Doc fix for `face-spec-set'.
[bpt/emacs.git] / doc / lispref / display.texi
1 @c -*-texinfo-*-
2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990-1995, 1998-2014 Free Software Foundation, Inc.
4 @c See the file elisp.texi for copying conditions.
5 @node Display
6 @chapter Emacs Display
7
8 This chapter describes a number of features related to the display
9 that Emacs presents to the user.
10
11 @menu
12 * Refresh Screen:: Clearing the screen and redrawing everything on it.
13 * Forcing Redisplay:: Forcing redisplay.
14 * Truncation:: Folding or wrapping long text lines.
15 * The Echo Area:: Displaying messages at the bottom of the screen.
16 * Warnings:: Displaying warning messages for the user.
17 * Invisible Text:: Hiding part of the buffer text.
18 * Selective Display:: Hiding part of the buffer text (the old way).
19 * Temporary Displays:: Displays that go away automatically.
20 * Overlays:: Use overlays to highlight parts of the buffer.
21 * Width:: How wide a character or string is on the screen.
22 * Line Height:: Controlling the height of lines.
23 * Faces:: A face defines a graphics style for text characters:
24 font, colors, etc.
25 * Fringes:: Controlling window fringes.
26 * Scroll Bars:: Controlling vertical scroll bars.
27 * Display Property:: Enabling special display features.
28 * Images:: Displaying images in Emacs buffers.
29 * Buttons:: Adding clickable buttons to Emacs buffers.
30 * Abstract Display:: Emacs's Widget for Object Collections.
31 * Blinking:: How Emacs shows the matching open parenthesis.
32 * Character Display:: How Emacs displays individual characters.
33 * Beeping:: Audible signal to the user.
34 * Window Systems:: Which window system is being used.
35 * Bidirectional Display:: Display of bidirectional scripts, such as
36 Arabic and Farsi.
37 @end menu
38
39 @node Refresh Screen
40 @section Refreshing the Screen
41
42 The function @code{redraw-frame} clears and redisplays the entire
43 contents of a given frame (@pxref{Frames}). This is useful if the
44 screen is corrupted.
45
46 @defun redraw-frame frame
47 This function clears and redisplays frame @var{frame}.
48 @end defun
49
50 Even more powerful is @code{redraw-display}:
51
52 @deffn Command redraw-display
53 This function clears and redisplays all visible frames.
54 @end deffn
55
56 In Emacs, processing user input takes priority over redisplay. If
57 you call these functions when input is available, they don't redisplay
58 immediately, but the requested redisplay does happen
59 eventually---after all the input has been processed.
60
61 On text terminals, suspending and resuming Emacs normally also
62 refreshes the screen. Some terminal emulators record separate
63 contents for display-oriented programs such as Emacs and for ordinary
64 sequential display. If you are using such a terminal, you might want
65 to inhibit the redisplay on resumption.
66
67 @defopt no-redraw-on-reenter
68 @cindex suspend (cf. @code{no-redraw-on-reenter})
69 @cindex resume (cf. @code{no-redraw-on-reenter})
70 This variable controls whether Emacs redraws the entire screen after it
71 has been suspended and resumed. Non-@code{nil} means there is no need
72 to redraw, @code{nil} means redrawing is needed. The default is @code{nil}.
73 @end defopt
74
75 @node Forcing Redisplay
76 @section Forcing Redisplay
77 @cindex forcing redisplay
78
79 Emacs normally tries to redisplay the screen whenever it waits for
80 input. With the following function, you can request an immediate
81 attempt to redisplay, in the middle of Lisp code, without actually
82 waiting for input.
83
84 @defun redisplay &optional force
85 This function tries immediately to redisplay. The optional argument
86 @var{force}, if non-@code{nil}, forces the redisplay to be performed,
87 instead of being preempted, even if input is pending and the variable
88 @code{redisplay-dont-pause} is @code{nil} (see below). If
89 @code{redisplay-dont-pause} is non-@code{nil} (the default), this
90 function redisplays in any case, i.e., @var{force} does nothing.
91
92 The function returns @code{t} if it actually tried to redisplay, and
93 @code{nil} otherwise. A value of @code{t} does not mean that
94 redisplay proceeded to completion; it could have been preempted by
95 newly arriving input.
96 @end defun
97
98 @defvar redisplay-dont-pause
99 If this variable is @code{nil}, arriving input events preempt
100 redisplay; Emacs avoids starting a redisplay, and stops any redisplay
101 that is in progress, until the input has been processed. In
102 particular, @code{(redisplay)} returns @code{nil} without actually
103 redisplaying, if there is pending input.
104
105 The default value is @code{t}, which means that pending input does not
106 preempt redisplay.
107 @end defvar
108
109 @defvar redisplay-preemption-period
110 If @code{redisplay-dont-pause} is @code{nil}, this variable specifies
111 how many seconds Emacs waits between checks for new input during
112 redisplay; if input arrives during this interval, redisplay stops and
113 the input is processed. The default value is 0.1; if the value is
114 @code{nil}, Emacs does not check for input during redisplay.
115
116 This variable has no effect when @code{redisplay-dont-pause} is
117 non-@code{nil} (the default).
118 @end defvar
119
120 Although @code{redisplay} tries immediately to redisplay, it does
121 not change how Emacs decides which parts of its frame(s) to redisplay.
122 By contrast, the following function adds certain windows to the
123 pending redisplay work (as if their contents had completely changed),
124 but does not immediately try to perform redisplay.
125
126 @defun force-window-update &optional object
127 This function forces some or all windows to be updated the next time
128 Emacs does a redisplay. If @var{object} is a window, that window is
129 to be updated. If @var{object} is a buffer or buffer name, all
130 windows displaying that buffer are to be updated. If @var{object} is
131 @code{nil} (or omitted), all windows are to be updated.
132
133 This function does not do a redisplay immediately; Emacs does that as
134 it waits for input, or when the function @code{redisplay} is called.
135 @end defun
136
137 @node Truncation
138 @section Truncation
139 @cindex line wrapping
140 @cindex line truncation
141 @cindex continuation lines
142 @cindex @samp{$} in display
143 @cindex @samp{\} in display
144
145 When a line of text extends beyond the right edge of a window, Emacs
146 can @dfn{continue} the line (make it ``wrap'' to the next screen
147 line), or @dfn{truncate} the line (limit it to one screen line). The
148 additional screen lines used to display a long text line are called
149 @dfn{continuation} lines. Continuation is not the same as filling;
150 continuation happens on the screen only, not in the buffer contents,
151 and it breaks a line precisely at the right margin, not at a word
152 boundary. @xref{Filling}.
153
154 On a graphical display, tiny arrow images in the window fringes
155 indicate truncated and continued lines (@pxref{Fringes}). On a text
156 terminal, a @samp{$} in the rightmost column of the window indicates
157 truncation; a @samp{\} on the rightmost column indicates a line that
158 ``wraps''. (The display table can specify alternate characters to use
159 for this; @pxref{Display Tables}).
160
161 @defopt truncate-lines
162 If this buffer-local variable is non-@code{nil}, lines that extend
163 beyond the right edge of the window are truncated; otherwise, they are
164 continued. As a special exception, the variable
165 @code{truncate-partial-width-windows} takes precedence in
166 @dfn{partial-width} windows (i.e., windows that do not occupy the
167 entire frame width).
168 @end defopt
169
170 @defopt truncate-partial-width-windows
171 @cindex partial-width windows
172 This variable controls line truncation in @dfn{partial-width} windows.
173 A partial-width window is one that does not occupy the entire frame
174 width (@pxref{Splitting Windows}). If the value is @code{nil}, line
175 truncation is determined by the variable @code{truncate-lines} (see
176 above). If the value is an integer @var{n}, lines are truncated if
177 the partial-width window has fewer than @var{n} columns, regardless of
178 the value of @code{truncate-lines}; if the partial-width window has
179 @var{n} or more columns, line truncation is determined by
180 @code{truncate-lines}. For any other non-@code{nil} value, lines are
181 truncated in every partial-width window, regardless of the value of
182 @code{truncate-lines}.
183 @end defopt
184
185 When horizontal scrolling (@pxref{Horizontal Scrolling}) is in use in
186 a window, that forces truncation.
187
188 @defvar wrap-prefix
189 If this buffer-local variable is non-@code{nil}, it defines a
190 @dfn{wrap prefix} which Emacs displays at the start of every
191 continuation line. (If lines are truncated, @code{wrap-prefix} is
192 never used.) Its value may be a string or an image (@pxref{Other
193 Display Specs}), or a stretch of whitespace such as specified by the
194 @code{:width} or @code{:align-to} display properties (@pxref{Specified
195 Space}). The value is interpreted in the same way as a @code{display}
196 text property. @xref{Display Property}.
197
198 A wrap prefix may also be specified for regions of text, using the
199 @code{wrap-prefix} text or overlay property. This takes precedence
200 over the @code{wrap-prefix} variable. @xref{Special Properties}.
201 @end defvar
202
203 @defvar line-prefix
204 If this buffer-local variable is non-@code{nil}, it defines a
205 @dfn{line prefix} which Emacs displays at the start of every
206 non-continuation line. Its value may be a string or an image
207 (@pxref{Other Display Specs}), or a stretch of whitespace such as
208 specified by the @code{:width} or @code{:align-to} display properties
209 (@pxref{Specified Space}). The value is interpreted in the same way
210 as a @code{display} text property. @xref{Display Property}.
211
212 A line prefix may also be specified for regions of text using the
213 @code{line-prefix} text or overlay property. This takes precedence
214 over the @code{line-prefix} variable. @xref{Special Properties}.
215 @end defvar
216
217 @ignore
218 If your buffer contains only very short lines, you might find it
219 advisable to set @code{cache-long-scans} to @code{nil}.
220
221 @defvar cache-long-scans
222 If this variable is non-@code{nil} (the default), various indentation
223 and motion functions, and Emacs redisplay, cache the results of
224 scanning the buffer, and consult the cache to avoid rescanning regions
225 of the buffer unless they are modified.
226
227 Turning off the cache speeds up processing of short lines somewhat.
228
229 This variable is automatically buffer-local in every buffer.
230 @end defvar
231 @end ignore
232
233 @node The Echo Area
234 @section The Echo Area
235 @cindex error display
236 @cindex echo area
237
238 @c FIXME: Why not use @xref{Minibuffers} directly? --xfq
239 The @dfn{echo area} is used for displaying error messages
240 (@pxref{Errors}), for messages made with the @code{message} primitive,
241 and for echoing keystrokes. It is not the same as the minibuffer,
242 despite the fact that the minibuffer appears (when active) in the same
243 place on the screen as the echo area. @xref{Minibuffer,, The
244 Minibuffer, emacs, The GNU Emacs Manual}.
245
246 Apart from the functions documented in this section, you can print
247 Lisp objects to the echo area by specifying @code{t} as the output
248 stream. @xref{Output Streams}.
249
250 @menu
251 * Displaying Messages:: Explicitly displaying text in the echo area.
252 * Progress:: Informing user about progress of a long operation.
253 * Logging Messages:: Echo area messages are logged for the user.
254 * Echo Area Customization:: Controlling the echo area.
255 @end menu
256
257 @node Displaying Messages
258 @subsection Displaying Messages in the Echo Area
259 @cindex display message in echo area
260
261 This section describes the standard functions for displaying
262 messages in the echo area.
263
264 @defun message format-string &rest arguments
265 This function displays a message in the echo area.
266 @var{format-string} is a format string, and @var{arguments} are the
267 objects for its format specifications, like in the @code{format}
268 function (@pxref{Formatting Strings}). The resulting formatted string
269 is displayed in the echo area; if it contains @code{face} text
270 properties, it is displayed with the specified faces (@pxref{Faces}).
271 The string is also added to the @file{*Messages*} buffer, but without
272 text properties (@pxref{Logging Messages}).
273
274 In batch mode, the message is printed to the standard error stream,
275 followed by a newline.
276
277 If @var{format-string} is @code{nil} or the empty string,
278 @code{message} clears the echo area; if the echo area has been
279 expanded automatically, this brings it back to its normal size. If
280 the minibuffer is active, this brings the minibuffer contents back
281 onto the screen immediately.
282
283 @example
284 @group
285 (message "Minibuffer depth is %d."
286 (minibuffer-depth))
287 @print{} Minibuffer depth is 0.
288 @result{} "Minibuffer depth is 0."
289 @end group
290
291 @group
292 ---------- Echo Area ----------
293 Minibuffer depth is 0.
294 ---------- Echo Area ----------
295 @end group
296 @end example
297
298 To automatically display a message in the echo area or in a pop-buffer,
299 depending on its size, use @code{display-message-or-buffer} (see below).
300 @end defun
301
302 @defmac with-temp-message message &rest body
303 This construct displays a message in the echo area temporarily, during
304 the execution of @var{body}. It displays @var{message}, executes
305 @var{body}, then returns the value of the last body form while restoring
306 the previous echo area contents.
307 @end defmac
308
309 @defun message-or-box format-string &rest arguments
310 This function displays a message like @code{message}, but may display it
311 in a dialog box instead of the echo area. If this function is called in
312 a command that was invoked using the mouse---more precisely, if
313 @code{last-nonmenu-event} (@pxref{Command Loop Info}) is either
314 @code{nil} or a list---then it uses a dialog box or pop-up menu to
315 display the message. Otherwise, it uses the echo area. (This is the
316 same criterion that @code{y-or-n-p} uses to make a similar decision; see
317 @ref{Yes-or-No Queries}.)
318
319 You can force use of the mouse or of the echo area by binding
320 @code{last-nonmenu-event} to a suitable value around the call.
321 @end defun
322
323 @defun message-box format-string &rest arguments
324 @anchor{message-box}
325 This function displays a message like @code{message}, but uses a dialog
326 box (or a pop-up menu) whenever that is possible. If it is impossible
327 to use a dialog box or pop-up menu, because the terminal does not
328 support them, then @code{message-box} uses the echo area, like
329 @code{message}.
330 @end defun
331
332 @defun display-message-or-buffer message &optional buffer-name not-this-window frame
333 This function displays the message @var{message}, which may be either a
334 string or a buffer. If it is shorter than the maximum height of the
335 echo area, as defined by @code{max-mini-window-height}, it is displayed
336 in the echo area, using @code{message}. Otherwise,
337 @code{display-buffer} is used to show it in a pop-up buffer.
338
339 Returns either the string shown in the echo area, or when a pop-up
340 buffer is used, the window used to display it.
341
342 If @var{message} is a string, then the optional argument
343 @var{buffer-name} is the name of the buffer used to display it when a
344 pop-up buffer is used, defaulting to @file{*Message*}. In the case
345 where @var{message} is a string and displayed in the echo area, it is
346 not specified whether the contents are inserted into the buffer anyway.
347
348 The optional arguments @var{not-this-window} and @var{frame} are as for
349 @code{display-buffer}, and only used if a buffer is displayed.
350 @end defun
351
352 @defun current-message
353 This function returns the message currently being displayed in the
354 echo area, or @code{nil} if there is none.
355 @end defun
356
357 @node Progress
358 @subsection Reporting Operation Progress
359 @cindex progress reporting
360
361 When an operation can take a while to finish, you should inform the
362 user about the progress it makes. This way the user can estimate
363 remaining time and clearly see that Emacs is busy working, not hung.
364 A convenient way to do this is to use a @dfn{progress reporter}.
365
366 Here is a working example that does nothing useful:
367
368 @smallexample
369 (let ((progress-reporter
370 (make-progress-reporter "Collecting mana for Emacs..."
371 0 500)))
372 (dotimes (k 500)
373 (sit-for 0.01)
374 (progress-reporter-update progress-reporter k))
375 (progress-reporter-done progress-reporter))
376 @end smallexample
377
378 @defun make-progress-reporter message &optional min-value max-value current-value min-change min-time
379 This function creates and returns a progress reporter object, which
380 you will use as an argument for the other functions listed below. The
381 idea is to precompute as much data as possible to make progress
382 reporting very fast.
383
384 When this progress reporter is subsequently used, it will display
385 @var{message} in the echo area, followed by progress percentage.
386 @var{message} is treated as a simple string. If you need it to depend
387 on a filename, for instance, use @code{format} before calling this
388 function.
389
390 The arguments @var{min-value} and @var{max-value} should be numbers
391 standing for the starting and final states of the operation. For
392 instance, an operation that ``scans'' a buffer should set these to the
393 results of @code{point-min} and @code{point-max} correspondingly.
394 @var{max-value} should be greater than @var{min-value}.
395
396 Alternatively, you can set @var{min-value} and @var{max-value} to
397 @code{nil}. In that case, the progress reporter does not report
398 process percentages; it instead displays a ``spinner'' that rotates a
399 notch each time you update the progress reporter.
400
401 If @var{min-value} and @var{max-value} are numbers, you can give the
402 argument @var{current-value} a numerical value specifying the initial
403 progress; if omitted, this defaults to @var{min-value}.
404
405 The remaining arguments control the rate of echo area updates. The
406 progress reporter will wait for at least @var{min-change} more
407 percents of the operation to be completed before printing next
408 message; the default is one percent. @var{min-time} specifies the
409 minimum time in seconds to pass between successive prints; the default
410 is 0.2 seconds. (On some operating systems, the progress reporter may
411 handle fractions of seconds with varying precision).
412
413 This function calls @code{progress-reporter-update}, so the first
414 message is printed immediately.
415 @end defun
416
417 @defun progress-reporter-update reporter &optional value
418 This function does the main work of reporting progress of your
419 operation. It displays the message of @var{reporter}, followed by
420 progress percentage determined by @var{value}. If percentage is zero,
421 or close enough according to the @var{min-change} and @var{min-time}
422 arguments, then it is omitted from the output.
423
424 @var{reporter} must be the result of a call to
425 @code{make-progress-reporter}. @var{value} specifies the current
426 state of your operation and must be between @var{min-value} and
427 @var{max-value} (inclusive) as passed to
428 @code{make-progress-reporter}. For instance, if you scan a buffer,
429 then @var{value} should be the result of a call to @code{point}.
430
431 This function respects @var{min-change} and @var{min-time} as passed
432 to @code{make-progress-reporter} and so does not output new messages
433 on every invocation. It is thus very fast and normally you should not
434 try to reduce the number of calls to it: resulting overhead will most
435 likely negate your effort.
436 @end defun
437
438 @defun progress-reporter-force-update reporter &optional value new-message
439 This function is similar to @code{progress-reporter-update} except
440 that it prints a message in the echo area unconditionally.
441
442 The first two arguments have the same meaning as for
443 @code{progress-reporter-update}. Optional @var{new-message} allows
444 you to change the message of the @var{reporter}. Since this function
445 always updates the echo area, such a change will be immediately
446 presented to the user.
447 @end defun
448
449 @defun progress-reporter-done reporter
450 This function should be called when the operation is finished. It
451 prints the message of @var{reporter} followed by word ``done'' in the
452 echo area.
453
454 You should always call this function and not hope for
455 @code{progress-reporter-update} to print ``100%''. Firstly, it may
456 never print it, there are many good reasons for this not to happen.
457 Secondly, ``done'' is more explicit.
458 @end defun
459
460 @defmac dotimes-with-progress-reporter (var count [result]) message body@dots{}
461 This is a convenience macro that works the same way as @code{dotimes}
462 does, but also reports loop progress using the functions described
463 above. It allows you to save some typing.
464
465 You can rewrite the example in the beginning of this node using
466 this macro this way:
467
468 @example
469 (dotimes-with-progress-reporter
470 (k 500)
471 "Collecting some mana for Emacs..."
472 (sit-for 0.01))
473 @end example
474 @end defmac
475
476 @node Logging Messages
477 @subsection Logging Messages in @file{*Messages*}
478 @cindex logging echo-area messages
479
480 Almost all the messages displayed in the echo area are also recorded
481 in the @file{*Messages*} buffer so that the user can refer back to
482 them. This includes all the messages that are output with
483 @code{message}. By default, this buffer is read-only and uses the major
484 mode @code{messages-buffer-mode}. Nothing prevents the user from
485 killing the @file{*Messages*} buffer, but the next display of a message
486 recreates it. Any Lisp code that needs to access the
487 @file{*Messages*} buffer directly and wants to ensure that it exists
488 should use the function @code{messages-buffer}.
489
490 @defun messages-buffer
491 This function returns the @file{*Messages*} buffer. If it does not
492 exist, it creates it, and switches it to @code{messages-buffer-mode}.
493 @end defun
494
495 @defopt message-log-max
496 This variable specifies how many lines to keep in the @file{*Messages*}
497 buffer. The value @code{t} means there is no limit on how many lines to
498 keep. The value @code{nil} disables message logging entirely. Here's
499 how to display a message and prevent it from being logged:
500
501 @example
502 (let (message-log-max)
503 (message @dots{}))
504 @end example
505 @end defopt
506
507 To make @file{*Messages*} more convenient for the user, the logging
508 facility combines successive identical messages. It also combines
509 successive related messages for the sake of two cases: question
510 followed by answer, and a series of progress messages.
511
512 A ``question followed by an answer'' means two messages like the
513 ones produced by @code{y-or-n-p}: the first is @samp{@var{question}},
514 and the second is @samp{@var{question}...@var{answer}}. The first
515 message conveys no additional information beyond what's in the second,
516 so logging the second message discards the first from the log.
517
518 A ``series of progress messages'' means successive messages like
519 those produced by @code{make-progress-reporter}. They have the form
520 @samp{@var{base}...@var{how-far}}, where @var{base} is the same each
521 time, while @var{how-far} varies. Logging each message in the series
522 discards the previous one, provided they are consecutive.
523
524 The functions @code{make-progress-reporter} and @code{y-or-n-p}
525 don't have to do anything special to activate the message log
526 combination feature. It operates whenever two consecutive messages
527 are logged that share a common prefix ending in @samp{...}.
528
529 @node Echo Area Customization
530 @subsection Echo Area Customization
531
532 These variables control details of how the echo area works.
533
534 @defvar cursor-in-echo-area
535 This variable controls where the cursor appears when a message is
536 displayed in the echo area. If it is non-@code{nil}, then the cursor
537 appears at the end of the message. Otherwise, the cursor appears at
538 point---not in the echo area at all.
539
540 The value is normally @code{nil}; Lisp programs bind it to @code{t}
541 for brief periods of time.
542 @end defvar
543
544 @defvar echo-area-clear-hook
545 This normal hook is run whenever the echo area is cleared---either by
546 @code{(message nil)} or for any other reason.
547 @end defvar
548
549 @defopt echo-keystrokes
550 This variable determines how much time should elapse before command
551 characters echo. Its value must be an integer or floating point number,
552 which specifies the
553 number of seconds to wait before echoing. If the user types a prefix
554 key (such as @kbd{C-x}) and then delays this many seconds before
555 continuing, the prefix key is echoed in the echo area. (Once echoing
556 begins in a key sequence, all subsequent characters in the same key
557 sequence are echoed immediately.)
558
559 If the value is zero, then command input is not echoed.
560 @end defopt
561
562 @defvar message-truncate-lines
563 Normally, displaying a long message resizes the echo area to display
564 the entire message. But if the variable @code{message-truncate-lines}
565 is non-@code{nil}, the echo area does not resize, and the message is
566 truncated to fit it.
567 @end defvar
568
569 The variable @code{max-mini-window-height}, which specifies the
570 maximum height for resizing minibuffer windows, also applies to the
571 echo area (which is really a special use of the minibuffer window;
572 @pxref{Minibuffer Misc}).
573
574 @node Warnings
575 @section Reporting Warnings
576 @cindex warnings
577
578 @dfn{Warnings} are a facility for a program to inform the user of a
579 possible problem, but continue running.
580
581 @menu
582 * Warning Basics:: Warnings concepts and functions to report them.
583 * Warning Variables:: Variables programs bind to customize their warnings.
584 * Warning Options:: Variables users set to control display of warnings.
585 * Delayed Warnings:: Deferring a warning until the end of a command.
586 @end menu
587
588 @node Warning Basics
589 @subsection Warning Basics
590 @cindex severity level
591
592 Every warning has a textual message, which explains the problem for
593 the user, and a @dfn{severity level} which is a symbol. Here are the
594 possible severity levels, in order of decreasing severity, and their
595 meanings:
596
597 @table @code
598 @item :emergency
599 A problem that will seriously impair Emacs operation soon
600 if you do not attend to it promptly.
601 @item :error
602 A report of data or circumstances that are inherently wrong.
603 @item :warning
604 A report of data or circumstances that are not inherently wrong, but
605 raise suspicion of a possible problem.
606 @item :debug
607 A report of information that may be useful if you are debugging.
608 @end table
609
610 When your program encounters invalid input data, it can either
611 signal a Lisp error by calling @code{error} or @code{signal} or report
612 a warning with severity @code{:error}. Signaling a Lisp error is the
613 easiest thing to do, but it means the program cannot continue
614 processing. If you want to take the trouble to implement a way to
615 continue processing despite the bad data, then reporting a warning of
616 severity @code{:error} is the right way to inform the user of the
617 problem. For instance, the Emacs Lisp byte compiler can report an
618 error that way and continue compiling other functions. (If the
619 program signals a Lisp error and then handles it with
620 @code{condition-case}, the user won't see the error message; it could
621 show the message to the user by reporting it as a warning.)
622
623 @c FIXME: Why use "(bytecomp)" instead of "'bytecomp" or simply
624 @c "bytecomp" here? The parens are part of warning-type-format but
625 @c not part of the warning type. --xfq
626 @cindex warning type
627 Each warning has a @dfn{warning type} to classify it. The type is a
628 list of symbols. The first symbol should be the custom group that you
629 use for the program's user options. For example, byte compiler
630 warnings use the warning type @code{(bytecomp)}. You can also
631 subcategorize the warnings, if you wish, by using more symbols in the
632 list.
633
634 @defun display-warning type message &optional level buffer-name
635 This function reports a warning, using @var{message} as the message
636 and @var{type} as the warning type. @var{level} should be the
637 severity level, with @code{:warning} being the default.
638
639 @var{buffer-name}, if non-@code{nil}, specifies the name of the buffer
640 for logging the warning. By default, it is @file{*Warnings*}.
641 @end defun
642
643 @defun lwarn type level message &rest args
644 This function reports a warning using the value of @code{(format
645 @var{message} @var{args}...)} as the message in the @file{*Warnings*}
646 buffer. In other respects it is equivalent to @code{display-warning}.
647 @end defun
648
649 @defun warn message &rest args
650 This function reports a warning using the value of @code{(format
651 @var{message} @var{args}...)} as the message, @code{(emacs)} as the
652 type, and @code{:warning} as the severity level. It exists for
653 compatibility only; we recommend not using it, because you should
654 specify a specific warning type.
655 @end defun
656
657 @node Warning Variables
658 @subsection Warning Variables
659
660 Programs can customize how their warnings appear by binding
661 the variables described in this section.
662
663 @defvar warning-levels
664 This list defines the meaning and severity order of the warning
665 severity levels. Each element defines one severity level,
666 and they are arranged in order of decreasing severity.
667
668 Each element has the form @code{(@var{level} @var{string}
669 @var{function})}, where @var{level} is the severity level it defines.
670 @var{string} specifies the textual description of this level.
671 @var{string} should use @samp{%s} to specify where to put the warning
672 type information, or it can omit the @samp{%s} so as not to include
673 that information.
674
675 The optional @var{function}, if non-@code{nil}, is a function to call
676 with no arguments, to get the user's attention.
677
678 Normally you should not change the value of this variable.
679 @end defvar
680
681 @defvar warning-prefix-function
682 If non-@code{nil}, the value is a function to generate prefix text for
683 warnings. Programs can bind the variable to a suitable function.
684 @code{display-warning} calls this function with the warnings buffer
685 current, and the function can insert text in it. That text becomes
686 the beginning of the warning message.
687
688 The function is called with two arguments, the severity level and its
689 entry in @code{warning-levels}. It should return a list to use as the
690 entry (this value need not be an actual member of
691 @code{warning-levels}). By constructing this value, the function can
692 change the severity of the warning, or specify different handling for
693 a given severity level.
694
695 If the variable's value is @code{nil} then there is no function
696 to call.
697 @end defvar
698
699 @defvar warning-series
700 Programs can bind this variable to @code{t} to say that the next
701 warning should begin a series. When several warnings form a series,
702 that means to leave point on the first warning of the series, rather
703 than keep moving it for each warning so that it appears on the last one.
704 The series ends when the local binding is unbound and
705 @code{warning-series} becomes @code{nil} again.
706
707 The value can also be a symbol with a function definition. That is
708 equivalent to @code{t}, except that the next warning will also call
709 the function with no arguments with the warnings buffer current. The
710 function can insert text which will serve as a header for the series
711 of warnings.
712
713 Once a series has begun, the value is a marker which points to the
714 buffer position in the warnings buffer of the start of the series.
715
716 The variable's normal value is @code{nil}, which means to handle
717 each warning separately.
718 @end defvar
719
720 @defvar warning-fill-prefix
721 When this variable is non-@code{nil}, it specifies a fill prefix to
722 use for filling each warning's text.
723 @end defvar
724
725 @defvar warning-type-format
726 This variable specifies the format for displaying the warning type
727 in the warning message. The result of formatting the type this way
728 gets included in the message under the control of the string in the
729 entry in @code{warning-levels}. The default value is @code{" (%s)"}.
730 If you bind it to @code{""} then the warning type won't appear at
731 all.
732 @end defvar
733
734 @node Warning Options
735 @subsection Warning Options
736
737 These variables are used by users to control what happens
738 when a Lisp program reports a warning.
739
740 @defopt warning-minimum-level
741 This user option specifies the minimum severity level that should be
742 shown immediately to the user. The default is @code{:warning}, which
743 means to immediately display all warnings except @code{:debug}
744 warnings.
745 @end defopt
746
747 @defopt warning-minimum-log-level
748 This user option specifies the minimum severity level that should be
749 logged in the warnings buffer. The default is @code{:warning}, which
750 means to log all warnings except @code{:debug} warnings.
751 @end defopt
752
753 @defopt warning-suppress-types
754 This list specifies which warning types should not be displayed
755 immediately for the user. Each element of the list should be a list
756 of symbols. If its elements match the first elements in a warning
757 type, then that warning is not displayed immediately.
758 @end defopt
759
760 @defopt warning-suppress-log-types
761 This list specifies which warning types should not be logged in the
762 warnings buffer. Each element of the list should be a list of
763 symbols. If it matches the first few elements in a warning type, then
764 that warning is not logged.
765 @end defopt
766
767 @node Delayed Warnings
768 @subsection Delayed Warnings
769
770 Sometimes, you may wish to avoid showing a warning while a command is
771 running, and only show it only after the end of the command. You can
772 use the variable @code{delayed-warnings-list} for this.
773
774 @defvar delayed-warnings-list
775 The value of this variable is a list of warnings to be displayed after
776 the current command has finished. Each element must be a list
777
778 @smallexample
779 (@var{type} @var{message} [@var{level} [@var{buffer-name}]])
780 @end smallexample
781
782 @noindent
783 with the same form, and the same meanings, as the argument list of
784 @code{display-warning} (@pxref{Warning Basics}). Immediately after
785 running @code{post-command-hook} (@pxref{Command Overview}), the Emacs
786 command loop displays all the warnings specified by this variable,
787 then resets it to @code{nil}.
788 @end defvar
789
790 Programs which need to further customize the delayed warnings
791 mechanism can change the variable @code{delayed-warnings-hook}:
792
793 @defvar delayed-warnings-hook
794 This is a normal hook which is run by the Emacs command loop, after
795 @code{post-command-hook}, in order to to process and display delayed
796 warnings.
797
798 Its default value is a list of two functions:
799
800 @smallexample
801 (collapse-delayed-warnings display-delayed-warnings)
802 @end smallexample
803
804 @findex collapse-delayed-warnings
805 @findex display-delayed-warnings
806 @noindent
807 The function @code{collapse-delayed-warnings} removes repeated entries
808 from @code{delayed-warnings-list}. The function
809 @code{display-delayed-warnings} calls @code{display-warning} on each
810 of the entries in @code{delayed-warnings-list}, in turn, and then sets
811 @code{delayed-warnings-list} to @code{nil}.
812 @end defvar
813
814 @node Invisible Text
815 @section Invisible Text
816
817 @cindex invisible text
818 You can make characters @dfn{invisible}, so that they do not appear on
819 the screen, with the @code{invisible} property. This can be either a
820 text property (@pxref{Text Properties}) or an overlay property
821 (@pxref{Overlays}). Cursor motion also partly ignores these
822 characters; if the command loop finds that point is inside a range of
823 invisible text after a command, it relocates point to the other side
824 of the text.
825
826 In the simplest case, any non-@code{nil} @code{invisible} property makes
827 a character invisible. This is the default case---if you don't alter
828 the default value of @code{buffer-invisibility-spec}, this is how the
829 @code{invisible} property works. You should normally use @code{t}
830 as the value of the @code{invisible} property if you don't plan
831 to set @code{buffer-invisibility-spec} yourself.
832
833 More generally, you can use the variable @code{buffer-invisibility-spec}
834 to control which values of the @code{invisible} property make text
835 invisible. This permits you to classify the text into different subsets
836 in advance, by giving them different @code{invisible} values, and
837 subsequently make various subsets visible or invisible by changing the
838 value of @code{buffer-invisibility-spec}.
839
840 Controlling visibility with @code{buffer-invisibility-spec} is
841 especially useful in a program to display the list of entries in a
842 database. It permits the implementation of convenient filtering
843 commands to view just a part of the entries in the database. Setting
844 this variable is very fast, much faster than scanning all the text in
845 the buffer looking for properties to change.
846
847 @defvar buffer-invisibility-spec
848 This variable specifies which kinds of @code{invisible} properties
849 actually make a character invisible. Setting this variable makes it
850 buffer-local.
851
852 @table @asis
853 @item @code{t}
854 A character is invisible if its @code{invisible} property is
855 non-@code{nil}. This is the default.
856
857 @item a list
858 Each element of the list specifies a criterion for invisibility; if a
859 character's @code{invisible} property fits any one of these criteria,
860 the character is invisible. The list can have two kinds of elements:
861
862 @table @code
863 @item @var{atom}
864 A character is invisible if its @code{invisible} property value is
865 @var{atom} or if it is a list with @var{atom} as a member; comparison
866 is done with @code{eq}.
867
868 @item (@var{atom} . t)
869 A character is invisible if its @code{invisible} property value is
870 @var{atom} or if it is a list with @var{atom} as a member; comparison
871 is done with @code{eq}. Moreover, a sequence of such characters
872 displays as an ellipsis.
873 @end table
874 @end table
875 @end defvar
876
877 Two functions are specifically provided for adding elements to
878 @code{buffer-invisibility-spec} and removing elements from it.
879
880 @defun add-to-invisibility-spec element
881 This function adds the element @var{element} to
882 @code{buffer-invisibility-spec}. If @code{buffer-invisibility-spec}
883 was @code{t}, it changes to a list, @code{(t)}, so that text whose
884 @code{invisible} property is @code{t} remains invisible.
885 @end defun
886
887 @defun remove-from-invisibility-spec element
888 This removes the element @var{element} from
889 @code{buffer-invisibility-spec}. This does nothing if @var{element}
890 is not in the list.
891 @end defun
892
893 A convention for use of @code{buffer-invisibility-spec} is that a
894 major mode should use the mode's own name as an element of
895 @code{buffer-invisibility-spec} and as the value of the
896 @code{invisible} property:
897
898 @example
899 ;; @r{If you want to display an ellipsis:}
900 (add-to-invisibility-spec '(my-symbol . t))
901 ;; @r{If you don't want ellipsis:}
902 (add-to-invisibility-spec 'my-symbol)
903
904 (overlay-put (make-overlay beginning end)
905 'invisible 'my-symbol)
906
907 ;; @r{When done with the invisibility:}
908 (remove-from-invisibility-spec '(my-symbol . t))
909 ;; @r{Or respectively:}
910 (remove-from-invisibility-spec 'my-symbol)
911 @end example
912
913 You can check for invisibility using the following function:
914
915 @defun invisible-p pos-or-prop
916 If @var{pos-or-prop} is a marker or number, this function returns a
917 non-@code{nil} value if the text at that position is invisible.
918
919 If @var{pos-or-prop} is any other kind of Lisp object, that is taken
920 to mean a possible value of the @code{invisible} text or overlay
921 property. In that case, this function returns a non-@code{nil} value
922 if that value would cause text to become invisible, based on the
923 current value of @code{buffer-invisibility-spec}.
924 @end defun
925
926 @vindex line-move-ignore-invisible
927 Ordinarily, functions that operate on text or move point do not care
928 whether the text is invisible. The user-level line motion commands
929 ignore invisible newlines if @code{line-move-ignore-invisible} is
930 non-@code{nil} (the default), but only because they are explicitly
931 programmed to do so.
932
933 However, if a command ends with point inside or at the boundary of
934 invisible text, the main editing loop relocates point to one of the
935 two ends of the invisible text. Emacs chooses the direction of
936 relocation so that it is the same as the overall movement direction of
937 the command; if in doubt, it prefers a position where an inserted char
938 would not inherit the @code{invisible} property. Additionally, if the
939 text is not replaced by an ellipsis and the command only moved within
940 the invisible text, then point is moved one extra character so as to
941 try and reflect the command's movement by a visible movement of the
942 cursor.
943
944 Thus, if the command moved point back to an invisible range (with the usual
945 stickiness), Emacs moves point back to the beginning of that range. If the
946 command moved point forward into an invisible range, Emacs moves point forward
947 to the first visible character that follows the invisible text and then forward
948 one more character.
949
950 Incremental search can make invisible overlays visible temporarily
951 and/or permanently when a match includes invisible text. To enable
952 this, the overlay should have a non-@code{nil}
953 @code{isearch-open-invisible} property. The property value should be a
954 function to be called with the overlay as an argument. This function
955 should make the overlay visible permanently; it is used when the match
956 overlaps the overlay on exit from the search.
957
958 During the search, such overlays are made temporarily visible by
959 temporarily modifying their invisible and intangible properties. If you
960 want this to be done differently for a certain overlay, give it an
961 @code{isearch-open-invisible-temporary} property which is a function.
962 The function is called with two arguments: the first is the overlay, and
963 the second is @code{nil} to make the overlay visible, or @code{t} to
964 make it invisible again.
965
966 @node Selective Display
967 @section Selective Display
968 @c @cindex selective display Duplicates selective-display
969
970 @dfn{Selective display} refers to a pair of related features for
971 hiding certain lines on the screen.
972
973 @cindex explicit selective display
974 The first variant, explicit selective display, was designed for use in a Lisp
975 program: it controls which lines are hidden by altering the text. This kind of
976 hiding is now obsolete; instead you can get the same effect with the
977 @code{invisible} property (@pxref{Invisible Text}).
978
979 In the second variant, the choice of lines to hide is made
980 automatically based on indentation. This variant is designed to be a
981 user-level feature.
982
983 The way you control explicit selective display is by replacing a
984 newline (control-j) with a carriage return (control-m). The text that
985 was formerly a line following that newline is now hidden. Strictly
986 speaking, it is temporarily no longer a line at all, since only
987 newlines can separate lines; it is now part of the previous line.
988
989 Selective display does not directly affect editing commands. For
990 example, @kbd{C-f} (@code{forward-char}) moves point unhesitatingly
991 into hidden text. However, the replacement of newline characters with
992 carriage return characters affects some editing commands. For
993 example, @code{next-line} skips hidden lines, since it searches only
994 for newlines. Modes that use selective display can also define
995 commands that take account of the newlines, or that control which
996 parts of the text are hidden.
997
998 When you write a selectively displayed buffer into a file, all the
999 control-m's are output as newlines. This means that when you next read
1000 in the file, it looks OK, with nothing hidden. The selective display
1001 effect is seen only within Emacs.
1002
1003 @defvar selective-display
1004 This buffer-local variable enables selective display. This means that
1005 lines, or portions of lines, may be made hidden.
1006
1007 @itemize @bullet
1008 @item
1009 If the value of @code{selective-display} is @code{t}, then the character
1010 control-m marks the start of hidden text; the control-m, and the rest
1011 of the line following it, are not displayed. This is explicit selective
1012 display.
1013
1014 @item
1015 If the value of @code{selective-display} is a positive integer, then
1016 lines that start with more than that many columns of indentation are not
1017 displayed.
1018 @end itemize
1019
1020 When some portion of a buffer is hidden, the vertical movement
1021 commands operate as if that portion did not exist, allowing a single
1022 @code{next-line} command to skip any number of hidden lines.
1023 However, character movement commands (such as @code{forward-char}) do
1024 not skip the hidden portion, and it is possible (if tricky) to insert
1025 or delete text in an hidden portion.
1026
1027 In the examples below, we show the @emph{display appearance} of the
1028 buffer @code{foo}, which changes with the value of
1029 @code{selective-display}. The @emph{contents} of the buffer do not
1030 change.
1031
1032 @example
1033 @group
1034 (setq selective-display nil)
1035 @result{} nil
1036
1037 ---------- Buffer: foo ----------
1038 1 on this column
1039 2on this column
1040 3n this column
1041 3n this column
1042 2on this column
1043 1 on this column
1044 ---------- Buffer: foo ----------
1045 @end group
1046
1047 @group
1048 (setq selective-display 2)
1049 @result{} 2
1050
1051 ---------- Buffer: foo ----------
1052 1 on this column
1053 2on this column
1054 2on this column
1055 1 on this column
1056 ---------- Buffer: foo ----------
1057 @end group
1058 @end example
1059 @end defvar
1060
1061 @defopt selective-display-ellipses
1062 If this buffer-local variable is non-@code{nil}, then Emacs displays
1063 @samp{@dots{}} at the end of a line that is followed by hidden text.
1064 This example is a continuation of the previous one.
1065
1066 @example
1067 @group
1068 (setq selective-display-ellipses t)
1069 @result{} t
1070
1071 ---------- Buffer: foo ----------
1072 1 on this column
1073 2on this column ...
1074 2on this column
1075 1 on this column
1076 ---------- Buffer: foo ----------
1077 @end group
1078 @end example
1079
1080 You can use a display table to substitute other text for the ellipsis
1081 (@samp{@dots{}}). @xref{Display Tables}.
1082 @end defopt
1083
1084 @node Temporary Displays
1085 @section Temporary Displays
1086
1087 Temporary displays are used by Lisp programs to put output into a
1088 buffer and then present it to the user for perusal rather than for
1089 editing. Many help commands use this feature.
1090
1091 @defmac with-output-to-temp-buffer buffer-name forms@dots{}
1092 This function executes @var{forms} while arranging to insert any output
1093 they print into the buffer named @var{buffer-name}, which is first
1094 created if necessary, and put into Help mode. Finally, the buffer is
1095 displayed in some window, but not selected. (See the similar
1096 form @code{with-temp-buffer-window} below.)
1097
1098 If the @var{forms} do not change the major mode in the output buffer,
1099 so that it is still Help mode at the end of their execution, then
1100 @code{with-output-to-temp-buffer} makes this buffer read-only at the
1101 end, and also scans it for function and variable names to make them
1102 into clickable cross-references. @xref{Docstring hyperlinks, , Tips
1103 for Documentation Strings}, in particular the item on hyperlinks in
1104 documentation strings, for more details.
1105
1106 The string @var{buffer-name} specifies the temporary buffer, which
1107 need not already exist. The argument must be a string, not a buffer.
1108 The buffer is erased initially (with no questions asked), and it is
1109 marked as unmodified after @code{with-output-to-temp-buffer} exits.
1110
1111 @code{with-output-to-temp-buffer} binds @code{standard-output} to the
1112 temporary buffer, then it evaluates the forms in @var{forms}. Output
1113 using the Lisp output functions within @var{forms} goes by default to
1114 that buffer (but screen display and messages in the echo area, although
1115 they are ``output'' in the general sense of the word, are not affected).
1116 @xref{Output Functions}.
1117
1118 Several hooks are available for customizing the behavior
1119 of this construct; they are listed below.
1120
1121 The value of the last form in @var{forms} is returned.
1122
1123 @example
1124 @group
1125 ---------- Buffer: foo ----------
1126 This is the contents of foo.
1127 ---------- Buffer: foo ----------
1128 @end group
1129
1130 @group
1131 (with-output-to-temp-buffer "foo"
1132 (print 20)
1133 (print standard-output))
1134 @result{} #<buffer foo>
1135
1136 ---------- Buffer: foo ----------
1137
1138 20
1139
1140 #<buffer foo>
1141
1142 ---------- Buffer: foo ----------
1143 @end group
1144 @end example
1145 @end defmac
1146
1147 @defopt temp-buffer-show-function
1148 If this variable is non-@code{nil}, @code{with-output-to-temp-buffer}
1149 calls it as a function to do the job of displaying a help buffer. The
1150 function gets one argument, which is the buffer it should display.
1151
1152 It is a good idea for this function to run @code{temp-buffer-show-hook}
1153 just as @code{with-output-to-temp-buffer} normally would, inside of
1154 @code{save-selected-window} and with the chosen window and buffer
1155 selected.
1156 @end defopt
1157
1158 @defvar temp-buffer-setup-hook
1159 This normal hook is run by @code{with-output-to-temp-buffer} before
1160 evaluating @var{body}. When the hook runs, the temporary buffer is
1161 current. This hook is normally set up with a function to put the
1162 buffer in Help mode.
1163 @end defvar
1164
1165 @defvar temp-buffer-show-hook
1166 This normal hook is run by @code{with-output-to-temp-buffer} after
1167 displaying the temporary buffer. When the hook runs, the temporary buffer
1168 is current, and the window it was displayed in is selected.
1169 @end defvar
1170
1171 @defmac with-temp-buffer-window buffer-or-name action quit-function forms@dots{}
1172 This macro is similar to @code{with-output-to-temp-buffer}.
1173 Like that construct, it executes @var{forms} while arranging to insert
1174 any output they print into the buffer named @var{buffer-or-name}.
1175 Finally, the buffer is displayed in some window, but not selected.
1176 Unlike @code{with-output-to-temp-buffer}, this does not switch to Help
1177 mode.
1178
1179 The argument @var{buffer-or-name} specifies the temporary buffer.
1180 It can be either a buffer, which must already exist, or a string,
1181 in which case a buffer of that name is created if necessary.
1182 The buffer is marked as unmodified and read-only when
1183 @code{with-temp-buffer-window} exits.
1184
1185 This macro does not call @code{temp-buffer-show-function}. Rather, it
1186 passes the @var{action} argument to @code{display-buffer} in order to
1187 display the buffer.
1188
1189 The value of the last form in @var{forms} is returned, unless the
1190 argument @var{quit-function} is specified. In that case,
1191 it is called with two arguments: the window showing the buffer
1192 and the result of @var{forms}. The final return value is then
1193 whatever @var{quit-function} returns.
1194
1195 @vindex temp-buffer-window-setup-hook
1196 @vindex temp-buffer-window-show-hook
1197 This macro uses the normal hooks @code{temp-buffer-window-setup-hook}
1198 and @code{temp-buffer-window-show-hook} in place of the analogous hooks
1199 run by @code{with-output-to-temp-buffer}.
1200 @end defmac
1201
1202 @defun momentary-string-display string position &optional char message
1203 This function momentarily displays @var{string} in the current buffer at
1204 @var{position}. It has no effect on the undo list or on the buffer's
1205 modification status.
1206
1207 The momentary display remains until the next input event. If the next
1208 input event is @var{char}, @code{momentary-string-display} ignores it
1209 and returns. Otherwise, that event remains buffered for subsequent use
1210 as input. Thus, typing @var{char} will simply remove the string from
1211 the display, while typing (say) @kbd{C-f} will remove the string from
1212 the display and later (presumably) move point forward. The argument
1213 @var{char} is a space by default.
1214
1215 The return value of @code{momentary-string-display} is not meaningful.
1216
1217 If the string @var{string} does not contain control characters, you can
1218 do the same job in a more general way by creating (and then subsequently
1219 deleting) an overlay with a @code{before-string} property.
1220 @xref{Overlay Properties}.
1221
1222 If @var{message} is non-@code{nil}, it is displayed in the echo area
1223 while @var{string} is displayed in the buffer. If it is @code{nil}, a
1224 default message says to type @var{char} to continue.
1225
1226 In this example, point is initially located at the beginning of the
1227 second line:
1228
1229 @example
1230 @group
1231 ---------- Buffer: foo ----------
1232 This is the contents of foo.
1233 @point{}Second line.
1234 ---------- Buffer: foo ----------
1235 @end group
1236
1237 @group
1238 (momentary-string-display
1239 "**** Important Message! ****"
1240 (point) ?\r
1241 "Type RET when done reading")
1242 @result{} t
1243 @end group
1244
1245 @group
1246 ---------- Buffer: foo ----------
1247 This is the contents of foo.
1248 **** Important Message! ****Second line.
1249 ---------- Buffer: foo ----------
1250
1251 ---------- Echo Area ----------
1252 Type RET when done reading
1253 ---------- Echo Area ----------
1254 @end group
1255 @end example
1256 @end defun
1257
1258 @node Overlays
1259 @section Overlays
1260 @cindex overlays
1261 @c FIXME: mention intervals in this section?
1262
1263 You can use @dfn{overlays} to alter the appearance of a buffer's text on
1264 the screen, for the sake of presentation features. An overlay is an
1265 object that belongs to a particular buffer, and has a specified
1266 beginning and end. It also has properties that you can examine and set;
1267 these affect the display of the text within the overlay.
1268
1269 @cindex scalability of overlays
1270 The visual effect of an overlay is the same as of the corresponding
1271 text property (@pxref{Text Properties}). However, due to a different
1272 implementation, overlays generally don't scale well (many operations
1273 take a time that is proportional to the number of overlays in the
1274 buffer). If you need to affect the visual appearance of many portions
1275 in the buffer, we recommend using text properties.
1276
1277 An overlay uses markers to record its beginning and end; thus,
1278 editing the text of the buffer adjusts the beginning and end of each
1279 overlay so that it stays with the text. When you create the overlay,
1280 you can specify whether text inserted at the beginning should be
1281 inside the overlay or outside, and likewise for the end of the overlay.
1282
1283 @menu
1284 * Managing Overlays:: Creating and moving overlays.
1285 * Overlay Properties:: How to read and set properties.
1286 What properties do to the screen display.
1287 * Finding Overlays:: Searching for overlays.
1288 @end menu
1289
1290 @node Managing Overlays
1291 @subsection Managing Overlays
1292
1293 This section describes the functions to create, delete and move
1294 overlays, and to examine their contents. Overlay changes are not
1295 recorded in the buffer's undo list, since the overlays are not
1296 part of the buffer's contents.
1297
1298 @defun overlayp object
1299 This function returns @code{t} if @var{object} is an overlay.
1300 @end defun
1301
1302 @defun make-overlay start end &optional buffer front-advance rear-advance
1303 This function creates and returns an overlay that belongs to
1304 @var{buffer} and ranges from @var{start} to @var{end}. Both @var{start}
1305 and @var{end} must specify buffer positions; they may be integers or
1306 markers. If @var{buffer} is omitted, the overlay is created in the
1307 current buffer.
1308
1309 The arguments @var{front-advance} and @var{rear-advance} specify the
1310 marker insertion type for the start of the overlay and for the end of
1311 the overlay, respectively. @xref{Marker Insertion Types}. If they
1312 are both @code{nil}, the default, then the overlay extends to include
1313 any text inserted at the beginning, but not text inserted at the end.
1314 If @var{front-advance} is non-@code{nil}, text inserted at the
1315 beginning of the overlay is excluded from the overlay. If
1316 @var{rear-advance} is non-@code{nil}, text inserted at the end of the
1317 overlay is included in the overlay.
1318 @end defun
1319
1320 @defun overlay-start overlay
1321 This function returns the position at which @var{overlay} starts,
1322 as an integer.
1323 @end defun
1324
1325 @defun overlay-end overlay
1326 This function returns the position at which @var{overlay} ends,
1327 as an integer.
1328 @end defun
1329
1330 @defun overlay-buffer overlay
1331 This function returns the buffer that @var{overlay} belongs to. It
1332 returns @code{nil} if @var{overlay} has been deleted.
1333 @end defun
1334
1335 @defun delete-overlay overlay
1336 This function deletes @var{overlay}. The overlay continues to exist as
1337 a Lisp object, and its property list is unchanged, but it ceases to be
1338 attached to the buffer it belonged to, and ceases to have any effect on
1339 display.
1340
1341 A deleted overlay is not permanently disconnected. You can give it a
1342 position in a buffer again by calling @code{move-overlay}.
1343 @end defun
1344
1345 @defun move-overlay overlay start end &optional buffer
1346 This function moves @var{overlay} to @var{buffer}, and places its bounds
1347 at @var{start} and @var{end}. Both arguments @var{start} and @var{end}
1348 must specify buffer positions; they may be integers or markers.
1349
1350 If @var{buffer} is omitted, @var{overlay} stays in the same buffer it
1351 was already associated with; if @var{overlay} was deleted, it goes into
1352 the current buffer.
1353
1354 The return value is @var{overlay}.
1355
1356 This is the only valid way to change the endpoints of an overlay. Do
1357 not try modifying the markers in the overlay by hand, as that fails to
1358 update other vital data structures and can cause some overlays to be
1359 ``lost''.
1360 @end defun
1361
1362 @defun remove-overlays &optional start end name value
1363 This function removes all the overlays between @var{start} and
1364 @var{end} whose property @var{name} has the value @var{value}. It can
1365 move the endpoints of the overlays in the region, or split them.
1366
1367 If @var{name} is omitted or @code{nil}, it means to delete all overlays in
1368 the specified region. If @var{start} and/or @var{end} are omitted or
1369 @code{nil}, that means the beginning and end of the buffer respectively.
1370 Therefore, @code{(remove-overlays)} removes all the overlays in the
1371 current buffer.
1372 @end defun
1373
1374 @defun copy-overlay overlay
1375 This function returns a copy of @var{overlay}. The copy has the same
1376 endpoints and properties as @var{overlay}. However, the marker
1377 insertion type for the start of the overlay and for the end of the
1378 overlay are set to their default values (@pxref{Marker Insertion
1379 Types}).
1380 @end defun
1381
1382 Here are some examples:
1383
1384 @example
1385 ;; @r{Create an overlay.}
1386 (setq foo (make-overlay 1 10))
1387 @result{} #<overlay from 1 to 10 in display.texi>
1388 (overlay-start foo)
1389 @result{} 1
1390 (overlay-end foo)
1391 @result{} 10
1392 (overlay-buffer foo)
1393 @result{} #<buffer display.texi>
1394 ;; @r{Give it a property we can check later.}
1395 (overlay-put foo 'happy t)
1396 @result{} t
1397 ;; @r{Verify the property is present.}
1398 (overlay-get foo 'happy)
1399 @result{} t
1400 ;; @r{Move the overlay.}
1401 (move-overlay foo 5 20)
1402 @result{} #<overlay from 5 to 20 in display.texi>
1403 (overlay-start foo)
1404 @result{} 5
1405 (overlay-end foo)
1406 @result{} 20
1407 ;; @r{Delete the overlay.}
1408 (delete-overlay foo)
1409 @result{} nil
1410 ;; @r{Verify it is deleted.}
1411 foo
1412 @result{} #<overlay in no buffer>
1413 ;; @r{A deleted overlay has no position.}
1414 (overlay-start foo)
1415 @result{} nil
1416 (overlay-end foo)
1417 @result{} nil
1418 (overlay-buffer foo)
1419 @result{} nil
1420 ;; @r{Undelete the overlay.}
1421 (move-overlay foo 1 20)
1422 @result{} #<overlay from 1 to 20 in display.texi>
1423 ;; @r{Verify the results.}
1424 (overlay-start foo)
1425 @result{} 1
1426 (overlay-end foo)
1427 @result{} 20
1428 (overlay-buffer foo)
1429 @result{} #<buffer display.texi>
1430 ;; @r{Moving and deleting the overlay does not change its properties.}
1431 (overlay-get foo 'happy)
1432 @result{} t
1433 @end example
1434
1435 Emacs stores the overlays of each buffer in two lists, divided
1436 around an arbitrary ``center position''. One list extends backwards
1437 through the buffer from that center position, and the other extends
1438 forwards from that center position. The center position can be anywhere
1439 in the buffer.
1440
1441 @defun overlay-recenter pos
1442 This function recenters the overlays of the current buffer around
1443 position @var{pos}. That makes overlay lookup faster for positions
1444 near @var{pos}, but slower for positions far away from @var{pos}.
1445 @end defun
1446
1447 A loop that scans the buffer forwards, creating overlays, can run
1448 faster if you do @code{(overlay-recenter (point-max))} first.
1449
1450 @node Overlay Properties
1451 @subsection Overlay Properties
1452
1453 Overlay properties are like text properties in that the properties that
1454 alter how a character is displayed can come from either source. But in
1455 most respects they are different. @xref{Text Properties}, for comparison.
1456
1457 Text properties are considered a part of the text; overlays and
1458 their properties are specifically considered not to be part of the
1459 text. Thus, copying text between various buffers and strings
1460 preserves text properties, but does not try to preserve overlays.
1461 Changing a buffer's text properties marks the buffer as modified,
1462 while moving an overlay or changing its properties does not. Unlike
1463 text property changes, overlay property changes are not recorded in
1464 the buffer's undo list.
1465
1466 Since more than one overlay can specify a property value for the
1467 same character, Emacs lets you specify a priority value of each
1468 overlay. You should not make assumptions about which overlay will
1469 prevail when there is a conflict and they have the same priority.
1470
1471 These functions read and set the properties of an overlay:
1472
1473 @defun overlay-get overlay prop
1474 This function returns the value of property @var{prop} recorded in
1475 @var{overlay}, if any. If @var{overlay} does not record any value for
1476 that property, but it does have a @code{category} property which is a
1477 symbol, that symbol's @var{prop} property is used. Otherwise, the value
1478 is @code{nil}.
1479 @end defun
1480
1481 @defun overlay-put overlay prop value
1482 This function sets the value of property @var{prop} recorded in
1483 @var{overlay} to @var{value}. It returns @var{value}.
1484 @end defun
1485
1486 @defun overlay-properties overlay
1487 This returns a copy of the property list of @var{overlay}.
1488 @end defun
1489
1490 See also the function @code{get-char-property} which checks both
1491 overlay properties and text properties for a given character.
1492 @xref{Examining Properties}.
1493
1494 Many overlay properties have special meanings; here is a table
1495 of them:
1496
1497 @table @code
1498 @item priority
1499 @kindex priority @r{(overlay property)}
1500 This property's value (which should be a non-negative integer number)
1501 determines the priority of the overlay. No priority, or @code{nil},
1502 means zero.
1503
1504 The priority matters when two or more overlays cover the same
1505 character and both specify the same property; the one whose
1506 @code{priority} value is larger overrides the other. For the
1507 @code{face} property, the higher priority overlay's value does not
1508 completely override the other value; instead, its face attributes
1509 override the face attributes of the lower priority @code{face}
1510 property.
1511
1512 Currently, all overlays take priority over text properties. Please
1513 avoid using negative priority values, as we have not yet decided just
1514 what they should mean.
1515
1516 @item window
1517 @kindex window @r{(overlay property)}
1518 If the @code{window} property is non-@code{nil}, then the overlay
1519 applies only on that window.
1520
1521 @item category
1522 @kindex category @r{(overlay property)}
1523 If an overlay has a @code{category} property, we call it the
1524 @dfn{category} of the overlay. It should be a symbol. The properties
1525 of the symbol serve as defaults for the properties of the overlay.
1526
1527 @item face
1528 @kindex face @r{(overlay property)}
1529 This property controls the appearance of the text (@pxref{Faces}).
1530 The value of the property can be the following:
1531
1532 @itemize @bullet
1533 @item
1534 A face name (a symbol or string).
1535
1536 @item
1537 An anonymous face: a property list of the form @code{(@var{keyword}
1538 @var{value} @dots{})}, where each @var{keyword} is a face attribute
1539 name and @var{value} is a value for that attribute.
1540
1541 @item
1542 A list of faces. Each list element should be either a face name or an
1543 anonymous face. This specifies a face which is an aggregate of the
1544 attributes of each of the listed faces. Faces occurring earlier in
1545 the list have higher priority.
1546
1547 @item
1548 A cons cell of the form @code{(foreground-color . @var{color-name})}
1549 or @code{(background-color . @var{color-name})}. This specifies the
1550 foreground or background color, similar to @code{(:foreground
1551 @var{color-name})} or @code{(:background @var{color-name})}. This
1552 form is supported for backward compatibility only, and should be
1553 avoided.
1554 @end itemize
1555
1556 @item mouse-face
1557 @kindex mouse-face @r{(overlay property)}
1558 This property is used instead of @code{face} when the mouse is within
1559 the range of the overlay. However, Emacs ignores all face attributes
1560 from this property that alter the text size (e.g., @code{:height},
1561 @code{:weight}, and @code{:slant}). Those attributes are always the
1562 same as in the unhighlighted text.
1563
1564 @item display
1565 @kindex display @r{(overlay property)}
1566 This property activates various features that change the
1567 way text is displayed. For example, it can make text appear taller
1568 or shorter, higher or lower, wider or narrower, or replaced with an image.
1569 @xref{Display Property}.
1570
1571 @item help-echo
1572 @kindex help-echo @r{(overlay property)}
1573 If an overlay has a @code{help-echo} property, then when you move the
1574 mouse onto the text in the overlay, Emacs displays a help string in the
1575 echo area, or in the tooltip window. For details see @ref{Text
1576 help-echo}.
1577
1578 @item field
1579 @kindex field @r{(overlay property)}
1580 @c Copied from Special Properties.
1581 Consecutive characters with the same @code{field} property constitute a
1582 @emph{field}. Some motion functions including @code{forward-word} and
1583 @code{beginning-of-line} stop moving at a field boundary.
1584 @xref{Fields}.
1585
1586 @item modification-hooks
1587 @kindex modification-hooks @r{(overlay property)}
1588 This property's value is a list of functions to be called if any
1589 character within the overlay is changed or if text is inserted strictly
1590 within the overlay.
1591
1592 The hook functions are called both before and after each change.
1593 If the functions save the information they receive, and compare notes
1594 between calls, they can determine exactly what change has been made
1595 in the buffer text.
1596
1597 When called before a change, each function receives four arguments: the
1598 overlay, @code{nil}, and the beginning and end of the text range to be
1599 modified.
1600
1601 When called after a change, each function receives five arguments: the
1602 overlay, @code{t}, the beginning and end of the text range just
1603 modified, and the length of the pre-change text replaced by that range.
1604 (For an insertion, the pre-change length is zero; for a deletion, that
1605 length is the number of characters deleted, and the post-change
1606 beginning and end are equal.)
1607
1608 If these functions modify the buffer, they should bind
1609 @code{inhibit-modification-hooks} to @code{t} around doing so, to
1610 avoid confusing the internal mechanism that calls these hooks.
1611
1612 Text properties also support the @code{modification-hooks} property,
1613 but the details are somewhat different (@pxref{Special Properties}).
1614
1615 @item insert-in-front-hooks
1616 @kindex insert-in-front-hooks @r{(overlay property)}
1617 This property's value is a list of functions to be called before and
1618 after inserting text right at the beginning of the overlay. The calling
1619 conventions are the same as for the @code{modification-hooks} functions.
1620
1621 @item insert-behind-hooks
1622 @kindex insert-behind-hooks @r{(overlay property)}
1623 This property's value is a list of functions to be called before and
1624 after inserting text right at the end of the overlay. The calling
1625 conventions are the same as for the @code{modification-hooks} functions.
1626
1627 @item invisible
1628 @kindex invisible @r{(overlay property)}
1629 The @code{invisible} property can make the text in the overlay
1630 invisible, which means that it does not appear on the screen.
1631 @xref{Invisible Text}, for details.
1632
1633 @item intangible
1634 @kindex intangible @r{(overlay property)}
1635 The @code{intangible} property on an overlay works just like the
1636 @code{intangible} text property. @xref{Special Properties}, for details.
1637
1638 @item isearch-open-invisible
1639 This property tells incremental search how to make an invisible overlay
1640 visible, permanently, if the final match overlaps it. @xref{Invisible
1641 Text}.
1642
1643 @item isearch-open-invisible-temporary
1644 This property tells incremental search how to make an invisible overlay
1645 visible, temporarily, during the search. @xref{Invisible Text}.
1646
1647 @item before-string
1648 @kindex before-string @r{(overlay property)}
1649 This property's value is a string to add to the display at the beginning
1650 of the overlay. The string does not appear in the buffer in any
1651 sense---only on the screen.
1652
1653 @item after-string
1654 @kindex after-string @r{(overlay property)}
1655 This property's value is a string to add to the display at the end of
1656 the overlay. The string does not appear in the buffer in any
1657 sense---only on the screen.
1658
1659 @item line-prefix
1660 This property specifies a display spec to prepend to each
1661 non-continuation line at display-time. @xref{Truncation}.
1662
1663 @item wrap-prefix
1664 This property specifies a display spec to prepend to each continuation
1665 line at display-time. @xref{Truncation}.
1666
1667 @item evaporate
1668 @kindex evaporate @r{(overlay property)}
1669 If this property is non-@code{nil}, the overlay is deleted automatically
1670 if it becomes empty (i.e., if its length becomes zero). If you give
1671 an empty overlay a non-@code{nil} @code{evaporate} property, that deletes
1672 it immediately.
1673
1674 @item keymap
1675 @cindex keymap of character (and overlays)
1676 @kindex keymap @r{(overlay property)}
1677 If this property is non-@code{nil}, it specifies a keymap for a portion of the
1678 text. This keymap is used when the character after point is within the
1679 overlay, and takes precedence over most other keymaps. @xref{Active Keymaps}.
1680
1681 @item local-map
1682 @kindex local-map @r{(overlay property)}
1683 The @code{local-map} property is similar to @code{keymap} but replaces the
1684 buffer's local map rather than augmenting existing keymaps. This also means it
1685 has lower precedence than minor mode keymaps.
1686 @end table
1687
1688 The @code{keymap} and @code{local-map} properties do not affect a
1689 string displayed by the @code{before-string}, @code{after-string}, or
1690 @code{display} properties. This is only relevant for mouse clicks and
1691 other mouse events that fall on the string, since point is never on
1692 the string. To bind special mouse events for the string, assign it a
1693 @code{keymap} or @code{local-map} text property. @xref{Special
1694 Properties}.
1695
1696 @node Finding Overlays
1697 @subsection Searching for Overlays
1698
1699 @defun overlays-at pos
1700 This function returns a list of all the overlays that cover the
1701 character at position @var{pos} in the current buffer. The list is in
1702 no particular order. An overlay contains position @var{pos} if it
1703 begins at or before @var{pos}, and ends after @var{pos}.
1704
1705 To illustrate usage, here is a Lisp function that returns a list of the
1706 overlays that specify property @var{prop} for the character at point:
1707
1708 @smallexample
1709 (defun find-overlays-specifying (prop)
1710 (let ((overlays (overlays-at (point)))
1711 found)
1712 (while overlays
1713 (let ((overlay (car overlays)))
1714 (if (overlay-get overlay prop)
1715 (setq found (cons overlay found))))
1716 (setq overlays (cdr overlays)))
1717 found))
1718 @end smallexample
1719 @end defun
1720
1721 @defun overlays-in beg end
1722 This function returns a list of the overlays that overlap the region
1723 @var{beg} through @var{end}. ``Overlap'' means that at least one
1724 character is contained within the overlay and also contained within the
1725 specified region; however, empty overlays are included in the result if
1726 they are located at @var{beg}, strictly between @var{beg} and @var{end},
1727 or at @var{end} when @var{end} denotes the position at the end of the
1728 buffer.
1729 @end defun
1730
1731 @defun next-overlay-change pos
1732 This function returns the buffer position of the next beginning or end
1733 of an overlay, after @var{pos}. If there is none, it returns
1734 @code{(point-max)}.
1735 @end defun
1736
1737 @defun previous-overlay-change pos
1738 This function returns the buffer position of the previous beginning or
1739 end of an overlay, before @var{pos}. If there is none, it returns
1740 @code{(point-min)}.
1741 @end defun
1742
1743 As an example, here's a simplified (and inefficient) version of the
1744 primitive function @code{next-single-char-property-change}
1745 (@pxref{Property Search}). It searches forward from position
1746 @var{pos} for the next position where the value of a given property
1747 @code{prop}, as obtained from either overlays or text properties,
1748 changes.
1749
1750 @smallexample
1751 (defun next-single-char-property-change (position prop)
1752 (save-excursion
1753 (goto-char position)
1754 (let ((propval (get-char-property (point) prop)))
1755 (while (and (not (eobp))
1756 (eq (get-char-property (point) prop) propval))
1757 (goto-char (min (next-overlay-change (point))
1758 (next-single-property-change (point) prop)))))
1759 (point)))
1760 @end smallexample
1761
1762 @node Width
1763 @section Width
1764
1765 Since not all characters have the same width, these functions let you
1766 check the width of a character. @xref{Primitive Indent}, and
1767 @ref{Screen Lines}, for related functions.
1768
1769 @defun char-width char
1770 This function returns the width in columns of the character
1771 @var{char}, if it were displayed in the current buffer (i.e., taking
1772 into account the buffer's display table, if any; @pxref{Display
1773 Tables}). The width of a tab character is usually @code{tab-width}
1774 (@pxref{Usual Display}).
1775 @end defun
1776
1777 @defun string-width string
1778 This function returns the width in columns of the string @var{string},
1779 if it were displayed in the current buffer and the selected window.
1780 @end defun
1781
1782 @defun truncate-string-to-width string width &optional start-column padding ellipsis
1783 This function returns the part of @var{string} that fits within
1784 @var{width} columns, as a new string.
1785
1786 If @var{string} does not reach @var{width}, then the result ends where
1787 @var{string} ends. If one multi-column character in @var{string}
1788 extends across the column @var{width}, that character is not included in
1789 the result. Thus, the result can fall short of @var{width} but cannot
1790 go beyond it.
1791
1792 The optional argument @var{start-column} specifies the starting column.
1793 If this is non-@code{nil}, then the first @var{start-column} columns of
1794 the string are omitted from the value. If one multi-column character in
1795 @var{string} extends across the column @var{start-column}, that
1796 character is not included.
1797
1798 The optional argument @var{padding}, if non-@code{nil}, is a padding
1799 character added at the beginning and end of the result string, to extend
1800 it to exactly @var{width} columns. The padding character is used at the
1801 end of the result if it falls short of @var{width}. It is also used at
1802 the beginning of the result if one multi-column character in
1803 @var{string} extends across the column @var{start-column}.
1804
1805 If @var{ellipsis} is non-@code{nil}, it should be a string which will
1806 replace the end of @var{string} (including any padding) if it extends
1807 beyond @var{width}, unless the display width of @var{string} is equal
1808 to or less than the display width of @var{ellipsis}. If
1809 @var{ellipsis} is non-@code{nil} and not a string, it stands for
1810 @code{"..."}.
1811
1812 @example
1813 (truncate-string-to-width "\tab\t" 12 4)
1814 @result{} "ab"
1815 (truncate-string-to-width "\tab\t" 12 4 ?\s)
1816 @result{} " ab "
1817 @end example
1818 @end defun
1819
1820 @node Line Height
1821 @section Line Height
1822 @cindex line height
1823 @cindex height of a line
1824
1825 The total height of each display line consists of the height of the
1826 contents of the line, plus optional additional vertical line spacing
1827 above or below the display line.
1828
1829 The height of the line contents is the maximum height of any
1830 character or image on that display line, including the final newline
1831 if there is one. (A display line that is continued doesn't include a
1832 final newline.) That is the default line height, if you do nothing to
1833 specify a greater height. (In the most common case, this equals the
1834 height of the default frame font.)
1835
1836 There are several ways to explicitly specify a larger line height,
1837 either by specifying an absolute height for the display line, or by
1838 specifying vertical space. However, no matter what you specify, the
1839 actual line height can never be less than the default.
1840
1841 @kindex line-height @r{(text property)}
1842 A newline can have a @code{line-height} text or overlay property
1843 that controls the total height of the display line ending in that
1844 newline.
1845
1846 If the property value is @code{t}, the newline character has no
1847 effect on the displayed height of the line---the visible contents
1848 alone determine the height. This is useful for tiling small images
1849 (or image slices) without adding blank areas between the images.
1850
1851 If the property value is a list of the form @code{(@var{height}
1852 @var{total})}, that adds extra space @emph{below} the display line.
1853 First Emacs uses @var{height} as a height spec to control extra space
1854 @emph{above} the line; then it adds enough space @emph{below} the line
1855 to bring the total line height up to @var{total}. In this case, the
1856 other ways to specify the line spacing are ignored.
1857
1858 @cindex height spec
1859 Any other kind of property value is a height spec, which translates
1860 into a number---the specified line height. There are several ways to
1861 write a height spec; here's how each of them translates into a number:
1862
1863 @table @code
1864 @item @var{integer}
1865 If the height spec is a positive integer, the height value is that integer.
1866 @item @var{float}
1867 If the height spec is a float, @var{float}, the numeric height value
1868 is @var{float} times the frame's default line height.
1869 @item (@var{face} . @var{ratio})
1870 If the height spec is a cons of the format shown, the numeric height
1871 is @var{ratio} times the height of face @var{face}. @var{ratio} can
1872 be any type of number, or @code{nil} which means a ratio of 1.
1873 If @var{face} is @code{t}, it refers to the current face.
1874 @item (nil . @var{ratio})
1875 If the height spec is a cons of the format shown, the numeric height
1876 is @var{ratio} times the height of the contents of the line.
1877 @end table
1878
1879 Thus, any valid height spec determines the height in pixels, one way
1880 or another. If the line contents' height is less than that, Emacs
1881 adds extra vertical space above the line to achieve the specified
1882 total height.
1883
1884 If you don't specify the @code{line-height} property, the line's
1885 height consists of the contents' height plus the line spacing.
1886 There are several ways to specify the line spacing for different
1887 parts of Emacs text.
1888
1889 On graphical terminals, you can specify the line spacing for all
1890 lines in a frame, using the @code{line-spacing} frame parameter
1891 (@pxref{Layout Parameters}). However, if the default value of
1892 @code{line-spacing} is non-@code{nil}, it overrides the
1893 frame's @code{line-spacing} parameter. An integer value specifies the
1894 number of pixels put below lines. A floating point number specifies
1895 the spacing relative to the frame's default line height.
1896
1897 @vindex line-spacing
1898 You can specify the line spacing for all lines in a buffer via the
1899 buffer-local @code{line-spacing} variable. An integer value specifies
1900 the number of pixels put below lines. A floating point number
1901 specifies the spacing relative to the default frame line height. This
1902 overrides line spacings specified for the frame.
1903
1904 @kindex line-spacing @r{(text property)}
1905 Finally, a newline can have a @code{line-spacing} text or overlay
1906 property that overrides the default frame line spacing and the buffer
1907 local @code{line-spacing} variable, for the display line ending in
1908 that newline.
1909
1910 One way or another, these mechanisms specify a Lisp value for the
1911 spacing of each line. The value is a height spec, and it translates
1912 into a Lisp value as described above. However, in this case the
1913 numeric height value specifies the line spacing, rather than the line
1914 height.
1915
1916 On text terminals, the line spacing cannot be altered.
1917
1918 @node Faces
1919 @section Faces
1920 @cindex faces
1921
1922 A @dfn{face} is a collection of graphical attributes for displaying
1923 text: font, foreground color, background color, optional underlining,
1924 etc. Faces control how Emacs displays text in buffers, as well as
1925 other parts of the frame such as the mode line.
1926
1927 @cindex anonymous face
1928 One way to represent a face is as a property list of attributes,
1929 like @code{(:foreground "red" :weight bold)}. Such a list is called
1930 an @dfn{anonymous face}. For example, you can assign an anonymous
1931 face as the value of the @code{face} text property, and Emacs will
1932 display the underlying text with the specified attributes.
1933 @xref{Special Properties}.
1934
1935 @cindex face name
1936 More commonly, a face is referred to via a @dfn{face name}: a Lisp
1937 symbol associated with a set of face attributes@footnote{For backward
1938 compatibility, you can also use a string to specify a face name; that
1939 is equivalent to a Lisp symbol with the same name.}. Named faces are
1940 defined using the @code{defface} macro (@pxref{Defining Faces}).
1941 Emacs comes with several standard named faces (@pxref{Basic Faces}).
1942
1943 Many parts of Emacs required named faces, and do not accept
1944 anonymous faces. These include the functions documented in
1945 @ref{Attribute Functions}, and the variable @code{font-lock-keywords}
1946 (@pxref{Search-based Fontification}). Unless otherwise stated, we
1947 will use the term @dfn{face} to refer only to named faces.
1948
1949 @defun facep object
1950 This function returns a non-@code{nil} value if @var{object} is a
1951 named face: a Lisp symbol or string which serves as a face name.
1952 Otherwise, it returns @code{nil}.
1953 @end defun
1954
1955 @menu
1956 * Face Attributes:: What is in a face?
1957 * Defining Faces:: How to define a face.
1958 * Attribute Functions:: Functions to examine and set face attributes.
1959 * Displaying Faces:: How Emacs combines the faces specified for a character.
1960 * Face Remapping:: Remapping faces to alternative definitions.
1961 * Face Functions:: How to define and examine faces.
1962 * Auto Faces:: Hook for automatic face assignment.
1963 * Basic Faces:: Faces that are defined by default.
1964 * Font Selection:: Finding the best available font for a face.
1965 * Font Lookup:: Looking up the names of available fonts
1966 and information about them.
1967 * Fontsets:: A fontset is a collection of fonts
1968 that handle a range of character sets.
1969 * Low-Level Font:: Lisp representation for character display fonts.
1970 @end menu
1971
1972 @node Face Attributes
1973 @subsection Face Attributes
1974 @cindex face attributes
1975
1976 @dfn{Face attributes} determine the visual appearance of a face.
1977 The following table lists all the face attributes, their possible
1978 values, and their effects.
1979
1980 Apart from the values given below, each face attribute can have the
1981 value @code{unspecified}. This special value means that the face
1982 doesn't specify that attribute directly. An @code{unspecified}
1983 attribute tells Emacs to refer instead to a parent face (see the
1984 description @code{:inherit} attribute below); or, failing that, to an
1985 underlying face (@pxref{Displaying Faces}). The @code{default} face
1986 must specify all attributes.
1987
1988 Some of these attributes are meaningful only on certain kinds of
1989 displays. If your display cannot handle a certain attribute, the
1990 attribute is ignored.
1991
1992 @table @code
1993 @item :family
1994 Font family or fontset (a string). @xref{Fonts,,, emacs, The GNU
1995 Emacs Manual}, for more information about font families. The function
1996 @code{font-family-list} (see below) returns a list of available family
1997 names. @xref{Fontsets}, for information about fontsets.
1998
1999 @item :foundry
2000 The name of the @dfn{font foundry} for the font family specified by
2001 the @code{:family} attribute (a string). @xref{Fonts,,, emacs, The
2002 GNU Emacs Manual}.
2003
2004 @item :width
2005 Relative character width. This should be one of the symbols
2006 @code{ultra-condensed}, @code{extra-condensed}, @code{condensed},
2007 @code{semi-condensed}, @code{normal}, @code{semi-expanded},
2008 @code{expanded}, @code{extra-expanded}, or @code{ultra-expanded}.
2009
2010 @item :height
2011 The height of the font. In the simplest case, this is an integer in
2012 units of 1/10 point.
2013
2014 The value can also be a floating point number or a function, which
2015 specifies the height relative to an @dfn{underlying face}
2016 (@pxref{Displaying Faces}). If the value is a floating point number,
2017 that specifies the amount by which to scale the height of the
2018 underlying face. If the value is a function, that function is called
2019 with one argument, the height of the underlying face, and returns the
2020 height of the new face. If the function is passed an integer
2021 argument, it must return an integer.
2022
2023 The height of the default face must be specified using an integer;
2024 floating point and function values are not allowed.
2025
2026 @item :weight
2027 Font weight---one of the symbols (from densest to faintest)
2028 @code{ultra-bold}, @code{extra-bold}, @code{bold}, @code{semi-bold},
2029 @code{normal}, @code{semi-light}, @code{light}, @code{extra-light}, or
2030 @code{ultra-light}. On text terminals which support
2031 variable-brightness text, any weight greater than normal is displayed
2032 as extra bright, and any weight less than normal is displayed as
2033 half-bright.
2034
2035 @cindex italic text
2036 @item :slant
2037 Font slant---one of the symbols @code{italic}, @code{oblique},
2038 @code{normal}, @code{reverse-italic}, or @code{reverse-oblique}. On
2039 text terminals that support variable-brightness text, slanted text is
2040 displayed as half-bright.
2041
2042 @item :foreground
2043 Foreground color, a string. The value can be a system-defined color
2044 name, or a hexadecimal color specification. @xref{Color Names}. On
2045 black-and-white displays, certain shades of gray are implemented by
2046 stipple patterns.
2047
2048 @item :distant-foreground
2049 Alternative foreground color, a string. This is like @code{:foreground}
2050 but the color is only used as a foreground when the background color is
2051 near to the foreground that would have been used. This is useful for
2052 example when marking text (i.e. the region face). If the text has a foreground
2053 that is visible with the region face, that foreground is used.
2054 If the foreground is near the region face background,
2055 @code{:distant-foreground} is used instead so the text is readable.
2056
2057 @item :background
2058 Background color, a string. The value can be a system-defined color
2059 name, or a hexadecimal color specification. @xref{Color Names}.
2060
2061 @cindex underlined text
2062 @item :underline
2063 Whether or not characters should be underlined, and in what
2064 way. The possible values of the @code{:underline} attribute are:
2065
2066 @table @asis
2067 @item @code{nil}
2068 Don't underline.
2069
2070 @item @code{t}
2071 Underline with the foreground color of the face.
2072
2073 @item @var{color}
2074 Underline in color @var{color}, a string specifying a color.
2075
2076 @item @code{(:color @var{color} :style @var{style})}
2077 @var{color} is either a string, or the symbol @code{foreground-color},
2078 meaning the foreground color of the face. Omitting the attribute
2079 @code{:color} means to use the foreground color of the face.
2080 @var{style} should be a symbol @code{line} or @code{wave}, meaning to
2081 use a straight or wavy line. Omitting the attribute @code{:style}
2082 means to use a straight line.
2083 @end table
2084
2085 @cindex overlined text
2086 @item :overline
2087 Whether or not characters should be overlined, and in what color.
2088 If the value is @code{t}, overlining uses the foreground color of the
2089 face. If the value is a string, overlining uses that color. The
2090 value @code{nil} means do not overline.
2091
2092 @cindex strike-through text
2093 @item :strike-through
2094 Whether or not characters should be strike-through, and in what
2095 color. The value is used like that of @code{:overline}.
2096
2097 @cindex 2D box
2098 @cindex 3D box
2099 @item :box
2100 Whether or not a box should be drawn around characters, its color, the
2101 width of the box lines, and 3D appearance. Here are the possible
2102 values of the @code{:box} attribute, and what they mean:
2103
2104 @table @asis
2105 @item @code{nil}
2106 Don't draw a box.
2107
2108 @item @code{t}
2109 Draw a box with lines of width 1, in the foreground color.
2110
2111 @item @var{color}
2112 Draw a box with lines of width 1, in color @var{color}.
2113
2114 @item @code{(:line-width @var{width} :color @var{color} :style @var{style})}
2115 This way you can explicitly specify all aspects of the box. The value
2116 @var{width} specifies the width of the lines to draw; it defaults to
2117 1. A negative width @var{-n} means to draw a line of width @var{n}
2118 that occupies the space of the underlying text, thus avoiding any
2119 increase in the character height or width.
2120
2121 The value @var{color} specifies the color to draw with. The default is
2122 the foreground color of the face for simple boxes, and the background
2123 color of the face for 3D boxes.
2124
2125 The value @var{style} specifies whether to draw a 3D box. If it is
2126 @code{released-button}, the box looks like a 3D button that is not being
2127 pressed. If it is @code{pressed-button}, the box looks like a 3D button
2128 that is being pressed. If it is @code{nil} or omitted, a plain 2D box
2129 is used.
2130 @end table
2131
2132 @item :inverse-video
2133 Whether or not characters should be displayed in inverse video. The
2134 value should be @code{t} (yes) or @code{nil} (no).
2135
2136 @item :stipple
2137 The background stipple, a bitmap.
2138
2139 The value can be a string; that should be the name of a file containing
2140 external-format X bitmap data. The file is found in the directories
2141 listed in the variable @code{x-bitmap-file-path}.
2142
2143 Alternatively, the value can specify the bitmap directly, with a list
2144 of the form @code{(@var{width} @var{height} @var{data})}. Here,
2145 @var{width} and @var{height} specify the size in pixels, and
2146 @var{data} is a string containing the raw bits of the bitmap, row by
2147 row. Each row occupies @math{(@var{width} + 7) / 8} consecutive bytes
2148 in the string (which should be a unibyte string for best results).
2149 This means that each row always occupies at least one whole byte.
2150
2151 If the value is @code{nil}, that means use no stipple pattern.
2152
2153 Normally you do not need to set the stipple attribute, because it is
2154 used automatically to handle certain shades of gray.
2155
2156 @item :font
2157 The font used to display the face. Its value should be a font object.
2158 @xref{Low-Level Font}, for information about font objects, font specs,
2159 and font entities.
2160
2161 When specifying this attribute using @code{set-face-attribute}
2162 (@pxref{Attribute Functions}), you may also supply a font spec, a font
2163 entity, or a string. Emacs converts such values to an appropriate
2164 font object, and stores that font object as the actual attribute
2165 value. If you specify a string, the contents of the string should be
2166 a font name (@pxref{Fonts,,, emacs, The GNU Emacs Manual}); if the
2167 font name is an XLFD containing wildcards, Emacs chooses the first
2168 font matching those wildcards. Specifying this attribute also changes
2169 the values of the @code{:family}, @code{:foundry}, @code{:width},
2170 @code{:height}, @code{:weight}, and @code{:slant} attributes.
2171
2172 @cindex inheritance, for faces
2173 @item :inherit
2174 The name of a face from which to inherit attributes, or a list of face
2175 names. Attributes from inherited faces are merged into the face like
2176 an underlying face would be, with higher priority than underlying
2177 faces (@pxref{Displaying Faces}). If a list of faces is used,
2178 attributes from faces earlier in the list override those from later
2179 faces.
2180 @end table
2181
2182 @defun font-family-list &optional frame
2183 This function returns a list of available font family names. The
2184 optional argument @var{frame} specifies the frame on which the text is
2185 to be displayed; if it is @code{nil}, the selected frame is used.
2186 @end defun
2187
2188 @defopt underline-minimum-offset
2189 This variable specifies the minimum distance between the baseline and
2190 the underline, in pixels, when displaying underlined text.
2191 @end defopt
2192
2193 @defopt x-bitmap-file-path
2194 This variable specifies a list of directories for searching
2195 for bitmap files, for the @code{:stipple} attribute.
2196 @end defopt
2197
2198 @defun bitmap-spec-p object
2199 This returns @code{t} if @var{object} is a valid bitmap specification,
2200 suitable for use with @code{:stipple} (see above). It returns
2201 @code{nil} otherwise.
2202 @end defun
2203
2204 @node Defining Faces
2205 @subsection Defining Faces
2206
2207 @cindex face spec
2208 The usual way to define a face is through the @code{defface} macro.
2209 This macro associates a face name (a symbol) with a default @dfn{face
2210 spec}. A face spec is a construct which specifies what attributes a
2211 face should have on any given terminal; for example, a face spec might
2212 specify one foreground color on high-color terminals, and a different
2213 foreground color on low-color terminals.
2214
2215 People are sometimes tempted to create a variable whose value is a
2216 face name. In the vast majority of cases, this is not necessary; the
2217 usual procedure is to define a face with @code{defface}, and then use
2218 its name directly.
2219
2220 @defmac defface face spec doc [keyword value]@dots{}
2221 This macro declares @var{face} as a named face whose default face spec
2222 is given by @var{spec}. You should not quote the symbol @var{face},
2223 and it should not end in @samp{-face} (that would be redundant). The
2224 argument @var{doc} is a documentation string for the face. The
2225 additional @var{keyword} arguments have the same meanings as in
2226 @code{defgroup} and @code{defcustom} (@pxref{Common Keywords}).
2227
2228 If @var{face} already has a default face spec, this macro does
2229 nothing.
2230
2231 The default face spec determines @var{face}'s appearance when no
2232 customizations are in effect (@pxref{Customization}). If @var{face}
2233 has already been customized (via Custom themes or via customizations
2234 read from the init file), its appearance is determined by the custom
2235 face spec(s), which override the default face spec @var{spec}.
2236 However, if the customizations are subsequently removed, the
2237 appearance of @var{face} will again be determined by its default face
2238 spec.
2239
2240 As an exception, if you evaluate a @code{defface} form with
2241 @kbd{C-M-x} in Emacs Lisp mode (@code{eval-defun}), a special feature
2242 of @code{eval-defun} overrides any custom face specs on the face,
2243 causing the face to reflect exactly what the @code{defface} says.
2244
2245 The @var{spec} argument is a @dfn{face spec}, which states how the
2246 face should appear on different kinds of terminals. It should be an
2247 alist whose elements each have the form
2248
2249 @example
2250 (@var{display} . @var{plist})
2251 @end example
2252
2253 @noindent
2254 @var{display} specifies a class of terminals (see below). @var{plist}
2255 is a property list of face attributes and their values, specifying how
2256 the face appears on such terminals. For backward compatibility, you
2257 can also write an element as @code{(@var{display} @var{plist})}.
2258
2259 The @var{display} part of an element of @var{spec} determines which
2260 terminals the element matches. If more than one element of @var{spec}
2261 matches a given terminal, the first element that matches is the one
2262 used for that terminal. There are three possibilities for
2263 @var{display}:
2264
2265 @table @asis
2266 @item @code{default}
2267 This element of @var{spec} doesn't match any terminal; instead, it
2268 specifies defaults that apply to all terminals. This element, if
2269 used, must be the first element of @var{spec}. Each of the following
2270 elements can override any or all of these defaults.
2271
2272 @item @code{t}
2273 This element of @var{spec} matches all terminals. Therefore, any
2274 subsequent elements of @var{spec} are never used. Normally @code{t}
2275 is used in the last (or only) element of @var{spec}.
2276
2277 @item a list
2278 If @var{display} is a list, each element should have the form
2279 @code{(@var{characteristic} @var{value}@dots{})}. Here
2280 @var{characteristic} specifies a way of classifying terminals, and the
2281 @var{value}s are possible classifications which @var{display} should
2282 apply to. Here are the possible values of @var{characteristic}:
2283
2284 @table @code
2285 @item type
2286 The kind of window system the terminal uses---either @code{graphic}
2287 (any graphics-capable display), @code{x}, @code{pc} (for the MS-DOS
2288 console), @code{w32} (for MS Windows 9X/NT/2K/XP), or @code{tty} (a
2289 non-graphics-capable display). @xref{Window Systems, window-system}.
2290
2291 @item class
2292 What kinds of colors the terminal supports---either @code{color},
2293 @code{grayscale}, or @code{mono}.
2294
2295 @item background
2296 The kind of background---either @code{light} or @code{dark}.
2297
2298 @item min-colors
2299 An integer that represents the minimum number of colors the terminal
2300 should support. This matches a terminal if its
2301 @code{display-color-cells} value is at least the specified integer.
2302
2303 @item supports
2304 Whether or not the terminal can display the face attributes given in
2305 @var{value}@dots{} (@pxref{Face Attributes}). @xref{Display Face
2306 Attribute Testing}, for more information on exactly how this testing
2307 is done.
2308 @end table
2309
2310 If an element of @var{display} specifies more than one @var{value} for
2311 a given @var{characteristic}, any of those values is acceptable. If
2312 @var{display} has more than one element, each element should specify a
2313 different @var{characteristic}; then @emph{each} characteristic of the
2314 terminal must match one of the @var{value}s specified for it in
2315 @var{display}.
2316 @end table
2317 @end defmac
2318
2319 For example, here's the definition of the standard face
2320 @code{highlight}:
2321
2322 @example
2323 (defface highlight
2324 '((((class color) (min-colors 88) (background light))
2325 :background "darkseagreen2")
2326 (((class color) (min-colors 88) (background dark))
2327 :background "darkolivegreen")
2328 (((class color) (min-colors 16) (background light))
2329 :background "darkseagreen2")
2330 (((class color) (min-colors 16) (background dark))
2331 :background "darkolivegreen")
2332 (((class color) (min-colors 8))
2333 :background "green" :foreground "black")
2334 (t :inverse-video t))
2335 "Basic face for highlighting."
2336 :group 'basic-faces)
2337 @end example
2338
2339 Internally, Emacs stores each face's default spec in its
2340 @code{face-defface-spec} symbol property (@pxref{Symbol Properties}).
2341 The @code{saved-face} property stores any face spec saved by the user
2342 using the customization buffer; the @code{customized-face} property
2343 stores the face spec customized for the current session, but not
2344 saved; and the @code{theme-face} property stores an alist associating
2345 the active customization settings and Custom themes with the face
2346 specs for that face. The face's documentation string is stored in the
2347 @code{face-documentation} property.
2348
2349 Normally, a face is declared just once, using @code{defface}, and
2350 any further changes to its appearance are applied using the Customize
2351 framework (e.g., via the Customize user interface or via the
2352 @code{custom-set-faces} function; @pxref{Applying Customizations}), or
2353 by face remapping (@pxref{Face Remapping}). In the rare event that
2354 you need to change a face spec directly from Lisp, you can use the
2355 @code{face-spec-set} function.
2356
2357 @defun face-spec-set face spec &optional spec-type
2358 This function applies @var{spec} as a face spec for @code{face}.
2359 @var{spec} should be a face spec, as described in the above
2360 documentation for @code{defface}.
2361
2362 This function also defines @var{face} as a valid face name if it is
2363 not already one, and (re)calculates its attributes on existing frames.
2364
2365 @cindex override spec @r{(for a face)}
2366 The argument @var{spec-type} determines which spec to set. If it is
2367 @code{nil} or @code{face-override-spec}, this function sets the
2368 @dfn{override spec}, which overrides over all other face specs on
2369 @var{face}. If it is @code{customized-face} or @code{saved-face},
2370 this function sets the customized spec or the saved custom spec. If
2371 it is @code{face-defface-spec}, this function sets the default face
2372 spec (the same one set by @code{defface}). If it is @code{reset},
2373 this function clears out all customization specs and override specs
2374 from @var{face} (in this case, the value of @var{spec} is ignored).
2375 Any other value of @var{spec-type} is reserved for internal use.
2376 @end defun
2377
2378 @node Attribute Functions
2379 @subsection Face Attribute Functions
2380
2381 This section describes functions for directly accessing and
2382 modifying the attributes of a named face.
2383
2384 @defun face-attribute face attribute &optional frame inherit
2385 This function returns the value of the @var{attribute} attribute for
2386 @var{face} on @var{frame}.
2387
2388 If @var{frame} is @code{nil}, that means the selected frame
2389 (@pxref{Input Focus}). If @var{frame} is @code{t}, this function
2390 returns the value of the specified attribute for newly-created frames
2391 (this is normally @code{unspecified}, unless you have specified some
2392 value using @code{set-face-attribute}; see below).
2393
2394 If @var{inherit} is @code{nil}, only attributes directly defined by
2395 @var{face} are considered, so the return value may be
2396 @code{unspecified}, or a relative value. If @var{inherit} is
2397 non-@code{nil}, @var{face}'s definition of @var{attribute} is merged
2398 with the faces specified by its @code{:inherit} attribute; however the
2399 return value may still be @code{unspecified} or relative. If
2400 @var{inherit} is a face or a list of faces, then the result is further
2401 merged with that face (or faces), until it becomes specified and
2402 absolute.
2403
2404 To ensure that the return value is always specified and absolute, use
2405 a value of @code{default} for @var{inherit}; this will resolve any
2406 unspecified or relative values by merging with the @code{default} face
2407 (which is always completely specified).
2408
2409 For example,
2410
2411 @example
2412 (face-attribute 'bold :weight)
2413 @result{} bold
2414 @end example
2415 @end defun
2416
2417 @c FIXME: Add an index for "relative face attribute", maybe here? --xfq
2418 @defun face-attribute-relative-p attribute value
2419 This function returns non-@code{nil} if @var{value}, when used as the
2420 value of the face attribute @var{attribute}, is relative. This means
2421 it would modify, rather than completely override, any value that comes
2422 from a subsequent face in the face list or that is inherited from
2423 another face.
2424
2425 @code{unspecified} is a relative value for all attributes. For
2426 @code{:height}, floating point and function values are also relative.
2427
2428 For example:
2429
2430 @example
2431 (face-attribute-relative-p :height 2.0)
2432 @result{} t
2433 @end example
2434 @end defun
2435
2436 @defun face-all-attributes face &optional frame
2437 This function returns an alist of attributes of @var{face}. The
2438 elements of the result are name-value pairs of the form
2439 @w{@code{(@var{attr-name} . @var{attr-value})}}. Optional argument
2440 @var{frame} specifies the frame whose definition of @var{face} to
2441 return; if omitted or @code{nil}, the returned value describes the
2442 default attributes of @var{face} for newly created frames.
2443 @end defun
2444
2445 @defun merge-face-attribute attribute value1 value2
2446 If @var{value1} is a relative value for the face attribute
2447 @var{attribute}, returns it merged with the underlying value
2448 @var{value2}; otherwise, if @var{value1} is an absolute value for the
2449 face attribute @var{attribute}, returns @var{value1} unchanged.
2450 @end defun
2451
2452 Normally, Emacs uses the face specs of each face to automatically
2453 calculate its attributes on each frame (@pxref{Defining Faces}). The
2454 function @code{set-face-attribute} can override this calculation by
2455 directly assigning attributes to a face, either on a specific frame or
2456 for all frames. This function is mostly intended for internal usage.
2457
2458 @defun set-face-attribute face frame &rest arguments
2459 This function sets one or more attributes of @var{face} for
2460 @var{frame}. The attributes specifies in this way override the face
2461 spec(s) belonging to @var{face}.
2462
2463 The extra arguments @var{arguments} specify the attributes to set, and
2464 the values for them. They should consist of alternating attribute
2465 names (such as @code{:family} or @code{:underline}) and values. Thus,
2466
2467 @example
2468 (set-face-attribute 'foo nil :weight 'bold :slant 'italic)
2469 @end example
2470
2471 @noindent
2472 sets the attribute @code{:weight} to @code{bold} and the attribute
2473 @code{:slant} to @code{italic}.
2474
2475
2476 If @var{frame} is @code{t}, this function sets the default attributes
2477 for newly created frames. If @var{frame} is @code{nil}, this function
2478 sets the attributes for all existing frames, as well as for newly
2479 created frames.
2480 @end defun
2481
2482 The following commands and functions mostly provide compatibility
2483 with old versions of Emacs. They work by calling
2484 @code{set-face-attribute}. Values of @code{t} and @code{nil} for
2485 their @var{frame} argument are handled just like
2486 @code{set-face-attribute} and @code{face-attribute}. The commands
2487 read their arguments using the minibuffer, if called interactively.
2488
2489 @deffn Command set-face-foreground face color &optional frame
2490 @deffnx Command set-face-background face color &optional frame
2491 These set the @code{:foreground} attribute (or @code{:background}
2492 attribute, respectively) of @var{face} to @var{color}.
2493 @end deffn
2494
2495 @deffn Command set-face-stipple face pattern &optional frame
2496 This sets the @code{:stipple} attribute of @var{face} to
2497 @var{pattern}.
2498 @end deffn
2499
2500 @deffn Command set-face-font face font &optional frame
2501 This sets the @code{:font} attribute of @var{face} to @var{font}.
2502 @end deffn
2503
2504 @defun set-face-bold face bold-p &optional frame
2505 This sets the @code{:weight} attribute of @var{face} to @var{normal}
2506 if @var{bold-p} is @code{nil}, and to @var{bold} otherwise.
2507 @end defun
2508
2509 @defun set-face-italic face italic-p &optional frame
2510 This sets the @code{:slant} attribute of @var{face} to @var{normal} if
2511 @var{italic-p} is @code{nil}, and to @var{italic} otherwise.
2512 @end defun
2513
2514 @defun set-face-underline face underline &optional frame
2515 This sets the @code{:underline} attribute of @var{face} to
2516 @var{underline}.
2517 @end defun
2518
2519 @defun set-face-inverse-video face inverse-video-p &optional frame
2520 This sets the @code{:inverse-video} attribute of @var{face} to
2521 @var{inverse-video-p}.
2522 @end defun
2523
2524 @deffn Command invert-face face &optional frame
2525 This swaps the foreground and background colors of face @var{face}.
2526 @end deffn
2527
2528 The following functions examine the attributes of a face. They
2529 mostly provide compatibility with old versions of Emacs. If you don't
2530 specify @var{frame}, they refer to the selected frame; @code{t} refers
2531 to the default data for new frames. They return @code{unspecified} if
2532 the face doesn't define any value for that attribute. If
2533 @var{inherit} is @code{nil}, only an attribute directly defined by the
2534 face is returned. If @var{inherit} is non-@code{nil}, any faces
2535 specified by its @code{:inherit} attribute are considered as well, and
2536 if @var{inherit} is a face or a list of faces, then they are also
2537 considered, until a specified attribute is found. To ensure that the
2538 return value is always specified, use a value of @code{default} for
2539 @var{inherit}.
2540
2541 @defun face-font face &optional frame
2542 This function returns the name of the font of face @var{face}.
2543 @end defun
2544
2545 @defun face-foreground face &optional frame inherit
2546 @defunx face-background face &optional frame inherit
2547 These functions return the foreground color (or background color,
2548 respectively) of face @var{face}, as a string.
2549 @end defun
2550
2551 @defun face-stipple face &optional frame inherit
2552 This function returns the name of the background stipple pattern of face
2553 @var{face}, or @code{nil} if it doesn't have one.
2554 @end defun
2555
2556 @defun face-bold-p face &optional frame inherit
2557 This function returns a non-@code{nil} value if the @code{:weight}
2558 attribute of @var{face} is bolder than normal (i.e., one of
2559 @code{semi-bold}, @code{bold}, @code{extra-bold}, or
2560 @code{ultra-bold}). Otherwise, it returns @code{nil}.
2561 @end defun
2562
2563 @defun face-italic-p face &optional frame inherit
2564 This function returns a non-@code{nil} value if the @code{:slant}
2565 attribute of @var{face} is @code{italic} or @code{oblique}, and
2566 @code{nil} otherwise.
2567 @end defun
2568
2569 @defun face-underline-p face &optional frame inherit
2570 This function returns non-@code{nil} if face @var{face} specifies
2571 a non-@code{nil} @code{:underline} attribute.
2572 @end defun
2573
2574 @defun face-inverse-video-p face &optional frame inherit
2575 This function returns non-@code{nil} if face @var{face} specifies
2576 a non-@code{nil} @code{:inverse-video} attribute.
2577 @end defun
2578
2579 @node Displaying Faces
2580 @subsection Displaying Faces
2581
2582 When Emacs displays a given piece of text, the visual appearance of
2583 the text may be determined by faces drawn from different sources. If
2584 these various sources together specify more than one face for a
2585 particular character, Emacs merges the attributes of the various
2586 faces. Here is the order in which Emacs merges the faces, from
2587 highest to lowest priority:
2588
2589 @itemize @bullet
2590 @item
2591 If the text consists of a special glyph, the glyph can specify a
2592 particular face. @xref{Glyphs}.
2593
2594 @item
2595 If the text lies within an active region, Emacs highlights it using
2596 the @code{region} face. @xref{Standard Faces,,, emacs, The GNU Emacs
2597 Manual}.
2598
2599 @item
2600 If the text lies within an overlay with a non-@code{nil} @code{face}
2601 property, Emacs applies the face(s) specified by that property. If
2602 the overlay has a @code{mouse-face} property and the mouse is ``near
2603 enough'' to the overlay, Emacs applies the face or face attributes
2604 specified by the @code{mouse-face} property instead. @xref{Overlay
2605 Properties}.
2606
2607 When multiple overlays cover one character, an overlay with higher
2608 priority overrides those with lower priority. @xref{Overlays}.
2609
2610 @item
2611 If the text contains a @code{face} or @code{mouse-face} property,
2612 Emacs applies the specified faces and face attributes. @xref{Special
2613 Properties}. (This is how Font Lock mode faces are applied.
2614 @xref{Font Lock Mode}.)
2615
2616 @item
2617 If the text lies within the mode line of the selected window, Emacs
2618 applies the @code{mode-line} face. For the mode line of a
2619 non-selected window, Emacs applies the @code{mode-line-inactive} face.
2620 For a header line, Emacs applies the @code{header-line} face.
2621
2622 @item
2623 If any given attribute has not been specified during the preceding
2624 steps, Emacs applies the attribute of the @code{default} face.
2625 @end itemize
2626
2627 At each stage, if a face has a valid @code{:inherit} attribute,
2628 Emacs treats any attribute with an @code{unspecified} value as having
2629 the corresponding value drawn from the parent face(s). @pxref{Face
2630 Attributes}. Note that the parent face(s) may also leave the
2631 attribute unspecified; in that case, the attribute remains unspecified
2632 at the next level of face merging.
2633
2634 @node Face Remapping
2635 @subsection Face Remapping
2636
2637 The variable @code{face-remapping-alist} is used for buffer-local or
2638 global changes in the appearance of a face. For instance, it is used
2639 to implement the @code{text-scale-adjust} command (@pxref{Text
2640 Scale,,, emacs, The GNU Emacs Manual}).
2641
2642 @defvar face-remapping-alist
2643 The value of this variable is an alist whose elements have the form
2644 @code{(@var{face} . @var{remapping})}. This causes Emacs to display
2645 any text having the face @var{face} with @var{remapping}, rather than
2646 the ordinary definition of @var{face}.
2647
2648 @var{remapping} may be any face spec suitable for a @code{face} text
2649 property: either a face (i.e., a face name or a property list of
2650 attribute/value pairs), or a list of faces. For details, see the
2651 description of the @code{face} text property in @ref{Special
2652 Properties}. @var{remapping} serves as the complete specification for
2653 the remapped face---it replaces the normal definition of @var{face},
2654 instead of modifying it.
2655
2656 If @code{face-remapping-alist} is buffer-local, its local value takes
2657 effect only within that buffer.
2658
2659 Note: face remapping is non-recursive. If @var{remapping} references
2660 the same face name @var{face}, either directly or via the
2661 @code{:inherit} attribute of some other face in @var{remapping}, that
2662 reference uses the normal definition of @var{face}. For instance, if
2663 the @code{mode-line} face is remapped using this entry in
2664 @code{face-remapping-alist}:
2665
2666 @example
2667 (mode-line italic mode-line)
2668 @end example
2669
2670 @noindent
2671 then the new definition of the @code{mode-line} face inherits from the
2672 @code{italic} face, and the @emph{normal} (non-remapped) definition of
2673 @code{mode-line} face.
2674 @end defvar
2675
2676 @cindex relative remapping, faces
2677 @cindex base remapping, faces
2678 The following functions implement a higher-level interface to
2679 @code{face-remapping-alist}. Most Lisp code should use these
2680 functions instead of setting @code{face-remapping-alist} directly, to
2681 avoid trampling on remappings applied elsewhere. These functions are
2682 intended for buffer-local remappings, so they all make
2683 @code{face-remapping-alist} buffer-local as a side-effect. They manage
2684 @code{face-remapping-alist} entries of the form
2685
2686 @example
2687 (@var{face} @var{relative-spec-1} @var{relative-spec-2} @var{...} @var{base-spec})
2688 @end example
2689
2690 @noindent
2691 where, as explained above, each of the @var{relative-spec-N} and
2692 @var{base-spec} is either a face name, or a property list of
2693 attribute/value pairs. Each of the @dfn{relative remapping} entries,
2694 @var{relative-spec-N}, is managed by the
2695 @code{face-remap-add-relative} and @code{face-remap-remove-relative}
2696 functions; these are intended for simple modifications like changing
2697 the text size. The @dfn{base remapping} entry, @var{base-spec}, has
2698 the lowest priority and is managed by the @code{face-remap-set-base}
2699 and @code{face-remap-reset-base} functions; it is intended for major
2700 modes to remap faces in the buffers they control.
2701
2702 @defun face-remap-add-relative face &rest specs
2703 This function adds the face spec in @var{specs} as relative
2704 remappings for face @var{face} in the current buffer. The remaining
2705 arguments, @var{specs}, should form either a list of face names, or a
2706 property list of attribute/value pairs.
2707
2708 The return value is a Lisp object that serves as a ``cookie''; you can
2709 pass this object as an argument to @code{face-remap-remove-relative}
2710 if you need to remove the remapping later.
2711
2712 @example
2713 ;; Remap the `escape-glyph' face into a combination
2714 ;; of the `highlight' and `italic' faces:
2715 (face-remap-add-relative 'escape-glyph 'highlight 'italic)
2716
2717 ;; Increase the size of the `default' face by 50%:
2718 (face-remap-add-relative 'default :height 1.5)
2719 @end example
2720 @end defun
2721
2722 @defun face-remap-remove-relative cookie
2723 This function removes a relative remapping previously added by
2724 @code{face-remap-add-relative}. @var{cookie} should be the Lisp
2725 object returned by @code{face-remap-add-relative} when the remapping
2726 was added.
2727 @end defun
2728
2729 @defun face-remap-set-base face &rest specs
2730 This function sets the base remapping of @var{face} in the current
2731 buffer to @var{specs}. If @var{specs} is empty, the default base
2732 remapping is restored, similar to calling @code{face-remap-reset-base}
2733 (see below); note that this is different from @var{specs} containing a
2734 single value @code{nil}, which has the opposite result (the global
2735 definition of @var{face} is ignored).
2736
2737 This overwrites the default @var{base-spec}, which inherits the global
2738 face definition, so it is up to the caller to add such inheritance if
2739 so desired.
2740 @end defun
2741
2742 @defun face-remap-reset-base face
2743 This function sets the base remapping of @var{face} to its default
2744 value, which inherits from @var{face}'s global definition.
2745 @end defun
2746
2747 @node Face Functions
2748 @subsection Functions for Working with Faces
2749
2750 Here are additional functions for creating and working with faces.
2751
2752 @defun face-list
2753 This function returns a list of all defined face names.
2754 @end defun
2755
2756 @defun face-id face
2757 This function returns the @dfn{face number} of face @var{face}. This
2758 is a number that uniquely identifies a face at low levels within
2759 Emacs. It is seldom necessary to refer to a face by its face number.
2760 @end defun
2761
2762 @defun face-documentation face
2763 This function returns the documentation string of face @var{face}, or
2764 @code{nil} if none was specified for it.
2765 @end defun
2766
2767 @defun face-equal face1 face2 &optional frame
2768 This returns @code{t} if the faces @var{face1} and @var{face2} have the
2769 same attributes for display.
2770 @end defun
2771
2772 @defun face-differs-from-default-p face &optional frame
2773 This returns non-@code{nil} if the face @var{face} displays
2774 differently from the default face.
2775 @end defun
2776
2777 @cindex face alias
2778 @cindex alias, for faces
2779 A @dfn{face alias} provides an equivalent name for a face. You can
2780 define a face alias by giving the alias symbol the @code{face-alias}
2781 property, with a value of the target face name. The following example
2782 makes @code{modeline} an alias for the @code{mode-line} face.
2783
2784 @example
2785 (put 'modeline 'face-alias 'mode-line)
2786 @end example
2787
2788 @defmac define-obsolete-face-alias obsolete-face current-face when
2789 This macro defines @code{obsolete-face} as an alias for
2790 @var{current-face}, and also marks it as obsolete, indicating that it
2791 may be removed in future. @var{when} should be a string indicating
2792 when @code{obsolete-face} was made obsolete (usually a version number
2793 string).
2794 @end defmac
2795
2796 @node Auto Faces
2797 @subsection Automatic Face Assignment
2798 @cindex automatic face assignment
2799 @cindex faces, automatic choice
2800
2801 This hook is used for automatically assigning faces to text in the
2802 buffer. It is part of the implementation of Jit-Lock mode, used by
2803 Font-Lock.
2804
2805 @defvar fontification-functions
2806 This variable holds a list of functions that are called by Emacs
2807 redisplay as needed, just before doing redisplay. They are called even
2808 when Font Lock Mode isn't enabled. When Font Lock Mode is enabled, this
2809 variable usually holds just one function, @code{jit-lock-function}.
2810
2811 The functions are called in the order listed, with one argument, a
2812 buffer position @var{pos}. Collectively they should attempt to assign
2813 faces to the text in the current buffer starting at @var{pos}.
2814
2815 The functions should record the faces they assign by setting the
2816 @code{face} property. They should also add a non-@code{nil}
2817 @code{fontified} property to all the text they have assigned faces to.
2818 That property tells redisplay that faces have been assigned to that text
2819 already.
2820
2821 It is probably a good idea for the functions to do nothing if the
2822 character after @var{pos} already has a non-@code{nil} @code{fontified}
2823 property, but this is not required. If one function overrides the
2824 assignments made by a previous one, the properties after the last
2825 function finishes are the ones that really matter.
2826
2827 For efficiency, we recommend writing these functions so that they
2828 usually assign faces to around 400 to 600 characters at each call.
2829 @end defvar
2830
2831 @node Basic Faces
2832 @subsection Basic Faces
2833
2834 If your Emacs Lisp program needs to assign some faces to text, it is
2835 often a good idea to use certain existing faces or inherit from them,
2836 rather than defining entirely new faces. This way, if other users
2837 have customized the basic faces to give Emacs a certain look, your
2838 program will ``fit in'' without additional customization.
2839
2840 Some of the basic faces defined in Emacs are listed below. In
2841 addition to these, you might want to make use of the Font Lock faces
2842 for syntactic highlighting, if highlighting is not already handled by
2843 Font Lock mode, or if some Font Lock faces are not in use.
2844 @xref{Faces for Font Lock}.
2845
2846 @table @code
2847 @item default
2848 The default face, whose attributes are all specified. All other faces
2849 implicitly inherit from it: any unspecified attribute defaults to the
2850 attribute on this face (@pxref{Face Attributes}).
2851
2852 @item bold
2853 @itemx italic
2854 @itemx bold-italic
2855 @itemx underline
2856 @itemx fixed-pitch
2857 @itemx variable-pitch
2858 These have the attributes indicated by their names (e.g., @code{bold}
2859 has a bold @code{:weight} attribute), with all other attributes
2860 unspecified (and so given by @code{default}).
2861
2862 @item shadow
2863 For ``dimmed out'' text. For example, it is used for the ignored
2864 part of a filename in the minibuffer (@pxref{Minibuffer File,,
2865 Minibuffers for File Names, emacs, The GNU Emacs Manual}).
2866
2867 @item link
2868 @itemx link-visited
2869 For clickable text buttons that send the user to a different
2870 buffer or ``location''.
2871
2872 @item highlight
2873 For stretches of text that should temporarily stand out. For example,
2874 it is commonly assigned to the @code{mouse-face} property for cursor
2875 highlighting (@pxref{Special Properties}).
2876
2877 @item match
2878 For text matching a search command.
2879
2880 @item error
2881 @itemx warning
2882 @itemx success
2883 For text concerning errors, warnings, or successes. For example,
2884 these are used for messages in @file{*Compilation*} buffers.
2885 @end table
2886
2887 @node Font Selection
2888 @subsection Font Selection
2889 @cindex font selection
2890 @cindex selecting a font
2891
2892 Before Emacs can draw a character on a graphical display, it must
2893 select a @dfn{font} for that character@footnote{In this context, the
2894 term @dfn{font} has nothing to do with Font Lock (@pxref{Font Lock
2895 Mode}).}. @xref{Fonts,,, emacs, The GNU Emacs Manual}. Normally,
2896 Emacs automatically chooses a font based on the faces assigned to that
2897 character---specifically, the face attributes @code{:family},
2898 @code{:weight}, @code{:slant}, and @code{:width} (@pxref{Face
2899 Attributes}). The choice of font also depends on the character to be
2900 displayed; some fonts can only display a limited set of characters.
2901 If no available font exactly fits the requirements, Emacs looks for
2902 the @dfn{closest matching font}. The variables in this section
2903 control how Emacs makes this selection.
2904
2905 @defopt face-font-family-alternatives
2906 If a given family is specified but does not exist, this variable
2907 specifies alternative font families to try. Each element should have
2908 this form:
2909
2910 @example
2911 (@var{family} @var{alternate-families}@dots{})
2912 @end example
2913
2914 If @var{family} is specified but not available, Emacs will try the other
2915 families given in @var{alternate-families}, one by one, until it finds a
2916 family that does exist.
2917 @end defopt
2918
2919 @defopt face-font-selection-order
2920 If there is no font that exactly matches all desired face attributes
2921 (@code{:width}, @code{:height}, @code{:weight}, and @code{:slant}),
2922 this variable specifies the order in which these attributes should be
2923 considered when selecting the closest matching font. The value should
2924 be a list containing those four attribute symbols, in order of
2925 decreasing importance. The default is @code{(:width :height :weight
2926 :slant)}.
2927
2928 Font selection first finds the best available matches for the first
2929 attribute in the list; then, among the fonts which are best in that
2930 way, it searches for the best matches in the second attribute, and so
2931 on.
2932
2933 The attributes @code{:weight} and @code{:width} have symbolic values in
2934 a range centered around @code{normal}. Matches that are more extreme
2935 (farther from @code{normal}) are somewhat preferred to matches that are
2936 less extreme (closer to @code{normal}); this is designed to ensure that
2937 non-normal faces contrast with normal ones, whenever possible.
2938
2939 One example of a case where this variable makes a difference is when the
2940 default font has no italic equivalent. With the default ordering, the
2941 @code{italic} face will use a non-italic font that is similar to the
2942 default one. But if you put @code{:slant} before @code{:height}, the
2943 @code{italic} face will use an italic font, even if its height is not
2944 quite right.
2945 @end defopt
2946
2947 @defopt face-font-registry-alternatives
2948 This variable lets you specify alternative font registries to try, if a
2949 given registry is specified and doesn't exist. Each element should have
2950 this form:
2951
2952 @example
2953 (@var{registry} @var{alternate-registries}@dots{})
2954 @end example
2955
2956 If @var{registry} is specified but not available, Emacs will try the
2957 other registries given in @var{alternate-registries}, one by one,
2958 until it finds a registry that does exist.
2959 @end defopt
2960
2961 @cindex scalable fonts
2962 Emacs can make use of scalable fonts, but by default it does not use
2963 them.
2964
2965 @defopt scalable-fonts-allowed
2966 This variable controls which scalable fonts to use. A value of
2967 @code{nil}, the default, means do not use scalable fonts. @code{t}
2968 means to use any scalable font that seems appropriate for the text.
2969
2970 Otherwise, the value must be a list of regular expressions. Then a
2971 scalable font is enabled for use if its name matches any regular
2972 expression in the list. For example,
2973
2974 @example
2975 (setq scalable-fonts-allowed '("iso10646-1$"))
2976 @end example
2977
2978 @noindent
2979 allows the use of scalable fonts with registry @code{iso10646-1}.
2980 @end defopt
2981
2982 @defvar face-font-rescale-alist
2983 This variable specifies scaling for certain faces. Its value should
2984 be a list of elements of the form
2985
2986 @example
2987 (@var{fontname-regexp} . @var{scale-factor})
2988 @end example
2989
2990 If @var{fontname-regexp} matches the font name that is about to be
2991 used, this says to choose a larger similar font according to the
2992 factor @var{scale-factor}. You would use this feature to normalize
2993 the font size if certain fonts are bigger or smaller than their
2994 nominal heights and widths would suggest.
2995 @end defvar
2996
2997 @node Font Lookup
2998 @subsection Looking Up Fonts
2999
3000 @defun x-list-fonts name &optional reference-face frame maximum width
3001 This function returns a list of available font names that match
3002 @var{name}. @var{name} should be a string containing a font name in
3003 either the Fontconfig, GTK, or XLFD format (@pxref{Fonts,,, emacs, The
3004 GNU Emacs Manual}). Within an XLFD string, wildcard characters may be
3005 used: the @samp{*} character matches any substring, and the @samp{?}
3006 character matches any single character. Case is ignored when matching
3007 font names.
3008
3009 If the optional arguments @var{reference-face} and @var{frame} are
3010 specified, the returned list includes only fonts that are the same
3011 size as @var{reference-face} (a face name) currently is on the frame
3012 @var{frame}.
3013
3014 The optional argument @var{maximum} sets a limit on how many fonts to
3015 return. If it is non-@code{nil}, then the return value is truncated
3016 after the first @var{maximum} matching fonts. Specifying a small
3017 value for @var{maximum} can make this function much faster, in cases
3018 where many fonts match the pattern.
3019
3020 The optional argument @var{width} specifies a desired font width. If
3021 it is non-@code{nil}, the function only returns those fonts whose
3022 characters are (on average) @var{width} times as wide as
3023 @var{reference-face}.
3024 @end defun
3025
3026 @defun x-family-fonts &optional family frame
3027 This function returns a list describing the available fonts for family
3028 @var{family} on @var{frame}. If @var{family} is omitted or @code{nil},
3029 this list applies to all families, and therefore, it contains all
3030 available fonts. Otherwise, @var{family} must be a string; it may
3031 contain the wildcards @samp{?} and @samp{*}.
3032
3033 The list describes the display that @var{frame} is on; if @var{frame} is
3034 omitted or @code{nil}, it applies to the selected frame's display
3035 (@pxref{Input Focus}).
3036
3037 Each element in the list is a vector of the following form:
3038
3039 @example
3040 [@var{family} @var{width} @var{point-size} @var{weight} @var{slant}
3041 @var{fixed-p} @var{full} @var{registry-and-encoding}]
3042 @end example
3043
3044 The first five elements correspond to face attributes; if you
3045 specify these attributes for a face, it will use this font.
3046
3047 The last three elements give additional information about the font.
3048 @var{fixed-p} is non-@code{nil} if the font is fixed-pitch.
3049 @var{full} is the full name of the font, and
3050 @var{registry-and-encoding} is a string giving the registry and
3051 encoding of the font.
3052 @end defun
3053
3054 @node Fontsets
3055 @subsection Fontsets
3056
3057 A @dfn{fontset} is a list of fonts, each assigned to a range of
3058 character codes. An individual font cannot display the whole range of
3059 characters that Emacs supports, but a fontset can. Fontsets have names,
3060 just as fonts do, and you can use a fontset name in place of a font name
3061 when you specify the ``font'' for a frame or a face. Here is
3062 information about defining a fontset under Lisp program control.
3063
3064 @defun create-fontset-from-fontset-spec fontset-spec &optional style-variant-p noerror
3065 This function defines a new fontset according to the specification
3066 string @var{fontset-spec}. The string should have this format:
3067
3068 @smallexample
3069 @var{fontpattern}, @r{[}@var{charset}:@var{font}@r{]@dots{}}
3070 @end smallexample
3071
3072 @noindent
3073 Whitespace characters before and after the commas are ignored.
3074
3075 The first part of the string, @var{fontpattern}, should have the form of
3076 a standard X font name, except that the last two fields should be
3077 @samp{fontset-@var{alias}}.
3078
3079 The new fontset has two names, one long and one short. The long name is
3080 @var{fontpattern} in its entirety. The short name is
3081 @samp{fontset-@var{alias}}. You can refer to the fontset by either
3082 name. If a fontset with the same name already exists, an error is
3083 signaled, unless @var{noerror} is non-@code{nil}, in which case this
3084 function does nothing.
3085
3086 If optional argument @var{style-variant-p} is non-@code{nil}, that says
3087 to create bold, italic and bold-italic variants of the fontset as well.
3088 These variant fontsets do not have a short name, only a long one, which
3089 is made by altering @var{fontpattern} to indicate the bold and/or italic
3090 status.
3091
3092 The specification string also says which fonts to use in the fontset.
3093 See below for the details.
3094 @end defun
3095
3096 The construct @samp{@var{charset}:@var{font}} specifies which font to
3097 use (in this fontset) for one particular character set. Here,
3098 @var{charset} is the name of a character set, and @var{font} is the font
3099 to use for that character set. You can use this construct any number of
3100 times in the specification string.
3101
3102 For the remaining character sets, those that you don't specify
3103 explicitly, Emacs chooses a font based on @var{fontpattern}: it replaces
3104 @samp{fontset-@var{alias}} with a value that names one character set.
3105 For the @acronym{ASCII} character set, @samp{fontset-@var{alias}} is replaced
3106 with @samp{ISO8859-1}.
3107
3108 In addition, when several consecutive fields are wildcards, Emacs
3109 collapses them into a single wildcard. This is to prevent use of
3110 auto-scaled fonts. Fonts made by scaling larger fonts are not usable
3111 for editing, and scaling a smaller font is not useful because it is
3112 better to use the smaller font in its own size, which Emacs does.
3113
3114 Thus if @var{fontpattern} is this,
3115
3116 @example
3117 -*-fixed-medium-r-normal-*-24-*-*-*-*-*-fontset-24
3118 @end example
3119
3120 @noindent
3121 the font specification for @acronym{ASCII} characters would be this:
3122
3123 @example
3124 -*-fixed-medium-r-normal-*-24-*-ISO8859-1
3125 @end example
3126
3127 @noindent
3128 and the font specification for Chinese GB2312 characters would be this:
3129
3130 @example
3131 -*-fixed-medium-r-normal-*-24-*-gb2312*-*
3132 @end example
3133
3134 You may not have any Chinese font matching the above font
3135 specification. Most X distributions include only Chinese fonts that
3136 have @samp{song ti} or @samp{fangsong ti} in the @var{family} field. In
3137 such a case, @samp{Fontset-@var{n}} can be specified as below:
3138
3139 @smallexample
3140 Emacs.Fontset-0: -*-fixed-medium-r-normal-*-24-*-*-*-*-*-fontset-24,\
3141 chinese-gb2312:-*-*-medium-r-normal-*-24-*-gb2312*-*
3142 @end smallexample
3143
3144 @noindent
3145 Then, the font specifications for all but Chinese GB2312 characters have
3146 @samp{fixed} in the @var{family} field, and the font specification for
3147 Chinese GB2312 characters has a wild card @samp{*} in the @var{family}
3148 field.
3149
3150 @defun set-fontset-font name character font-spec &optional frame add
3151 This function modifies the existing fontset @var{name} to use the font
3152 matching with @var{font-spec} for the character @var{character}.
3153
3154 If @var{name} is @code{nil}, this function modifies the fontset of the
3155 selected frame or that of @var{frame} if @var{frame} is not
3156 @code{nil}.
3157
3158 If @var{name} is @code{t}, this function modifies the default
3159 fontset, whose short name is @samp{fontset-default}.
3160
3161 @var{character} may be a cons; @code{(@var{from} . @var{to})}, where
3162 @var{from} and @var{to} are character codepoints. In that case, use
3163 @var{font-spec} for all characters in the range @var{from} and @var{to}
3164 (inclusive).
3165
3166 @var{character} may be a charset. In that case, use
3167 @var{font-spec} for all character in the charsets.
3168
3169 @var{character} may be a script name. In that case, use
3170 @var{font-spec} for all character in the charsets.
3171
3172 @var{font-spec} may be a cons; @code{(@var{family} . @var{registry})},
3173 where @var{family} is a family name of a font (possibly including a
3174 foundry name at the head), @var{registry} is a registry name of a font
3175 (possibly including an encoding name at the tail).
3176
3177 @var{font-spec} may be a font name string.
3178
3179 The optional argument @var{add}, if non-@code{nil}, specifies how to
3180 add @var{font-spec} to the font specifications previously set. If it
3181 is @code{prepend}, @var{font-spec} is prepended. If it is
3182 @code{append}, @var{font-spec} is appended. By default,
3183 @var{font-spec} overrides the previous settings.
3184
3185 For instance, this changes the default fontset to use a font of which
3186 family name is @samp{Kochi Gothic} for all characters belonging to
3187 the charset @code{japanese-jisx0208}.
3188
3189 @smallexample
3190 (set-fontset-font t 'japanese-jisx0208
3191 (font-spec :family "Kochi Gothic"))
3192 @end smallexample
3193 @end defun
3194
3195 @defun char-displayable-p char
3196 This function returns @code{t} if Emacs ought to be able to display
3197 @var{char}. More precisely, if the selected frame's fontset has a
3198 font to display the character set that @var{char} belongs to.
3199
3200 Fontsets can specify a font on a per-character basis; when the fontset
3201 does that, this function's value may not be accurate.
3202 @end defun
3203
3204 @node Low-Level Font
3205 @subsection Low-Level Font Representation
3206 @cindex font property
3207
3208 Normally, it is not necessary to manipulate fonts directly. In case
3209 you need to do so, this section explains how.
3210
3211 In Emacs Lisp, fonts are represented using three different Lisp
3212 object types: @dfn{font objects}, @dfn{font specs}, and @dfn{font
3213 entities}.
3214
3215 @defun fontp object &optional type
3216 Return @code{t} if @var{object} is a font object, font spec, or font
3217 entity. Otherwise, return @code{nil}.
3218
3219 The optional argument @var{type}, if non-@code{nil}, determines the
3220 exact type of Lisp object to check for. In that case, @var{type}
3221 should be one of @code{font-object}, @code{font-spec}, or
3222 @code{font-entity}.
3223 @end defun
3224
3225 @cindex font object
3226 A font object is a Lisp object that represents a font that Emacs has
3227 @dfn{opened}. Font objects cannot be modified in Lisp, but they can
3228 be inspected.
3229
3230 @defun font-at position &optional window string
3231 Return the font object that is being used to display the character at
3232 position @var{position} in the window @var{window}. If @var{window}
3233 is @code{nil}, it defaults to the selected window. If @var{string} is
3234 @code{nil}, @var{position} specifies a position in the current buffer;
3235 otherwise, @var{string} should be a string, and @var{position}
3236 specifies a position in that string.
3237 @end defun
3238
3239 @cindex font spec
3240 A font spec is a Lisp object that contains a set of specifications
3241 that can be used to find a font. More than one font may match the
3242 specifications in a font spec.
3243
3244 @defun font-spec &rest arguments
3245 Return a new font spec using the specifications in @var{arguments},
3246 which should come in @code{property}-@code{value} pairs. The possible
3247 specifications are as follows:
3248
3249 @table @code
3250 @item :name
3251 The font name (a string), in either XLFD, Fontconfig, or GTK format.
3252 @xref{Fonts,,, emacs, The GNU Emacs Manual}.
3253
3254 @item :family
3255 @itemx :foundry
3256 @itemx :weight
3257 @itemx :slant
3258 @itemx :width
3259 These have the same meanings as the face attributes of the same name.
3260 @xref{Face Attributes}.
3261
3262 @item :size
3263 The font size---either a non-negative integer that specifies the pixel
3264 size, or a floating point number that specifies the point size.
3265
3266 @item :adstyle
3267 Additional typographic style information for the font, such as
3268 @samp{sans}. The value should be a string or a symbol.
3269
3270 @cindex font registry
3271 @item :registry
3272 The charset registry and encoding of the font, such as
3273 @samp{iso8859-1}. The value should be a string or a symbol.
3274
3275 @item :script
3276 The script that the font must support (a symbol).
3277
3278 @item :otf
3279 @cindex OpenType font
3280 The font must be an OpenType font that supports these OpenType
3281 features, provided Emacs is compiled with support for @samp{libotf} (a
3282 library for performing complex text layout in certain scripts). The
3283 value must be a list of the form
3284
3285 @smallexample
3286 @code{(@var{script-tag} @var{langsys-tag} @var{gsub} @var{gpos})}
3287 @end smallexample
3288
3289 where @var{script-tag} is the OpenType script tag symbol;
3290 @var{langsys-tag} is the OpenType language system tag symbol, or
3291 @code{nil} to use the default language system; @code{gsub} is a list
3292 of OpenType GSUB feature tag symbols, or @code{nil} if none is
3293 required; and @code{gpos} is a list of OpenType GPOS feature tag
3294 symbols, or @code{nil} if none is required. If @code{gsub} or
3295 @code{gpos} is a list, a @code{nil} element in that list means that
3296 the font must not match any of the remaining tag symbols. The
3297 @code{gpos} element may be omitted.
3298 @end table
3299 @end defun
3300
3301 @defun font-put font-spec property value
3302 Set the font property @var{property} in the font-spec @var{font-spec}
3303 to @var{value}.
3304 @end defun
3305
3306 @cindex font entity
3307 A font entity is a reference to a font that need not be open. Its
3308 properties are intermediate between a font object and a font spec:
3309 like a font object, and unlike a font spec, it refers to a single,
3310 specific font. Unlike a font object, creating a font entity does not
3311 load the contents of that font into computer memory. Emacs may open
3312 multiple font objects of different sizes from a single font entity
3313 referring to a scalable font.
3314
3315 @defun find-font font-spec &optional frame
3316 This function returns a font entity that best matches the font spec
3317 @var{font-spec} on frame @var{frame}. If @var{frame} is @code{nil},
3318 it defaults to the selected frame.
3319 @end defun
3320
3321 @defun list-fonts font-spec &optional frame num prefer
3322 This function returns a list of all font entities that match the font
3323 spec @var{font-spec}.
3324
3325 The optional argument @var{frame}, if non-@code{nil}, specifies the
3326 frame on which the fonts are to be displayed. The optional argument
3327 @var{num}, if non-@code{nil}, should be an integer that specifies the
3328 maximum length of the returned list. The optional argument
3329 @var{prefer}, if non-@code{nil}, should be another font spec, which is
3330 used to control the order of the returned list; the returned font
3331 entities are sorted in order of decreasing ``closeness'' to that font
3332 spec.
3333 @end defun
3334
3335 If you call @code{set-face-attribute} and pass a font spec, font
3336 entity, or font name string as the value of the @code{:font}
3337 attribute, Emacs opens the best ``matching'' font that is available
3338 for display. It then stores the corresponding font object as the
3339 actual value of the @code{:font} attribute for that face.
3340
3341 The following functions can be used to obtain information about a
3342 font. For these functions, the @var{font} argument can be a font
3343 object, a font entity, or a font spec.
3344
3345 @defun font-get font property
3346 This function returns the value of the font property @var{property}
3347 for @var{font}.
3348
3349 If @var{font} is a font spec and the font spec does not specify
3350 @var{property}, the return value is @code{nil}. If @var{font} is a
3351 font object or font entity, the value for the @var{:script} property
3352 may be a list of scripts supported by the font.
3353 @end defun
3354
3355 @defun font-face-attributes font &optional frame
3356 This function returns a list of face attributes corresponding to
3357 @var{font}. The optional argument @var{frame} specifies the frame on
3358 which the font is to be displayed. If it is @code{nil}, the selected
3359 frame is used. The return value has the form
3360
3361 @smallexample
3362 (:family @var{family} :height @var{height} :weight @var{weight}
3363 :slant @var{slant} :width @var{width})
3364 @end smallexample
3365
3366 where the values of @var{family}, @var{height}, @var{weight},
3367 @var{slant}, and @var{width} are face attribute values. Some of these
3368 key-attribute pairs may be omitted from the list if they are not
3369 specified by @var{font}.
3370 @end defun
3371
3372 @defun font-xlfd-name font &optional fold-wildcards
3373 This function returns the XLFD (X Logical Font Descriptor), a string,
3374 matching @var{font}. @xref{Fonts,,, emacs, The GNU Emacs Manual}, for
3375 information about XLFDs. If the name is too long for an XLFD (which
3376 can contain at most 255 characters), the function returns @code{nil}.
3377
3378 If the optional argument @var{fold-wildcards} is non-@code{nil},
3379 consecutive wildcards in the XLFD are folded into one.
3380 @end defun
3381
3382 @node Fringes
3383 @section Fringes
3384 @cindex fringes
3385
3386 On graphical displays, Emacs draws @dfn{fringes} next to each
3387 window: thin vertical strips down the sides which can display bitmaps
3388 indicating truncation, continuation, horizontal scrolling, and so on.
3389
3390 @menu
3391 * Fringe Size/Pos:: Specifying where to put the window fringes.
3392 * Fringe Indicators:: Displaying indicator icons in the window fringes.
3393 * Fringe Cursors:: Displaying cursors in the right fringe.
3394 * Fringe Bitmaps:: Specifying bitmaps for fringe indicators.
3395 * Customizing Bitmaps:: Specifying your own bitmaps to use in the fringes.
3396 * Overlay Arrow:: Display of an arrow to indicate position.
3397 @end menu
3398
3399 @node Fringe Size/Pos
3400 @subsection Fringe Size and Position
3401
3402 The following buffer-local variables control the position and width
3403 of fringes in windows showing that buffer.
3404
3405 @defvar fringes-outside-margins
3406 The fringes normally appear between the display margins and the window
3407 text. If the value is non-@code{nil}, they appear outside the display
3408 margins. @xref{Display Margins}.
3409 @end defvar
3410
3411 @defvar left-fringe-width
3412 This variable, if non-@code{nil}, specifies the width of the left
3413 fringe in pixels. A value of @code{nil} means to use the left fringe
3414 width from the window's frame.
3415 @end defvar
3416
3417 @defvar right-fringe-width
3418 This variable, if non-@code{nil}, specifies the width of the right
3419 fringe in pixels. A value of @code{nil} means to use the right fringe
3420 width from the window's frame.
3421 @end defvar
3422
3423 Any buffer which does not specify values for these variables uses
3424 the values specified by the @code{left-fringe} and @code{right-fringe}
3425 frame parameters (@pxref{Layout Parameters}).
3426
3427 The above variables actually take effect via the function
3428 @code{set-window-buffer} (@pxref{Buffers and Windows}), which calls
3429 @code{set-window-fringes} as a subroutine. If you change one of these
3430 variables, the fringe display is not updated in existing windows
3431 showing the buffer, unless you call @code{set-window-buffer} again in
3432 each affected window. You can also use @code{set-window-fringes} to
3433 control the fringe display in individual windows.
3434
3435 @defun set-window-fringes window left &optional right outside-margins
3436 This function sets the fringe widths of window @var{window}.
3437 If @var{window} is @code{nil}, the selected window is used.
3438
3439 The argument @var{left} specifies the width in pixels of the left
3440 fringe, and likewise @var{right} for the right fringe. A value of
3441 @code{nil} for either one stands for the default width. If
3442 @var{outside-margins} is non-@code{nil}, that specifies that fringes
3443 should appear outside of the display margins.
3444 @end defun
3445
3446 @defun window-fringes &optional window
3447 This function returns information about the fringes of a window
3448 @var{window}. If @var{window} is omitted or @code{nil}, the selected
3449 window is used. The value has the form @code{(@var{left-width}
3450 @var{right-width} @var{outside-margins})}.
3451 @end defun
3452
3453
3454 @node Fringe Indicators
3455 @subsection Fringe Indicators
3456 @cindex fringe indicators
3457 @cindex indicators, fringe
3458
3459 @dfn{Fringe indicators} are tiny icons displayed in the window
3460 fringe to indicate truncated or continued lines, buffer boundaries,
3461 etc.
3462
3463 @defopt indicate-empty-lines
3464 @cindex fringes, and empty line indication
3465 @cindex empty lines, indicating
3466 When this is non-@code{nil}, Emacs displays a special glyph in the
3467 fringe of each empty line at the end of the buffer, on graphical
3468 displays. @xref{Fringes}. This variable is automatically
3469 buffer-local in every buffer.
3470 @end defopt
3471
3472 @defopt indicate-buffer-boundaries
3473 @cindex buffer boundaries, indicating
3474 This buffer-local variable controls how the buffer boundaries and
3475 window scrolling are indicated in the window fringes.
3476
3477 Emacs can indicate the buffer boundaries---that is, the first and last
3478 line in the buffer---with angle icons when they appear on the screen.
3479 In addition, Emacs can display an up-arrow in the fringe to show
3480 that there is text above the screen, and a down-arrow to show
3481 there is text below the screen.
3482
3483 There are three kinds of basic values:
3484
3485 @table @asis
3486 @item @code{nil}
3487 Don't display any of these fringe icons.
3488 @item @code{left}
3489 Display the angle icons and arrows in the left fringe.
3490 @item @code{right}
3491 Display the angle icons and arrows in the right fringe.
3492 @item any non-alist
3493 Display the angle icons in the left fringe
3494 and don't display the arrows.
3495 @end table
3496
3497 Otherwise the value should be an alist that specifies which fringe
3498 indicators to display and where. Each element of the alist should
3499 have the form @code{(@var{indicator} . @var{position})}. Here,
3500 @var{indicator} is one of @code{top}, @code{bottom}, @code{up},
3501 @code{down}, and @code{t} (which covers all the icons not yet
3502 specified), while @var{position} is one of @code{left}, @code{right}
3503 and @code{nil}.
3504
3505 For example, @code{((top . left) (t . right))} places the top angle
3506 bitmap in left fringe, and the bottom angle bitmap as well as both
3507 arrow bitmaps in right fringe. To show the angle bitmaps in the left
3508 fringe, and no arrow bitmaps, use @code{((top . left) (bottom . left))}.
3509 @end defopt
3510
3511 @defvar fringe-indicator-alist
3512 This buffer-local variable specifies the mapping from logical fringe
3513 indicators to the actual bitmaps displayed in the window fringes. The
3514 value is an alist of elements @code{(@var{indicator}
3515 . @var{bitmaps})}, where @var{indicator} specifies a logical indicator
3516 type and @var{bitmaps} specifies the fringe bitmaps to use for that
3517 indicator.
3518
3519 Each @var{indicator} should be one of the following symbols:
3520
3521 @table @asis
3522 @item @code{truncation}, @code{continuation}.
3523 Used for truncation and continuation lines.
3524
3525 @item @code{up}, @code{down}, @code{top}, @code{bottom}, @code{top-bottom}
3526 Used when @code{indicate-buffer-boundaries} is non-@code{nil}:
3527 @code{up} and @code{down} indicate a buffer boundary lying above or
3528 below the window edge; @code{top} and @code{bottom} indicate the
3529 topmost and bottommost buffer text line; and @code{top-bottom}
3530 indicates where there is just one line of text in the buffer.
3531
3532 @item @code{empty-line}
3533 Used to indicate empty lines when @code{indicate-empty-lines} is
3534 non-@code{nil}.
3535
3536 @item @code{overlay-arrow}
3537 Used for overlay arrows (@pxref{Overlay Arrow}).
3538 @c Is this used anywhere?
3539 @c @item Unknown bitmap indicator:
3540 @c @code{unknown}.
3541 @end table
3542
3543 Each @var{bitmaps} value may be a list of symbols @code{(@var{left}
3544 @var{right} [@var{left1} @var{right1}])}. The @var{left} and
3545 @var{right} symbols specify the bitmaps shown in the left and/or right
3546 fringe, for the specific indicator. @var{left1} and @var{right1} are
3547 specific to the @code{bottom} and @code{top-bottom} indicators, and
3548 are used to indicate that the last text line has no final newline.
3549 Alternatively, @var{bitmaps} may be a single symbol which is used in
3550 both left and right fringes.
3551
3552 @xref{Fringe Bitmaps}, for a list of standard bitmap symbols and how
3553 to define your own. In addition, @code{nil} represents the empty
3554 bitmap (i.e., an indicator that is not shown).
3555
3556 When @code{fringe-indicator-alist} has a buffer-local value, and
3557 there is no bitmap defined for a logical indicator, or the bitmap is
3558 @code{t}, the corresponding value from the default value of
3559 @code{fringe-indicator-alist} is used.
3560 @end defvar
3561
3562 @node Fringe Cursors
3563 @subsection Fringe Cursors
3564 @cindex fringe cursors
3565 @cindex cursor, fringe
3566
3567 When a line is exactly as wide as the window, Emacs displays the
3568 cursor in the right fringe instead of using two lines. Different
3569 bitmaps are used to represent the cursor in the fringe depending on
3570 the current buffer's cursor type.
3571
3572 @defopt overflow-newline-into-fringe
3573 If this is non-@code{nil}, lines exactly as wide as the window (not
3574 counting the final newline character) are not continued. Instead,
3575 when point is at the end of the line, the cursor appears in the right
3576 fringe.
3577 @end defopt
3578
3579 @defvar fringe-cursor-alist
3580 This variable specifies the mapping from logical cursor type to the
3581 actual fringe bitmaps displayed in the right fringe. The value is an
3582 alist where each element has the form @code{(@var{cursor-type}
3583 . @var{bitmap})}, which means to use the fringe bitmap @var{bitmap} to
3584 display cursors of type @var{cursor-type}.
3585
3586 Each @var{cursor-type} should be one of @code{box}, @code{hollow},
3587 @code{bar}, @code{hbar}, or @code{hollow-small}. The first four have
3588 the same meanings as in the @code{cursor-type} frame parameter
3589 (@pxref{Cursor Parameters}). The @code{hollow-small} type is used
3590 instead of @code{hollow} when the normal @code{hollow-rectangle}
3591 bitmap is too tall to fit on a specific display line.
3592
3593 Each @var{bitmap} should be a symbol specifying the fringe bitmap to
3594 be displayed for that logical cursor type.
3595 @iftex
3596 See the next subsection for details.
3597 @end iftex
3598 @ifnottex
3599 @xref{Fringe Bitmaps}.
3600 @end ifnottex
3601
3602 @c FIXME: I can't find the fringes-indicator-alist variable. Maybe
3603 @c it should be fringe-indicator-alist or fringe-cursor-alist? --xfq
3604 When @code{fringe-cursor-alist} has a buffer-local value, and there is
3605 no bitmap defined for a cursor type, the corresponding value from the
3606 default value of @code{fringes-indicator-alist} is used.
3607 @end defvar
3608
3609 @node Fringe Bitmaps
3610 @subsection Fringe Bitmaps
3611 @cindex fringe bitmaps
3612 @cindex bitmaps, fringe
3613
3614 The @dfn{fringe bitmaps} are the actual bitmaps which represent the
3615 logical fringe indicators for truncated or continued lines, buffer
3616 boundaries, overlay arrows, etc. Each bitmap is represented by a
3617 symbol.
3618 @iftex
3619 These symbols are referred to by the variables
3620 @code{fringe-indicator-alist} and @code{fringe-cursor-alist},
3621 described in the previous subsections.
3622 @end iftex
3623 @ifnottex
3624 These symbols are referred to by the variable
3625 @code{fringe-indicator-alist}, which maps fringe indicators to bitmaps
3626 (@pxref{Fringe Indicators}), and the variable
3627 @code{fringe-cursor-alist}, which maps fringe cursors to bitmaps
3628 (@pxref{Fringe Cursors}).
3629 @end ifnottex
3630
3631 Lisp programs can also directly display a bitmap in the left or
3632 right fringe, by using a @code{display} property for one of the
3633 characters appearing in the line (@pxref{Other Display Specs}). Such
3634 a display specification has the form
3635
3636 @example
3637 (@var{fringe} @var{bitmap} [@var{face}])
3638 @end example
3639
3640 @noindent
3641 @var{fringe} is either the symbol @code{left-fringe} or
3642 @code{right-fringe}. @var{bitmap} is a symbol identifying the bitmap
3643 to display. The optional @var{face} names a face whose foreground
3644 color is used to display the bitmap; this face is automatically merged
3645 with the @code{fringe} face.
3646
3647 Here is a list of the standard fringe bitmaps defined in Emacs, and
3648 how they are currently used in Emacs (via
3649 @code{fringe-indicator-alist} and @code{fringe-cursor-alist}):
3650
3651 @table @asis
3652 @item @code{left-arrow}, @code{right-arrow}
3653 Used to indicate truncated lines.
3654
3655 @item @code{left-curly-arrow}, @code{right-curly-arrow}
3656 Used to indicate continued lines.
3657
3658 @item @code{right-triangle}, @code{left-triangle}
3659 The former is used by overlay arrows. The latter is unused.
3660
3661 @item @code{up-arrow}, @code{down-arrow}, @code{top-left-angle} @code{top-right-angle}
3662 @itemx @code{bottom-left-angle}, @code{bottom-right-angle}
3663 @itemx @code{top-right-angle}, @code{top-left-angle}
3664 @itemx @code{left-bracket}, @code{right-bracket}, @code{top-right-angle}, @code{top-left-angle}
3665 Used to indicate buffer boundaries.
3666
3667 @item @code{filled-rectangle}, @code{hollow-rectangle}
3668 @itemx @code{filled-square}, @code{hollow-square}
3669 @itemx @code{vertical-bar}, @code{horizontal-bar}
3670 Used for different types of fringe cursors.
3671
3672 @item @code{empty-line}, @code{exclamation-mark}, @code{question-mark}, @code{exclamation-mark}
3673 Not used by core Emacs features.
3674 @end table
3675
3676 @noindent
3677 The next subsection describes how to define your own fringe bitmaps.
3678
3679 @defun fringe-bitmaps-at-pos &optional pos window
3680 This function returns the fringe bitmaps of the display line
3681 containing position @var{pos} in window @var{window}. The return
3682 value has the form @code{(@var{left} @var{right} @var{ov})}, where @var{left}
3683 is the symbol for the fringe bitmap in the left fringe (or @code{nil}
3684 if no bitmap), @var{right} is similar for the right fringe, and @var{ov}
3685 is non-@code{nil} if there is an overlay arrow in the left fringe.
3686
3687 The value is @code{nil} if @var{pos} is not visible in @var{window}.
3688 If @var{window} is @code{nil}, that stands for the selected window.
3689 If @var{pos} is @code{nil}, that stands for the value of point in
3690 @var{window}.
3691 @end defun
3692
3693 @node Customizing Bitmaps
3694 @subsection Customizing Fringe Bitmaps
3695 @cindex fringe bitmaps, customizing
3696
3697 @defun define-fringe-bitmap bitmap bits &optional height width align
3698 This function defines the symbol @var{bitmap} as a new fringe bitmap,
3699 or replaces an existing bitmap with that name.
3700
3701 The argument @var{bits} specifies the image to use. It should be
3702 either a string or a vector of integers, where each element (an
3703 integer) corresponds to one row of the bitmap. Each bit of an integer
3704 corresponds to one pixel of the bitmap, where the low bit corresponds
3705 to the rightmost pixel of the bitmap.
3706
3707 The height is normally the length of @var{bits}. However, you
3708 can specify a different height with non-@code{nil} @var{height}. The width
3709 is normally 8, but you can specify a different width with non-@code{nil}
3710 @var{width}. The width must be an integer between 1 and 16.
3711
3712 The argument @var{align} specifies the positioning of the bitmap
3713 relative to the range of rows where it is used; the default is to
3714 center the bitmap. The allowed values are @code{top}, @code{center},
3715 or @code{bottom}.
3716
3717 The @var{align} argument may also be a list @code{(@var{align}
3718 @var{periodic})} where @var{align} is interpreted as described above.
3719 If @var{periodic} is non-@code{nil}, it specifies that the rows in
3720 @code{bits} should be repeated enough times to reach the specified
3721 height.
3722 @end defun
3723
3724 @defun destroy-fringe-bitmap bitmap
3725 This function destroy the fringe bitmap identified by @var{bitmap}.
3726 If @var{bitmap} identifies a standard fringe bitmap, it actually
3727 restores the standard definition of that bitmap, instead of
3728 eliminating it entirely.
3729 @end defun
3730
3731 @defun set-fringe-bitmap-face bitmap &optional face
3732 This sets the face for the fringe bitmap @var{bitmap} to @var{face}.
3733 If @var{face} is @code{nil}, it selects the @code{fringe} face. The
3734 bitmap's face controls the color to draw it in.
3735
3736 @var{face} is merged with the @code{fringe} face, so normally
3737 @var{face} should specify only the foreground color.
3738 @end defun
3739
3740 @node Overlay Arrow
3741 @subsection The Overlay Arrow
3742 @c @cindex overlay arrow Duplicates variable names
3743
3744 The @dfn{overlay arrow} is useful for directing the user's attention
3745 to a particular line in a buffer. For example, in the modes used for
3746 interface to debuggers, the overlay arrow indicates the line of code
3747 about to be executed. This feature has nothing to do with
3748 @dfn{overlays} (@pxref{Overlays}).
3749
3750 @defvar overlay-arrow-string
3751 This variable holds the string to display to call attention to a
3752 particular line, or @code{nil} if the arrow feature is not in use.
3753 On a graphical display the contents of the string are ignored; instead a
3754 glyph is displayed in the fringe area to the left of the display area.
3755 @end defvar
3756
3757 @defvar overlay-arrow-position
3758 This variable holds a marker that indicates where to display the overlay
3759 arrow. It should point at the beginning of a line. On a non-graphical
3760 display the arrow text
3761 appears at the beginning of that line, overlaying any text that would
3762 otherwise appear. Since the arrow is usually short, and the line
3763 usually begins with indentation, normally nothing significant is
3764 overwritten.
3765
3766 The overlay-arrow string is displayed in any given buffer if the value
3767 of @code{overlay-arrow-position} in that buffer points into that
3768 buffer. Thus, it is possible to display multiple overlay arrow strings
3769 by creating buffer-local bindings of @code{overlay-arrow-position}.
3770 However, it is usually cleaner to use
3771 @code{overlay-arrow-variable-list} to achieve this result.
3772 @c !!! overlay-arrow-position: but the overlay string may remain in the display
3773 @c of some other buffer until an update is required. This should be fixed
3774 @c now. Is it?
3775 @end defvar
3776
3777 You can do a similar job by creating an overlay with a
3778 @code{before-string} property. @xref{Overlay Properties}.
3779
3780 You can define multiple overlay arrows via the variable
3781 @code{overlay-arrow-variable-list}.
3782
3783 @defvar overlay-arrow-variable-list
3784 This variable's value is a list of variables, each of which specifies
3785 the position of an overlay arrow. The variable
3786 @code{overlay-arrow-position} has its normal meaning because it is on
3787 this list.
3788 @end defvar
3789
3790 Each variable on this list can have properties
3791 @code{overlay-arrow-string} and @code{overlay-arrow-bitmap} that
3792 specify an overlay arrow string (for text terminals) or fringe bitmap
3793 (for graphical terminals) to display at the corresponding overlay
3794 arrow position. If either property is not set, the default
3795 @code{overlay-arrow-string} or @code{overlay-arrow} fringe indicator
3796 is used.
3797
3798 @node Scroll Bars
3799 @section Scroll Bars
3800 @cindex scroll bars
3801
3802 Normally the frame parameter @code{vertical-scroll-bars} controls
3803 whether the windows in the frame have vertical scroll bars, and
3804 whether they are on the left or right. The frame parameter
3805 @code{scroll-bar-width} specifies how wide they are (@code{nil}
3806 meaning the default). @xref{Layout Parameters}.
3807
3808 @defun frame-current-scroll-bars &optional frame
3809 This function reports the scroll bar type settings for frame
3810 @var{frame}. The value is a cons cell
3811 @code{(@var{vertical-type} .@: @var{horizontal-type})}, where
3812 @var{vertical-type} is either @code{left}, @code{right}, or @code{nil}
3813 (which means no scroll bar.) @var{horizontal-type} is meant to
3814 specify the horizontal scroll bar type, but since they are not
3815 implemented, it is always @code{nil}.
3816 @end defun
3817
3818 @vindex vertical-scroll-bar
3819 You can enable or disable scroll bars for a particular buffer,
3820 by setting the variable @code{vertical-scroll-bar}. This variable
3821 automatically becomes buffer-local when set. The possible values are
3822 @code{left}, @code{right}, @code{t}, which means to use the
3823 frame's default, and @code{nil} for no scroll bar.
3824
3825 You can also control this for individual windows. Call the function
3826 @code{set-window-scroll-bars} to specify what to do for a specific window:
3827
3828 @defun set-window-scroll-bars window width &optional vertical-type horizontal-type
3829 This function sets the width and type of scroll bars for window
3830 @var{window}.
3831
3832 @var{width} specifies the scroll bar width in pixels (@code{nil} means
3833 use the width specified for the frame). @var{vertical-type} specifies
3834 whether to have a vertical scroll bar and, if so, where. The possible
3835 values are @code{left}, @code{right} and @code{nil}, just like the
3836 values of the @code{vertical-scroll-bars} frame parameter.
3837
3838 The argument @var{horizontal-type} is meant to specify whether and
3839 where to have horizontal scroll bars, but since they are not
3840 implemented, it has no effect. If @var{window} is @code{nil}, the
3841 selected window is used.
3842 @end defun
3843
3844 @defun window-scroll-bars &optional window
3845 Report the width and type of scroll bars specified for @var{window}.
3846 If @var{window} is omitted or @code{nil}, the selected window is used.
3847 The value is a list of the form @code{(@var{width}
3848 @var{cols} @var{vertical-type} @var{horizontal-type})}. The value
3849 @var{width} is the value that was specified for the width (which may
3850 be @code{nil}); @var{cols} is the number of columns that the scroll
3851 bar actually occupies.
3852
3853 @var{horizontal-type} is not actually meaningful.
3854 @end defun
3855
3856 @defun window-scroll-bar-width &optional window
3857 This function returns the width of @var{window}'s vertical scrollbar,
3858 in pixels. @var{window} must be a live window. If @var{window} is
3859 @code{nil} or omitted, it will be the selected window.
3860 @end defun
3861
3862 If you don't specify these values for a window with
3863 @code{set-window-scroll-bars}, the buffer-local variables
3864 @code{scroll-bar-mode} and @code{scroll-bar-width} in the buffer being
3865 displayed control the window's vertical scroll bars. The function
3866 @code{set-window-buffer} examines these variables. If you change them
3867 in a buffer that is already visible in a window, you can make the
3868 window take note of the new values by calling @code{set-window-buffer}
3869 specifying the same buffer that is already displayed.
3870
3871 @defopt scroll-bar-mode
3872 This variable, always local in all buffers, controls whether and where
3873 to put scroll bars in windows displaying the buffer. The possible values
3874 are @code{nil} for no scroll bar, @code{left} to put a scroll bar on
3875 the left, and @code{right} to put a scroll bar on the right.
3876 @end defopt
3877
3878 @defun window-current-scroll-bars &optional window
3879 This function reports the scroll bar type for window @var{window}.
3880 If @var{window} is omitted or @code{nil}, the selected window is used.
3881 The value is a cons cell
3882 @code{(@var{vertical-type} .@: @var{horizontal-type})}. Unlike
3883 @code{window-scroll-bars}, this reports the scroll bar type actually
3884 used, once frame defaults and @code{scroll-bar-mode} are taken into
3885 account.
3886 @end defun
3887
3888 @defvar scroll-bar-width
3889 This variable, always local in all buffers, specifies the width of the
3890 buffer's scroll bars, measured in pixels. A value of @code{nil} means
3891 to use the value specified by the frame.
3892 @end defvar
3893
3894 @node Display Property
3895 @section The @code{display} Property
3896 @cindex display specification
3897 @kindex display @r{(text property)}
3898
3899 The @code{display} text property (or overlay property) is used to
3900 insert images into text, and to control other aspects of how text
3901 displays. The value of the @code{display} property should be a
3902 display specification, or a list or vector containing several display
3903 specifications. Display specifications in the same @code{display}
3904 property value generally apply in parallel to the text they cover.
3905
3906 If several sources (overlays and/or a text property) specify values
3907 for the @code{display} property, only one of the values takes effect,
3908 following the rules of @code{get-char-property}. @xref{Examining
3909 Properties}.
3910
3911 The rest of this section describes several kinds of
3912 display specifications and what they mean.
3913
3914 @menu
3915 * Replacing Specs:: Display specs that replace the text.
3916 * Specified Space:: Displaying one space with a specified width.
3917 * Pixel Specification:: Specifying space width or height in pixels.
3918 * Other Display Specs:: Displaying an image; adjusting the height,
3919 spacing, and other properties of text.
3920 * Display Margins:: Displaying text or images to the side of the main text.
3921 @end menu
3922
3923 @node Replacing Specs
3924 @subsection Display Specs That Replace The Text
3925
3926 Some kinds of display specifications specify something to display
3927 instead of the text that has the property. These are called
3928 @dfn{replacing} display specifications. Emacs does not allow the user
3929 to interactively move point into the middle of buffer text that is
3930 replaced in this way.
3931
3932 If a list of display specifications includes more than one replacing
3933 display specification, the first overrides the rest. Replacing
3934 display specifications make most other display specifications
3935 irrelevant, since those don't apply to the replacement.
3936
3937 For replacing display specifications, ``the text that has the
3938 property'' means all the consecutive characters that have the same
3939 Lisp object as their @code{display} property; these characters are
3940 replaced as a single unit. If two characters have different Lisp
3941 objects as their @code{display} properties (i.e., objects which are
3942 not @code{eq}), they are handled separately.
3943
3944 Here is an example which illustrates this point. A string serves as
3945 a replacing display specification, which replaces the text that has
3946 the property with the specified string (@pxref{Other Display Specs}).
3947 Consider the following function:
3948
3949 @smallexample
3950 (defun foo ()
3951 (dotimes (i 5)
3952 (let ((string (concat "A"))
3953 (start (+ i i (point-min))))
3954 (put-text-property start (1+ start) 'display string)
3955 (put-text-property start (+ 2 start) 'display string))))
3956 @end smallexample
3957
3958 @noindent
3959 This function gives each of the first ten characters in the buffer a
3960 @code{display} property which is a string @code{"A"}, but they don't
3961 all get the same string object. The first two characters get the same
3962 string object, so they are replaced with one @samp{A}; the fact that
3963 the display property was assigned in two separate calls to
3964 @code{put-text-property} is irrelevant. Similarly, the next two
3965 characters get a second string (@code{concat} creates a new string
3966 object), so they are replaced with one @samp{A}; and so on. Thus, the
3967 ten characters appear as five A's.
3968
3969 @node Specified Space
3970 @subsection Specified Spaces
3971 @cindex spaces, specified height or width
3972 @cindex variable-width spaces
3973
3974 To display a space of specified width and/or height, use a display
3975 specification of the form @code{(space . @var{props})}, where
3976 @var{props} is a property list (a list of alternating properties and
3977 values). You can put this property on one or more consecutive
3978 characters; a space of the specified height and width is displayed in
3979 place of @emph{all} of those characters. These are the properties you
3980 can use in @var{props} to specify the weight of the space:
3981
3982 @table @code
3983 @item :width @var{width}
3984 If @var{width} is an integer or floating point number, it specifies
3985 that the space width should be @var{width} times the normal character
3986 width. @var{width} can also be a @dfn{pixel width} specification
3987 (@pxref{Pixel Specification}).
3988
3989 @item :relative-width @var{factor}
3990 Specifies that the width of the stretch should be computed from the
3991 first character in the group of consecutive characters that have the
3992 same @code{display} property. The space width is the width of that
3993 character, multiplied by @var{factor}.
3994
3995 @item :align-to @var{hpos}
3996 Specifies that the space should be wide enough to reach @var{hpos}.
3997 If @var{hpos} is a number, it is measured in units of the normal
3998 character width. @var{hpos} can also be a @dfn{pixel width}
3999 specification (@pxref{Pixel Specification}).
4000 @end table
4001
4002 You should use one and only one of the above properties. You can
4003 also specify the height of the space, with these properties:
4004
4005 @table @code
4006 @item :height @var{height}
4007 Specifies the height of the space.
4008 If @var{height} is an integer or floating point number, it specifies
4009 that the space height should be @var{height} times the normal character
4010 height. The @var{height} may also be a @dfn{pixel height} specification
4011 (@pxref{Pixel Specification}).
4012
4013 @item :relative-height @var{factor}
4014 Specifies the height of the space, multiplying the ordinary height
4015 of the text having this display specification by @var{factor}.
4016
4017 @item :ascent @var{ascent}
4018 If the value of @var{ascent} is a non-negative number no greater than
4019 100, it specifies that @var{ascent} percent of the height of the space
4020 should be considered as the ascent of the space---that is, the part
4021 above the baseline. The ascent may also be specified in pixel units
4022 with a @dfn{pixel ascent} specification (@pxref{Pixel Specification}).
4023
4024 @end table
4025
4026 Don't use both @code{:height} and @code{:relative-height} together.
4027
4028 The @code{:width} and @code{:align-to} properties are supported on
4029 non-graphic terminals, but the other space properties in this section
4030 are not.
4031
4032 Note that space properties are treated as paragraph separators for
4033 the purposes of reordering bidirectional text for display.
4034 @xref{Bidirectional Display}, for the details.
4035
4036 @node Pixel Specification
4037 @subsection Pixel Specification for Spaces
4038 @cindex spaces, pixel specification
4039
4040 The value of the @code{:width}, @code{:align-to}, @code{:height},
4041 and @code{:ascent} properties can be a special kind of expression that
4042 is evaluated during redisplay. The result of the evaluation is used
4043 as an absolute number of pixels.
4044
4045 The following expressions are supported:
4046
4047 @smallexample
4048 @group
4049 @var{expr} ::= @var{num} | (@var{num}) | @var{unit} | @var{elem} | @var{pos} | @var{image} | @var{form}
4050 @var{num} ::= @var{integer} | @var{float} | @var{symbol}
4051 @var{unit} ::= in | mm | cm | width | height
4052 @end group
4053 @group
4054 @var{elem} ::= left-fringe | right-fringe | left-margin | right-margin
4055 | scroll-bar | text
4056 @var{pos} ::= left | center | right
4057 @var{form} ::= (@var{num} . @var{expr}) | (@var{op} @var{expr} ...)
4058 @var{op} ::= + | -
4059 @end group
4060 @end smallexample
4061
4062 The form @var{num} specifies a fraction of the default frame font
4063 height or width. The form @code{(@var{num})} specifies an absolute
4064 number of pixels. If @var{num} is a symbol, @var{symbol}, its
4065 buffer-local variable binding is used.
4066
4067 The @code{in}, @code{mm}, and @code{cm} units specify the number of
4068 pixels per inch, millimeter, and centimeter, respectively. The
4069 @code{width} and @code{height} units correspond to the default width
4070 and height of the current face. An image specification @code{image}
4071 corresponds to the width or height of the image.
4072
4073 The elements @code{left-fringe}, @code{right-fringe},
4074 @code{left-margin}, @code{right-margin}, @code{scroll-bar}, and
4075 @code{text} specify to the width of the corresponding area of the
4076 window.
4077
4078 The @code{left}, @code{center}, and @code{right} positions can be
4079 used with @code{:align-to} to specify a position relative to the left
4080 edge, center, or right edge of the text area.
4081
4082 Any of the above window elements (except @code{text}) can also be
4083 used with @code{:align-to} to specify that the position is relative to
4084 the left edge of the given area. Once the base offset for a relative
4085 position has been set (by the first occurrence of one of these
4086 symbols), further occurrences of these symbols are interpreted as the
4087 width of the specified area. For example, to align to the center of
4088 the left-margin, use
4089
4090 @example
4091 :align-to (+ left-margin (0.5 . left-margin))
4092 @end example
4093
4094 If no specific base offset is set for alignment, it is always relative
4095 to the left edge of the text area. For example, @samp{:align-to 0} in a
4096 header-line aligns with the first text column in the text area.
4097
4098 A value of the form @code{(@var{num} . @var{expr})} stands for the
4099 product of the values of @var{num} and @var{expr}. For example,
4100 @code{(2 . in)} specifies a width of 2 inches, while @code{(0.5 .
4101 @var{image})} specifies half the width (or height) of the specified
4102 image.
4103
4104 The form @code{(+ @var{expr} ...)} adds up the value of the
4105 expressions. The form @code{(- @var{expr} ...)} negates or subtracts
4106 the value of the expressions.
4107
4108 @node Other Display Specs
4109 @subsection Other Display Specifications
4110
4111 Here are the other sorts of display specifications that you can use
4112 in the @code{display} text property.
4113
4114 @table @code
4115 @item @var{string}
4116 Display @var{string} instead of the text that has this property.
4117
4118 Recursive display specifications are not supported---@var{string}'s
4119 @code{display} properties, if any, are not used.
4120
4121 @item (image . @var{image-props})
4122 This kind of display specification is an image descriptor (@pxref{Images}).
4123 When used as a display specification, it means to display the image
4124 instead of the text that has the display specification.
4125
4126 @item (slice @var{x} @var{y} @var{width} @var{height})
4127 This specification together with @code{image} specifies a @dfn{slice}
4128 (a partial area) of the image to display. The elements @var{y} and
4129 @var{x} specify the top left corner of the slice, within the image;
4130 @var{width} and @var{height} specify the width and height of the
4131 slice. Integer values are numbers of pixels. A floating point number
4132 in the range 0.0--1.0 stands for that fraction of the width or height
4133 of the entire image.
4134
4135 @item ((margin nil) @var{string})
4136 A display specification of this form means to display @var{string}
4137 instead of the text that has the display specification, at the same
4138 position as that text. It is equivalent to using just @var{string},
4139 but it is done as a special case of marginal display (@pxref{Display
4140 Margins}).
4141
4142 @item (left-fringe @var{bitmap} @r{[}@var{face}@r{]})
4143 @itemx (right-fringe @var{bitmap} @r{[}@var{face}@r{]})
4144 This display specification on any character of a line of text causes
4145 the specified @var{bitmap} be displayed in the left or right fringes
4146 for that line, instead of the characters that have the display
4147 specification. The optional @var{face} specifies the colors to be
4148 used for the bitmap. @xref{Fringe Bitmaps}, for the details.
4149
4150 @item (space-width @var{factor})
4151 This display specification affects all the space characters within the
4152 text that has the specification. It displays all of these spaces
4153 @var{factor} times as wide as normal. The element @var{factor} should
4154 be an integer or float. Characters other than spaces are not affected
4155 at all; in particular, this has no effect on tab characters.
4156
4157 @item (height @var{height})
4158 This display specification makes the text taller or shorter.
4159 Here are the possibilities for @var{height}:
4160
4161 @table @asis
4162 @item @code{(+ @var{n})}
4163 @c FIXME: Add an index for "step"? --xfq
4164 This means to use a font that is @var{n} steps larger. A ``step'' is
4165 defined by the set of available fonts---specifically, those that match
4166 what was otherwise specified for this text, in all attributes except
4167 height. Each size for which a suitable font is available counts as
4168 another step. @var{n} should be an integer.
4169
4170 @item @code{(- @var{n})}
4171 This means to use a font that is @var{n} steps smaller.
4172
4173 @item a number, @var{factor}
4174 A number, @var{factor}, means to use a font that is @var{factor} times
4175 as tall as the default font.
4176
4177 @item a symbol, @var{function}
4178 A symbol is a function to compute the height. It is called with the
4179 current height as argument, and should return the new height to use.
4180
4181 @item anything else, @var{form}
4182 If the @var{height} value doesn't fit the previous possibilities, it is
4183 a form. Emacs evaluates it to get the new height, with the symbol
4184 @code{height} bound to the current specified font height.
4185 @end table
4186
4187 @item (raise @var{factor})
4188 This kind of display specification raises or lowers the text
4189 it applies to, relative to the baseline of the line.
4190
4191 @var{factor} must be a number, which is interpreted as a multiple of the
4192 height of the affected text. If it is positive, that means to display
4193 the characters raised. If it is negative, that means to display them
4194 lower down.
4195
4196 If the text also has a @code{height} display specification, that does
4197 not affect the amount of raising or lowering, which is based on the
4198 faces used for the text.
4199 @end table
4200
4201 @c We put all the `@code{(when ...)}' on one line to encourage
4202 @c makeinfo's end-of-sentence heuristics to DTRT. Previously, the dot
4203 @c was at eol; the info file ended up w/ two spaces rendered after it.
4204 You can make any display specification conditional. To do that,
4205 package it in another list of the form
4206 @code{(when @var{condition} . @var{spec})}.
4207 Then the specification @var{spec} applies only when
4208 @var{condition} evaluates to a non-@code{nil} value. During the
4209 evaluation, @code{object} is bound to the string or buffer having the
4210 conditional @code{display} property. @code{position} and
4211 @code{buffer-position} are bound to the position within @code{object}
4212 and the buffer position where the @code{display} property was found,
4213 respectively. Both positions can be different when @code{object} is a
4214 string.
4215
4216 @node Display Margins
4217 @subsection Displaying in the Margins
4218 @cindex display margins
4219 @cindex margins, display
4220
4221 A buffer can have blank areas called @dfn{display margins} on the
4222 left and on the right. Ordinary text never appears in these areas,
4223 but you can put things into the display margins using the
4224 @code{display} property. There is currently no way to make text or
4225 images in the margin mouse-sensitive.
4226
4227 The way to display something in the margins is to specify it in a
4228 margin display specification in the @code{display} property of some
4229 text. This is a replacing display specification, meaning that the
4230 text you put it on does not get displayed; the margin display appears,
4231 but that text does not.
4232
4233 A margin display specification looks like @code{((margin
4234 right-margin) @var{spec})} or @code{((margin left-margin) @var{spec})}.
4235 Here, @var{spec} is another display specification that says what to
4236 display in the margin. Typically it is a string of text to display,
4237 or an image descriptor.
4238
4239 To display something in the margin @emph{in association with}
4240 certain buffer text, without altering or preventing the display of
4241 that text, put a @code{before-string} property on the text and put the
4242 margin display specification on the contents of the before-string.
4243
4244 Before the display margins can display anything, you must give
4245 them a nonzero width. The usual way to do that is to set these
4246 variables:
4247
4248 @defvar left-margin-width
4249 This variable specifies the width of the left margin, in character
4250 cell (a.k.a.@: ``column'') units. It is buffer-local in all buffers.
4251 A value of @code{nil} means no left marginal area.
4252 @end defvar
4253
4254 @defvar right-margin-width
4255 This variable specifies the width of the right margin, in character
4256 cell units. It is buffer-local in all buffers. A value of @code{nil}
4257 means no right marginal area.
4258 @end defvar
4259
4260 Setting these variables does not immediately affect the window. These
4261 variables are checked when a new buffer is displayed in the window.
4262 Thus, you can make changes take effect by calling
4263 @code{set-window-buffer}.
4264
4265 You can also set the margin widths immediately.
4266
4267 @defun set-window-margins window left &optional right
4268 This function specifies the margin widths for window @var{window}, in
4269 character cell units. The argument @var{left} controls the left
4270 margin, and @var{right} controls the right margin (default @code{0}).
4271 @end defun
4272
4273 @defun window-margins &optional window
4274 This function returns the width of the left and right margins of
4275 @var{window} as a cons cell of the form @w{@code{(@var{left}
4276 . @var{right})}}. If one of the two marginal areas does not exist,
4277 its width is returned as @code{nil}; if neither of the two margins exist,
4278 the function returns @code{(nil)}. If @var{window} is @code{nil}, the
4279 selected window is used.
4280 @end defun
4281
4282 @node Images
4283 @section Images
4284 @cindex images in buffers
4285
4286 To display an image in an Emacs buffer, you must first create an image
4287 descriptor, then use it as a display specifier in the @code{display}
4288 property of text that is displayed (@pxref{Display Property}).
4289
4290 Emacs is usually able to display images when it is run on a
4291 graphical terminal. Images cannot be displayed in a text terminal, on
4292 certain graphical terminals that lack the support for this, or if
4293 Emacs is compiled without image support. You can use the function
4294 @code{display-images-p} to determine if images can in principle be
4295 displayed (@pxref{Display Feature Testing}).
4296
4297 @menu
4298 * Image Formats:: Supported image formats.
4299 * Image Descriptors:: How to specify an image for use in @code{:display}.
4300 * XBM Images:: Special features for XBM format.
4301 * XPM Images:: Special features for XPM format.
4302 * PostScript Images:: Special features for PostScript format.
4303 * ImageMagick Images:: Special features available through ImageMagick.
4304 * Other Image Types:: Various other formats are supported.
4305 * Defining Images:: Convenient ways to define an image for later use.
4306 * Showing Images:: Convenient ways to display an image once it is defined.
4307 * Multi-Frame Images:: Some images contain more than one frame.
4308 * Image Cache:: Internal mechanisms of image display.
4309 @end menu
4310
4311 @node Image Formats
4312 @subsection Image Formats
4313 @cindex image formats
4314 @cindex image types
4315
4316 Emacs can display a number of different image formats. Some of
4317 these image formats are supported only if particular support libraries
4318 are installed. On some platforms, Emacs can load support libraries on
4319 demand; if so, the variable @code{dynamic-library-alist} can be used
4320 to modify the set of known names for these dynamic libraries.
4321 @xref{Dynamic Libraries}.
4322
4323 Supported image formats (and the required support libraries) include
4324 PBM and XBM (which do not depend on support libraries and are always
4325 available), XPM (@code{libXpm}), GIF (@code{libgif} or
4326 @code{libungif}), PostScript (@code{gs}), JPEG (@code{libjpeg}), TIFF
4327 (@code{libtiff}), PNG (@code{libpng}), and SVG (@code{librsvg}).
4328
4329 Each of these image formats is associated with an @dfn{image type
4330 symbol}. The symbols for the above formats are, respectively,
4331 @code{pbm}, @code{xbm}, @code{xpm}, @code{gif}, @code{postscript},
4332 @code{jpeg}, @code{tiff}, @code{png}, and @code{svg}.
4333
4334 Furthermore, if you build Emacs with ImageMagick
4335 (@code{libMagickWand}) support, Emacs can display any image format
4336 that ImageMagick can. @xref{ImageMagick Images}. All images
4337 displayed via ImageMagick have type symbol @code{imagemagick}.
4338
4339 @defvar image-types
4340 This variable contains a list of type symbols for image formats which
4341 are potentially supported in the current configuration.
4342
4343 ``Potentially'' means that Emacs knows about the image types, not
4344 necessarily that they can be used (for example, they could depend on
4345 unavailable dynamic libraries). To know which image types are really
4346 available, use @code{image-type-available-p}.
4347 @end defvar
4348
4349 @defun image-type-available-p type
4350 This function returns non-@code{nil} if images of type @var{type} can
4351 be loaded and displayed. @var{type} must be an image type symbol.
4352
4353 For image types whose support libraries are statically linked, this
4354 function always returns @code{t}. For image types whose support
4355 libraries are dynamically loaded, it returns @code{t} if the library
4356 could be loaded and @code{nil} otherwise.
4357 @end defun
4358
4359 @node Image Descriptors
4360 @subsection Image Descriptors
4361 @cindex image descriptor
4362
4363 An @dfn{image descriptor} is a list which specifies the underlying
4364 data for an image, and how to display it. It is typically used as the
4365 value of a @code{display} overlay or text property (@pxref{Other
4366 Display Specs}); but @xref{Showing Images}, for convenient helper
4367 functions to insert images into buffers.
4368
4369 Each image descriptor has the form @code{(image . @var{props})},
4370 where @var{props} is a property list of alternating keyword symbols
4371 and values, including at least the pair @code{:type @var{TYPE}} which
4372 specifies the image type.
4373
4374 The following is a list of properties that are meaningful for all
4375 image types (there are also properties which are meaningful only for
4376 certain image types, as documented in the following subsections):
4377
4378 @table @code
4379 @item :type @var{type}
4380 The image type.
4381 @ifnottex
4382 @xref{Image Formats}.
4383 @end ifnottex
4384 Every image descriptor must include this property.
4385
4386 @item :file @var{file}
4387 This says to load the image from file @var{file}. If @var{file} is
4388 not an absolute file name, it is expanded in @code{data-directory}.
4389
4390 @item :data @var{data}
4391 This specifies the raw image data. Each image descriptor must have
4392 either @code{:data} or @code{:file}, but not both.
4393
4394 For most image types, the value of a @code{:data} property should be a
4395 string containing the image data. Some image types do not support
4396 @code{:data}; for some others, @code{:data} alone is not enough, so
4397 you need to use other image properties along with @code{:data}. See
4398 the following subsections for details.
4399
4400 @item :margin @var{margin}
4401 This specifies how many pixels to add as an extra margin around the
4402 image. The value, @var{margin}, must be a non-negative number, or a
4403 pair @code{(@var{x} . @var{y})} of such numbers. If it is a pair,
4404 @var{x} specifies how many pixels to add horizontally, and @var{y}
4405 specifies how many pixels to add vertically. If @code{:margin} is not
4406 specified, the default is zero.
4407
4408 @item :ascent @var{ascent}
4409 This specifies the amount of the image's height to use for its
4410 ascent---that is, the part above the baseline. The value,
4411 @var{ascent}, must be a number in the range 0 to 100, or the symbol
4412 @code{center}.
4413
4414 If @var{ascent} is a number, that percentage of the image's height is
4415 used for its ascent.
4416
4417 If @var{ascent} is @code{center}, the image is vertically centered
4418 around a centerline which would be the vertical centerline of text drawn
4419 at the position of the image, in the manner specified by the text
4420 properties and overlays that apply to the image.
4421
4422 If this property is omitted, it defaults to 50.
4423
4424 @item :relief @var{relief}
4425 This adds a shadow rectangle around the image. The value,
4426 @var{relief}, specifies the width of the shadow lines, in pixels. If
4427 @var{relief} is negative, shadows are drawn so that the image appears
4428 as a pressed button; otherwise, it appears as an unpressed button.
4429
4430 @item :conversion @var{algorithm}
4431 This specifies a conversion algorithm that should be applied to the
4432 image before it is displayed; the value, @var{algorithm}, specifies
4433 which algorithm.
4434
4435 @table @code
4436 @item laplace
4437 @itemx emboss
4438 Specifies the Laplace edge detection algorithm, which blurs out small
4439 differences in color while highlighting larger differences. People
4440 sometimes consider this useful for displaying the image for a
4441 ``disabled'' button.
4442
4443 @item (edge-detection :matrix @var{matrix} :color-adjust @var{adjust})
4444 @cindex edge detection, images
4445 Specifies a general edge-detection algorithm. @var{matrix} must be
4446 either a nine-element list or a nine-element vector of numbers. A pixel
4447 at position @math{x/y} in the transformed image is computed from
4448 original pixels around that position. @var{matrix} specifies, for each
4449 pixel in the neighborhood of @math{x/y}, a factor with which that pixel
4450 will influence the transformed pixel; element @math{0} specifies the
4451 factor for the pixel at @math{x-1/y-1}, element @math{1} the factor for
4452 the pixel at @math{x/y-1} etc., as shown below:
4453 @iftex
4454 @tex
4455 $$\pmatrix{x-1/y-1 & x/y-1 & x+1/y-1 \cr
4456 x-1/y & x/y & x+1/y \cr
4457 x-1/y+1& x/y+1 & x+1/y+1 \cr}$$
4458 @end tex
4459 @end iftex
4460 @ifnottex
4461 @display
4462 (x-1/y-1 x/y-1 x+1/y-1
4463 x-1/y x/y x+1/y
4464 x-1/y+1 x/y+1 x+1/y+1)
4465 @end display
4466 @end ifnottex
4467
4468 The resulting pixel is computed from the color intensity of the color
4469 resulting from summing up the RGB values of surrounding pixels,
4470 multiplied by the specified factors, and dividing that sum by the sum
4471 of the factors' absolute values.
4472
4473 Laplace edge-detection currently uses a matrix of
4474 @iftex
4475 @tex
4476 $$\pmatrix{1 & 0 & 0 \cr
4477 0& 0 & 0 \cr
4478 0 & 0 & -1 \cr}$$
4479 @end tex
4480 @end iftex
4481 @ifnottex
4482 @display
4483 (1 0 0
4484 0 0 0
4485 0 0 -1)
4486 @end display
4487 @end ifnottex
4488
4489 Emboss edge-detection uses a matrix of
4490 @iftex
4491 @tex
4492 $$\pmatrix{ 2 & -1 & 0 \cr
4493 -1 & 0 & 1 \cr
4494 0 & 1 & -2 \cr}$$
4495 @end tex
4496 @end iftex
4497 @ifnottex
4498 @display
4499 ( 2 -1 0
4500 -1 0 1
4501 0 1 -2)
4502 @end display
4503 @end ifnottex
4504
4505 @item disabled
4506 Specifies transforming the image so that it looks ``disabled''.
4507 @end table
4508
4509 @item :mask @var{mask}
4510 If @var{mask} is @code{heuristic} or @code{(heuristic @var{bg})}, build
4511 a clipping mask for the image, so that the background of a frame is
4512 visible behind the image. If @var{bg} is not specified, or if @var{bg}
4513 is @code{t}, determine the background color of the image by looking at
4514 the four corners of the image, assuming the most frequently occurring
4515 color from the corners is the background color of the image. Otherwise,
4516 @var{bg} must be a list @code{(@var{red} @var{green} @var{blue})}
4517 specifying the color to assume for the background of the image.
4518
4519 If @var{mask} is @code{nil}, remove a mask from the image, if it has
4520 one. Images in some formats include a mask which can be removed by
4521 specifying @code{:mask nil}.
4522
4523 @item :pointer @var{shape}
4524 This specifies the pointer shape when the mouse pointer is over this
4525 image. @xref{Pointer Shape}, for available pointer shapes.
4526
4527 @item :map @var{map}
4528 @cindex image maps
4529 This associates an image map of @dfn{hot spots} with this image.
4530
4531 An image map is an alist where each element has the format
4532 @code{(@var{area} @var{id} @var{plist})}. An @var{area} is specified
4533 as either a rectangle, a circle, or a polygon.
4534
4535 A rectangle is a cons
4536 @code{(rect . ((@var{x0} . @var{y0}) . (@var{x1} . @var{y1})))}
4537 which specifies the pixel coordinates of the upper left and bottom right
4538 corners of the rectangle area.
4539
4540 A circle is a cons
4541 @code{(circle . ((@var{x0} . @var{y0}) . @var{r}))}
4542 which specifies the center and the radius of the circle; @var{r} may
4543 be a float or integer.
4544
4545 A polygon is a cons
4546 @code{(poly . [@var{x0} @var{y0} @var{x1} @var{y1} ...])}
4547 where each pair in the vector describes one corner in the polygon.
4548
4549 When the mouse pointer lies on a hot-spot area of an image, the
4550 @var{plist} of that hot-spot is consulted; if it contains a @code{help-echo}
4551 property, that defines a tool-tip for the hot-spot, and if it contains
4552 a @code{pointer} property, that defines the shape of the mouse cursor when
4553 it is on the hot-spot.
4554 @xref{Pointer Shape}, for available pointer shapes.
4555
4556 When you click the mouse when the mouse pointer is over a hot-spot, an
4557 event is composed by combining the @var{id} of the hot-spot with the
4558 mouse event; for instance, @code{[area4 mouse-1]} if the hot-spot's
4559 @var{id} is @code{area4}.
4560 @end table
4561
4562 @defun image-mask-p spec &optional frame
4563 This function returns @code{t} if image @var{spec} has a mask bitmap.
4564 @var{frame} is the frame on which the image will be displayed.
4565 @var{frame} @code{nil} or omitted means to use the selected frame
4566 (@pxref{Input Focus}).
4567 @end defun
4568
4569 @node XBM Images
4570 @subsection XBM Images
4571 @cindex XBM
4572
4573 To use XBM format, specify @code{xbm} as the image type. This image
4574 format doesn't require an external library, so images of this type are
4575 always supported.
4576
4577 Additional image properties supported for the @code{xbm} image type are:
4578
4579 @table @code
4580 @item :foreground @var{foreground}
4581 The value, @var{foreground}, should be a string specifying the image
4582 foreground color, or @code{nil} for the default color. This color is
4583 used for each pixel in the XBM that is 1. The default is the frame's
4584 foreground color.
4585
4586 @item :background @var{background}
4587 The value, @var{background}, should be a string specifying the image
4588 background color, or @code{nil} for the default color. This color is
4589 used for each pixel in the XBM that is 0. The default is the frame's
4590 background color.
4591 @end table
4592
4593 If you specify an XBM image using data within Emacs instead of an
4594 external file, use the following three properties:
4595
4596 @table @code
4597 @item :data @var{data}
4598 The value, @var{data}, specifies the contents of the image.
4599 There are three formats you can use for @var{data}:
4600
4601 @itemize @bullet
4602 @item
4603 A vector of strings or bool-vectors, each specifying one line of the
4604 image. Do specify @code{:height} and @code{:width}.
4605
4606 @item
4607 A string containing the same byte sequence as an XBM file would contain.
4608 You must not specify @code{:height} and @code{:width} in this case,
4609 because omitting them is what indicates the data has the format of an
4610 XBM file. The file contents specify the height and width of the image.
4611
4612 @item
4613 A string or a bool-vector containing the bits of the image (plus perhaps
4614 some extra bits at the end that will not be used). It should contain at
4615 least @var{width} * @code{height} bits. In this case, you must specify
4616 @code{:height} and @code{:width}, both to indicate that the string
4617 contains just the bits rather than a whole XBM file, and to specify the
4618 size of the image.
4619 @end itemize
4620
4621 @item :width @var{width}
4622 The value, @var{width}, specifies the width of the image, in pixels.
4623
4624 @item :height @var{height}
4625 The value, @var{height}, specifies the height of the image, in pixels.
4626 @end table
4627
4628 @node XPM Images
4629 @subsection XPM Images
4630 @cindex XPM
4631
4632 To use XPM format, specify @code{xpm} as the image type. The
4633 additional image property @code{:color-symbols} is also meaningful with
4634 the @code{xpm} image type:
4635
4636 @table @code
4637 @item :color-symbols @var{symbols}
4638 The value, @var{symbols}, should be an alist whose elements have the
4639 form @code{(@var{name} . @var{color})}. In each element, @var{name} is
4640 the name of a color as it appears in the image file, and @var{color}
4641 specifies the actual color to use for displaying that name.
4642 @end table
4643
4644 @node PostScript Images
4645 @subsection PostScript Images
4646 @cindex postscript images
4647
4648 To use PostScript for an image, specify image type @code{postscript}.
4649 This works only if you have Ghostscript installed. You must always use
4650 these three properties:
4651
4652 @table @code
4653 @item :pt-width @var{width}
4654 The value, @var{width}, specifies the width of the image measured in
4655 points (1/72 inch). @var{width} must be an integer.
4656
4657 @item :pt-height @var{height}
4658 The value, @var{height}, specifies the height of the image in points
4659 (1/72 inch). @var{height} must be an integer.
4660
4661 @item :bounding-box @var{box}
4662 The value, @var{box}, must be a list or vector of four integers, which
4663 specifying the bounding box of the PostScript image, analogous to the
4664 @samp{BoundingBox} comment found in PostScript files.
4665
4666 @example
4667 %%BoundingBox: 22 171 567 738
4668 @end example
4669 @end table
4670
4671 @node ImageMagick Images
4672 @subsection ImageMagick Images
4673 @cindex ImageMagick images
4674 @cindex images, support for more formats
4675
4676 If you build Emacs with ImageMagick support, you can use the
4677 ImageMagick library to load many image formats (@pxref{File
4678 Conveniences,,, emacs, The GNU Emacs Manual}). The image type symbol
4679 for images loaded via ImageMagick is @code{imagemagick}, regardless of
4680 the actual underlying image format.
4681
4682 @defun imagemagick-types
4683 This function returns a list of image file extensions supported by the
4684 current ImageMagick installation. Each list element is a symbol
4685 representing an internal ImageMagick name for an image type, such as
4686 @code{BMP} for @file{.bmp} images.
4687 @end defun
4688
4689 @defopt imagemagick-enabled-types
4690 The value of this variable is a list of ImageMagick image types which
4691 Emacs may attempt to render using ImageMagick. Each list element
4692 should be one of the symbols in the list returned by
4693 @code{imagemagick-types}, or an equivalent string. Alternatively, a
4694 value of @code{t} enables ImageMagick for all possible image types.
4695 Regardless of the value of this variable,
4696 @code{imagemagick-types-inhibit} (see below) takes precedence.
4697 @end defopt
4698
4699 @defopt imagemagick-types-inhibit
4700 The value of this variable lists the ImageMagick image types which
4701 should never be rendered using ImageMagick, regardless of the value of
4702 @code{imagemagick-enabled-types}. A value of @code{t} disables
4703 ImageMagick entirely.
4704 @end defopt
4705
4706 @defvar image-format-suffixes
4707 This variable is an alist mapping image types to file name extensions.
4708 Emacs uses this in conjunction with the @code{:format} image property
4709 (see below) to give a hint to the ImageMagick library as to the type
4710 of an image. Each element has the form @code{(@var{type}
4711 @var{extension})}, where @var{type} is a symbol specifying an image
4712 content-type, and @var{extension} is a string that specifies the
4713 associated file name extension.
4714 @end defvar
4715
4716 Images loaded with ImageMagick support the following additional
4717 image descriptor properties:
4718
4719 @table @code
4720 @item :background @var{background}
4721 @var{background}, if non-@code{nil}, should be a string specifying a
4722 color, which is used as the image's background color if the image
4723 supports transparency. If the value is @code{nil}, it defaults to the
4724 frame's background color.
4725
4726 @item :width @var{width}, :height @var{height}
4727 The @code{:width} and @code{:height} keywords are used for scaling the
4728 image. If only one of them is specified, the other one will be
4729 calculated so as to preserve the aspect ratio. If both are specified,
4730 aspect ratio may not be preserved.
4731
4732 @item :max-width @var{max-width}, :max-height @var{max-height}
4733 The @code{:max-width} and @code{:max-height} keywords are used for
4734 scaling if the size of the image of the image exceeds these values.
4735 If @code{:width} is set it will have precedence over @code{max-width},
4736 and if @code{:height} is set it will have precedence over
4737 @code{max-height}, but you can otherwise mix these keywords as you
4738 wish. @code{:max-width} and @code{:max-height} will always preserve
4739 the aspect ratio.
4740
4741 @item :format @var{type}
4742 The value, @var{type}, should be a symbol specifying the type of the
4743 image data, as found in @code{image-format-suffixes}. This is used
4744 when the image does not have an associated file name, to provide a
4745 hint to ImageMagick to help it detect the image type.
4746
4747 @item :rotation @var{angle}
4748 Specifies a rotation angle in degrees.
4749
4750 @item :index @var{frame}
4751 @c Doesn't work: http://debbugs.gnu.org/7978
4752 @xref{Multi-Frame Images}.
4753 @end table
4754
4755 @node Other Image Types
4756 @subsection Other Image Types
4757 @cindex PBM
4758
4759 For PBM images, specify image type @code{pbm}. Color, gray-scale and
4760 monochromatic images are supported. For mono PBM images, two additional
4761 image properties are supported.
4762
4763 @table @code
4764 @item :foreground @var{foreground}
4765 The value, @var{foreground}, should be a string specifying the image
4766 foreground color, or @code{nil} for the default color. This color is
4767 used for each pixel in the PBM that is 1. The default is the frame's
4768 foreground color.
4769
4770 @item :background @var{background}
4771 The value, @var{background}, should be a string specifying the image
4772 background color, or @code{nil} for the default color. This color is
4773 used for each pixel in the PBM that is 0. The default is the frame's
4774 background color.
4775 @end table
4776
4777 @noindent
4778 The remaining image types that Emacs can support are:
4779
4780 @table @asis
4781 @item GIF
4782 Image type @code{gif}.
4783 Supports the @code{:index} property. @xref{Multi-Frame Images}.
4784
4785 @item JPEG
4786 Image type @code{jpeg}.
4787
4788 @item PNG
4789 Image type @code{png}.
4790
4791 @item SVG
4792 Image type @code{svg}.
4793
4794 @item TIFF
4795 Image type @code{tiff}.
4796 Supports the @code{:index} property. @xref{Multi-Frame Images}.
4797 @end table
4798
4799 @node Defining Images
4800 @subsection Defining Images
4801
4802 The functions @code{create-image}, @code{defimage} and
4803 @code{find-image} provide convenient ways to create image descriptors.
4804
4805 @defun create-image file-or-data &optional type data-p &rest props
4806 This function creates and returns an image descriptor which uses the
4807 data in @var{file-or-data}. @var{file-or-data} can be a file name or
4808 a string containing the image data; @var{data-p} should be @code{nil}
4809 for the former case, non-@code{nil} for the latter case.
4810
4811 The optional argument @var{type} is a symbol specifying the image type.
4812 If @var{type} is omitted or @code{nil}, @code{create-image} tries to
4813 determine the image type from the file's first few bytes, or else
4814 from the file's name.
4815
4816 The remaining arguments, @var{props}, specify additional image
4817 properties---for example,
4818
4819 @c ':heuristic-mask' is not documented?
4820 @example
4821 (create-image "foo.xpm" 'xpm nil :heuristic-mask t)
4822 @end example
4823
4824 The function returns @code{nil} if images of this type are not
4825 supported. Otherwise it returns an image descriptor.
4826 @end defun
4827
4828 @defmac defimage symbol specs &optional doc
4829 This macro defines @var{symbol} as an image name. The arguments
4830 @var{specs} is a list which specifies how to display the image.
4831 The third argument, @var{doc}, is an optional documentation string.
4832
4833 Each argument in @var{specs} has the form of a property list, and each
4834 one should specify at least the @code{:type} property and either the
4835 @code{:file} or the @code{:data} property. The value of @code{:type}
4836 should be a symbol specifying the image type, the value of
4837 @code{:file} is the file to load the image from, and the value of
4838 @code{:data} is a string containing the actual image data. Here is an
4839 example:
4840
4841 @example
4842 (defimage test-image
4843 ((:type xpm :file "~/test1.xpm")
4844 (:type xbm :file "~/test1.xbm")))
4845 @end example
4846
4847 @code{defimage} tests each argument, one by one, to see if it is
4848 usable---that is, if the type is supported and the file exists. The
4849 first usable argument is used to make an image descriptor which is
4850 stored in @var{symbol}.
4851
4852 If none of the alternatives will work, then @var{symbol} is defined
4853 as @code{nil}.
4854 @end defmac
4855
4856 @defun find-image specs
4857 This function provides a convenient way to find an image satisfying one
4858 of a list of image specifications @var{specs}.
4859
4860 Each specification in @var{specs} is a property list with contents
4861 depending on image type. All specifications must at least contain the
4862 properties @code{:type @var{type}} and either @w{@code{:file @var{file}}}
4863 or @w{@code{:data @var{DATA}}}, where @var{type} is a symbol specifying
4864 the image type, e.g., @code{xbm}, @var{file} is the file to load the
4865 image from, and @var{data} is a string containing the actual image data.
4866 The first specification in the list whose @var{type} is supported, and
4867 @var{file} exists, is used to construct the image specification to be
4868 returned. If no specification is satisfied, @code{nil} is returned.
4869
4870 The image is looked for in @code{image-load-path}.
4871 @end defun
4872
4873 @defvar image-load-path
4874 This variable's value is a list of locations in which to search for
4875 image files. If an element is a string or a variable symbol whose
4876 value is a string, the string is taken to be the name of a directory
4877 to search. If an element is a variable symbol whose value is a list,
4878 that is taken to be a list of directory names to search.
4879
4880 The default is to search in the @file{images} subdirectory of the
4881 directory specified by @code{data-directory}, then the directory
4882 specified by @code{data-directory}, and finally in the directories in
4883 @code{load-path}. Subdirectories are not automatically included in
4884 the search, so if you put an image file in a subdirectory, you have to
4885 supply the subdirectory name explicitly. For example, to find the
4886 image @file{images/foo/bar.xpm} within @code{data-directory}, you
4887 should specify the image as follows:
4888
4889 @example
4890 (defimage foo-image '((:type xpm :file "foo/bar.xpm")))
4891 @end example
4892 @end defvar
4893
4894 @defun image-load-path-for-library library image &optional path no-error
4895 This function returns a suitable search path for images used by the
4896 Lisp package @var{library}.
4897
4898 The function searches for @var{image} first using @code{image-load-path},
4899 excluding @file{@code{data-directory}/images}, and then in
4900 @code{load-path}, followed by a path suitable for @var{library}, which
4901 includes @file{../../etc/images} and @file{../etc/images} relative to
4902 the library file itself, and finally in
4903 @file{@code{data-directory}/images}.
4904
4905 Then this function returns a list of directories which contains first
4906 the directory in which @var{image} was found, followed by the value of
4907 @code{load-path}. If @var{path} is given, it is used instead of
4908 @code{load-path}.
4909
4910 If @var{no-error} is non-@code{nil} and a suitable path can't be
4911 found, don't signal an error. Instead, return a list of directories as
4912 before, except that @code{nil} appears in place of the image directory.
4913
4914 Here is an example of using @code{image-load-path-for-library}:
4915
4916 @example
4917 (defvar image-load-path) ; shush compiler
4918 (let* ((load-path (image-load-path-for-library
4919 "mh-e" "mh-logo.xpm"))
4920 (image-load-path (cons (car load-path)
4921 image-load-path)))
4922 (mh-tool-bar-folder-buttons-init))
4923 @end example
4924 @end defun
4925
4926 @node Showing Images
4927 @subsection Showing Images
4928
4929 You can use an image descriptor by setting up the @code{display}
4930 property yourself, but it is easier to use the functions in this
4931 section.
4932
4933 @defun insert-image image &optional string area slice
4934 This function inserts @var{image} in the current buffer at point. The
4935 value @var{image} should be an image descriptor; it could be a value
4936 returned by @code{create-image}, or the value of a symbol defined with
4937 @code{defimage}. The argument @var{string} specifies the text to put
4938 in the buffer to hold the image. If it is omitted or @code{nil},
4939 @code{insert-image} uses @code{" "} by default.
4940
4941 The argument @var{area} specifies whether to put the image in a margin.
4942 If it is @code{left-margin}, the image appears in the left margin;
4943 @code{right-margin} specifies the right margin. If @var{area} is
4944 @code{nil} or omitted, the image is displayed at point within the
4945 buffer's text.
4946
4947 The argument @var{slice} specifies a slice of the image to insert. If
4948 @var{slice} is @code{nil} or omitted the whole image is inserted.
4949 Otherwise, @var{slice} is a list @code{(@var{x} @var{y} @var{width}
4950 @var{height})} which specifies the @var{x} and @var{y} positions and
4951 @var{width} and @var{height} of the image area to insert. Integer
4952 values are in units of pixels. A floating point number in the range
4953 0.0--1.0 stands for that fraction of the width or height of the entire
4954 image.
4955
4956 Internally, this function inserts @var{string} in the buffer, and gives
4957 it a @code{display} property which specifies @var{image}. @xref{Display
4958 Property}.
4959 @end defun
4960
4961 @cindex slice, image
4962 @cindex image slice
4963 @defun insert-sliced-image image &optional string area rows cols
4964 This function inserts @var{image} in the current buffer at point, like
4965 @code{insert-image}, but splits the image into @var{rows}x@var{cols}
4966 equally sized slices.
4967
4968 If an image is inserted ``sliced'', Emacs displays each slice as a
4969 separate image, and allow more intuitive scrolling up/down, instead of
4970 jumping up/down the entire image when paging through a buffer that
4971 displays (large) images.
4972 @end defun
4973
4974 @defun put-image image pos &optional string area
4975 This function puts image @var{image} in front of @var{pos} in the
4976 current buffer. The argument @var{pos} should be an integer or a
4977 marker. It specifies the buffer position where the image should appear.
4978 The argument @var{string} specifies the text that should hold the image
4979 as an alternative to the default.
4980
4981 The argument @var{image} must be an image descriptor, perhaps returned
4982 by @code{create-image} or stored by @code{defimage}.
4983
4984 The argument @var{area} specifies whether to put the image in a margin.
4985 If it is @code{left-margin}, the image appears in the left margin;
4986 @code{right-margin} specifies the right margin. If @var{area} is
4987 @code{nil} or omitted, the image is displayed at point within the
4988 buffer's text.
4989
4990 Internally, this function creates an overlay, and gives it a
4991 @code{before-string} property containing text that has a @code{display}
4992 property whose value is the image. (Whew!)
4993 @end defun
4994
4995 @defun remove-images start end &optional buffer
4996 This function removes images in @var{buffer} between positions
4997 @var{start} and @var{end}. If @var{buffer} is omitted or @code{nil},
4998 images are removed from the current buffer.
4999
5000 This removes only images that were put into @var{buffer} the way
5001 @code{put-image} does it, not images that were inserted with
5002 @code{insert-image} or in other ways.
5003 @end defun
5004
5005 @defun image-size spec &optional pixels frame
5006 @cindex size of image
5007 This function returns the size of an image as a pair
5008 @w{@code{(@var{width} . @var{height})}}. @var{spec} is an image
5009 specification. @var{pixels} non-@code{nil} means return sizes
5010 measured in pixels, otherwise return sizes measured in canonical
5011 character units (fractions of the width/height of the frame's default
5012 font). @var{frame} is the frame on which the image will be displayed.
5013 @var{frame} null or omitted means use the selected frame (@pxref{Input
5014 Focus}).
5015 @end defun
5016
5017 @defvar max-image-size
5018 This variable is used to define the maximum size of image that Emacs
5019 will load. Emacs will refuse to load (and display) any image that is
5020 larger than this limit.
5021
5022 If the value is an integer, it directly specifies the maximum
5023 image height and width, measured in pixels. If it is a floating
5024 point number, it specifies the maximum image height and width
5025 as a ratio to the frame height and width. If the value is
5026 non-numeric, there is no explicit limit on the size of images.
5027
5028 The purpose of this variable is to prevent unreasonably large images
5029 from accidentally being loaded into Emacs. It only takes effect the
5030 first time an image is loaded. Once an image is placed in the image
5031 cache, it can always be displayed, even if the value of
5032 @code{max-image-size} is subsequently changed (@pxref{Image Cache}).
5033 @end defvar
5034
5035 @node Multi-Frame Images
5036 @subsection Multi-Frame Images
5037 @cindex multi-frame images
5038
5039 @cindex animation
5040 @cindex image animation
5041 @cindex image frames
5042 Some image files can contain more than one image. We say that there
5043 are multiple ``frames'' in the image. At present, Emacs supports
5044 multiple frames for GIF, TIFF, and certain ImageMagick formats such as
5045 DJVM@.
5046
5047 The frames can be used either to represent multiple ``pages'' (this is
5048 usually the case with multi-frame TIFF files, for example), or to
5049 create animation (usually the case with multi-frame GIF files).
5050
5051 A multi-frame image has a property @code{:index}, whose value is an
5052 integer (counting from 0) that specifies which frame is being displayed.
5053
5054 @defun image-multi-frame-p image
5055 This function returns non-@code{nil} if @var{image} contains more than
5056 one frame. The actual return value is a cons @code{(@var{nimages}
5057 . @var{delay})}, where @var{nimages} is the number of frames and
5058 @var{delay} is the delay in seconds between them, or @code{nil}
5059 if the image does not specify a delay. Images that are intended to be
5060 animated usually specify a frame delay, whereas ones that are intended
5061 to be treated as multiple pages do not.
5062 @end defun
5063
5064 @defun image-current-frame image
5065 This function returns the index of the current frame number for
5066 @var{image}, counting from 0.
5067 @end defun
5068
5069 @defun image-show-frame image n &optional nocheck
5070 This function switches @var{image} to frame number @var{n}. It
5071 replaces a frame number outside the valid range with that of the end
5072 of the range, unless @var{nocheck} is non-@code{nil}. If @var{image}
5073 does not contain a frame with the specified number, the image displays
5074 as a hollow box.
5075 @end defun
5076
5077 @defun image-animate image &optional index limit
5078 This function animates @var{image}. The optional integer @var{index}
5079 specifies the frame from which to start (default 0). The optional
5080 argument @var{limit} controls the length of the animation. If omitted
5081 or @code{nil}, the image animates once only; if @code{t} it loops
5082 forever; if a number animation stops after that many seconds.
5083 @end defun
5084
5085 @vindex image-minimum-frame-delay
5086 @vindex image-default-frame-delay
5087 @noindent Animation operates by means of a timer. Note that Emacs imposes a
5088 minimum frame delay of 0.01 (@code{image-minimum-frame-delay}) seconds.
5089 If the image itself does not specify a delay, Emacs uses
5090 @code{image-default-frame-delay}.
5091
5092 @defun image-animate-timer image
5093 This function returns the timer responsible for animating @var{image},
5094 if there is one.
5095 @end defun
5096
5097
5098 @node Image Cache
5099 @subsection Image Cache
5100 @cindex image cache
5101
5102 Emacs caches images so that it can display them again more
5103 efficiently. When Emacs displays an image, it searches the image
5104 cache for an existing image specification @code{equal} to the desired
5105 specification. If a match is found, the image is displayed from the
5106 cache. Otherwise, Emacs loads the image normally.
5107
5108 @defun image-flush spec &optional frame
5109 This function removes the image with specification @var{spec} from the
5110 image cache of frame @var{frame}. Image specifications are compared
5111 using @code{equal}. If @var{frame} is @code{nil}, it defaults to the
5112 selected frame. If @var{frame} is @code{t}, the image is flushed on
5113 all existing frames.
5114
5115 In Emacs's current implementation, each graphical terminal possesses an
5116 image cache, which is shared by all the frames on that terminal
5117 (@pxref{Multiple Terminals}). Thus, refreshing an image in one frame
5118 also refreshes it in all other frames on the same terminal.
5119 @end defun
5120
5121 One use for @code{image-flush} is to tell Emacs about a change in an
5122 image file. If an image specification contains a @code{:file}
5123 property, the image is cached based on the file's contents when the
5124 image is first displayed. Even if the file subsequently changes,
5125 Emacs continues displaying the old version of the image. Calling
5126 @code{image-flush} flushes the image from the cache, forcing Emacs to
5127 re-read the file the next time it needs to display that image.
5128
5129 Another use for @code{image-flush} is for memory conservation. If
5130 your Lisp program creates a large number of temporary images over a
5131 period much shorter than @code{image-cache-eviction-delay} (see
5132 below), you can opt to flush unused images yourself, instead of
5133 waiting for Emacs to do it automatically.
5134
5135 @defun clear-image-cache &optional filter
5136 This function clears an image cache, removing all the images stored in
5137 it. If @var{filter} is omitted or @code{nil}, it clears the cache for
5138 the selected frame. If @var{filter} is a frame, it clears the cache
5139 for that frame. If @var{filter} is @code{t}, all image caches are
5140 cleared. Otherwise, @var{filter} is taken to be a file name, and all
5141 images associated with that file name are removed from all image
5142 caches.
5143 @end defun
5144
5145 If an image in the image cache has not been displayed for a specified
5146 period of time, Emacs removes it from the cache and frees the
5147 associated memory.
5148
5149 @defvar image-cache-eviction-delay
5150 This variable specifies the number of seconds an image can remain in
5151 the cache without being displayed. When an image is not displayed for
5152 this length of time, Emacs removes it from the image cache.
5153
5154 Under some circumstances, if the number of images in the cache grows
5155 too large, the actual eviction delay may be shorter than this.
5156
5157 If the value is @code{nil}, Emacs does not remove images from the cache
5158 except when you explicitly clear it. This mode can be useful for
5159 debugging.
5160 @end defvar
5161
5162 @node Buttons
5163 @section Buttons
5164 @cindex buttons in buffers
5165 @cindex clickable buttons in buffers
5166
5167 The Button package defines functions for inserting and manipulating
5168 @dfn{buttons} that can be activated with the mouse or via keyboard
5169 commands. These buttons are typically used for various kinds of
5170 hyperlinks.
5171
5172 A button is essentially a set of text or overlay properties,
5173 attached to a stretch of text in a buffer. These properties are
5174 called @dfn{button properties}. One of these properties, the
5175 @dfn{action property}, specifies a function which is called when the
5176 user invokes the button using the keyboard or the mouse. The action
5177 function may examine the button and use its other properties as
5178 desired.
5179
5180 In some ways, the Button package duplicates the functionality in the
5181 Widget package. @xref{Top, , Introduction, widget, The Emacs Widget
5182 Library}. The advantage of the Button package is that it is faster,
5183 smaller, and simpler to program. From the point of view of the user,
5184 the interfaces produced by the two packages are very similar.
5185
5186 @menu
5187 * Button Properties:: Button properties with special meanings.
5188 * Button Types:: Defining common properties for classes of buttons.
5189 * Making Buttons:: Adding buttons to Emacs buffers.
5190 * Manipulating Buttons:: Getting and setting properties of buttons.
5191 * Button Buffer Commands:: Buffer-wide commands and bindings for buttons.
5192 @end menu
5193
5194 @node Button Properties
5195 @subsection Button Properties
5196 @cindex button properties
5197
5198 Each button has an associated list of properties defining its
5199 appearance and behavior, and other arbitrary properties may be used
5200 for application specific purposes. The following properties have
5201 special meaning to the Button package:
5202
5203 @table @code
5204 @item action
5205 @kindex action @r{(button property)}
5206 The function to call when the user invokes the button, which is passed
5207 the single argument @var{button}. By default this is @code{ignore},
5208 which does nothing.
5209
5210 @item mouse-action
5211 @kindex mouse-action @r{(button property)}
5212 This is similar to @code{action}, and when present, will be used
5213 instead of @code{action} for button invocations resulting from
5214 mouse-clicks (instead of the user hitting @key{RET}). If not
5215 present, mouse-clicks use @code{action} instead.
5216
5217 @item face
5218 @kindex face @r{(button property)}
5219 This is an Emacs face controlling how buttons of this type are
5220 displayed; by default this is the @code{button} face.
5221
5222 @item mouse-face
5223 @kindex mouse-face @r{(button property)}
5224 This is an additional face which controls appearance during
5225 mouse-overs (merged with the usual button face); by default this is
5226 the usual Emacs @code{highlight} face.
5227
5228 @item keymap
5229 @kindex keymap @r{(button property)}
5230 The button's keymap, defining bindings active within the button
5231 region. By default this is the usual button region keymap, stored
5232 in the variable @code{button-map}, which defines @key{RET} and
5233 @key{mouse-2} to invoke the button.
5234
5235 @item type
5236 @kindex type @r{(button property)}
5237 The button type. @xref{Button Types}.
5238
5239 @item help-echo
5240 @kindex help-index @r{(button property)}
5241 A string displayed by the Emacs tool-tip help system; by default,
5242 @code{"mouse-2, RET: Push this button"}.
5243
5244 @item follow-link
5245 @kindex follow-link @r{(button property)}
5246 The follow-link property, defining how a @key{Mouse-1} click behaves
5247 on this button, @xref{Clickable Text}.
5248
5249 @item button
5250 @kindex button @r{(button property)}
5251 All buttons have a non-@code{nil} @code{button} property, which may be useful
5252 in finding regions of text that comprise buttons (which is what the
5253 standard button functions do).
5254 @end table
5255
5256 There are other properties defined for the regions of text in a
5257 button, but these are not generally interesting for typical uses.
5258
5259 @node Button Types
5260 @subsection Button Types
5261 @cindex button types
5262
5263 Every button has a @dfn{button type}, which defines default values
5264 for the button's properties. Button types are arranged in a
5265 hierarchy, with specialized types inheriting from more general types,
5266 so that it's easy to define special-purpose types of buttons for
5267 specific tasks.
5268
5269 @defun define-button-type name &rest properties
5270 Define a `button type' called @var{name} (a symbol).
5271 The remaining arguments
5272 form a sequence of @var{property value} pairs, specifying default
5273 property values for buttons with this type (a button's type may be set
5274 by giving it a @code{type} property when creating the button, using
5275 the @code{:type} keyword argument).
5276
5277 In addition, the keyword argument @code{:supertype} may be used to
5278 specify a button-type from which @var{name} inherits its default
5279 property values. Note that this inheritance happens only when
5280 @var{name} is defined; subsequent changes to a supertype are not
5281 reflected in its subtypes.
5282 @end defun
5283
5284 Using @code{define-button-type} to define default properties for
5285 buttons is not necessary---buttons without any specified type use the
5286 built-in button-type @code{button}---but it is encouraged, since
5287 doing so usually makes the resulting code clearer and more efficient.
5288
5289 @node Making Buttons
5290 @subsection Making Buttons
5291 @cindex making buttons
5292
5293 Buttons are associated with a region of text, using an overlay or
5294 text properties to hold button-specific information, all of which are
5295 initialized from the button's type (which defaults to the built-in
5296 button type @code{button}). Like all Emacs text, the appearance of
5297 the button is governed by the @code{face} property; by default (via
5298 the @code{face} property inherited from the @code{button} button-type)
5299 this is a simple underline, like a typical web-page link.
5300
5301 For convenience, there are two sorts of button-creation functions,
5302 those that add button properties to an existing region of a buffer,
5303 called @code{make-...button}, and those that also insert the button
5304 text, called @code{insert-...button}.
5305
5306 The button-creation functions all take the @code{&rest} argument
5307 @var{properties}, which should be a sequence of @var{property value}
5308 pairs, specifying properties to add to the button; see @ref{Button
5309 Properties}. In addition, the keyword argument @code{:type} may be
5310 used to specify a button-type from which to inherit other properties;
5311 see @ref{Button Types}. Any properties not explicitly specified
5312 during creation will be inherited from the button's type (if the type
5313 defines such a property).
5314
5315 The following functions add a button using an overlay
5316 (@pxref{Overlays}) to hold the button properties:
5317
5318 @defun make-button beg end &rest properties
5319 This makes a button from @var{beg} to @var{end} in the
5320 current buffer, and returns it.
5321 @end defun
5322
5323 @defun insert-button label &rest properties
5324 This insert a button with the label @var{label} at point,
5325 and returns it.
5326 @end defun
5327
5328 The following functions are similar, but using text properties
5329 (@pxref{Text Properties}) to hold the button properties. Such buttons
5330 do not add markers to the buffer, so editing in the buffer does not
5331 slow down if there is an extremely large numbers of buttons. However,
5332 if there is an existing face text property on the text (e.g., a face
5333 assigned by Font Lock mode), the button face may not be visible. Both
5334 of these functions return the starting position of the new button.
5335
5336 @defun make-text-button beg end &rest properties
5337 This makes a button from @var{beg} to @var{end} in the current buffer,
5338 using text properties.
5339 @end defun
5340
5341 @defun insert-text-button label &rest properties
5342 This inserts a button with the label @var{label} at point, using text
5343 properties.
5344 @end defun
5345
5346 @node Manipulating Buttons
5347 @subsection Manipulating Buttons
5348 @cindex manipulating buttons
5349
5350 These are functions for getting and setting properties of buttons.
5351 Often these are used by a button's invocation function to determine
5352 what to do.
5353
5354 Where a @var{button} parameter is specified, it means an object
5355 referring to a specific button, either an overlay (for overlay
5356 buttons), or a buffer-position or marker (for text property buttons).
5357 Such an object is passed as the first argument to a button's
5358 invocation function when it is invoked.
5359
5360 @defun button-start button
5361 Return the position at which @var{button} starts.
5362 @end defun
5363
5364 @defun button-end button
5365 Return the position at which @var{button} ends.
5366 @end defun
5367
5368 @defun button-get button prop
5369 Get the property of button @var{button} named @var{prop}.
5370 @end defun
5371
5372 @defun button-put button prop val
5373 Set @var{button}'s @var{prop} property to @var{val}.
5374 @end defun
5375
5376 @defun button-activate button &optional use-mouse-action
5377 Call @var{button}'s @code{action} property (i.e., invoke it). If
5378 @var{use-mouse-action} is non-@code{nil}, try to invoke the button's
5379 @code{mouse-action} property instead of @code{action}; if the button
5380 has no @code{mouse-action} property, use @code{action} as normal.
5381 @end defun
5382
5383 @defun button-label button
5384 Return @var{button}'s text label.
5385 @end defun
5386
5387 @defun button-type button
5388 Return @var{button}'s button-type.
5389 @end defun
5390
5391 @defun button-has-type-p button type
5392 Return @code{t} if @var{button} has button-type @var{type}, or one of
5393 @var{type}'s subtypes.
5394 @end defun
5395
5396 @defun button-at pos
5397 Return the button at position @var{pos} in the current buffer, or
5398 @code{nil}. If the button at @var{pos} is a text property button, the
5399 return value is a marker pointing to @var{pos}.
5400 @end defun
5401
5402 @defun button-type-put type prop val
5403 Set the button-type @var{type}'s @var{prop} property to @var{val}.
5404 @end defun
5405
5406 @defun button-type-get type prop
5407 Get the property of button-type @var{type} named @var{prop}.
5408 @end defun
5409
5410 @defun button-type-subtype-p type supertype
5411 Return @code{t} if button-type @var{type} is a subtype of @var{supertype}.
5412 @end defun
5413
5414 @node Button Buffer Commands
5415 @subsection Button Buffer Commands
5416 @cindex button buffer commands
5417
5418 These are commands and functions for locating and operating on
5419 buttons in an Emacs buffer.
5420
5421 @code{push-button} is the command that a user uses to actually `push'
5422 a button, and is bound by default in the button itself to @key{RET}
5423 and to @key{mouse-2} using a local keymap in the button's overlay or
5424 text properties. Commands that are useful outside the buttons itself,
5425 such as @code{forward-button} and @code{backward-button} are
5426 additionally available in the keymap stored in
5427 @code{button-buffer-map}; a mode which uses buttons may want to use
5428 @code{button-buffer-map} as a parent keymap for its keymap.
5429
5430 If the button has a non-@code{nil} @code{follow-link} property, and
5431 @code{mouse-1-click-follows-link} is set, a quick @key{Mouse-1} click
5432 will also activate the @code{push-button} command.
5433 @xref{Clickable Text}.
5434
5435 @deffn Command push-button &optional pos use-mouse-action
5436 Perform the action specified by a button at location @var{pos}.
5437 @var{pos} may be either a buffer position or a mouse-event. If
5438 @var{use-mouse-action} is non-@code{nil}, or @var{pos} is a
5439 mouse-event (@pxref{Mouse Events}), try to invoke the button's
5440 @code{mouse-action} property instead of @code{action}; if the button
5441 has no @code{mouse-action} property, use @code{action} as normal.
5442 @var{pos} defaults to point, except when @code{push-button} is invoked
5443 interactively as the result of a mouse-event, in which case, the mouse
5444 event's position is used. If there's no button at @var{pos}, do
5445 nothing and return @code{nil}, otherwise return @code{t}.
5446 @end deffn
5447
5448 @deffn Command forward-button n &optional wrap display-message
5449 Move to the @var{n}th next button, or @var{n}th previous button if
5450 @var{n} is negative. If @var{n} is zero, move to the start of any
5451 button at point. If @var{wrap} is non-@code{nil}, moving past either
5452 end of the buffer continues from the other end. If
5453 @var{display-message} is non-@code{nil}, the button's help-echo string
5454 is displayed. Any button with a non-@code{nil} @code{skip} property
5455 is skipped over. Returns the button found.
5456 @end deffn
5457
5458 @deffn Command backward-button n &optional wrap display-message
5459 Move to the @var{n}th previous button, or @var{n}th next button if
5460 @var{n} is negative. If @var{n} is zero, move to the start of any
5461 button at point. If @var{wrap} is non-@code{nil}, moving past either
5462 end of the buffer continues from the other end. If
5463 @var{display-message} is non-@code{nil}, the button's help-echo string
5464 is displayed. Any button with a non-@code{nil} @code{skip} property
5465 is skipped over. Returns the button found.
5466 @end deffn
5467
5468 @defun next-button pos &optional count-current
5469 @defunx previous-button pos &optional count-current
5470 Return the next button after (for @code{next-button}) or before (for
5471 @code{previous-button}) position @var{pos} in the current buffer. If
5472 @var{count-current} is non-@code{nil}, count any button at @var{pos}
5473 in the search, instead of starting at the next button.
5474 @end defun
5475
5476 @node Abstract Display
5477 @section Abstract Display
5478 @cindex ewoc
5479 @cindex display, abstract
5480 @cindex display, arbitrary objects
5481 @cindex model/view/controller
5482 @cindex view part, model/view/controller
5483
5484 The Ewoc package constructs buffer text that represents a structure
5485 of Lisp objects, and updates the text to follow changes in that
5486 structure. This is like the ``view'' component in the
5487 ``model/view/controller'' design paradigm. Ewoc means ``Emacs's
5488 Widget for Object Collections''.
5489
5490 An @dfn{ewoc} is a structure that organizes information required to
5491 construct buffer text that represents certain Lisp data. The buffer
5492 text of the ewoc has three parts, in order: first, fixed @dfn{header}
5493 text; next, textual descriptions of a series of data elements (Lisp
5494 objects that you specify); and last, fixed @dfn{footer} text.
5495 Specifically, an ewoc contains information on:
5496
5497 @itemize @bullet
5498 @item
5499 The buffer which its text is generated in.
5500
5501 @item
5502 The text's start position in the buffer.
5503
5504 @item
5505 The header and footer strings.
5506
5507 @item
5508 @cindex node, ewoc
5509 @c or "@cindex node, abstract display"?
5510 A doubly-linked chain of @dfn{nodes}, each of which contains:
5511
5512 @itemize
5513 @item
5514 A @dfn{data element}, a single Lisp object.
5515
5516 @item
5517 Links to the preceding and following nodes in the chain.
5518 @end itemize
5519
5520 @item
5521 A @dfn{pretty-printer} function which is responsible for
5522 inserting the textual representation of a data
5523 element value into the current buffer.
5524 @end itemize
5525
5526 Typically, you define an ewoc with @code{ewoc-create}, and then pass
5527 the resulting ewoc structure to other functions in the Ewoc package to
5528 build nodes within it, and display it in the buffer. Once it is
5529 displayed in the buffer, other functions determine the correspondence
5530 between buffer positions and nodes, move point from one node's textual
5531 representation to another, and so forth. @xref{Abstract Display
5532 Functions}.
5533
5534 @cindex encapsulation, ewoc
5535 @c or "@cindex encapsulation, abstract display"?
5536 A node @dfn{encapsulates} a data element much the way a variable
5537 holds a value. Normally, encapsulation occurs as a part of adding a
5538 node to the ewoc. You can retrieve the data element value and place a
5539 new value in its place, like so:
5540
5541 @lisp
5542 (ewoc-data @var{node})
5543 @result{} value
5544
5545 (ewoc-set-data @var{node} @var{new-value})
5546 @result{} @var{new-value}
5547 @end lisp
5548
5549 @noindent
5550 You can also use, as the data element value, a Lisp object (list or
5551 vector) that is a container for the ``real'' value, or an index into
5552 some other structure. The example (@pxref{Abstract Display Example})
5553 uses the latter approach.
5554
5555 When the data changes, you will want to update the text in the
5556 buffer. You can update all nodes by calling @code{ewoc-refresh}, or
5557 just specific nodes using @code{ewoc-invalidate}, or all nodes
5558 satisfying a predicate using @code{ewoc-map}. Alternatively, you can
5559 delete invalid nodes using @code{ewoc-delete} or @code{ewoc-filter},
5560 and add new nodes in their place. Deleting a node from an ewoc deletes
5561 its associated textual description from buffer, as well.
5562
5563 @menu
5564 * Abstract Display Functions:: Functions in the Ewoc package.
5565 * Abstract Display Example:: Example of using Ewoc.
5566 @end menu
5567
5568 @node Abstract Display Functions
5569 @subsection Abstract Display Functions
5570
5571 In this subsection, @var{ewoc} and @var{node} stand for the
5572 structures described above (@pxref{Abstract Display}), while
5573 @var{data} stands for an arbitrary Lisp object used as a data element.
5574
5575 @defun ewoc-create pretty-printer &optional header footer nosep
5576 This constructs and returns a new ewoc, with no nodes (and thus no data
5577 elements). @var{pretty-printer} should be a function that takes one
5578 argument, a data element of the sort you plan to use in this ewoc, and
5579 inserts its textual description at point using @code{insert} (and never
5580 @code{insert-before-markers}, because that would interfere with the
5581 Ewoc package's internal mechanisms).
5582
5583 Normally, a newline is automatically inserted after the header,
5584 the footer and every node's textual description. If @var{nosep}
5585 is non-@code{nil}, no newline is inserted. This may be useful for
5586 displaying an entire ewoc on a single line, for example, or for
5587 making nodes ``invisible'' by arranging for @var{pretty-printer}
5588 to do nothing for those nodes.
5589
5590 An ewoc maintains its text in the buffer that is current when
5591 you create it, so switch to the intended buffer before calling
5592 @code{ewoc-create}.
5593 @end defun
5594
5595 @defun ewoc-buffer ewoc
5596 This returns the buffer where @var{ewoc} maintains its text.
5597 @end defun
5598
5599 @defun ewoc-get-hf ewoc
5600 This returns a cons cell @code{(@var{header} . @var{footer})}
5601 made from @var{ewoc}'s header and footer.
5602 @end defun
5603
5604 @defun ewoc-set-hf ewoc header footer
5605 This sets the header and footer of @var{ewoc} to the strings
5606 @var{header} and @var{footer}, respectively.
5607 @end defun
5608
5609 @defun ewoc-enter-first ewoc data
5610 @defunx ewoc-enter-last ewoc data
5611 These add a new node encapsulating @var{data}, putting it, respectively,
5612 at the beginning or end of @var{ewoc}'s chain of nodes.
5613 @end defun
5614
5615 @defun ewoc-enter-before ewoc node data
5616 @defunx ewoc-enter-after ewoc node data
5617 These add a new node encapsulating @var{data}, adding it to
5618 @var{ewoc} before or after @var{node}, respectively.
5619 @end defun
5620
5621 @defun ewoc-prev ewoc node
5622 @defunx ewoc-next ewoc node
5623 These return, respectively, the previous node and the next node of @var{node}
5624 in @var{ewoc}.
5625 @end defun
5626
5627 @defun ewoc-nth ewoc n
5628 This returns the node in @var{ewoc} found at zero-based index @var{n}.
5629 A negative @var{n} means count from the end. @code{ewoc-nth} returns
5630 @code{nil} if @var{n} is out of range.
5631 @end defun
5632
5633 @defun ewoc-data node
5634 This extracts the data encapsulated by @var{node} and returns it.
5635 @end defun
5636
5637 @defun ewoc-set-data node data
5638 This sets the data encapsulated by @var{node} to @var{data}.
5639 @end defun
5640
5641 @defun ewoc-locate ewoc &optional pos guess
5642 This determines the node in @var{ewoc} which contains point (or
5643 @var{pos} if specified), and returns that node. If @var{ewoc} has no
5644 nodes, it returns @code{nil}. If @var{pos} is before the first node,
5645 it returns the first node; if @var{pos} is after the last node, it returns
5646 the last node. The optional third arg @var{guess}
5647 should be a node that is likely to be near @var{pos}; this doesn't
5648 alter the result, but makes the function run faster.
5649 @end defun
5650
5651 @defun ewoc-location node
5652 This returns the start position of @var{node}.
5653 @end defun
5654
5655 @defun ewoc-goto-prev ewoc arg
5656 @defunx ewoc-goto-next ewoc arg
5657 These move point to the previous or next, respectively, @var{arg}th node
5658 in @var{ewoc}. @code{ewoc-goto-prev} does not move if it is already at
5659 the first node or if @var{ewoc} is empty, whereas @code{ewoc-goto-next}
5660 moves past the last node, returning @code{nil}. Excepting this special
5661 case, these functions return the node moved to.
5662 @end defun
5663
5664 @defun ewoc-goto-node ewoc node
5665 This moves point to the start of @var{node} in @var{ewoc}.
5666 @end defun
5667
5668 @defun ewoc-refresh ewoc
5669 This function regenerates the text of @var{ewoc}. It works by
5670 deleting the text between the header and the footer, i.e., all the
5671 data elements' representations, and then calling the pretty-printer
5672 function for each node, one by one, in order.
5673 @end defun
5674
5675 @defun ewoc-invalidate ewoc &rest nodes
5676 This is similar to @code{ewoc-refresh}, except that only @var{nodes} in
5677 @var{ewoc} are updated instead of the entire set.
5678 @end defun
5679
5680 @defun ewoc-delete ewoc &rest nodes
5681 This deletes each node in @var{nodes} from @var{ewoc}.
5682 @end defun
5683
5684 @defun ewoc-filter ewoc predicate &rest args
5685 This calls @var{predicate} for each data element in @var{ewoc} and
5686 deletes those nodes for which @var{predicate} returns @code{nil}.
5687 Any @var{args} are passed to @var{predicate}.
5688 @end defun
5689
5690 @defun ewoc-collect ewoc predicate &rest args
5691 This calls @var{predicate} for each data element in @var{ewoc}
5692 and returns a list of those elements for which @var{predicate}
5693 returns non-@code{nil}. The elements in the list are ordered
5694 as in the buffer. Any @var{args} are passed to @var{predicate}.
5695 @end defun
5696
5697 @defun ewoc-map map-function ewoc &rest args
5698 This calls @var{map-function} for each data element in @var{ewoc} and
5699 updates those nodes for which @var{map-function} returns non-@code{nil}.
5700 Any @var{args} are passed to @var{map-function}.
5701 @end defun
5702
5703 @node Abstract Display Example
5704 @subsection Abstract Display Example
5705
5706 Here is a simple example using functions of the ewoc package to
5707 implement a ``color components display'', an area in a buffer that
5708 represents a vector of three integers (itself representing a 24-bit RGB
5709 value) in various ways.
5710
5711 @example
5712 (setq colorcomp-ewoc nil
5713 colorcomp-data nil
5714 colorcomp-mode-map nil
5715 colorcomp-labels ["Red" "Green" "Blue"])
5716
5717 (defun colorcomp-pp (data)
5718 (if data
5719 (let ((comp (aref colorcomp-data data)))
5720 (insert (aref colorcomp-labels data) "\t: #x"
5721 (format "%02X" comp) " "
5722 (make-string (ash comp -2) ?#) "\n"))
5723 (let ((cstr (format "#%02X%02X%02X"
5724 (aref colorcomp-data 0)
5725 (aref colorcomp-data 1)
5726 (aref colorcomp-data 2)))
5727 (samp " (sample text) "))
5728 (insert "Color\t: "
5729 (propertize samp 'face
5730 `(foreground-color . ,cstr))
5731 (propertize samp 'face
5732 `(background-color . ,cstr))
5733 "\n"))))
5734
5735 (defun colorcomp (color)
5736 "Allow fiddling with COLOR in a new buffer.
5737 The buffer is in Color Components mode."
5738 (interactive "sColor (name or #RGB or #RRGGBB): ")
5739 (when (string= "" color)
5740 (setq color "green"))
5741 (unless (color-values color)
5742 (error "No such color: %S" color))
5743 (switch-to-buffer
5744 (generate-new-buffer (format "originally: %s" color)))
5745 (kill-all-local-variables)
5746 (setq major-mode 'colorcomp-mode
5747 mode-name "Color Components")
5748 (use-local-map colorcomp-mode-map)
5749 (erase-buffer)
5750 (buffer-disable-undo)
5751 (let ((data (apply 'vector (mapcar (lambda (n) (ash n -8))
5752 (color-values color))))
5753 (ewoc (ewoc-create 'colorcomp-pp
5754 "\nColor Components\n\n"
5755 (substitute-command-keys
5756 "\n\\@{colorcomp-mode-map@}"))))
5757 (set (make-local-variable 'colorcomp-data) data)
5758 (set (make-local-variable 'colorcomp-ewoc) ewoc)
5759 (ewoc-enter-last ewoc 0)
5760 (ewoc-enter-last ewoc 1)
5761 (ewoc-enter-last ewoc 2)
5762 (ewoc-enter-last ewoc nil)))
5763 @end example
5764
5765 @cindex controller part, model/view/controller
5766 This example can be extended to be a ``color selection widget'' (in
5767 other words, the controller part of the ``model/view/controller''
5768 design paradigm) by defining commands to modify @code{colorcomp-data}
5769 and to ``finish'' the selection process, and a keymap to tie it all
5770 together conveniently.
5771
5772 @smallexample
5773 (defun colorcomp-mod (index limit delta)
5774 (let ((cur (aref colorcomp-data index)))
5775 (unless (= limit cur)
5776 (aset colorcomp-data index (+ cur delta)))
5777 (ewoc-invalidate
5778 colorcomp-ewoc
5779 (ewoc-nth colorcomp-ewoc index)
5780 (ewoc-nth colorcomp-ewoc -1))))
5781
5782 (defun colorcomp-R-more () (interactive) (colorcomp-mod 0 255 1))
5783 (defun colorcomp-G-more () (interactive) (colorcomp-mod 1 255 1))
5784 (defun colorcomp-B-more () (interactive) (colorcomp-mod 2 255 1))
5785 (defun colorcomp-R-less () (interactive) (colorcomp-mod 0 0 -1))
5786 (defun colorcomp-G-less () (interactive) (colorcomp-mod 1 0 -1))
5787 (defun colorcomp-B-less () (interactive) (colorcomp-mod 2 0 -1))
5788
5789 (defun colorcomp-copy-as-kill-and-exit ()
5790 "Copy the color components into the kill ring and kill the buffer.
5791 The string is formatted #RRGGBB (hash followed by six hex digits)."
5792 (interactive)
5793 (kill-new (format "#%02X%02X%02X"
5794 (aref colorcomp-data 0)
5795 (aref colorcomp-data 1)
5796 (aref colorcomp-data 2)))
5797 (kill-buffer nil))
5798
5799 (setq colorcomp-mode-map
5800 (let ((m (make-sparse-keymap)))
5801 (suppress-keymap m)
5802 (define-key m "i" 'colorcomp-R-less)
5803 (define-key m "o" 'colorcomp-R-more)
5804 (define-key m "k" 'colorcomp-G-less)
5805 (define-key m "l" 'colorcomp-G-more)
5806 (define-key m "," 'colorcomp-B-less)
5807 (define-key m "." 'colorcomp-B-more)
5808 (define-key m " " 'colorcomp-copy-as-kill-and-exit)
5809 m))
5810 @end smallexample
5811
5812 Note that we never modify the data in each node, which is fixed when the
5813 ewoc is created to be either @code{nil} or an index into the vector
5814 @code{colorcomp-data}, the actual color components.
5815
5816 @node Blinking
5817 @section Blinking Parentheses
5818 @cindex parenthesis matching
5819 @cindex blinking parentheses
5820 @cindex balancing parentheses
5821
5822 This section describes the mechanism by which Emacs shows a matching
5823 open parenthesis when the user inserts a close parenthesis.
5824
5825 @defvar blink-paren-function
5826 The value of this variable should be a function (of no arguments) to
5827 be called whenever a character with close parenthesis syntax is inserted.
5828 The value of @code{blink-paren-function} may be @code{nil}, in which
5829 case nothing is done.
5830 @end defvar
5831
5832 @defopt blink-matching-paren
5833 If this variable is @code{nil}, then @code{blink-matching-open} does
5834 nothing.
5835 @end defopt
5836
5837 @defopt blink-matching-paren-distance
5838 This variable specifies the maximum distance to scan for a matching
5839 parenthesis before giving up.
5840 @end defopt
5841
5842 @defopt blink-matching-delay
5843 This variable specifies the number of seconds for the cursor to remain
5844 at the matching parenthesis. A fraction of a second often gives
5845 good results, but the default is 1, which works on all systems.
5846 @end defopt
5847
5848 @deffn Command blink-matching-open
5849 This function is the default value of @code{blink-paren-function}. It
5850 assumes that point follows a character with close parenthesis syntax and
5851 moves the cursor momentarily to the matching opening character. If that
5852 character is not already on the screen, it displays the character's
5853 context in the echo area. To avoid long delays, this function does not
5854 search farther than @code{blink-matching-paren-distance} characters.
5855
5856 Here is an example of calling this function explicitly.
5857
5858 @smallexample
5859 @group
5860 (defun interactive-blink-matching-open ()
5861 "Indicate momentarily the start of parenthesized sexp before point."
5862 (interactive)
5863 @end group
5864 @group
5865 (let ((blink-matching-paren-distance
5866 (buffer-size))
5867 (blink-matching-paren t))
5868 (blink-matching-open)))
5869 @end group
5870 @end smallexample
5871 @end deffn
5872
5873 @node Character Display
5874 @section Character Display
5875
5876 This section describes how characters are actually displayed by
5877 Emacs. Typically, a character is displayed as a @dfn{glyph} (a
5878 graphical symbol which occupies one character position on the screen),
5879 whose appearance corresponds to the character itself. For example,
5880 the character @samp{a} (character code 97) is displayed as @samp{a}.
5881 Some characters, however, are displayed specially. For example, the
5882 formfeed character (character code 12) is usually displayed as a
5883 sequence of two glyphs, @samp{^L}, while the newline character
5884 (character code 10) starts a new screen line.
5885
5886 You can modify how each character is displayed by defining a
5887 @dfn{display table}, which maps each character code into a sequence of
5888 glyphs. @xref{Display Tables}.
5889
5890 @menu
5891 * Usual Display:: The usual conventions for displaying characters.
5892 * Display Tables:: What a display table consists of.
5893 * Active Display Table:: How Emacs selects a display table to use.
5894 * Glyphs:: How to define a glyph, and what glyphs mean.
5895 * Glyphless Chars:: How glyphless characters are drawn.
5896 @end menu
5897
5898 @node Usual Display
5899 @subsection Usual Display Conventions
5900
5901 Here are the conventions for displaying each character code (in the
5902 absence of a display table, which can override these
5903 @iftex
5904 conventions).
5905 @end iftex
5906 @ifnottex
5907 conventions; @pxref{Display Tables}).
5908 @end ifnottex
5909
5910 @cindex printable ASCII characters
5911 @itemize @bullet
5912 @item
5913 The @dfn{printable @acronym{ASCII} characters}, character codes 32
5914 through 126 (consisting of numerals, English letters, and symbols like
5915 @samp{#}) are displayed literally.
5916
5917 @item
5918 The tab character (character code 9) displays as whitespace stretching
5919 up to the next tab stop column. @xref{Text Display,,, emacs, The GNU
5920 Emacs Manual}. The variable @code{tab-width} controls the number of
5921 spaces per tab stop (see below).
5922
5923 @item
5924 The newline character (character code 10) has a special effect: it
5925 ends the preceding line and starts a new line.
5926
5927 @cindex ASCII control characters
5928 @item
5929 The non-printable @dfn{@acronym{ASCII} control characters}---character
5930 codes 0 through 31, as well as the @key{DEL} character (character code
5931 127)---display in one of two ways according to the variable
5932 @code{ctl-arrow}. If this variable is non-@code{nil} (the default),
5933 these characters are displayed as sequences of two glyphs, where the
5934 first glyph is @samp{^} (a display table can specify a glyph to use
5935 instead of @samp{^}); e.g., the @key{DEL} character is displayed as
5936 @samp{^?}.
5937
5938 If @code{ctl-arrow} is @code{nil}, these characters are displayed as
5939 octal escapes (see below).
5940
5941 This rule also applies to carriage return (character code 13), if that
5942 character appears in the buffer. But carriage returns usually do not
5943 appear in buffer text; they are eliminated as part of end-of-line
5944 conversion (@pxref{Coding System Basics}).
5945
5946 @cindex octal escapes
5947 @item
5948 @dfn{Raw bytes} are non-@acronym{ASCII} characters with codes 128
5949 through 255 (@pxref{Text Representations}). These characters display
5950 as @dfn{octal escapes}: sequences of four glyphs, where the first
5951 glyph is the @acronym{ASCII} code for @samp{\}, and the others are
5952 digit characters representing the character code in octal. (A display
5953 table can specify a glyph to use instead of @samp{\}.)
5954
5955 @item
5956 Each non-@acronym{ASCII} character with code above 255 is displayed
5957 literally, if the terminal supports it. If the terminal does not
5958 support it, the character is said to be @dfn{glyphless}, and it is
5959 usually displayed using a placeholder glyph. For example, if a
5960 graphical terminal has no font for a character, Emacs usually displays
5961 a box containing the character code in hexadecimal. @xref{Glyphless
5962 Chars}.
5963 @end itemize
5964
5965 The above display conventions apply even when there is a display
5966 table, for any character whose entry in the active display table is
5967 @code{nil}. Thus, when you set up a display table, you need only
5968 specify the characters for which you want special behavior.
5969
5970 The following variables affect how certain characters are displayed
5971 on the screen. Since they change the number of columns the characters
5972 occupy, they also affect the indentation functions. They also affect
5973 how the mode line is displayed; if you want to force redisplay of the
5974 mode line using the new values, call the function
5975 @code{force-mode-line-update} (@pxref{Mode Line Format}).
5976
5977 @defopt ctl-arrow
5978 @cindex control characters in display
5979 This buffer-local variable controls how control characters are
5980 displayed. If it is non-@code{nil}, they are displayed as a caret
5981 followed by the character: @samp{^A}. If it is @code{nil}, they are
5982 displayed as octal escapes: a backslash followed by three octal
5983 digits, as in @samp{\001}.
5984 @end defopt
5985
5986 @defopt tab-width
5987 The value of this buffer-local variable is the spacing between tab
5988 stops used for displaying tab characters in Emacs buffers. The value
5989 is in units of columns, and the default is 8. Note that this feature
5990 is completely independent of the user-settable tab stops used by the
5991 command @code{tab-to-tab-stop}. @xref{Indent Tabs}.
5992 @end defopt
5993
5994 @node Display Tables
5995 @subsection Display Tables
5996
5997 @cindex display table
5998 A display table is a special-purpose char-table
5999 (@pxref{Char-Tables}), with @code{display-table} as its subtype, which
6000 is used to override the usual character display conventions. This
6001 section describes how to make, inspect, and assign elements to a
6002 display table object.
6003
6004 @defun make-display-table
6005 This creates and returns a display table. The table initially has
6006 @code{nil} in all elements.
6007 @end defun
6008
6009 The ordinary elements of the display table are indexed by character
6010 codes; the element at index @var{c} says how to display the character
6011 code @var{c}. The value should be @code{nil} (which means to display
6012 the character @var{c} according to the usual display conventions;
6013 @pxref{Usual Display}), or a vector of glyph codes (which means to
6014 display the character @var{c} as those glyphs; @pxref{Glyphs}).
6015
6016 @strong{Warning:} if you use the display table to change the display
6017 of newline characters, the whole buffer will be displayed as one long
6018 ``line''.
6019
6020 The display table also has six ``extra slots'' which serve special
6021 purposes. Here is a table of their meanings; @code{nil} in any slot
6022 means to use the default for that slot, as stated below.
6023
6024 @table @asis
6025 @item 0
6026 The glyph for the end of a truncated screen line (the default for this
6027 is @samp{$}). @xref{Glyphs}. On graphical terminals, Emacs uses
6028 arrows in the fringes to indicate truncation, so the display table has
6029 no effect.
6030
6031 @item 1
6032 The glyph for the end of a continued line (the default is @samp{\}).
6033 On graphical terminals, Emacs uses curved arrows in the fringes to
6034 indicate continuation, so the display table has no effect.
6035
6036 @item 2
6037 The glyph for indicating a character displayed as an octal character
6038 code (the default is @samp{\}).
6039
6040 @item 3
6041 The glyph for indicating a control character (the default is @samp{^}).
6042
6043 @item 4
6044 A vector of glyphs for indicating the presence of invisible lines (the
6045 default is @samp{...}). @xref{Selective Display}.
6046
6047 @item 5
6048 The glyph used to draw the border between side-by-side windows (the
6049 default is @samp{|}). @xref{Splitting Windows}. This takes effect only
6050 when there are no scroll bars; if scroll bars are supported and in use,
6051 a scroll bar separates the two windows.
6052 @end table
6053
6054 For example, here is how to construct a display table that mimics
6055 the effect of setting @code{ctl-arrow} to a non-@code{nil} value
6056 (@pxref{Glyphs}, for the function @code{make-glyph-code}):
6057
6058 @example
6059 (setq disptab (make-display-table))
6060 (dotimes (i 32)
6061 (or (= i ?\t)
6062 (= i ?\n)
6063 (aset disptab i
6064 (vector (make-glyph-code ?^ 'escape-glyph)
6065 (make-glyph-code (+ i 64) 'escape-glyph)))))
6066 (aset disptab 127
6067 (vector (make-glyph-code ?^ 'escape-glyph)
6068 (make-glyph-code ?? 'escape-glyph)))))
6069 @end example
6070
6071 @defun display-table-slot display-table slot
6072 This function returns the value of the extra slot @var{slot} of
6073 @var{display-table}. The argument @var{slot} may be a number from 0 to
6074 5 inclusive, or a slot name (symbol). Valid symbols are
6075 @code{truncation}, @code{wrap}, @code{escape}, @code{control},
6076 @code{selective-display}, and @code{vertical-border}.
6077 @end defun
6078
6079 @defun set-display-table-slot display-table slot value
6080 This function stores @var{value} in the extra slot @var{slot} of
6081 @var{display-table}. The argument @var{slot} may be a number from 0 to
6082 5 inclusive, or a slot name (symbol). Valid symbols are
6083 @code{truncation}, @code{wrap}, @code{escape}, @code{control},
6084 @code{selective-display}, and @code{vertical-border}.
6085 @end defun
6086
6087 @defun describe-display-table display-table
6088 This function displays a description of the display table
6089 @var{display-table} in a help buffer.
6090 @end defun
6091
6092 @deffn Command describe-current-display-table
6093 This command displays a description of the current display table in a
6094 help buffer.
6095 @end deffn
6096
6097 @node Active Display Table
6098 @subsection Active Display Table
6099 @cindex active display table
6100
6101 Each window can specify a display table, and so can each buffer.
6102 The window's display table, if there is one, takes precedence over the
6103 buffer's display table. If neither exists, Emacs tries to use the
6104 standard display table; if that is @code{nil}, Emacs uses the usual
6105 character display conventions (@pxref{Usual Display}).
6106
6107 Note that display tables affect how the mode line is displayed, so
6108 if you want to force redisplay of the mode line using a new display
6109 table, call @code{force-mode-line-update} (@pxref{Mode Line Format}).
6110
6111 @defun window-display-table &optional window
6112 This function returns @var{window}'s display table, or @code{nil} if
6113 there is none. The default for @var{window} is the selected window.
6114 @end defun
6115
6116 @defun set-window-display-table window table
6117 This function sets the display table of @var{window} to @var{table}.
6118 The argument @var{table} should be either a display table or
6119 @code{nil}.
6120 @end defun
6121
6122 @defvar buffer-display-table
6123 This variable is automatically buffer-local in all buffers; its value
6124 specifies the buffer's display table. If it is @code{nil}, there is
6125 no buffer display table.
6126 @end defvar
6127
6128 @defvar standard-display-table
6129 The value of this variable is the standard display table, which is
6130 used when Emacs is displaying a buffer in a window with neither a
6131 window display table nor a buffer display table defined. Its default
6132 is @code{nil}.
6133 @end defvar
6134
6135 The @file{disp-table} library defines several functions for changing
6136 the standard display table.
6137
6138 @node Glyphs
6139 @subsection Glyphs
6140 @cindex glyph
6141
6142 @cindex glyph code
6143 A @dfn{glyph} is a graphical symbol which occupies a single
6144 character position on the screen. Each glyph is represented in Lisp
6145 as a @dfn{glyph code}, which specifies a character and optionally a
6146 face to display it in (@pxref{Faces}). The main use of glyph codes is
6147 as the entries of display tables (@pxref{Display Tables}). The
6148 following functions are used to manipulate glyph codes:
6149
6150 @defun make-glyph-code char &optional face
6151 This function returns a glyph code representing char @var{char} with
6152 face @var{face}. If @var{face} is omitted or @code{nil}, the glyph
6153 uses the default face; in that case, the glyph code is an integer. If
6154 @var{face} is non-@code{nil}, the glyph code is not necessarily an
6155 integer object.
6156 @end defun
6157
6158 @defun glyph-char glyph
6159 This function returns the character of glyph code @var{glyph}.
6160 @end defun
6161
6162 @defun glyph-face glyph
6163 This function returns face of glyph code @var{glyph}, or @code{nil} if
6164 @var{glyph} uses the default face.
6165 @end defun
6166
6167 @ifnottex
6168 You can set up a @dfn{glyph table} to change how glyph codes are
6169 actually displayed on text terminals. This feature is semi-obsolete;
6170 use @code{glyphless-char-display} instead (@pxref{Glyphless Chars}).
6171
6172 @defvar glyph-table
6173 The value of this variable, if non-@code{nil}, is the current glyph
6174 table. It takes effect only on character terminals; on graphical
6175 displays, all glyphs are displayed literally. The glyph table should
6176 be a vector whose @var{g}th element specifies how to display glyph
6177 code @var{g}, where @var{g} is the glyph code for a glyph whose face
6178 is unspecified. Each element should be one of the following:
6179
6180 @table @asis
6181 @item @code{nil}
6182 Display this glyph literally.
6183
6184 @item a string
6185 Display this glyph by sending the specified string to the terminal.
6186
6187 @item a glyph code
6188 Display the specified glyph code instead.
6189 @end table
6190
6191 Any integer glyph code greater than or equal to the length of the
6192 glyph table is displayed literally.
6193 @end defvar
6194 @end ifnottex
6195
6196 @node Glyphless Chars
6197 @subsection Glyphless Character Display
6198 @cindex glyphless characters
6199
6200 @dfn{Glyphless characters} are characters which are displayed in a
6201 special way, e.g., as a box containing a hexadecimal code, instead of
6202 being displayed literally. These include characters which are
6203 explicitly defined to be glyphless, as well as characters for which
6204 there is no available font (on a graphical display), and characters
6205 which cannot be encoded by the terminal's coding system (on a text
6206 terminal).
6207
6208 @defvar glyphless-char-display
6209 The value of this variable is a char-table which defines glyphless
6210 characters and how they are displayed. Each entry must be one of the
6211 following display methods:
6212
6213 @table @asis
6214 @item @code{nil}
6215 Display the character in the usual way.
6216
6217 @item @code{zero-width}
6218 Don't display the character.
6219
6220 @item @code{thin-space}
6221 Display a thin space, 1-pixel wide on graphical displays, or
6222 1-character wide on text terminals.
6223
6224 @item @code{empty-box}
6225 Display an empty box.
6226
6227 @item @code{hex-code}
6228 Display a box containing the Unicode codepoint of the character, in
6229 hexadecimal notation.
6230
6231 @item an @acronym{ASCII} string
6232 Display a box containing that string.
6233
6234 @item a cons cell @code{(@var{graphical} . @var{text})}
6235 Display with @var{graphical} on graphical displays, and with
6236 @var{text} on text terminals. Both @var{graphical} and @var{text}
6237 must be one of the display methods described above.
6238 @end table
6239
6240 @noindent
6241 The @code{thin-space}, @code{empty-box}, @code{hex-code}, and
6242 @acronym{ASCII} string display methods are drawn with the
6243 @code{glyphless-char} face.
6244
6245 The char-table has one extra slot, which determines how to display any
6246 character that cannot be displayed with any available font, or cannot
6247 be encoded by the terminal's coding system. Its value should be one
6248 of the above display methods, except @code{zero-width} or a cons cell.
6249
6250 If a character has a non-@code{nil} entry in an active display table,
6251 the display table takes effect; in this case, Emacs does not consult
6252 @code{glyphless-char-display} at all.
6253 @end defvar
6254
6255 @defopt glyphless-char-display-control
6256 This user option provides a convenient way to set
6257 @code{glyphless-char-display} for groups of similar characters. Do
6258 not set its value directly from Lisp code; the value takes effect only
6259 via a custom @code{:set} function (@pxref{Variable Definitions}),
6260 which updates @code{glyphless-char-display}.
6261
6262 Its value should be an alist of elements @code{(@var{group}
6263 . @var{method})}, where @var{group} is a symbol specifying a group of
6264 characters, and @var{method} is a symbol specifying how to display
6265 them.
6266
6267 @var{group} should be one of the following:
6268
6269 @table @code
6270 @item c0-control
6271 @acronym{ASCII} control characters @code{U+0000} to @code{U+001F},
6272 excluding the newline and tab characters (normally displayed as escape
6273 sequences like @samp{^A}; @pxref{Text Display,, How Text Is Displayed,
6274 emacs, The GNU Emacs Manual}).
6275
6276 @item c1-control
6277 Non-@acronym{ASCII}, non-printing characters @code{U+0080} to
6278 @code{U+009F} (normally displayed as octal escape sequences like
6279 @samp{\230}).
6280
6281 @item format-control
6282 Characters of Unicode General Category `Cf', such as @samp{U+200E}
6283 (Left-to-Right Mark), but excluding characters that have graphic
6284 images, such as @samp{U+00AD} (Soft Hyphen).
6285
6286 @item no-font
6287 Characters for there is no suitable font, or which cannot be encoded
6288 by the terminal's coding system.
6289 @end table
6290
6291 @c FIXME: this can also be `acronym', but that's not currently
6292 @c completely implemented; it applies only to the format-control
6293 @c group, and only works if the acronym is in `char-acronym-table'.
6294 The @var{method} symbol should be one of @code{zero-width},
6295 @code{thin-space}, @code{empty-box}, or @code{hex-code}. These have
6296 the same meanings as in @code{glyphless-char-display}, above.
6297 @end defopt
6298
6299 @node Beeping
6300 @section Beeping
6301 @cindex bell
6302
6303 This section describes how to make Emacs ring the bell (or blink the
6304 screen) to attract the user's attention. Be conservative about how
6305 often you do this; frequent bells can become irritating. Also be
6306 careful not to use just beeping when signaling an error is more
6307 appropriate (@pxref{Errors}).
6308
6309 @defun ding &optional do-not-terminate
6310 @cindex keyboard macro termination
6311 This function beeps, or flashes the screen (see @code{visible-bell} below).
6312 It also terminates any keyboard macro currently executing unless
6313 @var{do-not-terminate} is non-@code{nil}.
6314 @end defun
6315
6316 @defun beep &optional do-not-terminate
6317 This is a synonym for @code{ding}.
6318 @end defun
6319
6320 @defopt visible-bell
6321 This variable determines whether Emacs should flash the screen to
6322 represent a bell. Non-@code{nil} means yes, @code{nil} means no.
6323 This is effective on graphical displays, and on text terminals
6324 provided the terminal's Termcap entry defines the visible bell
6325 capability (@samp{vb}).
6326 @end defopt
6327
6328 @defvar ring-bell-function
6329 If this is non-@code{nil}, it specifies how Emacs should ``ring the
6330 bell''. Its value should be a function of no arguments. If this is
6331 non-@code{nil}, it takes precedence over the @code{visible-bell}
6332 variable.
6333 @end defvar
6334
6335 @node Window Systems
6336 @section Window Systems
6337
6338 Emacs works with several window systems, most notably the X Window
6339 System. Both Emacs and X use the term ``window'', but use it
6340 differently. An Emacs frame is a single window as far as X is
6341 concerned; the individual Emacs windows are not known to X at all.
6342
6343 @defvar window-system
6344 This terminal-local variable tells Lisp programs what window system
6345 Emacs is using for displaying the frame. The possible values are
6346
6347 @table @code
6348 @item x
6349 @cindex X Window System
6350 Emacs is displaying the frame using X.
6351 @item w32
6352 Emacs is displaying the frame using native MS-Windows GUI.
6353 @item ns
6354 Emacs is displaying the frame using the Nextstep interface (used on
6355 GNUstep and Mac OS X).
6356 @item pc
6357 Emacs is displaying the frame using MS-DOS direct screen writes.
6358 @item nil
6359 Emacs is displaying the frame on a character-based terminal.
6360 @end table
6361 @end defvar
6362
6363 @defvar initial-window-system
6364 This variable holds the value of @code{window-system} used for the
6365 first frame created by Emacs during startup. (When Emacs is invoked
6366 with the @option{--daemon} option, it does not create any initial
6367 frames, so @code{initial-window-system} is @code{nil}. @xref{Initial
6368 Options, daemon,, emacs, The GNU Emacs Manual}.)
6369 @end defvar
6370
6371 @defun window-system &optional frame
6372 This function returns a symbol whose name tells what window system is
6373 used for displaying @var{frame} (which defaults to the currently
6374 selected frame). The list of possible symbols it returns is the same
6375 one documented for the variable @code{window-system} above.
6376 @end defun
6377
6378 Do @emph{not} use @code{window-system} and
6379 @code{initial-window-system} as predicates or boolean flag variables,
6380 if you want to write code that works differently on text terminals and
6381 graphic displays. That is because @code{window-system} is not a good
6382 indicator of Emacs capabilities on a given display type. Instead, use
6383 @code{display-graphic-p} or any of the other @code{display-*-p}
6384 predicates described in @ref{Display Feature Testing}.
6385
6386 @defvar window-setup-hook
6387 This variable is a normal hook which Emacs runs after handling the
6388 initialization files. Emacs runs this hook after it has completed
6389 loading your init file, the default initialization file (if
6390 any), and the terminal-specific Lisp code, and running the hook
6391 @code{emacs-startup-hook}.
6392
6393 This hook is used for internal purposes: setting up communication with
6394 the window system, and creating the initial window. Users should not
6395 interfere with it.
6396 @end defvar
6397
6398 @node Bidirectional Display
6399 @section Bidirectional Display
6400 @cindex bidirectional display
6401 @cindex right-to-left text
6402
6403 Emacs can display text written in scripts, such as Arabic, Farsi,
6404 and Hebrew, whose natural ordering for horizontal text display runs
6405 from right to left. Furthermore, segments of Latin script and digits
6406 embedded in right-to-left text are displayed left-to-right, while
6407 segments of right-to-left script embedded in left-to-right text
6408 (e.g., Arabic or Hebrew text in comments or strings in a program
6409 source file) are appropriately displayed right-to-left. We call such
6410 mixtures of left-to-right and right-to-left text @dfn{bidirectional
6411 text}. This section describes the facilities and options for editing
6412 and displaying bidirectional text.
6413
6414 @cindex logical order
6415 @cindex reading order
6416 @cindex visual order
6417 @cindex unicode bidirectional algorithm
6418 @cindex UBA
6419 @cindex bidirectional reordering
6420 @cindex reordering, of bidirectional text
6421 Text is stored in Emacs buffers and strings in @dfn{logical} (or
6422 @dfn{reading}) order, i.e., the order in which a human would read
6423 each character. In right-to-left and bidirectional text, the order in
6424 which characters are displayed on the screen (called @dfn{visual
6425 order}) is not the same as logical order; the characters' screen
6426 positions do not increase monotonically with string or buffer
6427 position. In performing this @dfn{bidirectional reordering}, Emacs
6428 follows the Unicode Bidirectional Algorithm (a.k.a.@: @acronym{UBA}),
6429 which is described in Annex #9 of the Unicode standard
6430 (@url{http://www.unicode.org/reports/tr9/}). Emacs provides a ``Full
6431 Bidirectionality'' class implementation of the @acronym{UBA}.
6432
6433 @defvar bidi-display-reordering
6434 If the value of this buffer-local variable is non-@code{nil} (the
6435 default), Emacs performs bidirectional reordering for display. The
6436 reordering affects buffer text, as well as display strings and overlay
6437 strings from text and overlay properties in the buffer (@pxref{Overlay
6438 Properties}, and @pxref{Display Property}). If the value is
6439 @code{nil}, Emacs does not perform bidirectional reordering in the
6440 buffer.
6441
6442 The default value of @code{bidi-display-reordering} controls the
6443 reordering of strings which are not directly supplied by a buffer,
6444 including the text displayed in mode lines (@pxref{Mode Line Format})
6445 and header lines (@pxref{Header Lines}).
6446 @end defvar
6447
6448 @cindex unibyte buffers, and bidi reordering
6449 Emacs never reorders the text of a unibyte buffer, even if
6450 @code{bidi-display-reordering} is non-@code{nil} in the buffer. This
6451 is because unibyte buffers contain raw bytes, not characters, and thus
6452 lack the directionality properties required for reordering.
6453 Therefore, to test whether text in a buffer will be reordered for
6454 display, it is not enough to test the value of
6455 @code{bidi-display-reordering} alone. The correct test is this:
6456
6457 @example
6458 (if (and enable-multibyte-characters
6459 bidi-display-reordering)
6460 ;; Buffer is being reordered for display
6461 )
6462 @end example
6463
6464 However, unibyte display and overlay strings @emph{are} reordered if
6465 their parent buffer is reordered. This is because plain-@sc{ascii}
6466 strings are stored by Emacs as unibyte strings. If a unibyte display
6467 or overlay string includes non-@sc{ascii} characters, these characters
6468 are assumed to have left-to-right direction.
6469
6470 @cindex display properties, and bidi reordering of text
6471 Text covered by @code{display} text properties, by overlays with
6472 @code{display} properties whose value is a string, and by any other
6473 properties that replace buffer text, is treated as a single unit when
6474 it is reordered for display. That is, the entire chunk of text
6475 covered by these properties is reordered together. Moreover, the
6476 bidirectional properties of the characters in such a chunk of text are
6477 ignored, and Emacs reorders them as if they were replaced with a
6478 single character @code{U+FFFC}, known as the @dfn{Object Replacement
6479 Character}. This means that placing a display property over a portion
6480 of text may change the way that the surrounding text is reordered for
6481 display. To prevent this unexpected effect, always place such
6482 properties on text whose directionality is identical with text that
6483 surrounds it.
6484
6485 @cindex base direction of a paragraph
6486 Each paragraph of bidirectional text has a @dfn{base direction},
6487 either right-to-left or left-to-right. Left-to-right paragraphs are
6488 displayed beginning at the left margin of the window, and are
6489 truncated or continued when the text reaches the right margin.
6490 Right-to-left paragraphs are displayed beginning at the right margin,
6491 and are continued or truncated at the left margin.
6492
6493 By default, Emacs determines the base direction of each paragraph by
6494 looking at the text at its beginning. The precise method of
6495 determining the base direction is specified by the @acronym{UBA}; in a
6496 nutshell, the first character in a paragraph that has an explicit
6497 directionality determines the base direction of the paragraph.
6498 However, sometimes a buffer may need to force a certain base direction
6499 for its paragraphs. For example, buffers containing program source
6500 code should force all paragraphs to be displayed left-to-right. You
6501 can use following variable to do this:
6502
6503 @defvar bidi-paragraph-direction
6504 If the value of this buffer-local variable is the symbol
6505 @code{right-to-left} or @code{left-to-right}, all paragraphs in the
6506 buffer are assumed to have that specified direction. Any other value
6507 is equivalent to @code{nil} (the default), which means to determine
6508 the base direction of each paragraph from its contents.
6509
6510 @cindex @code{prog-mode}, and @code{bidi-paragraph-direction}
6511 Modes for program source code should set this to @code{left-to-right}.
6512 Prog mode does this by default, so modes derived from Prog mode do not
6513 need to set this explicitly (@pxref{Basic Major Modes}).
6514 @end defvar
6515
6516 @defun current-bidi-paragraph-direction &optional buffer
6517 This function returns the paragraph direction at point in the named
6518 @var{buffer}. The returned value is a symbol, either
6519 @code{left-to-right} or @code{right-to-left}. If @var{buffer} is
6520 omitted or @code{nil}, it defaults to the current buffer. If the
6521 buffer-local value of the variable @code{bidi-paragraph-direction} is
6522 non-@code{nil}, the returned value will be identical to that value;
6523 otherwise, the returned value reflects the paragraph direction
6524 determined dynamically by Emacs. For buffers whose value of
6525 @code{bidi-display-reordering} is @code{nil} as well as unibyte
6526 buffers, this function always returns @code{left-to-right}.
6527 @end defun
6528
6529 @cindex visual-order cursor motion
6530 Sometimes there's a need to move point in strict visual order,
6531 either to the left or to the right of its current screen position.
6532 Emacs provides a primitive to do that.
6533
6534 @defun move-point-visually direction
6535 This function moves point of the currently selected window to the
6536 buffer position that appears immediately to the right or to the left
6537 of point on the screen. If @var{direction} is positive, point will
6538 move one screen position to the right, otherwise it will move one
6539 screen position to the left. Note that, depending on the surrounding
6540 bidirectional context, this could potentially move point many buffer
6541 positions away. If invoked at the end of a screen line, the function
6542 moves point to the rightmost or leftmost screen position of the next
6543 or previous screen line, as appropriate for the value of
6544 @var{direction}.
6545
6546 The function returns the new buffer position as its value.
6547 @end defun
6548
6549 @cindex layout on display, and bidirectional text
6550 @cindex jumbled display of bidirectional text
6551 @cindex concatenating bidirectional strings
6552 Bidirectional reordering can have surprising and unpleasant effects
6553 when two strings with bidirectional content are juxtaposed in a
6554 buffer, or otherwise programmatically concatenated into a string of
6555 text. A typical problematic case is when a buffer consists of
6556 sequences of text ``fields'' separated by whitespace or punctuation
6557 characters, like Buffer Menu mode or Rmail Summary Mode. Because the
6558 punctuation characters used as separators have @dfn{weak
6559 directionality}, they take on the directionality of surrounding text.
6560 As result, a numeric field that follows a field with bidirectional
6561 content can be displayed @emph{to the left} of the preceding field,
6562 messing up the expected layout. There are several ways to avoid this
6563 problem:
6564
6565 @itemize @minus
6566 @item
6567 Append the special character @code{U+200E}, LEFT-TO-RIGHT MARK, or
6568 @acronym{LRM}, to the end of each field that may have bidirectional
6569 content, or prepend it to the beginning of the following field. The
6570 function @code{bidi-string-mark-left-to-right}, described below, comes
6571 in handy for this purpose. (In a right-to-left paragraph, use
6572 @code{U+200F}, RIGHT-TO-LEFT MARK, or @acronym{RLM}, instead.) This
6573 is one of the solutions recommended by the UBA.
6574
6575 @item
6576 Include the tab character in the field separator. The tab character
6577 plays the role of @dfn{segment separator} in bidirectional reordering,
6578 causing the text on either side to be reordered separately.
6579
6580 @cindex @code{space} display spec, and bidirectional text
6581 @item
6582 Separate fields with a @code{display} property or overlay with a
6583 property value of the form @code{(space . PROPS)} (@pxref{Specified
6584 Space}). Emacs treats this display specification as a @dfn{paragraph
6585 separator}, and reorders the text on either side separately.
6586 @end itemize
6587
6588 @defun bidi-string-mark-left-to-right string
6589 This function returns its argument @var{string}, possibly modified,
6590 such that the result can be safely concatenated with another string,
6591 or juxtaposed with another string in a buffer, without disrupting the
6592 relative layout of this string and the next one on display. If the
6593 string returned by this function is displayed as part of a
6594 left-to-right paragraph, it will always appear on display to the left
6595 of the text that follows it. The function works by examining the
6596 characters of its argument, and if any of those characters could cause
6597 reordering on display, the function appends the @acronym{LRM}
6598 character to the string. The appended @acronym{LRM} character is made
6599 invisible by giving it an @code{invisible} text property of @code{t}
6600 (@pxref{Invisible Text}).
6601 @end defun
6602
6603 The reordering algorithm uses the bidirectional properties of the
6604 characters stored as their @code{bidi-class} property
6605 (@pxref{Character Properties}). Lisp programs can change these
6606 properties by calling the @code{put-char-code-property} function.
6607 However, doing this requires a thorough understanding of the
6608 @acronym{UBA}, and is therefore not recommended. Any changes to the
6609 bidirectional properties of a character have global effect: they
6610 affect all Emacs frames and windows.
6611
6612 Similarly, the @code{mirroring} property is used to display the
6613 appropriate mirrored character in the reordered text. Lisp programs
6614 can affect the mirrored display by changing this property. Again, any
6615 such changes affect all of Emacs display.