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Using the Kernel Objects
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Now that we've reviewed the basics that apply t o all kernel objects, let's drill into each of the synchronization specific objects: mutexes, semaphores, auto- and manual-reset events, and waitable timers, in that order
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Mutex The mutex-also referred to as the mutant in the Windows kernel-is a ker nel object that is meant solely for synchronization purposes A mutex's pur pose is to facilitate building the mu tually exclusive (hence the abbreviated name mut-ex) critical regions of the kind that were introduced in 2, Synchronization and Time The mutual exclusion property is accomplished by the mutex object transitioning between the nonsignaled and signaled states atomically When a mutex is in the signaled state, it is available for acquisition; that is, there is no current owner A subsequent wait will atom ically transfer the mutex into a non signaled state It is atomic because the Windows kernel handles cases in which multiple threads wait on the same mutex simultaneously; that is, only one will be permitted to initiate the tran sition, while the other will see the mutex as nonsignaled When a mutex is nonsignaled, there is a single thread that currently owns the mutex Mutex ownership is based on the physical OS thread used to wait on the mutex in both native and managed code This allows Windows to provide errors in cases where a thread erroneously tries to release a mutex when it isn't the current owner In other synchronization primitives, such as events, this condition isn' t caught although it (usually, but not always) represents an error in the program For systems in which logical work might migrate between separate threads, or where multiple pieces of logical work might share the same physical thread, this can pose problems Such is the case for fibers, as described in 9, Fibers, because multiple fibers can be mul tiplexed onto the same OS thread and can even migrate between them over time The CLR denotes the acquisition and release of affinity through the use of the Th r e a d Beg i n T h r e a dAff i n i ty and E n d T h r e a dAff i n i ty APIs to notify hosts when affinity has been acquired and released, corresponding to the acquisition and release of a mutex object, respectively, allowing hosts to deal with this situation
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C h a pter 5 : W i n d ows Ke r n e l Syn c h ro n i z a t i o n
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As an illustration, here are two side-by-side code snippets that use a mutex to build a critical region: the left is written in C++ using Win32 and the right is C#
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HANDLE hMut ant
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C reateMutex ( ) ;
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Mutex mutant
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new Mutex ( ) ;
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Wait F o r S i ngleObj e c t ( hMut a n t , I N F I N I T E ) ; _t ry { I I The c ri t i c a l region } _fi n a l ly { R e l e a s eMut e x ( hMutant ) ; } CloseHand l e ( hMutant ) ;
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mutant WaitOne ( ) ; t ry { II The c rit i c a l region } f i n a l ly { mutant Relea seMutex ( ) ; } mutant C lose ( ) ;
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Notice that in native code, a mutex is referred to by its HAND L E, while in managed code, a mutex is referred to by an instance of the Mutex class The
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Mutex class derives from the common kernel object type System Thread i n g waitHa n d l e in the NET Framework All error checking has been omit
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ted from the native example for brevity, although a real program should check the return value of each API call Let's now review the mutex APIs in detail
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