#include <pcap/pcap.h> int pcap_get_selectable_fd(pcap_t *p);
Note that a descriptor on which a read can be done without blocking may, on some platforms, not have any packets to read if the packet buffer timeout has expired. A call to pcap_dispatch() will return 0 in this case, but will not block.
Note that in:
select() and poll() do not work correctly on BPF devices; pcap_get_selectable_fd() will return a file descriptor on most of those versions (the exceptions being FreeBSD 4.3 and 4.4), but a simple select() or poll() will not indicate that the descriptor is readable until a full buffer's worth of packets is received, even if the packet timeout expires before then. To work around this, an application that uses select() or poll() to wait for packets to arrive must put the pcap_t in non-blocking mode, and must arrange that the select() or poll() have a timeout less than or equal to the packet buffer timeout, and must try to read packets after that timeout expires, regardless of whether select() or poll() indicated that the file descriptor for the pcap_t is ready to be read or not. (That workaround will not work in FreeBSD 4.3 and later; however, in FreeBSD 4.6 and later, select() and poll() work correctly on BPF devices, so the workaround isn't necessary, although it does no harm.)
Note also that poll() doesn't work on character special files, including BPF devices, in Mac OS X 10.4 and 10.5, so, while select() can be used on the descriptor returned by pcap_get_selectable_fd(), poll() cannot be used on it those versions of Mac OS X. Kqueues also don't work on that descriptor. poll(), but not kqueues, work on that descriptor in Mac OS X releases prior to 10.4; poll() and kqueues work on that descriptor in Mac OS X 10.6 and later.
pcap_get_selectable_fd() is not available on Windows.