This example gets the region that needs to be updated and then repaints the invalid region, if necessary.

#define INCL_WINWINDOWMGR
#define INCL_GPIREGIONS
#include <OS2.H>

HWND hwnd;
HRGN hrgn; /* region handle. */

if (RGN_NULL != WinQueryUpdateRegion(hwnd, hrgn)) {
           /* repaint the invalid region  */
                .
                .
                .

}