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<h3 class="section">8.1 Fortran-interface routines</h3>
<p>Nearly all of the FFTW functions have Fortran-callable equivalents.
The name of the legacy 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>’.<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a> 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>’; quadruple precision (<code>real*16</code>)
is available on some systems as ‘<samp><span class="samp">fftwq_</span></samp>’ (see <a href="Precision.html#Precision">Precision</a>).
(Note that <code>long double</code> on x86 hardware is usually at most
80-bit extended precision, <em>not</em> quadruple precision.)
<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> everywhere,
since this should always be big enough.
<a name="index-portability-578"></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-579"></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>Legacy Fortran cannot use the <code>fftw_malloc</code> dynamic-allocation routine.
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
need to figure out some other way to ensure that your arrays are at
least 16-byte aligned.
<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>
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.
<li>The guru planner interface in Fortran does <em>not</em> do any automatic
translation between column-major and row-major; you are responsible
for setting the strides etcetera to correspond to your Fortran arrays.
However, as a slight bug that we are preserving for backwards
compatibility, the ‘<samp><span class="samp">plan_guru_r2r</span></samp>’ in Fortran <em>does</em> reverse the
order of its <code>kind</code> array parameter, so the <code>kind</code> array
of that routine should be in the reverse of the order of the iodim
arrays (see above).
</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.
In practice, this 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-582"></a>
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<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>
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