Functionality level
-------------------

The IEEE 1003.1e draft 17 ("POSIX.1e") describes a set of 28 functions.
These are grouped into three groups, based on their portability:

- first group, the most portable one. All systems which claim to support
  POSIX.1e should implement these:

     acl_delete_def_file(3), acl_dup(3), acl_free(3), acl_from_text(3),
     acl_get_fd(3), acl_get_file(3), acl_init(3), acl_set_fd(3),
     acl_set_file(3), acl_to_text(3), acl_valid(3)

- second group, containing the rest of the POSIX ACL functions. Systems
  which claim to fully implement POSIX.1e should implement these:

     acl_add_perm(3), acl_calc_mask(3), acl_clear_perms(3),
     acl_copy_entry(3), acl_copy_ext(3), acl_copy_int(3),
     acl_create_entry(3), acl_delete_entry(3), acl_delete_perm(3),
     acl_get_entry(3), acl_get_permset(3), acl_get_qualifier(3),
     acl_get_tag_type(3), acl_set_permset(3), acl_set_qualifier(3),
     acl_set_tag_type(3), acl_size(3)

- third group, containing extra functions implemented by each OS. These
  are non-portable version. Both Linux and FreeBSD implement some extra
  functions.

Thus we have the level of compliance. Depending on whether the system
library support the second group, you get some extra methods for the ACL
object.

The implementation of the second group of function can be tested by
checking the module-level constant HAS_ACL_ENTRY. The extra
functionality available on Linux can be tested by additional HAS_*
constants.

Internal structure
------------------

The POSIX draft has the following stuff (correct me if I'm wrong):

- an ACL is denoted by acl_t
- an ACL contains many acl_entry_t, these are the individual entries in
  the list; they always(!) belong to an acl_t
- each entry_t has a qualifier (think uid_t or gid_t), whose type is
  denoted by the acl_tag_t type, and an acl_permset_t
- the acl_permset_t can contain acl_perm_t value (ACL_READ, ACL_WRITE,
  ACL_EXECUTE, ACL_ADD, ACL_DELETE, ...)
- functions to manipulate all these, and functions to manipulate files