libtorrent blog

bittorrent development from the trenches

forward declarations and ABI

This post argues the C++ rule: One should never forward declare a name from a 3rd party library This rule is a generalisation of the same rule for the standard library. The standard says [namespace.std]: The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within . . .

building libtorrent for C.H.I.P.™

I recently bought a C.H.I.P. and naturally wanted to build libtorrent for it. I’m on a mac and chip runs arm, so first I needed to install the cross compiler toolchain. The target for chip is arm on linux, so the toolchain I’m looking for is “arm-linux-gnueabihf”. toolchain It seems like a fair number of people . . .

link compatibility

A major source of errors among users of libtorrent has traditionally been caused by binary incompatibility between the (built) libtorrent library and the client linking against it. Binary- or link compatibility is having two sides of a shared library (or translation unit) boundary have different understandings about the layout of objects or calling conventions. Consider the following library interface: . . .

DHT bootstrap node

With the release of libtorrent-1.1.1, libtorrent finally got its very own default DHT bootstrap node, dht.libtorrent.org:25401. This post gives some background on the work that went into setting it up.

unsigned integers

In this post I will talk about the use of unsigned (integral) types in C++, or perhaps more specifically the rationale for using them. In my experience, it is common to use unsigned types for any variable holding a value that cannot be negative. Say, the number of bytes in a buffer.

slow start

This post is a result of looking into a slow-start performance issue in uTP. Slow-start is a mechanism in TCP employed to discover the capacity of a link, before transitioning into the steady state regime of additive increase and multiplicative decrease. Slow start is employed on new connections and after time-outs (where the congestion window . . .

libtorrent alert queue

The main mechanism libtorrent uses to report events and errors to the client is via alerts. Alerts are messages as c++ objects with additional information depending on the type of message. Periodically clients poll for new alerts from a session object. In the next major release of libtorrent detailed peer logging will be available as . . .

bdecode parsers

I have recently revisited the bdecoder in libtorrent, and ended up implementing a new bdecoder that is two orders of magnitude faster than the original (naive) parser. This is the 3rd decoder in libtorrent’s history, and I would like to cover its evolution of parsing bencoded data.

a bittorrent filesystem

One of the main bottlenecks when downloading and seeding content over bittorrent is accessing the disk. This post explores the option to bypass traditional filesystems and use a block device as storage for torrents, in order to improve download performance. bittorrent protocol BitTorrent downloads conceptually divide up content into pieces, which are downloaded in rarest-first order. . . .

filenames

A .torrent file is very flexible in what it allows a path or filename to contain. Each directory name in a path is a length-prefixed utf-8 string. It can be an empty string, and it can have any character imaginable in it. The way a .torrent file represents filenames and paths is also agnostic to . . .