Upgrading to libtorrent 2.0

Author: Arvid Norberg, arvid@libtorrent.org

In libtorrent 2.0, some parts of the API has changed and some deprecated parts have been removed. This document summarizes the changes affecting library clients.

C++11 no longer supported

libtorrent 2.0 requires at least C++-14. To build with boost build, specify the C++ version using the cxxstd=14 build feature (14 is the default).

BitTorrent v2 support

Supporting bittorrent v2 come with some changes to the API. Specifically to support hybrid torrents. i.e. torrents that are compatible with v1-only bittorrent clients as well as supporting v2 features among the peers that support them.


With bittorrent v2 support, each torrent may now have two separate info hashes, one SHA-1 hash and one SHA-256 hash. These are bundled in a new type called info_hash_t. Many places that previously took an info-hash as sha1_hash now takes an info_hash_t. For backwards compatibility, info_hash_t is implicitly convertible to and from sha1_hash and is interpreted as the v1 info-hash. The implicit conversion is deprecated though.

Perhaps most noteworthy is that add_torrent_params::info_hash is now deprecated in favor of add_torrent_params::info_hashes which is an info_hash_t.

The alerts torrent_removed_alert, torrent_deleted_alert, torrent_delete_failed_alert all have info_hash members. Those members are now deprecated in favor of an info_hashes member, which is of type info_hash_t.

An info_hash_t object for a hybrid torrent will have both the v1 and v2 hashes set, it will compare false to a sha1_hash of just the v1 hash.

Calls to torrent_handle::info_hash() may need to be replaced by torrent_handle::info_hashes(), in order to get both v1 and v2 hashes.

announce_entry/tracker changes

On major change in the API is reporting of trackers. Since hybrid torrents announce once per info-hash (once for v1 and once for v2), the tracker results are also reported per bittorrent version.

Each tracker (announce_entry) has a list of endpoints. Each corresponding to a local listen socket. Each listen socket is announced independently. The announce_endpoint in turn has an array info_hashes, containing objects of type announce_infohash, for each bittorrent version. The array is indexed by the enum protocol_version. There are two members, V1 and V2.


std::vector<lt::announce_entry> tr = h.trackers();
for (lt::announce_entry const& ae : h.trackers()) {
    for (lt::announce_endpoint const& aep : ae.endpoints) {
        int version = 1;
        for (lt::announce_infohash const& ai : aep.info_hashes) {
            std::cout << "[V" << version << "] " << ae.tier << " " << ae.url
                << " " << (ih.updating ? "updating" : "")
                << " " << (ih.start_sent ? "start-sent" : "")
                << " fails: " << ih.fails
                << " msg: " << ih.message
                << "\n";

Merkle tree support removed

The old merkle tree torrent support has been removed, as BitTorrent v2 has better support for merkle trees, where each file has its own merkle tree.

This means add_torrent_params no longer has the merkle_tree member. Instead it has the new verified_leaf_hashes and merkle_trees members.

It also means the merkle flag for create_torrent has been removed. torrent_info no longer has set_merkle_tree() and merkle_tree() member functions.

create_torrent changes

The create_torrent class creates hybrid torrents by default. i.e. torrents compatible with both v1 and v2 bittorrent clients.

To create v1-only torrents use the v1_only flag. To create v2-only torrents, use the v2_only flag.

Perhaps the most important addition for v2 torrents is the new member function set_hash2(), which is similar to set_hash(), but for the v2-part of a torrent. One important difference is that v2 hashes are SHA-256 hashes, and they are set per file. In v2 torrents, each file forms a merkle tree and each v2 piece hash is the SHA-256 merkle root hash of the 16 kiB blocks in that piece.

All v2 torrents have pieces aligned to files, so the optimize_alignment flag is no longer relevant (as it's effectively always on). Similarly, the mutable_torrent_support flag is also always on.

pad_file_limit and alignment parameters to the create_torrent constructor have also been removed. The rules for padding and alignment is well defined for v2 torrents.

set_file_hash() and file_hash() functions are obsolete, as v2 torrents have a file_root() for each file.

on_unknown_torrent() plugin API

Since hybrid torrents have two info-hashes, the on_unknown_torrent() function on the plugin class now takes an info_hash_t instead of a sha1_hash.


There is a new enum class called socket_type_t used to identify different kinds of sockets. In previous versions of libtorrent this was exposed as plain int with subtly different sets of meanings.

Previously there was an enum value udp, which has been deprecated in favor of utp.

The socket type is exposed in the following alerts, which now use the socket_type_t enum instead of int:

  • peer_connect_alert
  • peer_disconnected_alert
  • incoming_connection_alert
  • listen_failed_alert
  • listen_succeeded_alert

DHT settings

DHT configuration options have previously been set separately from the main client settings. In libtorrent 2.0 they have been unified into the main settings_pack.

Hence, lt::dht::dht_settings is now deprecated, in favor of the new dht_* settings in settings_pack.

Deprecating dht_settings also causes an API change to the dht custom storage constructor (see session_params). Instead of taking a dht_settings object, it is now passed the full settings_pack. This is considered a niche interface, so there is no backward compatibility option provided.


The stats_alert is deprecated. Instead, call session::post_torrent_updates(). This will post a state_update_alert containing torrent_status of all torrents that have any updates since last time this function was called.

The new mechanism scales a lot better.

saving and restoring session state

The functions save_state() and load_state() on the session object have been deprecated in favor loading the session state up-front using read_session_params() and construct the session from it.

The session state can be acquired, in the form of a session_params object, by calling session::session_state().

The session_params object is passed to the session constructor, and will restore the state from a previous session.

Use read_session_params() and write_session_params() to serialize and de-serialize the session_params object.

As a result of this, plugins that wish to save and restore state or settings must now use the new overload of load_state(), that takes a std::map<std::string, std::string>. Similarly, for saving state, it now has to be saved to a std::map<std::string, std::string> via the new overload of save_state().

A lot of session constructors have been deprecated in favor of the ones that take a session_params object. The session_params object can be implicitly constructed from a settings_pack, to cover one of the now-deprecated constructors. However, to access this conversion libtorrent/session_params.hpp must be included.

userdata is no longer a void*

The userdata field in add_torrent_params is no longer a raw void pointer. Instead it is a type-safe client_data_t object. client_data_t is similar to std::any, it can hold a pointer of any type by assignment and can be cast back to that pointer via static_cast (explicit conversion). However, if the pointer type it is cast to is not identical to what was assigned, a nullptr is returned. Note that the type has to be identical in CV-qualifiers as well.

This userdata field affects the plugin APIs that has this field passed into it.

Additionally, there's now a way to as a torrent_handle for the userdata, so it is associated with the torrent itself.

Adding torrents by URL no longer supported

The URL covers 3 separate features, all deprecated in the previous version and removed in 2.0.

downloading over HTTP

One used to be able to add a torrent by specifying an HTTP URL in the add_torrent_params::url member. Libtorrent would download the file and attempt to load the file as a .torrent file. The torrent_handle in this mode would not represent a torrent, but a potential torrent. Its info-hash was the hash of the URL until the torrent file could be loaded, at which point the info hash changed. The corresponding torrent_update_alert has also been removed. In libtorrent 2.0 info-hashes cannot change. (Although they can be amended with bittorrent v1 or v2 info-hashes).

Instead of using this feature, clients should download the .torrent files themselves, possibly spawn their own threads, before adding them to the session.

async loading of .torrent files

The add_torrent_params::url field also supported file:// URLs. This would use a libtorrent thread to load the file from disk, asynchronously (in the case of async_add_torrent()). This feature has been removed. Clients should instead load their torrents from disk themselves, before adding them to the session. Possibly spawning their own threads.

Disk I/O overhaul

In libtorrent 2.0, the disk I/O subsystem underwent a significant update. In previous versions of libtorrent, each torrent has had its own, isolated, disk storage object. This was a customization point. In order to share things like a pool of open file handles across torrents (to have a global limit on open file descriptors) all storage objects would share a file_pool object passed in to them.

In libtorrent 2.0, the default disk I/O uses memory mapped files, which means a lot more of what used to belong in the disk caching subsystem is now handled by the kernel. This greatly simplifies the disk code and also has the potential of making a lot more efficient use of modern disks as well as physical memory.

In this new system, the customization point is the whole disk I/O subsystem. Instead of configuring a custom storage (implementing storage_interface) when adding a torrent, you can now configure a disk subsystem (implementing disk_interface) when creating a session.

Systems that don't support memory mapped files can still be used with a simple fopen()/fclose() family of functions. This disk subsystem is also not threaded and generally more primitive than the memory mapped file one.

Clients that need to customize storage should implement the disk_interface and configure it at session creation time instead of storage_interface configured in add_torrent_params. add_torrent_params no longer has a storage_constructor member.

As a consequence of this, get_storage_impl() has been removed from torrent_handle.

aio_threads and hashing_threads

In previous versions of libtorrent, the number of disk threads to use were configured by settings_pack::aio_threads. Every fourth thread was dedicated to run hash jobs, i.e. computing SHA-1 piece hashes to compare them against the expected hash.

This setting has now been split up to allow controlling the number of dedicated hash threads independently from the number of generic disk I/O threads. settings_pack::hashing_threads is now used to control the number of threads dedicated to computing hashes.


The cache_size setting is no longer used. The caching of disk I/O is handled by the operating system.

get_cache_info() get_cache_status()

Since libtorrent no longer manages the disk cache (except for a store-buffer), get_cache_info() and get_cache_status() on the session object has also been removed. They cannot return anything useful.

last remnants of RSS support removed

The rss_notification alert category flag has been removed, which has been unused and deprecated since libtorrent 1.2.

The uuid member of add_torrent_params has been removed. Torrents can no longer be added under a specific UUID. This feature was specifically meant for RSS feeds, which was removed in the previous version of libtorrent.