About DMA problem..please help me

besides , I changed the code to :

    /* Post the transmit request */
    if ((tc = alt_dma_txchan_send (txchan,
            tx_data,
            0x10,
            NULL,
            NULL)) < 0)
    {
        printf ("Failed to post transmit request, reason = %i\n", tc);
        exit (1);
    }
   else
    {
        printf ("Succeed to post transmit request when tc = %i\n",tc);
        //exit (1);
    }
    
    /* Post the receive request */
    if ((rc = alt_dma_rxchan_prepare (rxchan,
            rx_buffer,
            0x10,
            txrxDone,
            NULL)) < 0)
    {
        printf ("Failed to post read request, reason = %i\n", rc);
        exit (1);
    }
   else
    {
        printf ("Succeed to post read request when rc = %i\n",rc);
        printf ("txrx_done = %d",txrx_done);
    }