libtorrent
Description: A feature complete BitTorrent protocol implementation as a C++ library

Version: | 2.0.10

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Table of contents

- block_info
  - peer() set_peer()
  - enum block_state_t
- partial_piece_info
- torrent_handle
  - torrent_handle()
  - add_piece()
  - read_piece()
  - have_piece()
  - get_peer_info() post_peer_info()
  - status() post_status()
  - post_download_queue() get_download_queue()
  - set_piece_deadline() clear_piece_deadlines() reset_piece_deadline()
  - file_progress() post_file_progress()
  - file_status()
  - clear_error()
  - replace_trackers() post_trackers() add_tracker() trackers()
  - remove_url_seed() url_seeds() add_url_seed()
  - http_seeds() remove_http_seed() add_http_seed()
  - add_extension()
  - set_metadata()
  - is_valid()
  - pause() resume()
  - flags() unset_flags() set_flags()
  - flush_cache()
  - force_recheck()
  - save_resume_data()
  - need_save_resume_data()
  - queue_position_bottom() queue_position() queue_position_down() queue_position_top() queue_position_up()
  - queue_position_set()
  - set_ssl_certificate_buffer() set_ssl_certificate()
  - torrent_file_with_hashes() torrent_file()
  - piece_layers()
  - post_piece_availability() piece_availability()
  - get_piece_priorities() prioritize_pieces() piece_priority()
  - prioritize_files() file_priority() get_file_priorities()
  - force_dht_announce() force_reannounce() force_lsd_announce()
  - scrape_tracker()
  - set_download_limit() upload_limit() download_limit() set_upload_limit()
  - connect_peer()
  - clear_peers()
  - set_max_uploads() max_uploads()
  - set_max_connections() max_connections()
  - move_storage()
  - rename_file()
  - info_hash() info_hashes()
  - operator<() operator==() operator!=()
  - id()
  - native_handle()
  - userdata()
  - in_session()
- hash_value()

[report issue]

# block_info

Declared in "libtorrent/torrent_handle.hpp"

holds the state of a block in a piece. Who we requested it from and how far along we are at downloading it.

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

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

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

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## peer() set_peer()

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

The peer is the ip address of the peer this block was downloaded from.

[report issue]

## enum block_state_t

Declared in "libtorrent/torrent_handle.hpp"

name | value | description
--- | --- | ---
none | 0 | This block has not been downloaded or requested form any peer.
requested | 1 | The block has been requested, but not completely downloaded yet.
writing | 2 | The block has been downloaded and is currently queued for being written to disk.
finished | 3 | The block has been written to disk.

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bytes_progress
  the number of bytes that have been received for this block

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block_size
  the total number of bytes in this block.

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state
  the state this block is in (see block_state_t)

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num_peers
  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.

[report issue]

# partial_piece_info

Declared in "libtorrent/torrent_handle.hpp"

This class holds information about pieces that have outstanding requests or outstanding writes

    struct partial_piece_info
    {
       piece_index_t piece_index;
       int blocks_in_piece;
       int finished;
       int writing;
       int requested;
       block_info const* blocks;
    };

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piece_index
  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.

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blocks_in_piece
  the number of blocks in this piece

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finished
  the number of blocks that are in the finished state

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writing
  the number of blocks that are in the writing state

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requested
  the number of blocks that are in the requested state

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blocks

  this is an array of `blocks_in_piece` number of items. One for each block in the piece.

  Warning

  This is a pointer that points to an array that's owned by the session object. The next time get_download_queue() is called, it will be invalidated. In the case of piece_info_alert, these pointers point into the alert object itself, and will be invalidated when the alert destruct.

[report issue]

# torrent_handle

Declared in "libtorrent/torrent_handle.hpp"

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 querying 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.

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 system_error exception, in case the handle is no longer referring 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.

    struct torrent_handle
    {
       friend std::size_t hash_value (torrent_handle const& th);
       torrent_handle () noexcept = default;
       void add_piece (piece_index_t piece, char const* data, add_piece_flags_t flags = {}) const;
       void add_piece (piece_index_t piece, std::vector<char> data, add_piece_flags_t flags = {}) const;
       void read_piece (piece_index_t piece) const;
       bool have_piece (piece_index_t piece) const;
       void post_peer_info () const;
       void get_peer_info (std::vector<peer_info>& v) const;
       torrent_status status (status_flags_t flags = status_flags_t::all()) const;
       void post_status (status_flags_t flags = status_flags_t::all()) const;
       std::vector<partial_piece_info> get_download_queue () const;
       void get_download_queue (std::vector<partial_piece_info>& queue) const;
       void post_download_queue () const;
       void clear_piece_deadlines () const;
       void set_piece_deadline (piece_index_t index, int deadline, deadline_flags_t flags = {}) const;
       void reset_piece_deadline (piece_index_t index) const;
       void file_progress (std::vector<std::int64_t>& progress, file_progress_flags_t flags = {}) const;
       void post_file_progress (file_progress_flags_t flags) const;
       std::vector<std::int64_t> file_progress (file_progress_flags_t flags = {}) const;
       std::vector<open_file_state> file_status () const;
       void clear_error () const;
       void add_tracker (announce_entry const&) const;
       void replace_trackers (std::vector<announce_entry> const&) const;
       std::vector<announce_entry> trackers () const;
       void post_trackers () const;
       std::set<std::string> url_seeds () const;
       void add_url_seed (std::string const& url) const;
       void remove_url_seed (std::string const& url) const;
       std::set<std::string> http_seeds () const;
       void remove_http_seed (std::string const& url) const;
       void add_http_seed (std::string const& url) const;
       void add_extension (
          std::function<std::shared_ptr<torrent_plugin>(torrent_handle const&, client_data_t)> const& ext
          , client_data_t userdata = client_data_t{});
       bool set_metadata (span<char const> metadata) const;
       bool is_valid () const;
       void resume () const;
       void pause (pause_flags_t flags = {}) const;
       void unset_flags (torrent_flags_t flags) const;
       void set_flags (torrent_flags_t flags, torrent_flags_t mask) const;
       void set_flags (torrent_flags_t flags) const;
       torrent_flags_t flags () const;
       void flush_cache () const;
       void force_recheck () const;
       void save_resume_data (resume_data_flags_t flags = {}) const;
       bool need_save_resume_data () const;
       bool need_save_resume_data (resume_data_flags_t flags) const;
       void queue_position_top () const;
       void queue_position_up () const;
       queue_position_t queue_position () const;
       void queue_position_bottom () const;
       void queue_position_down () const;
       void queue_position_set (queue_position_t p) const;
       void set_ssl_certificate_buffer (std::string const& certificate
          , std::string const& private_key
          , std::string const& dh_params);
       void set_ssl_certificate (std::string const& certificate
          , std::string const& private_key
          , std::string const& dh_params
          , std::string const& passphrase = "");
       std::shared_ptr<torrent_info> torrent_file_with_hashes () const;
       std::shared_ptr<const torrent_info> torrent_file () const;
       std::vector<std::vector<sha256_hash>> piece_layers () const;
       void piece_availability (std::vector<int>& avail) const;
       void post_piece_availability () const;
       download_priority_t piece_priority (piece_index_t index) const;
       void prioritize_pieces (std::vector<std::pair<piece_index_t, download_priority_t>> const& pieces) const;
       void piece_priority (piece_index_t index, download_priority_t priority) const;
       void prioritize_pieces (std::vector<download_priority_t> const& pieces) const;
       std::vector<download_priority_t> get_piece_priorities () const;
       download_priority_t file_priority (file_index_t index) const;
       void file_priority (file_index_t index, download_priority_t priority) const;
       void prioritize_files (std::vector<download_priority_t> const& files) const;
       std::vector<download_priority_t> get_file_priorities () const;
       void force_dht_announce () const;
       void force_lsd_announce () const;
       void force_reannounce (int seconds = 0, int idx = -1, reannounce_flags_t = {}) const;
       void scrape_tracker (int idx = -1) const;
       void set_upload_limit (int limit) const;
       int download_limit () const;
       int upload_limit () const;
       void set_download_limit (int limit) const;
       void connect_peer (tcp::endpoint const& adr, peer_source_flags_t source = {}
          , pex_flags_t flags = pex_encryption | pex_utp | pex_holepunch) const;
       void clear_peers ();
       void set_max_uploads (int max_uploads) const;
       int max_uploads () const;
       void set_max_connections (int max_connections) const;
       int max_connections () const;
       void move_storage (std::string const& save_path
          , move_flags_t flags = move_flags_t::always_replace_files
          ) const;
       void rename_file (file_index_t index, std::string const& new_name) const;
       sha1_hash info_hash () const;
       info_hash_t info_hashes () const;
       bool operator!= (const torrent_handle& h) const;
       bool operator< (const torrent_handle& h) const;
       bool operator== (const torrent_handle& h) const;
       std::uint32_t id () const;
       std::shared_ptr<torrent> native_handle () const;
       client_data_t userdata () const;
       bool in_session () const;

       static constexpr add_piece_flags_t overwrite_existing  = 0_bit;
       static constexpr status_flags_t query_distributed_copies  = 0_bit;
       static constexpr status_flags_t query_accurate_download_counters  = 1_bit;
       static constexpr status_flags_t query_last_seen_complete  = 2_bit;
       static constexpr status_flags_t query_pieces  = 3_bit;
       static constexpr status_flags_t query_verified_pieces  = 4_bit;
       static constexpr status_flags_t query_torrent_file  = 5_bit;
       static constexpr status_flags_t query_name  = 6_bit;
       static constexpr status_flags_t query_save_path  = 7_bit;
       static constexpr deadline_flags_t alert_when_available  = 0_bit;
       static constexpr file_progress_flags_t piece_granularity  = 0_bit;
       static constexpr pause_flags_t graceful_pause  = 0_bit;
       static constexpr resume_data_flags_t flush_disk_cache  = 0_bit;
       static constexpr resume_data_flags_t save_info_dict  = 1_bit;
       static constexpr resume_data_flags_t only_if_modified  = 2_bit;
       static constexpr resume_data_flags_t if_counters_changed  = 3_bit;
       static constexpr resume_data_flags_t if_download_progress  = 4_bit;
       static constexpr resume_data_flags_t if_config_changed  = 5_bit;
       static constexpr resume_data_flags_t if_state_changed  = 6_bit;
       static constexpr resume_data_flags_t if_metadata_changed  = 7_bit;
       static constexpr reannounce_flags_t ignore_min_interval  = 0_bit;
    };

[report issue]

## torrent_handle()

    torrent_handle () noexcept = default;

constructs a torrent handle that does not refer to a torrent. i.e. is_valid() will return false.

[report issue]

## add_piece()

    void add_piece (piece_index_t piece, char const* data, add_piece_flags_t flags = {}) const;
    void add_piece (piece_index_t piece, std::vector<char> data, add_piece_flags_t flags = {}) const;

This function will write `data` to the storage as piece `piece`, as if it had been downloaded from a peer.

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 hash_failed_alert.

Adding pieces while the torrent is being checked (i.e. in torrent_status::checking_files state) is not supported.

The overload taking a raw pointer to the data is a blocking call. It won't return until the libtorrent thread has copied the data into its disk write buffer. `data` is expected to point to a buffer of as many bytes as the size of the specified piece. See file_storage::piece_size().

The data in the buffer is copied and passed on to the disk IO thread to be written at a later point.

The overload taking a `std::vector<char>` is not blocking, it will send the buffer to the main thread and return immediately.

[report issue]

## read_piece()

    void read_piece (piece_index_t 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 response 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.

[report issue]

## have_piece()

    bool have_piece (piece_index_t piece) const;

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

[report issue]

## get_peer_info() post_peer_info()

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

Query information about connected peers for this torrent. If the torrent_handle is invalid, it will throw a system_error exception.

`post_peer_info()` is asynchronous and will trigger the posting of a peer_info_alert. The alert contain a list of peer_info objects, one for each connected peer.

`get_peer_info()` is synchronous and 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. Each entry in the vector contains information about that particular peer. See peer_info.

[report issue]

## status() post_status()

    torrent_status status (status_flags_t flags = status_flags_t::all()) const;
    void post_status (status_flags_t flags = status_flags_t::all()) const;

`status()` will return a structure with information about the status of this torrent. If the torrent_handle is invalid, it will throw system_error 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.

The `status()` function will block until the internal libtorrent thread responds with the torrent_status object. To avoid blocking, instead call `post_status()`. It will trigger posting of a state_update_alert with a single torrent_status object for this torrent.

In order to get regular updates for torrents whose status changes, consider calling session::post_torrent_updates()`` instead.

By default everything is included. The flags you can use to decide what to include are defined in this class.

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## post_download_queue() get_download_queue()

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

`post_download_queue()` triggers a download_queue_alert to be posted. `get_download_queue()` is a synchronous call and returns a vector with information about pieces that are partially downloaded or not downloaded but partially requested. See partial_piece_info for the fields in the returned vector.

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## set_piece_deadline() clear_piece_deadlines() reset_piece_deadline()

    void clear_piece_deadlines () const;
    void set_piece_deadline (piece_index_t index, int deadline, deadline_flags_t flags = {}) const;
    void reset_piece_deadline (piece_index_t 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.

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 have the same effect 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.

`clear_piece_deadlines()` removes deadlines on all pieces in the torrent. As if reset_piece_deadline() was called on all pieces.

[report issue]

## file_progress() post_file_progress()

    void file_progress (std::vector<std::int64_t>& progress, file_progress_flags_t flags = {}) const;
    void post_file_progress (file_progress_flags_t flags) const;
    std::vector<std::int64_t> file_progress (file_progress_flags_t flags = {}) const;

This function fills in the supplied vector, or returns a vector, with 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 currently 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.

[report issue]

## file_status()

    std::vector<open_file_state> file_status () const;

This function returns a vector with status about files that are open for this torrent. Any file that is not open will not be reported in the vector, i.e. it's possible that the vector is empty when returning, if none of the files in the torrent are currently open.

See open_file_state

[report issue]

## 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.

[report issue]

## replace_trackers() post_trackers() add_tracker() trackers()

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

`trackers()` returns 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(). See announce_entry.

`post_trackers()` is the asynchronous version of `trackers()`. It will trigger a tracker_list_alert to be posted.

`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.

[report issue]

## remove_url_seed() url_seeds() add_url_seed()

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

[report issue]

## http_seeds() remove_http_seed() add_http_seed()

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

[report issue]

## add_extension()

    void add_extension (
          std::function<std::shared_ptr<torrent_plugin>(torrent_handle const&, client_data_t)> const& ext
          , client_data_t userdata = client_data_t{});

add the specified extension to this torrent. The `ext` argument is a function that will be called from within libtorrent's context passing in the internal torrent object and the specified userdata pointer. The function is expected to return a shared pointer to a torrent_plugin instance.

[report issue]

## set_metadata()

    bool set_metadata (span<char const> metadata) 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.

[report issue]

## 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 removed from the session AND destructed.

To tell if the torrent_handle is in the session, use torrent_handle::in_session(). This will return true before session_handle::remove_torrent() is called, and false afterward.

Clients should only use is_valid() to determine if the result of session::find_torrent() was successful.

Unlike other member functions which return a value, is_valid() completes immediately, without blocking on a result from the network thread. Also unlike other functions, it never throws the system_error exception.

[report issue]

## pause() resume()

    void resume () const;
    void pause (pause_flags_t flags = {}) 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.

For possible values of the `flags` parameter, see pause_flags_t.

To know if a torrent is paused or not, call `torrent_handle::flags()` and check for the `torrent_status::paused` flag.

Note

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 auto-managed first. Torrents are auto-managed by default when added to the session. For more information, see queuing.

[report issue]

## flags() unset_flags() set_flags()

    void unset_flags (torrent_flags_t flags) const;
    void set_flags (torrent_flags_t flags, torrent_flags_t mask) const;
    void set_flags (torrent_flags_t flags) const;
    torrent_flags_t flags () const;

sets and gets the torrent state flags. See torrent_flags_t. The `set_flags` overload that take a mask will affect all flags part of the mask, and set their values to what the `flags` argument is set to. This allows clearing and setting flags in a single function call. The `set_flags` overload that just takes flags, sets all the specified flags and leave any other flags unchanged. `unset_flags` clears the specified flags, while leaving any other flags unchanged.

The seed_mode flag is special, it can only be cleared once the torrent has been added, and it can only be set as part of the add_torrent_params flags, when adding the torrent.

[report issue]

## 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.

[report issue]

## 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. The torrent will be placed last in queue, i.e. its queue position will be the highest of all torrents in the session.

[report issue]

## save_resume_data()

    void save_resume_data (resume_data_flags_t flags = {}) const;

`save_resume_data()` asks libtorrent to generate fast-resume data for this torrent. The fast resume data (stored in an add_torrent_params object) can be used to resume a torrent in the next session without having to check all files for which pieces have been downloaded. It can also be used to save a .torrent file for a torrent_handle.

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

The operation will fail, and post a save_resume_data_failed_alert instead, in the following cases:

> 1. The torrent is in the process of being removed.
> 2. No torrent state has changed since the last saving of resume data, and the only_if_modified flag is set. metadata (see libtorrent's metadata from peers extension)

Note that some counters may be outdated by the time you receive the fast resume data

When saving resume data because of shutting down, make sure not to remove_torrent() before you receive the save_resume_data_alert. There's no need to pause the session or torrent when saving resume data.

The paused state of a torrent is saved in the resume data, so pausing all torrents before saving resume data will all torrents be restored in a paused state.

Note

It is typically a good idea to save resume data whenever a torrent is completed or paused. If you save resume data for torrents when they are paused, you can accelerate the shutdown process by not saving resume data again for those torrents. Completed torrents should have their resume data saved when they complete and on exit, since their statistics might be updated.

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();
    for (torrent_handle const& h : handles) try
    {
            h.save_resume_data(torrent_handle::only_if_modified);
            ++outstanding_resume_data;
    }
    catch (lt::system_error const& e)
    {
            // the handle was invalid, ignore this one and move to the next
    }

    while (outstanding_resume_data > 0)
    {
            alert const* a = ses.wait_for_alert(seconds(30));

            // if we don't get an alert within 30 seconds, abort
            if (a == nullptr) break;

            std::vector<alert*> alerts;
            ses.pop_alerts(&alerts);

            for (alert* i : alerts)
            {
                    if (alert_cast<save_resume_data_failed_alert>(i))
                    {
                            process_alert(i);
                            --outstanding_resume_data;
                            continue;
                    }

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

                    std::ofstream out((rd->params.save_path
                            + "/" + rd->params.name + ".fastresume").c_str()
                            , std::ios_base::binary);
                    std::vector<char> buf = write_resume_data_buf(rd->params);
                    out.write(buf.data(), buf.size());
                    --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.

[report issue]

## need_save_resume_data()

    bool need_save_resume_data () const;
    bool need_save_resume_data (resume_data_flags_t flags) const;

This function returns true if anything that is stored in the resume data has changed since the last time resume data was saved. The overload that takes `flags` let you ask if specific categories of properties have changed. These flags have the same behavior as in the save_resume_data() call.

This is a blocking call. It will wait for a response from libtorrent's main thread. A way to avoid blocking is to instead call save_resume_data() directly, specifying the conditions under which resume data should be saved.

Note

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

[report issue]

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

    void queue_position_top () const;
    void queue_position_up () const;
    queue_position_t queue_position () const;
    void queue_position_bottom () const;
    void queue_position_down () 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.

[report issue]

## queue_position_set()

    void queue_position_set (queue_position_t p) const;

updates the position in the queue for this torrent. The relative order of all other torrents remain intact but their numerical queue position shifts to make space for this torrent's new position

[report issue]

## set_ssl_certificate_buffer() set_ssl_certificate()

    void set_ssl_certificate_buffer (std::string const& certificate
          , std::string const& private_key
          , std::string const& dh_params);
    void set_ssl_certificate (std::string const& certificate
          , 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.

The set_ssl_certificate_buffer() overload takes the actual certificate, private key and DH params as strings, rather than paths to files.

`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.

[report issue]

## torrent_file_with_hashes() torrent_file()

    std::shared_ptr<torrent_info> torrent_file_with_hashes () const;
    std::shared_ptr<const torrent_info> torrent_file () const;

torrent_file() returns a pointer to the torrent_info object associated with this torrent. The torrent_info object may be a copy of the internal object. If the torrent doesn't have metadata, the pointer will not be initialized (i.e. a nullptr). The torrent may be in a state without metadata only if it was started without a .torrent file, e.g. by being added by magnet link.

Note that the torrent_info object returned here may be a different instance than the one added to the session, with different attributes like piece layers, dht nodes and trackers. A torrent_info object does not round-trip cleanly when added to a session.

If you want to save a .torrent file from the torrent_handle, instead call save_resume_data() and write_torrent_file() the add_torrent_params object passed back in the alert.

torrent_file_with_hashes() returns a copy of the internal torrent_info and piece layer hashes (if it's a v2 torrent). The piece layers will only be included if they are available. If this torrent was added from a .torrent file with piece layers or if it's seeding, the piece layers are available. This function is more expensive than torrent_file() since it needs to make copies of this information.

The torrent_file_with_hashes() is here for backwards compatibility when constructing a create_torrent object from a torrent_info that's in a session. Prefer save_resume_data() + write_torrent_file().

Note that a torrent added from a magnet link may not have the full merkle trees for all files, and hence not have the complete piece layers. In that state, you cannot create a .torrent file even from the torrent_info returned from torrent_file_with_hashes(). Once the torrent completes downloading all files, becoming a seed, you can make a .torrent file from it.

[report issue]

## piece_layers()

    std::vector<std::vector<sha256_hash>> piece_layers () const;

returns the piece layers for all files in the torrent. If this is a v1 torrent (and doesn't have any piece layers) it returns an empty vector. This is a blocking call that will synchronize with the libtorrent network thread.

[report issue]

## post_piece_availability() piece_availability()

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

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

`post_piece_availability()` will trigger a piece_availability_alert to be posted.

`piece_availability()` 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.

[report issue]

## get_piece_priorities() prioritize_pieces() piece_priority()

    download_priority_t piece_priority (piece_index_t index) const;
    void prioritize_pieces (std::vector<std::pair<piece_index_t, download_priority_t>> const& pieces) const;
    void piece_priority (piece_index_t index, download_priority_t priority) const;
    void prioritize_pieces (std::vector<download_priority_t> const& pieces) const;
    std::vector<download_priority_t> get_piece_priorities () const;

These functions are used to set and get the priority of individual pieces. By default all pieces have priority 4. 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 priority levels. 0 means not to download the piece at all. Otherwise, lower priority values means less likely to be picked. Piece priority takes precedence over piece availability. Every piece with priority 7 will be attempted to be picked before a priority 6 piece and so on.

The default priority of pieces is 4.

Piece priorities can not be changed for torrents that have not downloaded the metadata yet. Magnet links won't have metadata immediately. see the metadata_received_alert.

`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. The second overload of `prioritize_pieces` that takes a vector of pairs will update the priorities of only select pieces, and leave all other unaffected. Each pair is (piece, priority). That is, the first item is the piece index and the second item is the priority of that piece. Invalid entries, where the piece index or priority is out of range, are not allowed.

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

It's possible to cancel the effect of file priorities by setting the priorities for the affected pieces. Care has to be taken when mixing usage of file- and piece priorities.

[report issue]

## prioritize_files() file_priority() get_file_priorities()

    download_priority_t file_priority (file_index_t index) const;
    void file_priority (file_index_t index, download_priority_t priority) const;
    void prioritize_files (std::vector<download_priority_t> const& files) const;
    std::vector<download_priority_t> get_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.

`get_file_priorities()` returns a vector with the priorities of all files.

The priority values are the same as for piece_priority(). See download_priority_t.

Whenever a file priority is changed, all other piece priorities are reset to match the file priorities. In order to maintain special 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.

Since changing file priorities may involve disk operations (of moving files in- and out of the part file), the internal accounting of file priorities happen asynchronously. i.e. setting file priorities and then immediately querying them may not yield the same priorities just set. To synchronize with the priorities taking effect, wait for the file_prio_alert.

When combining file- and piece priorities, the resume file will record both. When loading the resume data, the file priorities will be applied first, then the piece priorities.

Moving data from a file into the part file is currently not supported. If a file has its priority set to 0 after it has already been created, it will not be moved into the partfile.

[report issue]

## force_dht_announce() force_reannounce() force_lsd_announce()

    void force_dht_announce () const;
    void force_lsd_announce () const;
    void force_reannounce (int seconds = 0, int idx = -1, reannounce_flags_t = {}) const;

`force_reannounce()` will force this torrent to do another tracker request, to receive new peers. The `seconds` argument specifies how many seconds from now to issue the tracker announces.

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.

The `tracker_index` argument specifies which tracker to re-announce. If set to -1 (which is the default), all trackers are re-announce.

The `flags` argument can be used to affect the re-announce. See ignore_min_interval.

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

`force_lsd_announce` will announce the torrent on LSD immediately.

[report issue]

## scrape_tracker()

    void scrape_tracker (int idx = -1) const;

`scrape_tracker()` will send a scrape request to a tracker. By default (`idx` = -1) it will scrape the last working tracker. If `idx` is >= 0, the tracker with the specified index will scraped.

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.

[report issue]

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

    void set_upload_limit (int limit) const;
    int download_limit () const;
    int upload_limit () const;
    void set_download_limit (int 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 (`settings_pack::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.

Local peers are not rate limited by default. see peer classes.

[report issue]

## connect_peer()

    void connect_peer (tcp::endpoint const& adr, peer_source_flags_t source = {}
          , pex_flags_t flags = pex_encryption | pex_utp | pex_holepunch) 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 system_error. The second (optional) argument will be bitwise 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.

For possible values of `flags`, see pex_flags_t.

[report issue]

## clear_peers()

    void clear_peers ();

This will disconnect all peers and clear the peer list for this torrent. New peers will have to be acquired before resuming, from trackers, DHT or local service discovery, for example.

[report issue]

## 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. This defaults to infinite. The primary setting controlling this is the global unchoke slots limit, set by unchoke_slots_limit in settings_pack.

`max_uploads()` returns the current settings.

[report issue]

## 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. There is also a global limit of the number of connections, set by `connections_limit` in settings_pack.

`max_connections()` returns the current settings.

[report issue]

## move_storage()

    void move_storage (std::string const& save_path
          , move_flags_t flags = move_flags_t::always_replace_files
          ) 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.

The `flags` argument determines the behavior of the copying/moving of the files in the torrent. see move_flags_t.

`always_replace_files` is the default and replaces any file that exist in both the source directory and the target directory.

`fail_if_exist` first check to see that none of the copy operations would cause an overwrite. If it would, it will fail. Otherwise it will proceed as if it was in `always_replace_files` mode. Note that there is an inherent race condition here. If the files in the target directory appear after the check but before the copy or move completes, they will be overwritten. When failing because of files already existing in the target path, the `error` of `move_storage_failed_alert` is set to `boost::system::errc::file_exists`.

The intention is that a client may use this as a probe, and if it fails, ask the user which mode to use. The client may then re-issue the `move_storage` call with one of the other modes.

`dont_replace` always keeps the existing file in the target directory, if there is one. The source files will still be removed in that case. Note that it won't automatically re-check files. If an incomplete torrent is moved into a directory with the complete files, pause, move, force-recheck and resume. Without the re-checking, the torrent will keep downloading and files in the new download directory will be overwritten.

Files that have been renamed to have absolute paths are not moved by this function. Keep in mind that files that don't belong to the torrent but are stored in the torrent's directory may be moved as well. This goes for files that have been renamed to absolute paths that still end up inside the save path.

When copying files, sparse regions are not likely to be preserved. This makes it proportionally more expensive to move a large torrent when only few pieces have been downloaded, since the files are then allocated with zeros in the destination directory.

[report issue]

## rename_file()

    void rename_file (file_index_t index, std::string const& new_name) 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.

[report issue]

## info_hash() info_hashes()

    sha1_hash info_hash () const;
    info_hash_t info_hashes () const;

returns the info-hash(es) of the torrent. If this handle is to a torrent that hasn't loaded yet (for instance by being added) by a URL, the returned value is undefined. The `info_hash()` returns the SHA-1 info-hash for v1 torrents and a truncated hash for v2 torrents. For the full v2 info-hash, use `info_hashes()` instead.

[report issue]

## operator<() operator==() operator!=()

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

comparison operators. The order of the torrents is unspecified but stable.

[report issue]

## id()

    std::uint32_t id () const;

returns a unique identifier for this torrent. It's not a dense index. It's not preserved across sessions.

[report issue]

## native_handle()

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

This function is intended only for use by plugins and the alert dispatch function. This type does not have a stable ABI and should be relied on as little as possible. Accessing the handle returned by this function is not thread safe outside of libtorrent's internal thread (which is used to invoke plugin callbacks). The `torrent` class is not only eligible for changing ABI across minor versions of libtorrent, its layout is also dependent on build configuration. This adds additional requirements on a client to be built with the exact same build configuration as libtorrent itself. i.e. the `TORRENT_` macros must match between libtorrent and the client builds.

[report issue]

## userdata()

    client_data_t userdata () const;

returns the userdata pointer as set in add_torrent_params

[report issue]

## in_session()

    bool in_session () const;

Returns true if the torrent is in the session. It returns true before session::remove_torrent() is called, and false afterward.

Note that this is a blocking function, unlike torrent_handle::is_valid() which returns immediately.

[report issue]

overwrite_existing
  instruct libtorrent to overwrite any data that may already have been downloaded with the data of the new piece being added. Using this flag when adding a piece that is actively being downloaded from other peers may have some unexpected consequences, as blocks currently being downloaded from peers may not be replaced.

[report issue]

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

[report issue]

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

[report issue]

query_last_seen_complete
  includes `last_seen_complete`.

[report issue]

query_pieces
  populate the `pieces` field in torrent_status.

[report issue]

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

[report issue]

query_torrent_file
  includes `torrent_file`, which is all the static information from the .torrent file.

[report issue]

query_name
  includes `name`, the name of the torrent. This is either derived from the .torrent file, or from the `&dn=` magnet link argument or possibly some other source. If the name of the torrent is not known, this is an empty string.

[report issue]

query_save_path
  includes `save_path`, the path to the directory the files of the torrent are saved to.

[report issue]

alert_when_available
  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.

[report issue]

piece_granularity
  only calculate file progress at piece granularity. This makes the file_progress() call cheaper and also only takes bytes that have passed the hash check into account, so progress cannot regress in this mode.

[report issue]

graceful_pause
  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.

[report issue]

flush_disk_cache
  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.

[report issue]

save_info_dict
  the resume data will contain the metadata from the torrent file as well. This is useful for clients that don't keep .torrent files around separately, or for torrents that were added via a magnet link.

[report issue]

only_if_modified
  this flag has the same behavior as the combination of: if_counters_changed | if_download_progress | if_config_changed | if_state_changed | if_metadata_changed

[report issue]

if_counters_changed
  save resume data if any counters has changed since the last time resume data was saved. This includes upload/download counters, active time counters and scrape data. A torrent that is not paused will have its active time counters incremented continuously.

[report issue]

if_download_progress
  save the resume data if any blocks have been downloaded since the last time resume data was saved. This includes: * checking existing files on disk * downloading a block from a peer

[report issue]

if_config_changed
  save the resume data if configuration options changed since last time the resume data was saved. This includes: * file- or piece priorities * upload- and download rate limits * change max-uploads (unchoke slots) * change max connection limit * enable/disable peer-exchange, local service discovery or DHT * enable/disable apply IP-filter * enable/disable auto-managed * enable/disable share-mode * enable/disable sequential-mode * files renamed * storage moved (save_path changed)

[report issue]

if_state_changed
  save the resume data if torrent state has changed since last time the resume data was saved. This includes: * upload mode * paused state * super-seeding * seed-mode

[report issue]

if_metadata_changed
  save the resume data if any metadata changed since the last time resume data was saved. This includes: * add/remove web seeds * add/remove trackers * receiving metadata for a magnet link

[report issue]

ignore_min_interval
  by default, force-reannounce will still honor the min-interval published by the tracker. If this flag is set, it will be ignored and the tracker is announced immediately.

[report issue]

# hash_value()

Declared in "libtorrent/torrent_handle.hpp"

    std::size_t hash_value (torrent_handle const& h);

for std::hash (and to support using this type in unordered_map etc.)