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lecture10-chapter05 - Operating Systems Operating Lecture...

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Operating Systems Operating Systems Lecture 10: Lecture 10: Semaphores Semaphores Anda Iamnitchi [email protected] 1
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Mutual Exclusion Mutual Exclusion Idea #3: Semaphores Idea #3: Semaphores 1) May be initialized to a nonnegative integer value 2) The semWait operation decrements the value 3) The semSignal operation increments the value
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Semaphore Primitives Semaphore Primitives
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Binary Semaphore Primitives Binary Semaphore Primitives
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Consequences Consequences Three consequences of the semaphore definition: There is no way to know before a process decrements a semaphore whether it will block or not. After a process increments a semaphore and another process gets woken up, both processes continue running concurrently. There is no way to know which process, if either, will continue immediately on a uniprocessor system. When you signal a semaphore, you don’t necessarily know whether another process is waiting, so the number of unblocked processes may be zero or one.
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Strong/Weak Semaphores Strong/Weak Semaphores A queue is used to hold processes waiting on the semaphore
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Example of Semaphore Example of Semaphore Mechanism Mechanism
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Mutual Exclusion Mutual Exclusion
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Shared Data Protected by a Shared Data Protected by a Semaphore Semaphore
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