Linux Access Control Lists library (libacl, -lacl).  The function
checks  the  ACL  referred  to by the argument for validity.  The
three required entries ACL_USER_OBJ, ACL_GROUP_OBJ, and ACL_OTHER
must exist exactly once in the  ACL.  If  the  ACL  contains  any
ACL_USER or ACL_GROUP entries, then an ACL_MASK entry is also re‐
quired. The ACL may contain at most one ACL_MASK entry.  The user
identifiers  must  be  unique among all entries of type ACL_USER.
The group identifiers must be unique among all  entries  of  type
ACL_GROUP.   If  any of the following conditions occur, the func‐
tion returns and sets to the corresponding value: The argument is
not a valid pointer to an ACL.  The argument does not point to  a
valid  ACL.   One  or  more  of  the  required ACL entries is not
present in The ACL contains entries that are  not  unique.   IEEE
Std  1003.1e  draft  17  (“POSIX.1e”, abandoned) Derived from the
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