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1 <html lang="en">
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2 <head>
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3 <title>Overview of Fortran interface - 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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7 <link title="Top" rel="start" href="index.html#Top">
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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="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" title="Calling FFTW from Modern Fortran">
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10 <link rel="next" href="Reversing-array-dimensions.html#Reversing-array-dimensions" title="Reversing array dimensions">
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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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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="Overview-of-Fortran-interface"></a>
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50 <p>
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51 Next: <a rel="next" accesskey="n" href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a>,
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52 Previous: <a rel="previous" accesskey="p" href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran">Calling FFTW from Modern 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">7.1 Overview of Fortran interface</h3>
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58
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59 <p>FFTW provides a file <code>fftw3.f03</code> that defines Fortran 2003
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60 interfaces for all of its C routines, except for the MPI routines
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61 described elsewhere, which can be found in the same directory as
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62 <code>fftw3.h</code> (the C header file). In any Fortran subroutine where
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63 you want to use FFTW functions, you should begin with:
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64
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65 <p><a name="index-iso_005fc_005fbinding-502"></a>
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66 <pre class="example"> use, intrinsic :: iso_c_binding
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67 include 'fftw3.f03'
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68 </pre>
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69 <p>This includes the interface definitions and the standard
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70 <code>iso_c_binding</code> module (which defines the equivalents of C
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71 types). You can also put the FFTW functions into a module if you
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72 prefer (see <a href="Defining-an-FFTW-module.html#Defining-an-FFTW-module">Defining an FFTW module</a>).
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73
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74 <p>At this point, you can now call anything in the FFTW C interface
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75 directly, almost exactly as in C other than minor changes in syntax.
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76 For example:
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77
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78 <p><a name="index-fftw_005fplan_005fdft_005f2d-503"></a><a name="index-fftw_005fexecute_005fdft-504"></a><a name="index-fftw_005fdestroy_005fplan-505"></a>
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79 <pre class="example"> type(C_PTR) :: plan
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80 complex(C_DOUBLE_COMPLEX), dimension(1024,1000) :: in, out
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81 plan = fftw_plan_dft_2d(1000,1024, in,out, FFTW_FORWARD,FFTW_ESTIMATE)
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82 ...
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83 call fftw_execute_dft(plan, in, out)
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84 ...
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85 call fftw_destroy_plan(plan)
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86 </pre>
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87 <p>A few important things to keep in mind are:
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88
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89 <ul>
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90 <li><a name="index-fftw_005fcomplex-506"></a><a name="index-C_005fPTR-507"></a><a name="index-C_005fINT-508"></a><a name="index-C_005fDOUBLE-509"></a><a name="index-C_005fDOUBLE_005fCOMPLEX-510"></a>FFTW plans are <code>type(C_PTR)</code>. Other C types are mapped in the
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91 obvious way via the <code>iso_c_binding</code> standard: <code>int</code> turns
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92 into <code>integer(C_INT)</code>, <code>fftw_complex</code> turns into
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93 <code>complex(C_DOUBLE_COMPLEX)</code>, <code>double</code> turns into
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94 <code>real(C_DOUBLE)</code>, and so on. See <a href="FFTW-Fortran-type-reference.html#FFTW-Fortran-type-reference">FFTW Fortran type reference</a>.
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95
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96 <li>Functions in C become functions in Fortran if they have a return value,
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97 and subroutines in Fortran otherwise.
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98
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99 <li>The ordering of the Fortran array dimensions must be <em>reversed</em>
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100 when they are passed to the FFTW plan creation, thanks to differences
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101 in array indexing conventions (see <a href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a>). This is <em>unlike</em> the legacy Fortran interface
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102 (see <a href="Fortran_002dinterface-routines.html#Fortran_002dinterface-routines">Fortran-interface routines</a>), which reversed the dimensions
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103 for you. See <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a>.
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104
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105 <li><a name="index-alignment-511"></a><a name="index-SIMD-512"></a>Using ordinary Fortran array declarations like this works, but may
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106 yield suboptimal performance because the data may not be not aligned
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107 to exploit SIMD instructions on modern proessors (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>). Better performance will often be obtained
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108 by allocating with ‘<samp><span class="samp">fftw_alloc</span></samp>’. See <a href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran">Allocating aligned memory in Fortran</a>.
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109
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110 <li><a name="index-fftw_005fexecute-513"></a>Similar to the legacy Fortran interface (see <a href="FFTW-Execution-in-Fortran.html#FFTW-Execution-in-Fortran">FFTW Execution in Fortran</a>), we currently recommend <em>not</em> using <code>fftw_execute</code>
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111 but rather using the more specialized functions like
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112 <code>fftw_execute_dft</code> (see <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>).
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113 However, you should execute the plan on the <code>same arrays</code> as the
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114 ones for which you created the plan, unless you are especially
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115 careful. See <a href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran">Plan execution in Fortran</a>. To prevent
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116 you from using <code>fftw_execute</code> by mistake, the <code>fftw3.f03</code>
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117 file does not provide an <code>fftw_execute</code> interface declaration.
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118
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119 <li><a name="index-flags-514"></a>Multiple planner flags are combined with <code>ior</code> (equivalent to ‘<samp><span class="samp">|</span></samp>’ in C). e.g. <code>FFTW_MEASURE | FFTW_DESTROY_INPUT</code> becomes <code>ior(FFTW_MEASURE, FFTW_DESTROY_INPUT)</code>. (You can also use ‘<samp><span class="samp">+</span></samp>’ as long as you don't try to include a given flag more than once.)
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120
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121 </ul>
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122
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123 <ul class="menu">
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124 <li><a accesskey="1" href="Extended-and-quadruple-precision-in-Fortran.html#Extended-and-quadruple-precision-in-Fortran">Extended and quadruple precision in Fortran</a>
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125 </ul>
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126
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127 </body></html>
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128
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