pthread_descr head;  acquire(&cond->c_spinlock); head = cond->c_waiting.head; release(&cond->c_spinlock); if (head != NULL) return EBUSY;
if (cond->c_waiting != NULL) return EBUSY;
int pthread_cond_destroy(pthread_cond_t *cond){  pthread_descr head;  acquire(&cond->c_spinlock);  head = cond->c_waiting.head;  release(&cond->c_spinlock);  if (head != NULL) return EBUSY;  return 0;}