Calling lua_pcall clear the lua_State

This is a simplified version of the code I'm testing:

int f(lua_State *L)
{
	std::cout << std::format("- Function call, stack size: {}, parameter value: {}\n", lua_gettop(L), lua_tointeger(L, 1));
	return 0;
}

int main()
{
	lua_State *const L = luaL_newstate();
	luaL_loadstring(L, "f1(1)");
	luaL_loadstring(L, "f2(2)");
	lua_register(L, "f1", f);
	lua_register(L, "f2", f);

	std::cout << "\nStack before call:\n";
	for (int b = 0, e = lua_gettop(L); b != e; ++b)
		std::cout << '\t' << lua_typename(L, lua_type(L, b + 1)) << '\n';

	for (int i = 0; lua_pcall(L, 0, LUA_MULTRET, 0) == 0; ++i)
	{
		std::cout << "\nStack on iteration " << i << '\n';
		for (int b = 0, e = lua_gettop(L); b != e; ++b)
			std::cout << '\t' << lua_typename(L, lua_type(L, b + 1)) << '\n';
	}

	std::cout << "Before exit: " << lua_tostring(L, -1);

	return 0;
}

I was expecting lua_pcall to call f twice each iteration of the loop, but this isn't how is it behaving, I'll paste the program output right below:

Stack before call:
        function
        function
- Function call, stack size: 1, parameter value: 2

Stack on iteration 0
        function
- Function call, stack size: 1, parameter value: 1

Stack on iteration 1
Before exit: attempt to call a nil value

I have two functions on the stack before the first call (they might be f1 and f2).

1. On the first call only f2 is called. f1 remains on the stack.
2. On the second call only f1 is called. The stack is empty.
3. On the third call, lua_pcall fails and ends the loop.

Looks like on each call one function is popped from the stack, so obviously I'm not understanding how the Lua library works. This is what I (wrongly) thought:

• Consecutive calls to luaL_loadstring over the same lua_State pointer, adds code to the lua state, as if we were concatenating scripts.
• Each call to lua_pcall over the same lua_State pointer, the state remains unchanged as if it was interpreting the loaded script.

What should I do in order to just execute (keep the state unchanged) the code I've added to the state with luaL_loadstring?

> Looks like on each call one function is popped from the stack

This is right, and your code is basically fine, the problem is after f1(1) is executed lua_gettop(L) becomes 0, so the next call to the lua_pcall function, you actually call an incorrect address. You may check the top first:

for (int i = 0; lua_gettop(L) > 0 && lua_pcall(L, 0, LUA_MULTRET, 0) == 0; ++i)

There is a api_checknelems line in the lua_pcall function, but by default it does nothing, you need to define LUAI_ASSERT while building the lua library.

For the same reason, you should not try to obtain a string anymore

std::cout << "Before exit: "; // << lua_tostring(L, -1);

> What should I do in order to just execute (keep the state unchanged) the code I've added to the state with luaL_loadstring?

You can use lua_pushvalue to duplicate a function before calling it.

for (int i = 0; lua_pushvalue(L, -1), lua_pcall(L, 0, LUA_MULTRET, 0) == 0; ++i)