findloop(struct node *start)
{
 struct node *ptr,*ptr1;
start=ptr;

 while(ptr!=NULL)
{
  ptr1=ptr->next;
  
  while(ptr1!=NULL)
  {
     if(ptr1->link==ptr->link)
     {
        printf("\nLoop found at %d node",ptr->data);
        exit(0);
       }
    ptr1=ptr1->link;
}
ptr=ptr->link;
}
printf("\nNo Loop found in list");
}

BOOL findloop(struct node *start)
{
 struct node *ptr,*ptr1;
 ptr=start;
 ptr1= start->next;
 while(ptr!=NULL && ptr->next!=null && ptr1->next!=null && 
ptr1->next->next!=null)
   {
     if(ptr==ptr1)
          return FALSE;
     ptr=ptr->next;
     ptr1=ptr1->next->next;
   }
  return TRUE;
}

the approach is to mark the nodes starting from the head.and
follow the links.
in the process if you find a node already marked then there
is loop.