Miscellaneous

C | Fortran-2008 | Fortran-90

MPI_UNDEFINED

Definition

MPI_UNDEFINED is a constant that is returned for instance when a process attempts to get its rank in a communicator in which it does not belong.

Example

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

/**
 * @brief Illustrate how to encounter a situation in which MPI_UNDEFINED is
 * returned.
 * @details This application is meant to be run with 4 processes. It creates a 
 * subgroup that will contain processes 0 and 1 only. Therefore, since processes
 * 2 and 3 do not belong to the subgroup, when they will try to get their rank
 * in that group, MPI_UNDEFINED will be returned.
 **/
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 subgroup
    int ranks_subgroup[2] = {0, 1};
    MPI_Group subgroup;
    MPI_Group_incl(world_group, 2, ranks_subgroup, &subgroup);

    // Get my rank in the world group and the subgroup
    int my_world_group_rank;
    MPI_Group_rank(world_group, &my_world_group_rank);
    int my_subgroup_rank;
    MPI_Group_rank(subgroup, &my_subgroup_rank);

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

    MPI_Finalize();

    return EXIT_SUCCESS;
}