This function sets a rectangle empty.

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

HAB       hab;       /*  Anchor-block handle. */
PRECTL    prclrect;  /*  Rectangle to be set empty. */
BOOL      rc;        /*  Success indicator. */

rc = WinSetRectEmpty(hab, prclrect);