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+@c -*-texinfo-*-
+@c This is part of the GNU Emacs Lisp Reference Manual.
+@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. 
+@c See the file elisp.texi for copying conditions.
+@setfilename ../info/backups
+@node Backups and Auto-Saving, Buffers, Files, Top
+@chapter Backups and Auto-Saving
+
+  Backup files and auto-save files are two methods by which Emacs tries
+to protect the user from the consequences of crashes or of the user's
+own errors.  Auto-saving preserves the text from earlier in the current
+editing session; backup files preserve file contents prior to the
+current session.
+
+@menu
+* Backup Files::   How backup files are made; how their names are chosen.
+* Auto-Saving::    How auto-save files are made; how their names are chosen.
+* Reverting::      @code{revert-buffer}, and how to customize what it does.
+@end menu
+
+@node Backup Files, Auto-Saving, Backups and Auto-Saving, Backups and Auto-Saving
+@section Backup Files
+@cindex backup file
+
+  A @dfn{backup file} is a copy of the old contents of a file you are
+editing.  Emacs makes a backup file the first time you save a buffer
+into its visited file.  Normally, this means that the backup file
+contains the contents of the file as it was before the current editing
+session.  The contents of the backup file normally remain unchanged once
+it exists.
+
+  Backups are usually made by renaming the visited file to a new name.
+Optionally, you can specify that backup files should be made by copying
+the visited file.  This choice makes a difference for files with
+multiple names; it also can affect whether the edited file remains owned
+by the original owner or becomes owned by the user editing it.
+
+  By default, Emacs makes a single backup file for each file edited.
+You can alternatively request numbered backups; then each new backup
+file gets a new name.  You can delete old numbered backups when you
+don't want them any more, or Emacs can delete them automatically.
+
+@menu
+* Making Backups::     How Emacs makes backup files, and when.
+* Rename or Copy::     Two alternatives: renaming the old file or copying it.
+* Numbered Backups::   Keeping multiple backups for each source file.
+* Backup Names::       How backup file names are computed; customization.
+@end menu
+
+@node Making Backups, Rename or Copy, Backup Files, Backup Files
+@subsection Making Backup Files
+
+@defun backup-buffer
+  This function makes a backup of the file visited by the current
+buffer, if appropriate.  It is called by @code{save-buffer} before
+saving the buffer the first time.
+@end defun
+
+@defvar buffer-backed-up
+  This buffer-local variable indicates whether this buffer's file has
+been backed up on account of this buffer.  If it is non-@code{nil}, then
+the backup file has been written.  Otherwise, the file should be backed
+up when it is next saved (if backup files are enabled).  This is a
+permanent local; @code{kill-local-variables} does not alter it.
+@end defvar
+
+@defopt make-backup-files
+  This variable determines whether or not to make backup files.  If it
+is non-@code{nil}, then Emacs creates a backup of each file when it is
+saved for the first time.
+
+  The following example shows how to change the @code{make-backup-files}
+variable only in the @file{RMAIL} buffer and not elsewhere.  Setting it
+@code{nil} stops Emacs from making backups of the @file{RMAIL} file,
+which may save disk space.  (You would put this code in your
+@file{.emacs} file.)
+
+@smallexample
+@group
+(add-hook 'rmail-mode-hook 
+          (function (lambda ()
+                      (make-local-variable 
+                       'make-backup-files)
+                      (setq make-backup-files nil))))
+@end group
+@end smallexample
+@end defopt
+
+@defvar backup-enable-predicate filename
+This variable's value is a function to be called on certain occasions to
+decide whether a there should be backup files for file name
+@var{filename}.  If it returns @code{nil}, backups are disabled.
+Otherwise, the other variables in this section say whether and how to
+make backups.
+
+The default value is this:
+
+@example
+(lambda (name)
+  (or (< (length name) 5)
+      (not (string-equal "/tmp/"
+                         (substring name 0 5)))))
+@end example
+@end defvar
+
+@defvar backup-inhibited
+If this variable is non-@code{nil}, backups are inhibited.  It records
+the result of testing @code{backup-enable-predicate} on the visited file
+name.  It can also coherently be used by other mechanisms that inhibit
+backups based on which file is visited.  Major modes should not set this
+variable.
+@end defvar
+
+@node Rename or Copy, Numbered Backups, Making Backups, Backup Files
+@subsection Backup by Renaming or by Copying?
+@cindex backup files, how to make them
+
+  There are two ways that Emacs can make a backup file: 
+
+@itemize @bullet
+@item
+Emacs can rename the original file so that it becomes a backup file, and
+then write the buffer being saved into a new file.  After this
+procedure, any other names (i.e., hard links) of the original file now
+refer to the backup file.  The new file is owned by the user doing the
+editing, and its group is the default for new files written by the user
+in that directory.
+
+@item
+Emacs can copy the original file into a backup file, and then overwrite
+the original file with new contents.  After this procedure, any other
+names (i.e., hard links) of the original file still refer to the current
+version of the file.  The file's owner and group will be unchanged.
+@end itemize
+
+  The first method, renaming, is the default.
+
+  The variable @code{backup-by-copying}, if non-@code{nil}, says to use
+the second method, which is to copy the original file and overwrite it
+with the new buffer contents.  The variable @code{file-precious-flag},
+if non-@code{nil}, also has this effect (as a sideline of its main
+significance).  @xref{Saving Buffers}.
+
+@defvar backup-by-copying
+If this variable is non-@code{nil}, Emacs always makes backup files by
+copying.
+@end defvar
+
+  The following two variables, when non-@code{nil}, cause the second
+method to be used in certain special cases.  They have no effect on the
+treatment of files that don't fall into the special cases.
+
+@defvar backup-by-copying-when-linked
+If this variable is non-@code{nil}, Emacs makes backups by copying for
+files with multiple names (hard links).
+
+This variable is significant only if @code{backup-by-copying} is
+@code{nil}, since copying is always used when that variable is
+non-@code{nil}.
+@end defvar
+
+@defvar backup-by-copying-when-mismatch
+If this variable is non-@code{nil}, Emacs makes backups by copying in cases
+where renaming would change either the owner or the group of the file.
+
+The value has no effect when renaming would not alter the owner or
+group of the file; that is, for files which are owned by the user and
+whose group matches the default for a new file created there by the
+user.
+
+This variable is significant only if @code{backup-by-copying} is
+@code{nil}, since copying is always used when that variable is
+non-@code{nil}.
+@end defvar
+
+@node Numbered Backups, Backup Names, Rename or Copy, Backup Files
+@subsection Making and Deleting Numbered Backup Files
+
+  If a file's name is @file{foo}, the names of its numbered backup
+versions are @file{foo.~@var{v}~}, for various integers @var{v}, like
+this: @file{foo.~1~}, @file{foo.~2~}, @file{foo.~3~}, @dots{},
+@file{foo.~259~}, and so on.
+
+@defopt version-control
+This variable controls whether to make a single non-numbered backup
+file or multiple numbered backups.
+
+@table @asis
+@item @code{nil}
+Make numbered backups if the visited file already has numbered backups;
+otherwise, do not.
+
+@item @code{never}
+Do not make numbered backups.
+
+@item @var{anything else}
+Do make numbered backups.
+@end table
+@end defopt
+
+  The use of numbered backups ultimately leads to a large number of
+backup versions, which must then be deleted.  Emacs can do this
+automatically.
+
+@defopt kept-new-versions
+The value of this variable is the number of oldest versions to keep
+when a new numbered backup is made.  The newly made backup is included
+in the count.  The default value is 2.
+@end defopt
+
+@defopt kept-old-versions
+The value of this variable is the number of oldest versions to keep
+when a new numbered backup is made.  The default value is 2.
+@end defopt
+
+  If there are backups numbered 1, 2, 3, 5, and 7, and both of these
+variables have the value 2, then the backups numbered 1 and 2 are kept
+as old versions and those numbered 5 and 7 are kept as new versions;
+backup version 3 is deleted.  The function @code{find-backup-file-name}
+(@pxref{Backup Names}) is responsible for determining which backup
+versions to delete, but does not delete them itself.
+
+@defopt trim-versions-without-asking
+If this variable is non-@code{nil}, then saving a file deletes excess
+backup versions silently.  Otherwise, it asks the user whether to delete
+them.
+@end defopt
+
+@defopt dired-kept-versions
+This variable specifies how many of the newest backup versions to keep
+in the Dired command @kbd{.} (@code{dired-clean-directory}).  That's the
+same thing @code{kept-new-versions} does when you make a new backup
+file.  The default value is 2.
+@end defopt
+
+@node Backup Names, , Numbered Backups, Backup Files
+@subsection Naming Backup Files
+
+  The functions in this section are documented mainly because you can
+customize the naming conventions for backup files by redefining them.
+If you change one, you probably need to change the rest.
+
+@defun backup-file-name-p filename
+This function returns a non-@code{nil} value if @var{filename} is a
+possible name for a backup file.  A file with the name @var{filename}
+need not exist; the function just checks the name.
+
+@smallexample
+@group
+(backup-file-name-p "foo")
+     @result{} nil
+@end group
+@group
+(backup-file-name-p "foo~")
+     @result{} 3
+@end group
+@end smallexample
+
+The standard definition of this function is as follows:
+
+@smallexample
+@group
+(defun backup-file-name-p (file)
+  "Return non-nil if FILE is a backup file \
+name (numeric or not)..."
+  (string-match "~$" file))
+@end group
+@end smallexample
+
+@noindent
+Thus, the function returns a non-@code{nil} value if the file name ends
+with a @samp{~}.  (We use a backslash to split the documentation
+string's first line into two lines in the text, but produce just one
+line in the string itself.)
+
+This simple expression is placed in a separate function to make it easy
+to redefine for customization.
+@end defun
+
+@defun make-backup-file-name filename
+This function returns a string which is the name to use for a
+non-numbered backup file for file @var{filename}.  On Unix, this is just
+@var{filename} with a tilde appended.
+
+The standard definition of this function is as follows:
+
+@smallexample
+@group
+(defun make-backup-file-name (file)
+  "Create the non-numeric backup file name for FILE.
+@dots{}"
+  (concat file "~"))
+@end group
+@end smallexample
+
+You can change the backup file naming convention by redefining this
+function.  The following example redefines @code{make-backup-file-name}
+to prepend a @samp{.} as well as appending a tilde:
+
+@smallexample
+@group
+(defun make-backup-file-name (filename)
+  (concat "." filename "~"))
+@end group
+
+@group
+(make-backup-file-name "backups.texi")
+     @result{} ".backups.texi~"
+@end group
+@end smallexample
+@end defun
+
+@defun find-backup-file-name filename
+This function computes the file name for a new backup file for
+@var{filename}.  It may also propose certain existing backup files for
+deletion.  @code{find-backup-file-name} returns a list whose @sc{car} is
+the name for the new backup file and whose @sc{cdr} is a list of backup
+files whose deletion is proposed.
+
+Two variables, @code{kept-old-versions} and @code{kept-new-versions},
+determine which backup versions should be kept.  This function keeps
+those versions by excluding them from the @sc{cdr} of the value.
+@xref{Numbered Backups}.
+
+In this example, the value says that @file{~rms/foo.~5~} is the name
+to use for the new backup file, and @file{~rms/foo.~3~} is an ``excess''
+version that the caller should consider deleting now.
+
+@smallexample
+@group
+(find-backup-file-name "~rms/foo")
+     @result{} ("~rms/foo.~5~" "~rms/foo.~3~")
+@end group
+@end smallexample
+@end defun
+
+@c Emacs 19 feature
+@defun file-newest-backup filename
+This function returns the name of the most recent backup file for
+@var{filename}, or @code{nil} that file has no backup files.
+
+Some file comparison commands use this function in order to compare
+a file by default with its most recent backup.
+@end defun 
+
+@node Auto-Saving, Reverting, Backup Files, Backups and Auto-Saving
+@section Auto-Saving
+@cindex auto-saving
+
+  Emacs periodically saves all files that you are visiting; this is
+called @dfn{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.  @xref{Auto-Save, Auto-Save, Auto-Saving: Protection Against
+Disasters, emacs, The GNU Emacs Manual}, for information on auto-save
+for users.  Here we describe the functions used to implement auto-saving
+and the variables that control them.
+
+@defvar buffer-auto-save-file-name
+This buffer-local variable is the name of the file used for
+auto-saving the current buffer.  It is @code{nil} if the buffer
+should not be auto-saved.
+
+@example
+@group
+buffer-auto-save-file-name
+=> "/xcssun/users/rms/lewis/#files.texi#"
+@end group
+@end example
+@end defvar
+
+@deffn 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 @var{arg}, the command turns auto-saving
+on if the value of @var{arg} is @code{t}, a nonempty list, or a positive
+integer.  Otherwise, it turns auto-saving off.
+@end deffn
+
+@defun auto-save-file-name-p filename
+This function returns a non-@code{nil} value if @var{filename} is a
+string that could be the name of an auto-save file.  It works based on
+knowledge of the naming convention for auto-save files: a name that
+begins and ends with hash marks (@samp{#}) is a possible auto-save file
+name.  The argument @var{filename} should not contain a directory part.
+
+@example
+@group
+(make-auto-save-file-name)
+     @result{} "/xcssun/users/rms/lewis/#files.texi#"
+@end group
+@group
+(auto-save-file-name-p "#files.texi#")
+     @result{} 0
+@end group
+@group
+(auto-save-file-name-p "files.texi")
+     @result{} nil
+@end group
+@end example
+
+The standard definition of this function is as follows:
+
+@example
+@group
+(defun auto-save-file-name-p (filename)
+  "Return non-nil if FILENAME can be yielded by..."
+  (string-match "^#.*#$" filename))
+@end group
+@end example
+
+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 @code{make-auto-save-file-name}
+correspondingly.
+@end defun
+
+@defun 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 (@samp{#}) appended
+and prepended to it.  This function does not look at the variable
+@code{auto-save-visited-file-name}; you should check that before calling
+this function.
+
+@example
+@group
+(make-auto-save-file-name)
+     @result{} "/xcssun/users/rms/lewis/#backup.texi#"
+@end group
+@end example
+
+The standard definition of this function is as follows:
+
+@example
+@group
+(defun make-auto-save-file-name ()
+  "Return file name to use for auto-saves \
+of current buffer.
+@dots{}"
+  (if buffer-file-name
+@end group
+@group
+      (concat
+       (file-name-directory buffer-file-name)
+       "#"
+       (file-name-nondirectory buffer-file-name)
+       "#")
+    (expand-file-name
+     (concat "#%" (buffer-name) "#"))))
+@end group
+@end example
+
+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 @code{auto-save-file-name-p} in a corresponding way.
+@end defun
+
+@defvar auto-save-visited-file-name
+If this variable is non-@code{nil}, Emacs auto-saves buffers in
+the files they are visiting.  That is, the auto-save is done in the same
+file which you are editing.  Normally, this variable is @code{nil}, so
+auto-save files have distinct names that are created by
+@code{make-auto-save-file-name}.
+
+When you change the value of this variable, the value does not take
+effect until the next time auto-save mode is reenabled in any given
+buffer.  If auto-save mode is already enabled, auto-saves continue to go
+in the same file name until @code{auto-save-mode} is called again.
+@end defvar
+
+@defun recent-auto-save-p
+This function returns @code{t} if the current buffer has been
+auto-saved since the last time it was read in or saved.
+@end defun
+
+@defun 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 @code{nil}.
+@end defun
+
+@defopt auto-save-interval
+The value of this variable is the number of characters that Emacs
+reads from the keyboard between auto-saves.  Each time this many more
+characters are read, auto-saving is done for all buffers in which it is
+enabled.
+@end defopt
+
+@defopt 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 auto-saves any buffers that need it.  (Actually, the specified
+timeout is multiplied by a factor depending on the size of the current
+buffer.)
+@end defopt
+
+@defvar auto-save-hook
+This normal hook is run whenever an auto-save is about to happen.
+@end defvar
+
+@defopt auto-save-default
+If this variable is non-@code{nil}, buffers that are visiting files
+have auto-saving enabled by default.  Otherwise, they do not.
+@end defopt
+
+@deffn Command do-auto-save &optional no-message
+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
+@samp{Auto-saving...} is displayed in the echo area while auto-saving is
+going on.  However, if @var{no-message} is non-@code{nil}, the message
+is inhibited.
+@end deffn
+
+@defun delete-auto-save-file-if-necessary
+This function deletes the current buffer's auto-save file if
+@code{delete-auto-save-files} is non-@code{nil}.  It is called every
+time a buffer is saved.
+@end defun
+
+@defvar delete-auto-save-files
+This variable is used by the function
+@code{delete-auto-save-file-if-necessary}.  If it is non-@code{nil},
+Emacs deletes auto-save files when a true save is done (in the visited
+file).  This saves disk space and unclutters your directory.
+@end defvar
+
+@defun 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.
+@end defun
+
+@node Reverting,  , Auto-Saving, Backups and Auto-Saving
+@section Reverting
+
+  If you have made extensive changes to a file and then change your mind
+about them, you can get rid of them by reading in the previous version
+of the file with the @code{revert-buffer} command.  @xref{Reverting, ,
+Reverting a Buffer, emacs, The GNU Emacs Manual}.
+
+@deffn Command revert-buffer &optional check-auto-save noconfirm
+This command replaces the buffer text with the text of the visited
+file on disk.  This action undoes all changes since the file was visited
+or saved.
+
+If the argument @var{check-auto-save} is non-@code{nil}, and the
+latest auto-save file is more recent than the visited file,
+@code{revert-buffer} asks the user whether to use that instead.
+Otherwise, it always uses the text of the visited file itself.
+Interactively, @var{check-auto-save} is set if there is a numeric prefix
+argument.
+
+Normally, @code{revert-buffer} asks for confirmation before it changes
+the buffer; but if the argument @var{noconfirm} is non-@code{nil},
+@code{revert-buffer} does not ask for confirmation.
+
+Reverting tries to preserve marker positions in the buffer by using the
+replacement feature of @code{insert-file-contents}.  If there is no
+actual difference between the buffer and the file, before reversion,
+this preserves all the markers.  If reversion does change the buffer,
+this preserves the markers in the unchanged text (if any) at the
+beginning and end of the buffer.  Preserving any additional markers
+would be problematical.
+
+If the value of the @code{revert-buffer-function} variable is
+non-@code{nil}, it is called as a function with no arguments to do the
+work.
+@end deffn
+
+@defvar revert-buffer-function
+The value of this variable is the function to use to revert this
+buffer; but if the value of this variable is @code{nil}, then the
+@code{revert-buffer} function carries out its default action.  Modes
+such as Dired mode, in which the text being edited does not consist of a
+file's contents but can be regenerated in some other fashion, give this
+variable a buffer-local value that is a function to regenerate the
+contents.
+@end defvar
+
+@defvar revert-buffer-insert-file-contents-function
+The value of this variable, if non-@code{nil}, is the function to use
+to insert contents when reverting this buffer.  The function receives
+two arguments, first the file name to use, and second, @code{t} if the
+user has asked to read the auto-save file.
+@end defvar
+
+@defvar before-revert-hook
+This normal hook is run by @code{revert-buffer} before actually
+inserting the modified contents---but only if
+@code{revert-buffer-function} is @code{nil}.
+
+Font Lock mode uses this hook to record that the buffer contents are no
+longer fontified.
+@end defvar
+
+@defvar after-revert-hook
+This normal hook is run by @code{revert-buffer} after actually inserting
+the modified contents---but only if @code{revert-buffer-function} is
+@code{nil}.
+
+Font Lock mode uses this hook to recompute the fonts for the updated
+buffer contents.
+@end defvar
+
+@deffn Command recover-file filename
+This function visits @var{filename}, but gets the contents from its
+last auto-save file.  This is useful after the system has crashed, to
+resume editing the same file without losing all the work done in the
+previous session.
+
+An error is signaled if there is no auto-save file for @var{filename},
+or if @var{filename} is newer than its auto-save file.  If
+@var{filename} does not exist, but its auto-save file does, then the
+auto-save file is read as usual.  This last situation may occur if you
+visited a nonexistent file and never actually saved it.
+@end deffn
+