Skip to content
An open-source C++ library developed and used at Facebook.
Branch: master
Clone or download
yfeldblum and facebook-github-bot A semaphore for testing
[Folly] A semaphore type for testing scenarios which need a basic portable semaphore. Supports FIFO and LIFO wake policies.

May be used when testing other, more sophisticated, semaphore types.

Reviewed By: strager

Differential Revision: D15843438

fbshipit-source-id: 9136e8e19ea79fce4191e69795d5fe848a482f03
Latest commit d12df6e Jun 17, 2019

Folly: Facebook Open-source Library

Build Status

What is 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 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.

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

Build Notes


folly requires gcc 5.1+ and 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 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 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 \

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 folly/build/ to build against master:


This will create a build directory _build 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 master.

Other Linux distributions

  • double-conversion (

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

    [double-conversion/] ln -s src double-conversion

    [folly/] mkdir build && cd build [folly/build/] cmake "-DCMAKE_INCLUDE_PATH=$DOUBLE_CONVERSION_HOME/include" "-DCMAKE_LIBRARY_PATH=$DOUBLE_CONVERSION_HOME/lib" ..

    [folly/build/] make

  • 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


    • libdwarf-dev
    • libelf-dev
    • libunwind8-dev
You can’t perform that action at this time.