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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
fd897522 GM |
3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999 |
4 | @c Free Software Foundation, Inc. | |
83ac6b45 RS |
5 | @c See the file elisp.texi for copying conditions. |
6 | @setfilename ../info/loading | |
f9f59935 | 7 | @node Loading, Byte Compilation, Customization, Top |
83ac6b45 RS |
8 | @chapter Loading |
9 | @cindex loading | |
10 | @cindex library | |
11 | @cindex Lisp library | |
12 | ||
13 | Loading a file of Lisp code means bringing its contents into the Lisp | |
14 | environment in the form of Lisp objects. Emacs finds and opens the | |
15 | file, reads the text, evaluates each form, and then closes the file. | |
16 | ||
17 | The load functions evaluate all the expressions in a file just | |
18 | as the @code{eval-current-buffer} function evaluates all the | |
19 | expressions in a buffer. The difference is that the load functions | |
20 | read and evaluate the text in the file as found on disk, not the text | |
21 | in an Emacs buffer. | |
22 | ||
23 | @cindex top-level form | |
24 | The loaded file must contain Lisp expressions, either as source code | |
78c71a98 RS |
25 | or as byte-compiled code. Each form in the file is called a |
26 | @dfn{top-level form}. There is no special format for the forms in a | |
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27 | loadable file; any form in a file may equally well be typed directly |
28 | into a buffer and evaluated there. (Indeed, most code is tested this | |
29 | way.) Most often, the forms are function definitions and variable | |
30 | definitions. | |
31 | ||
32 | A file containing Lisp code is often called a @dfn{library}. Thus, | |
33 | the ``Rmail library'' is a file containing code for Rmail mode. | |
34 | Similarly, a ``Lisp library directory'' is a directory of files | |
35 | containing Lisp code. | |
36 | ||
37 | @menu | |
38 | * How Programs Do Loading:: The @code{load} function and others. | |
a9f0a989 | 39 | * Library Search:: Finding a library to load. |
8241495d | 40 | * Loading Non-ASCII:: Non-@sc{ascii} characters in Emacs Lisp files. |
83ac6b45 RS |
41 | * Autoload:: Setting up a function to autoload. |
42 | * Repeated Loading:: Precautions about loading a file twice. | |
bfe721d1 | 43 | * Named Features:: Loading a library if it isn't already loaded. |
83ac6b45 RS |
44 | * Unloading:: How to ``unload'' a library that was loaded. |
45 | * Hooks for Loading:: Providing code to be run when | |
46 | particular libraries are loaded. | |
47 | @end menu | |
48 | ||
49 | @node How Programs Do Loading | |
50 | @section How Programs Do Loading | |
51 | ||
52 | Emacs Lisp has several interfaces for loading. For example, | |
f9f59935 RS |
53 | @code{autoload} creates a placeholder object for a function defined in a |
54 | file; trying to call the autoloading function loads the file to get the | |
83ac6b45 | 55 | function's real definition (@pxref{Autoload}). @code{require} loads a |
f9f59935 RS |
56 | file if it isn't already loaded (@pxref{Named Features}). Ultimately, |
57 | all these facilities call the @code{load} function to do the work. | |
83ac6b45 | 58 | |
a9f0a989 | 59 | @defun load filename &optional missing-ok nomessage nosuffix must-suffix |
83ac6b45 RS |
60 | This function finds and opens a file of Lisp code, evaluates all the |
61 | forms in it, and closes the file. | |
62 | ||
63 | To find the file, @code{load} first looks for a file named | |
64 | @file{@var{filename}.elc}, that is, for a file whose name is | |
65 | @var{filename} with @samp{.elc} appended. If such a file exists, it is | |
66 | loaded. If there is no file by that name, then @code{load} looks for a | |
78c71a98 | 67 | file named @file{@var{filename}.el}. If that file exists, it is loaded. |
83ac6b45 RS |
68 | Finally, if neither of those names is found, @code{load} looks for a |
69 | file named @var{filename} with nothing appended, and loads it if it | |
70 | exists. (The @code{load} function is not clever about looking at | |
71 | @var{filename}. In the perverse case of a file named @file{foo.el.el}, | |
72 | evaluation of @code{(load "foo.el")} will indeed find it.) | |
73 | ||
74 | If the optional argument @var{nosuffix} is non-@code{nil}, then the | |
75 | suffixes @samp{.elc} and @samp{.el} are not tried. In this case, you | |
f9f59935 RS |
76 | must specify the precise file name you want. By specifying the precise |
77 | file name and using @code{t} for @var{nosuffix}, you can prevent | |
78 | perverse file names such as @file{foo.el.el} from being tried. | |
83ac6b45 | 79 | |
a9f0a989 RS |
80 | If the optional argument @var{must-suffix} is non-@code{nil}, then |
81 | @code{load} insists that the file name used must end in either | |
82 | @samp{.el} or @samp{.elc}, unless it contains an explicit directory | |
83 | name. If @var{filename} does not contain an explicit directory name, | |
84 | and does not end in a suffix, then @code{load} insists on adding one. | |
85 | ||
83ac6b45 RS |
86 | If @var{filename} is a relative file name, such as @file{foo} or |
87 | @file{baz/foo.bar}, @code{load} searches for the file using the variable | |
88 | @code{load-path}. It appends @var{filename} to each of the directories | |
89 | listed in @code{load-path}, and loads the first file it finds whose name | |
90 | matches. The current default directory is tried only if it is specified | |
91 | in @code{load-path}, where @code{nil} stands for the default directory. | |
92 | @code{load} tries all three possible suffixes in the first directory in | |
93 | @code{load-path}, then all three suffixes in the second directory, and | |
a9f0a989 | 94 | so on. @xref{Library Search}. |
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95 | |
96 | If you get a warning that @file{foo.elc} is older than @file{foo.el}, it | |
97 | means you should consider recompiling @file{foo.el}. @xref{Byte | |
98 | Compilation}. | |
99 | ||
969fe9b5 RS |
100 | When loading a source file (not compiled), @code{load} performs |
101 | character set translation just as Emacs would do when visiting the file. | |
102 | @xref{Coding Systems}. | |
103 | ||
83ac6b45 RS |
104 | Messages like @samp{Loading foo...} and @samp{Loading foo...done} appear |
105 | in the echo area during loading unless @var{nomessage} is | |
106 | non-@code{nil}. | |
107 | ||
108 | @cindex load errors | |
109 | Any unhandled errors while loading a file terminate loading. If the | |
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110 | load was done for the sake of @code{autoload}, any function definitions |
111 | made during the loading are undone. | |
83ac6b45 RS |
112 | |
113 | @kindex file-error | |
114 | If @code{load} can't find the file to load, then normally it signals the | |
115 | error @code{file-error} (with @samp{Cannot open load file | |
116 | @var{filename}}). But if @var{missing-ok} is non-@code{nil}, then | |
117 | @code{load} just returns @code{nil}. | |
118 | ||
22697dac KH |
119 | You can use the variable @code{load-read-function} to specify a function |
120 | for @code{load} to use instead of @code{read} for reading expressions. | |
121 | See below. | |
122 | ||
83ac6b45 RS |
123 | @code{load} returns @code{t} if the file loads successfully. |
124 | @end defun | |
125 | ||
83ac6b45 | 126 | @deffn Command load-file filename |
f9f59935 RS |
127 | This command loads the file @var{filename}. If @var{filename} is a |
128 | relative file name, then the current default directory is assumed. | |
129 | @code{load-path} is not used, and suffixes are not appended. Use this | |
a9f0a989 | 130 | command if you wish to specify precisely the file name to load. |
83ac6b45 RS |
131 | @end deffn |
132 | ||
133 | @deffn Command load-library library | |
f9f59935 RS |
134 | This command loads the library named @var{library}. It is equivalent to |
135 | @code{load}, except in how it reads its argument interactively. | |
83ac6b45 | 136 | @end deffn |
83ac6b45 | 137 | |
a9f0a989 RS |
138 | @defvar load-in-progress |
139 | This variable is non-@code{nil} if Emacs is in the process of loading a | |
140 | file, and it is @code{nil} otherwise. | |
141 | @end defvar | |
142 | ||
143 | @defvar load-read-function | |
144 | This variable specifies an alternate expression-reading function for | |
145 | @code{load} and @code{eval-region} to use instead of @code{read}. | |
146 | The function should accept one argument, just as @code{read} does. | |
147 | ||
148 | Normally, the variable's value is @code{nil}, which means those | |
149 | functions should use @code{read}. | |
55607887 RS |
150 | |
151 | @strong{Note:} Instead of using this variable, it is cleaner to use | |
152 | another, newer feature: to pass the function as the @var{read-function} | |
153 | argument to @code{eval-region}. @xref{Eval}. | |
a9f0a989 RS |
154 | @end defvar |
155 | ||
1911e6e5 RS |
156 | For information about how @code{load} is used in building Emacs, see |
157 | @ref{Building Emacs}. | |
a9f0a989 RS |
158 | |
159 | @node Library Search | |
160 | @section Library Search | |
161 | ||
162 | When Emacs loads a Lisp library, it searches for the library | |
163 | in a list of directories specified by the variable @code{load-path}. | |
164 | ||
83ac6b45 RS |
165 | @defopt load-path |
166 | @cindex @code{EMACSLOADPATH} environment variable | |
167 | The value of this variable is a list of directories to search when | |
168 | loading files with @code{load}. Each element is a string (which must be | |
169 | a directory name) or @code{nil} (which stands for the current working | |
a9f0a989 RS |
170 | directory). |
171 | @end defopt | |
172 | ||
173 | The value of @code{load-path} is initialized from the environment | |
174 | variable @code{EMACSLOADPATH}, if that exists; otherwise its default | |
175 | value is specified in @file{emacs/src/paths.h} when Emacs is built. | |
176 | Then the list is expanded by adding subdirectories of the directories | |
177 | in the list. | |
83ac6b45 | 178 | |
a9f0a989 | 179 | The syntax of @code{EMACSLOADPATH} is the same as used for @code{PATH}; |
bfe721d1 KH |
180 | @samp{:} (or @samp{;}, according to the operating system) separates |
181 | directory names, and @samp{.} is used for the current default directory. | |
182 | Here is an example of how to set your @code{EMACSLOADPATH} variable from | |
183 | a @code{csh} @file{.login} file: | |
83ac6b45 | 184 | |
83ac6b45 | 185 | @smallexample |
f1e2c45e | 186 | setenv EMACSLOADPATH .:/user/bil/emacs:/usr/local/share/emacs/20.3/lisp |
83ac6b45 RS |
187 | @end smallexample |
188 | ||
a9f0a989 | 189 | Here is how to set it using @code{sh}: |
83ac6b45 RS |
190 | |
191 | @smallexample | |
192 | export EMACSLOADPATH | |
f1e2c45e | 193 | EMACSLOADPATH=.:/user/bil/emacs:/usr/local/share/emacs/20.3/lisp |
83ac6b45 RS |
194 | @end smallexample |
195 | ||
a40d4712 PR |
196 | Here is an example of code you can place in your init file (@pxref{Init |
197 | File}) to add several directories to the front of your default | |
198 | @code{load-path}: | |
83ac6b45 RS |
199 | |
200 | @smallexample | |
bda144f4 | 201 | @group |
83ac6b45 RS |
202 | (setq load-path |
203 | (append (list nil "/user/bil/emacs" | |
204 | "/usr/local/lisplib" | |
5e41cf03 | 205 | "~/emacs") |
83ac6b45 | 206 | load-path)) |
bda144f4 | 207 | @end group |
83ac6b45 RS |
208 | @end smallexample |
209 | ||
210 | @c Wordy to rid us of an overfull hbox. --rjc 15mar92 | |
211 | @noindent | |
212 | In this example, the path searches the current working directory first, | |
5e41cf03 RS |
213 | followed then by the @file{/user/bil/emacs} directory, the |
214 | @file{/usr/local/lisplib} directory, and the @file{~/emacs} directory, | |
83ac6b45 RS |
215 | which are then followed by the standard directories for Lisp code. |
216 | ||
a9f0a989 | 217 | Dumping Emacs uses a special value of @code{load-path}. If the value of |
c642171c RS |
218 | @code{load-path} at the end of dumping is unchanged (that is, still the |
219 | same special value), the dumped Emacs switches to the ordinary | |
cc8c51f1 | 220 | @code{load-path} value when it starts up, as described above. But if |
c642171c RS |
221 | @code{load-path} has any other value at the end of dumping, that value |
222 | is used for execution of the dumped Emacs also. | |
223 | ||
a9f0a989 | 224 | Therefore, if you want to change @code{load-path} temporarily for |
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225 | loading a few libraries in @file{site-init.el} or @file{site-load.el}, |
226 | you should bind @code{load-path} locally with @code{let} around the | |
227 | calls to @code{load}. | |
83ac6b45 | 228 | |
089e089d | 229 | The default value of @code{load-path}, when running an Emacs which has |
a9f0a989 RS |
230 | been installed on the system, includes two special directories (and |
231 | their subdirectories as well): | |
089e089d RS |
232 | |
233 | @smallexample | |
a9f0a989 | 234 | "/usr/local/share/emacs/@var{version}/site-lisp" |
089e089d RS |
235 | @end smallexample |
236 | ||
a9f0a989 RS |
237 | @noindent |
238 | and | |
239 | ||
240 | @smallexample | |
241 | "/usr/local/share/emacs/site-lisp" | |
242 | @end smallexample | |
243 | ||
244 | @noindent | |
245 | The first one is for locally installed packages for a particular Emacs | |
246 | version; the second is for locally installed packages meant for use with | |
247 | all installed Emacs versions. | |
089e089d RS |
248 | |
249 | There are several reasons why a Lisp package that works well in one | |
250 | Emacs version can cause trouble in another. Sometimes packages need | |
251 | updating for incompatible changes in Emacs; sometimes they depend on | |
252 | undocumented internal Emacs data that can change without notice; | |
253 | sometimes a newer Emacs version incorporates a version of the package, | |
254 | and should be used only with that version. | |
255 | ||
a9f0a989 RS |
256 | Emacs finds these directories' subdirectories and adds them to |
257 | @code{load-path} when it starts up. Both immediate subdirectories and | |
258 | subdirectories multiple levels down are added to @code{load-path}. | |
259 | ||
260 | Not all subdirectories are included, though. Subdirectories whose | |
261 | names do not start with a letter or digit are excluded. Subdirectories | |
8241495d RS |
262 | named @file{RCS} or @file{CVS} are excluded. Also, a subdirectory which |
263 | contains a file named @file{.nosearch} is excluded. You can use these | |
264 | methods to prevent certain subdirectories of the @file{site-lisp} | |
265 | directories from being searched. | |
a9f0a989 | 266 | |
089e089d RS |
267 | If you run Emacs from the directory where it was built---that is, an |
268 | executable that has not been formally installed---then @code{load-path} | |
269 | normally contains two additional directories. These are the @code{lisp} | |
270 | and @code{site-lisp} subdirectories of the main build directory. (Both | |
271 | are represented as absolute file names.) | |
272 | ||
f9f59935 RS |
273 | @deffn Command locate-library library &optional nosuffix path interactive-call |
274 | This command finds the precise file name for library @var{library}. It | |
275 | searches for the library in the same way @code{load} does, and the | |
276 | argument @var{nosuffix} has the same meaning as in @code{load}: don't | |
277 | add suffixes @samp{.elc} or @samp{.el} to the specified name | |
278 | @var{library}. | |
279 | ||
280 | If the @var{path} is non-@code{nil}, that list of directories is used | |
281 | instead of @code{load-path}. | |
282 | ||
283 | When @code{locate-library} is called from a program, it returns the file | |
284 | name as a string. When the user runs @code{locate-library} | |
285 | interactively, the argument @var{interactive-call} is @code{t}, and this | |
286 | tells @code{locate-library} to display the file name in the echo area. | |
287 | @end deffn | |
288 | ||
a9f0a989 | 289 | @node Loading Non-ASCII |
75708135 | 290 | @section Loading Non-@sc{ascii} Characters |
a9f0a989 | 291 | |
8241495d | 292 | When Emacs Lisp programs contain string constants with non-@sc{ascii} |
a9f0a989 RS |
293 | characters, these can be represented within Emacs either as unibyte |
294 | strings or as multibyte strings (@pxref{Text Representations}). Which | |
295 | representation is used depends on how the file is read into Emacs. If | |
296 | it is read with decoding into multibyte representation, the text of the | |
297 | Lisp program will be multibyte text, and its string constants will be | |
298 | multibyte strings. If a file containing Latin-1 characters (for | |
299 | example) is read without decoding, the text of the program will be | |
300 | unibyte text, and its string constants will be unibyte strings. | |
301 | @xref{Coding Systems}. | |
302 | ||
303 | To make the results more predictable, Emacs always performs decoding | |
304 | into the multibyte representation when loading Lisp files, even if it | |
305 | was started with the @samp{--unibyte} option. This means that string | |
8241495d | 306 | constants with non-@sc{ascii} characters translate into multibyte |
a9f0a989 RS |
307 | strings. The only exception is when a particular file specifies no |
308 | decoding. | |
309 | ||
310 | The reason Emacs is designed this way is so that Lisp programs give | |
311 | predictable results, regardless of how Emacs was started. In addition, | |
312 | this enables programs that depend on using multibyte text to work even | |
313 | in a unibyte Emacs. Of course, such programs should be designed to | |
314 | notice whether the user prefers unibyte or multibyte text, by checking | |
315 | @code{default-enable-multibyte-characters}, and convert representations | |
316 | appropriately. | |
317 | ||
8241495d | 318 | In most Emacs Lisp programs, the fact that non-@sc{ascii} strings are |
a9f0a989 RS |
319 | multibyte strings should not be noticeable, since inserting them in |
320 | unibyte buffers converts them to unibyte automatically. However, if | |
321 | this does make a difference, you can force a particular Lisp file to be | |
430f8c73 | 322 | interpreted as unibyte by writing @samp{-*-unibyte: t;-*-} in a |
a9f0a989 | 323 | comment on the file's first line. With that designator, the file will |
8241495d | 324 | unconditionally be interpreted as unibyte, even in an ordinary |
a9f0a989 RS |
325 | multibyte Emacs session. |
326 | ||
83ac6b45 RS |
327 | @node Autoload |
328 | @section Autoload | |
329 | @cindex autoload | |
330 | ||
331 | The @dfn{autoload} facility allows you to make a function or macro | |
bfe721d1 KH |
332 | known in Lisp, but put off loading the file that defines it. The first |
333 | call to the function automatically reads the proper file to install the | |
334 | real definition and other associated code, then runs the real definition | |
83ac6b45 RS |
335 | as if it had been loaded all along. |
336 | ||
337 | There are two ways to set up an autoloaded function: by calling | |
338 | @code{autoload}, and by writing a special ``magic'' comment in the | |
339 | source before the real definition. @code{autoload} is the low-level | |
340 | primitive for autoloading; any Lisp program can call @code{autoload} at | |
969fe9b5 | 341 | any time. Magic comments are the most convenient way to make a function |
a9f0a989 RS |
342 | autoload, for packages installed along with Emacs. These comments do |
343 | nothing on their own, but they serve as a guide for the command | |
969fe9b5 RS |
344 | @code{update-file-autoloads}, which constructs calls to @code{autoload} |
345 | and arranges to execute them when Emacs is built. | |
83ac6b45 | 346 | |
78c71a98 RS |
347 | @defun autoload function filename &optional docstring interactive type |
348 | This function defines the function (or macro) named @var{function} so as | |
83ac6b45 RS |
349 | to load automatically from @var{filename}. The string @var{filename} |
350 | specifies the file to load to get the real definition of @var{function}. | |
351 | ||
f9f59935 RS |
352 | If @var{filename} does not contain either a directory name, or the |
353 | suffix @code{.el} or @code{.elc}, then @code{autoload} insists on adding | |
354 | one of these suffixes, and it will not load from a file whose name is | |
355 | just @var{filename} with no added suffix. | |
356 | ||
83ac6b45 | 357 | The argument @var{docstring} is the documentation string for the |
f9f59935 | 358 | function. Normally, this should be identical to the documentation string |
83ac6b45 RS |
359 | in the function definition itself. Specifying the documentation string |
360 | in the call to @code{autoload} makes it possible to look at the | |
361 | documentation without loading the function's real definition. | |
362 | ||
969fe9b5 RS |
363 | If @var{interactive} is non-@code{nil}, that says @var{function} can be |
364 | called interactively. This lets completion in @kbd{M-x} work without | |
a9f0a989 RS |
365 | loading @var{function}'s real definition. The complete interactive |
366 | specification is not given here; it's not needed unless the user | |
367 | actually calls @var{function}, and when that happens, it's time to load | |
368 | the real definition. | |
83ac6b45 RS |
369 | |
370 | You can autoload macros and keymaps as well as ordinary functions. | |
371 | Specify @var{type} as @code{macro} if @var{function} is really a macro. | |
372 | Specify @var{type} as @code{keymap} if @var{function} is really a | |
373 | keymap. Various parts of Emacs need to know this information without | |
374 | loading the real definition. | |
375 | ||
bda144f4 MW |
376 | An autoloaded keymap loads automatically during key lookup when a prefix |
377 | key's binding is the symbol @var{function}. Autoloading does not occur | |
378 | for other kinds of access to the keymap. In particular, it does not | |
379 | happen when a Lisp program gets the keymap from the value of a variable | |
380 | and calls @code{define-key}; not even if the variable name is the same | |
381 | symbol @var{function}. | |
382 | ||
83ac6b45 | 383 | @cindex function cell in autoload |
78c71a98 | 384 | If @var{function} already has a non-void function definition that is not |
83ac6b45 | 385 | an autoload object, @code{autoload} does nothing and returns @code{nil}. |
78c71a98 | 386 | If the function cell of @var{function} is void, or is already an autoload |
83ac6b45 RS |
387 | object, then it is defined as an autoload object like this: |
388 | ||
389 | @example | |
390 | (autoload @var{filename} @var{docstring} @var{interactive} @var{type}) | |
391 | @end example | |
392 | ||
393 | For example, | |
394 | ||
395 | @example | |
bda144f4 | 396 | @group |
83ac6b45 RS |
397 | (symbol-function 'run-prolog) |
398 | @result{} (autoload "prolog" 169681 t nil) | |
bda144f4 | 399 | @end group |
83ac6b45 RS |
400 | @end example |
401 | ||
402 | @noindent | |
403 | In this case, @code{"prolog"} is the name of the file to load, 169681 | |
f9f59935 RS |
404 | refers to the documentation string in the |
405 | @file{emacs/etc/DOC-@var{version}} file (@pxref{Documentation Basics}), | |
406 | @code{t} means the function is interactive, and @code{nil} that it is | |
407 | not a macro or a keymap. | |
83ac6b45 RS |
408 | @end defun |
409 | ||
410 | @cindex autoload errors | |
411 | The autoloaded file usually contains other definitions and may require | |
412 | or provide one or more features. If the file is not completely loaded | |
413 | (due to an error in the evaluation of its contents), any function | |
414 | definitions or @code{provide} calls that occurred during the load are | |
415 | undone. This is to ensure that the next attempt to call any function | |
416 | autoloading from this file will try again to load the file. If not for | |
a9f0a989 RS |
417 | this, then some of the functions in the file might be defined by the |
418 | aborted load, but fail to work properly for the lack of certain | |
419 | subroutines not loaded successfully because they come later in the file. | |
83ac6b45 RS |
420 | |
421 | If the autoloaded file fails to define the desired Lisp function or | |
422 | macro, then an error is signaled with data @code{"Autoloading failed to | |
423 | define function @var{function-name}"}. | |
424 | ||
425 | @findex update-file-autoloads | |
426 | @findex update-directory-autoloads | |
a9f0a989 | 427 | A magic autoload comment consists of @samp{;;;###autoload}, on a line |
83ac6b45 RS |
428 | by itself, just before the real definition of the function in its |
429 | autoloadable source file. The command @kbd{M-x update-file-autoloads} | |
430 | writes a corresponding @code{autoload} call into @file{loaddefs.el}. | |
431 | Building Emacs loads @file{loaddefs.el} and thus calls @code{autoload}. | |
432 | @kbd{M-x update-directory-autoloads} is even more powerful; it updates | |
433 | autoloads for all files in the current directory. | |
434 | ||
435 | The same magic comment can copy any kind of form into | |
436 | @file{loaddefs.el}. If the form following the magic comment is not a | |
8241495d RS |
437 | function-defining form or a @code{defcustom} form, it is copied |
438 | verbatim. ``Function-defining forms'' include @code{define-skeleton}, | |
439 | @code{define-derived-mode}, @code{define-generic-mode} and | |
440 | @code{easy-mmode-define-minor-mode} as well as @code{defun} and | |
441 | @code{defmacro}. To save space, a @code{defcustom} form is converted to | |
442 | a @code{defvar} in @file{loaddefs.el}, with some additional information | |
443 | if it uses @code{:require}. | |
444 | ||
445 | You can also use a magic comment to execute a form at build time | |
446 | @emph{without} executing it when the file itself is loaded. To do this, | |
447 | write the form @emph{on the same line} as the magic comment. Since it | |
448 | is in a comment, it does nothing when you load the source file; but | |
449 | @kbd{M-x update-file-autoloads} copies it to @file{loaddefs.el}, where | |
450 | it is executed while building Emacs. | |
83ac6b45 RS |
451 | |
452 | The following example shows how @code{doctor} is prepared for | |
453 | autoloading with a magic comment: | |
454 | ||
455 | @smallexample | |
456 | ;;;###autoload | |
457 | (defun doctor () | |
458 | "Switch to *doctor* buffer and start giving psychotherapy." | |
459 | (interactive) | |
460 | (switch-to-buffer "*doctor*") | |
461 | (doctor-mode)) | |
462 | @end smallexample | |
463 | ||
464 | @noindent | |
465 | Here's what that produces in @file{loaddefs.el}: | |
466 | ||
467 | @smallexample | |
8241495d | 468 | (autoload 'doctor "doctor" "\ |
83ac6b45 RS |
469 | Switch to *doctor* buffer and start giving psychotherapy." |
470 | t) | |
471 | @end smallexample | |
472 | ||
473 | @noindent | |
474 | The backslash and newline immediately following the double-quote are a | |
8241495d | 475 | convention used only in the preloaded uncompiled Lisp files such as |
83ac6b45 RS |
476 | @file{loaddefs.el}; they tell @code{make-docfile} to put the |
477 | documentation string in the @file{etc/DOC} file. @xref{Building Emacs}. | |
8241495d | 478 | See also the commentary in @file{lib-src/make-docfile.c}. |
83ac6b45 RS |
479 | |
480 | @node Repeated Loading | |
83ac6b45 RS |
481 | @section Repeated Loading |
482 | @cindex repeated loading | |
483 | ||
a9f0a989 | 484 | You can load a given file more than once in an Emacs session. For |
83ac6b45 RS |
485 | example, after you have rewritten and reinstalled a function definition |
486 | by editing it in a buffer, you may wish to return to the original | |
487 | version; you can do this by reloading the file it came from. | |
488 | ||
489 | When you load or reload files, bear in mind that the @code{load} and | |
490 | @code{load-library} functions automatically load a byte-compiled file | |
491 | rather than a non-compiled file of similar name. If you rewrite a file | |
f9f59935 RS |
492 | that you intend to save and reinstall, you need to byte-compile the new |
493 | version; otherwise Emacs will load the older, byte-compiled file instead | |
494 | of your newer, non-compiled file! If that happens, the message | |
a9f0a989 | 495 | displayed when loading the file includes, @samp{(compiled; note, source is |
969fe9b5 | 496 | newer)}, to remind you to recompile it. |
83ac6b45 RS |
497 | |
498 | When writing the forms in a Lisp library file, keep in mind that the | |
f9f59935 RS |
499 | file might be loaded more than once. For example, think about whether |
500 | each variable should be reinitialized when you reload the library; | |
501 | @code{defvar} does not change the value if the variable is already | |
502 | initialized. (@xref{Defining Variables}.) | |
83ac6b45 RS |
503 | |
504 | The simplest way to add an element to an alist is like this: | |
505 | ||
506 | @example | |
507 | (setq minor-mode-alist | |
508 | (cons '(leif-mode " Leif") minor-mode-alist)) | |
509 | @end example | |
510 | ||
511 | @noindent | |
512 | But this would add multiple elements if the library is reloaded. | |
513 | To avoid the problem, write this: | |
514 | ||
515 | @example | |
516 | (or (assq 'leif-mode minor-mode-alist) | |
517 | (setq minor-mode-alist | |
518 | (cons '(leif-mode " Leif") minor-mode-alist))) | |
519 | @end example | |
520 | ||
a9f0a989 | 521 | To add an element to a list just once, you can also use @code{add-to-list} |
bfe721d1 KH |
522 | (@pxref{Setting Variables}). |
523 | ||
83ac6b45 RS |
524 | Occasionally you will want to test explicitly whether a library has |
525 | already been loaded. Here's one way to test, in a library, whether it | |
526 | has been loaded before: | |
527 | ||
528 | @example | |
969fe9b5 | 529 | (defvar foo-was-loaded nil) |
bfe721d1 | 530 | |
969fe9b5 RS |
531 | (unless foo-was-loaded |
532 | @var{execute-first-time-only} | |
533 | (setq foo-was-loaded t)) | |
83ac6b45 RS |
534 | @end example |
535 | ||
536 | @noindent | |
537 | If the library uses @code{provide} to provide a named feature, you can | |
969fe9b5 RS |
538 | use @code{featurep} earlier in the file to test whether the |
539 | @code{provide} call has been executed before. | |
37680279 | 540 | @ifnottex |
bfe721d1 | 541 | @xref{Named Features}. |
37680279 | 542 | @end ifnottex |
83ac6b45 | 543 | |
bfe721d1 | 544 | @node Named Features |
83ac6b45 RS |
545 | @section Features |
546 | @cindex features | |
547 | @cindex requiring features | |
548 | @cindex providing features | |
549 | ||
550 | @code{provide} and @code{require} are an alternative to | |
551 | @code{autoload} for loading files automatically. They work in terms of | |
552 | named @dfn{features}. Autoloading is triggered by calling a specific | |
553 | function, but a feature is loaded the first time another program asks | |
554 | for it by name. | |
555 | ||
556 | A feature name is a symbol that stands for a collection of functions, | |
557 | variables, etc. The file that defines them should @dfn{provide} the | |
558 | feature. Another program that uses them may ensure they are defined by | |
559 | @dfn{requiring} the feature. This loads the file of definitions if it | |
560 | hasn't been loaded already. | |
561 | ||
562 | To require the presence of a feature, call @code{require} with the | |
563 | feature name as argument. @code{require} looks in the global variable | |
564 | @code{features} to see whether the desired feature has been provided | |
565 | already. If not, it loads the feature from the appropriate file. This | |
78c71a98 | 566 | file should call @code{provide} at the top level to add the feature to |
83ac6b45 RS |
567 | @code{features}; if it fails to do so, @code{require} signals an error. |
568 | @cindex load error with require | |
569 | ||
83ac6b45 RS |
570 | For example, in @file{emacs/lisp/prolog.el}, |
571 | the definition for @code{run-prolog} includes the following code: | |
572 | ||
573 | @smallexample | |
574 | (defun run-prolog () | |
9e2b495b | 575 | "Run an inferior Prolog process, with I/O via buffer *prolog*." |
83ac6b45 RS |
576 | (interactive) |
577 | (require 'comint) | |
578 | (switch-to-buffer (make-comint "prolog" prolog-program-name)) | |
579 | (inferior-prolog-mode)) | |
580 | @end smallexample | |
581 | ||
582 | @noindent | |
583 | The expression @code{(require 'comint)} loads the file @file{comint.el} | |
584 | if it has not yet been loaded. This ensures that @code{make-comint} is | |
969fe9b5 RS |
585 | defined. Features are normally named after the files that provide them, |
586 | so that @code{require} need not be given the file name. | |
83ac6b45 RS |
587 | |
588 | The @file{comint.el} file contains the following top-level expression: | |
589 | ||
590 | @smallexample | |
591 | (provide 'comint) | |
592 | @end smallexample | |
593 | ||
594 | @noindent | |
595 | This adds @code{comint} to the global @code{features} list, so that | |
596 | @code{(require 'comint)} will henceforth know that nothing needs to be | |
597 | done. | |
598 | ||
599 | @cindex byte-compiling @code{require} | |
78c71a98 | 600 | When @code{require} is used at top level in a file, it takes effect |
83ac6b45 RS |
601 | when you byte-compile that file (@pxref{Byte Compilation}) as well as |
602 | when you load it. This is in case the required package contains macros | |
8241495d RS |
603 | that the byte compiler must know about. It also avoids byte-compiler |
604 | warnings for functions and variables defined in the file loaded with | |
605 | @code{require}. | |
83ac6b45 RS |
606 | |
607 | Although top-level calls to @code{require} are evaluated during | |
608 | byte compilation, @code{provide} calls are not. Therefore, you can | |
609 | ensure that a file of definitions is loaded before it is byte-compiled | |
610 | by including a @code{provide} followed by a @code{require} for the same | |
611 | feature, as in the following example. | |
612 | ||
613 | @smallexample | |
614 | @group | |
615 | (provide 'my-feature) ; @r{Ignored by byte compiler,} | |
616 | ; @r{evaluated by @code{load}.} | |
617 | (require 'my-feature) ; @r{Evaluated by byte compiler.} | |
618 | @end group | |
619 | @end smallexample | |
620 | ||
78c71a98 RS |
621 | @noindent |
622 | The compiler ignores the @code{provide}, then processes the | |
623 | @code{require} by loading the file in question. Loading the file does | |
624 | execute the @code{provide} call, so the subsequent @code{require} call | |
969fe9b5 | 625 | does nothing when the file is loaded. |
78c71a98 | 626 | |
83ac6b45 RS |
627 | @defun provide feature |
628 | This function announces that @var{feature} is now loaded, or being | |
629 | loaded, into the current Emacs session. This means that the facilities | |
630 | associated with @var{feature} are or will be available for other Lisp | |
631 | programs. | |
632 | ||
633 | The direct effect of calling @code{provide} is to add @var{feature} to | |
634 | the front of the list @code{features} if it is not already in the list. | |
635 | The argument @var{feature} must be a symbol. @code{provide} returns | |
636 | @var{feature}. | |
637 | ||
638 | @smallexample | |
639 | features | |
640 | @result{} (bar bish) | |
641 | ||
642 | (provide 'foo) | |
643 | @result{} foo | |
644 | features | |
645 | @result{} (foo bar bish) | |
646 | @end smallexample | |
647 | ||
bfe721d1 KH |
648 | When a file is loaded to satisfy an autoload, and it stops due to an |
649 | error in the evaluating its contents, any function definitions or | |
650 | @code{provide} calls that occurred during the load are undone. | |
651 | @xref{Autoload}. | |
83ac6b45 RS |
652 | @end defun |
653 | ||
b6954afd | 654 | @defun require feature &optional filename noerror |
83ac6b45 | 655 | This function checks whether @var{feature} is present in the current |
f9f59935 RS |
656 | Emacs session (using @code{(featurep @var{feature})}; see below). The |
657 | argument @var{feature} must be a symbol. | |
658 | ||
659 | If the feature is not present, then @code{require} loads @var{filename} | |
660 | with @code{load}. If @var{filename} is not supplied, then the name of | |
661 | the symbol @var{feature} is used as the base file name to load. | |
662 | However, in this case, @code{require} insists on finding @var{feature} | |
663 | with an added suffix; a file whose name is just @var{feature} won't be | |
664 | used. | |
83ac6b45 RS |
665 | |
666 | If loading the file fails to provide @var{feature}, @code{require} | |
667 | signals an error, @samp{Required feature @var{feature} was not | |
b6954afd | 668 | provided}, unless @var{noerror} is non-@code{nil}. |
83ac6b45 RS |
669 | @end defun |
670 | ||
671 | @defun featurep feature | |
672 | This function returns @code{t} if @var{feature} has been provided in the | |
969fe9b5 | 673 | current Emacs session (i.e., if @var{feature} is a member of |
83ac6b45 RS |
674 | @code{features}.) |
675 | @end defun | |
676 | ||
677 | @defvar features | |
678 | The value of this variable is a list of symbols that are the features | |
679 | loaded in the current Emacs session. Each symbol was put in this list | |
680 | with a call to @code{provide}. The order of the elements in the | |
681 | @code{features} list is not significant. | |
682 | @end defvar | |
683 | ||
684 | @node Unloading | |
685 | @section Unloading | |
686 | @cindex unloading | |
687 | ||
688 | @c Emacs 19 feature | |
689 | You can discard the functions and variables loaded by a library to | |
690 | reclaim memory for other Lisp objects. To do this, use the function | |
691 | @code{unload-feature}: | |
692 | ||
ee6bcc94 | 693 | @deffn Command unload-feature feature &optional force |
83ac6b45 | 694 | This command unloads the library that provided feature @var{feature}. |
78c71a98 | 695 | It undefines all functions, macros, and variables defined in that |
969fe9b5 RS |
696 | library with @code{defun}, @code{defalias}, @code{defsubst}, |
697 | @code{defmacro}, @code{defconst}, @code{defvar}, and @code{defcustom}. | |
698 | It then restores any autoloads formerly associated with those symbols. | |
699 | (Loading saves these in the @code{autoload} property of the symbol.) | |
ee6bcc94 | 700 | |
6582d61e RS |
701 | Before restoring the previous definitions, @code{unload-feature} runs |
702 | @code{remove-hook} to remove functions in the library from certain | |
703 | hooks. These hooks include variables whose names end in @samp{hook} or | |
704 | @samp{-hooks}, plus those listed in @code{loadhist-special-hooks}. This | |
705 | is to prevent Emacs from ceasing to function because important hooks | |
706 | refer to functions that are no longer defined. | |
707 | ||
708 | @vindex @var{feature}-unload-hook | |
709 | If these measures are not sufficient to prevent malfunction, a library | |
710 | can define an explicit unload hook. If @code{@var{feature}-unload-hook} | |
711 | is defined, it is run as a normal hook before restoring the previous | |
712 | definitions, @emph{instead of} the usual hook-removing actions. The | |
713 | unload hook ought to undo all the global state changes made by the | |
714 | library that might cease to work once the library is unloaded. | |
8241495d RS |
715 | @code{unload-feature} can cause problems with libraries that fail to do |
716 | this, so it should be used with caution. | |
6582d61e | 717 | |
ee6bcc94 RS |
718 | Ordinarily, @code{unload-feature} refuses to unload a library on which |
719 | other loaded libraries depend. (A library @var{a} depends on library | |
720 | @var{b} if @var{a} contains a @code{require} for @var{b}.) If the | |
721 | optional argument @var{force} is non-@code{nil}, dependencies are | |
722 | ignored and you can unload any library. | |
83ac6b45 RS |
723 | @end deffn |
724 | ||
725 | The @code{unload-feature} function is written in Lisp; its actions are | |
726 | based on the variable @code{load-history}. | |
727 | ||
728 | @defvar load-history | |
729 | This variable's value is an alist connecting library names with the | |
730 | names of functions and variables they define, the features they provide, | |
731 | and the features they require. | |
732 | ||
733 | Each element is a list and describes one library. The @sc{car} of the | |
734 | list is the name of the library, as a string. The rest of the list is | |
735 | composed of these kinds of objects: | |
736 | ||
737 | @itemize @bullet | |
738 | @item | |
78c71a98 | 739 | Symbols that were defined by this library. |
83ac6b45 RS |
740 | @item |
741 | Lists of the form @code{(require . @var{feature})} indicating | |
742 | features that were required. | |
743 | @item | |
744 | Lists of the form @code{(provide . @var{feature})} indicating | |
745 | features that were provided. | |
746 | @end itemize | |
747 | ||
748 | The value of @code{load-history} may have one element whose @sc{car} is | |
749 | @code{nil}. This element describes definitions made with | |
750 | @code{eval-buffer} on a buffer that is not visiting a file. | |
751 | @end defvar | |
752 | ||
753 | The command @code{eval-region} updates @code{load-history}, but does so | |
754 | by adding the symbols defined to the element for the file being visited, | |
55607887 | 755 | rather than replacing that element. @xref{Eval}. |
83ac6b45 | 756 | |
8241495d RS |
757 | Preloaded libraries don't contribute initially to @code{load-history}. |
758 | Instead, preloading writes information about preloaded libraries into a | |
05aea714 | 759 | file, which can be loaded later on to add information to |
8241495d RS |
760 | @code{load-history} describing the preloaded files. This file is |
761 | installed in @code{exec-directory} and has a name of the form | |
762 | @file{fns-@var{emacsversion}.el}. | |
763 | ||
764 | @findex symbol-file | |
765 | See the source for the function @code{symbol-file}, for an example of | |
766 | code that loads this file to find functions in preloaded libraries. | |
6582d61e | 767 | |
6582d61e RS |
768 | @defvar loadhist-special-hooks |
769 | This variable holds a list of hooks to be scanned before unloading a | |
770 | library, to remove functions defined in the library. | |
771 | @end defvar | |
772 | ||
83ac6b45 RS |
773 | @node Hooks for Loading |
774 | @section Hooks for Loading | |
775 | @cindex loading hooks | |
776 | @cindex hooks for loading | |
777 | ||
778 | You can ask for code to be executed if and when a particular library is | |
779 | loaded, by calling @code{eval-after-load}. | |
780 | ||
781 | @defun eval-after-load library form | |
782 | This function arranges to evaluate @var{form} at the end of loading the | |
d2e9ee06 RS |
783 | library @var{library}, if and when @var{library} is loaded. If |
784 | @var{library} is already loaded, it evaluates @var{form} right away. | |
83ac6b45 RS |
785 | |
786 | The library name @var{library} must exactly match the argument of | |
787 | @code{load}. To get the proper results when an installed library is | |
788 | found by searching @code{load-path}, you should not include any | |
789 | directory names in @var{library}. | |
790 | ||
791 | An error in @var{form} does not undo the load, but does prevent | |
792 | execution of the rest of @var{form}. | |
793 | @end defun | |
794 | ||
d2e9ee06 RS |
795 | In general, well-designed Lisp programs should not use this feature. |
796 | The clean and modular ways to interact with a Lisp library are (1) | |
797 | examine and set the library's variables (those which are meant for | |
cc8c51f1 | 798 | outside use), and (2) call the library's functions. If you wish to |
d2e9ee06 RS |
799 | do (1), you can do it immediately---there is no need to wait for when |
800 | the library is loaded. To do (2), you must load the library (preferably | |
801 | with @code{require}). | |
802 | ||
969fe9b5 RS |
803 | But it is OK to use @code{eval-after-load} in your personal |
804 | customizations if you don't feel they must meet the design standards for | |
805 | programs meant for wider use. | |
d2e9ee06 | 806 | |
83ac6b45 | 807 | @defvar after-load-alist |
8241495d RS |
808 | This variable holds an alist of expressions to evaluate if and when |
809 | particular libraries are loaded. Each element looks like this: | |
83ac6b45 RS |
810 | |
811 | @example | |
812 | (@var{filename} @var{forms}@dots{}) | |
813 | @end example | |
814 | ||
815 | The function @code{load} checks @code{after-load-alist} in order to | |
816 | implement @code{eval-after-load}. | |
817 | @end defvar | |
818 | ||
819 | @c Emacs 19 feature |