Setting a field optional when registering the header? (possible?)

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Thank you so much Graham for the answer. Although I have a question on the proto_tree_add_item thing. I have to register my header before I dissect yeah? So if I have different headers, can I only register the part of my header that is going to be that same and then dissect the other part according to the message using the proto_tree_add_item? Or do I register the whole header and then use proto_tree_add_item on the fields that I need to dissect?

Field registration is (normally) done before dissection, so register all fields, then dissect and add items as required.

I'm not quite sure I follow your question so this response may not be relevant but here it is anyway: keep in mind you're not registering an entire packet header (with the hf fields). You're registering individual fields. A given protocol header will (typically) consist of several (if not many) fields. For example a version field, a length field, etc.

Then you write C code which dissects these fields. Step 1 might be to dissect the version field. Once you know that you might call 2 different functions depending on the version. The v3 dissection function might dissect fields A, B, and C. The v4 dissection function might dissect fields A, X, Y, and Z (yes, a given field might be the same in both versions). What hf's you register must simply include all fields for all versions of the protocol.

Okay. So, from what I understand-

1. Register all fields in the header for all the types of header.
2. Dissect only the fields which I need until I get what type of packet I am dissecting.
3. Then dissect the packet accordingly.

Is that correct?

Also, another question from the Wireshark Developer Guide, the proto_tree_add_item, is it used to display the details on the screen yes?

The order you noted is correct, apart from the field registration should include all fields that might possibly be dissected.

Pedantic note, dissectors don't "display on the screen", they add items to the proto tree and\or columns that then allows whatever UI is being used, Qt, cli etc. to render the items accordingly. So yes, use proto_tree_add_item() to add protocol elements to the tree so that they can be displayed on screen.