module constants
    
   integer, parameter :: i4b = selected_int_kind(9)
   integer, parameter :: ndp = kind(1.0d0)
   
   real(ndp), parameter :: zero = 0.0d0
   real(ndp), parameter :: one = 1.0d0

end module constants

module common

   use constants

   implicit real(ndp) (a-h,o-z)
   implicit integer(i4b) (i-n)
   
end module common   

program main

   use constants
   use common
   use procedures
   
   implicit real(ndp) (a-h,o-z)
   implicit integer(i4b) (i-n)
   
   x = 2.0d0
   call callf(f,x,y)
   
   write(6,"(2f10.6)") x,y
   
end program main   
   
module procedures

   use constants

   implicit real(ndp) (a-h,o-z)
   implicit integer(i4b) (i-n)

   contains
   
   subroutine callf(fct,x,y)
      
      y = fct(x)
      
      return
      
   end subroutine callf   

   double precision function f(x)
   
      f = x*x
      
      return
      
   end function f

   subroutine wait
   
      write(6,"(' Waiting...')")
      read *

      return
      
   end subroutine wait
      
end module procedures