libtorrent Examples

Author: Arvid Norberg, arvid@libtorrent.org
Version: 1.2.1

examples

Except for the example programs in this manual, there's also a bigger example of a (little bit) more complete client, client_test. There are separate instructions for how to use it here if you'd like to try it.

simple client

This is a simple client. It doesn't have much output to keep it simple:

#include <cstdlib>
#include "libtorrent/entry.hpp"
#include "libtorrent/bencode.hpp"
#include "libtorrent/session.hpp"
#include "libtorrent/torrent_info.hpp"

#include <iostream>

int main(int argc, char* argv[]) try
{
  if (argc != 2) {
    std::cerr << "usage: ./simple_client torrent-file\n"
      "to stop the client, press return.\n";
    return 1;
  }

  lt::session s;
  lt::add_torrent_params p;
  p.save_path = "./";
  p.ti = std::make_shared<lt::torrent_info>(argv[1]);
  s.add_torrent(p);

  // wait for the user to end
  char a;
  int ret = std::scanf("%c\n", &a);
  (void)ret; // ignore
  return 0;
}
catch (std::exception const& e) {
  std::cerr << "ERROR: " << e.what() << "\n";
}

make_torrent

Shows how to create a torrent from a directory tree:

#include "libtorrent/entry.hpp"
#include "libtorrent/bencode.hpp"
#include "libtorrent/torrent_info.hpp"
#include "libtorrent/storage.hpp"
#include "libtorrent/create_torrent.hpp"

#include <functional>
#include <cstdio>
#include <sstream>
#include <fstream>
#include <iostream>

#ifdef TORRENT_WINDOWS
#include <direct.h> // for _getcwd
#endif

using namespace std::placeholders;

std::vector<char> load_file(std::string const& filename)
{
  std::fstream in;
  in.exceptions(std::ifstream::failbit);
  in.open(filename.c_str(), std::ios_base::in | std::ios_base::binary);
  in.seekg(0, std::ios_base::end);
  size_t const size = size_t(in.tellg());
  in.seekg(0, std::ios_base::beg);
  std::vector<char> ret(size);
  in.read(ret.data(), ret.size());
  return ret;
}

std::string branch_path(std::string const& f)
{
  if (f.empty()) return f;

#ifdef TORRENT_WINDOWS
  if (f == "\\\\") return "";
#endif
  if (f == "/") return "";

  int len = int(f.size());
  // if the last character is / or \ ignore it
  if (f[len-1] == '/' || f[len-1] == '\\') --len;
  while (len > 0) {
    --len;
    if (f[len] == '/' || f[len] == '\\')
      break;
  }

  if (f[len] == '/' || f[len] == '\\') ++len;
  return std::string(f.c_str(), len);
}

// do not include files and folders whose
// name starts with a .
bool file_filter(std::string const& f)
{
  if (f.empty()) return false;

  char const* first = f.c_str();
  char const* sep = strrchr(first, '/');
#if defined(TORRENT_WINDOWS) || defined(TORRENT_OS2)
  char const* altsep = strrchr(first, '\\');
  if (sep == nullptr || altsep > sep) sep = altsep;
#endif
  // if there is no parent path, just set 'sep'
  // to point to the filename.
  // if there is a parent path, skip the '/' character
  if (sep == nullptr) sep = first;
  else ++sep;

  // return false if the first character of the filename is a .
  if (sep[0] == '.') return false;

  std::cerr << f << "\n";
  return true;
}

void print_usage()
{
  std::cerr << R"(usage: make_torrent FILE [OPTIONS]

Generates a torrent file from the specified file
or directory and writes it to standard out


OPTIONS:
-m file       generate a merkle hash tree torrent.
              merkle torrents require client support
              the resulting full merkle tree is written to
              the specified file
-w url        adds a web seed to the torrent with
              the specified url
-t url        adds the specified tracker to the
              torrent. For multiple trackers, specify more
              -t options
-c comment    sets the comment to the specified string
-C creator    sets the created-by field to the specified string
-p bytes      enables padding files. Files larger
              than bytes will be piece-aligned
-s bytes      specifies a piece size for the torrent
              This has to be a multiple of 16 kiB
-l            Don't follow symlinks, instead encode them as
              links in the torrent file
-o file       specifies the output filename of the torrent file
              If this is not specified, the torrent file is
              printed to the standard out, except on windows
              where the filename defaults to a.torrent
-r file       add root certificate to the torrent, to verify
              the HTTPS tracker
-S info-hash  add a similar torrent by info-hash. The similar
              torrent is expected to share some files with this one
-L collection add a collection name to this torrent. Other torrents
              in the same collection is expected to share files
              with this one.
-M            make the torrent compatible with mutable torrents
              this means aligning large files and pad them in order
              for piece hashes to uniquely indentify a file without
              overlap
)";
}

int main(int argc_, char const* argv_[]) try
{
  lt::span<char const*> args(argv_, size_t(argc_));
  std::string creator_str = "libtorrent";
  std::string comment_str;

  if (args.size() < 2) {
    print_usage();
    return 1;
  }

  std::vector<std::string> web_seeds;
  std::vector<std::string> trackers;
  std::vector<std::string> collections;
  std::vector<lt::sha1_hash> similar;
  int pad_file_limit = -1;
  int piece_size = 0;
  lt::create_flags_t flags = {};
  std::string root_cert;

  std::string outfile;
  std::string merklefile;
#ifdef TORRENT_WINDOWS
  // don't ever write binary data to the console on windows
  // it will just be interpreted as text and corrupted
  outfile = "a.torrent";
#endif

  std::string full_path = args[1];
  args = args.subspan(2);

  for (; !args.empty(); args = args.subspan(1)) {
    if (args[0][0] != '-') {
      print_usage();
      return 1;
    }

    char const flag = args[0][1];

    switch (flag)
    {
      case 'M':
        flags |= lt::create_torrent::mutable_torrent_support;
        pad_file_limit = 0x4000;
        continue;
      case 'l':
        flags |= lt::create_torrent::symlinks;
        continue;
    }

    if (args.size() < 2) {
      print_usage();
      return 1;
    }

    switch (flag)
    {
      case 'w': web_seeds.push_back(args[1]); break;
      case 't': trackers.push_back(args[1]); break;
      case 's': piece_size = atoi(args[1]); break;
      case 'o': outfile = args[1]; break;
      case 'C': creator_str = args[1]; break;
      case 'c': comment_str = args[1]; break;
      case 'r': root_cert = args[1]; break;
      case 'L': collections.push_back(args[1]); break;
      case 'p':
        pad_file_limit = atoi(args[1]);
        flags |= lt::create_torrent::optimize_alignment;
        break;
      case 'm':
        merklefile = args[1];
        flags |= lt::create_torrent::merkle;
        break;
      case 'S': {
        if (strlen(args[1]) != 40) {
          std::cerr << "invalid info-hash for -S. "
            "Expected 40 hex characters\n";
          print_usage();
          return 1;
        }
        std::stringstream hash(args[1]);
        lt::sha1_hash ih;
        hash >> ih;
        if (hash.fail()) {
          std::cerr << "invalid info-hash for -S\n";
          print_usage();
          return 1;
        }
        similar.push_back(ih);
        break;
      }
      default:
        print_usage();
        return 1;
    }
    args = args.subspan(1);
  }

  lt::file_storage fs;
#ifdef TORRENT_WINDOWS
  if (full_path[1] != ':')
#else
  if (full_path[0] != '/')
#endif
  {
    char cwd[2048];
#ifdef TORRENT_WINDOWS
#define getcwd_ _getcwd
#else
#define getcwd_ getcwd
#endif

    char const* ret = getcwd_(cwd, sizeof(cwd));
    if (ret == nullptr) {
      std::cerr << "failed to get current working directory: "
        << strerror(errno) << "\n";
      return 1;
    }

#undef getcwd_
#ifdef TORRENT_WINDOWS
    full_path = cwd + ("\\" + full_path);
#else
    full_path = cwd + ("/" + full_path);
#endif
  }

  lt::add_files(fs, full_path, file_filter, flags);
  if (fs.num_files() == 0) {
    std::cerr << "no files specified.\n";
    return 1;
  }

  lt::create_torrent t(fs, piece_size, pad_file_limit, flags);
  int tier = 0;
  for (std::string const& tr : trackers) {
    t.add_tracker(tr, tier);
    ++tier;
  }

  for (std::string const& ws : web_seeds)
    t.add_url_seed(ws);

  for (std::string const& c : collections)
    t.add_collection(c);

  for (lt::sha1_hash const& s : similar)
    t.add_similar_torrent(s);

  auto const num = t.num_pieces();
  lt::set_piece_hashes(t, branch_path(full_path)
    , [num] (lt::piece_index_t const p) {
      std::cerr << "\r" << p << "/" << num;
    });

  std::cerr << "\n";
  t.set_creator(creator_str.c_str());
  if (!comment_str.empty()) {
    t.set_comment(comment_str.c_str());
  }

  if (!root_cert.empty()) {
    std::vector<char> const pem = load_file(root_cert);
    t.set_root_cert(std::string(&pem[0], pem.size()));
  }

  // create the torrent and print it to stdout
  std::vector<char> torrent;
  lt::bencode(back_inserter(torrent), t.generate());
  if (!outfile.empty()) {
    std::fstream out;
    out.exceptions(std::ifstream::failbit);
    out.open(outfile.c_str(), std::ios_base::out | std::ios_base::binary);
    out.write(torrent.data(), torrent.size());
  }
  else {
    std::cout.write(torrent.data(), torrent.size());
  }

  if (!merklefile.empty()) {
    std::fstream merkle;
    merkle.exceptions(std::ifstream::failbit);
    merkle.open(merklefile.c_str(), std::ios_base::out | std::ios_base::binary);
    auto const& tree = t.merkle_tree();
    merkle.write(reinterpret_cast<char const*>(tree.data()), tree.size() * 20);
  }
  return 0;
}
catch (std::exception& e) {
  std::cerr << "ERROR: " << e.what() << "\n";
  return 1;
}

dump_torrent

This is an example of a program that will take a torrent-file as a parameter and print information about it to std out:

#include <cstdio> // for snprintf
#include <cinttypes> // for PRId64 et.al.

#include "libtorrent/entry.hpp"
#include "libtorrent/bencode.hpp"
#include "libtorrent/torrent_info.hpp"
#include "libtorrent/announce_entry.hpp"
#include "libtorrent/bdecode.hpp"
#include "libtorrent/magnet_uri.hpp"

#include <fstream>
#include <iostream>

std::vector<char> load_file(std::string const& filename)
{
  std::fstream in;
  in.exceptions(std::ifstream::failbit);
  in.open(filename.c_str(), std::ios_base::in | std::ios_base::binary);
  in.seekg(0, std::ios_base::end);
  size_t const size = size_t(in.tellg());
  in.seekg(0, std::ios_base::beg);
  std::vector<char> ret(size);
  in.read(ret.data(), size);
  return ret;
}

int main(int argc, char* argv[]) try
{
  if (argc < 2 || argc > 4) {
    std::cerr << "usage: dump_torrent torrent-file [total-items-limit] [recursion-limit]\n";
    return 1;
  }

  int item_limit = 1000000;
  int depth_limit = 1000;

  if (argc > 2) item_limit = atoi(argv[2]);
  if (argc > 3) depth_limit = atoi(argv[3]);

  std::vector<char> buf = load_file(argv[1]);
  lt::bdecode_node e;
  int pos = -1;
  lt::error_code ec;
  std::cout << "decoding. recursion limit: " << depth_limit
    << " total item count limit: " << item_limit << "\n";
  int const ret = lt::bdecode(&buf[0], &buf[0] + buf.size(), e, ec, &pos
    , depth_limit, item_limit);

  std::printf("\n\n----- raw info -----\n\n%s\n", print_entry(e).c_str());

  if (ret != 0) {
    std::cerr << "failed to decode: '" << ec.message() << "' at character: " << pos<< "\n";
    return 1;
  }

  lt::torrent_info const t(e);
  e.clear();
  std::vector<char>().swap(buf);

  // print info about torrent
  std::printf("\n\n----- torrent file info -----\n\n"
    "nodes:\n");
  for (auto const& i : t.nodes())
    std::printf("%s: %d\n", i.first.c_str(), i.second);

  puts("trackers:\n");
  for (auto const& i : t.trackers())
    std::printf("%2d: %s\n", i.tier, i.url.c_str());

  std::stringstream ih;
  ih << t.info_hash();
  std::printf("number of pieces: %d\n"
    "piece length: %d\n"
    "info hash: %s\n"
    "comment: %s\n"
    "created by: %s\n"
    "magnet link: %s\n"
    "name: %s\n"
    "number of files: %d\n"
    "files:\n"
    , t.num_pieces()
    , t.piece_length()
    , ih.str().c_str()
    , t.comment().c_str()
    , t.creator().c_str()
    , make_magnet_uri(t).c_str()
    , t.name().c_str()
    , t.num_files());
  lt::file_storage const& st = t.files();
  for (auto const i : st.file_range())
  {
    auto const first = st.map_file(i, 0, 0).piece;
    auto const last = st.map_file(i, std::max(std::int64_t(st.file_size(i)) - 1, std::int64_t(0)), 0).piece;
    auto const flags = st.file_flags(i);
    std::stringstream file_hash;
    if (!st.hash(i).is_all_zeros())
      file_hash << st.hash(i);
    std::printf(" %8" PRIx64 " %11" PRId64 " %c%c%c%c [ %5d, %5d ] %7u %s %s %s%s\n"
      , st.file_offset(i)
      , st.file_size(i)
      , ((flags & lt::file_storage::flag_pad_file)?'p':'-')
      , ((flags & lt::file_storage::flag_executable)?'x':'-')
      , ((flags & lt::file_storage::flag_hidden)?'h':'-')
      , ((flags & lt::file_storage::flag_symlink)?'l':'-')
      , static_cast<int>(first)
      , static_cast<int>(last)
      , std::uint32_t(st.mtime(i))
      , file_hash.str().c_str()
      , st.file_path(i).c_str()
      , (flags & lt::file_storage::flag_symlink) ? "-> " : ""
      , (flags & lt::file_storage::flag_symlink) ? st.symlink(i).c_str() : "");
  }
  std::printf("web seeds:\n");
  for (auto const& ws : t.web_seeds())
  {
    std::printf("%s %s\n"
      , ws.type == lt::web_seed_entry::url_seed ? "BEP19" : "BEP17"
      , ws.url.c_str());
  }

  return 0;
}
catch (std::exception const& e)
{
  std::cerr << "ERROR: " << e.what() << "\n";
}