mpi_ring.c

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

int main(int argc, char** argv) {
  int world_rank, world_size, token;
  MPI_Init(NULL, NULL);
  MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
  MPI_Comm_size(MPI_COMM_WORLD, &world_size);
  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 {
    token = -1;
  }
  MPI_Send(&token, 1, MPI_INT, (world_rank + 1) % world_size, 0, MPI_COMM_WORLD);
  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();
}

/*
For 4 processes, output would look something like:

Process 0 received token -1 from process 3
Process 3 received token -1 from process 2
Process 1 received token -1 from process 0
Process 2 received token -1 from process 1
*/