CODEWITHSUNDEEP
c++c++11volatile
Šimon Tóth
Šimon Tóth

Reputation: 36451

Volatile in C++11 - follow-up

This is a follow-up to Volatile in C++11

In the question, I was told that the following code is exhibiting undefined behavior in C++11, which confused me greatly. Is there any material that I can read (sections of the standard perhaps) concerning the behavior of volatile in C++11?

Or can someone please explain where the problem lies?

#include <iostream>
#include <chrono>
#include <thread>
using namespace std;

volatile int notify;

void watcher()
{
    this_thread::sleep_for(chrono::seconds(2));
    notify = 1;
    cout << "Notification sent." << endl;
}

int main()
{
    thread(watcher).detach();

    notify = 0;
    while (!notify)
    {
        cout << "Waiting." << endl;
        this_thread::sleep_for(chrono::seconds(1));
    }

    cout << "Notification received." << endl;

    return 0;
}

Upvotes: 1

Views: 190

Answers (1)

Kerrek SB
Kerrek SB

Reputation: 477512

The standard describes the memory model, and in particular the notion of a "data race" (Section 1.10). It is fairly obvious that your code has a data race for the variable notify, and thus undefined behaviour.

To fix the problem, either guard access to notify by a lock, or make it an atomic variable such as an std::atomic<int>.

Upvotes: 7

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