multithreading · high

Producer-Consumer with Condition Variables

A bounded queue with a mutex + two condition variables: the producer blocks when full, the consumer when empty, and notify wakes the other.

The code

std::queue<T> q;
std::mutex m;
std::condition_variable not_full, not_empty;

void producer(T x) {
    std::unique_lock lk(m);
    not_full.wait(lk, [] { return q.size() < CAP; });
    q.push(x);
    not_empty.notify_one();
}
void consumer() {
    std::unique_lock lk(m);
    not_empty.wait(lk, [] { return !q.empty(); });
    x = q.front();
    q.pop();
    not_full.notify_one();
}

What this lesson walks through

1. 01A bounded queue shared by two threads
2. 02Producer pushes and notifies not_empty
3. 03One more push fills the queue
4. 04Full -> producer blocks on not_full
5. 05Consumer pops and notifies not_full
6. 06Producer wakes, re-checks, pushes E
7. 07Empty -> consumer blocks on not_empty
8. 08The whole protocol

A producer adds items to a fixed-size queue and a consumer removes them. They coordinate with one mutex and two condition variables: not_full and not_empty.