premiere-libtorrent/docs/tutorial.rst

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

:Author: Arvid Norberg, arvid@libtorrent.org
:Version: 1.2.0

.. contents:: Table of contents
  :depth: 2
  :backlinks: none

tutorial
========

The fundamental feature of starting and downloading torrents in libtorrent is
achieved by creating a *session*, which provides the context and a container for
torrents. This is done with via the session_ class, most of its interface is
documented under session_handle_ though.

To add a torrent to the session, you fill in an add_torrent_params_ object and
pass it either to `add_torrent()`_ or `async_add_torrent()`_.

``add_torrent()`` is a blocking call which returns a torrent_handle_.

For example:

.. code:: c++

	#include <libtorrent/session.hpp>
	#include <libtorrent/add_torrent_params.hpp>
	#include <libtorrent/torrent_handle.hpp>

	namespace lt = libtorrent;
	int main(int argc, char const* argv[])
	{
		if (argc != 2) {
			fprintf(stderr, "usage: %s <magnet-url>\n");
			return 1;
		}
		lt::session ses;

		lt::add_torrent_params atp;
		atp.url = argv[1];
		atp.save_path = "."; // save in current dir
		lt::torrent_handle h = ses.add_torrent(atp);

		// ...
	}

Once you have a torrent_handle_, you can affect it as well as querying status.
First, let's extend the example to print out messages from the bittorrent engine
about progress and events happening under the hood. libtorrent has a mechanism
referred to as *alerts* to communicate back information to the client application.

Clients can poll a session for new alerts via the `pop_alerts()`_ call. This
function fills in a vector of alert pointers with all new alerts since the last
call to this function. The pointers are owned by the session object at will
become invalidated by the next call to `pop_alerts()`_.

The alerts form a class hierarchy with alert_ as the root class. Each specific
kind of alert may include additional state, specific to the kind of message. All
alerts implement a message() function that prints out pertinent information
of the alert message. This can be convenient for simply logging events.

For programatically react to certain events, use `alert_cast<>`_ to attempt
a down cast of an alert object to a more specific type.

In order to print out events from libtorrent as well as exiting when the torrent
completes downloading, we can poll the session for alerts periodically and print
them out, as well as listening for the torrent_finished_alert_, which is posted
when a torrent completes.

.. include:: ../examples/bt-get.cpp
	:code: c++
	:tab-width: 2
	:start-after: */

alert masks
-----------

The output from this program will be quite verbose, which is probably a good
starting point to get some understanding of what's going on. Alerts are
categorized into alert categories. Each category can be enabled and disabled
independently via the *alert mask*.

The alert mask is a configuration option offered by libtorrent. There are many
configuration options, see settings_pack_. The alert_mask_ setting is an integer
of the `category flags`_ ORed together.

For instance, to only see the most pertinent alerts, the session can be
constructed like this:

.. code:: c++

	lt::settings_pack pack;
	pack.set_int(lt::settings_pack::alert_mask
		, lt::alert::error_notification
		| lt::alert::storage_notification
		| lt::alert::status_notification);

	lt::session ses(pack);

Configuration options can be updated after the session is started by calling
`apply_settings()`_. Some settings are best set before starting the session
though, like listen_interfaces_, to avoid race conditions. If you start the
session with the default settings and then immediately change them, there will
still be a window where the default settings apply.

Changing the settings may trigger listen sockets to close and re-open and
NAT-PMP, UPnP updates to be sent. For this reason, it's typically a good idea
to batch settings updates into a single call.

session destruction
-------------------

The session destructor is blocking by default. When shutting down, trackers
will need to be contacted to stop torrents and other outstanding operations
need to be cancelled. Shutting down can sometimes take several seconds,
primarily because of trackers that are unresponsive (and time out) and also
DNS servers that are unresponsive. DNS lookups are especially difficult to
abort when stalled.

In order to be able to start destruction an wait for it asynchronously, one
can call `session::abort()`_.

This call returns a session_proxy_ object, which is a handle keeping the session
state alive while destructing it. It deliberately does not provide any of the
session operations, since it's shutting down.

After having a session_proxy_ object, the session destructor does not block.
However, the session_proxy_ destructor *will*.

This can be used to shut down multiple sessions or other parts of the
application in parallel.

asynchronous operations
-----------------------

Essentially any call to a member function of session_ or torrent_handle_ that
returns a value is a blocking synchronous call. Meaning it will post a message
to the main libtorrent thread and wait for a response. Such calls may be
expensive, and in applications where stalls should be avoided (such as user
interface threads), blocking calls should be avoided.

In the example above, session::add_torrent() returns a torrent_handle_ and is
thus blocking. For higher efficiency, `async_add_torrent()`_ will post a message
to the main thread to add a torrent, and post the resulting torrent_handle_ back
in an alert (add_torrent_alert_). This is especially useful when adding a lot
of torrents in quick succession, as there's no stall in between calls.

In the example above, we don't actually use the torrent_handle_ for anything, so
converting it to use `async_add_torrent()`_ is just a matter of replacing the
`add_torrent()`_ call with `async_add_torrent()`_.

torrent_status_updates
----------------------

To get updates to the status of torrents, call `post_torrent_updates()`_ on the
session object. This will cause libtorrent to post a state_update_alert_
containing torrent_status_ objects for all torrents whose status has *changed*
since the last call to `post_torrent_updates()`_.

The state_update_alert_ looks something like this:

.. code:: c++

	struct state_update_alert : alert
	{
		virtual std::string message() const;
		std::vector<torrent_status> status;
	};

The ``status`` field only contains the torrent_status_ for torrents with
updates since the last call. It may be empty if no torrent has updated its
state. This feature is critical for scalability_.

See the torrent_status_ object for more information on what is in there.
Perhaps the most interesting fields are ``total_payload_download``,
``total_payload_upload``, ``num_peers`` and ``state``.

resuming torrents
-----------------

Since bittorrent downloads pieces of files in random order, it's not trivial to
resume a partial download. When resuming a download, the bittorrent engine must
restore the state of the downloading torrent, specifically which parts of the
file(s) are downloaded. There are two approaches to doing this:

1. read every piece of the downloaded files from disk and compare it against its
   expected hash.
2. save to disk the state of which pieces (and partial pieces) are downloaded,
   and load it back in again when resuming.

If no resume data is provided with a torrent that's added, libtorrent will
employ (1) by default.

To save resume data, call `save_resume_data()`_ on the torrent_handle_ object.
This will ask libtorrent to generate the resume data and post it back in
a save_resume_data_alert_. If generating the resume data fails for any reason,
a save_resume_data_failed_alert_ is posted instead.

Exactly one of those alerts will be posted for every call to
`save_resume_data()`_. This is an important property when shutting down a
session with multiple torrents, every resume alert must be handled before
resuming with shut down. Any torrent may fail to save resume data, so the client
would need to keep a count of the outstanding resume files, decremented on
either save_resume_data_alert_ or save_resume_data_failed_alert_.

The save_resume_data_alert_ looks something like this:

.. code:: c++

	struct save_resume_data_alert : torrent_alert
	{
		virtual std::string message() const;

		// points to the resume data.
		std::shared_ptr<entry> resume_data;
	};

``resume_data`` points to an entry_ object. This represents a node or a tree of
nodes in a bencoded_ structure, which is the native encoding scheme in
bittorrent. It can be encoded into a byte buffer or file using `bencode()`_.

When adding a torrent with resume data, set the `add_torrent_params::resume_data`_
to contain the bencoded buffer of the resume data.

example
-------

Here's an updated version of the above example with the following updates:

1. not using blocking calls
2. printing torrent status updates rather than the raw log
3. saving and loading resume files

.. include:: ../examples/bt-get2.cpp
	:code: c++
	:tab-width: 2
	:start-after: */

torrent files
-------------

To add torrent files to a session (as opposed to a magnet link), it must first
be loaded into a torrent_info_ object.

The torrent_info_ object can be created either by filename a buffer or a
bencoded structure. When adding by filename, there's a sanity check limit on the
size of the file, for adding arbitrarily large torrents, load the file outside
of the constructor.

The torrent_info_ object provides an opportunity to query information about the
.torrent file as well as mutating it before adding it to the session.

bencoding
---------

bencoded_ structures is the default data storage format used by bittorrent, such
as .torrent files, tracker announce and scrape responses and some wire protocol
extensions. libtorrent provides an efficient framework for decoding bencoded
data through `bdecode()`_ function.

There are two separate mechanisms for *encoding* and *decoding*. When decoding,
use the `bdecode()`_ function that returns a bdecode_node_. When encoding, use
`bencode()`_ taking an entry_ object.

The key property of `bdecode()`_ is that it does not copy any data out of the
buffer that was parsed. It builds the tree structures of references pointing
into the buffer. The buffer must stay alive and valid for as long as the
bdecode_node_ is in use.

For performance details on `bdecode()`_, see the `blog post about it`__.

__ http://blog.libtorrent.org/2015/03/bdecode-parsers/

.. _session: reference-Core.html#session
.. _session_handle: reference-Core.html#session_handle
.. _add_torrent_params: reference-Core.html#add_torrent_params
.. _`add_torrent()`: reference-Core.html#add_torrent()
.. _`async_add_torrent()`: reference-Core.html#add_torrent()
.. _torrent_handle: reference-Core.html#torrent_handle
.. _`pop_alerts()`: reference-Core.html#pop_alerts()
.. _`alert`: reference-Alerts.html#alert
.. _`alert_cast<>`: reference-Alerts.html#alert_cast()
.. _torrent_finished_alert: reference-Alerts.html#torrent-finished-alert
.. _listen_interfaces: reference-Settings.html#listen_interfaces
.. _`add_torrent_alert`: reference-Alerts.html#add-torrent-alert
.. _settings_pack: reference-Settings.html#settings_pack
.. _alert_mask: reference-Settings.html#alert_mask
.. _`category flags`: reference-Alerts.html#category_t
.. _`apply_settings()`: reference-Core.html#apply_settings()
.. _`session::abort()`: reference-Core.html#abort()
.. _session_proxy: reference-Core.html#session_proxy
.. _`post_torrent_updates()`: reference-Core.html#post_torrent_updates()
.. _torrent_status: reference-Core.html#torrent_status
.. _state_update_alert: reference-Alerts.html#state_update_alert
.. _scalability: http://blog.libtorrent.org/2011/11/scalable-interfaces/
.. _`save_resume_data()`: reference-Core.html#save_resume_data()
.. _save_resume_data_alert: reference-Alerts.html#save_resume_data_alert
.. _save_resume_data_failed_alert: reference-Alerts.html#save_resume_data_failed_alert
.. _bencoded: https://en.wikipedia.org/wiki/Bencode
.. _entry: reference-Bencoding.html#entry
.. _`bencode()`: reference-Bencoding.html#bencode()
.. _torrent_info: reference-Core.html#torrent_info
.. _`add_torrent_params::resume_data`: reference-Core.html#resume_data
.. _`bdecode()`: reference-Bdecoding.html#bdecode()
.. _bdecode_node: reference-Bdecoding.html#bdecode-node