菜鸟问问题( `NULL' undeclared)

chjcpu1

《UNIX环境高级编程》第一个程序，"ourdhr.h"也已经有了，编译程序出错
[root@localhost 1]# cc myls.c
myls.c: In function `main':
myls.c:12: `NULL' undeclared (first use in this function)
myls.c:12: (Each undeclared identifier is reported only once
myls.c:12: for each function it appears in.)
请帮忙解答一下，谢谢了
#include <sys/types.h>;
#include <dirent.h>;

int main(int argc, char *argv[])
{
        DIR     *dp;
        struct dirent *dirp;

        if (argc != 2)
                err_quit("a single argument (the directory name) is required";

        if ((dp = opendir(argv[1])) == NULL)
                err_sys("can't open %s", argv[1]);

        while ((dirp = readdir(dp)) != NULL)
                printf("%s\n", dirp->;d_name);

        closedir(dp);
        exit(0);
}

wujiajia

void main()//没有返回值
#include "stdio.h" // 宏定义 了 NULL