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From: "Jeffrey R. Carter" <spam.jrcarter.not@spam.acm.org>
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Subject: Re: Possible compiler bug with this simple program
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Xref: g2news1.google.com comp.lang.ada:1952
Date: 2008-09-05T17:09:56+00:00
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Ludovic Brenta wrote:
> 
> I know using System.Address may seem unholy at first sight but this
> implementation is explicit about what it does -- dangerous things with
> an address received from the unsafe "external world" outside the
> control of the compiler.
> 
> Comments?

There's no guarantee that a C pointer and Ada's System.Address are the same 
thing. Of course, to be safe, you shouldn't use Integer, Long_Float, or any 
other type not declared in Interfaces.C and its children, or otherwise declared 
convention C. So really this should be something like

type Double_Ptr is access all Interfaces.C.Double;
pragma Convention (C, Double_Ptr);

procedure Ada_Callback (Length : Interfaces.C.Int; First_Component : Double_Ptr);
pragma Convention (C, Ada_Callback);

And then mess with pointer arithmetic (:

But a constrained array type is just as safe:

type C_List is array (Interfaces.C.Int range 0 .. Interfaces.C.Int'Last) of 
aliased Interfaces.C.Double;
pragma Convention (C, C_List);

procedure Ada_Callback (Length : Interfaces.C.Int; List : C_List);
pragma Convention (C, Ada_Callback);

And much easier to use.

-- 
Jeff Carter
"You me on the head hitted."
Never Give a Sucker an Even Break
108