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1 <html lang="en">
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2 <head>
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3 <title>Upgrading from FFTW version 2 - FFTW 3.2.1</title>
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4 <meta http-equiv="Content-Type" content="text/html">
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5 <meta name="description" content="FFTW 3.2.1">
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7 <link title="Top" rel="start" href="index.html#Top">
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8 <link rel="prev" href="Calling-FFTW-from-Fortran.html#Calling-FFTW-from-Fortran" title="Calling FFTW from Fortran">
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9 <link rel="next" href="Installation-and-Customization.html#Installation-and-Customization" title="Installation and Customization">
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10 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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11 <!--
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12 This manual is for FFTW
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13 (version 3.2.1, 5 February 2009).
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14
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15 Copyright (C) 2003 Matteo Frigo.
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16
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17 Copyright (C) 2003 Massachusetts Institute of Technology.
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18
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19 Permission is granted to make and distribute verbatim copies of
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20 this manual provided the copyright notice and this permission
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21 notice are preserved on all copies.
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22
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23 Permission is granted to copy and distribute modified versions of
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24 this manual under the conditions for verbatim copying, provided
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25 that the entire resulting derived work is distributed under the
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26 terms of a permission notice identical to this one.
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27
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28 Permission is granted to copy and distribute translations of this
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29 manual into another language, under the above conditions for
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30 modified versions, except that this permission notice may be
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31 stated in a translation approved by the Free Software Foundation.
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32 -->
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33 <meta http-equiv="Content-Style-Type" content="text/css">
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34 <style type="text/css"><!--
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44 --></style>
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45 </head>
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46 <body>
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47 <div class="node">
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48 <p>
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49 <a name="Upgrading-from-FFTW-version-2"></a>
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50 Next: <a rel="next" accesskey="n" href="Installation-and-Customization.html#Installation-and-Customization">Installation and Customization</a>,
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51 Previous: <a rel="previous" accesskey="p" href="Calling-FFTW-from-Fortran.html#Calling-FFTW-from-Fortran">Calling FFTW from Fortran</a>,
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52 Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a>
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53 <hr>
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54 </div>
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55
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56 <h2 class="chapter">8 Upgrading from FFTW version 2</h2>
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57
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58 <p>In this chapter, we outline the process for updating codes designed for
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59 the older FFTW 2 interface to work with FFTW 3. The interface for FFTW
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60 3 is not backwards-compatible with the interface for FFTW 2 and earlier
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61 versions; codes written to use those versions will fail to link with
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62 FFTW 3. Nor is it possible to write “compatibility wrappers” to
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63 bridge the gap (at least not efficiently), because FFTW 3 has different
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64 semantics from previous versions. However, upgrading should be a
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65 straightforward process because the data formats are identical and the
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66 overall style of planning/execution is essentially the same.
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67
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68 <p>Unlike FFTW 2, there are no separate header files for real and complex
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69 transforms (or even for different precisions) in FFTW 3; all interfaces
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70 are defined in the <code><fftw3.h></code> header file.
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71
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72 <h3 class="heading">Numeric Types</h3>
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73
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74 <p>The main difference in data types is that <code>fftw_complex</code> in FFTW 2
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75 was defined as a <code>struct</code> with macros <code>c_re</code> and <code>c_im</code>
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76 for accessing the real/imaginary parts. (This is binary-compatible with
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77 FFTW 3 on any machine except perhaps for some older Crays in single
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78 precision.) The equivalent macros for FFTW 3 are:
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79
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80 <pre class="example"> #define c_re(c) ((c)[0])
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81 #define c_im(c) ((c)[1])
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82 </pre>
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83 <p>This does not work if you are using the C99 complex type, however,
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84 unless you insert a <code>double*</code> typecast into the above macros
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85 (see <a href="Complex-numbers.html#Complex-numbers">Complex numbers</a>).
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86
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87 <p>Also, FFTW 2 had an <code>fftw_real</code> typedef that was an alias for
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88 <code>double</code> (in double precision). In FFTW 3 you should just use
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89 <code>double</code> (or whatever precision you are employing).
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90
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91 <h3 class="heading">Plans</h3>
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92
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93 <p>The major difference between FFTW 2 and FFTW 3 is in the
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94 planning/execution division of labor. In FFTW 2, plans were found for a
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95 given transform size and type, and then could be applied to <em>any</em>
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96 arrays and for <em>any</em> multiplicity/stride parameters. In FFTW 3,
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97 you specify the particular arrays, stride parameters, etcetera when
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98 creating the plan, and the plan is then executed for <em>those</em> arrays
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99 (unless the guru interface is used) and <em>those</em> parameters
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100 <em>only</em>. (FFTW 2 had “specific planner” routines that planned for
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101 a particular array and stride, but the plan could still be used for
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102 other arrays and strides.) That is, much of the information that was
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103 formerly specified at execution time is now specified at planning time.
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104
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105 <p>Like FFTW 2's specific planner routines, the FFTW 3 planner overwrites
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106 the input/output arrays unless you use <code>FFTW_ESTIMATE</code>.
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107
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108 <p>FFTW 2 had separate data types <code>fftw_plan</code>, <code>fftwnd_plan</code>,
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109 <code>rfftw_plan</code>, and <code>rfftwnd_plan</code> for complex and real one- and
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110 multi-dimensional transforms, and each type had its own `<samp><span class="samp">destroy</span></samp>'
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111 function. In FFTW 3, all plans are of type <code>fftw_plan</code> and all are
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112 destroyed by <code>fftw_destroy_plan(plan)</code>.
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113
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114 <p>Where you formerly used <code>fftw_create_plan</code> and <code>fftw_one</code> to
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115 plan and compute a single 1d transform, you would now use
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116 <code>fftw_plan_dft_1d</code> to plan the transform. If you used the generic
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117 <code>fftw</code> function to execute the transform with multiplicity
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118 (<code>howmany</code>) and stride parameters, you would now use the advanced
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119 interface <code>fftw_plan_many_dft</code> to specify those parameters. The
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120 plans are now executed with <code>fftw_execute(plan)</code>, which takes all
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121 of its parameters (including the input/output arrays) from the plan.
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122
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123 <p>In-place transforms no longer interpret their output argument as scratch
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124 space, nor is there an <code>FFTW_IN_PLACE</code> flag. You simply pass the
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125 same pointer for both the input and output arguments. (Previously, the
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126 output <code>ostride</code> and <code>odist</code> parameters were ignored for
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127 in-place transforms; now, if they are specified via the advanced
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128 interface, they are significant even in the in-place case, although they
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129 should normally equal the corresponding input parameters.)
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130
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131 <p>The <code>FFTW_ESTIMATE</code> and <code>FFTW_MEASURE</code> flags have the same
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132 meaning as before, although the planning time will differ. You may also
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133 consider using <code>FFTW_PATIENT</code>, which is like <code>FFTW_MEASURE</code>
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134 except that it takes more time in order to consider a wider variety of
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135 algorithms.
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136
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137 <p>For multi-dimensional complex DFTs, instead of <code>fftwnd_create_plan</code>
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138 (or <code>fftw2d_create_plan</code> or <code>fftw3d_create_plan</code>), followed by
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139 <code>fftwnd_one</code>, you would use <code>fftw_plan_dft</code> (or
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140 <code>fftw_plan_dft_2d</code> or <code>fftw_plan_dft_3d</code>). followed by
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141 <code>fftw_execute</code>. If you used <code>fftwnd</code> to to specify strides
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142 etcetera, you would instead specify these via <code>fftw_plan_many_dft</code>.
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143
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144 <p>The analogues to <code>rfftw_create_plan</code> and <code>rfftw_one</code> with
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145 <code>FFTW_REAL_TO_COMPLEX</code> or <code>FFTW_COMPLEX_TO_REAL</code> directions
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146 are <code>fftw_plan_r2r_1d</code> with kind <code>FFTW_R2HC</code> or
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147 <code>FFTW_HC2R</code>, followed by <code>fftw_execute</code>. The stride etcetera
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148 arguments of <code>rfftw</code> are now in <code>fftw_plan_many_r2r</code>.
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149
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150 <p>Instead of <code>rfftwnd_create_plan</code> (or <code>rfftw2d_create_plan</code> or
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151 <code>rfftw3d_create_plan</code>) followed by
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152 <code>rfftwnd_one_real_to_complex</code> or
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153 <code>rfftwnd_one_complex_to_real</code>, you now use <code>fftw_plan_dft_r2c</code>
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154 (or <code>fftw_plan_dft_r2c_2d</code> or <code>fftw_plan_dft_r2c_3d</code>) or
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155 <code>fftw_plan_dft_c2r</code> (or <code>fftw_plan_dft_c2r_2d</code> or
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156 <code>fftw_plan_dft_c2r_3d</code>), respectively, followed by
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157 <code>fftw_execute</code>. As usual, the strides etcetera of
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158 <code>rfftwnd_real_to_complex</code> or <code>rfftwnd_complex_to_real</code> are no
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159 specified in the advanced planner routines,
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160 <code>fftw_plan_many_dft_r2c</code> or <code>fftw_plan_many_dft_c2r</code>.
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161
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162 <h3 class="heading">Wisdom</h3>
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163
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164 <p>In FFTW 2, you had to supply the <code>FFTW_USE_WISDOM</code> flag in order to
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165 use wisdom; in FFTW 3, wisdom is always used. (You could simulate the
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166 FFTW 2 wisdom-less behavior by calling <code>fftw_forget_wisdom</code> after
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167 every planner call.)
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168
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169 <p>The FFTW 3 wisdom import/export routines are almost the same as before
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170 (although the storage format is entirely different). There is one
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171 significant difference, however. In FFTW 2, the import routines would
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172 never read past the end of the wisdom, so you could store extra data
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173 beyond the wisdom in the same file, for example. In FFTW 3, the
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174 file-import routine may read up to a few hundred bytes past the end of
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175 the wisdom, so you cannot store other data just beyond it.<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>
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176
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177 <p>Wisdom has been enhanced by additional humility in FFTW 3: whereas FFTW
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178 2 would re-use wisdom for a given transform size regardless of the
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179 stride etc., in FFTW 3 wisdom is only used with the strides etc. for
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180 which it was created. Unfortunately, this means FFTW 3 has to create
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181 new plans from scratch more often than FFTW 2 (in FFTW 2, planning
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182 e.g. one transform of size 1024 also created wisdom for all smaller
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183 powers of 2, but this no longer occurs).
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184
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185 <p>FFTW 3 also has the new routine <code>fftw_import_system_wisdom</code> to
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186 import wisdom from a standard system-wide location.
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187
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188 <h3 class="heading">Memory allocation</h3>
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189
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190 <p>In FFTW 3, we recommend allocating your arrays with <code>fftw_malloc</code>
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191 and deallocating them with <code>fftw_free</code>; this is not required, but
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192 allows optimal performance when SIMD acceleration is used. (Those two
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193 functions actually existed in FFTW 2, and worked the same way, but were
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194 not documented.)
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195
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196 <p>In FFTW 2, there were <code>fftw_malloc_hook</code> and <code>fftw_free_hook</code>
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197 functions that allowed the user to replace FFTW's memory-allocation
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198 routines (e.g. to implement different error-handling, since by default
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199 FFTW prints an error message and calls <code>exit</code> to abort the program
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200 if <code>malloc</code> returns <code>NULL</code>). These hooks are not supported in
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201 FFTW 3; those few users who require this functionality can just
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202 directly modify the memory-allocation routines in FFTW (they are defined
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203 in <code>kernel/alloc.c</code>).
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204
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205 <h3 class="heading">Fortran interface</h3>
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206
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207 <p>In FFTW 2, the subroutine names were obtained by replacing `<samp><span class="samp">fftw_</span></samp>'
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208 with `<samp><span class="samp">fftw_f77</span></samp>'; in FFTW 3, you replace `<samp><span class="samp">fftw_</span></samp>' with
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209 `<samp><span class="samp">dfftw_</span></samp>' (or `<samp><span class="samp">sfftw_</span></samp>' or `<samp><span class="samp">lfftw_</span></samp>', depending upon the
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210 precision).
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211
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212 <p>In FFTW 3, we have begun recommending that you always declare the type
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213 used to store plans as <code>integer*8</code>. (Too many people didn't notice
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214 our instruction to switch from <code>integer</code> to <code>integer*8</code> for
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215 64-bit machines.)
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216
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217 <p>In FFTW 3, we provide a <code>fftw3.f</code> “header file” to include in
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218 your code (and which is officially installed on Unix systems). (In FFTW
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219 2, we supplied a <code>fftw_f77.i</code> file, but it was not installed.)
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220
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221 <p>Otherwise, the C-Fortran interface relationship is much the same as it
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222 was before (e.g. return values become initial parameters, and
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223 multi-dimensional arrays are in column-major order). Unlike FFTW 2, we
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224 do provide some support for wisdom import/export in Fortran
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225 (see <a href="Wisdom-of-Fortran_003f.html#Wisdom-of-Fortran_003f">Wisdom of Fortran?</a>).
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226
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227 <h3 class="heading">Threads</h3>
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228
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229 <p>Like FFTW 2, only the execution routines are thread-safe. All planner
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230 routines, etcetera, should be called by only a single thread at a time
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231 (see <a href="Thread-safety.html#Thread-safety">Thread safety</a>). <em>Unlike</em> FFTW 2, there is no special
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232 <code>FFTW_THREADSAFE</code> flag for the planner to allow a given plan to be
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233 usable by multiple threads in parallel; this is now the case by default.
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234
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235 <p>The multi-threaded version of FFTW 2 required you to pass the number of
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236 threads each time you execute the transform. The number of threads is
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237 now stored in the plan, and is specified before the planner is called by
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238 <code>fftw_plan_with_nthreads</code>. The threads initialization routine used
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239 to be called <code>fftw_threads_init</code> and would return zero on success;
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240 the new routine is called <code>fftw_init_threads</code> and returns zero on
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241 failure. See <a href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW">Multi-threaded FFTW</a>.
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242
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243 <p>There is no separate threads header file in FFTW 3; all the function
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244 prototypes are in <code><fftw3.h></code>. However, you still have to link to
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245 a separate library (<code>-lfftw3_threads -lfftw3 -lm</code> on Unix), as well as
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246 to the threading library (e.g. POSIX threads on Unix).
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247
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248 <!-- ************************************************************ -->
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249 <div class="footnote">
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250 <hr>
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251 <h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> We
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252 do our own buffering because GNU libc I/O routines are horribly slow for
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253 single-character I/O, apparently for thread-safety reasons (whether you
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254 are using threads or not).</p>
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255
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256 <p><hr></div>
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257
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258 </body></html>
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259
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