Extended attributes are arbitrarily named pieces of data associated with
a file and managed by the file system.

The names and the values for the attributes are freely chosen by
applications and/or users, with very few restrictions.  Many recent
systems and filesystems support extended attributes, but the interfaces
used to manage them are far from identical.

Linux, FreeBSD, and Mac OS X all provide similar extended attributes
functionality.

The pxattr package aims at providing a single interface to extended
attributes on all three systems.  It consists in a single C++ module
(one source and one include file), which is to be included in the
application source code, or can be built as a reference command-line
utility to manage user extended attributes in a uniform and cross-
platform way.  It includes an attributes archive/restore facility.

WWW: https://www.lesbonscomptes.com/pxattr/