This chapter describes the Fortran-callable interface to FFTW, which
differs from the C interface only in the prefix (‘dfftw_’ instead
of ‘fftw_’), and a few other minor details. The Fortran interface
is included in the FFTW libraries by default, unless a Fortran compiler
isn't found on your system or --disable-fortran is included in
the configure flags. We assume here that the reader is already
familiar with the usage of FFTW in C, as described elsewhere in this
manual.