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Date: Mon, 12 Feb 2007 11:00:38 +0800
From: Carlo Florendo <subscribermail AT gmail DOT com>
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Subject: Re: strange bug in gettimeofday function
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Andrew Makhorin wrote:
> Hi,
> 
> I detected a strange bug in the standard function gettimeofday.
> It *sometimes* reports the time which being expressed as the integer
> number of milliseconds is *less* than the time obtained *earlier* with
> the same function.

If you mean that you call gettimeofday twice and you get different 
values at each call, with a difference of about 100 ms, then this is all 
right.  Read on...


> 
> The expression 1000000 * tv.tv_sec + tv.tv_usec is calculated in
> 64-bit arithmetic, so overflow cannot happen. The negative difference
> in the time values on two successive calls is about 100 milliseconds.

Please keep in mind that windows is a pre-emptive multi-tasking OS. 
This means that there is a lot of time-sharing among multiple processes.

Windows could be accurate up to 15 ms or perhaps a little bit more. 
However, it is very difficult to achieve less than 15 ms or microsecond 
accuracy with windows due to the limitation on the OS itself.  Our 
extensive tests on windows clocks and timers reveal that windows cannot 
be accurate to the microsecond level or below 15 ms.

FWIW, The accuracy of gettimeofday may also depend on the processor speed.

In any case, in my experience, windows cannot be accurate with a 
precision of up to 15 milliseconds.

The time difference you are seeing seems normal to me (unless I have not 
understood you correctly, in which case, I apologize in advance.)

Thank you very much.


Best Regards,

Carlo


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Carlo Florendo
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