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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.

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 lastly just foo; the first one found is loaded. 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.)

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.

The Emacs Lisp load path is specified by the variable load-path. Its value should be a list of directory names (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. A list 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. (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")

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-auto-load to nil.

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