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C | FORTRAN-legacy | FORTRAN-2008

MPI_Group_difference

Definition

MPI_Group_difference creates a group made of all processes from the first group that are not in the second group. In case all the processes from the first group are in the second group, the difference group given in MPI_GROUP_EMPTY.

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int MPI_Group_difference(MPI_Group group_a,
                         MPI_Group group_b,
                         MPI_Group* difference_group);

Parameters

group_a
The first of the two groups to include in the difference.
group_b
The second of the two groups to include in the difference.
difference_group
The variable in which store the group representing the difference of the two groups given. If the difference is empty, MPI_GROUP_EMPTY is given.

Returned value

The error code returned from the group difference.

MPI_SUCCESS
The routine successfully completed.

Example

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

/**
 * @brief Illustrate how to create a group by difference.
 * @details This application is meant to be run with 4 processes. It creates two 
 * groups, named A and B, which contain processes 0,1 and 1,3 respectively. It
 * then creates a group that is the difference of groups A and B (all processes
 * belong to the first group that are not in the second one). It can be 
 * visualised as follows:
 * 
 *                    +---+---+---+---+
 *                    | 0 | 1 | 2 | 3 |
 * +------------------+---+---+---+---+
 * | Group A          | X | X |   |   |
 * | Group B          |   | X |   | X |
 * | Difference group | X |   |   |   |
 * +------------------+---+---+---+---+
 **/
int main(int argc, char* argv[])
{
    MPI_Init(&argc, &argv);

    // Check that 4 MPI processes are used
    int comm_size;
    MPI_Comm_size(MPI_COMM_WORLD, &comm_size);
    if(comm_size != 4)
    {
        printf("This application is meant to be run with 4 MPI processes.\n");
        MPI_Abort(MPI_COMM_WORLD, EXIT_FAILURE);
    }

    // Get the group or processes of the default communicator
    MPI_Group world_group;
    MPI_Comm_group(MPI_COMM_WORLD, &world_group);

    // Create the group A
    int ranks_group_a[2] = {0, 1};
    MPI_Group group_a;
    MPI_Group_incl(world_group, 2, ranks_group_a, &group_a);

    // Create the group B
    int ranks_group_b[2] = {1, 3};
    MPI_Group group_b;
    MPI_Group_incl(world_group, 2, ranks_group_b, &group_b);

    // Create the difference of groups A and B
    MPI_Group difference_group;
    MPI_Group_difference(group_a, group_b, &difference_group);

    // Get my rank in the world group and the difference group
    int my_world_group_rank;
    MPI_Group_rank(world_group, &my_world_group_rank);
    int my_difference_group_rank;
    MPI_Group_rank(difference_group, &my_difference_group_rank);

    if(my_difference_group_rank == MPI_UNDEFINED)
    {
        printf("I am process %d in world group but I am not part of the difference group.\n", my_world_group_rank);
    }
    else
    {
        printf("I am process %d in world group and process %d in difference group.\n", my_world_group_rank, my_difference_group_rank);
    }

    MPI_Finalize();

    return EXIT_SUCCESS;
}