FCP is a special protocol used by clients to interact with a Freenet node, so that they do not have to deal with the complex and changing network protocol of Freenet itself. If you want to write a Freenet client then FCP is almost certainly a much better idea than doing this or trying to hook into Freenet's internals directly.

Please see the documentation at : http://freenet.sourceforge.net/index.php?page=fcp&mode=developer