Review: Mutexes vs CRITICAL_SECTIONs in VS .NET

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Review: Mutexes vs CRITICAL_SECTIONs
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As stated several times, mutexes and CRITICAL_SECTIONs are very similar and solve the same set of problems In particular, both objects can be owned by a single thread, and other threads attempting to gain ownership will block until the object is released Mutexes do provide greater flexibility, but with a performance penalty In summary, the differences are as follows
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Mutexes, when abandoned by a terminated thread, are signaled so that other threads are not blocked forever Mutex waits can time out, whereas you can only poll a CS Mutexes can be named and are sharable by threads in different processes You can use WaitForMultipleObjects with mutexes, which is both a programming convenience and a way to avoid deadlocks if used properly The thread that creates a mutex can specify immediate ownership With a CS, several threads could race to acquire the CS CSs are usually, but not always, considerably faster than mutexes There will be more on this in 9
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Heap Synchronization
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A pair of functions for NTHeapLock and HeapUnlockis used to synchronize heap access ( 5) The heap handle is the only argument These functions are helpful when the HEAP_NO_SERIALIZE flag is used or when it is necessary for a thread to have exclusive access to a heap
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Semaphores
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Semaphores, the second of the three kernel synchronization objects, maintain a count, and the semaphore object is signaled when the count is greater than 0 The semaphore object is unsignaled when the count is 0 Threads or processes wait in the normal way, using one of the wait functions When a waiting thread is released, the semaphore's count is decremented by 1 The semaphore functions are CreateSemaphore, OpenSemaphore, and ReleaseSemaphore, which can increment the count by 1 or more These functions are similar to their mutex counterparts HANDLE CreateSemaphore ( LPSECURITY_ATTRIBUTES lpsa, LONG lSemInitial, LONG lSemMax, LPCTSTR lpSemName)
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lSemMax, which must be 1 or greater, is the maximum value for the semaphore lSemInitial, with 0 lSemInitial lSemMax, is the initial value, and the semaphore value is never allowed to go outside this range A NULL return value indicates failure It is possible to decrease the count only by 1 with any given wait operation, but a semaphore release can increment its count by any value up to the maximum BOOL ReleaseSemaphore ( HANDLE hSemaphore, LONG cReleaseCount, LPLONG lpPreviousCount)
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Notice that you can find the count preceding the release, but the pointer can be NULL if this value is not needed The release count must be greater than 0, but if it would cause the semaphore count to exceed the maximum, the call will fail, returning FALSE, and the count will remain unchanged Use the previous count value with caution as the semaphore count can also be changed by other threads Also, you cannot determine whether the count is at its maximum as there is no legal release count An example in the Web site code demonstrates using the previous count While it is tempting to think of a semaphore as a special case of a mutex with a maximum value of 1, this would be misleading because there is no ownership of a semaphore Any thread can release a semaphore, not just the one that performed the wait Likewise, since there is no ownership, there is no concept of an abandoned semaphore
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Using Semaphores
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The classic semaphore application regards the semaphore count as representing the number of available
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resources, such as the number of messages waiting in a queue The semaphore maximum then represents the maximum queue size Thus, a producer would place a message in the buffer and call ReleaseSemaphore, usually with a release count of 1 Consumer threads would wait on the semaphore, consuming a message and decrementing the semaphore count Another important use is described in the discussion following Program 9-1, where a semaphore can be used to limit the number of worker threads actually running at any one time, thereby decreasing contention between threads and, in some cases, improving performance 9 discusses this technique, using a semaphore throttle The potential race condition in sortMT (Program 7-2) illustrates another use of a semaphore to control the exact number of threads to wake up All the threads could be created without being suspended All of them would immediately wait on a semaphore initialized to 0 The boss thread, rather than resuming the threads, would simply call ReleaseSemaphore with a count of 4 (or whatever the number of threads is), and the four threads could then proceed While semaphores can be convenient, they are redundant in the sense that mutexes and events (described in the next major section), used together, are more powerful than semaphores See 10 for more information
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