In lwip_select call, if the data at receive side is not ready, it calls sys_sem_wait_timeout() on the select_scb.sem, which can only be signaled at event_callback() when the bit for the socket is set. Now if at the time the lwip_select is called and no data is ready, the bit for that socket will not be set at lwip_selscan(), so the question now is: who is going to set the bit for that socket when the data is ready? Is that socket waiting for another socket to call lwip_select() to get its bit updated in the readset and to get back from the wait state? What if this is the only select call? Anyone could answer this puzzle for me would be greatly appreciated. Thanks in advance.