WSAEINVAL error received when using select() with sockets

I'm using the select() function to prevent a blocking socket from hanging on recv() in the case of a dropped UDP packet.  The problem is that whenever I specify a timeout in my timeval structure that is less than necessary for data to be ready in my socket, instead of getting a return value of zero (in which case, I can pause or loop or something), I get SOCKET_ERROR, and WSAGetLastError() gives me 10022, which cooresponds to WSAEINVAL.  This should only occur if my timeout is an invalid value (I can't find anywhere that documents what is or is not valid), or if the three socket set structures are NULL.

If I set the timeout value high, all my operations complete successfully.  If I set it very low, it immediately fails with WSAEINVAL.  I initially thought that perhaps there was a minimum value, so I started setting it lower and lower to find the minimum, but I found at 15000 microseconds it wouldn't fail consistently.  In other words, I try to send 5 consecutive packets and sometimes it would fail on the fourth, sometimes the second, sometimes the first, sometimes not at all.  This is how I concluded that if it times out it is returning the error instead of 0.

So I'm not really sure what I'm doing wrong.  My ultimate goal is to determine how long I should reasonably wait for a response (by experimenting with the timeout value).  Then I can set a timeout value where I can resend the request packet if I don't receive a response in a reasonable amount of time.

Also, according to MSDN, a timeval of {0, 0} should cause select to block until a response is received.  But I still get the WSAEINVAL error.  Setting this parameter to NULL in the select() call works correctly and it blocks until a response is received.

Here is a code snippet:

send(*theSocket, (char *)request, msgSize, 0);


     //Wait for a response
     unsigned count = 0;
     FD_ZERO(&fdread);
     FD_SET(*theSocket, &fdread);
     fdTime.tv_sec = 0;
     fdTime.tv_usec = 10000;
     while (count < 100000)
     {
          if ((fdRet = select(0,&fdread,NULL,NULL,/*NULL*/&fdTime)) == SOCKET_ERROR)
          {
               if (WSAGetLastError() != 10022)          //this error means select timed out
               {
                    printf("Socket Error %d on select().",WSAGetLastError());
                    exit(1);
               }
          }
         
          printf("fdRet: %d\n",fdRet);

          if (fdRet == 1)
          {
               printf("response ready after %d select calls, with wait times of %d milliseconds.\n",count,fdTime.tv_usec);
               break;
          }

          count++;
     }

     responseSize = recv(*theSocket, (char *)response, 200, 0);