What is wrong in the stack implementation using linked lists? - c

Question

I am trying to implement a stack using linked list. Here is my code:
#include<stdio.h>
//implementation of stack
struct Node{
int data;
struct Node* next;
struct Node* prev;
};
struct Stack{
struct Node* headNode;
struct Node* presentNode;
int size;
};
struct Node* newNode()
{
struct Node* node;
return node;
}
struct Stack* newStack()
{
struct Stack* stack;
stack->headNode = newNode();
stack->presentNode = stack->headNode;
stack->size=0;
return stack;
}
int isempty(struct Stack* s)
{
if(s->headNode->next != NULL)
return 0;
return 1;
}
void push(struct Stack* s,int data)
{
struct Node* node = newNode();
node->data = data;
node->next = NULL;
s->presentNode->next = node;
node->prev = s->presentNode;
s->presentNode = node;
s->size ++;
}
int pop(struct Stack*s)
{
if(isempty(s)==1)
return 0;
int data = s->presentNode->data;
s->presentNode->prev->next = NULL;
s->presentNode = s->presentNode->prev;
s->size --;
return data;
}
int main()
{
struct Stack* stack = newStack();
int data = 0,type;
printf("Enter '1' if new element to be added or '0' if the latest element is to be deleted.\n");
while(data!=-1)//unbounded stack
//takes input until data==-1
{
scanf("%d",&type);
if(type)
{
printf("Enter the element:\t");
scanf("%d",&data);
if(data==-1)
continue;
push(stack,data);
}
else
printf("%d is popped out of the list!\n",pop(stack));
}
return 0;
}
But I am getting run-time error. Since I am new to pointers (I am a Java guy), I am pretty confused where my error is, although I am sure it is due to pointers.

Answer 1

You do not allocate memory for your new nodes:
struct Node* newNode()
{
struct Node* node;
return node;
}
Local variable node is an uninitialized pointer. As soon as you use the return value of this function, you have Undefined Behaviour.
Edit:
The same problem is true for newStack:
struct Stack* newStack()
{
struct Stack* stack;
stack->headNode = newNode();
...
return stack;
}
As soon as you dereference stack, you have Undefined Behaviour.

Answer 2

You have multiple problems. For starters
In your newStack() function, you do not have a return statement, so using the return value causes undefined behavior.
You need to allocate memory to pointers (i.e., make pointers to point to valid memory) before you can actually dereference them. In your case
struct Stack* stack;
stack->headNode
is an attempt to use uninitialized memory which again causes UB.