libuutil is a library of userland utilities originating from solaris

This library provides both a doubly linked-list implementation and a
AVL tree implementation.  This has been a private library best known
as a core component for ZFS and SMF.

The performance is considered excellent.  As this has always been a
private library, it is not well documented and there is no man page
for it.  The best documentation is located in the source code and
reading OpenSolaris/Illumos ZFS and SMF sources will help as well.