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@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2001,
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@c   2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010
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@c   Free Software Foundation, Inc.
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@c See the file elisp.texi for copying conditions.
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@setfilename ../../info/windows
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@node Windows, Frames, Buffers, Top
@chapter Windows

  This chapter describes most of the functions and variables related to
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Emacs windows.  @xref{Frames and Windows}, for how windows relate to
frames.  @xref{Display}, for information on how text is displayed in
windows.
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@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-level functions for displaying a buffer
                              and choosing a window for it.
* Choosing Window::	    How to choose a window for displaying a buffer.
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* Dedicated Windows::	    How to avoid displaying another buffer in
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                              a specific window.
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* Window Point::            Each window has its own location of point.
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* Window Start and End::    Buffer positions indicating which text is
                              on-screen in a window.
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* 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 Tree::             The layout and sizes of all windows in a frame.
* Window Configurations::   Saving and restoring the state of the screen.
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* Window Parameters::       Associating additional information with windows.
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* 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.

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  Emacs groups windows into frames; see @ref{Frames}.  A frame
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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.
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  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, but the other windows have ``non-selected'' cursors, normally
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less visible.  (@xref{Cursor Parameters}, for customizing this.)  At
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any time, one frame is the selected frame; and the window selected
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within that frame is @dfn{the selected window}.  The selected window's
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buffer is usually the current buffer (except when @code{set-buffer} has
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been used); see @ref{Current Buffer}.
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  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
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to it} from other Lisp objects; see @ref{Deleting Windows}.  Restoring a
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saved window configuration is the only way for a window no longer on the
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screen to come back to life; see @ref{Window Configurations}.
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@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
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which Emacs creates new windows (@pxref{Splitting Windows}) and resizes
them (@pxref{Resizing Windows}), not all conceivable tilings of windows
on an Emacs frame are actually possible.
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@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

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The functions described below are the primitives used to split a window
into two windows.  They do not accept a buffer as an argument.  Rather,
the two ``halves'' of the split window initially display the same buffer
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previously visible in the window that was split.

@deffn Command split-window &optional window size horizontal
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This function splits a new window out of @var{window}'s screen area.  It
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returns the new window.  The default for @var{window} is the selected
window.  When you split the selected window, it remains selected.
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If @var{horizontal} is non-@code{nil}, then @var{window} splits into two
side by side windows.  The original window keeps the leftmost @var{size}
columns, and gives the rest of the columns to the new window.
Otherwise, @var{window} splits into windows one above the other, the
original window keeps the upper @var{size} lines and gives the rest of
the lines to the new window.  The original window @var{window} is
therefore the left-hand or upper of the two, and the new window is the
right-hand or lower.
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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}.

If splitting would result in making a window that is smaller than
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@code{window-min-height} or @code{window-min-width} (@pxref{Resizing
Windows}), @code{split-window} signals an error and does not split the
window at all.
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The following example starts with one window on a screen that is 50
lines high by 80 columns wide; then it splits the window.

@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, split the top window 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
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with a scroll bar (@pxref{Scroll Bars}), or with @samp{|} characters.  The
display table can specify alternative border characters; see @ref{Display
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Tables}.
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@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
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gets @minus{}@var{size} lines and the upper window gets the rest, but
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the upper window is still the one selected.)  However, if
@code{split-window-keep-point} (see below) is @code{nil}, then either
window can be selected.

In other respects, this function is similar to @code{split-window}.
In particular, the upper window is the original one and the return
value is the new, lower window.
@end deffn

@defopt split-window-keep-point
If this variable is non-@code{nil} (the default), then
@code{split-window-vertically} behaves as described above.

If it is @code{nil}, then @code{split-window-vertically} adjusts point
in each of the two windows to avoid scrolling.  (This is useful on
slow terminals.)  It selects whichever window contains the screen line
that point was previously on.

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This variable affects the behavior of @code{split-window-vertically}
only.  It has no effect on the other functions described here.
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@end defopt

@deffn Command split-window-horizontally &optional size
This function splits the selected window into two windows
side-by-side, leaving the selected window on the left with @var{size}
columns.  If @var{size} is negative, the rightmost window gets
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@minus{}@var{size} columns, but the leftmost window still remains
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selected.

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)
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         (setq size (+ (window-width) size)))
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    (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
counted when 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}
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.

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  When you delete a window, the space it took up is given to one of
its sibling windows adjacent to it.
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@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
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This function removes @var{window} from display and returns @code{nil}.
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The default for @var{window} is the selected window.  An error is
signaled if @var{window} is the only window on its frame.
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@end deffn

@deffn Command delete-other-windows &optional window
This function makes @var{window} the only window on its frame, by
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deleting the other windows in that frame.  The default for @var{window}
is the selected window.  The return value is @code{nil}.
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@end deffn

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@deffn Command delete-windows-on &optional buffer-or-name frame
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This function deletes all windows showing @var{buffer-or-name}.  If
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there are no windows showing @var{buffer-or-name}, it does nothing.  The
optional argument @var{buffer-or-name} may be a buffer or the name of an
existing buffer and defaults to the current buffer.
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@code{delete-windows-on} operates frame by frame.  If a frame has
several windows showing different buffers, then those showing
@var{buffer-or-name} are removed, and the others expand to fill the
space.  If all windows in some frame are showing @var{buffer-or-name}
(including the case where there is only one window), then the frame
winds up with a single window showing another buffer chosen with
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@code{other-buffer} (@pxref{The Buffer List}).  If, however, the window
showing @var{buffer-or-name} is dedicated to its buffer
(@pxref{Dedicated Windows}), and there are other frames left, that
window's frame is deleted.
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The optional argument @var{frame} specifies 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:
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@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 a window

  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 &optional norecord
This function makes @var{window} the selected window.  The cursor then
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appears in @var{window} (after redisplay).  Unless @var{window} was
already selected, @code{select-window} makes @var{window}'s buffer the
current buffer.  The return value is @var{window}.
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Normally, @var{window}'s selected buffer is moved to the front of the
buffer list (@pxref{The Buffer List}) and @var{window} becomes the most
recently selected window.  But if @var{norecord} is non-@code{nil}, the
buffer list remains unchanged and @var{window} does not become the most
recently selected one.
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@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 frame, as well as the selected window
of each frame, executes @var{forms} in sequence, then restores the
earlier selected frame and windows.  It also saves and restores the
current buffer.  It returns the value of the last form in @var{forms}.

This macro does not save or restore anything about the sizes,
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arrangement or contents of windows; therefore, if @var{forms} change
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them, the change persists.  If the previously selected window of some
frame is no longer live at the time of exit from @var{forms}, that
frame's selected window is left alone.  If the previously selected
window is no longer live, then whatever window is selected at the end of
@var{forms} remains selected.  The current buffer is restored if and
only if it is still live when exiting @var{forms}.

This macro changes neither the ordering of recently selected windows nor
the buffer list.
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@end defmac

@defmac with-selected-window window forms@dots{}
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This macro selects @var{window}, executes @var{forms} in sequence, then
restores the previously selected window and current buffer.  The ordering
of recently selected windows and the buffer list remain unchanged unless
you deliberately change them within @var{forms}, for example, by calling
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@code{select-window} with argument @var{norecord} @code{nil}.
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@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 dedicated
This function returns the window least recently ``used'' (that is,
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selected) among a set of candidate windows.  If any full-width windows
are present, it only considers these.
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The selected window is returned if it is the only candidate.  A
minibuffer window is never a candidate.  A dedicated window
(@pxref{Dedicated Windows}) is never a candidate unless the optional
argument @var{dedicated} is non-@code{nil}.
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The optional argument @var{frame} specifies which windows are
considered.
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@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 dedicated
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
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with the most lines.  A minibuffer window is never a candidate.  A
dedicated window (@pxref{Dedicated Windows}) is never a candidate unless
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the optional argument @var{dedicated} is non-@code{nil}.
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If there are two candidate windows of the same size, this function
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prefers the one that comes first in the cyclic ordering of windows,
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starting from the selected window (@pxref{Cyclic Window Ordering}).
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The optional argument @var{frame} specifies which set of windows to
consider, see @code{get-lru-window} above.
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@end defun

@cindex window that satisfies a predicate
@cindex conditional selection of windows
@defun get-window-with-predicate predicate &optional minibuf all-frames default
This function returns a window satisfying @var{predicate}.  It cycles
through all visible windows using @code{walk-windows} (@pxref{Cyclic
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Window Ordering}), calling @var{predicate} on each one of them with that
window as its argument.  The function returns the first window for which
@var{predicate} returns a non-@code{nil} value; if that never happens,
it returns @var{default} (which defaults to @code{nil}).
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The optional arguments @var{minibuf} and @var{all-frames} specify the
set of windows to include in the scan.  See the description of
@code{next-window} in @ref{Cyclic Window Ordering}, for details.
@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
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some other window, it moves through the windows on the screen in a
specific order.  For any given configuration of windows, this order
never varies.  It is called the @dfn{cyclic ordering of windows}.
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  For a particular frame, 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 windows were split.
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  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.
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In general, within each set of siblings at any level in the window tree
(@pxref{Window Tree}), the order is left to right, or top to bottom.
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@defun next-window &optional window minibuf all-frames
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@cindex minibuffer window, and @code{next-window}
This function returns the window following @var{window} in the cyclic
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ordering of windows.  This is the window @kbd{C-x o} selects if typed
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when @var{window} is selected.  The default for @var{window} is the
selected window.
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The value of the optional argument @var{minibuf} specifies whether the
minibuffer is included in the window order.  Normally, when
@var{minibuf} is @code{nil}, the minibuffer is included only if it is
currently ``active''; this matches the behavior of @kbd{C-x o}.  (The
minibuffer window is active while the minibuffer is in use; see
@ref{Minibuffers}.)
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If @var{minibuf} is @code{t}, the cyclic ordering includes the
minibuffer window even if it is not active.  If @var{minibuf} is neither
@code{t} nor @code{nil}, the minibuffer window is not included even if
it is active.
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The optional argument @var{all-frames} specifies which frames to
consider.  Here are the possible values and their meanings:
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@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.  If the
minibuffer counts (as determined by @var{minibuf}), then all windows on
all frames that share that minibuffer count too.

@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 a frame
Consider all windows on that frame.

@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
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@end defun
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@defun previous-window &optional window minibuf all-frames
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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}.
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@end defun
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@deffn Command other-window count &optional all-frames
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This function selects another window in the cyclic ordering of windows.
@var{count} specifies the number of windows to skip in the ordering,
starting with the selected window, before making the selection.  If
@var{count} is a positive number, it skips @var{count} windows forwards.
@var{count} negative means skip @minus{}@var{count} windows backwards.
If @var{count} is zero, it does not skip any window, thus re-selecting
the selected window.  In an interactive call, @var{count} is the numeric
prefix argument.
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The optional argument @var{all-frames} has the same meaning as in
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@code{next-window}, but the @var{minibuf} argument of @code{next-window}
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is always effectively @code{nil}.  This function returns @code{nil}.
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@end deffn

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

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

@defun window-list &optional frame minibuf window
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This function returns a list of all windows on @var{frame}, starting
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with @var{window}.  The default for @var{frame} is the selected frame;
the default for @var{window} is the selected window.
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The value of @var{minibuf} specifies if the minibuffer window shall be
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included in the result list.  If @var{minibuf} is @code{t}, the result
always includes the minibuffer window.  If @var{minibuf} is @code{nil}
or omitted, that includes the minibuffer window if it is active.  If
@var{minibuf} is neither @code{nil} nor @code{t}, the result never
includes the minibuffer window.
@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.
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The functions described there are easier to use, but they employ
heuristics in choosing or creating a window; use the functions described
here when you need complete control.
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@defun set-window-buffer window buffer-or-name &optional keep-margins
This function makes @var{window} display @var{buffer-or-name} as its
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contents.  It returns @code{nil}.  The default for @var{window} is the
selected window.  The argument @var{buffer-or-name} must specify a
buffer or the name of an existing buffer.

@code{set-window-buffer} is the fundamental primitive for changing which
buffer is displayed in a window, and all ways of doing that call this
function.
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@example
@group
(set-window-buffer (selected-window) "foo")
     @result{} nil
@end group
@end example

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Normally, displaying @var{buffer-or-name} in @var{window} resets the
window's position, display margins, fringe widths, and scroll bar
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settings based on the local variables of that buffer.
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However, if @var{keep-margins} is non-@code{nil}, display margins and
fringe widths of @var{window} remain unchanged.  @xref{Fringes}.

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@code{set-window-buffer} signals an error when @var{window} is
@dfn{strongly} dedicated to its buffer (@pxref{Dedicated Windows}) and
does not already display @var{buffer-or-name}.
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Note that this function runs @code{window-scroll-functions} before
running @code{window-configuration-change-hook}.
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@end defun

@defvar buffer-display-count
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This buffer-local variable records the number of times a buffer has been
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displayed in a window.  It is incremented each time
@code{set-window-buffer} is called for the buffer.
@end defvar

@defun window-buffer &optional window
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This function returns the buffer that @var{window} is displaying.  The
default for @var{window} is the selected window.
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@example
@group
(window-buffer)
     @result{} #<buffer windows.texi>
@end group
@end example
@end defun

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@defun get-buffer-window &optional buffer-or-name all-frames
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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}.

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The argument @var{BUFFER-OR-NAME} may be a buffer or a buffer name and
defaults to the current buffer.  The optional argument @var{all-frames}
specifies which windows to consider:
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@itemize @bullet
@item
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@code{nil} means consider windows on the selected frame.
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@item
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@code{t} means consider windows on all existing frames.
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@item
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@code{visible} means consider windows on all visible frames.
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@item
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0 means consider windows on all visible or iconified frames.
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@item
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A frame means consider windows on that frame only.
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@end itemize
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Observe that the behavior of @code{get-buffer-window} may differ from
that of @code{next-window} (@pxref{Cyclic Window Ordering}) when
@var{all-frames} equals @code{nil} or any value not listed here.
Perhaps we will change @code{get-buffer-window} in the future to make it
compatible with the other functions.
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@end defun

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@defun get-buffer-window-list &optional buffer-or-name minibuf all-frames
This function returns a list of all windows currently displaying
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@var{buffer-or-name}.  The argument @var{buffer-or-name} may be a buffer
or the name of an existing buffer and defaults to the current buffer.
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The two remaining arguments work like the same-named arguments of
@code{next-window}; they are @emph{not} like the optional arguments of
@code{get-buffer-window}.
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@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
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low-level primitives that give you more precise control.  All of these
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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}
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the current buffer but does not display it in the selected window;
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see @ref{Current Buffer}.
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If @var{buffer-or-name} is @code{nil}, @code{switch-to-buffer} chooses a
buffer using @code{other-buffer}.  If @var{buffer-or-name} is a string
that 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
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the variable @code{major-mode}; see @ref{Auto Major Mode}.
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When the selected window is the minibuffer window or is strongly
dedicated to its buffer (@pxref{Dedicated Windows}), this function calls
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@code{pop-to-buffer} (see below) to display the buffer in some other
window.

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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
returns the buffer that it switched to.
@end deffn

The next two functions are similar to @code{switch-to-buffer}, except
for the described features.

@deffn Command switch-to-buffer-other-window buffer-or-name &optional norecord
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This function makes the buffer specified by @var{buffer-or-name} current
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and displays it in a window not currently selected, using the function
@code{pop-to-buffer} (see below).
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The currently selected window is absolutely never used to do the job.
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If the selected window already displays @var{buffer-or-name}, then it
continues to do so, but another window is nonetheless found to display
it in as well.
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This function updates the buffer list just like @code{switch-to-buffer}
unless @var{norecord} is non-@code{nil}.
@end deffn

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@deffn Command pop-to-buffer buffer-or-name &optional other-window norecord
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This command makes @var{buffer-or-name} the current buffer and switches
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to it in some window, preferably not the window previously selected.
The ``popped-to'' window becomes the selected window.  Its frame is
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given the X server's focus, if possible; see @ref{Input Focus}.  The
return value is the buffer that was switched to.
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If @var{buffer-or-name} is @code{nil}, that means to choose some other
buffer, but you don't specify which.  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
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variable @code{major-mode}.  @xref{Auto Major Mode}.
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If either of the variables @code{display-buffer-reuse-frames} or
@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 selects and returns that window.  If no such window exists and
@code{pop-up-frames} is non-@code{nil}, it creates a new frame and
displays the buffer in it.  Otherwise, @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.)
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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.
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For details, see @ref{Choosing Window}.
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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
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@code{nil}, then the selected window is considered sufficient for
displaying @var{buffer-or-name}, so that nothing needs to be done.
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All the variables that affect @code{display-buffer} affect
@code{pop-to-buffer} as well.  @xref{Choosing Window}.

This function updates the buffer list just like @code{switch-to-buffer}
unless @var{norecord} is non-@code{nil}.
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@end deffn
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@deffn Command replace-buffer-in-windows &optional buffer-or-name
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This function replaces @var{buffer-or-name} in all windows displaying
it with some other buffer.  It uses @code{other-buffer} to choose the
other buffer.  In the usual applications of this function, you
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don't care which other buffer is used; you just want to make sure that
@var{buffer-or-name} is no longer displayed.

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The argument @var{buffer-or-name} may be a buffer or the name of an
existing buffer and defaults to the current buffer.
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If a window displaying @var{buffer-or-name} is dedicated
(@pxref{Dedicated Windows}), and is not the only window on its frame,
that window is deleted.  If that window is the only window on its frame
and there are other frames left, the window's frame is deleted too.  If
there are no other frames left, some other buffer is displayed in that
window.
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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
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display a buffer in---@code{display-buffer}.  Higher-level functions and
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commands, like @code{switch-to-buffer} and @code{pop-to-buffer}, use this
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subroutine.  Here we describe how to use @code{display-buffer} and how
to customize it.
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@deffn Command display-buffer buffer-or-name &optional not-this-window frame
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This command makes @var{buffer-or-name} appear in some window, but it
does not select that window and does not make the buffer specified by
@var{buffer-or-name} current.  The identity of the selected window is
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unaltered by this function.  The argument @var{buffer-or-name} must be a
buffer or the name of an existing buffer.
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@var{not-this-window} non-@code{nil} means to display the specified
buffer in a window other than the selected one, even if it is already
displayed 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.
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@code{display-buffer} returns the window chosen to display
@var{buffer-or-name}.

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If the optional 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}:
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@itemize @bullet
@item
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@code{nil} means consider windows on the selected frame.
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(Actually, the last non-minibuffer frame.)
@item
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@code{t} means consider windows on all frames.
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@item
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@code{visible} means consider windows on all visible frames.
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@item
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0 means consider windows on all visible or iconified frames.
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@item
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A frame means consider windows on that frame only.
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@end itemize

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

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@defopt display-buffer-reuse-frames
If this variable is non-@code{nil}, @code{display-buffer} searches
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visible and iconified frames for a window displaying
@var{buffer-or-name}.  If there is such a window, @code{display-buffer}
makes that window's frame visible and raises it if necessary, and
returns the window.  If there is no such window or
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@code{display-buffer-reuse-frames} is @code{nil}, the behavior of
@code{display-buffer} is determined by the variables described next.
@end defopt

@defopt pop-up-windows
This variable specifies whether @code{display-buffer} is allowed to
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split (@pxref{Splitting Windows}) an existing window.  If this variable
is non-@code{nil}, @code{display-buffer} tries to split the largest or
least recently used window on the selected frame.  (If the selected
frame is a minibuffer-only frame, @code{display-buffer} tries to split a
window on another frame instead.)  If this variable is @code{nil} or the
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variable @code{pop-up-frames} (see below) is non-@code{nil},
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@code{display-buffer} does not split any window.
@end defopt

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@defopt split-window-preferred-function
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This variable must specify a function with one argument, which is a
window.  The @code{display-buffer} routines will call this function with
one or more candidate windows when they look for a window to split.  The
function is expected to split that window and return the new window.  If
the function returns @code{nil}, this means that the argument window
cannot (or shall not) be split.

The default value of @code{split-window-preferred-function} is the
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function @code{split-window-sensibly} described below.  If you
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customize this option, bear in mind that the @code{display-buffer}
routines may call your function up to two times when trying to split a
window.  The argument of the first call is the largest window on the
chosen frame (as returned by @code{get-largest-window}).  If that call
fails to return a live window, your function is called a second time
with the least recently used window on that frame (as returned by
@code{get-lru-window}).

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The function specified by this option may try to split any other window
instead of the argument window.  Note that the window selected at the
time @code{display-buffer} was invoked is still selected when your
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function is called.  Hence, you can split the selected window (instead
of the largest or least recently used one) by simply ignoring the window
argument in the body of your function.  You can even choose to not split
any window as long as the return value of your function specifies a live
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window or @code{nil}, but you are not encouraged to do so
unconditionally.  If you want @code{display-buffer} to never split any
windows, set @code{pop-up-windows} to @code{nil}.
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@end defopt
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@defun split-window-sensibly window
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This function takes a window as argument and tries to split that window
in a suitable way.  The two variables described next are useful for
tuning the behavior of this function.
@end defun

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@defopt split-height-threshold
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This variable specifies whether @code{split-window-sensibly} may split
windows vertically.  If it is an integer, @code{split-window-sensibly}
tries to vertically split a window only if it has at least this many
lines.  If the window has less lines, splitting fails, or the value of
this variable is @code{nil}, @code{split-window-sensibly} will try to
split the window horizontally, subject to restrictions of
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@code{split-width-threshold} (see below).  If splitting horizontally
fails too and the window is the only window on its frame,
@code{split-window-sensibly} will try to split the window vertically
disregarding the value of @code{split-height-threshold}.  If this fails
as well, @code{split-window-sensibly} returns @code{nil}.

@code{split-window-sensibly} does not split vertically a window whose
height is fixed (@pxref{Resizing Windows}).  Also, it vertically splits
a window only if the space taken up by that window can accommodate two
windows one above the other that are both at least
@code{window-min-height} lines tall.  Moreover, if the window that shall
be split has a mode line, @code{split-window-sensibly} does not split
the window unless the new window can accomodate a mode line too.
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@end defopt

@defopt split-width-threshold
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This variable specifies whether @code{split-window-sensibly} may split
windows horizontally.  If it is an integer, @code{split-window-sensibly}
tries to horizontally split a window only if it has at least this many
columns.  If it is @code{nil}, @code{split-window-sensibly} will not
split the window horizontally.  (It still might split the window
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vertically, though, see above.)

@code{split-window-sensibly} does not split horizontally a window if
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that window's width is fixed (@pxref{Resizing Windows}).  Also, it
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horizontally splits a window only if the space that window takes up can
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accommodate two windows side by side that are both at least
@code{window-min-width} columns wide.
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@end defopt

@defopt even-window-heights
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This variable specifies whether @code{display-buffer} should even out
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window heights if the buffer gets displayed in an existing window, above
or beneath another window.  If @code{even-window-heights} is
non-@code{nil}, the default, window heights will be evened out.  If
either of the involved window has fixed height (@pxref{Resizing
Windows}) or @code{even-window-heights} is @code{nil}, the original
window heights will be left alone.
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@end defopt

@c Emacs 19 feature
@defopt pop-up-frames
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This variable specifies whether @code{display-buffer} should make new
frames.  If it is non-@code{nil}, @code{display-buffer} looks for a
window already displaying @var{buffer-or-name} on any visible or
iconified frame.  If it finds such a window, it makes that window's
frame visible and raises it if necessary, and returns the window.
Otherwise it makes a new frame, unless the variable's value is
@code{graphic-only} and the selected frame is not on a graphic display.
@xref{Frames}, for more information.
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Note that the value of @code{pop-up-windows} does 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.
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@end defopt

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

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

@defopt pop-up-frame-alist
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This variable holds an alist specifying frame parameters used by the
default value of @code{pop-up-frame-function} for making new frames.
@xref{Frame Parameters}, for more information about frame parameters.
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@end defopt

@defopt special-display-buffer-names
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A list of buffer names identifying buffers that should be displayed
specially.  If the name of @var{buffer-or-name} is in this list,
@code{display-buffer} handles the buffer specially.  By default, special
display means to give the buffer a dedicated frame.
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If an element is a list, instead of a string, then the @sc{car} of that
list is the buffer name, and the rest of that list says how to create
the frame.  There are two possibilities for the rest of that list (its
@sc{cdr}): It can be an alist, specifying frame parameters, or it can
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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.)

For example:

@example
(("myfile" (minibuffer) (menu-bar-lines . 0)))
@end example

@noindent
specifies to display a buffer named @samp{myfile} in a dedicated frame
with specified @code{minibuffer} and @code{menu-bar-lines} parameters.

The list of frame parameters can also use the phony frame parameters
@code{same-frame} and @code{same-window}.  If the specified frame
parameters include @code{(same-window . @var{value})} and @var{value}
is non-@code{nil}, that means to display the buffer in the current
selected window.  Otherwise, if they include @code{(same-frame .
@var{value})} and @var{value} is non-@code{nil}, that means to display
the buffer in a new window in the currently selected frame.
@end defopt

@defopt special-display-regexps
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A list of regular expressions specifying buffers that should be
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displayed specially.  If the buffer's name matches any of the regular
expressions in this list, @code{display-buffer} handles the buffer
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specially.  By default, special display means to give the buffer a
dedicated frame.
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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
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create the frame.  See @code{special-display-buffer-names} above.
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@end defopt

@defun special-display-p buffer-name
This function returns non-@code{nil} if displaying a buffer
named @var{buffer-name} with @code{display-buffer} would
create a special frame.  The value is @code{t} if it would
use the default frame parameters, or else the specified list
of frame parameters.
@end defun

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@defopt special-display-function
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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
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which it is displayed.  The default value of this variable is
@code{special-display-popup-frame}, see below.
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@end defopt
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@defun special-display-popup-frame buffer &optional args
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This function tries to make @var{buffer} visible in a frame of its own.
If @var{buffer} is already displayed in some window, it makes that
window's frame visible and raises it.  Otherwise, it creates a frame
that is dedicated to @var{buffer}.  The return value is the window used
to display @var{buffer}.
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If @var{args} is an alist, it specifies frame parameters for the new
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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.
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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
@anchor{Definition of 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

@defun same-window-p buffer-name
This function returns @code{t} if displaying a buffer
named @var{buffer-name} with @code{display-buffer} would
put it in the selected window.
@end defun

@c Emacs 19 feature
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@defopt display-buffer-function
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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 first two arguments that @code{display-buffer}
received.  It should choose or create a window, display the specified
buffer in it, and then return the window.

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This variable takes precedence over all the other options described
above.
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@end defopt
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If all options described above fail to produce a suitable window,
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@code{display-buffer} tries to reuse an existing window.  As a last
resort, it will try to display @var{buffer-or-name} on a separate frame.
In that case, the value of @code{pop-up-frames} is disregarded.

@node Dedicated Windows
@section Dedicated Windows
@cindex dedicated window

Functions for displaying a buffer can be told to not use specific
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windows by marking these windows as @dfn{dedicated} to their buffers.
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@code{display-buffer} (@pxref{Choosing Window}) never uses a dedicated
window for displaying another buffer in it.  @code{get-lru-window} and
@code{get-largest-window} (@pxref{Selecting Windows}) do not consider
dedicated windows as candidates when their @var{dedicated} argument is
non-@code{nil}.  The behavior of @code{set-window-buffer}
(@pxref{Buffers and Windows}) with respect to dedicated windows is
slightly different, see below.

When @code{delete-windows-on} (@pxref{Deleting Windows}) wants to delete
a dedicated window and that window is the only window on its frame, it
deletes the window's frame too, provided there are other frames left.
@code{replace-buffer-in-windows} (@pxref{Displaying Buffers}) tries to
delete all dedicated windows showing its buffer argument.  When such a
window is the only window on its frame, that frame is deleted, provided
there are other frames left.  If there are no more frames left, some
other buffer is displayed in the window, and the window is marked as
non-dedicated.

When you kill a buffer (@pxref{Killing Buffers}) displayed in a
dedicated window, any such window usually gets deleted too, since
@code{kill-buffer} calls @code{replace-buffer-in-windows} for cleaning
up windows.  Burying a buffer (@pxref{The Buffer List}) deletes the
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selected window if it is dedicated to that buffer.  If, however, that
window is the only window on its frame, @code{bury-buffer} displays
another buffer in it and iconifies the frame.
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@defun window-dedicated-p &optional window
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This function returns non-@code{nil} if @var{window} is dedicated to its
buffer and @code{nil} otherwise.  More precisely, the return value is
the value assigned by the last call of @code{set-window-dedicated-p} for
@var{window} or @code{nil} if that function was never called with
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@var{window} as its argument.  The default for @var{window} is the
selected window.
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@end defun

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

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As a special case, if @var{flag} is @code{t}, @var{window} becomes
@dfn{strongly} dedicated to its buffer.  @code{set-window-buffer}
signals an error when the window it acts upon is strongly dedicated to
its buffer and does not already display the buffer it is asked to
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display.  Other functions do not treat @code{t} differently from any
non-@code{nil} value.
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@end defun
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@node Window Point
@section Windows and Point
@cindex window position
@cindex window point
@cindex position in window
@cindex point in window

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  Each window has its own value of point (@pxref{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.
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@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.
@end itemize

@cindex cursor
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   As far as the user is concerned, point is where the cursor is, and
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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
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window's buffer) if that window were selected.  The default for
@var{window} is the selected window.
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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.
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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.
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@end defun

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

If @var{window} is selected, and its buffer is current,
this simply does @code{goto-char}.
@end defun

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@defvar window-point-insertion-type
This variable specifies the marker insertion type (@pxref{Marker
Insertion Types}) of @code{window-point}.  The default is @code{nil},
so @code{window-point} will stay behind text inserted there.
@end defvar

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@node Window Start and End
@section The Window Start and End Positions
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@cindex window start position

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  Each window maintains a marker used to keep track of a buffer position
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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.

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  After switching windows or buffers, and in some other cases, if the
window start is in the middle of a line, Emacs adjusts the window
start to the start of a line.  This prevents certain operations from
leaving the window start at a meaningless point within a line.  This
feature may interfere with testing some Lisp code by executing it
using the commands of Lisp mode, because they trigger this
readjustment.  To test such code, put it into a command and bind the
command to a key.

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@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
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for the same buffer, or to @code{point-min} if the buffer doesn't have
any.
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Redisplay updates the window-start position (if you have not specified
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it explicitly since the previous redisplay)---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.
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For a realistic example of using @code{window-start}, see the
description of @code{count-lines}.  @xref{Definition of count-lines}.
@end defun

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@cindex window end position
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@defun window-end &optional window update
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This function returns the position where display of its buffer ends in
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@var{window}.  The default for @var{window} is the selected window.
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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
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up-to-date value for where display ends, based on the current
@code{window-start} value.  If a previously saved value of that position
is still valid, @code{window-end} returns that value; otherwise it
computes the correct value by scanning the buffer text.
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Even if @var{update} is non-@code{nil}, @code{window-end} does not
attempt to scroll the display if point has moved off the screen, the
way real redisplay would do.  It does not alter the
@code{window-start} value.  In effect, it reports where the displayed
text will end if scrolling is not required.
@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.

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For example, if point @w{is 1} and you set the start of the window
@w{to 37}, the start of the next line, point will 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:
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@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.
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---------- Buffer: foo ----------
@end group

@group
(set-window-start
 (selected-window)
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 (save-excursion
   (goto-char 1)
   (forward-line 1)
   (point)))
@result{} 37
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@end group

@group
;; @r{Here is what @samp{foo} looks like after executing}
;;   @r{the @code{set-window-start} expression.}
---------- Buffer: foo ----------
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@point{}4
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---------- 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 partially
This function returns non-@code{nil} if @var{position} is within the
range of text currently visible on the screen in @var{window}.  It
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returns @code{nil} if @var{position} is scrolled vertically out of view.
Locations that are partially obscured are not considered visible unless
@var{partially} is non-@code{nil}.  The argument @var{position} defaults
to the current position of point in @var{window}; @var{window}, to the
selected window.  If @var{position} is @code{t}, that means to check the
last visible position in @var{window}.
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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
non-@code{nil} anyway.  @xref{Horizontal Scrolling}.

If @var{position} is visible, @code{pos-visible-in-window-p} returns
@code{t} if @var{partially} is @code{nil}; if @var{partially} is
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non-@code{nil}, and the character following @var{position} is fully
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visible, it returns a list of the form @code{(@var{x} @var{y})}, where
@var{x} and @var{y} are the pixel coordinates relative to the top left
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corner of the window; otherwise it returns an extended list of the form
@code{(@var{x} @var{y} @var{rtop} @var{rbot} @var{rowh} @var{vpos})},
where @var{rtop} and @var{rbot} specify the number of off-window pixels
at the top and bottom of the row at @var{position}, @var{rowh} specifies
the visible height of that row, and @var{vpos} specifies the vertical
position (zero-based row number) of that row.
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Here is an example:

@example
@group
;; @r{If point is off the screen now, recenter it now.}
(or (pos-visible-in-window-p
     (point) (selected-window))
    (recenter 0))
@end group
@end example
@end defun

@defun window-line-height &optional line window
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This function returns the height of text line @var{line} in
@var{window}.  If @var{line} is one of @code{header-line} or
@code{mode-line}, @code{window-line-height} returns information about
the corresponding line of the window.  Otherwise, @var{line} is a text
line number starting from 0.  A negative number counts from the end of
the window.  The default for @var{line} is the current line in
@var{window}; the default for @var{window} is the selected window.
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