MPI_Comm_splitのcolorにMPI_UNDEFINEDを指定したときに返ってくるコミュニケータを確認する

mpi_comm_split_test.cpp

#include <mpi.h>
#include <unistd.h>
#include <iostream>

int main(int argc, char* argv[])
{
    MPI_Init(&argc, &argv);

    int rank, size;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    int color = (rank < 4) ? 0 : MPI_UNDEFINED;
    int key = 0;

    MPI_Comm newcomm;
    MPI_Comm_split(MPI_COMM_WORLD, color, key, &newcomm);

    MPI_Barrier(MPI_COMM_WORLD);
    for (int i=0; i<size; ++i)
    {
        if (rank == i)
            std::cout << rank << ':' << (newcomm == MPI_COMM_NULL) << ' ' << (newcomm == MPI_COMM_SELF) << std::endl;
        usleep(10000);
        MPI_Barrier(MPI_COMM_WORLD);
    }

    MPI_Finalize();
}