FFTW 3.3.8: Extended and quadruple precision in Fortran

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#### 7.1.1 Extended and quadruple precision in Fortran

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.