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<p>1) MPI 2.2 suggests that MPI_UNWEIGHTED is probably NULL in C. It also says that all participants must either provide weighs for edges they define or must use MPI_UNWEIGHTED. When some task has no edges to specify, the length of its weights arrays are 0. The natural thing to do is to pass NULL for the weights array address if its length is 0. How would the implementation know if the user intended to pass MPI_UNWEIGHTED vs ordinary NULL? <br>
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I suppose an allreduce that confirms all participants passed NULL (i.e. guess that NULL means MPI_UNWEIGHTED and test that guess) would work. If any process passed non-NULL then the local NULL is deduced to mean an empty array rather than MPI_UNWEIGHTED. Yuch. That requires treating the address of an array as significant, even if the user said it is empty . I.E. an empty array with a garbage address is OK if MPI_UNWEIGTHED was not intended but an error if it was (because the address != MPI_UNWEIGHTED)<br>
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--- I think the suggestion that MPI_UNWEIGTHED is probably NULL is inappropriate.<br>
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2) We say that MPI_UNWEIGHTED is the same as saying all mentioned edges have "(effectively no) weight". I would assume an edge that is not mentioned also has "(effectively no) weight" so why would I expect any edge that is mentioned as having "no weight" to be treated differently than another that has "no weight" but was not mentioned? <br>
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-- We should probably say "All specified edges have the same positive weight and any edge not specified is assumed to have no weight.". <br>
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Dick <br>
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Dick Treumann - MPI Team <br>
IBM Systems & Technology Group<br>
Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601<br>
Tele (845) 433-7846 Fax (845) 433-8363<br>
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