Linux man pages : setgid (2)
SETGID(2) Linux Programmer's Manual SETGID(2)
setgid - set group identity
int setgid(gid_t gid);
setgid sets the effective group ID of the current process. If the
caller is the superuser, the real and saved group ID's 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
On success, zero is returned. On error, -1 is returned, and errno is
EPERM The user is not the super-user (does not have the CAP_SETGID
capability), and gid does not match the effective group ID or
saved set-group-ID of the calling process.
getgid(2), setregid(2), setegid(2)
Linux 2.5 2002-03-09 SETGID(2)