MPI_SENDRECV_REPLACE(3)

MPI_Sendrecv_replace — 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.

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

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

    •   MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.

    •   MPI_ERRORS_ABORT An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.

    •   MPI_ERRORS_RETURN Returns an error code to the application.

MPI applications can also implement their own error handlers by calling:

    •   MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

    •   MPI_File_create_errhandler then MPI_File_set_errhandler

    •   MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

    •   MPI_Win_create_errhandler then MPI_Win_set_errhandler

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

Note that per the “Return Status” section in the “Point-to-Point Communication” chapter in the MPI Standard, MPI errors on messages received by 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 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.

SEE ALSO:

            •   MPI_Sendrecv

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