Code clarification on how to calculate NTP time

[yangliu.seu@xxxxxxxxx (liu yang)]

Hi experts,
I'm reading pjsip code, here the function pjmedia_rtcp_ntp_time makes
me confused. I'm confused why we need implement like this. I mean,
according to NTP spec, we can simply call "gettimeofday",and populate
second and fraction fields of ntp timestamp  with 'gettimeofday's sec
and usec>32/1000000  respectively.  Is there anything I overlook? I
think I am. please give me a clarification. Thanks in advance.


/*
 * Get NTP time.
 */
PJ_DEF(pj_status_t) pjmedia_rtcp_get_ntp_time(const pjmedia_rtcp_session *sess,
					      pjmedia_rtcp_ntp_rec *ntp)
{
/* Seconds between 1900-01-01 to 1970-01-01 */
#define JAN_1970  (2208988800UL)
    pj_timestamp ts;
    pj_status_t status;

    status = pj_get_timestamp(&ts);

    /* Fill up the high 32bit part */
    ntp->hi = (pj_uint32_t)((ts.u64 - sess->ts_base.u64) / sess->ts_freq.u64)
	      + sess->tv_base.sec + JAN_1970;

    /* Calculate seconds fractions */
    ts.u64 = (ts.u64 - sess->ts_base.u64) % sess->ts_freq.u64;
    pj_assert(ts.u64 < sess->ts_freq.u64);
    ts.u64 = (ts.u64 << 32) / sess->ts_freq.u64;

    /* Fill up the low 32bit part */
    ntp->lo = ts.u32.lo;


}



Thanks

Kandy