rl_start
rl->rl_start = offset;
if (rl->rl_start == offset && rl->rl_end == offset + length)
if (rl->rl_start < end && rl->rl_end > offset)
off_t rl_start;
static void rl_start(if_t);
if_setstartfn(ifp, rl_start);