With FUSE it is possible to implement a fully functional filesystem
in a userspace program.

There's also companion library: libfuse. This library along with
utilities is available from the FUSE homepage:
<http://fuse.sourceforge.net/>

See <a href="https://www.kernel.org/doc/Documentation/filesystems/fuse.txt">Documentation/filesystems/fuse.txt</a> for more information.
See <a href="https://www.kernel.org/doc/Documentation/Changes">Documentation/Changes</a> for needed library/utility version.

If you want to develop a userspace FS, or if you want to use
a filesystem based on FUSE, answer Y or M.

source code:

fs/fuse/