request = MPI_Irecv
MPI_Send (to the other process)
MPI_Barrier
flag = MPI_Test(request)
Without the barrier, there's a race and MPI_Test may or may not return
true, indicating whether the message has been received. With the
barrier, is it guaranteed that the message will have been received,
and MPI_Test will return true?
![[Flag]](../../../../../community/mirrors/flag-us.gif){kind=small}