MPI_Exscan(3) Open MPI MPI_Exscan(3)
NAME
MPI_Exscan, MPI_Iexscan - Computes an exclusive scan (partial reduc-
tion)
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 op-
erations 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 re-
strictions 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 partic-
ular, both processes 0 and 1 may participate in the computation even
though the results for both processes' recvbuf are degenerate. There-
fore, 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 un-
defined. 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 oper-
ations.
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 ei-
ther 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++ func-
tions 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
4.1.2 Nov 24, 2021 MPI_Exscan(3)
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