Re: New dissector: variable length of a fields

[Guy Harris <guy () alum mit edu>]

On Feb 16, 2014, at 2:09 AM, Dmitrij K <kdiman () live ru> wrote:

> I have corrected that by:
>
> // for sz
> tvb_memcpy(tvb, (guint8 *)&ldata, offset, 2);

That will either give you the wrong answer on a little-endian machine (such as a 32-bit x86 or x86-64 PC) or on a 
big-endian machine (such as a PowerPC/MIPS/SPARC-based box under most operating systems).

Either the field is:

        defined as big-endian, in which case you should use tvb_get_ntohs();

        defined as little-endian, in which case you should use tvb_get_letohs();

        big-endian in some packets and little-endian in others, in which case you need to somehow figure out what the 
byte order is.

It appears, from the other calls you're making, that the field is little-endian.

> proto_tree_add_item(my_tree, hf_hdr_sz, tvb, offset, 2, TRUE); offset += 2;

You appear to be using Wireshark 1.10.x (based on the line number in the assertion failure in your earlier message); in 
1.10.x, the last argument to proto_tree_add_item() isn't a Boolean, it's an ENC_ value, so that should be

        proto_tree_add_item(my_tree, hf_hdr_sz, tvb, offset, 2, ENC_LITTLE_ENDIAN); offset += 2;

> // for e_flags:
> proto_tree_add_item(my_tree, hf_hdr_e_flags, tvb, offset, 2, TRUE); offset += 2;

The same applies there:

        proto_tree_add_item(my_tree, hf_hdr_e_flags, tvb, offset, 2, ENC_LITTLE_ENDIAN); offset += 2;

> // for magic3:
> proto_tree_add_item(my_tree, hf_hdr_magic3, tvb, offset, 1, FALSE); offset += 1;

That's a one-byte numerical field; the ENC_ argument is currently ignored, but the convention is to say

        proto_tree_add_item(my_tree, hf_hdr_magic3, tvb, offset, 1, ENC_NA); offset += 1;

The key fix was replacing "-1" as the length field in the above call with "1", as per my other mail.

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