28. Major Modes

Emacs provides many alternative major modes, each of which customizes Emacs for editing text of a particular sort. The major modes are mutually exclusive, and each buffer has one major mode at any time. The mode line normally shows the name of the current major mode, in parentheses (see section The Mode Line).

The least specialized major mode is called 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 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. The ones that are changed frequently are TAB, DEL, and C-j. The prefix key 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. See section The Syntax Table.

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 (see section Dired, the Directory Editor), Mail mode for buffers made by C-x m (see section Sending Mail), and Shell mode for buffers used for communicating with an inferior shell process (see section Interactive Inferior Shell).

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

28.1 How Major Modes are Chosen

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 on special text in the file.

To explicitly select a new major, you use an M-x command. Take the name of a major mode and add -mode to get the name of the command to select that mode. Thus, you can enter Lisp mode by executing M-x lisp-mode.

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 `.c' are edited in C mode. The correspondence between file names and major modes is controlled by the variable auto-mode-alist. Its value is a list in which each element has this form,

(regexp . mode-function)

or this form,

(regexp mode-function flag)

For example, one element normally found in the list has the form ("\\.c\\'" . c-mode), and it is responsible for selecting C mode for files whose names end in `.c'. (Note that `\\' is needed in Lisp syntax to include a `\' in the string, which must be used to suppress the special meaning of `.' in regexps.) If the element has the form (regexp mode-function flag) and flag is non-nil, then after calling mode-function, Emacs discards the suffix that matched regexp and searches the list again for another match.

Sometimes the major mode is determined from the way the file's text begins. The variable magic-mode-alist controls this. Its value is a list of elements of these forms:

(regexp . mode-function)
(match-function . mode-function)

The first form looks like an element of auto-mode-alist, but it doesn't work the same: this regexp is matched against the text at the start of the buffer, not against the file name. Likewise, the second form calls match-function at the beginning of the buffer, and if the function returns non-nil, the mode-function is called. magic-mode-alist takes priority over auto-mode-alist.

You can specify the major mode to use for editing a certain file by special text in the first nonblank line of the file. The mode name should appear in this line both preceded and followed by `-*-'. Other text may appear on the line as well. For example,

;-*-Lisp-*-

tells Emacs to use Lisp mode. Such an explicit specification overrides any defaults based on the file name. Note how the semicolon is used to make Lisp treat this line as a comment.

Another format of mode specification is

-*- mode: modename;-*-

which allows you to specify local variables as well, like this:

-*- mode: modename; var: value; … -*-

See section Local Variables in Files, for more information about this.

On systems with case-insensitive file names, only a single case-insensitive search through the auto-mode-alist is made. On other systems, Emacs normally performs a single case-sensitive search through the alist, but if you set this variable to a non-nil value, Emacs will perform a second case-insensitive search if the first search fails.

When a file's contents begin with `#!', it 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.

When you visit such a file in Emacs, if the file's name does not specify a major mode, Emacs uses the interpreter name on the first line to choose a mode. If the first line is the name of a recognized interpreter program, such as `perl' or `tcl', Emacs uses a mode appropriate for programs for that interpreter. The variable interpreter-mode-alist specifies the correspondence between interpreter program names and major modes.

When the first line starts with `#!', you cannot (on many systems) use the `-*-' feature on the first line, because the system would get confused when running the interpreter. So Emacs looks for `-*-' on the second line in such files as well as on the first line.

When you visit a file that does not specify a major mode to use, or when you create a new buffer with C-x b, the variable default-major-mode specifies which major mode to use. Normally its value is the symbol fundamental-mode, which specifies Fundamental mode. If default-major-mode is nil, the major mode is taken from the previously current buffer.

If you change the major mode of a buffer, you can go back to the major mode Emacs would choose automatically: use the command M-x normal-mode to do this. This is the same function that find-file calls to choose the major mode. It also processes the file's `-*-' line or local variables list (if any). See section Local Variables in Files.

The commands C-x C-w and set-visited-file-name change to a new major mode if the new file name implies a mode (see section Saving Files). (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 change-major-mode-with-file-name to nil.

This document was generated by Mark Kaminski on July, 3 2008 using texi2html 1.70.