@c This is part of the Emacs manual.
@c Copyright (C) 1985, 1986, 1987, 1993, 1994, 1995, 1997, 2000, 2001,
@c   2002, 2003, 2004, 2005, 2006, 2007 Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Building, Maintaining, Programs, Top
@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 larger 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.      Creating Lisp programs to run in 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 noninteractive languages such as C and
Fortran as inferior processes, feeding the error log into an Emacs buffer.
It can also parse the error messages and show you the source lines where
compilation errors occurred.

@table @kbd
@item M-x compile
Run a compiler asynchronously under Emacs, with error messages going to
the @samp{*compilation*} buffer.
@item M-x recompile
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, do @kbd{M-x
compile}.  This command reads a shell command line using the minibuffer,
and then executes the command in an inferior shell, putting output in
the buffer named @samp{*compilation*}.  The current buffer's default
directory is used as the working directory for the execution of the
command; normally, therefore, the compilation happens in this
directory.

@vindex compile-command
  The default for the compilation command is normally @samp{make -k},
which is correct most of the time for nontrivial programs.
(@xref{Top,, Make, make, GNU Make Manual}.)  If you have done @kbd{M-x
compile} before, the default each time is the command you used the
previous time.  @code{compile} stores this command in the variable
@code{compile-command}, so setting that variable specifies the default
for the next use of @kbd{M-x compile}.  If a file specifies a file
local value for @code{compile-command}, that provides the default when
you type @kbd{M-x compile} in that file's buffer.  @xref{File
Variables}.

  Starting a compilation displays the buffer @samp{*compilation*} in
another window but does not select it.  The buffer's mode line tells
you whether compilation is finished, with the word @samp{run},
@samp{signal} or @samp{exit} inside the parentheses.  You do not have
to keep this buffer visible; compilation continues in any case.  While
a compilation is going on, the string @samp{Compiling} appears in the
mode lines of all windows.  When this string disappears, the
compilation is finished.

  If you want to watch the compilation transcript as it appears, switch
to the @samp{*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.  If point is not at the end of
the buffer, it remains fixed while more compilation output is added at
the end of the buffer.

@cindex compilation buffer, keeping point at end
@vindex compilation-scroll-output
  If you set the variable @code{compilation-scroll-output} to a
non-@code{nil} value, then the compilation buffer always scrolls to
follow output as it comes in.

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

  When the compiler process terminates, for whatever reason, the mode
line of the @samp{*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), instead of
@samp{run}.

@findex kill-compilation
  Starting a new compilation also kills any compilation already
running in @samp{*compilation*}, as the buffer can only handle one
compilation at any time.  However, @kbd{M-x compile} asks for
confirmation before actually killing a compilation that is running.
You can also kill the compilation process with @kbd{M-x
kill-compilation}.

  If you want to run two compilations at once, you should start the
first one, then rename the @samp{*compilation*} buffer (perhaps using
@code{rename-uniquely}; @pxref{Misc Buffer}), and start the other
compilation.  That will create a new @samp{*compilation*} buffer.

  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 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

  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

@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.

@node Compilation Mode
@section Compilation Mode

@cindex Compilation mode
@cindex mode, Compilation
  The @samp{*compilation*} buffer uses a special major mode,
Compilation mode, whose main feature is to provide a convenient way to
visit the source line corresponding to an error message.  These
commands are also available in other special buffers that list
locations in files, including those made by @kbd{M-x grep} and
@kbd{M-x occur}.

@table @kbd
@item M-g M-n
@itemx M-g n
@itemx C-x `
Visit the locus of the next error message or match.
@item M-g M-p
@itemx M-g p
Visit the locus of the previous error message or match.
@item @key{RET}
Visit the locus of the error message that point is on.
This command is used in the compilation buffer.
@item Mouse-2
Visit the locus of the error message that you click on.
@item M-n
Find and highlight the locus of the next error message, without
selecting the source buffer.
@item M-p
Find and highlight the locus of the previous error message, without
selecting the source buffer.
@item M-@}
Move point to the next error for a different file than the current
one.
@item M-@{
Move point to the previous error for a different file than the current
one.
@item C-c C-f
Toggle Next Error Follow minor mode, which makes cursor motion in the
compilation buffer produce automatic source display.
@end table

@findex compile-goto-error
  You can visit the source for any particular error message by moving
point in the @samp{*compilation*} buffer to that error message and
typing @key{RET} (@code{compile-goto-error}).  Alternatively, you can
click @kbd{Mouse-2} on the error message; you need not switch to the
@samp{*compilation*} buffer first.

@kindex M-g M-n
@kindex M-g n
@kindex C-x `
@findex next-error
@vindex next-error-highlight
  To parse the compiler error messages sequentially, type @kbd{C-x `}
(@code{next-error}).  The character following the @kbd{C-x} is the
backquote or ``grave accent,'' not the single-quote.  This command is
available in all buffers, not just in @samp{*compilation*}; it
displays the next error message at the top of one window and source
location of the error in another window.  It also temporarily
highlights the relevant source line, for a period controlled by the
variable @code{next-error-highlight}.

  The first time @w{@kbd{C-x `}} is used after the start of a compilation,
it moves to the first error's location.  Subsequent uses of @kbd{C-x
`} advance down to subsequent errors.  If you visit a specific error
message with @key{RET} or @kbd{Mouse-2}, subsequent @w{@kbd{C-x `}}
commands advance from there.  When @w{@kbd{C-x `}} gets to the end of the
buffer and finds no more error messages to visit, it fails and signals
an Emacs error.  @w{@kbd{C-u C-x `}} starts scanning from the beginning of
the compilation buffer, and goes to the first error's location.

@vindex compilation-skip-threshold
  By default, @w{@kbd{C-x `}} skips less important messages.  The variable
@code{compilation-skip-threshold} controls this.  If its value is 2,
@w{@kbd{C-x `}} skips anything less than error, 1 skips anything less
than warning, and 0 doesn't skip any messages.  The default is 1.

  When the window has a left fringe, an arrow in the fringe points to
the current message in the compilation buffer. The variable
@code{compilation-context-lines} controls the number of lines of
leading context to display before the current message.  Going to an
error message location scrolls the @samp{*compilation*} buffer to put
the message that far down from the top.  The value @code{nil} is
special: if there's a left fringe, the window doesn't scroll at all
if the message is already visible.  If there is no left fringe,
@code{nil} means display the message at the top of the window.

  If you're not in the compilation buffer when you run
@code{next-error}, Emacs will look for a buffer that contains error
messages.  First, it looks for one displayed in the selected frame,
then for one that previously had @code{next-error} called on it, and
then at the current buffer.  Finally, Emacs looks at all the remaining
buffers.  @code{next-error} signals an error if it can't find any such
buffer.

@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
formats of error messages and tells Emacs how to extract the source file
and the line number from the text of a message.  If your compiler isn't
supported, you can tailor Compilation mode to it by adding elements to
that list.  A similar variable @code{grep-regexp-alist} tells Emacs how
to parse output of a @code{grep} command.

@findex compilation-next-error
@findex compilation-previous-error
@findex compilation-next-file
@findex compilation-previous-file
  Compilation mode also redefines the keys @key{SPC} and @key{DEL} to
scroll by screenfuls, and @kbd{M-n} (@code{compilation-next-error})
and @kbd{M-p} (@code{compilation-previous-error}) to move to the next
or previous error message.  You can also use @kbd{M-@{}
(@code{compilation-next-file} and @kbd{M-@}}
(@code{compilation-previous-file}) to move up or down to an 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.  For instance, moving the
cursor to the next error message causes the location of that error to
be displayed immediately.

  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.  This defines the keys
@key{RET} and @kbd{Mouse-2}, as in the Compilation major mode.

  Compilation minor mode works in any buffer, as long as the contents
are in a format that it understands.  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

  Emacs 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 @samp{*compilation*}
buffer, it means you have made a mistake in your shell's init file by
setting the prompt unconditionally.  (This init file's name may be
@file{.bashrc}, @file{.profile}, @file{.cshrc}, @file{.shrc}, or
various other things, depending on the 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

  There may well be other things that your shell's init file
ought to do only for an interactive shell.  You can use the same
method to conditionalize them.

  The MS-DOS ``operating system'' does not support asynchronous
subprocesses; to work around this lack, @kbd{M-x compile} runs the
compilation command synchronously on MS-DOS.  As a consequence, you must
wait until the command finishes before you can do anything else in
Emacs.
@iftex
@inforef{MS-DOS,,emacs-xtra}.
@end iftex
@ifnottex
@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 @code{grep} and then visit
the lines on which matches were found.  This works by treating the
matches reported by @code{grep} as if they were ``errors.''  The
buffer of matches uses Grep mode, which is a variant of Compilation
mode (@pxref{Compilation Mode}).

@table @kbd
@item M-x grep
@item M-x lgrep
Run @code{grep} asynchronously under Emacs, with matching lines
listed in the buffer named @samp{*grep*}.
@item M-x grep-find
@itemx M-x find-grep
@itemx M-x rgrep
Run @code{grep} via @code{find}, with user-specified arguments, and
collect output in the buffer named @samp{*grep*}.
@item M-x kill-grep
Kill the running @code{grep} subprocess.
@end table

@findex grep
  To run @code{grep}, type @kbd{M-x grep}, then enter a command line
that specifies how to run @code{grep}.  Use the same arguments you
would give @code{grep} when running it normally: a @code{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 tag
(@pxref{Tags}) in the buffer around point, and puts that into the
default @code{grep} command.

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

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

  The output from @code{grep} goes in the @samp{*grep*} buffer.  You
can find the corresponding lines in the original files using @w{@kbd{C-x
`}}, @key{RET}, and so forth, just like compilation errors.

  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.

@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
@code{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
  The commands @kbd{M-x lgrep} (local grep) and @kbd{M-x rgrep}
(recursive grep) are more user-friendly versions of @code{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}.

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}.

Subdirectories listed in the variable
@code{grep-find-ignored-directories} such as those typically used by
various version control systems, like CVS and arch, are automatically
skipped by @code{rgrep}.

@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{}/La@TeX{}.  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 activate Flymake mode, type @kbd{M-x flymake-mode}.  You can move
to the errors spotted by Flymake mode with @kbd{M-x
flymake-goto-next-error} and @kbd{M-x flymake-goto-prev-error}.  To
display any error messages associated with the current line, use
@kbd{M-x flymake-display-err-menu-for-current-line}.

  For more details about using Flymake, see @ref{Top, Flymake,
Flymake, flymake, The Flymake Manual}.

@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

@c Do you believe in GUD?
The GUD (Grand Unified Debugger) library provides an interface to
various symbolic debuggers from within Emacs.  We recommend the
debugger GDB, which is free software, but GUD can also run DBX, SDB or
XDB.  GUD can also serve as an interface to Perl's debugging mode, the
Python debugger PDB, and to JDB, the Java Debugger.
@xref{Debugging,, The Lisp Debugger, elisp, the Emacs Lisp Reference
Manual}, for information on debugging Emacs Lisp programs.

@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 through
                        Emacs.
@end menu

@node Starting GUD
@subsection Starting GUD

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

@table @kbd
@item M-x gdb @key{RET} @var{file} @key{RET}
@findex gdb
Run GDB as a subprocess of Emacs.  By default, this uses an IDE-like
graphical interface; see @ref{GDB Graphical Interface}.  Only GDB
works with the graphical interface.

@item M-x dbx @key{RET} @var{file} @key{RET}
@findex dbx
Run DBX as a subprocess of Emacs.  Since Emacs does not implement a
graphical interface for DBX, communication with DBX works by typing
commands in the GUD interaction buffer.  The same is true for all
the other supported debuggers.

@item M-x xdb @key{RET} @var{file} @key{RET}
@findex xdb
@vindex gud-xdb-directories
Similar, but run XDB.  Use the variable
@code{gud-xdb-directories} to specify directories to search for source
files.

@item M-x sdb @key{RET} @var{file} @key{RET}
@findex sdb
Similar, but run SDB.

  Some versions of SDB do not mention source file names in their
messages.  When you use them, you need to have a valid tags table
(@pxref{Tags}) in order for GUD to find functions in the source code.
If you have not visited a tags table or the tags table doesn't list one
of the functions, you get a message saying @samp{The sdb support
requires a valid tags table to work}.  If this happens, generate a valid
tags table in the working directory and try again.

@item M-x perldb @key{RET} @var{file} @key{RET}
@findex perldb
Run the Perl interpreter in debug mode to debug @var{file}, a Perl program.

@item M-x jdb @key{RET} @var{file} @key{RET}
@findex jdb
Run the Java debugger to debug @var{file}.

@item M-x pdb @key{RET} @var{file} @key{RET}
@findex pdb
Run the Python debugger to debug @var{file}.
@end table

  Each of these commands takes one argument: a command line to invoke
the debugger.  In the simplest case, specify just the name of the
executable file you want to debug.  You may also use options that the
debugger supports.  However, shell wildcards and variables are not
allowed.  GUD assumes that the first argument not starting with a
@samp{-} is the executable file name.

@node Debugger Operation
@subsection Debugger Operation

@cindex fringes, and current execution line in GUD
  Generally when you run a debugger with GUD, the debugger uses an Emacs
buffer for its ordinary input and output.  This is called the GUD
buffer.  Input and output from the program you are debugging also use
this buffer.  We call this @dfn{text command mode}.  The GDB Graphical
Interface can use further buffers (@pxref{GDB Graphical Interface}).

  The debugger displays the source files of the program by visiting
them in Emacs buffers.  An arrow in the left fringe indicates the
current execution line.@footnote{On a text-only terminal, the arrow
appears as @samp{=>} and overlays the first two text columns.}  Moving
point in this buffer does not move the arrow.  The arrow is not part
of the file's text; it appears only on the screen.

  You can start editing these source files at any time in the buffers
that display them.  If you do modify a source file, keep in mind that
inserting or deleting lines will throw off the arrow's positioning;
GUD has no way of figuring out which line corresponded before your
changes to the line number in a debugger message.  Also, you'll
typically have to recompile and restart the program for your changes
to be reflected in the debugger's tables.

@cindex tooltips with GUD
@vindex tooltip-gud-modes
@vindex gud-tooltip-mode
@vindex gud-tooltip-echo-area
  The Tooltip facility (@pxref{Tooltips}) provides support for GUD@.
You activate this feature by turning on the minor mode
@code{gud-tooltip-mode}.  Then you can display a variable's value in a
tooltip simply by pointing at it with the mouse.  This operates in the
GUD buffer and in source buffers with major modes in the list
@code{gud-tooltip-modes}.  If the variable @code{gud-tooltip-echo-area}
is non-@code{nil} then the variable's value is displayed in the echo
area.  When debugging a C program using the GDB Graphical Interface, you
can also display macro definitions associated with an identifier when
the program is not executing.

  GUD tooltips are disabled when you use GDB in text command mode
(@pxref{GDB Graphical Interface}), because displaying an expression's
value in GDB can sometimes expand a macro and result in a side effect
that interferes with the program's operation.  The GDB graphical
interface supports GUD tooltips and assures they will not cause side
effects.

@node Commands of GUD
@subsection Commands of GUD

  The GUD interaction buffer uses a variant of Shell mode, so the
Emacs commands of Shell mode are available (@pxref{Shell Mode}).  All
the usual commands for your debugger are available, and you can use
the Shell mode history commands to repeat them.  If you wish, you can
control your debugger process entirely through this buffer.

  GUD mode also provides commands for setting and clearing
breakpoints, for selecting stack frames, and for stepping through the
program.  These commands are available both in the GUD buffer and
globally, but with different key bindings.  It also has its own tool
bar from which you can invoke the more common commands by clicking on
the appropriate icon.  This is particularly useful for repetitive
commands like @code{gud-next} and @code{gud-step}, and allows you to
keep the GUD buffer hidden.

  The breakpoint commands are normally used in source file buffers,
because that is the easiest way to specify where to set or clear the
breakpoint.  Here's the global command to set a breakpoint:

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

@kindex C-x C-a @r{(GUD)}
  Here are the other special commands provided by GUD@.  The keys
starting with @kbd{C-c} are available only in the GUD interaction
buffer.  The key bindings that start with @kbd{C-x C-a} are available
in the GUD interaction buffer and also in source files.  Some of these
commands are not available to all the supported 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 line referred to in the GUD
buffer (that is, the line indicated in the last location message).
This runs the command @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 a 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 a single line of code, stepping across entire function calls
at full speed (@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
  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{perldb-mode-hook}, for Perl debugging mode;
@code{pdb-mode-hook}, for PDB; @code{jdb-mode-hook}, for JDB.  You can
use these hooks to define custom key bindings for the debugger
interaction buffer.  @xref{Hooks}.

  Here is a convenient way to define a command that sends a particular
command string to the debugger, and set up a key binding for it in the
debugger 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

  By default, the command @code{gdb} starts GDB using a graphical
interface, using Emacs windows for display program state information.
In effect, this makes Emacs into an IDE (interactive development
environment).  With it, you do not need to use textual GDB commands;
you can control the debugging session with the mouse.  For example,
you can click in the fringe of a source buffer to set a breakpoint
there, or on a stack frame in the stack buffer to select that frame.

  This mode requires telling GDB that its ``screen size'' is
unlimited, so it sets the height and width accordingly.  For correct
operation you must not change these values during the GDB session.

@vindex gud-gdb-command-name
@findex gdba
  You can also run GDB in text command mode, like other debuggers.  To
do this, replace the GDB @code{"--annotate=3"} option with
@code{"--fullname"} either in the minibuffer for the current Emacs
session, or the custom variable @code{gud-gdb-command-name} for all
future sessions.  You need to use text command mode to debug multiple
programs within one Emacs session.  If you have customized
@code{gud-gdb-command-name} in this way, you can use @kbd{M-x gdba} to
invoke GDB in graphical mode.

@menu
* GDB-UI 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.
* Stack Buffer::                Select a frame from the call stack.
* Other GDB-UI Buffers::        Input/output, locals, registers,
                                assembler, threads and memory buffers.
* Watch Expressions::           Monitor variable values in the speedbar.
@end menu

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

@vindex gdb-many-windows
  If the variable @code{gdb-many-windows} is @code{nil} (the default
value) then @kbd{M-x gdb} normally displays only the GUD buffer.
However, if the variable @code{gdb-show-main} is also non-@code{nil},
it starts with two windows: one displaying the GUD 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 buffer (I/O of GDB)      |   Locals buffer                |
|--------------------------------+--------------------------------+
|   Primary Source buffer        |   I/O buffer for debugged pgm  |
|--------------------------------+--------------------------------+
|   Stack buffer                 |   Breakpoints buffer           |
+--------------------------------+--------------------------------+
@end group
@end smallexample

  However, if @code{gdb-use-separate-io-buffer} is @code{nil}, the I/O
buffer does not appear and the primary source buffer occupies the full
width of the frame.

@findex gdb-restore-windows
  If you change the window layout, for example, while editing and
re-compiling your program, then you can restore this standard window
layout with the command @code{gdb-restore-windows}.

@findex gdb-many-windows
  To switch between this standard layout and a simple layout
containing just the GUD buffer and a source file, type @kbd{M-x
gdb-many-windows}.

  You may also specify additional GDB-related buffers to display,
either in the same frame or a different one.  Select the buffers you
want with the @samp{GUD->GDB-windows} and @samp{GUD->GDB-Frames}
sub-menus.  If the menu-bar is unavailable, type @code{M-x
gdb-display-@var{buffertype}-buffer} or @code{M-x
gdb-frame-@var{buffertype}-buffer} respectively, where
@var{buffertype} is the relevant buffer type, such as
@samp{breakpoints}.  Most of these buffers are read-only, and typing
@kbd{q} in them kills them.

  When you finish debugging, kill the GUD 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 continue debugging.  When you
restart execution, GDB will automatically find your new executable.
Keeping the GUD 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 GDB commands in Fringe

@c @findex gdb-mouse-set-clear-breakpoint
@c @findex gdb-mouse-toggle-breakpoint
Many GDB commands can be entered using keybindings or the tool bar but
sometimes it is quicker to use the fringe.  These commands either
manipulate breakpoints or control program execution.  When there is no
fringe, you can use the margin but this is only present when the
source file already has a breakpoint.

You can click @kbd{Mouse-1} in the fringe or display margin of a
source buffer to set a breakpoint there and, on a graphical display, a
red bullet will appear on that line.  If a breakpoint already exists
on that line, the same click will remove it.  You can also enable or
disable a breakpoint by clicking @kbd{C-Mouse-1} on the bullet.

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 higher level
frames.

If you drag the arrow in the fringe with @kbd{Mouse-1}
(@code{gdb-mouse-until}), execution will continue to the line where
you release the button, provided it is still in the same frame.
Alternatively, you can click @kbd{Mouse-3} at some point in the fringe
of this buffer and execution will advance to there.  A similar command
(@code{gdb-mouse-jump}) allows you to jump to a source line without
executing the intermediate lines by clicking @kbd{C-Mouse-3}.  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.

@table @kbd
@item Mouse-1
Set or clear a breakpoint.

@item C-Mouse-1
Enable or disable a breakpoint.

@item Mouse-3
Continue execution to here.

@item C-Mouse-3
Jump to here.
@end table

If the variable @code{gdb-find-source-frame} is non-@code{nil} and
execution stops in a frame for which there is no source code e.g after
an interrupt, then Emacs finds and displays the first frame further up
stack for which there is source.  If it is @code{nil} then the source
buffer continues to display the last frame which maybe more useful,
for example, when re-setting a breakpoint.

@node Breakpoints Buffer
@subsubsection Breakpoints Buffer

  The breakpoints buffer shows the existing breakpoints, watchpoints and
catchpoints (@pxref{Breakpoints,,, gdb, The GNU debugger}).  It has
these special 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 the current breakpoint (@code{gdb-toggle-breakpoint}).
On a graphical display, this changes the color of a bullet in the
margin of a source buffer at the relevant line.  This is red when
the breakpoint is enabled and grey when it is disabled.  Text-only
terminals correspondingly display a @samp{B} or @samp{b}.

@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.
@end table

@node Stack Buffer
@subsubsection Stack Buffer

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

@findex gdb-frames-select
An arrow in the fringe points to the selected frame or, if the fringe is
not present, the number of the selected frame is displayed in reverse
contrast.  To select a frame in GDB, move point in the stack buffer to
that stack frame and type @key{RET} (@code{gdb-frames-select}), or click
@kbd{Mouse-2} on a stack frame.  If the locals buffer is visible,
selecting a stack frame updates it to display the local variables of the
new frame.

@node Other GDB-UI Buffers
@subsubsection Other Buffers

@table @asis
@item Input/Output Buffer
@vindex gdb-use-separate-io-buffer
If the variable @code{gdb-use-separate-io-buffer} is non-@code{nil},
the program being debugged takes its input and displays its output
here.  Otherwise it uses the GUD buffer for that.  To toggle whether
GUD mode uses this buffer, do @kbd{M-x gdb-use-separate-io-buffer}.
This takes effect when you next restart the program you are debugging.

The history and replay commands from Shell mode are available here,
as are the commands to send signals to the debugged program.
@xref{Shell Mode}.

@item Locals Buffer
The locals buffer displays the values of local variables of the
current 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,
move point to their name and press @key{RET}, or alternatively click
@kbd{Mouse-2} there, to examine their values.  With earlier versions
of GDB, use @kbd{Mouse-2} or @key{RET} on the type description
(@samp{[struct/union]} or @samp{[array]}).  @xref{Watch Expressions}.

@item Registers Buffer
@findex toggle-gdb-all-registers
The registers buffer displays the values held by the registers
(@pxref{Registers,,, gdb, The GNU debugger}).  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}.  With earlier versions of GDB, you can
press @key{SPC} to toggle the display of floating point registers
(@code{toggle-gdb-all-registers}).

@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.

@item Threads Buffer
@findex gdb-threads-select
The threads buffer displays a summary of all threads currently in your
program (@pxref{Threads, Threads, Debugging programs with multiple
threads, gdb, The GNU debugger}).  Move point to any thread in the
list and press @key{RET} to select it (@code{gdb-threads-select}) and
display the associated source in the primary source buffer.
Alternatively, click @kbd{Mouse-2} on a thread to select it.  If the
locals buffer is visible, its contents update to display the variables
that are local in the new thread.

@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.  Click @kbd{Mouse-3} on the header line to select the
display format or unit size for these data items.
@end table

@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.  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-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 Executing Lisp
@section Executing Lisp Expressions

  Emacs has several different major modes for Lisp and Scheme.  They are
the same in terms of editing commands, but differ in the commands for
executing Lisp expressions.  Each mode has its own purpose.

@table @asis
@item Emacs-Lisp mode
The mode for editing source files of programs to run in Emacs Lisp.
This mode defines @kbd{C-M-x} to evaluate the current defun.
@xref{Lisp Libraries}.
@item Lisp Interaction mode
The mode for an interactive session with Emacs Lisp.  It defines
@kbd{C-j} to evaluate the sexp 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.  This mode defines @kbd{C-M-x} to send the current defun
to an inferior Lisp process.  @xref{External Lisp}.
@item Inferior Lisp mode
The mode for an interactive session with an inferior Lisp process.
This mode combines the special features of Lisp mode and Shell mode
(@pxref{Shell Mode}).
@item Scheme mode
Like Lisp mode but for Scheme programs.
@item Inferior Scheme mode
The mode for an interactive session with an inferior Scheme process.
@end table

  Most editing commands for working with Lisp programs are in fact
available globally.  @xref{Programs}.

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

  Lisp code for Emacs editing commands is stored in files whose names
conventionally end in @file{.el}.  This ending tells Emacs to edit them in
Emacs-Lisp mode (@pxref{Executing Lisp}).

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

@findex load-file
  To execute a file of Emacs Lisp code, use @kbd{M-x load-file}.  This
command reads a file name using the minibuffer and then executes the
contents of that file as Lisp code.  It is not necessary to visit the
file first; in any case, this command reads the file as found on disk,
not text in an Emacs buffer.

@findex load
@findex load-library
  Once a file of Lisp code is installed in the Emacs Lisp library
directories, users can load it using @kbd{M-x load-library}.  Programs
can load it by calling @code{load}, a more primitive function that is
similar but accepts some additional arguments.

  @kbd{M-x load-library} differs from @kbd{M-x load-file} in that it
searches a sequence of directories and tries three file names in each
directory.  Suppose your argument is @var{lib}; the three names are
@file{@var{lib}.elc}, @file{@var{lib}.el}, and lastly just
@file{@var{lib}}.  If @file{@var{lib}.elc} exists, it is by convention
the result of compiling @file{@var{lib}.el}; it is better to load the
compiled file, since it will load and run faster.

  If @code{load-library} finds that @file{@var{lib}.el} is newer than
@file{@var{lib}.elc} file, it issues a warning, because it's likely
that somebody made changes to the @file{.el} file and forgot to
recompile it.  Nonetheless, it loads @file{@var{lib}.elc}.  This is
because people often leave unfinished edits the source file, and don't
recompile it until they think it is ready to use.

  Because the argument to @code{load-library} is usually not in itself
a valid file name, file name completion is not available.  Indeed, when
using this command, you usually do not know exactly what file name
will be used.

@vindex load-path
  The sequence of directories searched by @kbd{M-x load-library} is
specified by the variable @code{load-path}, a list of strings that are
directory names.  The default value of the list contains the directories where
the Lisp code for Emacs itself is stored.  If you have libraries of
your own, put them in a single directory and add that directory
to @code{load-path}.  @code{nil} in this list stands for the current default
directory, but it is probably not a good idea to put @code{nil} in the
list.  If you find yourself wishing that @code{nil} were in the list,
most likely what you really want to do is use @kbd{M-x load-file}
this once.

@cindex autoload
  Often you do not have to give any command to load a library, because
the commands defined in the library are set up to @dfn{autoload} that
library.  Trying to run any of those commands calls @code{load} to load
the library; this replaces the autoload definitions with the real ones
from the library.

@vindex load-dangerous-libraries
@cindex Lisp files byte-compiled by XEmacs
  By default, Emacs refuses to load compiled Lisp files which were
compiled with XEmacs, a modified versions of Emacs---they can cause
Emacs to crash.  Set the variable @code{load-dangerous-libraries} to
@code{t} if you want to try loading them.

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

@findex emacs-lisp-mode
  Lisp programs intended to be run in Emacs should be edited in
Emacs-Lisp mode; this happens automatically for file names ending in
@file{.el}.  By contrast, Lisp mode itself is used for editing Lisp
programs intended for other Lisp systems.  To switch to Emacs-Lisp mode
explicitly, use the command @kbd{M-x emacs-lisp-mode}.

  For testing of Lisp programs to run in Emacs, it is often useful to
evaluate part of the program as it is found in the Emacs buffer.  For
example, after changing the text of a Lisp function definition,
evaluating the definition installs the change for future calls to the
function.  Evaluation of Lisp expressions is also useful in any kind of
editing, for invoking noninteractive functions (functions that are
not commands).

@table @kbd
@item M-:
Read a single Lisp expression in the minibuffer, evaluate it, and print
the value in the echo area (@code{eval-expression}).
@item C-x C-e
Evaluate the Lisp expression before point, and print the value in the
echo area (@code{eval-last-sexp}).
@item C-M-x
Evaluate the defun containing or after point, and print the value in
the echo area (@code{eval-defun}).
@item M-x eval-region
Evaluate all the Lisp expressions in the region.
@item M-x eval-buffer
Evaluate all the 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-@key{colon}
@end ifinfo
@ifnotinfo
@kindex M-:
@end ifnotinfo
@findex eval-expression
  @kbd{M-:} (@code{eval-expression}) is the most basic command for evaluating
a Lisp expression interactively.  It reads the expression using the
minibuffer, so you can execute any expression on a buffer regardless of
what the buffer contains.  When the expression is evaluated, the current
buffer is once again the buffer that was current when @kbd{M-:} was
typed.

@kindex C-M-x @r{(Emacs-Lisp mode)}
@findex eval-defun
  In Emacs-Lisp mode, the key @kbd{C-M-x} is bound to the command
@code{eval-defun}, which parses the defun containing or following point
as a Lisp expression and evaluates it.  The value is printed in the echo
area.  This command is convenient for installing in the Lisp environment
changes that you have just made in the text of a function definition.

  @kbd{C-M-x} treats @code{defvar} expressions specially.  Normally,
evaluating a @code{defvar} expression does nothing if the variable it
defines already has a value.  But @kbd{C-M-x} unconditionally resets the
variable to the initial value specified in the @code{defvar} expression.
@code{defcustom} expressions are treated similarly.
This special feature is convenient for debugging Lisp programs.
Typing @kbd{C-M-x} on a @code{defface} expression reinitializes
the face according to the @code{defface} specification.

@kindex C-x C-e
@findex eval-last-sexp
  The command @kbd{C-x C-e} (@code{eval-last-sexp}) evaluates the Lisp
expression preceding point in the buffer, and displays the value in the
echo area.  It is available in all major modes, not just Emacs-Lisp
mode.  It does not treat @code{defvar} specially.

  When the result of an evaluation is an integer, you can type
@kbd{C-x C-e} a second time to display the value of the integer result
in additional formats (octal, hexadecimal, and character).

  If @kbd{C-x C-e}, or @kbd{M-:} is given a numeric argument, it
inserts the value into the current buffer at point, rather than
displaying it in the echo area.  The argument's value does not matter.
@kbd{C-M-x} with a numeric argument instruments the function
definition for Edebug (@pxref{Instrumenting, Instrumenting for Edebug,, elisp, the Emacs Lisp Reference Manual}).

@findex eval-region
@findex eval-buffer
  The most general command for evaluating Lisp expressions from a buffer
is @code{eval-region}.  @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.  This is a reasonable way to install the contents of a file of
Lisp code that you are ready to test.  Later, as you find bugs and
change individual functions, use @kbd{C-M-x} on each function that you
change.  This keeps the Lisp world in step with the source file.

@vindex eval-expression-print-level
@vindex eval-expression-print-length
@vindex eval-expression-debug-on-error
  The two customizable variables @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.  @code{eval-expression-debug-on-error} controls
whether evaluation errors invoke the debugger when these commands are
used; its default is @code{t}.

@node Lisp Interaction
@section Lisp Interaction Buffers

  The buffer @samp{*scratch*} which is selected when Emacs starts up is
provided for evaluating Lisp expressions interactively inside Emacs.

  The simplest way to use the @samp{*scratch*} buffer is to insert Lisp
expressions and type @kbd{C-j} after each expression.  This command
reads the Lisp expression before point, evaluates it, and inserts the
value in printed representation before point.  The result is a complete
typescript of the expressions you have evaluated and their values.

  The @samp{*scratch*} buffer's major mode is Lisp Interaction mode, which
is the same as Emacs-Lisp mode except for the binding of @kbd{C-j}.

@findex lisp-interaction-mode
  The rationale for this feature is that Emacs must have a buffer when
it starts up, but that buffer is not useful for editing files since a
new buffer is made for every file that you visit.  The Lisp interpreter
typescript is the most useful thing I can think of for the initial
buffer to do.  Type @kbd{M-x lisp-interaction-mode} to put the current
buffer in Lisp Interaction mode.

@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 @samp{*ielm*} buffer
which uses this mode.  For more information see that command's
documentation.

@node External Lisp
@section Running an External Lisp

  Emacs has facilities for running programs in other Lisp systems.  You can
run a Lisp process as an inferior of Emacs, and pass expressions to it to
be evaluated.  You can also pass changed function definitions directly from
the Emacs buffers in which you edit the Lisp programs to the inferior Lisp
process.

@findex run-lisp
@vindex inferior-lisp-program
@kindex C-x C-z
  To run an inferior Lisp process, type @kbd{M-x run-lisp}.  This runs
the program named @code{lisp}, the same program you would run by typing
@code{lisp} as a shell command, with both input and output going through
an Emacs buffer named @samp{*lisp*}.  That is to say, any ``terminal
output'' from Lisp will go into the buffer, advancing point, and any
``terminal input'' for Lisp comes from text in the buffer.  (You can
change the name of the Lisp executable file by setting the variable
@code{inferior-lisp-program}.)

  To give input to Lisp, go to the end of the buffer and type the input,
terminated by @key{RET}.  The @samp{*lisp*} buffer is in Inferior Lisp
mode, which combines the special characteristics of Lisp mode with most
of the features of Shell mode (@pxref{Shell Mode}).  The definition of
@key{RET} to send a line to a subprocess is one of the features of Shell
mode.

@findex lisp-mode
  For the source files of programs to run in external Lisps, use Lisp
mode.  You can switch to this mode with @kbd{M-x lisp-mode}, and it is
used automatically for files whose names end in @file{.l},
@file{.lsp}, or @file{.lisp}.

@kindex C-M-x @r{(Lisp mode)}
@findex lisp-eval-defun
  When you edit a function in a Lisp program you are running, the easiest
way to send the changed definition to the inferior Lisp process is the key
@kbd{C-M-x}.  In Lisp mode, this runs the function @code{lisp-eval-defun},
which finds the defun around or following point and sends it as input to
the Lisp process.  (Emacs can send input to any inferior process regardless
of what buffer is current.)

  Contrast the meanings of @kbd{C-M-x} in Lisp mode (for editing
programs to be run in another Lisp system) and Emacs-Lisp mode (for
editing Lisp programs to be run in Emacs; see @pxref{Lisp Eval}): in
both modes it has the effect of installing the function definition
that point is in, but the way of doing so is different according to
where the relevant Lisp environment is found.


@ignore
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@end ignore