Wireshark · Wireshark-dev: Re: [Wireshark-dev] [Wireshark-commits] rev 35393: /trunk/epan/dissectors/ /trunk/epan/dissectors/:

[Jeff Morriss <jeff.morriss.ws@xxxxxxxxx>]

Anders Broman wrote:
>   Martin Mathieson skrev 2011-01-06 17:03:
> > Thanks, I do believe that this is a special case - I wouldn't want to 
> > use tvb_get_ptr() anywhere else.
> >
> > Regards,
> > Martin
> >
> > On Thu, Jan 6, 2011 at 3:58 PM, Jeff Morriss <jeff.morriss.ws 
> > <http://jeff.morriss.ws>@gmail.com <http://gmail.com>> wrote:
> >
> >     Hi Martin,
> >
> >     Yeah, the code looked safe, I've just been on a mission to reduce
> >     the usage of tvb_get_ptr() (which I'd love to put in the category
> >     of "do not use!"--the only problem there being that it's used all
> >     over the place).  It did occur to me later that it might be
> >     slower;  I'll revert the change in a bit.
> >
> >     Regards,
> >     -Jeff
> >
> >     Martin Mathieson wrote:
> >
> >         Jeff,
> >         I made the change to use tvb_get_ptr() because a profile
> >         showed that getting the strings each time was quite slow.
> >         The reason I thought this is safe is that this protocol is
> >         really a header written out by the corresponding wiretap
> >         module, so it should be well-formed (if the file being read
> >         isn't well-formed that will be rejected by the wiretap module).
> >
> >         I can't remember how much slower it was, but it gets done
> >         several times for each frame read from the file.
> >
> >         Best regards,
> >         Martin
> Could tvb_get_ephemeral_stringz() be made more efficient?

I suppose that most of the work is fixed: allocating some (ep) memory, 
(in the case of the *z functions, finding the length of the string), 
then doing the memcpy().  I can't think of any meaningful optimizations.

Martin, I assume the pre-tvb_get_ptr() code here was similar to this 
change in that it only retrieved the string once?  (I ask since several 
of the strings are used multiple times.)

I suppose for cases like this a get_string function that uses the TVB as 
the backing store (and only pushes the tvb_get_strsize()+tvb_get_ptr() 
out of the dissector and into tvbuff.c) might be an option to help 
reduce the prevalence of tvb_get_ptr() in dissectors.  That may not be a 
bad idea because I think I saw some other dissectors doing the same 
thing as this one--and why take the performance hit?