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Date: Sun, 5 May 2013 18:24:19 +0200
From: Szabolcs Nagy <>
Subject: Re: [patch] add ether_(aton, ntoa)

* Strake <> [2013-05-05 09:02:04 -0500]:
> On 19/04/2013, Rich Felker <> wrote:
> > &x has the wrong type; strtoul requires char **, not const char **.
> > A separate temp var is required for the output, as in.
> >
> > 	char *y;
> > 	n = strtoul (x, &y, 16);
> > 	x = y;
> >
> > or similar. As far as I know the naive cast one could make to "fix"
> > the type mismatch is not valid; it would be an aliasing violation.
> What bad could happen?

the naive cast is strtoul(x, (char**)&x, 16)

(const char **) and (char **) are not compatible types
and they are not required to have the same representation
and alignment

so the conversion itself may invoke undefined behaviour

but even if the representation and alignment is the same
an object with effective type (const char*) cannot be
accessed through a (char*) lvalue expression within strtoul

so the aliasing rules are violated as well
(a compiler may reorder the loads from x and the stores
to *(char**)&x arbitarily)

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