curs_bkgd(3X)                    Library calls                   curs_bkgd(3X)

NAME

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

SYNOPSIS

     #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);

DESCRIPTION

     Every curses window has a background character property: in the library's
     non-wide-character configuration, it is a curses character (chtype) 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.

     o   The attribute part of the background character combines with all non-
         blank character cells in the window, as populated by the waddch(3X)
         and winsch(3X) families of functions (and those that call them).

     o   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.

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.

     o   The rendition of every character in the window changes to the new
         background rendition.

     o   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.

     o   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.

     o   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.

     o   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.

     o   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 (for example, if it is a
     control character), ncurses retains the existing 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(3X), ncurses ignores the new background
     character's color pair selection.

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

     o   the curses screen has not been initialized, or

     o   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 it
     is, 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", referring to the return value from
     wrefresh, which in SVr4 returns a count of characters written to the
     window if its immedok property is set; in ncurses, it does not.

     Neither X/Open Curses nor the SVr4 manual pages detail how the rendition
     of characters in the window updates when bkgd or wbkgd changes the
     background character.  ncurses, like SVr4 curses, does not (in its non-
     wide-character configuration) store the background and window attribute
     contributions to each character cell separately.

HISTORY

     SVr3.1 (1987) introduced these functions.

SEE ALSO

     curs_bkgrnd(3X) describes the corresponding functions in the wide
     configuration of ncurses.

     curses(3X), curs_addch(3X), curs_attr(3X)

ncurses 6.6                       2025-08-23                     curs_bkgd(3X)