## 3.2.7. Clarification of MPI_REDUCE

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The current text on p. 115, lines 25--28, from MPI-1.1 (June 12, 1995) says:

The datatype argument of MPI_REDUCE must be compatible with op. Predefined operators work only with the MPI types listed in Section 4.9.2 and Section 4.9.3. User-defined operators may operate on general, derived datatypes.

This text is changed to:

The datatype argument of MPI_REDUCE must be compatible with op. Predefined operators work only with the MPI types listed in Section 4.9.2 and Section 4.9.3. Furthermore, the datatype and op given for predefined operators must be the same on all processes.

Note that it is possible for users to supply different user-defined operations to MPI_REDUCE in each process. MPI does not define which operations are used on which operands in this case.

Advice to users.

Users should make no assumptions about how MPI_REDUCE is implemented. Safest is to ensure that the same function is passed to MPI_REDUCE by each process. ( End of advice to users.)

Overlapping datatypes are permitted in ``send'' buffers. Overlapping datatypes in ``receive'' buffers are erroneous and may give unpredictable results.

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