[Winpcap-users] pcap_get_selectable_fd for WinPcap
guy at alum.mit.edu
Sun Apr 26 14:40:59 PDT 2009
On Apr 26, 2009, at 12:33 PM, Andreas Rieke wrote:
> on unix systems, the function pcap_get_selectable_fd is very useful
> event loops with the select function. Unfortunately,
> pcap_get_selectable_fd is not contained in the windows
> implementation -
> at least not in WinPcap 4.0.2.
> Is it planned to implement the function in 4.1 or later versions? Or
> it already implemented? Or is it technically hard/impossible to
> implement that on windows systems?
As far as I know, the answer to the last question there is "yes".
In UN*X systems, file descriptors are the lowest-level entities use to
refer to objects that can supply events (with the exception of window
system events in Mac OS X/iPhone OS), and select() and/or poll() and/
or kqueues and/or epoll - all of which work with file descriptors -
are the lowest-level APIs for implementing event loops.
In Windows, it's a little more complicated. HANDLEs are the lowest-
level entities used to refer to many objects that can supply events
(not including window system events), and WaitForMultipleEvents() are
the lowest-level APIs for implementing event loops.
*However*, Windows has a separate set of networking APIs that use
SOCKETs rather than HANDLEs to refer to network connections - which
are objects that can supply events - including a select() call similar
to the UN*X select() call, except that it *only* works on sockets -
> Can I maybe - with some modifications
> - use the handle returned by pcap_fileno within the select loop?
No. pcap_fileno(), on Windows, returns a HANDLE cast to DWORD, not a
SOCKET - and, as far as I know, there's no way to turn a handle of
some other sort into a SOCKET.
> I do know that windows has a similar function WaitForMultipleEvents,
> things are much easier when different versions of pcap are compatible;
> additionally, it makes no sense to use select for some events and
> WaitForMultipleEvents for some other events.
I agree. Welcome to the wonderful world of Windows, wherein, even if
you *don't* use WinPcap, you have to, as far as I know, use select()
for some events (socket events) and WaitForMultipleEvents() for other
events (and MsgWaitForMultipleEvents() if you also need to wait for
window system events), unless there's some way to get a waitable
HANDLE for a socket, in which case you could use
WaitForMultipleEvents()/MsgWaitForMultipleEvents() for everything. It
appears that WSAAsyncSelect() lets you turn socket events into
*window* events, but that doesn't help for programs that aren't GUI
The only way you're going to be able to use the *same* code for an
event loop would be if libpcap/WinPcap provided a function that took a
set of pcap_t's as arguments and blocked waiting for a packet to
arrive on one of them or for a timeout; on UN*X, it'd use select(),
and, on Windows, it'd use WaitForMultipleEvents(). That wouldn't help
if you want to wait on a mixture of pcap_t's and other event sources;
as far as I know, *nothing* can help you there.
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