Merged from miles@gnu.org--gnu-2005 (patch 269)

[bpt/emacs.git] / lispref / modes.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999,
@c   2003, 2004, 2005 Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../info/modes
@node Modes, Documentation, Keymaps, Top
@chapter Major and Minor Modes
@cindex mode

  A @dfn{mode} is a set of definitions that customize Emacs and can be
turned on and off while you edit.  There are two varieties of modes:
@dfn{major modes}, which are mutually exclusive and used for editing
particular kinds of text, and @dfn{minor modes}, which provide features
that users can enable individually.

  This chapter describes how to write both major and minor modes, how to
indicate them in the mode line, and how they run hooks supplied by the
user.  For related topics such as keymaps and syntax tables, see
@ref{Keymaps}, and @ref{Syntax Tables}.

@menu
* Major Modes::        Defining major modes.
* Minor Modes::        Defining minor modes.
* Mode Line Format::   Customizing the text that appears in the mode line.
* Imenu::              How a mode can provide a menu
                         of definitions in the buffer.
* Font Lock Mode::     How modes can highlight text according to syntax.
* Desktop Save Mode::  How modes can have buffer state saved between
                         Emacs sessions.
* Hooks::              How to use hooks; how to write code that provides hooks.
@end menu

@node Major Modes
@section Major Modes
@cindex major mode
@cindex Fundamental mode

  Major modes specialize Emacs for editing particular kinds of text.
Each buffer has only one major mode at a time.  For each major mode
there is a function to switch to that mode in the current buffer; its
name should end in @samp{-mode}.  These functions work by setting
buffer-local variable bindings and other data associated with the
buffer, such as a local keymap.  The effect lasts until you switch
to another major mode in the same buffer.

  The least specialized major mode is called @dfn{Fundamental mode}.
This mode has no mode-specific definitions or variable settings, so each
Emacs command behaves in its default manner, and each option is in its
default state.  All other major modes redefine various keys and options.
For example, Lisp Interaction mode provides special key bindings for
@kbd{C-j} (@code{eval-print-last-sexp}), @key{TAB}
(@code{lisp-indent-line}), and other keys.

  When you need to write several editing commands to help you perform a
specialized editing task, creating a new major mode is usually a good
idea.  In practice, writing a major mode is easy (in contrast to
writing a minor mode, which is often difficult).

  If the new mode is similar to an old one, it is often unwise to modify
the old one to serve two purposes, since it may become harder to use and
maintain.  Instead, copy and rename an existing major mode definition
and alter the copy---or define a @dfn{derived mode} (@pxref{Derived
Modes}).  For example, Rmail Edit mode, which is in
@file{emacs/lisp/mail/rmailedit.el}, is a major mode that is very similar to
Text mode except that it provides two additional commands.  Its
definition is distinct from that of Text mode, but uses that of Text mode.

  Even if the new mode is not an obvious derivative of any other mode,
it is convenient to use @code{define-derived-mode} with a @code{nil}
parent argument, since it automatically enforces the most important
coding conventions for you.

@findex define-generic-mode
  For a very simple programming language major mode that handles
comments and fontification, you can use @code{define-generic-mode}
in @file{generic.el}.

  Rmail Edit mode offers an example of changing the major mode
temporarily for a buffer, so it can be edited in a different way (with
ordinary Emacs commands rather than Rmail commands).  In such cases, the
temporary major mode usually provides a command to switch back to the
buffer's usual mode (Rmail mode, in this case).  You might be tempted to
present the temporary redefinitions inside a recursive edit and restore
the usual ones when the user exits; but this is a bad idea because it
constrains the user's options when it is done in more than one buffer:
recursive edits must be exited most-recently-entered first.  Using an
alternative major mode avoids this limitation.  @xref{Recursive
Editing}.

  The standard GNU Emacs Lisp library directory tree contains the code
for several major modes, in files such as @file{text-mode.el},
@file{texinfo.el}, @file{lisp-mode.el}, @file{c-mode.el}, and
@file{rmail.el}.  They are found in various subdirectories of the
@file{lisp} directory.  You can study these libraries to see how modes
are written.  Text mode is perhaps the simplest major mode aside from
Fundamental mode.  Rmail mode is a complicated and specialized mode.

@menu
* Major Mode Conventions::  Coding conventions for keymaps, etc.
* Example Major Modes::     Text mode and Lisp modes.
* Auto Major Mode::         How Emacs chooses the major mode automatically.
* Mode Help::               Finding out how to use a mode.
* Derived Modes::           Defining a new major mode based on another major
                              mode.
* Mode Hooks::              Hooks run at the end of major mode functions.
@end menu

@node Major Mode Conventions
@subsection Major Mode Conventions

  The code for existing major modes follows various coding conventions,
including conventions for local keymap and syntax table initialization,
global names, and hooks.  Please follow these conventions when you
define a new major mode.

  This list of conventions is only partial, because each major mode
should aim for consistency in general with other Emacs major modes.
This makes Emacs as a whole more coherent.  It is impossible to list
here all the possible points where this issue might come up; if the
Emacs developers point out an area where your major mode deviates from
the usual conventions, please make it compatible.

@itemize @bullet
@item
Define a command whose name ends in @samp{-mode}, with no arguments,
that switches to the new mode in the current buffer.  This command
should set up the keymap, syntax table, and buffer-local variables in an
existing buffer, without changing the buffer's contents.

@item
Write a documentation string for this command that describes the
special commands available in this mode.  @kbd{C-h m}
(@code{describe-mode}) in your mode will display this string.

The documentation string may include the special documentation
substrings, @samp{\[@var{command}]}, @samp{\@{@var{keymap}@}}, and
@samp{\<@var{keymap}>}, which enable the documentation to adapt
automatically to the user's own key bindings.  @xref{Keys in
Documentation}.

@item
The major mode command should start by calling
@code{kill-all-local-variables}.  This is what gets rid of the
buffer-local variables of the major mode previously in effect.

@item
The major mode command should set the variable @code{major-mode} to the
major mode command symbol.  This is how @code{describe-mode} discovers
which documentation to print.

@item
The major mode command should set the variable @code{mode-name} to the
``pretty'' name of the mode, as a string.  This string appears in the
mode line.

@item
@cindex functions in modes
Since all global names are in the same name space, all the global
variables, constants, and functions that are part of the mode should
have names that start with the major mode name (or with an abbreviation
of it if the name is long).  @xref{Coding Conventions}.

@item
In a major mode for editing some kind of structured text, such as a
programming language, indentation of text according to structure is
probably useful.  So the mode should set @code{indent-line-function}
to a suitable function, and probably customize other variables
for indentation.

@item
@cindex keymaps in modes
The major mode should usually have its own keymap, which is used as the
local keymap in all buffers in that mode.  The major mode command should
call @code{use-local-map} to install this local map.  @xref{Active
Keymaps}, for more information.

This keymap should be stored permanently in a global variable named
@code{@var{modename}-mode-map}.  Normally the library that defines the
mode sets this variable.

@xref{Tips for Defining}, for advice about how to write the code to set
up the mode's keymap variable.

@item
The key sequences bound in a major mode keymap should usually start with
@kbd{C-c}, followed by a control character, a digit, or @kbd{@{},
@kbd{@}}, @kbd{<}, @kbd{>}, @kbd{:} or @kbd{;}.  The other punctuation
characters are reserved for minor modes, and ordinary letters are
reserved for users.

A major mode can also rebind the keys @kbd{M-n}, @kbd{M-p} and
@kbd{M-s}.  The bindings for @kbd{M-n} and @kbd{M-p} should normally
be some kind of ``moving forward and backward,'' but this does not
necessarily mean cursor motion.

It is legitimate for a major mode to rebind a standard key sequence if
it provides a command that does ``the same job'' in a way better
suited to the text this mode is used for.  For example, a major mode
for editing a programming language might redefine @kbd{C-M-a} to
``move to the beginning of a function'' in a way that works better for
that language.

It is also legitimate for a major mode to rebind a standard key
sequence whose standard meaning is rarely useful in that mode.  For
instance, minibuffer modes rebind @kbd{M-r}, whose standard meaning is
rarely of any use in the minibuffer.  Major modes such as Dired or
Rmail that do not allow self-insertion of text can reasonably redefine
letters and other printing characters as special commands.

@item
Major modes must not define @key{RET} to do anything other than insert
a newline.  The command to insert a newline and then indent is
@kbd{C-j}.  Please keep this distinction uniform for all major modes.

@item
Major modes should not alter options that are primarily a matter of user
preference, such as whether Auto-Fill mode is enabled.  Leave this to
each user to decide.  However, a major mode should customize other
variables so that Auto-Fill mode will work usefully @emph{if} the user
decides to use it.

@item
@cindex syntax tables in modes
The mode may have its own syntax table or may share one with other
related modes.  If it has its own syntax table, it should store this in
a variable named @code{@var{modename}-mode-syntax-table}.  @xref{Syntax
Tables}.

@item
If the mode handles a language that has a syntax for comments, it should
set the variables that define the comment syntax.  @xref{Options for
Comments,, Options Controlling Comments, emacs, The GNU Emacs Manual}.

@item
@cindex abbrev tables in modes
The mode may have its own abbrev table or may share one with other
related modes.  If it has its own abbrev table, it should store this
in a variable named @code{@var{modename}-mode-abbrev-table}.  If the
major mode command defines any abbrevs itself, it should pass @code{t}
for the @var{system-flag} argument to @code{define-abbrev}.
@xref{Abbrev Tables}.

@item
The mode should specify how to do highlighting for Font Lock mode, by
setting up a buffer-local value for the variable
@code{font-lock-defaults} (@pxref{Font Lock Mode}).

@item
The mode should specify how Imenu should find the definitions or
sections of a buffer, by setting up a buffer-local value for the
variable @code{imenu-generic-expression}, for the pair of variables
@code{imenu-prev-index-position-function} and
@code{imenu-extract-index-name-function}, or for the variable
@code{imenu-create-index-function} (@pxref{Imenu}).

@item
Use @code{defvar} or @code{defcustom} to set mode-related variables, so
that they are not reinitialized if they already have a value.  (Such
reinitialization could discard customizations made by the user.)

@item
@cindex buffer-local variables in modes
To make a buffer-local binding for an Emacs customization variable, use
@code{make-local-variable} in the major mode command, not
@code{make-variable-buffer-local}.  The latter function would make the
variable local to every buffer in which it is subsequently set, which
would affect buffers that do not use this mode.  It is undesirable for a
mode to have such global effects.  @xref{Buffer-Local Variables}.

With rare exceptions, the only reasonable way to use
@code{make-variable-buffer-local} in a Lisp package is for a variable
which is used only within that package.  Using it on a variable used by
other packages would interfere with them.

@item
@cindex mode hook
@cindex major mode hook
Each major mode should have a @dfn{mode hook} named
@code{@var{modename}-mode-hook}.  The major mode command should run that
hook, with @code{run-mode-hooks}, as the very last thing it
does.  @xref{Mode Hooks}.

@item
The major mode command may start by calling some other major mode
command (called the @dfn{parent mode}) and then alter some of its
settings.  A mode that does this is called a @dfn{derived mode}.  The
recommended way to define one is to use @code{define-derived-mode},
but this is not required.  Such a mode should use
@code{delay-mode-hooks} around its entire body (including the call to
the parent mode command) @emph{except} for the final call to
@code{run-mode-hooks}, which runs the derived mode's hook.  (Using
@code{define-derived-mode} does this automatically.)  @xref{Derived
Modes}, and @ref{Mode Hooks}.

@item
If something special should be done if the user switches a buffer from
this mode to any other major mode, this mode can set up a buffer-local
value for @code{change-major-mode-hook} (@pxref{Creating Buffer-Local}).

@item
If this mode is appropriate only for specially-prepared text, then the
major mode command symbol should have a property named @code{mode-class}
with value @code{special}, put on as follows:

@kindex mode-class @r{(property)}
@cindex @code{special}
@example
(put 'funny-mode 'mode-class 'special)
@end example

@noindent
This tells Emacs that new buffers created while the current buffer is
in Funny mode should not inherit Funny mode, in case
@code{default-major-mode} is @code{nil}.  Modes such as Dired, Rmail,
and Buffer List use this feature.

@item
If you want to make the new mode the default for files with certain
recognizable names, add an element to @code{auto-mode-alist} to select
the mode for those file names.  If you define the mode command to
autoload, you should add this element in the same file that calls
@code{autoload}.  Otherwise, it is sufficient to add the element in the
file that contains the mode definition.  @xref{Auto Major Mode}.

@item
In the comments that document the file, you should provide a sample
@code{autoload} form and an example of how to add to
@code{auto-mode-alist}, that users can include in their init files
(@pxref{Init File}).

@item
@cindex mode loading
The top-level forms in the file defining the mode should be written so
that they may be evaluated more than once without adverse consequences.
Even if you never load the file more than once, someone else will.
@end itemize

@node Example Major Modes
@subsection Major Mode Examples

  Text mode is perhaps the simplest mode besides Fundamental mode.
Here are excerpts from  @file{text-mode.el} that illustrate many of
the conventions listed above:

@smallexample
@group
;; @r{Create the syntax table for this mode.}
(defvar text-mode-syntax-table
  (let ((st (make-syntax-table)))
    (modify-syntax-entry ?\" ".   " st)
    (modify-syntax-entry ?\\ ".   " st)
    ;; We add `p' so that M-c on 'hello' leads to 'Hello' rather than 'hello'.
    (modify-syntax-entry ?' "w p" st)
    st)
  "Syntax table used while in `text-mode'.")
@end group

;; @r{Create the keymap for this mode.}
@group
(defvar text-mode-map
  (let ((map (make-sparse-keymap)))
    (define-key map "\e\t" 'ispell-complete-word)
    (define-key map "\es" 'center-line)
    (define-key map "\eS" 'center-paragraph)
    map)
  "Keymap for `text-mode'.
Many other modes, such as `mail-mode', `outline-mode' and `indented-text-mode',
inherit all the commands defined in this map.")
@end group
@end smallexample

  Here is how the actual mode command is defined now:

@smallexample
@group
(define-derived-mode text-mode nil "Text"
  "Major mode for editing text written for humans to read.
In this mode, paragraphs are delimited only by blank or white lines.
You can thus get the full benefit of adaptive filling
 (see the variable `adaptive-fill-mode').
\\@{text-mode-map@}
Turning on Text mode runs the normal hook `text-mode-hook'."
@end group
@group
  (make-local-variable 'text-mode-variant)
  (setq text-mode-variant t)
  ;; @r{These two lines are a feature added recently.}
  (set (make-local-variable 'require-final-newline)
       mode-require-final-newline)
  (set (make-local-variable 'indent-line-function) 'indent-relative))
@end group
@end smallexample

  But here is how it was defined formerly, before
@code{define-derived-mode} existed:

@smallexample
@group
;; @r{This isn't needed nowadays, since @code{define-derived-mode} does it.}
(defvar text-mode-abbrev-table nil
  "Abbrev table used while in text mode.")
(define-abbrev-table 'text-mode-abbrev-table ())
@end group

@group
(defun text-mode ()
  "Major mode for editing text intended for humans to read...
 Special commands: \\@{text-mode-map@}
@end group
@group
Turning on text-mode runs the hook `text-mode-hook'."
  (interactive)
  (kill-all-local-variables)
  (use-local-map text-mode-map)
@end group
@group
  (setq local-abbrev-table text-mode-abbrev-table)
  (set-syntax-table text-mode-syntax-table)
@end group
@group
  ;; @r{These four lines are absent from the current version}
  ;; @r{not because this is done some other way, but rather}
  ;; @r{because nowadays Text mode uses the normal definition of paragraphs.}
  (make-local-variable 'paragraph-start)
  (setq paragraph-start (concat "[ \t]*$\\|" page-delimiter))
  (make-local-variable 'paragraph-separate)
  (setq paragraph-separate paragraph-start)
  (make-local-variable 'indent-line-function)
  (setq indent-line-function 'indent-relative-maybe)
@end group
@group
  (setq mode-name "Text")
  (setq major-mode 'text-mode)
  (run-mode-hooks 'text-mode-hook)) ; @r{Finally, this permits the user to}
                                    ;   @r{customize the mode with a hook.}
@end group
@end smallexample

@cindex @file{lisp-mode.el}
  The three Lisp modes (Lisp mode, Emacs Lisp mode, and Lisp
Interaction mode) have more features than Text mode and the code is
correspondingly more complicated.  Here are excerpts from
@file{lisp-mode.el} that illustrate how these modes are written.

@cindex syntax table example
@smallexample
@group
;; @r{Create mode-specific table variables.}
(defvar lisp-mode-syntax-table nil "")
(defvar lisp-mode-abbrev-table nil "")
@end group

@group
(defvar emacs-lisp-mode-syntax-table
  (let ((table (make-syntax-table)))
    (let ((i 0))
@end group

@group
      ;; @r{Set syntax of chars up to @samp{0} to say they are}
      ;;   @r{part of symbol names but not words.}
      ;;   @r{(The digit @samp{0} is @code{48} in the @acronym{ASCII} character set.)}
      (while (< i ?0)
        (modify-syntax-entry i "_   " table)
        (setq i (1+ i)))
      ;; @r{@dots{} similar code follows for other character ranges.}
@end group
@group
      ;; @r{Then set the syntax codes for characters that are special in Lisp.}
      (modify-syntax-entry ?  "    " table)
      (modify-syntax-entry ?\t "    " table)
      (modify-syntax-entry ?\f "    " table)
      (modify-syntax-entry ?\n ">   " table)
@end group
@group
      ;; @r{Give CR the same syntax as newline, for selective-display.}
      (modify-syntax-entry ?\^m ">   " table)
      (modify-syntax-entry ?\; "<   " table)
      (modify-syntax-entry ?` "'   " table)
      (modify-syntax-entry ?' "'   " table)
      (modify-syntax-entry ?, "'   " table)
@end group
@group
      ;; @r{@dots{}likewise for many other characters@dots{}}
      (modify-syntax-entry ?\( "()  " table)
      (modify-syntax-entry ?\) ")(  " table)
      (modify-syntax-entry ?\[ "(]  " table)
      (modify-syntax-entry ?\] ")[  " table))
    table))
@end group
@group
;; @r{Create an abbrev table for lisp-mode.}
(define-abbrev-table 'lisp-mode-abbrev-table ())
@end group
@end smallexample

  Much code is shared among the three Lisp modes.  The following
function sets various variables; it is called by each of the major Lisp
mode functions:

@smallexample
@group
(defun lisp-mode-variables (lisp-syntax)
  (when lisp-syntax
    (set-syntax-table lisp-mode-syntax-table))
  (setq local-abbrev-table lisp-mode-abbrev-table)
  @dots{}
@end group
@end smallexample

  Functions such as @code{forward-paragraph} use the value of the
@code{paragraph-start} variable.  Since Lisp code is different from
ordinary text, the @code{paragraph-start} variable needs to be set
specially to handle Lisp.  Also, comments are indented in a special
fashion in Lisp and the Lisp modes need their own mode-specific
@code{comment-indent-function}.  The code to set these variables is the
rest of @code{lisp-mode-variables}.

@smallexample
@group
  (make-local-variable 'paragraph-start)
  (setq paragraph-start (concat page-delimiter "\\|$" ))
  (make-local-variable 'paragraph-separate)
  (setq paragraph-separate paragraph-start)
  @dots{}
@end group
@group
  (make-local-variable 'comment-indent-function)
  (setq comment-indent-function 'lisp-comment-indent))
  @dots{}
@end group
@end smallexample

  Each of the different Lisp modes has a slightly different keymap.  For
example, Lisp mode binds @kbd{C-c C-z} to @code{run-lisp}, but the other
Lisp modes do not.  However, all Lisp modes have some commands in
common.  The following code sets up the common commands:

@smallexample
@group
(defvar shared-lisp-mode-map ()
  "Keymap for commands shared by all sorts of Lisp modes.")

;; @r{Putting this @code{if} after the @code{defvar} is an older style.}
(if shared-lisp-mode-map
    ()
   (setq shared-lisp-mode-map (make-sparse-keymap))
   (define-key shared-lisp-mode-map "\e\C-q" 'indent-sexp)
   (define-key shared-lisp-mode-map "\177"
               'backward-delete-char-untabify))
@end group
@end smallexample

@noindent
And here is the code to set up the keymap for Lisp mode:

@smallexample
@group
(defvar lisp-mode-map ()
  "Keymap for ordinary Lisp mode...")

(if lisp-mode-map
    ()
  (setq lisp-mode-map (make-sparse-keymap))
  (set-keymap-parent lisp-mode-map shared-lisp-mode-map)
  (define-key lisp-mode-map "\e\C-x" 'lisp-eval-defun)
  (define-key lisp-mode-map "\C-c\C-z" 'run-lisp))
@end group
@end smallexample

  Finally, here is the complete major mode function definition for
Lisp mode.

@smallexample
@group
(defun lisp-mode ()
  "Major mode for editing Lisp code for Lisps other than GNU Emacs Lisp.
Commands:
Delete converts tabs to spaces as it moves back.
Blank lines separate paragraphs.  Semicolons start comments.
\\@{lisp-mode-map@}
Note that `run-lisp' may be used either to start an inferior Lisp job
or to switch back to an existing one.
@end group

@group
Entry to this mode calls the value of `lisp-mode-hook'
if that value is non-nil."
  (interactive)
  (kill-all-local-variables)
@end group
@group
  (use-local-map lisp-mode-map)          ; @r{Select the mode's keymap.}
  (setq major-mode 'lisp-mode)           ; @r{This is how @code{describe-mode}}
                                         ;   @r{finds out what to describe.}
  (setq mode-name "Lisp")                ; @r{This goes into the mode line.}
  (lisp-mode-variables t)                ; @r{This defines various variables.}
  (make-local-variable 'comment-start-skip)
  (setq comment-start-skip
        "\\(\\(^\\|[^\\\\\n]\\)\\(\\\\\\\\\\)*\\)\\(;+\\|#|\\) *")
  (make-local-variable 'font-lock-keywords-case-fold-search)
  (setq font-lock-keywords-case-fold-search t)
@end group
@group
  (setq imenu-case-fold-search t)
  (set-syntax-table lisp-mode-syntax-table)
  (run-mode-hooks 'lisp-mode-hook))           ; @r{This permits the user to use a}
                                         ;   @r{hook to customize the mode.}
@end group
@end smallexample

@node Auto Major Mode
@subsection How Emacs Chooses a Major Mode

  Based on information in the file name or in the file itself, Emacs
automatically selects a major mode for the new buffer when a file is
visited.  It also processes local variables specified in the file text.

@deffn Command fundamental-mode
  Fundamental mode is a major mode that is not specialized for anything
in particular.  Other major modes are defined in effect by comparison
with this one---their definitions say what to change, starting from
Fundamental mode.  The @code{fundamental-mode} function does @emph{not}
run any mode hooks; you're not supposed to customize it.  (If you want Emacs
to behave differently in Fundamental mode, change the @emph{global}
state of Emacs.)
@end deffn

@deffn Command normal-mode &optional find-file
This function establishes the proper major mode and buffer-local variable
bindings for the current buffer.  First it calls @code{set-auto-mode},
then it runs @code{hack-local-variables} to parse, and bind or
evaluate as appropriate, the file's local variables.

If the @var{find-file} argument to @code{normal-mode} is non-@code{nil},
@code{normal-mode} assumes that the @code{find-file} function is calling
it.  In this case, it may process a local variables list at the end of
the file and in the @samp{-*-} line.  The variable
@code{enable-local-variables} controls whether to do so.  @xref{File
variables, , Local Variables in Files, emacs, The GNU Emacs Manual}, for
the syntax of the local variables section of a file.

If you run @code{normal-mode} interactively, the argument
@var{find-file} is normally @code{nil}.  In this case,
@code{normal-mode} unconditionally processes any local variables list.

@cindex file mode specification error
@code{normal-mode} uses @code{condition-case} around the call to the
major mode function, so errors are caught and reported as a @samp{File
mode specification error},  followed by the original error message.
@end deffn

@defun set-auto-mode
@cindex visited file mode
  This function selects the major mode that is appropriate for the
current buffer.  It may base its decision on the value of the @w{@samp{-*-}}
line, on the visited file name (using @code{auto-mode-alist}), on the
@w{@samp{#!}} line (using @code{interpreter-mode-alist}), or on the
file's local variables list.  However, this function does not look for
the @samp{mode:} local variable near the end of a file; the
@code{hack-local-variables} function does that.  @xref{Choosing Modes, ,
How Major Modes are Chosen, emacs, The GNU Emacs Manual}.
@end defun

@defopt default-major-mode
This variable holds the default major mode for new buffers.  The
standard value is @code{fundamental-mode}.

If the value of @code{default-major-mode} is @code{nil}, Emacs uses
the (previously) current buffer's major mode for the major mode of a new
buffer.  However, if that major mode symbol has a @code{mode-class}
property with value @code{special}, then it is not used for new buffers;
Fundamental mode is used instead.  The modes that have this property are
those such as Dired and Rmail that are useful only with text that has
been specially prepared.
@end defopt

@defun set-buffer-major-mode buffer
This function sets the major mode of @var{buffer} to the value of
@code{default-major-mode}.  If that variable is @code{nil}, it uses
the current buffer's major mode (if that is suitable).

The low-level primitives for creating buffers do not use this function,
but medium-level commands such as @code{switch-to-buffer} and
@code{find-file-noselect} use it whenever they create buffers.
@end defun

@defvar initial-major-mode
@cindex @samp{*scratch*}
The value of this variable determines the major mode of the initial
@samp{*scratch*} buffer.  The value should be a symbol that is a major
mode command.  The default value is @code{lisp-interaction-mode}.
@end defvar

@defvar auto-mode-alist
This variable contains an association list of file name patterns
(regular expressions; @pxref{Regular Expressions}) and corresponding
major mode commands.  Usually, the file name patterns test for suffixes,
such as @samp{.el} and @samp{.c}, but this need not be the case.  An
ordinary element of the alist looks like @code{(@var{regexp} .
@var{mode-function})}.

For example,

@smallexample
@group
(("\\`/tmp/fol/" . text-mode)
 ("\\.texinfo\\'" . texinfo-mode)
 ("\\.texi\\'" . texinfo-mode)
@end group
@group
 ("\\.el\\'" . emacs-lisp-mode)
 ("\\.c\\'" . c-mode)
 ("\\.h\\'" . c-mode)
 @dots{})
@end group
@end smallexample

When you visit a file whose expanded file name (@pxref{File Name
Expansion}) matches a @var{regexp}, @code{set-auto-mode} calls the
corresponding @var{mode-function}.  This feature enables Emacs to select
the proper major mode for most files.

If an element of @code{auto-mode-alist} has the form @code{(@var{regexp}
@var{function} t)}, then after calling @var{function}, Emacs searches
@code{auto-mode-alist} again for a match against the portion of the file
name that did not match before.  This feature is useful for
uncompression packages: an entry of the form @code{("\\.gz\\'"
@var{function} t)} can uncompress the file and then put the uncompressed
file in the proper mode according to the name sans @samp{.gz}.

Here is an example of how to prepend several pattern pairs to
@code{auto-mode-alist}.  (You might use this sort of expression in your
init file.)

@smallexample
@group
(setq auto-mode-alist
  (append
   ;; @r{File name (within directory) starts with a dot.}
   '(("/\\.[^/]*\\'" . fundamental-mode)
     ;; @r{File name has no dot.}
     ("[^\\./]*\\'" . fundamental-mode)
     ;; @r{File name ends in @samp{.C}.}
     ("\\.C\\'" . c++-mode))
   auto-mode-alist))
@end group
@end smallexample
@end defvar

@defvar interpreter-mode-alist
This variable specifies major modes to use for scripts that specify a
command interpreter in a @samp{#!} line.  Its value is a list of
elements of the form @code{(@var{interpreter} . @var{mode})}; for
example, @code{("perl" . perl-mode)} is one element present by default.
The element says to use mode @var{mode} if the file specifies
an interpreter which matches @var{interpreter}.  The value of
@var{interpreter} is actually a regular expression.

This variable is applicable only when the @code{auto-mode-alist} does
not indicate which major mode to use.
@end defvar

@node Mode Help
@subsection Getting Help about a Major Mode
@cindex mode help
@cindex help for major mode
@cindex documentation for major mode

  The @code{describe-mode} function is used to provide information
about major modes.  It is normally called with @kbd{C-h m}.  The
@code{describe-mode} function uses the value of @code{major-mode},
which is why every major mode function needs to set the
@code{major-mode} variable.

@deffn Command describe-mode
This function displays the documentation of the current major mode.

The @code{describe-mode} function calls the @code{documentation}
function using the value of @code{major-mode} as an argument.  Thus, it
displays the documentation string of the major mode function.
(@xref{Accessing Documentation}.)
@end deffn

@defvar major-mode
This variable holds the symbol for the current buffer's major mode.
This symbol should have a function definition that is the command to
switch to that major mode.  The @code{describe-mode} function uses the
documentation string of the function as the documentation of the major
mode.
@end defvar

@node Derived Modes
@subsection Defining Derived Modes

  It's often useful to define a new major mode in terms of an existing
one.  An easy way to do this is to use @code{define-derived-mode}.

@defmac define-derived-mode variant parent name docstring body@dots{}
This construct defines @var{variant} as a major mode command, using
@var{name} as the string form of the mode name.

The new command @var{variant} is defined to call the function
@var{parent}, then override certain aspects of that parent mode:

@itemize @bullet
@item
The new mode has its own keymap, named @code{@var{variant}-map}.
@code{define-derived-mode} initializes this map to inherit from
@code{@var{parent}-map}, if it is not already set.

@item
The new mode has its own syntax table, kept in the variable
@code{@var{variant}-syntax-table}.
@code{define-derived-mode} initializes this variable by copying
@code{@var{parent}-syntax-table}, if it is not already set.

@item
The new mode has its own abbrev table, kept in the variable
@code{@var{variant}-abbrev-table}.
@code{define-derived-mode} initializes this variable by copying
@code{@var{parent}-abbrev-table}, if it is not already set.

@item
The new mode has its own mode hook, @code{@var{variant}-hook},
which it runs in standard fashion as the very last thing that it does.
(The new mode also runs the mode hook of @var{parent} as part
of calling @var{parent}.)
@end itemize

In addition, you can specify how to override other aspects of
@var{parent} with @var{body}.  The command @var{variant}
evaluates the forms in @var{body} after setting up all its usual
overrides, just before running @code{@var{variant}-hook}.

The argument @var{docstring} specifies the documentation string for the
new mode.  If you omit @var{docstring}, @code{define-derived-mode}
generates a documentation string.

Here is a hypothetical example:

@example
(define-derived-mode hypertext-mode
  text-mode "Hypertext"
  "Major mode for hypertext.
\\@{hypertext-mode-map@}"
  (setq case-fold-search nil))

(define-key hypertext-mode-map
  [down-mouse-3] 'do-hyper-link)
@end example

Do not write an @code{interactive} spec in the definition;
@code{define-derived-mode} does that automatically.
@end defmac

@node Mode Hooks
@subsection Mode Hooks

The two last things a major mode function does is to run its mode
hook and finally the mode independent normal hook
@code{after-change-major-mode-hook}.  If the major mode is a derived
mode, that is if it calls another major mode (the parent mode) in its
body, then the parent's mode hook is run just before the derived
mode's hook.  Neither the parent's mode hook nor
@code{after-change-major-mode-hook} are run at the end of the actual
call to the parent mode.  This applies recursively if the parent mode
has itself a parent.  That is, the mode hooks of all major modes called
directly or indirectly by the major mode function are all run in
sequence at the end, just before @code{after-change-major-mode-hook}.

If you are customizing a major mode, rather than defining one, the
above is all you need to know about the hooks run at the end of a
major mode.  This also applies if you use @code{define-derived-mode}
to define a major mode, because that macro will automatically
implement the above for you.

Programmers wishing to define a major mode without using
@code{define-derived-mode}, should make sure that their major mode
follows the above conventions.  @xref{Major Mode Conventions}, for how
this should be accomplished.  Below, we give some implementation
details.

@defun run-mode-hooks &rest hookvars
Major modes should run their mode hook using this function.  It is
similar to @code{run-hooks} (@pxref{Hooks}), but if run inside a
@code{delay-mode-hooks} form, this function does not run any hooks.
Instead, it arranges for @var{hookvars} to be run at a later call to
the function.  Otherwise, @code{run-mode-hooks} runs any delayed hooks
in order, then @var{hookvars} and finally
@code{after-change-major-mode-hook}.
@end defun

@defmac delay-mode-hooks body...
This macro executes @var{body} like @code{progn}, but all calls to
@code{run-mode-hooks} inside @var{body} delay running their hooks.
They will be run by the first call to @code{run-mode-hooks} after exit
from @code{delay-mode-hooks}.
@end defmac

@defvar after-change-major-mode-hook
Every major mode function should run this normal hook at its very end.
It normally does not need to do so explicitly.  Indeed, a major mode
function should normally run its mode hook with @code{run-mode-hooks}
as the very last thing it does and @code{run-mode-hooks} runs
@code{after-change-major-mode-hook} at its very end.
@end defvar

@node Minor Modes
@section Minor Modes
@cindex minor mode

  A @dfn{minor mode} provides features that users may enable or disable
independently of the choice of major mode.  Minor modes can be enabled
individually or in combination.  Minor modes would be better named
``generally available, optional feature modes,'' except that such a name
would be unwieldy.

  A minor mode is not usually meant as a variation of a single major mode.
Usually they are general and can apply to many major modes.  For
example, Auto Fill mode works with any major mode that permits text
insertion.  To be general, a minor mode must be effectively independent
of the things major modes do.

  A minor mode is often much more difficult to implement than a major
mode.  One reason is that you should be able to activate and deactivate
minor modes in any order.  A minor mode should be able to have its
desired effect regardless of the major mode and regardless of the other
minor modes in effect.

  Often the biggest problem in implementing a minor mode is finding a
way to insert the necessary hook into the rest of Emacs.  Minor mode
keymaps make this easier than it used to be.

@defvar minor-mode-list
The value of this variable is a list of all minor mode commands.
@end defvar

@menu
* Minor Mode Conventions::      Tips for writing a minor mode.
* Keymaps and Minor Modes::     How a minor mode can have its own keymap.
* Defining Minor Modes::        A convenient facility for defining minor modes.
@end menu

@node Minor Mode Conventions
@subsection Conventions for Writing Minor Modes
@cindex minor mode conventions
@cindex conventions for writing minor modes

  There are conventions for writing minor modes just as there are for
major modes.  Several of the major mode conventions apply to minor
modes as well: those regarding the name of the mode initialization
function, the names of global symbols, and the use of keymaps and
other tables.

  In addition, there are several conventions that are specific to
minor modes.  (The easiest way to follow all the conventions is to use
the macro @code{define-minor-mode}; @ref{Defining Minor Modes}.)

@itemize @bullet
@item
@cindex mode variable
Make a variable whose name ends in @samp{-mode} to control the minor
mode.  We call this the @dfn{mode variable}.  The minor mode command
should set this variable (@code{nil} to disable; anything else to
enable).

If possible, implement the mode so that setting the variable
automatically enables or disables the mode.  Then the minor mode command
does not need to do anything except set the variable.

This variable is used in conjunction with the @code{minor-mode-alist} to
display the minor mode name in the mode line.  It can also enable
or disable a minor mode keymap.  Individual commands or hooks can also
check the variable's value.

If you want the minor mode to be enabled separately in each buffer,
make the variable buffer-local.

@item
Define a command whose name is the same as the mode variable.
Its job is to enable and disable the mode by setting the variable.

The command should accept one optional argument.  If the argument is
@code{nil}, it should toggle the mode (turn it on if it is off, and
off if it is on).  It should turn the mode on if the argument is a
positive integer, the symbol @code{t}, or a list whose @sc{car} is one
of those.  It should turn the mode off if the argument is a negative
integer or zero, the symbol @code{-}, or a list whose @sc{car} is a
negative integer or zero.  The meaning of other arguments is not
specified.

Here is an example taken from the definition of @code{transient-mark-mode}.
It shows the use of @code{transient-mark-mode} as a variable that enables or
disables the mode's behavior, and also shows the proper way to toggle,
enable or disable the minor mode based on the raw prefix argument value.

@smallexample
@group
(setq transient-mark-mode
      (if (null arg) (not transient-mark-mode)
        (> (prefix-numeric-value arg) 0)))
@end group
@end smallexample

@item
Add an element to @code{minor-mode-alist} for each minor mode
(@pxref{Mode Line Variables}), if you want to indicate the minor mode in
the mode line.  This element should be a list of the following form:

@smallexample
(@var{mode-variable} @var{string})
@end smallexample

Here @var{mode-variable} is the variable that controls enabling of the
minor mode, and @var{string} is a short string, starting with a space,
to represent the mode in the mode line.  These strings must be short so
that there is room for several of them at once.

When you add an element to @code{minor-mode-alist}, use @code{assq} to
check for an existing element, to avoid duplication.  For example:

@smallexample
@group
(unless (assq 'leif-mode minor-mode-alist)
  (setq minor-mode-alist
        (cons '(leif-mode " Leif") minor-mode-alist)))
@end group
@end smallexample

@noindent
or like this, using @code{add-to-list} (@pxref{Setting Variables}):

@smallexample
@group
(add-to-list 'minor-mode-alist '(leif-mode " Leif"))
@end group
@end smallexample
@end itemize

  Global minor modes distributed with Emacs should if possible support
enabling and disabling via Custom (@pxref{Customization}).  To do this,
the first step is to define the mode variable with @code{defcustom}, and
specify @code{:type boolean}.

  If just setting the variable is not sufficient to enable the mode, you
should also specify a @code{:set} method which enables the mode by
invoking the mode command.  Note in the variable's documentation string that
setting the variable other than via Custom may not take effect.

  Also mark the definition with an autoload cookie (@pxref{Autoload}),
and specify a @code{:require} so that customizing the variable will load
the library that defines the mode.  This will copy suitable definitions
into @file{loaddefs.el} so that users can use @code{customize-option} to
enable the mode.  For example:

@smallexample
@group

;;;###autoload
(defcustom msb-mode nil
  "Toggle msb-mode.
Setting this variable directly does not take effect;
use either \\[customize] or the function `msb-mode'."
  :set (lambda (symbol value)
         (msb-mode (or value 0)))
  :initialize 'custom-initialize-default
  :version "20.4"
  :type    'boolean
  :group   'msb
  :require 'msb)
@end group
@end smallexample

@node Keymaps and Minor Modes
@subsection Keymaps and Minor Modes

  Each minor mode can have its own keymap, which is active when the mode
is enabled.  To set up a keymap for a minor mode, add an element to the
alist @code{minor-mode-map-alist}.  @xref{Active Keymaps}.

@cindex @code{self-insert-command}, minor modes
  One use of minor mode keymaps is to modify the behavior of certain
self-inserting characters so that they do something else as well as
self-insert.  In general, this is the only way to do that, since the
facilities for customizing @code{self-insert-command} are limited to
special cases (designed for abbrevs and Auto Fill mode).  (Do not try
substituting your own definition of @code{self-insert-command} for the
standard one.  The editor command loop handles this function specially.)

The key sequences bound in a minor mode should consist of @kbd{C-c}
followed by a punctuation character @emph{other than} @kbd{@{},
@kbd{@}}, @kbd{<}, @kbd{>}, @kbd{:}, and @kbd{;}.  (Those few punctuation
characters are reserved for major modes.)

@node Defining Minor Modes
@subsection Defining Minor Modes

  The macro @code{define-minor-mode} offers a convenient way of
implementing a mode in one self-contained definition.

@defmac define-minor-mode mode doc [init-value [lighter [keymap]]] keyword-args... body...
@tindex define-minor-mode
This macro defines a new minor mode whose name is @var{mode} (a
symbol).  It defines a command named @var{mode} to toggle the minor
mode, with @var{doc} as its documentation string.  It also defines a
variable named @var{mode}, which is set to @code{t} or @code{nil} by
enabling or disabling the mode.  The variable is initialized to
@var{init-value}.

The string @var{lighter} says what to display in the mode line
when the mode is enabled; if it is @code{nil}, the mode is not displayed
in the mode line.

The optional argument @var{keymap} specifies the keymap for the minor mode.
It can be a variable name, whose value is the keymap, or it can be an alist
specifying bindings in this form:

@example
(@var{key-sequence} . @var{definition})
@end example

The above three arguments @var{init-value}, @var{lighter}, and
@var{keymap} can be (partially) omitted when @var{keyword-args} are
used.  The @var{keyword-args} consist of keywords followed by
corresponding values.  A few keywords have special meanings:

@table @code
@item :group @var{group}
Custom group name to use in all generated @code{defcustom} forms.
Defaults to @var{mode} without the possible trailing @samp{-mode}.  Be
aware that this default may not be a valid customization group defined
with @code{defgroup}.  Please make sure it is.

@item :global @var{global}
If non-@code{nil} specifies that the minor mode should be global.
By default, minor modes are buffer-local.

@item :init-value @var{init-value}
This is equivalent to specifying @var{init-value} positionally.

@item :lighter @var{lighter}
This is equivalent to specifying @var{lighter} positionally.

@item :keymap @var{keymap}
This is equivalent to specifying @var{keymap} positionally.
@end table

Any other keyword arguments are passed passed directly to the
@code{defcustom} generated for the variable @var{mode}.

The command named @var{mode} finishes by executing the @var{body} forms,
if any, after it has performed the standard actions such as setting
the variable named @var{mode}.
@end defmac

@findex easy-mmode-define-minor-mode
  The name @code{easy-mmode-define-minor-mode} is an alias
for this macro.

  Here is an example of using @code{define-minor-mode}:

@smallexample
(define-minor-mode hungry-mode
  "Toggle Hungry mode.
With no argument, this command toggles the mode.
Non-null prefix argument turns on the mode.
Null prefix argument turns off the mode.

When Hungry mode is enabled, the control delete key
gobbles all preceding whitespace except the last.
See the command \\[hungry-electric-delete]."
 ;; The initial value.
 nil
 ;; The indicator for the mode line.
 " Hungry"
 ;; The minor mode bindings.
 '(("\C-\^?" . hungry-electric-delete))
 :group 'hunger)
@end smallexample

@noindent
This defines a minor mode named ``Hungry mode'', a command named
@code{hungry-mode} to toggle it, a variable named @code{hungry-mode}
which indicates whether the mode is enabled, and a variable named
@code{hungry-mode-map} which holds the keymap that is active when the
mode is enabled.  It initializes the keymap with a key binding for
@kbd{C-@key{DEL}}.  It puts the variable @code{hungry-mode} into
custom group @code{hunger}.  There are no @var{body} forms---many
minor modes don't need any.

  Here's an equivalent way to write it:

@smallexample
(define-minor-mode hungry-mode
  "Toggle Hungry mode.
With no argument, this command toggles the mode.
Non-null prefix argument turns on the mode.
Null prefix argument turns off the mode.

When Hungry mode is enabled, the control delete key
gobbles all preceding whitespace except the last.
See the command \\[hungry-electric-delete]."
 ;; The initial value.
 :initial-value nil
 ;; The indicator for the mode line.
 :lighter " Hungry"
 ;; The minor mode bindings.
 :keymap
 '(("\C-\^?" . hungry-electric-delete)
   ("\C-\M-\^?"
    . (lambda ()
        (interactive)
        (hungry-electric-delete t))))
 :group 'hunger)
@end smallexample

@node Mode Line Format
@section Mode-Line Format
@cindex mode line

  Each Emacs window (aside from minibuffer windows) typically has a mode
line at the bottom, which displays status information about the buffer
displayed in the window.  The mode line contains information about the
buffer, such as its name, associated file, depth of recursive editing,
and major and minor modes.  A window can also have a @dfn{header
line}, which is much like the mode line but appears at the top of the
window.

  This section describes how to control the contents of the mode line
and header line.  We include it in this chapter because much of the
information displayed in the mode line relates to the enabled major and
minor modes.

  @code{mode-line-format} is a buffer-local variable that holds a
template used to display the mode line of the current buffer.  All
windows for the same buffer use the same @code{mode-line-format}, so
their mode lines appear the same---except for scrolling percentages, and
line and column numbers, since those depend on point and on how the
window is scrolled.  @code{header-line-format} is used likewise for
header lines.

  For efficiency, Emacs does not recompute the mode line and header
line of a window in every redisplay.  It does so when circumstances
appear to call for it---for instance, if you change the window
configuration, switch buffers, narrow or widen the buffer, scroll, or
change the buffer's modification status.  If you modify any of the
variables referenced by @code{mode-line-format} (@pxref{Mode Line
Variables}), or any other variables and data structures that affect
how text is displayed (@pxref{Display}), you may want to force an
update of the mode line so as to display the new information or
display it in the new way.

@c Emacs 19 feature
@defun force-mode-line-update &optional all
Force redisplay of the current buffer's mode line and header line.
The next redisplay will update the mode line and header line based on
the latest values of all relevant variables.  With optional
non-@code{nil} @var{all}, force redisplay of all mode lines and header
lines.

This function also forces recomputation of the menu bar menus
and the frame title.
@end defun

  The selected window's mode line is usually displayed in a different
color using the face @code{mode-line}.  Other windows' mode lines
appear in the face @code{mode-line-inactive} instead.  @xref{Faces}.

  A window that is just one line tall does not display either a mode
line or a header line, even if the variables call for one.  A window
that is two lines tall cannot display both a mode line and a header
line at once; if the variables call for both, only the mode line
actually appears.

@menu
* Mode Line Data::        The data structure that controls the mode line.
* Mode Line Variables::   Variables used in that data structure.
* %-Constructs::          Putting information into a mode line.
* Properties in Mode::    Using text properties in the mode line.
* Header Lines::          Like a mode line, but at the top.
* Emulating Mode Line::   Formatting text as the mode line would.
@end menu

@node Mode Line Data
@subsection The Data Structure of the Mode Line
@cindex mode-line construct

  The mode-line contents are controlled by a data structure of lists,
strings, symbols, and numbers kept in buffer-local variables.  The data
structure is called a @dfn{mode-line construct}, and it is built in
recursive fashion out of simpler mode-line constructs.  The same data
structure is used for constructing frame titles (@pxref{Frame Titles})
and header lines (@pxref{Header Lines}).

@defvar mode-line-format
The value of this variable is a mode-line construct with overall
responsibility for the mode-line format.  The value of this variable
controls which other variables are used to form the mode-line text, and
where they appear.

If you set this variable to @code{nil} in a buffer, that buffer does not
have a mode line.
@end defvar

  A mode-line construct may be as simple as a fixed string of text, but
it usually specifies how to use other variables to construct the text.
Many of these variables are themselves defined to have mode-line
constructs as their values.

  The default value of @code{mode-line-format} incorporates the values
of variables such as @code{mode-line-position} and
@code{mode-line-modes} (which in turn incorporates the values of the
variables @code{mode-name} and @code{minor-mode-alist}).  Because of
this, very few modes need to alter @code{mode-line-format} itself.  For
most purposes, it is sufficient to alter some of the variables that
@code{mode-line-format} either directly or indirectly refers to.

  A mode-line construct may be a list, a symbol, or a string.  If the
value is a list, each element may be a list, a symbol, or a string.

  The mode line can display various faces, if the strings that control
it have the @code{face} property.  @xref{Properties in Mode}.  In
addition, the face @code{mode-line} is used as a default for the whole
mode line (@pxref{Standard Faces}).

@table @code
@cindex percent symbol in mode line
@item @var{string}
A string as a mode-line construct is displayed verbatim in the mode line
except for @dfn{@code{%}-constructs}.  Decimal digits after the @samp{%}
specify the field width for space filling on the right (i.e., the data
is left justified).  @xref{%-Constructs}.

@item @var{symbol}
A symbol as a mode-line construct stands for its value.  The value of
@var{symbol} is used as a mode-line construct, in place of @var{symbol}.
However, the symbols @code{t} and @code{nil} are ignored, as is any
symbol whose value is void.

There is one exception: if the value of @var{symbol} is a string, it is
displayed verbatim: the @code{%}-constructs are not recognized.

Unless @var{symbol} is marked as ``risky'' (i.e., it has a
non-@code{nil} @code{risky-local-variable} property), all properties in
any strings, as well as all @code{:eval} and @code{:propertize} forms in
the value of that symbol will be ignored.

@item (@var{string} @var{rest}@dots{}) @r{or} (@var{list} @var{rest}@dots{})
A list whose first element is a string or list means to process all the
elements recursively and concatenate the results.  This is the most
common form of mode-line construct.

@item (:eval @var{form})
A list whose first element is the symbol @code{:eval} says to evaluate
@var{form}, and use the result as a string to display.

@item (:propertize @var{elt} @var{props}@dots{})
A list whose first element is the symbol @code{:propertize} says to
process the mode-line construct @var{elt} recursively and add the text
properties specified by @var{props} to the result.  The argument
@var{props} should consist of zero or more pairs @var{text-property}
@var{value}.  (This feature is new as of Emacs 22.1.)

@item (@var{symbol} @var{then} @var{else})
A list whose first element is a symbol that is not a keyword specifies a
conditional.  Its meaning depends on the value of @var{symbol}.  If the
value is non-@code{nil}, the second element, @var{then}, is processed
recursively as a mode-line element.  But if the value of @var{symbol} is
@code{nil}, the third element, @var{else}, is processed recursively.
You may omit @var{else}; then the mode-line element displays nothing if
the value of @var{symbol} is @code{nil}.

@item (@var{width} @var{rest}@dots{})
A list whose first element is an integer specifies truncation or
padding of the results of @var{rest}.  The remaining elements
@var{rest} are processed recursively as mode-line constructs and
concatenated together.  Then the result is space filled (if
@var{width} is positive) or truncated (to @minus{}@var{width} columns,
if @var{width} is negative) on the right.

For example, the usual way to show what percentage of a buffer is above
the top of the window is to use a list like this: @code{(-3 "%p")}.
@end table

  If you do alter @code{mode-line-format} itself, the new value should
use the same variables that appear in the default value (@pxref{Mode
Line Variables}), rather than duplicating their contents or displaying
the information in another fashion.  This way, customizations made by
the user or by Lisp programs (such as @code{display-time} and major
modes) via changes to those variables remain effective.

@cindex Shell mode @code{mode-line-format}
  Here is an example of a @code{mode-line-format} that might be
useful for @code{shell-mode}, since it contains the host name and default
directory.

@example
@group
(setq mode-line-format
  (list "-"
   'mode-line-mule-info
   'mode-line-modified
   'mode-line-frame-identification
   "%b--"
@end group
@group
   ;; @r{Note that this is evaluated while making the list.}
   ;; @r{It makes a mode-line construct which is just a string.}
   (getenv "HOST")
@end group
   ":"
   'default-directory
   "   "
   'global-mode-string
   "   %[("
   '(:eval (mode-line-mode-name))
   'mode-line-process
   'minor-mode-alist
   "%n"
   ")%]--"
@group
   '(which-func-mode ("" which-func-format "--"))
   '(line-number-mode "L%l--")
   '(column-number-mode "C%c--")
   '(-3 "%p")
   "-%-"))
@end group
@end example

@noindent
(The variables @code{line-number-mode}, @code{column-number-mode}
and @code{which-func-mode} enable particular minor modes; as usual,
these variable names are also the minor mode command names.)

@node Mode Line Variables
@subsection Variables Used in the Mode Line

  This section describes variables incorporated by the
standard value of @code{mode-line-format} into the text of the mode
line.  There is nothing inherently special about these variables; any
other variables could have the same effects on the mode line if
@code{mode-line-format} were changed to use them.

@defvar mode-line-mule-info
This variable holds the value of the mode-line construct that displays
information about the language environment, buffer coding system, and
current input method.  @xref{Non-ASCII Characters}.
@end defvar

@defvar mode-line-modified
This variable holds the value of the mode-line construct that displays
whether the current buffer is modified.

The default value of @code{mode-line-modified} is @code{("%1*%1+")}.
This means that the mode line displays @samp{**} if the buffer is
modified, @samp{--} if the buffer is not modified, @samp{%%} if the
buffer is read only, and @samp{%*} if the buffer is read only and
modified.

Changing this variable does not force an update of the mode line.
@end defvar

@defvar mode-line-frame-identification
This variable identifies the current frame.  The default value is
@code{"  "} if you are using a window system which can show multiple
frames, or @code{"-%F  "} on an ordinary terminal which shows only one
frame at a time.
@end defvar

@defvar mode-line-buffer-identification
This variable identifies the buffer being displayed in the window.  Its
default value is @code{("%12b")}, which displays the buffer name, padded
with spaces to at least 12 columns.
@end defvar

@defvar mode-line-position
This variable indicates the position in the buffer.  Here is a
simplified version of its default value.  The actual default value
also specifies addition of the @code{help-echo} text property.

@example
@group
((-3 "%p")
 (size-indication-mode (8 " of %I"))
@end group
@group
 (line-number-mode
  ((column-number-mode
    (10 " (%l,%c)")
    (6 " L%l")))
  ((column-number-mode
    (5 " C%c")))))
@end group
@end example

This means that @code{mode-line-position} displays at least the buffer
percentage and possibly the buffer size, the line number and the column
number.
@end defvar

@defvar vc-mode
The variable @code{vc-mode}, buffer-local in each buffer, records
whether the buffer's visited file is maintained with version control,
and, if so, which kind.  Its value is a string that appears in the mode
line, or @code{nil} for no version control.
@end defvar

@defvar mode-line-modes
This variable displays the buffer's major and minor modes.  Here is a
simplified version of its default value.  The real default value also
specifies addition of text properties.

@example
@group
("%[(" mode-name
 mode-line-process minor-mode-alist
 "%n" ")%]--")
@end group
@end example

So @code{mode-line-modes} normally also displays the recursive editing
level, information on the process status and whether narrowing is in
effect.
@end defvar

  The following three variables are used in @code{mode-line-modes}:

@defvar mode-name
This buffer-local variable holds the ``pretty'' name of the current
buffer's major mode.  Each major mode should set this variable so that the
mode name will appear in the mode line.
@end defvar

@defvar mode-line-process
This buffer-local variable contains the mode-line information on process
status in modes used for communicating with subprocesses.  It is
displayed immediately following the major mode name, with no intervening
space.  For example, its value in the @samp{*shell*} buffer is
@code{(":%s")}, which allows the shell to display its status along
with the major mode as: @samp{(Shell:run)}.  Normally this variable
is @code{nil}.
@end defvar

@defvar minor-mode-alist
This variable holds an association list whose elements specify how the
mode line should indicate that a minor mode is active.  Each element of
the @code{minor-mode-alist} should be a two-element list:

@example
(@var{minor-mode-variable} @var{mode-line-string})
@end example

More generally, @var{mode-line-string} can be any mode-line spec.  It
appears in the mode line when the value of @var{minor-mode-variable}
is non-@code{nil}, and not otherwise.  These strings should begin with
spaces so that they don't run together.  Conventionally, the
@var{minor-mode-variable} for a specific mode is set to a
non-@code{nil} value when that minor mode is activated.

@code{minor-mode-alist} itself is not buffer-local.  Each variable
mentioned in the alist should be buffer-local if its minor mode can be
enabled separately in each buffer.
@end defvar

@defvar global-mode-string
This variable holds a mode-line spec that, by default, appears in the
mode line just after the @code{which-func-mode} minor mode if set,
else after @code{mode-line-modes}.  The command @code{display-time}
sets @code{global-mode-string} to refer to the variable
@code{display-time-string}, which holds a string containing the time
and load information.

The @samp{%M} construct substitutes the value of
@code{global-mode-string}, but that is obsolete, since the variable is
included in the mode line from @code{mode-line-format}.
@end defvar

  The variable @code{default-mode-line-format} is where
@code{mode-line-format} usually gets its value:

@defvar default-mode-line-format
This variable holds the default @code{mode-line-format} for buffers
that do not override it.  This is the same as @code{(default-value
'mode-line-format)}.

Here is a simplified version of the default value of
@code{default-mode-line-format}.  The real default value also
specifies addition of text properties.

@example
@group
("-"
 mode-line-mule-info
 mode-line-modified
 mode-line-frame-identification
 mode-line-buffer-identification
@end group
 "   "
 mode-line-position
 (vc-mode vc-mode)
 "   "
@group
 mode-line-modes
 (which-func-mode ("" which-func-format "--"))
 (global-mode-string ("--" global-mode-string))
 "-%-")
@end group
@end example
@end defvar

@node %-Constructs
@subsection @code{%}-Constructs in the Mode Line

  The following table lists the recognized @code{%}-constructs and what
they mean.  In any construct except @samp{%%}, you can add a decimal
integer after the @samp{%} to specify how many characters to display.

@table @code
@item %b
The current buffer name, obtained with the @code{buffer-name} function.
@xref{Buffer Names}.

@item %c
The current column number of point.

@item %f
The visited file name, obtained with the @code{buffer-file-name}
function.  @xref{Buffer File Name}.

@item %F
The title (only on a window system) or the name of the selected frame.
@xref{Window Frame Parameters}.

@item %i
The size of the accessible part of the current buffer; basically
@code{(- (point-max) (point-min))}.

@item %I
Like @samp{%i}, but the size is printed in a more readable way by using
@samp{k} for 10^3, @samp{M} for 10^6, @samp{G} for 10^9, etc., to
abbreviate.

@item %l
The current line number of point, counting within the accessible portion
of the buffer.

@item %n
@samp{Narrow} when narrowing is in effect; nothing otherwise (see
@code{narrow-to-region} in @ref{Narrowing}).

@item %p
The percentage of the buffer text above the @strong{top} of window, or
@samp{Top}, @samp{Bottom} or @samp{All}.  Note that the default
mode-line specification truncates this to three characters.

@item %P
The percentage of the buffer text that is above the @strong{bottom} of
the window (which includes the text visible in the window, as well as
the text above the top), plus @samp{Top} if the top of the buffer is
visible on screen; or @samp{Bottom} or @samp{All}.

@item %s
The status of the subprocess belonging to the current buffer, obtained with
@code{process-status}.  @xref{Process Information}.

@item %t
Whether the visited file is a text file or a binary file.  This is a
meaningful distinction only on certain operating systems (@pxref{MS-DOS
File Types}).

@item %*
@samp{%} if the buffer is read only (see @code{buffer-read-only}); @*
@samp{*} if the buffer is modified (see @code{buffer-modified-p}); @*
@samp{-} otherwise.  @xref{Buffer Modification}.

@item %+
@samp{*} if the buffer is modified (see @code{buffer-modified-p}); @*
@samp{%} if the buffer is read only (see @code{buffer-read-only}); @*
@samp{-} otherwise.  This differs from @samp{%*} only for a modified
read-only buffer.  @xref{Buffer Modification}.

@item %&
@samp{*} if the buffer is modified, and @samp{-} otherwise.

@item %[
An indication of the depth of recursive editing levels (not counting
minibuffer levels): one @samp{[} for each editing level.
@xref{Recursive Editing}.

@item %]
One @samp{]} for each recursive editing level (not counting minibuffer
levels).

@item %-
Dashes sufficient to fill the remainder of the mode line.

@item %%
The character @samp{%}---this is how to include a literal @samp{%} in a
string in which @code{%}-constructs are allowed.
@end table

The following two @code{%}-constructs are still supported, but they are
obsolete, since you can get the same results with the variables
@code{mode-name} and @code{global-mode-string}.

@table @code
@item %m
The value of @code{mode-name}.

@item %M
The value of @code{global-mode-string}.  Currently, only
@code{display-time} modifies the value of @code{global-mode-string}.
@end table

@node Properties in Mode
@subsection Properties in the Mode Line
@cindex text properties in the mode line

  Certain text properties are meaningful in the
mode line.  The @code{face} property affects the appearance of text; the
@code{help-echo} property associate help strings with the text, and
@code{local-map} can make the text mouse-sensitive.

  There are four ways to specify text properties for text in the mode
line:

@enumerate
@item
Put a string with a text property directly into the mode-line data
structure.

@item
Put a text property on a mode-line %-construct such as @samp{%12b}; then
the expansion of the %-construct will have that same text property.

@item
Use a @code{(:propertize @var{elt} @var{props}@dots{})} construct to
give @var{elt} a text property specified by @var{props}.

@item
Use a list containing @code{:eval @var{form}} in the mode-line data
structure, and make @var{form} evaluate to a string that has a text
property.
@end enumerate

  You use the @code{local-map} property to specify a keymap.  Like any
keymap, it can bind character keys and function keys; but that has no
effect, since it is impossible to move point into the mode line.  This
keymap can only take real effect for mouse clicks.

  When the mode line refers to a variable which does not have a
non-@code{nil} @code{risky-local-variable} property, any text
properties given or specified within that variable's values are
ignored.  This is because such properties could otherwise specify
functions to be called, and those functions could come from file
local variables.

@node Header Lines
@subsection Window Header Lines
@cindex header line (of a window)
@cindex window header line

  A window can have a @dfn{header line} at the
top, just as it can have a mode line at the bottom.  The header line
feature works just like the mode-line feature, except that it's
controlled by different variables.

@tindex header-line-format
@defvar header-line-format
This variable, local in every buffer, specifies how to display the
header line, for windows displaying the buffer.  The format of the value
is the same as for @code{mode-line-format} (@pxref{Mode Line Data}).
@end defvar

@tindex default-header-line-format
@defvar default-header-line-format
This variable holds the default @code{header-line-format} for buffers
that do not override it.  This is the same as @code{(default-value
'header-line-format)}.

It is normally @code{nil}, so that ordinary buffers have no header line.
@end defvar

@node Emulating Mode Line
@subsection Emulating Mode-Line Formatting

  You can use the function @code{format-mode-line} to compute
the text that would appear in a mode line or header line
based on certain mode-line specification.

@defun format-mode-line format &optional face window buffer
This function formats a line of text according to @var{format} as if
it were generating the mode line for @var{window}, but instead of
displaying the text in the mode line or the header line, it returns
the text as a string.  The argument @var{window} defaults to the
selected window.  If @var{buffer} is non-@code{nil}, all the
information used is taken from @var{buffer}; by default, it comes from
@var{window}'s buffer.

The value string normally has text properties that correspond to the
faces, keymaps, etc., that the mode line would have.  And any character
for which no @code{face} property is specified gets a default
value which is usually @var{face}.  (If @var{face} is @code{t},
that stands for either @code{mode-line} if @var{window} is selected,
otherwise @code{mode-line-inactive}.)

However, if @var{face} is an integer, the value has no text properties.

For example, @code{(format-mode-line header-line-format)} returns the
text that would appear in the selected window's header line (@code{""}
if it has no header line).  @code{(format-mode-line header-line-format
'header-line)} returns the same text, with each character
carrying the face that it will have in the header line itself.
@end defun

@node Imenu
@section Imenu

@cindex Imenu
  @dfn{Imenu} is a feature that lets users select a definition or
section in the buffer, from a menu which lists all of them, to go
directly to that location in the buffer.  Imenu works by constructing
a buffer index which lists the names and buffer positions of the
definitions, or other named portions of the buffer; then the user can
choose one of them and move point to it.  Major modes can add a menu
bar item to use Imenu using @code{imenu-add-to-menubar}.

@defun imenu-add-to-menubar name
This function defines a local menu bar item named @var{name}
to run Imenu.
@end defun

  The user-level commands for using Imenu are described in the Emacs
Manual (@pxref{Imenu,, Imenu, emacs, the Emacs Manual}).  This section
explains how to customize Imenu's method of finding definitions or
buffer portions for a particular major mode.

  The usual and simplest way is to set the variable
@code{imenu-generic-expression}:

@defvar imenu-generic-expression
This variable, if non-@code{nil}, is a list that specifies regular
expressions for finding definitions for Imenu.  Simple elements of
@code{imenu-generic-expression} look like this:

@example
(@var{menu-title} @var{regexp} @var{index})
@end example

Here, if @var{menu-title} is non-@code{nil}, it says that the matches
for this element should go in a submenu of the buffer index;
@var{menu-title} itself specifies the name for the submenu.  If
@var{menu-title} is @code{nil}, the matches for this element go directly
in the top level of the buffer index.

The second item in the list, @var{regexp}, is a regular expression
(@pxref{Regular Expressions}); anything in the buffer that it matches
is considered a definition, something to mention in the buffer index.
The third item, @var{index}, is a non-negative integer that indicates
which subexpression in @var{regexp} matches the definition's name.

An element can also look like this:

@example
(@var{menu-title} @var{regexp} @var{index} @var{function} @var{arguments}@dots{})
@end example

Like in the previous case, each match for this element creates an
index item.  However, if this index item is selected by the user, it
calls @var{function} with arguments consisting of the item name, the
buffer position, and @var{arguments}.

For Emacs Lisp mode, @code{imenu-generic-expression} could look like
this:

@c should probably use imenu-syntax-alist and \\sw rather than [-A-Za-z0-9+]
@example
@group
((nil "^\\s-*(def\\(un\\|subst\\|macro\\|advice\\)\
\\s-+\\([-A-Za-z0-9+]+\\)" 2)
@end group
@group
 ("*Vars*" "^\\s-*(def\\(var\\|const\\)\
\\s-+\\([-A-Za-z0-9+]+\\)" 2)
@end group
@group
 ("*Types*"
  "^\\s-*\
(def\\(type\\|struct\\|class\\|ine-condition\\)\
\\s-+\\([-A-Za-z0-9+]+\\)" 2))
@end group
@end example

Setting this variable makes it buffer-local in the current buffer.
@end defvar

@defvar imenu-case-fold-search
This variable controls whether matching against the regular
expressions in the value of @code{imenu-generic-expression} is
case-sensitive: @code{t}, the default, means matching should ignore
case.

Setting this variable makes it buffer-local in the current buffer.
@end defvar

@defvar imenu-syntax-alist
This variable is an alist of syntax table modifiers to use while
processing @code{imenu-generic-expression}, to override the syntax table
of the current buffer.  Each element should have this form:

@example
(@var{characters} . @var{syntax-description})
@end example

The @sc{car}, @var{characters}, can be either a character or a string.
The element says to give that character or characters the syntax
specified by @var{syntax-description}, which is passed to
@code{modify-syntax-entry} (@pxref{Syntax Table Functions}).

This feature is typically used to give word syntax to characters which
normally have symbol syntax, and thus to simplify
@code{imenu-generic-expression} and speed up matching.
For example, Fortran mode uses it this way:

@example
(setq imenu-syntax-alist '(("_$" . "w")))
@end example

The @code{imenu-generic-expression} regular expressions can then use
@samp{\\sw+} instead of @samp{\\(\\sw\\|\\s_\\)+}.  Note that this
technique may be inconvenient when the mode needs to limit the initial
character of a name to a smaller set of characters than are allowed in
the rest of a name.

Setting this variable makes it buffer-local in the current buffer.
@end defvar

  Another way to customize Imenu for a major mode is to set the
variables @code{imenu-prev-index-position-function} and
@code{imenu-extract-index-name-function}:

@defvar imenu-prev-index-position-function
If this variable is non-@code{nil}, its value should be a function that
finds the next ``definition'' to put in the buffer index, scanning
backward in the buffer from point.  It should return @code{nil} if it
doesn't find another ``definition'' before point.  Otherwise it should
leave point at the place it finds a ``definition,'' and return any
non-@code{nil} value.

Setting this variable makes it buffer-local in the current buffer.
@end defvar

@defvar imenu-extract-index-name-function
If this variable is non-@code{nil}, its value should be a function to
return the name for a definition, assuming point is in that definition
as the @code{imenu-prev-index-position-function} function would leave
it.

Setting this variable makes it buffer-local in the current buffer.
@end defvar

  The last way to customize Imenu for a major mode is to set the
variable @code{imenu-create-index-function}:

@defvar imenu-create-index-function
This variable specifies the function to use for creating a buffer
index.  The function should take no arguments, and return an index
alist for the current buffer.  It is called within
@code{save-excursion}, so where it leaves point makes no difference.

The index alist can have three types of elements.  Simple elements
look like this:

@example
(@var{index-name} . @var{index-position})
@end example

Selecting a simple element has the effect of moving to position
@var{index-position} in the buffer.  Special elements look like this:

@example
(@var{index-name} @var{index-position} @var{function} @var{arguments}@dots{})
@end example

Selecting a special element performs:

@example
(funcall @var{function}
         @var{index-name} @var{index-position} @var{arguments}@dots{})
@end example

A nested sub-alist element looks like this:

@example
(@var{menu-title} @var{sub-alist})
@end example

It creates the submenu @var{menu-title} specified by @var{sub-alist}.

The default value of @code{imenu-create-index-function} is
@code{imenu-default-create-index-function}.  This function uses
@code{imenu-prev-index-position-function} and
@code{imenu-extract-index-name-function} to produce the index alist.
However, if either of these two variables is @code{nil}, the default
function uses @code{imenu-generic-expression} instead.

Setting this variable makes it buffer-local in the current buffer.
@end defvar

@node Font Lock Mode
@section Font Lock Mode
@cindex Font Lock Mode

  @dfn{Font Lock mode} is a feature that automatically attaches
@code{face} properties to certain parts of the buffer based on their
syntactic role.  How it parses the buffer depends on the major mode;
most major modes define syntactic criteria for which faces to use in
which contexts.  This section explains how to customize Font Lock for a
particular major mode.

  Font Lock mode finds text to highlight in two ways: through syntactic
parsing based on the syntax table, and through searching (usually for
regular expressions).  Syntactic fontification happens first; it finds
comments and string constants, and highlights them using
@code{font-lock-comment-face} and @code{font-lock-string-face}
(@pxref{Faces for Font Lock}).  Search-based fontification follows.

@menu
* Font Lock Basics::            Overview of customizing Font Lock.
* Search-based Fontification::  Fontification based on regexps.
* Other Font Lock Variables::   Additional customization facilities.
* Levels of Font Lock::         Each mode can define alternative levels
                                  so that the user can select more or less.
* Precalculated Fontification:: How Lisp programs that produce the buffer
                                  contents can also specify how to fontify it.
* Faces for Font Lock::         Special faces specifically for Font Lock.
* Syntactic Font Lock::         Defining character syntax based on context
                                  using the Font Lock mechanism.
@end menu

@node Font Lock Basics
@subsection Font Lock Basics

  There are several variables that control how Font Lock mode highlights
text.  But major modes should not set any of these variables directly.
Instead, they should set @code{font-lock-defaults} as a buffer-local
variable.  The value assigned to this variable is used, if and when Font
Lock mode is enabled, to set all the other variables.

@defvar font-lock-defaults
This variable is set by major modes, as a buffer-local variable, to
specify how to fontify text in that mode.  The value should look like
this:

@example
(@var{keywords} @var{keywords-only} @var{case-fold}
 @var{syntax-alist} @var{syntax-begin} @var{other-vars}@dots{})
@end example

The first element, @var{keywords}, indirectly specifies the value of
@code{font-lock-keywords}.  It can be a symbol, a variable whose value
is the list to use for @code{font-lock-keywords}.  It can also be a list of
several such symbols, one for each possible level of fontification.  The
first symbol specifies how to do level 1 fontification, the second
symbol how to do level 2, and so on.

The second element, @var{keywords-only}, specifies the value of the
variable @code{font-lock-keywords-only}.  If this is non-@code{nil},
syntactic fontification (of strings and comments) is not performed.

The third element, @var{case-fold}, specifies the value of
@code{font-lock-keywords-case-fold-search}.  If it is non-@code{nil}, Font Lock
mode ignores case when searching as directed by
@code{font-lock-keywords}.

If the fourth element, @var{syntax-alist}, is non-@code{nil}, it should be
a list of cons cells of the form @code{(@var{char-or-string}
. @var{string})}.  These are used to set up a syntax table for
fontification (@pxref{Syntax Table Functions}).  The resulting syntax
table is stored in @code{font-lock-syntax-table}.

The fifth element, @var{syntax-begin}, specifies the value of
@code{font-lock-beginning-of-syntax-function} (see below).

All the remaining elements (if any) are collectively called
@var{other-vars}.  Each of these elements should have the form
@code{(@var{variable} . @var{value})}---which means, make @var{variable}
buffer-local and then set it to @var{value}.  You can use these
@var{other-vars} to set other variables that affect fontification,
aside from those you can control with the first five elements.
@end defvar

@node Search-based Fontification
@subsection Search-based Fontification

  The most important variable for customizing Font Lock mode is
@code{font-lock-keywords}.  It specifies the search criteria for
search-based fontification.

@defvar font-lock-keywords
This variable's value is a list of the keywords to highlight.  Be
careful when composing regular expressions for this list; a poorly
written pattern can dramatically slow things down!
@end defvar

  Each element of @code{font-lock-keywords} specifies how to find
certain cases of text, and how to highlight those cases.  Font Lock mode
processes the elements of @code{font-lock-keywords} one by one, and for
each element, it finds and handles all matches.  Ordinarily, once
part of the text has been fontified already, this cannot be overridden
by a subsequent match in the same text; but you can specify different
behavior using the @var{override} element of a @var{subexp-highlighter}.

  Each element of @code{font-lock-keywords} should have one of these
forms:

@table @code
@item @var{regexp}
Highlight all matches for @var{regexp} using
@code{font-lock-keyword-face}.  For example,

@example
;; @r{Highlight occurrences of the word @samp{foo}}
;; @r{using @code{font-lock-keyword-face}.}
"\\<foo\\>"
@end example

The function @code{regexp-opt} (@pxref{Regexp Functions}) is useful
for calculating optimal regular expressions to match a number of
different keywords.

@item @var{function}
Find text by calling @var{function}, and highlight the matches
it finds using @code{font-lock-keyword-face}.

When @var{function} is called, it receives one argument, the limit of
the search; it should begin searching at point, and not search beyond the
limit.  It should return non-@code{nil} if it succeeds, and set the
match data to describe the match that was found.  Returning @code{nil}
indicates failure of the search.

Fontification will call @var{function} repeatedly with the same limit,
and with point where the previous invocation left it, until
@var{function} fails.  On failure, @var{function} need not reset point
in any particular way.

@item (@var{matcher} . @var{subexp})
In this kind of element, @var{matcher} is either a regular
expression or a function, as described above.  The @sc{cdr},
@var{subexp}, specifies which subexpression of @var{matcher} should be
highlighted (instead of the entire text that @var{matcher} matched).

@example
;; @r{Highlight the @samp{bar} in each occurrence of @samp{fubar},}
;; @r{using @code{font-lock-keyword-face}.}
("fu\\(bar\\)" . 1)
@end example

If you use @code{regexp-opt} to produce the regular expression
@var{matcher}, then you can use @code{regexp-opt-depth} (@pxref{Regexp
Functions}) to calculate the value for @var{subexp}.

@item (@var{matcher} . @var{facespec})
In this kind of element, @var{facespec} is an expression whose value
specifies the face to use for highlighting.  In the simplest case,
@var{facespec} is a Lisp variable (a symbol) whose value is a face
name.

@example
;; @r{Highlight occurrences of @samp{fubar},}
;; @r{using the face which is the value of @code{fubar-face}.}
("fubar" . fubar-face)
@end example

However, @var{facespec} can also evaluate to a list of this form:

@example
(face @var{face} @var{prop1} @var{val1} @var{prop2} @var{val2}@dots{})
@end example

@noindent
to specify the face @var{face} and various additional text properties
to put on the text that matches.  If you do this, be sure to add the
other text property names that you set in this way to the value of
@code{font-lock-extra-managed-props} so that the properties will also
be cleared out when they are no longer appropriate.  Alternatively,
you can set the variable @code{font-lock-unfontify-region-function} to
a function that clears these properties.

@item (@var{matcher} . @var{subexp-highlighter})
In this kind of element, @var{subexp-highlighter} is a list
which specifies how to highlight matches found by @var{matcher}.
It has the form:

@example
(@var{subexp} @var{facespec} [[@var{override} [@var{laxmatch}]])
@end example

The @sc{car}, @var{subexp}, is an integer specifying which subexpression
of the match to fontify (0 means the entire matching text).  The second
subelement, @var{facespec}, is an expression whose value specifies the
face, as described above.

The last two values in @var{subexp-highlighter}, @var{override} and
@var{laxmatch}, are optional flags.  If @var{override} is @code{t},
this element can override existing fontification made by previous
elements of @code{font-lock-keywords}.  If it is @code{keep}, then
each character is fontified if it has not been fontified already by
some other element.  If it is @code{prepend}, the face specified by
@var{facespec} is added to the beginning of the @code{font-lock-face}
property.  If it is @code{append}, the face is added to the end of the
@code{font-lock-face} property.

If @var{laxmatch} is non-@code{nil}, it means there should be no error
if there is no subexpression numbered @var{subexp} in @var{matcher}.
Obviously, fontification of the subexpression numbered @var{subexp} will
not occur.  However, fontification of other subexpressions (and other
regexps) will continue.  If @var{laxmatch} is @code{nil}, and the
specified subexpression is missing, then an error is signaled which
terminates search-based fontification.

Here are some examples of elements of this kind, and what they do:

@smallexample
;; @r{Highlight occurrences of either @samp{foo} or @samp{bar}, using}
;; @r{@code{foo-bar-face}, even if they have already been highlighted.}
;; @r{@code{foo-bar-face} should be a variable whose value is a face.}
("foo\\|bar" 0 foo-bar-face t)

;; @r{Highlight the first subexpression within each occurrence}
;; @r{that the function @code{fubar-match} finds,}
;; @r{using the face which is the value of @code{fubar-face}.}
(fubar-match 1 fubar-face)
@end smallexample

@item (@var{matcher} . @var{anchored-highlighter})
In this kind of element, @var{anchored-highlighter} specifies how to
highlight text that follows a match found by @var{matcher}.  So a
match found by @var{matcher} acts as the anchor for further searches
specified by @var{anchored-highlighter}.  @var{anchored-highlighter}
is a list of the following form:

@example
(@var{anchored-matcher} @var{pre-form} @var{post-form}
                        @var{subexp-highlighters}@dots{})
@end example

Here, @var{anchored-matcher}, like @var{matcher}, is either a regular
expression or a function.  After a match of @var{matcher} is found,
point is at the end of the match.  Now, Font Lock evaluates the form
@var{pre-form}.  Then it searches for matches of
@var{anchored-matcher} and uses @var{subexp-highlighters} to highlight
these.  A @var{subexp-highlighter} is as described above.  Finally,
Font Lock evaluates @var{post-form}.

The forms @var{pre-form} and @var{post-form} can be used to initialize
before, and cleanup after, @var{anchored-matcher} is used.  Typically,
@var{pre-form} is used to move point to some position relative to the
match of @var{matcher}, before starting with @var{anchored-matcher}.
@var{post-form} might be used to move back, before resuming with
@var{matcher}.

After Font Lock evaluates @var{pre-form}, it does not search for
@var{anchored-matcher} beyond the end of the line.  However, if
@var{pre-form} returns a buffer position that is greater than the
position of point after @var{pre-form} is evaluated, then the position
returned by @var{pre-form} is used as the limit of the search instead.
It is generally a bad idea to return a position greater than the end
of the line; in other words, the @var{anchored-matcher} search should
not span lines.

For example,

@smallexample
;; @r{Highlight occurrences of the word @samp{item} following}
;; @r{an occurrence of the word @samp{anchor} (on the same line)}
;; @r{in the value of @code{item-face}.}
("\\<anchor\\>" "\\<item\\>" nil nil (0 item-face))
@end smallexample

Here, @var{pre-form} and @var{post-form} are @code{nil}.  Therefore
searching for @samp{item} starts at the end of the match of
@samp{anchor}, and searching for subsequent instances of @samp{anchor}
resumes from where searching for @samp{item} concluded.

@item (@var{matcher} @var{highlighters}@dots{})
This sort of element specifies several @var{highlighter} lists for a
single @var{matcher}.  A @var{highlighter} list can be of the type
@var{subexp-highlighter} or @var{anchored-highlighter} as described
above.

For example,

@smallexample
;; @r{Highlight occurrences of the word @samp{anchor} in the value}
;; @r{of @code{anchor-face}, and subsequent occurrences of the word}
;; @r{@samp{item} (on the same line) in the value of @code{item-face}.}
("\\<anchor\\>" (0 anchor-face)
                ("\\<item\\>" nil nil (0 item-face)))
@end smallexample

@item (eval . @var{form})
Here @var{form} is an expression to be evaluated the first time
this value of @code{font-lock-keywords} is used in a buffer.
Its value should have one of the forms described in this table.
@end table

@strong{Warning:} Do not design an element of @code{font-lock-keywords}
to match text which spans lines; this does not work reliably.  While
@code{font-lock-fontify-buffer} handles multi-line patterns correctly,
updating when you edit the buffer does not, since it considers text one
line at a time.  If you have patterns that typically only span one
line but can occasionally span two or three, such as
@samp{<title>...</title>}, you can ask Font Lock to be more careful by
setting @code{font-lock-multiline} to @code{t}.  But it still will not
work in all cases.

@node Other Font Lock Variables
@subsection Other Font Lock Variables

  This section describes additional variables that a major mode
can set by means of @code{font-lock-defaults}.

@defvar font-lock-keywords-only
Non-@code{nil} means Font Lock should not fontify comments or strings
syntactically; it should only fontify based on
@code{font-lock-keywords}.
@end defvar

@ignore
Other variables include those for buffer-specialized fontification functions,
`font-lock-fontify-buffer-function', `font-lock-unfontify-buffer-function',
`font-lock-fontify-region-function', `font-lock-unfontify-region-function',
`font-lock-inhibit-thing-lock' and `font-lock-maximum-size'.
@end ignore

@defvar font-lock-keywords-case-fold-search
Non-@code{nil} means that regular expression matching for the sake of
@code{font-lock-keywords} should be case-insensitive.
@end defvar

@defvar font-lock-syntax-table
This variable specifies the syntax table to use for fontification of
comments and strings.
@end defvar

@defvar font-lock-beginning-of-syntax-function
If this variable is non-@code{nil}, it should be a function to move
point back to a position that is syntactically at ``top level'' and
outside of strings or comments.  Font Lock uses this when necessary
to get the right results for syntactic fontification.

This function is called with no arguments.  It should leave point at the
beginning of any enclosing syntactic block.  Typical values are
@code{beginning-of-line} (i.e., the start of the line is known to be
outside a syntactic block), or @code{beginning-of-defun} for programming
modes or @code{backward-paragraph} for textual modes (i.e., the
mode-dependent function is known to move outside a syntactic block).

If the value is @code{nil}, the beginning of the buffer is used as a
position outside of a syntactic block.  This cannot be wrong, but it can
be slow.
@end defvar

@defvar font-lock-mark-block-function
If this variable is non-@code{nil}, it should be a function that is
called with no arguments, to choose an enclosing range of text for
refontification for the command @kbd{M-o M-o}
(@code{font-lock-fontify-block}).

The function should report its choice by placing the region around it.
A good choice is a range of text large enough to give proper results,
but not too large so that refontification becomes slow.  Typical values
are @code{mark-defun} for programming modes or @code{mark-paragraph} for
textual modes.
@end defvar

@defvar font-lock-extra-managed-props
Additional properties (other than @code{font-lock-face}) that are
being managed by Font Lock mode.  Font Lock mode normally manages only
the @code{font-lock-face} property; if you want it to manage others as
well, you must specify them in a @var{facespec} in
@code{font-lock-keywords} as well as adding them to this list.
@end defvar

@defvar font-lock-syntactic-face-function
A function to determine which face to use for a given syntactic
element (a string or a comment).  The function is called with one
argument, the parse state at point returned by
@code{parse-partial-sexp}, and should return a face.  The default
value returns @code{font-lock-comment-face} for comments and
@code{font-lock-string-face} for strings.

This can be used to highlighting different kinds of strings or
comments differently.  It is also sometimes abused together with
@code{font-lock-syntactic-keywords} to highlight elements that span
multiple lines, but this is too obscure to document in this manual.
@end defvar

@defvar font-lock-lines-before
This variable specifies the number of extra lines to consider when
refontifying the buffer after each text change.  Font lock begins
refontifying from that number of lines before the changed region.  The
default is 1, but using a larger value can be useful for coping with
multi-line patterns.
@end defvar

@node Levels of Font Lock
@subsection Levels of Font Lock

  Many major modes offer three different levels of fontification.  You
can define multiple levels by using a list of symbols for @var{keywords}
in @code{font-lock-defaults}.  Each symbol specifies one level of
fontification; it is up to the user to choose one of these levels.  The
chosen level's symbol value is used to initialize
@code{font-lock-keywords}.

  Here are the conventions for how to define the levels of
fontification:

@itemize @bullet
@item
Level 1: highlight function declarations, file directives (such as include or
import directives), strings and comments.  The idea is speed, so only
the most important and top-level components are fontified.

@item
Level 2: in addition to level 1, highlight all language keywords,
including type names that act like keywords, as well as named constant
values.  The idea is that all keywords (either syntactic or semantic)
should be fontified appropriately.

@item
Level 3: in addition to level 2, highlight the symbols being defined in
function and variable declarations, and all builtin function names,
wherever they appear.
@end itemize

@node Precalculated Fontification
@subsection Precalculated Fontification

  In addition to using @code{font-lock-defaults} for search-based
fontification, you may use the special character property
@code{font-lock-face} (@pxref{Special Properties}).  This property
acts just like the explicit @code{face} property, but its activation
is toggled when the user calls @kbd{M-x font-lock-mode}.  Using
@code{font-lock-face} is especially convenient for special modes
which construct their text programmatically, such as
@code{list-buffers} and @code{occur}.

If your mode does not use any of the other machinery of Font Lock
(i.e. it only uses the @code{font-lock-face} property), you can tell
Emacs not to load all of font-lock.el (unless it's already loaded), by
setting the variable @code{font-lock-core-only} to non-@code{nil} as
part of the @code{font-lock-defaults} settings.  Here is the canonical
way to do this:

@example
(set (make-local-variable 'font-lock-defaults)
     '(nil t nil nil nil (font-lock-core-only . t)))
@end example

@node Faces for Font Lock
@subsection Faces for Font Lock

  You can make Font Lock mode use any face, but several faces are
defined specifically for Font Lock mode.  Each of these symbols is both
a face name, and a variable whose default value is the symbol itself.
Thus, the default value of @code{font-lock-comment-face} is
@code{font-lock-comment-face}.  This means you can write
@code{font-lock-comment-face} in a context such as
@code{font-lock-keywords} where a face-name-valued expression is used.

@table @code
@item font-lock-comment-face
@vindex font-lock-comment-face
Used (typically) for comments.

@item font-lock-doc-face
@vindex font-lock-doc-face
Used (typically) for documentation strings in the code.

@item font-lock-string-face
@vindex font-lock-string-face
Used (typically) for string constants.

@item font-lock-keyword-face
@vindex font-lock-keyword-face
Used (typically) for keywords---names that have special syntactic
significance, like @code{for} and @code{if} in C.

@item font-lock-builtin-face
@vindex font-lock-builtin-face
Used (typically) for built-in function names.

@item font-lock-function-name-face
@vindex font-lock-function-name-face
Used (typically) for the name of a function being defined or declared,
in a function definition or declaration.

@item font-lock-variable-name-face
@vindex font-lock-variable-name-face
Used (typically) for the name of a variable being defined or declared,
in a variable definition or declaration.

@item font-lock-type-face
@vindex font-lock-type-face
Used (typically) for names of user-defined data types,
where they are defined and where they are used.

@item font-lock-constant-face
@vindex font-lock-constant-face
Used (typically) for constant names.

@item font-lock-preprocessor-face
@vindex font-lock-preprocessor-face
Used (typically) for preprocessor commands.

@item font-lock-warning-face
@vindex font-lock-warning-face
Used (typically) for constructs that are peculiar, or that greatly
change the meaning of other text.  For example, this is used for
@samp{;;;###autoload} cookies in Emacs Lisp, and for @code{#error}
directives in C.
@end table

@node Syntactic Font Lock
@subsection Syntactic Font Lock

  Font Lock mode can be used to update @code{syntax-table} properties
automatically.  This is useful in languages for which a single syntax
table by itself is not sufficient.

@defvar font-lock-syntactic-keywords
This variable enables and controls syntactic Font Lock.  It is
normally set via @code{font-lock-defaults}.  Its value should be a
list of elements of this form:

@example
(@var{matcher} @var{subexp} @var{syntax} @var{override} @var{laxmatch})
@end example

The parts of this element have the same meanings as in the corresponding
sort of element of @code{font-lock-keywords},

@example
(@var{matcher} @var{subexp} @var{facename} @var{override} @var{laxmatch})
@end example

However, instead of specifying the value @var{facename} to use for the
@code{face} property, it specifies the value @var{syntax} to use for
the @code{syntax-table} property.  Here, @var{syntax} can be a string
(as taken by @code{modify-syntax-entry}), a syntax table, a cons cell
(as returned by @code{string-to-syntax}), or an expression whose value
is one of those two types.  @var{override} cannot be @code{prepend} or
@code{append}.

For example, an element of the form:

@example
("\\$\\(#\\)" 1 ".")
@end example

highlights syntactically a hash character when following a dollar
character, with a SYNTAX of @code{"."} (meaning punctuation syntax).
Assuming that the buffer syntax table specifies hash characters to
have comment start syntax, the element will only highlight hash
characters that do not follow dollar characters as comments
syntactically.

An element of the form:

@example
 ("\\('\\).\\('\\)"
  (1 "\"")
  (2 "\""))
@end example

highlights syntactically both single quotes which surround a single
character, with a SYNTAX of @code{"\""} (meaning string quote syntax).
Assuming that the buffer syntax table does not specify single quotes
to have quote syntax, the element will only highlight single quotes of
the form @samp{'@var{c}'} as strings syntactically.  Other forms, such
as @samp{foo'bar} or @samp{'fubar'}, will not be highlighted as
strings.

@end defvar

@node Desktop Save Mode
@section Desktop Save Mode
@cindex desktop save mode

@dfn{Desktop Save Mode} is a feature to save the state of Emacs from
one session to another.  The user-level commands for using Desktop
Save Mode are described in the GNU Emacs Manual (@pxref{Saving Emacs
Sessions,,, emacs, the GNU Emacs Manual}).  Modes whose buffers visit
a file, don't have to do anything to use this feature.

For buffers not visiting a file to have their state saved, the major
mode must bind the buffer local variable @code{desktop-save-buffer} to
a non-@code{nil} value.

@defvar desktop-save-buffer
If this buffer-local variable is non-@code{nil}, the buffer will have
its state saved in the desktop file at desktop save.  If the value is
a function, it is called at desktop save with argument
@var{desktop-dirname}, and its value is saved in the desktop file along
with the state of the buffer for which it was called.  When file names
are returned as part of the auxiliary information, they should be
formatted using the call

@example
(desktop-file-name @var{file-name} @var{desktop-dirname})
@end example

@end defvar

For buffers not visiting a file to be restored, the major mode must
define a function to do the job, and that function must be listed in
the alist @code{desktop-buffer-mode-handlers}.

@defvar desktop-buffer-mode-handlers
Alist with elements

@example
(@var{major-mode} . @var{restore-buffer-function})
@end example

The function @var{restore-buffer-function} will be called with
argument list

@example
(@var{buffer-file-name} @var{buffer-name} @var{desktop-buffer-misc})
@end example

and it should return the restored buffer.
Here @var{desktop-buffer-misc} is the value returned by the function
optionally bound to @code{desktop-save-buffer}.

@end defvar

@node Hooks
@section Hooks
@cindex hooks

  A @dfn{hook} is a variable where you can store a function or functions
to be called on a particular occasion by an existing program.  Emacs
provides hooks for the sake of customization.  Most often, hooks are set
up in the init file (@pxref{Init File}), but Lisp programs can set them also.
@xref{Standard Hooks}, for a list of standard hook variables.

@cindex normal hook
  Most of the hooks in Emacs are @dfn{normal hooks}.  These variables
contain lists of functions to be called with no arguments.  When the
hook name ends in @samp{-hook}, that tells you it is normal.  We try to
make all hooks normal, as much as possible, so that you can use them in
a uniform way.

  Every major mode function is supposed to run a normal hook called the
@dfn{mode hook} as the last step of initialization.  This makes it easy
for a user to customize the behavior of the mode, by overriding the
buffer-local variable assignments already made by the mode.  But hooks
are used in other contexts too.  For example, the hook
@code{suspend-hook} runs just before Emacs suspends itself
(@pxref{Suspending Emacs}).

  The recommended way to add a hook function to a normal hook is by
calling @code{add-hook} (see below).  The hook functions may be any of
the valid kinds of functions that @code{funcall} accepts (@pxref{What
Is a Function}).  Most normal hook variables are initially void;
@code{add-hook} knows how to deal with this.  You can add hooks either
globally or buffer-locally with @code{add-hook}.

@cindex abnormal hook
  If the hook variable's name does not end with @samp{-hook}, that
indicates it is probably an @dfn{abnormal hook}.  Then you should look at its
documentation to see how to use the hook properly.

  If the variable's name ends in @samp{-functions} or @samp{-hooks},
then the value is a list of functions, but it is abnormal in that either
these functions are called with arguments or their values are used in
some way.  You can use @code{add-hook} to add a function to the list,
but you must take care in writing the function.  (A few of these
variables, notably those ending in @samp{-hooks}, are actually
normal hooks which were named before we established the convention of
using @samp{-hook} for them.)

  If the variable's name ends in @samp{-function}, then its value
is just a single function, not a list of functions.

  Here's an example that uses a mode hook to turn on Auto Fill mode when
in Lisp Interaction mode:

@example
(add-hook 'lisp-interaction-mode-hook 'turn-on-auto-fill)
@end example

  At the appropriate time, Emacs uses the @code{run-hooks} function to
run particular hooks.  This function calls the hook functions that have
been added with @code{add-hook}.

@defun run-hooks &rest hookvars
This function takes one or more normal hook variable names as
arguments, and runs each hook in turn.  Each argument should be a
symbol that is a normal hook variable.  These arguments are processed
in the order specified.

If a hook variable has a non-@code{nil} value, that value may be a
function or a list of functions.  (The former option is considered
obsolete.)  If the value is a function (either a lambda expression or
a symbol with a function definition), it is called.  If it is a list
that isn't a function, its elements are called, consecutively.  All
the hook functions are called with no arguments.
@end defun

@defun run-hook-with-args hook &rest args
This function is the way to run an abnormal hook and always call all
of the hook functions.  It calls each of the hook functions one by
one, passing each of them the arguments @var{args}.
@end defun

@defun run-hook-with-args-until-failure hook &rest args
This function is the way to run an abnormal hook until one of the hook
functions fails.  It calls each of the hook functions, passing each of
them the arguments @var{args}, until some hook function returns
@code{nil}.  It then stops and returns @code{nil}.  If none of the
hook functions return @code{nil}, it returns a non-@code{nil} value.
@end defun

@defun run-hook-with-args-until-success hook &rest args
This function is the way to run an abnormal hook until a hook function
succeeds.  It calls each of the hook functions, passing each of them
the arguments @var{args}, until some hook function returns
non-@code{nil}.  Then it stops, and returns whatever was returned by
the last hook function that was called.  If all hook functions return
@code{nil}, it returns @code{nil} as well.
@end defun

@defun add-hook hook function &optional append local
This function is the handy way to add function @var{function} to hook
variable @var{hook}.  You can use it for abnormal hooks as well as for
normal hooks.  @var{function} can be any Lisp function that can accept
the proper number of arguments for @var{hook}.  For example,

@example
(add-hook 'text-mode-hook 'my-text-hook-function)
@end example

@noindent
adds @code{my-text-hook-function} to the hook called @code{text-mode-hook}.

If @var{function} is already present in @var{hook} (comparing using
@code{equal}), then @code{add-hook} does not add it a second time.

It is best to design your hook functions so that the order in which they
are executed does not matter.  Any dependence on the order is ``asking
for trouble''.  However, the order is predictable: normally,
@var{function} goes at the front of the hook list, so it will be
executed first (barring another @code{add-hook} call).  If the optional
argument @var{append} is non-@code{nil}, the new hook function goes at
the end of the hook list and will be executed last.

If @var{local} is non-@code{nil}, that says to add @var{function} to
the buffer-local hook list instead of to the global hook list.  If
needed, this makes the hook buffer-local and adds @code{t} to the
buffer-local value.  The latter acts as a flag to run the hook
functions in the default value as well as in the local value.
@end defun

@defun remove-hook hook function &optional local
This function removes @var{function} from the hook variable
@var{hook}.  It compares @var{function} with elements of @var{hook}
using @code{equal}, so it works for both symbols and lambda
expressions.

If @var{local} is non-@code{nil}, that says to remove @var{function}
from the buffer-local hook list instead of from the global hook list.
@end defun

@ignore
   arch-tag: 4c7bff41-36e6-4da6-9e7f-9b9289e27c8e
@end ignore