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 process group of current process. Note that the
POSIX version of getpgrp() does not accept a
PID argument, so only
PID==0 is truly portable.