Hello
I have a big problem with one of my processes named "mbusd" ;it is an opensource modbus RTU/TCP gateway when I plug USB to serial convertor to it my laptop without this process linux makes virtual ttyUSB very fine and when I unplug it it removes except some times (SOME TIMES not all the times) that I run mbusd process to work with, at that time during mbusd process work when I unplug USB/serial converter the virtual ttyUSB does not disappear and mbusd does not exit too and it turnes in something like this when I get ps -aux: mbusd [defunc] at this time I can not even kill it with -9 or -15 signals and pluging back the converter does not solve the problem too and mbusd does not exit or start to run again. the only way to scape from this situation I found is to restarting the system that is an terrible solution. what can I do to solve this.

Thanks a lot.

A defunct (a.k.a. zombie) process is one that has exited (and thus cannot be killed, as it is already "dead") but still has an entry in the process table to allow the parent process to retrieve its exit status. The parent has not checked on ("wait"ed for) the child process, so the child's status and id remain in the process table. Normally this isn't a problem, but in your case it sounds like this improper signal handling is also leaving some cleanup undone, and new instances of mbusd won't run with old instances in the process table.

I'm not familiar with this software, so I don't know what the parent process is or why it's not responding properly to SIGCHLD. You may want to report this as a bug.

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hello
thank you for your reply
so how can I clean it from process table ? I checked it and I saw it's parent process is init. how can I clean a zombie process from my process list. I want to write a watchdog for this process that if it terminated I could start it again but as it does not leave process table I can not check this situation.

I found some thing else:
root 1620 32.9 0.0 0 0 ? Ds Oct19 6676:21 [mbusd]
that Ds stand for :
D Uninterruptible sleep (usually IO)
s is a session leader
I found that it is wainting for an I/O. BUT I unpluged the mod bus serial/USB converter and as I checked there is it's virtual tty:
ls /dev/ttyUSB*
/dev/ttyUSB0
how can I free it's I/O, if I could do it I think it will exit by itself.