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From: Stephen Leake <Stephen.Leake@gsfc.nasa.gov>
Subject: Re: How do I get this to work??
Date: 1999/01/07
Message-ID: <uhfu2bzmo.fsf@gsfc.nasa.gov>#1/1
X-Deja-AN: 429920101
References: <76s0dp$1v4$1@nntp3.uunet.ca> <76tbvv$ba5$1@nntp3.uunet.ca>
 <uiuelbj6r.fsf@gsfc.nasa.gov> <770ifd$qui$1@goblin.uunet.ca>
Organization: NASA Goddard Space Flight Center -- Greenbelt, Maryland USA
Newsgroups: comp.lang.ada
Date: 1999-01-07T00:00:00+00:00
List-Id: <comp.lang.ada>

"Chris Warwick" <chris.warwick@cdcgy.com> writes:

> Stephen Leake wrote in message ...
> 
> >Why does Close_Port need a pointer to a Session? If you are passing
> >Session to C code, you should probably use 'Address rather than
> >'access. Or, define Close_Port to take an access parameter:

As Robert Dewar pointed out, this was bad advice. Aparently too much
xmas turkey clogged the arteries to my brain :)

The current advice (I believe) is to declare the Ada procedure to take
an 'in' or 'in out' parameter, and let the compiler pass this to the C
code by reference.

This only works if the C code doesn't allow a null pointer as a valid
option; the correct way to pass a parameter that can either be a null
pointer or a pointer to an object is with an access type parameter,
and 'Unchecked_Access.

> I am indeed passing this to some C code, and I realize that what I am doing
> is cheating, just a little. The reason is I can't seem to get the stuff to
> the C function by value, if I pass it by reference its happy, but it barfs
> when I try to do it by value.

Post the code that fails to "pass by value"; maybe we can tell you
what's wrong with it.
 
> My old trick of translating pointers into Ada types by creating a function
> to return the record by value also doesn't work...
> 
> >function Close_Port (Session : access Session_Type);
> >
> >If you really, really need the interface you've given,
> >'unchecked_access will eliminate the error message.
> 
> 
> Thanks, it does indeed eliminate the error, but it leaves me with a bad
> taste. Somewhat like resorting to an unchecked_conversion.

I agree with the other comments here; the bad taste is from not
knowing what will happen to the pointer when you pass it to C. That's
why it is "unchecked".

> Is there a "proper" way to do this, or am I stuck with the unchecked
> solution?

If you don't need to be able to pass a null pointer, passing as an
'in' or 'in out' parameter should work, and would be the proper
solution.

-- Stephe