Semaphore (programming) - Library Analogy

Library Analogy

Suppose a library has 10 identical study rooms, intended to be used by one student at a time. To prevent disputes, students must request a room from the front counter if they wish to make use of a study room. When a student has finished using a room, the student must return to the counter and indicate that one room has become free. If no rooms are free, students wait at the counter until someone relinquishes a room.

The clerk at the front desk does not keep track of which room is occupied, only the number of free rooms available. When a student requests a room, the clerk decreases this number. When a student releases a room, the clerk increases this number. Once access to a room is granted, the room can be used for as long as desired, and so it is not possible to book rooms ahead of time.

In this scenario the front desk represents a semaphore, the rooms are the resources, and the students represent processes. The value of the semaphore in this scenario is initially 10. When a student requests a room he or she is granted access and the value of the semaphore is changed to 9. After the next student comes, it drops to 8, then 7 and so on. If someone requests a room and the resulting value of the semaphore is negative, they are forced to wait. When multiple people are waiting, they will either wait in a queue, or use Round-robin scheduling and race back to the counter when someone releases a room (depending on the nature of the semaphore).