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MPI_Sendrecv_replace.3.rst

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MPI_Sendrecv_replace

:ref:`MPI_Sendrecv_replace` |mdash| Sends and receives a message using a single buffer.

SYNTAX

C Syntax

#include <mpi.h>

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)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_SENDRECV_REPLACE(BUF, COUNT, DATATYPE, DEST, SENDTAG, SOURCE,
             RECVTAG, COMM, STATUS, IERROR)
     <type>  BUF(*)
     INTEGER COUNT, DATATYPE, DEST, SENDTAG
     INTEGER SOURCE, RECVTAG, COMM
     INTEGER STATUS(MPI_STATUS_SIZE), IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Sendrecv_replace(buf, count, datatype, dest, sendtag, source, recvtag,
             comm, status, ierror)
     TYPE(*), DIMENSION(..) :: buf
     INTEGER, INTENT(IN) :: count, dest, sendtag, source, recvtag
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Status) :: status
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT/OUTPUT PARAMETER

  • buf: Initial address of send and receive buffer (choice).

INPUT PARAMETERS

  • count: Number of elements in send and receive buffer (integer).
  • datatype: Type of elements to send and receive (handle).
  • dest: Rank of destination (integer).
  • sendtag: Send message tag (integer).
  • source: Rank of source (integer).
  • recvtag: Receive message tag (integer).
  • comm: Communicator (handle).

OUTPUT PARAMETERS

  • status: Status object (status).
  • ierror: Fortran only: Error status (integer).

DESCRIPTION

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.

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.

:ref:`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.

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.

ERRORS

Note that per the "Return Status" section in the "Point-to-Point Communication" chapter in the MPI Standard, MPI errors on messages received by :ref:`MPI_Sendrecv_replace` do not set the status.MPI_ERROR field in the returned status. The error code is always passed to the back-end error handler and may be passed back to the caller through the return value of :ref:`MPI_Sendrecv_replace` if the back-end error handler returns it. The pre-defined MPI error handler MPI_ERRORS_RETURN exhibits this behavior, for example.

.. seealso::
   * :ref:`MPI_Sendrecv`