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 the ACL referred to by is invalid, returns a posi‐
tive  error code that indicates which type of error was detected.
The following symbolic error codes are defined: The ACL  contains
multiple  entries  that  have  a  tag type that may occur at most
once.  The ACL contains multiple ACL_USER entries with  the  same
user ID, or multiple ACL_GROUP entries with the same group ID.  A
required entry is missing.  The ACL contains an invalid entry tag
type.   The function can be used to translate error codes to text
messages.  In addition, if the pointer is not assigns the  number
of  the  ACL  entry  at which the error was detected to the value
pointed to by Entries are numbered starting with zero, in the or‐
der in which they would be returned by the function.  If success‐
ful, the function returns if the ACL referred to by is valid, and
a positive error code if the ACL is invalid. Otherwise,  a  value
of is returned and the global variable is set to indicate the er‐
ror.   If any of the following conditions occur, the function re‐
turns and sets to the corresponding value: The argument is not  a
valid  pointer to an ACL.  This is a non‐portable, Linux specific
extension to the ACL manipulation functions defined in  IEEE  Std
1003.1e draft 17 (“POSIX.1e”, abandoned).  Written by