recvfrom() error 10035 using non-blocking sockets

Question

I am using ioctlsocket() function to make my socket non-blocking but when I call recvfrom(), I get the error 10035 ( WSAEWOULDBLOCK ).

u_long mode = 1;
ioctlsocket(newSocketIdentifier, FIONBIO, &mode);

while(1)
   {
      if((recv_len = recvfrom(newSocketIdentifier, receiveBuffer, sizeof(receiveBuffer), 0, (struct sockaddr *) &clientSocket, &clientSocketLength)) == SOCKET_ERROR)
      {
         char err[128];
         itoa(WSAGetLastError(),err,10);
         MessageBox( NULL,"Could not Receive Data",err,MB_ICONINFORMATION);
         BREAK;
      }
   }

Can anybody explain why this happens? :(

Answer 1

This is normal if no data is available. The code is WSAEWOULDBLOCK (see this table) and means, that on a blocking port the function would have to sit and wait until it could be served.

   while(1)
   {
      if((recv_len = recvfrom(newSocketIdentifier, receiveBuffer, sizeof(receiveBuffer), 0, (struct sockaddr *) &clientSocket, &clientSocketLength)) == SOCKET_ERROR)
      { 
         int ierr= WSAGetLastError();
         if (ierr==WSAEWOULDBLOCK) {  // currently no data available
             Sleep(50);  // wait and try again
             continue; 
         }

         // Other errors
         char err[128];
         itoa(ierr,err,10);
         MessageBox( NULL,"Could not Receive Data",err,MB_ICONINFORMATION);
         break;
      }
   }

Answer 2

I cannot agree that this is "normal" like posted above.

In your call of recvfrom you will receive an error in recv_len . I recommend to check that value - it will be SOCKET_ERROR and by calling WSAGetLastErrorenter you will see error WSAEWOULDBLOCK .

I am not a (Windows) socket expert, but based on my tests, I cannot use combination of ioctlsocket and recvfrom for receiving data via UDP in non-blocking mode (I did the same thing like you in your example).

I am planning to use combination of select and recvfrom with minimum possible timeout (1us). I do not know any other possibly better solution now.

Note: you should check also the return value of ioctlsocket for possible error.

I will provide my code sample later today.

UPDATE (adding code as promised):

/* define list of sockets for function select(..) */
fd_set readfds;
/* define timeout for function select(..) */
TIMEVAL tv;
/* timeout: 1us */
tv.tv_usec = 1;
/* timeout: 0s */
tv.tv_sec = 0;
/* just 1 socket is used */
readfds.fd_count = 1;
readfds.fd_array[0] = receivingSocket;
/* determine the status of one or more sockets with timeout */
int selectReturnValue = select(0, &readfds, 0, 0, &tv);
/* check return value of the call of function select(..) */
switch (selectReturnValue)
{
  /* select(..) function timeout */
  case 0:
    /* time limit expired */
    break;
  /* select(..) function error */
  case SOCKET_ERROR:
    /* check the error status for the last windows sockets operation */
    selectError(WSAGetLastError());
    break;
  /* no timeout and no error */
  default:
    /* receive data from UDP */
    resultOfrecvfrom = recvfrom(receivingSocket, receivingBuffer, sizeof(receivingBuffer), 0, (SOCKADDR *)&serverReceptionInfo, &serverReceptionInfoLength);
    /* check result of call of recvfrom(..) */
    switch (resultOfrecvfrom)
    {
      /* connection has been gracefully closed  */
      case 0:
        /* socket was closed */
        break;
      /* socket error occurred during last call of socket operation */
      case SOCKET_ERROR:
        /* check the error status for the last Windows Sockets operation */
        recvfromError(WSAGetLastError());
        break;
      /* resultOfrecvfrom amount of data received */
      default:            
        /* ... add your code here */
        break;
    }
    break;      
}

I just copied crucial part of my code if more is needed, let me know in comments.