Skip to content


Repository files navigation


CoArray Fortran Framework of Efficient Interfaces to Network Environments

Caffeine is a parallel runtime library that aims to support Fortran compilers with a programming-model-agnostic application interface to various communication libraries. Current work is on supporting the Parallel Runtime Interface for Fortran (PRIF) with the GASNet-EX exascale-ready networking middleware. Future plans include support for an alternative Message Passing Interface (MPI) back end.

Caffeine system stack diagram


Build prerequisites

The script uses the following packages:

The script will invoke these if present in a user's PATH. If not present, the script will ask permission to use Homebrew to install the relevant package.

Caffeine leverages the following non-parallel features of Fortran to simplify the writing of a portable, compact runtime-library that supports Fortran's parallel features:

Feature Introduced in
The iso_c_binding module Fortran 2003
The contiguous attribute Fortran 2008
Submodule support [1] Fortran 2008
do concurrent [2] Fortran 2008
The ISO_Fortran_binding.h C header file Fortran 2018
Assumed-type dummy arguments: type(*) Fortran 2018
Assumed-rank dummy arguments: array(..) Fortran 2018

[1] This feature simplifies development but is not essential to the package

[2] This feature is used to support only co_reduce and might become optional in a future release.

Download, build, and run an example

git clone
cd caffeine
FC=<Fortran-compiler-path> CC=<C-compiler-path> CXX=<C++-compiler-path> ./build/ run --example hello

Run tests

./build/ test


Citing Caffeine? Please use the following publication:

Damian Rouson, Dan Bonachea.
"Caffeine: CoArray Fortran Framework of Efficient Interfaces to Network Environments",
Proceedings of the Eighth Annual Workshop on the LLVM Compiler Infrastructure in HPC (LLVM-HPC2022), November 2022.
Talk Slides

Caffeine is an implementation of the Parallel Runtime Interface for Fortran (PRIF):

Dan Bonachea, Katherine Rasmussen, Brad Richardson, Damian Rouson.
"Parallel Runtime Interface for Fortran (PRIF) Specification, Revision 0.3",
Lawrence Berkeley National Laboratory Technical Report (LBNL-2001590), May 2024.


One of our continuous integration (CI) scripts generates up-to-date Caffeine documentation using ford. The CI script also deploys the generated documentation to the our GitHub Pages site. Alternatively, generate HTML documentation locally using ford as follows:


Open doc/html/index.html in a web browser.


The Computer Languages and Systems Software (CLaSS) Group at Berkeley Lab has developed Caffeine development on funding from the Exascale Computing Project (ECP) and the Stewardship for Programming Systems and Tools (S4PST) project.


See LICENSE.txt for usage terms and conditions.