Re: Question for data argument of dissector_try_heuristic

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

On Mar 25, 2013, at 1:57 PM, Roland Knall <rknall () gmail com> wrote:

> Under Mac OS X some warnings came up, and among them is a repeated
> warning about an unused *data parameter for every call to a heuristic
> dissector.

Or, rather, a repeated warning about an unused *data parameter in every *definition* of a heuristic dissector *that 
doesn't use it or explicitly flag it as unused*.

No existing dissectors use it, but they all flag it as unused, so the warnings do *not* come up when you compile the 
Wireshark trunk; they only show up if you're adding your own dissector, or have changed an existing dissector, and have 
a heuristic dissector that doesn't use the argument and doesn't flag it as unused.

> As I do use the heuristic dissector, but do not use the
> data argument, I looked up every occurence, where dissection is passed
> to a heuristic dissector. In every case, NULL is passed as *data
> argument.

To be precise, in the *current* Wireshark source tree, NULL is passed as the *data argument in all calls to 
dissector_try_heuristic().

> So my question is this: What is the *data arguments intention

To allow the calling dissector to pass private information to the called dissector, rather than using the 
pinfo->private_data variable; that way, you don't have to worry about, for example, saving and restoring private_data 
values.  (It replaces a quasi-global variable with an argument.)

> and who uses it actually?

No heuristic dissectors currently use it, but it's there for use in the future; we may convert heuristic dissectors to 
use it in the future.

> But from a software development point of view, this is faulty
> behaviour. If we do have an argument we should use it or throw it
> away.

In places where arbitrary functions can be registered in tables, or passed as callback routines, the function signature 
is fixed, but not all functions necessarily have a use for all the information passed to them, even if some functions 
do.
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