atan2 - arc tangent function of two variables
double atan2(double y, double x);
The atan2() function calculates
the arc tangent of the two variables x and y. It is similar to calculating
the arc tangent of y / x, except that the signs of both arguments are used
to determine the quadrant of the result.
The atan2() function
returns the result in radians, which is between -PI and PI (inclusive).
SVID 3, POSIX, BSD 4.3, ISO 9899