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.\" $Id: curs_bkgd.3x,v 1.59 2024/03/23 19:58:58 tom Exp $
.TH bkgd 3NCURSES 2024-03-23 "ncurses 6.4" "Library calls"
.ie \n(.g \{\
.ds `` \(lq
.ds '' \(rq
.\}
.el \{\
.ie t .ds `` ``
.el   .ds `` ""
.ie t .ds '' ''
.el   .ds '' ""
.\}
.
.de bP
.ie n  .IP \(bu 4
.el    .IP \(bu 2
..
.SH NAME
\fB\%bkgdset\fP,
\fB\%wbkgdset\fP,
\fB\%bkgd\fP,
\fB\%wbkgd\fP,
\fB\%getbkgd\fP \-
manipulate background of a \fIcurses\fR window of characters
.SH SYNOPSIS
.nf
\fB#include <curses.h>
.PP
\fBint bkgd(chtype \fIch\fP);
\fBint wbkgd(WINDOW *\fIwin\fP, chtype \fIch\fP);
.PP
\fBvoid bkgdset(chtype \fIch\fP);
\fBvoid wbkgdset(WINDOW *\fIwin\fP, chtype \fIch\fP);
.PP
\fBchtype getbkgd(WINDOW *\fIwin\fP);
.fi
.SH DESCRIPTION
The
.I background
of a
.I curses
window
(in the library's non-\*(``wide\*('' configuration)
is a
.I \%chtype
combining a set of attributes
(see \fB\%attr\fP(3NCURSES))
with a character called the
.I "blank character."
.PP
The blank character is a spacing character that populates a window's
character cells when their contents are erased without replacement.
The background's attributes are combined with all non-blank characters
written to the window,
as with the \fB\%waddch\fP(3NCURSES) and \fB\%winsch\fP(3NCURSES) families of
functions.
.PP
The blank character and attributes of the background combine with
characters written to the window as described below.
The background becomes a property of the character and moves with it
through any scrolling and insert/delete line/character operations.
.PP
To the extent possible on a given terminal,
the attribute part of the background is displayed as the graphic
rendition of the character put on the screen.
.SS "bkgd, wbkgd"
\fB\%bkgd\fP and \fB\%wbkgd\fP set the background property of
\fB\%stdscr\fP 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 colors.
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 blank
character;
if they match,
.I \%ncurses
writes the new blank character to the cell.
.bP
.I \%ncurses
then checks if 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
.I \%ncurses
treats a background character value of zero (0) as a blank character.
.PP
If the terminal does not support color,
or if color has not been initialized with \fB\%start_color\fP(3NCURSES),
.I \%ncurses
ignores the new background character's color attribute.
.SS "bkgdset, wbkgdset"
\fB\%bkgdset\fP and \fB\%wbkgdset\fP manipulate the background of
the applicable window,
without updating the character cells as \fB\%bkgd\fP and
\fB\%wbkgd\fP do;
only future writes reflect the updated background.
.SS getbkgd
\fB\%getbkgd\fP obtains the given window's background character and
attribute combination.
.SH RETURN VALUE
Functions returning an \fIint\fP return \fBOK\fP on success.
\fB\%bkgd\fP returns \fBERR\fP if the library has not been initialized.
\fB\%wbkgd\fP and \fB\%getbkgd\fP return \fBERR\fP if a \fI\%WINDOW\fP
pointer argument is null.
.PP
\fB\%bkgdset\fP and \fBwbkgdset\fP do not return a value.
.PP
\fB\%getbkgd\fP returns a window's background character and attribute
combination.
.SH NOTES
Unusually,
there is no \fB\%wgetbkgd\fP function;
\fB\%getbkgd\fP behaves as one would expect \fB\%wgetbkgd\fP to,
accepting a \fI\%WINDOW\fP pointer argument.
.PP
\fB\%bkgd\fP and
\fB\%bkgdset\fP
may be implemented as macros.
.PP
X/Open Curses mentions that the character part of the background must
be a single-byte value.
\fI\%ncurses\fP,
like SVr4 \fIcurses\fP,
checks to ensure that,
and will reuse the old background character if the check fails.
.SH PORTABILITY
X/Open Curses, Issue 4, describes these functions.
It specifies that
\fB\%bkgd\fP,
\fB\%wbkgd\fP,
and
\fB\%getbkgd\fP
return \fBERR\fP on failure
(in the case of the last,
this value is cast to
.IR \%chtype ),
but describes no failure conditions.
.PP
The SVr4.0 manual says that \fB\%bkgd\fP and \fB\%wbkgd\fP may return
\fBOK\fP
\*(``or a non-negative integer if \fB\%immedok\fP is set\*('',
which refers to the return value from \fB\%wrefresh\fP(3NCURSES),
used to implement the immediate repainting.
SVr4 \fIcurses\fP's \fB\%wrefresh\fP returns the number of characters
written to the screen during the refresh.
\fI\%ncurses\fP does not do that.
.PP
Neither X/Open Curses nor the SVr4 manual pages detail how the rendition
of characters on the screen updates when \fB\%bkgd\fP or \fB\%wbkgd\fP
changes the background character.
.IR \%ncurses ,
like SVr4
.IR curses ,
does not
(in its non-\*(``wide\*('' configuration)
store the background and window attribute contributions to each
character cell separately.
.SH SEE ALSO
\fB\%bkgrnd\fP(3NCURSES) describes the corresponding functions in the
\*(``wide\*('' configuration of
.IR \%ncurses .
.PP
\fB\%ncurses\fP(3NCURSES),
\fB\%addch\fP(3NCURSES),
\fB\%attr\fP(3NCURSES)