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If FFTW is compiled in long double (extended) precision
(see Installation and Customization), you may be able to call the
resulting fftwl_ routines (see Precision) from Fortran if
your compiler supports the C_LONG_DOUBLE_COMPLEX type code.
Because some Fortran compilers do not support
C_LONG_DOUBLE_COMPLEX, the fftwl_ declarations are
segregated into a separate interface file fftw3l.f03, which you
should include in addition to fftw3.f03 (which declares
precision-independent ‘FFTW_’ constants):
use, intrinsic :: iso_c_binding include 'fftw3.f03' include 'fftw3l.f03'
We also support using the nonstandard __float128
quadruple-precision type provided by recent versions of gcc on
32- and 64-bit x86 hardware (see Installation and Customization),
using the corresponding real(16) and complex(16) types
supported by gfortran. The quadruple-precision ‘fftwq_’
functions (see Precision) are declared in a fftw3q.f03
interface file, which should be included in addition to
fftw3l.f03, as above. You should also link with
-lfftw3q -lquadmath -lm as in C.