semaphorebased

Semaphore-based synchronization relies on semaphores to coordinate access to shared resources or to order activities among threads or processes. A semaphore is a small, integer-valued primitive with two atomic operations: wait (often called P) and signal (V). A counting semaphore tracks the number of available instances of a resource, while a binary semaphore (or mutex) is used to enforce mutual exclusion.

Initialization sets the starting count. When a thread executes wait, if the semaphore value is greater than zero, the value is decremented and the thread proceeds; if the value is zero, the thread blocks until another thread performs signal. A signal increments the value and may wake a blocked thread. These operations are designed to be atomic with respect to memory ordering.

Common patterns use semaphores to implement resource pools, producer–consumer queues, and critical sections. Binary semaphores provide mutual exclusion, while counting semaphores regulate access to a finite number of resources. Semaphores can coordinate threads across multiple producers and consumers, or separate readers and writers.

Limitations include potential deadlock if multiple semaphores are acquired in inconsistent orders, and priority inversion or starvation in some scheduling environments. Semaphores do not track ownership, unlike mutexes, which can complicate correct release semantics. They require careful design and discipline.

Historically, semaphore-based synchronization was introduced by Edsger Dijkstra and became a foundational primitive in operating systems and concurrent programming. Modern systems provide semaphores in standard libraries across languages and platforms, though higher-level abstractions such as monitors, condition variables, or task-based runtimes are often preferred for complex synchronization.