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Tidy up for ROLI
author Geogaddi\David <d.m.ronan@qmul.ac.uk>
date Tue, 17 May 2016 18:50:19 +0100
parents 636c989477e7
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1 <html lang="en">
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3 <title>Fortran-interface routines - FFTW 3.2.1</title>
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49 <p>
50 <a name="Fortran-interface-routines"></a>
51 <a name="Fortran_002dinterface-routines"></a>
52 Next:&nbsp;<a rel="next" accesskey="n" href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran">FFTW Constants in Fortran</a>,
53 Previous:&nbsp;<a rel="previous" accesskey="p" href="Calling-FFTW-from-Fortran.html#Calling-FFTW-from-Fortran">Calling FFTW from Fortran</a>,
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55 <hr>
56 </div>
57
58 <h3 class="section">7.1 Fortran-interface routines</h3>
59
60 <p>Nearly all of the FFTW functions have Fortran-callable equivalents. The
61 name of the Fortran routine is the same as that of the corresponding C
62 routine, but with the `<samp><span class="samp">fftw_</span></samp>' prefix replaced by `<samp><span class="samp">dfftw_</span></samp>'.
63 (The single and long-double precision versions use `<samp><span class="samp">sfftw_</span></samp>' and
64 `<samp><span class="samp">lfftw_</span></samp>', respectively, instead of `<samp><span class="samp">fftwf_</span></samp>' and
65 `<samp><span class="samp">fftwl_</span></samp>'.)<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>
66
67 <p>For the most part, all of the arguments to the functions are the same,
68 with the following exceptions:
69
70 <ul>
71 <li><code>plan</code> variables (what would be of type <code>fftw_plan</code> in C),
72 must be declared as a type that is at least as big as a pointer
73 (address) on your machine. We recommend using <code>integer*8</code>.
74 <a name="index-portability-332"></a>
75 <li>Any function that returns a value (e.g. <code>fftw_plan_dft</code>) is
76 converted into a <em>subroutine</em>. The return value is converted into
77 an additional <em>first</em> parameter of this subroutine.<a rel="footnote" href="#fn-2" name="fnd-2"><sup>2</sup></a>
78
79 <li><a name="index-column_002dmajor-333"></a>The Fortran routines expect multi-dimensional arrays to be in
80 <em>column-major</em> order, which is the ordinary format of Fortran
81 arrays (see <a href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a>). They do this
82 transparently and costlessly simply by reversing the order of the
83 dimensions passed to FFTW, but this has one important consequence for
84 multi-dimensional real-complex transforms, discussed below.
85
86 <li>Wisdom import and export is somewhat more tricky because one cannot
87 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>.
88
89 <li>Fortran cannot use the <code>fftw_malloc</code> dynamic-allocation routine.
90 If you want to exploit the SIMD FFTW (see <a href="Data-Alignment.html#Data-Alignment">Data Alignment</a>), you'll
91 need to figure out some other way to ensure that your arrays are at
92 least 16-byte aligned.
93
94 <li><a name="index-fftw_005fiodim-334"></a><a name="index-guru-interface-335"></a>Since Fortran 77 does not have data structures, the <code>fftw_iodim</code>
95 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
96 <code>fftw_iodim</code> array arguments in the C guru interface become three
97 integer array arguments (<code>n</code>, <code>is</code>, and <code>os</code>) in the
98 Fortran guru interface, all of whose lengths should be equal to the
99 corresponding <code>rank</code> argument.
100
101 <li>The guru planner interface in Fortran does <em>not</em> do any automatic
102 translation between column-major and row-major; you are responsible
103 for setting the strides etcetera to correspond to your Fortran arrays.
104 However, as a slight bug that we are preserving for backwards
105 compatibility, the `<samp><span class="samp">plan_guru_r2r</span></samp>' in Fortran <em>does</em> reverse the
106 order of its <code>kind</code> array parameter, so the <code>kind</code> array
107 of that routine should be in the reverse of the order of the iodim
108 arrays (see above).
109
110 </ul>
111
112 <p>In general, you should take care to use Fortran data types that
113 correspond to (i.e. are the same size as) the C types used by FFTW. If
114 your C and Fortran compilers are made by the same vendor, the
115 correspondence is usually straightforward (i.e. <code>integer</code>
116 corresponds to <code>int</code>, <code>real</code> corresponds to <code>float</code>,
117 etcetera). The native Fortran double/single-precision complex type
118 should be compatible with <code>fftw_complex</code>/<code>fftwf_complex</code>.
119 Such simple correspondences are assumed in the examples below.
120 <a name="index-portability-336"></a>
121 <!-- -->
122
123 <div class="footnote">
124 <hr>
125 <h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> Technically, Fortran 77 identifiers are
126 not allowed to have more than 6 characters, nor may they contain
127 underscores. Any compiler that enforces this limitation doesn't deserve
128 to link to FFTW.</p>
129
130 <p class="footnote"><small>[<a name="fn-2" href="#fnd-2">2</a>]</small> The
131 reason for this is that some Fortran implementations seem to have
132 trouble with C function return values, and vice versa.</p>
133
134 <p><hr></div>
135
136 </body></html>
137