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Replace these with versions built using an older toolset (so as to avoid ABI compatibilities when linking on Ubuntu 14.04 for packaging purposes)
author Chris Cannam
date Fri, 07 Feb 2020 11:51:13 +0000
parents d0c2a83c1364
children
rev   line source
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Chris@82 24 <head>
Chris@82 25 <title>FFTW 3.3.8: Fixed-size Arrays in C</title>
Chris@82 26
Chris@82 27 <meta name="description" content="FFTW 3.3.8: Fixed-size Arrays in C">
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Chris@82 36 <link href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" rel="up" title="Multi-dimensional Array Format">
Chris@82 37 <link href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C" rel="next" title="Dynamic Arrays in C">
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Chris@82 71 <a name="Fixed_002dsize-Arrays-in-C"></a>
Chris@82 72 <div class="header">
Chris@82 73 <p>
Chris@82 74 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@82 75 </div>
Chris@82 76 <hr>
Chris@82 77 <a name="Fixed_002dsize-Arrays-in-C-1"></a>
Chris@82 78 <h4 class="subsection">3.2.3 Fixed-size Arrays in C</h4>
Chris@82 79 <a name="index-C-multi_002ddimensional-arrays"></a>
Chris@82 80
Chris@82 81 <p>A multi-dimensional array whose size is declared at compile time in C
Chris@82 82 is <em>already</em> in row-major order. You don&rsquo;t have to do anything
Chris@82 83 special to transform it. For example:
Chris@82 84 </p>
Chris@82 85 <div class="example">
Chris@82 86 <pre class="example">{
Chris@82 87 fftw_complex data[N0][N1][N2];
Chris@82 88 fftw_plan plan;
Chris@82 89 ...
Chris@82 90 plan = fftw_plan_dft_3d(N0, N1, N2, &amp;data[0][0][0], &amp;data[0][0][0],
Chris@82 91 FFTW_FORWARD, FFTW_ESTIMATE);
Chris@82 92 ...
Chris@82 93 }
Chris@82 94 </pre></div>
Chris@82 95
Chris@82 96 <p>This will plan a 3d in-place transform of size <code>N0 x N1 x N2</code>.
Chris@82 97 Notice how we took the address of the zero-th element to pass to the
Chris@82 98 planner (we could also have used a typecast).
Chris@82 99 </p>
Chris@82 100 <p>However, we tend to <em>discourage</em> users from declaring their
Chris@82 101 arrays in this way, for two reasons. First, this allocates the array
Chris@82 102 on the stack (&ldquo;automatic&rdquo; storage), which has a very limited size on
Chris@82 103 most operating systems (declaring an array with more than a few
Chris@82 104 thousand elements will often cause a crash). (You can get around this
Chris@82 105 limitation on many systems by declaring the array as
Chris@82 106 <code>static</code> and/or global, but that has its own drawbacks.)
Chris@82 107 Second, it may not optimally align the array for use with a SIMD
Chris@82 108 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@82 109 using <code>fftw_malloc</code>, as described below.
Chris@82 110 </p>
Chris@82 111
Chris@82 112
Chris@82 113
Chris@82 114 </body>
Chris@82 115 </html>