Inherits from
Available in OS X v10.0 and later.
Declared in


Abstract class for all work-loop event sources.

The IOEventSource declares the abstract super class that all event sources must inherit from if an IOWorkLoop is to receive events from them.

An event source can represent any event that should cause the work-loop of a device to wake up and perform work. Two examples of event sources are the IOInterruptEventSource which delivers interrupt notifications and IOCommandGate which delivers command requests.

A kernel module can always use the work-loop model for serialising access to anything at all. The IOEventSource is used for communicating events to the work-loop, and the chain of event sources should be used to walk the possible event sources and demultipex them. Note a particular instance of an event source may only be a member of 1 linked list chain. If you need to move it between chains than make sure it is removed from the original chain before attempting to move it.

The IOEventSource makes no attempt to maintain the consistency of its internal data across multi-threading. It is assumed that the user of these basic tools will protect the data that these objects represent in some sort of device wide instance lock. For example the IOWorkLoop maintains the event chain by using an IOCommandGate and thus single threading access to its state.

All subclasses of IOEventSource that wish to perform work on the work-loop thread are expected to implement the checkForWork() member function. As of Mac OS X, 10.7 (Darwin 11), checkForWork is no longer pure virtual, and should not be overridden if there is no work to be done.

checkForWork() is the key method in this class. It is called by some work-loop when convienient and is expected to evaluate its internal state and determine if an event has occurred since the last call. In the case of an event having occurred then the instance defined target(owner)/action will be called. The action is stored as an ordinary C function pointer but the first parameter is always the owner. This means that a C++ member function can be used as an action function though this depends on the ABI.

Although the eventChainNext variable contains a reference to the next event source in the chain this reference is not retained. The list 'owner' i.e. the client that creates the event, not the work-loop, is expected to retain the source.



Instance Methods


Virtual member function used by IOWorkLoop for work scheduling.

virtual bool checkForWork();
Return Value

Return true if this function needs to be called again before all its outstanding events have been processed.


This function will be called to request a subclass to check its internal state for any work to do and then to call out the owner/action. If this event source never performs any work (e.g. IOCommandGate), this method should not be overridden. NOTE: This method is no longer declared pure virtual. A default implementation is provided in IOEventSource.


Disable event source.

virtual void disable();

A subclass implementation is expected to respect the enabled state when checkForWork is called.


Enable event source.

virtual void enable();

A subclass implementation is expected to respect the enabled state when checkForWork is called. Calling this function will cause the work-loop to be signalled so that a checkForWork is performed.


Get'ter for $link action variable.

virtual IOEventSource::Action getAction() const;
Return Value

value of action.


Get'ter for $link eventChainNext variable.

virtual IOEventSource *getNext() const;
Return Value

value of eventChainNext.


Get'ter for $link workLoop variable.

virtual IOWorkLoop *getWorkLoop() const;
Return Value

value of workLoop.


Primary initialiser for the IOEventSource class.

virtual bool init( OSObject *owner, IOEventSource::Action action = 0);

Owner of this instance of an event source. Used as the first parameter of the action callout. Owner will generally be an OSObject it doesn't have to be as no member functions will be called directly in it. It can just be a refcon for a client routine.


Pointer to C call out function. Action is a pointer to a C function that gets called when this event source has outstanding work. It will usually be called by the checkForWork member function. The first parameter of the action call out will always be the owner, this allows C++ member functions to be used as actions. Defaults to 0.

Return Value

true if the inherited classes and this instance initialise successfully.


Get'ter for $link enable variable.

virtual bool isEnabled() const;
Return Value

true if enabled.


Convenience function for workLoop->onThread.

virtual bool onThread() const;
Return Value

true if called on the work-loop thread.


Set'ter for $link action variable.

virtual void setAction( IOEventSource::Action action);

Pointer to a C function of type IOEventSource::Action.


Set'ter for $link eventChainNext variable.

virtual void setNext( IOEventSource *next);

Pointer to another IOEventSource instance.


Set'ter for $link workLoop variable.

virtual void setWorkLoop( IOWorkLoop *workLoop);

Target work-loop of this event source instance. A subclass of IOWorkLoop that at least reacts to signalWorkAvailable() and onThread functions.



typedef void ( *Action)(
   OSObject *owner,


Target of the function, can be used as a refcon. The owner is set during initialisation. Note if a C++ function was specified this parameter is implicitly the first paramter in the target member function's parameter list.


Placeholder type for C++ function overloading discrimination. As the all event sources require an action and it has to be stored somewhere and be of some type, this is that type.

Declared In


Miscellaneous Defines

#define IOEventSourceAction IOEventSource::Action

Backward compatibilty define for the old non-class scoped type definition. See $link IOEventSource::Action

Available in OS X v10.0 and later.

Declared in IOEventSource.h.


struct ExpansionData {
   struct IOEventSourceCounter *counter;
   void *iokitstatsReserved;

This structure will be used to expand the capablilties of the IOEventSource in the future.

Instance Variables


Action action;

The action method called when an event has been delivered


bool enabled;

Is this event source enabled to deliver requests to the work-loop.


IOEventSource *eventChainNext;

The next event source in the event chain. nil at end of chain.


OSObject *owner;

The owner object called when an event has been delivered.


void *refcon;

What ever the client wants to do, see $link setRefcon.


ExpansionData *reserved;

Reserved for future use. (Internal use only)


IOWorkLoop *workLoop;

What is the work-loop for this event source.