Semaphore is quiet easy and simple…

Lets take an example from our real life which makes semaphore a simple task for me….

when we are using a printer on lan network and ten’s of computer are connected to the same printer for printout so if anyone wants to get a printout he can give a command and get that but suppose there is a situation in which all the computers execute print command at the same time so how will printer control it so there need to be some protocol implementation to control the sequence otherwise what will happen if printer will access all the commands at the same time imagine yourself, It might be possible that all the picture get on the one paper. so we use some technique to control it. in this example critical condition is when computers are trying to access the printer. So there should be some condition before entering the critical region so that a single process will enter at one time and rest will be in the waiting condition till one is accessing so we implement the semaphore.

While implementing one need to have knowledge of three system calls:

semget();   //it is used to declare semaphore

semctl();    //it is used to initialize semaphore

semop();     // it is used to control condition before entering and leaving the semaphore

so now you need to think where to use what????… so we will implement according to its use as such if semget is for declaration so we need to declare it once and where you want to use semaphore.and also it returns sem_id that is generated by kernel which is required to access the semop command and for using semaphore in future.

and when it comes to semctl() it is used for initializing the semaphore and it will make the clients to see the condition and enter the semaphore so you need to enter it once at time of deceleration but it is not mandatory it may vary according to once need.

and semop() is used to decrease the operator value before executing statement after entering critical region and increment operator value again before leaning the critical region so that other can access it….

see figure it will also help you to understand.