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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  - the event
       queue and notifier interfaces

       #include <tcl.h>

       Tcl_CreateEventSource(setupProc, checkProc, clientData)

       Tcl_DeleteEventSource(setupProc, checkProc, clientData)


       Tcl_QueueEvent(evPtr, position)

       void								       │
       Tcl_ThreadQueueEvent(threadId, evPtr, position)			       │

       void								       │
       Tcl_ThreadAlert(threadId)					       │

       Tcl_ThreadId							       │
       Tcl_GetCurrentThread()						       │

       void								       │
       Tcl_DeleteEvents(deleteProc, clientData)				       │

       ClientData							       │
       Tcl_InitNotifier()						       │

       void								       │
       Tcl_FinalizeNotifier(clientData)					       │

       int								       │
       Tcl_WaitForEvent(timePtr)					       │

       void								       │
       Tcl_AlertNotifier(clientData)					       │

       void								       │
       Tcl_SetTimer(timePtr)						       │

       int								       │
       Tcl_ServiceAll()							       │

       int								       │
       Tcl_ServiceEvent(flags)						       │

       int								       │
       Tcl_GetServiceMode()						       │

       int								       │
       Tcl_EventSetupProc    *setupProc	    (in)      Procedure to  invoke  to
						      prepare  for  event wait
						      in Tcl_DoOneEvent.

       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  setupProc,
						      checkProc,	    or

       Tcl_Time		     *timePtr	    (in)      Indicates	  the  maximum
						      amount of time  to  wait
						      for  an  event.  This is
						      specified as an interval
						      (how  long to wait), not
						      an absolute  time	 (when
						      to   wakeup).    If  the
						      pointer	 passed	    to
						      Tcl_WaitForEvent	    is
						      NULL, it means there  is
						      no  maximum  wait	 time:
						      wait forever  if	neces‐

       Tcl_Event	     *evPtr	    (in)      An  event	 to add to the
						      event queue.  The	 stor‐
						      age  for	the event must
						      have been	 allocated  by
						      the     caller	 using
						      Tcl_Alloc 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

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

       int		     mode	    (in)			       │
						      Indicates whether events │
						      should  be  serviced  by │
						      Tcl_ServiceAll.  Must be │
						      one of  TCL_SERVICE_NONE │
						      or TCL_SERVICE_ALL.

       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
	      implements  the  Tcl_CreateTimerHandler  procedure and the after
	      command, 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 │
	      thread.							       │

       [3]								       │
	      The event loop: in order	to  detect  and	 process  events,  the │
	      application 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.				       │

NOTIFIER BASICS								       │
       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
	      invokes  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_Wait‐
	      ForEvent 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
	      immediately 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
       indicates what kinds of events should be considered; if the bit	corre‐
       sponding to this event source isn't set, the event source should return
       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
       describes 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 │
       returns.	 The next time an  event  wait	is  done  each	of  the	 event
       sources'	 setup	procedures  will be called again, and they can specify
       new information 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 │
       Tcl_ServiceAll.							       │

       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
			       insert  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
			       ungrab 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
       removed	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 indi‐
       cate  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 defer 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	isn't  set  in	flags.
       Another	example of deferring events happens in Tk if Tk_RestrictEvents
       has been invoked 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, eight of the procedures are │
       notifier-dependent:  Tcl_InitNotifier,  Tcl_AlertNotifier,   Tcl_Final‐ │
       izeNotifier, Tcl_SetTimer, Tcl_Sleep, Tcl_WaitForEvent, Tcl_CreateFile‐ │
       Handler and Tcl_DeleteFileHandler.  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 │
       argument	 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 isn't 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	 haven't  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 │
       return  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 │
       because	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 message is received;  under  Unix  it  happens  when  Tcl_Wait‐ │
       ForEvent	 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 │
       reduced.	 Tcl_SetTimer should arrange for the external  event  loop  to │
       invoke  Tcl_ServiceAll  after  the specified interval even if no events │
       have occurred.  This interface is needed because Tcl_WaitForEvent isn't │
       invoked	when  there  is	 an external event loop.  If the notifier will │
       only be used from Tcl_DoOneEvent, then Tcl_SetTimer need	 not  do  any‐ │
       thing.								       │

       On  Unix	 systems,  the	file  event source also needs support from the │
       notifier.  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 │
       respective 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.				       │

       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 │
       recursive 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 │
       application.  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 │
       application.  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,	     Tcl_DeleteFileHandler,	    Tcl_Sleep,
       Tcl_DoOneEvent, Thread(3)

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

Tcl				      8.1			   Notifier(3)
                             _         _         _ 
                            | |       | |       | |     
                            | |       | |       | |     
                         __ | | __ __ | | __ __ | | __  
                         \ \| |/ / \ \| |/ / \ \| |/ /  
                          \ \ / /   \ \ / /   \ \ / /   
                           \   /     \   /     \   /    
                            \_/       \_/       \_/ 
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