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Comment by nwellnhof | Hacker News Reader

#define ARENA_ALIGN 8

This is unportable. You really need to pass the required alignment to the allocation function.

Or you can just use alignof(max_align_t), and your pointers will have the same alignment guarantees as malloc.

And I might be wrong, but even if you do accept a required alignment `align`, I think it's impossible in portable C to align a pointer `p` to it, unless you assume something not-strictly-portable about `(uintptr_t)p`, or `align` is less than some predetermined alignment that you used to allocate the char array.

> 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().

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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