redhat man page for setgid

SETGID(2)						     Linux Programmer's Manual							 SETGID(2)

NAME
       setgid - set group identity

SYNOPSIS
       #include <sys/types.h>
       #include <unistd.h>

       int setgid(gid_t gid);

DESCRIPTION
       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  man-
       ner.

RETURN VALUE
       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS
       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.

CONFORMING TO
       SVr4, SVID.

SEE ALSO
       getgid(2), setregid(2), setegid(2)

Linux 2.5							    2002-03-09								 SETGID(2)