How does indirection work in this code?

Question

I was reading the answer to Merging two sorted linked list. The code:

#define SWAP_PTRS(a, b) do { void *t = (a); (a) = (b); (b) = t; } while (0)

Node* MergeLi         


        

Answer 1

pnext is a pointer to a pointer of Node and is meant to hold the address of the next field of the last node

so the first line sets the pointer to the next node (either list or the previous node->next)

the second line sets pnext to the next field of the current node

the third line advances list1 after just dealing with the head of it using the just assigned pnext for a micro-optimization that avoids dereferencing list1 again

you can also write it in terms of node->next:

#define SWAP_PTRS(a, b) do { void *t = (a); (a) = (b); (b) = t; } while (0)

Node* MergeLists(Node* list1, Node* list2) 
{
  Node *list = NULL, *node ;

  if (list2 == NULL)
    return list1;

  if (list1->data > list2->data)
    SWAP_PTRS(list1, list2);

  node=list=list1;

  list1=list1->next;

  while (list1 != NULL)
  {
    if (list1->data > list2->data)
      SWAP_PTRS(list1, list2);

    node->next = list1;  

    node = list1->next; 
    list1 = list1->next;     
  }

  node->next = list2;
  return list;
}