Why I can not use "p->data" instead of "head->data" in C?

Question

typedef struct DNode{
    int data;
    struct DNode *lLink,*rLink;
} DblNode;
typedef DblNode * DblList;

void DestroyDblList(DblList L){
    DblList head=L;
    DblList p=head->rLink;
    if(head->data){ //p->data is wrong?
        while(p->data){
            free(head);
            head=p;
            p=head->rLink;
         }
         free(head);
    }
}

At the line with comments, when I use if(p->data){ my computer tells me

Segmentation fault (core dumped)

but when I use if(head->data){ it is OK.

How did it happen,what is the cause of it?

Answer 1

In your code p->data is same as head->rLink->data, where head->rLink is not allocated memory. So, segmentation fault [as a side effect of undefined behavior.].

On other hand, head->data is a perfect access.

BTW, you understand that p->data and head->data are not same [at line 56], right?

Answer 2

p->data and head->data would be the same if p and head would be the same. In line 56 this can only be true if head is the same as head->rLink by line 55, i.e. head points to itself which contradicts the understanding of a double linked list.

So p->data and head->data must be different.

Answer 3

You should test the nullity of p, not p->data.