REAL, ALLOCATABLE :: u(:,:), f(:,:), r(:,:)

ALLOCATE(u(0:nx, 0:ny), f(0:nc, 0:ny), r(0:nx, 0:ny))

INTERFACE

SUBROUTINE residual(r,u,f)

REAL r(:,:), u(:,:), f(:,:)

END SUBROUTINE

END INTERFACE

CALL residual(r,u,f)

CALL residual(r(0:nx:2, 0:ny:2), &

   (0:nx:2, 0:ny:2), f(0:nx:2, 0:ny:2))