NAME

       MPI_Exscan, MPI_Iexscan - Computes an exclusive scan (partial reduction)

SYNTAX

C Syntax

       #include <mpi.h>
       int MPI_Exscan(const void *sendbuf, void *recvbuf, int count,
            MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)

       int MPI_Iexscan(const void *sendbuf, void *recvbuf, int count,
            MPI_Datatype datatype, MPI_Op op, MPI_Comm comm,
            MPI_Request *request)

Fortran Syntax

       INCLUDE 'mpif.h'
       MPI_EXSCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR)
            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   COUNT, DATATYPE, OP, COMM, IERROR

       MPI_IEXSCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, REQUEST, IERROR)
            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   COUNT, DATATYPE, OP, COMM, REQUEST, IERROR

C++ Syntax

       #include <mpi.h>
       void MPI::Intracomm::Exscan(const void* sendbuf, void* recvbuf,
            int count, const MPI::Datatype& datatype,
            const MPI::Op& op) const

INPUT PARAMETERS

       sendbuf   Send buffer (choice).

       count     Number of elements in input buffer (integer).

       datatype  Data type of elements of input buffer (handle).

       op        Operation (handle).

       comm      Communicator (handle).

OUTPUT PARAMETERS

       recvbuf   Receive buffer (choice).

       request   Request (handle, non-blocking only).

       IERROR    Fortran only: Error status (integer).

DESCRIPTION

       MPI_Exscan is used to perform an exclusive prefix reduction on data distributed across the
       calling processes. The operation returns, in the recvbuf of the process with rank  i,  the
       reduction  (calculated  according  to  the  function  op) of the values in the sendbufs of
       processes with ranks 0, ..., i-1. Compare this with the functionality of  MPI_Scan,  which
       calculates  over  the range 0, ..., i (inclusive). The type of operations supported, their
       semantics, and the constraints on send and receive buffers are as for MPI_Reduce.

       The value in recvbuf  on  process  0  is  undefined  and  unreliable  as  recvbuf  is  not
       significant  for  process  0.  The  value  of  recvbuf on process 1 is always the value in
       sendbuf on process 0.

USE OF IN-PLACE OPTION

       The `in place' option for intracommunicators is specified by passing MPI_IN_PLACE  in  the
       sendbuf  argument.  In  this  case,  the  input data is taken from the receive buffer, and
       replaced by the output data.

       Note that MPI_IN_PLACE is a special kind of value; it has the same restrictions on its use
       as MPI_BOTTOM.

       Because  the in-place option converts the receive buffer into a send-and-receive buffer, a
       Fortran binding that includes INTENT must mark these as INOUT, not OUT.

NOTES

       MPI does not specify which process computes which operation. In particular, both processes
       0  and  1  may  participate in the computation even though the results for both processes'
       recvbuf are degenerate. Therefore, all processes, including 0 and 1, must provide the same
       op.

       It  can  be  argued, from a mathematical perspective, that the definition of MPI_Exscan is
       unsatisfactory because the output at process 0 is undefined.  The "mathematically correct"
       output for process 0 would be the unit element of the reduction operation. However, such a
       definition of an exclusive scan would not work with user-defined op functions as there  is
       no way for MPI to "know" the unit value for these custom operations.

NOTES ON COLLECTIVE OPERATIONS

       The  reduction functions of type MPI_Op do not return an error value.  As a result, if the
       functions detect an error, all they can do is either call MPI_Abort or silently  skip  the
       problem. Thus, if the error handler is changed from MPI_ERRORS_ARE_FATAL to something else
       (e.g., MPI_ERRORS_RETURN), then no error may be indicated.

       The reason for this is the performance problems in ensuring that all  collective  routines
       return the same error value.

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

SEE ALSO

       MPI_Op_create
       MPI_Reduce
       MPI_Scan