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