The Mac OS 10.10 man page for initgroups(3) says:

"Processes should not use the group ID numbers from getgroups(2) to determine a user's group membership.  The list obtained from getgroups() may only be a partial list of a user's group membership.  Membership checks should use the mbr_gid_to_uuid(3), mbr_uid_to_uuid(3), and mbr_check_membership(3) functions."
(http://www.manpagez.com/man/3/initgroups/ -- not official Apple page, but it matches what I see in my OS.)

When the man page says, "The list obtained from getgroups() may only be a partial list of a user's group membership.", and the list from `id -G` is presumably a complete list, should we understand that Apple is saying their getgroups(2) implementation isn't POSIX-compliant? If so, maybe we should skip test_getgroups on Mac OS X systems?

Or, should we consider rewriting os_getgroups_impl() to use a Mac-specific implementation on Mac OS X?