ntp客户端源码

以下是一个简单的 NTP 客户端的示例代码，使用 C 语言编写：

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

#define NTP_TIMESTAMP_DELTA 2208988800ull

typedef struct {
    uint8_t li_vn_mode;      // Leap indicator, version and mode
    uint8_t stratum;         // Stratum level of the local clock
    uint8_t poll;            // Maximum interval between successive messages
    uint8_t precision;       // Precision of the local clock
    uint32_t rootDelay;      // Total round trip delay time
    uint32_t rootDispersion; // Max error aloud from primary clock source
    uint32_t refId;          // Reference clock identifier
    uint32_t refTm_s;        // Reference time-stamp seconds
    uint32_t refTm_f;        // Reference time-stamp fraction of a second
    uint32_t origTm_s;       // Originate time-stamp seconds
    uint32_t origTm_f;       // Originate time-stamp fraction of a second
    uint32_t rxTm_s;         // Received time-stamp seconds
    uint32_t rxTm_f;         // Received time-stamp fraction of a second
    uint32_t txTm_s;         // Transmit time-stamp seconds
    uint32_t txTm_f;         // Transmit time-stamp fraction of a second
} ntp_packet;

void error(const char *msg) {
    perror(msg);
    exit(1);
}

int main(int argc, char *argv[]) {
    int sockfd, n;
    struct sockaddr_in serv_addr;
    struct hostent *server;

    char buffer[256];
    ntp_packet packet;

    if (argc < 2) {
        fprintf(stderr,"usage %s hostname\n", argv[0]);
        exit(0);
    }

    sockfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    if (sockfd < 0) {
        error("ERROR opening socket");
    }

    server = gethostbyname(argv[1]);
    if (server == NULL) {
        fprintf(stderr,"ERROR, no such host\n");
        exit(0);
    }

    bzero((char *) &serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    bcopy((char *)server->h_addr,
          (char *)&serv_addr.sin_addr.s_addr,
          server->h_length);
    serv_addr.sin_port = htons(123);

    memset(&packet, 0, sizeof(ntp_packet));
    packet.li_vn_mode = (0x3 << 6) | (0x3 << 3) | 0x3; // NTPv4, client mode

    n = sendto(sockfd, (char *)&packet, sizeof(ntp_packet), 0, (struct sockaddr *)&serv_addr, sizeof(serv_addr));
    if (n < 0) {
        error("ERROR in sendto");
    }

    n = recvfrom(sockfd, (char *)&packet, sizeof(ntp_packet), 0, NULL, NULL);
    if (n < 0) {
        error("ERROR in recvfrom");
    }

    packet.txTm_s = ntohl(packet.txTm_s);
    packet.txTm_f = ntohl(packet.txTm_f);

    time_t txTm = (time_t)(packet.txTm_s - NTP_TIMESTAMP_DELTA);

    printf("Time: %s", ctime((const time_t *)&txTm));

    close(sockfd);
    return 0;
}

该代码中的 `ntp_packet` 结构体表示 NTP 协议中的数据包格式。在 `main` 函数中，它首先创建了一个 UDP 套接字，然后向指定的 NTP 服务器发送一个 NTP 数据包，并等待接收服务器返回的数据包。最后，从接收到的数据包中解析出服务器的时间戳，并将其转换为可读的时间格式，输出到标准输出中。