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Understanding Semaphores: Coordination and Synchronization in Concurrent Programming

Semaphores are crucial synchronization tools that allow multiple concurrent threads to coordinate through signaling. They support four main operations: initialize, get value, increment (signal/post), and decrement (wait). The thread that locks a semaphore isn't always the one that unlocks it, and blocked threads are maintained in a list. Binary semaphores restrict access to one thread at a time, akin to mutex locks, while counting semaphores can hold any integer value, facilitating complex synchronization scenarios. Examples include managing critical sections and the producer/consumer model.

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Understanding Semaphores: Coordination and Synchronization in Concurrent Programming

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  1. CS 311/350/550 Semaphores

  2. Semaphores – General Idea • Allows two or more concurrent threads to coordinate through signaling/waiting • Has four main operations • initialize, getvalue, increment (signal/post), decrement (wait) • The thread who “locks” is not necessarily the thread who “unlocks” • Those blocked-waiting are always put in a list

  3. Semaphores – Signal/Wait • Integer counter • coordination and synchronization depends on this value • sem_wait • decrements the counter • then blocked if counter is < 0 • thread placed in the blocked list • sem_post • increments the counter • unblocks the next thread in the blocked list

  4. Binary Semaphores • Binary Semaphores • Counter can only be one or zero • Access to the critical section is one at a time • Similar to a mutex lock

  5. Counting Semaphores • Counter can be any integer value at any time • Used for more complex synchronization • Examples • Prioritizing access to the critical section • Producer/Consumer model with a bound buffer • Multiple counting semaphores can be used to coordinate multiple Readers/Writers

  6. Semaphore Example - POSIX

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