[MPI3 Fortran] MPI Data types

Craig Rasmussen crasmussen at newmexicoconsortium.org
Thu May 7 13:02:17 CDT 2009

I'll try to be gentle to you C folk (except for Jeff  of course :-)

The MPI implementation will get an array descriptor instead of _just_  
a pointer.  Consider a simple MPI_send,

void MPI_send(CFI_desc_t * cdesc, int count, int * error)
    void * addr = cdesc->base_add;  /* this is the normal address C  
expects (given some qualifications) */

    /* now for some example error checking code, not real code */

   assert(cdesc->type == the_right_MPI_type_given_by_the_user);

   CFI_is_contiguous(cdesc, &isContiguous);  /* this function supplied  
by Fortran compiler */

   assert(MPI_lib_actual_size(cdesc) >= count*cdesc->elem_len);  /*  
this function supplied by MPI library */

   addr = MPI_lib_actual_addr_start(cdesc);  /* this function supplied  
by MPI library */

   /* now can send data */

   *error = ...

On May 7, 2009, at 10:54 AM, N.M. Maclaren wrote:

> On May 7 2009, Jeff Squyres wrote:
>> I don't understand the implications of what you are saying.  Be  
>> gentle  with me; I'm *NOT* a Fortran expert (or anywhere close).
> Sorry.
> C's array model is confused, but the issue is its semantic aspect.   
> A C
> array is simply a pointer to the first element of a contiguous  
> sequence
> of an unspecified number of elements.  Nothing more.  A void * pointer
> is the first byte of an unspecified length of storage of unspecified
> type.  It is assumed that it is of appropriate alignment and size  
> for how
> it is used, but that information is not associated with an array  
> argument.
> A Fortran array is strongly typed (the first difference), and it  
> comes in
> many forms.  Most of them are contiguous, but the most important  
> Fortran 90
> one (plain assumed size) is not.  All but assumed shape (a Fortran 77
> feature) have a strongly associated size.  It is the user's  
> responsibility
> to never pass a smaller array to a largerr argument.
> Pretty well all combinations are allowed in argument passing, and the
> compiler is expected to use copy-in/copy-out when needed.  In  
> particular,
> it is needed (in general) when passing an assumed-shape array to an  
> assumed
> size argument (which is the form that matches C's model).  That can be
> locked out, but that means that callers cannot use assumed shape  
> arrays,
> which (as I said) are the main Fortran 90 mechanism.
>>> I thought that we were discussing features to enable MPI-3 to   
>>> support things
>>> like Fortran array arguments that have no equivalent in C  
>>> (principally
>>> assumed shape).
>> I don't understand.  What does it matter if there is no equivalent   
>> data type in C?
> Not data type - semantic model.
>> MPI is about message passing -- as long as we can get a memory  
>> layout  description of the data to send / receive, then it doesn't  
>> matter what  language the data will be accessed from.
> Yes, but ....
> The data layout is part of the MPI datatype, so there is a problem  
> when
> passing a Fortran 90 assumed shape REAL array (for example).  If the  
> datatype is REAL, the MPI send / receive needs to handle discontiguous
> data (which it doesn't).  If the MPI datatype is an MPI derived  
> type, there
> is no direct equivalence between the Fortran and C interfaces.
> Regards,
> Nick Maclaren,
> University of Cambridge Computing Service,
> New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
> Email:  nmm1 at cam.ac.uk
> Tel.:  +44 1223 334761    Fax:  +44 1223 334679
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