update.

[chise/xemacs-chise.git-] / info / lispref.info-24

This is ../info/lispref.info, produced by makeinfo version 4.0 from
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INFO-DIR-SECTION XEmacs Editor
START-INFO-DIR-ENTRY
* Lispref: (lispref).           XEmacs Lisp Reference Manual.
END-INFO-DIR-ENTRY

   Edition History:

   GNU Emacs Lisp Reference Manual Second Edition (v2.01), May 1993 GNU
Emacs Lisp Reference Manual Further Revised (v2.02), August 1993 Lucid
Emacs Lisp Reference Manual (for 19.10) First Edition, March 1994
XEmacs Lisp Programmer's Manual (for 19.12) Second Edition, April 1995
GNU Emacs Lisp Reference Manual v2.4, June 1995 XEmacs Lisp
Programmer's Manual (for 19.13) Third Edition, July 1995 XEmacs Lisp
Reference Manual (for 19.14 and 20.0) v3.1, March 1996 XEmacs Lisp
Reference Manual (for 19.15 and 20.1, 20.2, 20.3) v3.2, April, May,
November 1997 XEmacs Lisp Reference Manual (for 21.0) v3.3, April 1998

   Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995 Free Software
Foundation, Inc.  Copyright (C) 1994, 1995 Sun Microsystems, Inc.
Copyright (C) 1995, 1996 Ben Wing.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the Foundation.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the section entitled "GNU General Public License" is included
exactly as in the original, and provided that the entire resulting
derived work is distributed under the terms of a permission notice
identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the section entitled "GNU General Public License"
may be included in a translation approved by the Free Software
Foundation instead of in the original English.

\1f
File: lispref.info,  Node: User Name Completion,  Prev: File Name Completion,  Up: File Names

User Name Completion
--------------------

   This section describes low-level subroutines for completing a user
name.  For other completion functions, see *Note Completion::.

 - Function: user-name-all-completions partial-username
     This function returns a list of all possible completions for a user
     whose name starts with PARTIAL-USERNAME.  The order of the
     completions is unpredictable and conveys no useful information.

     The argument PARTIAL-USERNAME must be a partial user name
     containing no tilde character and no slash.

 - Function: user-name-completion username
     This function completes the user name USERNAME.  It returns the
     longest prefix common to all user names that start with USERNAME.

     If only one match exists and USERNAME matches it exactly, the
     function returns `t'.  The function returns `nil' if no user name
     starting with USERNAME exists.

 - Function: user-name-completion-1 username
     This function completes the user name USERNAME, like
     `user-name-completion', differing only in the return value.  This
     function returns the cons of the completion returned by
     `user-name-completion', and a boolean indicating whether that
     completion was unique.

\1f
File: lispref.info,  Node: Contents of Directories,  Next: Create/Delete Dirs,  Prev: File Names,  Up: Files

Contents of Directories
=======================

   A directory is a kind of file that contains other files entered under
various names.  Directories are a feature of the file system.

   XEmacs can list the names of the files in a directory as a Lisp list,
or display the names in a buffer using the `ls' shell command.  In the
latter case, it can optionally display information about each file,
depending on the value of switches passed to the `ls' command.

 - Function: directory-files directory &optional full-name match-regexp
          nosort files-only
     This function returns a list of the names of the files in the
     directory DIRECTORY.  By default, the list is in alphabetical
     order.

     If FULL-NAME is non-`nil', the function returns the files'
     absolute file names.  Otherwise, it returns just the names
     relative to the specified directory.

     If MATCH-REGEXP is non-`nil', this function returns only those
     file names that contain that regular expression--the other file
     names are discarded from the list.

     If NOSORT is non-`nil', `directory-files' does not sort the list,
     so you get the file names in no particular order.  Use this if you
     want the utmost possible speed and don't care what order the files
     are processed in.  If the order of processing is visible to the
     user, then the user will probably be happier if you do sort the
     names.

     If FILES-ONLY is the symbol `t', then only the "files" in the
     directory will be returned; subdirectories will be excluded.  If
     FILES-ONLY is not `nil' and not `t', then only the subdirectories
     will be returned.  Otherwise, if FILES-ONLY is `nil' (the default)
     then both files and subdirectories will be returned.

          (directory-files "~lewis")
               => ("#foo#" "#foo.el#" "." ".."
                   "dired-mods.el" "files.texi"
                   "files.texi.~1~")

     An error is signaled if DIRECTORY is not the name of a directory
     that can be read.

 - Function: insert-directory file switches &optional wildcard
          full-directory-p
     This function inserts (in the current buffer) a directory listing
     for directory FILE, formatted with `ls' according to SWITCHES.  It
     leaves point after the inserted text.

     The argument FILE may be either a directory name or a file
     specification including wildcard characters.  If WILDCARD is
     non-`nil', that means treat FILE as a file specification with
     wildcards.

     If FULL-DIRECTORY-P is non-`nil', that means FILE is a directory
     and switches do not contain `-d', so that the listing should show
     the full contents of the directory.  (The `-d' option to `ls' says
     to describe a directory itself rather than its contents.)

     This function works by running a directory listing program whose
     name is in the variable `insert-directory-program'.  If WILDCARD is
     non-`nil', it also runs the shell specified by `shell-file-name',
     to expand the wildcards.

 - Variable: insert-directory-program
     This variable's value is the program to run to generate a
     directory listing for the function `insert-directory'.

\1f
File: lispref.info,  Node: Create/Delete Dirs,  Next: Magic File Names,  Prev: Contents of Directories,  Up: Files

Creating and Deleting Directories
=================================

   Most XEmacs Lisp file-manipulation functions get errors when used on
files that are directories.  For example, you cannot delete a directory
with `delete-file'.  These special functions exist to create and delete
directories.

 - Command: make-directory dirname &optional parents
     This function creates a directory named DIRNAME.  Interactively,
     the default choice of directory to create is the current default
     directory for file names.  That is useful when you have visited a
     file in a nonexistent directory.

     Non-interactively, optional argument PARENTS says whether to
     create parent directories if they don't exist. (Interactively, this
     always happens.)

 - Command: delete-directory dirname
     This function deletes the directory named DIRNAME.  The function
     `delete-file' does not work for files that are directories; you
     must use `delete-directory' in that case.

\1f
File: lispref.info,  Node: Magic File Names,  Next: Partial Files,  Prev: Create/Delete Dirs,  Up: Files

Making Certain File Names "Magic"
=================================

   You can implement special handling for certain file names.  This is
called making those names "magic".  You must supply a regular
expression to define the class of names (all those that match the
regular expression), plus a handler that implements all the primitive
XEmacs file operations for file names that do match.

   The variable `file-name-handler-alist' holds a list of handlers,
together with regular expressions that determine when to apply each
handler.  Each element has this form:

     (REGEXP . HANDLER)

All the XEmacs primitives for file access and file name transformation
check the given file name against `file-name-handler-alist'.  If the
file name matches REGEXP, the primitives handle that file by calling
HANDLER.

   The first argument given to HANDLER is the name of the primitive;
the remaining arguments are the arguments that were passed to that
operation.  (The first of these arguments is typically the file name
itself.)  For example, if you do this:

     (file-exists-p FILENAME)

and FILENAME has handler HANDLER, then HANDLER is called like this:

     (funcall HANDLER 'file-exists-p FILENAME)

   Here are the operations that a magic file name handler gets to
handle:

`add-name-to-file', `copy-file', `delete-directory', `delete-file',
`diff-latest-backup-file', `directory-file-name', `directory-files',
`dired-compress-file', `dired-uncache', `expand-file-name',
`file-accessible-directory-p', `file-attributes', `file-directory-p',
`file-executable-p', `file-exists-p', `file-local-copy', `file-modes',
`file-name-all-completions', `file-name-as-directory',
`file-name-completion', `file-name-directory', `file-name-nondirectory',
`file-name-sans-versions', `file-newer-than-file-p', `file-readable-p',
`file-regular-p', `file-symlink-p', `file-truename', `file-writable-p',
`get-file-buffer', `insert-directory', `insert-file-contents', `load',
`make-directory', `make-symbolic-link', `rename-file', `set-file-modes',
`set-visited-file-modtime', `unhandled-file-name-directory',
`verify-visited-file-modtime', `write-region'.

   Handlers for `insert-file-contents' typically need to clear the
buffer's modified flag, with `(set-buffer-modified-p nil)', if the
VISIT argument is non-`nil'.  This also has the effect of unlocking the
buffer if it is locked.

   The handler function must handle all of the above operations, and
possibly others to be added in the future.  It need not implement all
these operations itself--when it has nothing special to do for a
certain operation, it can reinvoke the primitive, to handle the
operation "in the usual way".  It should always reinvoke the primitive
for an operation it does not recognize.  Here's one way to do this:

     (defun my-file-handler (operation &rest args)
       ;; First check for the specific operations
       ;; that we have special handling for.
       (cond ((eq operation 'insert-file-contents) ...)
             ((eq operation 'write-region) ...)
             ...
             ;; Handle any operation we don't know about.
             (t (let ((inhibit-file-name-handlers
                      (cons 'my-file-handler
                            (and (eq inhibit-file-name-operation operation)
                                 inhibit-file-name-handlers)))
                     (inhibit-file-name-operation operation))
                  (apply operation args)))))

   When a handler function decides to call the ordinary Emacs primitive
for the operation at hand, it needs to prevent the primitive from
calling the same handler once again, thus leading to an infinite
recursion.  The example above shows how to do this, with the variables
`inhibit-file-name-handlers' and `inhibit-file-name-operation'.  Be
careful to use them exactly as shown above; the details are crucial for
proper behavior in the case of multiple handlers, and for operations
that have two file names that may each have handlers.

 - Variable: inhibit-file-name-handlers
     This variable holds a list of handlers whose use is presently
     inhibited for a certain operation.

 - Variable: inhibit-file-name-operation
     The operation for which certain handlers are presently inhibited.

 - Function: find-file-name-handler file operation
     This function returns the handler function for file name FILE, or
     `nil' if there is none.  The argument OPERATION should be the
     operation to be performed on the file--the value you will pass to
     the handler as its first argument when you call it.  The operation
     is needed for comparison with `inhibit-file-name-operation'.

 - Function: file-local-copy filename
     This function copies file FILENAME to an ordinary non-magic file,
     if it isn't one already.

     If FILENAME specifies a "magic" file name, which programs outside
     Emacs cannot directly read or write, this copies the contents to
     an ordinary file and returns that file's name.

     If FILENAME is an ordinary file name, not magic, then this function
     does nothing and returns `nil'.

 - Function: unhandled-file-name-directory filename
     This function returns the name of a directory that is not magic.
     It uses the directory part of FILENAME if that is not magic.
     Otherwise, it asks the handler what to do.

     This is useful for running a subprocess; every subprocess must
     have a non-magic directory to serve as its current directory, and
     this function is a good way to come up with one.

\1f
File: lispref.info,  Node: Partial Files,  Next: Format Conversion,  Prev: Magic File Names,  Up: Files

Partial Files
=============

* Menu:

* Intro to Partial Files::
* Creating a Partial File::
* Detached Partial Files::

\1f
File: lispref.info,  Node: Intro to Partial Files,  Next: Creating a Partial File,  Up: Partial Files

Intro to Partial Files
----------------------

   A "partial file" is a section of a buffer (called the "master
buffer") that is placed in its own buffer and treated as its own file.
Changes made to the partial file are not reflected in the master buffer
until the partial file is "saved" using the standard buffer save
commands.  Partial files can be "reverted" (from the master buffer)
just like normal files.  When a file part is active on a master buffer,
that section of the master buffer is marked as read-only.  Two file
parts on the same master buffer are not allowed to overlap.  Partial
file buffers are indicated by the words `File Part' in the modeline.

   The master buffer knows about all the partial files that are active
on it, and thus killing or reverting the master buffer will be handled
properly.  When the master buffer is saved, if there are any unsaved
partial files active on it then the user will be given the opportunity
to first save these files.

   When a partial file buffer is first modified, the master buffer is
automatically marked as modified so that saving the master buffer will
work correctly.

\1f
File: lispref.info,  Node: Creating a Partial File,  Next: Detached Partial Files,  Prev: Intro to Partial Files,  Up: Partial Files

Creating a Partial File
-----------------------

 - Function: make-file-part &optional start end name buffer
     Make a file part on buffer BUFFER out of the region.  Call it
     NAME.  This command creates a new buffer containing the contents
     of the region and marks the buffer as referring to the specified
     buffer, called the "master buffer".  When the file-part buffer is
     saved, its changes are integrated back into the master buffer.
     When the master buffer is deleted, all file parts are deleted with
     it.

     When called from a function, expects four arguments, START, END,
     NAME, and BUFFER, all of which are optional and default to the
     beginning of BUFFER, the end of BUFFER, a name generated from
     BUFFER name, and the current buffer, respectively.

\1f
File: lispref.info,  Node: Detached Partial Files,  Prev: Creating a Partial File,  Up: Partial Files

Detached Partial Files
----------------------

   Every partial file has an extent in the master buffer associated
with it (called the "master extent"), marking where in the master
buffer the partial file begins and ends.  If the text in master buffer
that is contained by the extent is deleted, then the extent becomes
"detached", meaning that it no longer refers to a specific region of
the master buffer.  This can happen either when the text is deleted
directly or when the master buffer is reverted.  Neither of these should
happen in normal usage because the master buffer should generally not be
edited directly.

   Before doing any operation that references a partial file's master
extent, XEmacs checks to make sure that the extent is not detached.  If
this is the case, XEmacs warns the user of this and the master extent is
deleted out of the master buffer, disconnecting the file part.  The file
part's filename is cleared and thus must be explicitly specified if the
detached file part is to be saved.

\1f
File: lispref.info,  Node: Format Conversion,  Next: Files and MS-DOS,  Prev: Partial Files,  Up: Files

File Format Conversion
======================

   The variable `format-alist' defines a list of "file formats", which
describe textual representations used in files for the data (text,
text-properties, and possibly other information) in an Emacs buffer.
Emacs performs format conversion if appropriate when reading and writing
files.

 - Variable: format-alist
     This list contains one format definition for each defined file
     format.

   Each format definition is a list of this form:

     (NAME DOC-STRING REGEXP FROM-FN TO-FN MODIFY MODE-FN)

   Here is what the elements in a format definition mean:

NAME
     The name of this format.

DOC-STRING
     A documentation string for the format.

REGEXP
     A regular expression which is used to recognize files represented
     in this format.

FROM-FN
     A function to call to decode data in this format (to convert file
     data into the usual Emacs data representation).

     The FROM-FN is called with two args, BEGIN and END, which specify
     the part of the buffer it should convert.  It should convert the
     text by editing it in place.  Since this can change the length of
     the text, FROM-FN should return the modified end position.

     One responsibility of FROM-FN is to make sure that the beginning
     of the file no longer matches REGEXP.  Otherwise it is likely to
     get called again.

TO-FN
     A function to call to encode data in this format (to convert the
     usual Emacs data representation into this format).

     The TO-FN is called with two args, BEGIN and END, which specify
     the part of the buffer it should convert.  There are two ways it
     can do the conversion:

        * By editing the buffer in place.  In this case, TO-FN should
          return the end-position of the range of text, as modified.

        * By returning a list of annotations.  This is a list of
          elements of the form `(POSITION . STRING)', where POSITION is
          an integer specifying the relative position in the text to be
          written, and STRING is the annotation to add there.  The list
          must be sorted in order of position when TO-FN returns it.

          When `write-region' actually writes the text from the buffer
          to the file, it intermixes the specified annotations at the
          corresponding positions.  All this takes place without
          modifying the buffer.

MODIFY
     A flag, `t' if the encoding function modifies the buffer, and
     `nil' if it works by returning a list of annotations.

MODE
     A mode function to call after visiting a file converted from this
     format.

   The function `insert-file-contents' automatically recognizes file
formats when it reads the specified file.  It checks the text of the
beginning of the file against the regular expressions of the format
definitions, and if it finds a match, it calls the decoding function for
that format.  Then it checks all the known formats over again.  It
keeps checking them until none of them is applicable.

   Visiting a file, with `find-file-noselect' or the commands that use
it, performs conversion likewise (because it calls
`insert-file-contents'); it also calls the mode function for each
format that it decodes.  It stores a list of the format names in the
buffer-local variable `buffer-file-format'.

 - Variable: buffer-file-format
     This variable states the format of the visited file.  More
     precisely, this is a list of the file format names that were
     decoded in the course of visiting the current buffer's file.  It
     is always local in all buffers.

   When `write-region' writes data into a file, it first calls the
encoding functions for the formats listed in `buffer-file-format', in
the order of appearance in the list.

 - Function: format-write-file file format
     This command writes the current buffer contents into the file FILE
     in format FORMAT, and makes that format the default for future
     saves of the buffer.  The argument FORMAT is a list of format
     names.

 - Function: format-find-file file format
     This command finds the file FILE, converting it according to
     format FORMAT.  It also makes FORMAT the default if the buffer is
     saved later.

     The argument FORMAT is a list of format names.  If FORMAT is
     `nil', no conversion takes place.  Interactively, typing just
     <RET> for FORMAT specifies `nil'.

 - Function: format-insert-file file format &optional beg end
     This command inserts the contents of file FILE, converting it
     according to format FORMAT.  If BEG and END are non-`nil', they
     specify which part of the file to read, as in
     `insert-file-contents' (*note Reading from Files::).

     The return value is like what `insert-file-contents' returns: a
     list of the absolute file name and the length of the data inserted
     (after conversion).

     The argument FORMAT is a list of format names.  If FORMAT is
     `nil', no conversion takes place.  Interactively, typing just
     <RET> for FORMAT specifies `nil'.

 - Function: format-find-file file format
     This command finds the file FILE, converting it according to
     format FORMAT.  It also makes FORMAT the default if the buffer is
     saved later.

     The argument FORMAT is a list of format names.  If FORMAT is
     `nil', no conversion takes place.  Interactively, typing just
     <RET> for FORMAT specifies `nil'.

 - Function: format-insert-file file format &optional beg end
     This command inserts the contents of file FILE, converting it
     according to format FORMAT.  If BEG and END are non-`nil', they
     specify which part of the file to read, as in
     `insert-file-contents' (*note Reading from Files::).

     The return value is like what `insert-file-contents' returns: a
     list of the absolute file name and the length of the data inserted
     (after conversion).

     The argument FORMAT is a list of format names.  If FORMAT is
     `nil', no conversion takes place.  Interactively, typing just
     <RET> for FORMAT specifies `nil'.

 - Variable: auto-save-file-format
     This variable specifies the format to use for auto-saving.  Its
     value is a list of format names, just like the value of
     `buffer-file-format'; but it is used instead of
     `buffer-file-format' for writing auto-save files.  This variable
     is always local in all buffers.

\1f
File: lispref.info,  Node: Files and MS-DOS,  Prev: Format Conversion,  Up: Files

Files and MS-DOS
================

   Emacs on MS-DOS makes a distinction between text files and binary
files.  This is necessary because ordinary text files on MS-DOS use a
two character sequence between lines: carriage-return and linefeed
(CRLF).  Emacs expects just a newline character (a linefeed) between
lines.  When Emacs reads or writes a text file on MS-DOS, it needs to
convert the line separators.  This means it needs to know which files
are text files and which are binary.  It makes this decision when
visiting a file, and records the decision in the variable
`buffer-file-type' for use when the file is saved.

   *Note MS-DOS Subprocesses::, for a related feature for subprocesses.

 - Variable: buffer-file-type
     This variable, automatically local in each buffer, records the
     file type of the buffer's visited file.  The value is `nil' for
     text, `t' for binary.

 - Function: find-buffer-file-type filename
     This function determines whether file FILENAME is a text file or a
     binary file.  It returns `nil' for text, `t' for binary.

 - User Option: file-name-buffer-file-type-alist
     This variable holds an alist for distinguishing text files from
     binary files.  Each element has the form (REGEXP . TYPE), where
     REGEXP is matched against the file name, and TYPE may be is `nil'
     for text, `t' for binary, or a function to call to compute which.
     If it is a function, then it is called with a single argument (the
     file name) and should return `t' or `nil'.

 - User Option: default-buffer-file-type
     This variable specifies the default file type for files whose names
     don't indicate anything in particular.  Its value should be `nil'
     for text, or `t' for binary.

 - Command: find-file-text filename
     Like `find-file', but treat the file as text regardless of its
     name.

 - Command: find-file-binary filename
     Like `find-file', but treat the file as binary regardless of its
     name.

\1f
File: lispref.info,  Node: Backups and Auto-Saving,  Next: Buffers,  Prev: Files,  Up: Top

Backups and Auto-Saving
***********************

   Backup files and auto-save files are two methods by which XEmacs
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::      `revert-buffer', and how to customize what it does.

\1f
File: lispref.info,  Node: Backup Files,  Next: Auto-Saving,  Up: Backups and Auto-Saving

Backup Files
============

   A "backup file" is a copy of the old contents of a file you are
editing.  XEmacs 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, XEmacs 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 XEmacs can delete them automatically.

* Menu:

* Making Backups::     How XEmacs 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.

\1f
File: lispref.info,  Node: Making Backups,  Next: Rename or Copy,  Up: Backup Files

Making Backup Files
-------------------

 - Function: backup-buffer
     This function makes a backup of the file visited by the current
     buffer, if appropriate.  It is called by `save-buffer' before
     saving the buffer the first time.

 - Variable: 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-`nil', then
     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; `kill-local-variables' does not alter it.

 - User Option: make-backup-files
     This variable determines whether or not to make backup files.  If
     it is non-`nil', then XEmacs creates a backup of each file when it
     is saved for the first time--provided that `backup-inhibited' is
     `nil' (see below).

     The following example shows how to change the `make-backup-files'
     variable only in the `RMAIL' buffer and not elsewhere.  Setting it
     `nil' stops XEmacs from making backups of the `RMAIL' file, which
     may save disk space.  (You would put this code in your `.emacs'
     file.)

          (add-hook 'rmail-mode-hook
                    (function (lambda ()
                                (make-local-variable
                                 'make-backup-files)
                                (setq make-backup-files nil))))

 - Variable: 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, a file name to consider.  If the
     function returns `nil', backups are disabled for that file.
     Otherwise, the other variables in this section say whether and how
     to make backups.

     The default value is this:

          (lambda (name)
            (or (< (length name) 5)
                (not (string-equal "/tmp/"
                                   (substring name 0 5)))))

 - Variable: backup-inhibited
     If this variable is non-`nil', backups are inhibited.  It records
     the result of testing `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-`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 `make-backup-files' instead.

\1f
File: lispref.info,  Node: Rename or Copy,  Next: Numbered Backups,  Prev: Making Backups,  Up: Backup Files

Backup by Renaming or by Copying?
---------------------------------

   There are two ways that XEmacs can make a backup file:

   * XEmacs 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.

   * XEmacs 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.

   The first method, renaming, is the default.

   The variable `backup-by-copying', if non-`nil', says to use the
second method, which is to copy the original file and overwrite it with
the new buffer contents.  The variable `file-precious-flag', if
non-`nil', also has this effect (as a sideline of its main
significance).  *Note Saving Buffers::.

 - Variable: backup-by-copying
     If this variable is non-`nil', XEmacs always makes backup files by
     copying.

   The following two variables, when non-`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.

 - Variable: backup-by-copying-when-linked
     If this variable is non-`nil', XEmacs makes backups by copying for
     files with multiple names (hard links).

     This variable is significant only if `backup-by-copying' is `nil',
     since copying is always used when that variable is non-`nil'.

 - Variable: backup-by-copying-when-mismatch
     If this variable is non-`nil', XEmacs 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 `backup-by-copying' is `nil',
     since copying is always used when that variable is non-`nil'.

\1f
File: lispref.info,  Node: Numbered Backups,  Next: Backup Names,  Prev: Rename or Copy,  Up: Backup Files

Making and Deleting Numbered Backup Files
-----------------------------------------

   If a file's name is `foo', the names of its numbered backup versions
are `foo.~V~', for various integers V, like this: `foo.~1~', `foo.~2~',
`foo.~3~', ..., `foo.~259~', and so on.

 - User Option: version-control
     This variable controls whether to make a single non-numbered backup
     file or multiple numbered backups.

    `nil'
          Make numbered backups if the visited file already has
          numbered backups; otherwise, do not.

    `never'
          Do not make numbered backups.

    ANYTHING ELSE
          Make numbered backups.

   The use of numbered backups ultimately leads to a large number of
backup versions, which must then be deleted.  XEmacs can do this
automatically or it can ask the user whether to delete them.

 - User Option: 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 2.

 - User Option: 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.

   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 `find-backup-file-name'
(*note Backup Names::) is responsible for determining which backup
versions to delete, but does not delete them itself.

 - User Option: delete-old-versions
     If this variable is non-`nil', then saving a file deletes excess
     backup versions silently.  Otherwise, it asks the user whether to
     delete them.

 - User Option: dired-kept-versions
     This variable specifies how many of the newest backup versions to
     keep in the Dired command `.' (`dired-clean-directory').  That's
     the same thing `kept-new-versions' specifies when you make a new
     backup file.  The default value is 2.

\1f
File: lispref.info,  Node: Backup Names,  Prev: Numbered Backups,  Up: Backup Files

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.

 - Function: backup-file-name-p filename
     This function returns a non-`nil' value if FILENAME is a possible
     name for a backup file.  A file with the name FILENAME need not
     exist; the function just checks the name.

          (backup-file-name-p "foo")
               => nil
          (backup-file-name-p "foo~")
               => 3

     The standard definition of this function is as follows:

          (defun backup-file-name-p (file)
            "Return non-nil if FILE is a backup file \
          name (numeric or not)..."
            (string-match "~$" file))

     Thus, the function returns a non-`nil' value if the file name ends
     with a `~'.  (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.

 - Function: make-backup-file-name filename
     This function returns a string that is the name to use for a
     non-numbered backup file for file FILENAME.  On Unix, this is just
     FILENAME with a tilde appended.

     The standard definition of this function is as follows:

          (defun make-backup-file-name (file)
            "Create the non-numeric backup file name for FILE.
          ..."
            (concat file "~"))

     You can change the backup-file naming convention by redefining this
     function.  The following example redefines `make-backup-file-name'
     to prepend a `.' in addition to appending a tilde:

          (defun make-backup-file-name (filename)
            (concat "." filename "~"))
          
          (make-backup-file-name "backups.texi")
               => ".backups.texi~"

 - Function: find-backup-file-name filename
     This function computes the file name for a new backup file for
     FILENAME.  It may also propose certain existing backup files for
     deletion.  `find-backup-file-name' returns a list whose CAR is the
     name for the new backup file and whose CDR is a list of backup
     files whose deletion is proposed.

     Two variables, `kept-old-versions' and `kept-new-versions',
     determine which backup versions should be kept.  This function
     keeps those versions by excluding them from the CDR of the value.
     *Note Numbered Backups::.

     In this example, the value says that `~rms/foo.~5~' is the name to
     use for the new backup file, and `~rms/foo.~3~' is an "excess"
     version that the caller should consider deleting now.

          (find-backup-file-name "~rms/foo")
               => ("~rms/foo.~5~" "~rms/foo.~3~")

 - Function: file-newest-backup filename
     This function returns the name of the most recent backup file for
     FILENAME, or `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.

\1f
File: lispref.info,  Node: Auto-Saving,  Next: Reverting,  Prev: Backup Files,  Up: Backups and Auto-Saving

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

   XEmacs 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: (emacs)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/#files.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 works based on
     knowledge of the 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/#files.texi#"
          (auto-save-file-name-p "#files.texi#")
               => 0
          (auto-save-file-name-p "files.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 (`#')
     appended and prepended to it.  This function does not look at the
     variable `auto-save-visited-file-name' (described below); you
     should check that before calling this function.

          (make-auto-save-file-name)
               => "/xcssun/users/rms/lewis/#backup.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', XEmacs 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 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 `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 is the number of characters that XEmacs
     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.

 - 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, XEmacs auto-saves any buffers that need it.  (Actually, the
     specified timeout is multiplied by a factor depending on the size
     of the current buffer.)

 - 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 as of the last time it was 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 deletion.  Explicitly saving the buffer
     stores a positive value in this variable, thus reenabling
     auto-saving.  Turning auto-save mode off or on also alters this
     variable.

 - 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 XEmacs 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.

     If XEmacs exits normally, it deletes this file.  If XEmacs
     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 these files.

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

\1f
File: lispref.info,  Node: Reverting,  Prev: Auto-Saving,  Up: Backups and Auto-Saving

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 `revert-buffer' command.  *Note Reverting
a Buffer: (emacs)Reverting.

 - 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 CHECK-AUTO-SAVE is non-`nil', and the latest
     auto-save file is more recent than the visited file,
     `revert-buffer' asks the user whether to use that instead.
     Otherwise, it always uses the text of the visited file itself.
     Interactively, CHECK-AUTO-SAVE is set if there is a numeric prefix
     argument.

     Normally, `revert-buffer' asks for confirmation before it changes
     the buffer; but if the argument NOCONFIRM is non-`nil',
     `revert-buffer' does not ask for confirmation.

     Reverting tries to preserve marker positions in the buffer by
     using the replacement feature of `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; then 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.

   You can customize how `revert-buffer' does its work by setting these
variables--typically, as buffer-local variables.

 - Variable: revert-buffer-function
     The value of this variable is the function to use to revert this
     buffer.  If non-`nil', it is called as a function with no
     arguments to do the work of reverting.  If the value is `nil',
     reverting works the usual way.

     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.

 - Variable: revert-buffer-insert-file-contents-function
     The value of this variable, if non-`nil', is 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, `t' if the user has asked to read the auto-save file.

 - Variable: before-revert-hook
     This normal hook is run by `revert-buffer' before actually
     inserting the modified contents--but only if
     `revert-buffer-function' is `nil'.

     Font Lock mode uses this hook to record that the buffer contents
     are no longer fontified.

 - Variable: after-revert-hook
     This normal hook is run by `revert-buffer' after actually inserting
     the modified contents--but only if `revert-buffer-function' is
     `nil'.

     Font Lock mode uses this hook to recompute the fonts for the
     updated buffer contents.

\1f
File: lispref.info,  Node: Buffers,  Next: Windows,  Prev: Backups and Auto-Saving,  Up: Top

Buffers
*******

   A "buffer" is a Lisp object containing text to be edited.  Buffers
are used to hold the contents of files that are being visited; there may
also be buffers that are not visiting files.  While several buffers may
exist at one time, exactly one buffer is designated the "current
buffer" at any time.  Most editing commands act on the contents of the
current buffer.  Each buffer, including the current buffer, may or may
not be displayed in any windows.

* Menu:

* Buffer Basics::       What is a buffer?
* Current Buffer::      Designating a buffer as current
                          so primitives will access its contents.
* Buffer Names::        Accessing and changing buffer names.
* Buffer File Name::    The buffer file name indicates which file is visited.
* Buffer Modification:: A buffer is "modified" if it needs to be saved.
* Modification Time::   Determining whether the visited file was changed
                         ``behind XEmacs's back''.
* Read Only Buffers::   Modifying text is not allowed in a read-only buffer.
* The Buffer List::     How to look at all the existing buffers.
* Creating Buffers::    Functions that create buffers.
* Killing Buffers::     Buffers exist until explicitly killed.
* Indirect Buffers::    An indirect buffer shares text with some other buffer.