This call obtains an update region of a window.

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

HWND    hwnd;         /*  Handle of window whose update region is to be queried. */
HRGN    hrgn;         /*  Handle of the window's update region. */
LONG    lComplexity;  /*  Complexity of resulting region/error indicator. */

lComplexity = WinQueryUpdateRegion(hwnd, hrgn);