Clamp framerate in Windows - c++

Question

I have a simple loop
LARGE_INTEGER ticks_per_second;
::QueryPerformanceFrequency(&ticks_per_second);
MSG msg = { 0 };
while (true)
{
if (msg.message == WM_QUIT)
exit(0);
if (::PeekMessageW(&msg, NULL, 0U, 0U, PM_REMOVE))
{
::TranslateMessage(&msg);
::DispatchMessageW(&msg);
continue;
}
static double last_time_s = 0;
LARGE_INTEGER cur_time_li;
::QueryPerformanceCounter(&cur_time_li);
double cur_time_s = (double)cur_time_li.QuadPart / (double)ticks_per_second.QuadPart;
double diff_s = cur_time_s - last_time_s;
double rate_s = 1 / 30.0f;
uint32_t slept_ms = 0;
if (diff_s < rate_s)
{
slept_ms = (uint32_t)((rate_s - diff_s) * 1000.0);
::Sleep(slept_ms);
}
update();
::printf("updated %f %u\n", diff_s, slept_ms);
last_time_s = cur_time_s;
}
And want update() to be called 30 times per second, but not more often
With this code it goes wrong, in console I getting something like this:
updated 0.031747 1
updated 0.001997 31
updated 0.031912 1
updated 0.001931 31
updated 0.031442 1
updated 0.002084 31
Which is seems to be correct only for first update, second one called too fast, and I can't understand why
I understand that update, PeekMessageW and etc. also wasting time, but even if I create a while (true) loop and comment update() out, it's still printing similar result
I using DirectX 11 with vsync turned off for rendering (rendering inside update function):
g_pSwapChain->Present(0, 0);
How do I fix code to make update() stable called 30 times in one second?

Answer 1

I don't think casting to double is good idea.I would run something like this:
static LARGE_INTEGER last_time_s = { 0 };
::QueryPerformanceCounter(&cur_time_li);
time_diff_microsec.QuadPart = cur_time_li.QuadPart - last_time_s.QuadPart;
// To avoid precision lost, convert to seconds *before* dividing by ticks-per-second.
time_diff_microsec.QuadPart *= 1000000;
time_diff_microsec.QuadPart /= ticks_per_second.QuadPart;
double rate_s = 1 / 30.0f;
uint32_t slept_ms = 0;
if (time_diff_microsec.QuadPart >= rate_s)// if (diff_s < rate_s)
{
// slept_ms = (uint32_t)(rate_s - time_diff_microsec.LowPart);// *1000.0);
// ::Sleep(slept_ms);
//}
//update();
::printf("updated %lld %u\n", time_diff_microsec.QuadPart, slept_ms);
}
last_time_s.QuadPart = time_diff_microsec.QuadPart/ 1000000;
}
Just brief "sketch". Not verified that calculations are correct though.