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1 <html lang="en">
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2 <head>
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3 <title>Thread safety - FFTW 3.3.4</title>
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4 <meta http-equiv="Content-Type" content="text/html">
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5 <meta name="description" content="FFTW 3.3.4">
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7 <link title="Top" rel="start" href="index.html#Top">
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8 <link rel="up" href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW" title="Multi-threaded FFTW">
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9 <link rel="prev" href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f" title="How Many Threads to Use?">
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10 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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11 <!--
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12 This manual is for FFTW
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13 (version 3.3.4, 20 September 2013).
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14
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15 Copyright (C) 2003 Matteo Frigo.
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16
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17 Copyright (C) 2003 Massachusetts Institute of Technology.
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18
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19 Permission is granted to make and distribute verbatim copies of
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20 this manual provided the copyright notice and this permission
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21 notice are preserved on all copies.
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22
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23 Permission is granted to copy and distribute modified versions of
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24 this manual under the conditions for verbatim copying, provided
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25 that the entire resulting derived work is distributed under the
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26 terms of a permission notice identical to this one.
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27
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28 Permission is granted to copy and distribute translations of this
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29 manual into another language, under the above conditions for
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30 modified versions, except that this permission notice may be
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31 stated in a translation approved by the Free Software Foundation.
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32 -->
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44 --></style>
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45 </head>
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46 <body>
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47 <div class="node">
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48 <a name="Thread-safety"></a>
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49 <p>
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50 Previous: <a rel="previous" accesskey="p" href="How-Many-Threads-to-Use_003f.html#How-Many-Threads-to-Use_003f">How Many Threads to Use?</a>,
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51 Up: <a rel="up" accesskey="u" href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW">Multi-threaded FFTW</a>
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52 <hr>
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53 </div>
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54
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55 <h3 class="section">5.4 Thread safety</h3>
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56
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57 <p><a name="index-threads-344"></a><a name="index-OpenMP-345"></a><a name="index-thread-safety-346"></a>Users writing multi-threaded programs (including OpenMP) must concern
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58 themselves with the <dfn>thread safety</dfn> of the libraries they
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59 use—that is, whether it is safe to call routines in parallel from
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60 multiple threads. FFTW can be used in such an environment, but some
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61 care must be taken because the planner routines share data
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62 (e.g. wisdom and trigonometric tables) between calls and plans.
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63
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64 <p>The upshot is that the only thread-safe (re-entrant) routine in FFTW is
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65 <code>fftw_execute</code> (and the new-array variants thereof). All other routines
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66 (e.g. the planner) should only be called from one thread at a time. So,
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67 for example, you can wrap a semaphore lock around any calls to the
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68 planner; even more simply, you can just create all of your plans from
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69 one thread. We do not think this should be an important restriction
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70 (FFTW is designed for the situation where the only performance-sensitive
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71 code is the actual execution of the transform), and the benefits of
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72 shared data between plans are great.
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73
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74 <p>Note also that, since the plan is not modified by <code>fftw_execute</code>,
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75 it is safe to execute the <em>same plan</em> in parallel by multiple
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76 threads. However, since a given plan operates by default on a fixed
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77 array, you need to use one of the new-array execute functions (see <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>) so that different threads compute the transform of different data.
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78
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79 <p>(Users should note that these comments only apply to programs using
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80 shared-memory threads or OpenMP. Parallelism using MPI or forked processes
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81 involves a separate address-space and global variables for each process,
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82 and is not susceptible to problems of this sort.)
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83
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84 <p>If you are configured FFTW with the <code>--enable-debug</code> or
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85 <code>--enable-debug-malloc</code> flags (see <a href="Installation-on-Unix.html#Installation-on-Unix">Installation on Unix</a>),
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86 then <code>fftw_execute</code> is not thread-safe. These flags are not
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87 documented because they are intended only for developing
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88 and debugging FFTW, but if you must use <code>--enable-debug</code> then you
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89 should also specifically pass <code>--disable-debug-malloc</code> for
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90 <code>fftw_execute</code> to be thread-safe.
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91
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92 </body></html>
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93
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