Today support for Mac OS X 10.4 ACLs was added to xar. Mac OS X has a rather bizarre ACL API.
It uses the same function names as POSIX draft ACLs implemented by Linux and FreeBSD, however the return values and arguments have different semantics.
For example, acl_get_entry() on Linux and FreeBSD returns 1 on success. The same call on Mac OS X returns 0 on success.
Additionally, Mac OS X uses a different ACL system using UUIDs to identify users instead of username/uid. Presumably this is because a uid/username is not considered unique on Mac OS X, due to 'mobile' environments. This means instead of using ACL_TYPE_DEFAULT or ACL_TYPE_ACCESS as an argument to acl_get_file(), on Mac OS X we must use ACL_TYPE_EXTENDED. These "extended" acls do not format the same when calling acl_to_text() so it prevents the acls from being portable to other systems, but at least acl_to_text() works on Mac OS X and is read by acl_from_text().