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<span id="Fixed_002dsize-Arrays-in-C"></span><div class="header">
<p>
Next: <a href="Dynamic-Arrays-in-C.html" accesskey="n" rel="next">Dynamic Arrays in C</a>, Previous: <a href="Column_002dmajor-Format.html" accesskey="p" rel="prev">Column-major Format</a>, Up: <a href="Multi_002ddimensional-Array-Format.html" accesskey="u" rel="up">Multi-dimensional Array Format</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
<span id="Fixed_002dsize-Arrays-in-C-1"></span><h4 class="subsection">3.2.3 Fixed-size Arrays in C</h4>
<span id="index-C-multi_002ddimensional-arrays"></span>
<p>A multi-dimensional array whose size is declared at compile time in C
is <em>already</em> in row-major order. You don&rsquo;t have to do anything
special to transform it. For example:
</p>
<div class="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></div>
<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>
<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_malloc</a>). Instead, we recommend
using <code>fftw_malloc</code>, as described below.
</p>
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