How do I stop child processes from becoming defunct?

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    How do I stop child processes from becoming defunct?

    Hello!

    I'm writing a shell and by now it works pretty well, even though I still haven't managed to incorporate ncurses/termios yet. But when I run a program from my shell using "&", and shutdown the program, the child process becomes defunct.

    As I understand it I need to handle the child process ending in some way, but how do I handle it without waiting for it? Or do I need two child processes, where the first starts the second and then waits for it, or would that just make the first child process defunct instead of the second?

    //Zarniwoop

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    spurious conceit MK27's Avatar
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    Quote Originally Posted by Zarniwoop View Post
    As I understand it I need to handle the child process ending in some way, but how do I handle it without waiting for it?
    The child should handle it's own ending just like a normal program. The parent does not have to wait for it -- that is the purpose of fork, &.

    They still use the same display, remember. If the child fails to do something (like exec), it should execute error handling code itself, including alerting the user. This could appear to interrupt output of a foreground or other backgrounded process, but that is what happens with bash.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

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    {Jaxom,Imriel,Liam}'s Dad Kennedy's Avatar
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    Are you talking about zombies? If you are, you can waitpid() with WNOHANG to keep from getting zombies, but at the same time you'll not pause your main thread either.

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    Quote Originally Posted by Kennedy View Post
    Are you talking about zombies? If you are, you can waitpid() with WNOHANG to keep from getting zombies, but at the same time you'll not pause your main thread either.
    To add to Kennedy's suggestion, I think if you call waitpid() after detecting a SIGCHLD, you can avoid calling waitpid() when there's no child ending.
    If you understand what you're doing, you're not learning anything.

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    Thank you for all the responses.

    itsme86: How do I detect "SIGCHLD" if I don't wait? Or can I check for incoming signals regularly? I assume that would be useful for ignoring Ctrl+C also?

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    The old-time standard procedure for creating proper daemons is to first fork. Let the parent die. The child will then have init (pid=1) as its parent. Use setsid() to put the child in a process group of its own. Then close all file descriptors (including 0, 1 and 2) and then open 0, 1 and 2 to some harmless device such as /dev/null. Then set up signal handlers (or simply ignore them) and you should be all set.

    Most *nix daemons use this method and this is also why you tend to see the pid increased by two each time you start a daemon.

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    I don't understand how you mean?

    What I want to do is the following. I have created a shell, and when the user start e.g. emacs using e.g. "emacs test.c &", he/she can continue using the shell. When the user shuts down emacs, I want to avoid making the process defunct which it becomes now. Actually thanks to running waitpid with WNOHANG every time the user presses enter in the shell I avoid this now, but how would I do what you and itsme86 suggest?

    If I create a parent process, which then forks and runs the program, wouldn't that process become defunct in place of the child process?

  8. #8
    spurious conceit MK27's Avatar
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    It is totally normal for a process to appear <defunct> for a short duration after exit.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

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