[MPICH] empty group returned by MPI_Group_excl

[Toon Knapen]

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
>>
>>
>