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).