title: Wisdom in FFTW doesn't import/export
body: <p>I am using FFTW for FFTs, it's all working well but the optimisation takes a long time with the <code>FFTW_PATIENT</code> flag.  However, according to <a href="http://www.fftw.org/doc/Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans" rel="nofollow">the FFTW docs</a>, I can improve on this by reusing wisdom between runs, which I can import and export to file.  (I am using the floating point fftw routines, hence the fftwf_ prefix below instead of fftw_)</p>
  
  <p>So, at the start of my <code>main()</code>, I have :</p>
  
  <pre><code>char wisdom_file[] = "optimise.fft";
  fftwf_import_wisdom_from_filename(wisdom_file);
  </code></pre>
  
  <p>and at the end, I have:</p>
  
  <pre><code>fftwf_export_wisdom_to_filename(wisdom_file);
  </code></pre>
  
  <p>(I've also got error-checking to check the return is non-zero, omitted for simplicity above, so I know the files are reading and writing correctly)</p>
  
  <p>After one run I get a file <code>optimise.fft</code> with what looks like ASCII wisdom.  However, subsequent runs do not get any faster, and if I create my plans with the <code>FFTW_WISDOM_ONLY</code> flag, I get a null plan, showing that it doesn't see any wisdom there.</p>
  
  <p>I am using 3 different FFTs (2 real to complex and 1 inverse complex to real), so have also tried import/export in each FFT, and to separate files, but that doesn't help.</p>
  
  <p>I am using FFTW-3.3.3, I can see that FFTW-2 seemed to need more setting up to reuse wisdom, but the above seems sufficient now- what am I doing wrong?</p>