Semaphore: Difference between revisions
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'''Semaphore''' is a synchronization object proposed by Edsger Dijkstra. A semaphore is characterized by a natural number ''k''. A [[task]] may atomically increase or decrease ''k''. When ''k'' reaches 0 the tasks attempting to decrease it are blocked. These are released in an unspecified order when other tasks increase ''k'', one per increment. |
'''Semaphore''' is a synchronization object proposed by Edsger Dijkstra. A semaphore is characterized by a natural number ''k''. A [[task]] may atomically increase or decrease ''k''. When ''k'' reaches 0 the tasks attempting to decrease it are blocked. These are released in an unspecified order when other tasks increase ''k'', one per increment. |
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The natural number ''k'' works like a count of available slots for resources. When you ( |
The natural number ''k'' works like a count of available slots for resources. When you (a task) want to use something (an object, a file, any resource) that can only be used by a limited number of tasks (usually one, but possibly more), you see if there are available slots (check the value of ''k''). If there are slots available (''k'' > 0), you take one (decrement ''k''). When you're done with the resource, you free your slot up (increment ''k''). If there were no slots available when you checked (''k'' = 0), you wait until one becomes available. |
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A semaphore is considered a low-level synchronization primitive. They are exposed to deadlocking. |
A semaphore is considered a low-level synchronization primitive. They are exposed to deadlocking. |
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Revision as of 18:15, 31 October 2008
Semaphore is a synchronization object proposed by Edsger Dijkstra. A semaphore is characterized by a natural number k. A task may atomically increase or decrease k. When k reaches 0 the tasks attempting to decrease it are blocked. These are released in an unspecified order when other tasks increase k, one per increment.
The natural number k works like a count of available slots for resources. When you (a task) want to use something (an object, a file, any resource) that can only be used by a limited number of tasks (usually one, but possibly more), you see if there are available slots (check the value of k). If there are slots available (k > 0), you take one (decrement k). When you're done with the resource, you free your slot up (increment k). If there were no slots available when you checked (k = 0), you wait until one becomes available.
A semaphore is considered a low-level synchronization primitive. They are exposed to deadlocking.
See also mutex, a variant of semaphore.