How does one call a Lua function from the C API?

https://www.lua.org/pil/25.2.html

// Push the function on to the stack
// Then push N arguments on to the stack
int top = lua_gettop(L);
int R = lua_call(L, N, LUA_MULTRET, 0);
int nresults = lua_gettop(L) - top;