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[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 Free Software Foundation, Inc. 
@c See the file elisp.texi for copying conditions.
@setfilename ../info/display
@node Display, Calendar, System Interface, 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.
* Screen Size::         How big is the Emacs screen.
* Truncation::          Folding or wrapping long text lines.
* The Echo Area::       Where messages are displayed.
* Selective Display::   Hiding part of the buffer text.
* 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.
* 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.  @xref{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

  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.  @xref{Suspending Emacs}.

@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 yes, @code{nil}
means no.
@end defvar

  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.

@node Screen Size
@section Screen Size
@cindex size of screen
@cindex screen size
@cindex display lines
@cindex display columns
@cindex resize redisplay

  The screen size functions report or tell Emacs the height or width of
the terminal.  When you are using multiple frames, they apply to the
selected frame (@pxref{Frames}).

@defun screen-height
This function returns the number of lines on the screen that are
available for display.

@example
@group
(screen-height)
     @result{} 50
@end group
@end example
@end defun

@defun screen-width
This function returns the number of columns on the screen that are
available for display.

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

@defun set-screen-height lines &optional not-actual-size
This function declares that the terminal can display @var{lines} lines.
The sizes of existing windows are altered proportionally to fit.

If @var{not-actual-size} is non-@code{nil}, then Emacs displays
@var{lines} lines of output, but does not change its value for the
actual height of the screen.  (Knowing the correct actual size may be
necessary for correct cursor positioning.)  Using a smaller height than
the terminal actually implements may be useful to reproduce behavior
observed on a smaller screen, or if the terminal malfunctions when using
its whole screen.

If @var{lines} is different from what it was previously, then the
entire screen is cleared and redisplayed using the new size.

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

@defun set-screen-width columns &optional not-actual-size
This function declares that the terminal can display @var{columns}
columns.  The details are as in @code{set-screen-height}.
@end defun

@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'' or is continued
onto the next 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

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

@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

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

@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 prints 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.

@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

@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

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

  @dfn{Selective display} is a class of minor modes in which specially
marked lines do not appear on the screen, or in which highly indented
lines do not appear.

  The first variant, explicit selective display, is designed for use in
a Lisp program.  The program controls which lines are hidden by altering
the text.  Outline mode uses this variant.  In the second variant, the
choice of lines to hide is made automatically based on indentation.
This variant is designed as 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 which
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.

@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 as an arrow, or @code{nil} if
the arrow feature is not in use.
@end defvar

@defvar overlay-arrow-position
This variable holds a marker which indicates where to display the 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 which 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

@node Temporary Displays
@section Temporary Displays

  Temporary displays are used by commands to put output into a buffer
and then present it to the user for perusal rather than for editing.
Many of the 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, if 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.

In Emacs versions 18 and earlier, this variable was called
@code{temp-buffer-show-hook}.
@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 @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.  An overlay is an object which 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 some respects, but the
differences are more important than the similarities.  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 propery changes, overlay changes are not recorded in the
buffer's undo list.

@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 face
@kindex face @r{(overlay property)}
This property controls the font and color of text.  @xref{Faces}, for
more information.  This feature is temporary; in the future, we may
replace it with other ways of specifying how to display text.

@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.  This feature may be temporary, like
@code{face}.

@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.  Each function receives three arguments: the
overlay, and the beginning and end of the part of the buffer being
modified.

@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
if text is inserted right at the beginning of the overlay.

@item insert-behind-hooks
@kindex insert-behind-hooks @r{(overlay property)}
This property's value is a list of functions to be called if text is
inserted right at the end of the overlay.

@item invisible
@kindex invisible @r{(overlay property)}
A non-@code{nil} @code{invisible} property means that the text in the
overlay does not appear on the screen.  This works much like selective
display.  Details of this feature are likely to change in future
versions, so check the @file{etc/NEWS} file in the version you are
using.

@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.  This is not yet implemented, but will be.

@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.  This is not yet implemented, but will be.
@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 @var{overlay} does not record any value for that
property, then 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
This function creates and returns an overlay which 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.
@end defun

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

@defun overlay-end overlay
This function returns the position at which @var{overlay} ends.
@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 part of the buffer it belonged to, and
ceases to have any effect on display.
@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 next-overlay-change pos
This function returns the buffer position of the next beginning or end
of an overlay, after @var{pos}.
@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 id number} which distinguishes faces at
low levels within Emacs.  However, for most purposes, you can refer to
faces in Lisp programs by their names.

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.
* 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 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}, whose id number is
zero, which is used for all text that doesn't somehow specify another
face.

@item
With text properties.  A character may have a @code{face} property; if so,
it's 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 special glyphs.  Each glyph can specify a particular face id
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 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 (respectively, background) color of
face @var{face} to @var{color}.  The argument @var{color} should be a
string, the name of a color.
@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.
@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.
@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 (respectively, background) color
of face @var{face}, as a string.
@end defun

@defun face-font face &optional frame
This function returns the name of the font 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-number face
This function returns the face id number of face @var{face}.
@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 id 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 id number of the face named @code{region}.
@end defvar

@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.

@vindex blink-paren-hook
@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.

@quotation
@strong{Please note:} this variable was named @code{blink-paren-hook} in
older Emacs versions, but since it is not called with the standard
convention for hooks, it was renamed to @code{blink-paren-function} in
version 19.
@end quotation
@end defvar

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

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

@defun 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 defun

@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 (under
X, this uses the face named @code{modeline}, which you can set as you
wish).  Otherwise, mode lines are displayed normally, just like text.
The default is @code{t}.
@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 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 code in octal.  (A display table can
specify a glyph to use instead of @samp{\}.)
@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 the characters for which you want unusual 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.

@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 from 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 256
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}.

@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.
* ISO Latin 1::                 How to use display tables
                                  to support the ISO Latin 1 character set.
@end menu

@node Display Table Format
@subsection Display Table Format

  A display table is actually an array of 261 elements.

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

  The first 256 elements correspond to character codes; the @var{n}th
element says how to display the character code @var{n}.  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}).

  The remaining five elements of a display table serve special purposes,
and @code{nil} means use the default stated below.

@table @asis
@item 256
The glyph for the end of a truncated screen line (the default for this
is @samp{$}).  @xref{Glyphs}.
@item 257
The glyph for the end of a continued line (the default is @samp{\}).
@item 258
The glyph for indicating a character displayed as an octal character
code (the default is @samp{\}).
@item 259
The glyph for indicating a control character (the default is @samp{^}).
@item 260
A vector of glyphs for indicating the presence of invisible lines (the
default is @samp{...}).  @xref{Selective Display}.
@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

@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 local in all buffers; its value in a
particular buffer is the display table for that buffer, or @code{nil} if
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 has none, its buffer has none, and
@code{standard-display-table} has none---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 @var
@item string
Send the characters in @var{string} to the terminal to output
this glyph.  This alternative is available on character terminals,
but not under X.

@item integer
Define this glyph code as an alias for 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 256
is the character to output.  With X, the glyph code mod 256 is the
character to output, and the glyph code divided by 256 specifies the
@dfn{face id number} to use while outputting it.  @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 ISO Latin 1
@subsection ISO Latin 1

If you have a terminal that can handle the entire ISO Latin 1 character
set, you can arrange to use that character set as follows:

@example
(require 'disp-table)
;; @r{Set char codes 160--255 to display as themselves.}
;; @r{(Codes 128--159 are the additional control characters.)}
(standard-display-8bit 160 255)
@end example

If you are editing buffers written in the ISO Latin 1 character set and
your terminal doesn't handle anything but @sc{ASCII}, you can load the file
@file{iso-ascii} to set up a display table which makes the other ISO
characters display as sequences of @sc{ASCII} characters.  For example, the
character ``o with umlaut'' displays as @samp{@{"o@}}.

Some European countries have terminals that don't support ISO Latin 1
but do support the special characters for that country's language.  You
can define a display table to work one language using such terminals.
For an example, see @file{lisp/iso-swed.el}, which handles certain
Swedish terminals.

You can load the appropriate display table for your terminal
automatically by writing a terminal-specific Lisp file for the terminal
type.

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

  You can make Emacs ring a 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 beeping alone
when signaling an error is appropriate.  (@xref{Errors}.)

@defun ding &optional dont-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{dont-terminate} is non-@code{nil}.
@end defun

@defun beep &optional dont-terminate
This is a synonym for @code{ding}.
@end defun

@defvar 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 only if the Termcap entry for the terminal in use has the
visible bell flag (@samp{vb}) set.
@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
@cindex X Window System
This variable tells Lisp programs what window system Emacs is running
under.  Its value should be a symbol such as @code{x} (if Emacs is
running under X) or @code{nil} (if Emacs is running on an ordinary
terminal).
@end defvar

@defvar window-system-version
This variable distinguishes between different versions of the X Window
System.  Its value is 10 or 11 when using X; @code{nil} otherwise.
@end defvar

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