How Can QueryPerformanceCounter Achieve Accurate Time Measurement in C ?

Implementing QueryPerformanceCounter for Accurate Time Measurement

QueryPerformanceCounter is a highly precise function provided by Microsoft Windows for measuring time intervals in microseconds. To implement it effectively in C , let's dissect its usage step by step.

Initially, you need to include the necessary header file:

#include <windows.h>

Next, declare two global variables to store the performance counter frequency and starting point:

double PCFreq = 0.0;
__int64 CounterStart = 0;

The StartCounter() function initializes the performance counter:

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

    PCFreq = double(li.QuadPart)/1000.0;

    QueryPerformanceCounter(&amp;li);
    CounterStart = li.QuadPart;
}

QueryPerformanceFrequency retrieves the counter's frequency, and QueryPerformanceCounter records the current value.

To measure time elapsed since StartCounter was called, use the GetCounter() function:

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

The function calculates the elapsed time by comparing the current counter value to the starting value, adjusting for the counter's frequency.

Example Usage:

StartCounter();
Sleep(1000);
cout << GetCounter() <<"\n";

This program should output a number close to 1000, demonstrating the accuracy of the counter for measuring time durations in microseconds.

You can customize the precision of the counter by modifying the PCFreq calculation:

• Seconds: PCFreq = double(li.QuadPart);
• Microseconds: PCFreq = double(li.QuadPart)/1000000.0;

The choice depends on your desired level of granularity.

The above is the detailed content of How Can QueryPerformanceCounter Achieve Accurate Time Measurement in C ?. For more information, please follow other related articles on the PHP Chinese website!