[MPICH] empty group returned by MPI_Group_excl

[William Gropp]

This is a good point, and one that deserves an errata item in the  
standard.  I would argue that for internal consistency,  
MPI_GROUP_EMPTY was intended by the Forum, and this case was overlooked.

Bill

On Jul 5, 2007, at 3:25 PM, Toon Knapen wrote:

> Indeed it makes sense that if Group_excl returns an empty group  
> that it returns MPI_GROUP_EMPTY and of course nothing in the  
> standard forbids this.
>
> But because you test explicitly on this, I just wanted to know if  
> this is standard-required or an mpich2-convention.
>
> thanks,
>
> toon
>
> Rajeev Thakur wrote:
>> It is not spelled out in the definition of MPI_Group_excl, but it  
>> is in
>> others. For example,
>> MPI_Group_difference: "The new group can be empty, that is, equal to
>> MPI_GROUP_EMPTY."
>> MPI_Group_incl: "If n = 0, then newgroup is MPI_GROUP_EMPTY." So I  
>> would guess that if MPI_Group_excl results in an empty group, one  
>> would
>> expect to get back MPI_GROUP_EMPTY.
>> Rajeev
>>> -----Original Message-----
>>> From: owner-mpich-discuss at mcs.anl.gov [mailto:owner-mpich- 
>>> discuss at mcs.anl.gov] On Behalf Of Toon Knapen
>>> Sent: Thursday, July 05, 2007 2:50 AM
>>> To: mpich-discuss at mcs.anl.gov
>>> Subject: [MPICH] empty group returned by MPI_Group_excl
>>>
>>> Hi all,
>>>
>>> I noticed that there is a test in test/mpi/group/grouptest.c of  
>>> mpich2-1.0.5p4 that verifies that a group of which all ranks are  
>>> excluded (using MPI_Group_excl) is equal to MPI_GROUP_EMPTY.
>>>
>>> Is this really required by the standard? AFAICT the standard does  
>>> not require that an empty group is equal to MPI_GROUP_EMPTY (both  
>>> groups of course will be MPI_IDENT).
>>>
>>> Thanks in advance,
>>>
>>> Toon Knapen
>>>
>>>
>