The setgid function can be used with POSIX style identifiers
    enabled or disabled.

    POSIX style IDs are supported on OpenVMS Version 7.3-2 and
    higher.

    With POSIX IDs disabled, the setgid function is implemented for
    program portability and serves no function. It returns 0 (to
    indicate success).

    With POSIX style IDs enabled:

    o  If the process has the IMPERSONATE privilege, the setgid
       function sets the real group ID, effective group ID, and the
       saved set-group-ID to gid.

    o  If the process does not have appropriate privileges but gid is
       equal to the real group ID or to the saved set-group-ID, then
       the setgid function sets the effective group ID to gid. The
       real group ID and saved set-group-ID remain unchanged.

    o  Any supplementary group IDs of the calling process remain
       unchanged.