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


On Mon, Oct 06, 2003 at 06:11:05PM +0200, Falk Hueffner wrote:
> Joe Buck <jbuck@synopsys.com> writes:
> > 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 catch.  It seems that the real problem is not negation of unsigned
values, but negation followed by extension to a potentially larger word size.
If you can figure out how to warn specifically about that, it might
be worth adding.  But it would be tricky.

 


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