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<h4 class="subsection">7.1.1 Extended and quadruple precision in Fortran</h4>
<p><a name="index-precision-515"></a>
If FFTW is compiled in <code>long double</code> (extended) precision
(see <a href="Installation-and-Customization.html#Installation-and-Customization">Installation and Customization</a>), you may be able to call the
resulting <code>fftwl_</code> routines (see <a href="Precision.html#Precision">Precision</a>) from Fortran if
your compiler supports the <code>C_LONG_DOUBLE_COMPLEX</code> type code.
<p>Because some Fortran compilers do not support
<code>C_LONG_DOUBLE_COMPLEX</code>, the <code>fftwl_</code> declarations are
segregated into a separate interface file <code>fftw3l.f03</code>, which you
should include <em>in addition</em> to <code>fftw3.f03</code> (which declares
precision-independent ‘<samp><span class="samp">FFTW_</span></samp>’ constants):
<p><a name="index-iso_005fc_005fbinding-516"></a>
<pre class="example"> use, intrinsic :: iso_c_binding
include 'fftw3.f03'
include 'fftw3l.f03'
</pre>
<p>We also support using the nonstandard <code>__float128</code>
quadruple-precision type provided by recent versions of <code>gcc</code> on
32- and 64-bit x86 hardware (see <a href="Installation-and-Customization.html#Installation-and-Customization">Installation and Customization</a>),
using the corresponding <code>real(16)</code> and <code>complex(16)</code> types
supported by <code>gfortran</code>. The quadruple-precision ‘<samp><span class="samp">fftwq_</span></samp>’
functions (see <a href="Precision.html#Precision">Precision</a>) are declared in a <code>fftw3q.f03</code>
interface file, which should be included in addition to
<code>fftw3l.f03</code>, as above. You should also link with
<code>-lfftw3q -lquadmath -lm</code> as in C.
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