get_free_unit Function

    function get_free_unit() result(free_unit)

        ! Returns:
        !    The first free file unit above 10 and less than or equal to
        !    the paramater max_unit (currently set to 200).
        !
        !    If max_unit is exceeded, the function returns -1

        integer, parameter :: max_unit = 100
        integer :: free_unit
        integer :: i
        logical :: t_open, t_exist

        free_unit = -1
        do i = 10, max_unit
            inquire(unit=i, opened=t_open, exist=t_exist)
            if(.not. t_open .and. t_exist) then
                free_unit = i
                exit
            end if
        end do
        if(i == max_unit + 1) call stop_all('get_free_unit', 'Cannot find a free unit below max_unit.')

    end function get_free_unit