MPI_Exscan is used to perform an exclusive prefix reduction on data
distributed across the calling processes. The operation returns, in
the \fIrecvbuf\fP of the process with rank i, the reduction
(calculated according to the function \fIop\fP) of the values in the
\fIsendbuf\fPs 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.
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The value in \fIrecvbuf\fP on process 0 is undefined and unreliable
as \fIrecvbuf\fP is not significant for process 0. The value of
\fIrecvbuf\fP on process 1 is always the value in \fIsendbuf\fP on
process 0.
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.SH USE OF IN-PLACE OPTION
The `in place' option for intracommunicators is specified by passing MPI_IN_PLACE in the \fIsendbuf\fP argument. In this case, the input data is taken from the receive buffer, and replaced by the output data.
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Note that MPI_IN_PLACE is a special kind of value; it has the same restrictions on its use as MPI_BOTTOM.
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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.
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.SH NOTES
.ft R
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' \fIrecvbuf\fP are
degenerate. Therefore, all processes, including 0 and 1, must provide
the same \fIop\fP.
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It can be argued, from a mathematical perspective, that the definition
of MPI_Exscan is unsatisfactory 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
\fIop\fP functions as there is no way for MPI to "know" the unit value
for these custom operations.
.SH NOTES ON COLLECTIVE OPERATIONS
.ft R
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.
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The reason for this is the performance problems in ensuring that
all collective routines return the same error value.
.SH ERRORS
.ft R
Almost all MPI routines return an error value; C routines as
