emacs/lispref/os.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999
@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.
* Special Keysyms::     Defining system-specific key symbols for X.
* 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 non-@code{nil}.  (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 non-@code{nil}, 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 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}.

@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 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 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 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
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{Command
Switches, , Command Line Switches and 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 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}.

@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}.

@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}, Emacs is not killed.
@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 runs the functions in
this 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 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 non-@code{nil}, 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 non-@code{nil}
(@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 cancelled")))))
     @result{} (lambda nil
          (or (y-or-n-p "Really suspend? ")
              (error "Suspend cancelled")))
@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 @sc{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 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 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 defvar

@deffn Command getenv var
@cindex environment variable access
This function returns the value of the environment variable @var{var},
as a string.  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 value
This command sets the value of the environment variable named
@var{variable} to @var{value}.  Both arguments should be strings.  This
function works by modifying @code{process-environment}; binding that
variable with @code{let} is also reasonable practice.
@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 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.

@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 @sc{id} of the Emacs process.
@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.
@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 non-@code{nil}, @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.  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 @sc{uid}, not the real @sc{uid}.

If you specify @var{uid}, the value is the user name that corresponds
to @var{uid} (which should be an integer).

@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
@sc{uid}.  This ignores the effective @sc{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 an integer (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 @sc{uid} of the user.

@example
@group
(user-real-uid)
     @result{} 19
@end group
@end example
@end defun

@defun user-uid
This function returns the effective @sc{uid} of the user.  
@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{File Attributes}).

@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 (local 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{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, both 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 be a cons cell
containing two integers, or a list whose first two elements are
integers.  Thus, you can use times obtained from @code{current-time}
(see above) and from @code{file-attributes} (@pxref{File Attributes}).
@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), and it also 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{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 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 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} 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 time
This function converts a time value into calendrical information.  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.
@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, 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.
@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.

@defun run-at-time time repeat function &rest args
This function arranges to call @var{function} with arguments @var{args}
at time @var{time}.  The argument @var{function} is a function to call
later, and @var{args} are the arguments to give it when it is called.
The time @var{time} is specified as a string.

Absolute times may be specified in a wide variety of formats; this
function tries to accept all the commonly used date formats.  Valid
formats include these two,

@example
@var{year}-@var{month}-@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, 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.

If @var{time} is a number (integer or floating point), that specifies a
relative time measured in seconds.

The argument @var{repeat} specifies how often to repeat the call.  If
@var{repeat} is @code{nil}, there are no repetitions; @var{function} is
called just once, at @var{time}.  If @var{repeat} is a number, it
specifies a repetition period measured in seconds.

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 defun

@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 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
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} (see below).
@end defun

@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.

@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 @code{run-at-time}; 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 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.  @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 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 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 bit mask:

@table @asis
@item 1
The @key{SHIFT} key.
@item 2
The @key{LOCK} key.
@item 4
The @key{CTL} key.
@item 8
The @key{META} key.
@end table

Each time the user types a keyboard key, it is altered as if the
modifier keys specified in the bit mask were held down.

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.
@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}.

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
non-@code{nil}, 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.
@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 @kbd{@key{ESC} 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'' @kbd{@key{ESC} 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
@code{key-translation-map} overrides actual key bindings.  For example,
if @kbd{C-x f} has a 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

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{Specifying Coding Systems}.  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.

See also @code{open-dribble-file} in @ref{Terminal 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 Special Keysyms
@section System-Specific 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

@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 @sc{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
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}).
@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, with @code{(set-input-mode nil t)}.

@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 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 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 the current buffer set
to a temporary buffer.  Each functions can use @code{insert} to add
Lisp code to this buffer.  At the end, Emacs saves the buffer in a
file that Emacs will load in order to restart the saved session.

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 *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