How to get position of a child window in window coordinates

#include "DIALOG.h"


/*********************************************************************
*
*   	Static code
*
**********************************************************************
*/
/*********************************************************************
*
*   	_cbWinParent
*/
static void _cbWinParent(WM_MESSAGE * pMsg) {
  WM_HWIN  hChild;
  GUI_RECT Rect;
  int  	xPos;
  int  	yPos;


  switch (pMsg->MsgId) {
  case WM_PAINT:
	GUI_SetBkColor(GUI_RED);
	GUI_Clear();
	hChild = WM_GetFirstChild(pMsg->hWin);
	xPos = WM_GetWindowOrgX(hChild) - WM_GetWindowOrgX(pMsg->hWin);
	yPos = WM_GetWindowOrgY(hChild) - WM_GetWindowOrgY(pMsg->hWin);
	break;
  default:
	WM_DefaultProc(pMsg);
	break;
  }
}


/*********************************************************************
*
*   	_cbBk
*/
static void _cbWinChild(WM_MESSAGE * pMsg) {
  switch (pMsg->MsgId) {
  case WM_PAINT:
	GUI_SetBkColor(GUI_GREEN);
	GUI_Clear();
	break;
  default:
	WM_DefaultProc(pMsg);
	break;
  }
}


/*********************************************************************
*
*   	Public code
*
**********************************************************************
*/
/*********************************************************************
*
*   	MainTask
*/
void MainTask(void) {
  WM_HWIN hWinParent;
  WM_HWIN hWinChild;


  GUI_Init();
  hWinParent = WM_CreateWindowAsChild(10, 10, 150, 150, WM_HBKWIN, WM_CF_SHOW, _cbWinParent, 0);
  hWinChild  = WM_CreateWindowAsChild(10, 10, 50, 50, hWinParent, WM_CF_SHOW, _cbWinChild, 0);
  while (1) {
	GUI_Delay(100);
  }
}