! This function is not thread-safe, because it writes to global variables.
Real Function f(x,y)
  Real,Intent(In) :: x,y
  Real :: z
  Common/c/z(2)
  Real,External :: sum_of_z
  z(1) = x
  z(2) = y
  f = sum_of_z()
End Function

! This function is thread-safe, as although it reads from global variables,
! it does not change any.
Real Function sum_of_z()
  Real z
  Common/c/z(2)
  sum_of_z = Sum(z)
End Function

! This function is thread-safe, as it has no side-effects.
Real Function g(x,y)
  Real x,y
  g = x + y**2
End Function