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FW: Combining winpcap packet wait with poll/select in cygwin

Hi Brian 

Finally got a change to try this (sorry it tooks so long). It looks like it does NOT work.

The fd returned from cygwin_attach_handle_to_fd() always returns
from the poll call with a POLLIN event even though no packets
are flowing on the network (and hence there should be no pcap
packets received).

The code I used is attached below. I tried both blocking and non blocking
fcntl calls. But the poll awakes immediately all the time. If I try
to do a read of the fd, read returns a -1.

I also found another thread:

I didn't know what exactly to pass as the name (i.e. the first argument
to cygwin_attach_handle_to_fd()) but I tried NULL, "", "/dev/fd").

Maybe I need to pass the right type of device for a windows socket. Just can't
find what it is. I've looked at all the fhandler_base classes & derivatives
but can't seem to locate the "name" field. But I don't know C++ that well.

I'd be happy to try any other suggestions. Thanks.

main (int argc, char *argv[])
  pcap_t *adhandle;
  int pcap_fd;
  struct pollfd pcap_pfd;
  int i, ret;

  [initialization for winpcap adhandle]

  pcap_fd = cygwin_attach_handle_to_fd("/dev/fd", -1, pcap_getevent(adhandle),
                                       O_BINARY, GENERIC_READ);

  fcntl(pcap_fd, F_SETFL, O_NONBLOCK);

  pcap_pfd.fd = pcap_fd; =  POLLIN;
  pcap_pfd.revents = 0;

  while (1) {
      pcap_pfd.fd = pcap_fd; =  POLLIN;
      pcap_pfd.revents = 0;

      ret = poll(&pcap_pfd, 1, -1);
      if (ret> 0) {
          if (pcap_pfd.revents) { fprintf(stdout, "fd %d is awake on events %x\n", pcap_pfd.fd, pcap_pfd.revents); }
      else if (ret == 0) {
          printf("poll timeout!\n");
      } else if (ret < 0) {
          printf("poll errno=%d, %s\n", errno, strerror(errno));


> Summary: Has anyone tried to have events (POLL_IN/POLL_OUT) on regular
>          fd descriptors combined with winpcap packet receive handles
>          incorporated into the same sleep event (i.e. poll/select)?

No, I haven't tried because it looked too fragile if it even works, and
Cygwin's poll/select (or the preamble to read/recv; I'm not sure yet) was
causing significant packet reception jitter in my real time application.

> I know very little of windows but I understand both winpcap & cygwin
> poll (winsup/cygwin/ right?) use a call "WaitForMultipleObjects
> ()" to sleep and wait for new receive events. I've seen some of the code
> in but is there an easy way to translate the fd's into the
> HANDLE w4 array so I can somehow combine it with the winpcap HANDLE)?

You could try get_osfhandle, but I'd suggest going the other way around by
using Cygwin's cygwin_attach_handle_to_fd.  I don't know if it will work
for you, but this might get you an fd that you can use in poll/select for
the winpcap handle.

> My only other options (none of which I like) are:
> 1. Use threads.

I went this route and it worked out well for me, but my application was
already threaded, and it didn't have this kind of interaction.

> 2. Use winpcap to also receive IP packets and thus handle the telnet
> protocol in my program. However this is unnnecessarily complex.

Why did you choose this strange localhost telnet IPC to begin with?
Couldn't it just be a library?  I guess it might be because of privilege

> Any recommendations on what to do or more code to look at are greatly
> welcomed. Thanks!

I would appreciate you letting us know how it turns out.

Brian Ford
Lead Realtime Software Engineer
VITAL - Visual Simulation Systems
FlightSafety International
the best safety device in any aircraft is a well-trained crew...


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