8.5.4. Communication Safety Example

The following example (Communication Safety 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.
Example

#define TAG_ARBITRARY 12345 
#define SOME_COUNT       50 
 
int main(int argc, char *argv[]) 
{ 
  int me; 
  MPI_Request request[2]; 
  MPI_Status status[2]; 
  MPI_Group group_world, subgroup; 
  int ranks[] = {2, 4, 6, 8}; 
  MPI_Comm the_comm; 
  ... 
  MPI_Init(&argc, &argv); 
  MPI_Comm_group(MPI_COMM_WORLD, &group_world); 
 
  MPI_Group_incl(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(&the_comm); 
  } 
 
  MPI_Group_free(&group_world); 
  MPI_Group_free(&subgroup); 
  MPI_Finalize(); 
  return 0; 
}