# previous code ...

err = LibPcap::PCAP_ERRBUF_SIZE.dup
cap = LibPcap.pcap_open_live("en0", 65535, 0, 1, pointerof(err))

raise Exception.new("An error happened: #{String.new(err)}") if cap.null?

loop do
  result = LibPcap.pcap_next_ex(cap, out pkt_header, out pkt_data)
  case result
  when 1 # success
    puts pkt_data.to_slice(pkt_header.value.caplen).hexdump
  when 0 # timeout
    next
  end
end