double hypot(double x, double y);

This function computes the length of the hypotenuse of a right-angled triangle whose other two sides have length x and y. Equivalently, it computes the length of the vector (x,y) in Cartesian coordinates. Using hypot(x,y) is better than sqrt(x*x+y*y) because some values of x and y could cause x * x + y * y to overflow even though its square root would not.

hypot returns an ERANGE error when the result does not fit in a double.