Coarray

A coarray is a programming construct used in partitioned global address space (PGAS) parallelism to expose distributed data as if it were part of a single global address space. Coarrays enable one-sided communication between multiple parallel images (processes or threads) without explicit message passing. Each image holds a local portion of a coarray, and the coarray reference syntax allows a program to read or write data on other images.

In practice, coarrays are most closely associated with Fortran’s coarray feature, integrated into the language from Fortran 2008 onward. A variable can be declared with a coarray segment by adding a coarray specifier in brackets, for example real :: A(10)[*]. Here, each image maintains its own local copy of A, while remote references can access elements on other images using syntax that designates the target image (various languages differ in exact syntax). Synchronization statements, such as sync all, are used to coordinate data movement and to ensure data consistency across images.

Coarrays aim to simplify parallel programming for certain classes of problems, such as stencil computations, reductions, or data-parallel kernels, by providing direct access to remote data and reducing explicit message-passing code. The programming model emphasizes a global view of memory, with the runtime handling data movement and synchronization between images.

Implementations exist primarily in languages with PGAS models. Fortran’s coarray feature is supported by multiple compilers and runtimes (e.g., OpenCoarrays), and is often used in conjunction with MPI in heterogeneous workflows. Similar concepts appear in other PGAS languages such as UPC and Chapel, which provide comparable capabilities under different syntax.