c 开发ntp远程校时程序

         ntp的协议程序是网上找的，ntp的服务器用的是上海交大的。因为udp是无连接的服务，所以原有的程序采样的是利用recvform()这个方法来接收远程的数据，可能是网络是原因当我发送请求的时候没有接收到回复的数据，所以程序就一直挂起。我在后期添加select操作来做超时的处理，超时之后打印超时信息否则正常接收数据。

/*t.  it is intended to show how to use a NTP server as a time

 * source for a simple network connected device.

 * This is the C version.  The orignal was in Perl

 *

 * For better clock management see the offical NTP info at:

 * http://www.eecis.udel.edu/~ntp/

 *

 * written by Tim Hogard ([email protected])

 * Thu Sep 26 13:35:41 EAST 2002

 * Converted to C Fri Feb 21 21:42:49 EAST 2003

 * this code is in the public domain.

 * it can be found here http://www.abnormal.com/~thogard/ntp/

 *

 */

#include <stdio.h>

#include <sys/types.h>

#include <sys/socket.h>

#include <netinet/in.h>

#include <arpa/inet.h>

#include <netdb.h>

#include <sys/time.h>

#include <string.h>



void ntpdate();



int main() {

    ntpdate();

    return 0;

}



void ntpdate() {

char *hostname="202.120.2.101";

int portno=123;     //NTP is port 123

int maxlen=1024;        //check our buffers

int i;          // misc var i

unsigned char msg[48]={010,0,0,0,0,0,0,0,0};    // the packet we send

unsigned long  buf[maxlen]; // the buffer we get back

//struct in_addr ipaddr;        //

struct protoent *proto;     //

struct sockaddr_in server_addr;

int s;  // socket

int tmit;   // the time -- This is a time_t sort of



//use Socket;

//

//#we use the system call to open a UDP socket

//socket(SOCKET, PF_INET, SOCK_DGRAM, getprotobyname("udp")) or die "socket: $!";

proto=getprotobyname("udp");

s=socket(PF_INET, SOCK_DGRAM, proto->p_proto);

perror("socket");

//

//#convert hostname to ipaddress if needed

//$ipaddr   = inet_aton($HOSTNAME);

memset( &server_addr, 0, sizeof( server_addr ));

server_addr.sin_family=AF_INET;

server_addr.sin_addr.s_addr = inet_addr(hostname);

//argv[1] );

//i   = inet_aton(hostname,&server_addr.sin_addr);

server_addr.sin_port=htons(portno);

//printf("ipaddr (in hex): %x\n",server_addr.sin_addr);



/*

 * build a message.  Our message is all zeros except for a one in the

 * protocol version field

 * msg[] in binary is 00 001 000 00000000

 * it should be a total of 48 bytes long

*/



// send the data

printf("sending data..\n");

i=sendto(s,msg,sizeof(msg),0,(struct sockaddr *)&server_addr,sizeof(server_addr));

perror("sendto");

// get the data back

struct sockaddr saddr;

socklen_t saddr_l = sizeof (saddr);

fd_set readfds, masterfds;

struct timeval timeout;

timeout.tv_sec = 1;                    /*set the timeout to 10 seconds*/

timeout.tv_usec = 0;

FD_ZERO(&masterfds);

FD_SET(s, &masterfds);

memcpy(&readfds, &masterfds, sizeof(fd_set));

if (select(s+1, &readfds, NULL, NULL, &timeout) < 0)

{

}

if(FD_ISSET(s,&readfds))

{

    i=recvfrom(s,buf,48,0,&saddr,&saddr_l);

    perror("recvfr:");



    //We get 12 long words back in Network order

    /*

    for(i=0;i<12;i++)

        printf("%d\t%-8x\n",i,ntohl(buf[i]));

    */



    /*

     * The high word of transmit time is the 10th word we get back

     * tmit is the time in seconds not accounting for network delays which

     * should be way less than a second if this is a local NTP server

     */



    tmit=ntohl((time_t)buf[10]);    //# get transmit time

    //printf("tmit=%d\n",tmit);



    /*

     * Convert time to unix standard time NTP is number of seconds since 0000

     * UT on 1 January 1900 unix time is seconds since 0000 UT on 1 January

     * 1970 There has been a trend to add a 2 leap seconds every 3 years.

     * Leap seconds are only an issue the last second of the month in June and

     * December if you don't try to set the clock then it can be ignored but

     * this is importaint to people who coordinate times with GPS clock sources.

     */



    tmit-= 2208988800U;

    //printf("tmit=%d\n",tmit);

    /* use unix library function to show me the local time (it takes care

     * of timezone issues for both north and south of the equator and places

     * that do Summer time/ Daylight savings time.

     */





    //#compare to system time

    printf("Time: %s",ctime(&tmit));

    i=time(0);

    //printf("%d-%d=%d\n",i,tmit,i-tmit);

    printf("System time is %d seconds off\n",i-tmit);

}

else{

    perror("socket 超时");

}

}