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Try a double-precision kissfft
author Chris Cannam
date Wed, 07 Sep 2016 10:40:32 +0100
parents 26056e866c29
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Chris@19 3 <title>Fixed-size Arrays in C - FFTW 3.3.4</title>
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Chris@19 49 <a name="Fixed-size-Arrays-in-C"></a>
Chris@19 50 <a name="Fixed_002dsize-Arrays-in-C"></a>
Chris@19 51 <p>
Chris@19 52 Next:&nbsp;<a rel="next" accesskey="n" href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C">Dynamic Arrays in C</a>,
Chris@19 53 Previous:&nbsp;<a rel="previous" accesskey="p" href="Column_002dmajor-Format.html#Column_002dmajor-Format">Column-major Format</a>,
Chris@19 54 Up:&nbsp;<a rel="up" accesskey="u" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a>
Chris@19 55 <hr>
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Chris@19 57
Chris@19 58 <h4 class="subsection">3.2.3 Fixed-size Arrays in C</h4>
Chris@19 59
Chris@19 60 <p><a name="index-C-multi_002ddimensional-arrays-120"></a>
Chris@19 61 A multi-dimensional array whose size is declared at compile time in C
Chris@19 62 is <em>already</em> in row-major order. You don't have to do anything
Chris@19 63 special to transform it. For example:
Chris@19 64
Chris@19 65 <pre class="example"> {
Chris@19 66 fftw_complex data[N0][N1][N2];
Chris@19 67 fftw_plan plan;
Chris@19 68 ...
Chris@19 69 plan = fftw_plan_dft_3d(N0, N1, N2, &amp;data[0][0][0], &amp;data[0][0][0],
Chris@19 70 FFTW_FORWARD, FFTW_ESTIMATE);
Chris@19 71 ...
Chris@19 72 }
Chris@19 73 </pre>
Chris@19 74 <p>This will plan a 3d in-place transform of size <code>N0 x N1 x N2</code>.
Chris@19 75 Notice how we took the address of the zero-th element to pass to the
Chris@19 76 planner (we could also have used a typecast).
Chris@19 77
Chris@19 78 <p>However, we tend to <em>discourage</em> users from declaring their
Chris@19 79 arrays in this way, for two reasons. First, this allocates the array
Chris@19 80 on the stack (&ldquo;automatic&rdquo; storage), which has a very limited size on
Chris@19 81 most operating systems (declaring an array with more than a few
Chris@19 82 thousand elements will often cause a crash). (You can get around this
Chris@19 83 limitation on many systems by declaring the array as
Chris@19 84 <code>static</code> and/or global, but that has its own drawbacks.)
Chris@19 85 Second, it may not optimally align the array for use with a SIMD
Chris@19 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
Chris@19 87 using <code>fftw_malloc</code>, as described below.
Chris@19 88
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