@c -*-texinfo-*-
@c This is part of the XEmacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993 Free Software Foundation, Inc. 
@c See the file lispref.texi for copying conditions.
@setfilename ../../info/buffers.info
@node Buffers, Windows, Backups and Auto-Saving, Top
@chapter Buffers
@cindex buffer

  A @dfn{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 which are not visiting files.  While several buffers may
exist at one time, exactly one buffer is designated the @dfn{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?
* Buffer Names::        Accessing and changing buffer names.
* Buffer File Name::    The buffer file name indicates which file is visited.
* Buffer Modification:: A buffer is @dfn{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.
* Current Buffer::      Designating a buffer as current
                          so primitives will access its contents.
@end menu

@node Buffer Basics
@section Buffer Basics

@ifinfo
  A @dfn{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 which are not visiting files.  While several buffers may
exist at one time, exactly one buffer is designated the @dfn{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.
@end ifinfo

  Buffers in XEmacs editing are objects which have distinct names and
hold text that can be edited.  Buffers appear to Lisp programs as a
special data type.  The contents of a buffer may be viewed as an
extendable string; insertions and deletions may occur in any part of the
buffer.  @xref{Text}.

  A Lisp buffer object contains numerous pieces of information.  Some of
this information is directly accessible to the programmer through
variables, while other information is only accessible through
special-purpose functions.  For example, the width of a tab character is
directly accessible through a variable, while the value of point is
accessible only through a primitive function.

  Buffer-specific information that is directly accessible is stored in
@dfn{buffer-local} variable bindings, which are variable values that are
effective only in a particular buffer.  This feature allows each buffer
to override the values of certain variables.  Most major modes override
variables such as @code{fill-column} or @code{comment-column} in this
way.  For more information about buffer-local variables and functions
related to them, see @ref{Buffer-Local Variables}.

  For functions and variables related to visiting files in buffers, see
@ref{Visiting Files} and @ref{Saving Buffers}.  For functions and
variables related to the display of buffers in windows, see
@ref{Buffers and Windows}.

@defun bufferp object
  This function returns @code{t} if @var{object} is a buffer,
@code{nil} otherwise.
@end defun

@node Buffer Names
@section Buffer Names
@cindex buffer names

  Each buffer has a unique name, which is a string.  Many of the
functions that work on buffers accept either a buffer or a buffer name
as an argument.  Any argument called @var{buffer-or-name} is of this
sort, and an error is signaled if it is neither a string nor a buffer.
Any argument called @var{buffer} is required to be an actual buffer
object, not a name.

  Buffers that are ephemeral and generally uninteresting to the user
have names starting with a space, which prevents them from being listed
by the @code{list-buffers} or @code{buffer-menu} commands.  (A name
starting with space also initially disables recording undo information;
see @ref{Undo}.)

@defun buffer-name &optional buffer
  This function returns the name of @var{buffer} as a string.  If
@var{buffer} is not supplied, it defaults to the current buffer.

  If @code{buffer-name} returns @code{nil}, it means that @var{buffer}
has been killed.  @xref{Killing Buffers}.

@example
@group
(buffer-name)
     @result{} "buffers.texi"
@end group

@group
(setq foo (get-buffer "temp"))
     @result{} #<buffer temp>
@end group
@group
(kill-buffer foo)
     @result{} nil
@end group
@group
(buffer-name foo)
     @result{} nil
@end group
@group
foo
     @result{} #<killed buffer>
@end group
@end example
@end defun

@deffn Command rename-buffer newname &optional unique
  This function renames the current buffer to @var{newname}.  An error
is signaled if @var{newname} is not a string, or if there is already a
buffer with that name.  The function returns @code{nil}.

@c Emacs 19 feature
  Ordinarily, @code{rename-buffer} signals an error if @var{newname} is
already in use.  However, if @var{unique} is non-@code{nil}, it modifies
@var{newname} to make a name that is not in use.  Interactively, you can
make @var{unique} non-@code{nil} with a numeric prefix argument.

  One application of this command is to rename the @samp{*shell*} buffer
to some other name, thus making it possible to create a second shell
buffer under the name @samp{*shell*}.
@end deffn

@defun get-buffer buffer-or-name
  This function returns the buffer specified by @var{buffer-or-name}.
If @var{buffer-or-name} is a string and there is no buffer with that
name, the value is @code{nil}.  If @var{buffer-or-name} is a buffer, it
is returned as given.  (That is not very useful, so the argument is usually 
a name.)  For example:

@example
@group
(setq b (get-buffer "lewis"))
     @result{} #<buffer lewis>
@end group
@group
(get-buffer b)
     @result{} #<buffer lewis>
@end group
@group
(get-buffer "Frazzle-nots")
     @result{} nil
@end group
@end example

See also the function @code{get-buffer-create} in @ref{Creating Buffers}.
@end defun

@c Emacs 19 feature
@c IGNORE is only in XEmacs
@defun generate-new-buffer-name starting-name &optional ignore
This function returns a name that would be unique for a new buffer---but
does not create the buffer.  It starts with @var{starting-name}, and
produces a name not currently in use for any buffer by appending a
number inside of @samp{<@dots{}>}.

If @var{ignore} is given, it specifies a name that is okay to use (if it
is in the sequence to be tried), even if a buffer with that name exists.

See the related function @code{generate-new-buffer} in @ref{Creating
Buffers}.
@end defun

@node Buffer File Name
@section Buffer File Name
@cindex visited file
@cindex buffer file name
@cindex file name of buffer

  The @dfn{buffer file name} is the name of the file that is visited in
that buffer.  When a buffer is not visiting a file, its buffer file name
is @code{nil}.  Most of the time, the buffer name is the same as the
nondirectory part of the buffer file name, but the buffer file name and
the buffer name are distinct and can be set independently.
@xref{Visiting Files}.

@defun buffer-file-name &optional buffer
  This function returns the absolute file name of the file that
@var{buffer} is visiting.  If @var{buffer} is not visiting any file,
@code{buffer-file-name} returns @code{nil}.  If @var{buffer} is not
supplied, it defaults to the current buffer.

@example
@group
(buffer-file-name (other-buffer))
     @result{} "/usr/user/lewis/manual/files.texi"
@end group
@end example
@end defun

@defvar buffer-file-name
This buffer-local variable contains the name of the file being visited
in the current buffer, or @code{nil} if it is not visiting a file.  It
is a permanent local, unaffected by @code{kill-local-variables}.

@example
@group
buffer-file-name
     @result{} "/usr/user/lewis/manual/buffers.texi"
@end group
@end example

It is risky to change this variable's value without doing various other
things.  See the definition of @code{set-visited-file-name} in
@file{files.el}; some of the things done there, such as changing the
buffer name, are not strictly necessary, but others are essential to
avoid confusing XEmacs.
@end defvar

@defvar buffer-file-truename
This buffer-local variable holds the truename of the file visited in the
current buffer, or @code{nil} if no file is visited.  It is a permanent
local, unaffected by @code{kill-local-variables}.  @xref{Truenames}.
@end defvar

@defvar buffer-file-number
This buffer-local variable holds the file number and directory device
number of the file visited in the current buffer, or @code{nil} if no
file or a nonexistent file is visited.  It is a permanent local,
unaffected by @code{kill-local-variables}.  @xref{Truenames}.

The value is normally a list of the form @code{(@var{filenum}
@var{devnum})}.  This pair of numbers uniquely identifies the file among
all files accessible on the system.  See the function
@code{file-attributes}, in @ref{File Attributes}, for more information
about them.
@end defvar

@defun get-file-buffer filename
  This function returns the buffer visiting file @var{filename}.  If
there is no such buffer, it returns @code{nil}.  The argument
@var{filename}, which must be a string, is expanded (@pxref{File Name
Expansion}), then compared against the visited file names of all live
buffers.

@example
@group
(get-file-buffer "buffers.texi")
    @result{} #<buffer buffers.texi>
@end group
@end example

  In unusual circumstances, there can be more than one buffer visiting
the same file name.  In such cases, this function returns the first
such buffer in the buffer list.
@end defun

@deffn Command set-visited-file-name filename
  If @var{filename} is a non-empty string, this function changes the
name of the file visited in current buffer to @var{filename}.  (If the
buffer had no visited file, this gives it one.)  The @emph{next time}
the buffer is saved it will go in the newly-specified file.  This
command marks the buffer as modified, since it does not (as far as XEmacs
knows) match the contents of @var{filename}, even if it matched the
former visited file.

  If @var{filename} is @code{nil} or the empty string, that stands for
``no visited file''.  In this case, @code{set-visited-file-name} marks
the buffer as having no visited file.

@c Wordy to avoid overfull hbox.  --rjc 16mar92
  When the function @code{set-visited-file-name} is called interactively, it
prompts for @var{filename} in the minibuffer.

  See also @code{clear-visited-file-modtime} and
@code{verify-visited-file-modtime} in @ref{Buffer Modification}.
@end deffn

@defvar list-buffers-directory
This buffer-local variable records a string to display in a buffer
listing in place of the visited file name, for buffers that don't have a
visited file name.  Dired buffers use this variable.
@end defvar

@node Buffer Modification
@section Buffer Modification
@cindex buffer modification
@cindex modification flag (of buffer)

  XEmacs keeps a flag called the @dfn{modified flag} for each buffer, to
record whether you have changed the text of the buffer.  This flag is
set to @code{t} whenever you alter the contents of the buffer, and
cleared to @code{nil} when you save it.  Thus, the flag shows whether
there are unsaved changes.  The flag value is normally shown in the
modeline (@pxref{Modeline Variables}), and controls saving
(@pxref{Saving Buffers}) and auto-saving (@pxref{Auto-Saving}).

  Some Lisp programs set the flag explicitly.  For example, the function
@code{set-visited-file-name} sets the flag to @code{t}, because the text
does not match the newly-visited file, even if it is unchanged from the
file formerly visited.

  The functions that modify the contents of buffers are described in
@ref{Text}.

@defun buffer-modified-p &optional buffer
  This function returns @code{t} if the buffer @var{buffer} has been modified
since it was last read in from a file or saved, or @code{nil}
otherwise.  If @var{buffer} is not supplied, the current buffer
is tested.
@end defun

@defun set-buffer-modified-p flag
  This function marks the current buffer as modified if @var{flag} is
non-@code{nil}, or as unmodified if the flag is @code{nil}.

  Another effect of calling this function is to cause unconditional
redisplay of the modeline for the current buffer.  In fact, the
function @code{redraw-modeline} works by doing this:

@example
@group
(set-buffer-modified-p (buffer-modified-p))
@end group
@end example
@end defun

@c ARG is only in XEmacs
@deffn Command not-modified &optional arg
  This command marks the current buffer as unmodified, and not needing
to be saved. (If @var{arg} is non-@code{nil}, the buffer is instead
marked as modified.) Don't use this function in programs, since it
prints a message in the echo area; use @code{set-buffer-modified-p}
(above) instead.
@end deffn

@c Emacs 19 feature
@defun buffer-modified-tick &optional buffer
This function returns @var{buffer}`s modification-count.  This is a
counter that increments every time the buffer is modified.  If
@var{buffer} is @code{nil} (or omitted), the current buffer is used.
@end defun

@node Modification Time
@section Comparison of Modification Time
@cindex comparison of modification time
@cindex modification time, comparison of 

  Suppose that you visit a file and make changes in its buffer, and
meanwhile the file itself is changed on disk.  At this point, saving the
buffer would overwrite the changes in the file.  Occasionally this may
be what you want, but usually it would lose valuable information.  XEmacs
therefore checks the file's modification time using the functions
described below before saving the file.

@defun verify-visited-file-modtime buffer
  This function compares XEmacs's record of the modification time for the
file that the buffer is visiting against the actual modification time of
the file as recorded by the operating system.  The two should be the same
unless some other process has written the file since XEmacs visited or
saved it.

  The function returns @code{t} if the last actual modification time and
XEmacs's recorded modification time are the same, @code{nil} otherwise.
@end defun

@defun clear-visited-file-modtime
  This function clears out the record of the last modification time of
the file being visited by the current buffer.  As a result, the next
attempt to save this buffer will not complain of a discrepancy in
file modification times.

  This function is called in @code{set-visited-file-name} and other
exceptional places where the usual test to avoid overwriting a changed
file should not be done.
@end defun

@c Emacs 19 feature
@defun set-visited-file-modtime &optional time
This function updates the buffer's record of the last modification time
of the visited file, to the value specified by @var{time} if @var{time}
is not @code{nil}, and otherwise to the last modification time of the
visited file.

If @var{time} is not @code{nil}, it should have the form
@code{(@var{high} . @var{low})} or @code{(@var{high} @var{low})}, in
either case containing two integers, each of which holds 16 bits of the
time.  (This is the same format that @code{file-attributes} uses to
return time values; see @ref{File Attributes}.)

This function is useful if the buffer was not read from the file
normally, or if the file itself has been changed for some known benign
reason.
@end defun

@c Emacs 19 feature
@defun visited-file-modtime
This function returns the buffer's recorded last file modification time,
as a list of the form @code{(@var{high} . @var{low})}.  Note that this
is not identical to the last modification time of the file that is
visited (though under normal circumstances the values are equal).
@end defun

@defun ask-user-about-supersession-threat fn
@cindex obsolete buffer
  This function is used to ask a user how to proceed after an attempt to
modify an obsolete buffer.  An @dfn{obsolete buffer} is an unmodified
buffer for which the associated file on disk is newer than the last
save-time of the buffer.  This means some other program has probably
altered the file.

  This function is called automatically by XEmacs on the proper
occasions.  It exists so you can customize XEmacs by redefining it.
See the file @file{userlock.el} for the standard definition.

@kindex file-supersession
  Depending on the user's answer, the function may return normally, in
which case the modification of the buffer proceeds, or it may signal a
@code{file-supersession} error with data @code{(@var{fn})}, in which
case the proposed buffer modification is not allowed.  

  See also the file locking mechanism in @ref{File Locks}.
@end defun

@node Read Only Buffers
@section Read-Only Buffers
@cindex read-only buffer
@cindex buffer, read-only

  A buffer may be designated as @dfn{read-only}.  This means that the
buffer's contents may not be modified, although you may change your view
of the contents by scrolling, narrowing, or widening, etc.

  Read-only buffers are used in two kinds of situations:

@itemize @bullet
@item
A buffer visiting a file is made read-only if the file is
write-protected.

Here, the purpose is to show the user that editing the buffer with the
aim of saving it in the file may be futile or undesirable.  The user who
wants to change the buffer text despite this can do so after clearing
the read-only flag with the function @code{toggle-read-only}.

@item
Modes such as Dired and Rmail make buffers read-only when altering the
contents with the usual editing commands is probably a mistake.

The special commands of the mode in question bind
@code{buffer-read-only} to @code{nil} (with @code{let}) around the
places where they change the text.
@end itemize

@defvar buffer-read-only
  This buffer-local variable specifies whether the buffer is read-only.
The buffer is read-only if this variable is non-@code{nil}.
@end defvar

@defvar inhibit-read-only
If this variable is non-@code{nil}, then read-only buffers and read-only
characters may be modified.  The value of @code{buffer-read-only} does
not matter when @code{inhibit-read-only} is non-@code{nil}.

If @code{inhibit-read-only} is @code{t}, all @code{read-only} text
properties have no effect (@pxref{Special Properties}).  If
@code{inhibit-read-only} is a list, then @code{read-only} text
properties are ignored if they are members of the list (comparison is
done with @code{eq}).
@end defvar

@deffn Command toggle-read-only
  This command changes whether the current buffer is read-only.  It is
intended for interactive use; don't use it in programs.  At any given
point in a program, you should know whether you want the read-only flag
on or off; so you can set @code{buffer-read-only} explicitly to the
proper value, @code{t} or @code{nil}.
@end deffn

@defun barf-if-buffer-read-only
@kindex buffer-read-only
  This function signals a @code{buffer-read-only} error if the current
buffer is read-only.  @xref{Interactive Call}, for another way to
signal an error if the current buffer is read-only.
@end defun

@node The Buffer List
@section The Buffer List
@cindex buffer list

  The @dfn{buffer list} is a list of all buffers that have not been
killed.  The order of the buffers in the list is based primarily on how
recently each buffer has been displayed in the selected window.  Several
functions, notably @code{other-buffer}, make use of this ordering.

@c XEmacs feature
  Every frame has its own order for the buffer list.  Switching to a
new buffer inside of a particular frame changes the buffer list order
for that frame, but does not affect the buffer list order of any other
frames.  In addition, there is a global, non-frame buffer list order
that is independent of the buffer list orders for any particular frame.

Note that the different buffer lists all contain the same elements.  It
is only the order of those elements that is different.

@defun buffer-list &optional frame
  This function returns a list of all buffers, including those whose
names begin with a space.  The elements are actual buffers, not their
names.  The order of the list is specific to @var{frame}, which
defaults to the current frame.  If @var{frame} is @code{t}, the
global, non-frame ordering is returned instead.

@example
@group
(buffer-list)
     @result{} (#<buffer buffers.texi>
         #<buffer  *Minibuf-1*> #<buffer buffer.c>
         #<buffer *Help*> #<buffer TAGS>)
@end group

@group
;; @r{Note that the name of the minibuffer}
;;   @r{begins with a space!}

(mapcar (function buffer-name) (buffer-list))
    @result{} ("buffers.texi" " *Minibuf-1*" 
         "buffer.c" "*Help*" "TAGS")
@end group
@end example

  Buffers appear earlier in the list if they were current more recently.

  This list is a copy of a list used inside XEmacs; modifying it has no
effect on the buffers.
@end defun

@defun other-buffer &optional buffer-or-name frame
  This function returns the first buffer in the buffer list other than
@var{buffer-or-name}, in @var{frame}'s ordering for the buffer list.
(@var{frame} defaults to the current frame.  If @var{frame} is
@code{t}, then the global, non-frame ordering is used.) Usually this is
the buffer most recently shown in the selected window, aside from
@var{buffer-or-name}.  Buffers are moved to the front of the list when
they are selected and to the end when they are buried.  Buffers whose
names start with a space are not even considered.

  If @var{buffer-or-name} is not supplied (or if it is not a buffer),
then some interesting buffer is returned.
@ignore
@c I'm not sure what really happens here, but the following is probably
@c not correct.
then @code{other-buffer} returns the first buffer on the buffer list
that is not visible in any window in a visible frame.
@end ignore

  Note that the @var{frame} argument has a different interpretation in
FSF Emacs 19.
@ignore  @c Different in XEmacs
@c Emacs 19 feature
  Normally, @code{other-buffer} avoids returning a buffer visible in any
window on any visible frame, except as a last resort.  However, if
@var{visible-ok} is non-@code{nil}, then a buffer displayed in some
window is admissible to return.
@end ignore

  If no suitable buffer exists, the buffer @samp{*scratch*} is returned
(and created, if necessary).
@end defun

@deffn Command list-buffers &optional files-only
  This function displays a listing of the names of existing buffers.  It
clears the buffer @samp{*Buffer List*}, then inserts the listing into
that buffer and displays it in a window.  @code{list-buffers} is
intended for interactive use, and is described fully in @cite{The XEmacs
Reference Manual}.  It returns @code{nil}.
@end deffn

@deffn Command bury-buffer &optional buffer-or-name
  This function puts @var{buffer-or-name} at the end of the buffer list
without changing the order of any of the other buffers on the list.
This buffer therefore becomes the least desirable candidate for
@code{other-buffer} to return, and appears last in the list displayed by
@code{list-buffers}.

  If @var{buffer-or-name} is @code{nil} or omitted, this means to bury
the current buffer.  In addition, this switches to some other buffer
(obtained using @code{other-buffer}) in the selected window.  If the
buffer is displayed in a window other than the selected one, it remains
there.

  If you wish to remove a buffer from all the windows that display it,
you can do so with a loop that uses @code{get-buffer-window}.
@xref{Buffers and Windows}.
@end deffn

@node Creating Buffers
@section Creating Buffers
@cindex creating buffers
@cindex buffers, creating

  This section describes the two primitives for creating buffers.
@code{get-buffer-create} creates a buffer if it finds no existing
buffer; @code{generate-new-buffer} always creates a new buffer, and
gives it a unique name.

  Other functions you can use to create buffers include
@code{with-output-to-temp-buffer} (@pxref{Temporary Displays}) and
@code{create-file-buffer} (@pxref{Visiting Files}).

@defun get-buffer-create name
  This function returns a buffer named @var{name}.  If such a buffer
already exists, it is returned.  If such a buffer does not exist,
one is created and returned.  The buffer does not become the current
buffer---this function does not change which buffer is current.

  An error is signaled if @var{name} is not a string.

@example
@group
(get-buffer-create "foo")
     @result{} #<buffer foo>
@end group
@end example

  The major mode for the new buffer is set by the value of
@code{default-major-mode}.  @xref{Auto Major Mode}.
@end defun

@defun generate-new-buffer name
  This function returns a newly created, empty buffer, but does not make
it current.  If there is no buffer named @var{name}, then that is the
name of the new buffer.  If that name is in use, this function adds
suffixes of the form @samp{<@var{n}>} are added to @var{name}, where
@var{n} is an integer.  It tries successive integers starting with 2
until it finds an available name.

  An error is signaled if @var{name} is not a string.

@example
@group
(generate-new-buffer "bar")
     @result{} #<buffer bar>
@end group
@group
(generate-new-buffer "bar")
     @result{} #<buffer bar<2>>
@end group
@group
(generate-new-buffer "bar")
     @result{} #<buffer bar<3>>
@end group
@end example

  The major mode for the new buffer is set by the value of
@code{default-major-mode}.  @xref{Auto Major Mode}.

See the related function @code{generate-new-buffer-name} in @ref{Buffer
Names}.
@end defun

@node Killing Buffers
@section Killing Buffers
@cindex killing buffers
@cindex buffers, killing

  @dfn{Killing a buffer} makes its name unknown to XEmacs and makes its
space available for other use.

  The buffer object for the buffer which has been killed remains in
existence as long as anything refers to it, but it is specially marked
so that you cannot make it current or display it.  Killed buffers retain
their identity, however; two distinct buffers, when killed, remain
distinct according to @code{eq}.

  If you kill a buffer that is current or displayed in a window, XEmacs
automatically selects or displays some other buffer instead.  This means
that killing a buffer can in general change the current buffer.
Therefore, when you kill a buffer, you should also take the precautions
associated with changing the current buffer (unless you happen to know
that the buffer being killed isn't current).  @xref{Current Buffer}.

  The @code{buffer-name} of a killed buffer is @code{nil}.  To test
whether a buffer has been killed, you can either use this feature
or the function @code{buffer-live-p}.

@defun buffer-live-p buffer
This function returns @code{nil} if @var{buffer} is deleted, and
@code{t} otherwise.
@end defun

@deffn Command kill-buffer buffer-or-name
  This function kills the buffer @var{buffer-or-name}, freeing all its
memory for use as space for other buffers.  (Emacs version 18 and older
was unable to return the memory to the operating system.)  It returns
@code{nil}.

  Any processes that have this buffer as the @code{process-buffer} are
sent the @code{SIGHUP} signal, which normally causes them to terminate.
(The usual meaning of @code{SIGHUP} is that a dialup line has been
disconnected.)  @xref{Deleting Processes}.

  If the buffer is visiting a file when @code{kill-buffer} is called and
the buffer has not been saved since it was last modified, the user is
asked to confirm before the buffer is killed.  This is done even if
@code{kill-buffer} is not called interactively.  To prevent the request
for confirmation, clear the modified flag before calling
@code{kill-buffer}.  @xref{Buffer Modification}.

@c Emacs 19 feature
  Just before actually killing the buffer, after asking all questions,
@code{kill-buffer} runs the normal hook @code{kill-buffer-hook}.  The
buffer to be killed is current when the hook functions run.
@xref{Hooks}.

  Killing a buffer that is already dead has no effect.

@smallexample
(kill-buffer "foo.unchanged")
     @result{} nil
(kill-buffer "foo.changed")

---------- Buffer: Minibuffer ----------
Buffer foo.changed modified; kill anyway? (yes or no) @kbd{yes}
---------- Buffer: Minibuffer ----------

     @result{} nil
@end smallexample
@end deffn

@node Current Buffer
@section The Current Buffer
@cindex selecting a buffer
@cindex changing to another buffer
@cindex current buffer

  There are, in general, many buffers in an XEmacs session.  At any time,
one of them is designated as the @dfn{current buffer}.  This is the
buffer in which most editing takes place, because most of the primitives
for examining or changing text in a buffer operate implicitly on the
current buffer (@pxref{Text}).  Normally the buffer that is displayed on
the screen in the selected window is the current buffer, but this is not
always so: a Lisp program can designate any buffer as current
temporarily in order to operate on its contents, without changing what
is displayed on the screen.

  The way to designate a current buffer in a Lisp program is by calling
@code{set-buffer}.  The specified buffer remains current until a new one
is designated.

  When an editing command returns to the editor command loop, the
command loop designates the buffer displayed in the selected window as
current, to prevent confusion: the buffer that the cursor is in, when
XEmacs reads a command, is the one to which the command will apply.
(@xref{Command Loop}.)  Therefore, @code{set-buffer} is not usable for
switching visibly to a different buffer so that the user can edit it.
For this, you must use the functions described in @ref{Displaying
Buffers}.

  However, Lisp functions that change to a different current buffer
should not leave it to the command loop to set it back afterwards.
Editing commands written in Emacs Lisp can be called from other programs
as well as from the command loop.  It is convenient for the caller if
the subroutine does not change which buffer is current (unless, of
course, that is the subroutine's purpose).  Therefore, you should
normally use @code{set-buffer} within a @code{save-excursion} that will
restore the current buffer when your program is done
(@pxref{Excursions}).  Here is an example, the code for the command
@code{append-to-buffer} (with the documentation string abridged):

@example
@group
(defun append-to-buffer (buffer start end)
  "Append to specified buffer the text of the region..."
  (interactive "BAppend to buffer: \nr")
  (let ((oldbuf (current-buffer)))
    (save-excursion
      (set-buffer (get-buffer-create buffer))
      (insert-buffer-substring oldbuf start end))))
@end group
@end example

@noindent
This function binds a local variable to the current buffer, and then
@code{save-excursion} records the values of point, the mark, and the
original buffer.  Next, @code{set-buffer} makes another buffer current.
Finally, @code{insert-buffer-substring} copies the string from the
original current buffer to the new current buffer.

  If the buffer appended to happens to be displayed in some window, then
the next redisplay will show how its text has changed.  Otherwise, you
will not see the change immediately on the screen.  The buffer becomes
current temporarily during the execution of the command, but this does
not cause it to be displayed.

  Changing the current buffer between the binding and unbinding of a
buffer-local variable can cause it to be bound in one buffer, and then
unbound in another!  You can avoid this problem by using save-excursion
to make sure that the buffer from which the variable was bound is
current again whenever the variable is unbound.

@example
@group
(let (buffer-read-only)
  (save-excursion
    (set-buffer @dots{})
    @dots{}))
@end group
@end example

@defun current-buffer
  This function returns the current buffer.

@example
@group
(current-buffer)
     @result{} #<buffer buffers.texi>
@end group
@end example
@end defun

@defun set-buffer buffer-or-name
  This function makes @var{buffer-or-name} the current buffer.  However,
it does not display the buffer in the currently selected window or in
any other window.  This means that the user cannot necessarily see the
buffer, but Lisp programs can in any case work on it.

   This function returns the buffer identified by @var{buffer-or-name}.
An error is signaled if @var{buffer-or-name} does not identify an
existing buffer.
@end defun
