GNU Emacs Manual - Lisp Libraries

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## Libraries of Lisp Code for Emacs

Lisp code for Emacs editing commands is stored in files whose names conventionally end in ``.el'`. This ending tells Emacs to edit them in Emacs-Lisp mode (see section Executing Lisp Expressions).

To execute a file of Emacs Lisp code, use `M-x load-file`. This command reads a file name using the minibuffer and then executes the contents of that file as Lisp code. It is not necessary to visit the file first; in any case, this command reads the file as found on disk, not text in an Emacs buffer.

Once a file of Lisp code is installed in the Emacs Lisp library directories, users can load it using `M-x load-library`. Programs can load it by calling `load-library`, or with `load`, a more primitive function that is similar but accepts some additional arguments.

`M-x load-library` differs from `M-x load-file` in that it searches a sequence of directories and tries three file names in each directory. Suppose your argument is lib; the three names are ``lib.elc'`, ``lib.el'`, and lastly just ``lib'`. If ``lib.elc'` exists, it is by convention the result of compiling ``lib.el'`; it is better to load the compiled file, since it will load and run faster.

If `load-library` finds that ``lib.el'` is newer than ``lib.elc'` file, it prints a warning, because it's likely that somebody made changes to the ``.el'` file and forgot to recompile it.

Because the argument to `load-library` is usually not in itself a valid file name, file name completion is not available. Indeed, when using this command, you usually do not know exactly what file name will be used.

The sequence of directories searched by `M-x load-library` is specified by the variable `load-path`, a list of strings that are directory names. The default value of the list contains the directory where the Lisp code for Emacs itself is stored. If you have libraries of your own, put them in a single directory and add that directory to `load-path`. `nil` in this list stands for the current default directory, but it is probably not a good idea to put `nil` in the list. If you find yourself wishing that `nil` were in the list, most likely what you really want to do is use `M-x load-file` this once.

Often you do not have to give any command to load a library, because the commands defined in the library are set up to autoload that library. Trying to run any of those commands calls `load` to load the library; this replaces the autoload definitions with the real ones from the library.

Emacs Lisp code can be compiled into byte-code which loads faster, takes up less space when loaded, and executes faster. See section `Byte Compilation' in the Emacs Lisp Reference Manual. By convention, the compiled code for a library goes in a separate file whose name consists of the library source file with ``c'` appended. Thus, the compiled code for ``foo.el'` goes in ``foo.elc'`. That's why `load-library` searches for ``.elc'` files first.

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