/sys$common/syshlp/helplib.hlb CC, Run-time functions, atan2  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Returns a value in the range -pi to pi, which is the arc tangent of
  x/y, where x and y are the two arguments.

  Syntax:

       #include <math.h>

       double atan2(double x, double y);