Why Do I Get Segmentation Faults in My Thread-Safe C 11 Queue Despite Using Condition Variables?

Thread-Safe C 11 Queues: Resolving Spurious Thread Wake-ups

In a multifaceted project, multiple threads simultaneously handle a list of files. Each thread can add files to the queue for processing, which should operate seamlessly and avoid race conditions. However, some unexpected segmentation faults have emerged, prompting an investigation into their origin.

The FileQueue class employs mutexes (qMutex) and condition variables (populatedNotifier) to coordinate queue operations between threads. When a thread adds a file to the queue (enqueue), it signals the waiting thread (populatedNotifier.notify_one()), and when a thread retrieves a file from the queue (dequeue), it waits for the queue to be populated (if necessary: populatedNotifier.wait_for()).

Despite these precautions, segmentation faults occasionally occur in the dequeue method, specifically within the if (...wait_for(lock, timeout) == std::cv_status::no_timeout) { } block. Examination of the code indicates that the queue is empty at the time of the crash. This behavior is paradoxical because wait_for is expected to only return cv_status::no_timeout when notified, implying that a file has been added to the queue.

How can this inexplicable fault occur?

The Culprit: Spurious Wake-ups

It turns out that condition variables can experience "spurious wake-ups" due to factors outside the program's control, such as system interrupts or reschedules. When this happens, a thread may be awakened even though no actual change in the monitored condition has occurred.

In the FileQueue dequeue method, the condition variable is used to wait for the arrival of a new file. However, because the condition is checked after the lock release, it is possible for the queue to become empty again before the thread re-acquires the lock. Consequently, the condition may no longer be valid.

The Solution: Inverse Condition and Lock Guard

A more robust condition-variable-based approach involves restructuring the loop to use the inverse condition and keep the lock throughout the entire operation:

<code class="cpp">while (q.empty()) {
    populatedNotifier.wait(lock);
}</code>

By checking for the empty queue before releasing the lock, the thread ensures that the condition remains valid throughout the critical section. If awakened spuriously, the thread re-checks the condition before proceeding.

Alternative Implementation: A Template for an Async Queue

In the spirit of a thread-safe queue implementation, here is a template that provides an alternative solution:

<code class="cpp">while (q.empty()) {
    populatedNotifier.wait(lock);
}</code>

This implementation employs a mutex and condition variable, as well as a while loop that ensures the condition (an empty queue) is checked within the lock and re-evaluated in case of a spurious wake-up.

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