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

问题描述

我在运行C ++ 11中的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的回溯记录: http://pastebin.com/5b5TN70c

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

那是怎么回事?

推荐答案

这与移动无关.

多个线程正在同一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.

可以使用 std::mutex 将呼叫同步到:

A std::mutex could be used to synchronize the calls to push_back():

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，这将导致竞争状态(可能的结果是将重复的int添加到vector中).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();

---

正如 bamboon 所评论的那样，更喜欢

---

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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