Which synchronization primitive allows threads to wait for a condition to become true, typically used with producers and consumers?

• A. Condition variable
• B. Mutex
• C. Spinlock
• D. Semaphore

Answer: (A)

A condition variable lets a thread sleep until a specific condition becomes true, which is exactly what producers and consumers need. In a producer–consumer setup, a consumer waits for the shared buffer to be non-empty and a producer signals when it enqueues an item. The condition variable is used with a mutex that protects the shared state. When the consumer finds nothing to consume, it waits on the condition variable; this releases the mutex and blocks the thread until a signal or broadcast occurs. When a producer adds an item, it signals the condition variable to wake a waiting thread (or broadcasts to wake all). The awakened thread re-acquires the mutex and rechecks the condition, looping if needed to handle spurious wakeups.

Compared to the alternatives, a mutex is only a lock for mutual exclusion and doesn’t provide a way to wait for a condition. A spinlock keeps polling in a busy-wait loop, wasting CPU cycles. A semaphore can coordinate resource counts but isn’t as natural or flexible for waiting on a specific condition tied to shared state; condition variables are designed for that pattern.