a.c

int trace(lua_State* L) {
  if (!lua_isstring( L, 1 )) return 1;
  lua_getglobal(L, "debug");
  if (!lua_istable(L, -1)) {
    lua_pop(L, 1);
    return 1;
  }
  lua_getfield(L, -1, "traceback");
  if (!lua_isfunction(L, -1)) {
    lua_pop(L, 2);
    return 1;
  }
  lua_pushvalue(L, 1);
  lua_pushinteger(L, 2);
  lua_call(L, 2, 1);
  return 1;
}

bool run(lua_State* L, int args, int results) {
  int base = lua_gettop(L) - args;
  lua_pushcfunction(L, trace);
  lua_insert(L, base);
  int status = lua_pcall(L, args, results, base);
  lua_remove(L, base);
  if (status != 0) {
    error(lua_tostring(L, -1));
    return false;
  }
  return true;
}

int main() {
  // some lua state setup here
  luaL_loadfile(L, "b.lua");
  run(L, 0, LUA_MULTRET);
  luaL_loadfile(L, "c.lua");
  run(L, 0, LUA_MULTRET);
}

a.lua

local a = { }
a.x = 0
a.y = 0
a.z = 0
a.__index = a

function a:new(x, y, z)
  return setmetatable( { x = x, y = y, z = z }, a )
end

a.foo = a:new(1, 2, 3)

return setmetatable(a, { __call = function(self, ...) return a:new(...) end } )

b.lua

local a = require 'a'

print(a.foo.x) -- prints 1
print(a.foo.y) -- prints 2
print(a.foo.z) -- prints 3

c.lua

local a = require 'a'

print(a.foo.x) -- prints 1
print(a.foo.y) -- prints 2
print(a.foo.z) -- prints <random garbage value>