Re: Unaligned memory access

On Sat, Feb 14, 2026 at 9:39 AM Roberto Ierusalimschy <
[email protected]> wrote:

>
> Lua ensures that the memory in a userdata is aligned with values
> in ISO C. If you have other objects that require other alignments
> (such as 16-bit values), you can redefine the macro LUAI_MAXALIGN, in
> luaconf.h. Its standard definition is like this:
>
>   #define LUAI_MAXALIGN  lua_Number n; double u; void *s; lua_Integer i;
> long l
>
> Just toss these other objects in this list.
>

This assumes that 'realloc' returns memory aligned on 16-byte boundaries. I
don't think that's guaranteed by the standard, at least I could not find it
in the C23 one.

I would replace the standard Lua allocator with one that when it creates a
uservalue uses the (post-C89) 'aligned_alloc' function or whatever the
system provides
to create a properly aligned object. And of course you'd still have to add
one of those Intel-specific types to LUAI_MAXALIGN.


-- 
Gé

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