Re: Size of a pointer to a tagged record

[Robert A Duff <bobduff@shell01.TheWorld.com>]

news01+Steven.Murdoch@cl.cam.ac.uk (Steven Murdoch) writes:

> 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:

All the Ada compilers I am familiar with represent access-to-tagged as a
single address.  The tag is stored in the tagged record, not in the
pointer.

I don't think it's even feasible to store the tag with the pointer,
because of a last-minute change to Ada 95 -- all tagged parameters are
aliased.

However, I don't understand what you're trying to do.  How can C code
produce a pointer to a tagged type?  I mean, the tag has to be
initialized on the Ada side.  And why must the *pointer* be heap
allocated?

If the tag *were* stored in the pointer, then you would not only have to
allocate space for it, but somehow set it to the right value.

> --
> 
> 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));
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
That code is wrong, so I'm not sure what you mean.

I presume Get_Void_Pointer is pragma Import(C).  And it does a "malloc",
passing sizeof(void *).  So it's returning a pointer to a pointer.  Is
that right?

Anyway, why not declare Get_Void_Pointer to return the access type?
I see no need for Address_To_Access_Conversions.

> 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.

You can find out the size of a pointer by writing a little test program
that prints out Parmeter_Access'Size, and running it on as many Ada
compilers as you can get your hands on.

- Bob