CODEWITHSUNDEEP
c++multithreading
mans
mans

Reputation: 18168

Can I write to a variable in a thread and read it in another thread in c++

I know that I can use semaphore or mutex and ..., but I like to know if there is any problem if I write to a variable in one thread and read it in another thread? something such as this:

#include <iostream>
#include <thread>

volatile int value=0;

int main()
{
     auto workerThread = new std::thread([&]()
        {
           for(int i=0;i<10000;++i)
           {
                value=i;
                std::this_thread::sleep_for(std::chrono::milliseconds(i*100));
            }
        }
     );

     while(true)
     {
            std::cout<<value<<std::endl;
            if(value >1000) break;
            std::this_thread::sleep_for(std::chrono::milliseconds(10));
     }
}

Is it a safe application and there is no smell in it?

Without any complexity, can I improve it?

Upvotes: 0

Views: 75

Answers (1)

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385144

No, it's not "safe".

Whoever told you that volatile was what you needed to make this safe was wrong.

Either wrap access to value in a mutex or semaphore, or make it an std::atomic<int>.

Also note that:

std::cout<<value<<std::endl;
if(value >1000) break;

may each read different value. You may want to read it only once:

int v = value;
std::cout << v << std::endl;
if(v > 1000) break;

Upvotes: 7

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