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You can install single and long-double precision versions of FFTW,
respectively (see Installation and Customization). To use these
-lfftw3linstead of (or in addition to)
-lfftw3. (You can link to the different-precision libraries simultaneously.)
fftwl_for single or long-double precision, respectively. (
FFTW_, remain the same.
long doublefor 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).