What is the problem with this piece of code? - c

Question

I am writing for deleting the last node of a doubly linked list. But, every time this function is giving me segmentation fault when I have 2 or more than 2 elements in the list.
void deleteEnd()
{
struct node *ptr;
if(head==NULL)
printf("\nList is empty.First add some numbers");
else if(head->next==NULL)
{
head = NULL;
free(head);
}
else
{
ptr = head;
while(ptr->next != NULL)
{
ptr = ptr -> next;
}
ptr -> prev -> next = NULL;
free(ptr);
}
}

Answer 1

Normally, when you are deleting a node from a linked list it is a good practice to pass a reference to the first node of the list as an argument of the function. In your case you are not showing us where the head is coming from, and I think that it could be quite a useful info, and I bet that the error hides there.
That is how the implementation could look like:
#include <stdio.h>
#include <stdlib.h>
struct node
{
int data;
struct node *next;
};
// insert end node: this is for testing purposes
struct node *insertEnd(struct node *head, int value) {
struct node *ptr, *new_node;
ptr = head;
new_node = (struct node *) malloc(sizeof(struct node));
new_node->data = value;
new_node->next = NULL;
// The list is empty
if (head == NULL) {
head = new_node;
return head;
}
// Non empty list
while (ptr->next != NULL) {
ptr = ptr->next;
}
ptr->next = new_node;
return head;
}
// delete end node
struct node *deleteEnd(struct node *head) {
struct node *ptr, *preptr;
ptr = head;
preptr = NULL;
// The list is empty
if (head == NULL) {
printf("The list is empty. Nothing to delete.\n");
return head;
}
while(ptr->next != NULL) {
preptr = ptr;
ptr= ptr->next;
}
free(ptr);
if (preptr == NULL) {
head = NULL;
}
else {
preptr->next = NULL;
}
return head;
}
int main(void) {
struct node *llist;
llist = NULL;
llist = insertEnd(llist, 10);
llist = insertEnd(llist, 20);
llist = insertEnd(llist, 30);
llist = deleteEnd(llist);
llist = deleteEnd(llist);
llist = deleteEnd(llist);
return 0;
}

Answer 2

It is always a great idea to maintain a global variable head and update it after every push/pop. See this:
struct node
{
int data;
struct node *next;
};
struct node *start = NULL;
struct node *deleteEnd(struct node *start)
{
struct node *ptr = start;
if(ptr==NULL)
{
printf("\nList is empty!!!");
return start;
}
else if(ptr->next == NULL)
{
free(ptr);
start = NULL;
return start;
}
else
{
while((ptr->next)->next!=NULL) //by doing this you dont have to maintain a prev pointer
{
ptr=ptr->next;
}
struct node *temp = ptr->next;
free(temp);
ptr->next = NULL;
return start;
}
}
Hope this helps!!!