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Notifier(3)		    Tcl	Library	Procedures		   Notifier(3)


       Tcl_CreateEventSource,	 Tcl_DeleteEventSource,	  Tcl_SetMaxBlockTime,
       Tcl_QueueEvent, Tcl_ThreadQueueEvent, Tcl_ThreadAlert,  Tcl_GetCurrent-
       Thread,	 Tcl_DeleteEvents,   Tcl_InitNotifier,	 Tcl_FinalizeNotifier,
       Tcl_WaitForEvent,  Tcl_AlertNotifier,   Tcl_SetTimer,   Tcl_ServiceAll,
       Tcl_ServiceEvent,  Tcl_GetServiceMode, Tcl_SetServiceMode, Tcl_Service-
       ModeHook, Tcl_SetNotifier - the event queue and notifier	interfaces

       #include	<tcl.h>

       Tcl_CreateEventSource(setupProc,	checkProc, clientData)

       Tcl_DeleteEventSource(setupProc,	checkProc, clientData)


       Tcl_QueueEvent(evPtr, position)

       Tcl_ThreadQueueEvent(threadId, evPtr, position)



       Tcl_DeleteEvents(deleteProc, clientData)












       Tcl_EventSetupProc *setupProc (in)		  Procedure to	invoke
							  to prepare for event
							  wait		    in

       Tcl_EventCheckProc *checkProc (in)		  Procedure	   for
							  Tcl_DoOneEvent    to
							  invoke after waiting
							  for events.	Checks
							  to see if any	events
							  have	occurred  and,
							  if so, queues	them.

       ClientData clientData (in)			  Arbitrary   one-word
							  value	to pass	to se-
							  tupProc,  checkProc,
							  or deleteProc.

       const Tcl_Time *timePtr (in)			  Indicates the	 maxi-
							  mum  amount  of time
							  to   wait   for   an
							  event.     This   is
							  specified as an  in-
							  terval  (how long to
							  wait), not an	 abso-
							  lute	time  (when to
							  wakeup).    If   the
							  pointer   passed  to
							  Tcl_WaitForEvent  is
							  NULL,	it means there
							  is no	 maximum  wait
							  time:	  wait forever
							  if necessary.

       Tcl_Event *evPtr	(in)				  An event to  add  to
							  the	event	queue.
							  The storage for  the
							  event	must have been
							  allocated   by   the
							  caller using Tcl_Al-
							  loc or ckalloc.

       Tcl_QueuePosition position (in)			  Where	to add the new
							  event	 in the	queue:
							  TCL_QUEUE_HEAD,   or

       Tcl_ThreadId threadId (in)			  A unique  identifier
							  for a	thread.

       Tcl_EventDeleteProc *deleteProc (in)		  Procedure  to	invoke
							  for	each	queued
							  event	 in  Tcl_Dele-

       int flags (in)					  What types of	events
							  to  service.	 These
							  flags	are  the  same
							  as  those  passed to

       int mode	(in)					  Indicates    whether
							  events   should   be
							  serviced by Tcl_Ser-
							  viceAll.    Must  be
							  one	of    TCL_SER-
							  VICE_NONE	    or

       Tcl_NotifierProcs* notifierProcPtr (in)		  Structure  of	 func-
							  tion	 pointers  de-
							  scribing    notifier
							  procedures  that are
							  to replace the  ones
							  installed in the ex-
							  ecutable.   See  RE-
							  for details.

       The interfaces described	here are used to customize the Tcl event loop.
       The two most common customizations are to add new sources of events and
       to merge	Tcl's event loop with some other event loop, such as one  pro-
       vided  by an application	in which Tcl is	embedded.  Each	of these tasks
       is described in a separate section below.

       The procedures in this manual entry are	the  building  blocks  out  of
       which the Tcl event notifier is constructed.  The event notifier	is the
       lowest layer in the Tcl event mechanism.	 It consists of	three things:

       [1]    Event sources: these represent the ways in which events  can  be
	      generated.   For example,	there is a timer event source that im-
	      plements the Tcl_CreateTimerHandler procedure and	the after com-
	      mand,  and  there	 is  a	file  event source that	implements the
	      Tcl_CreateFileHandler  procedure	on  Unix  systems.   An	 event
	      source must work with the	notifier to detect events at the right
	      times, record them on the	event  queue,  and  eventually	notify
	      higher-level  software  that they	have occurred.	The procedures
	      Tcl_CreateEventSource,   Tcl_DeleteEventSource,	and   Tcl_Set-
	      MaxBlockTime, Tcl_QueueEvent, and	Tcl_DeleteEvents are used pri-
	      marily by	event sources.

       [2]    The event	queue: for non-threaded	applications, there is a  sin-
	      gle queue	for the	whole application, containing events that have
	      been detected but	not yet	serviced.  Event sources place	events
	      onto  the	queue so that they may be processed in order at	appro-
	      priate times during the event loop. The event queue guarantees a
	      fair  discipline	of event handling, so that no event source can
	      starve the others.  It also allows events	to be saved  for  ser-
	      vicing  at a future time.	 Threaded applications work in a simi-
	      lar manner, except that there is a separate event	queue for each
	      thread  containing  a  Tcl  interpreter.	Tcl_QueueEvent is used
	      (primarily by event sources) to add events to  the  event	 queue
	      and  Tcl_DeleteEvents  is	 used  to remove events	from the queue
	      without	processing   them.    In   a   threaded	  application,
	      Tcl_QueueEvent  adds an event to the current thread's queue, and
	      Tcl_ThreadQueueEvent adds	an event to  a	queue  in  a  specific

       [3]    The  event  loop:	in order to detect and process events, the ap-
	      plication	enters a loop that waits for events to	occur,	places
	      them on the event	queue, and then	processes them.	 Most applica-
	      tions will do this  by  calling  the  procedure  Tcl_DoOneEvent,
	      which is described in a separate manual entry.

       Most  Tcl applications need not worry about any of the internals	of the
       Tcl notifier.  However, the notifier now	has enough flexibility	to  be
       retargeted  either  for a new platform or to use	an external event loop
       (such as	the Motif event	loop, when Tcl is embedded in a	Motif applica-
       tion).	The  procedures	Tcl_WaitForEvent and Tcl_SetTimer are normally
       implemented by Tcl, but may be replaced with new	versions  to  retarget
       the  notifier (the Tcl_InitNotifier, Tcl_AlertNotifier, Tcl_FinalizeNo-
       tifier,	Tcl_Sleep,  Tcl_CreateFileHandler,  and	 Tcl_DeleteFileHandler
       must  also be replaced; see CREATING A NEW NOTIFIER below for details).
       The procedures  Tcl_ServiceAll,	Tcl_ServiceEvent,  Tcl_GetServiceMode,
       and Tcl_SetServiceMode are provided to help connect Tcl's event loop to
       an external event loop such as Motif's.

       The easiest way to understand how the notifier  works  is  to  consider
       what happens when Tcl_DoOneEvent	is called.  Tcl_DoOneEvent is passed a
       flags argument that indicates what sort of events it is OK  to  process
       and   also   whether   or   not	to  block  if  no  events  are	ready.
       Tcl_DoOneEvent does the following things:

       [1]    Check the	event queue to see if it contains any events that  can
	      be serviced.  If so, service the first possible event, remove it
	      from the queue, and return.  It does this	 by  calling  Tcl_Ser-
	      viceEvent	and passing in the flags argument.

       [2]    Prepare  to  block for an	event.	To do this, Tcl_DoOneEvent in-
	      vokes a setup procedure in each event source.  The event	source
	      will  perform  event-source specific initialization and possibly
	      call Tcl_SetMaxBlockTime to limit	how long Tcl_WaitForEvent will
	      block if no new events occur.

       [3]    Call  Tcl_WaitForEvent.	This  procedure	is implemented differ-
	      ently on different platforms;  it	waits for an event  to	occur,
	      based  on	the information	provided by the	event sources.	It may
	      cause the	application to block if	timePtr	specifies an  interval
	      other  than 0.  Tcl_WaitForEvent returns when something has hap-
	      pened, such as a file becoming readable or the interval given by
	      timePtr  expiring.   If there are	no events for Tcl_WaitForEvent
	      to wait for, so that it would block forever, then	it returns im-
	      mediately	and Tcl_DoOneEvent returns 0.

       [4]    Call  a  check procedure in each event source.  The check	proce-
	      dure determines whether any events of interest  to  this	source
	      occurred.	 If so,	the events are added to	the event queue.

       [5]    Check  the event queue to	see if it contains any events that can
	      be serviced.  If so, service the first possible event, remove it
	      from the queue, and return.

       [6]    See  if  there  are idle callbacks pending. If so, invoke	all of
	      them and return.

       [7]    Either return 0 to indicate that no events  were	ready,	or  go
	      back to step [2] if blocking was requested by the	caller.

       An  event  source consists of three procedures invoked by the notifier,
       plus additional C procedures that are invoked by	higher-level  code  to
       arrange for event-driven	callbacks.  The	three procedures called	by the
       notifier	consist	of the setup and  check	 procedures  described	above,
       plus  an	 additional procedure that is invoked when an event is removed
       from the	event queue for	servicing.

       The procedure Tcl_CreateEventSource creates a new  event	 source.   Its
       arguments specify the setup procedure and check procedure for the event
       source.	SetupProc should match the following prototype:

	      typedef void Tcl_EventSetupProc(
		      ClientData clientData,
		      int flags);

       The clientData argument will be the same	as the clientData argument  to
       Tcl_CreateEventSource;  it is typically used to point to	private	infor-
       mation managed by the event source.  The	flags  argument	 will  be  the
       same as the flags argument passed to Tcl_DoOneEvent except that it will
       never be	0 (Tcl_DoOneEvent replaces 0 with TCL_ALL_EVENTS).  Flags  in-
       dicates	what  kinds  of	events should be considered; if	the bit	corre-
       sponding	to this	event source is	not set, the event source  should  re-
       turn  immediately  without doing	anything.  For example,	the file event
       source checks for the TCL_FILE_EVENTS bit.

       SetupProc's job is to make sure that  the  application  wakes  up  when
       events  of  the	desired	type occur.  This is typically done in a plat-
       form-dependent fashion.	For example, under Unix	an event source	 might
       call Tcl_CreateFileHandler; under Windows it might request notification
       with a Windows event.  For timer-driven event  sources  such  as	 timer
       events  or any polled event, the	event source can call Tcl_SetMaxBlock-
       Time to force the application to	wake up	after a	specified time even if
       no  events have occurred.  If no	event source calls Tcl_SetMaxBlockTime
       then Tcl_WaitForEvent will wait as long as necessary for	 an  event  to
       occur;  otherwise,  it  will only wait as long as the shortest interval
       passed to Tcl_SetMaxBlockTime by	one of the event sources.  If an event
       source knows that it already has	events ready to	report,	it can request
       a zero maximum block time.  For example,	the setup procedure for	the  X
       event source looks to see if there are events already queued.  If there
       are, it calls Tcl_SetMaxBlockTime with a	0 block	time so	that Tcl_Wait-
       ForEvent	 does  not  block if there is no new data on the X connection.
       The timePtr argument to Tcl_WaitForEvent	points to a structure that de-
       scribes a time interval in seconds and microseconds:

	      typedef struct Tcl_Time {
		  long sec;
		  long usec;
	      }	Tcl_Time;

       The usec	field should be	less than 1000000.

       Information  provided  to Tcl_SetMaxBlockTime is	only used for the next
       call to Tcl_WaitForEvent; it is discarded  after	 Tcl_WaitForEvent  re-
       turns.	The next time an event wait is done each of the	event sources'
       setup procedures	will be	called again, and they can specify new	infor-
       mation for that event wait.

       If   the	  application	uses   an  external  event  loop  rather  than
       Tcl_DoOneEvent, the event sources may need to call  Tcl_SetMaxBlockTime
       at other	times.	For example, if	a new event handler is registered that
       needs to	poll for events, the event source may call Tcl_SetMaxBlockTime
       to  set the block time to zero to force the external event loop to call
       Tcl.  In	this case, Tcl_SetMaxBlockTime invokes Tcl_SetTimer  with  the
       shortest	 interval  seen	 since	the  last  call	 to  Tcl_DoOneEvent or

       In addition to the generic procedure Tcl_SetMaxBlockTime,  other	 plat-
       form-specific  procedures may also be available for setupProc, if there
       is additional information needed	by Tcl_WaitForEvent on that  platform.
       For example, on Unix systems the	Tcl_CreateFileHandler interface	can be
       used to wait for	file events.

       The second procedure provided by	each event source is its check	proce-
       dure,  indicated	 by  the  checkProc argument to	Tcl_CreateEventSource.
       CheckProc must match the	following prototype:

	      typedef void Tcl_EventCheckProc(
		      ClientData clientData,
		      int flags);

       The arguments to	this procedure are the same as	those  for  setupProc.
       CheckProc  is invoked by	Tcl_DoOneEvent after it	has waited for events.
       Presumably at least one event source is now prepared to queue an	event.
       Tcl_DoOneEvent  calls  each  of	the event sources in turn, so they all
       have a chance to	queue any events that are ready.  The check  procedure
       does  two  things.   First,  it	must see if any	events have triggered.
       Different event sources do this in different ways.

       If an event source's check procedure detects an interesting  event,  it
       must  add the event to Tcl's event queue.  To do	this, the event	source
       calls Tcl_QueueEvent.  The evPtr	argument is a pointer to a dynamically
       allocated  structure  containing	the event (see below for more informa-
       tion on memory management issues).  Each	event source  can  define  its
       own event structure with	whatever information is	relevant to that event
       source.	However, the first element of the structure must be  a	struc-
       ture  of	type Tcl_Event,	and the	address	of this	structure is used when
       communicating between the event source and the rest of the notifier.  A
       Tcl_Event has the following definition:

	      typedef struct {
		  Tcl_EventProc	*proc;
		  struct Tcl_Event *nextPtr;
	      }	Tcl_Event;

       The  event source must fill in the proc field of	the event before call-
       ing Tcl_QueueEvent.  The	nextPtr	is used	to link	together the events in
       the queue and should not	be modified by the event source.

       An event	may be added to	the queue at any of three positions, depending
       on the position argument	to Tcl_QueueEvent:

       TCL_QUEUE_TAIL	       Add the event at	the back of the	queue, so that
			       all  other  pending  events  will  be  serviced
			       first.  This is almost always the  right	 place
			       for new events.

       TCL_QUEUE_HEAD	       Add  the	 event	at  the	front of the queue, so
			       that it	will  be  serviced  before  all	 other
			       queued events.

       TCL_QUEUE_MARK	       Add the event at	the front of the queue,	unless
			       there are other events at the front whose posi-
			       tion  is	 TCL_QUEUE_MARK;   if  so, add the new
			       event  just  after  all	other	TCL_QUEUE_MARK
			       events.	 This value of position	is used	to in-
			       sert an ordered sequence	of events at the front
			       of  the	queue,	such  as a series of Enter and
			       Leave events synthesized	during a grab  or  un-
			       grab operation in Tk.

       When it is time to handle an event from the queue (steps	1 and 4	above)
       Tcl_ServiceEvent	will invoke the	proc specified	in  the	 first	queued
       Tcl_Event structure.  Proc must match the following prototype:

	      typedef int Tcl_EventProc(
		      Tcl_Event	*evPtr,
		      int flags);

       The first argument to proc is a pointer to the event, which will	be the
       same as the first argument to the Tcl_QueueEvent	call  that  added  the
       event  to the queue.  The second	argument to proc is the	flags argument
       for the current call to Tcl_ServiceEvent;  this is used	by  the	 event
       source to return	immediately if its events are not relevant.

       It is up	to proc	to handle the event, typically by invoking one or more
       Tcl commands or C-level callbacks.  Once	the event source has  finished
       handling	 the  event it returns 1 to indicate that the event can	be re-
       moved from the queue.  If for some reason the event source decides that
       the  event  cannot be handled at	this time, it may return 0 to indicate
       that the	event should be	deferred for processing	later;	in  this  case
       Tcl_ServiceEvent	 will go on to the next	event in the queue and attempt
       to service it.  There are several reasons why an	event source might de-
       fer an event.  One possibility is that events of	this type are excluded
       by the flags argument.  For example, the	file event source will	always
       return 0	if the TCL_FILE_EVENTS bit is not set in flags.	 Another exam-
       ple of deferring	events happens in Tk if	Tk_RestrictEvents has been in-
       voked to	defer certain kinds of window events.

       When  proc  returns  1, Tcl_ServiceEvent	will remove the	event from the
       event queue and free its	storage.  Note that the	storage	for  an	 event
       must be allocated by the	event source (using Tcl_Alloc or the Tcl macro
       ckalloc)	before	calling	 Tcl_QueueEvent,  but  it  will	 be  freed  by
       Tcl_ServiceEvent, not by	the event source.

       Threaded	 applications work in a	similar	manner,	except that there is a
       separate	event queue for	each  thread  containing  a  Tcl  interpreter.
       Calling	Tcl_QueueEvent in a multithreaded application adds an event to
       the current thread's queue.  To add an event to another thread's	queue,
       use  Tcl_ThreadQueueEvent.  Tcl_ThreadQueueEvent	accepts	as an argument
       a Tcl_ThreadId argument,	which uniquely identifies a thread  in	a  Tcl
       application.   To  obtain  the Tcl_ThreadId for the current thread, use
       the Tcl_GetCurrentThread	procedure.  (A thread would then need to  pass
       this  identifier	 to  other threads for those threads to	be able	to add
       events to its queue.)  After adding an event to another thread's	queue,
       you  then  typically  need  to  call  Tcl_ThreadAlert to	"wake up" that
       thread's	notifier to alert it to	the new	event.

       Tcl_DeleteEvents	can be used to explicitly remove one  or  more	events
       from  the  event	 queue.	 Tcl_DeleteEvents calls	proc for each event in
       the queue, deleting those for with the procedure	returns	1.  Events for
       which the procedure returns 0 are left in the queue.  Proc should match
       the following prototype:

	      typedef int Tcl_EventDeleteProc(
		      Tcl_Event	*evPtr,
		      ClientData clientData);

       The clientData argument will be the same	as the clientData argument  to
       Tcl_DeleteEvents;  it is	typically used to point	to private information
       managed by the event source.  The evPtr will point to the next event in
       the queue.

       Tcl_DeleteEventSource  deletes  an event	source.	 The setupProc,	check-
       Proc, and clientData arguments must exactly match those provided	to the
       Tcl_CreateEventSource  for  the event source to be deleted.  If no such
       source exists, Tcl_DeleteEventSource has	no effect.

       The notifier consists of	all the	procedures described  in  this	manual
       entry,  plus  Tcl_DoOneEvent  and Tcl_Sleep, which are available	on all
       platforms, and Tcl_CreateFileHandler and	 Tcl_DeleteFileHandler,	 which
       are  Unix-specific.  Most of these procedures are generic, in that they
       are the same for	all notifiers.	However, none of  the  procedures  are
       notifier-dependent:   Tcl_InitNotifier,	Tcl_AlertNotifier,  Tcl_Final-
       izeNotifier, Tcl_SetTimer, Tcl_Sleep, Tcl_WaitForEvent, Tcl_CreateFile-
       Handler,	 Tcl_DeleteFileHandler	and Tcl_ServiceModeHook.  To support a
       new platform or to integrate Tcl	 with  an  application-specific	 event
       loop, you must write new	versions of these procedures.

       Tcl_InitNotifier	initializes the	notifier state and returns a handle to
       the notifier state.  Tcl	calls this procedure when initializing	a  Tcl
       interpreter.   Similarly, Tcl_FinalizeNotifier shuts down the notifier,
       and is called by	Tcl_Finalize when shutting down	a Tcl interpreter.

       Tcl_WaitForEvent	is the lowest-level procedure in the notifier;	it  is
       responsible  for	 waiting  for an "interesting" event to	occur or for a
       given time to elapse.  Before Tcl_WaitForEvent is invoked, each of  the
       event sources' setup procedure will have	been invoked.  The timePtr ar-
       gument to Tcl_WaitForEvent gives	the  maximum  time  to	block  for  an
       event,  based  on calls to Tcl_SetMaxBlockTime made by setup procedures
       and on other information	(such as the TCL_DONT_WAIT bit in flags).

       Ideally,	Tcl_WaitForEvent should	only wait for an event	to  occur;  it
       should  not actually process the	event in any way.  Later on, the event
       sources will process the	raw events and create Tcl_Events on the	 event
       queue  in their checkProc procedures.  However, on some platforms (such
       as Windows) this	is not possible; events	may be processed in  Tcl_Wait-
       ForEvent, including queuing Tcl_Events and more (for example, callbacks
       for native widgets may be invoked).  The	return	value  from  Tcl_Wait-
       ForEvent	 must  be  either  0,  1, or -1.  On platforms such as Windows
       where events get	processed in Tcl_WaitForEvent, a  return  value	 of  1
       means  that  there  may be more events still pending that have not been
       processed.  This	is a sign to the caller	that it	 must  call  Tcl_Wait-
       ForEvent	 again if it wants all pending events to be processed. A 0 re-
       turn value means	that calling Tcl_WaitForEvent again will not have  any
       effect:	either	this  is  a platform where Tcl_WaitForEvent only waits
       without doing any event processing, or Tcl_WaitForEvent knows for  sure
       that  there  are	 no additional events to process (e.g. it returned be-
       cause the time elapsed).	 Finally, a return value of -1 means that  the
       event loop is no	longer operational and the application should probably
       unwind and terminate.  Under Windows this happens when a	 WM_QUIT  mes-
       sage  is	 received;  under  Unix	it happens when	Tcl_WaitForEvent would
       have waited forever because there were no active	event sources and  the
       timeout was infinite.

       Tcl_AlertNotifier  is  used  in multithreaded applications to allow any
       thread to "wake up" the notifier	to alert  it  to  new  events  on  its
       queue.	Tcl_AlertNotifier  requires as an argument the notifier	handle
       returned	by Tcl_InitNotifier.

       If the notifier will be used with an external event loop, then it  must
       also  support  the  Tcl_SetTimer	interface.  Tcl_SetTimer is invoked by
       Tcl_SetMaxBlockTime whenever the	maximum	blocking  time	has  been  re-
       duced.	Tcl_SetTimer should arrange for	the external event loop	to in-
       voke Tcl_ServiceAll after the specified interval	even if	no events have
       occurred.  This interface is needed because Tcl_WaitForEvent is not in-
       voked when there	is an external event loop.  If the notifier will  only
       be used from Tcl_DoOneEvent, then Tcl_SetTimer need not do anything.

       Tcl_ServiceModeHook  is	called	by the platform-independent portion of
       the notifier when client	code makes a call to Tcl_SetServiceMode.  This
       hook  is	 provided  to  support	operating systems that require special
       event handling when the application is in a modal loop (the Windows no-
       tifier, for instance, uses this hook to create a	communication window).

       On  Unix	systems, the file event	source also needs support from the no-
       tifier.	The file event source consists	of  the	 Tcl_CreateFileHandler
       and  Tcl_DeleteFileHandler  procedures,	which  are  described  in  the
       Tcl_CreateFileHandler manual page.

       The Tcl_Sleep and Tcl_DoOneEvent	interfaces are described in their  re-
       spective	manual pages.

       The  easiest way	to create a new	notifier is to look at the code	for an
       existing	notifier, such as the files unix/tclUnixNotfy.c	or win/tclWin-
       Notify.c	in the Tcl source distribution.

       A notifier that has been	written	according to the conventions above can
       also be installed in a running process in place of the  standard	 noti-
       fier.   This  mechanism is used so that a single	executable can be used
       (with the standard notifier) as a stand-alone program and reused	 (with
       a  replacement notifier in a loadable extension)	as an extension	to an-
       other program, such as a	Web browser plugin.

       To do this, the extension makes a call  to  Tcl_SetNotifier  passing  a
       pointer	to  a Tcl_NotifierProcs	data structure.	 The structure has the
       following layout:

	      typedef struct Tcl_NotifierProcs {
		  Tcl_SetTimerProc *setTimerProc;
		  Tcl_WaitForEventProc *waitForEventProc;
		  Tcl_CreateFileHandlerProc *createFileHandlerProc;
		  Tcl_DeleteFileHandlerProc *deleteFileHandlerProc;
		  Tcl_InitNotifierProc *initNotifierProc;
		  Tcl_FinalizeNotifierProc *finalizeNotifierProc;
		  Tcl_AlertNotifierProc	*alertNotifierProc;
		  Tcl_ServiceModeHookProc *serviceModeHookProc;
	      }	Tcl_NotifierProcs;

       Following the call  to  Tcl_SetNotifier,	 the  pointers	given  in  the
       Tcl_NotifierProcs  structure  replace  whatever	notifier  had been in-
       stalled in the process.

       It is extraordinarily unwise to replace a running  notifier.  Normally,
       Tcl_SetNotifier	should be called at process initialization time	before
       the first call to Tcl_InitNotifier.

       The notifier interfaces are designed so that Tcl	can be	embedded  into
       applications  that  have	 their own private event loops.	 In this case,
       the application does not	call Tcl_DoOneEvent except in the case of  re-
       cursive	event loops such as calls to the Tcl commands update or	vwait.
       Most of the time	is spent in the	external event loop  of	 the  applica-
       tion.   In  this	 case the notifier must	arrange	for the	external event
       loop to call back into Tcl when something happens on  the  various  Tcl
       event  sources.	 These	callbacks  should  arrange for appropriate Tcl
       events to be placed on the Tcl event queue.

       Because the external event loop is not calling Tcl_DoOneEvent on	a reg-
       ular basis, it is up to the notifier to arrange for Tcl_ServiceEvent to
       be called whenever events are pending on	the Tcl	event queue.  The eas-
       iest  way  to  do  this	is to invoke Tcl_ServiceAll at the end of each
       callback	from the external event	loop.  This will ensure	 that  all  of
       the  event  sources are polled, any queued events are serviced, and any
       pending idle handlers are processed before returning control to the ap-
       plication.  In addition,	event sources that need	to poll	for events can
       call Tcl_SetMaxBlockTime	to force the external event loop to  call  Tcl
       even if no events are available on the system event queue.

       As  a  side  effect  of processing events detected in the main external
       event loop, Tcl may invoke Tcl_DoOneEvent to start  a  recursive	 event
       loop  in	 commands like vwait.  Tcl_DoOneEvent will invoke the external
       event loop, which will result in	callbacks as described in the  preced-
       ing  paragraph, which will result in calls to Tcl_ServiceAll.  However,
       in these	cases it is undesirable	to service events  in  Tcl_ServiceAll.
       Servicing  events there is unnecessary because control will immediately
       return to the external event loop and hence  to	Tcl_DoOneEvent,	 which
       can service the events itself.  Furthermore, Tcl_DoOneEvent is supposed
       to service only a single	event, whereas	Tcl_ServiceAll	normally  ser-
       vices  all  pending  events.   To handle	this situation,	Tcl_DoOneEvent
       sets a flag for Tcl_ServiceAll that causes it to	return without servic-
       ing  any	 events.  This flag is called the service mode;	Tcl_DoOneEvent
       restores	it to its previous value before	it returns.

       In some cases, however, it may be necessary for Tcl_ServiceAll to  ser-
       vice  events  even  when	it has been invoked from Tcl_DoOneEvent.  This
       happens when there is yet another recursive event loop invoked  via  an
       event  handler  called by Tcl_DoOneEvent	(such as one that is part of a
       native widget).	In this	case, Tcl_DoOneEvent may not have a chance  to
       service	events so Tcl_ServiceAll must service them all.	 Any recursive
       event loop that calls an	external event loop rather than	Tcl_DoOneEvent
       must  reset  the	 service  mode	so  that  all  events get processed in
       Tcl_ServiceAll.	This is	done by	invoking the Tcl_SetServiceMode	proce-
       dure.   If Tcl_SetServiceMode is	passed TCL_SERVICE_NONE, then calls to
       Tcl_ServiceAll will return immediately without processing  any  events.
       If Tcl_SetServiceMode is	passed TCL_SERVICE_ALL,	then calls to Tcl_Ser-
       viceAll will behave normally.  Tcl_SetServiceMode returns the  previous
       value  of the service mode, which should	be restored when the recursive
       loop exits.  Tcl_GetServiceMode returns the current value of  the  ser-
       vice mode.

       Tcl_CreateFileHandler(3),    Tcl_DeleteFileHandler(3),	 Tcl_Sleep(3),
       Tcl_DoOneEvent(3), Thread(3)

       event, notifier,	event queue, event sources, file events, timer,	 idle,
       service mode, threads

Tcl				      8.1			   Notifier(3)


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