A Case for New MPI Fortran Bindings

Abstract. The Fortran language has evolved substantially from the Fortran 77 bindings defined in the MPI-1 (Message Passing Interface) standard. Fortran 90 introduced interface blocks; subsequently, the MPI-2 standard defined Fortran 90 bindings with explicit Fortran interfaces to MPI routines. In this paper, we describe the Open MPI implementation of these two sets of Fortran bindings and point out particular issues related to them. In particular, we note that strong typing of the Fortran 90 MPI interfaces with user-choice buffers leads to an explosion of interface declarations; each choice buffer must be expanded to all possible combinations of Fortran type, kind, and array dimension. Because of this (and other reasons outlined in this paper), we propose a new set of Fortran MPI bindings that uses the intrinsic ISO C BINDING module in Fortran 2003. These new bindings will allow MPI interfaces to be defined in Fortran that directly invoke their corresponding MPI C implementation routines – no additional layer of software to marshall parameters between Fortran and C is required.