[Mpi-forum] MPI Count proposal from today's meeting

[N.M. Maclaren]

On Jun 17 2010, Supalov, Alexander wrote:
>
>Thanks. Why signed?

A few reasons:

    Many languages (including Fortran) do not support unsigned integers
and, of those that do, not all have compatible semantics.

    Some things specifiable by index values can have negative values in
some languages (e.g. Fortran 90), and it is ridiculous to limit MPI to
only languages with inferior array support.

    It is useful to be able to specify invalid or exceptional values as
out-of-band ones, and that isn't possible for any positive integer.  The
standard states that MPI_UNDEFINED is negative in one place, and that
can be returned from MPI_Get_count.

    MPI really, but REALLY, doesn't want to get snarled up in the bizarre
mess that is C unsigned support.  You did know that C90 and C99 are
subtly incompatible in that area, didn't you?  And C++ is different from
both, as far as I recall.


Regards,
Nick Maclaren.