In a condition variable, the thread invoking the wait operation is:

Question

BlackTom AI · Accepted Answer

A condition variable's wait causes the calling thread to be suspended until a signal (or broadcast) is delivered on that same condition variable.

1) Relevant knowledge points and concepts
- Condition variable: synchronization primitive used with a mutex to block a thread until a particular condition is met.
- wait operation: atomically releases the associated mutex and blocks the calling thread on the condition variable.
- signal (or notify) operation: wakes one waiting thread on that same condition variable; broadcast wakes all waiting threads.
- Upon wakeup, the thread reacquires the mutex before returning from wait, so the condition can be rechecked.

2) Step-by-step reasoning process
- A thread calls wait(cv, mutex). The runtime performs two atomic steps: release mutex and suspend the thread on cv.
- The thread remains suspended (blocked) and does not run until a thread calls signal(cv) or broadcast(cv).
- When another thread executes signal(cv), one suspended thread on cv is made runnable; when scheduled, it will re-acquire the mutex and continue.
- Therefore the waiting thread specifically waits for a signal on the same condition variable it used to wait on.

3) Intermediate steps shown explicitly
- Thread T: call wait(cv, m)
- Atomic action: release m; block T on cv
- Another Thread S: call signal(cv)
- Scheduler: move one blocked thread (T) from cv to ready state
- T resumes: reacquire m; return from wait

4) Why the correct answer is correct
- The canonical semantics of condition variables specify that wait suspends on the same condition variable and requires a matching signal/broadcast on that variable to wake the thread. This matches the answer that the thread is "suspended until another thread invokes the signal operation on the same condition variable."

5) Why the other options are incorrect
- "waiting for another thread to complete before it can itself call the signal operation": wait doesn't require the waiter to wait for another thread to call signal before it can call signal itself; it's the waiter that blocks until signaled.
- "stopped until any thread in the queue finishes execution": wakeup is tied to signal/broadcast on the condition variable, not to arbitrary thread termination.
- "suspended until another thread invokes the signal operation on a different condition variable": signaling a different condition variable does not affect threads blocked on this one.

6) Conclusion
This confirms the correct answer: suspended until another thread invokes the signal operation on the same condition variable.

In a condition variable, the thread invoking the wait operation is:单项选择题

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