Wrappers are delivered as source code, which you must compile to build the wrapper library. Then you can substitute the wrapper and Intel® oneAPI Math Kernel Library (oneMKL) libraries for the FFTW library. The source code for the wrappers, makefiles, and files with lists of wrappers are located in the .\interfaces\fftw2xfsubdirectory in the Intel® oneAPI Math Kernel Library (oneMKL) directory.