Inserting In a Doubly LinkedList - c

Question

I am trying to insert into a doubly linked list. I am then trying to print the list in both forward and reverse direction. I have created a head node and I am trying to insert another one, but I am unable to do so. The program shows a runtime error.
Please find my code below. Any help would be appreciated.
#include<stddef.h>
#include<stdio.h>
#include<stdlib.h>
struct node
{
int data;
struct node *next;
struct node *prev;
};
void insertAfter(struct node *node, int new_data){
if (node == NULL)
{
printf("the given previous node cannot be NULL");
return;
}
struct node* new_node = (struct node*)malloc(sizeof(struct node));
node->data = new_data;
node->next = new_node;
new_node->prev = node;
new_node->next - node->next;
if(new_node->next!=NULL)
new_node->next->prev = new_node;
}
void printlist(struct node *node){
struct node *last;
printf("Traversal in forward direction\n");
while(node!=NULL){
printf("%d\n",node->data);
last = node;
node=node->next;
}
printf("Traversal in backward direction\n");
while(last!=NULL){
printf("%d\n",last->data);
last=last->prev;
}
}
int main()
{
struct node *head;
struct node *tail;
head->data = 5;
tail->data = 10;
head->next = tail;
head->prev = NULL;
tail->next = NULL;
insertAfter(head, 8);
printf("\n Created DLL is: ");
printlist(head);
return 0;
}

Answer 1

There are several problems here.
First, as pointed out by #Igor, you are not allocating any memory for your head and tail nodes. You should also set tail->prev = head.
Second, the order in which insertAfter sets the link pointers causes node->next to be overwritten before it is used in setting new_node->next. This causes new_node->next to point back to new_node instead of to whatever was following node. You should set new_node->next and new_node->prev before you modify node. It also appears that you have used a minus sign instead of an equals in the "assignment" of new_node->next.
Third, in printlist, you should initialize last to NULL in case the list is empty; otherwise, you will attempt to walk the list backwards from an undefined starting (ending) point.

Answer 2

You want new_node->next to be the same as new_node?
if not, you'd better swap these two lines, in InsertAfter:
node->next = new_node;
new_node->next - node->next;

Answer 3

You need to allocate memory for your pointers head and tail.
int main()
{
struct node *head;
struct node *tail;
head = malloc(sizeof(struct node)); //allocating memory to head
tail = malloc(sizeof(struct node)); //allocating memory to tail
head->data = 5;
tail->data = 10;
head->next = tail;
head->prev = NULL;
tail->next = NULL;
insertAfter(head, 8);
printf("\n Created DLL is: ");
printlist(head);
return 0;
}
Also, never cast the return value of malloc, therefore change:
struct node* new_node = (struct node*)malloc(sizeof(struct node));
to
struct node* new_node = malloc(sizeof(struct node));