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Re^2: Best way to kill a child process

by Eliya (Vicar)
on Sep 21, 2011 at 13:58 UTC ( #927141=note: print w/ replies, xml ) Need Help??


in reply to Re: Best way to kill a child process
in thread Best way to kill a child process

$SIG{CHLD} = sub {while (waitpid(-1, WNOHANG) > 0){} };

AFAIK, on most platforms (where reaping children is of concern), setting

$SIG{CHLD} = 'IGNORE';

should have the same effect, as it will make Perl autoreap terminated child processes.  And it's less clutter (you don't need to load/import WNOHANG from POSIX ...).

See also perlipc.


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Re^3: Best way to kill a child process
by Marshall (Prior) on Sep 21, 2011 at 15:31 UTC
    Good point about POSIX. My Perl servers usually have:
    use IO::Socket; use POSIX ":sys_wait_h";
    Yes, on some platforms, setting up the sigaction stuff to a NULL handler will cause an "auto reap", but AFAIK that is not universal - I'm thinking about the low level C calls that Perl would use. In this case, this is an issue of how the OS deals with sigaction() handlers, not how Perl itself works. Perl cannot do what C cannot do.

    I guess where I'm at is that the code I suggested is going to work on all platforms all the time (AFIK). I agree that 'IGNORE' will work on almost all platforms. I'm just not sure about the difference between "almost all" and "all". This detail probably doesn't matter for this app - it doesn't sound like a "general purpose" application as far as the OP is concerned.

    So YMMV. Setting "IGNORE" is not "wrong" and it is "easier".

    We both agree on the main issue here:
    that the right way to deal with this is to explicitly do something with the CHLD signal: either a) explicitly ignore it which hopefully will cause the OS to "autoreap" the child or b)set a simple subroutine like I suggested. As long as one of these options "works", then it will work in all cases of child death: a) if the child kills itself (maybe a via a die statement) or b)I kill my own child or c)somebody else kills it.

      We both agree on the main issue here: ...

      I think we also do agree on the potential side issues related to portability :)

      Just for the record: a quick peek into the sources turned up this (in util.c):

      #if defined(SA_NOCLDWAIT) && !defined(BSDish) /* See [perl #18849] */ if (signo == SIGCHLD && handler == (Sighandler_t) SIG_IGN) act.sa_flags |= SA_NOCLDWAIT; #endif

      and the referenced #18849 is an interesting read.  Essentially, the issue seems to revolve around if and how the sigaction() flag SA_NOCLDWAIT is implemented, i.e. its interactions with the varieties of the wait calls (wait(), wait4(), waitpid() ). And - as I read it - the upshot of it is that if SA_NOCLDWAIT could lead to problems, it isn't needed anyway — which is why the current implemention "mostly" works...

      Anyhow, I've used $SIG{CHLD} = 'IGNORE' on quite a few versions and brands of Unix-ish systems (AIX, HP-UX, IRIX, Linux, Solaris) and haven't had any issues with it yet (which is not to say there might not be potential problems on some other platform, of course).

        I think we also do agree on the potential side issues related to portability :)

        Yes.

        I was thinking about a signal problem that a guy had about 6 months ago related to Apple OS X. We were using C and not Perl.

        Your one line of code will work at least 99% of the time!
        My 2 lines of code may work at a higher probability, but I don't think that it matters at all!

        The "right way" to deal with this is to have a CHLD signal handler. And either set that thing to "IGNORE' or a coderef to a subroutine that causes a waitpid loop. $SIG{CHLD}='IGNORE'; is far superior to doing nothing with the CHLD signal.

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