Defining an FFTW module (FFTW 3.3.10)

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### 7.7 Defining an FFTW module

Rather than using the `include` statement to include the `fftw3.f03` interface file in any subroutine where you want to use FFTW, you might prefer to define an FFTW Fortran module. FFTW does not install itself as a module, primarily because `fftw3.f03` can be shared between different Fortran compilers while modules (in general) cannot. However, it is trivial to define your own FFTW module if you want. Just create a file containing:

      module FFTW3
        use, intrinsic :: iso_c_binding
        include 'fftw3.f03'
      end module

Compile this file into a module as usual for your compiler (e.g. with `gfortran -c` you will get a file `fftw3.mod`). Now, instead of `include 'fftw3.f03'`, whenever you want to use FFTW routines you can just do:

      use FFTW3

as usual for Fortran modules. (You still need to link to the FFTW library, of course.)