MPI: what happens if send and receive sizes don't match?

Receive count开发者_如何学运维 is smaller than the matching send count? Send count is smaller than the matching receive count?

The MPI Standard 2.2 describes this in section 3.2.4. Basically the receive buffer (i.e., the count of the receiver) must be at least as big as the message being received (i.e., the count of the sender). Otherwise an overflow error occurs. If the count of the sender is less than the count of the receiver then the remaining elements of the receive buffer are unmodified.