Wireshark-dev: Re: [Wireshark-dev] New proto_add_bits function (Was: rev 21556:/trunk/epan//tr.
From: Richard van der Hoff <[email protected]>
Date: Tue, 15 Apr 2008 18:42:58 +0100
Hi all, and Anders in particular:

Anders Broman (AL/EAB) wrote:
Hi,
I have used it in the h263 dissector with a h263 prefix unfortunatly there is only short sequences
Beeing dissected most being the same in all h263 frames :(
Perhaps it can be used in the PER dissector but I didn't want to use it before we had agreed on a format to
avoid to many changes.
Regrads
Anders
I've just been investigating why packet colouring and display-filters 
weren't working on h263 picture headers. It turns out that the problem 
was that proto_tree_add_bits_ret_val extracts the value, returning 0 or 
1 (for a 1-bit field); proto_tree_set_uint then applies the bitmask 
specified in the hf_field definition - giving 0 for most fields.
It seems to me that the bitmask should be 0 for all fields intended for 
use with proto_tree_add_bits_*.
I've fixed this for h263 in r25050, but I'm concerned that there will 
other cases, both now (h264, maybe?) and in the future - and that such 
bugs will once again lie dormant until somebody tries to filter on the 
offending fields.
I'd therefore like to propose adding this to proto_tree_add_bits_ret_val:

+       if(hf_field -> bitmask != 0) {
+ REPORT_DISSECTOR_BUG(ep_strdup_printf("Incompatible use of proto_tree_add_bits_ret_val with field '%s' (%s) with bitmask != 0", + hf_field->abbrev, hf_field->name));
+       }
+

Comments?

Cheers

Richard