RTL_FM on the fly updates

Good evening - I am trying to make modifications to rtl_fm so that I can change frequencies, gains, modulation types, and squelch values on the fly (and thus avoiding a restart). Its unclear to me what functions need to be called in order to apply changes to the sdr device so that I can do the above. The rtl_fm code isnt documented so I dont have much to go off of... GQRX doesnt run reliably on any RPi3's I've tried so I hoping to use a modified rtl_fm in its place. Any help would be appreciated. Thanks :)