Time difference in C++

Question

Hi guys,

Does anyone know how to calculate time difference in C++ in miliseconds? I used difftime (time.h) but it's not enough precision for what I'm trying to measure.

Thanks, in advance. Alejo

Answer 1

You can use gettimeofday to get the number of microseconds since epoch. The seconds segment of the value returned by gettimeofday() is the same as that returned by time() and can be cast to a time_t and used in difftime. A millisecond is 1000 microseconds.

After you use difftime, calculate the difference in the microseconds field yourself.

Answer 2

You have to use one of the more specific time structures, either timeval (microsecond-resolution) or timespec (nanosecond-resolution), but you can do it manually fairly easily:

#include <time.h>

int diff_ms(timeval t1, timeval t2)
{
    return (((t1.tv_sec - t2.tv_sec) * 1000000) + 
            (t1.tv_usec - t2.tv_usec))/1000;
}

This obviously has some problems with integer overflow if the difference in times is really large (or if you have 16-bit ints), but that's probably not a common case.

Answer 3

The clock function gives you a millisecond timer, but it's not the greatest. Its real resolution is going to depend on your system. You can try

#include <time.h>

int clo = clock();
//do stuff
cout << (clock() - clo) << endl;

and see how your results are.

Answer 4

If you're looking to do benchmarking, you might want to see some of the other threads here on SO which discuss the topic.

Also, be sure you understand the difference between accuracy and precision.

Answer 5

I think you will have to use something platform-specific. Hopefully that won't matter? eg. On Windows, look at QueryPerformanceCounter() which will give you something much better than milliseconds.

Answer 6

You can get micro and nanosecond precision out of Boost.Date_Time.

Answer 7

Hi, if you are using win32 FILETIME is the most accurate that you can get: Contains a 64-bit value representing the number of 100-nanosecond intervals since January 1, 1601 (UTC).

So if you want to calculate the difference between two times in milliseconds you do the following:

UINT64 getTime()
{
    SYSTEMTIME st;
    GetSystemTime(&st);

    FILETIME ft;
    SystemTimeToFileTime(&st, &ft);  // converts to file time format
    ULARGE_INTEGER ui;
    ui.LowPart=ft.dwLowDateTime;
    ui.HighPart=ft.dwHighDateTime;

    return ui.QuadPart;
}

int _tmain(int argc, TCHAR* argv[], TCHAR* envp[])
{
    //! Start counting time
    UINT64   start, finish;

    start=getTime();

    //do something...

    //! Stop counting elapsed time
    finish = getTime();

    //now you can calculate the difference any way that you want
    //in seconds:
    _tprintf(_T("Time elapsed executing this code: %.03f seconds."), (((float)(finish-start))/((float)10000))/1000 );
    //or in miliseconds
    _tprintf(_T("Time elapsed executing this code: %I64d seconds."), (finish-start)/10000 );
}