Semaphores are synchronization primitives provided by the IPC subsystem. They deliver a protective mechanism for shared data structures or resources against concurrent access by processes in a multithreaded environment. At its core, each semaphore is composed of an integer counter that can be atomically accessed by a caller process. Semaphore implementations provide two operations, one for waiting on a semaphore variable and another to signal the semaphore variable. In other words, waiting on the semaphore decreases the counter by 1 and signaling the semaphore increases the counter by 1. Typically, when a process wants to access a shared resource, it tries to decrease the semaphore counter. This attempt is however handled by the kernel as it blocks the attempting process until the counter yields a positive value. Similarly, when a process relinquishes the resource, it increases the semaphore counter, which wakes up any process that is waiting for the resource.

Semaphore versions...