Title: Header field with scaling factor/units?
I have a common use case (hundreds to low thousands of data elements) where I need to take some data, encoded in an integer FT_UINT[8|16|32], sometimes has a bitmask applied, and needs to be multiplied by a scaling factor that may be an integer or floating point value, with an optional units string. I didn't see a use case in README.developer that directly handles this scenario.
I'm thinking about doing something like the following.
\code idea
proto_item *pi;
header_field_info *hf;
/* hf_index is the registered hf identifier */
pi = proto_tree_add_item(tree, hf_index, tvb, offset, length, ENC_BIG_ENDIAN);
hf = proto_registrar_get_nth(hf_index);
value = tvb_get_ntohX(tvb, offset);
tmpval = (value & hf->bitmask) >> hf->bitshift;
dblval = tmpval * scaling_factor;
if (units_str) {
proto_item_set_text(pi, "%s: %f %s", hf->name, dblval, units_str);
} else {
proto_item_set_text(pi, "%s: %f", hf->name, dblval);
}
\endcode
I can wrap this kind of code in one or more function(s), but I'm wondering if there is a recommended "Wireshark standard" solution.
Since at the moment it appears that I need to overwrite the item's text string to accomplish what I want, I was considering hijacking the 'strings' member to store the scaling factor and units strings. Then I could test for the existence of a scaling factor/units string in the hf->strings member. I'll probably have to package it into a VALS and use try_val_to_str to access the units string to remain compatible with 'proto_tree_add_item' before I rewrite the text representation. The scale factor code be encoded as a string where I'd have to convert it on the fly using some form of strto[d|l|ul]. Of course this could be just added inline with the dissector code, but it would be nice to have a place in the hf_register_info declaration that documents this information.
I would think it would be possible to extend the FT_ types with a constant, that informs the api that the scaling factor and units are encoded in 'hf->strings' as [{ 0, "0.25" } { 1, "pounds" }] with a new interface function or two to implement it.
Any thoughts on applying the proto_item_add_xxx interface to handle this use case?
How difficult would it be to allow a filter _expression_ to be able to search on a header field whose condition assumes that the scaling factor has been applied, i.e., the data is an integer and has a scaling factor of .25 and you want to filter its value using a floating point value (probably quite difficult I'm guessing)?
Thanks for any comments,
John Dill