FFTW 3.3.8: Advanced distributed-transpose interface

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#### 6.7.2 Advanced distributed-transpose interface

The above routines are for a transpose of a matrix of numbers (of type `double`), using FFTW’s default block sizes. More generally, one can perform transposes of tuples of numbers, with user-specified block sizes for the input and output:

    fftw_plan fftw_mpi_plan_many_transpose
                    (ptrdiff_t n0, ptrdiff_t n1, ptrdiff_t howmany,
                     ptrdiff_t block0, ptrdiff_t block1,
                     double *in, double *out, MPI_Comm comm, unsigned flags);

In this case, one is transposing an `n0` by `n1` matrix of `howmany`-tuples (e.g. `howmany = 2` for complex numbers). The input is distributed along the `n0` dimension with block size `block0`, and the `n1` by `n0` output is distributed along the `n1` dimension with block size `block1`. If `FFTW_MPI_DEFAULT_BLOCK` (0) is passed for a block size then FFTW uses its default block size. To get the local size of the data on each process, you should then call `fftw_mpi_local_size_many_transposed`.