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programming) An event object, like any other kernel object, is always in either the signaled or nonsignaled state In usual event terminology, these states map to set and reset, respectively I'll use the kernel object terminol ogy in subsequent chapters when referring to events abstractly I'll typically prefer to use the terms set and reset To summarize the differences between the two event types: when an auto reset has been signaled, only one thread will see this particular signal When a thread observes the signal by waiting on the event, it is automatically tran sitioned back to the nonsignaled state In this sense, an auto-reset event is like a mutex, with the sole difference being that auto-reset events have no notion of ownership and, hence, do not use thread affinity or recursion This means that any thread can subsequently set the event, unlike a mutex, which requires that only the owner thread release it If there are waiting threads when the auto-reset event transitions into a signaled state, Windows will select the first thread in the waiter queue to wake and will only wake up a single thread All of the previous information about fairness and FIFO order ing applies If there are no waiting threads at the time the signal arrives, then the first subsequent thread to wait on the object will return right away with out blocking, atomically transitioning the event to a nonsignaled state The manual-reset event, on the other hand, remains signaled until it is manually reset with an API call In other words, the event is "sticky" and persistent (just like a traditional latch) This allows multiple threads to wait on the same event and observe the same signal, which is often useful for one-time events All waiting threads are released at the time of a set As with mutex kernel objects, Win32 APIs are available to create and inter act with these objects through their HANDL Es, and the NET Framework exposes their capabilities through the Auto ResetEvent and Ma n u a l Reset Event classes, joined at the hip by the common (concrete) base class, System Threa d
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ing EventWa i tHa ndle EventWa i tHandle is a subclass of the abstract base class Wa i tHa n d l e You work with instances of the two separate events types with
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basically the same set of APIs-to create, open, set, reset, and wait on the event-although there are some substantial differences regarding how the separate object types respond to signals and waiting Note that the two subclasses of EventWa itHa n d l e are only there as a convenience: you can instantiate and deal with Eve ntWa i t H a n d l e objects directly if you prefer, as we'll see below
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Creating and opening events is identical to what we've already reviewed for semaphores and mutexes Like semaphores, we will review just the details specific to events in this section To create a new event object, or to find an existing one by name, you can use the C re a t e E v e n t , C r e a t e E v e n t E x, and Open Event APIs
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HAND L E WINAPI C reateEvent ( lPSECURITY_ATTR I BUTES I p EventAt t r i b u te s , BOO l bMa n u a lRe set , BOO l b l n it ialStat e , l PCTSTR IpName HAN D L E WINAPI C reateEvent E x ( lPSECURITY_ATTR I BUTES I p EventAtt ribute s , l PCTSTR I pName , DWORD dwF l a g s , DWORD dwDe s i redAc c e s s HAN D L E WINAPI O p e n E vent ( DWORD dwDe s i redAc c e s s , BOO l b l n heritHa n d l e , lPCTSTR IpName
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In the case of C re a t e E v e n t , the bMa n u a l Re s et argument specifies whether an auto-reset ( F A L S E ) or manual-reset (TRU E ) event should be created C re ate E v e n t E x (new to Windows Vista) uses the dwF l a g s bit flags argument to specify this same information: if the argument value contains C R EATE_EVENT_MANUAL_R E S E T, the event will be a manual-reset, and other wise it will be auto-reset This is the only valid flag that you can pass inside of dw F l a g s The b I n i t i a l S t a t e argument specifies whether the event should be created in the signaled (TRU E ) or nonsignaled (FALS E ) state The other parameters should be familiar by now: I p E v e ntAtt r i b ut e s for optional access control, I pN am e to optionally name the object, and dwDe s i redAc c e s s to specify the resulting HAN D L E ' s access rights, new to Windows Vista And Op e n E v e n t works the same way that OpenMutex, and so on do To create an event in managed code, you have an option An option is to instantiate one of the two derived classes Aut o R e s e t E v e n t and Ma n u a l R e s e t E v e n t Each has only a single constructor available
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