用c语言写一个ntp服务端的代码

以下是一个简单的NTP服务端的C语言代码示例：

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

#define NTP_TIMESTAMP_DELTA 2208988800ull
#define NTP_PACKET_SIZE 48

int main() {
    int sockfd;
    struct sockaddr_in servaddr;
    uint8_t packet[NTP_PACKET_SIZE];
    memset(packet, 0, sizeof(packet));

    packet[0] = 0x1b;

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

    memset(&servaddr, 0, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    servaddr.sin_port = htons(123);

    if (bind(sockfd, (struct sockaddr*)&servaddr, sizeof(servaddr)) < 0) {
        perror("bind");
        exit(1);
    }

    while (1) {
        struct sockaddr_in cliaddr;
        socklen_t len = sizeof(cliaddr);
        ssize_t n = recvfrom(sockfd, packet, sizeof(packet), 0, (struct sockaddr*)&cliaddr, &len);
        if (n < 0) {
            perror("recvfrom");
            continue;
        }

        if (n != NTP_PACKET_SIZE) {
            printf("Invalid NTP packet size %ld\n", n);
            continue;
        }

        uint32_t secs = ntohl(*((uint32_t*)&packet[40]));
        uint32_t frac = ntohl(*((uint32_t*)&packet[44]));

        printf("Received NTP request from %s\n", inet_ntoa(cliaddr.sin_addr));
        printf("Unix time: %u.%u\n", secs - NTP_TIMESTAMP_DELTA, frac);

        time_t t = time(NULL);
        uint32_t curr_secs = (uint32_t)t + NTP_TIMESTAMP_DELTA;
        uint32_t curr_frac = (uint32_t)(((double)(t - (time_t)curr_secs) / 1.0e9) * (1LL<<32));

        memset(packet, 0, sizeof(packet));
        packet[0] = 0x1c;
        *((uint32_t*)&packet[40]) = htonl(curr_secs);
        *((uint32_t*)&packet[44]) = htonl(curr_frac);

        n = sendto(sockfd, packet, sizeof(packet), 0, (struct sockaddr*)&cliaddr, len);
        if (n < 0) {
            perror("sendto");
            continue;
        }
    }

    return 0;
}

该代码创建了一个UDP套接字并将其绑定到本地主机上的UDP端口123。它接收来自客户端的NTP请求，解析请求中的时间戳，并向客户端发送一个响应，其中包含当前时间戳。请注意，这是一个非常简单的示例，没有进行任何错误检查或安全性检查。在实际使用中，请务必加以改进。