#include <stdio.h>
#include "mpi.h"
#define MSGLEN 2048         /* length of message in elements */
#define TAG_A 100
#define TAG_B 200

main( int argc, char **argv ) 
{
  float message1[MSGLEN], message2[MSGLEN];
  int rank, i;
  MPI_Status status;         /* status of communication */

  MPI_Init( &argc, &argv );
  MPI_Comm_rank( MPI_COMM_WORLD, &rank );
  printf ( " Task %d initialized\n", rank );

  for ( i=0; i<MSGLEN; i++ )  {
    message1[i] = 100; message2[i] = -100;
  }

  if ( rank == 0 )  {
      MPI_Send ( message1, MSGLEN, MPI_FLOAT, 1, TAG_A,
                MPI_COMM_WORLD );
      MPI_Recv ( message2, MSGLEN, MPI_FLOAT, 1, TAG_B,
                MPI_COMM_WORLD, &status );
  }
  else if ( rank == 1)  {
      MPI_Send ( message1, MSGLEN, MPI_FLOAT, 0, TAG_B,
                MPI_COMM_WORLD );
      MPI_Recv ( message2, MSGLEN, MPI_FLOAT, 0, TAG_A,
                MPI_COMM_WORLD, &status );
  }

  MPI_Finalize();
  return 0;
}
/*
 One Solution:
  MPI_Isend ( message1, MSGLEN, MPI_FLOAT, dest, send_tag,
             MPI_COMM_WORLD, &request);
  MPI_Recv ( message2, MSGLEN, MPI_FLOAT, source, recv_tag,
            MPI_COMM_WORLD, &status );
  MPI_Wait ( &request, &status);

 Other solutions include:
 1) Reordering sends and receives
 2) MPI_Sendrecv()
 3) Explicitly buffering: MPI_Bsend()
 */