Re: Headroom

[Stephen Donnelly <stephen () endace com>]

On Fri, 2006-10-06 at 13:46 -0400, Harley Stenzel wrote:
> Greetings--
>
> Has the idea of headroom for libpcap-based stack applications been
> discussed before?  The idea is to have a known amount of empty/usable
> space in front of the buffer given to the application by the
> pcap_dispatch callback, pcap_next(), or pcap_next_ex().
>
> It looks like it'd be a ((pcap_t *)p)->headroom, which would be added
> to p->offset and p->bufsize for malloc.
>
> p->offset would still have to guarantee alignment for for the data,
> not for the beginning of the buffer.
>
> Headroom would have to be specified at pcap_open_live() or
> pcap_open_offline() time, so it could alter the pcap_t initialization
> logic. I suppose the best approach would be to make an additional
> variant of those functions, so the prototype changes don't ripple to
> other apps.  Possibly have it be an additional options structure,
> which may be null to get the current behavior.
>
> Any thoughts?

Wouldn't work with Endace DAG cards, as libpcap accesses memory mapped
packet buffers directly.

If this feature is accepted into libpcap there would need to be a way to
'reject' the option, perhaps via a specific function call like
pcap_setnonblock()?

Stephen.
-- 
-----------------------------------------------------------------------
    Stephen Donnelly BCMS PhD           email: sfd () endace com
    Endace Technology Ltd               phone: +64 7 839 0540
    Hamilton, New Zealand               cell:  +64 21 1104378
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