Table as parameter in C function called from Lua - c++

Question

I'm dealing with Lua for a longer time but there is one point I can't achieve.
In a C function which is called from Lua I'm able to read a global Lua table using the table name like this:
C:
// get table
lua_getglobal(L, "tableName");
if (!lua_istable(L, -1))
break;
// count entries in table
ULONG numEntries = 0;
lua_pushnil(L);
while(lua_next(L,-2))
{
numEntries++;
lua_pop(L, 1);
}
but if I have a lua function which calls a C function like this:
Lua:
luaTable = { }
luaTable.Param1 = Value1
luaCallC("This is a Text", luaTable)
How do I access the table argument?
C:
// get table
// ???
// count entries in table
ULONG numEntries = 0;
lua_pushnil(L);
while(lua_next(L,-2))
{
numEntries++;
lua_pop(L, 1);
}

Answer 1

Arguments to a CFunction are pressed onto the virtual stack in the order that they are provided, and it is simply up to you to do the error checking required before you operate on these values.
Lua 5.3 Manual §4.8 - lua_CFunction:
In order to communicate properly with Lua, a C function must use the following protocol, which defines the way parameters and results are passed: a C function receives its arguments from Lua in its stack in direct order (the first argument is pushed first).
[ ... ]
The first argument (if any) is at index 1 and its last argument is at index lua_gettop(L). To return values to Lua, a C function just pushes them onto the stack, in direct order (the first result is pushed first), and returns the number of results
An example of exhaustively checking the number of elements in a table, with an arbitrary first argument.
int count (lua_State *L) {
luaL_checktype(L, 2, LUA_TTABLE);
puts(lua_tostring(L, 1));
size_t ec = 0;
lua_pushnil(L);
while (lua_next(L, 2)) {
lua_pop(L, 1);
ec++;
}
lua_pushinteger(L, (lua_Integer) ec);
return 1;
}
After registering the function for use in Lua:
count('foo', { 'a', 'b', 'c' }) -- 3