What is the function of fir_filter_ccc in gnuradio(synchronization process in ofdm example)?

Hi guys,

I am trying to figure out how fir_filter work. In particular for the
case
that using fir_filter to perform cross-correlation in synchronization
process. However, I read the code but still can not understand it.
Please
tell me if you know it. Thank you so much.

*Please refer to the following code in ofdm_sync_pnac.py

self.crosscorr_filter = gr.fir_filter_ccc(1, kstime) // kstime is the
known symbol

Cross-correlate input signal with known preamble

self.connect(self.input, self.crosscorr_filter)*

Best,
Kay