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Re: Suggested warning: "negating an expression of unsigned type does not yield a negative value"

From: Jamie Lokier <jamie at shareable dot org>
To: Falk Hueffner <falk dot hueffner at student dot uni-tuebingen dot de>
Cc: Joe Buck <jbuck at synopsys dot com>, gcc at gcc dot gnu dot org
Date: Mon, 6 Oct 2003 18:23:07 +0100
Subject: Re: Suggested warning: "negating an expression of unsigned type does not yield a negative value"
References: <87zngev8i4.fsf@student.uni-tuebingen.de> <20031006085958.A12894@synopsys.com> <87pthauz1i.fsf@student.uni-tuebingen.de>

Falk Hueffner wrote:
> > The C and C++ standards require that unsigned values obey modulo 2**N
> > arithmetic, so the value of -x is rigorously defined.
> 
> Sure it is. But it is not what is intended. Example: x = 5, then
> -x=4294967291, i.e., p will be advanced by 4294967291 bytes, which is
> way beyond the legal range of p, but happens to work anyway on 32 bit
> architectures (but not on 64 bit architectures).

Good point.  That's elusive enough that I saw ptr[-x] and thought it
was was fine; I am now humbly corrected.

-- Jamie

References:
- Suggested warning: "negating an expression of unsigned type does not yield a negative value"
  - From: Falk Hueffner
- Re: Suggested warning: "negating an expression of unsigned type does not yield a negative value"
  - From: Joe Buck
- Re: Suggested warning: "negating an expression of unsigned type does not yield a negative value"
  - From: Falk Hueffner

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