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(elisp)Auto-Saving

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Auto-Saving
===========

   Emacs periodically saves all files that you are visiting; this is
called "auto-saving".  Auto-saving prevents you from losing more than a
limited amount of work if the system crashes.  By default, auto-saves
happen every 300 keystrokes, or after around 30 seconds of idle time.
Note: Auto-Save, for information on auto-save for
users.  Here we describe the functions used to implement auto-saving
and the variables that control them.

 - Variable: buffer-auto-save-file-name
     This buffer-local variable is the name of the file used for
     auto-saving the current buffer.  It is `nil' if the buffer should
     not be auto-saved.

          buffer-auto-save-file-name
               => "/xcssun/users/rms/lewis/#backups.texi#"

 - Command: auto-save-mode arg
     When used interactively without an argument, this command is a
     toggle switch: it turns on auto-saving of the current buffer if it
     is off, and vice versa.  With an argument ARG, the command turns
     auto-saving on if the value of ARG is `t', a nonempty list, or a
     positive integer.  Otherwise, it turns auto-saving off.

 - Function: auto-save-file-name-p filename
     This function returns a non-`nil' value if FILENAME is a string
     that could be the name of an auto-save file.  It assumes the usual
     naming convention for auto-save files: a name that begins and ends
     with hash marks (`#') is a possible auto-save file name.  The
     argument FILENAME should not contain a directory part.

          (make-auto-save-file-name)
               => "/xcssun/users/rms/lewis/#backups.texi#"
          (auto-save-file-name-p "#backups.texi#")
               => 0
          (auto-save-file-name-p "backups.texi")
               => nil

     The standard definition of this function is as follows:

          (defun auto-save-file-name-p (filename)
            "Return non-nil if FILENAME can be yielded by..."
            (string-match "^#.*#$" filename))

     This function exists so that you can customize it if you wish to
     change the naming convention for auto-save files.  If you redefine
     it, be sure to redefine the function `make-auto-save-file-name'
     correspondingly.

 - Function: make-auto-save-file-name
     This function returns the file name to use for auto-saving the
     current buffer.  This is just the file name with hash marks (`#')
     prepended and appended to it.  This function does not look at the
     variable `auto-save-visited-file-name' (described below); callers
     of this function should check that variable first.

          (make-auto-save-file-name)
               => "/xcssun/users/rms/lewis/#backups.texi#"

     The standard definition of this function is as follows:

          (defun make-auto-save-file-name ()
            "Return file name to use for auto-saves \
          of current buffer.."
            (if buffer-file-name
                (concat
                 (file-name-directory buffer-file-name)
                 "#"
                 (file-name-nondirectory buffer-file-name)
                 "#")
              (expand-file-name
               (concat "#%" (buffer-name) "#"))))

     This exists as a separate function so that you can redefine it to
     customize the naming convention for auto-save files.  Be sure to
     change `auto-save-file-name-p' in a corresponding way.

 - Variable: auto-save-visited-file-name
     If this variable is non-`nil', Emacs auto-saves buffers in the
     files they are visiting.  That is, the auto-save is done in the
     same file that you are editing.  Normally, this variable is `nil',
     so auto-save files have distinct names that are created by
     `make-auto-save-file-name'.

     When you change the value of this variable, the new value does not
     take effect in an existing buffer until the next time auto-save
     mode is reenabled in it.  If auto-save mode is already enabled,
     auto-saves continue to go in the same file name until
     `auto-save-mode' is called again.

 - Function: recent-auto-save-p
     This function returns `t' if the current buffer has been
     auto-saved since the last time it was read in or saved.

 - Function: set-buffer-auto-saved
     This function marks the current buffer as auto-saved.  The buffer
     will not be auto-saved again until the buffer text is changed
     again.  The function returns `nil'.

 - User Option: auto-save-interval
     The value of this variable specifies how often to do auto-saving,
     in terms of number of input events.  Each time this many
     additional input events are read, Emacs does auto-saving for all
     buffers in which that is enabled.

 - User Option: auto-save-timeout
     The value of this variable is the number of seconds of idle time
     that should cause auto-saving.  Each time the user pauses for this
     long, Emacs does auto-saving for all buffers in which that is
     enabled.  (If the current buffer is large, the specified timeout
     is multiplied by a factor that increases as the size increases;
     for a million-byte buffer, the factor is almost 4.)

     If the value is zero or nil, then auto-saving is not done as a
     result of idleness, only after a certain number of input events as
     specified by `auto-save-interval'.

 - Variable: auto-save-hook
     This normal hook is run whenever an auto-save is about to happen.

 - User Option: auto-save-default
     If this variable is non-`nil', buffers that are visiting files
     have auto-saving enabled by default.  Otherwise, they do not.

 - Command: do-auto-save &optional no-message current-only
     This function auto-saves all buffers that need to be auto-saved.
     It saves all buffers for which auto-saving is enabled and that
     have been changed since the previous auto-save.

     Normally, if any buffers are auto-saved, a message that says
     `Auto-saving...' is displayed in the echo area while auto-saving is
     going on.  However, if NO-MESSAGE is non-`nil', the message is
     inhibited.

     If CURRENT-ONLY is non-`nil', only the current buffer is
     auto-saved.

 - Function: delete-auto-save-file-if-necessary
     This function deletes the current buffer's auto-save file if
     `delete-auto-save-files' is non-`nil'.  It is called every time a
     buffer is saved.

 - Variable: delete-auto-save-files
     This variable is used by the function
     `delete-auto-save-file-if-necessary'.  If it is non-`nil', Emacs
     deletes auto-save files when a true save is done (in the visited
     file).  This saves disk space and unclutters your directory.

 - Function: rename-auto-save-file
     This function adjusts the current buffer's auto-save file name if
     the visited file name has changed.  It also renames an existing
     auto-save file.  If the visited file name has not changed, this
     function does nothing.

 - Variable: buffer-saved-size
     The value of this buffer-local variable is the length of the
     current buffer, when it was last read in, saved, or auto-saved.
     This is used to detect a substantial decrease in size, and turn
     off auto-saving in response.

     If it is -1, that means auto-saving is temporarily shut off in
     this buffer due to a substantial decrease in size.  Explicitly
     saving the buffer stores a positive value in this variable, thus
     reenabling auto-saving.  Turning auto-save mode off or on also
     updates this variable, so that the substantial decrease in size is
     forgotten.

 - Variable: auto-save-list-file-name
     This variable (if non-`nil') specifies a file for recording the
     names of all the auto-save files.  Each time Emacs does
     auto-saving, it writes two lines into this file for each buffer
     that has auto-saving enabled.  The first line gives the name of
     the visited file (it's empty if the buffer has none), and the
     second gives the name of the auto-save file.

     When Emacs exits normally, it deletes this file; if Emacs crashes,
     you can look in the file to find all the auto-save files that
     might contain work that was otherwise lost.  The `recover-session'
     command uses this file to find them.

     The default name for this file specifies your home directory and
     starts with `.saves-'.  It also contains the Emacs process ID and
     the host name.

 - Variable: auto-save-list-file-prefix
     After Emacs reads your init file, it initializes
     `auto-save-list-file-name' (if you have not already set it
     non-`nil') based on this prefix, adding the host name and process
     ID.  If you set this to `nil' in your init file, then Emacs does
     not initialize `auto-save-list-file-name'.