old-cross-binutils/gdb/doc/gdbint.texinfo

\input texinfo
@setfilename gdbint.info
@c $Id$
@ifinfo
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 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 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, README, (dir), (dir)

@menu
* README::			The README File
* New Architectures::		Defining a New Host or Target Architecture
* Config::			Adding a New Configuration
* Host::			Adding a New Host
* Target::			Adding a New Target
* Languages::			Defining New Source Languages
* Releases::			Configuring GDB for Release
* BFD support for GDB::		How BFD and GDB interface
* Symbol Reading::		Defining New Symbol Readers
* Cleanups::			Cleanups
* Wrapping::			Wrapping Output Lines

@end menu

@node README, New Architectures, Top, Top
@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 New Architectures, Config, README, Top
@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''?

The @file{xconfig/*}, @file{xm-*.h} and @file{*-xdep.c} files are for
host support.  Similarly, the @file{tconfig/*}, @file{tm-*.h} and
@file{*-tdep.c} files are for target 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 @code{ptrace}, and such
are all in the host-dependent files.

@c FIXME so what kinds of things are target support?

This is still somewhat of a grey area; I (John Gilmore) didn't do the
@file{xm-*} and @file{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.


@node Config, Host, New Architectures, Top
@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:

At the top level, 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.

Then edit @file{include/sysdep.h}.  Add a new @code{#define} for
@samp{@var{xxx}_SYS}, with a numeric value not already in use.  Add a
new section that says

@example
#if HOST_SYS==@var{xxx}_SYS
#include <sys/h-@var{xxx}.h>
#endif
@end example

Now create a new file @file{include/sys/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 GDB.

Now, go to the @file{bfd} directory and edit @file{bfd/configure.in}.
Add shell script code to recognize your
@code{@var{xarch}-@var{xvend}-@var{xos}} configuration, and set
@code{bfd_host} to @var{xxx} when you recognize it.  Now create a file
@file{bfd/config/hmake-@var{xxx}}, which includes the line:

@example
HDEFINES=-DHOST_SYS=@var{xxx}_SYS
@end example

(If you have the binary utilities in the same tree, you'll have to do
the same thing to in the @file{binutils} directory as you've done in the
@file{bfd} directory.)

It's likely 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{hmake-@var{xxx}} file.  Then add @file{hmake-@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}.

Aha!  Now to configure GDB itself!  You shouldn't edit the
@code{configure} script itself to add hosts or targets; instead, edit
the script fragments in the file @code{configure.in}.  Modify
@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} ?

Then fold your changes into the @code{configure} script by using the
@code{+template} option, and specifying @code{configure} itself as the
template:
@example
configure +template=configure
@end example
@c If "configure" is the only workable option value for +template, it's
@c kind of silly to insist that it be provided.  If it isn't, somebody
@c please fill in here what are others... (then delete this comment!)

Finally, you'll need to specify and define the host- and
target-dependent files used for your configuration; the next two
chapters discuss those.


@node Host, Target, Config, Top
@chapter Adding a New Host

Once you have specified a new configuration for your host
(@pxref{Config,,Adding a New Configuration}), there are two 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}.

Add a file @file{gdb/xconfig/@var{xxx}} which specifies whatever object
files are needed as the makefile macro @samp{XDEPFILES=@dots{}}.  In the
same file, specify the header file to describe @var{xxx} as
@samp{XM_FILE= xm-@var{xxx}.h}.  You can also define @samp{CC},
@samp{REGEX} and @samp{REGEX1}, @samp{SYSV_DEFINE}, @samp{XM_CFLAGS},
etc. in there; see @file{Makefile.in}.

Create @file{gdb/xm-@var{xxx}.h} with the appropriate @code{#define}s
for your system (crib from existing @file{xm-*.h} files).

There are some ``generic'' versions of routines that can be used by
various host 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}.  

@subheading Generic Host Support Files

@table @code

@item infptrace.c
@code{ptrace} support.

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

@item coredep.c:register_addr()
If your @code{xm-@var{xxx}.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{xxx}-xdep.c} file, and be sure @code{@var{xxx}-xdep.o} is in
the @code{XDEPFILES} 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

Object files needed when the target system is an @var{xxx} are listed
in the file @file{tconfig/@var{xxx}}, in the makefile macro
@samp{TDEPFILES = }@dots{}.  The header file that defines the target
system should be called @file{tm-@var{xxx}.h}, and should be specified
as the value of @samp{TM_FILE} in @file{tconfig/@var{xxx}}.  You can
also define @samp{TM_CFLAGS}, @samp{TM_CLIBS}, and @samp{TM_CDEPS} in
there; see @file{Makefile.in}.

Now, from the top level (above @file{bfd}, @file{gdb}, etc), run:

@example
./configure -template=./configure
@end example

This will rebuild all your configure scripts, using the new
@file{configure.in} files that you modified.  (You can also run this command
at any subdirectory level.)  You are now ready to try configuring
GDB to compile for your system.  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

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

When hosting GDB 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-dep.c}; if not, it's in
your @file{@var{xxx}-xdep.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}
for more info about this.)


@node Target, Languages, Host, Top
@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.

Add a file @file{gdb/tconfig/@var{ttt}} which specifies whatever object
files are needed as the makefile macro @samp{TDEPFILES=@dots{}}.  In the
same file, specify the header file to describe @var{ttt} as
@samp{TM_FILE= tm-@var{ttt}.h}.  You can also define @samp{CC},
@samp{REGEX} and @samp{REGEX1}, @samp{SYSV_DEFINE}, @samp{XM_CFLAGS},
etc. in there; see @file{Makefile.in}.

Create @file{gdb/tm-@var{ttt}.h} with the appropriate @code{#define}s
for your system (crib from existing @file{tm-*.h} files).

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

The file @file{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.

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 are beyond the scope
of this document right now.  Look at the structure of the a.out
and COFF support, build a transfer vector (@code{xvec}) for your new format,
and start populating it with routines.  Add it to the list in 
@file{bfd/targets.c}.

If you are adding a new existing CPU chip (e.g. m68k family), you'll
need to define an @file{@var{xarch}-opcode.h} file, a
@file{tm-@var{xarch}.h} file that gives the basic layout of the chip
(registers, stack, etc), probably an @file{@var{xarch}-tdep.c} file that
has support routines for @file{tm-@var{xarch}.h}, etc.

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, Releases, Target, Top
@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, the following lines
@emph{must} be included at the top of the YACC parser:

@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
@end example

This will prevent name conflicts between the various parsers.

@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{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, BFD support for GDB, Languages, Top
@chapter Configuring GDB for Release

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

@samp{./configure none} is also a good way to rebuild the top level @file{Makefile}
after changing @file{Makefile.in}, @file{alldeps.mak}, 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 BFD support for GDB, Symbol Reading, Releases, Top
@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 Reading}.


@node Symbol Reading, Cleanups, BFD support for GDB, Top
@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, Wrapping, Symbol Reading, Top
@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 @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,  , Cleanups, Top
@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.


@contents
@bye