114. Exclusive Scan

Up: Scan Next: Example using MPI_SCAN Previous: Inclusive Scan

MPI_EXSCAN(sendbuf, recvbuf, count, datatype, op, comm)
IN sendbufstarting address of send buffer (choice)
OUT recvbufstarting address of receive buffer (choice)
IN countnumber of elements in input buffer (non-negative integer)
IN datatypedata type of elements of input buffer (handle)
IN opoperation (handle)
IN comm intracommunicator (handle)

int MPI_Exscan(void *sendbuf, void *recvbuf, int count, MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)

<type> SENDBUF(*), RECVBUF(*)

{ void MPI::Intracomm::Exscan(const void* sendbuf, void* recvbuf, int count, const MPI::Datatype& datatype, const MPI::Op& op) const (binding deprecated, see Section Deprecated since MPI-2.2 ) }

If comm is an intracommunicator, MPI_EXSCAN is used to perform a prefix reduction on data distributed across the group. The value in recvbuf on the process with rank 0 is undefined, and recvbuf is not signficant on process 0. The value in recvbuf on the process with rank 1 is defined as the value in sendbuf on the process with rank 0. For processes with rank i > 1, the operation returns, in the receive buffer of the process with rank i, the reduction of the values in the send buffers of processes with ranks 0,...,i-1 (inclusive). The type of operations supported, their semantics, and the constraints on send and receive buffers, are as for MPI_REDUCE.

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. The receive buffer on rank 0 is not changed by this operation. This operation is invalid for intercommunicators. 2.2

The exclusive scan is more general than the inclusive scan. Any inclusive scan operation can be achieved by using the exclusive scan and then locally combining the local contribution. Note that for non-invertable operations such as MPI_MAX, the exclusive scan cannot be computed with the inclusive scan. 2.2 ( End of rationale.)

Up: Scan Next: Example using MPI_SCAN Previous: Inclusive Scan

Return to MPI-2.2 Standard Index
Return to MPI Forum Home Page

(Unofficial) MPI-2.2 of September 4, 2009
HTML Generated on September 10, 2009