Build & Install

FleCSI can be configured to run with different distributed-memory runtimes, including Legion and MPI. FleCSI also has support for various fine-grained, node-level runtimes, including OpenMP, Kokkos, and the C++17 extensions for parallelism. Full documentation for FleCSI requires both Sphinx and Doxygen. These configuration options are listed to convey to the reader that the FleCSI build system has several paths that can be taken to tailor FleCSI to a given system and architecture.

Requirements & Prerequisites

The following list of requirements provides a complete set of build options, which might not be necessary for a particular build:


CI listings indicate specific versions used by FleCSI’s continuous integration tests. If nothing is indicated, there is no specific version tested.

  • Boost
    We require program_options and stacktrace.
    CI: 1.84.0

  • C++17 compliant compiler
    At the current time, FleCSI has been tested with GNU and Clang C++ compilers.
    CI: gcc 12.2.0, clang 17.0.6

  • Doxygen
    Doxygen is only required to build the interface documentation.

  • GASNet
    GASNet is only required if Legion support is enabled.

  • MPI
    The MPI implementation must have support for MPI_THREAD_MULTIPLE.
    CI: mpich 4.1.2, openmpi 5.0.2

  • Legion
    Requires Legion with control replication.
    CI: 24.03.0

  • Kokkos
    CI: 4.2.01

    CI: 4.0.3 (parMETIS), 5.1.0 (METIS)

  • CMake
    We currently require CMake version 3.23 or greater.
    CI: 3.27.9

  • Sphinx
    Sphinx is only required to build the web-based documentation. We also require the Sphinx RTD Theme. CI: 6.2.1

  • Python
    The documentation build requires Python 3.9 or greater.

Getting The Code


If you are a user and only want to install FleCSI, you can skip this step and use the instructions for installing FleCSI using Spack.

Clone the FleCSI git repository and create an out-of-source build area (FleCSI prohibits in-source builds):

$ git clone
$ cd flecsi
$ mkdir build
$ cd build


The preferred method for installing FleCSI and its dependencies is to use Spack. Spack is easy to install and configure:

$ git clone
$ source path/to/spack/repository/share/spack/

Once spack is installed, you can install FleCSI like:

$ spack install flecsi

FleCSI supports several different versions and variants, e.g.:

$ spack install flecsi@2 backend=legion +graphviz +kokkos

For a complete list of versions and variants, type:

$ spack info flecsi

More documentation and information on Spack is available here.

FleCSI Developers

If you are a developer, and would like to install only the dependencies of FleCSI (assuming that you will build FleCSI from source), you can use Spack’s --only option:

$ spack install --only dependencies flecsi backend=legion +hdf5 ^mpich

If you are developing against a particular branch of FleCSI, you can capture branch-specific spack dependencies by adding the FleCSI spack repo (before performing the above step):

$ spack repo add path/to/flecsi/spack-repo

This will prepend a spack repository path to your spack configuration, such that the specific branch of FleCSI can override the normal builtin spack dependencies to provide whatever features are required for a successful build.

Configuration & Build

Example configuration: MPI

$ cmake -DFLECSI_BACKEND=mpi ..

Example configuration: MPI + OpenMP


Example configuration: Legion

$ cmake -DFLECSI_BACKEND=legion ..

After configuration is complete, just use make to build:

$ make -j 16

Installation uses the normal make install and will install FleCSI in the directory specified by CMAKE_INSTALL_PREFIX:

$ make install

Building the Unit Tests

To build the FleCSI unit test suite, turn on the ENABLE_UNIT_TESTS option:


After building FleCSI, you can run the unit tests like:

$ make test

Building the Documentation (Developers)

FleCSI uses Doxygen for its API reference and Sphinx for user and developer documentation.

Doxygen can be installed with most Linux package managers. To install Sphinx, you can install pip3 and use it to install Sphinx and sphinx_rtd_theme. Your package manager should also have pip3; e.g., on Ubuntu, you can install all of these requirements like:

$ sudo apt install doxygen
$ sudo apt install python3-pip
$ pip3 install Sphinx
$ pip3 install sphinx_rtd_theme

To enable documentation, do this:


By default, this will enable Doxygen and Sphinx. Once you have properly configured FleCSI, you can build the documentation like:

$ make doxygen
$ make sphinx

Both of these targets will be built in your build directory under doc/: e.g., the main Doxygen index.html page will be located at doc/api/index.html. Similarly, the Sphinx main index.html page will be located at doc/index.html. You can open these in your browser with file:///path/to/your/build/directory/doc/api/index.html and file:///path/to/your/build/directory/doc/index.html.