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This is the `GNU Emacs Lisp Reference Manual' corresponding to Emacs
version 24.2.

Copyright (C) 1990-1996, 1998-2012 Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
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INFO-DIR-SECTION GNU Emacs Lisp
START-INFO-DIR-ENTRY
* Elisp: (elisp).       The Emacs Lisp Reference Manual.
END-INFO-DIR-ENTRY


File: elisp,  Node: Startup Summary,  Next: Init File,  Up: Starting Up

39.1.1 Summary: Sequence of Actions at Startup
----------------------------------------------

When Emacs is started up, it performs the following operations (see
`normal-top-level' in `startup.el'):

  1. It adds subdirectories to `load-path', by running the file named
     `subdirs.el' in each directory in the list.  Normally, this file
     adds the directory's subdirectories to the list, and those are
     scanned in their turn.  The files `subdirs.el' are normally
     generated automatically when Emacs is installed.

  2. It registers input methods by loading any `leim-list.el' file
     found in the `load-path'.

  3. It sets the variable `before-init-time' to the value of
     `current-time' (*note Time of Day::).  It also sets
     `after-init-time' to `nil', which signals to Lisp programs that
     Emacs is being initialized.

  4. It sets the language environment and the terminal coding system,
     if requested by environment variables such as `LANG'.

  5. It does some basic parsing of the command-line arguments.

  6. If not running in batch mode, it initializes the window system that
     the variable `initial-window-system' specifies (*note
     initial-window-system: Window Systems.).  The initialization
     function for each supported window system is specified by
     `window-system-initialization-alist'.  If the value of
     `initial-window-system' is WINDOWSYSTEM, then the appropriate
     initialization function is defined in the file
     `term/WINDOWSYSTEM-win.el'.  This file should have been compiled
     into the Emacs executable when it was built.

  7. It runs the normal hook `before-init-hook'.

  8. If appropriate, it creates a graphical frame.  This is not done if
     the options `--batch' or `--daemon' were specified.

  9. It initializes the initial frame's faces, and sets up the menu bar
     and tool bar if needed.  If graphical frames are supported, it
     sets up the tool bar even if the current frame is not a graphical
     one, since a graphical frame may be created later on.

 10. It use `custom-reevaluate-setting' to re-initialize the members of
     the list `custom-delayed-init-variables'.  These are any
     pre-loaded user options whose default value depends on the
     run-time, rather than build-time, context.  *Note
     custom-initialize-delay: Building Emacs.

 11. It loads the library `site-start', if it exists.  This is not done
     if the options `-Q' or `--no-site-file' were specified.  

 12. It loads your init file (*note Init File::).  This is not done if
     the options `-q', `-Q', or `--batch' were specified.  If the `-u'
     option was specified, Emacs looks for the init file in that user's
     home directory instead.

 13. It loads the library `default', if it exists.  This is not done if
     `inhibit-default-init' is non-`nil', nor if the options `-q',
     `-Q', or `--batch' were specified.  

 14. It loads your abbrevs from the file specified by
     `abbrev-file-name', if that file exists and can be read (*note
     abbrev-file-name: Abbrev Files.).  This is not done if the option
     `--batch' was specified.

 15. If `package-enable-at-startup' is non-`nil', it calls the function
     `package-initialize' to activate any optional Emacs Lisp package
     that has been installed.  *Note Packaging Basics::.

 16. It sets the variable `after-init-time' to the value of
     `current-time'.  This variable was set to `nil' earlier; setting
     it to the current time signals that the initialization phase is
     over, and, together with `before-init-time', provides the
     measurement of how long it took.

 17. It runs the normal hook `after-init-hook'.

 18. If the buffer `*scratch*' exists and is still in Fundamental mode
     (as it should be by default), it sets its major mode according to
     `initial-major-mode'.

 19. If started on a text terminal, it loads the terminal-specific Lisp
     library, which is specified by the variable `term-file-prefix'
     (*note Terminal-Specific::).  This is not done in `--batch' mode,
     nor if `term-file-prefix' is `nil'.

 20. It displays the initial echo area message, unless you have
     suppressed that with `inhibit-startup-echo-area-message'.

 21. It processes any command-line options that were not handled
     earlier.

 22. It now exits if the option `--batch' was specified.

 23. If `initial-buffer-choice' is a string, it visits the file with
     that name.  If the `*scratch*' buffer exists and is empty, it
     inserts `initial-scratch-message' into that buffer.

 24. It runs `emacs-startup-hook' and then `term-setup-hook'.

 25. It calls `frame-notice-user-settings', which modifies the
     parameters of the selected frame according to whatever the init
     files specify.

 26. It runs `window-setup-hook'.  *Note Window Systems::.

 27. It displays the "startup screen", which is a special buffer that
     contains information about copyleft and basic Emacs usage.  This is
     not done if `inhibit-startup-screen' or `initial-buffer-choice'
     are non-`nil', or if the `--no-splash' or `-Q' command-line
     options were specified.

 28. If the option `--daemon' was specified, it calls `server-start'
     and detaches from the controlling terminal.  *Note Emacs Server:
     (emacs)Emacs Server.

 29. If started by the X session manager, it calls
     `emacs-session-restore' passing it as argument the ID of the
     previous session.  *Note Session Management::.


The following options affect some aspects of the startup sequence.

 -- User Option: inhibit-startup-screen
     This variable, if non-`nil', inhibits the startup screen.  In that
     case, Emacs typically displays the `*scratch*' buffer; but see
     `initial-buffer-choice', below.

     Do not set this variable in the init file of a new user, or in a
     way that affects more than one user, as that would prevent new
     users from receiving information about copyleft and basic Emacs
     usage.

     `inhibit-startup-message' and `inhibit-splash-screen' are aliases
     for this variable.

 -- User Option: initial-buffer-choice
     If non-`nil', this variable is a string that specifies a file or
     directory for Emacs to display after starting up, instead of the
     startup screen.

 -- User Option: 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:

          (setq inhibit-startup-echo-area-message
                "YOUR-LOGIN-NAME")

     Emacs explicitly checks for an expression as shown above in your
     init file; your login name must appear in the expression as a Lisp
     string constant.  You can also use the Customize interface.  Other
     methods of setting `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.

 -- User Option: initial-scratch-message
     This variable, if non-`nil', should be a string, which is inserted
     into the `*scratch*' buffer when Emacs starts up.  If it is `nil',
     the `*scratch*' buffer is empty.

The following command-line options affect some aspects of the startup
sequence.  *Note Initial Options: (emacs)Initial Options.

`--no-splash'
     Do not display a splash screen.

`--batch'
     Run without an interactive terminal.  *Note Batch Mode::.

`--daemon'
     Do not initialize any display; just start a server in the
     background.

`--no-init-file'
`-Q'
     Do not load either the init file, or the `default' library.

`--no-site-file'
     Do not load the `site-start' library.

`--quick'
`-Q'
     Equivalent to `-q --no-site-file --no-splash'.


File: elisp,  Node: Init File,  Next: Terminal-Specific,  Prev: Startup Summary,  Up: Starting Up

39.1.2 The Init File
--------------------

When you start Emacs, it normally attempts to load your "init file".
This is either a file named `.emacs' or `.emacs.el' in your home
directory, or a file named `init.el' in a subdirectory named `.emacs.d'
in your home directory.

   The command-line switches `-q', `-Q', and `-u' control whether and
where to find the init file; `-q' (and the stronger `-Q') says not to
load an init file, while `-u USER' says to load USER's init file
instead of yours.  *Note Entering Emacs: (emacs)Entering Emacs.  If
neither option is specified, Emacs uses the `LOGNAME' environment
variable, or the `USER' (most systems) or `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.

   An Emacs installation may have a "default init file", which is a
Lisp library named `default.el'.  Emacs finds this file through the
standard search path for libraries (*note How Programs Do Loading::).
The Emacs distribution does not come with this file; it is intended for
local customizations.  If the default init file exists, it is loaded
whenever you start Emacs.  But your own personal init file, if any, is
loaded first; if it sets `inhibit-default-init' to a non-`nil' value,
then Emacs does not subsequently load the `default.el' file.  In batch
mode, or if you specify `-q' (or `-Q'), Emacs loads neither your
personal init file nor the default init file.

   Another file for site-customization is `site-start.el'.  Emacs loads
this _before_ the user's init file.  You can inhibit the loading of
this file with the option `--no-site-file'.

 -- User Option: site-run-file
     This variable specifies the site-customization file to load before
     the user's init file.  Its normal value is `"site-start"'.  The
     only way you can change it with real effect is to do so before
     dumping Emacs.

   *Note Init File Examples: (emacs)Init Examples, for examples of how
to make various commonly desired customizations in your `.emacs' file.

 -- User Option: inhibit-default-init
     If this variable is non-`nil', it prevents Emacs from loading the
     default initialization library file.  The default value is `nil'.

 -- Variable: before-init-hook
     This normal hook is run, once, just before loading all the init
     files (`site-start.el', your init file, and `default.el').  (The
     only way to change it with real effect is before dumping Emacs.)

 -- Variable: after-init-hook
     This normal hook is run, once, just after loading all the init
     files (`site-start.el', your init file, and `default.el'), before
     loading the terminal-specific library (if started on a text
     terminal) and processing the command-line action arguments.

 -- Variable: emacs-startup-hook
     This normal hook is run, once, just after handling the command line
     arguments, just before `term-setup-hook'.  In batch mode, Emacs
     does not run either of these hooks.

 -- Variable: user-init-file
     This variable holds the absolute file name of the user's init
     file.  If the actual init file loaded is a compiled file, such as
     `.emacs.elc', the value refers to the corresponding source file.

 -- Variable: user-emacs-directory
     This variable holds the name of the `.emacs.d' directory.  It is
     `~/.emacs.d' on all platforms but MS-DOS.


File: elisp,  Node: Terminal-Specific,  Next: Command-Line Arguments,  Prev: Init File,  Up: Starting Up

39.1.3 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 `term-file-prefix' and the
terminal type (specified by the environment variable `TERM').
Normally, `term-file-prefix' has the value `"term/"'; changing this is
not recommended.  Emacs finds the file in the normal manner, by
searching the `load-path' directories, and trying the `.elc' and `.el'
suffixes.

   The usual role of a terminal-specific library is to enable special
keys to send sequences that Emacs can recognize.  It may also need to
set or add to `input-decode-map' if the Termcap or Terminfo entry does
not specify all the terminal's function keys.  *Note Terminal Input::.

   When the name of the terminal type contains a hyphen or underscore,
and no library is found whose name is identical to the terminal's name,
Emacs strips from the terminal's name the last hyphen or underscore and
everything that follows it, and tries again.  This process is repeated
until Emacs finds a matching library, or until there are no more
hyphens or underscores in the name (i.e. there is no terminal-specific
library).  For example, if the terminal name is `xterm-256color' and
there is no `term/xterm-256color.el' library, Emacs tries to load
`term/xterm.el'.  If necessary, the terminal library can evaluate
`(getenv "TERM")' to find the full name of the terminal type.

   Your init file can prevent the loading of the terminal-specific
library by setting the variable `term-file-prefix' to `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 `term-setup-hook'.
This is a normal hook that Emacs runs at the end of its initialization,
after loading both your init file and any terminal-specific libraries.
You could use this hook to define initializations for terminals that do
not have their own libraries.  *Note Hooks::.

 -- Variable: term-file-prefix
     If the value of this variable is non-`nil', Emacs loads a
     terminal-specific initialization file as follows:

          (load (concat term-file-prefix (getenv "TERM")))

     You may set the `term-file-prefix' variable to `nil' in your init
     file if you do not wish to load the terminal-initialization file.

     On MS-DOS, Emacs sets the `TERM' environment variable to
     `internal'.

 -- Variable: 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 `term-setup-hook' to override the definitions made by a
     terminal-specific file.

     For a related feature, *note window-setup-hook: Window Systems.


File: elisp,  Node: Command-Line Arguments,  Prev: Terminal-Specific,  Up: Starting Up

39.1.4 Command-Line Arguments
-----------------------------

You can use command-line arguments to request various actions when you
start Emacs.  Note that the recommended way of using Emacs is to start
it just once, after logging in, and then do all editing in the same
Emacs session (*note Entering Emacs: (emacs)Entering Emacs.).  For this
reason, you might not use command-line arguments very often;
nonetheless, they can be useful when invoking Emacs from session
scripts or debugging Emacs.  This section describes how Emacs processes
command-line arguments.

 -- Function: command-line
     This function parses the command line that Emacs was called with,
     processes it, and (amongst other things) loads the user's init
     file and displays the startup messages.

 -- Variable: command-line-processed
     The value of this variable is `t' once the command line has been
     processed.

     If you redump Emacs by calling `dump-emacs', you may wish to set
     this variable to `nil' first in order to cause the new dumped Emacs
     to process its new command-line arguments.

 -- Variable: command-switch-alist
     This variable is an alist of user-defined command-line options and
     associated handler functions.  By default it is empty, but you can
     add elements if you wish.

     A "command-line option" is an argument on the command line, which
     has the form:

          -OPTION

     The elements of the `command-switch-alist' look like this:

          (OPTION . HANDLER-FUNCTION)

     The CAR, OPTION, is a string, the name of a command-line option
     (not including the initial hyphen).  The HANDLER-FUNCTION is
     called to handle 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 HANDLER-FUNCTION can find all the
     remaining command-line arguments in the variable
     `command-line-args-left'.  (The entire list of command-line
     arguments is in `command-line-args'.)

     The command-line arguments are parsed by the `command-line-1'
     function in the `startup.el' file.  See also *note Command Line
     Arguments for Emacs Invocation: (emacs)Emacs Invocation.

 -- Variable: command-line-args
     The value of this variable is the list of command-line arguments
     passed to Emacs.

 -- Variable: command-line-args-left
     The value of this variable is the list of command-line arguments
     that have not yet been processed.

 -- Variable: 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-`nil' value.

     These functions are called with no arguments.  They can access the
     command-line argument under consideration through the variable
     `argi', which is bound temporarily at this point.  The remaining
     arguments (not including the current one) are in the variable
     `command-line-args-left'.

     When a function recognizes and processes the argument in `argi', it
     should return a non-`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
     `command-line-args-left'.

     If all of these functions return `nil', then the argument is
     treated as a file name to visit.


File: elisp,  Node: Getting Out,  Next: System Environment,  Prev: Starting Up,  Up: System Interface

39.2 Getting Out of Emacs
=========================

There are two ways to get out of Emacs: you can kill the Emacs job,
which exits permanently, or you can suspend it, which permits you to
reenter the Emacs process later.  (In a graphical environment, you can
of course simply switch to another application without doing anything
special to Emacs, then switch back to Emacs when you want.)

* Menu:

* Killing Emacs::        Exiting Emacs irreversibly.
* Suspending Emacs::     Exiting Emacs reversibly.


File: elisp,  Node: Killing Emacs,  Next: Suspending Emacs,  Up: Getting Out

39.2.1 Killing Emacs
--------------------

Killing Emacs means ending the execution of the Emacs process.  If you
started Emacs from a terminal, the parent process normally resumes
control.  The low-level primitive for killing Emacs is `kill-emacs'.

 -- Command: kill-emacs &optional exit-data
     This command calls the hook `kill-emacs-hook', then exits the
     Emacs process and kills it.

     If EXIT-DATA is an integer, that is used as the exit status of the
     Emacs process.  (This is useful primarily in batch operation; see
     *note Batch Mode::.)

     If 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.

   The `kill-emacs' function is normally called via the higher-level
command `C-x C-c' (`save-buffers-kill-terminal').  *Note Exiting:
(emacs)Exiting.  It is also called automatically if Emacs receives a
`SIGTERM' or `SIGHUP' operating system signal (e.g. when the
controlling terminal is disconnected), or if it receives a `SIGINT'
signal while running in batch mode (*note Batch Mode::).

 -- Variable: kill-emacs-hook
     This normal hook is run by `kill-emacs', before it kills Emacs.

     Because `kill-emacs' can be called in situations where user
     interaction is impossible (e.g. when the terminal is disconnected),
     functions on this hook should not attempt to interact with the
     user.  If you want to interact with the user when Emacs is
     shutting down, use `kill-emacs-query-functions', described below.

   When Emacs is killed, all the information in the Emacs process,
aside from files that have been saved, is lost.  Because killing Emacs
inadvertently can lose a lot of work, the `save-buffers-kill-terminal'
command queries for confirmation if you have buffers that need saving
or subprocesses that are running.  It also runs the abnormal hook
`kill-emacs-query-functions':

 -- Variable: kill-emacs-query-functions
     When `save-buffers-kill-terminal' is killing Emacs, it calls the
     functions in this hook, after asking the standard questions and
     before calling `kill-emacs'.  The functions are called in order of
     appearance, with no arguments.  Each function can ask for
     additional confirmation from the user.  If any of them returns
     `nil', `save-buffers-kill-emacs' does not kill Emacs, and does not
     run the remaining functions in this hook.  Calling `kill-emacs'
     directly does not run this hook.


File: elisp,  Node: Suspending Emacs,  Prev: Killing Emacs,  Up: Getting Out

39.2.2 Suspending Emacs
-----------------------

On text terminals, it is possible to "suspend Emacs", which means
stopping Emacs temporarily and returning control to its superior
process, which is usually the shell.  This allows you to resume editing
later in the same Emacs process, with the same buffers, the same kill
ring, the same undo history, and so on.  To resume Emacs, use the
appropriate command in the parent shell--most likely `fg'.

   Suspending works only on a terminal device from which the Emacs
session was started.  We call that device the "controlling terminal" of
the session.  Suspending is not allowed if the controlling terminal is
a graphical terminal.  Suspending is usually not relevant in graphical
environments, since you can simply switch to another application
without doing anything special to Emacs.

   Some operating systems (those without `SIGTSTP', or MS-DOS) do not
support suspension of jobs; on these systems, "suspension" actually
creates a new shell temporarily as a subprocess of Emacs.  Then you
would exit the shell to return to Emacs.

 -- Command: suspend-emacs &optional string
     This function stops Emacs and returns control to the superior
     process.  If and when the superior process resumes Emacs,
     `suspend-emacs' returns `nil' to its caller in Lisp.

     This function works only on the controlling terminal of the Emacs
     session; to relinquish control of other tty devices, use
     `suspend-tty' (see below).  If the Emacs session uses more than
     one terminal, you must delete the frames on all the other terminals
     before suspending Emacs, or this function signals an error.  *Note
     Multiple Terminals::.

     If STRING is non-`nil', its characters are sent to Emacs's
     superior shell, to be read as terminal input.  The characters in
     STRING are not echoed by the superior shell; only the results
     appear.

     Before suspending, `suspend-emacs' runs the normal hook
     `suspend-hook'.  After the user resumes Emacs, `suspend-emacs'
     runs the normal hook `suspend-resume-hook'.  *Note Hooks::.

     The next redisplay after resumption will redraw the entire screen,
     unless the variable `no-redraw-on-reenter' is non-`nil'.  *Note
     Refresh Screen::.

     Here is an example of how you could use these hooks:

          (add-hook 'suspend-hook
                    (lambda () (or (y-or-n-p "Really suspend? ")
                                   (error "Suspend canceled"))))
          (add-hook 'suspend-resume-hook (lambda () (message "Resumed!")
                                           (sit-for 2)))

     Here is what you would see upon evaluating `(suspend-emacs "pwd")':

          ---------- Buffer: Minibuffer ----------
          Really suspend? y
          ---------- Buffer: Minibuffer ----------

          ---------- Parent Shell ----------
          bash$ /home/username
          bash$ fg

          ---------- Echo Area ----------
          Resumed!

     Note that `pwd' is not echoed after Emacs is suspended.  But it is
     read and executed by the shell.

 -- Variable: suspend-hook
     This variable is a normal hook that Emacs runs before suspending.

 -- Variable: suspend-resume-hook
     This variable is a normal hook that Emacs runs on resuming after a
     suspension.

 -- Function: suspend-tty &optional tty
     If TTY specifies a terminal device used by Emacs, this function
     relinquishes the device and restores it to its prior state.  Frames
     that used the device continue to exist, but are not updated and
     Emacs doesn't read input from them.  TTY can be a terminal object,
     a frame (meaning the terminal for that frame), or `nil' (meaning
     the terminal for the selected frame).  *Note Multiple Terminals::.

     If TTY is already suspended, this function does nothing.

     This function runs the hook `suspend-tty-functions', passing the
     terminal object as an argument to each function.

 -- Function: resume-tty &optional tty
     This function resumes the previously suspended terminal device
     TTY; where TTY has the same possible values as it does for
     `suspend-tty'.

     This function reopens the terminal device, re-initializes it, and
     redraws it with that terminal's selected frame.  It then runs the
     hook `resume-tty-functions', passing the terminal object as an
     argument to each function.

     If the same device is already used by another Emacs terminal, this
     function signals an error.  If TTY is not suspended, this function
     does nothing.

 -- Function: controlling-tty-p &optional tty
     This function returns non-`nil' if TTY is the controlling terminal
     of the Emacs session; TTY can be a terminal object, a frame
     (meaning the terminal for that frame), or `nil' (meaning the
     terminal for the selected frame).

 -- Command: suspend-frame
     This command "suspends" a frame.  For GUI frames, it calls
     `iconify-frame' (*note Visibility of Frames::); for frames on text
     terminals, it calls either `suspend-emacs' or `suspend-tty',
     depending on whether the frame is displayed on the controlling
     terminal device or not.


File: elisp,  Node: System Environment,  Next: User Identification,  Prev: Getting Out,  Up: System Interface

39.3 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 UID, and so on.

 -- Variable: system-configuration
     This variable holds the standard GNU configuration name for the
     hardware/software configuration of your system, as a string.  For
     example, a typical value for a 64-bit GNU/Linux system is
     `"x86_64-unknown-linux-gnu"'.

 -- Variable: system-type
     The value of this variable is a symbol indicating the type of
     operating system Emacs is running on.  The possible values are:

    `aix'
          IBM's AIX.

    `berkeley-unix'
          Berkeley BSD and its variants.

    `cygwin'
          Cygwin, a Posix layer on top of MS-Windows.

    `darwin'
          Darwin (Mac OS X).

    `gnu'
          The GNU system (using the GNU kernel, which consists of the
          HURD and Mach).

    `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.)

    `gnu/kfreebsd'
          A GNU (glibc-based) system with a FreeBSD kernel.

    `hpux'
          Hewlett-Packard HPUX operating system.

    `irix'
          Silicon Graphics Irix system.

    `ms-dos'
          Microsoft's DOS.  Emacs compiled with DJGPP for MS-DOS binds
          `system-type' to `ms-dos' even when you run it on MS-Windows.

    `usg-unix-v'
          AT&T Unix System V.

    `windows-nt'
          Microsoft Windows NT, 9X and later.  The value of
          `system-type' is always `windows-nt', e.g. even on Windows 7.


     We do not wish to add new symbols to make finer distinctions
     unless it is absolutely necessary!  In fact, we hope to eliminate
     some of these alternatives in the future.  If you need to make a
     finer distinction than `system-type' allows for, you can test
     `system-configuration', e.g. against a regexp.

 -- Function: system-name
     This function returns the name of the machine you are running on,
     as a string.

   The symbol `system-name' is a variable as well as a function.  In
fact, the function returns whatever value the variable `system-name'
currently holds.  Thus, you can set the variable `system-name' in case
Emacs is confused about the name of your system.  The variable is also
useful for constructing frame titles (*note Frame Titles::).

 -- User Option: mail-host-address
     If this variable is non-`nil', it is used instead of `system-name'
     for purposes of generating email addresses.  For example, it is
     used when constructing the default value of `user-mail-address'.
     *Note User Identification::.  (Since this is done when Emacs
     starts up, the value actually used is the one saved when Emacs was
     dumped.  *Note Building Emacs::.)

 -- Command: getenv var &optional frame
     This function returns the value of the environment variable VAR,
     as a string.  VAR should be a string.  If VAR is undefined in the
     environment, `getenv' returns `nil'.  It returns `""' if VAR is
     set but null.  Within Emacs, a list of environment variables and
     their values is kept in the variable `process-environment'.

          (getenv "USER")
               => "lewis"

     The shell command `printenv' prints all or part of the environment:

          bash$ printenv
          PATH=/usr/local/bin:/usr/bin:/bin
          USER=lewis
          TERM=xterm
          SHELL=/bin/bash
          HOME=/home/lewis
          ...

 -- Command: setenv variable &optional value substitute
     This command sets the value of the environment variable named
     VARIABLE to VALUE.  VARIABLE should be a string.  Internally,
     Emacs Lisp can handle any string.  However, normally 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 VARIABLE.  If VALUE is omitted or `nil'
     (or, interactively, with a prefix argument), `setenv' removes
     VARIABLE from the environment.  Otherwise, VALUE should be a
     string.

     If the optional argument SUBSTITUTE is non-`nil', Emacs calls the
     function `substitute-env-vars' to expand any environment variables
     in VALUE.

     `setenv' works by modifying `process-environment'; binding that
     variable with `let' is also reasonable practice.

     `setenv' returns the new value of VARIABLE, or `nil' if it removed
     VARIABLE from the environment.

 -- Variable: process-environment
     This variable is a list of strings, each describing one environment
     variable.  The functions `getenv' and `setenv' work by means of
     this variable.

          process-environment
          => ("PATH=/usr/local/bin:/usr/bin:/bin"
              "USER=lewis"
              "TERM=xterm"
              "SHELL=/bin/bash"
              "HOME=/home/lewis"
              ...)

     If `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.

 -- Variable: initial-environment
     This variable holds the list of environment variables Emacs
     inherited from its parent process when Emacs started.

 -- Variable: path-separator
     This variable holds a string that says which character separates
     directories in a search path (as found in an environment
     variable).  Its value is `":"' for Unix and GNU systems, and `";"'
     for MS systems.

 -- Function: parse-colon-path path
     This function takes a search path string such as the value of the
     `PATH' environment variable, and splits it at the separators,
     returning a list of directory names.  `nil' in this list means the
     current directory.  Although the function's name says "colon", it
     actually uses the value of `path-separator'.

          (parse-colon-path ":/foo:/bar")
               => (nil "/foo/" "/bar/")

 -- Variable: 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.

 -- Variable: invocation-directory
     This variable holds the directory from which the Emacs executable
     was invoked, or `nil' if that directory cannot be determined.

 -- Variable: installation-directory
     If non-`nil', this is a directory within which to look for the
     `lib-src' and `etc' subdirectories.  In an installed Emacs, it is
     normally `nil'.  It is non-`nil' when Emacs can't find those
     directories in their standard installed locations, but can find
     them in a directory related somehow to the one containing the
     Emacs executable (i.e., `invocation-directory').

 -- Function: load-average &optional use-float
     This function returns the current 1-minute, 5-minute, and 15-minute
     system load averages, in a list.  The load average indicates the
     number of processes trying to run on the system.

     By default, the values are integers that are 100 times the system
     load averages, but if USE-FLOAT is non-`nil', then they are
     returned as floating point numbers without multiplying by 100.

     If it is impossible to obtain the load average, this function
     signals an error.  On some platforms, access to load averages
     requires installing Emacs as setuid or setgid so that it can read
     kernel information, and that usually isn't advisable.

     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.

          (load-average)
               => (169 48 36)
          (load-average t)
               => (1.69 0.48 0.36)

     The shell command `uptime' returns similar information.

 -- Function: emacs-pid
     This function returns the process ID of the Emacs process, as an
     integer.

 -- Variable: tty-erase-char
     This variable holds the erase character that was selected in the
     system's terminal driver, before Emacs was started.


File: elisp,  Node: User Identification,  Next: Time of Day,  Prev: System Environment,  Up: System Interface

39.4 User Identification
========================

 -- Variable: init-file-user
     This variable says which user's init files should be used by
     Emacs--or `nil' if none.  `""' stands for the user who originally
     logged in.  The value reflects command-line options such as `-q'
     or `-u 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 `init-file-user' is `nil', meaning that the `-q'
     option was used, then Lisp packages should not load any
     customization files or user profile.

 -- User Option: 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.

 -- Function: user-login-name &optional uid
     This function returns the name under which the user is logged in.
     It uses the environment variables `LOGNAME' or `USER' if either is
     set.  Otherwise, the value is based on the effective UID, not the
     real UID.

     If you specify UID (a number), the result is the user name that
     corresponds to UID, or `nil' if there is no such user.

 -- Function: user-real-login-name
     This function returns the user name corresponding to Emacs's real
     UID.  This ignores the effective UID, and the environment
     variables `LOGNAME' and `USER'.

 -- Function: user-full-name &optional uid
     This function returns the full name of the logged-in user--or the
     value of the environment variable `NAME', if that is set.

     If the Emacs process's user-id does not correspond to any known
     user (and provided `NAME' is not set), the result is `"unknown"'.

     If UID is non-`nil', then it should be a number (a user-id) or a
     string (a login name).  Then `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 `nil'.

   The symbols `user-login-name', `user-real-login-name' and
`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 (*note Frame
Titles::).

 -- Function: user-real-uid
     This function returns the real UID of the user.  The value may be
     a floating point number, in the (unlikely) event that the UID is
     too large to fit in a Lisp integer.

 -- Function: user-uid
     This function returns the effective UID of the user.  The value
     may be a floating point number.


File: elisp,  Node: Time of Day,  Next: Time Conversion,  Prev: User Identification,  Up: System Interface

39.5 Time of Day
================

This section explains how to determine the current time and time zone.

   Most of these functions represent time as a list of either three
integers, `(SEC-HIGH SEC-LOW MICROSEC)', or of two integers, `(SEC-HIGH
SEC-LOW)'.  The integers SEC-HIGH and SEC-LOW give the high and low
bits of an integer number of seconds.  This integer number, HIGH *
2**16 + LOW, is the number of seconds from the "epoch" (0:00 January 1,
1970 UTC) to the specified time.  The third list element MICROSEC, if
present, gives the number of microseconds from the start of that second
to the specified time.

   The return value of `current-time' represents time using three
integers, while the timestamps in the return value of `file-attributes'
use two integers (*note Definition of file-attributes::).  In function
arguments, e.g. the TIME-VALUE argument to `current-time-string', both
two- and three-integer lists are accepted.  You can convert times from
the list representation into standard human-readable strings using
`current-time', or to other forms using the `decode-time' and
`format-time-string' functions documented in the following sections.

 -- Function: 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 `substring' to extract pieces of it.  You should count
     characters from the beginning of the string rather than from the
     end, as additional information may some day be added at the end.

     The argument TIME-VALUE, if given, specifies a time to format
     (represented as a list of integers), instead of the current time.

          (current-time-string)
               => "Wed Oct 14 22:21:05 1987"

 -- Function: current-time
     This function returns the current time, represented as a list of
     three integers `(SEC-HIGH SEC-LOW MICROSEC)'.  On systems with
     only one-second time resolutions, MICROSEC is 0.

 -- Function: float-time &optional time-value
     This function returns the current time as a floating-point number
     of seconds since the epoch.  The optional argument TIME-VALUE, if
     given, specifies a time (represented as a list of integers) to
     convert instead of the current time.

     _Warning_: Since the result is floating point, it may not be
     exact.  Do not use this function if precise time stamps are
     required.

 -- Function: 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 `(OFFSET NAME)'.  Here 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, NAME,
     is a string giving the name of the time zone.  Both elements
     change when daylight saving time begins or ends; if the user has
     specified a time zone that does not use a seasonal time
     adjustment, then the value is constant through time.

     If the operating system doesn't supply all the information
     necessary to compute the value, the unknown elements of the list
     are `nil'.

     The argument TIME-VALUE, if given, specifies a time (represented
     as a list of integers) to analyze instead of the current time.

   The current time zone is determined by the `TZ' environment
variable.  *Note System Environment::.  For example, you can tell Emacs
to use universal time with `(setenv "TZ" "UTC0")'.  If `TZ' is not in
the environment, Emacs uses a platform-dependent default time zone.


File: elisp,  Node: Time Conversion,  Next: Time Parsing,  Prev: Time of Day,  Up: System Interface

39.6 Time Conversion
====================

These functions convert time values (lists of two or three integers, as
explained in the previous section) into calendrical information and
vice versa.

   Many 32-bit operating systems are limited to time values containing
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,
64-bit and some 32-bit 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 -37
represents the Gregorian year 38 B.C.

 -- Function: decode-time &optional time
     This function converts a time value into calendrical information.
     If you don't specify TIME, it decodes the current time.  The return
     value is a list of nine elements, as follows:

          (SECONDS MINUTES HOUR DAY MONTH YEAR DOW DST ZONE)

     Here is what the elements mean:

    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.

    MINUTES
          The number of minutes past the hour, as an integer between 0
          and 59.

    HOUR
          The hour of the day, as an integer between 0 and 23.

    DAY
          The day of the month, as an integer between 1 and 31.

    MONTH
          The month of the year, as an integer between 1 and 12.

    YEAR
          The year, an integer typically greater than 1900.

    DOW
          The day of week, as an integer between 0 and 6, where 0
          stands for Sunday.

    DST
          `t' if daylight saving time is effect, otherwise `nil'.

    ZONE
          An integer indicating the time zone, as the number of seconds
          east of Greenwich.

     *Common Lisp Note:* Common Lisp has different meanings for DOW and
     ZONE.

 -- Function: encode-time seconds minutes hour day month year &optional
          zone
     This function is the inverse of `decode-time'.  It converts seven
     items of calendrical data into a time value.  For the meanings of
     the arguments, see the table above under `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 `encode-time'.

     The optional argument ZONE defaults to the current time zone and
     its daylight saving time rules.  If specified, it can be either a
     list (as you would get from `current-time-zone'), a string as in
     the `TZ' environment variable, `t' for Universal Time, or an
     integer (as you would get from `decode-time').  The specified zone
     is used without any further alteration for daylight saving time.

     If you pass more than seven arguments to `encode-time', the first
     six are used as SECONDS through YEAR, the last argument is used as
     ZONE, and the arguments in between are ignored.  This feature
     makes it possible to use the elements of a list returned by
     `decode-time' as the arguments to `encode-time', like this:

          (apply 'encode-time (decode-time ...))

     You can perform simple date arithmetic by using out-of-range
     values for the SECONDS, MINUTES, HOUR, DAY, and 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.


File: elisp,  Node: Time Parsing,  Next: Processor Run Time,  Prev: Time Conversion,  Up: System Interface

39.7 Parsing and Formatting Times
=================================

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

 -- Function: date-to-time string
     This function parses the time-string STRING and returns the
     corresponding time value.

 -- Function: format-time-string format-string &optional time universal
     This function converts TIME (or the current time, if TIME is
     omitted) to a string according to FORMAT-STRING.  The argument
     FORMAT-STRING may contain `%'-sequences which say to substitute
     parts of the time.  Here is a table of what the `%'-sequences mean:

    `%a'
          This stands for the abbreviated name of the day of week.

    `%A'
          This stands for the full name of the day of week.

    `%b'
          This stands for the abbreviated name of the month.

    `%B'
          This stands for the full name of the month.

    `%c'
          This is a synonym for `%x %X'.

    `%C'
          This has a locale-specific meaning.  In the default locale
          (named C), it is equivalent to `%A, %B %e, %Y'.

    `%d'
          This stands for the day of month, zero-padded.

    `%D'
          This is a synonym for `%m/%d/%y'.

    `%e'
          This stands for the day of month, blank-padded.

    `%h'
          This is a synonym for `%b'.

    `%H'
          This stands for the hour (00-23).

    `%I'
          This stands for the hour (01-12).

    `%j'
          This stands for the day of the year (001-366).

    `%k'
          This stands for the hour (0-23), blank padded.

    `%l'
          This stands for the hour (1-12), blank padded.

    `%m'
          This stands for the month (01-12).

    `%M'
          This stands for the minute (00-59).

    `%n'
          This stands for a newline.

    `%N'
          This stands for the nanoseconds (000000000-999999999).  To
          ask for fewer digits, use `%3N' for milliseconds, `%6N' for
          microseconds, etc.  Any excess digits are discarded, without
          rounding.  Currently Emacs time stamps are at best
          microsecond resolution so the last three digits generated by
          plain `%N' are always zero.

    `%p'
          This stands for `AM' or `PM', as appropriate.

    `%r'
          This is a synonym for `%I:%M:%S %p'.

    `%R'
          This is a synonym for `%H:%M'.

    `%S'
          This stands for the seconds (00-59).

    `%t'
          This stands for a tab character.

    `%T'
          This is a synonym for `%H:%M:%S'.

    `%U'
          This stands for the week of the year (01-52), assuming that
          weeks start on Sunday.

    `%w'
          This stands for the numeric day of week (0-6).  Sunday is day
          0.

    `%W'
          This stands for the week of the year (01-52), assuming that
          weeks start on Monday.

    `%x'
          This has a locale-specific meaning.  In the default locale
          (named `C'), it is equivalent to `%D'.

    `%X'
          This has a locale-specific meaning.  In the default locale
          (named `C'), it is equivalent to `%T'.

    `%y'
          This stands for the year without century (00-99).

    `%Y'
          This stands for the year with century.

    `%Z'
          This stands for the time zone abbreviation (e.g., `EST').

    `%z'
          This stands for the time zone numerical offset (e.g.,
          `-0500').

     You can also specify the field width and type of padding for any of
     these `%'-sequences.  This works as in `printf': you write the
     field width as digits in the middle of a `%'-sequences.  If you
     start the field width with `0', it means to pad with zeros.  If you
     start the field width with `_', it means to pad with spaces.

     For example, `%S' specifies the number of seconds since the minute;
     `%03S' means to pad this with zeros to 3 positions, `%_3S' to pad
     with spaces to 3 positions.  Plain `%3S' pads with zeros, because
     that is how `%S' normally pads to two positions.

     The characters `E' and `O' act as modifiers when used between `%'
     and one of the letters in the table above.  `E' specifies using
     the current locale's "alternative" version of the date and time.
     In a Japanese locale, for example, `%Ex' might yield a date format
     based on the Japanese Emperors' reigns.  `E' is allowed in `%Ec',
     `%EC', `%Ex', `%EX', `%Ey', and `%EY'.

     `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 UNIVERSAL is non-`nil', that means to describe the time as
     Universal Time; `nil' means describe it using what Emacs believes
     is the local time zone (see `current-time-zone').

     This function uses the C library function `strftime' (*note
     Formatting Calendar Time: (libc)Formatting Calendar Time.) to do
     most of the work.  In order to communicate with that function, it
     first encodes its argument using the coding system specified by
     `locale-coding-system' (*note Locales::); after `strftime' returns
     the resulting string, `format-time-string' decodes the string
     using that same coding system.

 -- Function: seconds-to-time seconds
     This function converts SECONDS, a floating point number of seconds
     since the epoch, to a time value and returns that.  To perform the
     inverse conversion, use `float-time'.

 -- Function: format-seconds format-string seconds
     This function converts its argument SECONDS into a string of
     years, days, hours, etc., according to FORMAT-STRING.  The
     argument FORMAT-STRING may contain `%'-sequences which control the
     conversion.  Here is a table of what the `%'-sequences mean:

    `%y'
    `%Y'
          The integer number of 365-day years.

    `%d'
    `%D'
          The integer number of days.

    `%h'
    `%H'
          The integer number of hours.

    `%m'
    `%M'
          The integer number of minutes.

    `%s'
    `%S'
          The integer number of seconds.

    `%z'
          Non-printing control flag.  When it is used, other specifiers
          must be given in the order of decreasing size, i.e. years
          before days, hours before minutes, etc.  Nothing will be
          produced in the result string to the left of `%z' until the
          first non-zero conversion is encountered.  For example, the
          default format used by `emacs-uptime' (*note emacs-uptime:
          Processor Run Time.)  `"%Y, %D, %H, %M, %z%S"' means that the
          number of seconds will always be produced, but years, days,
          hours, and minutes will only be shown if they are non-zero.

    `%%'
          Produces a literal `%'.

     Upper-case format sequences produce the units in addition to the
     numbers, lower-case formats produce only the numbers.

     You can also specify the field width by following the `%' with a
     number; shorter numbers will be padded with blanks.  An optional
     period before the width requests zero-padding instead.  For
     example, `"%.3Y"' might produce `"004 years"'.

     _Warning:_ This function works only with values of SECONDS that
     don't exceed `most-positive-fixnum' (*note most-positive-fixnum:
     Integer Basics.).


File: elisp,  Node: Processor Run Time,  Next: Time Calculations,  Prev: Time Parsing,  Up: System Interface

39.8 Processor Run time
=======================

Emacs provides several functions and primitives that return time, both
elapsed and processor time, used by the Emacs process.

 -- Command: emacs-uptime &optional format
     This function returns a string representing the Emacs
     "uptime"--the elapsed wall-clock time this instance of Emacs is
     running.  The string is formatted by `format-seconds' according to
     the optional argument FORMAT.  For the available format
     descriptors, see *note format-seconds: Time Parsing.  If FORMAT is
     `nil' or omitted, it defaults to `"%Y, %D, %H, %M, %z%S"'.

     When called interactively, it prints the uptime in the echo area.

 -- Function: get-internal-run-time
     This function returns the processor run time used by Emacs as a
     list of three integers: `(HIGH LOW MICROSEC)'.  The integers HIGH
     and LOW combine to give the number of seconds, which is HIGH *
     2**16 + LOW.

     The third element, MICROSEC, gives the microseconds (or 0 for
     systems that return time with the resolution of only one second).

     Note that the time returned by this function excludes the time
     Emacs was not using the processor, and if the Emacs process has
     several threads, the returned value is the sum of the processor
     times used up by all Emacs threads.

     If the system doesn't provide a way to determine the processor run
     time, `get-internal-run-time' returns the same time as
     `current-time'.

 -- Command: emacs-init-time
     This function returns the duration of the Emacs initialization
     (*note Startup Summary::) in seconds, as a string.  When called
     interactively, it prints the duration in the echo area.


File: elisp,  Node: Time Calculations,  Next: Timers,  Prev: Processor Run Time,  Up: System Interface

39.9 Time Calculations
======================

These functions perform calendrical computations using time values (the
kind of list that `current-time' returns).

 -- Function: time-less-p t1 t2
     This returns `t' if time value T1 is less than time value T2.

 -- Function: time-subtract t1 t2
     This returns the time difference T1 - T2 between two time values,
     in the same format as a time value.

 -- Function: 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:

          (time-add TIME (seconds-to-time SECONDS))

 -- Function: time-to-days time
     This function returns the number of days between the beginning of
     year 1 and TIME.

 -- Function: time-to-day-in-year time
     This returns the day number within the year corresponding to TIME.

 -- Function: date-leap-year-p year
     This function returns `t' if YEAR is a leap year.


File: elisp,  Node: Timers,  Next: Idle Timers,  Prev: Time Calculations,  Up: System Interface

39.10 Timers for Delayed Execution
==================================

You can set up a "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
`sit-for' or `read-event' which _can_ wait.  Therefore, a timer's
execution may be delayed if Emacs is busy.  However, the time of
execution is very precise if Emacs is idle.

   Emacs binds `inhibit-quit' to `t' before calling the timer function,
because quitting out of many timer functions can leave things in an
inconsistent state.  This is normally unproblematical because most
timer functions don't do a lot of work.  Indeed, for a timer to call a
function that takes substantial time to run is likely to be annoying.
If a timer function needs to allow quitting, it should use
`with-local-quit' (*note Quitting::).  For example, if a timer function
calls `accept-process-output' to receive output from an external
process, that call should be wrapped inside `with-local-quit', to
ensure that `C-g' works if the external process hangs.

   It is usually a bad idea for timer functions to alter buffer
contents.  When they do, they usually should call `undo-boundary' both
before and after changing the buffer, to separate the timer's changes
from user commands' changes and prevent a single undo entry from
growing to be quite large.

   Timer functions should also avoid calling functions that cause Emacs
to wait, such as `sit-for' (*note Waiting::).  This can lead to
unpredictable effects, since other timers (or even the same timer) can
run while waiting.  If a timer function needs to perform an action
after a certain time has elapsed, it can do this by scheduling a new
timer.

   If a timer function calls functions that can change the match data,
it should save and restore the match data.  *Note Saving Match Data::.

 -- Command: run-at-time time repeat function &rest args
     This sets up a timer that calls the function FUNCTION with
     arguments ARGS at time TIME.  If REPEAT is a number (integer or
     floating point), the timer is scheduled to run again every REPEAT
     seconds after TIME.  If REPEAT is `nil', the timer runs only once.

     TIME may specify an absolute or a relative time.

     Absolute times may be specified using a string with a limited
     variety of formats, and are taken to be times _today_, even if
     already in the past.  The recognized forms are `XXXX', `X:XX', or
     `XX:XX' (military time), and `XXam', `XXAM', `XXpm', `XXPM',
     `XX:XXam', `XX:XXAM', `XX:XXpm', or `XX:XXPM'.  A period can be
     used instead of a colon to separate the hour and minute parts.

     To specify a relative time as a string, use numbers followed by
     units.  For example:

    `1 min'
          denotes 1 minute from now.

    `1 min 5 sec'
          denotes 65 seconds from now.

    `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.

     For relative time values, Emacs considers a month to be exactly
     thirty days, and a year to be exactly 365.25 days.

     Not all convenient formats are strings.  If TIME is a number
     (integer or floating point), that specifies a relative time
     measured in seconds.  The result of `encode-time' can also be used
     to specify an absolute value for TIME.

     In most cases, REPEAT has no effect on when _first_ call takes
     place--TIME alone specifies that.  There is one exception: if TIME
     is `t', then the timer runs whenever the time is a multiple of
     REPEAT seconds after the epoch.  This is useful for functions like
     `display-time'.

     The function `run-at-time' returns a timer value that identifies
     the particular scheduled future action.  You can use this value to
     call `cancel-timer' (see below).

   A repeating timer nominally ought to run every REPEAT seconds, but
remember that any invocation of a timer can be late.  Lateness of one
repetition has no effect on the scheduled time of the next repetition.
For instance, if Emacs is busy computing for long enough to cover three
scheduled repetitions of the timer, and then starts to wait, it will
immediately call the timer function three times in immediate succession
(presuming no other timers trigger before or between them).  If you
want a timer to run again no less than N seconds after the last
invocation, don't use the REPEAT argument.  Instead, the timer function
should explicitly reschedule the timer.

 -- Variable: timer-max-repeats
     This variable's value specifies the maximum number of times to
     repeat calling a timer function in a row, when many previously
     scheduled calls were unavoidably delayed.

 -- Macro: with-timeout (seconds timeout-forms...) body...
     Execute BODY, but give up after SECONDS seconds.  If BODY finishes
     before the time is up, `with-timeout' returns the value of the
     last form in BODY.  If, however, the execution of BODY is cut
     short by the timeout, then `with-timeout' executes all the
     TIMEOUT-FORMS and returns the value of the last of them.

     This macro works by setting a timer to run after SECONDS seconds.
     If BODY finishes before that time, it cancels the timer.  If the
     timer actually runs, it terminates execution of BODY, then
     executes TIMEOUT-FORMS.

     Since timers can run within a Lisp program only when the program
     calls a primitive that can wait, `with-timeout' cannot stop
     executing BODY while it is in the midst of a computation--only
     when it calls one of those primitives.  So use `with-timeout' only
     with a BODY that waits for input, not one that does a long
     computation.

   The function `y-or-n-p-with-timeout' provides a simple way to use a
timer to avoid waiting too long for an answer.  *Note Yes-or-No
Queries::.

 -- Function: cancel-timer timer
     This cancels the requested action for TIMER, which should be a
     timer--usually, one previously returned by `run-at-time' or
     `run-with-idle-timer'.  This cancels the effect of that call to
     one of these functions; the arrival of the specified time will not
     cause anything special to happen.


File: elisp,  Node: Idle Timers,  Next: Terminal Input,  Prev: Timers,  Up: System Interface

39.11 Idle Timers
=================

Here is how to set up a timer that runs when Emacs is idle for a
certain length of time.  Aside from how to set them up, idle timers
work just like ordinary timers.

 -- Command: run-with-idle-timer secs repeat function &rest args
     Set up a timer which runs the next time Emacs is idle for SECS
     seconds.  The value of SECS may be an integer or a floating point
     number; a value of the type returned by `current-idle-time' is
     also allowed.

     If REPEAT is `nil', the timer runs just once, the first time Emacs
     remains idle for a long enough time.  More often REPEAT is
     non-`nil', which means to run the timer _each time_ Emacs remains
     idle for SECS seconds.

     The function `run-with-idle-timer' returns a timer value which you
     can use in calling `cancel-timer' (*note Timers::).

   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 REPEAT is non-`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.

 -- Function: current-idle-time
     If Emacs is idle, this function returns the length of time Emacs
     has been idle, as a list of three integers: `(SEC-HIGH SEC-LOW
     MICROSEC)', where HIGH and LOW are the high and low bits for the
     number of seconds and MICROSEC is the additional number of
     microseconds (*note Time of Day::).

     When Emacs is not idle, `current-idle-time' returns `nil'.  This
     is a convenient way to test whether Emacs is idle.

     The main use of this function is when an idle timer function wants
     to "take a break" for a while.  It can set up another idle timer to
     call the same function again, after a few seconds more idleness.
     Here's an example:

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

          (defun timer-function ()
            ;; If the user types a command while `resume-timer'
            ;; is active, the next time this function is called from
            ;; its main idle timer, deactivate `resume-timer'.
            (when resume-timer
              (cancel-timer resume-timer))
            ...DO THE WORK FOR A WHILE...
            (when TAKING-A-BREAK
              (setq resume-timer
                    (run-with-idle-timer
                      ;; Compute an idle time BREAK-LENGTH
                      ;; more than the current value.
                      (time-add (current-idle-time)
                                (seconds-to-time BREAK-LENGTH))
                      nil
                      'timer-function))))

   Do not write an idle timer function containing a loop which does a
certain amount of processing each time around, and exits when
`(input-pending-p)' is non-`nil'.  This approach seems very natural but
has two problems:

   * It blocks out all process output (since Emacs accepts process
     output only while waiting).

   * It blocks out any idle timers that ought to run during that time.

The correct approach is for the idle timer to reschedule itself after a
brief pause, using the method in the `timer-function' example above.


File: elisp,  Node: Terminal Input,  Next: Terminal Output,  Prev: Idle Timers,  Up: System Interface

39.12 Terminal Input
====================

This section describes functions and variables for recording or
manipulating terminal input.  See *note Display::, for related
functions.

* Menu:

* Input Modes::         Options for how input is processed.
* Recording Input::     Saving histories of recent or all input events.


File: elisp,  Node: Input Modes,  Next: Recording Input,  Up: Terminal Input

39.12.1 Input Modes
-------------------

 -- Function: set-input-mode interrupt flow meta &optional quit-char
     This function sets the mode for reading keyboard input.  If
     INTERRUPT is non-null, then Emacs uses input interrupts.  If it is
     `nil', then it uses CBREAK mode.  The default setting is
     system-dependent.  Some systems always use 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 FLOW is non-`nil', then Emacs uses XON/XOFF (`C-q', `C-s') flow
     control for output to the terminal.  This has no effect except in
     CBREAK mode.

     The argument META controls support for input character codes above
     127.  If META is `t', Emacs converts characters with the 8th bit
     set into Meta characters.  If META is `nil', Emacs disregards the
     8th bit; this is necessary when the terminal uses it as a parity
     bit.  If META is neither `t' nor `nil', Emacs uses all 8 bits of
     input unchanged.  This is good for terminals that use 8-bit
     character sets.

     If QUIT-CHAR is non-`nil', it specifies the character to use for
     quitting.  Normally this character is `C-g'.  *Note Quitting::.

   The `current-input-mode' function returns the input mode settings
Emacs is currently using.

 -- Function: current-input-mode
     This function returns the current mode for reading keyboard input.
     It returns a list, corresponding to the arguments of
     `set-input-mode', of the form `(INTERRUPT FLOW META QUIT)' in
     which:
    INTERRUPT
          is non-`nil' when Emacs is using interrupt-driven input.  If
          `nil', Emacs is using CBREAK mode.

    FLOW
          is non-`nil' if Emacs uses XON/XOFF (`C-q', `C-s') flow
          control for output to the terminal.  This value is meaningful
          only when INTERRUPT is `nil'.

    META
          is `t' if Emacs treats the eighth bit of input characters as
          the meta bit; `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.

    QUIT
          is the character Emacs currently uses for quitting, usually
          `C-g'.


File: elisp,  Node: Recording Input,  Prev: Input Modes,  Up: Terminal Input

39.12.2 Recording Input
-----------------------

 -- Function: recent-keys
     This function returns a vector containing the last 300 input
     events from the keyboard or mouse.  All input events are included,
     whether or not they were used as parts of key sequences.  Thus,
     you always get the last 100 input events, not counting events
     generated by keyboard macros.  (These are excluded because they
     are less interesting for debugging; it should be enough to see the
     events that invoked the macros.)

     A call to `clear-this-command-keys' (*note Command Loop Info::)
     causes this function to return an empty vector immediately
     afterward.

 -- Command: open-dribble-file filename
     This function opens a "dribble file" named 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 `<...>'.

     You close the dribble file by calling this function with an
     argument of `nil'.

     This function is normally used to record the input necessary to
     trigger an Emacs bug, for the sake of a bug report.

          (open-dribble-file "~/dribble")
               => nil

   See also the `open-termscript' function (*note Terminal Output::).


File: elisp,  Node: Terminal Output,  Next: Sound Output,  Prev: Terminal Input,  Up: System Interface

39.13 Terminal Output
=====================

The terminal output functions send output to a text terminal, or keep
track of output sent to the terminal.  The variable `baud-rate' tells
you what Emacs thinks is the output speed of the terminal.

 -- User Option: baud-rate
     This variable's value is the output speed of the terminal, as far
     as Emacs knows.  Setting this variable does not change the speed
     of actual data transmission, but the value is used for
     calculations such as padding.

     It also affects decisions about whether to scroll part of the
     screen or repaint on text terminals.  *Note Forcing Redisplay::,
     for the corresponding functionality on graphical terminals.

     The value is measured in baud.

   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 `baud-rate'.

 -- Function: send-string-to-terminal string &optional terminal
     This function sends STRING to TERMINAL without alteration.
     Control characters in STRING have terminal-dependent effects.
     This function operates only on text terminals.  TERMINAL may be a
     terminal object, a frame, or `nil' for the selected frame's
     terminal.  In batch mode, STRING is sent to `stdout' when TERMINAL
     is `nil'.

     One use of this function is to define function keys on terminals
     that have downloadable function key definitions.  For example,
     this is how (on certain terminals) to define function key 4 to
     move forward four characters (by transmitting the characters `C-u
     C-f' to the computer):

          (send-string-to-terminal "\eF4\^U\^F")
               => nil

 -- Command: open-termscript filename
     This function is used to open a "termscript file" that will record
     all the characters sent by Emacs to the terminal.  It returns
     `nil'.  Termscript files are useful for investigating problems
     where Emacs garbles the screen, problems that are due to incorrect
     Termcap entries or to undesirable settings of terminal options more
     often than to actual Emacs bugs.  Once you are certain which
     characters were actually output, you can determine reliably
     whether they correspond to the Termcap specifications in use.

     You close the termscript file by calling this function with an
     argument of `nil'.

     See also `open-dribble-file' in *note Recording Input::.

          (open-termscript "../junk/termscript")
               => nil


File: elisp,  Node: Sound Output,  Next: X11 Keysyms,  Prev: Terminal Output,  Up: System Interface

39.14 Sound Output
==================

To play sound using Emacs, use the function `play-sound'.  Only certain
systems are supported; if you call `play-sound' on a system which
cannot really do the job, it gives an error.

   The sound must be stored as a file in RIFF-WAVE format (`.wav') or
Sun Audio format (`.au').

 -- Function: play-sound sound
     This function plays a specified sound.  The argument, SOUND, has
     the form `(sound PROPERTIES...)', where the 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
     SOUND, and their meanings:

    `:file FILE'
          This specifies the file containing the sound to play.  If the
          file name is not absolute, it is expanded against the
          directory `data-directory'.

    `:data DATA'
          This specifies the sound to play without need to refer to a
          file.  The value, DATA, should be a string containing the
          same bytes as a sound file.  We recommend using a unibyte
          string.

    `:volume 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.

    `:device DEVICE'
          This specifies the system device on which to play the sound,
          as a string.  The default device is system-dependent.

     Before actually playing the sound, `play-sound' calls the
     functions in the list `play-sound-functions'.  Each function is
     called with one argument, SOUND.

 -- Command: play-sound-file file &optional volume device
     This function is an alternative interface to playing a sound FILE
     specifying an optional VOLUME and DEVICE.

 -- Variable: 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.


File: elisp,  Node: X11 Keysyms,  Next: Batch Mode,  Prev: Sound Output,  Up: System Interface

39.15 Operating on X11 Keysyms
==============================

To define system-specific X11 keysyms, set the variable
`system-key-alist'.

 -- Variable: 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 .
     SYMBOL)', where CODE is the numeric keysym code (not including the
     "vendor specific" bit, -2**28), and SYMBOL is the name for the
     function key.

     For example `(168 . mute-acute)' defines a system-specific key
     (used by HP X servers) whose numeric code is -2**28 + 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.  *Note Multiple Terminals::.

   You can specify which keysyms Emacs should use for the Meta, Alt,
Hyper, and Super modifiers by setting these variables:

 -- Variable: x-alt-keysym
 -- Variable: x-meta-keysym
 -- Variable: x-hyper-keysym
 -- Variable: x-super-keysym
     The name of the keysym that should stand for the Alt modifier
     (respectively, for Meta, Hyper, and Super).  For example, here is
     how to swap the Meta and Alt modifiers within Emacs:
          (setq x-alt-keysym 'meta)
          (setq x-meta-keysym 'alt)


File: elisp,  Node: Batch Mode,  Next: Session Management,  Prev: X11 Keysyms,  Up: System Interface

39.16 Batch Mode
================

The command-line option `-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 `-l FILE', which loads the library named FILE,
or `-f FUNCTION', which calls FUNCTION with no arguments, or `--eval
FORM'.

   Any Lisp program output that would normally go to the echo area,
either using `message', or using `prin1', etc., with `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.)

 -- Variable: noninteractive
     This variable is non-`nil' when Emacs is running in batch mode.


File: elisp,  Node: Session Management,  Next: Notifications,  Prev: Batch Mode,  Up: System Interface

39.17 Session Management
========================

Emacs supports the X Session Management Protocol, which is used to
suspend and restart applications.  In the X Window System, a program
called the "session manager" is responsible for keeping track of the
applications that are running.  When the X server shuts down, the
session manager asks applications to save their state, and delays the
actual shutdown until they respond.  An application can also cancel the
shutdown.

   When the session manager restarts a suspended session, it directs
these applications to individually reload their saved state.  It does
this by specifying a special command-line argument that says what saved
session to restore.  For Emacs, this argument is `--smid SESSION'.

 -- Variable: emacs-save-session-functions
     Emacs supports saving state via a hook called
     `emacs-save-session-functions'.  Emacs runs this hook when the
     session manager tells it that the window system is shutting down.
     The functions are called with no arguments, and with the current
     buffer set to a temporary buffer.  Each function can use `insert'
     to add Lisp code to this buffer.  At the end, Emacs saves the
     buffer in a file, called the "session file".

     Subsequently, when the session manager restarts Emacs, it loads the
     session file automatically (*note Loading::).  This is performed
     by a function named `emacs-session-restore', which is called during
     startup.  *Note Startup Summary::.

     If a function in `emacs-save-session-functions' returns non-`nil',
     Emacs tells the session manager to cancel the shutdown.

   Here is an example that just inserts some text into `*scratch*' when
Emacs is restarted by the session manager.

     (add-hook 'emacs-save-session-functions 'save-yourself-test)

     (defun save-yourself-test ()
       (insert "(save-current-buffer
       (switch-to-buffer \"*scratch*\")
       (insert \"I am restored\"))")
       nil)


File: elisp,  Node: Notifications,  Next: Dynamic Libraries,  Prev: Session Management,  Up: System Interface

39.18 Desktop Notifications
===========================

Emacs is able to send "notifications" on systems that support the
freedesktop.org Desktop Notifications Specification.  In order to use
this functionality, Emacs must have been compiled with D-Bus support,
and the `notifications' library must be loaded.

 -- Function: notifications-notify &rest params
     This function sends a notification to the desktop via D-Bus,
     consisting of the parameters specified by the PARAMS arguments.
     These arguments should consist of alternating keyword and value
     pairs.  The supported keywords and values are as follows:

    `:title TITLE'
          The notification title.

    `:body TEXT'
          The notification body text.  Depending on the implementation
          of the notification server, the text could contain HTML
          markups, like `"<b>bold text</b>"', or hyperlinks.

    `:app-name NAME'
          The name of the application sending the notification.  The
          default is `notifications-application-name'.

    `:replaces-id ID'
          The notification ID that this notification replaces.  ID must
          be the result of a previous `notifications-notify' call.

    `:app-icon ICON-FILE'
          The file name of the notification icon.  If set to `nil', no
          icon is displayed.  The default is
          `notifications-application-icon'.

    `:actions (KEY TITLE KEY TITLE ...)'
          A list of actions to be applied.  KEY and TITLE are both
          strings.  The default action (usually invoked by clicking the
          notification) should have a key named `"default"'.  The title
          can be anything, though implementations are free not to
          display it.

    `:timeout TIMEOUT'
          The timeout time in milliseconds since the display of the
          notification at which the notification should automatically
          close.  If -1, the notification's expiration time is
          dependent on the notification server's settings, and may vary
          for the type of notification.  If 0, the notification never
          expires.  Default value is -1.

    `:urgency URGENCY'
          The urgency level.  It can be `low', `normal', or `critical'.

    `:category CATEGORY'
          The type of notification this is, a string.

    `:desktop-entry FILENAME'
          This specifies the name of the desktop filename representing
          the calling program, like `"emacs"'.

    `:image-data (WIDTH HEIGHT ROWSTRIDE HAS-ALPHA BITS CHANNELS DATA)'
          This is a raw data image format that describes the width,
          height, rowstride, whether there is an alpha channel, bits
          per sample, channels and image data, respectively.

    `:image-path PATH'
          This is represented either as a URI (`file://' is the only URI
          schema supported right now) or a name in a
          freedesktop.org-compliant icon theme from
          `$XDG_DATA_DIRS/icons'.

    `:sound-file FILENAME'
          The path to a sound file to play when the notification pops
          up.

    `:sound-name NAME'
          A themable named sound from the freedesktop.org sound naming
          specification from `$XDG_DATA_DIRS/sounds', to play when the
          notification pops up.  Similar to the icon name, only for
          sounds. An example would be `"message-new-instant"'.

    `:suppress-sound'
          Causes the server to suppress playing any sounds, if it has
          that ability.

    `:x POSITION'
    `:y POSITION'
          Specifies the X, Y location on the screen that the
          notification should point to.  Both arguments must be used
          together.

    `:on-action FUNCTION'
          Function to call when an action is invoked.  The notification
          ID and the KEY of the action are passed as arguments to the
          function.

    `:on-close FUNCTION'
          Function to call when the notification has been closed by
          timeout or by the user.  The function receive the
          notification ID and the closing REASON as arguments:

             * `expired' if the notification has expired

             * `dismissed' if the notification was dismissed by the user

             * `close-notification' if the notification was closed by a
               call to `notifications-close-notification'

             * `undefined' if the notification server hasn't provided a
               reason

     This function returns a notification id, an integer, which can be
     used to manipulate the notification item with
     `notifications-close-notification' or the `:replaces-id' argument
     of another `notifications-notify' call.  For example:

          (defun my-on-action-function (id key)
            (message "Message %d, key \"%s\" pressed" id key))
               => my-on-action-function

          (defun my-on-close-function (id reason)
            (message "Message %d, closed due to \"%s\"" id reason))
               => my-on-close-function

          (notifications-notify
           :title "Title"
           :body "This is <b>important</b>."
           :actions '("Confirm" "I agree" "Refuse" "I disagree")
           :on-action 'my-on-action-function
           :on-close 'my-on-close-function)
               => 22

          A message window opens on the desktop.  Press "I agree"
               => Message 22, key "Confirm" pressed
                  Message 22, closed due to "dismissed"

 -- Function: notifications-close-notification id
     This function closes a notification with identifier ID.


File: elisp,  Node: Dynamic Libraries,  Prev: Notifications,  Up: System Interface

39.19 Dynamically Loaded Libraries
==================================

A "dynamically loaded library" is a library that is loaded on demand,
when its facilities are first needed.  Emacs supports such on-demand
loading of support libraries for some of its features.

 -- Variable: dynamic-library-alist
     This is an alist of dynamic libraries and external library files
     implementing them.

     Each element is a list of the form `(LIBRARY FILES...)', where the
     `car' is a symbol representing a supported external library, and
     the rest are strings giving alternate filenames for that library.

     Emacs tries to load the library from the files in the order they
     appear in the list; if none is found, the Emacs session won't have
     access to that library, and the features it provides will be
     unavailable.

     Image support on some platforms uses this facility.  Here's an
     example of setting this variable for supporting images on
     MS-Windows:

          (setq dynamic-library-alist
                '((xpm "libxpm.dll" "xpm4.dll" "libXpm-nox4.dll")
                  (png "libpng12d.dll" "libpng12.dll" "libpng.dll"
                       "libpng13d.dll" "libpng13.dll")
                  (jpeg "jpeg62.dll" "libjpeg.dll" "jpeg-62.dll"
                        "jpeg.dll")
                  (tiff "libtiff3.dll" "libtiff.dll")
                  (gif "giflib4.dll" "libungif4.dll" "libungif.dll")
                  (svg "librsvg-2-2.dll")
                  (gdk-pixbuf "libgdk_pixbuf-2.0-0.dll")
                  (glib "libglib-2.0-0.dll")
          	(gobject "libgobject-2.0-0.dll")))

     Note that image types `pbm' and `xbm' do not need entries in this
     variable because they do not depend on external libraries and are
     always available in Emacs.

     Also note that this variable is not meant to be a generic facility
     for accessing external libraries; only those already known by
     Emacs can be loaded through it.

     This variable is ignored if the given LIBRARY is statically linked
     into Emacs.


File: elisp,  Node: Packaging,  Next: Antinews,  Prev: System Interface,  Up: Top

40 Preparing Lisp code for distribution
***************************************

Emacs provides a standard way to distribute Emacs Lisp code to users.
A "package" is a collection of one or more files, formatted and bundled
in such a way that users can easily download, install, uninstall, and
upgrade it.

   The following sections describe how to create a package, and how to
put it in a "package archive" for others to download.  *Note Packages:
(emacs)Packages, for a description of user-level features of the
packaging system.

* Menu:

* Packaging Basics::        The basic concepts of Emacs Lisp packages.
* Simple Packages::         How to package a single .el file.
* Multi-file Packages::     How to package multiple files.
* Package Archives::        Maintaining package archives.


File: elisp,  Node: Packaging Basics,  Next: Simple Packages,  Up: Packaging

40.1 Packaging Basics
=====================

A package is either a "simple package" or a "multi-file package".  A
simple package is stored in a package archive as a single Emacs Lisp
file, while a multi-file package is stored as a tar file (containing
multiple Lisp files, and possibly non-Lisp files such as a manual).

   In ordinary usage, the difference between simple packages and
multi-file packages is relatively unimportant; the Package Menu
interface makes no distinction between them.  However, the procedure
for creating them differs, as explained in the following sections.

   Each package (whether simple or multi-file) has certain "attributes":

Name
     A short word (e.g. `auctex').  This is usually also the symbol
     prefix used in the program (*note Coding Conventions::).

Version
     A version number, in a form that the function `version-to-list'
     understands (e.g. `11.86').  Each release of a package should be
     accompanied by an increase in the version number.

Brief description
     This is shown when the package is listed in the Package Menu.  It
     should occupy a single line, ideally in 36 characters or less.

Long description
     This is shown in the buffer created by `C-h P'
     (`describe-package'), following the package's brief description
     and installation status.  It normally spans multiple lines, and
     should fully describe the package's capabilities and how to begin
     using it once it is installed.

Dependencies
     A list of other packages (possibly including minimal acceptable
     version numbers) on which this package depends.  The list may be
     empty, meaning this package has no dependencies.  Otherwise,
     installing this package also automatically installs its
     dependencies; if any dependency cannot be found, the package
     cannot be installed.

   Installing a package, either via the command `package-install-file',
or via the Package Menu, creates a subdirectory of `package-user-dir'
named `NAME-VERSION', where NAME is the package's name and VERSION its
version (e.g. `~/.emacs.d/elpa/auctex-11.86/').  We call this the
package's "content directory".  It is where Emacs puts the package's
contents (the single Lisp file for a simple package, or the files
extracted from a multi-file package).

   Emacs then searches every Lisp file in the content directory for
autoload magic comments (*note Autoload::).  These autoload definitions
are saved to a file named `NAME-autoloads.el' in the content directory.
They are typically used to autoload the principal user commands defined
in the package, but they can also perform other tasks, such as adding
an element to `auto-mode-alist' (*note Auto Major Mode::).  Note that a
package typically does _not_ autoload every function and variable
defined within it--only the handful of commands typically called to
begin using the package.  Emacs then byte-compiles every Lisp file in
the package.

   After installation, the installed package is "loaded": Emacs adds
the package's content directory to `load-path', and evaluates the
autoload definitions in `NAME-autoloads.el'.

   Whenever Emacs starts up, it automatically calls the function
`package-initialize' to load installed packages.  This is done after
loading the init file and abbrev file (if any) and before running
`after-init-hook' (*note Startup Summary::).  Automatic package loading
is disabled if the user option `package-enable-at-startup' is `nil'.

 -- Command: package-initialize &optional no-activate
     This function initializes Emacs' internal record of which packages
     are installed, and loads them.  The user option `package-load-list'
     specifies which packages to load; by default, all installed
     packages are loaded.  *Note Package Installation: (emacs)Package
     Installation.

     The optional argument NO-ACTIVATE, if non-`nil', causes Emacs to
     update its record of installed packages without actually loading
     them; it is for internal use only.


File: elisp,  Node: Simple Packages,  Next: Multi-file Packages,  Prev: Packaging Basics,  Up: Packaging

40.2 Simple Packages
====================

A simple package consists of a single Emacs Lisp source file.  The file
must conform to the Emacs Lisp library header conventions (*note
Library Headers::).  The package's attributes are taken from the
various headers, as illustrated by the following example:

     ;;; superfrobnicator.el --- Frobnicate and bifurcate flanges

     ;; Copyright (C) 2011 Free Software Foundation, Inc.

     ;; Author: J. R. Hacker <jrh@example.com>
     ;; Version: 1.3
     ;; Package-Requires: ((flange "1.0"))
     ;; Keywords: frobnicate

     ...

     ;;; Commentary:

     ;; This package provides a minor mode to frobnicate and/or
     ;; bifurcate any flanges you desire.  To activate it, just type
     ...

     ;;;###autoload
     (define-minor-mode superfrobnicator-mode
     ...

   The name of the package is the same as the base name of the file, as
written on the first line.  Here, it is `superfrobnicator'.

   The brief description is also taken from the first line.  Here, it
is `Frobnicate and bifurcate flanges'.

   The version number comes from the `Package-Version' header, if it
exists, or from the `Version' header otherwise.  One or the other
_must_ be present.  Here, the version number is 1.3.

   If the file has a `;;; Commentary:' section, this section is used as
the long description.  (When displaying the description, Emacs omits
the `;;; Commentary:' line, as well as the leading comment characters
in the commentary itself.)

   If the file has a `Package-Requires' header, that is used as the
package dependencies.  In the above example, the package depends on the
`flange' package, version 1.0 or higher.  *Note Library Headers::, for
a description of the `Package-Requires' header.  If the header is
omitted, the package has no dependencies.

   The file ought to also contain one or more autoload magic comments,
as explained in *note Packaging Basics::.  In the above example, a magic
comment autoloads `superfrobnicator-mode'.

   *Note Package Archives::, for a explanation of how to add a
single-file package to a package archive.


File: elisp,  Node: Multi-file Packages,  Next: Package Archives,  Prev: Simple Packages,  Up: Packaging

40.3 Multi-file Packages
========================

A multi-file package is less convenient to create than a single-file
package, but it offers more features: it can include multiple Emacs
Lisp files, an Info manual, and other file types (such as images).

   Prior to installation, a multi-file package is stored in a package
archive as a tar file.  The tar file must be named `NAME-VERSION.tar',
where NAME is the package name and VERSION is the version number.  Its
contents, once extracted, must all appear in a directory named
`NAME-VERSION', the "content directory" (*note Packaging Basics::).
Files may also extract into subdirectories of the content directory.

   One of the files in the content directory must be named
`NAME-pkg.el'.  It must contain a single Lisp form, consisting of a
call to the function `define-package', described below.  This defines
the package's version, brief description, and requirements.

   For example, if we distribute version 1.3 of the superfrobnicator as
a multi-file package, the tar file would be `superfrobnicator-1.3.tar'.
Its contents would extract into the directory `superfrobnicator-1.3',
and one of these would be the file `superfrobnicator-pkg.el'.

 -- Function: define-package name version &optional docstring
          requirements
     This function defines a package.  NAME is the package name, a
     string.  VERSION is the version, as a string of a form that can be
     understood by the function `version-to-list'.  DOCSTRING is the
     brief description.

     REQUIREMENTS is a list of required packages and their versions.
     Each element in this list should have the form `(DEP-NAME
     DEP-VERSION)', where DEP-NAME is a symbol whose name is the
     dependency's package name, and DEP-VERSION is the dependency's
     version (a string).

   If the content directory contains a file named `README', this file
is used as the long description.

   If the content directory contains a file named `dir', this is
assumed to be an Info directory file made with `install-info'.  *Note
Invoking install-info: (texinfo)Invoking install-info.  The relevant
Info files should also be present in the content directory.  In this
case, Emacs will automatically add the content directory to
`Info-directory-list' when the package is activated.

   Do not include any `.elc' files in the package.  Those are created
when the package is installed.  Note that there is no way to control
the order in which files are byte-compiled.

   Do not include any file named `NAME-autoloads.el'.  This file is
reserved for the package's autoload definitions (*note Packaging
Basics::).  It is created automatically when the package is installed,
by searching all the Lisp files in the package for autoload magic
comments.

   If the multi-file package contains auxiliary data files (such as
images), the package's Lisp code can refer to these files via the
variable `load-file-name' (*note Loading::).  Here is an example:

     (defconst superfrobnicator-base (file-name-directory load-file-name))

     (defun superfrobnicator-fetch-image (file)
       (expand-file-name file superfrobnicator-base))


File: elisp,  Node: Package Archives,  Prev: Multi-file Packages,  Up: Packaging

40.4 Creating and Maintaining Package Archives
==============================================

Via the Package Menu, users may download packages from "package
archives".  Such archives are specified by the variable
`package-archives', whose default value contains a single entry: the
archive hosted by the GNU project at `elpa.gnu.org'.  This section
describes how to set up and maintain a package archive.

 -- User Option: package-archives
     The value of this variable is an alist of package archives
     recognized by the Emacs package manager.

     Each alist element corresponds to one archive, and should have the
     form `(ID . LOCATION)', where ID is the name of the archive (a
     string) and LOCATION is its "base location" (a string).

     If the base location starts with `http:', it is treated as a HTTP
     URL, and packages are downloaded from this archive via HTTP (as is
     the case for the default GNU archive).

     Otherwise, the base location should be a directory name.  In this
     case, Emacs retrieves packages from this archive via ordinary file
     access.  Such "local" archives are mainly useful for testing.

   A package archive is simply a directory in which the package files,
and associated files, are stored.  If you want the archive to be
reachable via HTTP, this directory must be accessible to a web server.
How to accomplish this is beyond the scope of this manual.

   A convenient way to set up and update a package archive is via the
`package-x' library.  This is included with Emacs, but not loaded by
default; type `M-x load-library <RET> package-x <RET>' to load it, or
add `(require 'package-x)' to your init file.  *Note Lisp Libraries:
(emacs)Lisp Libraries.  Once loaded, you can make use of the following:

 -- User Option: package-archive-upload-base
     The value of this variable is the base location of a package
     archive, as a directory name.  The commands in the `package-x'
     library will use this base location.

     The directory name should be absolute.  You may specify a remote
     name, such as `/ssh:foo@example.com:/var/www/packages/', if the
     package archive is on a different machine.  *Note Remote Files:
     (emacs)Remote Files.

 -- Command: package-upload-file filename
     This command prompts for FILENAME, a file name, and uploads that
     file to `package-archive-upload-base'.  The file must be either a
     simple package (a `.el' file) or a multi-file package (a `.tar'
     file); otherwise, an error is raised.  The package attributes are
     automatically extracted, and the archive's contents list is
     updated with this information.

     If `package-archive-upload-base' does not specify a valid
     directory, the function prompts interactively for one.  If the
     directory does not exist, it is created.  The directory need not
     have any initial contents (i.e., you can use this command to
     populate an initially empty archive).

 -- Command: package-upload-buffer
     This command is similar to `package-upload-file', but instead of
     prompting for a package file, it uploads the contents of the
     current buffer.  The current buffer must be visiting a simple
     package (a `.el' file) or a multi-file package (a `.tar' file);
     otherwise, an error is raised.

After you create an archive, remember that it is not accessible in the
Package Menu interface unless it is in `package-archives'.


File: elisp,  Node: Antinews,  Next: GNU Free Documentation License,  Prev: Packaging,  Up: Top

Appendix A Emacs 23 Antinews
****************************

For those users who live backwards in time, here is information about
downgrading to Emacs version 23.4.  We hope you will enjoy the greater
simplicity that results from the absence of many Emacs 24.2 features.

A.1 Old Lisp Features in Emacs 23
=================================

   * Support for lexical scoping has been removed; all variables are
     dynamically scoped.  The `lexical-binding' variable has been
     removed, and so has the LEXICAL argument to `eval'.  The `defvar'
     and `defconst' forms no longer mark variables as dynamic, since
     all variables are dynamic.

     Having only dynamic binding follows the spirit of Emacs
     extensibility, for it allows any Emacs code to access any defined
     variable with a minimum of fuss.  But *Note Dynamic Binding
     Tips::, for tips to avoid making your programs hard to understand.

   * Calling a minor mode function from Lisp with a nil or omitted
     argument does not enable the minor mode unconditionally; instead,
     it toggles the minor mode--which is the straightforward thing to
     do, since that is the behavior when invoked interactively.  One
     downside is that it is more troublesome to enable minor modes from
     hooks; you have to do something like

          (add-hook 'foo-hook (lambda () (bar-mode 1)))

     or define `turn-on-bar-mode' and call that from the hook.

   * The `prog-mode' dummy major mode has been removed.  Instead of
     using it as a crutch to meet programming mode conventions, you
     should explicitly ensure that your mode follows those conventions.
     *Note Major Mode Conventions::.

   * Emacs no longer supports bidirectional display and editing.  Since
     there is no need to worry about the insertion of right-to-left text
     messing up how lines and paragraphs are displayed, the function
     `bidi-string-mark-left-to-right' has been removed; so have many
     other functions and variables related to bidirectional display.
     Unicode directionality characters like `U+200E' ("left-to-right
     mark") have no special effect on display.

   * Emacs windows now have most of their internal state hidden from
     Lisp.  Internal windows are no longer visible to Lisp; functions
     such as `window-parent', window parameters related to window
     arrangement, and window-local buffer lists have all been removed.
     Functions for resizing windows can delete windows if they become
     too small.

     The "action function" feature for controlling buffer display has
     been removed, including `display-buffer-overriding-action' and
     related variables, as well as the ACTION argument to
     `display-buffer' and other functions.  The way to programmatically
     control how Emacs chooses a window to display a buffer is to bind
     the right combination of `special-display-regexps',
     `pop-up-frames', and other variables.

   * The standard completion interface has been simplified, eliminating
     the `completion-extra-properties' variable, the `metadata' action
     flag for completion functions, and the concept of "completion
     categories".  Lisp programmers may now find the choice of methods
     for tuning completion less bewildering, but if a package finds the
     streamlined interface insufficient for its needs, it must
     implement its own specialized completion feature.

   * `copy-directory' now behaves the same whether or not the
     destination is an existing directory: if the destination exists,
     the _contents_ of the first directory are copied into it (with
     subdirectories handled recursively), rather than copying the first
     directory into a subdirectory.

   * The TRASH arguments for `delete-file' and `delete-directory' have
     been removed.  The variable `delete-by-moving-to-trash' must now
     be used with care; whenever it is non-`nil', all calls to
     `delete-file' or `delete-directory' use the trash.

   * Because Emacs no longer supports SELinux file contexts, the
     PRESERVE-SELINUX-CONTEXT argument to `copy-file' has been removed.
     The return value of `backup-buffer' no longer has an entry for the
     SELinux file context.

   * For mouse click input events in the text area, the Y pixel
     coordinate in the POSITION list (*note Click Events::) now counts
     from the top of the header line, if there is one, rather than the
     top of the text area.

   * Bindings in menu keymaps (*note Format of Keymaps::) now sometimes
     get an additional CACHE entry in their definitions, like this:

          (TYPE ITEM-NAME CACHE . BINDING)

     The CACHE entry is used internally by Emacs to record equivalent
     keyboard key sequences for invoking the same command; Lisp programs
     should never use it.

   * The `gnutls' library has been removed, and the function
     `open-network-stream' correspondingly simplified.  Lisp programs
     that want an encrypted network connection must now call external
     utilities such as `starttls' or `gnutls-cli'.

   * Tool bars can no longer display separators, which frees up several
     pixels of space on each graphical frame.

   * As part of the ongoing quest for simplicity, many other functions
     and variables have been eliminated.


File: elisp,  Node: GNU Free Documentation License,  Next: GPL,  Prev: Antinews,  Up: Top

Appendix B GNU Free Documentation License
*****************************************

                     Version 1.3, 3 November 2008

     Copyright (C) 2000, 2001, 2002, 2007, 2008, 2009 Free Software Foundation, Inc.
     `http://fsf.org/'

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

  0. PREAMBLE

     The purpose of this License is to make a manual, textbook, or other
     functional and useful document "free" in the sense of freedom: to
     assure everyone the effective freedom to copy and redistribute it,
     with or without modifying it, either commercially or
     noncommercially.  Secondarily, this License preserves for the
     author and publisher a way to get credit for their work, while not
     being considered responsible for modifications made by others.

     This License is a kind of "copyleft", which means that derivative
     works of the document must themselves be free in the same sense.
     It complements the GNU General Public License, which is a copyleft
     license designed for free software.

     We have designed this License in order to use it for manuals for
     free software, because free software needs free documentation: a
     free program should come with manuals providing the same freedoms
     that the software does.  But this License is not limited to
     software manuals; it can be used for any textual work, regardless
     of subject matter or whether it is published as a printed book.
     We recommend this License principally for works whose purpose is
     instruction or reference.

  1. APPLICABILITY AND DEFINITIONS

     This License applies to any manual or other work, in any medium,
     that contains a notice placed by the copyright holder saying it
     can be distributed under the terms of this License.  Such a notice
     grants a world-wide, royalty-free license, unlimited in duration,
     to use that work under the conditions stated herein.  The
     "Document", below, refers to any such manual or work.  Any member
     of the public is a licensee, and is addressed as "you".  You
     accept the license if you copy, modify or distribute the work in a
     way requiring permission under copyright law.

     A "Modified Version" of the Document means any work containing the
     Document or a portion of it, either copied verbatim, or with
     modifications and/or translated into another language.

     A "Secondary Section" is a named appendix or a front-matter section
     of the Document that deals exclusively with the relationship of the
     publishers or authors of the Document to the Document's overall
     subject (or to related matters) and contains nothing that could
     fall directly within that overall subject.  (Thus, if the Document
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     The "Invariant Sections" are certain Secondary Sections whose
     titles are designated, as being those of Invariant Sections, in
     the notice that says that the Document is released under this
     License.  If a section does not fit the above definition of
     Secondary then it is not allowed to be designated as Invariant.
     The Document may contain zero Invariant Sections.  If the Document
     does not identify any Invariant Sections then there are none.

     The "Cover Texts" are certain short passages of text that are
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     that says that the Document is released under this License.  A
     Front-Cover Text may be at most 5 words, and a Back-Cover Text may
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     A "Transparent" copy of the Document means a machine-readable copy,
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     Examples of suitable formats for Transparent copies include plain
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     The "Title Page" means, for a printed book, the title page itself,
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     The "publisher" means any person or entity that distributes copies
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     A section "Entitled XYZ" means a named subunit of the Document
     whose title either is precisely XYZ or contains XYZ in parentheses
     following text that translates XYZ in another language.  (Here XYZ
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     To "Preserve the Title" of such a section when you modify the
     Document means that it remains a section "Entitled XYZ" according
     to this definition.

     The Document may include Warranty Disclaimers next to the notice
     which states that this License applies to the Document.  These
     Warranty Disclaimers are considered to be included by reference in
     this License, but only as regards disclaiming warranties: any other
     implication that these Warranty Disclaimers may have is void and
     has no effect on the meaning of this License.

  2. VERBATIM COPYING

     You may copy and distribute the Document in any medium, either
     commercially or noncommercially, provided that this License, the
     copyright notices, and the license notice saying this License
     applies to the Document are reproduced in all copies, and that you
     add no other conditions whatsoever to those of this License.  You
     may not use technical measures to obstruct or control the reading
     or further copying of the copies you make or distribute.  However,
     you may accept compensation in exchange for copies.  If you
     distribute a large enough number of copies you must also follow
     the conditions in section 3.

     You may also lend copies, under the same conditions stated above,
     and you may publicly display copies.

  3. COPYING IN QUANTITY

     If you publish printed copies (or copies in media that commonly
     have printed covers) of the Document, numbering more than 100, and
     the Document's license notice requires Cover Texts, you must
     enclose the copies in covers that carry, clearly and legibly, all
     these Cover Texts: Front-Cover Texts on the front cover, and
     Back-Cover Texts on the back cover.  Both covers must also clearly
     and legibly identify you as the publisher of these copies.  The
     front cover must present the full title with all words of the
     title equally prominent and visible.  You may add other material
     on the covers in addition.  Copying with changes limited to the
     covers, as long as they preserve the title of the Document and
     satisfy these conditions, can be treated as verbatim copying in
     other respects.

     If the required texts for either cover are too voluminous to fit
     legibly, you should put the first ones listed (as many as fit
     reasonably) on the actual cover, and continue the rest onto
     adjacent pages.

     If you publish or distribute Opaque copies of the Document
     numbering more than 100, you must either include a
     machine-readable Transparent copy along with each Opaque copy, or
     state in or with each Opaque copy a computer-network location from
     which the general network-using public has access to download
     using public-standard network protocols a complete Transparent
     copy of the Document, free of added material.  If you use the
     latter option, you must take reasonably prudent steps, when you
     begin distribution of Opaque copies in quantity, to ensure that
     this Transparent copy will remain thus accessible at the stated
     location until at least one year after the last time you
     distribute an Opaque copy (directly or through your agents or
     retailers) of that edition to the public.

     It is requested, but not required, that you contact the authors of
     the Document well before redistributing any large number of
     copies, to give them a chance to provide you with an updated
     version of the Document.

  4. MODIFICATIONS

     You may copy and distribute a Modified Version of the Document
     under the conditions of sections 2 and 3 above, provided that you
     release the Modified Version under precisely this License, with
     the Modified Version filling the role of the Document, thus
     licensing distribution and modification of the Modified Version to
     whoever possesses a copy of it.  In addition, you must do these
     things in the Modified Version:

       A. Use in the Title Page (and on the covers, if any) a title
          distinct from that of the Document, and from those of
          previous versions (which should, if there were any, be listed
          in the History section of the Document).  You may use the
          same title as a previous version if the original publisher of
          that version gives permission.

       B. List on the Title Page, as authors, one or more persons or
          entities responsible for authorship of the modifications in
          the Modified Version, together with at least five of the
          principal authors of the Document (all of its principal
          authors, if it has fewer than five), unless they release you
          from this requirement.

       C. State on the Title page the name of the publisher of the
          Modified Version, as the publisher.

       D. Preserve all the copyright notices of the Document.

       E. Add an appropriate copyright notice for your modifications
          adjacent to the other copyright notices.

       F. Include, immediately after the copyright notices, a license
          notice giving the public permission to use the Modified
          Version under the terms of this License, in the form shown in
          the Addendum below.

       G. Preserve in that license notice the full lists of Invariant
          Sections and required Cover Texts given in the Document's
          license notice.

       H. Include an unaltered copy of this License.

       I. Preserve the section Entitled "History", Preserve its Title,
          and add to it an item stating at least the title, year, new
          authors, and publisher of the Modified Version as given on
          the Title Page.  If there is no section Entitled "History" in
          the Document, create one stating the title, year, authors,
          and publisher of the Document as given on its Title Page,
          then add an item describing the Modified Version as stated in
          the previous sentence.

       J. Preserve the network location, if any, given in the Document
          for public access to a Transparent copy of the Document, and
          likewise the network locations given in the Document for
          previous versions it was based on.  These may be placed in
          the "History" section.  You may omit a network location for a
          work that was published at least four years before the
          Document itself, or if the original publisher of the version
          it refers to gives permission.

       K. For any section Entitled "Acknowledgements" or "Dedications",
          Preserve the Title of the section, and preserve in the
          section all the substance and tone of each of the contributor
          acknowledgements and/or dedications given therein.

       L. Preserve all the Invariant Sections of the Document,
          unaltered in their text and in their titles.  Section numbers
          or the equivalent are not considered part of the section
          titles.

       M. Delete any section Entitled "Endorsements".  Such a section
          may not be included in the Modified Version.

       N. Do not retitle any existing section to be Entitled
          "Endorsements" or to conflict in title with any Invariant
          Section.

       O. Preserve any Warranty Disclaimers.

     If the Modified Version includes new front-matter sections or
     appendices that qualify as Secondary Sections and contain no
     material copied from the Document, you may at your option
     designate some or all of these sections as invariant.  To do this,
     add their titles to the list of Invariant Sections in the Modified
     Version's license notice.  These titles must be distinct from any
     other section titles.

     You may add a section Entitled "Endorsements", provided it contains
     nothing but endorsements of your Modified Version by various
     parties--for example, statements of peer review or that the text
     has been approved by an organization as the authoritative
     definition of a standard.

     You may add a passage of up to five words as a Front-Cover Text,
     and a passage of up to 25 words as a Back-Cover Text, to the end
     of the list of Cover Texts in the Modified Version.  Only one
     passage of Front-Cover Text and one of Back-Cover Text may be
     added by (or through arrangements made by) any one entity.  If the
     Document already includes a cover text for the same cover,
     previously added by you or by arrangement made by the same entity
     you are acting on behalf of, you may not add another; but you may
     replace the old one, on explicit permission from the previous
     publisher that added the old one.

     The author(s) and publisher(s) of the Document do not by this
     License give permission to use their names for publicity for or to
     assert or imply endorsement of any Modified Version.

  5. COMBINING DOCUMENTS

     You may combine the Document with other documents released under
     this License, under the terms defined in section 4 above for
     modified versions, provided that you include in the combination
     all of the Invariant Sections of all of the original documents,
     unmodified, and list them all as Invariant Sections of your
     combined work in its license notice, and that you preserve all
     their Warranty Disclaimers.

     The combined work need only contain one copy of this License, and
     multiple identical Invariant Sections may be replaced with a single
     copy.  If there are multiple Invariant Sections with the same name
     but different contents, make the title of each such section unique
     by adding at the end of it, in parentheses, the name of the
     original author or publisher of that section if known, or else a
     unique number.  Make the same adjustment to the section titles in
     the list of Invariant Sections in the license notice of the
     combined work.

     In the combination, you must combine any sections Entitled
     "History" in the various original documents, forming one section
     Entitled "History"; likewise combine any sections Entitled
     "Acknowledgements", and any sections Entitled "Dedications".  You
     must delete all sections Entitled "Endorsements."

  6. COLLECTIONS OF DOCUMENTS

     You may make a collection consisting of the Document and other
     documents released under this License, and replace the individual
     copies of this License in the various documents with a single copy
     that is included in the collection, provided that you follow the
     rules of this License for verbatim copying of each of the
     documents in all other respects.

     You may extract a single document from such a collection, and
     distribute it individually under this License, provided you insert
     a copy of this License into the extracted document, and follow
     this License in all other respects regarding verbatim copying of
     that document.

  7. AGGREGATION WITH INDEPENDENT WORKS

     A compilation of the Document or its derivatives with other
     separate and independent documents or works, in or on a volume of
     a storage or distribution medium, is called an "aggregate" if the
     copyright resulting from the compilation is not used to limit the
     legal rights of the compilation's users beyond what the individual
     works permit.  When the Document is included in an aggregate, this
     License does not apply to the other works in the aggregate which
     are not themselves derivative works of the Document.

     If the Cover Text requirement of section 3 is applicable to these
     copies of the Document, then if the Document is less than one half
     of the entire aggregate, the Document's Cover Texts may be placed
     on covers that bracket the Document within the aggregate, or the
     electronic equivalent of covers if the Document is in electronic
     form.  Otherwise they must appear on printed covers that bracket
     the whole aggregate.

  8. TRANSLATION

     Translation is considered a kind of modification, so you may
     distribute translations of the Document under the terms of section
     4.  Replacing Invariant Sections with translations requires special
     permission from their copyright holders, but you may include
     translations of some or all Invariant Sections in addition to the
     original versions of these Invariant Sections.  You may include a
     translation of this License, and all the license notices in the
     Document, and any Warranty Disclaimers, provided that you also
     include the original English version of this License and the
     original versions of those notices and disclaimers.  In case of a
     disagreement between the translation and the original version of
     this License or a notice or disclaimer, the original version will
     prevail.

     If a section in the Document is Entitled "Acknowledgements",
     "Dedications", or "History", the requirement (section 4) to
     Preserve its Title (section 1) will typically require changing the
     actual title.

  9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document
     except as expressly provided under this License.  Any attempt
     otherwise to copy, modify, sublicense, or distribute it is void,
     and will automatically terminate your rights under this License.

     However, if you cease all violation of this License, then your
     license from a particular copyright holder is reinstated (a)
     provisionally, unless and until the copyright holder explicitly
     and finally terminates your license, and (b) permanently, if the
     copyright holder fails to notify you of the violation by some
     reasonable means prior to 60 days after the cessation.

     Moreover, your license from a particular copyright holder is
     reinstated permanently if the copyright holder notifies you of the
     violation by some reasonable means, this is the first time you have
     received notice of violation of this License (for any work) from
     that copyright holder, and you cure the violation prior to 30 days
     after your receipt of the notice.

     Termination of your rights under this section does not terminate
     the licenses of parties who have received copies or rights from
     you under this License.  If your rights have been terminated and
     not permanently reinstated, receipt of a copy of some or all of
     the same material does not give you any rights to use it.

 10. FUTURE REVISIONS OF THIS LICENSE

     The Free Software Foundation may publish new, revised versions of
     the GNU Free Documentation License from time to time.  Such new
     versions will be similar in spirit to the present version, but may
     differ in detail to address new problems or concerns.  See
     `http://www.gnu.org/copyleft/'.

     Each version of the License is given a distinguishing version
     number.  If the Document specifies that a particular numbered
     version of this License "or any later version" applies to it, you
     have the option of following the terms and conditions either of
     that specified version or of any later version that has been
     published (not as a draft) by the Free Software Foundation.  If
     the Document does not specify a version number of this License,
     you may choose any version ever published (not as a draft) by the
     Free Software Foundation.  If the Document specifies that a proxy
     can decide which future versions of this License can be used, that
     proxy's public statement of acceptance of a version permanently
     authorizes you to choose that version for the Document.

 11. RELICENSING

     "Massive Multiauthor Collaboration Site" (or "MMC Site") means any
     World Wide Web server that publishes copyrightable works and also
     provides prominent facilities for anybody to edit those works.  A
     public wiki that anybody can edit is an example of such a server.
     A "Massive Multiauthor Collaboration" (or "MMC") contained in the
     site means any set of copyrightable works thus published on the MMC
     site.

     "CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0
     license published by Creative Commons Corporation, a not-for-profit
     corporation with a principal place of business in San Francisco,
     California, as well as future copyleft versions of that license
     published by that same organization.

     "Incorporate" means to publish or republish a Document, in whole or
     in part, as part of another Document.

     An MMC is "eligible for relicensing" if it is licensed under this
     License, and if all works that were first published under this
     License somewhere other than this MMC, and subsequently
     incorporated in whole or in part into the MMC, (1) had no cover
     texts or invariant sections, and (2) were thus incorporated prior
     to November 1, 2008.

     The operator of an MMC Site may republish an MMC contained in the
     site under CC-BY-SA on the same site at any time before August 1,
     2009, provided the MMC is eligible for relicensing.


ADDENDUM: How to use this License for your documents
====================================================

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.3
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
         the Front-Cover Texts being LIST, and with the Back-Cover Texts
         being LIST.

   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License, to
permit their use in free software.


File: elisp,  Node: GPL,  Next: Tips,  Prev: GNU Free Documentation License,  Up: Top

Appendix C GNU General Public License
*************************************

                        Version 3, 29 June 2007

     Copyright (C) 2007 Free Software Foundation, Inc. `http://fsf.org/'

     Everyone is permitted to copy and distribute verbatim copies of this
     license document, but changing it is not allowed.

Preamble
========

The GNU General Public License is a free, copyleft license for software
and other kinds of works.

   The licenses for most software and other practical works are designed
to take away your freedom to share and change the works.  By contrast,
the GNU General Public License is intended to guarantee your freedom to
share and change all versions of a program--to make sure it remains
free software for all its users.  We, the Free Software Foundation, use
the GNU General Public License for most of our software; it applies
also to any other work released this way by its authors.  You can apply
it to your programs, too.

   When we speak of free software, we are referring to freedom, not
price.  Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
them if you wish), that you receive source code or can get it if you
want it, that you can change the software or use pieces of it in new
free programs, and that you know you can do these things.

   To protect your rights, we need to prevent others from denying you
these rights or asking you to surrender the rights.  Therefore, you
have certain responsibilities if you distribute copies of the software,
or if you modify it: responsibilities to respect the freedom of others.

   For example, if you distribute copies of such a program, whether
gratis or for a fee, you must pass on to the recipients the same
freedoms that you received.  You must make sure that they, too, receive
or can get the source code.  And you must show them these terms so they
know their rights.

   Developers that use the GNU GPL protect your rights with two steps:
(1) assert copyright on the software, and (2) offer you this License
giving you legal permission to copy, distribute and/or modify it.

   For the developers' and authors' protection, the GPL clearly explains
that there is no warranty for this free software.  For both users' and
authors' sake, the GPL requires that modified versions be marked as
changed, so that their problems will not be attributed erroneously to
authors of previous versions.

   Some devices are designed to deny users access to install or run
modified versions of the software inside them, although the
manufacturer can do so.  This is fundamentally incompatible with the
aim of protecting users' freedom to change the software.  The
systematic pattern of such abuse occurs in the area of products for
individuals to use, which is precisely where it is most unacceptable.
Therefore, we have designed this version of the GPL to prohibit the
practice for those products.  If such problems arise substantially in
other domains, we stand ready to extend this provision to those domains
in future versions of the GPL, as needed to protect the freedom of
users.

   Finally, every program is threatened constantly by software patents.
States should not allow patents to restrict development and use of
software on general-purpose computers, but in those that do, we wish to
avoid the special danger that patents applied to a free program could
make it effectively proprietary.  To prevent this, the GPL assures that
patents cannot be used to render the program non-free.

   The precise terms and conditions for copying, distribution and
modification follow.

TERMS AND CONDITIONS
====================

  0. Definitions.

     "This License" refers to version 3 of the GNU General Public
     License.

     "Copyright" also means copyright-like laws that apply to other
     kinds of works, such as semiconductor masks.

     "The Program" refers to any copyrightable work licensed under this
     License.  Each licensee is addressed as "you".  "Licensees" and
     "recipients" may be individuals or organizations.

     To "modify" a work means to copy from or adapt all or part of the
     work in a fashion requiring copyright permission, other than the
     making of an exact copy.  The resulting work is called a "modified
     version" of the earlier work or a work "based on" the earlier work.

     A "covered work" means either the unmodified Program or a work
     based on the Program.

     To "propagate" a work means to do anything with it that, without
     permission, would make you directly or secondarily liable for
     infringement under applicable copyright law, except executing it
     on a computer or modifying a private copy.  Propagation includes
     copying, distribution (with or without modification), making
     available to the public, and in some countries other activities as
     well.

     To "convey" a work means any kind of propagation that enables other
     parties to make or receive copies.  Mere interaction with a user
     through a computer network, with no transfer of a copy, is not
     conveying.

     An interactive user interface displays "Appropriate Legal Notices"
     to the extent that it includes a convenient and prominently visible
     feature that (1) displays an appropriate copyright notice, and (2)
     tells the user that there is no warranty for the work (except to
     the extent that warranties are provided), that licensees may
     convey the work under this License, and how to view a copy of this
     License.  If the interface presents a list of user commands or
     options, such as a menu, a prominent item in the list meets this
     criterion.

  1. Source Code.

     The "source code" for a work means the preferred form of the work
     for making modifications to it.  "Object code" means any
     non-source form of a work.

     A "Standard Interface" means an interface that either is an
     official standard defined by a recognized standards body, or, in
     the case of interfaces specified for a particular programming
     language, one that is widely used among developers working in that
     language.

     The "System Libraries" of an executable work include anything,
     other than the work as a whole, that (a) is included in the normal
     form of packaging a Major Component, but which is not part of that
     Major Component, and (b) serves only to enable use of the work
     with that Major Component, or to implement a Standard Interface
     for which an implementation is available to the public in source
     code form.  A "Major Component", in this context, means a major
     essential component (kernel, window system, and so on) of the
     specific operating system (if any) on which the executable work
     runs, or a compiler used to produce the work, or an object code
     interpreter used to run it.

     The "Corresponding Source" for a work in object code form means all
     the source code needed to generate, install, and (for an executable
     work) run the object code and to modify the work, including
     scripts to control those activities.  However, it does not include
     the work's System Libraries, or general-purpose tools or generally
     available free programs which are used unmodified in performing
     those activities but which are not part of the work.  For example,
     Corresponding Source includes interface definition files
     associated with source files for the work, and the source code for
     shared libraries and dynamically linked subprograms that the work
     is specifically designed to require, such as by intimate data
     communication or control flow between those subprograms and other
     parts of the work.

     The Corresponding Source need not include anything that users can
     regenerate automatically from other parts of the Corresponding
     Source.

     The Corresponding Source for a work in source code form is that
     same work.

  2. Basic Permissions.

     All rights granted under this License are granted for the term of
     copyright on the Program, and are irrevocable provided the stated
     conditions are met.  This License explicitly affirms your unlimited
     permission to run the unmodified Program.  The output from running
     a covered work is covered by this License only if the output,
     given its content, constitutes a covered work.  This License
     acknowledges your rights of fair use or other equivalent, as
     provided by copyright law.

     You may make, run and propagate covered works that you do not
     convey, without conditions so long as your license otherwise
     remains in force.  You may convey covered works to others for the
     sole purpose of having them make modifications exclusively for
     you, or provide you with facilities for running those works,
     provided that you comply with the terms of this License in
     conveying all material for which you do not control copyright.
     Those thus making or running the covered works for you must do so
     exclusively on your behalf, under your direction and control, on
     terms that prohibit them from making any copies of your
     copyrighted material outside their relationship with you.

     Conveying under any other circumstances is permitted solely under
     the conditions stated below.  Sublicensing is not allowed; section
     10 makes it unnecessary.

  3. Protecting Users' Legal Rights From Anti-Circumvention Law.

     No covered work shall be deemed part of an effective technological
     measure under any applicable law fulfilling obligations under
     article 11 of the WIPO copyright treaty adopted on 20 December
     1996, or similar laws prohibiting or restricting circumvention of
     such measures.

     When you convey a covered work, you waive any legal power to forbid
     circumvention of technological measures to the extent such
     circumvention is effected by exercising rights under this License
     with respect to the covered work, and you disclaim any intention
     to limit operation or modification of the work as a means of
     enforcing, against the work's users, your or third parties' legal
     rights to forbid circumvention of technological measures.

  4. Conveying Verbatim Copies.

     You may convey verbatim copies of the Program's source code as you
     receive it, in any medium, provided that you conspicuously and
     appropriately publish on each copy an appropriate copyright notice;
     keep intact all notices stating that this License and any
     non-permissive terms added in accord with section 7 apply to the
     code; keep intact all notices of the absence of any warranty; and
     give all recipients a copy of this License along with the Program.

     You may charge any price or no price for each copy that you convey,
     and you may offer support or warranty protection for a fee.

  5. Conveying Modified Source Versions.

     You may convey a work based on the Program, or the modifications to
     produce it from the Program, in the form of source code under the
     terms of section 4, provided that you also meet all of these
     conditions:

       a. The work must carry prominent notices stating that you
          modified it, and giving a relevant date.

       b. The work must carry prominent notices stating that it is
          released under this License and any conditions added under
          section 7.  This requirement modifies the requirement in
          section 4 to "keep intact all notices".

       c. You must license the entire work, as a whole, under this
          License to anyone who comes into possession of a copy.  This
          License will therefore apply, along with any applicable
          section 7 additional terms, to the whole of the work, and all
          its parts, regardless of how they are packaged.  This License
          gives no permission to license the work in any other way, but
          it does not invalidate such permission if you have separately
          received it.

       d. If the work has interactive user interfaces, each must display
          Appropriate Legal Notices; however, if the Program has
          interactive interfaces that do not display Appropriate Legal
          Notices, your work need not make them do so.

     A compilation of a covered work with other separate and independent
     works, which are not by their nature extensions of the covered
     work, and which are not combined with it such as to form a larger
     program, in or on a volume of a storage or distribution medium, is
     called an "aggregate" if the compilation and its resulting
     copyright are not used to limit the access or legal rights of the
     compilation's users beyond what the individual works permit.
     Inclusion of a covered work in an aggregate does not cause this
     License to apply to the other parts of the aggregate.

  6. Conveying Non-Source Forms.

     You may convey a covered work in object code form under the terms
     of sections 4 and 5, provided that you also convey the
     machine-readable Corresponding Source under the terms of this
     License, in one of these ways:

       a. Convey the object code in, or embodied in, a physical product
          (including a physical distribution medium), accompanied by the
          Corresponding Source fixed on a durable physical medium
          customarily used for software interchange.

       b. Convey the object code in, or embodied in, a physical product
          (including a physical distribution medium), accompanied by a
          written offer, valid for at least three years and valid for
          as long as you offer spare parts or customer support for that
          product model, to give anyone who possesses the object code
          either (1) a copy of the Corresponding Source for all the
          software in the product that is covered by this License, on a
          durable physical medium customarily used for software
          interchange, for a price no more than your reasonable cost of
          physically performing this conveying of source, or (2) access
          to copy the Corresponding Source from a network server at no
          charge.

       c. Convey individual copies of the object code with a copy of
          the written offer to provide the Corresponding Source.  This
          alternative is allowed only occasionally and noncommercially,
          and only if you received the object code with such an offer,
          in accord with subsection 6b.

       d. Convey the object code by offering access from a designated
          place (gratis or for a charge), and offer equivalent access
          to the Corresponding Source in the same way through the same
          place at no further charge.  You need not require recipients
          to copy the Corresponding Source along with the object code.
          If the place to copy the object code is a network server, the
          Corresponding Source may be on a different server (operated
          by you or a third party) that supports equivalent copying
          facilities, provided you maintain clear directions next to
          the object code saying where to find the Corresponding Source.
          Regardless of what server hosts the Corresponding Source, you
          remain obligated to ensure that it is available for as long
          as needed to satisfy these requirements.

       e. Convey the object code using peer-to-peer transmission,
          provided you inform other peers where the object code and
          Corresponding Source of the work are being offered to the
          general public at no charge under subsection 6d.


     A separable portion of the object code, whose source code is
     excluded from the Corresponding Source as a System Library, need
     not be included in conveying the object code work.

     A "User Product" is either (1) a "consumer product", which means
     any tangible personal property which is normally used for personal,
     family, or household purposes, or (2) anything designed or sold for
     incorporation into a dwelling.  In determining whether a product
     is a consumer product, doubtful cases shall be resolved in favor of
     coverage.  For a particular product received by a particular user,
     "normally used" refers to a typical or common use of that class of
     product, regardless of the status of the particular user or of the
     way in which the particular user actually uses, or expects or is
     expected to use, the product.  A product is a consumer product
     regardless of whether the product has substantial commercial,
     industrial or non-consumer uses, unless such uses represent the
     only significant mode of use of the product.

     "Installation Information" for a User Product means any methods,
     procedures, authorization keys, or other information required to
     install and execute modified versions of a covered work in that
     User Product from a modified version of its Corresponding Source.
     The information must suffice to ensure that the continued
     functioning of the modified object code is in no case prevented or
     interfered with solely because modification has been made.

     If you convey an object code work under this section in, or with,
     or specifically for use in, a User Product, and the conveying
     occurs as part of a transaction in which the right of possession
     and use of the User Product is transferred to the recipient in
     perpetuity or for a fixed term (regardless of how the transaction
     is characterized), the Corresponding Source conveyed under this
     section must be accompanied by the Installation Information.  But
     this requirement does not apply if neither you nor any third party
     retains the ability to install modified object code on the User
     Product (for example, the work has been installed in ROM).

     The requirement to provide Installation Information does not
     include a requirement to continue to provide support service,
     warranty, or updates for a work that has been modified or
     installed by the recipient, or for the User Product in which it
     has been modified or installed.  Access to a network may be denied
     when the modification itself materially and adversely affects the
     operation of the network or violates the rules and protocols for
     communication across the network.

     Corresponding Source conveyed, and Installation Information
     provided, in accord with this section must be in a format that is
     publicly documented (and with an implementation available to the
     public in source code form), and must require no special password
     or key for unpacking, reading or copying.

  7. Additional Terms.

     "Additional permissions" are terms that supplement the terms of
     this License by making exceptions from one or more of its
     conditions.  Additional permissions that are applicable to the
     entire Program shall be treated as though they were included in
     this License, to the extent that they are valid under applicable
     law.  If additional permissions apply only to part of the Program,
     that part may be used separately under those permissions, but the
     entire Program remains governed by this License without regard to
     the additional permissions.

     When you convey a copy of a covered work, you may at your option
     remove any additional permissions from that copy, or from any part
     of it.  (Additional permissions may be written to require their own
     removal in certain cases when you modify the work.)  You may place
     additional permissions on material, added by you to a covered work,
     for which you have or can give appropriate copyright permission.

     Notwithstanding any other provision of this License, for material
     you add to a covered work, you may (if authorized by the copyright
     holders of that material) supplement the terms of this License
     with terms:

       a. Disclaiming warranty or limiting liability differently from
          the terms of sections 15 and 16 of this License; or

       b. Requiring preservation of specified reasonable legal notices
          or author attributions in that material or in the Appropriate
          Legal Notices displayed by works containing it; or

       c. Prohibiting misrepresentation of the origin of that material,
          or requiring that modified versions of such material be
          marked in reasonable ways as different from the original
          version; or

       d. Limiting the use for publicity purposes of names of licensors
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       e. Declining to grant rights under trademark law for use of some
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     NECESSARY SERVICING, REPAIR OR CORRECTION.

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END OF TERMS AND CONDITIONS
===========================

How to Apply These Terms to Your New Programs
=============================================

If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these
terms.

   To do so, attach the following notices to the program.  It is safest
to attach them to the start of each source file to most effectively
state the exclusion of warranty; and each file should have at least the
"copyright" line and a pointer to where the full notice is found.

     ONE LINE TO GIVE THE PROGRAM'S NAME AND A BRIEF IDEA OF WHAT IT DOES.
     Copyright (C) YEAR NAME OF AUTHOR

     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
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     your option) any later version.

     This program is distributed in the hope that it will be useful, but
     WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
     General Public License for more details.

     You should have received a copy of the GNU General Public License
     along with this program.  If not, see `http://www.gnu.org/licenses/'.

   Also add information on how to contact you by electronic and paper
mail.

   If the program does terminal interaction, make it output a short
notice like this when it starts in an interactive mode:

     PROGRAM Copyright (C) YEAR NAME OF AUTHOR
     This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
     This is free software, and you are welcome to redistribute it
     under certain conditions; type `show c' for details.

   The hypothetical commands `show w' and `show c' should show the
appropriate parts of the General Public License.  Of course, your
program's commands might be different; for a GUI interface, you would
use an "about box".

   You should also get your employer (if you work as a programmer) or
school, if any, to sign a "copyright disclaimer" for the program, if
necessary.  For more information on this, and how to apply and follow
the GNU GPL, see `http://www.gnu.org/licenses/'.

   The GNU General Public License does not permit incorporating your
program into proprietary programs.  If your program is a subroutine
library, you may consider it more useful to permit linking proprietary
applications with the library.  If this is what you want to do, use the
GNU Lesser General Public License instead of this License.  But first,
please read `http://www.gnu.org/philosophy/why-not-lgpl.html'.


File: elisp,  Node: Tips,  Next: GNU Emacs Internals,  Prev: GPL,  Up: Top

Appendix D Tips and Conventions
*******************************

This chapter describes no additional features of Emacs Lisp.  Instead
it gives advice on making effective use of the features described in the
previous chapters, and describes conventions Emacs Lisp programmers
should follow.

   You can automatically check some of the conventions described below
by running the command `M-x checkdoc RET' when visiting a Lisp file.
It cannot check all of the conventions, and not all the warnings it
gives necessarily correspond to problems, but it is worth examining them
all.

* Menu:

* Coding Conventions::        Conventions for clean and robust programs.
* Key Binding Conventions::   Which keys should be bound by which programs.
* Programming Tips::          Making Emacs code fit smoothly in Emacs.
* Compilation Tips::          Making compiled code run fast.
* Warning Tips::              Turning off compiler warnings.
* Documentation Tips::        Writing readable documentation strings.
* Comment Tips::              Conventions for writing comments.
* Library Headers::           Standard headers for library packages.


File: elisp,  Node: Coding Conventions,  Next: Key Binding Conventions,  Up: Tips

D.1 Emacs Lisp Coding Conventions
=================================

Here are conventions that you should follow when writing Emacs Lisp
code intended for widespread use:

   * Simply loading a package should not change Emacs's editing
     behavior.  Include a command or commands to enable and disable the
     feature, or to invoke it.

     This convention is mandatory for any file that includes custom
     definitions.  If fixing such a file to follow this convention
     requires an incompatible change, go ahead and make the
     incompatible change; don't postpone it.

   * You should choose a short word to distinguish your program from
     other Lisp programs.  The names of all global variables,
     constants, and functions in your program should begin with that
     chosen prefix.  Separate the prefix from the rest of the name with
     a hyphen, `-'.  This practice helps avoid name conflicts, since
     all global variables in Emacs Lisp share the same name space, and
     all functions share another name space(1).

     Occasionally, for a command name intended for users to use, it is
     more convenient if some words come before the package's name
     prefix.  And constructs that define functions, variables, etc.,
     work better if they start with `defun' or `defvar', so put the
     name prefix later on in the name.

     This recommendation applies even to names for traditional Lisp
     primitives that are not primitives in Emacs Lisp--such as
     `copy-list'.  Believe it or not, there is more than one plausible
     way to define `copy-list'.  Play it safe; append your name prefix
     to produce a name like `foo-copy-list' or `mylib-copy-list'
     instead.

     If you write a function that you think ought to be added to Emacs
     under a certain name, such as `twiddle-files', don't call it by
     that name in your program.  Call it `mylib-twiddle-files' in your
     program, and send mail to `bug-gnu-emacs@gnu.org' suggesting we add
     it to Emacs.  If and when we do, we can change the name easily
     enough.

     If one prefix is insufficient, your package can use two or three
     alternative common prefixes, so long as they make sense.

   * Put a call to `provide' at the end of each separate Lisp file.
     *Note Named Features::.

   * If a file requires certain other Lisp programs to be loaded
     beforehand, then the comments at the beginning of the file should
     say so.  Also, use `require' to make sure they are loaded.  *Note
     Named Features::.

   * If a file FOO uses a macro defined in another file BAR, but does
     not use any functions or variables defined in BAR, then FOO should
     contain the following expression:

          (eval-when-compile (require 'BAR))

     This tells Emacs to load BAR just before byte-compiling FOO, so
     that the macro definition is available during compilation.  Using
     `eval-when-compile' avoids loading BAR when the compiled version
     of FOO is _used_.  It should be called before the first use of the
     macro in the file.  *Note Compiling Macros::.

   * Avoid loading additional libraries at run time unless they are
     really needed.  If your file simply cannot work without some other
     library, then just `require' that library at the top-level and be
     done with it.  But if your file contains several independent
     features, and only one or two require the extra library, then
     consider putting `require' statements inside the relevant
     functions rather than at the top-level.  Or use `autoload'
     statements to load the extra library when needed.  This way people
     who don't use those aspects of your file do not need to load the
     extra library.

   * Please don't require the `cl' package of Common Lisp extensions at
     run time.  Use of this package is optional, and it is not part of
     the standard Emacs namespace.  If your package loads `cl' at run
     time, that could cause name clashes for users who don't use that
     package.

     However, there is no problem with using the `cl' package at
     compile time, with `(eval-when-compile (require 'cl))'.  That's
     sufficient for using the macros in the `cl' package, because the
     compiler expands them before generating the byte-code.

   * When defining a major mode, please follow the major mode
     conventions.  *Note Major Mode Conventions::.

   * When defining a minor mode, please follow the minor mode
     conventions.  *Note Minor Mode Conventions::.

   * If the purpose of a function is to tell you whether a certain
     condition is true or false, give the function a name that ends in
     `p' (which stands for "predicate").  If the name is one word, add
     just `p'; if the name is multiple words, add `-p'.  Examples are
     `framep' and `frame-live-p'.

   * If the purpose of a variable is to store a single function, give
     it a name that ends in `-function'.  If the purpose of a variable
     is to store a list of functions (i.e., the variable is a hook),
     please follow the naming conventions for hooks.  *Note Hooks::.

   * If loading the file adds functions to hooks, define a function
     `FEATURE-unload-hook', where FEATURE is the name of the feature
     the package provides, and make it undo any such changes.  Using
     `unload-feature' to unload the file will run this function.  *Note
     Unloading::.

   * It is a bad idea to define aliases for the Emacs primitives.
     Normally you should use the standard names instead.  The case
     where an alias may be useful is where it facilitates backwards
     compatibility or portability.

   * If a package needs to define an alias or a new function for
     compatibility with some other version of Emacs, name it with the
     package prefix, not with the raw name with which it occurs in the
     other version.  Here is an example from Gnus, which provides many
     examples of such compatibility issues.

          (defalias 'gnus-point-at-bol
            (if (fboundp 'point-at-bol)
                'point-at-bol
              'line-beginning-position))

   * Redefining or advising an Emacs primitive is a bad idea.  It may do
     the right thing for a particular program, but there is no telling
     what other programs might break as a result.

   * It is likewise a bad idea for one Lisp package to advise a
     function in another Lisp package (*note Advising Functions::).

   * Avoid using `eval-after-load' in libraries and packages (*note
     Hooks for Loading::).  This feature is meant for personal
     customizations; using it in a Lisp program is unclean, because it
     modifies the behavior of another Lisp file in a way that's not
     visible in that file.  This is an obstacle for debugging, much
     like advising a function in the other package.

   * If a file does replace any of the standard functions or library
     programs of Emacs, prominent comments at the beginning of the file
     should say which functions are replaced, and how the behavior of
     the replacements differs from that of the originals.

   * Constructs that define a function or variable should be macros,
     not functions, and their names should start with `define-'.  The
     macro should receive the name to be defined as the first argument.
     That will help various tools find the definition automatically.
     Avoid constructing the names in the macro itself, since that would
     confuse these tools.

   * In some other systems there is a convention of choosing variable
     names that begin and end with `*'.  We don't use that convention
     in Emacs Lisp, so please don't use it in your programs.  (Emacs
     uses such names only for special-purpose buffers.)  People will
     find Emacs more coherent if all libraries use the same conventions.

   * If your program contains non-ASCII characters in string or
     character constants, you should make sure Emacs always decodes
     these characters the same way, regardless of the user's settings.
     The easiest way to do this is to use the coding system
     `utf-8-emacs' (*note Coding System Basics::), and specify that
     coding in the `-*-' line or the local variables list.  *Note Local
     Variables in Files: (emacs)File Variables.

          ;; XXX.el  -*- coding: utf-8-emacs; -*-

   * Indent the file using the default indentation parameters.

   * Don't make a habit of putting close-parentheses on lines by
     themselves; Lisp programmers find this disconcerting.

   * Please put a copyright notice and copying permission notice on the
     file if you distribute copies.  *Note Library Headers::.


   ---------- Footnotes ----------

   (1) The benefits of a Common Lisp-style package system are
considered not to outweigh the costs.


File: elisp,  Node: Key Binding Conventions,  Next: Programming Tips,  Prev: Coding Conventions,  Up: Tips

D.2 Key Binding Conventions
===========================

   * Many special major modes, like Dired, Info, Compilation, and Occur,
     are designed to handle read-only text that contains "hyper-links".
     Such a major mode should redefine `mouse-2' and <RET> to follow
     the links.  It should also set up a `follow-link' condition, so
     that the link obeys `mouse-1-click-follows-link'.  *Note Clickable
     Text::.  *Note Buttons::, for an easy method of implementing such
     clickable links.

   * Don't define `C-c LETTER' as a key in Lisp programs.  Sequences
     consisting of `C-c' and a letter (either upper or lower case) are
     reserved for users; they are the *only* sequences reserved for
     users, so do not block them.

     Changing all the Emacs major modes to respect this convention was a
     lot of work; abandoning this convention would make that work go to
     waste, and inconvenience users.  Please comply with it.

   * Function keys <F5> through <F9> without modifier keys are also
     reserved for users to define.

   * Sequences consisting of `C-c' followed by a control character or a
     digit are reserved for major modes.

   * Sequences consisting of `C-c' followed by `{', `}', `<', `>', `:'
     or `;' are also reserved for major modes.

   * Sequences consisting of `C-c' followed by any other punctuation
     character are allocated for minor modes.  Using them in a major
     mode is not absolutely prohibited, but if you do that, the major
     mode binding may be shadowed from time to time by minor modes.

   * Don't bind `C-h' following any prefix character (including `C-c').
     If you don't bind `C-h', it is automatically available as a help
     character for listing the subcommands of the prefix character.

   * Don't bind a key sequence ending in <ESC> except following another
     <ESC>.  (That is, it is OK to bind a sequence ending in `<ESC>
     <ESC>'.)

     The reason for this rule is that a non-prefix binding for <ESC> in
     any context prevents recognition of escape sequences as function
     keys in that context.

   * Similarly, don't bind a key sequence ending in <C-g>, since that
     is commonly used to cancel a key sequence.

   * Anything that acts like a temporary mode or state that the user can
     enter and leave should define `<ESC> <ESC>' or `<ESC> <ESC> <ESC>'
     as a way to escape.

     For a state that accepts ordinary Emacs commands, or more
     generally any kind of state in which <ESC> followed by a function
     key or arrow key is potentially meaningful, then you must not
     define `<ESC> <ESC>', since that would preclude recognizing an
     escape sequence after <ESC>.  In these states, you should define
     `<ESC> <ESC> <ESC>' as the way to escape.  Otherwise, define
     `<ESC> <ESC>' instead.


File: elisp,  Node: Programming Tips,  Next: Compilation Tips,  Prev: Key Binding Conventions,  Up: Tips

D.3 Emacs Programming Tips
==========================

Following these conventions will make your program fit better into
Emacs when it runs.

   * Don't use `next-line' or `previous-line' in programs; nearly
     always, `forward-line' is more convenient as well as more
     predictable and robust.  *Note Text Lines::.

   * Don't call functions that set the mark, unless setting the mark is
     one of the intended features of your program.  The mark is a
     user-level feature, so it is incorrect to change the mark except
     to supply a value for the user's benefit.  *Note The Mark::.

     In particular, don't use any of these functions:

        * `beginning-of-buffer', `end-of-buffer'

        * `replace-string', `replace-regexp'

        * `insert-file', `insert-buffer'

     If you just want to move point, or replace a certain string, or
     insert a file or buffer's contents, without any of the other
     features intended for interactive users, you can replace these
     functions with one or two lines of simple Lisp code.

   * Use lists rather than vectors, except when there is a particular
     reason to use a vector.  Lisp has more facilities for manipulating
     lists than for vectors, and working with lists is usually more
     convenient.

     Vectors are advantageous for tables that are substantial in size
     and are accessed in random order (not searched front to back),
     provided there is no need to insert or delete elements (only lists
     allow that).

   * The recommended way to show a message in the echo area is with the
     `message' function, not `princ'.  *Note The Echo Area::.

   * When you encounter an error condition, call the function `error'
     (or `signal').  The function `error' does not return.  *Note
     Signaling Errors::.

     Don't use `message', `throw', `sleep-for', or `beep' to report
     errors.

   * An error message should start with a capital letter but should not
     end with a period.

   * A question asked in the minibuffer with `yes-or-no-p' or
     `y-or-n-p' should start with a capital letter and end with `? '.

   * When you mention a default value in a minibuffer prompt, put it
     and the word `default' inside parentheses.  It should look like
     this:

          Enter the answer (default 42):

   * In `interactive', if you use a Lisp expression to produce a list
     of arguments, don't try to provide the "correct" default values for
     region or position arguments.  Instead, provide `nil' for those
     arguments if they were not specified, and have the function body
     compute the default value when the argument is `nil'.  For
     instance, write this:

          (defun foo (pos)
            (interactive
             (list (if SPECIFIED SPECIFIED-POS)))
            (unless pos (setq pos DEFAULT-POS))
            ...)

     rather than this:

          (defun foo (pos)
            (interactive
             (list (if SPECIFIED SPECIFIED-POS
                       DEFAULT-POS)))
            ...)

     This is so that repetition of the command will recompute these
     defaults based on the current circumstances.

     You do not need to take such precautions when you use interactive
     specs `d', `m' and `r', because they make special arrangements to
     recompute the argument values on repetition of the command.

   * Many commands that take a long time to execute display a message
     that says something like `Operating...' when they start, and
     change it to `Operating...done' when they finish.  Please keep the
     style of these messages uniform: _no_ space around the ellipsis,
     and _no_ period after `done'.  *Note Progress::, for an easy way
     to generate such messages.

   * Try to avoid using recursive edits.  Instead, do what the Rmail `e'
     command does: use a new local keymap that contains a command
     defined to switch back to the old local keymap.  Or simply switch
     to another buffer and let the user switch back at will.  *Note
     Recursive Editing::.


File: elisp,  Node: Compilation Tips,  Next: Warning Tips,  Prev: Programming Tips,  Up: Tips

D.4 Tips for Making Compiled Code Fast
======================================

Here are ways of improving the execution speed of byte-compiled Lisp
programs.

   * Profile your program with the `elp' library.  See the file
     `elp.el' for instructions.

   * Check the speed of individual Emacs Lisp forms using the
     `benchmark' library.  See the functions `benchmark-run' and
     `benchmark-run-compiled' in `benchmark.el'.

   * Use iteration rather than recursion whenever possible.  Function
     calls are slow in Emacs Lisp even when a compiled function is
     calling another compiled function.

   * Using the primitive list-searching functions `memq', `member',
     `assq', or `assoc' is even faster than explicit iteration.  It can
     be worth rearranging a data structure so that one of these
     primitive search functions can be used.

   * Certain built-in functions are handled specially in byte-compiled
     code, avoiding the need for an ordinary function call.  It is a
     good idea to use these functions rather than alternatives.  To see
     whether a function is handled specially by the compiler, examine
     its `byte-compile' property.  If the property is non-`nil', then
     the function is handled specially.

     For example, the following input will show you that `aref' is
     compiled specially (*note Array Functions::):

          (get 'aref 'byte-compile)
               => byte-compile-two-args

     Note that in this case (and many others), you must first load the
     `bytecomp' library, which defines the `byte-compile' property.

   * If calling a small function accounts for a substantial part of your
     program's running time, make the function inline.  This eliminates
     the function call overhead.  Since making a function inline reduces
     the flexibility of changing the program, don't do it unless it
     gives a noticeable speedup in something slow enough that users
     care about the speed.  *Note Inline Functions::.


File: elisp,  Node: Warning Tips,  Next: Documentation Tips,  Prev: Compilation Tips,  Up: Tips

D.5 Tips for Avoiding Compiler Warnings
=======================================

   * Try to avoid compiler warnings about undefined free variables, by
     adding dummy `defvar' definitions for these variables, like this:

          (defvar foo)

     Such a definition has no effect except to tell the compiler not to
     warn about uses of the variable `foo' in this file.

   * Similarly, to avoid a compiler warning about an undefined function
     that you know _will_ be defined, use a `declare-function'
     statement (*note Declaring Functions::).

   * If you use many functions and variables from a certain file, you
     can add a `require' for that package to avoid compilation warnings
     for them.  For instance,

          (eval-when-compile
            (require 'foo))

   * If you bind a variable in one function, and use it or set it in
     another function, the compiler warns about the latter function
     unless the variable has a definition.  But adding a definition
     would be unclean if the variable has a short name, since Lisp
     packages should not define short variable names.  The right thing
     to do is to rename this variable to start with the name prefix
     used for the other functions and variables in your package.

   * The last resort for avoiding a warning, when you want to do
     something that is usually a mistake but you know is not a mistake
     in your usage, is to put it inside `with-no-warnings'.  *Note
     Compiler Errors::.


File: elisp,  Node: Documentation Tips,  Next: Comment Tips,  Prev: Warning Tips,  Up: Tips

D.6 Tips for Documentation Strings
==================================

Here are some tips and conventions for the writing of documentation
strings.  You can check many of these conventions by running the command
`M-x checkdoc-minor-mode'.

   * Every command, function, or variable intended for users to know
     about should have a documentation string.

   * An internal variable or subroutine of a Lisp program might as well
     have a documentation string.  Documentation strings take up very
     little space in a running Emacs.

   * Format the documentation string so that it fits in an Emacs window
     on an 80-column screen.  It is a good idea for most lines to be no
     wider than 60 characters.  The first line should not be wider than
     67 characters or it will look bad in the output of `apropos'.

     You can fill the text if that looks good.  However, rather than
     blindly filling the entire documentation string, you can often
     make it much more readable by choosing certain line breaks with
     care.  Use blank lines between sections if the documentation
     string is long.

   * The first line of the documentation string should consist of one
     or two complete sentences that stand on their own as a summary.
     `M-x apropos' displays just the first line, and if that line's
     contents don't stand on their own, the result looks bad.  In
     particular, start the first line with a capital letter and end it
     with a period.

     For a function, the first line should briefly answer the question,
     "What does this function do?"  For a variable, the first line
     should briefly answer the question, "What does this value mean?"

     Don't limit the documentation string to one line; use as many
     lines as you need to explain the details of how to use the
     function or variable.  Please use complete sentences for the rest
     of the text too.

   * When the user tries to use a disabled command, Emacs displays just
     the first paragraph of its documentation string--everything
     through the first blank line.  If you wish, you can choose which
     information to include before the first blank line so as to make
     this display useful.

   * The first line should mention all the important arguments of the
     function, and should mention them in the order that they are
     written in a function call.  If the function has many arguments,
     then it is not feasible to mention them all in the first line; in
     that case, the first line should mention the first few arguments,
     including the most important arguments.

   * When a function's documentation string mentions the value of an
     argument of the function, use the argument name in capital letters
     as if it were a name for that value.  Thus, the documentation
     string of the function `eval' refers to its first argument as
     `FORM', because the actual argument name is `form':

          Evaluate FORM and return its value.

     Also write metasyntactic variables in capital letters, such as
     when you show the decomposition of a list or vector into subunits,
     some of which may vary.  `KEY' and `VALUE' in the following example
     illustrate this practice:

          The argument TABLE should be an alist whose elements
          have the form (KEY . VALUE).  Here, KEY is ...

   * Never change the case of a Lisp symbol when you mention it in a doc
     string.  If the symbol's name is `foo', write "foo", not "Foo"
     (which is a different symbol).

     This might appear to contradict the policy of writing function
     argument values, but there is no real contradiction; the argument
     _value_ is not the same thing as the _symbol_ that the function
     uses to hold the value.

     If this puts a lower-case letter at the beginning of a sentence
     and that annoys you, rewrite the sentence so that the symbol is
     not at the start of it.

   * Do not start or end a documentation string with whitespace.

   * *Do not* indent subsequent lines of a documentation string so that
     the text is lined up in the source code with the text of the first
     line.  This looks nice in the source code, but looks bizarre when
     users view the documentation.  Remember that the indentation
     before the starting double-quote is not part of the string!

   * When a documentation string refers to a Lisp symbol, write it as it
     would be printed (which usually means in lower case), with
     single-quotes around it.  For example: `lambda'.  There are two
     exceptions: write t and nil without single-quotes.  (In this
     manual, we use a different convention, with single-quotes for all
     symbols.)

     Help mode automatically creates a hyperlink when a documentation
     string uses a symbol name inside single quotes, if the symbol has
     either a function or a variable definition.  You do not need to do
     anything special to make use of this feature.  However, when a
     symbol has both a function definition and a variable definition,
     and you want to refer to just one of them, you can specify which
     one by writing one of the words `variable', `option', `function',
     or `command', immediately before the symbol name.  (Case makes no
     difference in recognizing these indicator words.)  For example, if
     you write

          This function sets the variable `buffer-file-name'.

     then the hyperlink will refer only to the variable documentation of
     `buffer-file-name', and not to its function documentation.

     If a symbol has a function definition and/or a variable
     definition, but those are irrelevant to the use of the symbol that
     you are documenting, you can write the words `symbol' or `program'
     before the symbol name to prevent making any hyperlink.  For
     example,

          If the argument KIND-OF-RESULT is the symbol `list',
          this function returns a list of all the objects
          that satisfy the criterion.

     does not make a hyperlink to the documentation, irrelevant here,
     of the function `list'.

     Normally, no hyperlink is made for a variable without variable
     documentation.  You can force a hyperlink for such variables by
     preceding them with one of the words `variable' or `option'.

     Hyperlinks for faces are only made if the face name is preceded or
     followed by the word `face'.  In that case, only the face
     documentation will be shown, even if the symbol is also defined as
     a variable or as a function.

     To make a hyperlink to Info documentation, write the name of the
     Info node (or anchor) in single quotes, preceded by `info node',
     `Info node', `info anchor' or `Info anchor'.  The Info file name
     defaults to `emacs'.  For example,

          See Info node `Font Lock' and Info node `(elisp)Font Lock Basics'.

     Finally, to create a hyperlink to URLs, write the URL in single
     quotes, preceded by `URL'. For example,

          The home page for the GNU project has more information (see URL
          `http://www.gnu.org/').

   * Don't write key sequences directly in documentation strings.
     Instead, use the `\\[...]' construct to stand for them.  For
     example, instead of writing `C-f', write the construct
     `\\[forward-char]'.  When Emacs displays the documentation string,
     it substitutes whatever key is currently bound to `forward-char'.
     (This is normally `C-f', but it may be some other character if the
     user has moved key bindings.)  *Note Keys in Documentation::.

   * In documentation strings for a major mode, you will want to refer
     to the key bindings of that mode's local map, rather than global
     ones.  Therefore, use the construct `\\<...>' once in the
     documentation string to specify which key map to use.  Do this
     before the first use of `\\[...]'.  The text inside the `\\<...>'
     should be the name of the variable containing the local keymap for
     the major mode.

     It is not practical to use `\\[...]' very many times, because
     display of the documentation string will become slow.  So use this
     to describe the most important commands in your major mode, and
     then use `\\{...}' to display the rest of the mode's keymap.

   * For consistency, phrase the verb in the first sentence of a
     function's documentation string as an imperative--for instance,
     use "Return the cons of A and B." in preference to "Returns the
     cons of A and B."  Usually it looks good to do likewise for the
     rest of the first paragraph.  Subsequent paragraphs usually look
     better if each sentence is indicative and has a proper subject.

   * The documentation string for a function that is a yes-or-no
     predicate should start with words such as "Return t if", to
     indicate explicitly what constitutes "truth".  The word "return"
     avoids starting the sentence with lower-case "t", which could be
     somewhat distracting.

   * If a line in a documentation string begins with an
     open-parenthesis, write a backslash before the open-parenthesis,
     like this:

          The argument FOO can be either a number
          \(a buffer position) or a string (a file name).

     This prevents the open-parenthesis from being treated as the start
     of a defun (*note Defuns: (emacs)Defuns.).

   * Write documentation strings in the active voice, not the passive,
     and in the present tense, not the future.  For instance, use
     "Return a list containing A and B." instead of "A list containing
     A and B will be returned."

   * Avoid using the word "cause" (or its equivalents) unnecessarily.
     Instead of, "Cause Emacs to display text in boldface", write just
     "Display text in boldface".

   * Avoid using "iff" (a mathematics term meaning "if and only if"),
     since many people are unfamiliar with it and mistake it for a
     typo.  In most cases, the meaning is clear with just "if".
     Otherwise, try to find an alternate phrasing that conveys the
     meaning.

   * When a command is meaningful only in a certain mode or situation,
     do mention that in the documentation string.  For example, the
     documentation of `dired-find-file' is:

          In Dired, visit the file or directory named on this line.

   * When you define a variable that represents an option users might
     want to set, use `defcustom'.  *Note Defining Variables::.

   * The documentation string for a variable that is a yes-or-no flag
     should start with words such as "Non-nil means", to make it clear
     that all non-`nil' values are equivalent and indicate explicitly
     what `nil' and non-`nil' mean.


File: elisp,  Node: Comment Tips,  Next: Library Headers,  Prev: Documentation Tips,  Up: Tips

D.7 Tips on Writing Comments
============================

We recommend these conventions for comments:

`;'
     Comments that start with a single semicolon, `;', should all be
     aligned to the same column on the right of the source code.  Such
     comments usually explain how the code on that line does its job.
     For example:

          (setq base-version-list                 ; there was a base
                (assoc (substring fn 0 start-vn)  ; version to which
                       file-version-assoc-list))  ; this looks like
                                                  ; a subversion

`;;'
     Comments that start with two semicolons, `;;', should be aligned to
     the same level of indentation as the code.  Such comments usually
     describe the purpose of the following lines or the state of the
     program at that point.  For example:

          (prog1 (setq auto-fill-function
                       ...
                       ...
            ;; Update mode line.
            (force-mode-line-update)))

     We also normally use two semicolons for comments outside functions.

          ;; This Lisp code is run in Emacs when it is to operate as
          ;; a server for other processes.

     If a function has no documentation string, it should instead have a
     two-semicolon comment right before the function, explaining what
     the function does and how to call it properly.  Explain precisely
     what each argument means and how the function interprets its
     possible values.  It is much better to convert such comments to
     documentation strings, though.

`;;;'
     Comments that start with three semicolons, `;;;', should start at
     the left margin.  These are used, occasionally, for comments within
     functions that should start at the margin.  We also use them
     sometimes for comments that are between functions--whether to use
     two or three semicolons depends on whether the comment should be
     considered a "heading" by Outline minor mode.  By default,
     comments starting with at least three semicolons (followed by a
     single space and a non-whitespace character) are considered
     headings, comments starting with two or fewer are not.

     Another use for triple-semicolon comments is for commenting out
     lines within a function.  We use three semicolons for this
     precisely so that they remain at the left margin.  By default,
     Outline minor mode does not consider a comment to be a heading
     (even if it starts with at least three semicolons) if the
     semicolons are followed by at least two spaces.  Thus, if you add
     an introductory comment to the commented out code, make sure to
     indent it by at least two spaces after the three semicolons.

          (defun foo (a)
          ;;;  This is no longer necessary.
          ;;;  (force-mode-line-update)
            (message "Finished with %s" a))

     When commenting out entire functions, use two semicolons.

`;;;;'
     Comments that start with four semicolons, `;;;;', should be aligned
     to the left margin and are used for headings of major sections of a
     program.  For example:

          ;;;; The kill ring

Generally speaking, the `M-;' (`comment-dwim') command automatically
starts a comment of the appropriate type; or indents an existing
comment to the right place, depending on the number of semicolons.
*Note Manipulating Comments: (emacs)Comments.


File: elisp,  Node: Library Headers,  Prev: Comment Tips,  Up: Tips

D.8 Conventional Headers for Emacs Libraries
============================================

Emacs has conventions for using special comments in Lisp libraries to
divide them into sections and give information such as who wrote them.
Using a standard format for these items makes it easier for tools (and
people) to extract the relevant information.  This section explains
these conventions, starting with an example:

     ;;; foo.el --- Support for the Foo programming language

     ;; Copyright (C) 2010-2012 Your Name

     ;; Author: Your Name <yourname@example.com>
     ;; Maintainer: Someone Else <someone@example.com>
     ;; Created: 14 Jul 2010
     ;; Keywords: languages

     ;; This file is not part of GNU Emacs.

     ;; This file is free software...
     ...
     ;; along with this file.  If not, see <http://www.gnu.org/licenses/>.

   The very first line should have this format:

     ;;; FILENAME --- DESCRIPTION

The description should be contained in one line.  If the file needs a
`-*-' specification, put it after DESCRIPTION.  If this would make the
first line too long, use a Local Variables section at the end of the
file.

   The copyright notice usually lists your name (if you wrote the
file).  If you have an employer who claims copyright on your work, you
might need to list them instead.  Do not say that the copyright holder
is the Free Software Foundation (or that the file is part of GNU Emacs)
unless your file has been accepted into the Emacs distribution.  For
more information on the form of copyright and license notices, see the
guide on the GNU website (http://www.gnu.org/licenses/gpl-howto.html).

   After the copyright notice come several "header comment" lines, each
beginning with `;; HEADER-NAME:'.  Here is a table of the conventional
possibilities for HEADER-NAME:

`Author'
     This line states the name and email address of at least the
     principal author of the library.  If there are multiple authors,
     list them on continuation lines led by `;;' and whitespace (this
     is easier for tools to parse than having more than one author on
     one line).  We recommend including a contact email address, of the
     form `<...>'.  For example:

          ;; Author: Your Name <yourname@example.com>
          ;;      Someone Else <someone@example.com>
          ;;      Another Person <another@example.com>

`Maintainer'
     This header has the same format as the Author header.  It lists the
     person(s) who currently maintain(s) the file (respond to bug
     reports, etc.).

     If there is no maintainer line, the person(s) in the Author field
     is/are presumed to be the maintainers.  Some files in Emacs use
     `FSF' for the maintainer.  This means that the original author is
     no longer responsible for the file, and that it is maintained as
     part of Emacs.

`Created'
     This optional line gives the original creation date of the file,
     and is for historical interest only.

`Version'
     If you wish to record version numbers for the individual Lisp
     program, put them in this line.  Lisp files distributed with Emacs
     generally do not have a `Version' header, since the version number
     of Emacs itself serves the same purpose.  If you are distributing
     a collection of multiple files, we recommend not writing the
     version in every file, but only the main one.

`Keywords'
     This line lists keywords for the `finder-by-keyword' help command.
     Please use that command to see a list of the meaningful keywords.

     This field is how people will find your package when they're
     looking for things by topic.  To separate the keywords, you can
     use spaces, commas, or both.

     The name of this field is unfortunate, since people often assume
     it is the place to write arbitrary keywords that describe their
     package, rather than just the relevant Finder keywords.

`Package-Version'
     If `Version' is not suitable for use by the package manager, then
     a package can define `Package-Version'; it will be used instead.
     This is handy if `Version' is an RCS id or something else that
     cannot be parsed by `version-to-list'.  *Note Packaging Basics::.

`Package-Requires'
     If this exists, it names packages on which the current package
     depends for proper operation.  *Note Packaging Basics::.  This is
     used by the package manager both at download time (to ensure that
     a complete set of packages is downloaded) and at activation time
     (to ensure that a package is only activated if all its
     dependencies have been).

     Its format is a list of lists.  The `car' of each sub-list is the
     name of a package, as a symbol.  The `cadr' of each sub-list is
     the minimum acceptable version number, as a string.  For instance:

          ;; Package-Requires: ((gnus "1.0") (bubbles "2.7.2"))

     The package code automatically defines a package named `emacs'
     with the version number of the currently running Emacs.  This can
     be used to require a minimal version of Emacs for a package.

   Just about every Lisp library ought to have the `Author' and
`Keywords' header comment lines.  Use the others if they are
appropriate.  You can also put in header lines with other header
names--they have no standard meanings, so they can't do any harm.

   We use additional stylized comments to subdivide the contents of the
library file.  These should be separated from anything else by blank
lines.  Here is a table of them:

`;;; Commentary:'
     This begins introductory comments that explain how the library
     works.  It should come right after the copying permissions,
     terminated by a `Change Log', `History' or `Code' comment line.
     This text is used by the Finder package, so it should make sense
     in that context.

`;;; Change Log:'
     This begins an optional log of changes to the file over time.
     Don't put too much information in this section--it is better to
     keep the detailed logs in a separate `ChangeLog' file (as Emacs
     does), and/or to use a version control system.  `History' is an
     alternative to `Change Log'.

`;;; Code:'
     This begins the actual code of the program.

`;;; FILENAME ends here'
     This is the "footer line"; it appears at the very end of the file.
     Its purpose is to enable people to detect truncated versions of
     the file from the lack of a footer line.


File: elisp,  Node: GNU Emacs Internals,  Next: Standard Errors,  Prev: Tips,  Up: Top

Appendix E GNU Emacs Internals
******************************

This chapter describes how the runnable Emacs executable is dumped with
the preloaded Lisp libraries in it, how storage is allocated, and some
internal aspects of GNU Emacs that may be of interest to C programmers.

* Menu:

* Building Emacs::      How the dumped Emacs is made.
* Pure Storage::        Kludge to make preloaded Lisp functions shareable.
* Garbage Collection::  Reclaiming space for Lisp objects no longer used.
* Memory Usage::        Info about total size of Lisp objects made so far.
* Writing Emacs Primitives::   Writing C code for Emacs.
* Object Internals::    Data formats of buffers, windows, processes.


File: elisp,  Node: Building Emacs,  Next: Pure Storage,  Up: GNU Emacs Internals

E.1 Building Emacs
==================

This section explains the steps involved in building the Emacs
executable.  You don't have to know this material to build and install
Emacs, since the makefiles do all these things automatically.  This
information is pertinent to Emacs developers.

   Compilation of the C source files in the `src' directory produces an
executable file called `temacs', also called a "bare impure Emacs".  It
contains the Emacs Lisp interpreter and I/O routines, but not the
editing commands.

   The command `temacs -l loadup' would run `temacs' and direct it to
load `loadup.el'.  The `loadup' library loads additional Lisp
libraries, which set up the normal Emacs editing environment.  After
this step, the Emacs executable is no longer "bare".

   Because it takes some time to load the standard Lisp files, the
`temacs' executable usually isn't run directly by users.  Instead, as
one of the last steps of building Emacs, the command `temacs -batch -l
loadup dump' is run.  The special `dump' argument causes `temacs' to
dump out an executable program, called `emacs', which has all the
standard Lisp files preloaded.  (The `-batch' argument prevents
`temacs' from trying to initialize any of its data on the terminal, so
that the tables of terminal information are empty in the dumped Emacs.)

   The dumped `emacs' executable (also called a "pure" Emacs) is the
one which is installed.  The variable `preloaded-file-list' stores a
list of the Lisp files preloaded into the dumped Emacs.  If you port
Emacs to a new operating system, and are not able to implement dumping,
then Emacs must load `loadup.el' each time it starts.

   You can specify additional files to preload by writing a library
named `site-load.el' that loads them.  You may need to rebuild Emacs
with an added definition

     #define SITELOAD_PURESIZE_EXTRA N

to make N added bytes of pure space to hold the additional files; see
`src/puresize.h'.  (Try adding increments of 20000 until it is big
enough.)  However, the advantage of preloading additional files
decreases as machines get faster.  On modern machines, it is usually
not advisable.

   After `loadup.el' reads `site-load.el', it finds the documentation
strings for primitive and preloaded functions (and variables) in the
file `etc/DOC' where they are stored, by calling `Snarf-documentation'
(*note Accessing Documentation: Definition of Snarf-documentation.).

   You can specify other Lisp expressions to execute just before dumping
by putting them in a library named `site-init.el'.  This file is
executed after the documentation strings are found.

   If you want to preload function or variable definitions, there are
three ways you can do this and make their documentation strings
accessible when you subsequently run Emacs:

   * Arrange to scan these files when producing the `etc/DOC' file, and
     load them with `site-load.el'.

   * Load the files with `site-init.el', then copy the files into the
     installation directory for Lisp files when you install Emacs.

   * Specify a `nil' value for `byte-compile-dynamic-docstrings' as a
     local variable in each of these files, and load them with either
     `site-load.el' or `site-init.el'.  (This method has the drawback
     that the documentation strings take up space in Emacs all the
     time.)

   It is not advisable to put anything in `site-load.el' or
`site-init.el' that would alter any of the features that users expect
in an ordinary unmodified Emacs.  If you feel you must override normal
features for your site, do it with `default.el', so that users can
override your changes if they wish.  *Note Startup Summary::.

   In a package that can be preloaded, it is sometimes necessary (or
useful) to delay certain evaluations until Emacs subsequently starts
up.  The vast majority of such cases relate to the values of
customizable variables.  For example, `tutorial-directory' is a
variable defined in `startup.el', which is preloaded.  The default
value is set based on `data-directory'.  The variable needs to access
the value of `data-directory' when Emacs starts, not when it is dumped,
because the Emacs executable has probably been installed in a different
location since it was dumped.

 -- Function: custom-initialize-delay symbol value
     This function delays the initialization of SYMBOL to the next
     Emacs start.  You normally use this function by specifying it as
     the `:initialize' property of a customizable variable.  (The
     argument VALUE is unused, and is provided only for compatibility
     with the form Custom expects.)

   In the unlikely event that you need a more general functionality than
`custom-initialize-delay' provides, you can use `before-init-hook'
(*note Startup Summary::).

 -- Function: dump-emacs to-file from-file
     This function dumps the current state of Emacs into an executable
     file TO-FILE.  It takes symbols from FROM-FILE (this is normally
     the executable file `temacs').

     If you want to use this function in an Emacs that was already
     dumped, you must run Emacs with `-batch'.


File: elisp,  Node: Pure Storage,  Next: Garbage Collection,  Prev: Building Emacs,  Up: GNU Emacs Internals

E.2 Pure Storage
================

Emacs Lisp uses two kinds of storage for user-created Lisp objects:
"normal storage" and "pure storage".  Normal storage is where all the
new data created during an Emacs session are kept (*note Garbage
Collection::).  Pure storage is used for certain data in the preloaded
standard Lisp files--data that should never change during actual use of
Emacs.

   Pure storage is allocated only while `temacs' is loading the
standard preloaded Lisp libraries.  In the file `emacs', it is marked
as read-only (on operating systems that permit this), so that the
memory space can be shared by all the Emacs jobs running on the machine
at once.  Pure storage is not expandable; a fixed amount is allocated
when Emacs is compiled, and if that is not sufficient for the preloaded
libraries, `temacs' allocates dynamic memory for the part that didn't
fit.  The resulting image will work, but garbage collection (*note
Garbage Collection::) is disabled in this situation, causing a memory
leak.  Such an overflow normally won't happen unless you try to preload
additional libraries or add features to the standard ones.  Emacs will
display a warning about the overflow when it starts.  If this happens,
you should increase the compilation parameter `SYSTEM_PURESIZE_EXTRA'
in the file `src/puresize.h' and rebuild Emacs.

 -- Function: purecopy object
     This function makes a copy in pure storage of OBJECT, and returns
     it.  It copies a string by simply making a new string with the same
     characters, but without text properties, in pure storage.  It
     recursively copies the contents of vectors and cons cells.  It does
     not make copies of other objects such as symbols, but just returns
     them unchanged.  It signals an error if asked to copy markers.

     This function is a no-op except while Emacs is being built and
     dumped; it is usually called only in preloaded Lisp files.

 -- Variable: pure-bytes-used
     The value of this variable is the number of bytes of pure storage
     allocated so far.  Typically, in a dumped Emacs, this number is
     very close to the total amount of pure storage available--if it
     were not, we would preallocate less.

 -- Variable: purify-flag
     This variable determines whether `defun' should make a copy of the
     function definition in pure storage.  If it is non-`nil', then the
     function definition is copied into pure storage.

     This flag is `t' while loading all of the basic functions for
     building Emacs initially (allowing those functions to be shareable
     and non-collectible).  Dumping Emacs as an executable always writes
     `nil' in this variable, regardless of the value it actually has
     before and after dumping.

     You should not change this flag in a running Emacs.


File: elisp,  Node: Garbage Collection,  Next: Memory Usage,  Prev: Pure Storage,  Up: GNU Emacs Internals

E.3 Garbage Collection
======================

When a program creates a list or the user defines a new function (such
as by loading a library), that data is placed in normal storage.  If
normal storage runs low, then Emacs asks the operating system to
allocate more memory.  Different types of Lisp objects, such as
symbols, cons cells, markers, etc., are segregated in distinct blocks
in memory.  (Vectors, long strings, buffers and certain other editing
types, which are fairly large, are allocated in individual blocks, one
per object, while small strings are packed into blocks of 8k bytes.)

   It is quite common to use some storage for a while, then release it
by (for example) killing a buffer or deleting the last pointer to an
object.  Emacs provides a "garbage collector" to reclaim this abandoned
storage.  The garbage collector operates by finding and marking all
Lisp objects that are still accessible to Lisp programs.  To begin
with, it assumes all the symbols, their values and associated function
definitions, and any data presently on the stack, are accessible.  Any
objects that can be reached indirectly through other accessible objects
are also accessible.

   When marking is finished, all objects still unmarked are garbage.  No
matter what the Lisp program or the user does, it is impossible to refer
to them, since there is no longer a way to reach them.  Their space
might as well be reused, since no one will miss them.  The second
("sweep") phase of the garbage collector arranges to reuse them.

   The sweep phase puts unused cons cells onto a "free list" for future
allocation; likewise for symbols and markers.  It compacts the
accessible strings so they occupy fewer 8k blocks; then it frees the
other 8k blocks.  Vectors, buffers, windows, and other large objects are
individually allocated and freed using `malloc' and `free'.

     Common Lisp note: Unlike other Lisps, GNU Emacs Lisp does not call
     the garbage collector when the free list is empty.  Instead, it
     simply requests the operating system to allocate more storage, and
     processing continues until `gc-cons-threshold' bytes have been
     used.

     This means that you can make sure that the garbage collector will
     not run during a certain portion of a Lisp program by calling the
     garbage collector explicitly just before it (provided that portion
     of the program does not use so much space as to force a second
     garbage collection).

 -- Command: garbage-collect
     This command runs a garbage collection, and returns information on
     the amount of space in use.  (Garbage collection can also occur
     spontaneously if you use more than `gc-cons-threshold' bytes of
     Lisp data since the previous garbage collection.)

     `garbage-collect' returns a list containing the following
     information:

          ((USED-CONSES . FREE-CONSES)
           (USED-SYMS . FREE-SYMS)
           (USED-MISCS . FREE-MISCS)
           USED-STRING-CHARS
           USED-VECTOR-SLOTS
           (USED-FLOATS . FREE-FLOATS)
           (USED-INTERVALS . FREE-INTERVALS)
           (USED-STRINGS . FREE-STRINGS))

     Here is an example:

          (garbage-collect)
               => ((106886 . 13184) (9769 . 0)
                          (7731 . 4651) 347543 121628
                          (31 . 94) (1273 . 168)
                          (25474 . 3569))

     Here is a table explaining each element:

    USED-CONSES
          The number of cons cells in use.

    FREE-CONSES
          The number of cons cells for which space has been obtained
          from the operating system, but that are not currently being
          used.

    USED-SYMS
          The number of symbols in use.

    FREE-SYMS
          The number of symbols for which space has been obtained from
          the operating system, but that are not currently being used.

    USED-MISCS
          The number of miscellaneous objects in use.  These include
          markers and overlays, plus certain objects not visible to
          users.

    FREE-MISCS
          The number of miscellaneous objects for which space has been
          obtained from the operating system, but that are not
          currently being used.

    USED-STRING-CHARS
          The total size of all strings, in characters.

    USED-VECTOR-SLOTS
          The total number of elements of existing vectors.

    USED-FLOATS
          The number of floats in use.

    FREE-FLOATS
          The number of floats for which space has been obtained from
          the operating system, but that are not currently being used.

    USED-INTERVALS
          The number of intervals in use.  Intervals are an internal
          data structure used for representing text properties.

    FREE-INTERVALS
          The number of intervals for which space has been obtained
          from the operating system, but that are not currently being
          used.

    USED-STRINGS
          The number of strings in use.

    FREE-STRINGS
          The number of string headers for which the space was obtained
          from the operating system, but which are currently not in
          use.  (A string object consists of a header and the storage
          for the string text itself; the latter is only allocated when
          the string is created.)

     If there was overflow in pure space (*note Pure Storage::),
     `garbage-collect' returns `nil', because a real garbage collection
     cannot be done.

 -- User Option: garbage-collection-messages
     If this variable is non-`nil', Emacs displays a message at the
     beginning and end of garbage collection.  The default value is
     `nil'.

 -- Variable: post-gc-hook
     This is a normal hook that is run at the end of garbage collection.
     Garbage collection is inhibited while the hook functions run, so be
     careful writing them.

 -- User Option: gc-cons-threshold
     The value of this variable is the number of bytes of storage that
     must be allocated for Lisp objects after one garbage collection in
     order to trigger another garbage collection.  A cons cell counts
     as eight bytes, a string as one byte per character plus a few
     bytes of overhead, and so on; space allocated to the contents of
     buffers does not count.  Note that the subsequent garbage
     collection does not happen immediately when the threshold is
     exhausted, but only the next time the Lisp evaluator is called.

     The initial threshold value is 800,000.  If you specify a larger
     value, garbage collection will happen less often.  This reduces the
     amount of time spent garbage collecting, but increases total
     memory use.  You may want to do this when running a program that
     creates lots of Lisp data.

     You can make collections more frequent by specifying a smaller
     value, down to 10,000.  A value less than 10,000 will remain in
     effect only until the subsequent garbage collection, at which time
     `garbage-collect' will set the threshold back to 10,000.

 -- User Option: gc-cons-percentage
     The value of this variable specifies the amount of consing before a
     garbage collection occurs, as a fraction of the current heap size.
     This criterion and `gc-cons-threshold' apply in parallel, and
     garbage collection occurs only when both criteria are satisfied.

     As the heap size increases, the time to perform a garbage
     collection increases.  Thus, it can be desirable to do them less
     frequently in proportion.

   The value returned by `garbage-collect' describes the amount of
memory used by Lisp data, broken down by data type.  By contrast, the
function `memory-limit' provides information on the total amount of
memory Emacs is currently using.

 -- Function: memory-limit
     This function returns the address of the last byte Emacs has
     allocated, divided by 1024.  We divide the value by 1024 to make
     sure it fits in a Lisp integer.

     You can use this to get a general idea of how your actions affect
     the memory usage.

 -- Variable: memory-full
     This variable is `t' if Emacs is nearly out of memory for Lisp
     objects, and `nil' otherwise.

 -- Function: memory-use-counts
     This returns a list of numbers that count the number of objects
     created in this Emacs session.  Each of these counters increments
     for a certain kind of object.  See the documentation string for
     details.

 -- Variable: gcs-done
     This variable contains the total number of garbage collections
     done so far in this Emacs session.

 -- Variable: gc-elapsed
     This variable contains the total number of seconds of elapsed time
     during garbage collection so far in this Emacs session, as a
     floating point number.


File: elisp,  Node: Memory Usage,  Next: Writing Emacs Primitives,  Prev: Garbage Collection,  Up: GNU Emacs Internals

E.4 Memory Usage
================

These functions and variables give information about the total amount
of memory allocation that Emacs has done, broken down by data type.
Note the difference between these and the values returned by
`garbage-collect'; those count objects that currently exist, but these
count the number or size of all allocations, including those for
objects that have since been freed.

 -- Variable: cons-cells-consed
     The total number of cons cells that have been allocated so far in
     this Emacs session.

 -- Variable: floats-consed
     The total number of floats that have been allocated so far in this
     Emacs session.

 -- Variable: vector-cells-consed
     The total number of vector cells that have been allocated so far
     in this Emacs session.

 -- Variable: symbols-consed
     The total number of symbols that have been allocated so far in
     this Emacs session.

 -- Variable: string-chars-consed
     The total number of string characters that have been allocated so
     far in this session.

 -- Variable: misc-objects-consed
     The total number of miscellaneous objects that have been allocated
     so far in this session.  These include markers and overlays, plus
     certain objects not visible to users.

 -- Variable: intervals-consed
     The total number of intervals that have been allocated so far in
     this Emacs session.

 -- Variable: strings-consed
     The total number of strings that have been allocated so far in this
     Emacs session.


File: elisp,  Node: Writing Emacs Primitives,  Next: Object Internals,  Prev: Memory Usage,  Up: GNU Emacs Internals

E.5 Writing Emacs Primitives
============================

Lisp primitives are Lisp functions implemented in C.  The details of
interfacing the C function so that Lisp can call it are handled by a few
C macros.  The only way to really understand how to write new C code is
to read the source, but we can explain some things here.

   An example of a special form is the definition of `or', from
`eval.c'.  (An ordinary function would have the same general
appearance.)

     DEFUN ("or", For, Sor, 0, UNEVALLED, 0,
       doc: /* Eval args until one of them yields non-nil, then return
     that value.
     The remaining args are not evalled at all.
     If all args return nil, return nil.
     usage: (or CONDITIONS ...)  */)
       (Lisp_Object args)
     {
       register Lisp_Object val = Qnil;
       struct gcpro gcpro1;

       GCPRO1 (args);

       while (CONSP (args))
         {
           val = eval_sub (XCAR (args));
           if (!NILP (val))
             break;
           args = XCDR (args);
         }

       UNGCPRO;
       return val;
     }

   Let's start with a precise explanation of the arguments to the
`DEFUN' macro.  Here is a template for them:

     DEFUN (LNAME, FNAME, SNAME, MIN, MAX, INTERACTIVE, DOC)

LNAME
     This is the name of the Lisp symbol to define as the function
     name; in the example above, it is `or'.

FNAME
     This is the C function name for this function.  This is the name
     that is used in C code for calling the function.  The name is, by
     convention, `F' prepended to the Lisp name, with all dashes (`-')
     in the Lisp name changed to underscores.  Thus, to call this
     function from C code, call `For'.

SNAME
     This is a C variable name to use for a structure that holds the
     data for the subr object that represents the function in Lisp.
     This structure conveys the Lisp symbol name to the initialization
     routine that will create the symbol and store the subr object as
     its definition.  By convention, this name is always FNAME with `F'
     replaced with `S'.

MIN
     This is the minimum number of arguments that the function
     requires.  The function `or' allows a minimum of zero arguments.

MAX
     This is the maximum number of arguments that the function accepts,
     if there is a fixed maximum.  Alternatively, it can be `UNEVALLED',
     indicating a special form that receives unevaluated arguments, or
     `MANY', indicating an unlimited number of evaluated arguments (the
     equivalent of `&rest').  Both `UNEVALLED' and `MANY' are macros.
     If MAX is a number, it must be more than MIN but less than 8.

INTERACTIVE
     This is an interactive specification, a string such as might be
     used as the argument of `interactive' in a Lisp function.  In the
     case of `or', it is 0 (a null pointer), indicating that `or'
     cannot be called interactively.  A value of `""' indicates a
     function that should receive no arguments when called
     interactively.  If the value begins with a `(', the string is
     evaluated as a Lisp form.  For examples of the last two forms, see
     `widen' and `narrow-to-region' in `editfns.c'.

DOC
     This is the documentation string.  It uses C comment syntax rather
     than C string syntax because comment syntax requires nothing
     special to include multiple lines.  The `doc:' identifies the
     comment that follows as the documentation string.  The `/*' and
     `*/' delimiters that begin and end the comment are not part of the
     documentation string.

     If the last line of the documentation string begins with the
     keyword `usage:', the rest of the line is treated as the argument
     list for documentation purposes.  This way, you can use different
     argument names in the documentation string from the ones used in
     the C code.  `usage:' is required if the function has an unlimited
     number of arguments.

     All the usual rules for documentation strings in Lisp code (*note
     Documentation Tips::) apply to C code documentation strings too.

   After the call to the `DEFUN' macro, you must write the argument
list for the C function, including the types for the arguments.  If the
primitive accepts a fixed maximum number of Lisp arguments, there must
be one C argument for each Lisp argument, and each argument must be of
type `Lisp_Object'.  (Various macros and functions for creating values
of type `Lisp_Object' are declared in the file `lisp.h'.)  If the
primitive has no upper limit on the number of Lisp arguments, it must
have exactly two C arguments: the first is the number of Lisp
arguments, and the second is the address of a block containing their
values.  These have types `int' and `Lisp_Object *' respectively.

   Within the function `For' itself, note the use of the macros
`GCPRO1' and `UNGCPRO'.  These macros are defined for the sake of the
few platforms which do not use Emacs' default stack-marking garbage
collector.  The `GCPRO1' macro "protects" a variable from garbage
collection, explicitly informing the garbage collector that that
variable and all its contents must be as accessible.  GC protection is
necessary in any function which can perform Lisp evaluation by calling
`eval_sub' or `Feval' as a subroutine, either directly or indirectly.

   It suffices to ensure that at least one pointer to each object is
GC-protected.  Thus, a particular local variable can do without
protection if it is certain that the object it points to will be
preserved by some other pointer (such as another local variable that
has a `GCPRO').  Otherwise, the local variable needs a `GCPRO'.

   The macro `GCPRO1' protects just one local variable.  If you want to
protect two variables, use `GCPRO2' instead; repeating `GCPRO1' will
not work.  Macros `GCPRO3', `GCPRO4', `GCPRO5', and `GCPRO6' also
exist.  All these macros implicitly use local variables such as
`gcpro1'; you must declare these explicitly, with type `struct gcpro'.
Thus, if you use `GCPRO2', you must declare `gcpro1' and `gcpro2'.

   `UNGCPRO' cancels the protection of the variables that are protected
in the current function.  It is necessary to do this explicitly.

   You must not use C initializers for static or global variables unless
the variables are never written once Emacs is dumped.  These variables
with initializers are allocated in an area of memory that becomes
read-only (on certain operating systems) as a result of dumping Emacs.
*Note Pure Storage::.

   Defining the C function is not enough to make a Lisp primitive
available; you must also create the Lisp symbol for the primitive and
store a suitable subr object in its function cell.  The code looks like
this:

     defsubr (&SNAME);

Here SNAME is the name you used as the third argument to `DEFUN'.

   If you add a new primitive to a file that already has Lisp primitives
defined in it, find the function (near the end of the file) named
`syms_of_SOMETHING', and add the call to `defsubr' there.  If the file
doesn't have this function, or if you create a new file, add to it a
`syms_of_FILENAME' (e.g., `syms_of_myfile').  Then find the spot in
`emacs.c' where all of these functions are called, and add a call to
`syms_of_FILENAME' there.

   The function `syms_of_FILENAME' is also the place to define any C
variables that are to be visible as Lisp variables.  `DEFVAR_LISP'
makes a C variable of type `Lisp_Object' visible in Lisp.  `DEFVAR_INT'
makes a C variable of type `int' visible in Lisp with a value that is
always an integer.  `DEFVAR_BOOL' makes a C variable of type `int'
visible in Lisp with a value that is either `t' or `nil'.  Note that
variables defined with `DEFVAR_BOOL' are automatically added to the list
`byte-boolean-vars' used by the byte compiler.

   If you want to make a Lisp variables that is defined in C behave
like one declared with `defcustom', add an appropriate entry to
`cus-start.el'.

   If you define a file-scope C variable of type `Lisp_Object', you
must protect it from garbage-collection by calling `staticpro' in
`syms_of_FILENAME', like this:

     staticpro (&VARIABLE);

   Here is another example function, with more complicated arguments.
This comes from the code in `window.c', and it demonstrates the use of
macros and functions to manipulate Lisp objects.

     DEFUN ("coordinates-in-window-p", Fcoordinates_in_window_p,
       Scoordinates_in_window_p, 2, 2, 0,
       doc: /* Return non-nil if COORDINATES are in WINDOW.
       ...
       or `right-margin' is returned.  */)
       (register Lisp_Object coordinates, Lisp_Object window)
     {
       struct window *w;
       struct frame *f;
       int x, y;
       Lisp_Object lx, ly;

       CHECK_LIVE_WINDOW (window);
       w = XWINDOW (window);
       f = XFRAME (w->frame);
       CHECK_CONS (coordinates);
       lx = Fcar (coordinates);
       ly = Fcdr (coordinates);
       CHECK_NUMBER_OR_FLOAT (lx);
       CHECK_NUMBER_OR_FLOAT (ly);
       x = FRAME_PIXEL_X_FROM_CANON_X (f, lx) + FRAME_INTERNAL_BORDER_WIDTH(f);
       y = FRAME_PIXEL_Y_FROM_CANON_Y (f, ly) + FRAME_INTERNAL_BORDER_WIDTH(f);

       switch (coordinates_in_window (w, x, y))
         {
         case ON_NOTHING:            /* NOT in window at all. */
           return Qnil;

         ...

         case ON_MODE_LINE:          /* In mode line of window. */
           return Qmode_line;

         ...

         case ON_SCROLL_BAR:         /* On scroll-bar of window.  */
           /* Historically we are supposed to return nil in this case.  */
           return Qnil;

         default:
           abort ();
         }
     }

   Note that C code cannot call functions by name unless they are
defined in C.  The way to call a function written in Lisp is to use
`Ffuncall', which embodies the Lisp function `funcall'.  Since the Lisp
function `funcall' accepts an unlimited number of arguments, in C it
takes two: the number of Lisp-level arguments, and a one-dimensional
array containing their values.  The first Lisp-level argument is the
Lisp function to call, and the rest are the arguments to pass to it.
Since `Ffuncall' can call the evaluator, you must protect pointers from
garbage collection around the call to `Ffuncall'.

   The C functions `call0', `call1', `call2', and so on, provide handy
ways to call a Lisp function conveniently with a fixed number of
arguments.  They work by calling `Ffuncall'.

   `eval.c' is a very good file to look through for examples; `lisp.h'
contains the definitions for some important macros and functions.

   If you define a function which is side-effect free, update the code
in `byte-opt.el' that binds `side-effect-free-fns' and
`side-effect-and-error-free-fns' so that the compiler optimizer knows
about it.


File: elisp,  Node: Object Internals,  Prev: Writing Emacs Primitives,  Up: GNU Emacs Internals

E.6 Object Internals
====================

GNU Emacs Lisp manipulates many different types of data.  The actual
data are stored in a heap and the only access that programs have to it
is through pointers.  Each pointer is 32 bits wide on 32-bit machines,
and 64 bits wide on 64-bit machines; three of these bits are used for
the tag that identifies the object's type, and the remainder are used
to address the object.

   Because Lisp objects are represented as tagged pointers, it is always
possible to determine the Lisp data type of any object.  The C data type
`Lisp_Object' can hold any Lisp object of any data type.  Ordinary
variables have type `Lisp_Object', which means they can hold any type
of Lisp value; you can determine the actual data type only at run time.
The same is true for function arguments; if you want a function to
accept only a certain type of argument, you must check the type
explicitly using a suitable predicate (*note Type Predicates::).  

* Menu:

* Buffer Internals::    Components of a buffer structure.
* Window Internals::    Components of a window structure.
* Process Internals::   Components of a process structure.


File: elisp,  Node: Buffer Internals,  Next: Window Internals,  Up: Object Internals

E.6.1 Buffer Internals
----------------------

Two structures (see `buffer.h') are used to represent buffers in C.
The `buffer_text' structure contains fields describing the text of a
buffer; the `buffer' structure holds other fields.  In the case of
indirect buffers, two or more `buffer' structures reference the same
`buffer_text' structure.

   Here are some of the fields in `struct buffer_text':

`beg'
     The address of the buffer contents.

`gpt'
`gpt_byte'
     The character and byte positions of the buffer gap.  *Note Buffer
     Gap::.

`z'
`z_byte'
     The character and byte positions of the end of the buffer text.

`gap_size'
     The size of buffer's gap.  *Note Buffer Gap::.

`modiff'
`save_modiff'
`chars_modiff'
`overlay_modiff'
     These fields count the number of buffer-modification events
     performed in this buffer.  `modiff' is incremented after each
     buffer-modification event, and is never otherwise changed;
     `save_modiff' contains the value of `modiff' the last time the
     buffer was visited or saved; `chars_modiff' counts only
     modifications to the characters in the buffer, ignoring all other
     kinds of changes; and `overlay_modiff' counts only modifications
     to the overlays.

`beg_unchanged'
`end_unchanged'
     The number of characters at the start and end of the text that are
     known to be unchanged since the last complete redisplay.

`unchanged_modified'
`overlay_unchanged_modified'
     The values of `modiff' and `overlay_modiff', respectively, after
     the last complete redisplay.  If their current values match
     `modiff' or `overlay_modiff', that means `beg_unchanged' and
     `end_unchanged' contain no useful information.

`markers'
     The markers that refer to this buffer.  This is actually a single
     marker, and successive elements in its marker `chain' are the other
     markers referring to this buffer text.

`intervals'
     The interval tree which records the text properties of this buffer.

   Some of the fields of `struct buffer' are:

`header'
     A `struct vectorlike_header' structure where `header.next' points
     to the next buffer, in the chain of all buffers (including killed
     buffers).  This chain is used only for garbage collection, in
     order to collect killed buffers properly.  Note that vectors, and
     most kinds of objects allocated as vectors, are all on one chain,
     but buffers are on a separate chain of their own.

`own_text'
     A `struct buffer_text' structure that ordinarily holds the buffer
     contents.  In indirect buffers, this field is not used.

`text'
     A pointer to the `buffer_text' structure for this buffer.  In an
     ordinary buffer, this is the `own_text' field above.  In an
     indirect buffer, this is the `own_text' field of the base buffer.

`pt'
`pt_byte'
     The character and byte positions of point in a buffer.

`begv'
`begv_byte'
     The character and byte positions of the beginning of the accessible
     range of text in the buffer.

`zv'
`zv_byte'
     The character and byte positions of the end of the accessible
     range of text in the buffer.

`base_buffer'
     In an indirect buffer, this points to the base buffer.  In an
     ordinary buffer, it is null.

`local_flags'
     This field contains flags indicating that certain variables are
     local in this buffer.  Such variables are declared in the C code
     using `DEFVAR_PER_BUFFER', and their buffer-local bindings are
     stored in fields in the buffer structure itself.  (Some of these
     fields are described in this table.)

`modtime'
     The modification time of the visited file.  It is set when the
     file is written or read.  Before writing the buffer into a file,
     this field is compared to the modification time of the file to see
     if the file has changed on disk.  *Note Buffer Modification::.

`auto_save_modified'
     The time when the buffer was last auto-saved.

`last_window_start'
     The `window-start' position in the buffer as of the last time the
     buffer was displayed in a window.

`clip_changed'
     This flag indicates that narrowing has changed in the buffer.
     *Note Narrowing::.

`prevent_redisplay_optimizations_p'
     This flag indicates that redisplay optimizations should not be
     used to display this buffer.

`overlay_center'
     This field holds the current overlay center position.  *Note
     Managing Overlays::.

`overlays_before'
`overlays_after'
     These fields hold, respectively, a list of overlays that end at or
     before the current overlay center, and a list of overlays that end
     after the current overlay center.  *Note Managing Overlays::.
     `overlays_before' is sorted in order of decreasing end position,
     and `overlays_after' is sorted in order of increasing beginning
     position.

`name'
     A Lisp string that names the buffer.  It is guaranteed to be
     unique.  *Note Buffer Names::.

`save_length'
     The length of the file this buffer is visiting, when last read or
     saved.  This and other fields concerned with saving are not kept in
     the `buffer_text' structure because indirect buffers are never
     saved.

`directory'
     The directory for expanding relative file names.  This is the
     value of the buffer-local variable `default-directory' (*note File
     Name Expansion::).

`filename'
     The name of the file visited in this buffer, or `nil'.  This is
     the value of the buffer-local variable `buffer-file-name' (*note
     Buffer File Name::).

`undo_list'
`backed_up'
`auto_save_file_name'
`auto_save_file_format'
`read_only'
`file_format'
`file_truename'
`invisibility_spec'
`display_count'
`display_time'
     These fields store the values of Lisp variables that are
     automatically buffer-local (*note Buffer-Local Variables::), whose
     corresponding variable names have the additional prefix `buffer-'
     and have underscores replaced with dashes.  For instance,
     `undo_list' stores the value of `buffer-undo-list'.

`mark'
     The mark for the buffer.  The mark is a marker, hence it is also
     included on the list `markers'.  *Note The Mark::.

`local_var_alist'
     The association list describing the buffer-local variable bindings
     of this buffer, not including the built-in buffer-local bindings
     that have special slots in the buffer object.  (Those slots are
     omitted from this table.)  *Note Buffer-Local Variables::.

`major_mode'
     Symbol naming the major mode of this buffer, e.g., `lisp-mode'.

`mode_name'
     Pretty name of the major mode, e.g., `"Lisp"'.

`keymap'
`abbrev_table'
`syntax_table'
`category_table'
`display_table'
     These fields store the buffer's local keymap (*note Keymaps::),
     abbrev table (*note Abbrev Tables::), syntax table (*note Syntax
     Tables::), category table (*note Categories::), and display table
     (*note Display Tables::).

`downcase_table'
`upcase_table'
`case_canon_table'
     These fields store the conversion tables for converting text to
     lower case, upper case, and for canonicalizing text for case-fold
     search.  *Note Case Tables::.

`minor_modes'
     An alist of the minor modes of this buffer.

`pt_marker'
`begv_marker'
`zv_marker'
     These fields are only used in an indirect buffer, or in a buffer
     that is the base of an indirect buffer.  Each holds a marker that
     records `pt', `begv', and `zv' respectively, for this buffer when
     the buffer is not current.

`mode_line_format'
`header_line_format'
`case_fold_search'
`tab_width'
`fill_column'
`left_margin'
`auto_fill_function'
`truncate_lines'
`word_wrap'
`ctl_arrow'
`bidi_display_reordering'
`bidi_paragraph_direction'
`selective_display'
`selective_display_ellipses'
`overwrite_mode'
`abbrev_mode'
`mark_active'
`enable_multibyte_characters'
`buffer_file_coding_system'
`cache_long_line_scans'
`point_before_scroll'
`left_fringe_width'
`right_fringe_width'
`fringes_outside_margins'
`scroll_bar_width'
`indicate_empty_lines'
`indicate_buffer_boundaries'
`fringe_indicator_alist'
`fringe_cursor_alist'
`scroll_up_aggressively'
`scroll_down_aggressively'
`cursor_type'
`cursor_in_non_selected_windows'
     These fields store the values of Lisp variables that are
     automatically buffer-local (*note Buffer-Local Variables::), whose
     corresponding variable names have underscores replaced with
     dashes.  For instance, `mode_line_format' stores the value of
     `mode-line-format'.

`last_selected_window'
     This is the last window that was selected with this buffer in it,
     or `nil' if that window no longer displays this buffer.


File: elisp,  Node: Window Internals,  Next: Process Internals,  Prev: Buffer Internals,  Up: Object Internals

E.6.2 Window Internals
----------------------

The fields of a window (for a complete list, see the definition of
`struct window' in `window.h') include:

`frame'
     The frame that this window is on.

`mini_p'
     Non-`nil' if this window is a minibuffer window.

`parent'
     Internally, Emacs arranges windows in a tree; each group of
     siblings has a parent window whose area includes all the siblings.
     This field points to a window's parent.

     Parent windows do not display buffers, and play little role in
     display except to shape their child windows.  Emacs Lisp programs
     usually have no access to the parent windows; they operate on the
     windows at the leaves of the tree, which actually display buffers.

`hchild'
`vchild'
     These fields contain the window's leftmost child and its topmost
     child respectively.  `hchild' is used if the window is subdivided
     horizontally by child windows, and `vchild' if it is subdivided
     vertically.  In a live window, only one of `hchild', `vchild', and
     `buffer' (q.v.) is non-`nil'.

`next'
`prev'
     The next sibling and previous sibling of this window.  `next' is
     `nil' if the window is the right-most or bottom-most in its group;
     `prev' is `nil' if it is the left-most or top-most in its group.

`left_col'
     The left-hand edge of the window, measured in columns, relative to
     the leftmost column in the frame (column 0).

`top_line'
     The top edge of the window, measured in lines, relative to the
     topmost line in the frame (line 0).

`total_cols'
`total_lines'
     The width and height of the window, measured in columns and lines
     respectively.  The width includes the scroll bar and fringes,
     and/or the separator line on the right of the window (if any).

`buffer'
     The buffer that the window is displaying.

`start'
     A marker pointing to the position in the buffer that is the first
     character displayed in the window.

`pointm'
     This is the value of point in the current buffer when this window
     is selected; when it is not selected, it retains its previous
     value.

`force_start'
     If this flag is non-`nil', it says that the window has been
     scrolled explicitly by the Lisp program.  This affects what the
     next redisplay does if point is off the screen: instead of
     scrolling the window to show the text around point, it moves point
     to a location that is on the screen.

`frozen_window_start_p'
     This field is set temporarily to 1 to indicate to redisplay that
     `start' of this window should not be changed, even if point gets
     invisible.

`start_at_line_beg'
     Non-`nil' means current value of `start' was the beginning of a
     line when it was chosen.

`use_time'
     This is the last time that the window was selected.  The function
     `get-lru-window' uses this field.

`sequence_number'
     A unique number assigned to this window when it was created.

`last_modified'
     The `modiff' field of the window's buffer, as of the last time a
     redisplay completed in this window.

`last_overlay_modified'
     The `overlay_modiff' field of the window's buffer, as of the last
     time a redisplay completed in this window.

`last_point'
     The buffer's value of point, as of the last time a redisplay
     completed in this window.

`last_had_star'
     A non-`nil' value means the window's buffer was "modified" when the
     window was last updated.

`vertical_scroll_bar'
     This window's vertical scroll bar.

`left_margin_cols'
`right_margin_cols'
     The widths of the left and right margins in this window.  A value
     of `nil' means no margin.

`left_fringe_width'
`right_fringe_width'
     The widths of the left and right fringes in this window.  A value
     of `nil' or `t' means use the values of the frame.

`fringes_outside_margins'
     A non-`nil' value means the fringes outside the display margins;
     othersize they are between the margin and the text.

`window_end_pos'
     This is computed as `z' minus the buffer position of the last glyph
     in the current matrix of the window.  The value is only valid if
     `window_end_valid' is not `nil'.

`window_end_bytepos'
     The byte position corresponding to `window_end_pos'.

`window_end_vpos'
     The window-relative vertical position of the line containing
     `window_end_pos'.

`window_end_valid'
     This field is set to a non-`nil' value if `window_end_pos' is truly
     valid.  This is `nil' if nontrivial redisplay is pre-empted, since
     in that case the display that `window_end_pos' was computed for
     did not get onto the screen.

`cursor'
     A structure describing where the cursor is in this window.

`last_cursor'
     The value of `cursor' as of the last redisplay that finished.

`phys_cursor'
     A structure describing where the cursor of this window physically
     is.

`phys_cursor_type'
`phys_cursor_height'
`phys_cursor_width'
     The type, height, and width of the cursor that was last displayed
     on this window.

`phys_cursor_on_p'
     This field is non-zero if the cursor is physically on.

`cursor_off_p'
     Non-zero means the cursor in this window is logically off.  This is
     used for blinking the cursor.

`last_cursor_off_p'
     This field contains the value of `cursor_off_p' as of the time of
     the last redisplay.

`must_be_updated_p'
     This is set to 1 during redisplay when this window must be updated.

`hscroll'
     This is the number of columns that the display in the window is
     scrolled horizontally to the left.  Normally, this is 0.

`vscroll'
     Vertical scroll amount, in pixels.  Normally, this is 0.

`dedicated'
     Non-`nil' if this window is dedicated to its buffer.

`display_table'
     The window's display table, or `nil' if none is specified for it.

`update_mode_line'
     Non-`nil' means this window's mode line needs to be updated.

`base_line_number'
     The line number of a certain position in the buffer, or `nil'.
     This is used for displaying the line number of point in the mode
     line.

`base_line_pos'
     The position in the buffer for which the line number is known, or
     `nil' meaning none is known.  If it is a buffer, don't display the
     line number as long as the window shows that buffer.

`region_showing'
     If the region (or part of it) is highlighted in this window, this
     field holds the mark position that made one end of that region.
     Otherwise, this field is `nil'.

`column_number_displayed'
     The column number currently displayed in this window's mode line,
     or `nil' if column numbers are not being displayed.

`current_matrix'
`desired_matrix'
     Glyph matrices describing the current and desired display of this
     window.


File: elisp,  Node: Process Internals,  Prev: Window Internals,  Up: Object Internals

E.6.3 Process Internals
-----------------------

The fields of a process (for a complete list, see the definition of
`struct Lisp_Process' in `process.h') include:

`name'
     A string, the name of the process.

`command'
     A list containing the command arguments that were used to start
     this process.  For a network or serial process, it is `nil' if the
     process is running or `t' if the process is stopped.

`filter'
     If non-`nil', a function used to accept output from the process
     instead of a buffer.

`sentinel'
     If non-`nil', a function called whenever the state of the process
     changes.

`buffer'
     The associated buffer of the process.

`pid'
     An integer, the operating system's process ID.  Pseudo-processes
     such as network or serial connections use a value of 0.

`childp'
     A flag, `t' if this is really a child process.  For a network or
     serial connection, it is a plist based on the arguments to
     `make-network-process' or `make-serial-process'.

`mark'
     A marker indicating the position of the end of the last output
     from this process inserted into the buffer.  This is often but not
     always the end of the buffer.

`kill_without_query'
     If this is non-zero, killing Emacs while this process is still
     running does not ask for confirmation about killing the process.

`raw_status'
     The raw process status, as returned by the `wait' system call.

`status'
     The process status, as `process-status' should return it.

`tick'
`update_tick'
     If these two fields are not equal, a change in the status of the
     process needs to be reported, either by running the sentinel or by
     inserting a message in the process buffer.

`pty_flag'
     Non-`nil' if communication with the subprocess uses a PTY; `nil'
     if it uses a pipe.

`infd'
     The file descriptor for input from the process.

`outfd'
     The file descriptor for output to the process.

`tty_name'
     The name of the terminal that the subprocess is using, or `nil' if
     it is using pipes.

`decode_coding_system'
     Coding-system for decoding the input from this process.

`decoding_buf'
     A working buffer for decoding.

`decoding_carryover'
     Size of carryover in decoding.

`encode_coding_system'
     Coding-system for encoding the output to this process.

`encoding_buf'
     A working buffer for encoding.

`inherit_coding_system_flag'
     Flag to set `coding-system' of the process buffer from the coding
     system used to decode process output.

`type'
     Symbol indicating the type of process: `real', `network', `serial'.



File: elisp,  Node: Standard Errors,  Next: Standard Keymaps,  Prev: GNU Emacs Internals,  Up: Top

Appendix F Standard Errors
**************************

Here is a list of the more important error symbols in standard Emacs,
grouped by concept.  The list includes each symbol's message (on the
`error-message' property of the symbol) and a cross reference to a
description of how the error can occur.

   Each error symbol has an `error-conditions' property that is a list
of symbols.  Normally this list includes the error symbol itself and
the symbol `error'.  Occasionally it includes additional symbols, which
are intermediate classifications, narrower than `error' but broader
than a single error symbol.  For example, all the errors in accessing
files have the condition `file-error'.  If we do not say here that a
certain error symbol has additional error conditions, that means it has
none.

   As a special exception, the error symbol `quit' does not have the
condition `error', because quitting is not considered an error.

   Most of these error symbols are defined in C (mainly `data.c'), but
some are defined in Lisp.  For example, the file `userlock.el' defines
the `file-locked' and `file-supersession' errors.  Several of the
specialized Lisp libraries distributed with Emacs define their own
error symbols.  We do not attempt to list of all those here.

   *Note Errors::, for an explanation of how errors are generated and
handled.

`error'
     `"error"'
     *Note Errors::.

`quit'
     `"Quit"'
     *Note Quitting::.

`args-out-of-range'
     `"Args out of range"'
     This happens when trying to access an element beyond the range of a
     sequence or buffer.
     *Note Sequences Arrays Vectors::, *Note Text::.

`arith-error'
     `"Arithmetic error"'
     *Note Arithmetic Operations::.

`beginning-of-buffer'
     `"Beginning of buffer"'
     *Note Character Motion::.

`buffer-read-only'
     `"Buffer is read-only"'
     *Note Read Only Buffers::.

`circular-list'
     `"List contains a loop"'
     This happens when some operations (e.g. resolving face names)
     encounter circular structures.
     *Note Circular Objects::.

`cl-assertion-failed'
     `"Assertion failed"'
     This happens when the `assert' macro fails a test.
     *Note Assertions: (cl)Assertions.

`coding-system-error'
     `"Invalid coding system"'
     *Note Lisp and Coding Systems::.

`cyclic-function-indirection'
     `"Symbol's chain of function indirections contains a loop"'
     *Note Function Indirection::.

`cyclic-variable-indirection'
     `"Symbol's chain of variable indirections contains a loop"'
     *Note Variable Aliases::.

`dbus-error'
     `"D-Bus error"'
     This is only defined if Emacs was compiled with D-Bus support.
     *Note Errors and Events: (dbus)Errors and Events.

`end-of-buffer'
     `"End of buffer"'
     *Note Character Motion::.

`end-of-file'
     `"End of file during parsing"'
     Note that this is not a subcategory of `file-error', because it
     pertains to the Lisp reader, not to file I/O.
     *Note Input Functions::.

`file-already-exists'
     This is a subcategory of `file-error'.
     *Note Writing to Files::.

`file-date-error'
     This is a subcategory of `file-error'.  It occurs when `copy-file'
     tries and fails to set the last-modification time of the output
     file.
     *Note Changing Files::.

`file-error'
     We do not list the error-strings of this error and its
     subcategories, because the error message is normally constructed
     from the data items alone when the error condition `file-error' is
     present.  Thus, the error-strings are not very relevant.  However,
     these error symbols do have `error-message' properties, and if no
     data is provided, the `error-message' property _is_ used.
     *Note Files::.

`compression-error'
     This is a subcategory of `file-error', which results from problems
     handling a compressed file.
     *Note How Programs Do Loading::.

`file-locked'
     This is a subcategory of `file-error'.
     *Note File Locks::.

`file-supersession'
     This is a subcategory of `file-error'.
     *Note Modification Time::.

`ftp-error'
     This is a subcategory of `file-error', which results from problems
     in accessing a remote file using ftp.
     *Note Remote Files: (emacs)Remote Files.

`invalid-function'
     `"Invalid function"'
     *Note Function Indirection::.

`invalid-read-syntax'
     `"Invalid read syntax"'
     *Note Printed Representation::.

`invalid-regexp'
     `"Invalid regexp"'
     *Note Regular Expressions::.

`mark-inactive'
     `"The mark is not active now"'
     *Note The Mark::.

`no-catch'
     `"No catch for tag"'
     *Note Catch and Throw::.

`scan-error'
     `"Scan error"'
     This happens when certain syntax-parsing functions find invalid
     syntax or mismatched parentheses.
     *Note List Motion::, and *note Parsing Expressions::.

`search-failed'
     `"Search failed"'
     *Note Searching and Matching::.

`setting-constant'
     `"Attempt to set a constant symbol"'
     The values of the symbols `nil' and `t', and any symbols that
     start with `:', may not be changed.
     *Note Variables that Never Change: Constant Variables.

`text-read-only'
     `"Text is read-only"'
     This is a subcategory of `buffer-read-only'.
     *Note Special Properties::.

`undefined-color'
     `"Undefined color"'
     *Note Color Names::.

`void-function'
     `"Symbol's function definition is void"'
     *Note Function Cells::.

`void-variable'
     `"Symbol's value as variable is void"'
     *Note Accessing Variables::.

`wrong-number-of-arguments'
     `"Wrong number of arguments"'
     *Note Classifying Lists::.

`wrong-type-argument'
     `"Wrong type argument"'
     *Note Type Predicates::.

   The following kinds of error, which are classified as special cases
of `arith-error', can occur on certain systems for invalid use of
mathematical functions.  *Note Math Functions::.

`domain-error'
     `"Arithmetic domain error"'

`overflow-error'
     `"Arithmetic overflow error"'
     This is a subcategory of `domain-error'.

`range-error'
     `"Arithmetic range error"'

`singularity-error'
     `"Arithmetic singularity error"'
     This is a subcategory of `domain-error'.

`underflow-error'
     `"Arithmetic underflow error"'
     This is a subcategory of `domain-error'.


File: elisp,  Node: Standard Keymaps,  Next: Standard Hooks,  Prev: Standard Errors,  Up: Top

Appendix G Standard Keymaps
***************************

In this section we list some of the more general keymaps.  Many of
these exist when Emacs is first started, but some are loaded only when
the respective feature is accessed.

   There are many other, more specialized, maps than these; in
particular those associated with major and minor modes.  The minibuffer
uses several keymaps (*note Completion Commands::).  For more details on
keymaps, *note Keymaps::.

`2C-mode-map'
     A sparse keymap for subcommands of the prefix `C-x 6'.
     *Note Two-Column Editing: (emacs)Two-Column.

`abbrev-map'
     A sparse keymap for subcommands of the prefix `C-x a'.
     *Note Defining Abbrevs: (emacs)Defining Abbrevs.

`button-buffer-map'
     A sparse keymap useful for buffers containing buffers.
     You may want to use this as a parent keymap.  *Note Buttons::.

`button-map'
     A sparse keymap used by buttons.

`ctl-x-4-map'
     A sparse keymap for subcommands of the prefix `C-x 4'.

`ctl-x-5-map'
     A sparse keymap for subcommands of the prefix `C-x 5'.

`ctl-x-map'
     A full keymap for `C-x' commands.

`ctl-x-r-map'
     A sparse keymap for subcommands of the prefix `C-x r'.
     *Note Registers: (emacs)Registers.

`esc-map'
     A full keymap for `ESC' (or `Meta') commands.

`facemenu-keymap'
     A sparse keymap used for the `M-o' prefix key.

`function-key-map'
     The parent keymap of all `local-function-key-map' (q.v.) instances.

`global-map'
     The full keymap containing default global key bindings.
     Modes should not modify the Global map.

`goto-map'
     A sparse keymap used for the `M-g' prefix key.

`help-map'
     A sparse keymap for the keys following the help character `C-h'.
     *Note Help Functions::.

`Helper-help-map'
     A full keymap used by the help utility package.
     It has the same keymap in its value cell and in its function cell.

`input-decode-map'
     The keymap for translating keypad and function keys.
     If there are none, then it contains an empty sparse keymap.  *Note
     Translation Keymaps::.

`key-translation-map'
     A keymap for translating keys.  This one overrides ordinary key
     bindings, unlike `local-function-key-map'.  *Note Translation
     Keymaps::.

`kmacro-keymap'
     A sparse keymap for keys that follows the `C-x C-k' prefix search.
     *Note Keyboard Macros: (emacs)Keyboard Macros.

`local-function-key-map'
     The keymap for translating key sequences to preferred alternatives.
     If there are none, then it contains an empty sparse keymap.  *Note
     Translation Keymaps::.

`menu-bar-file-menu'
`menu-bar-edit-menu'
`menu-bar-options-menu'
`global-buffers-menu-map'
`menu-bar-tools-menu'
`menu-bar-help-menu'
     These keymaps display the main, top-level menus in the menu bar.
     Some of them contain sub-menus.  For example, the Edit menu
     contains `menu-bar-search-menu', etc.  *Note Menu Bar::.

`minibuffer-inactive-mode-map'
     A full keymap used in the minibuffer when it is not active.
     *Note Editing in the Minibuffer: (emacs)Minibuffer Edit.

`mode-line-coding-system-map'
`mode-line-input-method-map'
`mode-line-column-line-number-mode-map'
     These keymaps control various areas of the mode line.
     *Note Mode Line Format::.

`mode-specific-map'
     The keymap for characters following `C-c'.  Note, this is in the
     global map.  This map is not actually mode-specific: its name was
     chosen to be informative in `C-h b' (`display-bindings'), where it
     describes the main use of the `C-c' prefix key.

`mouse-appearance-menu-map'
     A sparse keymap used for the `S-mouse-1' key.

`mule-keymap'
     The global keymap used for the `C-x <RET>' prefix key.

`narrow-map'
     A sparse keymap for subcommands of the prefix `C-x n'.

`prog-mode-map'
     The keymap used by Prog mode.
     *Note Basic Major Modes::.

`query-replace-map'
`multi-query-replace-map'
     A sparse keymap used for responses in `query-replace' and related
     commands; also for `y-or-n-p' and `map-y-or-n-p'.  The functions
     that use this map do not support prefix keys; they look up one
     event at a time.  `multi-query-replace-map' extends
     `query-replace-map' for multi-buffer replacements.  *Note
     query-replace-map: Search and Replace.

`search-map'
     A sparse keymap that provides global bindings for search-related
     commands.

`special-mode-map'
     The keymap used by Special mode.
     *Note Basic Major Modes::.

`tool-bar-map'
     The keymap defining the contents of the tool bar.
     *Note Tool Bar::.

`universal-argument-map'
     A sparse keymap used while processing `C-u'.
     *Note Prefix Command Arguments::.

`vc-prefix-map'
     The global keymap used for the `C-x v' prefix key.

`x-alternatives-map'
     A sparse keymap used to map certain keys under graphical frames.
     The function `x-setup-function-keys' uses this.



File: elisp,  Node: Standard Hooks,  Next: Index,  Prev: Standard Keymaps,  Up: Top

Appendix H Standard Hooks
*************************

The following is a list of some hook variables that let you provide
functions to be called from within Emacs on suitable occasions.

   Most of these variables have names ending with `-hook'.  They are
"normal hooks", run by means of `run-hooks'.  The value of such a hook
is a list of functions; the functions are called with no arguments and
their values are completely ignored.  The recommended way to put a new
function on such a hook is to call `add-hook'.  *Note Hooks::, for more
information about using hooks.

   The variables whose names end in `-hooks' or `-functions' are
usually "abnormal hooks"; their values are lists of functions, but
these functions are called in a special way (they are passed arguments,
or their values are used). The variables whose names end in `-function'
have single functions as their values.

   This is not an exhaustive list, it only covers the more general
hooks.  For example, every major mode defines a hook named
`MODENAME-mode-hook'.  The major mode command runs this normal hook
with `run-mode-hooks' as the very last thing it does.  *Note Mode
Hooks::.  Most minor modes have mode hooks too.

   A special feature allows you to specify expressions to evaluate if
and when a file is loaded (*note Hooks for Loading::).  That feature is
not exactly a hook, but does a similar job.

`activate-mark-hook'
`deactivate-mark-hook'
     *Note The Mark::.

`after-change-functions'
`before-change-functions'
`first-change-hook'
     *Note Change Hooks::.

`after-change-major-mode-hook'
`change-major-mode-after-body-hook'
     *Note Mode Hooks::.

`after-init-hook'
`before-init-hook'
`emacs-startup-hook'
     *Note Init File::.

`after-insert-file-functions'
`write-region-annotate-functions'
`write-region-post-annotation-function'
     *Note Format Conversion::.

`after-make-frame-functions'
`before-make-frame-hook'
     *Note Creating Frames::.

`after-save-hook'
`before-save-hook'
`write-contents-functions'
`write-file-functions'
     *Note Saving Buffers::.

`after-setting-font-hook'
     Hook run after a frame's font changes.

`auto-save-hook'
     *Note Auto-Saving::.

`before-hack-local-variables-hook'
`hack-local-variables-hook'
     *Note File Local Variables::.

`buffer-access-fontify-functions'
     *Note Lazy Properties::.

`buffer-list-update-hook'
     Hook run when the buffer list changes.

`buffer-quit-function'
     Function to call to "quit" the current buffer.

`change-major-mode-hook'
     *Note Creating Buffer-Local::.

`command-line-functions'
     *Note Command-Line Arguments::.

`delayed-warnings-hook'
     The command loop runs this soon after `post-command-hook' (q.v.).

`delete-frame-functions'
     *Note Deleting Frames::.

`delete-terminal-functions'
     *Note Multiple Terminals::.

`display-buffer-function'
`pop-up-frame-function'
`special-display-function'
`split-window-preferred-function'
     *Note Choosing Window Options::.

`echo-area-clear-hook'
     *Note Echo Area Customization::.

`find-file-hook'
`find-file-not-found-functions'
     *Note Visiting Functions::.

`font-lock-extend-after-change-region-function'
     *Note Region to Refontify::.

`font-lock-extend-region-functions'
     *Note Multiline Font Lock::.

`font-lock-fontify-buffer-function'
`font-lock-fontify-region-function'
`font-lock-mark-block-function'
`font-lock-unfontify-buffer-function'
`font-lock-unfontify-region-function'
     *Note Other Font Lock Variables::.

`fontification-functions'
     *Note Automatic Face Assignment: Auto Faces.

`frame-auto-hide-function'
     *Note Quitting Windows::.

`kill-buffer-hook'
`kill-buffer-query-functions'
     *Note Killing Buffers::.

`kill-emacs-hook'
`kill-emacs-query-functions'
     *Note Killing Emacs::.

`menu-bar-update-hook'
     *Note Menu Bar::.

`minibuffer-setup-hook'
`minibuffer-exit-hook'
     *Note Minibuffer Misc::.

`mouse-leave-buffer-hook'
     Hook run when about to switch windows with a mouse command.

`mouse-position-function'
     *Note Mouse Position::.

`post-command-hook'
`pre-command-hook'
     *Note Command Overview::.

`post-gc-hook'
     *Note Garbage Collection::.

`post-self-insert-hook'
     *Note Keymaps and Minor Modes::.

`suspend-hook'
`suspend-resume-hook'
`suspend-tty-functions'
`resume-tty-functions'
     *Note Suspending Emacs::.

`syntax-begin-function'
`syntax-propertize-extend-region-functions'
`syntax-propertize-function'
`font-lock-syntactic-face-function'
     *Note Syntactic Font Lock::.  *Note Syntax Properties::.

`temp-buffer-setup-hook'
`temp-buffer-show-function'
`temp-buffer-show-hook'
     *Note Temporary Displays::.

`term-setup-hook'
     *Note Terminal-Specific::.

`window-configuration-change-hook'
`window-scroll-functions'
`window-size-change-functions'
     *Note Window Hooks::.

`window-setup-hook'
     *Note Window Systems::.

`window-text-change-functions'
     Functions to call in redisplay when text in the window might
     change.




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