#include <stddef.h>
#include <stdlib.h>
#include "mpi.h"

/* Root sends out a msg to every proc and receives answers back */

int main(int argc, char **argv ){
  char message[20];
  int i;
  int rank;
  int worker;
  int size;
  int tag = 0;
  MPI_Status status;

  MPI_Init(&argc, &argv);
  MPI_Comm_size(MPI_COMM_WORLD,&size);
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);

  if(rank == 0) {

    printf("Welcome to MPI\n");
    printf("The number of processes :: %d\n", size);
    strcpy(message, "Hello, world");

    /* Root sends out a msg to every proc */
    for (i=1; i<size; i++) 
      MPI_Send(message, 20, MPI_CHAR, i, tag, MPI_COMM_WORLD);

    printf("process ID :: %d\t%s\n", rank, message);

    /* Now expects an answer from everyone */
    for (i=1; i<size; i++) {
      MPI_Recv(&worker, 1, MPI_INT, MPI_ANY_SOURCE, tag, MPI_COMM_WORLD, &status);                 
      printf("process ID :: %d\tHello back\n", worker);
    }

  } else {
    
    /* Receives a msg from Root */
    MPI_Recv(message, 20, MPI_CHAR, 0, tag, MPI_COMM_WORLD, &status);
    /* Sends out its rank to Root */
    MPI_Send(&rank, 1, MPI_INT, 0, tag, MPI_COMM_WORLD);

  }

  MPI_Finalize();

  return 0;

}