Condition Variables and Predicate-Based Waiting

Condition variables let tasks wait for a shared-state predicate while coordinating through a lock. Recheck the predicate after waking.

Before you start

You should understand processes, threads, memory and basic file operations. Track ownership and state changes over time. For concurrent scenarios, write down at least two possible execution orders; the same instructions can behave differently when scheduling or resource availability changes.

The practical goal is to reason through this situation: A consumer waits while a queue is empty, then checks again before removing an item. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.

Step-by-step walkthrough

Step 1: Write the predicate

A consumer may proceed only when the shared queue is nonempty.

Step 2: Wait under the lock

Waiting coordinates release and reacquisition of the protecting lock.

Step 3: Recheck after wake

Another consumer or a spurious wake can leave the predicate false.

Worked scenario

A consumer waits while a queue is empty, then checks again before removing an item.

acquire lock
while queue is empty:
    condition.wait(lock)
item = queue.remove()
release lock

Text

This is pseudocode. A notification is a reason to check state, not a promise that this particular consumer owns an item. Producers modify the predicate’s state under the same synchronization discipline.

Common mistake

Assuming every wake guarantees an item ignores spurious wakeups or another consumer’s progress.

Verify the behavior

Wake two consumers for one item and verify only one consumes while the other waits safely.

Interview exercise

Write the wait pattern.

Answer and reasoning

Hold the lock, loop while the predicate is false, wait, then consume only after the predicate is true.

Continue learning

Compare the scenario with the Operating Systems interview questions and test your understanding with the Operating Systems MCQs. For terminology and implementation details, consult the reference material.