进程创建注意事项：函数及对应包

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