NTP:Example:C:01

/* This code will query a ntp server for the local time and display
 * it.  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/
 *
 * @ref <http://stackoverflow.com/questions/10757575/how-to-write-a-ntp-client>
 */
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <time.h>
#include <string.h>
#include <iostream>

void ntpdate();

int main() {
    ntpdate();
    return 0;
}

void ntpdate() {
    //char *hostname=(char *)"163.117.202.33";
    //char *hostname=(char *)"pool.ntp.br";
    char    *hostname=(char *)"200.20.186.76";
    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
    long 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);
    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]));
    long tmit2=ntohl((time_t)buf[i]);
    std::cout << "Round number " << i << " time is " << ctime(&tmit2)  << std::endl;
    }
    */
    /*
     * 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
    tmit=ntohl((time_t)buf[4]);    //# 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));

    std::cout << "time is " << ctime(&tmit)  << std::endl;
    i=time(0);
    //printf("%d-%d=%d\n",i,tmit,i-tmit);
    //printf("System time is %d seconds off\n",(i-tmit));
    std::cout << "System time is " << (i-tmit) << " seconds off" << std::endl;
}