linux下ntp实现

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

#define NTP_SERVER    	"clock.via.net"
#define NTP_PORT      		123

//
//rfc1305 defined from 1900 so also  2208988800 (1900 - 1970 ) seconds left
//
//timeval.tv_sec + JAN_1970 = timestamp.coarse
#define JAN_1970     		0x83aa7e80

//timeval.tv_usec=>timestamp.fine
#define NTPFRAC(x) (4294 * (x) + ((1981 * (x))>>11))
//timeval.tv_usec<=timestamp.fine
#define USEC(x) (((x) >> 12) - 759 * ((((x) >> 10) + 32768) >> 16))

#define Data(i) ntohl(((unsigned int *)data)[i])

#define LI 0
#define VN 3
#define MODE 3
#define STRATUM 0
#define POLL 4
#define PREC -6

struct ntptime
{
unsigned int coarse;
unsigned int fine;
};

void send_packet(int fd)
{
unsigned int data[12];
struct timeval now;
int ret;

if (sizeof(data) != 48)
{
fprintf(stderr,"size error/n");
return;
}

memset((char*)data, 0, sizeof(data));
data[0] = htonl((LI << 30) | (VN << 27) | (MODE << 24)
| (STRATUM << 16) | (POLL << 8) | (PREC & 0xff));
data[1] = htonl(1<<16);  /* Root Delay (seconds) */
data[2] = htonl(1<<16);  /* Root Dispersion (seconds) */
gettimeofday(&now, NULL);
data[10] = htonl(now.tv_sec + JAN_1970); /* Transmit Timestamp coarse */
data[11] = htonl(NTPFRAC(now.tv_usec));  /* Transmit Timestamp fine   */
send(fd, data, 48, 0);
}

void get_udp_arrival_local_timestamp(struct ntptime *udp_arrival_local_timestamp)
{
struct timeval udp_arrival_local_time;

gettimeofday(&udp_arrival_local_time, NULL);

//print just precise to second
printf("local time=>%s/n",ctime(&(udp_arrival_local_time.tv_sec)));
}

void get_new_time(unsigned int *data,struct timeval* new_time)
{
//int li, vn, mode, stratum, poll, prec;
//int delay, disp, refid;
//struct ntptime reftime, orgtime, rectime;
struct ntptime trantime;
/*li      = Data(0) >> 30 & 0x03;
vn      = Data(0) >> 27 & 0x07;
mode    = Data(0) >> 24 & 0x07;
stratum = Data(0) >> 16 & 0xff;
poll    = Data(0) >>  8 & 0xff;
prec    = Data(0)       & 0xff;
if (prec & 0x80) prec|=0xffffff00;
delay   = Data(1);
disp    = Data(2);
refid   = Data(3);
reftime.coarse = Data(4);
reftime.fine   = Data(5);
orgtime.coarse = Data(6);
orgtime.fine   = Data(7);
rectime.coarse = Data(8);
rectime.fine   = Data(9);*/
trantime.coarse = Data(10);
trantime.fine   = Data(11);

new_time->tv_sec = trantime.coarse - JAN_1970;
new_time->tv_usec = USEC(trantime.fine);

//print just precise to second
printf("server time=>%s/n",ctime(&(new_time->tv_sec)));
}

void set_local_time(struct timeval new_time)
{
/* need root user. */
if (0 != getuid() && 0 != geteuid())
{
printf("must be root user!:0)/n");
return;
}

settimeofday(&new_time, NULL);
}

int main(void)
{
int sockfd;
struct sockaddr_in addr_src,addr_dst;
fd_set fds_read;
int ret;
int receivebytes;
unsigned int buf[12];
int addr_len;

struct timeval timeout,new_time;
struct ntptime udp_arrival_local_timestamp;

struct hostent* host;

addr_len = sizeof(struct sockaddr_in);

memset(&addr_src, 0, addr_len);
addr_src.sin_family = AF_INET;
addr_src.sin_addr.s_addr = htonl(INADDR_ANY);
addr_src.sin_port = htons(0);

memset(&addr_dst, 0, addr_len);
addr_dst.sin_family = AF_INET;
host = gethostbyname(NTP_SERVER);
memcpy(&(addr_dst.sin_addr.s_addr), host->h_addr_list[0], 4);
addr_dst.sin_port = htons(NTP_PORT);

/* create socket. */
if(-1==(sockfd = socket(AF_INET,SOCK_DGRAM,IPPROTO_UDP)))
{
printf("create socket error!/n");
exit(1);
}

/* bind local address. */
ret = bind(sockfd, (struct sockaddr*)&addr_src, addr_len);
if(-1==ret)
{
printf("bind error!/n");
close(sockfd);
}

/* connect to ntp server. */
ret = connect(sockfd, (struct sockaddr*)&addr_dst, addr_len);
if(-1==ret)
{
printf("connect error!/n");
close(sockfd);
}
/*get and set time*/
while (1)
{
FD_ZERO(&fds_read);
FD_SET(sockfd, &fds_read);

timeout.tv_sec = 6;
timeout.tv_usec = 0;
ret = select(sockfd + 1, &fds_read, NULL, NULL, &timeout);
if (0 == ret || !FD_ISSET(sockfd, &fds_read))
{
/* send ntp protocol packet. */
send_packet(sockfd);
continue;
}
/* recv ntp server's response. */
receivebytes = recvfrom(sockfd, buf, sizeof(buf), 0, (struct sockaddr *)&addr_dst, &addr_len);
if(-1==receivebytes)
{
printf("recvfrom error/n");
close(sockfd);
exit(1);
}
/* get local timestamp. */
get_udp_arrival_local_timestamp(&udp_arrival_local_timestamp);
/* get server's time and print it. */
get_new_time(buf, &new_time);
/* set local time to the server's time, if you're a root user. */
set_local_time(new_time);
}

close(sockfd);
return 0;
}