What is a semaphore?

A semaphore is a synchronization primitive used in concurrent programming to control access to a shared resource. It is essentially a variable that is used to manage the number of threads that can access a particular resource simultaneously.

A semaphore maintains a count that represents the number of available resources. When a thread wants to access the resource, it must first acquire the semaphore. If the count is greater than zero, the thread is allowed to proceed and the count is decremented. If the count is zero, the thread is blocked until another thread releases the semaphore, increasing the count.

Semaphores can be used to solve various synchronization problems, such as preventing race conditions or coordinating access to a limited resource. They provide a way to enforce mutual exclusion and ensure that only a limited number of threads can access a resource at any given time.

In addition to the basic semaphore, there are also binary semaphores (also known as mutexes) that have a count of either 0 or 1. These are commonly used to implement critical sections, where only one thread can access a resource at a time.

Overall, semaphores are a fundamental tool in concurrent programming for managing shared resources and ensuring thread safety.