MPI_Exscan(3OpenMPI) MPI_Exscan(3OpenMPI)
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 (calcu-lated (calculated
lated 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 degener-ate. degenerate.
ate. 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 unsatisfac-tory unsatisfactory
tory 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
Open MPI 1.2 September 2006 MPI_Exscan(3OpenMPI)
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