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From: dewar@cs.nyu.edu (Robert Dewar)
Subject: Re: Changing discriminants at run-time: erroneous execution?
Date: 1996/08/07
Message-ID: <dewar.839448772@schonberg>#1/1
X-Deja-AN: 172762892
references: <lhmg25z8v2x.fsf@berlin.berlin.informatik.uni-stuttgart.de>
organization: Courant Institute of Mathematical Sciences
newsgroups: comp.lang.ada
Date: 1996-08-07T00:00:00+00:00
List-Id: <comp.lang.ada>


Andre asks about the program

    with Ada.Text_IO;
    use  Ada.Text_IO;

    procedure Example is

       type Buffer (Size : Natural := 3) is
          record
             Value : String (1..Size);
          end record;

       Message : Buffer;
       X       : Integer;

    begin
       Message := (5, "abcde");
       Put_Line (Message.Value);  -- prints "abcde"
       X := 1;
       Put_Line (Message.Value);  -- prints "abcd"
    end Example;

The warning should not be ignored. Creating of a value of type Buffer requires
4 gigabytes of storage. This might work on some 64 bit machines, but will
surely not work on a 32-bit machine.

So why don't you get the storage error -- that's easily explained, you
are using one of the versions of GNAT that does not yet support stack
checking, and so of course this object overflowed the stack.

This behavior is not justified by the RM! This is a simply a matter of
a (documented) missing feature in some implementations of GNAT (we are
gradually adding stack checking to additional verisons of GNAT, it's
one of the hard things to do for a highly portable technology like this,
because stack checking tends to be extremely target dependent).

Always read the file "features" to find out what is implemented and
what is not!