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Add FFTW3
author Chris Cannam
date Wed, 20 Mar 2013 15:35:50 +0000
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3 <title>Fixed-size Arrays in C - FFTW 3.3.3</title>
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51 <p>
52 Next:&nbsp;<a rel="next" accesskey="n" href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C">Dynamic Arrays in C</a>,
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57
58 <h4 class="subsection">3.2.3 Fixed-size Arrays in C</h4>
59
60 <p><a name="index-C-multi_002ddimensional-arrays-120"></a>
61 A multi-dimensional array whose size is declared at compile time in C
62 is <em>already</em> in row-major order. You don't have to do anything
63 special to transform it. For example:
64
65 <pre class="example"> {
66 fftw_complex data[N0][N1][N2];
67 fftw_plan plan;
68 ...
69 plan = fftw_plan_dft_3d(N0, N1, N2, &amp;data[0][0][0], &amp;data[0][0][0],
70 FFTW_FORWARD, FFTW_ESTIMATE);
71 ...
72 }
73 </pre>
74 <p>This will plan a 3d in-place transform of size <code>N0 x N1 x N2</code>.
75 Notice how we took the address of the zero-th element to pass to the
76 planner (we could also have used a typecast).
77
78 <p>However, we tend to <em>discourage</em> users from declaring their
79 arrays in this way, for two reasons. First, this allocates the array
80 on the stack (&ldquo;automatic&rdquo; storage), which has a very limited size on
81 most operating systems (declaring an array with more than a few
82 thousand elements will often cause a crash). (You can get around this
83 limitation on many systems by declaring the array as
84 <code>static</code> and/or global, but that has its own drawbacks.)
85 Second, it may not optimally align the array for use with a SIMD
86 FFTW (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>). Instead, we recommend
87 using <code>fftw_malloc</code>, as described below.
88
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