Overview of Fortran interface

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7.1 Overview of Fortran interface

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FFTW provides a file fftw3.f03 that defines Fortran 2003 Chris@10: interfaces for all of its C routines, except for the MPI routines Chris@10: described elsewhere, which can be found in the same directory as Chris@10: fftw3.h (the C header file). In any Fortran subroutine where Chris@10: you want to use FFTW functions, you should begin with: Chris@10: Chris@10:

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       use, intrinsic :: iso_c_binding
Chris@10:        include 'fftw3.f03'
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This includes the interface definitions and the standard Chris@10: iso_c_binding module (which defines the equivalents of C Chris@10: types). You can also put the FFTW functions into a module if you Chris@10: prefer (see Defining an FFTW module). Chris@10: Chris@10:

At this point, you can now call anything in the FFTW C interface Chris@10: directly, almost exactly as in C other than minor changes in syntax. Chris@10: For example: Chris@10: Chris@10:

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       type(C_PTR) :: plan
Chris@10:        complex(C_DOUBLE_COMPLEX), dimension(1024,1000) :: in, out
Chris@10:        plan = fftw_plan_dft_2d(1000,1024, in,out, FFTW_FORWARD,FFTW_ESTIMATE)
Chris@10:        ...
Chris@10:        call fftw_execute_dft(plan, in, out)
Chris@10:        ...
Chris@10:        call fftw_destroy_plan(plan)
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A few important things to keep in mind are: Chris@10: Chris@10:

FFTW plans are type(C_PTR). Other C types are mapped in the Chris@10: obvious way via the iso_c_binding standard: int turns Chris@10: into integer(C_INT), fftw_complex turns into Chris@10: complex(C_DOUBLE_COMPLEX), double turns into Chris@10: real(C_DOUBLE), and so on. See FFTW Fortran type reference. Chris@10: Chris@10:
Functions in C become functions in Fortran if they have a return value, Chris@10: and subroutines in Fortran otherwise. Chris@10: Chris@10:
The ordering of the Fortran array dimensions must be reversed Chris@10: when they are passed to the FFTW plan creation, thanks to differences Chris@10: in array indexing conventions (see Multi-dimensional Array Format). This is unlike the legacy Fortran interface Chris@10: (see Fortran-interface routines), which reversed the dimensions Chris@10: for you. See Reversing array dimensions. Chris@10: Chris@10:
Using ordinary Fortran array declarations like this works, but may Chris@10: yield suboptimal performance because the data may not be not aligned Chris@10: to exploit SIMD instructions on modern proessors (see SIMD alignment and fftw_malloc). Better performance will often be obtained Chris@10: by allocating with ‘fftw_alloc’. See Allocating aligned memory in Fortran. Chris@10: Chris@10:
Similar to the legacy Fortran interface (see FFTW Execution in Fortran), we currently recommend not using fftw_execute Chris@10: but rather using the more specialized functions like Chris@10: fftw_execute_dft (see New-array Execute Functions). Chris@10: However, you should execute the plan on the same arrays as the Chris@10: ones for which you created the plan, unless you are especially Chris@10: careful. See Plan execution in Fortran. To prevent Chris@10: you from using fftw_execute by mistake, the fftw3.f03 Chris@10: file does not provide an fftw_execute interface declaration. Chris@10: Chris@10:
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.) Chris@10: Chris@10:

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