Thread-safe GetTickCount64 implementation for Windows XP

Question

I'm targeting Windows XP, and I need a function similar to GetTickCount64, that does not overflow.

I couldn't find a decent solution that is correct and thread safe, so I tried to roll my own.

Here's what I came up with:

ULONGLONG MyGetTickCount64(void)
{
    static volatile DWORD dwHigh = 0;
    static volatile DWORD dwLastLow = 0;
    DWORD dwTickCount;

    dwTickCount = GetTickCount();
    if(dwTickCount < (DWORD)InterlockedExchange(&dwLastLow, dwTickCount))
    {
        InterlockedIncrement(&dwHigh);
    }

    return (ULONGLONG)dwTickCount | (ULONGLONG)dwHigh << 32;
}

Is it really thread safe?

Thread safety is difficult to check for correctness, so I'm not sure whether it's really correct in all cases.

Answer 1

On Windows the timer overflow problem in usually solved (in games) with using QueryPerformanceCounter() functions instead of GetTickCount():

double GetCycles() const
{
    LARGE_INTEGER T1;
    QueryPerformanceCounter( &T1 );
    return static_cast<double>( T1.QuadPart );
}

Then you can multiply this number by reciprocal number of cycles per second to convert cycles to seconds:

void Initialize()
{
    LARGE_INTEGER Freq;
    QueryPerformanceFrequency( &Freq );
    double CyclesPerSecond = static_cast<double>( Freq.QuadPart );
    RecipCyclesPerSecond = 1.0 / CyclesPerSecond;
}

After initialization, this code is thread safe:

double GetSeconds() const
{
    return GetCycles() * RecipCyclesPerSecond;
}

You can also checkout the full source code (portable between Windows and many other platforms) from our open-source Linderdaum Engine: http://www.linderdaum.com