QueryPerformanceFrequency

QueryPerformanceFrequency()函数获得机器内部计时器的时钟频率。

接着在需要严格计时的事件发生前和发生之后分别调用QueryPerformanceCounter()，利用两次获得的计 数之差和时钟频率，就可以计算出事件经历的精确时间。

    _LARGE_INTEGER time_start; 
    //结束时间
    _LARGE_INTEGER time_over;     
    //计时器频率
    double dqFreq;  

    //计时器频率
    LARGE_INTEGER f;            
    QueryPerformanceFrequency(&f);
    dqFreq=(double)f.QuadPart;
    QueryPerformanceCounter(&time_start);
    //循环耗时
    Sleep(1000);

    QueryPerformanceCounter(&time_over);

    //单位为秒，精度为1000 000/（cpu主频）微秒
    cout<<((time_over.QuadPart-time_start.QuadPart)/dqFreq)<<endl;