float atan2f(float y, float x);
atan2f returns the value, in radians, of the inverse circular tangent of y divided by x using the signs of x and y to compute the quadrant of the return value. The principal value lies in the interval [−½π, +½π] radians.
If x = y = 0, errno is set to EDOM and atan2f returns HUGE_VALF.
atan2f(x, NaN) is NaN.
atan2f(NaN, x) is NaN.
atan2f(±0, +(anything but NaN)) is ±0.
atan2f(±0, −(anything but NaN)) is ±π.
atan2f(±(anything but 0 and NaN), 0) is ±½π.
atan2f(±(anything but ∞ and NaN), +∞) is ±0.
atan2f(±(anything but ∞ and NaN), −∞) is ±π.
atan2f(±∞, +∞) is ±¼π.
atan2f(±∞, −∞) is ±¾π.
atan2f(±∞, (anything but 0, NaN, and ∞)) is ±½π.