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.\" $Id: curs_bkgrnd.3x,v 1.58 2025/02/15 18:40:23 tom Exp $
.TH curs_bkgrnd 3X 2025-02-15 "ncurses 6.5" "Library calls"
.de bP
.ie n  .IP \(bu 4
.el    .IP \(bu 2
..
.SH NAME
\fB\%bkgrnd\fP,
\fB\%wbkgrnd\fP,
\fB\%bkgrndset\fP,
\fB\%wbkgrndset\fP,
\fB\%getbkgrnd\fP,
\fB\%wgetbkgrnd\fP \-
manipulate background of a \fIcurses\fP window of wide characters
.SH SYNOPSIS
.nf
\fB#include <curses.h>
.PP
\fBint bkgrnd(const cchar_t *\fIwch\fP);
\fBint wbkgrnd(WINDOW *\fIwin\fP, const cchar_t *\fIwch\fP);
.PP
\fBvoid bkgrndset(const cchar_t *\fIwch\fP);
\fBvoid wbkgrndset(WINDOW *\fIwin\fP, const cchar_t *\fIwch\fP);
.PP
\fBint getbkgrnd(cchar_t *\fIwch\fP);
\fBint wgetbkgrnd(WINDOW *\fIwin\fP, cchar_t *\fIwch\fP);
.fi
.SH DESCRIPTION
Every
.I curses
window has a
.I "background character"
property:
in the library's wide configuration,
it is a
.I curses
complex character
.RI \%( cchar_t )
that combines a set of attributes
(and,
if colors are enabled,
a color pair identifier)
with a character code.
When erasing
(parts of)
a window,
.I curses
replaces the erased cells with the background character.
.PP
.I curses
also uses the background character when writing characters to a
populated window.
.bP
The attribute part of the background character combines with all
non-blank character cells in the window,
as populated by the \fB\%wadd_wch\fP(3X) and \fB\%wins_wch\fP(3X)
families of functions
(and those that call them).
.bP
Both the character code and attributes of the background character
combine with blank character cells in the window.
.PP
The background character's set of attributes becomes a property
of the character cell
and move with it through any scrolling
and insert/delete line/character operations.
To the extent possible on the terminal type,
.I curses
displays the attributes of the background character
as the graphic rendition of a character cell on the display.
.SS "bkgrnd, wbkgrnd"
.B \%bkgrnd
and
.B \%wbkgrnd
set the background property of
.B \%stdscr
or the specified window and then apply this setting to
every character cell in that window.
.bP
The rendition of every character in the window changes to the new
background rendition.
.bP
Wherever the former background character appears,
it changes to the new background character.
.PP
.I \%ncurses
updates the rendition of each character cell by comparing the character,
non-color attributes,
and color pair selection.
The library applies to following procedure to each cell in the window,
whether or not it is blank.
.bP
.I \%ncurses
first compares the cell's character to the previously specified
background character;
if they match,
.I \%ncurses
writes the new background character to the cell.
.bP
.I \%ncurses
then checks whether the cell uses color;
that is,
its color pair value is nonzero.
If not,
it simply replaces the attributes and color pair in the cell with those
from the new background character.
.bP
If the cell uses color,
and its background color matches that of the current window background,
.I \%ncurses
removes attributes that may have come from the current background and
adds those from the new background.
It finishes by setting the cell's background to use the new window
background color.
.bP
If the cell uses color,
and its background color does not match that of the current window
background,
.I \%ncurses
updates only the non-color attributes,
first removing those that may have come from the current background,
and then adding attributes from the new background.
.PP
If the new background's character is non-spacing,
.I \%ncurses
reuses the old background character,
except for one special case:
.I \%ncurses
treats a background character code of zero (0) as a space.
.PP
If the terminal does not support color,
or if color has not been initialized with \fB\%start_color\fP(3X),
.I \%ncurses
ignores the new background character's color pair selection.
.SS "bkgrndset, wbkgrndset"
.B \%bkgrndset
and
.B \%wbkgrndset
manipulate the background of the applicable window,
without updating the character cells as
.B \%bkgrnd
and
.B \%wbkgrnd
do;
only future writes reflect the updated background.
.SS "getbkgrnd, wgetbkgrnd"
.B \%getbkgrnd
and
.B \%wgetbkgrnd
respectively obtain
.BR \%stdscr 's
or the given window's background character,
attributes,
and color pair,
and store it in their
.I wch
argument.
.SH RETURN VALUE
.B \%bkgrndset
and
.B \%wbkgrndset
do not return a value.
.PP
Functions returning an
.I int
return
.B ERR
upon failure and
.B OK
upon success.
In
.IR \%ncurses ","
failure occurs if
.bP
the
.I curses
screen has not been initialized,
.bP
.I win
is
.IR NULL ","
or
.bP
.I wch
is
.IR NULL "."
.SH NOTES
.BR \%bkgrnd ","
.BR \%bkgrndset ","
and
.B \%getbkgrnd
may be implemented as macros.
.PP
Unlike their counterparts in the non-wide configuration of
.IR \%ncurses ","
.B \%getbkgrnd
and
.B \%wgetbkgrnd
store the background character in a modifiable
.I \%cchar_t
parameter,
rather than supplying it as the return value.
.SH PORTABILITY
X/Open Curses Issue\ 4 describes these functions.
It specifies no error conditions for them.
.PP
X/Open Curses does not provide details of how the rendition is updated.
.I \%ncurses
follows the approach used in SVr4
.IR curses 's
non-wide functions for manipulating the window background.
.SH HISTORY
X/Open Curses Issue\ 4 (1995) initially specified these functions.
.SH SEE ALSO
\fB\%curs_bkgd\fP(3X) describes the corresponding functions in the
non-wide configuration of
.IR \%ncurses "."
.PP
\fB\%curses\fP(3X),
\fB\%curs_add_wch\fP(3X),
\fB\%curs_attr\fP(3X)