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

Copyright 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
Contributed by Cygnus Support.  Written by John Gilmore.

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

@vskip 0pt plus 1filll
Copyright @copyright{} 1990, 1991, 1992, 1993, 1994 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 is
intended to document aspects of GDB which apply across many different
parts of GDB (for example, @pxref{Coding Style}), or which are global
aspects of design (for example, what are the major modules and which
files document them in detail?).  Information which pertains to specific
data structures, functions, variables, etc., should be put in comments
in the source code, not here.  It is more likely to get noticed and kept
up to date there.  Some of the information in this document should
probably be moved into comments.

@menu
* README::			The README File
* Getting Started::		Getting started working on GDB
* Debugging GDB::		Debugging GDB with itself
* New Architectures::		Defining a New Host or Target Architecture
* Config::			Adding a New Configuration
* Host::			Adding a New Host
* Native::			Adding a New Native Configuration
* Target::			Adding a New Target
* Languages::			Defining New Source Languages
* Releases::			Configuring GDB for Release
* Partial Symbol Tables::	How GDB reads symbols quickly at startup
* Types::			How GDB keeps track of types
* BFD support for GDB::		How BFD and GDB interface
* Symbol Reading::		Defining New Symbol Readers
* Cleanups::			Cleanups
* Wrapping::			Wrapping Output Lines
* Frames::			Keeping track of function calls
* Remote Stubs::		Code that runs in targets and talks to GDB
* Longjmp Support::		Stepping through longjmp's in the target
* Coding Style::		Strunk and White for GDB maintainers
* Clean Design::		Frank Lloyd Wright for GDB maintainers
* Submitting Patches::		How to get your changes into GDB releases
* Host Conditionals::		What features exist in the host
* Target Conditionals::		What features exist in the target
* Native Conditionals::		Conditionals for when host and target are same
* Obsolete Conditionals::	Conditionals that don't exist any more
* XCOFF::			The Object file format used on IBM's RS/6000
@end menu

@node README
@chapter The @file{README} File

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

@node Getting Started
@chapter Getting Started Working on GDB

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:

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

@item
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 (@pxref{Submitting Patches}).  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 has a list of macros (@pxref{Host Conditionals}, @pxref{Target
Conditionals}, @pxref{Native Conditionals}, and @pxref{Obsolete
Conditionals}) with some documentation.

@item
Start with the header files.  Once you 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.

@item
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.

@item
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.

@item
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}.

@item
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.
@end itemize

Good luck!

@node Debugging GDB
@chapter 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.

@node New Architectures
@chapter Defining a New Host or Target Architecture

When building support for a new host and/or target, much of the work you
need to do is handled by specifying configuration files;
@pxref{Config,,Adding a New Configuration}.  Further work can be
divided into ``host-dependent'' (@pxref{Host,,Adding a New Host}) and
``target-dependent'' (@pxref{Target,,Adding a New Target}).  The
following discussion is meant to explain the difference between hosts
and targets.

@heading What is considered ``host-dependent'' versus ``target-dependent''?

@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.

The dependent information in GDB is organized into files by naming
conventions.  

Host-Dependent Files
@table @file
@item	config/*/*.mh
Sets Makefile parameters
@item	config/*/xm-*.h
Global #include's and #define's and definitions
@item	*-xdep.c
Global variables and functions
@end table

Native-Dependent Files
@table @file
@item	config/*/*.mh
Sets Makefile parameters (for @emph{both} host and native)
@item	config/*/nm-*.h
#include's and #define's and definitions.  This file
is only included by the small number of modules that need it,
so beware of doing feature-test #define's from its macros.
@item	*-nat.c
global variables and functions
@end table

Target-Dependent Files
@table @file
@item	config/*/*.mt
Sets Makefile parameters
@item	config/*/tm-*.h
Global #include's and #define's and definitions
@item	*-tdep.c
Global variables and functions
@end table

At this writing, most supported hosts have had their host and native
dependencies sorted out properly.  There are a few stragglers, which
can be recognized by the absence of NATDEPFILES lines in their
@file{config/*/*.mh}.

@node Config
@chapter Adding a New Configuration

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{xxx} (e.g.
@samp{sun4}), and its full three-part configuration name is
@code{@var{xarch}-@var{xvend}-@var{xos}} (e.g.  @samp{sparc-sun-sunos4}).  In
particular:

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

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

Now, go to the @file{bfd} directory and 
create a new file @file{bfd/hosts/h-@var{xxx}.h}.  Examine the
other @file{h-*.h} files as templates, and create one that brings in the
right include files for your system, and defines any host-specific
macros needed by BFD, the Binutils, GNU LD, or the Opcodes directories.
(They all share the bfd @file{hosts} directory and the @file{configure.host}
file.)

Then edit @file{bfd/configure.host}.  Add a line to recognize your
@code{@var{xarch}-@var{xvend}-@var{xos}} configuration, and set
@code{my_host} to @var{xxx} when you recognize it.  This will cause your
file @file{h-@var{xxx}.h} to be linked to @file{sysdep.h} at configuration
time.  When creating the line that recognizes your configuration,
only match the fields that you really need to match; e.g. don't
match the architecture or manufacturer if the OS is sufficient
to distinguish the configuration that your @file{h-@var{xxx}.h} file supports.
Don't match the manufacturer name unless you really need to.
This should make future ports easier.

Also, if this host requires any changes to the Makefile, create a file
@file{bfd/config/@var{xxx}.mh}, which includes the required lines.

It's possible that the @file{libiberty} and @file{readline} directories
won't need any changes for your configuration, but if they do, you can
change the @file{configure.in} file there to recognize your system and
map to an @file{mh-@var{xxx}} file.  Then add @file{mh-@var{xxx}}
to the @file{config/} subdirectory, to set any makefile variables you
need.  The only current options in there are things like @samp{-DSYSV}.
(This @file{mh-@var{xxx}} naming convention differs from elsewhere
in GDB, by historical accident.  It should be cleaned up so that all
such files are called @file{@var{xxx}.mh}.)

Aha!  Now to configure GDB itself!  Edit
@file{gdb/configure.in} to recognize your system and set @code{gdb_host}
to @var{xxx}, and (unless your desired target is already available) also
set @code{gdb_target} to something appropriate (for instance,
@var{xxx}).  To handle new hosts, modify the segment after the comment
@samp{# per-host}; to handle new targets, modify after @samp{#
per-target}.
@c Would it be simpler to just use different per-host and per-target
@c *scripts*, and call them from {configure} ?

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; the next two chapters discuss those.


@node Host
@chapter Adding a New Host

Once you have specified a new configuration for your host
(@pxref{Config,,Adding a New Configuration}), there are three remaining
pieces to making GDB work on a new machine.  First, you have to make it
host on the new machine (compile there, handle that machine's terminals
properly, etc).  If you will be cross-debugging to some other kind of
system that's already supported, you are done.

If you want to use GDB to debug programs that run on the new machine,
you have to get it to understand the machine's object files, symbol
files, and interfaces to processes; @pxref{Target,,Adding a New Target}
and @pxref{Native,,Adding a New Native Configuration}

Several files control GDB's configuration for host systems:

@table @file
@item gdb/config/@var{arch}/@var{xxx}.mh
Specifies Makefile fragments needed when hosting on machine @var{xxx}.
In particular, this lists the required machine-dependent object files,
by defining @samp{XDEPFILES=@dots{}}.  Also
specifies the header file which describes host @var{xxx}, by defining
@code{XM_FILE= xm-@var{xxx}.h}.  You can also define @code{CC},
@code{REGEX} and @code{REGEX1}, @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{xxx}.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, ptrace support,
and core file structure.  Crib from existing @file{xm-*.h} files
to create a new one.

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

@table @file
@item ser-bsd.c
This contains serial line support for Berkeley-derived Unix systems.

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

@item ser-termios.c
This contains serial line support for System V-derived Unix systems.
@end table

Now, you are now ready to try configuring GDB to compile using your system
as its host.  From the top level (above @file{bfd}, @file{gdb}, etc), do:

@example
./configure @var{xxx} --target=vxworks960
@end example

This will configure your system to cross-compile for VxWorks on
the Intel 960, which is probably not what you really want, but it's
a test case that works at this stage.  (You haven't set up to be
able to debug programs that run @emph{on} @var{xxx} yet.)

If this succeeds, you can try building it all with:

@example
make
@end example

Repeat until the program configures, compiles, links, and runs.
When run, it won't be able to do much (unless you have a VxWorks/960
board on your network) but you will know that the host support is
pretty well done.

Good luck!  Comments and suggestions about this section are particularly
welcome; send them to @samp{bug-gdb@@prep.ai.mit.edu}.

@node Native
@chapter Adding a New Native Configuration

If you are making GDB run native on the @var{xxx} machine, you have
plenty more work to do.  Several files control GDB's configuration for
native support:

@table @file
@item gdb/config/@var{xarch}/@var{xxx}.mh
Specifies Makefile fragments needed when hosting @emph{or native}
on machine @var{xxx}.
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{xxx},
by defining @samp{NAT_FILE= nm-@var{xxx}.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{xxx}.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.
Crib from existing @file{nm-*.h} files to create a new one.

@item gdb/@var{xxx}-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

@subheading Generic Native 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{nm-@var{xxx}.h} file.  If these routines work for
the @var{xxx} 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{xxx}-nat.c}, and put @code{@var{xxx}-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.

@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{xxx}-nat.c} (or @code{REGISTER_U_ADDR} in @code{nm-@var{xxx}.h}).

@item core-aout.c::register_addr()
If your @code{nm-@var{xxx}.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{xxx}-nat.c} file, and be sure @code{@var{xxx}-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{xxx}-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.
(@xref{New Architectures,,Defining a New Host or Target Architecture},
for more info about this.)

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.

@node Target
@chapter Adding a New Target

For a new target called @var{ttt}, first specify the configuration as
described in @ref{Config,,Adding a New Configuration}.  If your new
target is the same as your new host, you've probably already done that.

A variety of files specify attributes of the GDB target environment:

@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{}}.
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},
and other Makefile variables here; see @file{Makefile.in}.

@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.
Crib from existing @file{tm-*.h} files when building a new one.

@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.

@item gdb/exec.c 
Defines functions for accessing files that are
executable on the target system.  These functions open and examine an
exec file, extract data from one, write data to one, print information
about one, etc.  Now that executable files are handled with BFD, every
target should be able to use the generic exec.c rather than its
own custom code.

@item gdb/@var{arch}-pinsn.c
Prints (disassembles) the target machine's instructions.
This file is usually shared with other target machines which use the
same processor, which is why it is @file{@var{arch}-pinsn.c} rather
than @file{@var{ttt}-pinsn.c}.

@item gdb/@var{arch}-opcode.h
Contains some large initialized
data structures describing the target machine's instructions.
This is a bit strange for a @file{.h} file, but it's OK since
it is only included in one place.  @file{@var{arch}-opcode.h} is shared
between the debugger and the assembler, if the GNU assembler has been
ported to the target machine.

@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{xxx}.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

When adding support for a new target machine, there are various areas
of support that might need change, or might be OK.

If you are using an existing object file format (a.out or COFF), 
there is probably little to be done.  See @file{bfd/doc/bfd.texinfo}
for more information on writing new a.out or COFF versions.

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 right now.  Basically
you must build a transfer vector (of type @code{bfd_target}), which will
mean writing all the required routines, and add it to the list in
@file{bfd/targets.c}.

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/libxxx.h}, which is included by GDB.

If you are adding a new operating system for an existing CPU chip, add a
@file{tm-@var{xos}.h} file that describes the operating system
facilities that are unusual (extra symbol table info; the breakpoint
instruction needed; etc).  Then write a
@file{tm-@var{xarch}-@var{xos}.h} that just @code{#include}s
@file{tm-@var{xarch}.h} and @file{tm-@var{xos}.h}.  (Now that we have
three-part configuration names, this will probably get revised to
separate the @var{xos} configuration from the @var{xarch}
configuration.)


@node Languages
@chapter 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 Releases
@chapter 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 Partial Symbol Tables
@chapter 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.  (@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.

@node Types
@chapter 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.

@node BFD support for GDB
@chapter Binary File Descriptor Library Support for GDB

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

@c The interface for symbol reading is described in @ref{Symbol
@c Reading,,Symbol Reading}.


@node Symbol Reading
@chapter 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 ``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 @file{symfile.c} using the BFD
library.  BFD identifies the type of the file by examining its header.
@code{symfile_init} 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{xxx}_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{xxx}_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{xxx}_symfile_init}, but it can call
@code{error} if it detects an unavoidable problem.

@item @var{xxx}_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{xxx}_symfile_init}, if a new symbol
table is being read, or may be called alone if all symbols are
simply being discarded.

@item @var{xxx}_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{xxx}_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{xxx}_symfile_read is called, these psymtabs will contain a pointer to
a function @code{@var{xxx}_psymtab_to_symtab}, which can be called from
any point in the GDB symbol-handling code.

@table @code
@item @var{xxx}_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


@node Cleanups
@chapter 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).


@node Wrapping
@chapter 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.


@node Frames
@chapter Frames

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

@table @code
@item 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.)

@item FRAME_CHAIN
Given a GDB frame, 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.
@end table

@node Remote Stubs
@chapter 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.

@node Longjmp Support
@chapter 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-xxx.h} file.  Look in @file{tm-sun4os4.h} and
@file{sparc-tdep.c} for examples of how to do this.

@node Coding Style
@chapter Coding Style

GDB is generally written using the GNU coding standards, as described in
@file{standards.texi}, which is available for anonymous FTP from GNU
archive sites.  There are some additional considerations for GDB
maintainers that reflect the unique environment and style of GDB
maintenance.  If you follow these guidelines, GDB will be more
consistent and easier to maintain.

GDB's policy on the use of prototypes is that prototypes are used
to @emph{declare} functions but never to @emph{define} them.  Simple
macros are used in the declarations, so that a non-ANSI compiler can
compile GDB without trouble.  The simple macro calls are used like
this:

@example @code
extern int
memory_remove_breakpoint PARAMS ((CORE_ADDR, char *));
@end example

Note the double parentheses around the parameter types.  This allows
an arbitrary number of parameters to be described, without freaking
out the C preprocessor.  When the function has no parameters, it
should be described like:

@example @code
void
noprocess PARAMS ((void));
@end example

The @code{PARAMS} macro expands to its argument in ANSI C, or to a simple
@code{()} in traditional C.

All external functions should have a @code{PARAMS} declaration in a
header file that callers include.  All static functions should have such
a declaration near the top of their source file.

We don't have a gcc option that will properly check that these rules
have been followed, but it's GDB policy, and we periodically check it
using the tools available (plus manual labor), and clean up any remnants.

@node Clean Design
@chapter 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.  In particular:

@table @bullet
@item
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}.

@item
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.

@item
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.

@item
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.

@item
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.

@item
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.

@item
@emph{Do} write code that doesn't 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.  In short, follow good 
programming practices for writing portable C code.

@end table

@node Submitting Patches
@chapter 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 two main problems with getting your patches in are,

@table @bullet
@item
The GDB maintainers will only install ``cleanly designed'' patches.
You may not always agree on what is clean design.
@pxref{Coding Style}, @pxref{Clean Design}.

@item
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.
@end table

I don't know how to get past these problems except by continuing to try.

There are two issues here -- technical and legal.

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 document for doing this by sending mail to
@code{gnu@@prep.ai.mit.edu} and asking for it.  I 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.  I can't start
merging changes until this paperwork is received by the FSF (their
rules, which I follow since I 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.texi or gdb/doc/gdbint.texi).  If there
are a lot of changes for a single feature, they can be split down
into multiple messages.

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

The reason to send each change in a separate message is that I will
not install some of the changes.  They'll be returned to you with
questions or comments.  If I'm doing my job, my 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 other changes (which I @emph{am} willing to accept) can be installed
while one or more changes are being reworked.  If multiple features
are sent in a single message, I 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 I get a lot
of bug reports and a lot of patches, and most of them don't get
installed because I 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 if and when
I scan back over the queue -- which can literally take months
sometimes.  It's in both our interests to make patch installation easy
-- you get your changes installed, and I make some forward progress on
GDB in a normal 12-hour day (instead of them having to wait until I
have a 14-hour or 16-hour day to spend cleaning up patches before I
can install them).

Please send patches to @code{bug-gdb@@prep.ai.mit.edu}, if they are less
than about 25,000 characters.  If longer than that, either make them
available somehow (e.g. anonymous FTP), and announce it on
@code{bug-gdb}, or send them directly to the GDB maintainers at
@code{gdb-patches@@cygnus.com}.

@node Host Conditionals
@chapter 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:

@emph{NOTE:  For now, both host and target conditionals are here.
Eliminate target conditionals from this list as they are identified.}

@table @code

@item BLOCK_ADDRESS_FUNCTION_RELATIVE
dbxread.c

@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_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 ADDITIONAL_OPTIONS
main.c
@item ADDITIONAL_OPTION_CASES
main.c
@item ADDITIONAL_OPTION_HANDLER
main.c
@item ADDITIONAL_OPTION_HELP
main.c

@item AIX_BUGGY_PTRACE_CONTINUE
infptrace.c

@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 CFRONT_PRODUCER
dwarfread.c
@item DBX_PARM_SYMBOL_CLASS
stabsread.c

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

@item DEV_TTY
symmisc.c
@item DO_REGISTERS_INFO
infcmd.c

@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.

@sc{ANSI} definition.

@item FILES_INFO_HOOK
target.c
@item FLOAT_INFO
infcmd.c

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

@item GCC2_COMPILED_FLAG_SYMBOL
dbxread.c
@item GCC_COMPILED_FLAG_SYMBOL
dbxread.c
@item GCC_MANGLE_BUG
symtab.c
@item GCC_PRODUCER
dwarfread.c

@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 GPLUS_PRODUCER
dwarfread.c

@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
inflow.c

@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 KERNEL_DEBUGGING
tm-ultra3.h

@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 LCC_PRODUCER
dwarfread.c

@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 will be defined automatically if GNU CC is used to compile GDB.

@item PRINTF_HAS_LONG_LONG
Define this if the host can handle printing of long long integers via a
format directive ``ll''.

@item LSEEK_NOT_LINEAR
source.c
@item L_LNNO32
coffread.c

@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 MAINTENANCE_CMDS
If the value of this is 1, then a number of optional maintenance commands
are compiled in.

@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 NOTICE_SIGNAL_HANDLING_CHANGE
infrun.c
@item NO_HIF_SUPPORT
remote-mm.c
@item NO_JOB_CONTROL
signals.h

@item NO_MMALLOC
GDB will use the @code{mmalloc} library for memory allocation for symbol
reading, unless this symbol is defined.  Define it on 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{NO_MMALLOC}, you will also have
to override the setting of @code{MMALLOC_LIB} to empty, in the Makefile.
Therefore, this define is usually set on the command line by overriding
@code{MMALLOC_DISABLE} in @file{config/*/*.mh}, rather than by defining
it in @file{xm-*.h}.

@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.

@item NO_SIGINTERRUPT
remote-adapt.c
@item NUMERIC_REG_NAMES
mips-tdep.c
@item N_SETV
dbxread.c
@item N_SET_MAGIC
hppabsd-tdep.c
@item ONE_PROCESS_WRITETEXT
breakpoint.c
@item O_BINARY
exec.c
@item O_RDONLY
xm-ultra3.h
@item PCC_SOL_BROKEN
dbxread.c
@item PC_LOAD_SEGMENT
stack.c
@item PRINT_RANDOM_SIGNAL
infcmd.c
@item PRINT_REGISTER_HOOK
infcmd.c
@item PROCESS_LINENUMBER_HOOK
buildsym.c
@item PROLOGUE_FIRSTLINE_OVERLAP
infrun.c
@item PUSH_ARGUMENTS
valops.c
@item PYRAMID_CONTROL_FRAME_DEBUGGING
pyr-xdep.c
@item PYRAMID_CORE
pyr-xdep.c
@item PYRAMID_PTRACE
pyr-xdep.c
@item REGISTER_BYTES
remote.c
@item REG_STACK_SEGMENT
exec.c
@item REG_STRUCT_HAS_ADDR
findvar.c
@item R_FP
dwarfread.c
@item R_OK
xm-altos.h
@item SEEK_END
state.c
@item SEEK_SET
state.c
@item SEM
coffread.c

@item SHELL_COMMAND_CONCAT
infrun.c
@item SHELL_FILE
infrun.c
@item SHIFT_INST_REGS
breakpoint.c
@item SIGTRAP_STOP_AFTER_LOAD
infrun.c
@item STACK_ALIGN
valops.c
@item STOP_SIGNAL
main.c
@item SUN4_COMPILER_FEATURE
infrun.c
@item SUN_FIXED_LBRAC_BUG
dbxread.c
@item SVR4_SHARED_LIBS
solib.c
@item SYMBOL_RELOADING_DEFAULT
symfile.c
@item TIOCGETC
inflow.c
@item TIOCGLTC
inflow.c
@item TIOCGPGRP
inflow.c
@item TIOCLGET
inflow.c
@item TIOCLSET
inflow.c
@item TIOCNOTTY
inflow.c
@item UPAGES
altos-xdep.c
@item USE_O_NOCTTY
inflow.c

@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 WRS_ORIG
remote-vx.c
@item alloca
defs.h
@item const
defs.h

@item lint
Define this to help lint in some stupid way.

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

@end table

Platform-specific host conditionals.

@table @code

@item ALTOS
altos-xdep.c
@item ALTOS_AS
xm-altos.h
@item MOTOROLA
xm-altos.h
@item NBPG
altos-xdep.c

@item BCS
tm-delta88.h

@item DELTA88
m88k-xdep.c
@item DGUX
m88k-xdep.c

@item F_OK
xm-ultra3.h

@end table

Regex conditionals.

@table @code

@item C_ALLOCA
regex.c
@item NFAILURES
regex.c
@item RE_NREGS
regex.h
@item SIGN_EXTEND_CHAR
regex.c
@item SWITCH_ENUM_BUG
regex.c
@item SYNTAX_TABLE
regex.c
@item Sword
regex.c
@item sparc
regex.c
@item test
regex.c

@end table

@node Target Conditionals
@chapter Target Conditionals

When GDB is configured and compiled, various macros are defined or left
undefined, to control compilation based on the attributes of the target
system.  These macros and their meanings are:

@emph{NOTE:  For now, both host and target conditionals are here.
Eliminate host conditionals from this list as they are identified.}

@table @code

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

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

@item BLOCK_ADDRESS_FUNCTION_RELATIVE
dbxread.c
@item PYRAMID_CONTROL_FRAME_DEBUGGING
pyr-xdep.c
@item ADDITIONAL_OPTIONS
main.c
@item ADDITIONAL_OPTION_CASES
main.c
@item ADDITIONAL_OPTION_HANDLER
main.c
@item ADDITIONAL_OPTION_HELP
main.c

@item ADDR_BITS_REMOVE (addr)
If a raw machine 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}.  For example, the two low-order
bits of a Motorola 88K address may be used by some kernels for their
own purposes, since addresses must always be 4-byte aligned, and so
are of no use for addressing.  Those bits should be filtered out with
an expression such as @code{((addr) & ~3)}.

@item ALIGN_STACK_ON_STARTUP
main.c
@item ALTOS
altos-xdep.c
@item ALTOS_AS
xm-altos.h
@item BCS
tm-delta88.h

@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.

@item BELIEVE_PCC_PROMOTION
coffread.c
@item BELIEVE_PCC_PROMOTION_TYPE
stabsread.c

@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 BLOCK_ADDRESS_ABSOLUTE
dbxread.c
@item BREAKPOINT
tm-m68k.h

@item CALL_DUMMY
valops.c
@item CALL_DUMMY_LOCATION
inferior.h
@item CALL_DUMMY_STACK_ADJUST
valops.c

@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 CFRONT_PRODUCER
dwarfread.c

@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 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
stabsread.c

@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 DELTA88
m88k-xdep.c
@item DEV_TTY
symmisc.c
@item DGUX
m88k-xdep.c

@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
infcmd.c

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

@item EXTRACT_RETURN_VALUE
tm-m68k.h
@item EXTRACT_STRUCT_VALUE_ADDRESS
values.c

@item EXTRA_FRAME_INFO
If defined, this must be a list of slots that may be inserted into
the @code{frame_info} structure defined in @code{frame.h}.

@item EXTRA_SYMTAB_INFO
If defined, this must be a list of slots that may be inserted into
the @code{symtab} structure defined in @code{symtab.h}.

@item FILES_INFO_HOOK
target.c
@item FLOAT_INFO
infcmd.c
@item FP0_REGNUM
a68v-xdep.c
@item FPC_REGNUM
mach386-xdep.c
@item FP_REGNUM
parse.c
@item FRAMELESS_FUNCTION_INVOCATION
blockframe.c
@item FRAME_ARGS_ADDRESS_CORRECT
stack.c

@item FRAME_CHAIN
Given FRAME, return a pointer to the calling frame.

@item FRAME_CHAIN_COMBINE
blockframe.c
@item FRAME_CHAIN_VALID
frame.h
@item FRAME_CHAIN_VALID_ALTERNATE
frame.h
@item FRAME_FIND_SAVED_REGS
stack.c
@item FRAME_GET_BASEREG_VALUE
frame.h

@item FRAME_NUM_ARGS (val, fi)
For the frame described by fi, set val to the number of arguments
that are being passed.

@item FRAME_SPECIFICATION_DYADIC
stack.c

@item FRAME_SAVED_PC
Given 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 GCC2_COMPILED_FLAG_SYMBOL
dbxread.c
@item GCC_COMPILED_FLAG_SYMBOL
dbxread.c
@item GCC_MANGLE_BUG
symtab.c
@item GCC_PRODUCER
dwarfread.c

@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 GDB_TARGET_IS_MACH386
mach386-xdep.c
@item GDB_TARGET_IS_SUN3
a68v-xdep.c
@item GDB_TARGET_IS_SUN386
sun386-xdep.c

@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}.
Currently this is only done for the a29k.

@item GPLUS_PRODUCER
dwarfread.c

@item GR64_REGNUM
Very a29k-specific.

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

@item IGNORE_SYMBOL type
This seems to be no longer used.

@item INIT_EXTRA_FRAME_INFO (fromleaf, fci)
If defined, this should be a C expression or statement that fills
in the @code{EXTRA_FRAME_INFO} slots of the given frame @var{fci}.

@item INIT_EXTRA_SYMTAB_INFO
symfile.c

@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
Define this to be either @code{<} if the target's stack grows
downward in memory, or @code{>} is the stack grows upwards.

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

@item SIGTRAMP_START
@item SIGTRAMP_END
Define these to be the start and end address of the sigtramp.
These will be used if defined, and @code{IN_SIGTRAMP} is not;
otherwise the name of the sigtramp will be assumed to be @code{_sigtramp}.

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

@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 KERNEL_DEBUGGING
tm-ultra3.h
@item LCC_PRODUCER
dwarfread.c
@item L_LNNO32
coffread.c
@item MIPSEL
mips-tdep.c
@item MOTOROLA
xm-altos.h
@item NBPG
altos-xdep.c

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

@item NOTICE_SIGNAL_HANDLING_CHANGE
infrun.c
@item NO_HIF_SUPPORT
remote-mm.c
@item NO_SIGINTERRUPT
remote-adapt.c

@item NO_SINGLE_STEP
Define this if the target does not support single-stepping.
If this is defined, you must supply, in @code{*-tdep.c}, the function
@code{single_step}, which takes a pid as argument and returns nothing.
It must insert breakpoints at each possible destinations of the next
instruction.  See @code{sparc-tdep.c} and @code{rs6000-tdep.c}
for examples.

@item NUMERIC_REG_NAMES
mips-tdep.c
@item N_SETV
dbxread.c
@item N_SET_MAGIC
hppabsd-tdep.c
@item ONE_PROCESS_WRITETEXT
breakpoint.c
@item PCC_SOL_BROKEN
dbxread.c
@item PC_IN_CALL_DUMMY
inferior.h
@item PC_LOAD_SEGMENT
stack.c

@item PC_REGNUM
If the program counter is kept in a register, then define this macro
to be the number of that register.
This need be defined only if @code{TARGET_WRITE_PC} is 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 PRINT_RANDOM_SIGNAL
infcmd.c
@item PRINT_REGISTER_HOOK
infcmd.c

@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
buildsym.c
@item PROLOGUE_FIRSTLINE_OVERLAP
infrun.c
@item PS_REGNUM
parse.c
@item PUSH_ARGUMENTS
valops.c
@item REGISTER_BYTES
remote.c

@item REGISTER_NAMES
Define this to expand into an initializer of an array of strings.
Each string is the name of a register.
[more detail]

@item REG_STACK_SEGMENT
exec.c
@item REG_STRUCT_HAS_ADDR
findvar.c
@item R_FP
dwarfread.c
@item R_OK
xm-altos.h

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

@item SEEK_END
state.c
@item SEEK_SET
state.c
@item SEM
coffread.c
@item SHELL_COMMAND_CONCAT
infrun.c
@item SHELL_FILE
infrun.c
@item SHIFT_INST_REGS
breakpoint.c
@item SIGTRAP_STOP_AFTER_LOAD
infrun.c

@item SKIP_PROLOGUE
A C statement that advances the PC across any function entry
prologue instructions so as to reach ``real'' code.

@item SKIP_PROLOGUE_FRAMELESS_P
A C statement 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
parse.c

@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
valops.c
@item STOP_SIGNAL
main.c

@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 SUN4_COMPILER_FEATURE
infrun.c
@item SUN_FIXED_LBRAC_BUG
dbxread.c
@item SVR4_SHARED_LIBS
solib.c
@item SYMBOL_RELOADING_DEFAULT
symfile.c

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

@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}.

@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}.

@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 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.

@item WRS_ORIG
remote-vx.c

@item test
(Define this to enable testing code in regex.c.)

@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

@node Native Conditionals
@chapter 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 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 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''; @pxref{Host}.

@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 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 Obsolete Conditionals
@chapter 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!

@end table

@node XCOFF
@chapter The XCOFF Object File Format

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, @xref{Top,,,stabs,The Stabs Debugging Format}, and search
for XCOFF.

The shared library scheme has a nice 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).

@contents
@bye