Asking Windows for real-time priority

Question

This code snippet is about asking Windows for the highest possible execution priority. Basically, we have to ask for real-time priority class for the entire process, after which each thread has to ask for real-time thread priorities. However, since Windows Vista, whatever is the program that needs the real-time priority, it must be asked to be run in administrator privilege level.

The demonstration program I provide simply loops for 7 seconds. During that period, your system should be frozen (try pressing Caps Lock; should not switch the Caps Lock light). Also, if you put on some audio, the sound card will repeat its contents of the sound buffer over and over again.

rtprio.h:

#pragma once
#include <Windows.h>

BOOL RealTimePriorityClass()
{
    return SetPriorityClass(GetCurrentProcess(), REALTIME_PRIORITY_CLASS);
}

BOOL RealTimeThreadPriority()
{
    return SetThreadPriority(GetCurrentThread(), THREAD_PRIORITY_TIME_CRITICAL);
}

main.cpp:

#include <windows.h>
#include "rtprio.h"

static const DWORD dwInterval = 7000; // millseconds.

DWORD WINAPI ThreadProc(LPVOID lpParams);

int WINAPI WinMain(HINSTANCE hInst,
    HINSTANCE hPrev,
    LPSTR lpCmdLine,
    int nShowCmd)
{
    DWORD       i;
    DWORD       dwInitTicks;
    DWORD       dwCores;
    SYSTEM_INFO sysInfo;

    GetSystemInfo(&sysInfo);
    dwCores = sysInfo.dwNumberOfProcessors;

    // Ask real-time priority class for the entire process.
    RealTimePriorityClass();

    for (i = 0; i < dwCores - 1; i++)
    {
        CreateThread(NULL,
                     0,
                     ThreadProc,
                     NULL,
                     0,
                     NULL);
    }

    RealTimeThreadPriority();

    dwInitTicks = GetTickCount();

    while (GetTickCount() - dwInitTicks < dwInterval);

    return 0;
}

DWORD WINAPI ThreadProc(LPVOID lpParams)
{
    // Ask for real-time priority for this thread.
    RealTimeThreadPriority();

    // Freeze the system for dwInterval milliseconds.
    while (TRUE) {}

    return 0;
}

Answer 1

Works as intended.
But I get a thread priority of 15 rather than the expected 31 for a REALTIME_PRIORITY_CLASS?
See SetThreadPriority.

It seems that the THREAD_PRIORITY_TIME_CRITICAL is defined to THREAD_BASE_PRIORITY_LOWRT - 1, which could be further elevated to 31 if needed.