Is it allowed to modify stack values as long as the stack stays balanced?

user_1197719 0
Viewed 99

Everybody knows it is good programming practice to keep the stack balanced. What I'm wondering, though, is whether I'm allowed to modify stack values in a C function called from a Lua script? Consider the following code:

int myfunc(lua_State *L)
{
    int arg1 = luaL_checkinteger(L, 1);
    int arg2 = luaL_checkinteger(L, 2);

    // pop arg 2
    lua_pop(L, 1);

    // this is to be our return value
    lua_newtable(L);

    ...do complicated stuff...

    // restore second parameter but set it to nil for convenience's sake
    lua_pushnil(L);
    lua_insert(L, 2);

    // return our table
    return 1;
}

So the code above replaces the second parameter with nil. Is this allowed or do I have to restore the original value, i.e. would I have to do

lua_pushinteger(L, arg2);

instead of

lua_pushnil(L);

? Or doesn't this matter as long as myfunc returns with the stack balanced?

lua
1 Answers

The stack values are the property of the C function being called. You can do whatever you wish with them. The only effect to the outside is the values returned by the function.

C functions called from Lua do not need to keep the stack balanced, that is, with the same contents or number of items it had on entry.

user_1070900
Related