使用条件变量而不是lock_guard

Question

I have a simple program in which I want to output the numbers 1-100, with one thread outputting all the odd numbers and the other one all even numbers. Using lock_guard, this is quite an easy task. Code follows:

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

std::mutex m;


void print_numbers(int i)
{
    for (i; i <= 100; i++)
    {
        std::lock_guard<std::mutex> locker(m);
        std::cout << i << std::endl;
        ++i;
    }
}

int main()
{
    std::thread t1(print_numbers, 0);
    std::thread t2(print_numbers, 1);
    t1.join();
    t2.join();
    return 0;
}

My question is, how can I use condition variables to do this, and perhaps make it a little more elegant?

Answer 1

There is no sharing whatsoever in your code. Each thread has a local variable i. A POSIX condition is used for inter-thread communication/signaling. The idea is simple:

1. a thread check a boolean predicates (depending on a shared state) and decide to "wait" when the predicate is false (pthread_cond_wait in POSIX)

2. another thread can modify the shared state and therefore impact the value of the boolean predicate. If this thread changes the state, it can notify the other thread (Thread #1) that it should wake up and re-check its predicate whose value might have now changed (pthread_cond_signal and pthread_cond_broadcast in POSIX).

Since there is a shared state, you still need a mutex to protect that shared state and "lock it" when you are modifying or accessing it.

The typical use case of a POSIX condition is a shared queue. A consumer wishes to "consumes" data items from the queue and, to do so, must wait on a boolean predicate [the queue is non-empty]. The producer generates data items that it deposits in the shared queue. As it adds items, it should notify any "waiters" that they should recheck.

Bottom line: conditions are not helpful here. You don't even need mutexes in your example. And conditions must be used with a mutex anyway (the call to pthread_cond_wait takes as input the address of a mutex to atomically release the lock and sleep on the condition). mutex are for mutual exclusion . Conditions are meant for signaling . Two very different purposes.