28. Fortran Binding Issues

Originally, MPI-1.1 provided bindings for Fortran 77. These bindings are retained, but they are now interpreted in the context of the Fortran 90 standard. MPI can still be used with most Fortran 77 compilers, as noted below. When the term Fortran is used it means Fortran 90.

All MPI names have an MPI_ prefix, and all characters are capitals. Programs must not declare variables, parameters, or functions with names beginning with the prefix MPI_. To avoid conflicting with the profiling interface, programs should also avoid functions with the prefix PMPI_. This is mandated to avoid possible name collisions.

All MPI Fortran subroutines have a return code in the last argument. A few MPI operations which are functions do not have the return code argument. The return code value for successful completion is MPI_SUCCESS. Other error codes are implementation dependent; see the error codes in Chapter MPI Environmental Management and Annex Language Bindings Summary . Constants representing the maximum length of a string are one smaller in Fortran than in C and C++ as discussed in Section Constants .

Handles are represented in Fortran as INTEGERs. Binary-valued variables are of type LOGICAL.

Array arguments are indexed from one.

The MPI Fortran binding is inconsistent with the Fortran 90 standard in several respects. These inconsistencies, such as register optimization problems, have implications for user codes that are discussed in detail in Section A Problem with Register Optimization . They are also inconsistent with Fortran 77.

• An MPI subroutine with a choice argument may be called with different argument types.
• An MPI subroutine with an assumed-size dummy argument may be passed an actual scalar argument.
• Many MPI routines assume that actual arguments are passed by address and that arguments are not copied on entrance to or exit from the subroutine.
• An MPI implementation may read or modify user data (e.g., communication buffers used by nonblocking communications) concurrently with a user program executing outside MPI calls.
• Several named ``constants,'' such as MPI_BOTTOM, MPI_STATUS_IGNORE, and MPI_ERRCODES_IGNORE, are not ordinary Fortran constants and require a special implementation. See Section Named Constants on page Named Constants for more information.

Additionally, MPI is inconsistent with Fortran 77 in a number of ways, as noted below.

• MPI identifiers exceed 6 characters.
• MPI identifiers may contain underscores after the first character.
• MPI requires an include file, mpif.h. On systems that do not support include files, the implementation should specify the values of named constants.
• Many routines in MPI have KIND-parameterized integers (e.g., MPI_ADDRESS_KIND and MPI_OFFSET_KIND) that hold address information. On systems that do not support Fortran 90-style parameterized types, INTEGER*8 or INTEGER should be used instead.
• The memory allocation routine MPI_ALLOC_MEM cannot be usefully used in Fortran without a language extension that allows the allocated memory to be associated with a Fortran variable.

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