Re: index 0 for stack
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- Subject: Re: index 0 for stack
- From: Roberto Ierusalimschy <roberto@...>
- Date: 2004年5月03日 16:45:48 -0300
> ~~~~~~~~~~~ Quote from section 3.4 ~~~~~~~~~~~
>
> lua_type returns the type of a value in the stack, or LUA_TNONE for a
> non-valid index (that is, if that stack position is "empty").
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The documentation should be more explicit here: lua_type returns
LUA_TNONE for any non-valid, *but acceptable*, index.
> So, some invalid indexes are rejected, others are treated as
> valid. That's not consistent.
Those that are treated as valid are exaclty the acceptable indices.
The manual states that "Most query functions accept as indices any value
inside the available stack space, [...]. Such indices are called
acceptable indices." "Note that 0 is never an acceptable index."
> Of course, if adding additional check for index being 0 is against
> certain policy, or is not compatible with older releases, then just
> forget about my question.
As already pointed out, the check is already there, you only need
to turn it on:
> static TObject *negindex (lua_State *L, int idx) {
> if (idx > LUA_REGISTRYINDEX) {
> api_check(L, idx != 0 && -idx <= L->top - L->base); <<<<<!!!
Define api_check to be assert and go ahead.
> how can I find out value of "stackspace" that is used in check for
> validity of index?
It is not used for validity, but for acceptability. You do not need to
ask for that value, because you set it: It is "the maximum stack size
you have set through lua_checkstack".
-- Roberto