Fix a typo.

[emacs.git] / lispref / windows.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999
@c   Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../info/windows
@node Windows, Frames, Buffers, Top
@chapter Windows

  This chapter describes most of the functions and variables related to
Emacs windows.  See @ref{Display}, for information on how text is
displayed in windows.

@menu
* Basic Windows::           Basic information on using windows.
* Splitting Windows::       Splitting one window into two windows.
* Deleting Windows::        Deleting a window gives its space to other windows.
* Selecting Windows::       The selected window is the one that you edit in.
* Cyclic Window Ordering::  Moving around the existing windows.
* Buffers and Windows::     Each window displays the contents of a buffer.
* Displaying Buffers::      Higher-lever functions for displaying a buffer
                              and choosing a window for it.
* Choosing Window::         How to choose a window for displaying a buffer.
* Window Point::            Each window has its own location of point.
* Window Start::            The display-start position controls which text
                              is on-screen in the window.
* Textual Scrolling::       Moving text up and down through the window.
* Vertical Scrolling::      Moving the contents up and down on the window.
* Horizontal Scrolling::    Moving the contents sideways on the window.
* Size of Window::          Accessing the size of a window.
* Resizing Windows::        Changing the size of a window.
* Coordinates and Windows:: Converting coordinates to windows.
* Window Configurations::   Saving and restoring the state of the screen.
* Window Hooks::            Hooks for scrolling, window size changes,
                              redisplay going past a certain point,
                              or window configuration changes.
@end menu

@node Basic Windows
@section Basic Concepts of Emacs Windows
@cindex window
@cindex selected window

  A @dfn{window} in Emacs is the physical area of the screen in which a
buffer is displayed.  The term is also used to refer to a Lisp object that
represents that screen area in Emacs Lisp.  It should be
clear from the context which is meant.

  Emacs groups windows into frames.  A frame represents an area of
screen available for Emacs to use.  Each frame always contains at least
one window, but you can subdivide it vertically or horizontally into
multiple nonoverlapping Emacs windows.

  In each frame, at any time, one and only one window is designated as
@dfn{selected within the frame}.  The frame's cursor appears in that
window.  At any time, one frame is the selected frame; and the window
selected within that frame is @dfn{the selected window}.  The selected
window's buffer is usually the current buffer (except when
@code{set-buffer} has been used).  @xref{Current Buffer}.

  For practical purposes, a window exists only while it is displayed in
a frame.  Once removed from the frame, the window is effectively deleted
and should not be used, @emph{even though there may still be references
to it} from other Lisp objects.  Restoring a saved window configuration
is the only way for a window no longer on the screen to come back to
life.  (@xref{Deleting Windows}.)

  Each window has the following attributes:

@itemize @bullet
@item
containing frame

@item
window height

@item
window width

@item
window edges with respect to the screen or frame

@item
the buffer it displays

@item
position within the buffer at the upper left of the window

@item
amount of horizontal scrolling, in columns

@item
point

@item
the mark

@item
how recently the window was selected
@end itemize

@cindex multiple windows
  Users create multiple windows so they can look at several buffers at
once.  Lisp libraries use multiple windows for a variety of reasons, but
most often to display related information.  In Rmail, for example, you
can move through a summary buffer in one window while the other window
shows messages one at a time as they are reached.

  The meaning of ``window'' in Emacs is similar to what it means in the
context of general-purpose window systems such as X, but not identical.
The X Window System places X windows on the screen; Emacs uses one or
more X windows as frames, and subdivides them into
Emacs windows.  When you use Emacs on a character-only terminal, Emacs
treats the whole terminal screen as one frame.

@cindex terminal screen
@cindex screen of terminal
@cindex tiled windows
  Most window systems support arbitrarily located overlapping windows.
In contrast, Emacs windows are @dfn{tiled}; they never overlap, and
together they fill the whole screen or frame.  Because of the way in
which Emacs creates new windows and resizes them, not all conceivable
tilings of windows on an Emacs frame are actually possible.
@xref{Splitting Windows}, and @ref{Size of Window}.

  @xref{Display}, for information on how the contents of the
window's buffer are displayed in the window.

@defun windowp object
This function returns @code{t} if @var{object} is a window.
@end defun

@node Splitting Windows
@section Splitting Windows
@cindex splitting windows
@cindex window splitting

  The functions described here are the primitives used to split a window
into two windows.  Two higher level functions sometimes split a window,
but not always: @code{pop-to-buffer} and @code{display-buffer}
(@pxref{Displaying Buffers}).

  The functions described here do not accept a buffer as an argument.
The two ``halves'' of the split window initially display the same buffer
previously visible in the window that was split.

@deffn Command split-window &optional window size horizontal
This function splits @var{window} into two windows.  The original
window @var{window} remains the selected window, but occupies only
part of its former screen area.  The rest is occupied by a newly created
window which is returned as the value of this function.

  If @var{horizontal} is non-@code{nil}, then @var{window} splits into
two side by side windows.  The original window @var{window} keeps the
leftmost @var{size} columns, and gives the rest of the columns to the
new window.  Otherwise, it splits into windows one above the other, and
@var{window} keeps the upper @var{size} lines and gives the rest of the
lines to the new window.  The original window is therefore the
left-hand or upper of the two, and the new window is the right-hand or
lower.

  If @var{window} is omitted or @code{nil}, then the selected window is
split.  If @var{size} is omitted or @code{nil}, then @var{window} is
divided evenly into two parts.  (If there is an odd line, it is
allocated to the new window.)  When @code{split-window} is called
interactively, all its arguments are @code{nil}.

  The following example starts with one window on a screen that is 50
lines high by 80 columns wide; then the window is split.

@smallexample
@group
(setq w (selected-window))
     @result{} #<window 8 on windows.texi>
(window-edges)          ; @r{Edges in order:}
     @result{} (0 0 80 50)     ;   @r{left--top--right--bottom}
@end group

@group
;; @r{Returns window created}
(setq w2 (split-window w 15))
     @result{} #<window 28 on windows.texi>
@end group
@group
(window-edges w2)
     @result{} (0 15 80 50)    ; @r{Bottom window;}
                        ;   @r{top is line 15}
@end group
@group
(window-edges w)
     @result{} (0 0 80 15)     ; @r{Top window}
@end group
@end smallexample

The screen looks like this:

@smallexample
@group
         __________
        |          |  line 0
        |    w     |
        |__________|
        |          |  line 15
        |    w2    |
        |__________|
                      line 50
 column 0   column 80
@end group
@end smallexample

Next, the top window is split horizontally:

@smallexample
@group
(setq w3 (split-window w 35 t))
     @result{} #<window 32 on windows.texi>
@end group
@group
(window-edges w3)
     @result{} (35 0 80 15)  ; @r{Left edge at column 35}
@end group
@group
(window-edges w)
     @result{} (0 0 35 15)   ; @r{Right edge at column 35}
@end group
@group
(window-edges w2)
     @result{} (0 15 80 50)  ; @r{Bottom window unchanged}
@end group
@end smallexample

@need 3000
Now, the screen looks like this:

@smallexample
@group
     column 35
         __________
        |   |      |  line 0
        | w |  w3  |
        |___|______|
        |          |  line 15
        |    w2    |
        |__________|
                      line 50
 column 0   column 80
@end group
@end smallexample

Normally, Emacs indicates the border between two side-by-side windows
with a scroll bar (@pxref{Window Frame Parameters,Scroll Bars}) or @samp{|}
characters.  The display table can specify alternative border
characters; see @ref{Display Tables}.
@end deffn

@deffn Command split-window-vertically &optional size
This function splits the selected window into two windows, one above the
other, leaving the upper of the two windows selected, with @var{size}
lines.  (If @var{size} is negative, then the lower of the two windows
gets @minus{} @var{size} lines and the upper window gets the rest, but
the upper window is still the one selected.)
@end deffn

@deffn Command split-window-horizontally &optional size
This function splits the selected window into two windows
side-by-side, leaving the selected window with @var{size} columns.

This function is basically an interface to @code{split-window}.
You could define a simplified version of the function like this:

@smallexample
@group
(defun split-window-horizontally (&optional arg)
  "Split selected window into two windows, side by side..."
  (interactive "P")
@end group
@group
  (let ((size (and arg (prefix-numeric-value arg))))
    (and size (< size 0)
         (setq size (+ (window-width) size)))
    (split-window nil size t)))
@end group
@end smallexample
@end deffn

@defun one-window-p &optional no-mini all-frames
This function returns non-@code{nil} if there is only one window.  The
argument @var{no-mini}, if non-@code{nil}, means don't count the
minibuffer even if it is active; otherwise, the minibuffer window is
included, if active, in the total number of windows, which is compared
against one.

The argument @var{all-frames} specifies which frames to consider.  Here
are the possible values and their meanings:

@table @asis
@item @code{nil}
Count the windows in the selected frame, plus the minibuffer used
by that frame even if it lies in some other frame.

@item @code{t}
Count all windows in all existing frames.

@item @code{visible}
Count all windows in all visible frames.

@item 0
Count all windows in all visible or iconified frames.

@item anything else
Count precisely the windows in the selected frame, and no others.
@end table
@end defun

@node Deleting Windows
@section Deleting Windows
@cindex deleting windows

A window remains visible on its frame unless you @dfn{delete} it by
calling certain functions that delete windows.  A deleted window cannot
appear on the screen, but continues to exist as a Lisp object until
there are no references to it.  There is no way to cancel the deletion
of a window aside from restoring a saved window configuration
(@pxref{Window Configurations}).  Restoring a window configuration also
deletes any windows that aren't part of that configuration.

  When you delete a window, the space it took up is given to one
adjacent sibling.

@c Emacs 19 feature
@defun window-live-p window
This function returns @code{nil} if @var{window} is deleted, and
@code{t} otherwise.

@strong{Warning:} Erroneous information or fatal errors may result from
using a deleted window as if it were live.
@end defun

@deffn Command delete-window &optional window
This function removes @var{window} from display, and returns @code{nil}.
If @var{window} is omitted, then the selected window is deleted.  An
error is signaled if there is only one window when @code{delete-window}
is called.
@end deffn

@deffn Command delete-other-windows &optional window
This function makes @var{window} the only window on its frame, by
deleting the other windows in that frame.  If @var{window} is omitted or
@code{nil}, then the selected window is used by default.

The return value is @code{nil}.
@end deffn

@deffn Command delete-windows-on buffer &optional frame
This function deletes all windows showing @var{buffer}.  If there are
no windows showing @var{buffer}, it does nothing.

@code{delete-windows-on} operates frame by frame.  If a frame has
several windows showing different buffers, then those showing
@var{buffer} are removed, and the others expand to fill the space.  If
all windows in some frame are showing @var{buffer} (including the case
where there is only one window), then the frame reverts to having a
single window showing another buffer chosen with @code{other-buffer}.
@xref{The Buffer List}.

The argument @var{frame} controls which frames to operate on.  This
function does not use it in quite the same way as the other functions
which scan all windows; specifically, the values @code{t} and @code{nil}
have the opposite of their meanings in other functions.  Here are the
full details:

@itemize @bullet
@item
If it is @code{nil}, operate on all frames.
@item
If it is @code{t}, operate on the selected frame.
@item
If it is @code{visible}, operate on all visible frames.
@item
If it is 0, operate on all visible or iconified frames.
@item
If it is a frame, operate on that frame.
@end itemize

This function always returns @code{nil}.
@end deffn

@node Selecting Windows
@section Selecting Windows
@cindex selecting windows

  When a window is selected, the buffer in the window becomes the current
buffer, and the cursor will appear in it.

@defun selected-window
This function returns the selected window.  This is the window in
which the cursor appears and to which many commands apply.
@end defun

@defun select-window window
This function makes @var{window} the selected window.  The cursor then
appears in @var{window} (on redisplay).  The buffer being displayed in
@var{window} is immediately designated the current buffer.

The return value is @var{window}.

@example
@group
(setq w (next-window))
(select-window w)
     @result{} #<window 65 on windows.texi>
@end group
@end example
@end defun

@defmac save-selected-window forms@dots{}
This macro records the selected window, executes @var{forms}
in sequence, then restores the earlier selected window.

This macro does not save or restore anything about the sizes, arrangement
or contents of windows; therefore, if the @var{forms} change them,
the change persists.

Each frame, at any time, has a window selected within the frame.  This
macro saves only @emph{the} selected window; it does not save anything
about other frames.  If the @var{forms} select some other frame and
alter the window selected within it, the change persists.
@end defmac

@cindex finding windows
  The following functions choose one of the windows on the screen,
offering various criteria for the choice.

@defun get-lru-window &optional frame
This function returns the window least recently ``used'' (that is,
selected).  The selected window is always the most recently used window.

The selected window can be the least recently used window if it is the
only window.  A newly created window becomes the least recently used
window until it is selected.  A minibuffer window is never a candidate.

The argument @var{frame} controls which windows are considered.

@itemize @bullet
@item
If it is @code{nil}, consider windows on the selected frame.
@item
If it is @code{t}, consider windows on all frames.
@item
If it is @code{visible}, consider windows on all visible frames.
@item
If it is 0, consider windows on all visible or iconified frames.
@item
If it is a frame, consider windows on that frame.
@end itemize
@end defun

@defun get-largest-window &optional frame
This function returns the window with the largest area (height times
width).  If there are no side-by-side windows, then this is the window
with the most lines.  A minibuffer window is never a candidate.

If there are two windows of the same size, then the function returns
the window that is first in the cyclic ordering of windows (see
following section), starting from the selected window.

The argument @var{frame} controls which set of windows to
consider.  See @code{get-lru-window}, above.
@end defun

@node Cyclic Window Ordering
@comment  node-name,  next,  previous,  up
@section Cyclic Ordering of Windows
@cindex cyclic ordering of windows
@cindex ordering of windows, cyclic
@cindex window ordering, cyclic

  When you use the command @kbd{C-x o} (@code{other-window}) to select
the next window, it moves through all the windows on the screen in a
specific cyclic order.  For any given configuration of windows, this
order never varies.  It is called the @dfn{cyclic ordering of windows}.

  This ordering generally goes from top to bottom, and from left to
right.  But it may go down first or go right first, depending on the
order in which the windows were split.

  If the first split was vertical (into windows one above each other),
and then the subwindows were split horizontally, then the ordering is
left to right in the top of the frame, and then left to right in the
next lower part of the frame, and so on.  If the first split was
horizontal, the ordering is top to bottom in the left part, and so on.
In general, within each set of siblings at any level in the window tree,
the order is left to right, or top to bottom.

@defun next-window &optional window minibuf all-frames
@cindex minibuffer window
This function returns the window following @var{window} in the cyclic
ordering of windows.  This is the window that @kbd{C-x o} would select
if typed when @var{window} is selected.  If @var{window} is the only
window visible, then this function returns @var{window}.  If omitted,
@var{window} defaults to the selected window.

The value of the argument @var{minibuf} determines whether the
minibuffer is included in the window order.  Normally, when
@var{minibuf} is @code{nil}, the minibuffer is included if it is
currently active; this is the behavior of @kbd{C-x o}.  (The minibuffer
window is active while the minibuffer is in use.  @xref{Minibuffers}.)

If @var{minibuf} is @code{t}, then the cyclic ordering includes the
minibuffer window even if it is not active.

If @var{minibuf} is neither @code{t} nor @code{nil}, then the minibuffer
window is not included even if it is active.

The argument @var{all-frames} specifies which frames to consider.  Here
are the possible values and their meanings:

@table @asis
@item @code{nil}
Consider all the windows in @var{window}'s frame, plus the minibuffer
used by that frame even if it lies in some other frame.

@item @code{t}
Consider all windows in all existing frames.

@item @code{visible}
Consider all windows in all visible frames.  (To get useful results, you
must ensure @var{window} is in a visible frame.)

@item 0
Consider all windows in all visible or iconified frames.

@item anything else
Consider precisely the windows in @var{window}'s frame, and no others.
@end table

This example assumes there are two windows, both displaying the
buffer @samp{windows.texi}:

@example
@group
(selected-window)
     @result{} #<window 56 on windows.texi>
@end group
@group
(next-window (selected-window))
     @result{} #<window 52 on windows.texi>
@end group
@group
(next-window (next-window (selected-window)))
     @result{} #<window 56 on windows.texi>
@end group
@end example
@end defun

@defun previous-window &optional window minibuf all-frames
This function returns the window preceding @var{window} in the cyclic
ordering of windows.  The other arguments specify which windows to
include in the cycle, as in @code{next-window}.
@end defun

@deffn Command other-window count &optional all-frames
This function selects the @var{count}th following window in the cyclic
order.  If count is negative, then it moves back @minus{}@var{count}
windows in the cycle, rather than forward.  It returns @code{nil}.

The argument @var{all-frames} has the same meaning as in
@code{next-window}, but the @var{minibuf} argument of @code{next-window}
is always effectively @code{nil}.

In an interactive call, @var{count} is the numeric prefix argument.
@end deffn

@c Emacs 19 feature
@defun walk-windows proc &optional minibuf all-frames
This function cycles through all windows, calling @code{proc}
once for each window with the window as its sole argument.

The optional arguments @var{minibuf} and @var{all-frames} specify the
set of windows to include in the scan.  See @code{next-window}, above,
for details.
@end defun

@node Buffers and Windows
@section Buffers and Windows
@cindex examining windows
@cindex windows, controlling precisely
@cindex buffers, controlled in windows

  This section describes low-level functions to examine windows or to
display buffers in windows in a precisely controlled fashion.
@iftex
See the following section for
@end iftex
@ifnottex
@xref{Displaying Buffers}, for
@end ifnottex
related functions that find a window to use and specify a buffer for it.
The functions described there are easier to use than these, but they
employ heuristics in choosing or creating a window; use these functions
when you need complete control.

@defun set-window-buffer window buffer-or-name
This function makes @var{window} display @var{buffer-or-name} as its
contents.  It returns @code{nil}.  This is the fundamental primitive
for changing which buffer is displayed in a window, and all ways
of doing that call this function.

@example
@group
(set-window-buffer (selected-window) "foo")
     @result{} nil
@end group
@end example
@end defun

@defun window-buffer &optional window
This function returns the buffer that @var{window} is displaying.  If
@var{window} is omitted, this function returns the buffer for the
selected window.

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

@defun get-buffer-window buffer-or-name &optional all-frames
This function returns a window currently displaying
@var{buffer-or-name}, or @code{nil} if there is none.  If there are
several such windows, then the function returns the first one in the
cyclic ordering of windows, starting from the selected window.
@xref{Cyclic Window Ordering}.

The argument @var{all-frames} controls which windows to consider.

@itemize @bullet
@item
If it is @code{nil}, consider windows on the selected frame.
@item
If it is @code{t}, consider windows on all frames.
@item
If it is @code{visible}, consider windows on all visible frames.
@item
If it is 0, consider windows on all visible or iconified frames.
@item
If it is a frame, consider windows on that frame.
@end itemize
@end defun

@defun get-buffer-window-list buffer-or-name &optional minibuf all-frames
This function returns a list of all the windows currently displaying
@var{buffer-or-name}.

The two optional arguments work like the optional arguments of
@code{next-window} (@pxref{Cyclic Window Ordering}); they are @emph{not}
like the single optional argument of @code{get-buffer-window}.  Perhaps
we should change @code{get-buffer-window} in the future to make it
compatible with the other functions.

The argument @var{all-frames} controls which windows to consider.

@itemize @bullet
@item
If it is @code{nil}, consider windows on the selected frame.
@item
If it is @code{t}, consider windows on all frames.
@item
If it is @code{visible}, consider windows on all visible frames.
@item
If it is 0, consider windows on all visible or iconified frames.
@item
If it is a frame, consider windows on that frame.
@end itemize
@end defun

@defvar buffer-display-time
This variable records the time at which a buffer was last made visible
in a window.  It is always local in each buffer; each time
@code{set-window-buffer} is called, it sets this variable to
@code{(current-time)} in the specified buffer (@pxref{Time of Day}).
When a buffer is first created, @code{buffer-display-time} starts out
with the value @code{nil}.
@end defvar

@node Displaying Buffers
@section Displaying Buffers in Windows
@cindex switching to a buffer
@cindex displaying a buffer

  In this section we describe convenient functions that choose a window
automatically and use it to display a specified buffer.  These functions
can also split an existing window in certain circumstances.  We also
describe variables that parameterize the heuristics used for choosing a
window.
@iftex
See the preceding section for
@end iftex
@ifnottex
@xref{Buffers and Windows}, for
@end ifnottex
low-level functions that give you more precise control.  All of these
functions work by calling @code{set-window-buffer}.

  Do not use the functions in this section in order to make a buffer
current so that a Lisp program can access or modify it; they are too
drastic for that purpose, since they change the display of buffers in
windows, which would be gratuitous and surprise the user.  Instead, use
@code{set-buffer} and @code{save-current-buffer} (@pxref{Current
Buffer}), which designate buffers as current for programmed access
without affecting the display of buffers in windows.

@deffn Command switch-to-buffer buffer-or-name &optional norecord
This function makes @var{buffer-or-name} the current buffer, and also
displays the buffer in the selected window.  This means that a human can
see the buffer and subsequent keyboard commands will apply to it.
Contrast this with @code{set-buffer}, which makes @var{buffer-or-name}
the current buffer but does not display it in the selected window.
@xref{Current Buffer}.

If @var{buffer-or-name} does not identify an existing buffer, then a new
buffer by that name is created.  The major mode for the new buffer is
set according to the variable @code{default-major-mode}.  @xref{Auto
Major Mode}.

Normally the specified buffer is put at the front of the buffer list
(both the selected frame's buffer list and the frame-independent buffer
list).  This affects the operation of @code{other-buffer}.  However, if
@var{norecord} is non-@code{nil}, this is not done.  @xref{The Buffer
List}.

The @code{switch-to-buffer} function is often used interactively, as
the binding of @kbd{C-x b}.  It is also used frequently in programs.  It
always returns @code{nil}.
@end deffn

@deffn Command switch-to-buffer-other-window buffer-or-name &optional norecord
This function makes @var{buffer-or-name} the current buffer and
displays it in a window not currently selected.  It then selects that
window.  The handling of the buffer is the same as in
@code{switch-to-buffer}.

The currently selected window is absolutely never used to do the job.
If it is the only window, then it is split to make a distinct window for
this purpose.  If the selected window is already displaying the buffer,
then it continues to do so, but another window is nonetheless found to
display it in as well.

This function updates the buffer list just like @code{switch-to-buffer}
unless @var{norecord} is non-@code{nil}.
@end deffn

@defun pop-to-buffer buffer-or-name &optional other-window norecord
This function makes @var{buffer-or-name} the current buffer and
switches to it in some window, preferably not the window previously
selected.  The ``popped-to'' window becomes the selected window within
its frame.

If the variable @code{pop-up-frames} is non-@code{nil},
@code{pop-to-buffer} looks for a window in any visible frame already
displaying the buffer; if there is one, it returns that window and makes
it be selected within its frame.  If there is none, it creates a new
frame and displays the buffer in it.

If @code{pop-up-frames} is @code{nil}, then @code{pop-to-buffer}
operates entirely within the selected frame.  (If the selected frame has
just a minibuffer, @code{pop-to-buffer} operates within the most
recently selected frame that was not just a minibuffer.)

If the variable @code{pop-up-windows} is non-@code{nil}, windows may
be split to create a new window that is different from the original
window.  For details, see @ref{Choosing Window}.

If @var{other-window} is non-@code{nil}, @code{pop-to-buffer} finds or
creates another window even if @var{buffer-or-name} is already visible
in the selected window.  Thus @var{buffer-or-name} could end up
displayed in two windows.  On the other hand, if @var{buffer-or-name} is
already displayed in the selected window and @var{other-window} is
@code{nil}, then the selected window is considered sufficient display
for @var{buffer-or-name}, so that nothing needs to be done.

All the variables that affect @code{display-buffer} affect
@code{pop-to-buffer} as well.  @xref{Choosing Window}.

If @var{buffer-or-name} is a string that does not name an existing
buffer, a buffer by that name is created.  The major mode for the new
buffer is set according to the variable @code{default-major-mode}.
@xref{Auto Major Mode}.

This function updates the buffer list just like @code{switch-to-buffer}
unless @var{norecord} is non-@code{nil}.
@end defun

@deffn Command replace-buffer-in-windows buffer
This function replaces @var{buffer} with some other buffer in all
windows displaying it.  The other buffer used is chosen with
@code{other-buffer}.  In the usual applications of this function, you
don't care which other buffer is used; you just want to make sure that
@var{buffer} is no longer displayed.

This function returns @code{nil}.
@end deffn

@node Choosing Window
@section Choosing a Window for Display

  This section describes the basic facility that chooses a window to
display a buffer in---@code{display-buffer}.  All the higher-level
functions and commands use this subroutine.  Here we describe how to use
@code{display-buffer} and how to customize it.

@deffn Command display-buffer buffer-or-name &optional not-this-window frame
This command makes @var{buffer-or-name} appear in some window, like
@code{pop-to-buffer}, but it does not select that window and does not
make the buffer current.  The identity of the selected window is
unaltered by this function.

If @var{not-this-window} is non-@code{nil}, it means to display the
specified buffer in a window other than the selected one, even if it is
already on display in the selected window.  This can cause the buffer to
appear in two windows at once.  Otherwise, if @var{buffer-or-name} is
already being displayed in any window, that is good enough, so this
function does nothing.

@code{display-buffer} returns the window chosen to display
@var{buffer-or-name}.

If the argument @var{frame} is non-@code{nil}, it specifies which frames
to check when deciding whether the buffer is already displayed.  If the
buffer is already displayed in some window on one of these frames,
@code{display-buffer} simply returns that window.  Here are the possible
values of @var{frame}:

@itemize @bullet
@item
If it is @code{nil}, consider windows on the selected frame.
@item
If it is @code{t}, consider windows on all frames.
@item
If it is @code{visible}, consider windows on all visible frames.
@item
If it is 0, consider windows on all visible or iconified frames.
@item
If it is a frame, consider windows on that frame.
@end itemize

Precisely how @code{display-buffer} finds or creates a window depends on
the variables described below.
@end deffn

@defopt display-buffer-reuse-frames
If this variable is non-@code{nil}, @code{display-buffer} searches
existing frames for a window displaying the buffer.  If the buffer is
already displayed in a window in some frame, @code{display-buffer} makes
the frame visible and raises it, to use that window.  If the buffer is
not already displayed, or if @code{display-buffer-reuse-frames} is
@code{nil}, @code{display-buffer}'s behavior is determined by other
variables, described below.
@end defopt

@defopt pop-up-windows
This variable controls whether @code{display-buffer} makes new windows.
If it is non-@code{nil} and there is only one window, then that window
is split.  If it is @code{nil}, then @code{display-buffer} does not
split the single window, but uses it whole.
@end defopt

@defopt split-height-threshold
This variable determines when @code{display-buffer} may split a window,
if there are multiple windows.  @code{display-buffer} always splits the
largest window if it has at least this many lines.  If the largest
window is not this tall, it is split only if it is the sole window and
@code{pop-up-windows} is non-@code{nil}.
@end defopt

@c Emacs 19 feature
@defopt pop-up-frames
This variable controls whether @code{display-buffer} makes new frames.
If it is non-@code{nil}, @code{display-buffer} looks for an existing
window already displaying the desired buffer, on any visible frame.  If
it finds one, it returns that window.  Otherwise it makes a new frame.
The variables @code{pop-up-windows} and @code{split-height-threshold} do
not matter if @code{pop-up-frames} is non-@code{nil}.

If @code{pop-up-frames} is @code{nil}, then @code{display-buffer} either
splits a window or reuses one.

@xref{Frames}, for more information.
@end defopt

@c Emacs 19 feature
@defvar pop-up-frame-function
This variable specifies how to make a new frame if @code{pop-up-frames}
is non-@code{nil}.

Its value should be a function of no arguments.  When
@code{display-buffer} makes a new frame, it does so by calling that
function, which should return a frame.  The default value of the
variable is a function that creates a frame using parameters from
@code{pop-up-frame-alist}.
@end defvar

@defopt pop-up-frame-alist
This variable holds an alist specifying frame parameters used when
@code{display-buffer} makes a new frame.  @xref{Frame Parameters}, for
more information about frame parameters.
@end defopt

@defopt special-display-buffer-names
A list of buffer names for buffers that should be displayed specially.
If the buffer's name is in this list, @code{display-buffer} handles the
buffer specially.

By default, special display means to give the buffer a dedicated frame.

If an element is a list, instead of a string, then the @sc{car} of the
list is the buffer name, and the rest of the list says how to create the
frame.  There are two possibilities for the rest of the list.  It can be
an alist, specifying frame parameters, or it can contain a function and
arguments to give to it.  (The function's first argument is always the
buffer to be displayed; the arguments from the list come after that.)
@end defopt

@defopt special-display-regexps
A list of regular expressions that specify buffers that should be
displayed specially.  If the buffer's name matches any of the regular
expressions in this list, @code{display-buffer} handles the buffer
specially.

By default, special display means to give the buffer a dedicated frame.

If an element is a list, instead of a string, then the @sc{car} of the
list is the regular expression, and the rest of the list says how to
create the frame.  See above, under @code{special-display-buffer-names}.
@end defopt

@defvar special-display-function
This variable holds the function to call to display a buffer specially.
It receives the buffer as an argument, and should return the window in
which it is displayed.

The default value of this variable is
@code{special-display-popup-frame}.
@end defvar

@defun special-display-popup-frame buffer &rest args
This function makes @var{buffer} visible in a frame of its own.  If
@var{buffer} is already displayed in a window in some frame, it makes
the frame visible and raises it, to use that window.  Otherwise, it
creates a frame that will be dedicated to @var{buffer}.

If @var{args} is an alist, it specifies frame parameters for the new
frame.

If @var{args} is a list whose @sc{car} is a symbol, then @code{(car
@var{args})} is called as a function to actually create and set up the
frame; it is called with @var{buffer} as first argument, and @code{(cdr
@var{args})} as additional arguments.

This function always uses an existing window displaying @var{buffer},
whether or not it is in a frame of its own; but if you set up the above
variables in your init file, before @var{buffer} was created, then
presumably the window was previously made by this function.
@end defun

@defopt special-display-frame-alist
This variable holds frame parameters for
@code{special-display-popup-frame} to use when it creates a frame.
@end defopt

@defopt same-window-buffer-names
A list of buffer names for buffers that should be displayed in the
selected window.  If the buffer's name is in this list,
@code{display-buffer} handles the buffer by switching to it in the
selected window.
@end defopt

@defopt same-window-regexps
A list of regular expressions that specify buffers that should be
displayed in the selected window.  If the buffer's name matches any of
the regular expressions in this list, @code{display-buffer} handles the
buffer by switching to it in the selected window.
@end defopt

@c Emacs 19 feature
@defvar display-buffer-function
This variable is the most flexible way to customize the behavior of
@code{display-buffer}.  If it is non-@code{nil}, it should be a function
that @code{display-buffer} calls to do the work.  The function should
accept two arguments, the same two arguments that @code{display-buffer}
received.  It should choose or create a window, display the specified
buffer, and then return the window.

This hook takes precedence over all the other options and hooks
described above.
@end defvar

@c Emacs 19 feature
@cindex dedicated window
A window can be marked as ``dedicated'' to its buffer.  Then
@code{display-buffer} will not try to use that window to display any
other buffer.

@defun window-dedicated-p window
This function returns @code{t} if @var{window} is marked as dedicated;
otherwise @code{nil}.
@end defun

@defun set-window-dedicated-p window flag
This function marks @var{window} as dedicated if @var{flag} is
non-@code{nil}, and nondedicated otherwise.
@end defun

@node Window Point
@section Windows and Point
@cindex window position
@cindex window point
@cindex position in window
@cindex point in window

  Each window has its own value of point, independent of the value of
point in other windows displaying the same buffer.  This makes it useful
to have multiple windows showing one buffer.

@itemize @bullet
@item
The window point is established when a window is first created; it is
initialized from the buffer's point, or from the window point of another
window opened on the buffer if such a window exists.

@item
Selecting a window sets the value of point in its buffer from the
window's value of point.  Conversely, deselecting a window sets the
window's value of point from that of the buffer.  Thus, when you switch
between windows that display a given buffer, the point value for the
selected window is in effect in the buffer, while the point values for
the other windows are stored in those windows.

@item
As long as the selected window displays the current buffer, the window's
point and the buffer's point always move together; they remain equal.

@item
@xref{Positions}, for more details on buffer positions.
@end itemize

  As far as the user is concerned, point is where the cursor is, and
when the user switches to another buffer, the cursor jumps to the
position of point in that buffer.

@defun window-point &optional window
This function returns the current position of point in @var{window}.
For a nonselected window, this is the value point would have (in that
window's buffer) if that window were selected.  If @var{window} is
@code{nil}, the selected window is used.

When @var{window} is the selected window and its buffer is also the
current buffer, the value returned is the same as point in that buffer.

Strictly speaking, it would be more correct to return the
``top-level'' value of point, outside of any @code{save-excursion}
forms.  But that value is hard to find.
@end defun

@defun set-window-point window position
This function positions point in @var{window} at position
@var{position} in @var{window}'s buffer.
@end defun

@node Window Start
@section The Window Start Position

  Each window contains a marker used to keep track of a buffer position
that specifies where in the buffer display should start.  This position
is called the @dfn{display-start} position of the window (or just the
@dfn{start}).  The character after this position is the one that appears
at the upper left corner of the window.  It is usually, but not
inevitably, at the beginning of a text line.

@defun window-start &optional window
@cindex window top line
This function returns the display-start position of window
@var{window}.  If @var{window} is @code{nil}, the selected window is
used.  For example,

@example
@group
(window-start)
     @result{} 7058
@end group
@end example

When you create a window, or display a different buffer in it, the
display-start position is set to a display-start position recently used
for the same buffer, or 1 if the buffer doesn't have any.

Redisplay updates the window-start position (if you have not specified
it explicitly since the previous redisplay)---for example, to make sure
point appears on the screen.  Nothing except redisplay automatically
changes the window-start position; if you move point, do not expect the
window-start position to change in response until after the next
redisplay.

For a realistic example of using @code{window-start}, see the
description of @code{count-lines} in @ref{Text Lines}.
@end defun

@defun window-end &optional window update
This function returns the position of the end of the display in window
@var{window}.  If @var{window} is @code{nil}, the selected window is
used.

Simply changing the buffer text or moving point does not update the
value that @code{window-end} returns.  The value is updated only when
Emacs redisplays and redisplay completes without being preempted.

If the last redisplay of @var{window} was preempted, and did not finish,
Emacs does not know the position of the end of display in that window.
In that case, this function returns @code{nil}.

If @var{update} is non-@code{nil}, @code{window-end} always returns
an up-to-date value for where the window ends.  If the saved value is
valid, @code{window-end} returns that; otherwise it computes the correct
value by scanning the buffer text.
@end defun

@defun set-window-start window position &optional noforce
This function sets the display-start position of @var{window} to
@var{position} in @var{window}'s buffer.  It returns @var{position}.

The display routines insist that the position of point be visible when a
buffer is displayed.  Normally, they change the display-start position
(that is, scroll the window) whenever necessary to make point visible.
However, if you specify the start position with this function using
@code{nil} for @var{noforce}, it means you want display to start at
@var{position} even if that would put the location of point off the
screen.  If this does place point off screen, the display routines move
point to the left margin on the middle line in the window.

For example, if point @w{is 1} and you set the start of the window @w{to
2}, then point would be ``above'' the top of the window.  The display
routines will automatically move point if it is still 1 when redisplay
occurs.  Here is an example:

@example
@group
;; @r{Here is what @samp{foo} looks like before executing}
;;   @r{the @code{set-window-start} expression.}
@end group

@group
---------- Buffer: foo ----------
@point{}This is the contents of buffer foo.
2
3
4
5
6
---------- Buffer: foo ----------
@end group

@group
(set-window-start
 (selected-window)
 (1+ (window-start)))
@result{} 2
@end group

@group
;; @r{Here is what @samp{foo} looks like after executing}
;;   @r{the @code{set-window-start} expression.}
---------- Buffer: foo ----------
his is the contents of buffer foo.
2
3
@point{}4
5
6
---------- Buffer: foo ----------
@end group
@end example

If @var{noforce} is non-@code{nil}, and @var{position} would place point
off screen at the next redisplay, then redisplay computes a new window-start
position that works well with point, and thus @var{position} is not used.
@end defun

@defun pos-visible-in-window-p &optional position window
This function returns @code{t} if @var{position} is within the range
of text currently visible on the screen in @var{window}.  It returns
@code{nil} if @var{position} is scrolled vertically out of view.  The
argument @var{position} defaults to the current position of point;
@var{window}, to the selected window.  Here is an example:

@example
@group
(or (pos-visible-in-window-p
     (point) (selected-window))
    (recenter 0))
@end group
@end example

The @code{pos-visible-in-window-p} function considers only vertical
scrolling.  If @var{position} is out of view only because @var{window}
has been scrolled horizontally, @code{pos-visible-in-window-p} returns
@code{t} anyway.  @xref{Horizontal Scrolling}.
@end defun

@node Textual Scrolling
@section Textual Scrolling
@cindex textual scrolling
@cindex scrolling textually

  @dfn{Textual scrolling} means moving the text up or down though a
window.  It works by changing the value of the window's display-start
location.  It may also change the value of @code{window-point} to keep
point on the screen.

  Textual scrolling was formerly called ``vertical scrolling,'' but we
changed its name to distinguish it from the new vertical fractional
scrolling feature (@pxref{Vertical Scrolling}).

  In the commands @code{scroll-up} and @code{scroll-down}, the directions
``up'' and ``down'' refer to the motion of the text in the buffer at which
you are looking through the window.  Imagine that the text is
written on a long roll of paper and that the scrolling commands move the
paper up and down.  Thus, if you are looking at text in the middle of a
buffer and repeatedly call @code{scroll-down}, you will eventually see
the beginning of the buffer.

  Some people have urged that the opposite convention be used: they
imagine that the window moves over text that remains in place.  Then
``down'' commands would take you to the end of the buffer.  This view is
more consistent with the actual relationship between windows and the
text in the buffer, but it is less like what the user sees.  The
position of a window on the terminal does not move, and short scrolling
commands clearly move the text up or down on the screen.  We have chosen
names that fit the user's point of view.

  The textual scrolling functions (aside from
@code{scroll-other-window}) have unpredictable results if the current
buffer is different from the buffer that is displayed in the selected
window.  @xref{Current Buffer}.

@deffn Command scroll-up &optional count
This function scrolls the text in the selected window upward
@var{count} lines.  If @var{count} is negative, scrolling is actually
downward.

If @var{count} is @code{nil} (or omitted), then the length of scroll
is @code{next-screen-context-lines} lines less than the usable height of
the window (not counting its mode line).

@code{scroll-up} returns @code{nil}.
@end deffn

@deffn Command scroll-down &optional count
This function scrolls the text in the selected window downward
@var{count} lines.  If @var{count} is negative, scrolling is actually
upward.

If @var{count} is omitted or @code{nil}, then the length of the scroll
is @code{next-screen-context-lines} lines less than the usable height of
the window (not counting its mode line).

@code{scroll-down} returns @code{nil}.
@end deffn

@deffn Command scroll-other-window &optional count
This function scrolls the text in another window upward @var{count}
lines.  Negative values of @var{count}, or @code{nil}, are handled
as in @code{scroll-up}.

You can specify which buffer to scroll by setting the variable
@code{other-window-scroll-buffer} to a buffer.  If that buffer isn't
already displayed, @code{scroll-other-window} displays it in some
window.

When the selected window is the minibuffer, the next window is normally
the one at the top left corner.  You can specify a different window to
scroll, when the minibuffer is selected, by setting the variable
@code{minibuffer-scroll-window}.  This variable has no effect when any
other window is selected.  @xref{Minibuffer Misc}.

When the minibuffer is active, it is the next window if the selected
window is the one at the bottom right corner.  In this case,
@code{scroll-other-window} attempts to scroll the minibuffer.  If the
minibuffer contains just one line, it has nowhere to scroll to, so the
line reappears after the echo area momentarily displays the message
``Beginning of buffer''.
@end deffn

@c Emacs 19 feature
@defvar other-window-scroll-buffer
If this variable is non-@code{nil}, it tells @code{scroll-other-window}
which buffer to scroll.
@end defvar

@defopt scroll-margin
This option specifies the size of the scroll margin---a minimum number
of lines between point and the top or bottom of a window.  Whenever
point gets within this many lines of the top or bottom of the window,
the window scrolls automatically (if possible) to move point out of the
margin, closer to the center of the window.
@end defopt

@defopt scroll-conservatively
This variable controls how scrolling is done automatically when point
moves off the screen (or into the scroll margin).  If the value is zero,
then redisplay scrolls the text to center point vertically in the
window.  If the value is a positive integer @var{n}, then redisplay
scrolls the window up to @var{n} lines in either direction, if that will
bring point back into view.  Otherwise, it centers point.  The default
value is zero.

A value of @code{nil} is equivalent to .5, since it centers point.  This
variable automatically becomes buffer-local when set in any fashion.
@end defopt

@defopt scroll-up-aggressively
@tindex scroll-up-aggressively
The value of this variable should be either @code{nil} or a fraction
@var{f} between 0 and 1.  If it is a fraction, that specifies where on
the screen to put point when scrolling upward.  More precisely, when a
window scrolls up because point is above the window start, the new start
position is chosen to put point @var{f} part of the window height from
the top.  The larger @var{f}, the more aggressive the scrolling.

A value of @code{nil} is equivalent to .5, since its effect is to center
point.  This variable automatically becomes buffer-local when set in any
fashion.
@end defopt

@defopt scroll-down-aggressively
@tindex scroll-down-aggressively
Likewise, for scrolling down.  The value, @var{f}, specifies how far
point should be placed from the bottom of the window; thus, as with
@code{scroll-up-aggressively}, a larger value scrolls more aggressively.
@end defopt

@defopt scroll-step
This variable is an older variant of @code{scroll-conservatively}.  The
difference is that it if its value is @var{n}, that permits scrolling
only by precisely @var{n} lines, not a smaller number.  This feature
does not work with @code{scroll-margin}.  The default value is zero.
@end defopt

@defopt scroll-preserve-screen-position
If this option is non-@code{nil}, the scroll functions move point so
that the vertical position of the cursor is unchanged, when that is
possible.
@end defopt

@defopt next-screen-context-lines
The value of this variable is the number of lines of continuity to
retain when scrolling by full screens.  For example, @code{scroll-up}
with an argument of @code{nil} scrolls so that this many lines at the
bottom of the window appear instead at the top.  The default value is
@code{2}.
@end defopt

@deffn Command recenter &optional count
@cindex centering point
This function scrolls the selected window to put the text where point
is located at a specified vertical position within the window.

If @var{count} is a nonnegative number, it puts the line containing
point @var{count} lines down from the top of the window.  If @var{count}
is a negative number, then it counts upward from the bottom of the
window, so that @minus{}1 stands for the last usable line in the window.
If @var{count} is a non-@code{nil} list, then it stands for the line in
the middle of the window.

If @var{count} is @code{nil}, @code{recenter} puts the line containing
point in the middle of the window, then clears and redisplays the entire
selected frame.

When @code{recenter} is called interactively, @var{count} is the raw
prefix argument.  Thus, typing @kbd{C-u} as the prefix sets the
@var{count} to a non-@code{nil} list, while typing @kbd{C-u 4} sets
@var{count} to 4, which positions the current line four lines from the
top.

With an argument of zero, @code{recenter} positions the current line at
the top of the window.  This action is so handy that some people make a
separate key binding to do this.  For example,

@example
@group
(defun line-to-top-of-window ()
  "Scroll current line to top of window.
Replaces three keystroke sequence C-u 0 C-l."
  (interactive)
  (recenter 0))

(global-set-key [kp-multiply] 'line-to-top-of-window)
@end group
@end example
@end deffn

@node Vertical Scrolling
@section Vertical Fractional Scrolling
@cindex Vertical Fractional Scrolling

  @dfn{Vertical fractional scrolling} means shifting the image in the
window up or down by a specified multiple or fraction of a line.
Starting in Emacs 21, each window has a @dfn{vertical scroll position},
which is a number, never less than zero.  It specifies how far to raise
the contents of the window.  Raising the window contents generally makes
all or part of some lines disappear off the top, and all or part of some
other lines appear at the bottom.  The usual value is zero.

  The vertical scroll position is measured in units of the normal line
height, which is the height of the default font.  Thus, if the value is
.5, that means the window contents are scrolled up half the normal line
height.  If it is 3.3, that means the window contents are scrolled up
somewhat over three times the normal line height.

  What fraction of a line the vertical scrolling covers, or how many
lines, depends on what the lines contain.  A value of .5 could scroll a
line whose height is very short off the screen, while a value of 3.3
could scroll just part of the way through a tall line or an image.

@defun window-vscroll &optional window
This function returns the current vertical scroll position of
@var{window}, If @var{window} is @code{nil}, the selected window is
used.

@example
@group
(window-vscroll)
     @result{} 0
@end group
@end example
@end defun

@defun set-window-vscroll window lines
This function sets @var{window}'s vertical scroll position to
@var{lines}.  The argument @var{lines} should be zero or positive; if
not, it is taken as zero.

The actual vertical scroll position must always correspond
to an integral number of pixels, so the value you specify
is rounded accordingly.

The return value is the result of this rounding.

@example
@group
(set-window-vscroll (selected-window) 1.2)
     @result{} 1.13
@end group
@end example
@end defun

@node Horizontal Scrolling
@section Horizontal Scrolling
@cindex horizontal scrolling

  @dfn{Horizontal scrolling} means shifting the image in the window left
or right by a specified multiple of the normal character width.  Each
window has a @dfn{vertical scroll position}, which is a number, never
less than zero.  It specifies how far to shift the contents left.
Shifting the window contents left generally makes all or part of some
characters disappear off the left, and all or part of some other
characters appear at the right.  The usual value is zero.

  The horizontal scroll position is measured in units of the normal
character width, which is the width of space in the default font.  Thus,
if the value is 5, that means the window contents are scrolled left by 5
times the the normal character width.  How many characters actually
disappear off to the left depends on their width, and could vary from
line to line.

  Because we read from side to side in the ``inner loop'', and from top
to bottom in the ``outer loop'', the effect of horizontal scrolling is
not like that of textual or vertical scrolling.  Textual scrolling
involves selection of a portion of text to display, and vertical
scrolling moves the window contents contiguously; but horizontal
scrolling causes part of @emph{each line} to go off screen.

  Usually, no horizontal scrolling is in effect; then the leftmost
column is at the left edge of the window.  In this state, scrolling to
the right is meaningless, since there is no data to the left of the edge
to be revealed by it; so this is not allowed.  Scrolling to the left is
allowed; it scrolls the first columns of text off the edge of the window
and can reveal additional columns on the right that were truncated
before.  Once a window has a nonzero amount of leftward horizontal
scrolling, you can scroll it back to the right, but only so far as to
reduce the net horizontal scroll to zero.  There is no limit to how far
left you can scroll, but eventually all the text will disappear off the
left edge.

  In Emacs 21, redisplay automatically alters the horizontal scrolling
of a window as necessary to ensure that point is always visible.
However, you can still set the horizontal scrolling value explicitly.
The value you specify will be used, provided it leaves point visible.

@deffn Command scroll-left &optional count
This function scrolls the selected window @var{count} columns to the
left (or to the right if @var{count} is negative).  The default
for @var{count} is the window width, minus 2.

The return value is the total amount of leftward horizontal scrolling in
effect after the change---just like the value returned by
@code{window-hscroll} (below).
@end deffn

@deffn Command scroll-right &optional count
This function scrolls the selected window @var{count} columns to the
right (or to the left if @var{count} is negative).  The default
for @var{count} is the window width, minus 2.

The return value is the total amount of leftward horizontal scrolling in
effect after the change---just like the value returned by
@code{window-hscroll} (below).

Once you scroll a window as far right as it can go, back to its normal
position where the total leftward scrolling is zero, attempts to scroll
any farther right have no effect.
@end deffn

@defun window-hscroll &optional window
This function returns the total leftward horizontal scrolling of
@var{window}---the number of columns by which the text in @var{window}
is scrolled left past the left margin.

The value is never negative.  It is zero when no horizontal scrolling
has been done in @var{window} (which is usually the case).

If @var{window} is @code{nil}, the selected window is used.

@example
@group
(window-hscroll)
     @result{} 0
@end group
@group
(scroll-left 5)
     @result{} 5
@end group
@group
(window-hscroll)
     @result{} 5
@end group
@end example
@end defun

@defun set-window-hscroll window columns
This function sets the number of columns from the left margin that
@var{window} is scrolled from the value of @var{columns}.  The argument
@var{columns} should be zero or positive; if not, it is taken as zero.
Fractional values of @var{columns} are not supported at present.

The value returned is @var{columns}.

@example
@group
(set-window-hscroll (selected-window) 10)
     @result{} 10
@end group
@end example
@end defun

  Here is how you can determine whether a given position @var{position}
is off the screen due to horizontal scrolling:

@example
@group
(defun hscroll-on-screen (window position)
  (save-excursion
    (goto-char position)
    (and
     (>= (- (current-column) (window-hscroll window)) 0)
     (< (- (current-column) (window-hscroll window))
        (window-width window)))))
@end group
@end example

@node Size of Window
@section The Size of a Window
@cindex window size
@cindex size of window

  An Emacs window is rectangular, and its size information consists of
the height (the number of lines) and the width (the number of character
positions in each line).  The mode line is included in the height.  But
the width does not count the scroll bar or the column of @samp{|}
characters that separates side-by-side windows.

  The following three functions return size information about a window:

@defun window-height &optional window
This function returns the number of lines in @var{window}, including its
mode line.  If @var{window} fills its entire frame, this is typically
one less than the value of @code{frame-height} on that frame (since the
last line is always reserved for the minibuffer).

If @var{window} is @code{nil}, the function uses the selected window.

@example
@group
(window-height)
     @result{} 23
@end group
@group
(split-window-vertically)
     @result{} #<window 4 on windows.texi>
@end group
@group
(window-height)
     @result{} 11
@end group
@end example
@end defun

@defun window-width &optional window
This function returns the number of columns in @var{window}.  If
@var{window} fills its entire frame, this is the same as the value of
@code{frame-width} on that frame.  The width does not include the
window's scroll bar or the column of @samp{|} characters that separates
side-by-side windows.

If @var{window} is @code{nil}, the function uses the selected window.

@example
@group
(window-width)
     @result{} 80
@end group
@end example
@end defun

@defun window-edges &optional window
This function returns a list of the edge coordinates of @var{window}.
If @var{window} is @code{nil}, the selected window is used.

The order of the list is @code{(@var{left} @var{top} @var{right}
@var{bottom})}, all elements relative to 0, 0 at the top left corner of
the frame.  The element @var{right} of the value is one more than the
rightmost column used by @var{window}, and @var{bottom} is one more than
the bottommost row used by @var{window} and its mode-line.

If a window has a scroll bar, the right edge value includes the width of
the scroll bar.  Otherwise, if the window has a neighbor on the right,
its right edge value includes the width of the separator line between
the window and that neighbor.  Since the width of the window does not
include this separator, the width does not usually equal the difference
between the right and left edges.

Here is the result obtained on a typical 24-line terminal with just one
window:

@example
@group
(window-edges (selected-window))
     @result{} (0 0 80 23)
@end group
@end example

@noindent
The bottom edge is at line 23 because the last line is the echo area.

If @var{window} is at the upper left corner of its frame, then
@var{bottom} is the same as the value of @code{(window-height)},
@var{right} is almost the same as the value of @code{(window-width)},
and @var{top} and @var{left} are zero.  For example, the edges of the
following window are @w{@samp{0 0 8 5}}.  Assuming that the frame has
more than 8 columns, the last column of the window (column 7) holds a
border rather than text.  The last row (row 4) holds the mode line,
shown here with @samp{xxxxxxxxx}.

@example
@group
           0
           _______
        0 |       |
          |       |
          |       |
          |       |
          xxxxxxxxx  4

                  7
@end group
@end example

In the following example, let's suppose that the frame is 7
columns wide.  Then the edges of the left window are @w{@samp{0 0 4 3}}
and the edges of the right window are @w{@samp{4 0 8 3}}.

@example
@group
           ___ ___
          |   |   |
          |   |   |
          xxxxxxxxx

           0  34  7
@end group
@end example
@end defun

@node Resizing Windows
@section Changing the Size of a Window
@cindex window resizing
@cindex changing window size
@cindex window size, changing

  The window size functions fall into two classes: high-level commands
that change the size of windows and low-level functions that access
window size.  Emacs does not permit overlapping windows or gaps between
windows, so resizing one window affects other windows.

@deffn Command enlarge-window size &optional horizontal
This function makes the selected window @var{size} lines taller,
stealing lines from neighboring windows.  It takes the lines from one
window at a time until that window is used up, then takes from another.
If a window from which lines are stolen shrinks below
@code{window-min-height} lines, that window disappears.

If @var{horizontal} is non-@code{nil}, this function makes
@var{window} wider by @var{size} columns, stealing columns instead of
lines.  If a window from which columns are stolen shrinks below
@code{window-min-width} columns, that window disappears.

If the requested size would exceed that of the window's frame, then the
function makes the window occupy the entire height (or width) of the
frame.

If there are various other windows from which lines or columns can be
stolen, and some of them specify fixed size (using
@code{window-size-fixed}, see below), they are left untouched while
other windows are ``robbed.''  If it would be necessary to alter the
size of a fixed-size window, @code{enlarge-window} gets an error
instead.

If @var{size} is negative, this function shrinks the window by
@minus{}@var{size} lines or columns.  If that makes the window smaller
than the minimum size (@code{window-min-height} and
@code{window-min-width}), @code{enlarge-window} deletes the window.

@code{enlarge-window} returns @code{nil}.
@end deffn

@deffn Command enlarge-window-horizontally columns
This function makes the selected window @var{columns} wider.
It could be defined as follows:

@example
@group
(defun enlarge-window-horizontally (columns)
  (enlarge-window columns t))
@end group
@end example
@end deffn

@deffn Command shrink-window size &optional horizontal
This function is like @code{enlarge-window} but negates the argument
@var{size}, making the selected window smaller by giving lines (or
columns) to the other windows.  If the window shrinks below
@code{window-min-height} or @code{window-min-width}, then it disappears.

If @var{size} is negative, the window is enlarged by @minus{}@var{size}
lines or columns.
@end deffn

@deffn Command shrink-window-horizontally columns
This function makes the selected window @var{columns} narrower.
It could be defined as follows:

@example
@group
(defun shrink-window-horizontally (columns)
  (shrink-window columns t))
@end group
@end example
@end deffn

@deffn Command shrink-window-if-larger-than-buffer &optional window
This command shrinks @var{window} to be as small as possible while still
showing the full contents of its buffer---but not less than
@code{window-min-height} lines.  If @var{window} is not given,
it defaults to the selected window.

However, the command does nothing if the window is already too small to
display the whole text of the buffer, or if part of the contents are
currently scrolled off screen, or if the window is not the full width of
its frame, or if the window is the only window in its frame.
@end deffn

@tindex window-size-fixed
@defvar window-size-fixed
If this variable is non-@code{nil}, in any given buffer,
then the size of any window displaying the buffer remains fixed
unless you explicitly change it or Emacs has no other choice.
(This feature is new in Emacs 21.)

If the value is @code{height}, then only the window's height is fixed;
if the value is @code{width}, then only the window's width is fixed.
Any other non-@code{nil} value fixes both the width and the height.

The usual way to use this variable is to give it a buffer-local value in
a particular buffer.  That way, the windows (but usually there is only
one) displaying that buffer have fixed size.

Explicit size-change functions such as @code{enlarge-window}
get an error if they would have to change a window size which is fixed.
Therefore, when you want to change the size of such a window,
you should bind @code{window-size-fixed} to @code{nil}, like this:

@example
(let ((window-size-fixed nil))
   (enlarge-window 10))
@end example

Note that changing the frame size will change the size of a
fixed-size window, if there is no other alternative.
@end defvar

@cindex minimum window size
  The following two variables constrain the window-size-changing
functions to a minimum height and width.

@defopt window-min-height
The value of this variable determines how short a window may become
before it is automatically deleted.  Making a window smaller than
@code{window-min-height} automatically deletes it, and no window may be
created shorter than this.  The absolute minimum height is two (allowing
one line for the mode line, and one line for the buffer display).
Actions that change window sizes reset this variable to two if it is
less than two.  The default value is 4.
@end defopt

@defopt window-min-width
The value of this variable determines how narrow a window may become
before it is automatically deleted.  Making a window smaller than
@code{window-min-width} automatically deletes it, and no window may be
created narrower than this.  The absolute minimum width is one; any
value below that is ignored.  The default value is 10.
@end defopt

@node Coordinates and Windows
@section Coordinates and Windows

This section describes how to relate screen coordinates to windows.

@defun window-at x y &optional frame
This function returns the window containing the specified cursor
position in the frame @var{frame}.  The coordinates @var{x} and @var{y}
are measured in characters and count from the top left corner of the
frame.  If they are out of range, @code{window-at} returns @code{nil}.

If you omit @var{frame}, the selected frame is used.
@end defun

@defun coordinates-in-window-p coordinates window
This function checks whether a particular frame position falls within
the window @var{window}.

The argument @var{coordinates} is a cons cell of the form @code{(@var{x}
. @var{y})}.  The coordinates @var{x} and @var{y} are measured in
characters, and count from the top left corner of the screen or frame.

The value returned by @code{coordinates-in-window-p} is non-@code{nil}
if the coordinates are inside @var{window}.  The value also indicates
what part of the window the position is in, as follows:

@table @code
@item (@var{relx} . @var{rely})
The coordinates are inside @var{window}.  The numbers @var{relx} and
@var{rely} are the equivalent window-relative coordinates for the
specified position, counting from 0 at the top left corner of the
window.

@item mode-line
The coordinates are in the mode line of @var{window}.

@item header-line
The coordinates are in the header line of @var{window}.

@item vertical-line
The coordinates are in the vertical line between @var{window} and its
neighbor to the right.  This value occurs only if the window doesn't
have a scroll bar; positions in a scroll bar are considered outside the
window for these purposes.

@item nil
The coordinates are not in any part of @var{window}.
@end table

The function @code{coordinates-in-window-p} does not require a frame as
argument because it always uses the frame that @var{window} is on.
@end defun

@node Window Configurations
@section Window Configurations
@cindex window configurations
@cindex saving window information

  A @dfn{window configuration} records the entire layout of one
frame---all windows, their sizes, which buffers they contain, what part
of each buffer is displayed, and the values of point and the mark.  You
can bring back an entire previous layout by restoring a window
configuration previously saved.

  If you want to record all frames instead of just one, use a frame
configuration instead of a window configuration.  @xref{Frame
Configurations}.

@defun current-window-configuration &optional frame
This function returns a new object representing @var{frame}'s
current window configuration, including the number of windows, their
sizes and current buffers, which window is the selected window, and for
each window the displayed buffer, the display-start position, and the
positions of point and the mark.  It also includes the values of
@code{window-min-height}, @code{window-min-width} and
@code{minibuffer-scroll-window}.  An exception is made for point in the
current buffer, whose value is not saved.

If @var{frame} is omitted, the selected frame is used.
@end defun

@defun set-window-configuration configuration
This function restores the configuration of windows and buffers as
specified by @var{configuration}, for the frame that @var{configuration}
was created for.

The argument @var{configuration} must be a value that was previously
returned by @code{current-window-configuration}.  This configuration is
restored in the frame from which @var{configuration} was made, whether
that frame is selected or not.  This always counts as a window size
change and triggers execution of the @code{window-size-change-functions}
(@pxref{Window Hooks}), because @code{set-window-configuration} doesn't
know how to tell whether the new configuration actually differs from the
old one.

If the frame which @var{configuration} was saved from is dead, all this
function does is restore the three variables @code{window-min-height},
@code{window-min-width} and @code{minibuffer-scroll-window}.

Here is a way of using this function to get the same effect
as @code{save-window-excursion}:

@example
@group
(let ((config (current-window-configuration)))
  (unwind-protect
      (progn (split-window-vertically nil)
             @dots{})
    (set-window-configuration config)))
@end group
@end example
@end defun

@defspec save-window-excursion forms@dots{}
This special form records the window configuration, executes @var{forms}
in sequence, then restores the earlier window configuration.  The window
configuration includes the value of point and the portion of the buffer
that is visible.  It also includes the choice of selected window.
However, it does not include the value of point in the current buffer;
use @code{save-excursion} also, if you wish to preserve that.

Don't use this construct when @code{save-selected-window} is all you need.

Exit from @code{save-window-excursion} always triggers execution of the
@code{window-size-change-functions}.  (It doesn't know how to tell
whether the restored configuration actually differs from the one in
effect at the end of the @var{forms}.)

The return value is the value of the final form in @var{forms}.
For example:

@example
@group
(split-window)
     @result{} #<window 25 on control.texi>
@end group
@group
(setq w (selected-window))
     @result{} #<window 19 on control.texi>
@end group
@group
(save-window-excursion
  (delete-other-windows w)
  (switch-to-buffer "foo")
  'do-something)
     @result{} do-something
     ;; @r{The screen is now split again.}
@end group
@end example
@end defspec

@defun window-configuration-p object
This function returns @code{t} if @var{object} is a window configuration.
@end defun

@defun compare-window-configurations config1 config2
This function compares two window configurations as regards the
structure of windows, but ignores the values of point and mark and the
saved scrolling positions---it can return @code{t} even if those
aspects differ.

The function @code{equal} can also compare two window configurations; it
regards configurations as unequal if they differ in any respect, even a
saved point or mark.
@end defun

  Primitives to look inside of window configurations would make sense,
but none are implemented.  It is not clear they are useful enough to be
worth implementing.

@node Window Hooks
@section Hooks for Window Scrolling and Changes

This section describes how a Lisp program can take action whenever a
window displays a different part of its buffer or a different buffer.
There are three actions that can change this: scrolling the window,
switching buffers in the window, and changing the size of the window.
The first two actions run @code{window-scroll-functions}; the last runs
@code{window-size-change-functions}.  The paradigmatic use of these
hooks is in the implementation of Lazy Lock mode; see @ref{Support
Modes, Lazy Lock, Font Lock Support Modes, emacs, The GNU Emacs Manual}.

@defvar window-scroll-functions
This variable holds a list of functions that Emacs should call before
redisplaying a window with scrolling.  It is not a normal hook, because
each function is called with two arguments: the window, and its new
display-start position.

Displaying a different buffer in the window also runs these functions.

These functions must be careful in using @code{window-end}
(@pxref{Window Start}); if you need an up-to-date value, you must use
the @var{update} argument to ensure you get it.
@end defvar

@defvar window-size-change-functions
This variable holds a list of functions to be called if the size of any
window changes for any reason.  The functions are called just once per
redisplay, and just once for each frame on which size changes have
occurred.

Each function receives the frame as its sole argument.  There is no
direct way to find out which windows on that frame have changed size, or
precisely how.  However, if a size-change function records, at each
call, the existing windows and their sizes, it can also compare the
present sizes and the previous sizes.

Creating or deleting windows counts as a size change, and therefore
causes these functions to be called.  Changing the frame size also
counts, because it changes the sizes of the existing windows.

It is not a good idea to use @code{save-window-excursion} (@pxref{Window
Configurations}) in these functions, because that always counts as a
size change, and it would cause these functions to be called over and
over.  In most cases, @code{save-selected-window} (@pxref{Selecting
Windows}) is what you need here.
@end defvar

@defvar redisplay-end-trigger-functions
This abnormal hook is run whenever redisplay in a window uses text that
extends past a specified end trigger position.  You set the end trigger
position with the function @code{set-window-redisplay-end-trigger}.  The
functions are called with two arguments: the window, and the end trigger
position.  Storing @code{nil} for the end trigger position turns off the
feature, and the trigger value is automatically reset to @code{nil} just
after the hook is run.
@end defvar

@defun set-window-redisplay-end-trigger window position
This function sets @var{window}'s end trigger position at
@var{position}.
@end defun

@defun window-redisplay-end-trigger &optional window
This function returns @var{window}'s current end trigger position.
@end defun

@defvar window-configuration-change-hook
A normal hook that is run every time you change the window configuration
of an existing frame.  This includes splitting or deleting windows,
changing the sizes of windows, or displaying a different buffer in a
window.  The frame whose window configuration has changed is the
selected frame when this hook runs.
@end defvar