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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 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 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.

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