Why Am I Getting Segfaults When Using a Condition Variable to Check for an Empty Queue in C ?

C 11 thread-safe queue: Confusion over condition variable use

In your code, you implemented a thread-safe queue using a mutex and a condition variable. However, you are encountering occasional segfaults within the code block controlled by the condition variable. Specifically, you notice that the segfaults occur when the queue is empty, which seems counterintuitive as the condition variable is intended to signal the availability of new items in the queue.

Incorrect Condition Variable Usage

Your misunderstanding stems from the incorrect use of the condition variable. The purpose of a condition variable is to signal a change in a shared state, in which other threads can then react to this change. However, in your code, you are using the condition variable to wait for a specific condition (the queue being empty) to become false. This is incorrect because when the condition becomes false (i.e., the queue is no longer empty), the thread will wake up, but it is not guaranteed that the condition remains false (i.e., the queue might have become empty again in the meantime).

Proper Condition Variable Usage

The correct way to utilize a condition variable is to have a loop that checks the condition (in this case, whether the queue is empty) and enters a wait state if the condition is not met. When the condition is signaled, the thread will wake up, but it will immediately recheck the condition before proceeding. This ensures that the thread only performs the desired operation if the condition is still true upon waking up.

Alternative Implementation

Here is an alternative implementation of a thread-safe queue that addresses the issue:

<code class="cpp">template <class T>
class SafeQueue {
public:
  SafeQueue() : q(), m(), cv() {}

  void enqueue(T t) {
    std::lock_guard<std::mutex> lock(m);
    q.push(t);
    cv.notify_one();
  }

  T dequeue() {
    std::unique_lock<std::mutex> lock(m);
    while (q.empty()) cv.wait(lock);
    T val = q.front();
    q.pop();
    return val;
  }

private:
  std::queue<T> q;
  mutable std::mutex m;
  std::condition_variable cv;
};</code>

In this implementation, the condition variable is correctly used to wait for the condition of the queue being empty (or non-empty for the enqueue method) to become false. The thread will only perform the desired operation (dequeueing an item) if the queue is non-empty after waking up.

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