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From: news01+Steven.Murdoch@cl.cam.ac.uk (Steven Murdoch)
Newsgroups: comp.lang.ada
Subject: Creating tagged records with C "malloc" rather than Ada "new"
Date: 7 Dec 2002 00:58:01 GMT
Organization: University of Cambridge, England
Sender: sjm217@solway.cl.cam.ac.uk (Steven Murdoch)
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Date: 2002-12-07T00:58:01+00:00
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This post is again about my binding from Ada95 to Lua. In the last
thread I proposed and asked for help for a way of giving Lua access
to tagged records by asking Lua to assign enough space for a pointer
to a tagged record, then storing the pointer in the memory location
provided by Lua. This works (thanks to all those who helped) but I
have another possibility and would greatly appreciate any advice on
it.

Lua provide a garbage collection facility, which I would like to use
(along with some other neat features), however I can only do this
if I get Lua to allocate the memory for the object. Currently I
get Lua to allocate space for the access type, but I allocate space
for the tagged record using new. In order to reclaim the space
allocated by new I would need to add a hook to the Lua garbage
collector to deallocate this space when the pointer is freed.

However I think I can simplify this by allocating space for the
tagged record in Lua, and hence it will deallocate this space at
the right time. I would like to know if this is possible to do this
reliably, and if it is relativly safe,

The code I am proposing would look something like as follows:

C:
/* Actually part of Lua, but this is equivalent */
void *allocate(int size) {
        return malloc(size);
}

Ada:
type Parameter is tagged private;
type Parameter_Access is access Parameter'Class;

function Allocate(Size: Storage_Unit) return Parameter_Access;
pragma Import(C, Allocate);

function Add_To_Lua(Param: in Parameter) return Parameter_Access
is
	Param_Ptr: Parameter_Access;
begin
        Param_Ptr:=Allocate(Parameter'Max_Size_In_Storage_Units);
        Param_Ptr.all:=Param; --[1]
        return Param_Ptr;
end;

Will this do what I want and is is comapritively safe. In particular
will statement [1] sort of the tag of Param properly? Are there any
other potential problems.

Thank you in advance,
Steven Murdoch.