Topology

C | Fortran-2008 | Fortran-90

MPI_Group_union

Parameters

group_a

The first of the two groups to include in the union.

group_b

The second of the two groups to include in the union.

union_group

The variable in which store the group representing the union of the two groups given.

Return value

The error code returned from the group union.

Example

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

/**
 * @brief Illustrates how to get the union of two groups of processes.
 * @details This code gets all processes of the default communicator and splits
 * them in two groups, designed to cover all cases: processes that belong to
 * both groups, one group or none.
 * It then gets the union of these two groups and creates a communicator
 * containing the processes of the union group. Each process then prints whether
 * it belongs to the communicator of the union group or not.
 * 
 * This application is meant to be run with 4 processes. The union can
 * be visualised as follows:
 *
 * +-----------+---+---+---+---+
 * | Processes | 0 | 1 | 2 | 3 |
 * +-----------+---+---+---+---+
 * | Group A   | X |   | X |   |
 * | Group B   |   |   | X | X |
 * | Union     | X |   | X | X |
 * +-----------+---+---+---+---+
 **/
int main(int argc, char* argv[])
{
    MPI_Init(&argc, &argv);

    // Check that the application is run with 4 processes.
    int size;
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    if(size != 4)
    {
        printf("Please run this application with 4 processes.\n");
        MPI_Abort(MPI_COMM_WORLD, EXIT_FAILURE);
    }

    // Get the group from the default communicator
    MPI_Group world_group;
    MPI_Comm_group(MPI_COMM_WORLD, &world_group);

    // Keep the processes 0 and 2 in the group A
    MPI_Group groupA;
    int groupAprocesses[2] = {0, 2};
    MPI_Group_incl(world_group, 2, groupAprocesses, &groupA);

    // Keep the processes 2 and 3 in the group B
    MPI_Group groupB;
    int groupBprocesses[2] = {2, 3};
    MPI_Group_incl(world_group, 2, groupBprocesses, &groupB);

    // Get the union of both groups
    MPI_Group union_group;
    MPI_Group_union(groupA, groupB, &union_group);

    // Get my rank in the communicator
    int my_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);

    // Create a communicator made of the processes in the union group
    MPI_Comm new_communicator;
    MPI_Comm_create(MPI_COMM_WORLD, union_group, &new_communicator);
    if(new_communicator == MPI_COMM_NULL)
    {
        // I am not part of the communicator created, so I am not part of the union group
        printf("Process %d is not part of the union group.\n", my_rank);
    }
    else
    {
        // I am part of the communicator created, so I am part of the union group
        printf("Process %d is part of the union group.\n", my_rank);
    }

    MPI_Finalize();

    return EXIT_SUCCESS;
}