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[bpt/emacs.git] / lispref / display.texi

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

  This chapter describes a number of features related to the display
that Emacs presents to the user.

@menu
* Refresh Screen::      Clearing the screen and redrawing everything on it.
* Truncation::          Folding or wrapping long text lines.
* The Echo Area::       Where messages are displayed.
* Invisible Text::      Hiding part of the buffer text.
* Selective Display::   Hiding part of the buffer text (the old way).
* Overlay Arrow::       Display of an arrow to indicate position.
* Temporary Displays::  Displays that go away automatically.
* Overlays::		Use overlays to highlight parts of the buffer.
* Width::               How wide is a character or string.
* Faces::		A face defines a graphics appearance: font, color, etc.
* Blinking::            How Emacs shows the matching open parenthesis.
* Inverse Video::	Specifying how the screen looks.
* Usual Display::	The usual conventions for displaying nonprinting chars.
* Display Tables::	How to specify other conventions.
* Beeping::             Audible signal to the user.
* Window Systems::      Which window system is being used.
@end menu

@node Refresh Screen
@section Refreshing the Screen

The function @code{redraw-frame} redisplays the entire contents of a
given frame (@pxref{Frames}).

@c Emacs 19 feature
@defun redraw-frame frame
This function clears and redisplays frame @var{frame}.
@end defun

Even more powerful is @code{redraw-display}:

@deffn Command redraw-display
This function clears and redisplays all visible frames.
@end deffn

  Processing user input takes absolute priority over redisplay.  If you
call these functions when input is available, they do nothing
immediately, but a full redisplay does happen eventually---after all the
input has been processed.

  Normally, suspending and resuming Emacs also refreshes the screen.
Some terminal emulators record separate contents for display-oriented
programs such as Emacs and for ordinary sequential display.  If you are
using such a terminal, you might want to inhibit the redisplay on
resumption.

@defvar no-redraw-on-reenter
@cindex suspend (cf. @code{no-redraw-on-reenter})
@cindex resume (cf. @code{no-redraw-on-reenter})
This variable controls whether Emacs redraws the entire screen after it
has been suspended and resumed.  Non-@code{nil} means there is no need
to redraw, @code{nil} means redrawing is needed.  The default is @code{nil}.
@end defvar

@node Truncation
@section Truncation
@cindex line wrapping
@cindex continuation lines
@cindex @samp{$} in display
@cindex @samp{\} in display

  When a line of text extends beyond the right edge of a window, the
line can either be continued on the next screen line, or truncated to
one screen line.  The additional screen lines used to display a long
text line are called @dfn{continuation} lines.  Normally, a @samp{$} in
the rightmost column of the window indicates truncation; a @samp{\} on
the rightmost column indicates a line that ``wraps'' onto the next line,
which is also called @dfn{continuing} the line.  (The display table can
specify alternative indicators; see @ref{Display Tables}.)

  Note that continuation is different from filling; continuation happens
on the screen only, not in the buffer contents, and it breaks a line
precisely at the right margin, not at a word boundary.  @xref{Filling}.

@defopt truncate-lines
This buffer-local variable controls how Emacs displays lines that extend
beyond the right edge of the window.  The default is @code{nil}, which
specifies continuation.  If the value is non-@code{nil}, then these
lines are truncated.

If the variable @code{truncate-partial-width-windows} is non-@code{nil},
then truncation is always used for side-by-side windows (within one
frame) regardless of the value of @code{truncate-lines}.
@end defopt

@defopt default-truncate-lines
This variable is the default value for @code{truncate-lines}, for
buffers that do not have buffer-local values for it.
@end defopt

@defopt truncate-partial-width-windows
This variable controls display of lines that extend beyond the right
edge of the window, in side-by-side windows (@pxref{Splitting Windows}).
If it is non-@code{nil}, these lines are truncated; otherwise,
@code{truncate-lines} says what to do with them.
@end defopt

  When horizontal scrolling (@pxref{Horizontal Scrolling}) is in use in
a window, that forces truncation.

  You can override the glyphs that indicate continuation or truncation
using the display table; see @ref{Display Tables}.

  If your buffer contains @emph{very} long lines, and you use
continuation to display them, just thinking about them can make Emacs
redisplay slow.  The column computation and indentation functions also
become slow.  Then you might find it advisable to set
@code{cache-long-line-scans} to @code{t}.

@defvar cache-long-line-scans
If this variable is non-@code{nil}, various indentation and motion
functions, and Emacs redisplay, cache the results of scanning the
buffer, and consult the cache to avoid rescanning regions of the buffer
unless they are modified.

Turning on the cache slows down processing of short lines somewhat.

This variable is automatically buffer-local in every buffer.
@end defvar

@node The Echo Area
@section The Echo Area
@cindex error display
@cindex echo area

The @dfn{echo area} is used for displaying messages made with the
@code{message} primitive, and for echoing keystrokes.  It is not the
same as the minibuffer, despite the fact that the minibuffer appears
(when active) in the same place on the screen as the echo area.  The
@cite{GNU Emacs Manual} specifies the rules for resolving conflicts
between the echo area and the minibuffer for use of that screen space
(@pxref{Minibuffer,, The Minibuffer, emacs, The GNU Emacs Manual}).
Error messages appear in the echo area; see @ref{Errors}.

You can write output in the echo area by using the Lisp printing
functions with @code{t} as the stream (@pxref{Output Functions}), or as
follows:

@defun message string &rest arguments
This function displays a one-line message in the echo area.  The
argument @var{string} is similar to a C language @code{printf} control
string.  See @code{format} in @ref{String Conversion}, for the details
on the conversion specifications.  @code{message} returns the
constructed string.

In batch mode, @code{message} prints the message text on the standard
error stream, followed by a newline.

@c Emacs 19 feature
If @var{string} is @code{nil}, @code{message} clears the echo area.  If
the minibuffer is active, this brings the minibuffer contents back onto
the screen immediately.

@example
@group
(message "Minibuffer depth is %d."
         (minibuffer-depth))
 @print{} Minibuffer depth is 0.
@result{} "Minibuffer depth is 0."
@end group

@group
---------- Echo Area ----------
Minibuffer depth is 0.
---------- Echo Area ----------
@end group
@end example
@end defun

@defun current-message
@tindex current-message
This function returns the message currently being displayed in the
echo area, or @code{nil} if there is none.
@end defun

@defvar cursor-in-echo-area
This variable controls where the cursor appears when a message is
displayed in the echo area.  If it is non-@code{nil}, then the cursor
appears at the end of the message.  Otherwise, the cursor appears at
point---not in the echo area at all.

The value is normally @code{nil}; Lisp programs bind it to @code{t}
for brief periods of time.
@end defvar

@defvar echo-area-clear-hook
@tindex echo-area-clear-hook
This normal hook is run whenever the echo area is cleared---either by
@code{(message nil)} or for any other reason.
@end defvar

Almost all the messages displayed in the echo area are also recorded
in the @samp{*Messages*} buffer.

@defopt message-log-max
This variable specifies how many lines to keep in the @samp{*Messages*}
buffer.  The value @code{t} means there is no limit on how many lines to
keep.  The value @code{nil} disables message logging entirely.  Here's
how to display a message and prevent it from being logged:

@example
(let (message-log-max)
  (message @dots{}))
@end example
@end defopt

@defvar echo-keystrokes
This variable determines how much time should elapse before command
characters echo.  Its value must be an integer, which specifies the
number of seconds to wait before echoing.  If the user types a prefix
key (such as @kbd{C-x}) and then delays this many seconds before
continuing, the prefix key is echoed in the echo area.  (Once echoing
begins in a key sequence, all subsequent characters in the same key
sequence are echoed immediately.)

If the value is zero, then command input is not echoed.
@end defvar

@node Invisible Text
@section Invisible Text

@cindex invisible text
You can make characters @dfn{invisible}, so that they do not appear on
the screen, with the @code{invisible} property.  This can be either a
text property (@pxref{Text Properties}) or a property of an overlay
(@pxref{Overlays}).

In the simplest case, any non-@code{nil} @code{invisible} property makes
a character invisible.  This is the default case---if you don't alter
the default value of @code{buffer-invisibility-spec}, this is how the
@code{invisible} property works.

More generally, you can use the variable @code{buffer-invisibility-spec}
to control which values of the @code{invisible} property make text
invisible.  This permits you to classify the text into different subsets
in advance, by giving them different @code{invisible} values, and
subsequently make various subsets visible or invisible by changing the
value of @code{buffer-invisibility-spec}.

Controlling visibility with @code{buffer-invisibility-spec} is
especially useful in a program to display the list of entries in a data
base.  It permits the implementation of convenient filtering commands to
view just a part of the entries in the data base.  Setting this variable
is very fast, much faster than scanning all the text in the buffer
looking for properties to change.

@defvar buffer-invisibility-spec
This variable specifies which kinds of @code{invisible} properties
actually make a character invisible.

@table @asis
@item @code{t}
A character is invisible if its @code{invisible} property is
non-@code{nil}.  This is the default.

@item a list
Each element of the list specifies a criterion for invisibility; if a
character's @code{invisible} property fits any one of these criteria,
the character is invisible.  The list can have two kinds of elements:

@table @code
@item @var{atom}
A character is invisible if its @code{invisible} property value
is @var{atom} or if it is a list with @var{atom} as a member.

@item (@var{atom} . t)
A character is invisible if its @code{invisible} property value
is @var{atom} or if it is a list with @var{atom} as a member.
Moreover, if this character is at the end of a line and is followed
by a visible newline, it displays an ellipsis.
@end table
@end table
@end defvar

  Two functions are specifically provided for adding elements to
@code{buffer-invisibility-spec} and removing elements from it.

@defun add-to-invisibility-spec element
@tindex add-to-invisibility-spec
Add the element @var{element} to @code{buffer-invisibility-spec}
(if it is not already present in that list).
@end defun

@defun remove-from-invisibility-spec element
@tindex remove-from-invisibility-spec
Remove the element @var{element} from @code{buffer-invisibility-spec}.
@end defun

  One convention about the use of @code{buffer-invisibility-spec} is
that a major mode should use the mode's own name as an element of
@code{buffer-invisibility-spec} and as the value of the @code{invisible}
property:

@example
;; @r{If you want to display an ellipsis:}
(add-to-invisibility-spec '(my-symbol . t)) 
;; @r{If you don't want ellipsis:}
(add-to-invisibility-spec 'my-symbol) 

(overlay-put (make-overlay beginning end)
             'invisible 'my-symbol)

;; @r{When done with the overlays:}
(remove-from-invisibility-spec '(my-symbol . t))
;; @r{Or respectively:}
(remove-from-invisibility-spec 'my-symbol)
@end example

@vindex line-move-ignore-invisible
  Ordinarily, commands that operate on text or move point do not care
whether the text is invisible.  The user-level line motion commands
explicitly ignore invisible newlines if
@code{line-move-ignore-invisible} is non-@code{nil}, but only because
they are explicitly programmed to do so.

  Incremental search can make invisible overlays visible temporarily
and/or permanently when a match includes invisible text.  To enable
this, the overlay should have a non-@code{nil}
@code{isearch-open-invisible} property.  The property value should be a
function to be called with the overlay as an argument.  This function
should make the overlay visible permanently; it is used when the match
overlaps the overlay on exit from the search.

  During the search, such overlays are made temporarily visible by
temporarily modifying their invisible and intangible properties.  If you
want this to be done differently for a certain overlay, give it a
@code{isearch-open-invisible-temporary} property which is a function.
The function is called with two arguments: the first is the overlay, and
the second is @code{t} to make the overlay visible, or @code{nil} to
make it invisible again.

@node Selective Display
@section Selective Display
@cindex selective display

  @dfn{Selective display} refers to a pair of related features for
hiding certain lines on the screen.

  The first variant, explicit selective display, is designed for use in
a Lisp program: it controls which lines are hidden by altering the text.
The invisible text feature (@pxref{Invisible Text}) has partially
replaced this feature.

  In the second variant, the choice of lines to hide is made
automatically based on indentation.  This variant is designed to be a
user-level feature.

  The way you control explicit selective display is by replacing a
newline (control-j) with a carriage return (control-m).  The text that
was formerly a line following that newline is now invisible.  Strictly
speaking, it is temporarily no longer a line at all, since only newlines
can separate lines; it is now part of the previous line.

  Selective display does not directly affect editing commands.  For
example, @kbd{C-f} (@code{forward-char}) moves point unhesitatingly into
invisible text.  However, the replacement of newline characters with
carriage return characters affects some editing commands.  For example,
@code{next-line} skips invisible lines, since it searches only for
newlines.  Modes that use selective display can also define commands
that take account of the newlines, or that make parts of the text
visible or invisible.

  When you write a selectively displayed buffer into a file, all the
control-m's are output as newlines.  This means that when you next read
in the file, it looks OK, with nothing invisible.  The selective display
effect is seen only within Emacs.

@defvar selective-display
This buffer-local variable enables selective display.  This means that
lines, or portions of lines, may be made invisible.  

@itemize @bullet
@item
If the value of @code{selective-display} is @code{t}, then any portion
of a line that follows a control-m is not displayed.  This is explicit
selective display.

@item
If the value of @code{selective-display} is a positive integer, then
lines that start with more than that many columns of indentation are not
displayed.
@end itemize

When some portion of a buffer is invisible, the vertical movement
commands operate as if that portion did not exist, allowing a single
@code{next-line} command to skip any number of invisible lines.
However, character movement commands (such as @code{forward-char}) do
not skip the invisible portion, and it is possible (if tricky) to insert
or delete text in an invisible portion.

In the examples below, we show the @emph{display appearance} of the
buffer @code{foo}, which changes with the value of
@code{selective-display}.  The @emph{contents} of the buffer do not
change.

@example
@group
(setq selective-display nil)
     @result{} nil

---------- Buffer: foo ----------
1 on this column
 2on this column
  3n this column
  3n this column
 2on this column
1 on this column
---------- Buffer: foo ----------
@end group

@group
(setq selective-display 2)
     @result{} 2

---------- Buffer: foo ----------
1 on this column
 2on this column
 2on this column
1 on this column
---------- Buffer: foo ----------
@end group
@end example
@end defvar

@defvar selective-display-ellipses
If this buffer-local variable is non-@code{nil}, then Emacs displays
@samp{@dots{}} at the end of a line that is followed by invisible text.
This example is a continuation of the previous one.

@example
@group
(setq selective-display-ellipses t)
     @result{} t

---------- Buffer: foo ----------
1 on this column
 2on this column ...
 2on this column
1 on this column
---------- Buffer: foo ----------
@end group
@end example

You can use a display table to substitute other text for the ellipsis
(@samp{@dots{}}).  @xref{Display Tables}.
@end defvar

@node Overlay Arrow
@section The Overlay Arrow
@cindex overlay arrow

  The @dfn{overlay arrow} is useful for directing the user's attention
to a particular line in a buffer.  For example, in the modes used for
interface to debuggers, the overlay arrow indicates the line of code
about to be executed.

@defvar overlay-arrow-string
This variable holds the string to display to call attention to a
particular line, or @code{nil} if the arrow feature is not in use.
@end defvar

@defvar overlay-arrow-position
This variable holds a marker that indicates where to display the overlay
arrow.  It should point at the beginning of a line.  The arrow text
appears at the beginning of that line, overlaying any text that would
otherwise appear.  Since the arrow is usually short, and the line
usually begins with indentation, normally nothing significant is
overwritten.

The overlay string is displayed only in the buffer that this marker
points into.  Thus, only one buffer can have an overlay arrow at any
given time.
@c !!! overlay-arrow-position: but the overlay string may remain in the display
@c of some other buffer until an update is required.  This should be fixed
@c now.  Is it?
@end defvar

  You can do a similar job by creating an overlay with a
@code{before-string} property.  @xref{Overlay Properties}.

@node Temporary Displays
@section Temporary Displays

  Temporary displays are used by Lisp programs to put output into a
buffer and then present it to the user for perusal rather than for
editing.  Many help commands use this feature.

@defspec with-output-to-temp-buffer buffer-name forms@dots{}
This function executes @var{forms} while arranging to insert any
output they print into the buffer named @var{buffer-name}.  The buffer
is then shown in some window for viewing, displayed but not selected.

The string @var{buffer-name} specifies the temporary buffer, which
need not already exist.  The argument must be a string, not a buffer.
The buffer is erased initially (with no questions asked), and it is
marked as unmodified after @code{with-output-to-temp-buffer} exits.

@code{with-output-to-temp-buffer} binds @code{standard-output} to the
temporary buffer, then it evaluates the forms in @var{forms}.  Output
using the Lisp output functions within @var{forms} goes by default to
that buffer (but screen display and messages in the echo area, although
they are ``output'' in the general sense of the word, are not affected).
@xref{Output Functions}.

The value of the last form in @var{forms} is returned.

@example
@group
---------- Buffer: foo ----------
 This is the contents of foo.
---------- Buffer: foo ----------
@end group

@group
(with-output-to-temp-buffer "foo"
    (print 20)
    (print standard-output))
@result{} #<buffer foo>

---------- Buffer: foo ----------
20

#<buffer foo>

---------- Buffer: foo ----------
@end group
@end example
@end defspec

@defvar temp-buffer-show-function
If this variable is non-@code{nil}, @code{with-output-to-temp-buffer}
calls it as a function to do the job of displaying a help buffer.  The
function gets one argument, which is the buffer it should display.

It is a good idea for this function to run @code{temp-buffer-show-hook}
just as @code{with-output-to-temp-buffer} normally would, inside of
@code{save-window-excursion} and with the chosen window and buffer
selected.
@end defvar

@defvar temp-buffer-show-hook
This normal hook is run by @code{with-output-to-temp-buffer} after
displaying the help buffer.  When the hook runs, the help buffer is
current, and the window it was displayed in is selected.
@end defvar

@defun momentary-string-display string position &optional char message
This function momentarily displays @var{string} in the current buffer at
@var{position}.  It has no effect on the undo list or on the buffer's
modification status.

The momentary display remains until the next input event.  If the next
input event is @var{char}, @code{momentary-string-display} ignores it
and returns.  Otherwise, that event remains buffered for subsequent use
as input.  Thus, typing @var{char} will simply remove the string from
the display, while typing (say) @kbd{C-f} will remove the string from
the display and later (presumably) move point forward.  The argument
@var{char} is a space by default.

The return value of @code{momentary-string-display} is not meaningful.

If the string @var{string} does not contain control characters, you can
do the same job in a more general way by creating (and then subsequently
deleting) an overlay with a @code{before-string} property.
@xref{Overlay Properties}.

If @var{message} is non-@code{nil}, it is displayed in the echo area
while @var{string} is displayed in the buffer.  If it is @code{nil}, a
default message says to type @var{char} to continue.

In this example, point is initially located at the beginning of the
second line:

@example
@group
---------- Buffer: foo ----------
This is the contents of foo.
@point{}Second line.
---------- Buffer: foo ----------
@end group

@group
(momentary-string-display
  "**** Important Message! ****"
  (point) ?\r
  "Type RET when done reading")
@result{} t
@end group

@group
---------- Buffer: foo ----------
This is the contents of foo.
**** Important Message! ****Second line.
---------- Buffer: foo ----------

---------- Echo Area ----------
Type RET when done reading
---------- Echo Area ----------
@end group
@end example
@end defun

@node Overlays
@section Overlays
@cindex overlays

You can use @dfn{overlays} to alter the appearance of a buffer's text on
the screen, for the sake of presentation features.  An overlay is an
object that belongs to a particular buffer, and has a specified
beginning and end.  It also has properties that you can examine and set;
these affect the display of the text within the overlay.

@menu
* Overlay Properties::	How to read and set properties.
			What properties do to the screen display.
* Managing Overlays::   Creating, moving, finding overlays.
@end menu

@node Overlay Properties
@subsection Overlay Properties

Overlay properties are like text properties in that the properties that
alter how a character is displayed can come from either source.  But in
most respects they are different.  Text properties are considered a part
of the text; overlays are specifically considered not to be part of the
text.  Thus, copying text between various buffers and strings preserves
text properties, but does not try to preserve overlays.  Changing a
buffer's text properties marks the buffer as modified, while moving an
overlay or changing its properties does not.  Unlike text property
changes, overlay changes are not recorded in the buffer's undo list.
@xref{Text Properties}, for comparison.

@table @code
@item priority
@kindex priority @r{(overlay property)}
This property's value (which should be a nonnegative number) determines
the priority of the overlay.  The priority matters when two or more
overlays cover the same character and both specify a face for display;
the one whose @code{priority} value is larger takes priority over the
other, and its face attributes override the face attributes of the lower
priority overlay.

Currently, all overlays take priority over text properties.  Please
avoid using negative priority values, as we have not yet decided just
what they should mean.

@item window
@kindex window @r{(overlay property)}
If the @code{window} property is non-@code{nil}, then the overlay
applies only on that window.

@item category
@kindex category @r{(overlay property)}
If an overlay has a @code{category} property, we call it the
@dfn{category} of the overlay.  It should be a symbol.  The properties
of the symbol serve as defaults for the properties of the overlay.

@item face
@kindex face @r{(overlay property)}
This property controls the way text is displayed---for example, which
font and which colors.  Its value is a face name or a list of face
names.  @xref{Faces}, for more information.

If the property value is a list, elements may also have the form
@code{(foreground-color . @var{color-name})} or @code{(background-color
. @var{color-name})}.  These elements specify just the foreground color
or just the background color; therefore, there is no need to create a
face for each color that you want to use.

@item mouse-face
@kindex mouse-face @r{(overlay property)}
This property is used instead of @code{face} when the mouse is within
the range of the overlay.

@item modification-hooks
@kindex modification-hooks @r{(overlay property)}
This property's value is a list of functions to be called if any
character within the overlay is changed or if text is inserted strictly
within the overlay.

The hook functions are called both before and after each change.
If the functions save the information they receive, and compare notes
between calls, they can determine exactly what change has been made
in the buffer text.

When called before a change, each function receives four arguments: the
overlay, @code{nil}, and the beginning and end of the text range to be
modified.

When called after a change, each function receives five arguments: the
overlay, @code{t}, the beginning and end of the text range just
modified, and the length of the pre-change text replaced by that range.
(For an insertion, the pre-change length is zero; for a deletion, that
length is the number of characters deleted, and the post-change
beginning and end are equal.)

@item insert-in-front-hooks
@kindex insert-in-front-hooks @r{(overlay property)}
This property's value is a list of functions to be called before and
after inserting text right at the beginning of the overlay.  The calling
conventions are the same as for the @code{modification-hooks} functions.

@item insert-behind-hooks
@kindex insert-behind-hooks @r{(overlay property)}
This property's value is a list of functions to be called before and
after inserting text right at the end of the overlay.  The calling
conventions are the same as for the @code{modification-hooks} functions.

@item invisible
@kindex invisible @r{(overlay property)}
The @code{invisible} property can make the text in the overlay
invisible, which means that it does not appear on the screen.
@xref{Invisible Text}, for details.

@item intangible
@kindex intangible @r{(overlay property)}
The @code{intangible} property on an overlay works just like the
@code{intangible} text property.  @xref{Special Properties}, for details.

@item isearch-open-invisible
This property tells incremental search how to make an invisible overlay
visible, permanently, if the final match overlaps it.  @xref{Invisible
Text}.

@item isearch-open-invisible-temporary
This property tells incremental search how to make an invisible overlay
visible, temporarily, during the search.  @xref{Invisible Text}.

@item before-string
@kindex before-string @r{(overlay property)}
This property's value is a string to add to the display at the beginning
of the overlay.  The string does not appear in the buffer in any
sense---only on the screen.  The string should contain only characters
that display as a single column---control characters, including tabs or
newlines, will give strange results.

@item after-string
@kindex after-string @r{(overlay property)}
This property's value is a string to add to the display at the end of
the overlay.  The string does not appear in the buffer in any
sense---only on the screen.  The string should contain only characters
that display as a single column---control characters, including tabs or
newlines, will give strange results.

@item evaporate
@kindex evaporate @r{(overlay property)}
If this property is non-@code{nil}, the overlay is deleted automatically
if it ever becomes empty (i.e., if it spans no characters).

@item local-map
@cindex keymap of character (and overlays)
@kindex local-map @r{(overlay property)}
If this property is non-@code{nil}, it specifies a keymap for a portion
of the text.  The property's value replaces the buffer's local map, when
the character after point is within the overlay.  @xref{Active Keymaps}.
@end table

  These are the functions for reading and writing the properties of an
overlay.

@defun overlay-get overlay prop
This function returns the value of property @var{prop} recorded in
@var{overlay}, if any.  If @var{overlay} does not record any value for
that property, but it does have a @code{category} property which is a
symbol, that symbol's @var{prop} property is used.  Otherwise, the value
is @code{nil}.
@end defun

@defun overlay-put overlay prop value
This function sets the value of property @var{prop} recorded in
@var{overlay} to @var{value}.  It returns @var{value}.
@end defun

  See also the function @code{get-char-property} which checks both
overlay properties and text properties for a given character.
@xref{Examining Properties}.

@node Managing Overlays
@subsection Managing Overlays

  This section describes the functions to create, delete and move
overlays, and to examine their contents.

@defun make-overlay start end &optional buffer front-advance rear-advance
This function creates and returns an overlay that belongs to
@var{buffer} and ranges from @var{start} to @var{end}.  Both @var{start}
and @var{end} must specify buffer positions; they may be integers or
markers.  If @var{buffer} is omitted, the overlay is created in the
current buffer.

The arguments @var{front-advance} and @var{rear-advance} specify the
insertion type for the start of the overlay and for the end of the
overlay.  @xref{Marker Insertion Types}.
@end defun

@defun overlay-start overlay
This function returns the position at which @var{overlay} starts,
as an integer.
@end defun

@defun overlay-end overlay
This function returns the position at which @var{overlay} ends,
as an integer.
@end defun

@defun overlay-buffer overlay
This function returns the buffer that @var{overlay} belongs to.
@end defun

@defun delete-overlay overlay
This function deletes @var{overlay}.  The overlay continues to exist as
a Lisp object, but ceases to be attached to the buffer it belonged to,
and ceases to have any effect on display.

A deleted overlay is not permanently useless.  You can give it
a new buffer position by calling @code{move-overlay}.
@end defun

@defun move-overlay overlay start end &optional buffer
This function moves @var{overlay} to @var{buffer}, and places its bounds
at @var{start} and @var{end}.  Both arguments @var{start} and @var{end}
must specify buffer positions; they may be integers or markers.  If
@var{buffer} is omitted, the overlay stays in the same buffer.

The return value is @var{overlay}.

This is the only valid way to change the endpoints of an overlay.  Do
not try modifying the markers in the overlay by hand, as that fails to
update other vital data structures and can cause some overlays to be
``lost''.
@end defun

@defun overlays-at pos
This function returns a list of all the overlays that contain position
@var{pos} in the current buffer.  The list is in no particular order.
An overlay contains position @var{pos} if it begins at or before
@var{pos}, and ends after @var{pos}.
@end defun

@defun overlays-in beg end
@tindex overlays-in
This function returns a list of the overlays that overlap the region
@var{beg} through @var{end}.  ``Overlap'' means that at least one
character is contained within the overlay and also contained within the
specified region; however, empty overlays are included in the result if
they are located at @var{beg} or between @var{beg} and @var{end}.
@end defun

@defun next-overlay-change pos
This function returns the buffer position of the next beginning or end
of an overlay, after @var{pos}.
@end defun

@defun previous-overlay-change pos
This function returns the buffer position of the previous beginning or
end of an overlay, before @var{pos}.
@end defun

@node Width
@section Width

Since not all characters have the same width, these functions let you
check the width of a character.  @xref{Primitive Indent}, and
@ref{Screen Lines}, for related functions.

@defun char-width char
@tindex char-width
This function returns the width in columns of the character @var{char},
if it were displayed in the current buffer and the selected window.
@end defun

@defun string-width string
@tindex string-width
This function returns the width in columns of the string @var{string},
if it were displayed in the current buffer and the selected window.
@end defun

@defun truncate-string-to-width string width &optional start-column padding
@tindex truncate-string-to-width
This function returns the part of @var{string} that fits within
@var{width} columns, as a new string.

If @var{string} does not reach @var{width}, then the result ends where
@var{string} ends.  If one multi-column character in @var{string}
extends across the column @var{width}, that character is not included in
the result.  Thus, the result can fall short of @var{width} but cannot
go beyond it.

The optional argument @var{start-column} specifies the starting column.
If this is non-@code{nil}, then the first @var{start-column} columns of
the string are omitted from the value.  If one multi-column character in
@var{string} extends across the column @var{start-column}, that
character is not included.

The optional argument @var{padding}, if non-@code{nil}, is a padding
character added at the beginning and end of the result string, to extend
it to exactly @var{width} columns.  The padding character is used at the
end of the result if it falls short of @var{width}.  It is also used at
the beginning of the result if one multi-column character in
@var{string} extends across the column @var{start-column}.

@example
(truncate-string-to-width "\tab\t" 12 4)
     @result{} "ab"
(truncate-string-to-width "\tab\t" 12 4 ?\ )
     @result{} "    ab  "
@end example
@end defun

@node Faces
@section Faces
@cindex face

A @dfn{face} is a named collection of graphical attributes: font,
foreground color, background color, and optional underlining.  Faces
control the display of text on the screen.

@cindex face id
Each face has its own @dfn{face number}, which distinguishes faces at
low levels within Emacs.  However, for most purposes, you can refer to
faces in Lisp programs by their names.

@defun facep object
This function returns @code{t} if @var{object} is a face name symbol (or
if it is a vector of the kind used internally to record face data).  It
returns @code{nil} otherwise.
@end defun

Each face name is meaningful for all frames, and by default it has the
same meaning in all frames.  But you can arrange to give a particular
face name a special meaning in one frame if you wish.

@menu
* Standard Faces::      The faces Emacs normally comes with.
* Defining Faces::      How to define a face with @code{defface}.
* Merging Faces::	How Emacs decides which face to use for a character.
* Face Functions::	How to define and examine faces.
@end menu

@node Standard Faces
@subsection Standard Faces

  This table lists all the standard faces and their uses.

@table @code
@item default
@kindex default @r{(face name)}
This face is used for ordinary text.

@item modeline
@kindex modeline @r{(face name)}
This face is used for mode lines and menu bars.

@item region
@kindex region @r{(face name)}
This face is used for highlighting the region in Transient Mark mode.

@item secondary-selection
@kindex secondary-selection @r{(face name)}
This face is used to show any secondary selection you have made.

@item highlight
@kindex highlight @r{(face name)}
This face is meant to be used for highlighting for various purposes.

@item underline
@kindex underline @r{(face name)}
This face underlines text.

@item bold
@kindex bold @r{(face name)}
This face uses a bold font, if possible.  It uses the bold variant of
the frame's font, if it has one.  It's up to you to choose a default
font that has a bold variant, if you want to use one.

@item italic
@kindex italic @r{(face name)}
This face uses the italic variant of the frame's font, if it has one.

@item bold-italic
@kindex bold-italic @r{(face name)}
This face uses the bold italic variant of the frame's font, if it has
one.
@end table

@node Defining Faces
@subsection Defining Faces

  The way to define a new face is with @code{defface}.  This creates a
kind of customization item (@pxref{Customization}) which the user can
customize using the Customization buffer (@pxref{Easy Customization,,,
emacs, The GNU Emacs Manual}).

@defmac defface face spec doc [keyword value]... 
@tindex defface
Declare @var{face} as a customizable face that defaults according to
@var{spec}.  Do not quote the symbol @var{face}.  The argument @var{doc}
specifies the face documentation.

When @code{defface} executes, it defines the face according to
@var{spec}, then uses any customizations that were read from the
@file{.emacs} file to override that specification.

The purpose of @var{spec} is to specify how the face should appear on
different kinds of terminals.  It should be an alist whose elements have
the form @code{(@var{display} @var{atts})}.  The element's @sc{car},
@var{display}, specifies a class of terminals.  The @sc{cdr},
@var{atts}, is a list of face attributes and their values; it specifies
what the face should look like on that kind of terminal.  The possible
attributes are defined in the value of @code{custom-face-attributes}.

The @var{display} part of an element of @var{spec} determines which
frames the element applies to.  If more than one element of @var{spec}
matches a given frame, the first matching element is the only one used
for that frame.  There are two possibilities for @var{display}:

@table @asis
@item @code{t}
This element of @var{spec} matches all frames.  Therefore, any
subsequent elements of @var{spec} are never used.  Normally
@code{t} is used in the last (or only) element of @var{spec}.

@item a list
If @var{display} is a list, each element should have the form
@code{(@var{characteristic} @var{value}@dots{})}.  Here
@var{characteristic} specifies a way of classifying frames, and the
@var{value}s are possible classifications which @var{display} should
apply to.  Here are the possible values of @var{characteristic}:

@table @code
@item type
The kind of window system the frame uses---either @code{x}, @code{pc}
(for the MS-DOS console), @code{w32} (for MS Windows 9X/NT), or
@code{tty}.

@item class
What kinds of colors the frame supports---either @code{color},
@code{grayscale}, or @code{mono}.

@item background
The kind of background---either @code{light} or @code{dark}.
@end table

If an element of @var{display} specifies more than one @var{value} for a
given @var{characteristic}, any of those values is acceptable.  If
@var{display} has more than one element, each element should specify a
different @var{characteristic}; then @emph{each} characteristic of the
frame must match one of the @var{value}s specified for it in
@var{display}.
@end table
@end defmac

  Here's how the standard face @code{region} could be defined
with @code{defface}:

@example
(defface region
         ((((class color) (background dark))
           (:background "blue"))
          (t (:background "gray")))
  "Used for displaying the region.")
@end example

  Internally, @code{defface} uses the symbol property
@code{face-defface-spec} to record the face attributes specified in
@code{defface}, @code{saved-face} for the attributes saved by the user
with the customization buffer, and @code{face-documentation} for the
documentation string.

@tindex frame-background-mode
@defopt frame-background-mode
This option, if non-@code{nil}, specifies the background type to use for
interpreting face definitions.  If it is @code{dark}, then Emacs treats
all frames as if they had a dark background, regardless of their actual
background colors.  If it is @code{light}, then Emacs treats all frames
as if they had a light background.
@end defopt

@node Merging Faces
@subsection Merging Faces for Display

  Here are all the ways to specify which face to use for display of text:

@itemize @bullet
@item
With defaults.  Each frame has a @dfn{default face}, which is used for
all text that doesn't somehow specify another face.  (We may change this
in a forthcoming Emacs version to serve as a default for all text.)

@item
With text properties.  A character may have a @code{face} property; if so,
it is displayed with that face.  @xref{Special Properties}.

If the character has a @code{mouse-face} property, that is used instead
of the @code{face} property when the mouse is ``near enough'' to the
character.

@item
With overlays.  An overlay may have @code{face} and @code{mouse-face}
properties too; they apply to all the text covered by the overlay.

@item
With a region that is active.  In Transient Mark mode, the region is
highlighted with a particular face (see @code{region-face}, below).

@item
With special glyphs.  Each glyph can specify a particular face 
number.  @xref{Glyphs}.
@end itemize

  If these various sources together specify more than one face for a
particular character, Emacs merges the attributes of the various faces
specified.  The attributes of the faces of special glyphs come first;
then comes the face for region highlighting, if appropriate;
then come attributes of faces from overlays, followed by those from text
properties, and last the default face.

  When multiple overlays cover one character, an overlay with higher
priority overrides those with lower priority.  @xref{Overlays}.

  If an attribute such as the font or a color is not specified in any of
the above ways, the frame's own font or color is used.

@node Face Functions
@subsection Functions for Working with Faces

  The attributes a face can specify include the font, the foreground
color, the background color, and underlining.  The face can also leave
these unspecified by giving the value @code{nil} for them.

  Here are the primitives for creating and changing faces.

@defun make-face name
This function defines a new face named @var{name}, initially with all
attributes @code{nil}.  It does nothing if there is already a face named
@var{name}.
@end defun

@defun face-list
This function returns a list of all defined face names.
@end defun

@defun copy-face old-face new-name &optional frame new-frame
This function defines the face @var{new-name} as a copy of the existing
face named @var{old-face}.  It creates the face @var{new-name} if that
doesn't already exist.

If the optional argument @var{frame} is given, this function applies
only to that frame.  Otherwise it applies to each frame individually,
copying attributes from @var{old-face} in each frame to @var{new-face}
in the same frame.

If the optional argument @var{new-frame} is given, then @code{copy-face}
copies the attributes of @var{old-face} in @var{frame} to @var{new-name}
in @var{new-frame}.
@end defun

  You can modify the attributes of an existing face with the following
functions.  If you specify @var{frame}, they affect just that frame;
otherwise, they affect all frames as well as the defaults that apply to
new frames.

@defun set-face-foreground face color &optional frame
@defunx set-face-background face color &optional frame
These functions set the foreground (or background, respectively) color
of face @var{face} to @var{color}.  The argument @var{color} should be a
string, the name of a color.

Certain shades of gray are implemented by stipple patterns on
black-and-white screens.
@end defun

@defun set-face-stipple face pattern &optional frame
This function sets the background stipple pattern of face @var{face} to
@var{pattern}.  The argument @var{pattern} should be the name of a
stipple pattern defined by the X server, or @code{nil} meaning don't use
stipple.

Normally there is no need to pay attention to stipple patterns, because
they are used automatically to handle certain shades of gray.
@end defun

@defun set-face-font face font &optional frame
This function sets the font of face @var{face}.  The argument @var{font}
should be a string, either a valid font name for your system or the name
of an Emacs fontset (@pxref{Fontsets}).  Note that if you set the font
explicitly, the bold and italic attributes cease to have any effect,
because the precise font that you specified is always used.
@end defun

@defun set-face-bold-p face bold-p &optional frame
@tindex set-face-bold-p
This function sets the bold attribute of face @var{face}.
Non-@code{nil} means bold; @code{nil} means non-bold.
@end defun

@defun set-face-italic-p face italic-p &optional frame
@tindex set-face-italic-p
This function sets the italic attribute of face @var{face}.
Non-@code{nil} means italic; @code{nil} means non-italic.
@end defun

@defun set-face-underline-p face underline-p &optional frame
This function sets the underline attribute of face @var{face}.
Non-@code{nil} means do underline; @code{nil} means don't.
@end defun

@defun invert-face face &optional frame
Swap the foreground and background colors of face @var{face}.  If the
face doesn't specify both foreground and background, then its foreground
and background are set to the default background and foreground,
respectively.
@end defun

  These functions examine the attributes of a face.  If you don't
specify @var{frame}, they refer to the default data for new frames.

@defun face-foreground face &optional frame
@defunx face-background face &optional frame
These functions return the foreground color (or background color,
respectively) of face @var{face}, as a string.
@end defun

@defun face-stipple face &optional frame
This function returns the name of the background stipple pattern of face
@var{face}, or @code{nil} if it doesn't have one.
@end defun

@defun face-font face &optional frame
This function returns the name of the font of face @var{face}.
@end defun

@defun face-bold-p face &optional frame
@tindex face-bold-p
This function returns the bold attribute of face @var{face}.
@end defun

@defun face-italic-p face &optional frame
@tindex face-italic-p
This function returns the italic attribute of face @var{face}.
@end defun

@defun face-underline-p face &optional frame
This function returns the underline attribute of face @var{face}.
@end defun

@defun face-id face
This function returns the face number of face @var{face}.
@end defun

@defun face-documentation face
@tindex face-documentation
This function returns the documentation string of face @var{face}, or
@code{nil} if none was specified for it.
@end defun

@defun face-equal face1 face2 &optional frame
This returns @code{t} if the faces @var{face1} and @var{face2} have the
same attributes for display.
@end defun

@defun face-differs-from-default-p face &optional frame
This returns @code{t} if the face @var{face} displays differently from
the default face.  A face is considered to be ``the same'' as the normal
face if each attribute is either the same as that of the default face or
@code{nil} (meaning to inherit from the default).
@end defun

@defvar region-face
This variable's value specifies the face number to use to display characters
in the region when it is active (in Transient Mark mode only).  The face
thus specified takes precedence over all faces that come from text
properties and overlays, for characters in the region.  @xref{The Mark},
for more information about Transient Mark mode.

Normally, the value is the face number of the face named @code{region}.
@end defvar

@tindex frame-update-face-colors
@defun frame-update-face-colors frame
This function updates the way faces display on @var{frame}, for a change
in @var{frame}'s foreground or background color.
@end defun

@node Blinking
@section Blinking Parentheses
@cindex parenthesis matching
@cindex blinking
@cindex balancing parentheses
@cindex close parenthesis

  This section describes the mechanism by which Emacs shows a matching
open parenthesis when the user inserts a close parenthesis.

@defvar blink-paren-function
The value of this variable should be a function (of no arguments) to
be called whenever a character with close parenthesis syntax is inserted.
The value of @code{blink-paren-function} may be @code{nil}, in which
case nothing is done.
@end defvar

@defopt blink-matching-paren
If this variable is @code{nil}, then @code{blink-matching-open} does
nothing.
@end defopt

@defopt blink-matching-paren-distance
This variable specifies the maximum distance to scan for a matching
parenthesis before giving up.
@end defopt

@defopt blink-matching-delay
This variable specifies the number of seconds for the cursor to remain
at the matching parenthesis.  A fraction of a second often gives
good results, but the default is 1, which works on all systems.
@end defopt

@deffn Command blink-matching-open
This function is the default value of @code{blink-paren-function}.  It
assumes that point follows a character with close parenthesis syntax and
moves the cursor momentarily to the matching opening character.  If that
character is not already on the screen, it displays the character's
context in the echo area.  To avoid long delays, this function does not
search farther than @code{blink-matching-paren-distance} characters.

Here is an example of calling this function explicitly.

@smallexample
@group
(defun interactive-blink-matching-open ()
@c Do not break this line! -- rms.
@c The first line of a doc string
@c must stand alone.
  "Indicate momentarily the start of sexp before point."
  (interactive)
@end group
@group
  (let ((blink-matching-paren-distance
         (buffer-size))
        (blink-matching-paren t))
    (blink-matching-open)))
@end group
@end smallexample
@end deffn

@node Inverse Video
@section Inverse Video
@cindex Inverse Video

@defopt inverse-video
@cindex highlighting
This variable controls whether Emacs uses inverse video for all text
on the screen.  Non-@code{nil} means yes, @code{nil} means no.  The
default is @code{nil}.
@end defopt

@defopt mode-line-inverse-video
This variable controls the use of inverse video for mode lines.  If it
is non-@code{nil}, then mode lines are displayed in inverse video.
Otherwise, mode lines are displayed normally, just like text.  The
default is @code{t}.

For window frames, this displays mode lines using the face named
@code{modeline}, which is normally the inverse of the default face
unless you change it.
@end defopt

@node Usual Display
@section Usual Display Conventions

  The usual display conventions define how to display each character
code.  You can override these conventions by setting up a display table
(@pxref{Display Tables}).  Here are the usual display conventions:

@itemize @bullet
@item
Character codes 32 through 126 map to glyph codes 32 through 126.
Normally this means they display as themselves.

@item
Character code 9 is a horizontal tab.  It displays as whitespace
up to a position determined by @code{tab-width}.

@item
Character code 10 is a newline.

@item
All other codes in the range 0 through 31, and code 127, display in one
of two ways according to the value of @code{ctl-arrow}.  If it is
non-@code{nil}, these codes map to sequences of two glyphs, where the
first glyph is the @sc{ASCII} code for @samp{^}.  (A display table can
specify a glyph to use instead of @samp{^}.)  Otherwise, these codes map
just like the codes in the range 128 to 255.

@item
Character codes 128 through 255 map to sequences of four glyphs, where
the first glyph is the @sc{ASCII} code for @samp{\}, and the others are
digit characters representing the character code in octal.  (A display
table can specify a glyph to use instead of @samp{\}.)

@item
Multibyte character codes above 256 are displayed as themselves, or as a
question mark or empty box if the terminal cannot display that
character.
@end itemize

  The usual display conventions apply even when there is a display
table, for any character whose entry in the active display table is
@code{nil}.  Thus, when you set up a display table, you need only
specify the characters for which you want special behavior.

  These variables affect the way certain characters are displayed on the
screen.  Since they change the number of columns the characters occupy,
they also affect the indentation functions.  These variables also affect
how the mode line is displayed; if you want to force redisplay of the
mode line using the new values, call the function
@code{force-mode-line-update} (@pxref{Mode Line Format}).

@defopt ctl-arrow
@cindex control characters in display
This buffer-local variable controls how control characters are
displayed.  If it is non-@code{nil}, they are displayed as a caret
followed by the character: @samp{^A}.  If it is @code{nil}, they are
displayed as a backslash followed by three octal digits: @samp{\001}.
@end defopt

@c Following may have overfull hbox.
@defvar default-ctl-arrow
The value of this variable is the default value for @code{ctl-arrow} in
buffers that do not override it.  @xref{Default Value}.
@end defvar

@defopt tab-width
The value of this variable is the spacing between tab stops used for
displaying tab characters in Emacs buffers.  The default is 8.  Note
that this feature is completely independent of the user-settable tab
stops used by the command @code{tab-to-tab-stop}.  @xref{Indent Tabs}.
@end defopt

@node Display Tables
@section Display Tables

@cindex display table
You can use the @dfn{display table} feature to control how all possible
character codes display on the screen.  This is useful for displaying
European languages that have letters not in the @sc{ASCII} character
set.

The display table maps each character code into a sequence of
@dfn{glyphs}, each glyph being an image that takes up one character
position on the screen.  You can also define how to display each glyph
on your terminal, using the @dfn{glyph table}.

Display tables affect how the mode line is displayed; if you want to
force redisplay of the mode line using a new display table, call
@code{force-mode-line-update} (@pxref{Mode Line Format}).

@menu
* Display Table Format::	What a display table consists of.
* Active Display Table::	How Emacs selects a display table to use.
* Glyphs::			How to define a glyph, and what glyphs mean.
@end menu

@node Display Table Format
@subsection Display Table Format

  A display table is actually a char-table (@pxref{Char-Tables}) with
@code{display-table} as its subtype.

@defun make-display-table
This creates and returns a display table.  The table initially has
@code{nil} in all elements.
@end defun

  The ordinary elements of the display table are indexed by character
codes; the element at index @var{c} says how to display the character
code @var{c}.  The value should be @code{nil} or a vector of glyph
values (@pxref{Glyphs}).  If an element is @code{nil}, it says to
display that character according to the usual display conventions
(@pxref{Usual Display}).

  If you use the display table to change the display of newline
characters, the whole buffer will be displayed as one long ``line.''

  The display table also has six ``extra slots'' which serve special
purposes.  Here is a table of their meanings; @code{nil} in any slot
means to use the default for that slot, as stated below.

@table @asis
@item 0
The glyph for the end of a truncated screen line (the default for this
is @samp{$}).  @xref{Glyphs}.
@item 1
The glyph for the end of a continued line (the default is @samp{\}).
@item 2
The glyph for indicating a character displayed as an octal character
code (the default is @samp{\}).
@item 3
The glyph for indicating a control character (the default is @samp{^}).
@item 4
A vector of glyphs for indicating the presence of invisible lines (the
default is @samp{...}).  @xref{Selective Display}.
@item 5
The glyph used to draw the border between side-by-side windows (the
default is @samp{|}).  @xref{Splitting Windows}.
@end table

  For example, here is how to construct a display table that mimics the
effect of setting @code{ctl-arrow} to a non-@code{nil} value:

@example
(setq disptab (make-display-table))
(let ((i 0))
  (while (< i 32)
    (or (= i ?\t) (= i ?\n)
        (aset disptab i (vector ?^ (+ i 64))))
    (setq i (1+ i)))
  (aset disptab 127 (vector ?^ ??)))
@end example

@defun display-table-slot display-table slot
@tindex display-table-slot
This function returns the value of the extra slot @var{slot} of
@var{display-table}.  The argument @var{slot} may be a number from 0 to
5 inclusive, or a slot name (symbol).  Valid symbols are
@code{truncation}, @code{wrap}, @code{escape}, @code{control},
@code{selective-display}, and @code{vertical-border}.
@end defun

@defun set-display-table-slot display-table slot value
@tindex set-display-table-slot
This function stores @var{value} in the extra slot @var{slot} of
@var{display-table}.  The argument @var{slot} may be a number from 0 to
5 inclusive, or a slot name (symbol).  Valid symbols are
@code{truncation}, @code{wrap}, @code{escape}, @code{control},
@code{selective-display}, and @code{vertical-border}.
@end defun

@node Active Display Table
@subsection Active Display Table
@cindex active display table

  Each window can specify a display table, and so can each buffer.  When
a buffer @var{b} is displayed in window @var{w}, display uses the
display table for window @var{w} if it has one; otherwise, the display
table for buffer @var{b} if it has one; otherwise, the standard display
table if any.  The display table chosen is called the @dfn{active}
display table.

@defun window-display-table window
This function returns @var{window}'s display table, or @code{nil}
if @var{window} does not have an assigned display table.
@end defun

@defun set-window-display-table window table
This function sets the display table of @var{window} to @var{table}.
The argument @var{table} should be either a display table or
@code{nil}.
@end defun

@defvar buffer-display-table
This variable is automatically buffer-local in all buffers; its value in
a particular buffer specifies the display table for that buffer.  If it
is @code{nil}, that means the buffer does not have an assigned display
table.
@end defvar

@defvar standard-display-table
This variable's value is the default display table, used whenever a
window has no display table and neither does the buffer displayed in
that window.  This variable is @code{nil} by default.
@end defvar

  If there is no display table to use for a particular window---that is,
if the window specifies none, its buffer specifies none, and
@code{standard-display-table} is @code{nil}---then Emacs uses the usual
display conventions for all character codes in that window.  @xref{Usual
Display}.

@node Glyphs
@subsection Glyphs

@cindex glyph
  A @dfn{glyph} is a generalization of a character; it stands for an
image that takes up a single character position on the screen.  Glyphs
are represented in Lisp as integers, just as characters are.

@cindex glyph table
  The meaning of each integer, as a glyph, is defined by the glyph
table, which is the value of the variable @code{glyph-table}.

@defvar glyph-table
The value of this variable is the current glyph table.  It should be a
vector; the @var{g}th element defines glyph code @var{g}.  If the value
is @code{nil} instead of a vector, then all glyphs are simple (see
below).
@end defvar

  Here are the possible types of elements in the glyph table:

@table @asis
@item @var{string}
Send the characters in @var{string} to the terminal to output
this glyph.  This alternative is available on character terminals,
but not under a window system.

@item @var{integer}
Define this glyph code as an alias for glyph code @var{integer}.  You
can use an alias to specify a face code for the glyph; see below.

@item @code{nil}
This glyph is simple.  On an ordinary terminal, the glyph code mod
524288 is the character to output.  In a window system, the glyph code
mod 524288 is the character to output, and the glyph code divided by
524288 specifies the face number (@pxref{Face Functions}) to use while
outputting it.  (524288 is
@ifinfo
2**19.)
@end ifinfo
@tex
$2^{19}$.)
@end tex
@xref{Faces}.
@end table

  If a glyph code is greater than or equal to the length of the glyph
table, that code is automatically simple.

@node Beeping
@section Beeping
@cindex beeping
@cindex bell

  This section describes how to make Emacs ring the bell (or blink the
screen) to attract the user's attention.  Be conservative about how
often you do this; frequent bells can become irritating.  Also be
careful not to use just beeping when signaling an error is more
appropriate.  (@xref{Errors}.)

@defun ding &optional do-not-terminate
@cindex keyboard macro termination
This function beeps, or flashes the screen (see @code{visible-bell} below).
It also terminates any keyboard macro currently executing unless
@var{do-not-terminate} is non-@code{nil}.
@end defun

@defun beep &optional do-not-terminate
This is a synonym for @code{ding}.
@end defun

@defopt visible-bell
This variable determines whether Emacs should flash the screen to
represent a bell.  Non-@code{nil} means yes, @code{nil} means no.  This
is effective on a window system, and on a character-only terminal
provided the terminal's Termcap entry defines the visible bell
capability (@samp{vb}).
@end defopt

@defvar ring-bell-function
@tindex ring-bell-function
If this is non-@code{nil}, it specifies how Emacs should ``ring the
bell.''  Its value should be a function of no arguments.
@end defvar

@node Window Systems
@section Window Systems

  Emacs works with several window systems, most notably the X Window
System.  Both Emacs and X use the term ``window'', but use it
differently.  An Emacs frame is a single window as far as X is
concerned; the individual Emacs windows are not known to X at all.

@defvar window-system
This variable tells Lisp programs what window system Emacs is running
under.  The possible values are

@table @code
@item x
@cindex X Window System
Emacs is displaying using X.
@item pc
Emacs is displaying using MSDOS.
@item w32
Emacs is displaying using Windows NT or Windows 95.
@item nil
Emacs is using a character-based terminal.
@end table
@end defvar

@defvar window-setup-hook
This variable is a normal hook which Emacs runs after handling the
initialization files.  Emacs runs this hook after it has completed
loading your @file{.emacs} file, the default initialization file (if
any), and the terminal-specific Lisp code, and running the hook
@code{term-setup-hook}.

This hook is used for internal purposes: setting up communication with
the window system, and creating the initial window.  Users should not
interfere with it.
@end defvar