27.8 Libraries of Lisp Code for Emacs

Emacs Lisp code is stored in files whose names conventionally end in .el. Such files are automatically visited in Emacs Lisp mode.

Emacs Lisp code can be compiled into byte-code, which loads faster, takes up less space, and executes faster. By convention, compiled Emacs Lisp code goes in a separate file whose name ends in ‘.elc’. For example, the compiled code for foo.el goes in foo.elc. See Byte Compilation in the Emacs Lisp Reference Manual.

To load an Emacs Lisp file, type M-x load-file. This command reads a file name using the minibuffer, and executes the contents of that file as Emacs Lisp code. It is not necessary to visit the file first; this command reads the file directly from disk, not from an existing Emacs buffer.

If an Emacs Lisp file is installed in the Emacs Lisp load path (defined below), you can load it by typing M-x load-library, instead of using M-x load-file. The M-x load-library command prompts for a library name rather than a file name; it searches through each directory in the Emacs Lisp load path, trying to find a file matching that library name. If the library name is ‘foo’, it tries looking for files named foo.elc, foo.el, and foo. The default behavior is to load the first file found. This command prefers .elc files over .el files because compiled files load and run faster. If it finds that lib.el is newer than lib.elc, it issues a warning, in case someone made changes to the .el file and forgot to recompile it, but loads the .elc file anyway. (Due to this behavior, you can save unfinished edits to Emacs Lisp source files, and not recompile until your changes are ready for use.) If you set the option load-prefer-newer to a non-nil value, however, then rather than the procedure described above, Emacs loads whichever version of the file is newest.

Emacs Lisp programs usually load Emacs Lisp files using the load function. This is similar to load-library, but is lower-level and accepts additional arguments. See How Programs Do Loading in the Emacs Lisp Reference Manual.

The Emacs Lisp load path is specified by the variable load-path. Its value should be a list of directories (strings). These directories are searched, in the specified order, by the M-x load-library command, the lower-level load function, and other Emacs functions that find Emacs Lisp libraries. An entry in load-path can also have the special value nil, which stands for the current default directory, but it is almost always a bad idea to use this, because its meaning will depend on the buffer that is current when load-path is used by Emacs. (If you find yourself wishing that nil were in the list, most likely what you really want is to use M-x load-file.)

The default value of load-path is a list of directories where the Lisp code for Emacs itself is stored. If you have libraries of your own in another directory, you can add that directory to the load path. Unlike most other variables described in this manual, load-path cannot be changed via the Customize interface (see Easy Customization), but you can add a directory to it by putting a line like this in your init file (see Init File):

(add-to-list 'load-path "/path/to/my/lisp/library")

It is customary to put locally installed libraries in the site-lisp directory that is already in the default value of load-path, or in some subdirectory of site-lisp. This way, you don’t need to modify the default value of load-path.

Some commands are autoloaded; when you run them, Emacs automatically loads the associated library first. For instance, the M-x compile command (see Compilation) is autoloaded; if you call it, Emacs automatically loads the compile library first. In contrast, the command M-x recompile is not autoloaded, so it is unavailable until you load the compile library.

Automatic loading can also occur when you look up the documentation of an autoloaded command (see Name Help), if the documentation refers to other functions and variables in its library (loading the library lets Emacs properly set up the hyperlinks in the *Help* buffer). To disable this feature, change the variable help-enable-autoload to nil.

Automatic loading also occurs when completing names for describe-variable and describe-function, based on the prefix being completed. To disable this feature, change the variable help-enable-completion-autoload to nil.

By default, Emacs refuses to load compiled Lisp files which were compiled with XEmacs, a modified version of Emacs—they can cause Emacs to crash. Set the variable load-dangerous-libraries to t if you want to try loading them.

Once you put your library in a directory where Emacs can find and load it, you may wish to make it available at startup. This is useful when the library defines features that should be available automatically on demand, and manually loading the library is thus inconvenient. In these cases, make sure the library will be loaded by adding suitable forms to your init file: either load or require (if you always need to load the library at startup), or autoload if you need Emacs to load the library when some command or function is invoked. For example:

 ;; Loads my-shining-package.elc unconditionally.
 (require 'my-shining-package)

 ;; Will load my-shining-package.elc when my-func is invoked.
 (autoload 'my-func "my-shining-package")

Note that installing a package using package-install (see Package Installation) takes care of placing the package’s Lisp files in a directory where Emacs will find it, and also writes the necessary initialization code into your init files, making the above manual customizations unnecessary.