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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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Chris@42 3 <!-- This manual is for FFTW
Chris@42 4 (version 3.3.5, 30 July 2016).
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Chris@42 6 Copyright (C) 2003 Matteo Frigo.
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Chris@42 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: Fortran-interface routines</title>
Chris@42 26
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Chris@42 36 <link href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" rel="up" title="Calling FFTW from Legacy Fortran">
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Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="Fortran_002dinterface-routines"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 75 Next: <a href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran" accesskey="n" rel="next">FFTW Constants in Fortran</a>, Previous: <a href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" accesskey="p" rel="prev">Calling FFTW from Legacy Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" accesskey="u" rel="up">Calling FFTW from Legacy Fortran</a> &nbsp; [<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>
Chris@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="Fortran_002dinterface-routines-1"></a>
Chris@42 79 <h3 class="section">8.1 Fortran-interface routines</h3>
Chris@42 80
Chris@42 81 <p>Nearly all of the FFTW functions have Fortran-callable equivalents.
Chris@42 82 The name of the legacy Fortran routine is the same as that of the
Chris@42 83 corresponding C routine, but with the &lsquo;<samp>fftw_</samp>&rsquo; prefix replaced by
Chris@42 84 &lsquo;<samp>dfftw_</samp>&rsquo;.<a name="DOCF9" href="#FOOT9"><sup>9</sup></a> The single and long-double precision
Chris@42 85 versions use &lsquo;<samp>sfftw_</samp>&rsquo; and &lsquo;<samp>lfftw_</samp>&rsquo;, respectively, instead of
Chris@42 86 &lsquo;<samp>fftwf_</samp>&rsquo; and &lsquo;<samp>fftwl_</samp>&rsquo;; quadruple precision (<code>real*16</code>)
Chris@42 87 is available on some systems as &lsquo;<samp>fftwq_</samp>&rsquo; (see <a href="Precision.html#Precision">Precision</a>).
Chris@42 88 (Note that <code>long double</code> on x86 hardware is usually at most
Chris@42 89 80-bit extended precision, <em>not</em> quadruple precision.)
Chris@42 90 </p>
Chris@42 91 <p>For the most part, all of the arguments to the functions are the same,
Chris@42 92 with the following exceptions:
Chris@42 93 </p>
Chris@42 94 <ul>
Chris@42 95 <li> <code>plan</code> variables (what would be of type <code>fftw_plan</code> in C),
Chris@42 96 must be declared as a type that is at least as big as a pointer
Chris@42 97 (address) on your machine. We recommend using <code>integer*8</code> everywhere,
Chris@42 98 since this should always be big enough.
Chris@42 99 <a name="index-portability-6"></a>
Chris@42 100
Chris@42 101 </li><li> Any function that returns a value (e.g. <code>fftw_plan_dft</code>) is
Chris@42 102 converted into a <em>subroutine</em>. The return value is converted into
Chris@42 103 an additional <em>first</em> parameter of this subroutine.<a name="DOCF10" href="#FOOT10"><sup>10</sup></a>
Chris@42 104
Chris@42 105 </li><li> <a name="index-column_002dmajor-2"></a>
Chris@42 106 The Fortran routines expect multi-dimensional arrays to be in
Chris@42 107 <em>column-major</em> order, which is the ordinary format of Fortran
Chris@42 108 arrays (see <a href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a>). They do this
Chris@42 109 transparently and costlessly simply by reversing the order of the
Chris@42 110 dimensions passed to FFTW, but this has one important consequence for
Chris@42 111 multi-dimensional real-complex transforms, discussed below.
Chris@42 112
Chris@42 113 </li><li> Wisdom import and export is somewhat more tricky because one cannot
Chris@42 114 easily pass files or strings between C and Fortran; see <a href="Wisdom-of-Fortran_003f.html#Wisdom-of-Fortran_003f">Wisdom of Fortran?</a>.
Chris@42 115
Chris@42 116 </li><li> Legacy Fortran cannot use the <code>fftw_malloc</code> dynamic-allocation routine.
Chris@42 117 If you want to exploit the SIMD FFTW (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>), you&rsquo;ll
Chris@42 118 need to figure out some other way to ensure that your arrays are at
Chris@42 119 least 16-byte aligned.
Chris@42 120
Chris@42 121 </li><li> <a name="index-fftw_005fiodim-2"></a>
Chris@42 122 <a name="index-guru-interface-4"></a>
Chris@42 123 Since Fortran 77 does not have data structures, the <code>fftw_iodim</code>
Chris@42 124 structure from the guru interface (see <a href="Guru-vector-and-transform-sizes.html#Guru-vector-and-transform-sizes">Guru vector and transform sizes</a>) must be split into separate arguments. In particular, any
Chris@42 125 <code>fftw_iodim</code> array arguments in the C guru interface become three
Chris@42 126 integer array arguments (<code>n</code>, <code>is</code>, and <code>os</code>) in the
Chris@42 127 Fortran guru interface, all of whose lengths should be equal to the
Chris@42 128 corresponding <code>rank</code> argument.
Chris@42 129
Chris@42 130 </li><li> The guru planner interface in Fortran does <em>not</em> do any automatic
Chris@42 131 translation between column-major and row-major; you are responsible
Chris@42 132 for setting the strides etcetera to correspond to your Fortran arrays.
Chris@42 133 However, as a slight bug that we are preserving for backwards
Chris@42 134 compatibility, the &lsquo;<samp>plan_guru_r2r</samp>&rsquo; in Fortran <em>does</em> reverse the
Chris@42 135 order of its <code>kind</code> array parameter, so the <code>kind</code> array
Chris@42 136 of that routine should be in the reverse of the order of the iodim
Chris@42 137 arrays (see above).
Chris@42 138
Chris@42 139 </li></ul>
Chris@42 140
Chris@42 141 <p>In general, you should take care to use Fortran data types that
Chris@42 142 correspond to (i.e. are the same size as) the C types used by FFTW.
Chris@42 143 In practice, this correspondence is usually straightforward
Chris@42 144 (i.e. <code>integer</code> corresponds to <code>int</code>, <code>real</code>
Chris@42 145 corresponds to <code>float</code>, etcetera). The native Fortran
Chris@42 146 double/single-precision complex type should be compatible with
Chris@42 147 <code>fftw_complex</code>/<code>fftwf_complex</code>. Such simple correspondences
Chris@42 148 are assumed in the examples below.
Chris@42 149 <a name="index-portability-7"></a>
Chris@42 150 </p>
Chris@42 151 <div class="footnote">
Chris@42 152 <hr>
Chris@42 153 <h4 class="footnotes-heading">Footnotes</h4>
Chris@42 154
Chris@42 155 <h3><a name="FOOT9" href="#DOCF9">(9)</a></h3>
Chris@42 156 <p>Technically, Fortran 77 identifiers are not
Chris@42 157 allowed to have more than 6 characters, nor may they contain
Chris@42 158 underscores. Any compiler that enforces this limitation doesn&rsquo;t
Chris@42 159 deserve to link to FFTW.</p>
Chris@42 160 <h3><a name="FOOT10" href="#DOCF10">(10)</a></h3>
Chris@42 161 <p>The
Chris@42 162 reason for this is that some Fortran implementations seem to have
Chris@42 163 trouble with C function return values, and vice versa.</p>
Chris@42 164 </div>
Chris@42 165 <hr>
Chris@42 166 <div class="header">
Chris@42 167 <p>
Chris@42 168 Next: <a href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran" accesskey="n" rel="next">FFTW Constants in Fortran</a>, Previous: <a href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" accesskey="p" rel="prev">Calling FFTW from Legacy Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" accesskey="u" rel="up">Calling FFTW from Legacy Fortran</a> &nbsp; [<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>
Chris@42 169 </div>
Chris@42 170
Chris@42 171
Chris@42 172
Chris@42 173 </body>
Chris@42 174 </html>