Re: Is this a valid use lua_pushexternalstring()

> Am 03.02.2026 um 16:58 schrieb Błażej Roszkowski <[email protected]>:
> 
> No, it's technically undefined behavior and you should not do that.
> 
> It happens to work because in case of integers or pointers the arguments and
> result are passed via registers on most calling conventions on most platforms
> you will run into.
> 
> In your case Lua's compiled C code will set 4 registers for ud, ptr, osize and
> nsize, jump to your function (free) and that function will only read the
> ud register (that you made contain the string too) so it's "okay". It will
> also not set the return register so it will contain some random temporary
> garbage when free returns to Lua. Luckily Lua doesn't check it.
> 
> If Lua checked that your dellocation function returns NULL, or if your
> deallocation function was using a different calling convention (one that
> uses different registers or uses or trashes the stack) your code
> wouldn't work anymore.
> 
> You also throw away type safety with that cast, so if Lua changed its alloc
> function to e.g. take extra argument lua_State as the first argument in a
> future release, your code would still compile, but then crash at runtime,
> instead of giving you a clear compile error about argument list mismatch.

Thank you for your comprehensive answer, which makes sense and was what I already suspected.

I now use this function to free external strings:

static void *
free_external_string(void *ud, void *ptr, size_t osize, size_t nsize)
{
        free(ud);
        return NULL;
}

And call lua_pushexternalstring() like so:

        lua_pushexternalstring(L, d->data, d->len,
            free_external_string, d->data);

- Marc

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