The send call described in Section 
used the standard communication mode. In this mode,
it is up to MPI to decide whether outgoing
messages will be buffered. MPI may
buffer outgoing messages. In such a case, the send call may complete
before a matching receive is invoked. On the other hand, buffer space may be
unavailable, or MPI may choose not to buffer
outgoing messages, for performance reasons. In this case,
the send call will not complete until a matching receive has been posted, and
the data has been moved to the receiver. (A blocking send completes when the
call returns; a nonblocking send completes when the matching Wait or Test call
returns successfully.)
Thus, a send in standard mode can be started whether or not a matching receive has been posted. It may complete before a matching receive is posted. The standard-mode send has non-local completion semantics, since successful completion of the send operation may depend on the occurrence of a matching receive.
modecommunication modes mode, standard mode, synchronoussynchronous mode mode, bufferedbuffered mode mode, readyready mode standard mode rendezvous
A buffered-mode send operation can be started whether or not a
matching receive has been posted.
It may complete before a matching receive is posted.
Buffered-mode send has local completion semantics: its
completion does not depend on the occurrence of a matching receive.
In order to complete the operation, it may be necessary to buffer the
outgoing message locally. For that purpose, buffer
space is provided by the application (Section ).
An error will occur if a buffered-mode send is called and
there is insufficient buffer space.
The buffer space occupied by the message is freed when the message is
transferred to its destination or when the buffered send is cancelled.
A synchronous-mode send can be started whether or not a matching receive was posted. However, the send will complete successfully only if a matching receive is posted, and the receive operation has started to receive the message sent by the synchronous send. Thus, the completion of a synchronous send not only indicates that the send buffer can be reused, but also indicates that the receiver has reached a certain point in its execution, namely that it has started executing the matching receive. Synchronous mode provides synchronous communication semantics: a communication does not complete at either end before both processes rendezvous at the communication. A synchronous-mode send has non-local completion semantics.
A ready-mode send may be started only if the matching receive has already been posted. Otherwise, the operation is erroneous and its outcome is undefined. On some systems, this allows the removal of a hand-shake operation and results in improved performance. A ready-mode send has the same semantics as a standard-mode send. In a correct program, therefore, a ready-mode send could be replaced by a standard-mode send with no effect on the behavior of the program other than performance.
Three additional send functions are provided for the three additional communication modes. The communication mode is indicated by a one letter prefix: B for buffered, S for synchronous, and R for ready. There is only one receive mode and it matches any of the send modes.
All send and receive operations use the buf, count, datatype, source, dest, tag, comm, status and request arguments in the same way as the standard-mode send and receive operations.