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[gnu-emacs] / lispref / buffers.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. 
@c See the file elisp.texi for copying conditions.
@setfilename ../info/buffers
@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 Emacs'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
@comment  node-name,  next,  previous,  up
@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 Emacs 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 visited file name 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} must be an actual buffer
object, not a name.

  Buffers that are ephemeral and generally uninteresting to the user
have names starting with a space, so that the @code{list-buffers} or
@code{buffer-menu} commands don't mention them.  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
@defun generate-new-buffer-name starting-name
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{}>}.

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 Emacs.
@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 Emacs
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)

  Emacs 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 mode
line (@pxref{Mode Line 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 mode line for the current buffer.  In fact, the
function @code{force-mode-line-update} works by doing this:

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

@deffn Command not-modified
This command marks the current buffer as unmodified, and not needing
to be saved.  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
@comment  node-name,  next,  previous,  up
@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.  Emacs
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 what @var{buffer} has recorded for the
modification time of its visited file 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 Emacs
visited or saved it.

The function returns @code{t} if the last actual modification time and
Emacs'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 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})}.  (This is the
same format that @code{file-attributes} uses to return time values; see
@ref{File Attributes}.)
@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 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

@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 Emacs on the proper
occasions.  It exists so you can customize Emacs 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

  If a buffer is @dfn{read-only}, then you cannot change its contents,
although you may change your view of the contents by scrolling and 
narrowing.

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

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

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 @kbd{C-M-q}.

@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 these modes bind @code{buffer-read-only} to
@code{nil} (with @code{let}) or bind @code{inhibit-read-only} to
@code{t} 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
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 live buffers.  Creating a
buffer adds it to this list, and killing a buffer deletes it.  The order
of the buffers in the list is based primarily on how recently each
buffer has been displayed in the selected window.  Buffers move to the
front of the list when they are selected and to the end when they are
buried.  Several functions, notably @code{other-buffer}, use this
ordering.  A buffer list displayed for the user also follows this order.

@defun buffer-list
This function returns a list of all buffers, including those whose names
begin with a space.  The elements are actual buffers, not their names.

@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

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

@defun other-buffer &optional buffer-or-name visible-ok
This function returns the first buffer in the buffer list other than
@var{buffer-or-name}.  Usually this is the buffer most recently shown in
the selected window, aside from @var{buffer-or-name}.  Buffers whose
names start with a space are not considered.

If @var{buffer-or-name} is not supplied (or if it is not a buffer),
then @code{other-buffer} returns the first buffer on the buffer list
that is not visible in any window in a visible frame.

@c Emacs 19 feature
If @var{visible-ok} is @code{nil}, @code{other-buffer} avoids returning
a buffer visible in any window on any visible frame, except as a last
resort.   If @var{visible-ok} is non-@code{nil}, then it does not matter
whether a buffer is displayed somewhere or not.

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

@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.

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 replace a buffer in all the windows that display it, use
@code{replace-buffer-in-windows}.  @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}).  Starting a
subprocess can also create a buffer (@pxref{Processes}).

@defun get-buffer-create name
This function returns a buffer named @var{name}.  It returns an existing
buffer with that name, if one exists; otherwise, it creates a new
buffer.  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 according to the variable
@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 Emacs 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, Emacs
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}.  You can use
this feature to test whether a buffer has been killed:

@example
@group
(defun buffer-killed-p (buffer)
  "Return t if BUFFER is killed."
  (not (buffer-name buffer)))
@end group
@end example

@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 basic meaning of @code{SIGHUP} is that a dialup line has been
disconnected.)  @xref{Deleting Processes}.

If the buffer is visiting a file and contains unsaved changes,
@code{kill-buffer} asks the user to confirm before the buffer is killed.
It does this even if not called interactively.  To prevent the request
for confirmation, clear the modified flag before calling
@code{kill-buffer}.  @xref{Buffer Modification}.

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

@defvar kill-buffer-query-functions
After confirming unsaved changes, @code{kill-buffer} calls the functions
in the list @code{kill-buffer-query-functions}, in order of appearance,
with no arguments.  The buffer being killed is the current buffer when
they are called.  The idea is that these functions ask for confirmation
from the user for various nonstandard reasons.  If any of them returns
non-@code{nil}, @code{kill-buffer} spares the buffer's life.
@end defvar

@defvar kill-buffer-hook
This is a normal hook run by @code{kill-buffer} after asking all the
questions it is going to ask, just before actually killing the buffer.
The buffer to be killed is current when the hook functions run.
@xref{Hooks}.
@end defvar

@defvar buffer-offer-save
This variable, if non-@code{nil} in a particular buffer, tells
@code{save-buffers-kill-emacs} and @code{save-some-buffers} to offer to
save that buffer, just as they offer to save file-visiting buffers.  The
variable @code{buffer-offer-save} automatically becomes buffer-local
when set for any reason.  @xref{Buffer-Local Variables}.
@end defvar

@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 Emacs 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
Emacs reads a command, is the one to which the command will apply.
(@xref{Command Loop}.)  Therefore, @code{set-buffer} is not the way to
switch 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 depend on 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 function 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.
@dots{}"
  (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, 
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.

  If you make local bindings (with @code{let} or function arguments) for
a variable that may also have buffer-local bindings, make sure that the
same buffer is current at the beginning and at the end of the local
binding's scope.  Otherwise you might bind it in one buffer and unbind
it in another!  There are two ways to do this.  In simple cases, you may
see that nothing ever changes the current buffer within the scope of the
binding.  Otherwise, use @code{save-excursion} to make sure that the
buffer current at the beginning is current again whenever the variable
is unbound.

  It is not reliable to change the current buffer back with
@code{set-buffer}, because that won't do the job if a quit happens while
the wrong buffer is current.  Here is what not to do:

@example
@group
(let (buffer-read-only
      (obuf (current-buffer)))
  (set-buffer @dots{})
  @dots{}
  (set-buffer obuf))
@end group
@end example

@noindent
Using  @code{save-excursion}, as shown below, handles quitting, errors
and @code{throw} as well as ordinary evaluation.

@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.  It does
not display the buffer in the currently selected window or in any other
window, so 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