C ++中的带有while循环的条件变量

Question

I'm trying to figure out how to use std::condition_variable in C++ implementing a "strange" producer and consumer program in which I had set a limit to the count variable.

The main thread ("producer") increments the count and must wait for this to return to zero to issue a new increment.

The other threads enters in a loop where they have to decrease the counter and issue the notification.

I am blocked because it is not clear to me how to conclude the program by orderly exiting the while loop inside the function of all threads.

Could someone give me some guidance on how to implement it, please?

Code

#include <iostream>

#include <thread>
#include <condition_variable>
#include <vector>

int main() {
        int n_core = std::thread::hardware_concurrency();
    std::vector<std::thread> workers;
    int max = 100;
    int count = 0;
    std::condition_variable cv;
    std::mutex mutex;
    int timecalled = 0;


    for (int i = 0; i < n_core; i++) {
        workers.emplace_back(std::thread{[&max, &count, &mutex, &cv]() {
            while (true) {
                std::unique_lock<std::mutex> lk{mutex};
                std::cout << std::this_thread::get_id() << " cv" << std::endl;
                cv.wait(lk, [&count]() { return count == 1; });
                std::cout << std::this_thread::get_id() << " - " << count << std::endl;
                count--;
                std::cout << std::this_thread::get_id() << " notify dec" << std::endl;
                cv.notify_all();
            }
        }});
    }

    while (max > 0) {
        std::unique_lock<std::mutex> lk{mutex};
        std::cout << std::this_thread::get_id() << " cv" << std::endl;
        cv.wait(lk, [&count]() { return count == 0; });
        std::cout << std::this_thread::get_id() << " created token" << std::endl;
        count++;
        max--;
        timecalled++;
        std::cout << std::this_thread::get_id() << " notify inc" << std::endl;

        cv.notify_all();
    }

    for (auto &w : workers) {
        w.join();
    }

    std::cout << timecalled << std::endl; // must be equal to max
    std::cout << count << std::endl; // must be zero

}

Problem

The program doesn't end because it is stuck on some final join .

Expected Result

The expected result must be:

100
0

Edits Made

EDIT 1 : I replaced max > 0 in the while with a true . Now the loops are unbounded, but using the solution of @prog-fh seems to work.

EDIT 2 : I added a variable to check the result in the end.

EDIT 3 : I changed while(true) to while(max >0) . Could this be a problem in concurrency because we are reading it without a lock?

Answer 1

The threads are waiting for something new in the call cv.wait() .
But the only change that can be observed with the provided lambda-closure is the value of count .
The value of max must be checked too in order to have a chance to leave this cv.wait() call.

A minimal change in your code could be

cv.wait(lk, [&max, &count]() { return count == 1 || max<=0; });
if(max<=0) break;

assuming that changes to max always occur under the control of the mutex.

---

An edit to clarify around the accesses to max .

If the loop run by the threads is now while(true) , then the max variable is only read in its body which is synchronised by mutex (thanks to lk ).
The loop run by the main program is while (max > 0) : max is read without synchronisation here but the only thread that can change this variable is the main program itself, so it's pure serial code from this perspective.
The whole body of this loop is synchronised by mutex (thanks to lk ) so it is safe to change the value of max here since the read operations in the threads are synchronised in the same way.

Answer 2

1. You're having race conditions: in your code max may be read by multiple threads, whilst it is being modified in main , which is a race condition according to C++ standard.

2. The predicates you are using in wait seems to be incorrect (you're using == ).