The autoload facility allows you to make a function or macro known in Lisp, but put off loading the file that defines it. The first call to the function automatically reads the proper file to install the real definition and other associated code, then runs the real definition as if it had been loaded all along.
There are two ways to set up an autoloaded function: by calling
autoload, and by writing a special “magic” comment in the
source before the real definition.
autoload is the low-level
primitive for autoloading; any Lisp program can call
any time. Magic comments are the most convenient way to make a function
autoload, for packages installed along with Emacs. These comments do
nothing on their own, but they serve as a guide for the command
update-file-autoloads, which constructs calls to
and arranges to execute them when Emacs is built.
This function defines the function (or macro) named function so as to load automatically from filename. The string filename specifies the file to load to get the real definition of function.
If filename does not contain either a directory name, or the suffix
autoloadinsists on adding one of these suffixes, and it will not load from a file whose name is just filename with no added suffix. (The variable
load-suffixesspecifies the exact required suffixes.)
The argument docstring is the documentation string for the function. Specifying the documentation string in the call to
autoloadmakes it possible to look at the documentation without loading the function's real definition. Normally, this should be identical to the documentation string in the function definition itself. If it isn't, the function definition's documentation string takes effect when it is loaded.
If interactive is non-
nil, that says function can be called interactively. This lets completion in M-x work without loading function's real definition. The complete interactive specification is not given here; it's not needed unless the user actually calls function, and when that happens, it's time to load the real definition.
You can autoload macros and keymaps as well as ordinary functions. Specify type as
macroif function is really a macro. Specify type as
keymapif function is really a keymap. Various parts of Emacs need to know this information without loading the real definition.
An autoloaded keymap loads automatically during key lookup when a prefix key's binding is the symbol function. Autoloading does not occur for other kinds of access to the keymap. In particular, it does not happen when a Lisp program gets the keymap from the value of a variable and calls
define-key; not even if the variable name is the same symbol function.
If function already has a non-void function definition that is not an autoload object,
autoloaddoes nothing and returns
nil. If the function cell of function is void, or is already an autoload object, then it is defined as an autoload object like this:(autoload filename docstring interactive type)
For example,(symbol-function 'run-prolog) ⇒ (autoload "prolog" 169681 t nil)
In this case,
"prolog"is the name of the file to load, 169681 refers to the documentation string in the emacs/etc/DOC-version file (see Documentation Basics),
tmeans the function is interactive, and
nilthat it is not a macro or a keymap.
The autoloaded file usually contains other definitions and may require
or provide one or more features. If the file is not completely loaded
(due to an error in the evaluation of its contents), any function
provide calls that occurred during the load are
undone. This is to ensure that the next attempt to call any function
autoloading from this file will try again to load the file. If not for
this, then some of the functions in the file might be defined by the
aborted load, but fail to work properly for the lack of certain
subroutines not loaded successfully because they come later in the file.
If the autoloaded file fails to define the desired Lisp function or
macro, then an error is signaled with data
"Autoloading failed to
define function function-name
A magic autoload comment (often called an autoload cookie)
consists of ‘;;;###autoload’, on a line by itself,
just before the real definition of the function in its
autoloadable source file. The command M-x update-file-autoloads
writes a corresponding
autoload call into loaddefs.el.
(The string that serves as the autoload cookie and the name of the
file generated by
update-file-autoloads can be changed from the
above defaults, see below.)
Building Emacs loads loaddefs.el and thus calls
M-x update-directory-autoloads is even more powerful; it updates
autoloads for all files in the current directory.
The same magic comment can copy any kind of form into
loaddefs.el. If the form following the magic comment is not a
function-defining form or a
defcustom form, it is copied
verbatim. “Function-defining forms” include
define-minor-mode as well as
defmacro. To save space, a
defcustom form is converted to
defvar in loaddefs.el, with some additional information
if it uses
You can also use a magic comment to execute a form at build time without executing it when the file itself is loaded. To do this, write the form on the same line as the magic comment. Since it is in a comment, it does nothing when you load the source file; but M-x update-file-autoloads copies it to loaddefs.el, where it is executed while building Emacs.
The following example shows how
doctor is prepared for
autoloading with a magic comment:
;;;###autoload (defun doctor () "Switch to *doctor* buffer and start giving psychotherapy." (interactive) (switch-to-buffer "*doctor*") (doctor-mode))
Here's what that produces in loaddefs.el:
(autoload (quote doctor) "doctor" "\ Switch to *doctor* buffer and start giving psychotherapy. \(fn)" t nil)
The backslash and newline immediately following the double-quote are a
convention used only in the preloaded uncompiled Lisp files such as
loaddefs.el; they tell
make-docfile to put the
documentation string in the etc/DOC file. See Building Emacs.
See also the commentary in lib-src/make-docfile.c. ‘(fn)’
in the usage part of the documentation string is replaced with the
function's name when the various help functions (see Help Functions) display it.
If you write a function definition with an unusual macro that is not
one of the known and recognized function definition methods, use of an
ordinary magic autoload comment would copy the whole definition into
loaddefs.el. That is not desirable. You can put the desired
autoload call into
loaddefs.el instead by writing this:
;;;###autoload (autoload 'foo "myfile") (mydefunmacro foo ...)
You can use a non-default string as the autoload cookie and have the corresponding autoload calls written into a file whose name is different from the default loaddefs.el. Emacs provides two variables to control this:
The value of this variable should be a string whose syntax is a Lisp comment. M-x update-file-autoloads copies the Lisp form that follows the cookie into the autoload file it generates. The default value of this variable is
The value of this variable names an Emacs Lisp file where the autoload calls should go. The default value is loaddefs.el, but you can override that, e.g., in the “Local Variables” section of a .el file (see File Local Variables). The autoload file is assumed to contain a trailer starting with a formfeed character.