Get local time in nanoseconds [duplicate]

问题:

Possible Duplicate:
C++ Timer function to provide time in nano seconds

I need to measure the duration of a function execution in nanoseconds resolution. Is it possible? Our ordinary computer hardware and software are able to give such a precision for time? If yes how to accomplish that with c++? Is it possible with Boost library?

回答1:

Yes, today most hardware supports this sort of resolution, and the C++ standard library has an API that can support it as well. Unfortunately not all implementations of C++ actually do provide it.

The API is the <chrono> library introduced in C++11:

#include <iostream>
#include <chrono>

int main() {
    auto start = std::chrono::high_resolution_clock::now();

    // operation to be timed ...

    auto finish = std::chrono::high_resolution_clock::now();
    std::cout << std::chrono::duration_cast<std::chrono::nanoseconds>(finish-start).count() << "ns\n";
}

The <chrono> implementation in libc++ for Darwin provides nanosecond resolution, but it seems the implementation in VS2012 only goes to tenths of milliseconds. The above code will still give times in terms of nanoseconds, but timings less than 100 microseconds will end up being zero nanoseconds.

Boost also provides an implemenation, boost::chrono, which does seem to use nanoseconds on Windows. It's also usable with C++03.

#include <boost/chrono.hpp>

int main() {
    boost::chrono::high_resolution_clock::time_point t1 =
        boost::chrono::high_resolution_clock::now();

    boost::chrono::high_resolution_clock::time_point t2 =
        boost::chrono::high_resolution_clock::now();

    std::cout << boost::chrono::duration_cast<boost::chrono::nanoseconds>(t2-t1) << "\n";
    // boost chrono has more advanced output, .count() isn't needed, nor is manually writing out the units
}

回答2:

About the best you can do with the Windows API is QueryPerformanceCounter. You can obtain its resolution with QueryPerformanceFrequency. Depending on the situation, this may be use the CPU's RDTSC instruction, in which case the frequency is basically the clock frequency of the CPU. In other cases, it uses a separate clock with a frequency of 1.024 MHz, so the resolution is basically one microsecond.

C++11 adds a chrono class that may be useful as well -- assuming you're using a compiler that already has it. If your compiler doesn't provide an implementation, Boost Chrono is a reasonable substitute that works with most existing compilers/libraries. Again, there's no guarantee that it'll provide nanosecond resolution, but I'd expect that on Windows it'll probably be a portable wrapper around QPC/QPF, so it'll probably give the same resolution they do, just more portably.

回答3:

I'm not sure if you're looking for clock or difftime, but one of these should be what you're looking for:

http://www.cplusplus.com/reference/clibrary/ctime/clock/

http://www.cplusplus.com/reference/clibrary/ctime/difftime/

You can use this before and after functions and compare the differences of runtimes to see the overall runtime of a function.

回答4:

And of course there is also the good old Boost::date_time :

#include <boost/date_time/posix_time/posix_time.hpp>

boost::posix_time::ptime date_time = boost::posix_time::microsec_clock::universal_time();

microsec only, though.