程序代码：

#include<stdio.h>
#include<stdlib.h>
typedef struct DuLNode
{
    char data;
    struct DuLNode *prior;
    struct DuLNode *next;
}DuLNode,*DuLinkList;

DuLinkList GetElem(DuLinkList *L,int i)
{
    DuLinkList p;
    int j;
    p = (*L)->next;
    j = 0;

    while(p && j<i-1)
    {
        p = p->next;
        ++j;
    }
    if(!p || j>i-1) return 0;
    else return p;
}
void ListInsert(DuLinkList *L,int i,char e)//插入
{
    //在带头结点的双链循环线性表L中第i位置之前插入元素e
    //i的合法值为1<=i<=表长+1  
    DuLinkList p,s;
    
    if((p=GetElem(L,i))<=0) //

        exit(1);
    printf("%d\n",p);
    if(!(s=(DuLinkList)malloc(sizeof(DuLNode)))) exit(1);

    s->data = e;
    s->prior = p->prior;
    p->prior->next = s;
    s->next = p;
    p->prior = s;
}

void ListDelete(DuLinkList *L,int i) //删除
{
    //删除带头结点的双链循环线性表L的第i个元素，
    DuLinkList p;

    if(!(p=GetElem(L,i))) //确定删除位置
        exit(1);
    p->prior->next = p->next;
    p->next->prior = p->prior;

    free(p);
}


void CreateList(DuLinkList *L,int n)
{
    int i;
    DuLinkList p;
    (*L) = (DuLinkList)malloc(sizeof(DuLNode));
    (*L)->next = NULL;
    for(i=n;i>0;--i)
    {
        p = (DuLinkList)malloc(sizeof(DuLNode));
        scanf("%c",&p->data);
        fflush(stdin);
        p->next = (*L)->next;
        (*L)->prior = p;
        (*L)->next = p;
    }
}
void display(DuLinkList *L)
{
    DuLinkList di;
    di = (*L)->next;
    while(di!=NULL)
    {
        printf("%c\t",di->data);
        di = di->next;
    }
    printf("\n");
}

void main()
{
    DuLinkList s1;
    CreateList(&s1,4);
    display(&s1);
    
    ListInsert(&s1,2,'e');  -->加上这句就会出错了,为什么呢?
}