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\input texinfo @c -*- texinfo -*-
@include gdb-cfg.texi
@settitle @value{GDBN} Internals
@setchapternewpage off
@dircategory Software development
* Gdb-Internals: (gdbint). The GNU debugger's internals.
@end direntry
Copyright @copyright{} 1990-1994, 1996, 1998-2006, 2008-2012 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 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
Texts. A copy of the license is included in the section entitled ``GNU
Free Documentation License''.
@end copying
This file documents the internals of the GNU debugger @value{GDBN}.
@end ifnottex
@syncodeindex vr fn
@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
\def\$#1${{#1}} % Kluge: collect RCS revision info without $...$
\xdef\manvers{\$Revision$} % For use in headers, footers too
\hfill Cygnus Solutions\par
\hfill \manvers\par
\hfill \TeX{}info \texinfoversion\par
@end tex
@vskip 0pt plus 1filll
@end titlepage
@node Top
@c Perhaps this should be the title of the document (but only for info,
@c not for TeX). Existing GNU manuals seem inconsistent on this point.
@top Scope of this Document
This document documents the internals of the GNU debugger, @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.
* Summary::
* Overall Structure::
* Algorithms::
* User Interface::
* libgdb::
* Values::
* Stack Frames::
* Symbol Handling::
* Language Support::
* Host Definition::
* Target Architecture Definition::
* Target Descriptions::
* Target Vector Definition::
* Native Debugging::
* Support Libraries::
* Coding Standards::
* Misc Guidelines::
* Porting GDB::
* Versions and Branches::
* Start of New Year Procedure::
* Releasing GDB::
* Testsuite::
* Hints::
* GDB Observers:: @value{GDBN} Currently available observers
* GNU Free Documentation License:: The license for this documentation
* Concept Index::
* Function and Variable Index::
@end menu
@node Summary
@chapter Summary
* Requirements::
* Contributors::
@end menu
@node Requirements
@section 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
@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 usually, 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}
@node Contributors
@section Contributors
The first edition of this document was written by John Gilmore of
Cygnus Solutions. The current second edition was written by Stan Shebs
of Cygnus Solutions, who continues to update the manual.
Over the years, many others have made additions and changes to this
document. This section attempts to record the significant contributors
to that effort. One of the virtues of free software is that everyone
is free to contribute to it; with regret, we cannot actually
acknowledge everyone here.
@emph{Plea:} This section has only been added relatively recently (four
years after publication of the second edition). Additions to this
section are particularly welcome. If you or your friends (or enemies,
to be evenhanded) have been unfairly omitted from this list, we would
like to add your names!
@end quotation
A document such as this relies on being kept up to date by numerous
small updates by contributing engineers as they make changes to the
code base. The file @file{ChangeLog} in the @value{GDBN} distribution
approximates a blow-by-blow account. The most prolific contributors to
this important, but low profile task are Andrew Cagney (responsible
for over half the entries), Daniel Jacobowitz, Mark Kettenis, Jim
Blandy and Eli Zaretskii.
Eli Zaretskii and Daniel Jacobowitz wrote the sections documenting
Jeremy Bennett updated the sections on initializing a new architecture
and register representation, and added the section on Frame Interpretation.
@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
@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. These are all calculated by @code{autoconf}
when the debugger is built.
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, calling @code{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 as the traditional native debugger
on a system, you will need to supply both target and native information.
@section Source Tree Structure
@cindex @value{GDBN} source tree structure
The @value{GDBN} source directory has a mostly flat structure---there
are only a few subdirectories. A file's name usually gives a hint as
to what it does; for example, @file{stabsread.c} reads stabs,
@file{dwarf2read.c} reads @sc{DWARF 2}, etc.
Files that are related to some common task have names that share
common substrings. For example, @file{*-thread.c} files deal with
debugging threads on various platforms; @file{*read.c} files deal with
reading various kinds of symbol and object files; @file{inf*.c} files
deal with direct control of the @dfn{inferior program} (@value{GDBN}
parlance for the program being debugged).
There are several dozens of files in the @file{*-tdep.c} family.
@samp{tdep} stands for @dfn{target-dependent code}---each of these
files implements debug support for a specific target architecture
(sparc, mips, etc). Usually, only one of these will be used in a
specific @value{GDBN} configuration (sometimes two, closely related).
Similarly, there are many @file{*-nat.c} files, each one for native
debugging on a specific system (e.g., @file{sparc-linux-nat.c} is for
native debugging of Sparc machines running the Linux kernel).
The few subdirectories of the source tree are:
@table @file
@item cli
Code that implements @dfn{CLI}, the @value{GDBN} Command-Line
Interpreter. @xref{User Interface, Command Interpreter}.
@item gdbserver
Code for the @value{GDBN} remote server.
@item gdbtk
Code for Insight, the @value{GDBN} TK-based GUI front-end.
@item mi
The @dfn{GDB/MI}, the @value{GDBN} Machine Interface interpreter.
@item signals
Target signal translation code.
@item tui
Code for @dfn{TUI}, the @value{GDBN} Text-mode full-screen User
Interface. @xref{User Interface, TUI}.
@end table
@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 Prologue Analysis
@cindex prologue analysis
@cindex call frame information
@cindex CFI (call frame information)
To produce a backtrace and allow the user to manipulate older frames'
variables and arguments, @value{GDBN} needs to find the base addresses
of older frames, and discover where those frames' registers have been
saved. Since a frame's ``callee-saves'' registers get saved by
younger frames if and when they're reused, a frame's registers may be
scattered unpredictably across younger frames. This means that
changing the value of a register-allocated variable in an older frame
may actually entail writing to a save slot in some younger frame.
Modern versions of GCC emit Dwarf call frame information (``CFI''),
which describes how to find frame base addresses and saved registers.
But CFI is not always available, so as a fallback @value{GDBN} uses a
technique called @dfn{prologue analysis} to find frame sizes and saved
registers. A prologue analyzer disassembles the function's machine
code starting from its entry point, and looks for instructions that
allocate frame space, save the stack pointer in a frame pointer
register, save registers, and so on. Obviously, this can't be done
accurately in general, but it's tractable to do well enough to be very
helpful. Prologue analysis predates the GNU toolchain's support for
CFI; at one time, prologue analysis was the only mechanism
@value{GDBN} used for stack unwinding at all, when the function
calling conventions didn't specify a fixed frame layout.
In the olden days, function prologues were generated by hand-written,
target-specific code in GCC, and treated as opaque and untouchable by
optimizers. Looking at this code, it was usually straightforward to
write a prologue analyzer for @value{GDBN} that would accurately
understand all the prologues GCC would generate. However, over time
GCC became more aggressive about instruction scheduling, and began to
understand more about the semantics of the prologue instructions
themselves; in response, @value{GDBN}'s analyzers became more complex
and fragile. Keeping the prologue analyzers working as GCC (and the
instruction sets themselves) evolved became a substantial task.
@cindex @file{prologue-value.c}
@cindex abstract interpretation of function prologues
@cindex pseudo-evaluation of function prologues
To try to address this problem, the code in @file{prologue-value.h}
and @file{prologue-value.c} provides a general framework for writing
prologue analyzers that are simpler and more robust than ad-hoc
analyzers. When we analyze a prologue using the prologue-value
framework, we're really doing ``abstract interpretation'' or
``pseudo-evaluation'': running the function's code in simulation, but
using conservative approximations of the values registers and memory
would hold when the code actually runs. For example, if our function
starts with the instruction:
addi r1, 42 # add 42 to r1
@end example
we don't know exactly what value will be in @code{r1} after executing
this instruction, but we do know it'll be 42 greater than its original
If we then see an instruction like:
addi r1, 22 # add 22 to r1
@end example
we still don't know what @code{r1's} value is, but again, we can say
it is now 64 greater than its original value.
If the next instruction were:
mov r2, r1 # set r2 to r1's value
@end example
then we can say that @code{r2's} value is now the original value of
@code{r1} plus 64.
It's common for prologues to save registers on the stack, so we'll
need to track the values of stack frame slots, as well as the
registers. So after an instruction like this:
mov (fp+4), r2
@end example
then we'd know that the stack slot four bytes above the frame pointer
holds the original value of @code{r1} plus 64.
And so on.
Of course, this can only go so far before it gets unreasonable. If we
wanted to be able to say anything about the value of @code{r1} after
the instruction:
xor r1, r3 # exclusive-or r1 and r3, place result in r1
@end example
then things would get pretty complex. But remember, we're just doing
a conservative approximation; if exclusive-or instructions aren't
relevant to prologues, we can just say @code{r1}'s value is now
``unknown''. We can ignore things that are too complex, if that loss of
information is acceptable for our application.
So when we say ``conservative approximation'' here, what we mean is an
approximation that is either accurate, or marked ``unknown'', but
never inaccurate.
Using this framework, a prologue analyzer is simply an interpreter for
machine code, but one that uses conservative approximations for the
contents of registers and memory instead of actual values. Starting
from the function's entry point, you simulate instructions up to the
current PC, or an instruction that you don't know how to simulate.
Now you can examine the state of the registers and stack slots you've
kept track of.
@itemize @bullet
To see how large your stack frame is, just check the value of the
stack pointer register; if it's the original value of the SP
minus a constant, then that constant is the stack frame's size.
If the SP's value has been marked as ``unknown'', then that means
the prologue has done something too complex for us to track, and
we don't know the frame size.
To see where we've saved the previous frame's registers, we just
search the values we've tracked --- stack slots, usually, but
registers, too, if you want --- for something equal to the register's
original value. If the calling conventions suggest a standard place
to save a given register, then we can check there first, but really,
anything that will get us back the original value will probably work.
@end itemize
This does take some work. But prologue analyzers aren't
quick-and-simple pattern patching to recognize a few fixed prologue
forms any more; they're big, hairy functions. Along with inferior
function calls, prologue analysis accounts for a substantial portion
of the time needed to stabilize a @value{GDBN} port. So it's
worthwhile to look for an approach that will be easier to understand
and maintain. In the approach described above:
@itemize @bullet
It's easier to see that the analyzer is correct: you just see
whether the analyzer properly (albeit conservatively) simulates
the effect of each instruction.
It's easier to extend the analyzer: you can add support for new
instructions, and know that you haven't broken anything that
wasn't already broken before.
It's orthogonal: to gather new information, you don't need to
complicate the code for each instruction. As long as your domain
of conservative values is already detailed enough to tell you
what you need, then all the existing instruction simulations are
already gathering the right data for you.
@end itemize
The file @file{prologue-value.h} contains detailed comments explaining
the framework and how to use it.
@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
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 anything 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 always 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 writable, 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
Basic breakpoint object handling is in @file{breakpoint.c}. However,
much of the interesting breakpoint action is in @file{infrun.c}.
@table @code
@cindex insert or remove software breakpoint
@findex target_remove_breakpoint
@findex target_insert_breakpoint
@item target_remove_breakpoint (@var{bp_tgt})
@itemx target_insert_breakpoint (@var{bp_tgt})
Insert or remove a software breakpoint at address
@code{@var{bp_tgt}->placed_address}. Returns zero for success,
non-zero for failure. On input, @var{bp_tgt} contains the address of the
breakpoint, and is otherwise initialized to zero. The fields of the
@code{struct bp_target_info} pointed to by @var{bp_tgt} are updated
to contain other information about the breakpoint on output. The field
@code{placed_address} may be updated if the breakpoint was placed at a
related address; the field @code{shadow_contents} contains the real
contents of the bytes where the breakpoint has been inserted,
if reading memory would return the breakpoint instead of the
underlying memory; the field @code{shadow_len} is the length of
memory cached in @code{shadow_contents}, if any; and the field
@code{placed_size} is optionally set and used by the target, if
it could differ from @code{shadow_len}.
For example, the remote target @samp{Z0} packet does not require
shadowing memory, so @code{shadow_len} is left at zero. However,
the length reported by @code{gdbarch_breakpoint_from_pc} is cached in
@code{placed_size}, so that a matching @samp{z0} packet can be
used to remove the breakpoint.
@cindex insert or remove hardware breakpoint
@findex target_remove_hw_breakpoint
@findex target_insert_hw_breakpoint
@item target_remove_hw_breakpoint (@var{bp_tgt})
@itemx target_insert_hw_breakpoint (@var{bp_tgt})
Insert or remove a hardware-assisted breakpoint at address
@code{@var{bp_tgt}->placed_address}. Returns zero for success,
non-zero for failure. See @code{target_insert_breakpoint} for
a description of the @code{struct bp_target_info} pointed to by
@var{bp_tgt}; the @code{shadow_contents} and
@code{shadow_len} members are not used for hardware breakpoints,
but @code{placed_size} may be.
@end table
@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}
@findex gdbarch_get_longjmp_target
To make this work, you need to define a function called
@code{gdbarch_get_longjmp_target}, which will examine the
@code{jmp_buf} structure and extract the @code{longjmp} target address.
Since @code{jmp_buf} is target specific and typically defined in a
target header not available to @value{GDBN}, you will need to
determine the offset of the PC manually and return that; many targets
define a @code{jb_pc_offset} field in the tdep structure to save the
value once calculated.
@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
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
The value of the expression to be watched depends on data held in
registers (as opposed to memory).
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.
No hardware-assisted watchpoints provided by the target
@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.
When the inferior stops, @value{GDBN} tries to establish, among other
possible reasons, whether it stopped due to a watchpoint being hit.
It first uses @code{STOPPED_BY_WATCHPOINT} to see if any watchpoint
was hit. If not, all watchpoint checking is skipped.
Then @value{GDBN} calls @code{target_stopped_data_address} exactly
once. This method returns the address of the watchpoint which
triggered, if the target can determine it. If the triggered address
is available, @value{GDBN} compares the address returned by this
method with each watched memory address in each active watchpoint.
For data-read and data-access watchpoints, @value{GDBN} announces
every watchpoint that watches the triggered address as being hit.
For this reason, data-read and data-access watchpoints
@emph{require} that the triggered address be available; if not, read
and access watchpoints will never be considered hit. For data-write
watchpoints, if the triggered address is available, @value{GDBN}
considers only those watchpoints which match that address;
otherwise, @value{GDBN} considers all data-write watchpoints. For
each data-write watchpoint that @value{GDBN} considers, it evaluates
the expression whose value is being watched, and tests whether the
watched value has changed. Watchpoints whose watched values have
changed are announced as hit.
@c FIXME move these to the main lists of target/native defns
@value{GDBN} uses several macros and primitives to support hardware
@table @code
@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).
@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}.
@cindex insert or remove hardware watchpoint
@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:
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 smallexample
These two macros should return 0 for success, non-zero for failure.
@findex target_stopped_data_address
@item target_stopped_data_address (@var{addr_p})
If the inferior has some watchpoint that triggered, place the address
associated with the watchpoint at the location pointed to by
@var{addr_p} and return non-zero. Otherwise, return zero. This
is required for data-read and data-access watchpoints. It is
not required for data-write watchpoints, but @value{GDBN} uses
it to improve handling of those also.
@value{GDBN} will only call this method once per watchpoint stop,
immediately after calling @code{STOPPED_BY_WATCHPOINT}. If the
target's watchpoint indication is sticky, i.e., stays set after
resuming, this method should clear it. For instance, the x86 debug
control register has sticky triggered flags.
@findex target_watchpoint_addr_within_range
@item target_watchpoint_addr_within_range (@var{target}, @var{addr}, @var{start}, @var{length})
Check whether @var{addr} (as returned by @code{target_stopped_data_address})
lies within the hardware-defined watchpoint region described by
@var{start} and @var{length}. This only needs to be provided if the
granularity of a watchpoint is greater than one byte, i.e., if the
watchpoint can also trigger on nearby addresses outside of the watched
If defined to a non-zero value, it is not necessary to disable a
watchpoint to step over it. Like @code{gdbarch_have_nonsteppable_watchpoint},
this is usually set when watchpoints trigger at the instruction
which will perform an interesting read or write. It should be
set if there is a temporary disable bit which allows the processor
to step over the interesting instruction without raising the
watchpoint exception again.
@findex gdbarch_have_nonsteppable_watchpoint
@item int gdbarch_have_nonsteppable_watchpoint (@var{gdbarch})
If it returns a non-zero value, @value{GDBN} should disable a
watchpoint to step the inferior over it. This is usually set when
watchpoints trigger at the instruction which will perform an
interesting read or write.
If defined to a non-zero value, it is possible to continue the
inferior after a watchpoint has been hit. This is usually set
when watchpoints trigger at the instruction following an interesting
read or write.
@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}.
Normally, this macro is defined to invoke the function pointed to by
the @code{to_stopped_by_watchpoint} member of the structure (of the
type @code{target_ops}, defined on @file{target.h}) that describes the
target-specific operations; @code{to_stopped_by_watchpoint} ignores
the @var{wait_status} argument.
@value{GDBN} does not require the non-zero value returned by
@code{STOPPED_BY_WATCHPOINT} to be 100% correct, so if a target cannot
determine for sure whether the inferior stopped due to a watchpoint,
it could return non-zero ``just in case''.
@end table
@subsection Watchpoints and Threads
@cindex watchpoints, with threads
@value{GDBN} only supports process-wide watchpoints, which trigger
in all threads. @value{GDBN} uses the thread ID to make watchpoints
act as if they were thread-specific, but it cannot set hardware
watchpoints that only trigger in a specific thread. Therefore, even
if the target supports threads, per-thread debug registers, and
watchpoints which only affect a single thread, it should set the
per-thread debug registers for all threads to the same value. On
@sc{gnu}/Linux native targets, this is accomplished by using
@code{ALL_LWPS} in @code{target_insert_watchpoint} and
@code{target_remove_watchpoint} and by using
@code{linux_set_new_thread} to register a handler for newly created
@value{GDBN}'s @sc{gnu}/Linux support only reports a single event
at a time, although multiple events can trigger simultaneously for
multi-threaded programs. When multiple events occur, @file{linux-nat.c}
queues subsequent events and returns them the next time the program
is resumed. This means that @code{STOPPED_BY_WATCHPOINT} and
@code{target_stopped_data_address} only need to consult the current
thread's state---the thread indicated by @code{inferior_ptid}. If
two threads have hit watchpoints simultaneously, those routines
will be called a second time for the second thread.
@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}.
(At present, the library functions read and write debug registers directly, and are
thus only available for native configurations.)
To use the generic x86 watchpoint support, a port should do the
@itemize @bullet
Define the macro @code{I386_USE_GENERIC_WATCHPOINTS} somewhere in the
target-dependent headers.
Include the @file{config/i386/nm-i386.h} header file @emph{after}
defining @code{I386_USE_GENERIC_WATCHPOINTS}.
Add @file{i386-nat.o} to the value of the Make variable
@code{NATDEPFILES} (@pxref{Native Debugging, NATDEPFILES}).
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
@table @code
@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
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
@findex i386_stopped_data_address
@item i386_stopped_data_address (@var{addr_p})
The target function
@code{target_stopped_data_address} is set to call this function.
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_stopped_by_watchpoint
@item i386_stopped_by_watchpoint (void)
is 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 true if any bit is set. Otherwise, false is
@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 mirrored
value to @var{addr}, sets the mirrored 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 mirrored value of the debug register and its read/write and
length bits in the mirrored 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{bp_tgt})
@itemx i386_remove_hw_breakpoint (@var{bp_tgt})
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.
The argument is a @code{struct bp_target_info *}, as described in
the documentation for @code{target_insert_breakpoint}.
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
@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
@enumerate 1
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.
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
@section Checkpoints
@cindex checkpoints
@cindex restart
In the abstract, a checkpoint is a point in the execution history of
the program, which the user may wish to return to at some later time.
Internally, a checkpoint is a saved copy of the program state, including
whatever information is required in order to restore the program to that
state at a later time. This can be expected to include the state of
registers and memory, and may include external state such as the state
of open files and devices.
There are a number of ways in which checkpoints may be implemented
in gdb, e.g.@: as corefiles, as forked processes, and as some opaque
method implemented on the target side.
A corefile can be used to save an image of target memory and register
state, which can in principle be restored later --- but corefiles do
not typically include information about external entities such as
open files. Currently this method is not implemented in gdb.
A forked process can save the state of user memory and registers,
as well as some subset of external (kernel) state. This method
is used to implement checkpoints on Linux, and in principle might
be used on other systems.
Some targets, e.g.@: simulators, might have their own built-in
method for saving checkpoints, and gdb might be able to take
advantage of that capability without necessarily knowing any
details of how it is done.
@section Observing changes in @value{GDBN} internals
@cindex observer pattern interface
@cindex notifications about changes in internals
In order to function properly, several modules need to be notified when
some changes occur in the @value{GDBN} internals. Traditionally, these
modules have relied on several paradigms, the most common ones being
hooks and gdb-events. Unfortunately, none of these paradigms was
versatile enough to become the standard notification mechanism in
@value{GDBN}. The fact that they only supported one ``client'' was also
a strong limitation.
A new paradigm, based on the Observer pattern of the @cite{Design
Patterns} book, has therefore been implemented. The goal was to provide
a new interface overcoming the issues with the notification mechanisms
previously available. This new interface needed to be strongly typed,
easy to extend, and versatile enough to be used as the standard
interface when adding new notifications.
See @ref{GDB Observers} for a brief description of the observers
currently implemented in GDB. The rationale for the current
implementation is also briefly discussed.
@node User Interface
@chapter User Interface
@value{GDBN} has several user interfaces, of which the traditional
command-line interface is perhaps the most familiar.
@section Command Interpreter
@cindex command interpreter
@cindex CLI
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.
@findex add_setshow_cmd
@findex add_setshow_cmd_full
To add paired @samp{set} and @samp{show} commands, use
@code{add_setshow_cmd} or @code{add_setshow_cmd_full}. The former is
a slightly simpler interface which is useful when you don't need to
further modify the new command structures, while the latter returns
the new command structures for manipulation.
@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 command 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.
@anchor{UI-Independent Output}
@section UI-Independent Output---the @code{ui_out} Functions
@c This section is based on the documentation written by Fernando
@c Nasser <>.
@cindex @code{ui_out} functions
The @code{ui_out} functions present an abstraction level for the
@value{GDBN} output code. They hide the specifics of different user
interfaces supported by @value{GDBN}, and thus free the programmer
from the need to write several versions of the same code, one each for
every UI, to produce output.
@subsection Overview and Terminology
In general, execution of each @value{GDBN} command produces some sort
of output, and can even generate an input request.
Output can be generated for the following purposes:
@itemize @bullet
to display a @emph{result} of an operation;
to convey @emph{info} or produce side-effects of a requested
to provide a @emph{notification} of an asynchronous event (including
progress indication of a prolonged asynchronous operation);
to display @emph{error messages} (including warnings);
to show @emph{debug data};
to @emph{query} or prompt a user for input (a special case).
@end itemize
This section mainly concentrates on how to build result output,
although some of it also applies to other kinds of output.
Generation of output that displays the results of an operation
involves one or more of the following:
@itemize @bullet
output of the actual data
formatting the output as appropriate for console output, to make it
easily readable by humans
machine oriented formatting--a more terse formatting to allow for easy
parsing by programs which read @value{GDBN}'s output
annotation, whose purpose is to help legacy GUIs to identify interesting
parts in the output
@end itemize
The @code{ui_out} routines take care of the first three aspects.
Annotations are provided by separate annotation routines. Note that use
of annotations for an interface between a GUI and @value{GDBN} is
Output can be in the form of a single item, which we call a @dfn{field};
a @dfn{list} consisting of identical fields; a @dfn{tuple} consisting of
non-identical fields; or a @dfn{table}, which is a tuple consisting of a
header and a body. In a BNF-like form:
@table @code
@item <table> @expansion{}
@code{<header> <body>}
@item <header> @expansion{}
@code{@{ <column> @}}
@item <column> @expansion{}
@code{<width> <alignment> <title>}
@item <body> @expansion{}
@end table
@subsection General Conventions
Most @code{ui_out} routines are of type @code{void}, the exceptions are
@code{ui_out_stream_new} (which returns a pointer to the newly created
object) and the @code{make_cleanup} routines.
The first parameter is always the @code{ui_out} vector object, a pointer
to a @code{struct ui_out}.
The @var{format} parameter is like in @code{printf} family of functions.
When it is present, there must also be a variable list of arguments
sufficient used to satisfy the @code{%} specifiers in the supplied
When a character string argument is not used in a @code{ui_out} function
call, a @code{NULL} pointer has to be supplied instead.
@subsection Table, Tuple and List Functions
@cindex list output functions
@cindex table output functions
@cindex tuple output functions
This section introduces @code{ui_out} routines for building lists,
tuples and tables. The routines to output the actual data items
(fields) are presented in the next section.
To recap: A @dfn{tuple} is a sequence of @dfn{fields}, each field
containing information about an object; a @dfn{list} is a sequence of
fields where each field describes an identical object.
Use the @dfn{table} functions when your output consists of a list of
rows (tuples) and the console output should include a heading. Use this
even when you are listing just one object but you still want the header.
@cindex nesting level in @code{ui_out} functions
Tables can not be nested. Tuples and lists can be nested up to a
maximum of five levels.
The overall structure of the table output code is something like this:
@end smallexample
Here is the description of table-, tuple- and list-related @code{ui_out}
@deftypefun void ui_out_table_begin (struct ui_out *@var{uiout}, int @var{nbrofcols}, int @var{nr_rows}, const char *@var{tblid})
The function @code{ui_out_table_begin} marks the beginning of the output
of a table. It should always be called before any other @code{ui_out}
function for a given table. @var{nbrofcols} is the number of columns in
the table. @var{nr_rows} is the number of rows in the table.
@var{tblid} is an optional string identifying the table. The string
pointed to by @var{tblid} is copied by the implementation of
@code{ui_out_table_begin}, so the application can free the string if it
was @code{malloc}ed.
The companion function @code{ui_out_table_end}, described below, marks
the end of the table's output.
@end deftypefun
@deftypefun void ui_out_table_header (struct ui_out *@var{uiout}, int @var{width}, enum ui_align @var{alignment}, const char *@var{colhdr})
@code{ui_out_table_header} provides the header information for a single
table column. You call this function several times, one each for every
column of the table, after @code{ui_out_table_begin}, but before
The value of @var{width} gives the column width in characters. The
value of @var{alignment} is one of @code{left}, @code{center}, and
@code{right}, and it specifies how to align the header: left-justify,
center, or right-justify it. @var{colhdr} points to a string that
specifies the column header; the implementation copies that string, so
column header strings in @code{malloc}ed storage can be freed after the
@end deftypefun
@deftypefun void ui_out_table_body (struct ui_out *@var{uiout})
This function delimits the table header from the table body.
@end deftypefun
@deftypefun void ui_out_table_end (struct ui_out *@var{uiout})
This function signals the end of a table's output. It should be called
after the table body has been produced by the list and field output
There should be exactly one call to @code{ui_out_table_end} for each
call to @code{ui_out_table_begin}, otherwise the @code{ui_out} functions
will signal an internal error.
@end deftypefun
The output of the tuples that represent the table rows must follow the
call to @code{ui_out_table_body} and precede the call to
@code{ui_out_table_end}. You build a tuple by calling
@code{ui_out_tuple_begin} and @code{ui_out_tuple_end}, with suitable
calls to functions which actually output fields between them.
@deftypefun void ui_out_tuple_begin (struct ui_out *@var{uiout}, const char *@var{id})
This function marks the beginning of a tuple output. @var{id} points
to an optional string that identifies the tuple; it is copied by the
implementation, and so strings in @code{malloc}ed storage can be freed
after the call.
@end deftypefun
@deftypefun void ui_out_tuple_end (struct ui_out *@var{uiout})
This function signals an end of a tuple output. There should be exactly
one call to @code{ui_out_tuple_end} for each call to
@code{ui_out_tuple_begin}, otherwise an internal @value{GDBN} error will
be signaled.
@end deftypefun
@deftypefun {struct cleanup *} make_cleanup_ui_out_tuple_begin_end (struct ui_out *@var{uiout}, const char *@var{id})
This function first opens the tuple and then establishes a cleanup
(@pxref{Misc Guidelines, Cleanups}) to close the tuple.
It provides a convenient and correct implementation of the
non-portable@footnote{The function cast is not portable ISO C.} code sequence:
struct cleanup *old_cleanup;
ui_out_tuple_begin (uiout, "...");
old_cleanup = make_cleanup ((void(*)(void *)) ui_out_tuple_end,
@end smallexample
@end deftypefun
@deftypefun void ui_out_list_begin (struct ui_out *@var{uiout}, const char *@var{id})
This function marks the beginning of a list output. @var{id} points to
an optional string that identifies the list; it is copied by the
implementation, and so strings in @code{malloc}ed storage can be freed
after the call.
@end deftypefun
@deftypefun void ui_out_list_end (struct ui_out *@var{uiout})
This function signals an end of a list output. There should be exactly
one call to @code{ui_out_list_end} for each call to
@code{ui_out_list_begin}, otherwise an internal @value{GDBN} error will
be signaled.
@end deftypefun
@deftypefun {struct cleanup *} make_cleanup_ui_out_list_begin_end (struct ui_out *@var{uiout}, const char *@var{id})
Similar to @code{make_cleanup_ui_out_tuple_begin_end}, this function
opens a list and then establishes cleanup
(@pxref{Misc Guidelines, Cleanups})
that will close the list.
@end deftypefun
@subsection Item Output Functions
@cindex item output functions
@cindex field output functions
@cindex data output
The functions described below produce output for the actual data
items, or fields, which contain information about the object.
Choose the appropriate function accordingly to your particular needs.
@deftypefun void ui_out_field_fmt (struct ui_out *@var{uiout}, char *@var{fldname}, char *@var{format}, ...)
This is the most general output function. It produces the
representation of the data in the variable-length argument list
according to formatting specifications in @var{format}, a
@code{printf}-like format string. The optional argument @var{fldname}
supplies the name of the field. The data items themselves are
supplied as additional arguments after @var{format}.
This generic function should be used only when it is not possible to
use one of the specialized versions (see below).
@end deftypefun
@deftypefun void ui_out_field_int (struct ui_out *@var{uiout}, const char *@var{fldname}, int @var{value})
This function outputs a value of an @code{int} variable. It uses the
@code{"%d"} output conversion specification. @var{fldname} specifies
the name of the field.
@end deftypefun
@deftypefun void ui_out_field_fmt_int (struct ui_out *@var{uiout}, int @var{width}, enum ui_align @var{alignment}, const char *@var{fldname}, int @var{value})
This function outputs a value of an @code{int} variable. It differs from
@code{ui_out_field_int} in that the caller specifies the desired @var{width} and @var{alignment} of the output.
@var{fldname} specifies
the name of the field.
@end deftypefun
@deftypefun void ui_out_field_core_addr (struct ui_out *@var{uiout}, const char *@var{fldname}, struct gdbarch *@var{gdbarch}, CORE_ADDR @var{address})
This function outputs an address as appropriate for @var{gdbarch}.
@end deftypefun
@deftypefun void ui_out_field_string (struct ui_out *@var{uiout}, const char *@var{fldname}, const char *@var{string})
This function outputs a string using the @code{"%s"} conversion
@end deftypefun
Sometimes, there's a need to compose your output piece by piece using
functions that operate on a stream, such as @code{value_print} or
@code{fprintf_symbol_filtered}. These functions accept an argument of
the type @code{struct ui_file *}, a pointer to a @code{ui_file} object
used to store the data stream used for the output. When you use one
of these functions, you need a way to pass their results stored in a
@code{ui_file} object to the @code{ui_out} functions. To this end,
you first create a @code{ui_stream} object by calling
@code{ui_out_stream_new}, pass the @code{stream} member of that
@code{ui_stream} object to @code{value_print} and similar functions,
and finally call @code{ui_out_field_stream} to output the field you
constructed. When the @code{ui_stream} object is no longer needed,
you should destroy it and free its memory by calling
@deftypefun {struct ui_stream *} ui_out_stream_new (struct ui_out *@var{uiout})
This function creates a new @code{ui_stream} object which uses the
same output methods as the @code{ui_out} object whose pointer is
passed in @var{uiout}. It returns a pointer to the newly created
@code{ui_stream} object.
@end deftypefun
@deftypefun void ui_out_stream_delete (struct ui_stream *@var{streambuf})
This functions destroys a @code{ui_stream} object specified by
@end deftypefun
@deftypefun void ui_out_field_stream (struct ui_out *@var{uiout}, const char *@var{fieldname}, struct ui_stream *@var{streambuf})
This function consumes all the data accumulated in
@code{streambuf->stream} and outputs it like
@code{ui_out_field_string} does. After a call to
@code{ui_out_field_stream}, the accumulated data no longer exists, but
the stream is still valid and may be used for producing more fields.
@end deftypefun
@strong{Important:} If there is any chance that your code could bail
out before completing output generation and reaching the point where
@code{ui_out_stream_delete} is called, it is necessary to set up a
cleanup, to avoid leaking memory and other resources. Here's a
skeleton code to do that:
struct ui_stream *mybuf = ui_out_stream_new (uiout);
struct cleanup *old = make_cleanup (ui_out_stream_delete, mybuf);
do_cleanups (old);
@end smallexample
If the function already has the old cleanup chain set (for other kinds
of cleanups), you just have to add your cleanup to it:
mybuf = ui_out_stream_new (uiout);
make_cleanup (ui_out_stream_delete, mybuf);
@end smallexample
Note that with cleanups in place, you should not call
@code{ui_out_stream_delete} directly, or you would attempt to free the
same buffer twice.
@subsection Utility Output Functions
@deftypefun void ui_out_field_skip (struct ui_out *@var{uiout}, const char *@var{fldname})
This function skips a field in a table. Use it if you have to leave
an empty field without disrupting the table alignment. The argument
@var{fldname} specifies a name for the (missing) filed.
@end deftypefun
@deftypefun void ui_out_text (struct ui_out *@var{uiout}, const char *@var{string})
This function outputs the text in @var{string} in a way that makes it
easy to be read by humans. For example, the console implementation of
this method filters the text through a built-in pager, to prevent it
from scrolling off the visible portion of the screen.
Use this function for printing relatively long chunks of text around
the actual field data: the text it produces is not aligned according
to the table's format. Use @code{ui_out_field_string} to output a
string field, and use @code{ui_out_message}, described below, to
output short messages.
@end deftypefun
@deftypefun void ui_out_spaces (struct ui_out *@var{uiout}, int @var{nspaces})
This function outputs @var{nspaces} spaces. It is handy to align the
text produced by @code{ui_out_text} with the rest of the table or
@end deftypefun
@deftypefun void ui_out_message (struct ui_out *@var{uiout}, int @var{verbosity}, const char *@var{format}, ...)
This function produces a formatted message, provided that the current
verbosity level is at least as large as given by @var{verbosity}. The
current verbosity level is specified by the user with the @samp{set
verbositylevel} command.@footnote{As of this writing (April 2001),
setting verbosity level is not yet implemented, and is always returned
as zero. So calling @code{ui_out_message} with a @var{verbosity}
argument more than zero will cause the message to never be printed.}
@end deftypefun
@deftypefun void ui_out_wrap_hint (struct ui_out *@var{uiout}, char *@var{indent})
This function gives the console output filter (a paging filter) a hint
of where to break lines which are too long. Ignored for all other
output consumers. @var{indent}, if non-@code{NULL}, is the string to
be printed to indent the wrapped text on the next line; it must remain
accessible until the next call to @code{ui_out_wrap_hint}, or until an
explicit newline is produced by one of the other functions. If
@var{indent} is @code{NULL}, the wrapped text will not be indented.
@end deftypefun
@deftypefun void ui_out_flush (struct ui_out *@var{uiout})
This function flushes whatever output has been accumulated so far, if
the UI buffers output.
@end deftypefun
@subsection Examples of Use of @code{ui_out} functions
@cindex using @code{ui_out} functions
@cindex @code{ui_out} functions, usage examples
This section gives some practical examples of using the @code{ui_out}
functions to generalize the old console-oriented code in
@value{GDBN}. The examples all come from functions defined on the
@file{breakpoints.c} file.
This example, from the @code{breakpoint_1} function, shows how to
produce a table.
The original code was:
if (!found_a_breakpoint++)
annotate_breakpoints_headers ();
annotate_field (0);
printf_filtered ("Num ");
annotate_field (1);
printf_filtered ("Type ");
annotate_field (2);
printf_filtered ("Disp ");
annotate_field (3);
printf_filtered ("Enb ");
if (addressprint)
annotate_field (4);
printf_filtered ("Address ");
annotate_field (5);
printf_filtered ("What\n");
annotate_breakpoints_table ();
@end smallexample
Here's the new version:
nr_printable_breakpoints = @dots{};
if (addressprint)
ui_out_table_begin (ui, 6, nr_printable_breakpoints, "BreakpointTable");
ui_out_table_begin (ui, 5, nr_printable_breakpoints, "BreakpointTable");
if (nr_printable_breakpoints > 0)
annotate_breakpoints_headers ();
if (nr_printable_breakpoints > 0)
annotate_field (0);
ui_out_table_header (uiout, 3, ui_left, "number", "Num"); /* 1 */
if (nr_printable_breakpoints > 0)
annotate_field (1);
ui_out_table_header (uiout, 14, ui_left, "type", "Type"); /* 2 */
if (nr_printable_breakpoints > 0)
annotate_field (2);
ui_out_table_header (uiout, 4, ui_left, "disp", "Disp"); /* 3 */
if (nr_printable_breakpoints > 0)
annotate_field (3);
ui_out_table_header (uiout, 3, ui_left, "enabled", "Enb"); /* 4 */
if (addressprint)
if (nr_printable_breakpoints > 0)
annotate_field (4);
if (print_address_bits <= 32)
ui_out_table_header (uiout, 10, ui_left, "addr", "Address");/* 5 */
ui_out_table_header (uiout, 18, ui_left, "addr", "Address");/* 5 */
if (nr_printable_breakpoints > 0)
annotate_field (5);
ui_out_table_header (uiout, 40, ui_noalign, "what", "What"); /* 6 */
ui_out_table_body (uiout);
if (nr_printable_breakpoints > 0)
annotate_breakpoints_table ();
@end smallexample
This example, from the @code{print_one_breakpoint} function, shows how
to produce the actual data for the table whose structure was defined
in the above example. The original code was:
annotate_record ();
annotate_field (0);
printf_filtered ("%-3d ", b->number);
annotate_field (1);
if ((int)b->type > (sizeof(bptypes)/sizeof(bptypes[0]))
|| ((int) b->type != bptypes[(int) b->type].type))
internal_error ("bptypes table does not describe type #%d.",
printf_filtered ("%-14s ", bptypes[(int)b->type].description);
annotate_field (2);
printf_filtered ("%-4s ", bpdisps[(int)b->disposition]);
annotate_field (3);
printf_filtered ("%-3c ", bpenables[(int)b->enable]);
@end smallexample
This is the new version:
annotate_record ();
ui_out_tuple_begin (uiout, "bkpt");
annotate_field (0);
ui_out_field_int (uiout, "number", b->number);
annotate_field (1);
if (((int) b->type > (sizeof (bptypes) / sizeof (bptypes[0])))
|| ((int) b->type != bptypes[(int) b->type].type))
internal_error ("bptypes table does not describe type #%d.",
(int) b->type);
ui_out_field_string (uiout, "type", bptypes[(int)b->type].description);
annotate_field (2);
ui_out_field_string (uiout, "disp", bpdisps[(int)b->disposition]);
annotate_field (3);
ui_out_field_fmt (uiout, "enabled", "%c", bpenables[(int)b->enable]);
@end smallexample
This example, also from @code{print_one_breakpoint}, shows how to
produce a complicated output field using the @code{print_expression}
functions which requires a stream to be passed. It also shows how to
automate stream destruction with cleanups. The original code was:
annotate_field (5);
print_expression (b->exp, gdb_stdout);
@end smallexample
The new version is:
struct ui_stream *stb = ui_out_stream_new (uiout);
struct cleanup *old_chain = make_cleanup_ui_out_stream_delete (stb);
annotate_field (5);
print_expression (b->exp, stb->stream);
ui_out_field_stream (uiout, "what", local_stream);
@end smallexample
This example, also from @code{print_one_breakpoint}, shows how to use
@code{ui_out_text} and @code{ui_out_field_string}. The original code
annotate_field (5);
if (b->dll_pathname == NULL)
printf_filtered ("<any library> ");
printf_filtered ("library \"%s\" ", b->dll_pathname);
@end smallexample
It became:
annotate_field (5);
if (b->dll_pathname == NULL)
ui_out_field_string (uiout, "what", "<any library>");
ui_out_spaces (uiout, 1);
ui_out_text (uiout, "library \"");
ui_out_field_string (uiout, "what", b->dll_pathname);
ui_out_text (uiout, "\" ");
@end smallexample
The following example from @code{print_one_breakpoint} shows how to
use @code{ui_out_field_int} and @code{ui_out_spaces}. The original
code was:
annotate_field (5);
if (b->forked_inferior_pid != 0)
printf_filtered ("process %d ", b->forked_inferior_pid);
@end smallexample
It became:
annotate_field (5);
if (b->forked_inferior_pid != 0)
ui_out_text (uiout, "process ");
ui_out_field_int (uiout, "what", b->forked_inferior_pid);
ui_out_spaces (uiout, 1);
@end smallexample
Here's an example of using @code{ui_out_field_string}. The original
code was:
annotate_field (5);
if (b->exec_pathname != NULL)
printf_filtered ("program \"%s\" ", b->exec_pathname);
@end smallexample
It became:
annotate_field (5);
if (b->exec_pathname != NULL)
ui_out_text (uiout, "program \"");
ui_out_field_string (uiout, "what", b->exec_pathname);
ui_out_text (uiout, "\" ");
@end smallexample
Finally, here's an example of printing an address. The original code:
annotate_field (4);
printf_filtered ("%s ",
hex_string_custom ((unsigned long) b->address, 8));
@end smallexample
It became:
annotate_field (4);
ui_out_field_core_addr (uiout, "Address", b->address);
@end smallexample
@section Console Printing
@section TUI
@node libgdb
@chapter libgdb
@section libgdb 1.0
@cindex @code{libgdb}
@code{libgdb} 1.0 was an abortive project of years ago. The theory was
to provide an API to @value{GDBN}'s functionality.
@section libgdb 2.0
@cindex @code{libgdb}
@code{libgdb} 2.0 is an ongoing effort to update @value{GDBN} so that is
better able to support graphical and other environments.
Since @code{libgdb} development is on-going, its architecture is still
evolving. The following components have so far been identified:
@itemize @bullet
Observer - @file{gdb-events.h}.
Builder - @file{ui-out.h}
Event Loop - @file{event-loop.h}
Library - @file{gdb.h}
@end itemize
The model that ties these components together is described below.
@section The @code{libgdb} Model
A client of @code{libgdb} interacts with the library in two ways.
@itemize @bullet
As an observer (using @file{gdb-events}) receiving notifications from
@code{libgdb} of any internal state changes (break point changes, run
state, etc).
As a client querying @code{libgdb} (using the @file{ui-out} builder) to
obtain various status values from @value{GDBN}.
@end itemize
Since @code{libgdb} could have multiple clients (e.g., a GUI supporting
the existing @value{GDBN} CLI), those clients must co-operate when
controlling @code{libgdb}. In particular, a client must ensure that
@code{libgdb} is idle (i.e.@: no other client is using @code{libgdb})
before responding to a @file{gdb-event} by making a query.
@section CLI support
At present @value{GDBN}'s CLI is very much entangled in with the core of
@code{libgdb}. Consequently, a client wishing to include the CLI in
their interface needs to carefully co-ordinate its own and the CLI's
It is suggested that the client set @code{libgdb} up to be bi-modal
(alternate between CLI and client query modes). The notes below sketch
out the theory:
@itemize @bullet
The client registers itself as an observer of @code{libgdb}.
The client create and install @code{cli-out} builder using its own
versions of the @code{ui-file} @code{gdb_stderr}, @code{gdb_stdtarg} and
@code{gdb_stdout} streams.
The client creates a separate custom @code{ui-out} builder that is only
used while making direct queries to @code{libgdb}.
@end itemize
When the client receives input intended for the CLI, it simply passes it
along. Since the @code{cli-out} builder is installed by default, all
the CLI output in response to that command is routed (pronounced rooted)
through to the client controlled @code{gdb_stdout} et.@: al.@: streams.
At the same time, the client is kept abreast of internal changes by
virtue of being a @code{libgdb} observer.
The only restriction on the client is that it must wait until
@code{libgdb} becomes idle before initiating any queries (using the
client's custom builder).
@section @code{libgdb} components
@subheading Observer - @file{gdb-events.h}
@file{gdb-events} provides the client with a very raw mechanism that can
be used to implement an observer. At present it only allows for one
observer and that observer must, internally, handle the need to delay
the processing of any event notifications until after @code{libgdb} has
finished the current command.
@subheading Builder - @file{ui-out.h}
@file{ui-out} provides the infrastructure necessary for a client to
create a builder. That builder is then passed down to @code{libgdb}
when doing any queries.
@subheading Event Loop - @file{event-loop.h}
@c There could be an entire section on the event-loop
@file{event-loop}, currently non-re-entrant, provides a simple event
loop. A client would need to either plug its self into this loop or,
implement a new event-loop that @value{GDBN} would use.
The event-loop will eventually be made re-entrant. This is so that
@value{GDBN} can better handle the problem of some commands blocking
instead of returning.
@subheading Library - @file{gdb.h}
@file{libgdb} is the most obvious component of this system. It provides
the query interface. Each function is parameterized by a @code{ui-out}
builder. The result of the query is constructed using that builder
before the query function returns.
@node Values
@chapter Values
@section Values
@cindex values
@cindex @code{value} structure
@value{GDBN} uses @code{struct value}, or @dfn{values}, as an internal
abstraction for the representation of a variety of inferior objects
and @value{GDBN} convenience objects.
Values have an associated @code{struct type}, that describes a virtual
view of the raw data or object stored in or accessed through the
A value is in addition discriminated by its lvalue-ness, given its
@code{enum lval_type} enumeration type:
@cindex @code{lval_type} enumeration, for values.
@table @code
@item @code{not_lval}
This value is not an lval. It can't be assigned to.
@item @code{lval_memory}
This value represents an object in memory.
@item @code{lval_register}
This value represents an object that lives in a register.
@item @code{lval_internalvar}
Represents the value of an internal variable.
@item @code{lval_internalvar_component}
Represents part of a @value{GDBN} internal variable. E.g., a
structure field.
@cindex computed values
@item @code{lval_computed}
These are ``computed'' values. They allow creating specialized value
objects for specific purposes, all abstracted away from the core value
support code. The creator of such a value writes specialized
functions to handle the reading and writing to/from the value's
backend data, and optionally, a ``copy operator'' and a
Pointers to these functions are stored in a @code{struct lval_funcs}
instance (declared in @file{value.h}), and passed to the
@code{allocate_computed_value} function, as in the example below.
static void
nil_value_read (struct value *v)
/* This callback reads data from some backend, and stores it in V.
In this case, we always read null data. You'll want to fill in
something more interesting. */
memset (value_contents_all_raw (v),
value_offset (v),
TYPE_LENGTH (value_type (v)));
static void
nil_value_write (struct value *v, struct value *fromval)
/* Takes the data from FROMVAL and stores it in the backend of V. */
to_oblivion (value_contents_all_raw (fromval),
value_offset (v),
TYPE_LENGTH (value_type (fromval)));
static struct lval_funcs nil_value_funcs =
struct value *
make_nil_value (void)
struct type *type;
struct value *v;
type = make_nils_type ();
v = allocate_computed_value (type, &nil_value_funcs, NULL);
return v;
@end smallexample
See the implementation of the @code{$_siginfo} convenience variable in
@file{infrun.c} as a real example use of lval_computed.
@end table
@node Stack Frames
@chapter Stack Frames
@cindex frame
@cindex call stack frame
A frame is a construct that @value{GDBN} uses to keep track of calling
and called functions.
@cindex unwind frame
@value{GDBN}'s frame model, a fresh design, was implemented with the
need to support @sc{dwarf}'s Call Frame Information in mind. In fact,
the term ``unwind'' is taken directly from that specification.
Developers wishing to learn more about unwinders, are encouraged to
read the @sc{dwarf} specification, available from
@findex frame_register_unwind
@findex get_frame_register
@value{GDBN}'s model is that you find a frame's registers by
``unwinding'' them from the next younger frame. That is,
@samp{get_frame_register} which returns the value of a register in
frame #1 (the next-to-youngest frame), is implemented by calling frame
#0's @code{frame_register_unwind} (the youngest frame). But then the
obvious question is: how do you access the registers of the youngest
frame itself?
@cindex sentinel frame
@findex get_frame_type
To answer this question, @value{GDBN} has the @dfn{sentinel} frame, the
``-1st'' frame. Unwinding registers from the sentinel frame gives you
the current values of the youngest real frame's registers. If @var{f}
is a sentinel frame, then @code{get_frame_type (@var{f}) @equiv{}
@section Selecting an Unwinder
@findex frame_unwind_prepend_unwinder
@findex frame_unwind_append_unwinder
The architecture registers a list of frame unwinders (@code{struct
frame_unwind}), using the functions
@code{frame_unwind_prepend_unwinder} and
@code{frame_unwind_append_unwinder}. Each unwinder includes a
sniffer. Whenever @value{GDBN} needs to unwind a frame (to fetch the
previous frame's registers or the current frame's ID), it calls
registered sniffers in order to find one which recognizes the frame.
The first time a sniffer returns non-zero, the corresponding unwinder
is assigned to the frame.
@section Unwinding the Frame ID
@cindex frame ID
Every frame has an associated ID, of type @code{struct frame_id}.
The ID includes the stack base and function start address for
the frame. The ID persists through the entire life of the frame,
including while other called frames are running; it is used to
locate an appropriate @code{struct frame_info} from the cache.
Every time the inferior stops, and at various other times, the frame
cache is flushed. Because of this, parts of @value{GDBN} which need
to keep track of individual frames cannot use pointers to @code{struct
frame_info}. A frame ID provides a stable reference to a frame, even
when the unwinder must be run again to generate a new @code{struct
frame_info} for the same frame.
The frame's unwinder's @code{this_id} method is called to find the ID.
Note that this is different from register unwinding, where the next
frame's @code{prev_register} is called to unwind this frame's
Both stack base and function address are required to identify the
frame, because a recursive function has the same function address for
two consecutive frames and a leaf function may have the same stack
address as its caller. On some platforms, a third address is part of
the ID to further disambiguate frames---for instance, on IA-64
the separate register stack address is included in the ID.
An invalid frame ID (@code{outer_frame_id}) returned from the
@code{this_id} method means to stop unwinding after this frame.
@code{null_frame_id} is another invalid frame ID which should be used
when there is no frame. For instance, certain breakpoints are attached
to a specific frame, and that frame is identified through its frame ID
(we use this to implement the "finish" command). Using
@code{null_frame_id} as the frame ID for a given breakpoint means
that the breakpoint is not specific to any frame. The @code{this_id}
method should never return @code{null_frame_id}.
@section Unwinding Registers
Each unwinder includes a @code{prev_register} method. This method
takes a frame, an associated cache pointer, and a register number.
It returns a @code{struct value *} describing the requested register,
as saved by this frame. This is the value of the register that is
current in this frame's caller.
The returned value must have the same type as the register. It may
have any lvalue type. In most circumstances one of these routines
will generate the appropriate value:
@table @code
@item frame_unwind_got_optimized
@findex frame_unwind_got_optimized
This register was not saved.
@item frame_unwind_got_register
@findex frame_unwind_got_register
This register was copied into another register in this frame. This
is also used for unchanged registers; they are ``copied'' into the
same register.
@item frame_unwind_got_memory
@findex frame_unwind_got_memory
This register was saved in memory.
@item frame_unwind_got_constant
@findex frame_unwind_got_constant
This register was not saved, but the unwinder can compute the previous
value some other way.
@item frame_unwind_got_address
@findex frame_unwind_got_address
Same as @code{frame_unwind_got_constant}, except that the value is a target
address. This is frequently used for the stack pointer, which is not
explicitly saved but has a known offset from this frame's stack
pointer. For architectures with a flat unified address space, this is
generally the same as @code{frame_unwind_got_constant}.
@end table
@node Symbol Handling
@chapter Symbol Handling
Symbols are a key part of @value{GDBN}'s operation. Symbols include
variables, functions, and types.
Symbol information for a large program can be truly massive, and
reading of symbol information is one of the major performance
bottlenecks in @value{GDBN}; it can take many minutes to process it
all. Studies have shown that nearly all the time spent is
computational, rather than file reading.
One of the ways for @value{GDBN} to provide a good user experience is
to start up quickly, taking no more than a few seconds. It is simply
not possible to process all of a program's debugging info in that
time, and so we attempt to handle symbols incrementally. For instance,
we create @dfn{partial symbol tables} consisting of only selected
symbols, and only expand them to full symbol tables when necessary.
@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. In
addition, it may bring in more symbols while loading shared
@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
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
Full symbol tables (@dfn{symtabs}). These contain the main
information about symbols and addresses.
@cindex psymtabs
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
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
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
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
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
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.
Whether or not psymtabs are created depends on the objfile's symbol
reader. The core of @value{GDBN} hides the details of partial symbols
and partial symbol tables behind a set of function pointers known as
the @dfn{quick symbol functions}. These are documented in
@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,
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. GNU's COFF-using configs often use stabs-type info,
encapsulated in special sections.
The COFF reader is in @file{coffread.c}.
@subsection ECOFF
@cindex ECOFF format
ECOFF is an extended COFF originally introduced for Mips and Alpha
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. Debugging info may be either stabs
encapsulated in ELF sections, or more commonly these days, DWARF.
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{somread.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.
@c mention DWARF 1 as a formerly-supported format
@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 Compressed DWARF 2
@cindex Compressed DWARF 2 debugging info
Compressed DWARF 2 is not technically a separate debugging format, but
merely DWARF 2 debug information that has been compressed. In this
format, every object-file section holding DWARF 2 debugging
information is compressed and prepended with a header. (The section
is also typically renamed, so a section called @code{.debug_info} in a
DWARF 2 binary would be called @code{.zdebug_info} in a compressed
DWARF 2 binary.) The header is 12 bytes long:
@itemize @bullet
4 bytes: the literal string ``ZLIB''
8 bytes: the uncompressed size of the section, in big-endian byte
@end itemize
The same reader is used for both compressed an normal DWARF 2 info.
Section decompression is done in @code{zlib_decompress_section} in
@subsection DWARF 3
@cindex DWARF 3 debugging info
DWARF 3 is an improved version of DWARF 2.
@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}.
@section Memory Management for Symbol Files
Most memory associated with a loaded symbol file is stored on
its @code{objfile_obstack}. This includes symbols, types,
namespace data, and other information produced by the symbol readers.
Because this data lives on the objfile's obstack, it is automatically
released when the objfile is unloaded or reloaded. Therefore one
objfile must not reference symbol or type data from another objfile;
they could be unloaded at different times.
User convenience variables, et cetera, have associated types. Normally
these types live in the associated objfile. However, when the objfile
is unloaded, those types are deep copied to global memory, so that
the values of the user variables and history items are not lost.
@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
@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
@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:
#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 smallexample
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
@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
@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 Edit @file{}
Add dependencies in @file{}. 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
@end table
@node Host Definition
@chapter Host Definition
With the advent of Autoconf, it's rarely necessary to have host
definition machinery anymore. The following information is provided,
mainly, as an historical reference.
@section Adding a New Host
@cindex adding a new host
@cindex host, adding
@value{GDBN}'s host configuration support normally happens via Autoconf.
New host-specific definitions should not be needed. Older hosts
@value{GDBN} still use the host-specific definitions and files listed
below, but these mostly exist for historical reasons, and will
eventually disappear.
@table @file
@item gdb/config/@var{arch}/@var{xyz}.mh
This file is a Makefile fragment that once contained both host and
native configuration information (@pxref{Native Debugging}) for the
machine @var{xyz}. The host configuration information is now handled
by Autoconf.
Host configuration information included definitions for @code{CC},
@code{SYSV_DEFINE}, @code{XM_CFLAGS}, @code{XM_ADD_FILES},
@code{XM_CLIBS}, @code{XM_CDEPS}, etc.; see @file{}.
New host-only configurations do not need this file.
@end table
(Files named @file{gdb/config/@var{arch}/xm-@var{xyz}.h} were once
used to define host-specific macros, but were no longer needed and
have all been removed.)
@subheading Generic Host Support Files
@cindex generic host support
There are some ``generic'' versions of routines that can be used by
various systems.
@table @file
@cindex remote debugging support
@cindex serial line support
@item ser-unix.c
This contains serial line support for Unix systems. It is included by
default on all Unix-like hosts.
@item ser-pipe.c
This contains serial pipe support for Unix systems. It is included by
default on all Unix-like hosts.
@item ser-mingw.c
This contains serial line support for 32-bit programs running under
Windows using MinGW.
@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. It is included by
default on all Unix-like hosts and with MinGW.
@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. While formerly they could be set in
host-specific header files, at present they can be changed only by
setting @code{CFLAGS} when building, or by editing the source code.
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)
@ftable @code
The default name of @value{GDBN}'s initialization file (normally
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.
Define this to expand into code that will define the function named by
the expansion of @code{SIGWINCH_HANDLER}.
@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"}.
@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 ISATTY
Substitute for isatty, if not available.
@item FOPEN_RB
Define this if binary files are opened the same way as text files.
@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.
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.
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 lint
Define this to help placate @code{lint} in some situations.
@item volatile
Define this to override the defaults of @code{__volatile__} or
@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.
The target architecture object is implemented as the C structure
@code{struct gdbarch *}. The structure, and its methods, are generated
using the Bourne shell script @file{}.
* OS ABI Variant Handling::
* Initialize New Architecture::
* Registers and Memory::
* Pointers and Addresses::
* Address Classes::
* Register Representation::
* Frame Interpretation::
* Inferior Call Setup::
* Adding support for debugging core files::
* Defining Other Architecture Features::
* Adding a New Target::
@end menu
@node OS ABI Variant Handling
@section Operating System ABI Variant Handling
@cindex OS ABI variants
@value{GDBN} provides a mechanism for handling variations in OS
ABIs. An OS ABI variant may have influence over any number of
variables in the target architecture definition. There are two major
components in the OS ABI mechanism: sniffers and handlers.
A @dfn{sniffer} examines a file matching a BFD architecture/flavour pair
(the architecture may be wildcarded) in an attempt to determine the
OS ABI of that file. Sniffers with a wildcarded architecture are considered
to be @dfn{generic}, while sniffers for a specific architecture are
considered to be @dfn{specific}. A match from a specific sniffer
overrides a match from a generic sniffer. Multiple sniffers for an
architecture/flavour may exist, in order to differentiate between two
different operating systems which use the same basic file format. The
OS ABI framework provides a generic sniffer for ELF-format files which
examines the @code{EI_OSABI} field of the ELF header, as well as note
sections known to be used by several operating systems.
@cindex fine-tuning @code{gdbarch} structure
A @dfn{handler} is used to fine-tune the @code{gdbarch} structure for the
selected OS ABI. There may be only one handler for a given OS ABI
for each BFD architecture.
The following OS ABI variants are defined in @file{defs.h}:
@table @code
Used for struct gdbarch_info if ABI is still uninitialized.
The ABI of the inferior is unknown. The default @code{gdbarch}
settings for the architecture will be used.
@findex GDB_OSABI_SVR4
UNIX System V Release 4.
GNU using the Hurd kernel.
Sun Solaris.
@findex GDB_OSABI_OSF1
OSF/1, including Digital UNIX and Compaq Tru64 UNIX.
GNU using the Linux kernel.
FreeBSD using the @code{a.out} executable format.
FreeBSD using the ELF executable format.
NetBSD using the @code{a.out} executable format.
NetBSD using the ELF executable format.
OpenBSD using the ELF executable format.
Windows CE.
@findex GDB_OSABI_GO32
@item GDB_OSABI_GO32
Interix (Posix layer for MS-Windows systems).
HP/UX using the ELF executable format.
HP/UX using the SOM executable format.
QNX Neutrino.
@end table
Here are the functions that make up the OS ABI framework:
@deftypefun {const char *} gdbarch_osabi_name (enum gdb_osabi @var{osabi})
Return the name of the OS ABI corresponding to @var{osabi}.
@end deftypefun
@deftypefun void gdbarch_register_osabi (enum bfd_architecture @var{arch}, unsigned long @var{machine}, enum gdb_osabi @var{osabi}, void (*@var{init_osabi})(struct gdbarch_info @var{info}, struct gdbarch *@var{gdbarch}))
Register the OS ABI handler specified by @var{init_osabi} for the
architecture, machine type and OS ABI specified by @var{arch},
@var{machine} and @var{osabi}. In most cases, a value of zero for the
machine type, which implies the architecture's default machine type,
will suffice.
@end deftypefun
@deftypefun void gdbarch_register_osabi_sniffer (enum bfd_architecture @var{arch}, enum bfd_flavour @var{flavour}, enum gdb_osabi (*@var{sniffer})(bfd *@var{abfd}))
Register the OS ABI file sniffer specified by @var{sniffer} for the
BFD architecture/flavour pair specified by @var{arch} and @var{flavour}.
If @var{arch} is @code{bfd_arch_unknown}, the sniffer is considered to
be generic, and is allowed to examine @var{flavour}-flavoured files for
any architecture.
@end deftypefun
@deftypefun {enum gdb_osabi} gdbarch_lookup_osabi (bfd *@var{abfd})
Examine the file described by @var{abfd} to determine its OS ABI.
The value @code{GDB_OSABI_UNKNOWN} is returned if the OS ABI cannot
be determined.
@end deftypefun
@deftypefun void gdbarch_init_osabi (struct gdbarch info @var{info}, struct gdbarch *@var{gdbarch}, enum gdb_osabi @var{osabi})
Invoke the OS ABI handler corresponding to @var{osabi} to fine-tune the
@code{gdbarch} structure specified by @var{gdbarch}. If a handler
corresponding to @var{osabi} has not been registered for @var{gdbarch}'s
architecture, a warning will be issued and the debugging session will continue
with the defaults already established for @var{gdbarch}.
@end deftypefun
@deftypefun void generic_elf_osabi_sniff_abi_tag_sections (bfd *@var{abfd}, asection *@var{sect}, void *@var{obj})
Helper routine for ELF file sniffers. Examine the file described by
@var{abfd} and look at ABI tag note sections to determine the OS ABI
from the note. This function should be called via
@end deftypefun
@node Initialize New Architecture
@section Initializing a New Architecture
* How an Architecture is Represented::
* Looking Up an Existing Architecture::
* Creating a New Architecture::
@end menu
@node How an Architecture is Represented
@subsection How an Architecture is Represented
@cindex architecture representation
@cindex representation of architecture
Each @code{gdbarch} is associated with a single @sc{bfd} architecture,
via a @code{bfd_arch_@var{arch}} in the @code{bfd_architecture}
enumeration. The @code{gdbarch} is registered by a call to
@code{register_gdbarch_init}, usually from the file's
@code{_initialize_@var{filename}} routine, which will be automatically
called during @value{GDBN} startup. The arguments are a @sc{bfd}
architecture constant and an initialization function.
@findex _initialize_@var{arch}_tdep
@cindex @file{@var{arch}-tdep.c}
A @value{GDBN} description for a new architecture, @var{arch} is created by
defining a global function @code{_initialize_@var{arch}_tdep}, by
convention in the source file @file{@var{arch}-tdep.c}. For example,
in the case of the OpenRISC 1000, this function is called
@code{_initialize_or1k_tdep} and is found in the file
@cindex @file{configure.tgt}
@cindex @code{gdbarch}
@findex gdbarch_register
The resulting object files containing the implementation of the
@code{_initialize_@var{arch}_tdep} function are specified in the @value{GDBN}
@file{configure.tgt} file, which includes a large case statement
pattern matching against the @code{--target} option of the
@code{configure} script. The new @code{struct gdbarch} is created
within the @code{_initialize_@var{arch}_tdep} function by calling
void gdbarch_register (enum bfd_architecture @var{architecture},
gdbarch_init_ftype *@var{init_func},
gdbarch_dump_tdep_ftype *@var{tdep_dump_func});
@end smallexample
The @var{architecture} will identify the unique @sc{bfd} to be
associated with this @code{gdbarch}. The @var{init_func} funciton is
called to create and return the new @code{struct gdbarch}. The
@var{tdep_dump_func} function will dump the target specific details
associated with this architecture.
For example the function @code{_initialize_or1k_tdep} creates its
architecture for 32-bit OpenRISC 1000 architectures by calling:
gdbarch_register (bfd_arch_or32, or1k_gdbarch_init, or1k_dump_tdep);
@end smallexample
@node Looking Up an Existing Architecture
@subsection Looking Up an Existing Architecture
@cindex @code{gdbarch} lookup
The initialization function has this prototype:
static struct gdbarch *
@var{arch}_gdbarch_init (struct gdbarch_info @var{info},
struct gdbarch_list *@var{arches})
@end smallexample
The @var{info} argument contains parameters used to select the correct
architecture, and @var{arches} is a list of architectures which
have already been created with the same @code{bfd_arch_@var{arch}}
The initialization function should first make sure that @var{info}
is acceptable, and return @code{NULL} if it is not. Then, it should
search through @var{arches} for an exact match to @var{info}, and
return one if found. Lastly, if no exact match was found, it should
create a new architecture based on @var{info} and return it.
@findex gdbarch_list_lookup_by_info
@cindex @code{gdbarch_info}
The lookup is done using @code{gdbarch_list_lookup_by_info}. It is
passed the list of existing architectures, @var{arches}, and the
@code{struct gdbarch_info}, @var{info}, and returns the first matching
architecture it finds, or @code{NULL} if none are found. If an
architecture is found it can be returned as the result from the
initialization function, otherwise a new @code{struct gdbach} will need
to be created.
The struct gdbarch_info has the following components:
struct gdbarch_info
const struct bfd_arch_info *bfd_arch_info;
int byte_order;
bfd *abfd;
struct gdbarch_tdep_info *tdep_info;
enum gdb_osabi osabi;
const struct target_desc *target_desc;
@end smallexample
@vindex bfd_arch_info
The @code{bfd_arch_info} member holds the key details about the
architecture. The @code{byte_order} member is a value in an
enumeration indicating the endianism. The @code{abfd} member is a
pointer to the full @sc{bfd}, the @code{tdep_info} member is
additional custom target specific information, @code{osabi} identifies
which (if any) of a number of operating specific ABIs are used by this
architecture and the @code{target_desc} member is a set of name-value
pairs with information about register usage in this target.
When the @code{struct gdbarch} initialization function is called, not
all the fields are provided---only those which can be deduced from the
@sc{bfd}. The @code{struct gdbarch_info}, @var{info} is used as a
look-up key with the list of existing architectures, @var{arches} to
see if a suitable architecture already exists. The @var{tdep_info},
@var{osabi} and @var{target_desc} fields may be added before this
lookup to refine the search.
Only information in @var{info} should be used to choose the new
architecture. Historically, @var{info} could be sparse, and
defaults would be collected from the first element on @var{arches}.
However, @value{GDBN} now fills in @var{info} more thoroughly,
so new @code{gdbarch} initialization functions should not take
defaults from @var{arches}.
@node Creating a New Architecture
@subsection Creating a New Architecture
@cindex @code{struct gdbarch} creation
@findex gdbarch_alloc
@cindex @code{gdbarch_tdep} when allocating new @code{gdbarch}
If no architecture is found, then a new architecture must be created,
by calling @code{gdbarch_alloc} using the supplied @code{@w{struct
gdbarch_info}} and any additional custom target specific
information in a @code{struct gdbarch_tdep}. The prototype for
@code{gdbarch_alloc} is:
struct gdbarch *gdbarch_alloc (const struct gdbarch_info *@var{info},
struct gdbarch_tdep *@var{tdep});
@end smallexample
@cindex @code{set_gdbarch} functions
@cindex @code{gdbarch} accessor functions
The newly created struct gdbarch must then be populated. Although
there are default values, in most cases they are not what is
For each element, @var{X}, there is are a pair of corresponding accessor
functions, one to set the value of that element,
@code{set_gdbarch_@var{X}}, the second to either get the value of an
element (if it is a variable) or to apply the element (if it is a
function), @code{gdbarch_@var{X}}. Note that both accessor functions
take a pointer to the @code{@w{struct gdbarch}} as first
argument. Populating the new @code{gdbarch} should use the
@code{set_gdbarch} functions.
The following sections identify the main elements that should be set
in this way. This is not the complete list, but represents the
functions and elements that must commonly be specified for a new
architecture. Many of the functions and variables are described in the
header file @file{gdbarch.h}.
This is the main work in defining a new architecture. Implementing the
set of functions to populate the @code{struct gdbarch}.
@cindex @code{gdbarch_tdep} definition
@code{struct gdbarch_tdep} is not defined within @value{GDBN}---it is up
to the user to define this struct if it is needed to hold custom target
information that is not covered by the standard @code{@w{struct
gdbarch}}. For example with the OpenRISC 1000 architecture it is used to
hold the number of matchpoints available in the target (along with other
If there is no additional target specific information, it can be set to
@node Registers and Memory
@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{gdbarch_register_name} and related functions.
@value{GDBN} can handle big-endian, little-endian, and bi-endian architectures.
@node Pointers and Addresses
@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.
@c D10V is gone from sources - more current example?
For example, the Renesas 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
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
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_address (struct value *@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
Here are two functions 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 unsigned byte addresses.
@deftypefun CORE_ADDR gdbarch_pointer_to_address (struct gdbarch *@var{gdbarch}, 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 deftypefun
@deftypefun void gdbarch_address_to_pointer (struct gdbarch *@var{gdbarch}, 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 deftypefun
@node Address Classes
@section Address Classes
@cindex address classes
@cindex DW_AT_byte_size
@cindex DW_AT_address_class
Sometimes information about different kinds of addresses is available
via the debug information. For example, some programming environments
define addresses of several different sizes. If the debug information
distinguishes these kinds of address classes through either the size
info (e.g, @code{DW_AT_byte_size} in @w{DWARF 2}) or through an explicit
address class attribute (e.g, @code{DW_AT_address_class} in @w{DWARF 2}), the
following macros should be defined in order to disambiguate these
types within @value{GDBN} as well as provide the added information to
a @value{GDBN} user when printing type expressions.
@deftypefun int gdbarch_address_class_type_flags (struct gdbarch *@var{gdbarch}, int @var{byte_size}, int @var{dwarf2_addr_class})
Returns the type flags needed to construct a pointer type whose size
is @var{byte_size} and whose address class is @var{dwarf2_addr_class}.
This function is normally called from within a symbol reader. See
@end deftypefun
@deftypefun {char *} gdbarch_address_class_type_flags_to_name (struct gdbarch *@var{gdbarch}, int @var{type_flags})
Given the type flags representing an address class qualifier, return
its name.
@end deftypefun
@deftypefun int gdbarch_address_class_name_to_type_flags (struct gdbarch *@var{gdbarch}, int @var{name}, int *@var{type_flags_ptr})
Given an address qualifier name, set the @code{int} referenced by @var{type_flags_ptr} to the type flags
for that address class qualifier.
@end deftypefun
Since the need for address classes is rather rare, none of
the address class functions are defined by default. Predicate
functions are provided to detect when they are defined.
Consider a hypothetical architecture in which addresses are normally
32-bits wide, but 16-bit addresses are also supported. Furthermore,
suppose that the @w{DWARF 2} information for this architecture simply
uses a @code{DW_AT_byte_size} value of 2 to indicate the use of one
of these "short" pointers. The following functions could be defined
to implement the address class functions:
somearch_address_class_type_flags (int byte_size,
int dwarf2_addr_class)
if (byte_size == 2)
return 0;
static char *
somearch_address_class_type_flags_to_name (int type_flags)
if (type_flags & TYPE_FLAG_ADDRESS_CLASS_1)
return "short";
return NULL;
somearch_address_class_name_to_type_flags (char *name,
int *type_flags_ptr)
if (strcmp (name, "short") == 0)
*type_flags_ptr = TYPE_FLAG_ADDRESS_CLASS_1;
return 1;
return 0;
@end smallexample
The qualifier @code{@@short} is used in @value{GDBN}'s type expressions
to indicate the presence of one of these ``short'' pointers. For
example if the debug information indicates that @code{short_ptr_var} is
one of these short pointers, @value{GDBN} might show the following
(gdb) ptype short_ptr_var
type = int * @@short
@end smallexample
@node Register Representation
@section Register Representation
* Raw and Cooked Registers::
* Register Architecture Functions & Variables::
* Register Information Functions::
* Register and Memory Data::
* Register Caching::
@end menu
@node Raw and Cooked Registers
@subsection Raw and Cooked Registers
@cindex raw register representation
@cindex cooked register representation
@cindex representations, raw and cooked registers
@value{GDBN} considers registers to be a set with members numbered
linearly from 0 upwards. The first part of that set corresponds to real
physical registers, the second part to any @dfn{pseudo-registers}.
Pseudo-registers have no independent physical existence, but are useful
representations of information within the architecture. For example the
OpenRISC 1000 architecture has up to 32 general purpose registers, which
are typically represented as 32-bit (or 64-bit) integers. However the
GPRs are also used as operands to the floating point operations, and it
could be convenient to define a set of pseudo-registers, to show the
GPRs represented as floating point values.
For any architecture, the implementer will decide on a mapping from
hardware to @value{GDBN} register numbers. The registers corresponding to real
hardware are referred to as @dfn{raw} registers, the remaining registers are
@dfn{pseudo-registers}. The total register set (raw and pseudo) is called
the @dfn{cooked} register set.
@node Register Architecture Functions & Variables
@subsection Functions and Variables Specifying the Register Architecture
@cindex @code{gdbarch} register architecture functions
These @code{struct gdbarch} functions and variables specify the number
and type of registers in the architecture.
@deftypefn {Architecture Function} CORE_ADDR read_pc (struct regcache *@var{regcache})
@end deftypefn
@deftypefn {Architecture Function} void write_pc (struct regcache *@var{regcache}, CORE_ADDR @var{val})
Read or write the program counter. The default value of both
functions is @code{NULL} (no function available). If the program
counter is just an ordinary register, it can be specified in
@code{struct gdbarch} instead (see @code{pc_regnum} below) and it will
be read or written using the standard routines to access registers. This
function need only be specified if the program counter is not an
ordinary register.
Any register information can be obtained using the supplied register
cache, @var{regcache}. @xref{Register Caching, , Register Caching}.
@end deftypefn
@deftypefn {Architecture Function} void pseudo_register_read (struct gdbarch *@var{gdbarch}, struct regcache *@var{regcache}, int @var{regnum}, const gdb_byte *@var{buf})
@end deftypefn
@deftypefn {Architecture Function} void pseudo_register_write (struct gdbarch *@var{gdbarch}, struct regcache *@var{regcache}, int @var{regnum}, const gdb_byte *@var{buf})
These functions should be defined if there are any pseudo-registers.
The default value is @code{NULL}. @var{regnum} is the number of the
register to read or write (which will be a @dfn{cooked} register
number) and @var{buf} is the buffer where the value read will be
placed, or from which the value to be written will be taken. The
value in the buffer may be converted to or from a signed or unsigned
integral value using one of the utility functions (@pxref{Register and
Memory Data, , Using Different Register and Memory Data
The access should be for the specified architecture,
@var{gdbarch}. Any register information can be obtained using the
supplied register cache, @var{regcache}. @xref{Register Caching, ,
Register Caching}.
@end deftypefn
@deftypevr {Architecture Variable} int sp_regnum
@vindex sp_regnum
@cindex stack pointer
@cindex @kbd{$sp}
This specifies the register holding the stack pointer, which may be a
raw or pseudo-register. It defaults to -1 (not defined), but it is an
error for it not to be defined.
The value of the stack pointer register can be accessed withing
@value{GDBN} as the variable @kbd{$sp}.
@end deftypevr
@deftypevr {Architecture Variable} int pc_regnum
@vindex pc_regnum
@cindex program counter
@cindex @kbd{$pc}
This specifies the register holding the program counter, which may be a
raw or pseudo-register. It defaults to -1 (not defined). If
@code{pc_regnum} is not defined, then the functions @code{read_pc} and
@code{write_pc} (see above) must be defined.
The value of the program counter (whether defined as a register, or
through @code{read_pc} and @code{write_pc}) can be accessed withing
@value{GDBN} as the variable @kbd{$pc}.
@end deftypevr
@deftypevr {Architecture Variable} int ps_regnum
@vindex ps_regnum
@cindex processor status register
@cindex status register
@cindex @kbd{$ps}
This specifies the register holding the processor status (often called
the status register), which may be a raw or pseudo-register. It
defaults to -1 (not defined).
If defined, the value of this register can be accessed withing
@value{GDBN} as the variable @kbd{$ps}.
@end deftypevr
@deftypevr {Architecture Variable} int fp0_regnum
@vindex fp0_regnum
@cindex first floating point register
This specifies the first floating point register. It defaults to
0. @code{fp0_regnum} is not needed unless the target offers support
for floating point.
@end deftypevr
@node Register Information Functions
@subsection Functions Giving Register Information
@cindex @code{gdbarch} register information functions
These functions return information about registers.
@deftypefn {Architecture Function} {const char *} register_name (struct gdbarch *@var{gdbarch}, int @var{regnum})
This function should convert a register number (raw or pseudo) to a
register name (as a C @code{const char *}). This is used both to
determine the name of a register for output and to work out the meaning
of any register names used as input. The function may also return
@code{NULL}, to indicate that @var{regnum} is not a valid register.
For example with the OpenRISC 1000, @value{GDBN} registers 0-31 are the
General Purpose Registers, register 32 is the program counter and
register 33 is the supervision register (i.e.@: the processor status
register), which map to the strings @code{"gpr00"} through
@code{"gpr31"}, @code{"pc"} and @code{"sr"} respectively. This means
that the @value{GDBN} command @kbd{print $gpr5} should print the value of
the OR1K general purpose register 5@footnote{
@cindex frame pointer
@cindex @kbd{$fp}
Historically, @value{GDBN} always had a concept of a frame pointer
register, which could be accessed via the @value{GDBN} variable,
@kbd{$fp}. That concept is now deprecated, recognizing that not all
architectures have a frame pointer. However if an architecture does
have a frame pointer register, and defines a register or
pseudo-register with the name @code{"fp"}, then that register will be
used as the value of the @kbd{$fp} variable.}.
The default value for this function is @code{NULL}, meaning
undefined. It should always be defined.
The access should be for the specified architecture, @var{gdbarch}.
@end deftypefn
@deftypefn {Architecture Function} {struct type *} register_type (struct gdbarch *@var{gdbarch}, int @var{regnum})
Given a register number, this function identifies the type of data it
may be holding, specified as a @code{struct type}. @value{GDBN} allows
creation of arbitrary types, but a number of built in types are
provided (@code{builtin_type_void}, @code{builtin_type_int32} etc),
together with functions to derive types from these.
Typically the program counter will have a type of ``pointer to
function'' (it points to code), the frame pointer and stack pointer
will have types of ``pointer to void'' (they point to data on the stack)
and all other integer registers will have a type of 32-bit integer or
64-bit integer.
This information guides the formatting when displaying register
information. The default value is @code{NULL} meaning no information is
available to guide formatting when displaying registers.
@end deftypefn
@deftypefn {Architecture Function} void print_registers_info (struct gdbarch *@var{gdbarch}, struct ui_file *@var{file}, struct frame_info *@var{frame}, int @var{regnum}, int @var{all})
Define this function to print out one or all of the registers for the
@value{GDBN} @kbd{info registers} command. The default value is the
function @code{default_print_registers_info}, which uses the register
type information (see @code{register_type} above) to determine how each
register should be printed. Define a custom version of this function
for fuller control over how the registers are displayed.
The access should be for the specified architecture, @var{gdbarch},
with output to the file specified by the User Interface
Independent Output file handle, @var{file} (@pxref{UI-Independent
Output, , UI-Independent Output---the @code{ui_out}
The registers should show their values in the frame specified by
@var{frame}. If @var{regnum} is -1 and @var{all} is zero, then all
the ``significant'' registers should be shown (the implementer should
decide which registers are ``significant''). Otherwise only the value of
the register specified by @var{regnum} should be output. If
@var{regnum} is -1 and @var{all} is non-zero (true), then the value of
all registers should be shown.
By default @code{default_print_registers_info} prints one register per
line, and if @var{all} is zero omits floating-point registers.
@end deftypefn
@deftypefn {Architecture Function} void print_float_info (struct gdbarch *@var{gdbarch}, struct ui_file *@var{file}, struct frame_info *@var{frame}, const char *@var{args})
Define this function to provide output about the floating point unit and
registers for the @value{GDBN} @kbd{info float} command respectively.
The default value is @code{NULL} (not defined), meaning no information
will be provided.
The @var{gdbarch} and @var{file} and @var{frame} arguments have the same
meaning as in the @code{print_registers_info} function above. The string
@var{args} contains any supplementary arguments to the @kbd{info float}
Define this function if the target supports floating point operations.
@end deftypefn
@deftypefn {Architecture Function} void print_vector_info (struct gdbarch *@var{gdbarch}, struct ui_file *@var{file}, struct frame_info *@var{frame}, const char *@var{args})
Define this function to provide output about the vector unit and
registers for the @value{GDBN} @kbd{info vector} command respectively.
The default value is @code{NULL} (not defined), meaning no information
will be provided.
The @var{gdbarch}, @var{file} and @var{frame} arguments have the
same meaning as in the @code{prin