This function adds a region to a window's update region.

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

HWND    hwnd;              /*  Handle of window whose update region is to be changed. */
HRGN    hrgn;              /*  Handle of the region to be added to the update region of the window. */
BOOL    fIncludeChildren;  /*  Invalidation-scope indicator. */
BOOL    rc;                /*  Success indicator. */

rc = WinInvalidateRegion(hwnd, hrgn, fIncludeChildren);