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.