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I am trying to write my own dissector and so far it works pretty well. However, I am using value_string but it does not work as expected. By studying other dissectors, I see that most value_string arrays look like the following:

static const value_string foo[] = {
    {1, "One"},
    {2, "Two"},
    {0, NULL} };

Instead of having integers as the value, I want to have characters, example:

static const value_string foo2[] = {
    {'A', "One"},
    {'B', "Two"},
    {0, NULL} };

My header field is the following:

{ &hf_field, /* supposed to be ampersand */
{ "Field", "proto.field", FT_STRING, STR_ASCII, VALS(foo2),0x00, NULL, HFILL }}

The idea behind having FT_STRING is that it should show the char value from foo2. However, this does not work. I get the following error message

"Err Field 'FIELDNAME' has a 'strings' value but is of type FT_STRING (which is not allowed to have strings)"

If I change from "FT_STRING, STR_ASCII" to "FT_UINT8, BASE_DEC" it works but it shows the ascii decimal value (example 65 instead of 'A').

Is there a way I can accomplish this? As previously mentioned, all dissectors I have studied only have integers as their values and not chars. Does that mean my idea is not possible?

I also want to point out that I read this which to me makes it sound like the value_string stuff does not support FT_STRING.

This question is marked "community wiki".

asked 21 Nov '16, 05:54

hokosha's gravatar image

accept rate: 0%

Indeed value_string arrays are designed to match a value (understand a number) and associate with it a string. FT_STRING is designed for fields containing a string, and cannot work with value_string arrays.

What you can eventually do is to use either the proto_tree_add_uint_format_value or proto_tree_add_string_format_value function to fully control the display. Note that it will not impact the filter content. Have a look at doc/README.dissector for more details.

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answered 21 Nov '16, 06:18

Pascal%20Quantin's gravatar image

Pascal Quantin
accept rate: 30%

You may also want to check out another type of value_string called a string_string. Unfortunately it seems this isn't yet supported in hf's (i.e., there's no VALS*() macro to allow your string_string to be put in the hf) but it should help...

(I wonder why string_string's aren't supported in hf's...)

(21 Nov '16, 08:20) JeffMorriss ♦

Note that master tree now supports a FT_CHAR, that could be used in this case:

"FT_CHAR An 8-bit ASCII character. It's treated similarly to an FT_UINT8, but is displayed as a C-style character constant."

(29 Nov '16, 03:24) Pascal Quantin
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question asked: 21 Nov '16, 05:54

question was seen: 1,023 times

last updated: 29 Nov '16, 03:24

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