2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990-1995, 1998-1999, 2001-2015 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 Silicon Graphics Irix system.
906 Google Native Client (@acronym{NaCl}) sandboxing system.
909 Microsoft's DOS@. Emacs compiled with DJGPP for MS-DOS binds
910 @code{system-type} to @code{ms-dos} even when you run it on MS-Windows.
916 Microsoft Windows NT, 9X and later. The value of @code{system-type}
917 is always @code{windows-nt}, e.g., even on Windows 10.
921 We do not wish to add new symbols to make finer distinctions unless it
922 is absolutely necessary! In fact, we hope to eliminate some of these
923 alternatives in the future. If you need to make a finer distinction
924 than @code{system-type} allows for, you can test
925 @code{system-configuration}, e.g., against a regexp.
929 This function returns the name of the machine you are running on, as a
933 @c FIXME seems like this section is not the best place for this option?
934 @defopt mail-host-address
935 If this variable is non-@code{nil}, it is used instead of
936 @code{system-name} for purposes of generating email addresses. For
937 example, it is used when constructing the default value of
938 @code{user-mail-address}. @xref{User Identification}. (Since this is
939 done when Emacs starts up, the value actually used is the one saved when
940 Emacs was dumped. @xref{Building Emacs}.)
941 @c FIXME sounds like should probably give this a :set-after and some
942 @c custom-initialize-delay voodoo.
945 @deffn Command getenv var &optional frame
946 @cindex environment variable access
947 This function returns the value of the environment variable @var{var},
948 as a string. @var{var} should be a string. If @var{var} is undefined
949 in the environment, @code{getenv} returns @code{nil}. It returns
950 @samp{""} if @var{var} is set but null. Within Emacs, a list of environment
951 variables and their values is kept in the variable @code{process-environment}.
960 The shell command @code{printenv} prints all or part of the environment:
965 PATH=/usr/local/bin:/usr/bin:/bin
977 @deffn Command setenv variable &optional value substitute
978 This command sets the value of the environment variable named
979 @var{variable} to @var{value}. @var{variable} should be a string.
980 Internally, Emacs Lisp can handle any string. However, normally
981 @var{variable} should be a valid shell identifier, that is, a sequence
982 of letters, digits and underscores, starting with a letter or
983 underscore. Otherwise, errors may occur if subprocesses of Emacs try
984 to access the value of @var{variable}. If @var{value} is omitted or
985 @code{nil} (or, interactively, with a prefix argument), @code{setenv}
986 removes @var{variable} from the environment. Otherwise, @var{value}
989 @c FIXME: Document 'substitute-env-vars'? --xfq
990 If the optional argument @var{substitute} is non-@code{nil}, Emacs
991 calls the function @code{substitute-env-vars} to expand any
992 environment variables in @var{value}.
994 @code{setenv} works by modifying @code{process-environment}; binding
995 that variable with @code{let} is also reasonable practice.
997 @code{setenv} returns the new value of @var{variable}, or @code{nil}
998 if it removed @var{variable} from the environment.
1001 @defvar process-environment
1002 This variable is a list of strings, each describing one environment
1003 variable. The functions @code{getenv} and @code{setenv} work by means
1009 @result{} ("PATH=/usr/local/bin:/usr/bin:/bin"
1020 If @code{process-environment} contains multiple elements that
1021 specify the same environment variable, the first of these elements
1022 specifies the variable, and the others are ignored.
1025 @defvar initial-environment
1026 This variable holds the list of environment variables Emacs inherited
1027 from its parent process when Emacs started.
1030 @defvar path-separator
1031 This variable holds a string that says which character separates
1032 directories in a search path (as found in an environment variable). Its
1033 value is @code{":"} for Unix and GNU systems, and @code{";"} for MS systems.
1036 @defun parse-colon-path path
1037 This function takes a search path string such as the value of
1038 the @env{PATH} environment variable, and splits it at the separators,
1039 returning a list of directory names. @code{nil} in this list means
1040 the current directory. Although the function's name says
1041 ``colon'', it actually uses the value of @code{path-separator}.
1044 (parse-colon-path ":/foo:/bar")
1045 @result{} (nil "/foo/" "/bar/")
1049 @defvar invocation-name
1050 This variable holds the program name under which Emacs was invoked. The
1051 value is a string, and does not include a directory name.
1054 @defvar invocation-directory
1055 This variable holds the directory from which the Emacs executable was
1056 invoked, or @code{nil} if that directory cannot be determined.
1059 @defvar installation-directory
1060 If non-@code{nil}, this is a directory within which to look for the
1061 @file{lib-src} and @file{etc} subdirectories. In an installed Emacs,
1062 it is normally @code{nil}. It is non-@code{nil}
1063 when Emacs can't find those directories in their standard installed
1064 locations, but can find them in a directory related somehow to the one
1065 containing the Emacs executable (i.e., @code{invocation-directory}).
1068 @defun load-average &optional use-float
1069 This function returns the current 1-minute, 5-minute, and 15-minute
1070 system load averages, in a list. The load average indicates the
1071 number of processes trying to run on the system.
1073 By default, the values are integers that are 100 times the system load
1074 averages, but if @var{use-float} is non-@code{nil}, then they are
1075 returned as floating-point numbers without multiplying by 100.
1077 If it is impossible to obtain the load average, this function signals
1078 an error. On some platforms, access to load averages requires
1079 installing Emacs as setuid or setgid so that it can read kernel
1080 information, and that usually isn't advisable.
1081 @c FIXME which platforms are these? Are they still relevant?
1083 If the 1-minute load average is available, but the 5- or 15-minute
1084 averages are not, this function returns a shortened list containing
1085 the available averages.
1090 @result{} (169 48 36)
1094 @result{} (1.69 0.48 0.36)
1098 The shell command @code{uptime} returns similar information.
1102 This function returns the process @acronym{ID} of the Emacs process,
1106 @defvar tty-erase-char
1107 This variable holds the erase character that was selected
1108 in the system's terminal driver, before Emacs was started.
1109 @c FIXME? Seems untrue since 23.1. For me, it is 0.
1110 @c The value is @code{nil} if Emacs is running under a window system.
1113 @node User Identification
1114 @section User Identification
1115 @cindex user identification
1117 @defvar init-file-user
1118 This variable says which user's init files should be used by
1119 Emacs---or @code{nil} if none. @code{""} stands for the user who
1120 originally logged in. The value reflects command-line options such as
1121 @samp{-q} or @samp{-u @var{user}}.
1123 Lisp packages that load files of customizations, or any other sort of
1124 user profile, should obey this variable in deciding where to find it.
1125 They should load the profile of the user name found in this variable.
1126 If @code{init-file-user} is @code{nil}, meaning that the @samp{-q},
1127 @samp{-Q}, or @samp{-batch} option was used, then Lisp packages should
1128 not load any customization files or user profile.
1131 @defopt user-mail-address
1132 This holds the nominal email address of the user who is using Emacs.
1133 Emacs normally sets this variable to a default value after reading your
1134 init files, but not if you have already set it. So you can set the
1135 variable to some other value in your init file if you do not
1136 want to use the default value.
1139 @defun user-login-name &optional uid
1140 This function returns the name under which the user is logged in.
1141 It uses the environment variables @env{LOGNAME} or @env{USER} if
1142 either is set. Otherwise, the value is based on the effective
1143 @acronym{UID}, not the real @acronym{UID}.
1145 If you specify @var{uid} (a number), the result is the user name that
1146 corresponds to @var{uid}, or @code{nil} if there is no such user.
1149 @defun user-real-login-name
1150 This function returns the user name corresponding to Emacs's real
1151 @acronym{UID}. This ignores the effective @acronym{UID}, and the
1152 environment variables @env{LOGNAME} and @env{USER}.
1155 @defun user-full-name &optional uid
1156 This function returns the full name of the logged-in user---or the value
1157 of the environment variable @env{NAME}, if that is set.
1159 If the Emacs process's user-id does not correspond to any known user (and
1160 provided @code{NAME} is not set), the result is @code{"unknown"}.
1162 If @var{uid} is non-@code{nil}, then it should be a number (a user-id)
1163 or a string (a login name). Then @code{user-full-name} returns the full
1164 name corresponding to that user-id or login name. If you specify a
1165 user-id or login name that isn't defined, it returns @code{nil}.
1168 @vindex user-full-name
1169 @vindex user-real-login-name
1170 @vindex user-login-name
1171 The symbols @code{user-login-name}, @code{user-real-login-name} and
1172 @code{user-full-name} are variables as well as functions. The functions
1173 return the same values that the variables hold. These variables allow
1174 you to fake out Emacs by telling the functions what to return. The
1175 variables are also useful for constructing frame titles (@pxref{Frame
1179 @defun user-real-uid
1180 This function returns the real @acronym{UID} of the user.
1181 The value may be floating point, in the (unlikely) event that
1182 the UID is too large to fit in a Lisp integer.
1186 This function returns the effective @acronym{UID} of the user.
1187 The value may be floating point.
1192 This function returns the effective @acronym{GID} of the Emacs process.
1193 The value may be floating point.
1196 @defun group-real-gid
1197 This function returns the real @acronym{GID} of the Emacs process.
1198 The value may be floating point.
1202 This function returns a list of strings, listing the user names on the
1203 system. If Emacs cannot retrieve this information, the return value
1204 is a list containing just the value of @code{user-real-login-name}.
1208 @defun system-groups
1209 This function returns a list of strings, listing the names of user
1210 groups on the system. If Emacs cannot retrieve this information, the
1211 return value is @code{nil}.
1216 @section Time of Day
1219 This section explains how to determine the current time and time
1223 Most of these functions represent time as a list of four integers
1224 @code{(@var{sec-high} @var{sec-low} @var{microsec} @var{picosec})}.
1225 This represents the number of seconds from the @dfn{epoch} (January
1226 1, 1970 at 00:00 UTC), using the formula:
1228 @var{high} * 2**16 + @var{low} + @var{micro} * 10**@minus{}6 +
1229 @var{pico} * 10**@minus{}12.
1232 $high*2^{16} + low + micro*10^{-6} + pico*10^{-12}$.
1234 The return value of @code{current-time} represents time using this
1235 form, as do the timestamps in the return values of other functions
1236 such as @code{file-attributes} (@pxref{Definition of
1237 file-attributes}). In some cases, functions may return two- or
1238 three-element lists, with omitted @var{microsec} and @var{picosec}
1239 components defaulting to zero.
1242 Function arguments, e.g., the @var{time} argument to
1243 @code{current-time-string}, accept a more-general @dfn{time value}
1244 format, which can be a list of integers as above, or a single number
1245 for seconds since the epoch, or @code{nil} for the current time. You
1246 can convert a time value into a human-readable string using
1247 @code{current-time-string} and @code{format-time-string}, into a list
1248 of integers using @code{seconds-to-time}, and into other forms using
1249 @code{decode-time} and @code{float-time}. These functions are
1250 described in the following sections.
1252 @defun current-time-string &optional time zone
1253 This function returns the current time and date as a human-readable
1254 string. The format does not vary for the initial part of the string,
1255 which contains the day of week, month, day of month, and time of day
1256 in that order: the number of characters used for these fields is
1257 always the same, so you can reliably
1258 use @code{substring} to extract them. You should count
1259 characters from the beginning of the string rather than from the end,
1260 as the year might not have exactly four digits, and additional
1261 information may some day be added at the end.
1263 The argument @var{time}, if given, specifies a time to format,
1264 instead of the current time. The optional argument @var{zone}
1265 defaults to the current time zone rule.
1269 (current-time-string)
1270 @result{} "Wed Oct 14 22:21:05 1987"
1276 This function returns the current time, represented as a list of four
1277 integers @code{(@var{sec-high} @var{sec-low} @var{microsec} @var{picosec})}.
1278 These integers have trailing zeros on systems that return time with
1279 lower resolutions. On all current machines @var{picosec} is a
1280 multiple of 1000, but this may change as higher-resolution clocks
1284 @defun float-time &optional time
1285 This function returns the current time as a floating-point number of
1286 seconds since the epoch. The optional argument @var{time}, if
1287 given, specifies a time to convert instead of the current time.
1289 @emph{Warning}: Since the result is floating point, it may not be
1290 exact. Do not use this function if precise time stamps are required.
1292 @code{time-to-seconds} is an alias for this function.
1295 @defun seconds-to-time time
1296 This function converts a time value to list-of-integer form.
1297 For example, if @var{time} is a number, @code{(time-to-seconds
1298 (seconds-to-time @var{time}))} equals the number unless overflow
1299 or rounding errors occur.
1302 @defun current-time-zone &optional time zone
1303 @cindex time zone, current
1304 This function returns a list describing the time zone that the user is
1307 The value has the form @code{(@var{offset} @var{name})}. Here
1308 @var{offset} is an integer giving the number of seconds ahead of UTC
1309 (east of Greenwich). A negative value means west of Greenwich. The
1310 second element, @var{name}, is a string giving the name of the time
1311 zone. Both elements change when daylight saving time begins or ends;
1312 if the user has specified a time zone that does not use a seasonal time
1313 adjustment, then the value is constant through time.
1315 If the operating system doesn't supply all the information necessary to
1316 compute the value, the unknown elements of the list are @code{nil}.
1318 The argument @var{time}, if given, specifies a time value to
1319 analyze instead of the current time. The optional argument @var{zone}
1320 defaults to the current time zone rule.
1323 @vindex TZ, environment variable
1324 The default time zone is determined by the @env{TZ} environment
1325 variable. @xref{System Environment}. For example, you can tell Emacs
1326 to default to universal time with @code{(setenv "TZ" "UTC0")}. If
1327 @env{TZ} is not in the environment, Emacs uses system wall clock time,
1328 which is a platform-dependent default time zone.
1330 @cindex time zone rule
1331 Functions that convert to and from local time accept an optional
1332 @dfn{time zone rule} argument, which specifies the conversion's time
1333 zone and daylight saving time history. If the time zone rule is
1334 omitted or @code{nil}, the conversion uses Emacs's default time zone.
1335 If it is @code{t}, the conversion uses Universal Time. If it is
1336 @code{wall}, the conversion uses the system wall clock time. If it is
1337 a string, the conversion uses the time zone rule equivalent to setting
1338 @env{TZ} to that string.
1340 @node Time Conversion
1341 @section Time Conversion
1342 @cindex calendrical information
1343 @cindex time conversion
1345 These functions convert time values (@pxref{Time of Day}) into
1346 calendrical information and vice versa.
1348 Many 32-bit operating systems are limited to system times containing
1349 32 bits of information in their seconds component; these systems
1350 typically handle only the times from 1901-12-13 20:45:52 UTC through
1351 2038-01-19 03:14:07 UTC@. However, 64-bit and some 32-bit operating
1352 systems have larger seconds components, and can represent times far in
1355 Time conversion functions always use the Gregorian calendar, even
1356 for dates before the Gregorian calendar was introduced. Year numbers
1357 count the number of years since the year 1 B.C., and do not skip zero
1358 as traditional Gregorian years do; for example, the year number
1359 @minus{}37 represents the Gregorian year 38 B.C@.
1361 @defun decode-time &optional time zone
1362 This function converts a time value into calendrical information. If
1363 you don't specify @var{time}, it decodes the current time, and similarly
1364 @var{zone} defaults to the current time zone rule. The return
1365 value is a list of nine elements, as follows:
1368 (@var{seconds} @var{minutes} @var{hour} @var{day} @var{month} @var{year} @var{dow} @var{dst} @var{utcoff})
1371 Here is what the elements mean:
1375 The number of seconds past the minute, as an integer between 0 and 59.
1376 On some operating systems, this is 60 for leap seconds.
1378 The number of minutes past the hour, as an integer between 0 and 59.
1380 The hour of the day, as an integer between 0 and 23.
1382 The day of the month, as an integer between 1 and 31.
1384 The month of the year, as an integer between 1 and 12.
1386 The year, an integer typically greater than 1900.
1388 The day of week, as an integer between 0 and 6, where 0 stands for
1391 @code{t} if daylight saving time is effect, otherwise @code{nil}.
1393 An integer indicating the UTC offset in seconds, i.e., the number of
1394 seconds east of Greenwich.
1397 @strong{Common Lisp Note:} Common Lisp has different meanings for
1398 @var{dow} and @var{utcoff}.
1401 @defun encode-time seconds minutes hour day month year &optional zone
1402 This function is the inverse of @code{decode-time}. It converts seven
1403 items of calendrical data into a list-of-integer time value. For the
1404 meanings of the arguments, see the table above under
1407 Year numbers less than 100 are not treated specially. If you want them
1408 to stand for years above 1900, or years above 2000, you must alter them
1409 yourself before you call @code{encode-time}.
1411 The optional argument @var{zone} defaults to the current time zone rule.
1412 In addition to the usual time zone rule values, it can also be a list
1413 (as you would get from @code{current-time-zone}) or an integer (as
1414 from @code{decode-time}), applied without any further alteration for
1415 daylight saving time.
1417 If you pass more than seven arguments to @code{encode-time}, the first
1418 six are used as @var{seconds} through @var{year}, the last argument is
1419 used as @var{zone}, and the arguments in between are ignored. This
1420 feature makes it possible to use the elements of a list returned by
1421 @code{decode-time} as the arguments to @code{encode-time}, like this:
1424 (apply 'encode-time (decode-time @dots{}))
1427 You can perform simple date arithmetic by using out-of-range values for
1428 the @var{seconds}, @var{minutes}, @var{hour}, @var{day}, and @var{month}
1429 arguments; for example, day 0 means the day preceding the given month.
1431 The operating system puts limits on the range of possible time values;
1432 if you try to encode a time that is out of range, an error results.
1433 For instance, years before 1970 do not work on some systems;
1434 on others, years as early as 1901 do work.
1438 @section Parsing and Formatting Times
1439 @cindex time parsing
1440 @cindex time formatting
1441 @cindex formatting time values
1443 These functions convert time values to text in a string, and vice versa.
1444 Time values are lists of two to four integers (@pxref{Time of Day}).
1446 @defun date-to-time string
1447 This function parses the time-string @var{string} and returns the
1448 corresponding time value.
1451 @defun format-time-string format-string &optional time zone
1453 This function converts @var{time} (or the current time, if
1454 @var{time} is omitted) to a string according to
1455 @var{format-string}. The conversion uses the time zone rule @var{zone}
1456 (or the current time zone rule, if omitted). The argument
1457 @var{format-string} may contain @samp{%}-sequences which say to
1458 substitute parts of the time. Here is a table of what the
1459 @samp{%}-sequences mean:
1463 This stands for the abbreviated name of the day of week.
1465 This stands for the full name of the day of week.
1467 This stands for the abbreviated name of the month.
1469 This stands for the full name of the month.
1471 This is a synonym for @samp{%x %X}.
1473 This has a locale-specific meaning. In the default locale (named C), it
1474 is equivalent to @samp{%A, %B %e, %Y}.
1476 This stands for the day of month, zero-padded.
1478 This is a synonym for @samp{%m/%d/%y}.
1480 This stands for the day of month, blank-padded.
1482 This is a synonym for @samp{%b}.
1484 This stands for the hour (00--23).
1486 This stands for the hour (01--12).
1488 This stands for the day of the year (001--366).
1490 This stands for the hour (0--23), blank padded.
1492 This stands for the hour (1--12), blank padded.
1494 This stands for the month (01--12).
1496 This stands for the minute (00--59).
1498 This stands for a newline.
1500 This stands for the nanoseconds (000000000--999999999). To ask for
1501 fewer digits, use @samp{%3N} for milliseconds, @samp{%6N} for
1502 microseconds, etc. Any excess digits are discarded, without rounding.
1504 This stands for @samp{AM} or @samp{PM}, as appropriate.
1506 This is a synonym for @samp{%I:%M:%S %p}.
1508 This is a synonym for @samp{%H:%M}.
1510 This stands for the seconds (00--59).
1512 This stands for a tab character.
1514 This is a synonym for @samp{%H:%M:%S}.
1516 This stands for the week of the year (01--52), assuming that weeks
1519 This stands for the numeric day of week (0--6). Sunday is day 0.
1521 This stands for the week of the year (01--52), assuming that weeks
1524 This has a locale-specific meaning. In the default locale (named
1525 @samp{C}), it is equivalent to @samp{%D}.
1527 This has a locale-specific meaning. In the default locale (named
1528 @samp{C}), it is equivalent to @samp{%T}.
1530 This stands for the year without century (00--99).
1532 This stands for the year with century.
1534 This stands for the time zone abbreviation (e.g., @samp{EST}).
1536 This stands for the time zone numerical offset (e.g., @samp{-0500}).
1539 You can also specify the field width and type of padding for any of
1540 these @samp{%}-sequences. This works as in @code{printf}: you write
1541 the field width as digits in the middle of a @samp{%}-sequences. If you
1542 start the field width with @samp{0}, it means to pad with zeros. If you
1543 start the field width with @samp{_}, it means to pad with spaces.
1545 For example, @samp{%S} specifies the number of seconds since the minute;
1546 @samp{%03S} means to pad this with zeros to 3 positions, @samp{%_3S} to
1547 pad with spaces to 3 positions. Plain @samp{%3S} pads with zeros,
1548 because that is how @samp{%S} normally pads to two positions.
1550 The characters @samp{E} and @samp{O} act as modifiers when used between
1551 @samp{%} and one of the letters in the table above. @samp{E} specifies
1552 using the current locale's alternative version of the date and time.
1553 In a Japanese locale, for example, @code{%Ex} might yield a date format
1554 based on the Japanese Emperors' reigns. @samp{E} is allowed in
1555 @samp{%Ec}, @samp{%EC}, @samp{%Ex}, @samp{%EX}, @samp{%Ey}, and
1558 @samp{O} means to use the current locale's alternative
1559 representation of numbers, instead of the ordinary decimal digits. This
1560 is allowed with most letters, all the ones that output numbers.
1562 If @var{universal} is non-@code{nil}, that means to describe the time as
1563 Universal Time; @code{nil} means describe it using what Emacs believes
1564 is the local time zone (see @code{current-time-zone}).
1566 This function uses the C library function @code{strftime}
1567 (@pxref{Formatting Calendar Time,,, libc, The GNU C Library Reference
1568 Manual}) to do most of the work. In order to communicate with that
1569 function, it first encodes its argument using the coding system
1570 specified by @code{locale-coding-system} (@pxref{Locales}); after
1571 @code{strftime} returns the resulting string,
1572 @code{format-time-string} decodes the string using that same coding
1576 @defun format-seconds format-string seconds
1577 This function converts its argument @var{seconds} into a string of
1578 years, days, hours, etc., according to @var{format-string}. The
1579 argument @var{format-string} may contain @samp{%}-sequences which
1580 control the conversion. Here is a table of what the
1581 @samp{%}-sequences mean:
1586 The integer number of 365-day years.
1589 The integer number of days.
1592 The integer number of hours.
1595 The integer number of minutes.
1598 The integer number of seconds.
1600 Non-printing control flag. When it is used, other specifiers must be
1601 given in the order of decreasing size, i.e., years before days, hours
1602 before minutes, etc. Nothing will be produced in the result string to
1603 the left of @samp{%z} until the first non-zero conversion is
1604 encountered. For example, the default format used by
1605 @code{emacs-uptime} (@pxref{Processor Run Time, emacs-uptime})
1606 @w{@code{"%Y, %D, %H, %M, %z%S"}} means that the number of seconds
1607 will always be produced, but years, days, hours, and minutes will only
1608 be shown if they are non-zero.
1610 Produces a literal @samp{%}.
1613 Upper-case format sequences produce the units in addition to the
1614 numbers, lower-case formats produce only the numbers.
1616 You can also specify the field width by following the @samp{%} with a
1617 number; shorter numbers will be padded with blanks. An optional
1618 period before the width requests zero-padding instead. For example,
1619 @code{"%.3Y"} might produce @code{"004 years"}.
1621 @emph{Warning:} This function works only with values of @var{seconds}
1622 that don't exceed @code{most-positive-fixnum} (@pxref{Integer Basics,
1623 most-positive-fixnum}).
1626 @node Processor Run Time
1627 @section Processor Run time
1628 @cindex processor run time
1629 @cindex Emacs process run time
1631 Emacs provides several functions and primitives that return time,
1632 both elapsed and processor time, used by the Emacs process.
1634 @deffn Command emacs-uptime &optional format
1635 @cindex uptime of Emacs
1636 This function returns a string representing the Emacs
1637 @dfn{uptime}---the elapsed wall-clock time this instance of Emacs is
1638 running. The string is formatted by @code{format-seconds} according
1639 to the optional argument @var{format}. For the available format
1640 descriptors, see @ref{Time Parsing, format-seconds}. If @var{format}
1641 is @code{nil} or omitted, it defaults to @code{"%Y, %D, %H, %M,
1644 When called interactively, it prints the uptime in the echo area.
1647 @defun get-internal-run-time
1648 This function returns the processor run time used by Emacs as a list
1649 of four integers: @code{(@var{sec-high} @var{sec-low} @var{microsec}
1650 @var{picosec})}, using the same format as @code{current-time}
1651 (@pxref{Time of Day}).
1653 Note that the time returned by this function excludes the time Emacs
1654 was not using the processor, and if the Emacs process has several
1655 threads, the returned value is the sum of the processor times used up
1656 by all Emacs threads.
1658 If the system doesn't provide a way to determine the processor run
1659 time, @code{get-internal-run-time} returns the same time as
1660 @code{current-time}.
1663 @deffn Command emacs-init-time
1664 This function returns the duration of the Emacs initialization
1665 (@pxref{Startup Summary}) in seconds, as a string. When called
1666 interactively, it prints the duration in the echo area.
1669 @node Time Calculations
1670 @section Time Calculations
1671 @cindex time calculations
1672 @cindex comparing time values
1673 @cindex calendrical computations
1675 These functions perform calendrical computations using time values
1676 (@pxref{Time of Day}).
1678 @defun time-less-p t1 t2
1679 This returns @code{t} if time value @var{t1} is less than time value
1683 @defun time-subtract t1 t2
1684 This returns the time difference @var{t1} @minus{} @var{t2} between
1685 two time values, as a time value.
1688 @defun time-add t1 t2
1689 This returns the sum of two time values, as a time value.
1690 One argument should represent a time difference rather than a point in time.
1691 Here is how to add a number of seconds to a time value:
1694 (time-add @var{time} @var{seconds})
1698 @defun time-to-days time-value
1699 This function returns the number of days between the beginning of year
1700 1 and @var{time-value}.
1703 @defun time-to-day-in-year time-value
1704 This returns the day number within the year corresponding to @var{time-value}.
1707 @defun date-leap-year-p year
1708 This function returns @code{t} if @var{year} is a leap year.
1712 @section Timers for Delayed Execution
1715 You can set up a @dfn{timer} to call a function at a specified
1716 future time or after a certain length of idleness.
1718 Emacs cannot run timers at any arbitrary point in a Lisp program; it
1719 can run them only when Emacs could accept output from a subprocess:
1720 namely, while waiting or inside certain primitive functions such as
1721 @code{sit-for} or @code{read-event} which @emph{can} wait. Therefore, a
1722 timer's execution may be delayed if Emacs is busy. However, the time of
1723 execution is very precise if Emacs is idle.
1725 Emacs binds @code{inhibit-quit} to @code{t} before calling the timer
1726 function, because quitting out of many timer functions can leave
1727 things in an inconsistent state. This is normally unproblematical
1728 because most timer functions don't do a lot of work. Indeed, for a
1729 timer to call a function that takes substantial time to run is likely
1730 to be annoying. If a timer function needs to allow quitting, it
1731 should use @code{with-local-quit} (@pxref{Quitting}). For example, if
1732 a timer function calls @code{accept-process-output} to receive output
1733 from an external process, that call should be wrapped inside
1734 @code{with-local-quit}, to ensure that @kbd{C-g} works if the external
1737 It is usually a bad idea for timer functions to alter buffer
1738 contents. When they do, they usually should call @code{undo-boundary}
1739 both before and after changing the buffer, to separate the timer's
1740 changes from user commands' changes and prevent a single undo entry
1741 from growing to be quite large.
1743 Timer functions should also avoid calling functions that cause Emacs
1744 to wait, such as @code{sit-for} (@pxref{Waiting}). This can lead to
1745 unpredictable effects, since other timers (or even the same timer) can
1746 run while waiting. If a timer function needs to perform an action
1747 after a certain time has elapsed, it can do this by scheduling a new
1750 If a timer function calls functions that can change the match data,
1751 it should save and restore the match data. @xref{Saving Match Data}.
1753 @deffn Command run-at-time time repeat function &rest args
1754 This sets up a timer that calls the function @var{function} with
1755 arguments @var{args} at time @var{time}. If @var{repeat} is a number
1756 (integer or floating point), the timer is scheduled to run again every
1757 @var{repeat} seconds after @var{time}. If @var{repeat} is @code{nil},
1758 the timer runs only once.
1760 @var{time} may specify an absolute or a relative time.
1762 Absolute times may be specified using a string with a limited variety
1763 of formats, and are taken to be times @emph{today}, even if already in
1764 the past. The recognized forms are @samp{@var{xxxx}},
1765 @samp{@var{x}:@var{xx}}, or @samp{@var{xx}:@var{xx}} (military time),
1766 and @samp{@var{xx}am}, @samp{@var{xx}AM}, @samp{@var{xx}pm},
1767 @samp{@var{xx}PM}, @samp{@var{xx}:@var{xx}am},
1768 @samp{@var{xx}:@var{xx}AM}, @samp{@var{xx}:@var{xx}pm}, or
1769 @samp{@var{xx}:@var{xx}PM}. A period can be used instead of a colon
1770 to separate the hour and minute parts.
1772 To specify a relative time as a string, use numbers followed by units.
1777 denotes 1 minute from now.
1779 denotes 65 seconds from now.
1780 @item 1 min 2 sec 3 hour 4 day 5 week 6 fortnight 7 month 8 year
1781 denotes exactly 103 months, 123 days, and 10862 seconds from now.
1784 For relative time values, Emacs considers a month to be exactly thirty
1785 days, and a year to be exactly 365.25 days.
1787 Not all convenient formats are strings. If @var{time} is a number
1788 (integer or floating point), that specifies a relative time measured in
1789 seconds. The result of @code{encode-time} can also be used to specify
1790 an absolute value for @var{time}.
1792 In most cases, @var{repeat} has no effect on when @emph{first} call
1793 takes place---@var{time} alone specifies that. There is one exception:
1794 if @var{time} is @code{t}, then the timer runs whenever the time is a
1795 multiple of @var{repeat} seconds after the epoch. This is useful for
1796 functions like @code{display-time}.
1798 The function @code{run-at-time} returns a timer value that identifies
1799 the particular scheduled future action. You can use this value to call
1800 @code{cancel-timer} (see below).
1803 A repeating timer nominally ought to run every @var{repeat} seconds,
1804 but remember that any invocation of a timer can be late. Lateness of
1805 one repetition has no effect on the scheduled time of the next
1806 repetition. For instance, if Emacs is busy computing for long enough
1807 to cover three scheduled repetitions of the timer, and then starts to
1808 wait, it will immediately call the timer function three times in
1809 immediate succession (presuming no other timers trigger before or
1810 between them). If you want a timer to run again no less than @var{n}
1811 seconds after the last invocation, don't use the @var{repeat} argument.
1812 Instead, the timer function should explicitly reschedule the timer.
1814 @defopt timer-max-repeats
1815 This variable's value specifies the maximum number of times to repeat
1816 calling a timer function in a row, when many previously scheduled
1817 calls were unavoidably delayed.
1820 @defmac with-timeout (seconds timeout-forms@dots{}) body@dots{}
1821 Execute @var{body}, but give up after @var{seconds} seconds. If
1822 @var{body} finishes before the time is up, @code{with-timeout} returns
1823 the value of the last form in @var{body}. If, however, the execution of
1824 @var{body} is cut short by the timeout, then @code{with-timeout}
1825 executes all the @var{timeout-forms} and returns the value of the last
1828 This macro works by setting a timer to run after @var{seconds} seconds. If
1829 @var{body} finishes before that time, it cancels the timer. If the
1830 timer actually runs, it terminates execution of @var{body}, then
1831 executes @var{timeout-forms}.
1833 Since timers can run within a Lisp program only when the program calls a
1834 primitive that can wait, @code{with-timeout} cannot stop executing
1835 @var{body} while it is in the midst of a computation---only when it
1836 calls one of those primitives. So use @code{with-timeout} only with a
1837 @var{body} that waits for input, not one that does a long computation.
1840 The function @code{y-or-n-p-with-timeout} provides a simple way to use
1841 a timer to avoid waiting too long for an answer. @xref{Yes-or-No
1844 @defun cancel-timer timer
1845 This cancels the requested action for @var{timer}, which should be a
1846 timer---usually, one previously returned by @code{run-at-time} or
1847 @code{run-with-idle-timer}. This cancels the effect of that call to
1848 one of these functions; the arrival of the specified time will not
1849 cause anything special to happen.
1853 @section Idle Timers
1856 Here is how to set up a timer that runs when Emacs is idle for a
1857 certain length of time. Aside from how to set them up, idle timers
1858 work just like ordinary timers.
1860 @deffn Command run-with-idle-timer secs repeat function &rest args
1861 Set up a timer which runs the next time Emacs is idle for @var{secs}
1862 seconds. The value of @var{secs} may be a number or a value of the type
1863 returned by @code{current-idle-time}.
1865 If @var{repeat} is @code{nil}, the timer runs just once, the first time
1866 Emacs remains idle for a long enough time. More often @var{repeat} is
1867 non-@code{nil}, which means to run the timer @emph{each time} Emacs
1868 remains idle for @var{secs} seconds.
1870 The function @code{run-with-idle-timer} returns a timer value which you
1871 can use in calling @code{cancel-timer} (@pxref{Timers}).
1875 Emacs becomes @dfn{idle} when it starts waiting for user input, and
1876 it remains idle until the user provides some input. If a timer is set
1877 for five seconds of idleness, it runs approximately five seconds after
1878 Emacs first becomes idle. Even if @var{repeat} is non-@code{nil},
1879 this timer will not run again as long as Emacs remains idle, because
1880 the duration of idleness will continue to increase and will not go
1881 down to five seconds again.
1883 Emacs can do various things while idle: garbage collect, autosave or
1884 handle data from a subprocess. But these interludes during idleness do
1885 not interfere with idle timers, because they do not reset the clock of
1886 idleness to zero. An idle timer set for 600 seconds will run when ten
1887 minutes have elapsed since the last user command was finished, even if
1888 subprocess output has been accepted thousands of times within those ten
1889 minutes, and even if there have been garbage collections and autosaves.
1891 When the user supplies input, Emacs becomes non-idle while executing the
1892 input. Then it becomes idle again, and all the idle timers that are
1893 set up to repeat will subsequently run another time, one by one.
1895 Do not write an idle timer function containing a loop which does a
1896 certain amount of processing each time around, and exits when
1897 @code{(input-pending-p)} is non-@code{nil}. This approach seems very
1898 natural but has two problems:
1902 It blocks out all process output (since Emacs accepts process output
1903 only while waiting).
1906 It blocks out any idle timers that ought to run during that time.
1910 Similarly, do not write an idle timer function that sets up another
1911 idle timer (including the same idle timer) with @var{secs} argument
1912 less than or equal to the current idleness time. Such a timer will
1913 run almost immediately, and continue running again and again, instead
1914 of waiting for the next time Emacs becomes idle. The correct approach
1915 is to reschedule with an appropriate increment of the current value of
1916 the idleness time, as described below.
1918 @defun current-idle-time
1919 If Emacs is idle, this function returns the length of time Emacs has
1920 been idle, as a list of four integers: @code{(@var{sec-high}
1921 @var{sec-low} @var{microsec} @var{picosec})}, using the same format as
1922 @code{current-time} (@pxref{Time of Day}).
1924 When Emacs is not idle, @code{current-idle-time} returns @code{nil}.
1925 This is a convenient way to test whether Emacs is idle.
1928 The main use of @code{current-idle-time} is when an idle timer
1929 function wants to ``take a break'' for a while. It can set up another
1930 idle timer to call the same function again, after a few seconds more
1931 idleness. Here's an example:
1934 (defvar my-resume-timer nil
1935 "Timer for `my-timer-function' to reschedule itself, or nil.")
1937 (defun my-timer-function ()
1938 ;; @r{If the user types a command while @code{my-resume-timer}}
1939 ;; @r{is active, the next time this function is called from}
1940 ;; @r{its main idle timer, deactivate @code{my-resume-timer}.}
1941 (when my-resume-timer
1942 (cancel-timer my-resume-timer))
1943 ...@var{do the work for a while}...
1944 (when @var{taking-a-break}
1945 (setq my-resume-timer
1946 (run-with-idle-timer
1947 ;; Compute an idle time @var{break-length}
1948 ;; more than the current value.
1949 (time-add (current-idle-time) @var{break-length})
1951 'my-timer-function))))
1954 @node Terminal Input
1955 @section Terminal Input
1956 @cindex terminal input
1958 This section describes functions and variables for recording or
1959 manipulating terminal input. See @ref{Display}, for related
1963 * Input Modes:: Options for how input is processed.
1964 * Recording Input:: Saving histories of recent or all input events.
1968 @subsection Input Modes
1970 @cindex terminal input modes
1972 @defun set-input-mode interrupt flow meta &optional quit-char
1973 This function sets the mode for reading keyboard input. If
1974 @var{interrupt} is non-@code{nil}, then Emacs uses input interrupts.
1975 If it is @code{nil}, then it uses @sc{cbreak} mode. The default
1976 setting is system-dependent. Some systems always use @sc{cbreak} mode
1977 regardless of what is specified.
1979 When Emacs communicates directly with X, it ignores this argument and
1980 uses interrupts if that is the way it knows how to communicate.
1982 If @var{flow} is non-@code{nil}, then Emacs uses @sc{xon/xoff}
1983 (@kbd{C-q}, @kbd{C-s}) flow control for output to the terminal. This
1984 has no effect except in @sc{cbreak} mode.
1986 The argument @var{meta} controls support for input character codes
1987 above 127. If @var{meta} is @code{t}, Emacs converts characters with
1988 the 8th bit set into Meta characters. If @var{meta} is @code{nil},
1989 Emacs disregards the 8th bit; this is necessary when the terminal uses
1990 it as a parity bit. If @var{meta} is neither @code{t} nor @code{nil},
1991 Emacs uses all 8 bits of input unchanged. This is good for terminals
1992 that use 8-bit character sets.
1994 If @var{quit-char} is non-@code{nil}, it specifies the character to
1995 use for quitting. Normally this character is @kbd{C-g}.
1999 The @code{current-input-mode} function returns the input mode settings
2000 Emacs is currently using.
2002 @defun current-input-mode
2003 This function returns the current mode for reading keyboard input. It
2004 returns a list, corresponding to the arguments of @code{set-input-mode},
2005 of the form @code{(@var{interrupt} @var{flow} @var{meta} @var{quit})} in
2009 is non-@code{nil} when Emacs is using interrupt-driven input. If
2010 @code{nil}, Emacs is using @sc{cbreak} mode.
2012 is non-@code{nil} if Emacs uses @sc{xon/xoff} (@kbd{C-q}, @kbd{C-s})
2013 flow control for output to the terminal. This value is meaningful only
2014 when @var{interrupt} is @code{nil}.
2016 is @code{t} if Emacs treats the eighth bit of input characters as
2017 the meta bit; @code{nil} means Emacs clears the eighth bit of every
2018 input character; any other value means Emacs uses all eight bits as the
2019 basic character code.
2021 is the character Emacs currently uses for quitting, usually @kbd{C-g}.
2025 @node Recording Input
2026 @subsection Recording Input
2027 @cindex recording input
2030 This function returns a vector containing the last 300 input events from
2031 the keyboard or mouse. All input events are included, whether or not
2032 they were used as parts of key sequences. Thus, you always get the last
2033 300 input events, not counting events generated by keyboard macros.
2034 (These are excluded because they are less interesting for debugging; it
2035 should be enough to see the events that invoked the macros.)
2037 A call to @code{clear-this-command-keys} (@pxref{Command Loop Info})
2038 causes this function to return an empty vector immediately afterward.
2041 @deffn Command open-dribble-file filename
2042 @cindex dribble file
2043 This function opens a @dfn{dribble file} named @var{filename}. When a
2044 dribble file is open, each input event from the keyboard or mouse (but
2045 not those from keyboard macros) is written in that file. A
2046 non-character event is expressed using its printed representation
2047 surrounded by @samp{<@dots{}>}. Be aware that sensitive information
2048 (such as passwords) may end up recorded in the dribble file.
2050 You close the dribble file by calling this function with an argument
2054 See also the @code{open-termscript} function (@pxref{Terminal Output}).
2056 @node Terminal Output
2057 @section Terminal Output
2058 @cindex terminal output
2060 The terminal output functions send output to a text terminal, or keep
2061 track of output sent to the terminal. The variable @code{baud-rate}
2062 tells you what Emacs thinks is the output speed of the terminal.
2065 This variable's value is the output speed of the terminal, as far as
2066 Emacs knows. Setting this variable does not change the speed of actual
2067 data transmission, but the value is used for calculations such as
2070 It also affects decisions about whether to scroll part of the
2071 screen or repaint on text terminals. @xref{Forcing Redisplay},
2072 for the corresponding functionality on graphical terminals.
2074 The value is measured in baud.
2077 If you are running across a network, and different parts of the
2078 network work at different baud rates, the value returned by Emacs may be
2079 different from the value used by your local terminal. Some network
2080 protocols communicate the local terminal speed to the remote machine, so
2081 that Emacs and other programs can get the proper value, but others do
2082 not. If Emacs has the wrong value, it makes decisions that are less
2083 than optimal. To fix the problem, set @code{baud-rate}.
2085 @defun send-string-to-terminal string &optional terminal
2086 This function sends @var{string} to @var{terminal} without alteration.
2087 Control characters in @var{string} have terminal-dependent effects.
2088 This function operates only on text terminals. @var{terminal} may be
2089 a terminal object, a frame, or @code{nil} for the selected frame's
2090 terminal. In batch mode, @var{string} is sent to @code{stdout} when
2091 @var{terminal} is @code{nil}.
2093 One use of this function is to define function keys on terminals that
2094 have downloadable function key definitions. For example, this is how (on
2095 certain terminals) to define function key 4 to move forward four
2096 characters (by transmitting the characters @kbd{C-u C-f} to the
2101 (send-string-to-terminal "\eF4\^U\^F")
2107 @deffn Command open-termscript filename
2108 @cindex termscript file
2109 This function is used to open a @dfn{termscript file} that will record
2110 all the characters sent by Emacs to the terminal. It returns
2111 @code{nil}. Termscript files are useful for investigating problems
2112 where Emacs garbles the screen, problems that are due to incorrect
2113 Termcap entries or to undesirable settings of terminal options more
2114 often than to actual Emacs bugs. Once you are certain which characters
2115 were actually output, you can determine reliably whether they correspond
2116 to the Termcap specifications in use.
2120 (open-termscript "../junk/termscript")
2125 You close the termscript file by calling this function with an
2126 argument of @code{nil}.
2128 See also @code{open-dribble-file} in @ref{Recording Input}.
2132 @section Sound Output
2135 To play sound using Emacs, use the function @code{play-sound}. Only
2136 certain systems are supported; if you call @code{play-sound} on a
2137 system which cannot really do the job, it gives an error.
2139 @c FIXME: Add indexes for Au and WAV? --xfq
2140 The sound must be stored as a file in RIFF-WAVE format (@samp{.wav})
2141 or Sun Audio format (@samp{.au}).
2143 @defun play-sound sound
2144 This function plays a specified sound. The argument, @var{sound}, has
2145 the form @code{(sound @var{properties}...)}, where the @var{properties}
2146 consist of alternating keywords (particular symbols recognized
2147 specially) and values corresponding to them.
2149 Here is a table of the keywords that are currently meaningful in
2150 @var{sound}, and their meanings:
2153 @item :file @var{file}
2154 This specifies the file containing the sound to play.
2155 If the file name is not absolute, it is expanded against
2156 the directory @code{data-directory}.
2158 @item :data @var{data}
2159 This specifies the sound to play without need to refer to a file. The
2160 value, @var{data}, should be a string containing the same bytes as a
2161 sound file. We recommend using a unibyte string.
2163 @item :volume @var{volume}
2164 This specifies how loud to play the sound. It should be a number in the
2165 range of 0 to 1. The default is to use whatever volume has been
2168 @item :device @var{device}
2169 This specifies the system device on which to play the sound, as a
2170 string. The default device is system-dependent.
2173 Before actually playing the sound, @code{play-sound}
2174 calls the functions in the list @code{play-sound-functions}.
2175 Each function is called with one argument, @var{sound}.
2178 @deffn Command play-sound-file file &optional volume device
2179 This function is an alternative interface to playing a sound @var{file}
2180 specifying an optional @var{volume} and @var{device}.
2183 @defvar play-sound-functions
2184 A list of functions to be called before playing a sound. Each function
2185 is called with one argument, a property list that describes the sound.
2189 @section Operating on X11 Keysyms
2192 To define system-specific X11 keysyms, set the variable
2193 @code{system-key-alist}.
2195 @defvar system-key-alist
2196 This variable's value should be an alist with one element for each
2197 system-specific keysym. Each element has the form @code{(@var{code}
2198 . @var{symbol})}, where @var{code} is the numeric keysym code (not
2199 including the vendor-specific bit,
2206 and @var{symbol} is the name for the function key.
2208 For example @code{(168 . mute-acute)} defines a system-specific key (used
2209 by HP X servers) whose numeric code is
2218 It is not crucial to exclude from the alist the keysyms of other X
2219 servers; those do no harm, as long as they don't conflict with the ones
2220 used by the X server actually in use.
2222 The variable is always local to the current terminal, and cannot be
2223 buffer-local. @xref{Multiple Terminals}.
2226 You can specify which keysyms Emacs should use for the Meta, Alt, Hyper, and Super modifiers by setting these variables:
2228 @defvar x-alt-keysym
2229 @defvarx x-meta-keysym
2230 @defvarx x-hyper-keysym
2231 @defvarx x-super-keysym
2232 The name of the keysym that should stand for the Alt modifier
2233 (respectively, for Meta, Hyper, and Super). For example, here is
2234 how to swap the Meta and Alt modifiers within Emacs:
2236 (setq x-alt-keysym 'meta)
2237 (setq x-meta-keysym 'alt)
2245 The command-line option @samp{-batch} causes Emacs to run
2246 noninteractively. In this mode, Emacs does not read commands from the
2247 terminal, it does not alter the terminal modes, and it does not expect
2248 to be outputting to an erasable screen. The idea is that you specify
2249 Lisp programs to run; when they are finished, Emacs should exit. The
2250 way to specify the programs to run is with @samp{-l @var{file}}, which
2251 loads the library named @var{file}, or @samp{-f @var{function}}, which
2252 calls @var{function} with no arguments, or @samp{--eval @var{form}}.
2254 Any Lisp program output that would normally go to the echo area,
2255 either using @code{message}, or using @code{prin1}, etc., with @code{t}
2256 as the stream, goes instead to Emacs's standard error descriptor when
2257 in batch mode. Similarly, input that would normally come from the
2258 minibuffer is read from the standard input descriptor.
2259 Thus, Emacs behaves much like a noninteractive
2260 application program. (The echo area output that Emacs itself normally
2261 generates, such as command echoing, is suppressed entirely.)
2263 @defvar noninteractive
2264 This variable is non-@code{nil} when Emacs is running in batch mode.
2267 @node Session Management
2268 @section Session Management
2269 @cindex session manager
2271 Emacs supports the X Session Management Protocol, which is used to
2272 suspend and restart applications. In the X Window System, a program
2273 called the @dfn{session manager} is responsible for keeping track of
2274 the applications that are running. When the X server shuts down, the
2275 session manager asks applications to save their state, and delays the
2276 actual shutdown until they respond. An application can also cancel
2279 When the session manager restarts a suspended session, it directs
2280 these applications to individually reload their saved state. It does
2281 this by specifying a special command-line argument that says what
2282 saved session to restore. For Emacs, this argument is @samp{--smid
2285 @defvar emacs-save-session-functions
2286 @cindex session file
2287 Emacs supports saving state via a hook called
2288 @code{emacs-save-session-functions}. Emacs runs this hook when the
2289 session manager tells it that the window system is shutting down. The
2290 functions are called with no arguments, and with the current buffer
2291 set to a temporary buffer. Each function can use @code{insert} to add
2292 Lisp code to this buffer. At the end, Emacs saves the buffer in a
2293 file, called the @dfn{session file}.
2295 @findex emacs-session-restore
2296 Subsequently, when the session manager restarts Emacs, it loads the
2297 session file automatically (@pxref{Loading}). This is performed by a
2298 function named @code{emacs-session-restore}, which is called during
2299 startup. @xref{Startup Summary}.
2301 If a function in @code{emacs-save-session-functions} returns
2302 non-@code{nil}, Emacs tells the session manager to cancel the
2306 Here is an example that just inserts some text into @file{*scratch*} when
2307 Emacs is restarted by the session manager.
2311 (add-hook 'emacs-save-session-functions 'save-yourself-test)
2315 (defun save-yourself-test ()
2316 (insert "(save-current-buffer
2317 (switch-to-buffer \"*scratch*\")
2318 (insert \"I am restored\"))")
2323 @node Desktop Notifications
2324 @section Desktop Notifications
2325 @cindex desktop notifications
2326 @cindex notifications, on desktop
2328 Emacs is able to send @dfn{notifications} on systems that support the
2329 freedesktop.org Desktop Notifications Specification and on MS-Windows.
2330 In order to use this functionality on Posix hosts, Emacs must have
2331 been compiled with D-Bus support, and the @code{notifications} library
2332 must be loaded. @xref{Top, , D-Bus,dbus,D-Bus integration in Emacs}.
2333 The following function is supported when D-Bus support is available:
2335 @defun notifications-notify &rest params
2336 This function sends a notification to the desktop via D-Bus,
2337 consisting of the parameters specified by the @var{params} arguments.
2338 These arguments should consist of alternating keyword and value pairs.
2339 The supported keywords and values are as follows:
2342 @item :bus @var{bus}
2343 The D-Bus bus. This argument is needed only if a bus other than
2344 @code{:session} shall be used.
2346 @item :title @var{title}
2347 The notification title.
2349 @item :body @var{text}
2350 The notification body text. Depending on the implementation of the
2351 notification server, the text could contain HTML markups, like
2352 @samp{"<b>bold text</b>"}, hyperlinks, or images. Special HTML
2353 characters must be encoded, as @samp{"Contact
2354 <postmaster@@localhost>!"}.
2356 @item :app-name @var{name}
2357 The name of the application sending the notification. The default is
2358 @code{notifications-application-name}.
2360 @item :replaces-id @var{id}
2361 The notification @var{id} that this notification replaces. @var{id}
2362 must be the result of a previous @code{notifications-notify} call.
2364 @item :app-icon @var{icon-file}
2365 The file name of the notification icon. If set to @code{nil}, no icon
2366 is displayed. The default is @code{notifications-application-icon}.
2368 @item :actions (@var{key} @var{title} @var{key} @var{title} ...)
2369 A list of actions to be applied. @var{key} and @var{title} are both
2370 strings. The default action (usually invoked by clicking the
2371 notification) should have a key named @samp{"default"}. The title can
2372 be anything, though implementations are free not to display it.
2374 @item :timeout @var{timeout}
2375 The timeout time in milliseconds since the display of the notification
2376 at which the notification should automatically close. If @minus{}1, the
2377 notification's expiration time is dependent on the notification
2378 server's settings, and may vary for the type of notification. If 0,
2379 the notification never expires. Default value is @minus{}1.
2381 @item :urgency @var{urgency}
2382 The urgency level. It can be @code{low}, @code{normal}, or @code{critical}.
2385 When this keyword is given, the @var{title} string of the actions is
2386 interpreted as icon name.
2388 @item :category @var{category}
2389 The type of notification this is, a string. See the
2390 @uref{http://developer.gnome.org/notification-spec/#categories,
2391 Desktop Notifications Specification} for a list of standard
2394 @item :desktop-entry @var{filename}
2395 This specifies the name of the desktop filename representing the
2396 calling program, like @samp{"emacs"}.
2398 @item :image-data (@var{width} @var{height} @var{rowstride} @var{has-alpha} @var{bits} @var{channels} @var{data})
2399 This is a raw data image format that describes the width, height,
2400 rowstride, whether there is an alpha channel, bits per sample,
2401 channels and image data, respectively.
2403 @item :image-path @var{path}
2404 This is represented either as a URI (@samp{file://} is the only URI
2405 schema supported right now) or a name in a freedesktop.org-compliant
2406 icon theme from @samp{$XDG_DATA_DIRS/icons}.
2408 @item :sound-file @var{filename}
2409 The path to a sound file to play when the notification pops up.
2411 @item :sound-name @var{name}
2412 A themable named sound from the freedesktop.org sound naming
2413 specification from @samp{$XDG_DATA_DIRS/sounds}, to play when the
2414 notification pops up. Similar to the icon name, only for sounds. An
2415 example would be @samp{"message-new-instant"}.
2417 @item :suppress-sound
2418 Causes the server to suppress playing any sounds, if it has that
2422 When set the server will not automatically remove the notification
2423 when an action has been invoked. The notification will remain resident
2424 in the server until it is explicitly removed by the user or by the
2425 sender. This hint is likely only useful when the server has the
2426 @code{:persistence} capability.
2429 When set the server will treat the notification as transient and
2430 by-pass the server's persistence capability, if it should exist.
2432 @item :x @var{position}
2433 @itemx :y @var{position}
2434 Specifies the X, Y location on the screen that the
2435 notification should point to. Both arguments must be used together.
2437 @item :on-action @var{function}
2438 Function to call when an action is invoked. The notification @var{id}
2439 and the @var{key} of the action are passed as arguments to the
2442 @item :on-close @var{function}
2443 Function to call when the notification has been closed by timeout or
2444 by the user. The function receive the notification @var{id} and the closing
2445 @var{reason} as arguments:
2448 @item @code{expired} if the notification has expired
2449 @item @code{dismissed} if the notification was dismissed by the user
2450 @item @code{close-notification} if the notification was closed by a call to
2451 @code{notifications-close-notification}
2452 @item @code{undefined} if the notification server hasn't provided a reason
2456 Which parameters are accepted by the notification server can be
2457 checked via @code{notifications-get-capabilities}.
2459 This function returns a notification id, an integer, which can be used
2460 to manipulate the notification item with
2461 @code{notifications-close-notification} or the @code{:replaces-id}
2462 argument of another @code{notifications-notify} call. For example:
2466 (defun my-on-action-function (id key)
2467 (message "Message %d, key \"%s\" pressed" id key))
2468 @result{} my-on-action-function
2472 (defun my-on-close-function (id reason)
2473 (message "Message %d, closed due to \"%s\"" id reason))
2474 @result{} my-on-close-function
2478 (notifications-notify
2480 :body "This is <b>important</b>."
2481 :actions '("Confirm" "I agree" "Refuse" "I disagree")
2482 :on-action 'my-on-action-function
2483 :on-close 'my-on-close-function)
2488 A message window opens on the desktop. Press ``I agree''.
2489 @result{} Message 22, key "Confirm" pressed
2490 Message 22, closed due to "dismissed"
2495 @defun notifications-close-notification id &optional bus
2496 This function closes a notification with identifier @var{id}.
2497 @var{bus} can be a string denoting a D-Bus connection, the default is
2501 @defun notifications-get-capabilities &optional bus
2502 Returns the capabilities of the notification server, a list of
2503 symbols. @var{bus} can be a string denoting a D-Bus connection, the
2504 default is @code{:session}. The following capabilities can be
2509 The server will provide the specified actions to the user.
2514 @item :body-hyperlinks
2515 The server supports hyperlinks in the notifications.
2518 The server supports images in the notifications.
2521 Supports markup in the body text.
2524 The server will render an animation of all the frames in a given image
2528 Supports display of exactly 1 frame of any given image array. This
2529 value is mutually exclusive with @code{:icon-multi}.
2532 The server supports persistence of notifications.
2535 The server supports sounds on notifications.
2538 Further vendor-specific caps start with @code{:x-vendor}, like
2539 @code{:x-gnome-foo-cap}.
2542 @defun notifications-get-server-information &optional bus
2543 Return information on the notification server, a list of strings.
2544 @var{bus} can be a string denoting a D-Bus connection, the default is
2545 @code{:session}. The returned list is @code{(@var{name} @var{vendor}
2546 @var{version} @var{spec-version})}.
2550 The product name of the server.
2553 The vendor name. For example, @samp{"KDE"}, @samp{"GNOME"}.
2556 The server's version number.
2559 The specification version the server is compliant with.
2562 If @var{spec_version} is @code{nil}, the server supports a
2563 specification prior to @samp{"1.0"}.
2566 @cindex tray notifications, MS-Windows
2567 When Emacs runs on MS-Windows as a GUI session, it supports a small
2568 subset of the D-Bus notifications functionality via a native
2571 @defun w32-notification-notify &rest params
2572 This function displays an MS-Windows tray notification as specified by
2573 @var{params}. MS-Windows tray notifications are displayed in a
2574 balloon from an icon in the notification area of the taskbar.
2576 Value is the integer unique ID of the notification that can be used to
2577 remove the notification using @code{w32-notification-close}, described
2578 below. If the function fails, the return value is @code{nil}.
2580 The arguments @var{params} are specified as keyword/value pairs. All the
2581 parameters are optional, but if no parameters are specified, the
2582 function will do nothing and return @code{nil}.
2584 The following parameters are supported:
2587 @item :icon @var{icon}
2588 Display @var{icon} in the system tray. If @var{icon} is a string, it
2589 should specify a file name from which to load the icon; the specified
2590 file should be a @file{.ico} Windows icon file. If @var{icon} is not
2591 a string, or if this parameter is not specified, the standard Emacs
2594 @item :tip @var{tip}
2595 Use @var{tip} as the tooltip for the notification. If @var{tip} is a
2596 string, this is the text of a tooltip that will be shown when the
2597 mouse pointer hovers over the tray icon added by the notification. If
2598 @var{tip} is not a string, or if this parameter is not specified, the
2599 default tooltip text is @samp{Emacs notification}. The tooltip text can
2600 be up to 127 characters long (63 on Windows versions before W2K).
2601 Longer strings will be truncated.
2603 @item :level @var{level}
2604 Notification severity level, one of @code{info}, @code{warning}, or
2605 @code{error}. If given, the value determines the icon displayed to the
2606 left of the notification title, but only if the @code{:title} parameter
2607 (see below) is also specified and is a string.
2609 @item :title @var{title}
2610 The title of the notification. If @var{title} is a string, it is
2611 displayed in a larger font immediately above the body text. The title
2612 text can be up to 63 characters long; longer text will be truncated.
2614 @item :body @var{body}
2615 The body of the notification. If @var{body} is a string, it specifies
2616 the text of the notification message. Use embedded newlines to
2617 control how the text is broken into lines. The body text can be up to
2618 255 characters long, and will be truncated if it's longer. Unlike
2619 with D-Bus, the body text should be plain text, with no markup.
2622 Note that versions of Windows before W2K support only @code{:icon} and
2623 @code{:tip}. The other parameters can be passed, but they will be
2624 ignored on those old systems.
2626 There can be at most one active notification at any given time. An
2627 active notification must be removed by calling
2628 @code{w32-notification-close} before a new one can be shown.
2631 To remove the notification and its icon from the taskbar, use the
2634 @defun w32-notification-close id
2635 This function removes the tray notification given by its unique
2639 @node File Notifications
2640 @section Notifications on File Changes
2641 @cindex file notifications
2642 @cindex watch, for filesystem events
2644 Several operating systems support watching of filesystems for changes
2645 of files. If configured properly, Emacs links a respective library
2646 like @file{inotify}, @file{kqueue}, @file{gfilenotify}, or
2647 @file{w32notify} statically. These libraries enable watching of
2648 filesystems on the local machine.
2650 It is also possible to watch filesystems on remote machines,
2651 @pxref{Remote Files,, Remote Files, emacs, The GNU Emacs Manual}
2652 This does not depend on one of the libraries linked to Emacs.
2654 Since all these libraries emit different events on notified file
2655 changes, there is the Emacs library @code{filenotify} which provides a
2658 @defun file-notify-add-watch file flags callback
2659 Add a watch for filesystem events pertaining to @var{file}. This
2660 arranges for filesystem events pertaining to @var{file} to be reported
2663 The returned value is a descriptor for the added watch. Its type
2664 depends on the underlying library, it cannot be assumed to be an
2665 integer as in the example below. It should be used for comparison by
2668 If the @var{file} cannot be watched for some reason, this function
2669 signals a @code{file-notify-error} error.
2671 Sometimes, mounted filesystems cannot be watched for file changes.
2672 This is not detected by this function, a non-@code{nil} return value
2673 does not guarantee that changes on @var{file} will be notified.
2675 @var{flags} is a list of conditions to set what will be watched for.
2676 It can include the following symbols:
2680 watch for file changes
2681 @item attribute-change
2682 watch for file attribute changes, like permissions or modification
2686 If @var{file} is a directory, changes for all files in that directory
2687 will be notified. This does not work recursively.
2689 When any event happens, Emacs will call the @var{callback} function
2690 passing it a single argument @var{event}, which is of the form
2693 (@var{descriptor} @var{action} @var{file} [@var{file1}])
2696 @var{descriptor} is the same object as the one returned by this
2697 function. @var{action} is the description of the event. It could be
2698 any one of the following symbols:
2702 @var{file} was created
2704 @var{file} was deleted
2706 @var{file}'s contents has changed; with @file{w32notify} library,
2707 reports attribute changes as well
2709 @var{file} has been renamed to @var{file1}
2710 @item attribute-changed
2711 a @var{file} attribute was changed
2713 watching @var{file} has been stopped
2716 Note that the @file{w32notify} library does not report
2717 @code{attribute-changed} events. When some file's attribute, like
2718 permissions or modification time, has changed, this library reports a
2719 @code{changed} event. Likewise, the @file{kqueue} library does not
2720 report reliably file attribute changes when watching a directory.
2722 The @code{stopped} event reports, that watching the file has been
2723 stopped. This could be because @code{file-notify-rm-watch} was called
2724 (see below), or because the file being watched was deleted, or due to
2725 another error reported from the underlying library.
2727 @var{file} and @var{file1} are the name of the file(s) whose event is
2728 being reported. For example:
2732 (require 'filenotify)
2733 @result{} filenotify
2737 (defun my-notify-callback (event)
2738 (message "Event %S" event))
2739 @result{} my-notify-callback
2743 (file-notify-add-watch
2744 "/tmp" '(change attribute-change) 'my-notify-callback)
2749 (write-region "foo" nil "/tmp/foo")
2750 @result{} Event (35025468 created "/tmp/.#foo")
2751 Event (35025468 created "/tmp/foo")
2752 Event (35025468 changed "/tmp/foo")
2753 Event (35025468 deleted "/tmp/.#foo")
2757 (write-region "bla" nil "/tmp/foo")
2758 @result{} Event (35025468 created "/tmp/.#foo")
2759 Event (35025468 changed "/tmp/foo")
2760 Event (35025468 deleted "/tmp/.#foo")
2764 (set-file-modes "/tmp/foo" (default-file-modes))
2765 @result{} Event (35025468 attribute-changed "/tmp/foo")
2769 Whether the action @code{renamed} is returned, depends on the used
2770 watch library. Otherwise, the actions @code{deleted} and
2771 @code{created} could be returned in a random order.
2775 (rename-file "/tmp/foo" "/tmp/bla")
2776 @result{} Event (35025468 renamed "/tmp/foo" "/tmp/bla")
2780 (delete-file "/tmp/bla")
2781 @result{} Event (35025468 deleted "/tmp/bla")
2786 @defun file-notify-rm-watch descriptor
2787 Removes an existing file watch specified by its @var{descriptor}.
2788 @var{descriptor} should be an object returned by
2789 @code{file-notify-add-watch}.
2792 @defun file-notify-valid-p descriptor
2793 Checks a watch specified by its @var{descriptor} for validity.
2794 @var{descriptor} should be an object returned by
2795 @code{file-notify-add-watch}.
2797 A watch can become invalid if the file or directory it watches is
2798 deleted, or if the watcher thread exits abnormally for any other
2799 reason. Removing the watch by calling @code{file-notify-rm-watch}
2800 also makes it invalid.
2804 (make-directory "/tmp/foo")
2805 @result{} Event (35025468 created "/tmp/foo")
2810 (file-notify-add-watch
2811 "/tmp/foo" '(change) 'my-notify-callback))
2816 (file-notify-valid-p desc)
2821 (write-region "bla" nil "/tmp/foo/bla")
2822 @result{} Event (11359632 created "/tmp/foo/.#bla")
2823 Event (11359632 created "/tmp/foo/bla")
2824 Event (11359632 changed "/tmp/foo/bla")
2825 Event (11359632 deleted "/tmp/foo/.#bla")
2829 ;; Deleting a file in the directory doesn't invalidate the watch.
2830 (delete-file "/tmp/foo/bla")
2831 @result{} Event (11359632 deleted "/tmp/foo/bla")
2835 (write-region "bla" nil "/tmp/foo/bla")
2836 @result{} Event (11359632 created "/tmp/foo/.#bla")
2837 Event (11359632 created "/tmp/foo/bla")
2838 Event (11359632 changed "/tmp/foo/bla")
2839 Event (11359632 deleted "/tmp/foo/.#bla")
2843 ;; Deleting the directory invalidates the watch.
2844 ;; Events arrive for different watch descriptors.
2845 (delete-directory "/tmp/foo" 'recursive)
2846 @result{} Event (35025468 deleted "/tmp/foo")
2847 Event (11359632 deleted "/tmp/foo/bla")
2848 Event (11359632 deleted "/tmp/foo")
2849 Event (11359632 stopped "/tmp/foo")
2853 (file-notify-valid-p desc)
2859 @node Dynamic Libraries
2860 @section Dynamically Loaded Libraries
2861 @cindex dynamic libraries
2863 A @dfn{dynamically loaded library} is a library that is loaded on
2864 demand, when its facilities are first needed. Emacs supports such
2865 on-demand loading of support libraries for some of its features.
2867 @defvar dynamic-library-alist
2868 This is an alist of dynamic libraries and external library files
2871 Each element is a list of the form
2872 @w{@code{(@var{library} @var{files}@dots{})}}, where the @code{car} is
2873 a symbol representing a supported external library, and the rest are
2874 strings giving alternate filenames for that library.
2876 Emacs tries to load the library from the files in the order they
2877 appear in the list; if none is found, the Emacs session won't have
2878 access to that library, and the features it provides will be
2881 Image support on some platforms uses this facility. Here's an example
2882 of setting this variable for supporting images on MS-Windows:
2885 (setq dynamic-library-alist
2886 '((xpm "libxpm.dll" "xpm4.dll" "libXpm-nox4.dll")
2887 (png "libpng12d.dll" "libpng12.dll" "libpng.dll"
2888 "libpng13d.dll" "libpng13.dll")
2889 (jpeg "jpeg62.dll" "libjpeg.dll" "jpeg-62.dll"
2891 (tiff "libtiff3.dll" "libtiff.dll")
2892 (gif "giflib4.dll" "libungif4.dll" "libungif.dll")
2893 (svg "librsvg-2-2.dll")
2894 (gdk-pixbuf "libgdk_pixbuf-2.0-0.dll")
2895 (glib "libglib-2.0-0.dll")
2896 (gobject "libgobject-2.0-0.dll")))
2899 Note that image types @code{pbm} and @code{xbm} do not need entries in
2900 this variable because they do not depend on external libraries and are
2901 always available in Emacs.
2903 Also note that this variable is not meant to be a generic facility for
2904 accessing external libraries; only those already known by Emacs can
2905 be loaded through it.
2907 This variable is ignored if the given @var{library} is statically
2911 @node Security Considerations
2912 @section Security Considerations
2916 Like any application, Emacs can be run in a secure environment, where
2917 the operating system enforces rules about access and the like. With
2918 some care, Emacs-based applications can also be part of a security
2919 perimeter that checks such rules. Although the default settings for
2920 Emacs work well for a typical software development environment, they
2921 may require adjustment in environments containing untrusted users that
2922 may include attackers. Here is a compendium of security issues that
2923 may be helpful if you are developing such applications. It is by no
2924 means complete; it is intended to give you an idea of the security
2925 issues involved, rather than to be a security checklist.
2928 @item File local variables
2929 @cindex file local variables
2930 A file that Emacs visits can contain variable settings that affects
2931 the buffer visiting that file; @xref{File Local Variables}.
2932 Similarly, a directory can specify local variable values common to all
2933 files in that directory; @xref{Directory Local Variables}. Although
2934 Emacs takes some effort to protect against misuse of these variables,
2935 a security hole can be created merely by a package setting
2936 @code{safe-local-variable} too optimistically, a problem that is all
2937 too common. To disable this feature for both files and directories,
2938 set @code{enable-local-variables} to @code{nil}.
2940 @item Access control
2941 Although Emacs normally respects access permissions of the underlying
2942 operating system, in some cases it handles accesses specially. For
2943 example, file names can have handlers that treat the files specially,
2944 with their own access checking. @xref{Magic File Names}. Also, a
2945 buffer can be read-only even if the corresponding file is writeable,
2946 and vice versa, which can result in messages such as @samp{File passwd
2947 is write-protected; try to save anyway? (yes or no)}. @xref{Read Only
2950 @item Authentication
2951 Emacs has several functions that deal with passwords, e.g.,
2952 @code{password-read}. Although these functions do not attempt to
2953 broadcast passwords to the world, their implementations are not proof
2954 against determined attackers with access to Emacs internals. For
2955 example, even if Elisp code attempts to scrub a password from
2956 its memory after using it, remnants of the password may still reside
2957 in the garbage-collected free list.
2959 @item Code injection
2960 Emacs can send commands to many other applications, and applications
2961 should take care that strings sent as operands of these commands are
2962 not misinterpreted as directives. For example, when sending a shell
2963 command to rename a file @var{a} to @var{b}, do not simply use the
2964 string @code{mv @var{a} @var{b}}, because either file name might start
2965 with @samp{-}, or might contain shell metacharacters like @samp{;}.
2966 Although functions like @code{shell-quote-argument} can help avoid
2967 this sort of problem, they are not panaceas; for example, on a POSIX
2968 platform @code{shell-quote-argument} quotes shell metacharacters but
2969 not leading @samp{-}. @xref{Shell Arguments}.
2971 @item Coding systems
2972 Emacs attempts to infer the coding systems of the files and network
2973 connections it accesses. If it makes a mistake, or if the other
2974 parties to the network connection disagree with Emacs's deductions,
2975 the resulting system could be unreliable. Also, even when it infers
2976 correctly, Emacs often can use bytes that other programs cannot. For
2977 example, although to Emacs the NUL (all bits zero) byte is just a
2978 character like any other, many other applications treat it as a string
2979 terminator and mishandle strings or files containing NUL bytes.
2981 @item Environment and configuration variables
2982 POSIX specifies several environment variables that can affect how
2983 Emacs behaves. Any environment variable whose name consists entirely
2984 of uppercase ASCII letters, digits, and the underscore may affect the
2985 internal behavior of Emacs. Emacs uses several such variables, e.g.,
2986 @env{EMACSLOADPATH}. @xref{Library Search}. On some platforms some
2987 environment variables (e.g., @env{PATH}, @env{POSIXLY_CORRECT},
2988 @env{SHELL}, @env{TMPDIR}) need to have properly-configured values in
2989 order to get standard behavior for any utility Emacs might invoke.
2990 Even seemingly-benign variables like @env{TZ} may have security
2993 Emacs has customization and other variables with similar
2994 considerations. For example, if the variable @code{shell-file-name}
2995 specifies a shell with nonstandard behavior, an Emacs-based
2996 application may misbehave.
2999 When Emacs is installed, if the installation directory hierarchy can
3000 be modified by untrusted users, the application cannot be trusted.
3001 This applies also to the directory hierarchies of the programs that
3002 Emacs uses, and of the files that Emacs reads and writes.
3004 @item Network access
3005 Emacs often accesses the network, and you may want to configure it to
3006 avoid network accesses that it would normally do. For example, unless
3007 you set @code{tramp-mode} to @code{nil}, file names using a certain
3008 syntax are interpreted as being network files, and are retrieved
3009 across the network. @xref{Top, The Tramp Manual,, tramp, The Tramp
3012 @item Race conditions
3013 Emacs applications have the same sort of race-condition issues that
3014 other applications do. For example, even when
3015 @code{(file-readable-p "foo.txt")} returns @code{t}, it could be that
3016 @file{foo.txt} is unreadable because some other program changed the
3017 file's permissions between the call to @code{file-readable-p} and now.
3019 @item Resource limits
3020 When Emacs exhausts memory or other operating system resources, its
3021 behavior can be less reliable, in that computations that ordinarily
3022 run to completion may abort back to the top level. This may cause
3023 Emacs to neglect operations that it normally would have done.