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@c -*-texinfo-*-
@c This is part of the SXEmacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
@c Copyright (C) 2005 Sebastian Freundt <hroptatyr@sxemacs.org>
@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 that 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 window.

@menu
* Buffer Basics::       What is a buffer?
* Current Buffer::      Designating a buffer as current
                          so primitives will access its contents.
* Buffer Names::        Accessing and changing buffer names.
* Buffer File Name::    The buffer file name indicates which file is visited.
* Buffer Modification:: A buffer is @dfn{modified} if it needs to be saved.
* Modification Time::   Determining whether the visited file was changed
                         ``behind SXEmacs'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.
@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 that 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 SXEmacs 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 an
extendible 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 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 a SXEmacs 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}).

  If you span your SXEmacs session on multiple displays it may happen
that you appear to have more than one current buffer, but it remains one
current buffer per display.  Spanning SXEmacs in this sense also spans
a scope where some (probably many) buffers are non-current and one among
them is current in this scope.

  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 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
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 SXEmacs 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 @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-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.

@var{buffer-or-name} must be a buffer or the name of an existing
buffer--else an error is signaled.  This function returns the buffer
identified by @var{buffer-or-name}.
@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} 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 @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 named @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 actually 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

@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 SXEmacs.
@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
SXEmacs 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)

  SXEmacs 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