Re: A question about lua_toclose()

[Roberto Ierusalimschy <roberto@...>]

> >>> What do you mean by "safe"? What is exactly your concerns?
> >> 
> >> I mean, if the C stack unwind before calling __close when raising an
> >> error, the pointer is unsafe. because it's on the C stack.
> > 
> [...]
> 
> int
> foobar(lua_State *L) {
>   struct mystruct foo;
>   lua_settop(L, 0);
> 
>   lua_newtable(L);   // index 1 <—— CLOSED
>   lua_pushlightuserdata(L, &foo);
>   [...]
> 
> Lua calls CLOSED.__close() after foobar() returns, but CLOSED._ptr is the address on the C stackframe of the foobar(). It fells like:
> 
> [...]

That is right! Sorry about my wrong answer. I did not realize that 'foo'
was a stack-allocated variable, not a pointer.

As Francisco pointed out, that code "smells fishy" :-) It is
always slippery to put pointers to stack-allocated variables into
heap-allocated ones. It may be worth adding a note in the manual about
this interaction between lua_toclose and stack-allocated variables.

-- Roberto