session()

session(fingerprint const& print
        = libtorrent::fingerprint("LT", 0, 1, 0, 0));
session(fingerprint const& print
        , std::pair<int, int> listen_port_range
        , char const* listen_interface = 0);

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.

~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().

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)(
        boost::intrusive_ptr<torrent_info const>, fs::path const&
        , file_pool&);

torrent_handle add_torrent(
        boost::intrusive_ptr<torrent_info> const& ti
        , boost::filesystem::path const& save_path
        , entry const& resume_data = entry()
        , storage_mode_t storage_mode = storage_mode_sparse
        , bool paused = false
        , storage_constructor_type sc = default_storage_constructor
        , void* userdata = 0);

torrent_handle add_torrent(
        char const* tracker_url
        , sha1_hash const& info_hash
        , char const* name
        , boost::filesystem::path const& save_path
        , entry const& resume_data = entry()
        , storage_mode_t storage_mode = storage_mode_sparse
        , bool paused = false
        , storage_constructor_type sc = default_storage_constructor
        , void* userdata = 0);

You add torrents through the add_torrent() function where you give an object representing the information found in the torrent file and the path where you want to save the files. The save_path will be prepended to the directory structure in the torrent-file.

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.

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 torrent_handle::write_resume_data(). See fast resume.

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

All pieces will be allocated, zeroes will be written to the files, before the data is downloaded and written to the file. This might be useful for filesystems that don't support sparse files.

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.

storage_constructor 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 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.

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

The second overload that takes a tracker url and an info-hash instead of metadata (torrent_info) can be used with torrents where (at least some) peers support the metadata extension. For the overload to be available, 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.

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()

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

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.

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.

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 session_status
{
        bool has_incoming_connections;

        float upload_rate;
        float download_rate;

        float payload_upload_rate;
        float payload_download_rate;

        size_type total_download;
        size_type total_upload;

        size_type total_payload_download;
        size_type total_payload_upload;

        int num_peers;
        int num_unchoked;
        int allowed_upload_slots;

        int dht_nodes;
        int dht_cache_nodes;
        int dht_torrents;
        int dht_global_nodes;
};

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, payload_download_rate and payload_upload_rate are the total download and upload rates accumulated from all torrents. The payload versions is the payload download only.

total_download and total_upload are the total number of bytes downloaded and uploaded to and from all torrents. total_payload_download and total_payload_upload are the same thing but where only the payload is considered.

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.

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.

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_severity_level()

std::auto_ptr<alert> pop_alert();
void set_severity_level(alert::severity_t s);

pop_alert() is used to ask the session if any errors or events has occurred. With set_severity_level() you can filter how serious the event has to be for you to receive it through pop_alert(). For information, see alerts.

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. 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.

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();
torrent_info(sha1_hash const& info_hash);
torrent_info(entry const& torrent_file);

The default constructor of torrent_info is used when creating torrent files. It will initialize the object to an empty torrent, containing no files. The info hash will be set to 0 when this constructor is used. To use the empty torrent_info object, add files and piece hashes, announce URLs and optionally a creator tag and comment. To do this you use the members set_comment(), set_piece_size(), set_creator(), set_hash() etc.

The constructor that takes an info-hash is identical to the default constructor with the exception that it will initialize the info-hash to the given value. 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 last constructor is the one that is used in most cases. It will create a torrent_info object from the information found in the given torrent_file. The entry represents a tree node in an bencoded file. To load an ordinary .torrent file into an entry, use bdecode(), see bdecode() bencode().

set_comment() set_piece_size() set_creator() set_hash() add_tracker() add_file()

void set_comment(char const* str);
void set_piece_size(int size);
void set_creator(char const* str);
void set_hash(int index, sha1_hash const& h);
void add_tracker(std::string const& url, int tier = 0);
void add_file(boost::filesystem::path file, size_type size);

These files are used when creating a torrent file. set_comment() will simply set the comment that belongs to this torrent. The comment can be retrieved with the comment() member. The string should be UTF-8 encoded.

set_piece_size() will set the size of each piece in this torrent. The piece size must be an even multiple of 2. i.e. usually something like 256 kiB, 512 kiB, 1024 kiB etc. The size is given in number of bytes.

set_creator() is an optional attribute that can be used to identify your application that was used to create the torrent file. The string should be UTF-8 encoded.

set_hash() writes the hash for the piece with the given piece-index. You have to call this function for every piece in the torrent. Usually the hasher is used to calculate the sha1-hash for a piece.

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().

add_file() adds a file to the torrent. The order in which you add files will determine the order in which they are placed in the torrent file. You have to add at least one file to the torrent. The path you give has to be a relative path from the root directory of the torrent. The size is given in bytes.

When you have added all the files and hashes to your torrent, you can generate an entry which then can be encoded as a .torrent file. You do this by calling create_torrent().

For a complete example of how to create a torrent from a file structure, see make_torrent.

create_torrent()

entry create_torrent();

Returns an entry representing the bencoded tree of data that makes up a .torrent file. You can save this data as a torrent file with bencode() (see bdecode() bencode()), for a complete example, see make_torrent.

This function is not const because it will also set the info-hash of the torrent_info object.

Note that a torrent file must include at least one file, and it must have at least one tracker url or at least one DHT node.

remap_files()

bool remap_files(std::vector<file_entry> const& map);

This call will create a new mapping of the data in this torrent to other files. The torrent_info maintains 2 views of the file storage. One that is true to the torrent file, and one that represents what is actually saved on disk. This call will change what the files on disk are called.

The each entry in the vector map is a file_entry. The only fields in this struct that are used in this case are path, size and file_base.

The return value indicates if the remap was successful or not. True means success and false means failure. The sum of all the files passed in through map has to be exactly the same as the total_size of the torrent. If the number of bytes that are mapped do not match, false will be returned (this is the only case this function may fail).

Changing this mapping for an existing torrent will not move or rename files. If some files should be renamed, this can be done before the torrent is added.

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

file_iterator begin_files(bool storage = false) const;
file_iterator end_files(bool storage = false) const;
reverse_file_iterator rbegin_files(bool storage = false) const;
reverse_file_iterator rend_files(bool storage = false) 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.

The storage parameter specifies which view of the files you want. The default is false, which means you will see the content of the torrent file. If set to true, you will see the file that the storage class uses to save the files to disk. Typically these views are the same, but in case the files have been remapped, they may differ. For more info, see remap_files().

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(bool storage = false) const;
file_entry const& file_at(int index, bool storage = false) const;

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

The storage parameter specifies which view of the files you want. The default is false, which means you will see the content of the torrent file. If set to true, you will see the file that the storage class uses to save the files to disk. Typically these views are the same, but in case the files have been remapped, they may differ. For more info, see remap_files().

map_block()

std::vector<file_slice> map_block(int piece, size_type offset
        , int size, bool storage = false) 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.

The storage parameter specifies which view of the files you want. The default is false, which means you will see the content of the torrent file. If set to true, you will see the file that the storage class uses to save the files to disk. Typically these views are the same, but in case the files have been remapped, they may differ. For more info, see remap_files().

map_file()

peer_request map_file(int file_index, size_type file_offset
        , int size, bool storage = false) 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.

The storage parameter specifies which view of the files you want. The default is false, which means you will see the content of the torrent file. If set to true, you will see the file that the storage class uses to save the files to disk. Typically these views are the same, but in case the files have been remapped, they may differ. For more info, see remap_files().

See HTTP seeding for more information.

print()

void print(std::ostream& os) const;

The print() function is there for debug purposes only. It will print the info from the torrent file to the given outstream. This function has been deprecated and will be removed from future releases.

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;
        int tier;
};

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

size_type total_size() const;
size_type piece_length() const;
size_type 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() info_hash()

size_type piece_size(unsigned int index) const;
sha1_hash const& hash_for_piece(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. info_hash() will only return a valid hash if the torrent_info was read from a .torrent file or if an entry was created from it (through create_torrent).

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() set_priv()

bool priv() const;
void set_priv(bool v);

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

set_priv() sets or clears the private flag on this torrent.

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.

piece_priority() prioritize_pieces() piece_priorities() prioritize_files()

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;
void prioritize_files(std::vector<int> const& files) 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.

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_progress()

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

This function fills in the supplied vector with the progress (a value in the range [0, 1]) describing the download progress 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.

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. This operation will only have the desired effect if the given save_path is located on the same drive as the original save path. 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.

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_sequenced_download_threshold()

void set_sequenced_download_threshold(int threshold);

sequenced-download threshold is the limit on how popular a piece has to be (popular == inverse of rarity) to be downloaded in sequence instead of in random (rarest first) order. It can be used to tweak disk performance in settings where the random download property is less necessary. For example, if the threshold is 10, all pieces which 10 or more peers have, will be downloaded in index order. This setting defaults to 100, which means that it is disabled in practice.

Setting this threshold to a very small value will affect the piece distribution negatively in the swarm. It should basically only be used in situations where the random seeks on the disk is the download bottleneck.

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.

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).

has_metadata()

bool has_metadata() const;

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_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()

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

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_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.

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()

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

write_resume_data()

entry write_resume_data() const;

write_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.

There are three cases where this function will just return an empty entry:

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 this function returns, the resume data may already be invalid if the torrent is still downloading! The recommended practice is to first pause the torrent, then generate the fast resume data, and then close it down. Since the disk IO is done in a separate thread, in order to synchronize, you shoule to wait for the torrent_paused_alert before you write the resume data.

In full allocation mode the reume data is never invalidated by subsequent writes to the files, since pieces won't move around. This means that you don't need to pause before writing resume data in full or sparse mode. If you don't, however, any data written to disk after you saved resume data and before the session closed is lost.

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.

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.

TODO: document storage

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.

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.

supports_sparse_files()

bool supports_sparse_files(boost::filesystem::path const&);

The path is expected to be the path to the directory where you will want to store sparse files. The return value is true if the file system supports sparse files or if it supports automatic zero filling of files. The main characteristics that is tested by this function is not the storage aspects of sparse files, but rather the support for seeking passed end of file and write data there, with expected behavior.

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 is generated as severity level fatal.

struct listen_failed_alert: alert
{
        listen_failed_alert(const std::string& msg);
        virtual std::auto_ptr<alert> clone() const;
};

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.

The alert is generated as severity warning, since it should be displayed to the user somehow, and could mean reduced preformance.

struct portmap_error_alert: alert
{
        portmap_error_alert(const std::string& msg);
        virtual std::auto_ptr<alert> clone() const;
};

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. This is merely an informational alert, and is generated at severity level info.

struct portmap_alert: alert
{
        portmap_alert(const std::string& msg);
        virtual std::auto_ptr<alert> clone() const;
};

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. It is generated as severity level fatal.

struct file_error_alert: torrent_alert
{
        file_error_alert(
                const torrent_handle& h
                , const std::string& msg);

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

tracker_announce_alert

This alert is generated each time a tracker announce is sent (or attempted to be sent). It is generated at severity level info.

struct tracker_announce_alert: torrent_alert
{
        tracker_announce_alert(
                const torrent_handle& h
                , const std::string& msg);

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

tracker_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. This alert is generated as severity level warning.

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_alert: torrent_alert
{
        tracker_alert(torrent_handle const& h, int times, int status
                , const std::string& msg);
        virtual std::auto_ptr<alert> clone() const;

        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. It is generated with severity level info.

struct tracker_reply_alert: torrent_alert
{
        tracker_reply_alert(const torrent_handle& h
                , int num_peers
                , const std::string& msg);

        int num_peers;

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

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.

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 message string in the alert will contain the warning message from the tracker. It is generated with severity level warning.

struct tracker_warning_alert: torrent_alert
{
        tracker_warning_alert(torrent_handle const& h
                , std::string const& msg);

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

scrape_reply_alert

struct scrape_reply_alert: torrent_alert
{
        scrape_reply_alert(torrent_handle const& h
                , int incomplete_
                , int complete_
                , std::string const& msg);

        int incomplete;
        int complete;

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

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.

scrape_failed_alert

struct scrape_failed_alert: torrent_alert
{
        scrape_failed_alert(torrent_handle const& h
                , std::string const& msg);

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

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.

url_seed_alert

This alert is generated when a HTTP seed name lookup fails. This alert is generated as severity level warning.

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

struct url_seed_alert: torrent_alert
{
        url_seed_alert(torrent_handle const& h, std::string const& url
                , const std::string& msg);
        virtual std::auto_ptr<alert> clone() const;

        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. This alert is generated as severity level info.

struct hash_failed_alert: torrent_alert
{
        hash_failed_alert(
                torrent_handle const& h
                , int index
                , const std::string& msg);

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

        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. It is generated at severity level info. The handle member is a torrent_handle to the torrent that this peer was a member of.

struct peer_ban_alert: torrent_alert
{
        peer_ban_alert(
                asio::ip::tcp::endpoint const& pip
                , torrent_handle h
                , const std::string& msg);

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

        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. This alert is generated as severity level debug.

struct peer_error_alert: alert
{
        peer_error_alert(
                asio::ip::tcp::endpoint const& pip
                , peer_id const& pid
                , const std::string& msg);

        virtual std::auto_ptr<alert> clone() const;
        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. The handle is a handle to the torrent the peer is a member of. Ïp is the address of the peer and the request is the actual incoming request from the peer. The alert is generated as severity level debug.

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. This alert is generated as severity level info.

struct torrent_finished_alert: torrent_alert
{
        torrent_finished_alert(
                const torrent_handle& h
                , const std::string& msg);

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

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. It is generated at severity level info.

struct metadata_failed_alert: torrent_alert
{
        metadata_failed_alert(
                torrent_handle const& h
                , std::string const& msg);

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

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). It is generated at severity level info.

struct metadata_received_alert: torrent_alert
{
        metadata_received_alert(
                torrent_handle const_& h
                , std::string const& msg);

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

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. It is generated at severity level warning.

struct fastresume_rejected_alert: torrent_alert
{
        fastresume_rejected_alert(torrent_handle const& h
                , std::string const& msg);

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

peer_blocked_alert

This alert is generated when a peer is blocked by the IP filter. It has the severity leve info. The ip member is the address that was blocked.

struct peer_blocked_alert: alert
{
        peer_blocked_alert(address const& ip_
                , std::string const& msg);

        address ip;

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

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.

struct storage_moved_alert: torrent_alert
{
        storage_moved_alert(torrent_handle const& h, std::string const& path);
        virtual std::auto_ptr<alert> clone() const;
};

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.

struct torrent_paused_alert: torrent_alert
{
        torrent_paused_alert(torrent_handle const& h, std::string const& msg);
        virtual std::auto_ptr<alert> clone() const;
};

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()

void 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.

read()

size_type 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()

void 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.

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().

verify_resume_data()

bool verify_resume_data(entry& 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.

write_resume_data( )

void 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.

move_slot()

void 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.

swap_slots()

void 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.

swap_slots3()

void 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.

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.

release_files()

void 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).

delete_files()

void delete_files() = 0;

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

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 checker 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.