How std::condition_variable and std::mutex works exactly?

1
votes

I'm reading this std::condition_variable example:

#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();
}

Let's focus on the beggining:

std::thread worker(worker_thread);

here, we start the worked, which will lock the mutex immediately:

std::unique_lock<std::mutex> lk(m);
cv.wait(lk, []{return ready;});

Then, we lock the mutex on main to change the value of ready

{
    std::lock_guard<std::mutex> lk(m);
    ready = true;
    std::cout << "main() signals data ready for processing\n";
}

but how is it possible that we even arrive at the line ready=true? The mutext m is blocked from the worked thread, so the line std::lock_guard<std::mutex> lk(m); will wait until the mutex m is unlocked. As I understand, when a mutex is locked and we try to lock it, we'll wait until it gets unlocked. However, it'll never get unlocked because the worked thread is waiting, therefore not releasing it.

2
c++multithreadingconcurrencymutexcondition-variable
std::condition_variable releases its lock while it's waiting. – Miles Budnek
the std::lock_gaurd is destroyed when it gets out of scoop that is - } then its unlock this is part of the RAII paradigm read the doc – yaodav
Read about wait: "Atomically unlocks lock, blocks the current executing thread, and adds it to the list of threads waiting on *this. " (Emphasis mine.) – molbdnilo

2 Answers

1
votes

In the link you have attached, note that the following

The wait operations atomically release the mutex and suspend the execution of the thread.

Hence the line

cv.wait(lk, []{return ready;});

releases the mutex and suspends the execution of the thread until the condition variable is notified, a timeout expires (with wit_for()), or a spurious wake-up occurs, hence the thread is awakened, and the mutex is atomically reacquired

0
votes

Here's another example that will give you the gist of the mechanism.

#include <cstdio>                                                                                                                                                                                                   
#include <thread>
#include <mutex>
#include <condition_variable>
#include <chrono>

std::mutex mtx;
std::condition_variable cv;
bool signal = 0;

void A()
{
  while (1)
  {
    {
      std::unique_lock<std::mutex> lock(mtx);
      cv.wait(lock, [](){return signal;});
      signal = 0;
    }

    printf("Lock release from th A\n");
  }
}

int main()
{
  std::thread th(A);

  for (unsigned i = 0; i < 100; i++)
  {
    {
      std::lock_guard<std::mutex> lock(mtx);
      signal = 1;
    }

    cv.notify_one();
    printf("Sending signal %i\n", i);
    std::this_thread::sleep_for(std::chrono::seconds(5));
  }

  th.join();
  return 0;
}