NAME

       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

       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

C++ Syntax

       #include <mpi.h>
       void Comm::Sendrecv_replace(void* buf, int count, const
            Datatype& datatype, int dest, int sendtag, int source,
            int recvtag, Status& status) const

       void Comm::Sendrecv_replace(void* buf, int count, const
            Datatype& datatype, int dest, int sendtag, int source,
            int recvtag) const

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  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.

SEE ALSO

       MPI_Sendrecv