Initial address of send and receive buffer (choice).
.SH INPUT PARAMETERS
.ft R
.TP 1i
count
Number of elements in send and receive buffer (integer).
.TP 1i
datatype
Type of elements to send and receive (handle).
.TP 1i
dest
Rank of destination (integer).
.TP 1i
sendtag
Send message tag (integer).
.TP 1i
source
Rank of source (integer).
.TP 1i
recvtag
Receive message tag (integer).
.TP 1i
comm
Communicator (handle).
.SH OUTPUT PARAMETERS
.ft R
.TP 1i
status
Status object (status).
.TP 1i
IERROR
Fortran only: Error status (integer).
.SH DESCRIPTION
.ft R
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 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) in order 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 6 of the MPI Standard, "Process Topologies," in order to perform shifts on various logical topologies. Also, a send-receive operation is useful for implementing remote procedure calls.
.sp
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.
.sp
MPI_Sendrecv_replace executes 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.
.sp
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.
Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ functions do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to throw an MPI::Exception object.
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.
