If the process has appropriate privileges, setgid() shall set the real
group ID, effective group ID, and the saved set-group-ID of the calling
process to gid.
If the process does not have appropriate privileges, but gid is equal to
the real group ID or the saved set-group-ID, setgid() shall set the
effective group ID to gid; the real group ID and saved set-group-ID
shall remain unchanged.
The setgid() function shall not affect the supplementary group list in
Any supplementary group IDs of the calling process shall remain unchanged.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html