path: root/man/curs_bkgd.3x

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.\" $Id: curs_bkgd.3x,v 1.76 2025/08/23 22:39:20 tom Exp $
.TH curs_bkgd 3X 2025-08-23 "ncurses @NCURSES_MAJOR@.@NCURSES_MINOR@" "Library calls"
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.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
Every
.I curses
window has a
.I "background character"
property:
in the library's non-wide-character configuration,
it is a
.I curses
character
.RI \%( chtype )
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\%waddch\fP(3X) and \fB\%winsch\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 "bkgd, wbkgd"
.B \%bkgd
and
.B \%wbkgd
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 the 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
(for example,
if it is a control character),
.I \%ncurses
retains the existing 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 "bkgdset, wbkgdset"
.B \%bkgdset
and
.B \%wbkgdset
manipulate the background of the applicable window,
without updating the character cells as
.B \%bkgd
and
.B \%wbkgd
do;
only future writes reflect the updated background.
.SS getbkgd
.B \%getbkgd
returns the given window's background character,
attributes,
and color pair as a
.IR \%chtype "."
.SH RETURN VALUE
.B \%bkgdset
and
.B \%wbkgdset
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,
or
.bP
.I win
is
.IR NULL "."
.PP
.BR \%getbkgd 's
return value is as described above.
.SH NOTES
Unusually,
there is no
.B \%wgetbkgd
function;
.B \%getbkgd
behaves as one would expect
.B \%wgetbkgd
to,
accepting a
.I \%WINDOW
pointer argument.
.PP
.B \%bkgd
and
.B \%bkgdset
may be implemented as macros.
.PP
X/Open Curses mentions that the character part of the background must
be a single-byte value.
.IR \%ncurses ","
like SVr4
.IR curses ","
checks to ensure that it is,
and retains the existing background character if the check fails.
.SH PORTABILITY
X/Open Curses Issue\ 4 describes these functions.
It indicates that
.IR bkgd ","
.IR wbkgd ","
and
.I getbkgd
return
.I ERR
on failure
(in the case of the last,
this value is cast to
.IR \%chtype ),
but specifies no error conditions for them.
.PP
SVr4 documentation
.\" SVID 4, vol. 3, p. 482
says that
.I \%bkgd
and
.I \%wbkgd
return
.I OK
\*(``or a non-negative integer if
.I \%immedok() \" Courier roman in source
is set\*('',
referring to the return value from
.IR \%wrefresh ","
which in SVr4 returns a count of characters
written to the window if its
.I \%immedok
property is set;
.\" ...though its wrefresh() man page says nothing about immedok()...
in
.IR \%ncurses ","
it does not.
.PP
Neither X/Open Curses nor the SVr4 manual pages detail how the rendition
of characters in the window updates when
.I \%bkgd
or
.I \%wbkgd
changes the background character.
.IR \%ncurses ","
like SVr4
.IR curses ","
does not
(in its non-wide-character configuration)
store the background and window attribute contributions to each
character cell separately.
.SH HISTORY
SVr3.1 (1987)
introduced these functions.
.SH SEE ALSO
\fB\%curs_bkgrnd\fP(3X) describes the corresponding functions in the
wide configuration of
.IR \%ncurses "."
.PP
\fB\%curses\fP(3X),
\fB\%curs_addch\fP(3X),
\fB\%curs_attr\fP(3X)