The IEEE 1003.1e draft 17 ("POSIX.1e") describes a set of 17 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 function. 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. 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 - 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, ...) - function to manipulate all these, and functions to manipulate files