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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Passing Ada Proc as a C Function Pointer
Newsgroups: comp.lang.ada
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Date: Wed, 4 Aug 2010 21:36:38 +0200
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Date: 2010-08-04T21:36:35+02:00
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On Wed, 04 Aug 2010 11:46:56 -0700, Jeffrey Carter wrote:

> procedure Thread_Proc :    out Thread_Context;
>                         Proc    : in     Thread_Proc;
>                         Stack   :    out Stack_Area)
> is
>     type C_Context is ...;
>     -- Something C-compatible that corresponds to the C
>     -- definition of avr_thread_context, using only
>     -- convention-C components.
>     pragma Convention (C, C_Context);
> 
>     procedure Local_Proc is
>        -- null;
>     begin -- Local_Proc
>        Proc.all;
>     end Local_Proc;
>     pragma Convention (C, Local_Proc);

I am not sure if this works everywhere. The problem is that Local_Proc
refers to a local variable Proc (formal parameter of Thread_Start). This
needs some special techniques on the compiler side to become C-conform,
e.g. a trampoline.

Local_Proc should be moved out of Thread_Proc.

Then a copy of Proc is created in the scope of. That would be not task safe
without a protected object around the copy of Proc.

Another way could be passing a copy of Proc on Thread_Context.

But IMO there is nothing with putting C conventions on Ada callbacks. All
bindings I know, use it without hesitation.

> Are Context and Stack really needed outside Thread_Start?

In any case they need to be collected upon thread completion. Therefore
they must come from outside (unless Thread_Proc could free them, which is
unlikely).

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de