Parallel Programming with Coarrays

Coarrays are a native parallel programming feature added to the Fortran standard in Fortran 2008 and extended in Fortran 2018. They let a program run as several replicated copies, called images, that execute the same code (the SPMD model, Single Program Multiple Data) but each hold their own private data. A coarray is declared by adding square-bracket codimensions after the normal array dimensions, for example real :: temperature(100)[*], and any image can read or write another image's copy of that variable directly using square-bracket syntax, without hand-written message-passing calls.

A coarray variable such as real :: x(10)[*] declares a rank-1 array of 10 reals that is replicated on every image, and the asterisk in the codimension means the number of images is determined at run time. To reference another image's copy, you append a bracket with the image index, as in x(:)[3], which reads or writes the array on image 3 from wherever the statement executes. Local access, x(:) with no brackets, always refers to the calling image's own copy and behaves exactly like ordinary Fortran array syntax, so existing array code is easy to extend into a coarray-aware program.

Coarray accesses are not automatically ordered between images, so the standard requires explicit synchronization whenever one image depends on data another image has written. sync all is a full barrier that blocks every image until all of them arrive; sync images(list) synchronizes only with a named subset, which is cheaper when only two or three images actually exchange data. The critical construct wraps a block of code so that only one image executes it at a time, which is essential for safely updating a shared accumulator or writing to a shared log file without interleaving.

Fortran 2018 added collective intrinsic subroutines — co_sum, co_max, co_min, co_reduce, and co_broadcast — that perform a reduction or broadcast across all images in a single call instead of requiring a hand-rolled loop of point-to-point coarray accesses. For example, call co_sum(partial, result) adds together the partial value held by every image and returns the total to result on every image (or to a chosen result_image). On the toolchain side, gfortran implements coarrays either as -fcoarray=single for serial testing or, more usefully, via the OpenCoarrays library with -fcoarray=lib, which maps images onto MPI processes under the hood, so a coarray program can be launched with cafrun -np 4 ./a.out much like an MPI job.