7.1 Overview of Fortran interface

FFTW provides a file fftw3.f03 that defines Fortran 2003 interfaces for all of its C routines, except for the MPI routines described elsewhere, which can be found in the same directory as fftw3.h (the C header file). In any Fortran subroutine where you want to use FFTW functions, you should begin with:

  use, intrinsic :: iso_c_binding 
  include 'fftw3.f03'

This includes the interface definitions and the standard iso_c_binding module (which defines the equivalents of C types). You can also put the FFTW functions into a module if you prefer (see section Defining an FFTW module).

At this point, you can now call anything in the FFTW C interface directly, almost exactly as in C other than minor changes in syntax. For example:

  type(C_PTR) :: plan
  complex(C_DOUBLE_COMPLEX), dimension(1024,1000) :: in, out
  plan = fftw_plan_dft_2d(1000,1024, in,out, FFTW_FORWARD,FFTW_ESTIMATE)
  ...
  call fftw_execute_dft(plan, in, out)
  ...
  call fftw_destroy_plan(plan)

A few important things to keep in mind are:

• FFTW plans are type(C_PTR). Other C types are mapped in the obvious way via the iso_c_binding standard: int turns into integer(C_INT), fftw_complex turns into complex(C_DOUBLE_COMPLEX), double turns into real(C_DOUBLE), and so on. See section FFTW Fortran type reference.
• Functions in C become functions in Fortran if they have a return value, and subroutines in Fortran otherwise.
• The ordering of the Fortran array dimensions must be reversed when they are passed to the FFTW plan creation, thanks to differences in array indexing conventions (see section Multi-dimensional Array Format). This is unlike the legacy Fortran interface (see section Fortran-interface routines), which reversed the dimensions for you. See section Reversing array dimensions.
• Using ordinary Fortran array declarations like this works, but may yield suboptimal performance because the data may not be not aligned to exploit SIMD instructions on modern proessors (see section SIMD alignment and fftw_malloc). Better performance will often be obtained by allocating with ‘fftw_alloc’. See section Allocating aligned memory in Fortran.
• Similar to the legacy Fortran interface (see section FFTW Execution in Fortran), we currently recommend not using fftw_execute but rather using the more specialized functions like fftw_execute_dft (see section New-array Execute Functions). However, you should execute the plan on the same arrays as the ones for which you created the plan, unless you are especially careful. See section Plan execution in Fortran. To prevent you from using fftw_execute by mistake, the fftw3.f03 file does not provide an fftw_execute interface declaration.
• Multiple planner flags are combined with ior (equivalent to ‘|’ in C). e.g. FFTW_MEASURE | FFTW_DESTROY_INPUT becomes ior(FFTW_MEASURE, FFTW_DESTROY_INPUT). (You can also use ‘+’ as long as you don’t try to include a given flag more than once.)

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