| MPI_EXSCAN(sendbuf, recvbuf, count, datatype, op, comm) | |
| IN sendbuf | starting address of send buffer (choice) |
| OUT recvbuf | starting address of receive buffer (choice) |
| IN count | number of elements in input buffer (non-negative integer) |
| IN datatype | datatype of elements of input buffer (handle) |
| IN op | operation (handle) |
| IN comm | intra-communicator (handle) |
If comm is an intra-communicator, MPI_EXSCAN is used to perform a prefix reduction on data distributed across the group. The value in recvbuf on the MPI process with rank 0 is undefined, and recvbuf is not signficant on that MPI process. The value in recvbuf on the MPI process with rank 1 is defined as the value in sendbuf on the MPI process with rank 0. For MPI processes with rank i > 1, the operation returns, in the receive buffer of the MPI process with rank i, the reduction of the values in the send buffers of MPI processes with ranks 0,...,i-1 (inclusive). The routine is called by all group members using the same arguments for count, datatype, op and comm, except that for user-defined operations, the same rules apply as for MPI_REDUCE. 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 intra-communicators 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 inter-communicators.
Rationale.
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 noninvertable operations such as
MPI_MAX, the exclusive scan cannot be computed with the
inclusive scan.
( End of rationale.)