PROGRAM ringnb

      IMPLICIT NONE

      include "mpif.h"

      INTEGER ierror, val, my_rank, nprocs
      INTEGER rightid, leftid
      INTEGER tmp, sum
      INTEGER wait_status(MPI_STATUS_SIZE)
      INTEGER recv_request

C     *----------------*
C     * Initialize MPI *
C     *----------------*

      CALL MPI_INIT(ierror)

C     *--------------------------------------------------------------------*
C     * Find out my rank and size using global communicator MPI_COMM_WORLD *
C     *--------------------------------------------------------------------*

      CALL MPI_COMM_RANK(MPI_COMM_WORLD, my_rank, ierror)
      CALL MPI_COMM_SIZE(MPI_COMM_WORLD, nprocs, ierror)

C     *--------------------*
C     * Find out neighbors *
C     *--------------------*

      rightid = my_rank + 1
      IF (rightid .EQ. nprocs) rightid = 0

      leftid = my_rank - 1
      IF (leftid .EQ. -1) leftid = nprocs-1

C     *---------------------------------------------------------------------*
C     * Send the process rank stored as val to the process on my right and  *
C     * receive a process rank from the process on my left and store as tmp *
C     *---------------------------------------------------------------------*

      sum = 0
      val = my_rank
 100  CONTINUE

      CALL MPI_IRECV(tmp, 1, MPI_INTEGER, leftid, 99,
     +     MPI_COMM_WORLD, recv_request, ierror)

      CALL MPI_SSEND(val, 1, MPI_INTEGER, rightid, 99,
     +     MPI_COMM_WORLD, ierror)

      CALL MPI_WAIT(recv_request,wait_status)

      sum = sum + tmp
      val = tmp

      IF(tmp .NE. my_rank) GOTO 100

C     *--------------------------*/
C     * Print the output message */
C     *--------------------------*/

      PRINT *, 'Proc ', my_rank, ' sum = ',sum

C     *-----------------------*/
C     * Exit and finalize MPI */
C     *-----------------------*/

      CALL MPI_FINALIZE(ierror)

      STOP
      END