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From: Simon Clubley <clubley@remove_me.eisner.decus.org-Earth.UFP>
Newsgroups: comp.lang.ada
Subject: Re: How do I write directly to a memory address?
Date: Mon, 7 Feb 2011 12:31:29 +0000 (UTC)
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Date: 2011-02-07T12:31:29+00:00
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On 2011-02-07, Niklas Holsti <niklas.holsti@tidorum.invalid> wrote:
> Shark8 wrote:
>> *In some languages a "constant" [rather literal] was a label/pointer 
>> to the location which held the value thereof and one could therefore
>> "change the value of '4'" with some dirty-tricks/black-magic.
>
> A former colleague of mine spent a considerable time debugging a FORTRAN 
> IV program's wrong results before he found that a COMMON area contained, 
> in order, a one-dimensional array and a variable that was supposed to 
> hold the value of pi, as set up at program start. An assignment to the 
> array with an index that was too large had changed "pi" to something 
> else, with unfortunate consequences for later trigonometric calculations.
>
> Unless your Ada program uses hardware (MMU) protection for its 
> memory-resident constants you can change the "constants" through 
> accesses that you get from address-to-access conversions.
>

I think the "change the value of 4" reference above is literally that and
does not mean changing a language accessible constant containing the value
of 4.

I made this mistake once with a old Fortran compiler when I was a student.

The constant would be placed into a memory location somewhere by the Fortran
compiler and then that memory location would be referenced at every point in
the code in which the literal value (the integer 4 in this example) was used
in a expression or a call to a subroutine.

In my case, I assigned (IIRC) to the calling argument within the subroutine
and the memory location containing the literal value (in this example, 4)
was changed. This meant that the value of 4 was changed for the rest of
the program.

I did not make this mistake a second time. :-)

Simon.

-- 
Simon Clubley, clubley@remove_me.eisner.decus.org-Earth.UFP
Microsoft: Bringing you 1980s technology to a 21st century world