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1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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2 <html>
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3 <!-- This manual is for FFTW
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4 (version 3.3.8, 24 May 2018).
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5
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6 Copyright (C) 2003 Matteo Frigo.
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7
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8 Copyright (C) 2003 Massachusetts Institute of Technology.
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9
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10 Permission is granted to make and distribute verbatim copies of this
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11 manual provided the copyright notice and this permission notice are
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12 preserved on all copies.
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13
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14 Permission is granted to copy and distribute modified versions of this
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15 manual under the conditions for verbatim copying, provided that the
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16 entire resulting derived work is distributed under the terms of a
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17 permission notice identical to this one.
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18
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19 Permission is granted to copy and distribute translations of this manual
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20 into another language, under the above conditions for modified versions,
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21 except that this permission notice may be stated in a translation
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22 approved by the Free Software Foundation. -->
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23 <!-- Created by GNU Texinfo 6.3, http://www.gnu.org/software/texinfo/ -->
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24 <head>
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25 <title>FFTW 3.3.8: FFTW MPI Fortran Interface</title>
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26
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27 <meta name="description" content="FFTW 3.3.8: FFTW MPI Fortran Interface">
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28 <meta name="keywords" content="FFTW 3.3.8: FFTW MPI Fortran Interface">
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33 <link href="index.html#Top" rel="start" title="Top">
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34 <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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36 <link href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" rel="up" title="Distributed-memory FFTW with MPI">
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37 <link href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" rel="next" title="Calling FFTW from Modern Fortran">
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38 <link href="MPI-Wisdom-Communication.html#MPI-Wisdom-Communication" rel="prev" title="MPI Wisdom Communication">
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39 <style type="text/css">
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64 -->
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65 </style>
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66
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67
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68 </head>
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69
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70 <body lang="en">
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71 <a name="FFTW-MPI-Fortran-Interface"></a>
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72 <div class="header">
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73 <p>
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74 Previous: <a href="FFTW-MPI-Reference.html#FFTW-MPI-Reference" accesskey="p" rel="prev">FFTW MPI Reference</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" accesskey="u" rel="up">Distributed-memory FFTW with MPI</a> [<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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75 </div>
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76 <hr>
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77 <a name="FFTW-MPI-Fortran-Interface-1"></a>
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78 <h3 class="section">6.13 FFTW MPI Fortran Interface</h3>
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79 <a name="index-Fortran-interface-1"></a>
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80
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81 <a name="index-iso_005fc_005fbinding"></a>
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82 <p>The FFTW MPI interface is callable from modern Fortran compilers
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83 supporting the Fortran 2003 <code>iso_c_binding</code> standard for calling
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84 C functions. As described in <a href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran">Calling FFTW from Modern Fortran</a>,
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85 this means that you can directly call FFTW’s C interface from Fortran
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86 with only minor changes in syntax. There are, however, a few things
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87 specific to the MPI interface to keep in mind:
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88 </p>
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89 <ul>
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90 <li> Instead of including <code>fftw3.f03</code> as in <a href="Overview-of-Fortran-interface.html#Overview-of-Fortran-interface">Overview of Fortran interface</a>, you should <code>include 'fftw3-mpi.f03'</code> (after
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91 <code>use, intrinsic :: iso_c_binding</code> as before). The
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92 <code>fftw3-mpi.f03</code> file includes <code>fftw3.f03</code>, so you should
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93 <em>not</em> <code>include</code> them both yourself. (You will also want to
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94 include the MPI header file, usually via <code>include 'mpif.h'</code> or
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95 similar, although though this is not needed by <code>fftw3-mpi.f03</code>
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96 <i>per se</i>.) (To use the ‘<samp>fftwl_</samp>’ <code>long double</code> extended-precision routines in supporting compilers, you should include <code>fftw3f-mpi.f03</code> in <em>addition</em> to <code>fftw3-mpi.f03</code>. See <a href="Extended-and-quadruple-precision-in-Fortran.html#Extended-and-quadruple-precision-in-Fortran">Extended and quadruple precision in Fortran</a>.)
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97
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98 </li><li> Because of the different storage conventions between C and Fortran,
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99 you reverse the order of your array dimensions when passing them to
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100 FFTW (see <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a>). This is merely a
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101 difference in notation and incurs no performance overhead. However,
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102 it means that, whereas in C the <em>first</em> dimension is distributed,
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103 in Fortran the <em>last</em> dimension of your array is distributed.
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104
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105 </li><li> <a name="index-MPI-communicator-3"></a>
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106 In Fortran, communicators are stored as <code>integer</code> types; there is
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107 no <code>MPI_Comm</code> type, nor is there any way to access a C
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108 <code>MPI_Comm</code>. Fortunately, this is taken care of for you by the
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109 FFTW Fortran interface: whenever the C interface expects an
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110 <code>MPI_Comm</code> type, you should pass the Fortran communicator as an
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111 <code>integer</code>.<a name="DOCF8" href="#FOOT8"><sup>8</sup></a>
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112
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113 </li><li> Because you need to call the ‘<samp>local_size</samp>’ function to find out
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114 how much space to allocate, and this may be <em>larger</em> than the
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115 local portion of the array (see <a href="MPI-Data-Distribution.html#MPI-Data-Distribution">MPI Data Distribution</a>), you should
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116 <em>always</em> allocate your arrays dynamically using FFTW’s allocation
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117 routines as described in <a href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran">Allocating aligned memory in Fortran</a>.
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118 (Coincidentally, this also provides the best performance by
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119 guaranteeding proper data alignment.)
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120
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121 </li><li> Because all sizes in the MPI FFTW interface are declared as
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122 <code>ptrdiff_t</code> in C, you should use <code>integer(C_INTPTR_T)</code> in
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123 Fortran (see <a href="FFTW-Fortran-type-reference.html#FFTW-Fortran-type-reference">FFTW Fortran type reference</a>).
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124
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125 </li><li> <a name="index-fftw_005fexecute_005fdft-1"></a>
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126 <a name="index-fftw_005fmpi_005fexecute_005fdft-1"></a>
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127 <a name="index-new_002darray-execution-3"></a>
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128 In Fortran, because of the language semantics, we generally recommend
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129 using the new-array execute functions for all plans, even in the
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130 common case where you are executing the plan on the same arrays for
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131 which the plan was created (see <a href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran">Plan execution in Fortran</a>).
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132 However, note that in the MPI interface these functions are changed:
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133 <code>fftw_execute_dft</code> becomes <code>fftw_mpi_execute_dft</code>,
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134 etcetera. See <a href="Using-MPI-Plans.html#Using-MPI-Plans">Using MPI Plans</a>.
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135
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136 </li></ul>
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137
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138 <p>For example, here is a Fortran code snippet to perform a distributed
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139 L × M
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140 complex DFT in-place. (This assumes you have already
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141 initialized MPI with <code>MPI_init</code> and have also performed
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142 <code>call fftw_mpi_init</code>.)
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143 </p>
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144 <div class="example">
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145 <pre class="example"> use, intrinsic :: iso_c_binding
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146 include 'fftw3-mpi.f03'
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147 integer(C_INTPTR_T), parameter :: L = ...
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148 integer(C_INTPTR_T), parameter :: M = ...
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149 type(C_PTR) :: plan, cdata
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150 complex(C_DOUBLE_COMPLEX), pointer :: data(:,:)
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151 integer(C_INTPTR_T) :: i, j, alloc_local, local_M, local_j_offset
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152
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153 ! <span class="roman">get local data size and allocate (note dimension reversal)</span>
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154 alloc_local = fftw_mpi_local_size_2d(M, L, MPI_COMM_WORLD, &
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155 local_M, local_j_offset)
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156 cdata = fftw_alloc_complex(alloc_local)
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157 call c_f_pointer(cdata, data, [L,local_M])
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158
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159 ! <span class="roman">create MPI plan for in-place forward DFT (note dimension reversal)</span>
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160 plan = fftw_mpi_plan_dft_2d(M, L, data, data, MPI_COMM_WORLD, &
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161 FFTW_FORWARD, FFTW_MEASURE)
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162
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163 ! <span class="roman">initialize data to some function</span> my_function(i,j)
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164 do j = 1, local_M
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165 do i = 1, L
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166 data(i, j) = my_function(i, j + local_j_offset)
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167 end do
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168 end do
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169
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170 ! <span class="roman">compute transform (as many times as desired)</span>
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171 call fftw_mpi_execute_dft(plan, data, data)
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172
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173 call fftw_destroy_plan(plan)
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174 call fftw_free(cdata)
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175 </pre></div>
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176
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177 <p>Note that when we called <code>fftw_mpi_local_size_2d</code> and
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178 <code>fftw_mpi_plan_dft_2d</code> with the dimensions in reversed order,
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179 since a L × M
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180 Fortran array is viewed by FFTW in C as a
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181 M × L
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182 array. This means that the array was distributed over
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183 the <code>M</code> dimension, the local portion of which is a
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184 L × local_M
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185 array in Fortran. (You must <em>not</em> use an
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186 <code>allocate</code> statement to allocate an L × local_M
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187 array,
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188 however; you must allocate <code>alloc_local</code> complex numbers, which
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189 may be greater than <code>L * local_M</code>, in order to reserve space for
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190 intermediate steps of the transform.) Finally, we mention that
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191 because C’s array indices are zero-based, the <code>local_j_offset</code>
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192 argument can conveniently be interpreted as an offset in the 1-based
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193 <code>j</code> index (rather than as a starting index as in C).
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194 </p>
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195 <p>If instead you had used the <code>ior(FFTW_MEASURE,
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196 FFTW_MPI_TRANSPOSED_OUT)</code> flag, the output of the transform would be a
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197 transposed M × local_L
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198 array, associated with the <em>same</em>
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199 <code>cdata</code> allocation (since the transform is in-place), and which
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200 you could declare with:
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201 </p>
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202 <div class="example">
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203 <pre class="example"> complex(C_DOUBLE_COMPLEX), pointer :: tdata(:,:)
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204 ...
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205 call c_f_pointer(cdata, tdata, [M,local_L])
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206 </pre></div>
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207
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208 <p>where <code>local_L</code> would have been obtained by changing the
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209 <code>fftw_mpi_local_size_2d</code> call to:
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210 </p>
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211 <div class="example">
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212 <pre class="example"> alloc_local = fftw_mpi_local_size_2d_transposed(M, L, MPI_COMM_WORLD, &
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213 local_M, local_j_offset, local_L, local_i_offset)
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214 </pre></div>
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215 <div class="footnote">
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216 <hr>
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217 <h4 class="footnotes-heading">Footnotes</h4>
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218
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219 <h3><a name="FOOT8" href="#DOCF8">(8)</a></h3>
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220 <p>Technically, this is because you aren’t
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221 actually calling the C functions directly. You are calling wrapper
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222 functions that translate the communicator with <code>MPI_Comm_f2c</code>
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223 before calling the ordinary C interface. This is all done
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224 transparently, however, since the <code>fftw3-mpi.f03</code> interface file
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225 renames the wrappers so that they are called in Fortran with the same
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226 names as the C interface functions.</p>
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227 </div>
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228 <hr>
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229 <div class="header">
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230 <p>
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231 Previous: <a href="FFTW-MPI-Reference.html#FFTW-MPI-Reference" accesskey="p" rel="prev">FFTW MPI Reference</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" accesskey="u" rel="up">Distributed-memory FFTW with MPI</a> [<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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232 </div>
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233
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234
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235
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236 </body>
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237 </html>
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