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);

The background of a curses window (in the library's non-“wide” configuration) is a chtype combining a set of attributes (see curs_attr(3X)) with a 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 waddch(3X) and winsch(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.

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.

  • 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.

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)

2024-04-20 ncurses 6.5