【Linux 应用】 ntp请求时间，以及时间设置方式

最近弄了一个开发版，每次开机都要重新使用date命令设置一次时间。开发版老是死机，想着就弄一个ntp获取的程序自动设置。

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一、整体思路

1、请求ntp服务器时间

2、设置时间

二、具体步骤

1、请求ntp服务器时间

1.1、选择可用ntp服务器

地址：119.28.183.184

1.2、请求步骤

1、连接服务器

正常socket连接

2、组包发送到ntp服务器

int get_ntp_packet(void *buf, size_t *size)  //构建并发送NTP请求报文
{
    struct ntphdr *ntp;
    struct timeval tv;


    if (!size || *sizentp_li = NTP_LI;
    ntp->ntp_vn = NTP_VN;
    ntp->ntp_mode = NTP_MODE;
    ntp->ntp_stratum = NTP_STRATUM;
    ntp->ntp_poll = NTP_POLL;
    ntp->ntp_precision = NTP_PRECISION;

    gettimeofday(&tv, NULL);  //把目前的时间用tv 结构体返回
    ntp->ntp_transts.intpart = htonl(tv.tv_sec + JAN_1970);
    ntp->ntp_transts.fracpart = htonl(USEC2FRAC(tv.tv_usec));

    *size = NTP_HLEN;

    return 0;
}

3、接受消息

正常socket读取消息

4、计算偏差（计算当前时间）

//计算C/S时间偏移量
gettimeofday(&recvtv, NULL);
offset = get_offset((struct ntphdr *) buf, &recvtv);
更新系统时间

gettimeofday(&tv, NULL);

tv.tv_sec += (int) offset +28800;
tv.tv_usec += offset - (int) offset;

2、设置系统时间

1、命令方式

strftime(dateBuf, 64, "%Y-%m-%d %H:%M:%S", local);
sprintf(cmd, "busybox date -s \"%s\"", dateBuf);

printf("%s \n", ctime((time_t *) &tv.tv_sec));
printf("dateBuf: %s \n", dateBuf);

int ret = system(cmd);
if(ret < 0){
    printf("system cmd error \n");
}

2、函数接口

settimeofday(&tv, NULL);

三、具体代码

#include 
#include 
#include 
#include 
//#include
#include 
#include 
#include
#include 
#include 
#include 
#include 
#include 

#define VERSION_3           3
#define VERSION_4           4

#define MODE_CLIENT         3
#define MODE_SERVER         4

#define NTP_LI              0
#define NTP_VN              VERSION_3
#define NTP_MODE            MODE_CLIENT
#define NTP_STRATUM         0
#define NTP_POLL            4
#define NTP_PRECISION       -6

#define NTP_HLEN            48

#define NTP_PORT            123
#define NTP_SERVER          "182.92.12.11"

#define TIMEOUT             10

#define BUFSIZE             1500

#define JAN_1970            0x83aa7e80

#define NTP_CONV_FRAC32(x)  (uint64_t) ((x) * ((uint64_t)1<<32))
#define NTP_REVE_FRAC32(x)  ((double) ((double) (x) / ((uint64_t)1<<32)))

#define NTP_CONV_FRAC16(x)  (uint32_t) ((x) * ((uint32_t)1<<16))
#define NTP_REVE_FRAC16(x)  ((double)((double) (x) / ((uint32_t)1<<16)))


#define USEC2FRAC(x)        ((uint32_t) NTP_CONV_FRAC32( (x) / 1000000.0 ))
#define FRAC2USEC(x)        ((uint32_t) NTP_REVE_FRAC32( (x) * 1000000.0 ))


#define NTP_LFIXED2DOUBLE(x)    ((double) ( ntohl(((struct l_fixedpt *) (x))->intpart) - JAN_1970 + FRAC2USEC(ntohl(((struct l_fixedpt *) (x))->fracpart)) / 1000000.0 ))

struct s_fixedpt {
    uint16_t    intpart;
    uint16_t    fracpart;
};

struct l_fixedpt {
    uint32_t    intpart;
    uint32_t    fracpart;
};


struct ntphdr {
#if __BYTE_ORDER == __BID_ENDIAN
    unsigned int    ntp_li:2;
    unsigned int    ntp_vn:3;
    unsigned int    ntp_mode:3;
#endif
#if __BYTE_ORDER == __LITTLE_ENDIAN
    unsigned int    ntp_mode:3;
    unsigned int    ntp_vn:3;
    unsigned int    ntp_li:2;
#endif
    uint8_t         ntp_stratum;
    uint8_t         ntp_poll;
    int8_t          ntp_precision;
    struct s_fixedpt    ntp_rtdelay;
    struct s_fixedpt    ntp_rtdispersion;
    uint32_t            ntp_refid;
    struct l_fixedpt    ntp_refts;
    struct l_fixedpt    ntp_orits;
    struct l_fixedpt    ntp_recvts;
    struct l_fixedpt    ntp_transts;
};


in_addr_t inet_host(const char *host)
{
    in_addr_t saddr;
    struct hostent *hostent;

    if ((saddr = inet_addr(host)) == INADDR_NONE) {
        if ((hostent = gethostbyname(host)) == NULL)
            return INADDR_NONE;

        memmove(&saddr, hostent->h_addr, hostent->h_length);
    }

    return saddr;
}


int get_ntp_packet(void *buf, size_t *size)  //构建并发送NTP请求报文
{
    struct ntphdr *ntp;
    struct timeval tv;


    if (!size || *sizentp_li = NTP_LI;
    ntp->ntp_vn = NTP_VN;
    ntp->ntp_mode = NTP_MODE;
    ntp->ntp_stratum = NTP_STRATUM;
    ntp->ntp_poll = NTP_POLL;
    ntp->ntp_precision = NTP_PRECISION;

    gettimeofday(&tv, NULL);  //把目前的时间用tv 结构体返回
    ntp->ntp_transts.intpart = htonl(tv.tv_sec + JAN_1970);
    ntp->ntp_transts.fracpart = htonl(USEC2FRAC(tv.tv_usec));

    *size = NTP_HLEN;

    return 0;
}



double get_rrt(const struct ntphdr *ntp, const struct timeval *recvtv)  //往返时延
{
    double t1, t2, t3, t4;

    t1 = NTP_LFIXED2DOUBLE(&ntp->ntp_orits);
    t2 = NTP_LFIXED2DOUBLE(&ntp->ntp_recvts);
    t3 = NTP_LFIXED2DOUBLE(&ntp->ntp_transts);
    t4 = recvtv->tv_sec + recvtv->tv_usec / 1000000.0;

    return (t4 - t1) - (t3 - t2);
}


double get_offset(const struct ntphdr *ntp, const struct timeval *recvtv)  //偏移量
{
    double t1, t2, t3, t4;

    t1 = NTP_LFIXED2DOUBLE(&ntp->ntp_orits);
    t2 = NTP_LFIXED2DOUBLE(&ntp->ntp_recvts);
    t3 = NTP_LFIXED2DOUBLE(&ntp->ntp_transts);
    t4 = recvtv->tv_sec + recvtv->tv_usec / 1000000.0;

    return ((t2 - t1) + (t3 - t4)) / 2;
}




int main(int argc, char *argv[])
{
    char dateBuf[64] = {0};
    char cmd[128] = {0};
    struct tm* local;
    char buf[BUFSIZE];
    size_t nbytes;
    int sockfd, maxfd1;
    struct sockaddr_in servaddr;
    fd_set readfds;
    struct timeval timeout, recvtv, tv;
    double offset;

    // if (argc != 2) {
        // usage();
        // exit(-1);


    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons(NTP_PORT);
    servaddr.sin_addr.s_addr = inet_host("119.28.183.184");

    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
        perror("socket error");
        exit(-1);
    }

    if (connect(sockfd, (struct sockaddr *) &servaddr, sizeof(struct sockaddr)) != 0) {
        perror("connect error");
        exit(-1);
    }
    nbytes = BUFSIZE;
    if (get_ntp_packet(buf, &nbytes) != 0) {
        fprintf(stderr, "construct ntp request error \n");
        exit(-1);
    }
    send(sockfd, buf, nbytes, 0);


    FD_ZERO(&readfds);
    FD_SET(sockfd, &readfds);
    maxfd1 = sockfd + 1;


    timeout.tv_sec = TIMEOUT;
    timeout.tv_usec = 0;

    if (select(maxfd1, &readfds, NULL, NULL, &timeout) > 0) {
        if (FD_ISSET(sockfd, &readfds))
        {
            if ((nbytes = recv(sockfd, buf, BUFSIZE, 0)) < 0)
            {
                perror("recv error");
                exit(-1);
            }

            //计算C/S时间偏移量
            gettimeofday(&recvtv, NULL);
            offset = get_offset((struct ntphdr *) buf, &recvtv);
            更新系统时间

            gettimeofday(&tv, NULL);

            tv.tv_sec += (int) offset +28800;
            tv.tv_usec += offset - (int) offset;

            local = localtime((time_t *) &tv.tv_sec);
            strftime(dateBuf, 64, "%Y-%m-%d %H:%M:%S", local);
            sprintf(cmd, "busybox date -s \"%s\"", dateBuf);

            printf("%s \n", ctime((time_t *) &tv.tv_sec));
            printf("dateBuf: %s \n", dateBuf);

            #if 0   //命令形式
            int ret = system(cmd);
            if(ret < 0){
                printf("system cmd error \n");
            }
            #else
            settimeofday(&tv, NULL);
            #endif
        }
    }

    close(sockfd);

    return 0;
}

四、总结

1、获取服务器时间

2、跟本地时间对比计算时间差

3、计算出当前时间、设置时间

如果有ntpdate命令，上面的方法都可以不用，直接使用这个命令即可。具体用法自己百度看下。