* gdbint.texinfo: Change Permissions to GFDL. Update Copyright.

[deliverable/binutils-gdb.git] / gdb / doc / gdbint.texinfo

\input texinfo   @c -*- texinfo -*-
@setfilename gdbint.info
@include gdb-cfg.texi
@dircategory Programming & development tools.
@direntry
START-INFO-DIR-ENTRY
* Gdb-Internals: (gdbint).      The GNU debugger's internals.
END-INFO-DIR-ENTRY
@end direntry

@ifinfo
This file documents the internals of the GNU debugger @value{GDBN}.
Copyright 1990,1991,1992,1993,1994,1996,1998,1999,2000,2001
   Free Software Foundation, Inc.
Contributed by Cygnus Solutions.  Written by John Gilmore.
Second Edition by Stan Shebs.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``Algorithms'' and ``Porting GDB'', with the
Front-Cover texts being ``A GNU Manual,'' and with the Back-Cover
Texts as in (a) below.

(a) The FSF's Back-Cover Text is: ``You have freedom to copy and modify
this GNU Manual, like GNU software.  Copies published by the Free
Software Foundation raise funds for GNU development.''
@end ifinfo

@setchapternewpage off
@settitle @value{GDBN} Internals

@syncodeindex fn cp
@syncodeindex vr cp

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

@vskip 0pt plus 1filll
Copyright @copyright{} 1990,1991,1992,1993,1994,1996,1998,1999,2000,2001
   Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``Algorithms'' and ``Porting GDB'', with the
Front-Cover texts being ``A GNU Manual,'' and with the Back-Cover
Texts as in (a) below.

(a) The FSF's Back-Cover Text is: ``You have freedom to copy and modify
this GNU Manual, like GNU software.  Copies published by the Free
Software Foundation raise funds for GNU development.''
@end titlepage

@c TeX can handle the contents at the start but makeinfo 3.12 can not
@iftex
@contents
@end iftex

@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, @value{GDBN}.  It
includes description of @value{GDBN}'s key algorithms and operations, as well
as the mechanisms that adapt @value{GDBN} to specific hosts and targets.

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

@node Requirements

@chapter Requirements
@cindex requirements for @value{GDBN}

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

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

@value{GDBN} is an interactive tool.  Although a batch mode is
available, @value{GDBN}'s primary role is to interact with a human
programmer.

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

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

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

@value{GDBN} should be able to run everywhere.  No other debugger is
available for even half as many configurations as @value{GDBN}
supports.


@node Overall Structure

@chapter Overall Structure

@value{GDBN} consists of three major subsystems: user interface,
symbol handling (the @dfn{symbol side}), and target system handling (the
@dfn{target side}).

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

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

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

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

@section The Symbol Side

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

@section The Target Side

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

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

@section Configurations

@cindex host
@cindex target
@dfn{Host} refers to attributes of the system where @value{GDBN} 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 @value{GDBN} work ``native'' on a particular machine, you
have to include all three kinds of information.


@node Algorithms

@chapter Algorithms
@cindex algorithms

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

@section Frames

@cindex frame
@cindex call stack frame
A frame is a construct that @value{GDBN} uses to keep track of calling
and called functions.

@findex create_new_frame
@vindex FRAME_FP
@code{FRAME_FP} in the machine description has no meaning to the
machine-independent part of @value{GDBN}, 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

@cindex frame pointer register
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.)

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

@section Breakpoint Handling

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

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

@cindex hardware breakpoints
@cindex program counter
Hardware breakpoints are sometimes available as a builtin debugging
features with some chips.  Typically these work by having dedicated
register into which the breakpoint address may be stored.  If the PC
(shorthand for @dfn{program counter})
ever matches a value in a breakpoint registers, the CPU raises an
exception and reports it to @value{GDBN}.

Another possibility is when an emulator is in use; many emulators
include circuitry that watches the address lines coming out from the
processor, and force it to stop if the address matches a breakpoint's
address.

A third possibility is that the target already has the ability to do
breakpoints somehow; for instance, a ROM monitor may do its own
software breakpoints.  So although these are not literally ``hardware
breakpoints'', from @value{GDBN}'s point of view they work the same;
@value{GDBN} need not do nothing more than set the breakpoint and wait
for something to happen.

Since they depend on hardware resources, hardware breakpoints may be
limited in number; when the user asks for more, @value{GDBN} will
start trying to set software breakpoints.  (On some architectures,
notably the 32-bit x86 platforms, @value{GDBN} cannot alsways know
whether there's enough hardware resources to insert all the hardware
breakpoints and watchpoints.  On those platforms, @value{GDBN} prints
an error message only when the program being debugged is continued.)

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

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

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

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

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

@section Single Stepping

@section Signal Handling

@section Thread Handling

@section Inferior Function Calls

@section Longjmp Support

@cindex @code{longjmp} debugging
@value{GDBN} 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 output of the @samp{maint info breakpoint}
command.

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

@section Watchpoints
@cindex watchpoints

Watchpoints are a special kind of breakpoints (@pxref{Algorithms,
breakpoints}) which break when data is accessed rather than when some
instruction is executed.  When you have data which changes without
your knowing what code does that, watchpoints are the silver bullet to
hunt down and kill such bugs.

@cindex hardware watchpoints
@cindex software watchpoints
Watchpoints can be either hardware-assisted or not; the latter type is
known as ``software watchpoints.''  @value{GDBN} always uses
hardware-assisted watchpoints if they are available, and falls back on
software watchpoints otherwise.  Typical situations where @value{GDBN}
will use software watchpoints are:

@itemize @bullet
@item
The watched memory region is too large for the underlying hardware
watchpoint support.  For example, each x86 debug register can watch up
to 4 bytes of memory, so trying to watch data structures whose size is
more than 16 bytes will cause @value{GDBN} to use software
watchpoints.

@item
The value of the expression to be watched depends on data held in
registers (as opposed to memory).

@item
Too many different watchpoints requested.  (On some architectures,
this situation is impossible to detect until the debugged program is
resumed.)  Note that x86 debug registers are used both for hardware
breakpoints and for watchpoints, so setting too many hardware
breakpoints might cause watchpoint insertion to fail.

@item
No hardware-assisted watchpoints provided by the target
implementation.
@end itemize

Software watchpoints are very slow, since @value{GDBN} needs to
single-step the program being debugged and test the value of the
watched expression(s) after each instruction.  The rest of this
section is mostly irrelevant for software watchpoints.

@value{GDBN} uses several macros and primitives to support hardware
watchpoints:

@table @code
@findex TARGET_HAS_HARDWARE_WATCHPOINTS
@item TARGET_HAS_HARDWARE_WATCHPOINTS
If defined, the target supports hardware watchpoints.

@findex TARGET_CAN_USE_HARDWARE_WATCHPOINT
@item TARGET_CAN_USE_HARDWARE_WATCHPOINT (@var{type}, @var{count}, @var{other})
Return the number of hardware watchpoints of type @var{type} that are
possible to be set.  The value is positive if @var{count} watchpoints
of this type can be set, zero if setting watchpoints of this type is
not supported, and negative if @var{count} is more than the maximum
number of watchpoints of type @var{type} that can be set.  @var{other}
is non-zero if other types of watchpoints are currently enabled (there
are architectures which cannot set watchpoints of different types at
the same time).

@findex TARGET_REGION_OK_FOR_HW_WATCHPOINT
@item TARGET_REGION_OK_FOR_HW_WATCHPOINT (@var{addr}, @var{len})
Return non-zero if hardware watchpoints can be used to watch a region
whose address is @var{addr} and whose length in bytes is @var{len}.

@findex TARGET_REGION_SIZE_OK_FOR_HW_WATCHPOINT
@item TARGET_REGION_SIZE_OK_FOR_HW_WATCHPOINT (@var{size})
Return non-zero if hardware watchpoints can be used to watch a region
whose size is @var{size}.  @value{GDBN} only uses this macro as a
fall-back, in case @code{TARGET_REGION_OK_FOR_HW_WATCHPOINT} is not
defined.

@findex TARGET_DISABLE_HW_WATCHPOINTS
@item TARGET_DISABLE_HW_WATCHPOINTS (@var{pid})
Disables watchpoints in the process identified by @var{pid}.  This is
used, e.g., on HP-UX which provides operations to disable and enable
the page-level memory protection that implements hardware watchpoints
on that platform.

@findex TARGET_ENABLE_HW_WATCHPOINTS
@item TARGET_ENABLE_HW_WATCHPOINTS (@var{pid})
Enables watchpoints in the process identified by @var{pid}.  This is
used, e.g., on HP-UX which provides operations to disable and enable
the page-level memory protection that implements hardware watchpoints
on that platform.

@findex TARGET_RANGE_PROFITABLE_FOR_HW_WATCHPOINT
@item TARGET_RANGE_PROFITABLE_FOR_HW_WATCHPOINT (@var{pid},@var{start},@var{len})
Some addresses may not be profitable to use hardware to watch, or may
be difficult to understand when the addressed object is out of scope,
and hence should not be watched with hardware watchpoints.  On some
targets, this may have severe performance penalties, such that we
might as well use regular watchpoints, and save (possibly precious)
hardware watchpoints for other locations.

@findex target_insert_watchpoint
@findex target_remove_watchpoint
@item target_insert_watchpoint (@var{addr}, @var{len}, @var{type})
@itemx target_remove_watchpoint (@var{addr}, @var{len}, @var{type})
Insert or remove a hardware watchpoint starting at @var{addr}, for
@var{len} bytes.  @var{type} is the watchpoint type, one of the
possible values of the enumerated data type @code{target_hw_bp_type},
defined by @file{breakpoint.h} as follows:

@example
 enum target_hw_bp_type
   @{
     hw_write   = 0, /* Common (write) HW watchpoint */
     hw_read    = 1, /* Read    HW watchpoint */
     hw_access  = 2, /* Access (read or write) HW watchpoint */
     hw_execute = 3  /* Execute HW breakpoint */
   @};
@end example

@noindent
These two macros should return 0 for success, non-zero for failure.

@cindex insert or remove hardware breakpoint
@findex target_remove_hw_breakpoint
@findex target_insert_hw_breakpoint
@item target_remove_hw_breakpoint (@var{addr}, @var{shadow})
@itemx target_insert_hw_breakpoint (@var{addr}, @var{shadow})
Insert or remove a hardware-assisted breakpoint at address @var{addr}.
Returns zero for success, non-zero for failure.  @var{shadow} is the
real contents of the byte where the breakpoint has been inserted; it
is generally not valid when hardware breakpoints are used, but since
no other code touches these values, the implementations of the above
two macros can use them for their internal purposes.

@findex target_stopped_data_address
@item target_stopped_data_address ()
If the inferior has some watchpoint that triggered, return the address
associated with that watchpoint.  Otherwise, return zero.

@findex DECR_PC_AFTER_HW_BREAK
@item DECR_PC_AFTER_HW_BREAK
If defined, @value{GDBN} decrements the program counter by the value
of @code{DECR_PC_AFTER_HW_BREAK} after a hardware break-point.  This
overrides the value of @code{DECR_PC_AFTER_BREAK} when a breakpoint
that breaks is a hardware-assisted breakpoint.

@findex HAVE_STEPPABLE_WATCHPOINT
@item HAVE_STEPPABLE_WATCHPOINT
If defined to a non-zero value, it is not necessary to disable a
watchpoint to step over it.

@findex HAVE_NONSTEPPABLE_WATCHPOINT
@item HAVE_NONSTEPPABLE_WATCHPOINT
If defined to a non-zero value, @value{GDBN} should disable a
watchpoint to step the inferior over it.

@findex HAVE_CONTINUABLE_WATCHPOINT
@item HAVE_CONTINUABLE_WATCHPOINT
If defined to a non-zero value, it is possible to continue the
inferior after a watchpoint has been hit.

@findex CANNOT_STEP_HW_WATCHPOINTS
@item CANNOT_STEP_HW_WATCHPOINTS
If this is defined to a non-zero value, @value{GDBN} will remove all
watchpoints before stepping the inferior.

@findex STOPPED_BY_WATCHPOINT
@item STOPPED_BY_WATCHPOINT (@var{wait_status})
Return non-zero if stopped by a watchpoint.  @var{wait_status} is of
the type @code{struct target_waitstatus}, defined by @file{target.h}.
@end table

@subsection x86 Watchpoints
@cindex x86 debug registers
@cindex watchpoints, on x86

The 32-bit Intel x86 (a.k.a.@: ia32) processors feature special debug
registers designed to facilitate debugging.  @value{GDBN} provides a
generic library of functions that x86-based ports can use to implement
support for watchpoints and hardware-assisted breakpoints.  This
subsection documents the x86 watchpoint facilities in @value{GDBN}.

To use the generic x86 watchpoint support, a port should do the
following:

@itemize @bullet
@findex I386_USE_GENERIC_WATCHPOINTS
@item
Define the macro @code{I386_USE_GENERIC_WATCHPOINTS} somewhere in the
target-dependent headers.

@item
Include the @file{config/i386/nm-i386.h} header file @emph{after}
defining @code{I386_USE_GENERIC_WATCHPOINTS}.

@item
Add @file{i386-nat.o} to the value of the Make variable
@code{NATDEPFILES} (@pxref{Native Debugging, NATDEPFILES}) or
@code{TDEPFILES} (@pxref{Target Architecture Definition, TDEPFILES}).

@item
Provide implementations for the @code{I386_DR_LOW_*} macros described
below.  Typically, each macro should call a target-specific function
which does the real work.
@end itemize

The x86 watchpoint support works by maintaining mirror images of the
debug registers.  Values are copied between the mirror images and the
real debug registers via a set of macros which each target needs to
provide:

@table @code
@findex I386_DR_LOW_SET_CONTROL
@item I386_DR_LOW_SET_CONTROL (@var{val})
Set the Debug Control (DR7) register to the value @var{val}.

@findex I386_DR_LOW_SET_ADDR
@item I386_DR_LOW_SET_ADDR (@var{idx}, @var{addr})
Put the address @var{addr} into the debug register number @var{idx}.

@findex I386_DR_LOW_RESET_ADDR
@item I386_DR_LOW_RESET_ADDR (@var{idx})
Reset (i.e.@: zero out) the address stored in the debug register
number @var{idx}.

@findex I386_DR_LOW_GET_STATUS
@item I386_DR_LOW_GET_STATUS
Return the value of the Debug Status (DR6) register.  This value is
used immediately after it is returned by
@code{I386_DR_LOW_GET_STATUS}, so as to support per-thread status
register values.
@end table

For each one of the 4 debug registers (whose indices are from 0 to 3)
that store addresses, a reference count is maintained by @value{GDBN},
to allow sharing of debug registers by several watchpoints.  This
allows users to define several watchpoints that watch the same
expression, but with different conditions and/or commands, without
wasting debug registers which are in short supply.  @value{GDBN}
maintains the reference counts internally, targets don't have to do
anything to use this feature.

The x86 debug registers can each watch a region that is 1, 2, or 4
bytes long.  The ia32 architecture requires that each watched region
be appropriately aligned: 2-byte region on 2-byte boundary, 4-byte
region on 4-byte boundary.  However, the x86 watchpoint support in
@value{GDBN} can watch unaligned regions and regions larger than 4
bytes (up to 16 bytes) by allocating several debug registers to watch
a single region.  This allocation of several registers per a watched
region is also done automatically without target code intervention.

The generic x86 watchpoint support provides the following API for the
@value{GDBN}'s application code:

@table @code
@findex i386_region_ok_for_watchpoint
@item i386_region_ok_for_watchpoint (@var{addr}, @var{len})
The macro @code{TARGET_REGION_OK_FOR_HW_WATCHPOINT} is set to call
this function.  It counts the number of debug registers required to
watch a given region, and returns a non-zero value if that number is
less than 4, the number of debug registers available to x86
processors.

@findex i386_stopped_data_address
@item i386_stopped_data_address (void)
The macros @code{STOPPED_BY_WATCHPOINT} and
@code{target_stopped_data_address} are set to call this function.  The
argument passed to @code{STOPPED_BY_WATCHPOINT} is ignored.  This
function examines the breakpoint condition bits in the DR6 Debug
Status register, as returned by the @code{I386_DR_LOW_GET_STATUS}
macro, and returns the address associated with the first bit that is
set in DR6.

@findex i386_insert_watchpoint
@findex i386_remove_watchpoint
@item i386_insert_watchpoint (@var{addr}, @var{len}, @var{type})
@itemx i386_remove_watchpoint (@var{addr}, @var{len}, @var{type})
Insert or remove a watchpoint.  The macros
@code{target_insert_watchpoint} and @code{target_remove_watchpoint}
are set to call these functions.  @code{i386_insert_watchpoint} first
looks for a debug register which is already set to watch the same
region for the same access types; if found, it just increments the
reference count of that debug register, thus implementing debug
register sharing between watchpoints.  If no such register is found,
the function looks for a vacant debug register, sets its mirrorred
value to @var{addr}, sets the mirrorred value of DR7 Debug Control
register as appropriate for the @var{len} and @var{type} parameters,
and then passes the new values of the debug register and DR7 to the
inferior by calling @code{I386_DR_LOW_SET_ADDR} and
@code{I386_DR_LOW_SET_CONTROL}.  If more than one debug register is
required to cover the given region, the above process is repeated for
each debug register.

@code{i386_remove_watchpoint} does the opposite: it resets the address
in the mirrorred value of the debug register and its read/write and
length bits in the mirrorred value of DR7, then passes these new
values to the inferior via @code{I386_DR_LOW_RESET_ADDR} and
@code{I386_DR_LOW_SET_CONTROL}.  If a register is shared by several
watchpoints, each time a @code{i386_remove_watchpoint} is called, it
decrements the reference count, and only calls
@code{I386_DR_LOW_RESET_ADDR} and @code{I386_DR_LOW_SET_CONTROL} when
the count goes to zero.

@findex i386_insert_hw_breakpoint
@findex i386_remove_hw_breakpoint
@item i386_insert_hw_breakpoint (@var{addr}, @var{shadow}
@itemx i386_remove_hw_breakpoint (@var{addr}, @var{shadow})
These functions insert and remove hardware-assisted breakpoints.  The
macros @code{target_insert_hw_breakpoint} and
@code{target_remove_hw_breakpoint} are set to call these functions.
These functions work like @code{i386_insert_watchpoint} and
@code{i386_remove_watchpoint}, respectively, except that they set up
the debug registers to watch instruction execution, and each
hardware-assisted breakpoint always requires exactly one debug
register.

@findex i386_stopped_by_hwbp
@item i386_stopped_by_hwbp (void)
This function returns non-zero if the inferior has some watchpoint or
hardware breakpoint that triggered.  It works like
@code{i386_stopped_data_address}, except that it doesn't return the
address whose watchpoint triggered.

@findex i386_cleanup_dregs
@item i386_cleanup_dregs (void)
This function clears all the reference counts, addresses, and control
bits in the mirror images of the debug registers.  It doesn't affect
the actual debug registers in the inferior process.
@end table

@noindent
@strong{Notes:}
@enumerate 1
@item
x86 processors support setting watchpoints on I/O reads or writes.
However, since no target supports this (as of March 2001), and since
@code{enum target_hw_bp_type} doesn't even have an enumeration for I/O
watchpoints, this feature is not yet available to @value{GDBN} running
on x86.

@item
x86 processors can enable watchpoints locally, for the current task
only, or globally, for all the tasks.  For each debug register,
there's a bit in the DR7 Debug Control register that determines
whether the associated address is watched locally or globally.  The
current implementation of x86 watchpoint support in @value{GDBN}
always sets watchpoints to be locally enabled, since global
watchpoints might interfere with the underlying OS and are probably
unavailable in many platforms.
@end enumerate

@node User Interface

@chapter User Interface

@value{GDBN} has several user interfaces.  Although the command-line interface
is the most common and most familiar, there are others.

@section Command Interpreter

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

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

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

@cindex deprecating commands
@findex deprecate_cmd
Before removing commands from the command set it is a good idea to
deprecate them for some time.  Use @code{deprecate_cmd} on commands or
aliases to set the deprecated flag.  @code{deprecate_cmd} takes a
@code{struct cmd_list_element} as it's first argument.  You can use the
return value from @code{add_com} or @code{add_cmd} to deprecate the
command immediately after it is created.

The first time a comamnd is used the user will be warned and offered a
replacement (if one exists). Note that the replacement string passed to
@code{deprecate_cmd} should be the full name of the command, i.e. the
entire string the user should type at the command line.

@section Console Printing

@section TUI

@section libgdb

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

@node Symbol Handling

@chapter Symbol Handling

Symbols are a key part of @value{GDBN}'s operation.  Symbols include variables,
functions, and types.

@section Symbol Reading

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

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

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

The functions supplied by each module are:

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

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

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

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

@item @var{xyz}_new_init()

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

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

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

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

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

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

Called from @code{psymtab_to_symtab} (or the @code{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, @code{pst->readin} should have been set to 1, and
@code{pst->symtab} should contain a pointer to the new corresponding symtab, or
zero if there were no symbols in that part of the symbol file.
@end table

@section Partial Symbol Tables

@value{GDBN} has three types of symbol tables:

@itemize @bullet
@cindex full symbol table
@cindex symtabs
@item
Full symbol tables (@dfn{symtabs}).  These contain the main
information about symbols and addresses.

@cindex psymtabs
@item
Partial symbol tables (@dfn{psymtabs}).  These contain enough
information to know when to read the corresponding part of the full
symbol table.

@cindex minimal symbol table
@cindex minsymtabs
@item
Minimal symbol tables (@dfn{msymtabs}).  These contain information
gleaned from non-debugging symbols.
@end itemize

@cindex partial symbol table
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 @value{GDBN} to start up very
quickly.  Later, as the detailed rereading occurs, it occurs in small
pieces, at various times, and the delay therefrom is mostly invisible to
the user.
@c (@xref{Symbol Reading}.)

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

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

@cindex finding a symbol
@cindex symbol lookup
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
@findex find_pc_function
@findex find_pc_line
@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.

@cindex lookup_symbol
@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 @value{GDBN} to behave one way when only a psymtab has
been read, and another way if the corresponding symtab has been read
in.  Such bugs are typically caused by a psymtab that does not contain
all the visible symbols, or which has the wrong instruction address
ranges.

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

@section Types

@unnumberedsubsec Fundamental Types (e.g., @code{FT_VOID}, @code{FT_BOOLEAN}).

@cindex fundamental types
These are the fundamental types that @value{GDBN} 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 @value{GDBN} knows about for all the languages that @value{GDBN}
knows about.

@unnumberedsubsec Type Codes (e.g., @code{TYPE_CODE_PTR}, @code{TYPE_CODE_ARRAY}).

@cindex type codes
Each time @value{GDBN} builds an internal type, it marks it with one
of these types.  The type may be a fundamental type, such as
@code{TYPE_CODE_INT}, or a derived type, such as @code{TYPE_CODE_PTR}
which is a pointer to another type.  Typically, several @code{FT_*}
types map to one @code{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.

@unnumberedsubsec Builtin Types (e.g., @code{builtin_type_void}, @code{builtin_type_char}).

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

@section Object File Formats
@cindex object file formats

@subsection a.out

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

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

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

@subsection COFF

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

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

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

@subsection ECOFF

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

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

@subsection XCOFF

@cindex XCOFF 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 @code{dbx}-style stabs whose strings are located in the
@code{.debug} section (rather than the string table).  For more
information, see @ref{Top,,,stabs,The Stabs Debugging Format}.

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

@subsection PE

@cindex PE-COFF format
Windows 95 and NT use the PE (@dfn{Portable Executable}) format for their
executables.  PE is basically COFF with additional headers.

While BFD includes special PE support, @value{GDBN} needs only the basic
COFF reader.

@subsection ELF

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

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

@subsection SOM

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

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

@subsection Other File Formats

@cindex Netware Loadable Module format
Other file formats that have been supported by @value{GDBN} include Netware
Loadable Modules (@file{nlmread.c}.

@section Debugging File Formats

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

@subsection stabs

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

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

@subsection COFF

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

@subsection Mips debug (Third Eye)

@cindex ECOFF debugging info
ECOFF includes a definition of a special debug format.

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

@subsection DWARF 1

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

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

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

@subsection DWARF 2

@cindex DWARF 2 debugging info
DWARF 2 is an improved but incompatible version of DWARF 1.

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

@subsection SOM

@cindex SOM debugging info
Like COFF, the SOM definition includes debugging information.

@section Adding a New Symbol Reader to @value{GDBN}

@cindex adding debugging info reader
If you are using an existing object file format (@code{a.out}, COFF, ELF, etc),
there is probably little to be done.

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

You must then arrange for the BFD code to provide access to the
debugging symbols.  Generally @value{GDBN} will have to call swapping routines
from BFD and a few other BFD internal routines to locate the debugging
information.  As much as possible, @value{GDBN} 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/lib@var{xyz}.h}, which is included by @value{GDBN}.


@node Language Support

@chapter Language Support

@cindex language support
@value{GDBN}'s language support is mainly driven by the symbol reader,
although it is possible for the user to set the source language
manually.

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

@section Adding a Source Language to @value{GDBN}

@cindex adding source language
To add other languages to @value{GDBN}'s expression parser, follow the
following steps:

@table @emph
@item Create the expression parser.

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

@cindex language parser
Since we can't depend upon everyone having Bison, and YACC produces
parsers that define a bunch of global names, the following lines
@strong{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 @value{GDBN}
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

@cindex expression evaluation routines
@findex evaluate_subexp
@findex prefixify_subexp
@findex length_of_subexp
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 the @code{evaluate_subexp} function
defined in the file @file{eval.c}.  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.

@vindex current_language
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.

@findex _initialize_language
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 @value{GDBN} is built for.

@findex allocate_symtab
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.

@findex print_subexp
@findex op_print_tab
Add helper code to @code{print_subexp} (in @file{expprint.c}) 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

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

@item Use macros to trim code

@cindex trimming language-dependent code
The user has the option of building @value{GDBN} for some or all of the
languages.  If the user decides to build @value{GDBN} 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 @value{GDBN}
is not build for @var{lang}, then @file{@var{lang}-exp.tab.o} (the
compiled form of your parser) is not linked into @value{GDBN} at all.

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

@item Edit @file{Makefile.in}

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


@node Host Definition

@chapter Host Definition

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

@section Adding a New Host

@cindex adding a new host
@cindex host, adding
Most of @value{GDBN}'s host configuration support happens via
Autoconf.  New host-specific definitions should be rarely needed.
@value{GDBN} still uses the host-specific definitions and files listed
below, but these mostly exist for historical reasons, and should
eventually disappear.

Several files control @value{GDBN}'s configuration for host systems:

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

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

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

@subheading Generic Host Support Files

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

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

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

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

@cindex TCP remote support
@item ser-tcp.c
This contains generic TCP support using sockets.
@end table

@section Host Conditionals

When @value{GDBN} 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:

@ftable @code
@item @value{GDBN}INIT_FILENAME
The default name of @value{GDBN}'s initialization file (normally
@file{.gdbinit}).

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

@item NO_STD_REGS
This macro is deprecated.

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

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

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

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

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

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

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

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

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

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

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

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

@item HOST_BYTE_ORDER
@cindex 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
@itemx INT_MIN
@itemx LONG_MAX
@itemx UINT_MAX
@itemx ULONG_MAX
Values for host-side constants.

@item ISATTY
Substitute for isatty, if not available.

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

@item CC_HAS_LONG_LONG
@cindex @code{long long} data type
Define this if the host C compiler supports @code{long long}.  This is set
by the @code{configure} script.

@item PRINTF_HAS_LONG_LONG
Define this if the host can handle printing of long long integers via
the printf format conversion specifier @code{ll}.  This is set by the
@code{configure} script.

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

@item PRINTF_HAS_LONG_DOUBLE
Define this if the host can handle printing of long double float-point
numbers via the printf format conversion specifier @code{Lg}.  This is
set by the @code{configure} script.

@item SCANF_HAS_LONG_DOUBLE
Define this if the host can handle the parsing of long double
float-point numbers via the scanf format conversion specifier
@code{Lg}.  This is set by the @code{configure} script.

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

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

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

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

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

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

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

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

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

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

@item NO_MMCHECK
@findex 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 @code{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 @code{mmalloc}, but must define
@code{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, @value{GDBN} will issue a
warning.  This is currently defined if you configure using
@samp{--with-mmalloc}.

@item NO_SIGINTERRUPT
@findex siginterrupt
Define this to indicate that @code{siginterrupt} is not available.

@item R_OK
Define if this is not in a system header file (typically, @file{unistd.h}).

@item SEEK_CUR
@itemx SEEK_SET
Define these to appropriate value for the system @code{lseek}, if not already
defined.

@item STOP_SIGNAL
This is the signal for stopping @value{GDBN}.  Defaults to
@code{SIGTSTP}.  (Only redefined for the Convex.)

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

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

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

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


@node Target Architecture Definition

@chapter Target Architecture Definition

@cindex target architecture definition
@value{GDBN}'s target architecture defines what sort of
machine-language programs @value{GDBN} can work with, and how it works
with them.

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

@section Registers and Memory

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

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

@value{GDBN} can handle big-endian, little-endian, and bi-endian architectures.

@section Pointers Are Not Always Addresses
@cindex pointer representation
@cindex address representation
@cindex word-addressed machines
@cindex separate data and code address spaces
@cindex spaces, separate data and code address
@cindex address spaces, separate data and code
@cindex code pointers, word-addressed
@cindex converting between pointers and addresses
@cindex D10V addresses

On almost all 32-bit architectures, the representation of a pointer is
indistinguishable from the representation of some fixed-length number
whose value is the byte address of the object pointed to.  On such
machines, the words ``pointer'' and ``address'' can be used interchangeably.
However, architectures with smaller word sizes are often cramped for
address space, so they may choose a pointer representation that breaks this
identity, and allows a larger code address space.

For example, the Mitsubishi D10V is a 16-bit VLIW processor whose
instructions are 32 bits long@footnote{Some D10V instructions are
actually pairs of 16-bit sub-instructions.  However, since you can't
jump into the middle of such a pair, code addresses can only refer to
full 32 bit instructions, which is what matters in this explanation.}.
If the D10V used ordinary byte addresses to refer to code locations,
then the processor would only be able to address 64kb of instructions.
However, since instructions must be aligned on four-byte boundaries, the
low two bits of any valid instruction's byte address are always
zero---byte addresses waste two bits.  So instead of byte addresses,
the D10V uses word addresses---byte addresses shifted right two bits---to
refer to code.  Thus, the D10V can use 16-bit words to address 256kb of
code space.

However, this means that code pointers and data pointers have different
forms on the D10V.  The 16-bit word @code{0xC020} refers to byte address
@code{0xC020} when used as a data address, but refers to byte address
@code{0x30080} when used as a code address.

(The D10V also uses separate code and data address spaces, which also
affects the correspondence between pointers and addresses, but we're
going to ignore that here; this example is already too long.)

To cope with architectures like this---the D10V is not the only
one!---@value{GDBN} tries to distinguish between @dfn{addresses}, which are
byte numbers, and @dfn{pointers}, which are the target's representation
of an address of a particular type of data.  In the example above,
@code{0xC020} is the pointer, which refers to one of the addresses
@code{0xC020} or @code{0x30080}, depending on the type imposed upon it.
@value{GDBN} provides functions for turning a pointer into an address
and vice versa, in the appropriate way for the current architecture.

Unfortunately, since addresses and pointers are identical on almost all
processors, this distinction tends to bit-rot pretty quickly.  Thus,
each time you port @value{GDBN} to an architecture which does
distinguish between pointers and addresses, you'll probably need to
clean up some architecture-independent code.

Here are functions which convert between pointers and addresses:

@deftypefun CORE_ADDR extract_typed_address (void *@var{buf}, struct type *@var{type})
Treat the bytes at @var{buf} as a pointer or reference of type
@var{type}, and return the address it represents, in a manner
appropriate for the current architecture.  This yields an address
@value{GDBN} can use to read target memory, disassemble, etc.  Note that
@var{buf} refers to a buffer in @value{GDBN}'s memory, not the
inferior's.

For example, if the current architecture is the Intel x86, this function
extracts a little-endian integer of the appropriate length from
@var{buf} and returns it.  However, if the current architecture is the
D10V, this function will return a 16-bit integer extracted from
@var{buf}, multiplied by four if @var{type} is a pointer to a function.

If @var{type} is not a pointer or reference type, then this function
will signal an internal error.
@end deftypefun

@deftypefun CORE_ADDR store_typed_address (void *@var{buf}, struct type *@var{type}, CORE_ADDR @var{addr})
Store the address @var{addr} in @var{buf}, in the proper format for a
pointer of type @var{type} in the current architecture.  Note that
@var{buf} refers to a buffer in @value{GDBN}'s memory, not the
inferior's.

For example, if the current architecture is the Intel x86, this function
stores @var{addr} unmodified as a little-endian integer of the
appropriate length in @var{buf}.  However, if the current architecture
is the D10V, this function divides @var{addr} by four if @var{type} is
a pointer to a function, and then stores it in @var{buf}.

If @var{type} is not a pointer or reference type, then this function
will signal an internal error.
@end deftypefun

@deftypefun CORE_ADDR value_as_pointer (value_ptr @var{val})
Assuming that @var{val} is a pointer, return the address it represents,
as appropriate for the current architecture.

This function actually works on integral values, as well as pointers.
For pointers, it performs architecture-specific conversions as
described above for @code{extract_typed_address}.
@end deftypefun

@deftypefun CORE_ADDR value_from_pointer (struct type *@var{type}, CORE_ADDR @var{addr})
Create and return a value representing a pointer of type @var{type} to
the address @var{addr}, as appropriate for the current architecture.
This function performs architecture-specific conversions as described
above for @code{store_typed_address}.
@end deftypefun


@value{GDBN} also provides functions that do the same tasks, but assume
that pointers are simply byte addresses; they aren't sensitive to the
current architecture, beyond knowing the appropriate endianness.

@deftypefun CORE_ADDR extract_address (void *@var{addr}, int len)
Extract a @var{len}-byte number from @var{addr} in the appropriate
endianness for the current architecture, and return it.  Note that
@var{addr} refers to @value{GDBN}'s memory, not the inferior's.

This function should only be used in architecture-specific code; it
doesn't have enough information to turn bits into a true address in the
appropriate way for the current architecture.  If you can, use
@code{extract_typed_address} instead.
@end deftypefun

@deftypefun void store_address (void *@var{addr}, int @var{len}, LONGEST @var{val})
Store @var{val} at @var{addr} as a @var{len}-byte integer, in the
appropriate endianness for the current architecture.  Note that
@var{addr} refers to a buffer in @value{GDBN}'s memory, not the
inferior's.

This function should only be used in architecture-specific code; it
doesn't have enough information to turn a true address into bits in the
appropriate way for the current architecture.  If you can, use
@code{store_typed_address} instead.
@end deftypefun


Here are some macros which architectures can define to indicate the
relationship between pointers and addresses.  These have default
definitions, appropriate for architectures on which all pointers are
simple byte addresses.

@deftypefn {Target Macro} CORE_ADDR POINTER_TO_ADDRESS (struct type *@var{type}, char *@var{buf})
Assume that @var{buf} holds a pointer of type @var{type}, in the
appropriate format for the current architecture.  Return the byte
address the pointer refers to.

This function may safely assume that @var{type} is either a pointer or a
C@t{++} reference type.
@end deftypefn

@deftypefn {Target Macro} void ADDRESS_TO_POINTER (struct type *@var{type}, char *@var{buf}, CORE_ADDR @var{addr})
Store in @var{buf} a pointer of type @var{type} representing the address
@var{addr}, in the appropriate format for the current architecture.

This function may safely assume that @var{type} is either a pointer or a
C@t{++} reference type.
@end deftypefn


@section Using Different Register and Memory Data Representations
@cindex raw representation
@cindex virtual representation
@cindex representations, raw and virtual
@cindex register data formats, converting
@cindex @code{struct value}, converting register contents to

Some architectures use one representation for a value when it lives in a
register, but use a different representation when it lives in memory.
In @value{GDBN}'s terminology, the @dfn{raw} representation is the one used in
the target registers, and the @dfn{virtual} representation is the one
used in memory, and within @value{GDBN} @code{struct value} objects.

For almost all data types on almost all architectures, the virtual and
raw representations are identical, and no special handling is needed.
However, they do occasionally differ.  For example:

@itemize @bullet
@item
The x86 architecture supports an 80-bit @code{long double} type.  However, when
we store those values in memory, they occupy twelve bytes: the
floating-point number occupies the first ten, and the final two bytes
are unused.  This keeps the values aligned on four-byte boundaries,
allowing more efficient access.  Thus, the x86 80-bit floating-point
type is the raw representation, and the twelve-byte loosely-packed
arrangement is the virtual representation.

@item
Some 64-bit MIPS targets present 32-bit registers to @value{GDBN} as 64-bit
registers, with garbage in their upper bits.  @value{GDBN} ignores the top 32
bits.  Thus, the 64-bit form, with garbage in the upper 32 bits, is the
raw representation, and the trimmed 32-bit representation is the
virtual representation.
@end itemize

In general, the raw representation is determined by the architecture, or
@value{GDBN}'s interface to the architecture, while the virtual representation
can be chosen for @value{GDBN}'s convenience.  @value{GDBN}'s register file,
@code{registers}, holds the register contents in raw format, and the
@value{GDBN} remote protocol transmits register values in raw format.

Your architecture may define the following macros to request
conversions between the raw and virtual format:

@deftypefn {Target Macro} int REGISTER_CONVERTIBLE (int @var{reg})
Return non-zero if register number @var{reg}'s value needs different raw
and virtual formats.

You should not use @code{REGISTER_CONVERT_TO_VIRTUAL} for a register
unless this macro returns a non-zero value for that register.
@end deftypefn

@deftypefn {Target Macro} int REGISTER_RAW_SIZE (int @var{reg})
The size of register number @var{reg}'s raw value.  This is the number
of bytes the register will occupy in @code{registers}, or in a @value{GDBN}
remote protocol packet.
@end deftypefn

@deftypefn {Target Macro} int REGISTER_VIRTUAL_SIZE (int @var{reg})
The size of register number @var{reg}'s value, in its virtual format.
This is the size a @code{struct value}'s buffer will have, holding that
register's value.
@end deftypefn

@deftypefn {Target Macro} struct type *REGISTER_VIRTUAL_TYPE (int @var{reg})
This is the type of the virtual representation of register number
@var{reg}.  Note that there is no need for a macro giving a type for the
register's raw form; once the register's value has been obtained, @value{GDBN}
always uses the virtual form.
@end deftypefn

@deftypefn {Target Macro} void REGISTER_CONVERT_TO_VIRTUAL (int @var{reg}, struct type *@var{type}, char *@var{from}, char *@var{to})
Convert the value of register number @var{reg} to @var{type}, which
should always be @code{REGISTER_VIRTUAL_TYPE (@var{reg})}.  The buffer
at @var{from} holds the register's value in raw format; the macro should
convert the value to virtual format, and place it at @var{to}.

Note that @code{REGISTER_CONVERT_TO_VIRTUAL} and
@code{REGISTER_CONVERT_TO_RAW} take their @var{reg} and @var{type}
arguments in different orders.

You should only use @code{REGISTER_CONVERT_TO_VIRTUAL} with registers
for which the @code{REGISTER_CONVERTIBLE} macro returns a non-zero
value.
@end deftypefn

@deftypefn {Target Macro} void REGISTER_CONVERT_TO_RAW (struct type *@var{type}, int @var{reg}, char *@var{from}, char *@var{to})
Convert the value of register number @var{reg} to @var{type}, which
should always be @code{REGISTER_VIRTUAL_TYPE (@var{reg})}.  The buffer
at @var{from} holds the register's value in raw format; the macro should
convert the value to virtual format, and place it at @var{to}.

Note that REGISTER_CONVERT_TO_VIRTUAL and REGISTER_CONVERT_TO_RAW take
their @var{reg} and @var{type} arguments in different orders.
@end deftypefn


@section Frame Interpretation

@section Inferior Call Setup

@section Compiler Characteristics

@section Target Conditionals

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

@table @code

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

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

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

@item ADDRESS_TO_POINTER (@var{type}, @var{buf}, @var{addr})
@findex ADDRESS_TO_POINTER
Store in @var{buf} a pointer of type @var{type} representing the address
@var{addr}, in the appropriate format for the current architecture.
This macro may safely assume that @var{type} is either a pointer or a
C@t{++} reference type.
@xref{Target Architecture Definition, , Pointers Are Not Always Addresses}.

@item BEFORE_MAIN_LOOP_HOOK
@findex 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, @value{GDBN} will probably not compile, let alone run
correctly.  This macro is currently used only for the unsupported i960 Nindy
target, and should not be used in any other configuration.

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

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

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

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

@code{BREAKPOINT} has been deprecated in favor of
@code{BREAKPOINT_FROM_PC}.

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

@code{BIG_BREAKPOINT} and @code{LITTLE_BREAKPOINT} have been deprecated in
favor of @code{BREAKPOINT_FROM_PC}.

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

@code{BIG_REMOTE_BREAKPOINT} and @code{LITTLE_REMOTE_BREAKPOINT} have been
deprecated in favor of @code{BREAKPOINT_FROM_PC}.

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

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

Replaces all the other @var{BREAKPOINT} macros.

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

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

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

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

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

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

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

@item CALL_DUMMY_LOCATION
@findex CALL_DUMMY_LOCATION
See the file @file{inferior.h}.

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

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

@item CALL_DUMMY_STACK_ADJUST_P
@findex CALL_DUMMY_STACK_ADJUST_P
Predicate for use of @code{CALL_DUMMY_STACK_ADJUST}.

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

@item CANNOT_FETCH_REGISTER (@var{regno})
@findex CANNOT_FETCH_REGISTER
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 (@var{regno})
@findex CANNOT_STORE_REGISTER
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,
@value{GDBN} will assume that all registers may be written.

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

Currently only implemented correctly for native Sparc configurations?

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

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

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

@item CPLUS_MARKER
@findex CPLUS_MARKERz
Define this to expand into the character that G@t{++} 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
@findex DBX_PARM_SYMBOL_CLASS
Hook for the @code{SYMBOL_CLASS} of a parameter when decoding DBX symbol
information.  In the i960, parameters can be stored as locals or as
args, depending on the type of the debug record.

@item DECR_PC_AFTER_BREAK
@findex 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
@code{BREAKPOINT}, though not always.  For most targets this value will be 0.

@item DECR_PC_AFTER_HW_BREAK
@findex DECR_PC_AFTER_HW_BREAK
Similarly, for hardware breakpoints.

@item DISABLE_UNSETTABLE_BREAK (@var{addr})
@findex DISABLE_UNSETTABLE_BREAK
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
@findex DO_REGISTERS_INFO
If defined, use this to print the value of a register or all registers.

@item DWARF_REG_TO_REGNUM
@findex DWARF_REG_TO_REGNUM
Convert DWARF register number into @value{GDBN} regnum.  If not defined,
no conversion will be performed.

@item DWARF2_REG_TO_REGNUM
@findex DWARF2_REG_TO_REGNUM
Convert DWARF2 register number into @value{GDBN} regnum.  If not
defined, no conversion will be performed.

@item ECOFF_REG_TO_REGNUM
@findex ECOFF_REG_TO_REGNUM
Convert ECOFF register number into @value{GDBN} regnum.  If not defined,
no conversion will be performed.

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

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

@item EXTRACT_STRUCT_VALUE_ADDRESS_P
@findex EXTRACT_STRUCT_VALUE_ADDRESS_P
Predicate for @code{EXTRACT_STRUCT_VALUE_ADDRESS}.

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

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

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

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

@item FRAME_ARGS_ADDRESS_CORRECT@item
@findex FRAME_ARGS_ADDRESS_CORRECT
See @file{stack.c}.

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

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

@item FRAME_CHAIN_VALID(@var{chain}, @var{thisframe})
@findex FRAME_CHAIN_VALID
Define this to be an expression that returns zero if the given frame is
an outermost frame, with no caller, and nonzero otherwise.  Several
common definitions are available:

@itemize @bullet
@item
@code{file_frame_chain_valid} is nonzero if the chain pointer is nonzero
and given frame's PC is not inside the startup file (such as
@file{crt0.o}).

@item
@code{func_frame_chain_valid} is nonzero if the chain
pointer is nonzero and the given frame's PC is not in @code{main} or a
known entry point function (such as @code{_start}).

@item
@code{generic_file_frame_chain_valid} and
@code{generic_func_frame_chain_valid} are equivalent implementations for
targets using generic dummy frames.
@end itemize

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

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

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

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

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

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

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

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

@item @value{GDBN}_MULTI_ARCH
@findex @value{GDBN}_MULTI_ARCH
If defined and non-zero, enables suport for multiple architectures
within @value{GDBN}.

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

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

@item GET_LONGJMP_TARGET
@findex 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
trhe header file @file{setjmp.h} is needed to define it.

This macro determines the target PC address that @code{longjmp} will jump to,
assuming that we have just stopped at a @code{longjmp} breakpoint.  It takes a
@code{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
@findex GET_SAVED_REGISTER
@findex get_saved_register
Define this if you need to supply your own definition for the function
@code{get_saved_register}.

@item HAVE_REGISTER_WINDOWS
@findex HAVE_REGISTER_WINDOWS
Define this if the target has register windows.

@item REGISTER_IN_WINDOW_P (@var{regnum})
@findex REGISTER_IN_WINDOW_P
Define this to be an expression that is 1 if the given register is in
the window.

@item IBM6000_TARGET
@findex 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 a haste and we are
repenting at leisure.

@item I386_USE_GENERIC_WATCHPOINTS
An x86-based target can define this to use the generic x86 watchpoint
support; see @ref{Algorithms, I386_USE_GENERIC_WATCHPOINTS}.

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

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

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

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

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

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

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

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

@item IN_SOLIB_CALL_TRAMPOLINE (@var{pc}, @var{name})
@findex IN_SOLIB_CALL_TRAMPOLINE
Define this to evaluate to nonzero if the program is stopped in the
trampoline that connects to a shared library.

@item IN_SOLIB_RETURN_TRAMPOLINE (@var{pc}, @var{name})
@findex IN_SOLIB_RETURN_TRAMPOLINE
Define this to evaluate to nonzero if the program is stopped in the
trampoline that returns from a shared library.

@item IN_SOLIB_DYNSYM_RESOLVE_CODE (@var{pc})
@findex IN_SOLIB_DYNSYM_RESOLVE_CODE
Define this to evaluate to nonzero if the program is stopped in the
dynamic linker.

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

@item IS_TRAPPED_INTERNALVAR (@var{name})
@findex IS_TRAPPED_INTERNALVAR
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 @value{GDBN}.  Currently only used by the h8500.  Note that this
could be either a host or target conditional.

@item NEED_TEXT_START_END
@findex NEED_TEXT_START_END
Define this if @value{GDBN} should determine the start and end addresses of the
text section.  (Seems dubious.)

@item NO_HIF_SUPPORT
@findex NO_HIF_SUPPORT
(Specific to the a29k.)

@item POINTER_TO_ADDRESS (@var{type}, @var{buf})
@findex POINTER_TO_ADDRESS
Assume that @var{buf} holds a pointer of type @var{type}, in the
appropriate format for the current architecture.  Return the byte
address the pointer refers to.
@xref{Target Architecture Definition, , Pointers Are Not Always Addresses}.

@item REGISTER_CONVERTIBLE (@var{reg})
@findex REGISTER_CONVERTIBLE
Return non-zero if @var{reg} uses different raw and virtual formats.
@xref{Target Architecture Definition, , Using Different Register and Memory Data Representations}.

@item REGISTER_RAW_SIZE (@var{reg})
@findex REGISTER_RAW_SIZE
Return the raw size of @var{reg}.
@xref{Target Architecture Definition, , Using Different Register and Memory Data Representations}.

@item REGISTER_VIRTUAL_SIZE (@var{reg})
@findex REGISTER_VIRTUAL_SIZE
Return the virtual size of @var{reg}.
@xref{Target Architecture Definition, , Using Different Register and Memory Data Representations}.

@item REGISTER_VIRTUAL_TYPE (@var{reg})
@findex REGISTER_VIRTUAL_TYPE
Return the virtual type of @var{reg}.
@xref{Target Architecture Definition, , Using Different Register and Memory Data Representations}.

@item REGISTER_CONVERT_TO_VIRTUAL(@var{reg}, @var{type}, @var{from}, @var{to})
@findex REGISTER_CONVERT_TO_VIRTUAL
Convert the value of register @var{reg} from its raw form to its virtual
form.
@xref{Target Architecture Definition, , Using Different Register and Memory Data Representations}.

@item REGISTER_CONVERT_TO_RAW(@var{type}, @var{reg}, @var{from}, @var{to})
@findex REGISTER_CONVERT_TO_RAW
Convert the value of register @var{reg} from its virtual form to its raw
form.
@xref{Target Architecture Definition, , Using Different Register and Memory Data Representations}.

@item RETURN_VALUE_ON_STACK(@var{type})
@findex RETURN_VALUE_ON_STACK
@cindex returning structures by value
@cindex structures, returning by value

Return non-zero if values of type TYPE are returned on the stack, using
the ``struct convention'' (i.e., the caller provides a pointer to a
buffer in which the callee should store the return value).  This
controls how the @samp{finish} command finds a function's return value,
and whether an inferior function call reserves space on the stack for
the return value.

The full logic @value{GDBN} uses here is kind of odd.  

@itemize @bullet
@item
If the type being returned by value is not a structure, union, or array,
and @code{RETURN_VALUE_ON_STACK} returns zero, then @value{GDBN}
concludes the value is not returned using the struct convention.

@item
Otherwise, @value{GDBN} calls @code{USE_STRUCT_CONVENTION} (see below).
If that returns non-zero, @value{GDBN} assumes the struct convention is
in use.
@end itemize

In other words, to indicate that a given type is returned by value using
the struct convention, that type must be either a struct, union, array,
or something @code{RETURN_VALUE_ON_STACK} likes, @emph{and} something
that @code{USE_STRUCT_CONVENTION} likes.

Note that, in C and C@t{++}, arrays are never returned by value.  In those
languages, these predicates will always see a pointer type, never an
array type.  All the references above to arrays being returned by value
apply only to other languages.

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

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

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

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

@code{SOFUN_ADDRESS_MAYBE_MISSING} means two things:

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

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

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

@item PC_IN_CALL_DUMMY
@findex PC_IN_CALL_DUMMY
See @file{inferior.h}.

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

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

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

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

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

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

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

@item PRINT_TYPELESS_INTEGER
@findex 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
@findex PROCESS_LINENUMBER_HOOK
A hook defined for XCOFF reading.

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

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

@item POP_FRAME
@findex POP_FRAME
@findex call_function_by_hand
@findex return_command
Used in @samp{call_function_by_hand} to remove an artificial stack
frame and in @samp{return_command} to remove a real stack frame.

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

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

@item REGISTER_BYTES
@findex REGISTER_BYTES
The total amount of space needed to store @value{GDBN}'s copy of the machine's
register state.

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

@item REGISTER_NAMES
@findex REGISTER_NAMES
Deprecated in favor of @code{REGISTER_NAME}.

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

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

@item SDB_REG_TO_REGNUM
@findex SDB_REG_TO_REGNUM
Define this to convert sdb register numbers into @value{GDBN} regnums.  If not
defined, no conversion will be done.

@item SHIFT_INST_REGS
@findex SHIFT_INST_REGS
(Only used for m88k targets.)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

At present this macro is not used.

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

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

At present this macro is not used.

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

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

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

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

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

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

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

@item TARGET_READ_PC
@findex TARGET_READ_PC
@itemx TARGET_WRITE_PC (@var{val}, @var{pid})
@findex TARGET_WRITE_PC
@itemx TARGET_READ_SP
@findex TARGET_READ_SP
@itemx TARGET_WRITE_SP
@findex TARGET_WRITE_SP
@itemx TARGET_READ_FP
@findex TARGET_READ_FP
@itemx TARGET_WRITE_FP
@findex TARGET_WRITE_FP
@findex read_pc
@findex write_pc
@findex read_sp
@findex write_sp
@findex read_fp
@findex 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.  @value{GDBN} will call the read
and write register functions with the relevant @code{_REGNUM} argument.

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

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

@item TARGET_HAS_HARDWARE_WATCHPOINTS
If non-zero, the target has support for hardware-assisted
watchpoints.  @xref{Algorithms, watchpoints}, for more details and
other related macros.

@item USE_STRUCT_CONVENTION (@var{gcc_p}, @var{type})
@findex USE_STRUCT_CONVENTION
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 (@var{desc}, @var{gcc_p})
@findex VARIABLES_INSIDE_BLOCK
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 @value{GDBN} 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 (@var{desc}, @var{gcc_p})
@findex OS9K_VARIABLES_INSIDE_BLOCK
Similarly, for OS/9000.  Defaults to 1.
@end table

Motorola M68K target conditionals.

@ftable @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 ftable

@section Adding a New Target

@cindex adding a target
The following files define a target to @value{GDBN}:

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

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

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

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

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

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

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


@node Target Vector Definition

@chapter Target Vector Definition
@cindex target vector

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

@section File Targets

Both executables and core files have target vectors.

@section Standard Protocol and Remote Stubs

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

The @value{GDBN} 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
@code{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
set up 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 @value{GDBN}.

@section ROM Monitor Interface

@section Custom Protocols

@section Transport Layer

@section Builtin Simulator


@node Native Debugging

@chapter Native Debugging
@cindex native debugging

Several files control @value{GDBN}'s configuration for native support:

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

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

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

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

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

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

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

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

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

@section Native core file Support
@cindex native core files

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

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

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

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

@section ptrace

@section /proc

@section win32

@section shared libraries

@section Native Conditionals
@cindex native conditionals

When @value{GDBN} 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
@findex ATTACH_DETACH
If defined, then @value{GDBN} will include support for the @code{attach} and
@code{detach} commands.

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

@item FILES_INFO_HOOK
@findex FILES_INFO_HOOK
(Only defined for Convex.)

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

@item GET_LONGJMP_TARGET
@findex 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
@file{setjmp.h} is needed to define it.

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

@item I386_USE_GENERIC_WATCHPOINTS
An x86-based machine can define this to use the generic x86 watchpoint
support; see @ref{Algorithms, I386_USE_GENERIC_WATCHPOINTS}.

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

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

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

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

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

@item PROC_NAME_FMT
@findex 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
@findex PTRACE_FP_BUG
See @file{mach386-xdep.c}.

@item PTRACE_ARG3_TYPE
@findex 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
@findex REGISTER_U_ADDR
Defines the offset of the registers in the ``u area''.

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

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

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

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

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

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

@item U_REGS_OFFSET
@findex 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 @code{u.u_ar0} @emph{is} the location of the registers.

@item CLEAR_SOLIB
@findex CLEAR_SOLIB
See @file{objfiles.c}.

@item DEBUG_PTRACE
@findex DEBUG_PTRACE
Define this to debug @code{ptrace} calls.
@end table


@node Support Libraries

@chapter Support Libraries

@section BFD
@cindex BFD library

BFD provides support for @value{GDBN} 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).  @value{GDBN} simply
calls BFD to read or write section @var{x} at byte offset @var{y} for
length @var{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
@value{GDBN} uses an internal interface of BFD to determine where to find the
symbol information in an executable file or symbol-file.  @value{GDBN} itself
handles the reading of symbols, since BFD does not ``understand'' debug
symbols, but @value{GDBN} uses BFD's cached information to find the symbols,
string table, etc.
@end table

@section opcodes
@cindex opcodes library

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

@section readline

@section mmalloc

@section libiberty

@section gnu-regex
@cindex regular expressions library

Regex conditionals.

@table @code
@item C_ALLOCA

@item NFAILURES

@item RE_NREGS

@item SIGN_EXTEND_CHAR

@item SWITCH_ENUM_BUG

@item SYNTAX_TABLE

@item Sword

@item sparc
@end table

@section include

@node Coding

@chapter Coding

This chapter covers topics that are lower-level than the major
algorithms of @value{GDBN}.

@section Cleanups
@cindex 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.

@findex make_cleanup
@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}.

@findex do_cleanups
@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

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

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

@section Wrapping Output Lines
@cindex line wrap in output

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

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

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

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

@section @value{GDBN} Coding Standards
@cindex coding standards

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

@value{GDBN} follows an additional set of coding standards specific to
@value{GDBN}, as described in the following sections.

@cindex compiler warnings
You can configure with @samp{--enable-build-warnings} or
@samp{--enable-gdb-build-warnings} to get GCC to check on a number of
these rules.  @value{GDBN} sources ought not to engender any complaints,
unless they are caused by bogus host systems.  (The exact set of enabled
warnings is currently @samp{-Wimplicit -Wreturn-type -Wcomment
-Wtrigraphs -Wformat -Wparentheses -Wpointer-arith -Wuninitialized}.

@subsection Formatting

@cindex source code formatting
The standard GNU recommendations for formatting must be followed
strictly.

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

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

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

@subsection Comments

@cindex comment formatting
The standard GNU requirements on comments must be followed strictly.

Block comments must appear in the following form, with no @samp{/*}- or
@samp{*/}-only lines, and no leading @samp{*}:

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

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

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

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

@subsection C Usage

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

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

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

@subsection Function Prototypes

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

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

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

@subsection Clean Design

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

@cindex assumptions about targets
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
@value{GDBN}, or one of the swap routines defined in @file{bfd.h},
such as @code{bfd_get_32}.

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

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

@cindex portability
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.

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

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

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

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

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

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

@noindent
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 @value{GDBN} 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 @value{GDBN} along a particular interface is with an
attribute struct.  For example, @value{GDBN} has been generalized to handle
multiple kinds of remote interfaces---not by @code{#ifdef}s everywhere, but
by defining the @code{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.,
@code{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 @value{GDBN}'s access to multiple source languages.

Please avoid duplicating code.  For example, in @value{GDBN} 3.x all
the code interfacing between @code{ptrace} and the rest of
@value{GDBN} was duplicated in @file{*-dep.c}, and so changing
something was very painful.  In @value{GDBN} 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, @code{#if defined}, etc.), and machines which are
radically different don't need to use @file{infptrace.c} at all.

Don't put debugging @code{printf}s in the code.

@node Porting GDB

@chapter Porting @value{GDBN}
@cindex porting to new machines

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

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

@example
./config.sub @var{xyz}
@end example

@noindent
and

@example
./config.sub @code{@var{arch}-@var{xvend}-@var{xos}}
@end example

@noindent
which should both respond with @code{@var{arch}-@var{xvend}-@var{xos}}
and no error messages.

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

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

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

@section Configuring @value{GDBN} for Release

@cindex preparing a release
@cindex making a distribution tarball
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

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

This procedure requires:

@itemize @bullet

@item 
symbolic links;

@item
@code{makeinfo} (texinfo2 level);

@item
@TeX{};

@item
@code{dvips};

@item
@code{yacc} or @code{bison}.
@end itemize

@noindent
@dots{} and the usual slew of utilities (@code{sed}, @code{tar}, etc.).

@subheading TEMPORARY RELEASE PROCEDURE FOR DOCUMENTATION

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

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

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

@node Testsuite

@chapter Testsuite
@cindex test suite

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

The @value{GDBN} testsuite uses the DejaGNU testing framework.
DejaGNU is built using @code{Tcl} and @code{expect}.  The tests
themselves are calls to various @code{Tcl} procs; the framework runs all the
procs and summarizes the passes and fails.

@section Using the Testsuite

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

@example
                === gdb Summary ===

# of expected passes            6016
# of unexpected failures        58
# of unexpected successes       5
# of expected failures          183
# of unresolved testcases       3
# of untested testcases         5
@end example

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

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

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

@section Testsuite Organization

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

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

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

@table @file
@item gdb.base
This is the base testsuite.  The tests in it should apply to all
configurations of @value{GDBN} (but generic native-only tests may live here).
The test programs should be in the subset of C that is valid K&R,
ANSI/ISO, and C++ (@code{#ifdef}s are allowed if necessary, for instance
for prototypes).

@item gdb.@var{lang}
Language-specific tests for any language @var{lang} besides C.  Examples are
@file{gdb.c++} and @file{gdb.java}.

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

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

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

@section Writing Tests
@cindex writing tests

In many areas, the @value{GDBN} tests are already quite comprehensive; you
should be able to copy existing tests to handle new cases.

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

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

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

@node Hints

@chapter Hints

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

@menu
* Getting Started::             Getting started working on @value{GDBN}
* Debugging GDB::               Debugging @value{GDBN} with itself
@end menu

@node Getting Started,,, Hints

@section Getting Started

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

This manual, the @value{GDBN} Internals manual, has information which applies
generally to many parts of @value{GDBN}.

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 @value{GDBN}; feel
free to submit a bug report, with a suggested comment if you can figure
out what the comment should say.  If you find a comment which is
actually wrong, be especially sure to report that.

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

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

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

Once you have a part of @value{GDBN} 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.

@cindex command implementation
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 @value{GDBN} 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 @value{GDBN} on itself and set a
breakpoint in @code{execute_command}.

@cindex @code{bug-gdb} mailing list
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
@value{GDBN}''---if we had some magic secret we would put it in this manual.
Suggestions for improving the manual are always welcome, of course.

@node Debugging GDB,,,Hints

@section Debugging @value{GDBN} with itself
@cindex debugging @value{GDBN}

If @value{GDBN} 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 @kbd{@w{./gdb
./gdb}}, which works on Suns and such, you can copy @file{gdb} to
@file{gdb2} and then type @kbd{@w{./gdb ./gdb2}}.

When you run @value{GDBN} in the @value{GDBN} 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 @value{GDBN} 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 @value{GDBN} with your local cc, or
have run @code{fixincludes} if you are compiling with gcc.

@section Submitting Patches

@cindex submitting patches
Thanks for thinking of offering your changes back to the community of
@value{GDBN} 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 @value{GDBN} maintainers will only install ``cleanly designed'' patches.
This manual summarizes what we believe to be clean design for @value{GDBN}.

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.

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

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

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

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

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

Please send patches directly to
@email{gdb-patches@@sourceware.cygnus.com, the @value{GDBN} maintainers}.

@section Obsolete Conditionals
@cindex obsolete code

Fragments of old code in @value{GDBN} 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 @value{GDBN}
gets the info portably from there.  The values in @value{GDBN}'s configuration
files should be moved into BFD configuration files (if needed there),
and deleted from all of @value{GDBN}'s config files.

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

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

@item REG_STACK_SEGMENT
exec.c

@end table

@node Index
@unnumbered Index

@printindex cp

@c TeX can handle the contents at the start but makeinfo 3.12 can not
@ifinfo
@contents
@end ifinfo
@ifhtml
@contents
@end ifhtml

@bye