23.2.3 How Emacs Chooses a Major Mode

Based on information in the file name or in the file itself, Emacs automatically selects a major mode for the new buffer when a file is visited. It also processes local variables specified in the file text.

Command: fundamental-mode

Fundamental mode is a major mode that is not specialized for anything in particular. Other major modes are defined in effect by comparison with this one—their definitions say what to change, starting from Fundamental mode. The fundamental-mode function does not run any mode hooks; you're not supposed to customize it. (If you want Emacs to behave differently in Fundamental mode, change the global state of Emacs.)

Command: normal-mode &optional find-file

This function establishes the proper major mode and buffer-local variable bindings for the current buffer. First it calls set-auto-mode (see below), then it runs hack-local-variables to parse, and bind or evaluate as appropriate, the file's local variables (voir la section File Local Variables).

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. Voir (emacs)File Variables section `Local Variables in Files' dans 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.

If normal-mode processes the local variables list and this list specifies a major mode, that mode overrides any mode chosen by set-auto-mode. If neither set-auto-mode nor hack-local-variables specify a major mode, the buffer stays in the major mode determined by default-major-mode (see below).

normal-mode uses condition-case around the call to the major mode function, 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 the major mode that is appropriate for the current buffer. It bases its decision (in order of precedence) on the ‘-*-’ line, 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). Voir (emacs)Choosing Modes section `How Major Modes are Chosen' dans The GNU Emacs Manual. However, this function does not look for the ‘mode:’ local variable near the end of a file; the hack-local-variables function does that. If enable-local-variables is nil, set-auto-mode does not check the ‘-*-’ line for a mode tag either.

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.

User Option: default-major-mode

This variable holds the default major mode for new buffers. The standard value is fundamental-mode.

If the value of default-major-mode is nil, Emacs uses the (previously) current buffer's major mode as the default major mode of a new buffer. However, if that major mode symbol has a mode-class property with value special, then it is not used for new buffers; Fundamental mode is used instead. The modes that have this property are those such as Dired and Rmail that are useful only with text that has been specially prepared.

Function: set-buffer-major-mode buffer

This function sets the major mode of buffer to the value of default-major-mode; if that variable 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 (interpreter . mode); for example, ("perl" . perl-mode) is one element present by default. The element says to use mode mode if the file specifies an interpreter which matches interpreter.

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 (voir la section Functions that Expand Filenames), with version numbers and backup suffixes removed using file-name-sans-versions (voir la section 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))

Ce document a été généré par Eric Reinbold le 13 Octobre 2007 en utilisant texi2html 1.78.