14. Monitors / Condition Variables

The problem with just a mutex: if a condition isn't met (e.g., buffer empty), a thread would have to spin — lock, check, unlock, repeat — wasting CPU (busy waiting).

Condition variable: lets a thread sleep efficiently until a condition becomes true.

• wait(&cond, &mutex) — atomically releases the mutex and sleeps; reacquires the mutex on wakeup.
• signal(&cond) — wakes one sleeping thread.
• broadcast(&cond) — wakes all sleeping threads.

The atomic release-and-sleep is critical — it prevents the missed wakeup problem, where a signal arrives just before the consumer actually falls asleep.

Why while****, not if (key interview point):

while (buffer.empty())
    cond_wait(&not_empty, &lock);

Spurious wakeups can occur without a real signal, and with multiple consumers, two might wake for one signaled item — the second must recheck and go back to sleep, or it will operate on an empty buffer.

Monitor: a higher-level construct bundling shared data, an implicit mutex, and condition variables, so only one thread is active inside at a time. Java's synchronized implements a monitor; wait()/notify()/notifyAll() map to cond_wait/cond_signal/cond_broadcast.

Aspect Condition Variable Semaphore
Used with	Must pair with a mutex	Standalone
State	Stateless (no count)	Has a count
Missed signal	Lost if nobody waiting	Remembered (count persists)
Spurious wakeups	Yes — must use while	No

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