Which of the following is not an interprocessor communication mechanism in UNIX?

Explanation

Pipes, message queues and shared memory are classical UNIX System V IPC facilities that actually carry application data between cooperating processes. A semaphore, by contrast, is purely a synchronization primitive used to control access to a shared resource or coordinate timing between processes; it does not itself transfer any data between processes. Therefore, among the given options, the Semaphore is not a data-communication mechanism, even though it is an IPC-related synchronization tool.