curs_bkgd(3x) Library calls curs_bkgd(3x)
bkgdset, wbkgdset, bkgd, wbkgd, getbkgd - curses window background
manipulation routines
#include <curses.h>
void bkgdset(chtype ch);
void wbkgdset(WINDOW *win, chtype ch);
int bkgd(chtype ch);
int wbkgd(WINDOW *win, chtype ch);
chtype getbkgd(WINDOW *win);
The bkgdset and wbkgdset routines set the background for a window. A
window's background is a chtype consisting of any combination of
attributes (i.e., rendition) and a character:
o The attribute part of the background is combined (OR'ed) with all
non-blank characters that are written into the window with waddch.
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 each character and moves with the
character through any scrolling and insert/delete line/character
operations.
To the extent possible on a particular terminal, the attribute part of
the background is displayed as the graphic rendition of the character
put on the screen.
The bkgd and wbkgd functions set the background property of the current
or specified window and then apply this setting to every character
position in that window. According to X/Open Curses, it should do
this:
o The rendition of every character on the screen is changed to the
new background rendition.
o Wherever the former background character appears, it is changed to
the new background character.
Neither X/Open Curses nor the SVr4 manual pages give details about the
way the rendition of characters on the screen is updated when bkgd or
wbkgd is used to change the background character.
This implementation, like SVr4 curses, does not store the background
and window attribute contributions to each cell separately. It updates
the rendition by comparing the character, non-color attributes and
colors contained in the background. For each cell in the window,
whether or not it is blank:
o The library first compares the character, and if it matches the
current character part of the background, it replaces that with the
new background character.
When bkgdset is used to set the background character, that does not
update each cell in the window. A subsequent call to bkgd will
only modify the character in cells which match the current
background character.
o The library then checks if the cell uses color, i.e., 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 that matches the color in the current
background, the library removes attributes which may have come from
the current background and adds attributes from the new background.
It finishes by setting the cell to use the color from the new
background.
o If the cell uses color, and that does not match the color in the
current background, the library updates only the non-color
attributes, first removing those which may have come from the
current background, and then adding attributes from the new
background.
If the background's character value is zero (0), a space is assumed.
If the terminal does not support color, or if color has not been
started with start_color, the new background character's color
attribute will be ignored.
The getbkgd function returns the given window's current background
character/attribute pair.
These functions are described in the XSI Curses standard, Issue 4. It
specifies that bkgd and wbkgd return ERR on failure, but gives no
failure conditions.
The routines bkgd and wbkgd return the integer OK, unless the library
has not been initialized.
In contrast, the SVr4.0 manual says bkgd and wbkgd may return OK "or a
non-negative integer if immedok is set", which refers to the return
value from wrefresh (used to implement the immediate repainting). The
SVr4 curses wrefresh returns the number of characters written to the
screen during the refresh. This implementation does not do that.
Note that bkgdset and bkgd may be macros.
X/Open Curses mentions that the character part of the background must
be a single-byte value. This implementation, like SVr4, checks to
ensure that, and will reuse the old background character if the check
fails.
These functions are described in the XSI Curses standard, Issue 4
(X/Open Curses).
curses(3x), curs_addch(3x), curs_attr(3x), curs_outopts(3x)
ncurses 6.4 2022-06-25 curs_bkgd(3x)