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From: kilgallen@eisner.decus.org (Larry Kilgallen)
Subject: Re: Linking an ADA routine to a program written in C
Date: 1996/09/19
Message-ID: <1996Sep19.075836.1@eisner>#1/1
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 <01bba5e0$cf06b040$328371a5@dhoossr.iquest.com>
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Date: 1996-09-19T00:00:00+00:00
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In article <01bba5e0$cf06b040$328371a5@dhoossr.iquest.com>, "David C. Hoos, Sr." <david.c.hoos.sr@ada95.com> writes:

> This is an implementation-dependent thing.  Generally what has to be done
> is to tell the Ada compiler that the routine is to be called by a C
> compilation unit.  This is done with a pragma such a EXPORT or EXTERNAL,
> depending on the implementation.  Consult your compiler documentation,
> looking especially for discussions of Interface Programming.
> 
> Further, the issue of "withed" packages pretty much requires that your main
> program be an Ada program.  If you already have a suitable "main" program
> written in C, you would need to put an Ada "wrapper" around the C main. 
> The C main might have to be renamed, depending on the Ada implementation,
> because some Ada systems use a routine called "main", which would confuse
> the linker of there were two "main" symbols declared.
> 
> The Ada main program also allows the elaboration code for each compilation
> unit to be called before the main program begins execution.
> 
> The only kind of Ada unit which can be called from C is one in which there
> is NO elaboration code in the unit, or in any of the other Ada units it
> "withs".

Although the restrictions listed may be overly broad.  Although I
don't really use C, I do use shareable libraries with DEC Ada on
VMS (both platforms), and from what I understand of the mechanism
using C, even as the main program, should not be a barrier to
elaboration.  Perhaps I have just been lucky, but with 160,000 lines
of code, I have never had to worry about "elaboration", it "just
happens".  It uses the LIB$INITIALIZE mechanism, which would surely
not be inhibited by intermixing C.

Larry Kilgallen