In which header file is the NULL macro defined?

#include<stdio.h>

void fun(void *p);
int i;

int main()
{
    void *vptr;
    vptr = &i;
    fun(vptr);
    return 0;
}
void fun(void *p)
{
    int **q;
    q = (int**)&p;
    printf("%d\n", **q);
}

#include<stdio.h>
#include<string.h>

int main()
{
    static char s[] = "Hello!";
    printf("%d\n", *(s+strlen(s)));
    return 0;
}

#include<stdio.h>

int main()
{
    int i=4, j=8;
    printf("%d, %d, %d\n", i|j&j|i, i|j&&j|i, i^j);
    return 0;
}

#include<stdio.h>

int main()
{
    int i;
    printf("%d\n", scanf("%d", &i));
    return 0;
}

int *ptr[30];

#include<stdio.h>

int main()
{
    char huge *near *far *ptr1;
    char near *far *huge *ptr2;
    char far *huge *near *ptr3;
    printf("%d, %d, %d\n", sizeof(ptr1), sizeof(ptr2), sizeof(ptr3));
    return 0;
}