Here is the code I use, posted by someone else here long ago.

DECLARE INTEGER CreateEvent IN WIN32API ;
   INTEGER lpEventAttributes, ;
   SHORT bManualReset, ;
   SHORT bInitialState, ;
   STRING @ lpName
DECLARE INTEGER GetLastError IN Win32API
DECLARE CloseHandle IN Win32API INTEGER hObject
nEh = CreateEvent(0,0,1, PROGRAM(0) + '.EVENT')
IF GetLastError() = 183 OR nEh = 0
   *  There's an instance running already (183) or the Event can't be defined
   *  So don't do it;  do release the handle, since it's harmless and should
   *  be done rather than relying on Windows to clean up after itself
   =CloseHandle(nEh)
   QUIT
ENDIF

Bob



>Dear Tore,
>Thank for your reply, but I want to ask what is
>the 'Mutex', I copy your code to my startup program,
>but I got an error for the class 'Mutex' not define.
>
>Please advise.
>
>Thanks.