std::chrono::system_clock::now() precision problem under windows .

windows_precision_issue.cc

#include <random>
#include <chrono>

int main(int argc, char** argv){

    char sz_time_buf[1024] = { 0 };

    std::random_device rd;
    std::mt19937 gen(rd());
    std::uniform_int_distribution<unsigned long> dis(1,1000);

    for (int i = 0; i < 20; ++i) {
        auto _now = std::chrono::duration_cast<std::chrono::microseconds>(std::chrono::system_clock::now().time_since_epoch());
        std::snprintf(sz_time_buf,sizeof(sz_time_buf),".%llu",_now.count());
        std::cout << "time:" << sz_time_buf << std::endl;

        std::this_thread::sleep_for(std::chrono::microseconds(dis(gen)));
    }

    return 0;
}

/*
1. mac output:
time:.1560357052	644		707
time:.1560357052	645		032
time:.1560357052	646		144
time:.1560357052	646		998
time:.1560357052	648		005
time:.1560357052	648		319
time:.1560357052	648		765
time:.1560357052	648		977
time:.1560357052	649		798
time:.1560357052	651		150
time:.1560357052	652		017
time:.1560357052	652		113
time:.1560357052	652		459
time:.1560357052	653		264
time:.1560357052	653		786
time:.1560357052	654		064
time:.1560357052	655		173
time:.1560357052	656		326
time:.1560357052	656		718
time:.1560357052	657		277 

Note: microseconds part discrimination of the timestamp is normal.

2. windows output:
time:.1560357132	557		687
time:.1560357132	560		688
time:.1560357132	563		689
time:.1560357132	565		692
time:.1560357132	567		689
time:.1560357132	570		689
time:.1560357132	572		688
time:.1560357132	574		689
time:.1560357132	576		692
time:.1560357132	579		687
time:.1560357132	582		693
time:.1560357132	585		688
time:.1560357132	588		687
time:.1560357132	590		687
time:.1560357132	592		688
time:.1560357132	595		690
time:.1560357132	597		688
time:.1560357132	599		689
time:.1560357132	601		689
time:.1560357132	603		690

Note: microseconds part discrimination of the timestamp is quite bad.
*/