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subroutine parallel_factorial(n, ctr)
use omp_lib
use iso_fortran_env
implicit none
integer, intent(in) :: n
integer(kind =int64), intent(inout) :: ctr
integer(kind =int64) :: local_ctr
integer :: i
local_ctr = 1
!$omp parallel private(i) reduction(*:local_ctr)
!$omp do
do i = 1, n
local_ctr = local_ctr * i
end do
!$omp end do
!$omp end parallel
ctr = ctr + local_ctr
end subroutine
program openmp_09
use omp_lib
use iso_fortran_env
integer, parameter :: n = 20
integer(kind =int64) :: ctr
integer(kind =int64) :: res
res = 2432902008176640000_int64 ! Factorial of 20
call omp_set_num_threads(8)
ctr = 0
call parallel_factorial(n, ctr)
print *, ctr
if(ctr /= res) error stop
end program
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