### 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 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 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 Multi-dimensional Array Format). This is unlike the legacy Fortran interface (see Fortran-interface routines), which reversed the dimensions for you. See 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 SIMD alignment and fftw_malloc). Better performance will often be obtained by allocating with ‘`fftw_alloc`’. See Allocating aligned memory in Fortran.
- Similar to the legacy Fortran interface (see FFTW Execution in Fortran), we currently recommend not using `fftw_execute` but rather using the more specialized functions like `fftw_execute_dft` (see 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 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.)