A minimal program (using C11 threads):

   #include <threads.h>
   int x;

   int do_thread(void *) {
     for (int i = 0; i < 10000; i++)
       x = i;
     return 0;
   }
   int main() {
     thrd_t t;
     thrd_create(&t, do_thread, NULL);
     do_thread(NULL);
     thrd_join(&t, NULL);
     return 0;
   }

There is an undefined data race on x, since it is simultaneously accessed from two threads without any synchronization. Replace the declaration of x with `_Atomic int x;`, and the resulting code is well-defined.