20.1.10. Problems With Fortran Bindings for MPI

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This section discusses a number of problems that may arise when using MPI in a Fortran program. It is intended as advice to users, and clarifies how MPI interacts with Fortran. It is intended to clarify, not add to, this standard.

As noted in the original MPI specification, the interface violates the Fortran standard in several ways. While these may cause few problems for Fortran 77 programs, they become more significant for Fortran 90 programs, so that users must exercise care when using new Fortran 90 features. With Fortran 2008 and the semantics defined in TS 29113, most violations are resolved, and this is hinted at in an addendum to each item. The violations were originally adopted and have been retained because they are important for the usability of MPI. The rest of this section describes the potential problems in detail.

The following MPI features are inconsistent with Fortran 90 and Fortran 77.

    1. An MPI subroutine with a choice argument may be called with different argument types. When using the mpi_f08 module together with a compiler that supports Fortran 2008 with TS 29113, this problem is resolved.
    2. An MPI subroutine with an assumed-size dummy argument may be passed an actual scalar argument. This is only solved for choice buffers through the use of DIMENSION(..).
    3. Nonblocking and split-collective MPI routines assume that actual arguments are passed by address or descriptor and that arguments and the associated data are not copied on entrance to or exit from the subroutine. This problem is solved with the use of the ASYNCHRONOUS attribute.
    4. An MPI implementation may read or modify user data (e.g., communication buffers used by nonblocking communications) concurrently with a user program that is executing outside of MPI calls. This problem is resolved by relying on the extended semantics of the ASYNCHRONOUS attribute as specified in TS 29113.
    5. Several named ``constants,'' such as MPI_BOTTOM, MPI_IN_PLACE, MPI_STATUS_IGNORE, MPI_STATUSES_IGNORE, MPI_ERRCODES_IGNORE, MPI_UNWEIGHTED, MPI_WEIGHTS_EMPTY, MPI_ARGV_NULL, and MPI_ARGVS_NULL are not ordinary Fortran constants and require a special implementation. See Section Named Constants for more information.
    6. The memory allocation routine MPI_ALLOC_MEM cannot be used from Fortran 77/90/95 without a language extension (for example, Cray pointers) that allows the allocated memory to be associated with a Fortran variable. Therefore, address sized integers were used in MPI-2.0 -- MPI-2.2. In Fortran 2003, TYPE(C_PTR) entities were added, which allow a standard-conforming implementation of the semantics of MPI_ALLOC_MEM. In MPI-3.0 and later, MPI_ALLOC_MEM has an additional, overloaded interface to support this language feature. The use of Cray pointers is deprecated. The mpi_f08 module only supports TYPE(C_PTR) pointers.
Additionally, MPI is inconsistent with Fortran 77 in a number of ways, as noted below.
MPI-1 contained several routines that take address-sized information as input or return address-sized information as output. In C such arguments were of type MPI_Aint and in Fortran of type INTEGER. On machines where integers are smaller than addresses, these routines can lose information. In MPI-2 the use of these functions has been deprecated and they have been replaced by routines taking INTEGER arguments of KIND=MPI_ADDRESS_KIND. A number of MPI-2 functions also take INTEGER arguments of nondefault KIND. See Section Language Binding and Section Type Constructors with Explicit Addresses for more information.

Sections Problems Due to Strong Typing through Permanent Data Movement describe several problems in detail that concern the interaction of MPI and Fortran as well as their solutions. Some of these solutions require special capabilities from the compilers. Major requirements are summarized in Section Requirements on Fortran Compilers.

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