Re: Compatibility Libpcap/Winpcap and timout of

[Benoit <benpaka.spam () gmail com>]

Okay so I should use something like this:

int ret=0;
if((ret =pcap_select_ms(ifhard->adhandle,ifhard->timeout)<=0)) {
if((ret=pcap_next_ex(ifhard->adhandle, &(ifhard->header), (const
u_char**)(&p_pkt))> 0) {
    ...
}
}
return ret;

This let me be sure that if no packet are received the pcap_select_ms() will
return 0 after timeout.
Then it will return a packet after the N ms (timeout setup during
open_live()).

This might work only on Windows and Linux (This is okay for me because we
don't use OSX in our lab).

Moreover sometimes I need to loop 10 times to be sure to receive
> correctly the packet,

Why?  Does pcap_next_ex() return 0 or a negative value?
it return 0 because the timeout is less than the time for: t(PC -> FPGA) +
t(FPGA setup) + t(FPGA -> PC).

Thanks for your help.

--
Benoit RAT
www.neub.co.nr


On Wed, Jan 28, 2009 at 1:26 AM, Guy Harris <guy () alum mit edu> wrote:

> On Jan 27, 2009, at 9:17 AM, Benoit wrote:
>
>  I've start a simple protocol to communicate with FPGA using only MAC
> > layer.
> > This software should run under linux and windows, however i've a problem
> > with the timeout of pcap_next_ex() function under linux.
>
> There is no guarantee that, with a timeout value of N milliseconds,
> pcap_next_ex() will return within N milliseconds.  It might *never* return
> if no packets arrive.
>
> The only *guarantee* that libpcap/WinPcap offers is that it will return
> within N milliseconds of the arrival of a packet.  On some platforms , the
> timer starts when pcap_next() or pcap_next_ex() or pcap_dispatch() or
> pcap_loop() is called.  On other platforms, the timer starts when the first
> packet arrives.  On other platforms, there isn't any timer.
>
> The timeout is *NOT* intended to be used to make sure you don't block
> forever waiting for a packet.  The timeout is intended to be used so that,
> on platforms where the underlying packet capture mechanism will wait some
> amount of time to try to deliver multiple packets in one read, rather than
> waking up the process for every packet (to reduce the number of wakeups and
> user->kernel and kernel->user transitions), it won't wait forever if at
> least one packet has arrived.
>
> If you want a timeout, so you don't wait forever for a packet to arrived,
> I'd suggest using select() or poll() on UN*X systems (other than *BSD and
> Mac OS X, where select() and poll() might not work correctly on BPF devices
> - and, in the case of Mac OS X, where poll() doesn't work *at all* on
> "character" devices such as BPF devices or ttys).
>
>  Moreover sometimes I need to loop 10 times to be sure to receive
> > correctly the packet,
>
> Why?  Does pcap_next_ex() return 0 or a negative value?
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