conditionvariable

A condition variable is a synchronization primitive used in concurrent programming. It allows threads to wait efficiently for a certain condition to become true. When a thread needs to wait, it atomically releases a mutex associated with the condition variable and goes to sleep. Another thread, after changing the state that might satisfy the condition, can signal or broadcast to wake up one or all of the waiting threads.

Threads waiting on a condition variable must also hold a mutex. This mutex is used to protect the shared data that represents the condition being waited upon. When a thread calls wait, it first acquires the mutex, then checks the condition. If the condition is not met, it calls wait, which atomically releases the mutex and puts the thread to sleep. When another thread signals or broadcasts on the condition variable, a waiting thread is woken up. The woken thread then reacquires the mutex before returning from the wait call. It's crucial for the woken thread to re-check the condition after waking, as the condition might no longer be true due to other threads or spurious wakeups.

Signaling a condition variable typically involves waking up at least one waiting thread. Broadcasting wakes up all threads currently waiting on the condition variable. The choice between signal and broadcast depends on the specific synchronization pattern. If only one thread can act on the condition, signal is appropriate. If multiple threads can potentially satisfy their waiting condition, broadcast might be used, although it can lead to increased contention as all woken threads will try to reacquire the mutex. Condition variables are fundamental for implementing many higher-level synchronization patterns, such as producer-consumer queues.