Autoload

The autoload facility lets you register the existence of a function or macro, but put off loading the file that defines it. The first call to the function automatically loads the proper library, in order to install the real definition and other associated code, then runs the real definition as if it had been loaded all along. Autoloading can also be triggered by looking up the documentation of the function or macro (see Documentation Basics), and completion of variable and function names (see Autoload by Prefix below).

There are two ways to set up an autoloaded function: by calling autoload, and by writing a “magic” comment in the source before the real definition. autoload is the low-level primitive for autoloading; any Lisp program can call autoload at 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 autoload and arranges to execute them when Emacs is built.

— Function: autoload function filename &optional docstring interactive type

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 .el or .elc, this function insists 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-suffixes specifies the exact required suffixes.)
The argument docstring is the documentation string for the function. Specifying the documentation string in the call to autoload makes 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 macro if function is really a macro. Specify type as keymap if 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, this function does nothing and returns nil. Otherwise, it constructs an autoload object (see Autoload Type), and stores it as the function definition for function. The autoload object has this form:
          (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 file (see Documentation Basics), t means the function is interactive, and nil that it is not a macro or a keymap.

— Function: autoloadp object

This function returns non-nil if object is an autoload object. For example, to check if run-prolog is defined as an autoloaded function, evaluate
          (autoloadp (symbol-function 'run-prolog))

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 definitions or 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 autoload. 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. The form following the magic comment is copied verbatim, except if it is one of the forms which the autoload facility handles specially (e.g., by conversion into an autoload call). The forms which are not copied verbatim are the following:

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:

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:

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:

— Variable: generate-autoload-cookie

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 ";;;###autoload".

— Variable: generated-autoload-file

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.

The following function may be used to explicitly load the library specified by an autoload object:

16.5 Autoload