pcap asynchronous packet capturing - asynchronous

Question

I am capturing IEEE802.11 packets with the pcap library. As yet i used pcap_loop and a callback function for receiving and then processing the packets. But now I have switch the wifi channel the device is listening on periodically. Unfortunately, pcap_loop is blocking so I can't call my function using a timeout.
Then I read about pcap_dispatch, but I don't really know how that should work asynchronous, because doing something like
while(1) {
int cnt = pcap_dispatch(handle, -1, callback, null);
}
wouldn't solve a thing.
So can anyone explain to me how to make pcap capture packets asynchronous / event based or in other words how to solve my dilemma?

Answer 1

To quote the pcap_get_selectable_fd() man page (this is the version from the trunk):
DESCRIPTION
pcap_get_selectable_fd() returns, on UNIX, a file descriptor number for
a file descriptor on which one can do a select() or poll() to wait for
it to be possible to read packets without blocking, if such a descrip‐
tor exists, or −1, if no such descriptor exists. Some network devices
opened with pcap_create() and pcap_activate(), or with
pcap_open_live(), do not support select() or poll() (for example, regu‐
lar network devices on FreeBSD 4.3 and 4.4, and Endace DAG devices), so
−1 is returned for those devices.
Note that in:
FreeBSD prior to FreeBSD 4.6;
NetBSD prior to NetBSD 3.0;
OpenBSD prior to OpenBSD 2.4;
Mac OS X prior to Mac OS X 10.7;
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 read
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 read 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, includ‐
ing 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.
In addition, the workaround won't work on Mac OS X 10.6, due to a bug in 10.6's BPF (introduced as part of a fix to another bug); the workaround will work on 10.7 and later, but isn't necessary.
So you could make it event-based if you have a select()/poll()/epoll()/kqueue/etc.-based event loop - add the descriptor you get from pcap_get_selectable_fd() as one of the file descriptors selected on in that event loop.