session

The session class has the following synopsis:

class session: public boost::noncopyable
{
        session(int listen_port, const fingerprint& print);
        session(int listen_port);

        torrent_handle add_torrent(const torrent_info& t, const std::string& save_path);
        void remove_torrent(const torrent_handle& h);

        void set_http_settings(const http_settings& settings);
        void set_upload_rate_limit(int bytes_per_second);
};

Once it's created, it will spawn the main thread that will do all the work. The main thread will be idle as long it doesn't have any torrents to participate in. 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. add_torrent will throw duplicate_torrent exception if the torrent already exists in the session.

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

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 difference between the two constructors is that one of them takes a fingerprint as argument. If this 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.

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).

The destructor of session will notify all trackers that our torrents has been shut down. If some trackers are down, they will timout. 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().

How to parse a torrent file and create a torrent_info object is described below.

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 constructor takes a listen port as argument, if the given port is busy it will increase the port number by one and try again. If it still fails it will continue increasing the port number until it succeeds or has failed 9 ports. This will change in the future to give more control of the listen-port.

parsing torrent files

The torrent files are bencoded. There are two functions in libtorrent that can encode and decode bencoded data. They are:

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

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.

entry

The entry class represents one node in a bencoded hierarchy. It works as a variant type, it can be either a list, a dictionary (std::map), an integer or a string. This is its synopsis:

class entry
{
public:

        typedef std::map<std::string, entry> dictionary_type;
        typedef std::string string_type;
        typedef std::vector<entry> list_type;
        typedef implementation-defined integer_type;

        enum data_type
        {
                int_t,
                string_t,
                list_t,
                dictionary_t,
                undefined_t
        };

        data_type type() const;

        entry();
        entry(data_type t);
        entry(const entry& e);

        void operator=(const entry& e);

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

        void print(std::ostream& os, int indent = 0) const;
};

The integer(), string(), list() and dict() functions are accessorts 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.

torrent_info

The torrent_info has the following synopsis:

class torrent_info
{
public:

        torrent_info(const entry& torrent_file)

        typedef std::vector>file>::const_iterator file_iterator;
        typedef std::vector<file>::const_reverse_iterator reverse_file_iterator;

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

        std::size_t num_files() const;
        const file& file_at(int index) const;

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

        int prioritize_tracker(int index);

        entry::integer_type total_size() const;
        entry::integer_type piece_length() const;
        std::size_t num_pieces() const;
        const sha1_hash& info_hash() const;
        const std::stirng& name() const;
        const std::string& comment() const;
        boost::posiz_time::ptime creation_date() const;


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

        entry::integer_type piece_size(unsigned int index) const;
        const sha1_hash& hash_for_piece(unsigned int index) 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.

struct file
{
        std::string path;
        std::string filename;
        entry::integer_type size;
};

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

The print() function is there for debug purposes only. It will print the info from the torrent file to the given outstream.

name() returns the name of the torrent.

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
{
        std::string url;
        int tier;
};

The prioritize_tracker() is used internally to move a tracker to the front of its tier group. i.e. It will never be moved pass a tracker with a different tier number. For more information about how multiple trackers are dealt with, see the specification.

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() 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.

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.

torrent_handle

You will usually have to store your torrent handles somewhere, since it's the object through which you retrieve infromation about the torrent and aborts the torrent. Its declaration looks like this:

struct torrent_handle
{
        torrent_handle();

        torrent_status status();
        void get_download_queue(std::vector<partial_piece_info>& queue);
        void get_peer_info(std::vector<peer_info>& v);
        const torrent_info& get_torrent_info();
        bool is_valid();

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

        void set_max_uploads(int max_uploads);

        sha1_hash info_hash() const;

        bool operator==(const torrent_handle&) const;
        bool operator!=(const torrent_handle&) const;
        bool operator<(const torrent_handle&) const;
};

The default constructor will initialize the handle to an invalid state. Which means you cannot perform any operation on it, unless you first assign it a valid handle. If you try to perform any operation on an uninitialized handle, it will throw invalid_handle.

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

info_hash() returns the info hash for the torrent.

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.

struct torrent_status
{
        enum state_t
        {
                invalid_handle,
                queued_for_checking,
                checking_files,
                downloading,
                seeding
        };

        state_t state;
        float progress;
        boost::posix_time::time_duration next_announce;
        std::size_t total_download;
        std::size_t total_upload;
        float download_rate;
        float upload_rate;
        std::vector<bool> pieces;
        std::size_t total_done;
};

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];
};

struct peer_info
{
        enum
        {
                interesting = 0x1,
                choked = 0x2,
                remote_interested = 0x4,
                remote_choked = 0x8
        };
        unsigned int flags;
        address ip;
        float up_speed;
        float down_speed;
        unsigned int total_download;
        unsigned int total_upload;
        peer_id id;
        std::vector<bool> pieces;
        int upload_limit;
        int upload_ceiling;

        int load_balancing;

        int downloading_piece_index;
        int downloading_block_index;
        int downloading_progress;
        int downloading_total;
};

address

The address class represents a name of a network endpoint (usually referred to as IP-address) and a port number. This is the same thing as a sockaddr_in would contain. Its declaration looks like this:

class address
{
public:
        address();
        address(unsigned char a
                , unsigned char b
                , unsigned char c
                , unsigned char d
                , unsigned short  port);
        address(unsigned int addr, unsigned short port);
        address(const std::string& addr, unsigned short port);
        address(const address& a);
        ~address();

        std::string as_string() const;
        unsigned int ip() const;
        unsigned short port() const;

        bool operator<(const address& a) const;
        bool operator!=(const address& a) const;
        bool operator==(const address& a) const;
};

It is less-than comparable to make it possible to use it as a key in a map. as_string() may block while it does the DNS lookup, it returns a string that points to the address represented by the object.

ip() will return the 32-bit ip-address as an integer. port() returns the port number.

http_settings

You have some control over tracker requests through the http_settings object. You create it and fill it with your settings and the use session::set_http_settings() to apply them. You have control over proxy and authorization settings and also the user-agent that will be sent to the tracker. The user-agent is a good way to identify your client.

struct http_settings
{
        http_settings();
        std::string proxy_ip;
        int proxy_port;
        std::string proxy_login;
        std::string proxy_password;
        std::string user_agent;
        int tracker_timeout;
        int tracker_maximum_response_length;
};

proxy_ip may be a hostname or ip to a http proxy to use. If this is an empty string, no http proxy will be used.

proxy_port is the port on which the http proxy listens. If proxy_ip is empty, this will be ignored.

proxy_login should be the login username for the http proxy, if this empty, the http proxy will be trid to be used without authentication.

proxy_password the password string for the http proxy.

user_agent this is the client identification to the tracker. It will be followed by the string "(libtorrent)" to identify that this library is being used. This should be set to your client's name and version number.

tracker_timeout is the number of seconds the tracker connection will wait until it considers the tracker to have timed-out. Default value is 10 seconds.

tracker_maximum_response_length is the maximum number of bytes in a tracker response. If a response size passes this number it will be rejected and the connection will be closed. On gzipped responses this size is measured on the uncompressed data. So, if you get 20 bytes of gzip response that'll expand to 2 megs, it will be interrupted before the entire response has been uncompressed (given your limit is lower than 2 megs). Default limit is 1 megabyte.

big_number

Both the peer_id and sha1_hash types are typedefs of the class big_number. It represents 20 bytes of data. Its synopsis follows:

class big_number
{
public:
        bool operator==(const big_number& n) const;
        bool operator!=(const big_number& n) const;
        bool operator<(const big_number& n) const;

        const unsigned char* begin() const;
        const unsigned char* end() const;

        unsigned char* begin();
        unsigned char* end();
};

The iterators gives you access to individual bytes.

hasher

This class creates sha1-hashes. Its declaration looks like this:

class hasher
{
public:
        hasher();

        void update(const char* data, unsigned int len);
        sha1_hash final();
        void reset();
};

You use it by first instantiating it, then call update() to feed it with data. i.e. you don't have to keep the entire buffer of which you want to create the hash in memory. You can feed the hasher parts of it at a time. When You have fed the hasher with all the data, you call final() and it will return the sha1-hash of the data.

If you want to reuse the hasher object once you have created a hash, you have to call reset() to reinitialize it.

The sha1-algorithm used was implemented by Steve Reid and released as public domain. For more info, see src/sha1.c.

fingerprint

The fingerprint class represents information about a client and its version. It is used to encode this information into the client's peer id.

This is the class declaration:

struct fingerprint
{
        fingerprint(const char* id_string, int major, int minor, int revision, int tag);

        std::string to_string() const;

        char id[2];
        char major_version;
        char minor_version;
        char revision_version;
        char tag_version;

};

The constructor takes a const char* that should point to a string constant containing exactly two characters. These are the characters that should be unique for your client. Make sure not to clash with anybody else. Here are some taken id's:

The major, minor, revision and tag parameters are used to identify the version of your client. All these numbers must be within the range [0, 9].

to_string() will generate the actual string put in the peer-id, and return it.

exceptions

There are a number of exceptions that can be thrown from different places in libtorrent, here's a complete list with description.

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

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

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

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

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

example usage

#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;
}

#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(6881);

                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);
                s.add_torrent(t, "");
                        
                // 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;
}