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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, 1994 Free Software Foundation, Inc. | |
4 | @c See the file elisp.texi for copying conditions. | |
5 | @setfilename ../info/buffers | |
6 | @node Buffers, Windows, Backups and Auto-Saving, Top | |
7 | @chapter Buffers | |
8 | @cindex buffer | |
9 | ||
10 | A @dfn{buffer} is a Lisp object containing text to be edited. Buffers | |
11 | are used to hold the contents of files that are being visited; there may | |
47ba05ac | 12 | also be buffers that are not visiting files. While several buffers may |
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13 | exist at one time, exactly one buffer is designated the @dfn{current |
14 | buffer} at any time. Most editing commands act on the contents of the | |
15 | current buffer. Each buffer, including the current buffer, may or may | |
16 | not be displayed in any windows. | |
17 | ||
18 | @menu | |
19 | * Buffer Basics:: What is a buffer? | |
22697dac KH |
20 | * Current Buffer:: Designating a buffer as current |
21 | so primitives will access its contents. | |
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22 | * Buffer Names:: Accessing and changing buffer names. |
23 | * Buffer File Name:: The buffer file name indicates which file is visited. | |
24 | * Buffer Modification:: A buffer is @dfn{modified} if it needs to be saved. | |
25 | * Modification Time:: Determining whether the visited file was changed | |
26 | ``behind Emacs's back''. | |
27 | * Read Only Buffers:: Modifying text is not allowed in a read-only buffer. | |
28 | * The Buffer List:: How to look at all the existing buffers. | |
29 | * Creating Buffers:: Functions that create buffers. | |
30 | * Killing Buffers:: Buffers exist until explicitly killed. | |
22697dac | 31 | * Indirect Buffers:: An indirect buffer shares text with some other buffer. |
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32 | @end menu |
33 | ||
34 | @node Buffer Basics | |
35 | @comment node-name, next, previous, up | |
36 | @section Buffer Basics | |
37 | ||
38 | @ifinfo | |
39 | A @dfn{buffer} is a Lisp object containing text to be edited. Buffers | |
40 | are used to hold the contents of files that are being visited; there may | |
47ba05ac | 41 | also be buffers that are not visiting files. While several buffers may |
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42 | exist at one time, exactly one buffer is designated the @dfn{current |
43 | buffer} at any time. Most editing commands act on the contents of the | |
44 | current buffer. Each buffer, including the current buffer, may or may | |
45 | not be displayed in any windows. | |
46 | @end ifinfo | |
47 | ||
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48 | Buffers in Emacs editing are objects that have distinct names and hold |
49 | text that can be edited. Buffers appear to Lisp programs as a special | |
50 | data type. You can think of the contents of a buffer as an extendable | |
51 | string; insertions and deletions may occur in any part of the buffer. | |
52 | @xref{Text}. | |
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53 | |
54 | A Lisp buffer object contains numerous pieces of information. Some of | |
55 | this information is directly accessible to the programmer through | |
47ba05ac | 56 | variables, while other information is accessible only through |
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57 | special-purpose functions. For example, the visited file name is |
58 | directly accessible through a variable, while the value of point is | |
59 | accessible only through a primitive function. | |
60 | ||
61 | Buffer-specific information that is directly accessible is stored in | |
62 | @dfn{buffer-local} variable bindings, which are variable values that are | |
63 | effective only in a particular buffer. This feature allows each buffer | |
64 | to override the values of certain variables. Most major modes override | |
65 | variables such as @code{fill-column} or @code{comment-column} in this | |
66 | way. For more information about buffer-local variables and functions | |
67 | related to them, see @ref{Buffer-Local Variables}. | |
68 | ||
69 | For functions and variables related to visiting files in buffers, see | |
70 | @ref{Visiting Files} and @ref{Saving Buffers}. For functions and | |
71 | variables related to the display of buffers in windows, see | |
72 | @ref{Buffers and Windows}. | |
73 | ||
74 | @defun bufferp object | |
75 | This function returns @code{t} if @var{object} is a buffer, | |
76 | @code{nil} otherwise. | |
77 | @end defun | |
78 | ||
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79 | @node Current Buffer |
80 | @section The Current Buffer | |
81 | @cindex selecting a buffer | |
82 | @cindex changing to another buffer | |
83 | @cindex current buffer | |
84 | ||
85 | There are, in general, many buffers in an Emacs session. At any time, | |
86 | one of them is designated as the @dfn{current buffer}. This is the | |
87 | buffer in which most editing takes place, because most of the primitives | |
88 | for examining or changing text in a buffer operate implicitly on the | |
89 | current buffer (@pxref{Text}). Normally the buffer that is displayed on | |
90 | the screen in the selected window is the current buffer, but this is not | |
91 | always so: a Lisp program can designate any buffer as current | |
92 | temporarily in order to operate on its contents, without changing what | |
93 | is displayed on the screen. | |
94 | ||
95 | The way to designate a current buffer in a Lisp program is by calling | |
96 | @code{set-buffer}. The specified buffer remains current until a new one | |
97 | is designated. | |
98 | ||
99 | When an editing command returns to the editor command loop, the | |
100 | command loop designates the buffer displayed in the selected window as | |
101 | current, to prevent confusion: the buffer that the cursor is in when | |
102 | Emacs reads a command is the buffer that the command will apply to. | |
103 | (@xref{Command Loop}.) Therefore, @code{set-buffer} is not the way to | |
104 | switch visibly to a different buffer so that the user can edit it. For | |
105 | this, you must use the functions described in @ref{Displaying Buffers}. | |
106 | ||
107 | However, Lisp functions that change to a different current buffer | |
108 | should not depend on the command loop to set it back afterwards. | |
109 | Editing commands written in Emacs Lisp can be called from other programs | |
110 | as well as from the command loop. It is convenient for the caller if | |
111 | the subroutine does not change which buffer is current (unless, of | |
112 | course, that is the subroutine's purpose). Therefore, you should | |
113 | normally use @code{set-buffer} within a @code{save-excursion} that will | |
114 | restore the current buffer when your function is done | |
115 | (@pxref{Excursions}). Here is an example, the code for the command | |
116 | @code{append-to-buffer} (with the documentation string abridged): | |
117 | ||
118 | @example | |
119 | @group | |
120 | (defun append-to-buffer (buffer start end) | |
121 | "Append to specified buffer the text of the region. | |
122 | @dots{}" | |
123 | (interactive "BAppend to buffer: \nr") | |
124 | (let ((oldbuf (current-buffer))) | |
125 | (save-excursion | |
126 | (set-buffer (get-buffer-create buffer)) | |
127 | (insert-buffer-substring oldbuf start end)))) | |
128 | @end group | |
129 | @end example | |
130 | ||
131 | @noindent | |
132 | This function binds a local variable to the current buffer, and then | |
133 | @code{save-excursion} records the values of point, the mark, and the | |
134 | original buffer. Next, @code{set-buffer} makes another buffer current. | |
135 | Finally, @code{insert-buffer-substring} copies the string from the | |
136 | original current buffer to the new current buffer. | |
137 | ||
138 | If the buffer appended to happens to be displayed in some window, | |
139 | the next redisplay will show how its text has changed. Otherwise, you | |
140 | will not see the change immediately on the screen. The buffer becomes | |
141 | current temporarily during the execution of the command, but this does | |
142 | not cause it to be displayed. | |
143 | ||
144 | If you make local bindings (with @code{let} or function arguments) for | |
145 | a variable that may also have buffer-local bindings, make sure that the | |
146 | same buffer is current at the beginning and at the end of the local | |
147 | binding's scope. Otherwise you might bind it in one buffer and unbind | |
148 | it in another! There are two ways to do this. In simple cases, you may | |
149 | see that nothing ever changes the current buffer within the scope of the | |
150 | binding. Otherwise, use @code{save-excursion} to make sure that the | |
151 | buffer current at the beginning is current again whenever the variable | |
152 | is unbound. | |
153 | ||
154 | It is not reliable to change the current buffer back with | |
155 | @code{set-buffer}, because that won't do the job if a quit happens while | |
156 | the wrong buffer is current. Here is what @emph{not} to do: | |
157 | ||
158 | @example | |
159 | @group | |
160 | (let (buffer-read-only | |
161 | (obuf (current-buffer))) | |
162 | (set-buffer @dots{}) | |
163 | @dots{} | |
164 | (set-buffer obuf)) | |
165 | @end group | |
166 | @end example | |
167 | ||
168 | @noindent | |
169 | Using @code{save-excursion}, as shown below, handles quitting, errors, | |
170 | and @code{throw}, as well as ordinary evaluation. | |
171 | ||
172 | @example | |
173 | @group | |
174 | (let (buffer-read-only) | |
175 | (save-excursion | |
176 | (set-buffer @dots{}) | |
177 | @dots{})) | |
178 | @end group | |
179 | @end example | |
180 | ||
181 | @defun current-buffer | |
182 | This function returns the current buffer. | |
183 | ||
184 | @example | |
185 | @group | |
186 | (current-buffer) | |
187 | @result{} #<buffer buffers.texi> | |
188 | @end group | |
189 | @end example | |
190 | @end defun | |
191 | ||
192 | @defun set-buffer buffer-or-name | |
193 | This function makes @var{buffer-or-name} the current buffer. It does | |
194 | not display the buffer in the currently selected window or in any other | |
195 | window, so the user cannot necessarily see the buffer. But Lisp | |
196 | programs can in any case work on it. | |
197 | ||
198 | This function returns the buffer identified by @var{buffer-or-name}. | |
199 | An error is signaled if @var{buffer-or-name} does not identify an | |
200 | existing buffer. | |
201 | @end defun | |
202 | ||
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203 | @node Buffer Names |
204 | @section Buffer Names | |
205 | @cindex buffer names | |
206 | ||
207 | Each buffer has a unique name, which is a string. Many of the | |
208 | functions that work on buffers accept either a buffer or a buffer name | |
209 | as an argument. Any argument called @var{buffer-or-name} is of this | |
210 | sort, and an error is signaled if it is neither a string nor a buffer. | |
211 | Any argument called @var{buffer} must be an actual buffer | |
212 | object, not a name. | |
213 | ||
214 | Buffers that are ephemeral and generally uninteresting to the user | |
bfe721d1 | 215 | have names starting with a space, so that the @code{list-buffers} and |
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216 | @code{buffer-menu} commands don't mention them. A name starting with |
217 | space also initially disables recording undo information; see | |
218 | @ref{Undo}. | |
219 | ||
220 | @defun buffer-name &optional buffer | |
221 | This function returns the name of @var{buffer} as a string. If | |
222 | @var{buffer} is not supplied, it defaults to the current buffer. | |
223 | ||
224 | If @code{buffer-name} returns @code{nil}, it means that @var{buffer} | |
225 | has been killed. @xref{Killing Buffers}. | |
226 | ||
227 | @example | |
228 | @group | |
229 | (buffer-name) | |
230 | @result{} "buffers.texi" | |
231 | @end group | |
232 | ||
233 | @group | |
234 | (setq foo (get-buffer "temp")) | |
235 | @result{} #<buffer temp> | |
236 | @end group | |
237 | @group | |
238 | (kill-buffer foo) | |
239 | @result{} nil | |
240 | @end group | |
241 | @group | |
242 | (buffer-name foo) | |
243 | @result{} nil | |
244 | @end group | |
245 | @group | |
246 | foo | |
247 | @result{} #<killed buffer> | |
248 | @end group | |
249 | @end example | |
250 | @end defun | |
251 | ||
252 | @deffn Command rename-buffer newname &optional unique | |
253 | This function renames the current buffer to @var{newname}. An error | |
254 | is signaled if @var{newname} is not a string, or if there is already a | |
b5ef0e92 | 255 | buffer with that name. The function returns @var{newname}. |
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256 | |
257 | @c Emacs 19 feature | |
258 | Ordinarily, @code{rename-buffer} signals an error if @var{newname} is | |
259 | already in use. However, if @var{unique} is non-@code{nil}, it modifies | |
260 | @var{newname} to make a name that is not in use. Interactively, you can | |
261 | make @var{unique} non-@code{nil} with a numeric prefix argument. | |
262 | ||
263 | One application of this command is to rename the @samp{*shell*} buffer | |
264 | to some other name, thus making it possible to create a second shell | |
265 | buffer under the name @samp{*shell*}. | |
266 | @end deffn | |
267 | ||
268 | @defun get-buffer buffer-or-name | |
269 | This function returns the buffer specified by @var{buffer-or-name}. | |
270 | If @var{buffer-or-name} is a string and there is no buffer with that | |
271 | name, the value is @code{nil}. If @var{buffer-or-name} is a buffer, it | |
272 | is returned as given. (That is not very useful, so the argument is usually | |
273 | a name.) For example: | |
274 | ||
275 | @example | |
276 | @group | |
277 | (setq b (get-buffer "lewis")) | |
278 | @result{} #<buffer lewis> | |
279 | @end group | |
280 | @group | |
281 | (get-buffer b) | |
282 | @result{} #<buffer lewis> | |
283 | @end group | |
284 | @group | |
285 | (get-buffer "Frazzle-nots") | |
286 | @result{} nil | |
287 | @end group | |
288 | @end example | |
289 | ||
290 | See also the function @code{get-buffer-create} in @ref{Creating Buffers}. | |
291 | @end defun | |
292 | ||
293 | @c Emacs 19 feature | |
294 | @defun generate-new-buffer-name starting-name | |
295 | This function returns a name that would be unique for a new buffer---but | |
296 | does not create the buffer. It starts with @var{starting-name}, and | |
297 | produces a name not currently in use for any buffer by appending a | |
298 | number inside of @samp{<@dots{}>}. | |
299 | ||
300 | See the related function @code{generate-new-buffer} in @ref{Creating | |
301 | Buffers}. | |
302 | @end defun | |
303 | ||
304 | @node Buffer File Name | |
305 | @section Buffer File Name | |
306 | @cindex visited file | |
307 | @cindex buffer file name | |
308 | @cindex file name of buffer | |
309 | ||
310 | The @dfn{buffer file name} is the name of the file that is visited in | |
311 | that buffer. When a buffer is not visiting a file, its buffer file name | |
312 | is @code{nil}. Most of the time, the buffer name is the same as the | |
313 | nondirectory part of the buffer file name, but the buffer file name and | |
314 | the buffer name are distinct and can be set independently. | |
315 | @xref{Visiting Files}. | |
316 | ||
317 | @defun buffer-file-name &optional buffer | |
318 | This function returns the absolute file name of the file that | |
319 | @var{buffer} is visiting. If @var{buffer} is not visiting any file, | |
320 | @code{buffer-file-name} returns @code{nil}. If @var{buffer} is not | |
321 | supplied, it defaults to the current buffer. | |
322 | ||
323 | @example | |
324 | @group | |
325 | (buffer-file-name (other-buffer)) | |
326 | @result{} "/usr/user/lewis/manual/files.texi" | |
327 | @end group | |
328 | @end example | |
329 | @end defun | |
330 | ||
331 | @defvar buffer-file-name | |
332 | This buffer-local variable contains the name of the file being visited | |
333 | in the current buffer, or @code{nil} if it is not visiting a file. It | |
334 | is a permanent local, unaffected by @code{kill-local-variables}. | |
335 | ||
336 | @example | |
337 | @group | |
338 | buffer-file-name | |
339 | @result{} "/usr/user/lewis/manual/buffers.texi" | |
340 | @end group | |
341 | @end example | |
342 | ||
343 | It is risky to change this variable's value without doing various other | |
344 | things. See the definition of @code{set-visited-file-name} in | |
345 | @file{files.el}; some of the things done there, such as changing the | |
346 | buffer name, are not strictly necessary, but others are essential to | |
347 | avoid confusing Emacs. | |
348 | @end defvar | |
349 | ||
350 | @defvar buffer-file-truename | |
351 | This buffer-local variable holds the truename of the file visited in the | |
352 | current buffer, or @code{nil} if no file is visited. It is a permanent | |
353 | local, unaffected by @code{kill-local-variables}. @xref{Truenames}. | |
354 | @end defvar | |
355 | ||
356 | @defvar buffer-file-number | |
357 | This buffer-local variable holds the file number and directory device | |
358 | number of the file visited in the current buffer, or @code{nil} if no | |
359 | file or a nonexistent file is visited. It is a permanent local, | |
360 | unaffected by @code{kill-local-variables}. @xref{Truenames}. | |
361 | ||
362 | The value is normally a list of the form @code{(@var{filenum} | |
363 | @var{devnum})}. This pair of numbers uniquely identifies the file among | |
364 | all files accessible on the system. See the function | |
365 | @code{file-attributes}, in @ref{File Attributes}, for more information | |
366 | about them. | |
367 | @end defvar | |
368 | ||
369 | @defun get-file-buffer filename | |
370 | This function returns the buffer visiting file @var{filename}. If | |
371 | there is no such buffer, it returns @code{nil}. The argument | |
372 | @var{filename}, which must be a string, is expanded (@pxref{File Name | |
373 | Expansion}), then compared against the visited file names of all live | |
374 | buffers. | |
375 | ||
376 | @example | |
377 | @group | |
378 | (get-file-buffer "buffers.texi") | |
379 | @result{} #<buffer buffers.texi> | |
380 | @end group | |
381 | @end example | |
382 | ||
383 | In unusual circumstances, there can be more than one buffer visiting | |
384 | the same file name. In such cases, this function returns the first | |
385 | such buffer in the buffer list. | |
386 | @end defun | |
387 | ||
388 | @deffn Command set-visited-file-name filename | |
389 | If @var{filename} is a non-empty string, this function changes the | |
390 | name of the file visited in current buffer to @var{filename}. (If the | |
391 | buffer had no visited file, this gives it one.) The @emph{next time} | |
392 | the buffer is saved it will go in the newly-specified file. This | |
393 | command marks the buffer as modified, since it does not (as far as Emacs | |
394 | knows) match the contents of @var{filename}, even if it matched the | |
395 | former visited file. | |
396 | ||
397 | If @var{filename} is @code{nil} or the empty string, that stands for | |
398 | ``no visited file''. In this case, @code{set-visited-file-name} marks | |
399 | the buffer as having no visited file. | |
400 | ||
401 | @c Wordy to avoid overfull hbox. --rjc 16mar92 | |
402 | When the function @code{set-visited-file-name} is called interactively, it | |
403 | prompts for @var{filename} in the minibuffer. | |
404 | ||
405 | See also @code{clear-visited-file-modtime} and | |
406 | @code{verify-visited-file-modtime} in @ref{Buffer Modification}. | |
407 | @end deffn | |
408 | ||
409 | @defvar list-buffers-directory | |
410 | This buffer-local variable records a string to display in a buffer | |
411 | listing in place of the visited file name, for buffers that don't have a | |
412 | visited file name. Dired buffers use this variable. | |
413 | @end defvar | |
414 | ||
415 | @node Buffer Modification | |
416 | @section Buffer Modification | |
417 | @cindex buffer modification | |
418 | @cindex modification flag (of buffer) | |
419 | ||
420 | Emacs keeps a flag called the @dfn{modified flag} for each buffer, to | |
421 | record whether you have changed the text of the buffer. This flag is | |
422 | set to @code{t} whenever you alter the contents of the buffer, and | |
423 | cleared to @code{nil} when you save it. Thus, the flag shows whether | |
424 | there are unsaved changes. The flag value is normally shown in the mode | |
425 | line (@pxref{Mode Line Variables}), and controls saving (@pxref{Saving | |
426 | Buffers}) and auto-saving (@pxref{Auto-Saving}). | |
427 | ||
428 | Some Lisp programs set the flag explicitly. For example, the function | |
429 | @code{set-visited-file-name} sets the flag to @code{t}, because the text | |
430 | does not match the newly-visited file, even if it is unchanged from the | |
431 | file formerly visited. | |
432 | ||
433 | The functions that modify the contents of buffers are described in | |
434 | @ref{Text}. | |
435 | ||
436 | @defun buffer-modified-p &optional buffer | |
437 | This function returns @code{t} if the buffer @var{buffer} has been modified | |
438 | since it was last read in from a file or saved, or @code{nil} | |
439 | otherwise. If @var{buffer} is not supplied, the current buffer | |
440 | is tested. | |
441 | @end defun | |
442 | ||
443 | @defun set-buffer-modified-p flag | |
444 | This function marks the current buffer as modified if @var{flag} is | |
445 | non-@code{nil}, or as unmodified if the flag is @code{nil}. | |
446 | ||
447 | Another effect of calling this function is to cause unconditional | |
448 | redisplay of the mode line for the current buffer. In fact, the | |
449 | function @code{force-mode-line-update} works by doing this: | |
450 | ||
451 | @example | |
452 | @group | |
453 | (set-buffer-modified-p (buffer-modified-p)) | |
454 | @end group | |
455 | @end example | |
456 | @end defun | |
457 | ||
458 | @deffn Command not-modified | |
b5ef0e92 RS |
459 | This command marks the current buffer as unmodified, and not needing to |
460 | be saved. With prefix arg, it marks the buffer as modified, so that it | |
461 | will be saved at the next suitable occasion. | |
462 | ||
463 | Don't use this function in programs, since it prints a message in the | |
464 | echo area; use @code{set-buffer-modified-p} (above) instead. | |
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465 | @end deffn |
466 | ||
467 | @c Emacs 19 feature | |
468 | @defun buffer-modified-tick &optional buffer | |
b5ef0e92 | 469 | This function returns @var{buffer}'s modification-count. This is a |
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470 | counter that increments every time the buffer is modified. If |
471 | @var{buffer} is @code{nil} (or omitted), the current buffer is used. | |
472 | @end defun | |
473 | ||
474 | @node Modification Time | |
475 | @comment node-name, next, previous, up | |
476 | @section Comparison of Modification Time | |
477 | @cindex comparison of modification time | |
478 | @cindex modification time, comparison of | |
479 | ||
480 | Suppose that you visit a file and make changes in its buffer, and | |
481 | meanwhile the file itself is changed on disk. At this point, saving the | |
482 | buffer would overwrite the changes in the file. Occasionally this may | |
483 | be what you want, but usually it would lose valuable information. Emacs | |
484 | therefore checks the file's modification time using the functions | |
485 | described below before saving the file. | |
486 | ||
487 | @defun verify-visited-file-modtime buffer | |
488 | This function compares what @var{buffer} has recorded for the | |
489 | modification time of its visited file against the actual modification | |
490 | time of the file as recorded by the operating system. The two should be | |
491 | the same unless some other process has written the file since Emacs | |
492 | visited or saved it. | |
493 | ||
494 | The function returns @code{t} if the last actual modification time and | |
495 | Emacs's recorded modification time are the same, @code{nil} otherwise. | |
496 | @end defun | |
497 | ||
498 | @defun clear-visited-file-modtime | |
499 | This function clears out the record of the last modification time of | |
500 | the file being visited by the current buffer. As a result, the next | |
501 | attempt to save this buffer will not complain of a discrepancy in | |
502 | file modification times. | |
503 | ||
504 | This function is called in @code{set-visited-file-name} and other | |
505 | exceptional places where the usual test to avoid overwriting a changed | |
506 | file should not be done. | |
507 | @end defun | |
508 | ||
509 | @c Emacs 19 feature | |
510 | @defun visited-file-modtime | |
511 | This function returns the buffer's recorded last file modification time, | |
512 | as a list of the form @code{(@var{high} . @var{low})}. (This is the | |
513 | same format that @code{file-attributes} uses to return time values; see | |
514 | @ref{File Attributes}.) | |
515 | @end defun | |
516 | ||
517 | @c Emacs 19 feature | |
518 | @defun set-visited-file-modtime &optional time | |
519 | This function updates the buffer's record of the last modification time | |
520 | of the visited file, to the value specified by @var{time} if @var{time} | |
521 | is not @code{nil}, and otherwise to the last modification time of the | |
522 | visited file. | |
523 | ||
524 | If @var{time} is not @code{nil}, it should have the form | |
525 | @code{(@var{high} . @var{low})} or @code{(@var{high} @var{low})}, in | |
526 | either case containing two integers, each of which holds 16 bits of the | |
527 | time. | |
528 | ||
529 | This function is useful if the buffer was not read from the file | |
530 | normally, or if the file itself has been changed for some known benign | |
531 | reason. | |
532 | @end defun | |
533 | ||
47ba05ac | 534 | @defun ask-user-about-supersession-threat filename |
b1b12a8e RS |
535 | @cindex obsolete buffer |
536 | This function is used to ask a user how to proceed after an attempt to | |
47ba05ac RS |
537 | modify an obsolete buffer visiting file @var{filename}. An |
538 | @dfn{obsolete buffer} is an unmodified buffer for which the associated | |
539 | file on disk is newer than the last save-time of the buffer. This means | |
540 | some other program has probably altered the file. | |
b1b12a8e RS |
541 | |
542 | @kindex file-supersession | |
543 | Depending on the user's answer, the function may return normally, in | |
544 | which case the modification of the buffer proceeds, or it may signal a | |
47ba05ac | 545 | @code{file-supersession} error with data @code{(@var{filename})}, in which |
b1b12a8e RS |
546 | case the proposed buffer modification is not allowed. |
547 | ||
47ba05ac RS |
548 | This function is called automatically by Emacs on the proper |
549 | occasions. It exists so you can customize Emacs by redefining it. | |
550 | See the file @file{userlock.el} for the standard definition. | |
551 | ||
b1b12a8e RS |
552 | See also the file locking mechanism in @ref{File Locks}. |
553 | @end defun | |
554 | ||
555 | @node Read Only Buffers | |
556 | @section Read-Only Buffers | |
557 | @cindex read-only buffer | |
558 | @cindex buffer, read-only | |
559 | ||
560 | If a buffer is @dfn{read-only}, then you cannot change its contents, | |
561 | although you may change your view of the contents by scrolling and | |
562 | narrowing. | |
563 | ||
564 | Read-only buffers are used in two kinds of situations: | |
565 | ||
566 | @itemize @bullet | |
567 | @item | |
568 | A buffer visiting a write-protected file is normally read-only. | |
569 | ||
570 | Here, the purpose is to show the user that editing the buffer with the | |
571 | aim of saving it in the file may be futile or undesirable. The user who | |
572 | wants to change the buffer text despite this can do so after clearing | |
bfe721d1 | 573 | the read-only flag with @kbd{C-x C-q}. |
b1b12a8e RS |
574 | |
575 | @item | |
576 | Modes such as Dired and Rmail make buffers read-only when altering the | |
577 | contents with the usual editing commands is probably a mistake. | |
578 | ||
579 | The special commands of these modes bind @code{buffer-read-only} to | |
580 | @code{nil} (with @code{let}) or bind @code{inhibit-read-only} to | |
581 | @code{t} around the places where they change the text. | |
582 | @end itemize | |
583 | ||
584 | @defvar buffer-read-only | |
585 | This buffer-local variable specifies whether the buffer is read-only. | |
586 | The buffer is read-only if this variable is non-@code{nil}. | |
587 | @end defvar | |
588 | ||
589 | @defvar inhibit-read-only | |
590 | If this variable is non-@code{nil}, then read-only buffers and read-only | |
47ba05ac RS |
591 | characters may be modified. Read-only characters in a buffer are those |
592 | that have non-@code{nil} @code{read-only} properties (either text | |
593 | properties or overlay properties). @xref{Special Properties}, for more | |
594 | information about text properties. @xref{Overlays}, for more | |
595 | information about overlays and their properties. | |
596 | ||
597 | If @code{inhibit-read-only} is @code{t}, all @code{read-only} character | |
598 | properties have no effect. If @code{inhibit-read-only} is a list, then | |
599 | @code{read-only} character properties have no effect if they are members | |
600 | of the list (comparison is done with @code{eq}). | |
b1b12a8e RS |
601 | @end defvar |
602 | ||
603 | @deffn Command toggle-read-only | |
604 | This command changes whether the current buffer is read-only. It is | |
605 | intended for interactive use; don't use it in programs. At any given | |
606 | point in a program, you should know whether you want the read-only flag | |
607 | on or off; so you can set @code{buffer-read-only} explicitly to the | |
608 | proper value, @code{t} or @code{nil}. | |
609 | @end deffn | |
610 | ||
611 | @defun barf-if-buffer-read-only | |
612 | This function signals a @code{buffer-read-only} error if the current | |
613 | buffer is read-only. @xref{Interactive Call}, for another way to | |
614 | signal an error if the current buffer is read-only. | |
615 | @end defun | |
616 | ||
617 | @node The Buffer List | |
618 | @section The Buffer List | |
619 | @cindex buffer list | |
620 | ||
621 | The @dfn{buffer list} is a list of all live buffers. Creating a | |
622 | buffer adds it to this list, and killing a buffer deletes it. The order | |
623 | of the buffers in the list is based primarily on how recently each | |
624 | buffer has been displayed in the selected window. Buffers move to the | |
625 | front of the list when they are selected and to the end when they are | |
626 | buried. Several functions, notably @code{other-buffer}, use this | |
627 | ordering. A buffer list displayed for the user also follows this order. | |
628 | ||
629 | @defun buffer-list | |
630 | This function returns a list of all buffers, including those whose names | |
631 | begin with a space. The elements are actual buffers, not their names. | |
632 | ||
633 | @example | |
634 | @group | |
635 | (buffer-list) | |
636 | @result{} (#<buffer buffers.texi> | |
637 | #<buffer *Minibuf-1*> #<buffer buffer.c> | |
638 | #<buffer *Help*> #<buffer TAGS>) | |
639 | @end group | |
640 | ||
641 | @group | |
642 | ;; @r{Note that the name of the minibuffer} | |
643 | ;; @r{begins with a space!} | |
644 | (mapcar (function buffer-name) (buffer-list)) | |
645 | @result{} ("buffers.texi" " *Minibuf-1*" | |
646 | "buffer.c" "*Help*" "TAGS") | |
647 | @end group | |
648 | @end example | |
b1b12a8e RS |
649 | @end defun |
650 | ||
9ba7f85c RS |
651 | The list that @code{buffer-list} returns is constructed specifically |
652 | by @code{buffer-list}; it is not an internal Emacs data structure, and | |
653 | modifying it has no effect on the order of buffers. If you want to | |
654 | change the order of buffers in the list, here is an easy way: | |
655 | ||
656 | @example | |
657 | (defun reorder-buffer-list (new-list) | |
658 | (while new-list | |
659 | (bury-buffer (car new-list)) | |
660 | (setq new-list (cdr new-list)))) | |
661 | @end example | |
662 | ||
663 | With this method, you can specify any order for the list, but there is | |
664 | no danger of losing a buffer or adding something that is not a valid | |
665 | live buffer. | |
666 | ||
fad7d361 | 667 | @defun other-buffer &optional buffer visible-ok |
b1b12a8e | 668 | This function returns the first buffer in the buffer list other than |
fad7d361 RS |
669 | @var{buffer}. Usually this is the buffer most recently shown in |
670 | the selected window, aside from @var{buffer}. Buffers whose | |
b1b12a8e RS |
671 | names start with a space are not considered. |
672 | ||
fad7d361 RS |
673 | If @var{buffer} is not supplied (or if it is not a buffer), then |
674 | @code{other-buffer} returns the first buffer on the buffer list that is | |
675 | not visible in any window in a visible frame. | |
b1b12a8e | 676 | |
22697dac KH |
677 | If the selected frame has a non-@code{nil} @code{buffer-predicate} |
678 | parameter, then @code{other-buffer} uses that predicate to decide which | |
679 | buffers to consider. It calls the predicate once for each buffer, and | |
680 | if the value is @code{nil}, that buffer is ignored. @xref{X Frame | |
681 | Parameters}. | |
682 | ||
b1b12a8e RS |
683 | @c Emacs 19 feature |
684 | If @var{visible-ok} is @code{nil}, @code{other-buffer} avoids returning | |
685 | a buffer visible in any window on any visible frame, except as a last | |
686 | resort. If @var{visible-ok} is non-@code{nil}, then it does not matter | |
687 | whether a buffer is displayed somewhere or not. | |
688 | ||
689 | If no suitable buffer exists, the buffer @samp{*scratch*} is returned | |
690 | (and created, if necessary). | |
691 | @end defun | |
692 | ||
693 | @deffn Command bury-buffer &optional buffer-or-name | |
694 | This function puts @var{buffer-or-name} at the end of the buffer list | |
695 | without changing the order of any of the other buffers on the list. | |
696 | This buffer therefore becomes the least desirable candidate for | |
697 | @code{other-buffer} to return. | |
698 | ||
47ba05ac RS |
699 | If @var{buffer-or-name} is @code{nil} or omitted, this means to bury the |
700 | current buffer. In addition, if the buffer is displayed in the selected | |
701 | window, this switches to some other buffer (obtained using | |
702 | @code{other-buffer}) in the selected window. But if the buffer is | |
703 | displayed in some other window, it remains displayed there. | |
b1b12a8e RS |
704 | |
705 | If you wish to replace a buffer in all the windows that display it, use | |
706 | @code{replace-buffer-in-windows}. @xref{Buffers and Windows}. | |
707 | @end deffn | |
708 | ||
709 | @node Creating Buffers | |
710 | @section Creating Buffers | |
711 | @cindex creating buffers | |
712 | @cindex buffers, creating | |
713 | ||
714 | This section describes the two primitives for creating buffers. | |
47ba05ac RS |
715 | @code{get-buffer-create} creates a buffer if it finds no existing buffer |
716 | with the specified name; @code{generate-new-buffer} always creates a new | |
717 | buffer and gives it a unique name. | |
b1b12a8e RS |
718 | |
719 | Other functions you can use to create buffers include | |
720 | @code{with-output-to-temp-buffer} (@pxref{Temporary Displays}) and | |
721 | @code{create-file-buffer} (@pxref{Visiting Files}). Starting a | |
722 | subprocess can also create a buffer (@pxref{Processes}). | |
723 | ||
724 | @defun get-buffer-create name | |
725 | This function returns a buffer named @var{name}. It returns an existing | |
726 | buffer with that name, if one exists; otherwise, it creates a new | |
727 | buffer. The buffer does not become the current buffer---this function | |
728 | does not change which buffer is current. | |
729 | ||
730 | An error is signaled if @var{name} is not a string. | |
731 | ||
732 | @example | |
733 | @group | |
734 | (get-buffer-create "foo") | |
735 | @result{} #<buffer foo> | |
736 | @end group | |
737 | @end example | |
738 | ||
22697dac KH |
739 | The major mode for the new buffer is set to Fundamental mode. The |
740 | variable @code{default-major-mode} is handled at a higher level. | |
741 | @xref{Auto Major Mode}. | |
b1b12a8e RS |
742 | @end defun |
743 | ||
744 | @defun generate-new-buffer name | |
745 | This function returns a newly created, empty buffer, but does not make | |
746 | it current. If there is no buffer named @var{name}, then that is the | |
747 | name of the new buffer. If that name is in use, this function adds | |
47ba05ac RS |
748 | suffixes of the form @samp{<@var{n}>} to @var{name}, where @var{n} is an |
749 | integer. It tries successive integers starting with 2 until it finds an | |
750 | available name. | |
b1b12a8e RS |
751 | |
752 | An error is signaled if @var{name} is not a string. | |
753 | ||
754 | @example | |
755 | @group | |
756 | (generate-new-buffer "bar") | |
757 | @result{} #<buffer bar> | |
758 | @end group | |
759 | @group | |
760 | (generate-new-buffer "bar") | |
761 | @result{} #<buffer bar<2>> | |
762 | @end group | |
763 | @group | |
764 | (generate-new-buffer "bar") | |
765 | @result{} #<buffer bar<3>> | |
766 | @end group | |
767 | @end example | |
768 | ||
22697dac KH |
769 | The major mode for the new buffer is set to Fundamental mode. The |
770 | variable @code{default-major-mode} is handled at a higher level. | |
771 | @xref{Auto Major Mode}. | |
b1b12a8e RS |
772 | |
773 | See the related function @code{generate-new-buffer-name} in @ref{Buffer | |
774 | Names}. | |
775 | @end defun | |
776 | ||
777 | @node Killing Buffers | |
778 | @section Killing Buffers | |
779 | @cindex killing buffers | |
780 | @cindex buffers, killing | |
781 | ||
782 | @dfn{Killing a buffer} makes its name unknown to Emacs and makes its | |
47ba05ac | 783 | text space available for other use. |
b1b12a8e | 784 | |
47ba05ac | 785 | The buffer object for the buffer that has been killed remains in |
b1b12a8e RS |
786 | existence as long as anything refers to it, but it is specially marked |
787 | so that you cannot make it current or display it. Killed buffers retain | |
788 | their identity, however; two distinct buffers, when killed, remain | |
789 | distinct according to @code{eq}. | |
790 | ||
791 | If you kill a buffer that is current or displayed in a window, Emacs | |
792 | automatically selects or displays some other buffer instead. This means | |
793 | that killing a buffer can in general change the current buffer. | |
794 | Therefore, when you kill a buffer, you should also take the precautions | |
795 | associated with changing the current buffer (unless you happen to know | |
796 | that the buffer being killed isn't current). @xref{Current Buffer}. | |
797 | ||
bfe721d1 KH |
798 | If you kill a buffer that is the base buffer of one or more indirect |
799 | buffers, the indirect buffers are automatically killed as well. | |
800 | ||
b1b12a8e RS |
801 | The @code{buffer-name} of a killed buffer is @code{nil}. You can use |
802 | this feature to test whether a buffer has been killed: | |
803 | ||
804 | @example | |
805 | @group | |
806 | (defun buffer-killed-p (buffer) | |
807 | "Return t if BUFFER is killed." | |
808 | (not (buffer-name buffer))) | |
809 | @end group | |
810 | @end example | |
811 | ||
812 | @deffn Command kill-buffer buffer-or-name | |
813 | This function kills the buffer @var{buffer-or-name}, freeing all its | |
b5ef0e92 RS |
814 | memory for other uses or to be returned to the operating system. It |
815 | returns @code{nil}. | |
b1b12a8e RS |
816 | |
817 | Any processes that have this buffer as the @code{process-buffer} are | |
818 | sent the @code{SIGHUP} signal, which normally causes them to terminate. | |
819 | (The basic meaning of @code{SIGHUP} is that a dialup line has been | |
820 | disconnected.) @xref{Deleting Processes}. | |
821 | ||
822 | If the buffer is visiting a file and contains unsaved changes, | |
823 | @code{kill-buffer} asks the user to confirm before the buffer is killed. | |
824 | It does this even if not called interactively. To prevent the request | |
825 | for confirmation, clear the modified flag before calling | |
826 | @code{kill-buffer}. @xref{Buffer Modification}. | |
827 | ||
b1b12a8e RS |
828 | Killing a buffer that is already dead has no effect. |
829 | ||
830 | @smallexample | |
831 | (kill-buffer "foo.unchanged") | |
832 | @result{} nil | |
833 | (kill-buffer "foo.changed") | |
834 | ||
835 | ---------- Buffer: Minibuffer ---------- | |
836 | Buffer foo.changed modified; kill anyway? (yes or no) @kbd{yes} | |
837 | ---------- Buffer: Minibuffer ---------- | |
838 | ||
839 | @result{} nil | |
840 | @end smallexample | |
841 | @end deffn | |
842 | ||
e944d8f1 RS |
843 | @defvar kill-buffer-query-functions |
844 | After confirming unsaved changes, @code{kill-buffer} calls the functions | |
845 | in the list @code{kill-buffer-query-functions}, in order of appearance, | |
846 | with no arguments. The buffer being killed is the current buffer when | |
847 | they are called. The idea is that these functions ask for confirmation | |
848 | from the user for various nonstandard reasons. If any of them returns | |
c25b40ed | 849 | @code{nil}, @code{kill-buffer} spares the buffer's life. |
e944d8f1 RS |
850 | @end defvar |
851 | ||
852 | @defvar kill-buffer-hook | |
853 | This is a normal hook run by @code{kill-buffer} after asking all the | |
854 | questions it is going to ask, just before actually killing the buffer. | |
855 | The buffer to be killed is current when the hook functions run. | |
856 | @xref{Hooks}. | |
857 | @end defvar | |
858 | ||
859 | @defvar buffer-offer-save | |
860 | This variable, if non-@code{nil} in a particular buffer, tells | |
861 | @code{save-buffers-kill-emacs} and @code{save-some-buffers} to offer to | |
862 | save that buffer, just as they offer to save file-visiting buffers. The | |
863 | variable @code{buffer-offer-save} automatically becomes buffer-local | |
864 | when set for any reason. @xref{Buffer-Local Variables}. | |
865 | @end defvar | |
866 | ||
22697dac KH |
867 | @node Indirect Buffers |
868 | @section Indirect Buffers | |
869 | @cindex indirect buffers | |
870 | @cindex base buffer | |
871 | ||
872 | An @dfn{indirect buffer} shares the text of some other buffer, which | |
873 | is called the @dfn{base buffer} of the indirect buffer. In some ways it | |
bfe721d1 | 874 | is the analogue, for buffers, of a symbolic link among files. The base |
22697dac KH |
875 | buffer may not itself be an indirect buffer. |
876 | ||
877 | The text of the indirect buffer is always identical to the text of its | |
878 | base buffer; changes made by editing either one are visible immediately | |
879 | in the other. This includes the text properties as well as the characters | |
880 | themselves. | |
881 | ||
882 | But in all other respects, the indirect buffer and its base buffer are | |
883 | completely separate. They have different names, different values of | |
884 | point, different narrowing, different markers and overlays (though | |
885 | inserting or deleting text in either buffer relocates the markers and | |
886 | overlays for both), different major modes, and different local | |
887 | variables. | |
888 | ||
889 | An indirect buffer cannot visit a file, but its base buffer can. If | |
890 | you try to save the indirect buffer, that actually works by saving the | |
891 | base buffer. | |
892 | ||
893 | Killing an indirect buffer has no effect on its base buffer. Killing | |
894 | the base buffer effectively kills the indirect buffer in that it cannot | |
895 | ever again be the current buffer. | |
896 | ||
897 | @deffn Command make-indirect-buffer base-buffer name | |
898 | This creates an indirect buffer named @var{name} whose base buffer | |
899 | is @var{base-buffer}. The argument @var{base-buffer} may be a buffer | |
900 | or a string. | |
901 | ||
902 | If @var{base-buffer} is an indirect buffer, its base buffer is used as | |
903 | the base for the new buffer. | |
904 | @end deffn | |
b1b12a8e | 905 | |
22697dac KH |
906 | @defun buffer-base-buffer buffer |
907 | This function returns the base buffer of @var{buffer}. If @var{buffer} | |
908 | is not indirect, the value is @code{nil}. Otherwise, the value is | |
909 | another buffer, which is never an indirect buffer. | |
b1b12a8e RS |
910 | @end defun |
911 |