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 property. In the library's non-
wide configuration, this property is a chtype which combines a set of
attributes with the background character (see curs_attr(3x)). The
background character is a spacing character.
When erasing parts of the screen, curses fills the cells with the
background character. curses also uses the window background when
writing characters to the screen:
o The attribute part of the background is combined with all non-blank
characters that are written into the window, as with the waddch(3x)
and winsch(3x) families of functions.
o Both the character and attribute parts of the background are
combined with blank characters that are written into the window.
The background becomes a property of the character and moves with it
through any scrolling and insert/delete line/character operations.
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.
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 colors. The library applies to
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 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.
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 nonspacing, ncurses reuses the old
background character, except for one special case: ncurses treats a
background character value 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 attribute.
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 obtains the given window's background character and attribute
combination.
Functions returning an int return OK on success. bkgd returns ERR if
the library has not been initialized. wbkgd and getbkgd return ERR if
a WINDOW pointer argument is null.
bkgdset and wbkgdset do not return a value.
getbkgd returns a window's background character and attribute
combination.
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, and will reuse the old background character if the check fails.
X/Open Curses, Issue 4 describes these functions. It specifies that
bkgd, wbkgd, and getbkgd return ERR on failure (in the case of the
last, this value is cast to chtype), but describes no failure
conditions.
The SVr4.0 manual says that bkgd and wbkgd may return OK "or a non-
negative integer if immedok is set", which refers to the return value
from wrefresh(3x), used to implement the immediate repainting. SVr4
curses's wrefresh returns the number of characters written to the
screen during the refresh. ncurses does not do that.
Neither X/Open Curses nor the SVr4 manual pages detail how the
rendition of characters on the screen 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.
curs_bkgrnd(3x) describes the corresponding functions in the wide
configuration of ncurses.
curses(3x), curs_addch(3x), curs_attr(3x)
ncurses 6.5 2024-09-22 curs_bkgd(3x)