20.1.6. MPI for Different Fortran Standard Versions

This section describes which Fortran interface functionality can be provided for different versions of the Fortran standard.

• For Fortran 77 with some extensions:
  • MPI identifiers may be up to 30 characters (31 with the profiling interface).
  • MPI identifiers may contain underscores after the first character.
  • An MPI subroutine with a choice argument may be called with different argument types.
  • Although not required by the MPI standard, the INCLUDE statement should be available for including mpif.h into the user application source code.
  
  Only MPI-1.1, MPI-1.2, and MPI-1.3 can be implemented. The use of absolute addresses from MPI_ADDRESS and MPI_BOTTOM may cause problems if an address does not fit into the memory space provided by an INTEGER. (In MPI-2.0 this problem is solved with MPI_GET_ADDRESS, but not for Fortran 77.)
• For Fortran 90:
  The major additional features that are needed from Fortran 90 are:
  • The MODULE and INTERFACE concept.
  • The KIND= and SELECTED_ XXX_KIND concept.
  • Fortran derived TYPEs and the SEQUENCE attribute.
  • The OPTIONAL attribute for dummy arguments.
  • Cray pointers, which are a nonstandard compiler extension, are needed for the use of MPI_ALLOC_MEM.
  
  With these features, MPI-1.1 -- MPI-2.2 can be implemented without restrictions. MPI-3.0 and later can be implemented with some restrictions. The Fortran support methods are abbreviated with S1 = the mpi_f08 module, S2 = the mpi module, and S3 = the mpif.f include file. If not stated otherwise, restrictions exist for each method that prevent implementing the complete semantics of MPI.
  • MPI_SUBARRAYS_SUPPORTED equals .FALSE., i.e., subscript triplets and noncontiguous subarrays cannot be used as buffers in nonblocking routines, RMA, or split-collective I/O.
  • S1, S2, and S3 can be implemented, but for S1, only a preliminary implementation is possible.
  • In this preliminary interface of S1, the following changes are necessary:
    • TYPE(*), DIMENSION(..) is substituted by nonstandardized extensions like !$PRAGMA IGNORE_TKR.
    • The ASYNCHRONOUS attribute is omitted.
    • PROCEDURE(...) callback declarations are substituted by EXTERNAL.
    
    
  • The specific procedure names are specified in Section Interface Specifications, Procedure Names, and the Profiling Interface.
  • Due to the rules specified in Section Interface Specifications, Procedure Names, and the Profiling Interface, choice buffer declarations should be implemented only with nonstandardized extensions like !$PRAGMA IGNORE_TKR (as long as F2008 with TS 29113 or Fortran 2018 is not available).

    In S2 and S3: Without such extensions, routines with choice buffers should be provided with an implicit interface, instead of overloading with a different MPI function for each possible buffer type (as mentioned in Section Problems Due to Strong Typing). Such overloading would also imply restrictions for passing Fortran derived types as choice buffer, see also Section Fortran Derived Types.

    Only in S1: The implicit interfaces for routines with choice buffer arguments imply that the ierror argument cannot be defined as OPTIONAL. For this reason, it is recommended not to provide the mpi_f08 module if such an extension is not available.

  • The ASYNCHRONOUS attribute can not be used in applications to protect buffers in nonblocking MPI calls ( S1-- S3).
  • The TYPE(C_PTR) binding of the MPI_ALLOC_MEM and MPI_WIN_ALLOCATE routines is not available.
  • In S1 and S2, the definition of the handle types (e.g., TYPE(MPI_Comm) and the status type TYPE(MPI_Status) must be modified: The SEQUENCE attribute must be used instead of BIND(C) (which is not available in Fortran 90/95). This restriction implies that the application must be fully recompiled if one switches to an MPI library for Fortran 2003 and later because the internal memory size of the handles may have changed. For this reason, an implementor may choose not to provide the mpi_f08 module for Fortran 90 compilers. In this case, the mpi_f08 handle types and all routines, constants and types related to TYPE(MPI_Status) (see Section Status) are also not available in the mpi module and mpif.h.
  
  
• For Fortran 95:
  The quality of the MPI interface and the restrictions are the same as with Fortran 90.
• For Fortran 2003:
  The major features that are needed from Fortran 2003 are:
  • Interoperability with C, i.e.,
    • BIND(C) derived types.
    • The ISO_C_BINDING intrinsic type C_PTR and routine C_F_POINTER.
    
    
  • The ability to define an ABSTRACT INTERFACE and to use it for PROCEDURE dummy arguments.
  • The ability to overload the operators .EQ. and .NE. to allow the comparison of derived types (used in MPI-3.0 and later for MPI handles).
  • The ASYNCHRONOUS attribute is available to protect Fortran asynchronous I/O. This feature is not yet used by MPI, but it is the basis for the enhancement for MPI communication in the TS 29113.
  
  With these features (but still without the features of TS 29113), MPI-1.1 -- MPI-2.2 can be implemented without restrictions, but with one enhancement:
  • The user application can use TYPE(C_PTR) together with MPI_ALLOC_MEM as long as MPI_ALLOC_MEM is defined with an implicit interface because a C_PTR and an INTEGER(KIND=MPI_ADDRESS_KIND) argument must both map to a void * argument.
  
  MPI-3.0 and later can be implemented with the following restrictions:
  • MPI_SUBARRAYS_SUPPORTED equals .FALSE..
  • For S1, only a preliminary implementation is possible. The following changes are necessary:
    • TYPE(*), DIMENSION(..) is substituted by nonstandardized extensions like !$PRAGMA IGNORE_TKR.
    
    
  • The specific procedure names are specified in Section Interface Specifications, Procedure Names, and the Profiling Interface.
  • With S1, the ASYNCHRONOUS is required as specified in the second Fortran interfaces. With S2 and S3 the implementation can also add this attribute if explicit interfaces are used.
  • The ASYNCHRONOUS Fortran attribute can be used in applications to try to protect buffers in nonblocking MPI calls, but the protection can work only if the compiler is able to protect asynchronous Fortran I/O and makes no difference between such asynchronous Fortran I/O and MPI communication.
  • The TYPE(C_PTR) binding of the MPI_ALLOC_MEM, MPI_WIN_ALLOCATE, MPI_WIN_ALLOCATE_SHARED, and MPI_WIN_SHARED_QUERY routines can be used only for Fortran types that are C compatible.
  • The same restriction as for Fortran 90 applies if nonstandardized extensions like !$PRAGMA IGNORE_TKR are not available.
  
  
• For Fortran 2008 with TS 29113 and later and
  For Fortran 2003 with TS 29113:
  The major features that are needed from TS 29113 are:
  • TYPE(*), DIMENSION(..) is available.
  • The ASYNCHRONOUS attribute is extended to protect also nonblocking MPI communication.
  • The array dummy argument of the ISO_C_BINDING intrinsic C_F_POINTER is not restricted to Fortran types for which a corresponding type in C exists.
  
  Using these features, MPI-3.0 and later can be implemented without any restrictions.
  • With S1, MPI_SUBARRAYS_SUPPORTED equals .TRUE.. The ASYNCHRONOUS attribute can be used to protect buffers in nonblocking MPI calls. The TYPE(C_PTR) binding of the MPI_ALLOC_MEM, MPI_WIN_ALLOCATE, MPI_WIN_ALLOCATE_SHARED, and MPI_WIN_SHARED_QUERY routines can be used for any Fortran type.
  • With S2 and S3, the value of MPI_SUBARRAYS_SUPPORTED is implementation dependent. A high quality implementation will also provide MPI_SUBARRAYS_SUPPORTED set to .TRUE. and will use the ASYNCHRONOUS attribute in the same way as in S1.
  • If nonstandardized extensions like !$PRAGMA IGNORE_TKR are not available then S2 must be implemented with TYPE(*), DIMENSION(..).
  
  

If MPI_SUBARRAYS_SUPPORTED=.FALSE., the choice argument may be implemented with an explicit interface using compiler directives.
Example Use of typical compiler directives to disable type, kind, and rank checks for choice buffer arguments.

INTERFACE 
  SUBROUTINE MPI_...(buf, ...) 
    !DEC$ ATTRIBUTES NO_ARG_CHECK :: buf 
    !$PRAGMA IGNORE_TKR buf 
    !DIR$ IGNORE_TKR buf 
    !IBM* IGNORE_TKR buf 
    REAL, DIMENSION(*) :: buf 
    ... ! declarations of the other arguments 
  END SUBROUTINE 
END INTERFACE