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@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
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@c Copyright (C) 1990-1995, 1998-2012  Free Software Foundation, Inc.
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@c See the file elisp.texi for copying conditions.
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@setfilename ../../info/display
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@node Display, System Interface, Processes, Top
@chapter Emacs Display

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

@menu
* Refresh Screen::      Clearing the screen and redrawing everything on it.
* Forcing Redisplay::   Forcing redisplay.
* Truncation::          Folding or wrapping long text lines.
* The Echo Area::       Displaying messages at the bottom of the screen.
* Warnings::            Displaying warning messages for the user.
* Invisible Text::      Hiding part of the buffer text.
* Selective Display::   Hiding part of the buffer text (the old way).
* Temporary Displays::  Displays that go away automatically.
* Overlays::            Use overlays to highlight parts of the buffer.
* Width::               How wide a character or string is on the screen.
* Line Height::         Controlling the height of lines.
* Faces::               A face defines a graphics style for text characters:
                          font, colors, etc.
* Fringes::             Controlling window fringes.
* Scroll Bars::         Controlling vertical scroll bars.
* Display Property::    Enabling special display features.
* Images::              Displaying images in Emacs buffers.
* Buttons::             Adding clickable buttons to Emacs buffers.
* Abstract Display::    Emacs' Widget for Object Collections.
* Blinking::            How Emacs 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.
* Window Systems::      Which window system is being used.
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* Bidirectional Display:: Display of bidirectional scripts, such as
                             Arabic and Farsi.
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* Glyphless Chars::     How glyphless characters are drawn.
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@end menu

@node Refresh Screen
@section Refreshing the Screen

  The function @code{redraw-frame} clears and redisplays the entire
contents of a given frame (@pxref{Frames}).  This is useful if the
screen is corrupted.

@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

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  In Emacs, processing user input takes priority over redisplay.  If
you call these functions when input is available, they don't redisplay
immediately, but the requested redisplay does happen
eventually---after all the input has been processed.
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  On text-only terminals, suspending and resuming Emacs normally 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.
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@defopt no-redraw-on-reenter
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@cindex suspend (cf. @code{no-redraw-on-reenter})
@cindex resume (cf. @code{no-redraw-on-reenter})
This variable controls whether Emacs redraws the entire screen after it
has been suspended and resumed.  Non-@code{nil} means there is no need
to redraw, @code{nil} means redrawing is needed.  The default is @code{nil}.
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@end defopt
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@node Forcing Redisplay
@section Forcing Redisplay
@cindex forcing redisplay

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  Emacs normally tries to redisplay the screen whenever it waits for
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input.  With the following function, you can request an immediate
attempt to redisplay, in the middle of Lisp code, without actually
waiting for input.
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@defun redisplay &optional force
This function tries immediately to redisplay, provided there are no
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pending input events.
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If the optional argument @var{force} is non-@code{nil}, it does all
pending redisplay work even if input is available, with no
pre-emption.

The function returns @code{t} if it actually tried to redisplay, and
@code{nil} otherwise.  A value of @code{t} does not mean that
redisplay proceeded to completion; it could have been pre-empted by
newly arriving terminal input.
@end defun

  @code{redisplay} with no argument tries immediately to redisplay,
but has no effect on the usual rules for what parts of the screen to
redisplay.  By contrast, the following function adds certain windows
to the pending redisplay work (as if their contents had completely
changed), but doesn't immediately try to do any redisplay work.

@defun force-window-update &optional object
This function forces some or all windows to be updated on next
redisplay.  If @var{object} is a window, it requires eventual
redisplay of that window.  If @var{object} is a buffer or buffer name,
it requires eventual redisplay of all windows displaying that buffer.
If @var{object} is @code{nil} (or omitted), it requires eventual
redisplay of all windows.
@end defun

  @code{force-window-update} does not do a redisplay immediately.
(Emacs will do that when it waits for input.)  Rather, its effect is
to put more work on the queue to be done by redisplay whenever there
is a chance.

@defvar redisplay-dont-pause
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If this variable is non-@code{nil}, pending input does not prevent or
halt redisplay; redisplay occurs, and finishes, regardless of whether
input is available.  If it is @code{nil}, Emacs redisplay stops if
input arrives, and does not happen at all if input is available before
it starts.  The default is @code{t}.
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@end defvar

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@defvar redisplay-preemption-period
This variable specifies how many seconds Emacs waits between checks
for new input during redisplay.  (The default is 0.1 seconds.)  If
input has arrived when Emacs checks, it pre-empts redisplay and
processes the available input before trying again to redisplay.

If this variable is @code{nil}, Emacs does not check for input during
redisplay, and redisplay cannot be preempted by input.

This variable is only obeyed on graphical terminals.  For
text terminals, see @ref{Terminal Output}.
@end defvar

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

  When a line of text extends beyond the right edge of a window, Emacs
can @dfn{continue} the line (make it ``wrap'' to the next screen
line), or @dfn{truncate} the line (limit it to one screen line).  The
additional screen lines used to display a long text line are called
@dfn{continuation} lines.  Continuation is not the same as 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}.

   On a graphical display, tiny arrow images in the window fringes
indicate truncated and continued lines (@pxref{Fringes}).  On a text
terminal, a @samp{$} in the rightmost column of the window indicates
truncation; a @samp{\} on the rightmost column indicates a line that
``wraps.''  (The display table can specify alternate characters to use
for this; @pxref{Display Tables}).

@defopt truncate-lines
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If this buffer-local variable is non-@code{nil}, lines that extend
beyond the right edge of the window are truncated; otherwise, they are
continued.  As a special exception, the variable
@code{truncate-partial-width-windows} takes precedence in
@dfn{partial-width} windows (i.e., windows that do not occupy the
entire frame width).
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@end defopt

@defopt truncate-partial-width-windows
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This variable controls line truncation in @dfn{partial-width} windows.
A partial-width window is one that does not occupy the entire frame
width (@pxref{Splitting Windows}).  If the value is @code{nil}, line
truncation is determined by the variable @code{truncate-lines} (see
above).  If the value is an integer @var{n}, lines are truncated if
the partial-width window has fewer than @var{n} columns, regardless of
the value of @code{truncate-lines}; if the partial-width window has
@var{n} or more columns, line truncation is determined by
@code{truncate-lines}.  For any other non-@code{nil} value, lines are
truncated in every partial-width window, regardless of the value of
@code{truncate-lines}.
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@end defopt

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

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@defvar wrap-prefix
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If this buffer-local variable is non-@code{nil}, it defines a
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``prefix'' that is prepended to every continuation line at
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display time.  (If lines are truncated, the wrap-prefix is never
used.)  It may be a string or an image (@pxref{Other Display Specs}),
or a stretch of whitespace such as specified by the @code{:width} or
@code{:align-to} display properties (@pxref{Specified Space}).  The
value is interpreted in the same way as a @code{display} text
property.  @xref{Display Property}.
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A wrap-prefix may also be specified for regions of text, using the
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@code{wrap-prefix} text or overlay property.  This takes precedence
over the @code{wrap-prefix} variable.  @xref{Special Properties}.
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@end defvar

@defvar line-prefix
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If this buffer-local variable is non-@code{nil}, it defines a
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``prefix'' that is prepended to every non-continuation line at
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display time.  It may be a string or an image (@pxref{Other Display
Specs}), or a stretch of whitespace such as specified by the
@code{:width} or @code{:align-to} display properties (@pxref{Specified
Space}).  The value is interpreted in the same way as a @code{display}
text property.  @xref{Display Property}.
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A line-prefix may also be specified for regions of text using the
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@code{line-prefix} text or overlay property.  This takes precedence
over the @code{line-prefix} variable.  @xref{Special Properties}.
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@end defvar

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

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

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

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

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

  The @dfn{echo area} is used for displaying error messages
(@pxref{Errors}), for 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}).

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

@menu
* Displaying Messages:: Explicitly displaying text in the echo area.
* Progress::            Informing user about progress of a long operation.
* Logging Messages::    Echo area messages are logged for the user.
* Echo Area Customization:: Controlling the echo area.
@end menu

@node Displaying Messages
@subsection Displaying Messages in the Echo Area
@cindex display message in echo area

  This section describes the functions for explicitly producing echo
area messages.  Many other Emacs features display messages there, too.

@defun message format-string &rest arguments
This function displays a message in the echo area.  The argument
@var{format-string} is similar to a C language @code{printf} format
string.  See @code{format} in @ref{Formatting Strings}, 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.

If @var{format-string}, or strings among the @var{arguments}, have
@code{face} text properties, these affect the way the message is displayed.

@c Emacs 19 feature
If @var{format-string} is @code{nil} or the empty string,
@code{message} clears the echo area; if the echo area has been
expanded automatically, this brings it back to its normal size.
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

To automatically display a message in the echo area or in a pop-buffer,
depending on its size, use @code{display-message-or-buffer} (see below).
@end defun

@defmac with-temp-message message &rest body
This construct displays a message in the echo area temporarily, during
the execution of @var{body}.  It displays @var{message}, executes
@var{body}, then returns the value of the last body form while restoring
the previous echo area contents.
@end defmac

@defun message-or-box format-string &rest arguments
This function displays a message like @code{message}, but may display it
in a dialog box instead of the echo area.  If this function is called in
a command that was invoked using the mouse---more precisely, if
@code{last-nonmenu-event} (@pxref{Command Loop Info}) is either
@code{nil} or a list---then it uses a dialog box or pop-up menu to
display the message.  Otherwise, it uses the echo area.  (This is the
same criterion that @code{y-or-n-p} uses to make a similar decision; see
@ref{Yes-or-No Queries}.)

You can force use of the mouse or of the echo area by binding
@code{last-nonmenu-event} to a suitable value around the call.
@end defun

@defun message-box format-string &rest arguments
@anchor{message-box}
This function displays a message like @code{message}, but uses a dialog
box (or a pop-up menu) whenever that is possible.  If it is impossible
to use a dialog box or pop-up menu, because the terminal does not
support them, then @code{message-box} uses the echo area, like
@code{message}.
@end defun

@defun display-message-or-buffer message &optional buffer-name not-this-window frame
This function displays the message @var{message}, which may be either a
string or a buffer.  If it is shorter than the maximum height of the
echo area, as defined by @code{max-mini-window-height}, it is displayed
in the echo area, using @code{message}.  Otherwise,
@code{display-buffer} is used to show it in a pop-up buffer.

Returns either the string shown in the echo area, or when a pop-up
buffer is used, the window used to display it.

If @var{message} is a string, then the optional argument
@var{buffer-name} is the name of the buffer used to display it when a
pop-up buffer is used, defaulting to @samp{*Message*}.  In the case
where @var{message} is a string and displayed in the echo area, it is
not specified whether the contents are inserted into the buffer anyway.

The optional arguments @var{not-this-window} and @var{frame} are as for
@code{display-buffer}, and only used if a buffer is displayed.
@end defun

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

@node Progress
@subsection Reporting Operation Progress
@cindex progress reporting

  When an operation can take a while to finish, you should inform the
user about the progress it makes.  This way the user can estimate
remaining time and clearly see that Emacs is busy working, not hung.
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A convenient way to do this is to use a @dfn{progress reporter}.
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  Here is a working example that does nothing useful:
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@smallexample
(let ((progress-reporter
       (make-progress-reporter "Collecting mana for Emacs..."
                               0  500)))
  (dotimes (k 500)
    (sit-for 0.01)
    (progress-reporter-update progress-reporter k))
  (progress-reporter-done progress-reporter))
@end smallexample

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@defun make-progress-reporter message &optional min-value max-value current-value min-change min-time
This function creates and returns a progress reporter object, which
you will use as an argument for the other functions listed below.  The
idea is to precompute as much data as possible to make progress
reporting very fast.
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When this progress reporter is subsequently used, it will display
@var{message} in the echo area, followed by progress percentage.
@var{message} is treated as a simple string.  If you need it to depend
on a filename, for instance, use @code{format} before calling this
function.

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The arguments @var{min-value} and @var{max-value} should be numbers
standing for the starting and final states of the operation.  For
instance, an operation that ``scans'' a buffer should set these to the
results of @code{point-min} and @code{point-max} correspondingly.
@var{max-value} should be greater than @var{min-value}.

Alternatively, you can set @var{min-value} and @var{max-value} to
@code{nil}.  In that case, the progress reporter does not report
process percentages; it instead displays a ``spinner'' that rotates a
notch each time you update the progress reporter.

If @var{min-value} and @var{max-value} are numbers, you can give the
argument @var{current-value} a numerical value specifying the initial
progress; if omitted, this defaults to @var{min-value}.

The remaining arguments control the rate of echo area updates.  The
progress reporter will wait for at least @var{min-change} more
percents of the operation to be completed before printing next
message; the default is one percent.  @var{min-time} specifies the
minimum time in seconds to pass between successive prints; the default
is 0.2 seconds.  (On some operating systems, the progress reporter may
handle fractions of seconds with varying precision).
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This function calls @code{progress-reporter-update}, so the first
message is printed immediately.
@end defun

@defun progress-reporter-update reporter value
This function does the main work of reporting progress of your
operation.  It displays the message of @var{reporter}, followed by
progress percentage determined by @var{value}.  If percentage is zero,
or close enough according to the @var{min-change} and @var{min-time}
arguments, then it is omitted from the output.

@var{reporter} must be the result of a call to
@code{make-progress-reporter}.  @var{value} specifies the current
state of your operation and must be between @var{min-value} and
@var{max-value} (inclusive) as passed to
@code{make-progress-reporter}.  For instance, if you scan a buffer,
then @var{value} should be the result of a call to @code{point}.

This function respects @var{min-change} and @var{min-time} as passed
to @code{make-progress-reporter} and so does not output new messages
on every invocation.  It is thus very fast and normally you should not
try to reduce the number of calls to it: resulting overhead will most
likely negate your effort.
@end defun

@defun progress-reporter-force-update reporter value &optional new-message
This function is similar to @code{progress-reporter-update} except
that it prints a message in the echo area unconditionally.

The first two arguments have the same meaning as for
@code{progress-reporter-update}.  Optional @var{new-message} allows
you to change the message of the @var{reporter}.  Since this functions
always updates the echo area, such a change will be immediately
presented to the user.
@end defun

@defun progress-reporter-done reporter
This function should be called when the operation is finished.  It
prints the message of @var{reporter} followed by word ``done'' in the
echo area.

You should always call this function and not hope for
@code{progress-reporter-update} to print ``100%.''  Firstly, it may
never print it, there are many good reasons for this not to happen.
Secondly, ``done'' is more explicit.
@end defun

@defmac dotimes-with-progress-reporter (var count [result]) message body@dots{}
This is a convenience macro that works the same way as @code{dotimes}
does, but also reports loop progress using the functions described
above.  It allows you to save some typing.

You can rewrite the example in the beginning of this node using
this macro this way:

@example
(dotimes-with-progress-reporter
    (k 500)
    "Collecting some mana for Emacs..."
  (sit-for 0.01))
@end example
@end defmac

@node Logging Messages
@subsection Logging Messages in @samp{*Messages*}
@cindex logging echo-area messages

  Almost all the messages displayed in the echo area are also recorded
in the @samp{*Messages*} buffer so that the user can refer back to
them.  This includes all the messages that are output with
@code{message}.

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

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

  To make @samp{*Messages*} more convenient for the user, the logging
facility combines successive identical messages.  It also combines
successive related messages for the sake of two cases: question
followed by answer, and a series of progress messages.

  A ``question followed by an answer'' means two messages like the
ones produced by @code{y-or-n-p}: the first is @samp{@var{question}},
and the second is @samp{@var{question}...@var{answer}}.  The first
message conveys no additional information beyond what's in the second,
so logging the second message discards the first from the log.

  A ``series of progress messages'' means successive messages like
those produced by @code{make-progress-reporter}.  They have the form
@samp{@var{base}...@var{how-far}}, where @var{base} is the same each
time, while @var{how-far} varies.  Logging each message in the series
discards the previous one, provided they are consecutive.

  The functions @code{make-progress-reporter} and @code{y-or-n-p}
don't have to do anything special to activate the message log
combination feature.  It operates whenever two consecutive messages
are logged that share a common prefix ending in @samp{...}.

@node Echo Area Customization
@subsection Echo Area Customization

  These variables control details of how the echo area works.

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

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

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

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@defopt echo-keystrokes
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This variable determines how much time should elapse before command
characters echo.  Its value must be an integer or floating point 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.  (Once echoing
begins in a key sequence, all subsequent characters in the same key
sequence are echoed immediately.)

If the value is zero, then command input is not echoed.
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@end defopt
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@defvar message-truncate-lines
Normally, displaying a long message resizes the echo area to display
the entire message.  But if the variable @code{message-truncate-lines}
is non-@code{nil}, the echo area does not resize, and the message is
truncated to fit it, as in Emacs 20 and before.
@end defvar

  The variable @code{max-mini-window-height}, which specifies the
maximum height for resizing minibuffer windows, also applies to the
echo area (which is really a special use of the minibuffer window.
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@xref{Minibuffer Misc}.).
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@node Warnings
@section Reporting Warnings
@cindex warnings

  @dfn{Warnings} are a facility for a program to inform the user of a
possible problem, but continue running.

@menu
* Warning Basics::      Warnings concepts and functions to report them.
* Warning Variables::   Variables programs bind to customize their warnings.
* Warning Options::     Variables users set to control display of warnings.
@end menu

@node Warning Basics
@subsection Warning Basics
@cindex severity level

  Every warning has a textual message, which explains the problem for
the user, and a @dfn{severity level} which is a symbol.  Here are the
possible severity levels, in order of decreasing severity, and their
meanings:

@table @code
@item :emergency
A problem that will seriously impair Emacs operation soon
if you do not attend to it promptly.
@item :error
A report of data or circumstances that are inherently wrong.
@item :warning
A report of data or circumstances that are not inherently wrong, but
raise suspicion of a possible problem.
@item :debug
A report of information that may be useful if you are debugging.
@end table

  When your program encounters invalid input data, it can either
signal a Lisp error by calling @code{error} or @code{signal} or report
a warning with severity @code{:error}.  Signaling a Lisp error is the
easiest thing to do, but it means the program cannot continue
processing.  If you want to take the trouble to implement a way to
continue processing despite the bad data, then reporting a warning of
severity @code{:error} is the right way to inform the user of the
problem.  For instance, the Emacs Lisp byte compiler can report an
error that way and continue compiling other functions.  (If the
program signals a Lisp error and then handles it with
@code{condition-case}, the user won't see the error message; it could
show the message to the user by reporting it as a warning.)

@cindex warning type
  Each warning has a @dfn{warning type} to classify it.  The type is a
list of symbols.  The first symbol should be the custom group that you
use for the program's user options.  For example, byte compiler
warnings use the warning type @code{(bytecomp)}.  You can also
subcategorize the warnings, if you wish, by using more symbols in the
list.

@defun display-warning type message &optional level buffer-name
This function reports a warning, using @var{message} as the message
and @var{type} as the warning type.  @var{level} should be the
severity level, with @code{:warning} being the default.

@var{buffer-name}, if non-@code{nil}, specifies the name of the buffer
for logging the warning.  By default, it is @samp{*Warnings*}.
@end defun

@defun lwarn type level message &rest args
This function reports a warning using the value of @code{(format
@var{message} @var{args}...)} as the message.  In other respects it is
equivalent to @code{display-warning}.
@end defun

@defun warn message &rest args
This function reports a warning using the value of @code{(format
@var{message} @var{args}...)} as the message, @code{(emacs)} as the
type, and @code{:warning} as the severity level.  It exists for
compatibility only; we recommend not using it, because you should
specify a specific warning type.
@end defun

@node Warning Variables
@subsection Warning Variables

  Programs can customize how their warnings appear by binding
the variables described in this section.

@defvar warning-levels
This list defines the meaning and severity order of the warning
severity levels.  Each element defines one severity level,
and they are arranged in order of decreasing severity.

Each element has the form @code{(@var{level} @var{string}
@var{function})}, where @var{level} is the severity level it defines.
@var{string} specifies the textual description of this level.
@var{string} should use @samp{%s} to specify where to put the warning
type information, or it can omit the @samp{%s} so as not to include
that information.

The optional @var{function}, if non-@code{nil}, is a function to call
with no arguments, to get the user's attention.

Normally you should not change the value of this variable.
@end defvar

@defvar warning-prefix-function
If non-@code{nil}, the value is a function to generate prefix text for
warnings.  Programs can bind the variable to a suitable function.
@code{display-warning} calls this function with the warnings buffer
current, and the function can insert text in it.  That text becomes
the beginning of the warning message.

The function is called with two arguments, the severity level and its
entry in @code{warning-levels}.  It should return a list to use as the
entry (this value need not be an actual member of
@code{warning-levels}).  By constructing this value, the function can
change the severity of the warning, or specify different handling for
a given severity level.

If the variable's value is @code{nil} then there is no function
to call.
@end defvar

@defvar warning-series
Programs can bind this variable to @code{t} to say that the next
warning should begin a series.  When several warnings form a series,
that means to leave point on the first warning of the series, rather
than keep moving it for each warning so that it appears on the last one.
The series ends when the local binding is unbound and
@code{warning-series} becomes @code{nil} again.

The value can also be a symbol with a function definition.  That is
equivalent to @code{t}, except that the next warning will also call
the function with no arguments with the warnings buffer current.  The
function can insert text which will serve as a header for the series
of warnings.

Once a series has begun, the value is a marker which points to the
buffer position in the warnings buffer of the start of the series.

The variable's normal value is @code{nil}, which means to handle
each warning separately.
@end defvar

@defvar warning-fill-prefix
When this variable is non-@code{nil}, it specifies a fill prefix to
use for filling each warning's text.
@end defvar

@defvar warning-type-format
This variable specifies the format for displaying the warning type
in the warning message.  The result of formatting the type this way
gets included in the message under the control of the string in the
entry in @code{warning-levels}.  The default value is @code{" (%s)"}.
If you bind it to @code{""} then the warning type won't appear at
all.
@end defvar

@node Warning Options
@subsection Warning Options

  These variables are used by users to control what happens
when a Lisp program reports a warning.

@defopt warning-minimum-level
This user option specifies the minimum severity level that should be
shown immediately to the user.  The default is @code{:warning}, which
means to immediately display all warnings except @code{:debug}
warnings.
@end defopt

@defopt warning-minimum-log-level
This user option specifies the minimum severity level that should be
logged in the warnings buffer.  The default is @code{:warning}, which
means to log all warnings except @code{:debug} warnings.
@end defopt

@defopt warning-suppress-types
This list specifies which warning types should not be displayed
immediately for the user.  Each element of the list should be a list
of symbols.  If its elements match the first elements in a warning
type, then that warning is not displayed immediately.
@end defopt

@defopt warning-suppress-log-types
This list specifies which warning types should not be logged in the
warnings buffer.  Each element of the list should be a list of
symbols.  If it matches the first few elements in a warning type, then
that warning is not logged.
@end defopt

@node Invisible Text
@section Invisible Text

@cindex invisible text
You can make characters @dfn{invisible}, so that they do not appear on
the screen, with the @code{invisible} property.  This can be either a
text property (@pxref{Text Properties}) or a property of an overlay
(@pxref{Overlays}).  Cursor motion also partly ignores these
characters; if the command loop finds point within them, it moves
point to the other side of them.

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

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
database.  It permits the implementation of convenient filtering
commands to view just a part of the entries in the database.  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.  Setting this variable makes it
buffer-local.

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

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

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

@item (@var{atom} . t)
A character is invisible if its @code{invisible} property value is
@var{atom} or if it is a list with @var{atom} as a member.  Moreover,
a sequence of such characters displays as an ellipsis.
@end table
@end table
@end defvar

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

@defun add-to-invisibility-spec element
This function adds the element @var{element} to
@code{buffer-invisibility-spec}.  If @code{buffer-invisibility-spec}
was @code{t}, it changes to a list, @code{(t)}, so that text whose
@code{invisible} property is @code{t} remains invisible.
@end defun

@defun remove-from-invisibility-spec element
This removes the element @var{element} from
@code{buffer-invisibility-spec}.  This does nothing if @var{element}
is not in the list.
@end defun

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

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

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

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

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  You can check for invisibility using the following function:

@defun invisible-p pos-or-prop
If @var{pos-or-prop} is a marker or number, this function returns a
non-@code{nil} value if the text at that position is invisible.

If @var{pos-or-prop} is any other kind of Lisp object, that is taken
to mean a possible value of the @code{invisible} text or overlay
property.  In that case, this function returns a non-@code{nil} value
if that value would cause text to become invisible, based on the
current value of @code{buffer-invisibility-spec}.
@end defun

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@vindex line-move-ignore-invisible
  Ordinarily, functions that operate on text or move point do not care
whether the text is invisible.  The user-level line motion commands
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ignore invisible newlines if @code{line-move-ignore-invisible} is
non-@code{nil} (the default), but only because they are explicitly
programmed to do so.
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  However, if a command ends with point inside or at the boundary of invisible
text, the main editing loop moves point to one of the two ends of the invisible
text.  Which end to move to is chosen based on the following factors: make sure
that the overall movement of the command is still in the same direction, and
prefer a position where an inserted char would not inherit the @code{invisible}
property.  Additionally, if the text is not replaced by an ellipsis and the
command only moved within the invisible text, then point is moved one extra
character so as to try and reflect the command's movement by a visible movement
of the cursor.

  Thus, if the command moved point back to an invisible range (with the usual
stickiness), Emacs moves point back to the beginning of that range.  If the
command moved point forward into an invisible range, Emacs moves point forward
to the first visible character that follows the invisible text and then forward
one more character.
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  Incremental search can make invisible overlays visible temporarily
and/or permanently when a match includes invisible text.  To enable
this, the overlay should have a non-@code{nil}
@code{isearch-open-invisible} property.  The property value should be a
function to be called with the overlay as an argument.  This function
should make the overlay visible permanently; it is used when the match
overlaps the overlay on exit from the search.

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

@node Selective Display
@section Selective Display
@c @cindex selective display   Duplicates selective-display

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

  The first variant, explicit selective display, is designed for use
in a Lisp program: it controls which lines are hidden by altering the
text.  This kind of hiding in some ways resembles the effect of the
@code{invisible} property (@pxref{Invisible Text}), but the two
features are different and do not work the same way.

  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 hidden.  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 hidden text.  However, the replacement of newline characters with
carriage return characters affects some editing commands.  For
example, @code{next-line} skips hidden lines, since it searches only
for newlines.  Modes that use selective display can also define
commands that take account of the newlines, or that control which
parts of the text are hidden.

  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 hidden.  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 hidden.

@itemize @bullet
@item
If the value of @code{selective-display} is @code{t}, then the character
control-m marks the start of hidden text; the control-m, and the rest
of the line following it, are not displayed.  This is explicit selective
display.

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

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

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@defopt selective-display-ellipses
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If this buffer-local variable is non-@code{nil}, then Emacs displays
@samp{@dots{}} at the end of a line that is followed by hidden 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}.
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@end defopt
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@node Temporary Displays
@section Temporary Displays

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

@defspec with-output-to-temp-buffer buffer-name forms@dots{}
This function executes @var{forms} while arranging to insert any output
they print into the buffer named @var{buffer-name}, which is first
created if necessary, and put into Help mode.  Finally, the buffer is
displayed in some window, but not selected.

If the @var{forms} do not change the major mode in the output buffer,
so that it is still Help mode at the end of their execution, then
@code{with-output-to-temp-buffer} makes this buffer read-only at the
end, and also scans it for function and variable names to make them
into clickable cross-references.  @xref{Docstring hyperlinks, , Tips
for Documentation Strings}, in particular the item on hyperlinks in
documentation strings, for more details.

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}.

Several hooks are available for customizing the behavior
of this construct; they are listed below.

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

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

It is a good idea for this function to run @code{temp-buffer-show-hook}
just as @code{with-output-to-temp-buffer} normally would, inside of
@code{save-selected-window} and with the chosen window and buffer
selected.
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@end defopt
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@defvar temp-buffer-setup-hook
This normal hook is run by @code{with-output-to-temp-buffer} before
evaluating @var{body}.  When the hook runs, the temporary buffer is
current.  This hook is normally set up with a function to put the
buffer in Help mode.
@end defvar

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

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

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

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

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

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

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

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

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

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

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

@node Overlays
@section Overlays
@cindex overlays

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

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@cindex scalability of overlays
The visual effect of an overlay is the same as of the corresponding
text property (@pxref{Text Properties}).  However, due to a different
implementation, overlays generally don't scale well (many operations
take a time that is proportional to the number of overlays in the
buffer).  If you need to affect the visual appearance of many portions
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in the buffer, we recommend using text properties.
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An overlay uses markers to record its beginning and end; thus,
editing the text of the buffer adjusts the beginning and end of each
overlay so that it stays with the text.  When you create the overlay,
you can specify whether text inserted at the beginning should be
inside the overlay or outside, and likewise for the end of the overlay.

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

@node Managing Overlays
@subsection Managing Overlays

  This section describes the functions to create, delete and move
overlays, and to examine their contents.  Overlay changes are not
recorded in the buffer's undo list, since the overlays are not
part of the buffer's contents.

@defun overlayp object
This function returns @code{t} if @var{object} is an overlay.
@end defun

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

The arguments @var{front-advance} and @var{rear-advance} specify the
marker insertion type for the start of the overlay and for the end of
the overlay, respectively.  @xref{Marker Insertion Types}.  If they
are both @code{nil}, the default, then the overlay extends to include
any text inserted at the beginning, but not text inserted at the end.
If @var{front-advance} is non-@code{nil}, text inserted at the
beginning of the overlay is excluded from the overlay.  If
@var{rear-advance} is non-@code{nil}, text inserted at the end of the
overlay is included in the overlay.
@end defun

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

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

@defun overlay-buffer overlay
This function returns the buffer that @var{overlay} belongs to.  It
returns @code{nil} if @var{overlay} has been deleted.
@end defun

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

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

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

If @var{buffer} is omitted, @var{overlay} stays in the same buffer it
was already associated with; if @var{overlay} was deleted, it goes into
the current 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 remove-overlays &optional start end name value
This function removes all the overlays between @var{start} and
@var{end} whose property @var{name} has the value @var{value}.  It can
move the endpoints of the overlays in the region, or split them.

If @var{name} is omitted or @code{nil}, it means to delete all overlays in
the specified region.  If @var{start} and/or @var{end} are omitted or
@code{nil}, that means the beginning and end of the buffer respectively.
Therefore, @code{(remove-overlays)} removes all the overlays in the
current buffer.
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@end defun

@defun copy-overlay overlay
This function returns a copy of @var{overlay}.  The copy has the same
endpoints and properties as @var{overlay}.  However, the marker
insertion type for the start of the overlay and for the end of the
overlay are set to their default values (@pxref{Marker Insertion
Types}).
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@end defun

  Here are some examples:

@example
;; @r{Create an overlay.}
(setq foo (make-overlay 1 10))
     @result{} #<overlay from 1 to 10 in display.texi>
(overlay-start foo)
     @result{} 1
(overlay-end foo)
     @result{} 10
(overlay-buffer foo)
     @result{} #<buffer display.texi>
;; @r{Give it a property we can check later.}
(overlay-put foo 'happy t)
     @result{} t
;; @r{Verify the property is present.}
(overlay-get foo 'happy)
     @result{} t
;; @r{Move the overlay.}
(move-overlay foo 5 20)
     @result{} #<overlay from 5 to 20 in display.texi>
(overlay-start foo)
     @result{} 5
(overlay-end foo)
     @result{} 20
;; @r{Delete the overlay.}
(delete-overlay foo)
     @result{} nil
;; @r{Verify it is deleted.}
foo
     @result{} #<overlay in no buffer>
;; @r{A deleted overlay has no position.}
(overlay-start foo)
     @result{} nil
(overlay-end foo)
     @result{} nil
(overlay-buffer foo)
     @result{} nil
;; @r{Undelete the overlay.}
(move-overlay foo 1 20)
     @result{} #<overlay from 1 to 20 in display.texi>
;; @r{Verify the results.}
(overlay-start foo)
     @result{} 1
(overlay-end foo)
     @result{} 20
(overlay-buffer foo)
     @result{} #<buffer display.texi>
;; @r{Moving and deleting the overlay does not change its properties.}
(overlay-get foo 'happy)
     @result{} t
@end example

  Emacs stores the overlays of each buffer in two lists, divided
around an arbitrary ``center position.''  One list extends backwards
through the buffer from that center position, and the other extends
forwards from that center position.  The center position can be anywhere
in the buffer.

@defun overlay-recenter pos
This function recenters the overlays of the current buffer around
position @var{pos}.  That makes overlay lookup faster for positions
near @var{pos}, but slower for positions far away from @var{pos}.
@end defun

  A loop that scans the buffer forwards, creating overlays, can run
faster if you do @code{(overlay-recenter (point-max))} first.

@node Overlay Properties
@subsection Overlay Properties

  Overlay properties are like text properties in that the properties that
alter how a character is displayed can come from either source.  But in
most respects they are different.  @xref{Text Properties}, for comparison.

  Text properties are considered a part of the text; overlays and
their properties are specifically considered not to be part of the
text.  Thus, copying text between various buffers and strings
preserves text properties, but does not try to preserve overlays.
Changing a buffer's text properties marks the buffer as modified,
while moving an overlay or changing its properties does not.  Unlike
text property changes, overlay property changes are not recorded in
the buffer's undo list.

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  Since more than one overlay can specify a property value for the
same character, Emacs lets you specify a priority value of each
overlay.  You should not make assumptions about which overlay will
prevail when there is a conflict and they have the same priority.

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  These functions read and set the properties of an overlay:

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

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

@defun overlay-properties overlay
This returns a copy of the property list of @var{overlay}.
@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}.

  Many overlay properties have special meanings; here is a table
of them:

@table @code
@item priority
@kindex priority @r{(overlay property)}
This property's value (which should be a nonnegative integer number)
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determines the priority of the overlay.  No priority, or @code{nil},
means zero.

The priority matters when two or more overlays cover the same
character and both specify the same property; the one whose
@code{priority} value is larger overrides the other.  For the
@code{face} property, the higher priority overlay's value does not
completely override the other value; instead, its face attributes
override the face attributes of the lower priority @code{face}
property.
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Currently, all overlays take priority over text properties.  Please
avoid using negative priority values, as we have not yet decided just
what they should mean.

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

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

@item face
@kindex face @r{(overlay property)}
This property controls the way text is displayed---for example, which
font and which colors.  @xref{Faces}, for more information.

In the simplest case, the value is a face name.  It can also be a list;
then each element can be any of these possibilities:

@itemize @bullet
@item
A face name (a symbol or string).

@item
A property list of face attributes.  This has the form (@var{keyword}
@var{value} @dots{}), where each @var{keyword} is a face attribute
name and @var{value} is a meaningful value for that attribute.  With
this feature, you do not need to create a face each time you want to
specify a particular attribute for certain text.  @xref{Face
Attributes}.

@item
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A cons cell, of the form @code{(foreground-color . @var{color-name})}
or @code{(background-color . @var{color-name})}.  These elements
specify just the foreground color or just the background color.
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@code{(foreground-color . @var{color-name})} has the same effect as
@code{(:foreground @var{color-name})}; likewise for the background.
@end itemize

@item mouse-face
@kindex mouse-face @r{(overlay property)}
This property is used instead of @code{face} when the mouse is within
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the range of the overlay.  However, Emacs ignores all face attributes
from this property that alter the text size (e.g.  @code{:height},
@code{:weight}, and @code{:slant}).  Those attributes are always the
same as in the unhighlighted text.
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@item display
@kindex display @r{(overlay property)}
This property activates various features that change the
way text is displayed.  For example, it can make text appear taller
or shorter, higher or lower, wider or narrower, or replaced with an image.
@xref{Display Property}.

@item help-echo
@kindex help-echo @r{(overlay property)}
If an overlay has a @code{help-echo} property, then when you move the
mouse onto the text in the overlay, Emacs displays a help string in the
echo area, or in the tooltip window.  For details see @ref{Text
help-echo}.

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

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

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

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

If these functions modify the buffer, they should bind
@code{inhibit-modification-hooks} to @code{t} around doing so, to
avoid confusing the internal mechanism that calls these hooks.

Text properties also support the @code{modification-hooks} property,
but the details are somewhat different (@pxref{Special Properties}).

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

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

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

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

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

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

@item before-string
@kindex before-string @r{(overlay property)}
This property's value is a string to add to the display at the beginning
of the overlay.  The string does not appear in the buffer in any
sense---only on the screen.

@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.

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@item line-prefix
This property specifies a display spec to prepend to each
non-continuation line at display-time.  @xref{Truncation}.

@itemx wrap-prefix
This property specifies a display spec to prepend to each continuation
line at display-time.  @xref{Truncation}.

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@item evaporate
@kindex evaporate @r{(overlay property)}
If this property is non-@code{nil}, the overlay is deleted automatically
if it becomes empty (i.e., if its length becomes zero).  If you give
an empty overlay a non-@code{nil} @code{evaporate} property, that deletes
it immediately.

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

@item keymap
@kindex keymap @r{(overlay property)}
The @code{keymap} property is similar to @code{local-map} but overrides the
buffer's local map (and the map specified by the @code{local-map}
property) rather than replacing it.
@end table

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The @code{local-map} and @code{keymap} properties do not affect a
string displayed by the @code{before-string}, @code{after-string}, or
@code{display} properties.  This is only relevant for mouse clicks and
other mouse events that fall on the string, since point is never on
the string.  To bind special mouse events for the string, assign it a
@code{local-map} or @code{keymap} text property.  @xref{Special
Properties}.

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@node Finding Overlays
@subsection Searching for Overlays

@defun overlays-at pos
This function returns a list of all the overlays that cover the
character at 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}.

To illustrate usage, here is a Lisp function that returns a list of the
overlays that specify property @var{prop} for the character at point:

@smallexample
(defun find-overlays-specifying (prop)
  (let ((overlays (overlays-at (point)))
        found)
    (while overlays
      (let ((overlay (car overlays)))
        (if (overlay-get overlay prop)
            (setq found (cons overlay found))))
      (setq overlays (cdr overlays)))
    found))
@end smallexample
@end defun

@defun overlays-in beg end
This function returns a list of the overlays that overlap the region
@var{beg} through @var{end}.  ``Overlap'' means that at least one
character is contained within the overlay and also contained within the
specified region; however, empty overlays are included in the result if
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they are located at @var{beg}, strictly between @var{beg} and @var{end},
or at @var{end} when @var{end} denotes the position at the end of the
buffer.
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@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}.  If there is none, it returns
@code{(point-max)}.
@end defun

@defun previous-overlay-change pos
This function returns the buffer position of the previous beginning or
end of an overlay, before @var{pos}.  If there is none, it returns
@code{(point-min)}.
@end defun

  As an example, here's a simplified (and inefficient) version of the
primitive function @code{next-single-char-property-change}
(@pxref{Property Search}).  It searches forward from position
@var{pos} for the next position where the value of a given property
@code{prop}, as obtained from either overlays or text properties,
changes.

@smallexample
(defun next-single-char-property-change (position prop)
  (save-excursion
    (goto-char position)
    (let ((propval (get-char-property (point) prop)))
      (while (and (not (eobp))
                  (eq (get-char-property (point) prop) propval))
        (goto-char (min (next-overlay-change (point))
                        (next-single-property-change (point) prop)))))
    (point)))
@end smallexample

@node Width
@section Width

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

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

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

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

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

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

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

If @var{ellipsis} is non-@code{nil}, it should be a string which will
replace the end of @var{str} (including any padding) if it extends
beyond @var{end-column}, unless the display width of @var{str} is
equal to or less than the display width of @var{ellipsis}.  If
@var{ellipsis} is non-@code{nil} and not a string, it stands for
@code{"..."}.

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

@node Line Height
@section Line Height
@cindex line height

  The total height of each display line consists of the height of the
contents of the line, plus optional additional vertical line spacing
above or below the display line.

  The height of the line contents is the maximum height of any
character or image on that display line, including the final newline
if there is one.  (A display line that is continued doesn't include a
final newline.)  That is the default line height, if you do nothing to
specify a greater height.  (In the most common case, this equals the
height of the default frame font.)

  There are several ways to explicitly specify a larger line height,
either by specifying an absolute height for the display line, or by
specifying vertical space.  However, no matter what you specify, the
actual line height can never be less than the default.

@kindex line-height @r{(text property)}
  A newline can have a @code{line-height} text or overlay property
that controls the total height of the display line ending in that
newline.

  If the property value is @code{t}, the newline character has no
effect on the displayed height of the line---the visible contents
alone determine the height.  This is useful for tiling small images
(or image slices) without adding blank areas between the images.

  If the property value is a list of the form @code{(@var{height}
@var{total})}, that adds extra space @emph{below} the display line.
First Emacs uses @var{height} as a height spec to control extra space
@emph{above} the line; then it adds enough space @emph{below} the line
to bring the total line height up to @var{total}.  In this case, the
other ways to specify the line spacing are ignored.

  Any other kind of property value is a height spec, which translates
into a number---the specified line height.  There are several ways to
write a height spec; here's how each of them translates into a number:

@table @code
@item @var{integer}
If the height spec is a positive integer, the height value is that integer.
@item @var{float}
If the height spec is a float, @var{float}, the numeric height value
is @var{float} times the frame's default line height.
@item (@var{face} . @var{ratio})
If the height spec is a cons of the format shown, the numeric height
is @var{ratio} times the height of face @var{face}.  @var{ratio} can
be any type of number, or @code{nil} which means a ratio of 1.
If @var{face} is @code{t}, it refers to the current face.
@item (nil . @var{ratio})
If the height spec is a cons of the format shown, the numeric height
is @var{ratio} times the height of the contents of the line.
@end table

  Thus, any valid height spec determines the height in pixels, one way
or another.  If the line contents' height is less than that, Emacs
adds extra vertical space above the line to achieve the specified
total height.

  If you don't specify the @code{line-height} property, the line's
height consists of the contents' height plus the line spacing.
There are several ways to specify the line spacing for different
parts of Emacs text.

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  On graphical terminals, you can specify the line spacing for all
lines in a frame, using the @code{line-spacing} frame parameter
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(@pxref{Layout Parameters}).  However, if the default value of
@code{line-spacing} is non-@code{nil}, it overrides the
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frame's @code{line-spacing} parameter.  An integer value specifies the
number of pixels put below lines.  A floating point number specifies
the spacing relative to the frame's default line height.
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@vindex line-spacing
  You can specify the line spacing for all lines in a buffer via the
buffer-local @code{line-spacing} variable.  An integer value specifies
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the number of pixels put below lines.  A floating point number
specifies the spacing relative to the default frame line height.  This
overrides line spacings specified for the frame.
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@kindex line-spacing @r{(text property)}
  Finally, a newline can have a @code{line-spacing} text or overlay
property that overrides the default frame line spacing and the buffer
local @code{line-spacing} variable, for the display line ending in
that newline.

  One way or another, these mechanisms specify a Lisp value for the
spacing of each line.  The value is a height spec, and it translates
into a Lisp value as described above.  However, in this case the
numeric height value specifies the line spacing, rather than the line
height.

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  On text-only terminals, the line spacing cannot be altered.

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@node Faces
@section Faces
@cindex faces

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  A @dfn{face} is a collection of graphical attributes for displaying
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text: font, foreground color, background color, optional underlining,
and so on.  Faces control how buffer text is displayed, and how some
parts of the frame, such as the mode-line, are displayed.
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@xref{Standard Faces,,, emacs, The GNU Emacs Manual}, for the list of
faces Emacs normally comes with.
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@cindex face id
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  For most purposes, you refer to a face in Lisp programs using its
@dfn{face name}.  This is either a string or (equivalently) a Lisp
symbol whose name is equal to that string.
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@defun facep object
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This function returns a non-@code{nil} value if @var{object} is a Lisp
symbol or string that names a face.  Otherwise, it returns @code{nil}.
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@end defun

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  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.
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@menu
* Defining Faces::      How to define a face with @code{defface}.
* Face Attributes::     What is in a face?
* Attribute Functions::  Functions to examine and set face attributes.
* Displaying Faces::     How Emacs combines the faces specified for a character.
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* Face Remapping::      Remapping faces to alternative definitions.
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* Face Functions::      How to define and examine faces.
* Auto Faces::          Hook for automatic face assignment.
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* Basic Faces::         Faces that are defined by default.
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* Font Selection::      Finding the best available font for a face.
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* Font Lookup::         Looking up the names of available fonts
                          and information about them.
* Fontsets::            A fontset is a collection of fonts
                          that handle a range of character sets.
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* Low-Level Font::      Lisp representation for character display fonts.
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@end menu

@node Defining Faces
@subsection Defining Faces

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

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  People are sometimes tempted to create variables whose values specify
which faces to use (for example, Font-Lock does this).  In the vast
majority of cases, this is not necessary, and simply using faces
directly is preferable.

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@defmac defface face spec doc [keyword value]@dots{}
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This declares @var{face} as a customizable face whose default
attributes are given by @var{spec}.  You should not quote the symbol
@var{face}, and it should not end in @samp{-face} (that would be
redundant).  The argument @var{doc} specifies the face documentation.
The keywords you can use in @code{defface} are the same as in
@code{defgroup} and @code{defcustom} (@pxref{Common Keywords}).
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When @code{defface} executes, it defines the face according to
@var{spec}, then uses any customizations that were read from the
init file (@pxref{Init File}) to override that specification.

When you evaluate a @code{defface} form with @kbd{C-M-x} in Emacs
Lisp mode (@code{eval-defun}), a special feature of @code{eval-defun}
overrides any customizations of the face.  This way, the face reflects
exactly what the @code{defface} says.

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

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

@table @asis
@item @code{default}
This element of @var{spec} doesn't match any frames; instead, it
specifies defaults that apply to all frames.  This kind of element, if
used, must be the first element of @var{spec}.  Each of the following
elements can override any or all of these defaults.

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

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

@table @code
@item type
The kind of window system the frame uses---either @code{graphic} (any
graphics-capable display), @code{x}, @code{pc} (for the MS-DOS console),
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@code{w32} (for MS Windows 9X/NT/2K/XP), or @code{tty} 
(a non-graphics-capable display).
<