typedef struct record
{
    char name[15];   
    char tel[15];   
    char address[50];
};
typedef struct node
{
    struct record data;
    struct node *next;
}Node;   
void insert(Node*l)            
{
    Node*p=NULL;            
    Node*r=l->next;         
    char ch=0;               
    char name[15],tel[15],adr[50];        
    while(1)
    {
        printf("输入姓名:");
        fgets(name,15,stdin);            
        rewind(stdin);                    
        DelN(name);                       
        printf("输入电话号码:");
        fgets(tel,15,stdin);            
        rewind(stdin);
        DelN(tel);
        printf("输入地址:");
        fgets(adr,50,stdin);              
        rewind(stdin);
        DelN(adr);
        p=(Node*)malloc(sizeof(Node));   
        if(!p)                           
        {
            printf("内存申请失败！\n");   
            return;                        
        }
        strcpy(p->data.name,name);         
        strcpy(p->data.tel,tel);        
        strcpy(p->data.address,adr);   
        if(CheckData(p,l)==0)   ？这里把l结构体赐给CheckData函数了   问题2     
        {
            free(p);                    
            printf("已有该记录，请按任意键后重新输入！");  
            getchar();                 
            continue;                                    
        }
                 p->next=r;      
        l->next=p;              
        r=p;        

        printf("是否继续输入?是Y,否N:");
        scanf("%c",&ch);               
        rewind(stdin);
        if(ch=='N'||ch=='n')            
            break;                        
    }
}
int CheckData(Node*p,Node*l)        
{
    Node*r=l->next;？这里结构体指针r所指向的应该是个空链表NULL    问题3   
    int result;            
    while(r)？可是这个地方r为真，表示r保存的不是空链表        问题4   
    {
        if ((strcmp(r->data.name,p->data.name)==0)  
            &&(strcmp(r->data.tel,p->data.tel)==0)
            &&(strcmp(r->data.address,p->data.address)==0))
        {
            result=0;        
            break;            
        }
        r=r->next;            
        result=1;            
    }
    return result;            
}
         
int main()
{
    Node*l;
    l=(Node*)malloc(sizeof(Node));         
    if (l==NULL)                        
    {
        printf("为节点申请内存失败!");  
        return 1;                          
    }
    l->next=NULL;   ？这里l->next保存的是NULL表示没有链表了    问题1        
    while(1)            
    {
        switch(menu2())   
        {
        case 1:            
            insert(l);  
            break;        
        
        case 7:            
            printf("程序结束\n");
            exit(0);            }
    }
    return 0;
}
此程序不是完整程序本人只是把有问题的代码发出来的
l->next其他代码都没有用链表和结构体
l->next一直都是NULL怎么到了后面就不是NULL了