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<title>FFTW 3.3.8: New-array Execute Functions</title>

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<a name="New_002darray-Execute-Functions"></a>
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Next: <a href="Wisdom.html#Wisdom" accesskey="n" rel="next">Wisdom</a>, Previous: <a href="Guru-Interface.html#Guru-Interface" accesskey="p" rel="prev">Guru Interface</a>, Up: <a href="FFTW-Reference.html#FFTW-Reference" accesskey="u" rel="up">FFTW Reference</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="New_002darray-Execute-Functions-1"></a>
<h3 class="section">4.6 New-array Execute Functions</h3>
<a name="index-execute-2"></a>
<a name="index-new_002darray-execution"></a>

<p>Normally, one executes a plan for the arrays with which the plan was
created, by calling <code>fftw_execute(plan)</code> as described in <a href="Using-Plans.html#Using-Plans">Using Plans</a>.
<a name="index-fftw_005fexecute-2"></a>
However, it is possible for sophisticated users to apply a given plan
to a <em>different</em> array using the &ldquo;new-array execute&rdquo; functions
detailed below, provided that the following conditions are met:
</p>
<ul>
<li> The array size, strides, etcetera are the same (since those are set by
the plan).

</li><li> 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.

</li><li> For split arrays, the separations between the real and imaginary
parts, <code>ii-ri</code> and <code>io-ro</code>, 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.)

</li><li> The <em>alignment</em> 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 <code>FFTW_UNALIGNED</code> flag.
<a name="index-FFTW_005fUNALIGNED-1"></a>
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 <code>fftw_malloc</code> are guaranteed to
be equally aligned (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).

</li></ul>

<a name="index-alignment-2"></a>
<p>The alignment issue is especially critical, because if you don&rsquo;t use
<code>fftw_malloc</code> then you may have little control over the alignment
of arrays in memory.  For example, neither the C++ <code>new</code> function
nor the Fortran <code>allocate</code> statement provide strong enough
guarantees about data alignment.  If you don&rsquo;t use <code>fftw_malloc</code>,
therefore, you probably have to use <code>FFTW_UNALIGNED</code> (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.
</p>
<a name="index-fftw_005falignment_005fof-1"></a>
<p>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 <code>fftw_alignment_of</code> function:
</p><div class="example">
<pre class="example">int fftw_alignment_of(double *p);
</pre></div>
<p>Two arrays have equivalent alignment (for the purposes of applying a
plan) if and only if <code>fftw_alignment_of</code> returns the same value
for the corresponding pointers to their data (typecast to <code>double*</code>
if necessary).
</p>
<p>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 <a href="Advanced-Interface.html#Advanced-Interface">Advanced Interface</a>)).
</p>
<p>The new-array execute functions are:
</p>
<div class="example">
<pre class="example">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);
</pre></div>
<a name="index-fftw_005fexecute_005fdft"></a>
<a name="index-fftw_005fexecute_005fsplit_005fdft"></a>
<a name="index-fftw_005fexecute_005fdft_005fr2c"></a>
<a name="index-fftw_005fexecute_005fsplit_005fdft_005fr2c"></a>
<a name="index-fftw_005fexecute_005fdft_005fc2r"></a>
<a name="index-fftw_005fexecute_005fsplit_005fdft_005fc2r"></a>
<a name="index-fftw_005fexecute_005fr2r"></a>

<p>These execute the <code>plan</code> 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 <code>plan</code>
is not modified, and these routines can be called as many times as
desired, or intermixed with calls to the ordinary <code>fftw_execute</code>.
</p>
<p>The <code>plan</code> <em>must</em> have been created for the transform type
corresponding to the execute function, e.g. it must be a complex-DFT
plan for <code>fftw_execute_dft</code>.  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.
</p>
<hr>
<div class="header">
<p>
Next: <a href="Wisdom.html#Wisdom" accesskey="n" rel="next">Wisdom</a>, Previous: <a href="Guru-Interface.html#Guru-Interface" accesskey="p" rel="prev">Guru Interface</a>, Up: <a href="FFTW-Reference.html#FFTW-Reference" accesskey="u" rel="up">FFTW Reference</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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author	Chris Cannam <cannam@all-day-breakfast.com>
date	Tue, 19 Nov 2019 14:52:55 +0000
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