c_bittorrent

An experimental bare-bones bittorrent client for linux written in C. For concurrency, it uses non-blocking sockets on a single thread. Implements only the most necessary things from this unofficial specification. Made mostly for learning purposes but works well enough with cooperative peers. Try downloading a popular open source software which provides torrents, like debian or libreoffice.

libraries needed

• curl (for sending GET request to the HTTP tracker)
• openssl (for computing SHA1 hashes)

usage

c_bittorrent TORRENTFILE [FILENAME] [OPTION]

-v --verbose    explain what is being done
-h --help       print this help message and exit

Downloads the torrent in the TORRENTFILE file and saves it to FILENAME, if specified, and to the name given in the torrent file otherwise.

design choices

I decided not to use threads because I wanted to learn how non-blocking (and blocking) sockets work in C. Beej's guide was a lot of help.

I used my own bencode parser for this project.

There's a macro in p2p.h called COMMTIMEOUT. It is used as the timeout (microseconds) for the select call in the main communication loop. Increasing it reduces the CPU usage but can slow down the download. I set it to 10000 microseconds as default. You can change it as per your needs. I found that setting it to 1000 or 100 works fairly well too.

In the same header file, there's also SIMULBLOCKS. It's the number of blocks we request to a peer simultaneously. Increasing it can increase the download speed but if you set it too high, peers are more likely to choke us. I used 5 as default because, apparently, that's what the clients used classically.

We download the pieces in random order. No special strategy (like the rarest-first) for selecting pieces has been used, which sometimes leads to the problem: when all the peers we are connected to (by the end) don't have a piece, that piece is just left undownloaded and the client hangs.

We don't send cancel messages to peers after we get a piece in the "endgame".

known issues

Someimes hangs when the peers connected have no pieces to download. See above.

limitations

• Does not support uploading pieces (only leeching)
• Supports only single-file torrents
• No support for magnet links (only .torrent files)
• Only supports HTTP trackers
• No resume file option
• Contacts the tracker only once