FT plans should use
<code>fftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
<code>fftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
use <code>fftw_execute_dft_c2r</code>.  The various r2r plans should use
<code>fftw_execute_r2r</code>.  Fortunately, if you use the wrong one you
will get a compile-time type-mismatch error (unlike legacy Fortran).

</li><li> You should normally pass the same input/output arrays that were used when
creating the plan.  This is always safe.

</