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

Version: | 2.0.10

Table of contents

- downloading and building
- building from git
- building with boost build
  - build commands
  - docker file
  - Step 1: Download boost
  - Step 2: Setup BBv2
  - Step 3: Building libtorrent
  - Build features
  - Step 4: Installing libtorrent
  - Custom build flags
  - Cross compiling
- building with cmake
  - Step 1: Generating the build system
  - Step 2: Building libtorrent
- building with VCPKG
- build configurations
- building openssl for windows
- list of macros

# downloading and building

To download the latest version of libtorrent, clone the github repository.

The build systems supported "out of the box" in libtorrent are boost-build cmake. If you still can't build after following these instructions, you can usually get help in the `#libtorrent` IRC channel on `irc.freenode.net`.

Warning

A common mistake when building and linking against libtorrent is to build with one set of configuration options (#defines) and link against it using a different set of configuration options. Since libtorrent has some code in header files, that code will not be compatible with the built library if they see different configurations.

Always make sure that the same TORRENT_* and BOOST_* macros are defined when you link against libtorrent as when you build it. The simplest way to see the full list of macros defined is to build libtorrent with `-n -a` switches added to `b2` command line, which output all compiler switches.

Boost-build supports propagating configuration options to dependencies.

# building from git

To build libtorrent from git you need to clone the libtorrent repository from github. Note that the git repository depends on other git repositories via submodules, which also need to be initialized and updated. If you downloaded a release tarball, you can skip this section.

    git clone --recurse-submodules https://github.com/arvidn/libtorrent.git

# building with boost build

The primary reason to use boost-build is that it will automatically build the dependent boost libraries with the correct compiler settings, in order to ensure that the build targets are link compatible (see boost guidelines for some details on this issue).

Since BBv2 will build the boost libraries for you, you need the full boost source package. Having boost installed via some package system is usually not enough (and even if it is enough, the necessary environment variables are usually not set by the package installer).

If you want to build against an installed copy of boost, you can skip directly to step 3 (assuming you also have boost build installed).

## build commands

Linux:

    sudo apt install libboost-tools-dev libboost-dev libboost-system-dev
    echo "using gcc ;" >>~/user-config.jam
    b2 crypto=openssl cxxstd=14 release

Mac OS:

    brew install boost-build boost openssl@1.1
    echo "using darwin ;" >>~/user-config.jam
    b2 crypto=openssl cxxstd=14 release

Windows (assuming the boost package is saved to `C:\boost_1_69_0`):

    set BOOST_ROOT=c:\boost_1_69_0
    set BOOST_BUILD_PATH=%BOOST_ROOT%\tools\build
    (cd %BOOST_ROOT% && .\bootstrap.bat)
    echo using msvc ; >>%HOMEDRIVE%%HOMEPATH%\user-config.jam
    %BOOST_ROOT%\b2.exe --hash cxxstd=14 release

## docker file

A Docker file is available that's used to build and run the fuzzers, at OSS-Fuzz.

## Step 1: Download boost

If you want to build against boost installed on your system, you can skip this strep. Just make sure to have BOOST_ROOT unset for the b2 invocation.

You'll find boost here.

Extract the archive to some directory where you want it. For the sake of this guide, let's assume you extract the package to `c:\boost_1_69_0`. You'll need at least version 1.67 of the boost library in order to build libtorrent.

## Step 2: Setup BBv2

If you have installed `boost-build` via a package manager, you can skip this step. If not, you need to build boost build from the boost source package.

First you need to build `b2`. You do this by opening a terminal (In windows, run `cmd`). Change directory to `c:\boost_1_69_0\tools\build`. Then run the script called `bootstrap.bat` or `bootstrap.sh` on a Unix system. This will build `b2` and place it in a directory `src/engine/bin.<architecture>`. Copy the `b2.exe` (or `b2` on a Unix system) to a place that's in you shell's `PATH`. On Linux systems a place commonly used may be `/usr/local/bin` or on Windows `c:\windows` (you can also add directories to the search paths by modifying the environment variable called `PATH`).

Now you have `b2` installed. `b2` can be considered an interpreter that the boost-build system is implemented on. So boost-build uses `b2`. So, to complete the installation you need to make two more things. You need to set the environment variable `BOOST_BUILD_PATH`. This is the path that tells `b2` where it can find boost-build, your configuration file and all the toolsets (descriptions used by boost-build to know how to use different compilers on different platforms). Assuming the boost install path above, set it to `c:\boost_1_69_0\tools\build`.

To set an environment variable in windows, type for example:

    set BOOST_BUILD_PATH=c:\boost_1_69_0\tools\build\v2

In a terminal window.

The last thing to do is to configure which compiler(s) to use. Create a file `user-config.jam` in your home directory. Depending on your platform and which compiler you're using, you should add a line for each compiler and compiler version you have installed on your system that you want to be able to use with BBv2. For example, if you're using Microsoft Visual Studio 14.2 (2019), just add a line:

    using msvc : 14.2 ;

If you use GCC, add the line:

    using gcc ;

If you have more than one version of GCC installed, you can add the command line used to invoke g++ after the version number, like this:

    using gcc : 6.0 : g++-6 ;
    using gcc : 7.0 : g++-7 ;

Another toolset worth mentioning is the `darwin` toolset (for macOS). From Tiger (10.4) macOS comes with both GCC 3.3 and GCC 4.0. Then you can use the following toolsets:

    using darwin : 3.3 : g++-3.3 ;
    using darwin : 4.0 : g++-4.0 ;

Note that the spaces around the semi-colons and colons are important!

Also see the boost-build documentation.

## Step 3: Building libtorrent

When building libtorrent, boost is either picked up from system installed locations or from a boost source package, if the `BOOST_ROOT` environment variable is set pointing to one. If you're building boost from source, set `BOOST_ROOT` to your boost directory, e.g. `c:\boost_1_69_0`.

Then the only thing left is simply to invoke `b2`. If you want to specify a specific toolset to use (compiler) you can just add that to the command line. For example:

    b2 msvc-14.2
    b2 gcc-7.0
    b2 darwin-4.0

Note

If the environment variable `BOOST_ROOT` is not set, the Jamfile will attempt to link against "installed" boost libraries. i.e. assume the headers and libraries are available in default search paths. In this case it's critical that you build your project with the same version of C++ and the same build flags as the system libraries were built with.

Note

Also see the Visual Studio versions.

To build different versions you can also just add the name of the build variant. Some default build variants in BBv2 are `release`, `debug`, `profile`.

You can build libtorrent as a DLL too, by typing `link=shared`, or `link=static` to build a static library.

If you want to explicitly say how to link against the runtime library, you can set the `runtime-link` feature on the command line, either to `shared` or `static`. Most operating systems will only allow linking shared against the runtime, but on windows you can do both. Example:

    b2 msvc-14.2 variant=release link=static runtime-link=static debug-symbols=on

Note

When building on windows, the path boost-build puts targets in may be too long. If you get an error message like: "The input line is long", try to pass --hash on the `b2` command line.

Warning

If you link statically to the runtime library, you cannot build libtorrent as a shared library (DLL), since you will get separate heaps in the library and in the client application. It will result in crashes and possibly link errors.

Note

When building on Solaris, you may have to specify `stdlib=sun-stlport` on the b2 command line.

The build targets are put in a directory called bin, and under it they are sorted in directories depending on the toolset and build variant used.

To build the examples, just change directory to the examples directory and invoke `b2` from there. To build and run the tests, go to the test directory and run `b2`.

Note that if you're building on windows using the `msvc` toolset, you cannot run it from a cygwin terminal, you'll have to run it from a `cmd` terminal. The same goes for cygwin, if you're building with gcc in cygwin you'll have to run it from a cygwin terminal. Also, make sure the paths are correct in the different environments. In cygwin, the paths (`BOOST_BUILD_PATH` and `BOOST_ROOT`) should be in the typical Unix-format (e.g. `/cygdrive/c/boost_1_69_0`). In the windows environment, they should have the typical windows format (`c:/boost_1_69_0`).

Note

In Jamfiles, spaces are separators. It's typically easiest to avoid spaces in path names. If you want spaces in your paths, make sure to quote them with double quotes (").

The `Jamfile` will define `NDEBUG` when it's building a release build. For more build configuration flags see Build configurations.

Jamfile will look in some default directory for the openssl headers and libraries. On macOS, it will look for the homebrew openssl package. On Windows, it will look in `C:\OpenSSL-Win32`, or `C:\OpenSSL-Win64` if compiling in 64-bit.

To customize the library path and include path for openssl, set the features `openssl-lib` and `openssl-include` respectively.

The option to link with wolfSSL (by setting the `crypto` feature to `wolfssl`), requires a custom build of wolfSSL using the following options: `--enable-asio --enable-sni --enable-nginx`.

To customize the library path and include path for wolfSSL, set the features `wolfssl-lib` and `wolfssl-include` respectively.

To disable linking against any SSL library, set the `crypto` build feature to `built-in`. This will use an embedded version if SHA-1.

## Build features

boost build feature | values
--- | ---
`cxxstd` | The version of C++ to use, e.g. `11`, `14`, `17`, `20`. The C++ version may affect the libtorrent ABI (the ambition is to avoid that).
`boost-link` | `static` - links statically against the boost libraries. `shared` - links dynamically against the boost libraries.
`openssl-lib` | can be used to specify the directory where libssl and libcrypto are installed (or the windows counterparts).
`openssl-include` | can be used to specify the include directory where the openssl headers are installed.
`logging` | `off` - logging alerts disabled. The reason to disable logging is to keep the binary size low where that matters. `on` - default. logging alerts available, still need to be enabled by the alert mask.
`lto` | `on` - enables link time optimization, also known as whole program optimization.
`alert-msg` | `on` - (default) return human readable messages from the `alert::message()` call. `off` - Always return empty strings from `alert::message()`, and save binary size.
`dht` | `on` - build with DHT support `off` - build without DHT support.
`asserts` | `auto` - asserts are on if in debug mode `on` - asserts are on, even in release mode `off` - asserts are disabled `production` - assertion failures are logged to `asserts.log` in the current working directory, but won't abort the process. The file they are logged to can be customized by setting the global pointer `extern char const* libtorrent_assert_log` to a different filename. `system` use the libc assert macro
`encryption` | `on` - encrypted bittorrent connections enabled. (Message Stream encryption).(default) `off` - turns off support for encrypted connections. The shipped public domain SHA-1 implementation is used.
`mutable-torrents` | `on` - mutable torrents are supported (BEP 38) (default). `off` - mutable torrents are not supported.
`crypto` | `openssl` - (default) links against openssl and libcrypto to use for SHA-1 hashing. This also enables HTTPS-tracker support and support for bittorrent over SSL. `built-in` - (default) uses built-in SHA-1 implementation. In macOS/iOS it uses CommonCrypto SHA-1 implementation. `wolfssl` - links against wolfssl to use it for SHA-1 hashing and HTTPS tracker support. `libcrypto` - links against libcrypto to use the SHA-1 implementation. (no SSL support) `gcrypt` - links against libgcrypt to use the SHA-1 implementation. (no SSL support)
`openssl-version` | This can be used on windows to link against the special OpenSSL library names used on windows prior to OpenSSL 1.1. `1.1` - link against the normal openssl library name. (default) `pre1.1` - link against the old windows names (i.e. `ssleay32` and `libeay32`.
`link` | `static` - builds libtorrent as a static library (.a / .lib) `shared` - builds libtorrent as a shared library (.so / .dll).
`runtime-link` | `static` - links statically against the run-time library (if available on your platform). `shared` - link dynamically against the run-time library (default).
`variant` | `debug` - builds libtorrent with debug information and invariant checks. `release` - builds libtorrent in release mode without invariant checks and with optimization. `profile` - builds libtorrent with profile information.
`invariant-checks` | This setting only affects debug builds (where `NDEBUG` is not defined). It defaults to `on`. `on` - internal invariant checks are enabled. `off` - internal invariant checks are disabled. The resulting executable will run faster than a regular debug build. `full` - turns on extra expensive invariant checks.
`debug-symbols` | `on` - default for debug builds. This setting is useful for building release builds with symbols. `off` - default for release builds.
`deprecated-functions` | `on` - default. Includes deprecated functions of the API (might produce warnings during build when deprecated functions are used). `off` - excludes deprecated functions from the API. Generates build errors when deprecated functions are used.
`i2p` | `on` - default. build with I2P support `off` - build without I2P support
`profile-calls` | `off` - default. No additional call profiling. `on` - Enable logging of stack traces of calls into libtorrent that are blocking. On session shutdown, a file `blocking_calls.txt` is written with stack traces of blocking calls ordered by the number of them.
`utp-log` | `off` - default. Do not print verbose uTP log. `on` - Print verbose uTP log, used to debug the uTP implementation.
`picker-debugging` | `off` - default. no extra invariant checks in piece picker. `on` - include additional invariant checks in piece picker. Used for testing the piece picker.
`extensions` | `on` - enable extensions to the bittorrent protocol.(default) `off` - disable bittorrent extensions.
`streaming` | `on` - enable streaming functionality. i.e. `set_piece_deadline()`. (default) `off` - disable streaming functionality.
`super-seeding` | `on` - enable super seeding feature. (default) `off` - disable super seeding feature
`share-mode` | `on` - enable share-mode feature. (default) `off` - disable share-mode feature
`predictive-pieces` | `on` - enable predictive piece announce feature. i.e. settings_pack::predictive_piece_announce (default) `off` - disable feature.
`fpic` | `off` - default. Build without specifying `-fPIC`. `on` - Force build with `-fPIC` (useful for building a static library to be linked into a shared library).
`mmap-disk-io` | `on` - default. Enable mmap disk storage (if available. `off` - disable mmap storage, and fall back to single-threaded, portable file operations.

The `variant` feature is implicit, which means you don't need to specify the name of the feature, just the value.

When building the example client on windows, you need to build with `link=static` otherwise you may get unresolved external symbols for some boost.program-options symbols.

For more information, see the Boost build v2 documentation, or more specifically the section on built-in features.

## Step 4: Installing libtorrent

To install libtorrent run `b2` with the `install` target:

    b2 install --prefix=/usr/local

Change the value of the `--prefix` argument to install it in a different location.

## Custom build flags

Custom build flags can be passed to the command line via the `cflags`, `cxxflags` and `linkflags` features. When specifying custom flags, make sure to build everything from scratch, to not accidentally mix incompatible flags. Example:

    b2 cxxflags=-msse4.1

Custom flags can also be configured in the toolset, in `~/user-config.jam`, `Jamroot.jam` or `project-config.jam`. Example:

    using gcc : sse41 : g++ : <cxxflags>-msse4.1 ;

## Cross compiling

To cross compile libtorrent, configure a new toolset for `b2` to use. Toolsets can be configured in `~/user-config.jam`, `Jamroot.jam` or `project-config.jam`. The last two live in the libtorrent root directory.

A toolset configuration is in this form:

    using toolset : version : command-line : features ;

Toolset is essentially the family of compiler you're setting up, choose from this list.

Perhaps the most common ones would be `gcc`, `clang`, `msvc` and `darwin` (Apple's version of clang).

The version can be left empty to be auto configured, or a custom name can be used to identify this toolset.

The command-line is what to execute to run the compiler. This is also an opportunity to insert a call to `ccache` for example.

features are boost-build features. Typical features to set here are `<compileflags>`, `<cflags>` and `<cxxflags>`. For the `gcc` toolset, the `<archiver>` can be set to specify which tool to use to create a static library/archive. This is especially handy when cross compiling.

Here's an example toolset for cross compiling for ARM Linux:

    using gcc : arm : arm-linux-gnueabihf-g++ : <archiver>arm-linux-gnueabihf-ar ;

To build using this toolset, specify `gcc-arm` as the toolset on the `b2` command line. For example:

    b2 toolset=gcc-arm

# building with cmake

First of all, you need to install `cmake`. Additionally you need a build system to actually schedule builds, for example `ninja`.

## Step 1: Generating the build system

Create a build directory for out-of-source build inside the libtorrent root directory:

    mkdir build

and `cd` there:

    cd build

Run `cmake` in the build directory, like this:

    cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_STANDARD=14 -G Ninja ..

The `CMAKE_CXX_STANDARD` has to be at least 14, but you may want to raise it to `17` if your project use a newer version of the C++ standard.

Warning

The detection of boost sometimes fail in subtle ways. If you have the `BOOST_ROOT` environment variable set, it may find the pre-built system libraries, but use the header files from your source package. To avoid this, invoke `cmake` with `BOOST_ROOT` set to an empty string: `BOOST_ROOT="" cmake ...`.

Other build options are:

`BUILD_SHARED_LIBS` | Defaults `ON`. Builds libtorrent as a shared library.
`static_runtime` | Defaults `OFF`. Link libtorrent statically against the runtime libraries.
`build_tests` | Defaults `OFF`. Also build the libtorrent tests.
`build_examples` | Defaults `OFF`. Also build the examples in the examples directory.
`build_tools` | Defaults `OFF`. Also build the tools in the tools directory.
`python-bindings` | Defaults `OFF`. Also build the python bindings in bindings/python directory.
`encryption` | Defaults `ON`. Support trackers and bittorrent over TLS, and obfuscated bittorrent connections.

Options are set on the `cmake` command line with the `-D` option or later on using `ccmake` or `cmake-gui` applications. `cmake` run outputs a summary of all available options and their current values.

## Step 2: Building libtorrent

In the terminal, run:

    ninja

If you enabled test in the configuration step, to run them, run:

    ctest

# building with VCPKG

You can download and install libtorrent using the vcpkg dependency manager:

    git clone https://github.com/Microsoft/vcpkg.git
    cd vcpkg
    ./bootstrap-vcpkg.sh
    ./vcpkg integrate install
    ./vcpkg install libtorrent

The libtorrent port in vcpkg is kept up to date by Microsoft team members and community contributors. If the version is out of date, please create an issue or pull request on the vcpkg repository.

# build configurations

By default libtorrent is built In debug mode, and will have pretty expensive invariant checks and asserts built into it. If you want to disable such checks (you want to do that in a release build) you can see the table below for which defines you can use to control the build. Make sure to define the same macros in your own code that compiles and links with libtorrent.

macro | description
--- | ---
`NDEBUG` | If you define this macro, all asserts, invariant checks and general debug code will be removed. Since there is quite a lot of code in in header files in libtorrent, it may be important to define the symbol consistently across compilation units, including the clients files. Potential problems is different compilation units having different views of structs and class layouts and sizes.
`TORRENT_DISABLE_LOGGING` | This macro will disable support for logging alerts, like log_alert, torrent_log_alert and peer_log_alert. With this build flag, you cannot enable those alerts.
`TORRENT_DISABLE_ALERT_MSG` | Human readable messages returned from the alert `message()` member functions will return empty strings.
`TORRENT_DISABLE_SUPERSEEDING` | This macro will disable support for super seeding. The settings will exist, but will not have an effect, when this macro is defined.
`TORRENT_DISABLE_SHARE_MODE` | This macro will disable support for share-mode. i.e. the mode to maximize upload/download ratio for a torrent.
`TORRENT_DISABLE_MUTABLE_TORRENTS` | Disables mutable torrent support (BEP 38)
`TORRENT_DISABLE_STREAMING` | Disables set_piece_deadline() and associated functionality.
`TORRENT_DISABLE_PREDICTIVE_PIECES` | Disables settings_pack::predictive_piece_announce feature.
`TORRENT_LINKING_SHARED` | If this is defined when including the libtorrent headers, the classes and functions will be tagged with `__declspec(dllimport)` on msvc and default visibility on GCC 4 and later. Set this in your project if you're linking against libtorrent as a shared library. (This is set by the Jamfile when `link=shared` is set).
`TORRENT_BUILDING_SHARED` | If this is defined, the functions and classes in libtorrent are marked with `__declspec(dllexport)` on msvc, or with default visibility on GCC 4 and later. This should be defined when building libtorrent as a shared library. (This is set by the Jamfile when `link=shared` is set).
`TORRENT_DISABLE_DHT` | If this is defined, the support for trackerless torrents will be disabled.
`TORRENT_DISABLE_ENCRYPTION` | This will disable any encryption support and the dependencies of a crypto library. Encryption support is the peer connection encrypted supported by clients such as uTorrent, Azureus and KTorrent. If this is not defined, either `TORRENT_USE_LIBCRYPTO` or `TORRENT_USE_LIBGCRYPT` must be defined.
`TORRENT_DISABLE_EXTENSIONS` | When defined, libtorrent plugin support is disabled along with support for the extension handshake (BEP 10).
`TORRENT_USE_INVARIANT_CHECKS` | If defined to non-zero, this will enable internal invariant checks in libtorrent. The invariant checks can sometimes be quite expensive, they typically don't scale very well.
`TORRENT_EXPENSIVE_INVARIANT_CHECKS` | This will enable extra expensive invariant checks. Useful for finding particular bugs or for running before releases.
`TORRENT_NO_DEPRECATE` | This will exclude all deprecated functions from the header files and source files.
`TORRENT_PRODUCTION_ASSERTS` | Define to either 0 or 1. Enables assert logging in release builds.
`TORRENT_USE_ASSERTS` | Define as 0 to disable asserts unconditionally.
`TORRENT_USE_SYSTEM_ASSERTS` | Uses the libc assert macro rather then the custom one.
`TORRENT_HAVE_MMAP` | Define as 0 to disable mmap support.
`TORRENT_USE_OPENSSL` | Link against `libssl` for SSL support. Must be combined with `TORRENT_USE_LIBCRYPTO`
`TORRENT_USE_GNUTLS` | Link against `libgnutls` for SSL support.
`TORRENT_USE_LIBCRYPTO` | Link against `libcrypto` for SHA-1 support and other hashing algorithms.
`TORRENT_USE_LIBGCRYPT` | Link against `libgcrypt` for SHA-1 support and other hashing algorithms.
`TORRENT_SSL_PEERS` | Define to enable support for SSL torrents, peers are connected over authenticated SSL streams.

If you experience that libtorrent uses unreasonable amounts of CPU, it will definitely help to define `NDEBUG`, since it will remove the invariant checks within the library.

# building openssl for windows

To build openssl for windows with Visual Studio 7.1 (2003) execute the following commands in a command shell:

    perl Configure VC-WIN32 --prefix="c:/openssl
    call ms\do_nasm
    call "C:\Program Files\Microsoft Visual Studio .NET 2003\vc7\bin\vcvars32.bat"
    nmake -f ms\nt.mak
    copy inc32\openssl "C:\Program Files\Microsoft Visual Studio .NET 2003\vc7\include\"
    copy out32\libeay32.lib "C:\Program Files\Microsoft Visual Studio .NET 2003\vc7\lib"
    copy out32\ssleay32.lib "C:\Program Files\Microsoft Visual Studio .NET 2003\vc7\lib"

This will also install the headers and library files in the visual studio directories to be picked up by libtorrent.

# list of macros

The following is a list of defines that libtorrent is built with: `BOOST_ALL_NO_LIB`, `BOOST_ASIO_ENABLE_CANCELIO`, `BOOST_ASIO_HAS_STD_CHRONO`, `BOOST_MULTI_INDEX_DISABLE_SERIALIZATION`, `BOOST_NO_DEPRECATED`, `BOOST_SYSTEM_NO_DEPRECATED`

Make sure you define the same at compile time for your code to avoid any runtime errors and other issues.

These might change in the future, so it's always best to verify these every time you upgrade to a new version of libtorrent. The simplest way to see the full list of macros defined is to build libtorrent with `-n -a` switches added to `b2` command line:

    b2 -n -a toolset=msvc-14.2 link=static runtime-link=static boost-link=static variant=release

This will output all compiler switches, including defines (such as `-DBOOST_ASIO_ENABLE_CANCELIO`).