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From: emery@aries.mitre.org (David Emery)
Newsgroups: comp.lang.ada
Subject: Re: Calling Ada routines from C programs.
Message-ID: <EMERY.91Feb19100558@aries.mitre.org>
Date: 19 Feb 91 15:05:58 GMT
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Karl Keyte noted "the Ada system requires its complete environment in
order to function".  He's almost right.  The real issue is
elaboration.  Before an Ada subprogram is called, it must be
elaborated, along with everything it needs.  Furthermore, library
units must be elaborated before the main program starts.  This is what
must be done before a C program can invoke an Ada operation.  On at
least one compiler, there is a jump table containing the addresses of
the elaboration routines for everything that needs to be elaborated
before the main routine is invoked.  This table is "sorted" in
elaboration order (e.g.  pragma ELABORATE, etc is taken into account.)
It would be sufficient to have the C "_main" execute this code before
(or during) the first call to any Ada routine.  But, of course, how to
do this is highly compiler-dependent.  

In the short run, Karl's technique (having an Ada wrapper invoke the C
"_main" entry) should work.

				dave emery
				emery@aries.mitre.org