Re: Typo(s) in sys/time.h ?

[Paul Goyette <paul%whooppee.com@localhost>]

On Thu, 5 Jan 2017, Patrick Welche wrote:

> On Thu, Jan 05, 2017 at 12:05:35PM +0800, Paul Goyette wrote:
> > On Thu, 5 Jan 2017, Paul Goyette wrote:
> >
> > > Currently, we have
> > >
> > > static __inline struct bintime
> > > ms2bintime(uint64_t ms)
> > > {
> > >        struct bintime bt;
> > >
> > >        bt.sec = (time_t)(ms / 1000U);
> > >        bt.frac = (((ms % 1000U) >> 32)/1000U) >> 32;
> > >
> > >        return bt;
> > > }
> > >
> > > As far as I can tell, this will result in bt.frac always set to zero,
> > > since the (ms % 1000U) portion is a number less than 1000, and if you
> > > shift a number less than 1000 to the right by 32 bits, you end up with
> > > zero!
> > >
> > > I suspect we should have a left shift here, rather than a right shift:
> > >
> > >        bt.frac = (((ms % 1000U) << 32)/1000U) >> 32;
> > >
> > > ---------------------------------^^           ^^
> >                                                 ^^
> > also here --------------------------------------^^
>
> Why also here? (Guessing shift left to help division, then shift back again?)

You're shifting left a total of 64 bits in order to normalize the 64-bit 
fraction.  Remember, for bintime, this is a (binary) fraction..

Do the math...

start with 500 ms

	( ms % 1000U ) --> 500(decimal) == 0x1f4

	0x1f4 << 32 --> 0x1f400000000

	Now divide that by 0x3e8 (ie, 1000(decimal)) --> 0x80000000

	Shift left again to get the fraction 0x8000000000000000 which
	is 0.5 in decimal!



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