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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
3 | @c Copyright (C) 1990, 1991, 1992, 1993 Free Software Foundation, Inc. | |
4 | @c See the file elisp.texi for copying conditions. | |
5 | @setfilename ../info/internals | |
6 | @node GNU Emacs Internals, Standard Errors, Tips, Top | |
7 | @comment node-name, next, previous, up | |
8 | @appendix GNU Emacs Internals | |
9 | ||
10 | This chapter describes how the runnable Emacs executable is dumped with | |
11 | the preloaded Lisp libraries in it, how storage is allocated, and some | |
12 | internal aspects of GNU Emacs that may be of interest to C programmers. | |
13 | ||
14 | @menu | |
15 | * Building Emacs:: How to preload Lisp libraries into Emacs. | |
16 | * Pure Storage:: A kludge to make preloaded Lisp functions sharable. | |
17 | * Garbage Collection:: Reclaiming space for Lisp objects no longer used. | |
18 | * Writing Emacs Primitives:: Writing C code for Emacs. | |
19 | * Object Internals:: Data formats of buffers, windows, processes. | |
20 | @end menu | |
21 | ||
22 | @node Building Emacs, Pure Storage, GNU Emacs Internals, GNU Emacs Internals | |
23 | @appendixsec Building Emacs | |
24 | @cindex building Emacs | |
25 | @pindex temacs | |
26 | ||
27 | This section explains the steps involved in building the Emacs | |
28 | executable. You don't have to know this material to build and install | |
29 | Emacs, since the makefiles do all these things automatically. This | |
30 | information is pertinent to Emacs maintenance. | |
31 | ||
32 | Compilation of the C source files in the @file{src} directory | |
33 | produces an executable file called @file{temacs}, also called a | |
34 | @dfn{bare impure Emacs}. It contains the Emacs Lisp interpreter and I/O | |
35 | routines, but not the editing commands. | |
36 | ||
37 | @cindex @file{loadup.el} | |
38 | The command @w{@samp{temacs -l loadup}} uses @file{temacs} to create | |
39 | the real runnable Emacs executable. These arguments direct | |
40 | @file{temacs} to evaluate the Lisp files specified in the file | |
41 | @file{loadup.el}. These files set up the normal Emacs editing | |
574efc83 | 42 | environment, resulting in an Emacs that is still impure but no longer |
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43 | bare. |
44 | ||
45 | It takes a substantial time to load the standard Lisp files. Luckily, | |
46 | you don't have to do this each time you run Emacs; @file{temacs} can | |
574efc83 | 47 | dump out an executable program called @file{emacs} that has these files |
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48 | preloaded. @file{emacs} starts more quickly because it does not need to |
49 | load the files. This is the Emacs executable that is normally | |
50 | installed. | |
51 | ||
52 | To create @file{emacs}, use the command @samp{temacs -batch -l loadup | |
53 | dump}. The purpose of @samp{-batch} here is to prevent @file{temacs} | |
54 | from trying to initialize any of its data on the terminal; this ensures | |
55 | that the tables of terminal information are empty in the dumped Emacs. | |
56 | The argument @samp{dump} tells @file{loadup.el} to dump a new executable | |
57 | named @file{emacs}. | |
58 | ||
59 | Some operating systems don't support dumping. On those systems, you | |
60 | must start Emacs with the @samp{temacs -l loadup} command each time you | |
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61 | use it. This takes a substantial time, but since you need to start |
62 | Emacs once a day at most---or once a week if you never log out---the | |
63 | extra time is not too severe a problem. | |
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64 | |
65 | @cindex @file{site-load.el} | |
66 | You can specify additional files to preload by writing a library named | |
2a664e73 RS |
67 | @file{site-load.el} that loads them. You may need to increase the value |
68 | of @code{PURESIZE}, in @file{src/puresize.h}, to make room for the | |
69 | additional data. (Try adding increments of 20000 until it is big | |
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70 | enough.) However, the advantage of preloading additional files |
71 | decreases as machines get faster. On modern machines, it is usually not | |
72 | advisable. | |
73 | ||
2a664e73 RS |
74 | After @file{loadup.el} reads @file{site-load.el}, it finds the |
75 | documentation strings for primitive and preloaded functions (and | |
76 | variables) in the file @file{etc/DOC} where they are stored, by calling | |
77 | @code{Snarf-documentation} (@pxref{Accessing Documentation}). | |
78 | ||
a44af9f2 | 79 | @cindex @file{site-init.el} |
a890e1b0 | 80 | You can specify other Lisp expressions to execute just before dumping |
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81 | by putting them in a library named @file{site-init.el}. This file is |
82 | executed after the documentation strings are found. | |
a44af9f2 | 83 | |
2a664e73 RS |
84 | If you want to preload function or variable definitions, there are |
85 | three ways you can do this and make their documentation strings | |
86 | accessible when you subsequently run Emacs: | |
87 | ||
88 | @itemize @bullet | |
89 | @item | |
90 | Arrange to scan these files when producing the @file{etc/DOC} file, | |
91 | and load them with @file{site-load.el}. | |
92 | ||
93 | @item | |
94 | Load the files with @file{site-init.el}, then copy the files into the | |
95 | installation directory for Lisp files when you install Emacs. | |
96 | ||
97 | @item | |
98 | Specify a non-@code{nil} value for | |
99 | @code{byte-compile-dynamic-docstrings} as a local variable in each these | |
100 | files, and load them with either @file{site-load.el} or | |
101 | @file{site-init.el}. (This method has the drawback that the | |
102 | documentation strings take up space in Emacs all the time.) | |
103 | @end itemize | |
104 | ||
105 | It is not advisable to put anything in @file{site-load.el} or | |
106 | @file{site-init.el} that would alter any of the features that users | |
107 | expect in an ordinary unmodified Emacs. If you feel you must override | |
108 | normal features for your site, do it with @file{default.el}, so that | |
109 | users can override your changes if they wish. @xref{Start-up Summary}. | |
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110 | |
111 | @defun dump-emacs to-file from-file | |
112 | @cindex unexec | |
cb017dde | 113 | This function dumps the current state of Emacs into an executable file |
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114 | @var{to-file}. It takes symbols from @var{from-file} (this is normally |
115 | the executable file @file{temacs}). | |
116 | ||
cb017dde RS |
117 | If you want to use this function in an Emacs that was already dumped, |
118 | you must run Emacs with @samp{-batch}. | |
a44af9f2 RS |
119 | @end defun |
120 | ||
121 | @deffn Command emacs-version | |
122 | This function returns a string describing the version of Emacs that is | |
123 | running. It is useful to include this string in bug reports. | |
124 | ||
125 | @example | |
126 | @group | |
127 | (emacs-version) | |
bfe721d1 KH |
128 | @result{} "GNU Emacs 19.29.1 (i386-debian-linux) \ |
129 | of Tue Jun 6 1995 on balloon" | |
a44af9f2 RS |
130 | @end group |
131 | @end example | |
132 | ||
133 | Called interactively, the function prints the same information in the | |
134 | echo area. | |
135 | @end deffn | |
136 | ||
137 | @defvar emacs-build-time | |
bfe721d1 | 138 | The value of this variable is the time at which Emacs was built at the |
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139 | local site. |
140 | ||
141 | @example | |
142 | @group | |
143 | emacs-build-time | |
bfe721d1 | 144 | @result{} "Tue Jun 6 14:55:57 1995" |
a44af9f2 RS |
145 | @end group |
146 | @end example | |
147 | @end defvar | |
148 | ||
149 | @defvar emacs-version | |
150 | The value of this variable is the version of Emacs being run. It is a | |
bfe721d1 | 151 | string such as @code{"19.29.1"}. |
a890e1b0 RS |
152 | @end defvar |
153 | ||
154 | The following two variables did not exist before Emacs version 19.23, | |
155 | which reduces their usefulness at present, but we hope they will be | |
156 | convenient in the future. | |
157 | ||
158 | @defvar emacs-major-version | |
574efc83 | 159 | The major version number of Emacs, as an integer. For Emacs version |
bfe721d1 | 160 | 19.29, the value is 19. |
a890e1b0 RS |
161 | @end defvar |
162 | ||
163 | @defvar emacs-minor-version | |
164 | The minor version number of Emacs, as an integer. For Emacs version | |
bfe721d1 | 165 | 19.29, the value is 29. |
a44af9f2 RS |
166 | @end defvar |
167 | ||
168 | @node Pure Storage, Garbage Collection, Building Emacs, GNU Emacs Internals | |
169 | @appendixsec Pure Storage | |
170 | @cindex pure storage | |
171 | ||
a890e1b0 RS |
172 | Emacs Lisp uses two kinds of storage for user-created Lisp objects: |
173 | @dfn{normal storage} and @dfn{pure storage}. Normal storage is where | |
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174 | all the new data created during an Emacs session is kept; see the |
175 | following section for information on normal storage. Pure storage is | |
176 | used for certain data in the preloaded standard Lisp files---data that | |
177 | should never change during actual use of Emacs. | |
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178 | |
179 | Pure storage is allocated only while @file{temacs} is loading the | |
180 | standard preloaded Lisp libraries. In the file @file{emacs}, it is | |
574efc83 | 181 | marked as read-only (on operating systems that permit this), so that |
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182 | the memory space can be shared by all the Emacs jobs running on the |
183 | machine at once. Pure storage is not expandable; a fixed amount is | |
184 | allocated when Emacs is compiled, and if that is not sufficient for the | |
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185 | preloaded libraries, @file{temacs} crashes. If that happens, you must |
186 | increase the compilation parameter @code{PURESIZE} in the file | |
a44af9f2 RS |
187 | @file{src/puresize.h}. This normally won't happen unless you try to |
188 | preload additional libraries or add features to the standard ones. | |
189 | ||
190 | @defun purecopy object | |
a890e1b0 | 191 | This function makes a copy of @var{object} in pure storage and returns |
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192 | it. It copies strings by simply making a new string with the same |
193 | characters in pure storage. It recursively copies the contents of | |
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194 | vectors and cons cells. It does not make copies of other objects such |
195 | as symbols, but just returns them unchanged. It signals an error if | |
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196 | asked to copy markers. |
197 | ||
c2cd5fb7 RS |
198 | This function is a no-op except while Emacs is being built and dumped; |
199 | it is usually called only in the file @file{emacs/lisp/loaddefs.el}, but | |
200 | a few packages call it just in case you decide to preload them. | |
a44af9f2 RS |
201 | @end defun |
202 | ||
203 | @defvar pure-bytes-used | |
a890e1b0 | 204 | The value of this variable is the number of bytes of pure storage |
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205 | allocated so far. Typically, in a dumped Emacs, this number is very |
206 | close to the total amount of pure storage available---if it were not, | |
207 | we would preallocate less. | |
208 | @end defvar | |
209 | ||
210 | @defvar purify-flag | |
a890e1b0 | 211 | This variable determines whether @code{defun} should make a copy of the |
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212 | function definition in pure storage. If it is non-@code{nil}, then the |
213 | function definition is copied into pure storage. | |
214 | ||
a890e1b0 | 215 | This flag is @code{t} while loading all of the basic functions for |
a44af9f2 | 216 | building Emacs initially (allowing those functions to be sharable and |
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217 | non-collectible). Dumping Emacs as an executable always writes |
218 | @code{nil} in this variable, regardless of the value it actually has | |
219 | before and after dumping. | |
a44af9f2 | 220 | |
a890e1b0 | 221 | You should not change this flag in a running Emacs. |
a44af9f2 RS |
222 | @end defvar |
223 | ||
224 | @node Garbage Collection, Writing Emacs Primitives, Pure Storage, GNU Emacs Internals | |
225 | @appendixsec Garbage Collection | |
226 | @cindex garbage collector | |
227 | ||
228 | @cindex memory allocation | |
229 | When a program creates a list or the user defines a new function (such | |
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230 | as by loading a library), that data is placed in normal storage. If |
231 | normal storage runs low, then Emacs asks the operating system to | |
a44af9f2 | 232 | allocate more memory in blocks of 1k bytes. Each block is used for one |
a890e1b0 RS |
233 | type of Lisp object, so symbols, cons cells, markers, etc., are |
234 | segregated in distinct blocks in memory. (Vectors, long strings, | |
235 | buffers and certain other editing types, which are fairly large, are | |
236 | allocated in individual blocks, one per object, while small strings are | |
237 | packed into blocks of 8k bytes.) | |
238 | ||
239 | It is quite common to use some storage for a while, then release it by | |
240 | (for example) killing a buffer or deleting the last pointer to an | |
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241 | object. Emacs provides a @dfn{garbage collector} to reclaim this |
242 | abandoned storage. (This name is traditional, but ``garbage recycler'' | |
243 | might be a more intuitive metaphor for this facility.) | |
244 | ||
a890e1b0 RS |
245 | The garbage collector operates by finding and marking all Lisp objects |
246 | that are still accessible to Lisp programs. To begin with, it assumes | |
247 | all the symbols, their values and associated function definitions, and | |
574efc83 | 248 | any data presently on the stack, are accessible. Any objects that can |
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249 | be reached indirectly through other accessible objects are also |
250 | accessible. | |
a44af9f2 | 251 | |
a890e1b0 | 252 | When marking is finished, all objects still unmarked are garbage. No |
a44af9f2 | 253 | matter what the Lisp program or the user does, it is impossible to refer |
a890e1b0 | 254 | to them, since there is no longer a way to reach them. Their space |
574efc83 RS |
255 | might as well be reused, since no one will miss them. The second |
256 | (``sweep'') phase of the garbage collector arranges to reuse them. | |
a44af9f2 RS |
257 | |
258 | @cindex free list | |
a890e1b0 RS |
259 | The sweep phase puts unused cons cells onto a @dfn{free list} |
260 | for future allocation; likewise for symbols and markers. It compacts | |
261 | the accessible strings so they occupy fewer 8k blocks; then it frees the | |
574efc83 | 262 | other 8k blocks. Vectors, buffers, windows, and other large objects are |
a890e1b0 | 263 | individually allocated and freed using @code{malloc} and @code{free}. |
a44af9f2 RS |
264 | |
265 | @cindex CL note---allocate more storage | |
266 | @quotation | |
574efc83 | 267 | @b{Common Lisp note:} Unlike other Lisps, GNU Emacs Lisp does not |
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268 | call the garbage collector when the free list is empty. Instead, it |
269 | simply requests the operating system to allocate more storage, and | |
270 | processing continues until @code{gc-cons-threshold} bytes have been | |
271 | used. | |
272 | ||
273 | This means that you can make sure that the garbage collector will not | |
274 | run during a certain portion of a Lisp program by calling the garbage | |
275 | collector explicitly just before it (provided that portion of the | |
276 | program does not use so much space as to force a second garbage | |
277 | collection). | |
278 | @end quotation | |
279 | ||
280 | @deffn Command garbage-collect | |
a890e1b0 | 281 | This command runs a garbage collection, and returns information on |
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282 | the amount of space in use. (Garbage collection can also occur |
283 | spontaneously if you use more than @code{gc-cons-threshold} bytes of | |
284 | Lisp data since the previous garbage collection.) | |
285 | ||
a890e1b0 | 286 | @code{garbage-collect} returns a list containing the following |
a44af9f2 RS |
287 | information: |
288 | ||
a890e1b0 | 289 | @example |
a44af9f2 RS |
290 | @group |
291 | ((@var{used-conses} . @var{free-conses}) | |
292 | (@var{used-syms} . @var{free-syms}) | |
a890e1b0 | 293 | @end group |
a44af9f2 RS |
294 | (@var{used-markers} . @var{free-markers}) |
295 | @var{used-string-chars} | |
296 | @var{used-vector-slots} | |
297 | (@var{used-floats} . @var{free-floats})) | |
298 | ||
a890e1b0 | 299 | @group |
a44af9f2 RS |
300 | (garbage-collect) |
301 | @result{} ((3435 . 2332) (1688 . 0) | |
302 | (57 . 417) 24510 3839 (4 . 1)) | |
303 | @end group | |
a890e1b0 | 304 | @end example |
a44af9f2 RS |
305 | |
306 | Here is a table explaining each element: | |
307 | ||
308 | @table @var | |
309 | @item used-conses | |
310 | The number of cons cells in use. | |
311 | ||
312 | @item free-conses | |
313 | The number of cons cells for which space has been obtained from the | |
314 | operating system, but that are not currently being used. | |
315 | ||
316 | @item used-syms | |
317 | The number of symbols in use. | |
318 | ||
319 | @item free-syms | |
320 | The number of symbols for which space has been obtained from the | |
321 | operating system, but that are not currently being used. | |
322 | ||
323 | @item used-markers | |
324 | The number of markers in use. | |
325 | ||
326 | @item free-markers | |
327 | The number of markers for which space has been obtained from the | |
328 | operating system, but that are not currently being used. | |
329 | ||
330 | @item used-string-chars | |
331 | The total size of all strings, in characters. | |
332 | ||
333 | @item used-vector-slots | |
334 | The total number of elements of existing vectors. | |
335 | ||
336 | @item used-floats | |
337 | @c Emacs 19 feature | |
338 | The number of floats in use. | |
339 | ||
340 | @item free-floats | |
341 | @c Emacs 19 feature | |
342 | The number of floats for which space has been obtained from the | |
343 | operating system, but that are not currently being used. | |
344 | @end table | |
345 | @end deffn | |
346 | ||
d64c55d8 RS |
347 | @defopt garbage-collection-messages |
348 | If this variable is non-@code{nil}, Emacs displays a message at the | |
349 | beginning and end of garbage collection. The default value is | |
350 | @code{nil}, meaning there are no such messages. | |
351 | @end defopt | |
352 | ||
a44af9f2 | 353 | @defopt gc-cons-threshold |
a890e1b0 | 354 | The value of this variable is the number of bytes of storage that must |
a44af9f2 | 355 | be allocated for Lisp objects after one garbage collection in order to |
a890e1b0 | 356 | trigger another garbage collection. A cons cell counts as eight bytes, |
a44af9f2 | 357 | a string as one byte per character plus a few bytes of overhead, and so |
a890e1b0 RS |
358 | on; space allocated to the contents of buffers does not count. Note |
359 | that the subsequent garbage collection does not happen immediately when | |
360 | the threshold is exhausted, but only the next time the Lisp evaluator is | |
a44af9f2 RS |
361 | called. |
362 | ||
bfe721d1 | 363 | The initial threshold value is 300,000. If you specify a larger |
a44af9f2 RS |
364 | value, garbage collection will happen less often. This reduces the |
365 | amount of time spent garbage collecting, but increases total memory use. | |
574efc83 | 366 | You may want to do this when running a program that creates lots of |
a44af9f2 RS |
367 | Lisp data. |
368 | ||
a890e1b0 | 369 | You can make collections more frequent by specifying a smaller value, |
a44af9f2 RS |
370 | down to 10,000. A value less than 10,000 will remain in effect only |
371 | until the subsequent garbage collection, at which time | |
372 | @code{garbage-collect} will set the threshold back to 10,000. | |
373 | @end defopt | |
374 | ||
375 | @c Emacs 19 feature | |
376 | @defun memory-limit | |
377 | This function returns the address of the last byte Emacs has allocated, | |
378 | divided by 1024. We divide the value by 1024 to make sure it fits in a | |
379 | Lisp integer. | |
380 | ||
381 | You can use this to get a general idea of how your actions affect the | |
382 | memory usage. | |
383 | @end defun | |
384 | ||
385 | @node Writing Emacs Primitives, Object Internals, Garbage Collection, GNU Emacs Internals | |
386 | @appendixsec Writing Emacs Primitives | |
387 | @cindex primitive function internals | |
388 | ||
389 | Lisp primitives are Lisp functions implemented in C. The details of | |
390 | interfacing the C function so that Lisp can call it are handled by a few | |
391 | C macros. The only way to really understand how to write new C code is | |
392 | to read the source, but we can explain some things here. | |
393 | ||
394 | An example of a special form is the definition of @code{or}, from | |
395 | @file{eval.c}. (An ordinary function would have the same general | |
396 | appearance.) | |
397 | ||
398 | @cindex garbage collection protection | |
399 | @smallexample | |
400 | @group | |
401 | DEFUN ("or", For, Sor, 0, UNEVALLED, 0, | |
9e2b495b | 402 | "Eval args until one of them yields non-nil; return that value.\n\ |
a44af9f2 RS |
403 | The remaining args are not evalled at all.\n\ |
404 | @end group | |
405 | @group | |
a890e1b0 | 406 | If all args return nil, return nil.") |
a44af9f2 RS |
407 | (args) |
408 | Lisp_Object args; | |
409 | @{ | |
410 | register Lisp_Object val; | |
411 | Lisp_Object args_left; | |
412 | struct gcpro gcpro1; | |
413 | @end group | |
414 | ||
415 | @group | |
a890e1b0 | 416 | if (NULL (args)) |
a44af9f2 RS |
417 | return Qnil; |
418 | ||
419 | args_left = args; | |
420 | GCPRO1 (args_left); | |
421 | @end group | |
422 | ||
423 | @group | |
424 | do | |
425 | @{ | |
426 | val = Feval (Fcar (args_left)); | |
427 | if (!NULL (val)) | |
428 | break; | |
429 | args_left = Fcdr (args_left); | |
430 | @} | |
a890e1b0 | 431 | while (!NULL (args_left)); |
a44af9f2 RS |
432 | @end group |
433 | ||
434 | @group | |
435 | UNGCPRO; | |
436 | return val; | |
437 | @} | |
438 | @end group | |
439 | @end smallexample | |
440 | ||
441 | Let's start with a precise explanation of the arguments to the | |
a890e1b0 | 442 | @code{DEFUN} macro. Here is a template for them: |
a44af9f2 RS |
443 | |
444 | @example | |
445 | DEFUN (@var{lname}, @var{fname}, @var{sname}, @var{min}, @var{max}, @var{interactive}, @var{doc}) | |
446 | @end example | |
447 | ||
448 | @table @var | |
449 | @item lname | |
a890e1b0 RS |
450 | This is the name of the Lisp symbol to define as the function name; in |
451 | the example above, it is @code{or}. | |
a44af9f2 RS |
452 | |
453 | @item fname | |
454 | This is the C function name for this function. This is | |
455 | the name that is used in C code for calling the function. The name is, | |
456 | by convention, @samp{F} prepended to the Lisp name, with all dashes | |
457 | (@samp{-}) in the Lisp name changed to underscores. Thus, to call this | |
458 | function from C code, call @code{For}. Remember that the arguments must | |
459 | be of type @code{Lisp_Object}; various macros and functions for creating | |
460 | values of type @code{Lisp_Object} are declared in the file | |
461 | @file{lisp.h}. | |
462 | ||
463 | @item sname | |
464 | This is a C variable name to use for a structure that holds the data for | |
465 | the subr object that represents the function in Lisp. This structure | |
466 | conveys the Lisp symbol name to the initialization routine that will | |
467 | create the symbol and store the subr object as its definition. By | |
468 | convention, this name is always @var{fname} with @samp{F} replaced with | |
469 | @samp{S}. | |
470 | ||
471 | @item min | |
a890e1b0 RS |
472 | This is the minimum number of arguments that the function requires. The |
473 | function @code{or} allows a minimum of zero arguments. | |
a44af9f2 RS |
474 | |
475 | @item max | |
a890e1b0 RS |
476 | This is the maximum number of arguments that the function accepts, if |
477 | there is a fixed maximum. Alternatively, it can be @code{UNEVALLED}, | |
478 | indicating a special form that receives unevaluated arguments, or | |
479 | @code{MANY}, indicating an unlimited number of evaluated arguments (the | |
480 | equivalent of @code{&rest}). Both @code{UNEVALLED} and @code{MANY} are | |
481 | macros. If @var{max} is a number, it may not be less than @var{min} and | |
482 | it may not be greater than seven. | |
a44af9f2 RS |
483 | |
484 | @item interactive | |
485 | This is an interactive specification, a string such as might be used as | |
486 | the argument of @code{interactive} in a Lisp function. In the case of | |
487 | @code{or}, it is 0 (a null pointer), indicating that @code{or} cannot be | |
a890e1b0 RS |
488 | called interactively. A value of @code{""} indicates a function that |
489 | should receive no arguments when called interactively. | |
a44af9f2 RS |
490 | |
491 | @item doc | |
492 | This is the documentation string. It is written just like a | |
493 | documentation string for a function defined in Lisp, except you must | |
494 | write @samp{\n\} at the end of each line. In particular, the first line | |
495 | should be a single sentence. | |
496 | @end table | |
497 | ||
a890e1b0 RS |
498 | After the call to the @code{DEFUN} macro, you must write the argument |
499 | name list that every C function must have, followed by ordinary C | |
500 | declarations for the arguments. For a function with a fixed maximum | |
501 | number of arguments, declare a C argument for each Lisp argument, and | |
574efc83 RS |
502 | give them all type @code{Lisp_Object}. When a Lisp function has no |
503 | upper limit on the number of arguments, its implementation in C actually | |
504 | receives exactly two arguments: the first is the number of Lisp | |
505 | arguments, and the second is the address of a block containing their | |
506 | values. They have types @code{int} and @w{@code{Lisp_Object *}}. | |
a44af9f2 RS |
507 | |
508 | Within the function @code{For} itself, note the use of the macros | |
509 | @code{GCPRO1} and @code{UNGCPRO}. @code{GCPRO1} is used to ``protect'' | |
510 | a variable from garbage collection---to inform the garbage collector that | |
511 | it must look in that variable and regard its contents as an accessible | |
512 | object. This is necessary whenever you call @code{Feval} or anything | |
513 | that can directly or indirectly call @code{Feval}. At such a time, any | |
514 | Lisp object that you intend to refer to again must be protected somehow. | |
515 | @code{UNGCPRO} cancels the protection of the variables that are | |
516 | protected in the current function. It is necessary to do this explicitly. | |
517 | ||
a890e1b0 RS |
518 | For most data types, it suffices to protect at least one pointer to |
519 | the object; as long as the object is not recycled, all pointers to it | |
520 | remain valid. This is not so for strings, because the garbage collector | |
521 | can move them. When the garbage collector moves a string, it relocates | |
522 | all the pointers it knows about; any other pointers become invalid. | |
523 | Therefore, you must protect all pointers to strings across any point | |
524 | where garbage collection may be possible. | |
525 | ||
526 | The macro @code{GCPRO1} protects just one local variable. If you want | |
527 | to protect two, use @code{GCPRO2} instead; repeating @code{GCPRO1} will | |
528 | not work. Macros @code{GCPRO3} and @code{GCPRO4} also exist. | |
529 | ||
530 | These macros implicitly use local variables such as @code{gcpro1}; you | |
531 | must declare these explicitly, with type @code{struct gcpro}. Thus, if | |
532 | you use @code{GCPRO2}, you must declare @code{gcpro1} and @code{gcpro2}. | |
533 | Alas, we can't explain all the tricky details here. | |
534 | ||
e610024b RS |
535 | You must not use C initializers for static or global variables unless |
536 | they are never written once Emacs is dumped. These variables with | |
537 | initializers are allocated in an area of memory that becomes read-only | |
538 | (on certain operating systems) as a result of dumping Emacs. @xref{Pure | |
539 | Storage}. | |
540 | ||
541 | Do not use static variables within functions---place all static | |
542 | variables at top level in the file. This is necessary because Emacs on | |
543 | some operating systems defines the keyword @code{static} as a null | |
544 | macro. (This definition is used because those systems put all variables | |
545 | declared static in a place that becomes read-only after dumping, whether | |
546 | they have initializers or not.) | |
77223f05 | 547 | |
a890e1b0 RS |
548 | Defining the C function is not enough to make a Lisp primitive |
549 | available; you must also create the Lisp symbol for the primitive and | |
550 | store a suitable subr object in its function cell. The code looks like | |
551 | this: | |
a44af9f2 RS |
552 | |
553 | @example | |
554 | defsubr (&@var{subr-structure-name}); | |
555 | @end example | |
556 | ||
557 | @noindent | |
a890e1b0 RS |
558 | Here @var{subr-structure-name} is the name you used as the third |
559 | argument to @code{DEFUN}. | |
560 | ||
561 | If you add a new primitive to a file that already has Lisp primitives | |
562 | defined in it, find the function (near the end of the file) named | |
563 | @code{syms_of_@var{something}}, and add the call to @code{defsubr} | |
564 | there. If the file doesn't have this function, or if you create a new | |
565 | file, add to it a @code{syms_of_@var{filename}} (e.g., | |
566 | @code{syms_of_myfile}). Then find the spot in @file{emacs.c} where all | |
567 | of these functions are called, and add a call to | |
568 | @code{syms_of_@var{filename}} there. | |
a44af9f2 | 569 | |
574efc83 RS |
570 | The function @code{syms_of_@var{filename}} is also the place to define |
571 | any C variables that are to be visible as Lisp variables. | |
a890e1b0 RS |
572 | @code{DEFVAR_LISP} makes a C variable of type @code{Lisp_Object} visible |
573 | in Lisp. @code{DEFVAR_INT} makes a C variable of type @code{int} | |
574 | visible in Lisp with a value that is always an integer. | |
575 | @code{DEFVAR_BOOL} makes a C variable of type @code{int} visible in Lisp | |
576 | with a value that is either @code{t} or @code{nil}. | |
a44af9f2 | 577 | |
a890e1b0 RS |
578 | Here is another example function, with more complicated arguments. |
579 | This comes from the code for the X Window System, and it demonstrates | |
580 | the use of macros and functions to manipulate Lisp objects. | |
a44af9f2 RS |
581 | |
582 | @smallexample | |
583 | @group | |
584 | DEFUN ("coordinates-in-window-p", Fcoordinates_in_window_p, | |
585 | Scoordinates_in_window_p, 2, 2, | |
586 | "xSpecify coordinate pair: \nXExpression which evals to window: ", | |
587 | "Return non-nil if POSITIONS is in WINDOW.\n\ | |
588 | \(POSITIONS is a list, (SCREEN-X SCREEN-Y)\)\n\ | |
589 | @end group | |
590 | @group | |
591 | Returned value is list of positions expressed\n\ | |
592 | relative to window upper left corner.") | |
593 | (coordinate, window) | |
594 | register Lisp_Object coordinate, window; | |
595 | @{ | |
596 | register Lisp_Object xcoord, ycoord; | |
597 | @end group | |
598 | ||
599 | @group | |
600 | if (!CONSP (coordinate)) wrong_type_argument (Qlistp, coordinate); | |
601 | CHECK_WINDOW (window, 2); | |
602 | xcoord = Fcar (coordinate); | |
603 | ycoord = Fcar (Fcdr (coordinate)); | |
604 | CHECK_NUMBER (xcoord, 0); | |
605 | CHECK_NUMBER (ycoord, 1); | |
606 | @end group | |
607 | @group | |
608 | if ((XINT (xcoord) < XINT (XWINDOW (window)->left)) | |
609 | || (XINT (xcoord) >= (XINT (XWINDOW (window)->left) | |
610 | + XINT (XWINDOW (window)->width)))) | |
a890e1b0 | 611 | return Qnil; |
a44af9f2 RS |
612 | XFASTINT (xcoord) -= XFASTINT (XWINDOW (window)->left); |
613 | @end group | |
614 | @group | |
615 | if (XINT (ycoord) == (screen_height - 1)) | |
616 | return Qnil; | |
617 | @end group | |
618 | @group | |
619 | if ((XINT (ycoord) < XINT (XWINDOW (window)->top)) | |
620 | || (XINT (ycoord) >= (XINT (XWINDOW (window)->top) | |
621 | + XINT (XWINDOW (window)->height)) - 1)) | |
a890e1b0 | 622 | return Qnil; |
a44af9f2 RS |
623 | @end group |
624 | @group | |
625 | XFASTINT (ycoord) -= XFASTINT (XWINDOW (window)->top); | |
626 | return (Fcons (xcoord, Fcons (ycoord, Qnil))); | |
627 | @} | |
628 | @end group | |
629 | @end smallexample | |
630 | ||
a890e1b0 RS |
631 | Note that C code cannot call functions by name unless they are defined |
632 | in C. The way to call a function written in Lisp is to use | |
633 | @code{Ffuncall}, which embodies the Lisp function @code{funcall}. Since | |
634 | the Lisp function @code{funcall} accepts an unlimited number of | |
635 | arguments, in C it takes two: the number of Lisp-level arguments, and a | |
636 | one-dimensional array containing their values. The first Lisp-level | |
637 | argument is the Lisp function to call, and the rest are the arguments to | |
638 | pass to it. Since @code{Ffuncall} can call the evaluator, you must | |
639 | protect pointers from garbage collection around the call to | |
640 | @code{Ffuncall}. | |
641 | ||
642 | The C functions @code{call0}, @code{call1}, @code{call2}, and so on, | |
643 | provide handy ways to call a Lisp function conveniently with a fixed | |
644 | number of arguments. They work by calling @code{Ffuncall}. | |
a44af9f2 RS |
645 | |
646 | @file{eval.c} is a very good file to look through for examples; | |
647 | @file{lisp.h} contains the definitions for some important macros and | |
648 | functions. | |
649 | ||
650 | @node Object Internals, , Writing Emacs Primitives, GNU Emacs Internals | |
651 | @appendixsec Object Internals | |
652 | @cindex object internals | |
653 | ||
654 | GNU Emacs Lisp manipulates many different types of data. The actual | |
655 | data are stored in a heap and the only access that programs have to it is | |
656 | through pointers. Pointers are thirty-two bits wide in most | |
657 | implementations. Depending on the operating system and type of machine | |
658 | for which you compile Emacs, twenty-four to twenty-six bits are used to | |
659 | address the object, and the remaining six to eight bits are used for a | |
660 | tag that identifies the object's type. | |
661 | ||
a890e1b0 RS |
662 | Because Lisp objects are represented as tagged pointers, it is always |
663 | possible to determine the Lisp data type of any object. The C data type | |
664 | @code{Lisp_Object} can hold any Lisp object of any data type. Ordinary | |
665 | variables have type @code{Lisp_Object}, which means they can hold any | |
666 | type of Lisp value; you can determine the actual data type only at run | |
667 | time. The same is true for function arguments; if you want a function | |
668 | to accept only a certain type of argument, you must check the type | |
669 | explicitly using a suitable predicate (@pxref{Type Predicates}). | |
a44af9f2 RS |
670 | @cindex type checking internals |
671 | ||
672 | @menu | |
673 | * Buffer Internals:: Components of a buffer structure. | |
674 | * Window Internals:: Components of a window structure. | |
675 | * Process Internals:: Components of a process structure. | |
676 | @end menu | |
677 | ||
678 | @node Buffer Internals, Window Internals, Object Internals, Object Internals | |
679 | @appendixsubsec Buffer Internals | |
680 | @cindex internals, of buffer | |
681 | @cindex buffer internals | |
682 | ||
683 | Buffers contain fields not directly accessible by the Lisp programmer. | |
684 | We describe them here, naming them by the names used in the C code. | |
685 | Many are accessible indirectly in Lisp programs via Lisp primitives. | |
686 | ||
687 | @table @code | |
688 | @item name | |
574efc83 | 689 | The buffer name is a string that names the buffer. It is guaranteed to |
a44af9f2 RS |
690 | be unique. @xref{Buffer Names}. |
691 | ||
692 | @item save_modified | |
693 | This field contains the time when the buffer was last saved, as an integer. | |
694 | @xref{Buffer Modification}. | |
695 | ||
696 | @item modtime | |
697 | This field contains the modification time of the visited file. It is | |
698 | set when the file is written or read. Every time the buffer is written | |
699 | to the file, this field is compared to the modification time of the | |
700 | file. @xref{Buffer Modification}. | |
701 | ||
702 | @item auto_save_modified | |
703 | This field contains the time when the buffer was last auto-saved. | |
704 | ||
705 | @item last_window_start | |
706 | This field contains the @code{window-start} position in the buffer as of | |
707 | the last time the buffer was displayed in a window. | |
708 | ||
a890e1b0 RS |
709 | @item undo_list |
710 | This field points to the buffer's undo list. @xref{Undo}. | |
a44af9f2 RS |
711 | |
712 | @item syntax_table_v | |
713 | This field contains the syntax table for the buffer. @xref{Syntax Tables}. | |
714 | ||
715 | @item downcase_table | |
716 | This field contains the conversion table for converting text to lower case. | |
717 | @xref{Case Table}. | |
718 | ||
719 | @item upcase_table | |
720 | This field contains the conversion table for converting text to upper case. | |
721 | @xref{Case Table}. | |
722 | ||
723 | @item case_canon_table | |
724 | This field contains the conversion table for canonicalizing text for | |
725 | case-folding search. @xref{Case Table}. | |
726 | ||
727 | @item case_eqv_table | |
728 | This field contains the equivalence table for case-folding search. | |
729 | @xref{Case Table}. | |
730 | ||
731 | @item display_table | |
732 | This field contains the buffer's display table, or @code{nil} if it doesn't | |
733 | have one. @xref{Display Tables}. | |
734 | ||
735 | @item markers | |
a890e1b0 RS |
736 | This field contains the chain of all markers that currently point into |
737 | the buffer. Deletion of text in the buffer, and motion of the buffer's | |
738 | gap, must check each of these markers and perhaps update it. | |
739 | @xref{Markers}. | |
a44af9f2 RS |
740 | |
741 | @item backed_up | |
574efc83 | 742 | This field is a flag that tells whether a backup file has been made |
a44af9f2 RS |
743 | for the visited file of this buffer. |
744 | ||
745 | @item mark | |
746 | This field contains the mark for the buffer. The mark is a marker, | |
747 | hence it is also included on the list @code{markers}. @xref{The Mark}. | |
748 | ||
a890e1b0 RS |
749 | @item mark_active |
750 | This field is non-@code{nil} if the buffer's mark is active. | |
751 | ||
a44af9f2 | 752 | @item local_var_alist |
a890e1b0 RS |
753 | This field contains the association list describing the variables local |
754 | in this buffer, and their values, with the exception of local variables | |
755 | that have special slots in the buffer object. (Those slots are omitted | |
756 | from this table.) @xref{Buffer-Local Variables}. | |
757 | ||
bfe721d1 KH |
758 | @item base_buffer |
759 | This field holds the buffer's base buffer (if it is an indirect buffer), | |
760 | or @code{nil}. | |
761 | ||
a890e1b0 RS |
762 | @item keymap |
763 | This field holds the buffer's local keymap. @xref{Keymaps}. | |
764 | ||
765 | @item overlay_center | |
766 | This field holds the current overlay center position. @xref{Overlays}. | |
767 | ||
768 | @item overlays_before | |
769 | This field holds a list of the overlays in this buffer that end at or | |
770 | before the current overlay center position. They are sorted in order of | |
771 | decreasing end position. | |
772 | ||
773 | @item overlays_after | |
774 | This field holds a list of the overlays in this buffer that end after | |
775 | the current overlay center position. They are sorted in order of | |
776 | increasing beginning position. | |
a44af9f2 RS |
777 | @end table |
778 | ||
779 | @node Window Internals, Process Internals, Buffer Internals, Object Internals | |
780 | @appendixsubsec Window Internals | |
781 | @cindex internals, of window | |
782 | @cindex window internals | |
783 | ||
784 | Windows have the following accessible fields: | |
785 | ||
786 | @table @code | |
787 | @item frame | |
a890e1b0 | 788 | The frame that this window is on. |
a44af9f2 RS |
789 | |
790 | @item mini_p | |
a890e1b0 | 791 | Non-@code{nil} if this window is a minibuffer window. |
a44af9f2 RS |
792 | |
793 | @item buffer | |
574efc83 | 794 | The buffer that the window is displaying. This may change often during |
a44af9f2 RS |
795 | the life of the window. |
796 | ||
797 | @item dedicated | |
a890e1b0 | 798 | Non-@code{nil} if this window is dedicated to its buffer. |
a44af9f2 RS |
799 | |
800 | @item pointm | |
801 | @cindex window point internals | |
a890e1b0 | 802 | This is the value of point in the current buffer when this window is |
a44af9f2 RS |
803 | selected; when it is not selected, it retains its previous value. |
804 | ||
a890e1b0 | 805 | @item start |
574efc83 | 806 | The position in the buffer that is the first character to be displayed |
a890e1b0 RS |
807 | in the window. |
808 | ||
809 | @item force_start | |
810 | If this flag is non-@code{nil}, it says that the window has been | |
811 | scrolled explicitly by the Lisp program. This affects what the next | |
812 | redisplay does if point is off the screen: instead of scrolling the | |
813 | window to show the text around point, it moves point to a location that | |
814 | is on the screen. | |
815 | ||
816 | @item last_modified | |
817 | The @code{modified} field of the window's buffer, as of the last time | |
818 | a redisplay completed in this window. | |
819 | ||
820 | @item last_point | |
821 | The buffer's value of point, as of the last time | |
822 | a redisplay completed in this window. | |
823 | ||
a44af9f2 | 824 | @item left |
a890e1b0 | 825 | This is the left-hand edge of the window, measured in columns. (The |
a44af9f2 RS |
826 | leftmost column on the screen is @w{column 0}.) |
827 | ||
828 | @item top | |
a890e1b0 | 829 | This is the top edge of the window, measured in lines. (The top line on |
a44af9f2 RS |
830 | the screen is @w{line 0}.) |
831 | ||
a890e1b0 RS |
832 | @item height |
833 | The height of the window, measured in lines. | |
834 | ||
835 | @item width | |
836 | The width of the window, measured in columns. | |
837 | ||
a44af9f2 | 838 | @item next |
a890e1b0 RS |
839 | This is the window that is the next in the chain of siblings. It is |
840 | @code{nil} in a window that is the rightmost or bottommost of a group of | |
841 | siblings. | |
a44af9f2 RS |
842 | |
843 | @item prev | |
a890e1b0 RS |
844 | This is the window that is the previous in the chain of siblings. It is |
845 | @code{nil} in a window that is the leftmost or topmost of a group of | |
846 | siblings. | |
a44af9f2 | 847 | |
a890e1b0 RS |
848 | @item parent |
849 | Internally, Emacs arranges windows in a tree; each group of siblings has | |
850 | a parent window whose area includes all the siblings. This field points | |
851 | to a window's parent. | |
852 | ||
853 | Parent windows do not display buffers, and play little role in display | |
854 | except to shape their child windows. Emacs Lisp programs usually have | |
855 | no access to the parent windows; they operate on the windows at the | |
574efc83 | 856 | leaves of the tree, which actually display buffers. |
a44af9f2 RS |
857 | |
858 | @item hscroll | |
a890e1b0 | 859 | This is the number of columns that the display in the window is scrolled |
a44af9f2 RS |
860 | horizontally to the left. Normally, this is 0. |
861 | ||
862 | @item use_time | |
a890e1b0 | 863 | This is the last time that the window was selected. The function |
a44af9f2 RS |
864 | @code{get-lru-window} uses this field. |
865 | ||
866 | @item display_table | |
a890e1b0 RS |
867 | The window's display table, or @code{nil} if none is specified for it. |
868 | ||
869 | @item update_mode_line | |
870 | Non-@code{nil} means this window's mode line needs to be updated. | |
871 | ||
872 | @item base_line_number | |
873 | The line number of a certain position in the buffer, or @code{nil}. | |
874 | This is used for displaying the line number of point in the mode line. | |
875 | ||
876 | @item base_line_pos | |
877 | The position in the buffer for which the line number is known, or | |
878 | @code{nil} meaning none is known. | |
879 | ||
880 | @item region_showing | |
881 | If the region (or part of it) is highlighted in this window, this field | |
882 | holds the mark position that made one end of that region. Otherwise, | |
883 | this field is @code{nil}. | |
a44af9f2 RS |
884 | @end table |
885 | ||
886 | @node Process Internals, , Window Internals, Object Internals | |
887 | @appendixsubsec Process Internals | |
888 | @cindex internals, of process | |
889 | @cindex process internals | |
890 | ||
891 | The fields of a process are: | |
892 | ||
893 | @table @code | |
894 | @item name | |
895 | A string, the name of the process. | |
896 | ||
897 | @item command | |
898 | A list containing the command arguments that were used to start this | |
899 | process. | |
900 | ||
901 | @item filter | |
902 | A function used to accept output from the process instead of a buffer, | |
903 | or @code{nil}. | |
904 | ||
905 | @item sentinel | |
906 | A function called whenever the process receives a signal, or @code{nil}. | |
907 | ||
908 | @item buffer | |
909 | The associated buffer of the process. | |
910 | ||
911 | @item pid | |
912 | An integer, the Unix process @sc{id}. | |
913 | ||
914 | @item childp | |
915 | A flag, non-@code{nil} if this is really a child process. | |
916 | It is @code{nil} for a network connection. | |
917 | ||
a44af9f2 | 918 | @item mark |
574efc83 RS |
919 | A marker indicating the position of the end of the last output from this |
920 | process inserted into the buffer. This is often but not always the end | |
921 | of the buffer. | |
a44af9f2 RS |
922 | |
923 | @item kill_without_query | |
a890e1b0 RS |
924 | If this is non-@code{nil}, killing Emacs while this process is still |
925 | running does not ask for confirmation about killing the process. | |
926 | ||
927 | @item raw_status_low | |
928 | @itemx raw_status_high | |
929 | These two fields record 16 bits each of the process status returned by | |
930 | the @code{wait} system call. | |
931 | ||
932 | @item status | |
933 | The process status, as @code{process-status} should return it. | |
934 | ||
935 | @item tick | |
936 | @itemx update_tick | |
937 | If these two fields are not equal, a change in the status of the process | |
938 | needs to be reported, either by running the sentinel or by inserting a | |
939 | message in the process buffer. | |
940 | ||
941 | @item pty_flag | |
942 | Non-@code{nil} if communication with the subprocess uses a @sc{pty}; | |
943 | @code{nil} if it uses a pipe. | |
944 | ||
945 | @item infd | |
946 | The file descriptor for input from the process. | |
947 | ||
948 | @item outfd | |
949 | The file descriptor for output to the process. | |
950 | ||
951 | @item subtty | |
952 | The file descriptor for the terminal that the subprocess is using. (On | |
953 | some systems, there is no need to record this, so the value is | |
954 | @code{nil}.) | |
bfe721d1 KH |
955 | |
956 | @item tty_name | |
957 | The name of the terminal that the subprocess is using, | |
958 | or @code{nil} if it is using pipes. | |
a44af9f2 | 959 | @end table |