C Program for Insertion at nth node in doubly linked list

#include<stdio.h>
#include<stdlib.h>

struct Node
{
  int data;
  struct Node *next;
  struct Node *prev;
};

void insertLast (struct Node **head, int data);
void insertStart (struct Node **head, int data);

// helper function for insertion after position
int calcSize (struct Node *node)
{
  int size = 0;

  while (node != NULL)
    {
      node = node->next;
      size++;
    }
  return size;
}

void insertPosition (int pos, int data, struct Node **head)
{
  int size = calcSize (*head);

  /*If pos is 0 then should use insertStart method
  If pos is less than or equal to 0 then can't enter at all
  If size is lesser than pos then bufferbound issue*/
  if (pos < 0 || size < pos)
    {
      printf ("Can't insert, %d is not a valid position\n", pos);
      return;
    }
// Insert after 0th position would mean insert at start 
  if (pos == 0)
    insertStart (head, data);
// insert after size'th position would mean insert at the last 
  else if (pos == size)
    insertLast (head, data);
  else
    {
      struct Node *temp = *head;
      struct Node *newNode = (struct Node *) malloc (sizeof (struct Node));
      newNode->data = data;
      newNode->next = NULL;
      while (--pos)
	temp = temp->next;	//temp2 = 10 as 12->next is 10 
      struct Node *temp2 = temp->next;	//(25)->next = 10 as 12->next is 10 
      newNode->next = temp->next;	//(25)->prev = 12 
      newNode->prev = temp;	// (12)->next = 25 
      temp->next = newNode;	// (10)->prev = 25 
      temp2->prev = newNode;	//new node added in b/w 12 and 10
    }
}

void insertLast (struct Node **head, int data)
{
  struct Node *newNode = (struct Node *) malloc (sizeof (struct Node));
  newNode->data = data;
  newNode->next = NULL;
  //need this if there is no node present in linked list at all 
  if (*head == NULL)
    {
      *head = newNode;
      newNode->prev = NULL;
      return;
    }
  struct Node *temp = *head;
  while (temp->next != NULL)
    temp = temp->next;
  temp->next = newNode;
  newNode->prev = temp;
}

void insertStart (struct Node **head, int data)
{
  struct Node *newNode = (struct Node *) malloc (sizeof (struct Node));
  newNode->data = data;
  newNode->next = *head;
  newNode->prev = NULL;		//If the linked list already had atleast 1 node 
  if (*head != NULL)
    (*head)->prev = newNode;
  // *head->prev = newNode; would not work it has (*head) must be used 
  //changing the new head to this freshly entered node 
  *head = newNode;

}

// function to print the doubly linked list 
void display (struct Node *node)
{
  struct Node *end;
  printf ("List in Forward direction: ");
  while (node != NULL)
    {
      printf (" %d ", node->data);
      end = node;
      node = node->next;
    }
  printf ("\nList in backward direction: ");
  while (end != NULL)
    {
      printf (" %d ", end->data);
      end = end->prev;
    }
}

int main ()
{
  /*Need & i.e. address as we need to change head address */
  struct Node *head = NULL;
  insertStart (&head, 12);
  insertStart (&head, 16);
  insertStart (&head, 20);
  insertLast (&head, 10);
  insertLast (&head, 14);
  insertLast (&head, 18);
  insertLast (&head, 11);
  //Inserts after 3rd position 
  printf ("linked before insertion at specific position\n");
  display (head);
  printf ("\n\nlinked after insertion at specific position\n");
  insertPosition (3, 25, &head);
  /*No need for & i.e. address as we do not need to change head address */
  display (head);
  return 0;
}