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23.2.2 How Emacs Chooses a Major Mode

When Emacs visits a file, it automatically selects a major mode for the buffer based on information in the file name or in the file itself. It also processes local variables specified in the file text.

command normal-mode \&optional find-file​

This function establishes the proper major mode and buffer-local variable bindings for the current buffer. It calls set-auto-mode (see below). As of Emacs 26.1, it no longer runs hack-local-variables, this now being done in run-mode-hooks at the initialization of major modes (see Mode Hooks).

If the find-file argument to normal-mode is non-nil, normal-mode assumes that the find-file function is calling it. In this case, it may process local variables in the ‘-*-’ line or at the end of the file. The variable enable-local-variables controls whether to do so. See Local Variables in Files in The GNU Emacs Manual, for the syntax of the local variables section of a file.

If you run normal-mode interactively, the argument find-file is normally nil. In this case, normal-mode unconditionally processes any file local variables.

The function calls set-auto-mode to choose and set a major mode. If this does not specify a mode, the buffer stays in the major mode determined by the default value of major-mode (see below).

normal-mode uses condition-case around the call to the major mode command, so errors are caught and reported as a ‘File mode specification error’, followed by the original error message.

function set-auto-mode \&optional keep-mode-if-same​

This function selects and sets the major mode that is appropriate for the current buffer. It bases its decision (in order of precedence) on the ‘-*-’ line, on any ‘mode:’ local variable near the end of a file, on the ‘#!’ line (using interpreter-mode-alist), on the text at the beginning of the buffer (using magic-mode-alist), and finally on the visited file name (using auto-mode-alist). See How Major Modes are Chosen in The GNU Emacs Manual. If enable-local-variables is nil, set-auto-mode does not check the ‘-*-’ line, or near the end of the file, for any mode tag.

There are some file types where it is not appropriate to scan the file contents for a mode specifier. For example, a tar archive may happen to contain, near the end of the file, a member file that has a local variables section specifying a mode for that particular file. This should not be applied to the containing tar file. Similarly, a tiff image file might just happen to contain a first line that seems to match the ‘-*-’ pattern. For these reasons, both these file extensions are members of the list inhibit-local-variables-regexps. Add patterns to this list to prevent Emacs searching them for local variables of any kind (not just mode specifiers).

If keep-mode-if-same is non-nil, this function does not call the mode command if the buffer is already in the proper major mode. For instance, set-visited-file-name sets this to t to avoid killing buffer local variables that the user may have set.

function set-buffer-major-mode buffer​

This function sets the major mode of buffer to the default value of major-mode; if that is nil, it uses the current buffer’s major mode (if that is suitable). As an exception, if buffer’s name is *scratch*, it sets the mode to initial-major-mode.

The low-level primitives for creating buffers do not use this function, but medium-level commands such as switch-to-buffer and find-file-noselect use it whenever they create buffers.

user option initial-major-mode​

The value of this variable determines the major mode of the initial *scratch* buffer. The value should be a symbol that is a major mode command. The default value is lisp-interaction-mode.

variable interpreter-mode-alist​

This variable specifies major modes to use for scripts that specify a command interpreter in a ‘#!’ line. Its value is an alist with elements of the form (regexp . mode); this says to use mode mode if the file specifies an interpreter which matches \\`regexp\\'. For example, one of the default elements is ("python[0-9.]*" . python-mode).

variable magic-mode-alist​

This variable’s value is an alist with elements of the form (regexp . function), where regexp is a regular expression and function is a function or nil. After visiting a file, set-auto-mode calls function if the text at the beginning of the buffer matches regexp and function is non-nil; if function is nil, auto-mode-alist gets to decide the mode.

variable magic-fallback-mode-alist​

This works like magic-mode-alist, except that it is handled only if auto-mode-alist does not specify a mode for this file.

variable auto-mode-alist​

This variable contains an association list of file name patterns (regular expressions) and corresponding major mode commands. Usually, the file name patterns test for suffixes, such as ‘.el’ and ‘.c’, but this need not be the case. An ordinary element of the alist looks like (regexp . mode-function).

For example,

(("\\`/tmp/fol/" . text-mode)
 ("\\.texinfo\\'" . texinfo-mode)
 ("\\.texi\\'" . texinfo-mode)
 ("\\.el\\'" . emacs-lisp-mode)
 ("\\.c\\'" . c-mode)
 ("\\.h\\'" . c-mode)
 …)

When you visit a file whose expanded file name (see File Name Expansion), with version numbers and backup suffixes removed using file-name-sans-versions (see File Name Components), matches a regexp, set-auto-mode calls the corresponding mode-function. This feature enables Emacs to select the proper major mode for most files.

If an element of auto-mode-alist has the form (regexp function t), then after calling function, Emacs searches auto-mode-alist again for a match against the portion of the file name that did not match before. This feature is useful for uncompression packages: an entry of the form ("\\.gz\\'" function t) can uncompress the file and then put the uncompressed file in the proper mode according to the name sans ‘.gz’.

Here is an example of how to prepend several pattern pairs to auto-mode-alist. (You might use this sort of expression in your init file.)

(setq auto-mode-alist
  (append
   ;; File name (within directory) starts with a dot.
   '(("/\\.[^/]*\\'" . fundamental-mode)
     ;; File name has no dot.
     ("/[^\\./]*\\'" . fundamental-mode)
     ;; File name ends in ‘.C’.
     ("\\.C\\'" . c++-mode))
   auto-mode-alist))