15. Loading

Loading a file of Lisp code means bringing its contents into the Lisp environment in the form of Lisp objects. Emacs finds and opens the file, reads the text, evaluates each form, and then closes the file.

The load functions evaluate all the expressions in a file just as the eval-current-buffer function evaluates all the expressions in a buffer. The difference is that the load functions read and evaluate the text in the file as found on disk, not the text in an Emacs buffer.

The loaded file must contain Lisp expressions, either as source code or as byte-compiled code. Each form in the file is called a top-level form. There is no special format for the forms in a loadable file; any form in a file may equally well be typed directly into a buffer and evaluated there. (Indeed, most code is tested this way.) Most often, the forms are function definitions and variable definitions.

A file containing Lisp code is often called a library. Thus, the "Rmail library" is a file containing code for Rmail mode. Similarly, a "Lisp library directory" is a directory of files containing Lisp code.

15.1 How Programs Do Loading

Emacs Lisp has several interfaces for loading. For example, autoload creates a placeholder object for a function defined in a file; trying to call the autoloading function loads the file to get the function's real definition (see section 15.4 Autoload). require loads a file if it isn't already loaded (see section 15.6 Features). Ultimately, all these facilities call the load function to do the work.

Function: load filename &optional missing-ok nomessage nosuffix must-suffix

This function finds and opens a file of Lisp code, evaluates all the forms in it, and closes the file.

To find the file, load first looks for a file named `filename.elc', that is, for a file whose name is filename with `.elc' appended. If such a file exists, it is loaded. If there is no file by that name, then load looks for a file named `filename.el'. If that file exists, it is loaded. Finally, if neither of those names is found, load looks for a file named filename with nothing appended, and loads it if it exists. (The load function is not clever about looking at filename. In the perverse case of a file named `foo.el.el', evaluation of (load "foo.el") will indeed find it.)

If the optional argument nosuffix is non-nil, then the suffixes `.elc' and `.el' are not tried. In this case, you must specify the precise file name you want. By specifying the precise file name and using t for nosuffix, you can prevent perverse file names such as `foo.el.el' from being tried.

If the optional argument must-suffix is non-nil, then load insists that the file name used must end in either `.el' or `.elc', unless it contains an explicit directory name. If filename does not contain an explicit directory name, and does not end in a suffix, then load insists on adding one.

If filename is a relative file name, such as `foo' or `baz/foo.bar', load searches for the file using the variable load-path. It appends filename to each of the directories listed in load-path, and loads the first file it finds whose name matches. The current default directory is tried only if it is specified in load-path, where nil stands for the default directory. load tries all three possible suffixes in the first directory in load-path, then all three suffixes in the second directory, and so on. See section 15.2 Library Search.

If you get a warning that `foo.elc' is older than `foo.el', it means you should consider recompiling `foo.el'. See section 16. Byte Compilation.

When loading a source file (not compiled), load performs character set translation just as Emacs would do when visiting the file. See section 33.10 Coding Systems.

Messages like `Loading foo...' and `Loading foo...done' appear in the echo area during loading unless nomessage is non-nil.

Any unhandled errors while loading a file terminate loading. If the load was done for the sake of autoload, any function definitions made during the loading are undone.

If load can't find the file to load, then normally it signals the error file-error (with `Cannot open load file filename'). But if missing-ok is non-nil, then load just returns nil.

You can use the variable load-read-function to specify a function for load to use instead of read for reading expressions. See below.

load returns t if the file loads successfully.

Command: load-file filename

This command loads the file filename. If filename is a relative file name, then the current default directory is assumed. load-path is not used, and suffixes are not appended. Use this command if you wish to specify precisely the file name to load.

Command: load-library library

This command loads the library named library. It is equivalent to load, except in how it reads its argument interactively.

Variable: load-in-progress

This variable is non-nil if Emacs is in the process of loading a file, and it is nil otherwise.

Variable: load-read-function

This variable specifies an alternate expression-reading function for load and eval-region to use instead of read. The function should accept one argument, just as read does.

Normally, the variable's value is nil, which means those functions should use read.

Note: Instead of using this variable, it is cleaner to use another, newer feature: to pass the function as the read-function argument to eval-region. See section 9.4 Eval.

For information about how load is used in building Emacs, see E.1 Building Emacs.

15.2 Library Search

When Emacs loads a Lisp library, it searches for the library in a list of directories specified by the variable load-path.

User Option: load-path

The value of this variable is a list of directories to search when loading files with load. Each element is a string (which must be a directory name) or nil (which stands for the current working directory).

The value of load-path is initialized from the environment variable EMACSLOADPATH, if that exists; otherwise its default value is specified in `emacs/src/paths.h' when Emacs is built. Then the list is expanded by adding subdirectories of the directories in the list.

The syntax of EMACSLOADPATH is the same as used for PATH; `:' (or `;', according to the operating system) separates directory names, and `.' is used for the current default directory. Here is an example of how to set your EMACSLOADPATH variable from a csh `.login' file:

setenv EMACSLOADPATH .:/user/bil/emacs:/usr/local/share/emacs/20.3/lisp

Here is how to set it using sh:

export EMACSLOADPATH
EMACSLOADPATH=.:/user/bil/emacs:/usr/local/share/emacs/20.3/lisp

Here is an example of code you can place in your init file (see section 40.1.2 The Init File, `.emacs') to add several directories to the front of your default load-path:

(setq load-path
      (append (list nil "/user/bil/emacs"
                    "/usr/local/lisplib"
                    "~/emacs")
              load-path))

In this example, the path searches the current working directory first, followed then by the `/user/bil/emacs' directory, the `/usr/local/lisplib' directory, and the `~/emacs' directory, which are then followed by the standard directories for Lisp code.

Dumping Emacs uses a special value of load-path. If the value of load-path at the end of dumping is unchanged (that is, still the same special value), the dumped Emacs switches to the ordinary load-path value when it starts up, as described above. But if load-path has any other value at the end of dumping, that value is used for execution of the dumped Emacs also.

Therefore, if you want to change load-path temporarily for loading a few libraries in `site-init.el' or `site-load.el', you should bind load-path locally with let around the calls to load.

The default value of load-path, when running an Emacs which has been installed on the system, includes two special directories (and their subdirectories as well):

"/usr/local/share/emacs/version/site-lisp"

and

"/usr/local/share/emacs/site-lisp"

The first one is for locally installed packages for a particular Emacs version; the second is for locally installed packages meant for use with all installed Emacs versions.

There are several reasons why a Lisp package that works well in one Emacs version can cause trouble in another. Sometimes packages need updating for incompatible changes in Emacs; sometimes they depend on undocumented internal Emacs data that can change without notice; sometimes a newer Emacs version incorporates a version of the package, and should be used only with that version.

Emacs finds these directories' subdirectories and adds them to load-path when it starts up. Both immediate subdirectories and subdirectories multiple levels down are added to load-path.

Not all subdirectories are included, though. Subdirectories whose names do not start with a letter or digit are excluded. Subdirectories named `RCS' or `CVS' are excluded. Also, a subdirectory which contains a file named `.nosearch' is excluded. You can use these methods to prevent certain subdirectories of the `site-lisp' directories from being searched.

If you run Emacs from the directory where it was built--that is, an executable that has not been formally installed--then load-path normally contains two additional directories. These are the lisp and site-lisp subdirectories of the main build directory. (Both are represented as absolute file names.)

Command: locate-library library &optional nosuffix path interactive-call

This command finds the precise file name for library library. It searches for the library in the same way load does, and the argument nosuffix has the same meaning as in load: don't add suffixes `.elc' or `.el' to the specified name library.

If the path is non-nil, that list of directories is used instead of load-path.

When locate-library is called from a program, it returns the file name as a string. When the user runs locate-library interactively, the argument interactive-call is t, and this tells locate-library to display the file name in the echo area.

15.3 Loading Non-ASCII Characters

When Emacs Lisp programs contain string constants with non-ASCII characters, these can be represented within Emacs either as unibyte strings or as multibyte strings (see section 33.1 Text Representations). Which representation is used depends on how the file is read into Emacs. If it is read with decoding into multibyte representation, the text of the Lisp program will be multibyte text, and its string constants will be multibyte strings. If a file containing Latin-1 characters (for example) is read without decoding, the text of the program will be unibyte text, and its string constants will be unibyte strings. See section 33.10 Coding Systems.

To make the results more predictable, Emacs always performs decoding into the multibyte representation when loading Lisp files, even if it was started with the `--unibyte' option. This means that string constants with non-ASCII characters translate into multibyte strings. The only exception is when a particular file specifies no decoding.

The reason Emacs is designed this way is so that Lisp programs give predictable results, regardless of how Emacs was started. In addition, this enables programs that depend on using multibyte text to work even in a unibyte Emacs. Of course, such programs should be designed to notice whether the user prefers unibyte or multibyte text, by checking default-enable-multibyte-characters, and convert representations appropriately.

In most Emacs Lisp programs, the fact that non-ASCII strings are multibyte strings should not be noticeable, since inserting them in unibyte buffers converts them to unibyte automatically. However, if this does make a difference, you can force a particular Lisp file to be interpreted as unibyte by writing `-*-unibyte: t;-*-' in a comment on the file's first line. With that designator, the file will unconditionally be interpreted as unibyte, even in an ordinary multibyte Emacs session. This can matter when making keybindings to non-ASCII characters written as ?vliteral.

15.4 Autoload

The autoload facility allows you to make a function or macro known in Lisp, but put off loading the file that defines it. The first call to the function automatically reads the proper file to install the real definition and other associated code, then runs the real definition as if it had been loaded all along.

There are two ways to set up an autoloaded function: by calling autoload, and by writing a special "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, then autoload 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 argument docstring is the documentation string for the function. Normally, this should be identical to the documentation string in the function definition itself. Specifying the documentation string in the call to autoload makes it possible to look at the documentation without loading the function's real definition.

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, autoload does nothing and returns nil. If the function cell of function is void, or is already an autoload object, then it is defined as an autoload object like this:

(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-version' file (see section 24.1 Documentation Basics), t means the function is interactive, and nil that it is not a macro or a keymap.

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 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'. 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'. If the form following the magic comment is not a function-defining form or a defcustom form, it is copied verbatim. "Function-defining forms" include define-skeleton, define-derived-mode, define-generic-mode and define-minor-mode as well as defun and defmacro. To save space, a defcustom form is converted to a defvar in `loaddefs.el', with some additional information if it uses :require.

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:

;;;###autoload
(defun doctor ()
  "Switch to *doctor* buffer and start giving psychotherapy."
  (interactive)
  (switch-to-buffer "*doctor*")
  (doctor-mode))

Here's what that produces in `loaddefs.el':

(autoload 'doctor "doctor" "\
Switch to *doctor* buffer and start giving psychotherapy."
  t)

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 section E.1 Building Emacs. See also the commentary in `lib-src/make-docfile.c'.

15.5 Repeated Loading

You can load a given file more than once in an Emacs session. For example, after you have rewritten and reinstalled a function definition by editing it in a buffer, you may wish to return to the original version; you can do this by reloading the file it came from.

When you load or reload files, bear in mind that the load and load-library functions automatically load a byte-compiled file rather than a non-compiled file of similar name. If you rewrite a file that you intend to save and reinstall, you need to byte-compile the new version; otherwise Emacs will load the older, byte-compiled file instead of your newer, non-compiled file! If that happens, the message displayed when loading the file includes, `(compiled; note, source is newer)', to remind you to recompile it.

When writing the forms in a Lisp library file, keep in mind that the file might be loaded more than once. For example, think about whether each variable should be reinitialized when you reload the library; defvar does not change the value if the variable is already initialized. (See section 11.5 Defining Global Variables.)

The simplest way to add an element to an alist is like this:

(setq minor-mode-alist
      (cons '(leif-mode " Leif") minor-mode-alist))

But this would add multiple elements if the library is reloaded. To avoid the problem, write this:

(or (assq 'leif-mode minor-mode-alist)
    (setq minor-mode-alist
          (cons '(leif-mode " Leif") minor-mode-alist)))

To add an element to a list just once, you can also use add-to-list (see section 11.8 How to Alter a Variable Value).

Occasionally you will want to test explicitly whether a library has already been loaded. Here's one way to test, in a library, whether it has been loaded before:

(defvar foo-was-loaded nil)

(unless foo-was-loaded
  execute-first-time-only
  (setq foo-was-loaded t))

If the library uses provide to provide a named feature, you can use featurep earlier in the file to test whether the provide call has been executed before. See section 15.6 Features.

15.6 Features

provide and require are an alternative to autoload for loading files automatically. They work in terms of named features. Autoloading is triggered by calling a specific function, but a feature is loaded the first time another program asks for it by name.

A feature name is a symbol that stands for a collection of functions, variables, etc. The file that defines them should provide the feature. Another program that uses them may ensure they are defined by requiring the feature. This loads the file of definitions if it hasn't been loaded already.

To require the presence of a feature, call require with the feature name as argument. require looks in the global variable features to see whether the desired feature has been provided already. If not, it loads the feature from the appropriate file. This file should call provide at the top level to add the feature to features; if it fails to do so, require signals an error.

For example, in `emacs/lisp/prolog.el', the definition for run-prolog includes the following code:

(defun run-prolog ()
  "Run an inferior Prolog process, with I/O via buffer *prolog*."
  (interactive)
  (require 'comint)
  (switch-to-buffer (make-comint "prolog" prolog-program-name))
  (inferior-prolog-mode))

The expression (require 'comint) loads the file `comint.el' if it has not yet been loaded. This ensures that make-comint is defined. Features are normally named after the files that provide them, so that require need not be given the file name.

The `comint.el' file contains the following top-level expression:

(provide 'comint)

This adds comint to the global features list, so that (require 'comint) will henceforth know that nothing needs to be done.

When require is used at top level in a file, it takes effect when you byte-compile that file (see section 16. Byte Compilation) as well as when you load it. This is in case the required package contains macros that the byte compiler must know about. It also avoids byte-compiler warnings for functions and variables defined in the file loaded with require.

Although top-level calls to require are evaluated during byte compilation, provide calls are not. Therefore, you can ensure that a file of definitions is loaded before it is byte-compiled by including a provide followed by a require for the same feature, as in the following example.

(provide 'my-feature)  ; Ignored by byte compiler,
                       ;   evaluated by load.
(require 'my-feature)  ; Evaluated by byte compiler.

The compiler ignores the provide, then processes the require by loading the file in question. Loading the file does execute the provide call, so the subsequent require call does nothing when the file is loaded.

Function: provide feature

This function announces that feature is now loaded, or being loaded, into the current Emacs session. This means that the facilities associated with feature are or will be available for other Lisp programs.

The direct effect of calling provide is to add feature to the front of the list features if it is not already in the list. The argument feature must be a symbol. provide returns feature.

features => (bar bish) (provide 'foo) => foo features => (foo bar bish)

When a file is loaded to satisfy an autoload, and it stops due to an error in the evaluating its contents, any function definitions or provide calls that occurred during the load are undone. See section 15.4 Autoload.

Function: require feature &optional filename noerror

This function checks whether feature is present in the current Emacs session (using (featurep feature); see below). The argument feature must be a symbol.

If the feature is not present, then require loads filename with load. If filename is not supplied, then the name of the symbol feature is used as the base file name to load. However, in this case, require insists on finding feature with an added suffix; a file whose name is just feature won't be used.

If loading the file fails to provide feature, require signals an error, `Required feature feature was not provided', unless noerror is non-nil.

Function: featurep feature

This function returns t if feature has been provided in the current Emacs session (i.e., if feature is a member of features.)

Variable: features

The value of this variable is a list of symbols that are the features loaded in the current Emacs session. Each symbol was put in this list with a call to provide. The order of the elements in the features list is not significant.

15.7 Unloading

You can discard the functions and variables loaded by a library to reclaim memory for other Lisp objects. To do this, use the function unload-feature:

Command: unload-feature feature &optional force

This command unloads the library that provided feature feature. It undefines all functions, macros, and variables defined in that library with defun, defalias, defsubst, defmacro, defconst, defvar, and defcustom. It then restores any autoloads formerly associated with those symbols. (Loading saves these in the autoload property of the symbol.)

Before restoring the previous definitions, unload-feature runs remove-hook to remove functions in the library from certain hooks. These hooks include variables whose names end in `hook' or `-hooks', plus those listed in loadhist-special-hooks. This is to prevent Emacs from ceasing to function because important hooks refer to functions that are no longer defined.

If these measures are not sufficient to prevent malfunction, a library can define an explicit unload hook. If feature-unload-hook is defined, it is run as a normal hook before restoring the previous definitions, instead of the usual hook-removing actions. The unload hook ought to undo all the global state changes made by the library that might cease to work once the library is unloaded. unload-feature can cause problems with libraries that fail to do this, so it should be used with caution.

Ordinarily, unload-feature refuses to unload a library on which other loaded libraries depend. (A library a depends on library b if a contains a require for b.) If the optional argument force is non-nil, dependencies are ignored and you can unload any library.

The unload-feature function is written in Lisp; its actions are based on the variable load-history.

Variable: load-history

This variable's value is an alist connecting library names with the names of functions and variables they define, the features they provide, and the features they require.

Each element is a list and describes one library. The CAR of the list is the name of the library, as a string. The rest of the list is composed of these kinds of objects:

• Symbols that were defined by this library.
• Cons cells of the form (require . feature) indicating features that were required.
• Cons cells of the form (provide . feature) indicating features that were provided.

The value of load-history may have one element whose CAR is nil. This element describes definitions made with eval-buffer on a buffer that is not visiting a file.

The command eval-region updates load-history, but does so by adding the symbols defined to the element for the file being visited, rather than replacing that element. See section 9.4 Eval.

Preloaded libraries don't contribute initially to load-history. Instead, preloading writes information about preloaded libraries into a file, which can be loaded later on to add information to load-history describing the preloaded files. This file is installed in exec-directory and has a name of the form `fns-emacsversion.el'.

See the source for the function symbol-file, for an example of code that loads this file to find functions in preloaded libraries.

Variable: loadhist-special-hooks

This variable holds a list of hooks to be scanned before unloading a library, to remove functions defined in the library.

15.8 Hooks for Loading

You can ask for code to be executed if and when a particular library is loaded, by calling eval-after-load.

Function: eval-after-load library form

This function arranges to evaluate form at the end of loading the library library, if and when library is loaded. If library is already loaded, it evaluates form right away.

The library name library must exactly match the argument of load. To get the proper results when an installed library is found by searching load-path, you should not include any directory names in library.

An error in form does not undo the load, but does prevent execution of the rest of form.

In general, well-designed Lisp programs should not use this feature. The clean and modular ways to interact with a Lisp library are (1) examine and set the library's variables (those which are meant for outside use), and (2) call the library's functions. If you wish to do (1), you can do it immediately--there is no need to wait for when the library is loaded. To do (2), you must load the library (preferably with require).

But it is OK to use eval-after-load in your personal customizations if you don't feel they must meet the design standards for programs meant for wider use.

Variable: after-load-alist

This variable holds an alist of expressions to evaluate if and when particular libraries are loaded. Each element looks like this:

(filename forms...)

The function load checks after-load-alist in order to implement eval-after-load.