Here's a very nice Gnutella program one in C - you should easily get samples there:
- gtk-gnutella - The Graphical Unix Gnutella Client

Also there's Shareaza in C++, but that one's a multinetwork and the last time I checked its Gnutella implementation wasn't that advanced (bad track record) and it's Windows only:
- Shareaza - Bringing P2P Together

Did you already read Gnutella for Users - Gnufu and GDF - LimeWire ?

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