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0 pointsmnarayan019y ago0 comments

So then I guess malloc can't return an allocation which actually goes to the end of the address space, but has to leave at least one extra byte to avoid overflow? That's pretty interesting, though I guess it certainly makes sense.

Edit: Also now that I think about it, I've written code that relied on that behavior...not sure if I'd heard it before and internalized and forgot it, or just was being foolish.

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cbsmith9y ago

Technically, this needn't impact malloc, because dereferencing the "one past the end" address is still undefined. All you need is logic in your pointer arithmetic that essentially treats the past the end address as a special value (which normally would never need to be represented).

int_19h9y ago

Not just pointer arithmetic; also pointer comparisons. One-past-the-end pointer must compare greater-than any other pointer into that array.

So an implementation that could stick an array at the very end of the address space, and do wraparound for one-past-the-end so that it's represented by all bits zero, would then need to special-case that zero value when performing any pointer comparisons.

cbsmith9y ago

Oh, I was assuming a runtime that didn't do wraparound. If you are doing wraparound, you already have a ton of special case work you'd have to do.

But yes, you'd need logic for pointer comparisons as well.

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0 pointsmnarayan019y ago0 comments

Edit: Also now that I think about it, I've written code that relied on that behavior...not sure if I'd heard it before and internalized and forgot it, or just was being foolish.

0 comments

cbsmith9y ago

int_19h9y ago

Not just pointer arithmetic; also pointer comparisons. One-past-the-end pointer must compare greater-than any other pointer into that array.

cbsmith9y ago

Oh, I was assuming a runtime that didn't do wraparound. If you are doing wraparound, you already have a ton of special case work you'd have to do.

But yes, you'd need logic for pointer comparisons as well.

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