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

       USE MPI
       ! or the older form: 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

Fortran 2008 Syntax

       USE mpi_f08
       MPI_Exscan(sendbuf, recvbuf, count, datatype, op, comm, ierror)
            TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
            TYPE(*), DIMENSION(..) :: recvbuf
            INTEGER, INTENT(IN) :: count
            TYPE(MPI_Datatype), INTENT(IN) :: datatype
            TYPE(MPI_Op), INTENT(IN) :: op
            TYPE(MPI_Comm), INTENT(IN) :: comm
            INTEGER, OPTIONAL, INTENT(OUT) :: ierror

       MPI_Iexscan(sendbuf, recvbuf, count, datatype, op, comm, request, ierror)
            TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
            TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
            INTEGER, INTENT(IN) :: count
            TYPE(MPI_Datatype), INTENT(IN) :: datatype
            TYPE(MPI_Op), INTENT(IN) :: op
            TYPE(MPI_Comm), INTENT(IN) :: comm
            TYPE(MPI_Request), INTENT(OUT) :: request
            INTEGER, OPTIONAL, INTENT(OUT) :: 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

4.1.6                                             Sep 30, 2023                                     MPI_Exscan(3)