This function adds a rectangle 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. */
PRECTL    pwrc;              /*  Update rectangle. */
BOOL      fIncludeChildren;  /*  Invalidation-scope indicator. */
BOOL      rc;                /*  Success indicator. */

rc = WinInvalidateRect(hwnd, pwrc, fIncludeChildren);