/* allow only one instance of a program to run */

DEFINE VARIABLE TheWindowHandle AS INTEGER NO-UNDO.

Run FindWindowA (0, "myApplication Window Title", OUTPUT TheWindowHandle).

IF TheWindowHandle NE 0 THEN DO:
    MESSAGE "The program is already running"
        VIEW-AS ALERT-BOX INFO BUTTONS OK.
    QUIT.
END.
ELSE
    RUN myApplication.w.

PROCEDURE FindWindowA EXTERNAL "USER32.DLL":
    DEFINE INPUT  PARAMETER LpClassName AS LONG      NO-UNDO.
    DEFINE INPUT  PARAMETER LpCaption   AS CHARACTER NO-UNDO.
    DEFINE RETURN PARAMETER LpHandle    AS LONG      NO-UNDO.
END PROCEDURE.