CODEWITHSUNDEEP
c++multithreading
noziar
noziar

Reputation: 1057

How to ensure wait_for is called before notify_one

Typical usages of condition variables look like this (see code below): http://en.cppreference.com/w/cpp/thread/condition_variable.

However, it seems that the main thread could potentially call notify_one before the worker thread calls wait, which would result in a deadlock. Am I mistaken? If not, what is the usual workaround for this?

#include <iostream>
#include <string>
#include <thread>
#include <mutex>
#include <condition_variable>

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);

    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();
}

Upvotes: 0

Views: 2605

Answers (1)

Cubbi
Cubbi

Reputation: 47408

Note the definition of wait that uses a condition (the only wait you should ever be using):

while (!pred()) {
    wait(lock);
}

if notify was already fired it means the condition is already true (that was sequenced before notify_one in the signalling thread). So when the receiver takes the mutex and looks at pred(), it will be true and it will proceed.

Upvotes: 4

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