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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 3 <!-- This manual is for FFTW
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 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: Overview of Fortran interface</title>
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Chris@42 37 <link href="Extended-and-quadruple-precision-in-Fortran.html#Extended-and-quadruple-precision-in-Fortran" rel="next" title="Extended and quadruple precision in Fortran">
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Chris@42 70
Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="Overview-of-Fortran-interface"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 75 Next: <a href="Reversing-array-dimensions.html#Reversing-array-dimensions" accesskey="n" rel="next">Reversing array dimensions</a>, Previous: <a href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" accesskey="p" rel="prev">Calling FFTW from Modern Fortran</a>, Up: <a href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" accesskey="u" rel="up">Calling FFTW from Modern Fortran</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="Overview-of-Fortran-interface-1"></a>
Chris@42 79 <h3 class="section">7.1 Overview of Fortran interface</h3>
Chris@42 80
Chris@42 81 <p>FFTW provides a file <code>fftw3.f03</code> that defines Fortran 2003
Chris@42 82 interfaces for all of its C routines, except for the MPI routines
Chris@42 83 described elsewhere, which can be found in the same directory as
Chris@42 84 <code>fftw3.h</code> (the C header file). In any Fortran subroutine where
Chris@42 85 you want to use FFTW functions, you should begin with:
Chris@42 86 </p>
Chris@42 87 <a name="index-iso_005fc_005fbinding-1"></a>
Chris@42 88 <div class="example">
Chris@42 89 <pre class="example"> use, intrinsic :: iso_c_binding
Chris@42 90 include 'fftw3.f03'
Chris@42 91 </pre></div>
Chris@42 92
Chris@42 93 <p>This includes the interface definitions and the standard
Chris@42 94 <code>iso_c_binding</code> module (which defines the equivalents of C
Chris@42 95 types). You can also put the FFTW functions into a module if you
Chris@42 96 prefer (see <a href="Defining-an-FFTW-module.html#Defining-an-FFTW-module">Defining an FFTW module</a>).
Chris@42 97 </p>
Chris@42 98 <p>At this point, you can now call anything in the FFTW C interface
Chris@42 99 directly, almost exactly as in C other than minor changes in syntax.
Chris@42 100 For example:
Chris@42 101 </p>
Chris@42 102 <a name="index-fftw_005fplan_005fdft_005f2d-2"></a>
Chris@42 103 <a name="index-fftw_005fexecute_005fdft-2"></a>
Chris@42 104 <a name="index-fftw_005fdestroy_005fplan-3"></a>
Chris@42 105 <div class="example">
Chris@42 106 <pre class="example"> type(C_PTR) :: plan
Chris@42 107 complex(C_DOUBLE_COMPLEX), dimension(1024,1000) :: in, out
Chris@42 108 plan = fftw_plan_dft_2d(1000,1024, in,out, FFTW_FORWARD,FFTW_ESTIMATE)
Chris@42 109 ...
Chris@42 110 call fftw_execute_dft(plan, in, out)
Chris@42 111 ...
Chris@42 112 call fftw_destroy_plan(plan)
Chris@42 113 </pre></div>
Chris@42 114
Chris@42 115 <p>A few important things to keep in mind are:
Chris@42 116 </p>
Chris@42 117 <ul>
Chris@42 118 <li> <a name="index-fftw_005fcomplex-2"></a>
Chris@42 119 <a name="index-C_005fPTR"></a>
Chris@42 120 <a name="index-C_005fINT"></a>
Chris@42 121 <a name="index-C_005fDOUBLE"></a>
Chris@42 122 <a name="index-C_005fDOUBLE_005fCOMPLEX"></a>
Chris@42 123 FFTW plans are <code>type(C_PTR)</code>. Other C types are mapped in the
Chris@42 124 obvious way via the <code>iso_c_binding</code> standard: <code>int</code> turns
Chris@42 125 into <code>integer(C_INT)</code>, <code>fftw_complex</code> turns into
Chris@42 126 <code>complex(C_DOUBLE_COMPLEX)</code>, <code>double</code> turns into
Chris@42 127 <code>real(C_DOUBLE)</code>, and so on. See <a href="FFTW-Fortran-type-reference.html#FFTW-Fortran-type-reference">FFTW Fortran type reference</a>.
Chris@42 128
Chris@42 129 </li><li> Functions in C become functions in Fortran if they have a return value,
Chris@42 130 and subroutines in Fortran otherwise.
Chris@42 131
Chris@42 132 </li><li> The ordering of the Fortran array dimensions must be <em>reversed</em>
Chris@42 133 when they are passed to the FFTW plan creation, thanks to differences
Chris@42 134 in array indexing conventions (see <a href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a>). This is <em>unlike</em> the legacy Fortran interface
Chris@42 135 (see <a href="Fortran_002dinterface-routines.html#Fortran_002dinterface-routines">Fortran-interface routines</a>), which reversed the dimensions
Chris@42 136 for you. See <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a>.
Chris@42 137
Chris@42 138 </li><li> <a name="index-alignment-4"></a>
Chris@42 139 <a name="index-SIMD-2"></a>
Chris@42 140 Using ordinary Fortran array declarations like this works, but may
Chris@42 141 yield suboptimal performance because the data may not be not aligned
Chris@42 142 to exploit SIMD instructions on modern proessors (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>). Better performance will often be obtained
Chris@42 143 by allocating with &lsquo;<samp>fftw_alloc</samp>&rsquo;. See <a href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran">Allocating aligned memory in Fortran</a>.
Chris@42 144
Chris@42 145 </li><li> <a name="index-fftw_005fexecute-5"></a>
Chris@42 146 Similar to the legacy Fortran interface (see <a href="FFTW-Execution-in-Fortran.html#FFTW-Execution-in-Fortran">FFTW Execution in Fortran</a>), we currently recommend <em>not</em> using <code>fftw_execute</code>
Chris@42 147 but rather using the more specialized functions like
Chris@42 148 <code>fftw_execute_dft</code> (see <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>).
Chris@42 149 However, you should execute the plan on the <code>same arrays</code> as the
Chris@42 150 ones for which you created the plan, unless you are especially
Chris@42 151 careful. See <a href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran">Plan execution in Fortran</a>. To prevent
Chris@42 152 you from using <code>fftw_execute</code> by mistake, the <code>fftw3.f03</code>
Chris@42 153 file does not provide an <code>fftw_execute</code> interface declaration.
Chris@42 154
Chris@42 155 </li><li> <a name="index-flags-8"></a>
Chris@42 156 Multiple planner flags are combined with <code>ior</code> (equivalent to &lsquo;<samp>|</samp>&rsquo; in C). e.g. <code>FFTW_MEASURE | FFTW_DESTROY_INPUT</code> becomes <code>ior(FFTW_MEASURE, FFTW_DESTROY_INPUT)</code>. (You can also use &lsquo;<samp>+</samp>&rsquo; as long as you don&rsquo;t try to include a given flag more than once.)
Chris@42 157
Chris@42 158 </li></ul>
Chris@42 159
Chris@42 160 <table class="menu" border="0" cellspacing="0">
Chris@42 161 <tr><td align="left" valign="top">&bull; <a href="Extended-and-quadruple-precision-in-Fortran.html#Extended-and-quadruple-precision-in-Fortran" accesskey="1">Extended and quadruple precision in Fortran</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
Chris@42 162 </td></tr>
Chris@42 163 </table>
Chris@42 164
Chris@42 165 <hr>
Chris@42 166 <div class="header">
Chris@42 167 <p>
Chris@42 168 Next: <a href="Reversing-array-dimensions.html#Reversing-array-dimensions" accesskey="n" rel="next">Reversing array dimensions</a>, Previous: <a href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" accesskey="p" rel="prev">Calling FFTW from Modern Fortran</a>, Up: <a href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" accesskey="u" rel="up">Calling FFTW from Modern Fortran</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@42 169 </div>
Chris@42 170
Chris@42 171
Chris@42 172
Chris@42 173 </body>
Chris@42 174 </html>