Question

How would I make a version of sleep in C spin so it uses cpu cycles?

Answer 1

What you want to do is a busy wait where you loop until a certain amount of time has elapsed. Just get the current time (using the highest precision timer available) and loop until the current time is a certain amount of time after you started.

Here’s a concrete example using the Windows API and the performance counter using the two associated functions, QueryPerformanceCounter() and QueryPerformanceFrequency().

void Sleep_spin(DWORD dwMilliseconds)
{
    LARGE_INTEGER freq, target, current;

    /* get the counts per second */
    if (!QueryPerformanceFrequency(&freq)) { /* handle error */ }

    /* set target to dwMilliseconds worth of counts */
    target.QuadPart = freq.QuadPart * dwMilliseconds / 1000;

    /* get the current count */    
    if (!QueryPerformanceCounter(&current)) { /* handle error */ }

    /* adjust target to get the ending count */
    target.QuadPart += current.QuadPart;

    /* loop until the count exceeds the target */
    do
    {
        if (!QueryPerformanceCounter(&current)) { /* handle error */ }
    } while (current.QuadPart < target.QuadPart);
}

Use the appropriate API in your case, whatever that may be.