source: CIVL/examples/mpi/dev/rowcol.c

/* 
   This program demonstrates how to build communicators using
   MPI_Comm_split. For each process it builds a new communicator
   for the processes in the same row and column in a two-dimensional
   process structure.
   You have to use a square number of processes to execute the
   program, for instance 4 or 9 processes.

   Compile the program with  'mpicc rowcol.c -o rowcol'
   Run the program on a square number of processes.
*/

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

int main(int argc, char *argv[]) {
  int id, ntasks, q, i;
  int row, col;
  int ntasks_row, id_row;       /* Nr of processes and rank in the row */
  int id_col;                   /* Rank in the column communicator */
  MPI_Comm row_comm, col_comm;  /* Communicators for row and column */
  int X[10];                    /* Send buffer */
  
  MPI_Init(&argc, &argv);       /* Initialize MPI */
  MPI_Comm_size(MPI_COMM_WORLD, &ntasks);       /* Get nr of tasks */
  MPI_Comm_rank(MPI_COMM_WORLD, &id);   /* Get id of this process */

  /* The process grid will be of size q*q */
  q = (int) sqrt((double) ntasks);

  /* Check that we have a square number of processes */
  if (q*q == ntasks) {
   
    /* Calculate on which row and column this process is */
    row = id/q;
    col = id%q;
    
    /* Build communicator for processes in the same row */
    MPI_Comm_split(MPI_COMM_WORLD, row, id, &row_comm);
    
    /* Build communicator for processes in the same column */
    MPI_Comm_split(MPI_COMM_WORLD, col, id, &col_comm);
    
    /* Get nr of tasks in the row communicator */
    MPI_Comm_size(row_comm, &ntasks_row);
    
    /* Get own rank in row communicator */
    MPI_Comm_rank(row_comm, &id_row);
    
    /* Get own rank in row communicator */
    MPI_Comm_rank(col_comm, &id_col);
    
    printf("Process %d has rank %d in its row communicator ", id, id_row);
    printf("and %d in the column communicator\n", id_col);
    
    /* The first process in each row assigns values to X */
    if (id_row==0) {
      for (i=0; i<10; i++) X[i]=i;  
    }
    MPI_Bcast(&X, 10, MPI_INT, 0, row_comm);
  }
  
  else { /* Not a square number of proceses */
    if (id == 0) {
      printf("You have to use a square number of processes\n");
      printf("Quitting\n");
    }
  } /* end of else-part */


  /* All proceses exit MPI and terminate */
  MPI_Finalize();
  exit(0);
  
}