What is a semaphore used for in concurrent programming?

• A. Data compression
• B. Scheduling decisions
• C. Synchronization to control access to shared resources
• D. Memory allocation

Answer: (C)

Semaphores are synchronization primitives that coordinate how threads or processes access shared resources. They rely on a counter and two operations: wait (decrement, potentially blocking) and signal (increment, potentially waking a blocked thread). If the counter is positive, a wait lets a thread proceed and reduces the available count. If the counter is zero, the thread waits until another thread signals, increasing the count again. This mechanism enforces how many threads can access a resource at once, preventing data races and keeping access orderly. A binary semaphore works like a lock for exclusive access, while a counting semaphore manages multiple identical resources.

Data compression, scheduling decisions, and memory allocation are unrelated to this coordination role. Data compression reduces data size, scheduling decisions come from the OS or runtime’s scheduler, and memory allocation handles reserving and managing memory blocks. The synchronization to control access to shared resources is what semaphores provide.