pcap_fileno - get the file descriptor for a live capture
int pcap_fileno(pcap_t *p);
If p refers to a network device that was opened for a live capture using
a combination of pcap_create(3PCAP) and pcap_activate(3PCAP), or
using pcap_open_live(3PCAP), pcap_fileno() returns the file
descriptor from which captured packets are read.
If p refers to a ``savefile'' that was opened using
functions such as pcap_open_offline(3PCAP) or
pcap_fopen_offline(3PCAP), a ``dead'' pcap_t opened using
pcap_open_dead(3PCAP), or a pcap_t that was created with
pcap_create() but that has not yet been activated with
pcap_activate(), it returns PCAP_ERROR.