[MPIWG Fortran] Question about named constants

[Junchao Zhang]

In OpenMPI F08, those predefined MPI datatypes are not "parameter
variables".  Thus OMPI failed to compile an MPICH F08 test, which has

  INTEGER, PARAMETER :: nb=2
  TYPE(MPI_DATATYPE) :: types(nb)=(/MPI_DOUBLE_PRECISION, MPI_CHAR/)

Jeff Squyres and I discussed in emails.  I don't know whether this is
correct according to the MPI spec.



--Junchao Zhang

On Thu, Jun 18, 2015 at 12:48 PM, Bill Long <longb at cray.com> wrote:

>
> On Jun 18, 2015, at 11:05 AM, Junchao Zhang <jczhang at mcs.anl.gov> wrote:
>
> > Hello, Fortran experts,
> >   I am a little puzzled by MPI-3.1 Chapter 2.5.4,
>
> Not surprising.  The (mis)use of terminology is confusing at best.
>
> >
> > All named constants, with the exceptions noted below for Fortran, can be
> used in
> > initialization expressions or assignments, but not necessarily in array
> declarations or as
> > labels in C switch or Fortran select/case statements.  This implies
> named constants
> > to be link-time but not necessarily compile-time constants.
>
> In Fortran “named constant” is a defined term.  They are declared with the
> PARAMETER attribute.  And can be used in most places where literal
> constants can be used, including array declarations.  They cannot be used
> as statement labels, though I think the reference to “Fortran select/case
> statements” intends to talk about case-values, and not labels.  Named
> constants can be used as case-values.
>
> >
> >  Then in F08, should MPI_INTEGER be declared as a parameter variable?
>
> Yes.  That’s what I did in my prototype mpi_f08 modules.
>
> >  If no, then I get an error when compiling this initialization
> expression with gfortran
> >           type(MPI_Datatype) :: dtype = MPI_INTEGER
> >
> >     Error: Parameter 'mpi_integer' at (1) has not been declared or is a
> variable, which does not reduce to a constant expression
> >
> > If yes, then of course I can use MPI_INTEGER in Fortran select/case.
>
> Right.
>
> >
> > Another question is what is the meaning of "link-time constants". I
> could not find in MPI. Does it mean I can use MPI_INTEGER after main before
> MPI_Init?
>
> I suspect this is a historical artifact where some special head standing
> was needed to correctly define MPI_INTEGER based on whether the code was
> compiled with -i4 or -i8 (or equivalent, depending on the compiler).  For
> the modern case of using the mpi_f08 module, you just compile the mpi_f08
> module twice (once with -i4 and once with -i8) and then have the compile
> command select the right actual module to reference based on how the using
> program unit is compiled.   Generally, values of objects are not set at
> link time;  addresses of objects are set at link time.   MPI objects that
> represent addresses  (MPI_BOTTOM ??) are examples.  If MPI_INTEGER is
> accessible, you can reference it.  If should not have anything to do with
> whether MPI_Init has been called.
>
> Perhaps some of the MPI historians can shed more light on this
> terminology.  I suspect it would be completely moot if we got rid of mpif.h.
>
> Cheers,
> Bill
>
>
> >
> > Thank you!
> >
> > --Junchao Zhang
> > _______________________________________________
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> > mpiwg-fortran at lists.mpi-forum.org
> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpiwg-fortran
>
> Bill Long
>      longb at cray.com
> Fortran Technical Support  &                                  voice:
> 651-605-9024
> Bioinformatics Software Development                     fax:  651-605-9142
> Cray Inc./ Cray Plaza, Suite 210/ 380 Jackson St./ St. Paul, MN 55101
>
>
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