atan2f
extern float atan2f(float y, float x);
float angle = atan2f(y, x);
__imag__ result = M_LOG10E * atan2f (__imag__ x, __real__ x);
__imag__ result = atan2f (__imag__ x, __real__ x);
return atan2f(cimagf(z), crealf(z));
t = 0.5f * atan2f(2.0f * x, a);