Semaphores

Introduction

• Semaphores were introduced by Edsger Dijkstra and are commonly used in operating systems and concurrent programming.

Definition

• Semaphores provide a mechanism for coordinating concurrent processes or threads and protecting shared resources from simultaneous access, thereby avoiding race conditions and ensuring orderly execution. 
• In computer science, a semaphore is a synchronization construct that allows multiple processes or threads to coordinate their activities and control access to shared resources in a concurrent system.

Characteristics

• They are usually applied in a concurrent or multi-threaded environment.
• It is a variable or abstract data type that is typically used for process or thread synchronization.
• They provide a way to enforce mutual exclusion and ensure that only one process or thread accesses a shared resource at a time.
• A semaphore is essentially a variable that is used to control access to a shared resource.

Advantages

• Semaphores provide a flexible synchronization mechanism.
• Semaphores can be used to solve various synchronization problems, such as controlling access to the critical section problem (ensuring mutually exclusive access to a shared resource) and coordinating the producer-consumer problem (synchronizing the interaction between producer and consumer processes), and preventing race conditions.

Disadvantages

• Semaphores require careful/correct usage to avoid potential issues such as deadlocks (where multiple processes are blocked indefinitely) or race conditions or starvation, where processes may get stuck indefinitely.
• Proper initialization, correct usage of wait and signal operations, and careful design are essential when working with semaphores to ensure proper synchronization in multi-threaded or concurrent systems.

Types of Semaphore Operations

• It can have an integer value and supports two primary operations:-
  • Acquire (Wait[P]) Operation:
    • It is also known as “acquire” or “down,” and the wait operation decreases the count of the semaphore by one, indicating that a resource is being acquired. 
    • This operation decreases the value of the semaphore. If the resulting value or count becomes/is negative, the process or thread requesting access to the resource is blocked and put into a waiting state until the semaphore becomes non-negative/a resource becomes available. 
  • Release (Signal[V]) Operation:
    • Also known as “release” or “up,” the signal operation increases the count of the semaphore by one, indicating that a resource is being released. If there are blocked processes or threads waiting for a resource, the signal operation will unblock one of them, allowing it to proceed.
    • This operation increases the value of the semaphore. If there are processes or threads waiting for the semaphore, one of them is selected to be unblocked and allowed to proceed.

Types of Semaphore

• There are two commonly used types of semaphores:

• • Binary Semaphore:
    • Also known as a mutex (mutual exclusion), a binary semaphore can take only two values: 0 and 1.
    • It is primarily used for mutual exclusion, where only one process or thread can access a shared resource at a time.
    • Binary semaphores can be used to implement locks.
  • Counting Semaphore:
    • A counting semaphore can take multiple non-negative integer values.
    • It can be used to control access to a resource with a limited capacity.
    • For example, if the semaphore has a value of 3, it means that three processes or threads can access the resource simultaneously.