linux下ntp协议的实现及代码（更正源代码两处错误）

虽然说原理上并不难，但是过程中却也遇到了些问题。重要的是，做完这个实验之后，搞清了一些平时并不太注意的东西。

另外，只计算了整数部分，小数部分没有计算。校正后的时间跟标准时间误差不超过一秒。

接下来复习并总结一下前一阵学过的内容，并且准备拟定下一个阶段的计划。

//colinluan 090613
//GMT时间：格林威治时间，国际标准时间，中国位于东八区，时间为GMT+8
//UTC时间: 通用协调时间，可以认为跟GMT是一个概念。
//时间戳：NTP服务器上的时间戳为从1900年1月1日0时0分开始到至今的秒数(UTC时间)
//    而PC机上的时间戳为1970年1月1日0时0分到至今的秒数(程序中的time(NULL)返回的是UTC时间)
//    所以，要对PC机的时间戳与服务器时间戳计算时，需在PC机的时间戳基础上加上1900-1970的秒数 3600s*24h*(365days*70years+17days)
//    客户机跟服务器的时间差=((T2-T1)+(T3-T4))>>1
//    传输延时=((T2-T1)-(T3-T4))>>1 (假设来跟去的传输延时是一样的)
//    T1 请求离开客户端的时间戳
//    T2 请求到达服务器的时间戳(由服务器返回)
//    T3 从服务器返回的时间戳(由服务器返回)
//    T4 返回数据包到达客户端的时间戳
//  客户端(请求)          服务器
//    T1 ---------------------------------> T2
//              (返回)
//    T4 <--------------------------------- T3
//    在计算时由于会碰到32位数的符号位问题，索性用64位数进行计算。

#include <stdio.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <arpa/inet.h>
//#include <netdb.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <sys/time.h>
#include <unistd.h>

#define  int8      char
#define  uint8     unsigned char
#define  uint32    unsigned int
#define  ulong32   unsigned long
#define  long32    long
#define  int32 int
#define  long64    long long

#define  debug

//3600s*24h*(365days*70years+17days)
#define  From00to70 0x83aa7e80U

#define  NTPSVR1 "132.163.4.102" //USA
#define  NTPSVR2 "132.163.135.132" //USA
#define  NTPSVR3 "192.53.103.103" //Germany

#define  NTPPORT  123
typedef struct NTPPACKET
{
  uint8     li_vn_mode;
  uint8     stratum;
  uint8     poll;
  uint8     precision;
  ulong32   root_delay;
  ulong32   root_dispersion;
  int8      ref_id[4];
  ulong32   reftimestamphigh;
  ulong32   reftimestamplow;
  ulong32   oritimestamphigh;
  ulong32   oritimestamplow;
  ulong32   recvtimestamphigh;
  ulong32   recvtimestamplow;
  ulong32   trantimestamphigh;
  ulong32   trantimestamplow;
}NTPPacket;

NTPPacket  ntppack,newpack;

//定义为long64,解决32位数的符号位问题
long64   firsttimestamp,finaltimestamp;
long64   diftime,delaytime;

void NTP_Init()
{
  bzero(&ntppack,sizeof(ntppack));
  ntppack.li_vn_mode=0x1b;//0|(3<<2)|(3<<5);
  //获取初始时间戳T1
  firsttimestamp=From00to70+time(NULL);//-8*3600;
  ntppack.oritimestamphigh=htonl(firsttimestamp);
}
int main()
{
// ulong32 clienttime;
// ulong32 diftime,firsttimestamp,finaltimestamp;
  fd_set  inset1;
  int32  sockfd;
  struct timeval tv,tv1;
  struct timezone tz;
  struct sockaddr_in addr;
// printf("%d,%d,%d\n",&ntppack.li_vn_mode,&ntppack.stratum,&ntppack.poll);
// printf("%d  %ld\n",sizeof(NTPPacket),sizeof(ntppack));

// printf("%ld\n",time(NULL));

  if((sockfd=socket(AF_INET,SOCK_DGRAM,0))<0)
    {
      perror("create socket error!\n");
      exit(1);
    }

  addr.sin_family=AF_INET; //IPV4协议
  addr.sin_port =htons(NTPPORT); //NTP专用的123端口
  addr.sin_addr.s_addr=inet_addr(NTPSVR1); //校时服务器
  bzero(&(addr.sin_zero),8); //清零

  //wait 5s
  tv.tv_sec=10; //select等待时间为10S
  tv.tv_usec=0;

  FD_ZERO(&inset1);
  FD_SET(sockfd,&inset1);

  NTP_Init();
  //发送数据请求包
  sendto(sockfd,&ntppack,sizeof(ntppack),0,(struct sockaddr *)&addr,sizeof(struct sockaddr));
  //select巡视
  if(select(sockfd+1,&inset1,NULL,NULL,&tv)<0)
  {
    perror("select error!\n");
    exit(1);
  }
  else
  {
  //printf("OK\n");
    if(FD_ISSET(sockfd,&inset1))
    {
  // printf("OK\n");
     if(recv(sockfd,&newpack,sizeof(newpack),0)<0) //接收数据在newpack中。
     {
      perror("recv error!\n");
      exit(1);
     }
    }
  }
  //到达客户机时间戳T4
  finaltimestamp=time(NULL)+From00to70;//-8*3600;

  //将网络上传送的大端数据改为小端形式。
 newpack.root_delay= ntohl(newpack.root_delay);
 newpack.root_dispersion= ntohl(newpack.root_dispersion);
 newpack.reftimestamphigh=ntohl(newpack.reftimestamphigh);
 newpack.reftimestamplow= ntohl(newpack.reftimestamplow);
 newpack.oritimestamphigh= ntohl(newpack.oritimestamphigh);
 newpack.oritimestamplow= ntohl(newpack.oritimestamplow);
 newpack.recvtimestamphigh= ntohl(newpack.recvtimestamphigh);
 newpack.recvtimestamplow= ntohl(newpack.recvtimestamplow);
 newpack.trantimestamphigh= ntohl(newpack.trantimestamphigh);
 newpack.trantimestamplow= ntohl(newpack.trantimestamplow);

 //求出客户机跟服务器的时间差=((T2-T1)+(T3-T4))/2
 diftime=((newpack.recvtimestamphigh-firsttimestamp)+(newpack.trantimestamphigh-finaltimestamp))>>1;
 //求出延时
 delaytime=((newpack.recvtimestamphigh-firsttimestamp)-(newpack.trantimestamphigh-finaltimestamp))>>1;
 //diftime=(5-9)>>1;

 //求出真正时间的时间戳
 tv1.tv_sec=time(NULL)+diftime+delaytime;
 tv1.tv_usec=0;
 //tz.

 #ifdef debug
  printf("\n\ndebug information ...\n\n");
  printf("time(NULL) is %ld\n",time(NULL));
  printf("different time is %ld\n",diftime);
  printf("delaytime is %ld\n",delaytime);
  printf("time(NULL)+diftime+delaytime=%ld\n",time(NULL)+diftime+delaytime);
  printf("tv1.tv_sec is %ld\n\n", tv1.tv_sec);
 #endif

  settimeofday(&tv1,NULL);
//diftime=diftime-From00to70;

 #ifdef debug
  //printf("different time is %ld\n",diftime);
  printf("delay  time is %ld\n",delaytime);
  //printf("firsttimestamp is %x\n",time(NULL));
  printf("newpack.tran is %ld\n",newpack.trantimestamphigh);
  printf("newpack.recv is %ld\n",newpack.recvtimestamphigh);
  printf("firsttimestamp is %ld\n",firsttimestamp);
  printf("finaltimestamp is %ld\n",finaltimestamp);
  printf("newpack.recv-firsttimestamp is %ld\n",newpack.recvtimestamphigh-firsttimestamp);
  printf("newpack.tran-finaltimestamp is %ld\n",newpack.trantimestamphigh-finaltimestamp);
  printf("(recv-first)+(ftran-final) is %ld\n",(newpack.recvtimestamphigh-firsttimestamp)+(newpack.trantimestamphigh-finaltimestamp));
  printf("((recv-first)+(ftran-final))>>1 is %ld\n",((newpack.recvtimestamphigh-firsttimestamp)+(newpack.trantimestamphigh-finaltimestamp))>>1);
  printf("different time is %ld\n\n",diftime);
  printf("sizeof(long long)  is:%d\n",sizeof(long long));
  printf("Current time is...\n");
  system("date");
 #endif
}

注：原文代码59行和117行错误已修改，目前可以编译通过并成功运行。

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