[MPICH] help self-defined datatype

[llwaeva at 21cn.com]

> For just exchanging rows, I would use vectors or subarrays instead.
> E.g. to create a type to represent a set of contiguous rows of a
> two-dimensional matrix in column-major order, use MPI_Type_vector
> (num_cols, num_rows_to_send, total_num_of_rows, MPI_DOUBLE,
> &new_type).
Actually, the problem is not that simple. Finally I will define a much
more complicated datatype which is based on the ROW datatype, so I must
make sure the row exchange is completely correct. But I can't find any
problem in the code I gave above :(