Книга: Microsoft Windows Embedded CE 6.0 Exam Preparation Kit

Mutexes

Mutexes

Whereas critical sections are limited to a single process, mutexes can coordinate mutually exclusive access to resources shared between multiple processes. A mutex is a kernel object that facilitates inter-process synchronization. Call the CreateMutex function to create a mutex. The creating thread can specify a name for the mutex object at creation time, although it is possible to create an unnamed mutex. Threads in other processes can also call CreateMutex and specify the same name. However, these subsequent calls do not create new kernel objects, but instead return a handle to the existing mutex. At this point, the threads in the separate processes can use the mutex object to synchronize access to the protected shared resource.

The state of a mutex object is signaled when no thread owns it and non-signaled when one thread has ownership. A thread must use one of the wait functions, WaitForSingleObject or WaitForMultipleObjects, to request ownership. You can specify a timeout value to resume thread processing along an alternative code path if the mutex does not become available during the wait interval. On the other hand, if the mutex becomes available and ownership is granted to the current thread, do not forget to call ReleaseMutex for each time that the mutex satisfied a wait in order to release the mutex object for other threads. This is important because a thread can call a wait function multiple times, such as in a loop, without blocking its own execution. The system does not block the owning thread to avoid a deadlock situation, but the thread must still call ReleaseMutex as many times as the wait function to release the mutex.

Table 3-12 lists the most important functions that you can use to work with mutex objects for thread synchronization purposes.

Table 3-12 Mutex API

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