getpgrp pid
Returns the current process group for the specified process
ID (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 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
POSIX getpgrp(2) system call, pid must be omitted or, if
supplied, must be 0.