On Feb 21, 2014, at 11:43 AM, Hadriel Kaplan <[email protected]> wrote:
> A different question though is why FT_UINT64 isn't in the same group as the
> other FT_UINT* ones.
Because Wireshark was developed in an era where the majority of platforms on
which it ran were 32-bit, so we made the integral types 32-bit (not all C
compilers, at the time, supported 64-bit integral data types on 32-bit
platforms), and later added 64-bit integral types - and we were probably
thinking with a 32-bit mindset when we did that, as I think that predated
x86-64, so the majority of platforms, at least by "number of seats" rather than
by counting each ISA, in both its 32-bit and 64-bit versions, as a platform,
were probably still 32-bit.
The "value" union member of a "struct fvalue" is a union of a number of types,
the biggest of which are, on an ILP32 system, 128 bits (IPv6 address, GUID), so
it's not as if representing all integral values as 64-bit would increase the
memory usage of protocol trees.
It might increase memory bandwidth usage (but, with caches in the way, maybe
not), and might lengthen code paths a bit, but I don't know how big of a hit
that'd be.
It might be interesting to try, for example, running a huge capture file
through TShark with separate union members for up-to-32-bit and 64-bit integral
values, on a system with a 32-bit processor, and see whether it makes a
difference worth caring about. If not, we might want to consider storing all
integral values as 64-bit, and having FT_{U}INT64 in the same groups as the
other FT_{u}INTn types.
> Also, what about FT_NONE? Lots of current duplicate fields have one of the
> duplicates as FT_NONE - why I don't know, but I don't think that breaks
> filtering input.
If foo.bar is of type FT_UINTn, you could do "foo.bar == 17"; I'm not sure you
can do anything with an FT_NONE field other than test for its existence (if you
want to compare it with something, make it FT_BYTES).
So I'm not sure what it'd mean if a dissector had both FT_something and FT_NONE
versions of a field.
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