Re: Re: open(KID, "-|") and wait()?

"Zombies are created when a child has exited, but the parent has not wait()ed for it yet."

We have a little bit misunderstanding of "zombie" here ;-) Zombie is actually a process still running, but we lost track of it. In other words, in the fork case, if the parent created a child that cannot exit on its own, say it has some sort of dead loop, whatever the dead loop is created by mistake or purposely, as a good practice, the parent process should kill the child process. Otherwise, the child becomes a zombie, as it will run forever, and it will have that seat in the process table forever.

If a process exited, it will be removed from the process table, doesn't matter whether it is created by a parent process, or it runs on its own, as it is a process any way.

Other than the zombie situation, there are also other cases you may want to waitpid on your child process. For example, a parent process keeps creating child processes, and delegating tasks to them at a high speed. If you don't waitpid() to control the number of child processes you created and still alive, you would soon see an error message saying that, you cannot create child process, as resource used up.

In this case, you would keep a counter of running child processes, and when it reaches a predefined max, stop creating child process, and start waitpid(), until the number of running child processes drop below a safe line.

Also a little comment about SIGCHLD. You cannot capture SIGCHLD on win32, so the approach is not fully portable.

Comment on Re: Re: open(KID, "-\|") and wait()?

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Re: Re: Re: open(KID, "-\|") and wait()? by virtualsue (Vicar) on Apr 11, 2003 at 09:30 UTC
Q. How do you kill a zombie? A. You can't, it's already dead. Zombies are not running. They have terminated, but they have a living parent who either hasn't waited for them, or hasn't told the kernel that it doesn't want to do that. Zombies are created because child processes run asynchronously from their parents, and often terminate long before the parent has had a chance to call wait/waitpid on them. The kernel has to keep some minimal information about such child processes in the table so that the parent can have a chance to collect its status, but releases the memory & other resources they might be holding	[reply]
Re: Re: Re: open(KID, "-\|") and wait()? by robartes (Priest) on Apr 11, 2003 at 05:43 UTC
Zombie is actually a process still running, but we lost track of it. In other words, in the fork case, if the parent created a child that cannot exit on its own, say it has some sort of dead loop, whatever the dead loop is created by mistake or purposely, as a good practice, the parent process should kill the child process. Otherwise, the child becomes a zombie, as it will run forever, and it will have that seat in the process table forever. Ehm, I beg to differ. A zombie process is not what you describe here. A zombie process is one that is no longer running (it has exited), but its parent has not yet collected its exit status, so the kernel keeps it around in the process table. What you describe is a runaway process, or even a perfectly normal daemon. Something like inetd or sendmail fits your description, and I do not think many people will call those zombies. If a process exited, it will be removed from the process table, doesn't matter whether it is created by a parent process, or it runs on its own, as it is a process any way. Except when its a child process - it exits but stays in the process table unless its parent collects its status. Update: Added point about daemons. YAU: After a chatterbox conversation with pg, we've decided that we're both right - different sources mean different things with 'zombie process'. YMMV. CU Robartes-	[reply]