vfork 后只能调用 exit, exec 系列函数, 其他都是未定义行为 ...
(From POSIX.1) The vfork() function has the same effect as fork(2), 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(2) or one of the exec(3) family of
functions.