libtorrent manual

Author: Arvid Norberg,
Version: 1.1.13

downloading and building

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

The build systems supported "out of the box" in libtorrent are boost-build v2 (BBv2) and autotools (for unix-like systems). If you still can't build after following these instructions, you can usually get help in the #libtorrent IRC channel on


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_* macros are defined when you link against libtorrent as when you build it.

Boost-build supports propagating configuration options to dependencies. When building using the makefiles, this is handled by setting the configuration options in the pkg-config file. Always use pkg-config when linking against libtorrent.

building from git

To build libtorrent from git you need to clone the libtorrent repo from github. If you downloaded a release tarball, you can skip this section.

git clone

building with BBv2

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).

Step 1: Download boost

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_64_0 (I'm using a windows path in this example since if you're on linux/unix you're more likely to use the autotools). You'll need at least version 1.49 of the boost library in order to build libtorrent.

Step 2: Setup BBv2

First you need to build bjam. You do this by opening a terminal (In windows, run cmd). Change directory to c:\boost_1_64_0\tools\jam\src. Then run the script called build.bat or on a unix system. This will build bjam and place it in a directory starting with bin. and then have the name of your platform. Copy the bjam.exe (or bjam 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 bjam installed. bjam can be considered an interpreter that the boost-build system is implemented on. So boost-build uses bjam. 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 bjam 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_64_0\tools\build\v2.

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

set BOOST_BUILD_PATH=c:\boost_1_64_0\tools\build\v2

In a terminal window.

The last thing to do to complete the setup of BBv2 is to modify your user-config.jam file. It is located in c:\boost_1_64_0\tools\build\v2. 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 12 (2013), just add a line:

using msvc : 12.0 ;

If you use GCC, add the line:

using gcc ;

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

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

Another toolset worth mentioning is the darwin toolset (For MacOS X). From Tiger (10.4) MacOS X 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 official installation instructions.

Step 3: Building libtorrent

When building libtorrent, the Jamfile expects the environment variable BOOST_ROOT to be set to the boost installation directory. It uses this to find the boost libraries it depends on, so they can be built and their headers files found. So, set this to c:\boost_1_64_0. You only need this if you're building against a source distribution of boost.

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

bjam msvc-7.1
bjam gcc-3.3
bjam darwin-4.0


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.

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 commandline, either to shared or static. Most operating systems will only allow linking shared against the runtime, but on windows you can do both. Example:

bjam msvc-7.1 link=static runtime-link=static


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 --abbreviate-paths on the bjam command line.


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.


With boost-build V2 (Milestone 11), the darwin toolset uses the -s linker option to strip debug symbols. This option is buggy in Apple's GCC, and will make the executable crash on startup. On Mac OS X, instead build your release executables with the debug-symbols=on option, and later strip your executable with strip.


Some linux systems requires linking against librt in order to access the POSIX clock functions. If you get an error complaining about a missing symbol clock_gettime, you have to give need-librt=yes on the bjam command line. This will make libtorrent link against librt.


When building on Solaris, you might have to specify stdlib=sun-stlport on the bjam 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 bjam from there. To build and run the tests, go to the test directory and run bjam.

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_64_0). In the windows environment, they should have the typical windows format (c:/boost_1_64_0).


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.

When enabling linking against openssl (by setting the crypto feature to openssl) the 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 and mingw in c:\OpenSSL-Win32.

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

Build features:

boost build feature values
  • 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.
  • 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.
  • on - build with DHT support
  • off - build without DHT support.
  • 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
  • on - encrypted bittorrent connections enabled. (Message Stream encryption).
  • off - turns off support for encrypted connections. The shipped public domain SHA-1 implementation is used.
  • on - mutable torrents are supported (BEP 38) (default).
  • off - mutable torrents are not supported.
  • built-in - (default) uses built-in SHA-1 implementation.
  • openssl - links against openssl and libcrypto to use for SHA-1 hashing. This also enables HTTPS-tracker support and support for bittorrent over SSL.
  • gcrypt - links against libgcrypt to use for SHA-1 hashing.

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.
  • pool - default, uses pool allocators for send buffers.
  • system - uses malloc() and free() instead. Might be useful to debug buffer issues with tools like electric fence or libgmalloc.
  • debug - instruments buffer usage to catch bugs in libtorrent.
  • static - builds libtorrent as a static library (.a / .lib)
  • shared - builds libtorrent as a shared library (.so / .dll).
  • static - links statically against the run-time library (if available on your platform).
  • shared - link dynamically against the run-time library (default).
  • 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.

This setting will only have an affect on windows. Other platforms are expected to support UTF-8.

  • unicode - The unicode version of the win32 API is used. This is default.
  • ansi - The ansi version of the win32 API is used.

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.
  • on - default for debug builds. This setting is useful for building release builds with symbols.
  • off - default for release builds.
  • 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.
  • auto - use iconv for string conversions for linux and mingw and other posix platforms.
  • on - force use of iconv
  • off - force not using iconv (disables locale awareness except on windows).
  • on - build with I2P support
  • off - build without I2P support
  • 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.

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

The logs created when building vlog or log mode are put in a directory called libtorrent_logs in the current working directory.

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 builtin features.

building with autotools

First of all, you need to install automake and autoconf. Many unix/linux systems comes with these preinstalled.

The prerequisites for building libtorrent are boost.system, boost.chrono and boost.random. Those are the compiled boost libraries needed. The headers-only libraries needed include (but is not necessarily limited to) boost.bind, boost.ref, boost.multi_index, boost.optional, boost.integer, boost.iterator, boost.tuple, boost.array, boost.function, boost.smart_ptr, boost.preprocessor, boost.static_assert.

If you want to build the client_test example, you'll also need boost.regex and boost.program_options.

Step 1: Generating the build system

No build system is present if libtorrent is checked out from CVS - it needs to be generated first. If you're building from a released tarball, you may skip directly to Step 2: Running configure.

Execute the following command to generate the build system:


Step 2: Running configure

In your shell, change directory to the libtorrent directory and run ./configure. This will look for libraries and C++ features that libtorrent is dependent on. If something is missing or can't be found it will print an error telling you what failed.

The most likely problem you may encounter is that the configure script won't find the boost libraries. Make sure you have boost installed on your system. The easiest way to install boost is usually to use the preferred package system on your platform. Usually libraries and headers are installed in standard directories where the compiler will find them, but sometimes that may not be the case. For example when installing boost on darwin using darwinports (the package system based on BSD ports) all libraries are installed to /opt/local/lib and headers are installed to /opt/local/include. By default the compiler will not look in these directories. You have to set the enviornment variables LDFLAGS and CXXFLAGS in order to make the compiler find those libs. In this example you'd set them like this:

export LDFLAGS=-L/opt/local/lib
export CXXFLAGS=-I/opt/local/include

It was observed on FreeBSD (release 6.0) that one needs to add '-lpthread' to LDFLAGS, as Boost::Thread detection will fail without it, even if Boost::Thread is installed.

If you need to set these variables, it may be a good idea to add those lines to your ~/.profile or ~/.tcshrc depending on your shell.

If the boost libraries are named with a suffix on your platform, you may use the --with-boost-thread= option to specify the suffix used for the thread library in this case. For more information about these options, run:

./configure --help

On gentoo the boost libraries that are built with multi-threading support have the suffix mt.

You know that the boost libraries were found if you see the following output from the configure script:

Checking for boost libraries:
checking for boostlib >= 1.53... yes
checking whether the Boost::System library is available... yes
checking for exit in -lboost_system... yes
checking whether the Boost::Chrono library is available... yes
checking for exit in -lboost_chrono-mt... yes
checking whether the Boost::Random library is available... yes
checking for exit in -lboost_random-mt... yes

Another possible source of problems may be if the path to your libtorrent directory contains spaces. Make sure you either rename the directories with spaces in their names to remove the spaces or move the libtorrent directory.

Creating a debug build

To tell configure to build a debug version (with debug info, asserts and invariant checks enabled), you have to run the configure script with the following option:

./configure --enable-debug=yes

Creating a release build

To tell the configure to build a release version (without debug info, asserts and invariant checks), you have to run the configure script with the following option:

./configure --enable-debug=no

The above option make use of -DNDEBUG, which is used throughout libtorrent.

Step 3: Building libtorrent

Once the configure script is run successfully, you just type make and libtorrent, the examples and the tests will be built.

When libtorrent is built it may be a good idea to run the tests, you do this by running make check.

If you want to build a release version (without debug info, asserts and invariant checks), you have to rerun the configure script and rebuild, like this:

./configure --disable-debug
make clean

building with other build systems

If you're building in MS Visual Studio, you may have to set the compiler options "force conformance in for loop scope", "treat wchar_t as built-in type" and "Enable Run-Time Type Info" to Yes.

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.

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_DISK_STATS This will create a log of all disk activity which later can parsed and graphed using
UNICODE If building on windows this will make sure the UTF-8 strings in pathnames are converted into UTF-16 before they are passed to the file operations.
TORRENT_DISABLE_POOL_ALLOCATOR Disables use of boost::pool<>.
TORRENT_DISABLE_MUTABLE_TORRENTS Disables mutable torrent support (BEP 38)
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_OPENSSL or TORRENT_USE_GCRYPT must be defined.
TORRENT_DISABLE_EXTENSIONS When defined, libtorrent plugin support is disabled along with support for the extension handskake (BEP 10).
_UNICODE On windows, this will cause the file IO use wide character API, to properly support non-ansi characters.
TORRENT_DISABLE_RESOLVE_COUNTRIES Defining this will disable the ability to resolve countries of origin for peer IPs.
TORRENT_DISABLE_INVARIANT_CHECKS This will disable internal invariant checks in libtorrent. The invariant checks can sometime be quite expensive, they typically don't scale very well. This option can be used to still build in debug mode, with asserts enabled, but make the resulting executable faster.
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 cpp 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.

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.