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, 1995, 1998 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 that are not visiting files.  While several buffers may
exist at one time, only 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?
* Current Buffer::      Designating a buffer as current
                          so that 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 @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.
* Indirect Buffers::    An indirect buffer shares text with some other buffer.
* Buffer Gap::          The gap in the buffer.
@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 that are not visiting files.  Although several buffers
normally exist, only 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 that have distinct names and hold
text that can be edited.  Buffers appear to Lisp programs as a special
data type.  You can think of the contents of a buffer as a string that
you can extend; 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 accessible only 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 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 temporarily designate any buffer as
current 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 buffer that the command will apply to.
(@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
that, you must use the functions described in @ref{Displaying Buffers}.

  @strong{Note:} 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-current-buffer} or
@code{save-excursion} (@pxref{Excursions}) form that will restore the
current buffer when your function is done.  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-current-buffer
      (set-buffer (get-buffer-create buffer))
      (insert-buffer-substring oldbuf start end))))
@end group
@end example

@noindent
This function binds a local variable to record the current buffer, and
then @code{save-current-buffer} arranges to make it current again.
Next, @code{set-buffer} makes the specified buffer current.  Finally,
@code{insert-buffer-substring} copies the string from the original
current buffer to the specified (and now 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-current-buffer} or
@code{save-excursion} to make sure that the buffer current at the
beginning is current again whenever the variable is unbound.

  Do not rely on using @code{set-buffer} to change the current buffer
back, because that won't do the job if a quit happens while the wrong
buffer is current.  Here is what @emph{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-current-buffer}, as shown here, handles quitting,
errors, and @code{throw}, as well as ordinary evaluation.

@example
@group
(let (buffer-read-only)
  (save-current-buffer
    (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.  This does
not display the buffer in any window, so the user cannot necessarily see
the buffer.  But Lisp programs will now operate 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

@defspec save-current-buffer body...
@tindex save-current-buffer
The @code{save-current-buffer} macro saves the identity of the current
buffer, evaluates the @var{body} forms, and finally restores that buffer
as current.  The return value is the value of the last form in
@var{body}.  The current buffer is restored even in case of an abnormal
exit via @code{throw} or error (@pxref{Nonlocal Exits}).

If the buffer that used to be current has been killed by the time of
exit from @code{save-current-buffer}, then it is not made current again,
of course.  Instead, whichever buffer was current just before exit
remains current.
@end defspec

@defmac with-current-buffer buffer body...
@tindex with-current-buffer
The @code{with-current-buffer} macro saves the identity of the current
buffer, makes @var{buffer} current, evaluates the @var{body} forms, and
finally restores the buffer.  The return value is the value of the last
form in @var{body}.  The current buffer is restored even in case of an
abnormal exit via @code{throw} or error (@pxref{Nonlocal Exits}).
@end defmac

@defmac with-temp-buffer body...
@tindex with-temp-buffer
The @code{with-temp-buffer} macro evaluates the @var{body} forms
with a temporary buffer as the current buffer.  It saves the identity of
the current buffer, creates a temporary buffer and makes it current,
evaluates the @var{body} forms, and finally restores the previous
current buffer while killing the temporary buffer.

The return value is the value of the last form in @var{body}.  You can
return the contents of the temporary buffer by using
@code{(buffer-string)} as the last form.

The current buffer is restored even in case of an abnormal exit via
@code{throw} or error (@pxref{Nonlocal Exits}).
@end defmac

See also @code{with-temp-file} in @ref{Writing to Files}.

@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} and
@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 @var{newname}.

@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.
(This is how the command @code{rename-uniquely} is implemented.)
@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 variable, unaffected by
@code{kill-all-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.  Normally it is better to use @code{set-visited-file-name} (see
below); 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-all-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-all-local-variables}.

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 &optional no-query along-with-file
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.

Normally, this function asks the user for confirmation if the specified
file already exists.  If @var{no-query} is non-@code{nil}, that prevents
asking this question.

If @var{along-with-file} is non-@code{nil}, that means to assume that the
former visited file has been renamed to @var{filename}.

@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.
@end deffn

@defvar list-buffers-directory
This buffer-local variable specifies a string to display in a buffer
listing where the visited file name would go, 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.  With prefix arg, it marks the buffer as modified, so that it
will be saved at the next suitable occasion.

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 filename
@cindex obsolete buffer
This function is used to ask a user how to proceed after an attempt to
modify an obsolete buffer visiting file @var{filename}.  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.

@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{filename})}, in which
case the proposed buffer modification is not allowed.  

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.

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 inform 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-x C-q}.

@item
Modes such as Dired and Rmail make buffers read-only when altering the
contents with the usual editing commands would probably be 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 themselves 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.  Read-only characters in a buffer are those
that have non-@code{nil} @code{read-only} properties (either text
properties or overlay properties).  @xref{Special Properties}, for more
information about text properties.  @xref{Overlays}, for more
information about overlays and their properties.

If @code{inhibit-read-only} is @code{t}, all @code{read-only} character
properties have no effect.  If @code{inhibit-read-only} is a list, then
@code{read-only} character properties have no effect 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; do not 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 excises 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 (see @code{bury-buffer}, below).  Several functions, notably
@code{other-buffer}, use this ordering.  A buffer list displayed for the
user also follows this order.

  In addition to the fundamental Emacs buffer list, each frame has its
own version of the buffer list, in which the buffers that have been
selected in that frame come first, starting with the buffers most
recently selected @emph{in that frame}.  (This order is recorded in
@var{frame}'s @code{buffer-list} frame parameter; see @ref{Window Frame
Parameters}.)  The buffers that were never selected in @var{frame} come
afterward, ordered according to the fundamental Emacs buffer list.

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

If @var{frame} is a frame, this returns @var{frame}'s buffer list.  If
@var{frame} is @code{nil}, the fundamental Emacs buffer list is used:
all the buffers appear in order of most recent selection, regardless of
which frames they were selected in.

@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
@end defun

  The list that @code{buffer-list} returns is constructed specifically
by @code{buffer-list}; it is not an internal Emacs data structure, and
modifying it has no effect on the order of buffers.  If you want to
change the order of buffers in the frame-independent buffer list, here
is an easy way:

@example
(defun reorder-buffer-list (new-list)
  (while new-list
    (bury-buffer (car new-list))
    (setq new-list (cdr new-list))))
@end example

  With this method, you can specify any order for the list, but there is
no danger of losing a buffer or adding something that is not a valid
live buffer.

  To change the order or value of a frame's buffer list, set the frame's
@code{buffer-list} frame parameter with @code{modify-frame-parameters}
(@pxref{Parameter Access}).

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

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

If @var{frame} has a non-@code{nil} @code{buffer-predicate} parameter,
then @code{other-buffer} uses that predicate to decide which buffers to
consider.  It calls the predicate once for each buffer, and if the value
is @code{nil}, that buffer is ignored.  @xref{Window Frame Parameters}.

@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.

@code{bury-buffer} operates on each frame's @code{buffer-list} parameter
as well as the frame-independent Emacs buffer list; therefore, the
buffer that you bury will come last in the value of @code{(buffer-list
@var{frame})} and in the value of @code{(buffer-list nil)}.

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

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
with the specified name; @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 to Fundamental mode.  The
variable @code{default-major-mode} is handled at a higher level.
@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}>} 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 to Fundamental mode.  The
variable @code{default-major-mode} is handled at a higher level.
@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
text space available for other use.

  The buffer object for the buffer that 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; if you kill two distinct buffers, they remain
distinct according to @code{eq} although both are dead.

  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}.

  If you kill a buffer that is the base buffer of one or more indirect
buffers, the indirect buffers are automatically killed as well.

  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 other uses or to be returned 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 of this feature is that these functions will
ask for confirmation from the user.  If any of them returns @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 Indirect Buffers
@section Indirect Buffers
@cindex indirect buffers
@cindex base buffer

  An @dfn{indirect buffer} shares the text of some other buffer, which
is called the @dfn{base buffer} of the indirect buffer.  In some ways it
is the analogue, for buffers, of a symbolic link among files.  The base
buffer may not itself be an indirect buffer.

  The text of the indirect buffer is always identical to the text of its
base buffer; changes made by editing either one are visible immediately
in the other.  This includes the text properties as well as the characters
themselves.

  In all other respects, the indirect buffer and its base buffer are
completely separate.  They have different names, different values of
point, different narrowing, different markers and overlays (though
inserting or deleting text in either buffer relocates the markers and
overlays for both), different major modes, and different buffer-local
variables.

  An indirect buffer cannot visit a file, but its base buffer can.  If
you try to save the indirect buffer, that actually saves the base
buffer.

  Killing an indirect buffer has no effect on its base buffer.  Killing
the base buffer effectively kills the indirect buffer in that it cannot
ever again be the current buffer.

@deffn Command make-indirect-buffer base-buffer name
This creates an indirect buffer named @var{name} whose base buffer
is @var{base-buffer}.  The argument @var{base-buffer} may be a buffer
or a string.
 
If @var{base-buffer} is an indirect buffer, its base buffer is used as
the base for the new buffer.
@end deffn

@defun buffer-base-buffer buffer
This function returns the base buffer of @var{buffer}.  If @var{buffer}
is not indirect, the value is @code{nil}.  Otherwise, the value is
another buffer, which is never an indirect buffer.
@end defun

@node Buffer Gap
@section The Buffer Gap

  Emacs buffers are implemented using an invisible @dfn{gap} to make
insertion and deletion faster.  Insertion works by filling in part of
the gap, and deletion adds to the gap.  Of course, this means that the
gap must first be moved to the locus of the insertion or deletion.
Emacs moves the gap only when you try to insert or delete.  This is why
your first editing command in one part of a large buffer, after
previously editing in another far-away part, sometimes involves a
noticeable delay.

  This mechanism works invisibly, and Lisp code should never be affected
by the gap's current location, but these functions are available for
getting information about the gap status.

@defun gap-position
This function returns the current gap position in the current buffer.
@end defun

@defun gap-size
This function returns the current gap size of the current buffer.
@end defun