[Mpi-comments] Behaviour of not-significant output arguments in MPI

[Christoph Niethammer]

Hello,

Studying the MPI 3.1. Standard I went over the question
if output arguments, which are not significant to a subgroup
of processes may be modified by the MPI call.

MPI-3.1, Sect. 2.3. on page 10 lines 42-44 only discusses input arguments,
and therefore, the meaning of "not significant" for output arguments
is somehow undefined.

I would expect, that the values of these arguments are unchanged by MPI calls.

An example: In the following code using MPI_Exscan, I cannot clearly say whether
the last code line is needed to guarantee the 0 in recv on process 0.

>  ...
>  int myrank;
>  int recv = 0;
>  MPI_Comm_rank(MPI_COMM_WORLD, &myRank);
>  MPI_Exscan(&send, &recv, 1, MPI_INT, MPI_SUM, MPI_COMM_WORLD);
>  if(myRank == 0) { recv = 0; } /* is this necessary according to MPI 3.1 ?*/

I expect, that all real MPI implementations guarantee the 0 on process 0.


Best regards
Christoph Niethammer

--

Christoph Niethammer
High Performance Computing Center Stuttgart (HLRS)
Nobelstrasse 19
70569 Stuttgart

Tel: ++49(0)711-685-87203
email: niethammer at hlrs.de
http://www.hlrs.de/people/niethammer