FFTW 3.3.8: New-array Execute Functions

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### 4.6 New-array Execute Functions

Normally, one executes a plan for the arrays with which the plan was created, by calling `fftw_execute(plan)` as described in Using Plans. However, it is possible for sophisticated users to apply a given plan to a different array using the “new-array execute” functions detailed below, provided that the following conditions are met:

- The array size, strides, etcetera are the same (since those are set by the plan).
- The input and output arrays are the same (in-place) or different (out-of-place) if the plan was originally created to be in-place or out-of-place, respectively.
- For split arrays, the separations between the real and imaginary parts, `ii-ri` and `io-ro`, are the same as they were for the input and output arrays when the plan was created. (This condition is automatically satisfied for interleaved arrays.)
- The alignment of the new input/output arrays is the same as that of the input/output arrays when the plan was created, unless the plan was created with the `FFTW_UNALIGNED` flag. Here, the alignment is a platform-dependent quantity (for example, it is the address modulo 16 if SSE SIMD instructions are used, but the address modulo 4 for non-SIMD single-precision FFTW on the same machine). In general, only arrays allocated with `fftw_malloc` are guaranteed to be equally aligned (see SIMD alignment and fftw_malloc).

The alignment issue is especially critical, because if you don’t use `fftw_malloc` then you may have little control over the alignment of arrays in memory. For example, neither the C++ `new` function nor the Fortran `allocate` statement provide strong enough guarantees about data alignment. If you don’t use `fftw_malloc`, therefore, you probably have to use `FFTW_UNALIGNED` (which disables most SIMD support). If possible, it is probably better for you to simply create multiple plans (creating a new plan is quick once one exists for a given size), or better yet re-use the same array for your transforms.

For rare circumstances in which you cannot control the alignment of allocated memory, but wish to determine where a given array is aligned like the original array for which a plan was created, you can use the `fftw_alignment_of` function:

    int fftw_alignment_of(double *p);

Two arrays have equivalent alignment (for the purposes of applying a plan) if and only if `fftw_alignment_of` returns the same value for the corresponding pointers to their data (typecast to `double*` if necessary).

If you are tempted to use the new-array execute interface because you want to transform a known bunch of arrays of the same size, you should probably go use the advanced interface instead (see Advanced Interface)).

The new-array execute functions are:

    void fftw_execute_dft(
         const fftw_plan p,
         fftw_complex *in, fftw_complex *out);

    void fftw_execute_split_dft(
         const fftw_plan p,
         double *ri, double *ii, double *ro, double *io);

    void fftw_execute_dft_r2c(
         const fftw_plan p,
         double *in, fftw_complex *out);

    void fftw_execute_split_dft_r2c(
         const fftw_plan p,
         double *in, double *ro, double *io);

    void fftw_execute_dft_c2r(
         const fftw_plan p,
         fftw_complex *in, double *out);

    void fftw_execute_split_dft_c2r(
         const fftw_plan p,
         double *ri, double *ii, double *out);

    void fftw_execute_r2r(
         const fftw_plan p,
         double *in, double *out);

These execute the `plan` to compute the corresponding transform on the input/output arrays specified by the subsequent arguments. The input/output array arguments have the same meanings as the ones passed to the guru planner routines in the preceding sections. The `plan` is not modified, and these routines can be called as many times as desired, or intermixed with calls to the ordinary `fftw_execute`.

The `plan` must have been created for the transform type corresponding to the execute function, e.g. it must be a complex-DFT plan for `fftw_execute_dft`. Any of the planner routines for that transform type, from the basic to the guru interface, could have been used to create the plan, however.

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