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

int main(int argc, char**argv) {
  int rank, size;
  char hostname[1024];

  MPI_Init(&argc, &argv);

  /* Get the rank of the current MPI task */
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);

  /* Get the total number of MPI tasks */
  MPI_Comm_size(MPI_COMM_WORLD, &size);

  /* Get the hostname */
  gethostname(hostname, 1024);

  if(rank>0) {
    /* Wait until rank N-1 prints its message */
    int n;
    MPI_Recv(&n, 1, MPI_INTEGER, rank-1, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
  }

  printf("Hello World from task %d / %d on %s\n", rank, size, hostname);

  if(rank<size-1) {
    /* Notify rank N+1 */
    MPI_Send(&rank, 1, MPI_INTEGER, rank+1, 0, MPI_COMM_WORLD);
  }

  MPI_Finalize();
  return 0;
}