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# Rasterbar Software

## Software developement and consulting

# libtorrent API Documentation

Author: | Arvid Norberg, arvid@rasterbar.com
Version: | 0.16.7

Table of contents

- overview
- things to keep in mind
- network primitives
- session
- entry
- torrent_info
- torrent_handle
- torrent_status
- peer_info
- feed_handle
- feed_item
- session customization
- pe_settings
- proxy_settings
- ip_filter
- big_number
- bitfield
- hasher
- fingerprint
- UPnP and NAT-PMP
- free functions
- alerts
- alert dispatcher
- exceptions
- error_code
- storage_interface
- magnet links
- queuing
- fast resume
- storage allocation
- extensions
- piece picker
- SSL torrents

# overview

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:

- construct a session

- load session state from settings file (see load_state() save_state())

- start extensions (see add_extension()).

- start DHT, LSD, UPnP, NAT-PMP etc (see start_dht() stop_dht() set_dht_settings() dht_state() is_dht_running() start_lsd() stop_lsd(), start_upnp() stop_upnp() and start_natpmp() stop_natpmp())

- parse .torrent-files and add them to the session (see bdecode() bencode() and async_add_torrent() add_torrent())

- 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, see save_resume_data())

- save session state (see load_state() save_state())

- destruct session object

Each class and function is described in this manual.

For a description on how to create torrent files, see make_torrent.

# things to keep in mind

A common problem developers are facing is torrents stopping without explanation. Here is a description on which conditions libtorrent will stop your torrents, how to find out about it and what to do about it.

Make sure to keep track of the paused state, the error state and the upload mode of your torrents. By default, torrents are auto-managed, which means libtorrent will pause them, unpause them, scrape them and take them out of upload-mode automatically.

Whenever a torrent encounters a fatal error, it will be stopped, and the `torrent_status::error` will describe the error that caused it. If a torrent is auto managed, it is scraped periodically and paused or resumed based on the number of downloaders per seed. This will effectively seed torrents that are in the greatest need of seeds.

If a torrent hits a disk write error, it will be put into upload mode. This means it will not download anything, but only upload. The assumption is that the write error is caused by a full disk or write permission errors. If the torrent is auto-managed, it will periodically be taken out of the upload mode, trying to write things to the disk again. This means torrent will recover from certain disk errors if the problem is resolved. If the torrent is not auto managed, you have to call set_upload_mode() to turn downloading back on again.

# network primitives

There are a few typedefs in the `libtorrent` namespace which pulls in network types from the `asio` namespace. These are:

    typedef asio::ip::address address;
    typedef asio::ip::address_v4 address_v4;
    typedef asio::ip::address_v6 address_v6;
    using asio::ip::tcp;
    using asio::ip::udp;

These are declared in the `<libtorrent/socket.hpp>` header.

The `using` statements will give easy access to:

    tcp::endpoint
    udp::endpoint

Which are the endpoint types used in libtorrent. An endpoint is an address with an associated port.

For documentation on these types, please refer to the asio documentation.

# session

The `session` class has the following synopsis:

    class session: public boost::noncopyable
    {

            session(fingerprint const& print
                    = libtorrent::fingerprint(
                    "LT", 0, 1, 0, 0)
                    , int flags = start_default_features
                            | add_default_plugins
                    , int alert_mask = alert::error_notification);

            session(
                    fingerprint const& print
                    , std::pair<int, int> listen_port_range
                    , char const* listen_interface = 0
                    , int flags = start_default_features
                            | add_default_plugins
                    , int alert_mask = alert::error_notification);

            enum save_state_flags_t
            {
                    save_settings = 0x001,
                    save_dht_settings = 0x002,
                    save_dht_state = 0x004,
                    save_proxy = 0x008,
                    save_i2p_proxy = 0x010,
                    save_encryption_settings = 0x020,
                    save_as_map = 0x040,
                    save_feeds = 0x080,
            };

            void load_state(lazy_entry const& e);
            void save_state(entry& e, boost::uint32_t flags) const;

            torrent_handle add_torrent(
                    add_torrent_params const& params);
            torrent_handle add_torrent(
                    add_torrent_params const& params
                    , error_code& ec);

            void async_add_torrent(add_torrent_params const& params);

            void pause();
            void resume();

            session_proxy abort();

            enum options_t
            {
                    none = 0,
                    delete_files = 1
            };

            enum session_flags_t
            {
                    add_default_plugins = 1,
                    start_default_features = 2
            };

            void remove_torrent(torrent_handle const& h
                    , int options = none);
            torrent_handle find_torrent(sha_hash const& ih);

            std::vector<torrent_handle> get_torrents() const;
            void get_torrent_status(std::vector<torrent_status>* ret
                    , boost::function<bool(torrent_status const&)> const& pred
                    , boost::uint32_t flags = 0) const;
            void refresh_torrent_status(std::vector<torrent_status>* ret
                    , boost::uint32_t flags) const;

            void set_settings(session_settings const& settings);
            session_settings settings() const;
            void set_pe_settings(pe_settings const& settings);

            void set_upload_rate_limit(int bytes_per_second);
            int upload_rate_limit() const;
            void set_download_rate_limit(int bytes_per_second);
            int download_rate_limit() const;

            void set_local_upload_rate_limit(int bytes_per_second);
            int local_upload_rate_limit() const;
            void set_local_download_rate_limit(int bytes_per_second);
            int local_download_rate_limit() const;

            void set_max_uploads(int limit);
            void set_max_connections(int limit);
            int max_connections() const;
            void set_max_half_open_connections(int limit);
            int max_half_open_connections() const;

            void set_proxy(proxy_settings const& s);
            proxy_settings proxy() const;

            int num_uploads() const;
            int num_connections() const;

            void load_asnum_db(char const* file);
            void load_asnum_db(wchar_t const* file);
            void load_country_db(char const* file);
            void load_country_db(wchar_t const* file);
            int as_for_ip(address const& adr);

            void set_ip_filter(ip_filter const& f);
            ip_filter get_ip_filter() const;

            session_status status() const;
            cache_status get_cache_status() const;

            bool is_listening() const;
            unsigned short listen_port() const;

            enum {
                    listen_reuse_address = 1,
                    listen_no_system_port = 2
            };

            void listen_on(
                    std::pair<int, int> const& port_range
                    , error_code& ec
                    , char const* interface = 0
                    , int flags = 0);

            std::auto_ptr<alert> pop_alert();
            alert const* wait_for_alert(time_duration max_wait);
            void set_alert_mask(int m);
            size_t set_alert_queue_size_limit(
                    size_t queue_size_limit_);
            void set_alert_dispatch(boost::function<void(std::auto_ptr<alert>)> const& fun);

            feed_handle session::add_feed(feed_settings const& feed);
            void session::remove_feed(feed_handle h);
            void session::get_feeds(std::vector<feed_handle>& f) const;

            void add_extension(boost::function<
                    boost::shared_ptr<torrent_plugin>(torrent*)> ext);

            void start_dht();
            void stop_dht();
            void set_dht_settings(
                    dht_settings const& settings);
            entry dht_state() const;
            void add_dht_node(std::pair<std::string
                    , int> const& node);
            void add_dht_router(std::pair<std::string
                    , int> const& node);
            bool is_dht_running() const;

            void start_lsd();
            void stop_lsd();

            upnp* start_upnp();
            void stop_upnp();

            natpmp* start_natpmp();
            void stop_natpmp();
    };

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(fingerprint const& print
>             = libtorrent::fingerprint("LT", 0, 1, 0, 0)
>             , int flags = start_default_features
>                     | add_default_plugins
>             , int alert_mask = alert::error_notification);

>     session(fingerprint const& print
>             , std::pair<int, int> listen_port_range
>             , char const* listen_interface = 0
>             , int flags = start_default_features
>                     | add_default_plugins
>             , int alert_mask = alert::error_notification);

If the fingerprint in the first overload is omited, the client will get a default fingerprint stating the version of libtorrent. The fingerprint is a short string that will be used in the peer-id to identify the client and the client's version. For more details see the fingerprint class. The constructor that only takes a fingerprint will not open a listen port for the session, to get it running you'll have to call `session::listen_on()`. The other constructor, that takes a port range and an interface as well as the fingerprint will automatically try to listen on a port on the given interface. For more information about the parameters, see `listen_on()` function.

The flags paramater can be used to start default features (upnp & nat-pmp) and default plugins (ut_metadata, ut_pex and smart_ban). The default is to start those things. If you do not want them to start, pass 0 as the flags parameter.

The `alert_mask` is the same mask that you would send to set_alert_mask().

## ~session()

The destructor of session will notify all trackers that our torrents have been shut down. If some trackers are down, they will time out. All this before the destructor of session returns. So, it's advised that any kind of interface (such as windows) are closed before destructing the session object. Because it can take a few second for it to finish. The timeout can be set with `set_settings()`.

## load_state() save_state()

>     void load_state(lazy_entry const& e);
>     void save_state(entry& e, boost::uint32_t flags) const;

loads and saves all session settings, including dht_settings, encryption settings and proxy settings. `save_state` writes all keys to the `entry` that's passed in, which needs to either not be initialized, or initialized as a dictionary.

`load_state` expects a `lazy_entry` which can be built from a bencoded buffer with lazy_bdecode().

The `flags` arguments passed in to `save_state` can be used to filter which parts of the session state to save. By default, all state is saved (except for the individual torrents). These are the possible flags. A flag that's set, means those settings are saved:

    enum save_state_flags_t
    {
            save_settings =     0x001,
            save_dht_settings = 0x002,
            save_dht_state =    0x004,
            save_proxy =        0x008,
            save_i2p_proxy =    0x010,
            save_encryption_settings = 0x020,
            save_as_map =       0x040,
            save_feeds =        0x080
    };

## pause() resume() is_paused()

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

Pausing the session has the same effect as pausing every torrent in it, except that torrents will not be resumed by the auto-manage mechanism. Resuming will restore the torrents to their previous paused state. i.e. the session pause state is separate from the torrent pause state. A torrent is inactive if it is paused or if the session is paused.

## abort()

>     session_proxy abort();

In case you want to destruct the session asynchrounously, you can request a session destruction proxy. If you don't do this, the destructor of the session object will block while the trackers are contacted. If you keep one `session_proxy` to the session when destructing it, the destructor will not block, but start to close down the session, the destructor of the proxy will then synchronize the threads. So, the destruction of the session is performed from the `session` destructor call until the `session_proxy` destructor call. The `session_proxy` does not have any operations on it (since the session is being closed down, no operations are allowed on it). The only valid operation is calling the destructor:

    class session_proxy
    {
    public:
            session_proxy();
            ~session_proxy()
    };

## async_add_torrent() add_torrent()

>     typedef boost::function<storage_interface*(file_storage const&
>             , file_storage const*, std::string const&, file_pool&
>             , std::vector<boost::uint8_t> const&) storage_constructor_type;

>     struct add_torrent_params
>     {
>             add_torrent_params(storage_constructor_type s);

>             enum flags_t
>             {
>                     flag_seed_mode = 0x001,
>                     flag_override_resume_data = 0x002,
>                     flag_upload_mode = 0x004,
>                     flag_share_mode = 0x008,
>                     flag_apply_ip_filter = 0x010,
>                     flag_paused = 0x020,
>                     flag_auto_managed = 0x040.
>                     flag_duplicate_is_error = 0x080,
>                     flag_merge_resume_trackers = 0x100,
>                     flag_update_subscribe = 0x200
>             };

>             int version;
>             boost::intrusive_ptr<torrent_info> ti;
>     #ifndef TORRENT_NO_DEPRECATE
>             char const* tracker_url;
>     #endif
>             std::vector<std::string> trackers;
>             std::vector<std::pair<std::string, int> > dht_nodes;
>             sha1_hash info_hash;
>             std::string name;
>             std::string save_path;
>             std::vector<char>* resume_data;
>             storage_mode_t storage_mode;
>             storage_constructor_type storage;
>             void* userdata;
>             std::vector<boost::uint8_t> const* file_priorities;
>             std::string trackerid;
>             std::string url;
>             std::string uuid;
>             std::string source_feed_url;
>             boost::uint64_t flags;
>     };

>     torrent_handle add_torrent(add_torrent_params const& params);
>     torrent_handle add_torrent(add_torrent_params const& params
>             , error_code& ec);
>     void async_add_torrent(add_torrent_params const& params);

You add torrents through the `add_torrent()` function where you give an object with all the parameters. The `add_torrent()` overloads will block until the torrent has been added (or failed to be added) and returns an error code and a `torrent_handle`. In order to add torrents more efficiently, consider using `async_add_torrent()` which returns immediately, without waiting for the torrent to add. Notification of the torrent being added is sent as add_torrent_alert.

The overload that does not take an `error_code` throws an exception on error and is not available when building without exception support.

The only mandatory parameters are `save_path` which is the directory where you want the files to be saved. You also need to specify either the `ti` (the torrent file), the `info_hash` (the info hash of the torrent) or the `url` (the URL to where to download the .torrent file from). If you specify the info-hash, the torrent file will be downloaded from peers, which requires them to support the metadata extension. For the metadata extension to work, libtorrent must be built with extensions enabled (`TORRENT_DISABLE_EXTENSIONS` must not be defined). It also takes an optional `name` argument. This may be left empty in case no name should be assigned to the torrent. In case it's not, the name is used for the torrent as long as it doesn't have metadata. See `torrent_handle::name`.

If the torrent doesn't have a tracker, but relies on the DHT to find peers, the `trackers` (or the deprecated `tracker_url`) can specify tracker urls that for the torrent.

If you specify a `url`, the torrent will be set in `downloading_metadata` state until the .torrent file has been downloaded. If there's any error while downloading, the torrent will be stopped and the torrent error state (`torrent_status::error`) will indicate what went wrong. The `url` may refer to a magnet link or a regular http URL.

`dht_nodes` is a list of hostname and port pairs, representing DHT nodes to be added to the session (if DHT is enabled). The hostname may be an IP address.

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 libtorrent_exception which derives from `std::exception` unless `duplicate_is_error` is set to false. In that case, `add_torrent` will return the handle to the existing torrent.

The optional parameter, `resume_data` can be given if up to date fast-resume data is available. The fast-resume data can be acquired from a running torrent by calling save_resume_data() on torrent_handle. See fast resume. The `vector` that is passed in will be swapped into the running torrent instance with `std::vector::swap()`.

The `storage_mode` parameter refers to the layout of the storage for this torrent. There are 3 different modes:

storage_mode_sparse
  All pieces will be written to the place where they belong and sparse files will be used. This is the recommended, and default mode.
storage_mode_allocate
  All pieces will be written to their final position, all files will be allocated in full when the torrent is first started. This is done with `fallocate()` and similar calls. This mode minimizes fragmentation.
storage_mode_compact
  this mode is deprecated and will be removed in future versions of libtorrent The storage will grow as more pieces are downloaded, and pieces are rearranged to finally be in their correct places once the entire torrent has been downloaded.

For more information, see storage allocation.

`storage` can be used to customize how the data is stored. The default storage will simply write the data to the files it belongs to, but it could be overridden to save everything to a single file at a specific location or encrypt the content on disk for instance. For more information about the `storage_interface` that needs to be implemented for a custom storage, see storage_interface.

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

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.

`file_priorities` can be set to control the initial file priorities when adding a torrent. The semantics are the same as for `torrent_handle::prioritize_files()`.

`version` is filled in by the constructor and should be left untouched. It is used for forward binary compatibility.

`trackerid` is the default tracker id to be used when announcing to trackers. By default this is empty, and no tracker ID is used, since this is an optional argument. If a tracker returns a tracker ID, that ID is used instead of this.

if `uuid` is specified, it is used to find duplicates. If another torrent is already running with the same UUID as the one being added, it will be considered a duplicate. This is mainly useful for RSS feed items which has UUIDs specified.

`source_feed_url` should point to the URL of the RSS feed this torrent comes from, if it comes from an RSS feed.

`flags` is a 64 bit integer used for flags controlling aspects of this torrent and how it's added. These are the flags:

    enum flags_t
    {
            flag_seed_mode = 0x001,
            flag_override_resume_data = 0x002,
            flag_upload_mode = 0x004,
            flag_share_mode = 0x008,
            flag_apply_ip_filter = 0x010,
            flag_paused = 0x020,
            flag_auto_managed = 0x040.
            flag_duplicate_is_error = 0x080,
            flag_merge_resume_trackers = 0x100,
            flag_update_subscribe = 0x200
    }

`flag_apply_ip_filter` determines if the IP filter should apply to this torrent or not. By default all torrents are subject to filtering by the IP filter (i.e. this flag is set by default). This is useful if certain torrents needs to be excempt for some reason, being an auto-update torrent for instance.

`flag_merge_resume_trackers` defaults to off and specifies whether tracker URLs loaded from resume data should be added to the trackers in the torrent or replace the trackers.

`flag_update_subscribe` is on by default and means that this torrent will be part of state updates when calling post_torrent_updates().

`flag_paused` specifies whether or not the torrent is to be started in a paused state. I.e. it won't connect to the tracker or any of the peers until it's resumed. This is typically a good way of avoiding race conditions when setting configuration options on torrents before starting them.

If you pass in resume data, the paused state of the torrent when the resume data was saved will override the paused state you pass in here. You can override this by setting `flag_override_resume_data`.

If the torrent is auto-managed (`flag_auto_managed`), the torrent may be resumed at any point, regardless of how it paused. If it's important to manually control when the torrent is paused and resumed, don't make it auto managed.

If `flag_auto_managed` is set, the torrent will be queued, started and seeded automatically by libtorrent. When this is set, the torrent should also be started as paused. The default queue order is the order the torrents were added. They are all downloaded in that order. For more details, see queuing.

If you pass in resume data, the auto_managed state of the torrent when the resume data was saved will override the auto_managed state you pass in here. You can override this by setting `override_resume_data`.

If `flag_seed_mode` is set, libtorrent will assume that all files are present for this torrent and that they all match the hashes in the torrent file. Each time a peer requests to download a block, the piece is verified against the hash, unless it has been verified already. If a hash fails, the torrent will automatically leave the seed mode and recheck all the files. The use case for this mode is if a torrent is created and seeded, or if the user already know that the files are complete, this is a way to avoid the initial file checks, and significantly reduce the startup time.

Setting `flag_seed_mode` on a torrent without metadata (a .torrent file) is a no-op and will be ignored.

If resume data is passed in with this torrent, the seed mode saved in there will override the seed mode you set here.

If `flag_override_resume_data` is set, the `paused` and `auto_managed` state of the torrent are not loaded from the resume data, but the states requested by the flags in `add_torrent_params` will override them.

If `flag_upload_mode` is set, the torrent will be initialized in upload-mode, which means it will not make any piece requests. This state is typically entered on disk I/O errors, and if the torrent is also auto managed, it will be taken out of this state periodically. This mode can be used to avoid race conditions when adjusting priorities of pieces before allowing the torrent to start downloading.

If the torrent is auto-managed (`flag_auto_managed`), the torrent will eventually be taken out of upload-mode, regardless of how it got there. If it's important to manually control when the torrent leaves upload mode, don't make it auto managed.

`flag_share_mode` determines if the torrent should be added in share mode or not. Share mode indicates that we are not interested in downloading the torrent, but merlely want to improve our share ratio (i.e. increase it). A torrent started in share mode will do its best to never download more than it uploads to the swarm. If the swarm does not have enough demand for upload capacity, the torrent will not download anything. This mode is intended to be safe to add any number of torrents to, without manual screening, without the risk of downloading more than is uploaded.

A torrent in share mode sets the priority to all pieces to 0, except for the pieces that are downloaded, when pieces are decided to be downloaded. This affects the progress bar, which might be set to "100% finished" most of the time. Do not change file or piece priorities for torrents in share mode, it will make it not work.

The share mode has one setting, the share ratio target, see `session_settings::share_mode_target` for more info.

## remove_torrent()

>     void remove_torrent(torrent_handle const& h, int options = none);

`remove_torrent()` will close all peer connections associated with the torrent and tell the tracker that we've stopped participating in the swarm. The optional second argument `options` can be used to delete all the files downloaded by this torrent. To do this, pass in the value `session::delete_files`. The removal of the torrent is asyncronous, there is no guarantee that adding the same torrent immediately after it was removed will not throw a libtorrent_exception exception. Once the torrent is deleted, a torrent_deleted_alert is posted.

## find_torrent() get_torrents()

>     torrent_handle find_torrent(sha_hash const& ih);
>     std::vector<torrent_handle> get_torrents() const;

`find_torrent()` looks for a torrent with the given info-hash. In case there is such a torrent in the session, a torrent_handle to that torrent is returned. In case the torrent cannot be found, an invalid torrent_handle is returned.

See `torrent_handle::is_valid()` to know if the torrent was found or not.

`get_torrents()` returns a vector of torrent_handles to all the torrents currently in the session.

## get_torrent_status() refresh_torrent_status()

>     void get_torrent_status(std::vector<torrent_status>* ret
>             , boost::function<bool(torrent_status const&)> const& pred
>             , boost::uint32_t flags = 0) const;
>     void refresh_torrent_status(std::vector<torrent_status>* ret
>             , boost::uint32_t flags = 0) const;

Note

these calls are potentially expensive and won't scale well with lots of torrents. If you're concerned about performance, consider using `post_torrent_updates()` instead.

`get_torrent_status` returns a vector of the `torrent_status` for every torrent which satisfies `pred`, which is a predicate function which determines if a torrent should be included in the returned set or not. Returning true means it should be included and false means excluded. The `flags` argument is the same as to `torrent_handle::status()`. Since `pred` is guaranteed to be called for every torrent, it may be used to count the number of torrents of different categories as well.

`refresh_torrent_status` takes a vector of `torrent_status` structs (for instance the same vector that was returned by `get_torrent_status()`) and refreshes the status based on the `handle` member. It is possible to use this function by first setting up a vector of default constructed `torrent_status` objects, only initializing the `handle` member, in order to request the torrent status for multiple torrents in a single call. This can save a significant amount of time if you have a lot of torrents.

Any `torrent_status` object whose `handle` member is not referring to a valid torrent are ignored.

## post_torrent_updates()

>     void post_torrent_updates();

This functions instructs the session to post the state_update_alert, containing the status of all torrents whose state changed since the last time this function was called.

Only torrents who has the state subscription flag set will be included. This flag is on by default. See `add_torrent_params` under async_add_torrent() add_torrent().

## load_asnum_db() load_country_db() as_for_ip()

>     void load_asnum_db(char const* file);
>     void load_asnum_db(wchar_t const* file);
>     void load_country_db(char const* file);
>     void load_country_db(wchar_t const* file);
>     int as_for_ip(address const& adr);

These functions are not available if `TORRENT_DISABLE_GEO_IP` is defined. They expects a path to the MaxMind ASN database and MaxMind GeoIP database respectively. This will be used to look up which AS and country peers belong to.

`as_for_ip` returns the AS number for the IP address specified. If the IP is not in the database or the ASN database is not loaded, 0 is returned.

The `wchar_t` overloads are for wide character paths.

## set_ip_filter()

>     void set_ip_filter(ip_filter const& filter);

Sets a filter that will be used to reject and accept incoming as well as outgoing connections based on their originating ip address. The default filter will allow connections to any ip address. To build a set of rules for which addresses are accepted and not, see ip_filter.

Each time a peer is blocked because of the IP filter, a peer_blocked_alert is generated.

## get_ip_filter()

>     ip_filter get_ip_filter() const;

Returns the ip_filter currently in the session. See ip_filter.

## status()

>     session_status status() const;

`status()` returns session wide-statistics and status. The `session_status` struct has the following members:

    struct dht_lookup
    {
            char const* type;
            int outstanding_requests;
            int timeouts;
            int responses;
            int branch_factor;
            int nodes_left;
            int last_sent;
            int first_timeout;
    };

    struct dht_routing_bucket
    {
            int num_nodes;
            int num_replacements;
            int last_active;
    };

    struct utp_status
    {
            int num_idle;
            int num_syn_sent;
            int num_connected;
            int num_fin_sent;
            int num_close_wait;
    };

    struct session_status
    {
            bool has_incoming_connections;

            int upload_rate;
            int download_rate;
            size_type total_download;
            size_type total_upload;

            int payload_upload_rate;
            int payload_download_rate;
            size_type total_payload_download;
            size_type total_payload_upload;

            int ip_overhead_upload_rate;
            int ip_overhead_download_rate;
            size_type total_ip_overhead_download;
            size_type total_ip_overhead_upload;

            int dht_upload_rate;
            int dht_download_rate;
            size_type total_dht_download;
            size_type total_dht_upload;

            int tracker_upload_rate;
            int tracker_download_rate;
            size_type total_tracker_download;
            size_type total_tracker_upload;

            size_type total_redundant_bytes;
            size_type total_failed_bytes;

            int num_peers;
            int num_unchoked;
            int allowed_upload_slots;

            int up_bandwidth_queue;
            int down_bandwidth_queue;

            int up_bandwidth_bytes_queue;
            int down_bandwidth_bytes_queue;

            int optimistic_unchoke_counter;
            int unchoke_counter;

            int disk_write_queue;
            int disk_read_queue;

            int dht_nodes;
            int dht_node_cache;
            int dht_torrents;
            size_type dht_global_nodes;
            std::vector<dht_lookup> active_requests;
            std::vector<dht_routing_table> dht_routing_table;
            int dht_total_allocations;

            utp_status utp_stats;
    };

`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` are the total download and upload rates accumulated from all torrents. This includes bittorrent protocol, DHT and an estimated TCP/IP protocol overhead.

`total_download` and `total_upload` are the total number of bytes downloaded and uploaded to and from all torrents. This also includes all the protocol overhead.

`payload_download_rate` and `payload_upload_rate` is the rate of the payload down- and upload only.

`total_payload_download` and `total_payload_upload` is the total transfers of payload only. The payload does not include the bittorrent protocol overhead, but only parts of the actual files to be downloaded.

`ip_overhead_upload_rate`, `ip_overhead_download_rate`, `total_ip_overhead_download` and `total_ip_overhead_upload` is the estimated TCP/IP overhead in each direction.

`dht_upload_rate`, `dht_download_rate`, `total_dht_download` and `total_dht_upload` is the DHT bandwidth usage.

`total_redundant_bytes` is the number of bytes that has been received more than once. This can happen if a request from a peer times out and is requested from a different peer, and then received again from the first one. To make this lower, increase the `request_timeout` and the `piece_timeout` in the session settings.

`total_failed_bytes` is the number of bytes that was downloaded which later failed the hash-check.

`num_peers` is the total number of peer connections this session has. This includes incoming connections that still hasn't sent their handshake or outgoing connections that still hasn't completed the TCP connection. This number may be slightly higher than the sum of all peers of all torrents because the incoming connections may not be assigned a torrent yet.

`num_unchoked` is the current number of unchoked peers. `allowed_upload_slots` is the current allowed number of unchoked peers.

`up_bandwidth_queue` and `down_bandwidth_queue` are the number of peers that are waiting for more bandwidth quota from the torrent rate limiter. `up_bandwidth_bytes_queue` and `down_bandwidth_bytes_queue` count the number of bytes the connections are waiting for to be able to send and receive.

`optimistic_unchoke_counter` and `unchoke_counter` tells the number of seconds until the next optimistic unchoke change and the start of the next unchoke interval. These numbers may be reset prematurely if a peer that is unchoked disconnects or becomes notinterested.

`disk_write_queue` and `disk_read_queue` are the number of peers currently waiting on a disk write or disk read to complete before it receives or sends any more data on the socket. It'a a metric of how disk bound you are.

`dht_nodes`, `dht_node_cache` and `dht_torrents` are only available when built with DHT support. They are all set to 0 if the DHT isn't running. When the DHT is running, `dht_nodes` is set to the number of nodes in the routing table. This number only includes active nodes, not cache nodes. The `dht_node_cache` is set to the number of nodes in the node cache. These nodes are used to replace the regular nodes in the routing table in case any of them becomes unresponsive.

`dht_torrents` are the number of torrents tracked by the DHT at the moment.

`dht_global_nodes` is an estimation of the total number of nodes in the DHT network.

`active_requests` is a vector of the currently running DHT lookups.

`dht_routing_table` contains information about every bucket in the DHT routing table.

`dht_total_allocations` is the number of nodes allocated dynamically for a particular DHT lookup. This represents roughly the amount of memory used by the DHT.

`utp_stats` contains statistics on the uTP sockets.

## get_cache_status()

>     cache_status get_cache_status() const;

Returns status of the disk cache for this session.

>     struct cache_status
>     {
>             size_type blocks_written;
>             size_type writes;
>             size_type blocks_read;
>             size_type blocks_read_hit;
>             size_type reads;
>             int cache_size;
>             int read_cache_size;
>             int total_used_buffers;
>             int average_queue_time;
>             int average_read_time;
>             int average_write_time;
>             int average_hash_time;
>             int average_cache_time;
>             int job_queue_length;
>     };

`blocks_written` is the total number of 16 KiB blocks written to disk since this session was started.

`writes` is the total number of write operations performed since this session was started.

The ratio (`blocks_written` - `writes`) / `blocks_written` represents the number of saved write operations per total write operations. i.e. a kind of cache hit ratio for the write cahe.

`blocks_read` is the number of blocks that were requested from the bittorrent engine (from peers), that were served from disk or cache.

`blocks_read_hit` is the number of blocks that were served from cache.

The ratio `blocks_read_hit` / `blocks_read` is the cache hit ratio for the read cache.

`cache_size` is the number of 16 KiB blocks currently in the disk cache. This includes both read and write cache.

`read_cache_size` is the number of 16KiB blocks in the read cache.

`total_used_buffers` is the total number of buffers currently in use. This includes the read/write disk cache as well as send and receive buffers used in peer connections.

`average_queue_time` is the number of microseconds an average disk I/O job has to wait in the job queue before it get processed.

`average_read_time` is the time read jobs takes on average to complete (not including the time in the queue), in microseconds. This only measures read cache misses.

`average_write_time` is the time write jobs takes to complete, on average, in microseconds. This does not include the time the job sits in the disk job queue or in the write cache, only blocks that are flushed to disk.

`average_hash_time` is the time hash jobs takes to complete on average, in microseconds. Hash jobs include running SHA-1 on the data (which for the most part is done incrementally) and sometimes reading back parts of the piece. It also includes checking files without valid resume data.

`average_cache_time` is the average amuount of time spent evicting cached blocks that have expired from the disk cache.

`job_queue_length` is the number of jobs in the job queue.

## get_cache_info()

>     void get_cache_info(sha1_hash const& ih
>             , std::vector<cached_piece_info>& ret) const;

`get_cache_info()` fills out the supplied vector with information for each piece that is currently in the disk cache for the torrent with the specified info-hash (`ih`).

>     struct cached_piece_info
>     {
>             int piece;
>             std::vector<bool> blocks;
>             ptime last_use;
>             enum kind_t { read_cache = 0, write_cache = 1 };
>             kind_t kind;
>     };

`piece` is the piece index for this cache entry.

`blocks` has one entry for each block in this piece. `true` represents the data for that block being in the disk cache and `false` means it's not.

`last_use` is the time when a block was last written to this piece. The older a piece is, the more likely it is to be flushed to disk.

`kind` specifies if this piece is part of the read cache or the write cache.

## is_listening() listen_port() listen_on()

>     bool is_listening() const;
>     unsigned short listen_port() const;

>     enum {
>             listen_reuse_address = 1,
>             listen_no_system_port = 2
>     };

>     void listen_on(
>             std::pair<int, int> const& port_range
>             , error_code& ec
>             , char const* interface = 0
>             , int flags = 0);

`is_listening()` will tell you whether or not the session has successfully opened a listening port. If it hasn't, this function will return false, and then you can use `listen_on()` to make another attempt.

`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 the error code of the operation in `ec`. If this indicates success, the session is listening on a port within the specified range. If it fails, it will also generate an appropriate alert (listen_failed_alert).

If all ports in the specified range fails to be opened for listening, libtorrent will try to use port 0 (which tells the operating system to pick a port that's free). If that still fails you may see a listen_failed_alert with port 0 even if you didn't ask to listen on it.

It is possible to prevent libtorrent from binding to port 0 by passing in the flag `session::no_system_port` in the `flags` argument.

If you don't specify an interface, libtorrent may attempt to listen on multiple interfaces (typically 0.0.0.0 and ::). This means that if your IPv6 interface doesn't work, you may still see a listen_failed_alert, even though the IPv4 port succeeded.

The `flags` parameter can either be 0 or `session::listen_reuse_address`, which will set the reuse address socket option on the listen socket(s). By default, the listen socket does not use reuse address. If you're running a service that needs to run on a specific port no matter if it's in use, set this flag.

If you're also starting the DHT, it is a good idea to do that after you've called `listen_on()`, since the default listen port for the DHT is the same as the tcp listen socket. If you start the DHT first, it will assume the tcp port is free and open the udp socket on that port, then later, when `listen_on()` is called, it may turn out that the tcp port is in use. That results in the DHT and the bittorrent socket listening on different ports. If the DHT is active when `listen_on` is called, the udp port will be rebound to the new port, if it was configured to use the same port as the tcp socket, and if the listen_on call failed to bind to the same port that the udp uses.

If you want the OS to pick a port for you, pass in 0 as both first and second.

The reason why it's a good idea to run the DHT and the bittorrent socket on the same port is because that is an assumption that may be used to increase performance. One way to accelerate the connecting of peers on windows may be to first ping all peers with a DHT ping packet, and connect to those that responds first. On windows one can only connect to a few peers at a time because of a built in limitation (in XP Service pack 2).

## set_alert_mask()

>     void set_alert_mask(int m);

Changes the mask of which alerts to receive. By default only errors are reported. `m` is a bitmask where each bit represents a category of alerts.

See alerts for mor information on the alert categories.

## pop_alerts() pop_alert() wait_for_alert()

>     std::auto_ptr<alert> pop_alert();
>     void pop_alerts(std::deque<alert*>* alerts);
>     alert const* wait_for_alert(time_duration max_wait);

`pop_alert()` is used to ask the session if any errors or events has occurred. With set_alert_mask() you can filter which alerts to receive through `pop_alert()`. For information about the alert categories, see alerts.

`pop_alerts()` pops all pending alerts in a single call. In high performance environments with a very high alert churn rate, this can save significant amount of time compared to popping alerts one at a time. Each call requires one round-trip to the network thread. If alerts are produced in a higher rate than they can be popped (when popped one at a time) it's easy to get stuck in an infinite loop, trying to drain the alert queue. Popping the entire queue at once avoids this problem.

However, the `pop_alerts` function comes with significantly more responsibility. You pass in an empty `std::dequeue<alert*>` to it. If it's not empty, all elements in it will be deleted and then cleared. All currently pending alerts are returned by being swapped into the passed in container. The responsibility of deleting the alerts is transferred to the caller. This means you need to call delete for each item in the returned dequeue. It's probably a good idea to delete the alerts as you handle them, to save one extra pass over the dequeue.

Alternatively, you can pass in the same container the next time you call `pop_alerts`.

`wait_for_alert` blocks until an alert is available, or for no more than `max_wait` time. If `wait_for_alert` returns because of the time-out, and no alerts are available, it returns 0. If at least one alert was generated, a pointer to that alert is returned. The alert is not popped, any subsequent calls to `wait_for_alert` will return the same pointer until the alert is popped by calling `pop_alert`. This is useful for leaving any alert dispatching mechanism independent of this blocking call, the dispatcher can be called and it can pop the alert independently.

In the python binding, `wait_for_alert` takes the number of milliseconds to wait as an integer.

To control the max number of alerts that's queued by the session, see `session_settings::alert_queue_size`.

`save_resume_data_alert` and `save_resume_data_failed_alert` are always posted, regardelss of the alert mask.

## set_alert_dispatch()

>     void set_alert_dispatch(boost::function<void(std::auto_ptr<alert>)> const& fun);

This sets a function to be called (from within libtorrent's netowrk thread) every time an alert is posted. Since the function (`fun`) is run in libtorrent's internal thread, it may not call any of libtorrent's external API functions. Doing so results in a dead lock.

The main intention with this function is to support integration with platform-dependent message queues or signalling systems. For instance, on windows, one could post a message to an HNWD or on linux, write to a pipe or an eventfd.

## add_feed()

>     feed_handle session::add_feed(feed_settings const& feed);

This adds an RSS feed to the session. The feed will be refreshed regularly and optionally add all torrents from the feed, as they appear. The feed is defined by the `feed_settings` object:

    struct feed_settings
    {
            feed_settings();

    std::string url;
            bool auto_download;
            bool auto_map_handles;
            int default_ttl;
            add_torrent_params add_args;
    };

By default `auto_download` is true, which means all torrents in the feed will be downloaded. Set this to false in order to manually add torrents to the session. You may react to the rss_alert when a feed has been updated to poll it for the new items in the feed when adding torrents manually. When torrents are added automatically, an add_torrent_alert is posted which includes the torrent handle as well as the error code if it failed to be added. You may also call `session::get_torrents()` to get the handles to the new torrents.

Before adding the feed, you must set the `url` field to the feed's url. It may point to an RSS or an atom feed.

`auto_map_handles` defaults to true and determines whether or not to set the `handle` field in the `feed_item`, returned as the feed status. If auto-download is enabled, this setting is ignored. If auto-download is not set, setting this to false will save one pass through all the feed items trying to find corresponding torrents in the session.

The `default_ttl` is the default interval for refreshing a feed. This may be overridden by the feed itself (by specifying the `<ttl>` tag) and defaults to 30 minutes. The field specifies the number of minutes between refreshes.

If torrents are added automatically, you may want to set the `add_args` to appropriate values for download directory etc. This object is used as a template for adding torrents from feeds, but some torrent specific fields will be overridden by the individual torrent being added. For more information on the `add_torrent_params`, see async_add_torrent() add_torrent().

The returned feed_handle is a handle which is used to interact with the feed, things like forcing a refresh or querying for information about the items in the feed. For more information, see feed_handle.

## remove_feed()

>     void session::remove_feed(feed_handle h);

Removes a feed from being watched by the session. When this call returns, the feed handle is invalid and won't refer to any feed.

## get_feeds()

>     void session::get_feeds(std::vector<feed_handle>& f) const;

Returns a list of all RSS feeds that are being watched by the session.

## add_extension()

>     void add_extension(boost::function<
>             boost::shared_ptr<torrent_plugin>(torrent*, void*)> ext);

This function adds an extension to this session. The argument is a function object that is called with a `torrent*` and which should return a `boost::shared_ptr<torrent_plugin>`. To write custom plugins, see libtorrent plugins. For the typical bittorrent client all of these extensions should be added. The main plugins implemented in libtorrent are:

metadata extension
  Allows peers to download the metadata (.torren files) from the swarm directly. Makes it possible to join a swarm with just a tracker and info-hash.

    #include <libtorrent/extensions/metadata_transfer.hpp>
    ses.add_extension(&libtorrent::create_metadata_plugin);

uTorrent metadata
  Same as `metadata extension` but compatible with uTorrent.

    #include <libtorrent/extensions/ut_metadata.hpp>
    ses.add_extension(&libtorrent::create_ut_metadata_plugin);

uTorrent peer exchange
  Exchanges peers between clients.

    #include <libtorrent/extensions/ut_pex.hpp>
    ses.add_extension(&libtorrent::create_ut_pex_plugin);

smart ban plugin
  A plugin that, with a small overhead, can ban peers that sends bad data with very high accuracy. Should eliminate most problems on poisoned torrents.

    #include <libtorrent/extensions/smart_ban.hpp>
    ses.add_extension(&libtorrent::create_smart_ban_plugin);

## set_settings() set_pe_settings()

>     void set_settings(session_settings const& settings);
>     void set_pe_settings(pe_settings const& settings);

Sets the session settings and the packet encryption settings respectively. See session_settings and pe_settings for more information on available options.

## set_proxy() proxy()

>     void set_proxy(proxy_settings const& s);
>     proxy_setting proxy() const;

These functions sets and queries the proxy settings to be used for the session.

For more information on what settings are available for proxies, see proxy_settings.

## set_i2p_proxy() i2p_proxy()

>     void set_i2p_proxy(proxy_settings const&);
>     proxy_settings const& i2p_proxy();

`set_i2p_proxy` sets the i2p proxy, and tries to open a persistant connection to it. The only used fields in the proxy settings structs are `hostname` and `port`.

`i2p_proxy` returns the current i2p proxy in use.

## start_dht() stop_dht() set_dht_settings() dht_state() is_dht_running()

>     void start_dht(entry const& startup_state);
>     void stop_dht();
>     void set_dht_settings(dht_settings const& settings);
>     entry dht_state() const;
>     bool is_dht_running() const;

These functions are not available in case `TORRENT_DISABLE_DHT` is defined. `start_dht` starts the dht node and makes the trackerless service available to torrents. The startup state is optional and can contain nodes and the node id from the previous session. The dht node state is a bencoded dictionary with the following entries:

`nodes`
  A list of strings, where each string is a node endpoint encoded in binary. If the string is 6 bytes long, it is an IPv4 address of 4 bytes, encoded in network byte order (big endian), followed by a 2 byte port number (also network byte order). If the string is 18 bytes long, it is 16 bytes of IPv6 address followed by a 2 bytes port number (also network byte order).
`node-id`
  The node id written as a readable string as a hexadecimal number.

`dht_state` will return the current state of the dht node, this can be used to start up the node again, passing this entry to `start_dht`. It is a good idea to save this to disk when the session is closed, and read it up again when starting.

If the port the DHT is supposed to listen on is already in use, and exception is thrown, `asio::error`.

`stop_dht` stops the dht node.

`add_dht_node` adds a node to the routing table. This can be used if your client has its own source of bootstrapping nodes.

`set_dht_settings` sets some parameters availavle to the dht node. The struct has the following members:

    struct dht_settings
    {
            int max_peers_reply;
            int search_branching;
            int max_fail_count;
            int max_torrents;
            bool restrict_routing_ips;
            bool restrict_search_ips;
    };

`max_peers_reply` is the maximum number of peers the node will send in response to a `get_peers` message from another node.

`search_branching` is the number of concurrent search request the node will send when announcing and refreshing the routing table. This parameter is called alpha in the kademlia paper.

`max_fail_count` is the maximum number of failed tries to contact a node before it is removed from the routing table. If there are known working nodes that are ready to replace a failing node, it will be replaced immediately, this limit is only used to clear out nodes that don't have any node that can replace them.

`max_torrents` is the total number of torrents to track from the DHT. This is simply an upper limit to make sure malicious DHT nodes cannot make us allocate an unbounded amount of memory.

`max_feed_items` is the total number of feed items to store from the DHT. This is simply an upper limit to make sure malicious DHT nodes cannot make us allocate an unbounded amount of memory.

`restrict_routing_ips` determines if the routing table entries should restrict entries to one per IP. This defaults to true, which helps mitigate some attacks on the DHT. It prevents adding multiple nodes with IPs with a very close CIDR distance.

`restrict_search_ips` determines if DHT searches should prevent adding nodes with IPs with very close CIDR distance. This also defaults to true and helps mitigate certain attacks on the DHT.

The `dht_settings` struct used to contain a `service_port` member to control which port the DHT would listen on and send messages from. This field is deprecated and ignored. libtorrent always tries to open the UDP socket on the same port as the TCP socket.

`is_dht_running()` returns true if the DHT support has been started and false otherwise.

## add_dht_node() add_dht_router()

>     void add_dht_node(std::pair<std::string, int> const& node);
>     void add_dht_router(std::pair<std::string, int> const& node);

`add_dht_node` takes a host name and port pair. That endpoint will be pinged, and if a valid DHT reply is received, the node will be added to the routing table.

`add_dht_router` adds the given endpoint to a list of DHT router nodes. If a search is ever made while the routing table is empty, those nodes will be used as backups. Nodes in the router node list will also never be added to the regular routing table, which effectively means they are only used for bootstrapping, to keep the load off them.

An example routing node that you could typically add is `router.bittorrent.com`.

## start_lsd() stop_lsd()

>     void start_lsd();
>     void stop_lsd();

Starts and stops Local Service Discovery. This service will broadcast the infohashes of all the non-private torrents on the local network to look for peers on the same swarm within multicast reach.

It is turned off by default.

## start_upnp() stop_upnp()

>     upnp* start_upnp();
>     void stop_upnp();

Starts and stops the UPnP service. When started, the listen port and the DHT port are attempted to be forwarded on local UPnP router devices.

The upnp object returned by `start_upnp()` can be used to add and remove arbitrary port mappings. Mapping status is returned through the portmap_alert and the portmap_error_alert. The object will be valid until `stop_upnp()` is called. See UPnP and NAT-PMP.

It is off by default.

## start_natpmp() stop_natpmp()

>     natpmp* start_natpmp();
>     void stop_natpmp();

Starts and stops the NAT-PMP service. When started, the listen port and the DHT port are attempted to be forwarded on the router through NAT-PMP.

The natpmp object returned by `start_natpmp()` can be used to add and remove arbitrary port mappings. Mapping status is returned through the portmap_alert and the portmap_error_alert. The object will be valid until `stop_natpmp()` is called. See UPnP and NAT-PMP.

It is off by default.

# entry

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

    class entry
    {
    public:

            typedef std::map<std::string, entry> dictionary_type;
            typedef std::string string_type;
            typedef std::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(dictionary_type const&);
            entry(string_type const&);
            entry(list_type const&);
            entry(integer_type const&);

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

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

            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);
            entry const& operator[](char const* key) const;
            entry const& 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 respective type. If the `entry` object isn't of the type you request, the accessor will throw libtorrent_exception (which derives from `std::runtime_error`). You can ask an `entry` for its type through the `type()` function.

The `print()` function is there for debug purposes only.

If you want to create an `entry` you give it the type you want it to have in its constructor, and then use one of the non-const accessors to get a reference which you then can assign the value you want it to have.

The typical code to get info from a torrent file will then look like this:

    entry torrent_file;
    // ...

    // throws if this is not a dictionary
    entry::dictionary_type const& dict = torrent_file.dict();
    entry::dictionary_type::const_iterator i;
    i = dict.find("announce");
    if (i != dict.end())
    {
            std::string tracker_url = i->second.string();
            std::cout << tracker_url << "\n";
    }

The following code is equivalent, but a little bit shorter:

    entry torrent_file;
    // ...

    // throws if this is not a dictionary
    if (entry* i = torrent_file.find_key("announce"))
    {
            std::string tracker_url = i->string();
            std::cout << tracker_url << "\n";
    }

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

## operator[]

>     entry& operator[](char const* key);
>     entry& operator[](std::string const& key);
>     entry const& operator[](char const* key) const;
>     entry const& operator[](std::string const& key) const;

All of these functions requires the entry to be a dictionary, if it isn't they will throw `libtorrent::type_error`.

The non-const versions of the `operator[]` will return a reference to either the existing element at the given key or, if there is no element with the given key, a reference to a newly inserted element at that key.

The const version of `operator[]` will only return a reference to an existing element at the given key. If the key is not found, it will throw `libtorrent::type_error`.

## find_key()

>     entry* find_key(char const* key);
>     entry const* find_key(char const* key) const;

These functions requires the entry to be a dictionary, if it isn't they will throw `libtorrent::type_error`.

They will look for an element at the given key in the dictionary, if the element cannot be found, they will return 0. If an element with the given key is found, the return a pointer to it.

# torrent_info

In previous versions of libtorrent, this class was also used for creating torrent files. This functionality has been moved to `create_torrent`, see make_torrent.

The `torrent_info` has the following synopsis:

    class torrent_info
    {
    public:

            // these constructors throws exceptions on error
            torrent_info(sha1_hash const& info_hash, int flags = 0);
            torrent_info(lazy_entry const& torrent_file, int flags = 0);
            torrent_info(char const* buffer, int size, int flags = 0);
            torrent_info(std::string const& filename, int flags = 0);
            torrent_info(std::wstring const& filename, int flags = 0);

            // these constructors sets the error code on error
            torrent_info(sha1_hash const& info_hash, error_code& ec, int flags = 0);
            torrent_info(lazy_entry const& torrent_file, error_code& ec, int flags = 0);
            torrent_info(char const* buffer, int size, error_code& ec, int flags = 0);
            torrent_info(fs::path const& filename, error_code& ec, int flags = 0);
            torrent_info(fs::wpath const& filename, error_code& ec, int flags = 0);

            void add_tracker(std::string const& url, int tier = 0);
            std::vector<announce_entry> const& trackers() const;

            file_storage const& files() const;
            file_storage const& orig_files() const;

            void remap_files(file_storage const& f);

            void rename_file(int index, std::string const& new_filename);
            void rename_file(int index, std::wstring const& new_filename);

            typedef file_storage::iterator file_iterator;
            typedef file_storage::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<file_slice> map_block(int piece, size_type offset
                    , int size) const;
            peer_request map_file(int file_index, size_type file_offset
                    , int size) const;

            bool priv() const;

            void add_url_seed(std::string const& url);
            void add_http_seed(std::string const& url);
            std::vector<web_seed_entry> const& web_seeds() const;

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

            std::vector<std::pair<std::string, int> > const& nodes() const;
            void add_node(std::pair<std::string, int> const& node);

            boost::optional<time_t> creation_date() const;

            int piece_size(unsigned int index) const;
            sha1_hash const& hash_for_piece(unsigned int index) const;
            char const* hash_for_piece_ptr(unsigned int index) const;

            std::vector<sha1_hash> const& merkle_tree() const;
            void set_merkle_tree(std::vector<sha1_hash>& h);

            boost::shared_array<char> metadata() const;
            int metadata_size() const;
    };

## torrent_info()

>     torrent_info(sha1_hash const& info_hash, int flags = 0);
>     torrent_info(lazy_entry const& torrent_file, int flags = 0);
>     torrent_info(char const* buffer, int size, int flags = 0);
>     torrent_info(std::string const& filename, int flags = 0);
>     torrent_info(std::wstring const& filename, int flags = 0);

>     torrent_info(sha1_hash const& info_hash, error_code& ec, int flags = 0);
>     torrent_info(lazy_entry const& torrent_file, error_code& ec, int flags = 0);
>     torrent_info(char const* buffer, int size, error_code& ec, int flags = 0);
>     torrent_info(fs::path const& filename, error_code& ec, int flags = 0);
>     torrent_info(fs::wpath const& filename, error_code& ec, int flags = 0);

The constructor that takes an info-hash will initialize the info-hash to the given value, but leave all other fields empty. This is used internally when downloading torrents without the metadata. The metadata will be created by libtorrent as soon as it has been downloaded from the swarm.

The constructor that takes a `lazy_entry` will create a `torrent_info` object from the information found in the given torrent_file. The `lazy_entry` represents a tree node in an bencoded file. To load an ordinary .torrent file into a `lazy_entry`, use lazy_bdecode().

The version that takes a buffer pointer and a size will decode it as a .torrent file and initialize the torrent_info object for you.

The version that takes a filename will simply load the torrent file and decode it inside the constructor, for convenience. This might not be the most suitable for applications that want to be able to report detailed errors on what might go wrong.

The overloads that takes an `error_code const&` never throws if an error occur, they will simply set the error code to describe what went wrong and not fully initialize the torrent_info object. The overloads that do not take the extra error_code parameter will always throw if an error occurs. These overloads are not available when building without exception support.

The `flags` argument is currently unused.

## add_tracker()

>     void add_tracker(std::string const& url, int tier = 0);

`add_tracker()` adds a tracker to the announce-list. The `tier` determines the order in which the trackers are to be tried. For more information see trackers().

## files() orig_files()

>     file_storage const& files() const;
>     file_storage const& orig_files() const;

The `file_storage` object contains the information on how to map the pieces to files. It is separated from the `torrent_info` object because when creating torrents a storage object needs to be created without having a torrent file. When renaming files in a storage, the storage needs to make its own copy of the `file_storage` in order to make its mapping differ from the one in the torrent file.

`orig_files()` returns the original (unmodified) file storage for this torrent. This is used by the web server connection, which needs to request files with the original names. Filename may be chaged using `torrent_info::rename_file()`.

For more information on the `file_storage` object, see the separate document on how to create torrents.

## remap_files()

>     void remap_files(file_storage const& f);

Remaps the file storage to a new file layout. This can be used to, for instance, download all data in a torrent to a single file, or to a number of fixed size sector aligned files, regardless of the number and sizes of the files in the torrent.

The new specified `file_storage` must have the exact same size as the current one.

## rename_file()

>     void rename_file(int index, std::string const& new_filename);
>     void rename_file(int index, std::wstring const& new_filename);

Renames a the file with the specified index to the new name. The new filename is reflected by the `file_storage` returned by `files()` but not by the one returned by `orig_files()`.

If you want to rename the base name of the torrent (for a multifile torrent), you can copy the `file_storage` (see files() orig_files()), change the name, and then use remap_files().

## 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 `internal_file_entry`, which is an internal type.

You can resolve it into the public representation of a file (`file_entry`) using the `file_storage::at` function, which takes an index and an iterator;

    struct file_entry
    {
            std::string path;
            size_type offset;
            size_type size;
            size_type file_base;
            time_t mtime;
            sha1_hash filehash;
            bool pad_file:1;
            bool hidden_attribute:1;
            bool executable_attribute:1;
            bool symlink_attribute:1;
    };

The `path` is the full path of this file. The paths are unicode strings encoded in UTF-8.

`size` is the size of the file (in bytes) and `offset` is the byte offset of the file within the torrent. i.e. the sum of all the sizes of the files before it in the list.

`file_base` is the offset in the file where the storage should start. The normal case is to have this set to 0, so that the storage starts saving data at the start if the file. In cases where multiple files are mapped into the same file though, the `file_base` should be set to an offset so that the different regions do not overlap. This is used when mapping "unselected" files into a so-called part file.

`mtime` is the modification time of this file specified in posix time.

`symlink_path` is the path which this is a symlink to, or empty if this is not a symlink. This field is only used if the `symlink_attribute` is set.

`filehash` is a sha-1 hash of the content of the file, or zeroes, if no file hash was present in the torrent file. It can be used to potentially find alternative sources for the file.

`pad_file` is set to true for files that are not part of the data of the torrent. They are just there to make sure the next file is aligned to a particular byte offset or piece boundry. These files should typically be hidden from an end user. They are not written to disk.

`hidden_attribute` is true if the file was marked as hidden (on windows).

`executable_attribute` is true if the file was marked as executable (posix)

`symlink_attribute` is true if the file was a symlink. If this is the case the `symlink_index` refers to a string which specifies the original location where the data for this file was found.

## num_files() file_at()

>     int num_files() const;
>     file_entry const& 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.

## map_block()

>     std::vector<file_slice> map_block(int piece, size_type offset
>             , int size) const;

This function will map a piece index, a byte offset within that piece and a size (in bytes) into the corresponding files with offsets where that data for that piece is supposed to be stored.

The file slice struct looks like this:

    struct file_slice
    {
            int file_index;
            size_type offset;
            size_type size;
    };

The `file_index` refers to the index of the file (in the torrent_info). To get the path and filename, use `file_at()` and give the `file_index` as argument. The `offset` is the byte offset in the file where the range starts, and `size` is the number of bytes this range is. The size + offset will never be greater than the file size.

## map_file()

>     peer_request map_file(int file_index, size_type file_offset
>             , int size) const;

This function will map a range in a specific file into a range in the torrent. The `file_offset` parameter is the offset in the file, given in bytes, where 0 is the start of the file. The `peer_request` structure looks like this:

    struct peer_request
    {
            int piece;
            int start;
            int length;
            bool operator==(peer_request const& r) const;
    };

`piece` is the index of the piece in which the range starts. `start` is the offset within that piece where the range starts. `length` is the size of the range, in bytes.

The input range is assumed to be valid within the torrent. `file_offset` + `size` is not allowed to be greater than the file size. `file_index` must refer to a valid file, i.e. it cannot be >= `num_files()`.

## add_url_seed() add_http_seed()

>     void add_url_seed(std::string const& url
>             , std::string const& extern_auth = std::string()
>             , web_seed_entry::headers_t const& extra_headers = web_seed_entry::headers_t());
>     void add_http_seed(std::string const& url
>             , std::string const& extern_auth = std::string()
>             , web_seed_entry::headers_t const& extra_headers = web_seed_entry::headers_t());
>     std::vector<web_seed_entry> const& web_seeds() const;

`web_seeds()` returns all url seeds and http seeds in the torrent. Each entry is a `web_seed_entry` and may refer to either a url seed or http seed.

`add_url_seed()` and `add_http_seed()` adds one url to the list of url/http seeds. Currently, the only transport protocol supported for the url is http.

The `extern_auth` argument can be used for other athorization schemese than basic HTTP authorization. If set, it will override any username and password found in the URL itself. The string will be sent as the HTTP authorization header's value (without specifying "Basic").

The `extra_headers` argument defaults to an empty list, but can be used to insert custom HTTP headers in the requests to a specific web seed.

See HTTP seeding for more information.

The `web_seed_entry` has the following members:

    struct web_seed_entry
    {
            enum type_t { url_seed, http_seed };

            typedef std::vector<std::pair<std::string, std::string> > headers_t;

            web_seed_entry(std::string const& url_, type_t type_
                    , std::string const& auth_ = std::string()
                    , headers_t const& extra_headers_ = headers_t());

            bool operator==(web_seed_entry const& e) const;
            bool operator<(web_seed_entry const& e) const;

            std::string url;
            type_t type;
            std::string auth;
            headers_t extra_headers;

            // ...
    };

## trackers()

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

The `trackers()` function will return a sorted vector of `announce_entry`. Each announce entry contains a string, which is the tracker url, and a tier index. The tier index is the high-level priority. No matter which trackers that works or not, the ones with lower tier will always be tried before the one with higher tier number.

    struct announce_entry
    {
            announce_entry(std::string const& url);
            std::string url;

            int next_announce_in() const;
            int min_announce_in() const;

            error_code last_error;

            std::string message;

            boost::uint8_t tier;
            boost::uint8_t fail_limit;
            boost::uint8_t fails;

            enum tracker_source
            {
                    source_torrent = 1,
                    source_client = 2,
                    source_magnet_link = 4,
                    source_tex = 8
            };
            boost::uint8_t source;

            bool verified:1;
            bool updating:1;
            bool start_sent:1;
            bool complete_sent:1;
    };

`next_announce_in()` returns the number of seconds to the next announce on this tracker. `min_announce_in()` returns the number of seconds until we are allowed to force another tracker update with this tracker.

If the last time this tracker was contacted failed, `last_error` is the error code describing what error occurred.

If the last time this tracker was contacted, the tracker returned a warning or error message, `message` contains that message.

`fail_limit` is the max number of failures to announce to this tracker in a row, before this tracker is not used anymore.

`fails` is the number of times in a row we have failed to announce to this tracker.

`source` is a bitmask specifying which sources we got this tracker from.

`verified` is set to true the first time we receive a valid response from this tracker.

`updating` is true while we're waiting for a response from the tracker.

`start_sent` is set to true when we get a valid response from an announce with event=started. If it is set, we won't send start in the subsequent announces.

`complete_sent` is set to true when we send a event=completed.

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

>     size_type total_size() const;
>     int piece_length() const;
>     int 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() hash_for_piece_ptr() info_hash()

>     size_type piece_size(unsigned int index) const;
>     sha1_hash const& hash_for_piece(unsigned int index) const;
>     char const* hash_for_piece_ptr(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. `hash_for_piece_ptr()` returns a pointer to the 20 byte sha1 digest for the piece. Note that the string is not null-terminated.

## merkle_tree() set_merkle_tree()

>     std::vector<sha1_hash> const& merkle_tree() const;
>     void set_merkle_tree(std::vector<sha1_hash>& h);

`merkle_tree()` returns a reference to the merkle tree for this torrent, if any.

`set_merkle_tree()` moves the passed in merkle tree into the torrent_info object. i.e. `h` will not be identical after the call. You need to set the merkle tree for a torrent that you've just created (as a merkle torrent). The merkle tree is retrieved from the `create_torrent::merkle_tree()` function, and need to be saved separately from the torrent file itself. Once it's added to libtorrent, the merkle tree will be persisted in the resume data.

## name() comment() creation_date() creator()

>     std::string const& name() const;
>     std::string const& comment() const;
>     std::string const& creator() const;
>     boost::optional<time_t> 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 the creation date of the torrent as time_t (posix time). If there's no time stamp in the torrent file, the optional object will be uninitialized.

Both the name and the comment is UTF-8 encoded strings.

`creator()` returns the creator string in the torrent. If there is no creator string it will return an empty string.

## priv()

>     bool priv() const;

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

## nodes()

>     std::vector<std::pair<std::string, int> > const& nodes() const;

If this torrent contains any DHT nodes, they are put in this vector in their original form (host name and port number).

## add_node()

>     void add_node(std::pair<std::string, int> const& node);

This is used when creating torrent. Use this to add a known DHT node. It may be used, by the client, to bootstrap into the DHT network.

## metadata() metadata_size()

>     boost::shared_array<char> metadata() const;
>     int metadata_size() const;

`metadata()` returns a the raw info section of the torrent file. The size of the metadata is returned by `metadata_size()`.

# torrent_handle

You will usually have to store your torrent handles somewhere, since it's the object through which you retrieve information about the torrent and aborts the torrent.

Warning

Any member function that returns a value or fills in a value has to be made synchronously. This means it has to wait for the main thread to complete the query before it can return. This might potentially be expensive if done from within a GUI thread that needs to stay responsive. Try to avoid quering for information you don't need, and try to do it in as few calls as possible. You can get most of the interesting information about a torrent from the `torrent_handle::status()` call.

Its declaration looks like this:

    struct torrent_handle
    {
            torrent_handle();

            enum status_flags_t
            {
                    query_distributed_copies = 1,
                    query_accurate_download_counters = 2,
                    query_last_seen_complete = 4,
                    query_pieces = 8,
                    query_verified_pieces = 16
            };

            torrent_status status(boost::uint32_t flags = 0xffffffff);
            void file_progress(std::vector<size_type>& fp, int flags = 0);
            void get_download_queue(std::vector<partial_piece_info>& queue) const;
            void get_peer_info(std::vector<peer_info>& v) const;
            torrent_info const& get_torrent_info() const;
            bool is_valid() const;

            std::string name() const;

            enum save_resume_flags_t { flush_disk_cache = 1, save_info_dict = 2 };
            void save_resume_data(int flags = 0) const;
            bool need_save_resume_data() const;
            void force_reannounce() const;
            void force_dht_announce() const;
            void force_reannounce(boost::posix_time::time_duration) const;
            void scrape_tracker() const;
            void connect_peer(asio::ip::tcp::endpoint const& adr, int source = 0) const;

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

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

            void add_url_seed(std::string const& url);
            void remove_url_seed(std::string const& url);
            std::set<std::string> url_seeds() const;

            void add_http_seed(std::string const& url);
            void remove_http_seed(std::string const& url);
            std::set<std::string> http_seeds() const;

            int max_uploads() const;
            void set_max_uploads(int max_uploads) const;
            void set_max_connections(int max_connections) const;
            int max_connections() const;
            void set_upload_limit(int limit) const;
            int upload_limit() const;
            void set_download_limit(int limit) const;
            int download_limit() const;
            void set_sequential_download(bool sd) const;
            bool is_sequential_download() const;

            int queue_position() const;
            void queue_position_up() const;
            void queue_position_down() const;
            void queue_position_top() const;
            void queue_position_bottom() const;

            void set_priority(int prio) const;

            void use_interface(char const* net_interface) const;

            enum pause_flags_t { graceful_pause = 1 };
            void pause(int flags = 0) const;
            void resume() const;
            bool is_seed() const;
            void force_recheck() const;
            void clear_error() const;
            void set_upload_mode(bool m) const;
            void set_share_mode(bool m) const;

            void apply_ip_filter(bool b) const;

            void flush_cache() const;

            void resolve_countries(bool r);
            bool resolve_countries() const;

            enum deadline_flags { alert_when_available = 1 };
            void set_piece_deadline(int index, int deadline, int flags = 0) const;
            void reset_piece_deadline(int index) const;

            void piece_availability(std::vector<int>& avail) const;
            void piece_priority(int index, int priority) const;
            int piece_priority(int index) const;
            void prioritize_pieces(std::vector<int> const& pieces) const;
            std::vector<int> piece_priorities() const;

            void file_priority(int index, int priority) const;
            int file_priority(int index) const;
            void prioritize_files(std::vector<int> const& files) const;
            std::vector<int> file_priorities() const;

            void auto_managed(bool m) const;

            bool set_metadata(char const* buf, int size) const;

            std::string save_path() const;
            void move_storage(std::string const& save_path) const;
            void move_storage(std::wstring const& save_path) const;
            void rename_file(int index, std::string) const;
            void rename_file(int index, std::wstring) const;
            storage_interface* get_storage_impl() const;

            void super_seeding(bool on) const;

            enum flags_t { overwrite_existing = 1 };
            void add_piece(int piece, char const* data, int flags = 0) const;
            void read_piece(int piece) const;
            bool have_piece(int piece) const;

            sha1_hash info_hash() const;

            void set_ssl_certificate(std::string const& cert
                    , std::string const& private_key
                    , std::string const& dh_params
                    , std::string const& passphrase = "");

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

            boost::shared_ptr<torrent> native_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`.

Warning

All operations on a `torrent_handle` may throw libtorrent_exception exception, in case the handle is no longer refering to a torrent. There is one exception `is_valid()` will never throw. Since the torrents are processed by a background thread, there is no guarantee that a handle will remain valid between two calls.

## set_piece_deadline() reset_piece_deadline()

>     enum deadline_flags { alert_when_available = 1 };
>     void set_piece_deadline(int index, int deadline, int flags = 0) const;
>     void reset_piece_deadline(int index) const;

This function sets or resets the deadline associated with a specific piece index (`index`). libtorrent will attempt to download this entire piece before the deadline expires. This is not necessarily possible, but pieces with a more recent deadline will always be prioritized over pieces with a deadline further ahead in time. The deadline (and flags) of a piece can be changed by calling this function again.

The `flags` parameter can be used to ask libtorrent to send an alert once the piece has been downloaded, by passing `alert_when_available`. When set, the read_piece_alert alert will be delivered, with the piece data, when it's downloaded.

If the piece is already downloaded when this call is made, nothing happens, unless the `alert_when_available` flag is set, in which case it will do the same thing as calling read_piece() for `index`.

`deadline` is the number of milliseconds until this piece should be completed.

`reset_piece_deadline` removes the deadline from the piece. If it hasn't already been downloaded, it will no longer be considered a priority.

## piece_availability()

>     void piece_availability(std::vector<int>& avail) const;

Fills the specified `std::vector<int>` with the availability for each piece in this torrent. libtorrent does not keep track of availability for seeds, so if the torrent is seeding the availability for all pieces is reported as 0.

The piece availability is the number of peers that we are connected that has advertized having a particular piece. This is the information that libtorrent uses in order to prefer picking rare pieces.

## piece_priority() prioritize_pieces() piece_priorities()

>     void piece_priority(int index, int priority) const;
>     int piece_priority(int index) const;
>     void prioritize_pieces(std::vector<int> const& pieces) const;
>     std::vector<int> piece_priorities() const;

These functions are used to set and get the prioritiy of individual pieces. By default all pieces have priority 1. That means that the random rarest first algorithm is effectively active for all pieces. You may however change the priority of individual pieces. There are 8 different priority levels:

> 0. piece is not downloaded at all
> 1. normal priority. Download order is dependent on availability
> 2. higher than normal priority. Pieces are preferred over pieces with the same availability, but not over pieces with lower availability
> 3. pieces are as likely to be picked as partial pieces.
> 4. pieces are preferred over partial pieces, but not over pieces with lower availability
> 5. currently the same as 4
> 6. piece is as likely to be picked as any piece with availability 1
> 7. maximum priority, availability is disregarded, the piece is preferred over any other piece with lower priority

The exact definitions of these priorities are implementation details, and subject to change. The interface guarantees that higher number means higher priority, and that 0 means do not download.

`piece_priority` sets or gets the priority for an individual piece, specified by `index`.

`prioritize_pieces` takes a vector of integers, one integer per piece in the torrent. All the piece priorities will be updated with the priorities in the vector.

`piece_priorities` returns a vector with one element for each piece in the torrent. Each element is the current priority of that piece.

## file_priority() prioritize_files() file_priorities()

>     void file_priority(int index, int priority) const;
>     int file_priority(int index) const;
>     void prioritize_files(std::vector<int> const& files) const;
>     std::vector<int> file_priorities() const;

`index` must be in the range [0, number_of_files).

`file_priority` queries or sets the priority of file `index`.

`prioritize_files` takes a vector that has at as many elements as there are files in the torrent. Each entry is the priority of that file. The function sets the priorities of all the pieces in the torrent based on the vector.

`file_priorities` returns a vector with the priorities of all files.

The priority values are the same as for `piece_priority`.

Whenever a file priority is changed, all other piece priorities are reset to match the file priorities. In order to maintain sepcial priorities for particular pieces, `piece_priority` has to be called again for those pieces.

You cannot set the file priorities on a torrent that does not yet have metadata or a torrent that is a seed. `file_priority(int, int)` and `prioritize_files()` are both no-ops for such torrents.

## file_progress()

>     void file_progress(std::vector<size_type>& fp, int flags = 0);

This function fills in the supplied vector with the the number of bytes downloaded of each file in this torrent. The progress values are ordered the same as the files in the torrent_info. This operation is not very cheap. Its complexity is O(n + mj). Where n is the number of files, m is the number of downloading pieces and j is the number of blocks in a piece.

The `flags` parameter can be used to specify the granularity of the file progress. If left at the default value of 0, the progress will be as accurate as possible, but also more expensive to calculate. If `torrent_handle::piece_granularity` is specified, the progress will be specified in piece granularity. i.e. only pieces that have been fully downloaded and passed the hash check count. When specifying piece granularity, the operation is a lot cheaper, since libtorrent already keeps track of this internally and no calculation is required.

## save_path()

>     std::string save_path() const;

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

## move_storage()

>     void move_storage(std::string const& save_path) const;
>     void move_storage(std::wstring const& save_path) const;

Moves the file(s) that this torrent are currently seeding from or downloading to. If the given `save_path` is not located on the same drive as the original save path, The files will be copied to the new drive and removed from their original location. This will block all other disk IO, and other torrents download and upload rates may drop while copying the file.

Since disk IO is performed in a separate thread, this operation is also asynchronous. Once the operation completes, the `storage_moved_alert` is generated, with the new path as the message. If the move fails for some reason, `storage_moved_failed_alert` is generated instead, containing the error message.

## rename_file()

>     void rename_file(int index, std::string) const;
>     void rename_file(int index, std::wstring) const;

Renames the file with the given index asynchronously. The rename operation is complete when either a `file_renamed_alert` or `file_rename_failed_alert` is posted.

## get_storage_impl()

>     storage_interface* get_storage_impl() const;

Returns the storage implementation for this torrent. This depends on the storage contructor function that was passed to `session::add_torrent`.

## super_seeding()

>     void super_seeding(bool on) const;

Enables or disabled super seeding/initial seeding for this torrent. The torrent needs to be a seed for this to take effect.

## add_piece()

>     enum flags_t { overwrite_existing = 1 };
>     void add_piece(int piece, char const* data, int flags = 0) const;

This function will write `data` to the storage as piece `piece`, as if it had been downloaded from a peer. `data` is expected to point to a buffer of as many bytes as the size of the specified piece. The data in the buffer is copied and passed on to the disk IO thread to be written at a later point.

By default, data that's already been downloaded is not overwritten by this buffer. If you trust this data to be correct (and pass the piece hash check) you may pass the `overwrite_existing` flag. This will instruct libtorrent to overwrite any data that may already have been downloaded with this data.

Since the data is written asynchronously, you may know that is passed or failed the hash check by waiting for `piece_finished_alert` or `has_failed_alert`.

## read_piece()

>     void read_piece(int piece) const;

This function starts an asynchronous read operation of the specified piece from this torrent. You must have completed the download of the specified piece before calling this function.

When the read operation is completed, it is passed back through an alert, read_piece_alert. Since this alert is a reponse to an explicit call, it will always be posted, regardless of the alert mask.

Note that if you read multiple pieces, the read operations are not guaranteed to finish in the same order as you initiated them.

## have_piece()

>     bool have_piece(int piece) const;

Returns true if this piece has been completely downloaded, and false otherwise.

## force_reannounce() force_dht_announce()

>     void force_reannounce() const;
>     void force_reannounce(boost::posix_time::time_duration) const;
>     void force_dht_announce() const;

`force_reannounce()` will force this torrent to do another tracker request, to receive new peers. The second overload of `force_reannounce` that takes a `time_duration` as argument will schedule a reannounce in that amount of time from now.

If the tracker's `min_interval` has not passed since the last announce, the forced announce will be scheduled to happen immediately as the `min_interval` expires. This is to honor trackers minimum re-announce interval settings.

`force_dht_announce` will announce the torrent to the DHT immediately.

## scrape_tracker()

>     void scrape_tracker() const;

`scrape_tracker()` will send a scrape request to the tracker. A scrape request queries the tracker for statistics such as total number of incomplete peers, complete peers, number of downloads etc.

This request will specifically update the `num_complete` and `num_incomplete` fields in the torrent_status struct once it completes. When it completes, it will generate a scrape_reply_alert. If it fails, it will generate a scrape_failed_alert.

## connect_peer()

>     void connect_peer(asio::ip::tcp::endpoint const& adr, int source = 0) const;

`connect_peer()` is a way to manually connect to peers that one believe is a part of the torrent. If the peer does not respond, or is not a member of this torrent, it will simply be disconnected. No harm can be done by using this other than an unnecessary connection attempt is made. If the torrent is uninitialized or in queued or checking mode, this will throw libtorrent_exception. The second (optional) argument will be bitwised ORed into the source mask of this peer. Typically this is one of the source flags in peer_info. i.e. `tracker`, `pex`, `dht` etc.

## name()

>     std::string name() const;

Returns the name of the torrent. i.e. the name from the metadata associated with it. In case the torrent was started without metadata, and hasn't completely received it yet, it returns the name given to it when added to the session. See `session::add_torrent`.

## set_upload_limit() set_download_limit() upload_limit() download_limit()

>     void set_upload_limit(int limit) const;
>     void set_download_limit(int limit) const;
>     int upload_limit() const;
>     int download_limit() const;

`set_upload_limit` will limit the upload bandwidth used by this particular torrent to the limit you set. It is given as the number of bytes per second the torrent is allowed to upload. `set_download_limit` works the same way but for download bandwidth instead of upload bandwidth. Note that setting a higher limit on a torrent then the global limit (`session::set_upload_rate_limit`) will not override the global rate limit. The torrent can never upload more than the global rate limit.

`upload_limit` and `download_limit` will return the current limit setting, for upload and download, respectively.

## set_sequential_download()

>     void set_sequential_download(bool sd);

`set_sequential_download()` enables or disables sequential download. When enabled, the piece picker will pick pieces in sequence instead of rarest first.

Enabling sequential download will affect the piece distribution negatively in the swarm. It should be used sparingly.

## pause() resume()

>     enum pause_flags_t { graceful_pause = 1 };
>     void pause(int flags) const;
>     void resume() 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. Torrents may be paused automatically if there is a file error (e.g. disk full) or something similar. See file_error_alert.

To know if a torrent is paused or not, call `torrent_handle::status()` and inspect `torrent_status::paused`.

The `flags` argument to pause can be set to `torrent_handle::graceful_pause` which will delay the disconnect of peers that we're still downloading outstanding requests from. The torrent will not accept any more requests and will disconnect all idle peers. As soon as a peer is done transferring the blocks that were requested from it, it is disconnected. This is a graceful shut down of the torrent in the sense that no downloaded bytes are wasted.

torrents that are auto-managed may be automatically resumed again. It does not make sense to pause an auto-managed torrent without making it not automanaged first. Torrents are auto-managed by default when added to the session. For more information, see queuing.

## flush_cache()

>     void flush_cache() const;

Instructs libtorrent to flush all the disk caches for this torrent and close all file handles. This is done asynchronously and you will be notified that it's complete through cache_flushed_alert.

Note that by the time you get the alert, libtorrent may have cached more data for the torrent, but you are guaranteed that whatever cached data libtorrent had by the time you called `torrent_handle::flush_cache()` has been written to disk.

## force_recheck()

>     void force_recheck() const;

`force_recheck` puts the torrent back in a state where it assumes to have no resume data. All peers will be disconnected and the torrent will stop announcing to the tracker. The torrent will be added to the checking queue, and will be checked (all the files will be read and compared to the piece hashes). Once the check is complete, the torrent will start connecting to peers again, as normal.

## clear_error()

>     void clear_error() const;

If the torrent is in an error state (i.e. `torrent_status::error` is non-empty), this will clear the error and start the torrent again.

## set_upload_mode()

    void set_upload_mode(bool m) const;

Explicitly sets the upload mode of the torrent. In upload mode, the torrent will not request any pieces. If the torrent is auto managed, it will automatically be taken out of upload mode periodically (see `session_settings::optimistic_disk_retry`). Torrents are automatically put in upload mode whenever they encounter a disk write error.

`m` should be true to enter upload mode, and false to leave it.

To test if a torrent is in upload mode, call `torrent_handle::status()` and inspect `torrent_status::upload_mode`.

## set_share_mode()

>     void set_share_mode(bool m) const;

Enable or disable share mode for this torrent. When in share mode, the torrent will not necessarily be downloaded, especially not the whole of it. Only parts that are likely to be distributed to more than 2 other peers are downloaded, and only if the previous prediction was correct.

## apply_ip_filter()

    void apply_ip_filter(bool b) const;

Set to true to apply the session global IP filter to this torrent (which is the default). Set to false to make this torrent ignore the IP filter.

## resolve_countries()

>     void resolve_countries(bool r);
>     bool resolve_countries() const;

Sets or gets the flag that derermines if countries should be resolved for the peers of this torrent. It defaults to false. If it is set to true, the peer_info structure for the peers in this torrent will have their `country` member set. See peer_info for more information on how to interpret this field.

## is_seed()

>     bool is_seed() const;

Returns true if the torrent is in seed mode (i.e. if it has finished downloading).

## auto_managed()

>     void auto_managed(bool m) const;

`auto_managed()` changes whether the torrent is auto managed or not. For more info, see queuing.

## set_metadata()

>     bool set_metadata(char const* buf, int size) const;

`set_metadata` expects the info section of metadata. i.e. The buffer passed in will be hashed and verified against the info-hash. If it fails, a `metadata_failed_alert` will be generated. If it passes, a `metadata_received_alert` is generated. The function returns true if the metadata is successfully set on the torrent, and false otherwise. If the torrent already has metadata, this function will not affect the torrent, and false will be returned.

## set_tracker_login()

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

`set_tracker_login()` sets a username and password that will be sent along in the HTTP-request of the tracker announce. Set this if the tracker requires authorization.

## trackers() replace_trackers() add_tracker()

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

`trackers()` will return the list of trackers for this torrent. The announce entry contains both a string `url` which specify the announce url for the tracker as well as an int `tier`, which is specifies the order in which this tracker is tried. If you want libtorrent to use another list of trackers for this torrent, you can use `replace_trackers()` which takes a list of the same form as the one returned from `trackers()` and will replace it. If you want an immediate effect, you have to call force_reannounce() force_dht_announce(). See trackers() for the definition of `announce_entry`.

`add_tracker()` will look if the specified tracker is already in the set. If it is, it doesn't do anything. If it's not in the current set of trackers, it will insert it in the tier specified in the announce_entry.

The updated set of trackers will be saved in the resume data, and when a torrent is started with resume data, the trackers from the resume data will replace the original ones.

## add_url_seed() remove_url_seed() url_seeds()

>     void add_url_seed(std::string const& url);
>     void remove_url_seed(std::string const& url);
>     std::set<std::string> url_seeds() const;

`add_url_seed()` adds another url to the torrent's list of url seeds. If the given url already exists in that list, the call has no effect. The torrent will connect to the server and try to download pieces from it, unless it's paused, queued, checking or seeding. `remove_url_seed()` removes the given url if it exists already. `url_seeds()` return a set of the url seeds currently in this torrent. Note that urls that fails may be removed automatically from the list.

See HTTP seeding for more information.

## add_http_seed() remove_http_seed() http_seeds()

>     void add_http_seed(std::string const& url);
>     void remove_http_seed(std::string const& url);
>     std::set<std::string> http_seeds() const;

These functions are identical as the `*_url_seed()` variants, but they operate on BEP 17 web seeds instead of BEP 19.

See HTTP seeding for more information.

## queue_position() queue_position_up() queue_position_down() queue_position_top() queue_position_bottom()

>     int queue_position() const;
>     void queue_position_up() const;
>     void queue_position_down() const;
>     void queue_position_top() const;
>     void queue_position_bottom() const;

Every torrent that is added is assigned a queue position exactly one greater than the greatest queue position of all existing torrents. Torrents that are being seeded have -1 as their queue position, since they're no longer in line to be downloaded.

When a torrent is removed or turns into a seed, all torrents with greater queue positions have their positions decreased to fill in the space in the sequence.

`queue_position()` returns the torrent's position in the download queue. The torrents with the smallest numbers are the ones that are being downloaded. The smaller number, the closer the torrent is to the front of the line to be started.

The queue position is also available in the `torrent_status`.

The `queue_position_*()` functions adjust the torrents position in the queue. Up means closer to the front and down means closer to the back of the queue. Top and bottom refers to the front and the back of the queue respectively.

## set_priority()

>     void set_priority(int prio) const;

This sets the bandwidth priority of this torrent. The priority of a torrent determines how much bandwidth its peers are assigned when distributing upload and download rate quotas. A high number gives more bandwidth. The priority must be within the range [0, 255].

The default priority is 0, which is the lowest priority.

To query the priority of a torrent, use the `torrent_handle::status()` call.

Torrents with higher priority will not nececcarily get as much bandwidth as they can consume, even if there's is more quota. Other peers will still be weighed in when bandwidth is being distributed. With other words, bandwidth is not distributed strictly in order of priority, but the priority is used as a weight.

Peers whose Torrent has a higher priority will take precedence when distributing unchoke slots. This is a strict prioritization where every interested peer on a high priority torrent will be unchoked before any other, lower priority, torrents have any peers unchoked.

## use_interface()

>     void use_interface(char const* net_interface) const;

`use_interface()` sets the network interface this torrent will use when it opens outgoing connections. By default, it uses the same interface as the session uses to listen on. The parameter must be a string containing one or more, comma separated, ip-address (either an IPv4 or IPv6 address). When specifying multiple interfaces, the torrent will round-robin which interface to use for each outgoing conneciton. This is useful for clients that are multi-homed.

## info_hash()

>     sha1_hash info_hash() const;

`info_hash()` returns the info-hash for the torrent.

## set_max_uploads() max_uploads()

>     void set_max_uploads(int max_uploads) const;
>     int max_uploads() const;

`set_max_uploads()` sets the maximum number of peers that's unchoked at the same time on this torrent. If you set this to -1, there will be no limit.

`max_uploads()` returns the current settings.

## set_max_connections() max_connections()

>     void set_max_connections(int max_connections) const;
>     int max_connections() const;

`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.

`max_connections()` returns the current settings.

## save_resume_data()

>     enum save_resume_flags_t { flush_disk_cache = 1, save_info_dict = 2 };
>     void save_resume_data(int flags = 0) const;

`save_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.

The `flags` argument is a bitmask of flags ORed together. If the flag `torrent_handle::flush_cache` is set, the disk cache will be flushed before creating the resume data. This avoids a problem with file timestamps in the resume data in case the cache hasn't been flushed yet.

If the flag `torrent_handle::save_info_dict` is set, the resume data will contain the metadata from the torrent file as well. This is default for any torrent that's added without a torrent file (such as a magnet link or a URL).

This operation is asynchronous, `save_resume_data` will return immediately. The resume data is delivered when it's done through an save_resume_data_alert.

The fast resume data will be empty in the following cases:

> 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 you receive the fast resume data, it may already be invalid if the torrent is still downloading! The recommended practice is to first pause the session, then generate the fast resume data, and then close it down. Make sure to not remove_torrent() before you receive the save_resume_data_alert though. There's no need to pause when saving intermittent resume data.

Warning

If you pause every torrent individually instead of pausing the session, every torrent will have its paused state saved in the resume data!

Warning

The resume data contains the modification timestamps for all files. If one file has been modified when the torrent is added again, the will be rechecked. When shutting down, make sure to flush the disk cache before saving the resume data. This will make sure that the file timestamps are up to date and won't be modified after saving the resume data. The recommended way to do this is to pause the torrent, which will flush the cache and disconnect all peers.

Note

It is typically a good idea to save resume data whenever a torrent is completed or paused. In those cases you don't need to pause the torrent or the session, since the torrent will do no more writing to its files. If you save resume data for torrents when they are paused, you can accelerate the shutdown process by not saving resume data again for paused torrents. Completed torrents should have their resume data saved when they complete and on exit, since their statistics might be updated.

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

It also means that if the resume data is out dated, libtorrent will not re-check the files, but assume that it is fairly recent. The assumption is that it's better to loose a little bit than to re-check the entire file.

It is still a good idea to save resume data periodically during download as well as when closing down.

Example code to pause and save resume data for all torrents and wait for the alerts:

    extern int outstanding_resume_data; // global counter of outstanding resume data
    std::vector<torrent_handle> handles = ses.get_torrents();
    ses.pause();
    for (std::vector<torrent_handle>::iterator i = handles.begin();
            i != handles.end(); ++i)
    {
            torrent_handle& h = *i;
            if (!h.is_valid()) continue;
            torrent_status s = h.status();
            if (!s.has_metadata) continue;
            if (!s.need_save_resume_data()) continue;

            h.save_resume_data();
            ++outstanding_resume_data;
    }

    while (outstanding_resume_data > 0)
    {
            alert const* a = ses.wait_for_alert(seconds(10));

            // if we don't get an alert within 10 seconds, abort
            if (a == 0) break;

            std::auto_ptr<alert> holder = ses.pop_alert();

            if (alert_cast<save_resume_data_failed_alert>(a))
            {
                    process_alert(a);
                    --outstanding_resume_data;
                    continue;
            }

            save_resume_data_alert const* rd = alert_cast<save_resume_data_alert>(a);
            if (rd == 0)
            {
                    process_alert(a);
                    continue;
            }

            torrent_handle h = rd->handle;
            std::ofstream out((h.save_path() + "/" + h.get_torrent_info().name() + ".fastresume").c_str()
                    , std::ios_base::binary);
            out.unsetf(std::ios_base::skipws);
            bencode(std::ostream_iterator<char>(out), *rd->resume_data);
            --outstanding_resume_data;
    }

Note

Note how `outstanding_resume_data` is a global counter in this example. This is deliberate, otherwise there is a race condition for torrents that was just asked to save their resume data, they posted the alert, but it has not been received yet. Those torrents would report that they don't need to save resume data again, and skipped by the initial loop, and thwart the counter otherwise.

## need_save_resume_data()

>     bool need_save_resume_data() const;

This function returns true if any whole chunk has been downloaded since the torrent was first loaded or since the last time the resume data was saved. When saving resume data periodically, it makes sense to skip any torrent which hasn't downloaded anything since the last time.

Note

A torrent's resume data is considered saved as soon as the alert is posted. It is important to make sure this alert is received and handled in order for this function to be meaningful.

## status()

>     torrent_status status(boost::uint32_t flags = 0xffffffff) const;

`status()` will return a structure with information about the status of this torrent. If the torrent_handle is invalid, it will throw libtorrent_exception exception. See torrent_status. The `flags` argument filters what information is returned in the torrent_status. Some information in there is relatively expensive to calculate, and if you're not interested in it (and see performance issues), you can filter them out.

By default everything is included. The flags you can use to decide what to include are:

- `query_distributed_copies`

    calculates `distributed_copies`, `distributed_full_copies` and `distributed_fraction`.

- `query_accurate_download_counters`

    includes partial downloaded blocks in `total_done` and `total_wanted_done`.

- `query_last_seen_complete`

    includes `last_seen_complete`.

- `query_pieces`

    includes `pieces`.

- `query_verified_pieces`

    includes `verified_pieces` (only applies to torrents in seed mode).

## get_download_queue()

>     void get_download_queue(std::vector<partial_piece_info>& queue) const;

`get_download_queue()` takes a non-const reference to a vector which it will fill with information about pieces that are partially downloaded or not downloaded at all but partially requested. The entry in the vector (`partial_piece_info`) looks like this:

    struct partial_piece_info
    {
            int piece_index;
            int blocks_in_piece;
            enum state_t { none, slow, medium, fast };
            state_t piece_state;
            block_info* blocks;
    };

`piece_index` is the index of the piece in question. `blocks_in_piece` is the number of blocks in this particular piece. This number will be the same for most pieces, but the last piece may have fewer blocks than the standard pieces.

`piece_state` is set to either `fast`, `medium`, `slow` or `none`. It tells which download rate category the peers downloading this piece falls into. `none` means that no peer is currently downloading any part of the piece. Peers prefer picking pieces from the same category as themselves. The reason for this is to keep the number of partially downloaded pieces down. Pieces set to `none` can be converted into any of `fast`, `medium` or `slow` as soon as a peer want to download from it.

    struct block_info
    {
            enum block_state_t
            { none, requested, writing, finished };

            void set_peer(tcp::endpoint const& ep);
            tcp::endpoint peer() const;

            unsigned bytes_progress:15;
            unsigned block_size:15;
            unsigned state:2;
            unsigned num_peers:14;
    };

The `blocks` field points to an array of `blocks_in_piece` elements. This pointer is only valid until the next call to `get_download_queue()` for any torrent in the same session. They all share the storaga for the block arrays in their session object.

The `block_info` array contains data for each individual block in the piece. Each block has a state (`state`) which is any of:

- `none` - This block has not been downloaded or requested form any peer.
- `requested` - The block has been requested, but not completely downloaded yet.
- `writing` - The block has been downloaded and is currently queued for being written to disk.
- `finished` - The block has been written to disk.

The `peer` field is the ip address of the peer this block was downloaded from. `num_peers` is the number of peers that is currently requesting this block. Typically this is 0 or 1, but at the end of the torrent blocks may be requested by more peers in parallel to speed things up. `bytes_progress` is the number of bytes that have been received for this block, and `block_size` is the total number of bytes in this block.

## get_peer_info()

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

`get_peer_info()` takes a reference to a vector that will be cleared and filled with one entry for each peer connected to this torrent, given the handle is valid. If the torrent_handle is invalid, it will throw libtorrent_exception exception. Each entry in the vector contains information about that particular peer. See peer_info.

## get_torrent_info()

>     torrent_info const& get_torrent_info() const;

Returns a const reference to the torrent_info object associated with this torrent. This reference is valid as long as the torrent_handle is valid, no longer. If the torrent_handle is invalid or if it doesn't have any metadata, libtorrent_exception 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.

## set_ssl_certificate()

>     void set_ssl_certificate(std::string const& cert, std::string const& private_key
>             , std::string const& dh_params, std::string const& passphrase = "");

For SSL torrents, use this to specify a path to a .pem file to use as this client's certificate. The certificate must be signed by the certificate in the .torrent file to be valid.

`cert` is a path to the (signed) certificate in .pem format corresponding to this torrent.

`private_key` is a path to the private key for the specified certificate. This must be in .pem format.

`dh_params` is a path to the Diffie-Hellman parameter file, which needs to be in .pem format. You can generate this file using the openssl command like this: `openssl dhparam -outform PEM -out dhparams.pem 512`.

`passphrase` may be specified if the private key is encrypted and requires a passphrase to be decrypted.

Note that when a torrent first starts up, and it needs a certificate, it will suspend connecting to any peers until it has one. It's typically desirable to resume the torrent after setting the ssl certificate.

If you receive a torrent_need_cert_alert, you need to call this to provide a valid cert. If you don't have a cert you won't be allowed to connect to any peers.

## native_handle()

>     boost::shared_ptr<torrent> native_handle() const;

This function is intended only for use by plugins and the alert dispatch function. Any code that runs in libtorrent's network thread may not use the public API of `torrent_handle`. Doing so results in a dead-lock. For such routines, the `native_handle` gives access to the underlying type representing the torrent. This type does not have a stable API and should be relied on as little as possible.

# torrent_status

It contains the following fields:

    struct torrent_status
    {
            enum state_t
            {
                    queued_for_checking,
                    checking_files,
                    downloading_metadata,
                    downloading,
                    finished,
                    seeding,
                    allocating,
                    checking_resume_data
            };

            torrent_handle handle;

            state_t state;
            bool paused;
            bool auto_managed;
            bool sequential_download;
            bool seeding;
            bool finished;
            float progress;
            int progress_ppm;
            std::string error;

            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;
            size_type total_redundant_bytes;

            int download_rate;
            int upload_rate;

            int download_payload_rate;
            int upload_payload_rate;

            int num_peers;

            int num_complete;
            int num_incomplete;

            int list_seeds;
            int list_peers;

            int connect_candidates;

            bitfield pieces;
            bitfield verified_pieces;

            int num_pieces;

            size_type total_done;
            size_type total_wanted_done;
            size_type total_wanted;

            int num_seeds;

            int distributed_full_copies;
            int distributed_fraction;

            float distributed_copies;

            int block_size;

            int num_uploads;
            int num_connections;
            int uploads_limit;
            int connections_limit;

            storage_mode_t storage_mode;

            int up_bandwidth_queue;
            int down_bandwidth_queue;

            size_type all_time_upload;
            size_type all_time_download;

            int active_time;
            int finished_time;
            int seeding_time;

            int seed_rank;

            int last_scrape;

            bool has_incoming;

            int sparse_regions;

            bool seed_mode;
            bool upload_mode;
            bool share_mode;
            bool super_seeding;

            int priority;

            time_t added_time;
            time_t completed_time;
            time_t last_seen_complete;

            int time_since_upload;
            int time_since_download;

            int queue_position;
            bool need_save_resume;
            bool ip_filter_applies;

            sha1_hash info_hash;

            int listen_port;
    };

`handle` is a handle to the torrent whose status the object represents.

`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.

`progress_ppm` reflects the same value as `progress`, but instead in a range [0, 1000000] (ppm = parts per million). When floating point operations are disabled, this is the only alternative to the floating point value in `progress`.

The torrent's current task is in the `state` member, it will be one of the following:

`checking_resume_data` | The torrent is currently checking the fastresume data and comparing it to the files on disk. This is typically completed in a fraction of a second, but if you add a large number of torrents at once, they will queue up.
`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.
`downloading_metadata` | The torrent is trying to download metadata from peers. This assumes the metadata_transfer extension is in use.
`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.
`finished` | In this state the torrent has finished downloading but still doesn't have the entire torrent. i.e. some pieces are filtered and won't get downloaded.
`seeding` | In this state the torrent has finished downloading and is a pure seeder.
`allocating` | If the torrent was started in full allocation mode, this indicates that the (disk) storage for the torrent is allocated.

When downloading, the progress is `total_wanted_done` / `total_wanted`. This takes into account files whose priority have been set to 0. They are not considered.

`paused` is set to true if the torrent is paused and false otherwise. It's only true if the torrent itself is paused. If the torrent is not running because the session is paused, this is still false. To know if a torrent is active or not, you need to inspect both `torrent_status::paused` and `session::is_paused()`.

`auto_managed` is set to true if the torrent is auto managed, i.e. libtorrent is responsible for determining whether it should be started or queued. For more info see queuing

`sequential_download` is true when the torrent is in sequential download mode. In this mode pieces are downloaded in order rather than rarest first.

`is_seeding` is true if all pieces have been downloaded.

`is_finished` is true if all pieces that have a priority > 0 are downloaded. There is only a distinction between finished and seeding if some pieces or files have been set to priority 0, i.e. are not downloaded.

`has_metadata` is 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 be false is when the torrent was started torrent-less (i.e. with just an info-hash and tracker ip, a magnet link for instance). Note that if the torrent doesn't have metadata, the member get_torrent_info() will throw.

`error` may be set to an error message describing why the torrent was paused, in case it was paused by an error. If the torrent is not paused or if it's paused but not because of an error, this string is empty.

`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. The session is considered to restart when a torrent is paused and restarted again. When a torrent is paused, these counters are reset to 0. If you want complete, persistent, stats, see `all_time_upload` and `all_time_download`.

`total_payload_download` and `total_payload_upload` counts the amount of bytes send and received this session, but only the actual payload 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.

`total_redundant_bytes` is the number of bytes that has been downloaded even though that data already was downloaded. The reason for this is that in some situations the same data can be downloaded by mistake. When libtorrent sends requests to a peer, and the peer doesn't send a response within a certain timeout, libtorrent will re-request that block. Another situation when libtorrent may re-request blocks is when the requests it sends out are not replied in FIFO-order (it will re-request blocks that are skipped by an out of order block). This is supposed to be as low as possible.

`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.

`verified_pieces` is a bitmask representing which pieces has had their hash checked. This only applies to torrents in seed mode. If the torrent is not in seed mode, this bitmask may be empty.

`num_pieces` is the number of pieces that has been downloaded. It is equivalent to: `std::accumulate(pieces->begin(), pieces->end())`. So you don't have to count yourself. This can be used to see if anything has updated since last time if you want to keep a graph of the pieces up to date.

`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 noticeable.

`num_peers` is the number of peers this torrent currently is connected to. Peer connections that are in the half-open state (is attempting to connect) or are queued for later connection attempt do not count. Although they are visible in the peer list when you call get_peer_info().

`num_complete` and `num_incomplete` are set to -1 if the tracker did not send any scrape data in its announce reply. This data is optional and may not be available from all trackers. If these are not -1, they are the total number of peers that are seeding (complete) and the total number of peers that are still downloading (incomplete) this torrent.

`list_seeds` and `list_peers` are the number of seeds in our peer list and the total number of peers (including seeds) respectively. We are not necessarily connected to all the peers in our peer list. This is the number of peers we know of in total, including banned peers and peers that we have failed to connect to.

`connect_candidates` is the number of peers in this torrent's peer list that is a candidate to be connected to. i.e. It has fewer connect attempts than the max fail count, it is not a seed if we are a seed, it is not banned etc. If this is 0, it means we don't know of any more peers that we can try.

`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_payload_download`).

`total_wanted_done` is the number of bytes we have downloaded, only counting the pieces that we actually want to download. i.e. excluding any pieces that we have but have priority 0 (i.e. not wanted).

`total_wanted` is the total number of bytes we want to download. This is also excluding pieces whose priorities have been set to 0.

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

`distributed_full_copies` is the number of distributed copies of the torrent. Note that one copy may be spread out among many peers. It tells how many copies there are currently of the rarest piece(s) among the peers this client is connected to.

`distributed_fraction` tells the share of pieces that have more copies than the rarest piece(s). Divide this number by 1000 to get the fraction.

For example, if `distributed_full_copies` is 2 and `distrbuted_fraction` is 500, it means 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.

If we are a seed, the piece picker is deallocated as an optimization, and piece availability is no longer tracked. In this case the distributed copies members are set to -1.

`distributed_copies` is a floating point representation of the `distributed_full_copies` as the integer part and `distributed_fraction` / 1000 as the fraction part. If floating point operations are disabled this value is always -1.

`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.

`num_uploads` is the number of unchoked peers in this torrent.

`num_connections` is the number of peer connections this torrent has, including half-open connections that hasn't completed the bittorrent handshake yet. This is always >= `num_peers`.

`uploads_limit` is the set limit of upload slots (unchoked peers) for this torrent.

`connections_limit` is the set limit of number of connections for this torrent.

`storage_mode` is one of `storage_mode_allocate`, `storage_mode_sparse` or `storage_mode_compact`. Identifies which storage mode this torrent is being saved with. See Storage allocation.

`up_bandwidth_queue` and `down_bandwidth_queue` are the number of peers in this torrent that are waiting for more bandwidth quota from the torrent rate limiter. This can determine if the rate you get from this torrent is bound by the torrents limit or not. If there is no limit set on this torrent, the peers might still be waiting for bandwidth quota from the global limiter, but then they are counted in the `session_status` object.

`all_time_upload` and `all_time_download` are accumulated upload and download payload byte counters. They are saved in and restored from resume data to keep totals across sessions.

`active_time`, `finished_time` and `seeding_time` are second counters. They keep track of the number of seconds this torrent has been active (not paused) and the number of seconds it has been active while being finished and active while being a seed. `seeding_time` should be <= `finished_time` which should be <= `active_time`. They are all saved in and restored from resume data, to keep totals across sessions.

`seed_rank` is a rank of how important it is to seed the torrent, it is used to determine which torrents to seed and which to queue. It is based on the peer to seed ratio from the tracker scrape. For more information, see queuing.

`last_scrape` is the number of seconds since this torrent acquired scrape data. If it has never done that, this value is -1.

`has_incoming` is true if there has ever been an incoming connection attempt to this torrent.'

`sparse_regions` the number of regions of non-downloaded pieces in the torrent. This is an interesting metric on windows vista, since there is a limit on the number of sparse regions in a single file there.

`seed_mode` is true if the torrent is in seed_mode. If the torrent was started in seed mode, it will leave seed mode once all pieces have been checked or as soon as one piece fails the hash check.

`upload_mode` is true if the torrent is blocked from downloading. This typically happens when a disk write operation fails. If the torrent is auto-managed, it will periodically be taken out of this state, in the hope that the disk condition (be it disk full or permission errors) has been resolved. If the torrent is not auto-managed, you have to explicitly take it out of the upload mode by calling set_upload_mode() on the torrent_handle.

`share_mode` is true if the torrent is currently in share-mode, i.e. not downloading the torrent, but just helping the swarm out.

`super_seeding` is true if the torrent is in super seeding mode.

`added_time` is the posix-time when this torrent was added. i.e. what `time(NULL)` returned at the time.

`completed_time` is the posix-time when this torrent was finished. If the torrent is not yet finished, this is 0.

`last_seen_complete` is the time when we, or one of our peers, last saw a complete copy of this torrent.

`time_since_upload` and `time_since_download` are the number of seconds since any peer last uploaded from this torrent and the last time a downloaded piece passed the hash check, respectively.

`queue_position` is the position this torrent has in the download queue. If the torrent is a seed or finished, this is -1.

`need_save_resume` is true if this torrent has unsaved changes to its download state and statistics since the last resume data was saved.

`ip_filter_applies` is true if the session global IP filter applies to this torrent. This defaults to true.

`info_hash` is the info-hash of the torrent.

`listen_port` is the listen port this torrent is listening on for new connections, if the torrent has its own listen socket. Only SSL torrents have their own listen sockets. If the torrent doesn't have one, and is accepting connections on the single listen socket, this is 0.

# 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,
                    handshake = 0x40,
                    connecting = 0x80,
                    queued = 0x100,
                    on_parole = 0x200,
                    seed = 0x400,
                    optimistic_unchoke = 0x800,
                    snubbed = 0x1000,
                    upload_only = 0x2000,
                    endgame_mode = 0x4000,
                    holepunched = 0x8000,
                    rc4_encrypted = 0x100000,
                    plaintext_encrypted = 0x200000
            };

            unsigned int flags;

            enum peer_source_flags
            {
                    tracker = 0x1,
                    dht = 0x2,
                    pex = 0x4,
                    lsd = 0x8
            };

            int source;

            // bitmask representing socket state
            enum bw_state { bw_idle = 0, bw_limit = 1, bw_network = 2, bw_disk = 4 };

            char read_state;
            char write_state;

            asio::ip::tcp::endpoint ip;
            int up_speed;
            int down_speed;
            int payload_up_speed;
            int payload_down_speed;
            size_type total_download;
            size_type total_upload;
            peer_id pid;
            bitfield pieces;
            int upload_limit;
            int download_limit;

            time_duration last_request;
            time_duration last_active;
            int request_timeout;

            int send_buffer_size;
            int used_send_buffer;

            int receive_buffer_size;
            int used_receive_buffer;

            int num_hashfails;

            char country[2];

            std::string inet_as_name;
            int inet_as;

            size_type load_balancing;

            int requests_in_buffer;
            int download_queue_length;
            int upload_queue_length;

            int failcount;

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

            std::string client;

            enum
            {
                    standard_bittorrent = 0,
                    web_seed = 1
            };
            int connection_type;

            int remote_dl_rate;

            int pending_disk_bytes;

            int send_quota;
            int receive_quota;

            int rtt;

            int num_pieces;

            int download_rate_peak;
            int upload_rate_peak;

            float progress;
            int progress_ppm;

            tcp::endpoint local_endpoint;
    };

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` | the peer is interested in us
`remote_choked` | 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 as in the address of this peer. If this flag is not set, this peer connection was opened by this peer connecting to us.
`handshake` | The connection is opened, and waiting for the handshake. Until the handshake is done, the peer cannot be identified.
`connecting` | The connection is in a half-open state (i.e. it is being connected).
`queued` | The connection is currently queued for a connection attempt. This may happen if there is a limit set on the number of half-open TCP connections.
`on_parole` | The peer has participated in a piece that failed the hash check, and is now "on parole", which means we're only requesting whole pieces from this peer until it either fails that piece or proves that it doesn't send bad data.
`seed` | This peer is a seed (it has all the pieces).
`optimistic_unchoke` | This peer is subject to an optimistic unchoke. It has been unchoked for a while to see if it might unchoke us in return an earn an upload/unchoke slot. If it doesn't within some period of time, it will be choked and another peer will be optimistically unchoked.
`snubbed` | This peer has recently failed to send a block within the request timeout from when the request was sent. We're currently picking one block at a time from this peer.
`upload_only` | This peer has either explicitly (with an extension) or implicitly (by becoming a seed) told us that it will not downloading anything more, regardless of which pieces we have.
`endgame_mode` | This means the last time this peer picket a piece, it could not pick as many as it wanted because there were not enough free ones. i.e. all pieces this peer has were already requested from other peers.
`holepunched` | This flag is set if the peer was in holepunch mode when the connection succeeded. This typically only happens if both peers are behind a NAT and the peers connect via the NAT holepunch mechanism.

`source` is a combination of flags describing from which sources this peer was received. The flags are:

`tracker` | The peer was received from the tracker.
`dht` | The peer was received from the kademlia DHT.
`pex` | The peer was received from the peer exchange extension.
`lsd` | The peer was received from the local service discovery (The peer is on the local network).
`resume_data` | The peer was added from the fast resume data.

`read_state` and `write_state` are bitmasks indicating what state this peer is in with regards to sending and receiving data. The states are declared in the `bw_state` enum and defines as follows:

`bw_idle` | The peer is not waiting for any external events to send or receive data.
`bw_limit` | The peer is waiting for the rate limiter.
`bw_network` | The peer has quota and is currently waiting for a network read or write operation to complete. This is the state all peers are in if there are no bandwidth limits.
`bw_disk` | The peer is waiting for the disk I/O thread to catch up writing buffers to disk before downloading more.

Note that `read_state` and `write_state` are bitmasks. A peer may be waiting on disk and on the network at the same time. `bw_idle` does not represent a bit, but is simply a name for no bit being set in the bitmask.

The `ip` field is the IP-address to this peer. The type is an asio endpoint. For more info, see the asio documentation.

`up_speed` and `down_speed` contains the current upload and download speed we have to and from this peer (including any protocol messages). The transfer rates of payload data only are found in `payload_up_speed` and `payload_down_speed`. These figures are updated approximately 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.

`pid` 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 bitfield, with one bit per piece in the torrent. Each bit tells you if the peer has that piece (if it's set to 1) or if the peer miss that piece (set to 0).

`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 local limit on the peer. The global limit and the torrent limit is always enforced anyway.

`download_limit` is the number of bytes per second this peer is allowed to receive. -1 means it's unlimited.

`last_request` and `last_active` is the time since we last sent a request to this peer and since any transfer occurred with this peer, respectively.

`request_timeout` is the number of seconds until the current front piece request will time out. This timeout can be adjusted through `session_settings::request_timeout`. -1 means that there is not outstanding request.

`send_buffer_size` and `used_send_buffer` is the number of bytes allocated and used for the peer's send buffer, respectively.

`receive_buffer_size` and `used_receive_buffer` are the number of bytes allocated and used as receive buffer, respectively.

`num_hashfails` is the number of pieces this peer has participated in sending us that turned out to fail the hash check.

`country` is the two letter ISO 3166 country code for the country the peer is connected from. If the country hasn't been resolved yet, both chars are set to 0. If the resolution failed for some reason, the field is set to "--". If the resolution service returns an invalid country code, it is set to "!!". The `countries.nerd.dk` service is used to look up countries. This field will remain set to 0 unless the torrent is set to resolve countries, see resolve_countries().

`inet_as_name` is the name of the AS this peer is located in. This might be an empty string if there is no name in the geo ip database.

`inet_as` is the AS number the peer is located in.

`load_balancing` is a measurement 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.

`requests_in_buffer` is the number of requests messages that are currently in the send buffer waiting to be sent.

`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.

`failcount` is the number of times this peer has "failed". i.e. failed to connect or disconnected us. The failcount is decremented when we see this peer in a tracker response or peer exchange message.

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.

`client` is a string describing the software at the other end of the connection. In some cases this information is not available, then it will contain a string that may give away something about which software is running in the other end. In the case of a web seed, the server type and version will be a part of this string.

`connection_type` can currently be one of:

type | meaning
--- | ---
`peer_info::standard_bittorrent` | Regular bittorrent connection over TCP
`peer_info::bittorrent_utp` | Bittorrent connection over uTP
`peer_info::web_sesed` | HTTP connection using the BEP 19 protocol
`peer_info::http_seed` | HTTP connection using the BEP 17 protocol

`remote_dl_rate` is an estimate of the rate this peer is downloading at, in bytes per second.

`pending_disk_bytes` is the number of bytes this peer has pending in the disk-io thread. Downloaded and waiting to be written to disk. This is what is capped by `session_settings::max_queued_disk_bytes`.

`send_quota` and `receive_quota` are the number of bytes this peer has been assigned to be allowed to send and receive until it has to request more quota from the bandwidth manager.

`rtt` is an estimated round trip time to this peer, in milliseconds. It is estimated by timing the the tcp `connect()`. It may be 0 for incoming connections.

`num_pieces` is the number of pieces this peer has.

`download_rate_peak` and `upload_rate_peak` are the highest download and upload rates seen on this connection. They are given in bytes per second. This number is reset to 0 on reconnect.

`progress` is the progress of the peer in the range [0, 1]. This is always 0 when floating point operations are diabled, instead use `progress_ppm`.

`progress_ppm` indicates the download progress of the peer in the range [0, 1000000] (parts per million).

`local_endpoint` is the IP and port pair the socket is bound to locally. i.e. the IP address of the interface it's going out over. This may be useful for multi-homed clients with multiple interfaces to the internet.

# feed_handle

The `feed_handle` refers to a specific RSS feed which is watched by the session. The `feed_item` struct is defined in `<libtorrent/rss.hpp>`. It has the following functions:

    struct feed_handle
    {
            feed_handle();
            void update_feed();
            feed_status get_feed_status() const;
            void set_settings(feed_settings const& s);
            feed_settings settings() const;
    };

## update_feed()

>     void update_feed();

Forces an update/refresh of the feed. Regular updates of the feed is managed by libtorrent, be careful to not call this too frequently since it may overload the RSS server.

## get_feed_status()

>     feed_status get_feed_status() const;

Queries the RSS feed for information, including all the items in the feed. The `feed_status` object has the following fields:

    struct feed_status
    {
            std::string url;
            std::string title;
            std::string description;
            time_t last_update;
            int next_update;
            bool updating;
            std::vector<feed_item> items;
            error_code error;
            int ttl;
    };

`url` is the URL of the feed.

`title` is the name of the feed (as specified by the feed itself). This may be empty if we have not recevied a response from the RSS server yet, or if the feed does not specify a title.

`description` is the feed description (as specified by the feed itself). This may be empty if we have not received a response from the RSS server yet, or if the feed does not specify a description.

`last_update` is the posix time of the last successful response from the feed.

`next_update` is the number of seconds, from now, when the feed will be updated again.

`updating` is true if the feed is currently being updated (i.e. waiting for DNS resolution, connecting to the server or waiting for the response to the HTTP request, or receiving the response).

`items` is a vector of all items that we have received from the feed. See feed_item for more information.

`error` is set to the appropriate error code if the feed encountered an error.

`ttl` is the current refresh time (in minutes). It's either the configured default ttl, or the ttl specified by the feed.

## set_settings() settings()

>     void set_settings(feed_settings const& s);
>     feed_settings settings() const;

Sets and gets settings for this feed. For more information on the available settings, see add_feed().

# feed_item

The `feed_item` struct is defined in `<libtorrent/rss.hpp>`.

>     struct feed_item
>     {
>             feed_item();
>             std::string url;
>             std::string uuid;
>             std::string title;
>             std::string description;
>             std::string comment;
>             std::string category;
>             size_type size;
>             torrent_handle handle;
>             sha1_hash info_hash;
>     };

`size` is the total size of the content the torrent refers to, or -1 if no size was specified by the feed.

`handle` is the handle to the torrent, if the session is already downloading this torrent.

`info_hash` is the info-hash of the torrent, or cleared (i.e. all zeroes) if the feed does not specify the info-hash.

All the strings are self explanatory and may be empty if the feed does not specify those fields.

# session customization

You have some control over session configuration through the `session_settings` object. You create it and fill it with your settings and then use `session::set_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 will also be used to identify the client with other peers.

## presets

The default values of the session settings are set for a regular bittorrent client running on a desktop system. There are functions that can set the session settings to pre set settings for other environments. These can be used for the basis, and should be tweaked to fit your needs better.

    session_settings min_memory_usage();
    session_settings high_performance_seed();

`min_memory_usage` returns settings that will use the minimal amount of RAM, at the potential expense of upload and download performance. It adjusts the socket buffer sizes, disables the disk cache, lowers the send buffer watermarks so that each connection only has at most one block in use at any one time. It lowers the outstanding blocks send to the disk I/O thread so that connections only have one block waiting to be flushed to disk at any given time. It lowers the max number of peers in the peer list for torrents. It performs multiple smaller reads when it hashes pieces, instead of reading it all into memory before hashing.

This configuration is inteded to be the starting point for embedded devices. It will significantly reduce memory usage.

`high_performance_seed` returns settings optimized for a seed box, serving many peers and that doesn't do any downloading. It has a 128 MB disk cache and has a limit of 400 files in its file pool. It support fast upload rates by allowing large send buffers.

## session_settings

    struct session_settings
    {
            session_settings();
            int version;
            std::string user_agent;
            int tracker_completion_timeout;
            int tracker_receive_timeout;
            int stop_tracker_timeout;
            int tracker_maximum_response_length;

            int piece_timeout;
            float request_queue_time;
            int max_allowed_in_request_queue;
            int max_out_request_queue;
            int whole_pieces_threshold;
            int peer_timeout;
            int urlseed_timeout;
            int urlseed_pipeline_size;
            int file_pool_size;
            bool allow_multiple_connections_per_ip;
            int max_failcount;
            int min_reconnect_time;
            int peer_connect_timeout;
            bool ignore_limits_on_local_network;
            int connection_speed;
            bool send_redundant_have;
            bool lazy_bitfields;
            int inactivity_timeout;
            int unchoke_interval;
            int optimistic_unchoke_interval;
            std::string announce_ip;
            int num_want;
            int initial_picker_threshold;
            int allowed_fast_set_size;

            enum { no_piece_suggestions = 0, suggest_read_cache = 1 };
            int suggest_mode;
            int max_queued_disk_bytes;
            int handshake_timeout;
            bool use_dht_as_fallback;
            bool free_torrent_hashes;
            bool upnp_ignore_nonrouters;
            int send_buffer_watermark;
            int send_buffer_watermark_factor;

    #ifndef TORRENT_NO_DEPRECATE
            bool auto_upload_slots;
            bool auto_upload_slots_rate_based;
    #endif

            enum choking_algorithm_t
            {
                    fixed_slots_choker,
                    auto_expand_choker,
                    rate_based_choker,
                    bittyrant_choker
            };

            int choking_algorithm;

            enum seed_choking_algorithm_t
            {
                    round_robin,
                    fastest_upload,
                    anti_leech
            };

            int seed_choking_algorithm;

            bool use_parole_mode;
            int cache_size;
            int cache_buffer_chunk_size;
            int cache_expiry;
            bool use_read_cache;
            bool explicit_read_cache;
            int explicit_cache_interval;

            enum io_buffer_mode_t
            {
                    enable_os_cache = 0,
                    disable_os_cache_for_aligned_files = 1,
                    disable_os_cache = 2
            };
            int disk_io_write_mode;
            int disk_io_read_mode;

            std::pair<int, int> outgoing_ports;
            char peer_tos;

            int active_downloads;
            int active_seeds;
            int active_dht_limit;
            int active_tracker_limit;
            int active_limit;
            bool auto_manage_prefer_seeds;
            bool dont_count_slow_torrents;
            int auto_manage_interval;
            float share_ratio_limit;
            float seed_time_ratio_limit;
            int seed_time_limit;
            int peer_turnover_interval;
            float peer_turnover;
            float peer_turnover_cutoff;
            bool close_redundant_connections;

            int auto_scrape_interval;
            int auto_scrape_min_interval;

            int max_peerlist_size;

            int min_announce_interval;

            bool prioritize_partial_pieces;
            int auto_manage_startup;

            bool rate_limit_ip_overhead;

            bool announce_to_all_trackers;
            bool announce_to_all_tiers;

            bool prefer_udp_trackers;
            bool strict_super_seeding;

            int seeding_piece_quota;

            int max_sparse_regions;

            bool lock_disk_cache;

            int max_rejects;

            int recv_socket_buffer_size;
            int send_socket_buffer_size;

            bool optimize_hashing_for_speed;

            int file_checks_delay_per_block;

            enum disk_cache_algo_t
            { lru, largest_contiguous, avoid_readback };

            disk_cache_algo_t disk_cache_algorithm;

            int read_cache_line_size;
            int write_cache_line_size;

            int optimistic_disk_retry;
            bool disable_hash_check;

            int max_suggest_pieces;

            bool drop_skipped_requests;

            bool low_prio_disk;
            int local_service_announce_interval;
            int dht_announce_interval;

            int udp_tracker_token_expiry;
            bool volatile_read_cache;
            bool guided_read_cache;
            bool default_min_cache_age;

            int num_optimistic_unchoke_slots;
            bool no_atime_storage;
            int default_est_reciprocation_rate;
            int increase_est_reciprocation_rate;
            int decrease_est_reciprocation_rate;
            bool incoming_starts_queued_torrents;
            bool report_true_downloaded;
            bool strict_end_game_mode;

            bool broadcast_lsd;

            bool enable_outgoing_utp;
            bool enable_incoming_utp;
            bool enable_outgoing_tcp;
            bool enable_incoming_tcp;
            int max_pex_peers;
            bool ignore_resume_timestamps;
            bool no_recheck_incomplete_resume;
            bool anonymous_mode;
            int tick_interval;
            int share_mode_target;

            int upload_rate_limit;
            int download_rate_limit;
            int local_upload_rate_limit;
            int local_download_rate_limit;
            int dht_upload_rate_limit;
            int unchoke_slots_limit;
            int half_open_limit;
            int connections_limit;

            int utp_target_delay;
            int utp_gain_factor;
            int utp_min_timeout;
            int utp_syn_resends;
            int utp_num_resends;
            int utp_connect_timeout;
            int utp_delayed_ack;
            bool utp_dynamic_sock_buf;
            int utp_loss_multiplier;

            enum bandwidth_mixed_algo_t
            {
                    prefer_tcp = 0,
                    peer_proportional = 1

            };
            int mixed_mode_algorithm;
            bool rate_limit_utp;

            int listen_queue_size;

            bool announce_double_nat;

            int torrent_connect_boost;
            bool seeding_outgoing_connections;

            bool no_connect_privileged_ports;
            int alert_queue_size;
            int max_metadata_size;
            bool smooth_connects;
            bool always_send_user_agent;
            bool apply_ip_filter_to_trackers;
            int read_job_every;
            bool use_disk_read_ahead;
            bool lock_files;

            int ssl_listen;

            int tracker_backoff;

            bool ban_web_seeds;
    };

`version` is automatically set to the libtorrent version you're using in order to be forward binary compatible. This field should not be changed.

`user_agent` this is the client identification to the tracker. The recommended format of this string is: "ClientName/ClientVersion libtorrent/libtorrentVersion". This name will not only be used when making HTTP requests, but also when sending extended headers to peers that support that extension.

`tracker_completion_timeout` is the number of seconds the tracker connection will wait from when it sent the request until it considers the tracker to have timed-out. Default value is 60 seconds.

`tracker_receive_timeout` is the number of seconds to wait to receive any data from the tracker. If no data is received for this number of seconds, the tracker will be considered as having timed out. If a tracker is down, this is the kind of timeout that will occur. The default value is 20 seconds.

`stop_tracker_timeout` is the time to wait for tracker responses when shutting down the session object. This is given in seconds. Default 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.

`piece_timeout` controls the number of seconds from a request is sent until it times out if no piece response is returned.

`request_queue_time` is the length of the request queue given in the number of seconds it should take for the other end to send all the pieces. i.e. the actual number of requests depends on the download rate and this number.

`max_allowed_in_request_queue` is the number of outstanding block requests a peer is allowed to queue up in the client. If a peer sends more requests than this (before the first one has been handled) the last request will be dropped. The higher this is, the faster upload speeds the client can get to a single peer.

`max_out_request_queue` is the maximum number of outstanding requests to send to a peer. This limit takes precedence over `request_queue_time`. i.e. no matter the download speed, the number of outstanding requests will never exceed this limit.

`whole_pieces_threshold` is a limit in seconds. if a whole piece can be downloaded in at least this number of seconds from a specific peer, the peer_connection will prefer requesting whole pieces at a time from this peer. The benefit of this is to better utilize disk caches by doing localized accesses and also to make it easier to identify bad peers if a piece fails the hash check.

`peer_timeout` is the number of seconds the peer connection should wait (for any activity on the peer connection) before closing it due to time out. This defaults to 120 seconds, since that's what's specified in the protocol specification. After half the time out, a keep alive message is sent.

`urlseed_timeout` is the same as `peer_timeout` but applies only to url seeds. This value defaults to 20 seconds.

`urlseed_pipeline_size` controls the pipelining with the web server. When using persistent connections to HTTP 1.1 servers, the client is allowed to send more requests before the first response is received. This number controls the number of outstanding requests to use with url-seeds. Default is 5.

`file_pool_size` is the the upper limit on the total number of files this session will keep open. The reason why files are left open at all is that some anti virus software hooks on every file close, and scans the file for viruses. deferring the closing of the files will be the difference between a usable system and a completely hogged down system. Most operating systems also has a limit on the total number of file descriptors a process may have open. It is usually a good idea to find this limit and set the number of connections and the number of files limits so their sum is slightly below it.

`allow_multiple_connections_per_ip` determines if connections from the same IP address as existing connections should be rejected or not. Multiple connections from the same IP address is not allowed by default, to prevent abusive behavior by peers. It may be useful to allow such connections in cases where simulations are run on the same machie, and all peers in a swarm has the same IP address.

`max_failcount` is the maximum times we try to connect to a peer before stop connecting again. If a peer succeeds, the failcounter is reset. If a peer is retrieved from a peer source (other than DHT) the failcount is decremented by one, allowing another try.

`min_reconnect_time` is the time to wait between connection attempts. If the peer fails, the time is multiplied by fail counter.

`peer_connect_timeout` the number of seconds to wait after a connection attempt is initiated to a peer until it is considered as having timed out. The default is 10 seconds. This setting is especially important in case the number of half-open connections are limited, since stale half-open connection may delay the connection of other peers considerably.

`ignore_limits_on_local_network`, if set to true, upload, download and unchoke limits are ignored for peers on the local network.

`connection_speed` is the number of connection attempts that are made per second. If a number < 0 is specified, it will default to 200 connections per second. If 0 is specified, it means don't make outgoing connections at all.

`send_redundant_have` controls if have messages will be sent to peers that already have the piece. This is typically not necessary, but it might be necessary for collecting statistics in some cases. Default is false.

`lazy_bitfields` prevents outgoing bitfields from being full. If the client is seed, a few bits will be set to 0, and later filled in with have-messages. This is to prevent certain ISPs from stopping people from seeding.

`inactivity_timeout`, if a peer is uninteresting and uninterested for longer than this number of seconds, it will be disconnected. Default is 10 minutes

`unchoke_interval` is the number of seconds between chokes/unchokes. On this interval, peers are re-evaluated for being choked/unchoked. This is defined as 30 seconds in the protocol, and it should be significantly longer than what it takes for TCP to ramp up to it's max rate.

`optimistic_unchoke_interval` is the number of seconds between each optimistic unchoke. On this timer, the currently optimistically unchoked peer will change.

`announce_ip` is the ip address passed along to trackers as the `&ip=` parameter. If left as the default (an empty string), that parameter is omitted.

`num_want` is the number of peers we want from each tracker request. It defines what is sent as the `&num_want=` parameter to the tracker.

`initial_picker_threshold` specifies the number of pieces we need before we switch to rarest first picking. This defaults to 4, which means the 4 first pieces in any torrent are picked at random, the following pieces are picked in rarest first order.

`allowed_fast_set_size` is the number of pieces we allow peers to download from us without being unchoked.

`suggest_mode` controls whether or not libtorrent will send out suggest messages to create a bias of its peers to request certain pieces. The modes are:

- `no_piece_suggestsions` which is the default and will not send out suggest messages.
- `suggest_read_cache` which will send out suggest messages for the most recent pieces that are in the read cache.

`max_queued_disk_bytes` is the number maximum number of bytes, to be written to disk, that can wait in the disk I/O thread queue. This queue is only for waiting for the disk I/O thread to receive the job and either write it to disk or insert it in the write cache. When this limit is reached, the peer connections will stop reading data from their sockets, until the disk thread catches up. Setting this too low will severly limit your download rate.

`handshake_timeout` specifies the number of seconds we allow a peer to delay responding to a protocol handshake. If no response is received within this time, the connection is closed.

`use_dht_as_fallback` determines how the DHT is used. If this is true, the DHT will only be used for torrents where all trackers in its tracker list has failed. Either by an explicit error message or a time out. This is false by default, which means the DHT is used by default regardless of if the trackers fail or not.

`free_torrent_hashes` determines whether or not the torrent's piece hashes are kept in memory after the torrent becomes a seed or not. If it is set to `true` the hashes are freed once the torrent is a seed (they're not needed anymore since the torrent won't download anything more). If it's set to false they are not freed. If they are freed, the torrent_info returned by get_torrent_info() will return an object that may be incomplete, that cannot be passed back to async_add_torrent() add_torrent() for instance.

`upnp_ignore_nonrouters` indicates whether or not the UPnP implementation should ignore any broadcast response from a device whose address is not the configured router for this machine. i.e. it's a way to not talk to other people's routers by mistake.

`send_buffer_watermark` is the upper limit of the send buffer low-watermark. if the send buffer has fewer bytes than this, we'll read another 16kB block onto it. If set too small, upload rate capacity will suffer. If set too high, memory will be wasted. The actual watermark may be lower than this in case the upload rate is low, this is the upper limit.

`send_buffer_watermark_factor` is multiplied to the peer's upload rate to determine the low-watermark for the peer. It is specified as a percentage, which means 100 represents a factor of 1. The low-watermark is still clamped to not exceed the `send_buffer_watermark` upper limit. This defaults to 50. For high capacity connections, setting this higher can improve upload performance and disk throughput. Setting it too high may waste RAM and create a bias towards read jobs over write jobs.

`auto_upload_slots` defaults to true. When true, if there is a global upload limit set and the current upload rate is less than 90% of that, another upload slot is opened. If the upload rate has been saturated for an extended period of time, on upload slot is closed. The number of upload slots will never be less than what has been set by `session::set_max_uploads()`. To query the current number of upload slots, see `session_status::allowed_upload_slots`.

When `auto_upload_slots_rate_based` is set, and `auto_upload_slots` is set, the max upload slots setting is used as a minimum number of unchoked slots. This algorithm is designed to prevent the peer from spreading its upload capacity too thin, but still open more slots in order to utilize the full capacity.

`choking_algorithm` specifies which algorithm to use to determine which peers to unchoke. This setting replaces the deprecated settings `auto_upload_slots` and `auto_upload_slots_rate_based`.

The options for choking algorithms are:

- `fixed_slots_choker` is the traditional choker with a fixed number of unchoke slots (as specified by `session::set_max_uploads()`).
- `auto_expand_choker` opens at least the number of slots as specified by `session::set_max_uploads()` but opens up more slots if the upload capacity is not saturated. This unchoker will work just like the `fixed_slot_choker` if there's no global upload rate limit set.
- `rate_based_choker` opens up unchoke slots based on the upload rate achieved to peers. The more slots that are opened, the marginal upload rate required to open up another slot increases.
- `bittyrant_choker` attempts to optimize download rate by finding the reciprocation rate of each peer individually and prefers peers that gives the highest return on investment. It still allocates all upload capacity, but shuffles it around to the best peers first. For this choker to be efficient, you need to set a global upload rate limit (`session::set_upload_rate_limit()`). For more information about this choker, see the paper.

`seed_choking_algorithm` controls the seeding unchoke behavior. The available options are:

- `round_robin` which round-robins the peers that are unchoked when seeding. This distributes the upload bandwidht uniformly and fairly. It minimizes the ability for a peer to download everything without redistributing it.
- `fastest_upload` unchokes the peers we can send to the fastest. This might be a bit more reliable in utilizing all available capacity.
- `anti_leech` prioritizes peers who have just started or are just about to finish the download. The intention is to force peers in the middle of the download to trade with each other.

`use_parole_mode` specifies if parole mode should be used. Parole mode means that peers that participate in pieces that fail the hash check are put in a mode where they are only allowed to download whole pieces. If the whole piece a peer in parole mode fails the hash check, it is banned. If a peer participates in a piece that passes the hash check, it is taken out of parole mode.

`cache_size` is the disk write and read cache. It is specified in units of 16 KiB blocks. Buffers that are part of a peer's send or receive buffer also count against this limit. Send and receive buffers will never be denied to be allocated, but they will cause the actual cached blocks to be flushed or evicted. If this is set to -1, the cache size is automatically set to the amount of physical RAM available in the machine divided by 8. If the amount of physical RAM cannot be determined, it's set to 1024 (= 16 MiB).

Disk buffers are allocated using a pool allocator, the number of blocks that are allocated at a time when the pool needs to grow can be specified in `cache_buffer_chunk_size`. This defaults to 16 blocks. Lower numbers saves memory at the expense of more heap allocations. It must be at least 1.

`cache_expiry` is the number of seconds from the last cached write to a piece in the write cache, to when it's forcefully flushed to disk. Default is 60 second.

`use_read_cache`, is set to true (default), the disk cache is also used to cache pieces read from disk. Blocks for writing pieces takes presedence.

`explicit_read_cache` defaults to 0. If set to something greater than 0, the disk read cache will not be evicted by cache misses and will explicitly be controlled based on the rarity of pieces. Rare pieces are more likely to be cached. This would typically be used together with `suggest_mode` set to `suggest_read_cache`. The value is the number of pieces to keep in the read cache. If the actual read cache can't fit as many, it will essentially be clamped.

`explicit_cache_interval` is the number of seconds in between each refresh of a part of the explicit read cache. Torrents take turns in refreshing and this is the time in between each torrent refresh. Refreshing a torrent's explicit read cache means scanning all pieces and picking a random set of the rarest ones. There is an affinity to pick pieces that are already in the cache, so that subsequent refreshes only swaps in pieces that are rarer than whatever is in the cache at the time.

`disk_io_write_mode` and `disk_io_read_mode` determines how files are opened when they're in read only mode versus read and write mode. The options are:

> - enable_os_cache

>     This is the default and files are opened normally, with the OS caching reads and writes.

> - disable_os_cache_for_aligned_files

>     This will open files in unbuffered mode for files where every read and write would be sector aligned. Using aligned disk offsets is a requirement on some operating systems.

> - disable_os_cache

>     This opens all files in unbuffered mode (if allowed by the operating system). Linux and Windows, for instance, require disk offsets to be sector aligned, and in those cases, this option is the same as `disable_os_caches_for_aligned_files`.

One reason to disable caching is that it may help the operating system from growing its file cache indefinitely. Since some OSes only allow aligned files to be opened in unbuffered mode, It is recommended to make the largest file in a torrent the first file (with offset 0) or use pad files to align all files to piece boundries.

`outgoing_ports`, if set to something other than (0, 0) is a range of ports used to bind outgoing sockets to. This may be useful for users whose router allows them to assign QoS classes to traffic based on its local port. It is a range instead of a single port because of the problems with failing to reconnect to peers if a previous socket to that peer and port is in `TIME_WAIT` state.

Warning

setting outgoing ports will limit the ability to keep multiple connections to the same client, even for different torrents. It is not recommended to change this setting. Its main purpose is to use as an escape hatch for cheap routers with QoS capability but can only classify flows based on port numbers.

`peer_tos` determines the TOS byte set in the IP header of every packet sent to peers (including web seeds). The default value for this is `0x0` (no marking). One potentially useful TOS mark is `0x20`, this represents the QBone scavenger service. For more details, see QBSS.

`active_downloads` and `active_seeds` controls how many active seeding and downloading torrents the queuing mechanism allows. The target number of active torrents is `min(active_downloads + active_seeds, active_limit)`. `active_downloads` and `active_seeds` are upper limits on the number of downloading torrents and seeding torrents respectively. Setting the value to -1 means unlimited.

For example if there are 10 seeding torrents and 10 downloading torrents, and `active_downloads` is 4 and `active_seeds` is 4, there will be 4 seeds active and 4 downloading torrents. If the settings are `active_downloads` = 2 and `active_seeds` = 4, then there will be 2 downloading torrents and 4 seeding torrents active. Torrents that are not auto managed are also counted against these limits. If there are non-auto managed torrents that use up all the slots, no auto managed torrent will be activated.

`auto_manage_prefer_seeds` specifies if libtorrent should prefer giving seeds active slots or downloading torrents. The default is `false`.

if `dont_count_slow_torrents` is true, torrents without any payload transfers are not subject to the `active_seeds` and `active_downloads` limits. This is intended to make it more likely to utilize all available bandwidth, and avoid having torrents that don't transfer anything block the active slots.

`active_limit` is a hard limit on the number of active torrents. This applies even to slow torrents.

`active_dht_limit` is the max number of torrents to announce to the DHT. By default this is set to 88, which is no more than one DHT announce every 10 seconds.

`active_tracker_limit` is the max number of torrents to announce to their trackers. By default this is 360, which is no more than one announce every 5 seconds.

`active_lsd_limit` is the max number of torrents to announce to the local network over the local service discovery protocol. By default this is 80, which is no more than one announce every 5 seconds (assuming the default announce interval of 5 minutes).

You can have more torrents active, even though they are not announced to the DHT, lsd or their tracker. If some peer knows about you for any reason and tries to connect, it will still be accepted, unless the torrent is paused, which means it won't accept any connections.

`auto_manage_interval` is the number of seconds between the torrent queue is updated, and rotated.

`share_ratio_limit` is the upload / download ratio limit for considering a seeding torrent have met the seed limit criteria. See queuing.

`seed_time_ratio_limit` is the seeding time / downloading time ratio limit for considering a seeding torrent to have met the seed limit criteria. See queuing.

`seed_time_limit` is the limit on the time a torrent has been an active seed (specified in seconds) before it is considered having met the seed limit criteria. See queuing.

`peer_turnover_interval` controls a feature where libtorrent periodically can disconnect the least useful peers in the hope of connecting to better ones. This settings controls the interval of this optimistic disconnect. It defaults to every 5 minutes, and is specified in seconds.

`peer_turnover` Is the fraction of the peers that are disconnected. This is a float where 1.f represents all peers an 0 represents no peers. It defaults to 4% (i.e. 0.04f)

`peer_turnover_cutoff` is the cut off trigger for optimistic unchokes. If a torrent has more than this fraction of its connection limit, the optimistic unchoke is triggered. This defaults to 90% (i.e. 0.9f).

`close_redundant_connections` specifies whether libtorrent should close connections where both ends have no utility in keeping the connection open. For instance if both ends have completed their downloads, there's no point in keeping it open. This defaults to `true`.

`auto_scrape_interval` is the number of seconds between scrapes of queued torrents (auto managed and paused torrents). Auto managed torrents that are paused, are scraped regularly in order to keep track of their downloader/seed ratio. This ratio is used to determine which torrents to seed and which to pause.

`auto_scrape_min_interval` is the minimum number of seconds between any automatic scrape (regardless of torrent). In case there are a large number of paused auto managed torrents, this puts a limit on how often a scrape request is sent.

`max_peerlist_size` is the maximum number of peers in the list of known peers. These peers are not necessarily connected, so this number should be much greater than the maximum number of connected peers. Peers are evicted from the cache when the list grows passed 90% of this limit, and once the size hits the limit, peers are no longer added to the list. If this limit is set to 0, there is no limit on how many peers we'll keep in the peer list.

`max_paused_peerlist_size` is the max peer list size used for torrents that are paused. This default to the same as `max_peerlist_size`, but can be used to save memory for paused torrents, since it's not as important for them to keep a large peer list.

`min_announce_interval` is the minimum allowed announce interval for a tracker. This is specified in seconds, defaults to 5 minutes and is used as a sanity check on what is returned from a tracker. It mitigates hammering misconfigured trackers.

If `prioritize_partial_pieces` is true, partial pieces are picked before pieces that are more rare. If false, rare pieces are always prioritized, unless the number of partial pieces is growing out of proportion.

`auto_manage_startup` is the number of seconds a torrent is considered active after it was started, regardless of upload and download speed. This is so that newly started torrents are not considered inactive until they have a fair chance to start downloading.

If `rate_limit_ip_overhead` is set to true, the estimated TCP/IP overhead is drained from the rate limiters, to avoid exceeding the limits with the total traffic

`announce_to_all_trackers` controls how multi tracker torrents are treated. If this is set to true, all trackers in the same tier are announced to in parallel. If all trackers in tier 0 fails, all trackers in tier 1 are announced as well. If it's set to false, the behavior is as defined by the multi tracker specification. It defaults to false, which is the same behavior previous versions of libtorrent has had as well.

`announce_to_all_tiers` also controls how multi tracker torrents are treated. When this is set to true, one tracker from each tier is announced to. This is the uTorrent behavior. This is false by default in order to comply with the multi-tracker specification.

`prefer_udp_trackers` is true by default. It means that trackers may be rearranged in a way that udp trackers are always tried before http trackers for the same hostname. Setting this to fails means that the trackers' tier is respected and there's no preference of one protocol over another.

`strict_super_seeding` when this is set to true, a piece has to have been forwarded to a third peer before another one is handed out. This is the traditional definition of super seeding.

`seeding_piece_quota` is the number of pieces to send to a peer, when seeding, before rotating in another peer to the unchoke set. It defaults to 3 pieces, which means that when seeding, any peer we've sent more than this number of pieces to will be unchoked in favour of a choked peer.

`max_sparse_regions` is a limit of the number of sparse regions in a torrent. A sparse region is defined as a hole of pieces we have not yet downloaded, in between pieces that have been downloaded. This is used as a hack for windows vista which has a bug where you cannot write files with more than a certain number of sparse regions. This limit is not hard, it will be exceeded. Once it's exceeded, pieces that will maintain or decrease the number of sparse regions are prioritized. To disable this functionality, set this to 0. It defaults to 0 on all platforms except windows.

`lock_disk_cache` if lock disk cache is set to true the disk cache that's in use, will be locked in physical memory, preventing it from being swapped out.

`max_rejects` is the number of piece requests we will reject in a row while a peer is choked before the peer is considered abusive and is disconnected.

`recv_socket_buffer_size` and `send_socket_buffer_size` specifies the buffer sizes set on peer sockets. 0 (which is the default) means the OS default (i.e. don't change the buffer sizes). The socket buffer sizes are changed using setsockopt() with SOL_SOCKET/SO_RCVBUF and SO_SNDBUFFER.

`optimize_hashing_for_speed` chooses between two ways of reading back piece data from disk when its complete and needs to be verified against the piece hash. This happens if some blocks were flushed to the disk out of order. Everything that is flushed in order is hashed as it goes along. Optimizing for speed will allocate space to fit all the the remaingin, unhashed, part of the piece, reads the data into it in a single call and hashes it. This is the default. If `optimizing_hashing_for_speed` is false, a single block will be allocated (16 kB), and the unhashed parts of the piece are read, one at a time, and hashed in this single block. This is appropriate on systems that are memory constrained.

`file_checks_delay_per_block` is the number of milliseconds to sleep in between disk read operations when checking torrents. This defaults to 0, but can be set to higher numbers to slow down the rate at which data is read from the disk while checking. This may be useful for background tasks that doesn't matter if they take a bit longer, as long as they leave disk I/O time for other processes.

`disk_cache_algorithm` tells the disk I/O thread which cache flush algorithm to use. The default algorithm is largest_contiguous. This flushes the entire piece, in the write cache, that was least recently written to. This is specified by the `session_settings::lru` enum value. `session_settings::largest_contiguous` will flush the largest sequences of contiguous blocks from the write cache, regarless of the piece's last use time. `session_settings::avoid_readback` will prioritize flushing blocks that will avoid having to read them back in to verify the hash of the piece once it's done. This is especially useful for high throughput setups, where reading from the disk is especially expensive.

`read_cache_line_size` is the number of blocks to read into the read cache when a read cache miss occurs. Setting this to 0 is essentially the same thing as disabling read cache. The number of blocks read into the read cache is always capped by the piece boundry.

When a piece in the write cache has `write_cache_line_size` contiguous blocks in it, they will be flushed. Setting this to 1 effectively disables the write cache.

`optimistic_disk_retry` is the number of seconds from a disk write errors occur on a torrent until libtorrent will take it out of the upload mode, to test if the error condition has been fixed.

libtorrent will only do this automatically for auto managed torrents.

You can explicitly take a torrent out of upload only mode using set_upload_mode().

`disable_hash_check` controls if downloaded pieces are verified against the piece hashes in the torrent file or not. The default is false, i.e. to verify all downloaded data. It may be useful to turn this off for performance profiling and simulation scenarios. Do not disable the hash check for regular bittorrent clients.

`max_suggest_pieces` is the max number of suggested piece indices received from a peer that's remembered. If a peer floods suggest messages, this limit prevents libtorrent from using too much RAM. It defaults to 10.

If `drop_skipped_requests` is set to true (it defaults to false), piece requests that have been skipped enough times when piece messages are received, will be considered lost. Requests are considered skipped when the returned piece messages are re-ordered compared to the order of the requests. This was an attempt to get out of dead-locks caused by BitComet peers silently ignoring some requests. It may cause problems at high rates, and high level of reordering in the uploading peer, that's why it's disabled by default.

`low_prio_disk` determines if the disk I/O should use a normal or low priority policy. This defaults to true, which means that it's low priority by default. Other processes doing disk I/O will normally take priority in this mode. This is meant to improve the overall responsiveness of the system while downloading in the background. For high-performance server setups, this might not be desirable.

`local_service_announce_interval` is the time between local network announces for a torrent. By default, when local service discovery is enabled a torrent announces itself every 5 minutes. This interval is specified in seconds.

`dht_announce_interval` is the number of seconds between announcing torrents to the distributed hash table (DHT). This is specified to be 15 minutes which is its default.

`dht_max_torrents` is the max number of torrents we will track in the DHT.

`udp_tracker_token_expiry` is the number of seconds libtorrent will keep UDP tracker connection tokens around for. This is specified to be 60 seconds, and defaults to that. The higher this value is, the fewer packets have to be sent to the UDP tracker. In order for higher values to work, the tracker needs to be configured to match the expiration time for tokens.

`volatile_read_cache`, if this is set to true, read cache blocks that are hit by peer read requests are removed from the disk cache to free up more space. This is useful if you don't expect the disk cache to create any cache hits from other peers than the one who triggered the cache line to be read into the cache in the first place.

`guided_read_cache` enables the disk cache to adjust the size of a cache line generated by peers to depend on the upload rate you are sending to that peer. The intention is to optimize the RAM usage of the cache, to read ahead further for peers that you're sending faster to.

`default_min_cache_age` is the minimum number of seconds any read cache line is kept in the cache. This defaults to one second but may be greater if `guided_read_cache` is enabled. Having a lower bound on the time a cache line stays in the cache is an attempt to avoid swapping the same pieces in and out of the cache in case there is a shortage of spare cache space.

`num_optimistic_unchoke_slots` is the number of optimistic unchoke slots to use. It defaults to 0, which means automatic. Having a higher number of optimistic unchoke slots mean you will find the good peers faster but with the trade-off to use up more bandwidth. When this is set to 0, libtorrent opens up 20% of your allowed upload slots as optimistic unchoke slots.

`no_atime_storage` this is a linux-only option and passes in the `O_NOATIME` to `open()` when opening files. This may lead to some disk performance improvements.

`default_est_reciprocation_rate` is the assumed reciprocation rate from peers when using the BitTyrant choker. This defaults to 14 kiB/s. If set too high, you will over-estimate your peers and be more altruistic while finding the true reciprocation rate, if it's set too low, you'll be too stingy and waste finding the true reciprocation rate.

`increase_est_reciprocation_rate` specifies how many percent the extimated reciprocation rate should be increased by each unchoke interval a peer is still choking us back. This defaults to 20%. This only applies to the BitTyrant choker.

`decrease_est_reciprocation_rate` specifies how many percent the estimated reciprocation rate should be decreased by each unchoke interval a peer unchokes us. This default to 3%. This only applies to the BitTyrant choker.

`incoming_starts_queued_torrents` defaults to false. If a torrent has been paused by the auto managed feature in libtorrent, i.e. the torrent is paused and auto managed, this feature affects whether or not it is automatically started on an incoming connection. The main reason to queue torrents, is not to make them unavailable, but to save on the overhead of announcing to the trackers, the DHT and to avoid spreading one's unchoke slots too thin. If a peer managed to find us, even though we're no in the torrent anymore, this setting can make us start the torrent and serve it.

When `report_true_downloaded` is true, the `&downloaded=` argument sent to trackers will include redundant downloaded bytes. It defaults to `false`, which means redundant bytes are not reported to the tracker.

`strict_end_game_mode` defaults to true, and controls when a block may be requested twice. If this is `true`, a block may only be requested twice when there's ay least one request to every piece that's left to download in the torrent. This may slow down progress on some pieces sometimes, but it may also avoid downloading a lot of redundant bytes. If this is `false`, libtorrent attempts to use each peer connection to its max, by always requesting something, even if it means requesting something that has been requested from another peer already.

if `broadcast_lsd` is set to true, the local peer discovery (or Local Service Discovery) will not only use IP multicast, but also broadcast its messages. This can be useful when running on networks that don't support multicast. Since broadcast messages might be expensive and disruptive on networks, only every 8th announce uses broadcast.

`enable_outgoing_utp`, `enable_incoming_utp`, `enable_outgoing_tcp`, `enable_incoming_tcp` all determines if libtorrent should attempt to make outgoing connections of the specific type, or allow incoming connection. By default all of them are enabled.

`ignore_resume_timestamps` determines if the storage, when loading resume data files, should verify that the file modification time with the timestamps in the resume data. This defaults to false, which means timestamps are taken into account, and resume data is less likely to accepted (torrents are more likely to be fully checked when loaded). It might be useful to set this to true if your network is faster than your disk, and it would be faster to redownload potentially missed pieces than to go through the whole storage to look for them.

`no_recheck_incomplete_resume` determines if the storage should check the whole files when resume data is incomplete or missing or whether it should simply assume we don't have any of the data. By default, this is determined by the existance of any of the files. By setting this setting to true, the files won't be checked, but will go straight to download mode.

`anonymous_mode` defaults to false. When set to true, the client tries to hide its identity to a certain degree. The peer-ID will no longer include the client's fingerprint. The user-agent will be reset to an empty string. Trackers will only be used if they are using a proxy server. The listen sockets are closed, and incoming connections will only be accepted through a SOCKS5 or I2P proxy (if a peer proxy is set up and is run on the same machine as the tracker proxy). Since no incoming connections are accepted, NAT-PMP, UPnP, DHT and local peer discovery are all turned off when this setting is enabled.

If you're using I2P, it might make sense to enable anonymous mode as well.

`tick_interval` specifies the number of milliseconds between internal ticks. This is the frequency with which bandwidth quota is distributed to peers. It should not be more than one second (i.e. 1000 ms). Setting this to a low value (around 100) means higher resolution bandwidth quota distribution, setting it to a higher value saves CPU cycles.

`share_mode_target` specifies the target share ratio for share mode torrents. This defaults to 3, meaning we'll try to upload 3 times as much as we download. Setting this very high, will make it very conservative and you might end up not downloading anything ever (and not affecting your share ratio). It does not make any sense to set this any lower than 2. For instance, if only 3 peers need to download the rarest piece, it's impossible to download a single piece and upload it more than 3 times. If the share_mode_target is set to more than 3, nothing is downloaded.

`upload_rate_limit`, `download_rate_limit`, `local_upload_rate_limit` and `local_download_rate_limit` sets the session-global limits of upload and download rate limits, in bytes per second. The local rates refer to peers on the local network. By default peers on the local network are not rate limited.

These rate limits are only used for local peers (peers within the same subnet as the client itself) and it is only used when `session_settings::ignore_limits_on_local_network` is set to true (which it is by default). These rate limits default to unthrottled, but can be useful in case you want to treat local peers preferentially, but not quite unthrottled.

A value of 0 means unlimited.

`dht_upload_rate_limit` sets the rate limit on the DHT. This is specified in bytes per second and defaults to 4000. For busy boxes with lots of torrents that requires more DHT traffic, this should be raised.

`unchoke_slots_limit` is the mac number of unchoked peers in the session.

The number of unchoke slots may be ignored depending on what `choking_algorithm` is set to.

`half_open_limit` sets the maximum number of half-open connections libtorrent will have when connecting to peers. A half-open connection is one where connect() has been called, but the connection still hasn't been established (nor failed). Windows XP Service Pack 2 sets a default, system wide, limit of the number of half-open connections to 10. So, this limit can be used to work nicer together with other network applications on that system. The default is to have no limit, and passing -1 as the limit, means to have no limit. When limiting the number of simultaneous connection attempts, peers will be put in a queue waiting for their turn to get connected.

`connections_limit` sets a global limit on the number of connections opened. The number of connections is set to a hard minimum of at least two per torrent, so if you set a too low connections limit, and open too many torrents, the limit will not be met.

`utp_target_delay` is the target delay for uTP sockets in milliseconds. A high value will make uTP connections more aggressive and cause longer queues in the upload bottleneck. It cannot be too low, since the noise in the measurements would cause it to send too slow. The default is 50 milliseconds.

`utp_gain_factor` is the number of bytes the uTP congestion window can increase at the most in one RTT. This defaults to 300 bytes. If this is set too high, the congestion controller reacts too hard to noise and will not be stable, if it's set too low, it will react slow to congestion and not back off as fast.

`utp_min_timeout` is the shortest allowed uTP socket timeout, specified in milliseconds. This defaults to 500 milliseconds. The timeout depends on the RTT of the connection, but is never smaller than this value. A connection times out when every packet in a window is lost, or when a packet is lost twice in a row (i.e. the resent packet is lost as well).

The shorter the timeout is, the faster the connection will recover from this situation, assuming the RTT is low enough.

`utp_syn_resends` is the number of SYN packets that are sent (and timed out) before giving up and closing the socket.

`utp_num_resends` is the number of times a packet is sent (and lossed or timed out) before giving up and closing the connection.

`utp_connect_timeout` is the number of milliseconds of timeout for the initial SYN packet for uTP connections. For each timed out packet (in a row), the timeout is doubled.

`utp_delayed_ack` is the number of milliseconds to delay ACKs the most. Delaying ACKs significantly helps reducing the amount of protocol overhead in the reverse direction from downloads. It defaults to 100 milliseconds. If set to 0, delayed ACKs are disabled and every incoming payload packet is ACKed. The granularity of this timer is capped by the tick interval (as specified by `tick_interval`).

`utp_dynamic_sock_buf` controls if the uTP socket manager is allowed to increase the socket buffer if a network interface with a large MTU is used (such as loopback or ethernet jumbo frames). This defaults to true and might improve uTP throughput. For RAM constrained systems, disabling this typically saves around 30kB in user space and probably around 400kB in kernel socket buffers (it adjusts the send and receive buffer size on the kernel socket, both for IPv4 and IPv6).

`utp_loss_multiplier` controls how the congestion window is changed when a packet loss is experienced. It's specified as a percentage multiplier for `cwnd`. By default it's set to 50 (i.e. cut in half). Do not change this value unless you know what you're doing. Never set it higher than 100.

The `mixed_mode_algorithm` determines how to treat TCP connections when there are uTP connections. Since uTP is designed to yield to TCP, there's an inherent problem when using swarms that have both TCP and uTP connections. If nothing is done, uTP connections would often be starved out for bandwidth by the TCP connections. This mode is `prefer_tcp`. The `peer_proportional` mode simply looks at the current throughput and rate limits all TCP connections to their proportional share based on how many of the connections are TCP. This works best if uTP connections are not rate limited by the global rate limiter (which they aren't by default).

`rate_limit_utp` determines if uTP connections should be throttled by the global rate limiter or not. By default they are.

`listen_queue_size` is the value passed in to listen() for the listen socket. It is the number of outstanding incoming connections to queue up while we're not actively waiting for a connection to be accepted. The default is 5 which should be sufficient for any normal client. If this is a high performance server which expects to receive a lot of connections, or used in a simulator or test, it might make sense to raise this number. It will not take affect until listen_on() is called again (or for the first time).

if `announce_double_nat` is true, the `&ip=` argument in tracker requests (unless otherwise specified) will be set to the intermediate IP address, if the user is double NATed. If ther user is not double NATed, this option has no affect.

`torrent_connect_boost` is the number of peers to try to connect to immediately when the first tracker response is received for a torrent. This is a boost to given to new torrents to accelerate them starting up. The normal connect scheduler is run once every second, this allows peers to be connected immediately instead of waiting for the session tick to trigger connections.

`seeding_outgoing_connections` determines if seeding (and finished) torrents should attempt to make outgoing connections or not. By default this is true. It may be set to false in very specific applications where the cost of making outgoing connections is high, and there are no or small benefits of doing so. For instance, if no nodes are behind a firewall or a NAT, seeds don't need to make outgoing connections.

if `no_connect_privileged_ports` is true (which is the default), libtorrent will not connect to any peers on priviliged ports (<= 1023). This can mitigate using bittorrent swarms for certain DDoS attacks.

`alert_queue_size` is the maximum number of alerts queued up internally. If alerts are not popped, the queue will eventually fill up to this level. This defaults to 1000.

`max_metadata_size` is the maximum allowed size (in bytes) to be received by the metadata extension, i.e. magnet links. It defaults to 1 MiB.

`smooth_connects` is true by default, which means the number of connection attempts per second may be limited to below the `connection_speed`, in case we're close to bump up against the limit of number of connections. The intention of this setting is to more evenly distribute our connection attempts over time, instead of attempting to connectin in batches, and timing them out in batches.

`always_send_user_agent` defaults to false. When set to true, web connections will include a user-agent with every request, as opposed to just the first request in a connection.

`apply_ip_filter_to_trackers` defaults to true. It determines whether the IP filter applies to trackers as well as peers. If this is set to false, trackers are exempt from the IP filter (if there is one). If no IP filter is set, this setting is irrelevant.

`read_job_every` is used to avoid starvation of read jobs in the disk I/O thread. By default, read jobs are deferred, sorted by physical disk location and serviced once all write jobs have been issued. In scenarios where the download rate is enough to saturate the disk, there's a risk the read jobs will never be serviced. With this setting, every x write job, issued in a row, will instead pick one read job off of the sorted queue, where x is `read_job_every`.

`use_disk_read_ahead` defaults to true and will attempt to optimize disk reads by giving the operating system heads up of disk read requests as they are queued in the disk job queue. This gives a significant performance boost for seeding.

`lock_files` determines whether or not to lock files which libtorrent is downloading to or seeding from. This is implemented using `fcntl(F_SETLK)` on unix systems and by not passing in `SHARE_READ` and `SHARE_WRITE` on windows. This might prevent 3rd party processes from corrupting the files under libtorrent's feet.

`ssl_listen` sets the listen port for SSL connections. If this is set to 0, no SSL listen port is opened. Otherwise a socket is opened on this port. This setting is only taken into account when opening the regular listen port, and won't re-open the listen socket simply by changing this setting.

It defaults to port 4433.

`tracker_backoff` determines how aggressively to back off from retrying failing trackers. This value determines x in the following formula, determining the number of seconds to wait until the next retry:

> delay = 5 + 5 * x / 100 * fails^2

It defaults to 250.

This setting may be useful to make libtorrent more or less aggressive in hitting trackers.

`ban_web_seeds` enables banning web seeds. By default, web seeds that send corrupt data are banned.

# pe_settings

The `pe_settings` structure is used to control the settings related to peer protocol encryption:

    struct pe_settings
    {
            pe_settings();

            enum enc_policy
            {
                    forced,
                    enabled,
                    disabled
            };

            enum enc_level
            {
                    plaintext,
                    rc4,
                    both
            };

            enc_policy out_enc_policy;
            enc_policy in_enc_policy;
            enc_level allowed_enc_level;
            bool prefer_rc4;
    };

`in_enc_policy` and `out_enc_policy` control the settings for incoming and outgoing connections respectively. The settings for these are:

> - `forced` - Only encrypted connections are allowed. Incoming connections that are not encrypted are closed and if the encrypted outgoing connection fails, a non-encrypted retry will not be made.
> - `enabled` - encrypted connections are enabled, but non-encrypted connections are allowed. An incoming non-encrypted connection will be accepted, and if an outgoing encrypted connection fails, a non- encrypted connection will be tried.
> - `disabled` - only non-encrypted connections are allowed.

`allowed_enc_level` determines the encryption level of the connections. This setting will adjust which encryption scheme is offered to the other peer, as well as which encryption scheme is selected by the client. The settings are:

> - `plaintext` - only the handshake is encrypted, the bulk of the traffic remains unchanged.
> - `rc4` - the entire stream is encrypted with RC4
> - `both` - both RC4 and plaintext connections are allowed.

`prefer_rc4` can be set to true if you want to prefer the RC4 encrypted stream.

# proxy_settings

The `proxy_settings` structs contains the information needed to direct certain traffic to a proxy.

>     struct proxy_settings
>     {
>             proxy_settings();

>             std::string hostname;
>             int port;

>             std::string username;
>             std::string password;

>             enum proxy_type
>             {
>                     none,
>                     socks4,
>                     socks5,
>                     socks5_pw,
>                     http,
>                     http_pw
>             };

>             proxy_type type;
>             bool proxy_hostnames;
>             bool proxy_peer_connections;
>     };

`hostname` is the name or IP of the proxy server. `port` is the port number the proxy listens to. If required, `username` and `password` can be set to authenticate with the proxy.

The `type` tells libtorrent what kind of proxy server it is. The following options are available:

> - `none` - This is the default, no proxy server is used, all other fields are ignored.
> - `socks4` - The server is assumed to be a SOCKS4 server that requires a username.
> - `socks5` - The server is assumed to be a SOCKS5 server (RFC 1928) that does not require any authentication. The username and password are ignored.
> - `socks5_pw` - The server is assumed to be a SOCKS5 server that supports plain text username and password authentication (RFC 1929). The username and password specified may be sent to the proxy if it requires.
> - `http` - The server is assumed to be an HTTP proxy. If the transport used for the connection is non-HTTP, the server is assumed to support the CONNECT method. i.e. for web seeds and HTTP trackers, a plain proxy will suffice. The proxy is assumed to not require authorization. The username and password will not be used.
> - `http_pw` - The server is assumed to be an HTTP proxy that requires user authorization. The username and password will be sent to the proxy.

`proxy_hostnames` defaults to true. It means that hostnames should be attempted to be resolved through the proxy instead of using the local DNS service. This is only supported by SOCKS5 and HTTP.

`proxy_peer_connections` determines whether or not to excempt peer and web seed connections from using the proxy. This defaults to true, i.e. peer connections are proxied by default.

# ip_filter

The `ip_filter` class is a set of rules that uniquely categorizes all ip addresses as allowed or disallowed. The default constructor creates a single rule that allows all addresses (0.0.0.0 - 255.255.255.255 for the IPv4 range, and the equivalent range covering all addresses for the IPv6 range).

>     template <class Addr>
>     struct ip_range
>     {
>             Addr first;
>             Addr last;
>             int flags;
>     };

>     class ip_filter
>     {
>     public:
>             enum access_flags { blocked = 1 };

>             ip_filter();
>             void add_rule(address first, address last, int flags);
>             int access(address const& addr) const;

>             typedef boost::tuple<std::vector<ip_range<address_v4> >
>                     , std::vector<ip_range<address_v6> > > filter_tuple_t;

>             filter_tuple_t export_filter() const;
>     };

## ip_filter()

>     ip_filter()

Creates a default filter that doesn't filter any address.

postcondition: `access(x) == 0` for every `x`

## add_rule()

>     void add_rule(address first, address last, int flags);

Adds a rule to the filter. `first` and `last` defines a range of ip addresses that will be marked with the given flags. The `flags` can currently be 0, which means allowed, or `ip_filter::blocked`, which means disallowed.

precondition: `first.is_v4() == last.is_v4() && first.is_v6() == last.is_v6()`

postcondition: `access(x) == flags` for every `x` in the range [`first`, `last`]

This means that in a case of overlapping ranges, the last one applied takes precedence.

## access()

>     int access(address const& addr) const;

Returns the access permissions for the given address (`addr`). The permission can currently be 0 or `ip_filter::blocked`. The complexity of this operation is O(`log` n), where n is the minimum number of non-overlapping ranges to describe the current filter.

## export_filter()

>     boost::tuple<std::vector<ip_range<address_v4> >
>             , std::vector<ip_range<address_v6> > > export_filter() const;

This function will return the current state of the filter in the minimum number of ranges possible. They are sorted from ranges in low addresses to high addresses. Each entry in the returned vector is a range with the access control specified in its `flags` field.

The return value is a tuple containing two range-lists. One for IPv4 addresses and one for IPv6 addresses.

# 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.

# bitfield

The bitfiled type stores any number of bits as a bitfield in an array.

    class bitfield
    {
            bitfield();
            bitfield(int bits);
            bitfield(int bits, bool val);
            bitfield(char const* bytes, int bits);
            bitfield(bitfield const& rhs);

            void borrow_bytes(char* bytes, int bits);
            ~bitfield();

            void assign(char const* bytes, int bits);

            bool operator[](int index) const;

            bool get_bit(int index) const;

            void clear_bit(int index);
            void set_bit(int index);

            std::size_t size() const;
            bool empty() const;

            char const* bytes() const;

            bitfield& operator=(bitfield const& rhs);

            int count() const;

            typedef const_iterator;
            const_iterator begin() const;
            const_iterator end() const;

            void resize(int bits, bool val);
            void set_all();
            void clear_all();
            void resize(int bits);
    };

# hasher

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

    class hasher
    {
    public:
            hasher();
            hasher(char const* data, unsigned int len);

            void update(char const* 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.

The constructor that takes a `char const*` and an integer will construct the sha1 context and feed it the data passed in.

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 name[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

There's currently an informal directory of client id's here.

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.

# UPnP and NAT-PMP

The `upnp` and `natpmp` classes contains the state for all UPnP and NAT-PMP mappings, by default 1 or two mappings are made by libtorrent, one for the listen port and one for the DHT port (UDP).

    class upnp
    {
    public:

            enum protocol_type { none = 0, udp = 1, tcp = 2 };
            int add_mapping(protocol_type p, int external_port, int local_port);
            void delete_mapping(int mapping_index);

            void discover_device();
            void close();

            std::string router_model();
    };

    class natpmp
    {
    public:

            enum protocol_type { none = 0, udp = 1, tcp = 2 };
            int add_mapping(protocol_type p, int external_port, int local_port);
            void delete_mapping(int mapping_index);

            void close();
            void rebind(address const& listen_interface);
    };

`discover_device()`, `close()` and `rebind()` are for internal uses and should not be called directly by clients.

## add_mapping()

>     int add_mapping(protocol_type p, int external_port, int local_port);

Attempts to add a port mapping for the specified protocol. Valid protocols are `upnp::tcp` and `upnp::udp` for the UPnP class and `natpmp::tcp` and `natpmp::udp` for the NAT-PMP class.

`external_port` is the port on the external address that will be mapped. This is a hint, you are not guaranteed that this port will be available, and it may end up being something else. In the portmap_alert notification, the actual external port is reported.

`local_port` is the port in the local machine that the mapping should forward to.

The return value is an index that identifies this port mapping. This is used to refer to mappings that fails or succeeds in the portmap_error_alert and portmap_alert respectively. If The mapping fails immediately, the return value is -1, which means failure. There will not be any error alert notification for mappings that fail with a -1 return value.

## delete_mapping()

>     void delete_mapping(int mapping_index);

This function removes a port mapping. `mapping_index` is the index that refers to the mapping you want to remove, which was returned from add_mapping().

## router_model()

>     std::string router_model();

This is only available for UPnP routers. If the model is advertized by the router, it can be queried through this function.

# 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.

## client_fingerprint()

>     boost::optional<fingerprint> client_fingerprint(peer_id const& p);

Returns an optional fingerprint if any can be identified from the peer id. This can be used to automate the identification of clients. It will not be able to identify peers with non- standard encodings. Only Azureus style, Shadow's style and Mainline style. This function is declared in the header `<libtorrent/identify_client.hpp>`.

## lazy_bdecode()

>     int lazy_bdecode(char const* start, char const* end, lazy_entry& ret
>             , error_code& ec, int* error_pos = 0, int depth_limit = 1000
>             , int item_limit = 1000000);

This function decodes bencoded data.

Whenever possible, `lazy_bdecode()` should be preferred over `bdecode()`. It is more efficient and more secure. It supports having constraints on the amount of memory is consumed by the parser.

lazy refers to the fact that it doesn't copy any actual data out of the bencoded buffer. It builds a tree of `lazy_entry` which has pointers into the bencoded buffer. This makes it very fast and efficient. On top of that, it is not recursive, which saves a lot of stack space when parsing deeply nested trees. However, in order to protect against potential attacks, the `depth_limit` and `item_limit` control how many levels deep the tree is allowed to get. With recursive parser, a few thousand levels would be enough to exhaust the threads stack and terminate the process. The `item_limit` protects against very large structures, not necessarily deep. Each bencoded item in the structure causes the parser to allocate some amount of memory, this memory is constant regardless of how much data actually is stored in the item. One potential attack is to create a bencoded list of hundreds of thousands empty strings, which would cause the parser to allocate a significant amount of memory, perhaps more than is available on the machine, and effectively provide a denial of service. The default item limit is set as a reasonable upper limit for desktop computers. Very few torrents have more items in them. The limit corresponds to about 25 MB, which might be a bit much for embedded systems.

`start` and `end` defines the bencoded buffer to be decoded. `ret` is the `lazy_entry` which is filled in with the whole decoded tree. `ec` is a reference to an `error_code` which is set to describe the error encountered in case the function fails. `error_pos` is an optional pointer to an int, which will be set to the byte offset into the buffer where an error occurred, in case the function fails.

## 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.

If possible, lazy_bdecode() should be preferred over `bdecode()`.

The entry class is the internal representation of the bencoded data and it can be used to retrieve information, an entry can also be build by the program and given to `bencode()` to encode it into the `OutIt` iterator.

The `OutIt` and `InIt` are iterators (InputIterator and OutputIterator respectively). They are templates and are usually instantiated as ostream_iterator, back_insert_iterator or istream_iterator. These functions will assume that the iterator refers to a character (`char`). So, if you want to encode entry `e` into a buffer in memory, you can do it like this:

    std::vector<char> buffer;
    bencode(std::back_inserter(buf), e);

If you want to decode a torrent file from a buffer in memory, you can do it like this:

    std::vector<char> buffer;
    // ...
    entry e = bdecode(buf.begin(), buf.end());

Or, if you have a raw char buffer:

    const char* buf;
    // ...
    entry e = bdecode(buf, buf + data_size);

Now we just need to know how to retrieve information from the entry.

If `bdecode()` encounters invalid encoded data in the range given to it it will throw libtorrent_exception.

## add_magnet_uri()

deprecated

>     torrent_handle add_magnet_uri(session& ses, std::string const& uri
>             add_torrent_params p);
>     torrent_handle add_magnet_uri(session& ses, std::string const& uri
>             add_torrent_params p, error_code& ec);

This function parses the magnet URI (`uri`) as a bittorrent magnet link, and adds the torrent to the specified session (`ses`). It returns the handle to the newly added torrent, or an invalid handle in case parsing failed. To control some initial settings of the torrent, sepcify those in the `add_torrent_params`, `p`. See async_add_torrent() add_torrent().

The overload that does not take an `error_code` throws an exception on error and is not available when building without exception support.

A simpler way to add a magnet link to a session is to pass in the link through `add_torrent_params::url` argument to `session::add_torrent()`.

For more information about magnet links, see magnet links.

## parse_magnet_uri()

>     void parse_magnet_uri(std::string const& uri, add_torrent_params& p, error_code& ec);

This function parses out information from the magnet link and populates the `add_torrent_params` object.

## make_magnet_uri()

>     std::string make_magnet_uri(torrent_handle const& handle);

Generates a magnet URI from the specified torrent. If the torrent handle is invalid, an empty string is returned.

For more information about magnet links, see magnet links.

# alerts

The `pop_alert()` function on session is the interface for retrieving alerts, warnings, messages and errors from libtorrent. If no alerts have been posted by libtorrent `pop_alert()` will return a default initialized `auto_ptr` object. If there is an alert in libtorrent's queue, the alert from the front of the queue is popped and returned. You can then use the alert object and query

By default, only errors are reported. set_alert_mask() can be used to specify which kinds of events should be reported. The alert mask is a bitmask with the following bits:

`error_notification` | Enables alerts that report an error. This includes: tracker errors tracker warnings file errors resume data failures web seed errors .torrent files errors listen socket errors port mapping errors
`peer_notification` | Enables alerts when peers send invalid requests, get banned or snubbed.
`port_mapping_notification` | Enables alerts for port mapping events. For NAT-PMP and UPnP.
`storage_notification` | Enables alerts for events related to the storage. File errors and synchronization events for moving the storage, renaming files etc.
`tracker_notification` | Enables all tracker events. Includes announcing to trackers, receiving responses, warnings and errors.
`debug_notification` | Low level alerts for when peers are connected and disconnected.
`status_notification` | Enables alerts for when a torrent or the session changes state.
`progress_notification` | Alerts for when blocks are requested and completed. Also when pieces are completed.
`ip_block_notification` | Alerts when a peer is blocked by the ip blocker or port blocker.
`performance_warning` | Alerts when some limit is reached that might limit the download or upload rate.
`stats_notification` | If you enable these alerts, you will receive a `stats_alert` approximately once every second, for every active torrent. These alerts contain all statistics counters for the interval since the lasts stats alert.
`dht_notification` | Alerts on events in the DHT node. For incoming searches or bootstrapping being done etc.
`rss_notification` | Alerts on RSS related events, like feeds being updated, feed error conditions and successful RSS feed updates. Enabling this categoty will make you receive `rss_alert` alerts.
`all_categories` | The full bitmask, representing all available categories.

Every alert belongs to one or more category. There is a small cost involved in posting alerts. Only alerts that belong to an enabled category are posted. Setting the alert bitmask to 0 will disable all alerts

When you get an alert, you can use `alert_cast<>` to attempt to cast the pointer to a more specific alert type, to be queried for more information about the alert. `alert_cast` has the followinf signature:

    template <T> T* alert_cast(alert* a);
    template <T> T const* alert_cast(alert const* a);

You can also use a alert 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 category_t
            {
                    error_notification = implementation defined,
                    peer_notification = implementation defined,
                    port_mapping_notification = implementation defined,
                    storage_notification = implementation defined,
                    tracker_notification = implementation defined,
                    debug_notification = implementation defined,
                    status_notification = implementation defined,
                    progress_notification = implementation defined,
                    ip_block_notification = implementation defined,
                    performance_warning = implementation defined,
                    dht_notification = implementation defined,
                    stats_notification = implementation defined,

                    all_categories = implementation defined
            };

            ptime timestamp() const;

            virtual ~alert();

            virtual int type() const = 0;
            virtual std::string message() const = 0;
            virtual char const* what() const = 0;
            virtual int category() const = 0;
            virtual bool discardable() const;
            virtual std::auto_ptr<alert> clone() const = 0;
    };

`type()` returns an integer that is unique to this alert type. It can be compared against a specific alert by querying a static constant called `alert_type` in the alert. It can be used to determine the run-time type of an alert* in order to cast to that alert type and access specific members.

e.g:

    std::auto_ptr<alert> a = ses.pop_alert();
    switch (a->type())
    {
            case read_piece_alert::alert_type:
            {
                    read_piece_alert* p = (read_piece_alert*)a.get();
                    // use p
                    break;
            }
            case file_renamed_alert::alert_type:
            {
                    // etc...
            }
    }

`what()` returns a string literal describing the type of the alert. It does not include any information that might be bundled with the alert.

`category()` returns a bitmask specifying which categories this alert belong to.

`clone()` returns a pointer to a copy of the alert.

`discardable()` determines whether or not an alert is allowed to be discarded when the alert queue is full. There are a few alerts which may not be discared, since they would break the user contract, such as `save_resume_data_alert`.

`message()` generate a string describing the alert and the information bundled with it. This is mainly intended for debug and development use. It is not suitable to use this for applications that may be localized. Instead, handle each alert type individually and extract and render the information from the alert depending on the locale.

There's another alert base class that most alerts derives from, all the alerts that are generated for a specific torrent are derived from:

    struct torrent_alert: alert
    {
            // ...
            torrent_handle handle;
    };

There's also a base class for all alerts referring to tracker events:

    struct tracker_alert: torrent_alert
    {
            // ...
            std::string url;
    };

The specific alerts are:

## torrent_added_alert

The `torrent_added_alert` is posted once every time a torrent is added. It doesn't contain any members of its own, but inherits the torrent handle from its base class. It's posted when the `status_notification` bit is set in the alert mask.

    struct torrent_added_alert: torrent_alert
    {
            // ...
    };

## add_torrent_alert

This alert is always posted when a torrent was added via `async_add_torrent()` and contains the return status of the add operation. The torrent handle of the new torrent can be found in the base class' `handle` member.

    struct add_torrent_alert: torrent_alert
    {
            // ...
            add_torrent_params params;
            error_code error;
    };

`params` is a copy of the parameters used when adding the torrent, it can be used to identify which invocation to `async_add_torrent()` caused this alert.

`error` is set to the error, if any, adding the torrent.

## torrent_removed_alert

The `torrent_removed_alert` is posted whenever a torrent is removed. Since the torrent handle in its baseclass will always be invalid (since the torrent is already removed) it has the info hash as a member, to identify it. It's posted when the `status_notification` bit is set in the alert mask.

    struct torrent_removed_alert: torrent_alert
    {
            // ...
            sha1_hash info_hash;
    };

## read_piece_alert

This alert is posted when the asynchronous read operation initiated by a call to read_piece() is completed. If the read failed, the torrent is paused and an error state is set and the buffer member of the alert is 0. If successful, `buffer` points to a buffer containing all the data of the piece. `piece` is the piece index that was read. `size` is the number of bytes that was read.

    struct read_piece_alert: torrent_alert
    {
            // ...
            boost::shared_ptr<char> buffer;
            int piece;
            int size;
    };

## external_ip_alert

Whenever libtorrent learns about the machines external IP, this alert is generated. The external IP address can be acquired from the tracker (if it supports that) or from peers that supports the extension protocol. The address can be accessed through the `external_address` member.

    struct external_ip_alert: alert
    {
            // ...
            address external_address;
    };

## listen_failed_alert

This alert is generated when none of the ports, given in the port range, to session can be opened for listening. The `endpoint` member is the interface and port that failed, `error` is the error code describing the failure.

libtorrent may sometimes try to listen on port 0, if all other ports failed. Port 0 asks the operating system to pick a port that's free). If that fails you may see a listen_failed_alert with port 0 even if you didn't ask to listen on it.

    struct listen_failed_alert: alert
    {
            // ...
            tcp::endpoint endpoint;
            error_code error;
    };

## listen_succeeded_alert

This alert is posted when the listen port succeeds to be opened on a particular interface. `endpoint` is the endpoint that successfully was opened for listening.

    struct listen_succeeded_alert: alert
    {
            // ...
            tcp::endpoint endpoint;
    };

## portmap_error_alert

This alert is generated when a NAT router was successfully found but some part of the port mapping request failed. It contains a text message that may help the user figure out what is wrong. This alert is not generated in case it appears the client is not running on a NAT:ed network or if it appears there is no NAT router that can be remote controlled to add port mappings.

`mapping` refers to the mapping index of the port map that failed, i.e. the index returned from add_mapping().

`map_type` is 0 for NAT-PMP and 1 for UPnP.

`error` tells you what failed.

    struct portmap_error_alert: alert
    {
            // ...
            int mapping;
            int type;
            error_code error;
    };

## portmap_alert

This alert is generated when a NAT router was successfully found and a port was successfully mapped on it. On a NAT:ed network with a NAT-PMP capable router, this is typically generated once when mapping the TCP port and, if DHT is enabled, when the UDP port is mapped.

`mapping` refers to the mapping index of the port map that failed, i.e. the index returned from add_mapping().

`external_port` is the external port allocated for the mapping.

`type` is 0 for NAT-PMP and 1 for UPnP.

    struct portmap_alert: alert
    {
            // ...
            int mapping;
            int external_port;
            int map_type;
    };

## portmap_log_alert

This alert is generated to log informational events related to either UPnP or NAT-PMP. They contain a log line and the type (0 = NAT-PMP and 1 = UPnP). Displaying these messages to an end user is only useful for debugging the UPnP or NAT-PMP implementation.

    struct portmap_log_alert: alert
    {
            //...
            int map_type;
            std::string msg;
    };

## 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.

`file` is the path to the file that was accessed when the error occurred.

`error` is the error code describing the error.

    struct file_error_alert: torrent_alert
    {
            // ...
            std::string file;
            error_code error;
    };

## torrent_error_alert

This is posted whenever a torrent is transitioned into the error state.

    struct torrent_error_alert: torrent_alert
    {
            // ...
            error_code error;
    };

The `error` specifies which error the torrent encountered.

## file_renamed_alert

This is posted as a response to a `torrent_handle::rename_file` call, if the rename operation succeeds.

    struct file_renamed_alert: torrent_alert
    {
            // ...
            std::string name;
            int index;
    };

The `index` member refers to the index of the file that was renamed, `name` is the new name of the file.

## file_rename_failed_alert

This is posted as a response to a `torrent_handle::rename_file` call, if the rename operation failed.

    struct file_rename_failed_alert: torrent_alert
    {
            // ...
            int index;
            error_code error;
    };

The `index` member refers to the index of the file that was supposed to be renamed, `error` is the error code returned from the filesystem.

## tracker_announce_alert

This alert is generated each time a tracker announce is sent (or attempted to be sent). There are no extra data members in this alert. The url can be found in the base class however.

    struct tracker_announce_alert: tracker_alert
    {
            // ...
            int event;
    };

Event specifies what event was sent to the tracker. It is defined as:

0. None
1. Completed
2. Started
3. Stopped

## tracker_error_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.

The `times_in_row` member says how many times in a row this tracker has failed. `status_code` is the code returned from the HTTP server. 401 means the tracker needs authentication, 404 means not found etc. If the tracker timed out, the code will be set to 0.

    struct tracker_error_alert: tracker_alert
    {
            // ...
            int times_in_row;
            int status_code;
    };

## tracker_reply_alert

This alert is only for informational purpose. It is generated when a tracker announce succeeds. It is generated regardless what kind of tracker was used, be it UDP, HTTP or the DHT.

    struct tracker_reply_alert: tracker_alert
    {
            // ...
            int num_peers;
    };

The `num_peers` tells how many peers were returned from the tracker. This is not necessarily all new peers, some of them may already be connected.

## tracker_warning_alert

This alert is triggered if the tracker reply contains a warning field. Usually this means that the tracker announce was successful, but the tracker has a message to the client. The `msg` string in the alert contains the warning message from the tracker.

    struct tracker_warning_alert: tracker_alert
    {
            // ...
            std::string msg;
    };

## scrape_reply_alert

This alert is generated when a scrape request succeeds. `incomplete` and `complete` is the data returned in the scrape response. These numbers may be -1 if the reponse was malformed.

    struct scrape_reply_alert: tracker_alert
    {
            // ...
            int incomplete;
            int complete;
    };

## scrape_failed_alert

If a scrape request fails, this alert is generated. This might be due to the tracker timing out, refusing connection or returning an http response code indicating an error. `msg` contains a message describing the error.

    struct scrape_failed_alert: tracker_alert
    {
            // ...
            std::string msg;
    };

## url_seed_alert

This alert is generated when a HTTP seed name lookup fails.

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

    struct url_seed_alert: torrent_alert
    {
            // ...
            std::string url;
    };

## hash_failed_alert

This alert is generated when a finished piece fails its hash check. You can get the handle to the torrent which got the failed piece and the index of the piece itself from the alert.

    struct hash_failed_alert: torrent_alert
    {
            // ...
            int piece_index;
    };

## peer_alert

The peer alert is a base class for alerts that refer to a specific peer. It includes all the information to identify the peer. i.e. `ip` and `peer-id`.

    struct peer_alert: torrent_alert
    {
            // ...
            tcp::endpoint ip;
            peer_id pid;
    };

## peer_connect_alert

This alert is posted every time an outgoing peer connect attempts succeeds.

    struct peer_connect_alert: peer_alert
    {
            // ...
    };

## peer_ban_alert

This alert is generated when a peer is banned because it has sent too many corrupt pieces to us. `ip` is the endpoint to the peer that was banned.

    struct peer_ban_alert: peer_alert
    {
            // ...
    };

## peer_snubbed_alert

This alert is generated when a peer is snubbed, when it stops sending data when we request it.

    struct peer_snubbed_alert: peer_alert
    {
            // ...
    };

## peer_unsnubbed_alert

This alert is generated when a peer is unsnubbed. Essentially when it was snubbed for stalling sending data, and now it started sending data again.

    struct peer_unsnubbed_alert: peer_alert
    {
            // ...
    };

## 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.

The `error_code` tells you what error caused this alert.

    struct peer_error_alert: peer_alert
    {
            // ...
            error_code error;
    };

## peer_connected_alert

This alert is generated when a peer is connected.

    struct peer_connected_alert: peer_alert
    {
            // ...
    };

## peer_disconnected_alert

This alert is generated when a peer is disconnected for any reason (other than the ones covered by `peer_error_alert`).

The `error_code` tells you what error caused peer to disconnect.

    struct peer_disconnected_alert: peer_alert
    {
            // ...
            error_code error;
    };

## invalid_request_alert

This is a debug alert that is generated by an incoming invalid piece request. `Ïp` is the address of the peer and the `request` is the actual incoming request from the peer.

    struct invalid_request_alert: peer_alert
    {
            // ...
            peer_request request;
    };

    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.

## request_dropped_alert

This alert is generated when a peer rejects or ignores a piece request.

    struct request_dropped_alert: peer_alert
    {
            // ...
            int block_index;
            int piece_index;
    };

## block_timeout_alert

This alert is generated when a block request times out.

    struct block_timeout_alert: peer_alert
    {
            // ...
            int block_index;
            int piece_index;
    };

## block_finished_alert

This alert is generated when a block request receives a response.

    struct block_finished_alert: peer_alert
    {
            // ...
            int block_index;
            int piece_index;
    };

## lsd_peer_alert

This alert is generated when we receive a local service discovery message from a peer for a torrent we're currently participating in.

    struct lsd_peer_alert: peer_alert
    {
            // ...
    };

## file_completed_alert

This is posted whenever an individual file completes its download. i.e. All pieces overlapping this file have passed their hash check.

    struct file_completed_alert: torrent_alert
    {
            // ...
            int index;
    };

The `index` member refers to the index of the file that completed.

## block_downloading_alert

This alert is generated when a block request is sent to a peer.

    struct block_downloading_alert: peer_alert
    {
            // ...
            int block_index;
            int piece_index;
    };

## unwanted_block_alert

This alert is generated when a block is received that was not requested or whose request timed out.

    struct unwanted_block_alert: peer_alert
    {
            // ...
            int block_index;
            int piece_index;
    };

## torrent_delete_failed_alert

This alert is generated when a request to delete the files of a torrent fails.

The `error_code` tells you why it failed.

    struct torrent_delete_failed_alert: torrent_alert
    {
            // ...
            error_code error;
    };

## torrent_deleted_alert

This alert is generated when a request to delete the files of a torrent complete.

The `info_hash` is the info-hash of the torrent that was just deleted. Most of the time the torrent_handle in the `torrent_alert` will be invalid by the time this alert arrives, since the torrent is being deleted. The `info_hash` member is hence the main way of identifying which torrent just completed the delete.

This alert is posted in the `storage_notification` category, and that bit needs to be set in the alert mask.

    struct torrent_deleted_alert: torrent_alert
    {
            // ...
            sha1_hash info_hash;
    };

## 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.

There are no additional data members in this alert.

## performance_alert

This alert is generated when a limit is reached that might have a negative impact on upload or download rate performance.

    struct performance_alert: torrent_alert
    {
            // ...

            enum performance_warning_t
            {
                    outstanding_disk_buffer_limit_reached,
                    outstanding_request_limit_reached,
                    upload_limit_too_low,
                    download_limit_too_low,
                    send_buffer_watermark_too_low,
                    too_many_optimistic_unchoke_slots,
                    too_high_disk_queue_limit,
                    too_few_outgoing_ports
            };

            performance_warning_t warning_code;
    };

outstanding_disk_buffer_limit_reached
  This warning means that the number of bytes queued to be written to disk exceeds the max disk byte queue setting (`session_settings::max_queued_disk_bytes`). This might restrict the download rate, by not queuing up enough write jobs to the disk I/O thread. When this alert is posted, peer connections are temporarily stopped from downloading, until the queued disk bytes have fallen below the limit again. Unless your `max_queued_disk_bytes` setting is already high, you might want to increase it to get better performance.
outstanding_request_limit_reached
  This is posted when libtorrent would like to send more requests to a peer, but it's limited by `session_settings::max_out_request_queue`. The queue length libtorrent is trying to achieve is determined by the download rate and the assumed round-trip-time (`session_settings::request_queue_time`). The assumed rount-trip-time is not limited to just the network RTT, but also the remote disk access time and message handling time. It defaults to 3 seconds. The target number of outstanding requests is set to fill the bandwidth-delay product (assumed RTT times download rate divided by number of bytes per request). When this alert is posted, there is a risk that the number of outstanding requests is too low and limits the download rate. You might want to increase the `max_out_request_queue` setting.
upload_limit_too_low
  This warning is posted when the amount of TCP/IP overhead is greater than the upload rate limit. When this happens, the TCP/IP overhead is caused by a much faster download rate, triggering TCP ACK packets. These packets eat into the rate limit specified to libtorrent. When the overhead traffic is greater than the rate limit, libtorrent will not be able to send any actual payload, such as piece requests. This means the download rate will suffer, and new requests can be sent again. There will be an equilibrium where the download rate, on average, is about 20 times the upload rate limit. If you want to maximize the download rate, increase the upload rate limit above 5% of your download capacity.
download_limit_too_low
  This is the same warning as `upload_limit_too_low` but referring to the download limit instead of upload. This suggests that your download rate limit is mcuh lower than your upload capacity. Your upload rate will suffer. To maximize upload rate, make sure your download rate limit is above 5% of your upload capacity.
send_buffer_watermark_too_low

  We're stalled on the disk. We want to write to the socket, and we can write but our send buffer is empty, waiting to be refilled from the disk. This either means the disk is slower than the network connection or that our send buffer watermark is too small, because we can send it all before the disk gets back to us. The number of bytes that we keep outstanding, requested from the disk, is calculated as follows:

      min(512, max(upload_rate * send_buffer_watermark_factor / 100, send_buffer_watermark))

  If you receive this alert, you migth want to either increase your `send_buffer_watermark` or `send_buffer_watermark_factor`.

too_many_optimistic_unchoke_slots
  If the half (or more) of all upload slots are set as optimistic unchoke slots, this warning is issued. You probably want more regular (rate based) unchoke slots.
too_high_disk_queue_limit
  If the disk write queue ever grows larger than half of the cache size, this warning is posted. The disk write queue eats into the total disk cache and leaves very little left for the actual cache. This causes the disk cache to oscillate in evicting large portions of the cache before allowing peers to download any more, onto the disk write queue. Either lower `max_queued_disk_bytes` or increase `cache_size`.
too_few_outgoing_ports
  This is generated if outgoing peer connections are failing because of address in use errors, indicating that `session_settings::outgoing_ports` is set and is too small of a range. Consider not using the `outgoing_ports` setting at all, or widen the range to include more ports.

## state_changed_alert

Generated whenever a torrent changes its state.

    struct state_changed_alert: torrent_alert
    {
            // ...

            torrent_status::state_t state;
            torrent_status::state_t prev_state;
    };

`state` is the new state of the torrent. `prev_state` is the previous state.

## metadata_failed_alert

This alert is generated when the metadata has been completely received and the info-hash failed to match it. i.e. the metadata that was received was corrupt. libtorrent will automatically retry to fetch it in this case. This is only relevant when running a torrent-less download, with the metadata extension provided by libtorrent.

There are no additional data members in this alert.

## 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).

There are no additional data members in this alert.

Typically, when receiving this alert, you would want to save the torrent file in order to load it back up again when the session is restarted. Here's an example snippet of code to do that:

    torrent_handle h = alert->handle();
    if (h.is_valid()) {
            torrent_info const& ti = h.get_torrent_info();
            create_torrent ct(ti);
            entry te = ct.generate();
            std::vector<char> buffer;
            bencode(std::back_inserter(buffer), te);
            FILE* f = fopen((to_hex(ti.info_hash().to_string()) + ".torrent").c_str(), "w+");
            if (f) {
                    fwrite(&buffer[0], 1, buffer.size(), f);
                    fclose(f);
            }
    }

## fastresume_rejected_alert

This alert is generated when a fastresume file has been passed to `add_torrent` but the files on disk did not match the fastresume file. The `error_code` explains the reason why the resume file was rejected.

    struct fastresume_rejected_alert: torrent_alert
    {
            // ...
            error_code error;
    };

## peer_blocked_alert

This alert is generated when a peer is blocked by the IP filter. The `ip` member is the address that was blocked.

    struct peer_blocked_alert: torrent_alert
    {
            // ...
            address ip;
    };

## storage_moved_alert

The `storage_moved_alert` is generated when all the disk IO has completed and the files have been moved, as an effect of a call to `torrent_handle::move_storage`. This is useful to synchronize with the actual disk. The `path` member is the new path of the storage.

    struct storage_moved_alert: torrent_alert
    {
            // ...
            std::string path;
    };

## storage_moved_failed_alert

The `storage_moved_failed_alert` is generated when an attempt to move the storage (via torrent_handle::move_storage()) fails.

    struct storage_moved_failed_alert: torrent_alert
    {
            // ...
            error_code error;
    };

## torrent_paused_alert

This alert is generated as a response to a `torrent_handle::pause` request. It is generated once all disk IO is complete and the files in the torrent have been closed. This is useful for synchronizing with the disk.

There are no additional data members in this alert.

## torrent_resumed_alert

This alert is generated as a response to a `torrent_handle::resume` request. It is generated when a torrent goes from a paused state to an active state.

There are no additional data members in this alert.

## save_resume_data_alert

This alert is generated as a response to a `torrent_handle::save_resume_data` request. It is generated once the disk IO thread is done writing the state for this torrent. The `resume_data` member points to the resume data.

    struct save_resume_data_alert: torrent_alert
    {
            // ...
            boost::shared_ptr<entry> resume_data;
    };

## save_resume_data_failed_alert

This alert is generated instead of `save_resume_data_alert` if there was an error generating the resume data. `error` describes what went wrong.

    struct save_resume_data_failed_alert: torrent_alert
    {
            // ...
            error_code error;
    };

## stats_alert

This alert is posted approximately once every second, and it contains byte counters of most statistics that's tracked for torrents. Each active torrent posts these alerts regularly.

    struct stats_alert: torrent_alert
    {
            // ...
            enum stats_channel
            {
                    upload_payload,
                    upload_protocol,
                    upload_ip_protocol,
                    upload_dht_protocol,
                    upload_tracker_protocol,
                    download_payload,
                    download_protocol,
                    download_ip_protocol,
                    download_dht_protocol,
                    download_tracker_protocol,
                    num_channels
            };

            int transferred[num_channels];
            int interval;
    };

`transferred` this is an array of samples. The enum describes what each sample is a measurement of. All of these are raw, and not smoothing is performed.

`interval` the number of milliseconds during which these stats were collected. This is typically just above 1000, but if CPU is limited, it may be higher than that.

## cache_flushed_alert

This alert is posted when the disk cache has been flushed for a specific torrent as a result of a call to flush_cache(). This alert belongs to the `storage_notification` category, which must be enabled to let this alert through. The alert is also posted when removing a torrent from the session, once the outstanding cache flush is complete and the torrent does no longer have any files open.

    struct flush_cached_alert: torrent_alert
    {
            // ...
    };

## torrent_need_cert_alert

This is always posted for SSL torrents. This is a reminder to the client that the torrent won't work unless torrent_handle::set_ssl_certificate() is called with a valid certificate. Valid certificates MUST be signed by the SSL certificate in the .torrent file.

    struct torrent_need_cert_alert: tracker_alert
    {
            // ...
    };

## dht_announce_alert

This alert is generated when a DHT node announces to an info-hash on our DHT node. It belongs to the `dht_notification` category.

    struct dht_announce_alert: alert
    {
            // ...
            address ip;
            int port;
            sha1_hash info_hash;
    };

## dht_get_peers_alert

This alert is generated when a DHT node sends a `get_peers` message to our DHT node. It belongs to the `dht_notification` category.

    struct dht_get_peers_alert: alert
    {
            // ...
            sha1_hash info_hash;
    };

## dht_reply_alert

This alert is generated each time the DHT receives peers from a node. `num_peers` is the number of peers we received in this packet. Typically these packets are received from multiple DHT nodes, and so the alerts are typically generated a few at a time.

    struct dht_reply_alert: tracker_alert
    {
            // ...
            int num_peers;
    };

## dht_bootstrap_alert

This alert is posted when the initial DHT bootstrap is done. There's no any other relevant information associated with this alert.

    struct dht_bootstrap_alert: alert
    {
            // ...
    };

## anonymous_mode_alert

This alert is posted when a bittorrent feature is blocked because of the anonymous mode. For instance, if the tracker proxy is not set up, no trackers will be used, because trackers can only be used through proxies when in anonymous mode.

    struct anonymous_mode_alert: tracker_alert
    {
            // ...
            enum kind_t
            {
                    tracker_not_anonymous = 1
            };
            int kind;
            std::string str;
    };

`kind` specifies what error this is, it's one of:

`tracker_not_anonymous` means that there's no proxy set up for tracker communication and the tracker will not be contacted. The tracker which this failed for is specified in the `str` member.

## rss_alert

This alert is posted on RSS feed events such as start of RSS feed updates, successful completed updates and errors during updates.

This alert is only posted if the `rss_notifications` category is enabled in the alert mask.

    struct rss_alert: alert
    {
            // ..
            virtual std::string message() const;

            enum state_t
            {
                    state_updating, state_updated, state_error
            };

            feed_handle handle;
            std::string url;
            int state;
            error_code error;
    };

`handle` is the handle to the feed which generated this alert.

`url` is a short cut to access the url of the feed, without having to call `get_settings()`.

`state` is one of:

`rss_alert::state_updating`
  An update of this feed was just initiated, it will either succeed or fail soon.
`rss_alert::state_updated`
  The feed just completed a successful update, there may be new items in it. If you're adding torrents manually, you may want to request the feed status of the feed and look through the `items` vector.
`rss_akert::state_error`
  An error just occurred. See the `error` field for information on what went wrong.

`error` is an error code used for when an error occurs on the feed.

## incoming_connection_alert

The incoming connection alert is posted every time we successfully accept an incoming connection, through any mean. The most straigh-forward ways of accepting incoming connections are through the TCP listen socket and the UDP listen socket for uTP sockets. However, connections may also be accepted ofer a Socks5 or i2p listen socket, or via a torrent specific listen socket for SSL torrents.

    struct incoming_connection_alert: alert
    {
            // ...
            virtual std::string message() const;

            int socket_type;
            tcp::endpoint ip;
    };

`socket_type` tells you what kind of socket the connection was accepted as:

value | type
--- | ---
0 | none (no socket instantiated)
1 | TCP
2 | Socks5
3 | HTTP
4 | uTP
5 | i2p
6 | SSL/TCP
7 | SSL/Socks5
8 | HTTPS (SSL/HTTP)
9 | SSL/uTP

`ip` is the IP address and port the connection came from.

## state_update_alert

This alert is only posted when requested by the user, by calling post_torrent_updates() on the session. It contains the torrent status of all torrents that changed since last time this message was posted. Its category is `status_notification`, but it's not subject to filtering, since it's only manually posted anyway.

    struct state_update_alert: alert
    {
            // ...
            std::vector<torrent_status> status;
    };

`status` contains the torrent status of all torrents that changed since last time this message was posted. Note that you can map a torrent status to a specific torrent via its `handle` member. The receiving end is suggested to have all torrents sorted by the `torrent_handle` or hashed by it, for efficient updates.

# alert dispatcher

The `handle_alert` class is defined in `<libtorrent/alert.hpp>`.

Examples usage:

    struct my_handler
    {
            void operator()(portmap_error_alert const& a) const
            {
                    std::cout << "Portmapper: " << a.msg << std::endl;
            }

            void operator()(tracker_warning_alert const& a) const
            {
                    std::cout << "Tracker warning: " << a.msg << std::endl;
            }

            void operator()(torrent_finished_alert const& a) const
            {
                    // write fast resume data
                    // ...

                    std::cout << a.handle.get_torrent_info().name() << "completed"
                            << std::endl;
            }
    };

    std::auto_ptr<alert> a;
    a = ses.pop_alert();
    my_handler h;
    while (a.get())
    {
            handle_alert<portmap_error_alert
                    , tracker_warning_alert
                    , torrent_finished_alert
            >::handle_alert(h, a);
            a = ses.pop_alert();
    }

In this example 3 alert types are used. You can use any number of template parameters to select between more types. If the number of types are more than 15, you can define `TORRENT_MAX_ALERT_TYPES` to a greater number before including `<libtorrent/alert.hpp>`.

# exceptions

Many functions in libtorrent have two versions, one that throws exceptions on errors and one that takes an `error_code` reference which is filled with the error code on errors.

There is one exception class that is used for errors in libtorrent, it is based on boost.system's `error_code` class to carry the error code.

## libtorrent_exception

    struct libtorrent_exception: std::exception
    {
            libtorrent_exception(error_code const& s);
            virtual const char* what() const throw();
            virtual ~libtorrent_exception() throw() {}
            boost::system::error_code error() const;
    };

# error_code

libtorrent uses boost.system's `error_code` class to represent errors. libtorrent has its own error category (`libtorrent::get_libtorrent_category()`) whith the following error codes:

code | symbol | description
--- | --- | ---
0 | no_error | Not an error
1 | file_collision | Two torrents has files which end up overwriting each other
2 | failed_hash_check | A piece did not match its piece hash
3 | torrent_is_no_dict | The .torrent file does not contain a bencoded dictionary at its top level
4 | torrent_missing_info | The .torrent file does not have an `info` dictionary
5 | torrent_info_no_dict | The .torrent file's `info` entry is not a dictionary
6 | torrent_missing_piece_length | The .torrent file does not have a `piece length` entry
7 | torrent_missing_name | The .torrent file does not have a `name` entry
8 | torrent_invalid_name | The .torrent file's name entry is invalid
9 | torrent_invalid_length | The length of a file, or of the whole .torrent file is invalid. Either negative or not an integer
10 | torrent_file_parse_failed | Failed to parse a file entry in the .torrent
11 | torrent_missing_pieces | The `pieces` field is missing or invalid in the .torrent file
12 | torrent_invalid_hashes | The `pieces` string has incorrect length
13 | too_many_pieces_in_torrent | The .torrent file has more pieces than is supported by libtorrent
14 | invalid_swarm_metadata | The metadata (.torrent file) that was received from the swarm matched the info-hash, but failed to be parsed
15 | invalid_bencoding | The file or buffer is not correctly bencoded
16 | no_files_in_torrent | The .torrent file does not contain any files
17 | invalid_escaped_string | The string was not properly url-encoded as expected
18 | session_is_closing | Operation is not permitted since the session is shutting down
19 | duplicate_torrent | There's already a torrent with that info-hash added to the session
20 | invalid_torrent_handle | The supplied torrent_handle is not referring to a valid torrent
21 | invalid_entry_type | The type requested from the entry did not match its type
22 | missing_info_hash_in_uri | The specified URI does not contain a valid info-hash
23 | file_too_short | One of the files in the torrent was unexpectadly small. This might be caused by files being changed by an external process
24 | unsupported_url_protocol | The URL used an unknown protocol. Currently `http` and `https` (if built with openssl support) are recognized. For trackers `udp` is recognized as well.
25 | url_parse_error | The URL did not conform to URL syntax and failed to be parsed
26 | peer_sent_empty_piece | The peer sent a 'piece' message of length 0
27 | parse_failed | A bencoded structure was currupt and failed to be parsed
28 | invalid_file_tag | The fast resume file was missing or had an invalid file version tag
29 | missing_info_hash | The fast resume file was missing or had an invalid info-hash
30 | mismatching_info_hash | The info-hash in the resume file did not match the torrent
31 | invalid_hostname | The URL contained an invalid hostname
32 | invalid_port | The URL had an invalid port
33 | port_blocked | The port is blocked by the port-filter, and prevented the connection
34 | expected_close_bracket_in_address | The IPv6 address was expected to end with ']'
35 | destructing_torrent | The torrent is being destructed, preventing the operation to succeed
36 | timed_out | The connection timed out
37 | upload_upload_connection | The peer is upload only, and we are upload only. There's no point in keeping the connection
38 | uninteresting_upload_peer | The peer is upload only, and we're not interested in it. There's no point in keeping the connection
39 | invalid_info_hash | The peer sent an unknown info-hash
40 | torrent_paused | The torrent is paused, preventing the operation from succeeding
41 | invalid_have | The peer sent an invalid have message, either wrong size or referring to a piece that doesn't exist in the torrent
42 | invalid_bitfield_size | The bitfield message had the incorrect size
43 | too_many_requests_when_choked | The peer kept requesting pieces after it was choked, possible abuse attempt.
44 | invalid_piece | The peer sent a piece message that does not correspond to a piece request sent by the client
45 | no_memory | memory allocation failed
46 | torrent_aborted | The torrent is aborted, preventing the operation to succeed
47 | self_connection | The peer is a connection to ourself, no point in keeping it
48 | invalid_piece_size | The peer sent a piece message with invalid size, either negative or greater than one block
49 | timed_out_no_interest | The peer has not been interesting or interested in us for too long, no point in keeping it around
50 | timed_out_inactivity | The peer has not said anything in a long time, possibly dead
51 | timed_out_no_handshake | The peer did not send a handshake within a reasonable amount of time, it might not be a bittorrent peer
52 | timed_out_no_request | The peer has been unchoked for too long without requesting any data. It might be lying about its interest in us
53 | invalid_choke | The peer sent an invalid choke message
54 | invalid_unchoke | The peer send an invalid unchoke message
55 | invalid_interested | The peer sent an invalid interested message
56 | invalid_not_interested | The peer sent an invalid not-interested message
57 | invalid_request | The peer sent an invalid piece request message
58 | invalid_hash_list | The peer sent an invalid hash-list message (this is part of the merkle-torrent extension)
59 | invalid_hash_piece | The peer sent an invalid hash-piece message (this is part of the merkle-torrent extension)
60 | invalid_cancel | The peer sent an invalid cancel message
61 | invalid_dht_port | The peer sent an invalid DHT port-message
62 | invalid_suggest | The peer sent an invalid suggest piece-message
63 | invalid_have_all | The peer sent an invalid have all-message
64 | invalid_have_none | The peer sent an invalid have none-message
65 | invalid_reject | The peer sent an invalid reject message
66 | invalid_allow_fast | The peer sent an invalid allow fast-message
67 | invalid_extended | The peer sent an invalid extesion message ID
68 | invalid_message | The peer sent an invalid message ID
69 | sync_hash_not_found | The synchronization hash was not found in the encrypted handshake
70 | invalid_encryption_constant | The encryption constant in the handshake is invalid
71 | no_plaintext_mode | The peer does not support plaintext, which is the selected mode
72 | no_rc4_mode | The peer does not support rc4, which is the selected mode
73 | unsupported_encryption_mode | The peer does not support any of the encryption modes that the client supports
74 | unsupported_encryption_mode_selected | The peer selected an encryption mode that the client did not advertise and does not support
75 | invalid_pad_size | The pad size used in the encryption handshake is of invalid size
76 | invalid_encrypt_handshake | The encryption handshake is invalid
77 | no_incoming_encrypted | The client is set to not support incoming encrypted connections and this is an encrypted connection
78 | no_incoming_regular | The client is set to not support incoming regular bittorrent connections, and this is a regular connection
79 | duplicate_peer_id | The client is already connected to this peer-ID
80 | torrent_removed | Torrent was removed
81 | packet_too_large | The packet size exceeded the upper sanity check-limit
82 | reserved |
83 | http_error | The web server responded with an error
84 | missing_location | The web server response is missing a location header
85 | invalid_redirection | The web seed redirected to a path that no longer matches the .torrent directory structure
86 | redirecting | The connection was closed becaused it redirected to a different URL
87 | invalid_range | The HTTP range header is invalid
88 | no_content_length | The HTTP response did not have a content length
89 | banned_by_ip_filter | The IP is blocked by the IP filter
90 | too_many_connections | At the connection limit
91 | peer_banned | The peer is marked as banned
92 | stopping_torrent | The torrent is stopping, causing the operation to fail
93 | too_many_corrupt_pieces | The peer has sent too many corrupt pieces and is banned
94 | torrent_not_ready | The torrent is not ready to receive peers
95 | peer_not_constructed | The peer is not completely constructed yet
96 | session_closing | The session is closing, causing the operation to fail
97 | optimistic_disconnect | The peer was disconnected in order to leave room for a potentially better peer
98 | torrent_finished | The torrent is finished
99 | no_router | No UPnP router found
100 | metadata_too_large | The metadata message says the metadata exceeds the limit
101 | invalid_metadata_request | The peer sent an invalid metadata request message
102 | invalid_metadata_size | The peer advertised an invalid metadata size
103 | invalid_metadata_offset | The peer sent a message with an invalid metadata offset
104 | invalid_metadata_message | The peer sent an invalid metadata message
105 | pex_message_too_large | The peer sent a peer exchange message that was too large
106 | invalid_pex_message | The peer sent an invalid peer exchange message
107 | invalid_lt_tracker_message | The peer sent an invalid tracker exchange message
108 | too_frequent_pex | The peer sent an pex messages too often. This is a possible attempt of and attack
109 | no_metadata | The operation failed because it requires the torrent to have the metadata (.torrent file) and it doesn't have it yet. This happens for magnet links before they have downloaded the metadata, and also torrents added by URL.
110 | invalid_dont_have | The peer sent an invalid `dont_have` message. The dont have message is an extension to allow peers to advertise that the no longer has a piece they previously had.
111 | requires_ssl_connection | The peer tried to connect to an SSL torrent without connecting over SSL.
112 | invalid_ssl_cert | The peer tried to connect to a torrent with a certificate for a different torrent.

NAT-PMP errors:

code | symbol | description
--- | --- | ---
120 | unsupported_protocol_version | The NAT-PMP router responded with an unsupported protocol version
121 | natpmp_not_authorized | You are not authorized to map ports on this NAT-PMP router
122 | network_failure | The NAT-PMP router failed because of a network failure
123 | no_resources | The NAT-PMP router failed because of lack of resources
124 | unsupported_opcode | The NAT-PMP router failed because an unsupported opcode was sent

fastresume data errors:

code | symbol | description
--- | --- | ---
130 | missing_file_sizes | The resume data file is missing the 'file sizes' entry
131 | no_files_in_resume_data | The resume data file 'file sizes' entry is empty
132 | missing_pieces | The resume data file is missing the 'pieces' and 'slots' entry
133 | mismatching_number_of_files | The number of files in the resume data does not match the number of files in the torrent
134 | mismatching_files_size | One of the files on disk has a different size than in the fast resume file
135 | mismatching_file_timestamp | One of the files on disk has a different timestamp than in the fast resume file
136 | not_a_dictionary | The resume data file is not a dictionary
137 | invalid_blocks_per_piece | The 'blocks per piece' entry is invalid in the resume data file
138 | missing_slots | The resume file is missing the 'slots' entry, which is required for torrents with compact allocation
139 | too_many_slots | The resume file contains more slots than the torrent
140 | invalid_slot_list | The 'slot' entry is invalid in the resume data
141 | invalid_piece_index | One index in the 'slot' list is invalid
142 | pieces_need_reorder | The pieces on disk needs to be re-ordered for the specified allocation mode. This happens if you specify sparse allocation and the files on disk are using compact storage. The pieces needs to be moved to their right position

HTTP errors:

150 | http_parse_error | The HTTP header was not correctly formatted
151 | http_missing_location | The HTTP response was in the 300-399 range but lacked a location header
152 | http_failed_decompress | The HTTP response was encoded with gzip or deflate but decompressing it failed

I2P errors:

160 | no_i2p_router | The URL specified an i2p address, but no i2p router is configured

tracker errors:

170 | scrape_not_available | The tracker URL doesn't support transforming it into a scrape URL. i.e. it doesn't contain "announce.
171 | invalid_tracker_response | invalid tracker response
172 | invalid_peer_dict | invalid peer dictionary entry. Not a dictionary
173 | tracker_failure | tracker sent a failure message
174 | invalid_files_entry | missing or invalid 'files' entry
175 | invalid_hash_entry | missing or invalid 'hash' entry
176 | invalid_peers_entry | missing or invalid 'peers' and 'peers6' entry
177 | invalid_tracker_response_length | udp tracker response packet has invalid size
178 | invalid_tracker_transaction_id | invalid transaction id in udp tracker response
179 | invalid_tracker_action | invalid action field in udp tracker response
190 | expected_string | expected string in bencoded string
191 | expected_colon | expected colon in bencoded string
192 | unexpected_eof | unexpected end of file in bencoded string
193 | expected_value | expected value (list, dict, int or string) in bencoded string
194 | depth_exceeded | bencoded recursion depth limit exceeded
195 | item_limit_exceeded | bencoded item count limit exceeded

The names of these error codes are declared in then `libtorrent::errors` namespace.

There is also another error category, `libtorrent::upnp_category`, defining errors retrned by UPnP routers. Here's a (possibly incomplete) list of UPnP error codes:

code | symbol | description
--- | --- | ---
0 | no_error | No error
402 | invalid_argument | One of the arguments in the request is invalid
501 | action_failed | The request failed
714 | value_not_in_array | The specified value does not exist in the array
715 | source_ip_cannot_be_wildcarded | The source IP address cannot be wild-carded, but must be fully specified
716 | external_port_cannot_be_wildcarded | The external port cannot be wildcarded, but must be specified
718 | port_mapping_conflict | The port mapping entry specified conflicts with a mapping assigned previously to another client
724 | internal_port_must_match_external | Internal and external port value must be the same
725 | only_permanent_leases_supported | The NAT implementation only supports permanent lease times on port mappings
726 | remote_host_must_be_wildcard | RemoteHost must be a wildcard and cannot be a specific IP addres or DNS name
727 | external_port_must_be_wildcard | ExternalPort must be a wildcard and cannot be a specific port

The UPnP errors are declared in the `libtorrent::upnp_errors` namespace.

HTTP errors are reported in the `libtorrent::http_category`, with error code enums in the `libtorrent::errors` namespace.

code | symbol
--- | ---
100 | cont
200 | ok
201 | created
202 | accepted
204 | no_content
300 | multiple_choices
301 | moved_permanently
302 | moved_temporarily
304 | not_modified
400 | bad_request
401 | unauthorized
403 | forbidden
404 | not_found
500 | internal_server_error
501 | not_implemented
502 | bad_gateway
503 | service_unavailable

## translating error codes

The error_code::message() function will typically return a localized error string, for system errors. That is, errors that belong to the generic or system category.

Errors that belong to the libtorrent error category are not localized however, they are only available in english. In order to translate libtorrent errors, compare the error category of the `error_code` object against `libtorrent::get_libtorrent_category()`, and if matches, you know the error code refers to the list above. You can provide your own mapping from error code to string, which is localized. In this case, you cannot rely on `error_code::message()` to generate your strings.

The numeric values of the errors are part of the API and will stay the same, although new error codes may be appended at the end.

Here's a simple example of how to translate error codes:

    std::string error_code_to_string(boost::system::error_code const& ec)
    {
            if (ec.category() != libtorrent::get_libtorrent_category())
            {
                    return ec.message();
            }
            // the error is a libtorrent error

            int code = ec.value();
            static const char const* swedish[] =
            {
                    "inget fel",
                    "en fil i torrenten kolliderar med en fil frÂn en annan torrent",
                    "hash check misslyckades",
                    "torrent filen ‰r inte en dictionary",
                    "'info'-nyckeln saknas eller ‰r korrupt i torrentfilen",
                    "'info'-f‰ltet ‰r inte en dictionary",
                    "'piece length' f‰ltet saknas eller ‰r korrupt i torrentfilen",
                    "torrentfilen saknar namnf‰ltet",
                    "ogiltigt namn i torrentfilen (kan vara en attack)",
                    // ... more strings here
            };

            // use the default error string in case we don't have it
            // in our translated list
            if (code < 0 || code >= sizeof(swedish)/sizeof(swedish[0]))
                    return ec.message();

            return swedish[code];
    }

# storage_interface

The storage interface is a pure virtual class that can be implemented to customize how and where data for a torrent is stored. The default storage implementation uses regular files in the filesystem, mapping the files in the torrent in the way one would assume a torrent is saved to disk. Implementing your own storage interface makes it possible to store all data in RAM, or in some optimized order on disk (the order the pieces are received for instance), or saving multifile torrents in a single file in order to be able to take advantage of optimized disk-I/O.

It is also possible to write a thin class that uses the default storage but modifies some particular behavior, for instance encrypting the data before it's written to disk, and decrypting it when it's read again.

The storage interface is based on slots, each slot is 'piece_size' number of bytes. All access is done by writing and reading whole or partial slots. One slot is one piece in the torrent, but the data in the slot does not necessarily correspond to the piece with the same index (in compact allocation mode it won't).

libtorrent comes with two built-in storage implementations; `default_storage` and `disabled_storage`. Their constructor functions are called `default_storage_constructor` and `disabled_storage_constructor` respectively. The disabled storage does just what it sounds like. It throws away data that's written, and it reads garbage. It's useful mostly for benchmarking and profiling purpose.

The interface looks like this:

    struct storage_interface
    {
            virtual bool initialize(bool allocate_files) = 0;
            virtual bool has_any_file() = 0;
            virtual void hint_read(int slot, int offset, int len);
            virtual int readv(file::iovec_t const* bufs, int slot, int offset, int num_bufs) = 0;
            virtual int writev(file::iovec_t const* bufs, int slot, int offset, int num_bufs) = 0;
            virtual int sparse_end(int start) const;
            virtual bool move_storage(fs::path save_path) = 0;
            virtual bool verify_resume_data(lazy_entry const& rd, error_code& error) = 0;
            virtual bool write_resume_data(entry& rd) const = 0;
            virtual bool move_slot(int src_slot, int dst_slot) = 0;
            virtual bool swap_slots(int slot1, int slot2) = 0;
            virtual bool swap_slots3(int slot1, int slot2, int slot3) = 0;
            virtual bool rename_file(int file, std::string const& new_name) = 0;
            virtual bool release_files() = 0;
            virtual bool delete_files() = 0;
            virtual void finalize_file(int index) {}
            virtual ~storage_interface() {}

            // non virtual functions

            disk_buffer_pool* disk_pool();
            void set_error(std::string const& file, error_code const& ec) const;
            error_code const& error() const;
            std::string const& error_file() const;
            void clear_error();
    };

## initialize()

>     bool initialize(bool allocate_files) = 0;

This function is called when the storage is to be initialized. The default storage will create directories and empty files at this point. If `allocate_files` is true, it will also `ftruncate` all files to their target size.

Returning `true` indicates an error occurred.

## has_any_file()

>     virtual bool has_any_file() = 0;

This function is called when first checking (or re-checking) the storage for a torrent. It should return true if any of the files that is used in this storage exists on disk. If so, the storage will be checked for existing pieces before starting the download.

## hint_read()

>     void hint_read(int slot, int offset, int len);

This function is called when a read job is queued. It gives the storage wrapper an opportunity to hint the operating system about this coming read. For instance, the storage may call `posix_fadvise(POSIX_FADV_WILLNEED)` or `fcntl(F_RDADVISE)`.

## readv() writev()

>     int readv(file::iovec_t const* buf, int slot, int offset, int num_bufs) = 0;
>     int write(const char* buf, int slot, int offset, int size) = 0;

These functions should read or write the data in or to the given `slot` at the given `offset`. It should read or write `num_bufs` buffers sequentially, where the size of each buffer is specified in the buffer array `bufs`. The file::iovec_t type has the following members:

    struct iovec_t
    {
            void* iov_base;
            size_t iov_len;
    };

The return value is the number of bytes actually read or written, or -1 on failure. If it returns -1, the error code is expected to be set to

Every buffer in `bufs` can be assumed to be page aligned and be of a page aligned size, except for the last buffer of the torrent. The allocated buffer can be assumed to fit a fully page aligned number of bytes though. This is useful when reading and writing the last piece of a file in unbuffered mode.

The `offset` is aligned to 16 kiB boundries most of the time, but there are rare exceptions when it's not. Specifically if the read cache is disabled/or full and a client requests unaligned data, or the file itself is not aligned in the torrent. Most clients request aligned data.

## sparse_end()

>     int sparse_end(int start) const;

This function is optional. It is supposed to return the first piece, starting at `start` that is fully contained within a data-region on disk (i.e. non-sparse region). The purpose of this is to skip parts of files that can be known to contain zeros when checking files.

## move_storage()

>     bool move_storage(fs::path save_path) = 0;

This function should move all the files belonging to the storage to the new save_path. The default storage moves the single file or the directory of the torrent.

Before moving the files, any open file handles may have to be closed, like `release_files()`.

Returning `false` indicates an error occurred.

## verify_resume_data()

>     bool verify_resume_data(lazy_entry const& rd, error_code& error) = 0;

This function should verify the resume data `rd` with the files on disk. If the resume data seems to be up-to-date, return true. If not, set `error` to a description of what mismatched and return false.

The default storage may compare file sizes and time stamps of the files.

Returning `false` indicates an error occurred.

## write_resume_data()

>     bool write_resume_data(entry& rd) const = 0;

This function should fill in resume data, the current state of the storage, in `rd`. The default storage adds file timestamps and sizes.

Returning `true` indicates an error occurred.

## move_slot()

>     bool move_slot(int src_slot, int dst_slot) = 0;

This function should copy or move the data in slot `src_slot` to the slot `dst_slot`. This is only used in compact mode.

If the storage caches slots, this could be implemented more efficient than reading and writing the data.

Returning `true` indicates an error occurred.

## swap_slots()

>     bool swap_slots(int slot1, int slot2) = 0;

This function should swap the data in `slot1` and `slot2`. The default storage uses a scratch buffer to read the data into, then moving the other slot and finally writing back the temporary slot's data

This is only used in compact mode.

Returning `true` indicates an error occurred.

## swap_slots3()

>     bool swap_slots3(int slot1, int slot2, int slot3) = 0;

This function should do a 3-way swap, or shift of the slots. `slot1` should move to `slot2`, which should be moved to `slot3` which in turn should be moved to `slot1`.

This is only used in compact mode.

Returning `true` indicates an error occurred.

## rename_file()

>     bool rename_file(int file, std::string const& new_name) = 0;

Rename file with index `file` to the thame `new_name`. If there is an error, `true` should be returned.

## release_files()

>     bool release_files() = 0;

This function should release all the file handles that it keeps open to files belonging to this storage. The default implementation just calls `file_pool::release_files(this)`.

Returning `true` indicates an error occurred.

## delete_files()

>     bool delete_files() = 0;

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

Returning `true` indicates an error occurred.

The `disk_buffer_pool` is used to allocate and free disk buffers. It has the following members:

    struct disk_buffer_pool : boost::noncopyable
    {
            char* allocate_buffer(char const* category);
            void free_buffer(char* buf);

            char* allocate_buffers(int blocks, char const* category);
            void free_buffers(char* buf, int blocks);

            int block_size() const { return m_block_size; }

            void release_memory();
    };

## finalize_file()

>     virtual void finalize_file(int index);

This function is called each time a file is completely downloaded. The storage implementation can perform last operations on a file. The file will not be opened for writing after this.

`index` is the index of the file that completed.

On windows the default storage implementation clears the sparse file flag on the specified file.

## example

This is an example storage implementation that stores all pieces in a `std::map`, i.e. in RAM. It's not necessarily very useful in practice, but illustrates the basics of implementing a custom storage.

    struct temp_storage : storage_interface
    {
            temp_storage(file_storage const& fs) : m_files(fs) {}
            virtual bool initialize(bool allocate_files) { return false; }
            virtual bool has_any_file() { return false; }
            virtual int read(char* buf, int slot, int offset, int size)
            {
                    std::map<int, std::vector<char> >::const_iterator i = m_file_data.find(slot);
                    if (i == m_file_data.end()) return 0;
                    int available = i->second.size() - offset;
                    if (available <= 0) return 0;
                    if (available > size) available = size;
                    memcpy(buf, &i->second[offset], available);
                    return available;
            }
            virtual int write(const char* buf, int slot, int offset, int size)
            {
                    std::vector<char>& data = m_file_data[slot];
                    if (data.size() < offset + size) data.resize(offset + size);
                    std::memcpy(&data[offset], buf, size);
                    return size;
            }
            virtual bool rename_file(int file, std::string const& new_name)
            { assert(false); return false; }
            virtual bool move_storage(std::string const& save_path) { return false; }
            virtual bool verify_resume_data(lazy_entry const& rd, error_code& error) { return false; }
            virtual bool write_resume_data(entry& rd) const { return false; }
            virtual bool move_slot(int src_slot, int dst_slot) { assert(false); return false; }
            virtual bool swap_slots(int slot1, int slot2) { assert(false); return false; }
            virtual bool swap_slots3(int slot1, int slot2, int slot3) { assert(false); return false; }
            virtual size_type physical_offset(int slot, int offset)
            { return slot * m_files.piece_length() + offset; };
            virtual sha1_hash hash_for_slot(int slot, partial_hash& ph, int piece_size)
            {
                    int left = piece_size - ph.offset;
                    assert(left >= 0);
                    if (left > 0)
                    {
                            std::vector<char>& data = m_file_data[slot];
                            // if there are padding files, those blocks will be considered
                            // completed even though they haven't been written to the storage.
                            // in this case, just extend the piece buffer to its full size
                            // and fill it with zeroes.
                            if (data.size() < piece_size) data.resize(piece_size, 0);
                            ph.h.update(&data[ph.offset], left);
                    }
                    return ph.h.final();
            }
            virtual bool release_files() { return false; }
            virtual bool delete_files() { return false; }

            std::map<int, std::vector<char> > m_file_data;
            file_storage m_files;
    };

    storage_interface* temp_storage_constructor(
            file_storage const& fs, file_storage const* mapped
            , std::string const& path, file_pool& fp
            , std::vector<boost::uint8_t> const& prio)
    {
            return new temp_storage(fs);
    }

# magnet links

Magnet links are URIs that includes an info-hash, a display name and optionally a tracker url. The idea behind magnet links is that an end user can click on a link in a browser and have it handled by a bittorrent application, to start a download, without any .torrent file.

The format of the magnet URI is:

magnet:?xt=urn:btih: Base32 encoded info-hash [ &dn= name of download ] [ &tr= tracker URL ]*

# queuing

libtorrent supports queuing. Which means it makes sure that a limited number of torrents are being downloaded at any given time, and once a torrent is completely downloaded, the next in line is started.

Torrents that are auto managed are subject to the queuing and the active torrents limits. To make a torrent auto managed, set `auto_managed` to true when adding the torrent (see async_add_torrent() add_torrent()).

The limits of the number of downloading and seeding torrents are controlled via `active_downloads`, `active_seeds` and `active_limit` in session_settings. These limits takes non auto managed torrents into account as well. If there are more non-auto managed torrents being downloaded than the `active_downloads` setting, any auto managed torrents will be queued until torrents are removed so that the number drops below the limit.

The default values are 8 active downloads and 5 active seeds.

At a regular interval, torrents are checked if there needs to be any re-ordering of which torrents are active and which are queued. This interval can be controlled via `auto_manage_interval` in session_settings. It defaults to every 30 seconds.

For queuing to work, resume data needs to be saved and restored for all torrents. See save_resume_data().

## downloading

Torrents that are currently being downloaded or incomplete (with bytes still to download) are queued. The torrents in the front of the queue are started to be actively downloaded and the rest are ordered with regards to their queue position. Any newly added torrent is placed at the end of the queue. Once a torrent is removed or turns into a seed, its queue position is -1 and all torrents that used to be after it in the queue, decreases their position in order to fill the gap.

The queue positions are always in a sequence without any gaps.

Lower queue position means closer to the front of the queue, and will be started sooner than torrents with higher queue positions.

To query a torrent for its position in the queue, or change its position, see: queue_position() queue_position_up() queue_position_down() queue_position_top() queue_position_bottom().

## seeding

Auto managed seeding torrents are rotated, so that all of them are allocated a fair amount of seeding. Torrents with fewer completed seed cycles are prioritized for seeding. A seed cycle is completed when a torrent meets either the share ratio limit (uploaded bytes / downloaded bytes), the share time ratio (time seeding / time downloaing) or seed time limit (time seeded).

The relevant settings to control these limits are `share_ratio_limit`, `seed_time_ratio_limit` and `seed_time_limit` in session_settings.

# 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 save_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 async_add_torrent() 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.
`pieces` | A string with piece flags, one character per piece. Bit 1 means we have that piece. Bit 2 means we have verified that this piece is correct. This only applies when the torrent is in seed_mode.
`slots` | list of integers. The list maps 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.
`total_uploaded` | integer. The number of bytes that have been uploaded in total for this torrent.
`total_downloaded` | integer. The number of bytes that have been downloaded in total for this torrent.
`active_time` | integer. The number of seconds this torrent has been active. i.e. not paused.
`seeding_time` | integer. The number of seconds this torrent has been active and seeding.
`num_seeds` | integer. An estimate of the number of seeds on this torrent when the resume data was saved. This is scrape data or based on the peer list if scrape data is unavailable.
`num_downloaders` | integer. An estimate of the number of downloaders on this torrent when the resume data was last saved. This is used as an initial estimate until we acquire up-to-date scrape info.
`upload_rate_limit` | integer. In case this torrent has a per-torrent upload rate limit, this is that limit. In bytes per second.
`download_rate_limit` | integer. The download rate limit for this torrent in case one is set, in bytes per second.
`max_connections` | integer. The max number of peer connections this torrent may have, if a limit is set.
`max_uploads` | integer. The max number of unchoked peers this torrent may have, if a limit is set.
`seed_mode` | integer. 1 if the torrent is in seed mode, 0 otherwise.
`file_priority` | list of integers. One entry per file in the torrent. Each entry is the priority of the file with the same index.
`piece_priority` | string of bytes. Each byte is interpreted as an integer and is the priority of that piece.
`auto_managed` | integer. 1 if the torrent is auto managed, otherwise 0.
`sequential_download` | integer. 1 if the torrent is in sequential download mode, 0 otherwise.
`paused` | integer. 1 if the torrent is paused, 0 otherwise.
`trackers` | list of lists of strings. The top level list lists all tracker tiers. Each second level list is one tier of trackers.
`mapped_files` | list of strings. If any file in the torrent has been renamed, this entry contains a list of all the filenames. In the same order as in the torrent file.
`url-list` | list of strings. List of url-seed URLs used by this torrent. The urls are expected to be properly encoded and not contain any illegal url characters.
`httpseeds` | list of strings. List of httpseed URLs used by this torrent. The urls are expected to be properly encoded and not contain any illegal url characters.
`merkle tree` | string. In case this torrent is a merkle torrent, this is a string containing the entire merkle tree, all nodes, including the root and all leaves. The tree is not necessarily complete, but complete enough to be able to send any piece that we have, indicated by the have bitmask.
`peers` | list of dictionaries. Each dictionary has the following layout:
`ip` | string, the ip address of the peer. This is not a binary representation of the ip address, but the string representation. It may be an IPv6 string or an IPv4 string.
`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`.
`file sizes` | list where each entry corresponds to a file in the file list in the metadata. Each entry has a list of two values, the first value is the size of the file in bytes, the second is the time stamp when the last time someone wrote to it. This information is used to compare with the files on disk. All the files must match exactly this information in order to consider the resume data as current. Otherwise a full re-check is issued.
`allocation` | The allocation mode for the storage. Can be either `full` or `compact`. If this is full, the file sizes and timestamps are disregarded. Pieces are assumed not to have moved around even if the files have been modified after the last resume data checkpoint.

# storage allocation

There are two modes in which storage (files on disk) are allocated in libtorrent.

1. The traditional full allocation mode, where the entire files are filled up with zeros before anything is downloaded. libtorrent will look for sparse files support in the filesystem that is used for storage, and use sparse files or file system zero fill support if present. This means that on NTFS, full allocation mode will only allocate storage for the downloaded pieces.
2. The sparse allocation, sparse files are used, and pieces are downloaded directly to where they belong. This is the recommended (and default) mode.

In previous versions of libtorrent, a 3rd mode was supported, compact allocation. Support for this is deprecated and will be removed in future versions of libtorrent. It's still described in here for completeness.

The allocation mode is selected when a torrent is started. It is passed as an argument to `session::add_torrent()` (see async_add_torrent() add_torrent()).

The decision to use full allocation or compact allocation typically depends on whether any files have priority 0 and if the filesystem supports sparse files.

## sparse allocation

On filesystems that supports sparse files, this allocation mode will only use as much space as has been downloaded.

> - It does not require an allocation pass on startup.
> - It supports skipping files (setting prioirty to 0 to not download).
> - Fast resume data will remain valid even when file time stamps are out of date.

## full allocation

When a torrent is started in full allocation mode, the disk-io thread will make sure that the entire storage is allocated, and fill any gaps with zeros. This will be skipped if the filesystem supports sparse files or automatic zero filling. It will of course still check for existing pieces and fast resume data. The main drawbacks of this mode are:

> - It may take longer to start the torrent, since it will need to fill the files with zeros on some systems. This delay is linearly dependent on the size of the download.
> - The download may occupy unnecessary disk space between download sessions. In case sparse files are not supported.
> - Disk caches usually perform extremely poorly with random access to large files and may slow down a download considerably.

The benefits of this mode are:

> - Downloaded pieces are written directly to their final place in the files and the total number of disk operations will be fewer and may also play nicer to filesystems' file allocation, and reduce fragmentation.
> - No risk of a download failing because of a full disk during download. Unless sparse files are being used.
> - The fast resume data will be more likely to be usable, regardless of crashes or out of date data, since pieces won't move around.
> - Can be used with prioritizing files to 0.

## compact allocation

Note that support for compact allocation is deprecated in libttorrent, and will be removed in future versions.

The compact allocation will only allocate as much storage as it needs to keep the pieces downloaded so far. This means that pieces will be moved around to be placed at their final position in the files while downloading (to make sure the completed download has all its pieces in the correct place). So, the main drawbacks are:

> - More disk operations while downloading since pieces are moved around.
> - Potentially more fragmentation in the filesystem.
> - Cannot be used while having files with priority 0.

The benefits though, are:

> - No startup delay, since the files don't need allocating.
> - The download will not use unnecessary disk space.
> - Disk caches perform much better than in full allocation and raises the download speed limit imposed by the disk.
> - Works well on filesystems that don't support sparse files.

The algorithm that is used when allocating pieces and slots isn't very complicated. For the interested, a description follows.

storing a piece:

1. let A be a newly downloaded piece, with index n.
2. let s be the number of slots allocated in the file we're downloading to. (the number of pieces it has room for).
3. if n >= s then allocate a new slot and put the piece there.
4. if n < s then allocate a new slot, move the data at slot n to the new slot and put A in slot n.

allocating a new slot:

1. if there's an unassigned slot (a slot that doesn't contain any piece), return that slot index.
2. append the new slot at the end of the file (or find an unused slot).
3. let i be the index of newly allocated slot
4. if we have downloaded piece index i already (to slot j) then
   1. move the data at slot j to slot i.
   2. return slot index j as the newly allocated free slot.
5. return i as the newly allocated slot.

# 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).

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.

## metadata from peers

Extension name: "LT_metadata"

This extension is deprecated in favor of the more widely supported `ut_metadata` extension, see BEP 9. 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.

## dont_have

Extension name: "lt_dont_have"

The `dont_have` extension message is used to tell peers that the client no longer has a specific piece. The extension message should be advertised in the `m` dictionary as `lt_dont_have`. The message format mimics the regular `HAVE` bittorrent message.

Just like all extension messages, the first 2 bytes in the mssage itself are 20 (the bittorrent extension message) and the message ID assigned to this extension in the `m` dictionary in the handshake.

size | name | description
--- | --- | ---
uint32_t | piece | index of the piece the peer no longer has.

The length of this message (including the extension message prefix) is 6 bytes, i.e. one byte longer than the normal `HAVE` message, because of the extension message wrapping.

## HTTP seeding

There are two kinds of HTTP seeding. One with that assumes a smart (and polite) client and one that assumes a smart server. These are specified in BEP 19 and BEP 17 respectively.

libtorrent supports both. In the libtorrent source code and API, BEP 19 urls are typically referred to as url seeds and BEP 17 urls are typically referred to as HTTP seeds.

The libtorrent implementation of BEP 19 assumes that, if the URL ends with a slash ('/'), the filename should be appended to it in order to request pieces from that file. The way this works is that if the torrent is a single-file torrent, only that filename is appended. If the torrent is a multi-file torrent, the torrent's name '/' the file name is appended. This is the same directory structure that libtorrent will download torrents into.

# piece picker

The piece picker in libtorrent has the following features:

- rarest first
- sequential download
- random pick
- reverse order picking
- parole mode
- prioritize partial pieces
- prefer whole pieces
- piece affinity by speed category
- piece priorities

## internal representation

It is optimized by, at all times, keeping a list of pieces ordered by rarity, randomly shuffled within each rarity class. This list is organized as a single vector of contigous memory in RAM, for optimal memory locality and to eliminate heap allocations and frees when updating rarity of pieces.

Expensive events, like a peer joining or leaving, are evaluated lazily, since it's cheaper to rebuild the whole list rather than updating every single piece in it. This means as long as no blocks are picked, peers joining and leaving is no more costly than a single peer joining or leaving. Of course the special cases of peers that have all or no pieces are optimized to not require rebuilding the list.

## picker strategy

The normal mode of the picker is of course rarest first, meaning pieces that few peers have are preferred to be downloaded over pieces that more peers have. This is a fundamental algorithm that is the basis of the performance of bittorrent. However, the user may set the piece picker into sequential download mode. This mode simply picks pieces sequentially, always preferring lower piece indices.

When a torrent starts out, picking the rarest pieces means increased risk that pieces won't be completed early (since there are only a few peers they can be downloaded from), leading to a delay of having any piece to offer to other peers. This lack of pieces to trade, delays the client from getting started into the normal tit-for-tat mode of bittorrent, and will result in a long ramp-up time. The heuristic to mitigate this problem is to, for the first few pieces, pick random pieces rather than rare pieces. The threshold for when to leave this initial picker mode is determined by `session_settings::initial_picker_threshold`.

## reverse order

An orthogonal setting is reverse order, which is used for snubbed peers. Snubbed peers are peers that appear very slow, and might have timed out a piece request. The idea behind this is to make all snubbed peers more likely to be able to do download blocks from the same piece, concentrating slow peers on as few pieces as possible. The reverse order means that the most common pieces are picked, instead of the rarest pieces (or in the case of sequential download, the last pieces, intead of the first).

## parole mode

Peers that have participated in a piece that failed the hash check, may be put in parole mode. This means we prefer downloading a full piece from this peer, in order to distinguish which peer is sending corrupt data. Whether to do this is or not is controlled by `session_settings::use_parole_mode`.

In parole mode, the piece picker prefers picking one whole piece at a time for a given peer, avoiding picking any blocks from a piece any other peer has contributed to (since that would defeat the purpose of parole mode).

## prioritize partial pieces

This setting determines if partially downloaded or requested pieces should always be preferred over other pieces. The benefit of doing this is that the number of partial pieces is minimized (and hence the turn-around time for downloading a block until it can be uploaded to others is minimized). It also puts less stress on the disk cache, since fewer partial pieces need to be kept in the cache. Whether or not to enable this is controlled by `session_settings::prioritize_partial_pieces`.

The main benefit of not prioritizing partial pieces is that the rarest first algorithm gets to have more influence on which pieces are picked. The picker is more likely to truly pick the rarest piece, and hence improving the performance of the swarm.

This setting is turned on automatically whenever the number of partial pieces in the piece picker exceeds the number of peers we're connected to times 1.5. This is in order to keep the waste of partial pieces to a minimum, but still prefer rarest pieces.

## prefer whole pieces

The prefer whole pieces setting makes the piece picker prefer picking entire pieces at a time. This is used by web connections (both http seeding standards), in order to be able to coalesce the small bittorrent requests to larger HTTP requests. This significantly improves performance when downloading over HTTP.

It is also used by peers that are downloading faster than a certain threshold. The main advantage is that these peers will better utilize the other peer's disk cache, by requesting all blocks in a single piece, from the same peer.

This threshold is controlled by `session_settings::whole_pieces_threshold`.

TODO: piece affinity by speed category TODO: piece priorities

# SSL torrents

Torrents may have an SSL root (CA) certificate embedded in them. Such torrents are called SSL torrents. An SSL torrent talks to all bittorrent peers over SSL. The protocols are layered like this:

    +-----------------------+
    | BitTorrent protocol   |
    +-----------------------+
    | SSL                   |
    +-----------+-----------+
    | TCP       | uTP       |
    |           +-----------+
    |           | UDP       |
    +-----------+-----------+

During the SSL handshake, both peers need to authenticate by providing a certificate that is signed by the CA certificate found in the .torrent file. These peer certificates are expected to be privided to peers through some other means than bittorrent. Typically by a peer generating a certificate request which is sent to the publisher of the torrent, and the publisher returning a signed certificate.

In libtorrent, set_ssl_certificate() in torrent_handle is used to tell libtorrent where to find the peer certificate and the private key for it. When an SSL torrent is loaded, the torrent_need_cert_alert is posted to remind the user to provide a certificate.

A peer connecting to an SSL torrent MUST provide the SNI TLS extension (server name indication). The server name is the hex encoded info-hash of the torrent to connect to. This is required for the client accepting the connection to know which certificate to present.

SSL connections are accepted on a separate socket from normal bittorrent connections. To pick which port the SSL socket should bind to, set `session_settings::ssl_listen` to a different port. It defaults to port 4433. This setting is only taken into account when the normal listen socket is opened (i.e. just changing this setting won't necessarily close and re-open the SSL socket). To not listen on an SSL socket at all, set `ssl_listen` to 0.

This feature is only available if libtorrent is build with openssl support (`TORRENT_USE_OPENSSL`) and requires at least openSSL version 1.0, since it needs SNI support.

Peer certificates must have at least one SubjectAltName field of type dNSName. At least one of the fields must exactly match the name of the torrent. This is a byte-by-byte comparison, the UTF-8 encoding must be identical (i.e. there's no unicode normalization going on). This is the recommended way of verifying certificates for HTTPS servers according to RFC 2818. Note the difference that for torrents only dNSName fields are taken into account (not IP address fields). The most specific (i.e. last) Common Name field is also taken into account if no SubjectAltName did not match.

If any of these fields contain a single asterisk ("*"), the certificate is considered covering any torrent, allowing it to be reused for any torrent.

The purpose of matching the torrent name with the fields in the peer certificate is to allow a publisher to have a single root certificate for all torrents it distributes, and issue separate peer certificates for each torrent. A peer receiving a certificate will not necessarily be able to access all torrents published by this root certificate (only if it has a "star cert").

## testing

To test incoming SSL connections to an SSL torrent, one can use the following openssl command:

    openssl s_client -cert <peer-certificate>.pem -key <peer-private-key>.pem -CAfile <torrent-cert>.pem -debug -connect 127.0.0.1:4433 -tls1 -servername <info-hash>

To create a root certificate, the Distinguished Name (DN) is not taken into account by bittorrent peers. You still need to specify something, but from libtorrent's point of view, it doesn't matter what it is. libtorrent only makes sure the peer certificates are signed by the correct root certificate.

One way to create the certificates is to use the `CA.sh` script that comes with openssl, like thisi (don't forget to enter a common Name for the certificate):

    CA.sh -newca
    CA.sh -newreq
    CA.sh -sign

The torrent certificate is located in `./demoCA/private/demoCA/cacert.pem`, this is the pem file to include in the .torrent file.

The peer's certificate is located in `./newcert.pem` and the certificate's private key in `./newkey.pem`.