source: timediff/timediff.c

/**
 * Rick van der Zwet <info@rickvanderzwet.nl>
 * Licence: BSD
 */

#include "timediff.h"
#include <errno.h>

void diep(char *s)
{
    perror(s);
    exit(1);
}

int main(int argc, char * argv[]) {
    printf("Size of .. : %li\n", sizeof(uint32_t));
    printf("Size of .. : %li\n", sizeof(struct timeval_t));
    printf("Size of .. : %li\n", sizeof(struct timeval));
    struct timeval tv, tv2, tv3, tv4, tv_delta, tv_delta_system;
    struct tm *tm, *tm2;
    struct timeval_t packet;
    gettimeofday(&tv, NULL);
    tm=localtime(&tv.tv_sec);
    printf("Started at: %d:%02d:%02d.%li \n", tm->tm_hour, tm->tm_min, tm->tm_sec, tv.tv_usec);

    struct sockaddr_in si_other, si_me;
    int s, r, i, slen=sizeof(si_other);
    char buf[BUFLEN];

    /* Set up receiving socket */
    if ((r=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))==-1)
        diep("socket");

    memset((char *) &si_me, 0, sizeof(si_me));
    si_me.sin_family = AF_INET;
    si_me.sin_port = htons(PORT_CLIENT);
    si_me.sin_addr.s_addr = htonl(INADDR_ANY);
    if (bind(r, (struct sockaddr *)&si_me, sizeof(si_me))==-1)
        diep("bind");

    if ((r=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))==-1)
        diep("socket");

    /* Prepare server connection */
    memset((char *) &si_other, 0, sizeof(si_other));
    si_other.sin_family = AF_INET;
    si_other.sin_port = htons(PORT);
    if (inet_aton(SRV_IP, &si_other.sin_addr)==0) {
        fprintf(stderr, "inet_aton() failed\n");
        exit(1);
    }

    if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))==-1)
        diep("socket");

    /* We don't care in slow packets anymore */
    struct timeval tv_timeout;
    tv_timeout.tv_sec = 1L;
    tv_timeout.tv_usec = 0L;
    if (setsockopt(s, SOL_SOCKET, SO_RCVTIMEO, &tv_timeout, sizeof(tv_timeout)) != 0)
        diep("setsockopt");


    for (i=0; i<NPACK; i++) {
        printf("Sending packet %d and waiting for response\n", i);
        gettimeofday(&tv, NULL);
        packet.tv_sec = tv.tv_sec;
        packet.tv_usec = tv.tv_usec;
        if (sendto(s, (const void *)&packet, sizeof(packet), 0, (struct sockaddr *)&si_other, slen)==-1)
            diep("sendto()");

        /* This will be generated more and less the same time as on the target side */
        gettimeofday(&tv3, NULL);

        packet.tv_usec = 0;
        packet.tv_sec = 0;

        if (recv(s, &packet, sizeof(packet), 0)==-1) {
            if (errno == EAGAIN) {
                perror("ERROR: No response received in time");
                continue;
            } else {
                diep("recvfrom()");
            }
        }
        gettimeofday(&tv4, NULL);
        tv2.tv_sec = packet.tv_sec;
        tv2.tv_usec = packet.tv_usec;

        printf("Received %li bytes\n", sizeof(packet));

        /* Display and calculations */
        tm=localtime(&tv.tv_sec);
        tm2=localtime(&tv2.tv_sec);
        printf("Send timestamp: %d:%02d:%02d.%06li \n", tm->tm_hour, tm->tm_min, tm->tm_sec, tv.tv_usec);
        printf("Received timestamp: %d:%02d:%02d.%06li \n", tm2->tm_hour, tm2->tm_min, tm2->tm_sec, tv2.tv_usec);

        tv_delta.tv_sec = tv4.tv_sec - tv.tv_sec;
        tv_delta.tv_usec = tv4.tv_usec - tv.tv_usec;
        printf("Time delta at local system (including transmit/receive): %02d.%06li\n",
                (int)(tv_delta.tv_sec), tv_delta.tv_usec);

        tv_delta_system.tv_sec = (tv2.tv_sec - tv.tv_sec) - tv_delta.tv_sec / 2;
        tv_delta_system.tv_usec = (tv2.tv_usec - tv.tv_usec) - tv_delta.tv_usec / 2;
        printf("Time delta between sender and target: %02d.%06li\n",
                (int)(tv_delta_system.tv_sec), tv_delta_system.tv_usec);
        printf("\n");
    }

    close(s);
    return 0;

    exit(0);
}