc}.  When defining
@code{USE_MMALLOC}, you will also have to set @code{MMALLOC} in the
Makefile, to point to the mmalloc library.  This define is set when you
configure with --with-mmalloc.

@item NO_MMCHECK
Define this if you are using @code{mmalloc}, but don't want the overhead
of checking the heap with @code{mmcheck}.  Note that on some systems,
the C runtime makes calls to malloc prior to calling @code{main}, and if
@code{free} is ever called with these pointers after calling
@code{mmcheck} to enable checking, a memory corruption abort is certain
to occur.  These systems can still use mmalloc, but must define
NO_MMCHECK.

@item MMCHECK_FORCE
Define this to 1 if the C runtime allocates memory prior to
@code{mmcheck} being called, but that memory is never freed so we don't
have to worry about it triggering a memory corruption abort.  The
default is 0, which means that @code{mmcheck} will only install the heap
checking functions if there has not yet been any memory allocation
calls, and if it fails to install the functions, gdb will issue a
warning.  This is currently defined if you configure using
--with-mmalloc.

@item NO_SIGINTERRUPT
Define this to indicate that siginterrupt() is not available.

@item R_OK
Define if this is not in a system .h file.

@item SEEK_CUR
@item SEEK_SET
Define these to appropriate value for the system lseek(), if not already
defined.

@item STOP_SIGNAL
This is the signal for stopping GDB.  Defaults to SIGTSTP.  (Only
redefined for the Convex.)

@item USE_O_NOCTTY
Define this if the interior's tty should be opened with the O_NOCTTY
flag.  (FIXME: This should be a native-only flag, but @file{inflow.c} is
always linked in.)

@item USG
Means that System V (prior to SVR4) include files are in use.  (FIXME:
This symbol is abused in @file{infrun.c}, @file{regex.c},
@file{remote-nindy.c}, and @file{utils.c} for other things, at the
moment.)

@item lint
Define this to help placate lint in some situations.

@item volatile
Define this to override the defaults of @code{__volatile__} or
@code{/**/}.

@end table


@node Target Architecture Definition

@chapter Target Architecture Definition

GDB's target architecture defines what sort of machine-language programs
GDB can work with, and how it works with them.

At present, the target architecture definition consists of a number of C
macros.

@section Registers and Memory

GDB's model of the target machine is rather simple.  GDB assumes the
machine includes a bank of registers and a block of memory.  Each
register may have a different size.

GDB does not have a magical way to match up with the compiler's idea of
which registers are which; however, it is critical that they do match up
accurately.  The only way to make this work is to get accurate
information about the order that the compiler uses, and to reflect that
in the @code{REGISTER_NAME} and related macros.

GDB can handle big-endian, little-endian, and bi-endian architectures.

@section Frame Interpretation

@section Inferior Call Setup

@section Compiler Characteristics

@section Target Conditionals

This section describes the macros that you can use to define the target
machine.

@table @code

@item ADDITIONAL_OPTIONS
@item ADDITIONAL_OPTION_CASES
@item ADDITIONAL_OPTION_HANDLER
@item ADDITIONAL_OPTION_HELP
These are a set of macros that allow the addition of additional command
line options to GDB.  They are currently used only for the unsupported
i960 Nindy target, and should not be used in any other configuration.

@item ADDR_BITS_REMOVE (addr)
If a raw machine instruction address includes any bits that are not
really part of the address, then define this macro to expand into an
expression that zeros those bits in @var{addr}.  This is only used for
addresses of instructions, and even then not in all contexts.

For example, the two low-order bits of the PC on the Hewlett-Packard PA
2.0 architecture contain the privilege level of the corresponding
instruction.  Since instructions must always be aligned on four-byte
boundaries, the processor masks out these bits to generate the actual
address of the instruction.  ADDR_BITS_REMOVE should filter out these
bits with an expression such as @code{((addr) & ~3)}.

@item BEFORE_MAIN_LOOP_HOOK
Define this to expand into any code that you want to execute before the
main loop starts.  Although this is not, strictly speaking, a target
conditional, that is how it is currently being used.  Note that if a
configuration were to define it one way for a host and a different way
for the target, GDB will probably not compile, let alone run correctly.
This is currently used only for the unsupported i960 Nindy target, and
should not be used in any other configuration.

@item BELIEVE_PCC_PROMOTION
Define if the compiler promotes a short or char parameter to an int, but
still reports the parameter as its original type, rather than the
promoted type.

@item BELIEVE_PCC_PROMOTION_TYPE
Define this if GDB should believe the type of a short argument when
compiled by pcc, but look within a full int space to get its value.
Only defined for Sun-3 at present.

@item BITS_BIG_ENDIAN
Define this if the numbering of bits in the targets does *not* match the
endianness of the target byte order.  A value of 1 means that the bits
are numbered in a big-endian order, 0 means little-endian.

@item BREAKPOINT
This is the character array initializer for the bit pattern to put into
memory where a breakpoint is set.  Although it's common to use a trap
instruction for a breakpoint, it's not required; for instance, the bit
pattern could be an invalid instruction.  The breakpoint must be no
longer than the shortest instruction of the architecture.

@var{BREAKPOINT} has been deprecated in favour of
@var{BREAKPOINT_FROM_PC}.

@item BIG_BREAKPOINT
@item LITTLE_BREAKPOINT
Similar to BREAKPOINT, but used for bi-endian targets.

@var{BIG_BREAKPOINT} and @var{LITTLE_BREAKPOINT} have been deprecated in
favour of @var{BREAKPOINT_FROM_PC}.

@item REMOTE_BREAKPOINT
@item LITTLE_REMOTE_BREAKPOINT
@item BIG_REMOTE_BREAKPOINT
Similar to BREAKPOINT, but used for remote targets.

@var{BIG_REMOTE_BREAKPOINT} and @var{LITTLE_REMOTE_BREAKPOINT} have been
deprecated in favour of @var{BREAKPOINT_FROM_PC}.

@item BREAKPOINT_FROM_PC (pcptr, lenptr)

Use the program counter to determine the contents and size of a
breakpoint instruction.  It returns a pointer to a string of bytes that
encode a breakpoint instruction, stores the length of the string to
*lenptr, and adjusts pc (if necessary) to point to the actual memory
location where the breakpoint should be inserted.

Although it is common to use a trap instruction for a breakpoint, it's
not required; for instance, the bit pattern could be an invalid
instruction.  The breakpoint must be no longer than the shortest
instruction of the architecture.

Replaces all the other @var{BREAKPOINT} macros.

@item MEMORY_INSERT_BREAKPOINT (addr, contents_cache)
@item MEMORY_REMOVE_BREAKPOINT (addr, contents_cache)

Insert or remove memory based breakpoints.  Reasonable defaults
(@code{default_memory_insert_breakpoint} and
@code{default_memory_remove_breakpoint} respectively) have been
provided so that it is not necessary to define these for most
architectures.  Architectures which may want to define
@var{MEMORY_INSERT_BREAKPOINT} and @var{MEMORY_REMOVE_BREAKPOINT} will
likely have instructions that are oddly sized or are not stored in a
conventional manner.

It may also be desirable (from an efficiency standpoint) to define
custom breakpoint insertion and removal routines if
@var{BREAKPOINT_FROM_PC} needs to read the target's memory for some
reason.

@item CALL_DUMMY_P
A C expresson that is non-zero when the target suports inferior function
calls.

@item CALL_DUMMY_WORDS
Pointer to an array of @var{LONGEST} words of data containing
host-byte-ordered @var{REGISTER_BYTES} sized values that partially
specify the sequence of instructions needed for an inferior function
call.

Should be deprecated in favour of a macro that uses target-byte-ordered
data.

@item SIZEOF_CALL_DUMMY_WORDS
The size of @var{CALL_DUMMY_WORDS}.  When @var{CALL_DUMMY_P} this must
return a positive value.  See also @var{CALL_DUMMY_LENGTH}.

@item CALL_DUMMY
A static initializer for @var{CALL_DUMMY_WORDS}.  Deprecated.

@item CALL_DUMMY_LOCATION
inferior.h

@item CALL_DUMMY_STACK_ADJUST
Stack adjustment needed when performing an inferior function call.

Should be deprecated in favor of something like @var{STACK_ALIGN}.

@item CALL_DUMMY_STACK_ADJUST_P
Predicate for use of @var{CALL_DUMMY_STACK_ADJUST}.

Should be deprecated in favor of something like @var{STACK_ALIGN}.

@item CANNOT_FETCH_REGISTER (regno)
A C expression that should be nonzero if @var{regno} cannot be fetched
from an inferior process.  This is only relevant if
@code{FETCH_INFERIOR_REGISTERS} is not defined.

@item CANNOT_STORE_REGISTER (regno)
A C expression that should be nonzero if @var{regno} should not be
written to the target.  This is often the case for program counters,
status words, and other special registers.  If this is not defined, GDB
will assume that all registers may be written.

@item DO_DEFERRED_STORES
@item CLEAR_DEFERRED_STORES
Define this to execute any deferred stores of registers into the inferior,
and to cancel any deferred stores.

Currently only implemented correctly for native Sparc configurations?

@item COERCE_FLOAT_TO_DOUBLE (@var{formal}, @var{actual})
If we are calling a function by hand, and the function was declared
(according to the debug info) without a prototype, should we
automatically promote floats to doubles?  This macro must evaluate to
non-zero if we should, or zero if we should leave the value alone.

The argument @var{actual} is the type of the value we want to pass to
the function.  The argument @var{formal} is the type of this argument,
as it appears in the function's definition.  Note that @var{formal} may
be zero if we have no debugging information for the function, or if
we're passing more arguments than are officially declared (for example,
varargs).  This macro is never invoked if the function definitely has a
prototype.

The default behavior is to promote only when we have no type information
for the formal parameter.  This is different from the obvious behavior,
which would be to promote whenever we have no prototype, just as the
compiler does.  It's annoying, but some older targets rely on this.  If
you want GDB to follow the typical compiler behavior --- to always
promote when there is no prototype in scope --- your gdbarch init
function can call @code{set_gdbarch_coerce_float_to_double} and select
the @code{standard_coerce_float_to_double} function.

@item CPLUS_MARKER
Define this to expand into the character that G++ uses to distinguish
compiler-generated identifiers from programmer-specified identifiers.
By default, this expands into @code{'$'}.  Most System V targets should
define this to @code{'.'}.

@item DBX_PARM_SYMBOL_CLASS
Hook for the @code{SYMBOL_CLASS} of a parameter when decoding DBX symbol
information.  In the i960, parameters can be stored as locals or as
args, depending on the type of the debug record.

@item DECR_PC_AFTER_BREAK
Define this to be the amount by which to decrement the PC after the
program encounters a breakpoint.  This is often the number of bytes in
BREAKPOINT, though not always.  For most targets this value will be 0.

@item DECR_PC_AFTER_HW_BREAK
Similarly, for hardware breakpoints.

@item DISABLE_UNSETTABLE_BREAK addr
If defined, this should evaluate to 1 if @var{addr} is in a shared
library in which breakpoints cannot be set and so should be disabled.

@item DO_REGISTERS_INFO
If defined, use this to print the value of a register or all registers.

@item END_OF_TEXT_DEFAULT
This is an expression that should designate the end of the text section
(? FIXME ?)

@item EXTRACT_RETURN_VALUE(type,regbuf,valbuf)
Define this to extract a function's return value of type @var{type} from
the raw register state @var{regbuf} and copy that, in virtual format,
into @var{valbuf}.
 
@item EXTRACT_STRUCT_VALUE_ADDRESS(regbuf)
When @var{EXTRACT_STRUCT_VALUE_ADDRESS_P} this is used to to extract
from an array @var{regbuf} (containing the raw register state) the
address in which a function should return its structure value, as a
CORE_ADDR (or an expression that can be used as one).

@item EXTRACT_STRUCT_VALUE_ADDRESS_P
Predicate for @var{EXTRACT_STRUCT_VALUE_ADDRESS}.

@item FLOAT_INFO
If defined, then the `info float' command will print information about
the processor's floating point unit.

@item FP_REGNUM
If the virtual frame pointer is kept in a register, then define this
macro to be the number (greater than or equal to zero) of that register.

This should only need to be defined if @code{TARGET_READ_FP} and
@code{TARGET_WRITE_FP} are not defined.

@item FRAMELESS_FUNCTION_INVOCATION(fi)
Define this to an expression that returns 1 if the function invocation
represented by @var{fi} does not have a stack frame associated with it.
Otherwise return 0.

@item FRAME_ARGS_ADDRESS_CORRECT
stack.c

@item FRAME_CHAIN(frame)
Given @var{frame}, return a pointer to the calling frame.

@item FRAME_CHAIN_COMBINE(chain,frame)
Define this to take the frame chain pointer and the frame's nominal
address and produce the nominal address of the caller's frame.
Presently only defined for HP PA.

@item FRAME_CHAIN_VALID(chain,thisframe)

Define this to be an expression that returns zero if the given frame is
an outermost frame, with no caller, and nonzero otherwise.  Several
common definitions are available. 

@code{file_frame_chain_valid} is nonzero if the chain pointer is nonzero
and given frame's PC is not inside the startup file (such as
@file{crt0.o}).  @code{func_frame_chain_valid} is nonzero if the chain
pointer is nonzero and the given frame's PC is not in @code{main()} or a
known entry point function (such as @code{_start()}).
@code{generic_file_frame_chain_valid} and
@code{generic_func_frame_chain_valid} are equivalent implementations for
targets using generic dummy frames.

@item FRAME_INIT_SAVED_REGS(frame)
See @file{frame.h}.  Determines the address of all registers in the
current stack frame storing each in @code{frame->saved_regs}.  Space for
@code{frame->saved_regs} shall be allocated by
@code{FRAME_INIT_SAVED_REGS} using either
@code{frame_saved_regs_zalloc} or @code{frame_obstack_alloc}.

@var{FRAME_FIND_SAVED_REGS} and @var{EXTRA_FRAME_INFO} are deprecated.

@item FRAME_NUM_ARGS (fi)
For the frame described by @var{fi} return the number of arguments that
are being passed.  If the number of arguments is not known, return
@code{-1}.

@item FRAME_SAVED_PC(frame)
Given @var{frame}, return the pc saved there.  That is, the return
address.

@item FUNCTION_EPILOGUE_SIZE
For some COFF targets, the @code{x_sym.x_misc.x_fsize} field of the
function end symbol is 0.  For such targets, you must define
@code{FUNCTION_EPILOGUE_SIZE} to expand into the standard size of a
function's epilogue.

@item FUNCTION_START_OFFSET
An integer, giving the offset in bytes from a function's address (as
used in the values of symbols, function pointers, etc.), and the
function's first genuine instruction.

This is zero on almost all machines: the function's address is usually
the address of its first instruction.  However, on the VAX, for example,
each function starts with two bytes containing a bitmask indicating
which registers to save upon entry to the function.  The VAX @code{call}
instructions check this value, and save the appropriate registers
automatically.  Thus, since the offset from the function's address to
its first instruction is two bytes, @code{FUNCTION_START_OFFSET} would
be 2 on the VAX.

@item GCC_COMPILED_FLAG_SYMBOL
@item GCC2_COMPILED_FLAG_SYMBOL
If defined, these are the names of the symbols that GDB will look for to
detect that GCC compiled the file.  The default symbols are
@code{gcc_compiled.} and @code{gcc2_compiled.}, respectively. (Currently
only defined for the Delta 68.)

@item GDB_MULTI_ARCH
If defined and non-zero, enables suport for multiple architectures
within GDB.

The support can be enabled at two levels.  At level one, only
definitions for previously undefined macros are provided; at level two,
a multi-arch definition of all architecture dependant macros will be
defined.

@item GDB_TARGET_IS_HPPA
This determines whether horrible kludge code in dbxread.c and
partial-stab.h is used to mangle multiple-symbol-table files from
HPPA's.  This should all be ripped out, and a scheme like elfread.c
used.

@item GET_LONGJMP_TARGET
For most machines, this is a target-dependent parameter.  On the
DECstation and the Iris, this is a native-dependent parameter, since
<setjmp.h> is needed to define it.

This macro determines the target PC address that longjmp() will jump to,
assuming that we have just stopped at a longjmp breakpoint.  It takes a
CORE_ADDR * as argument, and stores the target PC value through this
pointer.  It examines the current state of the machine as needed.

@item GET_SAVED_REGISTER
Define this if you need to supply your own definition for the function
@code{get_saved_register}.

@item HAVE_REGISTER_WINDOWS
Define this if the target has register windows.
@item REGISTER_IN_WINDOW_P (regnum)
Define this to be an expression that is 1 if the given register is in
the window.

@item IBM6000_TARGET
Shows that we are configured for an IBM RS/6000 target.  This
conditional should be eliminated (FIXME) and replaced by
feature-specific macros.  It was introduced in haste and we are
repenting at leisure.

@item SYMBOLS_CAN_START_WITH_DOLLAR
Some systems have routines whose names start with @samp{$}.  Giving this
macro a non-zero value tells GDB's expression parser to check for such
routines when parsing tokens that begin with @samp{$}.

On HP-UX, certain system routines (millicode) have names beginning with
@samp{$} or @samp{$$}.  For example, @code{$$dyncall} is a millicode
routine that handles inter-space procedure calls on PA-RISC.

@item IEEE_FLOAT
Define this if the target system uses IEEE-format floating point numbers.

@item INIT_EXTRA_FRAME_INFO (fromleaf, frame)
If additional information about the frame is required this should be
stored in @code{frame->extra_info}.  Space for @code{frame->extra_info}
is allocated using @code{frame_obstack_alloc}.

@item INIT_FRAME_PC (fromleaf, prev)
This is a C statement that sets the pc of the frame pointed to by
@var{prev}.  [By default...]

@item INNER_THAN (lhs,rhs)
Returns non-zero if stack address @var{lhs} is inner than (nearer to the
stack top) stack address @var{rhs}. Define this as @code{lhs < rhs} if
the target's stack grows downward in memory, or @code{lhs > rsh} if the
stack grows upward.

@item IN_SIGTRAMP (pc, name)
Define this to return true if the given @var{pc} and/or @var{name}
indicates that the current function is a sigtramp.

@item SIGTRAMP_START (pc)
@item SIGTRAMP_END (pc)
Define these to be the start and end address of the sigtramp for the
given @var{pc}.  On machines where the address is just a compile time
constant, the macro expansion will typically just ignore the supplied
@var{pc}.

@item IN_SOLIB_CALL_TRAMPOLINE pc name
Define this to evaluate to nonzero if the program is stopped in the
trampoline that connects to a shared library.

@item IN_SOLIB_RETURN_TRAMPOLINE pc name
Define this to evaluate to nonzero if the program is stopped in the
trampoline that returns from a shared library.

@item IN_SOLIB_DYNSYM_RESOLVE_CODE pc
Define this to evaluate to nonzero if the program is stopped in the
dynamic linker.

@item SKIP_SOLIB_RESOLVER pc
Define this to evaluate to the (nonzero) address at which execution
should continue to get past the dynamic linker's symbol resolution
function.  A zero value indicates that it is not important or necessary
to set a breakpoint to get through the dynamic linker and that single
stepping will suffice.

@item IS_TRAPPED_INTERNALVAR (name)
This is an ugly hook to allow the specification of special actions that
should occur as a side-effect of setting the value of a variable
internal to GDB.  Currently only used by the h8500.  Note that this
could be either a host or target conditional.

@item NEED_TEXT_START_END
Define this if GDB should determine the start and end addresses of the
text section.  (Seems dubious.)

@item NO_HIF_SUPPORT
(Specific to the a29k.)

@item SOFTWARE_SINGLE_STEP_P
Define this as 1 if the target does not have a hardware single-step
mechanism. The macro @code{SOFTWARE_SINGLE_STEP} must also be defined.

@item SOFTWARE_SINGLE_STEP(signal,insert_breapoints_p)
A function that inserts or removes (dependant on
@var{insert_breapoints_p}) breakpoints at each possible destinations of
the next instruction. See @code{sparc-tdep.c} and @code{rs6000-tdep.c}
for examples.

@item SOFUN_ADDRESS_MAYBE_MISSING

Somebody clever observed that, the more actual addresses you have in the
debug information, the more time the linker has to spend relocating
them.  So whenever there's some other way the debugger could find the
address it needs, you should omit it from the debug info, to make
linking faster.

@code{SOFUN_ADDRESS_MAYBE_MISSING} indicates that a particular set of
hacks of this sort are in use, affecting @code{N_SO} and @code{N_FUN}
entries in stabs-format debugging information.  @code{N_SO} stabs mark
the beginning and ending addresses of compilation units in the text
segment.  @code{N_FUN} stabs mark the starts and ends of functions.

@code{SOFUN_ADDRESS_MAYBE_MISSING} means two things:
@itemize @bullet

@item
@code{N_FUN} stabs have an address of zero.  Instead, you should find the
addresses where the function starts by taking the function name from
the stab, and then looking that up in the minsyms (the linker/
assembler symbol table).  In other words, the stab has the name, and
the linker / assembler symbol table is the only place that carries
the address.

@item
@code{N_SO} stabs have an address of zero, too.  You just look at the
@code{N_FUN} stabs that appear before and after the @code{N_SO} stab,
and guess the starting and ending addresses of the compilation unit from
them.

@end itemize

@item PCC_SOL_BROKEN
(Used only in the Convex target.)

@item PC_IN_CALL_DUMMY
inferior.h

@item PC_LOAD_SEGMENT
If defined, print information about the load segment for the program
counter.  (Defined only for the RS/6000.)

@item PC_REGNUM
If the program counter is kept in a register, then define this macro to
be the number (greater than or equal to zero) of that register.

This should only need to be defined if @code{TARGET_READ_PC} and
@code{TARGET_WRITE_PC} are not defined.

@item NPC_REGNUM
The number of the ``next program counter'' register, if defined.

@item NNPC_REGNUM
The number of the ``next next program counter'' register, if defined.
Currently, this is only defined for the Motorola 88K.

@item PARM_BOUNDARY
If non-zero, round arguments to a boundary of this many bits before
pushing them on the stack.

@item PRINT_REGISTER_HOOK (regno)
If defined, this must be a function that prints the contents of the
given register to standard output.

@item PRINT_TYPELESS_INTEGER
This is an obscure substitute for @code{print_longest} that seems to
have been defined for the Convex target.

@item PROCESS_LINENUMBER_HOOK
A hook defined for XCOFF reading.

@item PROLOGUE_FIRSTLINE_OVERLAP
(Only used in unsupported Convex configuration.)

@item PS_REGNUM
If defined, this is the number of the processor status register.  (This
definition is only used in generic code when parsing "$ps".)

@item POP_FRAME
Used in @samp{call_function_by_hand} to remove an artificial stack
frame.

@item PUSH_ARGUMENTS (nargs, args, sp, struct_return, struct_addr)
Define this to push arguments onto the stack for inferior function
call. Return the updated stack pointer value.

@item PUSH_DUMMY_FRAME
Used in @samp{call_function_by_hand} to create an artificial stack frame.

@item REGISTER_BYTES
The total amount of space needed to store GDB's copy of the machine's
register state.

@item REGISTER_NAME(i)
Return the name of register @var{i} as a string.  May return @var{NULL}
or @var{NUL} to indicate that register @var{i} is not valid.

@item REGISTER_NAMES
Deprecated in favor of @var{REGISTER_NAME}.

@item REG_STRUCT_HAS_ADDR (gcc_p, type)
Define this to return 1 if the given type will be passed by pointer
rather than directly.

@item SAVE_DUMMY_FRAME_TOS (sp)
Used in @samp{call_function_by_hand} to notify the target dependent code
of the top-of-stack value that will be passed to the the inferior code.
This is the value of the @var{SP} after both the dummy frame and space
for parameters/results have been allocated on the stack.

@item SDB_REG_TO_REGNUM
Define this to convert sdb register numbers into GDB regnums.  If not
defined, no conversion will be done.

@item SHIFT_INST_REGS
(Only used for m88k targets.)

@item SKIP_PERMANENT_BREAKPOINT
Advance the inferior's PC past a permanent breakpoint.  GDB normally
steps over a breakpoint by removing it, stepping one instruction, and
re-inserting the breakpoint.  However, permanent breakpoints are
hardwired into the inferior, and can't be removed, so this strategy
doesn't work.  Calling SKIP_PERMANENT_BREAKPOINT adjusts the processor's
state so that execution will resume just after the breakpoint.  This
macro does the right thing even when the breakpoint is in the delay slot
of a branch or jump.

@item SKIP_PROLOGUE (pc)
A C expression that returns the address of the ``real'' code beyond the
function entry prologue found at @var{pc}.

@item SKIP_PROLOGUE_FRAMELESS_P
A C expression that should behave similarly, but that can stop as soon
as the function is known to have a frame.  If not defined,
@code{SKIP_PROLOGUE} will be used instead.

@item SKIP_TRAMPOLINE_CODE (pc)
If the target machine has trampoline code that sits between callers and
the functions being called, then define this macro to return a new PC
that is at the start of the real function.

@item SP_REGNUM
If the stack-pointer is kept in a register, then define this macro to be
the number (greater than or equal to zero) of that register.

This should only need to be defined if @code{TARGET_WRITE_SP} and
@code{TARGET_WRITE_SP} are not defined.

@item STAB_REG_TO_REGNUM
Define this to convert stab register numbers (as gotten from `r'
declarations) into GDB regnums.  If not defined, no conversion will be
done.

@item STACK_ALIGN (addr)
Define this to adjust the address to the alignment required for the
processor's stack.

@item STEP_SKIPS_DELAY (addr)
Define this to return true if the address is of an instruction with a
delay slot.  If a breakpoint has been placed in the instruction's delay
slot, GDB will single-step over that instruction before resuming
normally.  Currently only defined for the Mips.

@item STORE_RETURN_VALUE (type, valbuf)
A C expression that stores a function return value of type @var{type},
where @var{valbuf} is the address of the value to be stored.

@item SUN_FIXED_LBRAC_BUG
(Used only for Sun-3 and Sun-4 targets.)

@item SYMBOL_RELOADING_DEFAULT
The default value of the `symbol-reloading' variable.  (Never defined in
current sources.)

@item TARGET_BYTE_ORDER_DEFAULT
The ordering of bytes in the target.  This must be either
@code{BIG_ENDIAN} or @code{LITTLE_ENDIAN}.  This macro replaces
@var{TARGET_BYTE_ORDER} which is deprecated.

@item TARGET_BYTE_ORDER_SELECTABLE_P
Non-zero if the target has both @code{BIG_ENDIAN} and
@code{LITTLE_ENDIAN} variants.  This macro replaces
@var{TARGET_BYTE_ORDER_SELECTABLE} which is deprecated.

@item TARGET_CHAR_BIT
Number of bits in a char; defaults to 8.

@item TARGET_COMPLEX_BIT
Number of bits in a complex number; defaults to @code{2 * TARGET_FLOAT_BIT}.

At present this macro is not used.

@item TARGET_DOUBLE_BIT
Number of bits in a double float; defaults to @code{8 * TARGET_CHAR_BIT}.

@item TARGET_DOUBLE_COMPLEX_BIT
Number of bits in a double complex; defaults to @code{2 * TARGET_DOUBLE_BIT}.

At present this macro is not used.

@item TARGET_FLOAT_BIT
Number of bits in a float; defaults to @code{4 * TARGET_CHAR_BIT}.

@item TARGET_INT_BIT
Number of bits in an integer; defaults to @code{4 * TARGET_CHAR_BIT}.

@item TARGET_LONG_BIT
Number of bits in a long integer; defaults to @code{4 * TARGET_CHAR_BIT}.

@item TARGET_LONG_DOUBLE_BIT
Number of bits in a long double float;
defaults to @code{2 * TARGET_DOUBLE_BIT}.

@item TARGET_LONG_LONG_BIT
Number of bits in a long long integer; defaults to @code{2 * TARGET_LONG_BIT}.

@item TARGET_PTR_BIT
Number of bits in a pointer; defaults to @code{TARGET_INT_BIT}.

@item TARGET_SHORT_BIT
Number of bits in a short integer; defaults to @code{2 * TARGET_CHAR_BIT}.

@item TARGET_READ_PC
@item TARGET_WRITE_PC (val, pid)
@item TARGET_READ_SP
@item TARGET_WRITE_SP
@item TARGET_READ_FP
@item TARGET_WRITE_FP
These change the behavior of @code{read_pc}, @code{write_pc},
@code{read_sp}, @code{write_sp}, @code{read_fp} and @code{write_fp}.
For most targets, these may be left undefined.  GDB will call the read
and write register functions with the relevant @code{_REGNUM} argument.

These macros are useful when a target keeps one of these registers in a
hard to get at place; for example, part in a segment register and part
in an ordinary register.

@item TARGET_VIRTUAL_FRAME_POINTER(pc,regp,offsetp)
Returns a @code{(register, offset)} pair representing the virtual 
frame pointer in use at the code address @code{"pc"}.  If virtual 
frame pointers are not used, a default definition simply returns 
@code{FP_REGNUM}, with an offset of zero.

@item USE_STRUCT_CONVENTION (gcc_p, type)
If defined, this must be an expression that is nonzero if a value of the
given @var{type} being returned from a function must have space
allocated for it on the stack.  @var{gcc_p} is true if the function
being considered is known to have been compiled by GCC; this is helpful
for systems where GCC is known to use different calling convention than
other compilers.

@item VARIABLES_INSIDE_BLOCK (desc, gcc_p)
For dbx-style debugging information, if the compiler puts variable
declarations inside LBRAC/RBRAC blocks, this should be defined to be
nonzero.  @var{desc} is the value of @code{n_desc} from the
@code{N_RBRAC} symbol, and @var{gcc_p} is true if GDB has noticed the
presence of either the @code{GCC_COMPILED_SYMBOL} or the
@code{GCC2_COMPILED_SYMBOL}.  By default, this is 0.

@item OS9K_VARIABLES_INSIDE_BLOCK (desc, gcc_p)
Similarly, for OS/9000.  Defaults to 1.

@end table

Motorola M68K target conditionals.

@table @code

@item BPT_VECTOR
Define this to be the 4-bit location of the breakpoint trap vector.  If
not defined, it will default to @code{0xf}.

@item REMOTE_BPT_VECTOR
Defaults to @code{1}.

@end table

@section Adding a New Target

The following files define a target to GDB:

@table @file

@item gdb/config/@var{arch}/@var{ttt}.mt
Contains a Makefile fragment specific to this target.  Specifies what
object files are needed for target @var{ttt}, by defining
@samp{TDEPFILES=@dots{}} and @samp{TDEPLIBS=@dots{}}.  Also specifies
the header file which describes @var{ttt}, by defining @samp{TM_FILE=
tm-@var{ttt}.h}.

You can also define @samp{TM_CFLAGS}, @samp{TM_CLIBS}, @samp{TM_CDEPS},
but these are now deprecated, replaced by autoconf, and may go away in
future versions of GDB.

@item gdb/config/@var{arch}/tm-@var{ttt}.h
(@file{tm.h} is a link to this file, created by configure).  Contains
macro definitions about the target machine's registers, stack frame
format and instructions.

@item gdb/@var{ttt}-tdep.c
Contains any miscellaneous code required for this target machine.  On
some machines it doesn't exist at all.  Sometimes the macros in
@file{tm-@var{ttt}.h} become very complicated, so they are implemented
as functions here instead, and the macro is simply defined to call the
function.  This is vastly preferable, since it is easier to understand
and debug.

@item gdb/config/@var{arch}/tm-@var{arch}.h
This often exists to describe the basic layout of the target machine's
processor chip (registers, stack, etc).  If used, it is included by
@file{tm-@var{ttt}.h}.  It can be shared among many targets that use the
same processor.

@item gdb/@var{arch}-tdep.c
Similarly, there are often common subroutines that are shared by all
target machines that use this particular architecture.

@end table

If you are adding a new operating system for an existing CPU chip, add a
@file{config/tm-@var{os}.h} file that describes the operating system
facilities that are unusual (extra symbol table info; the breakpoint
instruction needed; etc).  Then write a @file{@var{arch}/tm-@var{os}.h}
that just @code{#include}s @file{tm-@var{arch}.h} and
@file{config/tm-@var{os}.h}.


@node Target Vector Definition

@chapter Target Vector Definition

The target vector defines the interface between GDB's abstract handling
of target systems, and the nitty-gritty code that actually exercises
control over a process or a serial port.  GDB includes some 30-40
different target vectors; however, each configuration of GDB includes
only a few of them.

@section File Targets

Both executables and core files have target vectors.

@section Standard Protocol and Remote Stubs

GDB's file @file{remote.c} talks a serial protocol to code that runs in
the target system.  GDB provides several sample ``stubs'' that can be
integrated into target programs or operating systems for this purpose;
they are named @file{*-stub.c}.

The GDB user's manual describes how to put such a stub into your target
code.  What follows is a discussion of integrating the SPARC stub into a
complicated operating system (rather than a simple program), by Stu
Grossman, the author of this stub.

The trap handling code in the stub assumes the following upon entry to
trap_low:

@enumerate

@item %l1 and %l2 contain pc and npc respectively at the time of the trap

@item traps are disabled

@item you are in the correct trap window

@end enumerate

As long as your trap handler can guarantee those conditions, then there
is no reason why you shouldn't be able to `share' traps with the stub.
The stub has no requirement that it be jumped to directly from the
hardware trap vector.  That is why it calls @code{exceptionHandler()},
which is provided by the external environment.  For instance, this could
setup the hardware traps to actually execute code which calls the stub
first, and then transfers to its own trap handler.

For the most point, there probably won't be much of an issue with
`sharing' traps, as the traps we use are usually not used by the kernel,
and often indicate unrecoverable error conditions.  Anyway, this is all
controlled by a table, and is trivial to modify.  The most important
trap for us is for @code{ta 1}.  Without that, we can't single step or
do breakpoints.  Everything else is unnecessary for the proper operation
of the debugger/stub.

From reading the stub, it's probably not obvious how breakpoints work.
They are simply done by deposit/examine operations from GDB.

@section ROM Monitor Interface

@section Custom Protocols

@section Transport Layer

@section Builtin Simulator


@node Native Debugging

@chapter Native Debugging

Several files control GDB's configuration for native support:

@table @file

@item gdb/config/@var{arch}/@var{xyz}.mh
Specifies Makefile fragments needed when hosting @emph{or native} on
machine @var{xyz}.  In particular, this lists the required
native-dependent object files, by defining @samp{NATDEPFILES=@dots{}}.
Also specifies the header file which describes native support on
@var{xyz}, by defining @samp{NAT_FILE= nm-@var{xyz}.h}.  You can also
define @samp{NAT_CFLAGS}, @samp{NAT_ADD_FILES}, @samp{NAT_CLIBS},
@samp{NAT_CDEPS}, etc.; see @file{Makefile.in}.

@item gdb/config/@var{arch}/nm-@var{xyz}.h
(@file{nm.h} is a link to this file, created by configure).  Contains C
macro definitions describing the native system environment, such as
child process control and core file support.

@item gdb/@var{xyz}-nat.c
Contains any miscellaneous C code required for this native support of
this machine.  On some machines it doesn't exist at all.

@end table

There are some ``generic'' versions of routines that can be used by
various systems.  These can be customized in various ways by macros
defined in your @file{nm-@var{xyz}.h} file.  If these routines work for
the @var{xyz} host, you can just include the generic file's name (with
@samp{.o}, not @samp{.c}) in @code{NATDEPFILES}.

Otherwise, if your machine needs custom support routines, you will need
to write routines that perform the same functions as the generic file.
Put them into @code{@var{xyz}-nat.c}, and put @code{@var{xyz}-nat.o}
into @code{NATDEPFILES}.

@table @file

@item inftarg.c
This contains the @emph{target_ops vector} that supports Unix child
processes on systems which use ptrace and wait to control the child.

@item procfs.c
This contains the @emph{target_ops vector} that supports Unix child
processes on systems which use /proc to control the child.

@item fork-child.c
This does the low-level grunge that uses Unix system calls to do a "fork
and exec" to start up a child process.

@item infptrace.c
This is the low level interface to inferior processes for systems using
the Unix @code{ptrace} call in a vanilla way.

@end table

@section Native core file Support

@table @file

@item core-aout.c::fetch_core_registers()
Support for reading registers out of a core file.  This routine calls
@code{register_addr()}, see below.  Now that BFD is used to read core
files, virtually all machines should use @code{core-aout.c}, and should
just provide @code{fetch_core_registers} in @code{@var{xyz}-nat.c} (or
@code{REGISTER_U_ADDR} in @code{nm-@var{xyz}.h}).

@item core-aout.c::register_addr()
If your @code{nm-@var{xyz}.h} file defines the macro
@code{REGISTER_U_ADDR(addr, blockend, regno)}, it should be defined to
set @code{addr} to the offset within the @samp{user} struct of GDB
register number @code{regno}.  @code{blockend} is the offset within the
``upage'' of @code{u.u_ar0}.  If @code{REGISTER_U_ADDR} is defined,
@file{core-aout.c} will define the @code{register_addr()} function and
use the macro in it.  If you do not define @code{REGISTER_U_ADDR}, but
you are using the standard @code{fetch_core_registers()}, you will need
to define your own version of @code{register_addr()}, put it into your
@code{@var{xyz}-nat.c} file, and be sure @code{@var{xyz}-nat.o} is in
the @code{NATDEPFILES} list.  If you have your own
@code{fetch_core_registers()}, you may not need a separate
@code{register_addr()}.  Many custom @code{fetch_core_registers()}
implementations simply locate the registers themselves.@refill

@end table

When making GDB run native on a new operating system, to make it
possible to debug core files, you will need to either write specific
code for parsing your OS's core files, or customize
@file{bfd/trad-core.c}.  First, use whatever @code{#include} files your
machine uses to define the struct of registers that is accessible
(possibly in the u-area) in a core file (rather than
@file{machine/reg.h}), and an include file that defines whatever header
exists on a core file (e.g. the u-area or a @samp{struct core}).  Then
modify @code{trad_unix_core_file_p()} to use these values to set up the
section information for the data segment, stack segment, any other
segments in the core file (perhaps shared library contents or control
information), ``registers'' segment, and if there are two discontiguous
sets of registers (e.g.  integer and float), the ``reg2'' segment.  This
section information basically delimits areas in the core file in a
standard way, which the section-reading routines in BFD know how to seek
around in.

Then back in GDB, you need a matching routine called
@code{fetch_core_registers()}.  If you can use the generic one, it's in
@file{core-aout.c}; if not, it's in your @file{@var{xyz}-nat.c} file.
It will be passed a char pointer to the entire ``registers'' segment,
its length, and a zero; or a char pointer to the entire ``regs2''
segment, its length, and a 2.  The routine should suck out the supplied
register values and install them into GDB's ``registers'' array.

If your system uses @file{/proc} to control processes, and uses ELF
format core files, then you may be able to use the same routines for
reading the registers out of processes and out of core files.

@section ptrace

@section /proc

@section win32

@section shared libraries

@section Native Conditionals

When GDB is configured and compiled, various macros are defined or left
undefined, to control compilation when the host and target systems are
the same.  These macros should be defined (or left undefined) in
@file{nm-@var{system}.h}.

@table @code

@item ATTACH_DETACH
If defined, then GDB will include support for the @code{attach} and
@code{detach} commands.

@item CHILD_PREPARE_TO_STORE
If the machine stores all registers at once in the child process, then
define this to ensure that all values are correct.  This usually entails
a read from the child.

[Note that this is incorrectly defined in @file{xm-@var{system}.h} files
currently.]

@item FETCH_INFERIOR_REGISTERS
Define this if the native-dependent code will provide its own routines
@code{fetch_inferior_registers} and @code{store_inferior_registers} in
@file{@var{HOST}-nat.c}.  If this symbol is @emph{not} defined, and
@file{infptrace.c} is included in this configuration, the default
routines in @file{infptrace.c} are used for these functions.

@item FILES_INFO_HOOK
(Only defined for Convex.)

@item FP0_REGNUM
This macro is normally defined to be the number of the first floating
point register, if the machine has such registers.  As such, it would
appear only in target-specific code.  However, /proc support uses this
to decide whether floats are in use on this target.

@item GET_LONGJMP_TARGET
For most machines, this is a target-dependent parameter.  On the
DECstation and the Iris, this is a native-dependent parameter, since
<setjmp.h> is needed to define it.

This macro determines the target PC address that longjmp() will jump to,
assuming that we have just stopped at a longjmp breakpoint.  It takes a
CORE_ADDR * as argument, and stores the target PC value through this
pointer.  It examines the current state of the machine as needed.

@item KERNEL_U_ADDR
Define this to the address of the @code{u} structure (the ``user
struct'', also known as the ``u-page'') in kernel virtual memory.  GDB
needs to know this so that it can subtract this address from absolute
addresses in the upage, that are obtained via ptrace or from core files.
On systems that don't need this value, set it to zero.

@item KERNEL_U_ADDR_BSD
Define this to cause GDB to determine the address of @code{u} at
runtime, by using Berkeley-style @code{nlist} on the kernel's image in
the root directory.

@item KERNEL_U_ADDR_HPUX
Define this to cause GDB to determine the address of @code{u} at
runtime, by using HP-style @code{nlist} on the kernel's image in the
root directory.

@item ONE_PROCESS_WRITETEXT
Define this to be able to, when a breakpoint insertion fails, warn the
user that another process may be running with the same executable.

@item PREPARE_TO_PROCEED @var{select_it}
This (ugly) macro allows a native configuration to customize the way the
@code{proceed} function in @file{infrun.c} deals with switching between
threads.

In a multi-threaded task we may select another thread and then continue
or step.  But if the old thread was stopped at a breakpoint, it will
immediately cause another breakpoint stop without any execution (i.e. it
will report a breakpoint hit incorrectly).  So GDB must step over it
first.

If defined, @code{PREPARE_TO_PROCEED} should check the current thread
against the thread that reported the most recent event.  If a step-over
is required, it returns TRUE.  If @var{select_it} is non-zero, it should
reselect the old thread.

@item PROC_NAME_FMT
Defines the format for the name of a @file{/proc} device.  Should be
defined in @file{nm.h} @emph{only} in order to override the default
definition in @file{procfs.c}.

@item PTRACE_FP_BUG
mach386-xdep.c

@item PTRACE_ARG3_TYPE
The type of the third argument to the @code{ptrace} system call, if it
exists and is different from @code{int}.

@item REGISTER_U_ADDR
Defines the offset of the registers in the ``u area''.

@item SHELL_COMMAND_CONCAT
If defined, is a string to prefix on the shell command used to start the
inferior.

@item SHELL_FILE
If defined, this is the name of the shell to use to run the inferior.
Defaults to @code{"/bin/sh"}.

@item SOLIB_ADD (filename, from_tty, targ)
Define this to expand into an expression that will cause the symbols in
@var{filename} to be added to GDB's symbol table.

@item SOLIB_CREATE_INFERIOR_HOOK
Define this to expand into any shared-library-relocation code that you
want to be run just after the child process has been forked.

@item START_INFERIOR_TRAPS_EXPECTED
When starting an inferior, GDB normally expects to trap twice; once when
the shell execs, and once when the program itself execs.  If the actual
number of traps is something other than 2, then define this macro to
expand into the number expected.

@item SVR4_SHARED_LIBS
Define this to indicate that SVR4-style shared libraries are in use.

@item USE_PROC_FS
This determines whether small routines in @file{*-tdep.c}, which
translate register values between GDB's internal representation and the
/proc representation, are compiled.

@item U_REGS_OFFSET
This is the offset of the registers in the upage.  It need only be
defined if the generic ptrace register access routines in
@file{infptrace.c} are being used (that is, @file{infptrace.c} is
configured in, and @code{FETCH_INFERIOR_REGISTERS} is not defined).  If
the default value from @file{infptrace.c} is good enough, leave it
undefined.

The default value means that u.u_ar0 @emph{points to} the location of
the registers.  I'm guessing that @code{#define U_REGS_OFFSET 0} means
that u.u_ar0 @emph{is} the location of the registers.

@item CLEAR_SOLIB
objfiles.c

@item DEBUG_PTRACE
Define this to debug ptrace calls.

@end table


@node Support Libraries

@chapter Support Libraries

@section BFD

BFD provides support for GDB in several ways:

@table @emph

@item identifying executable and core files
BFD will identify a variety of file types, including a.out, coff, and
several variants thereof, as well as several kinds of core files.

@item access to sections of files
BFD parses the file headers to determine the names, virtual addresses,
sizes, and file locations of all the various named sections in files
(such as the text section or the data section).  GDB simply calls BFD to
read or write section X at byte offset Y for length Z.

@item specialized core file support
BFD provides routines to determine the failing command name stored in a
core file, the signal with which the program failed, and whether a core
file matches (i.e. could be a core dump of) a particular executable
file.

@item locating the symbol information
GDB uses an internal interface of BFD to determine where to find the
symbol information in an executable file or symbol-file.  GDB itself
handles the reading of symbols, since BFD does not ``understand'' debug
symbols, but GDB uses BFD's cached information to find the symbols,
string table, etc.

@end table

@section opcodes

The opcodes library provides GDB's disassembler.  (It's a separate
library because it's also used in binutils, for @file{objdump}).

@section readline

@section mmalloc

@section libiberty

@section gnu-regex

Regex conditionals.

@table @code

@item C_ALLOCA

@item NFAILURES

@item RE_NREGS

@item SIGN_EXTEND_CHAR

@item SWITCH_ENUM_BUG

@item SYNTAX_TABLE

@item Sword

@item sparc

@end table

@section include

@node Coding

@chapter Coding

This chapter covers topics that are lower-level than the major
algorithms of GDB.

@section Cleanups

Cleanups are a structured way to deal with things that need to be done
later.  When your code does something (like @code{malloc} some memory,
or open a file) that needs to be undone later (e.g. free the memory or
close the file), it can make a cleanup.  The cleanup will be done at
some future point: when the command is finished, when an error occurs,
or when your code decides it's time to do cleanups.

You can also discard cleanups, that is, throw them away without doing
what they say.  This is only done if you ask that it be done.

Syntax:

@table @code

@item struct cleanup *@var{old_chain};
Declare a variable which will hold a cleanup chain handle.

@item @var{old_chain} = make_cleanup (@var{function}, @var{arg});
Make a cleanup which will cause @var{function} to be called with
@var{arg} (a @code{char *}) later.  The result, @var{old_chain}, is a
handle that can be passed to @code{do_cleanups} or
@code{discard_cleanups} later.  Unless you are going to call
@code{do_cleanups} or @code{discard_cleanups} yourself, you can ignore
the result from @code{make_cleanup}.

@item do_cleanups (@var{old_chain});
Perform all cleanups done since @code{make_cleanup} returned
@var{old_chain}.  E.g.:
@example
make_cleanup (a, 0); 
old = make_cleanup (b, 0); 
do_cleanups (old);
@end example
@noindent
will call @code{b()} but will not call @code{a()}.  The cleanup that
calls @code{a()} will remain in the cleanup chain, and will be done
later unless otherwise discarded.@refill

@item discard_cleanups (@var{old_chain});
Same as @code{do_cleanups} except that it just removes the cleanups from
the chain and does not call the specified functions.

@end table

Some functions, e.g. @code{fputs_filtered()} or @code{error()}, specify
that they ``should not be called when cleanups are not in place''.  This
means that any actions you need to reverse in the case of an error or
interruption must be on the cleanup chain before you call these
functions, since they might never return to your code (they
@samp{longjmp} instead).

@section Wrapping Output Lines

Output that goes through @code{printf_filtered} or @code{fputs_filtered}
or @code{fputs_demangled} needs only to have calls to @code{wrap_here}
added in places that would be good breaking points.  The utility
routines will take care of actually wrapping if the line width is
exceeded.

The argument to @code{wrap_here} is an indentation string which is
printed @emph{only} if the line breaks there.  This argument is saved
away and used later.  It must remain valid until the next call to
@code{wrap_here} or until a newline has been printed through the
@code{*_filtered} functions.  Don't pass in a local variable and then
return!

It is usually best to call @code{wrap_here()} after printing a comma or
space.  If you call it before printing a space, make sure that your
indentation properly accounts for the leading space that will print if
the line wraps there.

Any function or set of functions that produce filtered output must
finish by printing a newline, to flush the wrap buffer, before switching
to unfiltered (``@code{printf}'') output.  Symbol reading routines that
print warnings are a good example.

@section GDB Coding Standards

GDB follows the GNU coding standards, as described in
@file{etc/standards.texi}.  This file is also available for anonymous
FTP from GNU archive sites.  GDB takes a strict interpretation of the
standard; in general, when the GNU standard recommends a practice but
does not require it, GDB requires it.

GDB follows an additional set of coding standards specific to GDB,
as described in the following sections.

You can configure with @samp{--enable-build-warnings} to get GCC to
check on a number of these rules.  GDB sources ought not to engender any
complaints, unless they are caused by bogus host systems.  (The exact
set of enabled warnings is currently @samp{-Wall -Wpointer-arith
-Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations}.

@subsection Formatting

The standard GNU recommendations for formatting must be followed
strictly.

Note that while in a definition, the function's name must be in column
zero; in a function declaration, the name must be on the same line as
the return type.

In addition, there must be a space between a function or macro name and
the opening parenthesis of its argument list (except for macro
definitions, as required by C).  There must not be a space after an open
paren/bracket or before a close paren/bracket.

While additional whitespace is generally helpful for reading, do not use
more than one blank line to separate blocks, and avoid adding whitespace
after the end of a program line (as of 1/99, some 600 lines had whitespace
after the semicolon).  Excess whitespace causes difficulties for diff and
patch.

@subsection Comments

The standard GNU requirements on comments must be followed strictly.

Block comments must appear in the following form, with no `/*'- or
'*/'-only lines, and no leading `*':

@example @code
/* Wait for control to return from inferior to debugger.  If inferior
   gets a signal, we may decide to start it up again instead of
   returning.  That is why there is a loop in this function.  When
   this function actually returns it means the inferior should be left
   stopped and GDB should read more commands.  */
@end example

(Note that this format is encouraged by Emacs; tabbing for a multi-line
comment works correctly, and M-Q fills the block consistently.)

Put a blank line between the block comments preceding function or
variable definitions, and the definition itself.

In general, put function-body comments on lines by themselves, rather
than trying to fit them into the 20 characters left at the end of a
line, since either the comment or the code will inevitably get longer
than will fit, and then somebody will have to move it anyhow.

@subsection C Usage

Code must not depend on the sizes of C data types, the format of the
host's floating point numbers, the alignment of anything, or the order
of evaluation of expressions.

Use functions freely.  There are only a handful of compute-bound areas
in GDB that might be affected by the overhead of a function call, mainly
in symbol reading.  Most of GDB's performance is limited by the target
interface (whether serial line or system call).

However, use functions with moderation.  A thousand one-line functions
are just as hard to understand as a single thousand-line function.

@subsection Function Prototypes

Prototypes must be used to @emph{declare} functions, and may be used to
@emph{define} them.  Prototypes for GDB functions must include both the
argument type and name, with the name matching that used in the actual
function definition.

All external functions should have a declaration in a header file that
callers include, except for @code{_initialize_*} functions, which must
be external so that @file{init.c} construction works, but shouldn't be
visible to random source files.

All static functions must be declared in a block near the top of the
source file.

@subsection Clean Design

In addition to getting the syntax right, there's the little question of
semantics.  Some things are done in certain ways in GDB because long
experience has shown that the more obvious ways caused various kinds of
trouble.

You can't assume the byte order of anything that comes from a target
(including @var{value}s, object files, and instructions).  Such things
must be byte-swapped using @code{SWAP_TARGET_AND_HOST} in GDB, or one of
the swap routines defined in @file{bfd.h}, such as @code{bfd_get_32}.

You can't assume that you know what interface is being used to talk to
the target system.  All references to the target must go through the
current @code{target_ops} vector.

You can't assume that the host and target machines are the same machine
(except in the ``native'' support modules).  In particular, you can't
assume that the target machine's header files will be available on the
host machine.  Target code must bring along its own header files --
written from scratch or explicitly donated by their owner, to avoid
copyright problems.

Insertion of new @code{#ifdef}'s will be frowned upon.  It's much better
to write the code portably than to conditionalize it for various
systems.

New @code{#ifdef}'s which test for specific compilers or manufacturers
or operating systems are unacceptable.  All @code{#ifdef}'s should test
for features.  The information about which configurations contain which
features should be segregated into the configuration files.  Experience
has proven far too often that a feature unique to one particular system
often creeps into other systems; and that a conditional based on some
predefined macro for your current system will become worthless over
time, as new versions of your system come out that behave differently
with regard to this feature.

Adding code that handles specific architectures, operating systems,
target interfaces, or hosts, is not acceptable in generic code.  If a
hook is needed at that point, invent a generic hook and define it for
your configuration, with something like:

@example
#ifdef	WRANGLE_SIGNALS
   WRANGLE_SIGNALS (signo);
#endif
@end example

In your host, target, or native configuration file, as appropriate,
define @code{WRANGLE_SIGNALS} to do the machine-dependent thing.  Take a
bit of care in defining the hook, so that it can be used by other ports
in the future, if they need a hook in the same place.

If the hook is not defined, the code should do whatever "most" machines
want.  Using @code{#ifdef}, as above, is the preferred way to do this,
but sometimes that gets convoluted, in which case use

@example
#ifndef SPECIAL_FOO_HANDLING
#define SPECIAL_FOO_HANDLING(pc, sp) (0)
#endif
@end example

where the macro is used or in an appropriate header file.

Whether to include a @dfn{small} hook, a hook around the exact pieces of
code which are system-dependent, or whether to replace a whole function
with a hook depends on the case.  A good example of this dilemma can be
found in @code{get_saved_register}.  All machines that GDB 2.8 ran on
just needed the @code{FRAME_FIND_SAVED_REGS} hook to find the saved
registers.  Then the SPARC and Pyramid came along, and
@code{HAVE_REGISTER_WINDOWS} and @code{REGISTER_IN_WINDOW_P} were
introduced.  Then the 29k and 88k required the @code{GET_SAVED_REGISTER}
hook.  The first three are examples of small hooks; the latter replaces
a whole function.  In this specific case, it is useful to have both
kinds; it would be a bad idea to replace all the uses of the small hooks
with @code{GET_SAVED_REGISTER}, since that would result in much
duplicated code.  Other times, duplicating a few lines of code here or
there is much cleaner than introducing a large number of small hooks.

Another way to generalize GDB along a particular interface is with an
attribute struct.  For example, GDB has been generalized to handle
multiple kinds of remote interfaces -- not by #ifdef's everywhere, but
by defining the "target_ops" structure and having a current target (as
well as a stack of targets below it, for memory references).  Whenever
something needs to be done that depends on which remote interface we are
using, a flag in the current target_ops structure is tested (e.g.
`target_has_stack'), or a function is called through a pointer in the
current target_ops structure.  In this way, when a new remote interface
is added, only one module needs to be touched -- the one that actually
implements the new remote interface.  Other examples of
attribute-structs are BFD access to multiple kinds of object file
formats, or GDB's access to multiple source languages.

Please avoid duplicating code.  For example, in GDB 3.x all the code
interfacing between @code{ptrace} and the rest of GDB was duplicated in
@file{*-dep.c}, and so changing something was very painful.  In GDB 4.x,
these have all been consolidated into @file{infptrace.c}.
@file{infptrace.c} can deal with variations between systems the same way
any system-independent file would (hooks, #if defined, etc.), and
machines which are radically different don't need to use infptrace.c at
all.

Don't put debugging printfs in the code.

@node Porting GDB

@chapter Porting GDB

Most of the work in making GDB compile on a new machine is in specifying
the configuration of the machine.  This is done in a dizzying variety of
header files and configuration scripts, which we hope to make more
sensible soon.  Let's say your new host is called an @var{xyz} (e.g.
@samp{sun4}), and its full three-part configuration name is
@code{@var{arch}-@var{xvend}-@var{xos}} (e.g.  @samp{sparc-sun-sunos4}).
In particular:

In the top level directory, edit @file{config.sub} and add @var{arch},
@var{xvend}, and @var{xos} to the lists of supported architectures,
vendors, and operating systems near the bottom of the file.  Also, add
@var{xyz} as an alias that maps to
@code{@var{arch}-@var{xvend}-@var{xos}}.  You can test your changes by
running

@example
./config.sub @var{xyz}
@end example
@noindent
and
@example
./config.sub @code{@var{arch}-@var{xvend}-@var{xos}}
@end example
@noindent
which should both respond with @code{@var{arch}-@var{xvend}-@var{xos}}
and no error messages.

You need to port BFD, if that hasn't been done already.  Porting BFD is
beyond the scope of this manual.

To configure GDB itself, edit @file{gdb/configure.host} to recognize
your system and set @code{gdb_host} to @var{xyz}, and (unless your
desired target is already available) also edit @file{gdb/configure.tgt},
setting @code{gdb_target} to something appropriate (for instance,
@var{xyz}).

Finally, you'll need to specify and define GDB's host-, native-, and
target-dependent @file{.h} and @file{.c} files used for your
configuration.

@section Configuring GDB for Release

From the top level directory (containing @file{gdb}, @file{bfd},
@file{libiberty}, and so on):
@example
make -f Makefile.in gdb.tar.gz
@end example

This will properly configure, clean, rebuild any files that are
distributed pre-built (e.g. @file{c-exp.tab.c} or @file{refcard.ps}),
and will then make a tarfile.  (If the top level directory has already
been configured, you can just do @code{make gdb.tar.gz} instead.)

This procedure requires:
@itemize @bullet
@item symbolic links
@item @code{makeinfo} (texinfo2 level)
@item @TeX{}
@item @code{dvips}
@item @code{yacc} or @code{bison}
@end itemize
@noindent
@dots{} and the usual slew of utilities (@code{sed}, @code{tar}, etc.).

@subheading TEMPORARY RELEASE PROCEDURE FOR DOCUMENTATION

@file{gdb.texinfo} is currently marked up using the texinfo-2 macros,
which are not yet a default for anything (but we have to start using
them sometime).

For making paper, the only thing this implies is the right generation of
@file{texinfo.tex} needs to be included in the distribution.

For making info files, however, rather than duplicating the texinfo2
distribution, generate @file{gdb-all.texinfo} locally, and include the
files @file{gdb.info*} in the distribution.  Note the plural;
@code{makeinfo} will split the document into one overall file and five
or so included files.

@node Testsuite

@chapter Testsuite

The testsuite is an important component of the GDB package.  While it is
always worthwhile to encourage user testing, in practice this is rarely
sufficient; users typically use only a small subset of the available
commands, and it has proven all too common for a change to cause a
significant regression that went unnoticed for some time.

The GDB testsuite uses the DejaGNU testing framework.  DejaGNU is built
using tcl and expect.  The tests themselves are calls to various tcl
procs; the framework runs all the procs and summarizes the passes and
fails.

@section Using the Testsuite

To run the testsuite, simply go to the GDB object directory (or to the
testsuite's objdir) and type @code{make check}.  This just sets up some
environment variables and invokes DejaGNU's @code{runtest} script.  While
the testsuite is running, you'll get mentions of which test file is in use,
and a mention of any unexpected passes or fails.  When the testsuite is
finished, you'll get a summary that looks like this:
@example
                === gdb Summary ===

# of expected passes            6016
# of unexpected failures        58
# of unexpected successes       5
# of expected failures          183
# of unresolved testcases       3
# of untested testcases         5
@end example
The ideal test run consists of expected passes only; however, reality
conspires to keep us from this ideal.  Unexpected failures indicate
real problems, whether in GDB or in the testsuite.  Expected failures
are still failures, but ones which have been decided are too hard to
deal with at the time; for instance, a test case might work everywhere
except on AIX, and there is no prospect of the AIX case being fixed in
the near future.  Expected failures should not be added lightly, since
you may be masking serious bugs in GDB.  Unexpected successes are expected
fails that are passing for some reason, while unresolved and untested
cases often indicate some minor catastrophe, such as the compiler being
unable to deal with a test program.

When making any significant change to GDB, you should run the testsuite
before and after the change, to confirm that there are no regressions.
Note that truly complete testing would require that you run the
testsuite with all supported configurations and a variety of compilers;
however this is more than really necessary.  In many cases testing with
a single configuration is sufficient.  Other useful options are to test
one big-endian (Sparc) and one little-endian (x86) host, a cross config
with a builtin simulator (powerpc-eabi, mips-elf), or a 64-bit host
(Alpha).

If you add new functionality to GDB, please consider adding tests for it
as well; this way future GDB hackers can detect and fix their changes
that break the functionality you added.  Similarly, if you fix a bug
that was not previously reported as a test failure, please add a test
case for it.  Some cases are extremely difficult to test, such as code
that handles host OS failures or bugs in particular versions of
compilers, and it's OK not to try to write tests for all of those.

@section Testsuite Organization

The testsuite is entirely contained in @file{gdb/testsuite}.  While the
testsuite includes some makefiles and configury, these are very minimal,
and used for little besides cleaning up, since the tests themselves
handle the compilation of the programs that GDB will run.  The file
@file{testsuite/lib/gdb.exp} contains common utility procs useful for
all GDB tests, while the directory @file{testsuite/config} contains
configuration-specific files, typically used for special-purpose
definitions of procs like @code{gdb_load} and @code{gdb_start}.

The tests themselves are to be found in @file{testsuite/gdb.*} and
subdirectories of those.  The names of the test files must always end
with @file{.exp}.  DejaGNU collects the test files by wildcarding
in the test directories, so both subdirectories and individual files
get chosen and run in alphabetical order.

The following table lists the main types of subdirectories and what they
are for.  Since DejaGNU finds test files no matter where they are
located, and since each test file sets up its own compilation and
execution environment, this organization is simply for convenience and
intelligibility.

@table @code

@item gdb.base

This is the base testsuite.  The tests in it should apply to all
configurations of GDB (but generic native-only tests may live here).
The test programs should be in the subset of C that is valid K&R,
ANSI/ISO, and C++ (ifdefs are allowed if necessary, for instance
for prototypes).

@item gdb.@var{lang}

Language-specific tests for all languages besides C.  Examples are
@file{gdb.c++} and @file{gdb.java}.

@item gdb.@var{platform}

Non-portable tests.  The tests are specific to a specific configuration
(host or target), such as HP-UX or eCos.  Example is @file{gdb.hp}, for
HP-UX.

@item gdb.@var{compiler}

Tests specific to a particular compiler.  As of this writing (June
1999), there aren't currently any groups of tests in this category that
couldn't just as sensibly be made platform-specific, but one could
imagine a gdb.gcc, for tests of GDB's handling of GCC extensions.

@item gdb.@var{subsystem}

Tests that exercise a specific GDB subsystem in more depth.  For
instance, @file{gdb.disasm} exercises various disassemblers, while
@file{gdb.stabs} tests pathways through the stabs symbol reader.

@end table

@section Writing Tests

In many areas, the GDB tests are already quite comprehensive; you
should be able to copy existing tests to handle new cases.

You should try to use @code{gdb_test} whenever possible, since it
includes cases to handle all the unexpected errors that might happen.
However, it doesn't cost anything to add new test procedures; for
instance, @file{gdb.base/exprs.exp} defines a @code{test_expr} that
calls @code{gdb_test} multiple times.

Only use @code{send_gdb} and @code{gdb_expect} when absolutely
necessary, such as when GDB has several valid responses to a command.

The source language programs do @emph{not} need to be in a consistent
style.  Since GDB is used to debug programs written in many different
styles, it's worth having a mix of styles in the testsuite; for
instance, some GDB bugs involving the display of source lines would
never manifest themselves if the programs used GNU coding style
uniformly.

@node Hints

@chapter Hints

Check the @file{README} file, it often has useful information that does not
appear anywhere else in the directory.

@menu
* Getting Started::		Getting started working on GDB
* Debugging GDB::		Debugging GDB with itself
@end menu

@node Getting Started,,, Hints

@section Getting Started

GDB is a large and complicated program, and if you first starting to
work on it, it can be hard to know where to start.  Fortunately, if you
know how to go about it, there are ways to figure out what is going on.

This manual, the GDB Internals manual, has information which applies
generally to many parts of GDB.

Information about particular functions or data structures are located in
comments with those functions or data structures.  If you run across a
function or a global variable which does not have a comment correctly
explaining what is does, this can be thought of as a bug in GDB; feel
free to submit a bug report, with a suggested comment if you can figure
out what the comment should say.  If you find a comment which is
actually wrong, be especially sure to report that.

Comments explaining the function of macros defined in host, target, or
native dependent files can be in several places.  Sometimes they are
repeated every place the macro is defined.  Sometimes they are where the
macro is used.  Sometimes there is a header file which supplies a
default definition of the macro, and the comment is there.  This manual
also documents all the available macros.
@c (@pxref{Host Conditionals}, @pxref{Target
@c Conditionals}, @pxref{Native Conditionals}, and @pxref{Obsolete
@c Conditionals})

Start with the header files.  Once you have some idea of how GDB's internal
symbol tables are stored (see @file{symtab.h}, @file{gdbtypes.h}), you
will find it much easier to understand the code which uses and creates
those symbol tables.

You may wish to process the information you are getting somehow, to
enhance your understanding of it.  Summarize it, translate it to another
language, add some (perhaps trivial or non-useful) feature to GDB, use
the code to predict what a test case would do and write the test case
and verify your prediction, etc.  If you are reading code and your eyes
are starting to glaze over, this is a sign you need to use a more active
approach.

Once you have a part of GDB to start with, you can find more
specifically the part you are looking for by stepping through each
function with the @code{next} command.  Do not use @code{step} or you
will quickly get distracted; when the function you are stepping through
calls another function try only to get a big-picture understanding
(perhaps using the comment at the beginning of the function being
called) of what it does.  This way you can identify which of the
functions being called by the function you are stepping through is the
one which you are interested in.  You may need to examine the data
structures generated at each stage, with reference to the comments in
the header files explaining what the data structures are supposed to
look like.

Of course, this same technique can be used if you are just reading the
code, rather than actually stepping through it.  The same general
principle applies---when the code you are looking at calls something
else, just try to understand generally what the code being called does,
rather than worrying about all its details.

A good place to start when tracking down some particular area is with a
command which invokes that feature.  Suppose you want to know how
single-stepping works.  As a GDB user, you know that the @code{step}
command invokes single-stepping.  The command is invoked via command
tables (see @file{command.h}); by convention the function which actually
performs the command is formed by taking the name of the command and
adding @samp{_command}, or in the case of an @code{info} subcommand,
@samp{_info}.  For example, the @code{step} command invokes the
@code{step_command} function and the @code{info display} command invokes
@code{display_info}.  When this convention is not followed, you might
have to use @code{grep} or @kbd{M-x tags-search} in emacs, or run GDB on
itself and set a breakpoint in @code{execute_command}.

If all of the above fail, it may be appropriate to ask for information
on @code{bug-gdb}.  But @emph{never} post a generic question like ``I was
wondering if anyone could give me some tips about understanding
GDB''---if we had some magic secret we would put it in this manual.
Suggestions for improving the manual are always welcome, of course.

@node Debugging GDB,,,Hints

@section Debugging GDB with itself

If GDB is limping on your machine, this is the preferred way to get it
fully functional.  Be warned that in some ancient Unix systems, like
Ultrix 4.2, a program can't be running in one process while it is being
debugged in another.  Rather than typing the command @code{@w{./gdb
./gdb}}, which works on Suns and such, you can copy @file{gdb} to
@file{gdb2} and then type @code{@w{./gdb ./gdb2}}.

When you run GDB in the GDB source directory, it will read a
@file{.gdbinit} file that sets up some simple things to make debugging
gdb easier.  The @code{info} command, when executed without a subcommand
in a GDB being debugged by gdb, will pop you back up to the top level
gdb.  See @file{.gdbinit} for details.

If you use emacs, you will probably want to do a @code{make TAGS} after
you configure your distribution; this will put the machine dependent
routines for your local machine where they will be accessed first by
@kbd{M-.}

Also, make sure that you've either compiled GDB with your local cc, or
have run @code{fixincludes} if you are compiling with gcc.

@section Submitting Patches

Thanks for thinking of offering your changes back to the community of
GDB users.  In general we like to get well designed enhancements.
Thanks also for checking in advance about the best way to transfer the
changes.

The GDB maintainers will only install ``cleanly designed'' patches.
This manual summarizes what we believe to be clean design for GDB.

If the maintainers don't have time to put the patch in when it arrives,
or if there is any question about a patch, it goes into a large queue
with everyone else's patches and bug reports.

The legal issue is that to incorporate substantial changes requires a
copyright assignment from you and/or your employer, granting ownership
of the changes to the Free Software Foundation.  You can get the
standard documents for doing this by sending mail to @code{gnu@@gnu.org}
and asking for it.  We recommend that people write in "All programs
owned by the Free Software Foundation" as "NAME OF PROGRAM", so that
changes in many programs (not just GDB, but GAS, Emacs, GCC, etc) can be
contributed with only one piece of legalese pushed through the
bureacracy and filed with the FSF.  We can't start merging changes until
this paperwork is received by the FSF (their rules, which we follow
since we maintain it for them).

Technically, the easiest way to receive changes is to receive each
feature as a small context diff or unidiff, suitable for "patch".  Each
message sent to me should include the changes to C code and header files
for a single feature, plus ChangeLog entries for each directory where
files were modified, and diffs for any changes needed to the manuals
(gdb/doc/gdb.texinfo or gdb/doc/gdbint.texinfo).  If there are a lot of
changes for a single feature, they can be split down into multiple
messages.

In this way, if we read and like the feature, we can add it to the
sources with a single patch command, do some testing, and check it in.
If you leave out the ChangeLog, we have to write one.  If you leave
out the doc, we have to puzzle out what needs documenting.  Etc.

The reason to send each change in a separate message is that we will not
install some of the changes.  They'll be returned to you with questions
or comments.  If we're doing our job correctly, the message back to you
will say what you have to fix in order to make the change acceptable.
The reason to have separate messages for separate features is so that
the acceptable changes can be installed while one or more changes are
being reworked.  If multiple features are sent in a single message, we
tend to not put in the effort to sort out the acceptable changes from
the unacceptable, so none of the features get installed until all are
acceptable.

If this sounds painful or authoritarian, well, it is.  But we get a lot
of bug reports and a lot of patches, and many of them don't get
installed because we don't have the time to finish the job that the bug
reporter or the contributor could have done.  Patches that arrive
complete, working, and well designed, tend to get installed on the day
they arrive.  The others go into a queue and get installed as time
permits, which, since the maintainers have many demands to meet, may not
be for quite some time.

Please send patches directly to the GDB maintainers at
@code{gdb-patches@@sourceware.cygnus.com}.

@section Obsolete Conditionals

Fragments of old code in GDB sometimes reference or set the following
configuration macros.  They should not be used by new code, and old uses
should be removed as those parts of the debugger are otherwise touched.

@table @code

@item STACK_END_ADDR
This macro used to define where the end of the stack appeared, for use
in interpreting core file formats that don't record this address in the
core file itself.  This information is now configured in BFD, and GDB
gets the info portably from there.  The values in GDB's configuration
files should be moved into BFD configuration files (if needed there),
and deleted from all of GDB's config files.

Any @file{@var{foo}-xdep.c} file that references STACK_END_ADDR
is so old that it has never been converted to use BFD.  Now that's old!

@item PYRAMID_CONTROL_FRAME_DEBUGGING
pyr-xdep.c
@item PYRAMID_CORE
pyr-xdep.c
@item PYRAMID_PTRACE
pyr-xdep.c

@item REG_STACK_SEGMENT
exec.c

@end table


@contents
@bye