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<h4 class="subsection">3.2.3 Fixed-size Arrays in C</h4>

<p><a name="index-C-multi_002ddimensional-arrays-120"></a>
A multi-dimensional array whose size is declared at compile time in C
is <em>already</em> in row-major order.  You don't have to do anything
special to transform it.  For example:

<pre class="example">     {
          fftw_complex data[N0][N1][N2];
          fftw_plan plan;
          ...
          plan = fftw_plan_dft_3d(N0, N1, N2, &amp;data[0][0][0], &amp;data[0][0][0],
                                  FFTW_FORWARD, FFTW_ESTIMATE);
          ...
     }
</pre>
   <p>This will plan a 3d in-place transform of size <code>N0 x N1 x N2</code>.
Notice how we took the address of the zero-th element to pass to the
planner (we could also have used a typecast).

   <p>However, we tend to <em>discourage</em> users from declaring their
arrays in this way, for two reasons.  First, this allocates the array
on the stack (&ldquo;automatic&rdquo; storage), which has a very limited size on
most operating systems (declaring an array with more than a few
thousand elements will often cause a crash).  (You can get around this
limitation on many systems by declaring the array as
<code>static</code> and/or global, but that has its own drawbacks.)
Second, it may not optimally align the array for use with a SIMD
FFTW (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).  Instead, we recommend
using <code>fftw_malloc</code>, as described below.

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author	Chris Cannam
date	Wed, 20 Mar 2013 15:35:50 +0000
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