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1 <html lang="en">
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2 <head>
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3 <title>Fixed-size Arrays in C - FFTW 3.3.3</title>
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4 <meta http-equiv="Content-Type" content="text/html">
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5 <meta name="description" content="FFTW 3.3.3">
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6 <meta name="generator" content="makeinfo 4.13">
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7 <link title="Top" rel="start" href="index.html#Top">
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8 <link rel="up" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" title="Multi-dimensional Array Format">
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9 <link rel="prev" href="Column_002dmajor-Format.html#Column_002dmajor-Format" title="Column-major Format">
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10 <link rel="next" href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C" title="Dynamic Arrays in C">
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11 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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12 <!--
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13 This manual is for FFTW
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14 (version 3.3.3, 25 November 2012).
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15
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16 Copyright (C) 2003 Matteo Frigo.
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17
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18 Copyright (C) 2003 Massachusetts Institute of Technology.
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19
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20 Permission is granted to make and distribute verbatim copies of
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21 this manual provided the copyright notice and this permission
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22 notice are preserved on all copies.
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23
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24 Permission is granted to copy and distribute modified versions of
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25 this manual under the conditions for verbatim copying, provided
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26 that the entire resulting derived work is distributed under the
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27 terms of a permission notice identical to this one.
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28
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29 Permission is granted to copy and distribute translations of this
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30 manual into another language, under the above conditions for
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31 modified versions, except that this permission notice may be
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32 stated in a translation approved by the Free Software Foundation.
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33 -->
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35 <style type="text/css"><!--
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44 span.sansserif { font-family:sans-serif; font-weight:normal; }
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45 --></style>
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46 </head>
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47 <body>
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48 <div class="node">
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49 <a name="Fixed-size-Arrays-in-C"></a>
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50 <a name="Fixed_002dsize-Arrays-in-C"></a>
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51 <p>
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52 Next: <a rel="next" accesskey="n" href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C">Dynamic Arrays in C</a>,
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53 Previous: <a rel="previous" accesskey="p" href="Column_002dmajor-Format.html#Column_002dmajor-Format">Column-major Format</a>,
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54 Up: <a rel="up" accesskey="u" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a>
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55 <hr>
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56 </div>
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57
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58 <h4 class="subsection">3.2.3 Fixed-size Arrays in C</h4>
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59
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60 <p><a name="index-C-multi_002ddimensional-arrays-120"></a>
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61 A multi-dimensional array whose size is declared at compile time in C
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62 is <em>already</em> in row-major order. You don't have to do anything
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63 special to transform it. For example:
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64
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65 <pre class="example"> {
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66 fftw_complex data[N0][N1][N2];
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67 fftw_plan plan;
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68 ...
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69 plan = fftw_plan_dft_3d(N0, N1, N2, &data[0][0][0], &data[0][0][0],
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70 FFTW_FORWARD, FFTW_ESTIMATE);
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71 ...
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72 }
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73 </pre>
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74 <p>This will plan a 3d in-place transform of size <code>N0 x N1 x N2</code>.
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75 Notice how we took the address of the zero-th element to pass to the
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76 planner (we could also have used a typecast).
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77
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78 <p>However, we tend to <em>discourage</em> users from declaring their
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79 arrays in this way, for two reasons. First, this allocates the array
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80 on the stack (“automatic” storage), which has a very limited size on
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81 most operating systems (declaring an array with more than a few
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82 thousand elements will often cause a crash). (You can get around this
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83 limitation on many systems by declaring the array as
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84 <code>static</code> and/or global, but that has its own drawbacks.)
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85 Second, it may not optimally align the array for use with a SIMD
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86 FFTW (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>). Instead, we recommend
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87 using <code>fftw_malloc</code>, as described below.
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88
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89 <!-- =========> -->
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90 </body></html>
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91
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