Size of a pointer to a tagged record

[news01+Steven.Murdoch@cl.cam.ac.uk (Steven Murdoch)]

I am writing a binding to the Lua (http://www.lua.org/)
scripting language (which is written in C). One thing I
need to do is store a pointer to a tagged record in a
memory location allocated by malloc. For this reason I need
to know the size.

If a pointer to a tagged record is equivalent to a
System.Address then I can store this in a memory location
of sizeof(void *), however if the tag is stored in the
pointer rather than the record then I would need to
allocate space for the tag as well.

If a pointer to a tagged record is equivalent to a
System.Address then I would use the following code:

--

type Parameter is tagged private;
type Parameter_Access is access all Parameter'Class;

package Convert is new
     System.Address_To_Access_Conversions(Parameter);

addr: System.Address;
param: Parameter_Access;

addr:=Get_Void_Pointer; --Void* from C
param:=Parameter_Access(User_Data_Convert.To_Pointer(Address));

--

At the end of this code I would use param as a normal pointer
to a tagged record. Does this look OK? Is there a better way
to achieve the same thing.

I am using GNAT 3.13p but I would like the code to be as
cross platform/compiler as possible.

Thanks in advance,
Steven Murdoch.