lwip的tftpserver.c文件，自己注释了下

/****************************************************************//**
 *
 * @file tftp_server.c
 *
 * @author   Logan Gunthorpe <[e-mail]>
 *           Dirk Ziegelmeier <[e-mail]>
 *
 * @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 <[e-mail]>
 *         Dirk Ziegelmeier <[e-mail]>
 *
 */

/**
 * @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)
{
  //结束对当前客户端的服务，把端口号置零，把IP地址设置为any
  tftp_state.port = 0;
  ip_addr_set_any(0, &tftp_state.addr);
  //释放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);
  
  //执行关闭文件操作
  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);
  //实际发送
  udp_sendto(tftp_state.upcb, p, addr, port);
  pbuf_free(p);
}

//发送确认帧
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);
  //实际发送
  udp_sendto(tftp_state.upcb, p, &tftp_state.addr, tftp_state.port);
  pbuf_free(p);
}

//发送数据
static void
resend_data(void)
{
  //为什么要新建一个pbuf，拷贝一次数据呢？为什么不用last_data直接发呢？
  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是有符号的变量
  int ret;

  //如果上一次用到的pbuf没有释放，则先释放
  if(tftp_state.last_data != NULL) {
    pbuf_free(tftp_state.last_data);
  }
  //申请一个pbuf，last_data是指向pbuf的指针
  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的数据段，填充tftp协议的头两个字节
  payload = (u16_t *) tftp_state.last_data->payload;
  payload[0] = PP_HTONS(TFTP_DATA);
  payload[1] = lwip_htons(tftp_state.blknum);

  //要求把数据填充到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);
  //只有小于0才是出错了，因为等于0的话也要再发一次数据，因为上一次发的是512个字节，客户端认为数据还没有传完，所以这次虽然0个字节，也要发
  if (ret < 0) {
    send_error(&tftp_state.addr, tftp_state.port, TFTP_ERROR_ACCESS_VIOLATION, "Error occured while reading the file.");
    close_handle();
    return;
  }
  //这里根据ret值，重新分配pbuf的大小，pbuf是从内存堆中分配的，所以可以缩减。在下面这个函数调整完后，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);
  
  //首先判断一下发来消息的客户端的IP和端口号是不是正在被服务的客户端，如果不是则不服务，如果是第一次来客户，则服务
  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;
  }
  //获得帧的操作码
  opcode = sbuf[0];

  tftp_state.last_pkt = tftp_state.timer;
  tftp_state.retries = 0;
  //如果是读请求帧或者写请求帧
  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);

      //提取请求帧中的文件名，复制到filename[]数组
	  /* 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：文件编码方式，复制到mode数组
      /* 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);
 
      //打开文件，根据操作码，读打开或写打开
	  tftp_state.handle = tftp_state.ctx->open(filename, mode, opcode == PP_HTONS(TFTP_WRQ));
      //把blknum设置为1，准备发送时用到
	  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));

      //把客户的IP和端口号写到tftp_state结构体
	  ip_addr_copy(tftp_state.addr, *addr);
      tftp_state.port = port;

      //如果是写操作，则返回确认号0，如果是读操作，则返回第一块数据
	  if (opcode == PP_HTONS(TFTP_WRQ)) {
        tftp_state.mode_write = 1;
        send_ack(0);
      } else {
        tftp_state.mode_write = 0;
        send_data();
      }

      break;
    }
    //如果是数据帧，是客户端给服务器发数据
    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;
      }
	  //如果不是写操作，那么客户端是不可以给服务器发数据帧的
      if (tftp_state.mode_write != 1) {
        send_error(addr, port, TFTP_ERROR_ACCESS_VIOLATION, "Not a write connection");
        break;
      }
	  //取得块号
      blknum = lwip_ntohs(sbuf[1]);
	  //移动pbuf的payload，向后移动，把帧头部分让过去，指向实际数据
      pbuf_header(p, -TFTP_HEADER_LENGTH);

      ret = tftp_state.ctx->write(tftp_state.handle, p);
      //如果写错误，发送差错帧
	  if (ret < 0) {
        send_error(addr, port, TFTP_ERROR_ACCESS_VIOLATION, "error writing file");
        close_handle();
      } else {
        //这里没有对数据进行判断，如果数据不对，也没有要求重发。而实际上如果数据部分没有格式，也无从判断数据是否有错
		send_ack(blknum);
      }
	  //如果pbuf中数据小于512个字节，表示这是最后一个数据帧了，后面关闭handle
      if (p->tot_len < TFTP_MAX_PAYLOAD_SIZE) {
        close_handle();
      }
      break;
    }
    //如果是确认帧，在客户段读文件时用的到
    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;
      }
	  //只有在读文件时才收确认帧
      if (tftp_state.mode_write != 0) {
        send_error(addr, port, TFTP_ERROR_ACCESS_VIOLATION, "Not a read connection");
        break;
      }
      //提取被确认的块号，如果和服务器上次发的块号不符，则返回错误帧
      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;
      //如果上次的数据不是512个字节，则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，则关闭handle。如果等于0就把blknum++,然后继续发数据。
      if (!lastpkt) {
        tftp_state.blknum++;
        send_data();
      } else {
        close_handle();
      }

      break;
    }
    //如果操作码不对，则发错误帧
    default:
      send_error(addr, port, TFTP_ERROR_ILLEGAL_OPERATION, "Unknown operation");
      break;
  }
  //用完的pbuf要释放掉
  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);
  //执行的是超时重传策略
  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"));
      //重传数据
	  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;
  //申请pcb
  struct udp_pcb *pcb = udp_new_ip_type(IPADDR_TYPE_ANY);
  if (pcb == NULL) {
    return ERR_MEM;
  }
  //绑定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;
  //注册接收函数，当手动udp数据帧时，执行recv函数
  udp_recv(pcb, recv, NULL);

  return ERR_OK;
}

#endif /* LWIP_UDP */

看代码前最后先了解下TFTP的传输流程