sets the effective group ID of the calling process.
If the caller is the
superuser, the real GID and saved set-group-ID are also set.
is implemented like the POSIX version with the
This allows a set-group-ID program that is not set-user-ID-root
to drop all of its group
privileges, do some un-privileged work, and then reengage the original
effective group ID in a secure manner.
On success, zero is returned.
On error, -1 is returned, and
is set appropriately.
The calling process is not privileged (does not have the
CAP_SETGID capability), and
does not match the real group ID or saved set-group-ID of
the calling process.