binutils-gdb/gdb/doc/gdbint.texinfo

\input texinfo
@setfilename gdb-internals
@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 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
* BFD support for GDB::		How BFD and GDB interface
* Host versus Target::		What features are in which files
* Symbol Reading::		Defining new symbol readers
@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 @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, 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{./configure 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{./configure none} is also a good way to rebuild the top level Makefile
after changing Makefile.in, alldeps.mak, etc.

@emph{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
texinfo.tex needs to be included in the distribution.

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

@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, BFD support for GDB, 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{}.  The header file that defines the host system should be
called xm-@var{ARCH}.h, and should be specified as the value of @samp{XM_FILE=}
in @file{xconfig/@var{ARCH}}.  
You can also define @samp{CC}, @samp{REGEX} and @samp{REGEX1},
@samp{SYSV_DEFINE}, @samp{XM_CFLAGS}, etc in there; see @file{Makefile.in}.

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.@refill
@end table

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

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 BFD support for GDB, Host versus Target, New Architectures, 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

The interface for symbol reading is described in @xref{Symbol Reading}.

@node Host versus Target, Symbol Reading, BFD support for GDB, 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.

@menu
* New Host::			Adding a New Host
* New Target::			Adding a New Target
* New Config::			Extending @code{configure}
@end menu

@node New Host, New Target, Host versus Target, Host versus Target
@section Adding a New Host

There are two halves 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, 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{New Target}.)

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 XXX (e.g. sun4),
and its full three-part configuration name is XARCH-XVEND-XOS (e.g.
sparc-sun-sunos4).  In particular:

At the top level, edit @file{config.sub} and add XARCH, XVEND, and
XOS to the lists of supported architectures, vendors, and operating systems
near the bottom of the file.  Also, add XXX as an alias that maps to
XARCH-XVEND-XOS.  You can test your changes by running

	./config.sub XXX
and	./config.sub XARCH-XVEND-XOS

which should both respond with XARCH-XVEND-XOS and no error messages.  

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

	#if HOST_SYS==XXX_SYS
	#include <sys/h-XXX.h>
	#endif

Now create a new file @file{include/sys/h-XXX.h}.  Examine the other
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 bfd directory and edit @file{bfd/configure.in}.  Add shell
script code to recognize your XARCH-XVEND-XOS configuration, and set
bfd_host to XXX when you recognize it.  Now create a file
@file{bfd/config/hmake-XXX}, which includes the line:

	HDEFINES=-DHOST_SYS=XXX_SYS

(If you have the binutils in the same tree, you'll have to do the same
thing to in the binutils directory as you've done in the bfd directory.)

It's likely that the libiberty and 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
hmake-XXX file.  Then add @file{hmake-XXX} to the @file{config/} subdirectory,
to set any makefile variables you need.  The only current options
in there are things like -DSYSV.

Aha!  Now to configure GDB itself!  Modify @file{gdb/configure.in} to
recognize your system and set gdb_host to XXX.  Add a file 
@file{gdb/xconfig/XXX} which specifies XDEPFILES=(whatever is needed),
and XM_FILE= xm-XXX.h.  Create @file{gdb/xm-XXX.h} with the appropriate
#define's for your system (crib from existing xm-*.h files).
If your machine needs custom support routines, you can put them in
a file @file{gdb/XXX-xdep.c}, and add XXX-xdep.o to the XDEPFILES=
line.  If not, you can use the generic routines for ptrace support
(infptrace.o) and core files (coredep.o).  These can be customized
in various ways by macros defined in your @file{xm-XXX.h} file.

Now, from the top level (above bfd, gdb, etc), run:

	./configure -template=./configure

This will rebuild all your configure scripts, using the new
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:

	./configure XXX +target=vxworks960

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} XXX yet.)

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

	make

Good luck!  Comments and suggestions about this section are particularly
welcome; send them to 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
bfd/trad-core.c.   First, use whatever #include files your machine uses
to define the struct of registers that is accessible (possibly in the
upage) in a core file (rather than <machine/reg.h>), and an include
file that defines whatever header exists on a core file (e.g. the
u-area or a "struct core").  Then modify @samp{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 fetch_core_registers.
If you can use the generic one, it's in core-dep.c; if not, it's in
your foobar-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 New Target, New Config, New Host, Host versus Target
@section Adding a New Target

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 are beyond the scope
of this document right now.  Look at the structure of the a.out
and COFF support, build a transfer vector (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 XARCH-opcode.h file, a tm-XARCH.h file that gives
the basic layout of the chip (registers, stack, etc), probably
an XARCH-tdep.c file that has support routines for tm-XARCH.h, etc.

If you are adding a new operating system for an existing CPU chip,
add a tm-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-XARCH-XOS.h}
that just #include's tm-XARCH.h and tm-XOS.h.  (Now that we have
three-part configuration names, this will probably get revised to
separate the OS configuration from the ARCH configuration.  FIXME.)

@node New Config,  , New Target, Host versus Target
@section Extending @code{configure}
Once you have added a new host, target, or both, you'll also need to
extend the @code{configure} script to recognize the new configuration
possibilities.  

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}.  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!)

@node Symbol Reading,  , Host versus Target, 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 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{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{XXX_symfile_init}, but it can call
@code{error} if it detects an unavoidable problem.

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

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

@table @code
@item 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

@contents
@bye