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## 6 Calling FFTW from Fortran

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.