This function subtracts a rectangle 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. */
PRECTL    prclRect;                 /*  Rectangle to be subtracted from the window's update region. */
BOOL      fIncludeClippedChildren;  /*  Validation-scope indicator. */
BOOL      rc;                       /*  Success indicator. */

rc = WinValidateRect(hwnd, prclRect, fIncludeClippedChildren);