struct bintime

[Dimitry Andric]

On 17 Oct 2014, at 00:15, Jeremie Le Hen <jlh at FreeBSD.org> wrote:
> I need to get microseconds from a struct bintime.  I found
> bintime2timeval() in sys/time.h which more or less does this, but I
> don't understand how the computation works.
> 
> Can someone explain it to me please?
> 
> static __inline void
> bintime2timeval(const struct bintime *_bt, struct timeval *_tv)
> {
> 
>        _tv->tv_sec = _bt->sec;
>        _tv->tv_usec = ((uint64_t)1000000 * (uint32_t)(_bt->frac >> 32)) >> 32;
> }

Maybe it's easier to grok if you read it as:

  usec = (10^6 * (frac / 2^32)) / 2^32

or even:

  usec = 10^6 * (frac / 2^64)

and the other way around:

  frac = (usec / 10^6) * 2^64

E.g., bintime divides a second into 2^64 slices, instead of 10^6 or
10^9.  And the division by 2^64 in bintime2timeval() is split up in two
shifts, to avoid losing precision.

-Dimitry

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