* doc/lispref/lists.texi (Sets And Lists): Mention cl provides union etc.

[bpt/emacs.git] / doc / lispref / windows.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990-1995, 1998-1999, 2001-2011
@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.  @xref{Frames and Windows}, for how windows relate to
frames.  @xref{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-level functions for displaying a buffer
                              and choosing a window for it.
* Choosing Window::         How to choose a window for displaying a buffer.
* Dedicated Windows::       How to avoid displaying another buffer in
                              a specific window.
* Window Point::            Each window has its own location of point.
* Window Start and End::    Buffer positions indicating which text is
                              on-screen in a 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 Tree::             The layout and sizes of all windows in a frame.
* Window Configurations::   Saving and restoring the state of the screen.
* Window Parameters::       Associating additional information with windows.
* 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; see @ref{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, but the other windows have ``non-selected'' cursors, normally
less visible.  (@xref{Cursor Parameters}, for customizing this.)  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); see @ref{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; see @ref{Deleting Windows}.  Restoring a
saved window configuration is the only way for a window no longer on the
screen to come back to life; see @ref{Window Configurations}.

@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 (@pxref{Splitting Windows}) and resizes
them (@pxref{Resizing Windows}), not all conceivable tilings of windows
on an Emacs frame are actually possible.

@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 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
previously visible in the window that was split.

@deffn Command split-window &optional window size horizontal
This function splits a new window out of @var{window}'s screen area.  It
returns the new window.  The default for @var{window} is the selected
window.  When you split the selected window, it remains selected.

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.

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
@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.

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
with a scroll bar (@pxref{Scroll Bars}), or with @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.)  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.

This variable affects the behavior of @code{split-window-vertically}
only.  It has no effect on the other functions described here.
@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
@minus{}@var{size} columns, but the leftmost window still remains
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)
         (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
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.

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

@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}.
The default for @var{window} is the selected window.  An error is
signaled if @var{window} is the only window on its frame.
@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.  The default for @var{window}
is the selected window.  The return value is @code{nil}.
@end deffn

@deffn Command delete-windows-on &optional buffer-or-name frame
This function deletes all windows showing @var{buffer-or-name}.  If
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.

@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
@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.

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:

@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
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}.

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.


@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,
arrangement or contents of windows; therefore, if @var{forms} change
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.
@end defmac

@defmac with-selected-window window forms@dots{}
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
@code{select-window} with argument @var{norecord} @code{nil}.
@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,
selected) among a set of candidate windows.  If any full-width windows
are present, it only considers these.

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}.

The optional argument @var{frame} specifies 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 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
with the most lines.  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}.

If there are two candidate windows of the same size, this function
prefers the one that comes first in the cyclic ordering of windows,
starting from the selected window (@pxref{Cyclic Window Ordering}).

The optional argument @var{frame} specifies which set of windows to
consider, see @code{get-lru-window} above.
@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
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}).

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
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}.

  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.

  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
(@pxref{Window Tree}), the order is left to right, or top to bottom.

@defun next-window &optional window minibuf all-frames
@cindex minibuffer window, and @code{next-window}
This function returns the window following @var{window} in the cyclic
ordering of windows.  This is the window @kbd{C-x o} selects if typed
when @var{window} is selected.  The default for @var{window} is the
selected window.

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}.)

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.

The optional 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.  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
@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 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.

The optional 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}.  This function returns @code{nil}.
@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
set of windows to include in the walk.  See @code{next-window}, above,
for details.
@end defun

@defun window-list &optional frame minibuf window
This function returns a list of all windows on @var{frame}, starting
with @var{window}.  The default for @var{frame} is the selected frame;
the default for @var{window} is the selected window.

The value of @var{minibuf} specifies if the minibuffer window shall be
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.
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.

@defun set-window-buffer window buffer-or-name &optional keep-margins
This function makes @var{window} display @var{buffer-or-name} as its
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.

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

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

@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}.

Note that this function runs @code{window-scroll-functions} before
running @code{window-configuration-change-hook}.
@end defun

@defvar buffer-display-count
This buffer-local variable records the number of times a buffer has been
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
This function returns the buffer that @var{window} is displaying.  The
default for @var{window} is the selected window.

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

@defun get-buffer-window &optional buffer-or-name 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{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:

@itemize @bullet
@item
@code{nil} means consider windows on the selected frame.
@item
@code{t} means consider windows on all existing frames.
@item
@code{visible} means consider windows on all visible frames.
@item
0 means consider windows on all visible or iconified frames.
@item
A frame means consider windows on that frame only.
@end itemize

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.
@end defun

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

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}.
@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 primitives 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;
see @ref{Current Buffer}.

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
the variable @code{major-mode}; see @ref{Auto Major Mode}.

When the selected window is the minibuffer window or is strongly
dedicated to its buffer (@pxref{Dedicated Windows}), this function calls
@code{pop-to-buffer} (see below) to display the buffer in some other
window.

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
This function makes the buffer specified by @var{buffer-or-name} current
and displays it in a window not currently selected, using the function
@code{pop-to-buffer} (see below).

The currently selected window is absolutely never used to do the job.
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.

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.  Its frame is
given the X server's focus, if possible; see @ref{Input Focus}.  The
return value is the buffer that was switched to.

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
variable @code{major-mode}.  @xref{Auto Major Mode}.

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.)

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 for
displaying @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}.

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 &optional buffer-or-name
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
don't care which other buffer is used; you just want to make sure that
@var{buffer-or-name} is no longer displayed.

The argument @var{buffer-or-name} may be a buffer or the name of an
existing buffer and defaults to the current buffer.

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.

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}.  Higher-level functions and
commands, like @code{switch-to-buffer} and @code{pop-to-buffer}, 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, 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
unaltered by this function.  The argument @var{buffer-or-name} must be a
buffer or the name of an existing buffer.

@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.

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

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}:

@itemize @bullet
@item
@code{nil} means consider windows on the selected frame.
(Actually, the last non-minibuffer frame.)
@item
@code{t} means consider windows on all frames.
@item
@code{visible} means consider windows on all visible frames.
@item
0 means consider windows on all visible or iconified frames.
@item
A frame means consider windows on that frame only.
@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
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
@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
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
variable @code{pop-up-frames} (see below) is non-@code{nil},
@code{display-buffer} does not split any window.
@end defopt

@defopt split-window-preferred-function
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
function @code{split-window-sensibly} described below.  If you
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}).

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
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
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}.
@end defopt

@defun split-window-sensibly window
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

@defopt split-height-threshold
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
@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 accommodate a mode line too.
@end defopt

@defopt split-width-threshold
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
vertically, though, see above.)

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

@defopt even-window-heights
This variable specifies whether @code{display-buffer} should even out
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.
@end defopt

@c Emacs 19 feature
@defopt pop-up-frames
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.

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.
@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}.

The value of this variable must 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 this
variable is a function that creates a frame using the parameters
specified by @code{pop-up-frame-alist} described next.
@end defopt

@defopt pop-up-frame-alist
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.
@end defopt

@defopt special-display-buffer-names
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.

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
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
A list of regular expressions specifying 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 @code{special-display-buffer-names} above.
@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

@defopt 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}, see below.
@end defopt

@defun special-display-popup-frame buffer &optional args
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}.

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
@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
@defopt 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 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.

This variable takes precedence over all the other options described
above.
@end defopt

If all options described above fail to produce a suitable window,
@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
windows by marking these windows as @dfn{dedicated} to their buffers.
@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
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.

@defun window-dedicated-p &optional window
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
@var{window} as its argument.  The default for @var{window} is the
selected window.
@end defun

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

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
display.  Other functions do not treat @code{t} differently from any
non-@code{nil} value.
@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 (@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.

@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
   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.  The default for
@var{window} is the selected window.

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.  It returns @var{position}.

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

@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

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

  Each window maintains 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.

  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.

@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 to @code{point-min} if the buffer doesn't have
any.

Redisplay updates the window-start position (if you have not specified
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.

For a realistic example of using @code{window-start}, see the
description of @code{count-lines}.  @xref{Definition of count-lines}.
@end defun

@cindex window end position
@defun window-end &optional window update
This function returns the position where display of its buffer ends in
@var{window}.  The default for @var{window} is the selected window.

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 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.

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.

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:

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

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

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
non-@code{nil}, and the character following @var{position} is fully
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
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.

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
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.

If the display is not up to date, @code{window-line-height} returns
@code{nil}.  In that case, @code{pos-visible-in-window-p} may be used
to obtain related information.

If there is no line corresponding to the specified @var{line},
@code{window-line-height} returns @code{nil}.  Otherwise, it returns
a list @code{(@var{height} @var{vpos} @var{ypos} @var{offbot})},
where @var{height} is the height in pixels of the visible part of the
line, @var{vpos} and @var{ypos} are the vertical position in lines and
pixels of the line relative to the top of the first text line, and
@var{offbot} is the number of off-window pixels at the bottom of the
text line.  If there are off-window pixels at the top of the (first)
text line, @var{ypos} is negative.
@end defun

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

  @dfn{Textual scrolling} means moving the text up or down through 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}.

  If the window contains a row which is taller than the height of the
window (for example in the presence of a large image), the scroll
functions will adjust the window's vertical scroll position to scroll
the partially visible row.  To disable this feature, Lisp code may bind
the variable @code{auto-window-vscroll} to @code{nil} (@pxref{Vertical
Scrolling}).

@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}, unless it gets an error
because it can't scroll any further.
@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}, unless it gets an error because
it can't scroll any further.
@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.  When it is non-@code{nil} and the
minibuffer is selected, it takes precedence over
@code{other-window-scroll-buffer}.  @xref{Definition of
minibuffer-scroll-window}.

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
@samp{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's window 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,
redisplay scrolls the text 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 a
positive integer @var{n}, then redisplay scrolls the text up to
@var{n} lines in either direction, if that will bring point back into
proper view.  This behavior is called @dfn{conservative scrolling}.
Otherwise, scrolling happens in the usual way, under the control of
other variables such as @code{scroll-up-aggressively} and
@code{scroll-down-aggressively}.

The default value is zero, which means that conservative scrolling
never happens.
@end defopt

@defopt scroll-down-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 down.  More precisely, when a
window scrolls down 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-up-aggressively
Likewise, for scrolling up.  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 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 @code{t}, scrolling which would move the current
point position out of the window chooses the new position of point
so that the vertical position of the cursor is unchanged, if possible.

If it is non-@code{nil} and not @code{t}, then the scrolling functions
always preserve the vertical position of point, if 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 text in the selected window so that point is
displayed at a specified vertical position within the window.  It does
not ``move point'' with respect to the text.

If @var{count} is a nonnegative number, that 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
@cindex vertical scroll position

   @dfn{Vertical fractional scrolling} means shifting text in a window
up or down by a specified multiple or fraction of a line.  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 pixels-p
This function returns the current vertical scroll position of
@var{window}.  The default for @var{window} is the selected window.
If @var{pixels-p} is non-@code{nil}, the return value is measured in
pixels, rather than in units of the normal line height.

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

@defun set-window-vscroll window lines &optional pixels-p
This function sets @var{window}'s vertical scroll position to
@var{lines}.  If @var{window} is @code{nil}, the selected window is
used.  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

If @var{pixels-p} is non-@code{nil}, @var{lines} specifies a number of
pixels.  In this case, the return value is @var{lines}.
@end defun

@defvar auto-window-vscroll
If this variable is non-@code{nil}, the line-move, scroll-up, and
scroll-down functions will automatically modify the vertical scroll
position to scroll through display rows that are taller than the height
of the window, for example in the presence of large images.
@end defvar

@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{horizontal 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 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.

@vindex auto-hscroll-mode
  If @code{auto-hscroll-mode} is set, 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 serves as a lower
bound for automatic scrolling, i.e. automatic scrolling will not
scroll a window to a column less than the specified one.

@deffn Command scroll-left &optional count set-minimum
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).

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.

If @var{set-minimum} is non-@code{nil}, the new scroll amount becomes
the lower bound for automatic scrolling; that is, automatic scrolling
will not scroll a window to a column less than the value returned by
this function.  Interactive calls pass non-@code{nil} for
@var{set-minimum}.
@end deffn

@deffn Command scroll-right &optional count set-minimum
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.  Aside from the direction
of scrolling, this works just like @code{scroll-left}.
@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 default for
@var{window} is the selected window.

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


@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 horizontal scrolling of @var{window}.  The value of
@var{columns} specifies the amount of scrolling, in terms of columns
from the left margin.  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.

Note that @code{set-window-hscroll} may appear not to work if you test
it by evaluating a call with @kbd{M-:} in a simple way.  What happens
is that the function sets the horizontal scroll value and returns, but
then redisplay adjusts the horizontal scrolling to make point visible,
and this overrides what the function did.  You can observe the
function's effect if you call it while point is sufficiently far from
the left margin that it will remain visible.

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} (by default
the selected window), including any mode line and header line.
The result is almost always less than the value of @code{frame-height}
for the associated frame, because the latter also includes any echo
area.  Depending on the toolkit in use, the frame height can also
include the menu bar and tool bar (@pxref{Size and Position}).
Therefore in general it is not straightforward to compare window and
frame heights (see @code{window-full-height-p} below).

@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-body-height &optional window
Like @code{window-height} but the value does not include the
mode line (if any) or the header line (if any).
@end defun

@defun window-full-height-p &optional window
This function returns non-@code{nil} if @var{window} is as tall as the
frame that contains it.  The default for @var{window} is the selected
window.
@end defun

@defun window-width &optional window
This function returns the number of columns in @var{window}.  The
default for @var{window} is the selected window.

The return value does not include the window's scroll bar or the column
of @samp{|} characters that separates side-by-side windows.  Moreover,
the return value does not include the space used for displaying fringes
and margins.  Hence you cannot, in general, compare the return values of
@code{window-width} and @code{frame-width} for equality to determine
whether a window is a wide as the containing frame.  Use the function
@code{window-full-width-p}, see below, instead.

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

@defun window-full-width-p &optional window
This function returns non-@code{nil} if @var{window} is as wide as the
frame that contains it; otherwise @code{nil}.  The default for
@var{window} is the selected window.
@end defun

@defun window-edges &optional window
This function returns a list of the edge coordinates of @var{window}.
The default for @var{window} is the selected window.

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.

The edges include the space used by the window's scroll bar, display
margins, fringes, header line, and mode line, if it has them.  Also,
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.
@end defun

@defun window-inside-edges &optional window
This is similar to @code{window-edges}, but the edge values
it returns include only the text area of the window.  They
do not include the header line, mode line, scroll bar or
vertical separator, fringes, or display margins.
@end defun

Here are the results obtained on a typical 24-line terminal with just
one window, with menu bar enabled:

@example
@group
(window-edges (selected-window))
     @result{} (0 1 80 23)
@end group
@group
(window-inside-edges (selected-window))
     @result{} (0 1 80 22)
@end group
@end example

@noindent
The bottom edge is at line 23 because the last line is the echo area.
The bottom inside edge is at line 22, which is the window's mode line.

If @var{window} is at the upper left corner of its frame, and there is
no menu bar, then @var{bottom} returned by @code{window-edges} 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 7 3}}.
The inside edges of the left window are @w{@samp{0 0 3 2}},
and the inside edges of the right window are @w{@samp{4 0 7 2}},

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

           0  34  7
@end group
@end example

@defun window-pixel-edges &optional window
This function is like @code{window-edges} except that, on a graphical
display, the edge values are measured in pixels instead of in
character lines and columns.
@end defun

@defun window-inside-pixel-edges &optional window
This function is like @code{window-inside-edges} except that, on a
graphical display, the edge values are measured in pixels instead of
in character lines and columns.
@end defun

@node Resizing Windows
@section Changing the Size of a Window
@cindex window resizing
@cindex resize window
@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 a window always affects at least one other window.

@deffn Command enlarge-window size &optional horizontal
This function makes the selected window @var{size} lines taller by
stealing lines from windows above or below.  In a first round, it takes
lines from one window at a time until that window is
@code{window-min-height} lines tall, then takes from another.  If, at
the end of the first round, the selected window is still not tall
enough, @code{enlarge-window} starts a second round, where it deletes
windows above or below the selected one.

If @var{horizontal} is non-@code{nil}, this function makes the window
@var{size} columns wider, 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 selected 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}), then @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)
  (interactive "p")
  (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)
  (interactive "p")
  (shrink-window columns t))
@end group
@end example
@end deffn

@defun adjust-window-trailing-edge window delta horizontal
This function makes the selected window @var{delta} lines taller or
@var{delta} columns wider, by moving the bottom or right edge.  This
function does not delete other windows; if it cannot make the
requested size adjustment, it signals an error.  On success, this
function returns @code{nil}.
@end defun

@deffn Command fit-window-to-buffer &optional window max-height min-height
This command makes @var{window} the right height to display its
contents exactly.  The default for @var{window} is the selected window.

The optional argument @var{max-height} specifies the maximum height the
window is allowed to be; @code{nil} means use the maximum permissible
height of a window on @var{window}'s frame.  The optional argument
@var{min-height} specifies the minimum height for the window; @code{nil}
means use @code{window-min-height}.  All these height values include the
mode line and/or header line.

This function can delete windows when their height shrinks below
@var{min-height}.  It returns non-@code{nil} if it orderly resized
@var{window}, and @code{nil} otherwise.
@end deffn

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

However, this 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.

This command returns non-@code{nil} if it actually shrank the window
and @code{nil} otherwise.
@end deffn

@cindex fixed-size window
@defvar window-size-fixed
If this variable is non-@code{nil}, in a given buffer, then the size of
any window displaying that buffer remains fixed unless you either
explicitly change it or Emacs has no other choice.

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.

This variable automatically becomes buffer-local when set.

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

Deleting an adjacent window or changing the frame size may 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-structure-changing
functions to a minimum height and width.

@defopt window-min-height
The value of this variable specifies 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 value is measured in line units.  When
the window wants a mode line and/or a header line, they are counted as
one line each.  The default value is @code{4}.  A value less than
@code{1} is ignored.
@end defopt

@defopt window-min-width
The value of this variable specifies 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 value is measured in characters and
includes any fringes or the scroll bar.  The default value is @code{10}.
A value less than @code{2} is ignored.
@end defopt

@cindex balancing window sizes
Emacs provides two functions to balance windows, that is, to even out
the sizes of windows on the same frame.  The minibuffer window and
fixed-size windows are not resized by these functions.

@deffn Command balance-windows &optional window-or-frame
This function balances windows in a way that gives more space to
full-width and/or full-height windows.  If @var{window-or-frame}
specifies a frame, it balances all windows on that frame.  If
@var{window-or-frame} specifies a window, it balances this window and
its ``siblings'' only.  Think of a sibling as the other (original or
new) window with respect to the present one, involved in the process of
splitting; see @ref{Splitting Windows}.  Since a sibling may have been
split again, a window can have more than one sibling.
@end deffn

@deffn Command balance-windows-area
This function attempts to give all windows on the selected frame
approximately the same share of the screen area.  This means that
full-width or full-height windows are not given more space than other
windows.
@end deffn

@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 left-fringe
@itemx right-fringe
The coordinates are in the left or right fringe of the window.

@item left-margin
@itemx right-margin
The coordinates are in the left or right margin of the window.

@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 Tree
@section The Window Tree
@cindex window tree

  A @dfn{window tree} specifies the layout, size, and relationship
between all windows in one frame.

@defun window-tree &optional frame
This function returns the window tree for frame @var{frame}.
If @var{frame} is omitted, the selected frame is used.

The return value is a list of the form @code{(@var{root} @var{mini})},
where @var{root} represents the window tree of the frame's
root window, and @var{mini} is the frame's minibuffer window.

If the root window is not split, @var{root} is the root window itself.
Otherwise, @var{root} is a list @code{(@var{dir} @var{edges} @var{w1}
@var{w2} ...)} where @var{dir} is @code{nil} for a horizontal split,
and @code{t} for a vertical split, @var{edges} gives the combined size and
position of the subwindows in the split, and the rest of the elements
are the subwindows in the split.  Each of the subwindows may again be
a window or a list representing a window split, and so on.  The
@var{edges} element is a list @code{(@var{left}@var{ top}@var{ right}@var{ bottom})}
similar to the value returned by @code{window-edges}.
@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, how those
buffers are scrolled, and their values of point and the mark; also their
fringes, margins, and scroll bar settings.  It also includes the value
of @code{minibuffer-scroll-window}.  As a special exception, the window
configuration does not record the value of point in the selected window
for the current buffer.  Also, the window configuration does not record
the values of window parameters; see @ref{Window Parameters}.

  You can bring back an entire frame layout by restoring a previously
saved window configuration.  If you want to record the layout of all
frames instead of just one, use a frame configuration instead of a
window configuration; see @ref{Frame Configurations}.

@defun current-window-configuration &optional frame
This function returns a new object representing @var{frame}'s current
window configuration.  The default for @var{frame} is the selected
frame.
@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}.  The 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}. In this
case, the function returns @code{nil}.  Otherwise, it returns @code{t}.

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, for each window, 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 sufficient.

Exit from @code{save-window-excursion} always triggers execution of
@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

@defun window-configuration-frame config
This function returns the frame for which the window configuration
@var{config} was made.
@end defun

  Other primitives to look inside of window configurations would make
sense, but are not implemented because we did not need them.  See the
file @file{winner.el} for some more operations on windows
configurations.

@node Window Parameters
@section Window Parameters
@cindex window parameters

This sections describes how window parameters can be used to associate
additional information with windows.

@defun window-parameter window parameter
This function returns @var{window}'s value for @var{parameter}.  The
default for @var{window} is the selected window.  If @var{window}
has no setting for @var{parameter}, this function returns @code{nil}.
@end defun

@defun window-parameters &optional window
This function returns all parameters of @var{window} and their values.
The default for @var{window} is the selected window.  The return value
is an association list of elements of the form @code{(@var{parameter}
. @var{value})}.
@end defun

@defun set-window-parameter window parameter value
This function sets @var{window}'s value of @var{parameter} to
@var{value} and returns @var{value}.  The default for @var{window}
is the selected window.
@end defun

Currently, window parameters are not saved in window configurations and
consequently not restored by @code{set-window-configuration}.  Hence,
any change of a parameter introduced via @code{set-window-parameter} can
be undone only by invoking @code{set-window-parameter} for the same
parameter again.  Since @code{save-window-excursion} relies on window
configurations (@pxref{Window Configurations}), window parameters are
not saved and restored by that special form, either.

@node Window Hooks
@section Hooks for Window Scrolling and Changes
@cindex hooks for window operations

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}.

@defvar window-scroll-functions
This variable holds a list of functions that Emacs should call before
redisplaying a window with scrolling.  Displaying a different buffer in
the window also runs these functions.

This variable is not a normal hook, because each function is called with
two arguments: the window, and its new display-start position.

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

@strong{Warning:} don't use this feature to alter the way the window
is scrolled.  It's not designed for that, and such use probably won't
work.
@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 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 buffer-local part of this hook is run once per each window on the
affected frame, with the relevant window selected and its buffer
current.  The global part is run once for the modified frame, with that
frame selected.
@end defvar

  In addition, you can use @code{jit-lock-register} to register a Font
Lock fontification function, which will be called whenever parts of a
buffer are (re)fontified because a window was scrolled or its size
changed.  @xref{Other Font Lock Variables}.