FFT without gui

Hi,
I’m interested in printing the output of an FFT sink to a file. Looking
at
the tutorial I’ve seen that it’s possible by using the
fftsink_new.fft_sink_c block and specifying the fft_file argument.
Unfortunately fft_sink_c is a gui block, I would like to do the same
with a block which does not need gui extension. Is it possible?

Thanks in advance
Valentina