Receiving UDP Data (zero copy interface)
Part of the FreeRTOS+TCP Networking Tutorial

static void vUDPReceivingUsingZeroCopyInterface( void *pvParameters )
{
int32_t lBytes;
uint8_t *pucUDPPayloadBuffer;
struct freertos_sockaddr xClient, xBindAddress;
uint32_t xClientLength = sizeof( xClient ), ulIPAddress;
Socket_t xListeningSocket;

   /* Attempt to open the socket. */
   xListeningSocket = FreeRTOS_socket( FREERTOS_AF_INET,
                                       FREERTOS_SOCK_DGRAM, /*FREERTOS_SOCK_DGRAM for UDP.*/
                                       FREERTOS_IPPROTO_UDP );

   /* Check the socket was created. */
   configASSERT( xListeningSocket != FREERTOS_INVALID_SOCKET );

   /* Bind to port 10000. */
   xBindAddress.sin_port = FreeRTOS_htons( 10000 );
   FreeRTOS_bind( xListeningSocket, &xBindAddress, sizeof( xBindAddress ) );

   for( ;; )
   {
       /* Receive data from the socket.  ulFlags has the zero copy bit set
       (FREERTOS_ZERO_COPY) indicating to the stack that a reference to the
       received data should be passed out to this RTOS task using the second
       parameter to the FreeRTOS_recvfrom() call.  When this is done the
       IP stack is no longer responsible for releasing the buffer, and
       the RTOS task must return the buffer to the stack when it is no longer
       needed.  By default the block time is portMAX_DELAY but it can be
       changed using FreeRTOS_setsockopt(). */
       lBytes = FreeRTOS_recvfrom( xListeningSocket,
                                   &pucUDPPayloadBuffer,
                                   0,
                                   FREERTOS_ZERO_COPY,
                                   &xClient,
                                   &xClientLength );

       if( lBytes > 0 )
       {
           /* Data was received and can be processed here. */
       }

       if( lBytes >= 0 )
       {
           /* The receive was successful so this RTOS task is now responsible for
           the buffer.  The buffer must be freed once it is no longer
           needed. */

           /*
            * The data can be processed here.
            */

           /* Return the buffer to the TCP/IP stack. */
           FreeRTOS_ReleaseUDPPayloadBuffer( pucUDPPayloadBuffer );
       }
   }
}