Re: Portability fix in kern_ntptime.c

[David Young <dyoung%pobox.com@localhost>]

On Thu, Jun 06, 2019 at 04:42:50AM +0700, Robert Elz wrote:
> Further, I'd never do it without a thorough review of the code,
> if you looked, you'd also see
> 
>                 freq = (ntv->freq * 1000LL) >> 16;
> 
> and
> 
>         ntv->ppsfreq = L_GINT((pps_freq / 1000LL) << 16);
> 
> (and perhaps more) - if one of those is a shift of a negative number,
> the others potentially are as well (not that shifts of negative numbers
> bother me at all if the code author understood what is happening, which
> I suspect that they did here.)

Unfortunately, if "undefined behavior" (UB) is invoked, you simply
cannot claim to understand what is happening, because C11-compliant
compilers have a lot of leeway in what code they generate when behavior
is undefined.

Dave

-- 
David Young
dyoung%pobox.com@localhost    Urbana, IL    (217) 721-9981