I don't think it was race condition in this case, because this code should work with and without SIG CHLD IGNORE statement.

Fair enough; your further testing speaks to that. I'd still recommend setting up the handler before the kill, as a matter of good practice if nothing else.

Well, for example I want to reap zombies while program run, but at the same time I want to wait all child processes at exit.

OK. Luckily, that's quite easy to do, with only one wait in your code. See perlipc for more info, but here's a handler inspired by that:

use POSIX ':sys_wait_h'; # WNOHANG
my %pids; # Parent's hash of child pids

$SIG{CHLD} = sub {
    local ($?, $!); # Don't change $! or $? outside handler
    until (-1 == (my $pid = waitpid(-1, WNOHANG))) {
        return if $pid == 0; # Processes still running
        next unless delete $pids{$pid};
        say "$pid exited with status $?";
    }
};

[download]

In your parent, just set $pids{$pid} = 1 every time you fork, and the SIGCHLD handler will clean them up. In your parent's main loop, you then know not to exit until %pids is empty. You need to track your pids somehow, if you intend to send signals to them, so you may as well take advantage of that.

In reply to Re^3: SIG CHLD IGNORE and wait at same time by rjt
in thread SIG CHLD IGNORE and wait at same time by vsespb

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