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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: Fixed-size Arrays in C</title>
Chris@42 26
Chris@42 27 <meta name="description" content="FFTW 3.3.5: Fixed-size Arrays in C">
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Chris@42 35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
Chris@42 36 <link href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" rel="up" title="Multi-dimensional Array Format">
Chris@42 37 <link href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C" rel="next" title="Dynamic Arrays in C">
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Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="Fixed_002dsize-Arrays-in-C"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 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>
Chris@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="Fixed_002dsize-Arrays-in-C-1"></a>
Chris@42 79 <h4 class="subsection">3.2.3 Fixed-size Arrays in C</h4>
Chris@42 80 <a name="index-C-multi_002ddimensional-arrays"></a>
Chris@42 81
Chris@42 82 <p>A multi-dimensional array whose size is declared at compile time in C
Chris@42 83 is <em>already</em> in row-major order. You don&rsquo;t have to do anything
Chris@42 84 special to transform it. For example:
Chris@42 85 </p>
Chris@42 86 <div class="example">
Chris@42 87 <pre class="example">{
Chris@42 88 fftw_complex data[N0][N1][N2];
Chris@42 89 fftw_plan plan;
Chris@42 90 ...
Chris@42 91 plan = fftw_plan_dft_3d(N0, N1, N2, &amp;data[0][0][0], &amp;data[0][0][0],
Chris@42 92 FFTW_FORWARD, FFTW_ESTIMATE);
Chris@42 93 ...
Chris@42 94 }
Chris@42 95 </pre></div>
Chris@42 96
Chris@42 97 <p>This will plan a 3d in-place transform of size <code>N0 x N1 x N2</code>.
Chris@42 98 Notice how we took the address of the zero-th element to pass to the
Chris@42 99 planner (we could also have used a typecast).
Chris@42 100 </p>
Chris@42 101 <p>However, we tend to <em>discourage</em> users from declaring their
Chris@42 102 arrays in this way, for two reasons. First, this allocates the array
Chris@42 103 on the stack (&ldquo;automatic&rdquo; storage), which has a very limited size on
Chris@42 104 most operating systems (declaring an array with more than a few
Chris@42 105 thousand elements will often cause a crash). (You can get around this
Chris@42 106 limitation on many systems by declaring the array as
Chris@42 107 <code>static</code> and/or global, but that has its own drawbacks.)
Chris@42 108 Second, it may not optimally align the array for use with a SIMD
Chris@42 109 FFTW (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>). Instead, we recommend
Chris@42 110 using <code>fftw_malloc</code>, as described below.
Chris@42 111 </p>
Chris@42 112
Chris@42 113
Chris@42 114
Chris@42 115 </body>
Chris@42 116 </html>