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@c This is part of the Emacs manual.
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@c Copyright (C) 1985,86,87,93,94,95,97,2000,2001 Free Software Foundation, Inc.
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@c See file emacs.texi for copying conditions.
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@node Building, Maintaining, Programs, Top
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@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 developing and maintaining programs.

@menu
* Compilation::         Compiling programs in languages other
                          than Lisp (C, Pascal, etc.).
* Grep Searching::      Running grep as if it were a compiler.
* Compilation Mode::    The mode for visiting compiler errors.
* Compilation Shell::   Customizing your shell properly
                          for use in the compilation buffer.
* 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.
* Interaction: Lisp Interaction.  Executing Lisp in an Emacs buffer.
* Eval: Lisp Eval.      Executing a single Lisp expression in Emacs.
* 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
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Run a compiler asynchronously under Emacs, with error messages going to
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the @samp{*compilation*} buffer.
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@item M-x recompile
Invoke a compiler with the same command as in the last invocation of
@kbd{M-x compile}.
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@item M-x grep
Run @code{grep} asynchronously under Emacs, with matching lines
listed in the buffer named @samp{*grep*}.
@item M-x grep-find
Run @code{grep} via @code{find}, with user-specified arguments, and
collect output in the buffer named @samp{*grep*}.
@item M-x kill-compilation
@itemx M-x kill-grep
Kill the running compilation or @code{grep} 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
  When the shell command line is read, the minibuffer appears containing
a default command line, which is the command you used the last time you
did @kbd{M-x compile}.  If you type just @key{RET}, the same command
line is used again.  For the first @kbd{M-x compile}, the default is
@samp{make -k}.  The default compilation command comes from the variable
@code{compile-command}; if the appropriate compilation command for a
file is something other than @samp{make -k}, it can be useful for the
file to specify a local value for @code{compile-command} (@pxref{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} 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.

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@cindex compilation buffer, keeping current position at the end
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@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 kill-compilation
  To kill the compilation process, do @kbd{M-x kill-compilation}.  When
the compiler process terminates, the mode line of the
@samp{*compilation*} buffer changes to say @samp{signal} instead of
@samp{run}.  Starting a new compilation also kills any running
compilation, as only one can exist at any time.  However, @kbd{M-x
compile} asks for confirmation before actually killing a compilation
that is running.

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@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}.

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@node Grep Searching
@section Searching with Grep under Emacs

@findex grep
  Just as you can run a compiler from Emacs and then visit the lines
where there were 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.''

  To do this, 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.  The output from
@code{grep} goes in the @samp{*grep*} buffer.  You can find the
corresponding lines in the original files using @kbd{C-x `} and
@key{RET}, as with compilation errors.

  If you specify a prefix argument for @kbd{M-x grep}, it figures out
the tag (@pxref{Tags}) around point, and puts that into the default
@code{grep} command.

@findex grep-find
  The command @kbd{M-x grep-find} 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}.

@node Compilation Mode
@section Compilation Mode

@findex compile-goto-error
@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 look at the
source line where the error happened.

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  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.

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@table @kbd
@item C-x `
Visit the locus of the next compiler error message or @code{grep} 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.
@end table

@kindex C-x `
@findex next-error
  You can visit the source for any particular error message by moving
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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.
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  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.

  The first time @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 @kbd{C-x `}
commands advance from there.  When @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.

  @kbd{C-u C-x `} starts scanning from the beginning of the compilation
buffer.  This is one way to process the same set of errors again.

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@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.

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  Compilation mode also redefines the keys @key{SPC} and @key{DEL} to
scroll by screenfuls, and @kbd{M-n} and @kbd{M-p} to move to the next or
previous error message.  You can also use @kbd{M-@{} and @kbd{M-@}} to
move up or down to an error message for a different source file.

  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.  In csh, here
is how to do it:

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

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

@example
if [ "$@{PS1+set@}" = set ]
then PS1=@dots{}
fi
@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.  @xref{MS-DOS}.

@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 you can also run DBX, SDB or XDB if you
have them.  GUD can also serve as an interface to the Perl's debugging
mode, the Python debugger PDB, and to JDB, the Java Debugger.
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@xref{Debugger,, The Lisp Debugger, elisp, the Emacs Lisp Reference Manual},
for information on debugging Emacs Lisp programs.
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@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.
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* GUD Tooltips::        Showing variable values by pointing with the mouse.
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@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
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Run GDB as a subprocess of Emacs.  This command creates a buffer
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for input and output to GDB, and switches to it.  If a GDB buffer
already exists, it just switches to that buffer.
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@item M-x dbx @key{RET} @var{file} @key{RET}
@findex dbx
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Similar, but run DBX instead of GDB.
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@item M-x xdb @key{RET} @var{file} @key{RET}
@findex xdb
@vindex gud-xdb-directories
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Similar, but run XDB instead of GDB.  Use the variable
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@code{gud-xdb-directories} to specify directories to search for source
files.

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

  Emacs can only run one debugger process at a time.

@node Debugger Operation
@subsection Debugger Operation

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@cindex fringes, and current execution line in GUD
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  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.  The
debugger displays the source files of the program by visiting them in
Emacs buffers.  An arrow (@samp{=>}) in one of these buffers indicates
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the current execution line.@footnote{Under a window system, the arrow
appears in the left fringe of the Emacs window.}  Moving point in this
buffer does not move the arrow.
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  You can start editing these source files at any time in the buffers
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that display them.  The arrow is not part of the file's
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text; it appears only on the screen.  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.

  If you wish, you can control your debugger process entirely through the
debugger buffer, which uses a variant of Shell mode.  All the usual
commands for your debugger are available, and you can use the Shell mode
history commands to repeat them.  @xref{Shell Mode}.

@node Commands of GUD
@subsection Commands of GUD

  The GUD interaction buffer uses a variant of Shell mode, so the
commands of Shell mode are available (@pxref{Shell Mode}).  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.

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  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:
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@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.

@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}).

@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.
If you use this command in the GUD interaction buffer,
it applies to the line where the program last stopped.
@end table

  The above commands are common to all supported debuggers.  If you are
using GDB or (some versions of) DBX, these additional commands are available:

@table @kbd
@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 @samp{up} command.

@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 @samp{down} command.
@end table

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

@table @kbd
@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, and requires
GDB versions 4.13 and later.

@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 until it
stops for some other reason).
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@item C-c C-j
@kindex C-c C-j @r{(GUD)}
@itemx C-x C-a C-j
@findex gud-jump
Only useful in a source buffer, (@code{gud-jump}) relocates the next
instruction to the current line at point in a source buffer.  If the
new execution line is in a different function from the previously one,
you will be prompted for confirmation since the results may be
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bizarre.  See the GDB manual entry regarding @code{jump} for details.
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@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;
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@code{pdb-mode-hook}, for PDB; @code{jdb-mode-hook}, for JDB.  You can
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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
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string @var{docstring}.  You can then use the command @var{function} in any
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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.
@c This said, ``the name of the file the program counter was in at the last breakpoint.''
@c But I suspect it is really the last stop file.

@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
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.
@end table

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@node GUD Tooltips
@subsection GUD Tooltips

@cindex tooltips with GUD
The Tooltip facility (@pxref{Tooltips}) provides support for GUD@.  If
GUD support is activated by customizing the @code{tooltip} group,
variable values can be displayed in tooltips by pointing at them with
the mouse in the GUD buffer or in source buffers with major modes in the
customizable list @code{tooltip-gud-modes}.

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@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}).

@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-library}, or with @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
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@file{@var{lib}.elc} file, it issues a warning, because it's likely that
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somebody made changes to the @file{.el} file and forgot to recompile
it.

  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 directory 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.

@cindex byte code
  Emacs Lisp code can be compiled into byte-code which loads faster,
takes up less space when loaded, 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 consists of the library source file with @samp{c} appended.
Thus, the compiled code for @file{foo.el} goes in @file{foo.elc}.
That's why @code{load-library} searches for @samp{.elc} files first.

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@vindex load-dangerous-libraries
@cindex Lisp files byte-compiled by XEmacs
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  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.
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@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-current-buffer
Evaluate all the Lisp expressions in the buffer.
@end table

@kindex M-:
@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.
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@code{defcustom} expressions are treated similarly.
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This special feature is convenient for debugging Lisp programs.

@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.

  If @kbd{C-M-x}, @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.

@findex eval-region
@findex eval-current-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-current-buffer} is similar but evaluates the entire
buffer.  This is a reasonable way to install the contents of a file of
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Lisp code that you are ready to test.  Later, as you find bugs and
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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.

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@vindex eval-expression-print-level
@vindex eval-expression-print-length
@vindex eval-expression-debug-on-error
The 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.

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@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.

@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.  This mode can be selected with @kbd{M-x lisp-mode}, and is used
automatically for files whose names end in @file{.l}, @file{.lsp}, or
@file{.lisp}, as most Lisp systems usually expect.

@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): 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.
@xref{Executing Lisp}.