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From: "Matthew Heaney" <matthew_heaney@acm.org>
Subject: Re: Function Calls by Address
Date: 1999/08/31
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In article <37CADE68.6AF06F5D@escmail.orl.lmco.com> , Craig Jameson 
<craigj@escmail.orl.lmco.com>  wrote:

> Does any one know how to make dynamic function calls in Ada83.
> I am writing an application using AIX Motif bindings and I
> need to create a dynamic list of functions to be called when
> a particular X event is dispatched.  Any ideas on how to
> create and use such a list?

We always used to use 'Address.  For example:

package P is

  procedure CB
    (W           : Widget;
     Client_Data : Addr_T;
     Call_Data   : Addr_T);

  pragma Interface (C, CB);
  pragma Interface_Name (CB, "pcb");

  pragma Export (CB, Name => "pcb");

(Or something like that.  You might not even need the pragma Export, because
you're referring to the subprogram by its address, not its name.  But I
don't remember.  Look in the vendor's compiler manuals about being able to
call Ada subprograms from another language.)

You can then say CB'Address, which you pass to the C side.

Make the subprogram is at library-level (though the compiler will probably
tell if it isn't).

Yes, this is vendor-specific, but I did X/Motif stuff in Ada83 for many
years (on VMS and SUN boxes), and never had a problem.

As others have pointed out, support for callbacks was added to the language
for Ada95.  The current debate is over support for some form of downward
closures.