This chapter describes a number of features related to the display that Emacs presents to the user.
The function redraw-frame redisplays the entire contents of a
given frame. See section Frames.
Even more powerful is redraw-display:
Normally, suspending and resuming Emacs also refreshes the screen. Some terminal emulators record separate contents for display-oriented programs such as Emacs and for ordinary sequential display. If you are using such a terminal, you might want to inhibit the redisplay on resumption. See section Suspending Emacs.
nil means yes, nil
means no.
Processing user input takes absolute priority over redisplay. If you call these functions when input is available, they do nothing immediately, but a full redisplay does happen eventually--after all the input has been processed.
The screen size functions report or tell Emacs the height or width of the terminal. When you are using multiple frames, they apply to the selected frame (see section Frames).
(screen-height)
=> 50
(screen-width)
=> 80
If not-actual-size is non-nil, then Emacs displays
lines lines of output, but does not change its value for the
actual height of the screen. (Knowing the correct actual size may be
necessary for correct cursor positioning.) Using a smaller height than
the terminal actually implements may be useful to reproduce behavior
observed on a smaller screen, or if the terminal malfunctions when using
its whole screen.
If lines is different from what it was previously, then the entire screen is cleared and redisplayed using the new size.
This function returns nil.
set-screen-height.
When a line of text extends beyond the right edge of a window, the line can either be continued on the next screen line, or truncated to one screen line. The additional screen lines used to display a long text line are called continuation lines. Normally, a `$' in the rightmost column of the window indicates truncation; a `\' on the rightmost column indicates a line that "wraps" or is continued onto the next line. (The display table can specify alternative indicators; see section Display Tables.)
Note that continuation is different from filling; continuation happens on the screen only, not in the buffer contents, and it breaks a line precisely at the right margin, not at a word boundary. See section Filling.
nil, which
specifies continuation. If the value is non-nil, then these
lines are truncated.
If the variable truncate-partial-width-windows is non-nil,
then truncation is always used for side-by-side windows (within one
frame) regardless of the value of truncate-lines.
truncate-lines, for
buffers that do not have local values for it.
nil, these lines are truncated; otherwise,
truncate-lines says what to do with them.
You can override the images that indicate continuation or truncation with the display table; see section Display Tables.
The echo area is used for displaying messages made with the
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
GNU Emacs Manual specifies the rules for resolving conflicts
between the echo area and the minibuffer for use of that screen space
(see section `The Minibuffer' in The GNU Emacs Manual).
Error messages appear in the echo area; see section Errors.
You can write output in the echo area by using the Lisp printing
functions with t as the stream (see section Output Functions), or as
follows:
printf control
string. See format in section Conversion of Characters and Strings, for the details
on the conversion specifications. message returns the
constructed string.
In batch mode, message prints the message text on the standard
error stream, followed by a newline.
If string is nil, message clears the echo area. If
the minibuffer is active, this brings the minibuffer contents back onto
the screen immediately.
(message "Minibuffer depth is %d."
(minibuffer-depth))
-| Minibuffer depth is 0.
=> "Minibuffer depth is 0."
---------- Echo Area ----------
Minibuffer depth is 0.
---------- Echo Area ----------
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 nil; Lisp programs bind it to t
for brief periods of time.
Selective display is a class of minor modes in which specially marked lines do not appear on the screen, or in which highly indented lines do not appear.
The first variant, explicit selective display, is designed for use in a Lisp program. The program controls which lines are hidden by altering the text. Outline mode uses this variant. In the second variant, the choice of lines to hide is made automatically based on indentation. This variant is designed as a user-level feature.
The way you control explicit selective display is by replacing a newline (control-j) with a carriage return (control-m). The text which was formerly a line following that newline is now invisible. Strictly speaking, it is temporarily no longer a line at all, since only newlines can separate lines; it is now part of the previous line.
Selective display does not directly affect editing commands. For
example, C-f (forward-char) moves point unhesitatingly into
invisible text. However, the replacement of newline characters with
carriage return characters affects some editing commands. For example,
next-line skips invisible lines, since it searches only for
newlines. Modes that use selective display can also define commands
that take account of the newlines, or that make parts of the text
visible or invisible.
When you write a selectively displayed buffer into a file, all the control-m's are output as newlines. This means that when you next read in the file, it looks OK, with nothing invisible. The selective display effect is seen only within Emacs.
selective-display is t, then any portion
of a line that follows a control-m is not displayed.
selective-display is a positive integer, then
lines that start with more than that many columns of indentation are not
displayed.
When some portion of a buffer is invisible, the vertical movement
commands operate as if that portion did not exist, allowing a single
next-line command to skip any number of invisible lines.
However, character movement commands (such as forward-char) do
not skip the invisible portion, and it is possible (if tricky) to insert
or delete text in an invisible portion.
In the examples below, we show the display appearance of the
buffer foo, which changes with the value of
selective-display. The contents of the buffer do not
change.
(setq selective-display nil)
=> 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 ----------
(setq selective-display 2)
=> 2
---------- Buffer: foo ----------
1 on this column
2on this column
2on this column
1 on this column
---------- Buffer: foo ----------
nil, then Emacs displays
`...' at the end of a line that is followed by invisible text.
This example is a continuation of the previous one.
(setq selective-display-ellipses t)
=> t
---------- Buffer: foo ----------
1 on this column
2on this column ...
2on this column
1 on this column
---------- Buffer: foo ----------
You can use a display table to substitute other text for the ellipsis (`...'). See section Display Tables.
The overlay arrow is useful for directing the user's attention to a particular line in a buffer. For example, in the modes used for interface to debuggers, the overlay arrow indicates the line of code about to be executed.
nil if
the arrow feature is not in use.
The overlay string is displayed only in the buffer which this marker points into. Thus, only one buffer can have an overlay arrow at any given time.
Temporary displays are used by commands to put output into a buffer and then present it to the user for perusal rather than for editing. Many of the help commands use this feature.
The string 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 with-output-to-temp-buffer exits.
with-output-to-temp-buffer binds standard-output to the
temporary buffer, then it evaluates the forms in forms. Output
using the Lisp output functions within 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).
See section Output Functions.
The value of the last form in forms is returned.
---------- Buffer: foo ----------
This is the contents of foo.
---------- Buffer: foo ----------
(with-output-to-temp-buffer "foo"
(print 20)
(print standard-output))
=> #<buffer foo>
---------- Buffer: foo ----------
20
#<buffer foo>
---------- Buffer: foo ----------
nil, with-output-to-temp-buffer
calls it as a function to do the job of displaying a help buffer. The
function gets one argument, which is the buffer it should display.
In Emacs versions 18 and earlier, this variable was called
temp-buffer-show-hook.
The momentary display remains until the next input event. If the next
input event is char, momentary-string-display ignores it
and returns. Otherwise, that event remains buffered for subsequent use
as input. Thus, typing char will simply remove the string from
the display, while typing (say) C-f will remove the string from
the display and later (presumably) move point forward. The argument
char is a space by default.
The return value of momentary-string-display is not meaningful.
If message is non-nil, it is displayed in the echo area
while string is displayed in the buffer. If it is nil, a
default message says to type char to continue.
In this example, point is initially located at the beginning of the second line:
---------- Buffer: foo ---------- This is the contents of foo. -!-Second line. ---------- Buffer: foo ---------- (momentary-string-display "**** Important Message! ****" (point) ?\r "Type RET when done reading") => t ---------- Buffer: foo ---------- This is the contents of foo. **** Important Message! ****Second line. ---------- Buffer: foo ---------- ---------- Echo Area ---------- Type RET when done reading ---------- Echo Area ----------
You can use overlays to alter the appearance of a buffer's text on the screen. An overlay is an object which belongs to a particular buffer, and has a specified beginning and end. It also has properties that you can examine and set; these affect the display of the text within the overlay.
Overlay properties are like text properties in some respects, but the differences are more important than the similarities. Text properties are considered a part of the text; overlays are specifically considered not to be part of the text. Thus, copying text between various buffers and strings preserves text properties, but does not try to preserve overlays. Changing a buffer's text properties marks the buffer as modified, while moving an overlay or changing its properties does not. Unlike text propery changes, overlay changes are not recorded in the buffer's undo list.
priority
priority value is larger takes priority over the
other, and its face attributes override the face attributes of the lower
priority overlay.
Currently, all overlays take priority over text properties. Please
avoid using negative priority values, as we have not yet decided just
what they should mean.
window
window property is non-nil, then the overlay
applies only on that window.
face
mouse-face
face when the mouse is within
the range of the overlay. This feature may be temporary, like
face.
modification-hooks
insert-in-front-hooks
insert-behind-hooks
invisible
nil invisible property means that the text in the
overlay does not appear on the screen. This works much like selective
display. Details of this feature are likely to change in future
versions, so check the `etc/NEWS' file in the version you are
using.
before-string
after-string
These are the functions for reading and writing the properties of an overlay.
nil.
See also the function get-char-property which checks both
overlay properties and text properties for a given character.
See section Examining Text Properties.
This section describes the functions to create, delete and move overlays, and to examine their contents.
The return value is 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".
A face is a named collection of graphical attributes: font, foreground color, background color and optional underlining. Faces control the display of text on the screen.
Each face has its own face id number which distinguishes faces at low levels within Emacs. However, for most purposes, you can refer to faces in Lisp programs by their names.
Each face name is meaningful for all frames, and by default it has the same meaning in all frames. But you can arrange to give a particular face name a special meaning in one frame if you wish.
This table lists all the standard faces and their uses.
default
modeline
region
secondary-selection
highlight
underline
bold
italic
bold-italic
Here are all the ways to specify which face to use for display of text:
face property; if so,
it's displayed with that face. See section Properties with Special Meanings.
If the character has a mouse-face property, that is used instead
of the face property when the mouse is "near enough" to the
character.
face and mouse-face
properties too; they apply to all the text covered by the overlay.
If these various sources together specify more than one face for a particular character, Emacs merges the attributes of the various faces specified. The attributes of the faces of special glyphs come first; then come attributes of faces from overlays, followed by those from text properties, and last the default face.
When multiple overlays cover one character, an overlay with higher priority overrides those with lower priority. See section Overlays.
If an attribute such as the font or a color is not specified in any of the above ways, the frame's own font or color is used.
The attributes a face can specify include the font, the foreground
color, the background color, and underlining. The face can also leave
these unspecified by giving the value nil for them.
Here are the primitives for creating and changing faces.
nil. It does nothing if there is already a face named
name.
If the optional argument frame is given, this function applies only to that frame. Otherwise it applies to each frame individually, copying attributes from old-face in each frame to new-face in the same frame.
If the optional argument new-frame is given, then copy-face
copies the attributes of old-face in frame to new-name
in new-frame.
You can modify the attributes of an existing face with the following functions. If you specify frame, they affect just that frame; otherwise, they affect all frames as well as the defaults that apply to new frames.
nil means do underline; nil means don't.
These functions examine the attributes of a face. If you don't specify frame, they refer to the default data for new frames.
t if the faces face1 and face2 have the
same attributes for display.
t if the face face displays differently from
the default face. A face is considered to be "the same" as the normal
face if each attribute is either the same as that of the default face or
nil (meaning to inherit from the default).
Normally, the value is the id number of the face named region.
This section describes the mechanism by which Emacs shows a matching open parenthesis when the user inserts a close parenthesis.
blink-paren-function may be nil, in which
case nothing is done.
Please note: this variable was named
blink-paren-hookin older Emacs versions, but since it is not called with the standard convention for hooks, it was renamed toblink-paren-functionin version 19.
nil, then blink-matching-open does
nothing.
blink-paren-function. It
assumes that point follows a character with close parenthesis syntax and
moves the cursor momentarily to the matching opening character. If that
character is not already on the screen, it displays the character's
context in the echo area. To avoid long delays, this function does not
search farther than blink-matching-paren-distance characters.
Here is an example of calling this function explicitly.
(defun interactive-blink-matching-open ()
"Indicate momentarily the start of sexp before point."
(interactive)
(let ((blink-matching-paren-distance
(buffer-size))
(blink-matching-paren t))
(blink-matching-open)))
nil means yes, nil means no. The
default is nil.
nil, then mode lines are displayed in inverse video (under
X, this uses the face named modeline, which you can set as you
wish). Otherwise, mode lines are displayed normally, just like text.
The default is t.
The usual display conventions define how to display each character code. You can override these conventions by setting up a display table (see section Display Tables). Here are the usual display conventions:
tab-width.
ctl-arrow. If it is is
non-nil, these codes map to sequences of two glyphs, where the
first glyph is the ASCII code for `^'. (A display table can
specify a glyph to use instead of `^'.) Otherwise, these codes map
just like the codes in the range 128 to 255.
The usual display conventions apply even when there is a display
table, for any character whose entry in the active display table is
nil. Thus, when you set up a display table, you need only
specify the the characters for which you want unusual behavior.
These variables affect the way certain characters are displayed on the screen. Since they change the number of columns the characters occupy, they also affect the indentation functions.
nil, they are displayed as a caret
followed by the character: `^A'. If it is nil, they are
displayed as a backslash followed by three octal digits: `\001'.
ctl-arrow in
buffers that do not override it. See section The Default Value of a Buffer-Local Variable.
tab-to-tab-stop. See section Adjustable "Tab Stops".
You can use the display table feature to control how all 256 possible character codes display on the screen. This is useful for displaying European languages that have letters not in the ASCII character set.
The display table maps each character code into a sequence of glyphs, each glyph being an image that takes up one character position on the screen. You can also define how to display each glyph on your terminal, using the glyph table.
A display table is actually an array of 261 elements.
nil in all elements.
The first 256 elements correspond to character codes; the nth
element says how to display the character code n. The value
should be nil or a vector of glyph values (see section Glyphs). If
an element is nil, it says to display that character according to
the usual display conventions (see section Usual Display Conventions).
The remaining five elements of a display table serve special purposes,
and nil means use the default stated below.
For example, here is how to construct a display table that mimics the
effect of setting ctl-arrow to a non-nil value:
(setq disptab (make-display-table))
(let ((i 0))
(while (< i 32)
(or (= i ?\t) (= i ?\n)
(aset disptab i (vector ?^ (+ i 64))))
(setq i (1+ i)))
(aset disptab 127 (vector ?^ ??)))
Each window can specify a display table, and so can each buffer. When a buffer b is displayed in window w, display uses the display table for window w if it has one; otherwise, the display table for buffer b if it has one; otherwise, the standard display table if any. The display table chosen is called the active display table.
nil
if window does not have an assigned display table.
nil.
nil if
the buffer does not have an assigned display table.
nil by default.
If there is no display table to use for a particular window--that is,
if the window has none, its buffer has none, and
standard-display-table has none--then Emacs uses the usual
display conventions for all character codes in that window. See section Usual Display Conventions.
A glyph is a generalization of a character; it stands for an image that takes up a single character position on the screen. Glyphs are represented in Lisp as integers, just as characters are.
The meaning of each integer, as a glyph, is defined by the glyph
table, which is the value of the variable glyph-table.
nil instead of a vector, then all glyphs are simple (see
below).
Here are the possible types of elements in the glyph table:
nil
If a glyph code is greater than or equal to the length of the glyph table, that code is automatically simple.
If you have a terminal that can handle the entire ISO Latin 1 character set, you can arrange to use that character set as follows:
(require 'disp-table) ;; Set char codes 160--255 to display as themselves. ;; (Codes 128--159 are the additional control characters.) (standard-display-8bit 160 255)
If you are editing buffers written in the ISO Latin 1 character set and your terminal doesn't handle anything but ASCII, you can load the file `iso-ascii' to set up a display table which makes the other ISO characters display as sequences of ASCII characters. For example, the character "o with umlaut" displays as `{"o}'.
Some European countries have terminals that don't support ISO Latin 1 but do support the special characters for that country's language. You can define a display table to work one language using such terminals. For an example, see `lisp/iso-swed.el', which handles certain Swedish terminals.
You can load the appropriate display table for your terminal automatically by writing a terminal-specific Lisp file for the terminal type.
You can make Emacs ring a bell (or blink the screen) to attract the user's attention. Be conservative about how often you do this; frequent bells can become irritating. Also be careful not to use beeping alone when signaling an error is appropriate. (See section Errors.)
visible-bell below).
It also terminates any keyboard macro currently executing unless
dont-terminate is non-nil.
ding.
nil means yes, nil means no. This
is effective only if the Termcap entry for the terminal in use has the
visible bell flag (`vb') set.
Emacs works with several window systems, most notably the X Window System. Both Emacs and X use the term "window", but use it differently. An Emacs frame is a single window as far as X is concerned; the individual Emacs windows are not known to X at all.
x (if Emacs is
running under X) or nil (if Emacs is running on an ordinary
terminal).
nil otherwise.
term-setup-hook.
This hook is used for internal purposes: setting up communication with the window system, and creating the initial window. Users should not interfere with it.