printf：＃include<stdio.h>
_exit(0):#include<unistd.h>
file(fd):#include<fcntl.h>
clone():#include<sched.h>
malloc():#include<malloc.h>
最重要的：#define _GNU_SOURCE
参考：
（1）未声明的CLONE_VM的解释：
http://stackoverflow.com/questions/22449342/clone-vm-undeclared-first-use-in-this-function
（2）＃define _GNU_SOURCE解释：
http://stackoverflow.com/questions/5582211/what-does-define-gnu-source-imply