lua_pushcclosure - does not put values on the stack - lua

Question

Why lua_pushcclosure does not work ?
static int my_callback(lua_State* L)
{
std::cout << "my_callback" << std::endl;
std::cout << "PARAM1:" << lua_tostring(L, lua_upvalueindex(1)) << std::endl; //empty
std::cout << "PARAM2:" << lua_tostring(L, lua_upvalueindex(2)) << std::endl; //empty
return 0;
}
lua_pushstring(L, "param1");
lua_pushstring(L, "param2");
lua_pushcclosure(L, my_callback, 2);
lua_getfield(L, index, "Set_Callback");
lua_pushcfunction(L, my_callback);
int status_lua_pcall = lua_pcall(L, 1, 0, 0);
Why lua_pushcclosure does not work ?
After all, I set parameters 1 and 2 for the my_callback function. The my_callback function is called, but parameters 1 and 2 are empty.
Please explain me - how I can make a closure from - param1, param2, Set_callback, my_callback and lua_pushcclosure?
Or how do I then pass additional parameters to the my_callback function, which is called by the Set_callback method??
Here's how it works on pure Lua:
function my_callbac_func(param1, param2)
--some code...
end
param_1 = "Hello1"
param_2 = "Hello2"
my_table = func_Lua_app() -- The function returns a table in which there is a Set_callback method that needs to be called and passed as a parameter to the my_callback function itself.
my_table:Set_callback(function()my_callbac(param1, param2) end)
That is, I need to make an analog of this Lua code, only on the Lua C api.
Good, I try like this:
static int my_callback(lua_State* L)
{
std::cout << "my_callback" << std::endl;
std::cout << "PARAM1:" << lua_tostring(L, lua_upvalueindex(1)) << std::endl; //empty
std::cout << "PARAM2:" << lua_tostring(L, lua_upvalueindex(2)) << std::endl; //empty
return 0;
}
lua_getfield(L, index, "Set_Callback");
lua_pushstring(L, "param1");
lua_pushstring(L, "param2");
lua_pushcclosure(L, my_callback, 2);
int status_lua_pcall = lua_pcall(L, 3, 0, 0);
It doesn't work that way either.

Answer 1

Here is how each call affects the stack:
// stack: empty
lua_pushstring(L, "param1");
// stack: "param1"
lua_pushstring(L, "param2");
// stack: "param1", "param2"
// ^^^^^^^^^^^^^^^^^^
// upvalues
lua_pushcclosure(L, my_callback, 2);
// stack: the closure with upvalues
lua_getfield(L, index, "Set_Callback");
// stack: the closure with upvalues, the Set_Callback function
lua_pushcfunction(L, my_callback);
// stack: the closure with upvalues, the Set_Callback function, the cfunction without upvalues
// ^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
// what gets called parameter
int status_lua_pcall = lua_pcall(L, 1, 0, 0);
// stack: the closure with upvalues
You made the closure with upvalues and didn't use it. And then you called Set_Callback and the parameter you gave to Set_Callback was a different cfunction which had no upvalues. The one with upvalues is still on the stack, waiting to be used for something.
You need the closure with upvalues to be on the top of the stack when you get to lua_pcall, so that the parameter is the closure with upvalues.