error: reduction variable ‘v1’ is private in outer context

Question

I have the following Fortran code

program hello

    use omp_lib
    implicit none
    
    integer :: num_threads = 2
    
    print*, "Display Hello world!"
    print*, "Number of threads used = ", num_threads
    
    call loop()
    
end program hello


subroutine loop()
    integer :: i,j,k,n
    real :: c0
    real, allocatable :: v1(:,:)

    n = 3
    c0 = 0.
    if (.not. allocated (v1)) allocate(v1(n,n))    
    v1 = c0
    
    !$omp do private(i, j, k) schedule(dynamic) reduction(+: v1)    
    do i = 1, n 
      do j = 1, n
        do k = 1, n
          v1(i,j) = v1(i,j) + k
        end do
        write (*,*) i, j, v1(i,j)
      end do
    end do  
    !$omp end do   

end subroutine

gfotran -fopenmp leads to

error: reduction variable ‘v1’ is private in outer context
     !$omp do private(i, j, k) schedule(dynamic) reduction(+: v1)

I checked reduction variable is private in outer context but still unsure the reason for my issue. v1 is only used inside the loop.

What's the reason for the error message reduction variable ‘v1’ is private in outer context ?

[Solved, by adding !$omp parallel and !$omp end parallel]

Answer 1

Thanks for Ian Bush's comment. By adding !$omp parallel and !$omp end parallel, i.e.,

program hello

    use omp_lib
    implicit none
    
    integer :: num_threads = 2
    
    print*, "Display Hello world!"
    print*, "Number of threads used = ", num_threads
          
    call loop()
    
end program hello


subroutine loop()
    integer :: i,j,k,n
    real :: c0
    real, allocatable :: v1(:,:)

    n = 3
    c0 = 0.
    if (.not. allocated (v1)) allocate(v1(n,n))    
    
    !$omp parallel
    !$omp do private(i, j, k) schedule(dynamic) reduction(+: v1)    
    do i = 1, n 
      do j = 1, n
        v1(i,j) = c0
        do k = 1, n
          v1(i,j) = v1(i,j) + k
        end do
        write (*,*) i, j, v1(i,j)
      end do
    end do  
    !$omp end do  
    !$omp end parallel

end subroutine

the code runs normally.