curs_bkgd(3x) Library calls curs_bkgd(3x)
bkgdset, wbkgdset, bkgd, wbkgd, getbkgd - manipulate background of a
curses window of characters
#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.
ncurses, 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.
Functions returning an int return OK on success. bkgd returns ERR if
the library has not been initialized. wbkgd and getbkgd return ERR if
the WINDOW pointer argument is null.
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). SVr4
curses wrefresh returns the number of characters written to the screen
during the refresh. ncurses does not do that.
Unusually, there is no wgetbkgd function; getbkgd behaves as one would
expect wgetbkgd to, accepting a WINDOW pointer argument.
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. ncurses, like SVr4 curses, 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. It
specifies that bkgd and wbkgd return ERR on failure, but gives no
failure conditions.
curses(3x), curs_addch(3x), curs_attr(3x), curs_outopts(3x)
ncurses 6.4 2023-12-02 curs_bkgd(3x)