What is a semaphore?

Answer Posted / sri

In programming, especially in Unix systems, semaphores are a
technique for coordinating or synchronizing activities in
which multiple processes compete for the same operating
system resources. A semaphore is a value in a designated
place in operating system (or kernel) storage that each
process can check and then change. Depending on the value
that is found, the process can use the resource or will find
that it is already in use and must wait for some period
before trying again. Semaphores can be binary (0 or 1) or
can have additional values. Typically, a process using
semaphores checks the value and then, if it using the
resource, changes the value to reflect this so that
subsequent semaphore users will know to wait.

Semaphores are commonly use for two purposes: to share a
common memory space and to share access to files. Semaphores
are one of the techniques for interprocess communication
(IPC). The C programming language provides a set of
interfaces or "functions" for managing semaphores.

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