This function checks whether a rectangle is empty.

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

HAB       hab;      /*  Anchor-block handle. */
PRECTL    prclprc;  /*  Rectangle to be checked. */
BOOL      rc;       /*  Empty indicator. */

rc = WinIsRectEmpty(hab, prclprc);