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 (thogard@abnormal.com)
 * 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
",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..
");
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	%-8x
",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
",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
",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
",i,tmit,i-tmit);
    printf("System time is %d seconds off
",i-tmit);
}
else{
    perror("socket 超时");
}
}