#include <sys/time.h>                // for gettimeofday() in Linux
                // QueryPerformanceCounter and QueryPerformanceFrequency for Windows


    timeval t1, t2;
    double elapsedTime;


        gettimeofday(&t1, NULL);

        gettimeofday(&t2, NULL);

        elapsedTime = (t2.tv_sec - t1.tv_sec) * 1000.0;      // sec to ms
        elapsedTime += (t2.tv_usec - t1.tv_usec) / 1000.0;   // us to ms

        cout << elapsedTime << " ms.\n";