C Main Loop without 100% cpu

Question

#include 

int main() {
  while(!DONE) {
    /* check for stuff */
  }
  return 0;
}

The above code sample uses 100% cpu until DONE

Answer 1

You have several choices:

1. Use sleep() to force the process to suspend periodically and allow other process to use the CPU
2. Run at a lower priority level - which will cause the OS to assign less CPU time
3. Use a mutex or other synchronization object to detect when work is available - which will keep the process from consuming any CPU time unless it is actually doing work
4. If you get work faster than you can process it - you may still need to use some sort of sleep/priority model to avoid completely consuming the CPU.

Option #2 can be tricky to do in a platform/OS neutral manner. Your best bet is to launch the process and change its priority in the runtime environment.

Answer 2

What exactly are you checking for?

If you're checking something volatile that is changed by hardware or another process, just call sleep in your loop.

If you are waiting on a file descriptor or a network socket descriptor, you will want to use select or poll in your loop to wait for the descriptor to have data ready for consumption.

Answer 3

Use yield().

Answer 4

On windows, you can use Sleep(int milliseconds), defined on windows.h.

Answer 5

Sleep(0); is enough

Answer 6

If I understood right, you said in the comments that DONE can be changed from other threads. If so, condition variables make sense. With pthreads, one would do:

In the thread that waits:

pthread_mutex_lock(&mutex);
while (!DONE) {
     pthread_cond_wait(&cond, &mutex);
}
pthread_mutex_unlock(&mutex);

In other threads, when DONE is changed:

pthread_mutex_lock(&mutex);
DONE = 1;
pthread_cond_signal(&cond);
pthread_mutex_unlock(&mutex);