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8.1 Fortran-interface routines

Nearly all of the FFTW functions have Fortran-callable equivalents. The name of the legacy Fortran routine is the same as that of the corresponding C routine, but with the ‘fftw_’ prefix replaced by ‘dfftw_’.9 The single and long-double precision versions use ‘sfftw_’ and ‘lfftw_’, respectively, instead of ‘fftwf_’ and ‘fftwl_’; quadruple precision (real*16) is available on some systems as ‘fftwq_’ (see Precision). (Note that long double on x86 hardware is usually at most 80-bit extended precision, not quadruple precision.)

For the most part, all of the arguments to the functions are the same, with the following exceptions:

In general, you should take care to use Fortran data types that correspond to (i.e. are the same size as) the C types used by FFTW. In practice, this correspondence is usually straightforward (i.e. integer corresponds to int, real corresponds to float, etcetera). The native Fortran double/single-precision complex type should be compatible with fftw_complex/fftwf_complex. Such simple correspondences are assumed in the examples below.



Technically, Fortran 77 identifiers are not allowed to have more than 6 characters, nor may they contain underscores. Any compiler that enforces this limitation doesn’t deserve to link to FFTW.


The reason for this is that some Fortran implementations seem to have trouble with C function return values, and vice versa.

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