The BMI interface does not allow partial completion of messages. However, it does allow for a sender to send less data than the receiver anticipated, resulting in what may be thought of as ``short'' messages from the receiver's point of view. Short messages do not indicate that another receive is needed to obtain the rest of the message. Instead it means that the sender does not have as much data to transmit as the receiver was expecting it to. In practice, this tends to occur in file systems when a read operation reaches EOF. It may also be a common occurance in request protocol operations, when requests may be of variable size and we do not wish to negotiate the correct size of messages before transmitting.
When a short send is posted, the sender must indicate the size that the receiver was expecting. This is necessary for the message to be matched properly between sender and receiver. When the receive completes, the caller is notified of how much data was actually present in the message.