I need to be clarified how lock and condition_variable work.
In the -slightly modified- code from here cplusplusreference
std::mutex m;
std::condition_variable cv;
std::string data;
bool ready = false;
bool processed = false;
void worker_thread()
{
// Wait until main() sends data
std::unique_lock<std::mutex> lk(m);
cv.wait(lk, []{return ready;});
// after the wait, we own the lock.
std::cout << "Worker thread is processing data\n";
data += " after processing";
// Send data back to main()
processed = true;
std::cout << "Worker thread signals data processing completed\n";
// Manual unlocking is done before notifying, to avoid waking up
// the waiting thread only to block again (see notify_one for details)
lk.unlock();
cv.notify_one();
}
int main()
{
std::thread worker(worker_thread);
std::this_thread::sleep_for(std::chrono::seconds(1));
data = "Example data";
// send data to the worker thread
{
std::lock_guard<std::mutex> lk(m);
ready = true;
std::cout << "main() signals data ready for processing\n";
}
cv.notify_one();
// wait for the worker
{
std::unique_lock<std::mutex> lk(m);
cv.wait(lk, []{return processed;});
}
std::cout << "Back in main(), data = " << data << '\n';
worker.join();
}
The thing that I was confused with was how main thread could lock the mutex if worker_thread had already locked it.
From this answer I saw that it is because cv.wait unlocks the mutex.
But now I am confused with this: So why do we need to lock it at all, if cv.wait will unlock it?
For example, could I do this?
std::unique_lock<std::mutex> lk(m, std::defer_lock);
So, I create the lock object because cv needs it, but I do not lock it when I create.
Is there any difference now?
I did not see why I get "runtime error" here in that case.