On 11/13/2012 08:31 AM US Central Time, Alejandro Allievi wrote:

However, when I want to apply the reduction operation using the new communicator as

MPI_Reduce(&test_newcomm, &sum_newcomm, 1, MPI_INT, MPI_SUM, 0, first_row_comm);

It compiles fine but throws an execution error:

Fatal error in PMPI_Reduce: Invalid communicator, error stack: PMPI_Reduce(1270): MPI_Reduce(sbuf=000000000022FE38, rbuf=000000000022FE30, count=1, MPI_INT, MPI_SUM, root=0, MPI_COMM_NULL) failed PMPI_Reduce(1160): Null communicator

But communicator " first_row_comm " is properly create. Is this a bug or I'm doing something wrong with the way I'm passing the new communicator argument in MPI_Reduce, or...??

It looks like you didn't create first_row_comm correctly. -- Pavan -- Pavan Balaji http://www.mcs.anl.gov/~balaji