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

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Folly: Facebook Open-source Library

What is

Logo Folly

Folly (acronymed loosely after Facebook Open Source Library) is a library of C++14 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

. 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
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.

). Good performance at large scale is a unifying theme in all of 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

, except of course macros. Macro names are ALLUPPERCASE and should be prefixed with `FOLLY
. Namespace
defines other internal namespaces
such as
detail`. User code should not depend on symbols in those namespaces.

Folly has an

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

used by Boost and others. The first directory serves as an installation root of the library (with possible versioning a la
), and the second is to distinguish the library when including files, e.g.

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

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
lest it may break when you update Folly.


subdirectory includes the unittests for all components, usually named
for each
. The
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

directory. You can also check the
for documentation, starting with the overview.

Folly is published on GitHub at

Build Notes

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

The simplest way to build folly is using the
script in the top-level of the repository.
can be used on Linux and MacOS, on Windows use the
script instead.

This script will download and build all of the necessary dependencies first, and will then build folly. This will help ensure that you build with recent versions of all of the dependent libraries, regardless of what versions are installed locally on your system.

By default this script will build and install folly and its dependencies in a scratch directory. You can also specify a

argument to control the location of the scratch directory used for the build. There are also
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
variable to allow CMake to find folly in this temporary installation directory when building your project.


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.)

folly requires a version of boost compiled with C++14 support.

googletest is required to build and run folly's tests. You can download it from The following commands can be used to download and install it:

wget && \
tar zxf release-1.8.0.tar.gz && \
rm -f release-1.8.0.tar.gz && \
cd googletest-release-1.8.0 && \
cmake . && \
make && \
make install

Finding dependencies in non-default locations

If you have boost, gtest, or other dependencies installed in a non-default location, you can use the

variables to make CMAKE look also look for header files and libraries in non-standard locations. For example, to also search the directories
for header files and the directories
for libraries, you can invoke
as follows:
cmake \
  -DCMAKE_INCLUDE_PATH=/alt/include/path1:/alt/include/path2 \
  -DCMAKE_LIBRARY_PATH=/alt/lib/path1:/alt/lib/path2 ...

Building tests

By default, building the tests is disabled as part of the CMake

target. To build the tests, specify
to CMake at configure time.

Ubuntu 16.04 LTS

The following packages are required (feel free to cut and paste the apt-get command below):

sudo apt-get install \
    g++ \
    cmake \
    libboost-all-dev \
    libevent-dev \
    libdouble-conversion-dev \
    libgoogle-glog-dev \
    libgflags-dev \
    libiberty-dev \
    liblz4-dev \
    liblzma-dev \
    libsnappy-dev \
    make \
    zlib1g-dev \
    binutils-dev \
    libjemalloc-dev \
    libssl-dev \
    pkg-config \

Folly relies on fmt which needs to be installed from source. The following commands will download, compile, and install fmt.

git clone && cd fmt

mkdir _build && cd _build cmake ..

make -j$(nproc) sudo make install

If advanced debugging functionality is required, use:

sudo apt-get install \
    libunwind8-dev \
    libelf-dev \

In the folly directory (e.g. the checkout root or the archive unpack root), run:

  mkdir _build && cd _build
  cmake ..
  make -j $(nproc)
  make install # with either sudo or DESTDIR as necessary

OS X (Homebrew)

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

brew install folly

You may also use

to build against

This will create a build directory

in the top-level.

OS X (MacPorts)

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
  cd double-conversion
  sudo make install

Download and install folly with the parameters listed below:

  git clone
  cd folly
  mkdir _build
  cd _build
  cmake ..
  sudo make install

Windows (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

Other Linux distributions

  • double-conversion (

Download and build double-conversion. You may need to tell cmake where to find it.


ln -s src double-conversion


mkdir build && cd build


  • additional platform specific dependencies:

Fedora >= 21 64-bit (last tested on Fedora 28 64-bit) - gcc - gcc-c++ - cmake - automake - boost-devel - libtool - lz4-devel - lzma-devel - snappy-devel - zlib-devel - glog-devel - gflags-devel - scons - double-conversion-devel - openssl-devel - libevent-devel - fmt-devel - libsodium-devel

Optional - libdwarf-devel - elfutils-libelf-devel - libunwind-devel

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