Re: [boost] [chrono] Possible bug in boost::chrono::process_real_cpu_clock::now() under Windows 32bits

Subject: Re: [boost] [chrono] Possible bug in boost::chrono::process_real_cpu_clock::now() under Windows 32bits
From: Howard Hinnant (howard.hinnant_at_[hidden])
Date: 2012-01-05 11:38:56

On Jan 5, 2012, at 10:30 AM, ivan.lelann_at_[hidden] wrote:

> Hi,
> 
> As of 1.48 Boost.Chrono contains code below for boost::chrono::process_real_cpu_clock::now() 
> (boost\chrono\detail\inlined\win\process_cpu_clocks.hpp)
> 
>    clock_t c = ::clock();
>    /* ... */
>    return time_point(
>      duration(c*(1000000000l/CLOCKS_PER_SEC))
>    );
> 
> duration::rep is int64/nanoseconds, clock_t is long. This is VS2008, Win XP 32bits.
> 
> I think "c" should be cast to duration::rep before being multiplied.
> I randomly get negative time_point and this seems to be fixed by replacing 
> "c" with "((duration::rep) (c))" in duration constructor :
> 
>        duration( ((duration::rep) (c)) *(1000000000l/CLOCKS_PER_SEC))
> 
> Can someone confirm my view and that the fix is correct ?

Your fix looks correct to me.  I'm not positive what value CLOCKS_PER_SEC has on your platform.  But here is a reformulation of your solution that might be a little safer (depending on what CLOCKS_PER_SEC actually is):

typedef ratio_divide<giga, ratio<CLOCKS_PER_SEC>>::type R;
return time_point(
  duration(static_cast<rep>(c)*R::num/R::den)
);

Using giga avoids mistakes in counting zeros.

Using ratio_divide reduces the fraction 1000000000l/CLOCKS_PER_SEC to lowest terms, no matter the value of CLOCKS_PER_SEC.

You can use rep as a shortcut for duration::rep, assuming this is inside of the clock's now() function.

static_cast<rep> is a little safer than a C cast.  I would expect the results to be the same, but it is good to code defensively.

If you know a-priori that R::den is 1, you might:

typedef ratio_divide<giga, ratio<CLOCKS_PER_SEC>>::type R;
static_assert(R::den == 1, "oops!");
return time_point(
  duration(static_cast<rep>(c)*R::num)
);

Howard