Re: Is this a valid use lua_pushexternalstring()

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.

On Tue, 3 Feb 2026 at 16:15, Marc Balmer <[email protected]> wrote:

> The following code „seems“ to work, but is it valid to simply pass free()
> as allocf?
>
> char *mystring;
>
> mystring = strdup(„just an example“);
>
> lua_pushexternalstring(L, mystring, strlen(mystring), (lua_Alloc)free,
> mystring);
>
>
>
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