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<a name="g_t64_002dbit-Guru-Interface"></a>
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Previous: <a href="Guru-Real_002dto_002dreal-Transforms.html#Guru-Real_002dto_002dreal-Transforms" accesskey="p" rel="prev">Guru Real-to-real Transforms</a>, Up: <a href="Guru-Interface.html#Guru-Interface" accesskey="u" rel="up">Guru Interface</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="g_t64_002dbit-Guru-Interface-1"></a>
<h4 class="subsection">4.5.6 64-bit Guru Interface</h4>
<a name="index-64_002dbit-architecture"></a>

<p>When compiled in 64-bit mode on a 64-bit architecture (where addresses
are 64 bits wide), FFTW uses 64-bit quantities internally for all
transform sizes, strides, and so on&mdash;you don&rsquo;t have to do anything
special to exploit this.  However, in the ordinary FFTW interfaces,
you specify the transform size by an <code>int</code> quantity, which is
normally only 32 bits wide.  This means that, even though FFTW is
using 64-bit sizes internally, you cannot specify a single transform
dimension larger than
2<sup><small>31</small></sup>&minus;1numbers.
</p>
<p>We expect that few users will require transforms larger than this, but,
for those who do, we provide a 64-bit version of the guru interface in
which all sizes are specified as integers of type <code>ptrdiff_t</code>
instead of <code>int</code>.  (<code>ptrdiff_t</code> is a signed integer type
defined by the C standard to be wide enough to represent address
differences, and thus must be at least 64 bits wide on a 64-bit
machine.)  We stress that there is <em>no performance advantage</em> to
using this interface&mdash;the same internal FFTW code is employed
regardless&mdash;and it is only necessary if you want to specify very
large transform sizes.
<a name="index-ptrdiff_005ft"></a>
</p>

<p>In particular, the 64-bit guru interface is a set of planner routines
that are exactly the same as the guru planner routines, except that
they are named with &lsquo;<samp>guru64</samp>&rsquo; instead of &lsquo;<samp>guru</samp>&rsquo; and they take
arguments of type <code>fftw_iodim64</code> instead of <code>fftw_iodim</code>.
For example, instead of <code>fftw_plan_guru_dft</code>, we have
<code>fftw_plan_guru64_dft</code>.
</p>
<div class="example">
<pre class="example">fftw_plan fftw_plan_guru64_dft(
     int rank, const fftw_iodim64 *dims,
     int howmany_rank, const fftw_iodim64 *howmany_dims,
     fftw_complex *in, fftw_complex *out,
     int sign, unsigned flags);
</pre></div>
<a name="index-fftw_005fplan_005fguru64_005fdft"></a>

<p>The <code>fftw_iodim64</code> type is similar to <code>fftw_iodim</code>, with the
same interpretation, except that it uses type <code>ptrdiff_t</code> instead
of type <code>int</code>.
</p>
<div class="example">
<pre class="example">typedef struct {
     ptrdiff_t n;
     ptrdiff_t is;
     ptrdiff_t os;
} fftw_iodim64;
</pre></div>
<a name="index-fftw_005fiodim64"></a>

<p>Every other &lsquo;<samp>fftw_plan_guru</samp>&rsquo; function also has a
&lsquo;<samp>fftw_plan_guru64</samp>&rsquo; equivalent, but we do not repeat their
documentation here since they are identical to the 32-bit versions
except as noted above.
</p>
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author	Chris Cannam <cannam@all-day-breakfast.com>
date	Mon, 02 Mar 2020 14:03:47 +0000
parents	7867fa7e1b6b
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