使用C语言写ntp客户端，对保文的详细处理步骤及代码

下面是使用C语言编写NTP客户端对保文的详细处理步骤及代码：

1. 创建UDP套接字

#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>

#define NTP_SERVER "time.windows.com"
#define NTP_PORT 123
#define NTP_PACKET_SIZE 48

int main(int argc, char **argv) {
    int sockfd;
    struct sockaddr_in servaddr;

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

    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = inet_addr(NTP_SERVER);
    servaddr.sin_port = htons(NTP_PORT);

    // 设置超时时间为5秒
    struct timeval timeout;
    timeout.tv_sec = 5;
    timeout.tv_usec = 0;
    setsockopt(sockfd, SOL_SOCKET, SO_RCVTIMEO, (char *)&timeout, sizeof(timeout));

    // 设置广播选项
    int broadcast = 1;
    setsockopt(sockfd, SOL_SOCKET, SO_BROADCAST, &broadcast, sizeof(broadcast));

    // ...
}

2. 构建NTP请求数据包

// 构建NTP请求数据包
unsigned char ntp_packet[NTP_PACKET_SIZE];
memset(ntp_packet, 0, NTP_PACKET_SIZE);
ntp_packet[0] = 0x1b; // LI, Version, Mode
// ...

3. 发送NTP请求数据包

// 发送NTP请求数据包
if (sendto(sockfd, ntp_packet, NTP_PACKET_SIZE, 0, (struct sockaddr *)&servaddr, sizeof(servaddr)) < 0) {
    perror("sendto");
    exit(1);
}

4. 接收NTP服务器返回的数据包

// 接收NTP服务器返回的数据包
unsigned char recv_buf[NTP_PACKET_SIZE];
struct sockaddr_in from;
socklen_t from_len = sizeof(from);
if (recvfrom(sockfd, recv_buf, NTP_PACKET_SIZE, 0, (struct sockaddr *)&from, &from_len) < 0) {
    perror("recvfrom");
    exit(1);
}

5. 解析NTP服务器返回的数据包，提取时间戳字段

// 解析NTP服务器返回的数据包，提取时间戳字段
unsigned char *timestamp = recv_buf + 40;
unsigned long long ntp_timestamp = 0;
for (int i = 0; i < 8; i++) {
    ntp_timestamp = (ntp_timestamp << 8) + timestamp[i];
}

6. 对时间戳进行格式化处理，转换为本地时间

// 对时间戳进行格式化处理，转换为本地时间
unsigned long long unix_timestamp = (ntp_timestamp - 2208988800ULL) & 0xffffffff;
struct tm *tm_time = localtime((time_t *)&unix_timestamp);
char time_buf[64];
strftime(time_buf, sizeof(time_buf), "%Y-%m-%d %H:%M:%S", tm_time);
printf("Local time: %s\n", time_buf);

7. 关闭UDP套接字，释放资源

// 关闭UDP套接字，释放资源
close(sockfd);

完整代码如下：

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

#define NTP_SERVER "time.windows.com"
#define NTP_PORT 123
#define NTP_PACKET_SIZE 48

int main(int argc, char **argv) {
    int sockfd;
    struct sockaddr_in servaddr;

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

    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = inet_addr(NTP_SERVER);
    servaddr.sin_port = htons(NTP_PORT);

    // 设置超时时间为5秒
    struct timeval timeout;
    timeout.tv_sec = 5;
    timeout.tv_usec = 0;
    setsockopt(sockfd, SOL_SOCKET, SO_RCVTIMEO, (char *)&timeout, sizeof(timeout));

    // 设置广播选项
    int broadcast = 1;
    setsockopt(sockfd, SOL_SOCKET, SO_BROADCAST, &broadcast, sizeof(broadcast));

    // 构建NTP请求数据包
    unsigned char ntp_packet[NTP_PACKET_SIZE];
    memset(ntp_packet, 0, NTP_PACKET_SIZE);
    ntp_packet[0] = 0x1b; // LI, Version, Mode

    // 发送NTP请求数据包
    if (sendto(sockfd, ntp_packet, NTP_PACKET_SIZE, 0, (struct sockaddr *)&servaddr, sizeof(servaddr)) < 0) {
        perror("sendto");
        exit(1);
    }

    // 接收NTP服务器返回的数据包
    unsigned char recv_buf[NTP_PACKET_SIZE];
    struct sockaddr_in from;
    socklen_t from_len = sizeof(from);
    if (recvfrom(sockfd, recv_buf, NTP_PACKET_SIZE, 0, (struct sockaddr *)&from, &from_len) < 0) {
        perror("recvfrom");
        exit(1);
    }

    // 解析NTP服务器返回的数据包，提取时间戳字段
    unsigned char *timestamp = recv_buf + 40;
    unsigned long long ntp_timestamp = 0;
    for (int i = 0; i < 8; i++) {
        ntp_timestamp = (ntp_timestamp << 8) + timestamp[i];
    }

    // 对时间戳进行格式化处理，转换为本地时间
    unsigned long long unix_timestamp = (ntp_timestamp - 2208988800ULL) & 0xffffffff;
    struct tm *tm_time = localtime((time_t *)&unix_timestamp);
    char time_buf[64];
    strftime(time_buf, sizeof(time_buf), "%Y-%m-%d %H:%M:%S", tm_time);
    printf("Local time: %s\n", time_buf);

    // 关闭UDP套接字，释放资源
    close(sockfd);

    return 0;
}