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<h4 class="subsection">4.1.2 Precision</h4>
<p><a name="index-precision-143"></a>
You can install single and long-double precision versions of FFTW,
which replace <code>double</code> with <code>float</code> and <code>long double</code>,
respectively (see <a href="Installation-and-Customization.html#Installation-and-Customization">Installation and Customization</a>). To use these
interfaces, you:
<ul>
<li>Link to the single/long-double libraries; on Unix, <code>-lfftw3f</code> or
<code>-lfftw3l</code> instead of (or in addition to) <code>-lfftw3</code>. (You
can link to the different-precision libraries simultaneously.)
<li>Include the <em>same</em> <code><fftw3.h></code> header file.
<li>Replace all lowercase instances of ‘<samp><span class="samp">fftw_</span></samp>’ with ‘<samp><span class="samp">fftwf_</span></samp>’ or
‘<samp><span class="samp">fftwl_</span></samp>’ for single or long-double precision, respectively.
(<code>fftw_complex</code> becomes <code>fftwf_complex</code>, <code>fftw_execute</code>
becomes <code>fftwf_execute</code>, etcetera.)
<li>Uppercase names, i.e. names beginning with ‘<samp><span class="samp">FFTW_</span></samp>’, remain the
same.
<li>Replace <code>double</code> with <code>float</code> or <code>long double</code> for
subroutine parameters.
</ul>
<p>Depending upon your compiler and/or hardware, <code>long double</code> may not
be any more precise than <code>double</code> (or may not be supported at all,
although it is standard in C99).
<a name="index-C99-144"></a>
<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>).
To use this type, link with <code>-lfftw3q -lquadmath -lm</code> (the
<code>libquadmath</code> library provided by <code>gcc</code> is needed for
quadruple-precision trigonometric functions) and use ‘<samp><span class="samp">fftwq_</span></samp>’
identifiers.
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