Setting userdata as namespace in Lua - c

Question

I have researched this subject and tried various approaches but I can't implement the behavior I have in mind (I'm not even sure it's possible). Basically, I have several userdata objects created in C that can be accessed by their metatable, like this:
Main.lua
config.display_width = 1280
What I'd like to do is to "force" the config namespace to a specific script. You've guessed it, I need to protect a configuration file so that users are restricted to deal only with the config metatable. Like this:
Config.lua
display_width = 1280
And I know I have to do something like this in C:
// Register the config metatable and its methods
luaL_loadfile(L, "my_config.cfg");
lua_getglobal(L, "config"); // Is this necessary?
lua_setfenv(L, -2); // I know this has to be used, but how?
lua_pcall(L, 0, 0, 0);
Thank you in advance, this one is driving me crazy!
PS: For the record, I really need to keep the config userdata as it is because it's binded to a C structure. In consequence, I'm not concerned about "losing" the Lua state or declared variables between different environments.
Adding the following information. This is how the config userdata is being created:
const struct luaL_Reg metaconfig[] =
{
{"__index", l_get},
{"__newindex", l_set},
{NULL, NULL}
};
lua_newuserdata(L, sizeof(void *));
luaL_newmetatable(L, "metaconfig");
luaL_register(L, NULL, metaconfig);
lua_setmetatable(L, -2);
lua_setglobal(L, "config");
So every time the user sets or gets values from the config userdata I update the C structure via the __index or__newindex methods.

Answer 1

you don't really need a global representing the config table, you can do with a lua_ref too.
Here this works as expected (I guess):
#include <lua.h>
#include <lauxlib.h>
#include <stdlib.h>
int main (void){
int idxConfig, res;
lua_State *L = luaL_newstate();
if ((res = luaL_loadfile(L,"my_config.cfg")) != 0){//Load file
printf("Got error code %d loading file my_config.cfg, exiting",res);
exit(-1);
}
lua_newtable(L); // new config table
lua_pushvalue(L,-1);// duplicate table
idxConfig = lua_ref(L,LUA_REGISTRYINDEX); // take a reference to the table (pops it)
lua_setfenv(L,-2); // pop table, set as environment for loaded chunk
lua_call(L,0,0); // load config -- nothing on stack
lua_rawgeti(L,LUA_REGISTRYINDEX,idxConfig); //push config table
lua_getfield(L,1,"display"); //read out "display"
lua_Integer disp_width = lua_tointeger(L,-1);
printf("Display width = %d",(int) disp_width);
lua_close(L);
exit(0);
}