Linux Access Control Lists library (libacl, -lacl).  The function
adds  the  permission contained in the argument (one of ACL_READ,
ACL_WRITE, ACL_EXECUTE) to the permission set referred to by  the
argument An attempt to add a permission that is already contained
in  the  permission set is not considered an error.  Any existing
descriptors that refer to continue to refer  to  that  permission
set.   If any of the following conditions occur, the function re‐
turns and sets to the corresponding value: The argument is not  a
valid  descriptor  for a permission set within an ACL entry.  The
argument does not contain a valid value.  IEEE Std 1003.1e  draft
17  (“POSIX.1e”, abandoned) Derived from the FreeBSD manual pages
written by and adapted for Linux by