cygwin and msvc

Note that if you're building on windows using the msvc toolset, you cannot run it from a cygwin terminal, you'll have to run it from a cmd terminal. The same goes for cygwin, if you're building with gcc (mingw) you'll have to run it from a cygwin terminal. Also, make sure the paths are correct in the different environments. In cygwin, the paths (BOOST_BUILD_PATH and BOOST_ROOT) should be in the typical unix-format (e.g. /cygdrive/c/boost_1_32_0). In the windows environment, they should have the typical windows format (c:/boost_1_32_0).

If you're building in developer studio, you may have to set the compiler options "force conformance in for loop scope", "treat wchar_t as built-in type" and "Enable Run-Time Type Info" to Yes.

release and debug builds

The Jamfile can build both a release and debug version of libtorrent. In debug mode, libtorrent will have pretty expensive invariant checks and asserts built into it. If you want to disable such checks (you want to do that in a release build) you can see the table below for which defines you can use to control the build. The Jamfile will define NDEBUG when it's building a release build.

If you experience that libtorrent uses unreasonable amounts of cpu, it will definately help to define NDEBUG, since it will remove the invariant checks within the library.

session()

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

If the fingerprint in the first overload is ommited, 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 adviced that any kind of interface (such as windows) are closed before destructing the sessoin object. Because it can take a few second for it to finish. The timeout can be set with set_http_settings().

add_torrent()

torrent_handle add_torrent(
        entry const& e
        , boost::filesystem::path const& save_path
        , entry const& resume_data = entry());
torrent_handle add_torrent(
        char const* tracker_url
        , sha1_hash const& info_hash
        , boost::filesystem::path const& save_path
        , entry const& resume_data = entry());

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 last 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 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 second overload that takes a tracker url and an info-hash instead of metadata (entry) 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_ENABLE_EXTENSIONS defined).

remove_torrent()

void remove_torrent(torrent_handle const& h);

remove_torrent() will close all peer connections associated with the torrent and tell the tracker that we've stopped participating in the swarm.

disable_extensions() enable_extension()

void disable_extensions();
void enable_extension(peer_connection::extension_index);

disable_extensions() will disable all extensions available in libtorrent. enable_extension() will enable a single extension. The available extensions are enumerated in the peer_connection class. These are the available extensions:

enum extension_index
{
        extended_chat_message,
        extended_metadata_message,
        extended_peer_exchange_message,
        extended_listen_port_message,
        num_supported_extensions
};

peer_exchange is not implemented yet

By default, all extensions are enabled. For more information about the extensions, see the extensions section.

set_upload_rate_limit() set_download_rate_limit()

void set_upload_rate_limit(int bytes_per_second);
void set_download_rate_limit(int bytes_per_second);

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.

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.

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;
};

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 have.

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 wether 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.

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 occured. 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.

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 respecive 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;
// ...

const entry::dictionary_type& 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";
}

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

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 size;
};

num_files() file_at()

int num_files() const;
const file_entry& 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.

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.

trackers()

const std::vector<announce_entry>& 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;
const sha1_hash& 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.

name() comment() creation_date()

const std::stirng& name() const;
const std::string& 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 timestamp in the torrent file, this will return a date of january 1:st 1970.

save_path()

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

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

move_storage()

bool move_storage(boost::filsystem::path const& save_path);

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. If the move operation fails, this function returns false, otherwise true. Post condition for successful operation is: save_path() == save_path.

force_reannounce()

void force_reannounce();

force_reannounce() will force this torrent to do another tracker request, to receive new peers. If the torrent is invalid, queued or in checking mode, this functions will throw invalid_handle.

connect_peer()

void connect_peer(address const& adr) 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.

set_ratio()

void set_ratio(float ratio);

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()

void set_upload_limit(int limit);
void set_download_limit(int limit);

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.

pause() resume() is_paused()

void pause();
void resume();
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 (eg. disk full) or something similar. See file_error_alert.

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);

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.

use_interface()

void use_interface(char const* net_interface);

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 can be a string containing an ip-address or a hostname.

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);
void set_max_connections(int max_connections);

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();

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.

metadata()

std::vector<char> const& metadata() const;

metadata() will return a reference to a buffer containing the exact info part of the .torrent file. This buffer will be valid as long as the torrent is still running. When hashed, it will produce the same hash as the info-hash.

status()

torrent_status status();

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);

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
{
        enum { max_blocks_per_piece };
        int piece_index;
        int blocks_in_piece;
        std::bitset<max_blocks_per_piece> requested_blocks;
        std::bitset<max_blocks_per_piece> finished_blocks;
        peer_id peer[max_blocks_per_piece];
        int num_downloads[max_blocks_per_piece];
};

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.

requested_blocks is a bitset with one bit per block in the piece. If a bit is set, it means that that block has been requested, but not necessarily fully downloaded yet. To know from whom the block has been requested, have a look in the peer array. The bit-index in the requested_blocks and finished_blocks correspons to the array-index into peers and num_downloads. The array of peers is contains the id of the peer the piece was requested from. If a piece hasn't been requested (the bit in requested_blocks is not set) the peer array entry will be undefined.

The finished_blocks is a bitset where each bit says if the block is fully downloaded or not. And the num_downloads array says how many times that block has been downloaded. When a piece fails a hash verification, single blocks may be redownloaded to see if the hash teast may pass then.

get_peer_info()

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

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();

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.

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.

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 retreive 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_insert_iterator<std::vector<char> >(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.

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;
};

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: alert
{
        file_error_alert(
                const torrent_handle& h
                , const std::string& msg);
                
        virtual std::auto_ptr<alert> clone() const;

        torrent_handle handle;
};

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.

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

        torrent_handle handle;
        int times_in_row;
};

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

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

        torrent_handle handle;
        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: alert
{
        peer_ban_alert(
                address const& pip
                , torrent_handle h
                , const std::string& msg);

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

        address ip;
        torrent_handle handle;
};

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(
                address const& pip
                , peer_id const& pid
                , const std::string& msg);

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

        address 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: alert
{
        invalid_request_alert(
                peer_request const& r
                , torrent_handle const& h
                , address const& send
                , peer_id const& pid
                , std::string const& msg);

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

        torrent_handle handle;
        address 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: alert
{
        torrent_finished_alert(
                const torrent_handle& h
                , const std::string& msg);

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

        torrent_handle handle;
};

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: alert
{
        metadata_received_alert(
                const torrent_handle& h
                , const std::string& msg);
                
        virtual std::auto_ptr<alert> clone() const;

        torrent_handle handle;
};

dispatcher

TODO: describe the dispatcher mechanism

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.

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();
};

dump_torrent

This is an example of a program that will take a torrent-file as a parameter and print information about it to std out:

#include <iostream>
#include <fstream>
#include <iterator>
#include <exception>
#include <iomanip>

#include "libtorrent/entry.hpp"
#include "libtorrent/bencode.hpp"
#include "libtorrent/torrent_info.hpp"


int main(int argc, char* argv[])
{
        using namespace libtorrent;

        if (argc != 2)
        {
                std::cerr << "usage: dump_torrent torrent-file\n";
                return 1;
        }

        try
        {
                std::ifstream in(argv[1], std::ios_base::binary);
                in.unsetf(std::ios_base::skipws);
                entry e = bdecode(std::istream_iterator<char>(in), std::istream_iterator<char>());
                torrent_info t(e);

                // print info about torrent
                std::cout << "\n\n----- torrent file info -----\n\n";
                std::cout << "trackers:\n";
                for (std::vector<announce_entry>::const_iterator i = t.trackers().begin();
                        i != t.trackers().end();
                        ++i)
                {
                        std::cout << i->tier << ": " << i->url << "\n";
                }

                std::cout << "number of pieces: " << t.num_pieces() << "\n";
                std::cout << "piece length: " << t.piece_length() << "\n";
                std::cout << "files:\n";
                for (torrent_info::file_iterator i = t.begin_files();
                        i != t.end_files();
                        ++i)
                {
                        std::cout << "  " << std::setw(11) << i->size
                        << "  " << i->path << " " << i->filename << "\n";
                }
                
        }
        catch (std::exception& e)
        {
                std::cout << e.what() << "\n";
        }

        return 0;
}

simple client

This is a simple client. It doesn't have much output to keep it simple:

#include <iostream>
#include <fstream>
#include <iterator>
#include <exception>

#include <boost/format.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>

#include "libtorrent/entry.hpp"
#include "libtorrent/bencode.hpp"
#include "libtorrent/session.hpp"
#include "libtorrent/http_settings.hpp"

int main(int argc, char* argv[])
{
        using namespace libtorrent;

        if (argc != 2)
        {
                std::cerr << "usage: ./simple_cient torrent-file\n"
                        "to stop the client, press return.\n";
                return 1;
        }

        try
        {
                session s;
                s.listen_on(std::make_pair(6881, 6889));

                std::ifstream in(argv[1], std::ios_base::binary);
                in.unsetf(std::ios_base::skipws);
                entry e = bdecode(std::istream_iterator<char>(in), std::istream_iterator<char>());
                s.add_torrent(e, "");
                        
                // wait for the user to end
                char a;
                std::cin.unsetf(std::ios_base::skipws);
                std::cin >> a;
        }
        catch (std::exception& e)
        {
                std::cout << e.what() << "\n";
        }
        return 0;
}

file format

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

chat messages

Extension name: "chat"

The payload in the packet is a bencoded dictionary with any combination of the following entries:

"msg"	This is a string that contains a message that should be displayed to the user.
"ctrl"	This is a control string that can tell a client that it is ignored (to make the user aware of that) and it can also tell a client that it is no longer ignored. These notifications are encoded as the strings: "ignored" and "not ignored". Any unrecognized strings should be ignored.

metadata from peers

Extension name: "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:

size	name	description
uint8_t	msg_type	Determines the kind of message this is 0 means 'request metadata'
uint8_t	start	The start of the metadata block that is requested. It is given in 256:ths of the total size of the metadata, since the requesting client don't know the size of the metadata.
uint8_t	size	The size of the metadata block that is requested. This is also given in 256:ths of the total size of the metadata. The size is given as size-1. That means that if this field is set 0, the request wants one 256:th of the metadata.

metadata:

size	name	description
uint8_t	msg_type	1 means 'metadata'
int32_t	total_size	The total size of the metadata, given in number of bytes.
int32_t	offset	The offset of where the metadata block in this message belongs in the final metadata. This is given in bytes.
uint8_t[]	metadata	The actual metadata block. The size of this part is given implicit by the length prefix in the bittorrent protocol packet.

Don't have metadata:

size	name	description
uint8_t	msg_type	2 means 'I don't have metadata'. This message is sent as a reply to a metadata request if the the client doesn't have any metadata.

The current implementation of this extension in libtorrent is experimental, and not optimal in any way.