if (!args)
  {
    /* unable to allocate memory for tftp args  */
    tftp_send_error_message(upcb, to, to_port, TFTP_ERR_NOTDEFINED);

    /* no need to use tftp_cleanup_rd because no 
            "tftp_connection_args" struct has been malloc'd   */
    tftp_cleanup_rd(upcb, args);

    return 0;
  }

  /* initialize connection structure  */
  args->op = TFTP_RRQ;	 //操作碼
  args->to_ip.addr = to->addr;	 //目的ip
  args->to_port = to_port;		 //to_port=0
  args->block = 1; //接收到讀請求時，塊號從1開始
  args->tot_bytes = 0;	 /* total number of bytes transferred */


  /* set callback for receives on this UDP PCB (Protocol Control Block) */
  udp_recv(upcb, rrq_recv_callback, args);	  //得到回復時調用rrq_recv_callback

  /* initiate the transaction by sending the first block of data
   * further blocks will be sent when ACKs are received
   *   - the receive callbacks need to get the proper state    */

  tftp_send_next_block(upcb, args, to, to_port);   //file_read要改

  return 1;
}

void wrq_recv_callback(void *_args, struct udp_pcb *upcb, struct pbuf *pkt_buf, struct ip_addr *addr, u16_t port)
{
  tftp_connection_args *args = (tftp_connection_args *)_args;
  int n = 0;

  if (pkt_buf->len != pkt_buf->tot_len)		
  {
    return;
  }

  /* Does this packet have any valid data to write? */
  if ((pkt_buf->len > TFTP_DATA_PKT_HDR_LEN) &&
      (tftp_extract_block(pkt_buf->payload) == (args->block + 1)))
  {
    /* write the received data to the file */ //將文件內容寫入
    n = file_write(&file_CR,
                   pkt_buf->len - TFTP_DATA_PKT_HDR_LEN,
                   (euint8*)pkt_buf->payload + TFTP_DATA_PKT_HDR_LEN);

    if (n <= 0)
    {
      tftp_send_error_message(upcb, addr, port, TFTP_ERR_FILE_NOT_FOUND);
      /* close the connection */
      tftp_cleanup_wr(upcb, args); /* close the connection */
    }

    /* update our block number to match the block number just received */
    args->block++;
    /* update total bytes  */
    (args->tot_bytes) += (pkt_buf->len - TFTP_DATA_PKT_HDR_LEN);  //加上數據部分長度

    /* This is a valid pkt but it has no data.  This would occur if the file being
       written is an exact multiple of 512 bytes.  In this case, the args->block
       value must still be updated, but we can skip everything else.    */
  }
  else if (tftp_extract_block(pkt_buf->payload) == (args->block + 1))
  {
    /* update our block number to match the block number just received  */
    args->block++;
  }

  /* SEndTransferthe appropriate ACK pkt (the block number sent in the ACK pkt echoes
   * the block number of the DATA pkt we just received - see RFC1350)
   * NOTE!: If the DATA pkt we received did not have the appropriate block
   * number, then the args->block (our block number) is never updated and
   * we simply sEndTransfera "duplicate ACK" which has the same block number as the
   * last ACK pkt we sent.  This lets the host know that we are still waiting
   * on block number args->block+1. */
  tftp_send_ack_packet(upcb, addr, port, args->block);

  /* If the last write returned less than the maximum TFTP data pkt length,
   * then we've received the whole file and so we can quit (this is how TFTP
   * signals the EndTransferof a transfer!)
   */
  if (pkt_buf->len < TFTP_DATA_PKT_LEN_MAX)
  {
    tftp_cleanup_wr(upcb, args);
    pbuf_free(pkt_buf);
  }
  else
  {
    pbuf_free(pkt_buf);
    return;
  }

}

int tftp_process_write(struct udp_pcb *upcb, struct ip_addr *to, int to_port, char *FileName)
{
  tftp_connection_args *args = NULL;

  /* If Could not open the file which will be transmitted  */
  if (file_fopen(&file_CR, &efs2.myFs, FileName, 'w') != 0)
  {
    tftp_send_error_message(upcb, to, to_port, TFTP_ERR_FILE_ALREADY_EXISTS);

    tftp_cleanup_wr(upcb, args);

    return 0;
  }

  /* This function is called from a callback,
   * therefore interrupts are disabled,
   * therefore we can use regular malloc   */
  args = mem_malloc(sizeof *args);
  if (!args)
  {
    tftp_send_error_message(upcb, to, to_port, TFTP_ERR_NOTDEFINED);

    tftp_cleanup_wr(upcb, args);

    return 0;
  }

  args->op = TFTP_WRQ;
  args->to_ip.addr = to->addr;
  args->to_port = to_port;
  /* the block # used as a positive response to a WRQ is _always_ 0!!! (see RFC1350)  */
  args->block = 0;		   //接收到的是寫入請求時，包號為0
  args->tot_bytes = 0;

  /* set callback for receives on this UDP PCB (Protocol Control Block) */
  udp_recv(upcb, wrq_recv_callback, args);

  /* initiate the write transaction by sending the first ack */
  tftp_send_ack_packet(upcb, to, to_port, args->block);

  return 0;
}

/* for each new request (data in p->payload) from addr:port,
 * create a new port to serve the response, and start the response
 * process
 */
void process_tftp_request(struct pbuf *pkt_buf, struct ip_addr *addr, u16_t port)
{
  tftp_opcode op = tftp_decode_op(pkt_buf->payload);   //取出操作碼
  char FileName[30];
  struct udp_pcb *upcb;
  err_t err;
  u32_t IPaddress;
  u8_t iptxt[20];
  u8_t iptab[4];
  
  IPaddress = addr->addr;
  printf("\n\rTFTP RRQ (read request) from: %d.%d.%d.%d\n\r", (u8_t)(IPaddress),
            (u8_t)(IPaddress >> 8),(u8_t)(IPaddress >> 16),(u8_t)(IPaddress >> 24));
  
  /* read its IP address */
  iptab[0] = (u8_t)(IPaddress >> 24);
  iptab[1] = (u8_t)(IPaddress >> 16);
  iptab[2] = (u8_t)(IPaddress >> 8);
  iptab[3] = (u8_t)(IPaddress);

  sprintf((char*)iptxt, "TFTP: %d.%d.%d.%d   ", iptab[3], iptab[2], iptab[1], iptab[0]);	  
  LCD_DisplayStringLine(Line7, iptxt);
  
  /* create new UDP PCB structure */
  upcb = udp_new();
  if (!upcb)
  {     /* Error creating PCB. Out of Memory  */
    return;
  }

  /* bind to port 0 to receive next available free port */
  /* NOTE:  This is how TFTP works.  There is a UDP PCB for the standard port
   * 69 which al transactions begin communication on, however, _all_ subsequent
   * transactions for a given "stream" occur on another port!  */
  err = udp_bind(upcb, IP_ADDR_ANY, 0);
  if (err != ERR_OK)
  {    /* Unable to bind to port   */
    return;
  }


  switch (op)
  {

    case TFTP_RRQ:    /* TFTP RRQ (read request)  */
      /* Read the name of the file asked by the client 
                            to be sent from the SD card */
      tftp_extract_filename(FileName, pkt_buf->payload);  //提取文件名

      printf("\n\rTFTP RRQ (read request)");
      printf("\n\rONLY EFS filesystem(NTFS in WinXp) is support");
      
      /* If Could not open filesystem */
      if (efs_init(&efs1, 0) != 0)
      {
        printf("\n\rIf Could not open filesystem");
        return;
      }
      printf("\n\rCould open filesystem\n\r");
      /* If Could not open the selected directory */
      if (ls_openDir(&list1, &(efs1.myFs), "/") != 0)
      {
        printf("\n\rIf Could not open the selected directory");
        return;
      }
      /* Start the TFTP read mode*/	  //addr為接收方的IP地址 ，port=0
      printf("\n\rStart the TFTP read mode....");
      tftp_process_read(upcb, addr, port, FileName); //根據文件名讀取SD卡中的文件，并發送
      break;

    case TFTP_WRQ:    /* TFTP WRQ (write request)   */
      /* Read the name of the file asked by the client 
                to received and writen in the SD card */
      tftp_extract_filename(FileName, pkt_buf->payload);

      /* If Could not open filesystem */
      if (efs_init(&efs2, 0) != 0)
      {
        return;
      }
      /* If Could not open the selected directory */
      if (ls_openDir(&list2, &(efs2.myFs), "/") != 0)
      {
        return;
      }

      /* Start the TFTP write mode*/
      tftp_process_write(upcb, addr, port, FileName);
      break;

    default:
      /* sEndTransfera generic access violation message */
      tftp_send_error_message(upcb, addr, port, TFTP_ERR_ACCESS_VIOLATION);
      /* TFTP unknown request op */
      /* no need to use tftp_cleanup_wr because no 
            "tftp_connection_args" struct has been malloc'd   */
      udp_remove(upcb);

      break;
  }
}

/* the recv_callback function is called when there is a packet received
 * on the main tftp server port (69)
 */
void recv_callback_tftp(void *arg, struct udp_pcb *upcb, struct pbuf *pkt_buf,
                        struct ip_addr *addr, u16_t port)
{
  /* process new connection request */
  process_tftp_request(pkt_buf, addr, port);

  pbuf_free(pkt_buf);
}

void tftpd_init(void)
{
  err_t err;
  unsigned port = 69;

  /* create a new UDP PCB structure  */
  UDPpcb = udp_new();
  if (!UDPpcb)
  {  /* Error creating PCB. Out of Memory  */
    return;
  }

  /* Bind this PCB to port 69  */
  err = udp_bind(UDPpcb, IP_ADDR_ANY, port);
  if (err != ERR_OK)
  {    /* Unable to bind to port  */
    return;
  }

  /* TFTP server start  */	//啟動TFTP服務器
  udp_recv(UDPpcb, recv_callback_tftp, NULL); //通知協議棧當69端口有連接請求時調用recv_callback_tftp，
}