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
acos(3), asin(3), atan(3),