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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, 1998-2014 Free Software Foundation, Inc.
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@c See the file elisp.texi for copying conditions.
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@node Display
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@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.
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* Size of Displayed Text::  How large displayed text is.
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* 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.
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* Window Dividers::     Separating windows visually.
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* Display Property::    Enabling special display features.
* Images::              Displaying images in Emacs buffers.
* Buttons::             Adding clickable buttons to Emacs buffers.
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* Abstract Display::    Emacs's Widget for Object Collections.
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* Blinking::            How Emacs shows the matching open parenthesis.
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* Character Display::   How Emacs displays individual characters.
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* 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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@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.

@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 terminals, suspending and resuming Emacs normally also
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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
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This function tries immediately to redisplay.  The optional argument
@var{force}, if non-@code{nil}, forces the redisplay to be performed,
instead of being preempted, even if input is pending and the variable
@code{redisplay-dont-pause} is @code{nil} (see below).  If
@code{redisplay-dont-pause} is non-@code{nil} (the default), this
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function redisplays in any case, i.e., @var{force} does nothing.
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The function returns @code{t} if it actually tried to redisplay, and
@code{nil} otherwise.  A value of @code{t} does not mean that
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redisplay proceeded to completion; it could have been preempted by
newly arriving input.
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@end defun

@defvar redisplay-dont-pause
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If this variable is @code{nil}, arriving input events preempt
redisplay; Emacs avoids starting a redisplay, and stops any redisplay
that is in progress, until the input has been processed.  In
particular, @code{(redisplay)} returns @code{nil} without actually
redisplaying, if there is pending input.

The default value is @code{t}, which means that pending input does not
preempt redisplay.
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@end defvar

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@defvar redisplay-preemption-period
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If @code{redisplay-dont-pause} is @code{nil}, this variable specifies
how many seconds Emacs waits between checks for new input during
redisplay; if input arrives during this interval, redisplay stops and
the input is processed.  The default value is 0.1; if the value is
@code{nil}, Emacs does not check for input during redisplay.

This variable has no effect when @code{redisplay-dont-pause} is
non-@code{nil} (the default).
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@end defvar

@defvar pre-redisplay-function
A function run just before redisplay.  It is called with one argument,
the set of windows to redisplay.
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@end defvar
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  Although @code{redisplay} tries immediately to redisplay, it does
not change how Emacs decides which parts of its frame(s) to redisplay.
By contrast, the following function adds certain windows to the
pending redisplay work (as if their contents had completely changed),
but does not immediately try to perform redisplay.
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@defun force-window-update &optional object
This function forces some or all windows to be updated the next time
Emacs does a redisplay.  If @var{object} is a window, that window is
to be updated.  If @var{object} is a buffer or buffer name, all
windows displaying that buffer are to be updated.  If @var{object} is
@code{nil} (or omitted), all windows are to be updated.

This function does not do a redisplay immediately; Emacs does that as
it waits for input, or when the function @code{redisplay} is called.
@end defun
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@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
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``wraps''.  (The display table can specify alternate characters to use
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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
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@dfn{partial-width} windows (i.e., windows that do not occupy the
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entire frame width).
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@end defopt

@defopt truncate-partial-width-windows
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@cindex 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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@dfn{wrap prefix} which Emacs displays at the start of every
continuation line.  (If lines are truncated, @code{wrap-prefix} is
never used.)  Its value 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}.

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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@dfn{line prefix} which Emacs displays at the start of every
non-continuation line.  Its value 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}.

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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@ignore
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  If your buffer contains only very short lines, you might find it
advisable to set @code{cache-long-scans} to @code{nil}.
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@defvar cache-long-scans
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If this variable is non-@code{nil} (the default), 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.
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Turning off the cache speeds up processing of short lines somewhat.
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This variable is automatically buffer-local in every buffer.
@end defvar
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@end ignore
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@node The Echo Area
@section The Echo Area
@cindex error display
@cindex echo area

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@c FIXME: Why not use @xref{Minibuffers} directly?  --xfq
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  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
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place on the screen as the echo area.  @xref{Minibuffer,, The
Minibuffer, emacs, The GNU Emacs Manual}.
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  Apart from the functions documented in this section, you can print
Lisp objects to the echo area by specifying @code{t} as the output
stream.  @xref{Output Streams}.
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@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

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  This section describes the standard functions for displaying
messages in the echo area.
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@defun message format-string &rest arguments
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This function displays a message in the echo area.
@var{format-string} is a format string, and @var{arguments} are the
objects for its format specifications, like in the @code{format}
function (@pxref{Formatting Strings}).  The resulting formatted string
is displayed in the echo area; if it contains @code{face} text
properties, it is displayed with the specified faces (@pxref{Faces}).
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The string is also added to the @file{*Messages*} buffer, but without
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text properties (@pxref{Logging Messages}).

In batch mode, the message is printed to the standard error stream,
followed by a newline.
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If @var{format-string} is @code{nil} or the empty string,
@code{message} clears the echo area; if the echo area has been
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expanded automatically, this brings it back to its normal size.  If
the minibuffer is active, this brings the minibuffer contents back
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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
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pop-up buffer is used, defaulting to @file{*Message*}.  In the case
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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

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@defun progress-reporter-update reporter &optional value
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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

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@defun progress-reporter-force-update reporter &optional value new-message
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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
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you to change the message of the @var{reporter}.  Since this function
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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
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@code{progress-reporter-update} to print ``100%''.  Firstly, it may
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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
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@subsection Logging Messages in @file{*Messages*}
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@cindex logging echo-area messages

  Almost all the messages displayed in the echo area are also recorded
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in the @file{*Messages*} buffer so that the user can refer back to
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them.  This includes all the messages that are output with
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@code{message}.  By default, this buffer is read-only and uses the major
mode @code{messages-buffer-mode}.  Nothing prevents the user from
killing the @file{*Messages*} buffer, but the next display of a message
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recreates it.  Any Lisp code that needs to access the
@file{*Messages*} buffer directly and wants to ensure that it exists
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should use the function @code{messages-buffer}.
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@defun messages-buffer
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This function returns the @file{*Messages*} buffer.  If it does not
exist, it creates it, and switches it to @code{messages-buffer-mode}.
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@end defun
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@defopt message-log-max
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This variable specifies how many lines to keep in the @file{*Messages*}
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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

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  To make @file{*Messages*} more convenient for the user, the logging
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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
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truncated to fit it.
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@end defvar

  The variable @code{max-mini-window-height}, which specifies the
maximum height for resizing minibuffer windows, also applies to the
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echo area (which is really a special use of the minibuffer window;
@pxref{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.
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* Delayed Warnings::    Deferring a warning until the end of a command.
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@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.)

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@c FIXME: Why use "(bytecomp)" instead of "'bytecomp" or simply
@c "bytecomp" here?  The parens are part of warning-type-format but
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@c not part of the warning type. --xfq
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@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
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for logging the warning.  By default, it is @file{*Warnings*}.
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@end defun

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

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@node Delayed Warnings
@subsection Delayed Warnings

Sometimes, you may wish to avoid showing a warning while a command is
running, and only show it only after the end of the command.  You can
use the variable @code{delayed-warnings-list} for this.

@defvar delayed-warnings-list
The value of this variable is a list of warnings to be displayed after
the current command has finished.  Each element must be a list

@smallexample
(@var{type} @var{message} [@var{level} [@var{buffer-name}]])
@end smallexample

@noindent
with the same form, and the same meanings, as the argument list of
@code{display-warning} (@pxref{Warning Basics}).  Immediately after
running @code{post-command-hook} (@pxref{Command Overview}), the Emacs
command loop displays all the warnings specified by this variable,
then resets it to @code{nil}.
@end defvar

  Programs which need to further customize the delayed warnings
mechanism can change the variable @code{delayed-warnings-hook}:

@defvar delayed-warnings-hook
This is a normal hook which is run by the Emacs command loop, after
@code{post-command-hook}, in order to to process and display delayed
warnings.

Its default value is a list of two functions:

@smallexample
(collapse-delayed-warnings display-delayed-warnings)
@end smallexample

@findex collapse-delayed-warnings
@findex display-delayed-warnings
@noindent
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The function @code{collapse-delayed-warnings} removes repeated entries
from @code{delayed-warnings-list}.  The function
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@code{display-delayed-warnings} calls @code{display-warning} on each
of the entries in @code{delayed-warnings-list}, in turn, and then sets
@code{delayed-warnings-list} to @code{nil}.
@end defvar

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@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
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text property (@pxref{Text Properties}) or an overlay property
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(@pxref{Overlays}).  Cursor motion also partly ignores these
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characters; if the command loop finds that point is inside a range of
invisible text after a command, it relocates point to the other side
of the text.
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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}
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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; comparison
is done with @code{eq}.
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@item (@var{atom} . t)
A character is invisible if its @code{invisible} property value is
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@var{atom} or if it is a list with @var{atom} as a member; comparison
is done with @code{eq}.  Moreover, a sequence of such characters
displays as an ellipsis.
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@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)

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;; @r{When done with the invisibility:}
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(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 relocates point to one of the
two ends of the invisible text.  Emacs chooses the direction of
relocation so that it is the same as the overall movement direction of
the command; if in doubt, it prefers 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.
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  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.

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@cindex explicit selective display
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  The first variant, explicit selective display, was designed for use in a Lisp
program: it controls which lines are hidden by altering the text.  This kind of
hiding is now obsolete; instead you can get the same effect with the
@code{invisible} property (@pxref{Invisible Text}).
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  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.

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@defmac with-output-to-temp-buffer buffer-name body@dots{}
This function executes the forms in @var{body} 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.  (See the similar
form @code{with-temp-buffer-window} below.)  Finally, the buffer is
displayed in some window, but that window is not selected.

If the forms in @var{body} 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
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documentation strings, for more details.

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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.
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@code{with-output-to-temp-buffer} binds @code{standard-output} to the
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temporary buffer, then it evaluates the forms in @var{body}.  Output
using the Lisp output functions within @var{body} goes by default to
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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.

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The value of the last form in @var{body} is returned.
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@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 ----------
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#<buffer foo>

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

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@defmac with-temp-buffer-window buffer-or-name action quit-function body@dots{}
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This macro is similar to @code{with-output-to-temp-buffer}.  Like that
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construct, it executes @var{body} while arranging to insert any output
it prints into the buffer named @var{buffer-or-name} and displays that
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buffer in some window.  Unlike @code{with-output-to-temp-buffer},
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however, it does not automatically switch that buffer to Help mode.
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Like @code{with-output-to-temp-buffer} it neither makes the buffer
specified by @var{buffer-or-name} current when executing @var{body}.
@findex with-current-buffer-window
The otherwise identical macro @code{with-current-buffer-window} can be
used to execute @var{body} with that buffer current.

The argument @var{buffer-or-name} specifies the temporary buffer.  It
can be either a buffer, which must already exist, or a string, in which
case a buffer of that name is created, if necessary.  The buffer is
marked as unmodified and read-only when @code{with-temp-buffer-window}
exits.
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This macro does not call @code{temp-buffer-show-function}.  Rather, it
passes the @var{action} argument to @code{display-buffer} in order to
display the buffer.

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The value of the last form in @var{body} is returned, unless the
argument @var{quit-function} is specified.  In that case, it is called
with two arguments: the window showing the buffer and the result of
@var{body}.  The final return value is then whatever
@var{quit-function} returns.
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@vindex temp-buffer-window-setup-hook
@vindex temp-buffer-window-show-hook
This macro uses the normal hooks @code{temp-buffer-window-setup-hook}
and @code{temp-buffer-window-show-hook} in place of the analogous hooks
run by @code{with-output-to-temp-buffer}.
@end defmac

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@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
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@c FIXME: mention intervals in this section?
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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
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``lost''.
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@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
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around an arbitrary ``center position''.  One list extends backwards
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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)}
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This property's value (which should be a non-negative 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)}
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This property controls the appearance of the text (@pxref{Faces}).
The value of the property can be the following:
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@itemize @bullet
@item
A face name (a symbol or string).

@item
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An anonymous face: a property list of the form @code{(@var{keyword}
@var{value} @dots{})}, where each @var{keyword} is a face attribute
name and @var{value} is a value for that attribute.
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@item
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A list of faces.  Each list element should be either a face name or an
anonymous face.  This specifies a face which is an aggregate of the
attributes of each of the listed faces.  Faces occurring earlier in
the list have higher priority.
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@item
A cons cell of the form @code{(foreground-color . @var{color-name})}
or @code{(background-color . @var{color-name})}.  This specifies the
foreground or background color, similar to @code{(:foreground
@var{color-name})} or @code{(:background @var{color-name})}.  This
form is supported for backward compatibility only, and should be
avoided.
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@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
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from this property that alter the text size (e.g., @code{:height},
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@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}.

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@item field
@kindex field @r{(overlay property)}
@c Copied from Special Properties.
Consecutive characters with the same @code{field} property constitute a
@emph{field}.  Some motion functions including @code{forward-word} and
@code{beginning-of-line} stop moving at a field boundary.
@xref{Fields}.

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@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}.

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@item wrap-prefix
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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 keymap
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@cindex keymap of character (and overlays)
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@kindex keymap @r{(overlay property)}
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If this property is non-@code{nil}, it specifies a keymap for a portion of the
text.  This keymap is used when the character after point is within the
overlay, and takes precedence over most other keymaps.  @xref{Active Keymaps}.

@item local-map
@kindex local-map @r{(overlay property)}
The @code{local-map} property is similar to @code{keymap} but replaces the
buffer's local map rather than augmenting existing keymaps.  This also means it
has lower precedence than minor mode keymaps.
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@end table

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The @code{keymap} and @code{local-map} properties do not affect a
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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
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@code{keymap} or @code{local-map} text property.  @xref{Special
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