虚拟化 Ubuntu 上的奇怪进程行为

Question

我使用 VirtualBox 在 Windows 8.1 上运行 Ubuntu 14.04 LTS。我正在接近操作系统，以下程序显示了一个奇怪的结果：

#include <stdio.h>
#include <unistd.h>
int main() {
printf("I'm the parent, my PID is %d, my parent is process %d\n",
getpid(), getppid());
fork();
printf("This sentence has been printed by process: %d my parent is process %d\n",
getpid(), getppid());
return 0;
}     

我用GCC编译并运行它。然后输出如下：

kocks@kocks-VirtualBox:~$ ./uno
I'm the parent, my PID is 3746, my parent is process 3507
This sentence has been printed by process: 3746 my parent is process 3507
kocks@kocks-VirtualBox:~$ This sentence has been printed by process: 3747 my parent is process 2857

问题出在第三句：我期望“我的父进程是3746”！特别是，数字 (2857) 始终相同，并且仅在重新启动 VM 后才会更改。有几点需要补充：

1. 如果我wait()在返回之前添加一个，它就可以完美工作
2. 当我尝试“拉皮条”虚拟机（比如给它两个处理器、更多的内存等）时，它有时会随机返回正确的输出

所以我认为问题出在虚拟机上，对吗？我怎么解决这个问题？

编辑 当在双引导下从 Ubuntu 运行程序时，输出如下：

I'm the parent, my PID is 3186, my parent is process 2454 
This sentence has been printed by process: 3186 my parent is process 2454
This sentence has been printed by process: 3187 my parent is process 3186

这怎么可能？

Answer 1

我在 2 个虚拟机设置上进行了测试，均使用 OS X 作为主机，并为每个客户端仅分配 1 个 CPU。

1 Ubuntu 14.04.2

在此设置中，在 X 窗口环境下，行为与 OP 匹配

john@U64D:~$ ./a
I'm the parent, my PID is 2682, my parent is process 2632
This sentence has been printed by process: 2682 my parent is process 2632
john@U64D:~$ This sentence has been printed by process: 2683 my parent is process 1673

父进程1673将保持不变，直到我注销并再次登录

john@U64D:~$ ./a
I'm the parent, my PID is 3787, my parent is process 3740
This sentence has been printed by process: 3787 my parent is process 3740
john@U64D:~$ This sentence has been printed by process: 3788 my parent is process 3107
./a
I'm the parent, my PID is 3790, my parent is process 3740
This sentence has been printed by process: 3790 my parent is process 3740
john@U64D:~$ This sentence has been printed by process: 3791 my parent is process 3107

第3107章初始化--用户

john@U64D:~$ ps -ef|grep 3107
john      3107  2911  0 15:07 ?        00:00:00 init --user

然而，在非 X 环境下（例如 tty1），结果将与 Barmar 答案匹配。

2 Debian 7.8

对于 X 和非 X 环境，此设置与 Barmar 答案相匹配。

约翰@debian:~$ ./a

I'm the parent, my PID is 3455, my parent is process 3406
This sentence has been printed by process: 3455 my parent is process 3406
john@debian:~$ This sentence has been printed by process: 3456 my parent is process 1

结论

OP 观察是 Ubuntu 在默认 X 环境中使用会话初始化的结果。可以找到session init的详细信息这里。

Answer 2

如果您不使用wait()，则父级可以在子级有机会运行之前完成。当子进程调用 时getppid()，父进程已经退出，因此子进程被 收养init。在正常的 Unix 实现上， 的 PIDinit是 1，所以你会期望它说我的父母是进程1当这个情况发生时。但显然 VirtualBox 改变了这一点。

如果您使用wait()，这会使父级等待子级完成后再退出。这样孩子就永远不会成为孤儿。