Different types of semaphores

Mumbai University > Information Technology > Sem 5 > Operating System

Marks: 5M

Year: May 2016

  • Dijkestra proposed a significant technique for managing concurrent processes for complex mutual exclusion problems.
  • He introduced a new synchronization tool called Semaphore.

Semaphores are of two types − 1. Binary semaphore 2. Counting semaphore

Binary semaphore

Binary semaphore can take the value 0 & 1 only.

Counting semaphore

Counting semaphore can take nonnegative integer values.

  • Two standard operations, wait and signal are defined on the semaphore. Entry to the critical section is controlled by the wait operation and exit from a critical region is taken care by signal operation.
  • The wait, signal operations are also called P and V operations. The manipulation of semaphore (S) takes place as following:
  • The wait command P(S) decrements the semaphore value by 1. If the resulting value becomes negative then P command is delayed until the condition is satisfied.
  • The V(S) i.e. signals operation increments the semaphore value by 1.
    • Mutual exclusion on the semaphore is enforced within P(S) and V(S).
    • If a number of processes attempt P(S) simultaneously, only one process will be allowed to proceed & the other processes will be waiting.These operations are defined as under −

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  • The semaphore operation are implemented as operating system services and so wait and signal are atomic in nature i.e. once started, execution of these operations cannot be interrupted.
  • Thus semaphore is a simple yet powerful mechanism to ensure mutual exclusion among concurrent processes.
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