./lua/addtest.lua:9: attempt to index local 'testobj' (a userdata value)]]

Question

test.exe call addTest.lua and set the lua_testobj to the table, and addTest.lua call testobj.dll, but testobj.dll can not get the "lua_testobj"

error msg is

addTest.lua:9 attempt to index local 'testobj' (a userdata value)

1. test.exe

   L = luaL_newstate();
   // link lua lib
   luaL_openlibs(L);
   //
   addLuaCPath( L, "./clib/?.dll" );
   //
   lua_pushlightuserdata(L, (void*)g_TestObj.get()); // g_TestObj is a global vars
   lua_setfield(L, LUA_REGISTRYINDEX, "lua_testobj");
   // 
   int err = 0;
   err = luaL_loadfile( L, "./lua/addTest.lua" );
   if( err != LUA_OK )
     printf("Failed to load addTest.lua![%s]", lua_tostring(L,-1));
   
   err =  lua_pcall( L, 0, 1, 0 );
   if( err != LUA_OK )
     printf("Failed to call addTest.lua![%s]", lua_tostring(L,-1));
   

2. the addtest.lua code is following

   local luapath = package.path
   local cpath = package.cpath
   
   print(luapath)
   print(cpath)
   
   local testobj= require "testobj"
   
   testobj.addTest()
   

3. and the testobj.dll source code is following

   static int laddTest(lua_State *L)
   {
     lua_getfield(L, LUA_REGISTRYINDEX, "lua_testobj");
     return 1;
   }
   
   extern "C" int __declspec(dllexport) 
   luaopen_testobj(lua_State *L)
   {
     luaL_Reg l[] = {
       { "addTest", laddTest },
       { NULL, NULL },
     };
   
     luaL_checkversion(L);
     luaL_newlib(L,l);
   
     lua_getfield(L, LUA_REGISTRYINDEX, "lua_testobj");
     CTestObj* pTestObj = static_cast<CTestObj*>( lua_touserdata(L,-1) );
   
     return 1;
   }
   

Answer 1

It looks like testobj.dll did actually return your lua_testobj successfully because the error you're getting:

addTest.lua:9 attempt to index local 'testobj' (a userdata value)

indicates lua sees testobj as a userdata. That's not where the problem is; the real issue is that you didn't associate any metatable with that userdata so lua can't really do anything with it when a script tries to use it.

I've modified your luaopen_testobj to create and register a metatable for your testobj:

extern "C" int __declspec(dllexport) 
luaopen_testobj(lua_State *L) 
{
  luaL_Reg l[] = 
  {
      { "addTest", laddTest },
      { NULL, NULL },
  };

  luaL_checkversion(L);
  lua_pushlightuserdata(L, (void*)g_TestObj.get());

  // g_TestObj, testobj_mt, {l}
  luaL_newmetatable(L, "lua_testobj");
  luaL_newlib(L, l);
  // testobj_mt.__index = {l}
  lua_setfield(L, -2, "__index");

  // return setmetatable(g_TestObj, testobj_mt)
  lua_setmetatable(L, -2);
  return 1;
}

This should allow you to access laddTest using testobj:addTest() from lua. laddtest should check that testobj is indeed the userdata you passed in, for example:

static int laddTest(lua_State *L)
{
    auto pTestObj = reinterpret_cast<CTestObj *> (luaL_checkudata(L, 1, "lua_testobj"));
    // do something ...
    return 1;
}

Answer 2

I've modified the luaopen_testobj function, add lua_pop(L, 1); before return

extern "C" int __declspec(dllexport) 
luaopen_testobj(lua_State *L)
{
  luaL_Reg l[] = {
    { "addTest", laddTest },
    { NULL, NULL },
  };

  luaL_checkversion(L);
  luaL_newlib(L,l);

  lua_getfield(L, LUA_REGISTRYINDEX, "lua_testobj");
  CTestObj* pTestObj = static_cast<CTestObj*>( lua_touserdata(L,-1) );

  lua_pop(L, 1);

  return 1;
}

Now, it can correctly to obtain the value of pTestObj, no longer appear "addTest.lua:9 attempt to index local 'testobj' (a userdata value)" wrong.

But I don't know what's the meaning of the error message