How to use QueryPerformanceCounter?

Question

I recently decided that I needed to change from using milliseconds to microseconds for my Timer class, and after some research I've decided that QueryPerformanceCounter is probably my safest best. (The warning on Boost::Posix that it may not works on Win32 API put me off a bit). However, I'm not really sure how to implement it.

What I'm doing is calling whatever GetTicks() esque function I'm using and assigning it to Timer's startingTicks variable. Then to find the amount of time passed I just subtract the function's return value from the startingTicks, and when I reset the timer I just call the function again and assign startingTicks to it. Unfortunately, from the code I've seen it isn't as simple as just calling QueryPerformanceCounter(), and I'm not sure what I'm supposed to pass as its argument.

Any assistance is appreciated, thanks.

Answer 1

#include <windows.h>

double PCFreq = 0.0;
__int64 CounterStart = 0;

void StartCounter()
{
    LARGE_INTEGER li;
    if(!QueryPerformanceFrequency(&li))
 cout << "QueryPerformanceFrequency failed!\n";

    PCFreq = double(li.QuadPart)/1000.0;

    QueryPerformanceCounter(&li);
    CounterStart = li.QuadPart;
}
double GetCounter()
{
    LARGE_INTEGER li;
    QueryPerformanceCounter(&li);
    return double(li.QuadPart-CounterStart)/PCFreq;
}

int main()
{
    StartCounter();
    Sleep(1000);
    cout << GetCounter() <<"\n";
    return 0;
}

This program should output a number close to 1000 (windows sleep isn't that accurate, but it should be like 999).

The StartCounter() function records the number of ticks the performance counter has in the CounterStart variable. The GetCounter() function returns the number of milliseconds since StartCounter() was last called as a double, so if GetCounter() returns 0.001 then it has been about 1 microsecond since StartCounter() was called.

If you want to have the timer use seconds instead then change

PCFreq = double(li.QuadPart)/1000.0;

to

PCFreq = double(li.QuadPart);

or if you want microseconds then use

PCFreq = double(li.QuadPart)/1000000.0;

But really it's about convenience since it returns a double.

Answer 2

Assuming you're on Windows (if so you should tag your question as such!), on this MSDN page you can find the source for a simple, useful HRTimer C++ class that wraps the needed system calls to do something very close to what you require (it would be easy to add a GetTicks() method to it, in particular, to do exactly what you require).

On non-Windows platforms, there's no QueryPerformanceCounter function, so the solution won't be directly portable. However, if you do wrap it in a class such as the above-mentioned HRTimer, it will be easier to change the class's implementation to use what the current platform is indeed able to offer (maybe via Boost or whatever!).