Create ntp time stamp from gettimeofday
- by krunk
I need to calculate an ntp time stamp using gettimeofday. Below is how I've done it with comments on method. Look good to you guys? (minus error checking). Also, here's a codepad link.
#include <unistd.h>
#include <sys/time.h>
const unsigned long EPOCH = 2208988800UL; // delta between epoch time and ntp time
const double NTP_SCALE_FRAC = 4294967295.0; // maximum value of the ntp fractional part
int main()
{
struct timeval tv;
uint64_t ntp_time;
uint64_t tv_ntp;
double tv_usecs;
gettimeofday(&tv, NULL);
tv_ntp = tv.tv_sec + EPOCH;
// convert tv_usec to a fraction of a second
// next, we multiply this fraction times the NTP_SCALE_FRAC, which represents
// the maximum value of the fraction until it rolls over to one. Thus,
// .05 seconds is represented in NTP as (.05 * NTP_SCALE_FRAC)
tv_usecs = (tv.tv_usec * 1e-6) * NTP_SCALE_FRAC;
// next we take the tv_ntp seconds value and shift it 32 bits to the left. This puts the
// seconds in the proper location for NTP time stamps. I recognize this method has an
// overflow hazard if used after around the year 2106
// Next we do a bitwise AND with the tv_usecs cast as a uin32_t, dropping the fractional
// part
ntp_time = ((tv_ntp << 32) & (uint32_t)tv_usecs);
}
