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Re^3: Check if forked process is dead

by Eliya (Vicar)
on Mar 02, 2011 at 23:10 UTC ( [id://891126]=note: print w/replies, xml ) Need Help??


in reply to Re^2: Check if forked process is dead
in thread Check if forked process is dead

You could inspect what the (non-blocking) waitpid returns. If it's the $pid, the process could be reaped successfully, which means it was no longer running.

In your case here, you're calling the waitpid just a tad too early, so the preceding kill TERM hasn't been delivered/succeeded yet.  And as the waitpid $pid => WNOHANG couldn't yet reap at that moment, the process still exists as a zombie at the time you call kill 0  (like in your original case).

Compare:

Child reacts to SIGTERM:

#!/usr/bin/perl -wl use strict; use POSIX; my $pid = fork; if ( $pid ) { kill TERM => $pid; # wait for kill TERM to take effect select undef, undef, undef, 0.01; my $reaped = waitpid $pid => WNOHANG; if ($reaped == $pid) { print "already gone."; # <--- } else { print "trying harder..."; kill 9 => $pid; } } else { sleep 3; }

Child ignores SIGTERM:

#!/usr/bin/perl -wl use strict; use POSIX; my $pid = fork; if ( $pid ) { # give child some time to set up its $SIG{TERM} handler select undef, undef, undef, 0.01; kill TERM => $pid; select undef, undef, undef, 0.01; my $reaped = waitpid $pid => WNOHANG; if ($reaped == $pid) { print "already gone."; } else { print "trying harder..."; # <--- kill 9 => $pid; } } else { $SIG{TERM} = 'IGNORE'; sleep 3; }

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Re^4: Check if forked process is dead
by petr999 (Acolyte) on Mar 03, 2011 at 01:42 UTC
    Hello,
    Great!
    I believe too I can save a syscall with returning value from waitpid instead of kill 0 trial.
    Special thanks for show that I can use select instead of usleep(). Is it about any advantages, besides few typing and probably that Time::HiRes is not yet included in Perl-5.8 distribution?
    I think I'd better use some specialized CPAN Test::* module for just to build FCGI::Spawn tests... ;-)
    Peter Vereshagin <peter@vereshagin.org>

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