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