Info: (fftw3) Overview of Fortran interface

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fftw3: Calling FFTW from Modern Fortran
fftw3: Reversing array dimensions
fftw3: Overview of Fortran interface

 
 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 (
 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.  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 (Multi-dimensional
      Array Format).  This is _unlike_ the legacy Fortran interface
      (Fortran-interface routines), which reversed the dimensions
      for you.  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 (
      SIMD alignment and fftw_malloc).  Better performance will often
      be obtained by allocating with 'fftw_alloc'.  Allocating
      aligned memory in Fortran.
 
    * Similar to the legacy Fortran interface (FFTW Execution in
      Fortran), we currently recommend _not_ using 'fftw_execute' but
      rather using the more specialized functions like 'fftw_execute_dft'
      (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.  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.)
fftw3: Overview of Fortran interface

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