[MPI3 Fortran] Proposing changes to Fortran 2008

[Jeff Squyres]

Ok.  But the point here is that you do not have to have an  
interoperable type to pass it to an MPI function as a choice buffer.

Specifically: if you have a derived type in Fortran and the compiler  
re-orders the fields in memory, that's no problem for MPI because the  
user application will have had to created an MPI datatype *at run  
time* that will have accurate offsets/types of each member.  So MPI  
has the true memory layout map, regardless of whether the Fortran  
compiler re-ordered the members or not.

On Mar 20, 2008, at 12:50 PM, Aleksandar Donev wrote:
> Craig Rasmussen wrote:
>> struct foo {
>>     char a;
>>     char b;
>>     int i;
>>     char c;
>>     char d;
>> }
>>
>> Don't you have to worry about the C compiler reordering the elements
>> of foo?
> No, the C standard mandates that the order be preserved (with possible
> padding). I don't know the C99 standard well-enough to search it
> efficiently, but I am pretty sure it is so. Fortran does not guarantee
> anything unless it is an interoperable type.
> Aleks
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-- 
Jeff Squyres
Cisco Systems