Major modes specialize Emacs for editing or interacting with particular kinds of text. Each buffer has exactly one major mode at a time. Every major mode is associated with a major mode command, whose name should end in ‘-mode’. This command takes care of switching to that mode in the current buffer, by setting various buffer-local variables such as a local keymap. See Major Mode Conventions. Note that unlike minor modes there is no way to “turn off” a major mode, instead the buffer must be switched to a different one. However, you can temporarily suspend a major mode and later restore the suspended mode, see below.
The least specialized major mode is called Fundamental mode, which has no mode-specific definitions or variable settings.
This is the major mode command for Fundamental mode. Unlike other mode commands, it does not run any mode hooks (see Major Mode Conventions), since you are not supposed to customize this mode.
This function works like fundamental-mode, in that it kills all
buffer-local variables, but it also records the major mode in effect,
so that it could subsequently be restored. This function and
major-mode-restore (described next) are useful when you need to
put a buffer under some specialized mode other than the one Emacs
chooses for it automatically (see How Emacs Chooses a Major Mode), but would also
like to be able to switch back to the original mode later.
This function restores the major mode recorded by
major-mode-suspend. If no major mode was recorded, this
function calls normal-mode (see normal-mode), but tries to force it not to choose any modes in
avoided-modes, if that argument is non-nil.
Changing the major mode clears out most local variables, but it
doesn’t remove all artifacts in the buffer (like text properties and
overlays). It’s rare to change a buffer from one major mode to
another (except from fundamental-mode to everything else), so
this is usually not a concern. It can sometimes be convenient (mostly
when debugging a problem in a buffer) to do a “full reset” of the
buffer, and that’s what the clean-mode major mode offers. It
will kill all local variables (even the permanently local ones), and
also removes all overlays and text properties.
The easiest way to write a major mode is to use the macro
define-derived-mode, which sets up the new mode as a variant of
an existing major mode. See Defining Derived Modes. We recommend using
define-derived-mode even if the new mode is not an obvious
derivative of another mode, as it automatically enforces many coding
conventions for you. See Basic Major Modes, for common modes to
derive from.
The standard GNU Emacs Lisp directory tree contains the code for several major modes, in files such as text-mode.el, texinfo.el, lisp-mode.el, and rmail.el. You can study these libraries to see how modes are written.
The buffer-local value of this variable holds the symbol for the current
major mode. Its default value holds the default major mode for new
buffers. The standard default value is fundamental-mode.
If the default value is nil, then whenever Emacs creates a new
buffer via a command such as C-x b (switch-to-buffer), the
new buffer is put in the major mode of the previously current buffer.
As an exception, if the major mode of the previous buffer has a
mode-class symbol property with value special, the new
buffer is put in Fundamental mode (see Major Mode Conventions).