double fmod(double x,
            double y);

fmod computes the floating-point remainder of x divided by y. #b #this returns the value xn y, for some integer n such that, if y is nonzero, the result has the same sign as x and magnitude less than the magnitude of y.

fmod (NaN, y) is NaN. fmod (x, NaN) is NaN. fmod (± 0, y) is ± 0 for y not zero.
fmod (∞, y) is NaN.
fmod (x, 0) is NaN.
fmod (x, ± ∞) is x for x not infinite.