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From: blakemor@software.ORG (Alex Blakemore)
Newsgroups: comp.lang.ada
Subject: (none)
Message-ID: <8709152214.AA00945@ucbvax.Berkeley.EDU>
Date: Tue, 15-Sep-87 15:39:18 EDT
Article-I.D.: ucbvax.8709152214.AA00945
Posted: Tue Sep 15 15:39:18 1987
Date-Received: Thu, 17-Sep-87 06:38:23 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
List-Id: <comp.lang.ada>


    Paul Attermier at Sandia National Labs
wanted to know about using Ada with X windows.

    You can pass a callback array easily in Ada,
assuming your compiler supports the most basic
features from Chapter 13 of the LRM. (Most do).
An example follows :

with system;
procedure main is

  type call_back_arrays is array (1 .. 2) of system.address;
  call_back_array : call_back_arrays;

  procedure X_routine (call_back : system.address); -- passed as system.address
  pragma interface (C, X_routine);                  -- due to Verdix limitation
 
  procedure user_routine_1 is
  begin ...

  procedure user_routine_2 is
  begin ...

begin
  call_back_array (1) := user_routine_1'address;
  call_back_array (2) := user_routine_2'address;
  X_routine (call_back_array'address);
end main;

    We've successfully used this technique to bind Ada programs
to our Apollo X substitutes.  I don't believe this violates
the spirit of Ada, but like most Chapt 13 stuff, it should be
well isolated. What IS difficult in Ada is to call a routine
from an address known only at run time (ie. make a callback).
However, that doesn't appear quite as often and you can always
use a layer of C.  Or maybe try some unchecked conversion and
generics with subprogram formal parameters ... ?

                            Alex Blakemore
                            Software Productivity Consortium
                            Reston, VA