http://www.perlmonks.org?node_id=891115


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

I'd like to know if I can check that process was not terminated by SIGTERM, so I'd SIGKILL it after some seconds. This doesn't work:
$ perl -Mstrict -MPOSIX -wE 'my $pid = fork; if( $pid ){ kill TERM => +$pid; waitpid $pid => WNOHANG; say kill 0 => $pid; } else { sleep 3; + exit; }' 1 $
But only 0 instead of WNOHANG shows me that process is ended.
Can I check if the process is ended in a limited amount of time?
Should I fork a new process only for a blocking waitpid $pid => 0 particularly?
Thank you.

Replies are listed 'Best First'.
Re^3: Check if forked process is dead
by Eliya (Vicar) on Mar 02, 2011 at 23:10 UTC

    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; }
      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>
Re^3: Check if forked process is dead
by sundialsvc4 (Abbot) on Mar 03, 2011 at 20:46 UTC

    You cannot avoid race conditions in such a quest ...