I would really appreciate if someone could explain this one to me. My code is:

#!/usr/bin/perl

sub nanny {
    print "ENTER\n";

    while ((my $cpid = waitpid(-1, WNOHANG)) > 0) {
        print "$cpid exited\n";
    }

    $SIG{CHLD} = \&nanny;

    print "EXIT\n";
}

$SIG{CHLD} = \&nanny;

my $c1 = open (C1, "-|");

if ($c1 == 0) {
    exec("sleep 2");
    die $!;
} elsif (!defined $c1) {
    die "Failed: $!\n";
}

my $c2 = open (C2, "-|");

if ($c2 == 0) {
    exec("sleep 7");
    die $!;
} elsif (!defined $c2) {
    die "Failed: $!\n";
}

my $c3 = open (C3, "-|");

if ($c3 == 0) {
    exec("sleep 10");
    die $!;
} elsif (!defined $c3) {
    die "Failed: $!\n";
}

sleep 20;
print "DONE\n";
[download]

When executed, I see:

ENTER
13173 exited
13174 exited
13175 exited
EXIT
DONE
ENTER
EXIT
[download]

This is Perl 5.16 on Linux.

Why is the signal handler not exiting after reaping the first (and only at the time) zombie child? If I replace the open with an explicit fork(), I get the same results. If I change the fork/exec to backticks, the handler gets called 3 times as expected, and each call fails to find any zombies, also as expected.

Incidentally, the script terminates after the third child exits without waiting for the sleep to complete. Same with fork(). Backticks let the sleep finish.

Thanks!