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Semaphores in Linux

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Multithreaded applications are part and parcel of day-to-day commercial application. It would be difficult to imagine any full fledged application running commercially that is not multithreaded. Applications must use the multithreaded approach to improve on the performance of the application or systems. However, most beautiful things in life do not come without a price. Likewise, if the multithreaded feature needs to be used by the application, then it comes with a set of issues, such as deadlocks, race conditions, incorrect behavior of threads, etc. To overcome these issues, the OS provides a set of tools like Mutex, semaphores, signals and barriers that are handy in solving multithreaded multiprocessed issues. This article discusses one of these tool, semaphores, and provides some insight about them.

Introduction to Semaphores

Semaphores can be thought of as simple counters that indicate the status of a resource. This counter is a protected variable and cannot be accessed by the user directly. The shield to this variable is provided by none other than the kernel. The usage of this semaphore variable is simple. If counter is greater that 0, then the resource is available, and if the counter is 0 or less, then that resource is busy or being used by someone else. This simple mechanism helps in synchronizing multithreaded and multiprocess based applications. Semaphores were invented and proposed by Edsger Dijkstra, and still used in operating systems today for synchronization purposes. The same mechanism is now available for application developers too. Its one of the most important aspects of interprocess communication.

Semaphores can be either binary or counting, depending on the number of shared resources.

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