Re: stack overflow?

[Rici Lake <lua@...>]

On 30-Aug-05, at 9:24 PM, Mike Pall wrote:

> Hi,
>
> Rici Lake wrote:
> > By the way, the largest possible Lua stack frame is 255 slots.
> > (LUA_MAXSTACK + LUA_EXTRA_STACK, but don't change these!)
>
> 255 is just the limit for the directly addressable slots
> (aka local variables). A Lua function will happily pass
> on any number for open operations (return f(), g(f()),
> {f()}, return ..., f(...), {...}).
>
> Examples (Lua 5.1 required):
>
> $ lua -e 'print(string.byte(string.rep("\0", 1000), 1, -1))'
> $ lua -e 'print(#{string.byte(string.rep("\0", 1000), 1, -1)})'
>
> [The limit here is LUAI_MAXCSTACK (2048).]

That's correct, but I think it is a shade of meaning. The extra stack 
slots here come from the following stack frame, in effect; they could 
not be there had they not been created in the following stack. So one 
reading of what is happening here is that the downstream stack frame is 
being merged with the current stack frame and the merger is being 
returned (or used as a table constructor). In any event, the size of 
the total stack cannot exceed the maximum recursion depth * the maximum 
stack size for each frame (calculated independently for Lua frames and 
C frames.)

I don't know if that was clear, but it made sense to me :)

> But we can do better:
>
>   local function foo(n, ...)
>     if n == 0 then return ... else return foo(n-1, 0, ...) end
>   end
>
>   print(#{foo(30000)})
>
> [This will take a long time.]

That is an exception I had not contemplated, indeed. I don't know that 
there is a limit to the number of varargs, but it really ought to be 
LUAI_MAXCSTACK.

> And unless I'm mistaken there is no relationship between
> MAXSTACK and EXTRA_STACK.

I never said there was. I just said that the sum of those two values in 
the default Lua happens to be 255. Or, rather, I meant to say that. :)

>  Only MAXSTACK has to be <= MAXARG_A
> (255). EXTRA_STACK is used for easier calling of metamethods.
> There are no arithmetics involved that would truncate the
> sum of those two to 8 bits.
>
> Please correct me if I'm wrong, because I increased EXTRA_STACK
> (for a good, but complicated to explain reason).
>
> Bye,
>      Mike