Convert consecutive FSF copyright years to ranges.

[gnu-emacs] / doc / lispref / os.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990-1995, 1998-1999, 2001-2011
@c   Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../../info/os
@node System Interface, Packaging, Display, 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.  @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 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.
* Dynamic Libraries::   On-demand loading of support libraries.
@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
(which are defined 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.

@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.

@vindex initial-window-system@r{, and startup}
@vindex window-system-initialization-alist
@item
It loads the initialization library for the window system specified by
the variable @code{initial-window-system} (@pxref{Window Systems,
initial-window-system}).  This library's name is
@file{term/@var{windowsystem}-win.el}, where @var{windowsystem} is the
value of @code{initial-window-system}.  From that library, it calls
the appropriate initialization function.  The initialization function
for each supported window system is specified by
@code{window-system-initialization-alist}.

@item
It sets the language environment and the terminal coding system,
if requested by environment variables such as @code{LANG}.

@item
It processes the initial options.  (Some of them are handled
even earlier than this.)

@item
It runs the normal hook @code{before-init-hook}.

@item
It initializes the window frame and faces, if appropriate, and turns
on the menu bar and tool bar, if the initial frame needs them.

@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.

@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 @samp{*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-only terminal, it loads the terminal-specific
Lisp library, which is specified by the variable
@code{term-file-prefix} (@pxref{Terminal-Specific}).  This is not done
in @code{--batch} mode, nor if @code{term-file-prefix} is @code{nil}.

@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 now exits if the option @code{--batch} was specified.

@item
If @code{initial-buffer-choice} is a string, it visits the file with
that name.  Furthermore, if the @samp{*scratch*} buffer exists and is
empty, it inserts @code{initial-scratch-message} into that buffer.

@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
If the option @code{--daemon} was specified, it calls
@code{server-start} and detaches from the controlling terminal.
@xref{Emacs Server,,, emacs, The GNU Emacs Manual}.

@item
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{initial-buffer-choice} or
@code{inhibit-startup-screen} are @code{nil}, nor if the
@samp{--no-splash} or @samp{-Q} command-line options were specified.

@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}.
@end enumerate

@defopt inhibit-startup-screen
This variable, if non-@code{nil}, inhibits the startup screen.  In
that case, Emacs typically displays the @samp{*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
This variable, if non-@code{nil}, determines a file or buffer for
Emacs to display after starting up, instead of the startup screen.  If
its value is @code{t}, Emacs displays the @samp{*scratch*} buffer.  If
its value is a string, that specifies the name of a file for Emacs to
visit.
@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

@defopt initial-scratch-message
This variable, if non-@code{nil}, should be a string, which is
inserted into the @samp{*scratch*} buffer when Emacs starts up.  If it
is @code{nil}, the @samp{*scratch*} buffer is empty.
@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}.  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.  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}.

  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 @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} (or @samp{-Q}) is specified.  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.

  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.
@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
This variable prevents Emacs from loading the default initialization
library file for your session of Emacs.  If its value is non-@code{nil},
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
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
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
ordinarily @file{~/.emacs.d}, but differs on some platforms.
@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.

@cindex Termcap
  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{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, and no library
is found whose name is identical to the terminal's name, Emacs strips
from the terminal's name the last hyphen 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 in the name (the
latter means the terminal doesn't have any library specific to it).
Thus, for example, if there are no @samp{aaa-48} and @samp{aaa-30}
libraries, Emacs will try the same library @file{term/aaa.el} for
terminal types @samp{aaa-48} and @samp{aaa-30-rv}.  If necessary, the
library can evaluate @code{(getenv "TERM")} to find the full name of
the terminal type.@refill

  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 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.  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
-@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}.  (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.  @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 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}.

@deffn Command kill-emacs &optional exit-data
This command 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 deffn

  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} 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

  On text-only 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.

  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.

@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.  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}.

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 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}; @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).

@vindex resume-tty-functions
This function reopens the terminal device, re-initializes it, and
redraws its 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.
@end defun

@defun controlling-tty-p &optional terminal
This function returns non-@code{nil} if @var{terminal} is the
controlling terminal of the Emacs session; @code{terminal} 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 text-only
frames, 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.  The
convenient way to test parts of this string is with
@code{string-match}.
@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 operating on.  Here is a table of the possible values:

@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 (Mac OS X).

@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 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 usg-unix-v
AT&T Unix System V.

@item windows-nt
Microsoft Windows NT and later.  The same executable supports Windows
9X, but the value of @code{system-type} is @code{windows-nt} in either
case.

@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}).

@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}.  (Since this is
done when Emacs starts up, the value actually used is the one saved when
Emacs was dumped.  @xref{Building Emacs}.)
@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}.  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 initial-environment
This variable holds the list of environment variables Emacs inherited
from its parent process.  It is computed during startup, see
@ref{Startup Summary}.
@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
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 non-@code{nil}, this is a directory within which to look for the
@file{lib-src} and @file{etc} subdirectories.  This 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.
@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 non-@code{nil}, 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

@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}
option was used, then Lisp packages should not load any customization
files or user profile.
@end defvar

@defopt 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 defopt

@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 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}).

@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.

The argument @var{time-value}, if given, specifies a time to format
instead of the current time.  This argument should have the same form
as the times obtained from @code{current-time} (see below) and from
@code{file-attributes} (@pxref{Definition of file-attributes}).  It
should be a list whose first two elements are integers; a third
(microsecond) element, if present, is ignored.  @var{time-value} can
also be a cons of two integers, but this usage is obsolete.

@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 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

@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 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-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} (@pxref{Definition of 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 calendrical information and vice versa.  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 B.C@.

@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 saving 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 saving 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 saving 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 Parsing
@section Parsing and Formatting Times

  These functions convert time values (lists of two or three integers)
to text in a string, and vice versa.

@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 (e.g., @samp{EST}).
@item %z
This stands for the time zone numerical offset (e.g., @samp{-0500}).
@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 non-@code{nil}, 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}
(@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 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 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
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 three integers: @code{(@var{high} @var{low} @var{microsec})}.  The
integers @var{high} and @var{low} combine to give the number of
seconds, 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 (or 0 for
systems that return time with the resolution of only one second).

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

  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.  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.

@defvar 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 defvar

@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

@node Idle Timers
@section 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 when Emacs has been idle for @var{secs}
seconds.  The value of @var{secs} may be an integer or a floating point
number; a value of the type returned by @code{current-idle-time}
is also allowed.

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 ``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.

@c Emacs 19 feature
@defun current-idle-time
If Emacs is idle, this function returns the length of time Emacs has
been idle, 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 of idleness, 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).

When Emacs is not idle, @code{current-idle-time} returns @code{nil}.
This is a convenient way to test whether Emacs is idle.

The main use of this function 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:

@smallexample
(defvar resume-timer nil
  "Timer that `timer-function' used to reschedule itself, or nil.")

(defun timer-function ()
  ;; @r{If the user types a command while @code{resume-timer}}
  ;; @r{is active, the next time this function is called from}
  ;; @r{its main idle timer, deactivate @code{resume-timer}.}
  (when resume-timer
    (cancel-timer resume-timer))
  ...@var{do the work for a while}...
  (when @var{taking-a-break}
    (setq resume-timer
          (run-with-idle-timer
            ;; Compute an idle time @var{break-length}
            ;; more than the current value.
            (time-add (current-idle-time)
                      (seconds-to-time @var{break-length}))
            nil
            'timer-function))))
@end smallexample
@end defun

  Some idle timer functions in user Lisp packages have a loop that
does a certain amount of processing each time around, and exits when
@code{(input-pending-p)} is non-@code{nil}.  That 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
To avoid these problems, don't use that technique.  Instead, write
such idle timers to reschedule themselves after a brief pause, using
the method in the @code{timer-function} example above.

@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-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 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.

@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 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.

@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 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
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 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.
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.

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}).

@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
This function is an alternative interface to playing a sound @var{file}
specifying an optional @var{volume} and @var{device}.
@end defun

@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
-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 Terminals}.
@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 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 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 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
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 @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-current-buffer
  (switch-to-buffer \"*scratch*\")
  (insert \"I am restored\"))")
  nil)
@end group
@end example

@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 running session of Emacs
won't have access to that library, and the features that depend on the
library 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:

@lisp
(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 lisp

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