DOKK / manpages / debian 13 / ncurses-doc / getbkgrnd.3ncurses.en
bkgrnd(3NCURSES) Library calls bkgrnd(3NCURSES)

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);

Every curses window has a background character property: in the library'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 window, 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 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 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 specified 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 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.

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 initialized with start_color(3NCURSES), ncurses ignores the new background character's color pair selection.

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 and wgetbkgrnd respectively obtain stdscr's or the given window's background character, attributes, and color pair, and store it in their wch argument.

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.

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.

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.

X/Open Curses Issue 4 (1995) initially specified these functions.

bkgd(3NCURSES) describes the corresponding functions in the non-wide configuration of ncurses.

ncurses(3NCURSES), add_wch(3NCURSES), attr(3NCURSES)

2025-02-15 ncurses 6.5