setgid(2) -- Linux man page
NAMEsetgid - set group identity
DESCRIPTIONsetgid 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 manner.
RETURN VALUEOn success, zero is returned. On error, -1 is returned, and errno is set appropriately.
- 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.
CONFORMING TOSVr4, SVID.
SEE ALSOgetgid(2), setregid(2), setegid(2)