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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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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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preserved on all copies.
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Permission is granted to copy and distribute modified versions of this
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manual under the conditions for verbatim copying, provided that the
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entire resulting derived work is distributed under the terms of a
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permission notice identical to this one.
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Permission is granted to copy and distribute translations of this manual
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approved by the Free Software Foundation. -->
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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>Overview of Fortran interface (FFTW 3.3.10)</title>
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<link href="index.html" rel="start" title="Top">
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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="Calling-FFTW-from-Modern-Fortran.html" rel="up" title="Calling FFTW from Modern Fortran">
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<link href="Extended-and-quadruple-precision-in-Fortran.html" rel="next" title="Extended and quadruple precision in Fortran">
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<span id="Overview-of-Fortran-interface"></span><div class="header">
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<p>
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Next: <a href="Reversing-array-dimensions.html" accesskey="n" rel="next">Reversing array dimensions</a>, Previous: <a href="Calling-FFTW-from-Modern-Fortran.html" accesskey="p" rel="prev">Calling FFTW from Modern Fortran</a>, Up: <a href="Calling-FFTW-from-Modern-Fortran.html" accesskey="u" rel="up">Calling FFTW from Modern Fortran</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="Overview-of-Fortran-interface-1"></span><h3 class="section">7.1 Overview of Fortran interface</h3>
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<p>FFTW provides a file <code>fftw3.f03</code> that defines Fortran 2003
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interfaces for all of its C routines, except for the MPI routines
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described elsewhere, which can be found in the same directory as
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<code>fftw3.h</code> (the C header file). In any Fortran subroutine where
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you want to use FFTW functions, you should begin with:
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</p>
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<span id="index-iso_005fc_005fbinding-1"></span>
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<div class="example">
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<pre class="example"> use, intrinsic :: iso_c_binding
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include 'fftw3.f03'
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</pre></div>
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<p>This includes the interface definitions and the standard
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<code>iso_c_binding</code> module (which defines the equivalents of C
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types). You can also put the FFTW functions into a module if you
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prefer (see <a href="Defining-an-FFTW-module.html">Defining an FFTW module</a>).
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</p>
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<p>At this point, you can now call anything in the FFTW C interface
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directly, almost exactly as in C other than minor changes in syntax.
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For example:
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</p>
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<span id="index-fftw_005fplan_005fdft_005f2d-2"></span>
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<span id="index-fftw_005fexecute_005fdft-2"></span>
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<span id="index-fftw_005fdestroy_005fplan-3"></span>
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<div class="example">
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<pre class="example"> type(C_PTR) :: plan
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complex(C_DOUBLE_COMPLEX), dimension(1024,1000) :: in, out
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plan = fftw_plan_dft_2d(1000,1024, in,out, FFTW_FORWARD,FFTW_ESTIMATE)
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...
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call fftw_execute_dft(plan, in, out)
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...
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call fftw_destroy_plan(plan)
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</pre></div>
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<p>A few important things to keep in mind are:
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</p>
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<ul>
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<li> <span id="index-fftw_005fcomplex-2"></span>
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<span id="index-C_005fPTR"></span>
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<span id="index-C_005fINT"></span>
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<span id="index-C_005fDOUBLE"></span>
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<span id="index-C_005fDOUBLE_005fCOMPLEX"></span>
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FFTW plans are <code>type(C_PTR)</code>. Other C types are mapped in the
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obvious way via the <code>iso_c_binding</code> standard: <code>int</code> turns
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into <code>integer(C_INT)</code>, <code>fftw_complex</code> turns into
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<code>complex(C_DOUBLE_COMPLEX)</code>, <code>double</code> turns into
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<code>real(C_DOUBLE)</code>, and so on. See <a href="FFTW-Fortran-type-reference.html">FFTW Fortran type reference</a>.
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</li><li> Functions in C become functions in Fortran if they have a return value,
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and subroutines in Fortran otherwise.
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</li><li> The ordering of the Fortran array dimensions must be <em>reversed</em>
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when they are passed to the FFTW plan creation, thanks to differences
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in array indexing conventions (see <a href="Multi_002ddimensional-Array-Format.html">Multi-dimensional Array Format</a>). This is <em>unlike</em> the legacy Fortran interface
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(see <a href="Fortran_002dinterface-routines.html">Fortran-interface routines</a>), which reversed the dimensions
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for you. See <a href="Reversing-array-dimensions.html">Reversing array dimensions</a>.
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</li><li> <span id="index-alignment-4"></span>
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<span id="index-SIMD-2"></span>
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Using ordinary Fortran array declarations like this works, but may
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yield suboptimal performance because the data may not be not aligned
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to exploit SIMD instructions on modern proessors (see <a href="SIMD-alignment-and-fftw_005fmalloc.html">SIMD alignment and fftw_malloc</a>). Better performance will often be obtained
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by allocating with ‘<samp>fftw_alloc</samp>’. See <a href="Allocating-aligned-memory-in-Fortran.html">Allocating aligned memory in Fortran</a>.
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</li><li> <span id="index-fftw_005fexecute-5"></span>
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Similar to the legacy Fortran interface (see <a href="FFTW-Execution-in-Fortran.html">FFTW Execution in Fortran</a>), we currently recommend <em>not</em> using <code>fftw_execute</code>
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but rather using the more specialized functions like
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<code>fftw_execute_dft</code> (see <a href="New_002darray-Execute-Functions.html">New-array Execute Functions</a>).
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However, you should execute the plan on the <code>same arrays</code> as the
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ones for which you created the plan, unless you are especially
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careful. See <a href="Plan-execution-in-Fortran.html">Plan execution in Fortran</a>. To prevent
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you from using <code>fftw_execute</code> by mistake, the <code>fftw3.f03</code>
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file does not provide an <code>fftw_execute</code> interface declaration.
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</li><li> <span id="index-flags-8"></span>
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Multiple planner flags are combined with <code>ior</code> (equivalent to ‘<samp>|</samp>’ in C). e.g. <code>FFTW_MEASURE | FFTW_DESTROY_INPUT</code> becomes <code>ior(FFTW_MEASURE, FFTW_DESTROY_INPUT)</code>. (You can also use ‘<samp>+</samp>’ as long as you don’t try to include a given flag more than once.)
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</li></ul>
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<table class="menu" border="0" cellspacing="0">
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<tr><td align="left" valign="top">• <a href="Extended-and-quadruple-precision-in-Fortran.html" accesskey="1">Extended and quadruple precision in Fortran</a></td><td> </td><td align="left" valign="top">
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</td></tr>
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</table>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Reversing-array-dimensions.html" accesskey="n" rel="next">Reversing array dimensions</a>, Previous: <a href="Calling-FFTW-from-Modern-Fortran.html" accesskey="p" rel="prev">Calling FFTW from Modern Fortran</a>, Up: <a href="Calling-FFTW-from-Modern-Fortran.html" accesskey="u" rel="up">Calling FFTW from Modern Fortran</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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</body>
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</html>
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