C++中怎么避免數據競爭?

這期內容當中小編將會給大家帶來有關C++中怎么避免數據競爭，文章內容豐富且以專業的角度為大家分析和敘述，閱讀完這篇文章希望大家可以有所收獲。

Reason（原因）

Unless you do, nothing is guaranteed to work and subtle errors will persist.

除非你做到了，否則沒有任何東西可以保證動作，微妙的錯誤還會繼續存在。

Note（注意）

In a nutshell, if two threads can access the same object concurrently (without synchronization), and at least one is a writer (performing a non-const operation), you have a data race. For further information of how to use synchronization well to eliminate data races, please consult a good book about concurrency.

簡而言之，如果兩個線程可以（不進行任何同步）并發訪問同一個對象，至少一個線程執行寫操作（執行非常量操作），就會發生數據競爭。為了獲得如何更好地使用同步以消除數據競爭的進一步信息，請查閱有關并發的經典書籍。

Example, bad（反面示例）

There are many examples of data races that exist, some of which are running in production software at this very moment. One very simple example:

有關數據競爭的例子非常多，有些就發生于正在運行的產品級軟件。下面是很簡單的例子：

int get_id()
{
  static int id = 1;
  return id++;
}

The increment here is an example of a data race. This can go wrong in many ways, including:

代碼中的增量操作就是數據競爭的例子。出錯的方式可以有很多種，包括：

• Thread A loads the value of id, the OS context switches A out for some period, during which other threads create hundreds of IDs. Thread A is then allowed to run again, and id is written back to that location as A's read of id plus one.

• 線程A獲取id的值之后操作系統上下文從A中退出一段時間，這時另外的線程生成了幾百個ID。接著線程A繼續運行，這時id重新被寫入，而值是A讀取的局部變量加1之后的結果。
  

• Thread A and B load id and increment it simultaneously. They both get the same ID.

• 線程A和B同時獲取id并加1。它們得到同樣的ID。
  

Local static variables are a common source of data races.

局部靜態變量是數據競爭的常見來源。

Example, bad（反面示例）:

void f(fstream&  fs, regex pattern)
{
    array<double, max> buf;
    int sz = read_vec(fs, buf, max);            // read from fs into buf
    gsl::span<double> s {buf};
    // ...
    auto h2 = async([&] { sort(std::execution::par, s); });     // spawn a task to sort
    // ...
    auto h3 = async([&] { return find_all(buf, sz, pattern); });   // spawn a task to find matches
    // ...
}

Here, we have a (nasty) data race on the elements of buf (sort will both read and write). All data races are nasty. Here, we managed to get a data race on data on the stack. Not all data races are as easy to spot as this one.

這里，保存在buf中的元素會發生（嚴重的）數據競爭（排序既包含讀操作也包含寫操作）。沒有哪個數據競爭是不嚴重的。代碼中的數據競爭發生在堆棧中的數據。不是所有的數據競爭都像本例這樣容易被發現。

Example, bad（反面示例）:

// code not controlled by a lock

unsigned val;

if (val < 5) {
    // ... other thread can change val here ...
    switch (val) {
    case 0: // ...
    case 1: // ...
    case 2: // ...
    case 3: // ...
    case 4: // ...
    }
}

Now, a compiler that does not know that val can change will most likely implement that switch using a jump table with five entries. Then, a val outside the [0..4] range will cause a jump to an address that could be anywhere in the program, and execution would proceed there. Really, "all bets are off" if you get a data race. Actually, it can be worse still: by looking at the generated code you may be able to determine where the stray jump will go for a given value; this can be a security risk.

現在，編譯器不知道val會被修改，因此編譯結果很可能是一個帶有五個分支的跳轉表。那么一旦val的值超越了范圍[0..4]，就有可能調轉到程序的任何位置并從那里繼續執行。真的，一旦發生了數據競爭，結果會怎么樣誰也不知道。實際上，還有可能更壞：通過檢查生成的代碼，你或許可以準確算出針對一個給定的值，程序可以跳轉到什么位置。這可能成為一個安全風險。

Enforcement（實施建議）

Some is possible, do at least something. There are commercial and open-source tools that try to address this problem, but be aware that solutions have costs and blind spots. Static tools often have many false positives and run-time tools often have a significant cost. We hope for better tools. Using multiple tools can catch more problems than a single one.

有一些是可能的，至少做點什么。有些商用和開源工具試圖定位這些問題，但是需要注意的是：解決方案需要成本并存在盲區。靜態工具會產生很多誤報，而運行時工具通常需要巨大的成本。我們希望出現更好的工具。使用多種工具會比單一工具捕捉更多的錯誤。

There are other ways you can mitigate the chance of data races:

存在另外的方法可以降低數據競爭的可能性：

• Avoid global data

• 避免全局數據
  

• Avoid static variables

• 避免靜態數據
  

• More use of value types on the stack (and don't pass pointers around too much)

• 在堆棧上更多地使用值類型（并且不要來回傳遞指針）
  

• More use of immutable data (literals, constexpr, and const)

• 更多地使用不可修改的數據（literals, constexpr, and const）

  
  

上述就是小編為大家分享的C++中怎么避免數據競爭了，如果剛好有類似的疑惑，不妨參照上述分析進行理解。如果想知道更多相關知識，歡迎關注億速云行業資訊頻道。