Question

I’m trying to understand this C code from a simple priority queue, especially – why does it need the struct qnode **first part:

int quedel(struct qnode **first, struct qnode **last, int *prio, int *val) {
 struct qnode *tmp = NULL;

 if((NULL == *last) && (*last == *first)) {
  fprintf(stderr, "Empty queue.....\n");
  return -1;
 }

 *val = (*first)->data, *prio = (*first)->prio;
 tmp = *first, *first = (*first)->next;

 if(*last == tmp)
  *last = (*last)->next;
 free(tmp);

 return 0;
}

Answer 1

Since C doesn’t have pass-by-reference, only pass-by-value, this approach is the only way to make this assignment:

*first = (*first)->next;

visible to the caller.

(If first were just a single pointer, and we wrote first = first->next, then the code that calls this function wouldn’t see the modification.)