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NNTP-Posting-Date: Tue, 11 Jan 2005 21:09:42 -0600
From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: Ada to C Interface: Passing Pointers Via System.Address
References: 
 <8e19522cfe30bb2ccbfe1393fdd6b235@localhost.talkaboutprogramming.com>
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Date: Tue, 11 Jan 2005 21:09:42 -0600
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Date: 2005-01-11T21:09:42-06:00
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> My C functions return pointers to data structures (very large structures).
> I need to then pass the pointer to the data struct to the Ada code and
> back into other C functions.  There is no interaction with the data
> structure in the Ada code except to pass the pointer from one C function
> to another.
  If the data being passed through Ada is a black box to Ada, why not
just declare a type of the appropriate size, eg
  type C_Thing is range 0 .. 2**32-1;
  for C_Thing'size use 32;
for the Ada code - Ada doesn't need to know if the thing is a pointer,
address, segment:offset, whatever.

> I have been trying to use the C interface pragma and return the pointers
> from C into an Ada System.Address back into the correct pointer type.
  If you declare something to be an Ada access type, and use a pragma
Convention C, the compiler should do the Right Thing.  Note that a
System.Address might not be the right thing (say if System.Address was a
segment:offset while the C pointer was just an offset in a fixed segment).

> So far I have not been able to get this to work properly.
  What kind of error are you getting?