lwip的tftpserver.c文件，自己註釋了下

/****************************************************************//**
 *
 * @file tftp_server.c
 *
 * @author   Logan Gunthorpe <[email protected]>
 *           Dirk Ziegelmeier <[email protected]>
 *
 * @brief    Trivial File Transfer Protocol (RFC 1350)
 *
 * Copyright (c) Deltatee Enterprises Ltd. 2013
 * All rights reserved.
 *
 ********************************************************************/

/* 
 * Redistribution and use in source and binary forms, with or without
 * modification,are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote products
 *    derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO
 * EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
 * TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * Author: Logan Gunthorpe <[email protected]>
 *         Dirk Ziegelmeier <[email protected]>
 *
 */

/**
 * @defgroup tftp TFTP server
 * @ingroup apps
 *
 * This is simple TFTP server for the lwIP raw API.
 */

#include "lwip/apps/tftp_server.h"

#if LWIP_UDP

#include "lwip/udp.h"
#include "lwip/timeouts.h"
#include "lwip/debug.h"

#define TFTP_MAX_PAYLOAD_SIZE 512
#define TFTP_HEADER_LENGTH    4

#define TFTP_RRQ   1
#define TFTP_WRQ   2
#define TFTP_DATA  3
#define TFTP_ACK   4
#define TFTP_ERROR 5

enum tftp_error {
  TFTP_ERROR_FILE_NOT_FOUND    = 1,
  TFTP_ERROR_ACCESS_VIOLATION  = 2,
  TFTP_ERROR_DISK_FULL         = 3,
  TFTP_ERROR_ILLEGAL_OPERATION = 4,
  TFTP_ERROR_UNKNOWN_TRFR_ID   = 5,
  TFTP_ERROR_FILE_EXISTS       = 6,
  TFTP_ERROR_NO_SUCH_USER      = 7
};

#include <string.h>

struct tftp_state {
  const struct tftp_context *ctx;
  void *handle;
  struct pbuf *last_data;
  struct udp_pcb *upcb;
  ip_addr_t addr;
  u16_t port;
  int timer;
  int last_pkt;
  u16_t blknum;
  u8_t retries;
  u8_t mode_write;
};

static struct tftp_state tftp_state;

static void tftp_tmr(void* arg);

static void
close_handle(void)
{
  //結束對當前客戶端的服務，把端口號置零，把IP地址設置爲any
  tftp_state.port = 0;
  ip_addr_set_any(0, &tftp_state.addr);
  //釋放last_data指向的pbuf
  if(tftp_state.last_data != NULL) {
    pbuf_free(tftp_state.last_data);
    tftp_state.last_data = NULL;
  }

  sys_untimeout(tftp_tmr, NULL);
  
  //執行關閉文件操作
  if (tftp_state.handle) {
    tftp_state.ctx->close(tftp_state.handle);
    tftp_state.handle = NULL;
    LWIP_DEBUGF(TFTP_DEBUG | LWIP_DBG_STATE, ("tftp: closing\n"));
  }
}

static void
send_error(const ip_addr_t *addr, u16_t port, enum tftp_error code, const char *str)
{
  int str_length = strlen(str);
  struct pbuf* p;
  u16_t* payload;
  
  p = pbuf_alloc(PBUF_TRANSPORT, (u16_t)(TFTP_HEADER_LENGTH + str_length + 1), PBUF_RAM);
  if(p == NULL) {
    return;
  }

  payload = (u16_t*) p->payload;
  payload[0] = PP_HTONS(TFTP_ERROR);
  payload[1] = lwip_htons(code);
  MEMCPY(&payload[2], str, str_length + 1);
  //實際發送
  udp_sendto(tftp_state.upcb, p, addr, port);
  pbuf_free(p);
}

//發送確認幀
static void
send_ack(u16_t blknum)
{
  struct pbuf* p;
  u16_t* payload;
  
  p = pbuf_alloc(PBUF_TRANSPORT, TFTP_HEADER_LENGTH, PBUF_RAM);
  if(p == NULL) {
    return;
  }
  payload = (u16_t*) p->payload;
  
  payload[0] = PP_HTONS(TFTP_ACK);
  payload[1] = lwip_htons(blknum);
  //實際發送
  udp_sendto(tftp_state.upcb, p, &tftp_state.addr, tftp_state.port);
  pbuf_free(p);
}

//發送數據
static void
resend_data(void)
{
  //爲什麼要新建一個pbuf，拷貝一次數據呢？爲什麼不用last_data直接發呢？
  struct pbuf *p = pbuf_alloc(PBUF_TRANSPORT, tftp_state.last_data->len, PBUF_RAM);
  if(p == NULL) {
    return;
  }
  
  if(pbuf_copy(p, tftp_state.last_data) != ERR_OK) {
    pbuf_free(p);
    return;
  }
    
  udp_sendto(tftp_state.upcb, p, &tftp_state.addr, tftp_state.port);
  pbuf_free(p);
}

static void
send_data(void)
{
  u16_t *payload;
  //注意ret是有符號的變量
  int ret;

  //如果上一次用到的pbuf沒有釋放，則先釋放
  if(tftp_state.last_data != NULL) {
    pbuf_free(tftp_state.last_data);
  }
  //申請一個pbuf，last_data是指向pbuf的指針
  tftp_state.last_data = pbuf_alloc(PBUF_TRANSPORT, TFTP_HEADER_LENGTH + TFTP_MAX_PAYLOAD_SIZE, PBUF_RAM);
  if(tftp_state.last_data == NULL) {
    return;
  }
  //指向pbuf的數據段，填充tftp協議的頭兩個字節
  payload = (u16_t *) tftp_state.last_data->payload;
  payload[0] = PP_HTONS(TFTP_DATA);
  payload[1] = lwip_htons(tftp_state.blknum);

  //要求把數據填充到pbuf，能不能移花接木，直接修改pbuf的payload指向的位置
  ret = tftp_state.ctx->read(tftp_state.handle, &payload[2], TFTP_MAX_PAYLOAD_SIZE);
  //只有小於0纔是出錯了，因爲等於0的話也要再發一次數據，因爲上一次發的是512個字節，客戶端認爲數據還沒有傳完，所以這次雖然0個字節，也要發
  if (ret < 0) {
    send_error(&tftp_state.addr, tftp_state.port, TFTP_ERROR_ACCESS_VIOLATION, "Error occured while reading the file.");
    close_handle();
    return;
  }
  //這裏根據ret值，重新分配pbuf的大小，pbuf是從內存堆中分配的，所以可以縮減。在下面這個函數調整完後，last_data指向的pbuf的len值被更新了
  pbuf_realloc(tftp_state.last_data, (u16_t)(TFTP_HEADER_LENGTH + ret));
  resend_data();
}

static void
recv(void *arg, struct udp_pcb *upcb, struct pbuf *p, const ip_addr_t *addr, u16_t port)
{
  u16_t *sbuf = (u16_t *) p->payload;
  int opcode;

  LWIP_UNUSED_ARG(arg);
  LWIP_UNUSED_ARG(upcb);
  
  //首先判斷一下發來消息的客戶端的IP和端口號是不是正在被服務的客戶端，如果不是則不服務，如果是第一次來客戶，則服務
  if (((tftp_state.port != 0) && (port != tftp_state.port)) ||
      (!ip_addr_isany_val(tftp_state.addr) && !ip_addr_cmp(&tftp_state.addr, addr))) {
    send_error(addr, port, TFTP_ERROR_ACCESS_VIOLATION, "Only one connection at a time is supported");
    pbuf_free(p);
    return;
  }
  //獲得幀的操作碼
  opcode = sbuf[0];

  tftp_state.last_pkt = tftp_state.timer;
  tftp_state.retries = 0;
  //如果是讀請求幀或者寫請求幀
  switch (opcode) {
    case PP_HTONS(TFTP_RRQ): /* fall through */
    case PP_HTONS(TFTP_WRQ):
    {
      const char tftp_null = 0;
      char filename[TFTP_MAX_FILENAME_LEN];
      char mode[TFTP_MAX_MODE_LEN];
      u16_t filename_end_offset;
      u16_t mode_end_offset;

      if(tftp_state.handle != NULL) {
        send_error(addr, port, TFTP_ERROR_ACCESS_VIOLATION, "Only one connection at a time is supported");
        break;
      }
      
      sys_timeout(TFTP_TIMER_MSECS, tftp_tmr, NULL);

      //提取請求幀中的文件名，複製到filename[]數組
	  /* find \0 in pbuf -> end of filename string */
      filename_end_offset = pbuf_memfind(p, &tftp_null, sizeof(tftp_null), 2);
      if((u16_t)(filename_end_offset-2) > sizeof(filename)) {
        send_error(addr, port, TFTP_ERROR_ACCESS_VIOLATION, "Filename too long/not NULL terminated");
        break;
      }
      pbuf_copy_partial(p, filename, filename_end_offset-2, 2);
      //提取mode：文件編碼方式，複製到mode數組
      /* find \0 in pbuf -> end of mode string */
      mode_end_offset = pbuf_memfind(p, &tftp_null, sizeof(tftp_null), filename_end_offset+1);
      if((u16_t)(mode_end_offset-filename_end_offset) > sizeof(mode)) {
        send_error(addr, port, TFTP_ERROR_ACCESS_VIOLATION, "Mode too long/not NULL terminated");
        break;
      }
      pbuf_copy_partial(p, mode, mode_end_offset-filename_end_offset, filename_end_offset+1);
 
      //打開文件，根據操作碼，讀打開或寫打開
	  tftp_state.handle = tftp_state.ctx->open(filename, mode, opcode == PP_HTONS(TFTP_WRQ));
      //把blknum設置爲1，準備發送時用到
	  tftp_state.blknum = 1;

      if (!tftp_state.handle) {
        send_error(addr, port, TFTP_ERROR_FILE_NOT_FOUND, "Unable to open requested file.");
        break;
      }

      LWIP_DEBUGF(TFTP_DEBUG | LWIP_DBG_STATE, ("tftp: %s request from ", (opcode == PP_HTONS(TFTP_WRQ)) ? "write" : "read"));
      ip_addr_debug_print(TFTP_DEBUG | LWIP_DBG_STATE, addr);
      LWIP_DEBUGF(TFTP_DEBUG | LWIP_DBG_STATE, (" for '%s' mode '%s'\n", filename, mode));

      //把客戶的IP和端口號寫到tftp_state結構體
	  ip_addr_copy(tftp_state.addr, *addr);
      tftp_state.port = port;

      //如果是寫操作，則返回確認號0，如果是讀操作，則返回第一塊數據
	  if (opcode == PP_HTONS(TFTP_WRQ)) {
        tftp_state.mode_write = 1;
        send_ack(0);
      } else {
        tftp_state.mode_write = 0;
        send_data();
      }

      break;
    }
    //如果是數據幀，是客戶端給服務器發數據
    case PP_HTONS(TFTP_DATA):
    {
      int ret;
      u16_t blknum;
      
      if (tftp_state.handle == NULL) {
        send_error(addr, port, TFTP_ERROR_ACCESS_VIOLATION, "No connection");
        break;
      }
	  //如果不是寫操作，那麼客戶端是不可以給服務器發數據幀的
      if (tftp_state.mode_write != 1) {
        send_error(addr, port, TFTP_ERROR_ACCESS_VIOLATION, "Not a write connection");
        break;
      }
	  //取得塊號
      blknum = lwip_ntohs(sbuf[1]);
	  //移動pbuf的payload，向後移動，把幀頭部分讓過去，指向實際數據
      pbuf_header(p, -TFTP_HEADER_LENGTH);

      ret = tftp_state.ctx->write(tftp_state.handle, p);
      //如果寫錯誤，發送差錯幀
	  if (ret < 0) {
        send_error(addr, port, TFTP_ERROR_ACCESS_VIOLATION, "error writing file");
        close_handle();
      } else {
        //這裏沒有對數據進行判斷，如果數據不對，也沒有要求重發。而實際上如果數據部分沒有格式，也無從判斷數據是否有錯
		send_ack(blknum);
      }
	  //如果pbuf中數據小於512個字節，表示這是最後一個數據幀了，後面關閉handle
      if (p->tot_len < TFTP_MAX_PAYLOAD_SIZE) {
        close_handle();
      }
      break;
    }
    //如果是確認幀，在客戶段讀文件時用的到
    case PP_HTONS(TFTP_ACK):
    {
      u16_t blknum;
      int lastpkt;

      if (tftp_state.handle == NULL) {
        send_error(addr, port, TFTP_ERROR_ACCESS_VIOLATION, "No connection");
        break;
      }
	  //只有在讀文件時才收確認幀
      if (tftp_state.mode_write != 0) {
        send_error(addr, port, TFTP_ERROR_ACCESS_VIOLATION, "Not a read connection");
        break;
      }
      //提取被確認的塊號，如果和服務器上次發的塊號不符，則返回錯誤幀
      blknum = lwip_ntohs(sbuf[1]);
      if (blknum != tftp_state.blknum) {
        send_error(addr, port, TFTP_ERROR_UNKNOWN_TRFR_ID, "Wrong block number");
        break;
      }

      lastpkt = 0;
      //如果上次的數據不是512個字節，則lastpkt = 1;
      if (tftp_state.last_data != NULL) {
        lastpkt = tftp_state.last_data->tot_len != (TFTP_MAX_PAYLOAD_SIZE + TFTP_HEADER_LENGTH);
      }
      //如果lastpkt = 1，則關閉handle。如果等於0就把blknum++,然後繼續發數據。
      if (!lastpkt) {
        tftp_state.blknum++;
        send_data();
      } else {
        close_handle();
      }

      break;
    }
    //如果操作碼不對，則發錯誤幀
    default:
      send_error(addr, port, TFTP_ERROR_ILLEGAL_OPERATION, "Unknown operation");
      break;
  }
  //用完的pbuf要釋放掉
  pbuf_free(p);
}

static void
tftp_tmr(void* arg)
{
  LWIP_UNUSED_ARG(arg);
  
  tftp_state.timer++;

  if (tftp_state.handle == NULL) {
    return;
  }

  sys_timeout(TFTP_TIMER_MSECS, tftp_tmr, NULL);
  //執行的是超時重傳策略
  if ((tftp_state.timer - tftp_state.last_pkt) > (TFTP_TIMEOUT_MSECS / TFTP_TIMER_MSECS)) {
    if ((tftp_state.last_data != NULL) && (tftp_state.retries < TFTP_MAX_RETRIES)) {
      LWIP_DEBUGF(TFTP_DEBUG | LWIP_DBG_STATE, ("tftp: timeout, retrying\n"));
      //重傳數據
	  resend_data();
      tftp_state.retries++;
    } else {
      LWIP_DEBUGF(TFTP_DEBUG | LWIP_DBG_STATE, ("tftp: timeout\n"));
      close_handle();
    }
  }
}

/** @ingroup tftp
 * Initialize TFTP server.
 * @param ctx TFTP callback struct
 */
err_t 
tftp_init(const struct tftp_context *ctx)
{
  err_t ret;
  //申請pcb
  struct udp_pcb *pcb = udp_new_ip_type(IPADDR_TYPE_ANY);
  if (pcb == NULL) {
    return ERR_MEM;
  }
  //綁定pcb
  ret = udp_bind(pcb, IP_ANY_TYPE, TFTP_PORT);
  if (ret != ERR_OK) {
    udp_remove(pcb);
    return ret;
  }

  tftp_state.handle    = NULL;
  tftp_state.port      = 0;
  tftp_state.ctx       = ctx;
  tftp_state.timer     = 0;
  tftp_state.last_data = NULL;
  tftp_state.upcb      = pcb;
  //註冊接收函數，當手動udp數據幀時，執行recv函數
  udp_recv(pcb, recv, NULL);

  return ERR_OK;
}

#endif /* LWIP_UDP */

看代碼前最後先了解下TFTP的傳輸流程