semaphore: a standalone construct

A synchronisation primitive for concurrent programs: an integer counter guarded so that two atomic operations, a wait that decrements it and blocks when it would go negative, and a signal that increments it and releases a waiting process, control how many processes may enter a protected region at once. Dijkstra introduced it in his 1968 Cooperating sequential processes, naming the two operations P and V, and distinguished a binary semaphore that acts as a simple lock from a counting semaphore that admits a fixed number of holders. The referent is an abstract counter inside a running program, measured by its value and the queue of blocked processes, and it borrowed its name by analogy from the arm signals below; it is not a machine that waves an arm on a hillside or beside a railway track.[1]

Unit

An integer counter with atomic wait and signal operations

Recovery means

The signal operation increments the counter and wakes one blocked process, restoring an available slot

Measured by

Observed by its current value, its maximum count, and the length of the queue of processes waiting on it