pcap_read_linux_mmap() not returning packets on time

[Cristian Morales Vega <cristian () samknows com>]

I am having a problem using libpcap in my program and I would like a
bit of guidance before I try to understand how Linux PACKET_MMAP
works.

Using an unpatched libpcap 1.4.0 (and a OpenWRT patched  1.1.1) I find
this call:

pcap_dispatch(pcap_descr, -1, placeholder, NULL);

after a blocking, successful, read() sometimes doesn't return any packet.

Adding a few printf() to libpcap I found that in
pcap_read_linux-mmap() sometimes,
a) The first pcap_get_ring_frame() returns NULL
b) poll() immediately returns 1 (pcap_descr is non blocking)
c) The pcap_get_ring_frame() in the while returns NULL in the first iteration

Since poll() says there is a packet, shouldn't pcap_get_ring_frame
return it? I mean, shouldn't be 100% guaranteed?

The behaviour I am seeing is that for some periods all pcap_dispatch()
calls return 1 without even calling poll(). But for some other
periods, that can last *minutes* hundreds of pcap_dispatch() calls
return 0 just to then get all these missing packets in a single
pcap_dispatch() call.


The program is running in one of these things:
http://wiki.openwrt.org/toh/tp-link/tl-wdr3600. I don't discard the
possibility of a problem in the network driver. But what kind of
problem would make read() and poll() see the packets, but not
pcap_get_ring_frame()?

There is a bug here (probably not in libpcap) or is all this expected?


Thanks.

-- 
Cristian Morales Vega

Email cristian () samknows com
Office +44 (0) 20 3111 4330
Web:  www.samknows.com


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