NTP协议应用（获取网络时间）

#include <stdio.h>#include <string.h>#include <sys/socket.h>#include <netdb.h>#include <time.h>#include <stdlib.h>#define debug 1#define TIMEOUT 3#define JAN_1970 0x83AA7E80#define NTP_SERVER_1 "114.80.81.1"    //cn.pool.ntp.org#define NTP_SERVER_2 "122.226.192.4"  //cn.pool.ntp.org#define NTP_SERVER_3 "218.75.4.130"   //cn.pool.ntp.orgvoid construct_ntp_packet(char content[]){long           timer;memset(content, 0, 48);content[0] = 0x1b; // LI = 0 ; VN = 3 ; Mode = 3 (client);time((time_t *)&timer);timer = htonl(timer + JAN_1970 );memcpy(content + 40, &timer, sizeof(timer));  //trans_timastamp}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;      //time out block_time.tv_usec = 0;construct_ntp_packet(content);if (sendto(sockfd, content, 48, 0, (struct sockaddr *)server_addr, addr_len) < 0) {#if debugperror("sendto error");#endifreturn (-1);}if(select(sockfd + 1, &sockfd_set, NULL, NULL, &block_time ) > 0) {if (recvfrom(sockfd, content, 256, 0, (struct sockaddr *)server_addr, (socklen_t *)&addr_len) < 0) {#if debugperror("recvfrom error");#endifreturn (-1);}else {memcpy(&temp, content + 40, 4);temp = (time_t)(ntohl(temp) - JAN_1970 );timet = (time_t)temp;memcpy(net_tm, gmtime(&timet), sizeof(struct tm));net_tm->tm_hour = net_tm->tm_hour + 8;  //beijing time zone}}else {return(-1);}return(0);}int main(){int                  sockfd, i;struct tm            *net_tm;struct sockaddr_in   addr;char                 ip[4][16]= { { NTP_SERVER_1 } , { NTP_SERVER_2 }, { NTP_SERVER_3 } };char                 date_buf[50];net_tm = (struct tm *)malloc(sizeof(struct tm));for (i = 0 ; i < 3 ; i++ ) {memset(&addr, 0, sizeof(addr));addr.sin_addr.s_addr = inet_addr( ip[i] );addr.sin_port = htons(123);if((sockfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1) {#if debugperror ("socket error");#endifreturn (-1);}if (get_ntp_time(sockfd, &addr, net_tm) == 0) {break;}close(sockfd);}strftime(date_buf, sizeof(date_buf), "date -s \"%F %T\"", net_tm);system(date_buf);return (0);}