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:
.IP • 4 The rendition of every character on the screen is changed to the new background rendition.
.IP • 4 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 character.
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:
.IP • 4 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.
.IP • 4 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 background character.
.IP • 4 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.
.IP • 4 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.
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.