A semaphore is a synchronization tool used in operating systems to manage concurrent access to shared resources. It is essentially a variable that can take on integer values and is used to signal resource availability between processes or threads.

A semaphore is a synchronization primitive used in operating systems for controlling access to a common resource by multiple processes in a concurrent system. Formally, a semaphore is defined as a variable or abstract data type that provides two atomic operations: wait (P), which decrements the semaphore’s value and blocks the calling process if the value is non-positive, and signal (V), which increments the semaphore’s value and wakes up a waiting process. Semaphores are used to manage resource allocation and avoid issues like race conditions and deadlock in concurrent processes.

Semaphores can be classified as binary semaphores (which take values 0 or 1) and counting semaphores (which take non-negative integer values). They help resolve issues like race conditions and maintain process synchronization by allowing or blocking access based on availability.

The primary operations on semaphores are wait (P) and signal (V). When a process encounters a wait operation, it decrements the semaphore value and may block if the value is zero. Signal operations increment the semaphore and wake up waiting processes if needed. Proper use of semaphores is crucial for maintaining system stability and efficiency.