When "volatile" becomes necessary

volatile.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>

static int shared;

static void *thread_func(void *args)
{
	int ID = *(int *)args;

	printf("Hello from thread %d!\n", ID);

	if (ID != 0) {
		// Forces the compiler to load from memory
		while (*(volatile int *)&shared == 0) ;
		printf("shared is %d\n", shared);
	} else {
		sleep(1);
		shared = 42;
	}

	printf("Bye from thread %d!\n", ID);

	return NULL;
}

int main(void)
{
	pthread_t thread;
	int IDs[2] = { 0, 1 };

	pthread_create(&thread, NULL, thread_func, &IDs[1]);

	thread_func(&IDs[0]);

	pthread_join(thread, NULL);

	return 0;
}