This function subtracts a region from the update region of an asynchronous paint window, marking that part of the window as visually valid.

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

HWND    hwnd;                     /*  Handle of window whose update region is changed. */
HRGN    hrgn;                     /*  Handle of subtracted region. */
BOOL    fIncludeClippedChildren;  /*  Validation-scope indicator. */
BOOL    rc;                       /*  Success indicator. */

rc = WinValidateRegion(hwnd, hrgn, fIncludeClippedChildren);