Doubly Linked List - Segmentation Faults - C - c

Question

I am trying to implement a doubly linked list that acts like a queue (I want it to act like queue).
[EDIT]
When I add nodes to the list (e.g 5 nodes) and empty the list (delete all elements) and try to add another node to the list again, it gives me a segmentation fault (core dumped) error.
linkedlist.h
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct node{
int d;
struct node *prev;
struct node *next;
}node;
typedef struct linkedlist{
int size;
struct node *first;
struct node *last;
}linkedlist;
linkedlist.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "linkedlist.h"
linkedlist* createList(){
linkedlist* myList = (linkedlist*)calloc(1,sizeof(linkedlist));
myList->first = NULL;
myList->last = NULL;
myList->size =0;
return myList;
}
static node* createNode(int n){
node *myNode = (node*)calloc(1,sizeof(node));
myNode->d = n;
myNode->prev = NULL;
myNode->next = NULL;
return myNode;
}
void insertNode(linkedlist* l, int num){
node *temp, *newNode;
newNode = createNode(num);
if (l->size == 0){
newNode->next = NULL;
newNode->prev = NULL;
l->first = newNode;
l->last = newNode;
l->size++;
}
else{
temp = l->first;
while (temp->next != NULL){
temp = temp->next;
}
newNode->prev = temp;
temp->next = newNode;
newNode->next = NULL;
l->size++;
}
}
int deleteNode(linkedlist* l){
node *temp = calloc(1,sizeof(node));
if (l->first ==NULL){
return -1;
}
else if (l->size ==1){
free(l->first);
l->first= NULL;
l->last = NULL;
l->size--;
}
else if (l->size > 1){
temp = l->first;
l->first = temp->next;
free(temp);
}
}
void display(linkedlist *l){
node *temp = calloc(1,sizeof(node));
temp = l->first;
if (temp == NULL){
printf("The list is empty\n");
}
while (temp != NULL) {
printf("-> %d ", temp->d);
temp = temp->next;
}
}
int main(){
linkedlist *myList = createList();
int choice, temp=0, numb;
printf("(1) Insert \n (2) Delete \n");
for (temp; temp<10; temp++){
printf("Choice :");
scanf ("%d", &choice);
switch(choice) {
case 1: {
printf("Enter a Number: ");
scanf("%d", &numb);
insertNode(myList, numb);
display(myList);
break;
}
case 2:{
deleteNode(myList);
display(myList);
break;
}
}
}
}

Answer 1

In your delete node function:
else if (l->size > 1){
temp = l->first;
l->first = NULL; //this is problem
l->first->next = NULL;
temp->next = l->first;
l->first->prev = NULL;
You are assigning l->first = NULL and then accessing it in next statement l->first->next = NULL;, which will fail and give you segmentation fault.
Also, when l->size == 1 you should also set l->first = NULL after freeing it.

Answer 2

The problem appears when access to a "NULL" position. Let's revise the code:
temp = l->first;
l->first = NULL; // here, you set l->first = 0
l->first->next = NULL; // here, you access to 0->next: this is not correct.
temp->next = l->first;
Change it for:
temp = l->first;
l->first = temp->next;
delete temp;

Answer 3

int deleteNode(linkedlist* l){
node *temp= (node*)malloc(sizeof(node)) ;
if (l->first ==NULL){
return -1;
}
else
{
temp= l->first;
l->first= temp->next;
l->first->previous= temp;
l->size--;
free(l->first->previous);
}
}

Answer 4

In the deleteNode you leave first pointing to freed memory if the size is 1
It should be:
else if (l->size ==1){
free(l->first);
l->first = NULL;
l->last = NULL;
l->size--;
}
Also temp is a pointer you don't need to allocate memory for it with malloc