C实现一个NTP客户端，可以从指定IP的NTP服务器获取时间戳

参考：：https://blog.csdn.net/dosthing/article/details/81588219

下面的代码测试通过

头文件

#include <sys/types.h>  
#include <sys/stat.h> 
#include <sys/ioctl.h> 
#include <sys/socket.h>
#include <sys/wait.h> 
#include <sys/time.h>
#include <netdb.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h> 
#include <stdio.h>  
#include <stdlib.h>
#include <string.h>  
#include <pthread.h>   
#include <dirent.h> 
#include <time.h>
#include <fcntl.h> 
#include <errno.h>
 
#define debugprintf 1
#ifdef debugprintf
    #define debugpri(mesg, args...) fprintf(stderr, "[NetRate print:%s:%d:] " mesg "\n", __FILE__, __LINE__, ##args) 
#else
    #define debugpri(mesg, args...)
#endif
 
#define JAN_1970             0x83aa7e80
#define NTPFRAC(x) (4294 * (x) + ((1981 * (x))>>11))
#define USEC(x) (((x) >> 12) - 759 * ((((x) >> 10) + 32768) >> 16))
#define Data(i) ntohl(((unsigned int *)data)[i])
#define LI 0
#define VN 3
#define MODE 3
#define STRATUM 0
#define POLL 4 
#define PREC -6


struct NtpTime 
{
    unsigned int coarse;
    unsigned int fine;
};

void sendPacket(int fd);
void getNewTime(unsigned int *data,struct timeval *ptimeval);
double getNtpTime(char * pIP);

源文件

#include "ntp1_so.h"

　int main()
 {
　　char myIP[20]="52.175.49.4"; //time.windows.com
　　double currentTime = getNtpTime(myIP);
　　printf("currentTime=%f\n",currentTime);

　　return 0;
 }

void sendPacket(int fd)
{
    unsigned int data[12];
    struct timeval now;
 
    if (sizeof(data) != 48) 
    {
            fprintf(stderr,"size error\n");
            return;
    }
 
    memset((char*)data, 0, sizeof(data));
    data[0] = htonl((LI << 30) | (VN << 27) | (MODE << 24) | (STRATUM << 16) | (POLL << 8) | (PREC & 0xff));
    data[1] = htonl(1<<16);
    data[2] = htonl(1<<16);
    gettimeofday(&now, NULL);
    data[10] = htonl(now.tv_sec + JAN_1970);
    data[11] = htonl(NTPFRAC(now.tv_usec));
    send(fd, data, 48, 0);
}

void getNewTime(unsigned int *data,struct timeval *ptimeval)
{
    struct NtpTime trantime;
    trantime.coarse = Data(10);
    trantime.fine   = Data(11);
    
    ptimeval->tv_sec = trantime.coarse - JAN_1970;
    ptimeval->tv_usec = USEC(trantime.fine);
}

 
double getNtpTime(char * pIP)
{
    int sockfd;
    struct sockaddr_in addr_src,addr_dst;
    fd_set fds;
    int ret;
    int recv_len;
    unsigned int buf[12];
    memset(buf,0,sizeof(buf));
    int addr_len;
    int count = 0;

    struct timeval recvtv;
    struct timeval *ptimeval;
    double offset;    
    struct timeval timeout;
 
    addr_len = sizeof(struct sockaddr_in);
     //local
    memset(&addr_src, 0, addr_len);
    addr_src.sin_family = AF_INET;
    addr_src.sin_addr.s_addr = htonl(INADDR_ANY);
    addr_src.sin_port = htons(0);
     //remote
    memset(&addr_dst, 0, addr_len);
    addr_dst.sin_family = AF_INET;
    addr_dst.sin_addr.s_addr=inet_addr(pIP);
    addr_dst.sin_port = htons(123);//ntp默认端口123
 
    if(-1==(sockfd = socket(AF_INET,SOCK_DGRAM,IPPROTO_UDP)))
    {        
        debugpri("create socket error!\n");
        return -1;
    }
 
    ret = bind(sockfd, (struct sockaddr*)&addr_src, addr_len);
    if(-1==ret)    
    {        
        debugpri("bind error!\n");        
        close(sockfd);        
        return -1;
    }
    
    ret = connect(sockfd, (struct sockaddr*)&addr_dst, addr_len);
    if(-1==ret)    
    {        
        debugpri("connect error!\n");        
        close(sockfd);        
        return -1;
    }

    sendPacket(sockfd);    
    while (count < 20)    
    {
        FD_ZERO(&fds);
        FD_SET(sockfd, &fds);
 
        timeout.tv_sec = 0;
        timeout.tv_usec = 100000;
        ret = select(sockfd + 1, &fds, NULL, NULL, &timeout);
        if (0 == ret)
        {
            count++;
            debugpri("ret == 0\n");
            sendPacket(sockfd);
            usleep(100*1000);
            continue;
        }
        if(FD_ISSET(sockfd, &fds))
        {
            recv_len = recvfrom(sockfd, buf, sizeof(buf), 0, (struct sockaddr *)&addr_dst, (socklen_t*)&addr_len);
            if(-1==recv_len)        
            {            
                debugpri("recvfrom error\n");            
                close(sockfd);            
                return -1;
            }
            else if(recv_len > 0)
            {
                debugpri("receiv data\n");
                getNewTime(buf,ptimeval);    
                debugpri("sec = %d usec = %d",ptimeval->tv_sec ,ptimeval->tv_usec);
                
                break;
            }
        }
        else
        {
            debugpri("count %d \n",count);
            usleep(50*1000);
            count ++;
        }
    }
    if(count >=20)
    {
        debugpri("getNewTime timeout ! \n");
        close(sockfd);
        return -1;
    }
    close(sockfd);
    return ptimeval->tv_sec;
}