luaL_len

luaL_len[-0, +0, e]

lua_Integer luaL_len (lua_State *L, int index);

Returns the "length" of the value at the given index as a number; it is equivalent to the '#' operator in Lua (see §3.4.7). Raises an error if the result of the operation is not an integer. (This case only can happen through metamethods.)