@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990-1995, 1999, 2001-2019 Free Software Foundation,
@c Inc.
@c See the file elisp.texi for copying conditions.
@node Backups and Auto-Saving
@chapter Backups and Auto-Saving
@cindex 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
@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.  Thus, normally, 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.

  For performance, the operating system may not write the backup
file's contents to secondary storage immediately, or may alias the
backup data with the original until one or the other is later
modified.  @xref{Files and Storage}.

@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
@subsection Making Backup Files
@cindex 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.

If a backup was made by renaming, the return value is a cons cell of
the form (@var{modes} @var{extra-alist} @var{backupname}), where
@var{modes} are the mode bits of the original file, as returned by
@code{file-modes} (@pxref{Testing Accessibility}), @var{extra-alist}
is an alist describing the original file's extended attributes, as
returned by @code{file-extended-attributes} (@pxref{Extended
Attributes}), and @var{backupname} is the name of the backup.

In all other cases (i.e., if a backup was made by copying or if no
backup was made), this function returns @code{nil}.
@end defun

@defvar buffer-backed-up
  This buffer-local variable says whether this buffer's file has
been backed up on account of this buffer.  If it is non-@code{nil},
the backup file has been written.  Otherwise, the file should be backed
up when it is next saved (if backups are enabled).  This is a
permanent local; @code{kill-all-local-variables} does not alter@tie{}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---provided that @code{backup-inhibited}
is @code{nil} (see below).

The following example shows how to change the @code{make-backup-files}
variable only in the Rmail buffers and not elsewhere.  Setting it
@code{nil} stops Emacs from making backups of these files, which may
save disk space.  (You would put this code in your init file.)

@smallexample
@group
(add-hook 'rmail-mode-hook
          (lambda () (setq-local make-backup-files nil)))
@end group
@end smallexample
@end defopt

@defvar backup-enable-predicate
This variable's value is a function to be called on certain occasions to
decide whether a file should have backup files.  The function receives
one argument, an absolute file name to consider.  If the function returns
@code{nil}, backups are disabled for that file.  Otherwise, the other
variables in this section say whether and how to make backups.

@findex normal-backup-enable-predicate
The default value is @code{normal-backup-enable-predicate}, which checks
for files in @code{temporary-file-directory} and
@code{small-temporary-file-directory}.
@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.  For example, VC sets this
variable non-@code{nil} to prevent making backups for files managed
with a version control system.

This is a permanent local, so that changing the major mode does not lose
its value.  Major modes should not set this variable---they should set
@code{make-backup-files} instead.
@end defvar

@defopt backup-directory-alist
This variable's value is an alist of filename patterns and backup
directories.  Each element looks like
@smallexample
(@var{regexp} . @var{directory})
@end smallexample

@noindent
Backups of files with names matching @var{regexp} will be made in
@var{directory}.  @var{directory} may be relative or absolute.  If it is
absolute, so that all matching files are backed up into the same
directory, the file names in this directory will be the full name of the
file backed up with all directory separators changed to @samp{!} to
prevent clashes.  This will not work correctly if your filesystem
truncates the resulting name.

For the common case of all backups going into one directory, the alist
should contain a single element pairing @samp{"."} with the appropriate
directory.

If this variable is @code{nil} (the default), or it fails to match a
filename, the backup is made in the original file's directory.

On MS-DOS filesystems without long names this variable is always
ignored.
@end defopt

@defopt make-backup-file-name-function
This variable's value is a function to use for making backup file names.
The function @code{make-backup-file-name} calls it.
@xref{Backup Names,, Naming Backup Files}.

This could be buffer-local to do something special for specific
files.  If you change it, you may need to change
@code{backup-file-name-p} and @code{file-name-sans-versions} too.
@end defopt


@node Rename or Copy
@subsection Backup by Renaming or by Copying?
@cindex backup files, rename or copy

  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 continue to refer to the
current (updated) 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}.

@defopt backup-by-copying
If this variable is non-@code{nil}, Emacs always makes backup files by
copying.  The default is @code{nil}.
@end defopt

  The following three 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.

@defopt backup-by-copying-when-linked
If this variable is non-@code{nil}, Emacs makes backups by copying for
files with multiple names (hard links).  The default is @code{nil}.

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 defopt

@defopt backup-by-copying-when-mismatch
If this variable is non-@code{nil} (the default), 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 defopt

@defopt backup-by-copying-when-privileged-mismatch
This variable, if non-@code{nil}, specifies the same behavior as
@code{backup-by-copying-when-mismatch}, but only for certain user-id
and group-id values: namely, those less than or equal to a certain number.
You set this variable to that number.

Thus, if you set @code{backup-by-copying-when-privileged-mismatch}
to 0, backup by copying is done for the superuser and group 0 only,
when necessary to prevent a change in the owner of the file.

The default is 200.
@end defopt

@node Numbered Backups
@subsection Making and Deleting Numbered Backup Files
@cindex numbered backups

  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.  This is the default.

@item @code{never}
Do not make numbered backups.

@item @var{anything else}
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 or it can ask the user whether to delete them.

@defopt kept-new-versions
The value of this variable is the number of newest versions to keep
when a new numbered backup is made.  The newly made backup is included
in the count.  The default value is@tie{}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@tie{}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 excess.  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 delete-old-versions
If this variable is @code{t}, then saving a file deletes excess
backup versions silently.  If it is @code{nil}, that means
to ask for confirmation before deleting excess backups.
Otherwise, they are not deleted at all.
@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} specifies when you make a new backup
file.  The default is@tie{}2.
@end defopt

@node Backup Names
@subsection Naming Backup Files
@cindex 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.  It just checks the name, not whether
a file with the name @var{filename} exists.

@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 that 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, on most operating systems, is
as follows:

@smallexample
@group
(defun make-backup-file-name (file)
  "Create the non-numeric backup file name for FILE..."
  (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{.} in addition to appending a tilde:

@smallexample
@group
(defun make-backup-file-name (filename)
  (expand-file-name
    (concat "." (file-name-nondirectory filename) "~")
    (file-name-directory filename)))
@end group

@group
(make-backup-file-name "backups.texi")
     @result{} ".backups.texi~"
@end group
@end smallexample

Some parts of Emacs, including some Dired commands, assume that backup
file names end with @samp{~}.  If you do not follow that convention, it
will not cause serious problems, but these commands may give
less-than-desirable results.
@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.  The value can also be @code{nil},
which means not to make a backup.

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} if that file has no backup files.

Some file comparison commands use this function so that they can
automatically compare a file with its most recent backup.
@end defun

@node Auto-Saving
@section Auto-Saving
@c @cindex auto-saving   Lots of symbols starting with auto-save here.

  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
     @result{} "/xcssun/users/rms/lewis/#backups.texi#"
@end group
@end example
@end defvar

@deffn Command auto-save-mode arg
This is the mode command for Auto Save mode, a buffer-local minor
mode.  When Auto Save mode is enabled, auto-saving is enabled in the
buffer.  The calling convention is the same as for other minor mode
commands (@pxref{Minor Mode Conventions}).

Unlike most minor modes, there is no @code{auto-save-mode} variable.
Auto Save mode is enabled if @code{buffer-auto-save-file-name} is
non-@code{nil} and @code{buffer-saved-size} (see below) is non-zero.
@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 assumes
the usual 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/#backups.texi#"
@end group
@group
(auto-save-file-name-p "#backups.texi#")
     @result{} 0
@end group
@group
(auto-save-file-name-p "backups.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{#}) prepended
and appended to it.  This function does not look at the variable
@code{auto-save-visited-file-name} (described below); callers of this
function should check that variable first.

@example
@group
(make-auto-save-file-name)
     @result{} "/xcssun/users/rms/lewis/#backups.texi#"
@end group
@end example

Here is a simplified version of the standard definition of this
function:

@example
@group
(defun make-auto-save-file-name ()
  "Return file name to use for auto-saves \
of current buffer.."
  (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

@defopt 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 that 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 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 @code{auto-save-mode} is
called again.

Note that setting this variable to a non-@code{nil} value does not
change the fact that auto-saving is different from saving the buffer;
e.g., the hooks described in @ref{Saving Buffers} are @emph{not} run
when a buffer is auto-saved.
@end defopt

@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 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.  Setting this to zero disables autosaving based on the
number of characters typed.
@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 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 @code{nil}, then auto-saving is not done as a
result of idleness, only after a certain number of input events as
specified by @code{auto-save-interval}.
@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 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.

If any buffers are auto-saved, @code{do-auto-save} normally displays a
message saying @samp{Auto-saving...} in the echo area while
auto-saving is going on.  However, if @var{no-message} is
non-@code{nil}, the message is inhibited.

If @var{current-only} is non-@code{nil}, only the current buffer
is auto-saved.
@end deffn

@defun delete-auto-save-file-if-necessary &optional force
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.

Unless @var{force} is non-@code{nil}, this function only deletes the
file if it was written by the current Emacs session since the last
true save.
@end defun

@defopt 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 defopt

@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 it was made in the current Emacs session.  If the visited
file name has not changed, this function does nothing.
@end defun

@defvar 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 @minus{}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.

If it is @minus{}2, that means this buffer should disregard changes in
buffer size; in particular, it should not shut off auto-saving
temporarily due to changes in buffer size.
@end defvar

@defvar auto-save-list-file-name
This variable (if non-@code{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 @code{recover-session} command uses
this file to find them.

The default name for this file specifies your home directory and starts
with @samp{.saves-}.  It also contains the Emacs process @acronym{ID} and the
host name.
@end defvar

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

@node Reverting
@section Reverting
@cindex reverting buffers

  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 ignore-auto noconfirm preserve-modes
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.

By default, if the latest auto-save file is more recent than the visited
file, and the argument @var{ignore-auto} is @code{nil},
@code{revert-buffer} asks the user whether to use that auto-save
instead.  When you invoke this command interactively, @var{ignore-auto}
is @code{t} if there is no numeric prefix argument; thus, the
interactive default is not to check the auto-save file.

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.

Normally, this command reinitializes the buffer's major and minor modes
using @code{normal-mode}.  But if @var{preserve-modes} is
non-@code{nil}, the modes remain unchanged.

Reverting tries to preserve marker positions in the buffer by using the
replacement feature of @code{insert-file-contents}.  If the buffer
contents and the file contents are identical before the revert
operation, reverting preserves all the markers.  If they are not
identical, reverting does change the buffer; in that case, it preserves
the markers in the unchanged text (if any) at the beginning and end of
the buffer.  Preserving any additional markers would be problematical.
@end deffn

@defvar revert-buffer-in-progress-p
@code{revert-buffer} binds this variable to a non-@code{nil} value
while it is working.
@end defvar

You can customize how @code{revert-buffer} does its work by setting
the variables described in the rest of this section.

@defopt revert-without-query
This variable holds a list of files that should be reverted without
query.  The value is a list of regular expressions.  If the visited file
name matches one of these regular expressions, and the file has changed
on disk but the buffer is not modified, then @code{revert-buffer}
reverts the file without asking the user for confirmation.
@end defopt

  Some major modes customize @code{revert-buffer} by making
buffer-local bindings for these variables:

@defvar revert-buffer-function
@anchor{Definition of revert-buffer-function}
The value of this variable is the function to use to revert this
buffer.  It should be a function with two optional
arguments to do the work of reverting.  The two optional arguments,
@var{ignore-auto} and @var{noconfirm}, are the arguments that
@code{revert-buffer} received.

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, can give this variable a buffer-local value that is a special
function to regenerate the contents.
@end defvar

@defvar revert-buffer-insert-file-contents-function
The value of this variable specifies the function to use to
insert the updated contents when reverting this buffer.  The function
receives two arguments: first the file name to use; second, @code{t} if
the user has asked to read the auto-save file.

The reason for a mode to change this variable instead of
@code{revert-buffer-function} is to avoid duplicating or replacing the
rest of what @code{revert-buffer} does: asking for confirmation,
clearing the undo list, deciding the proper major mode, and running the
hooks listed below.
@end defvar

@defvar before-revert-hook
This normal hook is run by the default @code{revert-buffer-function}
before inserting the modified contents.  A custom @code{revert-buffer-function}
may or may not run this hook.
@end defvar

@defvar after-revert-hook
This normal hook is run by the default @code{revert-buffer-function}
after inserting the modified contents.  A custom @code{revert-buffer-function}
may or may not run this hook.
@end defvar

Emacs can revert buffers automatically.  It does that by default for
buffers visiting files.  The following describes how to add support
for auto-reverting new types of buffers.

First, such buffers must have a suitable @code{revert-buffer-function}
and @code{buffer-stale-function} defined.

@defvar buffer-stale-function
The value of this variable specifies a function to call to check
whether a buffer needs reverting.  The default value only handles
buffers that are visiting files, by checking their modification time.
Buffers that are not visiting files require a custom function of one
optional argument @var{noconfirm}.  The function should return
non-@code{nil} if the buffer should be reverted.  The buffer is
current when this function is called.

While this function is mainly intended for use in auto-reverting, it
could be used for other purposes as well.  For instance, if
auto-reverting is not enabled, it could be used to warn the user that
the buffer needs reverting.  The idea behind the @var{noconfirm}
argument is that it should be @code{t} if the buffer is going to be
reverted without asking the user and @code{nil} if the function is
just going to be used to warn the user that the buffer is out of date.
In particular, for use in auto-reverting, @var{noconfirm} is @code{t}.
If the function is only going to be used for auto-reverting, you can
ignore the @var{noconfirm} argument.

If you just want to automatically auto-revert every
@code{auto-revert-interval} seconds (like the Buffer Menu), use:

@example
(setq-local buffer-stale-function
     #'(lambda (&optional noconfirm) 'fast))
@end example

@noindent
in the buffer's mode function.

The special return value @samp{fast} tells the caller that the need
for reverting was not checked, but that reverting the buffer is fast.
It also tells Auto Revert not to print any revert messages, even if
@code{auto-revert-verbose} is non-@code{nil}.  This is important, as
getting revert messages every @code{auto-revert-interval} seconds can
be very annoying.  The information provided by this return value could
also be useful if the function is consulted for purposes other than
auto-reverting.
@end defvar

Once the buffer has a suitable @code{revert-buffer-function} and
@code{buffer-stale-function}, several problems usually remain.

The buffer will only auto-revert if it is marked unmodified.  Hence,
you will have to make sure that various functions mark the buffer
modified if and only if either the buffer contains information that
might be lost by reverting, or there is reason to believe that the user
might be inconvenienced by auto-reverting, because he is actively
working on the buffer.  The user can always override this by manually
adjusting the modified status of the buffer.  To support this, calling
the @code{revert-buffer-function} on a buffer that is marked
unmodified should always keep the buffer marked unmodified.

It is important to assure that point does not continuously jump around
as a consequence of auto-reverting.  Of course, moving point might be
inevitable if the buffer radically changes.

You should make sure that the @code{revert-buffer-function} does not
print messages that unnecessarily duplicate Auto Revert's own messages,
displayed if @code{auto-revert-verbose} is @code{t}, and effectively
override a @code{nil} value for @code{auto-revert-verbose}.  Hence,
adapting a mode for auto-reverting often involves getting rid of such
messages.  This is especially important for buffers that automatically
revert every @code{auto-revert-interval} seconds.

If the new auto-reverting is part of Emacs, you should mention it
in the documentation string of @code{global-auto-revert-non-file-buffers}.

Similarly, you should document the additions in the Emacs manual.