求助大神，关于C语言数据结构中栈的问题

Question

为什么我的代码的p->next不能指向NULL?

#include <stdio.h>
#include <stdlib.h>

typedef struct T_Node{
    int d;
    struct T_Node *next;
} Node, *List;

void createlink(List *pf)
{
    int i;
    Node *p;

    *pf = (Node *)malloc(sizeof(Node));
    p = *pf;
    for(i = 1; i <= 99; i++)
    {
        p->d = i;
        p->next = (Node *)malloc(sizeof(Node));
        p = p->next;
    }
    p->d = 100;
    p->next = NULL;            //为什么编辑器上面报错Use of undeclared identifier 'NULL'
}

double getaverage(List f, int *nodenum){

    return 0;
}

int main(int argc, char* argv[])
{
    int n;
    List f;
    double a;
    createlink(&f);
    a = getaverage(f, &n);
    printf("%d  %lf\n",n, a);
    return 0;
 }

Answer 1

加上这个头文件然后再试试，#include < stddef.h >