bkgrnd(3NCURSES) Library calls bkgrnd(3NCURSES)
NAME
bkgrnd, wbkgrnd, bkgrndset, wbkgrndset, getbkgrnd, wgetbkgrnd - manipu-
late background of a curses window of wide characters
SYNOPSIS
#include <curses.h>
int bkgrnd(const cchar_t *wch);
int wbkgrnd(WINDOW *win, const cchar_t *wch);
void bkgrndset(const cchar_t *wch);
void wbkgrndset(WINDOW *win, const cchar_t *wch);
int getbkgrnd(cchar_t *wch);
int wgetbkgrnd(WINDOW *win, cchar_t *wch);
DESCRIPTION
Every curses window has a background character property: in the li-
brary's wide configuration, it is a curses complex character (cchar_t)
that combines a set of attributes (and, if colors are enabled, a color
pair identifier) with a character code. When erasing (parts of) a win-
dow, curses replaces the erased cells with the background character.
curses also uses the background character when writing characters to a
populated window.
• The attribute part of the background character combines with all
non-blank character cells in the window, as populated by the
wadd_wch(3NCURSES) and wins_wch(3NCURSES) families of functions (and
those that call them).
• 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.
bkgrnd, wbkgrnd
bkgrnd and wbkgrnd set the background property of stdscr or the speci-
fied window and then apply this setting to every character cell in that
window.
• The rendition of every character in the window changes to the new
background rendition.
• 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 to following procedure to each cell in the window, whether or
not it is blank.
• ncurses first compares the cell's character to the previously speci-
fied background character; if they match, ncurses writes the new
background character to the cell.
• 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 char-
acter.
• 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 back-
ground. It finishes by setting the cell's background to use the new
window background color.
• 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, ncurses reuses the old
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 ini-
tialized with start_color(3NCURSES), ncurses ignores the new background
character's color pair selection.
bkgrndset, wbkgrndset
bkgrndset and wbkgrndset manipulate the background of the applicable
window, without updating the character cells as bkgrnd and wbkgrnd do;
only future writes reflect the updated background.
getbkgrnd, wgetbkgrnd
getbkgrnd and wgetbkgrnd respectively obtain stdscr's or the given win-
dow's background character, attributes, and color pair, and store it in
their wch argument.
RETURN VALUE
bkgrndset and wbkgrndset do not return a value.
Functions returning an int return ERR upon failure and OK upon success.
In ncurses, failure occurs if
• the curses screen has not been initialized,
• win is NULL, or
• wch is NULL.
NOTES
bkgrnd, bkgrndset, and getbkgrnd may be implemented as macros.
Unlike their counterparts in the non-wide configuration of ncurses,
getbkgrnd and wgetbkgrnd store the background character in a modifiable
cchar_t parameter, rather than supplying it as the return value.
PORTABILITY
X/Open Curses Issue 4 describes these functions. It specifies no error
conditions for them.
X/Open Curses does not provide details of how the rendition is updated.
ncurses follows the approach used in SVr4 curses's non-wide functions
for manipulating the window background.
HISTORY
X/Open Curses Issue 4 (1995) initially specified these functions.
SEE ALSO
bkgd(3NCURSES) describes the corresponding functions in the non-wide
configuration of ncurses.
ncurses(3NCURSES), add_wch(3NCURSES), attr(3NCURSES)
ncurses 6.5 2025-02-15 bkgrnd(3NCURSES)
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