Hello Libtorrent : Publish Data (2) Create Torrent file

#libtorrent #bittorrent #cpp #p2p

In this section, I explain about how to create a file with metadata. These files usually have a torrent extension, like "xxx.torrent".

What is a torrent file.

These torrent files contain information for connecting other peer and about target data 's HASH and name etc

Tracker Server Address (Optional)
File Name or Directory Name
Piece Info
Data's Hash Info
Data Size etc

Create a Torrentfile with Libtorrent

Let's crate a torrent file with libtorrent.

Source Code

First of all. The completed code is below.

//
// Create a torrent file
//  LD_LIBRARY_PATH=/usr/local/lib/libtorrent-rasterbar.so g++ main_tiny_torrentfile_creator.cpp  -o torrentfile_creator.out -ltorrent-rasterbar -lpthread
//
#include<iostream>
#include<vector>
#include<unistd.h>

#include<libtorrent/bencode.hpp>
#include<libtorrent/torrent_info.hpp>
#include<libtorrent/create_torrent.hpp>

std::string to_absolute_path(std::string args);
std::string extract_parent_path(std::string filepath);


int main(int argc, char *argv[]) {
    std::string target_tracker_address = "<Your Tracker Address>";
    std::string target_file_path = "<Your Target File Path>";


    //
    // setup file storage
    std::string source_absolute_path = to_absolute_path(target_file_path);
    lt::file_storage fs;
    lt::create_flags_t flags = {};
    lt::add_files(fs, source_absolute_path, flags);

    // 
    // create torrent creator
    lt::create_torrent torrent(fs, 16*1024);
    if(target_tracker_address.length() > 0) {
        torrent.add_tracker(target_tracker_address);
    }
    //
    // add hash info
    lt::set_piece_hashes(torrent, extract_parent_path(source_absolute_path));

    //
    // generate metadata
    lt::entry entry_info = torrent.generate();

    //
    // convert to bencoding
    std::vector<char> torrentfile_source;
    lt::bencode(std::back_inserter(torrentfile_source), entry_info); // metainfo -> binary
    std::cout.write(torrentfile_source.data(),torrentfile_source.size()); // binary -> stdout

    return 0;
}

std::string to_absolute_path(std::string path) {
    if(path.length() == 0) {
        throw std::runtime_error("failed to current_dir");
    }
    if(path[0] == '/') {
        return path;
    }
    char current_dir_path[2056];
    auto ret = getcwd(current_dir_path, sizeof(current_dir_path));
    if(ret == NULL) {
        std::cerr << "failed to current_dir ";
        throw std::runtime_error("failed to current_dir");
    }
    if(0==path.compare(0, 2, "./")) {
        path = path.substr(2);
    }
    return std::string(current_dir_path) + "/" + path;
} 

std::string extract_parent_path(std::string filepath)
{
  if (filepath.empty()) {
    return filepath;
  }
  int pos = filepath.find_last_of("/");

  return filepath.substr(0,pos);
}

Setup target file info

Torrent files can contain multiple file information.
These structures can be described using file storage objects.

like this.

std::string source_absolute_path = to_absolute_path(target_file_path);
lt::file_storage fs;
lt::create_flags_t flags = {};
lt::add_files(fs, source_absolute_path, flags);

Setup Torrentfile Info

Torrent file contain tracker address and piece length and target file info.

like this.

lt::create_torrent torrent(fs, 16*1024);
torrent.add_tracker(target_tracker_address);
lt::set_piece_hashes(torrent, extract_parent_path(source_absolute_path));
lt::set_piece_hashes(torrent, extract_parent_path(source_absolute_path));

Generate binary data

Torrent files are written in bencode format.

like this.

lt::entry entry_info = torrent.generate();
std::vector<char> torrentfile_source;
lt::bencode(std::back_inserter(torrentfile_source), entry_info); // metainfo -> binary  
std::cout.write(torrentfile_source.data(),torrentfile_source.size()); // binary -> stdout

Good!! You can now create a torrent file.