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Solaris™ Internals: Core Kernel Components by Richard McDougall, Jim Mauro

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10.3. System V Semaphores

A semaphore, as defined in the dictionary, is a mechanical signalling device or a means of doing visual signalling. The analogy typically used is the railroad mechanism of signalling trains, where mechanical arms would swing down to block a train from a section of track that another train was currently using. When the track was free, the arm would swing up, and the waiting train could then proceed.

The notion of using semaphores as a means of synchronization in computer software was originated by a Dutch mathematician, E. W. Dijkstra, in 1965. Dijkstra's original work defined two semaphore operations, wait and signal (which correlate nicely to the railroad example). The operations were referred to as P and V operations. ...

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