This function returns the current process group for the specified
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
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.