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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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into another language, under the above conditions for modified versions,
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except that this permission notice may be stated in a translation
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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>FFTW MPI Fortran Interface (FFTW 3.3.10)</title>
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<meta name="description" content="FFTW MPI Fortran Interface (FFTW 3.3.10)">
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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="Distributed_002dmemory-FFTW-with-MPI.html" rel="up" title="Distributed-memory FFTW with MPI">
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<link href="Calling-FFTW-from-Modern-Fortran.html" rel="next" title="Calling FFTW from Modern Fortran">
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</head>
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<body lang="en">
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<span id="FFTW-MPI-Fortran-Interface"></span><div class="header">
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<p>
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Previous: <a href="FFTW-MPI-Reference.html" accesskey="p" rel="prev">FFTW MPI Reference</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html" 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" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="FFTW-MPI-Fortran-Interface-1"></span><h3 class="section">6.13 FFTW MPI Fortran Interface</h3>
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<span id="index-Fortran-interface-1"></span>
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<span id="index-iso_005fc_005fbinding"></span>
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<p>The FFTW MPI interface is callable from modern Fortran compilers
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supporting the Fortran 2003 <code>iso_c_binding</code> standard for calling
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C functions. As described in <a href="Calling-FFTW-from-Modern-Fortran.html">Calling FFTW from Modern Fortran</a>,
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this means that you can directly call FFTW’s C interface from Fortran
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with only minor changes in syntax. There are, however, a few things
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specific to the MPI interface to keep in mind:
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</p>
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<ul>
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<li> Instead of including <code>fftw3.f03</code> as in <a href="Overview-of-Fortran-interface.html">Overview of Fortran interface</a>, you should <code>include 'fftw3-mpi.f03'</code> (after
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<code>use, intrinsic :: iso_c_binding</code> as before). The
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<code>fftw3-mpi.f03</code> file includes <code>fftw3.f03</code>, so you should
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<em>not</em> <code>include</code> them both yourself. (You will also want to
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include the MPI header file, usually via <code>include 'mpif.h'</code> or
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similar, although though this is not needed by <code>fftw3-mpi.f03</code>
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<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</a>.)
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</li><li> Because of the different storage conventions between C and Fortran,
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you reverse the order of your array dimensions when passing them to
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FFTW (see <a href="Reversing-array-dimensions.html">Reversing array dimensions</a>). This is merely a
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difference in notation and incurs no performance overhead. However,
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it means that, whereas in C the <em>first</em> dimension is distributed,
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in Fortran the <em>last</em> dimension of your array is distributed.
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</li><li> <span id="index-MPI-communicator-3"></span>
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In Fortran, communicators are stored as <code>integer</code> types; there is
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no <code>MPI_Comm</code> type, nor is there any way to access a C
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<code>MPI_Comm</code>. Fortunately, this is taken care of for you by the
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FFTW Fortran interface: whenever the C interface expects an
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<code>MPI_Comm</code> type, you should pass the Fortran communicator as an
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<code>integer</code>.<a id="DOCF8" href="#FOOT8"><sup>8</sup></a>
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</li><li> Because you need to call the ‘<samp>local_size</samp>’ function to find out
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how much space to allocate, and this may be <em>larger</em> than the
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local portion of the array (see <a href="MPI-Data-Distribution.html">MPI Data Distribution</a>), you should
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<em>always</em> allocate your arrays dynamically using FFTW’s allocation
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routines as described in <a href="Allocating-aligned-memory-in-Fortran.html">Allocating aligned memory in Fortran</a>.
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(Coincidentally, this also provides the best performance by
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guaranteeding proper data alignment.)
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</li><li> Because all sizes in the MPI FFTW interface are declared as
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<code>ptrdiff_t</code> in C, you should use <code>integer(C_INTPTR_T)</code> in
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Fortran (see <a href="FFTW-Fortran-type-reference.html">FFTW Fortran type reference</a>).
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</li><li> <span id="index-fftw_005fexecute_005fdft-1"></span>
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<span id="index-fftw_005fmpi_005fexecute_005fdft-1"></span>
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<span id="index-new_002darray-execution-3"></span>
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In Fortran, because of the language semantics, we generally recommend
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using the new-array execute functions for all plans, even in the
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common case where you are executing the plan on the same arrays for
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which the plan was created (see <a href="Plan-execution-in-Fortran.html">Plan execution in Fortran</a>).
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However, note that in the MPI interface these functions are changed:
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<code>fftw_execute_dft</code> becomes <code>fftw_mpi_execute_dft</code>,
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etcetera. See <a href="Using-MPI-Plans.html">Using MPI Plans</a>.
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</li></ul>
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<p>For example, here is a Fortran code snippet to perform a distributed
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L × M
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complex DFT in-place. (This assumes you have already
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initialized MPI with <code>MPI_init</code> and have also performed
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<code>call fftw_mpi_init</code>.)
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</p>
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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-mpi.f03'
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integer(C_INTPTR_T), parameter :: L = ...
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integer(C_INTPTR_T), parameter :: M = ...
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type(C_PTR) :: plan, cdata
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complex(C_DOUBLE_COMPLEX), pointer :: data(:,:)
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integer(C_INTPTR_T) :: i, j, alloc_local, local_M, local_j_offset
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! <span class="roman">get local data size and allocate (note dimension reversal)</span>
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alloc_local = fftw_mpi_local_size_2d(M, L, MPI_COMM_WORLD, &
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local_M, local_j_offset)
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cdata = fftw_alloc_complex(alloc_local)
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call c_f_pointer(cdata, data, [L,local_M])
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! <span class="roman">create MPI plan for in-place forward DFT (note dimension reversal)</span>
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plan = fftw_mpi_plan_dft_2d(M, L, data, data, MPI_COMM_WORLD, &
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FFTW_FORWARD, FFTW_MEASURE)
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! <span class="roman">initialize data to some function</span> my_function(i,j)
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do j = 1, local_M
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do i = 1, L
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data(i, j) = my_function(i, j + local_j_offset)
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end do
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end do
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! <span class="roman">compute transform (as many times as desired)</span>
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call fftw_mpi_execute_dft(plan, data, data)
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call fftw_destroy_plan(plan)
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call fftw_free(cdata)
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</pre></div>
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<p>Note that when we called <code>fftw_mpi_local_size_2d</code> and
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<code>fftw_mpi_plan_dft_2d</code> with the dimensions in reversed order,
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since a L × M
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Fortran array is viewed by FFTW in C as a
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M × L
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array. This means that the array was distributed over
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the <code>M</code> dimension, the local portion of which is a
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L × local_M
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array in Fortran. (You must <em>not</em> use an
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<code>allocate</code> statement to allocate an L × local_M
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array,
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however; you must allocate <code>alloc_local</code> complex numbers, which
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may be greater than <code>L * local_M</code>, in order to reserve space for
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intermediate steps of the transform.) Finally, we mention that
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because C’s array indices are zero-based, the <code>local_j_offset</code>
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argument can conveniently be interpreted as an offset in the 1-based
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<code>j</code> index (rather than as a starting index as in C).
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</p>
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<p>If instead you had used the <code>ior(FFTW_MEASURE,
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FFTW_MPI_TRANSPOSED_OUT)</code> flag, the output of the transform would be a
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transposed M × local_L
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array, associated with the <em>same</em>
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<code>cdata</code> allocation (since the transform is in-place), and which
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you could declare with:
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</p>
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<div class="example">
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<pre class="example"> complex(C_DOUBLE_COMPLEX), pointer :: tdata(:,:)
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...
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call c_f_pointer(cdata, tdata, [M,local_L])
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</pre></div>
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<p>where <code>local_L</code> would have been obtained by changing the
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<code>fftw_mpi_local_size_2d</code> call to:
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</p>
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<div class="example">
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<pre class="example"> alloc_local = fftw_mpi_local_size_2d_transposed(M, L, MPI_COMM_WORLD, &
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local_M, local_j_offset, local_L, local_i_offset)
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</pre></div>
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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="FOOT8" href="#DOCF8">(8)</a></h3>
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<p>Technically, this is because you aren’t
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actually calling the C functions directly. You are calling wrapper
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functions that translate the communicator with <code>MPI_Comm_f2c</code>
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before calling the ordinary C interface. This is all done
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transparently, however, since the <code>fftw3-mpi.f03</code> interface file
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renames the wrappers so that they are called in Fortran with the same
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names as the C interface functions.</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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Previous: <a href="FFTW-MPI-Reference.html" accesskey="p" rel="prev">FFTW MPI Reference</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html" 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" 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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