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PONetwork receipt timer to ms resolution
 

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    <p>My scenario, I'm collecting network packets and if packets match a network filter I want to record the time difference between consecutive packets, this last part is the part that doesn't work. My problem is that I cant get accurate sub-second measurements no matter what C timer function I use. I've tried: gettimeofday(), clock_gettime(), and clock().</p> <p>I'm looking for assistance to figure out why my timing code isn't working properly.</p> <p>I'm running on a cygwin environment. Compile Options: gcc -Wall capture.c -o capture -lwpcap -lrt</p> <p>Code snippet :</p> <pre><code>/*globals*/ int first_time = 0; struct timespec start, end; double sec_diff = 0; main() { pcap_t *adhandle; const struct pcap_pkthdr header; const u_char *packet; int sockfd = socket(PF_INET, SOCK_STREAM, 0); .... (previous I create socket/connect - works fine) save_attr = tty_set_raw(); while (1) { packet = pcap_next(adhandle, &amp;header); // Receive a packet? Process it if (packet != NULL) { got_packet(&amp;header, packet, adhandle); } if (linux_kbhit()) { // User types message to channel kb_char = linux_getch(); // Get user-supplied character if (kb_char == 0x03) // Stop loop (exit channel) if user hits Ctrl+C break; } } tty_restore(save_attr); close(sockfd); pcap_close(adhandle); printf("\nCapture complete.\n"); } </code></pre> <p>In got_packet:</p> <pre><code>got_packet(const struct pcap_pkthdr *header, const u_char *packet, pcap_t * p){ ... { ....do some packet filtering to only handle my packets, set match = 1 if (match == 1) { if (first_time == 0) { clock_gettime( CLOCK_MONOTONIC, &amp;start ); first_time++; } else { clock_gettime( CLOCK_MONOTONIC, &amp;end ); sec_diff = (end.tv_sec - start.tv_sec) + ((end.tv_nsec - start.tv_nsec)/1000000000.0); // Packet difference in seconds printf("sec_diff: %ld,\tstart_nsec: %ld,\tend_nsec: %ld\n", (end.tv_sec - start.tv_sec), start.tv_nsec, end.tv_nsec); printf("sec_diffcalc: %ld,\tstart_sec: %ld,\tend_sec: %ld\n", sec_diff, start.tv_sec, end.tv_sec); start = end; // Set the current to the start for next match } } } </code></pre> <p>I record all packets with Wireshark to compare, so I expect the difference in my timer to be the same as Wireshark's, however that is never the case. My output for tv_sec will be correct, however tv_nsec is not even close. Say there is a 0.5 second difference in wireshark, my timer will say there is a 1.999989728 second difference.</p>
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