5.5.4 Example #4

The following example is meant to illustrate ``safety'' between point-to-point and collective communication. MPI guarantees that a single communicator can do safe point-to-point and collective communication.
   #define TAG_ARBITRARY 12345
   #define SOME_COUNT       50

   main(int argc, char **argv)
   {
     int me;
     MPI_Request request[2];
     MPI_Status status[2];
     MPI_Group MPI_GROUP_WORLD, subgroup;
     int ranks[] = {2, 4, 6, 8};
     MPI_Comm the_comm;
     ...
     MPI_Init(&argc, &argv);
     MPI_Comm_group(MPI_COMM_WORLD, &MPI_GROUP_WORLD);

     MPI_Group_incl(MPI_GROUP_WORLD, 4, ranks, &subgroup); /* local */
     MPI_Group_rank(subgroup, &me);     /* local */

     MPI_Comm_create(MPI_COMM_WORLD, subgroup, &the_comm);

     if(me != MPI_UNDEFINED)
     {
         MPI_Irecv(buff1, count, MPI_DOUBLE, MPI_ANY_SOURCE, TAG_ARBITRARY,
                           the_comm, request);
         MPI_Isend(buff2, count, MPI_DOUBLE, (me+1)%4, TAG_ARBITRARY,
                           the_comm, request+1);
     }

     for(i = 0; i < SOME_COUNT, i++)
       MPI_Reduce(..., the_comm);
     MPI_Waitall(2, request, status);

     MPI_Comm_free(t&he_comm);
     MPI_Group_free(&MPI_GROUP_WORLD);
     MPI_Group_free(&subgroup);
     MPI_Finalize();
   }

MPI-Standard for MARMOT