Doubly-linked list/Element insertion

Use the link structure defined in Doubly-Linked List (element) to define a procedure for inserting a link into a doubly-linked list. Call this procedure to insert element C into a list {A,B}, between elements A and B.

This is much like inserting into a Singly-Linked List, but with added assignments so that the backwards-pointing links remain correct.

C

Define the function:

void insert(link* anchor, link* newlink) {
  newlink->next = anchor->next;
  newlink->prev = anchor;
  (newlink->next)->prev = newlink;
  anchor->next = newlink;
}

Production code should also include checks that the passed links are valid (e.g. not null pointers). There should also be code to handle special cases, such as when *anchor is the end of the existing list (i.e. anchor->next is a null pointer).

To call the function:

Create links, and form the list:

link a, b, c;
a.next = &b;
a.prev = null;
a.data = 1;
b.next = null;
b.prev = &a;
b.data = 3;
c.data = 2;

This list is now {a,b}, and c is separate.

Now call the function:

insert(&a, &c);

This function call changes the list from {a,b} to {a,b,c}.