max = -1;
ret = r->pre_select(&rn, &rfds, &wfds, &timeout);
max = PARA_MAX(max, ret);
max = -1;
ret = r->pre_select(&rn, &rfds, &wfds, &timeout);
max = PARA_MAX(max, ret);
ret = para_select(max + 1, &rfds, &wfds, &timeout);
if (ret < 0) {
ret = -E_RECV_SELECT;
ret = para_select(max + 1, &rfds, &wfds, &timeout);
if (ret < 0) {
ret = -E_RECV_SELECT;