@c This is part of the Emacs manual.
@c Copyright (C) 1985--1987, 1993--1995, 1997, 2000--2026 Free Software
@c Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Building
@chapter Compiling and Testing Programs
@cindex building programs
@cindex program building
@cindex running Lisp functions

  The previous chapter discusses the Emacs commands that are useful
for making changes in programs.  This chapter deals with commands that
assist in the process of compiling and testing programs.

@menu
* Compilation::         Compiling programs in languages other
                          than Lisp (C, Pascal, etc.).
* Compilation Mode::    The mode for visiting compiler errors.
* Compilation Shell::   Customizing your shell properly
                          for use in the compilation buffer.
* Grep Searching::      Searching with grep.
* Flymake::             Finding syntax errors on the fly.
* Debuggers::           Running symbolic debuggers for non-Lisp programs.
* Executing Lisp::      Various modes for editing Lisp programs,
                          with different facilities for running
                          the Lisp programs.
* Libraries: Lisp Libraries.      How Lisp programs are loaded into Emacs.
* Eval: Lisp Eval.      Executing a single Lisp expression in Emacs.
* Interaction: Lisp Interaction.  Executing Lisp in an Emacs buffer.
* External Lisp::       Communicating through Emacs with a separate Lisp.
@end menu

@node Compilation
@section Running Compilations under Emacs
@cindex inferior process
@cindex make
@cindex compilation errors
@cindex error log

  Emacs can run compilers for languages such as C and Fortran, feeding
the compilation log into an Emacs buffer.  It can also parse the error
messages and show you where the errors occurred.

@table @kbd
@item M-x compile
Run a compiler asynchronously under Emacs, with error messages going to
the @file{*compilation*} buffer.

@item M-x recompile
@itemx g@r{ (Compilation mode)}
Invoke a compiler with the same command as in the last invocation of
@kbd{M-x compile}.

@item M-x kill-compilation
Kill the running compilation subprocess.
@end table

@findex compile
  To run @code{make} or another compilation command, type @kbd{M-x
compile}.  This reads a shell command line using the minibuffer, and
then executes the command by running a shell as a subprocess (or
@dfn{inferior process}) of Emacs.  The output is inserted in a buffer
named @file{*compilation*}.  The current buffer's default directory is
used as the working directory for the execution of the command, so by
default compilation takes place in that directory.

  When invoked with a prefix argument, the @file{*compilation*} buffer
is using Comint mode as its major mode (@pxref{Shell Mode}).  By default
Comint mode has the nice property of looking for any credential prompts
in its contents and make Emacs asks for a password if this happens.
This is useful should the compilation command need such a credential.

@vindex compile-command
  The default compilation command is @samp{make -k}, which is usually
correct for programs compiled using the @command{make} utility (the
@samp{-k} flag tells @command{make} to continue compiling as much as
possible after an error).  @xref{Top,, Make, make, GNU Make Manual}.
If you have done @kbd{M-x compile} before, the command that you
specified is automatically stored in the variable
@code{compile-command}; this is used as the default the next time you
type @kbd{M-x compile}.  A file can also specify a file-local value
for @code{compile-command} (@pxref{File Variables}).

  Starting a compilation displays the @file{*compilation*} buffer in
another window but does not select it.  While the compilation is
running, the word @samp{run} is shown in the major mode indicator for
the @file{*compilation*} buffer, and the word @samp{Compiling} appears
in all mode lines.  You do not have to keep the @file{*compilation*}
buffer visible while compilation is running; it continues in any case.
When the compilation ends, for whatever reason, the mode line of the
@file{*compilation*} buffer changes to say @samp{exit} (followed by
the exit code: @samp{[0]} for a normal exit), or @samp{signal} (if a
signal terminated the process).

  If you want to watch the compilation transcript as it appears,
switch to the @file{*compilation*} buffer and move point to the end of
the buffer.  When point is at the end, new compilation output is
inserted above point, which remains at the end.  Otherwise, point
remains fixed while compilation output is added at the end of the
buffer.

  While compilation proceeds, the mode line shows the number of
errors, warnings, and informational messages emitted by the compiler
so far.

@cindex compilation buffer, keeping point at end
@vindex compilation-scroll-output
  If you change the variable @code{compilation-scroll-output} to a
non-@code{nil} value, the @file{*compilation*} buffer scrolls
automatically to follow the output.  If the value is
@code{first-error}, scrolling stops when the first error appears,
leaving point at that error.  For any other non-@code{nil} value,
scrolling continues until there is no more output.

@findex recompile
  To rerun the last compilation with the same command, type @kbd{M-x
recompile}.  This reuses the compilation command from the last
invocation of @kbd{M-x compile}.  It also reuses the
@file{*compilation*} buffer and starts the compilation in its default
directory, which is the directory in which the previous compilation
was started.  In @file{*compilation*} buffers this command is bound to
@kbd{g}.

@findex kill-compilation
@vindex compilation-always-kill
  Starting a new compilation also kills any compilation already
running in @file{*compilation*}, as the buffer can only handle one
compilation at any time.  However, @kbd{M-x compile} and @kbd{M-x
recompile} ask for confirmation before actually killing a compilation
that is running; to always automatically kill the compilation without
asking, change the variable @code{compilation-always-kill} to
@code{t}.  You can also kill a compilation process with the command
@kbd{M-x kill-compilation}.

  To run two compilations at once, start the first one, then rename
the @file{*compilation*} buffer (perhaps using @code{rename-uniquely};
@pxref{Misc Buffer}), then switch buffers and start the other
compilation.  This will create a new @file{*compilation*} buffer.

@vindex compilation-environment
  You can control the environment passed to the compilation command
with the variable @code{compilation-environment}.  Its value is a list
of environment variable settings; each element should be a string of
the form @code{"@var{envvarname}=@var{value}"}.  These environment
variable settings override the usual ones.

@vindex compilation-max-output-line-length
  Displaying extremely long lines in compilation output can slow Emacs
down.  Lines that are longer than
@code{compilation-max-output-line-length} will have the portion that's
exceeds that limit hidden behind a button that can be clicked on to
reveal the hidden portion.  Set this variable to @code{nil} to never
hide anything.

@node Compilation Mode
@section Compilation Mode

@cindex Compilation mode
@cindex mode, Compilation
@cindex locus
  The @file{*compilation*} buffer uses a major mode called Compilation
mode.  Compilation mode turns each error message in the buffer into a
hyperlink; you can move point to it and type @key{RET}, or click on it
with the mouse (@pxref{Mouse References}), to visit the @dfn{locus} of
the error message in a separate window.  The locus is the specific
position in a file where that error occurred.

@cindex compilation mode faces
@vindex compilation-error
@vindex compilation-warning
  The appearance of the @file{*compilation*} buffer can be controlled
by customizing the faces which are used to highlight parts of the
@file{*compilation*} buffer, e.g., @code{compilation-error} or
@code{compilation-warning}, for error and warning messages
respectively.  Note that since those faces inherit from the
@code{error} and @code{warning} faces, it is also possible to
customize the parent face directly instead.

  Use @w{@kbd{M-x customize-group RET compilation}} to see the entire
list of customization variables and faces.

@findex compile-goto-error
@vindex compilation-auto-jump-to-first-error
  If you change the variable
@code{compilation-auto-jump-to-first-error} to a non-@code{nil} value,
Emacs automatically visits the locus of the first error message that
appears in the @file{*compilation*} buffer.  (This variable can also
have the values @code{if-location-known} and @code{first-known}, which
modify the conditions for automatically visiting the error locus.)

@vindex compilation-search-path
@vindex compilation-search-extra-path
  The actual locus of an error message is determined by
@code{compilation-search-path}, a list of directory names.  By default,
Emacs visits the file indicated in the error message under the directory
where the compilation happens, represented by the special directory name
@code{nil} in the default value for @code{compilation-search-path}
(i.e. @code{(nil)}).  To make Emacs try to find the file under a
different directory (e.g. when compiling a nested project), customize
@code{compilation-search-path} to include that directory.  For
directory-local customization (@pxref{Directory Variables}), prefer
setting @code{compilation-search-extra-path} instead, which Emacs
considers in addition to @code{compilation-search-path}.  Entries in
@code{compilation-search-extra-path} take precedence over entries in
@code{compilation-search-path}.

  Compilation mode provides the following additional commands.  These
commands can also be used in @file{*grep*} buffers, where the
hyperlinks are search matches rather than error messages (@pxref{Grep
Searching}).

@table @kbd
@item M-g M-n
@itemx M-g n
@itemx C-x `
Visit the locus of the next error message or match (@code{next-error}).
@item M-g M-p
@itemx M-g p
Visit the locus of the previous error message or match
(@code{previous-error}).
@item M-n
Move point to the next error message or match, without visiting its
locus (@code{compilation-next-error}).
@item M-p
Move point to the previous error message or match, without visiting
its locus (@code{compilation-previous-error}).
@item M-@}
Move point to the next error message or match occurring in a different
file (@code{compilation-next-file}).
@item M-@{
Move point to the previous error message or match occurring in a
different file (@code{compilation-previous-file}).
@item C-c C-f
Toggle Next Error Follow minor mode, which makes cursor motion in the
compilation buffer produce automatic source display.
@item g
Re-run the last command whose output is shown in the
@file{*compilation*} buffer.
@item M-x next-error-select-buffer
Select a buffer to be used by next invocation of @code{next-error} and
@code{previous-error}.
@end table

@kindex M-g M-n
@kindex M-g n
@kindex C-x `
@findex next-error
@vindex next-error-message-highlight
@vindex next-error-highlight
@vindex next-error-highlight-no-select
  To visit errors sequentially, type @w{@kbd{C-x `}}
(@code{next-error}), or equivalently @kbd{M-g M-n} or @kbd{M-g n}.
This command can be invoked from any buffer, not just a Compilation
mode buffer.  The first time you invoke it after a compilation, it
visits the locus of the first error message.  Each subsequent
@w{@kbd{M-g M-n}} visits the next error, in a similar fashion.  If you
visit a specific error with @key{RET} or a mouse click in the
@file{*compilation*} buffer, subsequent @w{@kbd{M-g M-n}} commands
advance from there.  When @w{@kbd{M-g M-n}} finds no more error messages
to visit, it signals an error.  @w{@kbd{C-u M-g M-n}} starts again from
the beginning of the compilation buffer, and visits the first locus.

  @kbd{M-g M-p} or @kbd{M-g p} (@code{previous-error}) iterates
through errors in the opposite direction.

@vindex next-error-find-buffer-function
@findex next-error-select-buffer
  The @code{next-error} and @code{previous-error} commands don't just
act on the errors or matches listed in @file{*compilation*} and
@file{*grep*} buffers; they also know how to iterate through error or
match lists produced by other commands, such as @kbd{M-x occur}
(@pxref{Other Repeating Search}).  If the current buffer contains
error messages or matches, these commands will iterate through them;
otherwise, Emacs looks for a buffer containing error messages or
matches amongst the windows of the selected frame (if the variable
@code{next-error-find-buffer-function} is customized to the value
@code{next-error-buffer-on-selected-frame}), then for a buffer used
previously by @code{next-error} or @code{previous-error}, and finally
all other buffers.  Any buffer these commands iterate through that is
not currently displayed in a window will be displayed.  You can use
the @command{next-error-select-buffer} command to switch to
a different buffer to be used by the subsequent invocation of
@code{next-error}.

@vindex compilation-skip-threshold
  By default, the @code{next-error} and @code{previous-error} commands
skip less important messages.  The variable
@code{compilation-skip-threshold} controls this.  The default value,
1, means to skip anything less important than a warning.  A value of 2
means to skip anything less important than an error, while 0 means not
to skip any messages.

  When Emacs visits the locus of an error message, it momentarily
highlights the relevant source line.  The duration of this highlight
is determined by the variable @code{next-error-highlight} for the locus
in the selected buffer, and @code{next-error-highlight-no-select} for
the locus in non-selected buffers.  Also you can customize the variable
@code{next-error-message-highlight} that defines how to highlight the
current error message in the buffer that contains messages.

@vindex compilation-context-lines
  If the @file{*compilation*} buffer is shown in a window with a left
fringe (@pxref{Fringes}), the locus-visiting commands put an arrow in
the fringe, pointing to the current error message.  If the window has
no left fringe, such as on a text terminal, these commands scroll the
window so that the current message is at the top of the window.  If
you change the variable @code{compilation-context-lines} to @code{t},
a visible arrow is inserted before column zero instead.  If you change
the variable to an integer value @var{n}, these commands scroll the
window so that the current error message is @var{n} lines from the
top, whether or not there is a fringe; the default value, @code{nil},
gives the behavior described above.

@vindex compilation-hidden-output
  Compilation output can sometimes be very verbose, and much of it isn't
of particular interest to a user.  The
@code{compilation-hidden-output} user option should either be a regexp
or a list of regexps, and output that matches will be made invisible.
For instance, to hide the verbose output from recursive makefiles, you
can say something like:

@lisp
(setq compilation-hidden-output
      '("^make[^\n]+\n"))
@end lisp

@vindex compilation-error-regexp-alist
@vindex grep-regexp-alist
  To parse messages from the compiler, Compilation mode uses the
variable @code{compilation-error-regexp-alist} which lists various
error message formats and tells Emacs how to extract the locus from
each.  A similar variable, @code{grep-regexp-alist}, tells Emacs how
to parse output from a @code{grep} command (@pxref{Grep Searching}).

@vindex compilation-transform-file-match-alist
@cindex compilation transform file
@cindex compilation find file
  Sometimes @code{compilation-error-regexp-alist} doesn't correctly
determine the filename that is the source of the error.  Use user option
@code{compilation-transform-file-match-alist} to make any necessary
adjustments, such as adding or changing a directory component, or even
considering certain compiler messages not error messages at all.

@findex compilation-next-error
@findex compilation-previous-error
@findex compilation-next-file
@findex compilation-previous-file
  Compilation mode also defines the keys @key{SPC} and @key{DEL} to
scroll by screenfuls; @kbd{M-n} (@code{compilation-next-error}) and
@kbd{M-p} (@code{compilation-previous-error}) to move to the next or
previous error message; and @kbd{M-@{} (@code{compilation-next-file})
and @kbd{M-@}} (@code{compilation-previous-file}) to move to the next
or previous error message for a different source file.

@cindex Next Error Follow mode
@findex next-error-follow-minor-mode
  You can type @kbd{C-c C-f} to toggle Next Error Follow mode.  In
this minor mode, ordinary cursor motion in the compilation buffer
automatically updates the source buffer, i.e., moving the cursor over
an error message causes the locus of that error to be displayed.

  The features of Compilation mode are also available in a minor mode
called Compilation Minor mode.  This lets you parse error messages in
any buffer, not just a normal compilation output buffer.  Type
@kbd{M-x compilation-minor-mode} to enable the minor mode.  For
instance, in an Rlogin buffer (@pxref{Remote Host}), Compilation minor
mode automatically accesses remote source files by FTP (@pxref{File
Names}).

@node Compilation Shell
@section Subshells for Compilation

  This section includes various techniques and advice for using a
shell and its features in compilation buffers.  This material is
specific to local compilations, and will most probably not work in (or
be irrelevant to) compilation buffers whose default directory is on
remote hosts.

  The @kbd{M-x compile} command uses a shell to run the compilation
command, but specifies the option for a noninteractive shell.  This
means, in particular, that the shell should start with no prompt.  If
you find your usual shell prompt making an unsightly appearance in the
@file{*compilation*} buffer, it means you have made a mistake in your
shell's init file by setting the prompt unconditionally.  (This init
file may be named @file{.bashrc}, @file{.profile}, @file{.cshrc},
@file{.shrc}, etc., depending on what shell you use.)  The shell init
file should set the prompt only if there already is a prompt.  Here's
how to do it in bash:

@example
if [ "$@{PS1+set@}" = set ]
then PS1=@dots{}
fi
@end example

@noindent
And here's how to do it in csh:

@example
if ($?prompt) set prompt = @dots{}
@end example

@vindex TERM@r{, environment variable, in compilation mode}
  If you want to customize the value of the @env{TERM} environment
variable passed to the compilation subshell, customize the variable
@code{comint-terminfo-terminal} (@pxref{Shell Options}).

  Emacs does not expect a compiler process to launch asynchronous
subprocesses; if it does, and they keep running after the main
compiler process has terminated, Emacs may kill them or their output
may not arrive in Emacs.  To avoid this problem, make the main
compilation process wait for its subprocesses to finish.  In a shell
script, you can do this using @samp{$!} and @samp{wait}, like this:

@example
(sleep 10; echo 2nd)& pid=$!  # @r{Record pid of subprocess}
echo first message
wait $pid                     # @r{Wait for subprocess}
@end example

@noindent
If the background process does not output to the compilation buffer,
so you only need to prevent it from being killed when the main
compilation process terminates, this is sufficient:

@example
nohup @var{command}; sleep 1
@end example

@ifnottex
  On MS-DOS, asynchronous subprocesses are
not supported, so @kbd{M-x compile} runs the compilation command
synchronously (i.e., you must wait until the command finishes before
you can do anything else in Emacs).  @xref{MS-DOS}.
@end ifnottex

@node Grep Searching
@section Searching with Grep under Emacs

  Just as you can run a compiler from Emacs and then visit the lines
with compilation errors, you can also run @command{grep} and then
visit the lines on which matches were found.  This works by treating
the matches reported by @command{grep} as if they were errors.
The output buffer uses Grep mode, which is a variant of Compilation
mode (@pxref{Compilation Mode}).

@table @kbd
@item M-x grep
@itemx M-x lgrep
Run @command{grep} asynchronously under Emacs, listing matching lines in
the buffer named @file{*grep*}.
@item M-x grep-find
@itemx M-x find-grep
@itemx M-x rgrep
Run @command{grep} via @code{find}, and collect output in the
@file{*grep*} buffer.
@item M-x zrgrep
Run @code{zgrep} and collect output in the @file{*grep*} buffer.
@item M-x kill-grep
Kill the running @command{grep} subprocess.
@end table

@findex grep
  To run @command{grep}, type @kbd{M-x grep}, then enter a command line
that specifies how to run @command{grep}.  Use the same arguments you
would give @command{grep} when running it normally: a @command{grep}-style
regexp (usually in single-quotes to quote the shell's special
characters) followed by file names, which may use wildcards.  If you
specify a prefix argument for @kbd{M-x grep}, it finds the identifier
(@pxref{Xref}) in the buffer around point, and puts that into the
default @command{grep} command.

  Your command need not simply run @command{grep}; you can use any shell
command that produces output in the same format.  For instance, you
can chain @command{grep} commands, like this:

@example
grep -nH -e foo *.el | grep bar | grep toto
@end example

  The output from @command{grep} goes in the @file{*grep*} buffer.  You
can find the corresponding lines in the original files using @w{@kbd{M-g
M-n}}, @key{RET}, and so forth, just like compilation errors.
@xref{Compilation Mode}, for detailed description of commands and key
bindings available in the @file{*grep*} buffer.

@vindex grep-match-regexp
  Some grep programs accept a @samp{--color} option to output special
markers around matches for the purpose of highlighting.  You can make
use of this feature by setting @code{grep-highlight-matches} to
@code{t}.  When displaying a match in the source buffer, the exact
match will be highlighted, instead of the entire source line.
Highlighting is provided via matching the @acronym{ANSI} escape
sequences emitted by @command{grep}.  The matching of the sequences is
controlled by @code{grep-match-regexp}, which can be customized to
accommodate different @command{grep} programs.

  As with compilation commands (@pxref{Compilation}), while the grep
command runs, the mode line shows the running number of matches found
and highlighted so far.

  The @command{grep} commands will offer to save buffers before
running.  This is controlled by the @code{grep-save-buffers} variable.
The possible values are either @code{nil} (don't save), @code{ask}
(ask before saving), or a function which will be used as a predicate
(and is called with the file name as the parameter and should return
non-@code{nil} if the buffer is to be saved).  Any other
non-@code{nil} value means that all buffers should be saved without
asking.  The default is @code{ask}.

@vindex grep-use-headings
@vindex grep-heading@r{ face}
  By default, grep matches are shown with the file names prefixed to
each line.  But if the variable @code{grep-use-headings} is customized
to a non-@code{nil} value, the matches are split into sections, one
section for each file with matches, and the file names are shown in the
section headings using a special face @code{grep-heading}.

@findex grep-find
@findex find-grep
  The command @kbd{M-x grep-find} (also available as @kbd{M-x
find-grep}) is similar to @kbd{M-x grep}, but it supplies a different
initial default for the command---one that runs both @code{find} and
@command{grep}, so as to search every file in a directory tree.  See also
the @code{find-grep-dired} command, in @ref{Dired and Find}.

@findex lgrep
@findex rgrep
@findex zrgrep
  The commands @kbd{M-x lgrep} (local grep) and @kbd{M-x rgrep}
(recursive grep) are more user-friendly versions of @command{grep} and
@code{grep-find}, which prompt separately for the regular expression
to match, the files to search, and the base directory for the search.
Case sensitivity of the search is controlled by the current value of
@code{case-fold-search}.  The command @kbd{M-x zrgrep} is similar to
@kbd{M-x rgrep}, but it calls @command{zgrep} instead of
@command{grep} to search the contents of gzipped files.

  These commands build the shell commands based on the variables
@code{grep-template} (for @code{lgrep}) and @code{grep-find-template}
(for @code{rgrep}).  The files to search can use aliases defined in
the variable @code{grep-files-aliases}.

@vindex grep-find-ignored-directories
  Directories listed in the variable
@code{grep-find-ignored-directories} are automatically skipped by
@kbd{M-x rgrep}.  The default value includes the data directories used
by various version control systems.

@vindex grep-find-abbreviate
@findex grep-find-toggle-abbreviation
  By default, the shell commands constructed for @code{lgrep},
@code{rgrep}, and @code{zgrep} are abbreviated for display by
concealing the part that contains a long list of files and directories
to ignore.  You can reveal the concealed part by clicking on the
button with ellipsis, which represents them.  You can also
interactively toggle viewing the concealed part by typing @kbd{M-x
grep-find-toggle-abbreviation}.  To disable this abbreviation of the
shell commands, customize the option @code{grep-find-abbreviate} to a
@code{nil} value.

@findex grep-change-to-grep-edit-mode
@cindex Grep Edit mode
@cindex mode, Grep Edit
  Typing @kbd{e} in the @file{*grep*} buffer makes the buffer writable
and enters the Grep Edit mode.  Similar to Occur Edit mode (@pxref{Other
Repeating Search}), you can edit the matching lines reported by
@code{grep} and have those changes reflected in the buffer visiting the
originating file.  Type @kbd{C-c C-c} to leave the Grep Edit mode and
return to the Grep mode.

@node Flymake
@section Finding Syntax Errors On The Fly
@cindex checking syntax

  Flymake mode is a minor mode that performs on-the-fly syntax
checking for many programming and markup languages, including C, C++,
Perl, HTML, and @TeX{}/@LaTeX{}.  It is somewhat analogous to Flyspell
mode, which performs spell checking for ordinary human languages in a
similar fashion (@pxref{Spelling}).  As you edit a file, Flymake mode
runs an appropriate syntax checking tool in the background, using a
temporary copy of the buffer.  It then parses the error and warning
messages, and highlights the erroneous lines in the buffer.  The
syntax checking tool used depends on the language; for example, for
C/C++ files this is usually the C compiler.  Flymake can also use
build tools such as @code{make} for checking complicated projects.

  To enable Flymake mode, type @kbd{M-x flymake-mode}.  You can jump
to the errors that it finds by using @w{@kbd{M-x
flymake-goto-next-error}} and @w{@kbd{M-x flymake-goto-prev-error}}.
To display a detailed overview of the diagnostics for the current
buffer, use the command @w{@kbd{M-x flymake-show-buffer-diagnostics}};
to display a similar overview of diagnostics for the entire project
(@pxref{Projects}), use @w{@kbd{M-x
flymake-show-project-diagnostics}}.

  For more details about using Flymake,
@ifnottex
see @ref{Top, Flymake, Flymake, flymake, The Flymake Manual}.
@end ifnottex
@iftex
see the Flymake Info manual, which is distributed with Emacs.
@end iftex

@node Debuggers
@section Running Debuggers Under Emacs
@cindex debuggers
@cindex GUD library
@cindex GDB
@cindex DBX
@cindex SDB
@cindex XDB
@cindex Perldb
@cindex JDB
@cindex PDB

The GUD (Grand Unified Debugger) library provides an Emacs interface
to a wide variety of symbolic debuggers.  It can run the GNU Debugger
(GDB), as well as LLDB, DBX, SDB, XDB, Guile REPL debug commands, Perl's
debugging mode, the Python debugger PDB, and the Java Debugger JDB.

  Emacs provides a special interface to GDB, which uses extra Emacs
windows to display the state of the debugged program.  @xref{GDB
Graphical Interface}.

  Emacs also has a built-in debugger for Emacs Lisp programs.
@xref{Debugging,, The Lisp Debugger, elisp, the Emacs Lisp Reference
Manual}.

@menu
* Starting GUD::        How to start a debugger subprocess.
* Debugger Operation::  Connection between the debugger and source buffers.
* Commands of GUD::     Key bindings for common commands.
* GUD Customization::   Defining your own commands for GUD.
* GDB Graphical Interface::  An enhanced mode that uses GDB features to
                        implement a graphical debugging environment.
@end menu

@node Starting GUD
@subsection Starting GUD

  There are several commands for starting a debugger subprocess, each
corresponding to a particular debugger program.

@table @kbd
@item M-x gdb
@findex gdb
Run GDB as a subprocess, and interact with it via an IDE-like Emacs
interface.  @xref{GDB Graphical Interface}, for more information about
this command.

@item M-x gud-gdb
@findex gud-gdb
Run GDB, using a GUD interaction buffer for input and output to the
GDB subprocess (@pxref{Debugger Operation}).  If such a buffer already
exists, switch to it; otherwise, create the buffer and switch to it.

The other commands in this list do the same, for other debugger
programs.

@item M-x lldb
@findex lldb
Run the LLDB debugger.

@item M-x perldb
@findex perldb
Run the Perl interpreter in debug mode.

@item M-x jdb
@findex jdb
Run the Java debugger.

@item M-x pdb
@findex pdb
Run the Python debugger.

@item M-x guiler
@findex guiler
Run Guile REPL for debugging Guile Scheme programs.

@item M-x dbx
@findex dbx
Run the DBX debugger.

@item M-x xdb
@findex xdb
@vindex gud-xdb-directories
Run the XDB debugger.

@item M-x sdb
@findex sdb
Run the SDB debugger.
@end table

  Each of these commands reads a command line to invoke the debugger,
using the minibuffer.  The minibuffer's initial contents contain the
standard executable name and options for the debugger, and sometimes
also a guess for the name of the executable file you want to debug.
For @code{pdb}, @code{perldb}, and @code{guiler}, if the current buffer
visits a file, pressing @kbd{M-n} (@code{next-history-element})
in the prompt suggests a full command line including that file name.
Shell wildcards and variables are not allowed in this command line.
Emacs assumes that the first command argument which does not start
with a @samp{-} is the executable file name.

@cindex remote host, debugging on
  Tramp provides a facility for remote debugging, whereby both the
debugger and the program being debugged are on the same remote host.
@xref{Running a debugger on a remote host,,, tramp, The Tramp Manual},
for details.  This is separate from GDB's remote debugging feature,
where the program and the debugger run on different machines
(@pxref{Remote Debugging,, Debugging Remote Programs, gdb, The GNU
debugger}).

@node Debugger Operation
@subsection Debugger Operation
@cindex GUD interaction buffer

  The @dfn{GUD interaction buffer} is an Emacs buffer which is used to
send text commands to a debugger subprocess, and record its output.
This is the basic interface for interacting with a debugger, used by
@kbd{M-x gud-gdb} and other commands listed in
@iftex
the preceding section.
@end iftex
@ifnottex
@ref{Starting GUD}.
@end ifnottex
The @kbd{M-x gdb} command extends this interface with additional
specialized buffers for controlling breakpoints, stack frames, and
other aspects of the debugger state (@pxref{GDB Graphical Interface}).

  The GUD interaction buffer uses a variant of Shell mode, so the
Emacs commands defined by Shell mode are available (@pxref{Shell
Mode}).  Completion is available for most debugger commands
(@pxref{Completion}), and you can use the usual Shell mode history
commands to repeat them.
@iftex
See the next section
@end iftex
@ifnottex
@xref{Commands of GUD},
@end ifnottex
for special commands that can be used in the GUD interaction buffer.

  As you debug a program, Emacs displays the relevant source files by
visiting them in Emacs buffers, with an arrow in the left fringe
indicating the current execution line.  (On a text terminal, the arrow
appears as @samp{=>}, overlaid on the first two text columns.)  Moving
point in such a buffer does not move the arrow.  You are free to edit
these source files, but note that inserting or deleting lines will
throw off the arrow's positioning, as Emacs has no way to figure out
which edited source line corresponds to the line reported by the
debugger subprocess.  To update this information, you typically have
to recompile and restart the program.

@cindex GUD and hl-line-mode
@cindex highlighting execution lines in GUD
@vindex gud-highlight-current-line
  Moreover, GUD is capable of visually demarcating the current
execution line within the window text itself in one of two fashions:
the first takes effect when the user option
@code{gud-highlight-current-line} is enabled, and displays that line
in an overlay whose appearance is provided by the face
@code{gud-highlight-current-line-face}.  The other takes effect when
HL Line Mode (@pxref{Cursor Display}) is enabled, and moves the
overlay introduced by HL Line Mode briefly to the execution line,
until a subsequent editing command repositions it back beneath the
cursor.

@cindex GUD Tooltip mode
@cindex mode, GUD Tooltip
@findex gud-tooltip-mode
  GUD Tooltip mode is a global minor mode that adds tooltip support to
GUD@.  To toggle this mode, type @kbd{M-x gud-tooltip-mode}.  It is
disabled by default.  If enabled, you can move the mouse pointer over a
variable, a function, or a macro (collectively called
@dfn{identifiers}) to show their values in tooltips
(@pxref{Tooltips}).  If just placing the mouse pointer over an
expression doesn't show the value of the expression you had in mind,
you can tell Emacs more explicitly what expression to evaluate by
dragging the mouse over the expression, then leaving the mouse inside
the marked area.  The GUD Tooltip mode takes effect in the GUD
interaction buffer, and in all source buffers with major modes listed
in the variable @code{gud-tooltip-modes}.  If you turned off the
tooltip mode, values are shown in the echo area instead of a tooltip.

  When using GUD Tooltip mode with @kbd{M-x gud-gdb}, displaying an
expression's value in GDB can sometimes expand a macro, potentially
causing side effects in the debugged program.  For that reason, using
tooltips in @code{gud-gdb} is disabled.  If you use the @kbd{M-x gdb}
interface, this problem does not occur, as there is special code to
avoid side-effects; furthermore, you can display macro definitions
associated with an identifier when the program is not executing.

@node Commands of GUD
@subsection Commands of GUD

  GUD provides commands for setting and clearing breakpoints,
selecting stack frames, and stepping through the program.

@table @kbd
@item C-x C-a C-b
@kindex C-x C-a C-b
Set a breakpoint on the source line that point is on.
@end table

  @kbd{C-x C-a C-b} (@code{gud-break}), when called in a source
buffer, sets a debugger breakpoint on the current source line.  This
command is available only after starting GUD@.  If you call it in a
buffer that is not associated with any debugger subprocess, it signals
an error.

@kindex C-x C-a @r{(GUD)}
  The following commands are available both in the GUD interaction
buffer and globally, but with different key bindings.  The keys
starting with @kbd{C-c} are available only in the GUD interaction
buffer, while those starting with @kbd{C-x C-a} are available
globally.  Some of these commands are also available via the tool bar;
some are not supported by certain debuggers.

@table @kbd
@item C-c C-l
@kindex C-c C-l @r{(GUD)}
@itemx C-x C-a C-l
@findex gud-refresh
Display, in another window, the last source line referred to in the
GUD interaction buffer (@code{gud-refresh}).

@item C-c C-s
@kindex C-c C-s @r{(GUD)}
@itemx C-x C-a C-s
@findex gud-step
Execute the next single line of code (@code{gud-step}).  If the line
contains a function call, execution stops after entering the called
function.

@item C-c C-n
@kindex C-c C-n @r{(GUD)}
@itemx C-x C-a C-n
@findex gud-next
Execute the next single line of code, stepping across function calls
without stopping inside the functions (@code{gud-next}).

@item C-c C-i
@kindex C-c C-i @r{(GUD)}
@itemx C-x C-a C-i
@findex gud-stepi
Execute a single machine instruction (@code{gud-stepi}).

@item C-c C-p
@kindex C-c C-p @r{(GUD)}
@itemx C-x C-a C-p
@findex gud-print
Evaluate the expression at point (@code{gud-print}).  If Emacs
does not print the exact expression that you want, mark it as a region
first.

@need 3000
@item C-c C-r
@kindex C-c C-r @r{(GUD)}
@itemx C-x C-a C-r
@findex gud-cont
Continue execution without specifying any stopping point.  The program
will run until it hits a breakpoint, terminates, or gets a signal that
the debugger is checking for (@code{gud-cont}).

@need 1000
@item C-c C-d
@kindex C-c C-d @r{(GUD)}
@itemx C-x C-a C-d
@findex gud-remove
Delete the breakpoint(s) on the current source line, if any
(@code{gud-remove}).  If you use this command in the GUD interaction
buffer, it applies to the line where the program last stopped.

@item C-c C-t
@kindex C-c C-t @r{(GUD)}
@itemx C-x C-a C-t
@findex gud-tbreak
Set a temporary breakpoint on the current source line, if any
(@code{gud-tbreak}).  If you use this command in the GUD interaction
buffer, it applies to the line where the program last stopped.

@item C-c <
@kindex C-c < @r{(GUD)}
@itemx C-x C-a <
@findex gud-up
Select the next enclosing stack frame (@code{gud-up}).  This is
equivalent to the GDB command @samp{up}.

@item C-c >
@kindex C-c > @r{(GUD)}
@itemx C-x C-a >
@findex gud-down
Select the next inner stack frame (@code{gud-down}).  This is
equivalent to the GDB command @samp{down}.

@item C-c C-u
@kindex C-c C-u @r{(GUD)}
@itemx C-x C-a C-u
@findex gud-until
Continue execution to the current line (@code{gud-until}).  The
program will run until it hits a breakpoint, terminates, gets a signal
that the debugger is checking for, or reaches the line on which the
cursor currently sits.

@item C-c C-f
@kindex C-c C-f @r{(GUD)}
@itemx C-x C-a C-f
@findex gud-finish
Run the program until the selected stack frame returns or
stops for some other reason (@code{gud-finish}).
@end table

  If you are using GDB, these additional key bindings are available:

@table @kbd
@item C-x C-a C-j
@kindex C-x C-a C-j @r{(GUD)}
@findex gud-jump
Only useful in a source buffer, @code{gud-jump} transfers the
program's execution point to the current line.  In other words, the
next line that the program executes will be the one where you gave the
command.  If the new execution line is in a different function from
the previously one, GDB prompts for confirmation since the results may
be bizarre.  See the GDB manual entry regarding @code{jump} for
details.

@item @key{TAB}
@kindex TAB @r{(GUD)}
@findex gud-gdb-complete-command
With GDB, complete a symbol name (@code{gud-gdb-complete-command}).
This key is available only in the GUD interaction buffer.
@end table

  These commands interpret a numeric argument as a repeat count, when
that makes sense.

  Because @key{TAB} serves as a completion command, you can't use it to
enter a tab as input to the program you are debugging with GDB@.
Instead, type @kbd{C-q @key{TAB}} to enter a tab.

@node GUD Customization
@subsection GUD Customization

@vindex gdb-mode-hook
@vindex dbx-mode-hook
@vindex sdb-mode-hook
@vindex xdb-mode-hook
@vindex perldb-mode-hook
@vindex pdb-mode-hook
@vindex jdb-mode-hook
@vindex guiler-mode-hook
  On startup, GUD runs one of the following hooks:
@code{gdb-mode-hook}, if you are using GDB; @code{dbx-mode-hook}, if
you are using DBX; @code{sdb-mode-hook}, if you are using SDB;
@code{xdb-mode-hook}, if you are using XDB; @code{guiler-mode-hook}
for Guile REPL debugging; @code{perldb-mode-hook}, for Perl debugging
mode; @code{pdb-mode-hook}, for PDB; @code{jdb-mode-hook}, for JDB@.
@xref{Hooks}.

  The @code{gud-def} Lisp macro (@pxref{Defining Macros,,, elisp, the
Emacs Lisp Reference Manual}) provides a convenient way to define an
Emacs command that sends a particular command string to the debugger,
and set up a key binding for in the GUD interaction buffer:

@findex gud-def
@example
(gud-def @var{function} @var{cmdstring} @var{binding} @var{docstring})
@end example

  This defines a command named @var{function} which sends
@var{cmdstring} to the debugger process, and gives it the documentation
string @var{docstring}.  You can then use the command @var{function} in any
buffer.  If @var{binding} is non-@code{nil}, @code{gud-def} also binds
the command to @kbd{C-c @var{binding}} in the GUD buffer's mode and to
@kbd{C-x C-a @var{binding}} generally.

  The command string @var{cmdstring} may contain certain
@samp{%}-sequences that stand for data to be filled in at the time
@var{function} is called:

@table @samp
@item %f
The name of the current source file.  If the current buffer is the GUD
buffer, then the current source file is the file that the program
stopped in.

@item %l
The number of the current source line.  If the current buffer is the GUD
buffer, then the current source line is the line that the program
stopped in.

@item %e
In transient-mark-mode the text in the region, if it is active.
Otherwise the text of the C lvalue or function-call expression at or
adjacent to point.

@item %a
The text of the hexadecimal address at or adjacent to point.

@item %p
The numeric argument of the called function, as a decimal number.  If
the command is used without a numeric argument, @samp{%p} stands for the
empty string.

If you don't use @samp{%p} in the command string, the command you define
ignores any numeric argument.

@item %d
The name of the directory of the current source file.

@item %c
Fully qualified class name derived from the expression surrounding point
(jdb only).
@end table

@node GDB Graphical Interface
@subsection GDB Graphical Interface

  The command @kbd{M-x gdb} starts GDB in an IDE-like interface, with
specialized buffers for controlling breakpoints, stack frames, and
other aspects of the debugger state.  It also provides additional ways
to control the debugging session with the mouse, such as clicking in
the fringe of a source buffer to set a breakpoint there.

@vindex gud-gdb-command-name
  To run GDB using just the GUD interaction buffer interface, without
these additional features, use @kbd{M-x gud-gdb} (@pxref{Starting
GUD}).

  Internally, @kbd{M-x gdb} informs GDB that its screen size is
unlimited; for correct operation, you must not change GDB's screen
height and width values during the debugging session.

@menu
* GDB User Interface Layout::   Control the number of displayed buffers.
* Source Buffers::              Use the mouse in the fringe/margin to
                                control your program.
* Breakpoints Buffer::          A breakpoint control panel.
* Threads Buffer::              Displays your threads.
* Stack Buffer::                Select a frame from the call stack.
* Other GDB Buffers::           Other buffers for controlling the GDB state.
* Watch Expressions::           Monitor variable values in the speedbar.
* Multithreaded Debugging::     Debugging programs with several threads.
@end menu

@node GDB User Interface Layout
@subsubsection GDB User Interface Layout
@cindex GDB User Interface layout

@vindex gdb-many-windows
@vindex gdb-show-main
  If the variable @code{gdb-many-windows} is @code{nil} (the default),
@kbd{M-x gdb} normally displays only the GUD interaction buffer.
However, if the variable @code{gdb-show-main} is also non-@code{nil},
it starts with two windows: one displaying the GUD interaction buffer,
and the other showing the source for the @code{main} function of the
program you are debugging.

  If @code{gdb-many-windows} is non-@code{nil}, then @kbd{M-x gdb}
displays the following frame layout:

@smallexample
@group
+--------------------------------+--------------------------------+
|   GUD interaction buffer       |   Locals/Registers buffer      |
|--------------------------------+--------------------------------+
|   Primary Source buffer        |   I/O buffer for debugged pgm  |
|--------------------------------+--------------------------------+
|   Stack buffer                 |   Breakpoints/Threads buffer   |
+--------------------------------+--------------------------------+
@end group
@end smallexample

@findex gdb-save-window-configuration
@findex gdb-load-window-configuration
@vindex gdb-default-window-configuration-file
@vindex gdb-window-configuration-directory
  You can customize the window layout based on the one above and save
that layout to a file using @code{gdb-save-window-configuration}.
Then you can later load this layout back using
@code{gdb-load-window-configuration}.  (Internally, Emacs uses the
term window configuration instead of window layout.)  You can set your
custom layout as the default one used by @code{gdb-many-windows} by
customizing @code{gdb-default-window-configuration-file}.  If it is
not an absolute file name, GDB looks under
@code{gdb-window-configuration-directory} for the file.
@code{gdb-window-configuration-directory} defaults to
@code{user-emacs-directory} (@pxref{Find Init}).


@findex gdb-restore-windows
@findex gdb-many-windows
@vindex gdb-restore-window-configuration-after-quit
  If you ever change the window layout, you can restore the default
layout by typing @kbd{M-x gdb-restore-windows}.  To toggle
between the many windows layout and a simple layout with just the GUD
interaction buffer and a source file, type @kbd{M-x gdb-many-windows}.

  If you have an elaborate window setup, and don't want
@code{gdb-many-windows} to disrupt that, it is better to invoke
@kbd{M-x gdb} in a separate frame to begin with, then the arrangement
of windows on your original frame will not be affected.  A separate
frame for GDB sessions can come in especially handy if you work on a
text-mode terminal, where the screen estate for windows could be at a
premium.  If you choose to start GDB in the same frame, consider
setting @code{gdb-restore-window-configuration-after-quit} to a
non-@code{nil} value.  Your original layout will then be restored
after GDB quits.  Use @code{t} to always restore; use
@code{if-gdb-many-windows} to restore only when
@code{gdb-many-windows} is non-@code{nil}; use @code{if-gdb-show-main}
to restore only when @code{gdb-show-main} is non-@code{nil}.

  You may also specify additional GDB-related buffers to display,
either in the same frame or a different one.  Select the buffers you
want by typing @kbd{M-x gdb-display-@var{buffertype}-buffer} or
@kbd{M-x gdb-frame-@var{buffertype}-buffer}, where @var{buffertype} is
the relevant buffer type, such as @samp{breakpoints} or @samp{io}.
You can do the same from the menu bar, with the @samp{GDB-Windows} and
@samp{GDB-Frames} sub-menus of the @samp{GUD} menu.

@vindex gdb-max-source-window-count
@vindex gdb-display-source-buffer-action
By default, GDB uses at most one window to display the source file.
You can make it use more windows by customizing
@code{gdb-max-source-window-count}.  You can also customize
@code{gdb-display-source-buffer-action} to control how GDB displays
source files.

  When you finish debugging, kill the GUD interaction buffer with
@kbd{C-x k}, which will also kill all the buffers associated with the
session.  However you need not do this if, after editing and
re-compiling your source code within Emacs, you wish to continue
debugging.  When you restart execution, GDB automatically finds the
new executable.  Keeping the GUD interaction buffer has the advantage
of keeping the shell history as well as GDB's breakpoints.  You do
need to check that the breakpoints in recently edited source files are
still in the right places.

@node Source Buffers
@subsubsection Source Buffers
@cindex fringes, for debugging

@table @asis
@item @kbd{mouse-1} (in fringe)
Set or clear a breakpoint on that line
(@code{gdb-mouse-set-clear-breakpoint}).

@item @kbd{C-mouse-1} (in fringe)
Enable or disable a breakpoint on that line
(@code{gdb-mouse-toggle-breakpoint-margin}).

@item @kbd{mouse-3} (in fringe)
Continue execution to that line (@code{gdb-mouse-until}).

@item @kbd{C-mouse-3} (in fringe)
Jump to that line (@code{gdb-mouse-jump}).
@end table

  On a graphical display, you can click @kbd{mouse-1} in the fringe of
a source buffer, to set a breakpoint on that line (@pxref{Fringes}).
A red dot appears in the fringe, where you clicked.  If a breakpoint
already exists there, the click removes it.  A @kbd{C-mouse-1} click
enables or disables an existing breakpoint; a breakpoint that is
disabled, but not unset, is indicated by a gray dot.

  On a text terminal, or when fringes are disabled, enabled
breakpoints are indicated with a @samp{B} character in the left margin
of the window.  Disabled breakpoints are indicated with @samp{b}.
(The margin is only displayed if a breakpoint is present.)

  A solid arrow in the left fringe of a source buffer indicates the
line of the innermost frame where the debugged program has stopped.  A
hollow arrow indicates the current execution line of a higher-level
frame.  If you drag the arrow in the fringe with @kbd{mouse-1}, that
causes execution to advance to the line where you release the button.
Alternatively, you can click @kbd{mouse-3} in the fringe to advance to
that line.  You can click @kbd{C-mouse-3} in the fringe to jump to
that line without executing the intermediate lines.  This command
allows you to go backwards, which can be useful for running through
code that has already executed, in order to examine its execution in
more detail.

@vindex gdb-mi-decode-strings
  By default, source file names and non-ASCII strings in the program
being debugged are decoded using the default coding-system.  If you
prefer a different decoding, perhaps because the program being
debugged uses a different character encoding, set the variable
@code{gdb-mi-decode-strings} to the appropriate coding-system, or to
@code{nil} to leave non-ASCII characters as undecoded octal escapes.

@node Breakpoints Buffer
@subsubsection Breakpoints Buffer

  The GDB Breakpoints buffer shows the breakpoints, watchpoints and
catchpoints in the debugger session.  @xref{Breakpoints,,, gdb, The
GNU debugger}.  It provides the following commands, which mostly apply
to the @dfn{current breakpoint} (the breakpoint which point is on):

@table @kbd
@item @key{SPC}
@kindex SPC @r{(GDB Breakpoints buffer)}
@findex gdb-toggle-breakpoint
Enable/disable current breakpoint (@code{gdb-toggle-breakpoint}).  On
a graphical display, this changes the color of the dot in the fringe
of the source buffer at that line.  The dot is red when the breakpoint
is enabled, and gray when it is disabled.

@item D
@kindex D @r{(GDB Breakpoints buffer)}
@findex gdb-delete-breakpoint
Delete the current breakpoint (@code{gdb-delete-breakpoint}).

@item @key{RET}
@kindex RET @r{(GDB Breakpoints buffer)}
@findex gdb-goto-breakpoint
Visit the source line for the current breakpoint
(@code{gdb-goto-breakpoint}).

@item mouse-2
@kindex mouse-2 @r{(GDB Breakpoints buffer)}
Visit the source line for the breakpoint you click on
(@code{gdb-goto-breakpoint}).
@end table

@vindex gdb-show-threads-by-default
  When @code{gdb-many-windows} is non-@code{nil}, the GDB Breakpoints
buffer shares its window with the GDB Threads buffer.  To switch from
one to the other click with @kbd{mouse-1} on the relevant button in
the header line.  If @code{gdb-show-threads-by-default} is
non-@code{nil}, the GDB Threads buffer is the one shown by default.

@node Threads Buffer
@subsubsection Threads Buffer

@findex gdb-select-thread
  The GDB Threads buffer displays a summary of the threads in the
debugged program.  @xref{Threads, Threads, Debugging programs with
multiple threads, gdb, The GNU debugger}.  To select a thread, move
point there and press @key{RET} (@code{gdb-select-thread}), or click on
it with @kbd{mouse-2}.  This also displays the associated source
buffer, and updates the contents of the other GDB buffers.

  You can customize variables in the @code{gdb-buffers} group to
select fields included in GDB Threads buffer.

@table @code
@item gdb-thread-buffer-verbose-names
@vindex gdb-thread-buffer-verbose-names
Show long thread names like @samp{Thread 0x4e2ab70 (LWP 1983)}.

@item gdb-thread-buffer-arguments
@vindex gdb-thread-buffer-arguments
Show arguments of thread top frames.

@item gdb-thread-buffer-locations
@vindex gdb-thread-buffer-locations
Show file information or library names.

@item gdb-thread-buffer-addresses
@vindex gdb-thread-buffer-addresses
Show addresses for thread frames in threads buffer.
@end table

  To view information for several threads simultaneously, use the
following commands from the GDB Threads buffer.

@table @kbd
@item d
@kindex d @r{(GDB threads buffer)}
@findex gdb-display-disassembly-for-thread
Display disassembly buffer for the thread at current line
(@code{gdb-display-disassembly-for-thread}).

@item f
@kindex f @r{(GDB threads buffer)}
@findex gdb-display-stack-for-thread
Display the GDB Stack buffer for the thread at current line
(@code{gdb-display-stack-for-thread}).

@item l
@kindex l @r{(GDB threads buffer)}
@findex gdb-display-locals-for-thread
Display the GDB Locals buffer for the thread at current line
(@code{gdb-display-locals-for-thread}).

@item r
@kindex r @r{(GDB threads buffer)}
@findex gdb-display-registers-for-thread
Display the GDB Registers buffer for the thread at current line
(@code{gdb-display-registers-for-thread}).
@end table

@noindent
Their upper-case counterparts, @kbd{D}, @kbd{F} ,@kbd{L} and @kbd{R},
display the corresponding buffer in a new frame.

  When you create a buffer showing information about some specific
thread, it becomes bound to that thread and keeps showing actual
information while you debug your program.  The mode indicator for each
GDB buffer shows the number of the thread whose information that
buffer displays.  The thread number is also included in the name of
each bound buffer.

  Further commands are available in the GDB Threads buffer which
depend on the mode of GDB that is used for controlling execution of
your program.  @xref{Multithreaded Debugging}.

@node Stack Buffer
@subsubsection Stack Buffer

  The GDB Stack buffer displays a @dfn{call stack}, with one line for
each of the nested subroutine calls (@dfn{stack frames}) in the
debugger session.  @xref{Backtrace,, Backtraces, gdb, The GNU
debugger}.

@findex gdb-frames-select
  On graphical displays, the selected stack frame is indicated by an
arrow in the fringe.  On text terminals, or when fringes are disabled,
the selected stack frame is displayed in reverse contrast.  To select
a stack frame, move point in its line and type @key{RET}
(@code{gdb-frames-select}), or click @kbd{mouse-2} on it.  Doing so
also updates the Locals buffer
@ifnottex
(@pxref{Other GDB Buffers}).
@end ifnottex
@iftex
(described in the next section).
@end iftex

@vindex gdb-stack-buffer-addresses
  If you want the frame address to be shown each stack frame,
customize the variable @code{gdb-stack-buffer-addresses} to a
non-@code{nil} value.

@node Other GDB Buffers
@subsubsection Other GDB Buffers

Other buffers provided by @kbd{M-x gdb} whose display you can
optionally request include:

@table @asis
@findex gdb-display-locals-buffer
@item Locals Buffer
This buffer displays the values of local variables of the current
stack frame for simple data types (@pxref{Frame Info, Frame Info,
Information on a frame, gdb, The GNU debugger}).  Press @key{RET} or
click @kbd{mouse-2} on the value if you want to edit it.

Arrays and structures display their type only.  With GDB 6.4 or later,
you can examine the value of the local variable at point by typing
@key{RET}, or with a @kbd{mouse-2} click.  With earlier versions of
GDB, use @key{RET} or @kbd{mouse-2} on the type description
(@samp{[struct/union]} or @samp{[array]}).  @xref{Watch Expressions}.

To display the Locals buffer, type @kbd{M-x gdb-display-locals-buffer}.

@findex gdb-display-io-buffer
@item I/O Buffer
If the program you are debugging uses standard input and output
streams for interaction with the user, or emits a significant amount
of output to its standard output, you may wish to separate its I/O
from interaction with GDB.  Use the command @w{@kbd{M-x
gdb-display-io-buffer}} to show a window with a buffer to which Emacs
redirects the input and output from the program you are debugging.
However, if the variable @code{gdb-display-io-buffer} is @code{nil},
Emacs will not create nor display a separate I/O buffer, but will
instead redirect the program's interaction to the GUD interaction
buffer.

@findex gdb-display-registers-buffer
@item Registers Buffer
This buffer displays the values held by the registers
(@pxref{Registers,,, gdb, The GNU debugger}).  Request the display of
this buffer with the command @kbd{M-x gdb-display-registers-buffer}.
Press @key{RET} or click @kbd{mouse-2} on a register if you want to
edit its value.  With GDB 6.4 or later, recently changed register
values display with @code{font-lock-warning-face}.

@findex gdb-display-disassembly-buffer
@item Assembler Buffer
The assembler buffer displays the current frame as machine code.  An
arrow points to the current instruction, and you can set and remove
breakpoints as in a source buffer.  Breakpoint icons also appear in
the fringe or margin.  To request the display of this buffer, use
@kbd{M-x gdb-display-disassembly-buffer}.

@findex gdb-display-memory-buffer
@item Memory Buffer
The memory buffer lets you examine sections of program memory
(@pxref{Memory, Memory, Examining memory, gdb, The GNU debugger}).
Click @kbd{mouse-1} on the appropriate part of the header line to
change the starting address or number of data items that the buffer
displays.  Alternatively, use @kbd{S} or @kbd{N} respectively.  Click
@kbd{mouse-3} on the header line to select the display format or unit
size for these data items.  Use @w{@kbd{M-x
gdb-display-memory-buffer}} to request display of this buffer.
@end table

When @code{gdb-many-windows} is non-@code{nil}, the locals buffer
shares its window with the registers buffer, just like breakpoints and
threads buffers.  To switch from one to the other, click with
@kbd{mouse-1} on the relevant button in the header line.

@node Watch Expressions
@subsubsection Watch Expressions
@cindex Watching expressions in GDB

@findex gud-watch
@kindex C-x C-a C-w @r{(GUD)}
  If you want to see how a variable changes each time your program
stops, move point into the variable name and click on the watch icon
in the tool bar (@code{gud-watch}) or type @kbd{C-x C-a C-w}.  If you
specify a prefix argument, you can enter the variable name in the
minibuffer.

  Each watch expression is displayed in the speedbar
(@pxref{Speedbar}).  Complex data types, such as arrays, structures
and unions are represented in a tree format.  Leaves and simple data
types show the name of the expression and its value and, when the
speedbar frame is selected, display the type as a tooltip.  Higher
levels show the name, type and address value for pointers and just the
name and type otherwise.  Root expressions also display the frame
address as a tooltip to help identify the frame in which they were
defined.

  To expand or contract a complex data type, click @kbd{mouse-2} or
press @key{SPC} on the tag to the left of the expression.  Emacs asks
for confirmation before expanding the expression if its number of
immediate children exceeds the value of the variable
@code{gdb-max-children}.

@kindex D @r{(GDB speedbar)}
@findex gdb-var-delete
  To delete a complex watch expression, move point to the root
expression in the speedbar and type @kbd{D} (@code{gdb-var-delete}).

@kindex RET @r{(GDB speedbar)}
@findex gdb-edit-value
  To edit a variable with a simple data type, or a simple element of a
complex data type, move point there in the speedbar and type @key{RET}
(@code{gdb-edit-value}).  Or you can click @kbd{mouse-2} on a value to
edit it.  Either way, this reads the new value using the minibuffer.

@vindex gdb-show-changed-values
  If you set the variable @code{gdb-show-changed-values} to
non-@code{nil} (the default value), Emacs uses
@code{font-lock-warning-face} to highlight values that have recently
changed and @code{shadow} face to make variables which have gone out of
scope less noticeable.  When a variable goes out of scope you can't
edit its value.

@vindex gdb-delete-out-of-scope
  If the variable @code{gdb-delete-out-of-scope} is non-@code{nil}
(the default value), Emacs automatically deletes watch expressions
which go out of scope.  Sometimes, when your program re-enters the
same function many times, it may be useful to set this value to
@code{nil} so that you don't need to recreate the watch expression.

@vindex gdb-use-colon-colon-notation
  If the variable @code{gdb-use-colon-colon-notation} is
non-@code{nil}, Emacs uses the @samp{@var{function}::@var{variable}}
format.  This allows the user to display watch expressions which share
the same variable name.  The default value is @code{nil}.

@vindex gdb-speedbar-auto-raise
To automatically raise the speedbar every time the display of watch
expressions updates, set @code{gdb-speedbar-auto-raise} to
non-@code{nil}.  This can be useful if you are debugging with a full
screen Emacs frame.

@node Multithreaded Debugging
@subsubsection Multithreaded Debugging
@cindex Multithreaded debugging in GDB
@cindex Non-stop debugging in GDB

  In GDB's @dfn{all-stop mode}, whenever your program stops, all
execution threads stop.  Likewise, whenever you restart the program,
all threads start executing.  @xref{All-Stop Mode, , All-Stop Mode,
gdb, The GNU debugger}.  For some multi-threaded targets, GDB supports
a further mode of operation, called @dfn{non-stop mode}, in which you
can examine stopped program threads in the debugger while other
threads continue to execute freely.  @xref{Non-Stop Mode, , Non-Stop
Mode, gdb, The GNU debugger}.  Versions of GDB prior to 7.0 do not
support non-stop mode, and it does not work on all targets.

@vindex gdb-non-stop-setting
  The variable @code{gdb-non-stop-setting} determines whether Emacs
runs GDB in all-stop mode or non-stop mode.  The default is @code{t},
which means it tries to use non-stop mode if that is available.  If
you change the value to @code{nil}, or if non-stop mode is
unavailable, Emacs runs GDB in all-stop mode.  The variable takes
effect when Emacs begins a debugging session; if you change its value,
you should restart any active debugging session.

@vindex gdb-switch-when-another-stopped
  When a thread stops in non-stop mode, Emacs usually switches to that
thread.  If you don't want Emacs to do this switch if another stopped
thread is already selected, change the variable
@code{gdb-switch-when-another-stopped} to @code{nil}.

@vindex gdb-switch-reasons
  Emacs can decide whether or not to switch to the stopped thread
depending on the reason which caused the stop.  Customize the variable
@code{gdb-switch-reasons} to select the stop reasons which will cause
a thread switch.

@vindex gdb-stopped-functions
  The variable @code{gdb-stopped-functions} allows you to execute your
functions whenever some thread stops.

  In non-stop mode, you can switch between different modes for GUD
execution control commands.

@vindex gdb-gud-control-all-threads
@table @dfn
@item Non-stop/A

  When @code{gdb-gud-control-all-threads} is @code{t} (the default
value), interruption and continuation commands apply to all threads,
so you can halt or continue all your threads with one command using
@code{gud-stop-subjob} and @code{gud-cont}, respectively.  The
@samp{Go} button is shown on the tool bar when at least one thread is
stopped, whereas @samp{Stop} button is shown when at least one thread
is running.

@item Non-stop/T

When @code{gdb-gud-control-all-threads} is @code{nil}, only the
current thread is stopped/continued.  @samp{Go} and @samp{Stop}
buttons on the GUD tool bar are shown depending on the state of
current thread.
@end table

You can change the current value of @code{gdb-gud-control-all-threads}
from the tool bar or from @samp{GUD->GDB-MI} menu.

  Stepping commands always apply to the current thread.

  In non-stop mode, you can interrupt/continue your threads without
selecting them.  Hitting @kbd{i} in threads buffer interrupts thread
under point, @kbd{c} continues it, @kbd{s} steps through.  More such
commands may be added in the future.

  Note that when you interrupt a thread, it stops with the
@samp{signal received} reason.  If that reason is included in your
@code{gdb-switch-reasons} (it is by default), Emacs will switch to
that thread.

@node Executing Lisp
@section Executing Lisp Expressions

  Emacs has major modes for several variants of Lisp.  They use the
same editing commands as other programming language modes
(@pxref{Programs}).  In addition, they provide special commands for
executing Lisp expressions.

@table @asis
@item Emacs Lisp mode
The mode for editing Emacs Lisp source files.  It defines @kbd{C-M-x}
to evaluate the current top-level Lisp expression.  @xref{Lisp Eval}.

@item Lisp Interaction mode
The mode for an interactive Emacs Lisp session.  It defines @kbd{C-j}
to evaluate the expression before point and insert its value in the
buffer.  @xref{Lisp Interaction}.

@item Lisp mode
The mode for editing source files of programs that run in Lisps other
than Emacs Lisp.  It defines @kbd{C-M-x} to evaluate the current
top-level expression in an external Lisp.  @xref{External Lisp}.

@item Inferior Lisp mode
The mode for an interactive session with an external Lisp which is
being run as a subprocess (or @dfn{inferior process}) of Emacs.
@ifnottex
@xref{External Lisp}.
@end ifnottex

@item Scheme mode
Like Lisp mode, but for Scheme programs.

@item Inferior Scheme mode
Like Inferior Lisp mode, but for Scheme.
@end table

@node Lisp Libraries
@section Libraries of Lisp Code for Emacs
@cindex libraries
@cindex loading Lisp code

  Emacs Lisp code is stored in files whose names conventionally end in
@file{.el}.  Such files are automatically visited in Emacs Lisp mode.

@cindex byte code
  Emacs Lisp code can be compiled into byte-code, which loads faster,
takes up less space, and executes faster.  By convention, compiled
Emacs Lisp code goes in a separate file whose name ends in
@samp{.elc}.  For example, the compiled code for @file{foo.el} goes in
@file{foo.elc}.  @xref{Byte Compilation,, Byte Compilation, elisp, the
Emacs Lisp Reference Manual}.

@cindex native compilation
  Emacs Lisp code can also be compiled into @dfn{native code}: machine
code not unlike the one produced by a C or Fortran compiler.  Native
code runs even faster than byte-code.  Natively compiled Emacs Lisp
code is stored in files whose names end in @samp{.eln}.  @xref{Native
Compilation,, Native Compilation, elisp, the Emacs Lisp Reference Manual}.

@findex load-file
  To @dfn{load} an Emacs Lisp file, type @kbd{M-x load-file}.  This
command reads a file name using the minibuffer, and executes the
contents of that file as Emacs Lisp code.  It is not necessary to
visit the file first; this command reads the file directly from disk,
not from an existing Emacs buffer.

@findex load
@findex load-library
@vindex load-prefer-newer
@cindex load path for Emacs Lisp
  If an Emacs Lisp file is installed in the Emacs Lisp @dfn{load path}
(defined below), you can load it by typing @kbd{M-x load-library},
instead of using @kbd{M-x load-file}.  The @kbd{M-x load-library}
command prompts for a @dfn{library name} rather than a file name; it
searches through each directory in the Emacs Lisp load path, trying to
find a file matching that library name.  If the library name is
@samp{@var{foo}}, it tries looking for files named
@file{@var{foo}.elc}, @file{@var{foo}.el}, and @file{@var{foo}}.  (If
Emacs was built with native compilation enabled, @code{load-library}
looks for a @samp{.eln} file that corresponds to @file{@var{foo}.el}
and loads it instead of @file{@var{foo}.elc}.)  The default behavior
is to load the first file found.  This command prefers @file{.eln}
files over @file{.elc} files, and prefers @file{.elc} files over
@file{.el} files, because compiled files load and run faster.  If it
finds that @file{@var{lib}.el} is newer than @file{@var{lib}.elc}, it
issues a warning, in case someone made changes to the @file{.el} file
and forgot to recompile it, but loads the @file{.elc} file anyway.
(Due to this behavior, you can save unfinished edits to Emacs Lisp
source files, and not recompile until your changes are ready for use.)
If you set the option @code{load-prefer-newer} to a non-@code{nil}
value, however, then rather than the procedure described above, Emacs
loads whichever version of the file is newest.  If Emacs was built
with native compilation, and it cannot find the @samp{.eln} file
corresponding to @file{@var{lib}.el}, it will load a
@file{@var{lib}.elc} and start native compilation of
@file{@var{lib}.el} in the background, then load the @samp{.eln} file
when it finishes compilation.

  Emacs Lisp programs usually load Emacs Lisp files using the
@code{load} function.  This is similar to @code{load-library}, but is
lower-level and accepts additional arguments.  @xref{How Programs Do
Loading,,, elisp, the Emacs Lisp Reference Manual}.

@vindex load-path
  The Emacs Lisp load path is specified by the variable
@code{load-path}.  Its value should be a list of directories
(strings).  These directories are searched, in the specified order, by
the @kbd{M-x load-library} command, the lower-level @code{load}
function, and other Emacs functions that find Emacs Lisp libraries.
An entry in @code{load-path} can also have the special value
@code{nil}, which stands for the current default directory, but it is
almost always a bad idea to use this, because its meaning will depend
on the buffer that is current when @code{load-path} is used by Emacs.
(If you find yourself wishing that @code{nil} were in the list, most
likely what you really want is to use @kbd{M-x load-file}.)

  The default value of @code{load-path} is a list of directories where
the Lisp code for Emacs itself is stored.  If you have libraries of
your own in another directory, you can add that directory to the load
path.  Unlike most other variables described in this manual,
@code{load-path} cannot be changed via the Customize interface
(@pxref{Easy Customization}), but you can add a directory to it by
putting a line like this in your init file (@pxref{Init File}):

@example
(add-to-list 'load-path "/path/to/my/lisp/library")
@end example

It is customary to put locally installed libraries in the
@file{site-lisp} directory that is already in the default value of
@code{load-path}, or in some subdirectory of @file{site-lisp}.  This
way, you don't need to modify the default value of @code{load-path}.

@vindex native-comp-eln-load-path
  Similarly to @code{load-path}, the list of directories where Emacs
looks for @file{*.eln} files with native-compiled Lisp code is
specified by the variable @code{native-comp-eln-load-path}.

@cindex autoload
  Some commands are @dfn{autoloaded}; when you run them, Emacs
automatically loads the associated library first.  For instance, the
@kbd{M-x compile} command (@pxref{Compilation}) is autoloaded; if you
call it, Emacs automatically loads the @code{compile} library first.
In contrast, the command @kbd{M-x recompile} is not autoloaded, so it
is unavailable until you load the @code{compile} library.

@vindex help-enable-autoload
  Automatic loading can also occur when you look up the documentation
of an autoloaded command (@pxref{Name Help}), if the documentation
refers to other functions and variables in its library (loading the
library lets Emacs properly set up the hyperlinks in the @file{*Help*}
buffer).  To disable this feature, change the variable
@code{help-enable-autoload} to @code{nil}.

@vindex help-enable-completion-autoload
Automatic loading also occurs when completing names for
@code{describe-variable} and @code{describe-function}, based on the
prefix being completed.  To disable this feature, change the variable
@code{help-enable-completion-autoload} to @code{nil}.

  Once you put your library in a directory where Emacs can find and
load it, you may wish to make it available at startup.  This is useful
when the library defines features that should be available
automatically on demand, and manually loading the library is thus
inconvenient.  In these cases, make sure the library will be loaded by
adding suitable forms to your init file: either @code{load} or
@code{require} (if you always need to load the library at startup), or
@code{autoload} if you need Emacs to load the library when some
command or function is invoked.  For example:

@smalllisp
@group
 ;; Loads @file{my-shining-package.elc} unconditionally.
 (require 'my-shining-package)
@end group
@group
 ;; Will load @file{my-shining-package.elc} when @code{my-func} is invoked.
 (autoload 'my-func "my-shining-package")
@end group
@end smalllisp

  Note that installing a package using @code{package-install}
(@pxref{Package Installation}) takes care of placing the package's
Lisp files in a directory where Emacs will find it, and also extends
@code{load-path} as needed, making the above manual customizations
unnecessary for such packages.

@node Lisp Eval
@section Evaluating Emacs Lisp Expressions
@cindex Emacs Lisp mode
@cindex mode, Emacs Lisp
@cindex evaluation, Emacs Lisp

@findex emacs-lisp-mode
  Emacs Lisp mode is the major mode for editing Emacs Lisp.  Its mode
command is @kbd{M-x emacs-lisp-mode}.

  Emacs provides several commands for evaluating Emacs Lisp
expressions.  You can use these commands in Emacs Lisp mode, to test
your Emacs Lisp code as it is being written.  For example, after
re-writing a function, you can evaluate the function definition to
make it take effect for subsequent function calls.  These commands are
also available globally, and can be used outside Emacs Lisp mode.

@table @asis
@item @kbd{M-:}
Read a single Emacs Lisp expression in the minibuffer, evaluate it,
and print the value in the echo area (@code{eval-expression}).
@item @kbd{C-x C-e}
Evaluate the Emacs Lisp expression before point, and print the value
in the echo area (@code{eval-last-sexp}).
@item @kbd{C-M-x} @r{(in Emacs Lisp mode)}
@itemx @kbd{M-x eval-defun}
Evaluate the defun containing or after point, and print the value in
the echo area (@code{eval-defun}).
@item @kbd{M-x eval-region}
Evaluate all the Emacs Lisp expressions in the region.
@item @kbd{M-x eval-buffer}
Evaluate all the Emacs Lisp expressions in the buffer.
@end table

@ifinfo
@c This uses 'colon' instead of a literal ':' because Info cannot
@c cope with a ':' in a menu.
@kindex M-colon
@end ifinfo
@ifnotinfo
@kindex M-:
@end ifnotinfo
@findex eval-expression
  @kbd{M-:} (@code{eval-expression}) reads an expression using the
minibuffer, and evaluates it.  (Before evaluating the expression, the
current buffer switches back to the buffer that was current when you
typed @kbd{M-:}, not the minibuffer into which you typed the
expression.)

@kindex C-x C-e
@findex eval-last-sexp
  The command @kbd{C-x C-e} (@code{eval-last-sexp}) evaluates the
Emacs Lisp expression preceding point in the buffer, and displays the
value in the echo area.  When the result of an evaluation is an
integer, it is displayed together with the value in other formats
(octal, hexadecimal, and character if
@code{eval-expression-print-maximum-character}, described below,
allows it).

  If @kbd{M-:} or @kbd{C-x C-e} is given a prefix argument, it inserts
the value into the current buffer at point, rather than displaying it
in the echo area.  If the prefix argument is zero, any integer output
is inserted together with its value in other formats (octal,
hexadecimal, and character).  Such a prefix argument also prevents
abbreviation of the output according to the variables
@code{eval-expression-print-level} and
@code{eval-expression-print-length} (see below).  Similarly, a prefix
argument of @code{-1} overrides the effect of
@code{eval-expression-print-length}.

 @kbd{C-x C-e} (@code{eval-last-sexp}) treats @code{defvar}
expressions specially.  Normally, evaluating a @code{defvar}
expression does nothing if the variable it defines already has a
value.  But this command unconditionally resets the variable to the
initial value specified by the @code{defvar}; this is convenient for
debugging Emacs Lisp programs.  @code{defcustom} and @code{defface}
expressions are treated similarly.  Note the other commands documented
in this section, except @code{eval-defun}, do not have this special
feature.

@kindex C-M-x @r{(Emacs Lisp mode)}
@findex eval-defun
  The @code{eval-defun} command is bound to @kbd{C-M-x} in Emacs Lisp
mode.  It evaluates the top-level Lisp expression containing or
following point, and prints the value in the echo area.  In this
context, a top-level expression is referred to as a ``defun'', but it
need not be an actual @code{defun} (function definition).

 This command handles @code{defvar}/@code{defcustom}/@code{defface}
forms the same way that @code{eval-last-sexp} does.

  With a prefix argument, @kbd{C-M-x} instruments the function
definition for Edebug, the Emacs Lisp Debugger.  @xref{Instrumenting,
Instrumenting for Edebug,, elisp, the Emacs Lisp Reference Manual}.

@findex eval-region
@findex eval-buffer
  The command @kbd{M-x eval-region} parses the text of the region as
one or more Lisp expressions, evaluating them one by one.  @kbd{M-x
eval-buffer} is similar but evaluates the entire buffer.

@vindex eval-expression-print-level
@vindex eval-expression-print-length
@vindex eval-expression-print-maximum-character
@vindex eval-expression-debug-on-error
  The options @code{eval-expression-print-level} and
@code{eval-expression-print-length} control the maximum depth and
length of lists to print in the result of the evaluation commands
before abbreviating them.  Supplying a zero prefix argument to
@code{eval-expression} or @code{eval-last-sexp} causes lists to be
printed in full.  @code{eval-expression-debug-on-error} controls
whether evaluation errors invoke the debugger when these commands are
used; its default is @code{t}.
@code{eval-expression-print-maximum-character} prevents integers which
are larger than it from being displayed as characters.

@node Lisp Interaction
@section Lisp Interaction Buffers

@findex lisp-interaction-mode
  When Emacs starts up, it contains a buffer named @file{*scratch*},
which is provided for evaluating Emacs Lisp expressions interactively.
Its major mode is Lisp Interaction mode.  You can also enable Lisp
Interaction mode by typing @kbd{M-x lisp-interaction-mode}.

@findex scratch-buffer
  If you kill the @file{*scratch*} buffer, you can recreate it with
the @kbd{M-x scratch-buffer} command.

@findex eval-print-last-sexp
@kindex C-j @r{(Lisp Interaction mode)}
  In the @file{*scratch*} buffer, and other Lisp Interaction mode
buffers, @kbd{C-j} (@code{eval-print-last-sexp}) evaluates the Lisp
expression before point, and inserts the value at point.  Thus, as you
type expressions into the buffer followed by @kbd{C-j} after each
expression, the buffer records a transcript of the evaluated
expressions and their values.  All other commands in Lisp Interaction
mode are the same as in Emacs Lisp mode.

@vindex initial-scratch-message
  At startup, the @file{*scratch*} buffer contains a short message, in
the form of a Lisp comment, that explains what it is for.  This
message is controlled by the variable @code{initial-scratch-message},
which should be either a documentation string, or @code{nil} (which means to
suppress the message).

@findex ielm
  An alternative way of evaluating Emacs Lisp expressions
interactively is to use Inferior Emacs Lisp mode, which provides an
interface rather like Shell mode (@pxref{Shell Mode}) for evaluating
Emacs Lisp expressions.  Type @kbd{M-x ielm} to create an
@file{*ielm*} buffer which uses this mode.  For more information, see
that command's documentation.

@cindex persistent *scratch* buffer
  By default, the @file{*scratch*} buffer is not associated with any
file.  Therefore, any text in the buffer is lost when you quit Emacs.
If you prefer, you can use Remember mode's notes buffer feature to make
Emacs save and reload the contents of @file{*scratch*}.  Customize the
variable @code{initial-buffer-choice} to the symbol
@code{remember-notes} and the variable @code{remember-notes-buffer-name}
to the string @code{"*scratch*"} to achieve this.  See also @xref{Quick
Start,,, remember, the Remember mode manual}.

@node External Lisp
@section Running an External Lisp
@cindex Lisp mode
@cindex mode, Lisp
@cindex Common Lisp

  Lisp mode is the major mode for editing programs written in
general-purpose Lisp dialects, such as Common Lisp.  Its mode command
is @kbd{M-x lisp-mode}.  Emacs uses Lisp mode automatically for files
whose names end in @file{.l}, @file{.lsp}, or @file{.lisp}.

@findex run-lisp
@vindex inferior-lisp-program
@kindex C-x C-z
  You can run an external Lisp session as a subprocess or
@dfn{inferior process} of Emacs, and pass expressions to it to be
evaluated.  To begin an external Lisp session, type @kbd{M-x
run-lisp}.  This runs the program named @command{lisp}, and sets it up
so that both input and output go through an Emacs buffer named
@file{*inferior-lisp*}.  To change the name of the Lisp program run by
@kbd{M-x run-lisp}, change the variable @code{inferior-lisp-program}.

  The major mode for the @file{*lisp*} buffer is Inferior Lisp mode,
which combines the characteristics of Lisp mode and Shell mode
(@pxref{Shell Mode}).  To send input to the Lisp session, go to the
end of the @file{*lisp*} buffer and type the input, followed by
@key{RET}.  Terminal output from the Lisp session is automatically
inserted in the buffer.

@kindex C-M-x @r{(Lisp mode)}
@findex lisp-eval-defun
  When you edit a Lisp program in Lisp mode, you can type @kbd{C-M-x}
(@code{lisp-eval-defun}) to send an expression from the Lisp mode
buffer to a Lisp session that you had started with @kbd{M-x run-lisp}.
The expression sent is the top-level Lisp expression at or following
point.  The resulting value goes as usual into the
@file{*inferior-lisp*} buffer.  Note that the effect of @kbd{C-M-x} in
Lisp mode is thus very similar to its effect in Emacs Lisp mode
(@pxref{Lisp Eval}), except that the expression is sent to a different
Lisp environment instead of being evaluated in Emacs.

@findex scheme-mode
@findex run-scheme
@cindex Scheme mode
@cindex mode, Scheme
@kindex C-M-x @r{(Scheme mode)}
  The facilities for editing Scheme code, and for sending expressions
to a Scheme subprocess, are very similar.  Scheme source files are
edited in Scheme mode, which can be explicitly enabled with @kbd{M-x
scheme-mode}.  You can initiate a Scheme session by typing @kbd{M-x
run-scheme} (the buffer for interacting with Scheme is named
@file{*scheme*}), and send expressions to it by typing @kbd{C-M-x}.
