[Frank Singleton <frank.singleton@xxxxxxxxxxxx>]

Hi,
Here is an example of how I organize my header and C files.
Similar to what has been proposed, but helps me understand
whats going on..

Any comments / Frank..


<snip from clcoip.h>
#undef NEW_INTERFACE		/* top dissector called with  tbvuffs */

#ifdef NEW_INTERFACE
void dissect_clcoip(tvbuff_t *, packet_info *, proto_tree *); /* new
interface */
#else
void dissect_clcoip(const u_char *, int, frame_data *, proto_tree *); /*
old interafce */
#endif;

<end snip from clcoip.h>

<snip from clcoip.c>


/* Code to actually dissect the packets */


#ifdef NEW_INTERFACE
void dissect_clcoip(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
  guint msgtype;		/* need here, as must update COL_INFO before */
				/* the if(tree) code */

#else
void dissect_clcoip(const u_char *pd, int offset, frame_data *fd,
proto_tree *tree) {
  tvbuff_t *tvb;
  packet_info *pinfo = &pi;
  guint msgtype;		/* need here, as must update COL_INFO before */
				/* the if(tree) code */

  tvb = tvb_new_subset(pinfo->compat_top_tvb, offset, 
	pinfo->captured_len - offset, pinfo->len - offset );

  /* or use tvb_create_from_top macro instead */

#endif
	 


Gilbert Ramirez wrote:
> 
> In a previous e-mail I suggested the usage of
> 
>         tvbuff_t *tvb = tvb_new_subset(pi.compat_top_tvb, offset, -1, -1);
> 
> to create a tvbuff for a dissector which did not accept a tvbuff in its
> argument list (all of the dissector-table dissectors).
> 
> This suggestion is wrong, because some dissectors (ethernet!) can modify
> the length of their packet data. That is, some ethernet packets have a trailer,
> so those trailing bytes are excluded from the next tvbuff
> (and are subtracted from pi.len and pi.captured_len).
> 
> The correct usage is more like:
> 
>         tvb_new_subset(pi.compat_top_tvb, offset, pi.captured_len - offset,
>                         pi.len - offset);
> 
> To make this easier to use, I have added a macro called tvb_create_from_top() to
> tvbuff.h (which is automatically included in each dissector through packet.h)
> 
> It takes one argument, the name of your offset variable. (Some authors use 'offset',
> some use 'o').
> 
> Here's an example from dissect_ipx():
> 
> #if 0
> void
> dissect_ipx(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
> {
> #else
> void
> dissect_ipx(const u_char *pd, int offset, frame_data *fd, proto_tree *tree)
> {
>         packet_info     *pinfo = &pi;
>         tvbuff_t        *tvb = tvb_create_from_top(offset);
> #endif
> 
> I have documented this change in doc/README.tvbuff, and have modifed all the
> dissectors that needed this change.
> 
> --gilbert

-- 
EUS/GN/V/Z Frank Singleton    ASO Americas BSS
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