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# Rasterbar Software

## Software developement and consulting

# creating torrents

Author: | Arvid Norberg, arvid@rasterbar.com

Table of contents

- overview
- high level example
- add_files
- set_piece_hashes()
- file_storage
  - add_file
- create_torrent
  - create_torrent()
  - generate()
  - set_comment()
  - set_creator()
  - set_hash()
  - add_url_seed()
  - add_node()
  - add_tracker()
  - set_priv() priv()

# overview

This section describes the functions and classes that are used to create torrent files. It is a layered API with low level classes and higher level convenience functions. A torrent is created in 4 steps:

1. first the files that will be part of the torrent are determined.
2. the torrent properties are set, such as tracker url, web seeds, DHT nodes etc.
3. Read through all the files in the torrent, SHA-1 all the data and set the piece hashes.
4. The torrent is bencoded into a file or buffer.

If there are a lot of files and or deep directoy hierarchies to traverse, step one can be time consuming.

Typically step 3 is by far the most time consuming step, since it requires to read all the bytes from all the files in the torrent.

All of these classes and functions are declared by including `libtorrent/create_torrent.hpp`.

# high level example

    file_storage fs;

    // recursively adds files in directories
    add_files(fs, "./my_torrent");

    create_torrent t(fs);
    t.add_tracker("http://my.tracker.com/announce");
    t.set_creator("libtorrent example");

    // reads the files and calculates the hashes
    set_piece_hashes(t, ".");

    ofstream out("my_torrent.torrent", std::ios_base::binary);
    bencode(std::ostream_iterator<char>(out), t.generate());

# add_files

>     template <class Pred>
>     void add_files(file_storage& fs, boost::filesystem::path const& path, Pred p);

>     void add_files(file_storage& fs, boost::filesystem::path const& path);

Adds the file specified by `path` to the `file_storage` object. In case `path` refers to a diretory, files will be added recursively from the directory.

If specified, the predicate `p` is called once for every file and directory that is encountered. files for which `p` returns true are added, and directories for which `p` returns true are traversed. `p` must have the following signature:

    bool Pred(boost::filesystem::path const& p);

The path that is passed in to the predicate is the full path of the file or directory. If no predicate is specified, all files are added, and all directories are traveresed.

The ".." directory is never traversed.

# set_piece_hashes()

>     template <class Fun>
>     void set_piece_hashes(create_torrent& t, boost::filesystem::path const& p, Fun f);

>     template <class Fun>
>     void set_piece_hashes(create_torrent& t, boost::filesystem::path const& p, Fun f, error_code& ec);

>     void set_piece_hashes(create_torrent& t, boost::filesystem::path const& p);

>     void set_piece_hashes(create_torrent& t, boost::filesystem::path const& p, error_code& ec);

This function will assume that the files added to the torrent file exists at path `p`, read those files and hash the content and set the hashes in the `create_torrent` object. The optional function `f` is called in between every hash that is set. `f` must have the following signature:

    void Fun(int);

The overloads that don't take an `error_code&` may throw an exception in case of a file error, the other overloads sets the error code to reflect the error, if any.

# file_storage

The `file_storage` class represents a file list and the piece size. Everything necessary to interpret a regular bittorrent storage file structure. Its synopsis:

    class file_storage
    {
    public:

            bool is_valid() const;

            void add_file(file_entry const& e);
            void add_file(fs::path const& p, size_type size);
            void rename_file(int index, std::string const& new_filename);

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

            typedef std::vector<file_entry>::const_iterator iterator;
            typedef std::vector<file_entry>::const_reverse_iterator reverse_iterator;

            iterator begin() const;
            iterator end() const;
            reverse_iterator rbegin();
            reverse_iterator rend() const;
            int num_files() const;

            file_entry const& at(int index) const;

            size_type total_size() const;
            void set_num_pieces(int n);
            int num_pieces() const;
            void set_piece_length(int l);
            int piece_length() const;
            int piece_size(int index) const;

            void set_name(std::string const& n);
            const std::string& name() const;

            void swap(file_storage& ti);
    }

## add_file

>     void add_file(file_entry const& e);
>     void add_file(fs::path const& p, size_type size);

Adds a file to the file storage. If more files than one are added, certain restrictions to their paths apply. In a multi-file file storage (torrent), all files must share the same root directory.

That is, the first path element of all files must be the same. This shared path element is also set to the name of the torrent. It can be changed by calling `set_name`.

The built in functions to traverse a directory to add files will make sure this requirement is fulfilled.

# create_torrent

The `create_torrent` class has the following synopsis:

    struct create_torrent
    {
            create_torrent(file_storage& fs, int piece_size);
            create_torrent(file_storage& fs);

            entry generate() const;

            file_storage const& files() const;

            void set_comment(char const* str);
            void set_creator(char const* str);
            void set_hash(int index, sha1_hash const& h);
            void add_url_seed(std::string const& url);
            void add_node(std::pair<std::string, int> const& node);
            void add_tracker(std::string const& url, int tier = 0);
            void set_priv(bool p);

            int num_pieces() const;
            int piece_length() const;
            int piece_size(int i) const;
            bool priv() const;
    };

## create_torrent()

>     create_torrent(file_storage& fs, int piece_size);
>     create_torrent(file_storage& fs);

The `piece_size` is the size of each piece in bytes. It must be a multiple of 16 kiB.

The constructor that does not take a piece_size will calculate a piece size such that the torrent file is roughly 40 kB.

## generate()

>     entry generate() const;

This function will generate the .torrent file as a bencode tree. In order to generate the flat file, use the bencode() function.

It may be useful to add custom entries to the torrent file before bencoding it and saving it to disk.

If anything goes wrong during torrent generation, this function will return an empty `entry` structure. You can test for this condition by querying the type of the entry:

    file_storage fs;
    // add file ...
    create_torrent t(fs);
    // add trackers and piece hashes ...
    e = t.generate();

    if (e.type() == entry::undefined_t)
    {
            // something went wrong
    }

For instance, you cannot generate a torrent with 0 files in it. If you don't add any files to the `file_storage`, torrent generation will fail.

## set_comment()

>     void set_comment(char const* str);

Sets the comment for the torrent. The string `str` should be utf-8 encoded. The comment in a torrent file is optional.

## set_creator()

>     void set_creator(char const* str);

Sets the creator of the torrent. The string `str` should be utf-8 encoded. This is optional.

## set_hash()

>     void set_hash(int index, sha1_hash const& h);

This sets the SHA-1 hash for the specified piece (`index`). You are required to set the hash for every piece in the torrent before generating it. If you have the files on disk, you can use the high level convenience function to do this. See set_piece_hashes().

## add_url_seed()

>     void add_url_seed(std::string const& url);

This adds a url seed to the torrent. You can have any number of url seeds. For a single file torrent, this should be an HTTP url, pointing to a file with identical content as the file of the torrent. For a multi-file torrent, it should point to a directory containing a directory with the same name as this torrent, and all the files of the torrent in it.

## add_node()

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

This adds a DHT node to the torrent. This especially useful if you're creating a tracker less torrent. It can be used by clients to bootstrap their DHT node from. The node is a hostname and a port number where there is a DHT node running. You can have any number of DHT nodes in a torrent.

## add_tracker()

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

Adds a tracker to the torrent. This is not strictly required, but most torrents use a tracker as their main source of peers. The url should be an http:// or udp:// url to a machine running a bittorrent tracker that accepts announces for this torrent's info-hash. The tier is the fallback priority of the tracker. All trackers with tier 0 are tried first (in any order). If all fail, trackers with tier 1 are tried. If all of those fail, trackers with tier 2 are tried, and so on.

## set_priv() priv()

>     void set_priv(bool p);
>     bool priv() const;

Sets and queries the private flag of the torrent.