@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990-1995, 1998-1999, 2001-2018 Free Software
@c Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node Windows
@chapter Windows

This chapter describes the functions and variables related to Emacs
windows.  @xref{Frames}, for how windows are assigned an area of screen
available for Emacs to use.  @xref{Display}, for information on how text
is displayed in windows.

@menu
* Basic Windows::           Basic information on using windows.
* Windows and Frames::      Relating windows to the frame they appear on.
* Window Sizes::            Accessing a window's size.
* Resizing Windows::        Changing the sizes of windows.
* Preserving Window Sizes:: Preserving the size of windows.
* Splitting Windows::       Creating a new window.
* Deleting Windows::        Removing a window from its frame.
* Recombining Windows::     Preserving the frame layout when splitting and
                              deleting 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.
* Switching Buffers::       Higher-level functions for switching to a buffer.
* Displaying Buffers::      Displaying a buffer in a suitable window.
* Window History::          Each window remembers the buffers displayed in it.
* Dedicated Windows::       How to avoid displaying another buffer in
                              a specific window.
* Quitting Windows::        How to restore the state prior to displaying a
                              buffer.
* Side Windows::            Special windows on a frame's sides.
* Atomic Windows::          Preserving parts of the window layout.
* 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.
* Coordinates and Windows:: Converting coordinates to windows.
* Mouse Window Auto-selection:: Automatically selecting windows with the mouse.
* 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

A @dfn{window} is an area of the screen that is used to display a buffer
(@pxref{Buffers}).  In Emacs Lisp, windows are represented by a special
Lisp object type.

@cindex multiple windows
  Windows are grouped into frames (@pxref{Frames}).  Each frame
contains at least one window; the user can subdivide it into multiple,
non-overlapping windows to view several buffers at once.  Lisp
programs can use multiple windows for a variety of purposes.  In
Rmail, for example, you can view a summary of message titles in one
window, and the contents of the selected message in another window.

@cindex terminal screen
@cindex screen of terminal
  Emacs uses the word ``window'' with a different meaning than in
graphical desktop environments and window systems, such as the X
Window System.  When Emacs is run on X, each of its graphical X
windows is an Emacs frame (containing one or more Emacs windows).
When Emacs is run on a text terminal, the frame fills the entire
terminal screen.

@cindex tiled windows
  Unlike X windows, Emacs windows are @dfn{tiled}; they never overlap
within the area of the frame.  When a window is created, resized, or
deleted, the change in window space is taken from or given to the
adjacent windows, so that the total area of the frame is unchanged.

@defun windowp object
This function returns @code{t} if @var{object} is a window (whether or
not it displays a buffer).  Otherwise, it returns @code{nil}.
@end defun

@cindex live windows
A @dfn{live window} is one that is actually displaying a buffer in a
frame.

@defun window-live-p object
This function returns @code{t} if @var{object} is a live window and
@code{nil} otherwise.  A live window is one that displays a buffer.
@end defun

@cindex internal windows
The windows in each frame are organized into a @dfn{window tree}.
@xref{Windows and Frames}.  The leaf nodes of each window tree are live
windows---the ones actually displaying buffers.  The internal nodes of
the window tree are @dfn{internal windows}, which are not live.

@cindex valid windows
   A @dfn{valid window} is one that is either live or internal.  A valid
window can be @dfn{deleted}, i.e., removed from its frame
(@pxref{Deleting Windows}); then it is no longer valid, but the Lisp
object representing it might be still referenced from other Lisp
objects.  A deleted window may be made valid again by restoring a saved
window configuration (@pxref{Window Configurations}).

   You can distinguish valid windows from deleted windows with
@code{window-valid-p}.

@defun window-valid-p object
This function returns @code{t} if @var{object} is a live window, or an
internal window in a window tree.  Otherwise, it returns @code{nil},
including for the case where @var{object} is a deleted window.
@end defun

@cindex selected window
@cindex window selected within a frame
  In each frame, at any time, exactly one Emacs window is designated
as @dfn{selected within the frame}.  For the selected frame, that
window is called the @dfn{selected window}---the one in which most
editing takes place, and in which the cursor for selected windows
appears (@pxref{Cursor Parameters}).  Keyboard input that inserts or
deletes text is also normally directed to this window.   The selected
window's buffer is usually also the current buffer, except when
@code{set-buffer} has been used (@pxref{Current Buffer}).  As for
non-selected frames, the window selected within the frame becomes the
selected window if the frame is ever selected.  @xref{Selecting
Windows}.

@defun selected-window
This function returns the selected window (which is always a live
window).
@end defun

@anchor{Window Group}Sometimes several windows collectively and
cooperatively display a buffer, for example, under the management of
Follow Mode (@pxref{Follow Mode,,, emacs}), where the windows together
display a bigger portion of the buffer than one window could alone.
It is often useful to consider such a @dfn{window group} as a single
entity.  Several functions such as @code{window-group-start}
(@pxref{Window Start and End}) allow you to do this by supplying, as
an argument, one of the windows as a stand in for the whole group.

@defun selected-window-group
@vindex selected-window-group-function
When the selected window is a member of a group of windows, this
function returns a list of the windows in the group, ordered such that
the first window in the list is displaying the earliest part of the
buffer, and so on.  Otherwise the function returns a list containing
just the selected window.

The selected window is considered part of a group when the buffer
local variable @code{selected-window-group-function} is set to a
function.  In this case, @code{selected-window-group} calls it with no
arguments and returns its result (which should be the list of windows
in the group).
@end defun


@node Windows and Frames
@section Windows and Frames

Each window belongs to exactly one frame (@pxref{Frames}).

@defun window-frame &optional window
This function returns the frame that the window @var{window} belongs
to.  If @var{window} is @code{nil}, it defaults to the selected
window.
@end defun

@defun window-list &optional frame minibuffer window
This function returns a list of live windows belonging to the frame
@var{frame}.  If @var{frame} is omitted or @code{nil}, it defaults to
the selected frame.

The optional argument @var{minibuffer} specifies whether to include
the minibuffer window in the returned list.  If @var{minibuffer} is
@code{t}, the minibuffer window is included.  If @var{minibuffer} is
@code{nil} or omitted, the minibuffer window is included only if it is
active.  If @var{minibuffer} is neither @code{nil} nor @code{t}, the
minibuffer window is never included.

The optional argument @var{window}, if non-@code{nil}, should be a live
window on the specified frame; then @var{window} will be the first
element in the returned list.  If @var{window} is omitted or @code{nil},
the window selected within the frame is the first element.
@end defun

@cindex window tree
@cindex root window
  Windows in the same frame are organized into a @dfn{window tree},
whose leaf nodes are the live windows.  The internal nodes of a window
tree are not live; they exist for the purpose of organizing the
relationships between live windows.  The root node of a window tree is
called the @dfn{root window}.  It can be either a live window (if the
frame has just one window), or an internal window.

  A minibuffer window (@pxref{Minibuffer Windows}) is not part of its
frame's window tree unless the frame is a minibuffer-only frame.
Nonetheless, most of the functions in this section accept the
minibuffer window as an argument.  Also, the function
@code{window-tree} described at the end of this section lists the
minibuffer window alongside the actual window tree.

@defun frame-root-window &optional frame-or-window
This function returns the root window for @var{frame-or-window}.  The
argument @var{frame-or-window} should be either a window or a frame;
if omitted or @code{nil}, it defaults to the selected frame.  If
@var{frame-or-window} is a window, the return value is the root window
of that window's frame.
@end defun

@cindex parent window
@cindex child window
@cindex sibling window
  When a window is split, there are two live windows where previously
there was one.  One of these is represented by the same Lisp window
object as the original window, and the other is represented by a
newly-created Lisp window object.  Both of these live windows become
leaf nodes of the window tree, as @dfn{child windows} of a single
internal window.  If necessary, Emacs automatically creates this
internal window, which is also called the @dfn{parent window}, and
assigns it to the appropriate position in the window tree.  A set of
windows that share the same parent are called @dfn{siblings}.

@cindex parent window
@defun window-parent &optional window
This function returns the parent window of @var{window}.  If
@var{window} is omitted or @code{nil}, it defaults to the selected
window.  The return value is @code{nil} if @var{window} has no parent
(i.e., it is a minibuffer window or the root window of its frame).
@end defun

  Each internal window always has at least two child windows.  If this
number falls to one as a result of window deletion, Emacs
automatically deletes the internal window, and its sole remaining
child window takes its place in the window tree.

  Each child window can be either a live window, or an internal window
(which in turn would have its own child windows).  Therefore, each
internal window can be thought of as occupying a certain rectangular
@dfn{screen area}---the union of the areas occupied by the live
windows that are ultimately descended from it.

@cindex window combination
@cindex vertical combination
@cindex horizontal combination
  For each internal window, the screen areas of the immediate children
are arranged either vertically or horizontally (never both).  If the
child windows are arranged one above the other, they are said to form
a @dfn{vertical combination}; if they are arranged side by side, they
are said to form a @dfn{horizontal combination}.  Consider the
following example:

@smallexample
@group
     ______________________________________
    | ______  ____________________________ |
    ||      || __________________________ ||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||____________W4____________|||
    ||      || __________________________ ||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||____________W5____________|||
    ||__W2__||_____________W3_____________ |
    |__________________W1__________________|

@end group
@end smallexample

@noindent
The root window of this frame is an internal window, @var{W1}.  Its
child windows form a horizontal combination, consisting of the live
window @var{W2} and the internal window @var{W3}.  The child windows
of @var{W3} form a vertical combination, consisting of the live
windows @var{W4} and @var{W5}.  Hence, the live windows in this
window tree are @var{W2}, @var{W4}, and @var{W5}.

  The following functions can be used to retrieve a child window of an
internal window, and the siblings of a child window.

@defun window-top-child &optional window
This function returns the topmost child window of @var{window}, if
@var{window} is an internal window whose children form a vertical
combination.  For any other type of window, the return value is
@code{nil}.
@end defun

@defun window-left-child &optional window
This function returns the leftmost child window of @var{window}, if
@var{window} is an internal window whose children form a horizontal
combination.  For any other type of window, the return value is
@code{nil}.
@end defun

@defun window-child window
This function returns the first child window of the internal window
@var{window}---the topmost child window for a vertical combination, or
the leftmost child window for a horizontal combination.  If
@var{window} is a live window, the return value is @code{nil}.
@end defun

@defun window-combined-p &optional window horizontal
This function returns a non-@code{nil} value if and only if
@var{window} is part of a vertical combination.  If @var{window} is
omitted or @code{nil}, it defaults to the selected one.

If the optional argument @var{horizontal} is non-@code{nil}, this
means to return non-@code{nil} if and only if @var{window} is part of
a horizontal combination.
@end defun

@defun window-next-sibling &optional window
This function returns the next sibling of the window @var{window}.  If
omitted or @code{nil}, @var{window} defaults to the selected window.
The return value is @code{nil} if @var{window} is the last child of
its parent.
@end defun

@defun window-prev-sibling &optional window
This function returns the previous sibling of the window @var{window}.
If omitted or @code{nil}, @var{window} defaults to the selected
window.  The return value is @code{nil} if @var{window} is the first
child of its parent.
@end defun

The functions @code{window-next-sibling} and
@code{window-prev-sibling} should not be confused with the functions
@code{next-window} and @code{previous-window}, which return the next
and previous window, respectively, in the cyclic ordering of windows
(@pxref{Cyclic Window Ordering}).

  The following functions can be useful to locate a window within its
frame.

@defun frame-first-window &optional frame-or-window
This function returns the live window at the upper left corner of the
frame specified by @var{frame-or-window}.  The argument
@var{frame-or-window} must denote a window or a live frame and defaults
to the selected frame.  If @var{frame-or-window} specifies a window,
this function returns the first window on that window's frame.  Under
the assumption that the frame from our canonical example is selected
@code{(frame-first-window)} returns @var{W2}.
@end defun

@defun window-at-side-p &optional window side
This function returns @code{t} if @var{window} is located at
@var{side} of its containing frame.  The argument @var{window} must be
a valid window and defaults to the selected one.  The argument
@var{side} can be any of the symbols @code{left}, @code{top},
@code{right} or @code{bottom}.  The default value @code{nil} is
handled like @code{bottom}.

Note that this function disregards the minibuffer window
(@pxref{Minibuffer Windows}).  Hence, with @var{side} equal to
@code{bottom} it may return @code{t} also when the minibuffer window
appears right below @var{window}.
@end defun

@cindex window in direction
@defun window-in-direction direction &optional window ignore sign wrap mini
This function returns the nearest live window in direction
@var{direction} as seen from the position of @code{window-point} in
window @var{window}.  The argument @var{direction} must be one of
@code{above}, @code{below}, @code{left} or @code{right}.  The optional
argument @var{window} must denote a live window and defaults to the
selected one.

This function does not return a window whose @code{no-other-window}
parameter is non-@code{nil} (@pxref{Window Parameters}).  If the nearest
window's @code{no-other-window} parameter is non-@code{nil}, this
function tries to find another window in the indicated direction whose
@code{no-other-window} parameter is @code{nil}.  If the optional
argument @var{ignore} is non-@code{nil}, a window may be returned even
if its @code{no-other-window} parameter is non-@code{nil}.

If the optional argument @var{sign} is a negative number, it means to
use the right or bottom edge of @var{window} as reference position
instead of @code{window-point}.  If @var{sign} is a positive number, it
means to use the left or top edge of @var{window} as reference position.

If the optional argument @var{wrap} is non-@code{nil}, this means to
wrap @var{direction} around frame borders.  For example, if @var{window}
is at the top of the frame and @var{direction} is @code{above}, then
this function usually returns the frame's minibuffer window if it's
active and a window at the bottom of the frame otherwise.

If the optional argument @var{mini} is @code{nil}, this means to return
the minibuffer window if and only if it is currently active.  If
@var{mini} is non-@code{nil}, this function may return the minibuffer
window even when it's not active.  However, if @var{wrap} is
non-@code{nil}, it always acts as if @var{mini} were @code{nil}.

If it doesn't find a suitable window, this function returns
@code{nil}.

Don't use this function to check whether there is @emph{no} window in
@var{direction}.  Calling @code{window-at-side-p} described above is a
much more efficient way to do that.
@end defun

The following function allows the entire window tree of a frame to be
retrieved:

@defun window-tree &optional frame
This function returns a list representing the window tree for frame
@var{frame}.  If @var{frame} is omitted or @code{nil}, it defaults to
the selected frame.

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 live, @var{root} is that 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
combination and @code{t} for a vertical combination, @var{edges} gives
the size and position of the combination, and the remaining elements
are the child windows.  Each child window may again be a window object
(for a live window) or a list with the same format as above (for an
internal window).  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} (@pxref{Coordinates and Windows}).
@end defun


@node Window Sizes
@section Window Sizes
@cindex window size
@cindex size of window

  The following schematic shows the structure of a live window:

@smallexample
@group
        ____________________________________________
       |______________ Header Line ______________|RD| ^
     ^ |LS|LM|LF|                       |RF|RM|RS|  | |
     | |  |  |  |                       |  |  |  |  | |
Window |  |  |  |       Text Area       |  |  |  |  | Window
Body | |  |  |  |     (Window Body)     |  |  |  |  | Total
Height |  |  |  |                       |  |  |  |  | Height
     | |  |  |  |<- Window Body Width ->|  |  |  |  | |
     v |__|__|__|_______________________|__|__|__|  | |
       |_________ Horizontal Scroll Bar _________|  | |
       |_______________ Mode Line _______________|__| |
       |_____________ Bottom Divider _______________| v
        <---------- Window Total Width ------------>

@end group
@end smallexample

@cindex window body
@cindex text area of a window
@cindex body of a window
  At the center of the window is the @dfn{text area}, or @dfn{body},
where the buffer text is displayed.  The text area can be surrounded by
a series of optional areas.  On the left and right, from innermost to
outermost, these are the left and right fringes, denoted by LF and RF
(@pxref{Fringes}); the left and right margins, denoted by LM and RM in
the schematic (@pxref{Display Margins}); the left or right vertical
scroll bar, only one of which is present at any time, denoted by LS and
RS (@pxref{Scroll Bars}); and the right divider, denoted by RD
(@pxref{Window Dividers}).  At the top of the window is the header line
(@pxref{Header Lines}).  At the bottom of the window are the horizontal
scroll bar (@pxref{Scroll Bars}); the mode line (@pxref{Mode Line
Format}); and the bottom divider (@pxref{Window Dividers}).

  Emacs provides miscellaneous functions for finding the height and
width of a window.  The return value of many of these functions can be
specified either in units of pixels or in units of lines and columns.
On a graphical display, the latter actually correspond to the height and
width of a default character specified by the frame's default font
as returned by @code{frame-char-height} and @code{frame-char-width}
(@pxref{Frame Font}).  Thus, if a window is displaying text with a
different font or size, the reported line height and column width for
that window may differ from the actual number of text lines or columns
displayed within it.

@cindex window height
@cindex height of a window
@cindex total height of a window
  The @dfn{total height} of a window is the number of lines comprising
the window's body, the header line, the horizontal scroll bar, the mode
line and the bottom divider (if any).

@defun window-total-height &optional window round
This function returns the total height, in lines, of the window
@var{window}.  If @var{window} is omitted or @code{nil}, it defaults to
the selected window.  If @var{window} is an internal window, the return
value is the total height occupied by its descendant windows.

  If a window's pixel height is not an integral multiple of its frame's
default character height, the number of lines occupied by the window is
rounded internally.  This is done in a way such that, if the window is a
parent window, the sum of the total heights of all its child windows
internally equals the total height of their parent.  This means that
although two windows have the same pixel height, their internal total
heights may differ by one line.  This means also, that if window is
vertically combined and has a next sibling, the topmost row of that
sibling can be calculated as the sum of this window's topmost row and
total height (@pxref{Coordinates and Windows})

  If the optional argument @var{round} is @code{ceiling}, this
function returns the smallest integer larger than @var{window}'s pixel
height divided by the character height of its frame; if it is
@code{floor}, it returns the largest integer smaller than said value;
with any other @var{round} it returns the internal value of
@var{windows}'s total height.
@end defun

@cindex window width
@cindex width of a window
@cindex total width of a window
The @dfn{total width} of a window is the number of lines comprising the
window's body, its margins, fringes, scroll bars and a right divider (if
any).

@defun window-total-width &optional window round
This function returns the total width, in columns, of the window
@var{window}.  If @var{window} is omitted or @code{nil}, it defaults to
the selected window.  If @var{window} is internal, the return value is
the total width occupied by its descendant windows.

  If a window's pixel width is not an integral multiple of its frame's
character width, the number of lines occupied by the window is rounded
internally.  This is done in a way such that, if the window is a parent
window, the sum of the total widths of all its children internally
equals the total width of their parent.  This means that although two
windows have the same pixel width, their internal total widths may
differ by one column.  This means also, that if this window is
horizontally combined and has a next sibling, the leftmost column of
that sibling can be calculated as the sum of this window's leftmost
column and total width (@pxref{Coordinates and Windows}).  The optional
argument @var{round} behaves as it does for @code{window-total-height}.
@end defun

@defun window-total-size &optional window horizontal round
This function returns either the total height in lines or the total
width in columns of the window @var{window}.  If @var{horizontal} is
omitted or @code{nil}, this is equivalent to calling
@code{window-total-height} for @var{window}; otherwise it is equivalent
to calling @code{window-total-width} for @var{window}.  The optional
argument @var{round} behaves as it does for @code{window-total-height}.
@end defun

The following two functions can be used to return the total size of a
window in units of pixels.

@cindex window pixel height
@cindex pixel height of a window
@cindex total pixel height of a window

@defun window-pixel-height &optional window
This function returns the total height of window @var{window} in pixels.
@var{window} must be a valid window and defaults to the selected one.

The return value includes mode and header line, a horizontal scroll bar
and a bottom divider, if any.  If @var{window} is an internal window,
its pixel height is the pixel height of the screen areas spanned by its
children.
@end defun

@defun window-pixel-height-before-size-change &optional Lisp_Object &optional window
This function returns the height of window @var{window} in pixels at the
time @code{window-size-change-functions} was run for the last time on
@var{window}'s frame (@pxref{Window Hooks}).
@end defun

@cindex window pixel width
@cindex pixel width of a window
@cindex total pixel width of a window

@defun window-pixel-width &optional window
This function returns the width of window @var{window} in pixels.
@var{window} must be a valid window and defaults to the selected one.

The return value includes the fringes and margins of @var{window} as
well as any vertical dividers or scroll bars belonging to @var{window}.
If @var{window} is an internal window, its pixel width is the width of
the screen areas spanned by its children.
@end defun

@defun window-pixel-width-before-size-change &optional Lisp_Object &optional window
This function returns the width of window @var{window} in pixels at the
time @code{window-size-change-functions} was run for the last time on
@var{window}'s frame (@pxref{Window Hooks}).
@end defun

@cindex full-width window
@cindex full-height window
  The following functions can be used to determine whether a given
window has any adjacent windows.

@defun window-full-height-p &optional window
This function returns non-@code{nil} if @var{window} has no other window
above or below it in its frame.  More precisely, this means that the
total height of @var{window} equals the total height of the root window
on that frame.  The minibuffer window does not count in this regard.  If
@var{window} is omitted or @code{nil}, it defaults to the selected
window.
@end defun

@defun window-full-width-p &optional window
This function returns non-@code{nil} if @var{window} has no other
window to the left or right in its frame, i.e., its total width equals
that of the root window on that frame.  If @var{window} is omitted or
@code{nil}, it defaults to the selected window.
@end defun

@cindex window body height
@cindex body height of a window
The @dfn{body height} of a window is the height of its text area, which
does not include a mode or header line, a horizontal scroll bar, or a
bottom divider.

@defun window-body-height &optional window pixelwise
This function returns the height, in lines, of the body of window
@var{window}.  If @var{window} is omitted or @code{nil}, it defaults to
the selected window; otherwise it must be a live window.

If the optional argument @var{pixelwise} is non-@code{nil}, this
function returns the body height of @var{window} counted in pixels.

If @var{pixelwise} is @code{nil}, the return value is rounded down to
the nearest integer, if necessary.  This means that if a line at the
bottom of the text area is only partially visible, that line is not
counted.  It also means that the height of a window's body can never
exceed its total height as returned by @code{window-total-height}.
@end defun

@cindex window body width
@cindex body width of a window
The @dfn{body width} of a window is the width of its text area, which
does not include the scroll bar, fringes, margins or a right divider.
Note that when one or both fringes are removed (by setting their width
to zero), the display engine reserves two character cells, one on each
side of the window, for displaying the continuation and truncation
glyphs, which leaves 2 columns less for text display.  (The function
@code{window-max-chars-per-line}, described below, takes this
peculiarity into account.)

@defun window-body-width &optional window pixelwise
This function returns the width, in columns, of the body of window
@var{window}.  If @var{window} is omitted or @code{nil}, it defaults to
the selected window; otherwise it must be a live window.

If the optional argument @var{pixelwise} is non-@code{nil}, this
function returns the body width of @var{window} in units of pixels.

If @var{pixelwise} is @code{nil}, the return value is rounded down to
the nearest integer, if necessary.  This means that if a column on the
right of the text area is only partially visible, that column is not
counted.  It also means that the width of a window's body can never
exceed its total width as returned by @code{window-total-width}.
@end defun

@cindex window body size
@cindex body size of a window
@defun window-body-size &optional window horizontal pixelwise
This function returns the body height or body width of @var{window}.  If
@var{horizontal} is omitted or @code{nil}, it is equivalent to calling
@code{window-body-height} for @var{window}; otherwise it is equivalent
to calling @code{window-body-width}.  In either case, the optional
argument @var{pixelwise} is passed to the function called.
@end defun

For compatibility with previous versions of Emacs,
@code{window-height} is an alias for @code{window-total-height}, and
@code{window-width} is an alias for @code{window-body-width}.  These
aliases are considered obsolete and will be removed in the future.

   The pixel heights of a window's mode and header line can be retrieved
with the functions given below.  Their return value is usually accurate
unless the window has not been displayed before: In that case, the
return value is based on an estimate of the font used for the window's
frame.

@defun window-mode-line-height &optional window
This function returns the height in pixels of @var{window}'s mode line.
@var{window} must be a live window and defaults to the selected one.  If
@var{window} has no mode line, the return value is zero.
@end defun

@defun window-header-line-height &optional window
This function returns the height in pixels of @var{window}'s header
line.  @var{window} must be a live window and defaults to the selected
one.  If @var{window} has no header line, the return value is zero.
@end defun

Functions for retrieving the height and/or width of window dividers
(@pxref{Window Dividers}), fringes (@pxref{Fringes}), scroll bars
(@pxref{Scroll Bars}), and display margins (@pxref{Display Margins}) are
described in the corresponding sections.

If your Lisp program needs to make layout decisions, you will find the
following function useful:

@defun window-max-chars-per-line &optional window face
This function returns the number of characters displayed in the
specified face @var{face} in the specified window @var{window} (which
must be a live window).  If @var{face} was remapped (@pxref{Face
Remapping}), the information is returned for the remapped face.  If
omitted or @code{nil}, @var{face} defaults to the default face, and
@var{window} defaults to the selected window.

Unlike @code{window-body-width}, this function accounts for the actual
size of @var{face}'s font, instead of working in units of the canonical
character width of @var{window}'s frame (@pxref{Frame Font}).  It also
accounts for space used by the continuation glyph, if @var{window} lacks
one or both of its fringes.
@end defun

@cindex fixed-size window
@vindex window-min-height
@vindex window-min-width
  Commands that change the size of windows (@pxref{Resizing Windows}),
or split them (@pxref{Splitting Windows}), obey the variables
@code{window-min-height} and @code{window-min-width}, which specify the
smallest allowable window height and width.  They also obey the variable
@code{window-size-fixed}, with which a window can be @dfn{fixed} in
size (@pxref{Preserving Window Sizes}).

@defopt window-min-height
This option specifies the minimum total height, in lines, of any window.
Its value has to accommodate at least one text line as well as a mode
and header line, a horizontal scroll bar and a bottom divider, if
present.
@end defopt

@defopt window-min-width
This option specifies the minimum total width, in columns, of any
window.  Its value has to accommodate two text columns as well as
margins, fringes, a scroll bar and a right divider, if present.
@end defopt

The following function tells how small a specific window can get taking
into account the sizes of its areas and the values of
@code{window-min-height}, @code{window-min-width} and
@code{window-size-fixed} (@pxref{Preserving Window Sizes}).

@defun window-min-size &optional window horizontal ignore pixelwise
This function returns the minimum size of @var{window}.  @var{window}
must be a valid window and defaults to the selected one.  The optional
argument @var{horizontal} non-@code{nil} means to return the minimum
number of columns of @var{window}; otherwise return the minimum number
of @var{window}'s lines.

The return value makes sure that all components of @var{window} remain
fully visible if @var{window}'s size were actually set to it.  With
@var{horizontal} @code{nil} it includes the mode and header line, the
horizontal scroll bar and the bottom divider, if present.  With
@var{horizontal} non-@code{nil} it includes the margins and fringes, the
vertical scroll bar and the right divider, if present.

The optional argument @var{ignore}, if non-@code{nil}, means ignore
restrictions imposed by fixed size windows, @code{window-min-height} or
@code{window-min-width} settings.  If @var{ignore} equals @code{safe},
live windows may get as small as @code{window-safe-min-height} lines and
@code{window-safe-min-width} columns.  If @var{ignore} is a window,
ignore restrictions for that window only.  Any other non-@code{nil}
value means ignore all of the above restrictions for all windows.

The optional argument @var{pixelwise} non-@code{nil} means to return the
minimum size of @var{window} counted in pixels.
@end defun


@node Resizing Windows
@section Resizing Windows
@cindex window resizing
@cindex resize window
@cindex changing window size
@cindex window size, changing

  This section describes functions for resizing a window without
changing the size of its frame.  Because live windows do not overlap,
these functions are meaningful only on frames that contain two or more
windows: resizing a window also changes the size of a neighboring
window.  If there is just one window on a frame, its size cannot be
changed except by resizing the frame (@pxref{Frame Size}).

  Except where noted, these functions also accept internal windows as
arguments.  Resizing an internal window causes its child windows to be
resized to fit the same space.

@defun window-resizable window delta &optional horizontal ignore pixelwise
This function returns @var{delta} if the size of @var{window} can be
changed vertically by @var{delta} lines.  If the optional argument
@var{horizontal} is non-@code{nil}, it instead returns @var{delta} if
@var{window} can be resized horizontally by @var{delta} columns.  It
does not actually change the window size.

If @var{window} is @code{nil}, it defaults to the selected window.

A positive value of @var{delta} means to check whether the window can be
enlarged by that number of lines or columns; a negative value of
@var{delta} means to check whether the window can be shrunk by that many
lines or columns.  If @var{delta} is non-zero, a return value of 0 means
that the window cannot be resized.

Normally, the variables @code{window-min-height} and
@code{window-min-width} specify the smallest allowable window size
(@pxref{Window Sizes}).  However, if the optional argument @var{ignore}
is non-@code{nil}, this function ignores @code{window-min-height} and
@code{window-min-width}, as well as @code{window-size-fixed}.  Instead,
it considers the minimum-height window to be one consisting of a header
and a mode line, a horizontal scrollbar and a bottom divider (if any),
plus a text area one line tall; and a minimum-width window as one
consisting of fringes, margins, a scroll bar and a right divider (if
any), plus a text area two columns wide.

If the optional argument @var{pixelwise} is non-@code{nil},
@var{delta} is interpreted as pixels.
@end defun

@defun window-resize window delta &optional horizontal ignore pixelwise
This function resizes @var{window} by @var{delta} increments.  If
@var{horizontal} is @code{nil}, it changes the height by @var{delta}
lines; otherwise, it changes the width by @var{delta} columns.  A
positive @var{delta} means to enlarge the window, and a negative
@var{delta} means to shrink it.

If @var{window} is @code{nil}, it defaults to the selected window.  If
the window cannot be resized as demanded, an error is signaled.

The optional argument @var{ignore} has the same meaning as for the
function @code{window-resizable} above.

If the optional argument @var{pixelwise} is non-@code{nil},
@var{delta} will be interpreted as pixels.

The choice of which window edges this function alters depends on the
values of the option @code{window-combination-resize} and the
combination limits of the involved windows; in some cases, it may alter
both edges.  @xref{Recombining Windows}.  To resize by moving only the
bottom or right edge of a window, use the function
@code{adjust-window-trailing-edge}.
@end defun

@c The commands enlarge-window, enlarge-window-horizontally,
@c shrink-window, and shrink-window-horizontally are documented in the
@c Emacs manual.  They are not preferred for calling from Lisp.

@defun adjust-window-trailing-edge window delta &optional horizontal pixelwise
This function moves @var{window}'s bottom edge by @var{delta} lines.
If optional argument @var{horizontal} is non-@code{nil}, it instead
moves the right edge by @var{delta} columns.  If @var{window} is
@code{nil}, it defaults to the selected window.

If the optional argument @var{pixelwise} is non-@code{nil},
@var{delta} is interpreted as pixels.

A positive @var{delta} moves the edge downwards or to the right; a
negative @var{delta} moves it upwards or to the left.  If the edge
cannot be moved as far as specified by @var{delta}, this function
moves it as far as possible but does not signal an error.

This function tries to resize windows adjacent to the edge that is
moved.  If this is not possible for some reason (e.g., if that adjacent
window is fixed-size), it may resize other windows.
@end defun

@cindex pixelwise, resizing windows
@defopt window-resize-pixelwise
If the value of this option is non-@code{nil}, Emacs resizes windows in
units of pixels.  This currently affects functions like
@code{split-window} (@pxref{Splitting Windows}), @code{maximize-window},
@code{minimize-window}, @code{fit-window-to-buffer},
@code{fit-frame-to-buffer} and
@code{shrink-window-if-larger-than-buffer} (all listed below).

Note that when a frame's pixel size is not a multiple of its character
size, at least one window may get resized pixelwise even if this
option is @code{nil}.  The default value is @code{nil}.
@end defopt

  The following commands resize windows in more specific ways.  When
called interactively, they act on the selected window.

@deffn Command fit-window-to-buffer &optional window max-height min-height max-width min-width preserve-size
This command adjusts the height or width of @var{window} to fit the text
in it.  It returns non-@code{nil} if it was able to resize @var{window},
and @code{nil} otherwise.  If @var{window} is omitted or @code{nil}, it
defaults to the selected window.  Otherwise, it should be a live window.

If @var{window} is part of a vertical combination, this function adjusts
@var{window}'s height.  The new height is calculated from the actual
height of the accessible portion of its buffer.  The optional argument
@var{max-height}, if non-@code{nil}, specifies the maximum total height
that this function can give @var{window}.  The optional argument
@var{min-height}, if non-@code{nil}, specifies the minimum total height
that it can give, which overrides the variable @code{window-min-height}.
Both @var{max-height} and @var{min-height} are specified in lines and
include mode and header line and a bottom divider, if any.

If @var{window} is part of a horizontal combination and the value of the
option @code{fit-window-to-buffer-horizontally} (see below) is
non-@code{nil}, this function adjusts @var{window}'s width.  The new
width of @var{window} is calculated from the maximum length of its
buffer's lines that follow the current start position of @var{window}.
The optional argument @var{max-width} specifies a maximum width and
defaults to the width of @var{window}'s frame.  The optional argument
@var{min-width} specifies a minimum width and defaults to
@code{window-min-width}.  Both @var{max-width} and @var{min-width} are
specified in columns and include fringes, margins and scrollbars, if
any.

The optional argument @var{preserve-size}, if non-@code{nil}, will
install a parameter to preserve the size of @var{window} during future
resize operations (@pxref{Preserving Window Sizes}).

If the option @code{fit-frame-to-buffer} (see below) is non-@code{nil},
this function will try to resize the frame of @var{window} to fit its
contents by calling @code{fit-frame-to-buffer} (see below).
@end deffn

@defopt fit-window-to-buffer-horizontally
If this is non-@code{nil}, @code{fit-window-to-buffer} can resize
windows horizontally.  If this is @code{nil} (the default)
@code{fit-window-to-buffer} never resizes windows horizontally.  If this
is @code{only}, it can resize windows horizontally only.  Any other
value means @code{fit-window-to-buffer} can resize windows in both
dimensions.
@end defopt

@defopt fit-frame-to-buffer
If this option is non-@code{nil}, @code{fit-window-to-buffer} can fit a
frame to its buffer.  A frame is fit if and only if its root window is a
live window and this option is non-@code{nil}.  If this is
@code{horizontally}, frames are fit horizontally only.  If this is
@code{vertically}, frames are fit vertically only.  Any other
non-@code{nil} value means frames can be resized in both dimensions.
@end defopt

If you have a frame that displays only one window, you can fit that
frame to its buffer using the command @code{fit-frame-to-buffer}.

@deffn Command fit-frame-to-buffer &optional frame max-height min-height max-width min-width only
This command adjusts the size of @var{frame} to display the contents of
its buffer exactly.  @var{frame} can be any live frame and defaults to
the selected one.  Fitting is done only if @var{frame}'s root window is
live.  The arguments @var{max-height}, @var{min-height}, @var{max-width}
and @var{min-width} specify bounds on the new total size of
@var{frame}'s root window.  @var{min-height} and @var{min-width} default
to the values of @code{window-min-height} and @code{window-min-width}
respectively.

If the optional argument @var{only} is @code{vertically}, this function
may resize the frame vertically only.  If @var{only} is
@code{horizontally}, it may resize the frame horizontally only.
@end deffn

The behavior of @code{fit-frame-to-buffer} can be controlled with the
help of the two options listed next.

@defopt fit-frame-to-buffer-margins
This option can be used to specify margins around frames to be fit by
@code{fit-frame-to-buffer}.  Such margins can be useful to avoid, for
example, that the resized frame overlaps the taskbar or parts of its
parent frame.

It specifies the numbers of pixels to be left free on the left, above,
the right, and below a frame that shall be fit.  The default specifies
@code{nil} for each which means to use no margins.  The value specified
here can be overridden for a specific frame by that frame's
@code{fit-frame-to-buffer-margins} parameter, if present.
@end defopt

@defopt fit-frame-to-buffer-sizes
This option specifies size boundaries for @code{fit-frame-to-buffer}.
It specifies the total maximum and minimum lines and maximum and minimum
columns of the root window of any frame that shall be fit to its buffer.
If any of these values is non-@code{nil}, it overrides the corresponding
argument of @code{fit-frame-to-buffer}.
@end defopt

@deffn Command shrink-window-if-larger-than-buffer &optional window
This command attempts to reduce @var{window}'s height as much as
possible while still showing its full buffer, but no less than
@code{window-min-height} lines.  The return value is non-@code{nil} if
the window was resized, and @code{nil} otherwise.  If @var{window} is
omitted or @code{nil}, it defaults to the selected window.  Otherwise,
it should be a live window.

This command does nothing if the window is already too short to
display all of its buffer, or if any of the buffer is scrolled
off-screen, or if the window is the only live window in its frame.

This command calls @code{fit-window-to-buffer} (see above) to do its
work.
@end deffn


@cindex balancing window sizes
@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 only that window
and its siblings (@pxref{Windows and Frames}).
@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.  Full-width or
full-height windows are not given more space than other windows.
@end deffn

@cindex maximizing windows
@deffn Command maximize-window &optional window
This function attempts to make @var{window} as large as possible, in
both dimensions, without resizing its frame or deleting other windows.
If @var{window} is omitted or @code{nil}, it defaults to the selected
window.
@end deffn

@cindex minimizing windows
@deffn Command minimize-window &optional window
This function attempts to make @var{window} as small as possible, in
both dimensions, without deleting it or resizing its frame.  If
@var{window} is omitted or @code{nil}, it defaults to the selected
window.
@end deffn


@node Preserving Window Sizes
@section Preserving Window Sizes
@cindex preserving window sizes

A window can get resized explicitly by using one of the functions from
the preceding section or implicitly, for example, when resizing an
adjacent window, when splitting or deleting a window (@pxref{Splitting
Windows}, @pxref{Deleting Windows}) or when resizing the window's frame
(@pxref{Frame Size}).

  It is possible to avoid implicit resizing of a specific window when
there are one or more other resizable windows on the same frame.  For
this purpose, Emacs must be advised to @dfn{preserve} the size of that
window.  There are two basic ways to do that.

@defvar window-size-fixed
If this buffer-local variable is non-@code{nil}, the size of any window
displaying the buffer cannot normally be changed.  Deleting a window or
changing the frame's size may still change the window's size, if there
is no 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.

If this variable is @code{nil}, this does not necessarily mean that any
window showing the buffer can be resized in the desired direction.  To
determine that, use the function @code{window-resizable}.
@xref{Resizing Windows}.
@end defvar

Often @code{window-size-fixed} is overly aggressive because it inhibits
any attempt to explicitly resize or split an affected window as well.
This may even happen after the window has been resized implicitly, for
example, when deleting an adjacent window or resizing the window's
frame.  The following function tries hard to never disallow resizing
such a window explicitly:

@defun window-preserve-size &optional window horizontal preserve
This function (un-)marks the height of window @var{window} as preserved
for future resize operations.  @var{window} must be a live window and
defaults to the selected one.  If the optional argument @var{horizontal}
is non-@code{nil}, it (un-)marks the width of @var{window} as preserved.

If the optional argument @var{preserve} is @code{t}, this means to
preserve the current height/width of @var{window}'s body.  The
height/width of @var{window} will change only if Emacs has no better
choice.  Resizing a window whose height/width is preserved by this
function never throws an error.

If @var{preserve} is @code{nil}, this means to stop preserving the
height/width of @var{window}, lifting any respective restraint induced
by a previous call of this function for @var{window}.  Calling
@code{enlarge-window}, @code{shrink-window} or
@code{fit-window-to-buffer} with @var{window} as argument may also
remove the respective restraint.
@end defun

@code{window-preserve-size} is currently invoked by the following
functions:

@table @code
@item fit-window-to-buffer
If the optional argument @var{preserve-size} of that function
(@pxref{Resizing Windows}) is non-@code{nil}, the size established by
that function is preserved.

@item display-buffer
If the @var{alist} argument of that function (@pxref{Choosing Window})
contains a @code{preserve-size} entry, the size of the window produced
by that function is preserved.
@end table

  @code{window-preserve-size} installs a window parameter (@pxref{Window
Parameters}) called @code{window-preserved-size} which is consulted by the
window resizing functions.  This parameter will not prevent resizing the
window when the window shows another buffer than the one when
@code{window-preserve-size} was invoked or if its size has changed since
then.

The following function can be used to check whether the height of a
particular window is preserved:

@defun window-preserved-size &optional window horizontal
This function returns the preserved height of window @var{window} in
pixels.  @var{window} must be a live window and defaults to the selected
one.  If the optional argument @var{horizontal} is non-@code{nil}, it
returns the preserved width of @var{window}.  It returns @code{nil} if
the size of @var{window} is not preserved.
@end defun


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

This section describes functions for creating a new window by
@dfn{splitting} an existing one.  Note that some windows are special in
the sense that these functions may fail to split them as described here.
Examples of such windows are side windows (@pxref{Side Windows}) and
atomic windows (@pxref{Atomic Windows}).

@defun split-window &optional window size side pixelwise
This function creates a new live window next to the window
@var{window}.  If @var{window} is omitted or @code{nil}, it defaults
to the selected window.  That window is split, and reduced in
size.  The space is taken up by the new window, which is returned.

The optional second argument @var{size} determines the sizes of
@var{window} and/or the new window.  If it is omitted or @code{nil},
both windows are given equal sizes; if there is an odd line, it is
allocated to the new window.  If @var{size} is a positive number,
@var{window} is given @var{size} lines (or columns, depending on the
value of @var{side}).  If @var{size} is a negative number, the new
window is given @minus{}@var{size} lines (or columns).

If @var{size} is @code{nil}, this function obeys the variables
@code{window-min-height} and @code{window-min-width} (@pxref{Window
Sizes}).  Thus, it signals an error if splitting would result in making
a window smaller than those variables specify.  However, a
non-@code{nil} value for @var{size} causes those variables to be
ignored; in that case, the smallest allowable window is considered to be
one that has space for a text area one line tall and/or two columns
wide.

Hence, if @var{size} is specified, it's the caller's responsibility to
check whether the emanating windows are large enough to encompass all
areas like a mode line or a scroll bar.  The function
@code{window-min-size} (@pxref{Window Sizes}) can be used to determine
the minimum requirements of @var{window} in this regard.  Since the new
window usually inherits areas like the mode line or the scroll bar
from @var{window}, that function is also a good guess for the minimum
size of the new window.  The caller should specify a smaller size only
if it correspondingly removes an inherited area before the next
redisplay.

The optional third argument @var{side} determines the position of the
new window relative to @var{window}.  If it is @code{nil} or
@code{below}, the new window is placed below @var{window}.  If it is
@code{above}, the new window is placed above @var{window}.  In both
these cases, @var{size} specifies a total window height, in lines.

If @var{side} is @code{t} or @code{right}, the new window is placed on
the right of @var{window}.  If @var{side} is @code{left}, the new
window is placed on the left of @var{window}.  In both these cases,
@var{size} specifies a total window width, in columns.

The optional fourth argument @var{pixelwise}, if non-@code{nil}, means
to interpret @var{size} in units of pixels, instead of lines and
columns.

If @var{window} is a live window, the new window inherits various
properties from it, including margins and scroll bars.  If
@var{window} is an internal window, the new window inherits the
properties of the window selected within @var{window}'s frame.

The behavior of this function may be altered by the window parameters
of @var{window}, so long as the variable
@code{ignore-window-parameters} is @code{nil}.  If the value of
the @code{split-window} window parameter is @code{t}, this function
ignores all other window parameters.  Otherwise, if the value of the
@code{split-window} window parameter is a function, that function is
called with the arguments @var{window}, @var{size}, and @var{side}, in
lieu of the usual action of @code{split-window}.  Otherwise, this
function obeys the @code{window-atom} or @code{window-side} window
parameter, if any.  @xref{Window Parameters}.
@end defun

  As an example, here is a sequence of @code{split-window} calls that
yields the window configuration discussed in @ref{Windows and Frames}.
This example demonstrates splitting a live window as well as splitting
an internal window.  We begin with a frame containing a single window
(a live root window), which we denote by @var{W4}.  Calling
@code{(split-window W4)} yields this window configuration:

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W4_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W5_________________||
    |__________________W3__________________|

@end group
@end smallexample

@noindent
The @code{split-window} call has created a new live window, denoted by
@var{W5}.  It has also created a new internal window, denoted by
@var{W3}, which becomes the root window and the parent of both
@var{W4} and @var{W5}.

  Next, we call @code{(split-window W3 nil 'left)}, passing the
internal window @var{W3} as the argument.  The result:

@smallexample
@group
     ______________________________________
    | ______  ____________________________ |
    ||      || __________________________ ||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||____________W4____________|||
    ||      || __________________________ ||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||____________W5____________|||
    ||__W2__||_____________W3_____________ |
    |__________________W1__________________|
@end group
@end smallexample

@noindent
A new live window @var{W2} is created, to the left of the internal
window @var{W3}.  A new internal window @var{W1} is created, becoming
the new root window.

   For interactive use, Emacs provides two commands which always split
the selected window.  These call @code{split-window} internally.

@deffn Command split-window-right &optional size
This function splits the selected window into two side-by-side
windows, putting the selected window on the left.  If @var{size} is
positive, the left window gets @var{size} columns; if @var{size} is
negative, the right window gets @minus{}@var{size} columns.
@end deffn

@deffn Command split-window-below &optional size
This function splits the selected window into two windows, one above
the other, leaving the upper window selected.  If @var{size} is
positive, the upper window gets @var{size} lines; if @var{size} is
negative, the lower window gets @minus{}@var{size} lines.
@end deffn

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

If it is @code{nil}, @code{split-window-below} adjusts point in each
of the two windows to minimize redisplay.  (This is useful on slow
terminals.)  It selects whichever window contains the screen line that
point was previously on.  Note that this only affects
@code{split-window-below}, not the lower-level @code{split-window}
function.
@end defopt


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

  @dfn{Deleting} a window removes it from the frame's window tree.  If
the window is a live window, it disappears from the screen.  If the
window is an internal window, its child windows are deleted too.

  Even after a window is deleted, it continues to exist as a Lisp
object, until there are no more references to it.  Window deletion can
be reversed, by restoring a saved window configuration (@pxref{Window
Configurations}).

@deffn Command delete-window &optional window
This function removes @var{window} from display and returns
@code{nil}.  If @var{window} is omitted or @code{nil}, it defaults to
the selected window.

If deleting the window would leave no more windows in the window tree
(e.g., if it is the only live window in the frame) or all remaining
windows on @var{window}'s frame are side windows (@pxref{Side Windows}),
an error is signaled.

By default, the space taken up by @var{window} is given to one of its
adjacent sibling windows, if any.  However, if the variable
@code{window-combination-resize} is non-@code{nil}, the space is
proportionally distributed among any remaining windows in the same
window combination.  @xref{Recombining Windows}.

The behavior of this function may be altered by the window parameters of
@var{window}, so long as the variable @code{ignore-window-parameters} is
@code{nil}.  If the value of the @code{delete-window} window parameter
is @code{t}, this function ignores all other window parameters.
Otherwise, if the value of the @code{delete-window} window parameter is
a function, that function is called with the argument @var{window}, in
lieu of the usual action of @code{delete-window}.  @xref{Window
Parameters}.
@end deffn

@deffn Command delete-other-windows &optional window
This function makes @var{window} fill its frame, deleting other windows
as necessary.  If @var{window} is omitted or @code{nil}, it defaults to
the selected window.  An error is signaled if @var{window} is a side
window (@pxref{Side Windows}).  The return value is @code{nil}.

The behavior of this function may be altered by the window parameters of
@var{window}, so long as the variable @code{ignore-window-parameters} is
@code{nil}.  If the value of the @code{delete-other-windows} window
parameter is @code{t}, this function ignores all other window
parameters.  Otherwise, if the value of the @code{delete-other-windows}
window parameter is a function, that function is called with the
argument @var{window}, in lieu of the usual action of
@code{delete-other-windows}.  @xref{Window Parameters}.

Also, if @code{ignore-window-parameters} is @code{nil}, this function
does not delete any window whose @code{no-delete-other-windows}
parameter is non-@code{nil}.
@end deffn

@deffn Command delete-windows-on &optional buffer-or-name frame
This function deletes all windows showing @var{buffer-or-name}, by
calling @code{delete-window} on those windows.  @var{buffer-or-name}
should be a buffer, or the name of a buffer; if omitted or @code{nil},
it defaults to the current buffer.  If there are no windows showing
the specified buffer, this function does nothing.  If the specified
buffer is a minibuffer, an error is signaled.

If there is a dedicated window showing the buffer, and that window is
the only one on its frame, this function also deletes that frame if it
is not the only frame on the terminal.

The optional argument @var{frame} specifies which frames to operate
on:

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

Note that this argument does not have the same meaning as in other
functions which scan all live windows (@pxref{Cyclic Window
Ordering}).  Specifically, the meanings of @code{t} and @code{nil} here
are the opposite of what they are in those other functions.
@end deffn


@node Recombining Windows
@section Recombining Windows
@cindex recombining windows
@cindex windows, recombining

When deleting the last sibling of a window @var{W}, its parent window
is deleted too, with @var{W} replacing it in the window tree.  This
means that @var{W} must be recombined with its parent's siblings to
form a new window combination (@pxref{Windows and Frames}).  In some
occasions, deleting a live window may even entail the deletion of two
internal windows.

@smallexample
@group
     ______________________________________
    | ______  ____________________________ |
    ||      || __________________________ ||
    ||      ||| ___________  ___________ |||
    ||      ||||           ||           ||||
    ||      ||||____W6_____||_____W7____||||
    ||      |||____________W4____________|||
    ||      || __________________________ ||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||____________W5____________|||
    ||__W2__||_____________W3_____________ |
    |__________________W1__________________|

@end group
@end smallexample

@noindent
Deleting @var{W5} in this configuration normally causes the deletion of
@var{W3} and @var{W4}.  The remaining live windows @var{W2},
@var{W6} and @var{W7} are recombined to form a new horizontal
combination with parent @var{W1}.

   Sometimes, however, it makes sense to not delete a parent window like
@var{W4}.  In particular, a parent window should not be removed when it
was used to preserve a combination embedded in a combination of the same
type.  Such embeddings make sense to assure that when you split a window
and subsequently delete the new window, Emacs reestablishes the layout
of the associated frame as it existed before the splitting.

   Consider a scenario starting with two live windows @var{W2} and
@var{W3} and their parent @var{W1}.

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W2_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W3_________________||
    |__________________W1__________________|

@end group
@end smallexample

@noindent
Split @var{W2} to make a new window @var{W4} as follows.

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W2_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W4_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W3_________________||
    |__________________W1__________________|

@end group
@end smallexample

@noindent
Now, when enlarging a window vertically, Emacs tries to obtain the
corresponding space from its lower sibling, provided such a window
exists.  In our scenario, enlarging @var{W4} will steal space from
@var{W3}.

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W2_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W4_________________||
    | ____________________________________ |
    ||_________________W3_________________||
    |__________________W1__________________|

@end group
@end smallexample

@noindent
Deleting @var{W4} will now give its entire space to @var{W2},
including the space earlier stolen from @var{W3}.

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W2_________________||
    | ____________________________________ |
    ||_________________W3_________________||
    |__________________W1__________________|

@end group
@end smallexample

@noindent
This can be counterintuitive, in particular if @var{W4} were used for
displaying a buffer only temporarily (@pxref{Temporary Displays}), and
you want to continue working with the initial layout.

The behavior can be fixed by making a new parent window when splitting
@var{W2}.  The variable described next allows that to be done.

@defopt window-combination-limit
This variable controls whether splitting a window shall make a new
parent window.  The following values are recognized:

@table @code
@item nil
This means that the new live window is allowed to share the existing
parent window, if one exists, provided the split occurs in the same
direction as the existing window combination (otherwise, a new internal
window is created anyway).

@item window-size
This means that @code{display-buffer} makes a new parent window when
it splits a window and is passed a @code{window-height} or
@code{window-width} entry in the @var{alist} argument (@pxref{Action
Functions}).  Otherwise, window splitting behaves as for a value of
@code{nil}.

@item temp-buffer-resize
In this case @code{with-temp-buffer-window} makes a new parent window
when it splits a window and @code{temp-buffer-resize-mode} is enabled
(@pxref{Temporary Displays}).  Otherwise, window splitting behaves as
for @code{nil}.

@item temp-buffer
In this case @code{with-temp-buffer-window} always makes a new parent
window when it splits an existing window (@pxref{Temporary Displays}).
Otherwise, window splitting behaves as for @code{nil}.

@item display-buffer
This means that when @code{display-buffer} (@pxref{Choosing Window})
splits a window it always makes a new parent window.  Otherwise, window
splitting behaves as for @code{nil}.

@item t
This means that splitting a window always creates a new parent window.
Thus, if the value of this variable is at all times @code{t}, then at
all times every window tree is a binary tree (a tree where each window
except the root window has exactly one sibling).
@end table

The default is @code{window-size}.  Other values are reserved for future
use.

If, as a consequence of this variable's setting, @code{split-window}
makes a new parent window, it also calls
@code{set-window-combination-limit} (see below) on the newly-created
internal window.  This affects how the window tree is rearranged when
the child windows are deleted (see below).
@end defopt

  If @code{window-combination-limit} is @code{t}, splitting @var{W2} in
the initial configuration of our scenario would have produced this:

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    || __________________________________ ||
    |||                                  |||
    |||________________W2________________|||
    || __________________________________ ||
    |||                                  |||
    |||________________W4________________|||
    ||_________________W5_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W3_________________||
    |__________________W1__________________|

@end group
@end smallexample

@noindent
A new internal window @var{W5} has been created; its children are
@var{W2} and the new live window @var{W4}.  Now, @var{W2} is the only
sibling of @var{W4}, so enlarging @var{W4} will try to shrink
@var{W2}, leaving @var{W3} unaffected.  Observe that @var{W5}
represents a vertical combination of two windows embedded in the
vertical combination @var{W1}.

@cindex window combination limit
@defun set-window-combination-limit window limit
This function sets the @dfn{combination limit} of the window
@var{window} to @var{limit}.  This value can be retrieved via the
function @code{window-combination-limit}.  See below for its effects;
note that it is only meaningful for internal windows.  The
@code{split-window} function automatically calls this function, passing
it @code{t} as @var{limit}, provided the value of the variable
@code{window-combination-limit} is @code{t} when it is called.
@end defun

@defun window-combination-limit window
This function returns the combination limit for @var{window}.

The combination limit is meaningful only for an internal window.  If it
is @code{nil}, then Emacs is allowed to automatically delete
@var{window}, in response to a window deletion, in order to group the
child windows of @var{window} with its sibling windows to form a new
window combination.  If the combination limit is @code{t}, the child
windows of @var{window} are never automatically recombined with its
siblings.

If, in the configuration shown at the beginning of this section, the
combination limit of @var{W4} (the parent window of @var{W6} and
@var{W7}) is @code{t}, deleting @var{W5} will not implicitly delete
@var{W4} too.
@end defun

Alternatively, the problems sketched above can be avoided by always
resizing all windows in the same combination whenever one of its windows
is split or deleted.  This also permits splitting windows that would be
otherwise too small for such an operation.

@defopt window-combination-resize
If this variable is @code{nil}, @code{split-window} can only split a
window (denoted by @var{window}) if @var{window}'s screen area is large
enough to accommodate both itself and the new window.

If this variable is @code{t}, @code{split-window} tries to resize all
windows that are part of the same combination as @var{window}, in order
to accommodate the new window.  In particular, this may allow
@code{split-window} to succeed even if @var{window} is a fixed-size
window or too small to ordinarily split.  Furthermore, subsequently
resizing or deleting @var{window} may resize all other windows in its
combination.

The default is @code{nil}.  Other values are reserved for future use.  A
specific split operation may ignore the value of this variable if it is
affected by a non-@code{nil} value of @code{window-combination-limit}.
@end defopt

  To illustrate the effect of @code{window-combination-resize}, consider
the following frame layout.

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W2_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W3_________________||
    |__________________W1__________________|

@end group
@end smallexample

@noindent
If @code{window-combination-resize} is @code{nil}, splitting window
@var{W3} leaves the size of @var{W2} unchanged:

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W2_________________||
    | ____________________________________ |
    ||                                    ||
    ||_________________W3_________________||
    | ____________________________________ |
    ||                                    ||
    ||_________________W4_________________||
    |__________________W1__________________|

@end group
@end smallexample

@noindent
If @code{window-combination-resize} is @code{t}, splitting @var{W3}
instead leaves all three live windows with approximately the same
height:

@smallexample
@group
     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W2_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W3_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||_________________W4_________________||
    |__________________W1__________________|

@end group
@end smallexample

@noindent
Deleting any of the live windows @var{W2}, @var{W3} or @var{W4} will
distribute its space proportionally among the two remaining live
windows.


@node Selecting Windows
@section Selecting Windows
@cindex selecting a window

@defun select-window window &optional norecord
This function makes @var{window} the selected window and the window
selected within its frame (@pxref{Basic Windows}), and selects that
frame.  It also makes @var{window}'s buffer (@pxref{Buffers and
Windows}) current and sets that buffer's value of @code{point} to the
value of @code{window-point} (@pxref{Window Point}) in @var{window}.
@var{window} must be a live window.  The return value is @var{window}.

By default, this function also moves @var{window}'s buffer to the front
of the buffer list (@pxref{Buffer List}) and makes @var{window} the most
recently selected window.  If the optional argument @var{norecord} is
non-@code{nil}, these additional actions are omitted.

In addition, this function by default also tells the display engine to
update the display of @var{window} when its frame gets redisplayed the
next time.  If @var{norecord} is non-@code{nil}, such updates are
usually not performed.  If, however, @var{norecord} equals the special
symbol @code{mark-for-redisplay}, the additional actions mentioned above
are omitted but @var{window} will be nevertheless updated.

Note that sometimes selecting a window is not enough to show it, or
make its frame the top-most frame on display: you may also need to
raise the frame or make sure input focus is directed to that frame.
@xref{Input Focus}.
@end defun

@cindex select window hook
@cindex running a hook when a window gets selected
For historical reasons, Emacs does not run a separate hook whenever a
window gets selected.  Applications and internal routines often
temporarily select a window to perform a few actions on it.  They do
that either to simplify coding---because many functions by default
operate on the selected window when no @var{window} argument is
specified---or because some functions did not (and still do not) take a
window as argument and always operate(d) on the selected window instead.
Running a hook every time a window gets selected for a short time and
once more when the previously selected window gets restored is not
useful.

  However, when its @var{norecord} argument is @code{nil},
@code{select-window} updates the buffer list and thus indirectly runs
the normal hook @code{buffer-list-update-hook} (@pxref{Buffer List}).
Consequently, that hook provides a reasonable way to run a function
whenever a window gets selected more ``permanently''.

  Since @code{buffer-list-update-hook} is also run by functions that are
not related to window management, it will usually make sense to save the
value of the selected window somewhere and compare it with the value of
@code{selected-window} while running that hook.  Also, to avoid false
positives when using @code{buffer-list-update-hook}, it is good practice
that every @code{select-window} call supposed to select a window only
temporarily passes a non-@code{nil} @var{norecord} argument.  If
possible, the macro @code{with-selected-window} (see below) should be
used in such cases.

@cindex most recently selected windows
  The sequence of calls to @code{select-window} with a non-@code{nil}
@var{norecord} argument determines an ordering of windows by their
selection time.  The function @code{get-lru-window} can be used to
retrieve the least recently selected live window (@pxref{Cyclic Window
Ordering}).

@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}.  Hence, this macro is the preferred way to temporarily work
with @var{window} as the selected window without needlessly running
@code{buffer-list-update-hook}.
@end defmac

@defun frame-selected-window &optional frame
This function returns the window on @var{frame} that is selected
within that frame.  @var{frame} should be a live frame; if omitted or
@code{nil}, it defaults to the selected frame.
@end defun

@defun set-frame-selected-window frame window &optional norecord
This function makes @var{window} the window selected within the frame
@var{frame}.  @var{frame} should be a live frame; if @code{nil}, it
defaults to the selected frame.  @var{window} should be a live window;
if @code{nil}, it defaults to the selected window.

If @var{frame} is the selected frame, this makes @var{window} the
selected window.

If the optional argument @var{norecord} is non-@code{nil}, this
function does not alter the list of most recently selected windows,
nor the buffer list.
@end defun

@cindex window use time
@cindex use time of window
@cindex window order by time of last use
@defun window-use-time &optional window
This functions returns the use time of window @var{window}.
@var{window} must be a live window and defaults to the selected one.

The @dfn{use time} of a window is not really a time value, but an
integer that does increase monotonically with each call of
@code{select-window} with a @code{nil} @var{norecord} argument.  The
window with the lowest use time is usually called the least recently
used window while the window with the highest use time is called the
most recently used one (@pxref{Cyclic Window Ordering}).
@end defun


@node Cyclic Window Ordering
@section Cyclic Ordering of Windows
@cindex cyclic ordering of windows
@cindex ordering of windows, cyclic
@cindex window ordering, cyclic

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

  The ordering is determined by a depth-first traversal of each frame's
window tree, retrieving the live windows which are the leaf nodes of the
tree (@pxref{Windows and Frames}).  If the minibuffer is active, the
minibuffer window is included too.  The ordering is cyclic, so the last
window in the sequence is followed by the first one.

@defun next-window &optional window minibuf all-frames
@cindex minibuffer window, and @code{next-window}
This function returns a live window, the one following @var{window} in
the cyclic ordering of windows.  @var{window} should be a live window;
if omitted or @code{nil}, it defaults to the selected window.

The optional argument @var{minibuf} specifies whether minibuffer windows
should be included in the cyclic ordering.  Normally, when @var{minibuf}
is @code{nil}, a minibuffer window is included only if it is currently
active; this matches the behavior of @w{@kbd{C-x o}}.  (Note that a
minibuffer window is active as long as its minibuffer is in use; see
@ref{Minibuffers}).

If @var{minibuf} is @code{t}, the cyclic ordering includes all
minibuffer windows.  If @var{minibuf} is neither @code{t} nor
@code{nil}, minibuffer windows are not included even if they are active.

The optional argument @var{all-frames} specifies which frames to
consider:

@itemize @bullet
@item @code{nil}
means to consider windows on @var{window}'s frame.  If the minibuffer
window is considered (as specified by the @var{minibuf} argument),
then frames that share the minibuffer window are considered too.

@item @code{t}
means to consider windows on all existing frames.

@item @code{visible}
means to consider windows on all visible frames.

@item 0
means to consider windows on all visible or iconified frames.

@item A frame
means to consider windows on that specific frame.

@item Anything else
means to consider windows on @var{window}'s frame, and no others.
@end itemize

If more than one frame is considered, the cyclic ordering is obtained
by appending the orderings for those frames, in the same order as the
list of all live frames (@pxref{Finding All Frames}).
@end defun

@defun previous-window &optional window minibuf all-frames
This function returns a live window, the one preceding @var{window} in
the cyclic ordering of windows.  The other arguments are handled like
in @code{next-window}.
@end defun

@deffn Command other-window count &optional all-frames
This function selects a live window, one @var{count} places from the
selected window in the cyclic ordering of windows.  If @var{count} is
a positive number, it skips @var{count} windows forwards; if
@var{count} is negative, it skips @minus{}@var{count} windows
backwards; if @var{count} is zero, that simply re-selects the selected
window.  When called interactively, @var{count} is the numeric prefix
argument.

The optional argument @var{all-frames} has the same meaning as in
@code{next-window}, like a @code{nil} @var{minibuf} argument to
@code{next-window}.

This function does not select a window that has a non-@code{nil}
@code{no-other-window} window parameter (@pxref{Window Parameters}).
@end deffn

@defun walk-windows fun &optional minibuf all-frames
This function calls the function @var{fun} once for each live window,
with the window as the argument.

It follows the cyclic ordering of windows.  The optional arguments
@var{minibuf} and @var{all-frames} specify the set of windows
included; these have the same arguments as in @code{next-window}.  If
@var{all-frames} specifies a frame, the first window walked is the
first window on that frame (the one returned by
@code{frame-first-window}), not necessarily the selected window.

If @var{fun} changes the window configuration by splitting or deleting
windows, that does not alter the set of windows walked, which is
determined prior to calling @var{fun} for the first time.
@end defun

@defun one-window-p &optional no-mini all-frames
This function returns @code{t} if the selected window is the only live
window, and @code{nil} otherwise.

If the minibuffer window is active, it is normally considered (so that
this function returns @code{nil}).  However, if the optional argument
@var{no-mini} is non-@code{nil}, the minibuffer window is ignored even
if active.  The optional argument @var{all-frames} has the same
meaning as for @code{next-window}.
@end defun

@cindex finding windows
  The following functions return a window which satisfies some
criterion, without selecting it:

@cindex least recently used window
@defun get-lru-window &optional all-frames dedicated not-selected
This function returns a live window which is heuristically the least
recently used.  The optional argument @var{all-frames} has
the same meaning as in @code{next-window}.

If any full-width windows are present, only those windows are
considered.  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 selected
window is never returned, unless it is the only candidate.  However, if
the optional argument @var{not-selected} is non-@code{nil}, this
function returns @code{nil} in that case.
@end defun

@cindex most recently used window
@defun get-mru-window &optional all-frames dedicated not-selected
This function is like @code{get-lru-window}, but it returns the most
recently used window instead.  The meaning of the arguments is the
same as described for @code{get-lru-window}.
@end defun

@cindex largest window
@defun get-largest-window &optional all-frames dedicated not-selected
This function returns the window with the largest area (height times
width).  The optional argument @var{all-frames} specifies the windows to
search, and has the same meaning as in @code{next-window}.

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 selected window is not
a candidate if the optional argument @var{not-selected} is
non-@code{nil}.  If the optional argument @var{not-selected} is
non-@code{nil} and the selected window is the only candidate, this
function returns @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.
@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 calls the function @var{predicate} for each of the
windows in the cyclic order of windows in turn, passing it the window
as an argument.  If the predicate returns non-@code{nil} for any
window, this function stops and returns that window.  If no such
window is found, the return value is @var{default} (which defaults to
@code{nil}).

The optional arguments @var{minibuf} and @var{all-frames} specify the
windows to search, and have the same meanings as in
@code{next-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 for examining and setting
the contents of windows.  @xref{Switching Buffers}, for higher-level
functions for displaying a specific buffer in a window.

@defun window-buffer &optional window
This function returns the buffer that @var{window} is displaying.  If
@var{window} is omitted or @code{nil} it defaults to the selected
window.  If @var{window} is an internal window, this function returns
@code{nil}.
@end defun

@defun set-window-buffer window buffer-or-name &optional keep-margins
This function makes @var{window} display @var{buffer-or-name}.
@var{window} should be a live window; if @code{nil}, it defaults to
the selected window.  @var{buffer-or-name} should be a buffer, or the
name of an existing buffer.  This function does not change which
window is selected, nor does it directly change which buffer is
current (@pxref{Current Buffer}).  Its return value is @code{nil}.

If @var{window} is @dfn{strongly dedicated} to a buffer and
@var{buffer-or-name} does not specify that buffer, this function
signals an error.  @xref{Dedicated Windows}.

By default, this function resets @var{window}'s position, display
margins, fringe widths, and scroll bar settings, based on the local
variables in the specified buffer.  However, if the optional argument
@var{keep-margins} is non-@code{nil}, it leaves @var{window}'s display
margins, fringes and scroll bar settings alone.

When writing an application, you should normally use the higher-level
functions described in @ref{Switching Buffers}, instead of calling
@code{set-window-buffer} directly.

This runs @code{window-scroll-functions}, followed by
@code{window-configuration-change-hook}.  @xref{Window Hooks}.
@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

@defvar buffer-display-time
This buffer-local variable records the time at which a buffer was last
displayed in a window.  The value is @code{nil} if the buffer has
never been displayed.  It is updated each time
@code{set-window-buffer} is called for the buffer, with the value
returned by @code{current-time} (@pxref{Time of Day}).
@end defvar

@defun get-buffer-window &optional buffer-or-name all-frames
This function returns the first window displaying @var{buffer-or-name}
in the cyclic ordering of windows, starting from the selected window
(@pxref{Cyclic Window Ordering}).  If no such window exists, the
return value is @code{nil}.

@var{buffer-or-name} should be a buffer or the name of a buffer; if
omitted or @code{nil}, it defaults to the current buffer.  The
optional argument @var{all-frames} specifies which windows to
consider:

@itemize @bullet
@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.
@item
Any other value means consider windows on the selected frame.
@end itemize

Note that these meanings differ slightly from those of the
@var{all-frames} argument to @code{next-window} (@pxref{Cyclic Window
Ordering}).  This function may be changed in a future version of Emacs
to eliminate this discrepancy.
@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}.  @var{buffer-or-name} should be a buffer or the
name of an existing buffer.  If omitted or @code{nil}, it defaults to
the current buffer.  If the currently selected window displays
@var{buffer-or-name}, it will be the first in the list returned by
this function.

The arguments @var{minibuf} and @var{all-frames} have the same
meanings as in the function @code{next-window} (@pxref{Cyclic Window
Ordering}).  Note that the @var{all-frames} argument does @emph{not}
behave exactly like in @code{get-buffer-window}.
@end defun

@deffn Command replace-buffer-in-windows &optional buffer-or-name
This command replaces @var{buffer-or-name} with some other buffer, in
all windows displaying it.  @var{buffer-or-name} should be a buffer, or
the name of an existing buffer; if omitted or @code{nil}, it defaults to
the current buffer.

The replacement buffer in each window is chosen via
@code{switch-to-prev-buffer} (@pxref{Window History}).  Any dedicated
window displaying @var{buffer-or-name} is deleted if possible
(@pxref{Dedicated Windows}).  If such a window is the only window on its
frame and there are other frames on the same terminal, the frame is
deleted as well.  If the dedicated window is the only window on the only
frame on its terminal, the buffer is replaced anyway.
@end deffn


@node Switching Buffers
@section Switching to a Buffer in a Window
@cindex switching to a buffer
@cindex displaying a buffer

This section describes high-level functions for switching to a specified
buffer in some window.  In general, ``switching to a buffer'' means to
(1) show the buffer in some window, (2) make that window the selected
window (and its frame the selected frame), and (3) make the buffer the
current buffer.

  Do @emph{not} use these functions to make a buffer temporarily
current just so a Lisp program can access or modify it.  They have
side-effects, such as changing window histories (@pxref{Window
History}), which will surprise the user if used that way.  If you want
to make a buffer current to modify it in Lisp, use
@code{with-current-buffer}, @code{save-current-buffer}, or
@code{set-buffer}.  @xref{Current Buffer}.

@deffn Command switch-to-buffer buffer-or-name &optional norecord force-same-window
This command attempts to display @var{buffer-or-name} in the selected
window and make it the current buffer.  It is often used interactively
(as the binding of @kbd{C-x b}), as well as in Lisp programs.  The
return value is the buffer switched to.

If @var{buffer-or-name} is @code{nil}, it defaults to the buffer
returned by @code{other-buffer} (@pxref{Buffer List}).  If
@var{buffer-or-name} is a string that is not the name of any existing
buffer, this function creates a new buffer with that name; the new
buffer's major mode is determined by the variable @code{major-mode}
(@pxref{Major Modes}).

Normally, the specified buffer is put at the front of the buffer
list---both the global buffer list and the selected frame's buffer
list (@pxref{Buffer List}).  However, this is not done if the
optional argument @var{norecord} is non-@code{nil}.

Sometimes, the selected window may not be suitable for displaying the
buffer.  This happens if the selected window is a minibuffer window, or
if the selected window is strongly dedicated to its buffer
(@pxref{Dedicated Windows}).  In such cases, the command normally tries
to display the buffer in some other window, by invoking
@code{pop-to-buffer} (see below).

If the optional argument @var{force-same-window} is non-@code{nil} and
the selected window is not suitable for displaying the buffer, this
function always signals an error when called non-interactively.  In
interactive use, if the selected window is a minibuffer window, this
function will try to use some other window instead.  If the selected
window is strongly dedicated to its buffer, the option
@code{switch-to-buffer-in-dedicated-window} described next can be used
to proceed.
@end deffn

@defopt switch-to-buffer-in-dedicated-window
This option, if non-@code{nil}, allows @code{switch-to-buffer} to
proceed when called interactively and the selected window is strongly
dedicated to its buffer.

The following values are respected:

@table @code
@item nil
Disallows switching and signals an error as in non-interactive use.

@item prompt
Prompts the user whether to allow switching.

@item pop
Invokes @code{pop-to-buffer} to proceed.

@item t
Marks the selected window as non-dedicated and proceeds.
@end table

This option does not affect non-interactive calls of
@code{switch-to-buffer}.
@end defopt

By default, @code{switch-to-buffer} tries to preserve
@code{window-point}.  This behavior can be tuned using the following
option.

@defopt switch-to-buffer-preserve-window-point
If this variable is @code{nil}, @code{switch-to-buffer} displays the
buffer specified by @var{buffer-or-name} at the position of that
buffer's @code{point}.  If this variable is @code{already-displayed}, it
tries to display the buffer at its previous position in the selected
window, provided the buffer is currently displayed in some other window
on any visible or iconified frame.  If this variable is @code{t},
@code{switch-to-buffer} unconditionally tries to display the buffer at
its previous position in the selected window.

This variable is ignored if the buffer is already displayed in the
selected window or never appeared in it before, or if
@code{switch-to-buffer} calls @code{pop-to-buffer} to display the
buffer.
@end defopt

The next two commands 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 displays the buffer specified by @var{buffer-or-name} in
some window other than the selected window.  It uses the function
@code{pop-to-buffer} internally (see below).

If the selected window already displays the specified buffer, it
continues to do so, but another window is nonetheless found to display
it as well.

The @var{buffer-or-name} and @var{norecord} arguments have the same
meanings as in @code{switch-to-buffer}.
@end deffn

@deffn Command switch-to-buffer-other-frame buffer-or-name &optional norecord
This function displays the buffer specified by @var{buffer-or-name} in a
new frame.  It uses the function @code{pop-to-buffer} internally (see
below).

If the specified buffer is already displayed in another window, in any
frame on the current terminal, this switches to that window instead of
creating a new frame.  However, the selected window is never used for
this.

The @var{buffer-or-name} and @var{norecord} arguments have the same
meanings as in @code{switch-to-buffer}.
@end deffn

The above commands use the function @code{pop-to-buffer}, which
flexibly displays a buffer in some window and selects that window for
editing.  In turn, @code{pop-to-buffer} uses @code{display-buffer} for
displaying the buffer.  Hence, all the variables affecting
@code{display-buffer} will affect it as well.  @xref{Choosing Window},
for the documentation of @code{display-buffer}.

@deffn Command pop-to-buffer buffer-or-name &optional action norecord
This function makes @var{buffer-or-name} the current buffer and
displays it in some window, preferably not the window currently
selected.  It then selects the displaying window.  If that window is
on a different graphical frame, that frame is given input focus if
possible (@pxref{Input Focus}).

If @var{buffer-or-name} is @code{nil}, it defaults to the buffer
returned by @code{other-buffer} (@pxref{Buffer List}).  If
@var{buffer-or-name} is a string that is not the name of any existing
buffer, this function creates a new buffer with that name; the new
buffer's major mode is determined by the variable @code{major-mode}
(@pxref{Major Modes}).  In any case, that buffer is made current and
returned, even when no suitable window was found to display it.

If @var{action} is non-@code{nil}, it should be a display action to
pass to @code{display-buffer} (@pxref{Choosing Window}).
Alternatively, a non-@code{nil}, non-list value means to pop to a
window other than the selected one---even if the buffer is already
displayed in the selected window.

Like @code{switch-to-buffer}, this function updates the buffer list
unless @var{norecord} is non-@code{nil}.
@end deffn


@node Displaying Buffers
@section Displaying a Buffer in a Suitable Window

In this section we describe how Emacs finds or creates a window
suitable for displaying a buffer.  We first introduce the function
@code{display-buffer}---the workhorse for choosing such a window.
Next, action functions---auxiliary functions called by
@code{display-buffer} to find or create a suitable window---are
presented.  Then we describe action alists---special association lists
used to fine-tune the behavior of action functions.

   We continue with the description of additional options to customize
the behavior of @code{display-buffer}.  Then a series of examples will
try to explain the precedence among action functions in a single call
of @code{display-buffer}.  We conclude this section with some
guidelines for managing the complexity of buffer display.

@menu
* Choosing Window::         How to choose a window for displaying a buffer.
* Action Functions::        Support functions for buffer display.
* Action Alists::           Alists for fine-tuning buffer display.
* Choosing Window Options:: Extra options affecting how buffers are displayed.
* Precedence of Action Functions:: Examples to explain the precedence of
                              action functions.
* The Zen of Buffer Display:: How to avoid that buffers get lost in between
                              windows.
@end menu


@node Choosing Window
@subsection Choosing a Window for Displaying a Buffer

The command @code{display-buffer} flexibly chooses a window for
display, and displays a specified buffer in that window.  It can be
called interactively, via the key binding @kbd{C-x 4 C-o}.  It is also
used as a subroutine by many functions and commands, including
@code{switch-to-buffer} and @code{pop-to-buffer} (@pxref{Switching
Buffers}).

@cindex display action
@cindex action function, for @code{display-buffer}
@cindex action alist, for @code{display-buffer}
  This command performs several complex steps to find a window to
display in.  These steps are described by means of @dfn{display
actions}, which have the form @code{(@var{function} . @var{alist})}.
Here, @var{function} is either a function or a list of functions,
which we refer to as @dfn{action functions}; @var{alist} is an
association list, which we refer to as an @dfn{action alist}.

  An action function accepts two arguments: the buffer to display and
an action alist.  It attempts to display the buffer in some window,
picking or creating a window according to its own criteria.  If
successful, it returns the window; otherwise, it returns @code{nil}.
@xref{Action Functions}, for a list of predefined action functions.
@xref{Action Alists}, for a list of action alist entries recognized by
these functions.

  @code{display-buffer} works by combining display actions from
several sources, and calling the action functions in turn, until one
of them manages to display the buffer and returns a non-@code{nil}
value.

@deffn Command display-buffer buffer-or-name &optional action frame
This command makes @var{buffer-or-name} appear in some window, without
selecting the window or making the buffer current.  The argument
@var{buffer-or-name} must be a buffer or the name of an existing
buffer.  The return value is the window chosen to display the buffer.
It is @code{nil} if no suitable window was found.

The optional argument @var{action}, if non-@code{nil}, should normally
be a display action (described above).  @code{display-buffer} builds a
list of action functions and an action alist, by consolidating display
actions from the following sources (in order of their precedence,
highest ranking first):

@itemize
@item
The variable @code{display-buffer-overriding-action}.

@item
The user option @code{display-buffer-alist}.

@item
The @var{action} argument.

@item
The user option @code{display-buffer-base-action}.

@item
The constant @code{display-buffer-fallback-action}.
@end itemize

@noindent
In practice this means that @code{display-buffer} builds a list of all
action functions specified by these display actions.  The first
element of this list is the first action function specified by
@code{display-buffer-overriding-action}, if any.  The last element of
this list is @code{display-buffer-pop-up-frame}---the last action
function specified by @code{display-buffer-fallback-action}.
Duplicates are not removed from this list---hence the same action
function may be called multiple times during one call of
@code{display-buffer}.

@code{display-buffer} calls the action functions specified by this
list in turn, passing the buffer as the first argument and the
combined action alist as the second argument, until one of the
functions returns non-@code{nil}.

Note that the second argument is always the list of @emph{all} action
alist entries specified by the sources named above.  Hence, the first
element of this list is the first action alist entry specified by
@code{display-buffer-overriding-action}, if any.  The last element of
this list is the last alist entry specified by
@code{display-buffer-base-action}, if any (the action alist specified
by @code{display-buffer-fallback-action} is empty).

Note also, that the combined action alist may contain duplicate
entries and entries for the same key with different values.  As a
rule, action functions always use the first association for each key
so users may not expect that the association found by the action
function is the one provided by the same display action,
see @ref{Precedence of Action Functions} for an example.

The argument @var{action} can also have a non-@code{nil}, non-list
value.  This has the special meaning that the buffer should be
displayed in a window other than the selected one, even if the
selected window is already displaying it.  If called interactively
with a prefix argument, @var{action} is @code{t}.

The optional argument @var{frame}, if non-@code{nil}, specifies which
frames to check when deciding whether the buffer is already displayed.
It is equivalent to adding an element @code{(reusable-frames
. @var{frame})} to the action alist of @var{action} (@pxref{Action
Alists}).
@end deffn

@defvar display-buffer-overriding-action
The value of this variable should be a display action, which is
treated with the highest priority by @code{display-buffer}.  The
default value is empty, i.e., @code{(nil . nil)}.
@end defvar

@defopt display-buffer-alist
The value of this option is an alist mapping conditions to display
actions.  Each condition may be either a regular expression matching a
buffer name or a function that takes two arguments: a buffer name and
the @var{action} argument passed to @code{display-buffer}.  If the name
of the buffer passed to @code{display-buffer} either matches a regular
expression in this alist or the function specified by a condition
returns non-@code{nil}, then @code{display-buffer} uses the
corresponding display action to display the buffer.
@end defopt

@defopt display-buffer-base-action
The value of this option should be a display action.  This option can
be used to define a standard display action for calls to
@code{display-buffer}.
@end defopt

@defvr Constant display-buffer-fallback-action
This display action specifies the fallback behavior for
@code{display-buffer} if no other display actions are given.
@end defvr


@node Action Functions
@subsection Action Functions for Buffer Display

The following basic action functions are defined in Emacs.  Each of
these functions takes two arguments: @var{buffer}, the buffer to
display, and @var{alist}, an action alist.  As a rule, each action
function is supposed to return a window displaying @var{buffer} if it
succeeds and @code{nil} if it fails (for the sole exception to this
rule see the last function listed
below---@code{display-buffer-no-window}).

@defun display-buffer-same-window buffer alist
This function tries to display @var{buffer} in the selected window.
It fails if the selected window is a minibuffer window or is dedicated
to another buffer (@pxref{Dedicated Windows}).  It also fails if
@var{alist} has a non-@code{nil} @code{inhibit-same-window} entry.
@end defun

@defun display-buffer-reuse-window buffer alist
This function tries to display @var{buffer} by finding a window that
is already displaying it.

If @var{alist} has a non-@code{nil} @code{inhibit-same-window} entry,
the selected window is not eligible for reuse.  The set of frames to
search for a window already displaying @var{buffer} can be specified
with the help of a @code{reusable-frames} action alist entry
(@pxref{Action Alists}).  If @var{alist} contains no
@code{reusable-frames} entry, this function searches just the selected
frame.

If this function chooses a window on another frame, it makes that
frame visible and, unless @var{alist} contains an
@code{inhibit-switch-frame} entry (@pxref{Action Alists}), raises that
frame if necessary.
@end defun

@defun display-buffer-reuse-mode-window buffer alist
This function tries to display @var{buffer} by finding a window
that is displaying a buffer in a given mode.

If @var{alist} contains a @code{mode} entry, its value specifes a
major mode (a symbol) or a list of major modes.  If @var{alist}
contains no @code{mode} entry, the current major mode of @var{buffer}
is used instead.  A window is a candidate if it displays a buffer
whose mode derives from one of the modes specified thusly.

The behavior is also controlled by alist entries for
@code{inhibit-same-window}, @code{reusable-frames} and
@code{inhibit-switch-frame} (@pxref{Action Alists}) as is done in the
function @code{display-buffer-reuse-window}.
@end defun

@defun display-buffer-pop-up-window buffer alist
This function tries to display @var{buffer} by splitting the largest
or least recently-used window (typically one on the selected frame).
It actually performs the split by calling the function specified by
@code{split-window-preferred-function} (@pxref{Choosing Window
Options}).

The size of the new window can be adjusted by supplying
@code{window-height} and @code{window-width} entries in @var{alist}
(@pxref{Action Alists}).  If @var{alist} contains a
@code{preserve-size} entry (@pxref{Action Alists}), Emacs will try to
preserve the size of the new window during future resize operations
(@pxref{Preserving Window Sizes}).

This function fails if no window can be split.  More often than not
this happens because no window is large enough to allow splitting.
Setting @code{split-height-threshold} or @code{split-width-threshold}
(@pxref{Choosing Window Options}) to a lower value may help in this
regard.  It also happens when the selected frame has an
@code{unsplittable} frame parameter; @pxref{Buffer Parameters}.
@end defun

@defun display-buffer-in-previous-window buffer alist
This function tries to display @var{buffer} in a window previously
showing it.  If @var{alist} has a non-@code{nil}
@code{inhibit-same-window} entry, the selected window is not eligible
for reuse.  If @var{alist} contains a @code{reusable-frames} entry,
its value determines which frames to search for a suitable window
(@pxref{Action Alists}).

If @var{alist} has a @code{previous-window} entry and the window
specified by that entry is live and not dedicated to another buffer,
that window will be preferred, even if it never showed @var{buffer}
before.
@end defun

@defun display-buffer-use-some-window buffer alist
This function tries to display @var{buffer} by choosing an existing
window and displaying the buffer in that window.  It can fail if all
windows are dedicated to another buffer (@pxref{Dedicated Windows}).
@end defun

@defun display-buffer-below-selected buffer alist
This function tries to display @var{buffer} in a window below the
selected window.  If there is a window below the selected one and that
window already displays @var{buffer}, it reuses that window.

If there is no such window, this function tries to create a new window
by splitting the selected one and display @var{buffer} there.  It will
also adjust that window's size provided @var{alist} contains a suitable
@code{window-height} or @code{window-width} entry, see above.

If splitting the selected window fails and there is a non-dedicated
window below the selected one showing some other buffer, it uses that
window for showing @var{buffer}.
@end defun

@defun display-buffer-at-bottom buffer alist
This function tries to display @var{buffer} in a window at the bottom
of the selected frame.

This either splits the window at the bottom of the frame or the
frame's root window, or reuses an existing window at the bottom of the
selected frame.
@end defun

@defun display-buffer-pop-up-frame buffer alist
This function creates a new frame, and displays the buffer in that
frame's window.  It actually performs the frame creation by calling
the function specified in @code{pop-up-frame-function}
(@pxref{Choosing Window Options}).  If @var{alist} contains a
@code{pop-up-frame-parameters} entry, the associated value
is added to the newly created frame's parameters.
@end defun

@defun display-buffer-in-child-frame buffer alist
This function tries to display @var{buffer} in a child frame
(@pxref{Child Frames}) of the selected frame, either reusing an existing
child frame or by making a new one.  If @var{alist} has a non-@code{nil}
@code{child-frame-parameters} entry, the corresponding value is an alist
of frame parameters to give the new frame.  A @code{parent-frame}
parameter specifying the selected frame is provided by default.  If the
child frame should be or become the child of another frame, a
corresponding entry must be added to @var{alist}.

The appearance of child frames is largely dependent on the parameters
provided via @var{alist}.  It is advisable to use at least ratios to
specify the size (@pxref{Size Parameters}) and the position
(@pxref{Position Parameters}) of the child frame and to add the
@code{keep-ratio} in order to make sure that the child frame remains
visible.  For other parameters that should be considered see @ref{Child
Frames}.
@end defun

@defun display-buffer-use-some-frame buffer alist
This function tries to display @var{buffer} by trying to find a
frame that meets a predicate (by default any frame other than the
selected frame).

If this function chooses a window on another frame, it makes that
frame visible and, unless @var{alist} contains an
@code{inhibit-switch-frame} entry (@pxref{Action Alists}), raises that
frame if necessary.

If @var{alist} has a non-@code{nil} @code{frame-predicate} entry, its
value is a function taking one argument (a frame), returning
non-@code{nil} if the frame is a candidate; this function replaces the
default predicate.

If @var{alist} has a non-@code{nil} @code{inhibit-same-window} entry,
the selected window is not used; thus if the selected frame has a
single window, it is not used.
@end defun

@defun display-buffer-no-window buffer alist
If @var{alist} has a non-@code{nil} @code{allow-no-window} entry, then
this function does not display @code{buffer} and returns the symbol
@code{fail}.  This will skip the excution of any further display
actions and cause @code{display-buffer} to return @code{nil}.

   It is assumed that when a call of @code{display-buffer} specifies a
non-@code{nil} @code{allow-no-window} entry, the caller can handle a
@code{nil} return value.
@end defun

If the @var{alist} argument of any of these functions contains a
@code{window-parameters} entry, @code{display-buffer} assigns the
elements of the associated value as window parameters of the chosen
window.


@node Action Alists
@subsection Action Alists for Buffer Display

An action alist (@pxref{Choosing Window}) is an association list
mapping predefined symbols recognized by action functions to values
these functions are supposed to interpret accordingly.  In each call,
@code{display-buffer} builds a new, possibly empty, action alist and
passes that entire list on to the action functions it calls.

   By design, action functions are free in their interpretation of
action alist entries.  In fact, some entries like
@code{allow-no-window} or @code{previous-window} have a meaning only
for one or a few action functions and are ignored by the rest.  Other
entries, like @code{inhibit-same-window} or @code{window-parameters},
are supposed to be respected by most action functions including those
provided by application programs and external packages.

   In the previous subsection we have described in detail how
individual action functions interpret the action alist entries they
care about.  Here we give a reference list of all known action alist
entries according to their symbols, together with their values and
action functions that recognize them.  Throughout this list, the terms
``buffer'' will refer to the buffer @code{display-buffer} is supposed
to display and ``value'' to the entry's value.

@table @code
@item allow-no-window
If the value is non-@code{nil}, @code{display-buffer} does not
necessarily have to display the buffer and the caller is prepared to
accept that.  This entry is not intended for user customizations since
there is no guarantee that an arbitrary caller of
@code{display-buffer} will be able to handle the case that no window
will display the buffer.  @code{display-buffer-no-window} is the only
action function that cares about this entry.

@item inhibit-same-window
If the value is non-@code{nil}, this signals that the selected window
must not be used for displaying the buffer.  All action functions that
(re-)use an existing window respect this entry.

@item previous-window
The value must specify a window that may have displayed the buffer
previously.  @code{display-buffer-in-previous-window} will give
preference to such a window provided it is still live and not
dedicated to another buffer.

@item mode
The value is either a major mode or a list of major modes.
@code{display-buffer-reuse-mode-window} may reuse a window whenever
the value specified by this entry matches the major mode of that
window's buffer.  Other action functions ignore such entries.

@item frame-predicate
The value must be a function taking one argument (a frame), supposed
to return non-@code{nil} if that frame is a candidate for displaying
the buffer.  This entry is used by
@code{display-buffer-use-some-frame}.

@item pop-up-frame-parameters
The value specifies an alist of frame parameters to give a new frame,
if one is created.  @code{display-buffer-pop-up-frame} is its one and
only one addressee.

@item reusable-frames
The value specifies the frame(s) to search for a window that can be
reused because it already displays the buffer.  It can be set as
follows:

@itemize @bullet
@item
@code{nil} means consider only windows on the selected frame.
(Actually, the last frame used that is not a minibuffer-only 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

Note that the meaning of @code{nil} differs slightly from that of the
@var{all-frames} argument to @code{next-window} (@pxref{Cyclic Window
Ordering}).

A major client of this is @code{display-buffer-reuse-window} but all
other action functions that try to reuse a window are affected as
well.

@item inhibit-switch-frame
A non-@code{nil} value prevents another frame from being raised or
selected, if the window chosen by @code{display-buffer} is displayed
there.  Primarily affected by this are
@code{display-buffer-use-some-frame} and
@code{display-buffer-reuse-window}.
@code{display-buffer-pop-up-frame} should be affected as well but
there is no guarantee that the window manager will not override it.

@item window-parameters
The value specifies an alist of window parameters to give the chosen
window.  All action functions that choose a window should process this
entry.

@item window-height
The value specifies whether and how to adjust the height of the chosen
window and can be provided as follows:

@itemize @bullet
@item
@code{nil} means to leave the height of the chosen window alone.

@item
A number specifies the desired height of the chosen window.  An
integer specifies the number of lines of the window.  A floating-point
number gives the fraction of the window's height with respect to the
height of the frame's root window.

@item
If the value specifies a function, that function is called with one
argument---the chosen window.  The function is supposed to adjust the
height of the window; its return value is ignored.  Suitable functions
are @code{shrink-window-if-larger-than-buffer} and
@code{fit-window-to-buffer}, see @ref{Resizing Windows}.
@end itemize

All action functions that choose a window should process this entry.

@item window-width
This entry is similar to the @code{window-height} entry described
before but can be used to adjust the chosen window's width instead.
The value can be one of the following:

@itemize @bullet
@item
@code{nil} means to leave the width of the chosen window alone.

@item
A number specifies the desired width of the chosen window.  An integer
specifies the number of columns of the window.  A floating-point
number gives the fraction of the window's width with respect to the
width of the frame's root window.

@item
If the value specifies a function, that function is called with one
argument---the chosen window.  The function is supposed to adjust the
width of the window; its return value is ignored.
@end itemize

All action functions that choose a window should process this entry.

@item preserve-size
If non-@code{nil} such an entry tells Emacs to preserve the size of
the window chosen (@pxref{Preserving Window Sizes}).  The value should
be either @code{(t . nil)} to preserve the width of the window,
@code{(nil . t)} to preserve its height or @code{(t . t)} to preserve
both, its width and its height.  All action functions that choose a
window should process this entry.

@item child-frame-parameters
The value specifies an alist of frame parameters used when the buffer
is displayed on a child frame.  This entry is used by
@code{display-buffer-in-child-frame} only.

@item side
The value denotes the side of the frame or window where a new window
displaying the buffer shall be created.  This entry is used by
@code{display-buffer-in-side-window} to indicate the side of the frame
where a new side window shall be placed (@pxref{Displaying Buffers in
Side Windows}).  It is also used by
@code{display-buffer-in-atom-window} to indicate the side of an
existing window where the new window shall be located (@pxref{Atomic
Windows}).

@item slot
If non-@code{nil}, the value specifies the slot of the side window
supposed to display the buffer.  This entry is used by
@code{display-buffer-in-side-window} only (@pxref{Displaying Buffers
in Side Windows}).

@item window
The value specifies a window that is in some way related to the window
chosen by @code{display-buffer}.  This entry is currently used by
@code{display-buffer-in-atom-window} to indicate the window on whose
side the new window shall be created (@pxref{Atomic Windows}).
@end table


@node Choosing Window Options
@subsection Additional Options for Displaying Buffers

The behavior of the display actions of @code{display-buffer}
(@pxref{Choosing Window}) can be modified by the following user
options.

@defopt pop-up-windows
If the value of this variable is non-@code{nil}, @code{display-buffer}
is allowed to split an existing window to make a new window for
displaying in.  This is the default.

This variable is provided for backward compatibility only.  It is
obeyed by @code{display-buffer} via a special mechanism in
@code{display-buffer-fallback-action}, which only calls the action
function @code{display-buffer-pop-up-window} (@pxref{Action
Functions}) when the value is @code{nil}.  It is not consulted by
@code{display-buffer-pop-up-window} itself, which the user may specify
directly in @code{display-buffer-alist} etc.
@end defopt

@defopt split-window-preferred-function
This variable specifies a function for splitting a window, in order to
make a new window for displaying a buffer.  It is used by the
@code{display-buffer-pop-up-window} action function to actually split
the window (@pxref{Action Functions}).

The default value is @code{split-window-sensibly}, which is documented
below.  The value must be a function that takes one argument, a window,
and return either a new window (which will be used to display the
desired buffer) or @code{nil} (which means the splitting failed).
@end defopt

@defun split-window-sensibly &optional window
This function tries to split @var{window}, and return the newly created
window.  If @var{window} cannot be split, it returns @code{nil}.  If
@var{window} is omitted or @code{nil}, it defaults to the selected
window.

This function obeys the usual rules that determine when a window may
be split (@pxref{Splitting Windows}).  It first tries to split by
placing the new window below, subject to the restriction imposed by
@code{split-height-threshold} (see below), in addition to any other
restrictions.  If that fails, it tries to split by placing the new
window to the right, subject to @code{split-width-threshold} (see
below).  If that fails, and the window is the only window on its
frame, this function again tries to split and place the new window
below, disregarding @code{split-height-threshold}.  If this fails as
well, this function gives up and returns @code{nil}.
@end defun

@defopt split-height-threshold
This variable specifies whether @code{split-window-sensibly} is
allowed to split the window placing the new window below.  If it is an
integer, that means to split only if the original window has at least
that many lines.  If it is @code{nil}, that means not to split this
way.
@end defopt

@defopt split-width-threshold
This variable specifies whether @code{split-window-sensibly} is
allowed to split the window placing the new window to the right.  If
the value is an integer, that means to split only if the original
window has at least that many columns.  If the value is @code{nil},
that means not to split this way.
@end defopt

@defopt even-window-sizes
This variable, if non-@code{nil}, causes @code{display-buffer} to even
window sizes whenever it reuses an existing window and that window is
adjacent to the selected one.

If its value is @code{width-only}, sizes are evened only if the reused
window is on the left or right of the selected one and the selected
window is wider than the reused one.  If its value is @code{height-only}
sizes are evened only if the reused window is above or beneath the
selected window and the selected window is higher than the reused one.
Any other non-@code{nil} value means to even sizes in any of these cases
provided the selected window is larger than the reused one in the sense
of their combination.
@end defopt

@defopt pop-up-frame-function
This variable specifies a function for creating a new frame, in order
to make a new window for displaying a buffer.  It is used by the
@code{display-buffer-pop-up-frame} action function (@pxref{Action
Functions}).

The value should be a function that takes no arguments and returns a
frame, or @code{nil} if no frame could be created.  The default value
is a function that creates a frame using the parameters specified by
@code{pop-up-frame-alist} (see below).
@end defopt

@defopt pop-up-frame-alist
This variable holds an alist of frame parameters (@pxref{Frame
Parameters}), which is used by the function specified by
@code{pop-up-frame-function} to make a new frame.  The default is
@code{nil}.

This option is provided for backward compatibility only.  Note,
however that when @code{display-buffer-pop-up-frame} calls the
function specified by @code{pop-up-frame-function}, it prepends the
value of all @code{pop-up-frame-parameters} action alist entries to
@code{pop-up-frame-alist} so that the values specified by the action
alist entry effectively override the corresponding values of
@code{pop-up-frame-alist}.

Hence, users should set up a @code{pop-up-frame-parameters} action
alist entry in @code{display-buffer-alist} instead of customizing
@code{pop-up-frame-alist}.  Only this will guarantee that the value of
a parameter specified by the user overrides the value of that
parameter specified by the caller of @code{display-buffer}.
@end defopt

   Many efforts in the design of @code{display-buffer} have been given
to maintain compatibility with code that uses older options like
@code{pop-up-windows}, @code{pop-up-frames},
@code{pop-up-frame-alist}, @code{same-window-buffer-names} and
@code{same-window-regexps}.  Applications and users should refrain
from using these options in future code.  Above we already warned
against customizing @code{pop-up-frame-alist}.  Here we describe how
to convert the remaining options to use display actions instead.

@table @code
@item pop-up-windows
This variable is @code{nil} by default.  Instead of customizing it to
@code{nil} and thus telling @code{display-buffer} what not to do, it's
much better to list in @code{display-buffer-base-action} the actions
@code{display-buffer} should do instead as, for example:

@example
@group
(customize-set-variable
 'display-buffer-base-action
 '((display-buffer-reuse-window display-buffer-pop-up-frame
    display-buffer-in-previous-window display-buffer-use-some-window)))
@end group
@end example

@item pop-up-frames
Instead of customizing this variable to @code{t}, customize
@code{display-buffer-base-action}, for example, as follows:

@example
@group
(customize-set-variable
 'display-buffer-base-action
 '((display-buffer-reuse-window display-buffer-pop-up-frame)
   (reusable-frames . 0)))
@end group
@end example

@item same-window-buffer-names
@itemx same-window-regexps
Instead of adding a buffer name or a regular expression to one of
these options use a @code{display-buffer-alist} entry for that buffer
specifying the action function @code{display-buffer-same-window}.
@example
@group
(customize-set-variable
 'display-buffer-alist
 (cons '("\\*foo\\*" (display-buffer-same-window)) display-buffer-alist))
@end group
@end example
@end table


@node Precedence of Action Functions
@subsection Precedence of Action Functions

From the past subsections we already know that @code{display-buffer}
must be supplied with a number of display actions (@pxref{Choosing
Window}) in order to display a buffer.  In a completely uncustomized
Emacs, these actions are specified by
@code{display-buffer-fallback-action} in the following order of
precedence: Reuse a window, pop up a new window on the same frame, use
a window previously showing the buffer, use some window and pop up a
new frame.  (The remaining actions in the default value of
@code{display-buffer-fallback-action} are void in an uncustomized
Emacs).

Let's now consider the following form:

@example
(display-buffer (get-buffer-create "*foo*"))
@end example

@noindent
Put this form into the buffer @file{*scratch*} of an uncustomized
Emacs session you just started and evaluate it: @code{display-buffer}
will fail to reuse a window showing @file{*foo*} but succeed in
popping up a new window.  If you evaluate the form again, nothing will
change---@code{display-buffer} reused the window already showing
@file{*foo*} because that action was applicable and had the highest
precedence among all applicable actions.

   Popping up a new window will fail if there is not enough space on
the selected frame.  In an uncustomized Emacs it typically fails when
there are already two windows on a frame.  For example, if you now
type @w{@kbd{C-x 1}} followed by @w{@kbd{C-x 2}} and evaluate the form
once more, @file{*foo*} should show up in the lower
window---@code{display-buffer} just used ``some'' window.  If, before
typing @w{@kbd{C-x 2}} you had typed @w{@kbd{C-x o}}, @file{*foo*}
would have been shown in the upper window because ``some'' window
stands for the ``least recently used'' window and the selected window
has been least recently used if and only if it is alone on its frame.

   Let's assume you did not type @w{@kbd{C-x o}} and @file{*foo*} is
shown in the lower window.  Type @w{@kbd{C-x o}} to get there followed
by @w{@kbd{C-x left}} and evaluate the form again.  This should
display @file{*foo*} in the same, lower window because that window had
already shown @file{*foo*} previously and was therefore chosen instead
of some other window (remember the precedence order prescribed by
@code{display-buffer-fallback-action} we sketched above).

  So far we have only observed default behaviors in an uncustomized
Emacs session.  Next let's see how these behaviors can be customized.
The first option we consider is @code{display-buffer-base-action}.  It
provides a very coarse customization which conceptually affects the
display of @emph{any} buffer.  It can be used to supplement the
actions supplied by @code{display-buffer-fallback-action} by
reordering them or by adding actions that are not present there but
fit more closely the user's editing practice.  It can be, however,
also used to change the default behavior in a more profound way.

   Let's consider a user who, as a rule, prefers to display buffers on
another frame.  Such a user might provide the following customization:

@example
@group
(customize-set-variable
 'display-buffer-base-action
 '((display-buffer-reuse-window display-buffer-pop-up-frame)
   (reusable-frames . 0)))
@end group
@end example

@noindent
This setting will cause@code{display-buffer} to first try to find a
window showing the buffer on a visible or iconified frame and, if no
such frame exists, pop up a new frame.  You can observe this behavior
on a graphical system by typing @w{@kbd{C-x 1}} in the window showing
@file{*scratch*} and evaluating the canonical @code{display-buffer}
form.  This will usually create (and give focus to) a new frame whose
root window shows @file{*foo*}.  Iconify that frame and evaluate the
canonical form again: @code{display-buffer} will reuse the window on
the new frame (usually raising the frame and giving it focus too).

   Only if creating a new frame fails, @code{display-buffer} will
apply the actions supplied by @code{display-buffer-fallback-action}
which means to again try to reuse a window, pop up a new window and so
on.  A trivial way to make frame creation fail is supplied by the
following form:

@example
@group
(let ((pop-up-frame-function 'ignore))
  (display-buffer (get-buffer-create "*foo*")))
@end group
@end example

@noindent
We will forget about that form immediately after observing that it
fails to create a new frame and uses a fallback action instead.

   Note that @code{display-buffer-reuse-window} appears redundant in
the customization of @code{display-buffer-base-action} because it is
already part of @code{display-buffer-fallback-action} and should be
tried there anyway.  However, that would fail because due to the
precedence of @code{display-buffer-base-action} over
@code{display-buffer-fallback-action}, at that time
@code{display-buffer-pop-up-frame} would have already won the race.
In fact

@example
@group
(customize-set-variable
 'display-buffer-base-action
 '(display-buffer-pop-up-frame (reusable-frames . 0)))
@end group
@end example

@noindent
would cause @code{display-buffer} to @emph{always} pop up a new frame
which is probably not what our user wants.

   So far, we have only shown how @emph{users} can customize the
default behavior of @code{display-buffer}.  Let us now see how
@emph{applications} can change the course of @code{display-buffer}.
The canonical way to do that is to use the @var{action} argument of
@code{display-buffer} or the function that eventually ends up calling
it.

   Suppose an application wants to display @file{*foo*} preferably
below the selected window (to immediately attract the attention of the
user to the new window) or, if that fails, in a window at the bottom
of the frame.  It could do that with a call like

@example
@group
(display-buffer
 (get-buffer-create "*foo*")
 '((display-buffer-below-selected display-buffer-at-bottom)))
@end group
@end example

@noindent
In order to see how this new, modified form works, delete any frame
showing @file{*foo*}, type @w{@kbd{C-x 1}} followed by @w{@kbd{C-x 2}} in the
window showing @file{*scratch*} and subsequently evaluate that form.
@code{display-buffer} should split the upper window and show
@file{*foo*} in the new window.  Alternatively, if after @w{@kbd{C-x 2}}
you had typed @w{@kbd{C-x o}}, @code{display-buffer} would have split the
window at the bottom instead.

   Suppose now that, before evaluating the new form, you have made the
selected window as small as possible, for example, by evaluating the
form @code{(fit-window-to-buffer)} in that window.  In that case,
@code{display-buffer} would have failed to split the selected window
and would have split the frame's root window instead, effectively
displaying @file{*foo*} at the bottom of the frame.

   In either case, evaluating the new form a second time now should
reuse the window already showing @file{*foo*} since both functions
supplied by the @var{action} argument try to reuse such a window
first.

   By setting the @var{action} argument, an application effectively
overrules any customization of @code{display-buffer-base-action}.  Our
user can now either accept the choice of the application or redouble
by customizing the option @code{display-buffer-alist} as follows:

@example
@group
(customize-set-variable
 'display-buffer-alist
 '(("\\*foo\\*"
    (display-buffer-reuse-window display-buffer-pop-up-frame))))
@end group
@end example

@noindent
Trying this with the new, modified form above in a configuration that
does not show @file{*foo*} anywhere, will display @file{*foo*} on a
separate frame, completely ignoring the @var{action} argument of
@code{display-buffer}.

   Note that we didn't care to specify a @code{reusable-frames} action
alist entry in our specification of @code{display-buffer-alist}.
@code{display-buffer} always takes the first one it finds---in our
case the one specified by @code{display-buffer-base-action}.  If we
wanted to use a different specification, for example, to exclude
iconified frames showing @file{*foo*} from the list of reusable ones,
we would have to specify that separately, however:

@example
@group
(customize-set-variable
 'display-buffer-alist
 '(("\\*foo\\*"
    (display-buffer-reuse-window display-buffer-pop-up-frame)
    (reusable-frames . visible))))
@end group
@end example

@noindent
If you try this, you will notice that repeated attempts to display
@file{*foo*} will succeed to reuse a frame only if that frame is
visible.

   The above example would allow the conclusion that users customize
@code{display-buffer-alist} for the sole purpose to overrule the
@var{action} argument chosen by applications.  Such a conclusion would
be incorrect.  @code{display-buffer-alist} is the standard option for
users to direct the course of display of specific buffers in a
preferred way regardless of whether the display is also guided by an
@var{action} argument.

   We can, however, reasonably conclude that customizing
@code{display-buffer-alist} differs from customizing
@code{display-buffer-base-action} in two major aspects: It is stronger
because it overrides the @var{action} argument of
@code{display-buffer}.  And it allows to explicitly specify all
affected buffers.  In fact displaying other buffers is not affected in
any way by a customization for @file{*foo*}.  For example,

@example
(display-buffer (get-buffer-create "*bar*"))
@end example

@noindent
continues being governed by the settings of
@code{display-buffer-base-action} and
@code{display-buffer-fallback-action} only.

   We could stop with our examples here but applications still have an
ace up their sleeves which they can use to overrule any customization
of @code{display-buffer-alist}.  It's the variable
@code{display-buffer-overriding-action} which they can bind around
@code{display-buffer} calls as follows:

@example
@group
(let ((display-buffer-overriding-action '((display-buffer-same-window))))
  (display-buffer
   (get-buffer-create "*foo*")
   '((display-buffer-below-selected display-buffer-at-bottom))))
@end group
@end example

@noindent
Evaluating this form will usually display @file{*foo*} in the selected
window regardless of the @var{action} argument and any user
customizations.  It might be illustrative to look at the list of
action functions @code{display-buffer} would have tried to display
@file{*foo*} with the customizations we provided earlier.  The list
(including comments explaining who added this and the subsequent
elements) is:

@example
@group
(display-buffer-same-window  ;; `display-buffer-overriding-action'
 display-buffer-reuse-window ;; `display-buffer-alist'
 display-buffer-pop-up-frame
 display-buffer-below-selected ;; ACTION argument
 display-buffer-at-bottom
 display-buffer-reuse-window ;; `display-buffer-base-action'
 display-buffer-pop-up-frame
 display-buffer--maybe-same-window ;; `display-buffer-fallback-action'
 display-buffer-reuse-window
 display-buffer--maybe-pop-up-frame-or-window
 display-buffer-in-previous-window
 display-buffer-use-some-window
 display-buffer-pop-up-frame)
@end group
@end example

@noindent
Among the internal functions listed here
@code{display-buffer--maybe-same-window} is effectively ignored while
@code{display-buffer--maybe-pop-up-frame-or-window} actually runs
@code{display-buffer-pop-up-window}.

The action alist passed in each function call is:

@example
@group
((reusable-frames . visible)
 (reusable-frames . 0))
@end group
@end example

@noindent
which shows that we have used the second specification of
@code{display-buffer-alist} above.  Suppose our user had written that
as

@example
@group
(customize-set-variable
 'display-buffer-alist
 '(("\\*foo\\*"
    (display-buffer-reuse-window display-buffer-pop-up-frame)
    (inhibit-same-window . t)
    (reusable-frames . visible))))
@end group
@end example

@noindent
In this case the @code{inhibit-same-window} alist entry will
successfully invalidate the @code{display-buffer-same-window}
specification from @code{display-buffer-overriding-action} and
@code{display-buffer} will show @file{*foo*} on another frame.

   This last example shows that while the precedence order of action
functions is fixed as described in @ref{Choosing Window}, an action
alist entry specified by a display action ranked lower in that order
can affect the execution of an higher ranked display action.


@node The Zen of Buffer Display
@subsection The Zen of Buffer Display

In its most simplistic form, a frame accommodates always one single
window that can be used for displaying a buffer.  As a consequence, it
is always the latest call of @code{display-buffer} that will have
succeeded in placing its buffer there.

   Since working with such a frame is not very practical, Emacs by
default allows for more complex frame layouts controlled by the
default values of the frame size and the @code{split-height-threshold}
and @code{split-width-threshold} options.  Displaying a buffer not yet
shown on a frame then either splits the single window on that frame or
(re-)uses one of its two windows.

   The default behavior is abandoned as soon as the user customizes
one of these thresholds or manually changes the frame's layout.  The
default behavior is also abandoned when calling @code{display-buffer}
with a non-@code{nil} @var{action} argument or the user customizes one
of the options mentioned in the previous subsections.  Mastering
@code{display-buffer} soon may become a frustrating experience due to
the plethora of applicable display actions and the resulting frame
layouts.

   However, refraining from using buffer display functions and falling
back on a split & delete windows metaphor is not a good idea either.
Buffer display functions give applications and users a framework to
reconcile their different needs; no comparable framework exists for
splitting and deleting windows.  They also allow to at least partially
restore the layout of a frame when removing a buffer from it later
(@pxref{Quitting Windows}).

   Below we will give a number of guidelines to redeem the frustration
mentioned above and thus to avoid that buffers literally get lost in
between the windows of a frame.

@table @asis
@item Write display actions without stress
Writing display actions can be a pain because one has to lump together
action functions and action alists in one huge list.  (Historical
reasons prevented us from having @code{display-buffer} support
separate arguments for these.)  It might help to memorize some basic
forms like the ones listed below:

@example
'(nil (inhibit-same-window . t))
@end example

@noindent
specifies an action alist entry only and no action function.  Its sole
purpose is to inhibit a @code{display-buffer-same-window} function
specified elsewhere from showing the buffer in the same window, see
also the last example of the preceding subsection.

@example
'(display-buffer-below-selected)
@end example

on the other hand specifies one action function and an empty action
alist.  To combine the effects of the above two specifications one
would write the form

@example
'(display-buffer-below-selected (inhibit-same-window . t))
@end example

to add another action function one would write

@example
@group
'((display-buffer-below-selected display-buffer-at-bottom)
  (inhibit-same-window . t))
@end group
@end example

and to add another alist entry one would write

@example
@group
'((display-buffer-below-selected display-buffer-at-bottom)
  (inhibit-same-window . t) (window-height . fit-window-to-buffer))
@end group
@end example

That last form can be used as @var{action} argument of
@code{display-buffer} in the following way:

@example
@group
(display-buffer
 (get-buffer-create "*foo*")
 '((display-buffer-below-selected display-buffer-at-bottom)
   (inhibit-same-window . t) (window-height . fit-window-to-buffer)))
@end group
@end example

In a customization of @code{display-buffer-alist} it would be used as
follows:

@example
@group
(customize-set-variable
 'display-buffer-alist
 '(("\\*foo\\*"
    (display-buffer-below-selected display-buffer-at-bottom)
    (inhibit-same-window . t) (window-height . fit-window-to-buffer))))
@end group
@end example

To add a customization for a second buffer one would then write:

@example
@group
(customize-set-variable
 'display-buffer-alist
 '(("\\*foo\\*"
    (display-buffer-below-selected display-buffer-at-bottom)
    (inhibit-same-window . t) (window-height . fit-window-to-buffer))
   ("\\*bar\\*"
    (display-buffer-reuse-window display-buffer-pop-up-frame)
    (reusable-frames . visible))))
@end group
@end example

@item Treat each other with respect
@code{display-buffer-alist} and @code{display-buffer-base-action} are
user options---applications must never set or rebind them.
@code{display-buffer-overriding-action}, on the other hand, is
reserved for applications---who seldom use that option and if they use
it, then with utmost care.

   Older implementations of @code{display-buffer} frequently caused
users and applications to fight over the settings of user options like
@code{pop-up-frames} and @code{pop-up-windows} (@pxref{Choosing Window
Options}).  This was one major reason for redesigning
@code{display-buffer}---to provide a clear rule dividing what users
and applications should be allowed to do.

   Applications must be prepared that a user's customizations may
cause buffers to get displayed in an unexpected way.  They should
never assume in their subsequent behavior, that the buffer has been
shown precisely the way they asked for in the @var{action} argument of
@code{display-buffer}.

   Users should not pose too many and too severe restrictions on how
arbitrary buffers gets displayed.  Otherwise, they will risk to lose
the characteristics of showing a buffer for a certain purpose.
Suppose an application has been written to compare different versions
of a buffer in two windows side-by-side.  If the customization of
@code{display-buffer-alist} prescribes that any such buffer should be
always shown in or below the selected window, the application will
have a hard time to set up the desired window configuration via
@code{display-buffer}.

   To specify a preference for showing an arbitrary buffer, users
should customize @code{display-buffer-base-action}.  An example of how
users who prefer working with multiple frames would do that was given
in the previous subsection.  @code{display-buffer-alist} should be
reserved for displaying specific buffers in a specific way.

@item Consider reusing a window that already shows the buffer
Generally, it's always a good idea for users and application
programmers to be prepared for the case that a window already shows
the buffer in question and to reuse that window.  In the preceding
subsection we have shown that failing to do so properly may cause
@code{display-buffer} to continuously pop up a new frame although a
frame showing that buffer existed already.  Only sometimes, it might
be undesirable to reuse a window, for example, when a different
portion of the buffer should be shown in that window.

   Hence, @code{display-buffer-reuse-window} is one action function
that should be used as often as possible, both in @var{action}
arguments and customizations.  An @code{inhibit-same-window} entry in
the @var{action} argument usually takes care of the most common case
where reusing a window showing the buffer should be avoided---that
where the window in question is the selected one.

@item Attract focus to the window chosen
This is a no-brainer for people working with multiple frames---the
frame showing the buffer will automatically raise and get focus unless
an @code{inhibit-switch-frame} entry forbids it.  For single frame
users this task can be considerably more difficult.  In particular,
@code{display-buffer-pop-up-window} and
@code{display-buffer-use-some-window} can become obtrusive in this
regard.  They split or use a seemingly arbitrary (often the largest or
least recently used) window, distracting the user's attention.

Some applications therefore try to choose a window at the bottom of
the frame, for example, in order to display the buffer in vicinity of
the minibuffer window where the user is expected to answer a question
related to the new window.  For non-input related actions
@code{display-buffer-below-selected} might be preferable because the
selected window usually already has the user's attention.

@item Handle subsequent invocations of @code{display-buffer}
@code{display-buffer} is not overly well suited for displaying several
buffers in sequence and making sure that all these buffers are shown
orderly in the resulting window configuration.  Again, the standard
action functions @code{display-buffer-pop-up-window} and
@code{display-buffer-use-some-window} are not very suited for this
purpose due to their somewhat chaotic nature in more complex
configurations.

   To produce a window configuration displaying multiple buffers (or
different views of one and the same buffer) in one and the same
display cycle, application programmers will unavoidably have to write
their own action functions.  A few tricks listed below might help in
this regard.

@itemize @bullet
@item
Making windows atomic (@pxref{Atomic Windows}) avoids that an
existing window composition gets broken when popping up a new window.
The new window will pop up outside the composition instead.

@item
Temporarily dedicating windows to their buffers (@pxref{Dedicated
Windows}) avoids that a window gets used for displaying a different
buffer.  A non-dedicated window will be used instead.

@item
Calling @code{window-preserve-size} (@pxref{Preserving Window Sizes})
will try to keep the size of the argument window unchanged when
popping up a new window.  You have to make sure that another window in
the same combination can be shrunk instead, though.

@item
Side windows (@pxref{Side Windows}) can be used for displaying
specific buffers always in a window at the same position of a frame.
This permits to group buffers that do not compete for being shown at
the same time on a frame and show any such buffer in the same window
without disrupting the display of other buffers.

@item
Child frames (@pxref{Child Frames}) can be used to display a buffer
within the screen estate of the selected frame without disrupting that
frame's window configuration and without the overhead associated with
full-fledged frames as inflicted by @code{display-buffer-pop-up-frame}.
@end itemize
@end table


@node Window History
@section Window History
@cindex window history

Each window remembers in a list the buffers it has previously
displayed, and the order in which these buffers were removed from it.
This history is used, for example, by @code{replace-buffer-in-windows}
(@pxref{Buffers and Windows}), and when quitting windows
(@pxref{Quitting Windows}).  The list is automatically maintained by
Emacs, but you can use the following functions to explicitly inspect
or alter it:

@defun window-prev-buffers &optional window
This function returns a list specifying the previous contents of
@var{window}.  The optional argument @var{window} should be a live
window and defaults to the selected one.

Each list element has the form @code{(@var{buffer} @var{window-start}
@var{window-pos})}, where @var{buffer} is a buffer previously shown in
the window, @var{window-start} is the window start position
(@pxref{Window Start and End}) when that buffer was last shown, and
@var{window-pos} is the point position (@pxref{Window Point}) when
that buffer was last shown in @var{window}.

The list is ordered so that earlier elements correspond to more
recently-shown buffers, and the first element usually corresponds to the
buffer most recently removed from the window.
@end defun

@defun set-window-prev-buffers window prev-buffers
This function sets @var{window}'s previous buffers to the value of
@var{prev-buffers}.  The argument @var{window} must be a live window
and defaults to the selected one.  The argument @var{prev-buffers}
should be a list of the same form as that returned by
@code{window-prev-buffers}.
@end defun

In addition, each buffer maintains a list of @dfn{next buffers}, which
is a list of buffers re-shown by @code{switch-to-prev-buffer} (see
below).  This list is mainly used by @code{switch-to-prev-buffer} and
@code{switch-to-next-buffer} for choosing buffers to switch to.

@defun window-next-buffers &optional window
This function returns the list of buffers recently re-shown in
@var{window} via @code{switch-to-prev-buffer}.  The @var{window}
argument must denote a live window or @code{nil} (meaning the selected
window).
@end defun

@defun set-window-next-buffers window next-buffers
This function sets the next buffer list of @var{window} to
@var{next-buffers}.  The @var{window} argument should be a live window
or @code{nil} (meaning the selected window).  The argument
@var{next-buffers} should be a list of buffers.
@end defun

The following commands can be used to cycle through the global buffer
list, much like @code{bury-buffer} and @code{unbury-buffer}.  However,
they cycle according to the specified window's history list, rather
than the global buffer list.  In addition, they restore
window-specific window start and point positions, and may show a
buffer even if it is already shown in another window.  The
@code{switch-to-prev-buffer} command, in particular, is used by
@code{replace-buffer-in-windows}, @code{bury-buffer} and
@code{quit-window} to find a replacement buffer for a window.

@deffn Command switch-to-prev-buffer &optional window bury-or-kill
This command displays the previous buffer in @var{window}.  The
argument @var{window} should be a live window or @code{nil} (meaning
the selected window).  If the optional argument @var{bury-or-kill} is
non-@code{nil}, this means that the buffer currently shown in
@var{window} is about to be buried or killed and consequently should
not be switched to in future invocations of this command.

The previous buffer is usually the buffer shown before the buffer
currently shown in @var{window}.  However, a buffer that has been buried
or killed, or has been already shown by a recent invocation of
@code{switch-to-prev-buffer}, does not qualify as previous buffer.

If repeated invocations of this command have already shown all buffers
previously shown in @var{window}, further invocations will show buffers
from the buffer list of the frame @var{window} appears on (@pxref{Buffer
List}), trying to skip buffers that are already shown in another window
on that frame.
@end deffn

@deffn Command switch-to-next-buffer &optional window
This command switches to the next buffer in @var{window}, thus undoing
the effect of the last @code{switch-to-prev-buffer} command in
@var{window}.  The argument @var{window} must be a live window and
defaults to the selected one.

If there is no recent invocation of @code{switch-to-prev-buffer} that
can be undone, this function tries to show a buffer from the buffer list
of the frame @var{window} appears on (@pxref{Buffer List}).
@end deffn

By default @code{switch-to-prev-buffer} and @code{switch-to-next-buffer}
can switch to a buffer that is already shown in another window on the
same frame.  The following option can be used to override this behavior.

@defopt switch-to-visible-buffer
If this variable is non-@code{nil}, @code{switch-to-prev-buffer} and
@code{switch-to-next-buffer} may switch to a buffer that is already
visible on the same frame, provided the buffer was shown in the
relevant window before.  If it is @code{nil},
@code{switch-to-prev-buffer} and @code{switch-to-next-buffer} always
try to avoid switching to a buffer that is already visible in another
window on the same frame.  The default is @code{t}.
@end defopt


@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{Cyclic Window Ordering}) 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.

   Functions supposed to remove a buffer from a window or a window from
a frame can behave specially when a window they operate on is dedicated.
We will distinguish three basic cases, namely where (1) the window is
not the only window on its frame, (2) the window is the only window on
its frame but there are other frames on the same terminal left, and (3)
the window is the only window on the only frame on the same terminal.

   In particular, @code{delete-windows-on} (@pxref{Deleting Windows})
handles case (2) by deleting the associated frame and case (3) by
showing another buffer in that frame's only window.  The function
@code{replace-buffer-in-windows} (@pxref{Buffers and Windows}) which is
called when a buffer gets killed, deletes the window in case (1) and
behaves like @code{delete-windows-on} otherwise.
@c FIXME: Does replace-buffer-in-windows _delete_ a window in case (1)?

   When @code{bury-buffer} (@pxref{Buffer List}) operates on the
selected window (which shows the buffer that shall be buried), it
handles case (2) by calling @code{frame-auto-hide-function}
(@pxref{Quitting Windows}) to deal with the selected frame.  The other
two cases are handled as with @code{replace-buffer-in-windows}.

@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 Quitting Windows
@section Quitting Windows

When you want to get rid of a window used for displaying a buffer, you
can call @code{delete-window} or @code{delete-windows-on}
(@pxref{Deleting Windows}) to remove that window from its frame.  If the
buffer is shown on a separate frame, you might want to call
@code{delete-frame} (@pxref{Deleting Frames}) instead.  If, on the other
hand, a window has been reused for displaying the buffer, you might
prefer showing the buffer previously shown in that window, by calling the
function @code{switch-to-prev-buffer} (@pxref{Window History}).
Finally, you might want to either bury (@pxref{Buffer List}) or kill
(@pxref{Killing Buffers}) the window's buffer.

   The following command uses information on how the window for
displaying the buffer was obtained in the first place, thus attempting
to automate the above decisions for you.

@deffn Command quit-window &optional kill window
This command quits @var{window} and buries its buffer.  The argument
@var{window} must be a live window and defaults to the selected one.
With prefix argument @var{kill} non-@code{nil}, it kills the buffer
instead of burying it.  It calls the function @code{quit-restore-window}
described next to deal with the window and its buffer.
@end deffn

@defun quit-restore-window &optional window bury-or-kill
This function handles @var{window} and its buffer after quitting.  The
optional argument @var{window} must be a live window and defaults to
the selected one. The function's behavior is determined by the four
elements of the @code{quit-restore} window parameter (@pxref{Window
Parameters}), which is set to @code{nil} afterwards.

The window is deleted entirely if: 1) the first element of the
@code{quit-restore} parameter is one of 'window or 'frame, 2) the
window has no history of previously-displayed buffers, and 3) the
displayed buffer matches the one in the fourth element of the
@code{quit-restore} parameter.  If @var{window} is the
only window on its frame and there are other frames on the frame's
terminal, the value of the optional argument @var{bury-or-kill}
determines how to proceed with the window.  If @var{bury-or-kill}
equals @code{kill}, the frame is deleted unconditionally.  Otherwise,
the fate of the frame is determined by calling
@code{frame-auto-hide-function} (see below) with that frame as sole
argument.

If the third element of the @code{quit-restore} parameter is a list of
buffer, window start (@pxref{Window Start and End}), and point
(@pxref{Window Point}), and that buffer is still live, the buffer will
be displayed, and start and point set accordingly.  If, in addition,
@var{window}'s buffer was temporarily resized, this function will also
try to restore the original height of @var{window}.

Otherwise, if @var{window} was previously used for displaying other
buffers (@pxref{Window History}), the most recent buffer in that
history will be displayed.

The optional argument @var{bury-or-kill} specifies how to deal with
@var{window}'s buffer.  The following values are handled:

@table @code
@item nil
This means to not deal with the buffer in any particular way.  As a
consequence, if @var{window} is not deleted, invoking
@code{switch-to-prev-buffer} will usually show the buffer again.

@item append
This means that if @var{window} is not deleted, its buffer is moved to
the end of @var{window}'s list of previous buffers, so it's less likely
that a future invocation of @code{switch-to-prev-buffer} will switch to
it.  Also, it moves the buffer to the end of the frame's buffer list.

@item bury
This means that if @var{window} is not deleted, its buffer is removed
from @var{window}'s list of previous buffers.  Also, it moves the buffer
to the end of the frame's buffer list.  This value provides the most
reliable remedy to not have @code{switch-to-prev-buffer} switch to this
buffer again without killing the buffer.

@item kill
This means to kill @var{window}'s buffer.
@end table

Typically, the display routines run by @code{display-buffer} will set
the @code{quit-restore} window parameter correctly.  It's also
possible to set it manually, using the following code for displaying
@var{buffer} in @var{window}:

@example
@group
(display-buffer-record-window type window buffer)

(set-window-buffer window buffer)

(set-window-prev-buffers window nil)
@end group
@end example

Setting the window history to @code{nil} ensures that a future call to
@code{quit-window} can delete the window altogether.

@end defun

The following option specifies how to deal with a frame containing just
one window that should be either quit, or whose buffer should be buried.

@defopt frame-auto-hide-function
The function specified by this option is called to automatically hide
frames.  This function is called with one argument---a frame.

The function specified here is called by @code{bury-buffer}
(@pxref{Buffer List}) when the selected window is dedicated and shows
the buffer to bury.  It is also called by @code{quit-restore-window}
(see above) when the frame of the window to quit has been specially
created for displaying that window's buffer and the buffer is not
killed.

The default is to call @code{iconify-frame} (@pxref{Visibility of
Frames}).  Alternatively, you may specify either @code{delete-frame}
(@pxref{Deleting Frames}) to remove the frame from its display,
@code{make-frame-invisible} to make the frame invisible, @code{ignore}
to leave the frame unchanged, or any other function that can take a
frame as its sole argument.

Note that the function specified by this option is called only if the
specified frame contains just one live window and there is at least one
other frame on the same terminal.

For a particular frame, the value specified here may be overridden by
that frame's @code{auto-hide-function} frame parameter (@pxref{Frame
Interaction Parameters}).
@end defopt


@node Side Windows
@section Side Windows
@cindex side windows
@cindex main window
@cindex main window of a frame

Side windows are special windows positioned at any of the four sides of
a frame's root window (@pxref{Windows and Frames}).  In practice, this
means that the area of the frame's root window is subdivided into a main
window and a number of side windows surrounding that main window.  The
main window is either a ``normal'' live window or specifies the area
containing all the normal windows.

   In their most simple form of use, side windows allow to display
specific buffers always in the same area of a frame.  Hence they can be
regarded as a generalization of the concept provided by
@code{display-buffer-at-bottom} (@pxref{Action Functions}) to
the remaining sides of a frame.  With suitable customizations, however,
side windows can be also used to provide frame layouts similar to those
found in so-called integrated development environments (IDEs).

@menu
* Displaying Buffers in Side Windows:: An action function for displaying
                              buffers in side windows.
* Side Window Options and Functions:: Further tuning of side windows.
* Frame Layouts with Side Windows:: Setting up frame layouts with side
                              windows.
@end menu


@node Displaying Buffers in Side Windows
@subsection Displaying Buffers in Side Windows

The following action function for @code{display-buffer} (@pxref{Action
Functions}) creates or reuses a side window for displaying the
specified buffer.

@defun display-buffer-in-side-window buffer alist
This function displays @var{buffer} in a side window of the selected
frame.  It returns the window used for displaying @var{buffer},
@code{nil} if no such window can be found or created.

@var{alist} is an association list of symbols and values as for
@code{display-buffer}.  The following symbols in @var{alist} are special
for this function:

@table @code
@item side
Denotes the side of the frame where the window shall be located.  Valid
values are @code{left}, @code{top}, @code{right} and @code{bottom}.  If
unspecified, the window is located at the bottom of the frame.

@item slot
Denotes a slot at the specified side where to locate the window.  A
value of zero means to preferably position the window in the middle of
the specified side.  A negative value means to use a slot preceding
(that is, above or on the left of) the middle slot.  A positive value
means to use a slot following (that is, below or on the right of) the
middle slot.  Hence, all windows on a specific side are ordered by their
@code{slot} value.  If unspecified, the window is located in the middle
of the specified side.
@end table

If you specify the same slot on the same side for two or more different
buffers, the buffer displayed last is shown in the corresponding window.
Hence, slots can be used for sharing the same side window between
buffers.

This function installs the @code{window-side} and @code{window-slot}
parameters (@pxref{Window Parameters}) and makes them persistent.  It
does not install any other window parameters unless they have been
explicitly provided via a @code{window-parameters} entry in @var{alist}.
@end defun

By default, side windows cannot be split via @code{split-window}
(@pxref{Splitting Windows}).  Also, a side window is not reused or split
by any buffer display action (@pxref{Action Functions}) unless
it is explicitly specified as target of that action.  Note also that
@code{delete-other-windows} cannot make a side window the only window on
its frame (@pxref{Deleting Windows}).

   Once set up, side windows also change the behavior of the commands
@code{switch-to-prev-buffer} and @code{switch-to-next-buffer}
(@pxref{Window History}).  In particular, these commands will refrain
from showing, in a side window, buffers that have not been displayed in
that window before.  They will also refrain from having a normal,
non-side window show a buffer that has been already displayed in a side
window.  A notable exception to the latter rule occurs when an
application, after displaying a buffer, resets that buffer's local
variables.


@node Side Window Options and Functions
@subsection Side Window Options and Functions

The following options provide additional control over the placement of
side windows.

@defopt window-sides-vertical
If non-@code{nil}, the side windows on the left and right of a frame
occupy the frame's full height.  Otherwise, the side windows on the top
and bottom of the frame occupy the frame's full width.
@end defopt

@defopt window-sides-slots
This option specifies the maximum number of side windows on each side of
a frame.  The value is a list of four elements specifying the number of
side window slots on (in this order) the left, top, right and bottom of
each frame.  If an element is a number, it means to display at most that
many windows on the corresponding side.  If an element is @code{nil}, it
means there's no bound on the number of slots on that side.

If any of the specified values is zero, no window can be created on the
corresponding side.  @code{display-buffer-in-side-window} will not
signal an error in that case, but will return @code{nil}.  If a specified
value just forbids the creation of an additional side window, the most
suitable window on that side is reused and may have its
@code{window-slot} parameter changed accordingly.
@end defopt

@defopt window-sides-reversed
This option specifies whether top/bottom side windows should appear in
reverse order.  When this is @code{nil}, side windows on the top and
bottom of a frame are always drawn from left to right with increasing
slot values.  When this is @code{t}, the drawing order is reversed and
side windows on the top and bottom of a frame are drawn from right to
left with increasing slot values.

When this is @code{bidi}, the drawing order is reversed if and only if
the value of @code{bidi-paragraph-direction} (@pxref{Bidirectional
Display}) is @code{right-to-left} in the buffer displayed in the window
most recently selected within the main window area of this frame.
Sometimes that window may be hard to find, so heuristics are used to
avoid that the drawing order changes inadvertently when another window
gets selected.

The layout of side windows on the left or right of a frame is not
affected by the value of this variable.
@end defopt

When a frame has side windows, the following function returns the main
window of that frame.

@defun window-main-window &optional frame
This function returns the main window of the specified @var{frame}.  The
optional argument @var{frame} must be a live frame and defaults to the
selected one.

If @var{frame} has no side windows, it returns @var{frame}'s root
window.  Otherwise, it returns either an internal non-side window such
that all other non-side windows on @var{frame} descend from it, or the
single live non-side window of @var{frame}.  Note that the main window
of a frame cannot be deleted via @code{delete-window}.
@end defun

The following command is handy to toggle the appearance of all side
windows on a specified frame.

@deffn Command window-toggle-side-windows &optional frame
This command toggles side windows on the specified @var{frame}.  The
optional argument @var{frame} must be a live frame and defaults to the
selected one.

If @var{frame} has at least one side window, this command saves the
state of @var{frame}'s root window in the @var{frame}'s
@code{window-state} frame parameter and deletes all side windows on
@var{frame} afterwards.

If @var{frame} has no side windows, but does have a @code{window-state}
parameter, this command uses that parameter's value to restore the side
windows on @var{frame} leaving @var{frame}'s main window alone.

An error is signaled if @var{frame} has no side windows and no saved
state is found for it.
@end deffn


@node Frame Layouts with Side Windows
@subsection Frame Layouts with Side Windows

Side windows can be used to create more complex frame layouts like those
provided by integrated development environments (IDEs).  In such
layouts, the area of the main window is where the normal editing
activities take place.  Side windows are not conceived for editing in
the usual sense.  Rather, they are supposed to display information
complementary to the current editing activity, like lists of files, tags
or buffers, help information, search or grep results or shell output.

   The layout of such a frame might appear as follows:

@smallexample
@group
     ___________________________________
    |          *Buffer List*            |
    |___________________________________|
    |     |                       |     |
    |  *  |                       |  *  |
    |  d  |                       |  T  |
    |  i  |                       |  a  |
    |  r  |   Main Window Area    |  g  |
    |  e  |                       |  s  |
    |  d  |                       |  *  |
    |  *  |                       |     |
    |_____|_______________________|_____|
    | *help*/*grep*/  |  *shell*/       |
    | *Completions*   |  *compilation*  |
    |_________________|_________________|
    |             Echo Area             |
    |___________________________________|


@end group
@end smallexample

The following example illustrates how window parameters (@pxref{Window
Parameters}) can be used with @code{display-buffer-in-side-window}
(@pxref{Displaying Buffers in Side Windows}) to set up code for
producing the frame layout sketched above.

@example
@group
(defvar parameters
  '(window-parameters . ((no-other-window . t)
                         (no-delete-other-windows . t))))

(setq fit-window-to-buffer-horizontally t)
(setq window-resize-pixelwise t)

(setq
 display-buffer-alist
 `(("\\*Buffer List\\*" display-buffer-in-side-window
    (side . top) (slot . 0) (window-height . fit-window-to-buffer)
    (preserve-size . (nil . t)) ,parameters)
   ("\\*Tags List\\*" display-buffer-in-side-window
    (side . right) (slot . 0) (window-width . fit-window-to-buffer)
    (preserve-size . (t . nil)) ,parameters)
   ("\\*\\(?:help\\|grep\\|Completions\\)\\*"
    display-buffer-in-side-window
    (side . bottom) (slot . -1) (preserve-size . (nil . t))
    ,parameters)
   ("\\*\\(?:shell\\|compilation\\)\\*" display-buffer-in-side-window
    (side . bottom) (slot . 1) (preserve-size . (nil . t))
    ,parameters)))
@end group
@end example

This specifies @code{display-buffer-alist} entries (@pxref{Choosing
Window}) for buffers with fixed names.  In particular, it asks for
showing @file{*Buffer List*} with adjustable height at the top of the
frame and @file{*Tags List*} with adjustable width on the frame's right.
It also asks for having the @file{*help*}, @file{*grep*} and
@file{*Completions*} buffers share a window on the bottom left side of
the frame and the @file{*shell*} and @file{*compilation*} buffers appear
in a window on the bottom right side of the frame.

   Note that the option @code{fit-window-to-buffer-horizontally} must
have a non-@code{nil} value in order to allow horizontal adjustment of
windows.  Entries are also added that ask for preserving the height of
side windows at the top and bottom of the frame and the width of side
windows at the left or right of the frame.  To assure that side windows
retain their respective sizes when maximizing the frame, the variable
@code{window-resize-pixelwise} is set to a non-@code{nil} value.
@xref{Resizing Windows}.

   The last form also makes sure that none of the created side windows
are accessible via @w{@kbd{C-x o}} by installing the @code{no-other-window}
parameter for each of these windows.  In addition, it makes sure that
side windows are not deleted via @w{@kbd{C-x 1}} by installing the
@code{no-delete-other-windows} parameter for each of these windows.

   Since @code{dired} buffers have no fixed names, we use a special
function @code{dired-default-directory-on-left} in order to display a
lean directory buffer on the left side of the frame.

@example
@group
(defun dired-default-directory-on-left ()
  "Display `default-directory' in side window on left, hiding details."
  (interactive)
  (let ((buffer (dired-noselect default-directory)))
    (with-current-buffer buffer (dired-hide-details-mode t))
    (display-buffer-in-side-window
     buffer `((side . left) (slot . 0)
              (window-width . fit-window-to-buffer)
              (preserve-size . (t . nil)) ,parameters))))
@end group
@end example

Evaluating the preceding forms and typing, in any order, @w{@kbd{M-x
list-buffers}}, @kbd{C-h f}, @kbd{M-x shell}, @w{@kbd{M-x list-tags}},
and @kbd{M-x dired-default-directory-on-left} should now reproduce the
frame layout sketched above.


@node Atomic Windows
@section Atomic Windows
@cindex atomic windows

Atomic windows are rectangular compositions of at least two live
windows.  They have the following distinctive characteristics:

@itemize @bullet
@item
The function @code{split-window} (@pxref{Splitting Windows}), when
applied to a constituent of an atomic window, will try to create the new
window outside of the atomic window.

@item
The function @code{delete-window} (@pxref{Deleting Windows}), when
applied to a constituent of an atomic window, will try to delete the
entire atomic window instead.

@item
The function @code{delete-other-windows} (@pxref{Deleting Windows}),
when applied to a constituent of an atomic window, will try to make the
atomic window fill its frame or main window (@pxref{Side Windows}).
@end itemize

This means that the basic groups of functions that alter the window
structure treat an atomic window like a live one, thus preserving the
internal structure of the atomic window.

   Atomic windows are useful to construct and preserve window layouts
that are meaningful only when all involved buffers are shown
simultaneously in a specific manner, such as when showing differences
between file revisions, or the same text in different languages or
markups.  They can also be used to permanently display information
pertinent to a specific window in bars on that window's sides.

@cindex root window of atomic window
   Atomic windows are implemented with the help of the reserved
@code{window-atom} window parameter (@pxref{Window Parameters}) and an
internal window (@pxref{Basic Windows}) called the root window of the
atomic window.  All windows that are part of the same atomic window have
this root window as their common ancestor and are assigned a
non-@code{nil} @code{window-atom} parameter.

  The following function returns the root of the atomic window a
specified window is part of:

@defun window-atom-root &optional window
This functions returns the root of the atomic window @var{window} is a
part of.  The specified @var{window} must be a valid window and defaults
to the selected one.  It returns @code{nil} if @var{window} is not part
of an atomic window.
@end defun

The most simple approach to make a new atomic window is to take an
existing internal window and apply the following function:

@defun window-make-atom window
This function converts @var{window} into an atomic window.  The
specified @var{window} must be an internal window.  All this function
does is to set the @code{window-atom} parameter of each descendant of
@var{window} to @code{t}.
@end defun

To create a new atomic window from an existing live window or to add a
new window to an existing atomic window, the following buffer display
action function (@pxref{Action Functions}) can be used:

@defun display-buffer-in-atom-window buffer alist
This function tries to display @var{buffer} in a new window that will be
combined with an existing window to form an atomic window.  If the
existing window is already part of an atomic window, it adds the new
window to that atomic window.

The specified @var{alist} is an association list of symbols and values.
The following symbols have a special meaning:

@table @code
@item window
The value of such an element specifies an existing window the new window
shall be combined with.  If it specifies an internal window, all
children of that window become part of the atomic window too.  If no
window is specified, the new window becomes a sibling of the selected
window.  The @code{window-atom} parameter of the existing window is set
to @code{main} provided that window is live and its @code{window-atom}
parameter was not already set.

@item side
The value of such an element denotes the side of the existing window
where the new window shall be located.  Valid values are @code{below},
@code{right}, @code{above} and @code{left}.  The default is
@code{below}.  The @code{window-atom} parameter of the new window is set
to this value.
@end table

The return value is the new window, @code{nil} when creating that window
failed.
@end defun

Note that the value of the @code{window-atom} parameter does not really
matter as long as it is non-@code{nil}.  The values assigned by
@code{display-buffer-in-atom-window} just allow for easy retrieval of
the original and the new window after that function has been applied.
Note also that the @code{window-atom} parameter is the only window
parameter assigned by @code{display-buffer-in-atom-window}.  Further
parameters have to be set by the application explicitly via a
@code{window-parameters} entry in @var{alist}.

   The following code snippet, when applied to a single-window frame,
first splits the selected window and makes the selected and the new
window constituents of an atomic window with their parent as root.  It
then displays the buffer @file{*Messages*} in a new window at the
frame's bottom and makes that new window part of the atomic window just
created.

@example
@group
(let ((window (split-window-right)))
  (window-make-atom (window-parent window))
  (display-buffer-in-atom-window
   (get-buffer-create "*Messages*")
   `((window . ,(window-parent window)) (window-height . 5))))
@end group
@end example

At this moment typing @w{@kbd{C-x 2}} in any window of that frame
produces a new window at the bottom of the frame.  Typing @w{@kbd{C-x
3}} instead will put the new window at the frame's right.  In either
case, typing now @w{@kbd{C-x 1}} in any window of the atomic window will
remove the new window only.  Typing @w{@kbd{C-x 0}} in any window of the
atomic window will make that new window fill the frame.


@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
   Emacs displays the cursor, by default as a rectangular block, in
each window at the position of that window's point.  When the user
switches to another buffer in a window, Emacs moves that window's
cursor to where point is in that buffer.  If the exact position of
point is hidden behind some display element, such as a display string
or an image, Emacs displays the cursor immediately before or after
that display element.

@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, the value returned is the
value of point in that window's 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, this simply does @code{goto-char} in
@var{window}'s buffer.
@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
@cindex display-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.

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.
@end defun

@defun window-group-start &optional window
@vindex window-group-start-function
This function is like @code{window-start}, except that when
@var{window} is a part of a group of windows (@pxref{Window Group}),
@code{window-group-start} returns the start position of the entire
group.  This condition holds when the buffer local variable
@code{window-group-start-function} is set to a function.  In this
case, @code{window-group-start} calls the function with the single
argument @var{window}, then returns its result.
@end defun

@cindex window end position
@cindex last visible position in a window
@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.  Note that the position it
returns might be only partially visible.
@end defun

@vindex window-group-end-function
@defun window-group-end &optional window update
This function is like @code{window-end}, except that when @var{window}
is a part of a group of windows (@pxref{Window Group}),
@code{window-group-end} returns the end position of the entire group.
This condition holds when the buffer local variable
@code{window-group-end-function} is set to a function.  In this case,
@code{window-group-end} calls the function with the two arguments
@var{window} and @var{update}, then returns its result.  The argument
@var{update} has the same meaning as in @code{window-end}.
@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

@vindex set-window-group-start-function
@defun set-window-group-start window position &optional noforce
This function is like @code{set-window-start}, except that when
@var{window} is a part of a group of windows (@pxref{Window Group}),
@code{set-window-group-start} sets the start position of the entire
group.  This condition holds when the buffer local variable
@code{set-window-group-start-function} is set to a function.  In this
case, @code{set-window-group-start} calls the function with the three
arguments @var{window}, @var{position}, and @var{noforce}, then
returns its result.  The arguments @var{position} and @var{noforce} in
this function have the same meaning as in @code{set-window-start}.
@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} defaults to the selected window.  If
@var{position} is @code{t}, that means to check either the first
visible position of the last screen line in @var{window}, or the
end-of-buffer position, whichever comes first.

This 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

@vindex pos-visible-in-window-group-p-function
@defun pos-visible-in-window-group-p &optional position window partially
This function is like @code{pos-visible-in-window-p}, except that when
@var{window} is a part of a group of windows (@pxref{Window Group}),
@code{pos-visible-in-window-group-p} tests the visibility of @var{pos}
in the entire group, not just in the single @var{window}.  This
condition holds when the buffer local variable
@code{pos-visible-in-window-group-p-function} is set to a function.
In this case @code{pos-visible-in-window-group-p} calls the function
with the three arguments @var{position}, @var{window}, and
@var{partially}, then returns its result.  The arguments
@var{position} and @var{partially} have the same meaning as in
@code{pos-visible-in-window-p}.
@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 window's display-start location.  It
may also change the value of @code{window-point} to keep point on the
screen (@pxref{Window Point}).

  The basic textual scrolling functions are @code{scroll-up} (which
scrolls forward) and @code{scroll-down} (which scrolls backward).  In
these function names, ``up'' and ``down'' refer to the direction of
motion of the buffer text relative to 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 the
middle of a buffer and repeatedly call @code{scroll-down}, you will
eventually see the beginning of the buffer.

  Unfortunately, this sometimes causes confusion, because some people
tend to think in terms of the opposite convention: they
imagine the window moving over text that remains in place, so that
``down'' commands take you to the end of the buffer.  This convention
is consistent with fact that such a command is bound to a key named
@key{PageDown} on modern keyboards.
@ignore
We have not switched to this convention as that is likely to break
existing Emacs Lisp code.
@end ignore

  Textual scrolling functions (aside from @code{scroll-other-window})
have unpredictable results if the current buffer is not the one
displayed in the selected window.  @xref{Current Buffer}.

  If the window contains a row 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.  Lisp callers can disable this feature by
binding the variable @code{auto-window-vscroll} to @code{nil}
(@pxref{Vertical Scrolling}).

@deffn Command scroll-up &optional count
This function scrolls forward by @var{count} lines in the selected
window.

If @var{count} is negative, it scrolls backward instead.  If
@var{count} is @code{nil} (or omitted), the distance scrolled is
@code{next-screen-context-lines} lines less than the height of the
window's text area.

If the selected window cannot be scrolled any further, this function
signals an error.  Otherwise, it returns @code{nil}.
@end deffn

@deffn Command scroll-down &optional count
This function scrolls backward by @var{count} lines in the selected
window.

If @var{count} is negative, it scrolls forward instead.  In other
respects, it behaves the same way as @code{scroll-up} does.
@end deffn

@deffn Command scroll-up-command &optional count
This behaves like @code{scroll-up}, except that if the selected window
cannot be scrolled any further and the value of the variable
@code{scroll-error-top-bottom} is @code{t}, it tries to move to the
end of the buffer instead.  If point is already there, it signals an
error.
@end deffn

@deffn Command scroll-down-command &optional count
This behaves like @code{scroll-down}, except that if the selected
window cannot be scrolled any further and the value of the variable
@code{scroll-error-top-bottom} is @code{t}, it tries to move to the
beginning of the buffer instead.  If point is already there, it
signals an error.
@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 leftmost one immediately above it.  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{End of buffer}.
@end deffn

@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 maximum-scroll-margin
This variable limits the effective value of @code{scroll-margin} to a
fraction of the current window line height.  For example, if the
current window has 20 lines and @code{maximum-scroll-margin} is 0.1,
then the scroll margins will never be larger than 2 lines, no matter
how big @code{scroll-margin} is.

@code{maximum-scroll-margin} itself has a maximum value of 0.5, which
allows setting margins large to keep the cursor at the middle line of
the window (or two middle lines if the window has an even number of
lines).  If it's set to a larger value (or any value other than a
float between 0.0 and 0.5) then the default value of 0.25 will be used
instead.
@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

@cindex @code{scroll-command} property
@defopt scroll-preserve-screen-position
If this option is @code{t}, whenever a scrolling command moves point
off-window, Emacs tries to adjust point to keep the cursor at its old
vertical position in the window, rather than the window edge.

If the value is non-@code{nil} and not @code{t}, Emacs adjusts point
to keep the cursor at the same vertical position, even if the
scrolling command didn't move point off-window.

This option affects all scroll commands that have a non-@code{nil}
@code{scroll-command} symbol property.
@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

@defopt scroll-error-top-bottom
If this option is @code{nil} (the default), @code{scroll-up-command}
and @code{scroll-down-command} simply signal an error when no more
scrolling is possible.

If the value is @code{t}, these commands instead move point to the
beginning or end of the buffer (depending on scrolling direction);
only if point is already on that position do they signal an error.
@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 non-negative 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 @code{nil} (or a non-@code{nil} list),
@code{recenter} puts the line containing point in the middle of the
window.  If @var{count} is @code{nil}, this function may redraw the
frame, according to the value of @code{recenter-redisplay}.

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.  The command @code{recenter-top-bottom} offers
a more convenient way to achieve this.
@end deffn

@vindex recenter-window-group-function
@defun recenter-window-group &optional count
This function is like @code{recenter}, except that when the selected
window is part of a group of windows (@pxref{Window Group}),
@code{recenter-window-group} scrolls the entire group.  This condition
holds when the buffer local variable
@code{recenter-window-group-function} is set to a function.  In this
case, @code{recenter-window-group} calls the function with the
argument @var{count}, then returns its result.  The argument
@var{count} has the same meaning as in @code{recenter}, but with
respect to the entire window group.
@end defun

@defopt recenter-redisplay
If this variable is non-@code{nil}, calling @code{recenter} with a
@code{nil} argument redraws the frame.  The default value is
@code{tty}, which means only redraw the frame if it is a tty frame.
@end defopt

@deffn Command recenter-top-bottom &optional count
This command, which is the default binding for @kbd{C-l}, acts like
@code{recenter}, except if called with no argument.  In that case,
successive calls place point according to the cycling order defined
by the variable @code{recenter-positions}.
@end deffn

@defopt recenter-positions
This variable controls how @code{recenter-top-bottom} behaves when
called with no argument.  The default value is @code{(middle top
bottom)}, which means that successive calls of
@code{recenter-top-bottom} with no argument cycle between placing
point at the middle, top, and bottom of the window.
@end defopt


@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 @code{line-move},
@code{scroll-up}, and @code{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.

  The default value of @code{auto-hscroll-mode} is @code{t}; setting
it to @code{current-line} activates a variant of automatic horizontal
scrolling whereby only the line showing the cursor is horizontally
scrolled to make point visible, the rest of the window is left either
unscrolled, or at the minimum scroll amount set by @code{scroll-left}
and @code{scroll-right}, see below.

@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).

Note that text in paragraphs whose base direction is right-to-left
(@pxref{Bidirectional Display}) moves in the opposite direction: e.g.,
it moves to the right when @code{scroll-left} is invoked with a
positive value of @var{count}.

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.  (In right-to-left paragraphs,
the value is the total amount of the rightward scrolling instead.)
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 (right margin in right-to-left paragraphs).  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:

@c FIXME: Maybe hscroll-on-screen-p is a better name?
@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 Coordinates and Windows
@section Coordinates and Windows
@cindex frame-relative coordinate
@cindex coordinate, relative to frame
@cindex window position

This section describes functions that report positions of and within a
window.  Most of these functions report positions relative to an origin
at the native position of the window's frame (@pxref{Frame Geometry}).
Some functions report positions relative to the origin of the display of
the window's frame.  In any case, the origin has the coordinates (0, 0)
and X and Y coordinates increase rightward and downward respectively.

  For the following functions, X and Y coordinates are reported in
integer character units, i.e., numbers of lines and columns
respectively.  On a graphical display, each ``line'' and ``column''
corresponds to the height and width of the default character specified by
the frame's default font (@pxref{Frame Font}).

@defun window-edges &optional window body absolute pixelwise
This function returns a list of the edge coordinates of @var{window}.
If @var{window} is omitted or @code{nil}, it defaults to the selected
window.

The return value has the form @code{(@var{left} @var{top} @var{right}
@var{bottom})}.  These list elements are, respectively, the X
coordinate of the leftmost column occupied by the window, the Y
coordinate of the topmost row, the X coordinate one column to the
right of the rightmost column, and the Y coordinate one row down from
the bottommost row.

Note that these are the actual outer edges of the window, including any
header line, mode line, scroll bar, fringes, window divider and display
margins.  On a text terminal, if the window has a neighbor on its right,
its right edge includes the separator line between the window and its
neighbor.

If the optional argument @var{body} is @code{nil}, this means to
return the edges corresponding to the total size of @var{window}.
@var{body} non-@code{nil} means to return the edges of @var{window}'s
body (aka text area).  If @var{body} is non-@code{nil}, @var{window}
must specify a live window.

If the optional argument @var{absolute} is @code{nil}, this means to
return edges relative to the native position of @var{window}'s frame.
@var{absolute} non-@code{nil} means to return coordinates relative to
the origin (0, 0) of @var{window}'s display.  On non-graphical systems
this argument has no effect.

If the optional argument @var{pixelwise} is @code{nil}, this means to
return the coordinates in terms of the default character width and
height of @var{window}'s frame (@pxref{Frame Font}), rounded if
necessary.  @var{pixelwise} non-@code{nil} means to return the
coordinates in pixels.  Note that the pixel specified by @var{right} and
@var{bottom} is immediately outside of these edges.  If @var{absolute}
is non-@code{nil}, @var{pixelwise} is implicitly non-@code{nil} too.
@end defun

@defun window-body-edges &optional window
This function returns the edges of @var{window}'s body (@pxref{Window
Sizes}).  Calling @code{(window-body-edges window)} is equivalent to
calling @code{(window-edges window t)}, see above.
@end defun

@comment The following two functions are confusing and hardly used.
@ignore
@defun window-left-column &optional window
This function returns the leftmost column of @var{window}.  This value
equals the @var{left} entry in the list returned by @code{(window-edges
window)} minus the number of columns occupied by the internal border of
@var{window}'s frame.
@end defun

@defun window-top-line &optional window
This function returns the topmost row of @var{window}.  This value is
equal to the @var{top} entry in the list returned by @code{(window-edges
window)} minus the number of lines occupied by the internal border of
@var{window}'s frame.
@end defun
@end ignore

  The following functions can be used to relate a set of
frame-relative coordinates to a window:

@defun window-at x y &optional frame
This function returns the live window at the coordinates @var{x} and
@var{y} given in default character sizes (@pxref{Frame Font}) relative
to the native position of @var{frame} (@pxref{Frame Geometry}).

If there is no window at that position, the return value is @code{nil}.
If @var{frame} is omitted or @code{nil}, it defaults to the selected
frame.
@end defun

@defun coordinates-in-window-p coordinates window
This function checks whether a window @var{window} occupies the frame
relative coordinates @var{coordinates}, and if so, which part of the
window that is.  @var{window} should be a live window.

@var{coordinates} should be a cons cell of the form @code{(@var{x}
. @var{y})}, where @var{x} and @var{y} are given in default character
sizes (@pxref{Frame Font}) relative to the native position of
@var{window}'s frame (@pxref{Frame Geometry}).

If there is no window at the specified position, the return value is
@code{nil} .  Otherwise, the return value is one of the following:

@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 right-divider
The coordinates are in the divider separating @var{window} from a
window on the right.

@item bottom-divider
The coordinates are in the divider separating @var{window} from a
window beneath.

@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

  The following functions return window positions in pixels, rather
than character units.  Though mostly useful on graphical displays,
they can also be called on text terminals, where the screen area of
each text character is taken to be one pixel.

@defun window-pixel-edges &optional window
This function returns a list of pixel coordinates for the edges of
@var{window}.  Calling @code{(window-pixel-edges window)} is equivalent
to calling @code{(window-edges window nil nil t)}, see above.
@end defun

@comment The following two functions are confusing and hardly used.
@ignore
@defun window-pixel-left &optional window
This function returns the left pixel edge of window @var{window}.  This
value equals the @var{left} entry in the list returned by
@code{(window-pixel-edges window)} minus the number of pixels occupied
by the internal border of @var{window}'s frame.  @var{window} must be a
valid window and defaults to the selected one.
@end defun

@defun window-pixel-top &optional window
This function returns the top pixel edge of window @var{window}.  This
value is equal to the @var{top} entry in the list returned by
@code{(window-pixel-edges window)} minus the number of pixels occupied
by the internal border of @var{window}'s frame.  @var{window} must be a
valid window and defaults to the selected one.
@end defun
@end ignore

@defun window-body-pixel-edges &optional window
This function returns the pixel edges of @var{window}'s body.  Calling
@code{(window-body-pixel-edges window)} is equivalent to calling
@code{(window-edges window t nil t)}, see above.
@end defun

  The following functions return window positions in pixels, relative to
the origin of the display screen rather than that of the frame:

@defun window-absolute-pixel-edges &optional window
This function returns the pixel coordinates of @var{window} relative to
an origin at (0, 0) of the display of @var{window}'s frame.  Calling
@code{(window-absolute-pixel-edges)} is equivalent to calling
@code{(window-edges window nil t t)}, see above.
@end defun

@defun window-absolute-body-pixel-edges &optional window
This function returns the pixel coordinates of @var{window}'s body
relative to an origin at (0, 0) of the display of @var{window}'s frame.
Calling @code{(window-absolute-body-pixel-edges window)} is equivalent
to calling @code{(window-edges window t t t)}, see above.

Combined with @code{set-mouse-absolute-pixel-position}, this function
can be used to move the mouse pointer to an arbitrary buffer position
visible in some window:

@example
@group
(let ((edges (window-absolute-body-pixel-edges))
      (position (pos-visible-in-window-p nil nil t)))
  (set-mouse-absolute-pixel-position
   (+ (nth 0 edges) (nth 0 position))
   (+ (nth 1 edges) (nth 1 position))))
@end group
@end example

On a graphical terminal this form ``warps'' the mouse cursor to the
upper left corner of the glyph at the selected window's point.  A
position calculated this way can be also used to show a tooltip window
there.
@end defun

The following function returns the screen coordinates of a buffer
position visible in a window:

@defun window-absolute-pixel-position &optional position window
If the buffer position @var{position} is visible in window @var{window},
this function returns the display coordinates of the upper/left corner
of the glyph at @var{position}.  The return value is a cons of the X-
and Y-coordinates of that corner, relative to an origin at (0, 0) of
@var{window}'s display.  It returns @code{nil} if @var{position} is not
visible in @var{window}.

@var{window} must be a live window and defaults to the selected
window.  @var{position} defaults to the value of @code{window-point}
of @var{window}.

This means that in order to move the mouse pointer to the position of
point in the selected window, it's sufficient to write:

@example
@group
(let ((position (window-absolute-pixel-position)))
  (set-mouse-absolute-pixel-position
   (car position) (cdr position)))
@end group
@end example
@end defun

The following function returns the largest rectangle that can be
inscribed in a window without covering text displayed in that window.

@defun window-largest-empty-rectangle &optional window count min-width min-height positions left
This function calculates the dimensions of the largest empty rectangle
that can be inscribed in the specified @var{window}'s text area.
@var{window} must be a live window and defaults to the selected one.

The return value is a triple of the width and the start and end
y-coordinates of the largest rectangle that can be inscribed into the
empty space (space not displaying any text) of the text area of
@var{window}.  No x-coordinates are returned by this function---any such
rectangle is assumed to end at the right edge of @var{window}'s text
area.  If no empty space can be found, the return value is @code{nil}.

The optional argument @var{count}, if non-@code{nil}, specifies a
maximum number of rectangles to return.  This means that the return
value is a list of triples specifying rectangles with the largest
rectangle first.  @var{count} can be also a cons cell whose car
specifies the number of rectangles to return and whose @sc{cdr}, if
non-@code{nil}, states that all rectangles returned must be disjoint.

The optional arguments @var{min-width} and @var{min-height}, if
non-@code{nil}, specify the minimum width and height of any rectangle
returned.

The optional argument @var{positions}, if non-@code{nil}, is a cons cell
whose @sc{car} specifies the uppermost and whose @sc{cdr} specifies the
lowermost pixel position that must be covered by any rectangle returned.
These positions measure from the start of the text area of @var{window}.

The optional argument @var{left}, if non-@code{nil}, means to return
values suitable for buffers displaying right to left text.  In that
case, any rectangle returned is assumed to start at the left edge of
@var{window}'s text area.

Note that this function has to retrieve the dimensions of each line of
@var{window}'s glyph matrix via @code{window-lines-pixel-dimensions}
(@pxref{Size of Displayed Text}).  Hence, this function may also return
@code{nil} when the current glyph matrix of @var{window} is not
up-to-date.
@end defun


@node Mouse Window Auto-selection
@section Mouse Window Auto-selection
@cindex window auto-selection
@cindex auto-selection of window
The following option allows to automatically select the window under the
mouse pointer.  This accomplishes a policy similar to that of window
managers that give focus to a frame (and thus trigger its subsequent
selection) whenever the mouse pointer enters its window-system window
(@pxref{Input Focus}).

@defopt mouse-autoselect-window
If this variable is non-@code{nil}, Emacs will try to automatically
select the window under the mouse pointer.  The following values are
meaningful:

@table @asis
@item A positive number
This specifies a delay in seconds after which auto-selection triggers.
The window under the mouse pointer is selected after the mouse has
remained in it for the entire duration of the delay.

@item A negative number
A negative number has a similar effect as a positive number, but selects
the window under the mouse pointer only after the mouse pointer has
remained in it for the entire duration of the absolute value of that
number and in addition has stopped moving.

@item Other value
Any other non-@code{nil} value means to select a window instantaneously
as soon as the mouse pointer enters it.
@end table

In either case, the mouse pointer must enter the text area of a window in
order to trigger its selection.  Dragging the scroll bar slider or the
mode line of a window conceptually should not cause its auto-selection.

Mouse auto-selection selects the minibuffer window only if it is active,
and never deselects the active minibuffer window.
@end defopt

Mouse auto-selection can be used to emulate a focus follows mouse policy
for child frames (@pxref{Child Frames}) which usually are not tracked by
the window manager.  This requires to set the value of
@code{focus-follows-mouse} (@pxref{Input Focus}) to a non-@code{nil}
value.  If the value of @code{focus-follows-mouse} is @code{auto-raise},
entering a child frame with the mouse will raise it automatically above
all other child frames of that frame's parent frame.


@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 value of point; 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.

  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.  @xref{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.  The variable @code{window-persistent-parameters} specifies
which window parameters (if any) are saved by this function.
@xref{Window Parameters}.
@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.  In some rare cases this may trigger
execution of the @code{window-size-change-functions} (@pxref{Window
Hooks}) even if the size of windows did not change at all.  The
@code{window-configuration-change-hook} functions will be called if and
only if at least one window was added to or deleted from the frame.

If the frame from which @var{configuration} was saved 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-below nil)
             @dots{})
    (set-window-configuration config)))
@end group
@end example
@end defun

@defmac save-window-excursion forms@dots{}
This macro records the window configuration of the selected frame,
executes @var{forms} in sequence, then restores the earlier window
configuration.  The return value is the value of the final form in
@var{forms}.

Most Lisp code should not use this macro; @code{save-selected-window}
is typically sufficient.  In particular, this macro cannot reliably
prevent the code in @var{forms} from opening new windows, because new
windows might be opened in other frames (@pxref{Choosing Window}), and
@code{save-window-excursion} only saves and restores the window
configuration on the current frame.

Do not use this macro in @code{window-size-change-functions}; exiting
the macro triggers execution of @code{window-size-change-functions},
leading to an endless loop.
@end defmac

@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 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.
@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.

  The objects returned by @code{current-window-configuration} die
together with the Emacs process.  In order to store a window
configuration on disk and read it back in another Emacs session, you
can use the functions described next.  These functions are also useful
to clone the state of a frame into an arbitrary live window
(@code{set-window-configuration} effectively clones the windows of a
frame into the root window of that very frame only).

@cindex window state
@defun window-state-get &optional window writable
This function returns the state of @var{window} as a Lisp object.  The
argument @var{window} must be a valid window and defaults to the root
window of the selected frame.

If the optional argument @var{writable} is non-@code{nil}, this means to
not use markers for sampling positions like @code{window-point} or
@code{window-start}.  This argument should be non-@code{nil} when the
state will be written to disk and read back in another session.

Together, the argument @var{writable} and the variable
@code{window-persistent-parameters} specify which window parameters are
saved by this function.  @xref{Window Parameters}.
@end defun

The value returned by @code{window-state-get} can be used in the same
session to make a clone of a window in another window.  It can be also
written to disk and read back in another session.  In either case, use
the following function to restore the state of the window.

@defun window-state-put state &optional window ignore
This function puts the window state @var{state} into @var{window}.
The argument @var{state} should be the state of a window returned by
an earlier invocation of @code{window-state-get}, see above.  The
optional argument @var{window} can be either a live window or an
internal window (@pxref{Windows and Frames}) and defaults to the
selected one.  If @var{window} is not live, it is replaced by a live
window before putting @var{state} into it.

If the optional argument @var{ignore} is non-@code{nil}, it means to ignore
minimum window sizes and fixed-size restrictions.  If @var{ignore}
is @code{safe}, this means windows can get as small as one line
and/or two columns.
@end defun

The functions @code{window-state-get} and @code{window-state-put} also
allow to exchange the contents of two live windows.  The following
function does precisely that:

@deffn Command window-swap-states &optional window-1 window-2 size
This command swaps the states of the two live windows @var{window-1} and
@var{window-2}.  @var{window-1} must specify a live window and defaults
to the selected one.  @var{window-2} must specify a live window and
defaults to the window following @var{window-1} in the cyclic ordering
of windows, excluding minibuffer windows and including live windows on
all visible frames.

Optional argument @var{size} non-@code{nil} means to try swapping the
sizes of @var{window-1} and @var{window-2} as well.  A value of
@code{height} means to swap heights only, a value of @code{width}
means to swap widths only, while @code{t} means to swap both widths
and heights, if possible.  Frames are not resized by this function.
@end deffn


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

This section 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 either @code{nil}, or an association list whose elements have 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

By default, the functions that save and restore window configurations or the
states of windows (@pxref{Window Configurations}) do not care about
window parameters.  This means that when you change the value of a
parameter within the body of a @code{save-window-excursion}, the
previous value is not restored when that macro exits.  It also means
that when you restore via @code{window-state-put} a window state saved
earlier by @code{window-state-get}, all cloned windows have their
parameters reset to @code{nil}.  The following variable allows you to
override the standard behavior:

@cindex persistent window parameters
@defvar window-persistent-parameters
This variable is an alist specifying which parameters get saved by
@code{current-window-configuration} and @code{window-state-get}, and
subsequently restored by @code{set-window-configuration} and
@code{window-state-put}.  @xref{Window Configurations}.

The @sc{car} of each entry of this alist is a symbol specifying the
parameter.  The @sc{cdr} should be one of the following:

@table @asis
@item @code{nil}
This value means the parameter is saved neither by
@code{window-state-get} nor by @code{current-window-configuration}.

@item @code{t}
This value specifies that the parameter is saved by
@code{current-window-configuration} and (provided its @var{writable}
argument is @code{nil}) by @code{window-state-get}.

@item @code{writable}
This means that the parameter is saved unconditionally by both
@code{current-window-configuration} and @code{window-state-get}.  This
value should not be used for parameters whose values do not have a read
syntax.  Otherwise, invoking @code{window-state-put} in another session
may fail with an @code{invalid-read-syntax} error.
@end table
@end defvar

Some functions (notably @code{delete-window},
@code{delete-other-windows} and @code{split-window}), may behave
specially when the window specified by their @var{window} argument has
a parameter whose name is equal to the function's name.  You can
override such special behavior by binding the following variable to a
non-@code{nil} value:

@defvar ignore-window-parameters
If this variable is non-@code{nil}, some standard functions do not
process window parameters.  The functions currently affected by this are
@code{split-window}, @code{delete-window}, @code{delete-other-windows},
and @code{other-window}.

An application can bind this variable to a non-@code{nil} value around
calls to these functions.  If it does so, the application is fully
responsible for correctly assigning the parameters of all involved
windows when exiting that function.
@end defvar

The following parameters are currently used by the window management
code:

@table @code
@item delete-window
@vindex delete-window@r{, a window parameter}
This parameter affects the execution of @code{delete-window}
(@pxref{Deleting Windows}).

@item delete-other-windows
@vindex delete-other-windows@r{, a window parameter}
This parameter affects the execution of @code{delete-other-windows}
(@pxref{Deleting Windows}).

@item no-delete-other-windows
@vindex no-delete-other-windows@r{, a window parameter}
This parameter marks the window as not deletable by
@code{delete-other-windows} (@pxref{Deleting Windows}).

@item split-window
@vindex split-window@r{, a window parameter}
This parameter affects the execution of @code{split-window}
(@pxref{Splitting Windows}).

@item other-window
@vindex other-window@r{, a window parameter}
This parameter affects the execution of @code{other-window}
(@pxref{Cyclic Window Ordering}).

@item no-other-window
@vindex no-other-window@r{, a window parameter}
This parameter marks the window as not selectable by @code{other-window}
(@pxref{Cyclic Window Ordering}).

@item clone-of
@vindex clone-of@r{, a window parameter}
This parameter specifies the window that this one has been cloned
from.  It is installed by @code{window-state-get} (@pxref{Window
Configurations}).

@item window-preserved-size
@vindex window-preserved-size@r{, a window parameter}
This parameter specifies a buffer, a direction where @code{nil} means
vertical and @code{t} horizontal, and a size in pixels.  If this window
displays the specified buffer and its size in the indicated direction
equals the size specified by this parameter, then Emacs will try to
preserve the size of this window in the indicated direction.  This
parameter is installed and updated by the function
@code{window-preserve-size} (@pxref{Preserving Window Sizes}).

@item quit-restore
@vindex quit-restore@r{, a window parameter}
This parameter is installed by the buffer display functions
(@pxref{Choosing Window}) and consulted by @code{quit-restore-window}
(@pxref{Quitting Windows}).  It contains four elements:

The first element is one of the symbols @code{window}, meaning that
the window has been specially created by @code{display-buffer};
@code{frame}, a separate frame has been created; @code{same}, the
window has only ever displayed this buffer; or @code{other}, the
window showed another buffer before.  @code{frame} and @code{window}
affect how the window is quit, while @code{same} and @code{other}
affect the redisplay of buffers previously shown in this window.

The second element is either one of the symbols @code{window} or
@code{frame}, or a list whose elements are the buffer shown in the
window before, that buffer's window start and window point positions,
and the window's height at that time.  If that buffer is still live
when the window is quit, then the function @code{quit-restore-window}
reuses the window to display the buffer.

The third element is the window selected at the time the parameter was
created.  If @code{quit-restore-window} deletes the window passed to
it as argument, it then tries to reselect this window.

The fourth element is the buffer whose display caused the creation of
this parameter.  @code{quit-restore-window} deletes the specified window
only if it still shows that buffer.

See the description of @code{quit-restore-window} in @ref{Quitting
Windows} for details.

@item window-side window-slot
@vindex window-side@r{, a window parameter}
@vindex window-slot@r{, a window parameter}
These parameters are used for implementing side windows (@pxref{Side
Windows}).

@item window-atom
@vindex window-atom@r{, a window parameter}
This parameter is used for implementing atomic windows, see @ref{Atomic
Windows}.

@item mode-line-format
@vindex mode-line-format@r{, a window parameter}
This parameter replaces the value of the buffer-local variable
@code{mode-line-format} (@pxref{Mode Line Basics}) of this window's
buffer whenever this window is displayed.  The symbol @code{none} means
to suppress display of a mode line for this window.  Display and
contents of the mode line on other windows showing this buffer are not
affected.

@item header-line-format
@vindex header-line-format@r{, a window parameter}
This parameter replaces the value of the buffer-local variable
@code{header-line-format} (@pxref{Mode Line Basics}) of this window's
buffer whenever this window is displayed.  The symbol @code{none} means
to suppress display of a header line for this window.  Display and
contents of the header line on other windows showing this buffer are not
affected.

@item min-margins
@vindex min-margins@r{, a window parameter}
The value of this parameter is a cons cell whose @sc{car} and @sc{cdr},
if non-@code{nil}, specify the minimum values (in columns) for the left
and right margin of this window.  When present, Emacs will use these
values instead of the actual margin widths for determining whether a
window can be split or shrunk horizontally.

Emacs never auto-adjusts the margins of any window after splitting or
resizing it.  It is the sole responsibility of any application setting
this parameter to adjust the margins of this window as well as those of
any new window that inherits this window's margins due to a split.
Both @code{window-configuration-change-hook} and
@code{window-size-change-functions} (@pxref{Window Hooks}) should be
employed for this purpose.

This parameter was introduced in Emacs version 25.1 to support
applications that use large margins to center buffer text within a
window and should be used, with due care, exclusively by those
applications.  It might be replaced by an improved solution in future
versions of Emacs.
@end table


@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.  At the
time of the call, the display-start position of the window argument is
already set to its new value, and the buffer to be displayed in the
window is already set as the current buffer.

These functions must take care when 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

@defun run-window-scroll-functions &optional window
This function calls @code{window-scroll-functions} for the specified
@var{window}, which defaults to the selected window.
@end defun

@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 once per
redisplay, and once for each frame on which size changes have occurred.

Each function receives the frame as its sole argument.  To find out
whether a specific window has changed size, compare the return values of
@code{window-pixel-width-before-size-change} and
@code{window-pixel-width} respectively
@code{window-pixel-height-before-size-change} and
@code{window-pixel-height} for that window (@pxref{Window Sizes}).

These function are usually only called when at least one window was
added or has changed size since the last time this hook was run for the
associated frame.  In some rare cases this hook also runs when a window
that was added intermittently has been deleted afterwards.  In these
cases none of the windows on the frame will appear to have changed its
size.

You may use @code{save-selected-window} in these functions
(@pxref{Selecting Windows}).  However, do not use
@code{save-window-excursion} (@pxref{Window Configurations}); exiting
that macro counts as a size change, which would cause these functions to
be called again.
@end defvar

@defvar window-configuration-change-hook
A normal hook that is run every time the window configuration of a frame
changes.  Window configuration changes include splitting and deleting
windows, and the display of a different buffer in a window.  Resizing the
frame or individual windows do not count as configuration changes.  Use
@code{window-size-change-functions}, see above, when you want to track
size changes that are not caused by the deletion or creation of windows.

The buffer-local value of this hook is run once for each window on the
affected frame, with the relevant window selected and its buffer
current.  The global value of this hook is run once for the modified
frame, with that frame selected.
@end defvar

@defun run-window-configuration-change-hook &optional frame
This function runs @code{window-configuration-change-hook} for the
specified @var{frame}, which defaults to the selected frame.
@end defun

  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}.