ring.c

// Author: Wes Kendall
// Copyright 2011 www.mpitutorial.com
// This code is provided freely with the tutorials on mpitutorial.com. Feel
// free to modify it for your own use. Any distribution of the code must
// either provide a link to www.mpitutorial.com or keep this header in tact.
//
// Example using MPI_Send and MPI_Recv to pass a message around in a ring.
//
#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char** argv) {
  // Initialize the MPI environment
  MPI_Init(NULL, NULL);
  // Find out rank, size
  int world_rank;
  MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
  int world_size;
  MPI_Comm_size(MPI_COMM_WORLD, &world_size);

  int token;
  // Receive from the lower process and send to the higher process. Take care
  // of the special case when you are the first process to prevent deadlock.
  if (world_rank != 0) {
    MPI_Recv(&token, 1, MPI_INT, world_rank - 1, 0, MPI_COMM_WORLD,
             MPI_STATUS_IGNORE);
    printf("Process %d received token %d from process %d\n", world_rank, token,
           world_rank - 1);
  } else {
    // Set the token's value if you are process 0
    token = -1;
  }
  MPI_Send(&token, 1, MPI_INT, (world_rank + 1) % world_size, 0,
           MPI_COMM_WORLD);
  // Now process 0 can receive from the last process. This makes sure that at
  // least one MPI_Send is initialized before all MPI_Recvs (again, to prevent
  // deadlock)
  if (world_rank == 0) {
    MPI_Recv(&token, 1, MPI_INT, world_size - 1, 0, MPI_COMM_WORLD,
             MPI_STATUS_IGNORE);
    printf("Process %d received token %d from process %d\n", world_rank, token,
           world_size - 1);
  }
  MPI_Finalize();
}