分段故障(堆芯倾倒）?它是什么，为什么会引起???(c++中的双循环链表程序）

我的程序（它还不完整，但我真的很想知道我的错误）：

#include <iostream.h>
using namespace std;
class dcllnode
{
private:
    int data;
    dcllnode *next;
    dcllnode *prev;
public:
    dcllnode(int ele)
    {
        data = ele;
        prev = next;
        next = prev;
    }
    friend class dcll;
};
class dcll
{
private:
    dcllnode *head;
public:
    dcll()
    {
        head = NULL;
    }
    void create();
    void inserts(int ele);
    void inserte(int ele);
    void inserta(int ele, int pos);
    void insertb(int ele, int pos);
    void dels();
    void dele();
    void dela();
    void display();
};
void dcll::create()
{
    if (head == NULL)
    {
        head = new dcllnode(0);
        cout << "list is created";
    }
    else
    {
        cout << "list has already been created";
    }
}
void dcll::inserts(int ele)
{
    if (head == NULL)
        cout << "please create the list first ";
    else
    {
        dcllnode *x;
        x = new dcllnode(ele);
        x->prev = head;
        x->next = head->next;
        (head->next)->prev = x;
        head->next = x;
        cout << "list is modified";
    }
}
void dcll::inserte(int ele)
{
    if (head == NULL)
        cout << "please create the list first ";
    else
    {
        dcllnode *x;
        x = new dcllnode(ele);
        x->next = head;
        x->prev = head->prev;
        (head->prev)->next = x;
        head->prev = x;
        cout << "list is modified";
    }
}
void dcll::dels()
{
    if (head == NULL)
        cout << "please create the list first ";
    else
    {
        dcllnode *x;
        head->next = x;
        head->next = x->next;
        (x->next)->prev = head;
        x->next = NULL;
        x->prev = NULL;
        delete(x);
        cout << "list is modified";
    }
}
void dcll::dele()
{
    if (head == NULL)
        cout << "please create the list first ";
    else
    {
        dcllnode *x;
        head->prev = x;
        head->prev = x->prev;
        (x->prev)->next = head;
        x->prev = NULL;
        x->next = NULL;
        delete(x);
        cout << "list is modified";
    }
}
void dcll::display()
{
    if (head == NULL)
        cout << "please create the list first ";
    else
    {
        dcllnode *p = head->next;
        while (p != head)
        {
            cout << "n" << p->data;
            p = p->next;
        }
    }
}
int main()
{
    dcll l1;
    l1.create();
    l1.inserte(10);
    l1.display();
    l1.inserts(20);
    return 0;
}

当我只使用create函数时，分割问题就不会发生。我想知道如何解决这些问题，以及如何避免未来出现分割错误。我们将提供任何信息和/或帮助。