More info. on Semaphores    

• A semaphore consists of the following
1. an integer value (e.g. int count)
2. two atomic (i.e. indivisible) operations: down (P) and up (V)
• Definition: down (P)

                      if (count == 0) sleep(); /* put process to sleep on rsource wait queue */
                      v--;  /* remove resource and use (e.g. enter the critical section) */
 
• Definition: up (V)

                      v++;    /* add resource */
                      wakeup();  /* wakeup all sleeping processes waiting for resource */

 

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This page is maintained by Christopher A. Gantz (cag@cs.du.edu).