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

  This chapter describes a number of other features related to the display
that XEmacs 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.
* Warnings::            Display of Warnings.
* 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.
* Blinking::            How XEmacs shows the matching open parenthesis.
* Usual Display::	The usual conventions for displaying nonprinting chars.
* Display Tables::	How to specify other conventions.
* Beeping::             Audible signal to the user.
@end menu

@node Refresh Screen
@section Refreshing the Screen

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

@defun redraw-frame &optional frame no-preempt
This function clears and redisplays frame @var{frame}.

@var{frame} defaults to the selected frame if omitted.

Normally, redisplay is preempted as normal if input arrives.  However,
if optional second arg @var{no-preempt} is non-@code{nil}, redisplay
will not stop for input and is guaranteed to proceed to completion.
@end defun

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

@deffn Command redraw-display &optional device
This function redraws all frames on @var{device} marked as having their
image garbled.  @var{device} defaults to the selected device.  If
@var{device} is @code{t}, all devices will have their frames checked.
@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 XEmacs also refreshes the screen.
Some terminal emulators record separate contents for display-oriented
programs such as XEmacs and for ordinary sequential display.  If you are
using such a terminal, you might want to inhibit the redisplay on
resumption.  @xref{Suspending XEmacs}.

@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 XEmacs redraws the entire screen after it
has been suspended and resumed.  Non-@code{nil} means yes, @code{nil}
means no.
@end defvar

@cindex display update
@cindex update display
@cindex refresh display
  The above functions do not actually cause the display to be updated;
rather, they clear out the internal display records that XEmacs
maintains, so that the next time the display is updated it will be
redrawn from scratch.  Normally this occurs the next time that
@code{next-event} or @code{sit-for} is called; however, a display update
will not occur if there is input pending.  @xref{Command Loop}.

@defun force-cursor-redisplay &optional frame
This function causes an immediate update of the cursor on @var{frame},
which defaults to the selected frame.
@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 truncated or continued on the next line.  When a line
is truncated, this is normally shown with a @samp{\} in the rightmost
column of the window on X displays, and with a @samp{$} on TTY devices.
When a line is continued or ``wrapped'' onto the next line, this is
shown with a curved arrow in the rightmost column of the window (or with
a @samp{\} on TTY devices).  The additional screen lines used to display
a long text line are called @dfn{continuation} lines.

  Normally, whenever line truncation is in effect for a particular
window, a horizontal scrollbar is displayed in that window if the
device supports scrollbars.  @xref{Scrollbars}.

  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 XEmacs displays lines that
extend beyond the right edge of the window.  If it is non-@code{nil},
then XEmacs does not display continuation lines; rather each line of
text occupies exactly one screen line, and a backslash appears at the
edge of any line that extends to or beyond the edge of the window.  The
default is @code{nil}.

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 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

  The backslash and curved arrow used to indicate truncated or continued
lines are only defaults, and can be changed.  These images are actually
glyphs (@pxref{Glyphs}).  XEmacs provides a great deal of flexibility
in how glyphs can be controlled. (This differs from FSF Emacs, which
uses display tables to control these images.)

  For details, @ref{Redisplay Glyphs}.

@ignore Not yet in XEmacs
  If your buffer contains @strong{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 local in every buffer.
@end defvar
@end ignore

@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{XEmacs Lisp Reference Manual} specifies the rules for resolving conflicts
between the echo area and the minibuffer for use of that screen space
(@pxref{Minibuffer,, The Minibuffer, xemacs, The XEmacs Lisp Reference Manual}).
Such a conflicts may be avoided at all as described in @ref{Customizing Message
Display}.

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

In addition to only displaying a message, XEmacs allows you to
@dfn{label} your messages, giving you fine-grained control of their
display.  Message label is a symbol denoting the message type.  Some
standard labels are:

@itemize @bullet
@item @code{message}---default label used by the @code{message}
function;

@item @code{error}---default label used for reporting errors;

@item @code{progress}---progress indicators like
@samp{Converting... 45%} (not logged by default);

@item @code{prompt}---prompt-like messages like @samp{Isearch: foo} (not
logged by default);

@item @code{command}---helper command messages like @samp{Mark set} (not
logged by default);

@item @code{no-log}---messages that should never be logged
@end itemize

Several messages may be stacked in the echo area at once.  Lisp programs
may access these messages, or remove them as appropriate, via the
message stack.

@defun display-message label message &optional frame stdout-p
This function displays @var{message} (a string) labeled as @var{label},
as described above.

The @var{frame} argument specifies the frame to whose minibuffer the
message should be printed.  This is currently unimplemented.  The
@var{stdout-p} argument is used internally.

@example
(display-message 'command "Mark set")
@end example
@end defun

@defun lmessage label string &rest arguments
This function displays a message @var{string} with label @var{label}.
It is similar to @code{message} in that it accepts a @code{printf}-like
strings and any number of arguments.

@example
@group
;; @r{Display a command message.}
(lmessage 'command "Comment column set to %d" comment-column)
@end group

@group
;; @r{Display a progress message.}
(lmessage 'progress "Fontifying %s... (%d)" buffer percentage)
@end group

@group
;; @r{Display a message that should not be logged.}
(lmessage 'no-log "Done")
@end group
@end example
@end defun

@defun clear-message &optional label frame stdout-p no-restore
This function remove any message with the given @var{label}
from the message-stack, erasing it from the echo area if it's currently
displayed there.

If a message remains at the head of the message-stack and
@var{no-restore} is @code{nil}, it will be displayed.  The string which
remains in the echo area will be returned, or @code{nil} if the
message-stack is now empty.  If @var{label} is @code{nil}, the entire
message-stack is cleared.

@example
;; @r{Show a message, wait for 2 seconds, and restore old minibuffer}
;; @r{contents.}
(message "A message")
 @print{} A message
@result{} "A Message"
(lmessage 'my-label "Newsflash!  Newsflash!")
 @print{} Newsflash!  Newsflash!
@result{} "Newsflash!  Newsflash!"
(sit-for 2)
(clear-message 'my-label)
 @print{} A message
@result{} "A message"
@end example

Unless you need the return value or you need to specify a label,
you should just use @code{(message nil)}.
@end defun

@defun current-message &optional frame
This function returns the current message in the echo area, or
@code{nil}.  The @var{frame} argument is currently unused.
@end defun

Some of the messages displayed in the echo area are also recorded in the
@samp{ *Message-Log*} buffer.  Exactly which messages will be recorded
can be tuned using the following variables.

@defopt log-message-max-size
This variable specifies the maximum size of the @samp{ *Message-log*}
buffer.
@end defopt

@defvar log-message-ignore-labels
This variable specifies the labels whose messages will not be logged.
It should be a list of symbols.
@end defvar

@defvar log-message-ignore-regexps
This variable specifies the regular expressions matching messages that
will not be logged.  It should be a list of regular expressions.

Normally, packages that generate messages that might need to be ignored
should label them with @code{progress}, @code{prompt}, or @code{no-log},
so they can be filtered by @code{log-message-ignore-labels}.
@end defvar

@defvar echo-keystrokes
This variable determines how much time should elapse before command
characters echo.  Its value must be a number, 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.  Any subsequent characters in the
same command will be echoed as well.

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

@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

@menu
* Customizing Message Display::
@end menu

@node Customizing Message Display
@subsection Customizing Message Display

Message display function specify message intended for echo area by
putting message text into @code{" *Echo Area*"} buffer.  When event
loop code decides to update display after displaying the message, text
of this buffer is erased.  How exactly the text will be displayed may
be affected by the following.

@findex redisplay-echo-area
@defvar redisplay-echo-area-function
The function called to display echo area text.  The default variable
value, @code{redisplay-echo-area} function, does that by displaying
the text in the same place on the screen as the echo area.  So does
other redisplay code.  User code can avoid this regardless of what
redisplay code will run afterwards by erasing text of @code{" *Echo
Area*"} buffer.
@end defvar

@defvar undisplay-echo-area-function
The variable value, if non-@code{nil}, is called by command loop after
erasing text of @code{" *Echo Area*"} buffer.  It must clean up data
created by @code{redisplay-echo-area-function} value.
@end defvar

@defvar minibuffer-echo-wait-function
The function is called by command loop only when minibuffer was active
and message was displayed (text appeared in @code{" *Echo Area*"}
buffer).  It must wait after displaying message so that user can read
it.  By default, when the variable value is @code{nil}, the equivalent
of @code{(sit-for 2)} is run.
@end defvar

@node Warnings
@section Warnings

XEmacs contains a facility for unified display of various warnings.
Unlike errors, warnings are displayed in the situations when XEmacs
encounters a problem that is recoverable, but which should be fixed for
safe future operation.

For example, warnings are printed by the startup code when it encounters
problems with X keysyms, when there is an error in @file{.emacs}, and in
other problematic situations.  Unlike messages, warnings are displayed
in a separate buffer, and include an explanatory message that may span
across several lines.  Here is an example of how a warning is displayed:

@example
(1) (initialization/error) An error has occurred while loading ~/.emacs:

Symbol's value as variable is void: bogus-variable

To ensure normal operation, you should investigate the cause of the error
in your initialization file and remove it.  Use the `-debug-init' option
to XEmacs to view a complete error backtrace.
@end example

Each warning has a @dfn{class} and a @dfn{priority level}.  The class is
a symbol describing what sort of warning this is, such as
@code{initialization}, @code{resource} or @code{key-mapping}.

The warning priority level specifies how important the warning is.  The
recognized warning levels, in increased order of priority, are:
@code{debug}, @code{info}, @code{notice}, @code{warning}, @code{error},
@code{critical}, @code{alert} and @code{emergency}.

@defun display-warning class message &optional level
This function displays a warning message @var{message} (a string).
@var{class} should be a warning class symbol, as described above, or a
list of such symbols.  @var{level} describes the warning priority level.
If unspecified, it default to @code{warning}.

@example
@group
(display-warning 'resource
  "Bad resource specification encountered:
something like

    Emacs*foo: bar

You should replace the * with a . in order to get proper behavior when
you use the specifier and/or `set-face-*' functions.")
@end group

@group
---------- Warning buffer ----------
(1) (resource/warning) Bad resource specification encountered:
something like

    Emacs*foo: bar

You should replace the * with a . in order to get proper behavior when
you use the specifier and/or `set-face-*' functions.
---------- Warning buffer ----------
@end group
@end example
@end defun

@defun lwarn class level message &rest args
This function displays a formatted labeled warning message.  As above,
@var{class} should be the warning class symbol, or a list of such
symbols, and @var{level} should specify the warning priority level
(@code{warning} by default).

Unlike in @code{display-warning}, @var{message} may be a formatted
message, which will be, together with the rest of the arguments, passed
to @code{format}.

@example
(lwarn 'message-log 'warning
  "Error caught in `remove-message-hook': %s"
  (error-message-string e))
@end example
@end defun

@defvar log-warning-minimum-level
This variable specifies the minimum level of warnings that should be
generated.  Warnings with level lower than defined by this variable are
completely ignored, as if they never happened.
@end defvar

@defvar display-warning-minimum-level
This variable specifies the minimum level of warnings that should be
displayed.  Unlike @code{log-warning-minimum-level}, setting this
function does not suppress warnings entirely---they are still generated
in the @samp{*Warnings*} buffer, only they are not displayed by default.
@end defvar

@defvar log-warning-suppressed-classes
This variable specifies a list of classes that should not be logged or
displayed.  If any of the class symbols associated with a warning is the
same as any of the symbols listed here, the warning will be completely
ignored, as it they never happened.
@end defvar

@defvar display-warning-suppressed-classes
This variable specifies a list of classes that should not be logged or
displayed.  If any of the class symbols associated with a warning is the
same as any of the symbols listed here, the warning will not be
displayed.  The warning will still logged in the *Warnings* buffer
(unless also contained in `log-warning-suppressed-classes'), but the
buffer will not be automatically popped up.
@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 or a property of an overlay.

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{invisibility} property works.  This feature is much like selective
display (@pxref{Selective Display}), but more general and cleaner.

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 makes certain characters invisible.
Ultimately, a character is invisible if any of the elements of this list
applies to it.  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

  Ordinarily, commands that operate on text or move point do not care
whether the text is invisible.  However, the user-level line motion
commands explicitly ignore invisible newlines.  Since this causes a
slow-down of these commands it is turned off by default, controlled by
the variable @code{line-move-ignore-invisible}.

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

  @dfn{Selective display} is a pair of features that hide certain
lines on the screen.

  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 has traditionally used this variant.  It has
been partially replaced by the invisible text feature (@pxref{Invisible
Text}); there is a new version of Outline mode which uses that instead.

  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 XEmacs.

@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 XEmacs 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.
Despite its name, the value of this variable can be either a string
or a glyph (@pxref{Glyphs}).
@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 the same job by creating an extent with a
@code{begin-glyph} property.  @xref{Extent Properties}.

@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.

@deffn {Special Operator} 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 deffn

@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.

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.

You can do the same job in a more general way by creating an extent
with a begin-glyph property.  @xref{Extent 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

  This function works by actually changing the text in the buffer.  As a
result, if you later undo in this buffer, you will see the message come
and go.
@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 XEmacs 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

@defvar blink-matching-paren-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 defvar

@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 ()
  "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 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 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 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{runes}, each rune being an image that takes up one character
position on the screen.  You can also define how to display each rune
on your terminal, using the @dfn{rune table}.

@menu
* Display Table Format::	What a display table consists of.
* Active Display Table::	How XEmacs selects a display table to use.
* Character Descriptors::	Format of an individual element of a
				  display table.
@end menu

@ignore Not yet working in XEmacs?
* ISO Latin 1::			How to use display tables
				  to support the ISO Latin 1 character set.
@end ignore

@node Display Table Format
@subsection Display Table Format

  A display table is an array of 256 elements. (In FSF Emacs, a display
table is 262 elements.  The six extra elements specify the truncation
and continuation glyphs, etc.  This method is very kludgey, and in
XEmacs the variables @code{truncation-glyph}, @code{continuation-glyph},
etc. are used.  @xref{Truncation}.)

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

  The 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}, a string, a glyph, or a vector of strings and
glyphs (@pxref{Character Descriptors}).  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.''

  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 (concat "^" (char-to-string (+ i 64)))))
    (setq i (1+ i)))
  (aset disptab 127 "^?"))
@end example

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

  The active display table is controlled by the variable
@code{current-display-table}.  This is a specifier, which means
that you can specify separate values for it in individual buffers,
windows, frames, and devices, as well as a global value.  It also
means that you cannot set this variable using @code{setq}; use
@code{set-specifier} instead.  @xref{Specifiers}. (FSF Emacs
uses @code{window-display-table}, @code{buffer-display-table},
@code{standard-display-table}, etc. to control the display table.
However, specifiers are a cleaner and more powerful way of doing
the same thing.  FSF Emacs also uses a different format for
the contents of a display table, using additional indirection
to a ``glyph table'' and such.  Note that ``glyph'' has a different
meaning in XEmacs.)

@defvar current-display-table

The display table currently in use.  This is a specifier.

Display tables are used to control how characters are displayed.  Each
time that redisplay processes a character, it is looked up in all the
display tables that apply (obtained by calling @code{specifier-instance}
on @code{current-display-table} and any overriding display tables
specified in currently active faces).  The first entry found that
matches the character determines how the character is displayed.  If
there is no matching entry, the default display method is
used. (Non-control characters are displayed as themselves and control
characters are displayed according to the buffer-local variable
@code{ctl-arrow}.  Control characters are further affected by
@code{control-arrow-glyph} and @code{octal-escape-glyph}.)

Each instantiator in this specifier and the display-table specifiers
in faces is a display table or a list of such tables.  If a list, each
table will be searched in turn for an entry matching a particular
character.  Each display table is one of

@itemize @bullet
@item
A vector, specifying values for characters starting at 0.
@item
A char table, either of type @code{char} or @code{generic}.
@item
A range table.
@end itemize

Each entry in a display table should be one of

@itemize @bullet
@item
nil (this entry is ignored and the search continues).
@item
A character (use this character; if it happens to be the same as
the original character, default processing happens, otherwise
redisplay attempts to display this character directly;
#### At some point recursive display-table lookup will be
implemented).
@item
A string (display each character in the string directly;
#### At some point recursive display-table lookup will be
implemented).
@item
A glyph (display the glyph;
#### At some point recursive display-table lookup will be
implemented when a string glyph is being processed).
@item
A cons of the form (format "@var{string}") where @var{string} is a
printf-like spec used to process the character. #### Unfortunately no
formatting directives other than %% are implemented.
@item
A vector (each element of the vector is processed recursively;
in such a case, nil elements in the vector are simply ignored).

#### At some point in the near future, display tables are likely to
be expanded to include other features, such as referencing characters
in particular fonts and allowing the character search to continue
all the way up the chain of specifier instantiators.  These features
are necessary to properly display Unicode characters.
@end itemize
@end defvar

  Individual faces can also specify an overriding display table;
this is set using @code{set-face-display-table}.  @xref{Faces}.

  If no display table can be determined for a particular window,
then XEmacs uses the usual display conventions.  @xref{Usual Display}.

@node Character Descriptors
@subsection Character Descriptors

@cindex character descriptor
  Each element of the display-table vector describes how to display
a particular character and is called a @dfn{character descriptor}.
A character descriptor can be:

@table @asis
@item a string
Display this particular string wherever the character is to be displayed.

@item a glyph
Display this particular glyph wherever the character is to be displayed.

@item a vector
The vector may contain strings and/or glyphs.  Display the elements of
the vector one after another wherever the character is to be displayed.

@item @code{nil}
Display according to the standard interpretation (@pxref{Usual Display}).
@end table

@ignore Not yet working in XEmacs?
@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 that displays the other
ISO characters as explanatory 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.
@end ignore

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

  You can make XEmacs ring a bell, play a sound, 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 sound device
@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}.  If @var{sound} is
specified, it should be a symbol specifying which sound to make.  This
sound will be played if @code{visible-bell} is @code{nil}. (This only
works if sound support was compiled into the executable and you are
running on the console of a Sun SparcStation, SGI, HP9000s700, or Linux
PC. Otherwise you just get a beep.) The optional third argument
specifies what device to make the sound on, and defaults to the selected
device.
@end defun

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

@defopt visible-bell
This variable determines whether XEmacs should flash the screen to
represent a bell.  Non-@code{nil} means yes, @code{nil} means no.  On
TTY devices, this is effective only if the Termcap entry for the
terminal type has the visible bell flag (@samp{vb}) set.
@end defopt

@defvar sound-alist
  This variable holds an alist associating names with sounds.  When
@code{beep} or @code{ding} is called with one of the name symbols, the
associated sound will be generated instead of the standard beep.

  Each element of @code{sound-alist} is a list describing a sound.  The
first element of the list is the name of the sound being defined.
Subsequent elements of the list are alternating keyword/value pairs:

@table @code
@item sound
A string of raw sound data, or the name of another sound to play.  The
symbol @code{t} here means use the default X beep.
@item volume
An integer from 0-100, defaulting to @code{bell-volume}.
@item pitch
If using the default X beep, the pitch (Hz) to generate.
@item duration
If using the default X beep, the duration (milliseconds).
@end table

For compatibility, elements of `sound-alist' may also be:

@itemize @bullet
@item
@code{( sound-name . <sound> )}
@item
@code{( sound-name <volume> <sound> )}
@end itemize

You should probably add things to this list by calling the function
@code{load-sound-file}.

Caveats:

@itemize @minus
@item
You can only play audio data if running on the console screen of a Sun
SparcStation, SGI, or HP9000s700.

@item
The pitch, duration, and volume options are available everywhere, but
many X servers ignore the `pitch' option.
@end itemize

The following beep-types are used by XEmacs itself:

@table @code
@item auto-save-error
when an auto-save does not succeed
@item command-error
when the XEmacs command loop catches an error
@item undefined-key
when you type a key that is undefined
@item undefined-click
when you use an undefined mouse-click combination
@item no-completion
during completing-read
@item y-or-n-p
when you type something other than 'y' or 'n'
@item yes-or-no-p
when you type something other than 'yes' or 'no'
@item default
used when nothing else is appropriate.
@end table

Other lisp packages may use other beep types, but these are the ones that
the C kernel of XEmacs uses.
@end defvar

@defopt bell-volume
This variable specifies the default volume for sounds, from 0 to 100.
@end defopt

@deffn Command load-default-sounds
This function loads and installs some sound files as beep-types.
@end deffn

@deffn Command load-sound-file filename sound-name &optional volume
This function reads in an audio file and adds it to @code{sound-alist}.
The sound file must be in the Sun/NeXT U-LAW format.  @var{sound-name}
should be a symbol, specifying the name of the sound.  If @var{volume}
is specified, the sound will be played at that volume; otherwise, the
value of @code{bell-volume} will be used.
@end deffn

@defun play-sound sound &optional volume device
This function plays sound @var{sound}, which should be a symbol
mentioned in @code{sound-alist}.  If @var{volume} is specified, it
overrides the value (if any) specified in @code{sound-alist}.
@var{device} specifies the device to play the sound on, and defaults
to the selected device.
@end defun

@deffn Command play-sound-file file &optional volume device
This function plays the named sound file at volume @var{volume}, which
defaults to @code{bell-volume}.  @var{device} specifies the device to
play the sound on, and defaults to the selected device.
@end deffn