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Copyright (C) 1988-2019 Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
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Invariant Sections being "Free Software" and "Free Software Needs Free
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   (a) The FSF's Back-Cover Text is: "You are free to copy and modify
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INFO-DIR-SECTION Software development
START-INFO-DIR-ENTRY
* Gdb: (gdb).                     The GNU debugger.
* gdbserver: (gdb) Server.        The GNU debugging server.
END-INFO-DIR-ENTRY

   This file documents the GNU debugger GDB.

   This is the Tenth Edition, of 'Debugging with GDB: the GNU
Source-Level Debugger' for GDB (GDB) Version 9.0.90.

   Copyright (C) 1988-2019 Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Free Software" and "Free Software Needs Free
Documentation", with the Front-Cover Texts being "A GNU Manual," and
with the Back-Cover Texts as in (a) below.

   (a) The FSF's Back-Cover Text is: "You are free to copy and modify
this GNU Manual.  Buying copies from GNU Press supports the FSF in
developing GNU and promoting software freedom."


File: gdb.info,  Node: Screen Size,  Next: Output Styling,  Prev: Command History,  Up: Controlling GDB

22.4 Screen Size
================

Certain commands to GDB may produce large amounts of information output
to the screen.  To help you read all of it, GDB pauses and asks you for
input at the end of each page of output.  Type <RET> when you want to
see one more page of output, 'q' to discard the remaining output, or 'c'
to continue without paging for the rest of the current command.  Also,
the screen width setting determines when to wrap lines of output.
Depending on what is being printed, GDB tries to break the line at a
readable place, rather than simply letting it overflow onto the
following line.

   Normally GDB knows the size of the screen from the terminal driver
software.  For example, on Unix GDB uses the termcap data base together
with the value of the 'TERM' environment variable and the 'stty rows'
and 'stty cols' settings.  If this is not correct, you can override it
with the 'set height' and 'set width' commands:

'set height LPP'
'set height unlimited'
'show height'
'set width CPL'
'set width unlimited'
'show width'
     These 'set' commands specify a screen height of LPP lines and a
     screen width of CPL characters.  The associated 'show' commands
     display the current settings.

     If you specify a height of either 'unlimited' or zero lines, GDB
     does not pause during output no matter how long the output is.
     This is useful if output is to a file or to an editor buffer.

     Likewise, you can specify 'set width unlimited' or 'set width 0' to
     prevent GDB from wrapping its output.

'set pagination on'
'set pagination off'
     Turn the output pagination on or off; the default is on.  Turning
     pagination off is the alternative to 'set height unlimited'.  Note
     that running GDB with the '--batch' option (*note -batch: Mode
     Options.) also automatically disables pagination.

'show pagination'
     Show the current pagination mode.


File: gdb.info,  Node: Output Styling,  Next: Numbers,  Prev: Screen Size,  Up: Controlling GDB

22.5 Output Styling
===================

GDB can style its output on a capable terminal.  This is enabled by
default on most systems, but disabled by default when in batch mode
(*note Mode Options::).  Various style settings are available; and
styles can also be disabled entirely.

'set style enabled 'on|off''
     Enable or disable all styling.  The default is host-dependent, with
     most hosts defaulting to 'on'.

'show style enabled'
     Show the current state of styling.

'set style sources 'on|off''
     Enable or disable source code styling.  This affects whether source
     code, such as the output of the 'list' command, is styled.  Note
     that source styling only works if styling in general is enabled,
     and if GDB was linked with the GNU Source Highlight library.  The
     default is 'on'.

'show style sources'
     Show the current state of source code styling.

   Subcommands of 'set style' control specific forms of styling.  These
subcommands all follow the same pattern: each style-able object can be
styled with a foreground color, a background color, and an intensity.

   For example, the style of file names can be controlled using the 'set
style filename' group of commands:

'set style filename background COLOR'
     Set the background to COLOR.  Valid colors are 'none' (meaning the
     terminal's default color), 'black', 'red', 'green', 'yellow',
     'blue', 'magenta', 'cyan', and'white'.

'set style filename foreground COLOR'
     Set the foreground to COLOR.  Valid colors are 'none' (meaning the
     terminal's default color), 'black', 'red', 'green', 'yellow',
     'blue', 'magenta', 'cyan', and'white'.

'set style filename intensity VALUE'
     Set the intensity to VALUE.  Valid intensities are 'normal' (the
     default), 'bold', and 'dim'.

   The 'show style' command and its subcommands are styling a style name
in their output using its own style.  So, use 'show style' to see the
complete list of styles, their characteristics and the visual aspect of
each style.

   The style-able objects are:
'filename'
     Control the styling of file names.  By default, this style's
     foreground color is green.

'function'
     Control the styling of function names.  These are managed with the
     'set style function' family of commands.  By default, this style's
     foreground color is yellow.

'variable'
     Control the styling of variable names.  These are managed with the
     'set style variable' family of commands.  By default, this style's
     foreground color is cyan.

'address'
     Control the styling of addresses.  These are managed with the 'set
     style address' family of commands.  By default, this style's
     foreground color is blue.

'title'
     Control the styling of titles.  These are managed with the 'set
     style title' family of commands.  By default, this style's
     intensity is bold.  Commands are using the title style to improve
     the readability of large output.  For example, the commands
     'apropos' and 'help' are using the title style for the command
     names.

'highlight'
     Control the styling of highlightings.  These are managed with the
     'set style highlight' family of commands.  By default, this style's
     foreground color is red.  Commands are using the highlight style to
     draw the user attention to some specific parts of their output.
     For example, the command 'apropos -v REGEXP' uses the highlight
     style to mark the documentation parts matching REGEXP.

'tui-border'
     Control the styling of the TUI border.  Note that, unlike other
     styling options, only the color of the border can be controlled via
     'set style'.  This was done for compatibility reasons, as TUI
     controls to set the border's intensity predated the addition of
     general styling to GDB.  *Note TUI Configuration::.

'tui-active-border'
     Control the styling of the active TUI border; that is, the TUI
     window that has the focus.


File: gdb.info,  Node: Numbers,  Next: ABI,  Prev: Output Styling,  Up: Controlling GDB

22.6 Numbers
============

You can always enter numbers in octal, decimal, or hexadecimal in GDB by
the usual conventions: octal numbers begin with '0', decimal numbers end
with '.', and hexadecimal numbers begin with '0x'.  Numbers that neither
begin with '0' or '0x', nor end with a '.' are, by default, entered in
base 10; likewise, the default display for numbers--when no particular
format is specified--is base 10.  You can change the default base for
both input and output with the commands described below.

'set input-radix BASE'
     Set the default base for numeric input.  Supported choices for BASE
     are decimal 8, 10, or 16.  The base must itself be specified either
     unambiguously or using the current input radix; for example, any of

          set input-radix 012
          set input-radix 10.
          set input-radix 0xa

     sets the input base to decimal.  On the other hand, 'set
     input-radix 10' leaves the input radix unchanged, no matter what it
     was, since '10', being without any leading or trailing signs of its
     base, is interpreted in the current radix.  Thus, if the current
     radix is 16, '10' is interpreted in hex, i.e. as 16 decimal, which
     doesn't change the radix.

'set output-radix BASE'
     Set the default base for numeric display.  Supported choices for
     BASE are decimal 8, 10, or 16.  The base must itself be specified
     either unambiguously or using the current input radix.

'show input-radix'
     Display the current default base for numeric input.

'show output-radix'
     Display the current default base for numeric display.

'set radix [BASE]'
'show radix'
     These commands set and show the default base for both input and
     output of numbers.  'set radix' sets the radix of input and output
     to the same base; without an argument, it resets the radix back to
     its default value of 10.


File: gdb.info,  Node: ABI,  Next: Auto-loading,  Prev: Numbers,  Up: Controlling GDB

22.7 Configuring the Current ABI
================================

GDB can determine the "ABI" (Application Binary Interface) of your
application automatically.  However, sometimes you need to override its
conclusions.  Use these commands to manage GDB's view of the current
ABI.

   One GDB configuration can debug binaries for multiple operating
system targets, either via remote debugging or native emulation.  GDB
will autodetect the "OS ABI" (Operating System ABI) in use, but you can
override its conclusion using the 'set osabi' command.  One example
where this is useful is in debugging of binaries which use an alternate
C library (e.g. UCLIBC for GNU/Linux) which does not have the same
identifying marks that the standard C library for your platform
provides.

   When GDB is debugging the AArch64 architecture, it provides a
"Newlib" OS ABI. This is useful for handling 'setjmp' and 'longjmp' when
debugging binaries that use the NEWLIB C library.  The "Newlib" OS ABI
can be selected by 'set osabi Newlib'.

'show osabi'
     Show the OS ABI currently in use.

'set osabi'
     With no argument, show the list of registered available OS ABI's.

'set osabi ABI'
     Set the current OS ABI to ABI.

   Generally, the way that an argument of type 'float' is passed to a
function depends on whether the function is prototyped.  For a
prototyped (i.e. ANSI/ISO style) function, 'float' arguments are passed
unchanged, according to the architecture's convention for 'float'.  For
unprototyped (i.e. K&R style) functions, 'float' arguments are first
promoted to type 'double' and then passed.

   Unfortunately, some forms of debug information do not reliably
indicate whether a function is prototyped.  If GDB calls a function that
is not marked as prototyped, it consults 'set coerce-float-to-double'.

'set coerce-float-to-double'
'set coerce-float-to-double on'
     Arguments of type 'float' will be promoted to 'double' when passed
     to an unprototyped function.  This is the default setting.

'set coerce-float-to-double off'
     Arguments of type 'float' will be passed directly to unprototyped
     functions.

'show coerce-float-to-double'
     Show the current setting of promoting 'float' to 'double'.

   GDB needs to know the ABI used for your program's C++ objects.  The
correct C++ ABI depends on which C++ compiler was used to build your
application.  GDB only fully supports programs with a single C++ ABI; if
your program contains code using multiple C++ ABI's or if GDB can not
identify your program's ABI correctly, you can tell GDB which ABI to
use.  Currently supported ABI's include "gnu-v2", for 'g++' versions
before 3.0, "gnu-v3", for 'g++' versions 3.0 and later, and "hpaCC" for
the HP ANSI C++ compiler.  Other C++ compilers may use the "gnu-v2" or
"gnu-v3" ABI's as well.  The default setting is "auto".

'show cp-abi'
     Show the C++ ABI currently in use.

'set cp-abi'
     With no argument, show the list of supported C++ ABI's.

'set cp-abi ABI'
'set cp-abi auto'
     Set the current C++ ABI to ABI, or return to automatic detection.


File: gdb.info,  Node: Auto-loading,  Next: Messages/Warnings,  Prev: ABI,  Up: Controlling GDB

22.8 Automatically loading associated files
===========================================

GDB sometimes reads files with commands and settings automatically,
without being explicitly told so by the user.  We call this feature
"auto-loading".  While auto-loading is useful for automatically adapting
GDB to the needs of your project, it can sometimes produce unexpected
results or introduce security risks (e.g., if the file comes from
untrusted sources).

* Menu:

* Init File in the Current Directory:: 'set/show/info auto-load local-gdbinit'
* libthread_db.so.1 file::             'set/show/info auto-load libthread-db'

* Auto-loading safe path::             'set/show/info auto-load safe-path'
* Auto-loading verbose mode::          'set/show debug auto-load'

   There are various kinds of files GDB can automatically load.  In
addition to these files, GDB supports auto-loading code written in
various extension languages.  *Note Auto-loading extensions::.

   Note that loading of these associated files (including the local
'.gdbinit' file) requires accordingly configured 'auto-load safe-path'
(*note Auto-loading safe path::).

   For these reasons, GDB includes commands and options to let you
control when to auto-load files and which files should be auto-loaded.

'set auto-load off'
     Globally disable loading of all auto-loaded files.  You may want to
     use this command with the '-iex' option (*note Option
     -init-eval-command::) such as:
          $ gdb -iex "set auto-load off" untrusted-executable corefile

     Be aware that system init file (*note System-wide configuration::)
     and init files from your home directory (*note Home Directory Init
     File::) still get read (as they come from generally trusted
     directories).  To prevent GDB from auto-loading even those init
     files, use the '-nx' option (*note Mode Options::), in addition to
     'set auto-load no'.

'show auto-load'
     Show whether auto-loading of each specific 'auto-load' file(s) is
     enabled or disabled.

          (gdb) show auto-load
          gdb-scripts:  Auto-loading of canned sequences of commands scripts is on.
          libthread-db:  Auto-loading of inferior specific libthread_db is on.
          local-gdbinit:  Auto-loading of .gdbinit script from current directory
                          is on.
          python-scripts:  Auto-loading of Python scripts is on.
          safe-path:  List of directories from which it is safe to auto-load files
                      is $debugdir:$datadir/auto-load.
          scripts-directory:  List of directories from which to load auto-loaded scripts
                              is $debugdir:$datadir/auto-load.

'info auto-load'
     Print whether each specific 'auto-load' file(s) have been
     auto-loaded or not.

          (gdb) info auto-load
          gdb-scripts:
          Loaded  Script
          Yes     /home/user/gdb/gdb-gdb.gdb
          libthread-db:  No auto-loaded libthread-db.
          local-gdbinit:  Local .gdbinit file "/home/user/gdb/.gdbinit" has been
                          loaded.
          python-scripts:
          Loaded  Script
          Yes     /home/user/gdb/gdb-gdb.py

   These are GDB control commands for the auto-loading:

*Note set auto-load off::.           Disable auto-loading globally.
*Note show auto-load::.              Show setting of all kinds of
                                     files.
*Note info auto-load::.              Show state of all kinds of files.
*Note set auto-load gdb-scripts::.   Control for GDB command scripts.
*Note show auto-load gdb-scripts::.  Show setting of GDB command
                                     scripts.
*Note info auto-load gdb-scripts::.  Show state of GDB command scripts.
*Note set auto-load python-scripts::.Control for GDB Python scripts.
*Note show auto-load python-scripts::.Show setting of GDB Python
                                     scripts.
*Note info auto-load python-scripts::.Show state of GDB Python scripts.
*Note set auto-load guile-scripts::. Control for GDB Guile scripts.
*Note show auto-load guile-scripts::.Show setting of GDB Guile scripts.
*Note info auto-load guile-scripts::.Show state of GDB Guile scripts.
*Note set auto-load scripts-directory::.Control for GDB auto-loaded
                                     scripts location.
*Note show auto-load scripts-directory::.Show GDB auto-loaded scripts
                                     location.
*Note add-auto-load-scripts-directory::.Add directory for auto-loaded
                                     scripts location list.
*Note set auto-load local-gdbinit::. Control for init file in the
                                     current directory.
*Note show auto-load local-gdbinit::.Show setting of init file in the
                                     current directory.
*Note info auto-load local-gdbinit::.Show state of init file in the
                                     current directory.
*Note set auto-load libthread-db::.  Control for thread debugging
                                     library.
*Note show auto-load libthread-db::. Show setting of thread debugging
                                     library.
*Note info auto-load libthread-db::. Show state of thread debugging
                                     library.
*Note set auto-load safe-path::.     Control directories trusted for
                                     automatic loading.
*Note show auto-load safe-path::.    Show directories trusted for
                                     automatic loading.
*Note add-auto-load-safe-path::.     Add directory trusted for
                                     automatic loading.


File: gdb.info,  Node: Init File in the Current Directory,  Next: libthread_db.so.1 file,  Up: Auto-loading

22.8.1 Automatically loading init file in the current directory
---------------------------------------------------------------

By default, GDB reads and executes the canned sequences of commands from
init file (if any) in the current working directory, see *note Init File
in the Current Directory during Startup::.

   Note that loading of this local '.gdbinit' file also requires
accordingly configured 'auto-load safe-path' (*note Auto-loading safe
path::).

'set auto-load local-gdbinit [on|off]'
     Enable or disable the auto-loading of canned sequences of commands
     (*note Sequences::) found in init file in the current directory.

'show auto-load local-gdbinit'
     Show whether auto-loading of canned sequences of commands from init
     file in the current directory is enabled or disabled.

'info auto-load local-gdbinit'
     Print whether canned sequences of commands from init file in the
     current directory have been auto-loaded.


File: gdb.info,  Node: libthread_db.so.1 file,  Next: Auto-loading safe path,  Prev: Init File in the Current Directory,  Up: Auto-loading

22.8.2 Automatically loading thread debugging library
-----------------------------------------------------

This feature is currently present only on GNU/Linux native hosts.

   GDB reads in some cases thread debugging library from places specific
to the inferior (*note set libthread-db-search-path::).

   The special 'libthread-db-search-path' entry '$sdir' is processed
without checking this 'set auto-load libthread-db' switch as system
libraries have to be trusted in general.  In all other cases of
'libthread-db-search-path' entries GDB checks first if 'set auto-load
libthread-db' is enabled before trying to open such thread debugging
library.

   Note that loading of this debugging library also requires accordingly
configured 'auto-load safe-path' (*note Auto-loading safe path::).

'set auto-load libthread-db [on|off]'
     Enable or disable the auto-loading of inferior specific thread
     debugging library.

'show auto-load libthread-db'
     Show whether auto-loading of inferior specific thread debugging
     library is enabled or disabled.

'info auto-load libthread-db'
     Print the list of all loaded inferior specific thread debugging
     libraries and for each such library print list of inferior PIDs
     using it.


File: gdb.info,  Node: Auto-loading safe path,  Next: Auto-loading verbose mode,  Prev: libthread_db.so.1 file,  Up: Auto-loading

22.8.3 Security restriction for auto-loading
--------------------------------------------

As the files of inferior can come from untrusted source (such as
submitted by an application user) GDB does not always load any files
automatically.  GDB provides the 'set auto-load safe-path' setting to
list directories trusted for loading files not explicitly requested by
user.  Each directory can also be a shell wildcard pattern.

   If the path is not set properly you will see a warning and the file
will not get loaded:

     $ ./gdb -q ./gdb
     Reading symbols from /home/user/gdb/gdb...done.
     warning: File "/home/user/gdb/gdb-gdb.gdb" auto-loading has been
              declined by your `auto-load safe-path' set
              to "$debugdir:$datadir/auto-load".
     warning: File "/home/user/gdb/gdb-gdb.py" auto-loading has been
              declined by your `auto-load safe-path' set
              to "$debugdir:$datadir/auto-load".

To instruct GDB to go ahead and use the init files anyway, invoke GDB
like this:

     $ gdb -q -iex "set auto-load safe-path /home/user/gdb" ./gdb

   The list of trusted directories is controlled by the following
commands:

'set auto-load safe-path [DIRECTORIES]'
     Set the list of directories (and their subdirectories) trusted for
     automatic loading and execution of scripts.  You can also enter a
     specific trusted file.  Each directory can also be a shell wildcard
     pattern; wildcards do not match directory separator - see
     'FNM_PATHNAME' for system function 'fnmatch' (*note fnmatch:
     (libc)Wildcard Matching.).  If you omit DIRECTORIES, 'auto-load
     safe-path' will be reset to its default value as specified during
     GDB compilation.

     The list of directories uses path separator (':' on GNU and Unix
     systems, ';' on MS-Windows and MS-DOS) to separate directories,
     similarly to the 'PATH' environment variable.

'show auto-load safe-path'
     Show the list of directories trusted for automatic loading and
     execution of scripts.

'add-auto-load-safe-path'
     Add an entry (or list of entries) to the list of directories
     trusted for automatic loading and execution of scripts.  Multiple
     entries may be delimited by the host platform path separator in
     use.

   This variable defaults to what '--with-auto-load-dir' has been
configured to (*note with-auto-load-dir::).  '$debugdir' and '$datadir'
substitution applies the same as for *note set auto-load
scripts-directory::.  The default 'set auto-load safe-path' value can be
also overriden by GDB configuration option '--with-auto-load-safe-path'.

   Setting this variable to '/' disables this security protection,
corresponding GDB configuration option is
'--without-auto-load-safe-path'.  This variable is supposed to be set to
the system directories writable by the system superuser only.  Users can
add their source directories in init files in their home directories
(*note Home Directory Init File::).  See also deprecated init file in
the current directory (*note Init File in the Current Directory during
Startup::).

   To force GDB to load the files it declined to load in the previous
example, you could use one of the following ways:

'~/.gdbinit': 'add-auto-load-safe-path ~/src/gdb'
     Specify this trusted directory (or a file) as additional component
     of the list.  You have to specify also any existing directories
     displayed by by 'show auto-load safe-path' (such as '/usr:/bin' in
     this example).

'gdb -iex "set auto-load safe-path /usr:/bin:~/src/gdb" ...'
     Specify this directory as in the previous case but just for a
     single GDB session.

'gdb -iex "set auto-load safe-path /" ...'
     Disable auto-loading safety for a single GDB session.  This assumes
     all the files you debug during this GDB session will come from
     trusted sources.

'./configure --without-auto-load-safe-path'
     During compilation of GDB you may disable any auto-loading safety.
     This assumes all the files you will ever debug with this GDB come
     from trusted sources.

   On the other hand you can also explicitly forbid automatic files
loading which also suppresses any such warning messages:

'gdb -iex "set auto-load no" ...'
     You can use GDB command-line option for a single GDB session.

'~/.gdbinit': 'set auto-load no'
     Disable auto-loading globally for the user (*note Home Directory
     Init File::).  While it is improbable, you could also use system
     init file instead (*note System-wide configuration::).

   This setting applies to the file names as entered by user.  If no
entry matches GDB tries as a last resort to also resolve all the file
names into their canonical form (typically resolving symbolic links) and
compare the entries again.  GDB already canonicalizes most of the
filenames on its own before starting the comparison so a canonical form
of directories is recommended to be entered.


File: gdb.info,  Node: Auto-loading verbose mode,  Prev: Auto-loading safe path,  Up: Auto-loading

22.8.4 Displaying files tried for auto-load
-------------------------------------------

For better visibility of all the file locations where you can place
scripts to be auto-loaded with inferior -- or to protect yourself
against accidental execution of untrusted scripts -- GDB provides a
feature for printing all the files attempted to be loaded.  Both
existing and non-existing files may be printed.

   For example the list of directories from which it is safe to
auto-load files (*note Auto-loading safe path::) applies also to
canonicalized filenames which may not be too obvious while setting it
up.

     (gdb) set debug auto-load on
     (gdb) file ~/src/t/true
     auto-load: Loading canned sequences of commands script "/tmp/true-gdb.gdb"
                for objfile "/tmp/true".
     auto-load: Updating directories of "/usr:/opt".
     auto-load: Using directory "/usr".
     auto-load: Using directory "/opt".
     warning: File "/tmp/true-gdb.gdb" auto-loading has been declined
              by your `auto-load safe-path' set to "/usr:/opt".

'set debug auto-load [on|off]'
     Set whether to print the filenames attempted to be auto-loaded.

'show debug auto-load'
     Show whether printing of the filenames attempted to be auto-loaded
     is turned on or off.


File: gdb.info,  Node: Messages/Warnings,  Next: Debugging Output,  Prev: Auto-loading,  Up: Controlling GDB

22.9 Optional Warnings and Messages
===================================

By default, GDB is silent about its inner workings.  If you are running
on a slow machine, you may want to use the 'set verbose' command.  This
makes GDB tell you when it does a lengthy internal operation, so you
will not think it has crashed.

   Currently, the messages controlled by 'set verbose' are those which
announce that the symbol table for a source file is being read; see
'symbol-file' in *note Commands to Specify Files: Files.

'set verbose on'
     Enables GDB output of certain informational messages.

'set verbose off'
     Disables GDB output of certain informational messages.

'show verbose'
     Displays whether 'set verbose' is on or off.

   By default, if GDB encounters bugs in the symbol table of an object
file, it is silent; but if you are debugging a compiler, you may find
this information useful (*note Errors Reading Symbol Files: Symbol
Errors.).

'set complaints LIMIT'
     Permits GDB to output LIMIT complaints about each type of unusual
     symbols before becoming silent about the problem.  Set LIMIT to
     zero to suppress all complaints; set it to a large number to
     prevent complaints from being suppressed.

'show complaints'
     Displays how many symbol complaints GDB is permitted to produce.

   By default, GDB is cautious, and asks what sometimes seems to be a
lot of stupid questions to confirm certain commands.  For example, if
you try to run a program which is already running:

     (gdb) run
     The program being debugged has been started already.
     Start it from the beginning? (y or n)

   If you are willing to unflinchingly face the consequences of your own
commands, you can disable this "feature":

'set confirm off'
     Disables confirmation requests.  Note that running GDB with the
     '--batch' option (*note -batch: Mode Options.) also automatically
     disables confirmation requests.

'set confirm on'
     Enables confirmation requests (the default).

'show confirm'
     Displays state of confirmation requests.

   If you need to debug user-defined commands or sourced files you may
find it useful to enable "command tracing".  In this mode each command
will be printed as it is executed, prefixed with one or more '+'
symbols, the quantity denoting the call depth of each command.

'set trace-commands on'
     Enable command tracing.
'set trace-commands off'
     Disable command tracing.
'show trace-commands'
     Display the current state of command tracing.


File: gdb.info,  Node: Debugging Output,  Next: Other Misc Settings,  Prev: Messages/Warnings,  Up: Controlling GDB

22.10 Optional Messages about Internal Happenings
=================================================

GDB has commands that enable optional debugging messages from various
GDB subsystems; normally these commands are of interest to GDB
maintainers, or when reporting a bug.  This section documents those
commands.

'set exec-done-display'
     Turns on or off the notification of asynchronous commands'
     completion.  When on, GDB will print a message when an asynchronous
     command finishes its execution.  The default is off.
'show exec-done-display'
     Displays the current setting of asynchronous command completion
     notification.
'set debug aarch64'
     Turns on or off display of debugging messages related to ARM
     AArch64.  The default is off.
'show debug aarch64'
     Displays the current state of displaying debugging messages related
     to ARM AArch64.
'set debug arch'
     Turns on or off display of gdbarch debugging info.  The default is
     off
'show debug arch'
     Displays the current state of displaying gdbarch debugging info.
'set debug aix-solib'
     Control display of debugging messages from the AIX shared library
     support module.  The default is off.
'show debug aix-thread'
     Show the current state of displaying AIX shared library debugging
     messages.
'set debug aix-thread'
     Display debugging messages about inner workings of the AIX thread
     module.
'show debug aix-thread'
     Show the current state of AIX thread debugging info display.
'set debug check-physname'
     Check the results of the "physname" computation.  When reading
     DWARF debugging information for C++, GDB attempts to compute each
     entity's name.  GDB can do this computation in two different ways,
     depending on exactly what information is present.  When enabled,
     this setting causes GDB to compute the names both ways and display
     any discrepancies.
'show debug check-physname'
     Show the current state of "physname" checking.
'set debug coff-pe-read'
     Control display of debugging messages related to reading of COFF/PE
     exported symbols.  The default is off.
'show debug coff-pe-read'
     Displays the current state of displaying debugging messages related
     to reading of COFF/PE exported symbols.
'set debug dwarf-die'
     Dump DWARF DIEs after they are read in.  The value is the number of
     nesting levels to print.  A value of zero turns off the display.
'show debug dwarf-die'
     Show the current state of DWARF DIE debugging.
'set debug dwarf-line'
     Turns on or off display of debugging messages related to reading
     DWARF line tables.  The default is 0 (off).  A value of 1 provides
     basic information.  A value greater than 1 provides more verbose
     information.
'show debug dwarf-line'
     Show the current state of DWARF line table debugging.
'set debug dwarf-read'
     Turns on or off display of debugging messages related to reading
     DWARF debug info.  The default is 0 (off).  A value of 1 provides
     basic information.  A value greater than 1 provides more verbose
     information.
'show debug dwarf-read'
     Show the current state of DWARF reader debugging.
'set debug displaced'
     Turns on or off display of GDB debugging info for the displaced
     stepping support.  The default is off.
'show debug displaced'
     Displays the current state of displaying GDB debugging info related
     to displaced stepping.
'set debug event'
     Turns on or off display of GDB event debugging info.  The default
     is off.
'show debug event'
     Displays the current state of displaying GDB event debugging info.
'set debug expression'
     Turns on or off display of debugging info about GDB expression
     parsing.  The default is off.
'show debug expression'
     Displays the current state of displaying debugging info about GDB
     expression parsing.
'set debug fbsd-lwp'
     Turns on or off debugging messages from the FreeBSD LWP debug
     support.
'show debug fbsd-lwp'
     Show the current state of FreeBSD LWP debugging messages.
'set debug fbsd-nat'
     Turns on or off debugging messages from the FreeBSD native target.
'show debug fbsd-nat'
     Show the current state of FreeBSD native target debugging messages.
'set debug frame'
     Turns on or off display of GDB frame debugging info.  The default
     is off.
'show debug frame'
     Displays the current state of displaying GDB frame debugging info.
'set debug gnu-nat'
     Turn on or off debugging messages from the GNU/Hurd debug support.
'show debug gnu-nat'
     Show the current state of GNU/Hurd debugging messages.
'set debug infrun'
     Turns on or off display of GDB debugging info for running the
     inferior.  The default is off.  'infrun.c' contains GDB's runtime
     state machine used for implementing operations such as
     single-stepping the inferior.
'show debug infrun'
     Displays the current state of GDB inferior debugging.
'set debug jit'
     Turn on or off debugging messages from JIT debug support.
'show debug jit'
     Displays the current state of GDB JIT debugging.
'set debug lin-lwp'
     Turn on or off debugging messages from the Linux LWP debug support.
'show debug lin-lwp'
     Show the current state of Linux LWP debugging messages.
'set debug linux-namespaces'
     Turn on or off debugging messages from the Linux namespaces debug
     support.
'show debug linux-namespaces'
     Show the current state of Linux namespaces debugging messages.
'set debug mach-o'
     Control display of debugging messages related to Mach-O symbols
     processing.  The default is off.
'show debug mach-o'
     Displays the current state of displaying debugging messages related
     to reading of COFF/PE exported symbols.
'set debug notification'
     Turn on or off debugging messages about remote async notification.
     The default is off.
'show debug notification'
     Displays the current state of remote async notification debugging
     messages.
'set debug observer'
     Turns on or off display of GDB observer debugging.  This includes
     info such as the notification of observable events.
'show debug observer'
     Displays the current state of observer debugging.
'set debug overload'
     Turns on or off display of GDB C++ overload debugging info.  This
     includes info such as ranking of functions, etc.  The default is
     off.
'show debug overload'
     Displays the current state of displaying GDB C++ overload debugging
     info.
'set debug parser'
     Turns on or off the display of expression parser debugging output.
     Internally, this sets the 'yydebug' variable in the expression
     parser.  *Note Tracing Your Parser: (bison)Tracing, for details.
     The default is off.
'show debug parser'
     Show the current state of expression parser debugging.
'set debug remote'
     Turns on or off display of reports on all packets sent back and
     forth across the serial line to the remote machine.  The info is
     printed on the GDB standard output stream.  The default is off.
'show debug remote'
     Displays the state of display of remote packets.

'set debug remote-packet-max-chars'
     Sets the maximum number of characters to display for each remote
     packet when 'set debug remote' is on.  This is useful to prevent
     GDB from displaying lengthy remote packets and polluting the
     console.

     The default value is '512', which means GDB will truncate each
     remote packet after 512 bytes.

     Setting this option to 'unlimited' will disable truncation and will
     output the full length of the remote packets.
'show debug remote-packet-max-chars'
     Displays the number of bytes to output for remote packet debugging.

'set debug separate-debug-file'
     Turns on or off display of debug output about separate debug file
     search.
'show debug separate-debug-file'
     Displays the state of separate debug file search debug output.

'set debug serial'
     Turns on or off display of GDB serial debugging info.  The default
     is off.
'show debug serial'
     Displays the current state of displaying GDB serial debugging info.
'set debug solib-frv'
     Turn on or off debugging messages for FR-V shared-library code.
'show debug solib-frv'
     Display the current state of FR-V shared-library code debugging
     messages.
'set debug symbol-lookup'
     Turns on or off display of debugging messages related to symbol
     lookup.  The default is 0 (off).  A value of 1 provides basic
     information.  A value greater than 1 provides more verbose
     information.
'show debug symbol-lookup'
     Show the current state of symbol lookup debugging messages.
'set debug symfile'
     Turns on or off display of debugging messages related to symbol
     file functions.  The default is off.  *Note Files::.
'show debug symfile'
     Show the current state of symbol file debugging messages.
'set debug symtab-create'
     Turns on or off display of debugging messages related to symbol
     table creation.  The default is 0 (off).  A value of 1 provides
     basic information.  A value greater than 1 provides more verbose
     information.
'show debug symtab-create'
     Show the current state of symbol table creation debugging.
'set debug target'
     Turns on or off display of GDB target debugging info.  This info
     includes what is going on at the target level of GDB, as it
     happens.  The default is 0.  Set it to 1 to track events, and to 2
     to also track the value of large memory transfers.
'show debug target'
     Displays the current state of displaying GDB target debugging info.
'set debug timestamp'
     Turns on or off display of timestamps with GDB debugging info.
     When enabled, seconds and microseconds are displayed before each
     debugging message.
'show debug timestamp'
     Displays the current state of displaying timestamps with GDB
     debugging info.
'set debug varobj'
     Turns on or off display of GDB variable object debugging info.  The
     default is off.
'show debug varobj'
     Displays the current state of displaying GDB variable object
     debugging info.
'set debug xml'
     Turn on or off debugging messages for built-in XML parsers.
'show debug xml'
     Displays the current state of XML debugging messages.


File: gdb.info,  Node: Other Misc Settings,  Prev: Debugging Output,  Up: Controlling GDB

22.11 Other Miscellaneous Settings
==================================

'set interactive-mode'
     If 'on', forces GDB to assume that GDB was started in a terminal.
     In practice, this means that GDB should wait for the user to answer
     queries generated by commands entered at the command prompt.  If
     'off', forces GDB to operate in the opposite mode, and it uses the
     default answers to all queries.  If 'auto' (the default), GDB tries
     to determine whether its standard input is a terminal, and works in
     interactive-mode if it is, non-interactively otherwise.

     In the vast majority of cases, the debugger should be able to guess
     correctly which mode should be used.  But this setting can be
     useful in certain specific cases, such as running a MinGW GDB
     inside a cygwin window.

'show interactive-mode'
     Displays whether the debugger is operating in interactive mode or
     not.


File: gdb.info,  Node: Extending GDB,  Next: Interpreters,  Prev: Controlling GDB,  Up: Top

23 Extending GDB
****************

GDB provides several mechanisms for extension.  GDB also provides the
ability to automatically load extensions when it reads a file for
debugging.  This allows the user to automatically customize GDB for the
program being debugged.

* Menu:

* Sequences::                Canned Sequences of GDB Commands
* Python::                   Extending GDB using Python
* Guile::                    Extending GDB using Guile
* Auto-loading extensions::  Automatically loading extensions
* Multiple Extension Languages:: Working with multiple extension languages
* Aliases::                  Creating new spellings of existing commands

   To facilitate the use of extension languages, GDB is capable of
evaluating the contents of a file.  When doing so, GDB can recognize
which extension language is being used by looking at the filename
extension.  Files with an unrecognized filename extension are always
treated as a GDB Command Files.  *Note Command files: Command Files.

   You can control how GDB evaluates these files with the following
setting:

'set script-extension off'
     All scripts are always evaluated as GDB Command Files.

'set script-extension soft'
     The debugger determines the scripting language based on filename
     extension.  If this scripting language is supported, GDB evaluates
     the script using that language.  Otherwise, it evaluates the file
     as a GDB Command File.

'set script-extension strict'
     The debugger determines the scripting language based on filename
     extension, and evaluates the script using that language.  If the
     language is not supported, then the evaluation fails.

'show script-extension'
     Display the current value of the 'script-extension' option.


File: gdb.info,  Node: Sequences,  Next: Python,  Up: Extending GDB

23.1 Canned Sequences of Commands
=================================

Aside from breakpoint commands (*note Breakpoint Command Lists: Break
Commands.), GDB provides two ways to store sequences of commands for
execution as a unit: user-defined commands and command files.

* Menu:

* Define::             How to define your own commands
* Hooks::              Hooks for user-defined commands
* Command Files::      How to write scripts of commands to be stored in a file
* Output::             Commands for controlled output
* Auto-loading sequences::  Controlling auto-loaded command files


File: gdb.info,  Node: Define,  Next: Hooks,  Up: Sequences

23.1.1 User-defined Commands
----------------------------

A "user-defined command" is a sequence of GDB commands to which you
assign a new name as a command.  This is done with the 'define' command.
User commands may accept an unlimited number of arguments separated by
whitespace.  Arguments are accessed within the user command via
'$arg0...$argN'.  A trivial example:

     define adder
       print $arg0 + $arg1 + $arg2
     end

To execute the command use:

     adder 1 2 3

This defines the command 'adder', which prints the sum of its three
arguments.  Note the arguments are text substitutions, so they may
reference variables, use complex expressions, or even perform inferior
functions calls.

   In addition, '$argc' may be used to find out how many arguments have
been passed.

     define adder
       if $argc == 2
         print $arg0 + $arg1
       end
       if $argc == 3
         print $arg0 + $arg1 + $arg2
       end
     end

   Combining with the 'eval' command (*note eval::) makes it easier to
process a variable number of arguments:

     define adder
       set $i = 0
       set $sum = 0
       while $i < $argc
         eval "set $sum = $sum + $arg%d", $i
         set $i = $i + 1
       end
       print $sum
     end

'define COMMANDNAME'
     Define a command named COMMANDNAME.  If there is already a command
     by that name, you are asked to confirm that you want to redefine
     it.  The argument COMMANDNAME may be a bare command name consisting
     of letters, numbers, dashes, dots, and underscores.  It may also
     start with any predefined or user-defined prefix command.  For
     example, 'define target my-target' creates a user-defined 'target
     my-target' command.

     The definition of the command is made up of other GDB command
     lines, which are given following the 'define' command.  The end of
     these commands is marked by a line containing 'end'.

'document COMMANDNAME'
     Document the user-defined command COMMANDNAME, so that it can be
     accessed by 'help'.  The command COMMANDNAME must already be
     defined.  This command reads lines of documentation just as
     'define' reads the lines of the command definition, ending with
     'end'.  After the 'document' command is finished, 'help' on command
     COMMANDNAME displays the documentation you have written.

     You may use the 'document' command again to change the
     documentation of a command.  Redefining the command with 'define'
     does not change the documentation.

'define-prefix COMMANDNAME'
     Define or mark the command COMMANDNAME as a user-defined prefix
     command.  Once marked, COMMANDNAME can be used as prefix command by
     the 'define' command.  Note that 'define-prefix' can be used with a
     not yet defined COMMANDNAME.  In such a case, COMMANDNAME is
     defined as an empty user-defined command.  In case you redefine a
     command that was marked as a user-defined prefix command, the
     subcommands of the redefined command are kept (and GDB indicates so
     to the user).

     Example:
          (gdb) define-prefix abc
          (gdb) define-prefix abc def
          (gdb) define abc def
          Type commands for definition of "abc def".
          End with a line saying just "end".
          >echo command initial def\n
          >end
          (gdb) define abc def ghi
          Type commands for definition of "abc def ghi".
          End with a line saying just "end".
          >echo command ghi\n
          >end
          (gdb) define abc def
          Keeping subcommands of prefix command "def".
          Redefine command "def"? (y or n) y
          Type commands for definition of "abc def".
          End with a line saying just "end".
          >echo command def\n
          >end
          (gdb) abc def ghi
          command ghi
          (gdb) abc def
          command def
          (gdb)

'dont-repeat'
     Used inside a user-defined command, this tells GDB that this
     command should not be repeated when the user hits <RET> (*note
     repeat last command: Command Syntax.).

'help user-defined'
     List all user-defined commands and all python commands defined in
     class COMMAND_USER. The first line of the documentation or
     docstring is included (if any).

'show user'
'show user COMMANDNAME'
     Display the GDB commands used to define COMMANDNAME (but not its
     documentation).  If no COMMANDNAME is given, display the
     definitions for all user-defined commands.  This does not work for
     user-defined python commands.

'show max-user-call-depth'
'set max-user-call-depth'
     The value of 'max-user-call-depth' controls how many recursion
     levels are allowed in user-defined commands before GDB suspects an
     infinite recursion and aborts the command.  This does not apply to
     user-defined python commands.

   In addition to the above commands, user-defined commands frequently
use control flow commands, described in *note Command Files::.

   When user-defined commands are executed, the commands of the
definition are not printed.  An error in any command stops execution of
the user-defined command.

   If used interactively, commands that would ask for confirmation
proceed without asking when used inside a user-defined command.  Many
GDB commands that normally print messages to say what they are doing
omit the messages when used in a user-defined command.


File: gdb.info,  Node: Hooks,  Next: Command Files,  Prev: Define,  Up: Sequences

23.1.2 User-defined Command Hooks
---------------------------------

You may define "hooks", which are a special kind of user-defined
command.  Whenever you run the command 'foo', if the user-defined
command 'hook-foo' exists, it is executed (with no arguments) before
that command.

   A hook may also be defined which is run after the command you
executed.  Whenever you run the command 'foo', if the user-defined
command 'hookpost-foo' exists, it is executed (with no arguments) after
that command.  Post-execution hooks may exist simultaneously with
pre-execution hooks, for the same command.

   It is valid for a hook to call the command which it hooks.  If this
occurs, the hook is not re-executed, thereby avoiding infinite
recursion.

   In addition, a pseudo-command, 'stop' exists.  Defining ('hook-stop')
makes the associated commands execute every time execution stops in your
program: before breakpoint commands are run, displays are printed, or
the stack frame is printed.

   For example, to ignore 'SIGALRM' signals while single-stepping, but
treat them normally during normal execution, you could define:

     define hook-stop
     handle SIGALRM nopass
     end

     define hook-run
     handle SIGALRM pass
     end

     define hook-continue
     handle SIGALRM pass
     end

   As a further example, to hook at the beginning and end of the 'echo'
command, and to add extra text to the beginning and end of the message,
you could define:

     define hook-echo
     echo <<<---
     end

     define hookpost-echo
     echo --->>>\n
     end

     (gdb) echo Hello World
     <<<---Hello World--->>>
     (gdb)


   You can define a hook for any single-word command in GDB, but not for
command aliases; you should define a hook for the basic command name,
e.g. 'backtrace' rather than 'bt'.  You can hook a multi-word command by
adding 'hook-' or 'hookpost-' to the last word of the command, e.g.
'define target hook-remote' to add a hook to 'target remote'.

   If an error occurs during the execution of your hook, execution of
GDB commands stops and GDB issues a prompt (before the command that you
actually typed had a chance to run).

   If you try to define a hook which does not match any known command,
you get a warning from the 'define' command.


File: gdb.info,  Node: Command Files,  Next: Output,  Prev: Hooks,  Up: Sequences

23.1.3 Command Files
--------------------

A command file for GDB is a text file made of lines that are GDB
commands.  Comments (lines starting with '#') may also be included.  An
empty line in a command file does nothing; it does not mean to repeat
the last command, as it would from the terminal.

   You can request the execution of a command file with the 'source'
command.  Note that the 'source' command is also used to evaluate
scripts that are not Command Files.  The exact behavior can be
configured using the 'script-extension' setting.  *Note Extending GDB:
Extending GDB.

'source [-s] [-v] FILENAME'
     Execute the command file FILENAME.

   The lines in a command file are generally executed sequentially,
unless the order of execution is changed by one of the _flow-control
commands_ described below.  The commands are not printed as they are
executed.  An error in any command terminates execution of the command
file and control is returned to the console.

   GDB first searches for FILENAME in the current directory.  If the
file is not found there, and FILENAME does not specify a directory, then
GDB also looks for the file on the source search path (specified with
the 'directory' command); except that '$cdir' is not searched because
the compilation directory is not relevant to scripts.

   If '-s' is specified, then GDB searches for FILENAME on the search
path even if FILENAME specifies a directory.  The search is done by
appending FILENAME to each element of the search path.  So, for example,
if FILENAME is 'mylib/myscript' and the search path contains
'/home/user' then GDB will look for the script
'/home/user/mylib/myscript'.  The search is also done if FILENAME is an
absolute path.  For example, if FILENAME is '/tmp/myscript' and the
search path contains '/home/user' then GDB will look for the script
'/home/user/tmp/myscript'.  For DOS-like systems, if FILENAME contains a
drive specification, it is stripped before concatenation.  For example,
if FILENAME is 'd:myscript' and the search path contains 'c:/tmp' then
GDB will look for the script 'c:/tmp/myscript'.

   If '-v', for verbose mode, is given then GDB displays each command as
it is executed.  The option must be given before FILENAME, and is
interpreted as part of the filename anywhere else.

   Commands that would ask for confirmation if used interactively
proceed without asking when used in a command file.  Many GDB commands
that normally print messages to say what they are doing omit the
messages when called from command files.

   GDB also accepts command input from standard input.  In this mode,
normal output goes to standard output and error output goes to standard
error.  Errors in a command file supplied on standard input do not
terminate execution of the command file--execution continues with the
next command.

     gdb < cmds > log 2>&1

   (The syntax above will vary depending on the shell used.)  This
example will execute commands from the file 'cmds'.  All output and
errors would be directed to 'log'.

   Since commands stored on command files tend to be more general than
commands typed interactively, they frequently need to deal with
complicated situations, such as different or unexpected values of
variables and symbols, changes in how the program being debugged is
built, etc.  GDB provides a set of flow-control commands to deal with
these complexities.  Using these commands, you can write complex scripts
that loop over data structures, execute commands conditionally, etc.

'if'
'else'
     This command allows to include in your script conditionally
     executed commands.  The 'if' command takes a single argument, which
     is an expression to evaluate.  It is followed by a series of
     commands that are executed only if the expression is true (its
     value is nonzero).  There can then optionally be an 'else' line,
     followed by a series of commands that are only executed if the
     expression was false.  The end of the list is marked by a line
     containing 'end'.

'while'
     This command allows to write loops.  Its syntax is similar to 'if':
     the command takes a single argument, which is an expression to
     evaluate, and must be followed by the commands to execute, one per
     line, terminated by an 'end'.  These commands are called the "body"
     of the loop.  The commands in the body of 'while' are executed
     repeatedly as long as the expression evaluates to true.

'loop_break'
     This command exits the 'while' loop in whose body it is included.
     Execution of the script continues after that 'while's 'end' line.

'loop_continue'
     This command skips the execution of the rest of the body of
     commands in the 'while' loop in whose body it is included.
     Execution branches to the beginning of the 'while' loop, where it
     evaluates the controlling expression.

'end'
     Terminate the block of commands that are the body of 'if', 'else',
     or 'while' flow-control commands.


File: gdb.info,  Node: Output,  Next: Auto-loading sequences,  Prev: Command Files,  Up: Sequences

23.1.4 Commands for Controlled Output
-------------------------------------

During the execution of a command file or a user-defined command, normal
GDB output is suppressed; the only output that appears is what is
explicitly printed by the commands in the definition.  This section
describes three commands useful for generating exactly the output you
want.

'echo TEXT'
     Print TEXT.  Nonprinting characters can be included in TEXT using C
     escape sequences, such as '\n' to print a newline.  *No newline is
     printed unless you specify one.*  In addition to the standard C
     escape sequences, a backslash followed by a space stands for a
     space.  This is useful for displaying a string with spaces at the
     beginning or the end, since leading and trailing spaces are
     otherwise trimmed from all arguments.  To print ' and foo = ', use
     the command 'echo \ and foo = \ '.

     A backslash at the end of TEXT can be used, as in C, to continue
     the command onto subsequent lines.  For example,

          echo This is some text\n\
          which is continued\n\
          onto several lines.\n

     produces the same output as

          echo This is some text\n
          echo which is continued\n
          echo onto several lines.\n

'output EXPRESSION'
     Print the value of EXPRESSION and nothing but that value: no
     newlines, no '$NN = '.  The value is not entered in the value
     history either.  *Note Expressions: Expressions, for more
     information on expressions.

'output/FMT EXPRESSION'
     Print the value of EXPRESSION in format FMT.  You can use the same
     formats as for 'print'.  *Note Output Formats: Output Formats, for
     more information.

'printf TEMPLATE, EXPRESSIONS...'
     Print the values of one or more EXPRESSIONS under the control of
     the string TEMPLATE.  To print several values, make EXPRESSIONS be
     a comma-separated list of individual expressions, which may be
     either numbers or pointers.  Their values are printed as specified
     by TEMPLATE, exactly as a C program would do by executing the code
     below:

          printf (TEMPLATE, EXPRESSIONS...);

     As in 'C' 'printf', ordinary characters in TEMPLATE are printed
     verbatim, while "conversion specification" introduced by the '%'
     character cause subsequent EXPRESSIONS to be evaluated, their
     values converted and formatted according to type and style
     information encoded in the conversion specifications, and then
     printed.

     For example, you can print two values in hex like this:

          printf "foo, bar-foo = 0x%x, 0x%x\n", foo, bar-foo

     'printf' supports all the standard 'C' conversion specifications,
     including the flags and modifiers between the '%' character and the
     conversion letter, with the following exceptions:

        * The argument-ordering modifiers, such as '2$', are not
          supported.

        * The modifier '*' is not supported for specifying precision or
          width.

        * The ''' flag (for separation of digits into groups according
          to 'LC_NUMERIC'') is not supported.

        * The type modifiers 'hh', 'j', 't', and 'z' are not supported.

        * The conversion letter 'n' (as in '%n') is not supported.

        * The conversion letters 'a' and 'A' are not supported.

     Note that the 'll' type modifier is supported only if the
     underlying 'C' implementation used to build GDB supports the 'long
     long int' type, and the 'L' type modifier is supported only if
     'long double' type is available.

     As in 'C', 'printf' supports simple backslash-escape sequences,
     such as '\n', '\t', '\\', '\"', '\a', and '\f', that consist of
     backslash followed by a single character.  Octal and hexadecimal
     escape sequences are not supported.

     Additionally, 'printf' supports conversion specifications for DFP
     ("Decimal Floating Point") types using the following length
     modifiers together with a floating point specifier.  letters:

        * 'H' for printing 'Decimal32' types.

        * 'D' for printing 'Decimal64' types.

        * 'DD' for printing 'Decimal128' types.

     If the underlying 'C' implementation used to build GDB has support
     for the three length modifiers for DFP types, other modifiers such
     as width and precision will also be available for GDB to use.

     In case there is no such 'C' support, no additional modifiers will
     be available and the value will be printed in the standard way.

     Here's an example of printing DFP types using the above conversion
     letters:
          printf "D32: %Hf - D64: %Df - D128: %DDf\n",1.2345df,1.2E10dd,1.2E1dl

'eval TEMPLATE, EXPRESSIONS...'
     Convert the values of one or more EXPRESSIONS under the control of
     the string TEMPLATE to a command line, and call it.


File: gdb.info,  Node: Auto-loading sequences,  Prev: Output,  Up: Sequences

23.1.5 Controlling auto-loading native GDB scripts
--------------------------------------------------

When a new object file is read (for example, due to the 'file' command,
or because the inferior has loaded a shared library), GDB will look for
the command file 'OBJFILE-gdb.gdb'.  *Note Auto-loading extensions::.

   Auto-loading can be enabled or disabled, and the list of auto-loaded
scripts can be printed.

'set auto-load gdb-scripts [on|off]'
     Enable or disable the auto-loading of canned sequences of commands
     scripts.

'show auto-load gdb-scripts'
     Show whether auto-loading of canned sequences of commands scripts
     is enabled or disabled.

'info auto-load gdb-scripts [REGEXP]'
     Print the list of all canned sequences of commands scripts that GDB
     auto-loaded.

   If REGEXP is supplied only canned sequences of commands scripts with
matching names are printed.


File: gdb.info,  Node: Python,  Next: Guile,  Prev: Sequences,  Up: Extending GDB

23.2 Extending GDB using Python
===============================

You can extend GDB using the Python programming language
(http://www.python.org/).  This feature is available only if GDB was
configured using '--with-python'.  GDB can be built against either
Python 2 or Python 3; which one you have depends on this configure-time
option.

   Python scripts used by GDB should be installed in
'DATA-DIRECTORY/python', where DATA-DIRECTORY is the data directory as
determined at GDB startup (*note Data Files::).  This directory, known
as the "python directory", is automatically added to the Python Search
Path in order to allow the Python interpreter to locate all scripts
installed at this location.

   Additionally, GDB commands and convenience functions which are
written in Python and are located in the
'DATA-DIRECTORY/python/gdb/command' or
'DATA-DIRECTORY/python/gdb/function' directories are automatically
imported when GDB starts.

* Menu:

* Python Commands::             Accessing Python from GDB.
* Python API::                  Accessing GDB from Python.
* Python Auto-loading::         Automatically loading Python code.
* Python modules::              Python modules provided by GDB.


File: gdb.info,  Node: Python Commands,  Next: Python API,  Up: Python

23.2.1 Python Commands
----------------------

GDB provides two commands for accessing the Python interpreter, and one
related setting:

'python-interactive [COMMAND]'
'pi [COMMAND]'
     Without an argument, the 'python-interactive' command can be used
     to start an interactive Python prompt.  To return to GDB, type the
     'EOF' character (e.g., 'Ctrl-D' on an empty prompt).

     Alternatively, a single-line Python command can be given as an
     argument and evaluated.  If the command is an expression, the
     result will be printed; otherwise, nothing will be printed.  For
     example:

          (gdb) python-interactive 2 + 3
          5

'python [COMMAND]'
'py [COMMAND]'
     The 'python' command can be used to evaluate Python code.

     If given an argument, the 'python' command will evaluate the
     argument as a Python command.  For example:

          (gdb) python print 23
          23

     If you do not provide an argument to 'python', it will act as a
     multi-line command, like 'define'.  In this case, the Python script
     is made up of subsequent command lines, given after the 'python'
     command.  This command list is terminated using a line containing
     'end'.  For example:

          (gdb) python
          >print 23
          >end
          23

'set python print-stack'
     By default, GDB will print only the message component of a Python
     exception when an error occurs in a Python script.  This can be
     controlled using 'set python print-stack': if 'full', then full
     Python stack printing is enabled; if 'none', then Python stack and
     message printing is disabled; if 'message', the default, only the
     message component of the error is printed.

   It is also possible to execute a Python script from the GDB
interpreter:

'source script-name'
     The script name must end with '.py' and GDB must be configured to
     recognize the script language based on filename extension using the
     'script-extension' setting.  *Note Extending GDB: Extending GDB.


File: gdb.info,  Node: Python API,  Next: Python Auto-loading,  Prev: Python Commands,  Up: Python

23.2.2 Python API
-----------------

You can get quick online help for GDB's Python API by issuing the
command 'python help (gdb)'.

   Functions and methods which have two or more optional arguments allow
them to be specified using keyword syntax.  This allows passing some
optional arguments while skipping others.  Example:
'gdb.some_function ('foo', bar = 1, baz = 2)'.

* Menu:

* Basic Python::                Basic Python Functions.
* Exception Handling::          How Python exceptions are translated.
* Values From Inferior::        Python representation of values.
* Types In Python::             Python representation of types.
* Pretty Printing API::         Pretty-printing values.
* Selecting Pretty-Printers::   How GDB chooses a pretty-printer.
* Writing a Pretty-Printer::    Writing a Pretty-Printer.
* Type Printing API::		Pretty-printing types.
* Frame Filter API::            Filtering Frames.
* Frame Decorator API::         Decorating Frames.
* Writing a Frame Filter::      Writing a Frame Filter.
* Unwinding Frames in Python::  Writing frame unwinder.
* Xmethods In Python::          Adding and replacing methods of C++ classes.
* Xmethod API::                 Xmethod types.
* Writing an Xmethod::          Writing an xmethod.
* Inferiors In Python::         Python representation of inferiors (processes)
* Events In Python::            Listening for events from GDB.
* Threads In Python::           Accessing inferior threads from Python.
* Recordings In Python::        Accessing recordings from Python.
* Commands In Python::          Implementing new commands in Python.
* Parameters In Python::        Adding new GDB parameters.
* Functions In Python::         Writing new convenience functions.
* Progspaces In Python::        Program spaces.
* Objfiles In Python::          Object files.
* Frames In Python::            Accessing inferior stack frames from Python.
* Blocks In Python::            Accessing blocks from Python.
* Symbols In Python::           Python representation of symbols.
* Symbol Tables In Python::     Python representation of symbol tables.
* Line Tables In Python::       Python representation of line tables.
* Breakpoints In Python::       Manipulating breakpoints using Python.
* Finish Breakpoints in Python:: Setting Breakpoints on function return
                                using Python.
* Lazy Strings In Python::      Python representation of lazy strings.
* Architectures In Python::     Python representation of architectures.


File: gdb.info,  Node: Basic Python,  Next: Exception Handling,  Up: Python API

23.2.2.1 Basic Python
.....................

At startup, GDB overrides Python's 'sys.stdout' and 'sys.stderr' to
print using GDB's output-paging streams.  A Python program which outputs
to one of these streams may have its output interrupted by the user
(*note Screen Size::).  In this situation, a Python 'KeyboardInterrupt'
exception is thrown.

   Some care must be taken when writing Python code to run in GDB.  Two
things worth noting in particular:

   * GDB install handlers for 'SIGCHLD' and 'SIGINT'.  Python code must
     not override these, or even change the options using 'sigaction'.
     If your program changes the handling of these signals, GDB will
     most likely stop working correctly.  Note that it is unfortunately
     common for GUI toolkits to install a 'SIGCHLD' handler.

   * GDB takes care to mark its internal file descriptors as
     close-on-exec.  However, this cannot be done in a thread-safe way
     on all platforms.  Your Python programs should be aware of this and
     should both create new file descriptors with the close-on-exec flag
     set and arrange to close unneeded file descriptors before starting
     a child process.

   GDB introduces a new Python module, named 'gdb'.  All methods and
classes added by GDB are placed in this module.  GDB automatically
'import's the 'gdb' module for use in all scripts evaluated by the
'python' command.

   Some types of the 'gdb' module come with a textual representation
(accessible through the 'repr' or 'str' functions).  These are offered
for debugging purposes only, expect them to change over time.

 -- Variable: gdb.PYTHONDIR
     A string containing the python directory (*note Python::).

 -- Function: gdb.execute (command [, from_tty [, to_string]])
     Evaluate COMMAND, a string, as a GDB CLI command.  If a GDB
     exception happens while COMMAND runs, it is translated as described
     in *note Exception Handling: Exception Handling.

     The FROM_TTY flag specifies whether GDB ought to consider this
     command as having originated from the user invoking it
     interactively.  It must be a boolean value.  If omitted, it
     defaults to 'False'.

     By default, any output produced by COMMAND is sent to GDB's
     standard output (and to the log output if logging is turned on).
     If the TO_STRING parameter is 'True', then output will be collected
     by 'gdb.execute' and returned as a string.  The default is 'False',
     in which case the return value is 'None'.  If TO_STRING is 'True',
     the GDB virtual terminal will be temporarily set to unlimited width
     and height, and its pagination will be disabled; *note Screen
     Size::.

 -- Function: gdb.breakpoints ()
     Return a sequence holding all of GDB's breakpoints.  *Note
     Breakpoints In Python::, for more information.  In GDB version 7.11
     and earlier, this function returned 'None' if there were no
     breakpoints.  This peculiarity was subsequently fixed, and now
     'gdb.breakpoints' returns an empty sequence in this case.

 -- Function: gdb.rbreak (regex [, minsyms [, throttle, [, symtabs ]]])
     Return a Python list holding a collection of newly set
     'gdb.Breakpoint' objects matching function names defined by the
     REGEX pattern.  If the MINSYMS keyword is 'True', all system
     functions (those not explicitly defined in the inferior) will also
     be included in the match.  The THROTTLE keyword takes an integer
     that defines the maximum number of pattern matches for functions
     matched by the REGEX pattern.  If the number of matches exceeds the
     integer value of THROTTLE, a 'RuntimeError' will be raised and no
     breakpoints will be created.  If THROTTLE is not defined then there
     is no imposed limit on the maximum number of matches and
     breakpoints to be created.  The SYMTABS keyword takes a Python
     iterable that yields a collection of 'gdb.Symtab' objects and will
     restrict the search to those functions only contained within the
     'gdb.Symtab' objects.

 -- Function: gdb.parameter (parameter)
     Return the value of a GDB PARAMETER given by its name, a string;
     the parameter name string may contain spaces if the parameter has a
     multi-part name.  For example, 'print object' is a valid parameter
     name.

     If the named parameter does not exist, this function throws a
     'gdb.error' (*note Exception Handling::).  Otherwise, the
     parameter's value is converted to a Python value of the appropriate
     type, and returned.

 -- Function: gdb.history (number)
     Return a value from GDB's value history (*note Value History::).
     The NUMBER argument indicates which history element to return.  If
     NUMBER is negative, then GDB will take its absolute value and count
     backward from the last element (i.e., the most recent element) to
     find the value to return.  If NUMBER is zero, then GDB will return
     the most recent element.  If the element specified by NUMBER
     doesn't exist in the value history, a 'gdb.error' exception will be
     raised.

     If no exception is raised, the return value is always an instance
     of 'gdb.Value' (*note Values From Inferior::).

 -- Function: gdb.convenience_variable (name)
     Return the value of the convenience variable (*note Convenience
     Vars::) named NAME.  NAME must be a string.  The name should not
     include the '$' that is used to mark a convenience variable in an
     expression.  If the convenience variable does not exist, then
     'None' is returned.

 -- Function: gdb.set_convenience_variable (name, value)
     Set the value of the convenience variable (*note Convenience
     Vars::) named NAME.  NAME must be a string.  The name should not
     include the '$' that is used to mark a convenience variable in an
     expression.  If VALUE is 'None', then the convenience variable is
     removed.  Otherwise, if VALUE is not a 'gdb.Value' (*note Values
     From Inferior::), it is is converted using the 'gdb.Value'
     constructor.

 -- Function: gdb.parse_and_eval (expression)
     Parse EXPRESSION, which must be a string, as an expression in the
     current language, evaluate it, and return the result as a
     'gdb.Value'.

     This function can be useful when implementing a new command (*note
     Commands In Python::), as it provides a way to parse the command's
     argument as an expression.  It is also useful simply to compute
     values.

 -- Function: gdb.find_pc_line (pc)
     Return the 'gdb.Symtab_and_line' object corresponding to the PC
     value.  *Note Symbol Tables In Python::.  If an invalid value of PC
     is passed as an argument, then the 'symtab' and 'line' attributes
     of the returned 'gdb.Symtab_and_line' object will be 'None' and 0
     respectively.  This is identical to
     'gdb.current_progspace().find_pc_line(pc)' and is included for
     historical compatibility.

 -- Function: gdb.post_event (event)
     Put EVENT, a callable object taking no arguments, into GDB's
     internal event queue.  This callable will be invoked at some later
     point, during GDB's event processing.  Events posted using
     'post_event' will be run in the order in which they were posted;
     however, there is no way to know when they will be processed
     relative to other events inside GDB.

     GDB is not thread-safe.  If your Python program uses multiple
     threads, you must be careful to only call GDB-specific functions in
     the GDB thread.  'post_event' ensures this.  For example:

          (gdb) python
          >import threading
          >
          >class Writer():
          > def __init__(self, message):
          >        self.message = message;
          > def __call__(self):
          >        gdb.write(self.message)
          >
          >class MyThread1 (threading.Thread):
          > def run (self):
          >        gdb.post_event(Writer("Hello "))
          >
          >class MyThread2 (threading.Thread):
          > def run (self):
          >        gdb.post_event(Writer("World\n"))
          >
          >MyThread1().start()
          >MyThread2().start()
          >end
          (gdb) Hello World

 -- Function: gdb.write (string [, stream])
     Print a string to GDB's paginated output stream.  The optional
     STREAM determines the stream to print to.  The default stream is
     GDB's standard output stream.  Possible stream values are:

     'gdb.STDOUT'
          GDB's standard output stream.

     'gdb.STDERR'
          GDB's standard error stream.

     'gdb.STDLOG'
          GDB's log stream (*note Logging Output::).

     Writing to 'sys.stdout' or 'sys.stderr' will automatically call
     this function and will automatically direct the output to the
     relevant stream.

 -- Function: gdb.flush ()
     Flush the buffer of a GDB paginated stream so that the contents are
     displayed immediately.  GDB will flush the contents of a stream
     automatically when it encounters a newline in the buffer.  The
     optional STREAM determines the stream to flush.  The default stream
     is GDB's standard output stream.  Possible stream values are:

     'gdb.STDOUT'
          GDB's standard output stream.

     'gdb.STDERR'
          GDB's standard error stream.

     'gdb.STDLOG'
          GDB's log stream (*note Logging Output::).

     Flushing 'sys.stdout' or 'sys.stderr' will automatically call this
     function for the relevant stream.

 -- Function: gdb.target_charset ()
     Return the name of the current target character set (*note
     Character Sets::).  This differs from
     'gdb.parameter('target-charset')' in that 'auto' is never returned.

 -- Function: gdb.target_wide_charset ()
     Return the name of the current target wide character set (*note
     Character Sets::).  This differs from
     'gdb.parameter('target-wide-charset')' in that 'auto' is never
     returned.

 -- Function: gdb.solib_name (address)
     Return the name of the shared library holding the given ADDRESS as
     a string, or 'None'.  This is identical to
     'gdb.current_progspace().solib_name(address)' and is included for
     historical compatibility.

 -- Function: gdb.decode_line ([expression])
     Return locations of the line specified by EXPRESSION, or of the
     current line if no argument was given.  This function returns a
     Python tuple containing two elements.  The first element contains a
     string holding any unparsed section of EXPRESSION (or 'None' if the
     expression has been fully parsed).  The second element contains
     either 'None' or another tuple that contains all the locations that
     match the expression represented as 'gdb.Symtab_and_line' objects
     (*note Symbol Tables In Python::).  If EXPRESSION is provided, it
     is decoded the way that GDB's inbuilt 'break' or 'edit' commands do
     (*note Specify Location::).

 -- Function: gdb.prompt_hook (current_prompt)

     If PROMPT_HOOK is callable, GDB will call the method assigned to
     this operation before a prompt is displayed by GDB.

     The parameter 'current_prompt' contains the current GDB prompt.
     This method must return a Python string, or 'None'.  If a string is
     returned, the GDB prompt will be set to that string.  If 'None' is
     returned, GDB will continue to use the current prompt.

     Some prompts cannot be substituted in GDB.  Secondary prompts such
     as those used by readline for command input, and annotation related
     prompts are prohibited from being changed.


File: gdb.info,  Node: Exception Handling,  Next: Values From Inferior,  Prev: Basic Python,  Up: Python API

23.2.2.2 Exception Handling
...........................

When executing the 'python' command, Python exceptions uncaught within
the Python code are translated to calls to GDB error-reporting
mechanism.  If the command that called 'python' does not handle the
error, GDB will terminate it and print an error message containing the
Python exception name, the associated value, and the Python call stack
backtrace at the point where the exception was raised.  Example:

     (gdb) python print foo
     Traceback (most recent call last):
       File "<string>", line 1, in <module>
     NameError: name 'foo' is not defined

   GDB errors that happen in GDB commands invoked by Python code are
converted to Python exceptions.  The type of the Python exception
depends on the error.

'gdb.error'
     This is the base class for most exceptions generated by GDB.  It is
     derived from 'RuntimeError', for compatibility with earlier
     versions of GDB.

     If an error occurring in GDB does not fit into some more specific
     category, then the generated exception will have this type.

'gdb.MemoryError'
     This is a subclass of 'gdb.error' which is thrown when an operation
     tried to access invalid memory in the inferior.

'KeyboardInterrupt'
     User interrupt (via 'C-c' or by typing 'q' at a pagination prompt)
     is translated to a Python 'KeyboardInterrupt' exception.

   In all cases, your exception handler will see the GDB error message
as its value and the Python call stack backtrace at the Python statement
closest to where the GDB error occured as the traceback.

   When implementing GDB commands in Python via 'gdb.Command', or
functions via 'gdb.Function', it is useful to be able to throw an
exception that doesn't cause a traceback to be printed.  For example,
the user may have invoked the command incorrectly.  GDB provides a
special exception class that can be used for this purpose.

'gdb.GdbError'
     When thrown from a command or function, this exception will cause
     the command or function to fail, but the Python stack will not be
     displayed.  GDB does not throw this exception itself, but rather
     recognizes it when thrown from user Python code.  Example:

          (gdb) python
          >class HelloWorld (gdb.Command):
          >  """Greet the whole world."""
          >  def __init__ (self):
          >    super (HelloWorld, self).__init__ ("hello-world", gdb.COMMAND_USER)
          >  def invoke (self, args, from_tty):
          >    argv = gdb.string_to_argv (args)
          >    if len (argv) != 0:
          >      raise gdb.GdbError ("hello-world takes no arguments")
          >    print "Hello, World!"
          >HelloWorld ()
          >end
          (gdb) hello-world 42
          hello-world takes no arguments


File: gdb.info,  Node: Values From Inferior,  Next: Types In Python,  Prev: Exception Handling,  Up: Python API

23.2.2.3 Values From Inferior
.............................

GDB provides values it obtains from the inferior program in an object of
type 'gdb.Value'.  GDB uses this object for its internal bookkeeping of
the inferior's values, and for fetching values when necessary.

   Inferior values that are simple scalars can be used directly in
Python expressions that are valid for the value's data type.  Here's an
example for an integer or floating-point value 'some_val':

     bar = some_val + 2

As result of this, 'bar' will also be a 'gdb.Value' object whose values
are of the same type as those of 'some_val'.  Valid Python operations
can also be performed on 'gdb.Value' objects representing a 'struct' or
'class' object.  For such cases, the overloaded operator (if present),
is used to perform the operation.  For example, if 'val1' and 'val2' are
'gdb.Value' objects representing instances of a 'class' which overloads
the '+' operator, then one can use the '+' operator in their Python
script as follows:

     val3 = val1 + val2

The result of the operation 'val3' is also a 'gdb.Value' object
corresponding to the value returned by the overloaded '+' operator.  In
general, overloaded operators are invoked for the following operations:
'+' (binary addition), '-' (binary subtraction), '*' (multiplication),
'/', '%', '<<', '>>', '|', '&', '^'.

   Inferior values that are structures or instances of some class can be
accessed using the Python "dictionary syntax".  For example, if
'some_val' is a 'gdb.Value' instance holding a structure, you can access
its 'foo' element with:

     bar = some_val['foo']

   Again, 'bar' will also be a 'gdb.Value' object.  Structure elements
can also be accessed by using 'gdb.Field' objects as subscripts (*note
Types In Python::, for more information on 'gdb.Field' objects).  For
example, if 'foo_field' is a 'gdb.Field' object corresponding to element
'foo' of the above structure, then 'bar' can also be accessed as
follows:

     bar = some_val[foo_field]

   A 'gdb.Value' that represents a function can be executed via inferior
function call.  Any arguments provided to the call must match the
function's prototype, and must be provided in the order specified by
that prototype.

   For example, 'some_val' is a 'gdb.Value' instance representing a
function that takes two integers as arguments.  To execute this
function, call it like so:

     result = some_val (10,20)

   Any values returned from a function call will be stored as a
'gdb.Value'.

   The following attributes are provided:

 -- Variable: Value.address
     If this object is addressable, this read-only attribute holds a
     'gdb.Value' object representing the address.  Otherwise, this
     attribute holds 'None'.

 -- Variable: Value.is_optimized_out
     This read-only boolean attribute is true if the compiler optimized
     out this value, thus it is not available for fetching from the
     inferior.

 -- Variable: Value.type
     The type of this 'gdb.Value'.  The value of this attribute is a
     'gdb.Type' object (*note Types In Python::).

 -- Variable: Value.dynamic_type
     The dynamic type of this 'gdb.Value'.  This uses the object's
     virtual table and the C++ run-time type information (RTTI) to
     determine the dynamic type of the value.  If this value is of class
     type, it will return the class in which the value is embedded, if
     any.  If this value is of pointer or reference to a class type, it
     will compute the dynamic type of the referenced object, and return
     a pointer or reference to that type, respectively.  In all other
     cases, it will return the value's static type.

     Note that this feature will only work when debugging a C++ program
     that includes RTTI for the object in question.  Otherwise, it will
     just return the static type of the value as in 'ptype foo' (*note
     ptype: Symbols.).

 -- Variable: Value.is_lazy
     The value of this read-only boolean attribute is 'True' if this
     'gdb.Value' has not yet been fetched from the inferior.  GDB does
     not fetch values until necessary, for efficiency.  For example:

          myval = gdb.parse_and_eval ('somevar')

     The value of 'somevar' is not fetched at this time.  It will be
     fetched when the value is needed, or when the 'fetch_lazy' method
     is invoked.

   The following methods are provided:

 -- Function: Value.__init__ (VAL)
     Many Python values can be converted directly to a 'gdb.Value' via
     this object initializer.  Specifically:

     Python boolean
          A Python boolean is converted to the boolean type from the
          current language.

     Python integer
          A Python integer is converted to the C 'long' type for the
          current architecture.

     Python long
          A Python long is converted to the C 'long long' type for the
          current architecture.

     Python float
          A Python float is converted to the C 'double' type for the
          current architecture.

     Python string
          A Python string is converted to a target string in the current
          target language using the current target encoding.  If a
          character cannot be represented in the current target
          encoding, then an exception is thrown.

     'gdb.Value'
          If 'val' is a 'gdb.Value', then a copy of the value is made.

     'gdb.LazyString'
          If 'val' is a 'gdb.LazyString' (*note Lazy Strings In
          Python::), then the lazy string's 'value' method is called,
          and its result is used.

 -- Function: Value.__init__ (VAL, TYPE)
     This second form of the 'gdb.Value' constructor returns a
     'gdb.Value' of type TYPE where the value contents are taken from
     the Python buffer object specified by VAL.  The number of bytes in
     the Python buffer object must be greater than or equal to the size
     of TYPE.

 -- Function: Value.cast (type)
     Return a new instance of 'gdb.Value' that is the result of casting
     this instance to the type described by TYPE, which must be a
     'gdb.Type' object.  If the cast cannot be performed for some
     reason, this method throws an exception.

 -- Function: Value.dereference ()
     For pointer data types, this method returns a new 'gdb.Value'
     object whose contents is the object pointed to by the pointer.  For
     example, if 'foo' is a C pointer to an 'int', declared in your C
     program as

          int *foo;

     then you can use the corresponding 'gdb.Value' to access what 'foo'
     points to like this:

          bar = foo.dereference ()

     The result 'bar' will be a 'gdb.Value' object holding the value
     pointed to by 'foo'.

     A similar function 'Value.referenced_value' exists which also
     returns 'gdb.Value' objects corresponding to the values pointed to
     by pointer values (and additionally, values referenced by reference
     values).  However, the behavior of 'Value.dereference' differs from
     'Value.referenced_value' by the fact that the behavior of
     'Value.dereference' is identical to applying the C unary operator
     '*' on a given value.  For example, consider a reference to a
     pointer 'ptrref', declared in your C++ program as

          typedef int *intptr;
          ...
          int val = 10;
          intptr ptr = &val;
          intptr &ptrref = ptr;

     Though 'ptrref' is a reference value, one can apply the method
     'Value.dereference' to the 'gdb.Value' object corresponding to it
     and obtain a 'gdb.Value' which is identical to that corresponding
     to 'val'.  However, if you apply the method
     'Value.referenced_value', the result would be a 'gdb.Value' object
     identical to that corresponding to 'ptr'.

          py_ptrref = gdb.parse_and_eval ("ptrref")
          py_val = py_ptrref.dereference ()
          py_ptr = py_ptrref.referenced_value ()

     The 'gdb.Value' object 'py_val' is identical to that corresponding
     to 'val', and 'py_ptr' is identical to that corresponding to 'ptr'.
     In general, 'Value.dereference' can be applied whenever the C unary
     operator '*' can be applied to the corresponding C value.  For
     those cases where applying both 'Value.dereference' and
     'Value.referenced_value' is allowed, the results obtained need not
     be identical (as we have seen in the above example).  The results
     are however identical when applied on 'gdb.Value' objects
     corresponding to pointers ('gdb.Value' objects with type code
     'TYPE_CODE_PTR') in a C/C++ program.

 -- Function: Value.referenced_value ()
     For pointer or reference data types, this method returns a new
     'gdb.Value' object corresponding to the value referenced by the
     pointer/reference value.  For pointer data types,
     'Value.dereference' and 'Value.referenced_value' produce identical
     results.  The difference between these methods is that
     'Value.dereference' cannot get the values referenced by reference
     values.  For example, consider a reference to an 'int', declared in
     your C++ program as

          int val = 10;
          int &ref = val;

     then applying 'Value.dereference' to the 'gdb.Value' object
     corresponding to 'ref' will result in an error, while applying
     'Value.referenced_value' will result in a 'gdb.Value' object
     identical to that corresponding to 'val'.

          py_ref = gdb.parse_and_eval ("ref")
          er_ref = py_ref.dereference ()       # Results in error
          py_val = py_ref.referenced_value ()  # Returns the referenced value

     The 'gdb.Value' object 'py_val' is identical to that corresponding
     to 'val'.

 -- Function: Value.reference_value ()
     Return a 'gdb.Value' object which is a reference to the value
     encapsulated by this instance.

 -- Function: Value.const_value ()
     Return a 'gdb.Value' object which is a 'const' version of the value
     encapsulated by this instance.

 -- Function: Value.dynamic_cast (type)
     Like 'Value.cast', but works as if the C++ 'dynamic_cast' operator
     were used.  Consult a C++ reference for details.

 -- Function: Value.reinterpret_cast (type)
     Like 'Value.cast', but works as if the C++ 'reinterpret_cast'
     operator were used.  Consult a C++ reference for details.

 -- Function: Value.format_string (...)
     Convert a 'gdb.Value' to a string, similarly to what the 'print'
     command does.  Invoked with no arguments, this is equivalent to
     calling the 'str' function on the 'gdb.Value'.  The representation
     of the same value may change across different versions of GDB, so
     you shouldn't, for instance, parse the strings returned by this
     method.

     All the arguments are keyword only.  If an argument is not
     specified, the current global default setting is used.

     'raw'
          'True' if pretty-printers (*note Pretty Printing::) should not
          be used to format the value.  'False' if enabled
          pretty-printers matching the type represented by the
          'gdb.Value' should be used to format it.

     'pretty_arrays'
          'True' if arrays should be pretty printed to be more
          convenient to read, 'False' if they shouldn't (see 'set print
          array' in *note Print Settings::).

     'pretty_structs'
          'True' if structs should be pretty printed to be more
          convenient to read, 'False' if they shouldn't (see 'set print
          pretty' in *note Print Settings::).

     'array_indexes'
          'True' if array indexes should be included in the string
          representation of arrays, 'False' if they shouldn't (see 'set
          print array-indexes' in *note Print Settings::).

     'symbols'
          'True' if the string representation of a pointer should
          include the corresponding symbol name (if one exists), 'False'
          if it shouldn't (see 'set print symbol' in *note Print
          Settings::).

     'unions'
          'True' if unions which are contained in other structures or
          unions should be expanded, 'False' if they shouldn't (see 'set
          print union' in *note Print Settings::).

     'deref_refs'
          'True' if C++ references should be resolved to the value they
          refer to, 'False' (the default) if they shouldn't.  Note that,
          unlike for the 'print' command, references are not
          automatically expanded when using the 'format_string' method
          or the 'str' function.  There is no global 'print' setting to
          change the default behaviour.

     'actual_objects'
          'True' if the representation of a pointer to an object should
          identify the _actual_ (derived) type of the object rather than
          the _declared_ type, using the virtual function table.
          'False' if the _declared_ type should be used.  (See 'set
          print object' in *note Print Settings::).

     'static_fields'
          'True' if static members should be included in the string
          representation of a C++ object, 'False' if they shouldn't (see
          'set print static-members' in *note Print Settings::).

     'max_elements'
          Number of array elements to print, or '0' to print an
          unlimited number of elements (see 'set print elements' in
          *note Print Settings::).

     'max_depth'
          The maximum depth to print for nested structs and unions, or
          '-1' to print an unlimited number of elements (see 'set print
          max-depth' in *note Print Settings::).

     'repeat_threshold'
          Set the threshold for suppressing display of repeated array
          elements, or '0' to represent all elements, even if repeated.
          (See 'set print repeats' in *note Print Settings::).

     'format'
          A string containing a single character representing the format
          to use for the returned string.  For instance, ''x'' is
          equivalent to using the GDB command 'print' with the '/x'
          option and formats the value as a hexadecimal number.

 -- Function: Value.string ([encoding[, errors[, length]]])
     If this 'gdb.Value' represents a string, then this method converts
     the contents to a Python string.  Otherwise, this method will throw
     an exception.

     Values are interpreted as strings according to the rules of the
     current language.  If the optional length argument is given, the
     string will be converted to that length, and will include any
     embedded zeroes that the string may contain.  Otherwise, for
     languages where the string is zero-terminated, the entire string
     will be converted.

     For example, in C-like languages, a value is a string if it is a
     pointer to or an array of characters or ints of type 'wchar_t',
     'char16_t', or 'char32_t'.

     If the optional ENCODING argument is given, it must be a string
     naming the encoding of the string in the 'gdb.Value', such as
     '"ascii"', '"iso-8859-6"' or '"utf-8"'.  It accepts the same
     encodings as the corresponding argument to Python's 'string.decode'
     method, and the Python codec machinery will be used to convert the
     string.  If ENCODING is not given, or if ENCODING is the empty
     string, then either the 'target-charset' (*note Character Sets::)
     will be used, or a language-specific encoding will be used, if the
     current language is able to supply one.

     The optional ERRORS argument is the same as the corresponding
     argument to Python's 'string.decode' method.

     If the optional LENGTH argument is given, the string will be
     fetched and converted to the given length.

 -- Function: Value.lazy_string ([encoding [, length]])
     If this 'gdb.Value' represents a string, then this method converts
     the contents to a 'gdb.LazyString' (*note Lazy Strings In
     Python::).  Otherwise, this method will throw an exception.

     If the optional ENCODING argument is given, it must be a string
     naming the encoding of the 'gdb.LazyString'.  Some examples are:
     'ascii', 'iso-8859-6' or 'utf-8'.  If the ENCODING argument is an
     encoding that GDB does recognize, GDB will raise an error.

     When a lazy string is printed, the GDB encoding machinery is used
     to convert the string during printing.  If the optional ENCODING
     argument is not provided, or is an empty string, GDB will
     automatically select the encoding most suitable for the string
     type.  For further information on encoding in GDB please see *note
     Character Sets::.

     If the optional LENGTH argument is given, the string will be
     fetched and encoded to the length of characters specified.  If the
     LENGTH argument is not provided, the string will be fetched and
     encoded until a null of appropriate width is found.

 -- Function: Value.fetch_lazy ()
     If the 'gdb.Value' object is currently a lazy value
     ('gdb.Value.is_lazy' is 'True'), then the value is fetched from the
     inferior.  Any errors that occur in the process will produce a
     Python exception.

     If the 'gdb.Value' object is not a lazy value, this method has no
     effect.

     This method does not return a value.


File: gdb.info,  Node: Types In Python,  Next: Pretty Printing API,  Prev: Values From Inferior,  Up: Python API

23.2.2.4 Types In Python
........................

GDB represents types from the inferior using the class 'gdb.Type'.

   The following type-related functions are available in the 'gdb'
module:

 -- Function: gdb.lookup_type (name [, block])
     This function looks up a type by its NAME, which must be a string.

     If BLOCK is given, then NAME is looked up in that scope.
     Otherwise, it is searched for globally.

     Ordinarily, this function will return an instance of 'gdb.Type'.
     If the named type cannot be found, it will throw an exception.

   If the type is a structure or class type, or an enum type, the fields
of that type can be accessed using the Python "dictionary syntax".  For
example, if 'some_type' is a 'gdb.Type' instance holding a structure
type, you can access its 'foo' field with:

     bar = some_type['foo']

   'bar' will be a 'gdb.Field' object; see below under the description
of the 'Type.fields' method for a description of the 'gdb.Field' class.

   An instance of 'Type' has the following attributes:

 -- Variable: Type.alignof
     The alignment of this type, in bytes.  Type alignment comes from
     the debugging information; if it was not specified, then GDB will
     use the relevant ABI to try to determine the alignment.  In some
     cases, even this is not possible, and zero will be returned.

 -- Variable: Type.code
     The type code for this type.  The type code will be one of the
     'TYPE_CODE_' constants defined below.

 -- Variable: Type.name
     The name of this type.  If this type has no name, then 'None' is
     returned.

 -- Variable: Type.sizeof
     The size of this type, in target 'char' units.  Usually, a target's
     'char' type will be an 8-bit byte.  However, on some unusual
     platforms, this type may have a different size.

 -- Variable: Type.tag
     The tag name for this type.  The tag name is the name after
     'struct', 'union', or 'enum' in C and C++; not all languages have
     this concept.  If this type has no tag name, then 'None' is
     returned.

 -- Variable: Type.objfile
     The 'gdb.Objfile' that this type was defined in, or 'None' if there
     is no associated objfile.

   The following methods are provided:

 -- Function: Type.fields ()
     For structure and union types, this method returns the fields.
     Range types have two fields, the minimum and maximum values.  Enum
     types have one field per enum constant.  Function and method types
     have one field per parameter.  The base types of C++ classes are
     also represented as fields.  If the type has no fields, or does not
     fit into one of these categories, an empty sequence will be
     returned.

     Each field is a 'gdb.Field' object, with some pre-defined
     attributes:
     'bitpos'
          This attribute is not available for 'enum' or 'static' (as in
          C++) fields.  The value is the position, counting in bits,
          from the start of the containing type.

     'enumval'
          This attribute is only available for 'enum' fields, and its
          value is the enumeration member's integer representation.

     'name'
          The name of the field, or 'None' for anonymous fields.

     'artificial'
          This is 'True' if the field is artificial, usually meaning
          that it was provided by the compiler and not the user.  This
          attribute is always provided, and is 'False' if the field is
          not artificial.

     'is_base_class'
          This is 'True' if the field represents a base class of a C++
          structure.  This attribute is always provided, and is 'False'
          if the field is not a base class of the type that is the
          argument of 'fields', or if that type was not a C++ class.

     'bitsize'
          If the field is packed, or is a bitfield, then this will have
          a non-zero value, which is the size of the field in bits.
          Otherwise, this will be zero; in this case the field's size is
          given by its type.

     'type'
          The type of the field.  This is usually an instance of 'Type',
          but it can be 'None' in some situations.

     'parent_type'
          The type which contains this field.  This is an instance of
          'gdb.Type'.

 -- Function: Type.array (N1 [, N2])
     Return a new 'gdb.Type' object which represents an array of this
     type.  If one argument is given, it is the inclusive upper bound of
     the array; in this case the lower bound is zero.  If two arguments
     are given, the first argument is the lower bound of the array, and
     the second argument is the upper bound of the array.  An array's
     length must not be negative, but the bounds can be.

 -- Function: Type.vector (N1 [, N2])
     Return a new 'gdb.Type' object which represents a vector of this
     type.  If one argument is given, it is the inclusive upper bound of
     the vector; in this case the lower bound is zero.  If two arguments
     are given, the first argument is the lower bound of the vector, and
     the second argument is the upper bound of the vector.  A vector's
     length must not be negative, but the bounds can be.

     The difference between an 'array' and a 'vector' is that arrays
     behave like in C: when used in expressions they decay to a pointer
     to the first element whereas vectors are treated as first class
     values.

 -- Function: Type.const ()
     Return a new 'gdb.Type' object which represents a 'const'-qualified
     variant of this type.

 -- Function: Type.volatile ()
     Return a new 'gdb.Type' object which represents a
     'volatile'-qualified variant of this type.

 -- Function: Type.unqualified ()
     Return a new 'gdb.Type' object which represents an unqualified
     variant of this type.  That is, the result is neither 'const' nor
     'volatile'.

 -- Function: Type.range ()
     Return a Python 'Tuple' object that contains two elements: the low
     bound of the argument type and the high bound of that type.  If the
     type does not have a range, GDB will raise a 'gdb.error' exception
     (*note Exception Handling::).

 -- Function: Type.reference ()
     Return a new 'gdb.Type' object which represents a reference to this
     type.

 -- Function: Type.pointer ()
     Return a new 'gdb.Type' object which represents a pointer to this
     type.

 -- Function: Type.strip_typedefs ()
     Return a new 'gdb.Type' that represents the real type, after
     removing all layers of typedefs.

 -- Function: Type.target ()
     Return a new 'gdb.Type' object which represents the target type of
     this type.

     For a pointer type, the target type is the type of the pointed-to
     object.  For an array type (meaning C-like arrays), the target type
     is the type of the elements of the array.  For a function or method
     type, the target type is the type of the return value.  For a
     complex type, the target type is the type of the elements.  For a
     typedef, the target type is the aliased type.

     If the type does not have a target, this method will throw an
     exception.

 -- Function: Type.template_argument (n [, block])
     If this 'gdb.Type' is an instantiation of a template, this will
     return a new 'gdb.Value' or 'gdb.Type' which represents the value
     of the Nth template argument (indexed starting at 0).

     If this 'gdb.Type' is not a template type, or if the type has fewer
     than N template arguments, this will throw an exception.
     Ordinarily, only C++ code will have template types.

     If BLOCK is given, then NAME is looked up in that scope.
     Otherwise, it is searched for globally.

 -- Function: Type.optimized_out ()
     Return 'gdb.Value' instance of this type whose value is optimized
     out.  This allows a frame decorator to indicate that the value of
     an argument or a local variable is not known.

   Each type has a code, which indicates what category this type falls
into.  The available type categories are represented by constants
defined in the 'gdb' module:

'gdb.TYPE_CODE_PTR'
     The type is a pointer.

'gdb.TYPE_CODE_ARRAY'
     The type is an array.

'gdb.TYPE_CODE_STRUCT'
     The type is a structure.

'gdb.TYPE_CODE_UNION'
     The type is a union.

'gdb.TYPE_CODE_ENUM'
     The type is an enum.

'gdb.TYPE_CODE_FLAGS'
     A bit flags type, used for things such as status registers.

'gdb.TYPE_CODE_FUNC'
     The type is a function.

'gdb.TYPE_CODE_INT'
     The type is an integer type.

'gdb.TYPE_CODE_FLT'
     A floating point type.

'gdb.TYPE_CODE_VOID'
     The special type 'void'.

'gdb.TYPE_CODE_SET'
     A Pascal set type.

'gdb.TYPE_CODE_RANGE'
     A range type, that is, an integer type with bounds.

'gdb.TYPE_CODE_STRING'
     A string type.  Note that this is only used for certain languages
     with language-defined string types; C strings are not represented
     this way.

'gdb.TYPE_CODE_BITSTRING'
     A string of bits.  It is deprecated.

'gdb.TYPE_CODE_ERROR'
     An unknown or erroneous type.

'gdb.TYPE_CODE_METHOD'
     A method type, as found in C++.

'gdb.TYPE_CODE_METHODPTR'
     A pointer-to-member-function.

'gdb.TYPE_CODE_MEMBERPTR'
     A pointer-to-member.

'gdb.TYPE_CODE_REF'
     A reference type.

'gdb.TYPE_CODE_RVALUE_REF'
     A C++11 rvalue reference type.

'gdb.TYPE_CODE_CHAR'
     A character type.

'gdb.TYPE_CODE_BOOL'
     A boolean type.

'gdb.TYPE_CODE_COMPLEX'
     A complex float type.

'gdb.TYPE_CODE_TYPEDEF'
     A typedef to some other type.

'gdb.TYPE_CODE_NAMESPACE'
     A C++ namespace.

'gdb.TYPE_CODE_DECFLOAT'
     A decimal floating point type.

'gdb.TYPE_CODE_INTERNAL_FUNCTION'
     A function internal to GDB.  This is the type used to represent
     convenience functions.

   Further support for types is provided in the 'gdb.types' Python
module (*note gdb.types::).


File: gdb.info,  Node: Pretty Printing API,  Next: Selecting Pretty-Printers,  Prev: Types In Python,  Up: Python API

23.2.2.5 Pretty Printing API
............................

A pretty-printer is just an object that holds a value and implements a
specific interface, defined here.  An example output is provided (*note
Pretty Printing::).

 -- Function: pretty_printer.children (self)
     GDB will call this method on a pretty-printer to compute the
     children of the pretty-printer's value.

     This method must return an object conforming to the Python iterator
     protocol.  Each item returned by the iterator must be a tuple
     holding two elements.  The first element is the "name" of the
     child; the second element is the child's value.  The value can be
     any Python object which is convertible to a GDB value.

     This method is optional.  If it does not exist, GDB will act as
     though the value has no children.

     For efficiency, the 'children' method should lazily compute its
     results.  This will let GDB read as few elements as necessary, for
     example when various print settings (*note Print Settings::) or
     '-var-list-children' (*note GDB/MI Variable Objects::) limit the
     number of elements to be displayed.

     Children may be hidden from display based on the value of 'set
     print max-depth' (*note Print Settings::).

 -- Function: pretty_printer.display_hint (self)
     The CLI may call this method and use its result to change the
     formatting of a value.  The result will also be supplied to an MI
     consumer as a 'displayhint' attribute of the variable being
     printed.

     This method is optional.  If it does exist, this method must return
     a string or the special value 'None'.

     Some display hints are predefined by GDB:

     'array'
          Indicate that the object being printed is "array-like".  The
          CLI uses this to respect parameters such as 'set print
          elements' and 'set print array'.

     'map'
          Indicate that the object being printed is "map-like", and that
          the children of this value can be assumed to alternate between
          keys and values.

     'string'
          Indicate that the object being printed is "string-like".  If
          the printer's 'to_string' method returns a Python string of
          some kind, then GDB will call its internal language-specific
          string-printing function to format the string.  For the CLI
          this means adding quotation marks, possibly escaping some
          characters, respecting 'set print elements', and the like.

     The special value 'None' causes GDB to apply the default display
     rules.

 -- Function: pretty_printer.to_string (self)
     GDB will call this method to display the string representation of
     the value passed to the object's constructor.

     When printing from the CLI, if the 'to_string' method exists, then
     GDB will prepend its result to the values returned by 'children'.
     Exactly how this formatting is done is dependent on the display
     hint, and may change as more hints are added.  Also, depending on
     the print settings (*note Print Settings::), the CLI may print just
     the result of 'to_string' in a stack trace, omitting the result of
     'children'.

     If this method returns a string, it is printed verbatim.

     Otherwise, if this method returns an instance of 'gdb.Value', then
     GDB prints this value.  This may result in a call to another
     pretty-printer.

     If instead the method returns a Python value which is convertible
     to a 'gdb.Value', then GDB performs the conversion and prints the
     resulting value.  Again, this may result in a call to another
     pretty-printer.  Python scalars (integers, floats, and booleans)
     and strings are convertible to 'gdb.Value'; other types are not.

     Finally, if this method returns 'None' then no further operations
     are peformed in this method and nothing is printed.

     If the result is not one of these types, an exception is raised.

   GDB provides a function which can be used to look up the default
pretty-printer for a 'gdb.Value':

 -- Function: gdb.default_visualizer (value)
     This function takes a 'gdb.Value' object as an argument.  If a
     pretty-printer for this value exists, then it is returned.  If no
     such printer exists, then this returns 'None'.


File: gdb.info,  Node: Selecting Pretty-Printers,  Next: Writing a Pretty-Printer,  Prev: Pretty Printing API,  Up: Python API

23.2.2.6 Selecting Pretty-Printers
..................................

GDB provides several ways to register a pretty-printer: globally, per
program space, and per objfile.  When choosing how to register your
pretty-printer, a good rule is to register it with the smallest scope
possible: that is prefer a specific objfile first, then a program space,
and only register a printer globally as a last resort.

 -- Variable: gdb.pretty_printers
     The Python list 'gdb.pretty_printers' contains an array of
     functions or callable objects that have been registered via
     addition as a pretty-printer.  Printers in this list are called
     'global' printers, they're available when debugging all inferiors.

   Each 'gdb.Progspace' contains a 'pretty_printers' attribute.  Each
'gdb.Objfile' also contains a 'pretty_printers' attribute.

   Each function on these lists is passed a single 'gdb.Value' argument
and should return a pretty-printer object conforming to the interface
definition above (*note Pretty Printing API::).  If a function cannot
create a pretty-printer for the value, it should return 'None'.

   GDB first checks the 'pretty_printers' attribute of each
'gdb.Objfile' in the current program space and iteratively calls each
enabled lookup routine in the list for that 'gdb.Objfile' until it
receives a pretty-printer object.  If no pretty-printer is found in the
objfile lists, GDB then searches the pretty-printer list of the current
program space, calling each enabled function until an object is
returned.  After these lists have been exhausted, it tries the global
'gdb.pretty_printers' list, again calling each enabled function until an
object is returned.

   The order in which the objfiles are searched is not specified.  For a
given list, functions are always invoked from the head of the list, and
iterated over sequentially until the end of the list, or a printer
object is returned.

   For various reasons a pretty-printer may not work.  For example, the
underlying data structure may have changed and the pretty-printer is out
of date.

   The consequences of a broken pretty-printer are severe enough that
GDB provides support for enabling and disabling individual printers.
For example, if 'print frame-arguments' is on, a backtrace can become
highly illegible if any argument is printed with a broken printer.

   Pretty-printers are enabled and disabled by attaching an 'enabled'
attribute to the registered function or callable object.  If this
attribute is present and its value is 'False', the printer is disabled,
otherwise the printer is enabled.


File: gdb.info,  Node: Writing a Pretty-Printer,  Next: Type Printing API,  Prev: Selecting Pretty-Printers,  Up: Python API

23.2.2.7 Writing a Pretty-Printer
.................................

A pretty-printer consists of two parts: a lookup function to detect if
the type is supported, and the printer itself.

   Here is an example showing how a 'std::string' printer might be
written.  *Note Pretty Printing API::, for details on the API this class
must provide.

     class StdStringPrinter(object):
         "Print a std::string"

         def __init__(self, val):
             self.val = val

         def to_string(self):
             return self.val['_M_dataplus']['_M_p']

         def display_hint(self):
             return 'string'

   And here is an example showing how a lookup function for the printer
example above might be written.

     def str_lookup_function(val):
         lookup_tag = val.type.tag
         if lookup_tag == None:
             return None
         regex = re.compile("^std::basic_string<char,.*>$")
         if regex.match(lookup_tag):
             return StdStringPrinter(val)
         return None

   The example lookup function extracts the value's type, and attempts
to match it to a type that it can pretty-print.  If it is a type the
printer can pretty-print, it will return a printer object.  If not, it
returns 'None'.

   We recommend that you put your core pretty-printers into a Python
package.  If your pretty-printers are for use with a library, we further
recommend embedding a version number into the package name.  This
practice will enable GDB to load multiple versions of your
pretty-printers at the same time, because they will have different
names.

   You should write auto-loaded code (*note Python Auto-loading::) such
that it can be evaluated multiple times without changing its meaning.
An ideal auto-load file will consist solely of 'import's of your printer
modules, followed by a call to a register pretty-printers with the
current objfile.

   Taken as a whole, this approach will scale nicely to multiple
inferiors, each potentially using a different library version.
Embedding a version number in the Python package name will ensure that
GDB is able to load both sets of printers simultaneously.  Then, because
the search for pretty-printers is done by objfile, and because your
auto-loaded code took care to register your library's printers with a
specific objfile, GDB will find the correct printers for the specific
version of the library used by each inferior.

   To continue the 'std::string' example (*note Pretty Printing API::),
this code might appear in 'gdb.libstdcxx.v6':

     def register_printers(objfile):
         objfile.pretty_printers.append(str_lookup_function)

And then the corresponding contents of the auto-load file would be:

     import gdb.libstdcxx.v6
     gdb.libstdcxx.v6.register_printers(gdb.current_objfile())

   The previous example illustrates a basic pretty-printer.  There are a
few things that can be improved on.  The printer doesn't have a name,
making it hard to identify in a list of installed printers.  The lookup
function has a name, but lookup functions can have arbitrary, even
identical, names.

   Second, the printer only handles one type, whereas a library
typically has several types.  One could install a lookup function for
each desired type in the library, but one could also have a single
lookup function recognize several types.  The latter is the conventional
way this is handled.  If a pretty-printer can handle multiple data
types, then its "subprinters" are the printers for the individual data
types.

   The 'gdb.printing' module provides a formal way of solving these
problems (*note gdb.printing::).  Here is another example that handles
multiple types.

   These are the types we are going to pretty-print:

     struct foo { int a, b; };
     struct bar { struct foo x, y; };

   Here are the printers:

     class fooPrinter:
         """Print a foo object."""

         def __init__(self, val):
             self.val = val

         def to_string(self):
             return ("a=<" + str(self.val["a"]) +
                     "> b=<" + str(self.val["b"]) + ">")

     class barPrinter:
         """Print a bar object."""

         def __init__(self, val):
             self.val = val

         def to_string(self):
             return ("x=<" + str(self.val["x"]) +
                     "> y=<" + str(self.val["y"]) + ">")

   This example doesn't need a lookup function, that is handled by the
'gdb.printing' module.  Instead a function is provided to build up the
object that handles the lookup.

     import gdb.printing

     def build_pretty_printer():
         pp = gdb.printing.RegexpCollectionPrettyPrinter(
             "my_library")
         pp.add_printer('foo', '^foo$', fooPrinter)
         pp.add_printer('bar', '^bar$', barPrinter)
         return pp

   And here is the autoload support:

     import gdb.printing
     import my_library
     gdb.printing.register_pretty_printer(
         gdb.current_objfile(),
         my_library.build_pretty_printer())

   Finally, when this printer is loaded into GDB, here is the
corresponding output of 'info pretty-printer':

     (gdb) info pretty-printer
     my_library.so:
       my_library
         foo
         bar


File: gdb.info,  Node: Type Printing API,  Next: Frame Filter API,  Prev: Writing a Pretty-Printer,  Up: Python API

23.2.2.8 Type Printing API
..........................

GDB provides a way for Python code to customize type display.  This is
mainly useful for substituting canonical typedef names for types.

   A "type printer" is just a Python object conforming to a certain
protocol.  A simple base class implementing the protocol is provided;
see *note gdb.types::.  A type printer must supply at least:

 -- Instance Variable of type_printer: enabled
     A boolean which is True if the printer is enabled, and False
     otherwise.  This is manipulated by the 'enable type-printer' and
     'disable type-printer' commands.

 -- Instance Variable of type_printer: name
     The name of the type printer.  This must be a string.  This is used
     by the 'enable type-printer' and 'disable type-printer' commands.

 -- Method on type_printer: instantiate (self)
     This is called by GDB at the start of type-printing.  It is only
     called if the type printer is enabled.  This method must return a
     new object that supplies a 'recognize' method, as described below.

   When displaying a type, say via the 'ptype' command, GDB will compute
a list of type recognizers.  This is done by iterating first over the
per-objfile type printers (*note Objfiles In Python::), followed by the
per-progspace type printers (*note Progspaces In Python::), and finally
the global type printers.

   GDB will call the 'instantiate' method of each enabled type printer.
If this method returns 'None', then the result is ignored; otherwise, it
is appended to the list of recognizers.

   Then, when GDB is going to display a type name, it iterates over the
list of recognizers.  For each one, it calls the recognition function,
stopping if the function returns a non-'None' value.  The recognition
function is defined as:

 -- Method on type_recognizer: recognize (self, type)
     If TYPE is not recognized, return 'None'.  Otherwise, return a
     string which is to be printed as the name of TYPE.  The TYPE
     argument will be an instance of 'gdb.Type' (*note Types In
     Python::).

   GDB uses this two-pass approach so that type printers can efficiently
cache information without holding on to it too long.  For example, it
can be convenient to look up type information in a type printer and hold
it for a recognizer's lifetime; if a single pass were done then type
printers would have to make use of the event system in order to avoid
holding information that could become stale as the inferior changed.


File: gdb.info,  Node: Frame Filter API,  Next: Frame Decorator API,  Prev: Type Printing API,  Up: Python API

23.2.2.9 Filtering Frames
.........................

Frame filters are Python objects that manipulate the visibility of a
frame or frames when a backtrace (*note Backtrace::) is printed by GDB.

   Only commands that print a backtrace, or, in the case of GDB/MI
commands (*note GDB/MI::), those that return a collection of frames are
affected.  The commands that work with frame filters are:

   'backtrace' (*note The backtrace command: backtrace-command.),
'-stack-list-frames' (*note The -stack-list-frames command:
-stack-list-frames.), '-stack-list-variables' (*note The
-stack-list-variables command: -stack-list-variables.),
'-stack-list-arguments' *note The -stack-list-arguments command:
-stack-list-arguments.) and '-stack-list-locals' (*note The
-stack-list-locals command: -stack-list-locals.).

   A frame filter works by taking an iterator as an argument, applying
actions to the contents of that iterator, and returning another iterator
(or, possibly, the same iterator it was provided in the case where the
filter does not perform any operations).  Typically, frame filters
utilize tools such as the Python's 'itertools' module to work with and
create new iterators from the source iterator.  Regardless of how a
filter chooses to apply actions, it must not alter the underlying GDB
frame or frames, or attempt to alter the call-stack within GDB.  This
preserves data integrity within GDB.  Frame filters are executed on a
priority basis and care should be taken that some frame filters may have
been executed before, and that some frame filters will be executed
after.

   An important consideration when designing frame filters, and well
worth reflecting upon, is that frame filters should avoid unwinding the
call stack if possible.  Some stacks can run very deep, into the tens of
thousands in some cases.  To search every frame when a frame filter
executes may be too expensive at that step.  The frame filter cannot
know how many frames it has to iterate over, and it may have to iterate
through them all.  This ends up duplicating effort as GDB performs this
iteration when it prints the frames.  If the filter can defer unwinding
frames until frame decorators are executed, after the last filter has
executed, it should.  *Note Frame Decorator API::, for more information
on decorators.  Also, there are examples for both frame decorators and
filters in later chapters.  *Note Writing a Frame Filter::, for more
information.

   The Python dictionary 'gdb.frame_filters' contains key/object
pairings that comprise a frame filter.  Frame filters in this dictionary
are called 'global' frame filters, and they are available when debugging
all inferiors.  These frame filters must register with the dictionary
directly.  In addition to the 'global' dictionary, there are other
dictionaries that are loaded with different inferiors via auto-loading
(*note Python Auto-loading::).  The two other areas where frame filter
dictionaries can be found are: 'gdb.Progspace' which contains a
'frame_filters' dictionary attribute, and each 'gdb.Objfile' object
which also contains a 'frame_filters' dictionary attribute.

   When a command is executed from GDB that is compatible with frame
filters, GDB combines the 'global', 'gdb.Progspace' and all
'gdb.Objfile' dictionaries currently loaded.  All of the 'gdb.Objfile'
dictionaries are combined, as several frames, and thus several object
files, might be in use.  GDB then prunes any frame filter whose
'enabled' attribute is 'False'.  This pruned list is then sorted
according to the 'priority' attribute in each filter.

   Once the dictionaries are combined, pruned and sorted, GDB creates an
iterator which wraps each frame in the call stack in a 'FrameDecorator'
object, and calls each filter in order.  The output from the previous
filter will always be the input to the next filter, and so on.

   Frame filters have a mandatory interface which each frame filter must
implement, defined here:

 -- Function: FrameFilter.filter (iterator)
     GDB will call this method on a frame filter when it has reached the
     order in the priority list for that filter.

     For example, if there are four frame filters:

          Name         Priority

          Filter1      5
          Filter2      10
          Filter3      100
          Filter4      1

     The order that the frame filters will be called is:

          Filter3 -> Filter2 -> Filter1 -> Filter4

     Note that the output from 'Filter3' is passed to the input of
     'Filter2', and so on.

     This 'filter' method is passed a Python iterator.  This iterator
     contains a sequence of frame decorators that wrap each 'gdb.Frame',
     or a frame decorator that wraps another frame decorator.  The first
     filter that is executed in the sequence of frame filters will
     receive an iterator entirely comprised of default 'FrameDecorator'
     objects.  However, after each frame filter is executed, the
     previous frame filter may have wrapped some or all of the frame
     decorators with their own frame decorator.  As frame decorators
     must also conform to a mandatory interface, these decorators can be
     assumed to act in a uniform manner (*note Frame Decorator API::).

     This method must return an object conforming to the Python iterator
     protocol.  Each item in the iterator must be an object conforming
     to the frame decorator interface.  If a frame filter does not wish
     to perform any operations on this iterator, it should return that
     iterator untouched.

     This method is not optional.  If it does not exist, GDB will raise
     and print an error.

 -- Variable: FrameFilter.name
     The 'name' attribute must be Python string which contains the name
     of the filter displayed by GDB (*note Frame Filter Management::).
     This attribute may contain any combination of letters or numbers.
     Care should be taken to ensure that it is unique.  This attribute
     is mandatory.

 -- Variable: FrameFilter.enabled
     The 'enabled' attribute must be Python boolean.  This attribute
     indicates to GDB whether the frame filter is enabled, and should be
     considered when frame filters are executed.  If 'enabled' is
     'True', then the frame filter will be executed when any of the
     backtrace commands detailed earlier in this chapter are executed.
     If 'enabled' is 'False', then the frame filter will not be
     executed.  This attribute is mandatory.

 -- Variable: FrameFilter.priority
     The 'priority' attribute must be Python integer.  This attribute
     controls the order of execution in relation to other frame filters.
     There are no imposed limits on the range of 'priority' other than
     it must be a valid integer.  The higher the 'priority' attribute,
     the sooner the frame filter will be executed in relation to other
     frame filters.  Although 'priority' can be negative, it is
     recommended practice to assume zero is the lowest priority that a
     frame filter can be assigned.  Frame filters that have the same
     priority are executed in unsorted order in that priority slot.
     This attribute is mandatory.  100 is a good default priority.


File: gdb.info,  Node: Frame Decorator API,  Next: Writing a Frame Filter,  Prev: Frame Filter API,  Up: Python API

23.2.2.10 Decorating Frames
...........................

Frame decorators are sister objects to frame filters (*note Frame Filter
API::).  Frame decorators are applied by a frame filter and can only be
used in conjunction with frame filters.

   The purpose of a frame decorator is to customize the printed content
of each 'gdb.Frame' in commands where frame filters are executed.  This
concept is called decorating a frame.  Frame decorators decorate a
'gdb.Frame' with Python code contained within each API call.  This
separates the actual data contained in a 'gdb.Frame' from the decorated
data produced by a frame decorator.  This abstraction is necessary to
maintain integrity of the data contained in each 'gdb.Frame'.

   Frame decorators have a mandatory interface, defined below.

   GDB already contains a frame decorator called 'FrameDecorator'.  This
contains substantial amounts of boilerplate code to decorate the content
of a 'gdb.Frame'.  It is recommended that other frame decorators inherit
and extend this object, and only to override the methods needed.

   'FrameDecorator' is defined in the Python module
'gdb.FrameDecorator', so your code can import it like:
     from gdb.FrameDecorator import FrameDecorator

 -- Function: FrameDecorator.elided (self)

     The 'elided' method groups frames together in a hierarchical
     system.  An example would be an interpreter, where multiple
     low-level frames make up a single call in the interpreted language.
     In this example, the frame filter would elide the low-level frames
     and present a single high-level frame, representing the call in the
     interpreted language, to the user.

     The 'elided' function must return an iterable and this iterable
     must contain the frames that are being elided wrapped in a suitable
     frame decorator.  If no frames are being elided this function may
     return an empty iterable, or 'None'.  Elided frames are indented
     from normal frames in a 'CLI' backtrace, or in the case of
     'GDB/MI', are placed in the 'children' field of the eliding frame.

     It is the frame filter's task to also filter out the elided frames
     from the source iterator.  This will avoid printing the frame
     twice.

 -- Function: FrameDecorator.function (self)

     This method returns the name of the function in the frame that is
     to be printed.

     This method must return a Python string describing the function, or
     'None'.

     If this function returns 'None', GDB will not print any data for
     this field.

 -- Function: FrameDecorator.address (self)

     This method returns the address of the frame that is to be printed.

     This method must return a Python numeric integer type of sufficient
     size to describe the address of the frame, or 'None'.

     If this function returns a 'None', GDB will not print any data for
     this field.

 -- Function: FrameDecorator.filename (self)

     This method returns the filename and path associated with this
     frame.

     This method must return a Python string containing the filename and
     the path to the object file backing the frame, or 'None'.

     If this function returns a 'None', GDB will not print any data for
     this field.

 -- Function: FrameDecorator.line (self):

     This method returns the line number associated with the current
     position within the function addressed by this frame.

     This method must return a Python integer type, or 'None'.

     If this function returns a 'None', GDB will not print any data for
     this field.

 -- Function: FrameDecorator.frame_args (self)

     This method must return an iterable, or 'None'.  Returning an empty
     iterable, or 'None' means frame arguments will not be printed for
     this frame.  This iterable must contain objects that implement two
     methods, described here.

     This object must implement a 'argument' method which takes a single
     'self' parameter and must return a 'gdb.Symbol' (*note Symbols In
     Python::), or a Python string.  The object must also implement a
     'value' method which takes a single 'self' parameter and must
     return a 'gdb.Value' (*note Values From Inferior::), a Python
     value, or 'None'.  If the 'value' method returns 'None', and the
     'argument' method returns a 'gdb.Symbol', GDB will look-up and
     print the value of the 'gdb.Symbol' automatically.

     A brief example:

          class SymValueWrapper():

              def __init__(self, symbol, value):
                  self.sym = symbol
                  self.val = value

              def value(self):
                  return self.val

              def symbol(self):
                  return self.sym

          class SomeFrameDecorator()
          ...
          ...
              def frame_args(self):
                  args = []
                  try:
                      block = self.inferior_frame.block()
                  except:
                      return None

                  # Iterate over all symbols in a block.  Only add
                  # symbols that are arguments.
                  for sym in block:
                      if not sym.is_argument:
                          continue
                      args.append(SymValueWrapper(sym,None))

                  # Add example synthetic argument.
                  args.append(SymValueWrapper(``foo'', 42))

                  return args

 -- Function: FrameDecorator.frame_locals (self)

     This method must return an iterable or 'None'.  Returning an empty
     iterable, or 'None' means frame local arguments will not be printed
     for this frame.

     The object interface, the description of the various strategies for
     reading frame locals, and the example are largely similar to those
     described in the 'frame_args' function, (*note The frame filter
     frame_args function: frame_args.).  Below is a modified example:

          class SomeFrameDecorator()
          ...
          ...
              def frame_locals(self):
                  vars = []
                  try:
                      block = self.inferior_frame.block()
                  except:
                      return None

                  # Iterate over all symbols in a block.  Add all
                  # symbols, except arguments.
                  for sym in block:
                      if sym.is_argument:
                          continue
                      vars.append(SymValueWrapper(sym,None))

                  # Add an example of a synthetic local variable.
                  vars.append(SymValueWrapper(``bar'', 99))

                  return vars

 -- Function: FrameDecorator.inferior_frame (self):

     This method must return the underlying 'gdb.Frame' that this frame
     decorator is decorating.  GDB requires the underlying frame for
     internal frame information to determine how to print certain values
     when printing a frame.


File: gdb.info,  Node: Writing a Frame Filter,  Next: Unwinding Frames in Python,  Prev: Frame Decorator API,  Up: Python API

23.2.2.11 Writing a Frame Filter
................................

There are three basic elements that a frame filter must implement: it
must correctly implement the documented interface (*note Frame Filter
API::), it must register itself with GDB, and finally, it must decide if
it is to work on the data provided by GDB.  In all cases, whether it
works on the iterator or not, each frame filter must return an iterator.
A bare-bones frame filter follows the pattern in the following example.

     import gdb

     class FrameFilter():

         def __init__(self):
             # Frame filter attribute creation.
             #
             # 'name' is the name of the filter that GDB will display.
             #
             # 'priority' is the priority of the filter relative to other
             # filters.
             #
             # 'enabled' is a boolean that indicates whether this filter is
             # enabled and should be executed.

             self.name = "Foo"
             self.priority = 100
             self.enabled = True

             # Register this frame filter with the global frame_filters
             # dictionary.
             gdb.frame_filters[self.name] = self

         def filter(self, frame_iter):
             # Just return the iterator.
             return frame_iter

   The frame filter in the example above implements the three
requirements for all frame filters.  It implements the API, self
registers, and makes a decision on the iterator (in this case, it just
returns the iterator untouched).

   The first step is attribute creation and assignment, and as shown in
the comments the filter assigns the following attributes: 'name',
'priority' and whether the filter should be enabled with the 'enabled'
attribute.

   The second step is registering the frame filter with the dictionary
or dictionaries that the frame filter has interest in.  As shown in the
comments, this filter just registers itself with the global dictionary
'gdb.frame_filters'.  As noted earlier, 'gdb.frame_filters' is a
dictionary that is initialized in the 'gdb' module when GDB starts.
What dictionary a filter registers with is an important consideration.
Generally, if a filter is specific to a set of code, it should be
registered either in the 'objfile' or 'progspace' dictionaries as they
are specific to the program currently loaded in GDB.  The global
dictionary is always present in GDB and is never unloaded.  Any filters
registered with the global dictionary will exist until GDB exits.  To
avoid filters that may conflict, it is generally better to register
frame filters against the dictionaries that more closely align with the
usage of the filter currently in question.  *Note Python Auto-loading::,
for further information on auto-loading Python scripts.

   GDB takes a hands-off approach to frame filter registration,
therefore it is the frame filter's responsibility to ensure registration
has occurred, and that any exceptions are handled appropriately.  In
particular, you may wish to handle exceptions relating to Python
dictionary key uniqueness.  It is mandatory that the dictionary key is
the same as frame filter's 'name' attribute.  When a user manages frame
filters (*note Frame Filter Management::), the names GDB will display
are those contained in the 'name' attribute.

   The final step of this example is the implementation of the 'filter'
method.  As shown in the example comments, we define the 'filter' method
and note that the method must take an iterator, and also must return an
iterator.  In this bare-bones example, the frame filter is not very
useful as it just returns the iterator untouched.  However this is a
valid operation for frame filters that have the 'enabled' attribute set,
but decide not to operate on any frames.

   In the next example, the frame filter operates on all frames and
utilizes a frame decorator to perform some work on the frames.  *Note
Frame Decorator API::, for further information on the frame decorator
interface.

   This example works on inlined frames.  It highlights frames which are
inlined by tagging them with an "[inlined]" tag.  By applying a frame
decorator to all frames with the Python 'itertools imap' method, the
example defers actions to the frame decorator.  Frame decorators are
only processed when GDB prints the backtrace.

   This introduces a new decision making topic: whether to perform
decision making operations at the filtering step, or at the printing
step.  In this example's approach, it does not perform any filtering
decisions at the filtering step beyond mapping a frame decorator to each
frame.  This allows the actual decision making to be performed when each
frame is printed.  This is an important consideration, and well worth
reflecting upon when designing a frame filter.  An issue that frame
filters should avoid is unwinding the stack if possible.  Some stacks
can run very deep, into the tens of thousands in some cases.  To search
every frame to determine if it is inlined ahead of time may be too
expensive at the filtering step.  The frame filter cannot know how many
frames it has to iterate over, and it would have to iterate through them
all.  This ends up duplicating effort as GDB performs this iteration
when it prints the frames.

   In this example decision making can be deferred to the printing step.
As each frame is printed, the frame decorator can examine each frame in
turn when GDB iterates.  From a performance viewpoint, this is the most
appropriate decision to make as it avoids duplicating the effort that
the printing step would undertake anyway.  Also, if there are many frame
filters unwinding the stack during filtering, it can substantially delay
the printing of the backtrace which will result in large memory usage,
and a poor user experience.

     class InlineFilter():

         def __init__(self):
             self.name = "InlinedFrameFilter"
             self.priority = 100
             self.enabled = True
             gdb.frame_filters[self.name] = self

         def filter(self, frame_iter):
             frame_iter = itertools.imap(InlinedFrameDecorator,
                                         frame_iter)
             return frame_iter

   This frame filter is somewhat similar to the earlier example, except
that the 'filter' method applies a frame decorator object called
'InlinedFrameDecorator' to each element in the iterator.  The 'imap'
Python method is light-weight.  It does not proactively iterate over the
iterator, but rather creates a new iterator which wraps the existing
one.

   Below is the frame decorator for this example.

     class InlinedFrameDecorator(FrameDecorator):

         def __init__(self, fobj):
             super(InlinedFrameDecorator, self).__init__(fobj)

         def function(self):
             frame = fobj.inferior_frame()
             name = str(frame.name())

             if frame.type() == gdb.INLINE_FRAME:
                 name = name + " [inlined]"

             return name

   This frame decorator only defines and overrides the 'function'
method.  It lets the supplied 'FrameDecorator', which is shipped with
GDB, perform the other work associated with printing this frame.

   The combination of these two objects create this output from a
backtrace:

     #0  0x004004e0 in bar () at inline.c:11
     #1  0x00400566 in max [inlined] (b=6, a=12) at inline.c:21
     #2  0x00400566 in main () at inline.c:31

   So in the case of this example, a frame decorator is applied to all
frames, regardless of whether they may be inlined or not.  As GDB
iterates over the iterator produced by the frame filters, GDB executes
each frame decorator which then makes a decision on what to print in the
'function' callback.  Using a strategy like this is a way to defer
decisions on the frame content to printing time.

Eliding Frames
--------------

It might be that the above example is not desirable for representing
inlined frames, and a hierarchical approach may be preferred.  If we
want to hierarchically represent frames, the 'elided' frame decorator
interface might be preferable.

   This example approaches the issue with the 'elided' method.  This
example is quite long, but very simplistic.  It is out-of-scope for this
section to write a complete example that comprehensively covers all
approaches of finding and printing inlined frames.  However, this
example illustrates the approach an author might use.

   This example comprises of three sections.

     class InlineFrameFilter():

         def __init__(self):
             self.name = "InlinedFrameFilter"
             self.priority = 100
             self.enabled = True
             gdb.frame_filters[self.name] = self

         def filter(self, frame_iter):
             return ElidingInlineIterator(frame_iter)

   This frame filter is very similar to the other examples.  The only
difference is this frame filter is wrapping the iterator provided to it
('frame_iter') with a custom iterator called 'ElidingInlineIterator'.
This again defers actions to when GDB prints the backtrace, as the
iterator is not traversed until printing.

   The iterator for this example is as follows.  It is in this section
of the example where decisions are made on the content of the backtrace.

     class ElidingInlineIterator:
         def __init__(self, ii):
             self.input_iterator = ii

         def __iter__(self):
             return self

         def next(self):
             frame = next(self.input_iterator)

             if frame.inferior_frame().type() != gdb.INLINE_FRAME:
                 return frame

             try:
                 eliding_frame = next(self.input_iterator)
             except StopIteration:
                 return frame
             return ElidingFrameDecorator(eliding_frame, [frame])

   This iterator implements the Python iterator protocol.  When the
'next' function is called (when GDB prints each frame), the iterator
checks if this frame decorator, 'frame', is wrapping an inlined frame.
If it is not, it returns the existing frame decorator untouched.  If it
is wrapping an inlined frame, it assumes that the inlined frame was
contained within the next oldest frame, 'eliding_frame', which it
fetches.  It then creates and returns a frame decorator,
'ElidingFrameDecorator', which contains both the elided frame, and the
eliding frame.

     class ElidingInlineDecorator(FrameDecorator):

         def __init__(self, frame, elided_frames):
             super(ElidingInlineDecorator, self).__init__(frame)
             self.frame = frame
             self.elided_frames = elided_frames

         def elided(self):
             return iter(self.elided_frames)

   This frame decorator overrides one function and returns the inlined
frame in the 'elided' method.  As before it lets 'FrameDecorator' do the
rest of the work involved in printing this frame.  This produces the
following output.

     #0  0x004004e0 in bar () at inline.c:11
     #2  0x00400529 in main () at inline.c:25
         #1  0x00400529 in max (b=6, a=12) at inline.c:15

   In that output, 'max' which has been inlined into 'main' is printed
hierarchically.  Another approach would be to combine the 'function'
method, and the 'elided' method to both print a marker in the inlined
frame, and also show the hierarchical relationship.


File: gdb.info,  Node: Unwinding Frames in Python,  Next: Xmethods In Python,  Prev: Writing a Frame Filter,  Up: Python API

23.2.2.12 Unwinding Frames in Python
....................................

In GDB terminology "unwinding" is the process of finding the previous
frame (that is, caller's) from the current one.  An unwinder has three
methods.  The first one checks if it can handle given frame ("sniff"
it).  For the frames it can sniff an unwinder provides two additional
methods: it can return frame's ID, and it can fetch registers from the
previous frame.  A running GDB mantains a list of the unwinders and
calls each unwinder's sniffer in turn until it finds the one that
recognizes the current frame.  There is an API to register an unwinder.

   The unwinders that come with GDB handle standard frames.  However,
mixed language applications (for example, an application running Java
Virtual Machine) sometimes use frame layouts that cannot be handled by
the GDB unwinders.  You can write Python code that can handle such
custom frames.

   You implement a frame unwinder in Python as a class with which has
two attributes, 'name' and 'enabled', with obvious meanings, and a
single method '__call__', which examines a given frame and returns an
object (an instance of 'gdb.UnwindInfo class)' describing it.  If an
unwinder does not recognize a frame, it should return 'None'.  The code
in GDB that enables writing unwinders in Python uses this object to
return frame's ID and previous frame registers when GDB core asks for
them.

   An unwinder should do as little work as possible.  Some otherwise
innocuous operations can cause problems (even crashes, as this code is
not not well-hardened yet).  For example, making an inferior call from
an unwinder is unadvisable, as an inferior call will reset GDB's stack
unwinding process, potentially causing re-entrant unwinding.

Unwinder Input
--------------

An object passed to an unwinder (a 'gdb.PendingFrame' instance) provides
a method to read frame's registers:

 -- Function: PendingFrame.read_register (reg)
     This method returns the contents of the register REG in the frame
     as a 'gdb.Value' object.  REG can be either a register number or a
     register name; the values are platform-specific.  They are usually
     found in the corresponding 'PLATFORM-tdep.h' file in the GDB source
     tree.  If REG does not name a register for the current
     architecture, this method will throw an exception.

     Note that this method will always return a 'gdb.Value' for a valid
     register name.  This does not mean that the value will be valid.
     For example, you may request a register that an earlier unwinder
     could not unwind--the value will be unavailable.  Instead, the
     'gdb.Value' returned from this method will be lazy; that is, its
     underlying bits will not be fetched until it is first used.  So,
     attempting to use such a value will cause an exception at the point
     of use.

     The type of the returned 'gdb.Value' depends on the register and
     the architecture.  It is common for registers to have a scalar
     type, like 'long long'; but many other types are possible, such as
     pointer, pointer-to-function, floating point or vector types.

   It also provides a factory method to create a 'gdb.UnwindInfo'
instance to be returned to GDB:

 -- Function: PendingFrame.create_unwind_info (frame_id)
     Returns a new 'gdb.UnwindInfo' instance identified by given
     FRAME_ID.  The argument is used to build GDB's frame ID using one
     of functions provided by GDB.  FRAME_ID's attributes determine
     which function will be used, as follows:

     'sp, pc'
          The frame is identified by the given stack address and PC. The
          stack address must be chosen so that it is constant throughout
          the lifetime of the frame, so a typical choice is the value of
          the stack pointer at the start of the function--in the DWARF
          standard, this would be the "Call Frame Address".

          This is the most common case by far.  The other cases are
          documented for completeness but are only useful in specialized
          situations.

     'sp, pc, special'
          The frame is identified by the stack address, the PC, and a
          "special" address.  The special address is used on
          architectures that can have frames that do not change the
          stack, but which are still distinct, for example the IA-64,
          which has a second stack for registers.  Both SP and SPECIAL
          must be constant throughout the lifetime of the frame.

     'sp'
          The frame is identified by the stack address only.  Any other
          stack frame with a matching SP will be considered to match
          this frame.  Inside gdb, this is called a "wild frame".  You
          will never need this.

     Each attribute value should be an instance of 'gdb.Value'.

Unwinder Output: UnwindInfo
---------------------------

Use 'PendingFrame.create_unwind_info' method described above to create a
'gdb.UnwindInfo' instance.  Use the following method to specify caller
registers that have been saved in this frame:

 -- Function: gdb.UnwindInfo.add_saved_register (reg, value)
     REG identifies the register.  It can be a number or a name, just as
     for the 'PendingFrame.read_register' method above.  VALUE is a
     register value (a 'gdb.Value' object).

Unwinder Skeleton Code
----------------------

GDB comes with the module containing the base 'Unwinder' class.  Derive
your unwinder class from it and structure the code as follows:

     from gdb.unwinders import Unwinder

     class FrameId(object):
         def __init__(self, sp, pc):
             self.sp = sp
             self.pc = pc


     class MyUnwinder(Unwinder):
         def __init__(....):
             super(MyUnwinder, self).__init___(<expects unwinder name argument>)

         def __call__(pending_frame):
             if not <we recognize frame>:
                 return None
             # Create UnwindInfo.  Usually the frame is identified by the stack
             # pointer and the program counter.
             sp = pending_frame.read_register(<SP number>)
             pc = pending_frame.read_register(<PC number>)
             unwind_info = pending_frame.create_unwind_info(FrameId(sp, pc))

             # Find the values of the registers in the caller's frame and
             # save them in the result:
             unwind_info.add_saved_register(<register>, <value>)
             ....

             # Return the result:
             return unwind_info


Registering a Unwinder
----------------------

An object file, a program space, and the GDB proper can have unwinders
registered with it.

   The 'gdb.unwinders' module provides the function to register a
unwinder:

 -- Function: gdb.unwinder.register_unwinder (locus, unwinder,
          replace=False)
     LOCUS is specifies an object file or a program space to which
     UNWINDER is added.  Passing 'None' or 'gdb' adds UNWINDER to the
     GDB's global unwinder list.  The newly added UNWINDER will be
     called before any other unwinder from the same locus.  Two
     unwinders in the same locus cannot have the same name.  An attempt
     to add a unwinder with already existing name raises an exception
     unless REPLACE is 'True', in which case the old unwinder is
     deleted.

Unwinder Precedence
-------------------

GDB first calls the unwinders from all the object files in no particular
order, then the unwinders from the current program space, and finally
the unwinders from GDB.


File: gdb.info,  Node: Xmethods In Python,  Next: Xmethod API,  Prev: Unwinding Frames in Python,  Up: Python API

23.2.2.13 Xmethods In Python
............................

"Xmethods" are additional methods or replacements for existing methods
of a C++ class.  This feature is useful for those cases where a method
defined in C++ source code could be inlined or optimized out by the
compiler, making it unavailable to GDB.  For such cases, one can define
an xmethod to serve as a replacement for the method defined in the C++
source code.  GDB will then invoke the xmethod, instead of the C++
method, to evaluate expressions.  One can also use xmethods when
debugging with core files.  Moreover, when debugging live programs,
invoking an xmethod need not involve running the inferior (which can
potentially perturb its state).  Hence, even if the C++ method is
available, it is better to use its replacement xmethod if one is
defined.

   The xmethods feature in Python is available via the concepts of an
"xmethod matcher" and an "xmethod worker".  To implement an xmethod, one
has to implement a matcher and a corresponding worker for it (more than
one worker can be implemented, each catering to a different overloaded
instance of the method).  Internally, GDB invokes the 'match' method of
a matcher to match the class type and method name.  On a match, the
'match' method returns a list of matching _worker_ objects.  Each worker
object typically corresponds to an overloaded instance of the xmethod.
They implement a 'get_arg_types' method which returns a sequence of
types corresponding to the arguments the xmethod requires.  GDB uses
this sequence of types to perform overload resolution and picks a
winning xmethod worker.  A winner is also selected from among the
methods GDB finds in the C++ source code.  Next, the winning xmethod
worker and the winning C++ method are compared to select an overall
winner.  In case of a tie between a xmethod worker and a C++ method, the
xmethod worker is selected as the winner.  That is, if a winning xmethod
worker is found to be equivalent to the winning C++ method, then the
xmethod worker is treated as a replacement for the C++ method.  GDB uses
the overall winner to invoke the method.  If the winning xmethod worker
is the overall winner, then the corresponding xmethod is invoked via the
'__call__' method of the worker object.

   If one wants to implement an xmethod as a replacement for an existing
C++ method, then they have to implement an equivalent xmethod which has
exactly the same name and takes arguments of exactly the same type as
the C++ method.  If the user wants to invoke the C++ method even though
a replacement xmethod is available for that method, then they can
disable the xmethod.

   *Note Xmethod API::, for API to implement xmethods in Python.  *Note
Writing an Xmethod::, for implementing xmethods in Python.


File: gdb.info,  Node: Xmethod API,  Next: Writing an Xmethod,  Prev: Xmethods In Python,  Up: Python API

23.2.2.14 Xmethod API
.....................

The GDB Python API provides classes, interfaces and functions to
implement, register and manipulate xmethods.  *Note Xmethods In
Python::.

   An xmethod matcher should be an instance of a class derived from
'XMethodMatcher' defined in the module 'gdb.xmethod', or an object with
similar interface and attributes.  An instance of 'XMethodMatcher' has
the following attributes:

 -- Variable: name
     The name of the matcher.

 -- Variable: enabled
     A boolean value indicating whether the matcher is enabled or
     disabled.

 -- Variable: methods
     A list of named methods managed by the matcher.  Each object in the
     list is an instance of the class 'XMethod' defined in the module
     'gdb.xmethod', or any object with the following attributes:

     'name'
          Name of the xmethod which should be unique for each xmethod
          managed by the matcher.

     'enabled'
          A boolean value indicating whether the xmethod is enabled or
          disabled.

     The class 'XMethod' is a convenience class with same attributes as
     above along with the following constructor:

      -- Function: XMethod.__init__ (self, name)
          Constructs an enabled xmethod with name NAME.

The 'XMethodMatcher' class has the following methods:

 -- Function: XMethodMatcher.__init__ (self, name)
     Constructs an enabled xmethod matcher with name NAME.  The
     'methods' attribute is initialized to 'None'.

 -- Function: XMethodMatcher.match (self, class_type, method_name)
     Derived classes should override this method.  It should return a
     xmethod worker object (or a sequence of xmethod worker objects)
     matching the CLASS_TYPE and METHOD_NAME.  CLASS_TYPE is a
     'gdb.Type' object, and METHOD_NAME is a string value.  If the
     matcher manages named methods as listed in its 'methods' attribute,
     then only those worker objects whose corresponding entries in the
     'methods' list are enabled should be returned.

   An xmethod worker should be an instance of a class derived from
'XMethodWorker' defined in the module 'gdb.xmethod', or support the
following interface:

 -- Function: XMethodWorker.get_arg_types (self)
     This method returns a sequence of 'gdb.Type' objects corresponding
     to the arguments that the xmethod takes.  It can return an empty
     sequence or 'None' if the xmethod does not take any arguments.  If
     the xmethod takes a single argument, then a single 'gdb.Type'
     object corresponding to it can be returned.

 -- Function: XMethodWorker.get_result_type (self, *args)
     This method returns a 'gdb.Type' object representing the type of
     the result of invoking this xmethod.  The ARGS argument is the same
     tuple of arguments that would be passed to the '__call__' method of
     this worker.

 -- Function: XMethodWorker.__call__ (self, *args)
     This is the method which does the _work_ of the xmethod.  The ARGS
     arguments is the tuple of arguments to the xmethod.  Each element
     in this tuple is a gdb.Value object.  The first element is always
     the 'this' pointer value.

   For GDB to lookup xmethods, the xmethod matchers should be registered
using the following function defined in the module 'gdb.xmethod':

 -- Function: register_xmethod_matcher (locus, matcher, replace=False)
     The 'matcher' is registered with 'locus', replacing an existing
     matcher with the same name as 'matcher' if 'replace' is 'True'.
     'locus' can be a 'gdb.Objfile' object (*note Objfiles In Python::),
     or a 'gdb.Progspace' object (*note Progspaces In Python::), or
     'None'.  If it is 'None', then 'matcher' is registered globally.


File: gdb.info,  Node: Writing an Xmethod,  Next: Inferiors In Python,  Prev: Xmethod API,  Up: Python API

23.2.2.15 Writing an Xmethod
............................

Implementing xmethods in Python will require implementing xmethod
matchers and xmethod workers (*note Xmethods In Python::).  Consider the
following C++ class:

     class MyClass
     {
     public:
       MyClass (int a) : a_(a) { }

       int geta (void) { return a_; }
       int operator+ (int b);

     private:
       int a_;
     };

     int
     MyClass::operator+ (int b)
     {
       return a_ + b;
     }

Let us define two xmethods for the class 'MyClass', one replacing the
method 'geta', and another adding an overloaded flavor of 'operator+'
which takes a 'MyClass' argument (the C++ code above already has an
overloaded 'operator+' which takes an 'int' argument).  The xmethod
matcher can be defined as follows:

     class MyClass_geta(gdb.xmethod.XMethod):
         def __init__(self):
             gdb.xmethod.XMethod.__init__(self, 'geta')

         def get_worker(self, method_name):
             if method_name == 'geta':
                 return MyClassWorker_geta()


     class MyClass_sum(gdb.xmethod.XMethod):
         def __init__(self):
             gdb.xmethod.XMethod.__init__(self, 'sum')

         def get_worker(self, method_name):
             if method_name == 'operator+':
                 return MyClassWorker_plus()


     class MyClassMatcher(gdb.xmethod.XMethodMatcher):
         def __init__(self):
             gdb.xmethod.XMethodMatcher.__init__(self, 'MyClassMatcher')
             # List of methods 'managed' by this matcher
             self.methods = [MyClass_geta(), MyClass_sum()]

         def match(self, class_type, method_name):
             if class_type.tag != 'MyClass':
                 return None
             workers = []
             for method in self.methods:
                 if method.enabled:
                     worker = method.get_worker(method_name)
                     if worker:
                         workers.append(worker)

             return workers

Notice that the 'match' method of 'MyClassMatcher' returns a worker
object of type 'MyClassWorker_geta' for the 'geta' method, and a worker
object of type 'MyClassWorker_plus' for the 'operator+' method.  This is
done indirectly via helper classes derived from 'gdb.xmethod.XMethod'.
One does not need to use the 'methods' attribute in a matcher as it is
optional.  However, if a matcher manages more than one xmethod, it is a
good practice to list the xmethods in the 'methods' attribute of the
matcher.  This will then facilitate enabling and disabling individual
xmethods via the 'enable/disable' commands.  Notice also that a worker
object is returned only if the corresponding entry in the 'methods'
attribute of the matcher is enabled.

   The implementation of the worker classes returned by the matcher
setup above is as follows:

     class MyClassWorker_geta(gdb.xmethod.XMethodWorker):
         def get_arg_types(self):
             return None

         def get_result_type(self, obj):
             return gdb.lookup_type('int')

         def __call__(self, obj):
             return obj['a_']


     class MyClassWorker_plus(gdb.xmethod.XMethodWorker):
         def get_arg_types(self):
             return gdb.lookup_type('MyClass')

         def get_result_type(self, obj):
             return gdb.lookup_type('int')

         def __call__(self, obj, other):
             return obj['a_'] + other['a_']

   For GDB to actually lookup a xmethod, it has to be registered with
it.  The matcher defined above is registered with GDB globally as
follows:

     gdb.xmethod.register_xmethod_matcher(None, MyClassMatcher())

   If an object 'obj' of type 'MyClass' is initialized in C++ code as
follows:

     MyClass obj(5);

then, after loading the Python script defining the xmethod matchers and
workers into 'GDBN', invoking the method 'geta' or using the operator
'+' on 'obj' will invoke the xmethods defined above:

     (gdb) p obj.geta()
     $1 = 5

     (gdb) p obj + obj
     $2 = 10

   Consider another example with a C++ template class:

     template <class T>
     class MyTemplate
     {
     public:
       MyTemplate () : dsize_(10), data_ (new T [10]) { }
       ~MyTemplate () { delete [] data_; }

       int footprint (void)
       {
         return sizeof (T) * dsize_ + sizeof (MyTemplate<T>);
       }

     private:
       int dsize_;
       T *data_;
     };

   Let us implement an xmethod for the above class which serves as a
replacement for the 'footprint' method.  The full code listing of the
xmethod workers and xmethod matchers is as follows:

     class MyTemplateWorker_footprint(gdb.xmethod.XMethodWorker):
         def __init__(self, class_type):
             self.class_type = class_type

         def get_arg_types(self):
             return None

         def get_result_type(self):
             return gdb.lookup_type('int')

         def __call__(self, obj):
             return (self.class_type.sizeof +
                     obj['dsize_'] *
                     self.class_type.template_argument(0).sizeof)


     class MyTemplateMatcher_footprint(gdb.xmethod.XMethodMatcher):
         def __init__(self):
             gdb.xmethod.XMethodMatcher.__init__(self, 'MyTemplateMatcher')

         def match(self, class_type, method_name):
             if (re.match('MyTemplate<[ \t\n]*[_a-zA-Z][ _a-zA-Z0-9]*>',
                          class_type.tag) and
                 method_name == 'footprint'):
                 return MyTemplateWorker_footprint(class_type)

   Notice that, in this example, we have not used the 'methods'
attribute of the matcher as the matcher manages only one xmethod.  The
user can enable/disable this xmethod by enabling/disabling the matcher
itself.


File: gdb.info,  Node: Inferiors In Python,  Next: Events In Python,  Prev: Writing an Xmethod,  Up: Python API

23.2.2.16 Inferiors In Python
.............................

Programs which are being run under GDB are called inferiors (*note
Inferiors and Programs::).  Python scripts can access information about
and manipulate inferiors controlled by GDB via objects of the
'gdb.Inferior' class.

   The following inferior-related functions are available in the 'gdb'
module:

 -- Function: gdb.inferiors ()
     Return a tuple containing all inferior objects.

 -- Function: gdb.selected_inferior ()
     Return an object representing the current inferior.

   A 'gdb.Inferior' object has the following attributes:

 -- Variable: Inferior.num
     ID of inferior, as assigned by GDB.

 -- Variable: Inferior.pid
     Process ID of the inferior, as assigned by the underlying operating
     system.

 -- Variable: Inferior.was_attached
     Boolean signaling whether the inferior was created using 'attach',
     or started by GDB itself.

 -- Variable: Inferior.progspace
     The inferior's program space.  *Note Progspaces In Python::.

   A 'gdb.Inferior' object has the following methods:

 -- Function: Inferior.is_valid ()
     Returns 'True' if the 'gdb.Inferior' object is valid, 'False' if
     not.  A 'gdb.Inferior' object will become invalid if the inferior
     no longer exists within GDB.  All other 'gdb.Inferior' methods will
     throw an exception if it is invalid at the time the method is
     called.

 -- Function: Inferior.threads ()
     This method returns a tuple holding all the threads which are valid
     when it is called.  If there are no valid threads, the method will
     return an empty tuple.

 -- Function: Inferior.architecture ()
     Return the 'gdb.Architecture' (*note Architectures In Python::) for
     this inferior.  This represents the architecture of the inferior as
     a whole.  Some platforms can have multiple architectures in a
     single address space, so this may not match the architecture of a
     particular frame (*note Frames In Python::).

 -- Function: Inferior.read_memory (address, length)
     Read LENGTH addressable memory units from the inferior, starting at
     ADDRESS.  Returns a buffer object, which behaves much like an array
     or a string.  It can be modified and given to the
     'Inferior.write_memory' function.  In Python 3, the return value is
     a 'memoryview' object.

 -- Function: Inferior.write_memory (address, buffer [, length])
     Write the contents of BUFFER to the inferior, starting at ADDRESS.
     The BUFFER parameter must be a Python object which supports the
     buffer protocol, i.e., a string, an array or the object returned
     from 'Inferior.read_memory'.  If given, LENGTH determines the
     number of addressable memory units from BUFFER to be written.

 -- Function: Inferior.search_memory (address, length, pattern)
     Search a region of the inferior memory starting at ADDRESS with the
     given LENGTH using the search pattern supplied in PATTERN.  The
     PATTERN parameter must be a Python object which supports the buffer
     protocol, i.e., a string, an array or the object returned from
     'gdb.read_memory'.  Returns a Python 'Long' containing the address
     where the pattern was found, or 'None' if the pattern could not be
     found.

 -- Function: Inferior.thread_from_handle (handle)
     Return the thread object corresponding to HANDLE, a thread library
     specific data structure such as 'pthread_t' for pthreads library
     implementations.

     The function 'Inferior.thread_from_thread_handle' provides the same
     functionality, but use of 'Inferior.thread_from_thread_handle' is
     deprecated.


File: gdb.info,  Node: Events In Python,  Next: Threads In Python,  Prev: Inferiors In Python,  Up: Python API

23.2.2.17 Events In Python
..........................

GDB provides a general event facility so that Python code can be
notified of various state changes, particularly changes that occur in
the inferior.

   An "event" is just an object that describes some state change.  The
type of the object and its attributes will vary depending on the details
of the change.  All the existing events are described below.

   In order to be notified of an event, you must register an event
handler with an "event registry".  An event registry is an object in the
'gdb.events' module which dispatches particular events.  A registry
provides methods to register and unregister event handlers:

 -- Function: EventRegistry.connect (object)
     Add the given callable OBJECT to the registry.  This object will be
     called when an event corresponding to this registry occurs.

 -- Function: EventRegistry.disconnect (object)
     Remove the given OBJECT from the registry.  Once removed, the
     object will no longer receive notifications of events.

   Here is an example:

     def exit_handler (event):
         print "event type: exit"
         print "exit code: %d" % (event.exit_code)

     gdb.events.exited.connect (exit_handler)

   In the above example we connect our handler 'exit_handler' to the
registry 'events.exited'.  Once connected, 'exit_handler' gets called
when the inferior exits.  The argument "event" in this example is of
type 'gdb.ExitedEvent'.  As you can see in the example the 'ExitedEvent'
object has an attribute which indicates the exit code of the inferior.

   The following is a listing of the event registries that are available
and details of the events they emit:

'events.cont'
     Emits 'gdb.ThreadEvent'.

     Some events can be thread specific when GDB is running in non-stop
     mode.  When represented in Python, these events all extend
     'gdb.ThreadEvent'.  Note, this event is not emitted directly;
     instead, events which are emitted by this or other modules might
     extend this event.  Examples of these events are
     'gdb.BreakpointEvent' and 'gdb.ContinueEvent'.

      -- Variable: ThreadEvent.inferior_thread
          In non-stop mode this attribute will be set to the specific
          thread which was involved in the emitted event.  Otherwise, it
          will be set to 'None'.

     Emits 'gdb.ContinueEvent' which extends 'gdb.ThreadEvent'.

     This event indicates that the inferior has been continued after a
     stop.  For inherited attribute refer to 'gdb.ThreadEvent' above.

'events.exited'
     Emits 'events.ExitedEvent' which indicates that the inferior has
     exited.  'events.ExitedEvent' has two attributes:
      -- Variable: ExitedEvent.exit_code
          An integer representing the exit code, if available, which the
          inferior has returned.  (The exit code could be unavailable
          if, for example, GDB detaches from the inferior.)  If the exit
          code is unavailable, the attribute does not exist.
      -- Variable: ExitedEvent.inferior
          A reference to the inferior which triggered the 'exited'
          event.

'events.stop'
     Emits 'gdb.StopEvent' which extends 'gdb.ThreadEvent'.

     Indicates that the inferior has stopped.  All events emitted by
     this registry extend StopEvent.  As a child of 'gdb.ThreadEvent',
     'gdb.StopEvent' will indicate the stopped thread when GDB is
     running in non-stop mode.  Refer to 'gdb.ThreadEvent' above for
     more details.

     Emits 'gdb.SignalEvent' which extends 'gdb.StopEvent'.

     This event indicates that the inferior or one of its threads has
     received as signal.  'gdb.SignalEvent' has the following
     attributes:

      -- Variable: SignalEvent.stop_signal
          A string representing the signal received by the inferior.  A
          list of possible signal values can be obtained by running the
          command 'info signals' in the GDB command prompt.

     Also emits 'gdb.BreakpointEvent' which extends 'gdb.StopEvent'.

     'gdb.BreakpointEvent' event indicates that one or more breakpoints
     have been hit, and has the following attributes:

      -- Variable: BreakpointEvent.breakpoints
          A sequence containing references to all the breakpoints (type
          'gdb.Breakpoint') that were hit.  *Note Breakpoints In
          Python::, for details of the 'gdb.Breakpoint' object.
      -- Variable: BreakpointEvent.breakpoint
          A reference to the first breakpoint that was hit.  This
          function is maintained for backward compatibility and is now
          deprecated in favor of the 'gdb.BreakpointEvent.breakpoints'
          attribute.

'events.new_objfile'
     Emits 'gdb.NewObjFileEvent' which indicates that a new object file
     has been loaded by GDB.  'gdb.NewObjFileEvent' has one attribute:

      -- Variable: NewObjFileEvent.new_objfile
          A reference to the object file ('gdb.Objfile') which has been
          loaded.  *Note Objfiles In Python::, for details of the
          'gdb.Objfile' object.

'events.clear_objfiles'
     Emits 'gdb.ClearObjFilesEvent' which indicates that the list of
     object files for a program space has been reset.
     'gdb.ClearObjFilesEvent' has one attribute:

      -- Variable: ClearObjFilesEvent.progspace
          A reference to the program space ('gdb.Progspace') whose
          objfile list has been cleared.  *Note Progspaces In Python::.

'events.inferior_call'
     Emits events just before and after a function in the inferior is
     called by GDB.  Before an inferior call, this emits an event of
     type 'gdb.InferiorCallPreEvent', and after an inferior call, this
     emits an event of type 'gdb.InferiorCallPostEvent'.

     'gdb.InferiorCallPreEvent'
          Indicates that a function in the inferior is about to be
          called.

           -- Variable: InferiorCallPreEvent.ptid
               The thread in which the call will be run.

           -- Variable: InferiorCallPreEvent.address
               The location of the function to be called.

     'gdb.InferiorCallPostEvent'
          Indicates that a function in the inferior has just been
          called.

           -- Variable: InferiorCallPostEvent.ptid
               The thread in which the call was run.

           -- Variable: InferiorCallPostEvent.address
               The location of the function that was called.

'events.memory_changed'
     Emits 'gdb.MemoryChangedEvent' which indicates that the memory of
     the inferior has been modified by the GDB user, for instance via a
     command like 'set *addr = value'.  The event has the following
     attributes:

      -- Variable: MemoryChangedEvent.address
          The start address of the changed region.

      -- Variable: MemoryChangedEvent.length
          Length in bytes of the changed region.

'events.register_changed'
     Emits 'gdb.RegisterChangedEvent' which indicates that a register in
     the inferior has been modified by the GDB user.

      -- Variable: RegisterChangedEvent.frame
          A gdb.Frame object representing the frame in which the
          register was modified.
      -- Variable: RegisterChangedEvent.regnum
          Denotes which register was modified.

'events.breakpoint_created'
     This is emitted when a new breakpoint has been created.  The
     argument that is passed is the new 'gdb.Breakpoint' object.

'events.breakpoint_modified'
     This is emitted when a breakpoint has been modified in some way.
     The argument that is passed is the new 'gdb.Breakpoint' object.

'events.breakpoint_deleted'
     This is emitted when a breakpoint has been deleted.  The argument
     that is passed is the 'gdb.Breakpoint' object.  When this event is
     emitted, the 'gdb.Breakpoint' object will already be in its invalid
     state; that is, the 'is_valid' method will return 'False'.

'events.before_prompt'
     This event carries no payload.  It is emitted each time GDB
     presents a prompt to the user.

'events.new_inferior'
     This is emitted when a new inferior is created.  Note that the
     inferior is not necessarily running; in fact, it may not even have
     an associated executable.

     The event is of type 'gdb.NewInferiorEvent'.  This has a single
     attribute:

      -- Variable: NewInferiorEvent.inferior
          The new inferior, a 'gdb.Inferior' object.

'events.inferior_deleted'
     This is emitted when an inferior has been deleted.  Note that this
     is not the same as process exit; it is notified when the inferior
     itself is removed, say via 'remove-inferiors'.

     The event is of type 'gdb.InferiorDeletedEvent'.  This has a single
     attribute:

      -- Variable: NewInferiorEvent.inferior
          The inferior that is being removed, a 'gdb.Inferior' object.

'events.new_thread'
     This is emitted when GDB notices a new thread.  The event is of
     type 'gdb.NewThreadEvent', which extends 'gdb.ThreadEvent'.  This
     has a single attribute:

      -- Variable: NewThreadEvent.inferior_thread
          The new thread.


File: gdb.info,  Node: Threads In Python,  Next: Recordings In Python,  Prev: Events In Python,  Up: Python API

23.2.2.18 Threads In Python
...........................

Python scripts can access information about, and manipulate inferior
threads controlled by GDB, via objects of the 'gdb.InferiorThread'
class.

   The following thread-related functions are available in the 'gdb'
module:

 -- Function: gdb.selected_thread ()
     This function returns the thread object for the selected thread.
     If there is no selected thread, this will return 'None'.

   To get the list of threads for an inferior, use the
'Inferior.threads()' method.  *Note Inferiors In Python::

   A 'gdb.InferiorThread' object has the following attributes:

 -- Variable: InferiorThread.name
     The name of the thread.  If the user specified a name using 'thread
     name', then this returns that name.  Otherwise, if an OS-supplied
     name is available, then it is returned.  Otherwise, this returns
     'None'.

     This attribute can be assigned to.  The new value must be a string
     object, which sets the new name, or 'None', which removes any
     user-specified thread name.

 -- Variable: InferiorThread.num
     The per-inferior number of the thread, as assigned by GDB.

 -- Variable: InferiorThread.global_num
     The global ID of the thread, as assigned by GDB. You can use this
     to make Python breakpoints thread-specific, for example (*note The
     Breakpoint.thread attribute: python_breakpoint_thread.).

 -- Variable: InferiorThread.ptid
     ID of the thread, as assigned by the operating system.  This
     attribute is a tuple containing three integers.  The first is the
     Process ID (PID); the second is the Lightweight Process ID (LWPID),
     and the third is the Thread ID (TID). Either the LWPID or TID may
     be 0, which indicates that the operating system does not use that
     identifier.

 -- Variable: InferiorThread.inferior
     The inferior this thread belongs to.  This attribute is represented
     as a 'gdb.Inferior' object.  This attribute is not writable.

   A 'gdb.InferiorThread' object has the following methods:

 -- Function: InferiorThread.is_valid ()
     Returns 'True' if the 'gdb.InferiorThread' object is valid, 'False'
     if not.  A 'gdb.InferiorThread' object will become invalid if the
     thread exits, or the inferior that the thread belongs is deleted.
     All other 'gdb.InferiorThread' methods will throw an exception if
     it is invalid at the time the method is called.

 -- Function: InferiorThread.switch ()
     This changes GDB's currently selected thread to the one represented
     by this object.

 -- Function: InferiorThread.is_stopped ()
     Return a Boolean indicating whether the thread is stopped.

 -- Function: InferiorThread.is_running ()
     Return a Boolean indicating whether the thread is running.

 -- Function: InferiorThread.is_exited ()
     Return a Boolean indicating whether the thread is exited.

 -- Function: InferiorThread.handle ()
     Return the thread object's handle, represented as a Python 'bytes'
     object.  A 'gdb.Value' representation of the handle may be
     constructed via 'gdb.Value(bufobj, type)' where BUFOBJ is the
     Python 'bytes' representation of the handle and TYPE is a
     'gdb.Type' for the handle type.


File: gdb.info,  Node: Recordings In Python,  Next: Commands In Python,  Prev: Threads In Python,  Up: Python API

23.2.2.19 Recordings In Python
..............................

The following recordings-related functions (*note Process Record and
Replay::) are available in the 'gdb' module:

 -- Function: gdb.start_recording ([method], [format])
     Start a recording using the given METHOD and FORMAT.  If no FORMAT
     is given, the default format for the recording method is used.  If
     no METHOD is given, the default method will be used.  Returns a
     'gdb.Record' object on success.  Throw an exception on failure.

     The following strings can be passed as METHOD:

        * '"full"'
        * '"btrace"': Possible values for FORMAT: '"pt"', '"bts"' or
          leave out for default format.

 -- Function: gdb.current_recording ()
     Access a currently running recording.  Return a 'gdb.Record' object
     on success.  Return 'None' if no recording is currently active.

 -- Function: gdb.stop_recording ()
     Stop the current recording.  Throw an exception if no recording is
     currently active.  All record objects become invalid after this
     call.

   A 'gdb.Record' object has the following attributes:

 -- Variable: Record.method
     A string with the current recording method, e.g. 'full' or
     'btrace'.

 -- Variable: Record.format
     A string with the current recording format, e.g. 'bt', 'pts' or
     'None'.

 -- Variable: Record.begin
     A method specific instruction object representing the first
     instruction in this recording.

 -- Variable: Record.end
     A method specific instruction object representing the current
     instruction, that is not actually part of the recording.

 -- Variable: Record.replay_position
     The instruction representing the current replay position.  If there
     is no replay active, this will be 'None'.

 -- Variable: Record.instruction_history
     A list with all recorded instructions.

 -- Variable: Record.function_call_history
     A list with all recorded function call segments.

   A 'gdb.Record' object has the following methods:

 -- Function: Record.goto (instruction)
     Move the replay position to the given INSTRUCTION.

   The common 'gdb.Instruction' class that recording method specific
instruction objects inherit from, has the following attributes:

 -- Variable: Instruction.pc
     An integer representing this instruction's address.

 -- Variable: Instruction.data
     A buffer with the raw instruction data.  In Python 3, the return
     value is a 'memoryview' object.

 -- Variable: Instruction.decoded
     A human readable string with the disassembled instruction.

 -- Variable: Instruction.size
     The size of the instruction in bytes.

   Additionally 'gdb.RecordInstruction' has the following attributes:

 -- Variable: RecordInstruction.number
     An integer identifying this instruction.  'number' corresponds to
     the numbers seen in 'record instruction-history' (*note Process
     Record and Replay::).

 -- Variable: RecordInstruction.sal
     A 'gdb.Symtab_and_line' object representing the associated symtab
     and line of this instruction.  May be 'None' if no debug
     information is available.

 -- Variable: RecordInstruction.is_speculative
     A boolean indicating whether the instruction was executed
     speculatively.

   If an error occured during recording or decoding a recording, this
error is represented by a 'gdb.RecordGap' object in the instruction
list.  It has the following attributes:

 -- Variable: RecordGap.number
     An integer identifying this gap.  'number' corresponds to the
     numbers seen in 'record instruction-history' (*note Process Record
     and Replay::).

 -- Variable: RecordGap.error_code
     A numerical representation of the reason for the gap.  The value is
     specific to the current recording method.

 -- Variable: RecordGap.error_string
     A human readable string with the reason for the gap.

   A 'gdb.RecordFunctionSegment' object has the following attributes:

 -- Variable: RecordFunctionSegment.number
     An integer identifying this function segment.  'number' corresponds
     to the numbers seen in 'record function-call-history' (*note
     Process Record and Replay::).

 -- Variable: RecordFunctionSegment.symbol
     A 'gdb.Symbol' object representing the associated symbol.  May be
     'None' if no debug information is available.

 -- Variable: RecordFunctionSegment.level
     An integer representing the function call's stack level.  May be
     'None' if the function call is a gap.

 -- Variable: RecordFunctionSegment.instructions
     A list of 'gdb.RecordInstruction' or 'gdb.RecordGap' objects
     associated with this function call.

 -- Variable: RecordFunctionSegment.up
     A 'gdb.RecordFunctionSegment' object representing the caller's
     function segment.  If the call has not been recorded, this will be
     the function segment to which control returns.  If neither the call
     nor the return have been recorded, this will be 'None'.

 -- Variable: RecordFunctionSegment.prev
     A 'gdb.RecordFunctionSegment' object representing the previous
     segment of this function call.  May be 'None'.

 -- Variable: RecordFunctionSegment.next
     A 'gdb.RecordFunctionSegment' object representing the next segment
     of this function call.  May be 'None'.

   The following example demonstrates the usage of these objects and
functions to create a function that will rewind a record to the last
time a function in a different file was executed.  This would typically
be used to track the execution of user provided callback functions in a
library which typically are not visible in a back trace.

     def bringback ():
         rec = gdb.current_recording ()
         if not rec:
             return

         insn = rec.instruction_history
         if len (insn) == 0:
             return

         try:
             position = insn.index (rec.replay_position)
         except:
             position = -1
         try:
             filename = insn[position].sal.symtab.fullname ()
         except:
             filename = None

         for i in reversed (insn[:position]):
     	try:
                 current = i.sal.symtab.fullname ()
     	except:
                 current = None

             if filename == current:
                 continue

             rec.goto (i)
             return

   Another possible application is to write a function that counts the
number of code executions in a given line range.  This line range can
contain parts of functions or span across several functions and is not
limited to be contiguous.

     def countrange (filename, linerange):
         count = 0

         def filter_only (file_name):
             for call in gdb.current_recording ().function_call_history:
                 try:
                     if file_name in call.symbol.symtab.fullname ():
                         yield call
                 except:
                     pass

         for c in filter_only (filename):
             for i in c.instructions:
                 try:
                     if i.sal.line in linerange:
                         count += 1
                         break;
                 except:
                         pass

         return count


File: gdb.info,  Node: Commands In Python,  Next: Parameters In Python,  Prev: Recordings In Python,  Up: Python API

23.2.2.20 Commands In Python
............................

You can implement new GDB CLI commands in Python.  A CLI command is
implemented using an instance of the 'gdb.Command' class, most commonly
using a subclass.

 -- Function: Command.__init__ (name, COMMAND_CLASS [, COMPLETER_CLASS
          [, PREFIX]])
     The object initializer for 'Command' registers the new command with
     GDB.  This initializer is normally invoked from the subclass' own
     '__init__' method.

     NAME is the name of the command.  If NAME consists of multiple
     words, then the initial words are looked for as prefix commands.
     In this case, if one of the prefix commands does not exist, an
     exception is raised.

     There is no support for multi-line commands.

     COMMAND_CLASS should be one of the 'COMMAND_' constants defined
     below.  This argument tells GDB how to categorize the new command
     in the help system.

     COMPLETER_CLASS is an optional argument.  If given, it should be
     one of the 'COMPLETE_' constants defined below.  This argument
     tells GDB how to perform completion for this command.  If not
     given, GDB will attempt to complete using the object's 'complete'
     method (see below); if no such method is found, an error will occur
     when completion is attempted.

     PREFIX is an optional argument.  If 'True', then the new command is
     a prefix command; sub-commands of this command may be registered.

     The help text for the new command is taken from the Python
     documentation string for the command's class, if there is one.  If
     no documentation string is provided, the default value "This
     command is not documented."  is used.

 -- Function: Command.dont_repeat ()
     By default, a GDB command is repeated when the user enters a blank
     line at the command prompt.  A command can suppress this behavior
     by invoking the 'dont_repeat' method.  This is similar to the user
     command 'dont-repeat', see *note dont-repeat: Define.

 -- Function: Command.invoke (argument, from_tty)
     This method is called by GDB when this command is invoked.

     ARGUMENT is a string.  It is the argument to the command, after
     leading and trailing whitespace has been stripped.

     FROM_TTY is a boolean argument.  When true, this means that the
     command was entered by the user at the terminal; when false it
     means that the command came from elsewhere.

     If this method throws an exception, it is turned into a GDB 'error'
     call.  Otherwise, the return value is ignored.

     To break ARGUMENT up into an argv-like string use
     'gdb.string_to_argv'.  This function behaves identically to GDB's
     internal argument lexer 'buildargv'.  It is recommended to use this
     for consistency.  Arguments are separated by spaces and may be
     quoted.  Example:

          print gdb.string_to_argv ("1 2\ \\\"3 '4 \"5' \"6 '7\"")
          ['1', '2 "3', '4 "5', "6 '7"]

 -- Function: Command.complete (text, word)
     This method is called by GDB when the user attempts completion on
     this command.  All forms of completion are handled by this method,
     that is, the <TAB> and <M-?> key bindings (*note Completion::), and
     the 'complete' command (*note complete: Help.).

     The arguments TEXT and WORD are both strings; TEXT holds the
     complete command line up to the cursor's location, while WORD holds
     the last word of the command line; this is computed using a
     word-breaking heuristic.

     The 'complete' method can return several values:
        * If the return value is a sequence, the contents of the
          sequence are used as the completions.  It is up to 'complete'
          to ensure that the contents actually do complete the word.  A
          zero-length sequence is allowed, it means that there were no
          completions available.  Only string elements of the sequence
          are used; other elements in the sequence are ignored.

        * If the return value is one of the 'COMPLETE_' constants
          defined below, then the corresponding GDB-internal completion
          function is invoked, and its result is used.

        * All other results are treated as though there were no
          available completions.

   When a new command is registered, it must be declared as a member of
some general class of commands.  This is used to classify top-level
commands in the on-line help system; note that prefix commands are not
listed under their own category but rather that of their top-level
command.  The available classifications are represented by constants
defined in the 'gdb' module:

'gdb.COMMAND_NONE'
     The command does not belong to any particular class.  A command in
     this category will not be displayed in any of the help categories.

'gdb.COMMAND_RUNNING'
     The command is related to running the inferior.  For example,
     'start', 'step', and 'continue' are in this category.  Type 'help
     running' at the GDB prompt to see a list of commands in this
     category.

'gdb.COMMAND_DATA'
     The command is related to data or variables.  For example, 'call',
     'find', and 'print' are in this category.  Type 'help data' at the
     GDB prompt to see a list of commands in this category.

'gdb.COMMAND_STACK'
     The command has to do with manipulation of the stack.  For example,
     'backtrace', 'frame', and 'return' are in this category.  Type
     'help stack' at the GDB prompt to see a list of commands in this
     category.

'gdb.COMMAND_FILES'
     This class is used for file-related commands.  For example, 'file',
     'list' and 'section' are in this category.  Type 'help files' at
     the GDB prompt to see a list of commands in this category.

'gdb.COMMAND_SUPPORT'
     This should be used for "support facilities", generally meaning
     things that are useful to the user when interacting with GDB, but
     not related to the state of the inferior.  For example, 'help',
     'make', and 'shell' are in this category.  Type 'help support' at
     the GDB prompt to see a list of commands in this category.

'gdb.COMMAND_STATUS'
     The command is an 'info'-related command, that is, related to the
     state of GDB itself.  For example, 'info', 'macro', and 'show' are
     in this category.  Type 'help status' at the GDB prompt to see a
     list of commands in this category.

'gdb.COMMAND_BREAKPOINTS'
     The command has to do with breakpoints.  For example, 'break',
     'clear', and 'delete' are in this category.  Type 'help
     breakpoints' at the GDB prompt to see a list of commands in this
     category.

'gdb.COMMAND_TRACEPOINTS'
     The command has to do with tracepoints.  For example, 'trace',
     'actions', and 'tfind' are in this category.  Type 'help
     tracepoints' at the GDB prompt to see a list of commands in this
     category.

'gdb.COMMAND_USER'
     The command is a general purpose command for the user, and
     typically does not fit in one of the other categories.  Type 'help
     user-defined' at the GDB prompt to see a list of commands in this
     category, as well as the list of gdb macros (*note Sequences::).

'gdb.COMMAND_OBSCURE'
     The command is only used in unusual circumstances, or is not of
     general interest to users.  For example, 'checkpoint', 'fork', and
     'stop' are in this category.  Type 'help obscure' at the GDB prompt
     to see a list of commands in this category.

'gdb.COMMAND_MAINTENANCE'
     The command is only useful to GDB maintainers.  The 'maintenance'
     and 'flushregs' commands are in this category.  Type 'help
     internals' at the GDB prompt to see a list of commands in this
     category.

   A new command can use a predefined completion function, either by
specifying it via an argument at initialization, or by returning it from
the 'complete' method.  These predefined completion constants are all
defined in the 'gdb' module:

'gdb.COMPLETE_NONE'
     This constant means that no completion should be done.

'gdb.COMPLETE_FILENAME'
     This constant means that filename completion should be performed.

'gdb.COMPLETE_LOCATION'
     This constant means that location completion should be done.  *Note
     Specify Location::.

'gdb.COMPLETE_COMMAND'
     This constant means that completion should examine GDB command
     names.

'gdb.COMPLETE_SYMBOL'
     This constant means that completion should be done using symbol
     names as the source.

'gdb.COMPLETE_EXPRESSION'
     This constant means that completion should be done on expressions.
     Often this means completing on symbol names, but some language
     parsers also have support for completing on field names.

   The following code snippet shows how a trivial CLI command can be
implemented in Python:

     class HelloWorld (gdb.Command):
       """Greet the whole world."""

       def __init__ (self):
         super (HelloWorld, self).__init__ ("hello-world", gdb.COMMAND_USER)

       def invoke (self, arg, from_tty):
         print "Hello, World!"

     HelloWorld ()

   The last line instantiates the class, and is necessary to trigger the
registration of the command with GDB.  Depending on how the Python code
is read into GDB, you may need to import the 'gdb' module explicitly.


File: gdb.info,  Node: Parameters In Python,  Next: Functions In Python,  Prev: Commands In Python,  Up: Python API

23.2.2.21 Parameters In Python
..............................

You can implement new GDB parameters using Python.  A new parameter is
implemented as an instance of the 'gdb.Parameter' class.

   Parameters are exposed to the user via the 'set' and 'show' commands.
*Note Help::.

   There are many parameters that already exist and can be set in GDB.
Two examples are: 'set follow fork' and 'set charset'.  Setting these
parameters influences certain behavior in GDB.  Similarly, you can
define parameters that can be used to influence behavior in custom
Python scripts and commands.

 -- Function: Parameter.__init__ (name, COMMAND-CLASS, PARAMETER-CLASS
          [, ENUM-SEQUENCE])
     The object initializer for 'Parameter' registers the new parameter
     with GDB.  This initializer is normally invoked from the subclass'
     own '__init__' method.

     NAME is the name of the new parameter.  If NAME consists of
     multiple words, then the initial words are looked for as prefix
     parameters.  An example of this can be illustrated with the 'set
     print' set of parameters.  If NAME is 'print foo', then 'print'
     will be searched as the prefix parameter.  In this case the
     parameter can subsequently be accessed in GDB as 'set print foo'.

     If NAME consists of multiple words, and no prefix parameter group
     can be found, an exception is raised.

     COMMAND-CLASS should be one of the 'COMMAND_' constants (*note
     Commands In Python::).  This argument tells GDB how to categorize
     the new parameter in the help system.

     PARAMETER-CLASS should be one of the 'PARAM_' constants defined
     below.  This argument tells GDB the type of the new parameter; this
     information is used for input validation and completion.

     If PARAMETER-CLASS is 'PARAM_ENUM', then ENUM-SEQUENCE must be a
     sequence of strings.  These strings represent the possible values
     for the parameter.

     If PARAMETER-CLASS is not 'PARAM_ENUM', then the presence of a
     fourth argument will cause an exception to be thrown.

     The help text for the new parameter is taken from the Python
     documentation string for the parameter's class, if there is one.
     If there is no documentation string, a default value is used.

 -- Variable: Parameter.set_doc
     If this attribute exists, and is a string, then its value is used
     as the help text for this parameter's 'set' command.  The value is
     examined when 'Parameter.__init__' is invoked; subsequent changes
     have no effect.

 -- Variable: Parameter.show_doc
     If this attribute exists, and is a string, then its value is used
     as the help text for this parameter's 'show' command.  The value is
     examined when 'Parameter.__init__' is invoked; subsequent changes
     have no effect.

 -- Variable: Parameter.value
     The 'value' attribute holds the underlying value of the parameter.
     It can be read and assigned to just as any other attribute.  GDB
     does validation when assignments are made.

   There are two methods that may be implemented in any 'Parameter'
class.  These are:

 -- Function: Parameter.get_set_string (self)
     If this method exists, GDB will call it when a PARAMETER's value
     has been changed via the 'set' API (for example, 'set foo off').
     The 'value' attribute has already been populated with the new value
     and may be used in output.  This method must return a string.  If
     the returned string is not empty, GDB will present it to the user.

     If this method raises the 'gdb.GdbError' exception (*note Exception
     Handling::), then GDB will print the exception's string and the
     'set' command will fail.  Note, however, that the 'value' attribute
     will not be reset in this case.  So, if your parameter must
     validate values, it should store the old value internally and reset
     the exposed value, like so:

          class ExampleParam (gdb.Parameter):
             def __init__ (self, name):
                super (ExampleParam, self).__init__ (name,
                             gdb.COMMAND_DATA,
                             gdb.PARAM_BOOLEAN)
                self.value = True
                self.saved_value = True
             def validate(self):
                return False
             def get_set_string (self):
                if not self.validate():
                  self.value = self.saved_value
                  raise gdb.GdbError('Failed to validate')
                self.saved_value = self.value

 -- Function: Parameter.get_show_string (self, svalue)
     GDB will call this method when a PARAMETER's 'show' API has been
     invoked (for example, 'show foo').  The argument 'svalue' receives
     the string representation of the current value.  This method must
     return a string.

   When a new parameter is defined, its type must be specified.  The
available types are represented by constants defined in the 'gdb'
module:

'gdb.PARAM_BOOLEAN'
     The value is a plain boolean.  The Python boolean values, 'True'
     and 'False' are the only valid values.

'gdb.PARAM_AUTO_BOOLEAN'
     The value has three possible states: true, false, and 'auto'.  In
     Python, true and false are represented using boolean constants, and
     'auto' is represented using 'None'.

'gdb.PARAM_UINTEGER'
     The value is an unsigned integer.  The value of 0 should be
     interpreted to mean "unlimited".

'gdb.PARAM_INTEGER'
     The value is a signed integer.  The value of 0 should be
     interpreted to mean "unlimited".

'gdb.PARAM_STRING'
     The value is a string.  When the user modifies the string, any
     escape sequences, such as '\t', '\f', and octal escapes, are
     translated into corresponding characters and encoded into the
     current host charset.

'gdb.PARAM_STRING_NOESCAPE'
     The value is a string.  When the user modifies the string, escapes
     are passed through untranslated.

'gdb.PARAM_OPTIONAL_FILENAME'
     The value is a either a filename (a string), or 'None'.

'gdb.PARAM_FILENAME'
     The value is a filename.  This is just like
     'PARAM_STRING_NOESCAPE', but uses file names for completion.

'gdb.PARAM_ZINTEGER'
     The value is an integer.  This is like 'PARAM_INTEGER', except 0 is
     interpreted as itself.

'gdb.PARAM_ZUINTEGER'
     The value is an unsigned integer.  This is like 'PARAM_INTEGER',
     except 0 is interpreted as itself, and the value cannot be
     negative.

'gdb.PARAM_ZUINTEGER_UNLIMITED'
     The value is a signed integer.  This is like 'PARAM_ZUINTEGER',
     except the special value -1 should be interpreted to mean
     "unlimited".  Other negative values are not allowed.

'gdb.PARAM_ENUM'
     The value is a string, which must be one of a collection string
     constants provided when the parameter is created.


File: gdb.info,  Node: Functions In Python,  Next: Progspaces In Python,  Prev: Parameters In Python,  Up: Python API

23.2.2.22 Writing new convenience functions
...........................................

You can implement new convenience functions (*note Convenience Vars::)
in Python.  A convenience function is an instance of a subclass of the
class 'gdb.Function'.

 -- Function: Function.__init__ (name)
     The initializer for 'Function' registers the new function with GDB.
     The argument NAME is the name of the function, a string.  The
     function will be visible to the user as a convenience variable of
     type 'internal function', whose name is the same as the given NAME.

     The documentation for the new function is taken from the
     documentation string for the new class.

 -- Function: Function.invoke (*ARGS)
     When a convenience function is evaluated, its arguments are
     converted to instances of 'gdb.Value', and then the function's
     'invoke' method is called.  Note that GDB does not predetermine the
     arity of convenience functions.  Instead, all available arguments
     are passed to 'invoke', following the standard Python calling
     convention.  In particular, a convenience function can have default
     values for parameters without ill effect.

     The return value of this method is used as its value in the
     enclosing expression.  If an ordinary Python value is returned, it
     is converted to a 'gdb.Value' following the usual rules.

   The following code snippet shows how a trivial convenience function
can be implemented in Python:

     class Greet (gdb.Function):
       """Return string to greet someone.
     Takes a name as argument."""

       def __init__ (self):
         super (Greet, self).__init__ ("greet")

       def invoke (self, name):
         return "Hello, %s!" % name.string ()

     Greet ()

   The last line instantiates the class, and is necessary to trigger the
registration of the function with GDB.  Depending on how the Python code
is read into GDB, you may need to import the 'gdb' module explicitly.

   Now you can use the function in an expression:

     (gdb) print $greet("Bob")
     $1 = "Hello, Bob!"


File: gdb.info,  Node: Progspaces In Python,  Next: Objfiles In Python,  Prev: Functions In Python,  Up: Python API

23.2.2.23 Program Spaces In Python
..................................

A program space, or "progspace", represents a symbolic view of an
address space.  It consists of all of the objfiles of the program.
*Note Objfiles In Python::.  *Note program spaces: Inferiors and
Programs, for more details about program spaces.

   The following progspace-related functions are available in the 'gdb'
module:

 -- Function: gdb.current_progspace ()
     This function returns the program space of the currently selected
     inferior.  *Note Inferiors and Programs::.  This is identical to
     'gdb.selected_inferior().progspace' (*note Inferiors In Python::)
     and is included for historical compatibility.

 -- Function: gdb.progspaces ()
     Return a sequence of all the progspaces currently known to GDB.

   Each progspace is represented by an instance of the 'gdb.Progspace'
class.

 -- Variable: Progspace.filename
     The file name of the progspace as a string.

 -- Variable: Progspace.pretty_printers
     The 'pretty_printers' attribute is a list of functions.  It is used
     to look up pretty-printers.  A 'Value' is passed to each function
     in order; if the function returns 'None', then the search
     continues.  Otherwise, the return value should be an object which
     is used to format the value.  *Note Pretty Printing API::, for more
     information.

 -- Variable: Progspace.type_printers
     The 'type_printers' attribute is a list of type printer objects.
     *Note Type Printing API::, for more information.

 -- Variable: Progspace.frame_filters
     The 'frame_filters' attribute is a dictionary of frame filter
     objects.  *Note Frame Filter API::, for more information.

   A program space has the following methods:

 -- Function: Progspace.block_for_pc (pc)
     Return the innermost 'gdb.Block' containing the given PC value.  If
     the block cannot be found for the PC value specified, the function
     will return 'None'.

 -- Function: Progspace.find_pc_line (pc)
     Return the 'gdb.Symtab_and_line' object corresponding to the PC
     value.  *Note Symbol Tables In Python::.  If an invalid value of PC
     is passed as an argument, then the 'symtab' and 'line' attributes
     of the returned 'gdb.Symtab_and_line' object will be 'None' and 0
     respectively.

 -- Function: Progspace.is_valid ()
     Returns 'True' if the 'gdb.Progspace' object is valid, 'False' if
     not.  A 'gdb.Progspace' object can become invalid if the program
     space file it refers to is not referenced by any inferior.  All
     other 'gdb.Progspace' methods will throw an exception if it is
     invalid at the time the method is called.

 -- Function: Progspace.objfiles ()
     Return a sequence of all the objfiles referenced by this program
     space.  *Note Objfiles In Python::.

 -- Function: Progspace.solib_name (address)
     Return the name of the shared library holding the given ADDRESS as
     a string, or 'None'.

   One may add arbitrary attributes to 'gdb.Progspace' objects in the
usual Python way.  This is useful if, for example, one needs to do some
extra record keeping associated with the program space.

   In this contrived example, we want to perform some processing when an
objfile with a certain symbol is loaded, but we only want to do this
once because it is expensive.  To achieve this we record the results
with the program space because we can't predict when the desired objfile
will be loaded.

     (gdb) python
     def clear_objfiles_handler(event):
         event.progspace.expensive_computation = None
     def expensive(symbol):
         """A mock routine to perform an "expensive" computation on symbol."""
         print "Computing the answer to the ultimate question ..."
         return 42
     def new_objfile_handler(event):
         objfile = event.new_objfile
         progspace = objfile.progspace
         if not hasattr(progspace, 'expensive_computation') or \
                 progspace.expensive_computation is None:
             # We use 'main' for the symbol to keep the example simple.
             # Note: There's no current way to constrain the lookup
             # to one objfile.
             symbol = gdb.lookup_global_symbol('main')
             if symbol is not None:
                 progspace.expensive_computation = expensive(symbol)
     gdb.events.clear_objfiles.connect(clear_objfiles_handler)
     gdb.events.new_objfile.connect(new_objfile_handler)
     end
     (gdb) file /tmp/hello
     Reading symbols from /tmp/hello...done.
     Computing the answer to the ultimate question ...
     (gdb) python print gdb.current_progspace().expensive_computation
     42
     (gdb) run
     Starting program: /tmp/hello
     Hello.
     [Inferior 1 (process 4242) exited normally]


File: gdb.info,  Node: Objfiles In Python,  Next: Frames In Python,  Prev: Progspaces In Python,  Up: Python API

23.2.2.24 Objfiles In Python
............................

GDB loads symbols for an inferior from various symbol-containing files
(*note Files::).  These include the primary executable file, any shared
libraries used by the inferior, and any separate debug info files (*note
Separate Debug Files::).  GDB calls these symbol-containing files
"objfiles".

   The following objfile-related functions are available in the 'gdb'
module:

 -- Function: gdb.current_objfile ()
     When auto-loading a Python script (*note Python Auto-loading::),
     GDB sets the "current objfile" to the corresponding objfile.  This
     function returns the current objfile.  If there is no current
     objfile, this function returns 'None'.

 -- Function: gdb.objfiles ()
     Return a sequence of objfiles referenced by the current program
     space.  *Note Objfiles In Python::, and *note Progspaces In
     Python::.  This is identical to
     'gdb.selected_inferior().progspace.objfiles()' and is included for
     historical compatibility.

 -- Function: gdb.lookup_objfile (name [, by_build_id])
     Look up NAME, a file name or build ID, in the list of objfiles for
     the current program space (*note Progspaces In Python::).  If the
     objfile is not found throw the Python 'ValueError' exception.

     If NAME is a relative file name, then it will match any source file
     name with the same trailing components.  For example, if NAME is
     'gcc/expr.c', then it will match source file name of
     '/build/trunk/gcc/expr.c', but not '/build/trunk/libcpp/expr.c' or
     '/build/trunk/gcc/x-expr.c'.

     If BY_BUILD_ID is provided and is 'True' then NAME is the build ID
     of the objfile.  Otherwise, NAME is a file name.  This is supported
     only on some operating systems, notably those which use the ELF
     format for binary files and the GNU Binutils.  For more details
     about this feature, see the description of the '--build-id'
     command-line option in *note Command Line Options: (ld)Options.

   Each objfile is represented by an instance of the 'gdb.Objfile'
class.

 -- Variable: Objfile.filename
     The file name of the objfile as a string, with symbolic links
     resolved.

     The value is 'None' if the objfile is no longer valid.  See the
     'gdb.Objfile.is_valid' method, described below.

 -- Variable: Objfile.username
     The file name of the objfile as specified by the user as a string.

     The value is 'None' if the objfile is no longer valid.  See the
     'gdb.Objfile.is_valid' method, described below.

 -- Variable: Objfile.owner
     For separate debug info objfiles this is the corresponding
     'gdb.Objfile' object that debug info is being provided for.
     Otherwise this is 'None'.  Separate debug info objfiles are added
     with the 'gdb.Objfile.add_separate_debug_file' method, described
     below.

 -- Variable: Objfile.build_id
     The build ID of the objfile as a string.  If the objfile does not
     have a build ID then the value is 'None'.

     This is supported only on some operating systems, notably those
     which use the ELF format for binary files and the GNU Binutils.
     For more details about this feature, see the description of the
     '--build-id' command-line option in *note Command Line Options:
     (ld)Options.

 -- Variable: Objfile.progspace
     The containing program space of the objfile as a 'gdb.Progspace'
     object.  *Note Progspaces In Python::.

 -- Variable: Objfile.pretty_printers
     The 'pretty_printers' attribute is a list of functions.  It is used
     to look up pretty-printers.  A 'Value' is passed to each function
     in order; if the function returns 'None', then the search
     continues.  Otherwise, the return value should be an object which
     is used to format the value.  *Note Pretty Printing API::, for more
     information.

 -- Variable: Objfile.type_printers
     The 'type_printers' attribute is a list of type printer objects.
     *Note Type Printing API::, for more information.

 -- Variable: Objfile.frame_filters
     The 'frame_filters' attribute is a dictionary of frame filter
     objects.  *Note Frame Filter API::, for more information.

   One may add arbitrary attributes to 'gdb.Objfile' objects in the
usual Python way.  This is useful if, for example, one needs to do some
extra record keeping associated with the objfile.

   In this contrived example we record the time when GDB loaded the
objfile.

     (gdb) python
     import datetime
     def new_objfile_handler(event):
         # Set the time_loaded attribute of the new objfile.
         event.new_objfile.time_loaded = datetime.datetime.today()
     gdb.events.new_objfile.connect(new_objfile_handler)
     end
     (gdb) file ./hello
     Reading symbols from ./hello...done.
     (gdb) python print gdb.objfiles()[0].time_loaded
     2014-10-09 11:41:36.770345

   A 'gdb.Objfile' object has the following methods:

 -- Function: Objfile.is_valid ()
     Returns 'True' if the 'gdb.Objfile' object is valid, 'False' if
     not.  A 'gdb.Objfile' object can become invalid if the object file
     it refers to is not loaded in GDB any longer.  All other
     'gdb.Objfile' methods will throw an exception if it is invalid at
     the time the method is called.

 -- Function: Objfile.add_separate_debug_file (file)
     Add FILE to the list of files that GDB will search for debug
     information for the objfile.  This is useful when the debug info
     has been removed from the program and stored in a separate file.
     GDB has built-in support for finding separate debug info files
     (*note Separate Debug Files::), but if the file doesn't live in one
     of the standard places that GDB searches then this function can be
     used to add a debug info file from a different place.

 -- Function: Objfile.lookup_global_symbol (name [, domain])
     Search for a global symbol named NAME in this objfile.  Optionally,
     the search scope can be restricted with the DOMAIN argument.  The
     DOMAIN argument must be a domain constant defined in the 'gdb'
     module and described in *note Symbols In Python::.  This function
     is similar to 'gdb.lookup_global_symbol', except that the search is
     limited to this objfile.

     The result is a 'gdb.Symbol' object or 'None' if the symbol is not
     found.

 -- Function: Objfile.lookup_static_symbol (name [, domain])
     Like 'Objfile.lookup_global_symbol', but searches for a global
     symbol with static linkage named NAME in this objfile.


File: gdb.info,  Node: Frames In Python,  Next: Blocks In Python,  Prev: Objfiles In Python,  Up: Python API

23.2.2.25 Accessing inferior stack frames from Python
.....................................................

When the debugged program stops, GDB is able to analyze its call stack
(*note Stack frames: Frames.).  The 'gdb.Frame' class represents a frame
in the stack.  A 'gdb.Frame' object is only valid while its
corresponding frame exists in the inferior's stack.  If you try to use
an invalid frame object, GDB will throw a 'gdb.error' exception (*note
Exception Handling::).

   Two 'gdb.Frame' objects can be compared for equality with the '=='
operator, like:

     (gdb) python print gdb.newest_frame() == gdb.selected_frame ()
     True

   The following frame-related functions are available in the 'gdb'
module:

 -- Function: gdb.selected_frame ()
     Return the selected frame object.  (*note Selecting a Frame:
     Selection.).

 -- Function: gdb.newest_frame ()
     Return the newest frame object for the selected thread.

 -- Function: gdb.frame_stop_reason_string (reason)
     Return a string explaining the reason why GDB stopped unwinding
     frames, as expressed by the given REASON code (an integer, see the
     'unwind_stop_reason' method further down in this section).

 -- Function: gdb.invalidate_cached_frames
     GDB internally keeps a cache of the frames that have been unwound.
     This function invalidates this cache.

     This function should not generally be called by ordinary Python
     code.  It is documented for the sake of completeness.

   A 'gdb.Frame' object has the following methods:

 -- Function: Frame.is_valid ()
     Returns true if the 'gdb.Frame' object is valid, false if not.  A
     frame object can become invalid if the frame it refers to doesn't
     exist anymore in the inferior.  All 'gdb.Frame' methods will throw
     an exception if it is invalid at the time the method is called.

 -- Function: Frame.name ()
     Returns the function name of the frame, or 'None' if it can't be
     obtained.

 -- Function: Frame.architecture ()
     Returns the 'gdb.Architecture' object corresponding to the frame's
     architecture.  *Note Architectures In Python::.

 -- Function: Frame.type ()
     Returns the type of the frame.  The value can be one of:
     'gdb.NORMAL_FRAME'
          An ordinary stack frame.

     'gdb.DUMMY_FRAME'
          A fake stack frame that was created by GDB when performing an
          inferior function call.

     'gdb.INLINE_FRAME'
          A frame representing an inlined function.  The function was
          inlined into a 'gdb.NORMAL_FRAME' that is older than this one.

     'gdb.TAILCALL_FRAME'
          A frame representing a tail call.  *Note Tail Call Frames::.

     'gdb.SIGTRAMP_FRAME'
          A signal trampoline frame.  This is the frame created by the
          OS when it calls into a signal handler.

     'gdb.ARCH_FRAME'
          A fake stack frame representing a cross-architecture call.

     'gdb.SENTINEL_FRAME'
          This is like 'gdb.NORMAL_FRAME', but it is only used for the
          newest frame.

 -- Function: Frame.unwind_stop_reason ()
     Return an integer representing the reason why it's not possible to
     find more frames toward the outermost frame.  Use
     'gdb.frame_stop_reason_string' to convert the value returned by
     this function to a string.  The value can be one of:

     'gdb.FRAME_UNWIND_NO_REASON'
          No particular reason (older frames should be available).

     'gdb.FRAME_UNWIND_NULL_ID'
          The previous frame's analyzer returns an invalid result.  This
          is no longer used by GDB, and is kept only for backward
          compatibility.

     'gdb.FRAME_UNWIND_OUTERMOST'
          This frame is the outermost.

     'gdb.FRAME_UNWIND_UNAVAILABLE'
          Cannot unwind further, because that would require knowing the
          values of registers or memory that have not been collected.

     'gdb.FRAME_UNWIND_INNER_ID'
          This frame ID looks like it ought to belong to a NEXT frame,
          but we got it for a PREV frame.  Normally, this is a sign of
          unwinder failure.  It could also indicate stack corruption.

     'gdb.FRAME_UNWIND_SAME_ID'
          This frame has the same ID as the previous one.  That means
          that unwinding further would almost certainly give us another
          frame with exactly the same ID, so break the chain.  Normally,
          this is a sign of unwinder failure.  It could also indicate
          stack corruption.

     'gdb.FRAME_UNWIND_NO_SAVED_PC'
          The frame unwinder did not find any saved PC, but we needed
          one to unwind further.

     'gdb.FRAME_UNWIND_MEMORY_ERROR'
          The frame unwinder caused an error while trying to access
          memory.

     'gdb.FRAME_UNWIND_FIRST_ERROR'
          Any stop reason greater or equal to this value indicates some
          kind of error.  This special value facilitates writing code
          that tests for errors in unwinding in a way that will work
          correctly even if the list of the other values is modified in
          future GDB versions.  Using it, you could write:
               reason = gdb.selected_frame().unwind_stop_reason ()
               reason_str =  gdb.frame_stop_reason_string (reason)
               if reason >=  gdb.FRAME_UNWIND_FIRST_ERROR:
                   print "An error occured: %s" % reason_str

 -- Function: Frame.pc ()
     Returns the frame's resume address.

 -- Function: Frame.block ()
     Return the frame's code block.  *Note Blocks In Python::.  If the
     frame does not have a block - for example, if there is no debugging
     information for the code in question - then this will throw an
     exception.

 -- Function: Frame.function ()
     Return the symbol for the function corresponding to this frame.
     *Note Symbols In Python::.

 -- Function: Frame.older ()
     Return the frame that called this frame.

 -- Function: Frame.newer ()
     Return the frame called by this frame.

 -- Function: Frame.find_sal ()
     Return the frame's symtab and line object.  *Note Symbol Tables In
     Python::.

 -- Function: Frame.read_register (register)
     Return the value of REGISTER in this frame.  The REGISTER argument
     must be a string (e.g., ''sp'' or ''rax'').  Returns a 'Gdb.Value'
     object.  Throws an exception if REGISTER does not exist.

 -- Function: Frame.read_var (variable [, block])
     Return the value of VARIABLE in this frame.  If the optional
     argument BLOCK is provided, search for the variable from that
     block; otherwise start at the frame's current block (which is
     determined by the frame's current program counter).  The VARIABLE
     argument must be a string or a 'gdb.Symbol' object; BLOCK must be a
     'gdb.Block' object.

 -- Function: Frame.select ()
     Set this frame to be the selected frame.  *Note Examining the
     Stack: Stack.


File: gdb.info,  Node: Blocks In Python,  Next: Symbols In Python,  Prev: Frames In Python,  Up: Python API

23.2.2.26 Accessing blocks from Python
......................................

In GDB, symbols are stored in blocks.  A block corresponds roughly to a
scope in the source code.  Blocks are organized hierarchically, and are
represented individually in Python as a 'gdb.Block'.  Blocks rely on
debugging information being available.

   A frame has a block.  Please see *note Frames In Python::, for a more
in-depth discussion of frames.

   The outermost block is known as the "global block".  The global block
typically holds public global variables and functions.

   The block nested just inside the global block is the "static block".
The static block typically holds file-scoped variables and functions.

   GDB provides a method to get a block's superblock, but there is
currently no way to examine the sub-blocks of a block, or to iterate
over all the blocks in a symbol table (*note Symbol Tables In Python::).

   Here is a short example that should help explain blocks:

     /* This is in the global block.  */
     int global;

     /* This is in the static block.  */
     static int file_scope;

     /* 'function' is in the global block, and 'argument' is
        in a block nested inside of 'function'.  */
     int function (int argument)
     {
       /* 'local' is in a block inside 'function'.  It may or may
          not be in the same block as 'argument'.  */
       int local;

       {
          /* 'inner' is in a block whose superblock is the one holding
             'local'.  */
          int inner;

          /* If this call is expanded by the compiler, you may see
             a nested block here whose function is 'inline_function'
             and whose superblock is the one holding 'inner'.  */
          inline_function ();
       }
     }

   A 'gdb.Block' is iterable.  The iterator returns the symbols (*note
Symbols In Python::) local to the block.  Python programs should not
assume that a specific block object will always contain a given symbol,
since changes in GDB features and infrastructure may cause symbols move
across blocks in a symbol table.  You can also use Python's "dictionary
syntax" to access variables in this block, e.g.:

     symbol = some_block['variable']  # symbol is of type gdb.Symbol

   The following block-related functions are available in the 'gdb'
module:

 -- Function: gdb.block_for_pc (pc)
     Return the innermost 'gdb.Block' containing the given PC value.  If
     the block cannot be found for the PC value specified, the function
     will return 'None'.  This is identical to
     'gdb.current_progspace().block_for_pc(pc)' and is included for
     historical compatibility.

   A 'gdb.Block' object has the following methods:

 -- Function: Block.is_valid ()
     Returns 'True' if the 'gdb.Block' object is valid, 'False' if not.
     A block object can become invalid if the block it refers to doesn't
     exist anymore in the inferior.  All other 'gdb.Block' methods will
     throw an exception if it is invalid at the time the method is
     called.  The block's validity is also checked during iteration over
     symbols of the block.

   A 'gdb.Block' object has the following attributes:

 -- Variable: Block.start
     The start address of the block.  This attribute is not writable.

 -- Variable: Block.end
     One past the last address that appears in the block.  This
     attribute is not writable.

 -- Variable: Block.function
     The name of the block represented as a 'gdb.Symbol'.  If the block
     is not named, then this attribute holds 'None'.  This attribute is
     not writable.

     For ordinary function blocks, the superblock is the static block.
     However, you should note that it is possible for a function block
     to have a superblock that is not the static block - for instance
     this happens for an inlined function.

 -- Variable: Block.superblock
     The block containing this block.  If this parent block does not
     exist, this attribute holds 'None'.  This attribute is not
     writable.

 -- Variable: Block.global_block
     The global block associated with this block.  This attribute is not
     writable.

 -- Variable: Block.static_block
     The static block associated with this block.  This attribute is not
     writable.

 -- Variable: Block.is_global
     'True' if the 'gdb.Block' object is a global block, 'False' if not.
     This attribute is not writable.

 -- Variable: Block.is_static
     'True' if the 'gdb.Block' object is a static block, 'False' if not.
     This attribute is not writable.


File: gdb.info,  Node: Symbols In Python,  Next: Symbol Tables In Python,  Prev: Blocks In Python,  Up: Python API

23.2.2.27 Python representation of Symbols
..........................................

GDB represents every variable, function and type as an entry in a symbol
table.  *Note Examining the Symbol Table: Symbols.  Similarly, Python
represents these symbols in GDB with the 'gdb.Symbol' object.

   The following symbol-related functions are available in the 'gdb'
module:

 -- Function: gdb.lookup_symbol (name [, block [, domain]])
     This function searches for a symbol by name.  The search scope can
     be restricted to the parameters defined in the optional domain and
     block arguments.

     NAME is the name of the symbol.  It must be a string.  The optional
     BLOCK argument restricts the search to symbols visible in that
     BLOCK.  The BLOCK argument must be a 'gdb.Block' object.  If
     omitted, the block for the current frame is used.  The optional
     DOMAIN argument restricts the search to the domain type.  The
     DOMAIN argument must be a domain constant defined in the 'gdb'
     module and described later in this chapter.

     The result is a tuple of two elements.  The first element is a
     'gdb.Symbol' object or 'None' if the symbol is not found.  If the
     symbol is found, the second element is 'True' if the symbol is a
     field of a method's object (e.g., 'this' in C++), otherwise it is
     'False'.  If the symbol is not found, the second element is
     'False'.

 -- Function: gdb.lookup_global_symbol (name [, domain])
     This function searches for a global symbol by name.  The search
     scope can be restricted to by the domain argument.

     NAME is the name of the symbol.  It must be a string.  The optional
     DOMAIN argument restricts the search to the domain type.  The
     DOMAIN argument must be a domain constant defined in the 'gdb'
     module and described later in this chapter.

     The result is a 'gdb.Symbol' object or 'None' if the symbol is not
     found.

 -- Function: gdb.lookup_static_symbol (name [, domain])
     This function searches for a global symbol with static linkage by
     name.  The search scope can be restricted to by the domain
     argument.

     NAME is the name of the symbol.  It must be a string.  The optional
     DOMAIN argument restricts the search to the domain type.  The
     DOMAIN argument must be a domain constant defined in the 'gdb'
     module and described later in this chapter.

     The result is a 'gdb.Symbol' object or 'None' if the symbol is not
     found.

     Note that this function will not find function-scoped static
     variables.  To look up such variables, iterate over the variables
     of the function's 'gdb.Block' and check that 'block.addr_class' is
     'gdb.SYMBOL_LOC_STATIC'.

     There can be multiple global symbols with static linkage with the
     same name.  This function will only return the first matching
     symbol that it finds.  Which symbol is found depends on where GDB
     is currently stopped, as GDB will first search for matching symbols
     in the current object file, and then search all other object files.
     If the application is not yet running then GDB will search all
     object files in the order they appear in the debug information.

 -- Function: gdb.lookup_global_symbol (name [, domain])
     This function searches for a global symbol by name.  The search
     scope can be restricted to by the domain argument.

     NAME is the name of the symbol.  It must be a string.  The optional
     DOMAIN argument restricts the search to the domain type.  The
     DOMAIN argument must be a domain constant defined in the 'gdb'
     module and described later in this chapter.

     The result is a 'gdb.Symbol' object or 'None' if the symbol is not
     found.

 -- Function: gdb.lookup_static_symbols (name [, domain])
     Similar to 'gdb.lookup_static_symbol', this function searches for
     global symbols with static linkage by name, and optionally
     restricted by the domain argument.  However, this function returns
     a list of all matching symbols found, not just the first one.

     NAME is the name of the symbol.  It must be a string.  The optional
     DOMAIN argument restricts the search to the domain type.  The
     DOMAIN argument must be a domain constant defined in the 'gdb'
     module and described later in this chapter.

     The result is a list of 'gdb.Symbol' objects which could be empty
     if no matching symbols were found.

     Note that this function will not find function-scoped static
     variables.  To look up such variables, iterate over the variables
     of the function's 'gdb.Block' and check that 'block.addr_class' is
     'gdb.SYMBOL_LOC_STATIC'.

   A 'gdb.Symbol' object has the following attributes:

 -- Variable: Symbol.type
     The type of the symbol or 'None' if no type is recorded.  This
     attribute is represented as a 'gdb.Type' object.  *Note Types In
     Python::.  This attribute is not writable.

 -- Variable: Symbol.symtab
     The symbol table in which the symbol appears.  This attribute is
     represented as a 'gdb.Symtab' object.  *Note Symbol Tables In
     Python::.  This attribute is not writable.

 -- Variable: Symbol.line
     The line number in the source code at which the symbol was defined.
     This is an integer.

 -- Variable: Symbol.name
     The name of the symbol as a string.  This attribute is not
     writable.

 -- Variable: Symbol.linkage_name
     The name of the symbol, as used by the linker (i.e., may be
     mangled).  This attribute is not writable.

 -- Variable: Symbol.print_name
     The name of the symbol in a form suitable for output.  This is
     either 'name' or 'linkage_name', depending on whether the user
     asked GDB to display demangled or mangled names.

 -- Variable: Symbol.addr_class
     The address class of the symbol.  This classifies how to find the
     value of a symbol.  Each address class is a constant defined in the
     'gdb' module and described later in this chapter.

 -- Variable: Symbol.needs_frame
     This is 'True' if evaluating this symbol's value requires a frame
     (*note Frames In Python::) and 'False' otherwise.  Typically, local
     variables will require a frame, but other symbols will not.

 -- Variable: Symbol.is_argument
     'True' if the symbol is an argument of a function.

 -- Variable: Symbol.is_constant
     'True' if the symbol is a constant.

 -- Variable: Symbol.is_function
     'True' if the symbol is a function or a method.

 -- Variable: Symbol.is_variable
     'True' if the symbol is a variable.

   A 'gdb.Symbol' object has the following methods:

 -- Function: Symbol.is_valid ()
     Returns 'True' if the 'gdb.Symbol' object is valid, 'False' if not.
     A 'gdb.Symbol' object can become invalid if the symbol it refers to
     does not exist in GDB any longer.  All other 'gdb.Symbol' methods
     will throw an exception if it is invalid at the time the method is
     called.

 -- Function: Symbol.value ([frame])
     Compute the value of the symbol, as a 'gdb.Value'.  For functions,
     this computes the address of the function, cast to the appropriate
     type.  If the symbol requires a frame in order to compute its
     value, then FRAME must be given.  If FRAME is not given, or if
     FRAME is invalid, then this method will throw an exception.

   The available domain categories in 'gdb.Symbol' are represented as
constants in the 'gdb' module:

'gdb.SYMBOL_UNDEF_DOMAIN'
     This is used when a domain has not been discovered or none of the
     following domains apply.  This usually indicates an error either in
     the symbol information or in GDB's handling of symbols.

'gdb.SYMBOL_VAR_DOMAIN'
     This domain contains variables, function names, typedef names and
     enum type values.

'gdb.SYMBOL_STRUCT_DOMAIN'
     This domain holds struct, union and enum type names.

'gdb.SYMBOL_LABEL_DOMAIN'
     This domain contains names of labels (for gotos).

'gdb.SYMBOL_MODULE_DOMAIN'
     This domain contains names of Fortran module types.

'gdb.SYMBOL_COMMON_BLOCK_DOMAIN'
     This domain contains names of Fortran common blocks.

   The available address class categories in 'gdb.Symbol' are
represented as constants in the 'gdb' module:

'gdb.SYMBOL_LOC_UNDEF'
     If this is returned by address class, it indicates an error either
     in the symbol information or in GDB's handling of symbols.

'gdb.SYMBOL_LOC_CONST'
     Value is constant int.

'gdb.SYMBOL_LOC_STATIC'
     Value is at a fixed address.

'gdb.SYMBOL_LOC_REGISTER'
     Value is in a register.

'gdb.SYMBOL_LOC_ARG'
     Value is an argument.  This value is at the offset stored within
     the symbol inside the frame's argument list.

'gdb.SYMBOL_LOC_REF_ARG'
     Value address is stored in the frame's argument list.  Just like
     'LOC_ARG' except that the value's address is stored at the offset,
     not the value itself.

'gdb.SYMBOL_LOC_REGPARM_ADDR'
     Value is a specified register.  Just like 'LOC_REGISTER' except the
     register holds the address of the argument instead of the argument
     itself.

'gdb.SYMBOL_LOC_LOCAL'
     Value is a local variable.

'gdb.SYMBOL_LOC_TYPEDEF'
     Value not used.  Symbols in the domain 'SYMBOL_STRUCT_DOMAIN' all
     have this class.

'gdb.SYMBOL_LOC_BLOCK'
     Value is a block.

'gdb.SYMBOL_LOC_CONST_BYTES'
     Value is a byte-sequence.

'gdb.SYMBOL_LOC_UNRESOLVED'
     Value is at a fixed address, but the address of the variable has to
     be determined from the minimal symbol table whenever the variable
     is referenced.

'gdb.SYMBOL_LOC_OPTIMIZED_OUT'
     The value does not actually exist in the program.

'gdb.SYMBOL_LOC_COMPUTED'
     The value's address is a computed location.

'gdb.SYMBOL_LOC_COMPUTED'
     The value's address is a symbol.  This is only used for Fortran
     common blocks.


File: gdb.info,  Node: Symbol Tables In Python,  Next: Line Tables In Python,  Prev: Symbols In Python,  Up: Python API

23.2.2.28 Symbol table representation in Python
...............................................

Access to symbol table data maintained by GDB on the inferior is exposed
to Python via two objects: 'gdb.Symtab_and_line' and 'gdb.Symtab'.
Symbol table and line data for a frame is returned from the 'find_sal'
method in 'gdb.Frame' object.  *Note Frames In Python::.

   For more information on GDB's symbol table management, see *note
Examining the Symbol Table: Symbols, for more information.

   A 'gdb.Symtab_and_line' object has the following attributes:

 -- Variable: Symtab_and_line.symtab
     The symbol table object ('gdb.Symtab') for this frame.  This
     attribute is not writable.

 -- Variable: Symtab_and_line.pc
     Indicates the start of the address range occupied by code for the
     current source line.  This attribute is not writable.

 -- Variable: Symtab_and_line.last
     Indicates the end of the address range occupied by code for the
     current source line.  This attribute is not writable.

 -- Variable: Symtab_and_line.line
     Indicates the current line number for this object.  This attribute
     is not writable.

   A 'gdb.Symtab_and_line' object has the following methods:

 -- Function: Symtab_and_line.is_valid ()
     Returns 'True' if the 'gdb.Symtab_and_line' object is valid,
     'False' if not.  A 'gdb.Symtab_and_line' object can become invalid
     if the Symbol table and line object it refers to does not exist in
     GDB any longer.  All other 'gdb.Symtab_and_line' methods will throw
     an exception if it is invalid at the time the method is called.

   A 'gdb.Symtab' object has the following attributes:

 -- Variable: Symtab.filename
     The symbol table's source filename.  This attribute is not
     writable.

 -- Variable: Symtab.objfile
     The symbol table's backing object file.  *Note Objfiles In
     Python::.  This attribute is not writable.

 -- Variable: Symtab.producer
     The name and possibly version number of the program that compiled
     the code in the symbol table.  The contents of this string is up to
     the compiler.  If no producer information is available then 'None'
     is returned.  This attribute is not writable.

   A 'gdb.Symtab' object has the following methods:

 -- Function: Symtab.is_valid ()
     Returns 'True' if the 'gdb.Symtab' object is valid, 'False' if not.
     A 'gdb.Symtab' object can become invalid if the symbol table it
     refers to does not exist in GDB any longer.  All other 'gdb.Symtab'
     methods will throw an exception if it is invalid at the time the
     method is called.

 -- Function: Symtab.fullname ()
     Return the symbol table's source absolute file name.

 -- Function: Symtab.global_block ()
     Return the global block of the underlying symbol table.  *Note
     Blocks In Python::.

 -- Function: Symtab.static_block ()
     Return the static block of the underlying symbol table.  *Note
     Blocks In Python::.

 -- Function: Symtab.linetable ()
     Return the line table associated with the symbol table.  *Note Line
     Tables In Python::.


File: gdb.info,  Node: Line Tables In Python,  Next: Breakpoints In Python,  Prev: Symbol Tables In Python,  Up: Python API

23.2.2.29 Manipulating line tables using Python
...............................................

Python code can request and inspect line table information from a symbol
table that is loaded in GDB.  A line table is a mapping of source lines
to their executable locations in memory.  To acquire the line table
information for a particular symbol table, use the 'linetable' function
(*note Symbol Tables In Python::).

   A 'gdb.LineTable' is iterable.  The iterator returns 'LineTableEntry'
objects that correspond to the source line and address for each line
table entry.  'LineTableEntry' objects have the following attributes:

 -- Variable: LineTableEntry.line
     The source line number for this line table entry.  This number
     corresponds to the actual line of source.  This attribute is not
     writable.

 -- Variable: LineTableEntry.pc
     The address that is associated with the line table entry where the
     executable code for that source line resides in memory.  This
     attribute is not writable.

   As there can be multiple addresses for a single source line, you may
receive multiple 'LineTableEntry' objects with matching 'line'
attributes, but with different 'pc' attributes.  The iterator is sorted
in ascending 'pc' order.  Here is a small example illustrating iterating
over a line table.

     symtab = gdb.selected_frame().find_sal().symtab
     linetable = symtab.linetable()
     for line in linetable:
        print "Line: "+str(line.line)+" Address: "+hex(line.pc)

   This will have the following output:

     Line: 33 Address: 0x4005c8L
     Line: 37 Address: 0x4005caL
     Line: 39 Address: 0x4005d2L
     Line: 40 Address: 0x4005f8L
     Line: 42 Address: 0x4005ffL
     Line: 44 Address: 0x400608L
     Line: 42 Address: 0x40060cL
     Line: 45 Address: 0x400615L

   In addition to being able to iterate over a 'LineTable', it also has
the following direct access methods:

 -- Function: LineTable.line (line)
     Return a Python 'Tuple' of 'LineTableEntry' objects for any entries
     in the line table for the given LINE, which specifies the source
     code line.  If there are no entries for that source code LINE, the
     Python 'None' is returned.

 -- Function: LineTable.has_line (line)
     Return a Python 'Boolean' indicating whether there is an entry in
     the line table for this source line.  Return 'True' if an entry is
     found, or 'False' if not.

 -- Function: LineTable.source_lines ()
     Return a Python 'List' of the source line numbers in the symbol
     table.  Only lines with executable code locations are returned.
     The contents of the 'List' will just be the source line entries
     represented as Python 'Long' values.


File: gdb.info,  Node: Breakpoints In Python,  Next: Finish Breakpoints in Python,  Prev: Line Tables In Python,  Up: Python API

23.2.2.30 Manipulating breakpoints using Python
...............................................

Python code can manipulate breakpoints via the 'gdb.Breakpoint' class.

   A breakpoint can be created using one of the two forms of the
'gdb.Breakpoint' constructor.  The first one accepts a string like one
would pass to the 'break' (*note Setting Breakpoints: Set Breaks.) and
'watch' (*note Setting Watchpoints: Set Watchpoints.) commands, and can
be used to create both breakpoints and watchpoints.  The second accepts
separate Python arguments similar to *note Explicit Locations::, and can
only be used to create breakpoints.

 -- Function: Breakpoint.__init__ (spec [, type ][, wp_class ][,
          internal ][, temporary ][, qualified ])
     Create a new breakpoint according to SPEC, which is a string naming
     the location of a breakpoint, or an expression that defines a
     watchpoint.  The string should describe a location in a format
     recognized by the 'break' command (*note Setting Breakpoints: Set
     Breaks.) or, in the case of a watchpoint, by the 'watch' command
     (*note Setting Watchpoints: Set Watchpoints.).

     The optional TYPE argument specifies the type of the breakpoint to
     create, as defined below.

     The optional WP_CLASS argument defines the class of watchpoint to
     create, if TYPE is 'gdb.BP_WATCHPOINT'.  If WP_CLASS is omitted, it
     defaults to 'gdb.WP_WRITE'.

     The optional INTERNAL argument allows the breakpoint to become
     invisible to the user.  The breakpoint will neither be reported
     when created, nor will it be listed in the output from 'info
     breakpoints' (but will be listed with the 'maint info breakpoints'
     command).

     The optional TEMPORARY argument makes the breakpoint a temporary
     breakpoint.  Temporary breakpoints are deleted after they have been
     hit.  Any further access to the Python breakpoint after it has been
     hit will result in a runtime error (as that breakpoint has now been
     automatically deleted).

     The optional QUALIFIED argument is a boolean that allows
     interpreting the function passed in 'spec' as a fully-qualified
     name.  It is equivalent to 'break''s '-qualified' flag (*note
     Linespec Locations:: and *note Explicit Locations::).

 -- Function: Breakpoint.__init__ ([ source ][, function ][, label ][,
          line ], ][ internal ][, temporary ][, qualified ])
     This second form of creating a new breakpoint specifies the
     explicit location (*note Explicit Locations::) using keywords.  The
     new breakpoint will be created in the specified source file SOURCE,
     at the specified FUNCTION, LABEL and LINE.

     INTERNAL, TEMPORARY and QUALIFIED have the same usage as explained
     previously.

   The available types are represented by constants defined in the 'gdb'
module:

'gdb.BP_BREAKPOINT'
     Normal code breakpoint.

'gdb.BP_WATCHPOINT'
     Watchpoint breakpoint.

'gdb.BP_HARDWARE_WATCHPOINT'
     Hardware assisted watchpoint.

'gdb.BP_READ_WATCHPOINT'
     Hardware assisted read watchpoint.

'gdb.BP_ACCESS_WATCHPOINT'
     Hardware assisted access watchpoint.

   The available watchpoint types represented by constants are defined
in the 'gdb' module:

'gdb.WP_READ'
     Read only watchpoint.

'gdb.WP_WRITE'
     Write only watchpoint.

'gdb.WP_ACCESS'
     Read/Write watchpoint.

 -- Function: Breakpoint.stop (self)
     The 'gdb.Breakpoint' class can be sub-classed and, in particular,
     you may choose to implement the 'stop' method.  If this method is
     defined in a sub-class of 'gdb.Breakpoint', it will be called when
     the inferior reaches any location of a breakpoint which
     instantiates that sub-class.  If the method returns 'True', the
     inferior will be stopped at the location of the breakpoint,
     otherwise the inferior will continue.

     If there are multiple breakpoints at the same location with a
     'stop' method, each one will be called regardless of the return
     status of the previous.  This ensures that all 'stop' methods have
     a chance to execute at that location.  In this scenario if one of
     the methods returns 'True' but the others return 'False', the
     inferior will still be stopped.

     You should not alter the execution state of the inferior (i.e.,
     step, next, etc.), alter the current frame context (i.e., change
     the current active frame), or alter, add or delete any breakpoint.
     As a general rule, you should not alter any data within GDB or the
     inferior at this time.

     Example 'stop' implementation:

          class MyBreakpoint (gdb.Breakpoint):
                def stop (self):
                  inf_val = gdb.parse_and_eval("foo")
                  if inf_val == 3:
                    return True
                  return False

 -- Function: Breakpoint.is_valid ()
     Return 'True' if this 'Breakpoint' object is valid, 'False'
     otherwise.  A 'Breakpoint' object can become invalid if the user
     deletes the breakpoint.  In this case, the object still exists, but
     the underlying breakpoint does not.  In the cases of watchpoint
     scope, the watchpoint remains valid even if execution of the
     inferior leaves the scope of that watchpoint.

 -- Function: Breakpoint.delete ()
     Permanently deletes the GDB breakpoint.  This also invalidates the
     Python 'Breakpoint' object.  Any further access to this object's
     attributes or methods will raise an error.

 -- Variable: Breakpoint.enabled
     This attribute is 'True' if the breakpoint is enabled, and 'False'
     otherwise.  This attribute is writable.  You can use it to enable
     or disable the breakpoint.

 -- Variable: Breakpoint.silent
     This attribute is 'True' if the breakpoint is silent, and 'False'
     otherwise.  This attribute is writable.

     Note that a breakpoint can also be silent if it has commands and
     the first command is 'silent'.  This is not reported by the
     'silent' attribute.

 -- Variable: Breakpoint.pending
     This attribute is 'True' if the breakpoint is pending, and 'False'
     otherwise.  *Note Set Breaks::.  This attribute is read-only.

 -- Variable: Breakpoint.thread
     If the breakpoint is thread-specific, this attribute holds the
     thread's global id.  If the breakpoint is not thread-specific, this
     attribute is 'None'.  This attribute is writable.

 -- Variable: Breakpoint.task
     If the breakpoint is Ada task-specific, this attribute holds the
     Ada task id.  If the breakpoint is not task-specific (or the
     underlying language is not Ada), this attribute is 'None'.  This
     attribute is writable.

 -- Variable: Breakpoint.ignore_count
     This attribute holds the ignore count for the breakpoint, an
     integer.  This attribute is writable.

 -- Variable: Breakpoint.number
     This attribute holds the breakpoint's number -- the identifier used
     by the user to manipulate the breakpoint.  This attribute is not
     writable.

 -- Variable: Breakpoint.type
     This attribute holds the breakpoint's type -- the identifier used
     to determine the actual breakpoint type or use-case.  This
     attribute is not writable.

 -- Variable: Breakpoint.visible
     This attribute tells whether the breakpoint is visible to the user
     when set, or when the 'info breakpoints' command is run.  This
     attribute is not writable.

 -- Variable: Breakpoint.temporary
     This attribute indicates whether the breakpoint was created as a
     temporary breakpoint.  Temporary breakpoints are automatically
     deleted after that breakpoint has been hit.  Access to this
     attribute, and all other attributes and functions other than the
     'is_valid' function, will result in an error after the breakpoint
     has been hit (as it has been automatically deleted).  This
     attribute is not writable.

 -- Variable: Breakpoint.hit_count
     This attribute holds the hit count for the breakpoint, an integer.
     This attribute is writable, but currently it can only be set to
     zero.

 -- Variable: Breakpoint.location
     This attribute holds the location of the breakpoint, as specified
     by the user.  It is a string.  If the breakpoint does not have a
     location (that is, it is a watchpoint) the attribute's value is
     'None'.  This attribute is not writable.

 -- Variable: Breakpoint.expression
     This attribute holds a breakpoint expression, as specified by the
     user.  It is a string.  If the breakpoint does not have an
     expression (the breakpoint is not a watchpoint) the attribute's
     value is 'None'.  This attribute is not writable.

 -- Variable: Breakpoint.condition
     This attribute holds the condition of the breakpoint, as specified
     by the user.  It is a string.  If there is no condition, this
     attribute's value is 'None'.  This attribute is writable.

 -- Variable: Breakpoint.commands
     This attribute holds the commands attached to the breakpoint.  If
     there are commands, this attribute's value is a string holding all
     the commands, separated by newlines.  If there are no commands,
     this attribute is 'None'.  This attribute is writable.


File: gdb.info,  Node: Finish Breakpoints in Python,  Next: Lazy Strings In Python,  Prev: Breakpoints In Python,  Up: Python API

23.2.2.31 Finish Breakpoints
............................

A finish breakpoint is a temporary breakpoint set at the return address
of a frame, based on the 'finish' command.  'gdb.FinishBreakpoint'
extends 'gdb.Breakpoint'.  The underlying breakpoint will be disabled
and deleted when the execution will run out of the breakpoint scope
(i.e. 'Breakpoint.stop' or 'FinishBreakpoint.out_of_scope' triggered).
Finish breakpoints are thread specific and must be create with the right
thread selected.

 -- Function: FinishBreakpoint.__init__ ([frame] [, internal])
     Create a finish breakpoint at the return address of the 'gdb.Frame'
     object FRAME.  If FRAME is not provided, this defaults to the
     newest frame.  The optional INTERNAL argument allows the breakpoint
     to become invisible to the user.  *Note Breakpoints In Python::,
     for further details about this argument.

 -- Function: FinishBreakpoint.out_of_scope (self)
     In some circumstances (e.g. 'longjmp', C++ exceptions, GDB 'return'
     command, ...), a function may not properly terminate, and thus
     never hit the finish breakpoint.  When GDB notices such a
     situation, the 'out_of_scope' callback will be triggered.

     You may want to sub-class 'gdb.FinishBreakpoint' and override this
     method:

          class MyFinishBreakpoint (gdb.FinishBreakpoint)
              def stop (self):
                  print "normal finish"
                  return True

              def out_of_scope ():
                  print "abnormal finish"

 -- Variable: FinishBreakpoint.return_value
     When GDB is stopped at a finish breakpoint and the frame used to
     build the 'gdb.FinishBreakpoint' object had debug symbols, this
     attribute will contain a 'gdb.Value' object corresponding to the
     return value of the function.  The value will be 'None' if the
     function return type is 'void' or if the return value was not
     computable.  This attribute is not writable.


File: gdb.info,  Node: Lazy Strings In Python,  Next: Architectures In Python,  Prev: Finish Breakpoints in Python,  Up: Python API

23.2.2.32 Python representation of lazy strings
...............................................

A "lazy string" is a string whose contents is not retrieved or encoded
until it is needed.

   A 'gdb.LazyString' is represented in GDB as an 'address' that points
to a region of memory, an 'encoding' that will be used to encode that
region of memory, and a 'length' to delimit the region of memory that
represents the string.  The difference between a 'gdb.LazyString' and a
string wrapped within a 'gdb.Value' is that a 'gdb.LazyString' will be
treated differently by GDB when printing.  A 'gdb.LazyString' is
retrieved and encoded during printing, while a 'gdb.Value' wrapping a
string is immediately retrieved and encoded on creation.

   A 'gdb.LazyString' object has the following functions:

 -- Function: LazyString.value ()
     Convert the 'gdb.LazyString' to a 'gdb.Value'.  This value will
     point to the string in memory, but will lose all the delayed
     retrieval, encoding and handling that GDB applies to a
     'gdb.LazyString'.

 -- Variable: LazyString.address
     This attribute holds the address of the string.  This attribute is
     not writable.

 -- Variable: LazyString.length
     This attribute holds the length of the string in characters.  If
     the length is -1, then the string will be fetched and encoded up to
     the first null of appropriate width.  This attribute is not
     writable.

 -- Variable: LazyString.encoding
     This attribute holds the encoding that will be applied to the
     string when the string is printed by GDB.  If the encoding is not
     set, or contains an empty string, then GDB will select the most
     appropriate encoding when the string is printed.  This attribute is
     not writable.

 -- Variable: LazyString.type
     This attribute holds the type that is represented by the lazy
     string's type.  For a lazy string this is a pointer or array type.
     To resolve this to the lazy string's character type, use the type's
     'target' method.  *Note Types In Python::.  This attribute is not
     writable.


File: gdb.info,  Node: Architectures In Python,  Prev: Lazy Strings In Python,  Up: Python API

23.2.2.33 Python representation of architectures
................................................

GDB uses architecture specific parameters and artifacts in a number of
its various computations.  An architecture is represented by an instance
of the 'gdb.Architecture' class.

   A 'gdb.Architecture' class has the following methods:

 -- Function: Architecture.name ()
     Return the name (string value) of the architecture.

 -- Function: Architecture.disassemble (START_PC [, END_PC [, COUNT]])
     Return a list of disassembled instructions starting from the memory
     address START_PC.  The optional arguments END_PC and COUNT
     determine the number of instructions in the returned list.  If both
     the optional arguments END_PC and COUNT are specified, then a list
     of at most COUNT disassembled instructions whose start address
     falls in the closed memory address interval from START_PC to END_PC
     are returned.  If END_PC is not specified, but COUNT is specified,
     then COUNT number of instructions starting from the address
     START_PC are returned.  If COUNT is not specified but END_PC is
     specified, then all instructions whose start address falls in the
     closed memory address interval from START_PC to END_PC are
     returned.  If neither END_PC nor COUNT are specified, then a single
     instruction at START_PC is returned.  For all of these cases, each
     element of the returned list is a Python 'dict' with the following
     string keys:

     'addr'
          The value corresponding to this key is a Python long integer
          capturing the memory address of the instruction.

     'asm'
          The value corresponding to this key is a string value which
          represents the instruction with assembly language mnemonics.
          The assembly language flavor used is the same as that
          specified by the current CLI variable 'disassembly-flavor'.
          *Note Machine Code::.

     'length'
          The value corresponding to this key is the length (integer
          value) of the instruction in bytes.


File: gdb.info,  Node: Python Auto-loading,  Next: Python modules,  Prev: Python API,  Up: Python

23.2.3 Python Auto-loading
--------------------------

When a new object file is read (for example, due to the 'file' command,
or because the inferior has loaded a shared library), GDB will look for
Python support scripts in several ways: 'OBJFILE-gdb.py' and
'.debug_gdb_scripts' section.  *Note Auto-loading extensions::.

   The auto-loading feature is useful for supplying application-specific
debugging commands and scripts.

   Auto-loading can be enabled or disabled, and the list of auto-loaded
scripts can be printed.

'set auto-load python-scripts [on|off]'
     Enable or disable the auto-loading of Python scripts.

'show auto-load python-scripts'
     Show whether auto-loading of Python scripts is enabled or disabled.

'info auto-load python-scripts [REGEXP]'
     Print the list of all Python scripts that GDB auto-loaded.

     Also printed is the list of Python scripts that were mentioned in
     the '.debug_gdb_scripts' section and were either not found (*note
     dotdebug_gdb_scripts section::) or were not auto-loaded due to
     'auto-load safe-path' rejection (*note Auto-loading::).  This is
     useful because their names are not printed when GDB tries to load
     them and fails.  There may be many of them, and printing an error
     message for each one is problematic.

     If REGEXP is supplied only Python scripts with matching names are
     printed.

     Example:

          (gdb) info auto-load python-scripts
          Loaded Script
          Yes    py-section-script.py
                 full name: /tmp/py-section-script.py
          No     my-foo-pretty-printers.py

   When reading an auto-loaded file or script, GDB sets the "current
objfile".  This is available via the 'gdb.current_objfile' function
(*note Objfiles In Python::).  This can be useful for registering
objfile-specific pretty-printers and frame-filters.


File: gdb.info,  Node: Python modules,  Prev: Python Auto-loading,  Up: Python

23.2.4 Python modules
---------------------

GDB comes with several modules to assist writing Python code.

* Menu:

* gdb.printing::       Building and registering pretty-printers.
* gdb.types::          Utilities for working with types.
* gdb.prompt::         Utilities for prompt value substitution.


File: gdb.info,  Node: gdb.printing,  Next: gdb.types,  Up: Python modules

23.2.4.1 gdb.printing
.....................

This module provides a collection of utilities for working with
pretty-printers.

'PrettyPrinter (NAME, SUBPRINTERS=None)'
     This class specifies the API that makes 'info pretty-printer',
     'enable pretty-printer' and 'disable pretty-printer' work.
     Pretty-printers should generally inherit from this class.

'SubPrettyPrinter (NAME)'
     For printers that handle multiple types, this class specifies the
     corresponding API for the subprinters.

'RegexpCollectionPrettyPrinter (NAME)'
     Utility class for handling multiple printers, all recognized via
     regular expressions.  *Note Writing a Pretty-Printer::, for an
     example.

'FlagEnumerationPrinter (NAME)'
     A pretty-printer which handles printing of 'enum' values.  Unlike
     GDB's built-in 'enum' printing, this printer attempts to work
     properly when there is some overlap between the enumeration
     constants.  The argument NAME is the name of the printer and also
     the name of the 'enum' type to look up.

'register_pretty_printer (OBJ, PRINTER, REPLACE=False)'
     Register PRINTER with the pretty-printer list of OBJ.  If REPLACE
     is 'True' then any existing copy of the printer is replaced.
     Otherwise a 'RuntimeError' exception is raised if a printer with
     the same name already exists.


File: gdb.info,  Node: gdb.types,  Next: gdb.prompt,  Prev: gdb.printing,  Up: Python modules

23.2.4.2 gdb.types
..................

This module provides a collection of utilities for working with
'gdb.Type' objects.

'get_basic_type (TYPE)'
     Return TYPE with const and volatile qualifiers stripped, and with
     typedefs and C++ references converted to the underlying type.

     C++ example:

          typedef const int const_int;
          const_int foo (3);
          const_int& foo_ref (foo);
          int main () { return 0; }

     Then in gdb:

          (gdb) start
          (gdb) python import gdb.types
          (gdb) python foo_ref = gdb.parse_and_eval("foo_ref")
          (gdb) python print gdb.types.get_basic_type(foo_ref.type)
          int

'has_field (TYPE, FIELD)'
     Return 'True' if TYPE, assumed to be a type with fields (e.g., a
     structure or union), has field FIELD.

'make_enum_dict (ENUM_TYPE)'
     Return a Python 'dictionary' type produced from ENUM_TYPE.

'deep_items (TYPE)'
     Returns a Python iterator similar to the standard
     'gdb.Type.iteritems' method, except that the iterator returned by
     'deep_items' will recursively traverse anonymous struct or union
     fields.  For example:

          struct A
          {
              int a;
              union {
                  int b0;
                  int b1;
              };
          };

     Then in GDB:
          (gdb) python import gdb.types
          (gdb) python struct_a = gdb.lookup_type("struct A")
          (gdb) python print struct_a.keys ()
          {['a', '']}
          (gdb) python print [k for k,v in gdb.types.deep_items(struct_a)]
          {['a', 'b0', 'b1']}

'get_type_recognizers ()'
     Return a list of the enabled type recognizers for the current
     context.  This is called by GDB during the type-printing process
     (*note Type Printing API::).

'apply_type_recognizers (recognizers, type_obj)'
     Apply the type recognizers, RECOGNIZERS, to the type object
     TYPE_OBJ.  If any recognizer returns a string, return that string.
     Otherwise, return 'None'.  This is called by GDB during the
     type-printing process (*note Type Printing API::).

'register_type_printer (locus, printer)'
     This is a convenience function to register a type printer PRINTER.
     The printer must implement the type printer protocol.  The LOCUS
     argument is either a 'gdb.Objfile', in which case the printer is
     registered with that objfile; a 'gdb.Progspace', in which case the
     printer is registered with that progspace; or 'None', in which case
     the printer is registered globally.

'TypePrinter'
     This is a base class that implements the type printer protocol.
     Type printers are encouraged, but not required, to derive from this
     class.  It defines a constructor:

      -- Method on TypePrinter: __init__ (self, name)
          Initialize the type printer with the given name.  The new
          printer starts in the enabled state.


File: gdb.info,  Node: gdb.prompt,  Prev: gdb.types,  Up: Python modules

23.2.4.3 gdb.prompt
...................

This module provides a method for prompt value-substitution.

'substitute_prompt (STRING)'
     Return STRING with escape sequences substituted by values.  Some
     escape sequences take arguments.  You can specify arguments inside
     "{}" immediately following the escape sequence.

     The escape sequences you can pass to this function are:

     '\\'
          Substitute a backslash.
     '\e'
          Substitute an ESC character.
     '\f'
          Substitute the selected frame; an argument names a frame
          parameter.
     '\n'
          Substitute a newline.
     '\p'
          Substitute a parameter's value; the argument names the
          parameter.
     '\r'
          Substitute a carriage return.
     '\t'
          Substitute the selected thread; an argument names a thread
          parameter.
     '\v'
          Substitute the version of GDB.
     '\w'
          Substitute the current working directory.
     '\['
          Begin a sequence of non-printing characters.  These sequences
          are typically used with the ESC character, and are not counted
          in the string length.  Example: "\[\e[0;34m\](gdb)\[\e[0m\]"
          will return a blue-colored "(gdb)" prompt where the length is
          five.
     '\]'
          End a sequence of non-printing characters.

     For example:

          substitute_prompt ("frame: \f, args: \p{print frame-arguments}")

will return the string:

          "frame: main, args: scalars"


File: gdb.info,  Node: Guile,  Next: Auto-loading extensions,  Prev: Python,  Up: Extending GDB

23.3 Extending GDB using Guile
==============================

You can extend GDB using the Guile implementation of the Scheme
programming language (http://www.gnu.org/software/guile/).  This feature
is available only if GDB was configured using '--with-guile'.

* Menu:

* Guile Introduction::     Introduction to Guile scripting in GDB
* Guile Commands::         Accessing Guile from GDB
* Guile API::              Accessing GDB from Guile
* Guile Auto-loading::     Automatically loading Guile code
* Guile Modules::          Guile modules provided by GDB


File: gdb.info,  Node: Guile Introduction,  Next: Guile Commands,  Up: Guile

23.3.1 Guile Introduction
-------------------------

Guile is an implementation of the Scheme programming language and is the
GNU project's official extension language.

   Guile support in GDB follows the Python support in GDB reasonably
closely, so concepts there should carry over.  However, some things are
done differently where it makes sense.

   GDB requires Guile version 2.0 or greater.  Older versions are not
supported.

   Guile scripts used by GDB should be installed in
'DATA-DIRECTORY/guile', where DATA-DIRECTORY is the data directory as
determined at GDB startup (*note Data Files::).  This directory, known
as the "guile directory", is automatically added to the Guile Search
Path in order to allow the Guile interpreter to locate all scripts
installed at this location.


File: gdb.info,  Node: Guile Commands,  Next: Guile API,  Prev: Guile Introduction,  Up: Guile

23.3.2 Guile Commands
---------------------

GDB provides two commands for accessing the Guile interpreter:

'guile-repl'
'gr'
     The 'guile-repl' command can be used to start an interactive Guile
     prompt or "repl".  To return to GDB, type ',q' or the 'EOF'
     character (e.g., 'Ctrl-D' on an empty prompt).  These commands do
     not take any arguments.

'guile [SCHEME-EXPRESSION]'
'gu [SCHEME-EXPRESSION]'
     The 'guile' command can be used to evaluate a Scheme expression.

     If given an argument, GDB will pass the argument to the Guile
     interpreter for evaluation.

          (gdb) guile (display (+ 20 3)) (newline)
          23

     The result of the Scheme expression is displayed using normal Guile
     rules.

          (gdb) guile (+ 20 3)
          23

     If you do not provide an argument to 'guile', it will act as a
     multi-line command, like 'define'.  In this case, the Guile script
     is made up of subsequent command lines, given after the 'guile'
     command.  This command list is terminated using a line containing
     'end'.  For example:

          (gdb) guile
          >(display 23)
          >(newline)
          >end
          23

   It is also possible to execute a Guile script from the GDB
interpreter:

'source script-name'
     The script name must end with '.scm' and GDB must be configured to
     recognize the script language based on filename extension using the
     'script-extension' setting.  *Note Extending GDB: Extending GDB.

'guile (load "script-name")'
     This method uses the 'load' Guile function.  It takes a string
     argument that is the name of the script to load.  See the Guile
     documentation for a description of this function.  (*note
     (guile)Loading::).


File: gdb.info,  Node: Guile API,  Next: Guile Auto-loading,  Prev: Guile Commands,  Up: Guile

23.3.3 Guile API
----------------

You can get quick online help for GDB's Guile API by issuing the command
'help guile', or by issuing the command ',help' from an interactive
Guile session.  Furthermore, most Guile procedures provided by GDB have
doc strings which can be obtained with ',describe PROCEDURE-NAME' or ',d
PROCEDURE-NAME' from the Guile interactive prompt.

* Menu:

* Basic Guile::              Basic Guile Functions
* Guile Configuration::      Guile configuration variables
* GDB Scheme Data Types::    Scheme representations of GDB objects
* Guile Exception Handling:: How Guile exceptions are translated
* Values From Inferior In Guile:: Guile representation of values
* Arithmetic In Guile::      Arithmetic in Guile
* Types In Guile::           Guile representation of types
* Guile Pretty Printing API:: Pretty-printing values with Guile
* Selecting Guile Pretty-Printers:: How GDB chooses a pretty-printer
* Writing a Guile Pretty-Printer:: Writing a pretty-printer
* Commands In Guile::        Implementing new commands in Guile
* Parameters In Guile::      Adding new GDB parameters
* Progspaces In Guile::      Program spaces
* Objfiles In Guile::        Object files in Guile
* Frames In Guile::          Accessing inferior stack frames from Guile
* Blocks In Guile::          Accessing blocks from Guile
* Symbols In Guile::         Guile representation of symbols
* Symbol Tables In Guile::   Guile representation of symbol tables
* Breakpoints In Guile::     Manipulating breakpoints using Guile
* Lazy Strings In Guile::    Guile representation of lazy strings
* Architectures In Guile::   Guile representation of architectures
* Disassembly In Guile::     Disassembling instructions from Guile
* I/O Ports in Guile::       GDB I/O ports
* Memory Ports in Guile::    Accessing memory through ports and bytevectors
* Iterators In Guile::       Basic iterator support


File: gdb.info,  Node: Basic Guile,  Next: Guile Configuration,  Up: Guile API

23.3.3.1 Basic Guile
....................

At startup, GDB overrides Guile's 'current-output-port' and
'current-error-port' to print using GDB's output-paging streams.  A
Guile program which outputs to one of these streams may have its output
interrupted by the user (*note Screen Size::).  In this situation, a
Guile 'signal' exception is thrown with value 'SIGINT'.

   Guile's history mechanism uses the same naming as GDB's, namely the
user of dollar-variables (e.g., $1, $2, etc.).  The results of
evaluations in Guile and in GDB are counted separately, '$1' in Guile is
not the same value as '$1' in GDB.

   GDB is not thread-safe.  If your Guile program uses multiple threads,
you must be careful to only call GDB-specific functions in the GDB
thread.

   Some care must be taken when writing Guile code to run in GDB.  Two
things are worth noting in particular:

   * GDB installs handlers for 'SIGCHLD' and 'SIGINT'.  Guile code must
     not override these, or even change the options using 'sigaction'.
     If your program changes the handling of these signals, GDB will
     most likely stop working correctly.  Note that it is unfortunately
     common for GUI toolkits to install a 'SIGCHLD' handler.

   * GDB takes care to mark its internal file descriptors as
     close-on-exec.  However, this cannot be done in a thread-safe way
     on all platforms.  Your Guile programs should be aware of this and
     should both create new file descriptors with the close-on-exec flag
     set and arrange to close unneeded file descriptors before starting
     a child process.

   GDB introduces a new Guile module, named 'gdb'.  All methods and
classes added by GDB are placed in this module.  GDB does not
automatically 'import' the 'gdb' module, scripts must do this
themselves.  There are various options for how to import a module, so
GDB leaves the choice of how the 'gdb' module is imported to the user.
To simplify interactive use, it is recommended to add one of the
following to your ~/.gdbinit.

     guile (use-modules (gdb))

     guile (use-modules ((gdb) #:renamer (symbol-prefix-proc 'gdb:)))

   Which one to choose depends on your preference.  The second one adds
'gdb:' as a prefix to all module functions and variables.

   The rest of this manual assumes the 'gdb' module has been imported
without any prefix.  See the Guile documentation for 'use-modules' for
more information (*note (guile)Using Guile Modules::).

   Example:

     (gdb) guile (value-type (make-value 1))
     ERROR: Unbound variable: value-type
     Error while executing Scheme code.
     (gdb) guile (use-modules (gdb))
     (gdb) guile (value-type (make-value 1))
     int
     (gdb)

   The '(gdb)' module provides these basic Guile functions.

 -- Scheme Procedure: execute command [#:from-tty boolean] [#:to-string
          boolean]
     Evaluate COMMAND, a string, as a GDB CLI command.  If a GDB
     exception happens while COMMAND runs, it is translated as described
     in *note Guile Exception Handling: Guile Exception Handling.

     FROM-TTY specifies whether GDB ought to consider this command as
     having originated from the user invoking it interactively.  It must
     be a boolean value.  If omitted, it defaults to '#f'.

     By default, any output produced by COMMAND is sent to GDB's
     standard output (and to the log output if logging is turned on).
     If the TO-STRING parameter is '#t', then output will be collected
     by 'execute' and returned as a string.  The default is '#f', in
     which case the return value is unspecified.  If TO-STRING is '#t',
     the GDB virtual terminal will be temporarily set to unlimited width
     and height, and its pagination will be disabled; *note Screen
     Size::.

 -- Scheme Procedure: history-ref number
     Return a value from GDB's value history (*note Value History::).
     The NUMBER argument indicates which history element to return.  If
     NUMBER is negative, then GDB will take its absolute value and count
     backward from the last element (i.e., the most recent element) to
     find the value to return.  If NUMBER is zero, then GDB will return
     the most recent element.  If the element specified by NUMBER
     doesn't exist in the value history, a 'gdb:error' exception will be
     raised.

     If no exception is raised, the return value is always an instance
     of '<gdb:value>' (*note Values From Inferior In Guile::).

     _Note:_ GDB's value history is independent of Guile's.  '$1' in
     GDB's value history contains the result of evaluating an expression
     from GDB's command line and '$1' from Guile's history contains the
     result of evaluating an expression from Guile's command line.

 -- Scheme Procedure: history-append! value
     Append VALUE, an instance of '<gdb:value>', to GDB's value history.
     Return its index in the history.

     Putting into history values returned by Guile extensions will allow
     the user convenient access to those values via CLI history
     facilities.

 -- Scheme Procedure: parse-and-eval expression
     Parse EXPRESSION as an expression in the current language, evaluate
     it, and return the result as a '<gdb:value>'.  The EXPRESSION must
     be a string.

     This function can be useful when implementing a new command (*note
     Commands In Guile::), as it provides a way to parse the command's
     arguments as an expression.  It is also is useful when computing
     values.  For example, it is the only way to get the value of a
     convenience variable (*note Convenience Vars::) as a '<gdb:value>'.


File: gdb.info,  Node: Guile Configuration,  Next: GDB Scheme Data Types,  Prev: Basic Guile,  Up: Guile API

23.3.3.2 Guile Configuration
............................

GDB provides these Scheme functions to access various configuration
parameters.

 -- Scheme Procedure: data-directory
     Return a string containing GDB's data directory.  This directory
     contains GDB's ancillary files.

 -- Scheme Procedure: guile-data-directory
     Return a string containing GDB's Guile data directory.  This
     directory contains the Guile modules provided by GDB.

 -- Scheme Procedure: gdb-version
     Return a string containing the GDB version.

 -- Scheme Procedure: host-config
     Return a string containing the host configuration.  This is the
     string passed to '--host' when GDB was configured.

 -- Scheme Procedure: target-config
     Return a string containing the target configuration.  This is the
     string passed to '--target' when GDB was configured.


File: gdb.info,  Node: GDB Scheme Data Types,  Next: Guile Exception Handling,  Prev: Guile Configuration,  Up: Guile API

23.3.3.3 GDB Scheme Data Types
..............................

The values exposed by GDB to Guile are known as "GDB objects".  There
are several kinds of GDB object, and each is disjoint from all other
types known to Guile.

 -- Scheme Procedure: gdb-object-kind object
     Return the kind of the GDB object, e.g., '<gdb:breakpoint>', as a
     symbol.

   GDB defines the following object types:

'<gdb:arch>'
     *Note Architectures In Guile::.

'<gdb:block>'
     *Note Blocks In Guile::.

'<gdb:block-symbols-iterator>'
     *Note Blocks In Guile::.

'<gdb:breakpoint>'
     *Note Breakpoints In Guile::.

'<gdb:command>'
     *Note Commands In Guile::.

'<gdb:exception>'
     *Note Guile Exception Handling::.

'<gdb:frame>'
     *Note Frames In Guile::.

'<gdb:iterator>'
     *Note Iterators In Guile::.

'<gdb:lazy-string>'
     *Note Lazy Strings In Guile::.

'<gdb:objfile>'
     *Note Objfiles In Guile::.

'<gdb:parameter>'
     *Note Parameters In Guile::.

'<gdb:pretty-printer>'
     *Note Guile Pretty Printing API::.

'<gdb:pretty-printer-worker>'
     *Note Guile Pretty Printing API::.

'<gdb:progspace>'
     *Note Progspaces In Guile::.

'<gdb:symbol>'
     *Note Symbols In Guile::.

'<gdb:symtab>'
     *Note Symbol Tables In Guile::.

'<gdb:sal>'
     *Note Symbol Tables In Guile::.

'<gdb:type>'
     *Note Types In Guile::.

'<gdb:field>'
     *Note Types In Guile::.

'<gdb:value>'
     *Note Values From Inferior In Guile::.

   The following GDB objects are managed internally so that the Scheme
function 'eq?' may be applied to them.

'<gdb:arch>'
'<gdb:block>'
'<gdb:breakpoint>'
'<gdb:frame>'
'<gdb:objfile>'
'<gdb:progspace>'
'<gdb:symbol>'
'<gdb:symtab>'
'<gdb:type>'


File: gdb.info,  Node: Guile Exception Handling,  Next: Values From Inferior In Guile,  Prev: GDB Scheme Data Types,  Up: Guile API

23.3.3.4 Guile Exception Handling
.................................

When executing the 'guile' command, Guile exceptions uncaught within the
Guile code are translated to calls to the GDB error-reporting mechanism.
If the command that called 'guile' does not handle the error, GDB will
terminate it and report the error according to the setting of the 'guile
print-stack' parameter.

   The 'guile print-stack' parameter has three settings:

'none'
     Nothing is printed.

'message'
     An error message is printed containing the Guile exception name,
     the associated value, and the Guile call stack backtrace at the
     point where the exception was raised.  Example:

          (gdb) guile (display foo)
          ERROR: In procedure memoize-variable-access!:
          ERROR: Unbound variable: foo
          Error while executing Scheme code.

'full'
     In addition to an error message a full backtrace is printed.

          (gdb) set guile print-stack full
          (gdb) guile (display foo)
          Guile Backtrace:
          In ice-9/boot-9.scm:
           157: 10 [catch #t #<catch-closure 2c76e20> ...]
          In unknown file:
             ?: 9 [apply-smob/1 #<catch-closure 2c76e20>]
          In ice-9/boot-9.scm:
           157: 8 [catch #t #<catch-closure 2c76d20> ...]
          In unknown file:
             ?: 7 [apply-smob/1 #<catch-closure 2c76d20>]
             ?: 6 [call-with-input-string "(display foo)" ...]
          In ice-9/boot-9.scm:
          2320: 5 [save-module-excursion #<procedure 2c2dc30 ... ()>]
          In ice-9/eval-string.scm:
            44: 4 [read-and-eval #<input: string 27cb410> #:lang ...]
            37: 3 [lp (display foo)]
          In ice-9/eval.scm:
           387: 2 [eval # ()]
           393: 1 [eval #<memoized foo> ()]
          In unknown file:
             ?: 0 [memoize-variable-access! #<memoized foo> ...]

          ERROR: In procedure memoize-variable-access!:
          ERROR: Unbound variable: foo
          Error while executing Scheme code.

   GDB errors that happen in GDB commands invoked by Guile code are
converted to Guile exceptions.  The type of the Guile exception depends
on the error.

   Guile procedures provided by GDB can throw the standard Guile
exceptions like 'wrong-type-arg' and 'out-of-range'.

   User interrupt (via 'C-c' or by typing 'q' at a pagination prompt) is
translated to a Guile 'signal' exception with value 'SIGINT'.

   GDB Guile procedures can also throw these exceptions:

'gdb:error'
     This exception is a catch-all for errors generated from within GDB.

'gdb:invalid-object'
     This exception is thrown when accessing Guile objects that wrap
     underlying GDB objects have become invalid.  For example, a
     '<gdb:breakpoint>' object becomes invalid if the user deletes it
     from the command line.  The object still exists in Guile, but the
     object it represents is gone.  Further operations on this
     breakpoint will throw this exception.

'gdb:memory-error'
     This exception is thrown when an operation tried to access invalid
     memory in the inferior.

'gdb:pp-type-error'
     This exception is thrown when a Guile pretty-printer passes a bad
     object to GDB.

   The following exception-related procedures are provided by the
'(gdb)' module.

 -- Scheme Procedure: make-exception key args
     Return a '<gdb:exception>' object given by its KEY and ARGS, which
     are the standard Guile parameters of an exception.  See the Guile
     documentation for more information (*note (guile)Exceptions::).

 -- Scheme Procedure: exception? object
     Return '#t' if OBJECT is a '<gdb:exception>' object.  Otherwise
     return '#f'.

 -- Scheme Procedure: exception-key exception
     Return the ARGS field of a '<gdb:exception>' object.

 -- Scheme Procedure: exception-args exception
     Return the ARGS field of a '<gdb:exception>' object.


File: gdb.info,  Node: Values From Inferior In Guile,  Next: Arithmetic In Guile,  Prev: Guile Exception Handling,  Up: Guile API

23.3.3.5 Values From Inferior In Guile
......................................

GDB provides values it obtains from the inferior program in an object of
type '<gdb:value>'.  GDB uses this object for its internal bookkeeping
of the inferior's values, and for fetching values when necessary.

   GDB does not memoize '<gdb:value>' objects.  'make-value' always
returns a fresh object.

     (gdb) guile (eq? (make-value 1) (make-value 1))
     $1 = #f
     (gdb) guile (equal? (make-value 1) (make-value 1))
     $1 = #t

   A '<gdb:value>' that represents a function can be executed via
inferior function call with 'value-call'.  Any arguments provided to the
call must match the function's prototype, and must be provided in the
order specified by that prototype.

   For example, 'some-val' is a '<gdb:value>' instance representing a
function that takes two integers as arguments.  To execute this
function, call it like so:

     (define result (value-call some-val 10 20))

   Any values returned from a function call are '<gdb:value>' objects.

   Note: Unlike Python scripting in GDB, inferior values that are simple
scalars cannot be used directly in Scheme expressions that are valid for
the value's data type.  For example, '(+ (parse-and-eval "int_variable")
2)' does not work.  And inferior values that are structures or instances
of some class cannot be accessed using any special syntax, instead
'value-field' must be used.

   The following value-related procedures are provided by the '(gdb)'
module.

 -- Scheme Procedure: value? object
     Return '#t' if OBJECT is a '<gdb:value>' object.  Otherwise return
     '#f'.

 -- Scheme Procedure: make-value value [#:type type]
     Many Scheme values can be converted directly to a '<gdb:value>'
     with this procedure.  If TYPE is specified, the result is a value
     of this type, and if VALUE can't be represented with this type an
     exception is thrown.  Otherwise the type of the result is
     determined from VALUE as described below.

     *Note Architectures In Guile::, for a list of the builtin types for
     an architecture.

     Here's how Scheme values are converted when TYPE argument to
     'make-value' is not specified:

     Scheme boolean
          A Scheme boolean is converted the boolean type for the current
          language.

     Scheme integer
          A Scheme integer is converted to the first of a C 'int',
          'unsigned int', 'long', 'unsigned long', 'long long' or
          'unsigned long long' type for the current architecture that
          can represent the value.

          If the Scheme integer cannot be represented as a target
          integer an 'out-of-range' exception is thrown.

     Scheme real
          A Scheme real is converted to the C 'double' type for the
          current architecture.

     Scheme string
          A Scheme string is converted to a string in the current target
          language using the current target encoding.  Characters that
          cannot be represented in the current target encoding are
          replaced with the corresponding escape sequence.  This is
          Guile's 'SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE' conversion
          strategy (*note (guile)Strings::).

          Passing TYPE is not supported in this case, if it is provided
          a 'wrong-type-arg' exception is thrown.

     '<gdb:lazy-string>'
          If VALUE is a '<gdb:lazy-string>' object (*note Lazy Strings
          In Guile::), then the 'lazy-string->value' procedure is
          called, and its result is used.

          Passing TYPE is not supported in this case, if it is provided
          a 'wrong-type-arg' exception is thrown.

     Scheme bytevector
          If VALUE is a Scheme bytevector and TYPE is provided, VALUE
          must be the same size, in bytes, of values of type TYPE, and
          the result is essentially created by using 'memcpy'.

          If VALUE is a Scheme bytevector and TYPE is not provided, the
          result is an array of type 'uint8' of the same length.

 -- Scheme Procedure: value-optimized-out? value
     Return '#t' if the compiler optimized out VALUE, thus it is not
     available for fetching from the inferior.  Otherwise return '#f'.

 -- Scheme Procedure: value-address value
     If VALUE is addressable, returns a '<gdb:value>' object
     representing the address.  Otherwise, '#f' is returned.

 -- Scheme Procedure: value-type value
     Return the type of VALUE as a '<gdb:type>' object (*note Types In
     Guile::).

 -- Scheme Procedure: value-dynamic-type value
     Return the dynamic type of VALUE.  This uses C++ run-time type
     information (RTTI) to determine the dynamic type of the value.  If
     the value is of class type, it will return the class in which the
     value is embedded, if any.  If the value is of pointer or reference
     to a class type, it will compute the dynamic type of the referenced
     object, and return a pointer or reference to that type,
     respectively.  In all other cases, it will return the value's
     static type.

     Note that this feature will only work when debugging a C++ program
     that includes RTTI for the object in question.  Otherwise, it will
     just return the static type of the value as in 'ptype foo'.  *Note
     ptype: Symbols.

 -- Scheme Procedure: value-cast value type
     Return a new instance of '<gdb:value>' that is the result of
     casting VALUE to the type described by TYPE, which must be a
     '<gdb:type>' object.  If the cast cannot be performed for some
     reason, this method throws an exception.

 -- Scheme Procedure: value-dynamic-cast value type
     Like 'value-cast', but works as if the C++ 'dynamic_cast' operator
     were used.  Consult a C++ reference for details.

 -- Scheme Procedure: value-reinterpret-cast value type
     Like 'value-cast', but works as if the C++ 'reinterpret_cast'
     operator were used.  Consult a C++ reference for details.

 -- Scheme Procedure: value-dereference value
     For pointer data types, this method returns a new '<gdb:value>'
     object whose contents is the object pointed to by VALUE.  For
     example, if 'foo' is a C pointer to an 'int', declared in your C
     program as

          int *foo;

     then you can use the corresponding '<gdb:value>' to access what
     'foo' points to like this:

          (define bar (value-dereference foo))

     The result 'bar' will be a '<gdb:value>' object holding the value
     pointed to by 'foo'.

     A similar function 'value-referenced-value' exists which also
     returns '<gdb:value>' objects corresponding to the values pointed
     to by pointer values (and additionally, values referenced by
     reference values).  However, the behavior of 'value-dereference'
     differs from 'value-referenced-value' by the fact that the behavior
     of 'value-dereference' is identical to applying the C unary
     operator '*' on a given value.  For example, consider a reference
     to a pointer 'ptrref', declared in your C++ program as

          typedef int *intptr;
          ...
          int val = 10;
          intptr ptr = &val;
          intptr &ptrref = ptr;

     Though 'ptrref' is a reference value, one can apply the method
     'value-dereference' to the '<gdb:value>' object corresponding to it
     and obtain a '<gdb:value>' which is identical to that corresponding
     to 'val'.  However, if you apply the method
     'value-referenced-value', the result would be a '<gdb:value>'
     object identical to that corresponding to 'ptr'.

          (define scm-ptrref (parse-and-eval "ptrref"))
          (define scm-val (value-dereference scm-ptrref))
          (define scm-ptr (value-referenced-value scm-ptrref))

     The '<gdb:value>' object 'scm-val' is identical to that
     corresponding to 'val', and 'scm-ptr' is identical to that
     corresponding to 'ptr'.  In general, 'value-dereference' can be
     applied whenever the C unary operator '*' can be applied to the
     corresponding C value.  For those cases where applying both
     'value-dereference' and 'value-referenced-value' is allowed, the
     results obtained need not be identical (as we have seen in the
     above example).  The results are however identical when applied on
     '<gdb:value>' objects corresponding to pointers ('<gdb:value>'
     objects with type code 'TYPE_CODE_PTR') in a C/C++ program.

 -- Scheme Procedure: value-referenced-value value
     For pointer or reference data types, this method returns a new
     '<gdb:value>' object corresponding to the value referenced by the
     pointer/reference value.  For pointer data types,
     'value-dereference' and 'value-referenced-value' produce identical
     results.  The difference between these methods is that
     'value-dereference' cannot get the values referenced by reference
     values.  For example, consider a reference to an 'int', declared in
     your C++ program as

          int val = 10;
          int &ref = val;

     then applying 'value-dereference' to the '<gdb:value>' object
     corresponding to 'ref' will result in an error, while applying
     'value-referenced-value' will result in a '<gdb:value>' object
     identical to that corresponding to 'val'.

          (define scm-ref (parse-and-eval "ref"))
          (define err-ref (value-dereference scm-ref))      ;; error
          (define scm-val (value-referenced-value scm-ref)) ;; ok

     The '<gdb:value>' object 'scm-val' is identical to that
     corresponding to 'val'.

 -- Scheme Procedure: value-field value field-name
     Return field FIELD-NAME from '<gdb:value>' object VALUE.

 -- Scheme Procedure: value-subscript value index
     Return the value of array VALUE at index INDEX.  The VALUE argument
     must be a subscriptable '<gdb:value>' object.

 -- Scheme Procedure: value-call value arg-list
     Perform an inferior function call, taking VALUE as a pointer to the
     function to call.  Each element of list ARG-LIST must be a
     <gdb:value> object or an object that can be converted to a value.
     The result is the value returned by the function.

 -- Scheme Procedure: value->bool value
     Return the Scheme boolean representing '<gdb:value>' VALUE.  The
     value must be "integer like".  Pointers are ok.

 -- Scheme Procedure: value->integer
     Return the Scheme integer representing '<gdb:value>' VALUE.  The
     value must be "integer like".  Pointers are ok.

 -- Scheme Procedure: value->real
     Return the Scheme real number representing '<gdb:value>' VALUE.
     The value must be a number.

 -- Scheme Procedure: value->bytevector
     Return a Scheme bytevector with the raw contents of '<gdb:value>'
     VALUE.  No transformation, endian or otherwise, is performed.

 -- Scheme Procedure: value->string value [#:encoding encoding]
          [#:errors errors] [#:length length]
     If VALUE> represents a string, then this method converts the
     contents to a Guile string.  Otherwise, this method will throw an
     exception.

     Values are interpreted as strings according to the rules of the
     current language.  If the optional length argument is given, the
     string will be converted to that length, and will include any
     embedded zeroes that the string may contain.  Otherwise, for
     languages where the string is zero-terminated, the entire string
     will be converted.

     For example, in C-like languages, a value is a string if it is a
     pointer to or an array of characters or ints of type 'wchar_t',
     'char16_t', or 'char32_t'.

     If the optional ENCODING argument is given, it must be a string
     naming the encoding of the string in the '<gdb:value>', such as
     '"ascii"', '"iso-8859-6"' or '"utf-8"'.  It accepts the same
     encodings as the corresponding argument to Guile's
     'scm_from_stringn' function, and the Guile codec machinery will be
     used to convert the string.  If ENCODING is not given, or if
     ENCODING is the empty string, then either the 'target-charset'
     (*note Character Sets::) will be used, or a language-specific
     encoding will be used, if the current language is able to supply
     one.

     The optional ERRORS argument is one of '#f', 'error' or
     'substitute'.  'error' and 'substitute' must be symbols.  If ERRORS
     is not specified, or if its value is '#f', then the default
     conversion strategy is used, which is set with the Scheme function
     'set-port-conversion-strategy!'.  If the value is ''error' then an
     exception is thrown if there is any conversion error.  If the value
     is ''substitute' then any conversion error is replaced with
     question marks.  *Note (guile)Strings::.

     If the optional LENGTH argument is given, the string will be
     fetched and converted to the given length.  The length must be a
     Scheme integer and not a '<gdb:value>' integer.

 -- Scheme Procedure: value->lazy-string value [#:encoding encoding]
          [#:length length]
     If this '<gdb:value>' represents a string, then this method
     converts VALUE to a '<gdb:lazy-string' (*note Lazy Strings In
     Guile::).  Otherwise, this method will throw an exception.

     If the optional ENCODING argument is given, it must be a string
     naming the encoding of the '<gdb:lazy-string'.  Some examples are:
     '"ascii"', '"iso-8859-6"' or '"utf-8"'.  If the ENCODING argument
     is an encoding that GDB does not recognize, GDB will raise an
     error.

     When a lazy string is printed, the GDB encoding machinery is used
     to convert the string during printing.  If the optional ENCODING
     argument is not provided, or is an empty string, GDB will
     automatically select the encoding most suitable for the string
     type.  For further information on encoding in GDB please see *note
     Character Sets::.

     If the optional LENGTH argument is given, the string will be
     fetched and encoded to the length of characters specified.  If the
     LENGTH argument is not provided, the string will be fetched and
     encoded until a null of appropriate width is found.  The length
     must be a Scheme integer and not a '<gdb:value>' integer.

 -- Scheme Procedure: value-lazy? value
     Return '#t' if VALUE has not yet been fetched from the inferior.
     Otherwise return '#f'.  GDB does not fetch values until necessary,
     for efficiency.  For example:

          (define myval (parse-and-eval "somevar"))

     The value of 'somevar' is not fetched at this time.  It will be
     fetched when the value is needed, or when the 'fetch-lazy'
     procedure is invoked.

 -- Scheme Procedure: make-lazy-value type address
     Return a '<gdb:value>' that will be lazily fetched from the target.
     The object of type '<gdb:type>' whose value to fetch is specified
     by its TYPE and its target memory ADDRESS, which is a Scheme
     integer.

 -- Scheme Procedure: value-fetch-lazy! value
     If VALUE is a lazy value ('(value-lazy? value)' is '#t'), then the
     value is fetched from the inferior.  Any errors that occur in the
     process will produce a Guile exception.

     If VALUE is not a lazy value, this method has no effect.

     The result of this function is unspecified.

 -- Scheme Procedure: value-print value
     Return the string representation (print form) of '<gdb:value>'
     VALUE.

