Create a doubly linked list or two-way circular linked list

#include <stdio.h>
#include <stdlib.h>
#define Len sizeof (struct list)
struct list
{
	int x;
	struct list *pre,*next;
};
struct list * Create ()//Create a linked list and return a pointer to the chain header
{
	struct list *p,*p1,*head;
	head=p= (struct list *) malloc (len);
	P->PRE=NULL;//1. The first element does not have a direct precursor
	scanf ("%d", &p->x);
	int n=0;
	while (P->x!=-1)
	{
		if (n==0)
			n++;
		else//understand clearly. Suggest drawing on the paper
			p1->next=p,p->pre=p1;
		p1=p;
		p= (struct list *) malloc (len);
		scanf ("%d", &p->x);
	}
	P1->next=null;//2. The last element has no direct successor
//	p1->next=head,head->pre=p1;//If 1 2 lines of code is deleted so that's a two-way loop list.
	return head;
}
int main ()
{
	struct list *l1;
	printf ("Please enter the list A, end with-1:");
	L1=create ();
	printf ("\ n");
	while (L1!=null)
	{
		printf ("%d", l1->x);
		l1=l1->next;
		if (l1!=null)
		printf ("%d", l1->pre->x);
	}
	printf ("\ n");
	return 0;
}