libnvpair is a name-value pair library originating from Solaris

The nvpair library was released in OpenSolaris and has been extended
by the Illumos project.  It exports a set of functions used for managing
name-value pairs.  This is a base library for Solaris and its descendents.
However, the kernel-specific code has been removed thus libnvpair is
strictly meant for the userland.  The XDR functionality has been masked
for now due to RPC differences between *BSD and Illumos.
