Re: Calling a dissector: Type for data parameter

[David Perry <boolean263 () gmail com>]

Sorry to drag up an old topic, but I've been thinking about this:

> Message: 5
> Date: Sat, 29 May 2021 09:32:29 +0200
> From: Anders Broman <a.broman58 () gmail com>
> To: Developer support list for Wireshark <wireshark-dev () wireshark org>
> Subject: Re: [Wireshark-dev] Calling a dissector: Type for data
>         parameter
> Message-ID:
>         <CAOpyz=zDycm33PXUwtBCTew7gTTEcSLiJ-f8SHW0L-863Q517A () mail gmail com>
> Content-Type: text/plain; charset="utf-8"
>
> Hi,
> Yes the method is fragile. At the time of development I think it was
> proposed to pass a struct containing a string and the void pointer where
> the string could be used as a identifier. But that was voted down.
> Regards
> Anders

I wasn't around for that discussion so I don't know the reasons, but how 
does this sound as a refined approach?:

* Define a `dissector_data_t` that has a `guint32` identifier field, and 
a `void *` data field.

* Replace the `void *data` parameter to dissectors with a pointer to a 
`dissector_data_t`.

* Either:

    * Easy way: maintain a static list of identifiers that map to 
expected data types, or

    * Have dissector X request an identifier in its registration 
function for the type of data it expects, and have dissector Y (which 
will call X) request, in its handoff function, the identifier of the 
type of data it needs to pass to X.

* Dissectors check for the right identifier in their `dissector_data_t` 
parameter and don't try to use it if it's wrong.

Thoughts?

David


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