MPI buffered send/receive order

Question

I\'m using MPI (with fortran but the question is more specific to the MPI standard than any given language), and specifically using the buffered send/receive functions isend and

Answer 1

Yes, the messages are received in the order they are sent. This is described by the standard as non-overtaking messages. See this MPI Standard section for more details, here's an excerpt:

Order Messages are non-overtaking: If a sender sends two messages in succession to the same destination, and both match the same receive, then this operation cannot receive the second message if the first one is still pending. If a receiver posts two receives in succession, and both match the same message, then the second receive operation cannot be satisfied by this message, if the first one is still pending. This requirement facilitates matching of sends to receives. It guarantees that message-passing code is deterministic, if processes are single-threaded and the wildcard MPI_ANY_SOURCE is not used in receives. (Some of the calls described later, such as MPI_CANCEL or MPI_WAITANY, are additional sources of nondeterminism.)