关于#c语言#的问题：include

include<stdio.h>
main()
{
    int **p = NULL;
    int *p1 = NULL;
    int a = 0;
    p1=&a;
    *p=p1;
    printf("%d\n",*p1);
    printf("%p\n",p1);
    printf("%p\n",p);
    printf("%p\n",*p);
    printf("%d\n",**p);
}

#include<stdio.h>
main()
{
    int **p = NULL;
    int *p1 = NULL;
    int a = 0;
    p1=&a;
    p=(int**)malloc(sizeof(int*));//
    *p=p1;
    printf("%d\n",*p1);
    printf("%p\n",p1);
    printf("%p\n",p);
    printf("%p\n",*p);
    printf("%d\n",**p);
} 

include<stdio.h>
main()
{
int **p = NULL;
int *p1 = NULL;
int a = 0;
p1=&a;
*p=&p1; //加上&即可
printf("%d\n",*p1);
printf("%p\n",p1);
printf("%p\n",p);
printf("%p\n",*p);
printf("%d\n",**p);
}

什么都打印不了