A pattern of
binary values which is combined
with some value using
bitwise AND with the result that bits
in the value in positions where the mask is zero are also set
to zero. For example, if, in
C, we want to test if bits 0
or 2 of x are set, we can write
int mask = 5; /* binary 101 */
if (x & mask) ...
A bit mask might also be used to set certain bits using
bitwise OR, or to invert them using bitwise
exclusive OR.