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Current fftw source
author Chris Cannam <cannam@all-day-breakfast.com>
date Tue, 18 Oct 2016 13:40:26 +0100
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cannam@127 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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cannam@127 3 <!-- This manual is for FFTW
cannam@127 4 (version 3.3.5, 30 July 2016).
cannam@127 5
cannam@127 6 Copyright (C) 2003 Matteo Frigo.
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cannam@127 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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cannam@127 24 <head>
cannam@127 25 <title>FFTW 3.3.5: Fixed-size Arrays in C</title>
cannam@127 26
cannam@127 27 <meta name="description" content="FFTW 3.3.5: Fixed-size Arrays in C">
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cannam@127 36 <link href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" rel="up" title="Multi-dimensional Array Format">
cannam@127 37 <link href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C" rel="next" title="Dynamic Arrays in C">
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cannam@127 70
cannam@127 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
cannam@127 72 <a name="Fixed_002dsize-Arrays-in-C"></a>
cannam@127 73 <div class="header">
cannam@127 74 <p>
cannam@127 75 Next: <a href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C" accesskey="n" rel="next">Dynamic Arrays in C</a>, Previous: <a href="Column_002dmajor-Format.html#Column_002dmajor-Format" accesskey="p" rel="prev">Column-major Format</a>, Up: <a href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" 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#Concept-Index" title="Index" rel="index">Index</a>]</p>
cannam@127 76 </div>
cannam@127 77 <hr>
cannam@127 78 <a name="Fixed_002dsize-Arrays-in-C-1"></a>
cannam@127 79 <h4 class="subsection">3.2.3 Fixed-size Arrays in C</h4>
cannam@127 80 <a name="index-C-multi_002ddimensional-arrays"></a>
cannam@127 81
cannam@127 82 <p>A multi-dimensional array whose size is declared at compile time in C
cannam@127 83 is <em>already</em> in row-major order. You don&rsquo;t have to do anything
cannam@127 84 special to transform it. For example:
cannam@127 85 </p>
cannam@127 86 <div class="example">
cannam@127 87 <pre class="example">{
cannam@127 88 fftw_complex data[N0][N1][N2];
cannam@127 89 fftw_plan plan;
cannam@127 90 ...
cannam@127 91 plan = fftw_plan_dft_3d(N0, N1, N2, &amp;data[0][0][0], &amp;data[0][0][0],
cannam@127 92 FFTW_FORWARD, FFTW_ESTIMATE);
cannam@127 93 ...
cannam@127 94 }
cannam@127 95 </pre></div>
cannam@127 96
cannam@127 97 <p>This will plan a 3d in-place transform of size <code>N0 x N1 x N2</code>.
cannam@127 98 Notice how we took the address of the zero-th element to pass to the
cannam@127 99 planner (we could also have used a typecast).
cannam@127 100 </p>
cannam@127 101 <p>However, we tend to <em>discourage</em> users from declaring their
cannam@127 102 arrays in this way, for two reasons. First, this allocates the array
cannam@127 103 on the stack (&ldquo;automatic&rdquo; storage), which has a very limited size on
cannam@127 104 most operating systems (declaring an array with more than a few
cannam@127 105 thousand elements will often cause a crash). (You can get around this
cannam@127 106 limitation on many systems by declaring the array as
cannam@127 107 <code>static</code> and/or global, but that has its own drawbacks.)
cannam@127 108 Second, it may not optimally align the array for use with a SIMD
cannam@127 109 FFTW (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>). Instead, we recommend
cannam@127 110 using <code>fftw_malloc</code>, as described below.
cannam@127 111 </p>
cannam@127 112
cannam@127 113
cannam@127 114
cannam@127 115 </body>
cannam@127 116 </html>