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curs_bkgd(3)                    Library calls                   curs_bkgd(3)


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

       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, 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 colors.  The library applies to
       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 blank character; if they match, ncurses writes the new
           blank character to the cell.

       o   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.

       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.

       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, 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 obtains the given window's background character and attribute
       combination.


RETURN VALUE

       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.


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 will reuse the old background character if the check fails.


PORTABILITY

       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.


SEE ALSO

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

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

ncurses 6.5                       2024-04-20                     curs_bkgd(3)

ncurses 6.5 - Generated Wed May 1 15:41:39 CDT 2024
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