3.10. Send-receive


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The send-receive operations combine in one call the sending of a message to one destination and the receiving of another message, from another process. The two (source and destination) are possibly the same. A send-receive operation is very useful for executing a shift operation across a chain of processes. If blocking sends and receives are used for such a shift, then one needs to order the sends and receives correctly (for example, even processes send, then receive, odd processes receive first, then send) so as to prevent cyclic dependencies that may lead to deadlock. When a send-receive operation is used, the communication subsystem takes care of these issues. The send-receive operation can be used in conjunction with the functions described in Chapter Process Topologies in order to perform shifts on various logical topologies. Also, a send-receive operation is useful for implementing remote procedure calls.

A message sent by a send-receive operation can be received by a regular receive operation or probed by a probe operation; a send-receive operation can receive a message sent by a regular send operation.

MPI_SENDRECV(sendbuf, sendcount, sendtype, dest, sendtag, recvbuf, recvcount, recvtype, source, recvtag, comm, status)
[ IN sendbuf] initial address of send buffer (choice)
[ IN sendcount] number of elements in send buffer (integer)
[ IN sendtype] type of elements in send buffer (handle)
[ IN dest] rank of destination (integer)
[ IN sendtag] send tag (integer)
[ OUT recvbuf] initial address of receive buffer (choice)
[ IN recvcount] number of elements in receive buffer (integer)
[ IN recvtype] type of elements in receive buffer (handle)
[ IN source] rank of source (integer)
[ IN recvtag] receive tag (integer)
[ IN comm] communicator (handle)
[ OUT status] status object (Status)

int MPI_Sendrecv(void *sendbuf, int sendcount, MPI_Datatype sendtype, int dest, int sendtag, void *recvbuf, int recvcount, MPI_Datatype recvtype, int source, MPI_Datatype recvtag, MPI_Comm comm, MPI_Status *status)

MPI_SENDRECV(SENDBUF, SENDCOUNT, SENDTYPE, DEST, SENDTAG, RECVBUF, RECVCOUNT, RECVTYPE, SOURCE, RECVTAG, COMM, STATUS, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNT, SENDTYPE, DEST, SENDTAG, RECVCOUNT, RECVTYPE, SOURCE, RECV TAG, COMM, STATUS(MPI_STATUS_SIZE), IERROR

Execute a blocking send and receive operation. Both send and receive use the same communicator, but possibly different tags. The send buffer and receive buffers must be disjoint, and may have different lengths and datatypes.

MPI_SENDRECV_REPLACE(buf, count, datatype, dest, sendtag, source, recvtag, comm, status)
[ INOUT buf] initial address of send and receive buffer (choice)
[ IN count] number of elements in send and receive buffer (integer)
[ IN datatype] type of elements in send and receive buffer (handle)
[ IN dest] rank of destination (integer)
[ IN sendtag] send message tag (integer)
[ IN source] rank of source (integer)
[ IN recvtag] receive message tag (integer)
[ IN comm] communicator (handle)
[ OUT status] status object (Status)

int MPI_Sendrecv_replace(void* buf, int count, MPI_Datatype datatype, int dest, int sendtag, int source, int recvtag, MPI_Comm comm, MPI_Status *status)

MPI_SENDRECV_REPLACE(BUF, COUNT, DATATYPE, DEST, SENDTAG, SOURCE, RECVTAG, COMM, STATUS, IERROR)
<type> BUF(*)
INTEGER COUNT, DATATYPE, DEST, SENDTAG, SOURCE, RECVTAG, COMM, STATUS(MPI_STATUS_SIZE), IERROR

Execute a blocking send and receive. The same buffer is used both for the send and for the receive, so that the message sent is replaced by the message received.

The semantics of a send-receive operation is what would be obtained if the caller forked two concurrent threads, one to execute the send, and one to execute the receive, followed by a join of these two threads.


[] Advice to implementors.

Additional intermediate buffering is needed for the ``replace'' variant. ( End of advice to implementors.)



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