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282 lines
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<title>Upgrading from FFTW version 2 (FFTW 3.3.10)</title>
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<meta name="description" content="Upgrading from FFTW version 2 (FFTW 3.3.10)">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Installation-and-Customization.html" rel="next" title="Installation and Customization">
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</head>
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<body lang="en">
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<span id="Upgrading-from-FFTW-version-2"></span><div class="header">
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<p>
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Next: <a href="Installation-and-Customization.html" accesskey="n" rel="next">Installation and Customization</a>, Previous: <a href="Calling-FFTW-from-Legacy-Fortran.html" accesskey="p" rel="prev">Calling FFTW from Legacy Fortran</a>, Up: <a href="index.html" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="Upgrading-from-FFTW-version-2-1"></span><h2 class="chapter">9 Upgrading from FFTW version 2</h2>
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<p>In this chapter, we outline the process for updating codes designed for
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the older FFTW 2 interface to work with FFTW 3. The interface for FFTW
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3 is not backwards-compatible with the interface for FFTW 2 and earlier
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versions; codes written to use those versions will fail to link with
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FFTW 3. Nor is it possible to write “compatibility wrappers” to
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bridge the gap (at least not efficiently), because FFTW 3 has different
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semantics from previous versions. However, upgrading should be a
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straightforward process because the data formats are identical and the
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overall style of planning/execution is essentially the same.
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</p>
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<p>Unlike FFTW 2, there are no separate header files for real and complex
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transforms (or even for different precisions) in FFTW 3; all interfaces
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are defined in the <code><fftw3.h></code> header file.
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</p>
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<span id="Numeric-Types"></span><h3 class="heading">Numeric Types</h3>
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<p>The main difference in data types is that <code>fftw_complex</code> in FFTW 2
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was defined as a <code>struct</code> with macros <code>c_re</code> and <code>c_im</code>
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for accessing the real/imaginary parts. (This is binary-compatible with
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FFTW 3 on any machine except perhaps for some older Crays in single
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precision.) The equivalent macros for FFTW 3 are:
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</p>
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<div class="example">
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<pre class="example">#define c_re(c) ((c)[0])
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#define c_im(c) ((c)[1])
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</pre></div>
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<p>This does not work if you are using the C99 complex type, however,
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unless you insert a <code>double*</code> typecast into the above macros
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(see <a href="Complex-numbers.html">Complex numbers</a>).
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</p>
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<p>Also, FFTW 2 had an <code>fftw_real</code> typedef that was an alias for
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<code>double</code> (in double precision). In FFTW 3 you should just use
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<code>double</code> (or whatever precision you are employing).
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</p>
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<span id="Plans"></span><h3 class="heading">Plans</h3>
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<p>The major difference between FFTW 2 and FFTW 3 is in the
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planning/execution division of labor. In FFTW 2, plans were found for a
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given transform size and type, and then could be applied to <em>any</em>
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arrays and for <em>any</em> multiplicity/stride parameters. In FFTW 3,
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you specify the particular arrays, stride parameters, etcetera when
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creating the plan, and the plan is then executed for <em>those</em> arrays
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(unless the guru interface is used) and <em>those</em> parameters
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<em>only</em>. (FFTW 2 had “specific planner” routines that planned for
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a particular array and stride, but the plan could still be used for
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other arrays and strides.) That is, much of the information that was
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formerly specified at execution time is now specified at planning time.
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</p>
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<p>Like FFTW 2’s specific planner routines, the FFTW 3 planner overwrites
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the input/output arrays unless you use <code>FFTW_ESTIMATE</code>.
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</p>
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<p>FFTW 2 had separate data types <code>fftw_plan</code>, <code>fftwnd_plan</code>,
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<code>rfftw_plan</code>, and <code>rfftwnd_plan</code> for complex and real one- and
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multi-dimensional transforms, and each type had its own ‘<samp>destroy</samp>’
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function. In FFTW 3, all plans are of type <code>fftw_plan</code> and all are
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destroyed by <code>fftw_destroy_plan(plan)</code>.
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</p>
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<p>Where you formerly used <code>fftw_create_plan</code> and <code>fftw_one</code> to
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plan and compute a single 1d transform, you would now use
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<code>fftw_plan_dft_1d</code> to plan the transform. If you used the generic
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<code>fftw</code> function to execute the transform with multiplicity
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(<code>howmany</code>) and stride parameters, you would now use the advanced
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interface <code>fftw_plan_many_dft</code> to specify those parameters. The
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plans are now executed with <code>fftw_execute(plan)</code>, which takes all
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of its parameters (including the input/output arrays) from the plan.
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</p>
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<p>In-place transforms no longer interpret their output argument as scratch
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space, nor is there an <code>FFTW_IN_PLACE</code> flag. You simply pass the
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same pointer for both the input and output arguments. (Previously, the
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output <code>ostride</code> and <code>odist</code> parameters were ignored for
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in-place transforms; now, if they are specified via the advanced
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interface, they are significant even in the in-place case, although they
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should normally equal the corresponding input parameters.)
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</p>
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<p>The <code>FFTW_ESTIMATE</code> and <code>FFTW_MEASURE</code> flags have the same
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meaning as before, although the planning time will differ. You may also
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consider using <code>FFTW_PATIENT</code>, which is like <code>FFTW_MEASURE</code>
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except that it takes more time in order to consider a wider variety of
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algorithms.
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</p>
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<p>For multi-dimensional complex DFTs, instead of <code>fftwnd_create_plan</code>
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(or <code>fftw2d_create_plan</code> or <code>fftw3d_create_plan</code>), followed by
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<code>fftwnd_one</code>, you would use <code>fftw_plan_dft</code> (or
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<code>fftw_plan_dft_2d</code> or <code>fftw_plan_dft_3d</code>). followed by
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<code>fftw_execute</code>. If you used <code>fftwnd</code> to to specify strides
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etcetera, you would instead specify these via <code>fftw_plan_many_dft</code>.
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</p>
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<p>The analogues to <code>rfftw_create_plan</code> and <code>rfftw_one</code> with
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<code>FFTW_REAL_TO_COMPLEX</code> or <code>FFTW_COMPLEX_TO_REAL</code> directions
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are <code>fftw_plan_r2r_1d</code> with kind <code>FFTW_R2HC</code> or
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<code>FFTW_HC2R</code>, followed by <code>fftw_execute</code>. The stride etcetera
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arguments of <code>rfftw</code> are now in <code>fftw_plan_many_r2r</code>.
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</p>
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<p>Instead of <code>rfftwnd_create_plan</code> (or <code>rfftw2d_create_plan</code> or
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<code>rfftw3d_create_plan</code>) followed by
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<code>rfftwnd_one_real_to_complex</code> or
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<code>rfftwnd_one_complex_to_real</code>, you now use <code>fftw_plan_dft_r2c</code>
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(or <code>fftw_plan_dft_r2c_2d</code> or <code>fftw_plan_dft_r2c_3d</code>) or
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<code>fftw_plan_dft_c2r</code> (or <code>fftw_plan_dft_c2r_2d</code> or
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<code>fftw_plan_dft_c2r_3d</code>), respectively, followed by
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<code>fftw_execute</code>. As usual, the strides etcetera of
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<code>rfftwnd_real_to_complex</code> or <code>rfftwnd_complex_to_real</code> are no
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specified in the advanced planner routines,
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<code>fftw_plan_many_dft_r2c</code> or <code>fftw_plan_many_dft_c2r</code>.
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</p>
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<span id="Wisdom-2"></span><h3 class="heading">Wisdom</h3>
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<p>In FFTW 2, you had to supply the <code>FFTW_USE_WISDOM</code> flag in order to
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use wisdom; in FFTW 3, wisdom is always used. (You could simulate the
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FFTW 2 wisdom-less behavior by calling <code>fftw_forget_wisdom</code> after
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every planner call.)
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</p>
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<p>The FFTW 3 wisdom import/export routines are almost the same as before
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(although the storage format is entirely different). There is one
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significant difference, however. In FFTW 2, the import routines would
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never read past the end of the wisdom, so you could store extra data
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beyond the wisdom in the same file, for example. In FFTW 3, the
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file-import routine may read up to a few hundred bytes past the end of
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the wisdom, so you cannot store other data just beyond it.<a id="DOCF11" href="#FOOT11"><sup>11</sup></a>
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</p>
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<p>Wisdom has been enhanced by additional humility in FFTW 3: whereas FFTW
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2 would re-use wisdom for a given transform size regardless of the
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stride etc., in FFTW 3 wisdom is only used with the strides etc. for
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which it was created. Unfortunately, this means FFTW 3 has to create
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new plans from scratch more often than FFTW 2 (in FFTW 2, planning
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e.g. one transform of size 1024 also created wisdom for all smaller
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powers of 2, but this no longer occurs).
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</p>
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<p>FFTW 3 also has the new routine <code>fftw_import_system_wisdom</code> to
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import wisdom from a standard system-wide location.
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</p>
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<span id="Memory-allocation"></span><h3 class="heading">Memory allocation</h3>
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<p>In FFTW 3, we recommend allocating your arrays with <code>fftw_malloc</code>
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and deallocating them with <code>fftw_free</code>; this is not required, but
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allows optimal performance when SIMD acceleration is used. (Those two
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functions actually existed in FFTW 2, and worked the same way, but were
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not documented.)
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</p>
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<p>In FFTW 2, there were <code>fftw_malloc_hook</code> and <code>fftw_free_hook</code>
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functions that allowed the user to replace FFTW’s memory-allocation
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routines (e.g. to implement different error-handling, since by default
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FFTW prints an error message and calls <code>exit</code> to abort the program
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if <code>malloc</code> returns <code>NULL</code>). These hooks are not supported in
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FFTW 3; those few users who require this functionality can just
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directly modify the memory-allocation routines in FFTW (they are defined
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in <code>kernel/alloc.c</code>).
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</p>
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<span id="Fortran-interface"></span><h3 class="heading">Fortran interface</h3>
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<p>In FFTW 2, the subroutine names were obtained by replacing ‘<samp>fftw_</samp>’
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with ‘<samp>fftw_f77</samp>’; in FFTW 3, you replace ‘<samp>fftw_</samp>’ with
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‘<samp>dfftw_</samp>’ (or ‘<samp>sfftw_</samp>’ or ‘<samp>lfftw_</samp>’, depending upon the
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precision).
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</p>
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<p>In FFTW 3, we have begun recommending that you always declare the type
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used to store plans as <code>integer*8</code>. (Too many people didn’t notice
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our instruction to switch from <code>integer</code> to <code>integer*8</code> for
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64-bit machines.)
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</p>
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<p>In FFTW 3, we provide a <code>fftw3.f</code> “header file” to include in
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your code (and which is officially installed on Unix systems). (In FFTW
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2, we supplied a <code>fftw_f77.i</code> file, but it was not installed.)
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</p>
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<p>Otherwise, the C-Fortran interface relationship is much the same as it
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was before (e.g. return values become initial parameters, and
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multi-dimensional arrays are in column-major order). Unlike FFTW 2, we
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do provide some support for wisdom import/export in Fortran
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(see <a href="Wisdom-of-Fortran_003f.html">Wisdom of Fortran?</a>).
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</p>
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<span id="Threads"></span><h3 class="heading">Threads</h3>
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<p>Like FFTW 2, only the execution routines are thread-safe. All planner
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routines, etcetera, should be called by only a single thread at a time
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(see <a href="Thread-safety.html">Thread safety</a>). <em>Unlike</em> FFTW 2, there is no special
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<code>FFTW_THREADSAFE</code> flag for the planner to allow a given plan to be
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usable by multiple threads in parallel; this is now the case by default.
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</p>
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<p>The multi-threaded version of FFTW 2 required you to pass the number of
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threads each time you execute the transform. The number of threads is
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now stored in the plan, and is specified before the planner is called by
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<code>fftw_plan_with_nthreads</code>. The threads initialization routine used
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to be called <code>fftw_threads_init</code> and would return zero on success;
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the new routine is called <code>fftw_init_threads</code> and returns zero on
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failure. The current number of threads used by the planner can be
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checked with <code>fftw_planner_nthreads</code>. See <a href="Multi_002dthreaded-FFTW.html">Multi-threaded FFTW</a>.
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</p>
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<p>There is no separate threads header file in FFTW 3; all the function
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prototypes are in <code><fftw3.h></code>. However, you still have to link to
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a separate library (<code>-lfftw3_threads -lfftw3 -lm</code> on Unix), as well as
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to the threading library (e.g. POSIX threads on Unix).
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</p>
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<div class="footnote">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h5><a id="FOOT11" href="#DOCF11">(11)</a></h3>
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<p>We
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do our own buffering because GNU libc I/O routines are horribly slow for
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single-character I/O, apparently for thread-safety reasons (whether you
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are using threads or not).</p>
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</div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Installation-and-Customization.html" accesskey="n" rel="next">Installation and Customization</a>, Previous: <a href="Calling-FFTW-from-Legacy-Fortran.html" accesskey="p" rel="prev">Calling FFTW from Legacy Fortran</a>, Up: <a href="index.html" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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