setuid - further details

setuid() sets the real-user-ID (ruid),effective-user-ID (euid), and/or saved-user-ID (suid) of the calling process. The super-user's euid is zero. The following conditions govern setuid's behavior:

• If the euid is zero, setuid() sets the ruid, euid, and suid to uid.
• If the euid is not zero, but the argument uid is equal to the ruid or the suid, setuid() sets the euid to uid; the ruid and suid remain unchanged. (If a set-user-ID program is not running as super-user, it can change its euid to match its ruid and reset itself to the previous euid value.)
• If euid is not zero, but the argument uid is equal to the euid, and the calling process is a member of a group that has the PRIV_SETRUGID privilege (see privgrp(4)), setuid() sets the ruid to uid; the euid and suid remain unchanged.