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

There's also a companion library: libfuse2. This library is available
from the FUSE homepage:
<http://fuse.sourceforge.net/>
although chances are your distribution already has that library
installed if you've installed the "fuse" package itself.

See Documentation/filesystems/fuse.txt for more information.
See Documentation/Changes 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/