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@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2001,
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@c   2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009  Free Software Foundation, Inc.
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@c See the file elisp.texi for copying conditions.
@setfilename ../../info/modes
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@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}.

* Hooks::              How to use hooks; how to write code that provides hooks.
* 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.
@end menu

@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.  By
convention, whenever 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 one of the last steps 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.  Most minor mode functions also run a mode hook at the end.
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}.  That means the hook
functions are called with arguments, or their return values are used
in some way.  The hook's documentation says how the functions are
called.  You can use @code{add-hook} to add a function to an abnormal
hook, but you must write the function to follow the hook's calling

  By convention, abnormal hook names end in @samp{-functions} or
@samp{-hooks}.  If the variable's name ends in @samp{-function}, then
its value is just a single function, not a list of functions.

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* Running Hooks::      How to run a hook.
* Setting Hooks::      How to put functions on a hook, or remove them.
@end menu
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@node Running Hooks
@subsection Running Hooks
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  At the appropriate times, Emacs uses the @code{run-hooks} function
and the other functions below to run particular hooks.
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@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 should be a
list of functions.  @code{run-hooks} calls all the functions, one by
one, with no arguments.

The hook variable's value can also be a single function---either a
lambda expression or a symbol with a function definition---which
@code{run-hooks} calls.  But this usage is obsolete.
@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

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@node Setting Hooks
@subsection Setting Hooks

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

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

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@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,

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

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.

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If @var{function} has a non-@code{nil} property
@code{permanent-local-hook}, then @code{kill-all-local-variables} (or
changing major modes) won't delete it from the hook variable's local

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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,
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@var{function} goes at the front of the hook list, so it will be
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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.
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@code{add-hook} can handle the cases where @var{hook} is void or its
value is a single function; it sets or changes the value to a list of

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

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

@node Major Modes
@section Major Modes
@cindex major 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.

* Major Mode Basics::
* Major Mode Conventions::  Coding conventions for keymaps, etc.
* 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
* Generic Modes::           Defining a simple major mode that supports
                              comment syntax and Font Lock mode.
* Mode Hooks::              Hooks run at the end of major mode functions.
* Example Major Modes::     Text mode and Lisp modes.
@end menu

@node Major Mode Basics
@subsection Major Mode Basics
@cindex Fundamental mode

  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
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definition and alter the copy---or use the @code{define-derived-mode}
macro to define a @dfn{derived mode} (@pxref{Derived Modes}).  For
example, Rmail Edit mode 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.
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  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.

  For a very simple programming language major mode that handles
comments and fontification, you can use @code{define-generic-mode}.
@xref{Generic Modes}.

  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

  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.

@node Major Mode Conventions
@subsection Major Mode Conventions
@cindex major mode conventions
@cindex conventions for writing major modes

  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.  (Fundamental mode is an exception to many
of these conventions, because its definition is to present the global
state of Emacs.)

  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
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.

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

The major mode command should start by calling
@code{kill-all-local-variables}.  This runs the normal hook
@code{change-major-mode-hook}, then gets rid of the buffer-local
variables of the major mode previously in effect.  @xref{Creating

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.

The major mode command should set the variable @code{mode-name} to the
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``pretty'' name of the mode, usually a string (but see @ref{Mode Line
Data}, for other possible forms).  The name of the mode appears
in the mode line.
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@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}.

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.

@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.

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.

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Major modes for editing text should not define @key{RET} to do
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anything other than insert a newline.  However, it is ok for
specialized modes for text that users don't directly edit, such as
Dired and Info modes, to redefine @key{RET} to do something entirely

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.

@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

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}.

@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{Defining Abbrevs}.

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}).

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 two 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}).

The mode can specify a local value for
@code{eldoc-documentation-function} to tell ElDoc mode how to handle
this mode.

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.)

@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.

@cindex mode hook
@cindex major mode hook
Each major mode should have a normal @dfn{mode hook} named
@code{@var{modename}-mode-hook}.  The very last thing the major mode command
should do is to call @code{run-mode-hooks}.  This runs the mode hook,
and then runs the normal hook @code{after-change-major-mode-hook}.
@xref{Mode Hooks}.

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
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recommended way to define one is to use the @code{define-derived-mode}
macro, but this is not required.  Such a mode should call the parent
mode command inside a @code{delay-mode-hooks} form.  (Using
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@code{define-derived-mode} does this automatically.)  @xref{Derived
Modes}, and @ref{Mode Hooks}.

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}).

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}
(put 'funny-mode 'mode-class 'special)
@end example

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

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The @code{define-derived-mode} macro automatically marks the derived
mode as special if the parent mode is special.  The special mode
@code{special-mode} provides a convenient parent for other special
modes to inherit from; it sets @code{buffer-read-only} to @code{t},
and does nothing else.

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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 (@pxref{Auto Major Mode}).  If you
define the mode command to autoload, you should add this element in
the same file that calls @code{autoload}.  If you use an autoload
cookie for the mode command, you can also use an autoload cookie for
the form that adds the element (@pxref{autoload cookie}).  If you do
not autoload the mode command, it is sufficient to add the element in
the file that contains the mode definition.

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}).

@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 Auto Major Mode
@subsection How Emacs Chooses a Major Mode
@cindex major mode, automatic selection

  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}
(see below), then it runs @code{hack-local-variables} to parse, and
bind or evaluate as appropriate, the file's local variables
(@pxref{File 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 local variables in the @samp{-*-}
line or at the end of the file.  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 file local variables.

If @code{normal-mode} processes the local variables list and this list
specifies a major mode, that mode overrides any mode chosen by
@code{set-auto-mode}.  If neither @code{set-auto-mode} nor
@code{hack-local-variables} specify a major mode, the buffer stays in
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the major mode determined by the default value of @code{major-mode}
(see below).
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@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 &optional keep-mode-if-same
@cindex visited file mode
  This function selects the major mode that is appropriate for the
current buffer.  It bases its decision (in order of precedence) on
the @w{@samp{-*-}} line, on the @w{@samp{#!}} line (using
@code{interpreter-mode-alist}), on the text at the beginning of the
buffer (using @code{magic-mode-alist}), and finally on the visited
file name (using @code{auto-mode-alist}).  @xref{Choosing Modes, , How
Major Modes are Chosen, emacs, The GNU Emacs Manual}.  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.
If @code{enable-local-variables} is @code{nil}, @code{set-auto-mode}
does not check the @w{@samp{-*-}} line for a mode tag either.

If @var{keep-mode-if-same} is non-@code{nil}, this function does not
call the mode command if the buffer is already in the proper major
mode.  For instance, @code{set-visited-file-name} sets this to
@code{t} to avoid killing buffer local variables that the user may
have set.
@end defun

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@defopt major-mode
The buffer-local value of this variable holds the major mode
currently active.  The default value of this variable holds the
default major mode for new buffers.  The standard default value is
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If the default value of @code{major-mode} is @code{nil}, Emacs uses
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the (previously) current buffer's major mode as the default 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
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This function sets the major mode of @var{buffer} to the default value of
@code{major-mode}; if that is @code{nil}, it uses the
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current buffer's major mode (if that is suitable).  As an exception,
if @var{buffer}'s name is @samp{*scratch*}, it sets the mode to

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

@defopt 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 defopt

@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 an alist with
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}.
@end defvar

@defvar magic-mode-alist
This variable's value is an alist with elements of the form
@code{(@var{regexp} .  @var{function})}, where @var{regexp} is a
regular expression and @var{function} is a function or @code{nil}.
After visiting a file, @code{set-auto-mode} calls @var{function} if
the text at the beginning of the buffer matches @var{regexp} and
@var{function} is non-@code{nil}; if @var{function} is @code{nil},
@code{auto-mode-alist} gets to decide the mode.
@end defvar

@defvar magic-fallback-mode-alist
This works like @code{magic-mode-alist}, except that it is handled
only if @code{auto-mode-alist} does not specify a mode for this file.
@end defvar

@defvar auto-mode-alist
This variable contains an association list of file name patterns
(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,

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

When you visit a file whose expanded file name (@pxref{File Name
Expansion}), with version numbers and backup suffixes removed using
@code{file-name-sans-versions} (@pxref{File Name Components}), 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.)

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

  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 keyword-args@dots{} body@dots{}
This macro defines @var{variant} as a major mode command, using
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@var{name} as the string form of the mode name.  @var{variant} and
@var{parent} should be unquoted symbols.

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

@itemize @bullet
The new mode has its own sparse keymap, named
@code{@var{variant}-map}.  @code{define-derived-mode}
makes the parent mode's keymap the parent of the new map, unless
@code{@var{variant}-map} is already set and already has a parent.

The new mode has its own syntax table, kept in the variable
@code{@var{variant}-syntax-table}, unless you override this using the
@code{:syntax-table} keyword (see below).  @code{define-derived-mode}
makes the parent mode's syntax-table the parent of
@code{@var{variant}-syntax-table}, unless the latter is already set
and already has a parent different from the standard syntax table.

The new mode has its own abbrev table, kept in the variable
@code{@var{variant}-abbrev-table}, unless you override this using the
@code{:abbrev-table} keyword (see below).

The new mode has its own mode hook, @code{@var{variant}-hook}.  It
runs this hook, after running the hooks of its ancestor modes, with
@code{run-mode-hooks}, as the last thing it does. @xref{Mode Hooks}.
@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 the mode hooks.

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If @var{parent} has a non-@code{nil} @code{mode-class} symbol
property, then @code{define-derived-mode} sets the @code{mode-class}
property of @var{variant} to the same value.  This ensures, for
example, that if @var{parent} is a special mode, then @var{variant} is
also a special mode (@pxref{Major Mode Conventions}).

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You can also specify @code{nil} for @var{parent}.  This gives the new
mode no parent.  Then @code{define-derived-mode} behaves as described
above, but, of course, omits all actions connected with @var{parent}.

The argument @var{docstring} specifies the documentation string for
the new mode.  @code{define-derived-mode} adds some general
information about the mode's hook, followed by the mode's keymap, at
the end of this docstring.  If you omit @var{docstring},
@code{define-derived-mode} generates a documentation string.

The @var{keyword-args} are pairs of keywords and values.  The values
are evaluated.  The following keywords are currently supported:

@table @code
@item :syntax-table
You can use this to explicitly specify a syntax table for the new
mode.  If you specify a @code{nil} value, the new mode uses the same
syntax table as @var{parent}, or the standard syntax table if
@var{parent} is @code{nil}.  (Note that this does @emph{not} follow
the convention used for non-keyword arguments that a @code{nil} value
is equivalent with not specifying the argument.)

@item :abbrev-table
You can use this to explicitly specify an abbrev table for the new
mode.  If you specify a @code{nil} value, the new mode uses the same
abbrev table as @var{parent}, or @code{fundamental-mode-abbrev-table}
if @var{parent} is @code{nil}.  (Again, a @code{nil} value is
@emph{not} equivalent to not specifying this keyword.)

@item :group
If this is specified, the value should be the customization group for
this mode.  (Not all major modes have one.)  Only the (still
experimental and unadvertised) command @code{customize-mode} currently
uses this.  @code{define-derived-mode} does @emph{not} automatically
define the specified customization group.
@end table

Here is a hypothetical example:

(define-derived-mode hypertext-mode
  text-mode "Hypertext"
  "Major mode for hypertext.
  (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 Generic Modes
@subsection Generic Modes
@cindex generic mode

  @dfn{Generic modes} are simple major modes with basic support for
comment syntax and Font Lock mode.  To define a generic mode, use the
macro @code{define-generic-mode}.  See the file @file{generic-x.el}
for some examples of the use of @code{define-generic-mode}.

@defmac define-generic-mode mode comment-list keyword-list font-lock-list auto-mode-list function-list &optional docstring
This macro defines a generic mode command named @var{mode} (a symbol,
not quoted).  The optional argument @var{docstring} is the
documentation for the mode command.  If you do not supply it,
@code{define-generic-mode} generates one by default.

The argument @var{comment-list} is a list in which each element is
either a character, a string of one or two characters, or a cons cell.
A character or a string is set up in the mode's syntax table as a
``comment starter.''  If the entry is a cons cell, the @sc{car} is set
up as a ``comment starter'' and the @sc{cdr} as a ``comment ender.''
(Use @code{nil} for the latter if you want comments to end at the end
of the line.)  Note that the syntax table mechanism has limitations
about what comment starters and enders are actually possible.
@xref{Syntax Tables}.

The argument @var{keyword-list} is a list of keywords to highlight
with @code{font-lock-keyword-face}.  Each keyword should be a string.
Meanwhile, @var{font-lock-list} is a list of additional expressions to
highlight.  Each element of this list should have the same form as an
element of @code{font-lock-keywords}.  @xref{Search-based

The argument @var{auto-mode-list} is a list of regular expressions to
add to the variable @code{auto-mode-alist}.  They are added by the execution
of the @code{define-generic-mode} form, not by expanding the macro call.

Finally, @var{function-list} is a list of functions for the mode
command to call for additional setup.  It calls these functions just
before it runs the mode hook variable @code{@var{mode}-hook}.
@end defmac

@node Mode Hooks
@subsection Mode Hooks

  Every major mode function should finish by running its mode hook and
the mode-independent normal hook @code{after-change-major-mode-hook}.
It does this by calling @code{run-mode-hooks}.  If the major mode is a
derived mode, that is if it calls another major mode (the parent mode)
in its body, it should do this inside @code{delay-mode-hooks} so that
the parent won't run these hooks itself.  Instead, the derived mode's
call to @code{run-mode-hooks} runs the parent's mode hook too.
@xref{Major Mode Conventions}.

  Emacs versions before Emacs 22 did not have @code{delay-mode-hooks}.
When user-implemented major modes have not been updated to use it,
they won't entirely follow these conventions: they may run the
parent's mode hook too early, or fail to run
@code{after-change-major-mode-hook}.  If you encounter such a major
mode, please correct it to follow these conventions.

  When you defined a major mode using @code{define-derived-mode}, it
automatically makes sure these conventions are followed.  If you
define a major mode ``by hand,'' not using @code{define-derived-mode},
use the following functions to handle these conventions automatically.

@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 it also runs

When this function is called during the execution of a
@code{delay-mode-hooks} form, it does not run the hooks immediately.
Instead, it arranges for the next call to @code{run-mode-hooks} to run
@end defun

@defmac delay-mode-hooks body@dots{}
When one major mode command calls another, it should do so inside of

This macro executes @var{body}, but tells all @code{run-mode-hooks}
calls during the execution of @var{body} to delay running their hooks.
The hooks will actually run during the next call to
@code{run-mode-hooks} after the end of the @code{delay-mode-hooks}
@end defmac

@defvar after-change-major-mode-hook
This is a normal hook run by @code{run-mode-hooks}.  It is run at the
very end of every properly-written major mode function.
@end defvar

@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:

;; @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)
    ;; Add `p' so M-c on `hello' leads to `Hello', not `hello'.
    (modify-syntax-entry ?' "w p" st)
  "Syntax table used while in `text-mode'.")
@end group

;; @r{Create the keymap for this mode.}
(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)
  "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:

(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').
Turning on Text mode runs the normal hook `text-mode-hook'."
@end 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)
  (set (make-local-variable 'indent-line-function) 'indent-relative))
@end group
@end smallexample

(The last line is redundant nowadays, since @code{indent-relative} is
the default value, and we'll delete it in a future version.)

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

;; @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

(defun text-mode ()
  "Major mode for editing text intended for humans to read...
 Special commands: \\@{text-mode-map@}
@end group
Turning on text-mode runs the hook `text-mode-hook'."
  (use-local-map text-mode-map)
@end group
  (setq local-abbrev-table text-mode-abbrev-table)
  (set-syntax-table text-mode-syntax-table)
@end 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
  (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
;; @r{Create mode-specific table variables.}
(defvar lisp-mode-syntax-table nil "")
(defvar lisp-mode-abbrev-table nil "")
@end group

(defvar emacs-lisp-mode-syntax-table
  (let ((table (make-syntax-table)))
    (let ((i 0))
@end 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)
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        (modify-syntax-entry i "_   " table)
        (setq i (1+ i)))
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      ;; @r{@dots{} similar code follows for other character ranges.}
@end 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
      ;; @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
      ;; @r{@dots{}likewise for many other characters@dots{}}
      (modify-syntax-entry ?\( "()  " table)
      (modify-syntax-entry ?\) ")(  " table)
      (modify-syntax-entry ?\[ "(]  " table)
      (modify-syntax-entry ?\] ")[  " table))
@end group
;; @r{Create an abbrev table for lisp-mode.}
(define-abbrev-table 'lisp-mode-abbrev-table ())
@end group
@end smallexample

  The three modes for Lisp share much of their code.  For instance,
each calls the following function to set various variables:

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

  In Lisp and most programming languages, we want the paragraph
commands to treat only blank lines as paragraph separators.  And the
modes should understand the Lisp conventions for comments.  The rest of
@code{lisp-mode-variables} sets this up:

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

(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"
@end group
@end smallexample

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

(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.

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

Entry to this mode calls the value of `lisp-mode-hook'
if that value is non-nil."
@end 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
  (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 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

* 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, the use of a hook at the end of
the initialization function, 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
@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

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.

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

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.

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

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

(@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:

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

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

(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 cookie}),
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:


(defcustom msb-mode nil
  "Toggle msb-mode.
Setting this variable directly does not take effect;
use either \\[customize] or the function `msb-mode'."
  :set 'custom-set-minor-mode
  :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{Definition of minor-mode-map-alist}.

@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 one of @kbd{.,/?`'"[]\|~!#$%^&*()-_+=}.  (The other
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@dots{} body@dots{}
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}.  Except in unusual circumstances (see below), this
value must be @code{nil}.

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:

(@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}.
@strong{Warning:} don't use this default group name unless you have
written a @code{defgroup} to define that group properly.  @xref{Group

@item :global @var{global}
If non-@code{nil}, this specifies that the minor mode should be global
rather than buffer-local.  It defaults to @code{nil}.

One of the effects of making a minor mode global is that the
@var{mode} variable becomes a customization variable.  Toggling it
through the Custom interface turns the mode on and off, and its value
can be saved for future Emacs sessions (@pxref{Saving
Customizations,,, emacs, The GNU Emacs Manual}.  For the saved
variable to work, you should ensure that the @code{define-minor-mode}
form is evaluated each time Emacs starts; for packages that are not
part of Emacs, the easiest way to do this is to specify a
@code{:require} keyword.

@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 directly to the
@code{defcustom} generated for the variable @var{mode}.

The command named @var{mode} first performs the standard actions such
as setting the variable named @var{mode} and then executes the
@var{body} forms, if any.  It finishes by running the mode hook
variable @code{@var{mode}-hook}.
@end defmac

  The initial value must be @code{nil} except in cases where (1) the
mode is preloaded in Emacs, or (2) it is painless for loading to
enable the mode even though the user did not request it.  For
instance, if the mode has no effect unless something else is enabled,
and will always be loaded by that time, enabling it by default is
harmless.  But these are unusual circumstances.  Normally, the
initial value must be @code{nil}.

@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}:

(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.
 ;; The indicator for the mode line.
 " Hungry"
 ;; The minor mode bindings.
 '(("\C-\^?" . hungry-electric-delete))
 :group 'hunger)
@end smallexample

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:

(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.
 :init-value nil
 ;; The indicator for the mode line.
 :lighter " Hungry"
 ;; The minor mode bindings.
 '(("\C-\^?" . hungry-electric-delete)
    . (lambda ()
        (hungry-electric-delete t))))
 :group 'hunger)
@end smallexample

@defmac define-globalized-minor-mode global-mode mode turn-on keyword-args@dots{}
This defines a global toggle named @var{global-mode} whose meaning is
to enable or disable the buffer-local minor mode @var{mode} in all
buffers.  To turn on the minor mode in a buffer, it uses the function
@var{turn-on}; to turn off the minor mode, it calls @code{mode} with
@minus{}1 as argument.

Globally enabling the mode also affects buffers subsequently created
by visiting files, and buffers that use a major mode other than
Fundamental mode; but it does not detect the creation of a new buffer
in Fundamental mode.

This defines the customization option @var{global-mode} (@pxref{Customization}),
which can be toggled in the Custom interface to turn the minor mode on
and off.  As with @code{define-minor-mode}, you should ensure that the
@code{define-globalized-minor-mode} form is evaluated each time Emacs
starts, for example by providing a @code{:require} keyword.

Use @code{:group @var{group}} in @var{keyword-args} to specify the
custom group for the mode variable of the global minor mode.
@end defmac

@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

  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.

* Base: Mode Line Basics. Basic ideas of mode line control.
* Data: Mode Line Data.   The data structure that controls the mode line.
* Top: Mode Line Top.     The top level variable, mode-line-format.
* 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 Basics
@subsection Mode Line Basics

  @code{mode-line-format} is a buffer-local variable that holds a
@dfn{mode line construct}, a kind of template, which controls what is
displayed on the mode line of the current buffer.  The value of
@code{header-line-format} specifies the buffer's header line in the
same way.  All windows for the same buffer use the same
@code{mode-line-format} and @code{header-line-format}.

  For efficiency, Emacs does not continuously recompute the mode
line and header line of a window.  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.

@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

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}.

@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 called a
@dfn{mode-line construct}, made up of lists, strings, symbols, and
numbers kept in buffer-local variables.  Each data type has a specific
meaning for the mode-line appearance, as described below.  The same
data structure is used for constructing frame titles (@pxref{Frame
Titles}) and header lines (@pxref{Header Lines}).

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

  Here are the meanings of various data types as mode-line constructs:

@table @code
@cindex percent symbol in mode line
@item @var{string}
A string as a mode-line construct appears verbatim except for
@dfn{@code{%}-constructs} in it.  These stand for substitution of
other data; see @ref{%-Constructs}.

If parts of the string have @code{face} properties, they control
display of the text just as they would text in the buffer.  Any
characters which have no @code{face} properties are displayed, by
default, in the face @code{mode-line} or @code{mode-line-inactive}
(@pxref{Standard Faces,,, emacs, The GNU Emacs Manual}).  The
@code{help-echo} and @code{local-map} properties in @var{string} have
special meanings.  @xref{Properties in Mode}.

@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 text
properties specified in @var{symbol}'s value are ignored.  This
includes the text properties of strings in @var{symbol}'s value, as
well as all @code{:eval} and @code{:propertize} forms in it.  (The
reason for this is security: non-risky variables could be set
automatically from file variables without prompting the user.)

@item (@var{string} @var{rest}@dots{})
@itemx (@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.  Make sure this
evaluation cannot load any files, as doing so could cause infinite

@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, then 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
@var{symbol} has a non-@code{nil} value, the second element,
@var{then}, is processed recursively as a mode-line element.
Otherwise, 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} or void.

@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.  When @var{width} is positive, the result is
space filled on the right if its width is less than @var{width}.  When
@var{width} is negative, the result is truncated on the right to
@minus{}@var{width} columns if its width exceeds @minus{}@var{width}.

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

@node Mode Line Top
@subsection The Top Level of Mode Line Control

  The variable in overall control of the mode line is

@defopt mode-line-format
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The value of this variable is a mode-line construct that controls the
contents of the mode-line.  It is always buffer-local in all buffers.

If you set this variable to @code{nil} in a buffer, that buffer does
not have a mode line.  (A window that is just one line tall never
displays a mode line.)
@end defopt
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  The default value of @code{mode-line-format} is designed to use the
values of other 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}).  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.

  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.

  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

(setq mode-line-format
  (list "-"
@end 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
   "   "
   "   %[("
   '(:eval (mode-line-mode-name))
   '(which-func-mode ("" which-func-format "--"))
   '(line-number-mode "L%l--")
   '(column-number-mode "C%c--")
   '(-3 "%p")
@end group
@end example

(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}'s value were changed to use them.  However,
various parts of Emacs set these variables on the understanding that
they will control parts of the mode line; therefore, practically
speaking, it is essential for the mode line 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
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whether the current buffer is modified.  Its default value 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.
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Changing this variable does not force an update of the mode line.
@end defvar

@defvar mode-line-frame-identification
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This variable identifies the current frame.  Its default value
displays @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.
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@end defvar

@defvar mode-line-buffer-identification
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This variable identifies the buffer being displayed in the window.
Its default value displays the buffer name, padded with spaces to at
least 12 columns.
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@end defvar

@defopt mode-line-position
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This variable indicates the position in the buffer.  Its default value
displays the buffer percentage and, optionally, the buffer size, the
line number and the column number.
@end defopt
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@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

@defopt mode-line-modes
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This variable displays the buffer's major and minor modes.  Its
default value also displays the recursive editing level, information
on the process status, and whether narrowing is in effect.
@end defopt
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  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
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buffer's major mode.  Each major mode should set this variable so that
the mode name will appear in the mode line.  The value does not have
to be a string, but can use any of the data types valid in a mode-line
construct (@pxref{Mode Line Data}).  To compute the string that will
identify the mode name in the mode line, use @code{format-mode-line}
(@pxref{Emulating Mode Line}).
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@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
@anchor{Definition of 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:

(@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

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

@end group
 "   "
 (vc-mode vc-mode)
 "   "
 (which-func-mode ("" which-func-format "--"))
 (global-mode-string ("--" global-mode-string))
@end group
@end example

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

  Strings used as mode-line constructs can use certain
@code{%}-constructs to substitute various kinds of data.  Here is a
list of the defined @code{%}-constructs, and what they mean.  In any
construct except @samp{%%}, you can add a decimal integer after the
@samp{%} to specify a minimum field width.  If the width is less, the
field is padded with spaces to the right.

@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 %e
When Emacs is nearly out of memory for Lisp objects, a brief message
saying so.  Otherwise, this is empty.

@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{Basic 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

@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 %z
The mnemonics of keyboard, terminal, and buffer coding systems.

@item %Z
Like @samp{%z}, but including the end-of-line format.

@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

@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}.
@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 associates 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

Put a string with a text property directly into the mode-line data

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.

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

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
@end enumerate

  You can use the @code{local-map} property to specify a keymap.  This
keymap only takes real effect for mouse clicks; binding character keys
and function keys to it has no effect,