Why Does My C 11 Thread-Safe Queue Segfault When Dequeuing from an Empty Queue?

C 11 Thread-Safe Queue Implementation with Conditional Variables

Problem:

While implementing a thread-safe queue, segfaults occurred when dequeuing from an empty queue. The issue stemmed from the condition variable wait_for function, which was expected to return only when notified. However, the queue sometimes remained empty after the wait_for function returned with cv_status::no_timeout.

Solution:

The correct approach is to invert the condition monitored by the condition variable. In this case, q.empty() should be inverted to !q.empty() as the desired condition is for the queue to have at least one element. Here's the modified dequeue method:

<code class="cpp">std::string FileQueue::dequeue(const std::chrono::milliseconds& timeout)
{
    std::unique_lock<std::mutex> lock(qMutex);
    while (q.empty()) {
        if (populatedNotifier.wait_for(lock, timeout) == std::cv_status::timeout) {
            return std::string();
        }
    }
    std::string ret = q.front();
    q.pop();
    return ret;
}</code>

Now, the wait_for function will only return cv_status::timeout if the queue remains empty after the timeout period, preventing an attempt to dequeue from an empty queue.

Additional Recommendations:

• Use a template for reusable thread-safe queues.
• Initialize the queue in the constructor of the class instead of inline with the declaration.
• Avoid using std::lock_guard in favor of std::unique_lock, as this ensures the mutex is unlocked even if an exception is thrown within the critical section.

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