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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: How do I write directly to a memory address?
Date: Tue, 8 Feb 2011 01:08:25 -0800 (PST)
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Date: 2011-02-08T01:08:25-08:00
List-Id: <comp.lang.ada>

Nasser M. Abbasi wrote on comp.lang.ada:
>> Georg Bauhaus wrote:
>>> void foo(int countdown)
>>> {
>>> =A0 =A0while (--countdown) {
>>> =A0 =A0 =A0fputc('.', stdout);
>>> =A0 =A0}
>>> =A0 =A0fputc('\n', stdout);
>>> }
>
>> BTW, like I said, *every* time I look at C code,
>
> This is funny, but I understand what you mean.
>
>> I see a bug. In your
>> case, foo has undefined behavior if countdown is negative.
>
> If I remember my C, since "while" will keep running until its argument
> becomes false, which in C means "zero", then this means if countdown is
> negative the above code will keep decrementing countdown until it hits
> -infinity? :) and the program will crash or do something else?

Actually, the bug also triggers if countdown =3D=3D 0 upon entry into foo,
because of the *prefix* decrement operator.

The behavior is undefined by the language but most C programmers
assume that counter will wrap around.  In fact, when GCC 4.5
introduced aggressive optimizations based on the knowledge that
overflow semantics was undefined in C, a *lot* of existing programs
were suddenly broken. In fact, only their incorrect assumptions
("hidden contracts") were broken.  The damage was so severe that the
GCC folks had to demote this aggressive optimization to a non-default
compiler switch :(

--
Ludovic Brenta.