An open-source C++ library developed and used at Facebook.

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Folly: Facebook Open-source Library

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What is folly?

Logo Folly

Folly (acronymed loosely after Facebook Open Source Library) is a library of C++17 components designed with practicality and efficiency in mind. Folly contains a variety of core library components used extensively at Facebook. In particular, it's often a dependency of Facebook's other open source C++ efforts and place where those projects can share code.

It complements (as opposed to competing against) offerings such as Boost and of course std. In fact, we embark on defining our own component only when something we need is either not available, or does not meet the needed performance profile. We endeavor to remove things from folly if or when std or Boost obsoletes them.

Performance concerns permeate much of Folly, sometimes leading to designs that are more idiosyncratic than they would otherwise be (see e.g. PackedSyncPtr.h, SmallLocks.h). Good performance at large scale is a unifying theme in all of Folly.

Check it out in the intro video

Explain Like I’m 5: Folly

Logical Design

Folly is a collection of relatively independent components, some as simple as a few symbols. There is no restriction on internal dependencies, meaning that a given folly module may use any other folly components.

All symbols are defined in the top-level namespace folly, except of course macros. Macro names are ALL_UPPERCASE and should be prefixed with FOLLY_. Namespace folly defines other internal namespaces such as internal or detail. User code should not depend on symbols in those namespaces.

Folly has an experimental directory as well. This designation connotes primarily that we feel the API may change heavily over time. This code, typically, is still in heavy use and is well tested.

Physical Design

At the top level Folly uses the classic "stuttering" scheme folly/folly used by Boost and others. The first directory serves as an installation root of the library (with possible versioning a la folly-1.0/), and the second is to distinguish the library when including files, e.g. #include <folly/FBString.h>.

The directory structure is flat (mimicking the namespace structure), i.e. we don't have an elaborate directory hierarchy (it is possible this will change in future versions). The subdirectory experimental contains files that are used inside folly and possibly at Facebook but not considered stable enough for client use. Your code should not use files in folly/experimental lest it may break when you update Folly.

The folly/folly/test subdirectory includes the unittests for all components, usually named ComponentXyzTest.cpp for each ComponentXyz.*. The folly/folly/docs directory contains documentation.

What's in it?

Because of folly's fairly flat structure, the best way to see what's in it is to look at the headers in top level folly/ directory. You can also check the docs folder for documentation, starting with the overview.

Folly is published on GitHub at https://github.com/facebook/folly.

Build Notes

Because folly does not provide any ABI compatibility guarantees from commit to commit, we generally recommend building folly as a static library.

folly supports gcc (5.1+), clang, or MSVC. It should run on Linux (x86-32, x86-64, and ARM), iOS, macOS, and Windows (x86-64). The CMake build is only tested on some of these platforms; at a minimum, we aim to support macOS and Linux (on the latest Ubuntu LTS release or newer.)


This script is used by many of Meta's OSS tools. It will download and build all of the necessary dependencies first, and will then invoke cmake etc to build folly. This will help ensure that you build with relevant versions of all of the dependent libraries, taking into account what versions are installed locally on your system.

It's written in python so you'll need python3.6 or later on your PATH. It works on Linux, macOS and Windows.

The settings for folly's cmake build are held in its getdeps manifest build/fbcode_builder/manifests/folly, which you can edit locally if desired.


If on Linux or MacOS (with homebrew installed) you can install system dependencies to save building them:

# Clone the repo
git clone https://github.com/facebook/folly
# Install dependencies
cd folly
sudo ./build/fbcode_builder/getdeps.py install-system-deps --recursive

If you'd like to see the packages before installing them:

./build/fbcode_builder/getdeps.py install-system-deps --dry-run --recursive

On other platforms or if on Linux and without system dependencies getdeps.py will mostly download and build them for you during the build step.

Some of the dependencies getdeps.py uses and installs are:

  • a version of boost compiled with C++14 support.
  • googletest is required to build and run folly's tests.


This script will download and build all of the necessary dependencies first, and will then invoke cmake etc to build folly. This will help ensure that you build with relevant versions of all of the dependent libraries, taking into account what versions are installed locally on your system.

getdeps.py currently requires python 3.6+ to be on your path.

getdeps.py will invoke cmake etc.

# Clone the repo
git clone https://github.com/facebook/folly
cd folly
# Build, using system dependencies if available
python3 ./build/fbcode_builder/getdeps.py --allow-system-packages build

It puts output in its scratch area:

  • installed/folly/lib/libfolly.a: Library

You can also specify a --scratch-path argument to control the location of the scratch directory used for the build. You can find the default scratch install location from logs or with python3 ./build/fbcode_builder/getdeps.py show-inst-dir.

There are also --install-dir and --install-prefix arguments to provide some more fine-grained control of the installation directories. However, given that folly provides no compatibility guarantees between commits we generally recommend building and installing the libraries to a temporary location, and then pointing your project's build at this temporary location, rather than installing folly in the traditional system installation directories. e.g., if you are building with CMake you can use the CMAKE_PREFIX_PATH variable to allow CMake to find folly in this temporary installation directory when building your project.

If you want to invoke cmake again to iterate, there is a helpful run_cmake.py script output in the scratch build directory. You can find the scratch build directory from logs or with python3 ./build/fbcode_builder/getdeps.py show-build-dir.

Run tests

By default getdeps.py will build the tests for folly. To run them:

cd folly
python3 ./build/fbcode_builder/getdeps.py --allow-system-packages test

build.sh/build.bat wrapper

build.sh can be used on Linux and MacOS, on Windows use the build.bat script instead. Its a wrapper around getdeps.py.

Build with cmake directly

If you don't want to let getdeps invoke cmake for you then by default, building the tests is disabled as part of the CMake all target. To build the tests, specify -DBUILD_TESTS=ON to CMake at configure time.

NB if you want to invoke cmake again to iterate on a getdeps.py build, there is a helpful run_cmake.py script output in the scratch-path build directory. You can find the scratch build directory from logs or with python3 ./build/fbcode_builder/getdeps.py show-build-dir.

Running tests with ctests also works if you cd to the build dir, e.g. (cd $(python3 ./build/fbcode_builder/getdeps.py show-build-dir) && ctest)

Finding dependencies in non-default locations

If you have boost, gtest, or other dependencies installed in a non-default location, you can use the CMAKE_INCLUDE_PATH and CMAKE_LIBRARY_PATH variables to make CMAKE look also look for header files and libraries in non-standard locations. For example, to also search the directories /alt/include/path1 and /alt/include/path2 for header files and the directories /alt/lib/path1 and /alt/lib/path2 for libraries, you can invoke cmake as follows:

cmake \
  -DCMAKE_INCLUDE_PATH=/alt/include/path1:/alt/include/path2 \
  -DCMAKE_LIBRARY_PATH=/alt/lib/path1:/alt/lib/path2 ...

Ubuntu LTS, CentOS Stream, Fedora

Use the getdeps.py approach above. We test in CI on Ubuntu LTS, and occasionally on other distros.

If you find the set of system packages is not quite right for your chosen distro, you can specify distro version specific overrides in the dependency manifests (e.g. https://github.com/facebook/folly/blob/main/build/fbcode_builder/manifests/boost ). You could probably make it work on most recent Ubuntu/Debian or Fedora/Redhat derived distributions.

At time of writing (Dec 2021) there is a build break on GCC 11.x based systems in lang_badge_test. If you don't need badge functionality you can work around by commenting it out from CMakeLists.txt (unfortunately fbthrift does need it)

Windows (Vcpkg)

Note that many tests are disabled for folly Windows builds, you can see them in the log from the cmake configure step, or by looking for WINDOWS_DISABLED in CMakeLists.txt

That said, getdeps.py builds work on Windows and are tested in CI.

If you prefer, you can try Vcpkg. folly is available in Vcpkg and releases may be built via vcpkg install folly:x64-windows.

You may also use vcpkg install folly:x64-windows --head to build against main.


getdeps.py builds work on macOS and are tested in CI, however if you prefer, you can try one of the macOS package managers


folly is available as a Formula and releases may be built via brew install folly.

You may also use folly/build/bootstrap-osx-homebrew.sh to build against main:


This will create a build directory _build in the top-level.


Install the required packages from MacPorts:

  sudo port install \
    boost \
    cmake \
    gflags \
    git \
    google-glog \
    libevent \
    libtool \
    lz4 \
    lzma \
    openssl \
    snappy \
    xz \

Download and install double-conversion:

  git clone https://github.com/google/double-conversion.git
  cd double-conversion
  sudo make install

Download and install folly with the parameters listed below:

  git clone https://github.com/facebook/folly.git
  cd folly
  mkdir _build
  cd _build
  cmake ..
  sudo make install