为什么一执行就停止运行啊

#include <stdio.h>
struct date{int price;
int attack;
struct date *next;//next 用来存放下一个结点的地址
};
int main()
{
 struct date a,b,c,*head;
    a.price=1;
 a.attack=100;
 b.price=2;
 b.attack=200;
 c.price=3;
 c.attack=300;
    head=&a;
 a.next=&b;
 b.next=&c;
    c.next=NULL;
 struct date *p;
 p=head;
 while(p=!NULL)
 {
  printf("%d,%d\n",p->price,p->attack);
  p=p->next;
 }
 return 0;
}