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

The FreeRTOS_sendto() TCP/IP stack API function is used to send data to a UDP socket. Data can only be sent after the socket has been created, configured, and optionally bound to a local port number.

As detailed on the FreeRTOS_sendto() API reference page, FreeRTOS_sendto() can be used with standard calling semantics, or zero copy calling semantics. This page demonstrates the zero copy calling semantics.

The source code below shows a RTOS task that creates a UDP socket before entering a loop that sends a string to the socket (using the standard calling semantics) every 1 second (1000ms). The comments in the source code example provide important information on how to use network buffers when the zero copy interface is used.

static void vUDPSendingUsingZeroCopyInterface( void *pvParameters )
Socket_t xSocket;
uint8_t *pucBuffer;
struct freertos_sockaddr xDestinationAddress;
BaseType_t lReturned;
uint32_t ulCount = 0UL;
const uint8_t *pucStringToSend = "Zero copy send message number ";
const TickType_t x1000ms = 1000UL / portTICK_PERIOD_MS;
/* 15 is added to ensure the number, \r\n and terminating zero fit. */
const size_t xStringLength = strlen( ( char * ) pucStringToSend ) + 15;

   /* Send strings to port 10000 on IP address */
   xDestinationAddress.sin_addr = FreeRTOS_inet_addr( "" );
   xDestinationAddress.sin_port = FreeRTOS_htons( 10000 );

   /* Create the socket. */
   xSocket = FreeRTOS_socket( FREERTOS_AF_INET,
                              FREERTOS_SOCK_DGRAM,/*FREERTOS_SOCK_DGRAM for UDP.*/
                              FREERTOS_IPPROTO_UDP );

   /* Check the socket was created. */

   /* NOTE: FreeRTOS_bind() is not called.  This will only work if
   ipconfigALLOW_SOCKET_SEND_WITHOUT_BIND is set to 1 in FreeRTOSIPConfig.h. */

   for( ;; )
       /* This RTOS task is going to send using the zero copy interface.  The
       data being sent is therefore written directly into a buffer that is
       passed into, rather than copied into, the FreeRTOS_sendto()

       First obtain a buffer of adequate length from the TCP/IP stack into which
       the string will be written. */
       pucBuffer = FreeRTOS_GetUDPPayloadBuffer( xStringLength, portMAX_DELAY );

       /* Check a buffer was obtained. */
       configASSERT( pucBuffer );

       /* Create the string that is sent. */
       memset( pucBuffer, 0x00, xStringLength );
       sprintf( pucBuffer, "%s%lu\r\n", ucStringToSend, ulCount );

       /* Pass the buffer into the send function.  ulFlags has the
       FREERTOS_ZERO_COPY bit set so the TCP/IP stack will take control of the
       buffer rather than copy data out of the buffer. */
       lReturned = FreeRTOS_sendto( xSocket,
                                   ( void * ) pucBuffer,
                                   strlen( ( const char * ) pucBuffer ) + 1,
                                   sizeof( xDestinationAddress ) );

       if( lReturned == 0 )
           /* The send operation failed, so this RTOS task is still responsible
           for the buffer obtained from the TCP/IP stack.  To ensure the buffer
           is not lost it must either be used again, or, as in this case,
           returned to the TCP/IP stack using FreeRTOS_ReleaseUDPPayloadBuffer().
           pucBuffer can be safely re-used after this call. */
           FreeRTOS_ReleaseUDPPayloadBuffer( ( void * ) pucBuffer );
           /* The send was successful so the TCP/IP stack is now managing the
           buffer pointed to by pucBuffer, and the TCP/IP stack will
           return the buffer once it has been sent.  pucBuffer can
           be safely re-used. */


       /* Wait until it is time to send again. */
       vTaskDelay( x1000ms );
Example using FreeRTOS_sendto() with the zero copy calling semantics

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