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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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Chris@42 4 (version 3.3.5, 30 July 2016).
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Chris@42 6 Copyright (C) 2003 Matteo Frigo.
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Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: Using Plans</title>
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Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="Using-Plans"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 75 Next: <a href="Basic-Interface.html#Basic-Interface" accesskey="n" rel="next">Basic Interface</a>, Previous: <a href="Data-Types-and-Files.html#Data-Types-and-Files" accesskey="p" rel="prev">Data Types and Files</a>, Up: <a href="FFTW-Reference.html#FFTW-Reference" accesskey="u" rel="up">FFTW Reference</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>
Chris@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="Using-Plans-1"></a>
Chris@42 79 <h3 class="section">4.2 Using Plans</h3>
Chris@42 80
Chris@42 81 <p>Plans for all transform types in FFTW are stored as type
Chris@42 82 <code>fftw_plan</code> (an opaque pointer type), and are created by one of the
Chris@42 83 various planning routines described in the following sections.
Chris@42 84 <a name="index-fftw_005fplan-1"></a>
Chris@42 85 An <code>fftw_plan</code> contains all information necessary to compute the
Chris@42 86 transform, including the pointers to the input and output arrays.
Chris@42 87 </p>
Chris@42 88 <div class="example">
Chris@42 89 <pre class="example">void fftw_execute(const fftw_plan plan);
Chris@42 90 </pre></div>
Chris@42 91 <a name="index-fftw_005fexecute-1"></a>
Chris@42 92
Chris@42 93 <p>This executes the <code>plan</code>, to compute the corresponding transform on
Chris@42 94 the arrays for which it was planned (which must still exist). The plan
Chris@42 95 is not modified, and <code>fftw_execute</code> can be called as many times as
Chris@42 96 desired.
Chris@42 97 </p>
Chris@42 98 <p>To apply a given plan to a different array, you can use the new-array execute
Chris@42 99 interface. See <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>.
Chris@42 100 </p>
Chris@42 101 <p><code>fftw_execute</code> (and equivalents) is the only function in FFTW
Chris@42 102 guaranteed to be thread-safe; see <a href="Thread-safety.html#Thread-safety">Thread safety</a>.
Chris@42 103 </p>
Chris@42 104 <p>This function:
Chris@42 105 </p><div class="example">
Chris@42 106 <pre class="example">void fftw_destroy_plan(fftw_plan plan);
Chris@42 107 </pre></div>
Chris@42 108 <a name="index-fftw_005fdestroy_005fplan-1"></a>
Chris@42 109 <p>deallocates the <code>plan</code> and all its associated data.
Chris@42 110 </p>
Chris@42 111 <p>FFTW&rsquo;s planner saves some other persistent data, such as the
Chris@42 112 accumulated wisdom and a list of algorithms available in the current
Chris@42 113 configuration. If you want to deallocate all of that and reset FFTW
Chris@42 114 to the pristine state it was in when you started your program, you can
Chris@42 115 call:
Chris@42 116 </p>
Chris@42 117 <div class="example">
Chris@42 118 <pre class="example">void fftw_cleanup(void);
Chris@42 119 </pre></div>
Chris@42 120 <a name="index-fftw_005fcleanup"></a>
Chris@42 121
Chris@42 122 <p>After calling <code>fftw_cleanup</code>, all existing plans become undefined,
Chris@42 123 and you should not attempt to execute them nor to destroy them. You can
Chris@42 124 however create and execute/destroy new plans, in which case FFTW starts
Chris@42 125 accumulating wisdom information again.
Chris@42 126 </p>
Chris@42 127 <p><code>fftw_cleanup</code> does not deallocate your plans, however. To prevent
Chris@42 128 memory leaks, you must still call <code>fftw_destroy_plan</code> before
Chris@42 129 executing <code>fftw_cleanup</code>.
Chris@42 130 </p>
Chris@42 131 <p>Occasionally, it may useful to know FFTW&rsquo;s internal &ldquo;cost&rdquo; metric
Chris@42 132 that it uses to compare plans to one another; this cost is
Chris@42 133 proportional to an execution time of the plan, in undocumented units,
Chris@42 134 if the plan was created with the <code>FFTW_MEASURE</code> or other
Chris@42 135 timing-based options, or alternatively is a heuristic cost function
Chris@42 136 for <code>FFTW_ESTIMATE</code> plans. (The cost values of measured and
Chris@42 137 estimated plans are not comparable, being in different units. Also,
Chris@42 138 costs from different FFTW versions or the same version compiled
Chris@42 139 differently may not be in the same units. Plans created from wisdom
Chris@42 140 have a cost of 0 since no timing measurement is performed for them.
Chris@42 141 Finally, certain problems for which only one top-level algorithm was
Chris@42 142 possible may have required no measurements of the cost of the whole
Chris@42 143 plan, in which case <code>fftw_cost</code> will also return 0.) The cost
Chris@42 144 metric for a given plan is returned by:
Chris@42 145 </p>
Chris@42 146 <div class="example">
Chris@42 147 <pre class="example">double fftw_cost(const fftw_plan plan);
Chris@42 148 </pre></div>
Chris@42 149 <a name="index-fftw_005fcost"></a>
Chris@42 150
Chris@42 151 <p>The following two routines are provided purely for academic purposes
Chris@42 152 (that is, for entertainment).
Chris@42 153 </p>
Chris@42 154 <div class="example">
Chris@42 155 <pre class="example">void fftw_flops(const fftw_plan plan,
Chris@42 156 double *add, double *mul, double *fma);
Chris@42 157 </pre></div>
Chris@42 158 <a name="index-fftw_005fflops"></a>
Chris@42 159
Chris@42 160 <p>Given a <code>plan</code>, set <code>add</code>, <code>mul</code>, and <code>fma</code> to an
Chris@42 161 exact count of the number of floating-point additions, multiplications,
Chris@42 162 and fused multiply-add operations involved in the plan&rsquo;s execution. The
Chris@42 163 total number of floating-point operations (flops) is <code>add + mul +
Chris@42 164 2*fma</code>, or <code>add + mul + fma</code> if the hardware supports fused
Chris@42 165 multiply-add instructions (although the number of FMA operations is only
Chris@42 166 approximate because of compiler voodoo). (The number of operations
Chris@42 167 should be an integer, but we use <code>double</code> to avoid overflowing
Chris@42 168 <code>int</code> for large transforms; the arguments are of type <code>double</code>
Chris@42 169 even for single and long-double precision versions of FFTW.)
Chris@42 170 </p>
Chris@42 171 <div class="example">
Chris@42 172 <pre class="example">void fftw_fprint_plan(const fftw_plan plan, FILE *output_file);
Chris@42 173 void fftw_print_plan(const fftw_plan plan);
Chris@42 174 char *fftw_sprint_plan(const fftw_plan plan);
Chris@42 175 </pre></div>
Chris@42 176 <a name="index-fftw_005ffprint_005fplan"></a>
Chris@42 177 <a name="index-fftw_005fprint_005fplan"></a>
Chris@42 178
Chris@42 179 <p>This outputs a &ldquo;nerd-readable&rdquo; representation of the <code>plan</code> to
Chris@42 180 the given file, to <code>stdout</code>, or two a newly allocated
Chris@42 181 NUL-terminated string (which the caller is responsible for deallocating
Chris@42 182 with <code>free</code>), respectively.
Chris@42 183 </p>
Chris@42 184 <hr>
Chris@42 185 <div class="header">
Chris@42 186 <p>
Chris@42 187 Next: <a href="Basic-Interface.html#Basic-Interface" accesskey="n" rel="next">Basic Interface</a>, Previous: <a href="Data-Types-and-Files.html#Data-Types-and-Files" accesskey="p" rel="prev">Data Types and Files</a>, Up: <a href="FFTW-Reference.html#FFTW-Reference" accesskey="u" rel="up">FFTW Reference</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>
Chris@42 188 </div>
Chris@42 189
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