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bkgd(3NCURSES) Library calls bkgd(3NCURSES)

NAME

bkgdset, wbkgdset, bkgd, wbkgd, getbkgd - manipulate background of a curses window of characters

SYNOPSIS

#include <ncursesw/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);

DESCRIPTION

Every curses window has a background property. In the library's non-wide configuration, this property is a chtype which combines a set of attributes with the background character (see attr(3NCURSES)). The background character is a spacing character.

When erasing parts of the screen, curses fills the cells with the background character. curses also uses the window background when writing characters to the screen.

  • The attribute part of the background combines with all non-blank characters written into the window, as with the waddch(3NCURSES) and winsch(3NCURSES) families of functions.
  • Both the character and attribute parts of the background combine with blank characters that are written into the window.

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, curses displays the attributes of the background as the graphic rendition of the character put on the screen.

bkgd, wbkgd

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 color pair selection. The library applies the 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 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.

If the new background's character is nonspacing, ncurses retains the existing background character, except for one special case: ncurses treats a background character value 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 attribute.

bkgdset, wbkgdset

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

getbkgd returns the given window's background character, attributes, and color pair as a chtype.

RETURN VALUE

bkgdset and wbkgdset do not return a value

Functions returning an int return ERR upon failure and OK upon success. In ncurses, failure occurs if

  • if the library has not been initialized, or
  • if win is NULL.

getbkgd's return value is as described above.

NOTES

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 retains the existing background character if the check fails.

PORTABILITY

X/Open Curses, Issue 4 describes these functions. It indicates that bkgd, wbkgd, and getbkgd return ERR on failure (in the case of the last, this value is cast to chtype), but specifies no error conditions for them.

SVr4 documentation says that bkgd and wbkgd 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's wrefresh returns the number of characters written to the screen during the refresh; that of ncurses does not.

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.

HISTORY

SVr3.1 (1987) introduced these functions.

SEE ALSO

bkgrnd(3NCURSES) describes the corresponding functions in the wide configuration of ncurses.

ncurses(3NCURSES), addch(3NCURSES), attr(3NCURSES)

2024-12-28 ncurses 6.5