Re: pointer conversion


"Richard Tobin" <richard@xxxxxxxxxxxxxxx> wrote in message news:fb9ran$1fap$1@xxxxxxxxxxxxxxxxxxxxxxxxxx
In article <_eidnezKNPZ67EXbnZ2dnUVZ_vShnZ2d@xxxxxxxxxxx>,
Eric Sosman <esosman@xxxxxxxxxxxxxxxxxxxx> wrote:

M> You will be OK. char is always 1 byte. So casting an arbitrary
M> pointer to a char *, adding an exact multiple of the size of
M> the original type, and casting back is guaranteed to preserve
M> alignment.

I am not so sure about that; would you care to cite C&V, please, if
you claim that it's guaranteed by the standard?

He's wrong: it's not guaranteed. Simple example:

int target = 42;
int *ptr = &target + 1; /* "an arbitrary pointer" */
ptr = (int*)((char*)ptr + sizeof *ptr); /* U.B. */

If the original pointer points at an actual object of its
type (so it's not "arbitrary"), the conversion is safe.

That seems to be excessive pedantry. He said it "preserves
alignment", not that it's legal. Presumably you would deny that
adding 2 to an int preserves it odd/even parity, because you might
choose INT_MAX.

Yes, to be a successful pedant you've got to be absolutely, precisely accurate.

Clearly if the pointer is misaligned, adding sizeof(*ptr) will preserve the (mis) alignment.

--
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