@c -*-texinfo-*- @c This is part of the GNU Emacs Lisp Reference Manual. @c Copyright (C) 1990-1995, 1998-1999, 2001-2018 Free Software @c Foundation, Inc. @c See the file elisp.texi for copying conditions. @node System Interface @chapter Operating System Interface This chapter is about starting and getting out of Emacs, access to values in the operating system environment, and terminal input, output. @xref{Building Emacs}, for related information. @xref{Display}, for additional operating system status information pertaining to the terminal and the screen. @menu * Starting Up:: Customizing Emacs startup processing. * Getting Out:: How exiting works (permanent or temporary). * System Environment:: Distinguish the name and kind of system. * User Identification:: Finding the name and user id of the user. * Time of Day:: Getting the current time. * Time Zone Rules:: Rules for time zones and daylight saving time. * Time Conversion:: Converting a time from numeric form to calendrical data and vice versa. * Time Parsing:: Converting a time from numeric form to text and vice versa. * Processor Run Time:: Getting the run time used by Emacs. * Time Calculations:: Adding, subtracting, comparing times, etc. * Timers:: Setting a timer to call a function at a certain time. * Idle Timers:: Setting a timer to call a function when Emacs has been idle for a certain length of time. * Terminal Input:: Accessing and recording terminal input. * Terminal Output:: Controlling and recording terminal output. * Sound Output:: Playing sounds on the computer's speaker. * X11 Keysyms:: Operating on key symbols for X Windows. * Batch Mode:: Running Emacs without terminal interaction. * Session Management:: Saving and restoring state with X Session Management. * Desktop Notifications:: Desktop notifications. * File Notifications:: File notifications. * Dynamic Libraries:: On-demand loading of support libraries. * Security Considerations:: Running Emacs in an unfriendly environment. @end menu @node Starting Up @section Starting Up Emacs This section describes what Emacs does when it is started, and how you can customize these actions. @menu * Startup Summary:: Sequence of actions Emacs performs at startup. * Init File:: Details on reading the init file. * Terminal-Specific:: How the terminal-specific Lisp file is read. * Command-Line Arguments:: How command-line arguments are processed, and how you can customize them. @end menu @node Startup Summary @subsection Summary: Sequence of Actions at Startup @cindex initialization of Emacs @cindex startup of Emacs @cindex @file{startup.el} When Emacs is started up, it performs the following operations (see @code{normal-top-level} in @file{startup.el}): @enumerate @item It adds subdirectories to @code{load-path}, by running the file named @file{subdirs.el} in each directory in the list. Normally, this file adds the directory's subdirectories to the list, and those are scanned in their turn. The files @file{subdirs.el} are normally generated automatically when Emacs is installed. @item It loads any @file{leim-list.el} that it finds in the @code{load-path} directories. This file is intended for registering input methods. The search is only for any personal @file{leim-list.el} files that you may have created; it skips the directories containing the standard Emacs libraries (these should contain only a single @file{leim-list.el} file, which is compiled into the Emacs executable). @vindex before-init-time @item It sets the variable @code{before-init-time} to the value of @code{current-time} (@pxref{Time of Day}). It also sets @code{after-init-time} to @code{nil}, which signals to Lisp programs that Emacs is being initialized. @c set-locale-environment @item It sets the language environment and the terminal coding system, if requested by environment variables such as @env{LANG}. @item It does some basic parsing of the command-line arguments. @vindex initial-window-system@r{, and startup} @vindex window-system-initialization-alist @item If not running in batch mode, it initializes the window system that the variable @code{initial-window-system} specifies (@pxref{Window Systems, initial-window-system}). The initialization function for each supported window system is specified by @code{window-system-initialization-alist}. If the value of @code{initial-window-system} is @var{windowsystem}, then the appropriate initialization function is defined in the file @file{term/@var{windowsystem}-win.el}. This file should have been compiled into the Emacs executable when it was built. @item It runs the normal hook @code{before-init-hook}. @item If appropriate, it creates a graphical frame. This is not done in batch (noninteractive) or daemon mode. @item It initializes the initial frame's faces, and sets up the menu bar and tool bar if needed. If graphical frames are supported, it sets up the tool bar even if the current frame is not a graphical one, since a graphical frame may be created later on. @item It use @code{custom-reevaluate-setting} to re-initialize the members of the list @code{custom-delayed-init-variables}. These are any pre-loaded user options whose default value depends on the run-time, rather than build-time, context. @xref{Building Emacs, custom-initialize-delay}. @c @item @c It registers the colors available for tty frames. @item It loads the library @file{site-start}, if it exists. This is not done if the options @samp{-Q} or @samp{--no-site-file} were specified. @cindex @file{site-start.el} @item It loads your init file (@pxref{Init File}). This is not done if the options @samp{-q}, @samp{-Q}, or @samp{--batch} were specified. If the @samp{-u} option was specified, Emacs looks for the init file in that user's home directory instead. @item It loads the library @file{default}, if it exists. This is not done if @code{inhibit-default-init} is non-@code{nil}, nor if the options @samp{-q}, @samp{-Q}, or @samp{--batch} were specified. @cindex @file{default.el} @item It loads your abbrevs from the file specified by @code{abbrev-file-name}, if that file exists and can be read (@pxref{Abbrev Files, abbrev-file-name}). This is not done if the option @samp{--batch} was specified. @item It calls the function @code{package-initialize} to activate any optional Emacs Lisp package that has been installed. @xref{Packaging Basics}. However, Emacs doesn't initialize packages when @code{package-enable-at-startup} is @code{nil} or when it's started with one of the options @samp{-q}, @samp{-Q}, or @samp{--batch}. To initialize packages in the latter case, @code{package-initialize} should be called explicitly (e.g., via the @samp{--funcall} option). @vindex after-init-time @item It sets the variable @code{after-init-time} to the value of @code{current-time}. This variable was set to @code{nil} earlier; setting it to the current time signals that the initialization phase is over, and, together with @code{before-init-time}, provides the measurement of how long it took. @item It runs the normal hook @code{after-init-hook}. @item If the buffer @file{*scratch*} exists and is still in Fundamental mode (as it should be by default), it sets its major mode according to @code{initial-major-mode}. @item If started on a text terminal, it loads the terminal-specific Lisp library (@pxref{Terminal-Specific}), and runs the hook @code{tty-setup-hook}. This is not done in @code{--batch} mode, nor if @code{term-file-prefix} is @code{nil}. @c Now command-line calls command-line-1. @item It displays the initial echo area message, unless you have suppressed that with @code{inhibit-startup-echo-area-message}. @item It processes any command-line options that were not handled earlier. @c This next one is back in command-line, but the remaining bits of @c command-line-1 are not done if noninteractive. @item It now exits if the option @code{--batch} was specified. @item If the @file{*scratch*} buffer exists and is empty, it inserts @code{(substitute-command-keys initial-scratch-message)} into that buffer. @item If @code{initial-buffer-choice} is a string, it visits the file (or directory) with that name. If it is a function, it calls the function with no arguments and selects the buffer that it returns. If one file is given as a command line argument, that file is visited and its buffer displayed alongside @code{initial-buffer-choice}. If more than one file is given, all of the files are visited and the @file{*Buffer List*} buffer is displayed alongside @code{initial-buffer-choice}. @ignore @c I do not think this should be mentioned. AFAICS it is just a dodge @c around inhibit-startup-screen not being settable on a site-wide basis. If it is @code{t}, it selects the @file{*scratch*} buffer. @end ignore @c To make things nice and confusing, the next three items can be @c called from two places. If displaying a startup screen, they are @c called in command-line-1 before the startup screen is shown. @c inhibit-startup-hooks is then set and window-setup-hook set to nil. @c If not displaying a startup screen, they are called in @c normal-top-level. @c FIXME? So it seems they can be called before or after the @c daemon/session restore step? @item It runs @code{emacs-startup-hook}. @item It calls @code{frame-notice-user-settings}, which modifies the parameters of the selected frame according to whatever the init files specify. @item It runs @code{window-setup-hook}. The only difference between this hook and @code{emacs-startup-hook} is that this one runs after the previously mentioned modifications to the frame parameters. @item @cindex startup screen It displays the @dfn{startup screen}, which is a special buffer that contains information about copyleft and basic Emacs usage. This is not done if @code{inhibit-startup-screen} or @code{initial-buffer-choice} are non-@code{nil}, or if the @samp{--no-splash} or @samp{-Q} command-line options were specified. @c End of command-line-1. @c Back to command-line from command-line-1. @c This is the point at which we actually exit in batch mode, but the @c last few bits of command-line-1 are not done in batch mode. @item If a daemon was requested, it calls @code{server-start}. (On POSIX systems, if a background daemon was requested, it then detaches from the controlling terminal.) @xref{Emacs Server,,, emacs, The GNU Emacs Manual}. @item If started by the X session manager, it calls @code{emacs-session-restore} passing it as argument the ID of the previous session. @xref{Session Management}. @c End of command-line. @c Back to normal-top-level from command-line. @end enumerate @noindent The following options affect some aspects of the startup sequence. @defopt inhibit-startup-screen This variable, if non-@code{nil}, inhibits the startup screen. In that case, Emacs typically displays the @file{*scratch*} buffer; but see @code{initial-buffer-choice}, below. Do not set this variable in the init file of a new user, or in a way that affects more than one user, as that would prevent new users from receiving information about copyleft and basic Emacs usage. @vindex inhibit-startup-message @vindex inhibit-splash-screen @code{inhibit-startup-message} and @code{inhibit-splash-screen} are aliases for this variable. @end defopt @defopt initial-buffer-choice If non-@code{nil}, this variable is a string that specifies a file or directory for Emacs to display after starting up, instead of the startup screen. If its value is a function, Emacs calls that function which must return a buffer which is then displayed. If its value is @code{t}, Emacs displays the @file{*scratch*} buffer. @end defopt @defopt inhibit-startup-echo-area-message This variable controls the display of the startup echo area message. You can suppress the startup echo area message by adding text with this form to your init file: @example (setq inhibit-startup-echo-area-message "@var{your-login-name}") @end example Emacs explicitly checks for an expression as shown above in your init file; your login name must appear in the expression as a Lisp string constant. You can also use the Customize interface. Other methods of setting @code{inhibit-startup-echo-area-message} to the same value do not inhibit the startup message. This way, you can easily inhibit the message for yourself if you wish, but thoughtless copying of your init file will not inhibit the message for someone else. @end defopt @defopt initial-scratch-message This variable, if non-@code{nil}, should be a string, which is treated as documentation to be inserted into the @file{*scratch*} buffer when Emacs starts up. If it is @code{nil}, the @file{*scratch*} buffer is empty. @end defopt @noindent The following command-line options affect some aspects of the startup sequence. @xref{Initial Options,,, emacs, The GNU Emacs Manual}. @table @code @item --no-splash Do not display a splash screen. @item --batch Run without an interactive terminal. @xref{Batch Mode}. @item --daemon @itemx --bg-daemon @itemx --fg-daemon Do not initialize any display; just start a server. (A ``background'' daemon automatically runs in the background.) @item --no-init-file @itemx -q Do not load either the init file, or the @file{default} library. @item --no-site-file Do not load the @file{site-start} library. @item --quick @itemx -Q Equivalent to @samp{-q --no-site-file --no-splash}. @c and --no-site-lisp, but let's not mention that here. @end table @node Init File @subsection The Init File @cindex init file @cindex @file{.emacs} @cindex @file{init.el} When you start Emacs, it normally attempts to load your @dfn{init file}. This is either a file named @file{.emacs} or @file{.emacs.el} in your home directory, or a file named @file{init.el} in a subdirectory named @file{.emacs.d} in your home directory. @ignore Whichever place you use, you can also compile the file (@pxref{Byte Compilation}); then the actual file loaded will be @file{.emacs.elc} or @file{init.elc}. @end ignore The command-line switches @samp{-q}, @samp{-Q}, and @samp{-u} control whether and where to find the init file; @samp{-q} (and the stronger @samp{-Q}) says not to load an init file, while @samp{-u @var{user}} says to load @var{user}'s init file instead of yours. @xref{Entering Emacs,,, emacs, The GNU Emacs Manual}. If neither option is specified, Emacs uses the @env{LOGNAME} environment variable, or the @env{USER} (most systems) or @env{USERNAME} (MS systems) variable, to find your home directory and thus your init file; this way, even if you have su'd, Emacs still loads your own init file. If those environment variables are absent, though, Emacs uses your user-id to find your home directory. @cindex default init file An Emacs installation may have a @dfn{default init file}, which is a Lisp library named @file{default.el}. Emacs finds this file through the standard search path for libraries (@pxref{How Programs Do Loading}). The Emacs distribution does not come with this file; it is intended for local customizations. If the default init file exists, it is loaded whenever you start Emacs. But your own personal init file, if any, is loaded first; if it sets @code{inhibit-default-init} to a non-@code{nil} value, then Emacs does not subsequently load the @file{default.el} file. In batch mode, or if you specify @samp{-q} (or @samp{-Q}), Emacs loads neither your personal init file nor the default init file. Another file for site-customization is @file{site-start.el}. Emacs loads this @emph{before} the user's init file. You can inhibit the loading of this file with the option @samp{--no-site-file}. @defopt site-run-file This variable specifies the site-customization file to load before the user's init file. Its normal value is @code{"site-start"}. The only way you can change it with real effect is to do so before dumping Emacs. @c So why even mention it here. I imagine it is almost never changed. @end defopt @xref{Init Examples,, Init File Examples, emacs, The GNU Emacs Manual}, for examples of how to make various commonly desired customizations in your @file{.emacs} file. @defopt inhibit-default-init If this variable is non-@code{nil}, it prevents Emacs from loading the default initialization library file. The default value is @code{nil}. @end defopt @defvar before-init-hook This normal hook is run, once, just before loading all the init files (@file{site-start.el}, your init file, and @file{default.el}). (The only way to change it with real effect is before dumping Emacs.) @end defvar @defvar after-init-hook This normal hook is run, once, just after loading all the init files (@file{site-start.el}, your init file, and @file{default.el}), before loading the terminal-specific library (if started on a text terminal) and processing the command-line action arguments. @end defvar @defvar emacs-startup-hook This normal hook is run, once, just after handling the command line arguments. In batch mode, Emacs does not run this hook. @end defvar @defvar window-setup-hook This normal hook is very similar to @code{emacs-startup-hook}. The only difference is that it runs slightly later, after setting of the frame parameters. @xref{Startup Summary, window-setup-hook}. @end defvar @defvar user-init-file This variable holds the absolute file name of the user's init file. If the actual init file loaded is a compiled file, such as @file{.emacs.elc}, the value refers to the corresponding source file. @end defvar @defvar user-emacs-directory This variable holds the name of the @file{.emacs.d} directory. It is @file{~/.emacs.d} on all platforms but MS-DOS. @end defvar @node Terminal-Specific @subsection Terminal-Specific Initialization @cindex terminal-specific initialization Each terminal type can have its own Lisp library that Emacs loads when run on that type of terminal. The library's name is constructed by concatenating the value of the variable @code{term-file-prefix} and the terminal type (specified by the environment variable @env{TERM}). Normally, @code{term-file-prefix} has the value @code{"term/"}; changing this is not recommended. If there is an entry matching @env{TERM} in the @code{term-file-aliases} association list, Emacs uses the associated value in place of @env{TERM}. Emacs finds the file in the normal manner, by searching the @code{load-path} directories, and trying the @samp{.elc} and @samp{.el} suffixes. @cindex Termcap The usual role of a terminal-specific library is to enable special keys to send sequences that Emacs can recognize. It may also need to set or add to @code{input-decode-map} if the Termcap or Terminfo entry does not specify all the terminal's function keys. @xref{Terminal Input}. When the name of the terminal type contains a hyphen or underscore, and no library is found whose name is identical to the terminal's name, Emacs strips from the terminal's name the last hyphen or underscore and everything that follows it, and tries again. This process is repeated until Emacs finds a matching library, or until there are no more hyphens or underscores in the name (i.e., there is no terminal-specific library). For example, if the terminal name is @samp{xterm-256color} and there is no @file{term/xterm-256color.el} library, Emacs tries to load @file{term/xterm.el}. If necessary, the terminal library can evaluate @code{(getenv "TERM")} to find the full name of the terminal type. Your init file can prevent the loading of the terminal-specific library by setting the variable @code{term-file-prefix} to @code{nil}. You can also arrange to override some of the actions of the terminal-specific library by using @code{tty-setup-hook}. This is a normal hook that Emacs runs after initializing a new text terminal. You could use this hook to define initializations for terminals that do not have their own libraries. @xref{Hooks}. @defopt term-file-prefix @cindex @env{TERM} environment variable If the value of this variable is non-@code{nil}, Emacs loads a terminal-specific initialization file as follows: @example (load (concat term-file-prefix (getenv "TERM"))) @end example @noindent You may set the @code{term-file-prefix} variable to @code{nil} in your init file if you do not wish to load the terminal-initialization file. On MS-DOS, Emacs sets the @env{TERM} environment variable to @samp{internal}. @end defopt @defopt term-file-aliases This variable is an association list mapping terminal types to their aliases. For example, an element of the form @code{("vt102" . "vt100")} means to treat a terminal of type @samp{vt102} like one of type @samp{vt100}. @end defopt @defvar tty-setup-hook This variable is a normal hook that Emacs runs after initializing a new text terminal. (This applies when Emacs starts up in non-windowed mode, and when making a tty @command{emacsclient} connection.) The hook runs after loading your init file (if applicable) and the terminal-specific Lisp file, so you can use it to adjust the definitions made by that file. For a related feature, @pxref{Init File, window-setup-hook}. @end defvar @node Command-Line Arguments @subsection Command-Line Arguments @cindex command-line arguments You can use command-line arguments to request various actions when you start Emacs. Note that the recommended way of using Emacs is to start it just once, after logging in, and then do all editing in the same Emacs session (@pxref{Entering Emacs,,, emacs, The GNU Emacs Manual}). For this reason, you might not use command-line arguments very often; nonetheless, they can be useful when invoking Emacs from session scripts or debugging Emacs. This section describes how Emacs processes command-line arguments. @defun command-line This function parses the command line that Emacs was called with, processes it, and (amongst other things) loads the user's init file and displays the startup messages. @end defun @defvar command-line-processed The value of this variable is @code{t} once the command line has been processed. If you redump Emacs by calling @code{dump-emacs} (@pxref{Building Emacs}), you may wish to set this variable to @code{nil} first in order to cause the new dumped Emacs to process its new command-line arguments. @end defvar @defvar command-switch-alist @cindex switches on command line @cindex options on command line @cindex command-line options This variable is an alist of user-defined command-line options and associated handler functions. By default it is empty, but you can add elements if you wish. A @dfn{command-line option} is an argument on the command line, which has the form: @example -@var{option} @end example The elements of the @code{command-switch-alist} look like this: @example (@var{option} . @var{handler-function}) @end example The @sc{car}, @var{option}, is a string, the name of a command-line option (not including the initial hyphen). The @var{handler-function} is called to handle @var{option}, and receives the option name as its sole argument. In some cases, the option is followed in the command line by an argument. In these cases, the @var{handler-function} can find all the remaining command-line arguments in the variable @code{command-line-args-left} (see below). (The entire list of command-line arguments is in @code{command-line-args}.) The command-line arguments are parsed by the @code{command-line-1} function in the @file{startup.el} file. See also @ref{Emacs Invocation, , Command Line Arguments for Emacs Invocation, emacs, The GNU Emacs Manual}. @end defvar @defvar command-line-args The value of this variable is the list of command-line arguments passed to Emacs. @end defvar @defvar command-line-args-left @vindex argv The value of this variable is the list of command-line arguments that have not yet been processed. @c Don't mention this, since it is a "bad name for a dynamically bound variable" @c @code{argv} is an alias for this. @end defvar @defvar command-line-functions This variable's value is a list of functions for handling an unrecognized command-line argument. Each time the next argument to be processed has no special meaning, the functions in this list are called, in order of appearance, until one of them returns a non-@code{nil} value. These functions are called with no arguments. They can access the command-line argument under consideration through the variable @code{argi}, which is bound temporarily at this point. The remaining arguments (not including the current one) are in the variable @code{command-line-args-left}. When a function recognizes and processes the argument in @code{argi}, it should return a non-@code{nil} value to say it has dealt with that argument. If it has also dealt with some of the following arguments, it can indicate that by deleting them from @code{command-line-args-left}. If all of these functions return @code{nil}, then the argument is treated as a file name to visit. @end defvar @node Getting Out @section Getting Out of Emacs @cindex exiting Emacs There are two ways to get out of Emacs: you can kill the Emacs job, which exits permanently, or you can suspend it, which permits you to reenter the Emacs process later. (In a graphical environment, you can of course simply switch to another application without doing anything special to Emacs, then switch back to Emacs when you want.) @menu * Killing Emacs:: Exiting Emacs irreversibly. * Suspending Emacs:: Exiting Emacs reversibly. @end menu @node Killing Emacs @subsection Killing Emacs @cindex killing Emacs Killing Emacs means ending the execution of the Emacs process. If you started Emacs from a terminal, the parent process normally resumes control. The low-level primitive for killing Emacs is @code{kill-emacs}. @deffn Command kill-emacs &optional exit-data This command calls the hook @code{kill-emacs-hook}, then exits the Emacs process and kills it. If @var{exit-data} is an integer, that is used as the exit status of the Emacs process. (This is useful primarily in batch operation; see @ref{Batch Mode}.) If @var{exit-data} is a string, its contents are stuffed into the terminal input buffer so that the shell (or whatever program next reads input) can read them. @end deffn @cindex SIGTERM @cindex SIGHUP @cindex SIGINT @cindex operating system signal The @code{kill-emacs} function is normally called via the higher-level command @kbd{C-x C-c} (@code{save-buffers-kill-terminal}). @xref{Exiting,,, emacs, The GNU Emacs Manual}. It is also called automatically if Emacs receives a @code{SIGTERM} or @code{SIGHUP} operating system signal (e.g., when the controlling terminal is disconnected), or if it receives a @code{SIGINT} signal while running in batch mode (@pxref{Batch Mode}). @defvar kill-emacs-hook This normal hook is run by @code{kill-emacs}, before it kills Emacs. Because @code{kill-emacs} can be called in situations where user interaction is impossible (e.g., when the terminal is disconnected), functions on this hook should not attempt to interact with the user. If you want to interact with the user when Emacs is shutting down, use @code{kill-emacs-query-functions}, described below. @end defvar When Emacs is killed, all the information in the Emacs process, aside from files that have been saved, is lost. Because killing Emacs inadvertently can lose a lot of work, the @code{save-buffers-kill-terminal} command queries for confirmation if you have buffers that need saving or subprocesses that are running. It also runs the abnormal hook @code{kill-emacs-query-functions}: @defopt kill-emacs-query-functions When @code{save-buffers-kill-terminal} is killing Emacs, it calls the functions in this hook, after asking the standard questions and before calling @code{kill-emacs}. The functions are called in order of appearance, with no arguments. Each function can ask for additional confirmation from the user. If any of them returns @code{nil}, @code{save-buffers-kill-emacs} does not kill Emacs, and does not run the remaining functions in this hook. Calling @code{kill-emacs} directly does not run this hook. @end defopt @node Suspending Emacs @subsection Suspending Emacs @cindex suspending Emacs On text terminals, it is possible to @dfn{suspend Emacs}, which means stopping Emacs temporarily and returning control to its superior process, which is usually the shell. This allows you to resume editing later in the same Emacs process, with the same buffers, the same kill ring, the same undo history, and so on. To resume Emacs, use the appropriate command in the parent shell---most likely @code{fg}. @cindex controlling terminal Suspending works only on a terminal device from which the Emacs session was started. We call that device the @dfn{controlling terminal} of the session. Suspending is not allowed if the controlling terminal is a graphical terminal. Suspending is usually not relevant in graphical environments, since you can simply switch to another application without doing anything special to Emacs. @c FIXME? Are there any systems Emacs still supports that do not @c have SIGTSTP? @cindex SIGTSTP Some operating systems (those without @code{SIGTSTP}, or MS-DOS) do not support suspension of jobs; on these systems, suspension actually creates a new shell temporarily as a subprocess of Emacs. Then you would exit the shell to return to Emacs. @deffn Command suspend-emacs &optional string This function stops Emacs and returns control to the superior process. If and when the superior process resumes Emacs, @code{suspend-emacs} returns @code{nil} to its caller in Lisp. This function works only on the controlling terminal of the Emacs session; to relinquish control of other tty devices, use @code{suspend-tty} (see below). If the Emacs session uses more than one terminal, you must delete the frames on all the other terminals before suspending Emacs, or this function signals an error. @xref{Multiple Terminals}. If @var{string} is non-@code{nil}, its characters are sent to Emacs's superior shell, to be read as terminal input. @c FIXME? It seems to me that shell does echo STRING. The characters in @var{string} are not echoed by the superior shell; only the results appear. Before suspending, @code{suspend-emacs} runs the normal hook @code{suspend-hook}. After the user resumes Emacs, @code{suspend-emacs} runs the normal hook @code{suspend-resume-hook}. @xref{Hooks}. The next redisplay after resumption will redraw the entire screen, unless the variable @code{no-redraw-on-reenter} is non-@code{nil}. @xref{Refresh Screen}. Here is an example of how you could use these hooks: @smallexample @group (add-hook 'suspend-hook (lambda () (or (y-or-n-p "Really suspend? ") (error "Suspend canceled")))) @end group (add-hook 'suspend-resume-hook (lambda () (message "Resumed!") (sit-for 2))) @end smallexample @c The sit-for prevents the @code{nil} that suspend-emacs returns @c hiding the message. Here is what you would see upon evaluating @code{(suspend-emacs "pwd")}: @smallexample @group ---------- Buffer: Minibuffer ---------- Really suspend? @kbd{y} ---------- Buffer: Minibuffer ---------- @end group @group ---------- Parent Shell ---------- bash$ /home/username bash$ fg @end group @group ---------- Echo Area ---------- Resumed! @end group @end smallexample @c FIXME? AFAICS, it is echoed. Note that @samp{pwd} is not echoed after Emacs is suspended. But it is read and executed by the shell. @end deffn @defvar suspend-hook This variable is a normal hook that Emacs runs before suspending. @end defvar @defvar suspend-resume-hook This variable is a normal hook that Emacs runs on resuming after a suspension. @end defvar @defun suspend-tty &optional tty If @var{tty} specifies a terminal device used by Emacs, this function relinquishes the device and restores it to its prior state. Frames that used the device continue to exist, but are not updated and Emacs doesn't read input from them. @var{tty} can be a terminal object, a frame (meaning the terminal for that frame), or @code{nil} (meaning the terminal for the selected frame). @xref{Multiple Terminals}. If @var{tty} is already suspended, this function does nothing. @vindex suspend-tty-functions This function runs the hook @code{suspend-tty-functions}, passing the terminal object as an argument to each function. @end defun @defun resume-tty &optional tty This function resumes the previously suspended terminal device @var{tty}; where @var{tty} has the same possible values as it does for @code{suspend-tty}. @vindex resume-tty-functions This function reopens the terminal device, re-initializes it, and redraws it with that terminal's selected frame. It then runs the hook @code{resume-tty-functions}, passing the terminal object as an argument to each function. If the same device is already used by another Emacs terminal, this function signals an error. If @var{tty} is not suspended, this function does nothing. @end defun @defun controlling-tty-p &optional tty This function returns non-@code{nil} if @var{tty} is the controlling terminal of the Emacs session; @var{tty} can be a terminal object, a frame (meaning the terminal for that frame), or @code{nil} (meaning the terminal for the selected frame). @end defun @deffn Command suspend-frame This command @dfn{suspends} a frame. For GUI frames, it calls @code{iconify-frame} (@pxref{Visibility of Frames}); for frames on text terminals, it calls either @code{suspend-emacs} or @code{suspend-tty}, depending on whether the frame is displayed on the controlling terminal device or not. @end deffn @node System Environment @section Operating System Environment @cindex operating system environment Emacs provides access to variables in the operating system environment through various functions. These variables include the name of the system, the user's @acronym{UID}, and so on. @defvar system-configuration This variable holds the standard GNU configuration name for the hardware/software configuration of your system, as a string. For example, a typical value for a 64-bit GNU/Linux system is @samp{"x86_64-unknown-linux-gnu"}. @end defvar @cindex system type and name @defvar system-type The value of this variable is a symbol indicating the type of operating system Emacs is running on. The possible values are: @table @code @item aix IBM's AIX. @item berkeley-unix Berkeley BSD and its variants. @item cygwin Cygwin, a POSIX layer on top of MS-Windows. @item darwin Darwin (macOS). @item gnu The GNU system (using the GNU kernel, which consists of the HURD and Mach). @item gnu/linux A GNU/Linux system---that is, a variant GNU system, using the Linux kernel. (These systems are the ones people often call ``Linux'', but actually Linux is just the kernel, not the whole system.) @item gnu/kfreebsd A GNU (glibc-based) system with a FreeBSD kernel. @item hpux Hewlett-Packard HPUX operating system. @item nacl Google Native Client (@acronym{NaCl}) sandboxing system. @item ms-dos Microsoft's DOS@. Emacs compiled with DJGPP for MS-DOS binds @code{system-type} to @code{ms-dos} even when you run it on MS-Windows. @item usg-unix-v AT&T Unix System V. @item windows-nt Microsoft Windows NT, 9X and later. The value of @code{system-type} is always @code{windows-nt}, e.g., even on Windows 10. @end table We do not wish to add new symbols to make finer distinctions unless it is absolutely necessary! In fact, we hope to eliminate some of these alternatives in the future. If you need to make a finer distinction than @code{system-type} allows for, you can test @code{system-configuration}, e.g., against a regexp. @end defvar @defun system-name This function returns the name of the machine you are running on, as a string. @end defun @c FIXME seems like this section is not the best place for this option? @defopt mail-host-address If this variable is non-@code{nil}, it is used instead of @code{system-name} for purposes of generating email addresses. For example, it is used when constructing the default value of @code{user-mail-address}. @xref{User Identification}. @end defopt @deffn Command getenv var &optional frame @cindex environment variable access This function returns the value of the environment variable @var{var}, as a string. @var{var} should be a string. If @var{var} is undefined in the environment, @code{getenv} returns @code{nil}. It returns @samp{""} if @var{var} is set but null. Within Emacs, a list of environment variables and their values is kept in the variable @code{process-environment}. @example @group (getenv "USER") @result{} "lewis" @end group @end example The shell command @code{printenv} prints all or part of the environment: @example @group bash$ printenv PATH=/usr/local/bin:/usr/bin:/bin USER=lewis @end group @group TERM=xterm SHELL=/bin/bash HOME=/home/lewis @end group @dots{} @end example @end deffn @deffn Command setenv variable &optional value substitute This command sets the value of the environment variable named @var{variable} to @var{value}. @var{variable} should be a string. Internally, Emacs Lisp can handle any string. However, normally @var{variable} should be a valid shell identifier, that is, a sequence of letters, digits and underscores, starting with a letter or underscore. Otherwise, errors may occur if subprocesses of Emacs try to access the value of @var{variable}. If @var{value} is omitted or @code{nil} (or, interactively, with a prefix argument), @code{setenv} removes @var{variable} from the environment. Otherwise, @var{value} should be a string. @c FIXME: Document 'substitute-env-vars'? --xfq If the optional argument @var{substitute} is non-@code{nil}, Emacs calls the function @code{substitute-env-vars} to expand any environment variables in @var{value}. @code{setenv} works by modifying @code{process-environment}; binding that variable with @code{let} is also reasonable practice. @code{setenv} returns the new value of @var{variable}, or @code{nil} if it removed @var{variable} from the environment. @end deffn @defvar process-environment This variable is a list of strings, each describing one environment variable. The functions @code{getenv} and @code{setenv} work by means of this variable. @smallexample @group process-environment @result{} ("PATH=/usr/local/bin:/usr/bin:/bin" "USER=lewis" @end group @group "TERM=xterm" "SHELL=/bin/bash" "HOME=/home/lewis" @dots{}) @end group @end smallexample If @code{process-environment} contains multiple elements that specify the same environment variable, the first of these elements specifies the variable, and the others are ignored. @end defvar @defvar initial-environment This variable holds the list of environment variables Emacs inherited from its parent process when Emacs started. @end defvar @defvar path-separator This variable holds a string that says which character separates directories in a search path (as found in an environment variable). Its value is @code{":"} for Unix and GNU systems, and @code{";"} for MS systems. @end defvar @defun parse-colon-path path This function takes a search path string such as the value of the @env{PATH} environment variable, and splits it at the separators, returning a list of directories. @code{nil} in this list means the current directory. Although the function's name says ``colon'', it actually uses the value of @code{path-separator}. @example (parse-colon-path ":/foo:/bar") @result{} (nil "/foo/" "/bar/") @end example @end defun @defvar invocation-name This variable holds the program name under which Emacs was invoked. The value is a string, and does not include a directory name. @end defvar @defvar invocation-directory This variable holds the directory in which the Emacs executable was located when it was run, or @code{nil} if that directory cannot be determined. @end defvar @defvar installation-directory If non-@code{nil}, this is a directory within which to look for the @file{lib-src} and @file{etc} subdirectories. In an installed Emacs, it is normally @code{nil}. It is non-@code{nil} when Emacs can't find those directories in their standard installed locations, but can find them in a directory related somehow to the one containing the Emacs executable (i.e., @code{invocation-directory}). @end defvar @defun load-average &optional use-float This function returns the current 1-minute, 5-minute, and 15-minute system load averages, in a list. The load average indicates the number of processes trying to run on the system. By default, the values are integers that are 100 times the system load averages, but if @var{use-float} is non-@code{nil}, then they are returned as floating-point numbers without multiplying by 100. If it is impossible to obtain the load average, this function signals an error. On some platforms, access to load averages requires installing Emacs as setuid or setgid so that it can read kernel information, and that usually isn't advisable. @c FIXME which platforms are these? Are they still relevant? If the 1-minute load average is available, but the 5- or 15-minute averages are not, this function returns a shortened list containing the available averages. @example @group (load-average) @result{} (169 48 36) @end group @group (load-average t) @result{} (1.69 0.48 0.36) @end group @end example The shell command @code{uptime} returns similar information. @end defun @defun emacs-pid This function returns the process @acronym{ID} of the Emacs process, as an integer. @end defun @defvar tty-erase-char This variable holds the erase character that was selected in the system's terminal driver, before Emacs was started. @c FIXME? Seems untrue since 23.1. For me, it is 0. @c The value is @code{nil} if Emacs is running under a window system. @end defvar @node User Identification @section User Identification @cindex user identification @defvar init-file-user This variable says which user's init files should be used by Emacs---or @code{nil} if none. @code{""} stands for the user who originally logged in. The value reflects command-line options such as @samp{-q} or @samp{-u @var{user}}. Lisp packages that load files of customizations, or any other sort of user profile, should obey this variable in deciding where to find it. They should load the profile of the user name found in this variable. If @code{init-file-user} is @code{nil}, meaning that the @samp{-q}, @samp{-Q}, or @samp{-batch} option was used, then Lisp packages should not load any customization files or user profile. @end defvar @defopt user-mail-address This holds the email address of the user who is using Emacs. @end defopt @defun user-login-name &optional uid This function returns the name under which the user is logged in. It uses the environment variables @env{LOGNAME} or @env{USER} if either is set. Otherwise, the value is based on the effective @acronym{UID}, not the real @acronym{UID}. If you specify @var{uid} (a number), the result is the user name that corresponds to @var{uid}, or @code{nil} if there is no such user. @end defun @defun user-real-login-name This function returns the user name corresponding to Emacs's real @acronym{UID}. This ignores the effective @acronym{UID}, and the environment variables @env{LOGNAME} and @env{USER}. @end defun @defun user-full-name &optional uid This function returns the full name of the logged-in user---or the value of the environment variable @env{NAME}, if that is set. If the Emacs process's user-id does not correspond to any known user (and provided @code{NAME} is not set), the result is @code{"unknown"}. If @var{uid} is non-@code{nil}, then it should be a number (a user-id) or a string (a login name). Then @code{user-full-name} returns the full name corresponding to that user-id or login name. If you specify a user-id or login name that isn't defined, it returns @code{nil}. @end defun @vindex user-full-name @vindex user-real-login-name @vindex user-login-name The symbols @code{user-login-name}, @code{user-real-login-name} and @code{user-full-name} are variables as well as functions. The functions return the same values that the variables hold. These variables allow you to fake out Emacs by telling the functions what to return. The variables are also useful for constructing frame titles (@pxref{Frame Titles}). @cindex UID @defun user-real-uid This function returns the real @acronym{UID} of the user. The value may be floating point, in the (unlikely) event that the UID is too large to fit in a Lisp integer. @end defun @defun user-uid This function returns the effective @acronym{UID} of the user. The value may be floating point. @end defun @cindex GID @defun group-gid This function returns the effective @acronym{GID} of the Emacs process. The value may be floating point. @end defun @defun group-real-gid This function returns the real @acronym{GID} of the Emacs process. The value may be floating point. @end defun @defun system-users This function returns a list of strings, listing the user names on the system. If Emacs cannot retrieve this information, the return value is a list containing just the value of @code{user-real-login-name}. @end defun @cindex user groups @defun system-groups This function returns a list of strings, listing the names of user groups on the system. If Emacs cannot retrieve this information, the return value is @code{nil}. @end defun @node Time of Day @section Time of Day @cindex time of day This section explains how to determine the current time and time zone. @cindex epoch Most of these functions represent time as a list of four integers @code{(@var{sec-high} @var{sec-low} @var{microsec} @var{picosec})}. This represents the number of seconds from the @dfn{epoch} (January 1, 1970 at 00:00 UTC), using the formula: @ifnottex @var{high} * 2**16 + @var{low} + @var{micro} * 10**@minus{}6 + @var{pico} * 10**@minus{}12. @end ifnottex @tex $high*2^{16} + low + micro*10^{-6} + pico*10^{-12}$. @end tex The return value of @code{current-time} represents time using this form, as do the timestamps in the return values of other functions such as @code{file-attributes} (@pxref{Definition of file-attributes}). In some cases, functions may return two- or three-element lists, with omitted @var{microsec} and @var{picosec} components defaulting to zero. @cindex time value Function arguments, e.g., the @var{time} argument to @code{current-time-string}, accept a more-general @dfn{time value} format, which can be a list of integers as above, or a single number for seconds since the epoch, or @code{nil} for the current time. You can convert a time value into a human-readable string using @code{current-time-string} and @code{format-time-string}, into a list of integers using @code{seconds-to-time}, and into other forms using @code{decode-time} and @code{float-time}. These functions are described in the following sections. @defun current-time-string &optional time zone This function returns the current time and date as a human-readable string. The format does not vary for the initial part of the string, which contains the day of week, month, day of month, and time of day in that order: the number of characters used for these fields is always the same, so you can reliably use @code{substring} to extract them. You should count characters from the beginning of the string rather than from the end, as the year might not have exactly four digits, and additional information may some day be added at the end. The argument @var{time}, if given, specifies a time to format, instead of the current time. The optional argument @var{zone} defaults to the current time zone rule. @xref{Time Zone Rules}. @example @group (current-time-string) @result{} "Wed Oct 14 22:21:05 1987" @end group @end example @end defun @defun current-time This function returns the current time, represented as a list of four integers @code{(@var{sec-high} @var{sec-low} @var{microsec} @var{picosec})}. These integers have trailing zeros on systems that return time with lower resolutions. On all current machines @var{picosec} is a multiple of 1000, but this may change as higher-resolution clocks become available. @end defun @defun float-time &optional time This function returns the current time as a floating-point number of seconds since the epoch. The optional argument @var{time}, if given, specifies a time to convert instead of the current time. @emph{Warning}: Since the result is floating point, it may not be exact. Do not use this function if precise time stamps are required. @code{time-to-seconds} is an alias for this function. @end defun @defun seconds-to-time time This function converts a time value to list-of-integer form. For example, if @var{time} is a number, @code{(time-to-seconds (seconds-to-time @var{time}))} equals the number unless overflow or rounding errors occur. @end defun @node Time Zone Rules @section Time Zone Rules @cindex time zone rules @vindex TZ, environment variable The default time zone is determined by the @env{TZ} environment variable. @xref{System Environment}. For example, you can tell Emacs to default to Universal Time with @code{(setenv "TZ" "UTC0")}. If @env{TZ} is not in the environment, Emacs uses system wall clock time, which is a platform-dependent default time zone. The set of supported @env{TZ} strings is system-dependent. GNU and many other systems support the tzdata database, e.g., @samp{"America/New_York"} specifies the time zone and daylight saving time history for locations near New York City. GNU and most other systems support POSIX-style @env{TZ} strings, e.g., @samp{"EST+5EDT,M4.1.0/2,M10.5.0/2"} specifies the rules used in New York from 1987 through 2006. All systems support the string @samp{"UTC0"} meaning Universal Time. @cindex time zone rule Functions that convert to and from local time accept an optional @dfn{time zone rule} argument, which specifies the conversion's time zone and daylight saving time history. If the time zone rule is omitted or @code{nil}, the conversion uses Emacs's default time zone. If it is @code{t}, the conversion uses Universal Time. If it is @code{wall}, the conversion uses the system wall clock time. If it is a string, the conversion uses the time zone rule equivalent to setting @env{TZ} to that string. If it is a list (@var{offset} @var{abbr}), where @var{offset} is an integer number of seconds east of Universal Time and @var{abbr} is a string, the conversion uses a fixed time zone with the given offset and abbreviation. An integer @var{offset} is treated as if it were (@var{offset} @var{abbr}), where @var{abbr} is a numeric abbreviation on POSIX-compatible platforms and is unspecified on MS-Windows. @defun current-time-zone &optional time zone @cindex time zone, current This function returns a list describing the time zone that the user is in. The value has the form @code{(@var{offset} @var{abbr})}. Here @var{offset} is an integer giving the number of seconds ahead of Universal Time (east of Greenwich). A negative value means west of Greenwich. The second element, @var{abbr}, is a string giving an abbreviation for the time zone, e.g., @samp{"CST"} for China Standard Time or for U.S. Central Standard Time. Both elements can change when daylight saving time begins or ends; if the user has specified a time zone that does not use a seasonal time adjustment, then the value is constant through time. If the operating system doesn't supply all the information necessary to compute the value, the unknown elements of the list are @code{nil}. The argument @var{time}, if given, specifies a time value to analyze instead of the current time. The optional argument @var{zone} defaults to the current time zone rule. @end defun @node Time Conversion @section Time Conversion @cindex calendrical information @cindex time conversion These functions convert time values (@pxref{Time of Day}) into calendrical information and vice versa. Many 32-bit operating systems are limited to system times containing 32 bits of information in their seconds component; these systems typically handle only the times from 1901-12-13 20:45:52 through 2038-01-19 03:14:07 Universal Time. However, 64-bit and some 32-bit operating systems have larger seconds components, and can represent times far in the past or future. Time conversion functions always use the Gregorian calendar, even for dates before the Gregorian calendar was introduced. Year numbers count the number of years since the year 1 B.C., and do not skip zero as traditional Gregorian years do; for example, the year number @minus{}37 represents the Gregorian year 38 B.C@. @defun decode-time &optional time zone This function converts a time value into calendrical information. If you don't specify @var{time}, it decodes the current time, and similarly @var{zone} defaults to the current time zone rule. @xref{Time Zone Rules}. The return value is a list of nine elements, as follows: @example (@var{seconds} @var{minutes} @var{hour} @var{day} @var{month} @var{year} @var{dow} @var{dst} @var{utcoff}) @end example Here is what the elements mean: @table @var @item seconds The number of seconds past the minute, as an integer between 0 and 59. On some operating systems, this is 60 for leap seconds. @item minutes The number of minutes past the hour, as an integer between 0 and 59. @item hour The hour of the day, as an integer between 0 and 23. @item day The day of the month, as an integer between 1 and 31. @item month The month of the year, as an integer between 1 and 12. @item year The year, an integer typically greater than 1900. @item dow The day of week, as an integer between 0 and 6, where 0 stands for Sunday. @item dst @code{t} if daylight saving time is effect, otherwise @code{nil}. @item utcoff An integer indicating the Universal Time offset in seconds, i.e., the number of seconds east of Greenwich. @end table @strong{Common Lisp Note:} Common Lisp has different meanings for @var{dow} and @var{utcoff}. @end defun @defun encode-time seconds minutes hour day month year &optional zone This function is the inverse of @code{decode-time}. It converts seven items of calendrical data into a list-of-integer time value. For the meanings of the arguments, see the table above under @code{decode-time}. Year numbers less than 100 are not treated specially. If you want them to stand for years above 1900, or years above 2000, you must alter them yourself before you call @code{encode-time}. The optional argument @var{zone} defaults to the current time zone rule. @xref{Time Zone Rules}. If you pass more than seven arguments to @code{encode-time}, the first six are used as @var{seconds} through @var{year}, the last argument is used as @var{zone}, and the arguments in between are ignored. This feature makes it possible to use the elements of a list returned by @code{decode-time} as the arguments to @code{encode-time}, like this: @example (apply 'encode-time (decode-time @dots{})) @end example You can perform simple date arithmetic by using out-of-range values for the @var{seconds}, @var{minutes}, @var{hour}, @var{day}, and @var{month} arguments; for example, day 0 means the day preceding the given month. The operating system puts limits on the range of possible time values; if you try to encode a time that is out of range, an error results. For instance, years before 1970 do not work on some systems; on others, years as early as 1901 do work. @end defun @node Time Parsing @section Parsing and Formatting Times @cindex time parsing @cindex time formatting @cindex formatting time values These functions convert time values to text in a string, and vice versa. Time values are lists of two to four integers (@pxref{Time of Day}). @defun date-to-time string This function parses the time-string @var{string} and returns the corresponding time value. @end defun @defun format-time-string format-string &optional time zone This function converts @var{time} (or the current time, if @var{time} is omitted or @code{nil}) to a string according to @var{format-string}. The conversion uses the time zone rule @var{zone}, which defaults to the current time zone rule. @xref{Time Zone Rules}. The argument @var{format-string} may contain @samp{%}-sequences which say to substitute parts of the time. Here is a table of what the @samp{%}-sequences mean: @table @samp @item %a This stands for the abbreviated name of the day of week. @item %A This stands for the full name of the day of week. @item %b This stands for the abbreviated name of the month. @item %B This stands for the full name of the month. @item %c This is a synonym for @samp{%x %X}. @item %C This stands for the century, that is, the year divided by 100, truncated toward zero. The default field width is 2. @item %d This stands for the day of month, zero-padded. @item %D This is a synonym for @samp{%m/%d/%y}. @item %e This stands for the day of month, blank-padded. @item %F This stands for the ISO 8601 date format, i.e., @samp{"%Y-%m-%d"}. @item %g This stands for the year corresponding to the ISO week within the century. @item %G This stands for the year corresponding to the ISO week. @item %h This is a synonym for @samp{%b}. @item %H This stands for the hour (00--23). @item %I This stands for the hour (01--12). @item %j This stands for the day of the year (001--366). @item %k This stands for the hour (0--23), blank padded. @item %l This stands for the hour (1--12), blank padded. @item %m This stands for the month (01--12). @item %M This stands for the minute (00--59). @item %n This stands for a newline. @item %N This stands for the nanoseconds (000000000--999999999). To ask for fewer digits, use @samp{%3N} for milliseconds, @samp{%6N} for microseconds, etc. Any excess digits are discarded, without rounding. @item %p This stands for @samp{AM} or @samp{PM}, as appropriate. @item %q This stands for the calendar quarter (1--4). @item %r This is a synonym for @samp{%I:%M:%S %p}. @item %R This is a synonym for @samp{%H:%M}. @item %s This stands for the integer number of seconds since the epoch. @item %S This stands for the second (00--59, or 00--60 on platforms that support leap seconds). @item %t This stands for a tab character. @item %T This is a synonym for @samp{%H:%M:%S}. @item %u This stands for the numeric day of week (1--7). Monday is day 1. @item %U This stands for the week of the year (01--52), assuming that weeks start on Sunday. @item %V This stands for the week of the year according to ISO 8601. @item %w This stands for the numeric day of week (0--6). Sunday is day 0. @item %W This stands for the week of the year (01--52), assuming that weeks start on Monday. @item %x This has a locale-specific meaning. In the default locale (named @samp{C}), it is equivalent to @samp{%D}. @item %X This has a locale-specific meaning. In the default locale (named @samp{C}), it is equivalent to @samp{%T}. @item %y This stands for the year without century (00--99). @item %Y This stands for the year with century. @item %Z This stands for the time zone abbreviation (e.g., @samp{EST}). @item %z This stands for the time zone numerical offset. The @samp{z} can be preceded by one, two, or three colons; if plain @samp{%z} stands for @samp{-0500}, then @samp{%:z} stands for @samp{-05:00}, @samp{%::z} stands for @samp{-05:00:00}, and @samp{%:::z} is like @samp{%::z} except it suppresses trailing instances of @samp{:00} so it stands for @samp{-05} in the same example. @item %% This stands for a single @samp{%}. @end table One or more flag characters can appear immediately after the @samp{%}. @samp{0} pads with zeros, @samp{_} pads with blanks, @samp{-} suppresses padding, @samp{^} upper-cases letters, and @samp{#} reverses the case of letters. You can also specify the field width and type of padding for any of these @samp{%}-sequences. This works as in @code{printf}: you write the field width as digits in a @samp{%}-sequence, after any flags. For example, @samp{%S} specifies the number of seconds since the minute; @samp{%03S} means to pad this with zeros to 3 positions, @samp{%_3S} to pad with spaces to 3 positions. Plain @samp{%3S} pads with zeros, because that is how @samp{%S} normally pads to two positions. The characters @samp{E} and @samp{O} act as modifiers when used after any flags and field widths in a @samp{%}-sequence. @samp{E} specifies using the current locale's alternative version of the date and time. In a Japanese locale, for example, @code{%Ex} might yield a date format based on the Japanese Emperors' reigns. @samp{E} is allowed in @samp{%Ec}, @samp{%EC}, @samp{%Ex}, @samp{%EX}, @samp{%Ey}, and @samp{%EY}. @samp{O} means to use the current locale's alternative representation of numbers, instead of the ordinary decimal digits. This is allowed with most letters, all the ones that output numbers. To help debug programs, unrecognized @samp{%}-sequences stand for themselves and are output as-is. Programs should not rely on this behavior, as future versions of Emacs may recognize new @samp{%}-sequences as extensions. This function uses the C library function @code{strftime} (@pxref{Formatting Calendar Time,,, libc, The GNU C Library Reference Manual}) to do most of the work. In order to communicate with that function, it first encodes its argument using the coding system specified by @code{locale-coding-system} (@pxref{Locales}); after @code{strftime} returns the resulting string, @code{format-time-string} decodes the string using that same coding system. @end defun @defun format-seconds format-string seconds This function converts its argument @var{seconds} into a string of years, days, hours, etc., according to @var{format-string}. The argument @var{format-string} may contain @samp{%}-sequences which control the conversion. Here is a table of what the @samp{%}-sequences mean: @table @samp @item %y @itemx %Y The integer number of 365-day years. @item %d @itemx %D The integer number of days. @item %h @itemx %H The integer number of hours. @item %m @itemx %M The integer number of minutes. @item %s @itemx %S The integer number of seconds. @item %z Non-printing control flag. When it is used, other specifiers must be given in the order of decreasing size, i.e., years before days, hours before minutes, etc. Nothing will be produced in the result string to the left of @samp{%z} until the first non-zero conversion is encountered. For example, the default format used by @code{emacs-uptime} (@pxref{Processor Run Time, emacs-uptime}) @w{@code{"%Y, %D, %H, %M, %z%S"}} means that the number of seconds will always be produced, but years, days, hours, and minutes will only be shown if they are non-zero. @item %% Produces a literal @samp{%}. @end table Upper-case format sequences produce the units in addition to the numbers, lower-case formats produce only the numbers. You can also specify the field width by following the @samp{%} with a number; shorter numbers will be padded with blanks. An optional period before the width requests zero-padding instead. For example, @code{"%.3Y"} might produce @code{"004 years"}. @emph{Warning:} This function works only with values of @var{seconds} that don't exceed @code{most-positive-fixnum} (@pxref{Integer Basics, most-positive-fixnum}). @end defun @node Processor Run Time @section Processor Run time @cindex processor run time @cindex Emacs process run time Emacs provides several functions and primitives that return time, both elapsed and processor time, used by the Emacs process. @deffn Command emacs-uptime &optional format @cindex uptime of Emacs This function returns a string representing the Emacs @dfn{uptime}---the elapsed wall-clock time this instance of Emacs is running. The string is formatted by @code{format-seconds} according to the optional argument @var{format}. For the available format descriptors, see @ref{Time Parsing, format-seconds}. If @var{format} is @code{nil} or omitted, it defaults to @code{"%Y, %D, %H, %M, %z%S"}. When called interactively, it prints the uptime in the echo area. @end deffn @defun get-internal-run-time This function returns the processor run time used by Emacs as a list of four integers: @code{(@var{sec-high} @var{sec-low} @var{microsec} @var{picosec})}, using the same format as @code{current-time} (@pxref{Time of Day}). Note that the time returned by this function excludes the time Emacs was not using the processor, and if the Emacs process has several threads, the returned value is the sum of the processor times used up by all Emacs threads. If the system doesn't provide a way to determine the processor run time, @code{get-internal-run-time} returns the same time as @code{current-time}. @end defun @deffn Command emacs-init-time This function returns the duration of the Emacs initialization (@pxref{Startup Summary}) in seconds, as a string. When called interactively, it prints the duration in the echo area. @end deffn @node Time Calculations @section Time Calculations @cindex time calculations @cindex comparing time values @cindex calendrical computations These functions perform calendrical computations using time values (@pxref{Time of Day}). A value of @code{nil} for any of their time-value arguments stands for the current system time, and a single integer number stands for the number of seconds since the epoch. @defun time-less-p t1 t2 This returns @code{t} if time value @var{t1} is less than time value @var{t2}. @end defun @defun time-subtract t1 t2 This returns the time difference @var{t1} @minus{} @var{t2} between two time values, as a time value. If you need the difference in units of elapsed seconds, use @code{float-time} (@pxref{Time of Day, float-time}) to convert the result into seconds. @end defun @defun time-add t1 t2 This returns the sum of two time values, as a time value. One argument should represent a time difference rather than a point in time, either as a list or as a single number of elapsed seconds. Here is how to add a number of seconds to a time value: @example (time-add @var{time} @var{seconds}) @end example @end defun @defun time-to-days time-value This function returns the number of days between the beginning of year 1 and @var{time-value}. @end defun @defun time-to-day-in-year time-value This returns the day number within the year corresponding to @var{time-value}. @end defun @defun date-leap-year-p year This function returns @code{t} if @var{year} is a leap year. @end defun @node Timers @section Timers for Delayed Execution @cindex timers You can set up a @dfn{timer} to call a function at a specified future time or after a certain length of idleness. A timer is a special object that stores the information about the next invocation times and the function to invoke. @defun timerp object This predicate function returns non-@code{nil} of @code{object} is a timer. @end defun Emacs cannot run timers at any arbitrary point in a Lisp program; it can run them only when Emacs could accept output from a subprocess: namely, while waiting or inside certain primitive functions such as @code{sit-for} or @code{read-event} which @emph{can} wait. Therefore, a timer's execution may be delayed if Emacs is busy. However, the time of execution is very precise if Emacs is idle. Emacs binds @code{inhibit-quit} to @code{t} before calling the timer function, because quitting out of many timer functions can leave things in an inconsistent state. This is normally unproblematical because most timer functions don't do a lot of work. Indeed, for a timer to call a function that takes substantial time to run is likely to be annoying. If a timer function needs to allow quitting, it should use @code{with-local-quit} (@pxref{Quitting}). For example, if a timer function calls @code{accept-process-output} to receive output from an external process, that call should be wrapped inside @code{with-local-quit}, to ensure that @kbd{C-g} works if the external process hangs. It is usually a bad idea for timer functions to alter buffer contents. When they do, they usually should call @code{undo-boundary} both before and after changing the buffer, to separate the timer's changes from user commands' changes and prevent a single undo entry from growing to be quite large. Timer functions should also avoid calling functions that cause Emacs to wait, such as @code{sit-for} (@pxref{Waiting}). This can lead to unpredictable effects, since other timers (or even the same timer) can run while waiting. If a timer function needs to perform an action after a certain time has elapsed, it can do this by scheduling a new timer. If a timer function calls functions that can change the match data, it should save and restore the match data. @xref{Saving Match Data}. @deffn Command run-at-time time repeat function &rest args This sets up a timer that calls the function @var{function} with arguments @var{args} at time @var{time}. If @var{repeat} is a number (integer or floating point), the timer is scheduled to run again every @var{repeat} seconds after @var{time}. If @var{repeat} is @code{nil}, the timer runs only once. @var{time} may specify an absolute or a relative time. Absolute times may be specified using a string with a limited variety of formats, and are taken to be times @emph{today}, even if already in the past. The recognized forms are @samp{@var{xxxx}}, @samp{@var{x}:@var{xx}}, or @samp{@var{xx}:@var{xx}} (military time), and @samp{@var{xx}am}, @samp{@var{xx}AM}, @samp{@var{xx}pm}, @samp{@var{xx}PM}, @samp{@var{xx}:@var{xx}am}, @samp{@var{xx}:@var{xx}AM}, @samp{@var{xx}:@var{xx}pm}, or @samp{@var{xx}:@var{xx}PM}. A period can be used instead of a colon to separate the hour and minute parts. To specify a relative time as a string, use numbers followed by units. For example: @table @samp @item 1 min denotes 1 minute from now. @item 1 min 5 sec denotes 65 seconds from now. @item 1 min 2 sec 3 hour 4 day 5 week 6 fortnight 7 month 8 year denotes exactly 103 months, 123 days, and 10862 seconds from now. @end table For relative time values, Emacs considers a month to be exactly thirty days, and a year to be exactly 365.25 days. Not all convenient formats are strings. If @var{time} is a number (integer or floating point), that specifies a relative time measured in seconds. The result of @code{encode-time} can also be used to specify an absolute value for @var{time}. In most cases, @var{repeat} has no effect on when @emph{first} call takes place---@var{time} alone specifies that. There is one exception: if @var{time} is @code{t}, then the timer runs whenever the time is a multiple of @var{repeat} seconds after the epoch. This is useful for functions like @code{display-time}. The function @code{run-at-time} returns a timer value that identifies the particular scheduled future action. You can use this value to call @code{cancel-timer} (see below). @end deffn A repeating timer nominally ought to run every @var{repeat} seconds, but remember that any invocation of a timer can be late. Lateness of one repetition has no effect on the scheduled time of the next repetition. For instance, if Emacs is busy computing for long enough to cover three scheduled repetitions of the timer, and then starts to wait, it will immediately call the timer function three times in immediate succession (presuming no other timers trigger before or between them). If you want a timer to run again no less than @var{n} seconds after the last invocation, don't use the @var{repeat} argument. Instead, the timer function should explicitly reschedule the timer. @defopt timer-max-repeats This variable's value specifies the maximum number of times to repeat calling a timer function in a row, when many previously scheduled calls were unavoidably delayed. @end defopt @defmac with-timeout (seconds timeout-forms@dots{}) body@dots{} Execute @var{body}, but give up after @var{seconds} seconds. If @var{body} finishes before the time is up, @code{with-timeout} returns the value of the last form in @var{body}. If, however, the execution of @var{body} is cut short by the timeout, then @code{with-timeout} executes all the @var{timeout-forms} and returns the value of the last of them. This macro works by setting a timer to run after @var{seconds} seconds. If @var{body} finishes before that time, it cancels the timer. If the timer actually runs, it terminates execution of @var{body}, then executes @var{timeout-forms}. Since timers can run within a Lisp program only when the program calls a primitive that can wait, @code{with-timeout} cannot stop executing @var{body} while it is in the midst of a computation---only when it calls one of those primitives. So use @code{with-timeout} only with a @var{body} that waits for input, not one that does a long computation. @end defmac The function @code{y-or-n-p-with-timeout} provides a simple way to use a timer to avoid waiting too long for an answer. @xref{Yes-or-No Queries}. @defun cancel-timer timer This cancels the requested action for @var{timer}, which should be a timer---usually, one previously returned by @code{run-at-time} or @code{run-with-idle-timer}. This cancels the effect of that call to one of these functions; the arrival of the specified time will not cause anything special to happen. @end defun @findex list-timers The @code{list-timers} command lists all the currently active timers. There's only one command available in the buffer displayed: @kbd{c} (@code{timer-list-cancel}) that will cancel the timer on the line under point. @node Idle Timers @section Idle Timers @cindex idle timers Here is how to set up a timer that runs when Emacs is idle for a certain length of time. Aside from how to set them up, idle timers work just like ordinary timers. @deffn Command run-with-idle-timer secs repeat function &rest args Set up a timer which runs the next time Emacs is idle for @var{secs} seconds. The value of @var{secs} may be a number or a value of the type returned by @code{current-idle-time}. If @var{repeat} is @code{nil}, the timer runs just once, the first time Emacs remains idle for a long enough time. More often @var{repeat} is non-@code{nil}, which means to run the timer @emph{each time} Emacs remains idle for @var{secs} seconds. The function @code{run-with-idle-timer} returns a timer value which you can use in calling @code{cancel-timer} (@pxref{Timers}). @end deffn @cindex idleness Emacs becomes @dfn{idle} when it starts waiting for user input, and it remains idle until the user provides some input. If a timer is set for five seconds of idleness, it runs approximately five seconds after Emacs first becomes idle. Even if @var{repeat} is non-@code{nil}, this timer will not run again as long as Emacs remains idle, because the duration of idleness will continue to increase and will not go down to five seconds again. Emacs can do various things while idle: garbage collect, autosave or handle data from a subprocess. But these interludes during idleness do not interfere with idle timers, because they do not reset the clock of idleness to zero. An idle timer set for 600 seconds will run when ten minutes have elapsed since the last user command was finished, even if subprocess output has been accepted thousands of times within those ten minutes, and even if there have been garbage collections and autosaves. When the user supplies input, Emacs becomes non-idle while executing the input. Then it becomes idle again, and all the idle timers that are set up to repeat will subsequently run another time, one by one. Do not write an idle timer function containing a loop which does a certain amount of processing each time around, and exits when @code{(input-pending-p)} is non-@code{nil}. This approach seems very natural but has two problems: @itemize @item It blocks out all process output (since Emacs accepts process output only while waiting). @item It blocks out any idle timers that ought to run during that time. @end itemize @noindent Similarly, do not write an idle timer function that sets up another idle timer (including the same idle timer) with @var{secs} argument less than or equal to the current idleness time. Such a timer will run almost immediately, and continue running again and again, instead of waiting for the next time Emacs becomes idle. The correct approach is to reschedule with an appropriate increment of the current value of the idleness time, as described below. @defun current-idle-time If Emacs is idle, this function returns the length of time Emacs has been idle, as a list of four integers: @code{(@var{sec-high} @var{sec-low} @var{microsec} @var{picosec})}, using the same format as @code{current-time} (@pxref{Time of Day}). When Emacs is not idle, @code{current-idle-time} returns @code{nil}. This is a convenient way to test whether Emacs is idle. @end defun The main use of @code{current-idle-time} is when an idle timer function wants to ``take a break'' for a while. It can set up another idle timer to call the same function again, after a few seconds more idleness. Here's an example: @example (defvar my-resume-timer nil "Timer for `my-timer-function' to reschedule itself, or nil.") (defun my-timer-function () ;; @r{If the user types a command while @code{my-resume-timer}} ;; @r{is active, the next time this function is called from} ;; @r{its main idle timer, deactivate @code{my-resume-timer}.} (when my-resume-timer (cancel-timer my-resume-timer)) ...@var{do the work for a while}... (when @var{taking-a-break} (setq my-resume-timer (run-with-idle-timer ;; Compute an idle time @var{break-length} ;; more than the current value. (time-add (current-idle-time) @var{break-length}) nil 'my-timer-function)))) @end example @node Terminal Input @section Terminal Input @cindex terminal input This section describes functions and variables for recording or manipulating terminal input. See @ref{Display}, for related functions. @menu * Input Modes:: Options for how input is processed. * Recording Input:: Saving histories of recent or all input events. @end menu @node Input Modes @subsection Input Modes @cindex input modes @cindex terminal input modes @defun set-input-mode interrupt flow meta &optional quit-char This function sets the mode for reading keyboard input. If @var{interrupt} is non-@code{nil}, then Emacs uses input interrupts. If it is @code{nil}, then it uses @sc{cbreak} mode. The default setting is system-dependent. Some systems always use @sc{cbreak} mode regardless of what is specified. When Emacs communicates directly with X, it ignores this argument and uses interrupts if that is the way it knows how to communicate. If @var{flow} is non-@code{nil}, then Emacs uses @sc{xon/xoff} (@kbd{C-q}, @kbd{C-s}) flow control for output to the terminal. This has no effect except in @sc{cbreak} mode. The argument @var{meta} controls support for input character codes above 127. If @var{meta} is @code{t}, Emacs converts characters with the 8th bit set into Meta characters. If @var{meta} is @code{nil}, Emacs disregards the 8th bit; this is necessary when the terminal uses it as a parity bit. If @var{meta} is neither @code{t} nor @code{nil}, Emacs uses all 8 bits of input unchanged. This is good for terminals that use 8-bit character sets. If @var{quit-char} is non-@code{nil}, it specifies the character to use for quitting. Normally this character is @kbd{C-g}. @xref{Quitting}. @end defun The @code{current-input-mode} function returns the input mode settings Emacs is currently using. @defun current-input-mode This function returns the current mode for reading keyboard input. It returns a list, corresponding to the arguments of @code{set-input-mode}, of the form @code{(@var{interrupt} @var{flow} @var{meta} @var{quit})} in which: @table @var @item interrupt is non-@code{nil} when Emacs is using interrupt-driven input. If @code{nil}, Emacs is using @sc{cbreak} mode. @item flow is non-@code{nil} if Emacs uses @sc{xon/xoff} (@kbd{C-q}, @kbd{C-s}) flow control for output to the terminal. This value is meaningful only when @var{interrupt} is @code{nil}. @item meta is @code{t} if Emacs treats the eighth bit of input characters as the meta bit; @code{nil} means Emacs clears the eighth bit of every input character; any other value means Emacs uses all eight bits as the basic character code. @item quit is the character Emacs currently uses for quitting, usually @kbd{C-g}. @end table @end defun @node Recording Input @subsection Recording Input @cindex recording input @defun recent-keys This function returns a vector containing the last 300 input events from the keyboard or mouse. All input events are included, whether or not they were used as parts of key sequences. Thus, you always get the last 300 input events, not counting events generated by keyboard macros. (These are excluded because they are less interesting for debugging; it should be enough to see the events that invoked the macros.) A call to @code{clear-this-command-keys} (@pxref{Command Loop Info}) causes this function to return an empty vector immediately afterward. @end defun @deffn Command open-dribble-file filename @cindex dribble file This function opens a @dfn{dribble file} named @var{filename}. When a dribble file is open, each input event from the keyboard or mouse (but not those from keyboard macros) is written in that file. A non-character event is expressed using its printed representation surrounded by @samp{<@dots{}>}. Be aware that sensitive information (such as passwords) may end up recorded in the dribble file. You close the dribble file by calling this function with an argument of @code{nil}. @end deffn See also the @code{open-termscript} function (@pxref{Terminal Output}). @node Terminal Output @section Terminal Output @cindex terminal output The terminal output functions send output to a text terminal, or keep track of output sent to the terminal. The variable @code{baud-rate} tells you what Emacs thinks is the output speed of the terminal. @defopt baud-rate This variable's value is the output speed of the terminal, as far as Emacs knows. Setting this variable does not change the speed of actual data transmission, but the value is used for calculations such as padding. It also affects decisions about whether to scroll part of the screen or repaint on text terminals. @xref{Forcing Redisplay}, for the corresponding functionality on graphical terminals. The value is measured in baud. @end defopt If you are running across a network, and different parts of the network work at different baud rates, the value returned by Emacs may be different from the value used by your local terminal. Some network protocols communicate the local terminal speed to the remote machine, so that Emacs and other programs can get the proper value, but others do not. If Emacs has the wrong value, it makes decisions that are less than optimal. To fix the problem, set @code{baud-rate}. @defun send-string-to-terminal string &optional terminal This function sends @var{string} to @var{terminal} without alteration. Control characters in @var{string} have terminal-dependent effects. (If you need to display non-ASCII text on the terminal, encode it using one of the functions described in @ref{Explicit Encoding}.) This function operates only on text terminals. @var{terminal} may be a terminal object, a frame, or @code{nil} for the selected frame's terminal. In batch mode, @var{string} is sent to @code{stdout} when @var{terminal} is @code{nil}. One use of this function is to define function keys on terminals that have downloadable function key definitions. For example, this is how (on certain terminals) to define function key 4 to move forward four characters (by transmitting the characters @kbd{C-u C-f} to the computer): @example @group (send-string-to-terminal "\eF4\^U\^F") @result{} nil @end group @end example @end defun @deffn Command open-termscript filename @cindex termscript file This function is used to open a @dfn{termscript file} that will record all the characters sent by Emacs to the terminal. It returns @code{nil}. Termscript files are useful for investigating problems where Emacs garbles the screen, problems that are due to incorrect Termcap entries or to undesirable settings of terminal options more often than to actual Emacs bugs. Once you are certain which characters were actually output, you can determine reliably whether they correspond to the Termcap specifications in use. @example @group (open-termscript "../junk/termscript") @result{} nil @end group @end example You close the termscript file by calling this function with an argument of @code{nil}. See also @code{open-dribble-file} in @ref{Recording Input}. @end deffn @node Sound Output @section Sound Output @cindex sound To play sound using Emacs, use the function @code{play-sound}. Only certain systems are supported; if you call @code{play-sound} on a system which cannot really do the job, it gives an error. @c FIXME: Add indexes for Au and WAV? --xfq The sound must be stored as a file in RIFF-WAVE format (@samp{.wav}) or Sun Audio format (@samp{.au}). @defun play-sound sound This function plays a specified sound. The argument, @var{sound}, has the form @code{(sound @var{properties}...)}, where the @var{properties} consist of alternating keywords (particular symbols recognized specially) and values corresponding to them. Here is a table of the keywords that are currently meaningful in @var{sound}, and their meanings: @table @code @item :file @var{file} This specifies the file containing the sound to play. If the file name is not absolute, it is expanded against the directory @code{data-directory}. @item :data @var{data} This specifies the sound to play without need to refer to a file. The value, @var{data}, should be a string containing the same bytes as a sound file. We recommend using a unibyte string. @item :volume @var{volume} This specifies how loud to play the sound. It should be a number in the range of 0 to 1. The default is to use whatever volume has been specified before. @item :device @var{device} This specifies the system device on which to play the sound, as a string. The default device is system-dependent. @end table Before actually playing the sound, @code{play-sound} calls the functions in the list @code{play-sound-functions}. Each function is called with one argument, @var{sound}. @end defun @deffn Command play-sound-file file &optional volume device This function is an alternative interface to playing a sound @var{file} specifying an optional @var{volume} and @var{device}. @end deffn @defvar play-sound-functions A list of functions to be called before playing a sound. Each function is called with one argument, a property list that describes the sound. @end defvar @node X11 Keysyms @section Operating on X11 Keysyms @cindex X11 keysyms To define system-specific X11 keysyms, set the variable @code{system-key-alist}. @defvar system-key-alist This variable's value should be an alist with one element for each system-specific keysym. Each element has the form @code{(@var{code} . @var{symbol})}, where @var{code} is the numeric keysym code (not including the vendor-specific bit, @ifnottex @minus{}2**28), @end ifnottex @tex $-2^{28}$), @end tex and @var{symbol} is the name for the function key. For example @code{(168 . mute-acute)} defines a system-specific key (used by HP X servers) whose numeric code is @ifnottex @minus{}2**28 @end ifnottex @tex $-2^{28}$ @end tex + 168. It is not crucial to exclude from the alist the keysyms of other X servers; those do no harm, as long as they don't conflict with the ones used by the X server actually in use. The variable is always local to the current terminal, and cannot be buffer-local. @xref{Multiple Terminals}. @end defvar You can specify which keysyms Emacs should use for the Control, Meta, Alt, Hyper, and Super modifiers by setting these variables: @defvar x-ctrl-keysym @defvarx x-alt-keysym @defvarx x-meta-keysym @defvarx x-hyper-keysym @defvarx x-super-keysym The name of the keysym that should stand for the Control modifier (respectively, for Alt, Meta, Hyper, and Super). For example, here is how to swap the Meta and Alt modifiers within Emacs: @lisp (setq x-alt-keysym 'meta) (setq x-meta-keysym 'alt) @end lisp @end defvar @node Batch Mode @section Batch Mode @cindex batch mode The command-line option @samp{-batch} causes Emacs to run noninteractively. In this mode, Emacs does not read commands from the terminal, it does not alter the terminal modes, and it does not expect to be outputting to an erasable screen. The idea is that you specify Lisp programs to run; when they are finished, Emacs should exit. The way to specify the programs to run is with @samp{-l @var{file}}, which loads the library named @var{file}, or @samp{-f @var{function}}, which calls @var{function} with no arguments, or @samp{--eval @var{form}}. Any Lisp program output that would normally go to the echo area, either using @code{message}, or using @code{prin1}, etc., with @code{t} as the stream, goes instead to Emacs's standard descriptors when in batch mode: @code{message} writes to the standard error descriptor, while @code{prin1} and other print functions write to the standard output. Similarly, input that would normally come from the minibuffer is read from the standard input descriptor. Thus, Emacs behaves much like a noninteractive application program. (The echo area output that Emacs itself normally generates, such as command echoing, is suppressed entirely.) Non-ASCII text written to the standard output or error descriptors is by default encoded using @code{locale-coding-system} (@pxref{Locales}) if it is non-@code{nil}; this can be overridden by binding @code{coding-system-for-write} to a coding system of you choice (@pxref{Explicit Encoding}). @defvar noninteractive This variable is non-@code{nil} when Emacs is running in batch mode. @end defvar @node Session Management @section Session Management @cindex session manager Emacs supports the X Session Management Protocol, which is used to suspend and restart applications. In the X Window System, a program called the @dfn{session manager} is responsible for keeping track of the applications that are running. When the X server shuts down, the session manager asks applications to save their state, and delays the actual shutdown until they respond. An application can also cancel the shutdown. When the session manager restarts a suspended session, it directs these applications to individually reload their saved state. It does this by specifying a special command-line argument that says what saved session to restore. For Emacs, this argument is @samp{--smid @var{session}}. @defvar emacs-save-session-functions @cindex session file Emacs supports saving state via a hook called @code{emacs-save-session-functions}. Emacs runs this hook when the session manager tells it that the window system is shutting down. The functions are called with no arguments, and with the current buffer set to a temporary buffer. Each function can use @code{insert} to add Lisp code to this buffer. At the end, Emacs saves the buffer in a file, called the @dfn{session file}. @findex emacs-session-restore Subsequently, when the session manager restarts Emacs, it loads the session file automatically (@pxref{Loading}). This is performed by a function named @code{emacs-session-restore}, which is called during startup. @xref{Startup Summary}. If a function in @code{emacs-save-session-functions} returns non-@code{nil}, Emacs tells the session manager to cancel the shutdown. @end defvar Here is an example that just inserts some text into @file{*scratch*} when Emacs is restarted by the session manager. @example @group (add-hook 'emacs-save-session-functions 'save-yourself-test) @end group @group (defun save-yourself-test () (insert "(save-current-buffer (switch-to-buffer \"*scratch*\") (insert \"I am restored\"))") nil) @end group @end example @node Desktop Notifications @section Desktop Notifications @cindex desktop notifications @cindex notifications, on desktop Emacs is able to send @dfn{notifications} on systems that support the freedesktop.org Desktop Notifications Specification and on MS-Windows. In order to use this functionality on POSIX hosts, Emacs must have been compiled with D-Bus support, and the @code{notifications} library must be loaded. @xref{Top, , D-Bus,dbus,D-Bus integration in Emacs}. The following function is supported when D-Bus support is available: @defun notifications-notify &rest params This function sends a notification to the desktop via D-Bus, consisting of the parameters specified by the @var{params} arguments. These arguments should consist of alternating keyword and value pairs. The supported keywords and values are as follows: @table @code @item :bus @var{bus} The D-Bus bus. This argument is needed only if a bus other than @code{:session} shall be used. @item :title @var{title} The notification title. @item :body @var{text} The notification body text. Depending on the implementation of the notification server, the text could contain HTML markups, like @samp{"bold text"}, hyperlinks, or images. Special HTML characters must be encoded, as @samp{"Contact <postmaster@@localhost>!"}. @item :app-name @var{name} The name of the application sending the notification. The default is @code{notifications-application-name}. @item :replaces-id @var{id} The notification @var{id} that this notification replaces. @var{id} must be the result of a previous @code{notifications-notify} call. @item :app-icon @var{icon-file} The file name of the notification icon. If set to @code{nil}, no icon is displayed. The default is @code{notifications-application-icon}. @item :actions (@var{key} @var{title} @var{key} @var{title} ...) A list of actions to be applied. @var{key} and @var{title} are both strings. The default action (usually invoked by clicking the notification) should have a key named @samp{"default"}. The title can be anything, though implementations are free not to display it. @item :timeout @var{timeout} The timeout time in milliseconds since the display of the notification at which the notification should automatically close. If @minus{}1, the notification's expiration time is dependent on the notification server's settings, and may vary for the type of notification. If 0, the notification never expires. Default value is @minus{}1. @item :urgency @var{urgency} The urgency level. It can be @code{low}, @code{normal}, or @code{critical}. @item :action-items When this keyword is given, the @var{title} string of the actions is interpreted as icon name. @item :category @var{category} The type of notification this is, a string. See the @uref{http://developer.gnome.org/notification-spec/#categories, Desktop Notifications Specification} for a list of standard categories. @item :desktop-entry @var{filename} This specifies the name of the desktop filename representing the calling program, like @samp{"emacs"}. @item :image-data (@var{width} @var{height} @var{rowstride} @var{has-alpha} @var{bits} @var{channels} @var{data}) This is a raw data image format that describes the width, height, rowstride, whether there is an alpha channel, bits per sample, channels and image data, respectively. @item :image-path @var{path} This is represented either as a URI (@samp{file://} is the only URI schema supported right now) or a name in a freedesktop.org-compliant icon theme from @samp{$XDG_DATA_DIRS/icons}. @item :sound-file @var{filename} The path to a sound file to play when the notification pops up. @item :sound-name @var{name} A themable named sound from the freedesktop.org sound naming specification from @samp{$XDG_DATA_DIRS/sounds}, to play when the notification pops up. Similar to the icon name, only for sounds. An example would be @samp{"message-new-instant"}. @item :suppress-sound Causes the server to suppress playing any sounds, if it has that ability. @item :resident When set the server will not automatically remove the notification when an action has been invoked. The notification will remain resident in the server until it is explicitly removed by the user or by the sender. This hint is likely only useful when the server has the @code{:persistence} capability. @item :transient When set the server will treat the notification as transient and by-pass the server's persistence capability, if it should exist. @item :x @var{position} @itemx :y @var{position} Specifies the X, Y location on the screen that the notification should point to. Both arguments must be used together. @item :on-action @var{function} Function to call when an action is invoked. The notification @var{id} and the @var{key} of the action are passed as arguments to the function. @item :on-close @var{function} Function to call when the notification has been closed by timeout or by the user. The function receive the notification @var{id} and the closing @var{reason} as arguments: @itemize @item @code{expired} if the notification has expired @item @code{dismissed} if the notification was dismissed by the user @item @code{close-notification} if the notification was closed by a call to @code{notifications-close-notification} @item @code{undefined} if the notification server hasn't provided a reason @end itemize @end table Which parameters are accepted by the notification server can be checked via @code{notifications-get-capabilities}. This function returns a notification id, an integer, which can be used to manipulate the notification item with @code{notifications-close-notification} or the @code{:replaces-id} argument of another @code{notifications-notify} call. For example: @example @group (defun my-on-action-function (id key) (message "Message %d, key \"%s\" pressed" id key)) @result{} my-on-action-function @end group @group (defun my-on-close-function (id reason) (message "Message %d, closed due to \"%s\"" id reason)) @result{} my-on-close-function @end group @group (notifications-notify :title "Title" :body "This is important." :actions '("Confirm" "I agree" "Refuse" "I disagree") :on-action 'my-on-action-function :on-close 'my-on-close-function) @result{} 22 @end group @group A message window opens on the desktop. Press ``I agree''. @result{} Message 22, key "Confirm" pressed Message 22, closed due to "dismissed" @end group @end example @end defun @defun notifications-close-notification id &optional bus This function closes a notification with identifier @var{id}. @var{bus} can be a string denoting a D-Bus connection, the default is @code{:session}. @end defun @defun notifications-get-capabilities &optional bus Returns the capabilities of the notification server, a list of symbols. @var{bus} can be a string denoting a D-Bus connection, the default is @code{:session}. The following capabilities can be expected: @table @code @item :actions The server will provide the specified actions to the user. @item :body Supports body text. @item :body-hyperlinks The server supports hyperlinks in the notifications. @item :body-images The server supports images in the notifications. @item :body-markup Supports markup in the body text. @item :icon-multi The server will render an animation of all the frames in a given image array. @item :icon-static Supports display of exactly 1 frame of any given image array. This value is mutually exclusive with @code{:icon-multi}. @item :persistence The server supports persistence of notifications. @item :sound The server supports sounds on notifications. @end table Further vendor-specific caps start with @code{:x-vendor}, like @code{:x-gnome-foo-cap}. @end defun @defun notifications-get-server-information &optional bus Return information on the notification server, a list of strings. @var{bus} can be a string denoting a D-Bus connection, the default is @code{:session}. The returned list is @code{(@var{name} @var{vendor} @var{version} @var{spec-version})}. @table @var @item name The product name of the server. @item vendor The vendor name. For example, @samp{"KDE"}, @samp{"GNOME"}. @item version The server's version number. @item spec-version The specification version the server is compliant with. @end table If @var{spec_version} is @code{nil}, the server supports a specification prior to @samp{"1.0"}. @end defun @cindex tray notifications, MS-Windows When Emacs runs on MS-Windows as a GUI session, it supports a small subset of the D-Bus notifications functionality via a native primitive: @defun w32-notification-notify &rest params This function displays an MS-Windows tray notification as specified by @var{params}. MS-Windows tray notifications are displayed in a balloon from an icon in the notification area of the taskbar. Value is the integer unique ID of the notification that can be used to remove the notification using @code{w32-notification-close}, described below. If the function fails, the return value is @code{nil}. The arguments @var{params} are specified as keyword/value pairs. All the parameters are optional, but if no parameters are specified, the function will do nothing and return @code{nil}. The following parameters are supported: @table @code @item :icon @var{icon} Display @var{icon} in the system tray. If @var{icon} is a string, it should specify a file name from which to load the icon; the specified file should be a @file{.ico} Windows icon file. If @var{icon} is not a string, or if this parameter is not specified, the standard Emacs icon will be used. @item :tip @var{tip} Use @var{tip} as the tooltip for the notification. If @var{tip} is a string, this is the text of a tooltip that will be shown when the mouse pointer hovers over the tray icon added by the notification. If @var{tip} is not a string, or if this parameter is not specified, the default tooltip text is @samp{Emacs notification}. The tooltip text can be up to 127 characters long (63 on Windows versions before W2K). Longer strings will be truncated. @item :level @var{level} Notification severity level, one of @code{info}, @code{warning}, or @code{error}. If given, the value determines the icon displayed to the left of the notification title, but only if the @code{:title} parameter (see below) is also specified and is a string. @item :title @var{title} The title of the notification. If @var{title} is a string, it is displayed in a larger font immediately above the body text. The title text can be up to 63 characters long; longer text will be truncated. @item :body @var{body} The body of the notification. If @var{body} is a string, it specifies the text of the notification message. Use embedded newlines to control how the text is broken into lines. The body text can be up to 255 characters long, and will be truncated if it's longer. Unlike with D-Bus, the body text should be plain text, with no markup. @end table Note that versions of Windows before W2K support only @code{:icon} and @code{:tip}. The other parameters can be passed, but they will be ignored on those old systems. There can be at most one active notification at any given time. An active notification must be removed by calling @code{w32-notification-close} before a new one can be shown. @end defun To remove the notification and its icon from the taskbar, use the following function: @defun w32-notification-close id This function removes the tray notification given by its unique @var{id}. @end defun @node File Notifications @section Notifications on File Changes @cindex file notifications @cindex watch, for filesystem events Several operating systems support watching of filesystems for changes of files. If configured properly, Emacs links a respective library like @file{inotify}, @file{kqueue}, @file{gfilenotify}, or @file{w32notify} statically. These libraries enable watching of filesystems on the local machine. It is also possible to watch filesystems on remote machines, @pxref{Remote Files,, Remote Files, emacs, The GNU Emacs Manual} This does not depend on one of the libraries linked to Emacs. Since all these libraries emit different events on notified file changes, there is the Emacs library @code{filenotify} which provides a unified interface. Lisp programs that want to receive file notifications should always use this library in preference to the native ones. @defun file-notify-add-watch file flags callback Add a watch for filesystem events pertaining to @var{file}. This arranges for filesystem events pertaining to @var{file} to be reported to Emacs. The returned value is a descriptor for the added watch. Its type depends on the underlying library, it cannot be assumed to be an integer as in the example below. It should be used for comparison by @code{equal} only. If the @var{file} cannot be watched for some reason, this function signals a @code{file-notify-error} error. Sometimes, mounted filesystems cannot be watched for file changes. This is not detected by this function, a non-@code{nil} return value does not guarantee that changes on @var{file} will be notified. @var{flags} is a list of conditions to set what will be watched for. It can include the following symbols: @table @code @item change watch for file changes @item attribute-change watch for file attribute changes, like permissions or modification time @end table If @var{file} is a directory, changes for all files in that directory will be notified. This does not work recursively. When any event happens, Emacs will call the @var{callback} function passing it a single argument @var{event}, which is of the form @lisp (@var{descriptor} @var{action} @var{file} [@var{file1}]) @end lisp @var{descriptor} is the same object as the one returned by this function. @var{action} is the description of the event. It could be any one of the following symbols: @table @code @item created @var{file} was created @item deleted @var{file} was deleted @item changed @var{file}'s contents has changed; with @file{w32notify} library, reports attribute changes as well @item renamed @var{file} has been renamed to @var{file1} @item attribute-changed a @var{file} attribute was changed @item stopped watching @var{file} has been stopped @end table Note that the @file{w32notify} library does not report @code{attribute-changed} events. When some file's attribute, like permissions or modification time, has changed, this library reports a @code{changed} event. Likewise, the @file{kqueue} library does not report reliably file attribute changes when watching a directory. The @code{stopped} event reports, that watching the file has been stopped. This could be because @code{file-notify-rm-watch} was called (see below), or because the file being watched was deleted, or due to another error reported from the underlying library. @var{file} and @var{file1} are the name of the file(s) whose event is being reported. For example: @example @group (require 'filenotify) @result{} filenotify @end group @group (defun my-notify-callback (event) (message "Event %S" event)) @result{} my-notify-callback @end group @group (file-notify-add-watch "/tmp" '(change attribute-change) 'my-notify-callback) @result{} 35025468 @end group @group (write-region "foo" nil "/tmp/foo") @result{} Event (35025468 created "/tmp/.#foo") Event (35025468 created "/tmp/foo") Event (35025468 changed "/tmp/foo") Event (35025468 deleted "/tmp/.#foo") @end group @group (write-region "bla" nil "/tmp/foo") @result{} Event (35025468 created "/tmp/.#foo") Event (35025468 changed "/tmp/foo") Event (35025468 deleted "/tmp/.#foo") @end group @group (set-file-modes "/tmp/foo" (default-file-modes)) @result{} Event (35025468 attribute-changed "/tmp/foo") @end group @end example Whether the action @code{renamed} is returned, depends on the used watch library. Otherwise, the actions @code{deleted} and @code{created} could be returned in a random order. @example @group (rename-file "/tmp/foo" "/tmp/bla") @result{} Event (35025468 renamed "/tmp/foo" "/tmp/bla") @end group @group (delete-file "/tmp/bla") @result{} Event (35025468 deleted "/tmp/bla") @end group @end example @end defun @defun file-notify-rm-watch descriptor Removes an existing file watch specified by its @var{descriptor}. @var{descriptor} should be an object returned by @code{file-notify-add-watch}. @end defun @defun file-notify-valid-p descriptor Checks a watch specified by its @var{descriptor} for validity. @var{descriptor} should be an object returned by @code{file-notify-add-watch}. A watch can become invalid if the file or directory it watches is deleted, or if the watcher thread exits abnormally for any other reason. Removing the watch by calling @code{file-notify-rm-watch} also makes it invalid. @example @group (make-directory "/tmp/foo") @result{} Event (35025468 created "/tmp/foo") @end group @group (setq desc (file-notify-add-watch "/tmp/foo" '(change) 'my-notify-callback)) @result{} 11359632 @end group @group (file-notify-valid-p desc) @result{} t @end group @group (write-region "bla" nil "/tmp/foo/bla") @result{} Event (11359632 created "/tmp/foo/.#bla") Event (11359632 created "/tmp/foo/bla") Event (11359632 changed "/tmp/foo/bla") Event (11359632 deleted "/tmp/foo/.#bla") @end group @group ;; Deleting a file in the directory doesn't invalidate the watch. (delete-file "/tmp/foo/bla") @result{} Event (11359632 deleted "/tmp/foo/bla") @end group @group (write-region "bla" nil "/tmp/foo/bla") @result{} Event (11359632 created "/tmp/foo/.#bla") Event (11359632 created "/tmp/foo/bla") Event (11359632 changed "/tmp/foo/bla") Event (11359632 deleted "/tmp/foo/.#bla") @end group @group ;; Deleting the directory invalidates the watch. ;; Events arrive for different watch descriptors. (delete-directory "/tmp/foo" 'recursive) @result{} Event (35025468 deleted "/tmp/foo") Event (11359632 deleted "/tmp/foo/bla") Event (11359632 deleted "/tmp/foo") Event (11359632 stopped "/tmp/foo") @end group @group (file-notify-valid-p desc) @result{} nil @end group @end example @end defun @node Dynamic Libraries @section Dynamically Loaded Libraries @cindex dynamic libraries A @dfn{dynamically loaded library} is a library that is loaded on demand, when its facilities are first needed. Emacs supports such on-demand loading of support libraries for some of its features. @defvar dynamic-library-alist This is an alist of dynamic libraries and external library files implementing them. Each element is a list of the form @w{@code{(@var{library} @var{files}@dots{})}}, where the @code{car} is a symbol representing a supported external library, and the rest are strings giving alternate filenames for that library. Emacs tries to load the library from the files in the order they appear in the list; if none is found, the Emacs session won't have access to that library, and the features it provides will be unavailable. Image support on some platforms uses this facility. Here's an example of setting this variable for supporting images on MS-Windows: @example (setq dynamic-library-alist '((xpm "libxpm.dll" "xpm4.dll" "libXpm-nox4.dll") (png "libpng12d.dll" "libpng12.dll" "libpng.dll" "libpng13d.dll" "libpng13.dll") (jpeg "jpeg62.dll" "libjpeg.dll" "jpeg-62.dll" "jpeg.dll") (tiff "libtiff3.dll" "libtiff.dll") (gif "giflib4.dll" "libungif4.dll" "libungif.dll") (svg "librsvg-2-2.dll") (gdk-pixbuf "libgdk_pixbuf-2.0-0.dll") (glib "libglib-2.0-0.dll") (gobject "libgobject-2.0-0.dll"))) @end example Note that image types @code{pbm} and @code{xbm} do not need entries in this variable because they do not depend on external libraries and are always available in Emacs. Also note that this variable is not meant to be a generic facility for accessing external libraries; only those already known by Emacs can be loaded through it. This variable is ignored if the given @var{library} is statically linked into Emacs. @end defvar @node Security Considerations @section Security Considerations @cindex security @cindex hardening Like any application, Emacs can be run in a secure environment, where the operating system enforces rules about access and the like. With some care, Emacs-based applications can also be part of a security perimeter that checks such rules. Although the default settings for Emacs work well for a typical software development environment, they may require adjustment in environments containing untrusted users that may include attackers. Here is a compendium of security issues that may be helpful if you are developing such applications. It is by no means complete; it is intended to give you an idea of the security issues involved, rather than to be a security checklist. @table @asis @item File local variables @cindex file local variables A file that Emacs visits can contain variable settings that affect the buffer visiting that file; @xref{File Local Variables}. Similarly, a directory can specify local variable values common to all files in that directory; see @ref{Directory Local Variables}. Although Emacs takes some effort to protect against misuse of these variables, a security hole can be created merely by a package setting @code{safe-local-variable} too optimistically, a problem that is all too common. To disable this feature for both files and directories, set @code{enable-local-variables} to @code{nil}. @item Access control Although Emacs normally respects access permissions of the underlying operating system, in some cases it handles accesses specially. For example, file names can have handlers that treat the files specially, with their own access checking. @xref{Magic File Names}. Also, a buffer can be read-only even if the corresponding file is writeable, and vice versa, which can result in messages such as @samp{File passwd is write-protected; try to save anyway? (yes or no)}. @xref{Read Only Buffers}. @item Authentication Emacs has several functions that deal with passwords, e.g., @code{read-passwd}. @xref{Reading a Password}. Although these functions do not attempt to broadcast passwords to the world, their implementations are not proof against determined attackers with access to Emacs internals. For example, even if Elisp code uses @code{clear-string} to scrub a password from its memory after using it, remnants of the password may still reside in the garbage-collected free list. @xref{Modifying Strings}. @item Code injection Emacs can send commands to many other applications, and applications should take care that strings sent as operands of these commands are not misinterpreted as directives. For example, when using a shell command to rename a file @var{a} to @var{b}, do not simply use the string @code{mv @var{a} @var{b}}, because either file name might start with @samp{-}, or might contain shell metacharacters like @samp{;}. Although functions like @code{shell-quote-argument} can help avoid this sort of problem, they are not panaceas; for example, on a POSIX platform @code{shell-quote-argument} quotes shell metacharacters but not leading @samp{-}. @xref{Shell Arguments}. Typically it is safer to use @code{call-process} than a subshell. @xref{Synchronous Processes}. And it is safer yet to use builtin Emacs functions; for example, use @code{(rename-file "@var{a}" "@var{b}" t)} instead of invoking @command{mv}. @xref{Changing Files}. @item Coding systems Emacs attempts to infer the coding systems of the files and network connections it accesses. @xref{Coding Systems}. If Emacs infers incorrectly, or if the other parties to the network connection disagree with Emacs's inferences, the resulting system could be unreliable. Also, even when it infers correctly, Emacs often can use bytes that other programs cannot. For example, although to Emacs the null byte is just a character like any other, many other applications treat it as a string terminator and mishandle strings or files containing null bytes. @item Environment and configuration variables POSIX specifies several environment variables that can affect how Emacs behaves. Any environment variable whose name consists entirely of uppercase ASCII letters, digits, and the underscore may affect the internal behavior of Emacs. Emacs uses several such variables, e.g., @env{EMACSLOADPATH}. @xref{Library Search}. On some platforms some environment variables (e.g., @env{PATH}, @env{POSIXLY_CORRECT}, @env{SHELL}, @env{TMPDIR}) need to have properly-configured values in order to get standard behavior for any utility Emacs might invoke. Even seemingly-benign variables like @env{TZ} may have security implications. @xref{System Environment}. Emacs has customization and other variables with similar considerations. For example, if the variable @code{shell-file-name} specifies a shell with nonstandard behavior, an Emacs-based application may misbehave. @item Installation When Emacs is installed, if the installation directory hierarchy can be modified by untrusted users, the application cannot be trusted. This applies also to the directory hierarchies of the programs that Emacs uses, and of the files that Emacs reads and writes. @item Network access Emacs often accesses the network, and you may want to configure it to avoid network accesses that it would normally do. For example, unless you set @code{tramp-mode} to @code{nil}, file names using a certain syntax are interpreted as being network files, and are retrieved across the network. @xref{Top, The Tramp Manual,, tramp, The Tramp Manual}. @item Race conditions Emacs applications have the same sort of race-condition issues that other applications do. For example, even when @code{(file-readable-p "foo.txt")} returns @code{t}, it could be that @file{foo.txt} is unreadable because some other program changed the file's permissions between the call to @code{file-readable-p} and now. @xref{Testing Accessibility}. @item Resource limits When Emacs exhausts memory or other operating system resources, its behavior can be less reliable, in that computations that ordinarily run to completion may abort back to the top level. This may cause Emacs to neglect operations that it normally would have done. @end table