单链表C程序中的分段错误

Question

In this code below, when I comment out the first and second call to display() function in main, It works fine. But without commenting, after adding 2nd element it says - segmentation fault .

I am new to pointers. Please suggest what might be wrong.

#include<stdio.h>
#include<malloc.h>
//-------------------------------------------------
struct node
{
int data;
struct node *next;
}*start=NULL;
//------------------------------------------------------------

void creat()
{

  struct node *new_node,*current;

  new_node=(struct node *)malloc(sizeof(struct node));

  printf("\nEnter the data : ");
  scanf("%d",&new_node->data);
  new_node->next=NULL;

  if(start==NULL)
  {
  start=new_node;
  current=new_node;
  }
  else
  {
  current->next=new_node;
  current=new_node;
  }
}
//------------------------------------------------------------------
void display()
{
struct node *new_node;
 printf("\nThe Linked List : ");
 new_node=start;
 while(new_node!=NULL)
   {
   printf("%d--->",new_node->data);
   new_node=new_node->next;
   }
  printf("NULL\n\n");
}
//----------------------------------------------------
void main()
{
creat();
display();
creat();
display();
creat();
display();
}

---

output ->

 $./a.out

Enter the data : 4

The Linked List : 4--->NULL


Enter the data : 6
Segmentation fault (core dumped)

Answer 1

This is happening because of current->next=new_node; in else block. When you enter data second time start is declared global so its value persists between function call but as current is declared within fuunction it's scope is limited to function only. So when you enter data second time current is null so accessing current->next causes segmentation fault.
So declare current node after start variable.

Answer 2

Second time you call creat , you are using an ininitialized current .

current_node is a local variable with auto storage. When you return from the function, it gets deleted.

Try adding

struct node* current = NULL;

right after you define start . Then, remove it from creat .

Answer 3

The offending code is:

       current->next=new_node;

You are dereferencing a NUll pointer which is illegal. What you should have done was to declare current as a global variable as well:

struct node
{
int data;
struct node *next;
}*start=NULL,*current=NULL;

This will fix your display issue as well.