并发环境中的 C++11 std::vector [英] C++11 std::vector in concurrent environment

问题描述

我在 C++11 中运行多线程代码时遇到问题(段错误).这是代码:

I had an issue (segfault) running a multithreaded code in C++11. Here it is the code:

#include <vector>
#include <thread>

std::vector<int> values;
int i;

void values_push_back()
{
    values.push_back(i);
}

int main()
{
    while(true)
    {
        std::vector<std::thread> threads;

        for(i=0; i<10; ++i)
        {
            std::thread t(values_push_back);
            threads.push_back(std::move(t));
        }
        for(i=0; i<10; ++i)
            threads[i].join();
    }

    return 0;
}

这里是 gdb 的回溯:http://pastebin.com/5b5TN70c

And here the backtrace on gdb: http://pastebin.com/5b5TN70c

这有什么问题?

推荐答案

这和搬家无关.

多个线程在同一个 vector 上执行 vector::push_back() 但是vector::push_back() 不是线程安全的.vector的修改需要同步.

Multiple threads are executing vector::push_back() on the same vector but vector::push_back() is not threadsafe. The modifications to the vector need to be synchronized.

std::mutex 可能是用于同步对 push_back() 的调用:

std::vector<int> values;
std::mutex values_mutex;

void values_push_back()
{
    values_mutex.lock();
    values.push_back(i);
    values_mutex.unlock();
}

此外，变量 i 在线程之间共享而没有同步，这将导致竞争条件(这可能的结果是重复的 int 添加到矢量).考虑将 int 值作为参数传递给线程以避免这种情况:

Also, the variable i is being shared among threads without synchronization which is will result in a race condition (a possible outcome of this is duplicate ints added to the vector). Consider passing the int value as an argument to the thread to avoid this:

std::vector<int> values;
std::mutex values_mutex;

void values_push_back(int i)
{
    values_mutex.lock();
    values.push_back(i);
    values_mutex.unlock();
}

for (int i = 0; i < 10; ++i)
{
    threads.push_back(std::thread(values_push_back, i));
}

for (auto& t: threads) t.join();

<小时>

正如 bamboon 所评论的，更喜欢 std::lock_guard 以确保如果 push_back() 抛出(在这种情况下可能只是 bad_alloc() 但如果 vector 更改以容纳具有抛出构造函数的更复杂的对象，则变得更加重要):

---

As commented by bamboon prefer std::lock_guard to ensure the lock is released if push_back() throws (which in this case could only be bad_alloc() but if the vector changes to hold more complex objects that have throwing constructors it becomes more important):

void values_push_back(int i)
{
    std::lock_guard<std::mutex> lk(values_mutex);
    values.push_back(i);
}

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