setgid sets the effective group ID of the current process. If the caller is the superuser, the real and saved group IDs are also set.
Under Linux, setgid is implemented like the POSIX version with the _POSIX_SAVED_IDS feature. This allows a setgid program that is not suid root to drop all of its group privileges, do some un-privileged work, and then re-engage the original effective group ID in a secure manner.
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.