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@setfilename gdb-internals
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This file documents the internals of the GNU debugger GDB.

Copyright (C) 1990, 1991 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 odd
@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 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, Cleanups, (dir), (dir)

@menu
* Cleanups::			Cleanups
* Wrapping::			Wrapping output lines
* Releases::			Configuring GDB for release
* README::			The README file
* New Architectures::		Defining a new host or target architecture
* Host versus Targt::		What features are in which files

@end menu

@node Cleanups, Wrapping, Top, Top
@chapter Cleanups

Cleanups are a structured way to deal with things that need to be done
later.  When your code does something (like 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 old_chain = make_cleanup (function, arg);
This makes a cleanup which will cause FUNCTION to be called with ARG
(a char *) later.  The result, OLD_CHAIN, is a handle that can be
passed to do_cleanups or discard_cleanups later.  Unless you are
going to call do_cleanups or discard_cleanups yourself,
you can ignore the result from make_cleanup.


@item do_cleanups (old_chain);
Performs all cleanups done since make_cleanup returned OLD_CHAIN.
E.g.:   make_cleanup (a, 0); old = make_cleanup (b, 0); do_cleanups (old);
will call b() but will not call a().  The cleanup that calls a() will remain
in the cleanup chain, and will be done later unless otherwise discarded.

@item discard_cleanups (old_chain);
Same as 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, Releases, Cleanups, Top
@chapter Wrapping output lines

Output that goes through printf_filtered or fputs_filtered or
fputs_demangled needs only to have calls to 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 wrap_here() is an indentation string which is printed
ONLY if the line breaks there.  This argument is saved away and used
later.  It must remain valid until the next call to wrap_here() or
until a newline has been printed through the *_filtered functions.
Don't pass in a local variable and then return!

It is usually best to call 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 ("printf") output.  Symbol reading routines that print
warnings are a good example.


@node Releases, README, Wrapping, Top
@chapter Configuring GDB for release


GDB should be released after doing @samp{config.gdb none} in the top level
directory.  This will leave a makefile there, but no tm- or xm- files.
The makefile is needed, for example, for @samp{make gdb.tar.Z}@dots{}  If you
have tm- or xm-files in the main source directory, C's include rules
cause them to be used in preference to tm- and xm-files in the 
subdirectories where the user will actually configure and build the
binaries.

@samp{config.gdb none} is also a good way to rebuild the top level Makefile
after changing Makefile.dist, alldeps.mak, etc.



@node README, New Architectures, Releases, Top
@chapter The README file


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



@node New Architectures, Host versus Target, README, Top
@chapter Defining a new host or target architecture


When building support for a new host and/or target, this will help you
organize where to put the various parts.  @var{ARCH} stands for the
architecture involved.

Object files needed when the host system is an @var{ARCH} are listed in
the file @file{xconfig/@var{ARCH}}, in the Makefile macro @samp{XDEPFILES
= }@dots{}.  You can also define XXXXXX in there.

There are some ``generic'' versions of routines that can be used by
various host systems.  If these routines work for the @var{ARCH} host,
you can just include the generic file's name (with .o, not .c) in
@code{XDEPFILES}.  Otherwise, you will need to write routines that
perform the same functions as the generic file, put them into
@code{@var{ARCH}-xdep.c}, and put @code{@var{ARCH}-xdep.o} into
@code{XDEPFILES}.  These generic host support files include:

@example
	coredep.c, coredep.o
@end example

@table @code
@item fetch_core_registers()
Support for reading registers out of a core file.  This routine calls
@code{register_addr(}), see below.

@item register_addr()
If your @code{xm-@var{ARCH}.h} file defines the macro @code{REGISTER_U_ADDR(reg)} to be the
offset within the @samp{user} struct of a register (represented as a GDB
register number), @file{coredep.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{ARCH}-xdep.c} file, and be sure @code{@var{ARCH}-xdep.o} is in the @code{XDEPFILES} list.
If you have your own @code{fetch_core_registers}, you only need to define
@code{register_addr} if your @code{fetch_core_registers} calls it.  Many custom
@code{fetch_core_registers} implementations simply locate the registers
themselves.
@end table

Files needed when the target system is an @var{ARCH} are listed in the file
@file{tconfig/@var{ARCH}}, in the @code{Makefile} macro @samp{TDEPFILES = }@dots{}.  You can also
define XXXXXX in there.

Similar generic support files for target systems are:

@example
	exec.c, exec.o:
@end example

This file 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 architecture
should be able to use the generic exec.c rather than its own custom code.

@node Host versus Target, , README, Top
@chapter What is considered ``host-dependent'' versus ``target-dependent''?

The xconfig/*, xm-*.h and *-xdep.c files are for host support.  The
question is, what features or aspects of a debugging or cross-debugging
environment are considered to be ``host'' support.

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.

Unix child process support is considered an aspect of the host.  Since
when you fork on the host you are still on the host, the various macros
needed for finding the registers in the upage, running ptrace, and such
are all in the host-dependent files.

This is still somewhat of a grey area; I (John Gilmore) didn't do the
xm- and tm- split for gdb (it was done by Jim Kingdon) so I have had to
figure out the grounds on which it was split, and make my own choices
as I evolve it.  I have moved many things out of the xdep files
actually, partly as a result of BFD and partly by removing duplicated
code.

@contents
@bye