premiere-libtorrent/docs/manual.rst

=================
libtorrent manual
=================

.. contents::

introduction
============

libtorrent is a C++ library that aims to be a good alternative to all the
other bittorrent implementations around. It is a
library and not a full featured client, although it comes with a working
example client.

The main goals of libtorrent are:

	* to be cpu efficient
	* to be memory efficient
	* to be very easy to use

libtorrent is not finished. It is an ongoing project (including this documentation).
The current state includes the following features:

	* multitracker extension support (as `described by John Hoffman`__)
	* serves multiple torrents on a single port and a single thread
	* supports http proxies and proxy authentication
	* gzipped tracker-responses
	* piece picking on block-level like in Azureus_ (as opposed to piece-level).
	* queues torrents for file check, instead of checking all of them in parallel.
	* uses separate threads for checking files and for main downloader, with a fool-proof
	  thread-safe library interface. (i.e. There's no way for the user to cause a deadlock).
	* can limit the upload and download bandwidth usage and the maximum number of unchoked peers
	* piece-wise, unordered, file allocation
	* implements fair trade. User settable trade-ratio, must at least be 1:1,
	  but one can choose to trade 1 for 2 or any other ratio that isn't unfair to the other
	  party. (i.e. real tit for tat)
	* fast resume support, a way to get rid of the costly piece check at the start
	  of a resumed torrent. Saves the storage state, piece_picker state as well as all local
	  peers in a separate fast-resume file.
	* supports the extension protocol `described by Nolar`__. See extensions_.
	* supports files > 2 gigabytes.
	* supports the ``no_peer_id=1`` extension that will ease the load off trackers.
	* supports the `udp-tracker protocol`__ by Olaf van der Spek.
	* possibility to limit the number of connections.
	* delays have messages if there's no other outgoing traffic to the peer, and doesn't
	  send have messages to peers that already has the piece. This saves bandwidth.
	* does not have any requirements on the piece order in a torrent that it resumes. This
	  means it can resume a torrent downloaded by any client.
	* adjusts the length of the request queue depending on download rate.
	* supports the ``compact=1`` tracker parameter.

__ http://home.elp.rr.com/tur/multitracker-spec.txt
.. _Azureus: http://azureus.sourceforge.net
__ http://nolar.com/azureus/extended.htm
__ udp_tracker_protocol.html


Functions that are yet to be implemented:

	* better identification of peers that send bad data
	* ip-filters
	* file-level priority

libtorrent is portable at least among windows, macosx, and other UNIX-systems. It uses Boost.Thread,
Boost.Filesystem, Boost.Date_time and various other boost libraries as well as zlib.

libtorrent has been successfully compiled and tested on:

	* Windows 2000 vc7.1
	* Linux x86 GCC 3.0.4, GCC 3.2.3, GCC 3.4.2
	* MacOS X, GCC 3.3
	* SunOS 5.8 GCC 3.1
	* Cygwin GCC 3.3.3

Fails on:

	* GCC 2.95.4 (``std::ios_base`` is missing)
	* msvc6 sp5

libtorrent is released under the BSD-license_.

.. _BSD-license: http://www.opensource.org/licenses/bsd-license.php


downloading and building
========================

To acquire the latest version of libtorrent, you'll have to grab it from CVS. You'll find instructions
on how to do this here__ (see Anonymous CVS access).

__ http://sourceforge.net/cvs/?group_id=79942

The easiest way to build libtorrent is probably to use `boost-build`_. Make sure you install it
correctly by setting the environment variable ``BOOST_BUILD_PATH`` to point to your boost build
installation. Also you have to modify the ``user_config.jam`` to reflect the toolsets you have installed.
(if you're building with gcc, uncomment the line "using gcc ;")

.. _`boost-build`: http://sourceforge.net/project/showfiles.php?group_id=7586&package_id=80982&release_id=278763

You also need to install boost__ (at least version 1.31.0).

__ http://sourceforge.net/project/showfiles.php?group_id=7586&package_id=8041&release_id=284047

Before you invoke ``bjam`` you have to set the environment variable ``BOOST_ROOT`` to the
path where you installed boost. This will be used to build and link against the required
boost libraries as well as be used as include path for boost headers.

To build you just have to run::

	bjam <toolset> link=static

in the libtorrent directory.

If you're building on a platform where dlls share the same heap, you can build libtorrent
as a dll too, by typing ``link=shared`` instead of ``link=static``.

To build on MacOS X, you need the latest version of boost-build, from the `boost cvs`__.

__ http://sourceforge.net/cvs/?group_id=7586

If you're making your own project file, note that there are two versions of the file
abstraction. There's one ``file_win.cpp`` which relies on windows file API that supports
files larger than 2 Gigabytes. This does not work in vc6 for some reason, possibly because
it may require windows NT and above. The other file, ``file.cpp`` is the default
implementation that simply relies on the standard low level io routines (read, write etc.),
this is the preferred implementation that should be used in all cases. The ``file_win.cpp``
have had some problems with failing seeks (I don't know why), so I advise everyone to use
the other file.


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.

+--------------------------------+-------------------------------------------------+
| macro                          | description                                     |
+================================+=================================================+
| ``NDEBUG``                     | If you define this macro, all asserts,          |
|                                | invariant checks and general debug code will be |
|                                | removed. This option takes precedence over      |
|                                | other debug settings.                           |
+--------------------------------+-------------------------------------------------+
| ``TORRENT_VERBOSE_LOGGING``    | If you define this macro, every peer connection |
|                                | will log its traffic to a log file.             |
|                                | This setting requires a debug build, i.e.       |
|                                | if you set ``NDEBUG`` aswell, no logs will be   |
|                                | written.                                        |
+--------------------------------+-------------------------------------------------+
| ``TORRENT_STORAGE_DEBUG``      | This will enable extra expensive invariant      |
|                                | checks in the storage, including logging of     |
|                                | piece sorting.                                  |
+--------------------------------+-------------------------------------------------+


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.


using
=====

The interface of libtorrent consists of a few classes. The main class is
the ``session``, it contains the main loop that serves all torrents.

The basic usage is as follows:

* conststruct a session
* parse .torrent-files and add them to the session (see `bdecode() bencode()`_)
* main loop (see session_)
	* query the torrent_handles for progress (see torrent_handle_)
	* query the session for information
	* add and remove torrents from the session at run-time
* save resume data for all torrent_handles (optional)
* destruct session object

Each class and function is described in this manual.



session
=======

The ``session`` class has the following synopsis::

	class session: public boost::noncopyable
	{

		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);

		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());

		void remove_torrent(torrent_handle const& h);

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

		void set_http_settings(const http_settings& settings);

		void set_upload_rate_limit(int bytes_per_second);
		void set_download_rate_limit(int bytes_per_second);
		void set_max_uploads(int limit);
		void set_max_connections(int limit);

		session_status status() const;

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


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

Once it's created, the session object 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.

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_.



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::list<std::pair<std::string, entry> > dictionary_type;
		typedef std::string string_type;
		typedef std::list<entry> list_type;
		typedef size_type integer_type;

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

		data_type type() const;

		entry(const dictionary_type&);
		entry(const string_type&);
		entry(const list_type&);
		entry(const integer_type&);

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

		void operator=(const entry& e);
		void operator=(const dictionary_type&);
		void operator=(const string_type&);
		void operator=(const list_type&);
		void operator=(const integer_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;

		// these functions requires that the entry
		// is a dictionary, otherwise they will throw	
		entry& operator[](char const* key);
		entry& operator[](std::string const& key);
		const entry& operator[](char const* key) const;
		const entry& operator[](std::string const& key) const;
		entry* find_key(char const* key);
		entry const* find_key(char const* key) const;
		
		void print(std::ostream& os, int indent = 0) const;
	};

*TODO: finish documentation of entry.*

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.



torrent_info
============

The ``torrent_info`` has the following synopsis::

	class torrent_info
	{
	public:

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

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

		typedef std::vector<file_entry>::const_iterator file_iterator;
		typedef std::vector<file_entry>::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;

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

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

		size_type total_size() const;
		size_type piece_length() const;
		int num_pieces() const;
		const sha1_hash& info_hash() const;
		const std::stirng& name() const;
		const std::string& comment() const;

		boost::optional<boost::posix_time::ptime>
		creation_date() const;


		void print(std::ostream& os) const;
	
		size_type piece_size(unsigned int index) const;
		const sha1_hash& hash_for_piece(unsigned int index) const;
	};


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.

__ http://www.boost.org/libs/date_time/doc/class_ptime.html




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);
		torrent_info const& get_torrent_info();
		bool is_valid();

		entry write_resume_data();
		std::vector<char> const& metadata() const;
		void force_reannounce();
		void connect_peer(address const& adr) const;

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

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

		void set_ratio(float ratio);
		void set_max_uploads(int max_uploads);
		void set_max_connections(int max_connections);
		void set_upload_limit(int limit);
		void set_download_limit(int limit);
		void use_interface(char const* net_interface);

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

		int num_complete() const;
		int num_incomplete() const;

		bool has_metadata() const;

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

		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``.

**TODO: document trackers() and replace_trackers()**
**TODO: document how to create a .torrent**

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).


num_complete() num_incomplete()
-------------------------------

	::

		int num_complete() const;
		int num_incomplete() const;

These members returns the optional scrape data returned by the tracker in the announce response.
If the tracker did not return any scrape data the return value of these functions are -1. Note
that in some cases the tracker can return some scrape data, so there is no guarantee that all
functions returns -1 just because one of them do. ``num_complete()`` is the total number of
seeds in the swarm and ``num_incomplete()`` is the number of downloaders in the swarm.


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.



torrent_status
==============

It contains the following fields::

	struct torrent_status
	{
		enum state_t
		{
			queued_for_checking,
			checking_files,
			connecting_to_tracker,
			downloading,
			seeding
		};
	
		state_t state;
		bool paused;
		float progress;
		boost::posix_time::time_duration next_announce;
		boost::posix_time::time_duration announce_interval;

		std::string current_tracker;

		size_type total_download;
		size_type total_upload;

		size_type total_payload_download;
		size_type total_payload_upload;

		size_type total_failed_bytes;

		float download_rate;
		float upload_rate;

		float download_payload_rate;
		float upload_payload_rate;

		int num_peers;

		const std::vector<bool>* pieces;
		size_type total_done;

		int num_seeds;
		float distributed_copies;

		int block_size;
	};

``progress`` is a value in the range [0, 1], that represents the progress of the
torrent's current task. It may be checking files or downloading. The torrent's
current task is in the ``state`` member, it will be one of the following:

+--------------------------+----------------------------------------------------------+
|``queued_for_checking``   |The torrent is in the queue for being checked. But there  |
|                          |currently is another torrent that are being checked.      |
|                          |This torrent will wait for its turn.                      |
|                          |                                                          |
+--------------------------+----------------------------------------------------------+
|``checking_files``        |The torrent has not started its download yet, and is      |
|                          |currently checking existing files.                        |
|                          |                                                          |
+--------------------------+----------------------------------------------------------+
|``connecting_to_tracker`` |The torrent has sent a request to the tracker and is      |
|                          |currently waiting for a response                          |
|                          |                                                          |
+--------------------------+----------------------------------------------------------+
|``downloading``           |The torrent is being downloaded. This is the state        |
|                          |most torrents will be in most of the time. The progress   |
|                          |meter will tell how much of the files that has been       |
|                          |downloaded.                                               |
|                          |                                                          |
+--------------------------+----------------------------------------------------------+
|``seeding``               |In this state the torrent has finished downloading and    |
|                          |is a pure seeder.                                         |
|                          |                                                          |
+--------------------------+----------------------------------------------------------+

``paused`` is set to true if the torrent is paused and false otherwise.

``next_announce`` is the time until the torrent will announce itself to the tracker. And
``announce_interval`` is the time the tracker want us to wait until we announce ourself
again the next time.

``current_tracker`` is the URL of the last working tracker. If no tracker request has
been successful yet, it's set to an empty string.

``total_download`` and ``total_upload`` is the number of bytes downloaded and
uploaded to all peers, accumulated, *this session* only.

``total_payload_download`` and ``total_payload_upload`` counts the amount of bytes
send and received this session, but only the actual oayload data (i.e the interesting
data), these counters ignore any protocol overhead.

``total_failed_bytes`` is the number of bytes that has been downloaded and that
has failed the piece hash test. In other words, this is just how much crap that
has been downloaded.

``pieces`` is the bitmask that represents which pieces we have (set to true) and
the pieces we don't have. It's a pointer and may be set to 0 if the torrent isn't
downloading or seeding.

``download_rate`` and ``upload_rate`` are the total rates for all peers for this
torrent. These will usually have better precision than summing the rates from
all peers. The rates are given as the number of bytes per second. The
``download_payload_rate`` and ``upload_payload_rate`` respectively is the
total transfer rate of payload only, not counting protocol chatter. This might
be slightly smaller than the other rates, but if projected over a long time
(e.g. when calculating ETA:s) the difference may be noticable.

``num_peers`` is the number of peers this torrent currently is connected to.

``total_done`` is the total number of bytes of the file(s) that we have. All
this does not necessarily has to be downloaded during this session (that's
``total_download_payload``).

``num_seeds`` is the number of peers that are seeding that this client is
currently connected to.

``distributed_copies`` is the number of distributed copies of the torrent.
Note that one copy may be spread out among many peers. The integer part
tells how many copies there are currently of the rarest piece(s) among the
peers this client is connected to. The fractional part tells the share of
pieces that have more copies than the rarest piece(s). For example: 2.5 would
mean that the rarest pieces have only 2 copies among the peers this torrent is
connected to, and that 50% of all the pieces have more than two copies.

``block_size`` is the size of a block, in bytes. A block is a sub piece, it
is the number of bytes that each piece request asks for and the number of
bytes that each bit in the ``partial_piece_info``'s bitset represents
(see `get_download_queue()`_). This is typically 16 kB, but it may be
larger if the pieces are larger.

peer_info
=========

It contains the following fields::

	struct peer_info
	{
		enum
		{
			interesting = 0x1,
			choked = 0x2,
			remote_interested = 0x4,
			remote_choked = 0x8,
			supports_extensions = 0x10,
			local_connection = 0x20
		};
		unsigned int flags;
		address ip;
		float up_speed;
		float down_speed;
		size_type total_download;
		size_type total_upload;
		peer_id id;
		std::vector<bool> pieces;
		bool seed;
		int upload_limit;
		int upload_ceiling;

		size_type load_balancing;

		int download_queue_length;
		int upload_queue_length;

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

The ``flags`` attribute tells you in which state the peer is. It is set to
any combination of the enums above. The following table describes each flag:

+-------------------------+-------------------------------------------------------+
| ``interesting``         | we are interested in pieces from this peer.           |
+-------------------------+-------------------------------------------------------+
| ``choked``              | **we** have choked this peer.                         |
+-------------------------+-------------------------------------------------------+
| ``remote_interested``   | means the same thing but that the peer is interested  |
| ``remote_choked``       | in pieces from us and the peer has choked **us**.     |
+-------------------------+-------------------------------------------------------+
| ``support_extensions``  | means that this peer supports the                     |
|                         | `extension protocol`__.                               |
+-------------------------+-------------------------------------------------------+
| ``local_connection``    | The connection was initiated by us, the peer has a    |
|                         | listen port open, and that port is the same is in the |
|                         | address_ of this peer. If this flag is not set, this  |
|                         | peer connection was opened by this peer connecting to |
|                         | us.                                                   |
+-------------------------+-------------------------------------------------------+

__ http://nolar.com/azureus/extended.htm

The ``ip`` field is the IP-address to this peer. Its type is a wrapper around the
actual address and the port number. See address_ class.

``up_speed`` and ``down_speed`` is the current upload and download speed
we have to and from this peer. These figures are updated aproximately once every second.

``total_download`` and ``total_upload`` are the total number of bytes downloaded
from and uploaded to this peer. These numbers do not include the protocol chatter, but only
the payload data.

``id`` is the peer's id as used in the bit torrent protocol. This id can be used to
extract 'fingerprints' from the peer. Sometimes it can tell you which client the peer
is using. See identify_client()_

``pieces`` is a vector of booleans that has as many entries as there are pieces
in the torrent. Each boolean tells you if the peer has that piece (if it's set to true)
or if the peer miss that piece (set to false).

``seed`` is true if this peer is a seed.

``upload_limit`` is the number of bytes per second we are allowed to send to this
peer every second. It may be -1 if there's no limit. The upload limits of all peers
should sum up to the upload limit set by ``session::set_upload_limit``.

``upload_ceiling`` is the current maximum allowed upload rate given the cownload
rate and share ratio. If the global upload rate is inlimited, the ``upload_limit``
for every peer will be the same as their ``upload_ceiling``.

``load_balancing`` is a measurment of the balancing of free download (that we get)
and free upload that we give. Every peer gets a certain amount of free upload, but
this member says how much *extra* free upload this peer has got. If it is a negative
number it means that this was a peer from which we have got this amount of free
download.

``download_queue_length`` is the number of piece-requests we have sent to this peer
that hasn't been answered with a piece yet.

``upload_queue_length`` is the number of piece-requests we have received from this peer
that we haven't answered with a piece yet.

You can know which piece, and which part of that piece, that is currently being
downloaded from a specific peer by looking at the next four members.
``downloading_piece_index`` is the index of the piece that is currently being downloaded.
This may be set to -1 if there's currently no piece downloading from this peer. If it is
>= 0, the other three members are valid. ``downloading_block_index`` is the index of the
block (or sub-piece) that is being downloaded. ``downloading_progress`` is the number
of bytes of this block we have received from the peer, and ``downloading_total`` is
the total number of bytes in this block.



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 then 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.cpp``.


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 ``char const*`` 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:

+----------+-----------------------+
| id chars | client                |
+==========+=======================+
| 'AZ'     | Azureus               |
+----------+-----------------------+
| 'LT'     | libtorrent (default)  |
+----------+-----------------------+
| 'BX'     | BittorrentX           |
+----------+-----------------------+
| 'MT'     | Moonlight Torrent     |
+----------+-----------------------+
| 'TS'     | Torrent Storm         |
+----------+-----------------------+
| 'SS'     | Swarm Scope           |
+----------+-----------------------+
| 'XT'     | Xan Torrent           |
+----------+-----------------------+


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.


free functions
==============

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.

.. _bencoded: http://wiki.theory.org/index.php/BitTorrentSpecification

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);

.. _InputIterator: http://www.sgi.com/tech/stl/InputIterator.html
.. _OutputIterator: http://www.sgi.com/tech/stl/OutputIterator.html
.. _ostream_iterator: http://www.sgi.com/tech/stl/ostream_iterator.html
.. _back_insert_iterator: http://www.sgi.com/tech/stl/back_insert_iterator.html
.. _istream_iterator: http://www.sgi.com/tech/stl/istream_iterator.html

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_.



alerts
======

The ``pop_alert()`` function on session is the interface for retrieving
alerts, warnings, messages and errors from libtorrent. If there hasn't
occured any errors (matching your severity level) ``pop_alert()`` will
return a zero pointer. If there has been some error, it will return a pointer
to an alert object describing it. You can then use the alert object and query
it for information about the error or message. To retrieve any alerts, you have
to select a severity level using ``session::set_severity_level()``. It defaults to
``alert::none``, which means that you don't get any messages at all, ever.
You have the following levels to select among:

+--------------+----------------------------------------------------------+
| ``none``     | No alert will ever have this severity level, which       |
|              | effectively filters all messages.                        |
|              |                                                          |
+--------------+----------------------------------------------------------+
| ``fatal``    | Fatal errors will have this severity level. Examples can |
|              | be disk full or something else that will make it         |
|              | impossible to continue normal execution.                 |
|              |                                                          |
+--------------+----------------------------------------------------------+
| ``critical`` | Signals errors that requires user interaction or         |
|              | messages that almost never should be ignored. For        |
|              | example, a chat message received from another peer is    |
|              | announced as severity ``critical``.                      |
|              |                                                          |
+--------------+----------------------------------------------------------+
| ``warning``  | Messages with the warning severity can be a tracker that |
|              | times out or responds with invalid data. It will be      |
|              | retried automatically, and the possible next tracker in  |
|              | a multitracker sequence will be tried. It does not       |
|              | require any user interaction.                            |
|              |                                                          |
+--------------+----------------------------------------------------------+
| ``info``     | Events that can be considered normal, but still deserves |
|              | an event. This could be a piece hash that fails.         |
|              |                                                          |
+--------------+----------------------------------------------------------+
| ``debug``    | This will include alot of debug events that can be used  |
|              | both for debugging libtorrent but also when debugging    |
|              | other clients that are connected to libtorrent. It will  |
|              | report strange behaviors among the connected peers.      |
|              |                                                          |
+--------------+----------------------------------------------------------+

When setting a severity level, you will receive messages of that severity and all
messages that are more sever. If you set ``alert::none`` (the default) you will not recieve
any events at all.

When you set a severuty level other than ``none``, you have the responsibility to call
``pop_alert()`` from time to time. If you don't do that, the alert queue will just grow.

When you get an alert, you can use ``typeid()`` or ``dynamic_cast<>`` to get more detailed
information on exactly which type it is. i.e. what kind of error it is. You can also use a
dispatcher_ mechanism that's available in libtorrent.

All alert types are defined in the ``<libtorrent/alert_types.hpp>`` header file.

The ``alert`` class is the base class that specific messages are derived from. This
is its synopsis::

	class alert
	{
	public:

		enum severity_t { debug, info, warning, critital, fatal, none };

		alert(severity_t severity, const std::string& msg);
		virtual ~alert();

		std::string const& msg() const;
		severity_t severity() const;

		virtual std::auto_ptr<alert> clone() const = 0;
	};

This means that all alerts have at least a string describing it. They also
have a severity level that can be used to sort them or present them to the
user in different ways.

The specific alerts, that all derives from ``alert``, are:


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. ``<EFBFBD>p`` 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;
	};



.. chat_message_alert
	------------------

	This alert is generated when you receive a chat message from another peer. Chat messages
	are supported as an extension ("chat"). It is generated as severity level ``critical``,
	even though it doesn't necessarily require any user intervention, it's high priority
	since you would almost never want to ignore such a message. The alert class contain
	a torrent_handle_ to the torrent in which the sender-peer is a member and the ip
	of the sending peer.

	::

		struct chat_message_alert: alert
		{
			chat_message_alert(const torrent_handle& h
				, const address& sender
				, const std::string& msg);
	
			virtual std::auto_ptr<alert> clone() const;
	
			torrent_handle handle;
			address ip;
		};


dispatcher
----------

*TODO: describe the dispatcher mechanism*



exceptions
==========

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


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();
	};


examples
========

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;
	}


fast resume
===========

The fast resume mechanism is a way to remember which pieces are downloaded and where they
are put between sessions. You can generate fast resume data by calling
``torrent_handle::write_resume_data()`` on torrent_handle_. You can then save this data
to disk and use it when resuming the torrent. libtorrent will not check the piece hashes
then, and rely on the information given in the fast-resume data. The fast-resume data
also contains information about which blocks, in the unfinished pieces, were downloaded,
so it will not have to start from scratch on the partially downloaded pieces.

To use the fast-resume data you simply give it to `add_torrent()`_, and it
will skip the time consuming checks. It may have to do the checking anyway, if the
fast-resume data is corrupt or doesn't fit the storage for that torrent, then it will
not trust the fast-resume data and just do the checking.

file format
-----------

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

+----------------------+--------------------------------------------------------------+
| ``file-format``      | string: "libtorrent resume file"                             |
|                      |                                                              |
+----------------------+--------------------------------------------------------------+
| ``file-version``     | integer: 1                                                   |
|                      |                                                              |
+----------------------+--------------------------------------------------------------+
| ``info-hash``        | string, the info hash of the torrent this data is saved for. |
|                      |                                                              |
+----------------------+--------------------------------------------------------------+
| ``blocks per piece`` | integer, the number of blocks per piece. Must be: piece_size |
|                      | / (16 * 1024). Clamped to be within the range [1, 256]. It   |
|                      | is the number of blocks per (normal sized) piece. Usually    |
|                      | each block is 16 * 1024 bytes in size. But if piece size is  |
|                      | greater than 4 megabytes, the block size will increase.      |
|                      |                                                              |
+----------------------+--------------------------------------------------------------+
| ``slots``            | list of integers. The list mappes slots to piece indices. It |
|                      | tells which piece is on which slot. If piece index is -2 it  |
|                      | means it is free, that there's no piece there. If it is -1,  |
|                      | means the slot isn't allocated on disk yet. The pieces have  |
|                      | to meet the following requirement:                           |
|                      |                                                              |
|                      | If there's a slot at the position of the piece index,        |
|                      | the piece must be located in that slot.                      |
|                      |                                                              |
+----------------------+--------------------------------------------------------------+
| ``peers``            | list of dictionaries. Each dictionary has the following      |
|                      | layout:                                                      |
|                      |                                                              |
|                      | +----------+-----------------------------------------------+ |
|                      | | ``ip``   | string, the ip address of the peer.           | |
|                      | +----------+-----------------------------------------------+ |
|                      | | ``port`` | integer, the listen port of the peer          | |
|                      | +----------+-----------------------------------------------+ |
|                      |                                                              |
|                      | These are the local peers we were connected to when this     |
|                      | fast-resume data was saved.                                  |
|                      |                                                              |
+----------------------+--------------------------------------------------------------+
| ``unfinished``       | list of dictionaries. Each dictionary represents an          |
|                      | piece, and has the following layout:                         |
|                      |                                                              |
|                      | +-------------+--------------------------------------------+ |
|                      | | ``piece``   | integer, the index of the piece this entry | |
|                      | |             | refers to.                                 | |
|                      | +-------------+--------------------------------------------+ |
|                      | | ``bitmask`` | string, a binary bitmask representing the  | |
|                      | |             | blocks that have been downloaded in this   | |
|                      | |             | piece.                                     | |
|                      | +-------------+--------------------------------------------+ |
|                      | | ``adler32`` | The adler32 checksum of the data in the    | |
|                      | |             | blocks specified by ``bitmask``.           | |
|                      | |             |                                            | |
|                      | +-------------+--------------------------------------------+ |
|                      |                                                              |
+----------------------+--------------------------------------------------------------+


extensions
==========

These extensions all operates within the `extension protocol`__. The
name of the extension is the name used in the extension-list packets,
and the payload is the data in the extended message (not counting the
length-prefix, message-id nor extension-id).

__ http://nolar.com/azureus/extended.html

Note that since this protocol relies on one of the reserved bits in the
handshake, it may be incompatible with future versions of the mainline
bittorrent client.

These are the extensions that are currently implemented.

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.


filename checks
===============

Boost.Filesystem will by default check all its paths to make sure they conform
to filename requirements on many platforms. If you don't want this check, you can
set it to either only check for native filesystem requirements or turn it off
alltogether. You can use::

	boost::filesystem::path::default_name_check(boost::filesystem::native);

for example. For more information, see the `Boost.Filesystem docs`__.

__ http://www.boost.org/libs/filesystem/doc/index.htm


acknowledgements
================

Written by Arvid Norberg. Copyright (c) 2003

Contributions by Magnus Jonsson and Daniel Wallin

Thanks to Reimond Retz for bugfixes, suggestions and testing

Thanks to `University of Ume<6D>`__ for providing development and
test hardware.

Project is hosted by sourceforge.

|sf_logo|__

__ http://www.cs.umu.se
.. |sf_logo| image:: http://sourceforge.net/sflogo.php?group_id=7994
__ http://sourceforge.net