Question

I have a problem with the following code:

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

#define NUM_SPAWNS 2
// Based on the example from: http://mpi.deino.net/mpi_functions/MPI_Comm_spawn.html
void MPI_messenger(int stuff, int dest)
{
    MPI_Send(&stuff, 1, MPI_INT, dest, 1,intercomm);
}

int main( int argc, char *argv[] )
{
    int my_rank;
    int size;
    int np = NUM_SPAWNS;
    int errcodes[NUM_SPAWNS];
    MPI_Comm parentcomm, intercomm, testcomm;
    MPI_Init( &argc, &argv );
    MPI_Status stat;
    MPI_Comm_get_parent( &parentcomm );
    if (parentcomm == MPI_COMM_NULL)
    {
        MPI_Comm_spawn( "spawn_example4", MPI_ARGV_NULL, np, MPI_INFO_NULL, 0, MPI_COMM_WORLD, &intercomm, errcodes );
        MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
        MPI_Comm_size(MPI_COMM_WORLD, &size);
        int lol = 10;
        MPI_messenger(lol,0);
        MPI_messenger(lol,1);
    }
    else
    {
        MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
        MPI_Comm_size(MPI_COMM_WORLD, &size);
        int lol;
        MPI_Recv(&lol, 1, MPI_INT,0,1,parentcomm, &stat);


        std::cout << lol << "\n";
    }
    fflush(stdout);
    MPI_Finalize();
    return 0;
}

Of course the intercommunicator intercomm is not defined in the scope of the function MPI_messenger. I wanted to know if and how I can get this intercommunicator inside the function without passing it as an argument.

Answer 1

The same way you would any other variable: declare it globally, or in some other common scope visible to both main() and MPI_messenger() (e.g. within the same class as a member).