what is a stub routine?

In C\C++ we can have it as a safe mechanism when calling another function

void stub(void (*func)(int), int arg) {
   (*func)(arg);
   thread_exit(0);
}

The reason to not just call func is to ensure that once it's done running it doesn't return to the random value stored at the top of the stack especially without calling the thread_exit(0) function. So instead we can return to stub, which then calls thread_exit(0). func itself won't call thread_exit(0) let stub do it.