Wireshark-dev: Re: [Wireshark-dev] Bitfield handling in proto_tree_add_uint_format()
From: Jaap Keuter <[email protected]>
Date: Mon, 19 Nov 2007 17:30:41 +0100
Hi,

From a code point of view (epan/proto.c) I can see why this is happening. From a API point of view this looks wrong. If I'm serving the function the value to work with I expect it to work with that value, not its own interpretation of it.
Let's have a look at what happens if we change this.

Thanx,
Jaap

Alexey Neyman wrote:
Hi all,

I have the following question: in the dissector I am writing, there is a bitfield occupying bits [2..7] of a byte. I have defined it as follows:
{ &hf_x, { "X", "p.x", FT_UINT8, BASE_HEX, NULL, 0xfc, "", HFILL }}

Everything is okay if I add that field using proto_tree_add_item(). However, the following code fails:
  x = tvb_get_guint8(tvb, 0) >> 2;
  proto_tree_add_uint_format(tree, hf_x, tvb, 0, 1, x,
	"the value of X formatted in some way");

The problem is that the value assigned to the "p.x" variable for packet matching is shifted 2 bits right one more time in proto_tree_set_uint(). Workaround is easy: for such fields, the bitmask could be specified as zero in header_field_info. I looked for other dissectors which might have faced such issue; in fact, epan/dissectors/packet-cimd.c just employs such workaround.
However, I wonder if this is a known and desired side-effect of 
proto_tree_add_uint_format() that makes its behavior different from 
proto_tree_add_item(). The doc/README.developer does not appear to 
mention that proto_tree_uint_format() takes not "final value" for fields 
with a bitmask, but rather raw, "unshifted" value.
Best regards,
Alexey Neyman.
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