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[Emacs-diffs] Changes to emacs/lispref/os.texi [gnus-5_10-branch]
From: |
Miles Bader |
Subject: |
[Emacs-diffs] Changes to emacs/lispref/os.texi [gnus-5_10-branch] |
Date: |
Sat, 04 Sep 2004 08:30:06 -0400 |
Index: emacs/lispref/os.texi
diff -c /dev/null emacs/lispref/os.texi:1.64.2.1
*** /dev/null Sat Sep 4 12:02:45 2004
--- emacs/lispref/os.texi Sat Sep 4 12:01:14 2004
***************
*** 0 ****
--- 1,2148 ----
+ @c -*-texinfo-*-
+ @c This is part of the GNU Emacs Lisp Reference Manual.
+ @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2004
+ @c Free Software Foundation, Inc.
+ @c See the file elisp.texi for copying conditions.
+ @setfilename ../info/os
+ @node System Interface, Antinews, Calendar, Top
+ @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,
+ and flow control.
+
+ @xref{Building Emacs}, for related information. See also
+ @ref{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 Conversion:: Converting a time from numeric form to a string, or
+ to calendrical data (or vice versa).
+ * Time Calculations:: Adding, subtracting, comparing times, etc.
+ * Timers:: Setting a timer to call a function at a certain time.
+ * Terminal Input:: Recording terminal input for debugging.
+ * Terminal Output:: Recording terminal output for debugging.
+ * Sound Output:: Playing sounds on the computer's speaker.
+ * X11 Keysyms:: Operating on key symbols for X Windows
+ * Flow Control:: How to turn output flow control on or off.
+ * Batch Mode:: Running Emacs without terminal interaction.
+ * Session Management:: Saving and restoring state with X Session Management.
+ @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 (@file{.emacs}).
+ * 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
+ @cindex startup of Emacs
+ @cindex @file{startup.el}
+
+ The order of operations performed (in @file{startup.el}) by Emacs when
+ it is started up is as follows:
+
+ @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 these will be
+ scanned in their turn. The files @file{subdirs.el} are normally
+ generated automatically by Emacs installation.
+
+ @item
+ It sets the language environment and the terminal coding system,
+ if requested by environment variables such as @code{LANG}.
+
+ @item
+ It loads the initialization library for the window system, if you are
+ using a window system. This library's name is
+ @file{term/@var{windowsystem}-win.el}.
+
+ @item
+ It processes the initial options. (Some of them are handled
+ even earlier than this.)
+
+ @item
+ It initializes the window frame and faces, if appropriate.
+
+ @item
+ It runs the normal hook @code{before-init-hook}.
+
+ @item
+ It loads the library @file{site-start}, unless the option
+ @samp{-no-site-file} was specified. The library's file name is usually
+ @file{site-start.el}.
+ @cindex @file{site-start.el}
+
+ @item
+ It loads your init file (usually @file{~/.emacs}), unless @samp{-q},
+ @samp{-no-init-file}, or @samp{-batch} was specified on the command line.
+ The @samp{-u} option can specify another user whose home directory
+ should be used instead of @file{~}.
+
+ @item
+ It loads the library @file{default}, unless @code{inhibit-default-init}
+ is address@hidden (This is not done in @samp{-batch} mode or if
+ @samp{-q} was specified on the command line.) The library's file name
+ is usually @file{default.el}.
+ @cindex @file{default.el}
+
+ @item
+ It runs the normal hook @code{after-init-hook}.
+
+ @item
+ It sets the major mode according to @code{initial-major-mode}, provided
+ the buffer @samp{*scratch*} is still current and still in Fundamental
+ mode.
+
+ @item
+ It loads the terminal-specific Lisp file, if any, except when in batch
+ mode or using a window system.
+
+ @item
+ It displays the initial echo area message, unless you have suppressed
+ that with @code{inhibit-startup-echo-area-message}.
+
+ @item
+ It processes the action arguments from the command line.
+
+ @item
+ It runs @code{emacs-startup-hook} and then @code{term-setup-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}. @xref{Window Systems}.
+
+ @item
+ It displays copyleft, nonwarranty, and basic use information, provided
+ there were no remaining command-line arguments (a few steps above),
+ the value of @code{inhibit-startup-message} is @code{nil}, and the
+ buffer is still empty.
+ @end enumerate
+
+ @defopt inhibit-startup-message
+ This variable inhibits the initial startup messages (the nonwarranty,
+ etc.). If it is address@hidden, then the messages are not printed.
+
+ This variable exists so you can set it in your personal init file, once
+ you are familiar with the contents of the startup message. Do not set
+ this variable in the init file of a new user, or in a way that affects
+ more than one user, because that would prevent new users from receiving
+ the information they are supposed to see.
+ @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. 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
+
+ @node Init File
+ @subsection The Init File, @file{.emacs}
+ @cindex init file
+ @cindex @file{.emacs}
+
+ When you start Emacs, it normally attempts to load your @dfn{init
+ file}, a file in your home directory. Its normal name is @file{.emacs},
+ but you can alternatively call it @file{.emacs.el}, which enables you to
+ byte-compile it (@pxref{Byte Compilation}); then the actual file loaded
+ will be @file{.emacs.elc}.
+
+ The command-line switches @samp{-q} and @samp{-u} control whether and
+ where to find the init file; @samp{-q} says not to load an init file,
+ and @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 @code{LOGNAME} environment
+ variable, or the @code{USER} (most systems) or @code{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
+ A site may have a @dfn{default init file}, which is the library named
+ @file{default.el}. Emacs finds the @file{default.el} file through the
+ standard search path for libraries (@pxref{How Programs Do Loading}).
+ The Emacs distribution does not come with this file; sites may provide
+ one for local customizations. If the default init file exists, it is
+ loaded whenever you start Emacs, except in batch mode or if @samp{-q} is
+ specified. But your own personal init file, if any, is loaded first; if
+ it sets @code{inhibit-default-init} to a address@hidden value, then
+ Emacs does not subsequently load the @file{default.el} 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}.
+
+ @defvar 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.
+ @end defvar
+
+ @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
+ This variable prevents Emacs from loading the default initialization
+ library file for your session of Emacs. If its value is address@hidden,
+ then the default library is not loaded. 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
+ (the user's init file, @file{default.el}, and/or @file{site-start.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
+ (the user's init file, @file{default.el}, and/or @file{site-start.el}),
+ before loading the terminal-specific library and processing the
+ command-line action arguments.
+ @end defvar
+
+ @defvar emacs-startup-hook
+ @tindex emacs-startup-hook
+ This normal hook is run, once, just after handling the command line
+ arguments, just before @code{term-setup-hook}.
+ @end defvar
+
+ @defvar user-init-file
+ @tindex 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
+
+ @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 @code{TERM}).
+ Normally, @code{term-file-prefix} has the value
+ @code{"term/"}; changing this is not recommended. Emacs finds the file
+ in the normal manner, by searching the @code{load-path} directories, and
+ trying the @samp{.elc} and @samp{.el} suffixes.
+
+ The usual function 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{function-key-map} if the Termcap entry does not
+ specify all the terminal's function keys. @xref{Terminal Input}.
+
+ @cindex Termcap
+ When the name of the terminal type contains a hyphen, only the part of
+ the name before the first hyphen is significant in choosing the library
+ name. Thus, terminal types @samp{aaa-48} and @samp{aaa-30-rv} both use
+ the @file{term/aaa} library. If necessary, the library can evaluate
+ @code{(getenv "TERM")} to find the full name of the terminal
+ address@hidden
+
+ Your init file can prevent the loading of the
+ terminal-specific library by setting the variable
+ @code{term-file-prefix} to @code{nil}. This feature is useful when
+ experimenting with your own peculiar customizations.
+
+ You can also arrange to override some of the actions of the
+ terminal-specific library by setting the variable
+ @code{term-setup-hook}. This is a normal hook which Emacs runs using
+ @code{run-hooks} at the end of Emacs initialization, after loading both
+ your init file and any terminal-specific libraries. You can
+ use this variable to define initializations for terminals that do not
+ have their own libraries. @xref{Hooks}.
+
+ @defvar term-file-prefix
+ @cindex @code{TERM} environment variable
+ If the @code{term-file-prefix} variable is address@hidden, 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. To do this, put the following in
+ your init file: @code{(setq term-file-prefix nil)}.
+
+ On MS-DOS, if the environment variable @code{TERM} is not set, Emacs
+ uses @samp{internal} as the terminal type.
+ @end defvar
+
+ @defvar term-setup-hook
+ This variable is a normal hook that Emacs runs after loading your
+ init file, the default initialization file (if any) and the
+ terminal-specific Lisp file.
+
+ You can use @code{term-setup-hook} to override the definitions made by a
+ terminal-specific file.
+ @end defvar
+
+ See @code{window-setup-hook} in @ref{Window Systems}, for a related
+ feature.
+
+ @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. Since you do not need to start Emacs more than once per
+ day, and will often leave your Emacs session running longer than that,
+ command-line arguments are hardly ever used. As a practical matter, it
+ is best to avoid making the habit of using them, since this habit would
+ encourage you to kill and restart Emacs unnecessarily often. These
+ options exist for two reasons: to be compatible with other editors (for
+ invocation by other programs) and to enable shell scripts to run
+ specific Lisp programs.
+
+ This section describes how Emacs processes command-line arguments,
+ and how you can customize them.
+
+ @ignore
+ (Note that some other editors require you to start afresh each time
+ you want to edit a file. With this kind of editor, you will probably
+ specify the file as a command-line argument. The recommended way to
+ use GNU Emacs is to start it only once, just after you log in, and do
+ all your editing in the same Emacs process. Each time you want to edit
+ a different file, you visit it with the existing Emacs, which eventually
+ comes to have many files in it ready for editing. Usually you do not
+ kill the Emacs until you are about to log out.)
+ @end ignore
+
+ @defun command-line
+ This function parses the command line that Emacs was called with,
+ processes it, 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}, 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
+ The value of this variable is an alist of user-defined command-line
+ options and associated handler functions. This variable exists so you
+ can add elements to it.
+
+ A @dfn{command-line option} is an argument on the command line, which
+ has the form:
+
+ @example
+ address@hidden
+ @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}. (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{Command
+ Arguments, , Command Line Arguments, 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-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 address@hidden
+ 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 address@hidden 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 used
+ 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. As a practical matter, you seldom kill
+ Emacs---only when you are about to log out. Suspending is much more
+ common.
+
+ @menu
+ * Killing Emacs:: Exiting Emacs irreversibly.
+ * Suspending Emacs:: Exiting Emacs reversibly.
+ @end menu
+
+ @node Killing Emacs
+ @comment node-name, next, previous, up
+ @subsection Killing Emacs
+ @cindex killing Emacs
+
+ Killing Emacs means ending the execution of the Emacs process. The
+ parent process normally resumes control. The low-level primitive for
+ killing Emacs is @code{kill-emacs}.
+
+ @defun kill-emacs &optional exit-data
+ This function exits the Emacs process and kills it.
+
+ If @var{exit-data} is an integer, then it 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 defun
+
+ All the information in the Emacs process, aside from files that have
+ been saved, is lost when the Emacs process is killed. Because killing
+ Emacs inadvertently can lose a lot of work, Emacs queries for
+ confirmation before actually terminating if you have buffers that need
+ saving or subprocesses that are running. This is done in the function
+ @code{save-buffers-kill-emacs}, the higher level function from which
+ @code{kill-emacs} is usually called.
+
+ @defvar kill-emacs-query-functions
+ After asking the standard questions, @code{save-buffers-kill-emacs}
+ calls the functions in the list @code{kill-emacs-query-functions}, in
+ order of appearance, with no arguments. These functions 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 defvar
+
+ @defvar kill-emacs-hook
+ This variable is a normal hook; once @code{save-buffers-kill-emacs} is
+ finished with all file saving and confirmation, it calls
+ @code{kill-emacs} which runs the functions in this hook.
+ @code{kill-emacs} does not run this hook in batch mode.
+
+ @code{kill-emacs} may be invoked directly (that is not via
+ @code{save-buffers-kill-emacs}) if the terminal is disconnected, or in
+ similar situations where interaction with the user is not possible.
+ Thus, if your hook needs to interact with the user, put it on
+ @code{kill-emacs-query-functions}; if it needs to run regardless of
+ how Emacs is killed, put it on @code{kill-emacs-hook}.
+ @end defvar
+
+ @node Suspending Emacs
+ @subsection Suspending Emacs
+ @cindex suspending Emacs
+
+ @dfn{Suspending Emacs} 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}.
+
+ Some operating systems 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.
+
+ Suspension is not useful with window systems, because the Emacs job
+ may not have a parent that can resume it again, and in any case you can
+ give input to some other job such as a shell merely by moving to a
+ different window. Therefore, suspending is not allowed when Emacs is using
+ a window system (X or MS Windows).
+
+ @defun 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.
+
+ If @var{string} is address@hidden, its characters are sent to be read
+ as terminal input by Emacs's superior shell. 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 address@hidden
+ (@pxref{Refresh Screen}).
+
+ In the following example, note that @samp{pwd} is not echoed after
+ Emacs is suspended. But it is read and executed by the shell.
+
+ @smallexample
+ @group
+ (suspend-emacs)
+ @result{} nil
+ @end group
+
+ @group
+ (add-hook 'suspend-hook
+ (function (lambda ()
+ (or (y-or-n-p
+ "Really suspend? ")
+ (error "Suspend canceled")))))
+ @result{} (lambda nil
+ (or (y-or-n-p "Really suspend? ")
+ (error "Suspend canceled")))
+ @end group
+ @group
+ (add-hook 'suspend-resume-hook
+ (function (lambda () (message "Resumed!"))))
+ @result{} (lambda nil (message "Resumed!"))
+ @end group
+ @group
+ (suspend-emacs "pwd")
+ @result{} nil
+ @end group
+ @group
+ ---------- Buffer: Minibuffer ----------
+ Really suspend? @kbd{y}
+ ---------- Buffer: Minibuffer ----------
+ @end group
+
+ @group
+ ---------- Parent Shell ----------
+ lewis@@slug[23] % /user/lewis/manual
+ lewis@@slug[24] % fg
+ @end group
+
+ @group
+ ---------- Echo Area ----------
+ Resumed!
+ @end group
+ @end smallexample
+ @end defun
+
+ @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
+
+ @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 GNU configuration name for the hardware/software
+ configuration of your system, as a string. The convenient way to test
+ parts of this string is with @code{string-match}.
+ @end defvar
+
+ @defvar system-type
+ The value of this variable is a symbol indicating the type of operating
+ system Emacs is operating on. Here is a table of the possible values:
+
+ @table @code
+ @item alpha-vms
+ VMS on the Alpha.
+
+ @item aix-v3
+ AIX.
+
+ @item berkeley-unix
+ Berkeley BSD.
+
+ @item cygwin
+ Cygwin.
+
+ @item dgux
+ Data General DGUX operating system.
+
+ @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 hpux
+ Hewlett-Packard HPUX operating system.
+
+ @item irix
+ Silicon Graphics Irix system.
+
+ @item ms-dos
+ Microsoft MS-DOS ``operating system.'' Emacs compiled with DJGPP for
+ MS-DOS binds @code{system-type} to @code{ms-dos} even when you run it on
+ MS-Windows.
+
+ @item next-mach
+ NeXT Mach-based system.
+
+ @item rtu
+ Masscomp RTU, UCB universe.
+
+ @item unisoft-unix
+ UniSoft UniPlus.
+
+ @item usg-unix-v
+ AT&T System V.
+
+ @item vax-vms
+ VAX VMS.
+
+ @item windows-nt
+ Microsoft windows NT. The same executable supports Windows 9X, but the
+ value of @code{system-type} is @code{windows-nt} in either case.
+
+ @item xenix
+ SCO Xenix 386.
+ @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. We recommend using
+ @code{system-configuration} to distinguish between different operating
+ systems.
+ @end defvar
+
+ @defun system-name
+ This function returns the name of the machine you are running on.
+ @example
+ (system-name)
+ @result{} "www.gnu.org"
+ @end example
+ @end defun
+
+ The symbol @code{system-name} is a variable as well as a function. In
+ fact, the function returns whatever value the variable
+ @code{system-name} currently holds. Thus, you can set the variable
+ @code{system-name} in case Emacs is confused about the name of your
+ system. The variable is also useful for constructing frame titles
+ (@pxref{Frame Titles}).
+
+ @defvar mail-host-address
+ If this variable is address@hidden, 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}. (Since this is
+ done when Emacs starts up, the value actually used is the one saved when
+ Emacs was dumped. @xref{Building Emacs}.)
+ @end defvar
+
+ @deffn Command getenv var
+ @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}. If returns
+ @samp{""} if @var{var} is set but null. Within Emacs, the environment
+ variable values are kept in the Lisp variable @code{process-environment}.
+
+ @example
+ @group
+ (getenv "USER")
+ @result{} "lewis"
+ @end group
+
+ @group
+ lewis@@slug[10] % printenv
+ PATH=.:/user/lewis/bin:/usr/bin:/usr/local/bin
+ USER=lewis
+ @end group
+ @group
+ TERM=ibmapa16
+ SHELL=/bin/csh
+ HOME=/user/lewis
+ @end group
+ @end example
+ @end deffn
+
+ @c Emacs 19 feature
+ @deffn Command setenv variable &optional value
+ 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}, @code{setenv} removes @var{variable} from the environment.
+ Otherwise, @var{value} should be a string.
+
+ @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{} ("l=/usr/stanford/lib/gnuemacs/lisp"
+ "PATH=.:/user/lewis/bin:/usr/class:/nfsusr/local/bin"
+ "USER=lewis"
+ @end group
+ @group
+ "TERM=ibmapa16"
+ "SHELL=/bin/csh"
+ "HOME=/user/lewis")
+ @end group
+ @end smallexample
+
+ If @code{process-environment} contains ``duplicate'' elements that
+ specify the same environment variable, the first of these elements
+ specifies the variable, and the other ``duplicates'' are ignored.
+ @end defvar
+
+ @defvar path-separator
+ This variable holds a string which 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-DOS
+ and MS-Windows.
+ @end defvar
+
+ @defun parse-colon-path path
+ @tindex parse-colon-path
+ This function takes a search path string such as would be the value of
+ the @code{PATH} environment variable, and splits it at the separators,
+ returning a list of directory names. @code{nil} in this list stands for
+ ``use 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 from which the Emacs executable was
+ invoked, or perhaps @code{nil} if that directory cannot be determined.
+ @end defvar
+
+ @defvar installation-directory
+ If address@hidden, this is a directory within which to look for the
+ @file{lib-src} and @file{etc} subdirectories. This is address@hidden
+ 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.
+ @end defvar
+
+ @defun load-average &optional use-float
+ This function returns the current 1-minute, 5-minute, and 15-minute load
+ averages, in a list.
+
+ By default, the values are integers that are 100 times the system load
+ averages, which indicate the average number of processes trying to run.
+ If @var{use-float} is address@hidden, then they are returned
+ as floating point numbers and 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.
+
+ 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
+
+ @group
+ lewis@@rocky[5] % uptime
+ 11:55am up 1 day, 19:37, 3 users,
+ load average: 1.69, 0.48, 0.36
+ @end group
+ @end example
+ @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.
+ The value is @code{nil} if Emacs is running under a window system.
+ @end defvar
+
+ @defun setprv privilege-name &optional setp getprv
+ This function sets or resets a VMS privilege. (It does not exist on
+ other systems.) The first argument is the privilege name, as a string.
+ The second argument, @var{setp}, is @code{t} or @code{nil}, indicating
+ whether the privilege is to be turned on or off. Its default is
+ @code{nil}. The function returns @code{t} if successful, @code{nil}
+ otherwise.
+
+ If the third argument, @var{getprv}, is address@hidden, @code{setprv}
+ does not change the privilege, but returns @code{t} or @code{nil}
+ indicating whether the privilege is currently enabled.
+ @end defun
+
+ @node User Identification
+ @section 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}
+ option was used, then Lisp packages should not load any customization
+ files or user profile.
+ @end defvar
+
+ @defvar user-mail-address
+ This holds the nominal email address of the user who is using Emacs.
+ Emacs normally sets this variable to a default value after reading your
+ init files, but not if you have already set it. So you can set the
+ variable to some other value in your init file if you do not
+ want to use the default value.
+ @end defvar
+
+ @defun user-login-name &optional uid
+ If you don't specify @var{uid}, this function returns the name under
+ which the user is logged in. If the environment variable @code{LOGNAME}
+ is set, that value is used. Otherwise, if the environment variable
+ @code{USER} is set, that value is used. Otherwise, the value is based
+ on the effective @acronym{UID}, not the real @acronym{UID}.
+
+ If you specify @var{uid}, the value is the user name that corresponds
+ to @var{uid} (which should be an integer), or @code{nil} if there is
+ no such user.
+
+ @example
+ @group
+ (user-login-name)
+ @result{} "lewis"
+ @end group
+ @end example
+ @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 ignores the
+ environment variables @code{LOGNAME} and @code{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 @code{NAME}, if that is set.
+
+ @c "Bil" is the correct spelling.
+ @example
+ @group
+ (user-full-name)
+ @result{} "Bil Lewis"
+ @end group
+ @end example
+
+ If the Emacs job's user-id does not correspond to any known user (and
+ provided @code{NAME} is not set), the value is @code{"unknown"}.
+
+ If @var{uid} is address@hidden, 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}).
+
+ @defun user-real-uid
+ This function returns the real @acronym{UID} of the user.
+ The value may be a floating point number.
+
+ @example
+ @group
+ (user-real-uid)
+ @result{} 19
+ @end group
+ @end example
+ @end defun
+
+ @defun user-uid
+ This function returns the effective @acronym{UID} of the user.
+ The value may be a floating point number.
+ @end defun
+
+ @node Time of Day
+ @section Time of Day
+
+ This section explains how to determine the current time and the time
+ zone.
+
+ @defun current-time-string &optional time-value
+ This function returns the current time and date as a human-readable
+ string. The format of the string is unvarying; the number of characters
+ used for each part is always the same, so you can reliably use
+ @code{substring} to extract pieces of it. It is wise to count the
+ characters from the beginning of the string rather than from the end, as
+ additional information may some day be added at the end.
+
+ @c Emacs 19 feature
+ The argument @var{time-value}, if given, specifies a time to format
+ instead of the current time. The argument should be a list whose first
+ two elements are integers. Thus, you can use times obtained from
+ @code{current-time} (see below) and from @code{file-attributes}
+ (@pxref{Definition of file-attributes}). @var{time-value} can also be
+ a cons of two integers, but this is considered obsolete.
+
+ @example
+ @group
+ (current-time-string)
+ @result{} "Wed Oct 14 22:21:05 1987"
+ @end group
+ @end example
+ @end defun
+
+ @c Emacs 19 feature
+ @defun current-time
+ This function returns the system's time value as a list of three
+ integers: @code{(@var{high} @var{low} @var{microsec})}. The integers
+ @var{high} and @var{low} combine to give the number of seconds since
+ 0:00 January 1, 1970 UTC (Coordinated Universal Time), which is
+ @ifnottex
+ @var{high} * 2**16 + @var{low}.
+ @end ifnottex
+ @tex
+ $high*2^{16}+low$.
+ @end tex
+
+ The third element, @var{microsec}, gives the microseconds since the
+ start of the current second (or 0 for systems that return time with
+ the resolution of only one second).
+
+ The first two elements can be compared with file time values such as you
+ get with the function @code{file-attributes}.
+ @xref{Definition of file-attributes}.
+ @end defun
+
+ @c Emacs 19 feature
+ @defun current-time-zone &optional time-value
+ This function returns a list describing the time zone that the user is
+ in.
+
+ The value has the form @code{(@var{offset} @var{name})}. Here
+ @var{offset} is an integer giving the number of seconds ahead of UTC
+ (east of Greenwich). A negative value means west of Greenwich. The
+ second element, @var{name}, is a string giving the name of the time
+ zone. Both elements change when daylight savings 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-value}, if given, specifies a time to analyze
+ instead of the current time. The argument should have the same form
+ as for @code{current-time-string} (see above). Thus, you can use
+ times obtained from @code{current-time} (see above) and from
+ @code{file-attributes}. @xref{Definition of file-attributes}.
+ @end defun
+
+ @defun set-time-zone-rule tz
+ This function specifies the local time zone according to @var{tz}. If
+ @var{tz} is @code{nil}, that means to use an implementation-defined
+ default time zone. If @var{tz} is @code{t}, that means to use
+ Universal Time. Otherwise, @var{tz} should be a string specifying a
+ time zone rule.
+ @end defun
+
+ @defun float-time &optional time-value
+ This function returns the current time as a floating-point number of
+ seconds since the epoch. The argument @var{time-value}, if given,
+ specifies a time to convert instead of the current time. The argument
+ should have the same form as for @code{current-time-string} (see
+ above). Thus, it accepts the output of @code{current-time} and
+ @code{file-attributes}.
+
+ @emph{Warning}: Since the result is floating point, it may not be
+ exact. Do not use this function if precise time stamps are required.
+ @end defun
+
+ @node Time Conversion
+ @section Time Conversion
+
+ These functions convert time values (lists of two or three integers)
+ to strings or to calendrical information. There is also a function to
+ convert calendrical information to a time value. You can get time
+ values from the functions @code{current-time} (@pxref{Time of Day}) and
+ @code{file-attributes} (@pxref{Definition of file-attributes}).
+
+ Many operating systems are limited to time values that contain 32 bits
+ of information; these systems typically handle only the times from
+ 1901-12-13 20:45:52 UTC through 2038-01-19 03:14:07 UTC. However, some
+ operating systems have larger time values, 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 address@hidden
+
+ @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 universal
+ This function converts @var{time} (or the current time, if @var{time} is
+ omitted) to a string according to @var{format-string}. 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 has a locale-specific meaning. In the default locale (named C), it
+ is equivalent to @samp{%A, %B %e, %Y}.
+ @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 %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 %p
+ This stands for @samp{AM} or @samp{PM}, as appropriate.
+ @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 seconds (00-59).
+ @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 week of the year (01-52), assuming that weeks
+ start on Sunday.
+ @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.
+ @end table
+
+ 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 the middle of a @samp{%}-sequences. If you
+ start the field width with @samp{0}, it means to pad with zeros. If you
+ start the field width with @samp{_}, it means to pad with spaces.
+
+ 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 between
+ @samp{%} and one of the letters in the table above. @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.
+
+ If @var{universal} is address@hidden, that means to describe the time as
+ Universal Time; @code{nil} means describe it using what Emacs believes
+ is the local time zone (see @code{current-time-zone}).
+
+ This function uses the C library function @code{strftime} 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 seconds-to-time seconds
+ This function converts @var{seconds}, a floating point number of
+ seconds since the epoch, to a time value and returns that. To perform
+ the inverse conversion, use @code{float-time}.
+ @end defun
+
+ @defun decode-time &optional time
+ This function converts a time value into calendrical information. If
+ you don't specify @var{time}, it decodes the current time. 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{zone})
+ @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 savings time is effect, otherwise @code{nil}.
+ @item zone
+ An integer indicating the time zone, as the number of seconds east of
+ Greenwich.
+ @end table
+
+ @strong{Common Lisp Note:} Common Lisp has different meanings for
+ @var{dow} and @var{zone}.
+ @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 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 and
+ its daylight savings time rules. If specified, it can be either a list
+ (as you would get from @code{current-time-zone}), a string as in the
+ @code{TZ} environment variable, @code{t} for Universal Time, or an
+ integer (as you would get from @code{decode-time}). The specified
+ zone is used without any further alteration for daylight savings time.
+
+ 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 Calculations
+ @section Time Calculations
+
+ These functions perform calendrical computations using time values
+ (the kind of list that @code{current-time} returns).
+
+ @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, in the same format as a time value.
+ @end defun
+
+ @defun time-add t1 t2
+ This returns the sum of two time values, one of which ought to
+ represent a time difference rather than a point in time.
+ Here is how to add a number of seconds to a time value:
+
+ @example
+ (time-add @var{time} (seconds-to-time @var{seconds}))
+ @end example
+ @end defun
+
+ @defun time-to-days time
+ This function returns the number of days between the beginning of year
+ 1 and @var{time}.
+ @end defun
+
+ @defun time-to-day-in-year time
+ This returns the day number within the year corresponding to @var{time}.
+ @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 timer
+
+ You can set up a @dfn{timer} to call a function at a specified
+ future time or after a certain length of idleness.
+
+ 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.
+
+ @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 also runs every @var{repeat}
+ seconds after that. 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 in a wide variety of formats; this
+ function tries to accept all the commonly used date formats. The most
+ convenient formats are strings. Valid such formats include these two,
+
+ @example
+ @address@hidden@var{day} @var{hour}:@var{min}:@var{sec} @var{timezone}
+
+ @var{hour}:@var{min}:@var{sec} @var{timezone} @var{month}/@var{day}/@var{year}
+ @end example
+
+ @noindent
+ where in both examples all fields are numbers; the format that
+ @code{current-time-string} returns is also allowed, and many others
+ as well.
+
+ 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.
+
+ In most cases, @var{repeat} has no effect on when @emph{first} call
+ takes address@hidden 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
+
+ @defmac with-timeout (seconds address@hidden) address@hidden
+ 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}.
+
+ @deffn Command run-with-idle-timer secs repeat function &rest args
+ Set up a timer which runs when Emacs has been idle for @var{secs}
+ seconds. The value of @var{secs} may be an integer or a floating point
+ number.
+
+ 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
+ address@hidden, 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} (see below).
+ @end deffn
+
+ @cindex idleness
+ Emacs becomes ``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 address@hidden, 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.
+
+ @defun cancel-timer timer
+ Cancel the requested action for @var{timer}, which should be a value
+ 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
+
+ @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.
+ * Translating Input:: Low level conversion of some characters or events
+ into others.
+ * 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-null, 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 address@hidden, 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. @xref{Flow Control}.
+
+ @c Emacs 19 feature
+ 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.
+
+ @c Emacs 19 feature
+ If @var{quit-char} is address@hidden, 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.
+
+ @c Emacs 19 feature
+ @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 address@hidden when Emacs is using interrupt-driven input. If
+ @code{nil}, Emacs is using @sc{cbreak} mode.
+ @item flow
+ is address@hidden 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 Translating Input
+ @subsection Translating Input Events
+ @cindex translating input events
+
+ This section describes features for translating input events into
+ other input events before they become part of key sequences. These
+ features apply to each event in the order they are described here: each
+ event is first modified according to @code{extra-keyboard-modifiers},
+ then translated through @code{keyboard-translate-table} (if applicable),
+ and finally decoded with the specified keyboard coding system. If it is
+ being read as part of a key sequence, it is then added to the sequence
+ being read; then subsequences containing it are checked first with
+ @code{function-key-map} and then with @code{key-translation-map}.
+
+ @c Emacs 19 feature
+ @defvar extra-keyboard-modifiers
+ This variable lets Lisp programs ``press'' the modifier keys on the
+ keyboard. The value is a character. Only the modifiers of the
+ character matter. Each time the user types a keyboard key, it is
+ altered as if those modifier keys were held down. For instance, if
+ you bind @code{extra-keyboard-modifiers} to @code{?\C-\M-a}, then all
+ keyboard input characters typed during the scope of the binding will
+ have the control and meta modifiers applied to them. The character
+ @code{?\C-@@}, equivalent to the integer 0, does not count as a control
+ character for this purpose, but as a character with no modifiers.
+ Thus, setting @code{extra-keyboard-modifiers} to zero cancels any
+ modification.
+
+ When using a window system, the program can ``press'' any of the
+ modifier keys in this way. Otherwise, only the @key{CTL} and @key{META}
+ keys can be virtually pressed.
+
+ Note that this variable applies only to events that really come from
+ the keyboard, and has no effect on mouse events or any other events.
+ @end defvar
+
+ @defvar keyboard-translate-table
+ This variable is the translate table for keyboard characters. It lets
+ you reshuffle the keys on the keyboard without changing any command
+ bindings. Its value is normally a char-table, or else @code{nil}.
+ (It can also be a string or vector, but this is considered obsolete.)
+
+ If @code{keyboard-translate-table} is a char-table
+ (@pxref{Char-Tables}), then each character read from the keyboard is
+ looked up in this char-table. If the value found there is
+ address@hidden, then it is used instead of the actual input character.
+
+ In the example below, we set @code{keyboard-translate-table} to a
+ char-table. Then we fill it in to swap the characters @kbd{C-s} and
+ @kbd{C-\} and the characters @kbd{C-q} and @kbd{C-^}. Subsequently,
+ typing @kbd{C-\} has all the usual effects of typing @kbd{C-s}, and vice
+ versa. (@xref{Flow Control}, for more information on this subject.)
+
+ @cindex flow control example
+ @example
+ @group
+ (defun evade-flow-control ()
+ "Replace C-s with C-\ and C-q with C-^."
+ (interactive)
+ @end group
+ @group
+ (setq keyboard-translate-table
+ (make-char-table 'keyboard-translate-table nil))
+ @end group
+ @group
+ ;; @r{Swap @kbd{C-s} and @kbd{C-\}.}
+ (aset keyboard-translate-table ?\034 ?\^s)
+ (aset keyboard-translate-table ?\^s ?\034)
+ @end group
+ @group
+ ;; @r{Swap @kbd{C-q} and @kbd{C-^}.}
+ (aset keyboard-translate-table ?\036 ?\^q)
+ (aset keyboard-translate-table ?\^q ?\036))
+ @end group
+ @end example
+
+ Note that this translation is the first thing that happens to a
+ character after it is read from the terminal. Record-keeping features
+ such as @code{recent-keys} and dribble files record the characters after
+ translation.
+
+ Note also that this translation is done before the characters are
+ supplied to input methods (@pxref{Input Methods}). Use
+ @code{translation-table-for-input} (@pxref{Translation of Characters}),
+ if you want to translate characters after input methods operate.
+ @end defvar
+
+ @defun keyboard-translate from to
+ This function modifies @code{keyboard-translate-table} to translate
+ character code @var{from} into character code @var{to}. It creates
+ the keyboard translate table if necessary.
+ @end defun
+
+ The remaining translation features translate subsequences of key
+ sequences being read. They are implemented in @code{read-key-sequence}
+ and have no effect on input read with @code{read-event}.
+
+ @defvar function-key-map
+ This variable holds a keymap that describes the character sequences sent
+ by function keys on an ordinary character terminal. This keymap has the
+ same structure as other keymaps, but is used differently: it specifies
+ translations to make while reading key sequences, rather than bindings
+ for key sequences.
+
+ If @code{function-key-map} ``binds'' a key sequence @var{k} to a vector
+ @var{v}, then when @var{k} appears as a subsequence @emph{anywhere} in a
+ key sequence, it is replaced with the events in @var{v}.
+
+ For example, VT100 terminals send @address@hidden O P} when the
+ keypad @key{PF1} key is pressed. Therefore, we want Emacs to translate
+ that sequence of events into the single event @code{pf1}. We accomplish
+ this by ``binding'' @address@hidden O P} to @code{[pf1]} in
+ @code{function-key-map}, when using a VT100.
+
+ Thus, typing @kbd{C-c @key{PF1}} sends the character sequence @kbd{C-c
+ @key{ESC} O P}; later the function @code{read-key-sequence} translates
+ this back into @kbd{C-c @key{PF1}}, which it returns as the vector
+ @code{[?\C-c pf1]}.
+
+ Entries in @code{function-key-map} are ignored if they conflict with
+ bindings made in the minor mode, local, or global keymaps. The intent
+ is that the character sequences that function keys send should not have
+ command bindings in their own right---but if they do, the ordinary
+ bindings take priority.
+
+ The value of @code{function-key-map} is usually set up automatically
+ according to the terminal's Terminfo or Termcap entry, but sometimes
+ those need help from terminal-specific Lisp files. Emacs comes with
+ terminal-specific files for many common terminals; their main purpose is
+ to make entries in @code{function-key-map} beyond those that can be
+ deduced from Termcap and Terminfo. @xref{Terminal-Specific}.
+ @end defvar
+
+ @defvar key-translation-map
+ This variable is another keymap used just like @code{function-key-map}
+ to translate input events into other events. It differs from
+ @code{function-key-map} in two ways:
+
+ @itemize @bullet
+ @item
+ @code{key-translation-map} goes to work after @code{function-key-map} is
+ finished; it receives the results of translation by
+ @code{function-key-map}.
+
+ @item
+ Non-prefix bindings in @code{key-translation-map} override actual key
+ bindings. For example, if @kbd{C-x f} has a non-prefix binding in
+ @code{key-translation-map}, that translation takes effect even though
+ @kbd{C-x f} also has a key binding in the global map.
+ @end itemize
+
+ Note however that actual key bindings can have an effect on
+ @code{key-translation-map}, even though they are overridden by it.
+ Indeed, actual key bindings override @code{function-key-map} and thus
+ may alter the key sequence that @code{key-translation-map} receives.
+ Clearly, it is better to avoid to avoid this type of situation.
+
+ The intent of @code{key-translation-map} is for users to map one
+ character set to another, including ordinary characters normally bound
+ to @code{self-insert-command}.
+ @end defvar
+
+ @cindex key translation function
+ You can use @code{function-key-map} or @code{key-translation-map} for
+ more than simple aliases, by using a function, instead of a key
+ sequence, as the ``translation'' of a key. Then this function is called
+ to compute the translation of that key.
+
+ The key translation function receives one argument, which is the prompt
+ that was specified in @code{read-key-sequence}---or @code{nil} if the
+ key sequence is being read by the editor command loop. In most cases
+ you can ignore the prompt value.
+
+ If the function reads input itself, it can have the effect of altering
+ the event that follows. For example, here's how to define @kbd{C-c h}
+ to turn the character that follows into a Hyper character:
+
+ @example
+ @group
+ (defun hyperify (prompt)
+ (let ((e (read-event)))
+ (vector (if (numberp e)
+ (logior (lsh 1 24) e)
+ (if (memq 'hyper (event-modifiers e))
+ e
+ (add-event-modifier "H-" e))))))
+
+ (defun add-event-modifier (string e)
+ (let ((symbol (if (symbolp e) e (car e))))
+ (setq symbol (intern (concat string
+ (symbol-name symbol))))
+ @end group
+ @group
+ (if (symbolp e)
+ symbol
+ (cons symbol (cdr e)))))
+
+ (define-key function-key-map "\C-ch" 'hyperify)
+ @end group
+ @end example
+
+ Finally, if you have enabled keyboard character set decoding using
+ @code{set-keyboard-coding-system}, decoding is done after the
+ translations listed above. @xref{Terminal I/O Encoding}. In future
+ Emacs versions, character set decoding may be done before the other
+ translations.
+
+ @node Recording Input
+ @subsection Recording Input
+
+ @defun recent-keys
+ This function returns a vector containing the last 100 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
+ 100 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{}>}.
+
+ You close the dribble file by calling this function with an argument
+ of @code{nil}.
+
+ This function is normally used to record the input necessary to
+ trigger an Emacs bug, for the sake of a bug report.
+
+ @example
+ @group
+ (open-dribble-file "~/dribble")
+ @result{} nil
+ @end group
+ @end example
+ @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 the 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.
+
+ @defvar 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---even when using a window system. (We designed it
+ this way despite the fact that a window system has no true ``output
+ speed'', to give you a way to tune these decisions.)
+
+ The value is measured in baud.
+ @end defvar
+
+ 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 baud-rate
+ This obsolete function returns the value of the variable
+ @code{baud-rate}.
+ @end defun
+
+ @defun send-string-to-terminal string
+ This function sends @var{string} to the terminal without alteration.
+ Control characters in @var{string} have terminal-dependent effects.
+
+ 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.
+
+ 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}.
+
+ @example
+ @group
+ (open-termscript "../junk/termscript")
+ @result{} nil
+ @end group
+ @end example
+ @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. Emacs version 20 and
+ earlier did not support sound at all.
+
+ The sound must be stored as a file in RIFF-WAVE format (@samp{.wav})
+ or Sun Audio format (@samp{.au}).
+
+ @tindex play-sound
+ @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
+
+ @defun play-sound-file file &optional volume device
+ @tindex play-sound-file
+ This function is an alternative interface to playing a sound @var{file}
+ specifying an optional @var{volume} and @var{device}.
+ @end defun
+
+ @tindex play-sound-functions
+ @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
+
+ 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
+ -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
+ -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 Displays}.
+ @end defvar
+
+ You can specify which keysyms Emacs should use for the Meta, Alt, Hyper, and
Super modifiers by setting these variables:
+
+ @defvar 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 Alt modifier
+ (respectively, for 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 Flow Control
+ @section Flow Control
+ @cindex flow control characters
+
+ This section attempts to answer the question ``Why does Emacs use
+ flow-control characters in its command character set?'' For a second
+ view on this issue, read the comments on flow control in the
+ @file{emacs/INSTALL} file from the distribution; for help with Termcap
+ entries and DEC terminal concentrators, see @file{emacs/etc/TERMS}.
+
+ @cindex @kbd{C-s}
+ @cindex @kbd{C-q}
+ At one time, most terminals did not need flow control, and none used
+ @code{C-s} and @kbd{C-q} for flow control. Therefore, the choice of
+ @kbd{C-s} and @kbd{C-q} as command characters for searching and quoting
+ was natural and uncontroversial. With so many commands needing key
+ assignments, of course we assigned meanings to nearly all @acronym{ASCII}
+ control characters.
+
+ Later, some terminals were introduced which required these characters
+ for flow control. They were not very good terminals for full-screen
+ editing, so Emacs maintainers ignored them. In later years, flow
+ control with @kbd{C-s} and @kbd{C-q} became widespread among terminals,
+ but by this time it was usually an option. And the majority of Emacs
+ users, who can turn flow control off, did not want to switch to less
+ mnemonic key bindings for the sake of flow control.
+
+ So which usage is ``right''---Emacs's or that of some terminal and
+ concentrator manufacturers? This question has no simple answer.
+
+ One reason why we are reluctant to cater to the problems caused by
+ @kbd{C-s} and @kbd{C-q} is that they are gratuitous. There are other
+ techniques (albeit less common in practice) for flow control that
+ preserve transparency of the character stream. Note also that their use
+ for flow control is not an official standard. Interestingly, on the
+ model 33 teletype with a paper tape punch (around 1970), @kbd{C-s} and
+ @kbd{C-q} were sent by the computer to turn the punch on and off!
+
+ As window systems and PC terminal emulators replace character-only
+ terminals, the flow control problem is gradually disappearing. For the
+ mean time, Emacs provides a convenient way of enabling flow control if
+ you want it: call the function @code{enable-flow-control}.
+
+ @deffn Command enable-flow-control &optional arg
+ When @var{arg} is a positive integer, this function enables use of
+ @kbd{C-s} and @kbd{C-q} for output flow control, and provides the
+ characters @kbd{C-\} and @kbd{C-^} as aliases for them using
+ @code{keyboard-translate-table} (@pxref{Translating Input}).
+
+ When @var{arg} is a negative integer or zero, it disables these
+ features. When @var{arg} is @code{nil} or omitted, it toggles.
+ Interactively, @var{arg} is the prefix argument. If address@hidden,
+ its numeric value is used.
+ @end deffn
+
+ You can use the function @code{enable-flow-control-on} in your
+ init file to enable flow control automatically on certain
+ terminal types.
+
+ @defun enable-flow-control-on &rest termtypes
+ This function enables flow control, and the aliases @kbd{C-\} and @kbd{C-^},
+ if the terminal type is one of @var{termtypes}. For example:
+
+ @smallexample
+ (enable-flow-control-on "vt200" "vt300" "vt101" "vt131")
+ @end smallexample
+ @end defun
+
+ Here is how @code{enable-flow-control} does its job:
+
+ @enumerate
+ @item
+ @cindex @sc{cbreak}
+ It sets @sc{cbreak} mode for terminal input, and tells the operating
+ system to handle flow control. This is done using @code{set-input-mode}.
+
+ @item
+ It sets up @code{keyboard-translate-table} to translate @kbd{C-\} and
+ @kbd{C-^} into @kbd{C-s} and @kbd{C-q}. Except at its very
+ lowest level, Emacs never knows that the characters typed were anything
+ but @kbd{C-s} and @kbd{C-q}, so you can in effect type them as @kbd{C-\}
+ and @kbd{C-^} even when they are input for other commands.
+ @xref{Translating Input}.
+ @end enumerate
+
+ If the terminal is the source of the flow control characters, then once
+ you enable kernel flow control handling, you probably can make do with
+ less padding than normal for that terminal. You can reduce the amount
+ of padding by customizing the Termcap entry. You can also reduce it by
+ setting @code{baud-rate} to a smaller value so that Emacs uses a smaller
+ speed when calculating the padding needed. @xref{Terminal Output}.
+
+ @node Batch Mode
+ @section Batch Mode
+ @cindex batch mode
+ @cindex noninteractive use
+
+ 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}, and @samp{-f @var{function}}, which
+ calls @var{function} with no arguments.
+
+ 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 error descriptor when
+ in batch mode. 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.)
+
+ @defvar noninteractive
+ This variable is address@hidden 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 for suspension and
+ restart of applications. In the X Window System, a program called the
+ @dfn{session manager} has the responsibility to keep track of the
+ applications that are running. During shutdown, 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
+ @tindex emacs-save-session-functions
+ Emacs supports saving state by using a hook called
+ @code{emacs-save-session-functions}. Each function in this hook is
+ called when the session manager tells Emacs 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 that a subsequent Emacs invocation will
+ load in order to restart the saved session.
+
+ If a function in @code{emacs-save-session-functions} returns
+ address@hidden, Emacs tells the session manager to cancel the
+ shutdown.
+ @end defvar
+
+ Here is an example that just inserts some text into @samp{*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-excursion
+ (switch-to-buffer \"*scratch*\")
+ (insert \"I am restored\"))")
+ nil)
+ @end group
+ @end example
+
+ @ignore
+ arch-tag: 8378814a-30d7-467c-9615-74a80b9988a7
+ @end ignore
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