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NNTP-Posting-Date: Fri, 14 Jan 2005 14:05:04 -0600
From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: Ada equivalent for C-pointer?
References: <mailman.52.1105722023.527.comp.lang.ada@ada-france.org>
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Date: Fri, 14 Jan 2005 14:05:04 -0600
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Date: 2005-01-14T14:05:04-06:00
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>a System.Address is what you would expect a pointer to be: a (eg) 32 bit
>number referring to a memory address, like a C pointer.
  Wrong.
>Most access types
>will be the same size as, and equivalent to, a System.Address, but some
  Wrong.
There is no requirement that a System.Address have any particular
representation.  For instance, quoting from the manual for a compiler
that handles the general segment:offset form of addressing in
Intel 8086 descended chips:
  type Address is record
    Offset : Offset_Type;
    Segment: Word;
  end record;
which is 48 bits.

>some access types may be bigger ("fat pointers") because ...
 Right.  Two different access types may have two different
representations, and neither is necessarily the same as a C pointer.

>       pragma Convention (C, C_Pointer);
  Right.  In Ada, one asks the compiler to handle such low-level stuff,
rather than worrying about it or trying to sneakily do it yourself.
That's why there is a pragma Convention to tell the compiler you want
something to be compatible with C (or Fortran, or whatever).

  Separately, and probably irrelevantly for the OP, there is a
package System.Address_to_Access_Conversions to let you tell the
compiler to convert between whatever form it is using for a particular
access type, and whatever form it is using for a System.Address.