Re: [Boost-users] [Chrono] Negative real time

Subject: Re: [Boost-users] [Chrono] Negative real time
From: Wilfried Kirschenmann (wilfried.kirschenmann_at_[hidden])
Date: 2012-05-23 03:39:31

On Tue, May 22, 2012 at 9:41 PM, Vicente J. Botet Escriba <
vicente.botet_at_[hidden]> wrote:

>  Le 22/05/12 18:55, Wilfried Kirschenmann a écrit :
>
> Dear chrono maintainer team,
>
> Using the following example :
>
> int main(int argc, char **argv)
> {
>        boost::chrono::process_real_cpu_clock::time_point start =
> boost::chrono::process_real_cpu_clock::now();
>
>        for ( long i = 0; i < 1000; ++i )  std::sqrt( 123.456L ); // burn
> some time
>        Sleep(10000); //idle some time
>
>        boost::chrono::process_real_cpu_clock::time_point end =
> boost::chrono::process_real_cpu_clock::now();
>        std::cout << end << std::endl;
>
>         boost::chrono::process_real_cpu_clock::duration elapsed = (end -
> start);
>        std::cout << "took real     : " << elapsed  << "nanoseconds\n";
>
>        return 0;
> }
>
> I obtain negative times.
>
> I run boost 1.49 win32 on Windows 7 x64 with Visual studio 2010.
>
> Studying the behavior of the timer, I think that the error is one of these
> :
> - process_real_cpu_clock::now() only stores the current time in a 32 bit
> integer instead of a 64 bit integer
>
>
> I don't know from where are you getting this conclusion.
> process_real_cpu_clock is using boost::int_least64_t, as representation.
>
>     typedef duration<boost::int_least64_t, nano> nanoseconds;    // at
> least 64 bits needed
>     class BOOST_CHRONO_DECL process_real_cpu_clock {
>     public:
>         typedef nanoseconds                          duration;
>         typedef duration::rep                        rep;
>         typedef duration::period                     period;
>         typedef chrono::time_point<process_real_cpu_clock>    time_point;
>         BOOST_STATIC_CONSTEXPR bool is_steady =             true;
>
>         static BOOST_CHRONO_INLINE time_point now() BOOST_NOEXCEPT;
> #if !defined BOOST_CHRONO_DONT_PROVIDE_HYBRID_ERROR_HANDLING
>         static BOOST_CHRONO_INLINE time_point now(system::error_code & ec
> );
> #endif
>
>     };
>
>
> From the fact that it behaves as if there were a 32bit overflow. Which is
in fact the case : https://svn.boost.org/trac/boost/ticket/6361
The problem is fixed in 1.50 and I no longer get negative times.

Wilfried K.