session()

session(fingerprint const& print
        = libtorrent::fingerprint("LT", 0, 1, 0, 0)
        , int flags = start_default_features | add_default_plugins
        , int alert_mask = alert::error_notification);

session(fingerprint const& print
        , std::pair<int, int> listen_port_range
        , char const* listen_interface = 0
        , int flags = start_default_features | add_default_plugins
        , int alert_mask = alert::error_notification);

If the fingerprint in the first overload is omited, the client will get a default fingerprint stating the version of libtorrent. The fingerprint is a short string that will be used in the peer-id to identify the client and the client's version. For more details see the fingerprint class. The constructor that only takes a fingerprint will not open a listen port for the session, to get it running you'll have to call session::listen_on(). The other constructor, that takes a port range and an interface as well as the fingerprint will automatically try to listen on a port on the given interface. For more information about the parameters, see listen_on() function.

The flags paramater can be used to start default features (upnp & nat-pmp) and default plugins (ut_metadata, ut_pex and smart_ban). The default is to start those things. If you do not want them to start, pass 0 as the flags parameter.

The alert_mask is the same mask that you would send to set_alert_mask.

~session()

The destructor of session will notify all trackers that our torrents have been shut down. If some trackers are down, they will time out. All this before the destructor of session returns. So, it's advised that any kind of interface (such as windows) are closed before destructing the session object. Because it can take a few second for it to finish. The timeout can be set with set_settings().

pause() resume() is_paused()

void pause();
void resume();
bool is_paused() const;

Pausing the session has the same effect as pausing every torrent in it. Resuming will restore the torrents to their previous paused state. i.e. the session pause state is separate from the torrent pause state. A torrent is inactive if it is paused or if the session is paused.

abort()

session_proxy abort();

In case you want to destruct the session asynchrounously, you can request a session destruction proxy. If you don't do this, the destructor of the session object will block while the trackers are contacted. If you keep one session_proxy to the session when destructing it, the destructor will not block, but start to close down the session, the destructor of the proxy will then synchronize the threads. So, the destruction of the session is performed from the session destructor call until the session_proxy destructor call. The session_proxy does not have any operations on it (since the session is being closed down, no operations are allowed on it). The only valid operation is calling the destructor:

class session_proxy
{
public:
        session_proxy();
        ~session_proxy()
};

add_torrent()

typedef storage_interface* (&storage_constructor_type)(
        file_storage const&, fs::path const&, file_pool&);

struct add_torrent_params
{
        add_torrent_params(storage_constructor_type s);

        boost::intrusive_ptr<torrent_info> ti;
        char const* tracker_url;
        sha1_hash info_hash;
        char const* name;
        fs::path save_path;
        std::vector<char>* resume_data;
        storage_mode_t storage_mode;
        bool paused;
        bool auto_managed;
        bool duplicate_is_error;
        storage_constructor_type storage;
        void* userdata;
};

torrent_handle add_torrent(add_torrent_params const& params);

You add torrents through the add_torrent() function where you give an object with all the parameters.

The only mandatory parameter is save_path which is the directory where you want the files to be saved. You also need to specify either the ti (the torrent file) or info_hash (the info hash of the torrent). If you specify the info-hash, the torrent file will be downloaded from peers, which requires them to support the metadata extension. For the metadata extension to work, libtorrent must be built with extensions enabled (TORRENT_DISABLE_EXTENSIONS must not be defined). It also takes an optional name argument. This may be 0 in case no name should be assigned to the torrent. In case it's not 0, the name is used for the torrent as long as it doesn't have metadata. See torrent_handle::name.

If the torrent doesn't have a tracker, but relies on the DHT to find peers, the tracker_url can be 0, otherwise you might specify a tracker url that tracks this torrent.

If the torrent you are trying to add already exists in the session (is either queued for checking, being checked or downloading) add_torrent() will throw duplicate_torrent which derives from std::exception unless duplicate_is_error is set to false. In that case, add_torrent will return the handle to the existing torrent.

The optional parameter, resume_data can be given if up to date fast-resume data is available. The fast-resume data can be acquired from a running torrent by calling save_resume_data() on torrent_handle. See fast resume. The vector that is passed in will be swapped into the running torrent instance with std::vector::swap().

The storage_mode parameter refers to the layout of the storage for this torrent. There are 3 different modes:

storage_mode_sparse

All pieces will be written to the place where they belong and sparse files will be used. This is the recommended, and default mode.

storage_mode_allocate

Same as storage_mode_sparse except that files will be ftruncated on startup (SetEndOfFile() on windows). For filesystem that supports sparse files, this is in all practical aspects identical to sparse mode. For filesystems that don't, it will allocate the data for the files. The mac filesystem HFS+ doesn't support sparse files, it will allocate the files with zeroes.

storage_mode_compact

The storage will grow as more pieces are downloaded, and pieces are rearranged to finally be in their correct places once the entire torrent has been downloaded.

For more information, see storage allocation.

paused is a boolean that specifies whether or not the torrent is to be started in a paused state. I.e. it won't connect to the tracker or any of the peers until it's resumed. This is typically a good way of avoiding race conditions when setting configuration options on torrents before starting them.

If auto_managed is true, this torrent will be queued, started and seeded automatically by libtorrent. When this is set, the torrent should also be started as paused. The default queue order is the order the torrents were added. They are all downloaded in that order. For more details, see queuing.

storage can be used to customize how the data is stored. The default storage will simply write the data to the files it belongs to, but it could be overridden to save everything to a single file at a specific location or encrypt the content on disk for instance. For more information about the storage_interface that needs to be implemented for a custom storage, see storage_interface.

The userdata parameter is optional and will be passed on to the extension constructor functions, if any (see add_extension()).

The torrent_handle returned by add_torrent() can be used to retrieve information about the torrent's progress, its peers etc. It is also used to abort a torrent.

remove_torrent()

void remove_torrent(torrent_handle const& h, int options = none);

remove_torrent() will close all peer connections associated with the torrent and tell the tracker that we've stopped participating in the swarm. The optional second argument options can be used to delete all the files downloaded by this torrent. To do this, pass in the value session::delete_files. The removal of the torrent is asyncronous, there is no guarantee that adding the same torrent immediately after it was removed will not throw a duplicate_torrent exception.

find_torrent() get_torrents()

torrent_handle find_torrent(sha_hash const& ih);
std::vector<torrent_handle> get_torrents() const;

find_torrent() looks for a torrent with the given info-hash. In case there is such a torrent in the session, a torrent_handle to that torrent is returned. In case the torrent cannot be found, an invalid torrent_handle is returned.

See torrent_handle::is_valid() to know if the torrent was found or not.

get_torrents() returns a vector of torrent_handles to all the torrents currently in the session.

set_upload_rate_limit() set_download_rate_limit() upload_rate_limit() download_rate_limit()

void set_upload_rate_limit(int bytes_per_second);
void set_download_rate_limit(int bytes_per_second);
int upload_rate_limit() const;
int download_rate_limit() const;

set_upload_rate_limit() set the maximum number of bytes allowed to be sent to peers per second. This bandwidth is distributed among all the peers. If you don't want to limit upload rate, you can set this to -1 (the default). set_download_rate_limit() works the same way but for download rate instead of upload rate. download_rate_limit() and upload_rate_limit() returns the previously set limits.

set_max_uploads() set_max_connections() max_connections()

void set_max_uploads(int limit);
void set_max_connections(int limit);
int max_connections() const;

These functions will set a global limit on the number of unchoked peers (uploads) and the number of connections opened. The number of connections is set to a hard minimum of at least two connections per torrent, so if you set a too low connections limit, and open too many torrents, the limit will not be met. The number of uploads is at least one per torrent.

max_connections() returns the current setting.

num_uploads() num_connections()

int num_uploads() const;
int num_connections() const;

Returns the number of currently unchoked peers and the number of connections (including half-open ones) respectively.

set_max_half_open_connections() max_half_open_connections()

void set_max_half_open_connections(int limit);
int max_half_open_connections() const;

Sets the maximum number of half-open connections libtorrent will have when connecting to peers. A half-open connection is one where connect() has been called, but the connection still hasn't been established (nor failed). Windows XP Service Pack 2 sets a default, system wide, limit of the number of half-open connections to 10. So, this limit can be used to work nicer together with other network applications on that system. The default is to have no limit, and passing -1 as the limit, means to have no limit. When limiting the number of simultaneous connection attempts, peers will be put in a queue waiting for their turn to get connected.

max_half_open_connections() returns the set limit. This limit defaults to 8 on windows.

load_asnum_db() load_country_db() int as_for_ip()

bool load_asnum_db(char const* file);
bool load_country_db(char const* file);
int as_for_ip(address const& adr);

These functions are not available if TORRENT_DISABLE_GEO_IP is defined. They expects a path to the MaxMind ASN database and MaxMind GeoIP database respectively. This will be used to look up which AS and country peers belong to.

as_for_ip returns the AS number for the IP address specified. If the IP is not in the database or the ASN database is not loaded, 0 is returned.

load_state() state()

void load_state(entry const& ses_state);
entry state() const;

These functions loads and save session state. Currently, the only state that's stored is peak download rates for ASes. This map is used to determine which order to connect to peers.

set_ip_filter()

void set_ip_filter(ip_filter const& filter);

Sets a filter that will be used to reject and accept incoming as well as outgoing connections based on their originating ip address. The default filter will allow connections to any ip address. To build a set of rules for which addresses are accepted and not, see ip_filter.

Each time a peer is blocked because of the IP filter, a peer_blocked_alert is generated.

status()

session_status status() const;

status() returns session wide-statistics and status. The session_status struct has the following members:

struct dht_lookup
{
        char const* type;
        int outstanding_requests;
        int timeouts;
        int responses;
        int branch_factor;
};

struct session_status
{
        bool has_incoming_connections;

        float upload_rate;
        float download_rate;
        size_type total_download;
        size_type total_upload;

        float payload_upload_rate;
        float payload_download_rate;
        size_type total_payload_download;
        size_type total_payload_upload;

        float ip_overhead_upload_rate;
        float ip_overhead_download_rate;
        size_type total_ip_overhead_download;
        size_type total_ip_overhead_upload;

        float dht_upload_rate;
        float dht_download_rate;
        size_type total_dht_download;
        size_type total_dht_upload;

        float tracker_upload_rate;
        float tracker_download_rate;
        size_type total_tracker_download;
        size_type total_tracker_upload;

        size_type total_redundant_bytes;
        size_type total_failed_bytes;

        int num_peers;
        int num_unchoked;
        int allowed_upload_slots;

        int optimistic_unchoke_counter;
        int unchoke_counter;

        int dht_nodes;
        int dht_cache_nodes;
        int dht_torrents;
        int dht_global_nodes;
        std::vector<dht_lookup> active_requests;
};

has_incoming_connections is false as long as no incoming connections have been established on the listening socket. Every time you change the listen port, this will be reset to false.

upload_rate, download_rate are the total download and upload rates accumulated from all torrents. This includes bittorrent protocol, DHT and an estimated TCP/IP protocol overhead.

total_download and total_upload are the total number of bytes downloaded and uploaded to and from all torrents. This also includes all the protocol overhead.

payload_download_rate and payload_upload_rate is the rate of the payload down- and upload only.

total_payload_download and total_payload_upload is the total transfers of payload only. The payload does not include the bittorrent protocol overhead, but only parts of the actual files to be downloaded.

ip_overhead_upload_rate, ip_overhead_download_rate, total_ip_overhead_download and total_ip_overhead_upload is the estimated TCP/IP overhead in each direction.

dht_upload_rate, dht_download_rate, total_dht_download and total_dht_upload is the DHT bandwidth usage.

total_redundant_bytes is the number of bytes that has been received more than once. This can happen if a request from a peer times out and is requested from a different peer, and then received again from the first one. To make this lower, increase the request_timeout and the piece_timeout in the session settings.

total_failed_bytes is the number of bytes that was downloaded which later failed the hash-check.

num_peers is the total number of peer connections this session has. This includes incoming connections that still hasn't sent their handshake or outgoing connections that still hasn't completed the TCP connection. This number may be slightly higher than the sum of all peers of all torrents because the incoming connections may not be assigned a torrent yet.

num_unchoked is the current number of unchoked peers. allowed_upload_slots is the current allowed number of unchoked peers.

optimistic_unchoke_counter and unchoke_counter tells the number of seconds until the next optimistic unchoke change and the start of the next unchoke interval. These numbers may be reset prematurely if a peer that is unchoked disconnects or becomes notinterested.

dht_nodes, dht_cache_nodes and dht_torrents are only available when built with DHT support. They are all set to 0 if the DHT isn't running. When the DHT is running, dht_nodes is set to the number of nodes in the routing table. This number only includes active nodes, not cache nodes. The dht_cache_nodes is set to the number of nodes in the node cache. These nodes are used to replace the regular nodes in the routing table in case any of them becomes unresponsive.

dht_torrents are the number of torrents tracked by the DHT at the moment.

dht_global_nodes is an estimation of the total number of nodes in the DHT network.

active_requests is a vector of the currently running DHT lookups.

get_cache_status()

cache_status get_cache_status() const;

Returns status of the disk cache for this session.

struct cache_status
{
        size_type blocks_written;
        size_type writes;
        size_type blocks_read;
        size_type blocks_read_hit;
        size_type reads;
        int cache_size;
        int read_cache_size;
};

blocks_written is the total number of 16 KiB blocks written to disk since this session was started.

writes is the total number of write operations performed since this session was started.

The ratio (blocks_written - writes) / blocks_written represents the number of saved write operations per total write operations. i.e. a kind of cache hit ratio for the write cahe.

blocks_read is the number of blocks that were requested from the bittorrent engine (from peers), that were served from disk or cache.

blocks_read_hit is the number of blocks that were served from cache.

The ratio blocks_read_hit / blocks_read is the cache hit ratio for the read cache.

cache_size is the number of 16 KiB blocks currently in the disk cache. This includes both read and write cache.

read_cache_size is the number of 16KiB blocks in the read cache.

get_cache_info()

void get_cache_info(sha1_hash const& ih
        , std::vector<cached_piece_info>& ret) const;

get_cache_info() fills out the supplied vector with information for each piece that is currently in the disk cache for the torrent with the specified info-hash (ih).

struct cached_piece_info
{
        int piece;
        std::vector<bool> blocks;
        ptime last_use;
        enum kind_t { read_cache = 0, write_cache = 1 };
        kind_t kind;
};

piece is the piece index for this cache entry.

blocks has one entry for each block in this piece. true represents the data for that block being in the disk cache and false means it's not.

last_use is the time when a block was last written to this piece. The older a piece is, the more likely it is to be flushed to disk.

kind specifies if this piece is part of the read cache or the write cache.

is_listening() listen_port() listen_on()

bool is_listening() const;
unsigned short listen_port() const;
bool listen_on(
        std::pair<int, int> const& port_range
        , char const* interface = 0);

is_listening() will tell you whether or not the session has successfully opened a listening port. If it hasn't, this function will return false, and then you can use listen_on() to make another try.

listen_port() returns the port we ended up listening on. Since you just pass a port-range to the constructor and to listen_on(), to know which port it ended up using, you have to ask the session using this function.

listen_on() will change the listen port and/or the listen interface. If the session is already listening on a port, this socket will be closed and a new socket will be opened with these new settings. The port range is the ports it will try to listen on, if the first port fails, it will continue trying the next port within the range and so on. The interface parameter can be left as 0, in that case the os will decide which interface to listen on, otherwise it should be the ip-address of the interface you want the listener socket bound to. listen_on() returns true if it managed to open the socket, and false if it failed. If it fails, it will also generate an appropriate alert (listen_failed_alert).

The interface parameter can also be a hostname that will resolve to the device you want to listen on.

If you're also starting the DHT, it is a good idea to do that after you've called listen_on(), since the default listen port for the DHT is the same as the tcp listen socket. If you start the DHT first, it will assume the tcp port is free and open the udp socket on that port, then later, when listen_on() is called, it may turn out that the tcp port is in use. That results in the DHT and the bittorrent socket listening on different ports. If the DHT is active when listen_on is called, the udp port will be rebound to the new port, if it was configured to use the same port as the tcp socket, and if the listen_on call failed to bind to the same port that the udp uses.

The reason why it's a good idea to run the DHT and the bittorrent socket on the same port is because that is an assumption that may be used to increase performance. One way to accelerate the connecting of peers on windows may be to first ping all peers with a DHT ping packet, and connect to those that responds first. On windows one can only connect to a few peers at a time because of a built in limitation (in XP Service pack 2).

pop_alert() set_alert_mask() wait_for_alert() set_alert_queue_size_limit()

std::auto_ptr<alert> pop_alert();
alert const* wait_for_alert(time_duration max_wait);
void set_alert_mask(int m);
size_t set_alert_queue_size_limit(size_t queue_size_limit_);

pop_alert() is used to ask the session if any errors or events has occurred. With set_alert_mask() you can filter which alerts to receive through pop_alert(). For information about the alert categories, see alerts.

wait_for_alert blocks until an alert is available, or for no more than max_wait time. If wait_for_alert returns because of the time-out, and no alerts are available, it returns 0. If at least one alert was generated, a pointer to that alert is returned. The alert is not popped, any subsequent calls to wait_for_alert will return the same pointer until the alert is popped by calling pop_alert. This is useful for leaving any alert dispatching mechanism independent of this blocking call, the dispatcher can be called and it can pop the alert independently.

set_alert_queue_size_limit() you can specify how many alerts can be awaiting for dispatching. If this limit is reached, new incoming alerts can not be received until alerts are popped by calling pop_alert. Default value is 1000.

add_extension()

void add_extension(boost::function<
        boost::shared_ptr<torrent_plugin>(torrent*, void*)> ext);

This function adds an extension to this session. The argument is a function object that is called with a torrent* and which should return a boost::shared_ptr<torrent_plugin>. To write custom plugins, see libtorrent plugins. For the typical bittorrent client all of these extensions should be added. The main plugins implemented in libtorrent are:

metadata extension

Allows peers to download the metadata (.torren files) from the swarm directly. Makes it possible to join a swarm with just a tracker and info-hash.

#include <libtorrent/extensions/metadata_transfer.hpp>
ses.add_extension(&libtorrent::create_metadata_plugin);

uTorrent metadata

Same as metadata extension but compatible with uTorrent.

#include <libtorrent/extensions/ut_metadata.hpp>
ses.add_extension(&libtorrent::create_ut_metadata_plugin);

uTorrent peer exchange

Exchanges peers between clients.

#include <libtorrent/extensions/ut_pex.hpp>
ses.add_extension(&libtorrent::create_ut_pex_plugin);

smart ban plugin

A plugin that, with a small overhead, can ban peers that sends bad data with very high accuracy. Should eliminate most problems on poisoned torrents.

#include <libtorrent/extensions/smart_ban.hpp>
ses.add_extension(&libtorrent::create_smart_ban_plugin);

set_settings() set_pe_settings()

void set_settings(session_settings const& settings);
void set_pe_settings(pe_settings const& settings);

Sets the session settings and the packet encryption settings respectively. See session_settings and pe_settings for more information on available options.

set_peer_proxy() set_web_seed_proxy() set_tracker_proxy() set_dht_proxy()

void set_peer_proxy(proxy_settings const& s);
void set_web_seed_proxy(proxy_settings const& s);
void set_tracker_proxy(proxy_settings const& s);
void set_dht_proxy(proxy_settings const& s);

The set_dht_proxy is not available when DHT is disabled. These functions sets the proxy settings for different kinds of connections, bittorrent peers, web seeds, trackers and the DHT traffic.

set_peer_proxy affects regular bittorrent peers. set_web_seed_proxy affects only web seeds. see HTTP seeding.

set_tracker_proxy only affects HTTP tracker connections (UDP tracker connections are affected if the given proxy supports UDP, e.g. SOCKS5).

set_dht_proxy affects the DHT messages. Since they are sent over UDP, it only has any effect if the proxy supports UDP.

For more information on what settings are available for proxies, see proxy_settings.

peer_proxy() web_seed_proxy() tracker_proxy() dht_proxy()

proxy_settings const& peer_proxy() const;
proxy_settings const& web_seed_proxy() const;
proxy_settings const& tracker_proxy() const;
proxy_settings const& dht_proxy() const;

These functions returns references to their respective current settings.

The dht_proxy is not available when DHT is disabled.

start_dht() stop_dht() set_dht_settings() dht_state()

void start_dht(entry const& startup_state);
void stop_dht();
void set_dht_settings(dht_settings const& settings);
entry dht_state() const;

These functions are not available in case TORRENT_DISABLE_DHT is defined. start_dht starts the dht node and makes the trackerless service available to torrents. The startup state is optional and can contain nodes and the node id from the previous session. The dht node state is a bencoded dictionary with the following entries:

nodes

A list of strings, where each string is a node endpoint encoded in binary. If the string is 6 bytes long, it is an IPv4 address of 4 bytes, encoded in network byte order (big endian), followed by a 2 byte port number (also network byte order). If the string is 18 bytes long, it is 16 bytes of IPv6 address followed by a 2 bytes port number (also network byte order).

node-id

The node id written as a readable string as a hexadecimal number.

dht_state will return the current state of the dht node, this can be used to start up the node again, passing this entry to start_dht. It is a good idea to save this to disk when the session is closed, and read it up again when starting.

If the port the DHT is supposed to listen on is already in use, and exception is thrown, asio::error.

stop_dht stops the dht node.

add_dht_node adds a node to the routing table. This can be used if your client has its own source of bootstrapping nodes.

set_dht_settings sets some parameters availavle to the dht node. The struct has the following members:

struct dht_settings
{
        int max_peers_reply;
        int search_branching;
        int service_port;
        int max_fail_count;
};

max_peers_reply is the maximum number of peers the node will send in response to a get_peers message from another node.

search_branching is the number of concurrent search request the node will send when announcing and refreshing the routing table. This parameter is called alpha in the kademlia paper.

service_port is the udp port the node will listen to. This will default to 0, which means the udp listen port will be the same as the tcp listen port. This is in general a good idea, since some NAT implementations reserves the udp port for any mapped tcp port, and vice versa. NAT-PMP guarantees this for example.

max_fail_count is the maximum number of failed tries to contact a node before it is removed from the routing table. If there are known working nodes that are ready to replace a failing node, it will be replaced immediately, this limit is only used to clear out nodes that don't have any node that can replace them.

add_dht_node() add_dht_router()

void add_dht_node(std::pair<std::string, int> const& node);
void add_dht_router(std::pair<std::string, int> const& node);

add_dht_node takes a host name and port pair. That endpoint will be pinged, and if a valid DHT reply is received, the node will be added to the routing table.

add_dht_router adds the given endpoint to a list of DHT router nodes. If a search is ever made while the routing table is empty, those nodes will be used as backups. Nodes in the router node list will also never be added to the regular routing table, which effectively means they are only used for bootstrapping, to keep the load off them.

An example routing node that you could typically add is router.bittorrent.com.

start_lsd() stop_lsd()

void start_lsd();
void stop_lsd();

Starts and stops Local Service Discovery. This service will broadcast the infohashes of all the non-private torrents on the local network to look for peers on the same swarm within multicast reach.

It is turned off by default.

start_upnp() stop_upnp()

upnp* start_upnp();
void stop_upnp();

Starts and stops the UPnP service. When started, the listen port and the DHT port are attempted to be forwarded on local UPnP router devices.

The upnp object returned by start_upnp() can be used to add and remove arbitrary port mappings. Mapping status is returned through the portmap_alert and the portmap_error_alert. The object will be valid until stop_upnp() is called. See UPnP and NAT-PMP.

It is off by default.

start_natpmp() stop_natpmp()

natpmp* start_natpmp();
void stop_natpmp();

Starts and stops the NAT-PMP service. When started, the listen port and the DHT port are attempted to be forwarded on the router through NAT-PMP.

The natpmp object returned by start_natpmp() can be used to add and remove arbitrary port mappings. Mapping status is returned through the portmap_alert and the portmap_error_alert. The object will be valid until stop_natpmp() is called. See UPnP and NAT-PMP.

It is off by default.

integer() string() list() dict() type()

integer_type& integer();
integer_type const& integer() const;
string_type& string();
string_type const& string() const;
list_type& list();
list_type const& list() const;
dictionary_type& dict();
dictionary_type const& dict() const;

The integer(), string(), list() and dict() functions are accessors that return the respective type. If the entry object isn't of the type you request, the accessor will throw type_error (which derives from std::runtime_error). You can ask an entry for its type through the type() function.

The print() function is there for debug purposes only.

If you want to create an entry you give it the type you want it to have in its constructor, and then use one of the non-const accessors to get a reference which you then can assign the value you want it to have.

The typical code to get info from a torrent file will then look like this:

entry torrent_file;
// ...

// throws if this is not a dictionary
entry::dictionary_type const& dict = torrent_file.dict();
entry::dictionary_type::const_iterator i;
i = dict.find("announce");
if (i != dict.end())
{
        std::string tracker_url = i->second.string();
        std::cout << tracker_url << "\n";
}

The following code is equivalent, but a little bit shorter:

entry torrent_file;
// ...

// throws if this is not a dictionary
if (entry* i = torrent_file.find_key("announce"))
{
        std::string tracker_url = i->string();
        std::cout << tracker_url << "\n";
}

To make it easier to extract information from a torrent file, the class torrent_info exists.

operator[]

entry& operator[](char const* key);
entry& operator[](std::string const& key);
entry const& operator[](char const* key) const;
entry const& operator[](std::string const& key) const;

All of these functions requires the entry to be a dictionary, if it isn't they will throw libtorrent::type_error.

The non-const versions of the operator[] will return a reference to either the existing element at the given key or, if there is no element with the given key, a reference to a newly inserted element at that key.

The const version of operator[] will only return a reference to an existing element at the given key. If the key is not found, it will throw libtorrent::type_error.

find_key()

entry* find_key(char const* key);
entry const* find_key(char const* key) const;

These functions requires the entry to be a dictionary, if it isn't they will throw libtorrent::type_error.

They will look for an element at the given key in the dictionary, if the element cannot be found, they will return 0. If an element with the given key is found, the return a pointer to it.

torrent_info()

torrent_info(sha1_hash const& info_hash);
torrent_info(lazy_entry const& torrent_file);
torrent_info(char const* buffer, int size);
torrent_info(boost::filesystem::path const& filename);

The constructor that takes an info-hash will initialize the info-hash to the given value, but leave all other fields empty. This is used internally when downloading torrents without the metadata. The metadata will be created by libtorrent as soon as it has been downloaded from the swarm.

The constructor that takes a lazy_entry will create a torrent_info object from the information found in the given torrent_file. The lazy_entry represents a tree node in an bencoded file. To load an ordinary .torrent file into a lazy_entry, use lazy_bdecode(), see bdecode() bencode().

The version that takes a buffer pointer and a size will decode it as a .torrent file and initialize the torrent_info object for you.

The version that takes a filename will simply load the torrent file and decode it inside the constructor, for convenience. This might not be the most suitable for applications that want to be able to report detailed errors on what might go wrong.

add_tracker()

void add_tracker(std::string const& url, int tier = 0);

add_tracker() adds a tracker to the announce-list. The tier determines the order in which the trackers are to be tried. For more information see trackers().

files()

file_storage const& file() const;

The file_storage object contains the information on how to map the pieces to files. It is separated from the torrent_info object because when creating torrents a storage object needs to be created without having a torrent file. When renaming files in a storage, the storage needs to make its own copy of the file_storage in order to make its mapping differ from the one in the torrent file.

For more information on the file_storage object, see the separate document on how to create torrents.

begin_files() end_files() rbegin_files() rend_files()

file_iterator begin_files() const;
file_iterator end_files() const;
reverse_file_iterator rbegin_files() const;
reverse_file_iterator rend_files() const;

This class will need some explanation. First of all, to get a list of all files in the torrent, you can use begin_files(), end_files(), rbegin_files() and rend_files(). These will give you standard vector iterators with the type file_entry.

struct file_entry
{
        boost::filesystem::path path;
        size_type offset;
        size_type size;
        size_type file_base;
        boost::shared_ptr<const boost::filesystem::path> orig_path;
};

The path is the full (relative) path of each file. i.e. if it is a multi-file torrent, all the files starts with a directory with the same name as torrent_info::name(). The filenames are encoded with UTF-8.

size is the size of the file (in bytes) and offset is the byte offset of the file within the torrent. i.e. the sum of all the sizes of the files before it in the list.

file_base is the offset in the file where the storage should start. The normal case is to have this set to 0, so that the storage starts saving data at the start if the file. In cases where multiple files are mapped into the same file though, the file_base should be set to an offset so that the different regions do not overlap. This is used when mapping "unselected" files into a so-called part file.

orig_path is set to 0 in case the path element is an exact copy of that found in the metadata. In case the path in the original metadata was incorrectly encoded, and had to be fixed in order to be acceptable utf-8, the original string is preserved in orig_path. The reason to keep it is to be able to reproduce the info-section exactly, with the correct info-hash.

num_files() file_at()

int num_files() const;
file_entry const& file_at(int index) const;

If you need index-access to files you can use the num_files() and file_at() to access files using indices.

map_block()

std::vector<file_slice> map_block(int piece, size_type offset
        , int size) const;

This function will map a piece index, a byte offset within that piece and a size (in bytes) into the corresponding files with offsets where that data for that piece is supposed to be stored.

The file slice struct looks like this:

struct file_slice
{
        int file_index;
        size_type offset;
        size_type size;
};

The file_index refers to the index of the file (in the torrent_info). To get the path and filename, use file_at() and give the file_index as argument. The offset is the byte offset in the file where the range starts, and size is the number of bytes this range is. The size + offset will never be greater than the file size.

map_file()

peer_request map_file(int file_index, size_type file_offset
        , int size) const;

This function will map a range in a specific file into a range in the torrent. The file_offset parameter is the offset in the file, given in bytes, where 0 is the start of the file. The peer_request structure looks like this:

struct peer_request
{
        int piece;
        int start;
        int length;
        bool operator==(peer_request const& r) const;
};

piece is the index of the piece in which the range starts. start is the offset within that piece where the range starts. length is the size of the range, in bytes.

The input range is assumed to be valid within the torrent. file_offset + size is not allowed to be greater than the file size. file_index must refer to a valid file, i.e. it cannot be >= num_files().

url_seeds() add_url_seed()

std::vector<std::string> const& url_seeds() const;
void add_url_seed(std::string const& url);

If there are any url-seeds in this torrent, url_seeds() will return a vector of those urls. If you're creating a torrent file, add_url_seed() adds one url to the list of url-seeds. Currently, the only transport protocol supported for the url is http.

See HTTP seeding for more information.

trackers()

std::vector<announce_entry> const& trackers() const;

The trackers() function will return a sorted vector of announce_entry. Each announce entry contains a string, which is the tracker url, and a tier index. The tier index is the high-level priority. No matter which trackers that works or not, the ones with lower tier will always be tried before the one with higher tier number.

struct announce_entry
{
        announce_entry(std::string const& url);
        std::string url;
        boost::uint8_t tier;
        boost::uint8_t fail_limit;
        boost::uint8_t fails;

        enum tracker_source
        {
                source_torrent = 1,
                source_client = 2,
                source_magnet_link = 4,
                source_tex = 8
        };
        boost::uint8_t source;

        bool verified:1;
        bool updating:1;
        bool start_sent:1;
        bool complete_sent:1;
};

fail_limit is the max number of failures to announce to this tracker in a row, before this tracker is not used anymore.

fails is the number of times in a row we have failed to announce to this tracker.

source is a bitmask specifying which sources we got this tracker from.

verified is set to true the first time we receive a valid response from this tracker.

updating is true while we're waiting for a response from the tracker.

start_sent is set to true when we get a valid response from an announce with event=started. If it is set, we won't send start in the subsequent announces.

complete_sent is set to true when we send a event=completed.

total_size() piece_length() piece_size() num_pieces()

size_type total_size() const;
int piece_length() const;
int piece_size(unsigned int index) const;
int num_pieces() const;

total_size(), piece_length() and num_pieces() returns the total number of bytes the torrent-file represents (all the files in it), the number of byte for each piece and the total number of pieces, respectively. The difference between piece_size() and piece_length() is that piece_size() takes the piece index as argument and gives you the exact size of that piece. It will always be the same as piece_length() except in the case of the last piece, which may be smaller.

hash_for_piece() hash_for_piece_ptr() info_hash()

size_type piece_size(unsigned int index) const;
sha1_hash const& hash_for_piece(unsigned int index) const;
char const* hash_for_piece_ptr(unsigned int index) const;

hash_for_piece() takes a piece-index and returns the 20-bytes sha1-hash for that piece and info_hash() returns the 20-bytes sha1-hash for the info-section of the torrent file. For more information on the sha1_hash, see the big_number class. hash_for_piece_ptr() returns a pointer to the 20 byte sha1 digest for the piece. Note that the string is not null-terminated.

name() comment() creation_date() creator()

std::string const& name() const;
std::string const& comment() const;
boost::optional<boost::posix_time::ptime> creation_date() const;

name() returns the name of the torrent.

comment() returns the comment associated with the torrent. If there's no comment, it will return an empty string. creation_date() returns a boost::posix_time::ptime object, representing the time when this torrent file was created. If there's no time stamp in the torrent file, this will return a date of January 1:st 1970.

Both the name and the comment is UTF-8 encoded strings.

creator() returns the creator string in the torrent. If there is no creator string it will return an empty string.

priv()

bool priv() const;

priv() returns true if this torrent is private. i.e., it should not be distributed on the trackerless network (the kademlia DHT).

nodes()

std::vector<std::pair<std::string, int> > const& nodes() const;

If this torrent contains any DHT nodes, they are put in this vector in their original form (host name and port number).

add_node()

void add_node(std::pair<std::string, int> const& node);

This is used when creating torrent. Use this to add a known DHT node. It may be used, by the client, to bootstrap into the DHT network.

metadata() metadata_size()

boost::shared_array<char> metadata() const;
int metadata_size() const;

metadata() returns a the raw info section of the torrent file. The size of the metadata is returned by metadata_size().

piece_priority() prioritize_pieces() piece_priorities()

void piece_priority(int index, int priority) const;
int piece_priority(int index) const;
void prioritize_pieces(std::vector<int> const& pieces) const;
std::vector<int> piece_priorities() const;

These functions are used to set and get the prioritiy of individual pieces. By default all pieces have priority 1. That means that the random rarest first algorithm is effectively active for all pieces. You may however change the priority of individual pieces. There are 8 different priority levels:

0. piece is not downloaded at all
1. normal priority. Download order is dependent on availability
2. higher than normal priority. Pieces are preferred over pieces with the same availability, but not over pieces with lower availability
3. pieces are as likely to be picked as partial pieces.
4. pieces are preferred over partial pieces, but not over pieces with lower availability
5. currently the same as 4
6. piece is as likely to be picked as any piece with availability 1
7. maximum priority, availability is disregarded, the piece is preferred over any other piece with lower priority

The exact definitions of these priorities are implementation details, and subject to change. The interface guarantees that higher number means higher priority, and that 0 means do not download.

piece_priority sets or gets the priority for an individual piece, specified by index.

prioritize_pieces takes a vector of integers, one integer per piece in the torrent. All the piece priorities will be updated with the priorities in the vector.

piece_priorities returns a vector with one element for each piece in the torrent. Each element is the current priority of that piece.

file_priority() prioritize_files() file_priorities()

void file_priority(int index, int priority) const;
int file_priority(int index) const;
void prioritize_files(std::vector<int> const& files) const;
std::vector<int> file_priorities() const;

index must be in the range [0, number_of_files).

file_priority queries or sets the priority of file index.

prioritize_files takes a vector that has at as many elements as there are files in the torrent. Each entry is the priority of that file. The function sets the priorities of all the pieces in the torrent based on the vector.

file_priorities returns a vector with the priorities of all files.

The priority values are the same as for piece_priority.

Whenever a file priority is changed, all other piece priorities are reset to match the file priorities. In order to maintain sepcial priorities for particular pieces, piece_priority has to be called again for those pieces.

file_progress()

void file_progress(std::vector<size_type>& fp);

This function fills in the supplied vector with the the number of bytes downloaded of each file in this torrent. The progress values are ordered the same as the files in the torrent_info. This operation is not very cheap. Its complexity is O(n + mj). Where n is the number of files, m is the number of downloading pieces and j is the number of blocks in a piece.

save_path()

boost::filesystem::path save_path() const;

save_path() returns the path that was given to add_torrent() when this torrent was started.

move_storage()

void move_storage(boost::filesystem::path const& save_path) const;

Moves the file(s) that this torrent are currently seeding from or downloading to. If the given save_path is not located on the same drive as the original save path, The files will be copied to the new drive and removed from their original location. This will block all other disk IO, and other torrents download and upload rates may drop while copying the file.

Since disk IO is performed in a separate thread, this operation is also asynchronous. Once the operation completes, the storage_moved_alert is generated, with the new path as the message.

rename_file()

void rename_file(int index, boost::filesystem::path) const;

Renames the file with the given index asynchronously. The rename operation is complete when either a file_renamed_alert or file_rename_failed_alert is posted.

get_storage_impl()

storage_interface* get_storage_impl() const;

Returns the storage implementation for this torrent. This depends on the storage contructor function that was passed to session::add_torrent.

force_reannounce()

void force_reannounce() const;
void force_reannounce(boost::posix_time::time_duration) const;

force_reannounce() will force this torrent to do another tracker request, to receive new peers. The second overload of force_reannounce that takes a time_duration as argument will schedule a reannounce in that amount of time from now.

scrape_tracker()

void scrape_tracker() const;

scrape_tracker() will send a scrape request to the tracker. A scrape request queries the tracker for statistics such as total number of incomplete peers, complete peers, number of downloads etc.

This request will specifically update the num_complete and num_incomplete fields in the torrent_status struct once it completes. When it completes, it will generate a scrape_reply_alert. If it fails, it will generate a scrape_failed_alert.

connect_peer()

void connect_peer(asio::ip::tcp::endpoint const& adr, int source = 0) const;

connect_peer() is a way to manually connect to peers that one believe is a part of the torrent. If the peer does not respond, or is not a member of this torrent, it will simply be disconnected. No harm can be done by using this other than an unnecessary connection attempt is made. If the torrent is uninitialized or in queued or checking mode, this will throw invalid_handle. The second (optional) argument will be bitwised ORed into the source mask of this peer. Typically this is one of the source flags in peer_info. i.e. tracker, pex, dht etc.

name()

std::string name() const;

Returns the name of the torrent. i.e. the name from the metadata associated with it. In case the torrent was started without metadata, and hasn't completely received it yet, it returns the name given to it when added to the session. See session::add_torrent.

set_ratio()

void set_ratio(float ratio) const;

set_ratio() sets the desired download / upload ratio. If set to 0, it is considered being infinite. i.e. the client will always upload as much as it can, no matter how much it gets back in return. With this setting it will work much like the standard clients.

Besides 0, the ratio can be set to any number greater than or equal to 1. It means how much to attempt to upload in return for each download. e.g. if set to 2, the client will try to upload 2 bytes for every byte received. The default setting for this is 0, which will make it work as a standard client.

set_upload_limit() set_download_limit() upload_limit() download_limit()

void set_upload_limit(int limit) const;
void set_download_limit(int limit) const;
int upload_limit() const;
int download_limit() const;

set_upload_limit will limit the upload bandwidth used by this particular torrent to the limit you set. It is given as the number of bytes per second the torrent is allowed to upload. set_download_limit works the same way but for download bandwidth instead of upload bandwidth. Note that setting a higher limit on a torrent then the global limit (session::set_upload_rate_limit) will not override the global rate limit. The torrent can never upload more than the global rate limit.

upload_limit and download_limit will return the current limit setting, for upload and download, respectively.

set_sequential_download() is_sequential_download()

void set_sequential_download(bool sd);
bool is_sequential_download() const;

set_sequential_download() enables or disables sequential download. When enabled, the piece picker will pick pieces in sequence instead of rarest first.

Enabling sequential download will affect the piece distribution negatively in the swarm. It should be used sparingly.

is_sequential_download() returns true if this torrent is downloading in sequence, and false otherwise.

set_peer_upload_limit() set_peer_download_limit()

void set_peer_upload_limit(asio::ip::tcp::endpoint ip, int limit) const;
void set_peer_download_limit(asio::ip::tcp::endpoint ip, int limit) const;

Works like set_upload_limit and set_download_limit respectively, but controls individual peer instead of the whole torrent.

pause() resume() is_paused()

void pause() const;
void resume() const;
bool is_paused() const;

pause(), and resume() will disconnect all peers and reconnect all peers respectively. When a torrent is paused, it will however remember all share ratios to all peers and remember all potential (not connected) peers. You can use is_paused() to determine if a torrent is currently paused. Torrents may be paused automatically if there is a file error (e.g. disk full) or something similar. See file_error_alert.

is_paused() only returns true if the torrent itself is paused. If the torrent is not running because the session is paused, this still returns false. To know if a torrent is active or not, you need to inspect both torrent_handle::is_paused() and session::is_paused().

force_recheck()

void force_recheck() const;

force_recheck puts the torrent back in a state where it assumes to have no resume data. All peers will be disconnected and the torrent will stop announcing to the tracker. The torrent will be added to the checking queue, and will be checked (all the files will be read and compared to the piece hashes). Once the check is complete, the torrent will start connecting to peers again, as normal.

clear_error()

void clear_error() const;

If the torrent is in an error state (i.e. torrent_status::error is non-empty), this will clear the error and start the torrent again.

resolve_countries()

void resolve_countries(bool r);
bool resolve_countries() const;

Sets or gets the flag that derermines if countries should be resolved for the peers of this torrent. It defaults to false. If it is set to true, the peer_info structure for the peers in this torrent will have their country member set. See peer_info for more information on how to interpret this field.

is_seed()

bool is_seed() const;

Returns true if the torrent is in seed mode (i.e. if it has finished downloading).

is_auto_managed() auto_managed()

bool is_auto_managed() const;
void auto_managed(bool m) const;

is_auto_managed() returns true if this torrent is currently auto managed. auto_managed() changes whether the torrent is auto managed or not. For more info, see queuing.

has_metadata() set_metadata()

bool has_metadata() const;
bool set_metadata(char const* buf, int size) const;

has_metadata returns true if this torrent has metadata (either it was started from a .torrent file or the metadata has been downloaded). The only scenario where this can return false is when the torrent was started torrent-less (i.e. with just an info-hash and tracker ip). Note that if the torrent doesn't have metadata, the member get_torrent_info() will throw.

set_metadata expects the info section of metadata. i.e. The buffer passed in will be hashed and verified against the info-hash. If it fails, a metadata_failed_alert will be generated. If it passes, a metadata_received_alert is generated. The function returns true if the metadata is successfully set on the torrent, and false otherwise. If the torrent already has metadata, this function will not affect the torrent, and false will be returned.

set_tracker_login()

void set_tracker_login(std::string const& username
        , std::string const& password) const;

set_tracker_login() sets a username and password that will be sent along in the HTTP-request of the tracker announce. Set this if the tracker requires authorization.

trackers() replace_trackers() add_tracker()

std::vector<announce_entry> const& trackers() const;
void replace_trackers(std::vector<announce_entry> const&) const;
void add_tracker(announc_entry const& url);

trackers() will return the list of trackers for this torrent. The announce entry contains both a string url which specify the announce url for the tracker as well as an int tier, which is specifies the order in which this tracker is tried. If you want libtorrent to use another list of trackers for this torrent, you can use replace_trackers() which takes a list of the same form as the one returned from trackers() and will replace it. If you want an immediate effect, you have to call force_reannounce().

add_tracker() will look if the specified tracker is already in the set. If it is, it doesn't do anything. If it's not in the current set of trackers, it will insert it in the tier specified in the announce_entry.

add_url_seed() remove_url_seed() url_seeds()

void add_url_seed(std::string const& url);
void remove_url_seed(std::string const& url);
std::set<std::string> url_seeds() const;

add_url_seed() adds another url to the torrent's list of url seeds. If the given url already exists in that list, the call has no effect. The torrent will connect to the server and try to download pieces from it, unless it's paused, queued, checking or seeding. remove_url_seed() removes the given url if it exists already. url_seeds() return a set of the url seeds currently in this torrent. Note that urls that fails may be removed automatically from the list.

See HTTP seeding for more information.

queue_position() queue_position_up() queue_position_down() queue_position_top() queue_position_bottom()

int queue_position() const;
void queue_position_up() const;
void queue_position_down() const;
void queue_position_top() const;
void queue_position_bottom() const;

Every torrent that is added is assigned a queue position exactly one greater than the greatest queue position of all existing torrents. Torrents that are being seeded have -1 as their queue position, since they're no longer in line to be downloaded.

When a torrent is removed or turns into a seed, all torrents with greater queue positions have their positions decreased to fill in the space in the sequence.

queue_position() returns the torrent's position in the download queue. The torrents with the smallest numbers are the ones that are being downloaded. The smaller number, the closer the torrent is to the front of the line to be started.

The queue_position_*() functions adjust the torrents position in the queue. Up means closer to the front and down means closer to the back of the queue. Top and bottom refers to the front and the back of the queue respectively.

use_interface()

void use_interface(char const* net_interface) const;

use_interface() sets the network interface this torrent will use when it opens outgoing connections. By default, it uses the same interface as the session uses to listen on. The parameter must be a string containing an ip-address (either an IPv4 or IPv6 address). If the string does not conform to this format and exception is thrown.

info_hash()

sha1_hash info_hash() const;

info_hash() returns the info-hash for the torrent.

set_max_uploads() set_max_connections() max_connections()

void set_max_uploads(int max_uploads) const;
void set_max_connections(int max_connections) const;
int max_connections() const;

set_max_uploads() sets the maximum number of peers that's unchoked at the same time on this torrent. If you set this to -1, there will be no limit.

set_max_connections() sets the maximum number of connection this torrent will open. If all connections are used up, incoming connections may be refused or poor connections may be closed. This must be at least 2. The default is unlimited number of connections. If -1 is given to the function, it means unlimited.

max_connections() returns the current setting.

save_resume_data()

void save_resume_data() const;

save_resume_data() generates fast-resume data and returns it as an entry. This entry is suitable for being bencoded. For more information about how fast-resume works, see fast resume.

This operation is asynchronous, save_resume_data will return immediately. The resume data is delivered when it's done through an save_resume_data_alert.

The fast resume data will be empty in the following cases:

1. The torrent handle is invalid.
2. The torrent is checking (or is queued for checking) its storage, it will obviously not be ready to write resume data.
3. The torrent hasn't received valid metadata and was started without metadata (see libtorrent's metadata from peers extension)

Note that by the time you receive the fast resume data, it may already be invalid if the torrent is still downloading! The recommended practice is to first pause the session, then generate the fast resume data, and then close it down. Make sure to not remove_torrent() before you receive the save_resume_data_alert though. There's no need to pause when saving intermittent resume data.

It also means that if the resume data is out dated, libtorrent will not re-check the files, but assume that it is fairly recent. The assumption is that it's better to loose a little bit than to re-check the entire file.

It is still a good idea to save resume data periodically during download as well as when closing down.

Example code to pause and save resume data for all torrents and wait for the alerts:

int num_resume_data = 0;
std::vector<torrent_handle> handles = ses.get_torrents();
ses.pause();
for (std::vector<torrent_handle>::iterator i = handles.begin();
        i != handles.end(); ++i)
{
        torrent_handle& h = *i;
        if (!h.has_metadata()) continue;
        if (!h.is_valid()) continue;

        h.save_resume_data();
        ++num_resume_data;
}

while (num_resume_data > 0)
{
        alert const* a = ses.wait_for_alert(seconds(10));

        // if we don't get an alert within 10 seconds, abort
        if (a == 0) break;

        std::auto_ptr<alert> holder = ses.pop_alert();

        if (dynamic_cast<save_resume_data_failed_alert const*>(a))
        {
                process_alert(a);
                --num_resume_data;
                continue;
        }

        save_resume_data_alert const* rd = dynamic_cast<save_resume_data_alert const*>(a);
        if (rd == 0)
        {
                process_alert(a);
                continue;
        }

        torrent_handle h = rd->handle;
        boost::filesystem::ofstream out(h.save_path()
                / (h.get_torrent_info().name() + ".fastresume"), std::ios_base::binary);
        out.unsetf(std::ios_base::skipws);
        bencode(std::ostream_iterator<char>(out), *rd->resume_data);
        --num_resume_data;
}

status()

torrent_status status() const;

status() will return a structure with information about the status of this torrent. If the torrent_handle is invalid, it will throw invalid_handle exception. See torrent_status.

get_download_queue()

void get_download_queue(std::vector<partial_piece_info>& queue) const;

get_download_queue() takes a non-const reference to a vector which it will fill with information about pieces that are partially downloaded or not downloaded at all but partially requested. The entry in the vector (partial_piece_info) looks like this:

struct partial_piece_info
{
        int piece_index;
        int blocks_in_piece;
        block_info blocks[256];
        enum state_t { none, slow, medium, fast };
        state_t piece_state;
};

piece_index is the index of the piece in question. blocks_in_piece is the number of blocks in this particular piece. This number will be the same for most pieces, but the last piece may have fewer blocks than the standard pieces.

piece_state is set to either fast, medium, slow or none. It tells which download rate category the peers downloading this piece falls into. none means that no peer is currently downloading any part of the piece. Peers prefer picking pieces from the same category as themselves. The reason for this is to keep the number of partially downloaded pieces down. Pieces set to none can be converted into any of fast, medium or slow as soon as a peer want to download from it.

struct block_info
{
        enum block_state_t
        { none, requested, writing, finished };

        tcp::endpoint peer;
        unsigned state:2;
        unsigned num_peers:14;
};

The block_info array contains data for each individual block in the piece. Each block has a state (state) which is any of:

• none - This block has not been downloaded or requested form any peer.
• requested - The block has been requested, but not completely downloaded yet.
• writing - The block has been downloaded and is currently queued for being written to disk.
• finished - The block has been written to disk.

The peer field is the ip address of the peer this block was downloaded from. num_peers is the number of peers that is currently requesting this block. Typically this is 0 or 1, but at the end of the torrent blocks may be requested by more peers in parallel to speed things up.

get_peer_info()

void get_peer_info(std::vector<peer_info>&) const;

get_peer_info() takes a reference to a vector that will be cleared and filled with one entry for each peer connected to this torrent, given the handle is valid. If the torrent_handle is invalid, it will throw invalid_handle exception. Each entry in the vector contains information about that particular peer. See peer_info.

get_torrent_info()

torrent_info const& get_torrent_info() const;

Returns a const reference to the torrent_info object associated with this torrent. This reference is valid as long as the torrent_handle is valid, no longer. If the torrent_handle is invalid or if it doesn't have any metadata, invalid_handle exception will be thrown. The torrent may be in a state without metadata only if it was started without a .torrent file, i.e. by using the libtorrent extension of just supplying a tracker and info-hash.

is_valid()

bool is_valid() const;

Returns true if this handle refers to a valid torrent and false if it hasn't been initialized or if the torrent it refers to has been aborted. Note that a handle may become invalid after it has been added to the session. Usually this is because the storage for the torrent is somehow invalid or if the filenames are not allowed (and hence cannot be opened/created) on your filesystem. If such an error occurs, a file_error_alert is generated and all handles that refers to that torrent will become invalid.

ip_filter()

ip_filter()

Creates a default filter that doesn't filter any address.

postcondition: access(x) == 0 for every x

add_rule()

void add_rule(address first, address last, int flags);

Adds a rule to the filter. first and last defines a range of ip addresses that will be marked with the given flags. The flags can currently be 0, which means allowed, or ip_filter::blocked, which means disallowed.

precondition: first.is_v4() == last.is_v4() && first.is_v6() == last.is_v6()

postcondition: access(x) == flags for every x in the range [first, last]

This means that in a case of overlapping ranges, the last one applied takes precedence.

access()

int access(address const& addr) const;

Returns the access permissions for the given address (addr). The permission can currently be 0 or ip_filter::blocked. The complexity of this operation is O(log n), where n is the minimum number of non-overlapping ranges to describe the current filter.

export_filter()

boost::tuple<std::vector<ip_range<address_v4> >
        , std::vector<ip_range<address_v6> > > export_filter() const;

This function will return the current state of the filter in the minimum number of ranges possible. They are sorted from ranges in low addresses to high addresses. Each entry in the returned vector is a range with the access control specified in its flags field.

The return value is a tuple containing two range-lists. One for IPv4 addresses and one for IPv6 addresses.

add_mapping

int add_mapping(protocol_type p, int external_port, int local_port);

Attempts to add a port mapping for the specified protocol. Valid protocols are upnp::tcp and upnp::udp for the UPnP class and natpmp::tcp and natpmp::udp for the NAT-PMP class.

external_port is the port on the external address that will be mapped. This is a hint, you are not guaranteed that this port will be available, and it may end up being something else. In the portmap_alert notification, the actual external port is reported.

local_port is the port in the local machine that the mapping should forward to.

The return value is an index that identifies this port mapping. This is used to refer to mappings that fails or succeeds in the portmap_error_alert and portmap_alert respectively. If The mapping fails immediately, the return value is -1, which means failure. There will not be any error alert notification for mappings that fail with a -1 return value.

delete_mapping

void delete_mapping(int mapping_index);

This function removes a port mapping. mapping_index is the index that refers to the mapping you want to remove, which was returned from add_mapping.

router_model()

std::string router_model();

This is only available for UPnP routers. If the model is advertized by the router, it can be queried through this function.

identify_client()

std::string identify_client(peer_id const& id);

This function is declared in the header <libtorrent/identify_client.hpp>. It can can be used to extract a string describing a client version from its peer-id. It will recognize most clients that have this kind of identification in the peer-id.

client_fingerprint()

boost::optional<fingerprint> client_fingerprint(peer_id const& p);

Returns an optional fingerprint if any can be identified from the peer id. This can be used to automate the identification of clients. It will not be able to identify peers with non- standard encodings. Only Azureus style, Shadow's style and Mainline style. This function is declared in the header <libtorrent/identify_client.hpp>.

bdecode() bencode()

template<class InIt> entry bdecode(InIt start, InIt end);
template<class OutIt> void bencode(OutIt out, const entry& e);

These functions will encode data to bencoded or decode bencoded data.

The entry class is the internal representation of the bencoded data and it can be used to retrieve information, an entry can also be build by the program and given to bencode() to encode it into the OutIt iterator.

The OutIt and InIt are iterators (InputIterator and OutputIterator respectively). They are templates and are usually instantiated as ostream_iterator, back_insert_iterator or istream_iterator. These functions will assume that the iterator refers to a character (char). So, if you want to encode entry e into a buffer in memory, you can do it like this:

std::vector<char> buffer;
bencode(std::back_inserter(buf), e);

If you want to decode a torrent file from a buffer in memory, you can do it like this:

std::vector<char> buffer;
// ...
entry e = bdecode(buf.begin(), buf.end());

Or, if you have a raw char buffer:

const char* buf;
// ...
entry e = bdecode(buf, buf + data_size);

Now we just need to know how to retrieve information from the entry.

If bdecode() encounters invalid encoded data in the range given to it it will throw invalid_encoding.

add_magnet_uri()

torrent_handle add_magnet_uri(session& ses, std::string const& uri
        add_torrent_params p);

This function parses the magnet URI (uri) as a bittorrent magnet link, and adds the torrent to the specified session (ses). It returns the handle to the newly added torrent, or an invalid handle in case parsing failed. To control some initial settings of the torrent, sepcify those in the add_torrent_params, p. See add_torrent().

For more information about magnet links, see magnet links.

make_magnet_uri()

std::string make_magnet_uri(torrent_handle const& handle);

Generates a magnet URI from the specified torrent. If the torrent handle is invalid, an empty string is returned.

For more information about magnet links, see magnet links.

external_ip_alert

Whenever libtorrent learns about the machines external IP, this alert is generated. The external IP address can be acquired from the tracker (if it supports that) or from peers that supports the extension protocol. The address can be accessed through the external_address member.

struct external_ip_alert: alert
{
        // ...
        address external_address;
};

listen_failed_alert

This alert is generated when none of the ports, given in the port range, to session can be opened for listening. This alert doesn't have any extra data members.

portmap_error_alert

This alert is generated when a NAT router was successfully found but some part of the port mapping request failed. It contains a text message that may help the user figure out what is wrong. This alert is not generated in case it appears the client is not running on a NAT:ed network or if it appears there is no NAT router that can be remote controlled to add port mappings.

mapping refers to the mapping index of the port map that failed, i.e. the index returned from add_mapping.

type is 0 for NAT-PMP and 1 for UPnP.

struct portmap_error_alert: alert
{
        // ...
        int mapping;
        int type;
};

portmap_alert

This alert is generated when a NAT router was successfully found and a port was successfully mapped on it. On a NAT:ed network with a NAT-PMP capable router, this is typically generated once when mapping the TCP port and, if DHT is enabled, when the UDP port is mapped.

mapping refers to the mapping index of the port map that failed, i.e. the index returned from add_mapping.

external_port is the external port allocated for the mapping.

type is 0 for NAT-PMP and 1 for UPnP.

struct portmap_alert: alert
{
        // ...
        int mapping;
        int external_port;
        int type;
};

portmap_log_alert

This alert is generated to log informational events related to either UPnP or NAT-PMP. They contain a log line and the type (0 = NAT-PMP and 1 = UPnP). Displaying these messages to an end user is only useful for debugging the UPnP or NAT-PMP implementation.

struct portmap_log_alert: alert
{
        //...
        int type;
        std::string msg;
};

file_error_alert

If the storage fails to read or write files that it needs access to, this alert is generated and the torrent is paused.

file is the path to the file that was accessed when the error occurred.

msg is the error message received from the OS.

struct file_error_alert: torrent_alert
{
        // ...
        std::string file;
        std::string msg;
};

tracker_announce_alert

This alert is generated each time a tracker announce is sent (or attempted to be sent). There are no extra data members in this alert. The url can be found in the base class however.

struct tracker_announce_alert: tracker_alert
{
        // ...
        int event;
};

Event specifies what event was sent to the tracker. It is defined as:

0. None
1. Completed
2. Started
3. Stopped

tracker_error_alert

This alert is generated on tracker time outs, premature disconnects, invalid response or a HTTP response other than "200 OK". From the alert you can get the handle to the torrent the tracker belongs to.

The times_in_row member says how many times in a row this tracker has failed. status_code is the code returned from the HTTP server. 401 means the tracker needs authentication, 404 means not found etc. If the tracker timed out, the code will be set to 0.

struct tracker_error_alert: tracker_alert
{
        // ...
        int times_in_row;
        int status_code;
};

tracker_reply_alert

This alert is only for informational purpose. It is generated when a tracker announce succeeds. It is generated regardless what kind of tracker was used, be it UDP, HTTP or the DHT.

struct tracker_reply_alert: tracker_alert
{
        // ...
        int num_peers;
};

The num_peers tells how many peers were returned from the tracker. This is not necessarily all new peers, some of them may already be connected.

dht_reply_alert

This alert is generated each time the DHT receives peers from a node. num_peers is the number of peers we received in this packet. Typically these packets are received from multiple DHT nodes, and so the alerts are typically generated a few at a time.

struct dht_reply_alert: tracker_alert
{
        // ...
        int num_peers;
};

tracker_warning_alert

This alert is triggered if the tracker reply contains a warning field. Usually this means that the tracker announce was successful, but the tracker has a message to the client. The msg string in the alert contains the warning message from the tracker.

struct tracker_warning_alert: tracker_alert
{
        // ...
        std::string msg;
};

scrape_reply_alert

This alert is generated when a scrape request succeeds. incomplete and complete is the data returned in the scrape response. These numbers may be -1 if the reponse was malformed.

struct scrape_reply_alert: tracker_alert
{
        // ...
        int incomplete;
        int complete;
};

scrape_failed_alert

If a scrape request fails, this alert is generated. This might be due to the tracker timing out, refusing connection or returning an http response code indicating an error. msg contains a message describing the error.

struct scrape_failed_alert: tracker_alert
{
        // ...
        std::string msg;
};

url_seed_alert

This alert is generated when a HTTP seed name lookup fails.

It contains url to the HTTP seed that failed along with an error message.

struct url_seed_alert: torrent_alert
{
        // ...
        std::string url;
};

hash_failed_alert

This alert is generated when a finished piece fails its hash check. You can get the handle to the torrent which got the failed piece and the index of the piece itself from the alert.

struct hash_failed_alert: torrent_alert
{
        // ...
        int piece_index;
};

peer_ban_alert

This alert is generated when a peer is banned because it has sent too many corrupt pieces to us. ip is the endpoint to the peer that was banned.

struct peer_ban_alert: torrent_alert
{
        // ...
        asio::ip::tcp::endpoint ip;
};

peer_error_alert

This alert is generated when a peer sends invalid data over the peer-peer protocol. The peer will be disconnected, but you get its ip address from the alert, to identify it.

struct peer_error_alert: torrent_alert
{
        // ...
        asio::ip::tcp::endpoint ip;
        peer_id id;
};

invalid_request_alert

This is a debug alert that is generated by an incoming invalid piece request. ìp is the address of the peer and the request is the actual incoming request from the peer.

struct invalid_request_alert: torrent_alert
{
        invalid_request_alert(
                peer_request const& r
                , torrent_handle const& h
                , asio::ip::tcp::endpoint const& send
                , peer_id const& pid
                , std::string const& msg);

        virtual std::auto_ptr<alert> clone() const;

        asio::ip::tcp::endpoint ip;
        peer_request request;
        peer_id id;
};


struct peer_request
{
        int piece;
        int start;
        int length;
        bool operator==(peer_request const& r) const;
};

The peer_request contains the values the client sent in its request message. piece is the index of the piece it want data from, start is the offset within the piece where the data should be read, and length is the amount of data it wants.

torrent_finished_alert

This alert is generated when a torrent switches from being a downloader to a seed. It will only be generated once per torrent. It contains a torrent_handle to the torrent in question.

There are no additional data members in this alert.

performance_alert

This alert is generated when a limit is reached that might have a negative impact on upload or download rate performance.

struct performance_alert: torrent_alert
{
        // ...

        enum performance_warning_t
        {
                outstanding_disk_buffer_limit_reached,
                outstanding_request_limit_reached,
                upload_limit_too_low,
                download_limit_too_low
        };

        performance_warning_t warning_code;
};

metadata_failed_alert

This alert is generated when the metadata has been completely received and the info-hash failed to match it. i.e. the metadata that was received was corrupt. libtorrent will automatically retry to fetch it in this case. This is only relevant when running a torrent-less download, with the metadata extension provided by libtorrent.

There are no additional data members in this alert.

metadata_received_alert

This alert is generated when the metadata has been completely received and the torrent can start downloading. It is not generated on torrents that are started with metadata, but only those that needs to download it from peers (when utilizing the libtorrent extension).

There are no additional data members in this alert.

fastresume_rejected_alert

This alert is generated when a fastresume file has been passed to add_torrent but the files on disk did not match the fastresume file. The string explains the reason why the resume file was rejected.

struct fastresume_rejected_alert: torrent_alert
{
        // ...
        std::string msg;
};

peer_blocked_alert

This alert is generated when a peer is blocked by the IP filter. The ip member is the address that was blocked.

struct peer_blocked_alert: alert
{
        // ...
        address ip;
};

storage_moved_alert

The storage_moved_alert is generated when all the disk IO has completed and the files have been moved, as an effect of a call to torrent_handle::move_storage. This is useful to synchronize with the actual disk. The path member is the new path of the storage.

struct storage_moved_alert: torrent_alert
{
        // ...
        std::string path;
};

torrent_paused_alert

This alert is generated as a response to a torrent_handle::pause request. It is generated once all disk IO is complete and the files in the torrent have been closed. This is useful for synchronizing with the disk.

There are no additional data members in this alert.

torrent_resumed_alert

This alert is generated as a response to a torrent_handle::resume request. It is generated when a torrent goes from a paused state to an active state.

There are no additional data members in this alert.

save_resume_data_alert

This alert is generated as a response to a torrent_handle::save_resume_data request. It is generated once the disk IO thread is done writing the state for this torrent. The resume_data member points to the resume data.

struct save_resume_data_alert: torrent_alert
{
        // ...
        boost::shared_ptr<entry> resume_data;
};

save_resume_data_failed_alert

This alert is generated instead of save_resume_data_alert if there was an error generating the resume data. msg describes what went wrong.

struct save_resume_data_failed_alert: torrent_alert
{
        // ...
        std::string msg;
};

dht_announce_alert

This alert is generated when a DHT node announces to an info-hash on our DHT node. It belongs to the dht_notification category.

struct dht_announce_alert: alert
{
        // ...
        address ip;
        int port;
        sha1_hash info_hash;
};

dht_get_peers_alert

This alert is generated when a DHT node sends a get_peers message to our DHT node. It belongs to the dht_notification category.

struct dht_get_peers_alert: alert
{
        // ...
        sha1_hash info_hash;
};

dispatcher

The handle_alert class is defined in <libtorrent/alert.hpp>.

Examples usage:

struct my_handler
{
        void operator()(portmap_error_alert const& a)
        {
                std::cout << "Portmapper: " << a.msg << std::endl;
        }

        void operator()(tracker_warning_alert const& a)
        {
                std::cout << "Tracker warning: " << a.msg << std::endl;
        }

        void operator()(torrent_finished_alert const& a)
        {
                // write fast resume data
                // ...

                std::cout << a.handle.get_torrent_info().name() << "completed"
                        << std::endl;
        }
};

std::auto_ptr<alert> a;
a = ses.pop_alert();
my_handler h;
while (a.get())
{
        handle_alert<portmap_error_alert
                , tracker_warning_alert
                , torrent_finished_alert
        >::handle_alert(h, a);
        a = ses.pop_alert();
}

In this example 3 alert types are used. You can use any number of template parameters to select between more types. If the number of types are more than 15, you can define TORRENT_MAX_ALERT_TYPES to a greater number before including <libtorrent/alert.hpp>.

invalid_handle

This exception is thrown when querying information from a torrent_handle that hasn't been initialized or that has become invalid.

struct invalid_handle: std::exception
{
        const char* what() const throw();
};

duplicate_torrent

This is thrown by add_torrent() if the torrent already has been added to the session. Since remove_torrent() is asynchronous, this exception may be thrown if the torrent is removed and then immediately added again.

struct duplicate_torrent: std::exception
{
        const char* what() const throw();
};

invalid_encoding

This is thrown by bdecode() if the input data is not a valid bencoding.

struct invalid_encoding: std::exception
{
        const char* what() const throw();
};

type_error

This is thrown from the accessors of entry if the data type of the entry doesn't match the type you want to extract from it.

struct type_error: std::runtime_error
{
        type_error(const char* error);
};

invalid_torrent_file

This exception is thrown from the constructor of torrent_info if the given bencoded information doesn't meet the requirements on what information has to be present in a torrent file.

struct invalid_torrent_file: std::exception
{
        const char* what() const throw();
};

initialize()

bool initialize(bool allocate_files) = 0;

This function is called when the storage is to be initialized. The default storage will create directories and empty files at this point. If allocate_files is true, it will also ftruncate all files to their target size.

Returning true indicates an error occurred.

read()

int read(char* buf, int slot, int offset, int size) = 0;

This function should read the data in the given slot and at the given offset and size number of bytes. The data is to be copied to buf.

The return value is the number of bytes actually read.

write()

int write(const char* buf, int slot, int offset, int size) = 0;

This function should write the data in buf to the given slot (slot) at offset offset in that slot. The buffer size is size.

The return value is the number of bytes actually written.

move_storage()

bool move_storage(fs::path save_path) = 0;

This function should move all the files belonging to the storage to the new save_path. The default storage moves the single file or the directory of the torrent.

Before moving the files, any open file handles may have to be closed, like release_files().

Returning true indicates an error occurred.

verify_resume_data()

bool verify_resume_data(lazy_entry const& rd, std::string& error) = 0;

This function should verify the resume data rd with the files on disk. If the resume data seems to be up-to-date, return true. If not, set error to a description of what mismatched and return false.

The default storage may compare file sizes and time stamps of the files.

Returning true indicates an error occurred.

write_resume_data()

bool write_resume_data(entry& rd) const = 0;

This function should fill in resume data, the current state of the storage, in rd. The default storage adds file timestamps and sizes.

Returning true indicates an error occurred.

move_slot()

bool move_slot(int src_slot, int dst_slot) = 0;

This function should copy or move the data in slot src_slot to the slot dst_slot. This is only used in compact mode.

If the storage caches slots, this could be implemented more efficient than reading and writing the data.

Returning true indicates an error occurred.

swap_slots()

bool swap_slots(int slot1, int slot2) = 0;

This function should swap the data in slot1 and slot2. The default storage uses a scratch buffer to read the data into, then moving the other slot and finally writing back the temporary slot's data

This is only used in compact mode.

Returning true indicates an error occurred.

swap_slots3()

bool swap_slots3(int slot1, int slot2, int slot3) = 0;

This function should do a 3-way swap, or shift of the slots. slot1 should move to slot2, which should be moved to slot3 which in turn should be moved to slot1.

This is only used in compact mode.

Returning true indicates an error occurred.

hash_for_slot()

sha1_hash hash_for_slot(int slot, partial_hash& h, int piece_size) = 0;

The function should read the remaining bytes of the slot and hash it with the sha-1 state in partion_hash. The partial_hash struct looks like this:

struct partial_hash
{
        partial_hash();
        int offset;
        hasher h;
};

offset is the number of bytes in the slot that has already been hashed, and h is the sha-1 state of that hash. piece_size is the size of the piece that is stored in the given slot.

The function should return the hash of the piece stored in the slot.

rename_file()

bool rename_file(int file, std::string const& new_name) = 0;

Rename file with index file to the thame new_name. If there is an error, true should be returned.

release_files()

bool release_files() = 0;

This function should release all the file handles that it keeps open to files belonging to this storage. The default implementation just calls file_pool::release_files(this).

Returning true indicates an error occurred.

delete_files()

bool delete_files() = 0;

This function should delete all files and directories belonging to this storage.

Returning true indicates an error occurred.

downloading

Torrents that are currently being downloaded or incomplete (with bytes still to download) are queued. The torrents in the front of the queue are started to be actively downloaded and the rest are ordered with regards to their queue position. Any newly added torrent is placed at the end of the queue. Once a torrent is removed or turns into a seed, its queue position is -1 and all torrents that used to be after it in the queue, decreases their position in order to fill the gap.

The queue positions are always in a sequence without any gaps.

Lower queue position means closer to the front of the queue, and will be started sooner than torrents with higher queue positions.

To query a torrent for its position in the queue, or change its position, see: queue_position() queue_position_up() queue_position_down() queue_position_top() queue_position_bottom().

seeding

Auto managed seeding torrents are rotated, so that all of them are allocated a fair amount of seeding. Torrents with fewer completed seed cycles are prioritized for seeding. A seed cycle is completed when a torrent meets either the share ratio limit (uploaded bytes / downloaded bytes), the share time ratio (time seeding / time downloaing) or seed time limit (time seeded).

The relevant settings to control these limits are share_ratio_limit, seed_time_ratio_limit and seed_time_limit in session_settings.

file format

The file format is a bencoded dictionary containing the following fields:

sparse allocation

On filesystems that supports sparse files, this allocation mode will only use as much space as has been downloaded.

• It does not require an allocation pass on startup.
• It supports skipping files (setting prioirty to 0 to not download).
• Fast resume data will remain valid even when file time stamps are out of date.

full allocation

When a torrent is started in full allocation mode, the disk-io thread (see threads) will make sure that the entire storage is allocated, and fill any gaps with zeros. This will be skipped if the filesystem supports sparse files or automatic zero filling. It will of course still check for existing pieces and fast resume data. The main drawbacks of this mode are:

• It may take longer to start the torrent, since it will need to fill the files with zeros on some systems. This delay is linearly dependent on the size of the download.
• The download may occupy unnecessary disk space between download sessions. In case sparse files are not supported.
• Disk caches usually perform extremely poorly with random access to large files and may slow down a download considerably.

The benefits of this mode are:

• Downloaded pieces are written directly to their final place in the files and the total number of disk operations will be fewer and may also play nicer to filesystems' file allocation, and reduce fragmentation.
• No risk of a download failing because of a full disk during download. Unless sparse files are being used.
• The fast resume data will be more likely to be usable, regardless of crashes or out of date data, since pieces won't move around.
• Can be used with the filter files feature.

compact allocation

The compact allocation will only allocate as much storage as it needs to keep the pieces downloaded so far. This means that pieces will be moved around to be placed at their final position in the files while downloading (to make sure the completed download has all its pieces in the correct place). So, the main drawbacks are:

• More disk operations while downloading since pieces are moved around.
• Potentially more fragmentation in the filesystem.
• Cannot be used while filtering files.

The benefits though, are:

• No startup delay, since the files doesn't need allocating.
• The download will not use unnecessary disk space.
• Disk caches perform much better than in full allocation and raises the download speed limit imposed by the disk.
• Works well on filesystems that doesn't support sparse files.

The algorithm that is used when allocating pieces and slots isn't very complicated. For the interested, a description follows.

storing a piece:

1. let A be a newly downloaded piece, with index n.
2. let s be the number of slots allocated in the file we're downloading to. (the number of pieces it has room for).
3. if n >= s then allocate a new slot and put the piece there.
4. if n < s then allocate a new slot, move the data at slot n to the new slot and put A in slot n.

allocating a new slot:

1. if there's an unassigned slot (a slot that doesn't contain any piece), return that slot index.
2. append the new slot at the end of the file (or find an unused slot).
3. let i be the index of newly allocated slot
4. if we have downloaded piece index i already (to slot j) then
   1. move the data at slot j to slot i.
   2. return slot index j as the newly allocated free slot.
5. return i as the newly allocated slot.

metadata from peers

Extension name: "LT_metadata"

The point with this extension is that you don't have to distribute the metadata (.torrent-file) separately. The metadata can be distributed through the bittorrent swarm. The only thing you need to download such a torrent is the tracker url and the info-hash of the torrent.

It works by assuming that the initial seeder has the metadata and that the metadata will propagate through the network as more peers join.

There are three kinds of messages in the metadata extension. These packets are put as payload to the extension message. The three packets are:

• request metadata
• metadata
• don't have metadata

request metadata:

metadata:

Don't have metadata:

HTTP seeding

The HTTP seed extension implements this specification.

The libtorrent implementation assumes that, if the URL ends with a slash ('/'), the filename should be appended to it in order to request pieces from that file. The way this works is that if the torrent is a single-file torrent, only that filename is appended. If the torrent is a multi-file torrent, the torrent's name '/' the file name is appended. This is the same directory structure that libtorrent will download torrents into.