* term.c: Fix minor fdopen-related file descriptor leaks.

[emacs.git] / doc / emacs / modes.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985-1987, 1993-1995, 1997, 2000-2013 Free Software
@c Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Modes
@chapter Major and Minor Modes

  Emacs contains many @dfn{editing modes} that alter its basic
behavior in useful ways.  These are divided into @dfn{major modes} and
@dfn{minor modes}.

  Major modes provide specialized facilities for working on a
particular file type, such as a C source file (@pxref{Programs}), or a
particular type of non-file buffer, such as a shell buffer
(@pxref{Shell}).  Major modes are mutually exclusive; each buffer has
one and only one major mode at any time.

  Minor modes are optional features which you can turn on or off, not
necessarily specific to a type of file or buffer.  For example, Auto
Fill mode is a minor mode in which @key{SPC} breaks lines between
words as you type (@pxref{Auto Fill}).  Minor modes are independent of
one another, and of the selected major mode.

@menu
* Major Modes::         Text mode vs. Lisp mode vs. C mode...
* Minor Modes::         Each minor mode is a feature you can turn on
                          independently of any others.
* Choosing Modes::      How modes are chosen when visiting files.
@end menu

@node Major Modes
@section Major Modes
@cindex major modes
@cindex mode, major
@kindex TAB @r{(and major modes)}
@kindex DEL @r{(and major modes)}
@kindex C-j @r{(and major modes)}

  Every buffer possesses a major mode, which determines the editing
behavior of Emacs while that buffer is current.  The mode line
normally shows the name of the current major mode, in parentheses
(@pxref{Mode Line}).

  The least specialized major mode is called @dfn{Fundamental mode}.
This mode has no mode-specific redefinitions or variable settings, so
that each Emacs command behaves in its most general manner, and each
user option variable is in its default state.

  For editing text of a specific type that Emacs knows about, such as
Lisp code or English text, you typically use a more specialized major
mode, such as Lisp mode or Text mode.  Most major modes fall into
three major groups.  The first group contains modes for normal text,
either plain or with mark-up.  It includes Text mode, HTML mode, SGML
mode, @TeX{} mode and Outline mode.  The second group contains modes
for specific programming languages.  These include Lisp mode (which
has several variants), C mode, Fortran mode, and others.  The third
group consists of major modes that are not associated directly with
files; they are used in buffers created for specific purposes by
Emacs, such as Dired mode for buffers made by Dired (@pxref{Dired}),
Message mode for buffers made by @kbd{C-x m} (@pxref{Sending Mail}),
and Shell mode for buffers used to communicate with an inferior shell
process (@pxref{Interactive Shell}).

  Usually, the major mode is automatically set by Emacs, when you
first visit a file or create a buffer (@pxref{Choosing Modes}).  You
can explicitly select a new major mode by using an @kbd{M-x} command.
Take the name of the mode and add @code{-mode} to get the name of the
command to select that mode (e.g., @kbd{M-x lisp-mode} enters Lisp mode).

@vindex major-mode
  The value of the buffer-local variable @code{major-mode} is a symbol
with the same name as the major mode command (e.g., @code{lisp-mode}).
This variable is set automatically; you should not change it yourself.

  The default value of @code{major-mode} determines the major mode to
use for files that do not specify a major mode, and for new buffers
created with @kbd{C-x b}.  Normally, this default value is the symbol
@code{fundamental-mode}, which specifies Fundamental mode.  You can
change this default value via the Customization interface (@pxref{Easy
Customization}), or by adding a line like this to your init file
(@pxref{Init File}):

@example
(setq-default major-mode 'text-mode)
@end example

@noindent
If the default value of @code{major-mode} is @code{nil}, the major
mode is taken from the previously current buffer.

  Specialized major modes often change the meanings of certain keys to
do something more suitable for the mode.  For instance, programming
language modes bind @key{TAB} to indent the current line according to
the rules of the language (@pxref{Indentation}).  The keys that are
commonly changed are @key{TAB}, @key{DEL}, and @kbd{C-j}.  Many modes
also define special commands of their own, usually bound in the prefix
key @kbd{C-c}.  Major modes can also alter user options and variables;
for instance, programming language modes typically set a buffer-local
value for the variable @code{comment-start}, which determines how
source code comments are delimited (@pxref{Comments}).

@findex describe-mode
@kindex C-h m
  To view the documentation for the current major mode, including a
list of its key bindings, type @code{C-h m} (@code{describe-mode}).

@cindex mode hook
@vindex text-mode-hook
@vindex prog-mode-hook
  Every major mode, apart from Fundamental mode, defines a @dfn{mode
hook}, a customizable list of Lisp functions to run each time the mode
is enabled in a buffer.  @xref{Hooks}, for more information about
hooks.  Each mode hook is named after its major mode, e.g., Fortran
mode has @code{fortran-mode-hook}.  Furthermore, all text-based major
modes run @code{text-mode-hook}, and all programming language modes
run @code{prog-mode-hook}, prior to running their own mode hooks.
Hook functions can look at the value of the variable @code{major-mode}
to see which mode is actually being entered.

  Mode hooks are commonly used to enable minor modes (@pxref{Minor
Modes}).  For example, you can put the following lines in your init
file to enable Flyspell minor mode in all text-based major modes
(@pxref{Spelling}), and Eldoc minor mode in Emacs Lisp mode
(@pxref{Lisp Doc}):

@example
(add-hook 'text-mode-hook 'flyspell-mode)
(add-hook 'emacs-lisp-mode-hook 'eldoc-mode)
@end example

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

  A minor mode is an optional editing mode that alters the behavior of
Emacs in some well-defined way.  Unlike major modes, any number of
minor modes can be in effect at any time.  Some minor modes are
@dfn{buffer-local}, and can be turned on (enabled) in certain buffers
and off (disabled) in others.  Other minor modes are @dfn{global}:
while enabled, they affect everything you do in the Emacs session, in
all buffers.  Most minor modes are disabled by default, but a few are
enabled by default.

  Most buffer-local minor modes say in the mode line when they are
enabled, just after the major mode indicator.  For example,
@samp{Fill} in the mode line means that Auto Fill mode is enabled.
@xref{Mode Line}.

@cindex mode commands for minor modes
  Like major modes, each minor mode is associated with a @dfn{mode
command}, whose name consists of the mode name followed by
@samp{-mode}.  For instance, the mode command for Auto Fill mode is
@code{auto-fill-mode}.  But unlike a major mode command, which simply
enables the mode, the mode command for a minor mode can either enable
or disable it:

@itemize
@item
If you invoke the mode command directly with no prefix argument
(either via @kbd{M-x}, or by binding it to a key and typing that key;
@pxref{Key Bindings}), that @dfn{toggles} the minor mode.  The minor
mode is turned on if it was off, and turned off if it was on.

@item
If you invoke the mode command with a prefix argument, the minor mode
is unconditionally turned off if that argument is zero or negative;
otherwise, it is unconditionally turned on.

@item
If the mode command is called via Lisp, the minor mode is
unconditionally turned on if the argument is omitted or @code{nil}.
This makes it easy to turn on a minor mode from a major mode's mode
hook (@pxref{Major Modes}).  A non-@code{nil} argument is handled like
an interactive prefix argument, as described above.
@end itemize

  Most minor modes also have a @dfn{mode variable}, with the same name
as the mode command.  Its value is non-@code{nil} if the mode is
enabled, and @code{nil} if it is disabled.  In general, you should not
try to enable or disable the mode by changing the value of the mode
variable directly in Lisp; you should run the mode command instead.
However, setting the mode variable through the Customize interface
(@pxref{Easy Customization}) will always properly enable or disable
the mode, since Customize automatically runs the mode command for you.

  The following is a list of some buffer-local minor modes:

@itemize @bullet
@item
Abbrev mode automatically expands text based on pre-defined
abbreviation definitions.  @xref{Abbrevs}.

@item
Auto Fill mode inserts newlines as you type to prevent lines from
becoming too long.  @xref{Filling}.

@item
Auto Save mode saves the buffer contents periodically to reduce the
amount of work you can lose in case of a crash.  @xref{Auto Save}.

@item
Enriched mode enables editing and saving of formatted text.
@xref{Enriched Text}.

@item
Flyspell mode automatically highlights misspelled words.
@xref{Spelling}.

@item
Font-Lock mode automatically highlights certain textual units found in
programs.  It is enabled globally by default, but you can disable it
in individual buffers.  @xref{Faces}.

@findex linum-mode
@cindex Linum mode
@item
Linum mode displays each line's line number in the window's left margin.

@item
Outline minor mode provides similar facilities to the major mode
called Outline mode.  @xref{Outline Mode}.

@cindex Overwrite mode
@cindex mode, Overwrite
@findex overwrite-mode
@kindex INSERT
@item
Overwrite mode causes ordinary printing characters to replace existing
text instead of shoving it to the right.  For example, if point is in
front of the @samp{B} in @samp{FOOBAR}, then in Overwrite mode typing
a @kbd{G} changes it to @samp{FOOGAR}, instead of producing
@samp{FOOGBAR} as usual.  In Overwrite mode, the command @kbd{C-q}
inserts the next character whatever it may be, even if it is a
digit---this gives you a way to insert a character instead of
replacing an existing character.  The mode command,
@code{overwrite-mode}, is bound to the @key{Insert} key.

@findex binary-overwrite-mode
@item
Binary Overwrite mode is a variant of Overwrite mode for editing
binary files; it treats newlines and tabs like other characters, so
that they overwrite other characters and can be overwritten by them.
In Binary Overwrite mode, digits after @kbd{C-q} specify an octal
character code, as usual.

@item
Visual Line mode performs ``word wrapping'', causing long lines to be
wrapped at word boundaries.  @xref{Visual Line Mode}.
@end itemize

@noindent
And here are some useful global minor modes:

@itemize @bullet
@item
Column Number mode enables display of the current column number in the
mode line.  @xref{Mode Line}.

@item
Delete Selection mode causes text insertion to first delete the text
in the region, if the region is active.  @xref{Using Region}.

@item
Icomplete mode displays an indication of available completions when
you are in the minibuffer and completion is active.  @xref{Completion
Options}.

@item
Line Number mode enables display of the current line number in the
mode line.  It is enabled by default.  @xref{Mode Line}.

@item
Menu Bar mode gives each frame a menu bar.  It is enabled by default.
@xref{Menu Bars}.

@item
Scroll Bar mode gives each window a scroll bar.  It is enabled by
default, but the scroll bar is only displayed on graphical terminals.
@xref{Scroll Bars}.

@item
Tool Bar mode gives each frame a tool bar.  It is enabled by default,
but the tool bar is only displayed on graphical terminals.  @xref{Tool
Bars}.

@item
Transient Mark mode highlights the region, and makes many Emacs
commands operate on the region when the mark is active.  It is enabled
by default.  @xref{Mark}.
@end itemize

@node Choosing Modes
@section Choosing File Modes

@cindex choosing a major mode
@cindex choosing a minor mode
@vindex auto-mode-alist
  When you visit a file, Emacs chooses a major mode automatically.
Normally, it makes the choice based on the file name---for example,
files whose names end in @samp{.c} are normally edited in C mode---but
sometimes it chooses the major mode based on special text in the file.
This special text can also be used to enable buffer-local minor modes.

  Here is the exact procedure:

  First, Emacs checks whether the file contains file-local mode
variables.  @xref{File Variables}.  If there is a file-local variable
that specifies a major mode, then Emacs uses that major mode, ignoring
all other criteria.  There are several methods to specify a major mode
using a file-local variable; the simplest is to put the mode name in
the first nonblank line, preceded and followed by @samp{-*-}.  Other
text may appear on the line as well.  For example,

@example
; -*-Lisp-*-
@end example

@noindent
tells Emacs to use Lisp mode.  Note how the semicolon is used to make
Lisp treat this line as a comment.  You could equivalently write

@example
; -*- mode: Lisp;-*-
@end example

@noindent
You can also use file-local variables to specify buffer-local minor
modes, by using @code{eval} specifications.  For example, this first
nonblank line puts the buffer in Lisp mode and enables Auto-Fill mode:

@example
; -*- mode: Lisp; eval: (auto-fill-mode 1); -*-
@end example

@noindent
Note, however, that it is usually inappropriate to enable minor modes
this way, since most minor modes represent individual user
preferences.  If you personally want to use a minor mode for a
particular file type, it is better to enable the minor mode via a
major mode hook (@pxref{Major Modes}).

@vindex interpreter-mode-alist
  Second, if there is no file variable specifying a major mode, Emacs
checks whether the file's contents begin with @samp{#!}.  If so, that
indicates that the file can serve as an executable shell command,
which works by running an interpreter named on the file's first line
(the rest of the file is used as input to the interpreter).
Therefore, Emacs tries to use the interpreter name to choose a mode.
For instance, a file that begins with @samp{#!/usr/bin/perl} is opened
in Perl mode.  The variable @code{interpreter-mode-alist} specifies
the correspondence between interpreter program names and major modes.

  When the first line starts with @samp{#!}, you usually cannot use
the @samp{-*-} feature on the first line, because the system would get
confused when running the interpreter.  So Emacs looks for @samp{-*-}
on the second line in such files as well as on the first line.  The
same is true for man pages which start with the magic string
@samp{'\"} to specify a list of troff preprocessors.

@vindex magic-mode-alist
  Third, Emacs tries to determine the major mode by looking at the
text at the start of the buffer, based on the variable
@code{magic-mode-alist}.  By default, this variable is @code{nil} (an
empty list), so Emacs skips this step; however, you can customize it
in your init file (@pxref{Init File}).  The value should be a list of
elements of the form

@example
(@var{regexp} . @var{mode-function})
@end example

@noindent
where @var{regexp} is a regular expression (@pxref{Regexps}), and
@var{mode-function} is a major mode command.  If the text at the
beginning of the file matches @var{regexp}, Emacs chooses the major
mode specified by @var{mode-function}.

Alternatively, an element of @code{magic-mode-alist} may have the form

@example
(@var{match-function} . @var{mode-function})
@end example

@noindent
where @var{match-function} is a Lisp function that is called at the
beginning of the buffer; if the function returns non-@code{nil}, Emacs
set the major mode with @var{mode-function}.

  Fourth---if Emacs still hasn't found a suitable major mode---it
looks at the file's name.  The correspondence between file names and
major modes is controlled by the variable @code{auto-mode-alist}.  Its
value is a list in which each element has this form,

@example
(@var{regexp} . @var{mode-function})
@end example

@noindent
or this form,

@example
(@var{regexp} @var{mode-function} @var{flag})
@end example

@noindent
For example, one element normally found in the list has the form
@code{(@t{"\\.c\\'"} . c-mode)}, and it is responsible for selecting C
mode for files whose names end in @file{.c}.  (Note that @samp{\\} is
needed in Lisp syntax to include a @samp{\} in the string, which must
be used to suppress the special meaning of @samp{.} in regexps.)  If
the element has the form @code{(@var{regexp} @var{mode-function}
@var{flag})} and @var{flag} is non-@code{nil}, then after calling
@var{mode-function}, Emacs discards the suffix that matched
@var{regexp} and searches the list again for another match.

@vindex auto-mode-case-fold
  On GNU/Linux and other systems with case-sensitive file names, Emacs
performs a case-sensitive search through @code{auto-mode-alist}; if
this search fails, it performs a second case-insensitive search
through the alist.  To suppress the second search, change the variable
@code{auto-mode-case-fold} to @code{nil}.  On systems with
case-insensitive file names, such as Microsoft Windows, Emacs performs
a single case-insensitive search through @code{auto-mode-alist}.

@vindex magic-fallback-mode-alist
  Finally, if Emacs @emph{still} hasn't found a major mode to use, it
compares the text at the start of the buffer to the variable
@code{magic-fallback-mode-alist}.  This variable works like
@code{magic-mode-alist}, described above, except that is consulted
only after @code{auto-mode-alist}.  By default,
@code{magic-fallback-mode-alist} contains forms that check for image
files, HTML/XML/SGML files, and PostScript files.

@findex normal-mode
  If you have changed the major mode of a buffer, you can return to
the major mode Emacs would have chosen automatically, by typing
@kbd{M-x normal-mode}.  This is the same function that
@code{find-file} calls to choose the major mode.  It also processes
the file's @samp{-*-} line or local variables list (if any).
@xref{File Variables}.

@vindex change-major-mode-with-file-name
  The commands @kbd{C-x C-w} and @code{set-visited-file-name} change to
a new major mode if the new file name implies a mode (@pxref{Saving}).
(@kbd{C-x C-s} does this too, if the buffer wasn't visiting a file.)
However, this does not happen if the buffer contents specify a major
mode, and certain ``special'' major modes do not allow the mode to
change.  You can turn off this mode-changing feature by setting
@code{change-major-mode-with-file-name} to @code{nil}.