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C | FORTRAN-legacy | FORTRAN-2008

MPI_COMM_NULL

Definition

MPI_COMM_NULL is the value given by MPI_Comm_create or MPI_Cart_create to processes that are not part of the new communicator created.

Example

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#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <mpi.h>

/**
 * @brief Illustrates how to check whether a process is part of a new
 * communicator.
 * @details This code creates a new communicator from a cartesian grid designed
 * to contain only 1 process. By running this application with multiple
 * processes, all but one will not belong to the new communicator created.
 **/
int main(int argc, char* argv[])
{
    MPI_Init(&argc, &argv);

    // Size of the default communicator
    int size;
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    // Rank in the default communicator
    int my_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);

    // Use a cartesian grid of 1 process in total
    int dims[2] = {1, 1};

    // Make both dimensions periodic
    int periods[2] = {true, true};

    // Let MPI assign arbitrary ranks if it deems it necessary
    int reorder = true;

    // Create a communicator given the 2D torus topology.
    MPI_Comm new_communicator;
    MPI_Cart_create(MPI_COMM_WORLD, 2, dims, periods, reorder, &new_communicator);

    // Check if I am part of the new communicator
    if(new_communicator == MPI_COMM_NULL)
    {
        printf("Process %d is not part of the new communicator.\n", my_rank);
    }
    else
    {
        printf("Process %d is part of the new communicator.\n", my_rank);
    }

    MPI_Finalize();

    return EXIT_SUCCESS;
}