my $countfile = './COUNTFILE';

# Not diropen()... plain old "open" on a directory:
open(DIRLOCK, '.') || die "open .: $!";
flock(DIRLOCK, LOCK_EX) || die "Lock failed: $!";

sysopen(COUNT, $countfile, O_RDWR|O_CREAT)
  || die "open $countfile: $!";
my $num = <COUNT> || 0;
chomp $num;
seek(COUNT, 0, 0) || die "rewind: $!";
truncate(COUNT, 0) || die "truncate: $!";

if ($operation eq 'checkin') {
    ++$num;
    warn "Checkin $num";
}
else {
    --$num;
    warn "Checkout $num";
}

if ($num) {
    print COUNT $num;
}
else { # count is 0; get rid of the file
    unlink $countfile;
}

close(DIRLOCK) || die "close .: $!";