2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990-1995, 1998-1999, 2001-2016 Free Software
5 @c See the file elisp.texi for copying conditions.
7 @chapter Operating System Interface
9 This chapter is about starting and getting out of Emacs, access to
10 values in the operating system environment, and terminal input, output.
12 @xref{Building Emacs}, for related information. @xref{Display}, for
13 additional operating system status information pertaining to the
14 terminal and the screen.
17 * Starting Up:: Customizing Emacs startup processing.
18 * Getting Out:: How exiting works (permanent or temporary).
19 * System Environment:: Distinguish the name and kind of system.
20 * User Identification:: Finding the name and user id of the user.
21 * Time of Day:: Getting the current time.
22 * Time Conversion:: Converting a time from numeric form to
23 calendrical data and vice versa.
24 * Time Parsing:: Converting a time from numeric form to text
26 * Processor Run Time:: Getting the run time used by Emacs.
27 * Time Calculations:: Adding, subtracting, comparing times, etc.
28 * Timers:: Setting a timer to call a function at a certain time.
29 * Idle Timers:: Setting a timer to call a function when Emacs has
30 been idle for a certain length of time.
31 * Terminal Input:: Accessing and recording terminal input.
32 * Terminal Output:: Controlling and recording terminal output.
33 * Sound Output:: Playing sounds on the computer's speaker.
34 * X11 Keysyms:: Operating on key symbols for X Windows.
35 * Batch Mode:: Running Emacs without terminal interaction.
36 * Session Management:: Saving and restoring state with X Session Management.
37 * Desktop Notifications:: Desktop notifications.
38 * File Notifications:: File notifications.
39 * Dynamic Libraries:: On-demand loading of support libraries.
40 * Security Considerations:: Running Emacs in an unfriendly environment.
44 @section Starting Up Emacs
46 This section describes what Emacs does when it is started, and how you
47 can customize these actions.
50 * Startup Summary:: Sequence of actions Emacs performs at startup.
51 * Init File:: Details on reading the init file.
52 * Terminal-Specific:: How the terminal-specific Lisp file is read.
53 * Command-Line Arguments:: How command-line arguments are processed,
54 and how you can customize them.
58 @subsection Summary: Sequence of Actions at Startup
59 @cindex initialization of Emacs
60 @cindex startup of Emacs
61 @cindex @file{startup.el}
63 When Emacs is started up, it performs the following operations
64 (see @code{normal-top-level} in @file{startup.el}):
68 It adds subdirectories to @code{load-path}, by running the file named
69 @file{subdirs.el} in each directory in the list. Normally, this file
70 adds the directory's subdirectories to the list, and those are scanned
71 in their turn. The files @file{subdirs.el} are normally generated
72 automatically when Emacs is installed.
75 It loads any @file{leim-list.el} that it finds in the @code{load-path}
76 directories. This file is intended for registering input methods.
77 The search is only for any personal @file{leim-list.el} files that you
78 may have created; it skips the directories containing the standard Emacs
79 libraries (these should contain only a single @file{leim-list.el} file,
80 which is compiled into the Emacs executable).
82 @vindex before-init-time
84 It sets the variable @code{before-init-time} to the value of
85 @code{current-time} (@pxref{Time of Day}). It also sets
86 @code{after-init-time} to @code{nil}, which signals to Lisp programs
87 that Emacs is being initialized.
89 @c set-locale-environment
91 It sets the language environment and the terminal coding system,
92 if requested by environment variables such as @env{LANG}.
95 It does some basic parsing of the command-line arguments.
97 @vindex initial-window-system@r{, and startup}
98 @vindex window-system-initialization-alist
100 If not running in batch mode, it initializes the window system that
101 the variable @code{initial-window-system} specifies (@pxref{Window
102 Systems, initial-window-system}). The initialization function for
103 each supported window system is specified by
104 @code{window-system-initialization-alist}. If the value
105 of @code{initial-window-system} is @var{windowsystem}, then the
106 appropriate initialization function is defined in the file
107 @file{term/@var{windowsystem}-win.el}. This file should have been
108 compiled into the Emacs executable when it was built.
111 It runs the normal hook @code{before-init-hook}.
114 If appropriate, it creates a graphical frame. This is not done if the
115 options @samp{--batch} or @samp{--daemon} were specified.
118 It initializes the initial frame's faces, and sets up the menu bar
119 and tool bar if needed. If graphical frames are supported, it sets up
120 the tool bar even if the current frame is not a graphical one, since a
121 graphical frame may be created later on.
124 It use @code{custom-reevaluate-setting} to re-initialize the members
125 of the list @code{custom-delayed-init-variables}. These are any
126 pre-loaded user options whose default value depends on the run-time,
127 rather than build-time, context.
128 @xref{Building Emacs, custom-initialize-delay}.
131 @c It registers the colors available for tty frames.
134 It loads the library @file{site-start}, if it exists. This is not
135 done if the options @samp{-Q} or @samp{--no-site-file} were specified.
136 @cindex @file{site-start.el}
139 It loads your init file (@pxref{Init File}). This is not done if the
140 options @samp{-q}, @samp{-Q}, or @samp{--batch} were specified. If
141 the @samp{-u} option was specified, Emacs looks for the init file in
142 that user's home directory instead.
145 It loads the library @file{default}, if it exists. This is not done
146 if @code{inhibit-default-init} is non-@code{nil}, nor if the options
147 @samp{-q}, @samp{-Q}, or @samp{--batch} were specified.
148 @cindex @file{default.el}
151 It loads your abbrevs from the file specified by
152 @code{abbrev-file-name}, if that file exists and can be read
153 (@pxref{Abbrev Files, abbrev-file-name}). This is not done if the
154 option @samp{--batch} was specified.
157 If @code{package-enable-at-startup} is non-@code{nil}, it calls the
158 function @code{package-initialize} to activate any optional Emacs Lisp
159 package that has been installed. @xref{Packaging Basics}.
161 @vindex after-init-time
163 It sets the variable @code{after-init-time} to the value of
164 @code{current-time}. This variable was set to @code{nil} earlier;
165 setting it to the current time signals that the initialization phase
166 is over, and, together with @code{before-init-time}, provides the
167 measurement of how long it took.
170 It runs the normal hook @code{after-init-hook}.
173 If the buffer @file{*scratch*} exists and is still in Fundamental mode
174 (as it should be by default), it sets its major mode according to
175 @code{initial-major-mode}.
178 If started on a text terminal, it loads the terminal-specific
179 Lisp library (@pxref{Terminal-Specific}), and runs the hook
180 @code{tty-setup-hook}. This is not done
181 in @code{--batch} mode, nor if @code{term-file-prefix} is @code{nil}.
183 @c Now command-line calls command-line-1.
186 It displays the initial echo area message, unless you have suppressed
187 that with @code{inhibit-startup-echo-area-message}.
190 It processes any command-line options that were not handled earlier.
192 @c This next one is back in command-line, but the remaining bits of
193 @c command-line-1 are not done if noninteractive.
195 It now exits if the option @code{--batch} was specified.
198 If the @file{*scratch*} buffer exists and is empty, it inserts
199 @code{(substitute-command-keys initial-scratch-message)} into that buffer.
202 If @code{initial-buffer-choice} is a string, it visits the file (or
203 directory) with that name. If it is a function, it calls the function
204 with no arguments and selects the buffer that it returns. If one file
205 is given as a command line argument, that file is visited and its
206 buffer displayed alongside @code{initial-buffer-choice}. If more than
207 one file is given, all of the files are visited and the @file{*Buffer
208 List*} buffer is displayed alongside @code{initial-buffer-choice}.
211 @c I do not think this should be mentioned. AFAICS it is just a dodge
212 @c around inhibit-startup-screen not being settable on a site-wide basis.
213 If it is @code{t}, it selects the @file{*scratch*} buffer.
216 @c To make things nice and confusing, the next three items can be
217 @c called from two places. If displaying a startup screen, they are
218 @c called in command-line-1 before the startup screen is shown.
219 @c inhibit-startup-hooks is then set and window-setup-hook set to nil.
220 @c If not displaying a startup screen, they are are called in
222 @c FIXME? So it seems they can be called before or after the
223 @c daemon/session restore step?
226 It runs @code{emacs-startup-hook}.
229 It calls @code{frame-notice-user-settings}, which modifies the
230 parameters of the selected frame according to whatever the init files
234 It runs @code{window-setup-hook}. The only difference between this
235 hook and @code{emacs-startup-hook} is that this one runs after the
236 previously mentioned modifications to the frame parameters.
239 @cindex startup screen
240 It displays the @dfn{startup screen}, which is a special buffer that
241 contains information about copyleft and basic Emacs usage. This is
242 not done if @code{inhibit-startup-screen} or @code{initial-buffer-choice}
243 are non-@code{nil}, or if the @samp{--no-splash} or @samp{-Q} command-line
244 options were specified.
246 @c End of command-line-1.
248 @c Back to command-line from command-line-1.
250 @c This is the point at which we actually exit in batch mode, but the
251 @c last few bits of command-line-1 are not done in batch mode.
254 If the option @code{--daemon} was specified, it calls
255 @code{server-start}, and on Posix systems also detaches from the
256 controlling terminal. @xref{Emacs Server,,, emacs, The GNU Emacs
260 If started by the X session manager, it calls
261 @code{emacs-session-restore} passing it as argument the ID of the
262 previous session. @xref{Session Management}.
264 @c End of command-line.
266 @c Back to normal-top-level from command-line.
271 The following options affect some aspects of the startup sequence.
273 @defopt inhibit-startup-screen
274 This variable, if non-@code{nil}, inhibits the startup screen. In
275 that case, Emacs typically displays the @file{*scratch*} buffer; but
276 see @code{initial-buffer-choice}, below.
278 Do not set this variable in the init file of a new user, or in a way
279 that affects more than one user, as that would prevent new users from
280 receiving information about copyleft and basic Emacs usage.
282 @vindex inhibit-startup-message
283 @vindex inhibit-splash-screen
284 @code{inhibit-startup-message} and @code{inhibit-splash-screen} are
285 aliases for this variable.
288 @defopt initial-buffer-choice
289 If non-@code{nil}, this variable is a string that specifies a file or
290 directory for Emacs to display after starting up, instead of the
292 If its value is a function, Emacs calls that function which must
293 return a buffer which is then displayed.
294 If its value is @code{t}, Emacs displays the @file{*scratch*} buffer.
297 @defopt inhibit-startup-echo-area-message
298 This variable controls the display of the startup echo area message.
299 You can suppress the startup echo area message by adding text with this
300 form to your init file:
303 (setq inhibit-startup-echo-area-message
304 "@var{your-login-name}")
307 Emacs explicitly checks for an expression as shown above in your init
308 file; your login name must appear in the expression as a Lisp string
309 constant. You can also use the Customize interface. Other methods of
310 setting @code{inhibit-startup-echo-area-message} to the same value do
311 not inhibit the startup message. This way, you can easily inhibit the
312 message for yourself if you wish, but thoughtless copying of your init
313 file will not inhibit the message for someone else.
316 @defopt initial-scratch-message
317 This variable, if non-@code{nil}, should be a string, which is
318 treated as documentation to be
319 inserted into the @file{*scratch*} buffer when Emacs starts up. If it
320 is @code{nil}, the @file{*scratch*} buffer is empty.
324 The following command-line options affect some aspects of the startup
325 sequence. @xref{Initial Options,,, emacs, The GNU Emacs Manual}.
329 Do not display a splash screen.
332 Run without an interactive terminal. @xref{Batch Mode}.
335 Do not initialize any display; just start a server in the background.
339 Do not load either the init file, or the @file{default} library.
342 Do not load the @file{site-start} library.
346 Equivalent to @samp{-q --no-site-file --no-splash}.
347 @c and --no-site-lisp, but let's not mention that here.
352 @subsection The Init File
354 @cindex @file{.emacs}
355 @cindex @file{init.el}
357 When you start Emacs, it normally attempts to load your @dfn{init
358 file}. This is either a file named @file{.emacs} or @file{.emacs.el}
359 in your home directory, or a file named @file{init.el} in a
360 subdirectory named @file{.emacs.d} in your home directory.
362 Whichever place you use, you can also compile the file (@pxref{Byte
363 Compilation}); then the actual file loaded will be @file{.emacs.elc}
367 The command-line switches @samp{-q}, @samp{-Q}, and @samp{-u}
368 control whether and where to find the init file; @samp{-q} (and the
369 stronger @samp{-Q}) says not to load an init file, while @samp{-u
370 @var{user}} says to load @var{user}'s init file instead of yours.
371 @xref{Entering Emacs,,, emacs, The GNU Emacs Manual}. If neither
372 option is specified, Emacs uses the @env{LOGNAME} environment
373 variable, or the @env{USER} (most systems) or @env{USERNAME} (MS
374 systems) variable, to find your home directory and thus your init
375 file; this way, even if you have su'd, Emacs still loads your own init
376 file. If those environment variables are absent, though, Emacs uses
377 your user-id to find your home directory.
379 @cindex default init file
380 An Emacs installation may have a @dfn{default init file}, which is a
381 Lisp library named @file{default.el}. Emacs finds this file through
382 the standard search path for libraries (@pxref{How Programs Do
383 Loading}). The Emacs distribution does not come with this file; it is
384 intended for local customizations. If the default init file exists,
385 it is loaded whenever you start Emacs. But your own personal init
386 file, if any, is loaded first; if it sets @code{inhibit-default-init}
387 to a non-@code{nil} value, then Emacs does not subsequently load the
388 @file{default.el} file. In batch mode, or if you specify @samp{-q}
389 (or @samp{-Q}), Emacs loads neither your personal init file nor
390 the default init file.
392 Another file for site-customization is @file{site-start.el}. Emacs
393 loads this @emph{before} the user's init file. You can inhibit the
394 loading of this file with the option @samp{--no-site-file}.
396 @defopt site-run-file
397 This variable specifies the site-customization file to load before the
398 user's init file. Its normal value is @code{"site-start"}. The only
399 way you can change it with real effect is to do so before dumping
401 @c So why even mention it here. I imagine it is almost never changed.
404 @xref{Init Examples,, Init File Examples, emacs, The GNU Emacs Manual}, for
405 examples of how to make various commonly desired customizations in your
408 @defopt inhibit-default-init
409 If this variable is non-@code{nil}, it prevents Emacs from loading the
410 default initialization library file. The default value is @code{nil}.
413 @defvar before-init-hook
414 This normal hook is run, once, just before loading all the init files
415 (@file{site-start.el}, your init file, and @file{default.el}).
416 (The only way to change it with real effect is before dumping Emacs.)
419 @defvar after-init-hook
420 This normal hook is run, once, just after loading all the init files
421 (@file{site-start.el}, your init file, and @file{default.el}),
422 before loading the terminal-specific library (if started on a text
423 terminal) and processing the command-line action arguments.
426 @defvar emacs-startup-hook
427 This normal hook is run, once, just after handling the command line
428 arguments. In batch mode, Emacs does not run this hook.
431 @defvar window-setup-hook
432 This normal hook is very similar to @code{emacs-startup-hook}.
433 The only difference is that it runs slightly later, after setting
434 of the frame parameters. @xref{Startup Summary, window-setup-hook}.
437 @defvar user-init-file
438 This variable holds the absolute file name of the user's init file. If the
439 actual init file loaded is a compiled file, such as @file{.emacs.elc},
440 the value refers to the corresponding source file.
443 @defvar user-emacs-directory
444 This variable holds the name of the @file{.emacs.d} directory. It is
445 @file{~/.emacs.d} on all platforms but MS-DOS.
448 @node Terminal-Specific
449 @subsection Terminal-Specific Initialization
450 @cindex terminal-specific initialization
452 Each terminal type can have its own Lisp library that Emacs loads when
453 run on that type of terminal. The library's name is constructed by
454 concatenating the value of the variable @code{term-file-prefix} and the
455 terminal type (specified by the environment variable @env{TERM}).
456 Normally, @code{term-file-prefix} has the value @code{"term/"};
457 changing this is not recommended. If there is an entry matching
458 @env{TERM} in the @code{term-file-aliases} association list,
459 Emacs uses the associated value in place of @env{TERM}.
460 Emacs finds the file in the normal manner, by searching the
461 @code{load-path} directories, and trying the @samp{.elc} and
465 The usual role of a terminal-specific library is to enable special
466 keys to send sequences that Emacs can recognize. It may also need to
467 set or add to @code{input-decode-map} if the Termcap or Terminfo entry
468 does not specify all the terminal's function keys. @xref{Terminal Input}.
470 When the name of the terminal type contains a hyphen or underscore,
471 and no library is found whose name is identical to the terminal's
472 name, Emacs strips from the terminal's name the last hyphen or
473 underscore and everything that follows
474 it, and tries again. This process is repeated until Emacs finds a
475 matching library, or until there are no more hyphens or underscores in the name
476 (i.e., there is no terminal-specific library). For example, if the
477 terminal name is @samp{xterm-256color} and there is no
478 @file{term/xterm-256color.el} library, Emacs tries to load
479 @file{term/xterm.el}. If necessary, the terminal library can evaluate
480 @code{(getenv "TERM")} to find the full name of the terminal type.
482 Your init file can prevent the loading of the terminal-specific
483 library by setting the variable @code{term-file-prefix} to @code{nil}.
485 You can also arrange to override some of the actions of the
486 terminal-specific library by using @code{tty-setup-hook}. This is
487 a normal hook that Emacs runs after initializing a new text terminal.
488 You could use this hook to define initializations for terminals that do not
489 have their own libraries. @xref{Hooks}.
491 @defopt term-file-prefix
492 @cindex @env{TERM} environment variable
493 If the value of this variable is non-@code{nil}, Emacs loads a
494 terminal-specific initialization file as follows:
497 (load (concat term-file-prefix (getenv "TERM")))
501 You may set the @code{term-file-prefix} variable to @code{nil} in your
502 init file if you do not wish to load the
503 terminal-initialization file.
505 On MS-DOS, Emacs sets the @env{TERM} environment variable to @samp{internal}.
508 @defopt term-file-aliases
509 This variable is an an association list mapping terminal types to
510 their aliases. For example, an element of the form @code{("vt102"
511 . "vt100")} means to treat a terminal of type @samp{vt102} like one of
515 @defvar tty-setup-hook
516 This variable is a normal hook that Emacs runs after initializing a
517 new text terminal. (This applies when Emacs starts up in non-windowed
518 mode, and when making a tty @command{emacsclient} connection.) The
519 hook runs after loading your init file (if applicable) and the
520 terminal-specific Lisp file, so you can use it to adjust the
521 definitions made by that file.
523 For a related feature, @pxref{Init File, window-setup-hook}.
526 @node Command-Line Arguments
527 @subsection Command-Line Arguments
528 @cindex command-line arguments
530 You can use command-line arguments to request various actions when
531 you start Emacs. Note that the recommended way of using Emacs is to
532 start it just once, after logging in, and then do all editing in the same
533 Emacs session (@pxref{Entering Emacs,,, emacs, The GNU Emacs Manual}).
534 For this reason, you might not use command-line arguments very often;
535 nonetheless, they can be useful when invoking Emacs from session
536 scripts or debugging Emacs. This section describes how Emacs
537 processes command-line arguments.
540 This function parses the command line that Emacs was called with,
541 processes it, and (amongst other things) loads the user's init file and
542 displays the startup messages.
545 @defvar command-line-processed
546 The value of this variable is @code{t} once the command line has been
549 If you redump Emacs by calling @code{dump-emacs} (@pxref{Building
550 Emacs}), you may wish to set this variable to @code{nil} first in
551 order to cause the new dumped Emacs to process its new command-line
555 @defvar command-switch-alist
556 @cindex switches on command line
557 @cindex options on command line
558 @cindex command-line options
559 This variable is an alist of user-defined command-line options and
560 associated handler functions. By default it is empty, but you can
561 add elements if you wish.
563 A @dfn{command-line option} is an argument on the command line, which
570 The elements of the @code{command-switch-alist} look like this:
573 (@var{option} . @var{handler-function})
576 The @sc{car}, @var{option}, is a string, the name of a command-line
577 option (not including the initial hyphen). The @var{handler-function}
578 is called to handle @var{option}, and receives the option name as its
581 In some cases, the option is followed in the command line by an
582 argument. In these cases, the @var{handler-function} can find all the
583 remaining command-line arguments in the variable
584 @code{command-line-args-left} (see below). (The entire list of
585 command-line arguments is in @code{command-line-args}.)
587 The command-line arguments are parsed by the @code{command-line-1}
588 function in the @file{startup.el} file. See also @ref{Emacs
589 Invocation, , Command Line Arguments for Emacs Invocation, emacs, The
593 @defvar command-line-args
594 The value of this variable is the list of command-line arguments passed
598 @defvar command-line-args-left
600 The value of this variable is the list of command-line arguments that
601 have not yet been processed.
602 @c Don't mention this, since it is a "bad name for a dynamically bound variable"
603 @c @code{argv} is an alias for this.
606 @defvar command-line-functions
607 This variable's value is a list of functions for handling an
608 unrecognized command-line argument. Each time the next argument to be
609 processed has no special meaning, the functions in this list are called,
610 in order of appearance, until one of them returns a non-@code{nil}
613 These functions are called with no arguments. They can access the
614 command-line argument under consideration through the variable
615 @code{argi}, which is bound temporarily at this point. The remaining
616 arguments (not including the current one) are in the variable
617 @code{command-line-args-left}.
619 When a function recognizes and processes the argument in @code{argi}, it
620 should return a non-@code{nil} value to say it has dealt with that
621 argument. If it has also dealt with some of the following arguments, it
622 can indicate that by deleting them from @code{command-line-args-left}.
624 If all of these functions return @code{nil}, then the argument is treated
625 as a file name to visit.
629 @section Getting Out of Emacs
630 @cindex exiting Emacs
632 There are two ways to get out of Emacs: you can kill the Emacs job,
633 which exits permanently, or you can suspend it, which permits you to
634 reenter the Emacs process later. (In a graphical environment, you can
635 of course simply switch to another application without doing anything
636 special to Emacs, then switch back to Emacs when you want.)
639 * Killing Emacs:: Exiting Emacs irreversibly.
640 * Suspending Emacs:: Exiting Emacs reversibly.
644 @subsection Killing Emacs
645 @cindex killing Emacs
647 Killing Emacs means ending the execution of the Emacs process.
648 If you started Emacs from a terminal, the parent process normally
649 resumes control. The low-level primitive for killing Emacs is
652 @deffn Command kill-emacs &optional exit-data
653 This command calls the hook @code{kill-emacs-hook}, then exits the
654 Emacs process and kills it.
656 If @var{exit-data} is an integer, that is used as the exit status of
657 the Emacs process. (This is useful primarily in batch operation; see
660 If @var{exit-data} is a string, its contents are stuffed into the
661 terminal input buffer so that the shell (or whatever program next reads
662 input) can read them.
668 @cindex operating system signal
669 The @code{kill-emacs} function is normally called via the
670 higher-level command @kbd{C-x C-c}
671 (@code{save-buffers-kill-terminal}). @xref{Exiting,,, emacs, The GNU
672 Emacs Manual}. It is also called automatically if Emacs receives a
673 @code{SIGTERM} or @code{SIGHUP} operating system signal (e.g., when the
674 controlling terminal is disconnected), or if it receives a
675 @code{SIGINT} signal while running in batch mode (@pxref{Batch Mode}).
677 @defvar kill-emacs-hook
678 This normal hook is run by @code{kill-emacs}, before it kills Emacs.
680 Because @code{kill-emacs} can be called in situations where user
681 interaction is impossible (e.g., when the terminal is disconnected),
682 functions on this hook should not attempt to interact with the user.
683 If you want to interact with the user when Emacs is shutting down, use
684 @code{kill-emacs-query-functions}, described below.
687 When Emacs is killed, all the information in the Emacs process,
688 aside from files that have been saved, is lost. Because killing Emacs
689 inadvertently can lose a lot of work, the
690 @code{save-buffers-kill-terminal} command queries for confirmation if
691 you have buffers that need saving or subprocesses that are running.
692 It also runs the abnormal hook @code{kill-emacs-query-functions}:
694 @defvar kill-emacs-query-functions
695 When @code{save-buffers-kill-terminal} is killing Emacs, it calls the
696 functions in this hook, after asking the standard questions and before
697 calling @code{kill-emacs}. The functions are called in order of
698 appearance, with no arguments. Each function can ask for additional
699 confirmation from the user. If any of them returns @code{nil},
700 @code{save-buffers-kill-emacs} does not kill Emacs, and does not run
701 the remaining functions in this hook. Calling @code{kill-emacs}
702 directly does not run this hook.
705 @node Suspending Emacs
706 @subsection Suspending Emacs
707 @cindex suspending Emacs
709 On text terminals, it is possible to @dfn{suspend Emacs}, which
710 means stopping Emacs temporarily and returning control to its superior
711 process, which is usually the shell. This allows you to resume
712 editing later in the same Emacs process, with the same buffers, the
713 same kill ring, the same undo history, and so on. To resume Emacs,
714 use the appropriate command in the parent shell---most likely
717 @cindex controlling terminal
718 Suspending works only on a terminal device from which the Emacs
719 session was started. We call that device the @dfn{controlling
720 terminal} of the session. Suspending is not allowed if the
721 controlling terminal is a graphical terminal. Suspending is usually
722 not relevant in graphical environments, since you can simply switch to
723 another application without doing anything special to Emacs.
725 @c FIXME? Are there any systems Emacs still supports that do not
728 Some operating systems (those without @code{SIGTSTP}, or MS-DOS) do
729 not support suspension of jobs; on these systems, suspension
730 actually creates a new shell temporarily as a subprocess of Emacs.
731 Then you would exit the shell to return to Emacs.
733 @deffn Command suspend-emacs &optional string
734 This function stops Emacs and returns control to the superior process.
735 If and when the superior process resumes Emacs, @code{suspend-emacs}
736 returns @code{nil} to its caller in Lisp.
738 This function works only on the controlling terminal of the Emacs
739 session; to relinquish control of other tty devices, use
740 @code{suspend-tty} (see below). If the Emacs session uses more than
741 one terminal, you must delete the frames on all the other terminals
742 before suspending Emacs, or this function signals an error.
743 @xref{Multiple Terminals}.
745 If @var{string} is non-@code{nil}, its characters are sent to Emacs's
746 superior shell, to be read as terminal input.
747 @c FIXME? It seems to me that shell does echo STRING.
748 The characters in @var{string} are not echoed by the superior shell;
749 only the results appear.
751 Before suspending, @code{suspend-emacs} runs the normal hook
752 @code{suspend-hook}. After the user resumes Emacs,
753 @code{suspend-emacs} runs the normal hook @code{suspend-resume-hook}.
756 The next redisplay after resumption will redraw the entire screen,
757 unless the variable @code{no-redraw-on-reenter} is non-@code{nil}.
758 @xref{Refresh Screen}.
760 Here is an example of how you could use these hooks:
764 (add-hook 'suspend-hook
765 (lambda () (or (y-or-n-p "Really suspend? ")
766 (error "Suspend canceled"))))
768 (add-hook 'suspend-resume-hook (lambda () (message "Resumed!")
771 @c The sit-for prevents the @code{nil} that suspend-emacs returns
772 @c hiding the message.
774 Here is what you would see upon evaluating @code{(suspend-emacs "pwd")}:
778 ---------- Buffer: Minibuffer ----------
779 Really suspend? @kbd{y}
780 ---------- Buffer: Minibuffer ----------
784 ---------- Parent Shell ----------
790 ---------- Echo Area ----------
795 @c FIXME? AFAICS, it is echoed.
796 Note that @samp{pwd} is not echoed after Emacs is suspended. But it
797 is read and executed by the shell.
801 This variable is a normal hook that Emacs runs before suspending.
804 @defvar suspend-resume-hook
805 This variable is a normal hook that Emacs runs on resuming
809 @defun suspend-tty &optional tty
810 If @var{tty} specifies a terminal device used by Emacs, this function
811 relinquishes the device and restores it to its prior state. Frames
812 that used the device continue to exist, but are not updated and Emacs
813 doesn't read input from them. @var{tty} can be a terminal object, a
814 frame (meaning the terminal for that frame), or @code{nil} (meaning
815 the terminal for the selected frame). @xref{Multiple Terminals}.
817 If @var{tty} is already suspended, this function does nothing.
819 @vindex suspend-tty-functions
820 This function runs the hook @code{suspend-tty-functions}, passing the
821 terminal object as an argument to each function.
824 @defun resume-tty &optional tty
825 This function resumes the previously suspended terminal device
826 @var{tty}; where @var{tty} has the same possible values as it does
827 for @code{suspend-tty}.
829 @vindex resume-tty-functions
830 This function reopens the terminal device, re-initializes it, and
831 redraws it with that terminal's selected frame. It then runs the
832 hook @code{resume-tty-functions}, passing the terminal object as an
833 argument to each function.
835 If the same device is already used by another Emacs terminal, this
836 function signals an error. If @var{tty} is not suspended, this
837 function does nothing.
840 @defun controlling-tty-p &optional tty
841 This function returns non-@code{nil} if @var{tty} is the
842 controlling terminal of the Emacs session; @var{tty} can be a
843 terminal object, a frame (meaning the terminal for that frame), or
844 @code{nil} (meaning the terminal for the selected frame).
847 @deffn Command suspend-frame
848 This command @dfn{suspends} a frame. For GUI frames, it calls
849 @code{iconify-frame} (@pxref{Visibility of Frames}); for frames on
850 text terminals, it calls either @code{suspend-emacs} or
851 @code{suspend-tty}, depending on whether the frame is displayed on the
852 controlling terminal device or not.
855 @node System Environment
856 @section Operating System Environment
857 @cindex operating system environment
859 Emacs provides access to variables in the operating system environment
860 through various functions. These variables include the name of the
861 system, the user's @acronym{UID}, and so on.
863 @defvar system-configuration
864 This variable holds the standard GNU configuration name for the
865 hardware/software configuration of your system, as a string. For
866 example, a typical value for a 64-bit GNU/Linux system is
867 @samp{"x86_64-unknown-linux-gnu"}.
870 @cindex system type and name
872 The value of this variable is a symbol indicating the type of operating
873 system Emacs is running on. The possible values are:
880 Berkeley BSD and its variants.
883 Cygwin, a Posix layer on top of MS-Windows.
889 The GNU system (using the GNU kernel, which consists of the HURD and Mach).
892 A GNU/Linux system---that is, a variant GNU system, using the Linux
893 kernel. (These systems are the ones people often call ``Linux'', but
894 actually Linux is just the kernel, not the whole system.)
897 A GNU (glibc-based) system with a FreeBSD kernel.
900 Hewlett-Packard HPUX operating system.
903 Google Native Client (@acronym{NaCl}) sandboxing system.
906 Microsoft's DOS@. Emacs compiled with DJGPP for MS-DOS binds
907 @code{system-type} to @code{ms-dos} even when you run it on MS-Windows.
913 Microsoft Windows NT, 9X and later. The value of @code{system-type}
914 is always @code{windows-nt}, e.g., even on Windows 10.
918 We do not wish to add new symbols to make finer distinctions unless it
919 is absolutely necessary! In fact, we hope to eliminate some of these
920 alternatives in the future. If you need to make a finer distinction
921 than @code{system-type} allows for, you can test
922 @code{system-configuration}, e.g., against a regexp.
926 This function returns the name of the machine you are running on, as a
930 @c FIXME seems like this section is not the best place for this option?
931 @defopt mail-host-address
932 If this variable is non-@code{nil}, it is used instead of
933 @code{system-name} for purposes of generating email addresses. For
934 example, it is used when constructing the default value of
935 @code{user-mail-address}. @xref{User Identification}. (Since this is
936 done when Emacs starts up, the value actually used is the one saved when
937 Emacs was dumped. @xref{Building Emacs}.)
938 @c FIXME sounds like should probably give this a :set-after and some
939 @c custom-initialize-delay voodoo.
942 @deffn Command getenv var &optional frame
943 @cindex environment variable access
944 This function returns the value of the environment variable @var{var},
945 as a string. @var{var} should be a string. If @var{var} is undefined
946 in the environment, @code{getenv} returns @code{nil}. It returns
947 @samp{""} if @var{var} is set but null. Within Emacs, a list of environment
948 variables and their values is kept in the variable @code{process-environment}.
957 The shell command @code{printenv} prints all or part of the environment:
962 PATH=/usr/local/bin:/usr/bin:/bin
974 @deffn Command setenv variable &optional value substitute
975 This command sets the value of the environment variable named
976 @var{variable} to @var{value}. @var{variable} should be a string.
977 Internally, Emacs Lisp can handle any string. However, normally
978 @var{variable} should be a valid shell identifier, that is, a sequence
979 of letters, digits and underscores, starting with a letter or
980 underscore. Otherwise, errors may occur if subprocesses of Emacs try
981 to access the value of @var{variable}. If @var{value} is omitted or
982 @code{nil} (or, interactively, with a prefix argument), @code{setenv}
983 removes @var{variable} from the environment. Otherwise, @var{value}
986 @c FIXME: Document 'substitute-env-vars'? --xfq
987 If the optional argument @var{substitute} is non-@code{nil}, Emacs
988 calls the function @code{substitute-env-vars} to expand any
989 environment variables in @var{value}.
991 @code{setenv} works by modifying @code{process-environment}; binding
992 that variable with @code{let} is also reasonable practice.
994 @code{setenv} returns the new value of @var{variable}, or @code{nil}
995 if it removed @var{variable} from the environment.
998 @defvar process-environment
999 This variable is a list of strings, each describing one environment
1000 variable. The functions @code{getenv} and @code{setenv} work by means
1006 @result{} ("PATH=/usr/local/bin:/usr/bin:/bin"
1017 If @code{process-environment} contains multiple elements that
1018 specify the same environment variable, the first of these elements
1019 specifies the variable, and the others are ignored.
1022 @defvar initial-environment
1023 This variable holds the list of environment variables Emacs inherited
1024 from its parent process when Emacs started.
1027 @defvar path-separator
1028 This variable holds a string that says which character separates
1029 directories in a search path (as found in an environment variable). Its
1030 value is @code{":"} for Unix and GNU systems, and @code{";"} for MS systems.
1033 @defun parse-colon-path path
1034 This function takes a search path string such as the value of
1035 the @env{PATH} environment variable, and splits it at the separators,
1036 returning a list of directory names. @code{nil} in this list means
1037 the current directory. Although the function's name says
1038 ``colon'', it actually uses the value of @code{path-separator}.
1041 (parse-colon-path ":/foo:/bar")
1042 @result{} (nil "/foo/" "/bar/")
1046 @defvar invocation-name
1047 This variable holds the program name under which Emacs was invoked. The
1048 value is a string, and does not include a directory name.
1051 @defvar invocation-directory
1052 This variable holds the directory from which the Emacs executable was
1053 invoked, or @code{nil} if that directory cannot be determined.
1056 @defvar installation-directory
1057 If non-@code{nil}, this is a directory within which to look for the
1058 @file{lib-src} and @file{etc} subdirectories. In an installed Emacs,
1059 it is normally @code{nil}. It is non-@code{nil}
1060 when Emacs can't find those directories in their standard installed
1061 locations, but can find them in a directory related somehow to the one
1062 containing the Emacs executable (i.e., @code{invocation-directory}).
1065 @defun load-average &optional use-float
1066 This function returns the current 1-minute, 5-minute, and 15-minute
1067 system load averages, in a list. The load average indicates the
1068 number of processes trying to run on the system.
1070 By default, the values are integers that are 100 times the system load
1071 averages, but if @var{use-float} is non-@code{nil}, then they are
1072 returned as floating-point numbers without multiplying by 100.
1074 If it is impossible to obtain the load average, this function signals
1075 an error. On some platforms, access to load averages requires
1076 installing Emacs as setuid or setgid so that it can read kernel
1077 information, and that usually isn't advisable.
1078 @c FIXME which platforms are these? Are they still relevant?
1080 If the 1-minute load average is available, but the 5- or 15-minute
1081 averages are not, this function returns a shortened list containing
1082 the available averages.
1087 @result{} (169 48 36)
1091 @result{} (1.69 0.48 0.36)
1095 The shell command @code{uptime} returns similar information.
1099 This function returns the process @acronym{ID} of the Emacs process,
1103 @defvar tty-erase-char
1104 This variable holds the erase character that was selected
1105 in the system's terminal driver, before Emacs was started.
1106 @c FIXME? Seems untrue since 23.1. For me, it is 0.
1107 @c The value is @code{nil} if Emacs is running under a window system.
1110 @node User Identification
1111 @section User Identification
1112 @cindex user identification
1114 @defvar init-file-user
1115 This variable says which user's init files should be used by
1116 Emacs---or @code{nil} if none. @code{""} stands for the user who
1117 originally logged in. The value reflects command-line options such as
1118 @samp{-q} or @samp{-u @var{user}}.
1120 Lisp packages that load files of customizations, or any other sort of
1121 user profile, should obey this variable in deciding where to find it.
1122 They should load the profile of the user name found in this variable.
1123 If @code{init-file-user} is @code{nil}, meaning that the @samp{-q},
1124 @samp{-Q}, or @samp{-batch} option was used, then Lisp packages should
1125 not load any customization files or user profile.
1128 @defopt user-mail-address
1129 This holds the nominal email address of the user who is using Emacs.
1130 Emacs normally sets this variable to a default value after reading your
1131 init files, but not if you have already set it. So you can set the
1132 variable to some other value in your init file if you do not
1133 want to use the default value.
1136 @defun user-login-name &optional uid
1137 This function returns the name under which the user is logged in.
1138 It uses the environment variables @env{LOGNAME} or @env{USER} if
1139 either is set. Otherwise, the value is based on the effective
1140 @acronym{UID}, not the real @acronym{UID}.
1142 If you specify @var{uid} (a number), the result is the user name that
1143 corresponds to @var{uid}, or @code{nil} if there is no such user.
1146 @defun user-real-login-name
1147 This function returns the user name corresponding to Emacs's real
1148 @acronym{UID}. This ignores the effective @acronym{UID}, and the
1149 environment variables @env{LOGNAME} and @env{USER}.
1152 @defun user-full-name &optional uid
1153 This function returns the full name of the logged-in user---or the value
1154 of the environment variable @env{NAME}, if that is set.
1156 If the Emacs process's user-id does not correspond to any known user (and
1157 provided @code{NAME} is not set), the result is @code{"unknown"}.
1159 If @var{uid} is non-@code{nil}, then it should be a number (a user-id)
1160 or a string (a login name). Then @code{user-full-name} returns the full
1161 name corresponding to that user-id or login name. If you specify a
1162 user-id or login name that isn't defined, it returns @code{nil}.
1165 @vindex user-full-name
1166 @vindex user-real-login-name
1167 @vindex user-login-name
1168 The symbols @code{user-login-name}, @code{user-real-login-name} and
1169 @code{user-full-name} are variables as well as functions. The functions
1170 return the same values that the variables hold. These variables allow
1171 you to fake out Emacs by telling the functions what to return. The
1172 variables are also useful for constructing frame titles (@pxref{Frame
1176 @defun user-real-uid
1177 This function returns the real @acronym{UID} of the user.
1178 The value may be floating point, in the (unlikely) event that
1179 the UID is too large to fit in a Lisp integer.
1183 This function returns the effective @acronym{UID} of the user.
1184 The value may be floating point.
1189 This function returns the effective @acronym{GID} of the Emacs process.
1190 The value may be floating point.
1193 @defun group-real-gid
1194 This function returns the real @acronym{GID} of the Emacs process.
1195 The value may be floating point.
1199 This function returns a list of strings, listing the user names on the
1200 system. If Emacs cannot retrieve this information, the return value
1201 is a list containing just the value of @code{user-real-login-name}.
1205 @defun system-groups
1206 This function returns a list of strings, listing the names of user
1207 groups on the system. If Emacs cannot retrieve this information, the
1208 return value is @code{nil}.
1213 @section Time of Day
1216 This section explains how to determine the current time and time
1220 Most of these functions represent time as a list of four integers
1221 @code{(@var{sec-high} @var{sec-low} @var{microsec} @var{picosec})}.
1222 This represents the number of seconds from the @dfn{epoch} (January
1223 1, 1970 at 00:00 UTC), using the formula:
1225 @var{high} * 2**16 + @var{low} + @var{micro} * 10**@minus{}6 +
1226 @var{pico} * 10**@minus{}12.
1229 $high*2^{16} + low + micro*10^{-6} + pico*10^{-12}$.
1231 The return value of @code{current-time} represents time using this
1232 form, as do the timestamps in the return values of other functions
1233 such as @code{file-attributes} (@pxref{Definition of
1234 file-attributes}). In some cases, functions may return two- or
1235 three-element lists, with omitted @var{microsec} and @var{picosec}
1236 components defaulting to zero.
1239 Function arguments, e.g., the @var{time} argument to
1240 @code{current-time-string}, accept a more-general @dfn{time value}
1241 format, which can be a list of integers as above, or a single number
1242 for seconds since the epoch, or @code{nil} for the current time. You
1243 can convert a time value into a human-readable string using
1244 @code{current-time-string} and @code{format-time-string}, into a list
1245 of integers using @code{seconds-to-time}, and into other forms using
1246 @code{decode-time} and @code{float-time}. These functions are
1247 described in the following sections.
1249 @defun current-time-string &optional time zone
1250 This function returns the current time and date as a human-readable
1251 string. The format does not vary for the initial part of the string,
1252 which contains the day of week, month, day of month, and time of day
1253 in that order: the number of characters used for these fields is
1254 always the same, so you can reliably
1255 use @code{substring} to extract them. You should count
1256 characters from the beginning of the string rather than from the end,
1257 as the year might not have exactly four digits, and additional
1258 information may some day be added at the end.
1260 The argument @var{time}, if given, specifies a time to format,
1261 instead of the current time. The optional argument @var{zone}
1262 defaults to the current time zone rule.
1266 (current-time-string)
1267 @result{} "Wed Oct 14 22:21:05 1987"
1273 This function returns the current time, represented as a list of four
1274 integers @code{(@var{sec-high} @var{sec-low} @var{microsec} @var{picosec})}.
1275 These integers have trailing zeros on systems that return time with
1276 lower resolutions. On all current machines @var{picosec} is a
1277 multiple of 1000, but this may change as higher-resolution clocks
1281 @defun float-time &optional time
1282 This function returns the current time as a floating-point number of
1283 seconds since the epoch. The optional argument @var{time}, if
1284 given, specifies a time to convert instead of the current time.
1286 @emph{Warning}: Since the result is floating point, it may not be
1287 exact. Do not use this function if precise time stamps are required.
1289 @code{time-to-seconds} is an alias for this function.
1292 @defun seconds-to-time time
1293 This function converts a time value to list-of-integer form.
1294 For example, if @var{time} is a number, @code{(time-to-seconds
1295 (seconds-to-time @var{time}))} equals the number unless overflow
1296 or rounding errors occur.
1299 @defun current-time-zone &optional time zone
1300 @cindex time zone, current
1301 This function returns a list describing the time zone that the user is
1304 The value has the form @code{(@var{offset} @var{name})}. Here
1305 @var{offset} is an integer giving the number of seconds ahead of Universal Time
1306 (east of Greenwich). A negative value means west of Greenwich. The
1307 second element, @var{name}, is a string giving the name of the time
1308 zone. Both elements change when daylight saving time begins or ends;
1309 if the user has specified a time zone that does not use a seasonal time
1310 adjustment, then the value is constant through time.
1312 If the operating system doesn't supply all the information necessary to
1313 compute the value, the unknown elements of the list are @code{nil}.
1315 The argument @var{time}, if given, specifies a time value to
1316 analyze instead of the current time. The optional argument @var{zone}
1317 defaults to the current time zone rule.
1320 @vindex TZ, environment variable
1321 The default time zone is determined by the @env{TZ} environment
1322 variable. @xref{System Environment}. For example, you can tell Emacs
1323 to default to Universal Time with @code{(setenv "TZ" "UTC0")}. If
1324 @env{TZ} is not in the environment, Emacs uses system wall clock time,
1325 which is a platform-dependent default time zone.
1327 @cindex time zone rule
1328 Functions that convert to and from local time accept an optional
1329 @dfn{time zone rule} argument, which specifies the conversion's time
1330 zone and daylight saving time history. If the time zone rule is
1331 omitted or @code{nil}, the conversion uses Emacs's default time zone.
1332 If it is @code{t}, the conversion uses Universal Time. If it is
1333 @code{wall}, the conversion uses the system wall clock time. If it is
1334 a string, the conversion uses the time zone rule equivalent to setting
1335 @env{TZ} to that string.
1337 @node Time Conversion
1338 @section Time Conversion
1339 @cindex calendrical information
1340 @cindex time conversion
1342 These functions convert time values (@pxref{Time of Day}) into
1343 calendrical information and vice versa.
1345 Many 32-bit operating systems are limited to system times containing
1346 32 bits of information in their seconds component; these systems
1347 typically handle only the times from 1901-12-13 20:45:52 through
1348 2038-01-19 03:14:07 Universal Time. However, 64-bit and some 32-bit operating
1349 systems have larger seconds components, and can represent times far in
1352 Time conversion functions always use the Gregorian calendar, even
1353 for dates before the Gregorian calendar was introduced. Year numbers
1354 count the number of years since the year 1 B.C., and do not skip zero
1355 as traditional Gregorian years do; for example, the year number
1356 @minus{}37 represents the Gregorian year 38 B.C@.
1358 @defun decode-time &optional time zone
1359 This function converts a time value into calendrical information. If
1360 you don't specify @var{time}, it decodes the current time, and similarly
1361 @var{zone} defaults to the current time zone rule. The return
1362 value is a list of nine elements, as follows:
1365 (@var{seconds} @var{minutes} @var{hour} @var{day} @var{month} @var{year} @var{dow} @var{dst} @var{utcoff})
1368 Here is what the elements mean:
1372 The number of seconds past the minute, as an integer between 0 and 59.
1373 On some operating systems, this is 60 for leap seconds.
1375 The number of minutes past the hour, as an integer between 0 and 59.
1377 The hour of the day, as an integer between 0 and 23.
1379 The day of the month, as an integer between 1 and 31.
1381 The month of the year, as an integer between 1 and 12.
1383 The year, an integer typically greater than 1900.
1385 The day of week, as an integer between 0 and 6, where 0 stands for
1388 @code{t} if daylight saving time is effect, otherwise @code{nil}.
1390 An integer indicating the Universal Time offset in seconds, i.e., the number of
1391 seconds east of Greenwich.
1394 @strong{Common Lisp Note:} Common Lisp has different meanings for
1395 @var{dow} and @var{utcoff}.
1398 @defun encode-time seconds minutes hour day month year &optional zone
1399 This function is the inverse of @code{decode-time}. It converts seven
1400 items of calendrical data into a list-of-integer time value. For the
1401 meanings of the arguments, see the table above under
1404 Year numbers less than 100 are not treated specially. If you want them
1405 to stand for years above 1900, or years above 2000, you must alter them
1406 yourself before you call @code{encode-time}.
1408 The optional argument @var{zone} defaults to the current time zone rule.
1409 In addition to the usual time zone rule values, it can also be a list
1410 (as you would get from @code{current-time-zone}) or an integer (as
1411 from @code{decode-time}), applied without any further alteration for
1412 daylight saving time.
1414 If you pass more than seven arguments to @code{encode-time}, the first
1415 six are used as @var{seconds} through @var{year}, the last argument is
1416 used as @var{zone}, and the arguments in between are ignored. This
1417 feature makes it possible to use the elements of a list returned by
1418 @code{decode-time} as the arguments to @code{encode-time}, like this:
1421 (apply 'encode-time (decode-time @dots{}))
1424 You can perform simple date arithmetic by using out-of-range values for
1425 the @var{seconds}, @var{minutes}, @var{hour}, @var{day}, and @var{month}
1426 arguments; for example, day 0 means the day preceding the given month.
1428 The operating system puts limits on the range of possible time values;
1429 if you try to encode a time that is out of range, an error results.
1430 For instance, years before 1970 do not work on some systems;
1431 on others, years as early as 1901 do work.
1435 @section Parsing and Formatting Times
1436 @cindex time parsing
1437 @cindex time formatting
1438 @cindex formatting time values
1440 These functions convert time values to text in a string, and vice versa.
1441 Time values are lists of two to four integers (@pxref{Time of Day}).
1443 @defun date-to-time string
1444 This function parses the time-string @var{string} and returns the
1445 corresponding time value.
1448 @defun format-time-string format-string &optional time zone
1450 This function converts @var{time} (or the current time, if
1451 @var{time} is omitted) to a string according to
1452 @var{format-string}. The conversion uses the time zone rule @var{zone}
1453 (or the current time zone rule, if omitted). The argument
1454 @var{format-string} may contain @samp{%}-sequences which say to
1455 substitute parts of the time. Here is a table of what the
1456 @samp{%}-sequences mean:
1460 This stands for the abbreviated name of the day of week.
1462 This stands for the full name of the day of week.
1464 This stands for the abbreviated name of the month.
1466 This stands for the full name of the month.
1468 This is a synonym for @samp{%x %X}.
1470 This has a locale-specific meaning. In the default locale (named C), it
1471 is equivalent to @samp{%A, %B %e, %Y}.
1473 This stands for the day of month, zero-padded.
1475 This is a synonym for @samp{%m/%d/%y}.
1477 This stands for the day of month, blank-padded.
1479 This is a synonym for @samp{%b}.
1481 This stands for the hour (00--23).
1483 This stands for the hour (01--12).
1485 This stands for the day of the year (001--366).
1487 This stands for the hour (0--23), blank padded.
1489 This stands for the hour (1--12), blank padded.
1491 This stands for the month (01--12).
1493 This stands for the minute (00--59).
1495 This stands for a newline.
1497 This stands for the nanoseconds (000000000--999999999). To ask for
1498 fewer digits, use @samp{%3N} for milliseconds, @samp{%6N} for
1499 microseconds, etc. Any excess digits are discarded, without rounding.
1501 This stands for @samp{AM} or @samp{PM}, as appropriate.
1503 This is a synonym for @samp{%I:%M:%S %p}.
1505 This is a synonym for @samp{%H:%M}.
1507 This stands for the seconds (00--59).
1509 This stands for a tab character.
1511 This is a synonym for @samp{%H:%M:%S}.
1513 This stands for the week of the year (01--52), assuming that weeks
1516 This stands for the numeric day of week (0--6). Sunday is day 0.
1518 This stands for the week of the year (01--52), assuming that weeks
1521 This has a locale-specific meaning. In the default locale (named
1522 @samp{C}), it is equivalent to @samp{%D}.
1524 This has a locale-specific meaning. In the default locale (named
1525 @samp{C}), it is equivalent to @samp{%T}.
1527 This stands for the year without century (00--99).
1529 This stands for the year with century.
1531 This stands for the time zone abbreviation (e.g., @samp{EST}).
1533 This stands for the time zone numerical offset (e.g., @samp{-0500}).
1536 You can also specify the field width and type of padding for any of
1537 these @samp{%}-sequences. This works as in @code{printf}: you write
1538 the field width as digits in the middle of a @samp{%}-sequences. If you
1539 start the field width with @samp{0}, it means to pad with zeros. If you
1540 start the field width with @samp{_}, it means to pad with spaces.
1542 For example, @samp{%S} specifies the number of seconds since the minute;
1543 @samp{%03S} means to pad this with zeros to 3 positions, @samp{%_3S} to
1544 pad with spaces to 3 positions. Plain @samp{%3S} pads with zeros,
1545 because that is how @samp{%S} normally pads to two positions.
1547 The characters @samp{E} and @samp{O} act as modifiers when used between
1548 @samp{%} and one of the letters in the table above. @samp{E} specifies
1549 using the current locale's alternative version of the date and time.
1550 In a Japanese locale, for example, @code{%Ex} might yield a date format
1551 based on the Japanese Emperors' reigns. @samp{E} is allowed in
1552 @samp{%Ec}, @samp{%EC}, @samp{%Ex}, @samp{%EX}, @samp{%Ey}, and
1555 @samp{O} means to use the current locale's alternative
1556 representation of numbers, instead of the ordinary decimal digits. This
1557 is allowed with most letters, all the ones that output numbers.
1559 This function uses the C library function @code{strftime}
1560 (@pxref{Formatting Calendar Time,,, libc, The GNU C Library Reference
1561 Manual}) to do most of the work. In order to communicate with that
1562 function, it first encodes its argument using the coding system
1563 specified by @code{locale-coding-system} (@pxref{Locales}); after
1564 @code{strftime} returns the resulting string,
1565 @code{format-time-string} decodes the string using that same coding
1569 @defun format-seconds format-string seconds
1570 This function converts its argument @var{seconds} into a string of
1571 years, days, hours, etc., according to @var{format-string}. The
1572 argument @var{format-string} may contain @samp{%}-sequences which
1573 control the conversion. Here is a table of what the
1574 @samp{%}-sequences mean:
1579 The integer number of 365-day years.
1582 The integer number of days.
1585 The integer number of hours.
1588 The integer number of minutes.
1591 The integer number of seconds.
1593 Non-printing control flag. When it is used, other specifiers must be
1594 given in the order of decreasing size, i.e., years before days, hours
1595 before minutes, etc. Nothing will be produced in the result string to
1596 the left of @samp{%z} until the first non-zero conversion is
1597 encountered. For example, the default format used by
1598 @code{emacs-uptime} (@pxref{Processor Run Time, emacs-uptime})
1599 @w{@code{"%Y, %D, %H, %M, %z%S"}} means that the number of seconds
1600 will always be produced, but years, days, hours, and minutes will only
1601 be shown if they are non-zero.
1603 Produces a literal @samp{%}.
1606 Upper-case format sequences produce the units in addition to the
1607 numbers, lower-case formats produce only the numbers.
1609 You can also specify the field width by following the @samp{%} with a
1610 number; shorter numbers will be padded with blanks. An optional
1611 period before the width requests zero-padding instead. For example,
1612 @code{"%.3Y"} might produce @code{"004 years"}.
1614 @emph{Warning:} This function works only with values of @var{seconds}
1615 that don't exceed @code{most-positive-fixnum} (@pxref{Integer Basics,
1616 most-positive-fixnum}).
1619 @node Processor Run Time
1620 @section Processor Run time
1621 @cindex processor run time
1622 @cindex Emacs process run time
1624 Emacs provides several functions and primitives that return time,
1625 both elapsed and processor time, used by the Emacs process.
1627 @deffn Command emacs-uptime &optional format
1628 @cindex uptime of Emacs
1629 This function returns a string representing the Emacs
1630 @dfn{uptime}---the elapsed wall-clock time this instance of Emacs is
1631 running. The string is formatted by @code{format-seconds} according
1632 to the optional argument @var{format}. For the available format
1633 descriptors, see @ref{Time Parsing, format-seconds}. If @var{format}
1634 is @code{nil} or omitted, it defaults to @code{"%Y, %D, %H, %M,
1637 When called interactively, it prints the uptime in the echo area.
1640 @defun get-internal-run-time
1641 This function returns the processor run time used by Emacs as a list
1642 of four integers: @code{(@var{sec-high} @var{sec-low} @var{microsec}
1643 @var{picosec})}, using the same format as @code{current-time}
1644 (@pxref{Time of Day}).
1646 Note that the time returned by this function excludes the time Emacs
1647 was not using the processor, and if the Emacs process has several
1648 threads, the returned value is the sum of the processor times used up
1649 by all Emacs threads.
1651 If the system doesn't provide a way to determine the processor run
1652 time, @code{get-internal-run-time} returns the same time as
1653 @code{current-time}.
1656 @deffn Command emacs-init-time
1657 This function returns the duration of the Emacs initialization
1658 (@pxref{Startup Summary}) in seconds, as a string. When called
1659 interactively, it prints the duration in the echo area.
1662 @node Time Calculations
1663 @section Time Calculations
1664 @cindex time calculations
1665 @cindex comparing time values
1666 @cindex calendrical computations
1668 These functions perform calendrical computations using time values
1669 (@pxref{Time of Day}).
1671 @defun time-less-p t1 t2
1672 This returns @code{t} if time value @var{t1} is less than time value
1676 @defun time-subtract t1 t2
1677 This returns the time difference @var{t1} @minus{} @var{t2} between
1678 two time values, as a time value.
1681 @defun time-add t1 t2
1682 This returns the sum of two time values, as a time value.
1683 One argument should represent a time difference rather than a point in time.
1684 Here is how to add a number of seconds to a time value:
1687 (time-add @var{time} @var{seconds})
1691 @defun time-to-days time-value
1692 This function returns the number of days between the beginning of year
1693 1 and @var{time-value}.
1696 @defun time-to-day-in-year time-value
1697 This returns the day number within the year corresponding to @var{time-value}.
1700 @defun date-leap-year-p year
1701 This function returns @code{t} if @var{year} is a leap year.
1705 @section Timers for Delayed Execution
1708 You can set up a @dfn{timer} to call a function at a specified
1709 future time or after a certain length of idleness.
1711 Emacs cannot run timers at any arbitrary point in a Lisp program; it
1712 can run them only when Emacs could accept output from a subprocess:
1713 namely, while waiting or inside certain primitive functions such as
1714 @code{sit-for} or @code{read-event} which @emph{can} wait. Therefore, a
1715 timer's execution may be delayed if Emacs is busy. However, the time of
1716 execution is very precise if Emacs is idle.
1718 Emacs binds @code{inhibit-quit} to @code{t} before calling the timer
1719 function, because quitting out of many timer functions can leave
1720 things in an inconsistent state. This is normally unproblematical
1721 because most timer functions don't do a lot of work. Indeed, for a
1722 timer to call a function that takes substantial time to run is likely
1723 to be annoying. If a timer function needs to allow quitting, it
1724 should use @code{with-local-quit} (@pxref{Quitting}). For example, if
1725 a timer function calls @code{accept-process-output} to receive output
1726 from an external process, that call should be wrapped inside
1727 @code{with-local-quit}, to ensure that @kbd{C-g} works if the external
1730 It is usually a bad idea for timer functions to alter buffer
1731 contents. When they do, they usually should call @code{undo-boundary}
1732 both before and after changing the buffer, to separate the timer's
1733 changes from user commands' changes and prevent a single undo entry
1734 from growing to be quite large.
1736 Timer functions should also avoid calling functions that cause Emacs
1737 to wait, such as @code{sit-for} (@pxref{Waiting}). This can lead to
1738 unpredictable effects, since other timers (or even the same timer) can
1739 run while waiting. If a timer function needs to perform an action
1740 after a certain time has elapsed, it can do this by scheduling a new
1743 If a timer function calls functions that can change the match data,
1744 it should save and restore the match data. @xref{Saving Match Data}.
1746 @deffn Command run-at-time time repeat function &rest args
1747 This sets up a timer that calls the function @var{function} with
1748 arguments @var{args} at time @var{time}. If @var{repeat} is a number
1749 (integer or floating point), the timer is scheduled to run again every
1750 @var{repeat} seconds after @var{time}. If @var{repeat} is @code{nil},
1751 the timer runs only once.
1753 @var{time} may specify an absolute or a relative time.
1755 Absolute times may be specified using a string with a limited variety
1756 of formats, and are taken to be times @emph{today}, even if already in
1757 the past. The recognized forms are @samp{@var{xxxx}},
1758 @samp{@var{x}:@var{xx}}, or @samp{@var{xx}:@var{xx}} (military time),
1759 and @samp{@var{xx}am}, @samp{@var{xx}AM}, @samp{@var{xx}pm},
1760 @samp{@var{xx}PM}, @samp{@var{xx}:@var{xx}am},
1761 @samp{@var{xx}:@var{xx}AM}, @samp{@var{xx}:@var{xx}pm}, or
1762 @samp{@var{xx}:@var{xx}PM}. A period can be used instead of a colon
1763 to separate the hour and minute parts.
1765 To specify a relative time as a string, use numbers followed by units.
1770 denotes 1 minute from now.
1772 denotes 65 seconds from now.
1773 @item 1 min 2 sec 3 hour 4 day 5 week 6 fortnight 7 month 8 year
1774 denotes exactly 103 months, 123 days, and 10862 seconds from now.
1777 For relative time values, Emacs considers a month to be exactly thirty
1778 days, and a year to be exactly 365.25 days.
1780 Not all convenient formats are strings. If @var{time} is a number
1781 (integer or floating point), that specifies a relative time measured in
1782 seconds. The result of @code{encode-time} can also be used to specify
1783 an absolute value for @var{time}.
1785 In most cases, @var{repeat} has no effect on when @emph{first} call
1786 takes place---@var{time} alone specifies that. There is one exception:
1787 if @var{time} is @code{t}, then the timer runs whenever the time is a
1788 multiple of @var{repeat} seconds after the epoch. This is useful for
1789 functions like @code{display-time}.
1791 The function @code{run-at-time} returns a timer value that identifies
1792 the particular scheduled future action. You can use this value to call
1793 @code{cancel-timer} (see below).
1796 A repeating timer nominally ought to run every @var{repeat} seconds,
1797 but remember that any invocation of a timer can be late. Lateness of
1798 one repetition has no effect on the scheduled time of the next
1799 repetition. For instance, if Emacs is busy computing for long enough
1800 to cover three scheduled repetitions of the timer, and then starts to
1801 wait, it will immediately call the timer function three times in
1802 immediate succession (presuming no other timers trigger before or
1803 between them). If you want a timer to run again no less than @var{n}
1804 seconds after the last invocation, don't use the @var{repeat} argument.
1805 Instead, the timer function should explicitly reschedule the timer.
1807 @defopt timer-max-repeats
1808 This variable's value specifies the maximum number of times to repeat
1809 calling a timer function in a row, when many previously scheduled
1810 calls were unavoidably delayed.
1813 @defmac with-timeout (seconds timeout-forms@dots{}) body@dots{}
1814 Execute @var{body}, but give up after @var{seconds} seconds. If
1815 @var{body} finishes before the time is up, @code{with-timeout} returns
1816 the value of the last form in @var{body}. If, however, the execution of
1817 @var{body} is cut short by the timeout, then @code{with-timeout}
1818 executes all the @var{timeout-forms} and returns the value of the last
1821 This macro works by setting a timer to run after @var{seconds} seconds. If
1822 @var{body} finishes before that time, it cancels the timer. If the
1823 timer actually runs, it terminates execution of @var{body}, then
1824 executes @var{timeout-forms}.
1826 Since timers can run within a Lisp program only when the program calls a
1827 primitive that can wait, @code{with-timeout} cannot stop executing
1828 @var{body} while it is in the midst of a computation---only when it
1829 calls one of those primitives. So use @code{with-timeout} only with a
1830 @var{body} that waits for input, not one that does a long computation.
1833 The function @code{y-or-n-p-with-timeout} provides a simple way to use
1834 a timer to avoid waiting too long for an answer. @xref{Yes-or-No
1837 @defun cancel-timer timer
1838 This cancels the requested action for @var{timer}, which should be a
1839 timer---usually, one previously returned by @code{run-at-time} or
1840 @code{run-with-idle-timer}. This cancels the effect of that call to
1841 one of these functions; the arrival of the specified time will not
1842 cause anything special to happen.
1846 The @code{timer-list} command lists all the currently active timers.
1847 There's only one command available in the buffer displayed: @kbd{c}
1848 (@code{timer-list-cancel}) that will cancel the timer on the line
1852 @section Idle Timers
1855 Here is how to set up a timer that runs when Emacs is idle for a
1856 certain length of time. Aside from how to set them up, idle timers
1857 work just like ordinary timers.
1859 @deffn Command run-with-idle-timer secs repeat function &rest args
1860 Set up a timer which runs the next time Emacs is idle for @var{secs}
1861 seconds. The value of @var{secs} may be a number or a value of the type
1862 returned by @code{current-idle-time}.
1864 If @var{repeat} is @code{nil}, the timer runs just once, the first time
1865 Emacs remains idle for a long enough time. More often @var{repeat} is
1866 non-@code{nil}, which means to run the timer @emph{each time} Emacs
1867 remains idle for @var{secs} seconds.
1869 The function @code{run-with-idle-timer} returns a timer value which you
1870 can use in calling @code{cancel-timer} (@pxref{Timers}).
1874 Emacs becomes @dfn{idle} when it starts waiting for user input, and
1875 it remains idle until the user provides some input. If a timer is set
1876 for five seconds of idleness, it runs approximately five seconds after
1877 Emacs first becomes idle. Even if @var{repeat} is non-@code{nil},
1878 this timer will not run again as long as Emacs remains idle, because
1879 the duration of idleness will continue to increase and will not go
1880 down to five seconds again.
1882 Emacs can do various things while idle: garbage collect, autosave or
1883 handle data from a subprocess. But these interludes during idleness do
1884 not interfere with idle timers, because they do not reset the clock of
1885 idleness to zero. An idle timer set for 600 seconds will run when ten
1886 minutes have elapsed since the last user command was finished, even if
1887 subprocess output has been accepted thousands of times within those ten
1888 minutes, and even if there have been garbage collections and autosaves.
1890 When the user supplies input, Emacs becomes non-idle while executing the
1891 input. Then it becomes idle again, and all the idle timers that are
1892 set up to repeat will subsequently run another time, one by one.
1894 Do not write an idle timer function containing a loop which does a
1895 certain amount of processing each time around, and exits when
1896 @code{(input-pending-p)} is non-@code{nil}. This approach seems very
1897 natural but has two problems:
1901 It blocks out all process output (since Emacs accepts process output
1902 only while waiting).
1905 It blocks out any idle timers that ought to run during that time.
1909 Similarly, do not write an idle timer function that sets up another
1910 idle timer (including the same idle timer) with @var{secs} argument
1911 less than or equal to the current idleness time. Such a timer will
1912 run almost immediately, and continue running again and again, instead
1913 of waiting for the next time Emacs becomes idle. The correct approach
1914 is to reschedule with an appropriate increment of the current value of
1915 the idleness time, as described below.
1917 @defun current-idle-time
1918 If Emacs is idle, this function returns the length of time Emacs has
1919 been idle, as a list of four integers: @code{(@var{sec-high}
1920 @var{sec-low} @var{microsec} @var{picosec})}, using the same format as
1921 @code{current-time} (@pxref{Time of Day}).
1923 When Emacs is not idle, @code{current-idle-time} returns @code{nil}.
1924 This is a convenient way to test whether Emacs is idle.
1927 The main use of @code{current-idle-time} is when an idle timer
1928 function wants to ``take a break'' for a while. It can set up another
1929 idle timer to call the same function again, after a few seconds more
1930 idleness. Here's an example:
1933 (defvar my-resume-timer nil
1934 "Timer for `my-timer-function' to reschedule itself, or nil.")
1936 (defun my-timer-function ()
1937 ;; @r{If the user types a command while @code{my-resume-timer}}
1938 ;; @r{is active, the next time this function is called from}
1939 ;; @r{its main idle timer, deactivate @code{my-resume-timer}.}
1940 (when my-resume-timer
1941 (cancel-timer my-resume-timer))
1942 ...@var{do the work for a while}...
1943 (when @var{taking-a-break}
1944 (setq my-resume-timer
1945 (run-with-idle-timer
1946 ;; Compute an idle time @var{break-length}
1947 ;; more than the current value.
1948 (time-add (current-idle-time) @var{break-length})
1950 'my-timer-function))))
1953 @node Terminal Input
1954 @section Terminal Input
1955 @cindex terminal input
1957 This section describes functions and variables for recording or
1958 manipulating terminal input. See @ref{Display}, for related
1962 * Input Modes:: Options for how input is processed.
1963 * Recording Input:: Saving histories of recent or all input events.
1967 @subsection Input Modes
1969 @cindex terminal input modes
1971 @defun set-input-mode interrupt flow meta &optional quit-char
1972 This function sets the mode for reading keyboard input. If
1973 @var{interrupt} is non-@code{nil}, then Emacs uses input interrupts.
1974 If it is @code{nil}, then it uses @sc{cbreak} mode. The default
1975 setting is system-dependent. Some systems always use @sc{cbreak} mode
1976 regardless of what is specified.
1978 When Emacs communicates directly with X, it ignores this argument and
1979 uses interrupts if that is the way it knows how to communicate.
1981 If @var{flow} is non-@code{nil}, then Emacs uses @sc{xon/xoff}
1982 (@kbd{C-q}, @kbd{C-s}) flow control for output to the terminal. This
1983 has no effect except in @sc{cbreak} mode.
1985 The argument @var{meta} controls support for input character codes
1986 above 127. If @var{meta} is @code{t}, Emacs converts characters with
1987 the 8th bit set into Meta characters. If @var{meta} is @code{nil},
1988 Emacs disregards the 8th bit; this is necessary when the terminal uses
1989 it as a parity bit. If @var{meta} is neither @code{t} nor @code{nil},
1990 Emacs uses all 8 bits of input unchanged. This is good for terminals
1991 that use 8-bit character sets.
1993 If @var{quit-char} is non-@code{nil}, it specifies the character to
1994 use for quitting. Normally this character is @kbd{C-g}.
1998 The @code{current-input-mode} function returns the input mode settings
1999 Emacs is currently using.
2001 @defun current-input-mode
2002 This function returns the current mode for reading keyboard input. It
2003 returns a list, corresponding to the arguments of @code{set-input-mode},
2004 of the form @code{(@var{interrupt} @var{flow} @var{meta} @var{quit})} in
2008 is non-@code{nil} when Emacs is using interrupt-driven input. If
2009 @code{nil}, Emacs is using @sc{cbreak} mode.
2011 is non-@code{nil} if Emacs uses @sc{xon/xoff} (@kbd{C-q}, @kbd{C-s})
2012 flow control for output to the terminal. This value is meaningful only
2013 when @var{interrupt} is @code{nil}.
2015 is @code{t} if Emacs treats the eighth bit of input characters as
2016 the meta bit; @code{nil} means Emacs clears the eighth bit of every
2017 input character; any other value means Emacs uses all eight bits as the
2018 basic character code.
2020 is the character Emacs currently uses for quitting, usually @kbd{C-g}.
2024 @node Recording Input
2025 @subsection Recording Input
2026 @cindex recording input
2029 This function returns a vector containing the last 300 input events from
2030 the keyboard or mouse. All input events are included, whether or not
2031 they were used as parts of key sequences. Thus, you always get the last
2032 300 input events, not counting events generated by keyboard macros.
2033 (These are excluded because they are less interesting for debugging; it
2034 should be enough to see the events that invoked the macros.)
2036 A call to @code{clear-this-command-keys} (@pxref{Command Loop Info})
2037 causes this function to return an empty vector immediately afterward.
2040 @deffn Command open-dribble-file filename
2041 @cindex dribble file
2042 This function opens a @dfn{dribble file} named @var{filename}. When a
2043 dribble file is open, each input event from the keyboard or mouse (but
2044 not those from keyboard macros) is written in that file. A
2045 non-character event is expressed using its printed representation
2046 surrounded by @samp{<@dots{}>}. Be aware that sensitive information
2047 (such as passwords) may end up recorded in the dribble file.
2049 You close the dribble file by calling this function with an argument
2053 See also the @code{open-termscript} function (@pxref{Terminal Output}).
2055 @node Terminal Output
2056 @section Terminal Output
2057 @cindex terminal output
2059 The terminal output functions send output to a text terminal, or keep
2060 track of output sent to the terminal. The variable @code{baud-rate}
2061 tells you what Emacs thinks is the output speed of the terminal.
2064 This variable's value is the output speed of the terminal, as far as
2065 Emacs knows. Setting this variable does not change the speed of actual
2066 data transmission, but the value is used for calculations such as
2069 It also affects decisions about whether to scroll part of the
2070 screen or repaint on text terminals. @xref{Forcing Redisplay},
2071 for the corresponding functionality on graphical terminals.
2073 The value is measured in baud.
2076 If you are running across a network, and different parts of the
2077 network work at different baud rates, the value returned by Emacs may be
2078 different from the value used by your local terminal. Some network
2079 protocols communicate the local terminal speed to the remote machine, so
2080 that Emacs and other programs can get the proper value, but others do
2081 not. If Emacs has the wrong value, it makes decisions that are less
2082 than optimal. To fix the problem, set @code{baud-rate}.
2084 @defun send-string-to-terminal string &optional terminal
2085 This function sends @var{string} to @var{terminal} without alteration.
2086 Control characters in @var{string} have terminal-dependent effects.
2087 (If you need to display non-ASCII text on the terminal, encode it
2088 using one of the functions described in @ref{Explicit Encoding}.)
2089 This function operates only on text terminals. @var{terminal} may be
2090 a terminal object, a frame, or @code{nil} for the selected frame's
2091 terminal. In batch mode, @var{string} is sent to @code{stdout} when
2092 @var{terminal} is @code{nil}.
2094 One use of this function is to define function keys on terminals that
2095 have downloadable function key definitions. For example, this is how (on
2096 certain terminals) to define function key 4 to move forward four
2097 characters (by transmitting the characters @kbd{C-u C-f} to the
2102 (send-string-to-terminal "\eF4\^U\^F")
2108 @deffn Command open-termscript filename
2109 @cindex termscript file
2110 This function is used to open a @dfn{termscript file} that will record
2111 all the characters sent by Emacs to the terminal. It returns
2112 @code{nil}. Termscript files are useful for investigating problems
2113 where Emacs garbles the screen, problems that are due to incorrect
2114 Termcap entries or to undesirable settings of terminal options more
2115 often than to actual Emacs bugs. Once you are certain which characters
2116 were actually output, you can determine reliably whether they correspond
2117 to the Termcap specifications in use.
2121 (open-termscript "../junk/termscript")
2126 You close the termscript file by calling this function with an
2127 argument of @code{nil}.
2129 See also @code{open-dribble-file} in @ref{Recording Input}.
2133 @section Sound Output
2136 To play sound using Emacs, use the function @code{play-sound}. Only
2137 certain systems are supported; if you call @code{play-sound} on a
2138 system which cannot really do the job, it gives an error.
2140 @c FIXME: Add indexes for Au and WAV? --xfq
2141 The sound must be stored as a file in RIFF-WAVE format (@samp{.wav})
2142 or Sun Audio format (@samp{.au}).
2144 @defun play-sound sound
2145 This function plays a specified sound. The argument, @var{sound}, has
2146 the form @code{(sound @var{properties}...)}, where the @var{properties}
2147 consist of alternating keywords (particular symbols recognized
2148 specially) and values corresponding to them.
2150 Here is a table of the keywords that are currently meaningful in
2151 @var{sound}, and their meanings:
2154 @item :file @var{file}
2155 This specifies the file containing the sound to play.
2156 If the file name is not absolute, it is expanded against
2157 the directory @code{data-directory}.
2159 @item :data @var{data}
2160 This specifies the sound to play without need to refer to a file. The
2161 value, @var{data}, should be a string containing the same bytes as a
2162 sound file. We recommend using a unibyte string.
2164 @item :volume @var{volume}
2165 This specifies how loud to play the sound. It should be a number in the
2166 range of 0 to 1. The default is to use whatever volume has been
2169 @item :device @var{device}
2170 This specifies the system device on which to play the sound, as a
2171 string. The default device is system-dependent.
2174 Before actually playing the sound, @code{play-sound}
2175 calls the functions in the list @code{play-sound-functions}.
2176 Each function is called with one argument, @var{sound}.
2179 @deffn Command play-sound-file file &optional volume device
2180 This function is an alternative interface to playing a sound @var{file}
2181 specifying an optional @var{volume} and @var{device}.
2184 @defvar play-sound-functions
2185 A list of functions to be called before playing a sound. Each function
2186 is called with one argument, a property list that describes the sound.
2190 @section Operating on X11 Keysyms
2193 To define system-specific X11 keysyms, set the variable
2194 @code{system-key-alist}.
2196 @defvar system-key-alist
2197 This variable's value should be an alist with one element for each
2198 system-specific keysym. Each element has the form @code{(@var{code}
2199 . @var{symbol})}, where @var{code} is the numeric keysym code (not
2200 including the vendor-specific bit,
2207 and @var{symbol} is the name for the function key.
2209 For example @code{(168 . mute-acute)} defines a system-specific key (used
2210 by HP X servers) whose numeric code is
2219 It is not crucial to exclude from the alist the keysyms of other X
2220 servers; those do no harm, as long as they don't conflict with the ones
2221 used by the X server actually in use.
2223 The variable is always local to the current terminal, and cannot be
2224 buffer-local. @xref{Multiple Terminals}.
2227 You can specify which keysyms Emacs should use for the Meta, Alt, Hyper, and Super modifiers by setting these variables:
2229 @defvar x-alt-keysym
2230 @defvarx x-meta-keysym
2231 @defvarx x-hyper-keysym
2232 @defvarx x-super-keysym
2233 The name of the keysym that should stand for the Alt modifier
2234 (respectively, for Meta, Hyper, and Super). For example, here is
2235 how to swap the Meta and Alt modifiers within Emacs:
2237 (setq x-alt-keysym 'meta)
2238 (setq x-meta-keysym 'alt)
2246 The command-line option @samp{-batch} causes Emacs to run
2247 noninteractively. In this mode, Emacs does not read commands from the
2248 terminal, it does not alter the terminal modes, and it does not expect
2249 to be outputting to an erasable screen. The idea is that you specify
2250 Lisp programs to run; when they are finished, Emacs should exit. The
2251 way to specify the programs to run is with @samp{-l @var{file}}, which
2252 loads the library named @var{file}, or @samp{-f @var{function}}, which
2253 calls @var{function} with no arguments, or @samp{--eval @var{form}}.
2255 Any Lisp program output that would normally go to the echo area,
2256 either using @code{message}, or using @code{prin1}, etc., with
2257 @code{t} as the stream, goes instead to Emacs's standard descriptors
2258 when in batch mode: @code{message} writes to the standard error
2259 descriptor, while @code{prin1} and other print functions write to the
2260 standard output. Similarly, input that would normally come from the
2261 minibuffer is read from the standard input descriptor. Thus, Emacs
2262 behaves much like a noninteractive application program. (The echo
2263 area output that Emacs itself normally generates, such as command
2264 echoing, is suppressed entirely.)
2266 Non-ASCII text written to the standard output or error descriptors is
2267 by default encoded using @code{locale-coding-system} (@pxref{Locales})
2268 if it is non-@code{nil}; this can be overridden by binding
2269 @code{coding-system-for-write} to a coding system of you choice
2270 (@pxref{Explicit Encoding}).
2272 @defvar noninteractive
2273 This variable is non-@code{nil} when Emacs is running in batch mode.
2276 @node Session Management
2277 @section Session Management
2278 @cindex session manager
2280 Emacs supports the X Session Management Protocol, which is used to
2281 suspend and restart applications. In the X Window System, a program
2282 called the @dfn{session manager} is responsible for keeping track of
2283 the applications that are running. When the X server shuts down, the
2284 session manager asks applications to save their state, and delays the
2285 actual shutdown until they respond. An application can also cancel
2288 When the session manager restarts a suspended session, it directs
2289 these applications to individually reload their saved state. It does
2290 this by specifying a special command-line argument that says what
2291 saved session to restore. For Emacs, this argument is @samp{--smid
2294 @defvar emacs-save-session-functions
2295 @cindex session file
2296 Emacs supports saving state via a hook called
2297 @code{emacs-save-session-functions}. Emacs runs this hook when the
2298 session manager tells it that the window system is shutting down. The
2299 functions are called with no arguments, and with the current buffer
2300 set to a temporary buffer. Each function can use @code{insert} to add
2301 Lisp code to this buffer. At the end, Emacs saves the buffer in a
2302 file, called the @dfn{session file}.
2304 @findex emacs-session-restore
2305 Subsequently, when the session manager restarts Emacs, it loads the
2306 session file automatically (@pxref{Loading}). This is performed by a
2307 function named @code{emacs-session-restore}, which is called during
2308 startup. @xref{Startup Summary}.
2310 If a function in @code{emacs-save-session-functions} returns
2311 non-@code{nil}, Emacs tells the session manager to cancel the
2315 Here is an example that just inserts some text into @file{*scratch*} when
2316 Emacs is restarted by the session manager.
2320 (add-hook 'emacs-save-session-functions 'save-yourself-test)
2324 (defun save-yourself-test ()
2325 (insert "(save-current-buffer
2326 (switch-to-buffer \"*scratch*\")
2327 (insert \"I am restored\"))")
2332 @node Desktop Notifications
2333 @section Desktop Notifications
2334 @cindex desktop notifications
2335 @cindex notifications, on desktop
2337 Emacs is able to send @dfn{notifications} on systems that support the
2338 freedesktop.org Desktop Notifications Specification and on MS-Windows.
2339 In order to use this functionality on Posix hosts, Emacs must have
2340 been compiled with D-Bus support, and the @code{notifications} library
2341 must be loaded. @xref{Top, , D-Bus,dbus,D-Bus integration in Emacs}.
2342 The following function is supported when D-Bus support is available:
2344 @defun notifications-notify &rest params
2345 This function sends a notification to the desktop via D-Bus,
2346 consisting of the parameters specified by the @var{params} arguments.
2347 These arguments should consist of alternating keyword and value pairs.
2348 The supported keywords and values are as follows:
2351 @item :bus @var{bus}
2352 The D-Bus bus. This argument is needed only if a bus other than
2353 @code{:session} shall be used.
2355 @item :title @var{title}
2356 The notification title.
2358 @item :body @var{text}
2359 The notification body text. Depending on the implementation of the
2360 notification server, the text could contain HTML markups, like
2361 @samp{"<b>bold text</b>"}, hyperlinks, or images. Special HTML
2362 characters must be encoded, as @samp{"Contact
2363 <postmaster@@localhost>!"}.
2365 @item :app-name @var{name}
2366 The name of the application sending the notification. The default is
2367 @code{notifications-application-name}.
2369 @item :replaces-id @var{id}
2370 The notification @var{id} that this notification replaces. @var{id}
2371 must be the result of a previous @code{notifications-notify} call.
2373 @item :app-icon @var{icon-file}
2374 The file name of the notification icon. If set to @code{nil}, no icon
2375 is displayed. The default is @code{notifications-application-icon}.
2377 @item :actions (@var{key} @var{title} @var{key} @var{title} ...)
2378 A list of actions to be applied. @var{key} and @var{title} are both
2379 strings. The default action (usually invoked by clicking the
2380 notification) should have a key named @samp{"default"}. The title can
2381 be anything, though implementations are free not to display it.
2383 @item :timeout @var{timeout}
2384 The timeout time in milliseconds since the display of the notification
2385 at which the notification should automatically close. If @minus{}1, the
2386 notification's expiration time is dependent on the notification
2387 server's settings, and may vary for the type of notification. If 0,
2388 the notification never expires. Default value is @minus{}1.
2390 @item :urgency @var{urgency}
2391 The urgency level. It can be @code{low}, @code{normal}, or @code{critical}.
2394 When this keyword is given, the @var{title} string of the actions is
2395 interpreted as icon name.
2397 @item :category @var{category}
2398 The type of notification this is, a string. See the
2399 @uref{http://developer.gnome.org/notification-spec/#categories,
2400 Desktop Notifications Specification} for a list of standard
2403 @item :desktop-entry @var{filename}
2404 This specifies the name of the desktop filename representing the
2405 calling program, like @samp{"emacs"}.
2407 @item :image-data (@var{width} @var{height} @var{rowstride} @var{has-alpha} @var{bits} @var{channels} @var{data})
2408 This is a raw data image format that describes the width, height,
2409 rowstride, whether there is an alpha channel, bits per sample,
2410 channels and image data, respectively.
2412 @item :image-path @var{path}
2413 This is represented either as a URI (@samp{file://} is the only URI
2414 schema supported right now) or a name in a freedesktop.org-compliant
2415 icon theme from @samp{$XDG_DATA_DIRS/icons}.
2417 @item :sound-file @var{filename}
2418 The path to a sound file to play when the notification pops up.
2420 @item :sound-name @var{name}
2421 A themable named sound from the freedesktop.org sound naming
2422 specification from @samp{$XDG_DATA_DIRS/sounds}, to play when the
2423 notification pops up. Similar to the icon name, only for sounds. An
2424 example would be @samp{"message-new-instant"}.
2426 @item :suppress-sound
2427 Causes the server to suppress playing any sounds, if it has that
2431 When set the server will not automatically remove the notification
2432 when an action has been invoked. The notification will remain resident
2433 in the server until it is explicitly removed by the user or by the
2434 sender. This hint is likely only useful when the server has the
2435 @code{:persistence} capability.
2438 When set the server will treat the notification as transient and
2439 by-pass the server's persistence capability, if it should exist.
2441 @item :x @var{position}
2442 @itemx :y @var{position}
2443 Specifies the X, Y location on the screen that the
2444 notification should point to. Both arguments must be used together.
2446 @item :on-action @var{function}
2447 Function to call when an action is invoked. The notification @var{id}
2448 and the @var{key} of the action are passed as arguments to the
2451 @item :on-close @var{function}
2452 Function to call when the notification has been closed by timeout or
2453 by the user. The function receive the notification @var{id} and the closing
2454 @var{reason} as arguments:
2457 @item @code{expired} if the notification has expired
2458 @item @code{dismissed} if the notification was dismissed by the user
2459 @item @code{close-notification} if the notification was closed by a call to
2460 @code{notifications-close-notification}
2461 @item @code{undefined} if the notification server hasn't provided a reason
2465 Which parameters are accepted by the notification server can be
2466 checked via @code{notifications-get-capabilities}.
2468 This function returns a notification id, an integer, which can be used
2469 to manipulate the notification item with
2470 @code{notifications-close-notification} or the @code{:replaces-id}
2471 argument of another @code{notifications-notify} call. For example:
2475 (defun my-on-action-function (id key)
2476 (message "Message %d, key \"%s\" pressed" id key))
2477 @result{} my-on-action-function
2481 (defun my-on-close-function (id reason)
2482 (message "Message %d, closed due to \"%s\"" id reason))
2483 @result{} my-on-close-function
2487 (notifications-notify
2489 :body "This is <b>important</b>."
2490 :actions '("Confirm" "I agree" "Refuse" "I disagree")
2491 :on-action 'my-on-action-function
2492 :on-close 'my-on-close-function)
2497 A message window opens on the desktop. Press ``I agree''.
2498 @result{} Message 22, key "Confirm" pressed
2499 Message 22, closed due to "dismissed"
2504 @defun notifications-close-notification id &optional bus
2505 This function closes a notification with identifier @var{id}.
2506 @var{bus} can be a string denoting a D-Bus connection, the default is
2510 @defun notifications-get-capabilities &optional bus
2511 Returns the capabilities of the notification server, a list of
2512 symbols. @var{bus} can be a string denoting a D-Bus connection, the
2513 default is @code{:session}. The following capabilities can be
2518 The server will provide the specified actions to the user.
2523 @item :body-hyperlinks
2524 The server supports hyperlinks in the notifications.
2527 The server supports images in the notifications.
2530 Supports markup in the body text.
2533 The server will render an animation of all the frames in a given image
2537 Supports display of exactly 1 frame of any given image array. This
2538 value is mutually exclusive with @code{:icon-multi}.
2541 The server supports persistence of notifications.
2544 The server supports sounds on notifications.
2547 Further vendor-specific caps start with @code{:x-vendor}, like
2548 @code{:x-gnome-foo-cap}.
2551 @defun notifications-get-server-information &optional bus
2552 Return information on the notification server, a list of strings.
2553 @var{bus} can be a string denoting a D-Bus connection, the default is
2554 @code{:session}. The returned list is @code{(@var{name} @var{vendor}
2555 @var{version} @var{spec-version})}.
2559 The product name of the server.
2562 The vendor name. For example, @samp{"KDE"}, @samp{"GNOME"}.
2565 The server's version number.
2568 The specification version the server is compliant with.
2571 If @var{spec_version} is @code{nil}, the server supports a
2572 specification prior to @samp{"1.0"}.
2575 @cindex tray notifications, MS-Windows
2576 When Emacs runs on MS-Windows as a GUI session, it supports a small
2577 subset of the D-Bus notifications functionality via a native
2580 @defun w32-notification-notify &rest params
2581 This function displays an MS-Windows tray notification as specified by
2582 @var{params}. MS-Windows tray notifications are displayed in a
2583 balloon from an icon in the notification area of the taskbar.
2585 Value is the integer unique ID of the notification that can be used to
2586 remove the notification using @code{w32-notification-close}, described
2587 below. If the function fails, the return value is @code{nil}.
2589 The arguments @var{params} are specified as keyword/value pairs. All the
2590 parameters are optional, but if no parameters are specified, the
2591 function will do nothing and return @code{nil}.
2593 The following parameters are supported:
2596 @item :icon @var{icon}
2597 Display @var{icon} in the system tray. If @var{icon} is a string, it
2598 should specify a file name from which to load the icon; the specified
2599 file should be a @file{.ico} Windows icon file. If @var{icon} is not
2600 a string, or if this parameter is not specified, the standard Emacs
2603 @item :tip @var{tip}
2604 Use @var{tip} as the tooltip for the notification. If @var{tip} is a
2605 string, this is the text of a tooltip that will be shown when the
2606 mouse pointer hovers over the tray icon added by the notification. If
2607 @var{tip} is not a string, or if this parameter is not specified, the
2608 default tooltip text is @samp{Emacs notification}. The tooltip text can
2609 be up to 127 characters long (63 on Windows versions before W2K).
2610 Longer strings will be truncated.
2612 @item :level @var{level}
2613 Notification severity level, one of @code{info}, @code{warning}, or
2614 @code{error}. If given, the value determines the icon displayed to the
2615 left of the notification title, but only if the @code{:title} parameter
2616 (see below) is also specified and is a string.
2618 @item :title @var{title}
2619 The title of the notification. If @var{title} is a string, it is
2620 displayed in a larger font immediately above the body text. The title
2621 text can be up to 63 characters long; longer text will be truncated.
2623 @item :body @var{body}
2624 The body of the notification. If @var{body} is a string, it specifies
2625 the text of the notification message. Use embedded newlines to
2626 control how the text is broken into lines. The body text can be up to
2627 255 characters long, and will be truncated if it's longer. Unlike
2628 with D-Bus, the body text should be plain text, with no markup.
2631 Note that versions of Windows before W2K support only @code{:icon} and
2632 @code{:tip}. The other parameters can be passed, but they will be
2633 ignored on those old systems.
2635 There can be at most one active notification at any given time. An
2636 active notification must be removed by calling
2637 @code{w32-notification-close} before a new one can be shown.
2640 To remove the notification and its icon from the taskbar, use the
2643 @defun w32-notification-close id
2644 This function removes the tray notification given by its unique
2648 @node File Notifications
2649 @section Notifications on File Changes
2650 @cindex file notifications
2651 @cindex watch, for filesystem events
2653 Several operating systems support watching of filesystems for changes
2654 of files. If configured properly, Emacs links a respective library
2655 like @file{inotify}, @file{kqueue}, @file{gfilenotify}, or
2656 @file{w32notify} statically. These libraries enable watching of
2657 filesystems on the local machine.
2659 It is also possible to watch filesystems on remote machines,
2660 @pxref{Remote Files,, Remote Files, emacs, The GNU Emacs Manual}
2661 This does not depend on one of the libraries linked to Emacs.
2663 Since all these libraries emit different events on notified file
2664 changes, there is the Emacs library @code{filenotify} which provides a
2667 @defun file-notify-add-watch file flags callback
2668 Add a watch for filesystem events pertaining to @var{file}. This
2669 arranges for filesystem events pertaining to @var{file} to be reported
2672 The returned value is a descriptor for the added watch. Its type
2673 depends on the underlying library, it cannot be assumed to be an
2674 integer as in the example below. It should be used for comparison by
2677 If the @var{file} cannot be watched for some reason, this function
2678 signals a @code{file-notify-error} error.
2680 Sometimes, mounted filesystems cannot be watched for file changes.
2681 This is not detected by this function, a non-@code{nil} return value
2682 does not guarantee that changes on @var{file} will be notified.
2684 @var{flags} is a list of conditions to set what will be watched for.
2685 It can include the following symbols:
2689 watch for file changes
2690 @item attribute-change
2691 watch for file attribute changes, like permissions or modification
2695 If @var{file} is a directory, changes for all files in that directory
2696 will be notified. This does not work recursively.
2698 When any event happens, Emacs will call the @var{callback} function
2699 passing it a single argument @var{event}, which is of the form
2702 (@var{descriptor} @var{action} @var{file} [@var{file1}])
2705 @var{descriptor} is the same object as the one returned by this
2706 function. @var{action} is the description of the event. It could be
2707 any one of the following symbols:
2711 @var{file} was created
2713 @var{file} was deleted
2715 @var{file}'s contents has changed; with @file{w32notify} library,
2716 reports attribute changes as well
2718 @var{file} has been renamed to @var{file1}
2719 @item attribute-changed
2720 a @var{file} attribute was changed
2722 watching @var{file} has been stopped
2725 Note that the @file{w32notify} library does not report
2726 @code{attribute-changed} events. When some file's attribute, like
2727 permissions or modification time, has changed, this library reports a
2728 @code{changed} event. Likewise, the @file{kqueue} library does not
2729 report reliably file attribute changes when watching a directory.
2731 The @code{stopped} event reports, that watching the file has been
2732 stopped. This could be because @code{file-notify-rm-watch} was called
2733 (see below), or because the file being watched was deleted, or due to
2734 another error reported from the underlying library.
2736 @var{file} and @var{file1} are the name of the file(s) whose event is
2737 being reported. For example:
2741 (require 'filenotify)
2742 @result{} filenotify
2746 (defun my-notify-callback (event)
2747 (message "Event %S" event))
2748 @result{} my-notify-callback
2752 (file-notify-add-watch
2753 "/tmp" '(change attribute-change) 'my-notify-callback)
2758 (write-region "foo" nil "/tmp/foo")
2759 @result{} Event (35025468 created "/tmp/.#foo")
2760 Event (35025468 created "/tmp/foo")
2761 Event (35025468 changed "/tmp/foo")
2762 Event (35025468 deleted "/tmp/.#foo")
2766 (write-region "bla" nil "/tmp/foo")
2767 @result{} Event (35025468 created "/tmp/.#foo")
2768 Event (35025468 changed "/tmp/foo")
2769 Event (35025468 deleted "/tmp/.#foo")
2773 (set-file-modes "/tmp/foo" (default-file-modes))
2774 @result{} Event (35025468 attribute-changed "/tmp/foo")
2778 Whether the action @code{renamed} is returned, depends on the used
2779 watch library. Otherwise, the actions @code{deleted} and
2780 @code{created} could be returned in a random order.
2784 (rename-file "/tmp/foo" "/tmp/bla")
2785 @result{} Event (35025468 renamed "/tmp/foo" "/tmp/bla")
2789 (delete-file "/tmp/bla")
2790 @result{} Event (35025468 deleted "/tmp/bla")
2795 @defun file-notify-rm-watch descriptor
2796 Removes an existing file watch specified by its @var{descriptor}.
2797 @var{descriptor} should be an object returned by
2798 @code{file-notify-add-watch}.
2801 @defun file-notify-valid-p descriptor
2802 Checks a watch specified by its @var{descriptor} for validity.
2803 @var{descriptor} should be an object returned by
2804 @code{file-notify-add-watch}.
2806 A watch can become invalid if the file or directory it watches is
2807 deleted, or if the watcher thread exits abnormally for any other
2808 reason. Removing the watch by calling @code{file-notify-rm-watch}
2809 also makes it invalid.
2813 (make-directory "/tmp/foo")
2814 @result{} Event (35025468 created "/tmp/foo")
2819 (file-notify-add-watch
2820 "/tmp/foo" '(change) 'my-notify-callback))
2825 (file-notify-valid-p desc)
2830 (write-region "bla" nil "/tmp/foo/bla")
2831 @result{} Event (11359632 created "/tmp/foo/.#bla")
2832 Event (11359632 created "/tmp/foo/bla")
2833 Event (11359632 changed "/tmp/foo/bla")
2834 Event (11359632 deleted "/tmp/foo/.#bla")
2838 ;; Deleting a file in the directory doesn't invalidate the watch.
2839 (delete-file "/tmp/foo/bla")
2840 @result{} Event (11359632 deleted "/tmp/foo/bla")
2844 (write-region "bla" nil "/tmp/foo/bla")
2845 @result{} Event (11359632 created "/tmp/foo/.#bla")
2846 Event (11359632 created "/tmp/foo/bla")
2847 Event (11359632 changed "/tmp/foo/bla")
2848 Event (11359632 deleted "/tmp/foo/.#bla")
2852 ;; Deleting the directory invalidates the watch.
2853 ;; Events arrive for different watch descriptors.
2854 (delete-directory "/tmp/foo" 'recursive)
2855 @result{} Event (35025468 deleted "/tmp/foo")
2856 Event (11359632 deleted "/tmp/foo/bla")
2857 Event (11359632 deleted "/tmp/foo")
2858 Event (11359632 stopped "/tmp/foo")
2862 (file-notify-valid-p desc)
2868 @node Dynamic Libraries
2869 @section Dynamically Loaded Libraries
2870 @cindex dynamic libraries
2872 A @dfn{dynamically loaded library} is a library that is loaded on
2873 demand, when its facilities are first needed. Emacs supports such
2874 on-demand loading of support libraries for some of its features.
2876 @defvar dynamic-library-alist
2877 This is an alist of dynamic libraries and external library files
2880 Each element is a list of the form
2881 @w{@code{(@var{library} @var{files}@dots{})}}, where the @code{car} is
2882 a symbol representing a supported external library, and the rest are
2883 strings giving alternate filenames for that library.
2885 Emacs tries to load the library from the files in the order they
2886 appear in the list; if none is found, the Emacs session won't have
2887 access to that library, and the features it provides will be
2890 Image support on some platforms uses this facility. Here's an example
2891 of setting this variable for supporting images on MS-Windows:
2894 (setq dynamic-library-alist
2895 '((xpm "libxpm.dll" "xpm4.dll" "libXpm-nox4.dll")
2896 (png "libpng12d.dll" "libpng12.dll" "libpng.dll"
2897 "libpng13d.dll" "libpng13.dll")
2898 (jpeg "jpeg62.dll" "libjpeg.dll" "jpeg-62.dll"
2900 (tiff "libtiff3.dll" "libtiff.dll")
2901 (gif "giflib4.dll" "libungif4.dll" "libungif.dll")
2902 (svg "librsvg-2-2.dll")
2903 (gdk-pixbuf "libgdk_pixbuf-2.0-0.dll")
2904 (glib "libglib-2.0-0.dll")
2905 (gobject "libgobject-2.0-0.dll")))
2908 Note that image types @code{pbm} and @code{xbm} do not need entries in
2909 this variable because they do not depend on external libraries and are
2910 always available in Emacs.
2912 Also note that this variable is not meant to be a generic facility for
2913 accessing external libraries; only those already known by Emacs can
2914 be loaded through it.
2916 This variable is ignored if the given @var{library} is statically
2920 @node Security Considerations
2921 @section Security Considerations
2925 Like any application, Emacs can be run in a secure environment, where
2926 the operating system enforces rules about access and the like. With
2927 some care, Emacs-based applications can also be part of a security
2928 perimeter that checks such rules. Although the default settings for
2929 Emacs work well for a typical software development environment, they
2930 may require adjustment in environments containing untrusted users that
2931 may include attackers. Here is a compendium of security issues that
2932 may be helpful if you are developing such applications. It is by no
2933 means complete; it is intended to give you an idea of the security
2934 issues involved, rather than to be a security checklist.
2937 @item File local variables
2938 @cindex file local variables
2939 A file that Emacs visits can contain variable settings that affects
2940 the buffer visiting that file; @xref{File Local Variables}.
2941 Similarly, a directory can specify local variable values common to all
2942 files in that directory; @xref{Directory Local Variables}. Although
2943 Emacs takes some effort to protect against misuse of these variables,
2944 a security hole can be created merely by a package setting
2945 @code{safe-local-variable} too optimistically, a problem that is all
2946 too common. To disable this feature for both files and directories,
2947 set @code{enable-local-variables} to @code{nil}.
2949 @item Access control
2950 Although Emacs normally respects access permissions of the underlying
2951 operating system, in some cases it handles accesses specially. For
2952 example, file names can have handlers that treat the files specially,
2953 with their own access checking. @xref{Magic File Names}. Also, a
2954 buffer can be read-only even if the corresponding file is writeable,
2955 and vice versa, which can result in messages such as @samp{File passwd
2956 is write-protected; try to save anyway? (yes or no)}. @xref{Read Only
2959 @item Authentication
2960 Emacs has several functions that deal with passwords, e.g.,
2961 @code{read-passwd}. @xref{Reading a Password}.
2962 Although these functions do not attempt to
2963 broadcast passwords to the world, their implementations are not proof
2964 against determined attackers with access to Emacs internals. For
2965 example, even if Elisp code uses @code{clear-string} to scrub a password from
2966 its memory after using it, remnants of the password may still reside
2967 in the garbage-collected free list. @xref{Modifying Strings}.
2969 @item Code injection
2970 Emacs can send commands to many other applications, and applications
2971 should take care that strings sent as operands of these commands are
2972 not misinterpreted as directives. For example, when using a shell
2973 command to rename a file @var{a} to @var{b}, do not simply use the
2974 string @code{mv @var{a} @var{b}}, because either file name might start
2975 with @samp{-}, or might contain shell metacharacters like @samp{;}.
2976 Although functions like @code{shell-quote-argument} can help avoid
2977 this sort of problem, they are not panaceas; for example, on a POSIX
2978 platform @code{shell-quote-argument} quotes shell metacharacters but
2979 not leading @samp{-}. @xref{Shell Arguments}. Typically it is safer
2980 to use @code{call-process} than a subshell. @xref{Synchronous
2981 Processes}. And it is safer yet to use builtin Emacs functions; for
2982 example, use @code{(rename-file "@var{a}" "@var{b}" t)} instead of
2983 invoking @command{mv}. @xref{Changing Files}.
2985 @item Coding systems
2986 Emacs attempts to infer the coding systems of the files and network
2987 connections it accesses. @xref{Coding Systems}.
2988 If Emacs infers incorrectly, or if the other
2989 parties to the network connection disagree with Emacs's inferences,
2990 the resulting system could be unreliable. Also, even when it infers
2991 correctly, Emacs often can use bytes that other programs cannot. For
2992 example, although to Emacs the null byte is just a
2993 character like any other, many other applications treat it as a string
2994 terminator and mishandle strings or files containing null bytes.
2996 @item Environment and configuration variables
2997 POSIX specifies several environment variables that can affect how
2998 Emacs behaves. Any environment variable whose name consists entirely
2999 of uppercase ASCII letters, digits, and the underscore may affect the
3000 internal behavior of Emacs. Emacs uses several such variables, e.g.,
3001 @env{EMACSLOADPATH}. @xref{Library Search}. On some platforms some
3002 environment variables (e.g., @env{PATH}, @env{POSIXLY_CORRECT},
3003 @env{SHELL}, @env{TMPDIR}) need to have properly-configured values in
3004 order to get standard behavior for any utility Emacs might invoke.
3005 Even seemingly-benign variables like @env{TZ} may have security
3006 implications. @xref{System Environment}.
3008 Emacs has customization and other variables with similar
3009 considerations. For example, if the variable @code{shell-file-name}
3010 specifies a shell with nonstandard behavior, an Emacs-based
3011 application may misbehave.
3014 When Emacs is installed, if the installation directory hierarchy can
3015 be modified by untrusted users, the application cannot be trusted.
3016 This applies also to the directory hierarchies of the programs that
3017 Emacs uses, and of the files that Emacs reads and writes.
3019 @item Network access
3020 Emacs often accesses the network, and you may want to configure it to
3021 avoid network accesses that it would normally do. For example, unless
3022 you set @code{tramp-mode} to @code{nil}, file names using a certain
3023 syntax are interpreted as being network files, and are retrieved
3024 across the network. @xref{Top, The Tramp Manual,, tramp, The Tramp
3027 @item Race conditions
3028 Emacs applications have the same sort of race-condition issues that
3029 other applications do. For example, even when
3030 @code{(file-readable-p "foo.txt")} returns @code{t}, it could be that
3031 @file{foo.txt} is unreadable because some other program changed the
3032 file's permissions between the call to @code{file-readable-p} and now.
3033 @xref{Testing Accessibility}.
3035 @item Resource limits
3036 When Emacs exhausts memory or other operating system resources, its
3037 behavior can be less reliable, in that computations that ordinarily
3038 run to completion may abort back to the top level. This may cause
3039 Emacs to neglect operations that it normally would have done.