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<title>Fortran-interface routines - FFTW 3.1.2</title>
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<a name="Fortran-interface-routines"></a>
<a name="Fortran_002dinterface-routines"></a>
Next: <a rel="next" accesskey="n" href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran">FFTW Constants in Fortran</a>,
Previous: <a rel="previous" accesskey="p" href="Calling-FFTW-from-Fortran.html#Calling-FFTW-from-Fortran">Calling FFTW from Fortran</a>,
Up: <a rel="up" accesskey="u" href="Calling-FFTW-from-Fortran.html#Calling-FFTW-from-Fortran">Calling FFTW from Fortran</a>
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<h3 class="section">6.1 Fortran-interface routines</h3>
<p>Nearly all of the FFTW functions have Fortran-callable equivalents. The
name of the Fortran routine is the same as that of the corresponding C
routine, but with the `<samp><span class="samp">fftw_</span></samp>' prefix replaced by `<samp><span class="samp">dfftw_</span></samp>'.
(The single and long-double precision versions use `<samp><span class="samp">sfftw_</span></samp>' and
`<samp><span class="samp">lfftw_</span></samp>', respectively, instead of `<samp><span class="samp">fftwf_</span></samp>' and
`<samp><span class="samp">fftwl_</span></samp>'.)<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>
<p>For the most part, all of the arguments to the functions are the same,
with the following exceptions:
<ul>
<li><code>plan</code> variables (what would be of type <code>fftw_plan</code> in C),
must be declared as a type that is at least as big as a pointer
(address) on your machine. We recommend using <code>integer*8</code>.
<a name="index-portability-320"></a>
<li>Any function that returns a value (e.g. <code>fftw_plan_dft</code>) is
converted into a <em>subroutine</em>. The return value is converted into
an additional <em>first</em> parameter of this subroutine.<a rel="footnote" href="#fn-2" name="fnd-2"><sup>2</sup></a>
<li><a name="index-column_002dmajor-321"></a>The Fortran routines expect multi-dimensional arrays to be in
<em>column-major</em> order, which is the ordinary format of Fortran
arrays (see <a href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a>). They do this
transparently and costlessly simply by reversing the order of the
dimensions passed to FFTW, but this has one important consequence for
multi-dimensional real-complex transforms, discussed below.
<li>Wisdom import and export is somewhat more tricky because one cannot
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>.
<li>Fortran cannot use the <code>fftw_malloc</code> dynamic-allocation routine.
If you want to exploit the SIMD FFTW (see <a href="Data-Alignment.html#Data-Alignment">Data Alignment</a>), you'll
need to figure out some other way to ensure that your arrays are at
least 16-byte aligned.
<li><a name="index-fftw_005fiodim-322"></a><a name="index-guru-interface-323"></a>Since Fortran 77 does not have data structures, the <code>fftw_iodim</code>
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
<code>fftw_iodim</code> array arguments in the C guru interface become three
integer array arguments (<code>n</code>, <code>is</code>, and <code>os</code>) in the
Fortran guru interface, all of whose lengths should be equal to the
corresponding <code>rank</code> argument.
</ul>
<p>In general, you should take care to use Fortran data types that
correspond to (i.e. are the same size as) the C types used by FFTW. If
your C and Fortran compilers are made by the same vendor, the
correspondence is usually straightforward (i.e. <code>integer</code>
corresponds to <code>int</code>, <code>real</code> corresponds to <code>float</code>,
etcetera). The native Fortran double/single-precision complex type
should be compatible with <code>fftw_complex</code>/<code>fftwf_complex</code>.
Such simple correspondences are assumed in the examples below.
<a name="index-portability-324"></a>
<!-- -->
<div class="footnote">
<hr>
<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> Technically, Fortran 77 identifiers are
not allowed to have more than 6 characters, nor may they contain
underscores. Any compiler that enforces this limitation doesn't deserve
to link to FFTW.</p>
<p class="footnote"><small>[<a name="fn-2" href="#fnd-2">2</a>]</small> The
reason for this is that some Fortran implementations seem to have
trouble with C function return values, and vice versa.</p>
<p><hr></div>
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