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<h3 class="section">7.1 Overview of Fortran interface</h3>

<p>FFTW provides a file <code>fftw3.f03</code> that defines Fortran 2003
interfaces for all of its C routines, except for the MPI routines
described elsewhere, which can be found in the same directory as
<code>fftw3.h</code> (the C header file).  In any Fortran subroutine where
you want to use FFTW functions, you should begin with:

   <p><a name="index-iso_005fc_005fbinding-502"></a>
<pre class="example">       use, intrinsic :: iso_c_binding
       include 'fftw3.f03'
</pre>
   <p>This includes the interface definitions and the standard
<code>iso_c_binding</code> module (which defines the equivalents of C
types).  You can also put the FFTW functions into a module if you
prefer (see <a href="Defining-an-FFTW-module.html#Defining-an-FFTW-module">Defining an FFTW module</a>).

   <p>At this point, you can now call anything in the FFTW C interface
directly, almost exactly as in C other than minor changes in syntax.
For example:

   <p><a name="index-fftw_005fplan_005fdft_005f2d-503"></a><a name="index-fftw_005fexecute_005fdft-504"></a><a name="index-fftw_005fdestroy_005fplan-505"></a>
<pre class="example">       type(C_PTR) :: plan
       complex(C_DOUBLE_COMPLEX), dimension(1024,1000) :: in, out
       plan = fftw_plan_dft_2d(1000,1024, in,out, FFTW_FORWARD,FFTW_ESTIMATE)
       ...
       call fftw_execute_dft(plan, in, out)
       ...
       call fftw_destroy_plan(plan)
</pre>
   <p>A few important things to keep in mind are:

     <ul>
<li><a name="index-fftw_005fcomplex-506"></a><a name="index-C_005fPTR-507"></a><a name="index-C_005fINT-508"></a><a name="index-C_005fDOUBLE-509"></a><a name="index-C_005fDOUBLE_005fCOMPLEX-510"></a>FFTW plans are <code>type(C_PTR)</code>.  Other C types are mapped in the
obvious way via the <code>iso_c_binding</code> standard: <code>int</code> turns
into <code>integer(C_INT)</code>, <code>fftw_complex</code> turns into
<code>complex(C_DOUBLE_COMPLEX)</code>, <code>double</code> turns into
<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>.

     <li>Functions in C become functions in Fortran if they have a return value,
and subroutines in Fortran otherwise.

     <li>The ordering of the Fortran array dimensions must be <em>reversed</em>
when they are passed to the FFTW plan creation, thanks to differences
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
(see <a href="Fortran_002dinterface-routines.html#Fortran_002dinterface-routines">Fortran-interface routines</a>), which reversed the dimensions
for you.  See <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a>.

     <li><a name="index-alignment-511"></a><a name="index-SIMD-512"></a>Using ordinary Fortran array declarations like this works, but may
yield suboptimal performance because the data may not be not aligned
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
by allocating with &lsquo;<samp><span class="samp">fftw_alloc</span></samp>&rsquo;. See <a href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran">Allocating aligned memory in Fortran</a>.

     <li><a name="index-fftw_005fexecute-513"></a>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>
but rather using the more specialized functions like
<code>fftw_execute_dft</code> (see <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>).
However, you should execute the plan on the <code>same arrays</code> as the
ones for which you created the plan, unless you are especially
careful.  See <a href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran">Plan execution in Fortran</a>.  To prevent
you from using <code>fftw_execute</code> by mistake, the <code>fftw3.f03</code>
file does not provide an <code>fftw_execute</code> interface declaration.

     <li><a name="index-flags-514"></a>Multiple planner flags are combined with <code>ior</code> (equivalent to &lsquo;<samp><span class="samp">|</span></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><span class="samp">+</span></samp>&rsquo; as long as you don't try to include a given flag more than once.)

   </ul>

<ul class="menu">
<li><a accesskey="1" href="Extended-and-quadruple-precision-in-Fortran.html#Extended-and-quadruple-precision-in-Fortran">Extended and quadruple precision in Fortran</a>
</ul>

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author	Chris Cannam <cannam@all-day-breakfast.com>
date	Wed, 20 Mar 2013 15:35:50 +0000
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