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Scientific Programming
Volume 15 (2007), Issue 1, Pages 9-26

Co-arrays in the Next Fortran Standard

John Reid1 and Robert W. Numrich2

1JKR Associates, Convener, ISO/IEC Fortran Committee WG5, UK
2Minnesota Supercomputing Institute, University of Minnesota, Minneapolis, MN, USA

Received 22 May 2007; Accepted 22 May 2007

Copyright © 2007 Hindawi Publishing Corporation. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


The WG5 committee, at its meeting in Delft, May 2005, decided to include co-arrays in the next Fortran Standard. A Fortran program containing co-arrays is interpreted as if it were replicated a fixed number of times and all copies were executed asynchronously. Each copy has its own set of data objects and is called an image. The array syntax of Fortran is extended with additional trailing subscripts in square brackets to give a clear and straightforward representation of access to data on other images. References without square brackets are to local data, so code that can run independently is uncluttered. Any occurrence of square brackets is a warning about communication between images. The additional syntax requires support in the compiler, but it has been designed to be easy to implement and to give the compiler scope both to apply its optimizations within each image and to optimize the communication between images. The extension includes execution control statements for synchronizing images and intrinsic procedures to return the number of images, to return the index of the current image, and to perform collective operations. The paper does not attempt to describe the full details of the feature as it now appears in the draft of the new standard. Instead, we describe a subset and demonstrate the use of this subset with examples.