实例：
这么一个简单的小程序，当把里面的exit(0),改为return 0，就会出现未定义的结果。
then Why?
The vfork() function shall be equivalent to fork(), except that the behavior is undefined if the process created by vfork() either modifies any data other
than a variable of type pid_t used to store the return value from vfork(), or returns from the function in which vfork() was called, or calls any other function before successfully calling _exit() or one of the exec family of functions.
调用vfork()之后，子进程要么_exit()，要么调用某个exec()，否则都是未定义行为！
注意：
This function is marked obsolescent.（这个函数式荒废了的），呵呵，所以不必纠结了。。。。。。。。。