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1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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2 <html>
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3 <!-- This manual is for FFTW
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4 (version 3.3.8, 24 May 2018).
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5
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6 Copyright (C) 2003 Matteo Frigo.
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7
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8 Copyright (C) 2003 Massachusetts Institute of Technology.
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9
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10 Permission is granted to make and distribute verbatim copies of this
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11 manual provided the copyright notice and this permission notice are
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12 preserved on all copies.
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13
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14 Permission is granted to copy and distribute modified versions of this
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15 manual under the conditions for verbatim copying, provided that the
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16 entire resulting derived work is distributed under the terms of a
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17 permission notice identical to this one.
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18
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19 Permission is granted to copy and distribute translations of this manual
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20 into another language, under the above conditions for modified versions,
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21 except that this permission notice may be stated in a translation
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22 approved by the Free Software Foundation. -->
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23 <!-- Created by GNU Texinfo 6.3, http://www.gnu.org/software/texinfo/ -->
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24 <head>
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25 <title>FFTW 3.3.8: Allocating aligned memory in Fortran</title>
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26
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27 <meta name="description" content="FFTW 3.3.8: Allocating aligned memory in Fortran">
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28 <meta name="keywords" content="FFTW 3.3.8: Allocating aligned memory in Fortran">
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33 <link href="index.html#Top" rel="start" title="Top">
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34 <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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36 <link href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" rel="up" title="Calling FFTW from Modern Fortran">
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37 <link href="Accessing-the-wisdom-API-from-Fortran.html#Accessing-the-wisdom-API-from-Fortran" rel="next" title="Accessing the wisdom API from Fortran">
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38 <link href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran" rel="prev" title="Plan execution in Fortran">
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64 -->
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65 </style>
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66
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67
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68 </head>
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69
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70 <body lang="en">
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71 <a name="Allocating-aligned-memory-in-Fortran"></a>
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72 <div class="header">
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73 <p>
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74 Next: <a href="Accessing-the-wisdom-API-from-Fortran.html#Accessing-the-wisdom-API-from-Fortran" accesskey="n" rel="next">Accessing the wisdom API from Fortran</a>, Previous: <a href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran" accesskey="p" rel="prev">Plan execution in Fortran</a>, Up: <a href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" accesskey="u" rel="up">Calling FFTW from Modern Fortran</a> [<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>
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75 </div>
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76 <hr>
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77 <a name="Allocating-aligned-memory-in-Fortran-1"></a>
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78 <h3 class="section">7.5 Allocating aligned memory in Fortran</h3>
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79
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80 <a name="index-alignment-5"></a>
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81 <a name="index-fftw_005falloc_005freal-5"></a>
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82 <a name="index-fftw_005falloc_005fcomplex-5"></a>
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83 <p>In order to obtain maximum performance in FFTW, you should store your
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84 data in arrays that have been specially aligned in memory (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>). Enforcing alignment also permits you to
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85 safely use the new-array execute functions (see <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>) to apply a given plan to more than one pair of in/out
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86 arrays. Unfortunately, standard Fortran arrays do <em>not</em> provide
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87 any alignment guarantees. The <em>only</em> way to allocate aligned
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88 memory in standard Fortran is to allocate it with an external C
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89 function, like the <code>fftw_alloc_real</code> and
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90 <code>fftw_alloc_complex</code> functions. Fortunately, Fortran 2003 provides
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91 a simple way to associate such allocated memory with a standard Fortran
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92 array pointer that you can then use normally.
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93 </p>
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94 <p>We therefore recommend allocating all your input/output arrays using
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95 the following technique:
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96 </p>
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97 <ol>
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98 <li> Declare a <code>pointer</code>, <code>arr</code>, to your array of the desired type
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99 and dimensions. For example, <code>real(C_DOUBLE), pointer :: a(:,:)</code>
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100 for a 2d real array, or <code>complex(C_DOUBLE_COMPLEX), pointer ::
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101 a(:,:,:)</code> for a 3d complex array.
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102
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103 </li><li> The number of elements to allocate must be an
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104 <code>integer(C_SIZE_T)</code>. You can either declare a variable of this
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105 type, e.g. <code>integer(C_SIZE_T) :: sz</code>, to store the number of
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106 elements to allocate, or you can use the <code>int(..., C_SIZE_T)</code>
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107 intrinsic function. e.g. set <code>sz = L * M * N</code> or use
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108 <code>int(L * M * N, C_SIZE_T)</code> for an L × M × N
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109 array.
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110
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111 </li><li> Declare a <code>type(C_PTR) :: p</code> to hold the return value from
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112 FFTW’s allocation routine. Set <code>p = fftw_alloc_real(sz)</code> for a real array, or <code>p = fftw_alloc_complex(sz)</code> for a complex array.
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113
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114 </li><li> <a name="index-c_005ff_005fpointer-2"></a>
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115 Associate your pointer <code>arr</code> with the allocated memory <code>p</code>
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116 using the standard <code>c_f_pointer</code> subroutine: <code>call
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117 c_f_pointer(p, arr, [...dimensions...])</code>, where
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118 <code>[...dimensions...])</code> are an array of the dimensions of the array
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119 (in the usual Fortran order). e.g. <code>call c_f_pointer(p, arr,
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120 [L,M,N])</code> for an L × M × N
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121 array. (Alternatively, you can
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122 omit the dimensions argument if you specified the shape explicitly
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123 when declaring <code>arr</code>.) You can now use <code>arr</code> as a usual
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124 multidimensional array.
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125
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126 </li><li> When you are done using the array, deallocate the memory by <code>call
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127 fftw_free(p)</code> on <code>p</code>.
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128
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129 </li></ol>
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130
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131 <p>For example, here is how we would allocate an L × M
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132 2d real array:
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133 </p>
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134 <div class="example">
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135 <pre class="example"> real(C_DOUBLE), pointer :: arr(:,:)
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136 type(C_PTR) :: p
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137 p = fftw_alloc_real(int(L * M, C_SIZE_T))
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138 call c_f_pointer(p, arr, [L,M])
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139 <em>...use arr and arr(i,j) as usual...</em>
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140 call fftw_free(p)
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141 </pre></div>
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142
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143 <p>and here is an L × M × N
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144 3d complex array:
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145 </p>
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146 <div class="example">
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147 <pre class="example"> complex(C_DOUBLE_COMPLEX), pointer :: arr(:,:,:)
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148 type(C_PTR) :: p
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149 p = fftw_alloc_complex(int(L * M * N, C_SIZE_T))
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150 call c_f_pointer(p, arr, [L,M,N])
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151 <em>...use arr and arr(i,j,k) as usual...</em>
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152 call fftw_free(p)
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153 </pre></div>
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154
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155 <p>See <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a> for an example allocating a
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156 single array and associating both real and complex array pointers with
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157 it, for in-place real-to-complex transforms.
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158 </p>
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159 <hr>
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160 <div class="header">
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161 <p>
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162 Next: <a href="Accessing-the-wisdom-API-from-Fortran.html#Accessing-the-wisdom-API-from-Fortran" accesskey="n" rel="next">Accessing the wisdom API from Fortran</a>, Previous: <a href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran" accesskey="p" rel="prev">Plan execution in Fortran</a>, Up: <a href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" accesskey="u" rel="up">Calling FFTW from Modern Fortran</a> [<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>
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163 </div>
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164
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165
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166
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167 </body>
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168 </html>
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