Linked List In Data Structures

Question

Suppose temp is a pointer to structure node. temp->next is NULL. So what will be the value of temp->next->next?

In short what is the value of NULL->next? Is it compiler dependent because I have seen different result in ubuntu and code blocks(windows)?

What will be the output of the program below?

 struct node
 {
  int data;
   struct node *next;
 };

 main()
 {
   struct node *temp,*p;
   int c=0;
   temp=(struct node *)malloc(sizeof(struct node));
   temp->data=50;
   temp->next=NULL;
   p=temp;
   if(p->next->next==NULL)//will it enter the if loop?
      c++;
   printf("%d",c);
  }

Not necessarily. Especially in Windows there is no segfaults... It's an undefined behaviour. It can blow up the Moon, for example, so please don't do this. We still need it. — Eugene Sh., Aug 12 '15 at 14:47
[Please do not cast the result of `malloc()`](http://stackoverflow.com/a/605858/3233393). — Quentin, Aug 12 '15 at 15:01
I'm voting to close this question as off-topic because it's yet another 'I know it's UB but I still want SO contributors to waste time on it' — Martin James, Aug 12 '15 at 16:13

score 1 · Answer 1 · answered Aug 12 '15 at 14:45

1

If temp->next is NULL, dereferencing it to get temp->next->next is undefined behavior. A crash is likely, but other things could happen. In principle, anything could happen.

Don't dereference null pointers.

answered Aug 12 '15 at 14:45

Wyzard

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score 1 · Accepted Answer · answered Aug 12 '15 at 14:47

1

NULL->next must give you a segfault.

You may want to have something like :

if(p->next != NULL && p->next->next==NULL)

or

if(p->next == NULL || p->next->next==NULL)

answered Aug 12 '15 at 14:47

Vouze

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Linked List In Data Structures

2 Answers2