This function returns the current process group for the specified PID
(use a PID of 0 for the current process).
Invoking getpgrp will produce a fatal error if used on a machine
that doesn't implement getpgrp(2). If PID is omitted, the
function returns the process group of the current process (the same as
using a PID of 0). On systems implementing this operator with the
POSIX getpgrp(2) system call, PID must be omitted or, if
supplied, must be 0.