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
    };

- time_points only prints the 32 lowest bits.
I came to this conclusion by observing the results printed before they got negatives (printing in the loop with the sqrt): 2019000000 nanoseconds is the last value printed before getting negative.

I guess that you have included chrono_io.hpp. Could you confim this? If not, you will need to get the number of nanoseconds using count()

std::cout << "took real : " << elapsed.count() << "nanoseconds\n";

If you have included it, the output should contain nanoseconds twice. E.g.

2383000 nanoseconds since process start-up
took real : 37000 nanosecondsnanoseconds

Is there any mistake in my way of using this chronometer ?

It depends of whether you have included chrono_io.hpp or not. If not, could you post the complete program?

HTH,
Vicente