Condition Variable vs Semaphore

Semaphore

Worldview:

resource synchronization

Example:

filled
empty

State exists inside semaphore.

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Condition Variable

Worldview:

condition synchronization

Example:

while (!predicate)
    pthread_cond_wait()

State exists outside condvar.

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Predicate Loop

Correct:

while (!condition)
    wait()

Wrong:

if (!condition)
    wait()

Reason:

• spurious wakeup
• broadcast
• race avoidance

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Comparison

Item	Semaphore	Condvar
Store count	Yes	No
Store state	Internal	External
Queue	Excellent	Good
State machine	Moderate	Excellent
Timeout	Yes	Yes

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Conclusion

Semaphore waits for resources.

Condition variable waits for state transitions.