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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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+<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
+<html>
+<!-- This manual is for FFTW
+(version 3.3.5, 30 July 2016).
+
+Copyright (C) 2003 Matteo Frigo.
+
+Copyright (C) 2003 Massachusetts Institute of Technology.
+
+Permission is granted to make and distribute verbatim copies of this
+manual provided the copyright notice and this permission notice are
+preserved on all copies.
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+entire resulting derived work is distributed under the terms of a
+permission notice identical to this one.
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+approved by the Free Software Foundation. -->
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+<head>
+<title>FFTW 3.3.5: Usage of Multi-threaded FFTW</title>
+
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+<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
+<link href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW" rel="up" title="Multi-threaded FFTW">
+<link href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f" rel="next" title="How Many Threads to Use?">
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+<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
+<a name="Usage-of-Multi_002dthreaded-FFTW"></a>
+<div class="header">
+<p>
+Next: <a href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f" accesskey="n" rel="next">How Many Threads to Use?</a>, Previous: <a href="Installation-and-Supported-Hardware_002fSoftware.html#Installation-and-Supported-Hardware_002fSoftware" accesskey="p" rel="prev">Installation and Supported Hardware/Software</a>, Up: <a href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW" accesskey="u" rel="up">Multi-threaded FFTW</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>
+</div>
+<hr>
+<a name="Usage-of-Multi_002dthreaded-FFTW-1"></a>
+<h3 class="section">5.2 Usage of Multi-threaded FFTW</h3>
+
+<p>Here, it is assumed that the reader is already familiar with the usage
+of the uniprocessor FFTW routines, described elsewhere in this manual.
+We only describe what one has to change in order to use the
+multi-threaded routines.
+</p>
+<a name="index-OpenMP-1"></a>
+<p>First, programs using the parallel complex transforms should be linked
+with <code>-lfftw3_threads -lfftw3 -lm</code> on Unix, or <code>-lfftw3_omp
+-lfftw3 -lm</code> if you compiled with OpenMP. You will also need to link
+with whatever library is responsible for threads on your system
+(e.g. <code>-lpthread</code> on GNU/Linux) or include whatever compiler flag
+enables OpenMP (e.g. <code>-fopenmp</code> with gcc).
+<a name="index-linking-on-Unix"></a>
+</p>
+
+<p>Second, before calling <em>any</em> FFTW routines, you should call the
+function:
+</p>
+<div class="example">
+<pre class="example">int fftw_init_threads(void);
+</pre></div>
+<a name="index-fftw_005finit_005fthreads"></a>
+
+<p>This function, which need only be called once, performs any one-time
+initialization required to use threads on your system.  It returns zero
+if there was some error (which should not happen under normal
+circumstances) and a non-zero value otherwise.
+</p>
+<p>Third, before creating a plan that you want to parallelize, you should
+call:
+</p>
+<div class="example">
+<pre class="example">void fftw_plan_with_nthreads(int nthreads);
+</pre></div>
+<a name="index-fftw_005fplan_005fwith_005fnthreads"></a>
+
+<p>The <code>nthreads</code> argument indicates the number of threads you want
+FFTW to use (or actually, the maximum number).  All plans subsequently
+created with any planner routine will use that many threads.  You can
+call <code>fftw_plan_with_nthreads</code>, create some plans, call
+<code>fftw_plan_with_nthreads</code> again with a different argument, and
+create some more plans for a new number of threads.  Plans already created
+before a call to <code>fftw_plan_with_nthreads</code> are unaffected.  If you
+pass an <code>nthreads</code> argument of <code>1</code> (the default), threads are
+disabled for subsequent plans.
+</p>
+<a name="index-OpenMP-2"></a>
+<p>With OpenMP, to configure FFTW to use all of the currently running
+OpenMP threads (set by <code>omp_set_num_threads(nthreads)</code> or by the
+<code>OMP_NUM_THREADS</code> environment variable), you can do:
+<code>fftw_plan_with_nthreads(omp_get_max_threads())</code>. (The &lsquo;<samp>omp_</samp>&rsquo;
+OpenMP functions are declared via <code>#include &lt;omp.h&gt;</code>.)
+</p>
+<a name="index-thread-safety"></a>
+<p>Given a plan, you then execute it as usual with
+<code>fftw_execute(plan)</code>, and the execution will use the number of
+threads specified when the plan was created.  When done, you destroy
+it as usual with <code>fftw_destroy_plan</code>.  As described in
+<a href="Thread-safety.html#Thread-safety">Thread safety</a>, plan <em>execution</em> is thread-safe, but plan
+creation and destruction are <em>not</em>: you should create/destroy
+plans only from a single thread, but can safely execute multiple plans
+in parallel.
+</p>
+<p>There is one additional routine: if you want to get rid of all memory
+and other resources allocated internally by FFTW, you can call:
+</p>
+<div class="example">
+<pre class="example">void fftw_cleanup_threads(void);
+</pre></div>
+<a name="index-fftw_005fcleanup_005fthreads"></a>
+
+<p>which is much like the <code>fftw_cleanup()</code> function except that it
+also gets rid of threads-related data.  You must <em>not</em> execute any
+previously created plans after calling this function.
+</p>
+<p>We should also mention one other restriction: if you save wisdom from a
+program using the multi-threaded FFTW, that wisdom <em>cannot be used</em>
+by a program using only the single-threaded FFTW (i.e. not calling
+<code>fftw_init_threads</code>).  See <a href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans">Words of Wisdom-Saving Plans</a>.
+</p>
+<hr>
+<div class="header">
+<p>
+Next: <a href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f" accesskey="n" rel="next">How Many Threads to Use?</a>, Previous: <a href="Installation-and-Supported-Hardware_002fSoftware.html#Installation-and-Supported-Hardware_002fSoftware" accesskey="p" rel="prev">Installation and Supported Hardware/Software</a>, Up: <a href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW" accesskey="u" rel="up">Multi-threaded FFTW</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>
+</div>
+
+
+
+</body>
+</html>