Semaphore: Difference between revisions
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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. |
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, like in the problem of [[dining philosophers]]. |
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See also [[mutex]], a variant of semaphore. |
See also [[mutex]], a variant of semaphore. |
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Revision as of 10:58, 1 November 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, like in the problem of dining philosophers.
See also mutex, a variant of semaphore.