This function returns the rectangle that bounds the update region of a specified window.

#define INCL_WINWINDOWMGR /* Or use INCL_WIN, INCL_PM, */
#include <os2.h>

HWND      hwnd;     /*  Handle of window whose update rectangle is to be queried. */
PRECTL    prclPrc;  /*  Update region that bounds the rectangle (in window coordinates). */
BOOL      rc;       /*  Success indicator. */

rc = WinQueryUpdateRect(hwnd, prclPrc);