一個簡單的NTP客戶端demo

// 一個簡單的NTP客戶端demo
// NTP協議參考：http://www.enhan.com.cn/blog/index.php/tech/520
// 代碼參考：http://blog.csdn.net/kviling/article/details/7088943

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

#define SECONDS_1900_1970 0x83AA7E80 // 1900/1/1 至 1970/1/1 的秒數

#define NTP_SRV_PORT 123
#define NTP_PACKET_LEN 48

#define TIMEOUT 1

// 一些可用的NTP服務器
#define NTP_SERVER_0 "218.75.4.130"
#define NTP_SERVER_1 "114.80.81.1"
#define NTP_SERVER_2 "202.120.2.101"
#define NTP_SERVER_3 "210.72.145.44"

static void construct_ntp_packet(char content[])
{
    memset(content, 0, NTP_PACKET_LEN);
    content[0] = 0x1b; // LI(2bit) = 0; VN(3bit) = 3; Mode(3bit) = 3 (client); 00 011 011 -> 00011011 -> 0x1b
}

int get_ntp_time(int sockfd, struct sockaddr_in *server_addr, struct tm *net_tm)
{
    char content[256];
    time_t timet;
    long temp;
    int addr_len = 16;
    struct timeval block_time;
    fd_set sockfd_set;

    FD_ZERO(&sockfd_set);
    FD_SET(sockfd, &sockfd_set);
    block_time.tv_sec = TIMEOUT; //recv timeout
    block_time.tv_usec = 0;

    construct_ntp_packet(content);
    if (sendto(sockfd, content, NTP_PACKET_LEN, 0, (struct sockaddr *)server_addr, addr_len) < 0)
    {
        perror("sendto error");
        return (-1);
    }

    if (select(sockfd + 1, &sockfd_set, NULL, NULL, &block_time ) > 0)
    {
        memset(content, 0, sizeof(content));
        if (recvfrom(sockfd, content, sizeof(content), 0, (struct sockaddr *)server_addr, (socklen_t *)&addr_len) < 0)
        {
            perror("recvfrom error");
            return (-1);
        }
        else
        {
            memcpy(&temp, content + 40, 4); //提取服務器端設置的transmit_timastamp

            temp = (time_t)(ntohl(temp) - SECONDS_1900_1970);
            timet = (time_t)temp;
            memcpy(net_tm, gmtime(&timet), sizeof(struct tm));
            net_tm->tm_hour = net_tm->tm_hour + 8; //北京時間，東八區
        }
    }
    else
    {
        return (-1);
    }

    return (0);
}

int main(void)
{
    int sockfd, i;
    struct tm *net_tm;
    struct sockaddr_in addr;
    char ip[4][16]= {{NTP_SERVER_0}, {NTP_SERVER_1}, {NTP_SERVER_2}, {NTP_SERVER_3}};

    net_tm = (struct tm *)malloc(sizeof(struct tm));

    for (i = 0; i < 4; i++)
    {
        memset(&addr, 0, sizeof(addr));
        addr.sin_addr.s_addr = inet_addr(ip[i]);
        addr.sin_port = htons(NTP_SRV_PORT);

        if ((sockfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1)
        {
            perror("socket error");
            return (-1);
        }

        printf("IP:[%s]\n", ip[i]);
        memset(net_tm, 0, sizeof(struct tm));
        if (get_ntp_time(sockfd, &addr, net_tm) == 0)
        {
            printf("NTP:[%d/%d/%d %d:%d:%d]\n\n",
                   net_tm->tm_year+1900,
                   net_tm->tm_mon+1,
                   net_tm->tm_mday,
                   net_tm->tm_hour,
                   net_tm->tm_min,
                   net_tm->tm_sec);
        }
        else
        {
            printf("failed!!!\n\n");
        }

        close(sockfd);
    }

    return (0);
}