Windows sockets: WSAWaitForMultipleEvents returning WSA_WAIT_EVENT_0, on an FD_READ event, with no data available to read.

Posted on 2006-10-26
Last Modified: 2012-06-21
I'm writing a simple client - server communication application.  My question here is in regards to the server side.

After the client establishes a connection, it then writes 8 bytes.

The following (server side) code section successfully receives the 8 bytes.  The problem is that future calls to WSAWaitForMultipleEvents seem to all return signalled, even though there is no data to read.  To illustrate, I put a for-next loop around the same WSAWaitForMultipleEvents...recv sequence.  The first iteration through the code successfully reads the 8 bytes, but on the 2nd iteration through, the WSAWaitForMultipleEvents call still returns signalled, even though the FD_READ event should have been cleared by the recv call.  See the output below.
Any thoughts on what the problem is would be greatly appreciated.  This is the only worker thread in the application.  

            int iError=WSAEventSelect(m_Sockets[ClientIndex], m_Events[ClientIndex], FD_READ);
            if (iError) Message6(RED,"WSAEventSelect Failed"); else Message6(GREEN,"WSAEventSelect Successful");

            DWORD dwResult;
            for (int k=1; k<4; k++)
                  Message6(BLACK,"Iteration #%d",k);
                  dwResult = WSAWaitForMultipleEvents(1,&m_Events[ClientIndex],FALSE,500,FALSE);
                  if (dwResult==WSA_WAIT_EVENT_0)
                  {      Message6(BLACK,"First WSAWaitForMultipleEvents returning WSA_WAIT_EVENT_0 (signalled).  mask=%x",FD_READ);

                        int iResult = recv(m_Sockets[ClientIndex], (char*)(&dData), 8, 0);
                        if (iResult == SOCKET_ERROR)
                              int iErr=WSAGetLastError();
                              if (iErr==WSAEWOULDBLOCK)
                                    Message6(BLACK,"WSAEWOULDBLOCK on receive.");
                                    DisplayWinSockErrorMessage("recv error: ",iErr);
                              Message6(GREEN,"Just received %d bytes of data. %f",iResult,dData);


2006.10.26 14:57:59.90851 WSAEventSelect Successful
2006.10.26 14:57:59.90852 Iteration #1
2006.10.26 14:57:59.91472 First WSAWaitForMultipleEvents returning WSA_WAIT_EVENT_0 (signalled).  mask=1
2006.10.26 14:57:59.91475 Just received 8 bytes of data.
2006.10.26 14:57:59.91476 Iteration #2
2006.10.26 14:57:59.91477 First WSAWaitForMultipleEvents returning WSA_WAIT_EVENT_0 (signalled).  mask=1
2006.10.26 14:57:59.91478 WSAEWOULDBLOCK on receive.
2006.10.26 14:57:59.91479 Iteration #3
2006.10.26 14:57:59.91480 First WSAWaitForMultipleEvents returning WSA_WAIT_EVENT_0 (signalled).  mask=1
2006.10.26 14:57:59.91481 WSAEWOULDBLOCK on receive.
Question by:dcShaver
1 Comment
LVL 48

Accepted Solution

AlexFM earned 500 total points
ID: 17814649

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