plucky (3) bkgd.3ncurses.gz

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 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.

       •   The attribute part of the background character combines with all non-blank  character  cells  in  the
           window,  as  populated  by the waddch(3NCURSES) and winsch(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.

   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 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 (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(3NCURSES),
       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

       •   the curses screen has not been initialized, or

       •   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  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)