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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 binary interface (ABI) to various communication libraries. Current work is on supporting the ABI 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


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 [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 requirement simplifies development but might be removed in a future release.

[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
./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


One of our continous 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 leads Caffeine development under funding from the Exascale Computing Project (ECP).


See LICENSE.txt for usage terms and conditions.