curs_bkgrnd(3X) Library calls curs_bkgrnd(3X)

bkgrnd, wbkgrnd, bkgrndset, wbkgrndset, getbkgrnd, wgetbkgrnd - manipulate background of a curses window of wide characters

#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);

The background of a curses window (in the library's “wide” configuration) is a cchar_t combining a set of attributes (see curs_attr(3X)) with a complex character called the blank character.

The blank character is a spacing character that populates a window's character cells when their contents are erased without replacement. The background's attributes are combined with all non-blank characters written to the window, as with the wadd_wch(3X) and wins_wch(3X) families of functions.

The blank character and attributes of the background combine with characters written to the window as described below. 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.

bkgrnd and wbkgrnd set the background property of stdscr or the specified 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 colors. 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 specified blank character; if they match, ncurses writes the new blank character to the cell.
  • 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.
  • 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.
  • 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.

ncurses treats a background character value of zero (0) as a blank character.

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.

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.

The getbkgrnd and wgetbkgrnd functions obtain the background character and attribute pair of stdscr or the specified window and store it via the wch pointer.

bkgrndset and wbkgrndset do not return a value.

The other functions return ERR upon failure and OK upon success. In ncurses, failure occurs if

  • a WINDOW pointer win is null, or
  • a cchar_t pointer wch is null.

bkgrnd, bkgrndset, and getbkgrnd may be implemented as macros.

Unlike their counterparts in the non-“wide” configuration of ncurses, getbkgrnd and wgetbkgrnd supply the background character and attribute in a modifiable cchar_t parameter, not as the return value.

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. This implementation follows the approach used in SVr4 curses.

curs_bkgd(3X) describes the corresponding functions in the non-“wide” configuration of ncurses.

curses(3X), curs_add_wch(3X), curs_attr(3X)

2024-04-20 ncurses 6.5