How to execute a while loop precisely every 10 seconds in windows vc++ - visual-c++

Question

Please help me in running the following loop precisely every 10 seconds in windows vc++.
Initially It should start at something like say 12:12:40:000, It should neglect the milliseconds it takes to do some work commented, and restart the next loop at 12:12:50:000 and so on every 10 seconds precisely.
void controlloop()
{
struct timeb start, end;
while(1)
{
ftime(&start);
if(start.time %10 == 0)
break;
else
Sleep(100);
}
while(1)
{
ftime(&start);
if(start.time %10 == 0)
{
// some work here which will roughly take 100 ms
ftime(&end);
elapsedtime = (int) (1000.0 * (end.time - start.time) + (end.millitm - start.millitm));
if(elapsedtime > 10000)
{
sleeptime = 0;
}
else
{
sleeptime = 10000-(elapsedtime);
}
}
Sleep(sleeptime);
}//1
}

Answer 1

The Sleep approach only guarantees you sleep at least 10 seconds. After that your thread is considered eligible for scheduling and on the next quanta it will be considered again. You are still subject to the priority of any other threads on the system, the number of logical cores, etc. You are also still subject to the resolution of the threading quanta which is by default ~15 ms. You can change it with timeBeginPeriod, but that has system-wide power implications.
For more information on Windows scheduling see Microsoft Docs. For more on the power issues, see this blog post.
For Windows the best option is to use the high-frequency performance counter via QueryPerformanceCounter. You use QueryPerformanceFrequency to convert between cycles and seconds.
LARGE_INTEGER qpcFrequency;
QueryPerformanceFrequency(&qpcFrequency);
LARGE_INTEGER startTime;
QueryPerformanceCounter(&startTime);
LARGE_INTEGER tenSeconds;
tenSeconds.QuadPart = startTime .QuadPart + qpcFrequency.QuadPart * 10;
while (true)
{
LARGE_INTEGER currentTime;
QueryPerformanceCounter(&currentTime);
if (currentTime.QuadPart >= tenSeconds.QuadPart)
break;
}
The timer resolution for QPC is typically close the cycle speed of your CPU processor.
If you want to run a thread for as close to 10 seconds as you can while still yielding the processor use:
LARGE_INTEGER qpcFrequency;
QueryPerformanceFrequency(&qpcFrequency);
LARGE_INTEGER startTime;
QueryPerformanceCounter(&startTime);
LARGE_INTEGER tenSeconds;
tenSeconds.QuadPart = startTime .QuadPart + qpcFrequency.QuadPart * 10;
while (true)
{
LARGE_INTEGER currentTIme;
QueryPerformanceCounter(&currentTIme);
if (currentTime.QuadPart >= tenSeconds.QuadPart)
{
// do a thing
tenSeconds.QuadPart = currentTime.QuadPart + qpcFrequency.QuadPart * 10;
SwitchToThread();
}
This is not really the most efficient way to do a periodic timer, but you asked for precision not efficiency.
If you are using VS 2015 or later, you can use the C++11 type high_resolution_clock which uses QPC for it’s implementation. In older versions of Visual C++ used ‘file system time’ which is back to your original resolution problem with ftime.