Re: Packet arrival order

["roy hills" <royhills () hotmail com>]

> > I'll need to check on how select() is behaving, but it is working, and
> > I'm only calling pcap_dispatch() if select() indicates that the pcap
> > fd is readable.
>
> Yeah, that's what I do too. However, if I remember correctly, I found
> out that select() was not blocking (as it should) when there was no
> data available, so I have a bit of extra CPU work. To test whether this
> is happening to you as well you can just put a printf() right after
> select()

I've done that.  What I find is that select() returns when there is any
data received on the network interface; not just data that will match
the pcap filter.  In other words, just because select() returned indicating
that the pcap file descriptor was ready to read does not mean that
there is interesting data.

I think this is because the DLPI implementation filters in userspace, not
in the kernel, unlike BPF and Linux.

> > but I then had to alter the code for BSD (BPF) to prevent buffering
>
> You mean setting BIOCIMMEDIATE on the BSDs, right?

Yes, that's what I do for BSD/BPF.

> > and work around select() issue,
>
> Would this be the issue of select() returning "there's data available
> to be read" only once? In other words, new data arrives and select()
> returns "there's data available". However, if more data arrives before
> you've read the data that was previously available, next time you call
> select() it won't tell you that there's data available.

It's that on some BPF implementations, the pcap file descriptor
is not selectable, so select() will always time out.  To work around
this, I call pcap_dispatch() after select times out for BSD platforms.

I use select() as both a timing loop for sending packets and also
to see when something is waiting, so this works OK for my application
because my timeout is only a few milliseconds.

Roy

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