How does ofdm sync work

GNU Radio
1

Hi,
Does any one know how does ofdm in gnuradio do sync?
What I found in the gr_ofdm_frame_sink.cc is just check sig[0],
which
is just the second bytes of input_items. However, I didn’t see any
reason
that the (const char *) input_items[1] will be NULL.

Thanks,
Bin Zan