In programming, especially in UNIX systems, semaphores are a method for coordinating or synchronizing activities in which multiple processes compete for the similar operating system resources. It is a value in a designated place in operating system (or kernel) storage that every process can check and then change. Based on the value that is found, the process can utilizes the resource or will discover that it is already in use and have to wait for some period before trying again. It can be binary (0 or 1) or can have additional values. Normally, a process using semaphores checks the value and then, if it using the resource modifies the value to reflect this so that subsequent semaphore users will know to wait.

Semaphores are devices utilized to help with synchronization. If multiple processes share a common resource, they require a way to be able to use that resource without disrupting each other. You desire each process to be able to read from and write to that resource uninterrupted.

A semaphore will either permit or prohibit access to the resource, based on how it is set up. One instance setup would be a semaphore which allowed any number of processes to read from the resource, but just one could ever be in the process of writing to that resource at a time.

A semaphore is or a software or hardware tag variable whose value denote the status of a common resource. Its cause is to lock the resource being utilized. A process that needs the resource will check the semaphore for seeking the status of the resource followed by the decision for proceeding. In multitasking operating systems, the activities are synchronized through the semaphore techniques.

Commonly semaphores are used for two purposes: to share a common memory space and to share access to files. Semaphores are one of the methods for inter-process communication (IPC). The C programming language contains a set of interfaces or "functions" for managing semaphores.

There are 2 types of semaphores:

Binary semaphores:

It has 2 methods linked with it. (Up, down / lock, unlock). It can take only 2 values (0/1). They are utilized to acquire locks. While a resource is obtainable, the process in charge set the semaphore to 1 else 0.

Counting Semaphore:

It may have value to be greater than one, normally used to allocate resources through a pool of identical resources.

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