QueryPerformanceCounter throwing incorrect numbers - c++

Question

I am using QueryPerformanceCounter to measure the time of some functions/operations. It used to give me correct numbers, for instance, I could test Sleep(1000) and it would return a time very close to 1 second. Now, it returns a time very different. I am not sure what the issue is as the code hasn't changed at all. Here is the code:
Expected Output:
duration : 1.000 seconds
Actual Output:
duration : 187.988 seconds
Code:
#include <windows.h>
#pragma comment (lib, "winmm.lib")
struct Clock{
Clock(){}
virtual ~Clock(){}
void start();
void stop();
long double duration();
LARGE_INTEGER _start, _end, _freq;
};
void Clock::start(){
_start.QuadPart = 0;
QueryPerformanceCounter(&_freq);
QueryPerformanceCounter(&_start);
}
void Clock::stop(){
QueryPerformanceCounter(&_end);
}
long double Clock::duration(){
//microseconds
LARGE_INTEGER delta;
delta.QuadPart = (_end.QuadPart - _start.QuadPart) * 1000000;
long double the_duration = ((long double)delta.QuadPart) / _freq.QuadPart;
std::cout << "duration : " << the_duration << " seconds" << std::endl;
return the_duration;
}
void main(){
Clock clock;
clock.start();
Sleep(1000);
clock.stop();
clock.duration();
std::cin.get();
}

Answer 1

You are assuming that the frequency from the performance counter is exactly 1000000 Hz.
You need to call QueryPerformanceFrequency instead, as the frequency can vary (some kernels use the motherboard's 1.024 MHz timer, others use the CPUs time-stamp-counter, which runs at approximately the CPU's clock frequency).

Answer 2

I use it in VBA. Sometimes I get 8976543566 ms instead of 40
I deal with it by calling it twice in the beginning of procedure:
If gbolShowRunTime = True Then
QueryPerformanceCounter curFrequencyStartLoop
End If
If gbolShowBFTime = True Then
QueryPerformanceCounter curFrequencyEndLoop 'Get the end time
QueryPerformanceCounter curFrequencyStartLoop ' reseed it
End If
Thereafter it works fine.