From: "Claudio Bley" <bley@cs.uni-magdeburg.de>
To: Florian Weimer <fw@deneb.enyo.de>
Cc: "Joshua Nye" <josh@boxcarmedia.com>,
"Steve Dondley" <s@dondley.com>, <gcc-help@gcc.gnu.org>
Subject: Re: Avoiding "assignment from incompatible pointer type" warning
Date: Mon, 21 Oct 2002 06:48:00 -0000 [thread overview]
Message-ID: <15796.1484.410694.966062@wh2-19.st.uni-magdeburg.de> (raw)
In-Reply-To: <87hefguj01.fsf@deneb.enyo.de>
>>>>> "Florian" == Florian Weimer <fw@deneb.enyo.de> writes:
Florian> "Claudio Bley" <bley@cs.uni-magdeburg.de> writes:
>> >> What do you consider legal and why wouldn't it be?
>>
Florian> Casting a pointer from float to int can result in
Florian> undefined behavior on some implementations.
>> I can't see why. Can you elaborate on that?
Florian> Well, it's all in the standard, although it's a bit
Florian> scattered.
>> I mean, casting on the machine code level does just nothing -
>> it's just "syntactic sugar" to convince the compiler to be
>> quiet.
Florian> It's not. In C, pointers are NOT machine addresses.
Florian> Pointers are an abstract concept, and often, something
Florian> which would work if they were machine addresses, fails
Florian> miserably. For example, even if two pointers compare
Florian> equal (because they point to the same object), it's
Florian> possible that an operation on one of the two is
Florian> undefined, while it is defined for the other one.
>> I think casting from one pointer type to another does no harm
>> at all. Am I wrong?
Florian> Yes, you are, I'm afraid.
I'm afraid too! ;-) Really. I'm using C for quite a while now, read a
few books but I never realized that. I was thinking that a cast just
meant "pretend these bits have a different type, and treat them
accordingly". Also, as Mathew Moore mentioned, I was not thinking
about alignment issues.
Regards.
--
Claudio Bley ASCII ribbon campaign (")
Debian GNU/Linux advocate - against HTML email X
http://www.cs.uni-magdeburg.de/~bley/ & vCards / \
next prev parent reply other threads:[~2002-10-21 13:48 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-10-13 9:08 Steve Dondley
2002-10-13 9:28 ` Joshua Nye
2002-10-13 9:34 ` Steve Dondley
2002-10-13 15:09 ` Florian Weimer
2002-10-13 15:19 ` Joshua Nye
2002-10-15 11:24 ` Joshua Nye
2002-10-21 3:12 ` Florian Weimer
2002-10-21 4:22 ` Claudio Bley
2002-10-21 4:30 ` Florian Weimer
2002-10-21 6:48 ` Claudio Bley [this message]
2002-10-21 4:42 ` Sebastian Huber
2002-10-21 6:43 ` John Love-Jensen
2002-10-21 6:52 ` Florian Weimer
2002-10-13 13:37 Moore, Mathew L
2002-10-13 15:10 ` Florian Weimer
2002-10-21 6:07 Moore, Mathew L
2002-10-21 7:04 Moore, Mathew L
2002-10-21 7:17 ` Florian Weimer
2002-10-21 7:43 ` John Love-Jensen
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