Re: Acceptable indicies

[Josh Simmons <simmons.44@...>]

On Wed, Aug 31, 2011 at 11:15 PM, oliver <oliver.schoenborn@gmail.com> wrote:
>
>
> On Tue, Aug 30, 2011 at 7:23 PM, Josh Simmons <simmons.44@gmail.com> wrote:
>>
>> On Wed, Aug 31, 2011 at 4:22 AM, oliver <oliver.schoenborn@gmail.com>
>> wrote:
>> >
>> >
>> > On Tue, Aug 30, 2011 at 11:39 AM, Robert Raschke
>> > <rtrlists@googlemail.com>
>> > wrote:
>> >>
>> >> On Tue, Aug 30, 2011 at 4:22 PM, oliver <oliver.schoenborn@gmail.com>
>> >> wrote:
>> >>>
>> >>> On Tue, Aug 30, 2011 at 10:58 AM, Robert Raschke
>> >>> <rtrlists@googlemail.com> wrote:
>> >>>>
>> >>>> On Tue, Aug 30, 2011 at 3:52 PM, Robert Raschke
>> >>>> <rtrlists@googlemail.com> wrote:
>> >>>>>
>> >>>>> On Tue, Aug 30, 2011 at 3:22 PM, oliver
>> >>>>> <oliver.schoenborn@gmail.com>
>> >>>>> wrote:
>> >>>>>>>
>> >>>>>>> Would it be correct to say, "all functions which take a stack
>> >>>>>>> index
>> >>>>>>> require that it is inside the available stack space",
>> >>>>>>> ...
>> >>>>>>> or when stackspace equals X
>> >>>>>>>    luaL_checkany(L,X+1) is undefined.
>> >>>>>>
>> >>>>>> BTW, is there a way of knowing "available stack space"? Alternately
>> >>>>>> a
>> >>>>>> "bool lua_isacceptable(L, index)" would be a nice function to have.
>> >>>>>> Oliver
>> >>>>>
>> >>>>> int lua_checkstack(lua_State *L, int extra);
>> >>>>>
>> >>>>> http://www.lua.org/manual/5.1/manual.html#lua_checkstack
>> >>>>>
>> >>>>
>> >>>> Actually, on reflection that's not exactly what you're asking for.
>> >>>> But
>> >>>> lua_gettop (http://www.lua.org/manual/5.1/manual.html#lua_gettop)
>> >>>> gets you
>> >>>> the size of the stack. So these two give you what you need, or not?
>> >>>>
>> >>>> Robby
>> >>>>
>> >>> I was under the impression that the stack has a certain size (say, 20,
>> >>> defined when the lua lib is compiled), and that gettop just returns
>> >>> the
>> >>> index of the top-most element on that stack. So if top=10, an index is
>> >>> still
>> >>> acceptable if it refers to 11, but not if it refers to 21. Is this
>> >>> defined
>> >>> rigorously anywhere?
>> >>> Oliver
>> >>
>> >> I'm might be misunderstanding what you are trying to do. If you want to
>> >> know if you can push more elements on to the stack then you use
>> >> lua_checkstack(). If you want to know how many elements currently
>> >> reside on
>> >> the stack, you use lua_gettop(). I fail to see why you might want to
>> >> know
>> >> how many currently unused, but allocated, stack spaces lie above the
>> >> current
>> >> top element within it.
>> >>
>> >> Robby
>> >>
>> >
>> > Because as Liam mentioned in another post in this thread, acceptable
>> > index
>> > is
>> > (index < 0 && abs(index) <= top) ||
>> >     (index > 0 && index <= stackspace)
>> > (which I had seen in several other places). The index is known, top is
>> > from
>> > lua_gettop, but what is stackspace? I can't just assume it is 20 since
>> > it
>> > might have been changed by whoever built the lua library.
>> > Oliver
>> >
>>
>> This is all a bit meta because there's no place you actually need to
>> validate an index without actually having to use it, i.e. there's no
>> case I'm aware of that you'd be using a positive stack index without
>> already knowing the height of the stack or having called checkstack.
>>
>> So why isn't there a lua_isacceptable function? Because there's no
>> case you actually need to use one (that I can think of, anyway).
>>
> Checkstack is only useful when you push stuff onto the stack. But how do you
> validate, say, an index you give to a C function that queries the Lua stack?
> For instance:
> int someFunc(int stackIndex) {
>     return lua_tonumber(L, stackIndex);
> }
> The lua_tonumber does not check that index is acceptable; if it is not, it
> can return garbage (0, NaN, whatever) and the caller wouldn't know that app
> state is now undefined.
> The only solution is to assert that stackIndex != 0 and abs(stackIndex) <=
> top, or?
> Oliver
>

When exactly have you defined a function that uses this paradigm. In
my experience this kind of function exists solely in internal helper
routines where the additional checking would be in error.

It just seems to be a bit of an invented problem from the limited
experience I have with the Lua API.