Thread-Safe and Re-entrancy
===

這篇起源自前一場面試，因為筆者這個嘴砲性格，遇到不如意總會說個幾句
於是有這篇詳述當場面試官的「錯誤」

一句話概括這個標題：「不是所有 thread-safe 都是 reentrant」，千萬不要誤把 thread-safe 就當作 reentrance。

- 前情背景
- 會讓筆者下定決心寫這邊部落格，除了面試官觀念錯誤之外，還對筆者**搖頭嘆氣**。
- 其實以前就有寫過[文章](https://hackmd.io/@25077667/Hky5QyIXu)，但當時沒有寫好重寫會更好。

## 定義
Reentrancy:
- The function executed by the same thread, or simultaneously executed by another thread and still correctly complete the original execution.
- 註：C++ 標準對 reentrancy 的定義並不 formal: [§16.4.6.9](https://eel.is/c++draft/reentrancy)
- [Dictionary](https://dictionary.cambridge.org/dictionary/english-chinese-traditional/re-entrant)
- Able to be run on several processors at the same time and be interrupted and called again without affecting what is happening on the other processors.

Thread-safe:
- Implementation is guaranteed to be free of race conditions when accessed by multiple threads simultaneously.

## 四個例子

### Thread-Safe and Reentrant

unsigned long long fib(unsigned long long index) {
    if (!index)
        return 0;
    else if (index == 1 || index == 2)
        return 1;
        
    return fib(index - 1) + fib(index - 2);
}

### Not Thread-Safe and Reentrant

static int data = 0;

int foo() {
    data++;

    return 42;
}

這邊需要注意到，reentrancy 保證到的是重入執行結果的正確性，但不會保證中間過程的狀態。
其他只能說是總結 reentrancy 的常見屬性，不是 reentrancy 的 requirements.

### Thread-Safe and Not Reentrant

void write_file(const std::string &filename, const std::vector<char> data) {
    static std::mutex m_;
    std::lock_guard<std::mutex> lock(m_);
    
    auto ofs = std::ofstream(filename, std::ios::out | std::ios::binary);
    for (const auto& byte : data)
        ofs << byte;
}

### Not Thread-Safe and Not Reentrant

static int g_data;

void write_file(const std::string &filename, const std::vector<char> data) {
    g_data++; // manipulate the g_data
    
    static std::mutex m_;
    std::lock_guard<std::mutex> lock(m_);
    
    auto ofs = std::ofstream(filename, std::ios::out | std::ios::binary);
    for (const auto& byte : data)
        ofs << byte;
}