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GENERAL: WHAT ARE SEMAPHORES?

QUESTION

What are semaphores and how would I use them?

ANSWER

There are several types of semaphores (the basic idea behind each type is the same):

  • Binary
  • Counting
  • Mutex

Semaphores are typically used in one of two ways:

  1. To control access to a shared device between tasks. A printer is a good example. You don't want 2 tasks sending to the printer at once, so you create a binary semaphore to control printer access. When a device wishes to print, it attempts to "take" the semaphore. If the semaphore is available, the task gets to print. If the semaphore is not available, the task will have to wait for the printer.
  2. Task synchronization. By tasks taking and giving the same semaphore, you can force them to perform operations in a desired order.

Counting semaphores are used when you might have multiple devices (like 3 printers or multiple memory buffers).

Binary semaphores are used to gain exclusive access to a single resource (like the serial port, a non-reentrant library routine, or a hard disk drive). A counting semaphore that has a maximum value of 1 is equivalent to a binary semaphore (because the semaphore's value can only be 0 or 1).

Mutex semaphores are optimized for use in controlling mutually exclusive access to a resource. There are several implementations of this type of semaphore.

SEE ALSO

Last Reviewed: Thursday, March 01, 2007


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