Tcl_CreateEventSource(), Tcl_DeleteEventSource(), Tcl_SetMaxBlockTime(), Tcl_QueueEvent(), Tcl_ThreadQueueEvent(), Tcl_ThreadAlert(), Tcl_GetCurrentThread(), 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 

Tcl Library Procedures


#include <tcl.h>

void Tcl_CreateEventSource(setupProc, checkProc, clientData)

void Tcl_DeleteEventSource(setupProc, checkProc, clientData)

void Tcl_SetMaxBlockTime(timePtr)

void Tcl_QueueEvent(evPtr, position)

void Tcl_ThreadQueueEvent(threadId, evPtr, position)

void Tcl_ThreadAlert(threadId, clientData)

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_SetServiceMode(mode)


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 deleteProc.

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 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_Alloc() or ckalloc.

Tcl_QueuePosition position (in) 

Where to add the new event in the queue: TCL_QUEUE_TAIL, TCL_QUEUE_HEAD, or TCL_QUEUE_MARK.

Tcl_ThreadId threadId (in) 

A unique identifier for a thread.

Tcl_EventDeleteProc *deleteProc (in) 

Procedure to invoke for each queued event in Tcl_DeleteEvents().

int flags (in) 

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

int mode (in) 

Inidicates 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 provided by an application in which Tcl is embedded. Each of these tasks is described in a separate section below.

The procedures in this reference page 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_SetMaxBlockTime(), Tcl_QueueEvent(), and Tcl_DeleteEvents() are used primarily by event sources.

  2. The event queue: for non-threaded applications, there is a single 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 appropriate 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 servicing at a future time. Threaded applications work in a similar 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 applications will do this by calling the procedure Tcl_DoOneEvent(), which is described in a separate reference page.

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 a Motif event loop, when Tcl is embedded in a Motif application). 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_FinalizeNotifier(), 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_ServiceEvent() 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_WaitForEvent() will block if no new events occur.

  3. Call Tcl_WaitForEvent(). This procedure is implemented differently 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 happened, 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 procedure 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 information 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 corresponding 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 platform-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_SetMaxBlockTime() 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_WaitForEvent() 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 platform-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 procedure, 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 information 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 structure 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 calling 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():


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.


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


Add the event at the front of the queue, unless there are other events at the front whose position 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 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 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, checkProc, 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 reference page, 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_FinalizeNotifier(), Tcl_SetTimer(), Tcl_Sleep(), Tcl_WaitForEvent(), Tcl_CreateFileHandler() 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_WaitForEvent(), including queuing Tcl_Events and more (for example, callbacks for native widgets may be invoked). The return value from Tcl_WaitForEvent() 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_WaitForEvent() 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 (for example, 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_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 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 anything.

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() reference page.

The Tcl_Sleep() and Tcl_DoOneEvent() interfaces are described in their respective reference 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/tclWinNotify.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 regular 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 easiest 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 preceding 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 services all pending events. To handle this situation, Tcl_DoOneEvent() sets a flag for Tcl_ServiceAll() that causes it to return without servicing 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 service 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() procedure. 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_ServiceAll() 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 service mode.


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

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