Advanced · Module 11 · Lesson 4 of 6

Condition Variables: Waiting for Work

Park consumer threads until a producer enqueues work, using a mutex, a condition variable and a predicate loop that survives spurious wakeups.

ProAbout 12 minutes

By the end you can

Write the wait-in-a-predicate-loop pattern correctly
Explain why notify must happen with the queue already updated
Shut a queue down so no consumer waits forever

Builds on Deadlock: What It Is and How std::lock Avoids It.

01 · A question first

Three producers file books while two consumers index them. Consumers must sleep when the shelf is empty and wake the instant work arrives, then all stop cleanly when the last producer finishes. How do threads sleep without spinning, and who wakes whom?

Make a guess before you read on.

02 · The idea

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What's inside

Start with the problem
How the idea works
A kitchen pass
Why it is correct
What it costs
Pitfalls and variations
Where it is used

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03 · Practice

Asked in interviews

Why must std::condition_variable::wait always check its condition in a predicate loop, and how do you shut down a blocking queue cleanly?

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