Semaphores in operating systems: Which statement best describes the primary purpose of semaphores when coordinating access to shared (critical) resources among concurrent processes/threads?

Introduction / Context:
Semaphores are classic synchronization primitives used in operating systems and concurrent programming to coordinate access to shared data. They help ensure that only a safe number of processes or threads enter a critical section at once, thereby avoiding race conditions and data corruption. This question tests your understanding of the core purpose of semaphores.

Given Data / Assumptions:

• Multiple processes or threads may attempt to access a shared resource.
• Critical sections require mutual exclusion to preserve correctness.
• Semaphore operations are atomic: wait (P/down) and signal (V/up).

Concept / Approach:

A binary semaphore (value 0 or 1) enforces mutual exclusion; a counting semaphore controls access to a limited resource with capacity greater than one. Their goal is to prevent contention (simultaneous, unsafe access), which in turn prevents race conditions. While semaphores can be used in protocols that avoid deadlocks, their mere presence does not automatically prevent all deadlocks—it depends on how they are used.

Step-by-Step Solution:

Verification / Alternative check:

Standard OS texts present semaphore-based mutual exclusion and producer–consumer solutions where contention is controlled by semaphore counters; deadlock avoidance requires additional policy (ordering, timeouts, or detection).

Why Other Options Are Wrong:

• synchronize critical resources to prevent deadlock: Semaphores alone do not ensure deadlock freedom; design matters.
• are used to do I/O: Semaphores are not I/O mechanisms.
• are used for memory management: They are not allocation or paging tools.
• None of the above: Incorrect because preventing contention is correct.

Common Pitfalls:

Assuming semaphores automatically eliminate deadlock; forgetting to signal, which can cause starvation; using multiple semaphores in inconsistent order leading to deadlock.

Final Answer:

synchronize critical resources to prevent contention