#### 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).