Also if using signal 0 (zero) to poll processes how do you poll the group with a negative signal!?

I was going to suggest killpg() instead, but there are two problems with that: first, POSIX.pm doesn't have it (though it probably should, where available); second, SUSv3 documents killpg(pgrp, sig) as being the same as kill(-pgrp, sig) when sig is > 1 and undefined otherwise.

That leads me to suspect that polling a process group is considered an unuseful feature. What would it mean?