Wrap an existing C struct with Data_Wrap_Struct

I have a C extension I'm working on where I would like to take an
existing C struct (it's already been allocated) and wrap it for Ruby so
I can handle marking other objects.

I know about the Data_Wrap_Struct function. What I would like to do is
take an existing VALUE that's been created by a previous call to
Data_Wrap_Struct and add a reference to a mark function for it.

Is this possible?

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--
Darryl L. Pierce <mcpierce@gmail.com>
http://mcpierce.blogspot.com/
Famous last words:
   "I wonder what happens if we do it this way?"