libspnav is a free (BSD-licensed) alternative to the proprietary
3Dconnexion libraries for accessing 3D input devices such as the
SpaceNavigator, SpaceMouse and similar 6-degrees-of-freedom (6dof)
devices. It provides a simple C API (spnav.h) for receiving motion and
button events.

At runtime the library communicates with the spacenavd daemon (its
native protocol) or, when built with X11 support (the default here),
with the proprietary 3Dconnexion daemon (3dxsrv). To actually use a
device you will need the spacenavd daemon installed and running, which
is packaged separately.