segmentation fault

Question

Could anyone tell me why am i getting error when I try to push

   #include <stdio.h>


typedef struct Element
{
  struct Element *next;
  void *data;
}Element;

bool createStack(Element **stack)
{
  *stack = NULL;
  return true;
}

bool push (Element **stack, void *data)
{
  Element *new_element = new Element;

  if(!new_element)
  {
    printf("Memory allocation error in push");

    return false;
  }

  new_element->data = data;
  new_element->next = *stack;
  *stack            = new_element;
  return true;

}

bool pop (Element **stack, void *popped_data)
{
  if(!*stack)
  {
    printf("Stack empty");

    return false;
  }


  Element *new_head = new Element;

  popped_data   = (*stack)->data;
  new_head      = (*stack)->next;
  delete *stack;
  return true;

}

bool emptyStack(Element **stack)
{
  if(!*stack)
  {
    printf("Stack empty");

    return false;
  }

  Element *delete_ele;

  while(*stack)
  {
    delete_ele=*stack;
    *stack = delete_ele->next;
    delete delete_ele;
  }

  return true;

}

int main()
{

  int i,*j;
  Element *stacka = new Element;

  while(i!=5)
  {
    printf("Enter ur choice \n");
    scanf("%d",&i);

    if(i==1)
    {
      if(createStack(&stacka))
      {
        printf("yes");

      }
    }

    if(i==2)
    {
      *j=2;
      if(push(&stacka,j))
      {
        printf("yes");

      }
    }

    if(i==3)
    {
      if(pop(&stacka,j))
      {

        printf("yes %d",*j);


      }
    }

    if(i==4)
    {
      if(emptyStack(&stacka))
      {
        printf("yes");

      }
    }

  }
return 0;
}

Thanks for the help running it on ubuntu

Answer 1

When you declare int i,*j;, j is just an uninitialized pointer that doesn't point to a valid memory location. Later, when you say *j=2;, you dereference that pointer, which results in undefined behaviour.

You have to assign a meaningful location to j, like so:

int j_content;
int *j = &j_content;

Answer 2

It's in this line

*j = 2;

j is uninitialized at that point.

You should either push &k where k is an int, or initialize j = new int. Memory leak avoidance is up to you in the latter case.