Usage

• Counting and binary semaphores. The value of a counting semaphore can range over an unrestricted domain.
• The value of a binary semaphore can range only between 0 and 1. On some systems, binary semaphores are known as mutex locks, as they are locks that provide mutual exclusion.
• We can use binary semaphores to deal with the critical-section problem for multiple processes. The $n$ processes share a semaphore, mutex, initialized to 1. Each process $P_i$ is organized as shown in Fig. 6.7.

  Figure 6.7: Mutual-exclusion implementation with semaphores.

  
• Counting semaphores can be used to control access to a given resource consisting of a finite number of instances. The semaphore is initialized to the number of resources available.
• Each process that wishes to use a resource performs a $wait()$ operation on the semaphore (thereby decrementing the $count$). When a process releases a resource, it performs a $signal()$ operation (incrementing the $count$).
• When the $count$ for the semaphore goes to 0, all resources are being used. After that, processes that wish to use a resource will block until the count becomes greater than 0.