text: /*
   * Copyright (c) 2004-2006 The Trustees of Indiana University and Indiana
   *                         University Research and Technology
   *                         Corporation.  All rights reserved.
   * Copyright (c) 2006      Cisco Systems, Inc.  All rights reserved.
   *
   * Simple ring test program
   */
  
  #include <stdio.h>
  #include "mpi.h"
  
  int main(int argc, char *argv[])
  {
      int rank, size, next, prev, message, tag = 201;
  
      /* Start up MPI */
  
      MPI_Init(&argc, &argv);
      MPI_Comm_rank(MPI_COMM_WORLD, &rank);
      MPI_Comm_size(MPI_COMM_WORLD, &size);
   
      /* Calculate the rank of the next process in the ring.  Use the
         modulus operator so that the last process "wraps around" to
         rank zero. */
  
      next = (rank + 1) % size;
      prev = (rank + size - 1) % size;
  
      /* If we are the "master" process (i.e., MPI_COMM_WORLD rank 0),
         put the number of times to go around the ring in the
         message. */
  
      if (0 == rank) {
          message = 10;
  
          printf("Process 0 sending %d to %d, tag %d (%d processes in ring)\n", 
                 message, next, tag, size);
          MPI_Send(&message, 1, MPI_INT, next, tag, MPI_COMM_WORLD); 
          printf("Process 0 sent to %d\n", next);
      }
  
      /* Pass the message around the ring.  The exit mechanism works as
         follows: the message (a positive integer) is passed around the
         ring.  Each time it passes rank 0, it is decremented.  When
         each processes receives a message containing a 0 value, it
         passes the message on to the next process and then quits.  By
         passing the 0 message first, every process gets the 0 message
         and can quit normally. */
  
      while (1) {
          MPI_Recv(&message, 1, MPI_INT, prev, tag, MPI_COMM_WORLD, 
                   MPI_STATUS_IGNORE);
  
          if (0 == rank) {
              --message;
              printf("Process 0 decremented value: %d\n", message);
          }
  
          MPI_Send(&message, 1, MPI_INT, next, tag, MPI_COMM_WORLD);
          if (0 == message) {
              printf("Process %d exiting\n", rank);
              break;
          }
      }
  
      /* The last process does one extra send to process 0, which needs
         to be received before the program can exit */
  
      if (0 == rank) {
          MPI_Recv(&message, 1, MPI_INT, prev, tag, MPI_COMM_WORLD,
                   MPI_STATUS_IGNORE);
      }
      
      /* All done */
  
      MPI_Finalize();
      return 0;
  }
  