Last Updated: 2023-06-19 09:41:37 Monday
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僵尸进程，zombie or defunct，是指一个已经结束但还没有被父进程reap的子进程。reap的方式，就是使用wait系统调用。
如下示例：
xinlin 311016 ... S+ 15:03 0:00 python3 test_dup2.py
xinlin 311017 ... Z+ 15:03 0:00 [python3] <defunct>
311017是311016的子进程，但311016不调用wait，也没有结束，因此311017不幸成为了zombie(Z+)!
// wait, waitpid, waitid - wait for process to change state
// man 2 wait
#include <sys/wait.h>
wait接口比较简单，它就是单纯的等待任一个子进程的结束，
pid_t wait(NULL);
等待某个子进程的结束，不需要子进程的status，不关心子进程的pid，不设超时时间（死等）。如果有多个子进程，只要其中任一个结束，就满足打破wait的条件（wait返回，程序继续）。
下面是较复杂的用法：
pid_t wait(int *wstatus);
既可以得到结束的子进程的status，还有pid。
子进程自己执行结束了，可为什么父进程还要wait呢？
这需要我们理解僵尸进程，不要被僵尸这个名字吓到了，僵尸进程一点都不可怕。manpage的信息，足够我们理解zombie process：
All of these system calls are used to wait for state changes in a child of the calling process, and obtain information about the child whose state has changed. A state change is considered to be: the child terminated; the child was stopped by a signal; or the child was resumed by a signal.
子进程结束，子进程被信号暂停，子进程被信号重新启动，都属于state change，都可以wait。
In the case of a terminated child, performing a wait allows the system to release the resources associated with the child; if a wait is not performed, then the terminated child remains in a "zombie" state.
对于已结束的子进程，父进程的wait系统调用，可以release一些与child绑定的资源，如果父进程没有wait，已结束的子进程就进入zombie状态。
A child that terminates, but has not been waited for becomes a "zombie". The kernel maintains a minimal set of information about the zombie process (PID, termination status, resource usage information) in order to allow the parent to later perform a wait to obtain information about the child. As long as a zombie is not removed from the system via a wait, it will consume a slot in the kernel process table, and if this table fills, it will not be possible to create further processes.
wait的目的，是让父进程能够获取子进程的信息（PID，结束状态信息，资源使用信息），既然父进程还没有调用wait，子进程的相关信息就还是会保留在系统中。僵尸进程也是进程，也会占用一点系统资源。
If a parent process terminates, then its "zombie" children (if any) are adopted by init(1), (or by the nearest "subreaper" process as defined through the use of the prctl(2) PR_SET_CHILD_SUBREAPER operation); init(1) automatically performs a wait to remove the zombies.
如果父进程没有调用wait就结束，僵死进程变为孤儿进程，被1号进程领养，然后被wait掉。
不断创建子进程的程序，一定要注意调用wait，否则运行时间长了，zombie太多，部分资源得不到释放，系统慢慢就异常了.....
waitpid接口就要复杂一些，有更多的参数，更丰富的控制。man 2 wait最后有一段代码，需仔细体会。Python的os模块中提供的接口，几乎就是系统调用上面的一层薄薄的封装。
本文链接：https://cs.pynote.net/sf/linux/prog/202207142/
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