Different behaviour of system_clock on windows and linux

Question

For the given program I'm getting different results on Windows(VS 17) compare to linux machine(gcc 4.8).

#include "CrossDevelopment.h"
using namespace std;

int main()
{

    for (auto i = 0; i < 3; i++)
    {
//chrono::high_resolution_clock::time_point start_time = chrono::high_resolution_clock::now();
chrono::system_clock::time_point start_time = chrono::system_clock::now();

        for (auto i = 0; i < 50; i++) {
        int a = 10;
        int b = 5;
        int c = a + b;
                c += 10;
                c *= a;
                a *= b;

    }
//chrono::high_resolution_clock::time_point end_time = chrono::high_resolution_clock::now();
chrono::system_clock::time_point end_time = chrono::system_clock::now();
    auto elapsed_time = chrono::duration<double, micro>(end_time - start_time);
    

cout << "Difference of time " << elapsed_time.count() << " " << (end_time - start_time).count() 
<< " " << (chrono::duration_cast<chrono::nanoseconds>(end_time - start_time)).count() << endl;
}

getchar();
return 0;
}

Output On Windows machine

Difference of time 1 10 1000

Difference of time 0.7 7 700

Difference of time 0.7 7 700

On Linux machine

Difference of time 0.806 806 806

Difference of time 0.6 600 600

Difference of time 0.542 542 542

---

If you see the last columns you will observe the difference. Which is not in case of high_resolution_clock.

Answer 1

The precision of system_clock::time-point is not portable across platforms. But one can easily inspect it, and/or convert it to a known precision as you have done in your question.

The easiest way to inspect it is to use my date.h header:

#include "date/date.h"
#include <iostream>

int
main()
{
    using namespace std;
    using namespace std::chrono;
    using date::operator<<;
    auto start_time = system_clock::now();
    auto end_time = system_clock::now();
    cout << end_time - start_time << '\n';
}

On gcc this is going to output something like:

1730ns

On Windows:

17[1/10000000]s

On macOS:

1µs

Explanation:

On gcc, system_clock::time_point has nanosecond precision, on Windows, it has precision 1/10'000'000 (100ns), and on macOS, microsecond precision.

You can inspect the precision without date.h header by looking at system_clock::duration::period::num and system_clock::duration::period::den, which are the numerator and denominator of a fraction that specifies the length in fractions of a second that each tick represents (1 and 10'000'000 on Windows).

The ability to print out durations with their units (like date.h allows), is currently now in the C++20 specification.