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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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3 <!-- This manual is for FFTW
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25 <title>FFTW 3.3.5: Fixed-size Arrays in C</title>
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36 <link href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" rel="up" title="Multi-dimensional Array Format">
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72 <a name="Fixed_002dsize-Arrays-in-C"></a>
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74 <p>
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>
76 </div>
77 <hr>
78 <a name="Fixed_002dsize-Arrays-in-C-1"></a>
79 <h4 class="subsection">3.2.3 Fixed-size Arrays in C</h4>
80 <a name="index-C-multi_002ddimensional-arrays"></a>
81
82 <p>A multi-dimensional array whose size is declared at compile time in C
83 is <em>already</em> in row-major order. You don&rsquo;t have to do anything
84 special to transform it. For example:
85 </p>
86 <div class="example">
87 <pre class="example">{
88 fftw_complex data[N0][N1][N2];
89 fftw_plan plan;
90 ...
91 plan = fftw_plan_dft_3d(N0, N1, N2, &amp;data[0][0][0], &amp;data[0][0][0],
92 FFTW_FORWARD, FFTW_ESTIMATE);
93 ...
94 }
95 </pre></div>
96
97 <p>This will plan a 3d in-place transform of size <code>N0 x N1 x N2</code>.
98 Notice how we took the address of the zero-th element to pass to the
99 planner (we could also have used a typecast).
100 </p>
101 <p>However, we tend to <em>discourage</em> users from declaring their
102 arrays in this way, for two reasons. First, this allocates the array
103 on the stack (&ldquo;automatic&rdquo; storage), which has a very limited size on
104 most operating systems (declaring an array with more than a few
105 thousand elements will often cause a crash). (You can get around this
106 limitation on many systems by declaring the array as
107 <code>static</code> and/or global, but that has its own drawbacks.)
108 Second, it may not optimally align the array for use with a SIMD
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
110 using <code>fftw_malloc</code>, as described below.
111 </p>
112
113
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