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@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
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@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2001,
@c   2002, 2003, 2004, 2005, 2006, 2007  Free Software Foundation, Inc.
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@c See the file elisp.texi for copying conditions.
@setfilename ../info/os
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@node System Interface, Antinews, Display, Top
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@chapter Operating System Interface

  This chapter is about starting and getting out of Emacs, access to
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values in the operating system environment, and terminal input, output,
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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
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* Starting Up::         Customizing Emacs startup processing.
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* 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.
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* 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.
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* Processor Run Time::  Getting the run time used by Emacs.
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* Time Calculations::   Adding, subtracting, comparing times, etc.
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* Timers::		Setting a timer to call a function at a certain time.
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* Idle Timers::         Setting a timer to call a function when Emacs has
                          been idle for a certain length of time.
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* Terminal Input::      Accessing and recording terminal input.
* Terminal Output::     Controlling and recording terminal output.
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* Sound Output::        Playing sounds on the computer's speaker.
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* X11 Keysyms::         Operating on key symbols for X Windows
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* Batch Mode::          Running Emacs without terminal interaction.
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* Session Management::  Saving and restoring state with X Session Management.
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@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
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* Startup Summary::         Sequence of actions Emacs performs at startup.
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* Init File::               Details on reading the init file (@file{.emacs}).
* Terminal-Specific::       How the terminal-specific Lisp file is read.
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* Command-Line Arguments::  How command-line arguments are processed,
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                              and how you can customize them.
@end menu

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@node Startup Summary
@subsection Summary: Sequence of Actions at Startup
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@cindex initialization of Emacs
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@cindex startup of Emacs
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@cindex @file{startup.el}

   The order of operations performed (in @file{startup.el}) by Emacs when
it is started up is as follows:

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

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@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}.

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

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@item
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It initializes the window frame and faces, if appropriate.
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@item
It runs the normal hook @code{before-init-hook}.

@item
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It loads the library @file{site-start} (if any), unless the option
@samp{-Q} (or @samp{--no-site-file}) was specified.  The library's file
name is usually @file{site-start.el}.
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@cindex @file{site-start.el}

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@item
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It loads your init file (usually @file{~/.emacs}), unless the option
@samp{-q} (or @samp{--no-init-file}), @samp{-Q}, 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{~}.
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@item
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It loads the library @file{default} (if any), unless
@code{inhibit-default-init} is non-@code{nil}.  (This is not done in
@samp{-batch} mode, or if @samp{-Q} or @samp{-q} was specified on the
command line.)  The library's file name is usually @file{default.el}.
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@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.

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@item
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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
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that with @code{inhibit-startup-echo-area-message}.
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@item
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It processes the action arguments from the command line.
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@item
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It runs @code{emacs-startup-hook} and then @code{term-setup-hook}.
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@item
It calls @code{frame-notice-user-settings}, which modifies the
parameters of the selected frame according to whatever the init files
specify.

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@item
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It runs @code{window-setup-hook}.  @xref{Window Systems}.

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@item
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It displays copyleft, nonwarranty, and basic use information, provided
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the value of @code{inhibit-startup-message} is @code{nil}, you didn't
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specify @samp{--no-splash} or @samp{-Q}.
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@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
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form to your init file:
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@example
(setq inhibit-startup-echo-area-message
      "@var{your-login-name}")
@end example

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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
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@code{inhibit-startup-echo-area-message} to the same value do not
inhibit the startup message.
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This way, you can easily inhibit the message for yourself if you wish,
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but thoughtless copying of your init file will not inhibit the message
for someone else.
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@end defopt

@node Init File
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@subsection The Init File, @file{.emacs}
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@cindex init file
@cindex @file{.emacs}

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  When you start Emacs, it normally attempts to load your @dfn{init
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file}, a file in your home directory.  Its normal name is
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@file{.emacs}, but you can also call it @file{.emacs.el}.
Alternatively, you can use a file named @file{init.el} in a
subdirectory @file{.emacs.d}.  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}.
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  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
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variable, or the @code{USER} (most systems) or @code{USERNAME} (MS
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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.
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@cindex default init file
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  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.
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  Another file for site-customization is @file{site-start.el}.  Emacs
loads this @emph{before} the user's init file.  You can inhibit the
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loading of this file with the option @samp{--no-site-file}.
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@defvar site-run-file
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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.
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@end defvar

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  @xref{Init Examples,, Init File Examples, emacs, The GNU Emacs Manual}, for
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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
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This normal hook is run, once, just before loading all the init files
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(the user's init file, @file{default.el}, and/or @file{site-start.el}).
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(The only way to change it with real effect is before dumping Emacs.)
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@end defvar

@defvar after-init-hook
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This normal hook is run, once, just after loading all the init files
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(the user's init file, @file{default.el}, and/or @file{site-start.el}),
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before loading the terminal-specific library and processing the
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command-line action arguments.
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@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
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This variable holds the absolute file name of the user's init file.  If the
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actual init file loaded is a compiled file, such as @file{.emacs.elc},
the value refers to the corresponding source file.
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@end defvar

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@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.
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@end defvar
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@node Terminal-Specific
@subsection Terminal-Specific Initialization
@cindex terminal-specific initialization

  Each terminal type can have its own Lisp library that Emacs loads when
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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
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terminal type (specified by the environment variable @code{TERM}).
Normally, @code{term-file-prefix} has the value
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@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.
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@cindex Termcap
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  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 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
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  Your init file can prevent the loading of the
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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
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your init file and any terminal-specific libraries.  You can
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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
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init file if you do not wish to load the
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terminal-initialization file.  To do this, put the following in
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your init file: @code{(setq term-file-prefix nil)}.
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On MS-DOS, if the environment variable @code{TERM} is not set, Emacs
uses @samp{internal} as the terminal type.
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@end defvar

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@defvar term-setup-hook
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This variable is a normal hook that Emacs runs after loading your
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init file, the default initialization file (if any) and the
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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.

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@node Command-Line Arguments
@subsection Command-Line Arguments
@cindex command-line arguments
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  You can use command-line arguments to request various actions when you
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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,
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command-line arguments are hardly ever used.  As a practical matter, it
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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.

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  This section describes how Emacs processes command-line arguments,
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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
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specify the file as a command-line argument.  The recommended way to
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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
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This function parses the command line that Emacs was called with,
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processes it, loads the user's init file and displays the
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startup messages.
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@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
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to process its new command-line arguments.
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@end defvar

@defvar command-switch-alist
@cindex switches on command line
@cindex options on command line
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@cindex command-line options
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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.

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A @dfn{command-line option} is an argument on the command line, which
has the form:
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@example
-@var{option}
@end example

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The elements of the @code{command-switch-alist} look like this:
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@example
(@var{option} . @var{handler-function})
@end example

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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.
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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}.)

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The command-line arguments are parsed by the @code{command-line-1}
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function in the @file{startup.el} file.  See also @ref{Emacs
Invocation, , Command Line Arguments for Emacs Invocation, emacs, The
GNU Emacs Manual}.
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@end defvar

@defvar command-line-args
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The value of this variable is the list of command-line arguments passed
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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,
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in order of appearance, until one of them returns a non-@code{nil}
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value.

These functions are called with no arguments.  They can access the
command-line argument under consideration through the variable
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@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}.
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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
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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
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@code{save-buffers-kill-emacs}, the higher level function from which
@code{kill-emacs} is usually called.
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@defvar kill-emacs-query-functions
After asking the standard questions, @code{save-buffers-kill-emacs}
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calls the functions in the list @code{kill-emacs-query-functions}, in
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order of appearance, with no arguments.  These functions can ask for
additional confirmation from the user.  If any of them returns
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@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.
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@end defvar

@defvar kill-emacs-hook
This variable is a normal hook; once @code{save-buffers-kill-emacs} is
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finished with all file saving and confirmation, it calls
@code{kill-emacs} which runs the functions in this hook.
@code{kill-emacs} does not run this hook in batch mode.

@code{kill-emacs} may be invoked directly (that is not via
@code{save-buffers-kill-emacs}) if the terminal is disconnected, or in
similar situations where interaction with the user is not possible.
Thus, if your hook needs to interact with the user, put it on
@code{kill-emacs-query-functions}; if it needs to run regardless of
how Emacs is killed, put it on @code{kill-emacs-hook}.
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@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.

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  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
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a window system (X, MS Windows, or Mac).
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@defun suspend-emacs &optional string
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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
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@code{suspend-hook}.
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After the user resumes Emacs, @code{suspend-emacs} runs the normal hook
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@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? ")
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                          (error "Suspend canceled")))))
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     @result{} (lambda nil
          (or (y-or-n-p "Really suspend? ")
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              (error "Suspend canceled")))
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@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
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This variable is a normal hook that Emacs runs before suspending.
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@end defvar

@defvar suspend-resume-hook
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This variable is a normal hook that Emacs runs on resuming
after a suspension.
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@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
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system, the user's @acronym{UID}, and so on.
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@defvar system-configuration
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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}.
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@end defvar

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@cindex system type and name
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@defvar system-type
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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:
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@table @code
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@item alpha-vms
VMS on the Alpha.

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@item aix-v3
AIX.

@item berkeley-unix
Berkeley BSD.

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@item cygwin
Cygwin.

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@item dgux
Data General DGUX operating system.

@item gnu
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the GNU system (using the GNU kernel, which consists of the HURD and Mach).
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@item gnu/linux
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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.)
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@item hpux
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Hewlett-Packard HPUX operating system.
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@item irix
Silicon Graphics Irix system.

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@item ms-dos
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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.
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@item next-mach
NeXT Mach-based system.
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@item rtu
Masscomp RTU, UCB universe.

@item unisoft-unix
UniSoft UniPlus.

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

@item vax-vms
VAX VMS.

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@item windows-nt
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Microsoft windows NT.  The same executable supports Windows 9X, but the
value of @code{system-type} is @code{windows-nt} in either case.
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@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)
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     @result{} "www.gnu.org"
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@end example
@end defun

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  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

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@deffn Command getenv var
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@cindex environment variable access
This function returns the value of the environment variable @var{var},
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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}.
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@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
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@end deffn
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@c Emacs 19 feature
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@deffn Command setenv variable &optional value
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This command sets the value of the environment variable named
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@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.
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@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"
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    "USER=lewis"
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@end group
@group
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    "TERM=ibmapa16"
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    "SHELL=/bin/csh"
    "HOME=/user/lewis")
@end group
@end smallexample
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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.
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@end defvar

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@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
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and MS-Windows.
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@end defvar

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@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

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@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
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locations, but can find them in a directory related somehow to the one
containing the Emacs executable.
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@end defvar

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@defun load-average &optional use-float
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This function returns the current 1-minute, 5-minute, and 15-minute load
averages, in a list.
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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
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as floating point numbers and without multiplying by 100.
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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.
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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.

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@example
@group
(load-average)
     @result{} (169 48 36)
@end group
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@group
(load-average t)
     @result{} (1.69 0.48 0.36)
@end group
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@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
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This function returns the process @acronym{ID} of the Emacs process,
as an integer.
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@end defun

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@defvar tty-erase-char
This variable holds the erase character that was selected
in the system's terminal driver, before Emacs was started.
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The value is @code{nil} if Emacs is running under a window system.
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@end defvar

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@defun setprv privilege-name &optional setp getprv
This function sets or resets a VMS privilege.  (It does not exist on
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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.
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If the third argument, @var{getprv}, is non-@code{nil}, @code{setprv}
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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
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@cindex user identification
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@defvar init-file-user
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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
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@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

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@defvar user-mail-address
This holds the nominal email address of the user who is using Emacs.
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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
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variable to some other value in your init file if you do not
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want to use the default value.
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@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
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on the effective @acronym{UID}, not the real @acronym{UID}.
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If you specify @var{uid}, the value is the user name that corresponds
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to @var{uid} (which should be an integer), or @code{nil} if there is
no such user.
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@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
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@acronym{UID}.  This ignores the effective @acronym{UID} and ignores the
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environment variables @code{LOGNAME} and @code{USER}.
@end defun

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@defun user-full-name &optional uid
This function returns the full name of the logged-in user---or the value
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of the environment variable @code{NAME}, if that is set.
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@c "Bil" is the correct spelling.
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@example
@group
(user-full-name)
     @result{} "Bil Lewis"
@end group
@end example
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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"}.

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If @var{uid} is non-@code{nil}, then it should be a number (a user-id)
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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}.
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@end defun

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@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}).

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@defun user-real-uid
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This function returns the real @acronym{UID} of the user.
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The value may be a floating point number.
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@example
@group
(user-real-uid)
     @result{} 19
@end group
@end example
@end defun

@defun user-uid
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This function returns the effective @acronym{UID} of the user.
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The value may be a floating point number.
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@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
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This function returns the current time and date as a human-readable
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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
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@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
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additional information may some day be added at the end.
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@c Emacs 19 feature
The argument @var{time-value}, if given, specifies a time to format
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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}
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(@pxref{Definition of file-attributes}).  @var{time-value} can also be
a cons of two integers, but this is considered obsolete.
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@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
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0:00 January 1, 1970 UTC (Coordinated Universal Time), which is
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@ifnottex
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@var{high} * 2**16 + @var{low}.
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@end ifnottex
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@tex
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$high*2^{16}+low$.
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@end tex

The third element, @var{microsec}, gives the microseconds since the
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start of the current second (or 0 for systems that return time with
the resolution of only one second).
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The first two elements can be compared with file time values such as you
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get with the function @code{file-attributes}.
@xref{Definition of file-attributes}.
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@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
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second element, @var{name}, is a string giving the name of the time
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zone.  Both elements change when daylight saving time begins or ends;
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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
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compute the value, the unknown elements of the list are @code{nil}.
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The argument @var{time-value}, if given, specifies a time to analyze
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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}.
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@end defun

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@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
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Universal Time.  Otherwise, @var{tz} should be a string specifying a
time zone rule.
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@end defun

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@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
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above).  Thus, it accepts the output of @code{current-time} and
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@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

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@node Time Conversion
@section Time Conversion

  These functions convert time values (lists of two or three integers)
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to calendrical information and vice versa.  You can get time values
from the functions @code{current-time} (@pxref{Time of Day}) and
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@code{file-attributes} (@pxref{Definition of file-attributes}).
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  Many operating systems are limited to time values that contain 32 bits
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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.

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  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
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@code{t} if daylight saving time is effect, otherwise @code{nil}.
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@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
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its daylight saving time rules.  If specified, it can be either a list
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(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
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zone is used without any further alteration for daylight saving time.
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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.
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@defun date-to-time string
This function parses the time-string @var{string} and returns the
corresponding time value.
@end defun

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@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:
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@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
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This has a locale-specific meaning.  In the default locale (named C), it
is equivalent to @samp{%A, %B %e, %Y}.
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@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
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This stands for the hour (01-12).
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@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
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This stands for the seconds (00-59).
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@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
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This has a locale-specific meaning.  In the default locale (named
@samp{C}), it is equivalent to @samp{%D}.
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@item %X
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This has a locale-specific meaning.  In the default locale (named
@samp{C}), it is equivalent to @samp{%T}.
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@item %y
This stands for the year without century (00-99).
@item %Y
This stands for the year with century.
@item %Z