Returns the current process group for the specified process
Use a pid of
0 for the current process.
getpgrp will produce a fatal error if used on a machine
that doesn't implement the
getpgrp system call.
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
getpgrp(2) system call, pid must be omitted or, if
supplied, must be