Determines whether the sign of its argument value is negative.
int signbit(real-floating x); (C99)
If the sign of the argument is negative, a value other than 0 is returned.
If the sign of the argument is positive, 0 is returned.
This macro determines whether the sign of its argument value is negative.
If a type other than the floating-point type is passed to the argument, correct operation is not guaranteed.