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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Convert between C "void*" pointer and an access
Date: Wed, 11 Oct 2017 17:32:35 -0500
Organization: JSA Research & Innovation
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Date: 2017-10-11T17:32:35-05:00
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"Victor Porton" <porton@narod.ru> wrote in message 
news:orm3qe$1r0c$1@gioia.aioe.org...
> What is the right way to convert between C "void*" pointer and an access 
> to
> a tagged or class-wide type?

I doubt that there is a *right* way, there just are several possibilities.

> Ada.Unchecked_Conversion seems to be what I need, but the access type may 
> be
> "fat" and thus have another format than void*.

Right, but not very likely. I'd expect this to work in most cases.

But our solution in the Claw libraries was simply to use the appropriate 
C-convention access type in the interface definitions and avoid making any 
conversions at all. I believe that on the C side, void* and <anything-else>* 
have to use the same representation, so C convention should work properly 
for any pointer type.

That is, if the C interface contains a void* parameter, we just used an 
appropriate C convention access type in its place in the Ada parameter 
definition. (Using overloading if we needed multiple such pointers - but 
that was very rare.) There's no counterpart to void* in Ada anyway, so one 
has to do something like that in the interfacing definitions.

                                                Randy.