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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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Permission is granted to copy and distribute modified versions of this
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manual under the conditions for verbatim copying, provided that the
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entire resulting derived work is distributed under the terms of a
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permission notice identical to this one.
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<head>
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<title>Fortran-interface routines (FFTW 3.3.10)</title>
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Calling-FFTW-from-Legacy-Fortran.html" rel="up" title="Calling FFTW from Legacy Fortran">
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<link href="FFTW-Constants-in-Fortran.html" rel="next" title="FFTW Constants in Fortran">
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</head>
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<body lang="en">
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<span id="Fortran_002dinterface-routines"></span><div class="header">
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<p>
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Next: <a href="FFTW-Constants-in-Fortran.html" accesskey="n" rel="next">FFTW Constants in Fortran</a>, Previous: <a href="Calling-FFTW-from-Legacy-Fortran.html" accesskey="p" rel="prev">Calling FFTW from Legacy Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html" accesskey="u" rel="up">Calling FFTW from Legacy Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="Fortran_002dinterface-routines-1"></span><h3 class="section">8.1 Fortran-interface routines</h3>
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<p>Nearly all of the FFTW functions have Fortran-callable equivalents.
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The name of the legacy Fortran routine is the same as that of the
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corresponding C routine, but with the ‘<samp>fftw_</samp>’ prefix replaced by
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‘<samp>dfftw_</samp>’.<a id="DOCF9" href="#FOOT9"><sup>9</sup></a> The single and long-double precision
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versions use ‘<samp>sfftw_</samp>’ and ‘<samp>lfftw_</samp>’, respectively, instead of
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‘<samp>fftwf_</samp>’ and ‘<samp>fftwl_</samp>’; quadruple precision (<code>real*16</code>)
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is available on some systems as ‘<samp>fftwq_</samp>’ (see <a href="Precision.html">Precision</a>).
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(Note that <code>long double</code> on x86 hardware is usually at most
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80-bit extended precision, <em>not</em> quadruple precision.)
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</p>
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<p>For the most part, all of the arguments to the functions are the same,
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with the following exceptions:
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</p>
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<ul>
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<li> <code>plan</code> variables (what would be of type <code>fftw_plan</code> in C),
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must be declared as a type that is at least as big as a pointer
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(address) on your machine. We recommend using <code>integer*8</code> everywhere,
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since this should always be big enough.
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<span id="index-portability-6"></span>
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</li><li> Any function that returns a value (e.g. <code>fftw_plan_dft</code>) is
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converted into a <em>subroutine</em>. The return value is converted into
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an additional <em>first</em> parameter of this subroutine.<a id="DOCF10" href="#FOOT10"><sup>10</sup></a>
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</li><li> <span id="index-column_002dmajor-2"></span>
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The Fortran routines expect multi-dimensional arrays to be in
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<em>column-major</em> order, which is the ordinary format of Fortran
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arrays (see <a href="Multi_002ddimensional-Array-Format.html">Multi-dimensional Array Format</a>). They do this
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transparently and costlessly simply by reversing the order of the
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dimensions passed to FFTW, but this has one important consequence for
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multi-dimensional real-complex transforms, discussed below.
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</li><li> Wisdom import and export is somewhat more tricky because one cannot
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easily pass files or strings between C and Fortran; see <a href="Wisdom-of-Fortran_003f.html">Wisdom of Fortran?</a>.
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</li><li> Legacy Fortran cannot use the <code>fftw_malloc</code> dynamic-allocation routine.
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If you want to exploit the SIMD FFTW (see <a href="SIMD-alignment-and-fftw_005fmalloc.html">SIMD alignment and fftw_malloc</a>), you’ll
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need to figure out some other way to ensure that your arrays are at
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least 16-byte aligned.
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</li><li> <span id="index-fftw_005fiodim-2"></span>
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<span id="index-guru-interface-4"></span>
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Since Fortran 77 does not have data structures, the <code>fftw_iodim</code>
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structure from the guru interface (see <a href="Guru-vector-and-transform-sizes.html">Guru vector and transform sizes</a>) must be split into separate arguments. In particular, any
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<code>fftw_iodim</code> array arguments in the C guru interface become three
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integer array arguments (<code>n</code>, <code>is</code>, and <code>os</code>) in the
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Fortran guru interface, all of whose lengths should be equal to the
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corresponding <code>rank</code> argument.
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</li><li> The guru planner interface in Fortran does <em>not</em> do any automatic
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translation between column-major and row-major; you are responsible
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for setting the strides etcetera to correspond to your Fortran arrays.
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However, as a slight bug that we are preserving for backwards
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compatibility, the ‘<samp>plan_guru_r2r</samp>’ in Fortran <em>does</em> reverse the
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order of its <code>kind</code> array parameter, so the <code>kind</code> array
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of that routine should be in the reverse of the order of the iodim
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arrays (see above).
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</li></ul>
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<p>In general, you should take care to use Fortran data types that
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correspond to (i.e. are the same size as) the C types used by FFTW.
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In practice, this correspondence is usually straightforward
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(i.e. <code>integer</code> corresponds to <code>int</code>, <code>real</code>
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corresponds to <code>float</code>, etcetera). The native Fortran
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double/single-precision complex type should be compatible with
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<code>fftw_complex</code>/<code>fftwf_complex</code>. Such simple correspondences
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are assumed in the examples below.
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<span id="index-portability-7"></span>
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</p>
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<div class="footnote">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h5><a id="FOOT9" href="#DOCF9">(9)</a></h3>
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<p>Technically, Fortran 77 identifiers are not
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allowed to have more than 6 characters, nor may they contain
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underscores. Any compiler that enforces this limitation doesn’t
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deserve to link to FFTW.</p>
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<h5><a id="FOOT10" href="#DOCF10">(10)</a></h3>
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<p>The
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reason for this is that some Fortran implementations seem to have
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trouble with C function return values, and vice versa.</p>
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</div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="FFTW-Constants-in-Fortran.html" accesskey="n" rel="next">FFTW Constants in Fortran</a>, Previous: <a href="Calling-FFTW-from-Legacy-Fortran.html" accesskey="p" rel="prev">Calling FFTW from Legacy Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html" accesskey="u" rel="up">Calling FFTW from Legacy Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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