### 6.13 FFTW MPI Fortran Interface ¶

The FFTW MPI interface is callable from modern Fortran compilers supporting the Fortran 2003 `iso_c_binding` standard for calling C functions. As described in Calling FFTW from Modern Fortran, this means that you can directly call FFTW’s C interface from Fortran with only minor changes in syntax. There are, however, a few things specific to the MPI interface to keep in mind:

- Instead of including `fftw3.f03` as in Overview of Fortran interface, you should `include 'fftw3-mpi.f03'` (after `use, intrinsic :: iso_c_binding` as before). The `fftw3-mpi.f03` file includes `fftw3.f03`, so you should not `include` them both yourself. (You will also want to include the MPI header file, usually via `include 'mpif.h'` or similar, although though this is not needed by `fftw3-mpi.f03` per se.) (To use the ‘`fftwl_`’ `long double` extended-precision routines in supporting compilers, you should include `fftw3f-mpi.f03` in addition to `fftw3-mpi.f03`. See Extended and quadruple precision in Fortran.)
- Because of the different storage conventions between C and Fortran, you reverse the order of your array dimensions when passing them to FFTW (see Reversing array dimensions). This is merely a difference in notation and incurs no performance overhead. However, it means that, whereas in C the first dimension is distributed, in Fortran the last dimension of your array is distributed.
- In Fortran, communicators are stored as `integer` types; there is no `MPI_Comm` type, nor is there any way to access a C `MPI_Comm`. Fortunately, this is taken care of for you by the FFTW Fortran interface: whenever the C interface expects an `MPI_Comm` type, you should pass the Fortran communicator as an `integer`.8
- Because you need to call the ‘`local_size`’ function to find out how much space to allocate, and this may be larger than the local portion of the array (see MPI Data Distribution), you should always allocate your arrays dynamically using FFTW’s allocation routines as described in Allocating aligned memory in Fortran. (Coincidentally, this also provides the best performance by guaranteeding proper data alignment.)
- Because all sizes in the MPI FFTW interface are declared as `ptrdiff_t` in C, you should use `integer(C_INTPTR_T)` in Fortran (see FFTW Fortran type reference).
- In Fortran, because of the language semantics, we generally recommend using the new-array execute functions for all plans, even in the common case where you are executing the plan on the same arrays for which the plan was created (see Plan execution in Fortran). However, note that in the MPI interface these functions are changed: `fftw_execute_dft` becomes `fftw_mpi_execute_dft`, etcetera. See Using MPI Plans.

For example, here is a Fortran code snippet to perform a distributed L × M complex DFT in-place. (This assumes you have already initialized MPI with `MPI_init` and have also performed `call fftw_mpi_init`.)

      use, intrinsic :: iso_c_binding
      include 'fftw3-mpi.f03'
      integer(C_INTPTR_T), parameter :: L = ...
      integer(C_INTPTR_T), parameter :: M = ...
      type(C_PTR) :: plan, cdata
      complex(C_DOUBLE_COMPLEX), pointer :: data(:,:)
      integer(C_INTPTR_T) :: i, j, alloc_local, local_M, local_j_offset

    !   get local data size and allocate (note dimension reversal)
      alloc_local = fftw_mpi_local_size_2d(M, L, MPI_COMM_WORLD, &
                                           local_M, local_j_offset)
      cdata = fftw_alloc_complex(alloc_local)
      call c_f_pointer(cdata, data, [L,local_M])

    !   create MPI plan for in-place forward DFT (note dimension reversal)
      plan = fftw_mpi_plan_dft_2d(M, L, data, data, MPI_COMM_WORLD, &
                                  FFTW_FORWARD, FFTW_MEASURE)

    ! initialize data to some function my_function(i,j)
      do j = 1, local_M
        do i = 1, L
          data(i, j) = my_function(i, j + local_j_offset)
        end do
      end do

    ! compute transform (as many times as desired)
      call fftw_mpi_execute_dft(plan, data, data)

      call fftw_destroy_plan(plan)
      call fftw_free(cdata)

Note that when we called `fftw_mpi_local_size_2d` and `fftw_mpi_plan_dft_2d` with the dimensions in reversed order, since a L × M Fortran array is viewed by FFTW in C as a M × L array. This means that the array was distributed over the `M` dimension, the local portion of which is a L × local_M array in Fortran. (You must not use an `allocate` statement to allocate an L × local_M array, however; you must allocate `alloc_local` complex numbers, which may be greater than `L * local_M`, in order to reserve space for intermediate steps of the transform.) Finally, we mention that because C’s array indices are zero-based, the `local_j_offset` argument can conveniently be interpreted as an offset in the 1-based `j` index (rather than as a starting index as in C).

If instead you had used the `ior(FFTW_MEASURE, FFTW_MPI_TRANSPOSED_OUT)` flag, the output of the transform would be a transposed M × local_L array, associated with the same `cdata` allocation (since the transform is in-place), and which you could declare with:

      complex(C_DOUBLE_COMPLEX), pointer :: tdata(:,:)
      ...
      call c_f_pointer(cdata, tdata, [M,local_L])

where `local_L` would have been obtained by changing the `fftw_mpi_local_size_2d` call to:

      alloc_local = fftw_mpi_local_size_2d_transposed(M, L, MPI_COMM_WORLD, &
                               local_M, local_j_offset, local_L, local_i_offset)

#### Footnotes

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Technically, this is because you aren’t actually calling the C functions directly. You are calling wrapper functions that translate the communicator with `MPI_Comm_f2c` before calling the ordinary C interface. This is all done transparently, however, since the `fftw3-mpi.f03` interface file renames the wrappers so that they are called in Fortran with the same names as the C interface functions.