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3 <title>Allocating aligned memory in Fortran - FFTW 3.3.4</title>
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5 <meta name="description" content="FFTW 3.3.4">
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8 <link rel="up" href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" title="Calling FFTW from Modern Fortran">
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9 <link rel="prev" href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran" title="Plan execution in Fortran">
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10 <link rel="next" href="Accessing-the-wisdom-API-from-Fortran.html#Accessing-the-wisdom-API-from-Fortran" title="Accessing the wisdom API from Fortran">
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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 3.3.4, 20 September 2013).
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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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47 <body>
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48 <div class="node">
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49 <a name="Allocating-aligned-memory-in-Fortran"></a>
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50 <p>
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51 Next: <a rel="next" accesskey="n" href="Accessing-the-wisdom-API-from-Fortran.html#Accessing-the-wisdom-API-from-Fortran">Accessing the wisdom API from Fortran</a>,
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52 Previous: <a rel="previous" accesskey="p" href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran">Plan execution in Fortran</a>,
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53 Up: <a rel="up" accesskey="u" href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran">Calling FFTW from Modern Fortran</a>
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54 <hr>
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55 </div>
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56
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57 <h3 class="section">7.5 Allocating aligned memory in Fortran</h3>
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58
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59 <p><a name="index-alignment-563"></a><a name="index-fftw_005falloc_005freal-564"></a><a name="index-fftw_005falloc_005fcomplex-565"></a>In order to obtain maximum performance in FFTW, you should store your
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60 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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61 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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62 arrays. Unfortunately, standard Fortran arrays do <em>not</em> provide
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63 any alignment guarantees. The <em>only</em> way to allocate aligned
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64 memory in standard Fortran is to allocate it with an external C
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65 function, like the <code>fftw_alloc_real</code> and
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66 <code>fftw_alloc_complex</code> functions. Fortunately, Fortran 2003 provides
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67 a simple way to associate such allocated memory with a standard Fortran
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68 array pointer that you can then use normally.
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69
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70 <p>We therefore recommend allocating all your input/output arrays using
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71 the following technique:
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72
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73 <ol type=1 start=1>
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74
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75 <li>Declare a <code>pointer</code>, <code>arr</code>, to your array of the desired type
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76 and dimensions. For example, <code>real(C_DOUBLE), pointer :: a(:,:)</code>
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77 for a 2d real array, or <code>complex(C_DOUBLE_COMPLEX), pointer ::
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78 a(:,:,:)</code> for a 3d complex array.
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79
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80 <li>The number of elements to allocate must be an
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81 <code>integer(C_SIZE_T)</code>. You can either declare a variable of this
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82 type, e.g. <code>integer(C_SIZE_T) :: sz</code>, to store the number of
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83 elements to allocate, or you can use the <code>int(..., C_SIZE_T)</code>
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84 intrinsic function. e.g. set <code>sz = L * M * N</code> or use
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85 <code>int(L * M * N, C_SIZE_T)</code> for an L × M × N array.
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86
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87 <li>Declare a <code>type(C_PTR) :: p</code> to hold the return value from
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88 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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89
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90 <li><a name="index-c_005ff_005fpointer-566"></a>Associate your pointer <code>arr</code> with the allocated memory <code>p</code>
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91 using the standard <code>c_f_pointer</code> subroutine: <code>call
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92 c_f_pointer(p, arr, [...dimensions...])</code>, where
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93 <code>[...dimensions...])</code> are an array of the dimensions of the array
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94 (in the usual Fortran order). e.g. <code>call c_f_pointer(p, arr,
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95 [L,M,N])</code> for an L × M × N array. (Alternatively, you can
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96 omit the dimensions argument if you specified the shape explicitly
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97 when declaring <code>arr</code>.) You can now use <code>arr</code> as a usual
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98 multidimensional array.
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99
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100 <li>When you are done using the array, deallocate the memory by <code>call
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101 fftw_free(p)</code> on <code>p</code>.
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102
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103 </ol>
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104
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105 <p>For example, here is how we would allocate an L × M 2d real array:
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106
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107 <pre class="example"> real(C_DOUBLE), pointer :: arr(:,:)
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108 type(C_PTR) :: p
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109 p = fftw_alloc_real(int(L * M, C_SIZE_T))
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110 call c_f_pointer(p, arr, [L,M])
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111 <em>...use arr and arr(i,j) as usual...</em>
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112 call fftw_free(p)
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113 </pre>
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114 <p>and here is an L × M × N 3d complex array:
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115
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116 <pre class="example"> complex(C_DOUBLE_COMPLEX), pointer :: arr(:,:,:)
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117 type(C_PTR) :: p
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118 p = fftw_alloc_complex(int(L * M * N, C_SIZE_T))
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119 call c_f_pointer(p, arr, [L,M,N])
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120 <em>...use arr and arr(i,j,k) as usual...</em>
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121 call fftw_free(p)
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122 </pre>
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123 <p>See <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a> for an example allocating a
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124 single array and associating both real and complex array pointers with
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125 it, for in-place real-to-complex transforms.
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126
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127 <!-- -->
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128 </body></html>
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129
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