109. Exclusive Scan

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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

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.

No ``in place'' option is supported.

This operation is invalid for intercommunicators.
Advice to users.

As for MPI_SCAN, MPI does not specify which processes may call the operation, only that the result be correctly computed. In particular, note that the process with rank 1 need not call the MPI_Op, since all it needs to do is to receive the value from the process with rank 0. However, all processes, even the processes with ranks zero and one, must provide the same op. ( End of advice to users.)


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.

No in-place version is specified for MPI_EXSCAN because it is not clear what this means for the process with rank zero. ( End of rationale.)

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