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Emacs Display

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

Refreshing the Screen

The function redraw-frame redisplays the entire contents of a given frame. See section Frames.

Function: redraw-frame frame

This function clears and redisplays frame frame.

Even more powerful is redraw-display.

Command: redraw-display

This function clears and redisplays all visible frames.

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.

Variable: no-redraw-on-reenter

This variable controls whether Emacs redraws the entire screen after it has been suspended and resumed. Non-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.

Screen Size

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).

Function: screen-height

This function returns the number of lines on the screen that are available for display.

     => 50

Function: screen-width

This function returns the number of columns on the screen that are available for display.

     => 80

Function: set-screen-height lines &optional not-actual-size

This function declares that the terminal can display lines lines. The sizes of existing windows are altered proportionally to fit.

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.

Function: set-screen-width columns &optional not-actual-size

This function declares that the terminal can display columns columns. The details are as in set-screen-height.


When a line of text extends beyond the right edge of a window, the line can either be truncated or continued on the next line. When a line is truncated, this is shown with a `$' in the rightmost column of the window. When a line is continued or "wrapped" onto the next line, this is shown with a `\' on the rightmost column of the window. The additional screen lines used to display a long text line are called continuation lines. (Note that wrapped lines are not filled; filling has nothing to do with continuation and truncation. See section Filling.)

User Option: truncate-lines

This buffer-local variable controls how Emacs displays lines that extend beyond the right edge of the window. If it is non-nil, then Emacs does not display continuation lines; rather each line of text occupies exactly one screen line, and a dollar sign appears at the edge of any line that extends to or beyond the edge of the window. The default is nil.

If the variable truncate-partial-width-windows is non-nil, then truncation is used for windows that are not the full width of the screen, regardless of the value of truncate-lines.

Variable: default-truncate-lines

This variable is the default value for truncate-lines in buffers that do not have local values for it.

User Option: truncate-partial-width-windows

This variable determines how lines that are too wide to fit on the screen are displayed in side-by-side windows (see section Splitting Windows). If it is non-nil, then wide lines are truncated (with a `$' at the end of the line); otherwise they wrap to the next screen line (with a `\' at the end of the line).

You can override the images that indicate continuation or truncation with the display table; see section Display Tables.

The Echo Area

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:

Function: message string &rest arguments

This function prints a one-line message in the echo area. The argument string is similar to a C language printf control string. See format in section Conversion of Characters and Strings, for the details on the conversion specifications. message returns the constructed string.

If string is nil, message clears the echo area. If the minibuffer is active, this brings the minibuffer contents back onto the screen immediately.

 "Minibuffer depth is %d."
=> "Minibuffer depth is 0."

---------- Echo Area ----------
Minibuffer depth is 0.
---------- Echo Area ----------

Variable: cursor-in-echo-area

This variable controls where the cursor appears when a message is displayed in the echo area. If it is non-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

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 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 space. 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 which make parts of the text visible or invisible.

When you write a selectively displayed buffer into a file, all the control-m's are replaced by their original 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.

Variable: selective-display

This buffer-local variable enables selective display. This means that lines, or portions of lines, may be made invisible.

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, what is shown is the display 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 ----------

Variable: selective-display-ellipses

If this buffer-local variable is non-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.

Overlay Arrow

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.

Variable: overlay-arrow-string

This variable holds the string to display as an arrow, or nil if the arrow feature is not in use.

Variable: overlay-arrow-position

This variable holds a marker which indicates where to display the arrow. It should point at the beginning of a line. The arrow text is displayed at the beginning of that line, overlaying any text that would otherwise appear. Since the arrow is usually short, and the line usually begins with indentation, normally nothing significant is overwritten.

The overlay string is displayed only in the buffer which this marker points into. Thus, only one buffer can have an overlay arrow at any given time.

Temporary Displays

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

Special Form: with-output-to-temp-buffer buffer-name forms...

This function executes forms while arranging to insert any output they print into the buffer named buffer-name. The buffer is then shown in some window for viewing, displayed but not selected.

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 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 ----------

#<buffer foo>

---------- Buffer: foo ----------

Variable: temp-buffer-show-function

The value of this variable, if non-nil, is called as a function to display a help buffer. This variable is used by with-output-to-temp-buffer.

In Emacs versions 18 and earlier, this variable was called temp-buffer-show-hook.

Function: momentary-string-display string position &optional char message

This function momentarily displays string in the current buffer at position (which is a character offset from the beginning of the buffer). The display remains until the next character is typed.

If the next character the user types is char, Emacs ignores it. Otherwise, that character 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, then instructions to type char are displayed there, e.g., `Type RET to continue editing'.

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 ----------

   "**** 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 ----------

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


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 which you can examine and set; these affect the display of the text within the overlay.

Overlay Properties

Overlay properties are like text properties in some respects, but the differences are more important than the similarities. Text properties are considered a part of the text; overlays are specifically considered not to be part of the text. Thus, copying text between various buffers and strings preserves text properties, but does not try to preserve overlays. Changing a buffer's text properties marks the buffer as modified, while moving an overlay or changing its properties does not.

This property controls the font and color of text. See section Faces, for more information. This feature is temporary; in the future, we may replace it with other ways of specifying how to display text.

This property is used instead of face when the mouse is within the range of the overlay. This feature is not yet implemented, and may be temporary. It is documented here because we are likely to implement it this way at least for a while.

This property's value (which should be a nonnegative number) determines the priority of the overlay. The priority matters when two or more overlays cover the same character and both specify a face for display; the one whose 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.

If the window property is non-nil, then the overlay applies only on that window.

This property's value is a string to add to the display at the beginning of the overlay. The string does not appear in the buffer in any sense--only on the screen. This is not yet implemented, but will be.

This property's value is a string to add to the display at the end of the overlay. The string does not appear in the buffer in any sense--only on the screen. This is not yet implemented, but will be.

This property's value is a list of functions to be called if any character within the overlay is changed or if text is inserted strictly within the overlay. Each function receives two arguments: the beginning and end of the part of the buffer being modified.

This property's value is a list of functions to be called if text is inserted right at the beginning of the overlay.

This property's value is a list of functions to be called if text is inserted right at the end of the overlay.

These are the functions for reading and writing the properties of an overlay.

Function: overlay-get overlay prop

This function returns the value of property prop recorded in overlay. If overlay does not record any value for that property, then the value is nil.

Function: overlay-put overlay prop value

This function set the value of property prop recorded in overlay to value. It returns value.

Managing Overlays

Function: make-overlay start end &optional buffer

This function creates and returns an overlay which belongs to buffer and ranges from start to end. Both start and end must specify buffer positions; they may be integers or markers. If buffer is omitted, the overlay is created in the current buffer.

The return value is the overlay itself.

Function: overlay-start overlay

This function returns the position at which overlay starts.

Function: overlay-end overlay

This function returns the position at which overlay ends.

Function: overlay-buffer overlay

This function returns the buffer that overlay belongs to.

Function: delete-overlay overlay

This function deletes overlay. The overlay continues to exist as a Lisp object, but ceases to be part of the buffer it belonged to, and ceases to have any effect on display.

Function: move-overlay overlay start end &optional buffer

This function moves overlay to buffer, and places its bounds at start and end. Both arguments start and end must specify buffer positions; they may be integers or markers. If buffer is omitted, the overlay stays in the same buffer.

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".

Function: overlays-at pos

This function returns a list of all the overlays that contain position pos in the current buffer. The list is in no particular order. An overlay contains position pos if it begins at or before pos, and ends after pos.

Function: next-overlay-change pos

This function returns the buffer position of the next beginning or end of an overlay, after pos.


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.

The face named default is used for ordinary text. The face named modeline is used for displaying the mode line and menu bars. The face named region is used for highlighting the region (in Transient Mark mode only).

Merging Faces for Display

Here are all the ways to specify which face to use for display of text:

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.

Functions for Working with Faces

The attributes a face can specify include the font, the foreground color, the background color, and underlining. The face can also leave these unspecified by giving the value nil for them.

Here are the primitives for creating and changing faces.

Function: make-face name

This function defines a new face named name, initially with all attributes nil. It does nothing if there is already a face named name.

Function: face-list

This function returns a list of all defined face names.

Function: copy-face old-face new-name &optional frame new-frame

This function defines a new face named new-name which is a copy of the existing face named old-face. If there is already a face named new-name, then it alters the face to have the same attributes as old-face.

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 that 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.

Function: set-face-foreground face color &optional frame

Function: set-face-background face color &optional frame

These functions set the foreground (respectively, background) color of face face to color. The argument color color should be a string, the name of a color.

Function: set-face-font face font &optional frame

This function sets the font of face face. The argument font should be a string.

Function: set-face-underline-p face underline-p &optional frame

This function sets the underline attribute of face face.

Function: invert-face face &optional frame

Swap the foreground and background colors of face face. If the face doesn't specify both foreground and background, then its foreground and background are set to the default background and foreground.

These functions examine the attributes of a face. If you don't specify frame, they refer to the default data for new frames.

Function: face-foreground face &optional frame

Function: face-background face &optional frame

These functions return the foreground (respectively, background) color of face face. The argument color color should be a string, the name of a color.

Function: face-font face &optional frame

This function returns the name of the font of face face.

Function: face-underline-p face &optional frame

This function returns the underline attribute of face face.

Function: face-id-number face

This function returns the id number of face face.

Function: face-equal face1 face2 &optional frame

This returns t if the faces face1 and face2 have the same attributes for display.

Function: face-differs-from-default-p face &optional frame

This returns 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).

Variable: region-face

This variable's value specifies the face id to use to display characters in the region when it is active (in Transient Mark mode only). The face thus specified takes precedence over all faces that come from text properties and overlays, for characters in the region. See section The Mark, for more information about Transient Mark mode.

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.

Variable: blink-paren-function

The value of this variable should be a function (of no arguments) to be called whenever a char with close parenthesis syntax is inserted. The value of blink-paren-function may be nil, in which case nothing is done.

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

Variable: blink-matching-paren

If this variable is nil, then blink-matching-open does nothing.

Variable: blink-matching-paren-distance

This variable specifies the maximum distance to scan for a matching parenthesis before giving up.

Function: blink-matching-open

This function is the default value of 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, then its context is shown by displaying it 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."
  (let ((blink-matching-paren-distance
        (blink-matching-paren t))

Inverse Video

User Option: inverse-video

This variable controls whether Emacs uses inverse video for all text on the screen. Non-nil means yes, nil means no. The default is nil.

User Option: mode-line-inverse-video

This variable controls the use of inverse video for mode lines. If it is non-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.

Usual Display Conventions

The usual display conventions define how to display each character code. You can override these conventions by setting up a display table (see section Display Tables). Here are the usual display conventions:

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.

User Option: ctl-arrow

This buffer-local variable controls how control characters are displayed. If it is non-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'.

Variable: default-ctl-arrow

The value of this variable is the default value for ctl-arrow in buffers that do not override it. This is the same as executing the following expression:

(default-value 'ctl-arrow)

See section The Default Value of a Buffer-Local Variable.

User Option: tab-width

The value of this variable is the spacing between tab stops used for displaying tab characters in Emacs buffers. The default is 8. Note that this feature is completely independent from the user-settable tab stops used by the command tab-to-tab-stop. See section Adjustable "Tab Stops".

Display Tables

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.

Display Table Format

A display table is actually an array of 261 elements.

Function: make-display-table

This creates and returns a display table. The table initially has 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.

The glyph for the end of a truncated screen line (the default for this is `$'). See section Glyphs.
The glyph for the end of a continued line (the default is `\').
The glyph for indicating a character displayed as an octal character code (the default is `\').
The glyph for indicating a control character (the default is `^').
A vector of glyphs for indicating the presence of invisible lines (the default is `...'). See section Selective Display.

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 ?^ ??)))

Active Display Table

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.

Function: window-display-table window

This function returns window's display table, or nil if window does not have an assigned display table.

Function: set-window-display-table window table

This function sets the display table of window to table. The argument table should be either a display table or nil.

Variable: buffer-display-table

This variable is automatically local in all buffers; its value in a particular buffer is the display table for that buffer, or nil if the buffer does not have any assigned display table.

Variable: standard-display-table

This variable's value is the default display table, used when neither the current buffer nor the window displaying it has an assigned display table. This variable is nil by default.

If neither the selected window nor the current buffer has a display table, and if the variable standard-display-table is nil, then Emacs uses the usual display conventions. 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.

Variable: glyph-table

The value of this variable is the current glyph table. It should be a vector; the gth element defines glyph code g. If the value is nil instead of a vector, then all glyphs are simple (see below).

Here are the possible types of elements in the glyph table:

Define this glyph code as an alias for code integer. This is used with X Windows to specify a face code.

Send the characters in string to the terminal to output this glyph. This alternative is available on character terminals, but not under X.

This glyph is simple. On an ordinary terminal, the glyph code mod 256 is the character to output. With X, the glyph code mod 256 is character to output, and the glyph code divided by 256 specifies the face id number to use while outputting it. See section Faces.

If a glyph code is greater than or equal to the length of the glyph table, that code is automatically simple.

ISO Latin 1

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

(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.)

Function: ding &optional dont-terminate

This function beeps, or flashes the screen (see visible-bell below). It also terminates any keyboard macro currently executing unless dont-terminate is non-nil.

Function: beep &optional dont-terminate

This is a synonym for ding.

Variable: visible-bell

This variable determines whether Emacs should flash the screen to represent a bell. Non-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.

Window Systems

Emacs works with several window systems, most notably the X Window Syste,. Note that 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.

Variable: window-system

This variable tells Lisp programs what window system Emacs is running under. Its value should be a symbol such as x (if Emacs is running under X) or nil (if Emacs is running on an ordinary terminal).

Variable: window-system-version

This variable distinguishes between different versions of the X Window System. Its value is 10 or 11 when using X; nil otherwise.

Variable: window-setup-hook

This variable is a normal hook which Emacs runs after loading your `.emacs' file and the default initialization file (if any), after loading terminal-specific Lisp code, and after running the hook 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.

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