Re: Not one stack but several? I'm mixed up

[Jerome Vuarand <jerome.vuarand@...>]

2013/1/6 Patrick <patrick@spellingbeewinnars.org>:
> So I am making progress learning the C API but there is something
> fundamental I am still mixed up about.
>
> After we create a Lua state most of the functions take a pointer to it:
>
> lua_DoSomething( Lua_State *L )
>
> I assume that this is a complex struct that contains the stack mechanisms.
>
> However rereading this in PIL2, on Page 241:
> An important concept here is the stack is not a global structure, each
> function has it's own private local stack
>
> If I push a string onto the stack and then push a number are they not
> sitting on the same data structure? In the Factor language it looks like
> they are but they are in fact not, from what I remember it is some sort of
> overloaded facility.

If you push a string and then a number, they both sit on the same data
structure. The following would print 2:

    L = luaL_newstate();
    lua_pushstring(L, "foo");
    lua_pushnumber(L, 42);
    printf("stack size: %d\n", lua_gettop(L));
    lua_close(L);

However if you push a string, and then call a function, and that
function push a number, they don't sit on the same stack. The
following would print 1:

static int f(lua_State* L)
{
    lua_pushnumber(L, 42);
    printf("stack size: %d\n", lua_gettop(L));
    return 0;
}

int main()
{
    lua_State* L;
    L = luaL_newstate();
    lua_pushstring(L, "foo");
    lua_pushcfunction(L, f);
    lua_pcall(L, 0, 0, 0);
    lua_close(L);
    return 0;
}

In that example, the function f doesn't have access to the stack of
the function main, and cannot access the string "foo". Note that the
functions need to be different in the Lua sense, not the C sense. You
need to use lua_call or lua_pcall to tell Lua that you are calling a
new Lua function. If you call directly f from main in C, they would
then share the same stack.

> Factors core is in C++, Lua is not, so I don't get how
> it could be overloaded and I don't understand how each function has it's own
> stack but "the stack"(as in singular) gets this value or that value if that
> is the case.
>
> Could someone straighten me out?

There is no need for overloading, since you have to explicitly use Lua
API functions (lua_call/lua_pcall) to trigger that change in the data
structure.

I hope this clarifies things for you.