[Winpcap-users] Multithreaded programming with WinPCap

[Guy Harris]

Loris Degioanni wrote:
> Prachyabrued Mores wrote:
>> Surprisingly, -1 value (no wait, return immediately) is not mentioned 
>> in current version of WinPCap help file. I have to browse through the 
>> archives.
> 
> For pcap functions, we use the libpcap documentation from tcpdump.org, 
> which is not very complete.

...but it *does* mention "pcap_setnonblock()", which is the right way to 
request non-blocking mode (with WinPcap, it sets the timeout to -1 - and 
saves it, so that if you turn non-blocking mode off, it can put the old 
timeout back; on UN*Xes, it sets O_NONBLOCK); the libpcap documentation 
wouldn't mention -1, as that's not a generic feature of pcap_open_live().

> Note that setting the timeout to -1 essentially switches your 
> architecture to polling, which is efficient but wastes a lot of CPU.

Yup - the pcap_setnonblock() documentation says that:

	pcap_setnonblock() puts a capture descriptor, opened with 
pcap_open_live(), into "non-blocking" mode, or takes it out of 
"non-blocking" mode, depending on whether the nonblock argument is 
non-zero or zero. It has no effect on "savefiles". If there is an error, 
-1 is returned and errbuf is filled in with an appropriate error 
message; otherwise, 0 is returned. In "non-blocking" mode, an attempt to 
read from the capture descriptor with pcap_dispatch() will, if no 
packets are currently available to be read, return 0 immediately rather 
than blocking waiting for packets to arrive. pcap_loop() and pcap_next() 
will not work in "non-blocking" mode.