获取网络时间协议（NTP）时间戳

Question

gcc (GCC) 4.6.1

I am creating some sdp using RTF 4566 and I want to get the NTP time stamp so that I can use it for the session ID. It is recommended to use the Network Time Protocol (NTP) format timestamp be used to ensure uniqueness.

However, is there any function that will return the ntp time?

Many thanks for any suggestions,

Answer 1

If anyone is interested, this is how I solved my problem.

/* Get the time in micro seconds to create an unique session id */
time_usec = apr_time_usec(apr_time_now());
apr_snprintf(session_id, MAX_STRING_LEN, "%lu", (unsigned long)time_usec);

Hope this helps someone,

Answer 2

The NTP time stamp consists of a 32bit wide value representing the seconds since 1.1.1900 and a 32bit wide value adding a fractional part of value*1/(2^32) seconds to it. 1/(2^32) of a seconds is less then a nano second (233 pico seconds to be exact).

Getting a value of such precision could be achieved using clock_gettime() . Anyhow one should use clock_getres() to test whether the clocks resolution is high enough to serve valid values.

Answer 3

surely there is a better way to do it but this one should works.

#include <stdio.h>
#include <sys/time.h> //gettimeofday()
    int main()
    {
    struct timeval NTP_value;

    gettimeofday(&NTP_value, NULL);

    printf("\nsince January 1, 1970: %ld.%ld s\n", (long int)NTP_value.tv_sec,(long int)NTP_value.tv_usec);
    // that value is since 1970. So you just have to add 70years and 17 days 
    printf("\nsince January 1, 1900: %.6f s\n", (float)NTP_value.tv_sec+ (float)2208988800 + (float)NTP_value.tv_usec/1000000);
    return 0;
    }