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104 lines
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HTML
104 lines
4.5 KiB
HTML
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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preserved on all copies.
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Permission is granted to copy and distribute modified versions of this
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manual under the conditions for verbatim copying, provided that the
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entire resulting derived work is distributed under the terms of a
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permission notice identical to this one.
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Permission is granted to copy and distribute translations of this manual
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approved by the Free Software Foundation. -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<title>Fixed-size Arrays in C (FFTW 3.3.10)</title>
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<meta name="description" content="Fixed-size Arrays in C (FFTW 3.3.10)">
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Multi_002ddimensional-Array-Format.html" rel="up" title="Multi-dimensional Array Format">
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<link href="Dynamic-Arrays-in-C.html" rel="next" title="Dynamic Arrays in C">
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<span id="Fixed_002dsize-Arrays-in-C"></span><div class="header">
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<p>
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Next: <a href="Dynamic-Arrays-in-C.html" accesskey="n" rel="next">Dynamic Arrays in C</a>, Previous: <a href="Column_002dmajor-Format.html" accesskey="p" rel="prev">Column-major Format</a>, Up: <a href="Multi_002ddimensional-Array-Format.html" accesskey="u" rel="up">Multi-dimensional Array Format</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="Fixed_002dsize-Arrays-in-C-1"></span><h4 class="subsection">3.2.3 Fixed-size Arrays in C</h4>
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<span id="index-C-multi_002ddimensional-arrays"></span>
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<p>A multi-dimensional array whose size is declared at compile time in C
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is <em>already</em> in row-major order. You don’t have to do anything
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special to transform it. For example:
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</p>
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<div class="example">
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<pre class="example">{
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fftw_complex data[N0][N1][N2];
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fftw_plan plan;
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...
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plan = fftw_plan_dft_3d(N0, N1, N2, &data[0][0][0], &data[0][0][0],
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FFTW_FORWARD, FFTW_ESTIMATE);
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...
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}
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</pre></div>
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<p>This will plan a 3d in-place transform of size <code>N0 x N1 x N2</code>.
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Notice how we took the address of the zero-th element to pass to the
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planner (we could also have used a typecast).
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</p>
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<p>However, we tend to <em>discourage</em> users from declaring their
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arrays in this way, for two reasons. First, this allocates the array
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on the stack (“automatic” storage), which has a very limited size on
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most operating systems (declaring an array with more than a few
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thousand elements will often cause a crash). (You can get around this
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limitation on many systems by declaring the array as
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<code>static</code> and/or global, but that has its own drawbacks.)
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Second, it may not optimally align the array for use with a SIMD
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FFTW (see <a href="SIMD-alignment-and-fftw_005fmalloc.html">SIMD alignment and fftw_malloc</a>). Instead, we recommend
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using <code>fftw_malloc</code>, as described below.
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</p>
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</body>
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</html>
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