Semaphore Lab 8 Semaphores Introduction to Semaphores 1...

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Semaphores Introduction to Semaphores 1. Semaphores are used to avoid race condition . Therefore, Semaphores are control mechanism. 2. Race condition occurs when two or more processes are in their critical section . 3. Critical section is a part of code in which a process accesses a shared resource. 4. So whenever two or more processes in their critical sections , their may be chances that they might go in race condition . 5. A semaphore is a data structure, which can be shared by a group of processes. 6. When several processes compete for same resource (e.g. a file , a message queue , or a shared memory ), their operations may be synchroniz ed with the use of semaphores, so that they do not interfere with each other.
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Semaphores 6. Semaphores ensure that only one process will be granted exclusive access of a particular resource. 7. All other processes requiring the same resource will be suspended by Linux kernel. 8. Once a process releases the resource, the kernel will grant exclusive access of the resource to a suspended process. 9. Semaphores are often used in pair in order to lock (protect) a resource and to subsequently unlock (release) a resource.
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Semaphores Controlling the synchronization using an abstract data type called a semaphore was proposes by Dijkstra in1965. A Semaphore S is a an integer variable that can be accesses only through two atomic operations. i.e. these operation can not be interrupted and can be treated as an indivisible step. Operation Semaphore Wait P Signal V
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Semaphores At its simplest a semaphore is variable S which takes one of two values: 0 when a resource is lock (protected), and should not be accessed by other processes, 1 when the resource is unlocked (released). Such semaphore, which could take two values 0 or 1, is called binary semaphore . But in Linux implementation the two values are 1 , which is our P operation to wait for a semaphore to become available +1 , which is our V operation to signal that a semaphore is now available.
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Semaphore System Calls #include <semaphore.h> int sem_init(); int sem_wait(); int sem_trywait(); int sem_post(); int sem_destroy();
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Create a semaphore
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  • Winter '18
  • Dr Sibt
  • Semaphore

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