我用vc的编译器调试的，虽然编译没有错误，但第二个num输入后，编译器
告知我调试错误，我翻阅书本，查不出为什么？程序如下：
#include<stdio.h>
#include<malloc.h>
#define NULL 0
#define len sizeof(struct student)
struct student
{
    int num;
    struct student *next;
};
struct student *main(void)//main可有其他函数代替，我只是测试而已
{
    struct student *p,*pback,a,*head;
    int i=1;
    pback=p=(struct student *)malloc(len);
    scanf("%d",&a.num);
    p->num=a.num;
    while(p->num!=0)
    {
        if(i==1)
            head=pback;
        else
            pback->next=p;
        pback=pback->next;
        p=(struct student *)malloc(len);
        scanf("%d",&a.num);
        p->num=a.num;
        i++;
    }
    pback->next=NULL;
       return head;
    
}