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Comment by teo_zero | Hacker News Reader

> unless you assume something not-strictly-portable about `(uintptr_t)p`

What's not strictly portable about it?

  padding = -(uintptr_t)p & (align - 1)

The only assumption here is that align is a power of 2, which is guaranteed if it's the result of an alignof().

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It'll work in reality, but I don't think C makes any guarantees about how the pointer p is represented in (uintptr_t)p, only that a roundtrip pointer -> uintptr_t -> pointer will give back p. So (as far as I know) you can't assume that the offset needed to make (uintptr_t)p a multiple of `align` is the same as the offset needed to align p to `align`.

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