MPI_Exscan(3)                      Open MPI                      MPI_Exscan(3)



NAME
       MPI_Exscan - Computes an exclusive scan (partial reduction)


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


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

       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.

       No MPI_IN_PLACE operation is supported.


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




1.4.1                            Jan 14, 2010                    MPI_Exscan(3)
