sockets - Select function allways returns 0

Question

I have Server-Client communication, I am trying to make a UDP communication with ACK of data that is transmitted. My client data is sent with sendto function and recieved at server side with nonblocking recvfrom using select function on recieving socket, then my code tries to send ACK using sento to the same socket and same address, and my client tries to recieve the ACK on his socket using select, but this select is allways 0. any help?

iResult = select(0 /* ignored */, &set, NULL, NULL, &timeVal);
    if (iResult == SOCKET_ERROR)
    {
        return -1;
    }

    if (iResult == 0)
    {
        SequenceNumToSend -= SentWithoutAck;
        continue;
    }

    iResult = recvfrom(*clientSocket,
        (char*)recievedAck,
        sizeof(int),
        0,
        (SOCKADDR *)&address,
        &addressLength);
    *recievedAck = ntohl(*recievedAck);

Answer 1

等待大神答复