On Wed, Aug 31, 2011 at 9:25 AM, Josh Simmons
<simmons.44@gmail.com> wrote:
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
>
... In
my experience this kind of function exists solely in internal helper
routines where the additional checking would be in error.
Programmers make mistakes when using library functions/methods, utility or not. For optimal productivity, a library should assert its preconditions, at least in debug build. It is an error *not* to check. Since some of us have to (for variety of reasons) use pre-built release-mode Lua DLL, where preconditions are therefore not asserted, and given that the "is-acceptable-index" formula is not trivial, and that this precondition is so prevalent in Lua lib, there should at least be a Lua lib function that allows to validate stack indices.
Oliver
