The bkgdset and wbkgdset routines manipulate the background of
the named window. The window background is a chtype consisting
of any combination of attributes (i.e., rendition) and a
character. The attribute part of the background is combined
(OR'ed) with all non-blank characters that are written into the
window with waddch. Both the character and attribute parts of
the background are combined with the blank characters. The
background becomes a property of the character and moves with the
character through any scrolling and insert/delete line/character
To the extent possible on a particular terminal, the attribute
part of the background is displayed as the graphic rendition of
the character put on the screen.
The bkgd and wbkgd functions set the background property of the
current or specified window and then apply this setting to every
character position in that window. According to X/Open Curses,
it should do this:
• The rendition of every character on the screen is changed to
the new background rendition.
• Wherever the former background character appears, it is
changed to the new background character.
Neither X/Open Curses nor the SVr4 manual pages give details
about the way the rendition of characters on the screen is
updated when bkgd or wbkgd is used to change the background
This implementation, like SVr4 curses, does not store the
background and window attribute contributions to each cell
separately. It updates the rendition by comparing the character,
non-color attributes and colors contained in the background. For
each cell in the window, whether or not it is blank:
• The library first compares the character, and if it matches
the current character part of the background, it replaces
that with the new background character.
• The library then checks if the cell uses color, i.e., its
color pair value is nonzero. If not, it simply replaces the
attributes and color pair in the cell with those from the new
• If the cell uses color, and that matches the color in the
current background, the library removes attributes which may
have come from the current background and adds attributes
from the new background. It finishes by setting the cell to
use the color from the new background.
• If the cell uses color, and that does not match the color in
the current background, the library updates only the non-
color attributes, first removing those which may have come
from the current background, and then adding attributes from
the new background.
If the background's character value is zero, a space is assumed.
If the terminal does not support color, or if color has not been
started with start_color, the new background character's color
attribute will be ignored.
The getbkgd function returns the given window's current
background character/attribute pair.
These functions are described in the XSI Curses standard, Issue
4. It specifies that bkgd and wbkgd return ERR on failure, but
gives no failure conditions.
The routines bkgd and wbkgd return the integer OK, unless the
library has not been initialized.
In contrast, the SVr4.0 manual says bkgd and wbkgd may 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). The SVr4 curses wrefresh returns the number of
characters written to the screen during the refresh. This
implementation does not do that.
Note that bkgdset and bkgd may be macros.
X/Open Curses mentions that the character part of the background
must be a single-byte value. This implementation, like SVr4,
checks to ensure that, and will reuse the old background
character if the check fails.
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