FFTW 3.3.8: Precision

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#### 4.1.2 Precision

You can install single and long-double precision versions of FFTW, which replace `double` with `float` and `long double`, respectively (see Installation and Customization). To use these interfaces, you:

- Link to the single/long-double libraries; on Unix, `-lfftw3f` or `-lfftw3l` instead of (or in addition to) `-lfftw3`. (You can link to the different-precision libraries simultaneously.)
- Include the same `<fftw3.h>` header file.
- Replace all lowercase instances of ‘`fftw_`’ with ‘`fftwf_`’ or ‘`fftwl_`’ for single or long-double precision, respectively. (`fftw_complex` becomes `fftwf_complex`, `fftw_execute` becomes `fftwf_execute`, etcetera.)
- Uppercase names, i.e. names beginning with ‘`FFTW_`’, remain the same.
- Replace `double` with `float` or `long double` for subroutine parameters.

Depending upon your compiler and/or hardware, `long double` may not be any more precise than `double` (or may not be supported at all, although it is standard in C99).

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). To use this type, link with `-lfftw3q -lquadmath -lm` (the `libquadmath` library provided by `gcc` is needed for quadruple-precision trigonometric functions) and use ‘`fftwq_`’ identifiers.