Semaphore

solution to critical section problem Semaphore

1. Semaphore Prepared By: Mr. Sangram A. Patil Assistant Professor PVPIT,Budhgaon

2. Semaphore Provides solution to impose Mutual exclusion among concurrently executing processes. Semaphore consists of an integer variable S S is shared by concurrently executing processes S protected variable: accessed and modified by two operations wait() and signal(). Value of S denotes number shared resources Wait() is indicated by symbol P Signal() is indicated by V P and V are called as semaphore primitives Each semaphore has a queue associated with it called as semaphore queue.

3. semaphore Each P operation decrements value of count by 1. Each V operation increments value of count by 1. P and V ensures that only one process enters in its critical section. All other processes waiting to enter in their critical section wait in semaphore queue Semaphore ensures that no process go into busy waiting. Moving the process to semaphore queue is called as block operation. In which process changes state from running state to waiting state. Removing process from semaphore queue and place into ready queue called as wakeup operation.

4. Semaphores Types of Semaphores There are two main types of semaphores i.e. counting semaphores and binary semaphores Counting Semaphores These are integer value semaphores and have an unrestricted value domain. These semaphores are used to coordinate the resource access, where the semaphore count is the number of available resources. If the resources are added, semaphore count automatically incremented and if the resources are removed, the count is decremented .

5. Semaphore Binary Semaphores The binary semaphores are like counting semaphores but their value is restricted to 0 and 1. The wait operation only works when the semaphore is 1 and the signal operation succeeds when semaphore is 0. It is sometimes easier to implement binary semaphores than counting semaphores.

6. semaphore Semaphore is defined as typedef struct { int count; struct processqueue queue; }semaphore;

7. Counting Semaphore Wait is defined as wait(semaphore S) { S.count --; if(S. count<0) { //Perform block operation and move the process to semaphore queue block(); } } signal is defined as signal(semaphore S) { S.count ++; if(S. count<=0 ) { //Perform wakeup operation and move the process to ready queue wakeup(); } }

8. semaphore Critical section can be accessed by while(TRUE) { wait(S); //Critical Section signal(S); //Remainder section }

9. Binary Semaphore Wait operation wait(Semaphore S) { if( S.count ==1) { s.count =0 } else { //block this process and place into Semphore queue block(); } } Signal operation signal(Semaphore S) { if(Semaphore queue is empty) { s.count =1 } else { //select the process from semaphore queue and wakeup(); block(); } }

10. Binary Semaphore w hile(TRUE) { wait(S); //Critical Section signal(S); //Remainder Section }

11. Disadvantages Semaphore with waiting queue may result in situation where two or more processes waiting indefinitely. These processes are called as deadlocked. Priority inversion occurs: Low priority process executes on high priority process.