program pingpong                          !!!     10-June-2005
	
c....... This program send the own processor number to the other processor
c....... (run with only 2 processors!)

        include 'mpif.h'

	integer status(MPI_STATUS_SIZE)
	
c........initialize MPI
        call mpi_init(ierr)
        call mpi_comm_rank(mpi_comm_world,iam,ierr)
        call mpi_comm_size(mpi_comm_world,nproc,ierr)


c........set an ID value for the message tag 
	itag=99

c........have the first task [task zero] send then receive
	if (iam.eq.0) then

c........set values for the send command
c........idest is where the message is going
c........icount is the number of items being sent
         idest=1
         icount=1

c........set values for the recv command
c........source is where to expect a message from
         isource=1

c........send a message containing the task number to task 1
c........place the task ID in the variable sbuf
	 isbuf = iam
         CALL MPI_SEND(isbuf, icount, MPI_INTEGER, idest, itag, 
     1                 MPI_COMM_WORLD, ierr)

c........receive the message from task 1 into irbuf
         CALL MPI_RECV(irbuf, icount, MPI_INTEGER, isource, itag,
     1                 MPI_COMM_WORLD, status, ierr)

	endif

c........have the second task [task one] receive then send.  This constitutes
c........"blocking" message passing.
	if (iam.eq.1) then

c........set values for the send command
c........idest is where the message is going
c........icount is the number of items being sent
         idest=0
         icount=1

c........set values for the recv command
c........isource is where to expect a message from
         isource=0

c........Receive the message from task 0 into irbuf
         CALL MPI_RECV(irbuf, icount, MPI_INTEGER, isource, itag,  
     1                 MPI_COMM_WORLD, status, ierr)

c........Send a message containing the task number to task 0
c........place the task ID in the variable sbuf
	 isbuf = iam
         CALL MPI_SEND(isbuf, icount, MPI_INTEGER, idest, itag,  
     1                 MPI_COMM_WORLD, ierr)

   	endif

c........have each task print both what it sent and received
      PRINT*, "TASK #", iam, " sent ", isbuf
      PRINT*, "TASK #", iam, " received ", irbuf 


c........close out MPI
	call mpi_finalize(ierr)

        stop
        end