Why Does My Multithreaded Program Hang When Compiled with Optimization?

Multithreading Program Stuck in Optimized Mode

Problem:

A simple multithreading program behaves unexpectedly when compiled in optimized modes. While the program runs normally in debug mode or with -O0, it becomes stuck when compiled with -O1, -O2, or -O3.

Solution:

The issue lies in the non-atomic access to the finished variable. In a multithreaded environment, two threads accessing a non-guarded, non-atomic variable can lead to undefined behavior. To fix this, the finished variable should be made atomic.

Fix:

#include <iostream>
#include <future>
#include <atomic>

static std::atomic<bool> finished = false;

int func()
{
    size_t i = 0;
    while (!finished)
        ++i;
    return i;
}

int main()
{
    auto result = std::async(std::launch::async, func);
    std::this_thread::sleep_for(std::chrono::seconds(1));
    finished = true;
    std::cout << "result = " << result.get() << std::endl;
    std::cout << "\nmain thread>

Explanation:

By using std::atomic, we ensure that the finished variable is accessed and modified in an atomic manner, preventing race conditions and undefined behavior.

Additional Notes:

• Using volatile for this purpose is not recommended, as it is not guaranteed to provide the necessary synchronization.
• The compiler can optimize the execution of code, including variable access patterns. It is important to use appropriate synchronization primitives to ensure correct program behavior.

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