[子程序] 定义外部函数要怎么调用？

program main
    implicit none
    real,external::vvmat(2,2)
    real dh0(2,2),v1(2)
    integer n
    n=2
    dh0=vvmat(v1,v1,n)
end program main

function vvmat(a,b,n)
implicit none
    integer n,i,j
    real(kind=dp) a(n),b(n),vvmat(n,n)
    do i=1,n
        do j=1,n
            vvmat(i,j)=a(i)*b(j)
        end do
    end do
end function vvmat

real,external::vvmat