Returns the current process group for the specified PID. Use a PID of 0 to get the current process group for the current process. Will raise an exception if used on a machine that doesn't implement getpgrp(2). If PID is omitted, returns the process group of the current process.
Some very old machines may not support PID != 0 and will throw an exception if PID != 0.
Portability issues: "getpgrp" in perlport.