I have an error when loading a third party .so file: I have libfftw3.so installed (Ubuntu fftw3 package), but it does not define fftwf_plan_dft_r2c_1d symbol. Instead of this it has: (prefix fftw_ instead of fftwf_), according to doc it should be fftw_ http://www.fftw.org/doc/Real_002ddata-DFTs.html, but where I can find the library which defines fftwf_plan_dft_r2c_1d ? Answer You need to link with