Volatile Variables in Optimized C

Ah, forgetting the volatile type qualifier when compiling with optimization enabled:

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#include <pthread.h>
// compile with: gcc -O3 -lpthread volatile.c -o volatile
// Fix by using volatile:
// volatile int flag = 0;
int flag = 0;
void *update() {
    flag = 1;
}
int main() {
    pthread_t update_thread;
    pthread_create(&update_thread, NULL, update, NULL);
    while (!flag) {} // This will loop forever
    pthread_join(update_thread, NULL);
}