An open-source C++ library developed and used at Facebook.
C++ M4 C Makefile Python Shell Other
Latest commit b7e1694 Oct 27, 2016 @andrewjcg andrewjcg committed with Facebook Github Bot folly/experimental/hazptr: mark static local as "library-local"
When using `-fvisibility-inlines-hidden` or using C/C++ shared libs
transitively from a Python binary, static locals in inline functions
can potentially have more than one definition at runtime.  This diff
explicitly marks this static local as safe for this scenario.

Reviewed By: djwatson

Differential Revision: D4082831

fbshipit-source-id: 75616ac26a39f44691ef59b889ea555e9daa391b

Folly: Facebook Open-source Library

What is folly?

Folly (acronymed loosely after Facebook Open Source Library) is a library of C++11 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 4.8+ and a version of boost compiled with C++11 support.

Please download googletest from and unzip it in the folly/test subdirectory.

Ubuntu 12.04

This release is old, requiring many upgrades. However, since Travis CI runs on 12.04, folly/build/ is provided, and upgrades all the required packages.

Ubuntu 13.10

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

sudo apt-get install \
    g++ \
    automake \
    autoconf \
    autoconf-archive \
    libtool \
    libboost-all-dev \
    libevent-dev \
    libdouble-conversion-dev \
    libgoogle-glog-dev \
    libgflags-dev \
    liblz4-dev \
    liblzma-dev \
    libsnappy-dev \
    make \
    zlib1g-dev \
    binutils-dev \
    libjemalloc-dev \

If advanced debugging functionality is required

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

Ubuntu 14.04 LTS

The packages listed above for Ubuntu 13.10 are required, as well as:

sudo apt-get install \

The above packages are sufficient for Ubuntu 13.10 and Ubuntu 14.04.

In the folly directory, run

  autoreconf -ivf
  make check
  sudo make install

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:

  cd folly
  make check

OS X (MacPorts)

Install the required packages from MacPorts:

  sudo port install \
    autoconf \
    automake \
    boost \
    gflags \
    git \
    google-glog \
    libevent \
    libtool \
    lz4 \
    lzma \
    scons \
    snappy \

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/folly
  autoreconf -ivf
  ./configure CPPFLAGS="-I/opt/local/include" LDFLAGS="-L/opt/local/lib"
  sudo make install

Other Linux distributions

  • double-conversion (

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

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



  • additional platform specific dependencies:

    Fedora 21 64-bit

    • gcc
    • gcc-c++
    • autoconf
    • autoconf-archive
    • 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