Login problem of chatting program

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I was programming a chatting software tools. But there is a problem on login. Client has a receiving threads to receive server`s message, When the client receives the message, A login sign will be marked (the sign is initializated '-1', if login failed it will be '0'. if login success it will be '1'). I want the client check this login sign from time to time . I would like to ask how to deal with this problem.

 
int g_iLoginACK = -1;
................................
 extern int g_iLoginACK;
 while( 1 != g_iLoginACK )
 {
  //Show LoginDialog
  CLoginDlg *dlgLogin = new CLoginDlg();
  int i = 2;
  if( dlgLogin->DoModal() == IDOK ) 
  {
   // Get ID and Password
   CString strID;
   CString strPassword;
   dlgLogin->GetIDAndPassword(strID, strPassword);

   CString strSend;
   strSend = "001"+strID+strPassword+"1";
   m_sockP2P.SendTo(strSend, m_strServerIP, m_strServerPort);

   DWORD   dwStart   =   GetTickCount(); 
   while(  -1 == g_iLoginACK )
   {
		// Sleep(300);
   }
   // received login ACK
   if( 1 == g_iLoginACK )
   {
	delete dlgLogin;
	return true;
   }
   else
   {
	MessageBox("ID or Password Error", "Login Error!", MB_ICONERROR);
	g_iLoginACK = -1;
	continue;
	delete dlgLogin;
   }
  }
  else
  {
   delete dlgLogin;
   return false;
  }
 }
 return true;
}