Passing an FFTW plan to a Fortran subroutine

Question

I have the following, seemingly simple issue, but I haven't been able to find any information on it:

In fortran90, is it possible to pass an FFTW plan to a subroutine, so as to allow reusing a plan in different "places" of the code? The naive approach of simply passing it as an INTEGER*8 variable yields mayhem, suggesting the code needs more than the plain integer plan to execute the Fourier Transform.

Answer 1

To this end, I usually write a Fortran module in which I put plans and allocatable arrays that will contain the in and out of FFTW3. Then, I can simply USE the module wherever I want.

Here is an example for 3dimensional FFT of real data:

MODULE modfft  

    IMPLICIT NONE  

    TYPE :: fftw3Needs  
        INTEGER(i4b) :: plan_forward, plan_backward ! plans for forward and inverse fft
        REAL(dp), ALLOCATABLE, DIMENSION (:,:,:) :: in_forward, out_backward ! input of FFT and output (result) of inverse FFT  
        COMPLEX(dp), ALLOCATABLE, DIMENSION (:,:,:) :: out_forward, in_backward ! output (result) of FFT and input of inverse FFT          
    END TYPE  
    TYPE (fftw3Needs) :: fftw3  

CONTAINS

    SUBROUTINE init
        CALL init_fftw3_plans! Where you initialize your plans with, e.g., dfftw_plan_dft_r2c_3d.
    END SUBROUTINE init

    SUBROUTINE deallocate_everything_fft
        CALL dfftw_destroy_plan ( fftw3%plan_forward ) ! destroy FFTW3 plans 
        CALL dfftw_destroy_plan ( fftw3%plan_backward ) ! destroy FFTW3 plans 
    END SUBROUTINE deallocate_everything_fft


END MODULE modfft

Note that this uses the old Fortran interface.