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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.


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