@node Major Modes, Indentation, Mule, Top
@chapter Major Modes
@cindex major modes
@kindex TAB
@kindex DEL
@kindex LFD

  Emacs has many different @dfn{major modes}, each of which customizes
Emacs for editing text of a particular sort.  The major modes are mutually
exclusive;  at any time, each buffer has one major mode.  The mode line
normally contains the name of the current major mode in parentheses.
@xref{Mode Line}.

  The least specialized major mode is called @dfn{Fundamental mode}.  This
mode has no mode-specific redefinitions or variable settings.  Each
Emacs command behaves in its most general manner, and each option is in its
default state.  For editing any specific type of text, such as Lisp code or
English text, you should switch to the appropriate major mode, such as Lisp
mode or Text mode.

  Selecting a major mode changes the meanings of a few keys to become
more specifically adapted to the language being edited.  @key{TAB},
@key{DEL}, and @key{LFD} are changed frequently.  In addition, commands
which handle comments use the mode to determine how to delimit comments.
Many major modes redefine the syntactical properties of characters
appearing in the buffer.  @xref{Syntax}.

  The major modes fall into three major groups.  Programming modes
(@pxref{Programs}) are for specific programming languages.  Text modes
(like Nroff mode, @TeX{} mode, Outline mode, XML mode, etc.@:) are for
editing human readable text.  The remaining major modes are not intended
for direct use in editing user files; they are used in buffers created
by Emacs for specific purposes. Examples of such modes include Dired
mode which is used for buffers made by Dired (@pxref{Dired}), Mail 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 so that the paragraph commands remain 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.  @xref{Indentation}.

@menu
* Choosing Modes::     How major modes are specified or chosen.
* Mode Hooks::         Customizing a major mode
@end menu

@node Choosing Modes, Mode Hooks, Major Modes, Major Modes
@section Choosing Major Modes

  You can select a major mode explicitly for the current buffer, but
most of the time Emacs determines which mode to use based on the file
name or some text in the file.

  Use a @kbd{M-x} command to explicitly select a new major mode.  Add
@code{-mode} to the name of a major mode to get the name of a command to
select that mode.  For example, to enter Lisp mode, execute @kbd{M-x
lisp-mode}.

@vindex auto-mode-alist
  When you visit a file, Emacs usually chooses the right major mode
based on the file's name.  For example, files whose names end in
@code{.c} are edited in C mode.  The variable @code{auto-mode-alist}
controls the correspondence between file names and major mode.  Its value
is a list in which each element has the form:

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

@noindent
For example, one element normally found in the list has the form
@code{(@t{"\\.c$"} . c-mode)}. 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 is needed to
suppress the special meaning of @samp{.} in regexps.)  The only practical
way to change this variable is with Lisp code.

  You can specify which major mode should be used for editing a certain
file by a special sort of text in the first non-blank line of the file.
The mode name should appear in this line both 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.  Such an explicit specification overrides any
default mode based on the file name.

  Another format of mode specification is:

@example
-*-Mode: @var{modename};-*-
@end example

@noindent
which allows other things besides the major mode name to be specified.
However, Emacs does not look for anything except the mode name.

The major mode can also be specified in a local variables list.
@xref{File Variables}.

@vindex default-major-mode
  When you visit a file that does not specify a major mode to use, or
when you create a new buffer with @kbd{C-x b}, Emacs uses the major mode
specified by the variable @code{default-major-mode}.  Normally this
value is the symbol @code{fundamental-mode}, which specifies Fundamental
mode.  If @code{default-major-mode} is @code{nil}, the major mode is
taken from the previously selected buffer.

@node Mode Hooks,  , Choosing Modes, Major Modes
@section Mode Hook Variables

@cindex Hook variables
@cindex mode hook
@findex add-hook
@findex remove-hook
@vindex lisp-mode-hook
@vindex emacs-lisp-mode-hook
@vindex lisp-interaction-mode-hook
@vindex scheme-mode-hook

  The last step taken by a major mode, by convention, is to invoke a
list of user supplied functions that are stored in a ``hook'' variable.
This allows a user to further customize the major mode, and is
particularly convenient for setting up buffer local variables
(@pxref{Locals}).

  The name of the hook variable is created by appending the string
@code{-hook} to the name of the major mode.  For example, the hook
variable used by @code{text-mode} would be named @code{text-mode-hook}.
By convention the mode hook function receives no arguments. If a hook
variable does not exist, or it has the value @code{nil}, the major mode
simply ignores it.

  The recommended way to add functions to a hook variable is with the
@code{add-hook} function.  For example, to automatically turn on the
Auto Fill mode when Text mode is invoked the following code can be used in
the initialization file (@pxref{Init File})

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

The @code{add-hook} function will check that the function is not already
listed in the hook variable before adding it. It will also create a hook
variable with the value @code{nil} if one does not exist before adding
the function. @code{add-hook} adds functions to the front of the hook
variable list. This means that the last hook added is run first by the
major mode. It is considered very poor style to write hook functions
that depend on the order that hooks are executed.

Hooks can be removed from hook variables with @code{remove-hook}.