emacs-gpu/doc/emacs/modes.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985-1987, 1993-1995, 1997, 2000-2011
@c   Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Modes, Indentation, International, Top
@chapter Editing Modes

  Emacs contains many @dfn{editing modes}, each of which alters 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.
@xref{Mode Line}.

  Usually, the major mode is automatically set by Emacs, when you
first visit a file or create a buffer.  @xref{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.  Thus, you can enter Lisp mode with
@kbd{M-x lisp-mode}.

  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.  Such major modes change the
meanings of some keys to become more specifically adapted to the
language being edited.  The ones that are commonly changed are
@key{TAB}, @key{DEL}, and @kbd{C-j}.  The prefix key @kbd{C-c}
normally contains mode-specific commands.  In addition, the commands
which handle comments use the mode to determine how comments are to be
delimited.  Many major modes redefine the syntactical properties of
characters appearing in the buffer.

  The 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 remaining major modes are not
intended for use on users' 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 for
communicating with an inferior shell process (@pxref{Interactive
Shell}).

  Most programming-language major modes specify that only blank lines
separate paragraphs.  This is to make the paragraph commands useful.
(@xref{Paragraphs}.)  They also cause Auto Fill mode to use the
definition of @key{TAB} to indent the new lines it creates.  This is
because most lines in a program are usually indented
(@pxref{Indentation}).

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

  A minor mode is an optional editing modes 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}: they apply only to the current buffer, so you can
enable the mode in certain buffers and not others.  Other minor modes
are @dfn{global}: while enabled, they affect everything you do in the
Emacs session, in all buffers.  Some global minor modes are enabled by
default.

  Most 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}.

  Each minor mode is associated with a command, called the @dfn{mode
command}, which turns it on or off.  The name of this command consists
of the name of the minor mode, followed by @samp{-mode}; for instance,
the mode command for Auto Fill mode is @code{auto-fill-mode}.  Calling
the minor mode command with no prefix argument @dfn{toggles} the mode,
turning it on if it was off, and off if it was on.  A positive
argument always turns the mode on, and a zero or negative argument
always turns it off.  Mode commands are usually invoked with
@kbd{M-x}, but you can bind keys to them if you wish (@pxref{Key
Bindings}).

  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 some minor modes---but
not all---the value of the variable alone determines whether the mode
is active: the mode command works simply by setting the variable, and
changing the value of the variable has the same effect as calling the
mode command.  Because not all minor modes work this way, we recommend
that you avoid changing the mode variables directly; use the mode
commands instead.

  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{Formatted 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.  Its mode command is @code{linum-mode}.

@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

  Here are some useful global minor modes.  Since Line Number mode and
Transient Mark mode can be enabled or disabled just by setting the
value of the minor mode variable, you @emph{can} set them differently
for particular buffers, by explicitly making the corresponding
variable local in those buffers.  @xref{Locals}.

@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.  Alternatively, you could write

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

@noindent
The latter format allows you to specify local variables as well, like
this:

@example
; -*- mode: Lisp; tab-width: 4; -*-
@end example

  If a file variable specifies a buffer-local minor mode, Emacs
enables that minor mode in the buffer.

@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 Lisp function that toggles a major mode.  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 wit @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.

@vindex major-mode
  Once a major mode is chosen, Emacs sets the value of the variable
@code{major-mode} to the symbol for that major mode (e.g.,
@code{text-mode} for Text mode).  This is a per-buffer variable
(@pxref{Locals}); its buffer-local value is set automatically, and 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 it via the Customization interface (@pxref{Easy
Customization}), or by adding a line like this to your init file
(@pxref{Init File}):

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

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

@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}.