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author Chris Cannam <cannam@all-day-breakfast.com>
date Wed, 20 Mar 2013 15:35:50 +0000
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3 <title>Fortran-interface routines - FFTW 3.3.3</title>
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49 <a name="Fortran-interface-routines"></a>
50 <a name="Fortran_002dinterface-routines"></a>
51 <p>
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-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran">Calling FFTW from Legacy Fortran</a>,
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55 <hr>
56 </div>
57
58 <h3 class="section">8.1 Fortran-interface routines</h3>
59
60 <p>Nearly all of the FFTW functions have Fortran-callable equivalents.
61 The name of the legacy Fortran routine is the same as that of the
62 corresponding C routine, but with the &lsquo;<samp><span class="samp">fftw_</span></samp>&rsquo; prefix replaced by
63 &lsquo;<samp><span class="samp">dfftw_</span></samp>&rsquo;.<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a> The single and long-double precision
64 versions use &lsquo;<samp><span class="samp">sfftw_</span></samp>&rsquo; and &lsquo;<samp><span class="samp">lfftw_</span></samp>&rsquo;, respectively, instead of
65 &lsquo;<samp><span class="samp">fftwf_</span></samp>&rsquo; and &lsquo;<samp><span class="samp">fftwl_</span></samp>&rsquo;; quadruple precision (<code>real*16</code>)
66 is available on some systems as &lsquo;<samp><span class="samp">fftwq_</span></samp>&rsquo; (see <a href="Precision.html#Precision">Precision</a>).
67 (Note that <code>long double</code> on x86 hardware is usually at most
68 80-bit extended precision, <em>not</em> quadruple precision.)
69
70 <p>For the most part, all of the arguments to the functions are the same,
71 with the following exceptions:
72
73 <ul>
74 <li><code>plan</code> variables (what would be of type <code>fftw_plan</code> in C),
75 must be declared as a type that is at least as big as a pointer
76 (address) on your machine. We recommend using <code>integer*8</code> everywhere,
77 since this should always be big enough.
78 <a name="index-portability-578"></a>
79 <li>Any function that returns a value (e.g. <code>fftw_plan_dft</code>) is
80 converted into a <em>subroutine</em>. The return value is converted into
81 an additional <em>first</em> parameter of this subroutine.<a rel="footnote" href="#fn-2" name="fnd-2"><sup>2</sup></a>
82
83 <li><a name="index-column_002dmajor-579"></a>The Fortran routines expect multi-dimensional arrays to be in
84 <em>column-major</em> order, which is the ordinary format of Fortran
85 arrays (see <a href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a>). They do this
86 transparently and costlessly simply by reversing the order of the
87 dimensions passed to FFTW, but this has one important consequence for
88 multi-dimensional real-complex transforms, discussed below.
89
90 <li>Wisdom import and export is somewhat more tricky because one cannot
91 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>.
92
93 <li>Legacy Fortran cannot use the <code>fftw_malloc</code> dynamic-allocation routine.
94 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'll
95 need to figure out some other way to ensure that your arrays are at
96 least 16-byte aligned.
97
98 <li><a name="index-fftw_005fiodim-580"></a><a name="index-guru-interface-581"></a>Since Fortran 77 does not have data structures, the <code>fftw_iodim</code>
99 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
100 <code>fftw_iodim</code> array arguments in the C guru interface become three
101 integer array arguments (<code>n</code>, <code>is</code>, and <code>os</code>) in the
102 Fortran guru interface, all of whose lengths should be equal to the
103 corresponding <code>rank</code> argument.
104
105 <li>The guru planner interface in Fortran does <em>not</em> do any automatic
106 translation between column-major and row-major; you are responsible
107 for setting the strides etcetera to correspond to your Fortran arrays.
108 However, as a slight bug that we are preserving for backwards
109 compatibility, the &lsquo;<samp><span class="samp">plan_guru_r2r</span></samp>&rsquo; in Fortran <em>does</em> reverse the
110 order of its <code>kind</code> array parameter, so the <code>kind</code> array
111 of that routine should be in the reverse of the order of the iodim
112 arrays (see above).
113
114 </ul>
115
116 <p>In general, you should take care to use Fortran data types that
117 correspond to (i.e. are the same size as) the C types used by FFTW.
118 In practice, this correspondence is usually straightforward
119 (i.e. <code>integer</code> corresponds to <code>int</code>, <code>real</code>
120 corresponds to <code>float</code>, etcetera). The native Fortran
121 double/single-precision complex type should be compatible with
122 <code>fftw_complex</code>/<code>fftwf_complex</code>. Such simple correspondences
123 are assumed in the examples below.
124 <a name="index-portability-582"></a>
125 <!-- -->
126
127 <div class="footnote">
128 <hr>
129 <h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> Technically, Fortran 77 identifiers are not
130 allowed to have more than 6 characters, nor may they contain
131 underscores. Any compiler that enforces this limitation doesn't
132 deserve to link to FFTW.</p>
133
134 <p class="footnote"><small>[<a name="fn-2" href="#fnd-2">2</a>]</small> The
135 reason for this is that some Fortran implementations seem to have
136 trouble with C function return values, and vice versa.</p>
137
138 <hr></div>
139
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141