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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
Chris@82 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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Chris@82 24 <head>
Chris@82 25 <title>FFTW 3.3.8: Dynamic Arrays in C-The Wrong Way</title>
Chris@82 26
Chris@82 27 <meta name="description" content="FFTW 3.3.8: Dynamic Arrays in C-The Wrong Way">
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Chris@82 35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans" rel="next" title="Words of Wisdom-Saving Plans">
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Chris@82 68 </head>
Chris@82 69
Chris@82 70 <body lang="en">
Chris@82 71 <a name="Dynamic-Arrays-in-C_002dThe-Wrong-Way"></a>
Chris@82 72 <div class="header">
Chris@82 73 <p>
Chris@82 74 Previous: <a href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C" accesskey="p" rel="prev">Dynamic Arrays in C</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="Dynamic-Arrays-in-C_002d_002d_002dThe-Wrong-Way"></a>
Chris@82 78 <h4 class="subsection">3.2.5 Dynamic Arrays in C&mdash;The Wrong Way</h4>
Chris@82 79
Chris@82 80 <p>A different method for allocating multi-dimensional arrays in C is
Chris@82 81 often suggested that is incompatible with FFTW: <em>using it will
Chris@82 82 cause FFTW to die a painful death</em>. We discuss the technique here,
Chris@82 83 however, because it is so commonly known and used. This method is to
Chris@82 84 create arrays of pointers of arrays of pointers of &hellip;etcetera.
Chris@82 85 For example, the analogue in this method to the example above is:
Chris@82 86 </p>
Chris@82 87 <div class="example">
Chris@82 88 <pre class="example">int i,j;
Chris@82 89 fftw_complex ***a_bad_array; /* <span class="roman">another way to make a 5x12x27 array</span> */
Chris@82 90
Chris@82 91 a_bad_array = (fftw_complex ***) malloc(5 * sizeof(fftw_complex **));
Chris@82 92 for (i = 0; i &lt; 5; ++i) {
Chris@82 93 a_bad_array[i] =
Chris@82 94 (fftw_complex **) malloc(12 * sizeof(fftw_complex *));
Chris@82 95 for (j = 0; j &lt; 12; ++j)
Chris@82 96 a_bad_array[i][j] =
Chris@82 97 (fftw_complex *) malloc(27 * sizeof(fftw_complex));
Chris@82 98 }
Chris@82 99 </pre></div>
Chris@82 100
Chris@82 101 <p>As you can see, this sort of array is inconvenient to allocate (and
Chris@82 102 deallocate). On the other hand, it has the advantage that the
Chris@82 103 <em>(i,j,k)</em>-th element can be referenced simply by
Chris@82 104 <code>a_bad_array[i][j][k]</code>.
Chris@82 105 </p>
Chris@82 106 <p>If you like this technique and want to maximize convenience in accessing
Chris@82 107 the array, but still want to pass the array to FFTW, you can use a
Chris@82 108 hybrid method. Allocate the array as one contiguous block, but also
Chris@82 109 declare an array of arrays of pointers that point to appropriate places
Chris@82 110 in the block. That sort of trick is beyond the scope of this
Chris@82 111 documentation; for more information on multi-dimensional arrays in C,
Chris@82 112 see the <code>comp.lang.c</code>
Chris@82 113 <a href="http://c-faq.com/aryptr/dynmuldimary.html">FAQ</a>.
Chris@82 114 </p>
Chris@82 115
Chris@82 116
Chris@82 117
Chris@82 118 </body>
Chris@82 119 </html>