https://invisible-island.net/ncurses/man/
curs_bkgd(3x) Library calls curs_bkgd(3x)
bkgdset, wbkgdset, bkgd, wbkgd, getbkgd - manipulate background of a
curses window of characters
#include <curses.h>
int bkgd(chtype ch);
int wbkgd(WINDOW *win, chtype ch);
void bkgdset(chtype ch);
void wbkgdset(WINDOW *win, chtype ch);
chtype getbkgd(WINDOW *win);
Every curses window has a background character property: in the
library's non-wide configuration, it is a curses character (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, curses replaces the erased cells with the background character.
curses also uses the background character when writing characters to a
populated window.
o The attribute part of the background character combines with all
non-blank character cells in the window, as populated by the
waddch(3x) and winsch(3x) families of functions (and those that
call them).
o Both the character code and attributes of the background character
combine with blank character cells in the window.
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, curses displays the attributes of the background character as the
graphic rendition of a character cell on the display.
bkgd and wbkgd set the background property of stdscr or the specified
window and then apply this setting to every character cell in that
window.
o The rendition of every character in the window changes to the new
background rendition.
o Wherever the former background character appears, it changes to the
new background character.
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.
o ncurses first compares the cell's character to the previously
specified background character; if they match, ncurses writes the
new background character to the cell.
o 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.
o If the cell uses color, and its background color matches that of
the current window background, 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.
o If the cell uses color, and its background color does not match
that of the current window background, 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.
If the new background's character is non-spacing (for example, if it is
a control character), ncurses retains the existing background
character, except for one special case: ncurses treats a background
character code of zero (0) as a space.
If the terminal does not support color, or if color has not been
initialized with start_color(3x), ncurses ignores the new background
character's color pair selection.
bkgdset and wbkgdset manipulate the background of the applicable
window, without updating the character cells as bkgd and wbkgd do; only
future writes reflect the updated background.
getbkgd returns the given window's background character, attributes,
and color pair as a chtype.
bkgdset and wbkgdset do not return a value.
Functions returning an int return ERR upon failure and OK upon success.
In ncurses, failure occurs if
o the curses screen has not been initialized, or
o win is NULL.
getbkgd's return value is as described above.
Unusually, there is no wgetbkgd function; getbkgd behaves as one would
expect wgetbkgd to, accepting a WINDOW pointer argument.
bkgd and bkgdset may be implemented as macros.
X/Open Curses mentions that the character part of the background must
be a single-byte value. ncurses, like SVr4 curses, checks to ensure
that it is, and retains the existing background character if the check
fails.
X/Open Curses Issue 4 describes these functions. It indicates that
bkgd, wbkgd, and getbkgd return ERR on failure (in the case of the
last, this value is cast to chtype), but specifies no error conditions
for them.
SVr4 documentation says that bkgd and wbkgd return OK "or a non-
negative integer if immedok() is set", referring to the return value
from wrefresh, which in SVr4 returns a count of characters written to
the window if its immedok property is set; in ncurses, it does not.
Neither X/Open Curses nor the SVr4 manual pages detail how the
rendition of characters in the window updates when bkgd or wbkgd
changes the background character. ncurses, like SVr4 curses, does not
(in its non-wide configuration) store the background and window
attribute contributions to each character cell separately.
SVr3.1 (1987) introduced these functions.
curs_bkgrnd(3x) describes the corresponding functions in the wide
configuration of ncurses.
curses(3x), curs_addch(3x), curs_attr(3x)
ncurses 6.5 2025-01-18 curs_bkgd(3x)