Login problem of chatting program

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.

Code:

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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;
}

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