A semaphore is a synchronization primitive used in concurrent programming to control access to a shared resource or a critical section by multiple processes or threads. It is a variable or an abstract data type that is used for signaling between different processes or threads to avoid race conditions and ensure mutual exclusion.
Key concepts related to semaphores include:
Mutex Semaphore: A binary semaphore, often referred to as a mutex (short for mutual exclusion), has two states: 0 and 1. It is used to control access to a critical section by allowing only one process or thread to enter the critical section at a time.
Counting Semaphore: A counting semaphore can have an integer value greater than 1. It is used to control access to a resource where multiple instances of the resource are available, and the semaphore value represents the number of available instances.
Operations on Semaphores:
Semaphores support two fundamental operations:
Binary Semaphore as a Mutex: In the context of mutual exclusion, a binary semaphore with an initial value of 1 is often used as a mutex. The "Wait" operation corresponds to acquiring the mutex, and the "Signal" operation corresponds to releasing the mutex.
Semaphore Implementation: Semaphores can be implemented using various mechanisms provided by the operating system, such as hardware instructions, software-based atomic operations, or a combination of both.
Use Cases: Semaphores are commonly used in scenarios where multiple processes or threads need to coordinate access to shared resources, such as shared memory, files, or critical sections in code. They are essential for preventing race conditions and ensuring the orderly execution of concurrent programs.
