[Winpcap-users] Strange behavior of pcap_next_ex

Jens Munk j.munk at maccor.com
Fri Dec 5 23:06:05 GMT 2008

Thank you so much. Now it works and give full speed.

I have an additional question. The data can be pulled out with both 
"pcap_next_ex" and a call back function loaded with "pcap_loop". Is 
there any advantage of one over the other? My main concern is lost 
packages. I do really, really need to get them all.


>> In my application, I am using pcap_sendpacket to send a raw packagage to
>> a networked microcontroller that is within a millisecond or so sending
>> another raw package as a reply (old microcontroller -> not IP), and I
>> can verify with the WireShark that this is working just fine. However,
>> the behavior of pcap_next_ex is not the way I expect. Due to its
>> blocking nature I have it sitting in a separate thread, so that should
>> be OK, but it doesn't return anything before it sees some kind of an IP
>> message like an ARP. Then, however, it keeps returning all the raw
>> messages since the last appearance of an IP message.
>> Is this the way it is supposed to be or am I doing something wrong?
> Please remember that there is a parameter called mintocopy. 
> pcap_next_ex will not return until
> - the timeout elapses (timeout that you chose in pcap_open_live)
> - at least mintocopy bytes have been received (default value = 
> 16000bytes)
> (whatever happens first).
> I would try to change the mintocopy value (pcap_setmintocopy) to a 
> very small value and see what happens.
> Have a nice day
> GV
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